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  • 11
    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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  • 12
    Publication Date: 2012-07-06
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 13
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    Elsevier
    In:  Earth and Planetary Science Letters, 237 (3-4). pp. 561-576.
    Publication Date: 2016-12-21
    Description: Rifted margins are commonly characterised by an extension discrepancy: the amount of extension measurable from the observed faulting is far less than that required to explain the crustal thinning. It is shown here that polyphase faulting may provide a simple explanation for this paradox, but can be very hard to recognise on seismic sections. However at the west Galicia rifted margin (the Galicia Interior Basin between the Galicia Bank and the mainland, and the deep Galicia margin to the west of the Galicia Bank), a combination of high quality depth images, seismic velocity information and stratigraphic control through ODP drilling and submersible sampling, provide complementary evidence for polyphase faulting. Berriasian–Hauterivian rifting in the Galicia Interior Basin occurred along two sets of faults: the first unroofed deep crustal rocks, evidenced by high seismic velocities close to top basement; the second cut and dismembered these early faults. Further rifting (up until the Aptian) then focussed west of the Galicia Bank, where two further phases of faulting can be inferred from the diachronous nature of seismostratigraphic units tilted within fault blocks. Removal of the latest phase of faulting aligns discontinuous reflections within the fault blocks into an anastomosing network of earlier faults; restoration along these brings the crust back to its early Hauterivian state, similar to the present structure of the Interior Basin.
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  • 14
    Publication Date: 2012-02-23
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 15
    Publication Date: 2012-02-23
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  • 16
    Publication Date: 2012-02-23
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 17
    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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  • 18
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    Unknown
    In:  [Other] In: AGU Fall Meeting, 11.12, San Francisco, California, USA .
    Publication Date: 2012-02-23
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  • 19
    Publication Date: 2018-07-10
    Description: We present results of a seismic refraction experiment which determines the crustal and upper-mantle structure of an oceanic core complex (OCC) and its conjugate side located south of the 5°S ridge–transform intersection at the Mid-Atlantic Ridge. The core complex with a corrugated surface has been split by a change in location of active seafloor spreading, resulting in two massifs on either side of the current spreading axis. We applied a joint tomographic inversion of wide-angle reflected and refracted phases for five intersecting seismic profiles. The obtained velocity models are used to constrain the magmatic evolution of the core complex from the analysis of seismic layer 3 and crustal thickness. An abrupt increase of crustal velocities at shallow depth coincides with the onset of the seafloor corrugations at the exposed footwall. The observed velocity structure is consistent with the presence of gabbros directly beneath the corrugated fault surface. The thickness of the high-velocity body is constrained by PmP reflections to vary along and across axis between 〈3 and 5 km. The thickest crust is associated with the central phase of detachment faulting at the higher-elevated northern portion of the massif. Beneath the breakaway of the OCC the crust is 2.5 km thick and reveals significantly lower velocities. This implies that the fault initially exhumed low-velocity material overlying the gabbro plutons. In contrast, crust formed at the conjugate side during OCC formation is characterized by an up to 2-km-thick seismic layer 2 overlying a 1.7-km-thick seismic layer 3. Obtained upper-mantle velocities range from 7.3 to 7.9 km s−1 and seem to increase with distance from the median valley. However, velocities of 7.3–7.5 km s−1 beneath the older portions of the OCC may derive from deep fluid circulation and related hydrothermal alteration, which may likely be facilitated by the subsequent rifting. Our velocity models reveal a strongly asymmetric velocity structure across the ridge axis, associated with the accretion of gabbros into the footwall of the detachment fault and upper-crustal portions concentrated at the conjugate side. Our results do not support a substantial increase in the axial ridge's melt supply related to the final phase of detachment faulting. Hence, the footwall rifting at 5°S may be a generic mechanism of detachment termination under very low melt conditions, as predicted by recent numerical models of Tucholke et al.
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  • 20
    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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