Paradox Basin: A Model Pull-Apart Basin of Pennsylvanian Age: ABSTRACT

Abstract

The Paradox basin of the east-central Colorado Plateau province is an elongate, roughly rhombic salt basin of Middle Pennsylvanian age. it is bounded on the northeast by the Uncompahgre-San Luis segments of the Ancestral Rockies. The writers have demonstrated previously that the End_Page 950------------------------------ basin sagged along basement rift zones by strong east-west extension during the Desmoinesian. The dominant zone of weakness was the northwest-trending Olympic-Wichita basement lineament that lies along the eastern margin of the Paradox salt basin and the southwestern edge of the Uncompahgre-San Luis uplifts. Less prominent northwest and northeast shear zones are ubiquitous, but are especially well developed in basement and Paleozoic rocks underlying the San Juan basin at the southeast termination of the Paradox basin. J. C. Crowell's classic model of a pull-apart basin along anastomosing transform fault zones is directly applicable to the Paradox basin, with the one exception that the Paradox is an intracratonic basin developed on continental crust. The primary zone of weakness, the Olympic-Wichita lineament, marks the abrupt eastern margin of the basin. The southwestern margin is less well defined along a broad zone of basement faults that trend northwesterly across the San Juan basin, through the southern margin of the salt basin, across the Monument upwarp at the anomalous Fish Creek structure and the Mille Crag Bend fracture zone, and on o the northwest through the Henry Mountains intrusives and the Fremont sag. The northwest termination is the expected irregular compressional (convergent) marg n at the Emery uplift (San Rafael swell), and the southeast limit of the basin is an irregular margin of normal faults and stretched attenuated floor (divergence) lying between the Hogback monocline and the House Creek fault. The complex intersections lying at the rhombic corners of the basin are in the San Juan Mountains on the southeast, the Defiance uplift on the southwest, the Fremont sag on the northwest, and the Oquirrh sag on the northeast. As the Paradox basin episodically deepened during the Middle Pennsylvanian by rejuvenation of basement faults, it was being filled contemporaneously with salt, which may have reached a thickness of 6,000-8,000 ft (1,800-2,400 m), and arkoses of 15,000-20,000 ft (4,600-6,100 m) thickness along the Uncompahgre front. A pull-apart of only about 5% of extension would account for a basin of this magnitude. By about mid-Desmoinesian time, the wrenching pull-apart was nearly completed. Folding caused by minor wrench movements formed shoaling conditions along the southwest shallow shelf of the basin where algal bioherms developed. Meanwhile, pull-apart stretching of the basin floor may have triggered salt flowage and diapirism in the eastern, deepest part of the basin. From the late Desmoines an through Permian, the basin filled with marine and nonmarine sediments as the wrench tectonism subsided. End_of_Article - Last_Page 951------------