Unstable fault slip induced by lawsonite dehydraon in blueschist: Keishi Okazaki ([email protected]) and Greg Hirth ([email protected]) Implicaon for the seismicity in the subducng oceanic crusts Department of Earth, Environmental & Planetary Sciences, Brown University Intraslab seismicity in subducon zones Temperature ramping experiments Stress-displacement curves Hot subducon zone (Matsubara et al., 2009) Lawsonite: unloading slope is similar to the effecve 5 Hot subducon zones: 6 x 10 6 x 102 unloading sffness 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 enre range of T/ε tested. Moho ⇒few crustal earthquakes → unstable fault slip ⇒a number of slow EQs Cold subducon zones: Angorite: 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., sck-slip) but Cold subducon zone (Tsuji et al., 2008) ⇒no slow earthquake Abers et al., 2013 stable slow slip in angorite, consistent with the differences in AE. Cold subducon zones: Hot subducon zones: . . Lawsonite blueschist, Greenschist & epidote amphibolite, Blueschist-eclogite transion Time-independent thermal-mechanical factor T/ε (Hacker, 2008) many crustal EQs few crustal EQs Lawsonite-out reacon in MORB system Unstable slip If we assume: (Poli & Schmidt, 1995) (C&H) Thermal gradient = 10 ˚C/km, subducing velocity = 10 cm/year, subducng angle of 45˚, –13 –15 range of strain rate from 10. . to 10 1/s, T/ε in natural subducon 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 subducon zone systems. Can dehydraon of lawsonite directly trigger seismic Sffness of the natural fault system:

fault slip in the subducng 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 lile gouge. ν: the Poisson’s rao (~0.25), Sample 1: lawsonite, a common mineral in “cold” subducng crusts → similar to observaons from high pressure faulng L: the length of the slipping region. • lawsonite vein within a lawsonite-blueschist block from the associated with the olivine-spinel transion (Schubnel et al., If we assume L≈ 10 m (i.e., for a M1 earthquake, Franciscan Complex in CA, USA 2014). the sffness of the natural systems Kf ~2 MPa/mm. • almost pure lawsonite (~98%), with minor staurolite and XRD analysis: glaucophane Unstable slip resulng 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 friconal behaviour has result of dehydraon during experiments. been observed during dehydraon (Chernak & Hirth, 2011; Proctor & Hirth, subducon 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 angorite (~98%) with minor diopside, spinel and zone, subparallel to the slickenside surface; anorthite 1. Unstable fault slip (i.e., sck-slip) occurred during dehydraon reacons in

magnete Blue lines: phase diagram of the CASH syetem (Newton & Kennedy, 1963) 1: Stability field of angorite at high fH2OUlmer&Trommsdorff, 1995 has a somewhat higher Si content (~44 wt%) than the lawsonite gouge layer and AE signals were connuously observed under

2: Stability field of angorite at low fH2O (Perrillat et al., 2003) lawsonite (38.3 wt%) and zoisite (~39.7 wt%). the differenal stress. Experimental condions:

Confining pressure (Pc): 1 GPa 2. The unloading slope during the unstable slip follows the sffness of the apparatus at all experimental condions, 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 esmated for natural subducon zones. Temperature ramping: 3. Lawsonite is one of the few minerals that exhibit brile deformaon resulng 300˚C → 600˚C for lawsonite in unstable fault slip at high pressure and temperature condions. 400˚C → 700˚C for angorite 4. Dehydraon of lawsonite can directly trigger unstable fault slip in the Temp. ramping rate (T): 0.05 – 0.5 ˚C/s subducng oceanic crust.