Beschreibung: Re-examination of marine geophysical data from the continental margin of West Morocco reveals a broad zone characterized by deformation, active faults and updoming offshore the High Atlas (Morocco margin), situated next to the Tafelney Plateau. Both seismic reflection and swath-bathymetric data, acquired during Mirror marine geophysical survey in 2011, indicate recent uplift of the margin including uplift of the basement. This deformation, which we propose to name the Atlantic Atlas tectonic arch, is interpreted to result largely through uplift of the basement, which originated during the Central Atlantic rifting stage - or even during phases of Hercynian deformation. This has produced a large number of closely spaced normal and reverse faults, “piano key faults”, originating from the basement and affecting the entire sedimentary sequence, as well as the seafloor. The presence of four terraces in the Essaouira canyon system at about 3500 meters water depth and “piano key faults” and the fact that these also affect the seafloor, indicate that the Atlantic Atlas is still active north of Agadir canyon. We propose that recent uplift is causing morphogenesis of four terraces in the Essaouira canyon system. In this paper the role of both Canary plume migration and ongoing convergence between the African and Eurasian plates in the formation of the Atlantic Atlas are discussed as possibilities to explain the presence of a tectonic arch in the region. The process of reactivation of passive margins is still not well understood. The region north of Agadir canyon represents a key area to better understand this process.

Beschreibung: Highlights • We document marine forearc deformation in the Northern Chile seismic gap. • Upper-plate normal faulting off Northern Chile locally extends close to the trench. • Normal faults indicate that past earthquakes may reached the shallow plate-boundary. Abstract Seismic rupture of the shallow plate-boundary can result in large tsunamis with tragic socio-economic consequences, as exemplified by the 2011 Tohoku-Oki earthquake. To better understand the processes involved in shallow earthquake rupture in seismic gaps (where megathrust earthquakes are expected), and investigate the tsunami hazard, it is important to assess whether the region experienced shallow earthquake rupture in the past. However, there are currently no established methods to elucidate whether a margin segment has repeatedly experienced shallow earthquake rupture, with the exception of mechanical studies on subducted fault-rocks. Here we combine new swath bathymetric data, unpublished seismic reflection images, and inter-seismic seismicity to evaluate if the pattern of permanent deformation in the marine forearc of the Northern Chile seismic gap allows inferences on past earthquake behavior. While the tectonic configuration of the middle and upper slope remains similar over hundreds of kilometers along the North Chilean margin, we document permanent extensional deformation of the lower slope localized to the region 20.8°S–22°S. Critical taper analyses, the comparison of permanent deformation to inter-seismic seismicity and plate-coupling models, as well as recent observations from other subduction-zones, including the area that ruptured during the 2011 Tohoku-Oki earthquake, suggest that the normal faults at the lower slope may have resulted from shallow, possibly near-trench breaking earthquake ruptures in the past. In the adjacent margin segments, the 1995 Antofagasta, 2007 Tocopilla, and 2014 Iquique earthquakes were limited to the middle and upper-slope and the terrestrial forearc, and so are upper-plate normal faults. Our findings suggest a seismo-tectonic segmentation of the North Chilean margin that seems to be stable over multiple earthquake cycles. If our interpretations are correct, they indicate a high tsunami hazard posed by the yet un-ruptured southern segment of the seismic gap.

Beschreibung: A 550-km-long transect across the Ninetyeast Ridge, a major Indian ocean hotspot trail, provided seismic refraction and wide-angle reflection data recorded on 60 ocean bottom instruments. About 24 000 crustal and 15 000 upper mantle arrivals have been picked and used to derive an image of the hotspot track. Two approaches have been chosen: (i) a first-arrival tomographic inversion yielding crustal properties; and (ii) forward modelling of mantle phases revealing the structure at the crust–mantle boundary region and of the uppermost mantle. Away from the volcanic edifice, seismic recordings show the typical phases from oceanic crust, that is, two crustal refraction branches (Pg), a wide-angle reflection from the crust–mantle boundary (PmP) and a wave group turning within the upper mantle (Pn). Approaching the edifice, three additional phases have been detected. We interpret these arrivals as a wide-angle reflection from the base of material trapped under the pre-hotspot crust (Pm2P) and as a wide-angle reflection (PnP) and its associated refraction branch (PN) from a layered upper mantle. The resulting models indicate normal oceanic crust to the west and east of the edifice. Crustal thickness averages 6.5–7 km. Wide-angle reflections from both the pre-hotspot and the post-hotspot crust–mantle boundary suggest that the crust under the ridge has been bent downwards by loading the lithosphere, and hotspot volcanism has underplated the pre-existing crust with material characterized by seismic velocities intermediate between those of mafic lower crustal and ultramafic upper mantle rocks (7.5–7.6 km s−1). In total, the crust is up to ≈ 24 km thick. The ratio between the volume of subcrustal plutonism forming the underplate and extrusive and intrusive volcanism forming the edifice is about 0.7. An important observation is that underplating continued to the east under the Wharton Basin. During the shield-building phase, however, Ninetyeast Ridge was located adjacent to the Broken Ridge and was subsequently pulled apart along a transform fault boundary. Therefore, underplating eastwards of the fracture zone separating the edifice from the Wharton Basin suggests that prolonged crustal growth by subcrustal plutonism occurred over millions of years after the major shield-building stage. This fact, however, requires mantle flow along the fossil hotspot trail. The occurrence of PnP and PN arrivals is probably associated with a layered and anisotropic upper mantle due to the preferential alignment of olivine crystals and may have formed by rising plume material which spread away under the base of the lithosphere.

Beschreibung: About 1000 km of the South Chilean margin were ruptured in 1960 by the Mw 9.5 Great Chile Earthquake. Early in 2010 the immediate area to the north was affected by the Mw 8.8 Maule Earthquake. In the area of the rupture boundary three giant Pleistocene submarine slope failures are observed in bathymetric and reflection seismic data. The slope failures each shifted volumes between 253 km$ and 472 km$ of slope sediments, compacted accretionary wedge material and continental framework rock from the continental slope into the trench. Seismic reflection data image an undisturbed well layered sedimentary trench fill and a continuous décollement in the areas where no slope failures are observed. However, at the exact locations of the slope failures, which coincide with the boundaries of the 1960 and 2010 ruptures, chaotic slide deposits compose the lower part of the trench-fill. At these locations no continuous décollement has developed. We speculate that the underthrusting of the highly inhomogeneous slide deposits prevents the development of a continuous décollement and thus the buildup of a thin (few millimeters) slip zone that is continuous in space as necessary for earthquake rupture propagation. Thus the 1960 Great Chile – 2010 Maule earthquake rupture boundary seems to be controlled by the underthrusting of products of giant submarine slope failures which impeded further propagation of earthquake rupture during both events. Our results emphasize that upper plate mass wasting, if it impacts on the internal structure and composition of the subduction channel rocks, can play a key role in defining seismotectonic segmentation at convergent plate boundaries.

Beschreibung: The southern central Chilean margin at the site of the largest historically recorded earthquake in the Valdivia region, in 1960 (Mw = 9.5), is part of the 5000-km-long active subduction system whose geodynamic evolution is controversially debated and poorly understood. Covering the area between 36° and 40°S, the oceanic crust is segmented by prominent fracture zones. The offshore forearc and its onshore continuation show a complex image with segments of varying geophysical character, and several fault systems active during the past 24 m.y. In autumn 2001, the project SPOC was organized to study the Subduction Processes Off Chile, with a focus on the seismogenic coupling zone and the forearc. The acquired seismic data crossing the Chilean subduction system were gathered in a combined offshore-onshore survey and provide new insights into the lithospheric structure and evolution of active margins with insignificant frontal accretion.

Beschreibung: Seismic investigations across the convergent Sunda margin off Indonesia provide a detailed image of the crustal architecture of the Sunda plate boundary. The combined analysis and interpretation of wide-angle and reflection seismic data along two coincident profiles across the subduction zone are complemented by additional lines within the forearc domain, which yield some three-dimensional (3-D) constraints on the velocity-depth structure across the margin. A detailed cross section of the subduction zone is presented, which is confirmed by supplementary gravity modeling. The Sunda convergence zone is a prime example of an accretionary margin, where sediment accretion has led to the formation of a massive accretionary prism, with a total width of 〉110 km between the trench and the forearc basin. It is composed of a frontal wedge which documents ongoing accretion and a fossil part behind the present backstop structure which constitutes the outer high. Moderate seismic velocities derived from wide-angle modeling indicate a sedimentary composition of the outer high. The subducting oceanic slab is traced to a depth of almost 30 km underneath the accretionary prism. The adjacent forearc domain is characterized by a pronounced morphological basin which is underlain by a layer of increased seismic velocities and a shallow upper plate Moho at 16 km depth. We speculate that remnant fragments of oceanic crust might be involved in the formation of this oceanic-type crust found at the leading edge of the upper plate beneath the forearc basin.