P-T-deformation paths of regional metamorphic belts at convergent plate boundaries

本计画的目的在于：（1）建立一个预测板块间变质带三维变形及热构造的数值模式，以预测板块运动及变形的可变驱动力;（2）建立一个预测物质循环的数值模式（特别是水）在与山脊俯冲有关的扰动情况下，这被认为是变质作用和岩浆作用的关键，（3）将模型结果与变质岩和火成岩记录的P-T和变形历史进行对比，以获得一个透视图。根据（1）到（3），我们得到以下结果：（A）在可能的变形场中（三维碰撞、仰冲、角流和/或弧前楔挤压流以及这些流的组合），平原应变或扁长应变占主导地位，而长应变则在日本弧白垩纪区域变质带中观察到（B）日本岛弧白垩纪区域变质带是由弧前楔的三维角流产生的，与白垩纪的洋脊俯冲有关，（C）考虑到玄武质和橄榄岩的含水相关系，水循环仅限于浅部（深度<50 km），没有显著的向深部携水，当洋脊俯冲发生时，（D）与平均年龄的板块俯冲有关的水循环也仅限于相对较浅的部分由于主要含水矿物相的脱水作用，地幔底部（深度<200 km）的水含量很低，而名义上无水矿物（如橄榄石）会将少量水带到更深的地幔。

This project aims at (1)constructing a numerical model for predicting 3-D deformation of metamorphic belts intercalated between the plates, as well as thermal structures, for variable driving forces from plate motions and deformations, (2)constructing a numerical model for predicting material cycling (in particular water) under perturbed situations associated with ridge subduction, which is thought to be critical for metamorphism and magmatism, (3)comparing the model results and the P-T and deformation history recorded in metamorphic and igneous rocks, in order to obtain a perspective view. According to (1) to (3), we obtain the following results : (A)Among the possible deformation fields (3-D collision, obduction, corner flow and/or squeezing flow in the forearc wedge, and combination of these flows), plain strain or oblate strain is dominant, whereas prolate strain, that is observed in the Cretaceous regional metamorphic belts of the Japan arcs (Ryoke and Sambagawa belts), Franciscan belt in the western United States, is reproduced only in the 3-D corner flow, (B)the Cretaceous regional metamorphic belts of the Japan arcs is deduced to have been produced by 3-D corner flow in the forearc wedge, associated with ridge subduction in Cretaceous, which had supplied both heat and water to the forearc wedge to heat up and soften the region, (C)considering the phase relationships of the hydrous basaltic and peridotitic rocks, the water cycling is thought to be limited to a shallow part (<50 km depth), without significantly carrying water to the deeper part, when the ridge subduction occurs, (D)the water cycling associated with subduction of plates with an average age is also limited to relatively shallow part (<200 km depth) due to dehydration of the major hydrous mineral phases, whereas nominally anhydrous minerals (such as olivine) bring a small amount of water to the deeper mantle.