COMPLEX FOLD-THRUST SYSTEM IN THE INDIO MOUNTAINS: ONGOING MAPPING OF THE INVERTED RIFT BASIN

The Indio Mountains in west Texas are an Early Cretaceous rift basin that was inverted by thin-skinned Laramide thrust systems. In this type of setting, out-of-sequence thrusts are likely where the orogenic wedge is disrupted form its critical taper by complex topography during thrust motion as well as complex fold-fault interactions at reactivated basement margin faults. Studies in the area have been minimal since the development of modern fold-thrust concepts.

This study reports on ongoing mapping of a part of the Indio Mountains where we have begun to recognize both out-of-sequence thrust systems and complex thrust stacking. In the mapped area, fold–thrust systems show abnormal trends of ~90 degrees from the regional trends. In addition, Neogene extension has provided a unique three-dimensional view of the Laramide trust system; with three NW trending Neogene normal faults that form a horst-graben system. The floor of the graben is exposed as an angular unconformity beneath Tertiary volcanics, and landslide deposits date the unconformity to the Eocene (~40Ma). Numerous klippe and fensters in the region assist the interpretation of fold-thrust systems, but stratigraphic complexities complicate correlation of individual thrust. Recent field work has shown at least two major flat-on-flat and ramp-flat thrust structures within the basal part of the section, with one of these structures clearly recoding out-of-sequence thrusting by decapitation of footwall folds.

Ongoing field and lab work are in progress to gain an understanding of the kinematics. Two hypotheses are being evaluated: 1) that unusual structural trends are the result of a subsurface lateral ramp along a subsurface duplex system, or 2) an out-of-sequence thrust produced from a propagating duplex system related to inversion of rift basin fault.