The beginning of modern style subduction tectonics is still a matter of debate. Although some authors (Stern, 2005) place it during the Neoproterozoic (ca. 1 Ga or younger), there is still paucity of evidence in the literature of subduction and accretion complexes that are older than Mesozoic in age. 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 preliminary 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
Mechanisms of accretion during building of the Khomas accretionary complex, Damara Belt, Central Namibia: consequences of a late Neoproterozoic ridge-trench interaction
MENEGHINI, FRANCESCA;
2012-01-01
Abstract
The beginning of modern style subduction tectonics is still a matter of debate. Although some authors (Stern, 2005) place it during the Neoproterozoic (ca. 1 Ga or younger), there is still paucity of evidence in the literature of subduction and accretion complexes that are older than Mesozoic in age. 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 preliminary 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 problematicI documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
