Evolution of the Pennsylvanian−early Permian Orogrande basin revealed through structural analysis and geohistory curves with implications for the Ancestral Rocky Mountains orogen

Intracratonic deformation in the southwestern part of the Laurentian block of Laurussia produced N-NW−trending, Precambrian-cored basement uplifts known as the Ancestral Rocky Mountains that were structurally offset from adjacent basins. A central debate concerns whether the Ancestral Rocky Mountains evolved as a coherent, synchronous orogen or through diachronous wrenching with various structural styles. Despite preserving well-dated Pennsylvanian−early Permian strata, the structural style of the Orogrande basin in southern New Mexico (USA) remains poorly documented. This study presents new detailed geologic mapping of structures and subsidence analysis from three stratigraphic sections along the basin’s eastern margin near the Pedernal uplift to reconstruct basin history. Subsidence results reveal a stairstep, convex-up pattern typical of fixed-load flexural basins. Subsidence was broadly synchronous, characterized by an initial pulse from late Morrowan to early Desmoinesian time followed by peak subsidence in the Late Pennsylvanian. In the Virgilian−early Wolfcampian, shelf strata were faulted, folded, uplifted, and eroded. Faulting in the northern part of the basin is best characterized as high-angle reverse faults. North-trending, west-vergent, asymmetric folding is consistent with folding atop a blind thrust. Cross-section restoration indicates shortening accommodated by these folds of ∼70−300 m, consistent with high-angle, basement-involved reverse faults observed elsewhere across the Ancestral Rocky Mountains. The Pennsylvanian subsidence and structural history of the Orogrande basin and adjacent Pedernal uplift mimics the rest of the Ancestral Rocky Mountains, suggesting a coherent, synchronous orogen. By mid-Wolfcampian time, Ancestral Rocky Mountains−related deformation had ceased, and the basin and former uplift subsided together in response to regional epeirogenic processes. This epeirogenic subsidence is observed along the southern realm of the orogen and may represent the geodynamic response to initiation of subduction along the southwestern margin of Laurentia, continued convergence from the amalgamation of Gondwana to Laurussia as deformation continued from the Marathon belt to the Sonora belt, or a combination of both.