[1] Two TCDP boreholes A and B were drilled in the northern part of the Chelungpu thrust fault where the Chi-Chi earthquake (21 September 1999, Mw 7.6) showed large displacement, low ground acceleration, and high slip velocity. In this paper, we describe the microstructures of the Chi-Chi Principal Slip Zone (PSZ) within black gouges localized at 1111 m depth in Hole A and at 1136 m depth in Hole B. In the FZA1111 the PSZ is a 2 cm-thick isotropic clay-rich gouge which contains aggregates formed by central clasts coated by clay cortex (clay-clast aggregates (CCAs)) and fragments of older gouges segregated in the top third of the PSZ. In FZB1136 the PSZ is 3 mm thick and is characterized by a foliated gouge displaying an alternation of clay-rich and clast-rich layers. The presence of CCAs, plucked underlying gouge fragments, gouge injections, and the occurrence of reverse grain size segregation of large clasts in the FZA1111 isotropic gouge suggest that the gouge was fluidized as a result of frictional heating and thermal pressurization. The foliated gouge in FZB1136 may be one locus of strain localization and related heat production. Small calcite veins present above the isotropic FZA1111 PSZ gouge and, characterized by an increasing strain with increasing distance away from the PSZ, are attributed to coseismic fluid escape from the pressurized gouge. The observed microstructures are interpreted in view of their seismic implications for the Chi-Chi earthquake in terms of slip weakening mechanisms by thermal pressurization, gouge fluidization, coseismic fluid distribution, and postseismic slip. Above the PSZ, several layers of compacted gouges containing deformed CCAs and gouge fragments correspond to several PSZ of past earthquakes similar to the Chi-Chi earthquake and display a fault-parallel cleavage resulting from a low strain rate pressure solution deformation mechanism that may be correlated to the interseismic periods.
New pseudosection modelling was applied to better constrain the P-T conditions and evolution of glaucophane-bearing rocks in the Tamayen block of the Yuli belt, recognized as the world's youngest known blueschist complex. Based on the predominant clinoamphibole, textural relationships, and mineral compositions, these glaucophane-bearing high-P rocks can be divided into four types. We focused on the three containing garnet. The chief phase assemblages are (in decreasing mode):(Type-II), and amphibole + quartz + albite + epidote + garnet + rutile + hematite + titanite (Type-III). Amphibole exhibits compositional zoning from core to rim as follows: glaucophane ? pargasitic amphibole ? actinolite (Type-I), barroisite ? Mg-katophorite/taramite ? Fe-glaucophane (Type-II), glaucophane ? winchite (Type-III). Using petrographic data, mineral compositions and Perple_X modelling (pseudosections and superimposed isopleths), peak P-T conditions were determined as 13 AE 1 kbar and 550 AE 40°C for Type-I, 10.5 AE 0.5 kbar and 560 AE 30°C for Type-II (thermal peak) and 11 AE 1 kbar and 530 AE 30°C for Type-III. The calculations yield higher pressures and temperatures than previously thought; the difference is~1-6 kbar and 50-200°C. The three rock types record similar P-T retrograde paths with clockwise trajectories; all rocks followed trajectories with substantial pressure decrease under near-isothermal conditions (Type-I and Type-III), with the probable exception of Type-II where decompression followed colder geotherms. The P-T paths suggest a tectonic environment in which the rocks were exhumed from maximum depths of~45 km within a subduction channel along a relative cold geothermal gradient of~11-14°C km À1 .
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