Brittle reactivation of inherited late Neoproterozoic/Cambrian lithospheric-scale structures during extension and break-up of Madagascar from Africa

We compare the temporal and structural evolution of the adjacent Morondava and Mahajanga sedimentary basins in western Madagascar in the light of reactivation of lithospheric and crustal-scale structures. Both basins formed during Mesozoic extension and break-up of Madagascar from Africa and are underlain and bordered by highly reworked Precambrian basement. Rifting in the Morondava basin was partly guided by brittle reactivation of inherited Neoproterozoic/Cambrian ductile structures [1]. Thus, brittle faults in both basin and basement trend parallel (I) to the borders of the basin, i.e. N-S and NW-SE, and (II) to shear zones in the basement to the east. In contrast, the border of the Mahajanga basin and most of the brittle faults within the basin run NE-SW, at a high angle to abundant N-S and WNW-ESE trending brittle lineaments in the basement to the east. Recent shear-wave splitting results from a seismic network across the entire island of southern Madagascar from SW to NE mimic the surface orientation of shear zones in the central part of the array and indicate that the Neoproterozoic/Cambrian structural trends follow lithospheric-scale anisotropies [2]. Further, this implies that these anisotropies persisted in this area since the latest stages of convergence during the East African orogeny and thus the location of rifting in the Morondava basin was likely controlled by the inherited lithospheric structure. NE-SW trending anisotropy has been identified in the seismic network below the Antananarivo domain of SE Madagascar, the same basement domain bordering and underlying (part of) the Mahajanga basin. The reason for this orientation of anisotropy at depth is not entirely clear, but [2] propose crystallographic preferred orientation of mantle minerals due to absolute plate motion. However, the contribution of inherited lithospheric scale tectonic structures is also feasible. [1] Giese, J., Seward, D., Schreurs, G., 2012. Tectonics 31, doi:10.1029/2011TC002921 [2] Reiss, M.C., Rümpker, G., Tilmann, F., Yuan, X., Giese, J., Rindraharisaona, E.J., in review. J. Geophys. Res.