Abstract
We present a quantitative examination of the liberation and subsequent deposition of silica at the subduction zone plate interface in the Mugi mélange, an exhumed accretionary complex in the Shimanto Belt of southwest Japan. Frequency and thickness measurements indicate that mineralized veins hosted in deformed shales make up approximately 0.4% of the volume of this exposure. In addition, whole-rock geochemical evidence suggests that the net volume of SiO2 liberated from the mélange at temperatures of < 200°C was as much as 35%, with up to 40% of the SiO2 loss related to the smectite-illite (S-I) conversion reaction, and the rest attributable to the pressure solution of detrital quartz and feldspar. Kinetic modeling of the S-I reaction indicates active liberation of SiO2 at approximately 70°C to 200°C, with peak SiO2 loss at around 100°C, although these estimates should be slightly shifted toward lower temperature conditions based on X-ray diffraction (XRD) analyses of mixed-layer S-I in the Mugi mélange. The onset of pressure solution was not fully constrained, but has been documented to occur at around 150°C in the study area. The deposition in deformed shales of quartz liberated by pressure solution and the S-I reaction is probably linked to seismogenic behavior along the plate interface by (1) progressively enhanced velocity-weakening properties, which are favorable for unstable seismogenic faulting, including very-low-frequency earthquakes and (2) increasing intrinsic frictional strength, which leads to a step-down of the plate boundary décollement into oceanic basalt.
Highlights
The deposition of quartz as pore-filling cement and/or veins is a common diagenetic process during burial sedimentation, and can cause dramatic modifications of bulkrock mechanics, including changes in stiffness (Laubach et al 2004), consolidation behavior (Karig and Morgan 1994), and frictional properties (Lockner and Byerlee 1986)
Quartz cementation starts at approximately 90°C (McBride 1989), whereas in shales, evidence of pore-filling cementation is rare, with mineralized veining being a much more common form of quartz deposition
At subduction margins, incoming sedimentary deposits are buried along the plate boundary thrust. The underthrusting causes these sediments to undergo several diagenetic reactions during burial. These processes might have a first-order influence on the mechanical transition from aseismic shallower deformation to unstable seismic slip in deeper parts of the subduction zone (Moore and Saffer 2001); quartz cementation and/or veining have been invoked as a cause of the formation of seismogenic plate boundaries (Byrne 1998; Moore and Saffer 2001; Moore et al 2007)
Summary
The deposition of quartz as pore-filling cement and/or veins is a common diagenetic process during burial sedimentation, and can cause dramatic modifications of bulkrock mechanics, including changes in stiffness (Laubach et al 2004), consolidation behavior (Karig and Morgan 1994), and frictional properties (Lockner and Byerlee 1986). At subduction margins, incoming sedimentary deposits are buried along the plate boundary thrust The underthrusting causes these sediments to undergo several diagenetic reactions during burial. These processes might have a first-order influence on the mechanical transition from aseismic shallower deformation to unstable seismic slip in deeper parts of the subduction zone (Moore and Saffer 2001); quartz cementation and/or veining have been invoked as a cause of the formation of seismogenic plate boundaries (Byrne 1998; Moore and Saffer 2001; Moore et al 2007). As the majority of plate boundary faults are located in smectite-rich horizons (Underwood 2007), the S-I reaction could supply significant amounts of silica, this factor has not been assessed in previous research
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