FOLD-THRUST SYSTEM IN THE INDIO MOUNTIANS, WEST TEXAS: OUT-OF-SEQUENCE THRUST IN AN INVERTED RIFT BASIN?

Out-of-sequence thrust are relatively common in fold thrust systems where the orogenic wedge is disrupted from its critical taper, and an inverted rift basin is one setting where out-of-sequence thrust are particularly common. The Indio Mountains region of west Texas lies on the northern margin of the Cretaceous Chihuahua trough rift basin that was inverted in the Laramide suggesting this is a site where structures of this type are likely. Nonetheless, there has been little work in the area since the development of the modern concepts of fold-thrust systems.

This study reports on new mapping in the central Indio Mountains where two NW trending Neogene normal faults form a graben, producing an unusually three-dimensional view of the Laramide thrust system. An angular unconformity beneath Tertiary volcanics and landslide deposits constrains structural levels at ~50 Ma. Multiple klippe and fensters in the region aid interpretation, particularly one small fenster in the footwall of the western normal fault. The fold-thrust system in the area also show unusual trends, differing ~90° from the regional trends.

The kinematics of this odd fold-thrust system is still uncertain, however two hypotheses are being evaluated: 1) trends are the result of an out-of-sequence thrust resulted from a propagating duplex system, related to inversion of the rift basin fault, and 2) the fold-thrust system contains a subsurface lateral ramp along a duplex system. Field work and thrust restorations in progress are evaluating these alternative hypotheses.