Seismicity, Metamorphism, Metasomatism and Fluid Evolution in the Crustal Forearc of Warm Subduction Zones | INSTITUT DE PHYSIQUE DU GLOBE DE PARIS


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  Seismicity, Metamorphism, Metasomatism and Fluid Evolution in the Crustal Forearc of Warm Subduction Zones

Mardi 03 Avril 2018
Séminaires communs Tectonique-Sismologie
Michael Bostock
(University of British Columbia)

Microseismicity (Mw<4) in the warm Cascadia subduction zone is represented in three distinct categories: 1) intraslab events within the subducting Juan de Fuca plate that occur at depths < 100 km, 2) crustal seismicity in the overriding North American plate, and 3) tremor (low frequency earthquakes) that accompanies slow slip episodes. The spatial distribution of tremor is the most regular with epicenters occuring in a band 40-80 km in width, paralleling the coast from northern Vancouver Island to northern California. Deep (> 40 km) intraslab events are largely restricted to the Georgia Strait/Puget Sound region with shallower concentrations along western Vancouver Island and northern California where the subducting plate is youngest. Crustal earthquake epicenters are more diffusely distributed but remain concentrated in Washington State and northern California, immediately landward of tremor. I will review evidence that suggests this distribution is largely the result of 2 factors: a deviatoric slab stress field governed by variable slab pull along strike, and the presence of an impermeable seal atop the plate boundary that coincides with and enables tremor occurrence. Warm slab temperatures and the downdip breach of seal creates a unique hydrological environment within the landward crustal forearc. Pronounced expulsion of fluids at relatively shallow depths (40-80 km) subjects northern Cascadia forearc crust to sustained (duration >10 Ma) fluid exposure and leads to low volcanic output. Much of the crustal seismicity is clustered in swarms indicating a fluid influence. A regional tomographic study across southern Vancouver Island / northern Washington reveals clustered crustal seismicity to depths of 25 km (temperatures ~400C) associated with low Poisson's ratios (< 0.24) that is interpreted as due to quartz concentration through metasomatism. Microseismicity, greenschist metamorphism, metasomatism and quartz vein networks are characteristic features of orogenic gold deposits leading to speculation that that preferential formation of such deposits during supercontinent cycles from late Archean through Phanerozoic is a consequence of increased incidence of warm subduction.