Structural evolution of the Warwick Hills, Marathon Basin, West Texas
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A detailed structural analysis was conducted of the Warwick Hills at the northeast tip of the doubly-plunging Dagger Flat anticlinorium, Marathon Basin, west Texas. Field work delineated a folded duplex structure composed of three horses. Thrust transport was towards the northwest and resulted in a hinterland-dipping duplex. Initial thrusting In the Warwick Hills shortened the area by 2.2:1 (54%). Post-thrusting, the duplex underwent nearly isoclinal folding creating two anticlines and a syncline, second-order folds to the Dagger Flat anticlinoium. Folding combined with thrusting brought the total shortening of the rock package to 6.5:1 (85%). Earlier estimates gave a shortening for the Warwick Hills of 3:1. Finally, the folded duplex was extended by oblique tear faulting that offset the folded thrusts accommodating extension of the major folds in a northeast direction. These tear faults occurred post-plunging of the folds and were the last deformational movements that affected the Warwick Hills. The Ordovician Maravillas and Devonian Caballos Formations acted in the Warwick Hills as a structurally competent couplet. Addition or subtraction of this couplet, or units in this couplet, controlled the location of the major and minor thrusts, the style and shape of folds, and the location of the fold hinges. Bounding the couplet are incompetent shales of the Ordovician Woods Hollow and the Mississippian Tesnus Formations. Thrusts in the Warwick Hills duplex have a basal décollement in the Woods Hollow shale and ramp up through the Maravillas/Caballos couplet with an upper décollement in the Tesnus shale. The entire duplex was primarily folded by flexural slip (i.e. concentric folds) as evidenced by slickensides oriented parallel to bedding and perpendicular to fold axes, the constant thickness of the competent layers and the change in fold shape with depth. Fold wavelength, as determined from the couplet in the lowest thrust sheet, averages ~1,300 m and the average fold axis for the Warwick Hills, as determined stereographically, plunges ~54° N90°E. Shale in the Woods Hollow and Tesnus Formations bounding the couplet, flowed passively during folding into the cavities that were created by the bending of the more competent units. Lower and upper boundaries of disharmonic folding developed in the Woods Hollow and Tesnus Formations respectively. Unique to this area when compared to the rest of the anticlinorium are the presence of tightly folded thrusts and steep east-trending fold axes. The anticlinorium plunges in the Warwick Hills because it drapes off a down-to-the-northeast basement fault. Folds were "dragged" or diverted to the east during thrusting of the duplex over this transversely-oriented paleotopographic fault scarp, or were diverted subsequent to thrusting of the duplex by strike-slip movements at depth along the basement fault.
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