ALBERT

Description: Water transported in subducting oceanic plates plays a key role in a number of phenomena, including intraslab seismicity and arc magmatism. However, the locus of plate hydration and water distribution in crust and mantle of plates entering subduction zones is debated. We present evidence for anomalously low seismic velocities and densities of the crust and upper mantle of the Nazca plate at the north Chile trench. Crustal seismic velocities at the trench are lower than velocities of mature fast-spreading crust and even lower than velocities of highly extended slow-spreading crust. In addition, the Nazca plate at the north Chile trench may contain an ∼20-km-thick upper-mantle layer with ∼17% serpentine, which implies ∼2.5 wt% water. These results document pervasive rock alteration by water percolation linked to bending-related extensional faulting.

Description: Erosion by high stress abrasion of convergent margins from horsts and grabens on the subducting plate is not shown in seismic images. In a proposed model, the frontal sediment prism is a dynamic mass that elevates pore-fluid pressure. Overpressured fluid invades fractures in the upper plate and separates fragments that are dragged into a subduction channel along the plate interface. Removed fragments are smaller than surface ship seismic techniques have resolved and beyond the reach of past scientific ocean drilling; however, current drill capability and downhole geophysics can test the model.

Description: Erosion by high stress abrasion of convergent margins from horsts and grabens on the subducting plate is not shown in seismic images. In a proposed model, the frontal sediment prism is a dynamic mass that elevates pore-fluid pressure. Overpressured fluid invades fractures in the upper plate and separates fragments that are dragged into a subduction channel along the plate interface. Removed fragments are smaller than surface ship seismic techniques have resolved and beyond the reach of past scientific ocean drilling; however, current drill capability and downhole geophysics can test the model.

Description: A large number of mound-shaped structures that originated from mud extrusions is present along the convergent continental margin off Costa Rica and Nicaragua. Active fluid venting is indicated by the existence of CH4- and H2S-rich pore fluids as well as associated benthic fauna and authigenic carbonates. End-member fluid samples from all mounds are significantly depleted in dissolved Cl and other major elements, suggesting a general process of freshwater addition and thus a common source of the fluids. Our data clearly rule out dilution by gas hydrate dissociation as a dominant source of the freshwater. Enrichments of the fluids in B (up to 2 mmol/L) and inversely correlated δ18O vs. δD values point to clay-mineral dehydration as the cause for these anomalies. Calculations assuming a δ18O vs. δD equilibrium between the pore fluid and clay minerals at depth of formation indicate temperatures of dehydration between 85 and 130 °C. This temperature range is in agreement with the B enrichments and the presence of thermogenically formed CH4. Because temperatures above 50 °C are not reached within the sediment cover of the upper plate, the fluids most likely form within the subducted sediments and flow upward along deep-seated faults from ≥12 km depth. Mound-related fluid expulsion may contribute significantly to the recycling of mineral-bound water.

Description: The Gulf of Cadiz seismicity is characterized by persistent low to intermediate magnitude earthquakes, occasionally punctuated by high magnitude events such as the M ~ 8.7 1755 Great Lisbon earthquake and the M = 7.9 event of February 28th, 1969. Micro-seismicity was recorded during 11 months by a temporary network of 25 ocean bottom seismometers (OBSs) in an area of high seismic activity, encompassing the potential source areas of the mentioned large magnitude earthquakes. We combined micro-seismicity analysis with processing and interpretation of deep crustal seismic reflection profiles and available refraction data to investigate the possible tectonic control of the seismicity in the Gulf of Cadiz area. Three controlling mechanisms are explored: i) active tectonic structures, ii) transitions between different lithospheric domains and inherited Mesozoic structures, and iii) fault weakening mechanisms. Our results show that micro-seismicity is mostly located in the upper mantle and is associated with tectonic inversion of extensional rift structures and to the transition between different lithospheric/rheological domains. Even though the crustal structure is well imaged in the seismic profiles and in the bathymetry, crustal faults show low to negligible seismic activity. A possible explanation for this is that the crustal thrusts are thin-skinned structures rooting in relatively shallow sub-horizontal décollements associated with (aseismic) serpentinization levels at the top of the lithospheric mantle. Therefore, co-seismic slip along crustal thrusts may only occur during large magnitude events, while for most of the inter-seismic cycle these thrusts remain locked, or slip aseismically. We further speculate that high magnitude earthquake's ruptures may only nucleate in the lithospheric mantle and then propagate into the crust across the serpentinized layers.