10:45 〜 12:15
[SCG45-P17] Serpentinite carbonation during trench-directed shearing in the Sanbagawa belt: a record of deep episodic tremor and slip?
キーワード：earthquakes, slow slip, episodic tremor and slip, sanbagawa, serpentinite carbonation, fluid/rock interaction
Fluids assume a key role during in controlling fast and slow earthquakes, not only because they lower the effective stress, but also for their role as catalysers of mineral reactions and in transporting chemical elements. Ultramafic rocks, like mantle wedge serpentinites, are particularly reactive in the presence of aqueous CO2-bearing fluids and the subsequent volume change and H2O-release can easily keep carbonation reactions active as long as serpentinites that can be altered as present. This process produces a mixed, polymineralic rheology, consisting of talc, chlorite, carbonates, and quartz, starting from a monomineralic, serpentine-dominated rheology. The key questions are: is this mixed rheology able to produce tremor and slow? And, is carbonation able to sustain slow earthquakes in the same rock volume over geologic time scales?
We present preliminary structural and microstructural observations of sheared chlorite-talc schists (carbonated serpentinites) that formed during coupling between mantle peridotites and subducted metasediments in the garnet zone of the Sanbagawa belt (Shikoku, Japan). Tectonic coupling was associated with overall top-to-S (trench-directed) shearing. RSCM data on associated schists constraints the T of deformation to ~ 450–550 °C at a paleodepth of 35–45 km, corresponding to the current conditions of episodic tremor and slip in the Nankai and Cascadia subduction zones.
The carbonation process was marked by the formation of talc-rich and chlorite-rich patches, the latter associated with alteration of spinel to Cr-magnetite, and the opening of talc, chlorite, and dolomite + quartz shear veins. Deformation was marked by a generally semibrittle style, with the opening of multiple and mutually cross-cutting veins, accompanied by mylonitic shearing concentrated along talc-rich bands. Punctuated events of brecciation interrupted this first-order deformation style producing massive carbonate veins with blocks of chlorite talc schists.
We propose that the carbonation process was responsible for the development of talc and hence overall strain softening, which enabled slow slip, but also for the release of aqueous fluids that caused pore fluid pressure to build up and fracture the rock volume. The development of a polyphase mixture of talc and chlorite, related to the presence of Al in the rock, and the continuous precipitation of carbonates and quartz as a result of the carbonation process kept the rheology mixed, preventing the full development of talc bands that would have allowed to localize aseismic creep. The episodic brecciation and formation of massive carbonate veins can be explained as momentaneous high-strain events, such as seismic events or fluid overpressures, and are reminiscent of deformation in the subduction interface, where slow slip can locally be associated with fast earthquakes.
We present preliminary structural and microstructural observations of sheared chlorite-talc schists (carbonated serpentinites) that formed during coupling between mantle peridotites and subducted metasediments in the garnet zone of the Sanbagawa belt (Shikoku, Japan). Tectonic coupling was associated with overall top-to-S (trench-directed) shearing. RSCM data on associated schists constraints the T of deformation to ~ 450–550 °C at a paleodepth of 35–45 km, corresponding to the current conditions of episodic tremor and slip in the Nankai and Cascadia subduction zones.
The carbonation process was marked by the formation of talc-rich and chlorite-rich patches, the latter associated with alteration of spinel to Cr-magnetite, and the opening of talc, chlorite, and dolomite + quartz shear veins. Deformation was marked by a generally semibrittle style, with the opening of multiple and mutually cross-cutting veins, accompanied by mylonitic shearing concentrated along talc-rich bands. Punctuated events of brecciation interrupted this first-order deformation style producing massive carbonate veins with blocks of chlorite talc schists.
We propose that the carbonation process was responsible for the development of talc and hence overall strain softening, which enabled slow slip, but also for the release of aqueous fluids that caused pore fluid pressure to build up and fracture the rock volume. The development of a polyphase mixture of talc and chlorite, related to the presence of Al in the rock, and the continuous precipitation of carbonates and quartz as a result of the carbonation process kept the rheology mixed, preventing the full development of talc bands that would have allowed to localize aseismic creep. The episodic brecciation and formation of massive carbonate veins can be explained as momentaneous high-strain events, such as seismic events or fluid overpressures, and are reminiscent of deformation in the subduction interface, where slow slip can locally be associated with fast earthquakes.