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  • AGU (American Geophysical Union)  (11)
  • Springer  (1)
  • 1
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    Microearthquake seismicity of the Mid-Atlantic Ridge at 5°S: A view of tectonic extension (2004)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Solid Earth, 109 . B06102.
    Details
    Publication Date: 2018-04-25
    Description: We report measurements made with an ocean bottom array which was operated for 10 days on the Mid-Atlantic Ridge just south of the 5°S transform fault/fracture zone. A total of 148 locatable earthquakes with magnitudes ∼0.5–2.8 were recorded; seismic activity appears to be concentrated within the western half of the median valley. The median valley seismic zone is bounded in along-axis direction by the transform fault to the north and the tip of the axial volcanic ridge to the south. A few scattered events occurred within the inside corner high, on the transform fault, and in the western sidewall close to the segment center. Earthquakes reach a maximum depth of 8 km below the median valley floor and appear to be predominantly in the mantle, although a few crustal earthquakes also occurred. The presence of earthquakes in the mantle indicates that it is not strongly serpentinized. We infer the median valley seismic activity to primarily arise from normal faulting.
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  • 2
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    Imaging and quantification of gas hydrate and free gas at the Storegga Slide offshore Norway (2005)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 32 . L04308.
    Details
    Publication Date: 2018-03-28
    Description: Wide–angle reflection seismic experiments were performed at the Storegga slide offshore Norway in 2002 with the goal to quantify the amount of gas hydrate and free gas in the sediment. Twenty‐two stations with Ocean Bottom Hydrophones (OBH) and Seismometers (OBS) were deployed for a 2D and a 3D experiment. Kirchhoff depth migration is used to transform the seismic wide–angle data into images of the sediment layers and to obtain P wave velocity–depth functions. The gas hydrate and free gas saturations are estimated from the elastic properties of the sediment on the basis of the Frenkel–Gassmann equations. There is 5–15% gas hydrate in the pore space of the sediment in the gas hydrate stability zone (GHSZ). The free gas saturation takes the value of 0.8% for a homogeneous distribution of gas in the pore water and 7% for the model of a patchy gas distribution.
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  • 3
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    Mid-depth internal wave energy off the Iberian Peninsula estimated from seismic reflection data (2008)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 113 . C12016.
    Details
    Publication Date: 2019-09-23
    Description: Energy levels of internal waves are estimated from seismic reflection data. Three legacy seismic sections from 1993 and 1997 obtained off the Iberian Peninsula have been analyzed for acoustic reflections within the water column. The reflections are aligned continuously for up to several kilometers over large parts of the sections and in the depth interval from 200 to 2000 m. Depth variations of these reflections are thought to be caused by the background internal wave field. From the variations we derive horizontal wave number spectra of normalized internal wave displacement. The general slope of the power density spectra is remarkably consistent for all sections and agrees well with model spectra for internal waves. Significant differences within the sections can be found when sufficiently large subsections are averaged. The spatial variation of the energy level indicates increasing internal wave activity with shallower water depths as well as near a subsurface eddy.
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  • 4
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    Lithospheric extension from rifting to continental breakup at magma-poor margins: rheology, serpentinisation and symmetry (2007)
    Springer
    In:  International Journal of Earth Sciences, 96 (6). pp. 1033-1046.
    Details
    Publication Date: 2017-05-18
    Description: The symmetry or asymmetry of the process of continental breakup has been much debated over the last 20 years, with various authors proposing asymmetric simple shear models, others advocating more symmetric, pure shear models and some combinations of the two. The unroofing of vast expanses of sub-continental mantle at non-volcanic margins has led some authors to argue in favour of simple shear models, but supporting evidence is lacking. Subsidence evidence from conjugate margin pairs is equivocal, and the detailed crustal and lithospheric structure of such pairs not generally well enough known to draw firm conclusions. In the Porcupine Basin, where the final stages of break-up are preserved, the development of structural asymmetry is demonstrable, and apparently related to late stage coupling of the crust to the mantle following the complete embrittlement of the crust. This agrees with theoretical modelling results, which predict that asymmetric models can develop only on a lithospheric scale when the crust and mantle are tightly coupled. However, whether such asymmetry is maintained during continued exhumation of the mantle is unclear.
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  • 5
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    Continental hyperextension, mantle exhumation and thin oceanic crust at the continent-ocean transition, West Iberia: new insights from wide-angle seismic (2016)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Solid Earth, 121 (5). pp. 3177-3199.
    Details
    Publication Date: 2019-04-08
    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.
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  • 6
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    Heterogeneous deformation in the Cascadia convergent margin and its relation to thermal gradient (Washington, NW USA) (2008)
    AGU (American Geophysical Union)
    In:  Tectonics, 27 (TC4005).
    Details
    Publication Date: 2017-05-12
    Description: We combine structural balancing with thermal and strength-envelope analysis of the Cascadia accretionary wedge to determine the influence thermal gradient has on the structure of the prism. BSR-derived heat flow in the Cascadia accretionary margin decreases from 90–110 mW/m2 at the deformation front to 45–70 mW/m2 in the upper slope. Extension of the thermal gradient to the top of the oceanic crust shows that the base of the prism reaches temperatures between 150–200°C and 250–300°C at the deformation front and the base of the upper slope, respectively. This high thermal gradient favors the development of a vertical strain gradient, which is accommodated by heterogeneous deformation of the accretionary prism. This process produces two overlying thrust wedges, a basal duplex and an overlying landward- or seaward-vergent imbricate stack. The thermal structure also influences the deformation distribution and structural style along the shortening direction. Initiation of plastic deformation at the base of the prism below the Cascadia upper slope affects the wedge geometry, changing its taper angle and favoring the development of a midcrustal duplex structure that propagates seaward as a dynamic backstop. Uplift related with this underplating process is accompanied with deep incision of submarine canyons, sliding and normal faulting in the upper slope. Heterogeneous deformation accommodated by the development of transfer faults separating landward-vergent from seaward-vergent domains is also observed along the margin. Landward-vergent areas accommodate 30–40% shortening at the front of the wedge, while in the narrower and thicker seaward-vergent segments shortening occurs mostly by underplating below the upper slope.
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  • 7
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    Microseismicity of the Mid-Atlantic Ridge at 7°S-8°15′S and at the Logatchev Massif oceanic core complex at 14°40′N-14°50′N (2013)
    AGU (American Geophysical Union) | Wiley
    In:  Geochemistry, Geophysics, Geosystems, 14 (9). pp. 3532-3554.
    Details
    Publication Date: 2018-02-28
    Description: Lithospheric formation at slow spreading rates is heterogeneous with multiple modalities, favoring symmetric spreading where magmatism dominates or core complex and inside corner high formation where tectonics dominate. We report microseismicity from three deployments of seismic networks at the Mid-Atlantic Ridge (MAR). Two networks surveyed the MAR near 7 degrees S in the vicinity of the Ascension transform fault. Three inside corner high settings were investigated. However, they remained seismically largely inactive and major seismic activity occurred along the center of the median valley. In contrast, at the Logatchev Massif core complex at 14 degrees 45N seismicity was sparse within the center of the median valley but concentrated along the eastern rift mountains just west of the serpentine hosted Logatchev hydrothermal vent field. To the north and south of the massif, however, seismic activity occurred along the ridge axis, emphasizing the asymmetry of seismicity at the Logatchev segment. Focal mechanisms indicated a large number of reverse faulting events occurring in the vicinity of the vent field at 3-5 km depth, which we interpret to reflect volume expansion accompanying serpentinization. At shallower depth of 2-4 km, some earthquakes in the vicinity of the vent field showed normal faulting behavior, suggesting that normal faults facilitates hydrothermal circulation feeding the vent field. Further, a second set of cross-cutting faults occurred, indicating that the surface location of the field is controlled by local fault systems.
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  • 8
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    Waveform inversion of the S reflector west of Spain: Fine structure of a detachment fault (2005)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 32 (22). L22304.
    Details
    Publication Date: 2018-03-28
    Description: The S reflection west of Iberia has been interpreted as a low‐angle detachment fault separating crustal fault blocks from partially serpentinized mantle. We apply full waveform inversion to investigate the fine structure of S. Our results confirm that S is largely a step increase in velocity (and density), probably from crustal rocks to partially serpentinized mantle peridotites. A ∼50 m thick low velocity zone above S might represent a main fault zone of highly serpentinized peridotites or hydrofractured and altered crustal rocks above the main fault zone. Both interpretations imply focused fluid flow along S, raising the possibility that low‐angle movement along S was aided by the development of local, transient high fluid pressures.
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  • 9
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    Movement along a low-angle normal fault: The S reflector west of Spain (2007)
    AGU (American Geophysical Union)
    In:  Geochemistry, Geophysics, Geosystems, 8 (Q06002).
    Details
    Publication Date: 2018-03-01
    Description: [1] The existence of normal faults that moved at low angles (less than 20°) has long been debated. One possible low-angle fault is the S detachment at the west Galicia (Spain) margin and thought to occur at the top of serpentinized mantle. It is unlikely that S was a large submarine slide as it was probably active over several million years without the development of any compressional features such as toe thrusts, it appears to have rooted beneath the conjugate Flemish Cap margin, and it is similar to structures elsewhere that also appear to be rooted detachments. Here we analyze depth images to identify synrift sediment packages above S and use the geometry of these synrift packages to constrain the angle at which S both formed and remained active. We find that S must have remained active at angles below 15°, too low to be explained simply by the low friction coefficient of partially serpentinized peridotites. Instead, we suggest that transient high fluid pressures must have developed within the serpentinites and propose a model in which anastomosing fault strands are alternately active and sealed, enabling moderately high fluid pressures to develop.
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  • 10
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    Along- and across-axis variations in crustal thickness and structure at the Mid-Atlantic Ridge at 5°S obtained from wide-angle seismic tomography: implications for ridge segmentation (2009)
    AGU (American Geophysical Union)
    Details
    Publication Date: 2022-03-07
    Description: Two end‐member styles of crustal accretion are observed at two adjacent spreading segments at the Mid‐Atlantic Ridge at 5°S: focused accretion to the segment center with rapid crustal thinning toward the transform in the northern segment and crustal thickening toward the transform at an oceanic core complex in the southern segment. Our results were obtained by tomographic inversion of wide‐angle seismic reflection and refraction data collected along three intersecting profiles. The segment north of the 5°S fracture zone is characterized by a well‐developed median valley with a pronounced seafloor bulge in the segment center. A discrete portion of anomalously low velocities (−0.4 to −0.5 km/s relative to average off‐axis structure) at depths of ∼2.5 km beneath this bulge is possibly related to the presence of elevated temperatures and perhaps small portions of partial melt. This suggests that this segment is currently in a magmatically active period, which is confirmed by the observation of fresh lava flows and ongoing high‐temperature hydrothermal activity at the seafloor. Close to the current spreading axis, the crust thins rapidly from 8.5 km beneath the segment center to less than 3 km beneath the transform fault which indicates that melt supply here is strongly focused to the segment center. The reduction in crustal thickness is almost exclusively accommodated by the thinning of velocity portions indicative of seismic layer 3. The transform fault is characterized by more uniform velocity gradients throughout the entire crustal section and very low upper mantle velocities of 7.2–7.3 km/s indicating that serpentinization could be as much as 25% at 3.5 km depth. In contrast, ∼4.1 Ma old crust of the northern segment shows only minor thinning from the segment center toward the segment end. Here, the transform is characterized by a normal seismic layer 2/3 transition suggesting robust melt supply to the segment end at those times. In the adjacent southern segment, the crust thickens from ∼2.5 km beneath the flank of an oceanic core complex to ∼5.0 km at the segment boundary. The observed changes in crustal thickness show a significant temporal and lateral variability in melt supply and suggest a more complex crustal emplacement process than predicted by models of focused melt supply to the segment centers.
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