The Early Life of a Stable Continental Craton: Mechanisms of Accretion as Consequences of a Late Neoproterozoic Ridge-Trench Interaction, Khomas Complex, Damara Belt, Namibia.

Accretion at subduction margins is a tectonic process that provides movement and transfer of surface sediments and rocks through an ever-increasing pressure-temperature-strain regime, at depths ranging from 1 km to around 80 km. The resulting accretionary wedge is then the external expression of one of the main tectonic mechanisms of continental crust growing. The late-Neoproterozoic/early-Phanerozoic Damara Belt in central Namibia records the convergence and eventual collision of the Kalahari and Congo Cratons between ca. 580 and 540-510 Ma and shows a remarkable preservation of its original architecture. We show here the results of field observations in the Khomas Complex, tentatively interpreted in the past as the accretionary prism of the belt (Kukla, 1992). The mapping and structural analysis of two traverses across the complex, where slivers of oceanic rocks (”Matchless Amphibolite”) are imbricated with trench and abyssal plane sediments are presented here. Together with comparisons with Meso-Cenozoic analogues, these data allow us to reconstruct the deformation history and to interpret it as being related to events of progressive offscraping and underplating in the context of an accretionary prism. We also document evidence for ridge subduction, in the form of primary contacts between trench sediments and lower plate mafic rocks (sills and lava flow in sediments). Although ridge subduction is an inevitable consequence of subduction tectonics, it is mainly documented in recent circum-Pacific examples. It has only rarely been recognized in the geological record, with most well documented examples limited to Meso-and Cenozoic convergent margins. To our knowledge, this is the first Proterozoic documented example of a ridge-trench encounter. Ridge subduction could explain a number of features recorded on the upper plate (the Central Zone, Congo Craton) that have, hitherto, been regarded as being problematic.