New Mexico Geological Society Annual Spring Meeting — Abstracts

Ladron Peak, a flanking uplift pounding part of the central Rio Grande rift, is located in a complex region where the late Miocene to Holocene rift widens south of the Colorado Plateau. East of Ladron is the 80 km-wide Belen basin (extended 13 km), formed by two west-tilted, nested half-grabens underlain by a west-dipping crustal detachment (imaged by COCORP). Directly south of Ladron, the rift is a broader (130 km-wide) zone of extension formed by three west-tilted, nested half-grabens. The arrangement of these grabens suggests that they sole into an east-dipping detachment.

The Ladron horst block is located in the apex where two arcuate half-grabens are linked end-to-end. We interpret the Ladron horst as an isostatically uplifted footwall block. The COCORP-imaged, subsurface intrarift horst northeast of Ladron is disconnected from the Ladron block by listric normal faults. The intrarift horst may be an accommodation zone (an area of compressional deformation and/or distributed shear resulting from interference of half-grabens) produced by end-to-end linking or nesting of similar polarity half-grabens.

The boundary between the relatively unextended Plateau west of Ladron and the rifted terrane to the south is a zone of distributed shear. A NE-SW directed sinisttal shear couple is expressed by north-trending, en echelon, high-angle normal faults, which may be reactivated from an Oligocene phase of extension. Extension along these structures has nearly broken Ladron Peak Valley from the Colorado Plateau.