Transient switching between creep and fracture at and below the base of the megathrust seismogenic zone.
Abstract
The physical mechanisms responsible for episodic tremor and slow slip (ETS) remain enigmatic. Exhumed plate interface shear zones exposed at three localities on Kyushu, SW Japan, preserve deformation structures developed in oceanic metabasalt and metasediment at temperatures of 300 - 500 oC. They therefore allow direct investigation of the region at and below the base of the megathrust seismogenic zone, where deep ETS is commonly located.
Each shear zone exposure preserves well developed penetrative solution cleavages, indications of sliding along phyllosilicate laminae and dislocation creep fabrics. These macroscopically ductile fabrics are cut by quartz veins, which are themselves deformed by thrust-sense shear. Therefore, bulk deformation was accomplished by dissolution-precipitation creep, dislocation creep, and frictional sliding but transiently punctuated by fracture and quartz vein growth. Quartz grain size piezometry indicates that quartz veins deformed at shear stresses of 43 - 95 MPa at ~ 370 oC, and 10 - 30 MPa at ~ 500 oC. Quartz flow laws suggest shear strain rates of ~10-11 s-1 at these stresses. Quartz veins that precipitated and deformed in the temperature range 300 - 500 oC have d 18O values in the range 17.9 - 15.0 ‰. Using quartz-water fractionation factors determined by Matsuhisa (1979), the d 18O values of the vein-forming fluids range from 10.9 - 12.0 ‰ for quartz precipitated at 300- 370 oC and 12.7 - 13.8 ‰ for quartz precipitated at ~500 oC. Progressive mineral dehydration in the subducting oceanic crust and sediments is thought to be the source of the vein forming fluids. Our results indicate that fluids derived from mineral dehydration, and the anisotropic permeability of phyllosilicate-rich schists give rise to high fluid pressures that allow a local, transient switch from viscous to brittle deformation at and below the base of the seismogenic zone. In addition to locally and transiently lowering effective stresses, metamorphic fluids enable the formation of frictionally weak phyllosilicates and facilitate dissolution-precipitation creep, lowering bulk rock viscosity over a wide pressure-temperature range. These interpretations are consistent with interpretations from seismic data that indicate low elastic stresses and fluid presence in the ETS source region.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T51G0374T
- Keywords:
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- 8045 Role of fluids;
- STRUCTURAL GEOLOGY;
- 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICS;
- 8163 Rheology and friction of fault zones;
- TECTONOPHYSICS;
- 8170 Subduction zone processes;
- TECTONOPHYSICS