Abstract
AbstractKnowledge of the seismological signature of serpentinites during deformation is fundamental for interpreting seismic observations in subduction zones, but this has yet to be experimentally constrained. We measured compressional and shear wave velocities during brittle deformation in polycrystalline antigorite, at room temperature and varying confining pressures up to 150 MPa. Ultrasonic velocity measurements, at varying directions to the compression axis, were combined with mechanical measurements of axial and volumetric strain, during direct loading and cyclic loading triaxial deformation tests. An additional deformation experiment was conducted on a specimen of Westerly granite for comparison. At all confining pressures, brittle deformation in antigorite is associated with a spectacular absence of stress‐induced anisotropy and with no noticeable dependence of wave velocities on axial compressive stress, prior to rock failure. The strength of antigorite samples is comparable to that of granite, but the mechanical behavior is elastic up to high stress ( of rock strength) and nondilatant. Microcracking is only observed in antigorite specimens taken to failure and not in those loaded even at 90–95% of their compressive strength. Microcrack damage is extremely localized near the fault and consists of shear microcracks that form exclusively along the cleavage plane of antigorite crystals. Our observations demonstrate that brittle deformation in antigorite occurs entirely by “mode II” shear microcracking. This is all the more remarkable than the preexisting microcrack population in antigorite, is comparable to that in granite. The mechanical behavior and seismic signature of antigorite brittle deformation thus appears to be unique within crystalline rocks.
Highlights
Serpentinites form by hydrothermal alteration of ultramafic rocks from the oceanic lithosphere and are commonly found in and around mid-ocean ridges, transform faults, obducted ophiolites, and in the subducting slabs and the overriding mantle wedge within subduction zones
The behavior of the Westerly granite (WG) sample is brittle at Pc = 100 MPa; the yield point in granite occurs at about 50% of rock strength, which is a much lower percentage than observed for antigorite
We have presented the results of a broad experimental study that combines new measurements of P and S wave velocities, at four different orientations to the compression axis, with measurements of axial and volumetric rock strain during brittle deformation of antigorite-rich (> 95%) serpentinite specimens
Summary
Serpentinites form by hydrothermal alteration of ultramafic rocks from the oceanic lithosphere and are commonly found in and around mid-ocean ridges, transform faults, obducted ophiolites, and in the subducting slabs and the overriding mantle wedge within subduction zones. As such, they play a major role in controlling lithospheric strength (e.g., Escartín et al, 1997; Hyndman & Peacock, 2003), rheological behavior (e.g., Amiguet et al, 2012; Auzende et al, 2015; Hilairet et al, 2007; Hirauchi & Katayama, 2013), frictional properties (e.g., Moore et al, 1997; Reinen et al, 1994), and mechanical anisotropy (e.g., Padrón-Navarta et al, 2012) in subduction zones. For an extended summary of the physical properties of antigorite, see Reynard (2013)
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