Thermal evolution of continental rifting in Corsica (France)

Seymour, Nikki Marie
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Despite advances in understanding the structural development of magma-poor rift margins, the temporal and thermal evolution of lithospheric hyperextension during rifting remains poorly understood. In contrast to classic pure-shear models, multi-stage rift models that include depth-dependent thinning predict significant conductive crustal reheating during the necking phase due to buoyant rise of the asthenosphere. The Santa Lucia nappe of NE Corsica is an ideal laboratory to test for lower- and upper-crustal reheating as it preserves both Permian lower crust exhumed from granulitic conditions during Tethyan rifting and upper crust in contact with syn- and post-rift Mesozoic strata. This study uses novel zircon, rutile, and apatite LA-ICP-MS depth-profile U-Pb petro-and thermochronology coupled with garnet trace-element thermometry to constrain the syn-rift thermal evolution of this lower-crustal section during progressive lithospheric necking hyperextension, providing compelling evidence for significant early reheating due to depth-dependent thinning. Jurassic reheating is recorded in the footwall of the Belli Piani Shear Zone (BPSZ), where 200-180 Ma zircon U-Pb overgrowths on Permian core populations and preservation of stranded-diffusion profiles in garnets resorbed during D2 deformation imply that the dominant footwall fabric formed as a result of large-magnitude ductile thinning and reheating to ~800°C during Jurassic hyperextension. Conductive reheating of the upper crust in the lower hanging wall to 500-550°C due to either juxtaposition against the hot BPSZ footwall during lower crustal exhumation or wholesale crustal reheating due to conductive steepening of geothermal gradients during early rifting. Rapid post-reheating cooling in response to extensional unroofing of the footwall underscores the importance of ductile shearing and thinning during crustal hyperextension. The results of this study suggest that the thermal evolution of magma-poor rifted margins mimics their multi-phase structural evolution, beginning with diffuse rifting and tectonic subsidence. Depth-dependent thinning triggers dramatic crustal reheating at the onset of necking and hyperextension, demonstrated here in the footwall and lower hanging wall of the Belli Piani subunit. Rapid cooling and exhumation of the lower crust and extreme crustal attenuation during continued hyperextension culminates in mantle exhumation, followed by thermal relaxation and subsidence to a steady-state thermal field coeval with the start of sea-floor spreading.