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Models of the Earth’s crust from controlled-source seismology — Where we stand and where we go?
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Vladivostok: Dalnauka, P. Kravchinsky V. Evolution of the Mongol-Okhotsk ocean as constrained by new palaeomagnetic data from the Mongol-Okhotsk suture zone, Siberis. Licensed under a Creative Commons Attribution 4. About The Authors R. Article Tools Print this article. Indexing metadata. How to cite item. Finding References. Review policy. Email this article Login required. Email the author Login required. Post a Comment Login required. User Username Password Remember me. The dashed green line is the basement reflection. The continuous green line is the reflection on the crustal damage zone at the base of the crust, the ship being See text for computations of out-of-plane reflections projected along Profile WG3 in-plane reflections.
In b , the light blue and light red areas represent travel time ranges corresponding to different mantle velocity distributions See text for detailed explanations. The top and the base of the oceanic crust are well imaged along Profile WG3 Fig. Interpretation of the WG3 seismic profile with identification of mantle features thin light grey lines interpreted as faults by Qin and Singh 4. Out-of-plane mantle reflections are computed along Profile WG3 from They are coincident with five mantle features identified by Qin and Singh 4.
Out-of-plane crustal reflections are computed along Profile WG3 from 9. They are coincident with at least six mantle features identified by Qin and Singh 4.
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Note the overlap between out-of-plane crustal and mantle reflections between However, these mantle features interpreted as faults Fig. Why can no systematic offsets be observed at the intersections of mantle features of opposite dips? Why do mantle features reach up close to the Moho interface with almost no extension into the oceanic crust and why are they disconnected from oblique crustal features?
Qin and Singh 4 proposed explanations for some of the above observations discussed in detail in the following sections but not for all of them. We suggest that most of these mantle features are not faults but out-of-plane reflections which occur perpendicularly to sub-vertical FZs acting as vertical mirrors for seismic rays, a case study not explored by Carton et al.
The serpentinization process Fischer-Tropsch reaction is a slow chemical reaction during which seawater transforms mantle peridotite into serpentine by hydration of olivine, ortho- and clinopyroxene. Methane and talc, which reduce the strength along faults 17 , are also formed during this reaction. Numerous examples of faults, carrying seawater across the brittle crust down to the upper mantle, exist in different tectonic environments.
For example, during rifting seawater penetrates along active normal faults and serpentinizes the underlying mantle peridotite if the crust is entirely brittle 18 , In the Alps, peridotites are systematically serpentinized along mantle portions of faults Observed low mantle velocities result from a combination of seawater-filled fractures, which serpentinize mantle peridotite 7.
The water access in mantle peridotites is controlled by the porosity and permeability of the medium 7. The serpentinization process increases the volume of rocks and therefore reduces its porosity considerably. However, as the volume expansion also increases the permeability, the water flow might use these cracks to further develop the serpentinization process In consequence it can be assumed that the mantle part of sub-vertical re-activated FZs consists of serpentinized peridotite with lower seismic velocities than the surrounding pristine peridotite.
As the damage zone in the mantle is sub-vertical and a few hundred meters wide, MCS profiles will not record such sub-vertical features. However, Korenaga 22 suggests that serpentinization might be less than assumed by all the authors cited above. These small crack-like porosities produced by thermal cracking and enhanced by bending-related faulting, do not necessarily lead to the substantial mantle hydration because of the high confining pressure.
Therefore, mantle serpentinization would still occur but to a lesser extent Within the oceanic crust, the damage zone consists of crushed rocks of brittle oceanic crust giving rise to mylonites, i. As for the mantle damage zones, such a narrow sub-vertical zone cannot be imaged by MCS data.
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On both sides of the damage zone, the nature of brittle oceanic crust is identical and therefore a horizontally travelling seismic ray crossing perpendicularly to the FZ will be reflected without changing polarity. Thus, only sedimentary deformation and associated sediment-buried basement fault scarps can help to approximately locate the position of the crustal and mantle portions of FZs.
The computation of crustal and upper mantle out-of-plane reflections occurring perpendicularly to FZs is performed by using the RayInvr software 23 and then projected along the plane of MCS Profile WG3 in-plane reflections. In a first approximation, the geometry of oceanic crust and sedimentary layers only slightly varies along Profile WG3 Fig.
The 1D velocity model is used to compute travel-times along DE Fig. We assume that the ABC ray path is reflecting perpendicularly to the low velocity serpentinized damage zone acting as a mirror in the upper mantle Fig. The trend of these out-of-plane reflections on FZs can be computed either in time or in space domains. As Profile WG3 and its interpretation, are displayed in time sections 4 Fig. Out-of-plane reflections perpendicular to the FZ direction and the resulting projections on Profile WG3 are computed using the RayInvr software 23 , which is developed to process wide-angle ocean bottom seismometers data, and adapted here to the geometry of MCS data acquisition See Methods section.
Out-of-plane mantle reflections are recorded from Out-of-plane crustal reflections occur from 9.
Thus, the reflections identified in Fig. A significant overlap exists in the upper mantle between crustal and mantle reflection phases from In this depth range, it is impossible to distinguish between out-of-plane crustal and mantle reflections, except if a polarity change is observed for mantle reflections.
The location of re-activated FZs can be identified in gravity data Fig.