Abstract The Chiapas Fold and Thrust Belt (CFTB) of southern Mexico underwent widespread contraction during the well-documented middle Miocene ‘Chiapanecan’ Orogeny. However, earlier phases of folding have been documented in the region and might have affected the belt before the Miocene. We carried out a stratigraphic review and a structural analysis of the belt, complemented by 40 Ar– 39 Ar dating of synorogenic illite to identify the successive pulses of deformation. Reliable radiometric ages were obtained in three folds from the southern portion of the belt, and in one thrust gouge near the front of the belt. Authigenic illite-rich samples were located in Upper Cretaceous limestone successions that experienced bed-parallel shear during folding and thrusting. They yielded ages ranging from 35 to 40 Ma in the folds and 54.5 ± 7 Ma in the thrust, documenting thin-skinned folding and thrusting in the belt mostly in Eocene time. Some of the basement faults of the belt may have been active during this phase. However, most of the pervasive lateral faults that cross-cut the regional folds in the belt were mostly active from the Miocene onwards, causing transpression and fold amplification.

Abstract Although the southern Front Range in Colorado Springs and Manitou Springs contains a near-complete record of Rocky Mountains geological evolution from Proterozoic to Present, there nevertheless are persistent geological problems that have eluded understanding for as much as 125 years. In keeping with the 2013 GSA Annual Meeting theme, “Celebrating Advances in Geoscience,” this field trip visits long-known elements of Front Range geology that merit reexamination within the context of new paleoenvironmental and geochronology data. Of note are: (1) the Great Unconformity and its chemically weathered substrate that correspond to a time of profound changes in global ocean chemistry; (2) lower Paleozoic strata that record sea-level fluctuations, attributable in part to regional tectonism; (3) an array of granite-hosted sandstone dikes, for which a new emplacement model is proposed; and (4) the Front Range monocline at Garden of the Gods Park, examined from the standpoint of its temporal evolution, newly bracketed by results of 40 Ar/ 39 Ar age analysis of illite generated by shear upon bedding-parallel faults.

Abstract The continental interior of Mexico is characterized by a Late Cretaceous prominent fold-thrust belt that shows characteristics of an eastward-tapering orogenic wedge. According to structural data and geothermometry of the deformation, this wedge is the result of horizontal stresses directed from the west (Pacific domain). The orogenic wedge is bounded to the west by the Guerrero Terrane, which is the second largest juvenile terrane accreted to the North American Cordillera. The possible linkage between the accretion of the Guerrero Terrane and the regional shortening in the Mexican interior is examined in detail in the region comprised between the Sierra de Guanajuato and the Peña de Bernal—Tamazunchale areas. In order to test the accretion hypothesis, we present key stratigraphic, structural, and geochronologic data from the Mexican Cordillera in central Mexico, and discuss the problems that exist in connecting the accretion of the Guerrero Terrane to the orogenic deformation of the Mexican continental interior.

Abstract 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.