DECIPHERING A STRUCTURAL CHANGE AT 11 MA IN THE EVOLUTION OF A CLASSIC TRANSFER FAULT ZONE, THE SOUTHERN EMBUDO FAULT SYSTEM IN NORTH-CENTRAL NEW MEXICO

In the late Neogene, the Embudo fault system (EFS) served as a classic transfer structure in accommodating extensional strain between the oppositely tilted San Luis and Española Basins of the northern Rio Grande rift. We emphasize a major change in structural configuration that occurred ca. 11 Ma on the southern half of this fault system, which accompanied a previously documented inward focusing of extensional strain from border faults of the northwestern Española Basin (EB) onto the southern EFS. Prior to 11 Ma, what became the EFS in the northern EB consisted of the NE-trending Velarde graben (VG) bounded by normal or normal-oblique faults, at least one of which was active in early rifting. After 11 Ma, throw rates of VG bounding faults and stratal accumulation rates within the graben decreased. Concomitant with these changes, we postulate that a mapped NE-striking, left-lateral oblique (NW-down) fault developed along the axis of the VG. This fault terminates southward at a E-W syncline, where there is a 2 km right step-over to a ESE-down, normal-oblique fault (Santa Clara fault of the EFS) that continues southward and forms the northeast margin of the Santa Clara graben. On the northern Santa Clara fault, stratigraphic markers indicate lower throw rates during 11-8 Ma compared to 8.5-3.5 Ma. We propose that previously reported high extensional strain rates at 15-10 Ma promoted NE-directed growth of VG eastern border faults along a pre-existing, NE-trending crustal weakness (Jemez lineament) and eventual hard linkage with the master fault(s) of the San Luis Basin half graben. This created an effective transfer fault following what previously may have been a more complex, broader transverse accommodation zone. Strain was then concentrated on the southern EFS from the northwestern EB border faults and from VG border faults -- resulting in a ca. 11 Ma unconformity and 11-9 Ma compressed stratigraphic section on the immediate footwalls of the northern Santa Clara fault and the western bounding fault of the VG. Increased strain rates on the southern EFS may also have facilitated extensive basaltic volcanism and the associated Rio del Oso dike swarm southwest of the VG; intrusion of the latter possibly reduced local extensional stresses and temporarily decreased throw rates on the northern Santa Clara fault until 10-8 Ma.