Search

Crystallographic preferred orientation (CPO) and the associated seismic anisotropy of serpentinites are important factors for the understanding of tectonic settings involving hydrated Earth´s mantle, for example, at slow-spreading mid-ocean ridges. CPO of lizardite and magnetite in low-grade metamorphic serpentinites from the Atlantis Massif oceanic core complex (Mid-Atlantic Ridge, 30°N) were determined using synchrotron high energy X-ray diffraction in combination with Rietveld texture analysis. Serpentinite mesh structures show weak CPO while deformed samples show a single (0001) maximum perpendicular to the foliation. Seismic anisotropies calculated from CPO show up to >11% anisotropy for compressional waves (Vp) and shear wave splitting up to 0.38 km/s in the deformed samples. This indicates that deformation in shear zones controls elastic anisotropy and highlights its importance in defining the seismic signature of hydrated upper mantle.

Mediterranean tectonics since the Lower Cretaceous has been characterized by a multi‐phase subduction and collision history with temporally and spatially‐variable, small‐scale plate configurations. A new shear‐wave velocity model of the Mediterranean upper mantle (MeRE2020), constrained by a very large set of over 200,000 broadband (8‐350 s), inter‐station, Rayleigh‐wave, phase‐velocity curves, illuminates the complex structure and fragmentation of the subducting slabs. Phase‐velocity maps computed using these measurements were inverted for depth‐dependent, shear‐wave velocities using a stochastic particle‐swarm‐optimization algorithm (PSO). The resulting three‐dimensional (3‐D) model makes possible an inventory of slab segments across the Mediterranean. Fourteen slab segments of 200‐800 km length along‐strike are identified. We distinguish three categories of subducted slabs: attached slabs reaching down to the bottom of the model; shallow slabs of shorter length in down‐dip direction, terminating shallower than 300 km depth; and detached slab segments. The location of slab segments are consistent with and validated by the intermediate‐depth seismicity, where it is present. The new high‐resolution tomography demonstrates the intricate relationships between slab fragmentation and the evolution of the relatively small and highly curved subduction zones and collisional orogens characteristic of the Mediterranean realm.

Large amounts of gas hydrate are present in marine sediments offshore Taitao Peninsula, near the Chile Triple Junction. Here, marine sediments on the forearc contain carbon that is converted to methane in a regime of very high heat flow and intense rock deformation above the downgoing oceanic spreading ridge separating the Nazca and Antarctic plates. This regime enables vigorous fluid migration. Here, we present an analysis of the spatial distribution, concentration, estimate of gas-phases (gas hydrate and free gas) and geothermal gradients in the accretionary prism, and forearc sediments offshore Taitao (45.5°–47° S). Velocity analysis of Seismic Profile RC2901-751 indicates gas hydrate concentration values <10% of the total rock volume and extremely high geothermal gradients (<190 °C·km−1). Gas hydrates are located in shallow sediments (90–280 m below the seafloor). The large amount of hydrate and free gas estimated (7.21 × 1011 m3 and 4.1 × 1010 m3; respectively), the high seismicity, the mechanically unstable nature of the sediments, and the anomalous conditions of the geothermal gradient set the stage for potentially massive releases of methane to the ocean, mainly through hydrate dissociation and/or migration directly to the seabed through faults. We conclude that the Chile Triple Junction is an important methane seepage area and should be the focus of novel geological, oceanographic, and ecological research.

Key Points: Multibeam bathymetric and seismic reflection data image the structure of the North Chilean marine forearc and the oceanic Nazca plate The structural character and tectonic configuration of the offshore forearc and the oceanic plate change significantly along the margin The derived pattern of permanent deformation may hold information for studying seismicity or other types of short term deformation New multibeam bathymetry allows an unprecedented view of the tectonic regime and its along‐strike heterogeneity of the North Chilean marine forearc and the oceanic Nazca Plate between 19‐22.75°S. Combining bathymetric and backscatter information from the multibeam data with sub‐bottom profiler and published and previously unpublished legacy seismic reflection lines, we derive a tectonic map. The new map reveals a middle and upper‐slope configuration dominated by pervasive extensional faulting, with some faults outlining a >500 km long ridge that may represent the remnants of a Jurassic or pre‐Jurassic magmatic arc. Lower slope deformation is more variable and includes slope‐failures, normal faulting, re‐entrant embayments, and NW‐SE trending anticlines and synclines. This complex pattern likely results from the combination of subducting lower‐plate topography, gravitational forearc collapse, and the accumulation of permanent deformation over multiple earthquake cycles. We find little evidence for widespread fluid seepage despite a highly faulted upper‐plate. An explanation could be a lack of fluid sources due to the sediment starved nature of the trench and most of the upper‐plate in vicinity of the hyper‐arid Atacama Desert. Changes in forearc architecture partly correlate to structural variations of the oceanic Nazca Plate, which is dominated by the spreading‐related abyssal hill fabric and is regionally overprinted by the Iquique Ridge. The ridge collides with the forearc around 20‐21°S. South of the ridge‐forearc intersection, bending‐related horst‐and‐grab

Pockmarks are variably sized crater-like structures that occur in young continental margin sediments. They are formed by gas eruptions and/or long-term release of fluid or gas. So far no pockmarks were known from the Pacific coast of South America between 51°S and 55°S. This article documents an extensive and previously unknown pockmark field in the Seno Otway (Otway Sound, 52°S) with multibeam bathymetry and parametric echosounding as well as sediment drill cores. Up to 31 pockmarks per square kilometer occur in water depths of 50 to >100 m in late glacial and Holocene sediments. They are up to 150 m wide and 10 m deep. Below and near the pockmarks, echosounder profiles image acoustic blanking as well as gas chimneys often crosscutting the 20 to >30 m thick glacial sediments above the acoustic basement, in particular along fault zones. Upward-migrating gas is trapped within the sediment strata, forming dome-like features. Two 5 m long piston cores from inside and outside a typical pockmark give no evidence for gas storage within the uppermost sediments. The inside core recovered poorly sorted glacial sediment, indicating reworking and re-deposition after several explosive events. The outside core documents an undisturbed stratigraphic sequence since ~15 ka. Many buried paleo-pockmarks occur directly below a prominent seismic reflector marking the mega-outflow event of the Seno Otway at 14.3 ka, lowering the proglacial lake level by about 80 m. This decompression would have led to frequent eruptions of gas trapped in reservoirs below the glacial sediments. However, the sediment fill of pockmarks formed after this event suggests recurrent events throughout the Holocene until today. Most pockmarks occur above folded hydrocarbon-bearing Upper Cretaceous and Paleogene rocks near the western margin of the Magallanes Basin, constraining them as likely source rocks for thermogenic gas.

Highlights • Bulk rock elastic moduli of gneiss, amphibolite and marble using different techniques. • Neutron diffraction texture analysis and modeling of rock physical properties. • Measurement of seismic velocity anisotropy under pressures of up to 600 MPa. • Extrapolation of experimental data to higher pressures of 1000 MPa (crack free rock). • Comparison of modeled, experimental and extrapolated elastic anisotropy data. Abstract In this study elastic moduli of three different rock types of simple (calcite marble) and more complex (amphibolite, micaschist) mineralogical compositions were determined by modeling of elastic moduli using texture (crystallographic preferred orientation; CPO) data, experimental investigation and extrapolation. 3D models were calculated using single crystal elastic moduli, and CPO measured using time-of-flight neutron diffraction at the SKAT diffractometer in Dubna (Russia) and subsequently analyzed using Rietveld Texture Analysis. To define extrinsic factors influencing elastic behaviour, P-wave and S-wave velocity anisotropies were experimentally determined at 200, 400 and 600 MPa confining pressure. Functions describing variations of the elastic moduli with confining pressure were then used to predict elastic properties at 1000 MPa, revealing anisotropies in a supposedly crack-free medium. In the calcite marble elastic anisotropy is dominated by the CPO. Velocities continuously increase, while anisotropies decrease from measured, over extrapolated to CPO derived data. Differences in velocity patterns with sample orientation suggest that the foliation forms an important mechanical anisotropy. The amphibolite sample shows similar magnitudes of extrapolated and CPO derived velocities, however the pattern of CPO derived velocity is closer to that measured at 200 MPa. Anisotropy decreases from the extrapolated to the CPO derived data. In the micaschist, velocities are higher and anisotropies are lower in the extrapolated data, in comparis

New multibeam bathymetry allows an unprecedented view of the tectonic regime and its along-strike heterogeneity of the North Chilean marine forearc and the oceanic Nazca Plate between 19 and 22.75°S. Combining bathymetric and backscatter information from the multibeam data with subbottom profiler and published and previously unpublished legacy seismic reflection lines, we derive a tectonic map. The new map reveals a middle and upper slope configuration dominated by pervasive extensional faulting, with some faults outlining a >500-km-long ridge that may represent the remnants of a Jurassic or pre-Jurassic magmatic arc. Lower slope deformation is more variable and includes slope-failures, normal faulting, re-entrant embayments, and NW-SE trending anticlines and synclines. This complex pattern likely results from the combination of subducting lower-plate topography, gravitational forearc collapse, and the accumulation of permanent deformation over multiple earthquake cycles. We find little evidence for widespread fluid seepage despite a highly faulted upper-plate. An explanation could be a lack of fluid sources due to the sediment starved nature of the trench and most of the upper-plate in vicinity of the hyperarid Atacama Desert. Changes in forearc architecture partly correlate to structural variations of the oceanic Nazca Plate, which is dominated by the spreading-related abyssal hill fabric and is regionally overprinted by the Iquique Ridge. The ridge collides with the forearc around 20–21°S. South of the ridge-forearc intersection, bending-related horst-and-grabens result in vertical seafloor offsets of hundreds of meters. To the north, plate-bending is accommodated by reactivation of the paleo-spreading fabric and new horst-and-grabens do not develop

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 s

Although only recognized in the middle of the last century, oceanic trenches are among the most spectacular structural and morphological features in the deep oceans. Caused by the collision and subduction of tectonic plates and shaped by the interplay of tectonic and sedimentary processes, the morphology of oceanic trenches can be manifold. In this chapter we discriminate between sediment starved trenches, partly sediment filled trenches, and sediment flooded trenches. In sediments starved trenches the tectonic signature is usually well preserved everywhere in the trench, including at the outer slope, the depression, and the inner slope. In contrast, in sediment flooded trenches the outer slope and the trench depression usually correspond to a flat seafloor that results from the deposition of thick sedimentary sequences that overprint all fault scarps. Here, a tectonic signature is only found at the trench inner slope where accretion of trench sediments results in thrust faulting. The remarkable differences in trench morphologies underline that for a comprehensive understanding of the structural evolution of a convergent margin, detailed knowledge on the sedimentary and tectonic history of the adjacent oceanic trench is necessary.