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[MIS13-17] Ultrahigh-pressure pseudotachylytes: a link between petrology and seismology
Keywords:Ultrahigh-pressure pseudotachylytes, carbonated serpentinite, intermediate-depth earthquakes, subduction zone, naodiamond
Earthquakes in the subduction zone are important subjects not only for scientific research but also for disaster prevention. Unfortunately, however, the mechanism of earthquakes has been largely unknown. Earthquakes at shallow depths (less than 10 km depth) can be explained by brittle fracturing of rocks. However, brittle fracturing hardly occurs in rocks at deeper part of the earth. The cause of intermediate-depth (up to 200 km depth) earthquakes can be either dehydration embrittlement, transformational faulting, or shear instability, any of which does not have strong evidence in field geology, and has been the subject of much debate. This time we found pseudotachylytes in a carbonated serpentinite within a palaeo-subduction complex. The pseudotachylytes consist mainly of magnesite and quartz, and contains microdiamonds, nanodiamond aggregates, and an assemblage of magnesite+calcite (possibly former aragonite). The occurrence of these peculiar minerals and a mineral assemblage implies that the pseudotachylyte formed under ultrahigh-pressure more than 2.8 GPa. Here we present a new model for the cause of the intermediate-depth earthquakes as follows. Decarbonation reactions in a subduction complex evolve CO2 which reacts with serpentinite of the overlying wedge-mantle to form carbonated serpentinite, in which brittle fracturing can occur because of high rigidity of the rock and generate intermediate-depth earthquakes. The key of this model is the formation of the carbonated serpentinite of which mineral assemblage (magnesite+quartz) is widely stable in the mantle (Kakizawa et al., 2015). Pseudotachylyte has been considered as a fossil of earthquake formed at relatively shallow depth up to 20 or 30 km, therefore, our finding of an ultrahigh-pressure (UHP) pseudotachylyte gives direct evidence of earthquakes at deeper depth, which has never been reported in the world. The UHP pseudotachylyte contains pulverized rutile and zircon, which gives information for the fracturing process during the intermediate-depth earthquakes. This study examines the pressure-temperature condition of the formation of the UHP pseudotachylyte and considers the corresponding tectonic position in the subduction zone. This study also examines energetics of the intermediate-depth earthquakes recorded in this UHP pseudotachylyte.
Occurrence of the UHP pseudotachylyte and its formation conditions
The UHP pseudotachylyte has been found from the Nishisonogi unit of the Nagasaki Metamorphic Complex, western Kyushu, Japan. The Nishisonogi unit is a Cretaceous subduction complex showing the pressure-temperature (P-T) condition of 1.0-1.5 GPa and ~450 ℃. Recently, Nishiyama et al. (2020) reported the limited occurrence of an UHP metamorphic rocks (>2.8 GPa) containing microdiamonds from the serpentinite mélange at Yukinoura, a member of the Nishisonogi unit, at the western margin of the Nishisonogi Peninsula. This is the first report of UHP metamorphic rocks from Japanese metamorphic terranes, and the second one from the oceanic subduction complexes in the world following that from Lago di Chignana. The Yukinoura mélange develops at the bottom of a sheet-like body of massive serpentinite (about 50 m thick), which is emplaced horizontally within pelitic schists. It crops out some hundreds of meters along the coast, in which tectonic blocks of metabasites and metapelites occur in serpentinite. Carbonated serpentinites occur along magnesite+quartz veins in serpentinite and also as meter-size tectonic blocks, in the latter of which the UHP pseudotachylytes occur. The peak metamorphic temperature of the pelitic schist is estimated at about 450 ℃ by Raman microspectroscopy (Mori et al., 2019). The peak pressure recorded in the Yukinoura mélange is more than 2.8 GPa as indicated by the occurrence of microdiamonds in pelitic schists, chromitites, and pseudotachylytes (Nishiyama et al., 2020). The pressure-temperature condition indicates a cold geotherm in the subduction zone. This result together with occurrence of microdiamonds clearly indicate that the pseudotachylytes represent a brittle fracturing of the carbonated serpentinites at the deeper part of the subduction zone.
Occurrence of the UHP pseudotachylyte and its formation conditions
The UHP pseudotachylyte has been found from the Nishisonogi unit of the Nagasaki Metamorphic Complex, western Kyushu, Japan. The Nishisonogi unit is a Cretaceous subduction complex showing the pressure-temperature (P-T) condition of 1.0-1.5 GPa and ~450 ℃. Recently, Nishiyama et al. (2020) reported the limited occurrence of an UHP metamorphic rocks (>2.8 GPa) containing microdiamonds from the serpentinite mélange at Yukinoura, a member of the Nishisonogi unit, at the western margin of the Nishisonogi Peninsula. This is the first report of UHP metamorphic rocks from Japanese metamorphic terranes, and the second one from the oceanic subduction complexes in the world following that from Lago di Chignana. The Yukinoura mélange develops at the bottom of a sheet-like body of massive serpentinite (about 50 m thick), which is emplaced horizontally within pelitic schists. It crops out some hundreds of meters along the coast, in which tectonic blocks of metabasites and metapelites occur in serpentinite. Carbonated serpentinites occur along magnesite+quartz veins in serpentinite and also as meter-size tectonic blocks, in the latter of which the UHP pseudotachylytes occur. The peak metamorphic temperature of the pelitic schist is estimated at about 450 ℃ by Raman microspectroscopy (Mori et al., 2019). The peak pressure recorded in the Yukinoura mélange is more than 2.8 GPa as indicated by the occurrence of microdiamonds in pelitic schists, chromitites, and pseudotachylytes (Nishiyama et al., 2020). The pressure-temperature condition indicates a cold geotherm in the subduction zone. This result together with occurrence of microdiamonds clearly indicate that the pseudotachylytes represent a brittle fracturing of the carbonated serpentinites at the deeper part of the subduction zone.