Rock melting in slow rifts: the role of surface processes and the case of Victoria Land Basin, Antarctica.

During the evolution of extensional basins, the area interested by tensile stresses is commonly characterized by volcanic products with mantle-sourcing geochemical imprint. In this context, erosion of the rift shoulders and sedimentation in the basin can affect the stress and thermal fields at depth thereby promoting or inhibiting rock melting, but the tectonics/climatic boundary conditions that allow for such surface forcing on extensional magmatism are elusive.

Here, we use a bi-dimensional coupled thermo-mechanical and landscape evolution numerical model to quantify melt production changes in slowly stretching rift basins due to changes in deposition rates. The model combines visco-elasto-plastic deformation of the lithosphere and underlying mantle during extension, partial rock melting and linear hillslope diffusion of the surface topography. The parametric study covers a range of slow extension rates, crustal thicknesses, mantle potential temperatures and diffusion coefficient (corresponding to sedimentation rates of 0.01-0.1-1 mm/y).

We use the Victoria Land Basin (VLB) on the western side of the Ross Sea Embayment in Antarctica, one of the largest, long-lasting and slowest continental rifts on Earth, as a natural term of comparison for our modeling results. We particularly aim at quantifying the contribution of surface processes to rock melting in slow extensional settings, so to assess the sensitivity of extensional magmatic systems to surface processes.

The VLB area is characterized by several episodes of extension from the Cretaceous to recent time, resulting in wide rifting across the West Antarctic Rift Region (WARS), and a more localized narrow rift in the VLB. The multi-phase rifting behavior of the WARS is described by seismic reflection data displaying up to 14 km-thick sediment infilling from the Lower Paleogene in the VLB and the wide to narrow rift transition marked by Ross Sea unconformities. Miocene climate cooling deeply affected the production and transport of sediments in the basin with a tenfold decrease in sedimentation rate from the M1 glaciation to the post-Mid-Miocene Climate Transition well-visible in the sedimentary record of the youngest basin, the Terror Rift.  The volcanic features in the VLB and its flanks are represented by the Meander Intrusive Complex (48 to 18 Ma) and the McMurdo Volcanic Group (since 18 Ma and still active).

Our models reproduce the 200 km-extension of the VLB and the lithospheric necking with up to 20 Ma of asthenospheric melt production before oceanization. Surface processes inhibit mantle decompression melting and delay the crust breakup. These results suggest that the VLB magmatic history has been significantly affected by sediment deposition within the basin, which acted as a primary melt-controlling parameter. Mutual feedbacks between surface and deep-seated processes in the VLB and other extensional basins with different tectonostratigraphic histories are also supported by our models.