Beschreibung: We examine micro-earthquake records from a dense temporary array of ocean bottom seismometers (OBS) and hydrophones that has been installed from September to November 2005 at the trench outer rise offshore Nicaragua. Approximately 1.5 locatable earthquakes per day within the array of 110 × 120 km show the high seismic activity in this region. Seismicity is restricted to the upper ∼15 km of the mantle and hence where temperatures reach 350–400 °C, which is smaller than values observed for large mantle intraplate events (650 °C). Determination of moment tensor solutions suggest a change of the stress region from tensional in the upper layers of the oceanic plate to compressional beneath. The neutral plane between both regimes is located at ∼6–9 km beneath Moho and thus very shallow. Fluids, which are thought to travel through the tensional fault system into the upper mantle, may not be able to penetrate any deeper. The earthquake catalogue, which seems to be complete for magnitudes above Mw = 1.6–1.8, suggests a strong change of the lithospheric rheology when approaching the trench. And b-factors, that is the ratio between small and large earthquakes increase significantly in the closest 20 km to the trench axis, implying that the crust and upper mantle is massively weakened and hence ruptures more frequently but under less release of stress. We explain this with a partly serpentinized upper mantle.

Beschreibung: The Galápagos volcanic province (GVP) includes several aseismic ridges resulting from the interaction between the Galápagos hotspot (GHS) and the Cocos–Nazca spreading centre (CNSC). The most prominent are the Cocos, Carnegie and Malpelo ridges. In this work, we investigate the seismic structure of the Carnegie ridge along two profiles acquired during the South American Lithospheric Transects Across Volcanic Ridges (SALIERI) 2001 experiment. Maximum crustal thickness is ∼19 km in the central Carnegie profile, located at ∼85°W over a 19–20 Myr old oceanic crust, and only ∼13 km in the eastern Carnegie profile, located at ∼82°W over a 11–12 Myr old oceanic crust. The crustal velocity models are subsequently compared with those obtained in a previous work along three other profiles over the Cocos and Malpelo ridges, two of which are located at the conjugate positions of the Carnegie ones. Oceanic layer 2 thickness is quite uniform along the five profiles regardless of the total crustal thickness variations, hence crustal thickening is mainly accommodated by layer 3. Lower crustal velocities are systematically lower where the crust is thicker, thus contrary to what would be expected from melting of a hotter than normal mantle. The velocity-derived crustal density models account for the gravity and depth anomalies considering uniform and normal mantle densities (3300 kg m−3), which confirms that velocity models are consistent with gravity and topography data, and indicates that the ridges are isostatically compensated at the base of the crust. Finally, a two-dimensional (2-D) steady-state mantle melting model is developed and used to illustrate that the crust of the ridges does not seem to be the product of anomalous mantle temperatures, even if hydrous melting coupled with vigorous subsolidus upwelling is considered in the model. In contrast, we show that upwelling of a normal temperature but fertile mantle source that may result from recycling of oceanic crust prior to melting, accounts more easily for the estimated seismic structure as well as for isotopic, trace element and major element patterns of the GVP basalts.

Beschreibung: Several trench-outer rise settings in subduction zones worldwide are characterized by a high degree of alteration, fracturing and hydration. These processes are induced by bending-related faulting in the upper part of the oceanic plate prior to its subduction. Mapping of P- and S-wave velocity structures in this complex tectonic setting provides crucial information for understanding the evolution of the incoming oceanic lithosphere, and serves as a baseline for comparison with seismic measurements elsewhere. Active source seismic investigations at the outer rise off Southern Central Chile (∼43°S) were carried out in order to study the seismic structure of the oceanic Nazca Plate. Seismic wide-angle data were used to derive 2-D velocity models of two seismic profiles located seaward of the trench axis on 14.5 Ma old crust; P01a approximately parallel to the direction of spreading and P03 approximately parallel to the spreading ridge and trench axes. We determined P- and S-velocity models using 2-D traveltime tomography. We found that the Poisson's ratio in the upper crust (layer 2) ranges between ∼0.33 at the top of the crust to ∼0.28 at the layer 2/3 interface, while in the lowermost crust and uppermost mantle it reaches values of ∼0.26 and ∼0.29, respectively. These features can be explained by an oceanic crust significantly weathered, altered and fractured. Relative high Poisson's ratios in the uppermost mantle may be likely related to partially hydrated mantle and hence serpentinization. Thus, the seismic structure of the oceanic lithosphere at the Southern Central Chile outer rise exhibits notable differences from the classic ophiolite seismic model (‘normal’ oceanic crust). These differences are primarily attributed to fracturing and hydration of the entire ocean crust, which are direct consequences of strong bending-related faulting at the outer rise. On the other hand, the comparison of the uppermost mantle P-wave velocities at the crossing point between the perpendicular profiles (∼90 km oceanward from the trench axis) reveals a low degree of Pn anisotropy (〈2 per cent).