Petrological modeling is a powerful technique to address different types of geological problems via phase-equilibria predictions at different pressure-temperature-composition conditions. Here, we show the versatility of this technique by (1) performing thermobarometrical calculations using phase equilibrium diagrams to explore the petrological evolution of high-pressure (HP) metabasites from the Renge and Sanbagawa belts, Japan and (2) forward-modeling the mineral-melt evolution of the subducted fresh and altered oceanic crust along the Nankai subduction zone geotherm at the Kii peninsula, Japan. In the first case, we selected three representative samples from these metamorphic belts: a glaucophane eclogite and a garnet glaucophane schist from the Renge belt (Omi area) and a quartz eclogite from the Sanbagawa belt (Besshi area). We calculated the peak metamorphic conditions at 2.0-2.3 GPa and 550-630 C for the HP metabasites from the Renge belt, whereas for the quartz eclogite, the peak equilibrium conditions were calculated at 2.5-2.8 GPa and 640-750 C.According to our models, the quartz eclogite experienced partial melting after peak metamorphism. In terms of the petrological evolution of the subducted uppermost portion of the oceanic crust along the warm Nankai geotherm, our models show that fluid release occurs at 20-60 km, likely promoting high pore-fluid pressure, and thus, seismicity at these depths; dehydration is controlled by chlorite breakdown.Our petrological models predict partial melting at >60 km, mainly driven by phengite and amphibole breakdown. According to our models, the melt proportion is relatively small, suggesting that slab anatexis is not an efficient mechanism for generating voluminous magmatism at these conditions. Modeled melt compositions correspond to high-SiO 2 adakites; these are similar to compositions found in the Daisen and Sambe volcanoes, in southwest Japan, suggesting that the modeled melts may serve as an analog to explain adakite petrogenesis.
The young oceanic Cocos plate (<20 Myr; Müller et al., 2008), in western Mexico, subducts underneath North America's continental lithosphere at an angle of ∼15° at the Guerrero trench (Pérez-Campos et al., 2008). Inland ∼125 km, subduction becomes near-horizontal for 150 km, before steepening to ∼75° and extending to ∼550 km depth (Pérez-Campos et al., 2008). Historically, earthquakes in Mexico occur along the Pacific coast megathrust (Figure 1), including the devastating 19 September 1985 Mw8.5 Michoacán earthquake. The 19 September 2017 Mw7.1 Puebla-Morelos earthquake is considered atypical due to its location as it nucleated inland (∼250 km from the trench; Figure 1) near the end of the Cocos flat slab segment where the subduction dip angle steepens, with a hypocenter located at ∼57 km depth, within the Cocos lithospheric mantle (Servicio Sismológico Nacional-SSN, 2020). This earthquake caused an unprecedented tragedy in the modern history of Mexico, causing 246 deaths and the collapse of 44 buildings (Mayoral et al., 2017).Seismicity can occur within the slab and at greater depths, ∼50-300 km (i.e., intermediate-depth (ID) seismicity; Green & Houston, 1995;Yamasaki & Seno, 2003). The origin of ID earthquakes has been hypothesized to be related to the metamorphic dehydration of the subducting oceanic lithosphere as the location of these earthquakes correlates with the depth where dehydration reactions occur within the slab (
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