ALBERT

Beschreibung: We present an integrated approach of the seismic structure and activity along the offshore SW Hellenic subduction from combined observations of marine and land seismic stations. Our imaging of the slab top topography from teleseismic receiver function analysis at Ocean Bottom Seismometers supports a trenchward continuation of the along-dip slab faults beneath the Peloponnesus. We further show that their morpho-structural control accounts for the backstepping of the thrust contact of the Mediterranean Ridge accretionary wedge over the upper plate. Local seismic activity offshore SW Peloponnesus constrained by OBS observations reveals a correlation with specific features of the forearc: the Matapan Troughs. We study the Mw 6.8 14.02.2008 interplate earthquake offshore SW Peloponnesus and show that its nucleation, rupture zone and aftershocks sequence are confined to one slab panel between two adjacent along-dip faults and are thus controlled by the offshore slab top segmentation, but also the upper plate sea-bottom morphology.

Beschreibung: At convergent margins, the structure of the subducting oceanic plate is one of the key factors controlling the morphology of the upper plate. We use high-resolution seafloor mapping and multichannel seismic reflection data along the accretionary Sumatra trench system to investigate the morphotectonic response of the upper plate to the subduction of lower plate fabric. Upper plate segmentation is reflected in varying modes of mass transfer. The deformation front in the southern Enggano segment is characterized by neotectonic formation of a broad and shallow fold-and-thrust belt consistent with the resumption of frontal sediment accretion in the wake of oceanic relief subduction. Conversely, surface erosion increasingly shapes the morphology of the lower slope and accretionary prism towards the north where significant oceanic relief is subducted. Subduction of the Investigator Fracture Zone and the fossil Wharton spreading centre in the Siberut segment exemplifies this. Such features also correlate with an irregularly trending deformation front suggesting active frontal erosion of the upper plate. Lower plate fabric extensively modulates upper plate morphology and the large-scale morphotectonic segmentation of the Sumatra trench system is linked to the subduction of reactivated fracture zones and aseismic ridges of the Wharton Basin. In general, increasing intensity of mass-wasting processes, from south to north, correlates with the extent of oversteepening of the lower slope (lower slope angle of 3.8° in the south compared with 7.6° in the north), probably in response to alternating phases of frontal accretion and sediment underthrusting. Accretionary mechanics thus pose a second-order factor in shaping upper plate morphology near the trench. © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists.