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

该项目的目标是(1)建立一个数值模型，用于预测板块之间夹层变质带的三维变形以及热结构，以反映板块运动和变形的各种驱动力；(2)建立一个数值模型，用于预测与山脊俯冲有关的扰动情况下的物质循环(特别是水)，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中记录的P-T和变形历史进行比较，以获得前瞻性的观点。根据(1)至(3)，我们得到以下结果：(A)在可能的变形场(弧前楔形体内的三维碰撞、逆冲、角流和挤压流，以及这些流的组合)中，平面应变或扁平应变占主导地位，而在日本弧白垩纪区域变质带(Ryoke和Sambagawa带)、美国西部的方济各斯带观察到的长应变仅在三维角流中再现。(2)推测日本弧白垩纪区域变质带是由弧前楔体中的三维角流形成的，与白垩纪的山脊俯冲有关，为弧前楔体提供了热和水，使区域升温和软化；(3)考虑含水玄武岩和橄榄岩的相态关系，认为水循环仅限于浅部(&lt；当海脊俯冲发生时，(D)与平均年龄的板块俯冲有关的水循环也限于相对较浅的部分(&lt；200公里深)，这是由于主要含水矿物相的脱水，而名义上无水矿物(如橄榄石)将少量水带到较深的地幔。

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.