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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. / text

Detailed mapping (1:6,000) of Lower Ordovician through Lower Pennsylvanian strata, exposed in the vicinity of the Sunshine Springs thrust fault, shows that the thrust ramps up-section twice in a direction parallel or subparallel with the thrusting, and that the geometry of folds can be attributed to their proximity to the two closely spaced ramps. The lower ramp is a frontal ramp which originated as a forelimb thrust through the overturned limb of a tight anticline-syncline fold couplet. The upper ramp cuts up-section through a thin, upper Paleozoic flysch sequence where the Sunshine Springs thrust becomes imbricated. Directly above the lower ramp, in the upper plate, is a broad symmetrical anticline which has a geometry similar to a fault-bend fold. Forward of the lower ramp is a large wavelength, flat-bottomed syncline, and behind the lower ramp is a series of tight to isoclinal overturned folds. As a result of fault-bend folding and continued shortening of the ramp region, upper plate folds characteristically have a larger amplitude than folds of the lower plate. Just forward of the lower ramp in the footwall is the tightly folded and truncated syncline of the syncline-anticline fold couplet. The rest of the lower plate section is only mildly deformed. A composite, down-structure cross section drawn parallel with the direction of thrusting shows that the Peña Colorada synclinorium has been transported along the Sunshine Springs thrust approximately 3.8 km. Shortening, as deduced from folding in this study alone, is 20 percent, and when the shortening by the thrust is also considered, the total amount of shortening equals 52 percent. A major left-lateral strike-slip system, trending WNW, approximately parallel with the thrusting direction, offsets the Sunshine Springs thrust fault. Strike-slip and dip-slip displacements can be calculated from a displaced fold axis of the lower plate syncline, and are 335 m and 90 m, respectively. In the vicinity of this strike-slip system, the axial traces of folds change from a dominantly southwesterly trend to a more southerly trend. The regional extent of the fault system within the Marathon Basin, and its correspondence with the change in major fold axes orientations suggests that the fault zone is a regional tear which formed in response to the impingement of the Marathon thrust front against the Diablo Platform during the Pennsylvanian Period. / text