ALBERT

Description: The dataset contains the raw .segy files of the ocean bottom seismometers/hydrophones (OBS/H) that recorded wide-angle seismic data along 6 profiles in the Porcupine Basin. The active-source seismic survey was conducted by GEOMAR in 2004. The cruise report, navigation files for each profile, and geographical coordinates of each OBS/H are also included in this dataset.

Description: The Porcupine Basin is a Mesozoic failed rift located in the North Atlantic margin (SW Ireland). Here, we present two sets of tomographic images obtained with travel-time tomography of two different active-source seismic data sets: ocean bottom seismic (OBS) data and long-streamer data. The study provides new insights into geological processes that occurred at different scales and geological stages during the formation of the Porcupine Basin. OBS-derived images show the Vp structure of the uppermost lithosphere and the geometry of the Moho across and along the basin axis, providing insights into formation processes that occurred during lithospheric extension in the Mesozoic. In particular, these tomographic results together with neighboring seismic reflection lines provide crustal stretching (βc) estimates of ∼2.5 in the north at 52.5N and 〉 10 in the south at 51.7N. These values suggest that no crustal embrittlement occurred in the northernmost region, and that rifting has potentially reached crustal breakup in the southern part of the study area. Tomographic images reveal that mantle velocities decrease across the basin axis from east to west. These variations occur in a region where βc is within the range at which crustal embrittlement and serpentinisation are possible (βc 3-4). Across the basin axis, the lowest seismic velocity in the mantle spatially coincides with the maximum amount of crustal faulting, indicating fault-controlled mantle hydration. Mantle velocities also suggest that the degree of serpentinisation, together with the amount of crustal faulting, increases southwards along the basin axis. Seismic reflection lines show a major detachment fault surface that grows southwards along the basin axis and is only visible where the inferred degree of serpentinisation is 〉 15 %. This is consistent with laboratory measurements that show that at this degree of serpentinisation, mantle rocks are sufficiently weak to allow low-angle normal faulting. In contrast, the long-streamer tomographic image shows the Vp structure of the post-rift section in much more detail than OBS-derived images providing insights into basin-scale processes that occurred after lithospheric extension during the Cenozoic. The tomographic image reveals faster vertical velocity gradient in the center of the basin than in the flanks. This variation together with a relatively thick sediment accumulation in the center of the basin suggests higher overburden pressure and compaction compared to the margins. This suggests fluid flow driven by differential compaction towards the margins of the basin. The model also reveals two prominent vertical velocity anomalies located at the western margin of the basin, coinciding with the location of a reactivated basin-bounding fault. Comparing the corresponding time-migrated seismic section with the tomographic model, we observe that the hanging wall of the basin-bounding fault is not significantly affected by major normal faulting and yet is associated with comparatively lower seismic velocities. This result together with exploration well data suggests high effective porosities within the hanging wall suggesting potential overpressured areas. Our results suggest that the western basin-bounding fault is acting as a barrier for fluid migration causing overpressured areas over the western flank.

Description: Highlights • Magmatism, detachment faulting and changing symmetry of crustal accretion at a ridge segment end. • PmPs from an OCC argue for constant magma production during detachment faulting. • Refraction seismic modelling and PmP events reveal very thin (4 km) oceanic crust at a segment end. A wide-angle seismic section across the Mid-Atlantic Ridge just south of the Ascension transform system reveals laterally varying crustal thickness, and to the east a strongly distorted Moho that appears to result from slip along a large-offset normal fault, termed an oceanic detachment fault. Gravity modelling supports the inferred crustal structure. We investigate the interplay between magmatism, detachment faulting and the changing asymmetry of crustal accretion, and consider several possible scenarios. The one that appears most likely is remarkably simple: an episode of detachment faulting which accommodates all plate divergence and results in the westward migration of the ridge axis, is interspersed with dominantly magmatic and moderately asymmetric (most on the western side) spreading which moves the spreading axis back towards the east. Following the runaway weakening of a normal fault and its development into an oceanic detachment fault, magma both intrudes the footwall to the fault, producing a layer of gabbro (subsequently partially exhumed).

Description: Hyperextension of continental crust at the Deep Galicia rifted margin in the North Atlantic has been accommodated by the rotation of continental fault blocks, which are underlain by the S reflector, an interpreted detachment fault, along which exhumed and serpentinized mantle peridotite is observed. West of these features, the enigmatic Peridotite Ridge has been inferred to delimit the western extent of the continent-ocean transition. An outstanding question at this margin is where oceanic crust begins, with little existing data to constrain this boundary and a lack of clear seafloor spreading magnetic anomalies. Here we present results from a 160 km long wide-angle seismic profile (Western Extension 1). Travel time tomography models of the crustal compressional velocity structure reveal highly thinned and rotated crustal blocks separated from the underlying mantle by the S reflector. The S reflector correlates with the 6.0–7.0 km s−1 velocity contours, corresponding to peridotite serpentinization of 60–30%, respectively. West of the Peridotite Ridge, shallow and sparse Moho reflections indicate the earliest formation of an anomalously thin oceanic crustal layer, which increases in thickness from ~0.5 km at ~20 km west of the Peridotite Ridge to ~1.5 km, 35 km further west. P wave velocities increase smoothly and rapidly below top basement, to a depth of 2.8–3.5 km, with an average velocity gradient of 1.0 s−1. Below this, velocities slowly increase toward typical mantle velocities. Such a downward increase into mantle velocities is interpreted as decreasing serpentinization of mantle rock with depth.

Description: Water and carbon are transferred from the ocean to the mantle in a process that alters mantle peridotite to create serpentinite and supports diverse ecosystems1. Serpentinized mantle rocks are found beneath the sea floor at slow- to ultraslow-spreading mid-ocean ridges1 and are thought to be present at about half the world’s rifted margins2, 3. Serpentinite is also inferred to exist in the downgoing plate at subduction zones4, where it may trigger arc magmatism or hydrate the deep Earth. Water is thought to reach the mantle via active faults3, 4. Here we show that serpentinization at the rifted continental margin offshore from western Spain was probably initiated when the whole crust cooled to become brittle and deformation was focused along large normal faults. We use seismic tomography to image the three-dimensional distribution of serpentinization in the mantle and find that the local volume of serpentinite beneath thinned, brittle crust is related to the amount of displacement along each fault. This implies that sea water reaches the mantle only when the faults are active. We estimate the fluid flux along the faults and find it is comparable to that inferred for mid-ocean ridge hydrothermal systems. We conclude that brittle processes in the crust may ultimately control the global flux of sea water into the Earth.