Post-rift seaward downwarping at passive margins: New insights from southern Oman using stratigraphy to constrain apatite fission-track and (U-Th)/He dating

Abstract

The plateau edge of southern Oman is used as a natural laboratory to independently test apatite fission-track analysis, (U-Th)/He dating, and stratigraphy as methods for quantifying post-rift erosion depths and lithospheric response at passive margins. A mappable unconformity between the Proterozoic basement and the low-conductivity, pre-rift sediment cover links residual buttes preserved at the coast to the escarpment, and therefore imposes tight limitations on parameter choices for modeling the Cenozoic topographic evolution. A mean geothermal gradient of ∼33 °C km⁻¹ ensures the best fit between the thermochronologic data and the stratigraphically defined coast-to-scarp maximum denudation depth of ∼1.75 km. With a lithospheric elastic thickness of 7 km, the resulting flexural response generates a retreat and uplift of the escarpment associated with a seaward downwarp of the unconformity, made possible because denudation minima are inboard of the escarpment and sediment loading occurs offshore. This closed experiment confirms the value of low-temperature thermochronology as a tool for quantifying long-term erosion, but also highlights the benefits of including independent stratigraphic information in terms of obtaining more plausible solutions. This study suggests that post-rift downwarping of eroded rift shoulders can occur, and may have not been commonly detected because low-temperature thermochronology data sets lack both suitable geologic data and intrinsic sensitivity.