Bull Volcanol (2012) 74:1187–1211 DOI 10.1007/s00445-012-0594-0

RESEARCH ARTICLE

Reconstruction of the volcanic history of the Tacámbaro-Puruarán area (Michoacán, México) reveals high frequency of Holocene monogenetic eruptions Marie-Noëlle Guilbaud & Claus Siebe & Paul Layer & Sergio Salinas

Received: 6 August 2011 / Accepted: 2 March 2012 / Published online: 13 May 2012 # Springer-Verlag 2012

Abstract The 690 km2 Tacámbaro-Puruarán area located at the arc-front part of the Michoácan-Guanajuato volcanic field in the Trans-Mexican Volcanic Belt (TMVB) records a protracted history of volcanism that culminated with intense monogenetic activity in the Holocene. Geologic mapping, 40Ar/39Ar and 14C radiometric dating, and whole-rock chemical analyses of volcanic products provide insights to that history. Eocene volcanics (55–40 Ma) exposed at uplifted blocks are related to a magmatic arc that preceded the TMVB. Early TMVB products are represented by poorly exposed Pliocene silicic domes (5–2 Ma). Quaternary (<2 Ma) volcanoes (114 mapped) are mainly scoria cones with lavas (49 vol.%), viscous lava flows (22 vol.%), and lava shields (22 vol.%). Erupted products are dominantly either basaltic andesites (37 vol. %), or andesites (17 vol.%), or span across both compositions (28 vol.%). Basalts (9 vol.%), dacites (4 vol.%), shoshonites (2 vol.%), and other alkali-rich rocks (<3 vol.%) occur subordinately. Early-Pleistocene volcanism was bimodal (dacites and basalts) and voluminous while since 1 Ma smallvolume eruptions of intermediate magmas have dominated.

Higher rates of lithospheric extension in the Quaternary may have allowed a larger number of small, poorly evolved dikes to reach the surface during this period. Eruptive centers as old as 1.7 Ma are aligned in a NE direction parallel to both, basement faults and the direction of regional compressive stress, implying structural control on volcanic activity. Data suggest that volcanism was strongly pulsatory and fed by localized lowdegree partial melting of mantle sources. In the Holocene, at least 13 eruptions occurred (average recurrence interval of 800 years). These produced ~3.8 km3 of basaltic andesitic to andesitic magma and included four eruptions dated at ~1,000; 4,000; 8,000; and 11,000 years BC (calibrated 14C ages). To date, this is one of the highest monogenetic eruption frequencies detected within such a small area in a subduction-related arc-setting. These anomalous rates of monogenetic activity in an area with thick crust (>30 km) may be related to high rates of magma production at depth and a favorable tectonic setting. Keywords Radiocarbon dating . Ar–Ar dating . Structural control . Morphometry . Trans-Mexican Volcanic Belt . Volcanic hazard

Editorial responsibility: J.D.L. White Electronic supplementary material The online version of this article (doi:10.1007/s00445-012-0594-0) contains supplementary material, which is available to authorized users. M.-N. Guilbaud (*) : C. Siebe : S. Salinas Departamento de Vulcanología, Instituto de Geofísica, Universidad Nacional Autónoma de México, Coyoacán, México D.F., México e-mail: [email protected] P. Layer Geophysical Institute and Department of Geology and Geophysics, University of Alaska Fairbanks, Fairbanks, AK, USA

Introduction The Trans-Mexican Volcanic Belt (TMVB) is an active volcanic arc related to the subduction of the Cocos Plate underneath the North American Plate along the Middle America trench (Fig. 1). This arc is peculiar because, rather than being parallel to the deep-sea trench, its long axis is oblique by ca. 15º with respect to the trench; in addition, it is highly irregular in width (Fig. 1). Both features may be related to the shallow and changing dip of the subducting plate (Pardo and Suárez 1995) and to the active deformation of the overlying continent (Tibaldi 1992). Another

1188

Bull Volcanol (2012) 74:1187–1211

CSP

NORTH AMERICA PLATE

TF

21°

Guadalajara

P. Vallarta

Colima graben

20°

RIVERA PLATE 19°

MGVF

X

Paricutin

Tancítaro

México City

Tacámbaro

Tzitzio Gap

Colima Rive

ra F ract ure Z

Jorullo BA L

one

SA

S

SIE

RR

AM

DE

laO

PR

AD

18°

Ch ap a

RE

ax a

ES

SIO

DE

N

LS

PACIFIC PLATE

Study area

ca fau lt

zo ne

UR

17°

COCOS PLATE Strato-volcanoes

107°

105°

Coastal intrusives

103°

Inland Tertiary volcanics

101°

99°

Fig. 1 Location of the study area (yellow rectangle including the city of Tacámbaro) in the Trans-Mexican Volcanic belt (TMVB). Background image is a digital elevation model of Mexico showing relative elevation in different scales of grey. The TMVB crosses the country at ca. 19.5°N and coincides with the southern part of the elevated dark-

grey area (Mexican Altiplano). The high concentration of small cones in the Michoacán-Guanajuato volcanic field (MVGF) is noticeable. Other volcanic fields mentioned in the text are: CSP (Ceboruco-San Pedro), TF (Tequila), ZVB (Zitácuaro-Valle del Bravo), SCVF (Sierra Chichinautzin), and X (Xalapa)

particularity of the TMVB is the relative scarcity and irregular distribution of stratovolcanoes, which contrasts with the high abundance of scoria cones and other small monogenetic eruption centers (ca. 3000) (Demant 1978; Tibaldi 1992). The highest concentration of monogenetic volcanoes occurs in the western–central part of the TMVB where the arc reaches a maximum width of almost 200 km and where there is only one stratovolcano, the probably extinct (~100 km3) Tancítaro (Fig. 1). This ~40,000 km2 area, named the MichoacánGuanajuato Volcanic Field (MGVF) by Hasenaka and Carmichael (1985), hosts the only two monogenetic volcanoes of the TMVB born in historical times: Jorullo (1759–1774) and Paricutin (1943–1952) (Fig. 1). Both eruptions caused serious social, environmental, and economic disruptions (Luhr and Simkin 1993; Guilbaud et al. 2009a). The hazard of future monogenetic activity in this area has not yet been adequately evaluated, and reliable estimates of eruption recurrence rates are not available. Furthermore, the relationships between the distribution of the volcanoes with respect to their age, eruptive style, composition, and the fracturing pattern of the crust still need to be clarified in order to establish the modus operandi of monogenetic volcanism in this region and thus facilitate hazard assessments and development of monitoring strategies. Geochronology data are especially needed to define the

spatio-temporal behavior of the volcanism (e.g., Bebbington and Cronin 2011). This information is also critical to constrain petrogenetic models, and thus contribute to a better understanding of this volcanic arc in particular and of monogenetic volcanism in arc settings worldwide in general. This paper focuses on the Tacámbaro-Puruarán area, a region located directly to the NE of the historic Jorullo volcano, near the southern margin of the MGVF. This area was chosen because satellite images show a large quantity of young eruptive centers and sparsely vegetated lava flows in a region that had not been studied in detail before and hence promised to yield a wealth of interesting new results that could be of relevance to hazard assessment. Note that this area is not considered as representative of the entire MGVF but instead may be one of the most, if not the most, active part of the MGVF in the Holocene. Based on detailed mapping, volumes of erupted products were estimated with the aid of a digital elevation model (DEM). In addition, compositional and new geochronologic data allowed us to estimate magma eruption rates and to determine variations in the relative proportion of different magma types erupted during the course of time. Exposure of the basement rocks provides insight into the role of pre-existing upper-crustal structures on Plio-Quaternary volcanic activity. All data

Bull Volcanol (2012) 74:1187–1211

1189

were integrated to reconstruct the volcanic history of this area since the Eocene. Results are compared with those obtained previously by similar studies in other areas within the MGVF, as well as other volcanic fields along the TMVB and other volcanic arcs in the world. These comparisons provide insights on the main factors that control eruption rates at monogenetic fields. C. LA MAGDALENA

to Ario C. LA CRUZ

0.05 ± 0.04 Ma

98

50

0.12 ± 0.14 Ma

C.TECARIO

43

47 48

11190+190/-185 yr BP

2.68 ± 0.03 Ma

old cone covered

8715+145/-140 yr BP 39

41 40

36 37

C. LAS CARRETAS C. ZIHUATZIO

73

C. LA VENTANA 2

0.59 ± 0.44 Ma 88 89

78

0.38 ± 0.18 Ma C. LAS CRUCES

83

to Ario

S

82

51.9 ± 0.6 Ma

64 A

65

66 26

1

C. NOMBRE DE DIOS

67

UZ CR

L C.

21

C. LA VENTANA 1

96

1

56 57

C. EL PINO

0.60 ± 0.05 Ma

20

Puruarán C. PETEMBO

M. CIENEGUILLAS 79 C. LAS CUEVAS

28

Cahulote

M. LOS MORELOS

LOBO

22

1.70 ± 0.02 Ma

C. DON NATO

M. EL MALPAIS C. LOS

0.38 ± 0.01 Ma

63

1.64 ± 0.04 Ma 30

77 M. EL CURATO

AL

84

24 25

31

Ojo de Agua de Chupio

23

0.32 ± 0.02 Ma

C. COLORADO 1

C. ALTO

C. EL LINDERO

19

C. ZIHUATANEJO C. EL TIGRE 2

87 85

55

18

34

76

1.74 ± 0.11 Ma Pedernales

CIN

3505+115/-110 yr BP SC SE 2815 ± 125 yr BP 86 LA

6 17

33

61

EN

LAS

IL OB

0.98 ± 0.06 Ma

C. LA TINAJA

Los Hacheros

C. EL CAJETE

LOS 103 Parocho

ERIL OTR CS. P

0.11 ± 0.04 Ma 16 14

32

60

62

EL

75

C. EL CAPUCIN

13

PUERTO C. LA PALMA C. COLORADO 5 LOS ATES

C. GRANDE C. EL ZOYATE

MESA DE LA MUERTA

M.

74 C. CIPRES C. LAS CANALEJAS

0.96 ± 0.07 Ma

35

38

C. LAS VEGAS

C. LOS COYOTES

12

C. EL JABALI

15 42

0.08 ± 0.04 Ma

C. MIRADOR Cutzaróndiro

C. EL COCO

C. EL LEÓN

C. ANIL

1.81 ± 0.03 Ma

9 10 11

C. EL TIGRE 1

C. LAS TABLAS

0.53 ± 0.01 Ma

68 44 54

3

5325 ± 130 yr BP

101

Cucha

53

C. LA LAGUNA

4

MALPAIS DE CUTZARÓNDIRO

100

102

C. PINO SOLO C. EL SALITRILLO

5115 ± 145 yr BP

71

4.18 ± 0.08 Ma

C.HUECO

52 2

0.06 ± 0.05 Ma

8

C. MARGARO

51

C. LA CRUZ 2

0.62 ± 0.03 Ma

Tacámbaro

Tecario

0.73 ± 0.02 Ma

C. LAS FLORES

C. PARTIDO

5

C. CARLTZIO 7

C. SAN JOSE

99 C. LA BARRA

1.51 ± 0.04 Ma

45

46

0.22 ± 0.04 Ma

72 49

70

69

C.COLORADO 3

0.78 ± 0.02 Ma

C. SOPOMIO C. LAS ANIMAS

C. ALEBINO

to Morelia

C. COLORADO 4

0.73 ± 0.04 Ma

0.78 ± 0.39 Ma

C. EL GATO

The small city of Tacámbaro (Lat. N19°14’, Long. W101°27’, Alt. 1755 m) is located 40 km NE of the Jorullo volcano, near the southern margin of the MGVF and at the limit between the Mexican Altiplano to the north and the Balsas Depression to the south (Fig. 1). The study area (N19°00’ to N19°15’, W101°

C. EL MALACATE C. EL CALABAZO

97 El Tepamal

Geographic, tectonic, and geologic settings

29

C. EL DIVISADERO

-0.01 ± 0.03 Ma

27

M. EL CARACOL

MESA CALZADA

C. CHATO

59

C. EL PIÑO

C. CANALES

0.34 ± 0.04 Ma

Tavera

95

92

C. LA PALMA

C. VERDE

80

-0.05 ± 0.15 Ma

C. EL CARACOL

94

93

91

MESA TURICATO

Turicato

C. EL TECOLOTE LOS POZOS

0.06 ± 0.02 Ma

Los 81 Puentes

58

C. COLORADO 2

C. SAN JOSE

C. EL SOSAL

M. EL BURRO

42.2 ± 1.3 Ma

90

105 STA ANA

C. EL ROSATE

C. EL CASCALETE

M. EL ATRAVESANO

CHUPADERITO M. LA CADENA

C. EL METATE

0.51 ± 0.04 Ma

EL CHOCOLATE

104

Sampling locations 2815 ± 125 yr BP Radiocarbon date of paleosol underlying ash-fallout deposit Ar-Ar date of lava

4.18 ± 0.08 Ma (whole rock,

plateau age) Paved road Tacambaro and village

Scoria cone Lava dome

Quaternary lavas Holocene (<11 ka)

Pliocene rocks

Late-Pleistocene (100-11 ka)

Uplifted blocks (thick line = break in slope)

Mid-Pleistocene (1-0.1 Ma)

Hydrothermally-altered rocks Pre-Pliocene sedimentary sequence

Maar Inferred lava flow direction Recent sediments

Fig. 2 a Geological map of the Tacámbaro area. New radiometric dates and sample locations (see text) are also indicated. Location of the study area is shown on Fig. 1. b Simplified version of a showing

Tertiary volcanics

Early-Pleistocene (2-1 Ma) Early-to-Mid Pleistocene lavas covered by younger volcanics and sediments

N 3 km

faults and vent alignments. Note that each alignment may not correspond to the surface expression of a single crustal fault but of a series of fault segments with similar directions. Legend same as a

1190

Bull Volcanol (2012) 74:1187–1211 C.COLORADO 3 C. SOPOMIO

Tectonic features

C.TECARIO

Fault

C. LAS ANIMAS

Vent alignments C. SAN JOSE

C. LAS FLORES MALPAIS DE CUTZARÓNDIRO

C. MARGARO

C. ANIL

19°10

MESA DE LA MUERTA C. LA VENTANA 2 C. CIPRES

PUERTO LOS ATES

C. EL CAJETE

C. LA TINAJA C. LA PALMA

C. EL ZOYATE C. EL CAPUCIN

C. EL PINO C. ALTO

C. LA VENTANA 1

19°05

C. PETEMBO

N

C. CHATO C. EL PIÑO

C. VERDE

3 km

C. EL CARACOL

19°00

C. EL TECOLOTE

101° 35

101° 30

101° 25

Fig. 2 (continued)

40’ to W101°20’) is bordered to the south by an elevated block capped by a lava plateau consisting mostly of an old (Eocene) volcanic sequence (Fig. 2). Farther south, at the base of this same block, Jorullo volcano was born in 1759 (Guilbaud et al. 2011). This early Tertiary volcanic sequence belongs to a belt of plutonic and volcanic rocks that borders the Pacific coast of southwestern Mexico (areas in light-grey on Fig. 1) and formed during a major magmatic episode that preceded the TMVB (Schaaf et al. 1995; Morán-Zenteno et al. 1999). The eastern border of the area corresponds to a major indentation in the TMVB known as the Tzitzio gap (e.g., Demant 1978; Blatter and Hammersley 2010) (Fig. 1). This gap in the recent volcanic cover coincides with the exposure of partly metamorphosed Jurassic to early Cretaceous sediments forming the NNWoriented Tzitzio anticline (Pasquaré et al. 1991; Garduño-

Monroy and Gutiérrez-Negrín 1992; Garduño-Monroy et al. 1999). The tectonic setting of the area is further described by Guilbaud et al. (2011), who also provide a discussion of the southern part of the MGVF (Morelia-Jorullo-Paricutin area) which includes the Tacámbaro-Puruarán area and features prominent NE-striking volcano-alignments and east–northeast (ENE)-oriented faults.

Methodology A 690 km2 area covering the 1:50,000 topographic sheet of Tacámbaro (E14A42) published by the Instituto Nacional de Estadística Geografía e Informática (INEGI) was mapped using aerial photos (also available from INEGI),

Bull Volcanol (2012) 74:1187–1211 Fig. 3 Stratigraphic sections of ash fallout exposures from which radiocarbon dates were obtained. Location of these sections is indicated on Fig. 2

1191

C. La Tinaja fallout TAC-0709

GPS: N19°11'21.1" W101°30'09.4" 1415 m

40 cm

TAC-08100

GPS: N19°12'06.9" W101°31'08.7" 1567 m

10 cm

immature light-brown soil

thin silty (immature) soil

130 cm

laminated fine to coarse ash scoriaceous fallout from C. La Tinaja

176 cm laminated fine to coarse ash scoriaceous fallout from C. La Tinaja 2-16 cm

indurated fine silt with vegetation casts

1 cm

5325 +/- 130 yrs BP (Lab no. A-14700, d13C=-21.1‰)

2 cm

5115 +/- 145 yrs BP (Lab no. A-14875, d13C=-19.4‰)

ochre-brown, well-structured paleosol

> 59 cm

ochre-brown, well-structured paleosol

TAC-08100

C. El Zoyate fallout TAC-0885

GPS: 19°06’59.5’’ 101°38’25.8” 1982 m

GPS: 19°07’09.7’’ 101°38’17.7” 1983 m reworked ash

TAC-0886

reworked ash grading into present silty-sandy soil 10 cm

10 cm

alternating fine scoria lapilli layers and fine to coarse ash layers

70 cm

100 cm

indurated fine ash with abundant casts of vegetation

3 cm

1 cm

> 20 cm

B: 2835 +/- 45 yrs BP (Lab no. AA81913, d13C=-22.9‰) ochre-brown paleosol

alternating fine scoria lapilli layers and fine to coarse ash layers

4 cm 2 cm

> 100 cm

A: 2815 +/- 125 yrs BP (Lab no. A-14870, d13C=-23.3‰)

indurated fine ash with abundant plant casts B: 2195 +/- 40 yrs BP (Lab no. AA81914, d13C=-22.5‰) red, clayey, mature paleosol

A: 3505 +115 /-110 yrs BP (Lab no. A-14872, d13C=-23.4‰)

paleosol TAC-0886

C. Grande (?) fallout TAC-0739

GPS: 19°10’15.8’’ 101°36’25.6” 2205 m brown silty soil

60 cm coarse to fine ash fallout, partly reworked

65 cm

8715 +145/-140 yrs BP (Lab no. A-14749, d13C=-23.4‰)

3 cm

>77 cm red, mature, clayey soil

paleosol TAC-0739

C. El Gato (?) fallout TAC-0898

GPS: 19°13’04.1’’ 101°36’15.4” 2128 m modern soil

>50 cm

altered laminated fine ash to lapilli scoria fallout 190 cm 11,190+190/-185 yrs BP (Lab no. A-14874, d13C=-24.1‰) >50 cm

2 cm

red, mature, clayey paleosol

paleosol TAC-0898

paleosol

1192

satellite images (including those available from Google Earth), a 20 m-resolution DEM, and 2 months of field work (Fig. 2). Thirty-three volcanic rock samples were dated by the 40 Ar/39Ar method at the Geophysical Institute, University of Alaska, Fairbanks. Procedures are described in Layer (2000), Guilbaud et al. (2011), and the Appendix. Six paleosol samples underlying young ash fallout deposits and two indurated ash samples holding plant casts (Fig. 3) were radiocarbondated using the conventional and accelerator mass spectrometer techniques, respectively, at the Radiocarbon Laboratory, Department of Geosciences, Tucson, Arizona. The ages obtained (years BP) were converted into calendar years (years BC) using the program Calib 6.0.1 (Stuiver and Reimer 1993). Compositional data (minor and major elements) were acquired at Activation Laboratories, Ancaster, Canada (for analytical procedures, see Agustín-Flores et al. 2011). Magma types were defined from the major element data recalculated to 100 %, following recommendations from the International Union of Geological Sciences (Le Maitre 2002). The complete chemical dataset, which includes SrNd-Pb isotopic data, will be presented in another paper discussing the petrogenesis of the magmas. The area and volume of exposed Plio-Quaternary volcanic products were quantified using a specially constructed 10 m-resolution DEM and ARCGIS and ILWIS GIS software (ITC, International Institute for Aerospace Survey and Earth Science 2001). The DEM was created with ILWIS by rasterizing the 10 m contour levels from the 1:50,000 scale topographic map of INEGI. Errors linked to the interpolation technique are ca. 10 m in vertical elevation (Pérez-Vega and François-Mas 2009) and considered null horizontally (INEGI does not report errors in topographic data). The volume of cones and domes was obtained by substracting the topography located below a certain level taken as the base of the edifices (see formula 8 in Table 1 of Salinas and López-Blanco 2010). When the edifice is built on inclined ground (i.e., the base is at different elevations around the edifice), an average level was considered. The volume of lavas (including lava shields) was calculated by multiplying their area of coverage (see formula 4 in Table 1 of Salinas and López-Blanco 2010) by their average thickness. Average thicknesses were derived from field measurements and refined by drawing multiple profiles from the DEM. Systematic errors in the measurements were estimated to be 3 % for the area covered by lavas, 5 m for their average thicknesses, 1 % for the area occupied by cones or domes, and 20 % for volumes calculated by digital truncation (cones, domes, and some lava shields; see Table DR2). Based on somewhat crude estimates from field observations, these volumes were converted to dense rock equivalent (DRE) assuming a 30 vol.% of empty spaces for scoria cones and maars and 10 vol.% vesicularity for cone-associated lavas, while the

Bull Volcanol (2012) 74:1187–1211

vesicularity of viscous flows and domes was neglected. The propagation of errors was considered when computing total errors based on these estimates. Morphometric parameters (cone and crater height and width, slope angles) were calculated for dated scoria cones, in order to test a possible evolution of these parameters with time. The morphology of the dated cones was also characterized qualitatively using several parameters indicative of their degree of degradation (e.g., soil thickness above lava, width and density of gullies, sharpness of crater rims and lava margins). Morphometric measurements were made on the 10 m-elevation DEM using ILWIS. Basic parameters were measured as shown in Fig. 4 (note that Hco corresponds here to cone maximum height). A map showing slope angles was generated for the entire area using the Shape Tool (formula 9 in Table 1 of Salinas and LópezBlanco 2010). The data obtained from ca. 15 radial profiles across the outer slope of each cone was used to derive mean, median, and maximum slope. Inspection of the slope map indicated that the maximum slope typically corresponds to the upper part of the cones. Some of these parameters were already determined before in the work of Hasenaka and Carmichael (1985), but these authors did not provide any age control for the cones in this area, which is crucial for detecting evolutionary trends.

Geochronology and radiometric dating A geological map integrating dating information is presented in Fig. 2a. Volcanic alignments and faults are shown separately in Fig. 2b. Photos showing the diversity of volcanic morphologies encountered in the study area are shown in Fig. 5. Geological divisions are based on the 2007 USGS recommendations (http://pubs.usgs.gov/fs/2007/3015/ fs2007-3015.pdf). Radiometric dating A summary of the 40Ar/39Ar dating is provided in Table 1. Representative age plots are given in Fig. 6. Detailed information can be seen in the Appendix and Table DR1.xls (Online resource). Two samples representing Eocene volcanism were dated (Fig. 6a). Age spectra show evidence for argon loss and some alteration (as seen by younger ages). For Plio-Pleistocene rocks, age spectra (Fig. 6b) are flat with wellconstrained plateaus. Isochron ages are concordant, and initial 40 Ar/36Ar ratios were within 2-sigma of the atmospheric value. For samples older than about 100 ka, plateau ages are generally more precise, and these are used in the following discussion. For the youngest lavas (Late Pleistocene to Holocene), isochron ages are used (Fig. 6c, d). Only two of these samples (C. La Laguna and C. Sosal; Fig. 6c) had

Bull Volcanol (2012) 74:1187–1211 Table 1 Summary of Source

1193

40

Ar/39Ar radiometric dates

Sample #

Lat (°N) Long (°W) Integrated age (Ma)

Plateau age (Ma)

39

Ar (%) N MSWD Isochron age (Ma)

Init

N

MSWD

Eocene C. La Cruz 1 C. San José

0856 WR 08105 WR

19.120 19.029

101.424 101.424

49.4±0.4 40.9±0.4

51.9±0.6 42.2±1.3

58 88

3 5

1.25 13.7

C. La Cruz 2 C. La Cruz 2

08102 WR 08102 PL

19.221 19.221

101.404 101.404

4.00±0.07 4.95±0.05

4.18±0.08 5.29±0.13

72 68

5 4

1.4 4.76

C. La Cruz 2

08102 BI

19.221

101.404

4.52±0.08

4.80±0.11

62

6

C. El Hueco 0853 WR Early Pleistocene

19.214

101.453

2.64±0.07

2.68±0.03

93

5

– 44.2±0.8

244±15

6

7.10

4.37±0.13 275±5 –

7

0.53

0.37

5.17±0.22 278±7

7

0.53

1.06

2.70±0.03 289±6

6

1

Pliocene

C. El Salitrillo

0844 WR

19.210

101.421

1.79±0.03

1.81±0.03

80

3

0.69

1.76±0.04 297±8

5

2.31

M. El Encinal C. Petembo

0855 WR 0867 WR

19.148 19.105

101.443 101.485

1.74±0.11 1.71±0.03

1.74±0.11 1.70±0.02

100 92

7 8

0.82 0.37

2.54±0.41 275±11 1.68±0.03 298±4

7 9

0.30 0.35

C. La Ventana

0864 WR

19.123

101.479

1.63±0.04

1.64±0.04

98

8

1.67

1.66±0.16 296±6

9

1.54

C. Colorado Mid-Pleistocene

0869 WR

19.248

101.489

1.50±0.03

1.51±0.04

96

5

1.62

1.52±0.03 293±6

6

1.76

C. Potrerillos

08103 WR

19.170

101.480

0.88±0.23

0.98±0.06

93

7

0.47

1.06±0.10 294±3

10

0.43

C. Jabalí C. Colorado 5

0736 WR 0845 WR

19.167 19.238

101.572 101.528

0.81±0.06 0.78±0.02

0.96±0.07 0.78±0.02

70 98

3 9

1.90 0.33

1.09±0.05 286±3 0.80±0.05 293±5

8 10

0.48 0.30

C. Las Flores C. Tecario

0849 WR 0846 WR

19.212 19.236

101.583 101.543

0.75±0.02 0.70±0.03

0.73±0.02 0.73±0.04

75 76

6 4

1.95 0.77

0.70±0.05 302±10 0.75±0.05 291±10

8 4

2.32 0.97

C. Partido C. Cántaro C. La Barra

0705 WR 0896 WR 0851 WR

19.243 19.110 19.189

101.455 101.433 101.644

0.61±0.03 0.57±0.04 0.50±0.02

0.62±0.03 0.60±0.05 0.53±0.01

92 89 87

4 8 5

1.49 0.86 0.45

0.57±0.08 303±9 6 0.75±0.28 288±14 10 0.55±0.02 291±5 6

1.69 1.55 0.46

C. El Chocolate 08104 WR C. El Mirador 0879 WR C. Verde 0892 WR

19.004 19.095 19.062

101.379 101.560 101.489

0.33±0.05 0.40±0.01 0.35±0.03

0.51±0.04 0.38±0.01 0.34±0.04

56 72 79

3 3 5

1.31 1.15 0.85

0.58±0.03 290±3 0.37±0.03 298±10 0.54±0.12 280±10

4 3 7

0.90 2.16 0.37

C. Flogopitario C. Flogopitario C. Flogopitario

0863 WR 0863 BI#1 0863 BI#2

19.124 19.124 19.124

101.478 101.478 101.478

0.22±0.05 0.43±0.08 0.35±0.07

0.32±0.02 0.40±0.07 0.34±0.06

66 95 82

5 9 6

0.46 0.17 0.65

0.34±0.04 292±4 0.37±0.12 297±3 0.36±0.08 296±3

6 13 11

0.50 0.20 0.46

Cone on side of 0872 WR C. San José Late Pleistocene to Holocene C. El Malacatea 0897 WR C. Zihuatanejoa 0731 WR 0889 WR C. Don Natoa a 0715 WR C. El Coco C. La Laguna 0703 WR C. Sosal 0891 WR C. Las Animas 0743 WR C. Márgaro 0899 WR C. El Caracol 0727 WR C. Los Lobos 0882 WR La Alberca 0702 WR

19.225

101.56

0.25±0.04

0.22±0.04

94

3

0.15

0.18±0.08 299±5

4

0.64

19.245 19.121 19.103 19.166 19.204 19.041 19.230 19.205 19.070 19.073 19.215

101.593 101.533 101.616 101.530 101.470 101.590 101.576 101.616 101.541 101.648 101.459

–0.05±0.11 0.08±0.09 0.32±0.20 0.09±0.18 −0.22±0.11 −0.11±0.16 −0.003±0.031 0.02±0.03 0.07±0.02 0.06±0.02 0.049±0.009 0.05±0.01 0.01±0.02 0.02±0.02 −0.008±0.035 0.03±0.04 −0.02±0.05 −0.07±0.08 −0.08±0.03 −0.03±0.03 −0.10±0.05 0.01±0.04

75 60 73 59 96 92 92 91 88 85 76

5 5 7 4 3 4 4 5 5 4 4

0.92 0.83 1.87 2.02 1.76 0.25 0.22 0.27 2.49 0.51 0.21

280±9 288±4 282±6 281±4 292±6 292±6 292±6 289±9 298±6 290±2 290±2

8 10 7 6 4 6 4 7 7 15 15

0.54 2.47 0.48 0.81 2.37 0.19 0.06 0.50 2.79 0.90 0.90

0.78±0.39 0.59±0.44 0.38±0.18 0.11±0.04 0.08±0.04 0.06±0.02 0.05±0.04 0.12±0.14 −0.05±0.15 −0.01±0.03 0.06±0.05

All errors quoted at the 1-sigma level Preferred age for each sample is in bold (see text for justification) Sample type: WR whole Rrck, PL Plagioclase, BI Biotite; N number of steps used in plateau or isochron, MSWD mean square of weighted deviates a

These volcanoes were considered as of mid-Pleistocene age, despite their low plateau ages, on the basis of the degraded morphology of their cone or dome and the thick soil covering their lavas

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Bull Volcanol (2012) 74:1187–1211

Dcr Wcr

Hco

Wco Fig. 4 Method followed to measure basic cone shape morphometric parameters: Hco 0 cone height, Wco 0 cone width, Wcr 0 crater width, Dcr 0 crater depth. Note that in the case of a sloping underlying ground (varying elevation of cone base around the volcano), Hco was taken as the maximum height of the cone over the surrounding basement

cone directly to the SW of C. Grande. Possible sources of the 11,000 years BC deposit could be C. Márgaro or C. El Gato. However, lavas produced by C. Carretas are mugearites and C. Márgaro erupted an andesitic lava flow (loc. # 99), whereas the source cones and lavas associated to the dated ash layers are expected to be compositionally similar to the ash (medium-K basaltic andesites). Products from the other possible source cones were unfortunately not chemically analyzed. Hence, based on the available information, the best possible source for the 8,000 years BC tephra might be C. Grande, while C. Gato is the best candidate for the 11,000 years BC tephra. Relative dating: morphometric data

isochron ages that were significantly non-zero. The others are considered as having very young ages that are irresolvable using the technique due to the small amounts of radiogenic argon. Two (C. Zihuatanejo and C. El Malacate) had very little isotopic variability, and isochron ages are not precise (1-sigma error greater than 0.3 Ma). Based on their morphology, their age was defined as mid-Pleistocene. Of the other seven samples, two have negative isochron ages, but all are statistically “zero aged”, so, although the ages cannot be well constrained, our data indicate that they are most likely younger than 100 ka and some are likely younger than 50 ka. Of note is that many initial ratios were less than 295.5. We do not know if this is due to some initial fractionation of 36Ar in the lava or due to laboratory-induced fractionation. Conventional and calibrated radiocarbon ages are presented in Table 2. Note that, because atmospheric 14C concentrations fluctuated during certain periods of time, several possible calibrated ages exist for some of the conventional ages. Eight different calibrated ages are thus obtained for C. El Zoyate. The distribution of ages (Fig. 7) suggests that this eruption most likely occurred around 1,000 years BC. Five ages are obtained for C. La Tinaja (3,640–4,450 cal years BC), indicating that this eruption occurred around 4,000 years BC. Two other eruptions occurred at around 8,000 and 11,000 years BC, respectively (Fig. 7). The source of these two eruptions can be debated. Given that the thickness of the dated ash deposits at outcrop localities is 65 and 190 cm, respectively, and considering the isopach map of the Paricutin eruption as a typical case (Segerstrom 1950), the sources of the ashes should not be located more than 2.5 and 2 km away, respectively, from their location of exposure. The young age and grain size distribution of the tephra (mostly coarse ash and fine lapilli) suggest that the sources should be poorly eroded scoria cones possibly associated with lavas covered by poorly developed soils. Taking into account these constraints, the ca. 8,000 years BC ash fallout may have derived from C. Carretas, C. Grande, or the nameless

Morphometric data for dated Quaternary cones are reported in Table 3. Cone-height/cone-width ratios (Hco/Wco) and cone maximum slope (Smax) decrease noticeably from the Holocene–Late Pleistocene (Hco/Wco=0.20±0.03; Smax, 47 ± 7°) to the mid-Pleistocene (Hco/Wco = 0.18 ± 0.05; Smax=42±9°) and to the early Pleistocene (Hco/Wco= 0.14±0.05; Smax=37±3°) periods, but the values show considerable scatter when plotted as a function of age (linear regression coefficients of ca. 0.2). The mean and median slopes are usually higher for the Holocene–Late Pleistocene period (ca. 22±3°) than for older periods (18–19±4–6°), and average crater width/cone width ratios decrease slightly over time (0.33±0.07 to 0.32±0.1 and to 0.30±0.05), but differences are small, and values scatter widely. All these parameters do not vary systematically against cone volume or cone basal area. The concomitant decrease in Hco/Wco and Smax is consistent with a general degradation of the cones through time by progressive erosion of their upper slopes and deposition of the eroded material at their base. The scatter in the data is, however, too large, and the magnitude of the changes too small for establishing a definite empirical relation between cone morphometry and age. This wide scatter in the data can be partly explained by errors in the measurements. Systematic absolute errors amount to <0.02 for the Hco/Wco ratio (Table 3) and are null for Wcr and Wco due to the absence of declared errors in topographic maps. Additional complications for making accurate and comparable morphometric measurements stem from cone imperfections related to breaching, elongation along fissures, construction on inclined slopes, and partial burial by lava flows (i.e., Favalli et al. 2009). All of these can multiply this error by a factor of up to 4. This type of error is particularly significant in this area due to the high density of volcanoes and to the irregularity of the terrain. In addition, large spatial and temporal variations of weathering rates across the area are to be expected due to: (1) observed differences in the granulometry of the upper cone layers

Bull Volcanol (2012) 74:1187–1211

1195

CR

CP

a

PE

b

VE

CA

c

d

CB

TA

MC

CT

e TA CP

PA

f

CG ZO

LF

SH

g

Fig. 5 Variety of volcanic morphologies encountered in the TacámbaroPuruarán area (see Fig. 2 for location). a Aerial view towards the ENE showing Cerro La Cruz 2 (CR) in the middle ground and the city of Tacámbaro in the foreground. Note the advanced state of degradation of the volcano which reflects its Pliocene age (radiometric age of 4.18± 0.08 Ma, loc. no. 102 on Fig. 2a). Photo taken December 14, 2008. b View from sampling point no. 30 towards the SSE with Early Pleistocene Cerro El Pino scoria cone (CP) in the middle and the range of Eocene volcanics in the background. Photo taken October 21, 2007. c Aerial view towards the W showing the ca. 1.7 Ma C. Petembo (PE) and La Ventana (VE) lava shields in the middle and the margin of the Mexican Altiplano in the background. Photo taken December 15, 2008. d View from Tavera towards the SE with late-Pleistocene C. El Caracol (CA) scoria cone in the middle and fault-bounded range of Eocene volcanics in the background.

h Photo taken April 4, 2008. e Aerial view towards the W with the crater of Cerro Las Tablas (CT) in the foreground and C. La Barra (CB) dome (ca. 0.53 Ma) in the background. Photo taken December 15, 2008. f View from Tacámbaro toward the SW showing the Malpaís de Cutzaróndiro (MC) and gullied slopes of C. La Tinaja (TA) scoria cone (ca. 4000 years bc). Photo taken February 7, 2007. g Aerial view towards the N showing the margins of ca. 1.7 Ma Cerro Petembo and C. La Ventana shields (SH), the Holocene C. La Palma lava flow (PA), the ~4,000 years BC La Tinaja scoria cone (TA), and the ca. 1 Ma Cerro Potrerillos range (CP). Photo taken December 14, 2008. h Aerial view toward the SE showing ~1,000 years BC Cerro El Zoyate scoria cone (ZO) and associated lava flow (LF) and ~8,000 years BC Cerro Grande (CG) scoria cone. Photo taken December 14, 2008. All photos by C.S.

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(from lapilli to bomb-dominated) that affect how cones degrade (Valentine et al. 2006, 2007); (2) the short distance between cones in some areas that favors periods of increased

a

60

60

C. La Cruz

50

50

40

40

Weighted mean age: 42.2 +/- 1.3 Ma 30

20

10

10 0.2 0.4 0.6 0.8 Fraction of 39Ar Released

0 0.0

1.0

4

0.8

1.0

C. Potrerillos

Age in Ma

Age in Ma

0.2 0.4 0.6 Fraction of 39Ar Released

2

C. El Hueco

2

Weighted mean age: 51.9 +/- 0.6 Ma

30

20

0 0.0

b

Age in Ma

Age in Ma

C. San José

Plateau age: 2.68 +/- 0.03 Ma

1

Plateau age: 0.98 +/- 0.06 Ma

0 0.0 3

0.2 0.4 0.6 0.8 Fraction of 39Ar Released

0 0.0

1.0

1.0

C. Colorado

Age in Ma

Age in Ma

0.2 0.4 0.6 0.8 Fraction of 39Ar Released

2

C. Petembo

1.5 Plateau age: 1.70 +/- 0.02 Ma

1

Plateau age: 0.78 +/- 0.02 Ma

0 0.0

0.2 0.4 0.6 0.8 Fraction of 39Ar Released

0 0.0

1.0

3

2

C. La Ventana

1.5 Plateau age: 1.64 +/- 0.04 Ma

0 0.0

0.2 0.4 0.6 0.8 Fraction of 39Ar Released

0.2 0.4 0.6 Fraction of 39Ar Released

0.8

1.0

0.2 0.4 0.6 0.8 Fraction of 39Ar Released

1.0

C. El Mirador

Age in Ma

Age in Ma

Fig. 6 a 40Ar/39Ar age spectra of Eocene samples. Weighted mean ages of plateau-like fractions are shown. b 40Ar/39Ar age spectra and plateau ages of representative Plio-Pleistocene samples. c 40Ar/39Ar age spectrum and inverse isochron plot for sample C. Sosal illustrating that a precise, non-zero age can be obtained from a 60 ka sample (isochron age). d 40Ar/39Ar inverse isochron plots from representative samples of ‘zero aged’ lavas. Ages quoted are±1σ

cone erosion rates due to blanketing by ash from nearby activity and consequent destruction of the vegetation (Segerstrom 1950; Wood 1980); (3) variations of the climate

1.0

1 Plateau age: 0.38 +/- 0.01 Ma

0 0.0

Bull Volcanol (2012) 74:1187–1211 Fig. 6 (continued)

1197

c 0.2

.004

C. Sosal

36Ar/40Ar

Age in Ma

C. Sosal

Plateau age: 0.05 +/- 0.01 Ma

0.1

zero age reference line (atmospheric argon)

.003

isochron age: 0.06 +/- 0.02 Ma

0 0.0

0.2 0.4 0.6 Fraction of 39Ar Released

0.8

0

1.0

.1

0.3

0.2 39Ar/40Ar

d .004

.004

C. El Caracol

zero age reference line (atmospheric argon)

36Ar/40Ar

36Ar/40Ar

C. Don Nato

zero age reference line

(atmospheric argon)

.003

.003

isochron age: -0.05 +/- 0.15 Ma

isochron age: 0.38 +/- 0.18 Ma 0

0.01

0.02

0.03

0.04

0

0.05

0.01

0.02

39Ar/40Ar

0.03

0.04

0.05

39Ar/40Ar

.004

.004

C. Los Lobos

36Ar/40Ar

36Ar/40Ar

La Alberca

zero age reference line (atmospheric argon)

.003

.003

isochron age: 0.06 +/- 0.05 Ma

0

0.01

0.02

0.03

39Ar/40Ar

across the area from humid and temperate in the NW (e.g., Tacámbaro, 1,755 m asl; 17 °C mean annual temperature; 1,146 mm annual precipitation) down to hotter and drier in the SE (e.g., Turicato, 740 m asl; 24 °C mean annual temperature; 706 mm annual precipitation) (data from García 2004); (4) strong climate variations in this region during the Quaternary (Vásquez-Selem 2003; Metcalfe 2006). Identifying the relative impact of each of these factors on cone morphometry at any location results impossible, and thus the method of dating by morphometry is considered unreliable in many cases in this area.

0.04

zero age reference line (atmospheric argon)

0 .05

isochron age: -0.01 +/- 0.03 Ma

0

0.05

0.1

0.15

0.2

39Ar/40Ar

Relative dating: morphological classification As a result of the above considerations, a scheme based on comparative morphology was established for dating volcanoes whose age was not directly determined by radiometric analysis or constrained by stratigraphic relations with dated products (Table 4). This classification combines a number of parameters that best describe the degradation state of the volcanic products of different age periods. This method provides results more reliable in our case than the simple morphometry method because it not only considers changes

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Table 2 Radiocarbon dates Sample no.

Lab. no.

C. El Zoyate TAC-0885A A-14870 TAC-0885B

TAC-0709

TAC-08100

Long. (W)

19º06′59.5” 101º38′25.8”

Alt (m) Conventional age, years BP

Calibrated age below Relative area δ13C probability curve intervala, ‰ cal BC (2σ)

1982

−23.3

2,815±125

0.98 0.018

1,130–890 850–870

0.98 0.017 0.0006 0.964

−22.9

3,505+115/−110

−23.4

1,186–1,186 2,140–1,600 1,590–1,530

0.035

AA-81914 19º07′09.7” 101º38’17.7” 1983

2,195±40

−22.5

380–170

1

A-14700

5,325±130

−21.1

1982

19º07′09.7” 101º38’17.7” 1983

19º11′21.1”

101º30′09.4”

1415

5,115±145

4,400–3,935

0.95

4,450–4,420 3,870–3,810

0.01 0.03

−19.4

4,260–3,640 4,308–4,305

0.998 0.001

−23.4

8,220–7,550

1

11,440–10,720

1

A-14875

19º12′06.9” 101º31′08.7”

1567

C. Grande (?) TAC-0739 A-14749 C. El Gato (?)

19º10′15.8” 101º36′25.6”

2205

8715+145/−140

19º13′04.1” 101º36′15.4”

2128

11,190+190/−185 −24.1

TAC-0898

740–1,320 1,380–1,340

2,835±45

AA-81913 19º06′59.5” 101º38′25.8”

TAC-0886A A-14872 TAC-0886B C. La Tinaja

Lat. (N)

A-14874

Lab. no. laboratory number, A conventional technique, AA accelerator mass spectrometer technique a

Rounded to nearest 10 years except when ranges <20 years, 2σ 2 standard deviation (95.4 % confidence interval). Names of source volcanoes in bold. (?): uncertain origin of the ash fallout overlying the dated paleosol (see text for more details)

in cone shape but also in lava surface morphology and thus can be applied to a wider range of volcanic landforms. In addition, several other parameters describing the shape of a cone (e.g., gullying style, crater rim sharpness) are good age

Sample number 0

Calibrated ages (yrs BC) 2 σ

1000 2000

C. EL ZOYATE

3000 4000

C. LA TINAJA

Geochronology of eruptive activity

5000 6000 7000 8000

C. GRANDE (?)

9000 10000 11000

indicators (in particular, in the case of young cones) but are difficult to quantify by morphomometric means. This is especially true in our case, given the low resolution of the DEM. Considering the climatic variations in the area and the subjectiveness of some criteria, the classification was not used indiscriminately, but instead each volcano was considered separately, and its morphology carefully compared with the nearest radiometrically dated volcano. Details on the age determination and related uncertainties are reported for each volcano in Table DR2. Errors linked to uncertainty in determining the age of some volcanoes are discussed below.

C. El GATO (?)

12000

Fig. 7 Distribution of calibrated radiocarbon ages obtained in this study. Note that the regular time spacing between the eruptive events is only apparent because stratigraphic relations indicate that other eruptions punctuated the time gaps

The oldest rocks exposed in the study area consist of a sequence of slightly tilted volcanic rocks (lavas and pyroclastic deposits) that form two elevated plateaus (green area in Fig. 2). The largest plateau is also the highest (1,300– 1,700 m asl) and bounds the study area to the south (picture in Fig. 5b, d). A trachytic lava collected at the base of this plateau was dated at 42.2±1.30 Ma, indicating an Eocene age. The eastern plateau stands at lower elevation (900– 1,300 m asl) and is composed of distinctly older rocks (rhyolite lava at the top dated at 51.9±0.6 Ma). The elevation of these plateaus above surrounding ground (400 m in

Bull Volcanol (2012) 74:1187–1211

1199

Table 3 Morphometric parameters of dated scoria cones in the Tacámbaro-Puruarán area Area (km2)

Vol (km3)

Hco (m)

Wco (m)

Hco/Wco

Error

Wcr (m)

Dcr (m)

Wcr/Wco

Max slope Smax

Mean slope Smean

Median slope Smedian

Jorullo 10.80 Late Pleistocene to Holocene

0.36

385

1407

0.27

0.01

448

90

0.32

55

24

26

Volcano

C. El Zoyate

1.13

0.12

282

1310

0.22

0.01

423

42

0.32

45

18

18

La Tinaja C. Grandea C. Gatoa C. Los Lobosa C. El Caracola C. El Sosal C. La Laguna

2.54 0.38 0.67 0.35 0.62

0.43 0.04 0.06 0.02 0.05

392 180 160 140 180

1793 760 950 730 1243

0.22 0.24 0.17 0.19 0.14

0.01 0.02 0.01 0.02 0.01

534 260 470 237 408

53 90 110 57 60

0.30 0.34 0.49 0.32 0.33

56 58 41 44 47

20 28 21 22 21

21 26 20 21 22

0.80 0.60

0.06 0.04

140 215

742 1036

0.19 0.21

0.02 0.01

207 283

14 8

0.28 0.27

36 49

21 22

24 23

Avg

0.89

0.10

211

1071

0.20

353

39

0.33

47

22

22

0.71

0.14

87

367

0.03

121

23

0.07

7

3

2

SD Mid-Pleistocene C. Las Animas

0.79

0.09

140

670

0.21

0.02

270

45

0.40

48

25

27

Zihuatanejo Cono al lado del C. San Joséa C. Verde

0.39 0.21

0.02 0.01

120 120

680 540

0.18 0.22

0.02 0.03

nd 154

nd 60

nd 0.29

59 35

18 23

15 22

0.18 1.14 0.55 0.77

0.01 0.07 0.03 0.06

95 120 140 170

581 867 880 1241

0.16 0.14 0.16 0.14

0.02 0.02 0.02 0.01

150 320 122 256

nd 25 14 nd

0.26 0.37 0.14 0.21

47 33 36 42

18 17 21 21

20 18 20 20

C. Colorado 3a

0.51 0.46 0.12 0.14

0.05 0.07 0.01 0.00

102 235 72 84

912 931 400 623

0.11 0.25 0.18 0.13

0.02 0.02 0.03 0.02

264 332 177 203

22 23 12 5

0.29 0.36 0.44 0.33

45 46 28 35

17 14 13 14

12 12 12 15

C. El Malacate

0.90

0.07

250

994

0.25

0.01

473

25

0.48

49

23

20

Avg SD Early Pleistocene

0.48 0.35

0.04 0.03

137 56

777 234

0.18 0.05

247 102

25 11

0.32 0.10

42 9

19 4

18 5

C. Colorado 4a La Ventana 1a C. Petembo

0.92 0.34 0.37

0.02 0.03 0.02

178 147 148

958 1023 874

0.19 0.14 0.17

0.01 0.01 0.02

222 310 288

24 nd nd

0.23 0.30 0.33

34 37 35

20 15 15

22 16 12

C. Salitrillo Avg SD

1.23 0.71 0.43

0.10 0.04 0.04

138 153 17

1868 1181 462

0.07 0.14 0.05

0.01

654 369 194

84 – –

0.35 0.30 0.05

41 37 3

26 19 5

25 19 6

C. Nombre de Diosa C. El Chocolatea C. La Barra C. Partido C. Las Flores C. Tecario

Jorullo volcano shown for comparison In bold: volcanoes located in the SE dry sector of study area. See text for more information on error calculation Area cone basal area, Vol volume, Wco cone width, Hco maximum cone height (error014 m, corresponds to propagation of 10 m error on upper and basal elevations), Wcr crater width, Dcr crater depth, Max máximum, Avg average, SD standard deviation, nd not determined (filled crater) a

Breached cone

both cases) and their steeply dipping longitudinal margins indicate that they represent fault-bounded uplifted blocks. The southern plateau is dissected by many sub-parallel NE to NNE strike–slip faults (Fig. 2) formed at a stage postdating the uplift. Plio-Quaternary volcanic rocks originated from small vents located along faults bordering or cutting across the plateaus (Fig. 2).

A set of ~N–S elongated narrow ridges made of thoroughly altered volcanic rocks (pink area on the map, Fig. 2) likely represent the hydrothermally altered roots of this Eocene volcanic range. Faults bounding these ridges may be relatively deep since hot springs (48.5 °C) issue from the base of one of them (“El Salitre”, location no. 20 on Fig. 2). The origin of the hydrothermally altered, recrystallized Qz-

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Table 4 Morphological characteristics of volcanoes classified by age Relative age

Morphological characteristics of volcanic products

Pliocene (5–2 Ma)

Cone: completely eroded; dome: irregular shape

Early Pleistocene (2–1 Ma)

Cone: irregular shape when originally large, to barely recognizable low relief when small Lavas: poorly defined low-sloping margins

Mid-Pleistocene (1–0.1 Ma)

Late Pleistocene (100–11 ka)

Holocene (<11 ka)

Lavas: barely recognizable margins (unless when very steep) Typical volcanoes: C. Hueco, M. El Encinal

Typical volcanoes: C. Petembo, C. La Ventana, C. El Salitrillo Cone: partly to completely filled cone crater, poorly defined rim and amphitheater walls, thick overlying soil, thoroughly altered scoria layers Lavas: poorly defined margins, steep slopes cut by river channels, surface structures absent Products often extensively covered by younger ones Typical volcanoes: C. La Barra, C. Las Flores, C. Partido Cone: widened base, lowered outer slopes, deeply incised gullies, thick overlying soil, partly filled cone crater, degraded amphitheater walls Lavas: cultivated or with extensive vegetation cover; subdued internal flow structures Typical volcanoes: C. Zihuatanejo, C. El Sosal, C. La Laguna Cone: fresh cone rim and amphitheater walls (when breached), steep outer slopes, dense and shallow gullies for large cones, absent to thin overlying soil Lavas: pristine margins and internal flow structures such as pressure ridges or channels; patchy tree cover (cultivated when covered by thick ash) Typical volcanoes: C. La Tinaja, C. Alto, M. El Malpaís

rich rocks forming the faulted hill north of Pedernales (loc. 19 on Fig. 2) is not clear yet but may possibly also be related to the Eocene volcanic range. A thick sedimentary sequence crops out in the SE corner of the study area (M. Turicato, Fig. 2). This formation is known as the “Angostura Conglomerate” formed in the Late Pliocene to Early Pleistocene (Pasquaré et al. 1991; Garduño-Monroy et al. 1999). In the study area, it is overlain by Pliocene volcanics. The earliest TMVB activity is represented by Pliocene (5–2 Ma) products that crop out mainly along the eastern border of the study area. They cover an area of 118 km2 and amount to a volume of ~9 km3 (Table DR2.xls, Online resource) and include a 2.68±0.03 Ma andesitic dome and a 4.18±0.08 Ma large intensely altered dacitic shield (picture on Fig. 5a). A N-S range of old silicic volcanic rocks

(nine viscous flows and domes) bounds the study area to the east, overlapping conglomerates of the Angostura formation (Fig. 2). Poor access to this area precluded the collection of samples for radiometric dating and compositional analysis. The highly degraded state of the volcanics suggests Late Pliocene to Early Pleistocene (3–1.5 Ma) ages. Quaternary (<2 Ma) volcanics cover most of the NW half of the study area and occur as sparse volcanic centers in the SE corner (Fig. 2). Geochronologic (Table 1), morphologic (Tables 3 and 4), and stratigraphic data suggest that the different Quaternary volcanic structures can be either assigned to the early Pleistocene (2–1 Ma), the midPleistocene (1–0.1 Ma), late-Pleistocene (100–11 ka), or the Holocene (<11 ka) periods.

Characterization of Quaternary volcanoes Number and volume A total of 114 Quaternary volcanoes were identified and mapped (Fig. 2). They erupted a total of ca. 22 km3 DRE of magma (see details and errors on Table DR2). Seventeen volcanoes are estimated to be early-Pleistocene (total volume, ~4.8 km3 DRE), 61 mid-Pleistocene (~12 km3 DRE), 18 latePleistocene (~1.4 km3 DRE), and 18 Holocene (~3.9 km3 DRE) (Table DR2). Uncertainties in the age of some volcanoes amount to 2–7 % of the total volumes reported for each geological time period older than Holocene (details on Table DR2). For the Holocene, stratigraphic and radiometric data indicate that a minimum of 13 volcanoes formed. Based on their pristine morphology, five additional volcanoes probably also erupted within this period. This yields a total erupted volume that ranges between 3.3 and 3.9 km3. The above volume estimates do no take into account ash fallout, which can amount to as much as three to eight times the cone volume in extreme cases where activity was strongly explosive (e.g., Lathrop Wells, Valentine et al. 2007; Paricutin, Fries 1953). This case might apply to C. La Tinaja and C. El Zoyate volcanoes (both have large cone volumes and are surrounded by thick fallout deposits), but the volume of ash produced by most other cones is likely to be smaller (
Bull Volcanol (2012) 74:1187–1211 9

a

Trachyandesite

8

Trachyte

Basaltic trachyandesite

Rhyolite

7 Na2O + K2O wt.%

Fig. 8 Whole-rock composition of volcanic products classified by age. Limits between fields after Le Maitre (2002). MGVF data (open circles) from compilation by Gómez-Tuena et al. (2007). Data was normalized to 100 % on anhydrous basis. a Na2O+ K2O versus SiO2 in weight percent. b K2O versus SiO2 in weight percent

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Trachybasalt

6

MGVF

Dacite

Pliocene

5

Early-Pleistocene Basalt

Mid-Pleistocene

4 Basaltic andesite

Late-Pleistocene

Andesite

Holocene

3 40

50

60

70

80

4

b

High-K

K2O wt.%

3

Medium-K

2

1 Low-K

0 40

are particularly abundant among Holocene products (~50 vol.%) while scoria cones and associated lavas dominate all periods (32–75 vol.%). Shields are absent in the Holocene and late-Pleistocene but represent important volumetric proportions in older periods (ca. 30 vol.%). Chemical composition Products from 61 volcanoes of all age periods and representing a total volume of ~16 km3 were chemically analyzed. Only whole-rock compositions are reported here. Following the procedures proposed by Le Maitre (2002), bulk rock compositions were plotted on a (Na2O+K2O) vs. SiO2 and a K2O vs. SiO2 diagram (Fig. 8) in order to define magma types. Accordingly, magma types belong mostly to the medium-K series (Fig. 8a). Products range from basalt to dacite, some having high-alkali and high-K contents (Fig. 8a, b). Basaltic andesite is the most voluminous product (37 vol.%), followed by andesite (17 vol.%), basalt (9 vol.%), dacite (4 vol.%), shoshonite (2 vol.%), and minor amounts (<3 vol.% in total) of latite, mugearite, high-K trachyandesite, and high-K basalt (one sample only), in

50

60 SiO2 wt.%

70

80

order of decreasing volume proportion. Single volcanoes whose products spread over a wider range of these compositions (basaltic andesite to andesite, mainly) represent ~28 vol.% and are all scoria cones. In contrast, viscous silicic flows have much more homogeneous compositions (range in SiO2 within ±0.5 wt.%). The relative proportion of the different magma types varies systematically with time (Fig. 9). With decreasing age, andesites increase continuously in abundance, basaltic andesites increase until ~1 Ma ago, and then decrease until present; dacites decrease and are absent since 100 ka ago until present. Basalts erupted only during the 2–1 Ma period forming a large lava plateau. High total alkali magmas (shoshonites, latites, mugearites) occur in small proportions (~4–7 vol.%) in all periods, except for the Holocene where they represent <1 vol.% (only two samples of a total of 36). Distribution and age Volcanoes are typically aligned along directions ranging from ENE to NE, except for eastern formations that trend NNW– SSE (i.e., lava plateau M. La Calzada, Pliocene volcanic

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Fig. 9 Variation of volcanic rock compositions with time over the last 5 Ma Relative proportions (vol.%)

80

60

Basalt Basaltic andesite Basaltic andesite to andesite (varied)

40

Andesite High-K and alkali-rich 20

Dacite

0 2

1

Early-Pleist.

range) (Fig. 2b). Note that alignments are not strictly linear but slightly curved (Fig. 2b). The distribution of volcanoes of different types and compositions along the alignments appears to be random. There is no evidence to indicate that multiple aligned cones may have formed during single eruptions, or that any volcano may have experienced more than one eruption. Thus, it is inferred that every volcano represents a single eruption and is hence truly monogenetic. Groups of volcanoes that formed within a relatively short period of time (<10 of 1,000 s of years) tend to be aligned along single trends (Fig. 2a, b). This applies especially to the Holocene volcanic centers that define three main sub-parallel trends passing through C. Alto, C. El Zoyate, and C. La Tinaja, respectively (Fig. 2b). It may also apply to latePleistocene volcanoes in the NW corner of the study area (C. Márgaro, C. Las Ánimas, and others in between), midPleistocene cones in the same area (0.78 to 0.73 Ma old C. Las Flores, C. Tecario, and C. Colorado 5), at the central-western border (C. El Capuchin, etc.), and in the SE corner (C. Verde, etc.), and Early Pleistocene volcanoes in the SE corner (C. El Pino–C. La Ventana 2) (Fig. 2b). Unfortunately, the available age data and stratigraphic constraints are not precise enough to reach definitive conclusions. Holocene volcanoes are located in such close proximity to each other that their relative ages can be determined from stratigraphic relationships alone (see also Table DR2). Lavas from C. La Tinaja and Puerto de los Ates are partly covered by younger C. Palma lavas, and lavas from M. de la Muerta also overlap lavas from C. La Tinaja. Lavas from Malpaís de Cutzaróndiro overlap lavas from M. La Muerta and directly cover ash fallout from La Tinaja without displaying a paleosol in between (quarry at loc. no. 8 on Fig. 2). Thus, among this group of volcanoes, Malpaís de Cutzaróndiro and C. La Palma are the youngest ones and C. La Tinaja and Puerto de los Ates, the oldest. Also, Malpaís de Cutzaróndiro erupted soon after C. La Tinaja, and M. de la Muerta formed during the time that elapsed between these two eruptions. Hence,

Mid-Pleist.

0.1

0.011

Late-Pleist.

Holoc.

time (in Ma)

no systematic shift of the activity with time towards one end or the other of the directional trend can be recognized. This also applies to the volcanoes in the vicinity of C. El Zoyate. Several eruptions (e.g., Las Escobillas cone, and viscous lava flows directly SE and SW of C. El Tigre, respectively) postdate the ~1,000 years BC C. El Zoyate eruption, implying very recent activity in this area. These eruptions may have produced high ash clouds that deposited fine ash layers all over the region, as occurred during the Paricutin and Jorullo eruptions (Luhr and Simkin 1993; Guilbaud et al. 2009a). Fallout deposits are however easily eroded, making it difficult to estimate their volume (see “Discussion” section). Nevertheless, young (<17,000 years BP) ash layers have been reported in sediment cores from the lakes of Pátzcuaro, Zirahuén, and Zacapu (Newton et al. 2005) located 35 to 80 km NW to NNW of Tacámbaro, respectively. The age of some of these ash layers coincides with that of fallout deposits dated in this study (i.e., C4/T287 and CA1/T37 dated at 2,765±70 and 2,840±90 years BP, respectively; CA1/T114 dated at 5,140±70 years BP; C4/ T464 dated at 8,345±50 years BP). However, the chemical analyses reported for the ash layers in the cores (major elements in tephra glasses) are unfortunately not sufficient to ascertain their origin, given the similarity of major element whole-rock compositions of many volcanoes in the area (Fig. 8). Eruptive activity in pre-historical times in the area is suggested by many Purhepecha (language spoken by the Tarascan indians) toponyms that make reference to volcanic phenomena. Examples are Puruarán (place of boiling water) or Cutzaróndiro (where coarse ash is abundant) and indicate that the youngest eruptions in the area were probably witnessed by pre-Hispanic populations. This is in accordance with the writings of the Belgian geographer Jules Leclercq (1885), who visited the area in 1883. He mentions that, before the eruption of Jorullo volcano in 1759, a local Indian legend referred to mythical volcanic activity near

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Cutzaróndiro, a small village located NE of La Tinaja volcano (see also Fig. 2).

Discussion Reconstruction of the volcanic history Volcanic activity started in the Eocene forming thick sequences of volcanics that are related to a volcanic arc that paralleled the Pacific coast of México in the Early Tertiary (e.g., MoránZenteno et al. 1999). During the following 25 Myr, major tectonic adjustments suppressed the generation of magmas in this area. Instead, crustal deformation, uplift, and erosion resulted in the exposure of the Early Tertiary volcanics and their plutonic roots as uplifted fault-bounded blocks (more details in Guilbaud et al. 2011). This mosaic of blocks forms the basement of the young volcanic cover in the study area. Volcanic activity resumed in the Pliocene (5–2 Ma), as indicated by very poorly exposed volcanic centers. In the Quaternary, the activity spread across a major part of the area. Eruption rates can be estimated for each period based on the volume of the exposed products, but they are not reliable for the oldest time period (especially >1 Ma) because the volume of buried flows is not known. Since 1 Ma, eruption rates have been at least 0.017 km3/kyr. For the Holocene, eruption rates can be better constrained. Total volume estimates (including ash fallouts) range from 3.8 to 4.3 km3, yielding production rates of 0.34 to 0.39 km3/kyr. Thirteen to eighteen eruptions occurred, indicating an eruption every 600 to 800 years on average. Evidence indicates that eruptions did not occur at regular intervals of time, but instead, many volcanoes erupted within close time intervals forming groups along limited segments of crustal faults. No systematic shift of the activity with time occurred along these structures, however (see previous paragraph), as has been described elsewhere (Conway et al. 1997). Hazard assessment and factors controlling eruption explosivity Results of the present study can be used for assessing hazards. The high frequency of eruptions in the Holocene indicates that more eruptions are likely to occur in this area. It is also likely that the next eruption will take place along one of the three main alignments on which most of the previous Holocene eruptions have occurred (Fig. 2b), although the whole area should be considered potentially active. Half of the Holocene eruptions formed scoria cones and relatively long (<9 km) lava flows, while the other half produced thick and short (<3 km) viscous flows that erupted from fissures without forming cones. Both scenarios should thus be considered for the next eruption. Clearly, scoria

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cone-forming activity would be the more hazardous case in this area because of the greater length of the associated lava flows that may threaten villages or disrupt communication systems but more particularly because of the extensive damage to agriculture by associated ash fallouts. Two of the youngest eruptions (La Tinaja and El Zoyate) built large cones and deposited thick fallout deposits implying strongly explosive episodes during their activity. These eruptions produced less-evolved and chemically diverse basaltic andesitic to andesitic magmas compared with the less explosive and less voluminous eruptions of degassed homogeneous andesite that produced only lava flows. Interestingly, magma compositions similar to those at La Tinaja and El Zoyate were involved in the long (9–15 years) and violent-Strombolian Jorullo and Paricutin eruptions (Pioli et al. 2008; Guilbaud et al. 2009a). Melt inclusion data from these and other scoria cones in the TMVB indicate that these less-evolved magmas (i.e., of the basaltic andesitic type) contain up to 5 vol.% H2O that drove the explosive activity (e.g., Pioli et al. 2008; Johnson et al. 2008; Guilbaud et al. 2009b; Roberge et al. 2011). These lessevolved but volatile rich magmas thus represent a potential hazard in this area. Controls on volcano distribution Volcanoes in the Tacámbaro area were fed by small magma batches that ascended through the crust as dikes. The alignment of volcanoes of different ages within the study area suggests that feeder dikes repetitively used specific fault planes cutting the local shallow basement before reaching the surface. The surface expression of these faults is poor due to ample cover by young volcanic products. It is recognized that dikes rising as self-propagating fractures in the deep crust are likely to occupy pre-existing fault planes when nearing the surface (Valentine and Gregg 2008). Mechanical constraints ensure that fault planes that are steep and oriented parallel to the direction of maximum horizontal stress are most likely to be occupied by rising magma because these conditions minimize the magma pressure required to dilate the cracks (Delaney et al. 1986; Gaffney et al. 2007). Requirements for dikes to ascend along preexisting fault planes in the upper crust are met in the study area where the young volcanics cover a mosaic of blocks bordered and cut by normal faults (i.e., the southern lava plateaus). In addition, many of these upper crustal faults strike NE to NNE, in the direction of the maximum horizontal stress imposed by the subduction process along the southern margin of Mexico (e.g., Guilbaud et al. 2011). Volcano alignments are curved in the same way as exposed faults, and some alignments form the continuation of outcropping basement faults in the SE part of the study area (Fig. 2b). In short, basement fault planes seem to control

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volcano alignments in the Tacámbaro area. The lateral dispersion of volcanoes along the main trends may be related to the inclination of the fault planes at depth (see Fig. 4 in Connor et al. 2000). The alignment of volcanoes near the eastern limit of the study area along NNW–SSE trends (Fig. 2b) can be related to the distinct faulting of the Eocene basement in this area (i.e., the NNW-faulted plateau and hydrothermally altered ridges). Dikes that propagated independently of faults may have fed some of the volcanoes lying apart from the main alignments. Despite the probable relation of volcano alignments with crustal faults, volcano distribution in general should primarily reflect the location and geometry of the magma sources in the mantle (i.e., the magma “footprint” of Valentine and Perry 2006) since steep faults should not be able to significantly deviate the rising magma batches from their ascent paths. Accordingly, the observed focus of Holocene activity along <10 km long segments of crustal faults suggests the existence of small-scale magma source zones. The diversity of magma compositions erupted along each fault (basaltic andesitic to andesitic) can be in part explained by smallscale heterogeneities in the mantle source and/or variations in the rise velocities (and thus extent of differentiation and contamination) of magmas ascending along the fault. Alternatively, the delivery of partial melts to the lower crust may be constant and homogeneous across the entire area, and the spatial focusing of the activity might only reflect the distribution of favorably oriented structures that allow magmas to rise and reach the surface in particular zones. These processes are further discussed in the following section. Magma generation and lithospheric deformation Valentine and Perry (2007) propose to distinguish two types of magmatism and related volcanism in monogenetic settings: (1) The case where the generation, ascent, and eruption of the magmas is fundamentally controlled by tectonic strain in the lithosphere (time-predictable low magma flux fields) and (2) another case where the flux of magma produced at depth controls the eruptive rate and the distribution of volcanoes (volume-predictable high magma flux fields). Discussing how volcanism in the study area and the MGVF relates to this model requires a better understanding of the processes of melt generation and lithospheric strain in this region. Available geochemical data on MGVF products and thermal models indicate that the mantle below this region is compositionally heterogeneous and rich in H2O (Johnson et al. 2009). Apparently, volatiles originate largely from dehydration of the subducting slab (Blatter and Hammersley 2010; Johnson et al. 2009). Recent activity has been documented in many parts of the MGVF and not only along the volcanic front (Hasenaka and Carmichael 1985), so we infer that the MGVF is underlain by a hydrated

Bull Volcanol (2012) 74:1187–1211

mantle source that is being melted to variable amounts (9– 15 wt.%) by the aid of fluids released from the subducting slab (Johnson et al. 2009). Compositional heterogeneities in the primitive melts stem from complex and poorly known modifications of the mantle prior to TMVB volcanism and to a variable extent from contamination by sediment-derived fluid components (Johnson et al. 2009). Guilbaud et al. (2011) review what is known about the intensity and pattern of lithospheric deformation in the MGVF. Seismic activity within the field is absent or of low magnitude, suggesting low strain rates. Faults cutting young volcanic rocks are rare across the field. This might indicate that rising dikes accommodate a significant part of the strain (case of high-flux fields according to Valentine and Perry 2007), but this can also simply be the result of rapid burial of active faults under the extensive young volcanic cover in this area. On the other hand, the boundaries of the MGVF are clearly defined by active regional faults. Thus, lithospheric deformation at a regional scale must play a role in regard to the distribution of volcanism. The alignment of volcanoes in the area suggests an important structural control at a local scale. In conclusion, magmatism in the MGVF appears to be governed both by processes of fluid-induced melting in the mantle wedge and the distribution of stress within the overlying crust, and the relative importance of each mechanism cannot be defined at present. It is clear that the MGVF is not a “low magma flux field”, but the existing evidence does not fit the requirements for the “high magma-flux controlled field” either (as described in the scheme of Valentine and Perry 2007), since some tectonic control is obvious. The MGVF may thus be an intermediate case that does not follow simple time- or volume-predictable trends (i.e., Valentine and Perry 2007). Instead, the available timevolume data in the Tacámbaro area suggest that volcanism was pulsatory, clustered in time and space, and fed by the repeated ascent of small batches of heterogeneous magma through an anisotropic crust. Temporal trends in magma compositions The dataset reveals systematic compositional variations of the magmas erupted in the area since the Pliocene (Fig. 9). Pliocene activity, though its products are poorly exposed, was essentially silicic and followed by voluminous bimodal volcanism in the Early Pleistocene (basalt and dacite). In contrast, since 1 Ma volcanism was intermediate in composition and mainly erupted calc-alkaline basaltic andesites and rarely alkaline products, whereas Holocene products are dominantly andesites and alkaline products are absent. This peculiarity of the MGVF was noticed before by Hasenaka and Carmichael (1987) for volcanoes older than 40 ka.

Bull Volcanol (2012) 74:1187–1211

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Silicic volcanism therefore marks the early stages of the TMVB in this part of Mexico, while intermediate magmas typify its activity since 1 Ma, in agreement with general trends across the entire arc (Gómez-Tuena et al. 2007). The origin of this major compositional change could be caused by important changes in the configuration of the subduction segment nearest to the MGVF that occurred during this period of time. Ban et al. (1992) suggest that the volcanic front has migrated southwards since 3 Ma, inspiring the hypothesis of Johnson et al. (2009) who postulate a change from flat-slab to a moderately dipping slab configuration for this period. However, the data of Ban et al. (1992) is flawed by sampling only a few volcanoes across the field, and the herein-presented data along with data in Guilbaud et al. (2011) indicate that volcanism was already underway at the southern part of the MGVF more than three million years ago. Hence, the postulated southward migration might be an artifact of an overall higher magma production rate and hence abundance of young products in the south and resulting better exposure of older volcanoes in the north. Instead of important changes in the subduction geometry, we thus favor the hypothesis of an increase in the lithospheric extension rate since 5 Ma. This can explain both the suggested increase in the amount of magma produced with time in our study area (although this is also partly biased by burial of old products) and the more primitive nature of the younger <1 Ma products, by allowing an increased number of smaller (poorly evolved) dikes to reach the surface and by limiting crustal magma Late Pleistocene Pleistocene to Holocene Early Mid

Pliocene Tacambaro ages

n = 31

Jorullo ages

n = 11

relative probability

0.0

1.0

Miocene

2.0

3.0 Age in Ma

4.0

5.0

storage. This can also explain the absence of lava shields fed by unique vents in young (<0.5 Ma) products, whose formation suggests the establishment of relatively large (1–3 km3) crustal reservoirs (see also, Hasenaka 1994). The recent (<100 ka) evolution towards more andesitic and less alkaline compositions is difficult to explain by changes in crustal deformation rates but may instead reflect changes in mantle source processes. Petrogenetic models still need to be developed in order to explain this trend. Comparison with the Jorullo area The timing and composition of volcanism in the TacámbaroPuruarán area (TAC) contrast with those in the ~330 km2 Jorullo area (JOR) located directly to the SW (Guilbaud et al. 2011; Fig. 10). In the TAC, old (>2 Ma) volcanics consist of basaltic lavas and viscous dacitic domes, while in the JOR basaltic to andesitic lavas predominate during this time. Average eruption rates in the JOR and the TAC for the last 1 Myr are similar (~3.1 vs ~2.5 km3/Myr/100 km2), but radiometric dates suggest that the activity progressively decreased in frequency in the JOR, whereas it increased in the TAC (Fig. 10). In particular, <100 ka activity was more frequent and more voluminous in the TAC than in the JOR. Also, since 1 Ma, andesites dominate in the JOR (~61 vol.%) in contrast to basaltic andesites (~48 vol.%) in the TAC, while basalts are absent in the TAC but present in the JOR (~10 vol.%). These differences indicate that volcanic activity was spatially and temporally heterogeneous along a ~50 km segment of the southernmost volcanic front of the MGVF during the last 5 Myr. Part of these differences may relate to differences in the composition and structure of the basement underlying both areas (thick volcanic sequence in the TAC compared with granodiorite batholiths in the JOR), but these cannot explain all of the observed variations. Heterogeneities in the composition and amount of partial melting of the mantle source may also play a role, and petrogenetic models based on further geochemical data should seek to explain these features. Comparison with the entire MGVF and other monogenetic volcanic fields in México

6.0

Fig. 10 Probability distribution plot of Holocene to Pliocene 40 Ar/39Ar ages from Tacámbaro (this study; Table 1) and Jorullo (Guilbaud et al. 2011). The data plotted in this diagram are preferred ages obtained on whole-rock chips only (see Table 1). Young, lowprecision ages reported in Table 1 are included

Considering the presently available data, the TacámbaroPuruarán region was the most active area of the MGVF during the past million years. Its average volcano density (16/100 km2) is more than six times that of the average MGVF (2.5/100 km2), and the average eruption rate in the last 100 ka is four times higher than the rate estimated for the last 40 kyr in the MGVF (0.008 vs. 0.002 km3/kyr/ 100 km2) (Hasenaka and Carmichael 1985). Reasons for this contrast may be various. On one hand, Hasenaka and Carmichael (1985) probably underestimated the number of young volcanoes within the field, and thus the average

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eruption rates for the MGVF may actually be higher than their estimate. On the other hand, reliable (i.e., with a sufficiently high resolution) estimates of crustal thickness in the Michoacán region do not exist yet, thus the hypothesis of a particularly thin crust in the Tacámbaro area that would allow higher eruption rates cannot be tested. Another possible cause is that of larger amounts of volatiles released at depth and hence higher melt generation at this location. Estimates of magma H2O contents based on trace-element whole-rock data (assuming a linear correlation between the two parameters) suggest low variations between volcanoes in the Tacámbaro area (15 data points from Hasenaka and Carmichael 1987) and the surrounding areas (Johnson et al. 2009). A more detailed analysis of the distribution and geochemical characteristics of recent activity within the MGVF are however needed to test this hypothesis and settle this issue. Comparison of the results of the present study with the limited number of similar datasets for other fields in Mexico (location of fields shown in Fig. 1) allows us to evaluate how eruption rates in monogenetic fields vary across the belt and how these variations can be broadly explained by changes in crustal thickness, style of crustal deformation, rates of magma input, and the related degree of focusing of the magmatic activity at any location that is reflected by the size and rates of activity of nearby polygenetic volcanoes. The following comments are somewhat speculative, given the wide range of often poorly known parameters involved. Such reasoning may however shed light on major trends. Eruption rate per area (or accumulation rate) over the last 1 Myr in the TAC is >2.5 km3/Myr/100 km2. This value is higher in the monogenetic area surrounding the Tancítaro stratovolcano to the NW (~4.2 km3/Myr/100 km2; Ownby et al. 2011), but this estimate includes the volume of buried flows (<70 % of the total volume estimate), which was not considered in the present study. Eruption rates in both regions may thus be similar. The eruption rate per area in the Zitácuaro-Valle de Bravo field (ZVB) to the east is lower (~1.7 km3/Myr/100 km2; Blatter et al. 2001) and may be related to a thicker crust in this region (~40 km compared with ~30 km for the TAC after Blatter and Hammersley (2010) and gravity data by Urrutia-Fucugauchi and FloresRuiz (1996)) that promotes longer crustal magma storage and generates larger amounts of crystal-rich dacites (see below). Located much further east, the Xalapa monogenetic field lies on the flank of a massive compound volcano (Cofre de Perote) whose activity preceded the onset of monogenetic volcanism (Carrasco-Núñez et al. 2010). The low eruption rate of the monogenetic field (~0.2 km3/Myr/ 100 km2; Rodríguez et al. 2010) may be related to its setting far from the trench (Fig. 1). By contrast, published eruption rates are markedly high for areas encompassing composite volcanoes and surrounding monogenetic vents in western

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Mexico (~5 km3/Myr/100 km2 for the Ceboruco-San-Pedro field, Frey et al. (2004) and ~8 km3/Myr/100 km2 for the Tequila field, Lewis-Kenedy et al. (2005)). In these regions, composite volcanoes represent >50 vol.% of the total volume of magma erupted (and hence related eruption rates). The subducting slab (the Rivera Plate) dips at a higher angle (~50° compared with ~30° for the Cocos Plate below the Michoacán region, Pardo and Suárez 1995), and volcanic activity concentrates in narrow, fault-bounded extensional basins (e.g., Tepic graben). These factors both favor high eruption rates and melt coalescence into crustal reservoirs feeding the composite volcanoes. In comparison, volcanism occurring far outside these grabens in this region is monogenetic and not very voluminous (ca. 7 km3 in 2.4 Myr in the Mascota region, Ownby et al. 2008). Monogenetic activity near the massive Colima–Nevado stratovolcano complex to the south is notably small (ca. 1.2 km 3 /Myr, Carmichael et al. 2006), implying that melts generated in this area are preferentially drawn into a large reservoir below the stratovolcano. Eruption rates per area in the Holocene in the Tacámbaro area are among the highest yet estimated in the TMVB, possibly only comparable to activity in the Sierra Chichinautzin Volcanic field (SCVF). There, >15 events producing >10 km3 of magma occurred since 14 ka (Agustín-Flores et al. 2011; Siebe et al. 2004a, 2005; Cervantes and Molinero 1995; Bloomfield 1975). This yields an eruption frequency and eruption rate per area that is lower than in the TAC (0.6 vs. 1.9–2.6 events/100 km 2 and 0.4 vs. 0.5–0.6 km 3 /100 km 2 ). Nevertheless, comprehensive mapping and identification of all Holocene volcanoes across the SCVF is still pending, precluding definite conclusions. In comparison, only ~1.5 km3 of magma was erupted from monogenetic vents around the Ceboruco stratovolcano since 12 ka, probably due to abundant activity at the stratovolcano during that period (Sieron and Siebe 2008). Monogenetic centers in the Xalapa region erupted only ~2 km3 of magma since 40 ka (Siebert and Carrasco-Núñez 2002), probably linked to overall low magma supplies in that area (see above). In conclusion, the high eruption rates in the Tacámbaro area may be related to a lower than average crustal thickness (~30 km) and extensional tectonics, the spatial dispersion of the magma input and associated large distance from stratovolcanoes (>75 km from the Tancítaro), and relatively high melt supply in this area. The dominant magma type erupted at any area may provide further constraints on magma generation and ascent characteristics. Note that, since 1 Ma, the TAC is the only area among those mentioned above to have produced mainly basaltic andesites. Dacites are dominant in the ZVB field, whereas andesites are the most common products in the Tequila and Ceboruco-San-Pedro fields (references as above). The relative proportions of andesites are similar in the TAC and the SCVF

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(ca. 60 vol.%) during the Holocene, but basalts represent up to 30 vol.% of magmas erupted in the SCVF during the last 14 kyr (references as above) whereas they are essentially absent in the TAC during the same time period. Among the fields mentioned above, the absence of <1 Ma basalts in the TAC is only shared by the CSP field. A general consensus exists that basalts and basaltic andesites in the TMVB are derived from hydrated mantle melts that underwent little modification by crystal fractionation and/or crustal contamination during transit to the surface (e.g., Moore and Carmichael 1998; Ownby et al. 2008, 2011; Carmichael 2002; Weber et al. 2011). Their predominance in the Tacámbaro area is thus compatible with the observed high magma eruption rates, the elevated H2Ocontents of primitive melts generated in the southern part of the MGVF (Johnson et al. 2009) and the existence of crustal structures that favor rapid magma ascent. By contrast, the process of generation of the crystal-poor andesites typically produced by monogenetic vents in the TMVB remains debated. Ownby et al. (2008, 2011) propose an origin by partial melting of the deep crust, whereas other authors favor a process of polybaric fractionation±contamination of the more primitive, mantle-derived melts (Hasenaka and Carmichael 1987; Moore and Carmichael 1998; Siebe et al. 2004b; Schaaf et al. 2005). Clearly, additional geochemical data (especially radiometric dating) are needed to address the origin of temporal and spatial variations in the proportion of andesites and other rock types across the belt. On the other hand, as mentioned above, the dominance of dacites in the Valle del Bravo area may relate to a thicker crust underlying this region (see above), whereas the relative abundance of basalts in the SCVF suggests tectonically favored conditions of melt ascent from the mantle. Comparison with monogenetic fields in other subduction-related volcanic arcs Comparison of results from the present study with those from other regions of the world is hampered by the lack of geological information (especially geochronological data) on monogenetic fields in subduction settings. Nonetheless, examination of the list of Holocene volcanic fields compiled by Siebert and Simkin (2002–2011), inspection of satellite images using GoogleEarth, as well as a review of recent papers on selected areas, provide some insight into the characteristics of young monogenetic fields in volcanic arcs wordwide (here, we only consider groups of >5 volcanoes with published Holocene ages). Many of these fields occur in extensional back-arc settings (e.g., Auckland volcanic field, New Zealand, Molloy et al. 2009; Los Volcanes field, Argentina, Germa et al. 2010; Sredinny Range, Kamchatka, Ponomareva et al. 2007) and only a few near the arc front

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(e.g., Andahua, Perú, Delacour et al. 2007; Sand Mountain, Oregon, Wood and Kienle 1990; Tolbachik, Kamchatka, Braitseva et al. 1984). The TMVB is certainly unique in the world by having such large numbers of monogenetic centers not only in the back but also along the front (e.g., Sierra Chichinautzin, Jorullo, Tacámbaro-Puruarán; Fig. 1) of the volcanic arc. Furthermore, not a single monogenetic field is as large (ca. 40,000 km2) and characterized by such a dense clustering of young vents as the MGVF. Along most island and continental arcs, Holocene monogenetic volcanoes occur in much smaller fields (typically <10 volcanoes) often aligned along defined fault zones forming tectonic depressions, or distributed around the base of larger composite volcanoes. The very young 875 km2 basaltic Tolbachik cinder cone field at the volcanic front in central Kamchatka has produced 71 km3 of lava during the last 10,000 years (Braitseva et al. 1984), compared with 4.5 km3 in the Tacámbaro-Puruarán area, but its activity is strongly related to that of the Ploskiy-Ostryy shield directly to the NE, implying a very different setting than vents of the Tacámbaro area. The large basaltic–andesitic Tolmachev Dol volcanic field in SW Kamchatka might be more comparable, but geological data is scarce (Ponomareva et al. 2007). Hence, based on the available data, it can be concluded that the TacámbaroPuruarán region contains the highest density of “independent” arc-related monogenetic Holocene vents whose ages, volumes, and compositions are known. One may speculate about the reasons behind such an anomalously high monogenetic activity in a specific area of the MGVF. Clearly, the TMVB itself is peculiar among other volcanic arcs for having such a high proportion of monogenetic versus polygenetic vents (Fig. 1, sea also Tibaldi 1992). This seems to be related to the shallow angle of dip of the subducting slab along most of the arc (Pardo and Suárez 1995) and the notorious extensional deformation of the upper crust along the central part of the volcanic belt (occurrence of normal faulting, lake basins, etc.) that has been taking place at least since the beginning of the Quaternary (Suter et al. 2001). These factors altogether favor the ascent of mantle melts generated above the subducted slab as physically separated magma batches that feed monogenetic eruptions. Furthermore, high rates of magma production at the mantle source are being favored by subduction-related hydrated conditions (Johnson et al. 2009) and may also explain the observed high frequency of monogenetic activity in the MGVF in general and in the Tacámbaro area in particular.

Summary and conclusions A comprehensive reconstruction of the volcanic geology of the small Tacámbaro-Puruarán area lying at the

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southernmost arc-front part of the Michoacán-Guanajuato Volcanic Field was conducted. The age, distribution, volume, type, and composition of volcanoes were determined in order to constrain the volcanic history of this previously unstudied part of the TMVB. Faulted and tilted blocks of Eocene volcanic rocks exposed in the south part of the study area form the basement of the dominantly young (<1 Ma) volcanic cover exposed to the north. Early TMVB volcanic activity goes back to ca. 5 Ma in this area. Early Pliocene to Early Pleistocene products are mainly bimodal (basalts and dacites), while <1 Ma volcanic rocks are intermediate in character (basaltic andesites and andesites). Eruption rates during the last 1 Myr were constrained to >0.017 km3/kyr. Eruptions are not strictly periodical but cluster in time and space. In the Holocene, 13 to 18 eruptions occurred, indicating an eruption every 600 to 800 years on average and production rates of 0.34 to 0.39 km3/kyr. This high eruption rate suggests that further eruptions are likely to occur, and the following two scenarios are the most plausible. (1) The next eruption may be mostly effusive and form thick and short (<3 km) viscous flows without constructing a cone (e.g., the <4,000 years BC Malpaís de Cutzaróndiro eruption), or (2) it may involve intermittent explosive and effusive activity building a scoria cone, depositing thick ash fallout, and emitting long (<9 km) lava flows (e.g., the 4,000 years BC La Tinaja and the 1,000 years BC El Zoyate eruptions). Clearly, the latter case would be the most hazardous. Volcano alignments are parallel to the strike of exposed faults and the regional minimum horizontal stress, implying a shallow structural control on the ascent of dikes feeding volcanic activity. These observations and available geochemical and structural data on the MGVF indicate that volcanism in the TAC and the MGVF are both tectonically and magmatically controlled, thus falling in an intermediate position in the model proposed by Valentine and Perry (2007). Temporal trends in magma compositions in the TAC can be explained by a higher crustal extension rate since <1 Ma while the origin of recent (<100 ka) compositional changes remains unexplained. A compilation of results from other areas in the TMVB shows that monogenetic volcanism is highly diverse along, as well as across, the entire TMVB. In particular, differences displayed between the Tacámbaro and Jorullo area directly to the SW, imply short-scale (<50 km) heterogeneities that may be related to changes in the nature of the basement or variations in the thermal structure or composition of the mantle source below these regions. Broad differences in eruption rates and proportions of magma compositional types between monogenetic fields across the belt can be explained by changes in the structure and thickness of the crust, the configuration of the subduction zone, the flux of melts generated at depth, and the related extent of focusing

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of the volcanic activity expressed by the spacing and size of large polygenetic volcanoes between the monogenetic areas. Comparison with existing data from other subductionrelated fields worldwide shows that the frequency of Holocene activity in the TAC area is one of the highest yet detected. This is rather surprising given the high (though as yet poorly constrained) crustal thickness in this region (>30 km). The ultimate causes of this high eruption frequency may be sought in the peculiarities of the geometry of the subduction zone and the specific tectonic setting of the MGVF, characterized by extensional normal faulting of the upper crust and high magma production rates in the underlying mantle wedge. These hypotheses should be tested when more detailed geochronological, geochemical, and geophysical studies of other regions within the MGVF, the TMVB, and other subduction-related monogenetic fields in the world become available. Acknowledgments The authors wish to thank Gabriel Valdés, Renato Castro-Govea, and Victor Hugo Garduño for participating in fieldwork. V.H. Garduño also helped in mapping some faults. Capitán Fernando Valencia is thanked for skillful and safe flights over the study area. Field and laboratory costs were defrayed from projects funded by the Consejo Nacional de Ciencia y Tecnología (CONACyT-P167231 and 152294) and the Dirección General de Asuntos del Personal Académico, UNAM (DGAPA IN-109412 and IA-101011) granted to C.S. and M.N. The Humboldt Foundation in Germany is also thanked for supporting this project. Part of the work was done as the first author completed a post-doctorate fellowship at the Instituto de Geografía, UNAM, sponsored by the Instituto de Ciencia y Tecnología del Distrito Federal (ICyTDF). L. Vázquez Selem, I. Alcántara Ayala, and others at Instituto de Geografía are thanked for their institutional support during that stay. Editorial handling by J. White and detailed reviews by G. Valentine and S. Cronin were very helpful and are greatly appreciated.

Appendix:

40

Ar/39Ar dating

Samples were crushed, washed, sieved, and hand-picked for biotite crystals (samples 63 and 102), plagioclase crystals (sample 102), and for small whole-rock chips (all samples) suitable for dating. The monitor mineral TCR-2 with an age of 27.87 Ma (Lanphere and Dalrymple 2000) was used to monitor neutron flux and calculate the irradiation parameter, J, for all samples. The samples and standards were wrapped in aluminum foil and loaded into aluminum cans of 2.5 cm diameter and 6 cm height. All samples were irradiated in position 5c of the uranium-enriched research reactor of McMaster University in Hamilton, Ontario, Canada, for 0.5 Mwatt-h. Upon their return from the reactor, the whole rock chips and grains of the monitor mineral were loaded into 2 mm diameter holes in a copper tray that was then loaded in an ultra-high vacuum extraction line. The monitors were fused and samples heated, using a 8-watt argon-

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ion laser following the technique described in York et al. (1981), Layer et al. (1987), and Layer (2000). Multiple holes were heated at the same time to improve the signal. Argon purification was achieved using a liquid nitrogen cold trap and a SAES Zr-Al getter at 400 °C for 20 min. The samples were analyzed in a VG-3600 mass spectrometer controlled by a Visual Basic operating program written in-house. The measured argon isotopes were corrected for system blank and mass discrimination, and for the irradiated samples, calcium, potassium, and chlorine interference reactions, following procedures outlined in McDougall and Harrison (1999). System blanks generally were 2×10-16 mol 40 Ar and 2×10-18 mol 36Ar, which are five to 50 times smaller than fraction volumes. Mass discrimination was monitored by running both calibrated air shots and a zero-age glass sample. These measurements were made on a weekly to monthly basis to check for changes in mass discrimination. Two runs of each sample were done, and the results were stacked to calculate composite isochron and plateau ages using the constants of Steiger and Jaeger (1977). References Agustín-Flores J, Siebe C, Guilbaud M-N (2011) Geology and geochemistry of Pelagatos, Cerro del Agua, and Dos Cerros monogenetic volcanoes in the Sierra Chichinautzin volcanic field, south of México City. J Volcanol Geotherm Res 201:143–162 Ban M, Hasenaka T, Delgado Granados H, Takaoka N (1992) K-Ar ages of lavas from shield volcanoes in the Michoacán-Guanajuato volcanic field, Mexico. Geofis Int 31:467–473 Bebbington MS, Cronin SJ (2011) Spatio-temporal hazard estimation in the Auckland Volcanic Field, New Zealand, with a new eventorder model. Bull Volcanol 73:55–72 Behncke B, Neri M, Pecora E, Zanon V (2006) The exceptional activity and growth of the Southeast Crater, Mount Etna (Italy), between 1996 and 2001. Bull Volcanol 69:149–173 Blatter DL, Hammersley L (2010) Impact of the Orozco Fracture Zone on the central Mexican Volcanic Belt. J Volcanol Geotherm Res 197:67–84 Blatter DL, Carmichael ISE, Deino AL, Renne PR (2001) Neogene volcanism at the front of the central Mexican volcanic belt: basaltic andesites to dacites, with contemporaneous shoshonites and high-TiO2 lava. Geol Soc Am Bull 113:1324–1342 Bloomfield K (1975) A late-Quaternary monogenetic volcano field in central Mexico. Geologische Rundsch 64:476–497 Braitseva OA, Melekestsev IV, Flerov GB, Ponomareva VV, Sulerzhitsky LD, Litasova SN (1984) Holocene volcanism of the Tolbachik regional zone of cinder cones. In: Fedotov SA (ed) Great Tolbachik fissure eruption, Kamchatka, 1975–1976. Nauka, Moscow, pp 177–209, In Russian Carmichael ISE (2002) The andesite aqueduct: perspectives on the evolution of intermediate magmatism in west-central (105– 99_W) Mexico. Contrib Mineral Petrol 143:641–663 Carmichael ISE, Frey HM, Lange RA, Hall CM (2006) The Pleistocene cinder cones surrounding Volcán Colima, Mexico re-visited: eruption ages and volumes, oxidation states, and sulfur content. Bull Volcanol 68:407–419 Carrasco-Núñez G, Siebert L, Díaz-Castellón R, Vásquez-Selem L, Capra L (2010) Evolution and hazards of a long-quiescent

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