Detachment faulting duriing slow spreading and during magma-poor continental breakup
IPGP - Îlot Cuvier
Séminaires Géosciences Marines
University of Birmingham
The ocean basins form by the breakup and continued divergence of the continents. The tectonic and structural processes of extension leading to breakup are preserved at magma-poor margins and are well-imaged by seismic profiling. These margins, are characterised by continental fault blocks that decrease in size and increase in complexity oceanward, underlain by mantle with a reduced seismic velocity, indicative of mantle serpentinisation. We focus on the Porcupine Basin west of Ireland where extreme crustal thinning resembles the two conjugate margins without an intervening ocean, and on those margins that are sediment-starved (hence accessible to sampling by drilling), such as the Iberia Abyssal Plain and the Galicia margins west of Portugal and Spain. In all three, the continental fault blocks are separated from the underlying serpentinised mantle by bright reflections (the P, H and S reflector respectively), interpreted as detachment faults. In 2013 we collected a 3D seismic survey measuring 65x20km across the Galicia margin, imaging in 3D the S reflector and the overlying fault blocks. The seismic image reveal previously only postulated complexity within the fault blocks that require the presence of multiple phases of faulting. The S detachment can be traced on the 3D seismic to a breakaway in the east, indicating that motion on S was probably top to the west, but cannot be traced oceanward beyond the last continental fault block. Instead, as with the H detachment further south and the Lower Tasna detachment in the Alps, S appears to be truncated by a younger generation of detachments that exhume mantle further oceanward. Successive detachment, dominantly operating in a flip-flop style of alternating polarity unroof a broad expanse of mantle: where extension is focused on a single large offset detachment (exhuming a broad expanse of footwall), the active portion of the fault moves with the hangingwall, over the ridge axis until the exhumed footwall is cut by a later fault. Similar structures are likely to control the unroofing of mantle at ultraslow and very slow spreading ridges. At the Southwest Indian Ridge for instance, we proposed in 2011 that the smooth seafloor was unroofed by the same mechanism: a succession of detachments of alternating polarity. At slow spreading ridges, detachments seem to be more restricted in time and space, largely being associated with the formation of oceanic core complexes. At 5 S, such an OCC appears to have been split apart by later faulting: just as at magma poor margins, the original detachment moved with the footwall across the ridge / rift axis to be cut by later faulting. The rapid movement of the active fault root towards and then across the ridge axis has also been inferred elsewhere (e.g. at 13N), suggesting that oceanic detachments are simply spatially restricted extremely weak faults that locally and temporarily accommodate most of the plate divergence. However it is possible that detachments are not so spatially restricted and may continue laterally away from the OCC, but beneath a layer of fault blocks sliced sequentially off the hangingwall of the fault and transferred to the footwall. In 2016, we will investigate the structure and in particular the lateral continuity of oceanic detachment faults in the region of 1320N through a combined seismic refraction and seismic reflection survey. Our objectives are to distinguish between the various models for the formation of oceanic detachments and hence for oceanic core complexes, in the process shedding light on the non-magmatic processes that contribute to seafloor spreading.