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    Solid earth sciences

    Rheology and stress in subduction zones around the aseismic/seismic transition

    Platt J, Xia H, Schmidt W L

    Dynamically recrystallized grain-size, Dislocation creep, Pressure solution, Tremor, Slow slip

    Relationship between the seismic, transition, and creep zones in a subduction zone, and the processes associated with each. The transition zone is the source of tectonic tremor and slow slip; and is also the likely locus of underplating, where the metamorphosed edimentary and volcanic cover of the subducting plate is progressively accreted to the upper plate. This produces significant off-fault deformation, and the active slip surface migrates downwards. In the creep zone no discrete displacement surface exists; ductile deformation is distributed in a subduction channel between the two plates. Inset shows folds with axial-planar cleavage of the type shown in Figure 11; we suggest that the hierarchy of structures at different scales can explain LFEs, tremor bursts, and slow slip events.

    Subduction channels are commonly occupied by deformed and metamorphosed basaltic rocks, together with clastic and pelagic sediments, which form a zone up to several kilometers thick to depths of at least 40 km. At temperatures above ~ 350 °C (corresponding to depths of > 25–35 km), the subduction zone undergoes a transition to aseismic behavior, and much of the relative motion is accommodated by ductile deformation in the subduction channel. Microstructures in metagreywacke suggest deformation occurs mainly by solution-redeposition creep in quartz. Interlayered metachert shows evidence for dislocation creep at relatively low stresses (8–13 MPa shear stress). Metachert is likely to be somewhat stronger than metagreywacke, so this value may be an upper limit for the shear stress in the channel as a whole. Metabasaltic rocks deform mainly by transformation-assisted diffusional creep during low-temperature metamorphism and, when dry, are somewhat stronger than metachert. Quartz flow laws for dislocation and solution-redeposition creep suggest strain rates of ~ 10−12 s−1 at 500 °C and 10 MPa shear stress: this is sufficient to accommodate a 100 mm/yr. convergence rate within a 1 km wide ductile shear zone.

    The up-dip transition into the seismic zone occurs through a region where deformation is still distributed over a thickness of several kilometers, but occurs by a combination of microfolding, dilational microcracking, and solution-redeposition creep. This process requires a high fluid flux, released by dehydration reactions down-dip, and produces a highly differentiated deformational fabric with alternating millimeter-scale quartz and phyllosilicate-rich bands, and very abundant quartz veins. Bursts of dilational microcracking in zones 100–200 m thick may cause cyclic fluctuations in fluid pressure and may be associated with episodic tremor and slow slip events. Shear stress estimates from dislocation creep microstructures in dynamically recrystallized metachert are ~ 10 MPa.