The mechanism of continental extension and breakup, and the role of detachment faults in these processes, are currently the subject of intense debate. One possible detachment fault is the S reflector, imaged as an undulating (in time), locally discontinuous reflection on existing seismic profiles west of Iberia. Here we present new images in depth of the four marginnormal profiles across the west Galicia rifted margin, where the S reflector was originally defined and is best imaged. It is shown that faults bounding wedge-shaped units of late synrift sediments, which hence were active during rifting immediately prior to breakup, appear to detach at shallow levels onto the S reflector. S itself appears as a continuous, locally domal feature and does not generally appear offset. The waveform of S is compatible with a reflection from a single interface such as a sharp tectonic boundary. The depth sections show that S was active at 1-3 km below the seafloor during final rifting; S is interpreted as a brittle detachment fault which controlled the final breakup of the continent west of Galicia. Furthermore, the data provide constraints on the sense of shear of S: analogy with detachment terranes, the present, synrift and structural dips of S, and the identification of a breakaway to S imply that S accommodated top-tothe-west shear. Toward the east of the profiles, S becomes more complex, possibly because of different phases of detachment faulting and the development of both incisement and excisement structures. From the geometry of wedge-shaped sedimentary units deposited during faulting above S it also appears that S was active at an angle of 20 ø or less and hence may be considered a genuine low-angle normal fault.
Structure sections across the Japan Trench from prestack depth migrated seismic data define the accretionary prism, a wedge‐shaped seaward end of the continental framework, and a tectonized middle slope between them. The 10‐km‐wide accretionary prism is underthrust by sediment remaining with the subducted oceanic crust. Structure beneath the steep middle slope more nearly resembles the adjacent continental margin than the accreted mass. This deformed middle slope domain forms a buffer between the compliant accretionary wedge and a more rigid older continental framework. Along the plate boundary, subducted strata thicken beneath the middle and upper slopes and, where last imaged, are four times the input thickness beneath the accretionary prism. This interplate stratified layer appears to control interplate friction because the layer is imaged 45 km landward from the trench axis to 12‐km depth and in that expanse, few upper plate earthquakes are recorded. Erosion of the base of the upper plate beneath the middle continental slope is indicated by margin subsidence and material flux. This erosion is concurrent with accretion at the front of the margin.
[1] 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.
Three multichannel seismic reflection records across the Kuril convergent margin provide the first deep data in this area. The records are located across the southern tip of Kamchatka, the central Kuril arc, and the northern extension of Hokkaido Island platform which show three distinct tectonic regimes. The lower slope contains a wedge‐shaped buttress surrounded by low velocity sediments. Underplating sediment uplifts the buttress, as indicated by faults that displace its upper surface. The middle slope is a block of acoustic basement, which has a rough surface with significant arcward dipping faults. The middle slope is separated from the upper slope along a steep arcward dipping reflection, the “middle‐slope boundary.” The upper slope structure off Kamchatka is different from that off Hokkaido. Off Kamchatka a regular stratified sediment section has been uplifted, tilted, and dips seaward. Off Hokkaido, stronger uplift has tilted regional blocks seaward and arcward. Along the southern line north of Hokkaido, well‐studied nonthrust earthquakes with lateral motions occur beneath the middle slope boundary and on the boundary of the subducting plate. Thrust earthquakes occur under the middle and upper slope, whereas tsunami earthquakes occur under the lower slope. High‐amplitude reflections along the lower boundary of the wedge‐shaped buttress and along the active decollement, indicate high fluid concentrations which reduce the friction along the tectonic units so that through the weak coupling, slow rupture may extend up to the seafloor from a tsunami earthquake.
[1] The observation of spatial and temporal dynamics of the ocean is fundamental to understand global and regional aspects of water mixing. Physical oceanography has traditionally observed ocean structures with in situ measurements, often limited in temporal and/or spatial resolution. In exploration seismology a set of techniques has been developed over the last decades to image and characterize the physical properties of sub-seafloor structures by inversion methods at high horizontal resolution. The two different fields have made contact in seismic oceanography where the well developed methods of marine reflection seismology have been applied to the dynamic ocean. However, one aspect, so far ignored in seismic oceanography, is the dynamical, temporally varying nature of water structures. Here we show that it is possible to estimate temporal variations of reflectors in water structures as an inversion parameter. The new dynamic property reflector movement velocity gives an additional parameter to characterize ocean water dynamics.
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