STRATIGRAPHY AND STRUCTURAL FEATURES IN CENTRAL SANTIAGO RIVER BASIN AND ITS RELATION WITH TEPIC-CHAPALA GRABEN, NAYARIT STATE, MEXICO

During 2002 feasibility geological studies were carried out for the Hydroelectric Project El Cajón, located 47 km SE of Tepic City, up-stream of Aguamilpa Power Plant.

Regional mapping (1:50 000 scale) covered an area up to 600 km2 and a detailed exploration, covering 30 km2 (1: 5 000 to 1: 100 scales) including underground exploration (bore holes and geophysical survey), was also done. Laboratory studies included petrography and geochronologic analysis. The regional stratigraphy consists of meta-vulcanosedimentary pre-Cenozoic rocks, Tertiary andesitic and basaltic flows, locally named Ignimbrita El Cajón (where main facilities will be built), which is about 450 m thick; volcano-sedimentary deposits up to 280 m thick, dacitic tuffs, porphyritic andesite, conglomerates and Quaternary diabasic dikes, basaltic flows and unconsolidated deposits.

The study area is located in the southwestern portion of the physiographic province of the Sierra Madre Occidental (SMO) at the boundary with the Faja Volcánica Transmexicana (FVT) Province. It is located in the Las Palomas Mountain Range wich has N-S direction, is about 50 km long, reaches elevations up to 1600 m above sea level and have big canyons. There are also several conic-shaped volcanic structures, calderic collapse, and young basaltic lava flows filling partially some stream valleys. The main regional faults are NW-SE (the oldest), E-W and N-S (youngest system) and dip up to 28°E or W in a basin and range structural style. Secondary faults have NE-SW and E-W directions; among them, the Sobaco and Perro de Agua faults are locally the most important.

New K/Ar and Ar/Ar ages of 20,5 and 2,0 to 1,0 Ma indicate Miocenic basaltic-andesitic rocks and Plio-Pleistocenic basalts, which are younger than the ones reported for the Río Santiago Group described near Guadalajara City.

Structural data and rock ages so far obtained are very important to understand tectonic behavior in the region as part of the Tepic-Chapala Graben, for a better knowledge about the boundary between the SMO and FVT as well as for the definition of a Volcanic Group in the Santiago River Basin.