2016 Volume 122 Issue 9 Pages 461-475
We found meter-wide, NNW-trending sinistral shear zones in the Poroshiri ophiolite which has been dextrally sheared during thrusting with the Hidaka metamorphic rocks. Metagabbro in the Poroshiri ophiolite was transformed into porphyroclastic amphibolite and dextrally sheared during the amphibolitization, the latter of which in turn was mylonitized in the sinistral shear zones through dynamic recrystallization of plagioclase and metamorphic reactions to produce fine-grained hornblende and plagioclase. Foliation deflection, σ- and d-type mantled plagioclase porphyroclasts, shear bands, and shape-preferred orientations of hornblende porphyroclasts as well as of dynamically recrystallized plagioclase grains oblique to mylonitic foliation all indicate a sinistral sense of shear. Hornblende-plagioclase geothermometry suggests the metamorphic temperatures of porphyroclastic amphibolite and amphibolite mylonite to be 708±7℃ and 678±6℃, respectively. Thus the mylonitization occurred during the retrograde metamorphism. In contrast to the preceding dextral shearing with a thrust component, the sinistral shearing has a normal slip component, which possibly occurred by local backward movement during the retrograde metamorphism.