Changes in detrital garnet assemblages related to transpressive uplifting associated with strike–slip faulting: An example from the Cretaceous System in Kii Peninsula, Southwest Japan
Abstract The chemical characteristics of detrital garnet have served as relatively reliable and definitive indicators of the major tectonic settings of sedimentary units in many parts of the world. In Southwest Japan a multidisciplinary approach combining sandstone modal, heavy mineral and garnet compositional analyses of the Cretaceous upper continental slope to trench deposits of the Kurosegawa and Shimanto belts combined with a comparison of the detrital garnet features in and around Kii Peninsula revealed the following characteristics. The garnet assemblages show significant changes with age. The Lower Cretaceous Yuasa Formation is characterized by pyrope-rich almandine garnets derived from granulite-facies metamorphic rocks. These are associated with garnet grains that have pyrope–almandine compositions and high grossular contents suggesting an eclogite-origin. Garnet assemblages of the lower Upper Cretaceous Yukawa Formation are dominated by grains with chemical characteristics indicating an origin as low P / T type metamorphic rocks. There are also spessartine–almandine garnets derived from acidic igneous rocks, some with a high P / T type metamorphic origin and others with a grandite composition. Garnet assemblages of the upper Upper Cretaceous Miyama Formation are similar to those of the Yukawa Formation, with the exception that grandite garnets are absent. The above compositional changes of garnets in the deposits allow us to characterize the corresponding provenances and by combining these results with stratigraphic ages temporal changes of these provenances can also be determined. The source area for the granulite- facies metamorphic rocks that continuously supplied detritus throughout the Cretaceous might be Precambrian continental basement in East Asia. In early Late Cretaceous time, the detritus was derived from terranes distributed along the Median Tectonic Line, a major strike–slip fault stretching throughout Southwest Japan. The garnet compositions suggest the source area included eclogite in the Kurosegawa belt that forms the basement to the Cretaceous strata, and the low P / T type metamorphic rocks and granitic rocks in the Paleo-Ryoke belt including the Higo belt. Studies of the Miyama Formation show that at this time widespread acidic volcanism began and this was the source for much of the volcanic sediment in the formation. The provenance change between the Lower and lower Upper Cretaceous Formations is likely to have been caused by uplifting and unroofing associated with Cretaceous transpression expressed by the strike–slip movement along the Median Tectonic Line and other lateral faults in the Chichibu belt.