A REVISED TECTONIC HISTORY OF THE RIO GRANDE RIFT IN THE TAOS REGION, NEW MEXICO: INTEGRATION OF GEOPHYSICAL, GEOLOGICAL, AND BOREHOLE INFORMATION

The ~64-km long, Embudo and related faults represents a classic transfer zone in the central Rio Grande rift, forming the link between the east-down western border of the northern Española Basin and the west-down eastern border of the southern San Luis Basin. In the southern San Luis Basin, the NE- to E-striking, left-oblique Embudo fault zone transitions to the N-striking, west-down, normal Sangre de Cristo fault along an embayment in the mountain front near the Town of Taos. We integrate geophysical, borehole, and geologic information to interpret the subsurface configuration of the rift margins formed by these faults and the geometry of the subbasin within the Taos embayment. From these interpretations, we can infer relations between faulting and flows of Pliocene Servilleta Basalt and older, buried basaltic rocks that, combined with geologic mapping, suggest a revised rift history involving shifts in the locus of fault activity as the Taos subbasin developed. We speculate that faults related to north-striking grabens at the end of Laramide time formed the first west-down master faults. The Embudo fault may have initiated in early Miocene southwest of the Taos region. Normal-oblique slip on these early fault strands likely transitioned in space and time to dominantly left-lateral slip as the Embudo fault propagated to the northeast. During and shortly after eruption of Servilleta Basalt, proto-Embudo fault strands were active along and parallel to the modern, NE-aligned Rio Pueblo de Taos, about 4–7 km basinward of the modern, mapped Embudo fault zone. Faults along the northeastern subbasin margin had northwest strikes for most of the period of subbasin formation and were located about 5–7 km basinward of the modern Sangre de Cristo fault. The locus of fault activity shifted to more northerly striking faults within 2 km of the modern range front sometime after Servilleta volcanism had ceased. The northerly faults may have linked with the northeasterly proto-Embudo faults at this time, concurrent with the development of the N-striking Los Cordovas normal faults within the interior of the subbasin. By Middle Pleistocene(?) time, the Los Cordovas faults had become inactive and the linked Embudo-Sangre de Cristo fault system migrated to the south, to the modern range front.