New Mexico Geological Society Annual Spring Meeting — Abstracts

The contractional deformation associated with the Laramide Orogeny in the southern Basin and Range province is obscured by widespread, Cenozoic extensional faulting and volcanism. As a result, debate continues about the structural style and tectonic evolution of the region. There are two end-member structural models that have been proposed for structural deformation: 1) high-angle reverse faulting and basement block uplift and 2) thin-skinned faulting and development of a thrust belt. We undertook new geologic mapping, cross-section construction and restoration, and structural analysis in Sierra Rica, in the bootheel region of southern New Mexico, to help evaluate these hypotheses.We have identified a major low-angle to sub-horizontal thrust fault that places Ordovician through Pennsylvanian carbonate rocks (El Paso Group, Escabrosa Formation, and Horquilla Formation) structurally above clastic rocks belonging to the Cretaceous Mojado Formation, an older-on-younger structural relationship. A hanging wall ramp is preserved in the thrust sheet and indicates a ramp-flat thrust fault geometry. Sedimentary structures, including trough cross-bedding, indicate that the units are upright and not overturned. No basement rocks are exposed in the map area and do not appear to be involved in faulting in Sierra Rica. The thrust fault has been locally offset and tilted by Cenozoic normal faulting, but it is interpreted to be close to its original orientation. Several small granitic intrusions and dikes locally cross-cut the thrust fault and the intrusive rocks are themselves cross-cut by normal faults. We collected samples of these intrusive rocks for U-Pb geochronology.We separated zircon from the rocks using standard crushing, density and magnetic separation techniques, and then individually picked, mounted, and polished the zircon to expose the interior of the crystals. Next, the mounts were imaged using backscatter electron (BSE) detector to examine the crystals for mineral inclusions or other defects. The zircon will be analyzed for U-Pb isotopes using laser ablation (LA)-inductively coupled plasma mass spectrometry (LA-ICPMS). With this data we will be able to constrain the timing of deformation in the area.