Abstract Rincón de Parangueo is a Quaternary maar that has been recently desiccated. The crater was partially occupied by a soda lake, and near the shoreline microbialites have formed. Evaporites (mainly trona and halite) precipitated as the water level dropped. Active subsidence of the lake floor ( c. 24 m since 1980) produced countless structures close to the lakeshore, where deformation is extensional. Closer to the depocentre, in the western half of the basin, gliding/spreading produced folds and mud-injection domes. The most remarkable structure throughout the basin is a monocline that forms a ring-like, nearly continuous scarp, approximately 15 m high, which in the eastern half of the basin was produced as a fault-propagation fold developed above the buried diatreme–country rock boundary. A more diffuse (wider) monocline, locally associated with compressive structures, occurs in the western half of the basin. These structures are interpreted as having developed above a gently inclined, irregular lake sediment–country rock (andesite) interphase. The monocline was modified by high-angle extensional faults/fractures with large heaves/apertures. In the eastern half of the basin, there is a second (outer) scarp, approximately 13 m high, formed by a high-angle, listric, normal fault. Rollover antiforms occur in the hanging wall of this structure. Rincón is an example of centripetal gravitational gliding/spreading.

Abstract The modern Mexico Megacity occupies almost a third of the surface of the Valley of Mexico, and it is exposed to natural and man-induced hazards affecting many aspects of urban development. Land subsidence is a geo-hazard imposing important constraints in the urban development by the gradual decrease in elevation of the land surface. This is caused either naturally, by the extraction of water, oil, minerals, or gas from the subsurface, or by the interaction between natural and anthropogenic forces. In this field trip guide we examine regional land subsidence and the vulnerability to fracturing of the lacustrine soils. Groundwater has been over-exploited for human consumption in Mexico City during the past 70 years, leading to a dramatic decline of piezometric levels and the associated land and subsoil deformation. Interdisciplinary research from geologists and engineers may play an important role in understanding the relationship between geological processes and the suitability of land for urban use.

In southwestern Mexico, Late Cretaceous to Early Tertiary deformation has been generally associated with the Laramide orogeny of the Cordillera. Several alternative models consider the deformation to result from the accretion of the Guerrero terrane, formed by the Zihuatanejo, Arcelia, and Teloloapan intraoceanic island arcs, to the continental margin of the North American plate. Here, we present a detailed geologic and structural study and new 40 Ar/ 39 Ar and U-Pb ages for a broad region in the central-eastern part of the Guerrero terrane that allow the accretion model to be tested. In the Huetamo–Ciudad Altamirano part of the region, an almost complete Cretaceous-Paleogene succession records the transition from an early Cretaceous shallow-marine environment to continental conditions that began in Santonian times, followed by the development of a major continental Eocene magmatic arc. Folding of the marine and transitional successions signifies a shortening episode between the late Cenomanian and the Santonian, and a subsequent, out-of-sequence, coaxial refolding event in Maastrichtian-Paleocene time amplified the previous structures. A major left-lateral shear zone postdates the contractional deformation, and it passively controlled the geographic distribution of Eocene silicic volcanism. Minor transcurrent faulting followed. Our results indicate that the Huetamo–Ciudad Altamirano region, which has been considered part of the Zihuatanejo subterrane, was in proximity to a continent during most of the Mesozoic. We found continental recycled material at various stratigraphic levels of the Huetamo Cretaceous succession and Grenvillian inherited ages in zircons from the ca. 120 Ma Placeres del Oro pluton. More importantly, detrital zircon ages from the pre-Cretaceous basement of the Huetamo succession (Tzitzio metaflysch) and the pre–Early Jurassic basement of the Arcelia subterrane (Tejupilco suite) yield very similar Late Permian and Ordovician age peaks. These ages are typical of the Acatlán complex, onto which the Guerrero terrane has been proposed to have been accreted in the Late Cretaceous. Similarly, Paleozoic and Precambrian ages are reported in detrital zircons from the volcano-sedimentary successions of the Zihuatanejo, Arcelia, and Teloloapan subterranes. Models considering this part of the Guerrero terrane as having formed by intraoceanic island arcs separated by one or more subduction zones cannot explain the ubiquitous presence of older continental material in the Mesozoic succession. We favor a model in which most of the Guerrero terrane consisted of autochthonous or parautochthonous units deposited on the thinned continental margin of the North American plate and where the Mesozoic magmatic and sedimentary record is explained in the framework of an enduring west-facing migrating arc and related extensional backarc and forearc basins. The results presented here exclude the accretion of allochthonous terranes as the cause for Laramide deformation and require an alternative driving force to explain the generation of the Late Cretaceous–early Tertiary shortening and shearing on the southern margin of the North American plate.

Abstract We present analogue models that illustrate the tectonic evolution of the continental margin of southwestern Mexico and the Early Cenozoic deformation of the Xolapa complex. Together with geological data they suggest that oblique convergence caused distributed deformation and mountain building near the present-day margin of southern Mexico in a general left-lateral transpressional regime. A similar deformation is also observed north of the Xolapa complex in Maastrichtian to Paleocene sedimentary and volcanic rock units. Since post-Oligocene exhumation of middle crust does not significantly affect Late Eocene to Oligocene volcanic rocks, we infer that the evolution of the transform margin led to the formation of discrete boundaries that eventually decoupled exhumed mid-lower crust from the onshore upper-crust sequences since the Late Eocene.

Recent advances in the knowledge of the Cenozoic structure and stratigraphy of southern México reveal a geological evolution characterized by Upper Cretaceous orogenic deformation, followed by truncation of the continental margin and gradual extinction of arc magmatism in the Sierra Madre del Sur, prior to the onset of magmatism in the Trans-Mexican Volcanic Belt. Orogenic deformation began in the Late Cretaceous and was coeval with the Laramide orogeny with structures of similar orientation. Deformation consisted of E-W shortening that migrated to the east with time and with a general easterly vergence. Models that relate the Laramide deformation to a decrease in the angle of subduction of the Farallon plate, which was converging in western México, cannot be applied in southern México because Paleocene to upper Eocene arc magmatism occurs near the inferred paleotrench. An alternative possible origin due to collision of an insular arc against the western margin of México suffers from an absence of features and petrogenetic associations indicating the closure of an oceanic basin. In light of recent geochronological data, the general pattern of magmatic extinction from Upper Cretaceous–Paleocene in Colima and Jalisco to the middle Miocene in central and southeastern Oaxaca presents some variations inconsistent with a simple pattern of extinction toward the E-SE. Maastrichtian to lower Paleocene plutonism recognized in the Jalisco block and Manzanillo areas is contemporaneous with a magmatic episode that has some documented adakitic affinities in the central part of the Sierra Madre del Sur. Magmatism from the Paleocene to middle Eocene seems to be concentrated in the Presa del Infiernillo area, although isolated centers existed in areas such as Taxco or the eastern Jalisco block. Finally, the main axis of magmatism between the middle Eocene and Oligocene developed along what is the present-day continental margin and extends 200 km inland as a broad band. In general, the geochemical characteristics of this magmatism indicate a low degree of continental crustal assimilation. Two episodes of principally sinistral lateral faulting that activated NW-SE– and later N-S–oriented faults, with variations in time and space, have been documented during the Eocene and lower Oligocene. The N-S set of faults was active only in the north of the Sierra Madre del Sur, whereas the activity of the NW-SE set continued during the Oligocene along the Oaxaca continental margin. The recognition of these deformational episodes suggests that extensional directions related to lateral faulting changed from NNW-SSE to NE-SW, and locally produced normal displacements on preexisting discontinuities. Fundamental problems still exist in the interpretation of the plate tectonic processes that produced the stress regimes acting on the different sets of faults, as well as in the determination of the factors influencing the migration of magmatism. Some of the arguments used to postulate the presence of the Chortis block off the southwestern Mexican continental margin during the early Cenozoic are uncertain. On the other hand, models that explain restricted displacements of the Chortis block with respect to the Maya block—without juxtaposition with the southwestern margin of México—suggest that continental truncation was essentially caused by subduction erosion and leave open the interpretation of the observed magmatic migration.

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.