Tectonic evolution of Cretaceous accretionary complex in the Idonnappu Zone, Urakawa area, central Hokkaido, Northern Japan : with reference to radiolarian ages and thermal structure

Abstract

Cretaceous accretionary complexes of the Idonnappu Zone in the Urakawa area consist of five lithological units (B-, MN-, MH-, PT- and T-units), which are combined into two geologic complexes : the greenstone-dominant Naizawa Complex and the clastic-dominant Horobetsugawa Complex. Radiolarian ages were determined for each unit. The Naizawa Complex is an alternating stack of slabs of tholeiitic greenstone (B-Unit), and fragments of Triassic seamount with Triassic - Lower Cretaceous ocean floor to trench-fill deposits (MN-Unit). The Horobetsugawa Complex is a pile of Upper Cretaceous ocean floor to trench-fill deposits (PT-Unit), with intercalated Lower Cretaceous to Upper Cretaceous submarine slide and debris flow deposits (MH-Unit) and latest Cretaceous - Paleocene slope basin deposits (T-Unit).Original disposition and relative tectonic movement of each unit were determined using illite crystallinity compared with fossil and K-Ar ages. These show post-accretion uplift, thrusting, and rearrangement by later right-lateral duplexing. A near-vertical section of the entire accretionary complex is exhumed in the Idonnappu Zone.The Naizawa Complex was formed by Early Cretaceous collision of subducting seamount with forearc ophiolite. The Horobetsugawa Complex formed as a clastic-dominant accretionary body in the Late Cretaceous, synchronous with the wane of forearc-basin sedimentation.