5:15 PM - 6:45 PM
[SCG41-P05] Reverse faulting stress in the Pliocene detected from microcracks in quartz of the Tanigawa-dake granitic rocks
Keywords:fluid inclusion analyses, microcracks, Northern Fossa Magna, paleostress, Pliocene, Tanigawa-dake granitic rocks
The Tanigawa-dake area is located at the northeastern part of the junction area of the northeast Japan, the southwest Japan, and the Izu-Bonin-Mariana arcs. Granites younger than ~6 Ma intruded (e.g., Harayama, 1992; Minami et al., 2021) in this area. Considering that the general emplacement depth of granites is deeper than several kilometers, the Tanigawa-dake area could have been uplifted/exhumated at high speed. One of the causes of such diastrophism might be the change of stress field around the Tanigawa-dake area triggered by some tectonic incident, such as the collision of the Tanzawa and the Izu block and/or changes in the directions of Philippine Sea subduction. This study aims to elucidate the stress history of the Tanigawa-dake area by conducting (1) stress analysis by observing microcracks in quartz of granites and (2) microthermometry analysis to estimate the formation temperatures of microcracks.
Orientation-restorable samples were collected at two locations in the Tanigawa body of the late Miocene to Pliocene Tanigawa-dake granites (Samples A and B). In three orthogonal thin sections for each sample, attitudes of healed microcracks (HC) and sealed microcracks (SC) were measured using the universal rotating stage. Orientation distributions were analyzed by mixed Bingham distribution method (Yamaji and Sato, 2011). Subsequently, heating/freezing experiments were conducted for fluid inclusions of orientation-measured HCs in Sample A, and salinities and formation temperatures of HC were estimated.
As a result of the orientation analysis, reverse faulting stresses with E–W σ1 axes (Stress H) were detected from HCs, and reverse faulting stresses with ENE–WSW σ1 axes (Stress S) were detected from SCs for both samples. Cross-cutting relationships between microcracks suggest that SCs were formed after HCs. Homogenized temperatures of fluid inclusion in Sample A were ~210 ℃ or ~260 ℃. Minami et al. (2022) reported ~3.3 Ma of zircon U–Pb age (with >~900 ℃ of closure temperature) and ~2.6 Ma of zircon (U-Th)/He age (~180 ℃) for Sample A. Therefore, Stress H could have been exerted between ~2.6 and ~3.3 Ma. Note that stresses H and S have not yet been distinguished as whether they represent stresses at different stress stages or mere fluctuation in stress at the same stage.
Takeuchi (1980) measured orientations of igneous dikes intruded at the southern part of the northeast Japan arc from the late Miocene to the present. Pliocene dikes suggest ENE–WSW or E–W σHmax direction and normal faulting or strike-slip faulting stress regime. Stresses H and S are consistent with them in terms of σHmax directions, whereas they are inconsistent in terms of stress regime. Tanigawa-dake granites are located at the NE extension of the Takai-Utsukushigahara belt (Kosaka, 1984), the northern Fossa Magna. These granites have a northeastward younging trend about intrusion ages (Kawano, 2017), suggesting more rapid denudation at the northeast than the southwest side by assuming the same emplacement depths. Thus, strain rates in the northern Fossa Magna might have spatial variation after the Pliocene. The difference in the stress regime between stresses detected from the igneous dikes and the microcracks may reflect such a spatial variation. In the future, we will carry out paleostress and microthermometry analyses for microcracks in the granites of different intrusion ages to estimate spatiotemporal stress variation all over the Tanigawa-dake granites.
<Reference> Harayama (1992) Geology, 20, 657–660. Kawano (2017) Earth Sci. (Chikyu Kagaku), 71, 75–86. Kosaka (1984) J. Fac. Sci., Shinshu Univ., 19, 121–141. Minami et al. (2021) Earth Planets Space, 73, 231. Minami et al. (2022) Fission Track News Letter, 35, 22–26. Takeuchi (1980) J. Geosci. Osaka City Univ., 23, 1–64. Yamaji and Sato (2011) J. Struct. Geol., 33, 1148–1157.
<Acknowledgement> This study was financially supported by the Fukada Geological Institute.
Orientation-restorable samples were collected at two locations in the Tanigawa body of the late Miocene to Pliocene Tanigawa-dake granites (Samples A and B). In three orthogonal thin sections for each sample, attitudes of healed microcracks (HC) and sealed microcracks (SC) were measured using the universal rotating stage. Orientation distributions were analyzed by mixed Bingham distribution method (Yamaji and Sato, 2011). Subsequently, heating/freezing experiments were conducted for fluid inclusions of orientation-measured HCs in Sample A, and salinities and formation temperatures of HC were estimated.
As a result of the orientation analysis, reverse faulting stresses with E–W σ1 axes (Stress H) were detected from HCs, and reverse faulting stresses with ENE–WSW σ1 axes (Stress S) were detected from SCs for both samples. Cross-cutting relationships between microcracks suggest that SCs were formed after HCs. Homogenized temperatures of fluid inclusion in Sample A were ~210 ℃ or ~260 ℃. Minami et al. (2022) reported ~3.3 Ma of zircon U–Pb age (with >~900 ℃ of closure temperature) and ~2.6 Ma of zircon (U-Th)/He age (~180 ℃) for Sample A. Therefore, Stress H could have been exerted between ~2.6 and ~3.3 Ma. Note that stresses H and S have not yet been distinguished as whether they represent stresses at different stress stages or mere fluctuation in stress at the same stage.
Takeuchi (1980) measured orientations of igneous dikes intruded at the southern part of the northeast Japan arc from the late Miocene to the present. Pliocene dikes suggest ENE–WSW or E–W σHmax direction and normal faulting or strike-slip faulting stress regime. Stresses H and S are consistent with them in terms of σHmax directions, whereas they are inconsistent in terms of stress regime. Tanigawa-dake granites are located at the NE extension of the Takai-Utsukushigahara belt (Kosaka, 1984), the northern Fossa Magna. These granites have a northeastward younging trend about intrusion ages (Kawano, 2017), suggesting more rapid denudation at the northeast than the southwest side by assuming the same emplacement depths. Thus, strain rates in the northern Fossa Magna might have spatial variation after the Pliocene. The difference in the stress regime between stresses detected from the igneous dikes and the microcracks may reflect such a spatial variation. In the future, we will carry out paleostress and microthermometry analyses for microcracks in the granites of different intrusion ages to estimate spatiotemporal stress variation all over the Tanigawa-dake granites.
<Reference> Harayama (1992) Geology, 20, 657–660. Kawano (2017) Earth Sci. (Chikyu Kagaku), 71, 75–86. Kosaka (1984) J. Fac. Sci., Shinshu Univ., 19, 121–141. Minami et al. (2021) Earth Planets Space, 73, 231. Minami et al. (2022) Fission Track News Letter, 35, 22–26. Takeuchi (1980) J. Geosci. Osaka City Univ., 23, 1–64. Yamaji and Sato (2011) J. Struct. Geol., 33, 1148–1157.
<Acknowledgement> This study was financially supported by the Fukada Geological Institute.