Ancestral Rocky Mountain deformation in south-central New Mexico, USA: Insights from detrital zircon U–Pb and (U–Th)/He dating in the Orogrande Basin

Abstract Late Paleozoic intracratonic shortening in southwestern Laurentia during the Ancestral Rocky Mountains (ARM) orogeny resulted in the formation of uplifted basement-cored fault blocks and flexural subsidence of adjacent basins. ARM deformation occurred during the final closure of the Rheic Ocean and the amalgamation of western equatorial Pangea, culminating in the Ouachita-Marathon-Sonora (OMS) orogeny. While the two orogenic events have long been linked kinematically, the temporal relationship and the geodynamic linkage between ARM and OMS deformation have become debated in recent years. This study investigates the syntectonic deposition and provenance evolution of the late Paleozoic Orogrande Basin and the tectonic unroofing of the basement-cored Pedernal faulted block in response to the ARM shortening in south-central New Mexico. Detailed detrital zircon (DZ) U–Pb ages (n = 3966 from 30 samples) and zircon (U–Th)/He double dating (n = 50 from five samples) help document a three-stage depositional history for the proximal Orogrande Basin in relation to reverse faulting along the western margin of the Pedernal block. The basal clastic section, represented by incised channels in the lower Gobbler Formation (Morrowan–Atokan), exhibits a pre-ARM Appalachian-type provenance signature, characterized by recycled Grenville (950–1300 Ma) and subordinate early Paleozoic DZ U–Pb ages. Upsection, these DZ components become diluted by locally derived Yavapai–Mazatzal and Granite–Rhyolite sources, signaling the early stages of the Pedernal unroofing. DZ provenance signatures rapidly shifted to a synorogenic unroofing signature linked to the Pedernal fault block, dominated by Yavapai–Mazatzal and Granite–Rhyolite and Grenvillian De Baca Group, defined by an age mode of ca. 1.25 Ga. This dominant DZ age signature derived from the Pedernal block remains uniform from the upper Gobbler (Desmoinesian) through the lower Abo formations (lower Wolfcampian) and constrains the timing of ARM shortening in the Sacramento Mountains from the Early Pennsylvanian to the earliest Permian. The upper part of the Abo Formation is marked by a sharp shift to a unimodal ca. 1.25 Ga DZ age signature, implying early Permian burial of the Pedernal block and cessation of ARM reverse faulting. DZ (U–Th)/He (DZHe) double dating of selected samples tracks the tectonic exhumation of the Pedernal fault block. While DZHe ages broadly scatter from the Pennsylvanian to the Mesoproterozoic, a young Carboniferous age mode (ca. 323 Ma) at the base of the section suggests rapid exhumation of a ca. 1.67 Ga ARM basement source. Upsection, the DZHe ages define a trend of progressively older cooling ages, typical of a reverse unroofing sequence of a sedimentary cover succession, characterized by the progressive erosion of un-reset lower Paleozoic strata followed by Pedernal crystalline basement. Overall, these DZ UPb and He ages from the proximal Orogrande Basin constrain the unroofing of the ARM-related Pedernal uplift from the Early Pennsylvanian to the earliest Permian, predating the Marathon fold-and-thrust belt deformation, the more proximal segment of the OMS orogen. Hence, these new constraints support a late Paleozoic transpressional margin in southwestern Laurentia as the driving mechanism for the synchronous ARM midcontinent deformation that largely predates shortening in the proximal Marathon fold-and-thrust belt.