New Mexico Geological Society Annual Spring Meeting — Abstracts

The current debate between advocates of north-south strike-slip faulting and east-west thrust faulting during Laramide deformation in New Mexico has centered on attempts to limit strike-slip displacements using sedimentary trends as pin lines because many of Laramide faults are poorly exposed and reactivated by subsequent Rio Grande extension. Unfortunately, the prevalence of Precambrian and Paleozoic structural weaknesses in the southern Rocky Mountains makes the assumption of linear pre-Laramide sedimentation trends suspect at best.

Another approach is to look at minor faults which reflect the regional strains and stresses. Minor fault planes (n=1363) and associated slickenlines were measured at 44 outcrops of Permian to Eocene sandstones between Albuquerque and Las Vegas, New Mexico. The latest stage of Rio Grande rifting is shown by normal faults (n=125) which indicate N70E extension. Laramide thrust and strike-slip faults show two directions of horizontal shortening and compression. An early stage of N83E shortening and compression is recorded by a combination of north-striking thrust faults (n=141), ENE-striking right-lateral faults (n=147) and ESE-striking left-lateral faults (n=146). These faults are commonly cut by faults which indicate a second stage of N20E shortening and compression. N-striking right-lateral faults (n=302) dominate these faults, with NE-striking left-lateral faults (n=209) commonly reactivating earlier right-lateral faults from stage 1 and relatively few thrust faults (n=28). Both Laramide fault sets appear to cut the Eocene Galisteo Formation and neither cut the 27.2 + 1.1 Ma Galisteo mafic dike.

The early stage of east-west compression is consistent with fault data from the eastern Front Range and Wet Mountains of Colorado. The second stage,however, is weakly indicated or lacking in minor fault populations studied to date in Colorado. The NNE-orientation of this second stage of deformation suggests that it may be the result of late-stage encroachment of the Laramide foreland by the thrust belt to the south-southwest. Thus, both the east-west thrust and north-south strike-slip hypotheses for Laramide deformation in New Mexico appear to be valid, although neither is adequate in exclusion of the other. Future challenges are to map the extent of these orogenic pulses, trace their sources, and determine their relative importance in generating the Laramide deformation of New Mexico.