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Los pliegues son estructuras geológicas que se presentan en muchos medios geológicos. Analizar su geometría nos permite obtener una aproximación de la deformación que acomodaron. Los pliegues tipo chevron son útiles para hacer estimaciones de acortamiento en sistemas orogénicos debido a su particular geometría. A pesar de esto, no se ha propuesto un método para calcular la deformación total (dinámica y cinemática) que experimentan estas estructuras. En el presente trabajo se expone un método para el cálculo del acortamiento en pliegues chevron que han sufrido una deformación post flexión o post pandeo, el cual toma como base el modelo propuesto por Ramsay en 1974. El método propuesto es conceptualmente sencillo y general, lo que permite su aplicación para estimar el acortamiento en sistemas orogénicos alrededor del mundo.

In this paper we report U Pb zircon ages from a syn-tectonic magmatism related to a contractional thick-skinned deformation pulse in the Mexican Fold and Thrust Belt. The study area corresponds to the Concepcion del Oro Block (COB), which is located in northern Zacatecas, Mexico. The COB is a NW-SE trending elongated tectonic structure located south of the Parras Transversal Sector in the Mexican Fold Thrust Belt (MFTB) in north-central Mexico. In this area, regional thin-skinned folds were formed between the late Turonian and Campanian and they define the core of a major northeastward-convex orocline around Concepcion del Oro mining district, Zacatecas, Mexico. The regional folds were cut by four Eocene-early Oligocene plutons (43-32 Ma; U Pb zircon), which in places modified the older thin-skinned structures. Magma was emplaced along thrust faults and modified the attitude of inverted limbs of overturned folds, and the orientation or position of the fold axis at the core of anticlines, causing the formation of complex emplacement-related deformation. Two NW-SE trending lineaments bound the COB, and both cut the periclinal ends of the thin-skinned older folds, at different time. The eastern lineament is here interpreted as a high angle reverse fault, which tilted polymictic conglomeratic continental succession that includes cobbles derived from one of the intrusive bodies and an andesitic syn-tectonic lava flow (40.7 ± 0.6 Ma; U Pb zircon). The western lineament is a normal fault generated by later regional tectonic extension related to the Basin and Range province. Altogether suggests that a late Eocene-early Oligocene contractional thick-skinned deformation pulse affected the COB. This pulse was contemporaneous with the Eocene-Oligocene magmatism in the Concepcion del Oro, Zacatecas region, and is similar in style and age to some of the structures produced by the Laramide orogeny in the southern Rocky Mountains and in some isolated areas in northern Mexico.

Few methods exist for directly dating orogenic events in the upper crust. Here we report in-situ dates from (∼10 μm diameter) monazite grown in quartz-calcite fibrous veins, which precipitated at very low-grade metamorphic conditions (about 250 °C) in volcaniclastic and calcareous rocks of the Early Cretaceous Trancas Fm. in the Zimapan Basin, Central Mexico. Our integrated study includes regional and local structural and kinematic analysis of vein and folds, detailed textural and mineralogical characterization of vein-bearing minerals, and texturally-controlled Th/Pb geochronologic analysis of monazite by LA-MC-ICPMS. We find that monazite in the veins preferentially crystallized at the interface between vein-forming calcite and quartz fibers, as well as at the vein-host rock interface. Monazite ages range from 154 to 68 Ma, with the youngest sub-population yielding a weighted mean age of 76.8 ± 0.8 Ma, coincident with the age of basin-wide folding as earlier constrained by Ar/Ar illite dating. The oldest monazite ages coincide with the timing of deposition of the volcaniclastic host-rock. Given that veins are ubiquitous in shallow crustal rocks, this approach to dating of veins with enclosed monazite has great potential to improve the constraints on temporal resolution of fluid-rock interaction during of deformation.

The impact of frictional crustal heterogeneities and surface processes on the evolution of the ancient thin-skinned Mexican fold-and-thrust belt (MFTB) is investigated and quantified using numerical finite-element simulations. These simulations explore the evolution, total shortening, and structural styles of deforming orogenic wedges. The models include spatial lateral variations in the internal friction, and the topographic erodibility ( K ), using the power law incision rule. The MFTB exhibits paleogeographical elements manifested as lateral facies changes across two platforms units and two sedimentary basins that show different degrees of deformation. The observed kinematics of the MFTB requires, based on the parameters considered in this study, a rheologically heterogeneous crust, with at least laterally varying internal friction. Yet only limited syntectonic and post-tectonic erosion are necessary to explain the geometries and internal total shortening constrained by field observations and interpretations. Application of the analytical and homogeneous critical Coulomb wedge (CCW) theory to the MFTB closely predicts the external wedge geometry but fails to predict the observed shortening of the individual litho-tectonic units and structural styles.

The Mexican orogen is the expression in Mexico of the Cordilleran orogenic system. The orogen extends the length of Mexico, a distance of 2000 km from the state of Sonora in the northwest to the state of Oaxaca in the south. The Mexican orogen consists of (1) a western hinterland of accreted oceanic basinal rocks and magmatic arc rocks generally known as the Guerrero volcanic superterrane, (2) a foreland orogenic wedge , commonly termed the Mexican fold and thrust belt (MFTB), composed of imbricated and folded Upper Jurassic-Lower Cretaceous carbonate rocks and Upper Cretaceous foreland-basin strata, and (3) an assemblage of variably folded and inverted Late Cretaceous to Eocene foreland basins that lie northeast and east of the MFTB. The Mexican orogen encompasses the entire country, spanning several physiographic provinces and deformational domains that display both thin-skinned and thick-skinned structural styles determined by inherited crustal structure and contrasting pre-kinematic sedimentary sections. The orogen contains kinematic characteristics of both the Sevier and Laramide orogens in the United States (U.S.), and deformation in the Mexican orogen spanned the deformational history of those U.S. orogens. The overall trend of the Mexican orogen is NW-SE, although it displays local trend variations. At presently exposed levels, the orogen consists of folded and reverse-faulted Mesozoic-Eocene strata. Lower Cretaceous strata of the deformed foreland are dominated by carbonate rocks, whereas time-equivalent strata in the hinterland consist of deformed plutons belonging to one or more magmatic arcs, as well as turbidites, pillow lavas and altered mafic rocks deposited in an offshore basin prior to consolidation of fringing arc systems to mainland Mexico. Upper Cretaceous syntectonic strata of the foreland orogenic wedge constitute siliciclastic turbidite successions that grade eastward to carbonate pelagites of the distal foreland basin, which was starved of siliciclastic sediment input. Uppermost Cretaceous and Paleogene strata of the foreland basin constitute a shelfal, deltaic and coastal plain fluvial succession in northeastern Mexico and a succession of turbidites in the Tampico-Misantla basin east of the MFTB. Structural geometry of the orogen was controlled by the spatial distribution of pre-Cretaceous crustal elements, such as Jurassic extensional basins and basement blocks, and detachment horizons at varying stratigraphic levels, as well as the direction of structural vergence, which is dominated by NE-directed tectonic transport throughout the belt. Jurassic evaporite horizons and Upper Jurassic carbonaceous shale units provide detachment surfaces in some parts of the orogen. The structural style of the MFTB is generally thin-skinned, although high-angle faults are present at several localities, where the steep faults cut thin-skinned, shallowly-dipping faults, detachment horizons and associated folds. Strain magnitude decreases toward the foreland and generally satisfies critical wedge predictions. Values of shortening > 70% are present in the hinterland of central Mexico; these decrease systematically to values Shortening history in the Mexican orogen approximately spanned Late Cretaceous-Eocene time. Deformation timing has been constrained using Ar-Ar systematics on illite generated by layer-parallel slip in the limbs of chevron folds. Estimates of deformation timing are in good agreement with the age of synorogenic sedimentary successions, and with ages of syn-tectonic plutons. Published data from central Mexico suggest episodic pulses of deformation between 93–80 Ma, 75–64 Ma and 55–43 Ma, which postdate the closure of the Arperos basin. Each of these shortening events affects rock units lying progressively farther to the east to yield a temporal eastward advance of deformation and sedimentation. Effects of successively younger shortening were superimposed on the westernmost exposures of the thrust belt and are evidenced on a map scale by abrupt trend variations in orogen-interior folds, compared to generally linear or broadly arcuate axial traces of frontal folds. Although potential tectonic mechanisms for shortening in the Mexican orogen remain debated, our analysis indicates that orogenic wedge development took place in a retroarc setting that postdated consolidation of the hinterland oceanic assemblages, which lay offshore western Mexico during Albian time. Orogen development followed a protracted period of early Mesozoic extension that affected most of the Mexico due to the combined effects of Laurentia-Gondwana separation and long-term Triassic-Jurassic rollback of a paleo-Farallon plate. Slab rollback ultimately resulted in the development of a marginal basin, the Arperos basin, between a rifted Late Jurassic magmatic arc and mainland Mexico. Initial shortening in the Mexican orogen, which followed Arperos basin closure and Guerrero superterrane accretion by ~ 5–10 Ma, was coeval with voluminous magmatism on the Pacific margin of Mexico, drowning of the western carbonate platforms and onset of foreland-basin sedimentation in Cenomanian time. Subduction of the Farallon slab from early Late Cretaceous to Eocene time was thus the primary driving mechanism of shortening in the Mexican orogen.

"La Falla de San Marcos (FSM) es una estructura que atraviesa al estado de Coahuila con rumbo general NW-SE. Esta estructura forma parte de un sistema de fallas normales en el Jurásico Medio con una reactivación en el Jurásico Tardío, condicionó el depósito de unidades carbonatadas durante el Cretácico Temprano y se reactivó como falla inversa posiblemente en el Paleógeno. El límite nororiental de la plataforma de Coahuila coincide con la traza de la FSM y en la localidad de Las Palomas, Coahuila, es donde mejor se expone. En este sitio las capas de la Formación Cupido cabalgan sobre lutitas calcáreas pelágicas, a las cuales nos referimos informalmente como Capas Cerro La Bruja (CCLB). Esta unidad se correlaciona con la Formación Eagle Ford en la cuenca de Sabinas al norte y con la Formación Indidura en la cuenca de Parras, al sur. Las CCLB se observan sub-horizontales sobre la plataforma de Coahuila, ya que sólo se acortaron en los 400 metros más próximos a la traza de la FSM. La actividad de la falla provocó el desarrollo de pliegues tipo chevron, los cuales se disponen en una cuña de deformación limitada por una falla basal que separa a las rocas plegadas de aquellas no plegadas. Un análisis detallado de pliegues permitió estimar un acortamiento horizontal de alrededor de 500 metros. Considerando que sólo hay una generación de pliegues en las CCLB y que el acortamiento fue causado por la actividad de la FSM. Basado en la estimación de este acortamiento, se propone un nuevo método para estimar el desplazamiento mínimo de la traza frontal del sistema de la FSM, el cual toma en cuenta el acortamiento de las CCLB y la inclinación de la falla. Dicho método permitió estimar un desplazamiento mínimo de 1300 m sobre el plano frontal de la falla, lo cual es consistente con las estimaciones de más de 3000 m, realizadas por autores previos para todo el sistema de fallas. La estimación de desplaza- miento a lo largo de fallas mayores siempre ha sido un reto en el análisis estructural, por lo cual consideramos que nuestro nuevo método puede ser una valiosa herramienta para determinar desplazamiento sobre fallas mayores."

Due to their minute size, 40Ar/39Ar analysis of illite faces significant analytical challenges, including mineral characterization and, especially, effects of grain size and crystallography on 39Ar recoil. Quantifying the effects of 39Ar recoil requires the use of sample vacuum encapsulation during irradiation, which permits the measurement of the fraction of recoiled 39Ar as well as the 39Ar and 40Ar∗ retained within illite crystals that are released during step heating. Total-Gas Ages (TGA) are calculated by using both recoiled and retained argon, which is functionally equivalent to K–Ar ages, while Retention Ages (RA) only involve retained Ar in the crystal. Natural applications have shown that TGA fits stratigraphic constraints of geological processes when the average illite crystallite thickness (ICT) is smaller than 10 nm, and that RA matches these constraints for ICTs larger than 50 nm. We propose a new age correction method that takes into account the average ICT and corresponding recoiled 39Ar for a sample, with X-ray Corrected Ages (XCA) lying between Total-Gas and Retention Ages depending on ICT. This correction is particularly useful in samples containing authigenic illite formed in the anchizone, with typical ICT values between 10 and 50 nm. In three samples containing authigenic illite from Cretaceous carbonates in the Monterrey Salient in northern Mexico, there is a range in TGAs among the different size-fractions of 46–49, 36–43 and 40–52 Ma, while RAs range from 54–64, 47–52 and 53–54 Ma, respectively. XCA calculations produce tighter age ranges for these samples of 52.5–56, 45.5–48.5 and 49–52.5 Ma, respectively. In an apparent age vs ICT or %2M 1illite plot, authigenic illite grains show a slope that is in general slightly positive for TGA, slightly negative for RA, but close to zero for XCA, with thinner crystallites showing more dispersion than thicker ones. In order to test if dispersion is due to a different formation history or the result of retention capability, degassing spectra were modeled for site XCA averages and overall XCA average. Modeling shows that local site age average best match the measured spectra, instead of a global average age, indicating that illite growth reflects local deformation, and is not the result of regional metamorphism. Modeling also shows that Ar-degassing spectra are very sensitive to grain size, such that age interpretation based on Ar-plateaus is meaningless for most fine-grained clays.

Growth of magnetite has been variably linked to fluid-bearing events or clay diagenesis, and the development of a chemical remagnetization as a result of such events. In this study we examine remagnetized carbonate rocks from the central Sierra Madre Oriental (the Mexican fold-thrust belt) in order to develop a method for dating synfolding remagnetizations. By combining 40 Ar/ 39 Ar deformation ages with new paleomagnetic results, we present a quantitative method for absolute dating of synfolding remagnetization. We find that the history of the central Sierra Madre Oriental involved two separate remagnetization events in our study area; synfolding remanence acquisition ca. 77 Ma (Late Cretaceous) in the Zimapan Basin and a younger synfolding remagnetization event ca. 44 Ma (mid-Eocene) in the Tampico-Misantla Basin. The growth of magnetite leading to chemical remagnetization detected in these limestones is interpreted as the result of rock interactions with an Fe-bearing fluid.

We used illite Ar/Ar dating to obtain absolute ages of folds and shear zones formed within the Mexican Fold–Thrust Belt (MFTB). The methodology takes advantage of illite dating in folded, clay-bearing layers and the ability to obtain accurate ages from small-size fractions of illite using encapsulated Ar analysis. We applied our approach to a cross-section that involves folded Aptian–Cenomanian shale-bentonitic layers interbedded with carbonates of the Zimapan (ZB) and Tampico–Misantla (TMB) Cretaceous basins in central-eastern Mexico. Basinal carbonates were buried by syn-tectonic turbidites and inverted during the formation of the MFTB in the Late Cretaceous. Results from folds and shear zones record different pulses of deformation within this thin-skinned orogenic wedge. Mineralogical compositions, variations in illite polytypes, illite crystallite size (CS), and Ar/Ar ages were obtained from several size fractions in limbs and hinges of the folds and in the shear zones. 1Md-illite polytype (with CS of 6–9 nm) dominates in two folds in the TMB while 2M1-illlite (with CS of 14–30 nm) dominates in the third fold, in the ZB, and in the fold/shear zone. From west (higher grade) to east (lower grade): Ar retention ages indicate shearing occurred at ~84 Ma in the westernmost shear zone, folding at ~82 Ma in the ZB with subsequent localized shearing at ~77 Ma, and Ar total gas ages constrain the time of folding at ~64 Ma on the west side of the TMB and ~44 Ma on the eastern edge. These results are consistent with the age and distribution of syn-tectonic turbidites and indicate episodic progression of deformation from west to east.

We propose a deformation dating method that combines XRD quantification and Ar chronology of submicroscopic illite to determine the absolute ages of folds that contain clay-bearing layers. Two folds in the frontal segment of the Mexican Fold-Thrust Belt (MFTB), which was deformed from Late Cretaceous to Eocene, are used to illustrate the method and its future potential. Variations in mineral composition, illite-polytype, crystallite-size (CS) and Ar total gas ages were analyzed in the limbs and hinge of two mesoscopic folds. This analysis examines potential effects of strain variation on illitization and the Ar isotopic system along folded layers, versus possible regional thermal overprints. The Ar total-gas ages for 9 samples in Fold 1 vary between 48.4 and 43.9 Ma. The % of 2M1 (detrital) illite vs. Ar total-gas ages tightly constrains the age of folding at 43.5 � 0.3 Ma. Nine ages from three samples in Fold 2 range from 76.2 to 62.7 Ma, which results in a folding age of 63.9 � 2.2 Ma. Both ages are in excellent agreement with more broadly constrained stratigraphic timing. The method offers a novel approach to radiometric dating of clay-bearing folds formed at very low-grade metamorphic conditions, and has the potential to constrain dates and rates of regional and local deformation along and across foreland orogenic belts.

espanolSe analizan las variaciones en la composicion isotopica de agua meteorica a lo largo de una seccion en el Cinturon de Pliegues y Cabalgaduras Mexicano, ubicado en la seccion centro–noreste de la Sierra Madre Oriental. El objetivo de ello es determinar dichas variaciones en funcion de:1) el incremento en la elevacion topografica y 2) la distancia de las precipitaciones al interior del continente, desde que se produjo la evaporacion de agua oceanica del Golfo de Mexico. Se consideraron las caracteristicas topograficas y climaticas de la seccion estudiada, seleccionada por presentar las ventajas detalladas como sigue. La deformacion en esta seccion genero un rasgo de cuna orogenica, que le confiere una pendiente promedio entre 1o y 1.5o, sin la presencia de barreras topograficas que dificulten la libre circulacion de vientos. Ello favorece evaluar variaciones significativas en la composicion isotopica de H y O en funcion de la elevacion topografica. La seccion se ubica a una latitud practicamente constante entre 21o y 22o N, y la temperatura media anual decrece gradualmente desde el antepais hacia el transpais de la seccion durante los periodos de precipitacion. El muestreo de agua meteorica se efectuo mediante la colocacion de 12 colectores de agua de lluvia en lugares elevados, a alturas entre 2740 y 56 msnm a lo largo de la seccion estudiada. Asimismo, se tomaron muestras de agua de rios y manantiales durante la temporada de lluvia de 2012 en zonas de escorrentia constante, de 7 localidades diferentes. Los valores obtenidos en agua de lluvia varian entre -4.89 y -11.75 ‰ para δ18O y entre -25.35 y -80.34 ‰ para δ2 H (respecto a VSMOW). Asi, se determino una Linea de Agua Meteorica local (LAML) definida por la ecuacion δ2 H = δ18O * 8.15 + 15. La tasa de fraccionamiento obtenida para O y H (δ18O y δ2 H) respecto a la elevacion, fue de 2.19 ‰ km-1 y 17.75 ‰ km–1, respectivamente. La LAML y las tasas de fraccionamiento obtenidos en este estudio podran ser utilizados como valores de referencia a nivel regional para evaluar el papel del agua meteorica involucrada en procesos atmosfericos y corticales superficiales. EnglishWe analyze the variations in isotopic composition of meteoric water along a transect in the Mexican FoldThrust Belt, located in the central–northeastern part of the Sierra Madre Oriental. The aim is to determine such variations as a function of 1) increasing topographic elevation and 2) the distance of rain inside the continent since evaporation occurrence of oceanic water from the Gulf of Mexico. Topographic and climate characteristics were considered in the studied section, which was chosen because it presents some advantages as follows. Deformation in this section exhibits orogenic wedge features, which generated an average slope between 1° and 1.5°, with no topographic barriers that obstruct the free circulation of wind. This allows us to evaluate significant isotopic variations of H and O as a function of topographic elevation. The section is located at constant latitude between 21° and 22° N. Furthermore, the average annual temperature decreases gradually from the foreland to the hinterland of the section during precipitation periods. Sampling of meteoric water was carried out by installing 12 rainwater collectors at heights between 2740 and 56 masl along the transect. Additionally, water samples were collected in rivers and springs during the 2012 rain season in zones of constant water runoff in 7 different localities. Obtained values from rain water vary between -4.89 and -11.75% for δ18O and between -25.35 and -80.34 ‰ for δ2 H (relative to VSMOW)— These allowed to determine a Local Meteoric Water Line (LMWL) defined by the equation δ2 H = δ18O * 8.15 + 15. With this, fractionation rate relative to elevation for O and H (δ18O y δ2 H) were obtained at 2.19 ‰ km-1 and 17.75 ‰ km-1, respectively. The LMWL and fractionation rates obtained in this study can be used as regional-scale references in order to evaluate the role of meteoric water involved in atmospheric and shallow cortical processes

The Mexican fold-and-thrust belt in central Mexico has overall characteristics that fit the critical tectonic wedge model. It is thin-skinned, forward propagating, tapers toward the toe (the east), and displays an overall decrease in deformation toward the toe. The internal structures and heterogeneity of deformation are not typical of fold-and-thrust belts, however, due to the presence of two large carbonate platforms, flanked by more thinly bedded basinal carbonates. Kilometer-scale thrusts dominate deformation in the platform carbonates (a more brittle behavior), and mesoscopic buckle folds and associated cleavage dominate deformation in the basinal carbonates (a more ductile behavior). Total shortening across the belt, including both platforms and basins, is ∼55%–65%, with higher values in the basins than in the platforms and a concentration of deformation near the platform borders. The dominant mechanism of folding in the basinal rocks is buckling, with thin chert horizons behaving as single layers and limestone and shaly limestone interbeds buckling as multilayers, with a dominant chevron style. A significant shear component of the deformation is indicated by monoclinic fold symmetry, with a consistent sense of vergence of top toward the foreland. We estimated strain and strain history from mesoscopic analysis of fold geometry and internal strain distribution at several locations across the basin and used this information used to assess the overall kinematics and progressive deformation in the basins, which involve both shortening and shear components. The implications of this for the kinematics of the fold-and-thrust belt are discussed.

Analysis of mesoscopic structures and related veins allows the history of deformation and role of fluids to be established for part of the central Mexican Fold-Thrust Belt (MFTB). The MFTB developed in mostly carbonate rocks with prominent lateral facies changes associated with two platforms and two basins. Fluids played a key role in the deformation, both physically and chemically, by reducing strength and inducing extensional fracturing through raised pore pressure and by providing the medium for solution transfer on the grain-scale. Veins preserve portions of water related to deformation as fluid inclusions. Lithology and lithological variations strongly control deformation styles, with thrusts dominant in the platforms and folds dominant in the basins, and also influence fluid behavior by controlling both porosity and permeability. Structural observations allow distinguishing veins (dominantly calcite) of several generations, emplaced early, during and late/after deformation (V1–V3 respectively). δ13C and δ18O analyses in calcite from veins and host rock show that the veins confined within thrust slices are isotopically buffered by the host rock and differ in isotopic composition from veins emplaced along major thrusts or crosscutting thrust slices. δD analyses in fluid inclusions and clay minerals strongly suggest rock interaction with meteoric fluids in the west (hinterland) and with fluids close to SMOW in the less deformed eastern (foreland) side of the cross-section. By focusing on a single stratigraphic interval exposed across the width of the fold-thrust belt, we propose a conceptual model that explains the differences in vein-rock isotopic composition, the differences of in isotopic composition of aqueous fluids active during deformation, and the progression of clay dehydration reactions as being related to variations in temperature and intensity of deformation in a growing tectonic wedge.

Despite the fact that most fold–thrust belts around the world share many features, successfully explained by the critical wedge model, the details of their geometric evolution and tectonic style development are poorly understood. In the classic section of the southern Canadian Rocky Mountains the dominant tectonic style consists of imbricate thrust sheets with relatively little internal deformation of the individual slices. In the Mexican fold–thrust Belt (Central Mexico), the age of deformation, the overall structural pattern and the total amount of shortening are similar, but the individual thrust sheets exhibit much more internal deformation as manifest by metre-scale buckle folds. One of the differences between these localities is the lateral variation of facies resulting in massive platform limestone separated by thinly-bedded basinal limestone in the Central Mexico section. Strain is concentrated toward the margins between platforms and basins. In Canada, thick platform carbonates form continuous resistant units across the Front Range. Possible reasons for the differences in tectonic style between the two sections include the dominant lithology, distribution of lithologies, taper angle of the tectonic wedges, amount of friction along the basal detachment and the degree of anisotropy of the basin facies rocks. Fold–thrust belts are common features that developed on the edges of orogenic belts worldwide. Many have been well studied and their characteristic features established (Dahlstrom 1970; Chapple 1978; Boyer & Elliott 1982; Price & Fermor 1985; McClay 1992). These features include the classic notions of forward propagation of thrusts, piggyback style, a basal decollement, and decreasing intensity of deformation towards the foreland. Most fold–thrust belts are wedge-shaped, and since the mid 1980s, the critical taper model of their development (Davis et al. 1983; Dahlen et al. 1984; Dahlen 1990) has become generally accepted. Analogue and numerical models have been developed that are able to capture many of the basic features of fold–thrust belts and that are broadly consistent with the critical taper theory (e.g. Huiqi et al. 1992; Stockmal et al. 2007). From early studies it has been clear that most of the large folds in fold–thrust belts are intimately related genetically to the thrusts, and extensive studies of thrust-related folds, in the field and in analogue and numerical models, have established the nature of this relationship (e.g. Dahlstrom 1969, 1970; Elliott 1976; Suppe 1983; McClay 1992; Wilkerson et al. 2002; Spratt et al. 2004). Despite the many similarities among fold–thrust belts in general, there are significant differences among belts and along strike within individual belts. These reflect, among other things, differences in the strength of the basal detachment and the composition and distribution of the rocks in the fold– thrust wedge. In this contribution we compare basic elements of the structures developed roughly synchronously in two widely separated parts of a single orogenic belt: the North American Cordillera. The sections we compare are a well-known section across the southern Canadian Rocky Mountains and a new section across the Sierra Madre Oriental in central Mexico. The dimensions of the fold– thrust belts in these two locations are similar, and both sections are dominated by carbonates. The structural style, however, is quite different. We compare structures in the two locations and speculate on what may be responsible for the differences.

We combine structural, fluid-inclusion microthermometry, illite-crystallinity, X-Ray Diffraction (XRD) and O and H stable isotope analyses of authigenic illite to determine the source of local fluids interacting with rock during folding in anchizonal shales of the Mexican Fold-Thrust Belt (MFTB). A well-exposed train of mesoscopic, asymmetrical folds in a sequence of Cretaceous limestones interbedded with shale was targeted for this study. We test the hypothesis that syn-folding vein minerals and clay minerals were formed from the same fluids by comparing the δ 2 H composition of inclusion fluids in calcite and quartz from veins, and from illite concentrates from sheared shale layers, and the sources of that fluid. Five clay size-fractions ( 2 μm ) were separated from eight shale samples. In the 40 clay grain-size fractions analyzed, illite, calcite, kaolinite, smectite, chlorite and minor quartz were identified by XRD analysis. Most samples show different proportions of various clay minerals, except for the finer fractions in two of the samples (BL3 and BL4) were illite is the only clay phase present. The discriminating potential of δ 18 O values of clay is generally masked by the abundance of calcite in all samples. In contrast, samples containing chlorite and smectite show very low values in δ 2 H (−75.9 to −53.9‰), while samples containing illite and kaolinite or pure illite show relatively high δ 2 H values (−33.1 to −50.1‰). The latter fall within the δ 2 H range (−39 to −49‰) determined in fluid inclusions of syntectonic veins, indicating isotopic equilibrium between water, veins fillings and illitic clay during deformation, according to fractionation factors at these temperatures (220–250 °C). The δ 2 H values and fluid inclusion salinities in the sampled rocks indicate that water active during folding was partly marine and partly meteoric and that the amount of such pore-water represented a small fraction of the deformed rock volume. Our study demonstrates that the H-isotopic composition of anchizonal illitic clays, commonly present in exhumed fold-thrust belts, can be used to determine the source(s) of fluids that were active during deformation.

En el área de Valle de Bravo, Estado de México, afloran dos ensambles de rocas cretácicas con metamorfismo de muy bajo grado: (1) rocas metasedimentarias (EMS), y (2) rocas metavolcánicas (EMV). Estas unidades forman parte de la secuencia mesozoica volcanosedimentaria metamorfizada de Ixtapan de la Sal – Teloloapan. La historia de deformación en la zona está definida por tres eventos de acortamiento subhorizontales (D1, D2 y D3), que coinciden en rumbo NE-SW, pero con direcciones alternadas de transporte tectónico. D1 tiene una vergencia general hacia 043°, D2 hacia 218° y D3 hacia 045°. Estos tres eventos han sido identificados en diversas localidades de los estados de Guerrero y Michoacán, representando importantes fenómenos de deformación a escala regional. D1 muestra deformación dúctil a escala de grano, mientras que D2 es dúctil-quebradizo y D3 es netamente quebradizo. Mediante el análisis de microestructuras en lámina delgada, a escala de afloramiento y con la determinación cuantitativo-comparativa de vetas mediante análisis de imágenes, se propone que D1 representa el pico metamórfico y deformacional y que D2 y D3 ocurrieron durante la exhumación de las rocas. Asociadas a los eventos D1 y D2, se desarrollaron dos generaciones de vetas, V1 y V2, cuya abundancia depende del grado de penetratividad de la deformación; es decir, de los fenómenos de deformación a escala microscópica (principalmente disolución por presión/reprecipitación) que movilizan material soluble que se deposita en forma de vetas en etapas tardías de la deformación. Adicionalmente a las vetas V1 y V2, se identificó también un sistema de vetas tardías (V3) asociadas a fallas normales que cortan estructuras D3. Análisis petrográfico y microtermométrico de inclusiones fluidas en cristales de cuarzo y calcita se realizaron en las tres generaciones de vetas en un área de 15 km2 en el EMS. Las temperaturas de homogeneización obtenidas fueron, en promedio, de 250°C para V1, entre 167 y 202°C para V2, y 220°C para V3. Las correspondientes salinidades aparentes obtenidas fueron, en promedio, de entre 6.1 y 7.4 (V1), 5.2 (V2), y 2.6 y 4.6 (V3)% en peso de NaCl equivalente. Los datos obtenidos en las vetas están de acuerdo con una exhumación progresiva de las rocas del área de estudio, ya que tanto las temperaturas de homogeneización como las salinidades asociadas a deformación son menores en V2 que en V1. Mientras que las temperaturas de V3 indican que fluidos relativamente más calientes circularon a lo largo de fallas normales posteriormente a D1, D2 y D3