(55–25 Ma) volcanism in central Mexico
Transcription
(55–25 Ma) volcanism in central Mexico
Tectonophysics 471 (2009) 136–152 Contents lists available at ScienceDirect Tectonophysics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t e c t o Post-Laramide and pre-Basin and Range deformation and implications for Paleogene (55–25 Ma) volcanism in central Mexico: A geological basis for a volcano-tectonic stress model Margarito Tristán-González a,b,1, Gerardo J. Aguirre-Díaz b,⁎, Guillermo Labarthe-Hernández a, José Ramón Torres-Hernández a, Hervé Bellon c a b c Instituto de Geología/DES Ingeniería, Universidad Autónoma de San Luis Potosí, Av. Dr. Manuel Nava 5, Zona Universitaria, 78240, San Luis Potosí, Mexico Centro de Geociencias, Universidad Nacional Autónoma de México, Campus UNAM-Juriquilla, Querétaro, Querétaro, 76230, Mexico UMR 6538, Domaines Océaniques, IUEM, Université de Bretagne Occidentale, 6, Av. Le Gorgeu, C.S. 93837, F-29238 Brest Cedex 3, France a r t i c l e i n f o Article history: Received 3 May 2008 Accepted 23 December 2008 Available online 13 January 2009 Keywords: Volcano-tectonics Paleocene–Oligocene Basin and Range extension Sierra Madre Occidental volcanism Mexico a b s t r a c t At central-eastern Mexico, in the Mesa Central province, there are several ranges that were formed after the K/T Laramide compression but before the Basin and Range peak extensional episodes at middle–late Oligocene. Two important volcano-tectonic events happened during this time interval, 1) uplift of crustal blocks exhuming the Triassic–Jurassic metamorphic sequence and formation of basins that were filled with red beds and volcanic sequences, and 2) normal faulting and tilting to the NE of these blocks and fanglomerate filling of graben and half-graben structures. The first event, from late Paleocene to early Eocene, was related to NNE and NNW oriented dextral strike-slip faults. These faults were combined with NW–SE en echelon faulting in these blocks through which plutonism and volcanism occurred. The second event lasted from early Oligocene to early Miocene and coincided with Basin and Range extension. Intense volcanic activity occurred synchronously with the newly-formed or reactivated old fault systems, producing thick sequences of silicic pyroclastic rocks and large domes. Volcano-tectonic peaks occurred in three main episodes during the middle–late Oligocene in this part of Mexico, at about 32–30 Ma, 30–28 Ma, and 26– 25 Ma. The objectives of this work is to summarize the volcano-tectonic events that occurred after the end of the Laramide orogeny and before the peak episodes of Basin and Range faulting and Sierra Madre Occidental Oligocene volcanism, and to discuss the influence of these events on the following Oligocene–Miocene volcano-tectonic peak episodes that formed the voluminous silicic volcanism in the Mesa Central, and hence, in the Sierra Madre Occidental. A model based upon geological observations summarizes the volcanictectonic evolution of this part of Mexico from the late Paleocene to the Early Miocene. © 2009 Elsevier B.V. All rights reserved. 1. Introduction An elevated plateau in central Mexico, with an average elevation of about 2000 m above sea level (Fig. 1), includes some of the best mapped Tertiary volcanic areas of Mexico. It is known as Mesa Central, which is described as part of the southern Basin and Range extensional province (Henry and Aranda-Gómez, 1992; Stewart, 1998; Nieto-Samaniego et al., 1999; Aranda-Gómez et al., 2000; Nieto-Samaniego et al., 2005) and the Sierra Madre Occidental volcanic province (McDowell and Clabaugh, 1979; Aguirre-Díaz and ⁎ Corresponding author. E-mail addresses: mtristan@uaslp.mx (M. Tristán-González), ger@geociencias.unam.mx (G.J. Aguirre-Díaz). 1 Tel.: +52 444 8171039. 0040-1951/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2008.12.021 McDowell, 1991; Ferarri et al., 2005). According to Aguirre-Díaz and Labarthe-Hernández (2003) these two geologic provinces overlap in space and time throughout their extent across Mexico, including Mesa Central. The eastern border of Mesa Central is marked by the Sierra Madre Oriental folded belt (Fig. 2), which is composed of Mesozoic marine sediments deformed during the Laramide orogeny at late Cretaceous–early Paleocene (De Cserna, 1956; Tardy et al., 1975; Padilla, 1985; Chávez-Cabello et al., 2004 –Fig. 2). Other ranges, faultbounded and with Triassic metamorphosed basement cores, can be observed near this eastern margin and towards the interior of the Mesa Central (Fig. 2). These ranges have been interpreted also as caused by the Laramide orogeny (Martínez-Pérez, 1972; AguillónRobles- Tristán-González, 1981; Labarthe-Hernández et al., 1982a,b; Gallo-Padilla et al., 1993; Gómez-Luna et al., 1998), but our data presented here indicates that they were apparently formed after Laramide orogeny. M. Tristán-González et al. / Tectonophysics 471 (2009) 136–152 137 Fig. 1. Index map of the Sierra Madre Occidental and Mesa Central provinces indicating the location of the study area. Most of the studies in this area have focused either on the Laramide compression-related structures and Mesozoic stratigraphy or on the Basin and Range extension-related structures and Oligocene Sierra Madre Occidental volcanism. In contrast, little is known on the faultbounded structures with Triassic and Jurassic cores mentioned above because the available works have been published in local internal geological reports (e.g., Labarthe-Hernández et al., 1982a,b, 1995; Tristán-González and Torres-Hernández, 1992; Tristán-González et al., 1995). From these reports, it can be inferred that important volcanotectonic events occurred between the late Paleocene and the late Oligocene that developed these fault-bounded ranges and some faultbounded basins as well, synchronously with plutonism and volcanism. The main purpose of this study is to summarize the volcanotectonic events that occurred between the end of the Laramide orogeny and the initiation of Basin and Range faulting and Sierra Madre Occidental Oligocene volcanism. We discuss the influence of these events on the following Oligocene–Miocene volcano-tectonic peak episodes that formed the voluminous silicic volcanism in the Mesa Central, and hence, in the Sierra Madre Occidental. In order to achieve these goals, the stratigraphy, geochronology and structure of three representative areas of the Mesa Central are briefly described, 1) La Ballena-Peñón Blanco range, 2) Las Minas range and 3), Ahualulco basin (Fig. 2). The first and second cases represent fault-bounded ranges with Triassic and Lower Cretaceous basement cores, respectively, and the third one, a listric-fault basin that initiated as a pullapart basin filled with a volcano-clastic sequence. This work provides a volcano-tectonic evolution model of a large area in central Mexico (Figs. 1, 2), based upon rigorous geological observations, which can be used as a case study to test experimental or mathematical volcano-tectonic stress models of continental crust that was first submitted to an intense compressive stress regime (Laramide orogeny), then to a crustal relaxation and trans-tension stress period, and finally to an intense extensional regime (Basin and Range extension); all of these occurring at the final stages of a long-lasting continental-margin subduction regime (Aguirre-Díaz and McDowell, 1991). Similar situations have been reported in other places and different geologic times with the result of an intense period of rhyolitic– andesitic volcanism in the form of domes and/or stratovolcanoes and ignimbrites; for instance, at the Catalan Pyrenees, where Permian– Carboniferous ignimbrites are apparently related to calderas influenced by the strike-slip tectonics (Martí, 1991), or at the Taupo Volcanic Zone, where silicic caldera volcanism and andesitic stratovolcanoes can be associated with rift-extension and trans-tension respectively (Spinks et al., 2004). Using the particular case of the Sierra Madre Occidental, Aguirre-Díaz et al. (2007, 2008) have coined the term of graben calderas for these types of volcano-tectonic caldera structures. 2. Tectonic framework The northern, northeastern and eastern limits of the Mesa Central are formed by the ranges of the Sierra Madre Oriental folded belt (Fig. 3). Several studies have been undertaken in this belt to understand the tectonic shortening at this area during the Upper Cretaceous–Early Tertiary, and in particular, on the portion where the belt makes a turn to the west at the Monterrey salient or “Curvatura de 138 M. Tristán-González et al. / Tectonophysics 471 (2009) 136–152 Fig. 2. Digital elevation model showing the main tectonic structures in the eastern and southeastern part of Mesa Central. 1—Sierra de Catorce, 2—Sierra de Coronado, 3—Sierra de Charcas, 4—Sierra Santa Catarina, 5—Sierra de Guanamé, 6—Sierra Las Minas, 7—Sierra La Ballena-Peñón Blanco, 8—Sierra de Zacatecas; A—Ahualulco Basin, B—Coronado Basin; C— Matehuala-El Huizache Basin; D—Villa de Arista Basin; E—Peotillos Basin; F—Aguascalientes Graben; G—Villa de Reyes Graben; SLPVF—San Luis Potosí Volcanic Field; MC—Monterrey Curvature. Monterrey” (Fig. 3, De Cserna, 1956; Tardy et al., 1975; Padilla, 1985; Chávez-Cabello et al., 2004). This deformation is characterized by folding and thrusting of the upper crust with an ENE transport direction, as well as by transcurrent faulting associated to these displacements. The Monterrey salient has been interpreted as the result of an orthogonal flexural folding that occurred in the late stage of the Laramide orogeny, and this regional deformation has been related to a “décollement” that produced the detachment of the upper carbonated and clastic sequence over the Minas Viejas evaporites (Padilla, 1985; Fischer and Jackson, 1999; Marrett and Aranda; 1999; Chávez-Cabello et al., 2004). The fold and thrust belt continues southward from the Monterrey salient and forms the eastern limit of the Mesa Central. From this eastern boundary and towards the inner parts of the Mesa Central the folded belt changes to fault-bounded ranges with NNE-trending and NW-trending patterns, some of which (mostly the NNE-trending) expose Triassic metamorphosed basement and that are separated by flat valleys (Fig. 4). The faults that bound the ranges are either strike-slip or normal faults, and apparently both lateral and lateral displacements occurred in the same faults juxtaposing different faulting episodes. In some cases, such as La Ballena-Peñón Blanco, Sierra Real de Catorce, Coronado, and Zacatecas, the ranges are bounded along one side by NNE normal faults causing M. Tristán-González et al. / Tectonophysics 471 (2009) 136–152 139 Fig. 3. Main regional tectonic structures for northeastern and central Mexico, based on satellite image interpretation and field geological studies (modified after Vélez-Scholvink, 1990). tilting to the east and exposing basement cores with Triassic rocks (Fig. 4). In other cases, such as Charcas, Santa Catarina, Las Minas and Guanamé ranges (Fig. 4), the ranges are outstanding blocks limited by NW and SE normal faults on both sides, which indicate relative vertical uplift with little or no tilting (Fig. 4). The ranges of Santa Catarina, Sierra Las Minas, La Parada as well as the Ahualulco basin form part of a large crustal block named here as the Pinos-Moctezuma block, with dimensions of at least 100 by 40 km, and that is limited by two large parallel NE trending lineaments that could be interpreted as fault systems. Strike-slip dextral displacements are inferred along these lineaments from en-echelon patterns, lateral displacement of units and from direct observation of a few cinematic indicators that were not erased by posterior vertical displacement on the same faults. These lineaments are named by us as La Pendencia (the western lineament) which extends for at least 100 km, from Villa García to Charcas, and Ahualulco (eastern lineament), extending ~ 85 km from Pino Suárez to Villa Arista (Fig. 4). Adjacent to this large block and to the west there is another NE-oriented series of aligned ranges that form another large crustal block, named here the Salinas-Charcas block, which is parallel to the Pinos-Moctezuma block and separated by the La Pendencia lineament (Fig. 4). The right-lateral movements along the La Pendencia and Ahualulco lineaments have been interpreted as caused by a dextral simple shear under transpression as described by Wilcox et al. (1973), or an oblique simple shear following Jones and Holdsworth (1998). In general, all the elevated ranges of the eastern Mesa Central are cut internally, i.e., within the range, by NW–SE normal faults that were formed at late Paleocene–early Eocene, an age constrained from plutonic and volcanic rocks that were emplaced through these faults (more details are described below). Parallel to these elongated ranges and at the eastern part of the area there are a series of basins that have been filled up with continental clastic sediments (red beds), such as the basins of Matehuala-Huizache, Coronado, Villa Arista, Ahualulco, and Peotillos (Fig. 2). All these basins include early to middle Tertiary volcanic rocks, too. At some of them, intrusive and volcanic rocks were emplaced through their fault-bounded margins, suggesting that these igneous rocks were tectonically controlled. Following is a brief geologic and structural description of the three representative ranges of La Ballena-Peñón Blanco, Las Minas and the Ahualulco basin, with their respective simplified geologic maps. Due to the size reduction of these maps because of publication purposes, many details from the original maps (scales 1:25,000 and 1:50, 000) have been omitted, but we do show the most relevant information. Fig. 4. Geologic map of the southeastern portion of the Mesa Central showing late Paleocene–early Eocene structures. 1—Sierra de Charcas, 2—Sierra de Coronado, 3—Sierra de Guanamé, 4—Sierra La Ballena-Peñón Blanco, 5—Sierra Santa Catarina, 6—Sierra Las Minas, 7—Sierra La Parada, 8—Sierra La Tapona, 9—Ahualulco Basin. Interpreted geologic crosssections for three different types of uplifts in the region are shown at bottom of Fig. 1) A–A′ vertical with exhumed core (Sierra de Charcas). 2) listric B–B′ at eastern sector (Sierra de Coronado). 3) c–c′ vertical without exhumed core (Sierra de Guanamé). 4) D–D′ listhric at eastern sector. Base map based on LANDSAT Thematic mapper image bands 1, 4, 7. Original scale 1:250, 000. 140 M. Tristán-González et al. / Tectonophysics 471 (2009) 136–152 M. Tristán-González et al. / Tectonophysics 471 (2009) 136–152 141 Fig. 5. Geological map of the La Ballena-Peñón Blanco range (modified after Labarthe-Hernández et al., 1982a,b). 2.1. La Ballena-Peñón Blanco range The Sierra La Ballena-Peñón Blanco forms the southeasternmost end of the Salinas-Charcas block (Fig. 4). This range includes the oldest rocks of the area (Late Triassic) and Eocene Peñón Blanco granite, with 2700 m above sea level, is one the highest peaks in the region. The Ballena-Peñón Blanco range is 30 km long and 5 km wide bounded to the west by a listhric normal fault with a NNE strike (Fig. 5). The range is internally segmented in five parts separated by four normal faults with an average strike of N60°–70°W. A series of granitic intrusions are exposed along the faults within the central parts, the main of which is Peñón Blanco granite of middle Eocene age (45.5 ±1.1 Ma, Table 1). Several granitic dikes and small apophyses were also emplaced along these faults. In some of the faults it was 142 M. Tristán-González et al. / Tectonophysics 471 (2009) 136–152 Table 1 New K–Ar ages from the Mesa Central. Volcanic unit Las Joyas basalt (Tbj) Upper Panalillo ignimbrite (Trp) Riolitic domes (Tdr) Riolitic domes (Tdr) Portezuelo latite (Tlp) Portezuelo latite (Tlp) Portezuelo latite (botton of ignimbrite) Jacavaquero dacite (Tdj) Zapatero riodacite (Trz) Casita Blanca andesite (Tcb) Casita Blanca andesite (Tcb) Peñon Blanco granite (Tgr) Sample 01–25 01–24 01–32 01–28 01–22 01–26 01–30 01–21 01–29 01–33 01–31 01–14 Coordinates 40 Arb Age a Latitud N Longitud W ± 1σ 2499 631 2487 607 2476 050 2480 728 2492 003 2504 063 2494 836 2490 725 2479 946 2493 349 2499 923 2493 748 283 207 287 070 269 295 270 404 281 659 280 774 280 240 276 673 274 610 271 210 276 778 224 449 01.5 ± 0.8 25.4 ± 0.6 31.0 ± 0.7 31.6 ± 0.8 31.0 ± 0.7 31.0 ± 0.7 32.2 ± 0.8 31.6 ± 0.8 31.2 ± 0.7 44.4 ± 1.0 45.5 ± 1.1 45.3 ± 1.1 40 ArRc K2O 0.99 49.26 54.59 49.30 47.68 47.60 63.35 47.50 59.49 28.50 26.60 145.90 2.06 6.04 5.41 4.78 5.22 4.71 6.09 4.70 5.86 1.79 1.79 9.86 (%) 18.1 81.7 82.7 49.1 68.7 91.6 79.6 75.4 86.6 82.4 72.5 77.90 Fractiond (wt.%) WR WR WR m m WR WR WR WR WR WR mu Ages performed in the laboratory of geochronology at Université de Bretagne Occidentale at Brest France. Coordinates in UTM system, 14Q Zone, using NAD27 projection. a Error at one σ was calculated with the equation given by Cox and Dalrymple (1967). b 40 Ar; radiogenic argon content of sample, in percent of total. c 40 ArR; radiogenic argon in sample is expressed in 10−7 cm3/g. d Dated material; WR-whole rock, m-matrix, mu-muscovite. possible to observe fault surfaces with striae indicating right-lateral displacement (Fig. 5). The Peñón Blanco granite did not form a prominent contact metamorphism aureole, but makes a sharp contact with the country rock, Jurassic on one side, and Cretaceous on the other due to the fault displacement before the granite emplacement (Fig. 4). The La Ballena-Peñón Blanco range is bounded to the west by a listhric normal fault inferred from stratigraphic and geologic relations and is named here as La Ballena fault (Figs. 4, 5). This fault has an average N 05°E strike and tilted the Mesozoic sedimentary sequence 15° to 18° to the SE. The tilting caused further exposure of Late Triassic basement (Zacatecas Formation). The Peñón Blanco granite as well as smaller contemporaneous granitic plutons (dikes and apophyses) were emplaced parallel to the trace of this fault. Plateaus formed by the Panalillo ignimbrite in the southern portion of this range are completely horizontal and thus they were not affected by the La Ballena fault, indicating that principal fault movement predated the 25.4 ± 0.6 Ma Panalillo ignimbrite (Table 1). However, at other sites at the eastern part of the study area, Panalillo ignimbrite is affected by normal faulting of the late Basin and Range extension. 2.2. Las Minas range The Las Minas range is located at the northernmost part of the San Luis Potosí Volcanic Field (Figs. 2, 4). It is a small range with a rhombohedral shape that covers an area of about 50 km2. It is considered as a plunging anticline developed during the Laramide orogeny at the end of Cretaceous (Aguillón-Robles and TristánGonzález, 1981). This range outstands from an alluvial plain with structural windows showing Maastrichtian marine sedimentary rocks and remnants of Oligocene volcanic rocks (Fig. 6). It shows the typical folding of the Laramide orogeny, with folds axes trending N–S (Fig. 5—1), fold vergence to the east and an average tectonic transport azimuth of 95°, with slicken-sides on So fault planes (equal area net 2 of Fig. 6). This deformation style is similar to that observed in other ranges in the region with Lower Cretaceous rocks (Fig. 4). However, field evidence indicates that Las Minas range is a horst and not an anticline. It was up-lifted in the early Tertiary (after the Laramide orogeny) and is limited by two main faults striking N40°W, dipping to the NE and SW. At its southern end the eastern fault changes strike to a more N–S direction (Fig. 6). Dextral strike-slip faults within the horst have a strike of N60°–80°W. Therefore, this range was apparently formed after Laramide deformation by a trans-pressional tectonics. This hypothesis is based upon the bounding and the internal faults observed in the range; that is, the uplifting occurred from a trans-pressional deformation that pushed-up the block along its marginal faults, following the oblique simple shear model of Jones and Holdsworth (1998). Besides, a few kilometers to the south of Las Minas range there are faults with left-lateral displacement affecting Upper Cretaceous rocks (Fig. 4). These are small faults (not shown in Fig. 6 because of the scale) within the Pinos-Moctezuma block that are here interpreted as antithetic strike-slip faults, which resulted from a clockwise rotation of the Pinos-Moctezuma block (Fig. 4). Therefore, these observations favor the trans-pression interpretation for Las Minas range. Andesitic dikes of late Eocene associated to this trans-pressional episode are found in the NE part of the range; thus, suggesting that these conditions were also favorable for magma ascension and/or near-surface emplacement as was in the case of La Ballena-Peñón Blanco range. 2.3. Ahualulco Basin This basin is located at the northeastern part of the PinosMoctezuma block (Fig. 4). It also includes the northern part of the San Luis Potosí Volcanic Field (Fig. 7). The Ahualulco basin has been defined as a tectonic depression related to Oligocene Basin and Range normal faulting (Labarthe-Hernández and Tristán-González, 1981; Labarthe-Hernández et al., 1982b, 1995; Martínez-Ruíz, 1994). The basin has a rough rhombohedral shape with dimensions of 45 by 15 km (Fig. 8). The floor of the basin consists of Upper Cretaceous sedimentary rocks. The basin fill includes Eocene andesitic lavas, Oligocene red beds, volcano-clastic sediments, Oligocene rhyodacitic domes and Oligocene rhyolitic ignimbrites, for a total thickness of about 800 m (Fig. 9). Capping the basin fill as well as the rocks outside the basin, is a younger sequence made by the late Oligocene Panalillo ignimbrite, Miocene–Pliocene? continental conglomerates and Quaternary basalts. Small graben-forming faults that affect the intra-basin rocks can be constrained at 31–28 Ma from cross-cutting relationships. Thus, this second faulting event is interpreted here as an episode different from the original one that formed the Ahualulco basin, which occurred at the early Eocene, based upon the first lavas that filled the basin (50–42 Ma). However, conglomerate deposits covering the Panalillo ignimbrite over the Ahualulco basin are tilted but the Quaternary basalt is not, indicating that faulting continued after Panalillo ignimbrite for an unknown time but ended before the eruption of the Quaternary basalt, possibly during late Miocene. We interpret the Ahualulco basin as a pull-apart graben instead of a simple Basin and Range graben as previously believed. This interpretation is based upon several observations, 1) the intra-basin M. Tristán-González et al. / Tectonophysics 471 (2009) 136–152 143 Fig. 6. Geological map of the Sierra Las Minas range. This range is 300 m higher with respect the adjacent plain (mostly of Upper Cretaceous rocks). Note that the core is formed by Lower Cretaceous rocks. Lower hemisphere equal area projections are shown in the bottom of Fig. 1) Laramide fold axis poles and slicken-sides lineation on So beds. 2) Laramide tectonic transport direction (modified after Aguillón-Robles and Tristán-González., 1981). faults have a braided arrangement with a general strike of NW–SE and an average dip of 82° SW, (equal area net 1, 2 of Fig. 10), 2) the slickensides on the intra-basin fault planes are oblique and horizontal indicating lateral displacements, 3) the rhombohedral shape of the basin and 4), the dextral strike-slip inferred movement along the La Pendencia and Ahualulco regional faults (Fig. 4). Some of these early Eocene strike-slip faults were overprinted with vertical-slip displacements that occurred at the late Oligocene during the Basin and Range tectonics, causing the misinterpretation of Ahualulco basin as just another typical graben of the Basin and Range that does not takes into account its initial strike-slip stage. This early stage was coeval with the regional strike-slip displacements that occurred in all the area during the late Paleocene–early Eocene. All the sequence that filled the Ahualulco basin is tilted to the NE due to a large listric fault (Section D–D′ of Fig. 4) that reactivated the fault that originally bounded the basin at the west (Fig. 8). Tilting increases from the youngest to the oldest rocks of the sequence, from 26° in 31.5 Ma volcanic rocks (Portezuelo dacite) to 40° in middle Eocene andesites (Cenicera Formation). This gradual tilting change with time indicates that tilting accumulated during episodic faulting events from the late Eocene to the Miocene (Fig. 10). The volcanic rocks that filled the basin were apparently synchronous with basin development, as they accumulated within the basin as subsidence was occurring. These rocks are more voluminous within the basin, and are rather sparse outside the basin. Emplacement of some andesitic dikes contemporaneous with the volcanic rocks of the filling sequence confirms this hypothesis. 3. Summary of volcano-tectonic-sedimentary events from Laramide orogeny to Basin and Range extension in the eastern Mesa Central The final stage of the Laramide orogeny in central-eastern Mexico occurred by the end of the late Paleocene–early Eocene. The latter 144 M. Tristán-González et al. / Tectonophysics 471 (2009) 136–152 Fig. 7. Geologic map of the northern sector of the San Luis Potosi Volcanic Field, included Ahualulco Basin and the Sierra Las Minas sectors (modified after Labarthe-Hernández et al., 1982a,b). according to the ages of several intrusive bodies that do not show compressive deformation, the latest of which is dated at 55 Ma (Table 2). Following Aguirre-Díaz and McDowell (1991), it is summarized graphically the different volcanic, tectonic and sedimentary events that occurred in the Mesa Central after the end of the Laramide orogeny (Fig. 11). These events are correlated with the main tectonic and magmatic regimes affecting Mexico between 60 and 20 Ma, from subduction-related (subduction of the Farallon plate beneath North America) to extensional-related (Basin and Range extension). After the Laramide orogeny, and along the limit between the crustal blocks of the Valles-San Luis Potosí Platform and the Mesozoic Basin of Central Mexico, a shear zone oriented NNE was developed. Dextral strike-slip movement between these crustal blocks produced a series of en echelon folds and uplift of smaller blocks with basement nuclei as old as Triassic that formed high ranges within the shear zone. These ranges were then displaced by high-angle NW–SE normal faults at about middle Eocene, which served as conduits for intrusive and volcanic rocks of this age. At the same time, subsidence occurred in the corresponding NW–SE grabens in which continental clastic deposits (red beds) and volcanic rocks were accumulated (Fig. 11). Several andesitic NNW dikes and small cones aligned NNW were emplaced within these basins. This fissural volcanism still show a subalkaline composition that can be associated to the subduction regime of the Farallon–North American plates, which should have continued active by this time along western Mexico (Fig. 11; Atwater, 1989; Aguirre-Díaz and McDowell, 1991; Ferrari et al., 1999). The fact that the middle Eocene andesitic volcanism was fed from dikes, suggests that an incipient extensional regime was starting (transitional volcanism, Fig. 11). Extensional activity increased with time, and by 32 Ma it was already relatively intense (Fig.11). At 32– 30 Ma there was a peak in the extension in northern Mexico including the study area, which can be related with the regional tectonics change caused by the collision of the East Pacific Rise with North America (Fig. 11; Atwater, 1989, Aguirre-Díaz and McDowell, 1991). This peak extensional episode was accompanied by a syn-extensional volcanic episode that produced voluminous felsic ignimbrites and lava domes dated at 32–29 Ma (Fig. 11). After this peak event, the magmatic conditions changed from predominantly felsic and subalkaline to a bimodal style of high-silica rhyolites and alkalic basalts. These bi-modal M. Tristán-González et al. / Tectonophysics 471 (2009) 136–152 145 Fig. 8. Structural map of Ahualulco Basin at the northern portion of the San Luis Potosi Volcanic Field showing the curvilinear pattern of faults. 1—Equal-area net showing attitude of normal, high-angle NW–SE faults in the western sector of the basin. 2—Equal-area net sowing attitude of normal faults in the northeastern sector of the basin. B, C, D and E represent the equal area net from tilting of the Tertiary sequence shown in Fig. 10. 146 M. Tristán-González et al. / Tectonophysics 471 (2009) 136–152 Fig. 9. Composite stratigraphic column for the northern sector of the San Luis Potosi Volcanic Field and the Ahualulco Basin (K–Ar ages data are shown in Table 1). volcanic events occurred mainly at 28–26 Ma (Fig. 11), and were fed from fissures related to the high-angle faults that formed the NNE and NNW grabens and half-grabens of the study area (Torres-Aguilera y Rodríguez-Ríos, 2005; Aguirre-Díaz et al., 2008). The same faults were reactivated later as several discrete episodes until Quaternary (1 Ma). Some times these reactivations were accompanied with basaltic volcanism with alkaline composition that shows a clear intra-plate signature (basanites, alkalic basalts, hawaiites; Luhr et al., 1995). The shear zone that was formed along the limits of the crustal blocks of the Valles-San Luis Potosí Platform and the Mesozoic Basin of Central Mexico, represent a crustal weakness zone (Fig. 12). This zone was developed following the Matehuala-San Luis lineament (Fig. 4). At 32–31 Ma, dacitic and trachytic lava domes were emplaced following the high-angle normal faults formed during the Eocene and Oligocene (Fig. 12A). At 31–28 Ma, high-angle extension and Oligocene synextensional volcanism episodes of rhyolitic lava emissions and pyroclastic rocks occurred along the SW margin of the MatehualaSan Luis lineament (Fig. 12B). At 28–26 Ma, there was another intense extensional episode that affected only the narrow zone along the Matehuala-San Luis lineament, producing low angle listric faulting that tilted the affected blocks up to 50° to the NE. The zone that concentrates the maximum extension follows a NNE trend and is about 30–50 km wide (Fig. 12C). Due to these characteristics, it is named here as the Matehuala-San Luis Maximum Extension Zone. At 28–25 Ma, syn-extensional pyroclastic volcanism occurred along some of the faults of the Matehuala-San Luis Maximum Extension Zone. 4. Tectono-magmatic evolution model for the eastern Mesa Central for the 55–25 Ma period It is presented here a geologic model that summarizes the tectono-volcanic evolution of the central-eastern Mesa Central from late Paleocene to late Oligocene (Fig. 13). The timing for the final events related to the Laramide compression in the area can be deduced from the ages of the non-compressionally-deformed plutons, which indicates that this compressive regime finished at the early Paleocene (Fig. 13A). This compression shortened the Mesozoic sedimentary sequence eastward, and formed numerous recumbent folds, thrusts and inverse faults (napes and decollements) that culminated with the Monterrey salient to the northeast and outside the Mesa Central. After the compressive phase of the Laramide orogeny, a major tectonic change took place during the Paleocene at the central-eastern Mesa Central that could be visualized as a crustal relaxation period that followed after a long- M. Tristán-González et al. / Tectonophysics 471 (2009) 136–152 147 Fig. 10. Lower hemisphere equal-area projection showing attitude of tilt direction of the Tertiary sequence in Ahualulco Basin. A) Attitude of tilt direction of Cenicera Formation in the northwestern part of the basin. B) Data from the ignimbrite underlying Portezuelo latite in the northwestern portion of the basin. C) Attitude of tilt direction in the San Nicolás epiclastics. D) Attitude of tilt direction of Upper Panalillo ignimbrite at the western part of the basin. E) Tilt direction of upper conglomerate at the eastern part of the basin. lasting intense compression, similarly as has been proposed farther north in the central-eastern Sierra Madre Occidental (Aguirre-Díaz and McDowell, 1991). During this period large listric faults were developed together with several basins and strike-slip accommodation faults at the eastern Mesa Central (Fig. 13B), whereas at the central portion of the Mesa Central large blocks were vertically uplifted as crustal wedges due to space accommodation (Fig. 13B). At the beginning of the Eocene (58–45 Ma), several plutons and andesitic lavas were emplaced in or next to the uplifted blocks, using the marginal and internal faults of the blocks as conduits during ascension (Fig. 13C). In this time, basins developed next to the uplifted blocks were filled by continental clastic sediments (Fig. 13C). At Oligocene (32–30 Ma) took place the most voluminous volcanic event in the area, consisting of both explosive and effusive eruptions that formed sequences of silicic pyroclastic rocks and lava domes. This volcanism occurred simultaneously with reactivation of old 148 M. Tristán-González et al. / Tectonophysics 471 (2009) 136–152 Table 2 Structural data of the faults of the central and the north portion of Ahualulco Basin. No 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 Dip direction Pitch 055°/88° 055°/88° 195°/75° 210°/85° 240°/45° 205°/60° 090°/75° 270°/50° 065°/60° 220°/85° 220°/80° 060°/55° 060°/55° 200°/75° 155°/85° 250°/70° 220°/88° 220°/60° 250°/70° 250°/65° 245°/75° 250°/85° 295°/50° 090°/65° 245°/55° 200°/75° 200°/75° 200°/70° 220°/65° 205°/70° 220°/80° 050°/80° 220°/80° 250°/65° 305°/80° 270°/80° 090°/75a 190°70° 040°/88° 210°/50° 202°/70° 280°/60° 075°/60° 270°/53° 170°/85° 075°/42° 200°/63° 225°/53° 085°/68° 240°/65° 245°/60° 240°/50° 220°/55° 252°765° 080°/50° 065°/35° 250°/75° 106°/65° 270°/60° 280°/46° 090°/56° 205°/55° 300°/75° O95°/80° 273°/70° 250°/65° 270°/50° 240°/63° 240°/70° 210°/70° 195°/74° 245°/76° 340°/88° 335°/87° 40° SE 40° SE 00° 90° 90° 60° SE 80° N 90° 10° SE 90° 90° 65° SE 80° SE 60° NW 45° SW 90° 90° 90° 70° NW 90° 90° 20° NW 90° 90° 70° SE 25° NW 80° NW 90° 35° NW 80° SE 10° NW 90° 00° R 00° R 90° 90° 90° 90° 60° SE 90° 60° SE 90° 70° SE 0° 0° 90° 73° NW 90° 90° 90° 90° 53° NW 90° 90° 90° 90° 90° 66° NE 90° 90° 0° 90° 0° 90° 90° 90° 90° 90° 90° 90° 90° 38° NW 0° 0° Table 2 (continued) No Dip direction Pitch 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 220°/65° 255°/55° 260°/68° 040°/70° 050°65° 215°/65° 220°/65° 215°/65° 225°/60° 240°/65° 270°/57° 280°/60° 263°/74° 110°/35° 170°/70° 050°/80 297°/53° 075°/87° 040°/89° 310°/50° 070°/70° 216°/80° 215°/78° 195°/43° 090°/75° 090°/70° 360°/70° 245°/87° 190°/75° 245°/62° 270°/755° 210°/70° 060°/65° 114°/47° 160°/88° 230°/43° 240°/65° 270°/55° 210°/60° 055°/81° 250°/60° 0° 90° 0° 0° 90° 90° 90° 90° 90° 90° 90° 90° 90° 90° 90° 90° 90° 90° 0° 90° 90° 90° 90° 44° NW 90° 0° 90° 90° 50° SE 90° 90° 90° 90° 90° 45° SW 90° 90° 90° 90° 90° 90° Coordinates North East 2494669 2494669 2494581 2494581 2494552 2494552 2494441 2494406 2494406 2494542 2495442 2495815 2495815 2491928 2491804 2491328 2490596 2490596 2490488 2490517 2490730 2491540 2491661 2492211 2491819 2491516 2491516 2491535 2491278 2491306 2491458 2495186 2495151 2495203 2494938 2495654 249598 2495213 2495440 2495323 2494887 2494617 2494427 2494825 2494825 2496668 2496704 2496704 2496534 2496534 2496411 2496496 2496131 2495750 2496637 2496468 2496023 2495823 2495459 2495737 2493283 2494023 2494006 2494082 2493955 2493374 2494087 2492044 2492020 2492124 2492315 2491993 2491164 2491071 276964 276964 277059 277059 277216 277225 277303 277353 277353 277408 277765 276949 276949 275579 276395 276395 276381 276381 276217 276075 275927 274213 274159 276652 277014 277345 277345 277480 278493 278053 277804 280997 280914 280850 280908 281679 281763 282265 281881 281285 280702 279682 279937 280696 280696 278298 278501 278501 278611 278641 278721 279038 279134 279233 278122 278153 278338 278196 278402 278402 275602 277933 277970 278137 278233 280062 278009 281698 281609 281484 281065 280416 281614 281527 Coordinates North East 2490839 2491460 2489081 2489599 2489545 2489770 2489897 2489964 2490023 2489739 2489799 2493010 2492469 2496551 2495213 2494911 2494941 2491583 2491675 2492699 2494573 2494163 2495352 2495 352 2492705 2492456 2496144 2496016 2496016 2496307 2496922 2496765 2492135 2492724 2492818 2492785 2489601 2490133 2487362 2487680 2486797 281280 280417 280158 281832 281657 281678 281380 281387 281263 280931 280340 271565 271356 280481 282265 281964 281716 282308 282308 283097 285889 284421 282541 282541 286434 286170 283747 283924 283924 283125 283007 283088 282547 286422 283533 283475 287268 287174 287780 286984 286566 Notes: Coordinates in UTM system, 14Q Zone, using NAD27 projection. faults related to the first-developed basins. Both the pyroclastic deposits and the lava domes were emplaced within these basins (Fig. 13D). Volcanism of this period was rhyodacitic and the vents related to the effusive products (domes) formed NNW chains parallel to the basins' main orientations. At 30–28 Ma, syn-extensional volcanism developed chains of elongated lava domes with a high-silica rhyolitic composition. Extension at this time marks the beginning of the Basin and Range event in this area. These domes were controlled by NWoriented normal faults, which were used as conduits of these magmas (Fig. 13E). Then, between 28 and 26 Ma Basin and Range extension reaches its peak in intensity in this area, and formed NW-oriented fault systems and grabens, reactivating older faults and creating new fault systems. At 26–25 Ma, widespread ignimbrite-forming pyroclastic flows were erupted through these faults that filled the graben and half-graben structures produced in this extensional event (Fig. 13F). Finally, these depressions were further filled with conglomerates and epiclastic rocks that were tilted during Basin and Range faulting activity that continued at least until Miocene. 5. Conclusions Uplifting of blocks associated with basin development related to right-lateral strike-slip tectonics characterized the late Paleocene–early Eocene interval of central-eastern Mexico. Early Eocene large plutons and volcanic rocks were emplaced within these blocks by means of an en-echelon fault system affecting these blocks. The uplifted blocks and M. Tristán-González et al. / Tectonophysics 471 (2009) 136–152 149 Fig. 11. Summary of tectonic, volcanic, and sedimentary events in the Mesa Central for the time range between 60 and 20 Ma (after Aguirre-Díaz and McDowell, 1991). See text for explanation. associated plutons formed some of the highest ranges observed in the eastern Mesa Central province, such as Peñón Blanco and Sierra del Catorce. At the same time, thick sequences of continental clastic deposits as well as silicic-andesitic lavas accumulated in pull-apart basins, as occurred in Ahualulco. During the early to middle Oligocene, intense volcanic activity occurred synchronously with the activity of newly-formed faults, reactivated old fault systems and produced thick sequences of silicic lava domes and pyroclastic rocks. Lava domes composition changed with time, from rhyodacitic to rhyolitic, and were formed contemporaneously with faulting episodes. The pyroclastic rocks are directly related with the faulting events, using these faults as their principal vents. NE oriented Basin and Range extension initiated at about the same time of rhyodacitic dome emplacement (32–30 Ma), and continued during emplacement of rhyolitic lava domes and pyroclastic rocks (30–28 Ma). Rhyolitic explosive volcanism continued in the area about ~26–25 Ma, which filled the contemporaneously formed graben. This study provides a volcano-tectonic evolution model based upon geological observations, which can be used as a case study to test experimental or mathematical volcano-tectonic stress models on continental crust affected by intense compression, then by a relaxation and trans-tension stage, and finally by an intense extensional regime, combined with a long-term subduction. Acknowledgements We thank to Adelina Geyer and Hiroaki Komuro for reviewing this work and for their comments, which substantially improved the manuscript. The authors also thank the comments of Elena Centeno and Scott Bryan on an earlier version. We also thank to Alfredo Aguillón Robles and Rodolfo Rodríguez for their suggestions and support during the development of this work. We appreciate the Principal Office of the Instituto de Geología of the Universidad Autónoma de San Luis Potosí, through Director Dr. Rafael Barboza, for financial and logistic support to this study. We recognize the help of Gildardo González and Ana Lizbeth Quevedo in the digitization of maps and figures. We are grateful to “Programa de Formación de Profesores” (PROMEP) for a 3-year scholarship to the first author. This study was financially supported in part by grants to GJAD from CONACYT No. 46005-P and from UNAMPAPIIT No. IN-115302. 150 M. Tristán-González et al. / Tectonophysics 471 (2009) 136–152 Fig. 12. Schematic diagram showing the different stages (A to D, see text for explanation) for the formation and development of the shear zone (SZ) between the crustal blocks of Valles-San Luis Potosí Platform and the Mesozoic Basin of Central Mexico at the end of the Laramide orogeny, which was later affected by high-normal faulting and syn-extensional volcanism during the Oligocene, and then affected by listric extension and syn-extensional volcanism after 28 Ma to develop the Matehuala-San Luis Maximum Extension Zone. Explanation of symbols, 1—pre-Tertiary crust, 2—magma of intermediate composition, 3—magma of rhyolitic composition, 4: Valles-San Luis Potosí Platform, 5—Mesozoic Basin of Central Mexico, 6—dacitic–trachytic lava domes, 7—silicic lava domes, 8—silicic pyroclastic rocks, 9—granitic batholiths. M. Tristán-González et al. / Tectonophysics 471 (2009) 136–152 151 Fig. 13. Tectono-volcanic schematic model of the southern and southeastern Mesa Central for the period between the end of the Laramide orogeny (late-Cretaceous–late Paleocene) and the onset of Basin and Range faulting (late Oligocene). See text for explanation of phases A to F. 152 M. Tristán-González et al. / Tectonophysics 471 (2009) 136–152 References Aguillón-Robles, A., Tristán-González, M., 1981. Cartografía Geológica Hoja Moctezuma. Univ. Autón. San Luis Potosí, Inst. Geol. Metal. Folleto Técnico, vol. 74. 30 pp. Aguirre-Díaz, G.J., McDowell, F.W., 1991. 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