INITIATION AND DESTRUCTION OF THE PENNSYLVANIAN-EARLY PERMIAN OROGRANDE BASIN REVEALED THROUGH GEOHISTORY CURVES AND STRUCTURAL ANALYSIS: IMPLICATIONS FOR ANCESTRAL ROCKY MOUNTAINS TECTONISM

Intracratonic western Laurentia experienced deformation consisting of N-NW trending Precambrian-cored uplifts termed the Ancestral Rocky Mountains (ARM), which were structurally offset from adjacent basins. ARM tectonic models remain unresolved, and debate over tectonic origins has continued largely because definitive ARM structures are scarce and detailed quantitative subsidence studies are lacking. The Orogrande basin (OB) of modern southern New Mexico provides this key missing data lacking elsewhere in the ARM province because it features deformed strata with structures that are unequivocally of Pennsylvanian-early Permian origin. This study examines the tectonic subsidence of the OB through backstripping analysis of three Pennsylvanian to early Permian sections along the basin margin adjacent to the Pedernal uplift (PU) of the ARM, and generated tectonic subsidence curves demonstrate initiation of basin subsidence, period of peak basin subsidence, and demise of the basin.

Magnitudes of tectonic subsidence are plotted against decompacted time units determined by conodont or fusulinid biostratigraphy to illustrate basin evolution over time. Results show pulses in tectonic activity, and a stepwise tectonic subsidence pattern. Subsidence along the eastern OB margin was roughly synchronous, with a first phase of subsidence lasting from late Morrowan through early Desmoinesian time. Following a quiescent phase, geohistory curves indicate a second subsidence pulse from late Desmoinesian through Missourian time. Virgilian to early Wolfcampian time marks a transition to a different phase of basin evolution, with faulting and folding of OB strata evident during this interval. By mid-Wolfcampian time, classical ARM tectonism had ceased in the region, and the OB and former PU subsided in unison to accommodate early Permian sediment deposition. Peak tectonic subsidence intervals were mid-Missourian in the northern part of the study area and early Desmoinesian southward. The geometry of tectonic subsidence curves suggests that the two phases of classical ARM tectonism here represent flexural foreland basin development in response to a migrating tectonic load, with potential strike-slip influence as well. More than one tectonic driver is likely necessary to explain the geologic history of the OB.