ANCESTRAL ROCKY MOUNTAIN UNROOFING HISTORY RECORDED IN THE OROGRANDE BASIN, NM

The Ancestral Rocky Mountains (ARM) orogen is a late Paleozoic intra-continental shortening belt characterized by basement-cored uplifts and adjacent basins in SW Laurentia. While traditionally believed to result from far-field shortening in the foreland of the diachronous Marathon-Ouachita (MO) thrust belt during final assembly of Pangea, this view has recently been challenged in light of alternative proto-Cordilleran tectonic models. Much of the uncertainty hinges on the paucity of temporal constraints on deformation and basin formation. This study investigates the deposition and provenance evolution of the Orogrande Basin (OrB) and tectonic unroofing of the Pedernal fault block. The OrB is one of the southernmost ARM basins and is separated from the Pedernal Uplift by a series of late Paleozoic reversal faults. Geometry and progression of reverse faulting and the syn-tectonic sedimentary record are well exposed due to Cenozoic Basin and Range normal faulting and offer the opportunity to investigate the timing of ARM tectonism. Clastic sedimentation initiated with the deposition of the early Pennsylvanian Gobbler Formation, continued with upper Pennsylvanian mixed carbonate and siliciclastic Holder and Bursum formations and early Permian Abo Formation. Detailed new detrital zircon (DZ) U-Pb geochronology from the Gobbler, Holder, Bursum and Abo formations show a 3-fold pre-, syn, post-tectonic depositional history. While lower Gobbler samples exhibit a pre-tectonic regional provenance signature likely derived from the Ouachita foreland, DZ provenance quickly shifted to a locally-derived signature linked to unroofing of the Pedernal Uplift, dominated by Yavapai-Mazatzal, granite-rhyolite and recycled Grenville DZ ages and signaling the onset of ARM shortening. From the upper Gobbler through the lower Abo formations, DZ patterns remain very constant and dominate ages derived from the Pedernal block. The upper part of Abo Formation is marked by a sharp shift to only 1.1 Ga DZ age, indicating post-tectonic early Permian burial of the Pedernal uplift. Overall, these DZ patterns in the proximal OrB indicate that ARM unroofing of the Pedernal Uplift started in the early Pennsylvanian and continued until the early Permian and hence largely predated shortening in Marathon fold-and-thrust belt.