Patterns of Cenozoic Dynamic Topography: An Example from the North American Middle Atlantic Margin
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Global eustatic sea-level histories generated by backstripping stratigraphy are predicated upon the lithosphere having a well understood tectonic history. However, sub-plate processes exert important control on lithospheric vertical motions with timescales and amplitudes akin to eustasy, which are difficult to isolate in the geologic record. We integrate stratigraphic and geophysical observations with simple isostatic models to disentangle histories of sub-plate support and eustasy. We focus on the passive margin of Eastern North America. Negative long wavelength free-air gravity anomalies, residual ocean-age depth estimates, fast upper mantle shear wave velocities, and geodynamic models suggest that Cenozoic evolution of this passive margin has been influenced by upper mantle drawdown. We build on existing analyses to backstrip sixteen wells, which, combined with seismic data, constrain timing and extent of Cenozoic post-rift subsidence. Results indicate up to ~600 m of water-loaded subsidence between ~20–0 Ma centered on the Baltimore Canyon Trough. Seismic data from the trough shows Neogene prograding and aggrading clinoforms, with little evidence for faulting or growth strata, indicating that Neogene lithospheric strain rates were low. Amplitude and spatial extent of Neogene subsidence are difficult to explain by glacio-eustasy or glacio-isostatic adjustment. Instead, we suggest that upper mantle drawdown was responsible for the subsidence of the margin. Sub-plate support calculated from conversion of shear wave velocities to temperature and isostatic calculations corroborate this conclusion. Our results highlight how multiple datasets can be combined to resolve climatic and tectonic controls of passive margins, and evaluating mantle and surface process interactions in deep time.