Insights into exhumation and mantle hydration processes at the Deep Galicia margin from a 3D high-resolution seismic velocity model

High-resolution velocity models developed using full-waveform inversion (FWI) can image fine details of the nature and structure of the subsurface. Using a 3D FWI velocity model of hyper-thinned crust at the Deep Galicia Margin (DGM) west of Iberia, we constrain the nature of the crust at this margin by comparing its velocity structure with those in other similar tectonic settings. Velocities representative of both the upper and lower continental crust are present, but there is no clear evidence for distinct upper and lower crustal layers within the hyper-thinned crust. Our velocity model supports exhumation of the lower crust under the footwalls of fault blocks to accommodate the extension. We used our model to generate a serpentinization map for the uppermost mantle at the DGM, at a depth of 100 ms (~340m) below the S-reflector, a low-angle detachment that marks the base of the crust at this margin. We find a good alignment between serpentinized areas and the overlying major block bounding faults on our map, suggesting that those faults played an important role in transporting water to the upper mantle. Further, we observe a weak correlation between fault heaves and serpentinization beneath the hanging-wall blocks, indicating that serpentinization was controlled by a complex faulting during rifting. A good match between topographic highs of the S and local highly serpentinized areas of the mantle suggests that the morphology of the S was affected by the volume-increasing process of serpentinization and deformation of the overlying crust

Insights into exhumation and mantle hydration processes at the Deep Galicia margin from a 3D high-resolution seismic velocity model

High-resolution velocity models developed using full waveform inversion (FWI) are capable of imaging fine details of the nature and structure of the subsurface. Using a 3D FWI velocity model of hyper-thinned crust at the Deep Galicia Margin (DGM), we constrain the nature of the crust at this margin by comparing its velocity structure with those in other similar tectonic settings. Velocities representative of both the upper and lower continental crust are present in this hyper-thinned crust. However, unlike in many other rifted margin settings, there is no clear evidence for distinct upper and lower crustal layers within the hyperextended crust. Our velocity model also shows evidence for exhumation of the lower crust under the footwalls of fault blocks to accommodate the extension. We used our model to generate a serpentinization map for the uppermost mantle at the DGM, at a depth of 100 ms (~340m) below the S-reflector, a low-angle detachment that marks the base of the crust at this margin. Based on this map, we propose that serpentinization began during rifting and continued into a post-rift phase until the faults were sealed. We find a weak correlation between the fault heaves and the degree of serpentinization beneath the hanging-wall blocks, indicating that serpentinization was controlled by a complex crosscutting and unrecognized faulting during and after rifting. A good match between topographic highs of S and local highly serpentinized areas of mantle suggests that the serpentinization process resulted in variable uplift of the S-surface

The Western Tyrrhenian Sea revisited: New evidence for a rifted basin during the Messinian Salinity Crisis

International audienceIn the last fifty years, the Messinian Salinity Crisis (MSC) has been widely investigated in the Mediterranean Sea, but a major basin remains fewly explored in terms of MSC thematic: the Western Tyrrhenian Basin. The rifting of this back-arc basin is considered to occur between the Middle-Miocene and the Early-Pliocene, thus including the MSC, giving a unique opportunity to study the crisis in a context of active geodynamics. However the MSC seismic markers in the Western part of the Tyrrhenian Sea have only been investigated in the early eighties and the MSC event in the Western Tyrrhenian Basin remains poorly studied and unclear.In this study, we revisit the MSC in the Western Tyrrhenian Basin, i.e. along the Eastern Sardinian margin. We present results from the interpretation of a 2400 km long HR seismic-reflection dataset, acquired along the margin during the “METYSS” research cruises in 2009 and 2011. The maps of the MSC seismic markers reveal that the Eastern Sardinian margin was already dissected in structurals highs and lows during the MSC. We also demonstrate that the MSC markers constitute powerfull time-markers to refine the age of the rifting, which ended earlier than expected in the East-Sardinian Basin and the Cornaglia Terrace. These results allow us to discuss the palaeo water-depth of the Western Tyrrhenian Basin during the MSC, as well as implications for possibles scenarios of the Messinian Salinity Crisis across the Eastern Sardinian margi

3D imaging of the rifting and breakup west of Spain and the nature of the S detachment

American Geophysical Union Fall Meeting, 12-16 December 2016, San FranciscoThe west Galicia margin (NW Spain) is a magma-poor margin with limited sedimentary cover, providing ideal conditions to study the processes of continental extension and break-up through seismic imaging. In 2013, we collected a 65km x 20km 3D multi-channel seismic dataset extending from the edge of the Galicia Bank over the feather edge of the continental crust to beyond the Peridotite Ridge to the west. The volume has been processed through to 3D prestack time migration and 3D depth conversion and provides 3D images of the hyper-extended continental crust, consisting of well-defined but internally intensely deformed rotated faults blocks with associated syn-kinematic sedimentary wedges. The rotated fault blocks contain possible low-angle normal faults that in places appear to define and elsewhere to offset top basement; we interpret them as early faults that predate the formation of the fault blocks. The early faults are in places cut and offset by the later block-bounding faults that appear to detach downwards onto a bright reflection, the S reflector, a detachment fault and locally the crust-mantle boundary. S is corrugated on both depth and time structure maps, with the corrugations sweeping in an arc from E-W proximally to ESE-WNW towards the ocean. We interpret the corrugations as forming during slip on S and representing the displacement (and hence extension) direction on the detachment leading to breakup. Early synrift sediment within the most recent faults blocks are intensely deformed, representing the internal deformation of the fault blocks. However, in places simple synrift wedges can be seen to thicken towards the block-bounding faults. The angular relationship of the top of these wedges to the block-bounding faults demonstrates that they were deposited during slip on the faults and low-angle slip on the underlying S detachment. Although beneath the continental crust S appears continuous and shows only minor distortion in the vicinity of the block-bounding faults, just east of the Peridotite Ridge, S appears to have been offset by landward-dipping faults associated with the development of the Ridge. We conclude that rifting to breakup was a complex, time variant, 3D process involving slip at low-angles, multiple generations of faults and the unroofing of mantle along later landward-dipping structuresPeer Reviewe

Resolving the fine-scale velocity structure of continental hyperextension at the Deep Galicia Margin using full-waveform inversion

Continental hyperextension during magma-poor rifting at the Deep Galicia Margin is characterised by a complex pattern of faulting, thin continental fault blocks, and the serpentinisation, with local exhumation, of mantle peridotites along the S-reflector, interpreted as a detachment surface. In order to understand fully the evolution of these features, it is important to image seismically the structure and to model the velocity structure to the greatest resolution possible. Travel-time tomography models have revealed the long-wavelength velocity structure of this hyperextended domain, but are often insufficient to match accurately the short-wavelength structure observed in reflection seismic imaging. Here we demonstrate the application of two-dimensional (2D) time-domain acoustic full-waveform inversion to deep water seismic data collected at the Deep Galicia Margin, in order to attain a high resolution velocity model of continental hyperextension. We have used several quality assurance procedures to assess the velocity model, including comparison of the observed and modelled waveforms, checkerboard tests, testing of parameter and inversion strategy, and comparison with the migrated reflection image. Our final model exhibits an increase in the resolution of subsurface velocities, with particular improvement observed in the westernmost continental fault blocks, with a clear rotation of the velocity field to match steeply dipping reflectors. Across the S-reflector there is a sharpening in the velocity contrast, with lower velocities beneath S indicative of preferential mantle serpentinisation. This study supports the hypothesis that normal faulting acts to hydrate the upper mantle peridotite, observed as a systematic decrease in seismic velocities, consistent with increased serpentinisation. Our results confirm the feasibility of applying the full-waveform inversion method to sparse, deep water crustal datasets

Three-dimensional P-wave velocity structure of the Deep Galicia rifted margin

European Geosciences Union (EGU) General Assembly, 7-12 April 2019, Vienna, Austria.-- 1 pageIn 2013, we carried out the Galicia-3D controlled-source reflection-refraction seismic experiment at the Deep Galicia rifted margin in the northeast Atlantic Ocean, west of Spain. We acquired more than 3200 km of seismic profiles within a three-dimensional (3D) box measuring 64 by 20 km (1280 km2). The main features within this box are: the peridotite ridge (PR), composed of serpentinized peridotite; a series of fault bounded, rotated basement blocks; and the S reflector, which has been interpreted across most of the box as a low angle detachment fault forming the crust-mantle boundary. Shots from two 3300 cu.in airgun arrays fired alternately in a flip-flop configuration, with a shot spacing of 37.5 m, were recorded by four six-kilometer streamers of R/V Marcus Langseth, as well as by 72 Ocean Bottom Seismometers (OBS) of the Galicia-3D network. We present tomographic results obtained using three well known controlled-source seismic tomography codes: the three-dimensional (3D) first-arrival time seismic tomography code FAST, and the 3D joint refraction-reflection travel-time tomography codes TOMO3D and JIVE3D. Results obtained with three different codes were compared with the seismic reflection images of the area, and this comparison confirms that the main features of the area are accurately represented in all three results. The 3.5, 5 and 6.5 km/s contours match well the top of the acoustic and crystalline basements and the S-reflector, respectively.Largest differences between results ofdifferent codes occur, as expected, in poorly resolved areas. n addition, due to differences in parameterisations and inversion schemes, the same structure is expressed in three different ways in the results. Below the S-reflector, P-wave velocities indicate a degree of serpentinisation of 45 to 30 %. The distribution of the serpentinisation allows us to estimate the amount of water reaching the mantle, and suggests that the water reaches the mantle when faults are active. Our models allow us to explore the relationship between extension inferred from fault heaves and crustal thickness variations in 3

Rifted Margins: State of the Art and Future Challenges

8 pages, 1 figureImprovements in seismic imaging, computing capabilities, and analytical methods, as well as a number of industry deep-water wells sampling distal offshore settings, have underpinned new concepts for rifted margin evolution developed in the last two decades; these mark significant progress in our understanding of extensional systems. For example, the tectonic, sedimentary, and magmatic processes linked to the formation of rifted margins have been overhauled, giving rise to more quantitative approaches and new concepts. However, these processes cannot be understood in isolation, requiring consideration of the continuum in which inheritance and physical processes are integrated within a plate tectonic framework. The major progress and fundamental developments of past research in rifted margins have been made hand-in-hand with other domains of Earth Sciences and have fundamental implications for the understanding of key geological systems such as active rifts, the ocean lithosphere, subduction zones, and collisional orogens. The “IMAGinING RIFTING” workshop, organized in Pontresina-Switzerland in September 2017, gathered researchers from all disciplines working on rifts and rifted margins, and included participants from academia and industry. This contribution summarizes the workshop discussions, in addition to outlining our state-of-the-art knowledge of rifted margins. We highlight future challenges in unraveling the processes and conditions under which these extensional systems form and, ultimately, how tectonic plates rupture and new oceans are born. Our aims here are to provide a framework for future research endeavors and to promote collaboration not only within the rift and rifted margins communities, but across other Earth Science discipline