Abstract

MORAN-ZENTENO, Dante Jaime; CERCA, Mariano  and  KEPPIE, John Duncan. Cenozoic geologic evolution of southwestern Mexico: advances and problems of interpretation. Bol. Soc. Geol. Mex [online]. 2005, vol.57, n.3, pp.319-341. ISSN 1405-3322.  https://doi.org/10.18268/bsgm2005v57n3a4.

Recent advances in the knowledge of the Cenozoic structure and stratigraphy of southern México reveal a geologi-cal evolution characterized by Late Cretaceous orogenic deformation, followed by truncation of the continental margin and gradual extinction of arc magmatism in the Sierra Madre del Sur, previous to the onset of magmatism in the Trans-Mexican Volcanic Belt.

Orogenic deformation beginning in the Late Cretaceous was coeval with the Laramide orogeny and displays struc-tures with a similar orientation. Deformation consisted of E-W shortening with a general eastward vergence and a well defined migration to the east. Models that associate the origin of Laramide deformation to a decrease in the angle of subduction of the Farallon plate, which was converging in western Mexico, cannot be applied in this region because of the occurrence of Paleocene to early Eocene arc magmatism near the inferredpaleotrench. A possible origin related to the collision of an insular arc against the western margin of Mexico does not seem plausible due the absence of features and petrogenetic associations indicating the closure of an oceanic basin.

In light of recentgeochronological data, the generalpattern of magmatism extinction, from Late Cretaceous-Paleocene in Colima and Jalisco to the middle Miocene in central and southeastern Oaxaca, presents variations inconsistent with a simple pattern of extinction toward the east-southeast.

Maastrichtian to early Paleocene plutonism recognized in the Jalisco block and Manzanillo areas is contemporane-ous with a magmatic episode with some documented adakite 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 now the present day continental margin and also within a band located 200 km inland. 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 the beginning of the 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 deformation episodes suggests that extension directions related to lateral faulting changed from NNW-SSE to NE-SW, and locally activated normal displacements on pre-existing discontinuities.

Fundamentalproblems still exist in the interpretation of the plate tectonic processes that originated the stress regimes acting on the different sets offaults, as well as in the knowledge of the factors influencing the migration of magmatism. Some of the arguments used to postulate the presence of the Chortis block off the southwestern Mexico 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 Mexico suggest that continental truncation was essentially caused by subduction erosion but leave open the interpretation of the observed magmatic migration.

Keywords : Southern Mexico; arc-magmatism; deformation; tectonics; Cenozoic.

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