A middle Miocene post‐rift stress regime revealed by dikes and mesoscale faults in the Kakunodate area, NE Japan

• Published in: The island arc 2019-07, Vol.28 (4), p.n/a
• Main Authors: Haji, Toshiki, Hosoi, Jun, Yamaji, Atsushi
• Format: Article
• Language: English
• Subjects: Deformation; Embankments; Faults; fault‐slip analysis; fission‐track age; Igneous dikes; Mesoscale phenomena; Miocene; mixed Bingham distribution; Neogene; paleostress; post‐rift stress; Radiometric dating; Ratios; Rifting; Rock intrusions; Stratigraphy; Stress; Stress history; Stresses; tectonics; U–Pb age
• ISSN: 1038-4871; 1440-1738
• DOI: 10.1111/iar.12304

Significant advances were made in the last century in the investigations of the Neogene stress history of the NE Japan arc. However, previous studies have failed to fully resolve middle Miocene post‐rift stress conditions owing to their assumption of Andersonian faulting and an inability to determine maximum and intermediate stress axes from dike orientations. We applied the latest methods of paleostress analysis in this study to igneous dikes and mesoscale faults in the Kakunodate area of the NE Japan arc to elucidate post‐rift stress conditions. Stratigraphic constraints and U–Pb dating indicate that the doleritic and dacitic dikes were formed at 16–12 Ma and 15–12 Ma, respectively. Dolerite and dacite dikes yielded NW–SE extensional stresses with intermediate and low stress ratios, respectively. Mesoscale faults in the middle Miocene formations of the studied area indicated similar stresses. We suggest the sluggish deformations resulting in the dike intrusion and faulting in the normal‐faulting stress regime after the termination of intra‐arc rifting at ca. 15 Ma. 約15 Ma 以降のポストリフト期の東北日本では，地質図規模の断層活動がなくなる.  この時期のテクトニクスを理解するには，地殻が大変形せずともできる，岩脈や小断層が鍵となる.  しかし以前の研究では，平行性の悪い岩脈群や，非Anderson型断層を扱えなかった.  そのため，ポストリフト期の応力状態は不明確だった. 本研究では，秋田県角館地域の中新統塩手沢層・砂子渕層・鵜養層から28枚のドレライト岩脈，10枚のデイサイト岩脈，59条の小断層のデータを採取し，最新の応力解析法を適用した.  層位的制約と放射年代から，岩脈の貫入年代はそれぞれ約16–12 Maと約15–12 Maと推定された.  岩脈と小断層からともに，σ3軸がNW‐SE方向の正断層型応力を得た.  小断層の活動時期は不明確だが，岩脈から得られた応力との類似性から，大部分が貫入と同時期だろう.  ポストリフト期には，地質図規模の変形がほとんどない状況で，正断層型応力が継続していたことが分かった. We applied the latest methods of paleostress analyses to igneous dikes and mesoscale faults in the Kakunodate area, NE Japan arc to understand the post‐rift stress condition. We found the normal faulting regime of stress indicating NW–SE extension. We suggest that the normal faulting regime of stress kept after the intra‐arc rifting was abounded at ~ 15 Ma and extensional deformation became sluggish in the post‐rift phase.