From sink to source: Using offshore thermochronometric data to extract onshore erosion signals in Namibia

Products of onshore passive continental margin erosion are best preserved in offshore sedimentary basins. Therefore, these basins potentially hold a recoverable record of the onshore erosion history. Here, we present apatite fission track (AFT) data for 13 samples from a borehole in the southern Walvis basin, offshore Namibia. All samples show AFT central ages older or similar to their respective stratigraphic ages, while many single grain ages are older, implying none of the samples has been totally annealed post-deposition. Furthermore, large dispersion in single grain ages in some samples suggests multiple age components related to separate source regions. Using Bayesian mixture modelling we classify single grain ages from a given sample to particular age components to create 'subsamples' and then jointly invert the entire dataset to obtain a thermal history. For each sample, the post-depositional thermal history is required to be the same for all age components, but each component ('subsample') has an independent pre-depositional thermal history. With this approach we can resolve pre- and post-depositional thermal events and identify changes in sediment provenance in response to the syn- and post-rift tectonic evolution of Namibia and southern Africa. Apatite U-Pb and compositional data obtained during the acquisition of LA-ICP-MS FT data are also presented to help track changes in provenance with time. We constrain multiple thermal events linked to the exhumation and burial history of the continental and offshore sectors of the margin over a longer timescale than has been possible using only onshore AFT thermochronological data.

Automated summary

Products of onshore passive continental margin erosion are well preserved in offshore sedimentary basins, providing a recoverable record of onshore erosion history. Analysis of apatite fission track (AFT) data and Bayesian mixture modeling can help resolve thermal histories and changes in sediment provenance in response to tectonic evolution. Additionally, tracking changes in provenance with time using apatite U-Pb and compositional data aids in understanding the exhumation and burial history of the margin over a longer timescale.