GEOLOGIC EVOLUTION OF THE VALLE DE OAXACA GRABEN

It lies between 16°00'-17°15' N Lat. and 96°20'-96°55'W Long, its shape is as an inverted Y centered around Oaxaca City. The Middle Proterozoic Oaxacan Complex makes up the western horst, and part of the northern horst, which also includes Mesozoic metamorphics, Cretaceous units, and a ?Middle Cenozoic thick, volcanic pile, partly pierced by granitoid stocks. The southern horst includes the Cretaceous units and the volcanics; both horsts are severely deformed.

The sedimentary sequence in the N arm chiefly includes a basal conglomerate, and the ~750 m thick, 20 Ma Suchilquitongo Formation [silicified lacustrine limestone, tuffaceous sandstone, and numerous tuff strata]; it shows a 20° dip to the NE. Quaternary alluvium and soil blanket the valley. The sequence in the SE arm includes the 2,000 m thick, 15 Ma Mitla Tuff, the ~2,000 m thick Matatlán Formation [similar to the Suchilquitongo; it intertongues the Mitla Tuff]. Both units show a 20° dip to the SW. Quaternary alluvium and soil cover the valley. The sequence in the S arm includes the Quaternary alluvium/soil blanket; scarce latest Tertiary clastics are present.

Early Paleogene regional tectonic activity reactivated the N-S Oaxaca Fault, and formed a narrow graben, where an incipient clastic system was set. By earliest Miocene, tectonic/magmatic activity caused deep faulting, which widened the graben, induced pyroclastics emplacement, and the deposition of the Suchilquitongo Formation. No further sedimentation occurred. Later faulting (age unknown) closed tectonic activity here. The same fate happened in the S arm, given the similar tectonic setting of both S and N arms. By Middle Miocene, the tectonic activity shifted SE, producing a similar set of events, with some variants: The volcanic activity and the subsidence rate were much greater; the later faulting allowed development of a large graben, where the Matatlán unit is chiefly preserved.

Orthodoxy on Southeastern Mexico's tectonic evolution links it to Cocos Plate subduction under the Pacific margin as the Chortis Block moved east. The Valle de Oaxaca Graben evolution does not conform in timing and stress field geometry to this paradigm. Perhaps subduction caused displacement of deep seated cortical blocks, which reflects to the surface; once this energy is spent, surface activity ceases.