U-Pb zircon data from three undeformed to slightly deformed, megacrystic, granitoid plutons in the northern Acatlán Complex of southern México has indicated that all three are part of a larger suite of late Ordovician plutons. 40 Ar/ 39 Ar data from hornblende and biotite show mainly disturbed spectra, but biotite from the Palo Liso and Los Hornos plutons yields plateaus with ages of 305 ± 26 Ma and 157 ± 12 Ma, respectively. These thermal events may be correlated, respectively, with Permo-Triassic and Jurassic tectonothermal events recorded elsewhere in the Acatlán Complex. All three plutons are peraluminous with calc-alkaline affinities, characteristics that are consistent with inherited zircon ages and together suggest a source in Mesoproterozoic calc-alkaline rocks similar to those exposed in the neighboring Oaxaca terrane. We interpret these granites to be related to the early Ordovician separation of peri-Gondwanan terranes from Gondwana during the opening of the Rheic Ocean. Elsewhere in the Acatlán Complex, Ordovician megacrystic granitoids of the Piaxtla Suite were subjected to high-grade metamorphism, which we infer to be related to subduction along the Gondwanan margin during the Devonian–Carboniferous. The three plutons reported here were not affected by Devono-Carboniferous metamorphism and thus are inferred to have remained outside the subduction zone.

Abstract Analogue models of polyphase deformation involving crustal differences in strength, thickness and density give insights into lateral and vertical strain propagation during Late Cretaceous shortening and Early Tertiary left-lateral shearing related to the early development of the North America–Caribbean plate boundary in southern Mexico. Analogue models reproduce a two-phase deformation characterized by a first stage of compression orthogonal to the plate boundary, simulating deformation induced by the Laramide orogeny, followed by a later stage of left-lateral transpression associated with the transfer of the Chortis block from the North American to the Caribbean plate during the early stage of development of the new plate boundary in Early Tertiary times. Based on detailed structural observations in the Guerrero–Morelos platform and the western part of the Mixteco terrane of southern Mexico, we document that a transpressive regime affected continental red bed sequences of Early Paleocene to Late Eocene, and rotated and refolded Laramide structures during this second phase. Our model ends before the transtensional regime that affected the region, which is marked by a volcanic episode of Late Eocene–Oligocene. This change in the deformation regime records the passage of the NW tip of the Chortis block (North America-Cocos-Caribbean triple junction), when subduction replaced transform faulting along the southern Mexico margin. The models focus on the structures formed around the flanks of a thicker/more rigid crustal block that simulates the rock assemblages of the Palaeozoic orogens of southern Mexico (Mixteco–Oaxaca–Juarez block, MOJB). The comparison of the mechanism of deformation of three different analogue models with the natural prototype explains most of the structures observed around the MOJB. Counterclockwise vertical-axis rotations of pre-existing structures in the western flank of the MOJB observed in the Guerrero–Morelos platform are consistent with the modelled structures. Vertical movements of the modelled MOJB induced by the transpressive regime can explain the Papalutla thrust and the basement upheaval and gravitational sliding of the cover in the Tentzo Ranges observed at the western and northern margins of the MOJB, respectively. The growth and propagation of thrusting controlled by the geometry of the block along the eastern margin also correlates with the Vista Hermosa fault. The propagation of strain to the north increases with higher contrast in strength of the thick block with respect to the adjacent modelled crust. Analogue modelling failed to reproduce all the structural details of southern Mexico and, specifically, the structures observed inside the MOJB. The latter, however, are controlled by pre-existing discontinuities, which are not simulated in the model. As a whole, the results demonstrate that crustal heterogeneity in a developing left-lateral plate boundary zone produces a stronger vertical coupling between ductile and brittle crust and a widening of the deformation zone along the margin of the North America plate in southern Mexico.

Abstract Transect H-3 extends 460 krn across southern Mexico from the Pacific Ocean to the Gulf of Mexico, far south of cratonic North America. The transect has two segments: one trending N7°E from the axis of the Acapulco Trench to central Oaxaca and another extending N53°W from central Oaxaca to the Gulf of Mexico. The cross sections lie near the middle of the transect and coincide with a line connecting Puerto Escondido at the Pacific coast, the city of Oaxaca, and San Martin Tuxtlas Volcano near the NE end of the transect The mosaic of geologic provinces along the corridor made it convenient to use a tectonostratigraphic approach for the analysis of the geologic history. Terrane nomenclature in this report is derived from names of pre-Hispanic cultures dominant in each respective region. The geologic description of terranes and the bases of their discrimination are given below.

The Caltepec shear zone is a dextral transpressional tectonic boundary between the Oaxacan and Acatlán Complexes, which are crystalline basements of the Zapo-teco and Mixteco terranes in southern México, respectively. The terrane boundary (2–6 km wide) reveals protracted and polyphase tectonic activity from at least Early Permian to the present. The major tectonothermal event in the Caltepec fault zone was related to the oblique collision of the metamorphic complexes during the amalgamation of Pangea. An anatectic leucosome and the resulting syntectonic granite (Cozahuico Granite) in the fault zone yielded U-Pb zircon ages of 275.6 ± 1.0 Ma and 270.4 ± 2.6 Ma, respectively. The initial 87 Sr/ 86 Sr ratios (0.70435–0.70686) and Sm-Nd model ages (T DM) (1.0–1.6 Ga) for the Cozahuico Granite and leucosome indicate a magmatic mixture that originated from melted Proterozoic crust and a component of depleted mantle. The Leonardian age of the cover (Matzitzi Formation) and a 40 Ar/ 39 Ar cooling age (muscovite) of 268.59 ± 1.27 Ma for mylonitic mica schist at the base of the thrust imply high cooling rates (∼180 °C/Ma) and uplift during the Permian. The adjacent sedimentological record indicates intense tectonic reactivation during Early Cretaceous, Paleogene, and Neogene along the long-lived Caltepec fault zone, alternating with periods of relative tectonic quiescence during Triassic, Jurassic, and Mid-Cretaceous times. The trend of the Caltepec fault zone parallel to the Oaxaca fault, 50 km to the east, is interpreted as part of a synchronous and dynamically coupled tectonic system that has been releasing tectonic stresses associated with the rupture of Pangea and the evolution of the Pacific margin of southern México from Jurassic to Holocene times.

Abstract The Permian marine and continental sedimentary sequences in south central México and the biota contained in them are analyzed in this paper. The fossil flora and fauna are contained in Permian strata that correspond to the Guacamaya Formation in the states of Hidalgo and Veracruz; to the Patlanoaya, Los Hornos, Cuxtepeque, and Matzitzi Formations in the state of Puebla; to the Olinalá Formation in the state of Guerrero; to the Ihualtepec, Ixtaltepec, and Yododeñe Formations in the state of Oaxaca; and to the Paso Hondo Formation in the state of Chiapas. The microfaunistic association of Olinalá fusulinids can be correlated with the Guadalupe Mountains and the La Mar Formation of Texas in the U.S.A., and with the Difunta Formation of Coahuila and strata of the states of Sonora, Guerrero, and Oaxaca in Mexico. Fusulinids from the Olinalá, Ihualtepec, and Yododeñe Formations suggest that during the Late Permian (Wordian-Capitanian), these organisms were part of the same paleogeographic province that comprised Texas, Arizona, New Mexico, and California in the United States, and northern Sonora, Chihuahua, Coahuila, extending into central Mexico.