The reasons for, and mechanisms of, continental margin truncation in SW Mexico where Mesozoic-Cenozoic plutons are situated directly on the Pacific coast, are not yet well understood. Large-scale dextral and/or sinistral displacements of the continental margin terranes, now forming parts of Baja California or the Chortis block, have been proposed. The well-defined along-coast NW-SE decreasing granitoid intrusion age trend (-q.2 cm/yr in the 100 Ma-40 Ma time interval) between Puerto Vallarta and Zihuatanejo is interpreted by us to be a geometric artifact of oblique continental margin truncation rather than the consequence of a sinistral offset of the Chortis block from those latitudes toward the SE. Changes in the dip and velocity of the NNW-SSE trending Cretaceous-Tertiary subduction zone resulted in a landward migration of the magmatic arc. Taking into account certain stratigraphic affinities of Chortis and the Oaxaca and Mixteca terranes, together with the known displacement rates along the North America-Caribbean Plate boundary, the northwesternmost paleoposition of the Chortis block with respect to SW Mexico was near Zihuatanejo. In contrast, between Zihuatanejo and the Isthmus of Tehuantepec, the cessation of the Tertiary magmatism decreased more rapidly (-7.7 cm/yr), although the trend is not so obvious. Starting in the late Eocene, Chortis moved about 1100 km to the SE along a transform boundary associated with the opening of the Cayman Trough. Based on our geochronological data and structural relationships between mylonite zones and plutons in the Acapulco-Tehuantepec area, we propose an approximately 650 km SE movement of Chortis from about 40-25 Ma, with a velocity of 6.5-4.3 cm/yr. Since this is considerably slower than the decreasing age trend obtained by us using the geochronological data, we consider batholith formation in this segment to predate and postdate the offshore passage of the North America-Farallon-Caribbean triple junction. Geological observations and paleomagnetic data do not give strong support for large-scale right-lateral displacements of crustal blocks like the Baja California. Given the isotopic data presented, the 1Now at Instituto de Geologia, Universidad Naeional Aut6noma de M6xieo, Mexico City. Paper number 95TC01928. 0278-7407/95/95 TC-01928510.00 continental margin truncation in SW Mexico seems to be the consequence of an interaction of mechanisms. Of these, we regard tectonic erosion associated with the subduction process to be the most important in the northwestern segment. On the other hand, the lateral removal of material associated with the displacement of Chortis is more important in the southeastern segment. The tectonic evolution of the SW Mexican Pacific coast has been linked to the development of the Caribbean Plate and the displacement of the Chortis block to its present position [Malfait and Dinkelman, 1972; Pindell and Dewey, 1982; Anderson and Schmidt, 1983], including southeastern Guatemala, all of Honduras, E1 Salvador, and probably most of Nicaragua [Donnell...

No abstract

No abstract

No abstract

No abstract

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 Zimapán (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. 1M d -illite polytype (with CS of 6-9 nm) dominates in two folds in the TMB while 2M 1 -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.

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.

ResumenA lo largo de la costa de Oaxaca, entre Puerto Ángel y Santiago Astata, afl oran rocas metamórfi cas del Complejo Xolapa intrusionadas por rocas ígneas de edad Paleógeno a Mioceno sin metamorfi smo regional. Ambas unidades se encuentran en contacto tectónico a lo largo de zonas de cizalla con rocas proterozoicas del Complejo Oaxaqueño al norte de Pochutla y con rocas sedimentarias mesozoicas discordantes que sobreyacen los gneises. La principal de estas zonas de cizalla en el área de estudio es la falla Chacalapa, expuesta al norte de Pochutla, que está constituida por ultramilonitas, milonitas, protomilonitas, pseudotaquilitas, fi lonitas y cataclasitas en orden cronológico de desarrollo. La cinemática de esta zona de cizalla vertical es predominantemente lateral-izquierda.Las rocas miloníticas de la falla Chacalapa registran temperaturas de recristalización dinámica de ~500ºC (deformación cristal-plástica de feldespatos) que disminuyen sistemáticamente hasta el régimen netamente quebradizo. Asociadas a la recristalización dinámica se desarrollaron texturas cristalográfi cas de cuarzo en las rocas miloníticas que, cuando son asimétricas, exhiben un sentido de cizalla izquierdo. Las rocas del régimen quebradizo también registran una cinemática izquierda, al igual que las fallas activas.La edad de las rocas miloníticas de la falla Chacalapa se ubica entre los 29±0.2 y los 23.7±1.2 Ma. El límite de edad superior lo constituye la edad del intrusivo Huatulco (U-Pb de circones, Herrmann et al. 1994) que se observa milonitizado en su margen septentrional y el límite de edad inferior corresponde a la edad K-Ar de hornblenda procedente de diques granodioríticos porfídicos que truncan la milonita.Palabras clave: Falla, México, Oaxaca, milonita, pseudotaquilita, Xolapa. Abstract Along the coast of Oaxaca between Puerto Angel and Santiago Astata a suite of rocks is exposed corresponding to the Xolapa metamorphic complex which is intruded by granitoid igneous rocks of