Basin subsidence & chronology of southern Rio Grande rift stratigraphy: Comparing different estimates of rapid subsidence

This study uses the southern Rio Grande rift to investigate rift development by comparing independent records of lithospheric deformation during continental rifting. New zircon U-Pb data from reworked tuffaceous intervals and volcaniclastic sandstones from Rio Grande rift basin stratigraphy north of the Lemitar Mountains yield maximum depositional ages that indicate rapid subsidence (>1.2 km/Myr) during the middle to late Miocene (approximately 118 Ma). This episode of rapid subsidence was followed by a period of erosion/nondeposition, which is expressed by the Miocene-Pliocene unconformity, and then returned to a period of deposition, with significantly lower rates of subsidence (< 0.05 km/Myr). This new estimate of rapid subsidence during Rio Grande rifting is several million years younger than previously published estimates. Further, because extension in the upper crust is the primary driver of rift basin subsidence, our estimate of rapid subsidence predicts relatively coeval rapid extension, but the proposed timing of activity between these methods also does not match. It is important to consider, however, that these discrepancies may be due to comparing different chronometers (e.g., zircon U-Pb maximum depositional age versus sanidine 40Ar/39Ar), lag times between different rift-related phenomena, deposition at different locations along the basin profile, and/or deposition within unidentified subbasins. Hence, we compare our new subsidence models with 1) previously published estimates of rift basin subsidence and 2) published time-Temperature models of adjacent footwall exhumation. We evaluate these records by comparing estimated time intervals of rapid rift activity, and attempt to reconcile their discrepancies with geologic contextual data and consideration of the different systematics of applied chronometers.