INFLUENCE OF LARAMIDE DEFORMATION ON THE PROTEROZOIC BASEMENT IN THE SOUTHERN SANGRE DE CRISTO MOUNTAINS, NEW MEXICO

The Las Vegas Range of the southern Sangre de Cristo Mountains is part of the southern-most subrange of the Rocky Mountains west of Las Vegas, New Mexico. Structures associated with the late Mississippian to early Permian (320-270 Ma) Ancestral Rocky Mountain orogenic event first deformed the Paleozoic cover strata that were deposited into a series of intracratonic basement uplifts. Evidence of Ancestral Rocky Mountain deformation are shown by the N-NW to S-SE striking structures throughout Colorado and New Mexico; however, many of these structures are overprinted by ones associated with the Late Cretaceous (80-55 Ma) Laramide Orogeny which resulted in broad folds, sedimentary basin development, and numerous range-frontal reverse faults. Uplift of these basement blocks and deformation of the Paleozoic-Mesozoic cover has implications for intraplate deformation, reactivation of older structures, and understanding the response of the continental lithosphere to compressive stress. The purpose of this study is to determine if, and how much, Laramide deformation affected the Proterozoic basement blocks. We hypothesize that the basement rocks were uplifted as largely intact and unrotated blocks. We further submit that Laramide deformation was accommodated in the basement by cleavage development and shearing at the Proterozoic contact - the Great Unconformity. The degree of cleavage development provides some evidence on the amount of strain the rocks have experienced. This can be seen by grain alignments discerned via the anisotropy of magnetic susceptibility (AMS) technique to find the preferred orientation of magnetic mineral phases. Paleomagnetic data will constrain the timing of deformation by determining the age of the magnetization (i.e. Late Paleozoic versus Late Cretaceous). AMS will map the preferred fabric orientation confirming any strain patterns observed in outcrops in the field. To accomplish this, macrostructures (fractures, foliations, and cleavages) have been measured in the field and compared to original basement structures. Preliminary results infer that the original undeformed basement regional structures trend NW - SE while at the Great Unconformity contact, fractures trend NE to SW which supports formation during Laramide deformation.