Description: The Alboran Basin in the westernmost Mediterranean hosts the orogenic boundary between the Iberian and African plates. Although numerous geophysical studies of crustal structure onshore Iberia have been carried out during the last decade, the crustal structure of the Alboran Basin has comparatively been poorly studied. We analyze crustal‐scale images of a grid of new and reprocessed multichannel seismic profiles showing the tectonic structure and variations in the reflective character of the crust of the basin. The nature of the distinct domains has been ground‐truthed using available basement samples from drilling and dredging. Our results reveal four different crustal types ‐domains‐ of the Alboran Basin: a) a thin continental crust underneath the West Alboran and Malaga basins, which transitions to b) a magmatic arc crust in the central part of the Alboran Sea and the East Alboran Basin, c) the North‐African continental crust containing the Pytheas and Habibas sub‐basins, and d) the oceanic crust in the transition towards the Algero‐Balearic Basin. The Alboran Basin crust is configured in a fore‐arc basin (West Alboran and Malaga basins), a magmatic arc (central and East Alboran), and a back‐arc system in the easternmost part of the East Alboran Basin and mainly Algero‐Balearic Basin. The North‐African continental crust is influenced by arc‐related magmatism along its edge, and was probably affected by strike‐slip tectonics during westward migration of the Miocene subduction system. The distribution of active tectonic structures in the current compressional setting generally corresponds to boundaries between domains, possibly representing inherited lithospheric‐scale weak structures.

Description: We use seismic oceanography to document and analyze oceanic thermohaline finestructure across the Tyrrhenian Sea. Multichannel seismic (MCS) reflection data were acquired during the MEDiterranean OCcidental survey in April-May 2010. We deployed along-track expendable bathythermograph probes simultaneous with MCS acquisition. At nearby locations we gathered conductivity-temperature-depth data. An autonomous glider survey added in-situ measurements of oceanic properties. The seismic reflectivity clearly delineates thermohaline finestructure in the upper 2,000 m of the water column, indicating the interfaces between Atlantic Water/Winter Intermediate Water, Levantine Intermediate Water, and Tyrrhenian Deep Water. We observe the Northern Tyrrhenian Anticyclone, a near-surface meso-scale eddy, plus laterally and vertically extensive thermohaline staircases. Using MCS we are able to fully image the anticyclone to a depth of 800 m and to confirm the horizontal continuity of the thermohaline staircases of more than 200 km. The staircases show the clearest step-like gradients in the center of the basin while they become more diffuse towards the periphery and bottom, where impedance gradients become too small to be detected by MCS. We quantify the internal wave field and find it to be weak in the region of the eddy and in the center of the staircases, while it is stronger near the coastlines. Our results indicate this is because of the influence of the boundary currents, which disrupt the formation of staircases by preventing diffusive convection. In the interior of the basin the staircases are clearer and the internal wave field weaker, suggesting that other mixing processes such as double-diffusion prevail. Synopsis We studied the internal temperature and salinity structure of the Tyrrhenian Sea (Mediterranean) using the multichannel seismic reflection method (the same used in the hydrocarbon industry). Low frequency sound (seismic) waves are produced at the surface with an explosive air source and recorded by a towed cable containing hydrophones (underwater microphones). The data are processed to reveal 'stratigraphy' that result from contrasts in density that are themselves caused by changes in temperature and salinity. In this way we can map ocean circulation in two-dimensions. We also deployed in situ oceanographic probes to measure temperature and salinity in order to corroborate and optimize the processing of the seismic data. We then quantified the internal gravity wave field by tracking the peaks of seismic trace wavelets. Our results show that the interior of the Tyrrhenian Sea is largely isolated from internal waves that are generated by a large cyclonic boundary current that contains waters from the Atlantic ocean and other parts of the Mediterranean. This isolation allows the thermohaline finestructure to form, where small scale vertical mixing processes are at play. Understanding these mixing processes will aid researchers study global ocean circulation and to add constraints that can help improve climate models.

Description: 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‐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.

Description: We present 2-D seismic velocity models and coincident multichannel seismic reflection images of the overriding plate and the inter-plate boundary of the Nicaragua convergent margin along two wide-angle seismic profiles parallel and normal to the trench acquired in the rupture area of the 1992 tsunami earthquake. The trench-perpendicular profile runs over a seamount subducting under the margin slope, at the location where seismological observations predict large coseismic slip. Along this profile, the igneous basement shows increasing velocity both with depth and away from the trench, reflecting a progressive decrease in upper-plate rock degree of fracturing. Upper mantle-like velocities are obtained at approximate to 10 km depth beneath the fore-arc Sandino basin, indicating a shallow mantle wedge. A mismatch of the inter-plate reflector in the velocity models and along coincident multichannel seismic profiles under the slope is best explained by approximate to 15% velocity anisotropy, probably caused by subvertical open fractures that may be related to fluid paths feeding known seafloor seepage sites. The presence of a shallow, partially serpentinized mantle wedge, and the fracture-related anisotropy are supported by gravity analysis of velocity-derived density models. The downdip limit of inter-plate seismicity occurs near the tip of the inferred mantle wedge, suggesting that seismicity could be controlled by the presence of serpentinite group minerals at the fault gouge. Near the trench, the inferred local increase of normal stress produced by the subducting seamount in the plate boundary may have made this fault segment unstable during earthquake rupture, which could explain its tsunamigenic character.

Description: Extension of the continental lithosphere leads to the formation of rift basins or rifted continental margins if breakup occurs. Seismic investigations have repeatedly shown that conjugate margins have asymmetric tectonic structures and different amount of extension and crustal thinning. Here we compare two coincident wide-angle and multichannel seismic profiles across the northern Tyrrhenian rift system sampling crust that underwent different stages of extension from north to south and from the flanks to the basin center. Tomographic inversion reveals that the crust has thinned homogeneously from ~24 km to ~17 km between the Corsica Margin and the Latium Margin implying a β factor of ~1.3–1.5. On the transect 80 km to the south, the crust thinned from ~24 km beneath Sardinia to a maximum of ~11 km in the eastern region near the Campania Margin (β factor of ~2.2). The increased crustal thinning is accompanied by a zone of reduced velocities in the upper crust that expands progressively toward the southeast. We interpret that the velocity reduction is related to rock fracturing caused by a higher degree of brittle faulting, as observed on multichannel seismic images. Locally, basalt flows are imaged intruding sediment in this zone, and heat flow values locally exceed 100 mW/m2. Velocities within the entire crust range 4.0–6.7 km/s, which are typical for continental rocks and indicate that significant rift-related magmatic underplating may not be present. The characteristics of the pre-tectonic, syn-tectonic and post-tectonic sedimentary units allow us to infer the spatial and temporal evolution of active rifting. In the western part of the southern transect, thick postrift sediments were deposited in half grabens that are bounded by large fault blocks. Fault spacing and block size diminish to the east as crustal thinning increases. Recent tectonic activity is expressed by faults cutting the seafloor in the east, near the mainland of Italy. The two transects show the evolution from the less extended rift in the north with a fairly symmetric conjugate structure to the asymmetric margins farther south. This structural evolution is consistent with W-E rift propagation and southward increasing extension rates.

Description: We investigate potential relations between variations in seafloor relief and age of the incoming plate and interplate seismicity. Westward from Osa Peninsula in Costa Rica, a major change in the character of the incoming Cocos Plate is displayed by abrupt lateral variations in seafloor depth and thermal structure. Here a Mw 6.4 thrust earthquake was followed by three aftershock clusters in June 2002. Initial relocations indicate that the main shock occurred fairly trenchward of most large earthquakes along the Middle America Trench off central Costa Rica. The earthquake sequence occurred while a temporary network of OBH and land stations ∼80 km to the northwest were deployed. By adding readings from permanent local stations, we obtain uncommon P wave coverage of a large subduction zone earthquake. We relocate this catalog using a nonlinear probabilistic approach within both, a 1-D and a 3-D P wave velocity models. The main shock occurred ∼25 km from the trench and probably along the plate interface at 5–10 km depth. We analyze teleseismic data to further constrain the rupture process of the main shock. The best depth estimates indicate that most of the seismic energy was radiated at shallow depth below the continental slope, supporting the nucleation of the Osa earthquake at ∼6 km depth. The location and depth coincide with the plate boundary imaged in prestack depth-migrated reflection lines shot near the nucleation area. Aftershocks propagated downdip to the area of a 1999 Mw 6.9 sequence and partially overlapped it. The results indicate that underthrusting of the young and buoyant Cocos Ridge has created conditions for interplate seismogenesis shallower and closer to the trench axis than elsewhere along the central Costa Rica margin.

Description: We present results from a seismic refraction and wide-angle experiment surveying an oceanic core complex on the Mid-Atlantic Ridge at 22°19′N. Oceanic core complexes are settings where petrological sampling found exposed lower crustal and upper mantle rocks, exhumed by asymmetric crustal accretion involving detachment faulting at magmatically starved ridge sections. Tomographic inversion of our seismic data yielded lateral variations of P wave velocity within the upper 3 to 4 km of the lithosphere across the median valley. A joint modeling procedure of seismic P wave travel times and marine gravity field data was used to constrain crustal thickness variations and the structure of the uppermost mantle. A gradual increase of seismic velocities from the median valley to the east is connected to aging of the oceanic crust, while a rapid change of seismic velocities at the western ridge flank indicates profound differences in lithology between conjugated ridge flanks, caused by un-roofing lower crust rocks. Under the core complex crust is approximately 40% thinner than in the median valley and under the conjugated eastern flank. Clear PmP reflections turning under the western ridge flank suggest the creation of a Moho boundary and hence continuous magmatic accretion during core complex formation.

Description: Active ridge propagation frequently occurs along spreading ridges and profoundly affects ridge crest segmentation over time. The mechanisms controlling ridge propagation, however, are poorly understood. At the slow spreading Mid-Atlantic Ridge at 21.5°N a seismic refraction and wide-angle reflection profile surveyed the crustal structure along a segment controlled by rapid ridge propagation. Tomographic traveltime inversion of seismic data suggests that the crustal structure along the ridge axis is controlled by melt supply; thus, crust is thickest, 8 km, at the domed segment center and decreases in thickness toward both segment ends. However, thicker crust is formed in the direction of ridge propagation, suggesting that melt is preferentially transferred toward the propagating ridge tip. Further, while seismic layer 2 remains constant along axis, seismic layer 3 shows profound changes in thickness, governing variations in total crustal thickness. This feature supports mantle upwelling at the segment center. Thus, fluid basaltic melt is redistributed easily laterally, while more viscose gabbroic melt tends to crystallize and accrete nearer to the locus of melt supply. The onset of propagation seems to have coincided with the formation of thicker crust, suggesting that propagation initiation might be due to changes in the melt supply. After a rapid initiation a continuous process of propagation was established. The propagation rate seems to be controlled by the amount of magma that reaches the segment ends. The strength of upwelling may govern the evolution of ridge segments and hence ultimately controls the propagation length.

Description: Water transported by slabs into the mantle at subduction zones plays key roles in tectonics, magmatism, fluid and volatiles fluxes, and most likely in the chemical evolution of the Earth's oceans and mantle. Yet, incorporation of water into oceanic plates before subduction is a poorly understood process. Several studies suggest that plates may acquire most water at subduction trenches because the ocean crust and uppermost mantle there are intensely faulted caused by bending and/or slab pull, and display anomalously low seismic velocities. The low velocities are interpreted to arise from a combination of fluid-filled fractures associated to normal faulting and mineral transformation by hydration. Mantle hydration by transformation of nominally dry peridotite to water-rich serpentinite could potentially create the largest fluid reservoir in slabs and is therefore the most relevant for the transport of water in the deep mantle. The depth of fracturing by normal-fault earthquakes is usually not well constrained, but could potentially create deep percolation paths for water that might hydrate up to tens of kilometers into the mantle, restrained only by serpentine stability. Yet, interpretation of deep intraplate mineral alteration remains speculative because active-source seismic experiments have sampled only the uppermost few kilometers of mantle, leaving the depth-extent of anomalous velocities and their relation to faulting unconstrained. Here we use a joint inversion of active-source seismic data, and both local and regional earthquakes to map the three dimensional distribution of anomalous velocities under a seismic network deployed at the trench seafloor. We found that anomalous velocities are restrained to the depth of normal-fault micro-earthquake activity recorded in the network, and are considerably shallower than either the rupture depth of teleseismic, normal-fault earthquakes, or the limit of serpentine stability. Extensional micro-earthquakes indicate that each fault in the region slips every 2–3 months which may facilitate regular water percolation. Deeper, teleseismic earthquakes are comparatively infrequent, and possibly do not cause significant fracturing that remains open long enough to promote alteration detectable with our seismic study. Our results show that the stability field of serpentine does not constrain the depth of potential mantle hydration.

Description: Quantification of fluid fluxes from cold seeps depends on accurate estimates of the spatial validity of flux measurements. These estimates are strongly influenced by the choice of geoacoustic mapping tools. Multibeam bathymetry, side-scan sonar, and Chirp subbottom profiler data of several mound-shaped cold seeps offshore central Costa Rica show great variety in morphology and structure although the features are only a few kilometers apart. Mound 11 (a 35 m high and 1000 m in diameter structure), situated in the SE of the study area, has an irregular morphology but a smooth surface on side-scan sonar data, while mound 12 (30 m high, 600 m across) is a cone of more regular outline but with a rough surface, and mound Grillo (5 m high, 500 m across) shows the same rough surface as mound 12 but without relief. Video observations and sediment cores indicate that the structures are formed by the precipitation of authigenic carbonates and indications for extensive mud extrusion are absent, except for one possible mudflow at mound 11. Different sonar frequencies result in variable estimates of the extent of these mounds with low frequencies suggesting much wider cold seeps, consequently overestimating fluid fluxes. The absence of mud volcanism compared to accretionary prisms where mud volcanism occurs is related to different tectonic styles: strong sediment overpressure and thrust faulting in typical accretionary prisms can generate mud volcanism, while subduction erosion and normal faulting (extension) of the overriding plate at the Costa Rican margin result in fluid venting driven by only slight fluid overpressures.