Unstable fault slip induced by lawsonite dehydraon in blueschist: Keishi Okazaki ([email protected]) and Greg Hirth ([email protected]) Implicaon for the seismicity in the subduc8ng oceanic crusts Department of Earth, Environmental & Planetary Sciences, Brown University Intraslab seismicity in subduc1on zones Temperature ramping experiments Stress-displacement curves Hot subduc8on zone (Matsubara et al., 2009) Lawsonite: unloading slope is similar to the effecDve 5 Hot subduc8on zones: 6 x 10 6 x 102 unloading sDffness of the apparatus (~280kN/mm, Slow earthquakes? -8.3 -8.4 Slab surface (e.g., Cascadia, SW Japan) 8.8 GPa/mm for the sample dimension used in this Oceanic crust Single seismic zone study) over the enDre range of T/ε tested. Moho ⇒few crustal earthquakes → unstable fault slip ⇒a number of slow EQs Cold subducon zones: An8gorite: the unloading slope is controlled by the . temperature ramping rate and strain rate. (e.g., NE Japan) T/ε = 6 x 103 ˚C 6 x 103 -8.1 Double seismic zone dσd/dx = -4.9 GPa/mm → reacon-controlled stable sliding ⇒a number of crustal earthquakes These observaons indicate dehydraon induced unstable fault slip in lawsonite (i.e., sDck-slip) but Cold subducon zone (Tsuji et al., 2008) ⇒no slow earthquake Abers et al., 2013 stable slow slip in anDgorite, consistent with the differences in AE. Cold subducon zones: Hot subduc8on zones: . Lawsonite blueschist, Greenschist & epidote amphibolite, Blueschist-eclogite transiDon Time-independent thermal-mechanical factor T/ε (Hacker, 2008) many crustal EQs few crustal EQs Lawsonite-out reacDon in MORB system Unstable slip If we assume: (Poli & Schmidt, 1995) (C&H) Thermal gradient = 10 ˚C/km, subducing velocity = 10 cm/year, subducDng angle of 45˚, –13 –15 range of strain rate from 10. to 10 1/s, T/ε in natural subducDon zone systems will range from ~102 to 104 ˚C. →The values of T/ε imposed in our experiments (6 2 5 Hacker et al., 2003 × 10 to 6 × 10 ˚C) cover the range expected for modified from Abers et al., 2013 subducDon zone systems. Can dehydra1on of lawsonite directly trigger seismic SDffness of the natural fault system: fault slip in the subduc1ng oceanic crust? Recovered samples of lawsonite: Kf = G/2/(1–ν)/L (Scholz, 2002) Strain is highly localised and the fault surface shows G: the shear modulus (~30GPa), Sample and experiment mirror-like slickensides with very liUle gouge. ν: the Poisson’s rao (~0.25), Sample 1: lawsonite, a common mineral in “cold” subduc8ng crusts → similar to observaons from high pressure faulDng L: the length of the slipping region. • lawsonite vein within a lawsonite-blueschist block from the associated with the olivine-spinel transiDon (Schubnel et al., If we assume L≈ 10 m (i.e., for a M1 earthquake, Franciscan Complex in CA, USA 2014). the sDffness of the natural systems Kf ~2 MPa/mm. • almost pure lawsonite (~98%), with minor staurolite and XRD analysis: glaucophane Unstable slip resulDng from dehydraon of A small amount of anorthite (<5 vol%) formed as a lawsonite will occur even more easily in natural Sample 2: angorite serpenne, stable fric8onal behaviour has result of dehydraon during experiments. been observed during dehydraon (Chernak & Hirth, 2011; Proctor & Hirth, subducDon zones than in the laboratory. 2015) EPMA chemical analysis: • from Nomo metamorphic rocks in Nagasaki, Japan Slightly higher concentraon of Si along the shear Summary: • predominantly anDgorite (~98%) with minor diopside, spinel and zone, subparallel to the slickenside surface; anorthite 1. Unstable fault slip (i.e., sDck-slip) occurred during dehydraon reacDons in magneDte Blue lines: phase diagram of the CASH syetem (Newton & Kennedy, 1963) 1: Stability field of anDgorite at high fH2OUlmer&Trommsdorff, 1995 has a somewhat higher Si content (~44 wt%) than the lawsonite gouge layer and AE signals were conDnuously observed under 2: Stability field of anDgorite at low fH2O (Perrillat et al., 2003) lawsonite (38.3 wt%) and zoisite (~39.7 wt%). the differenDal stress. Experimental condions: Confining pressure (Pc): 1 GPa 2. The unloading slope during the unstable slip follows the sDffness of the apparatus at all experimental condiDons, regardless of the strain rate and Strain rate (ε): 10-4 – 10-6 1/s temperature ramping rate. A thermal-mechanical scaling factor for the (ca. 500 – 5 cm/yr) experiments covers the range esDmated for natural subducDon zones. Temperature ramping: 3. Lawsonite is one of the few minerals that exhibit briUle deformaon resulDng 300˚C → 600˚C for lawsonite in unstable fault slip at high pressure and temperature condiDons. 400˚C → 700˚C for anDgorite 4. Dehydraon of lawsonite can directly trigger unstable fault slip in the Temp. ramping rate (T): 0.05 – 0.5 ˚C/s subducDng oceanic crust. .