Palaeobathymetry and anomalous subsidence at rifted margins: Observations from the magma-rich and magma-poor Nova Scotian margin

The isostatic evolution and bathymetry of rifted margins depends on thinning of continental crust, the volume of magmatic additions, lithosphere thermal perturbation during rifting and its post-rift re-equilibration, and sediment loading. Additionally, at some margins, bathymetric evolution may also be affected by basin isolation, where eustatic variations are not controlled by global sea-level changes, and mantle plume dynamic uplift and its collapse. The relative influence of these contributors to rifted margin bathymetric evolution varies from example to example.

Here we investigate the parameters controlling the palaeobathymetric evolution of the Nova Scotian rifted margin during the early stages of the opening of the Central Atlantic Ocean, following Triassic rifting, salt deposition and early Jurassic continental breakup. We use a 3D flexural backstripping technique which incorporates decompaction and post-breakup reverse thermal subsidence modelling to provide palaeobathymetric predictions through the Cretaceous down to the Late Triassic base salt.

Quantitative analysis of seismic reflection and gravity anomaly data together with residual depth anomaly analyses have also been used to determine variations of crustal thickness and crustal type as well as volumes of magmatic addition emplaced during rifting and continental breakup. We show the magma-rich to magma-poor transition of the Nova Scotian margin, characterized by seaward dipping reflectors (SDRs) in the SW, while in the NE mantle is possibly exhumed.

Comparison of our palaeobathymetric predictions with seismic observations and palaeoenvironments deduced from biostratigraphy of drill samples are in good agreement over the continental shelf. As expected, discrepancies exist more distally related to salt withdrawal and sediment gravity-driven sliding. Palaeobathymetries predicted seaward, on the first oceanic crust, range from 2 to 2.5 km; values in the range of those observed at young oceanic ridges.

The oceanic crust of the SW Nova Scotian margin shows well developed sequences of SDRs. Their morphology resembles that of inner SDRs of volcanic margins like the Norwegian and Greenland margins (North Atlantic), where drilling results indicate that they correspond to lava-flows emplaced near or above sea-level. Our predicted palaeobathymetry of top SDRs at breakup is nearly ~2km deeper than the expected near sea-level. This discrepancy suggests that the subsidence of this thick oceanic crust with SDRs requires an additional mechanism in addition to post-rift thermal subsidence.

Mantle plume uplift and collapse likely occurs at volcanic margins and has a long wavelength of the order of 500 km or more. However, the subsidence discrepancy we observe has a shorter wavelength and seems focused along the nascent spreading axis. Thinning of the thick oceanic crust after SDR emplacement by oceanward lateral flow of molten and ductile lower crust is an alternative possibility and may be a common occurrence at volcanic rifted margins after continental breakup.