FOLD-THRUST SYSTEMS OVERPRINTING SYN-RIFT STRUCTURES on THE MARGIN OF AN INVERTED RIFT BASIN: INDIO MOUNTAINS, WEST TEXAS

Detailed mapping in the Indio Mountains in west Texas, reveal a complex stacking of thrust sheets that record inversion of the Mesozoic Chihuahua Trough rift basin. Important newly recognized structural complexities include both folding of older thrust sheets and development of younger, out-of-sequence thrusts that cut across older folds; detachment fold horizons within thrust sheets; and a regional anticlinorium that exposes two deeper thrust sheets in a structural half-window bounded on the west by a Neogene normal fault. The Chihuahua trough originated as part of the late Mesozoic border rift system and was inverted during Laramide thrusting. Remnants of Mesozoic rifting are preserved in Cretaceous strata as facies and stratigraphic thickness variations between different thrust sheets as well as presence of numerous syn-depositional normal faults that involve strata as young as the Middle Albian. Statigraphic thickness variations are particularly prominent in the Yucca Formation. A well exposed transgression divides the upper yucca into a fluvial lower interval and a coastal upper interval. The Upper Yucca is over 600m thick in the hanging wall of the Squaw Peak thrust and 262 m in the structurally lowest thrust sheet. Klippe-fenster relationships indicate a minimum displacement of 7-10km for the structurally highest thrust (Shaw Peak thrust). However, we are evaluating a hypothesis that the Squaw Peak thrust sheet overlies a duplex that produced the regional anticlinorium and the half window. Restorations of this thrust sheet should allow testing of this hypothesis but will almost certainly show totally shortening significantly greater than 10km.