Relationship between melange fabrics of accretionary complex and relative convergence along the plate boundary

在日本四万十带的岩石中发现混杂岩组构的系统变化。几个剪切指标：包括S-C构造、Riedel剪切和褶皱，在露头和微观尺度上都表现出一致的剪切作用。独立颗粒流和压溶变形机制表明，变形开始于未成岩条件下，经历应变硬化的岩化作用，岩化作用后局部发生碎裂变形，但早期变形特征基本保持。这一过程似乎是沿着剥离作用发生在刮下的增生棱柱体之下。在Campanian混杂岩中观察到左旋反向剪切，而在Shimanto带的早始新世混杂岩中保留了右旋反向剪切。这种结构的变化被解释为与俯冲板块和覆盖板块之间相对会聚的变化有关。晚白垩世至早第三纪西太平洋边缘有三种不同的板块模型：1）库拉-太平洋海岭沿着大陆边缘俯冲，2）85 Ma以来太平洋板块俯冲，3）43 Ma后库拉-太平洋海岭的俯冲。其预测了在适当的时间从左旋到右旋的相对会聚的变化。

Systematic changes in fabrics of melanges have been detected in rocks of the Shimanto Belt, Japan. Several shear indicators : including S-C structures, Riedel shears and folds, in the melanges indicate consistent sense of shear both in outcrop and microscopic scales. The deformation mechanisms, independent particulate flow and pressure solution, suggest that the deformation started under unlithified condition and progressed through lithification due to strain hardening.After lithification, cataclastic deformation occurred locally but most of the earlier deformation features are preserved. This process appears to have taken place along the decollement beneath the off-scraped accretionary prism. Sinistral reverse sense of shear is observed in the Campanian melange, whereas a dextral reverse sense of shear is preserved in the Early Eocene melange in the Shimanto Belt. This change in fabric is interpreted to be linked to a change in relative convergence between the subducting and overriding plates.Three different plate models have been proposed for the Late Cretaceous to Early Tertiary western Pacific margin :1) subduction of the Kula-Pacific ridge along the continental margin,2) subduction of the Pacific Plate since 85 Ma, and3) subduction of the aborted Kula-Pacific ridge after 43 Ma.The obseved change in fabric of melange is consistent only with the third model, which predicts a change in relative convergence from sinistral to dextral at the appropriate time.