Two-Stage Laramide Orogeny in Southwestern United States: Tectonics and Sedimentation: ABSTRACT

Abstract

The Laramide orogeny (80-40 Ma) in the southwestern United States is usually thought of as a single tectonic event with attention concentrated on its early stage because of its dramatic expression in the sedimentary record. The sharpest pulse of deformation, however, occurred in the latest Paleocene-early Eocene and was separated from the early stage by a tectonic lull and development of widespread lateritic weathering profiles, remnants of which are preserved in some early Tertiary basins. The first stage correlates plate tectonically with opening of the North Atlantic and Labrador Sea, the second stage with opening of the Norwegian Sea and Eurasian basin in the eastern Arctic. Rapid convergence between the North American and Farallon plates decreased the dip of the subd cting Farallon plate, until by early Eocene (55 Ma), strong viscous coupling was occurring between the Farallon plate and the overlying lithosphere. Change in direction of convergence from west-southwest-east-northeast during the early stage to southwest-northeast during the late stage brought the first-order shear direction into near parallelism with the north-northeast-trending southern Rocky Mountain deformed belt in New Mexico. This parallelism allowed the Colorado Plateau to decouple from the craton along right-lateral wrench faults. The Colorado Plateau was translated 100-130 km north-northeast with a comparable shortening across the Wyoming province. Both sedimentation rates and coarseness of synorogenic sediments increased dramatically from Wyoming to the Gulf Coast of Texas beginning in the early Eocene. Simultaneously, an en echelon series of strike-slip basins formed along the zone of decoupling from southern New Mexico to southern Wyoming. Deformation in the Wyoming province also increased sharply, resulting in as much as 21 km of overhang on range-front thrusts and up to 15 km of structural relief between adjacent uplifts and basins. As much as 2,500 m of Eocene orogenic sediments were deposited in rapidly subsiding basins. Deformation was so rapid that surface drainage was disrupted and runoff was impounded in huge lakes in which as much as 1,000 m of lacustrine sediments accumulated to form the oil shale province. Wrench faulting along the eastern margin of the Colorado Plateau was distributed across a 100-km wide belt that followed zones of weakness inherited from Precambrian and late Paleozoic deformation. Wrenching changed from nearly parallel in New Mexico to strongly convergent in Colorado and southern Wyoming because of a change in structural grain from north-northeast in New Mexico to north-northwest in Colorado. At least 25 km of shortening across the zone of decoupling during convergent wrenching caused conspicuous low-angle thrusting, which has tended to mask the lateral component of movement. Major north-northeast shortening across the Wyoming province and a lack of such shortening on the High Plains demand right-lateral wrench faulting. Offset of transverse aeromagnetic anomalies ac oss the zone of decoupling in Colorado and New Mexico, offset of Precambrian metavolcanic belts and age province boundaries, and offset of northeast-trending lineaments and distinctive rock types provide independent measures of the magnitude of right slip. Northward translation of the Colorado Plateau was a relatively minor part of regional right-lateral shear that extended through much of the North American cordillera in the early Tertiary. Recognition of the wrench-faulted nature of the eastern margin of the Colorado Plateau and Wyoming province creates new opportunities for petroleum exploration. These can be divided into: (1) subthrust plays beneath low-angle faults caused by convergent wrenching; (2) structural, stratigraphic, and fracture-controlled traps along wrench faults; and (3) fracture reservoirs along northeast-trending lineaments dilated during wrenching. Recognition of wrench faults paralleling the structural grain of the southern Rocky Mountains also means that isopach and facies maps drawn smoothly across the zone of decoupling should be reevaluated with careful attention to control points. End_of_Article - Last_Page 2044------------