Paleomagnetism, rock magnetism, and fission track dating of the Kurihashi Granodiorite in northeast Japan: Evidence for a Cretaceous hydrothermal event and differential rotation along the eastern Eurasian margin

Abstract

Samples from the Late Cretaceous Kurihashi Granodiorite on the Pacific coast of northeast Japan are divided into two groups based on paleomagnetic data. One group (A) preserves stable single thermoremanent magnetizations (TRM) with large westerly deflected declinations. The other group (B) has two or three distinct components of natural remanent magnetization (NRM): components with high, middle, and low unblocking temperatures show westerly declinations (nearly identical with the stable TRM directions of group A), as well as northerly and easterly declinations. Rock magnetic experiments indicate that the dominant magnetic mineral of both groups is Ti‐poor titanomagnetite. Group A samples contain superparamagnetic (SP) titanomagnetite because they show a frequency dependence with the initial susceptibility measurements: Hysteresis parameters of group B samples are consistent with a mixture of single‐domain (SD) and multidomain (MD) grains. Electron microscopy confirms the presence of MD size magnetite in the group B samples. Fission track thermochronology indicates that the Kurihashi Granodiorite underwent a post cooling hydrothermal event during the Late Cretaceous; thus the northerly and easterly NRM components seem to have a thermoviscous origin. Many samples have anomalously high unblocking temperatures (up to 560°C) compared to predictions of SD theory and probably result from long thermal demagnetization “tails” of thermoviscous remanent magnetizations (TVRM) carried by MD magnetite. A strong TVRM was acquired during the hydrothermal event and is correlated with the long normal‐polarity superchron of the geomagnetic field. Because highly oblique (left‐lateral) subduction was active along the eastern Eurasian margin during the same period, the primary TRM and TVRM components record counterclockwise rotation (∼120°) of the Kurihashi Granodiorite and remarkable wrench deformation along the continental margin.