Progressive segmentation and systematic block rotation within a plutonic body: palaeomagnetism of the Cretaceous Kurihashi granodiorite in northeast Japan

Abstract

SUMMARY Intensive palaeomagnetic and rock magnetic studies clarify the history of multiple block rotational events within an active plate margin. The Cretaceous Kurihashi Granodiorite in northeast Japan preserves multi-component remanent magnetization: high-unblocking-temperature (TUB) thermoremanent magnetization (TRM) acquired during the initial cooling of the granodiorite, and middle-/low-TUB thermoviscous remanent magnetization (TVRM) acquired during succeeding hydrothermal activities. The observed directional difference between the three magnetic components requires the first counter-clockwise rotation event within a part (>1 km) of northeast Japan as a result of left-lateral plate motion in the Late Cretaceous. Later clockwise rotation, probably related with right-lateral faulting during the course of the Miocene backarc spreading of the Japan Sea, is indicated by significant easterly deflections in the low-TUB component. On the other hand, scatter in the high-TUB TRM declinations implies that the second deformation phase was achieved as non-rigid rotation of fault-bounded blocks as wide as 100 m. The latest and most confined rotation events are confirmed by complicated deflections of the TRM and TVRM in a fault zone. A shear zone as wide as 30 m with dense fractures near the main fault is sharply divided into three compartments, which have been rotated around three orthogonal poles. Systematic block rotations may accommodate differential slips on fault planes within a plutonic body, genetically containing orthogonal fractures as reactivated intrinsic cooling joints. We interpret our results to show that the Kurihashi granodiorite has been progressively segmented since the Cretaceous under varied stresses.