TECTONIC CONTROL ON THE EMPLACEMENT OF CRETACEOUS TO PALEOGENE GRANITIC BATHOLITHS IN THE SOUTHWESTERN JAPAN ARC

On the basis of the compilation of geologic studies on granitic batholiths in the southwestern Japan Arc, tectonic control on their emplacement was examined. The batholiths are divided into the Ryoke, San-yo (Hiroshima), and San-in belts from south to north, and the belts are juxtaposed along arc. The ages of the Ryoke and San-yo batholiths range from 95 to 70 Ma. The Ryoke batholith is closely associated with HT-LP metamorphic rocks without coeval volcanic rocks, but the San-yo batholith commonly occurs with coeval rhyolitic volcanic rocks. The San-in batholith has the ages ranging mostly from 65 to 30 Ma, and occurs with coeval dacitic to andesitic volcanic rocks. The San-yo batholith occurs mostly along S to SW dipping low-angle thrust faults, and the Ryoke batholith, in particular foliated deeper units, thrusted up partly on the San-yo batholith from the south (Hayashi, 1995 Bull. Fac. School Edu., Hiroshima Univ., Part II, 17, 95-150). The field occurrences of the San-in batholith also suggest that they intruded mostly along sinistral shear faults or thrust faults. The boundaries between the batholiths and wall rocks are always sharp without significant structural disturbance. The field evidence above indicates that their intrusive structure was basically formed under N-S to NE-SW compressional stress fields. The compressional stress fields would be caused by not plate subduction but back-arc spreading, because the S-SW dipping of the thrust faults is the opposite direction from the duplex structure of accretionary complex. The thick Shimanto accretionary complex formed concurrently with the granitic batholiths would act a role of a backstop for the crust of the back-arc side, and thus the Ryoke belt can be regarded as a collision zone. The faulting regions were moved gradually from the vicinity of the backstop to the back-arc side, resulting in the regional migration of plutonism from the Ryoke, San-yo to San-in belts. The tectonic control above is similar to that of North American Cordillera (Hyndman et al., 2005 GSA today 15-2, 4-10); it would be a common style of plutonism in the continental arcs.