A CENOZOIC, TECTONICALLY BOUNDED VOLCANIC FIELD IN THE YOLOMECATL-TLAXIACO SUBREGION, NORTHWESTERN OAXACA, SOUTHEASTERN MEXICO, AND ITS GEOLOGIC SIGNIFICANCE

Cenozoic volcanics in Mexico cover about 1/5 of the territory, yet most of it has not been stratigraphically differentiated nor discriminated in volcanic fields. The Sierra Madre del Sur [SMS hereafter], is no exception. Current multidisciplinary research carried out by the authors in northwestern Oaxaca, allows us to contribute filling this gap, discriminating and characterizing one such field, as reported here.

This field makes most of the Yolomécatl-Tlaxiaco Area, which lies between 17°15’-17°32” N Lat., and 97°28’-97°45” W Long., and between 2,000-3,250 masl, covering 936 sq. km of rugged territory, including three massifs [Northwestern, Southern and Eastern], and scattered hills; the fluvial network defines three drainage basins [western, the largest, northcentral, and southeastern].

The oldest units, Acatlán Complex [Middle to Late Paleozoic] and Taberna-Zorrillo Formations [continental, Middle Jurassic], crop out in the southwest, whereas without space continuity, the next oldest units lie in the east and southeast: two unnamed [marine, Late Jurassic], and the Teposcolula Limestone [Albian-Cenomanian]. The Cenozoic sequence covers most of the area, it includes the “Tamazulapam Conglomerate” [calcilithitic, Early Cenozoic], the “Yanhuitlán Formation” [fluvial, bears a Paleogenic fauna], both occupy a small part in the northeast; and a widely extent, thick volcanic succession [silicic pyroclastic in the lower part (San Martín Tuff), and andesitic lavic in the upper (Nicananduta Andesite)].

Preliminary geomagnetic/geochronometric dating indicates that the succession is Early Oligocene; it is affected and bounded by faults [some of subregional extent] that define a previously unrecognized, enormous field [~ 18 000 sq km, if its extension beyond the area in the north and west is included], herewith designated Yolomecatl-Tlaxiaco Volcanic Field. The close space/time occurrence of volcanism and structural deformation in this subregion [as well as and in others within SMS Morphotectonic Province], strongly suggests a genetic relationship, whereby tectonic activity generates the weak zones through which magma is issued. Thus, proper characterizing of volcanic fields could help to better understand the pattern, tempo and mode of Cenozoic tectonic evolution in Southeastern Mexico.