Description: Highlights • The basement at the mid-Norwegian Møre Margin is dominantly felsic in composition. • A lower crustal body is interpreted as a mixture of continental blocks and eclogite. • The thickness of the outer lower crustal body is twice as thick on the East Greenland Margin. • The thinning during this first phase of post-Caledonian extension was highest for proto Norway. Abstract The inner part of the volcanic, passive Møre Margin, mid-Norway, expresses an unusual abrupt thinning from high onshore topography with a thick crust to an offshore basin with thin crystalline crust. Previous P-wave modeling of wide-angle seismic data revealed the presence of a high-velocity (7.7–8.0 km/s) body in the lower crust in this transitional region. These velocities are too high to be readily interpreted as Early Cenozoic intrusions, a model often invoked to explain lower crustal high-velocity bodies in the region. We present a Vp/Vs model, derived from the modeling of wide-angle seismic data, acquired by use of Ocean Bottom Seismograph horizontal components. The modeling suggests dominantly felsic composition of the crust. An average Vp/Vs value for the lower crustal body is modeled at 1.77, which is compatible with a mixture of continental blocks and Caledonian eclogites. The results are compiled with earlier results into a transect extending from onshore Norway to onshore Greenland. Back-stripping of the transect to Early Cenozoic indicates asymmetric conjugate magmatism related to the continental break-up. Further back-stripping to the time when most of the Caledonian mountain range had collapsed indicates that the thinning during the first phase of extension was about 25% higher for proto Norway than proto Greenland.

Description: In April and May 1996, a geophysical study of the Cascadia continental margin off Oregon and Washington was carried out aboard the German RV Sonne as a cooperative experiment between GEOMAR, the USGS and COAS. Offshore central Oregon, which is the subject of this study, the experiment involved the collection of wide-angle refraction and reflection data along three profiles across the continental margin using ocean-bottom seismometers (OBS) and hydrophones (OBH) as well as land recorders. Two-dimensional modelling of the travel times provides a detailed velocity structure beneath these profiles. The subducting oceanic crust of the Juan de Fuca plate can be traced from the trench to its position some 10 km landward of the coastline. At the coastline, the Moho has a depth of 30 km. The dip of the plate changes from 1.5° westward of the trench to about 6.5° below the accretionary complex and to about 16° further eastward below the coast. The backstop forming western edge of the Siletz terrane, an oceanic plateau that was accreted to North America about 50 Ma ago, is well defined by the observations. It is located about 60 km to the east of the deformation front and has a seaward dip of 40°. At its seaward edge, the base of the Siletz terrane seems to be in contact with the subducting oceanic crust implying that sediments are unlikely to be subducted to greater depths. The upper oceanic crust is thinner to the east of this contact than to the west. At depths greater than 18 km, the top of the oceanic crust is the origin of pre-critical reflections observable in several land recordings and in the data of one ocean bottom instrument. These reflections are most likely caused by fluids that are released from the oceanic crust by metamorphic facies transition.

Description: Based on a compilation of published and new seismic refraction and multichannel seismic reflection data along the south central Chile margin (33°–46°S), we study the processes of sediment accretion and subduction and their implications on megathrust seismicity. In terms of the frontal accretionary prism (FAP) size, the marine south central Chile fore arc can be divided in two main segments: (1) the Maule segment (south of the Juan Fernández Ridge and north of the Mocha block) characterized by a relative large FAP (20–40 km wide) and (2) the Chiloé segment (south of the Mocha block and north of the Nazca-Antarctic-South America plates junction) characterized by a small FAP (≤10 km wide). In addition, the Maule and Chiloé segments correlate with a thin (〈1 km thick) and thick (∼1.5 km thick) subduction channel, respectively. The Mocha block lies between ∼37.5° and 40°S and is configured by the Chile trench, Mocha and Valdivia fracture zones. This region separates young (0–25 Ma) oceanic lithosphere in the south from old (30–35 Ma) oceanic lithosphere in the north, and it represents a fundamental tectonic boundary separating two different styles of sediment accretion and subduction, respectively. A process responsible for this segmentation could be related to differences in initial angles of subduction which in turn depend on the amplitude of the down-deflected oceanic lithosphere under trench sediment loading. On the other hand, a small FAP along the Chiloé segment is coincident with the rupture area of the trans-Pacific tsunamigenic 1960 earthquake (Mw = 9.5), while a relatively large FAP along the Maule segment is coincident with the rupture area of the 2010 earthquake (Mw = 8.8). Differences in earthquake and tsunami magnitudes between these events can be explained in terms of the FAP size along the Chiloé and Maule segments that control the location of the updip limit of the seismogenic zone. The rupture area of the 1960 event also correlates with a thick subduction channel (Chiloé segment) that may provide enough smoothness at the subduction interface allowing long lateral earthquake rupture propagation.

Description: We present the first detailed 2D seismic tomographic image of the trench-outer rise, fore- and back-arc of the Tonga subduction zone. The study area is located approximately 100 km north of the collision between the Louisville hot spot track and the overriding Indo-Australian plate where ~80 Ma old oceanic Pacific plate subducts at the Tonga Trench. In the outer rise region, the upper oceanic plate is pervasively fractured and most likely hydrated as demonstrated by extensional bending-related faults, anomalously large horst and graben structures, and a reduction of both crustal and mantle velocities. The 2D velocity model presented shows uppermost mantle velocities of ~7.3 km/s, ~10% lower than typical for mantle peridotite (~30% mantle serpentinization). In the model, Tonga arc crust ranges between 7 and 20 km in thickness, and velocities are typical of arc-type igneous basement with uppermost and lowermost crustal velocities of ~3.5 and ~7.1 km/s, respectively. Beneath the inner trench slope, however, the presence of a low velocity zone (4.0–5.5 km/s) suggests that the outer fore-arc is probably fluid-saturated, metamorphosed and disaggregated by fracturing as a consequence of frontal and basal erosion. Tectonic erosion has, most likely, been accelerated by the subduction of the Louisville Ridge, causing crustal thinning and subsidence of the outer fore-arc. Extension in the outer fore-arc is evidenced by (1) trenchward-dipping normal faults and (2) the presence of a giant scarp (~2 km offset and several hundred kilometers long) indicating gravitational collapse of the outermost fore-arc block. In addition, the contact between the subducting slab and the overriding arc crust is only 20 km wide, and the mantle wedge is characterized by low velocities of ~7.5 km/s, suggesting upper mantle serpentinization or the presence of melts frozen in the mantle.

Description: The highly active subduction zone of southern Chile was the source region of the 1960 Valdivia megathrust earthquake (Mw= 9.5), the largest earthquake ever recorded.This region is currently under investigation by the multidisciplinary TIPTEQ (From the Incoming Plate to Mega-Thrust Earthquake Processes) project, which is studying the structure, state, and deformation of the subduction zone lithosphere. Over 90 days, from December 2004 to February 2005,TIPTEQ scientists on cruise S0181 of the German research vessel (R/V Sonne acquired a broad variety of geophysical and geological data in the research area offshore Chile between 35°S and 48°S (Figure 1).These data include active and passive source seismics, heat flow probing, magnetics, magnetotellurics for studying Earth conductivity, highresolution multibeam bathymetry, and sediment probes from gravity cores.

Description: 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.

Description: High-resolution seismic experiments, employing arrays of closely spaced, four-component ocean-bottom seismic recorders, were conducted at a site off western Svalbard and a site on the northern margin of the Storegga slide, off Norway to investigate how well seismic data can be used to determine the concentration of methane hydrate beneath the seabed. Data from P-waves and from S-waves generated by P–S conversion on reflection were inverted for P- and S-wave velocity (Vp and Vs), using 3D travel-time tomography, 2D ray-tracing inversion and 1D waveform inversion. At the NW Svalbard site, positive Vp anomalies above a sea-bottom-simulating reflector (BSR) indicate the presence of gas hydrate. A zone containing free gas up to 150-m thick, lying immediately beneath the BSR, is indicated by a large reduction in Vp without significant reduction in Vs. At the Storegga site, the lateral and vertical variation in Vp and Vs and the variation in amplitude and polarity of reflectors indicate a heterogeneous distribution of hydrate that is related to a stratigraphically mediated distribution of free gas beneath the BSR. Derivation of hydrate content from Vp and Vs was evaluated, using different models for how hydrate affects the seismic properties of the sediment host and different approaches for estimating the background-velocity of the sediment host. The error in the average Vp of an interval of 20-m thickness is about 2.5%, at 95% confidence, and yields a resolution of hydrate concentration of about 3%, if hydrate forms a connected framework, or about 7%, if it is both pore-filling and framework-forming. At NW Svalbard, in a zone about 90-m thick above the BSR, a Biot-theory-based method predicts hydrate concentrations of up to 11% of pore space, and an effective-medium-based method predicts concentrations of up to 6%, if hydrate forms a connected framework, or 12%, if hydrate is both pore-filling and framework-forming. At Storegga, hydrate concentrations of up to 10% or 20% were predicted, depending on the hydrate model, in a zone about 120-m thick above a BSR. With seismic techniques alone, we can only estimate with any confidence the average hydrate content of broad intervals containing more than one layer, not only because of the uncertainty in the layer-by-layer variation in lithology, but also because of the negative correlation in the errors of estimation of velocity between adjacent layers. In this investigation, an interval of about 20-m thickness (equivalent to between 2 and 5 layers in the model used for waveform inversion) was the smallest within which one could sensibly estimate the hydrate content. If lithological layering much thinner than 20-m thickness controls hydrate content, then hydrate concentrations within layers could significantly exceed or fall below the average values derived from seismic data.