Implications of Long-Term Reactivation of Faults Normal to Rift Axis for Coarse-Grained Clastic Systems and Structural Segmentation in the Niigata Basin, Japan
Kurita, Hiroshi¹; Toyoshima, Tsuyoshi¹; Ishikawa, Yukako²
¹Faculty of Science, Niigata University, Niigata, Japan.
²Niigata University (present affiliation: JX Nippon Oil & Gas Exploration Corp., Tokyo), Niigata, Japan.
In the study of the Niigata back-arc basin, central Japan, considerable attention has been paid to the principal trend of the NNE-SSW to N-S rift axis that dominated the Miocene rifting as well as the post-Pliocene tectonic inversion. This study investigates the significance of faults approximately normal to the rift axis (NE-SW to NW-SE; rift-normal faults herein), by performing surface mapping, sedimentary facies analysis, and fault rock analysis on late Early-early Middle Miocene (ca. 17-15 Ma) clastic rocks and basement granitoids in the northern part of the basin.
The basal part of the Miocene includes thick breccia facies of fan delta systems, rich in debris flow deposits. This coarse-grained facies occurs only in narrow areas each of which is bordered by rift-normal faults. It should be noted that, typically, coeval sediments are very thin and fine-grained. These lateral changes in the sediments occur abruptly across the faults. Paleocurrent data of the breccia facies showed clastic supply from the east along the rift-normal faults. These faults thus, during the deposition of the basal Miocene, developed as growth faults, and constrained the loci of prominent lateral clastic supply.
Most of these rift-normal faults are characterized by fault gouges and breccias, with a rare presence of cataclasites. The thickness of the fault rocks suggests that the net slip of each fault is up to several tens of meters. Deformation structures of the faults indicate changes in the sense of shear from reverse to sinistral and finally dextral. These changes, as well as the net slip, suggest a long-term history of reactivation of the rift-normal fault system. Moreover, the largest rift-normal fault separates two structural segments in the study area. This segmentation is likely to have developed during the inversion-related deformation. A similar segmentation with the NW-SE faults has also been estimated within the upper crust by the observation of a recent earthquake in the other area of the basin.
Therefore, we emphasize the importance of structural elements normal to the rift axis of the Niigata basin. Within the long-term reactivation history, these elements functioned as tectonically controlled pathways of lateral clastic supply to the graben during the Miocene rifting. Then, they bordered structural segments, which may be analogous to the geological constraints of the present intra-crustal seismicity in the back-arc region.
AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012