sediments within the Rio Grande embayment, and Belcher. (1979), Wilson (1990) ... Sierra de San Carlos region (Robin and Tournon, 1978) (Fig. 1). PREVIOUS ...
Geological Socieiy of America Special Paper 340 1999
Crustal structure of northeastern Mexico revealed through the analysis ofgravity data Kevin Mickus Department of Geosciences, Southwest Missouri State University, Spi-ingfleld, Missouri 65804, United States Carlos Montana Department of Geological Sciences, University of Texas, El Paso, Texas 79968, United States
ABSTRACT An analysis of gravity data is used to compliment regional geological and tectonic studies of northeastern Mexico to determine a general crustal structure of the region. To determine a general crustal structure and the influence of tectonic events ranging from Precambrian to recent on the present-day gravity field, gravity anomaly maps, includ ing Bouguer, low- and band-pass filtered, were constructed. Band-pass filtered gravity anomaly maps, in addition to showing gravity maxima and minima correlating with known Mesozoic and Cenozoic tectonic features, are interpreted to indicate the possible existence of Triassic-Jurassic rift basins and a northern extension of the high-grade met amorphic rocks exposed at the Huizachal-Peregrina anticlinorium. Low-pass filtered gravity anomaly maps are interpreted in conjunction with published geological infor mation to indicate the existence of a magmatic arc accreted to the North American con tinent in the Jurassic. Two northeast-trending, regional gravity models constrained by previous geologic mapping, regional seismic studies, and well data indicate that the crustal thickness decreases from 41 kin near Zacatecas to 35 km along the Gulf coastal plain. The Jurassic magmatic arc as interpreted from gravity modeling and low-pass filtered gravity maps is located within the Sierra Madre Oriental from 25.5°N, 1O1.5°W, to 23.75°N, 100.O°W. Gravity maxima associated with the Sierra de Tamaulipas are interpreted to have been caused either by granitic intrusions andlor by denser transi tional upper crust formed during the opening of the Gulf of Mexico.
INTRODUCTION The general geologic and tectonic framework of north eastern Mexico is known in general terms from numerous geo logical studies and compilations (e.g., Guzmán and de Cserna, 1963; Padilla y Sanchez, 1986; Winker and Buffler. 1988; de Cserna, 1989; Salvador. l99la: Sedlock et al.. 1993; Wilson and Ward, 1993; Ortega-Gutiérrez. et al.. 1994). Major exposed and subsurface geologic features are shown in Fig ures 1 and 2 as interpreted by de Cserna (1989), Ewing and Lopez (1991), Salvador (1991b), and Henry and Aranda Gómez (1992). However, the region’s general crustal struc ture. particularly the deeper crustal structure, remains
relatively unknown due to the lack of published deep drill holes and geophysical studies. Northeastern Mexico has been affected by numerous tec tonic events that are reflected in the present-day gravity field. These events include possible Grenvillian age tectonics (Ruiz et al., 1988), Early Mississippian terrane accretion in the state of Tamaulipas (Stewart et al.. 1993; Boucot et al., 1997), develop ment of a Permian-Triassic magmatic arc (Grajales-Nishimura et al.. 1992; Torres-Vargas et al.. 1993), early Mesozoic rifting and subsequent opening of the Gulf of Mexico in Jurassic time (Buffler and Sawyer, 1985), possible Mesozoic and Cenozoic strike-slip faulting (Anderson and Schmidt, 1983; McKee et al., 1984; Longoria, 1985; Nowicki et a!., 1993), Mesozoic conver
Mickus. K.. and Montana, C., 1999, Crustal structure of northeastern Mexico revealed through the analysis of gravity data, in Bartolini, C., Wilson, J. L., and Lawton, T. F., eds,. Mesozoic Sedimentary and Tectonic History of North-Central Mexico: Boulder. Colorado, Geological Society of America Special Paper 340.
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Figure 1. Location of major surface geologic features in northeastern Mexico and southwestern Texas. Arches or uplifts include EBA—El Burro—Salado, PEA—Peyotes, PIA—Picachos, SSC—Sierra de San Carlos, ST—Sierra de Tamaulipas. Basins include PB—Parras, LPB—La Popa, MB—Magiscatzin, TMB—Tampico-Misantla. Other features include VSLP— Valles—San Luis PotosI carbonate platform. Bold lines with triangles represent major thrust faults within Sierra Madre Ori ental (de Cserna, 1989). Features are based on maps and interpretations of de Cserna (1989), Ewing and Lopez (1991), and Henry and Aranda-Gómez (1992).
gent tectonics culminating with the Laramide orogeny (Suter, 1984, 1987; Campa, 1985), and Cenozoic Basin and Range ex tensional tectonics and magmatism (Campa, 1985: Henry and Aranda-Gómez, 1992; Aguirre-Dfaz and McDowell. 1993). The exact nature, timing, and existence of these events are contro versial because much geological, geochemical. and geophysical needs to be done before the complete tectonic history of north eastern Mexico is understood. The purpose of this chapter is to examine the general crustal structure of northeastern Mexico through the analysis of gravity data. Series of Bouguer and filtered gravity anomaly maps were produced to delineate the general tectonic features of the region. Two regional gravity models, integrated with outcrop and seis mic data, will be used to interpret the general structure of this region. This interpretation will support future regional geologic and tectonic models.
GEOLOGICAL BACKGROUND Northeastern Mexico has been affected by numerous tec tonic events and only the general tectonic framework is known. We present an overview of the region’s tectonic history: see the references cited in the introduction for more detailed discussions. The oldest rocks within the study area are late Precambrian gran ulite, quartzite, and gneiss exposed at and encountered in drill holes east of the Huizachal-Peregrina anticlinorium (Figs. I and 2). These rocks have been interpreted as Grenvillian in age (Garrison et al., 1980; Woods et al., 1991; RamIrez-RamIrez, 1992) and may have originally formed in the eastern United States and moved to their present position during the late Paleo zoic (Ortega-Gutidrrez et al., 1995). Paleozoic tectonic events remain sketchy and open to differ ent interpretations due to the lack of outcrops and deep drill
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holes. Paleozoic sedimentary and low-grade metamorphic rocks exposed at and encountered in drill holes surrounding the Huizachal-Peregrina anticlinorium (Figs. I and 2) were originally interpreted to be a southern extension of the Ouachita orogenic belt (Flawn et al., 1961; RamIrez-RamIrez, 1992); however, recent geological and paleontological studies (Stewart et al., 1993; Boucot et al., 1997) proposed that these rocks were part of a microcontinent that formed south of the Paleozoic continental margin of North America and was accreted to North America by Early Mississippian time. The Mesozoic Era marked the start of the two most promi nent tectonic events in northeastern Mexico: (1) the breakup of the Pangean supercontinent and the subsequent rifting that led to the opening of the Gulf of Mexico (Buffler and Sawyer, 1985; Salvador, 1987; Wilson, 1990; Ewing, 1991) and (2) the eastward subduction of the Farallon plate in western Mexico (Coney, 1978; Dickinson, 1981; Damon and Shafiquillah, 1991; Grajales Nishimura et al., 1992). Onshore evidence for the earliest open ing of the Gulf of Mexico is found in a series of Triassic-Jurassic nonmarmne, clastic deposits deposited in rift basins along the east-
em front of the Sierra Madre Oriental (Carrillo-Bravo, 1961; Salvador, 1987, 1991a; Michalzik, 1991). The continued opening of the Gulf of Mexico in the Mesozoic and early Cenozoic formed a classic passive margin sedimentary sequence, which includes marginal marine Middle Jurassic(?) evaporites (Laudon, 1984), Cretaceous carbonate platforms (e.g., Coahuila and Valles-San Luis PotosI) (Wilson, 1990; Wilson and Ward, 1993), and Cretaceous through Eocene basinal carbonates and clastic strata (Imlay et al., 1948; McFarlan and Menes, 1991; SohI et al., 1991). The thicknesses of these deposits vary but can be as much as 5 km in the northeast section of the study area. Subduction in western Mexico created a magmatic arc that is represented by a Permian-Triassic granitoid belt (Wilson, 1990; Damon and Shafiquillah, 1991; Grajales-Nishimura et al., 1992; Ruiz et al., 1993; Torres-Vargas et al., 1993). Permian Triassic granites have been encountered in numerous wells throughout the study area, including the Sierra de Tamaulipas (Byerly, 1991) and northwest of Monterrey, Nuevo Leon (Padilla y Sanchez, 1986) (Fig. 2). Continuous subduction in western Mexico culminated with the Laramide orogeny at the end of the
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Cretaceous. This convergent tectonic event produced the Sierra Madre Oriental fold and thrust belt (Campa, 1985; Suter, 1987; de Cserna, 1989) and related foreland basins and uplifts east and northeast of the belt (Fig. 1). The basins and uplifts may have dif ferent origins; Hunt (1992) suggested that the Sierra de Tamauli pas may have been produced flexurally by the weight of the sediments within the Rio Grande embayment, and Belcher (1979), Wilson (1990), and Ewing (1991) proposed that although the present expressions of these tectonic features are Laramide in age, some of the uplifts in northeastern Mexico were positive features as early as the Jurassic. The exact nature of the Mesozoic tectonic regimes and the formation of the related structural features has generated much controversy. The structural patterns in northeast Mexico are prob ably the product of the Laramide orogeny (Humphrey. 1956: Campa, 1985; Padilla y Sanchez, 1986); howevei other workers (Silver and Anderson, 1974; Gose et al., 1982; Anderson and Schmidt, 1983; Longoria, 1985) suggested that northeastern Mexico has been affected by numerous strike-slip faults (mega shears) that were active in Early Jurassic time. These transpres sive structures have been used to explain local variations in the trend of the Sierra Madre Oriental fold and thrust belt (Now icki et al.. 1993), the origin of the Sabinas basin (Longoria, 1984), and the juxtaposition of rocks of varying ages (Silver and Anderson, 1974; Anderson and Schmidt, 1983). In addition, Longoria (1985) used transpression tectonics to explain the origin of most of the Mesozoic tectonic features of northeastern Mexico: how ever, transpression tectonics is still debated. The last major tectonic event in the western sections of the study area is represented by middle to late Cenozoic Basin and Range extension and related magmatism (Campa, 1985; Henry and Aranda-Gómez. 1992; Luhr et al., 1995) and Oligocene Miocene alkaline volcanism in the Sierra de Tamaulipas and Sierra de San Carlos region (Robin and Tournon, 1978) (Fig. 1).
PREVIOUS GEOPHYSICAL INVESTIGATIONS Despite the fact that northeastern Mexico is an important piece of the puzzle in the evolution of North America and there is significant oil exploration in the region, there have been few pub lished geophysical investigations. The most important studies have been gravity and magnetic investigations by McDonnell (1987), Cancienne (1987), Schellhom (1987), and Hunt (1992). Schellhorn (1987) and Hunt (1992) performed regional gravity and magnetic study analyses of northern Mexico to determine its general lithospheric structure by constructing a series of isostatic gravity anomaly maps based on the variable Bouguer reduction datum technique, and a series of two and one-half dimensional gravity and magnetic models. Combining the isostatic anomaly maps and the gravity models, they postulated that numerous crustal discontinuities (megashears) exist in northern Mexico. McDonnell (1987) and Cancienne (1987) collected and analyzed gravity and magnetic data over the Parras basin. Coahuila. to determine the general configuration of the basin and the distribu
tion of evaporates within the basin. They found that the basin is not associated with a gravity minimum even though it contains as much as 5.4 km of sediments (McBride et al., 1974). Gravity modeling (McDonnell. 1987) showed that granitic intrusions and possible high-grade metamorphic rocks may be obscuring any potential gravity minimum due to the basin fill. Seismic studies (e.g., surface wave, refraction, reflection) usually provide the most information about the crustal structure within a region. Unfortunately, there has been only one known published seismic study within the study area; Keller and Shurbet (1975) performed a surface-wave analysis to determine an aver age crustal thickness of 32 km within the Rio Grande embay ment. Nearby seismic analyses, including surface-wave studies to the west of the study area (Gomberg et al., 1988) and to the north (Pinkerton, 1978), determined an average crustal thickness of —45 km. A seismic refraction survey by Dorman et al. (1972) near Victoria, Texas, northeast of the study area, indicated an average crustal thickness of —35—36 km. The most important seismic studies near the study area were by Winker and Buffler (1988), who analyzed several seismic reflection profiles along the Gulf of Mexico coastal plain to determine the general struc ture and thickness of the Mesozoic and Cenozoic sedimentary rocks. They found that the strata vary between 1 and 3 km in thickness along the Mexican portion of the Gulf of Mexico coastal plain.
GRAVITY DATA AND PROCESSING The gravity data used in this study were obtained from the University of Texas at Dallas, University of Texas at El Paso, and from Cancienne (1987). All the data were merged into one coherent database of —12,000 points (Fig. 3) and processed into simple Bouguer gravity anomaly values. Terrain corrections were only available for a portion of the data, precluding the cal culation of complete Bouguer gravity anomaly values for the entire data set. These data were gridded at a 3 km interval using a minimum curvature algorithm (Briggs, 1974), and the result ant grid was used to construct a Bouguer gravity anomaly map (Fig. 4) and filtered gravity anomaly maps. The resultant grid included a 150 km rind surrounding the area shown in Figure 4 to reduce edge effects when performing wavelength filtering. The wide gravity station spacing in numerous regions (Fig. 3) precluded a detailed interpretation of the crustal structure of the study area. We concentrate on a regional crustal interpretation of the Bouguer gravity anomalies shown in Figure 4.
GRAVITY DATA ANALYSIS The Bouguer gravity anomaly values depicted in Figure 4 are interpreted to reflect the combined effect of tectonic events ranging from the Precambrian to recent. To use gravity data in the interpretation of the Earth’s crust, the source of any given anom aly should be isolated and ultimately determined. Techniques to accomplish this include map analysis (e.g., wavelength filtering,
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Figure 4. Bouguer gravity anomaly map of study area. Contour interval is 5 mGal. Relative gravity minima are shown by hatchured con tour lines. Also shown are major surface geologic features (see Fig. 1 for explanation).
Crustal structure of northeastern Mexico revealed through gra vita data analysis upward continuation, polynomial trend-surface fitting) and com puter modeling (e.g., two-dimensional and three-dimensional for ward modeling). We used wavelength filtering (Peeples et a!., 1986) to create a series of low-pass and band-pass filtered gravity anomaly maps to qualitatively interpret the region’s crustal struc ture. These maps were created in an attempt to isolate anomalies of certain wavelengths that may be interpreted to be caused by particular geologic features. In general, low-pass filtered maps highlight longer wavelength anomalies (which may be called regional gravity anomalies), which are most commonly caused by deep-seated features. Due to the gravity’s nonuniqueness, a large dimension, shallow, low-density body may also cause a long-wavelength anomaly. After producing a series of low-pass and band-pass filtered maps, a low-pass filter that passed wave lengths greater than 150 km (Fig. 5) and a band-pass filter that passed wavelengths between 50 and 150 km (Fig. 6) were selected for further analysis. The Bouguer gravity anomaly values range from —225 mGal in the southeast portion of Figure 4 to .-—lO mGal in the eastern portion. This decrease in the Bouguer gravity field is due to the thinning of the crust from the Trans Mexican volcanic belt of central Mexico to the Gulf of Mexico (Gomberg et al., 1988; Winker and Buffler, 1988). The most prominent gravity anomaly in Figure 4 is a northwest-trending gradient that generally follows the trend of the thrust front of the Sierra Madre Oriental fold and thrust belt. This gradient is caused by the abrupt thinning of the crust (Winker and Buffler, 1988) and the thick sedimentary pile within the Rio Grande embayment, which is east of the Sierra Madre Oriental fold and thrust belt. This gradient masks any potential gravity anomalies due to smaller scale features (e.g., the Picachos uplift and the Magiscatzin basin) and band-pass filter ing is used to isolate anomalies due to these features. As the gra dient trends toward the northwest, it breaks into two segments: (1) one segment follows the trend of the Sierra Madre Oriental fold and thrust belt across the Parras basin and (2) a second seg ment trends to the northwest and is caused by the edge of the thick pile of low-density sediments within the Rio Grande embayment. The source of the gradient across the Parras basin is more subtle and is discussed in the following. To emphasize upper crustal anomalies, a band-pass filtered gravity anomaly map was created (Fig. 6). Many combinations of wavelengths were tried and the wavelengths chosen (50—150 km) do not have a special significance, as most combinations be tween 30 and 200 km show the same general patterns as those in Figure 6. The filtered map does not show the steep gravity gra dient discussed here but instead indicates a series of gravity min ima and maxima that can be correlated with the basins and uplifts in the region. The Magiscatzin basin contains as much as 1.5 km of Cretaceous sediments (McFarlan and Menes, 1991) and produces a linear gravity anomaly (anomaly I) with an amplitude of—ID mGal. A linear gravity maximum (anomaly 2) northeast of the Magiscatzin basin (Fig. 6) is associated with the granite-cored uplifts of the Sierra de Tamaulipas, Sierra de San Carlos, and the Picachos arch (Lopez-Ramos, 1982). Anomaly 2
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may be caused by granitic intrusions and/or higher densities within the upper crust related to transitional crust formed during the opening of the Gulf of Mexico. To the west and northwest of the Magiscatzin basin, a linear gravity maximum (anomaly 3) occurs over the Huizachal-Peregrina anticlinorium and along the thrust front of the Sierra Madre Oriental fold and thrust belt. The origin of this anomaly is speculative, but may be caused by the high-grade metamorphic rocks that crop out at the Huizachal Peregrina anticlinorium (Fig. 2). High-grade metamorphic rocks have been encountered in wells along the Sierra Madre Oriental thrust front, south of the study area (Ewing, 1991). On the basis of the trend of anomaly 3, high-grade metamorphic rocks may be present to the north of the Huizachal-Peregrina anticlinorium along this anomaly. Triassic-Jurassic rift grabens that contain 1.5—2.0 km of sed iments are evident from scattered outcrops and wells along and east of the Sierra Madre Oriental thrust front (Carrillo-Bravo, 1961; Salvador, 1987, 199la). Similar grabens are found along the Paleozoic continental margin of the southern and eastern United States (Salvador, 1987) and these grabens in general pro duce only small-amplitude gravity anomalies that are often obscured by steep gravity gradients (Mickus et a!., 1988; Mickus and Keller, 1991). The Bouguer gravity anomaly and filtered maps (Figs. 4 and 6) do not indicate any small-scale gravity min ima that could be related to grabens in the vicinity of the Sierra Madre Oriental fold and thrust belt. This could be due to the grabens containing little sedimentary fill or to the density of the sediments being nearly the same as the surrounding rocks. How ever, anomaly 4 may be due to the Mesozoic grabens, because this low-amplitude anomaly (—8 to —10 mGal) occurs along strike between known occurrences of Triassic-Jurassic rift sedimentary rocks in Mexico and east-central Texas (Salvador, 1991a). Alter native sources for anomaly 4 include either Permian-Triassic granites (northwest of the anomaly; Fig. 2) intruding the Precam brian basement or variations in sediment thickness within the Rio Grande embankment. The Sabinas, La Popa, and Parra foreland basins (Fig. 2) are associated with small or nonexistent gravity anomalies (Figs. 4 and 6). The Sabinas basin contains as much as 5 km of Jurassic and older sandstones, shales, and evaporites; the southern end of the basin is within the study area. Anomaly 5 is a small-ampli tude gravity minimum (——6 mGal) (Fig. 6) that corresponds to —3 km of sediment fill within the Sabinas basin (Salvador, 199lb). The deeper sections of the basin are north of the study area and are associated with higher amplitude gravity minima (Hunt, 1992). The Parras and La Popa basins do not have gravity minima associated with them (Figs. 4 and 6), and any potential gravity minima are obscured by the regional gravity gradient dis cussed here. To determine why 4—5.5 km of sedimentary rocks (McBride et al., 1974) do not produce a gravity minimum, McDonnell (1987) and Cancienne (1987) performed a detailed gravity and magnetic study to show that granitic intrusions and possible high-grade metamorphic rocks are responsible for obscuring any potential gravity minima.
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Northwest of the Parras basin is the highest amplitude gravity minimum (—21 rnGaI) (anomaly 6) within the study area. This anomaly occurs over the Coahuila platform, which contains rela tively thin layers of Jurassic-Cretaceous sedimentary strata and is thought to have been a positive element (Coahuila peninsula) dur ing the Late Jurassic—Early Cretaceous (Jones et al., 1984). This area also contains exposures of granitic rocks and widely scattered wells encountering granitic rocks, which may be the source of anomaly 6 (Handschy et al., 1987). Anomaly 6 is located at the southern end of a more regionally extensive gravity minimum, which Handschy et al. (1987) used as partial evidence to name an accreted terrane (Coahuila terrane) formed during the Ouachita orogeny. In addition, detailed gravity modeling by Moreno et al. (1994) indicated that this regional gravity minimum may be caused by thickening of the continental crust. The interior of the Sierra Madre Oriental fold and thrust belt and the surrounding Basin and Range province (Henry and Aranda-Górnez. 1992) to the west are characterized by numerous short wavelength gravity maxima and minima (Figs. 1,4, and 6). A discussion of the individual anomalies is beyond the scope of this chapter. Possible anomaly sources include Jurassic and Ter tiary volcanic rock outcrops and probable plutons beneath the volcanic and sedimentary cover (Jones et al., 1995; Luhr et al.. 1995). Precambrian high-grade metamorphic rocks exposed in the southern section of the study area (Patchett and Ruiz, 1987), and horsts and grabens related to Basin and Range extension in the western region of the study area (Henry and Aranda-Gdmez, 1992). The deeper crustal structure of this region is interpreted from the low-pass filtered gravity map (Fig. 5), which indicates a gravity minimum occurring over most of the Sierra Madre Ori ental fold and thrust belt. This region is the site of numerous Jurassic volcanic rocks, which Hunt (1992) and Jones et al. (1995) inferred to be related to a Cordilleran magmatic arc. The presence of a large-scale gravity minimum provides additional evidence that the area may be a magmatic arc and possibly an accreted terrane (analogous to the Coahuila terrane mentioned here). Jones et al. (1995) suggested that the magmatic arc origi nated to the north in the United States and was brought to its pres ent position by the Mohave megashear. Schellhorn (1987) used the truncation of low-pass filtered gravity anomalies at the pro posed location of Mojave megashear in northwestern Mexico as evidence for its existence. However, regional gravity (Schellhorn, 1987; Hunt, 1992) and magnetic studies (Hunt, 1992) in north eastern and eastern Mexico do not provide evidence as strong as that in northwestern Mexico for the location of proposed strikeslip faults and a southward translation of the magmatic arc. The possibility remains that the magmatic arc was accreted to North America at its present position (Grajales-Nishimura et al., 1992).
MODELING AND DISCUSSION The above discussion showed that the gravity anomalies of northeastern Mexico could be caused by numerous geologic features of varying age. However, the majority of the gravity
anomalies are probably caused by geologic features formed by Mesozoic compressional and/or extensional tectonic events. By only qualitatively interpreting the various anomaly maps, one may misinterpret the location, depth, and geometry of the anomaly sources. To obtain a more quantitative representation of the subsurface structure of northeastern Mexico, regional gravity models were constructed along profiles A-A’ and B-B’ (Fig. 3). The locations of these profiles were based on the amount of gravity data available along a potential profile and the location of geologic features of interest. The models were derived using a 2.5-dimensional, forward-modeling algorithm (Lai, 1984), where the calculated gravity anomalies were deter mined using the gravity station elevations. Because gravity models are nonunique, constraints must be used to determine geologically meaningful models, and possible constraints include rock densities, depth to various rock units, and lateral distribution of the rock units. The first two constraints are not readily available in northeastern Mexico; however, constraints were available from regional seismic studies (Dorman et al., 1972; Keller and Shurbet, 1975; Pinkerton. 1978: Gomberg et al., 1988; Winker and Buffler, 1988), compilations of previ ous geological mapping (Dillman, 1985: Padilla y Sanchez. 1986; Ewing and Lopez, 1991; Salvador, 1991b; Henry and Aranda-Gómez. 1992; RamIrez-RamIrez, 1992), and gravity studies (Cancienne, 1987; Hunt, 1992). These studies provided depth to the crust-mantle boundary, thickness of the coastal plain sediments and other sedimentary basins, densities of upper crustal units, and locations of outcrops. A number of the studies (e.g., Padilla y Sanchez. 1982, 1986; Dillman, 1985; McDonnell, 1987; Wilson, 1990: Basánez-Loyola et al., 1993) included well data that provided depths to the top of various upper crustal formations. The final models (Figs. 7 and 8) were obtained through a trial and error process until the calculated gravity values matched the observed gravity values using the described constraints. The final models are not unique; one could change the densities, geometries, and/or depth to a given body within the bounds of the constraints and still obtain a reasonable observed and/or calcu lated fit. The final models shown in Figures 7 and 8 represent what we believe are reasonable geological cross sections with the given constraints. The main points the modeling addresses are the locations of the possible Jurassic magmatic arc and buried Permian-Trias sic(?) granites, and the thicknesses of the crust, thrusted units along the Sierra Madre Oriental thrust front, the Parras basin, and the Huizachal-Peregrina anticlinorium. A regional gravity mini mum possibly related to an accreted Jurassic magmatic arc is shown in Figure 5. This gravity minimum is present on profile B-B’ (Fig. 8) and is modeled as a low-density upper crustal body extending laterally to —120 km on model B-B’. The gravity min imum on profile A-A’ subtler (Fig. 7), and a similar low-density body was modeled at the southwestern end of model A-A’. How ever, an equal fit to the observed data could be obtained with or without this low-density body. This implies that the accreted
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body may not extend to this latitude, or if it does, the body is smaller. An alternative explanation is that the density of the mag matic arc is higher in this region. Permian-Triassic granites are known from exposures and wells throughout northeastern Mexico (Jones et al., 1984; Padilla y Sanchez, 1986; Handschy et al., 1987; Byerly, 1991) and prob ably are the source for some of the gravity minima in Figures 4 and 6. However, only subtle gravity minima are apparent along profile A-A’. These gravity minima were modeled with relatively small granitic intrusions, i.e., the body at 300 km located at the eastern margin of the Coahuila platform, and may be part of the Coahuila terrane described by Handschy et al (1987). The crustal thicknesses on both models decrease toward the northeast, agreeing with the scant seismic information in and
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surrounding the study area (e.g., Keller and Shurbet. 1975; Gomberg et al., 1988; Winker and Buffler, 1988). Both models (Figs. 7 and 8) have crustal thicknesses ranging from 41 km in the southwest to 35 km in the northeast, which agree above the seis mic studies. Model B-B’ has the upper crust along the northeast end of the profile, modeled with a higher density (2.75 gm/cm ) 3 than the remaining upper crust. This part of the model occurs over a gravity maximum (anomaly 2 in Fig. 6) associated with the Sierra de Tamaulipas, which is known to be granite cored (Byerly. 1991). The granite may be part of the source of the gravity maxi mum along profile B-Bc but the modeling process indicated that it could account only for a small portion of the observed anomaly. So, the gravity maximum is modeled as a higher density upper crust containing granitic intrusions. This higher density upper
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rnr i C H(h-1 3 Figure 7. Two and one-half dimensional gravity model along profile A-A’ (see Fig. 3). This model crosses Sierra Madre Ori ental fold and thrust belt (SMO) and Parras basin. SED—Cretaceous and younger sediments. Numbers in parentheses rep resent body’s average density (in gm/cm ). 3
K. Mickus and C. Montana
368
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