10:45 〜 12:15
[SIT20-P01] Contrasting deformation histories in Taiwan and SW Japan: Evidence for CCW rotation of the convergence vector < 1 Ma
キーワード：exhumation, oblique convergence, plate motions, Philippine Sea Plate
Geologists have long recognized the possible significance of a change in motion of the Philippine Sea Plate (PSP) in the middle to late Cenozoic (Huchon, 1983; Seno and Maruyama, 1984). However, understanding and constraining the timing of this change has been challenging because the PSP is surrounded by trenches, which tell no tales of their demise and transform boundaries where slip is difficult to quantify. Here, we show that contrasting deformation histories in SW Japan and Taiwan argue for a CCW change in motion of the PSP wrt Eurasia < 1.0 Ma.
In Taiwan, before ~1.0 Ma, deformation in the orogenic core was partitioned into LL strike-slip shear and west-directed thrusting along a generally N-S trending plate boundary. Ho et al. (2022) have proposed that this orogen-scale strain partitioning reflected highly oblique plate convergence. Recent mapping also identified a subhorizontal fabric, “S3,” that developed relatively late during oblique plate convergence. More importantly, these metamorphic fabrics have been folded and rotated in the northern Backbone Range as spreading in the Okinawa Trough propagated into the Ilan Plain ~ 1.0 Ma, signaling an end to highly oblique and partitioned plate convergence and the start of more orthogonal convergence. Structural and sedimentological data also suggest rapid uplift and erosion ~1 Ma (Hsieh et al., 2020; Lai et al., 2021; 2022). Additionally, limited low-temperature geochronologic data suggest an acceleration in exhumation cooling <1 Ma (Hsu et al., 2016).
In SW Japan, geological and geophysical studies suggest a change in kinematic regimes from nearly orthogonal plate convergence to RL oblique convergence ~1.0 Ma. For example, geologic data from the Median Tectonic Line south of Osaka document a change from thrusting to right-lateral strike-slip faulting at about ~0.8 Ma (Sato et al., 2015). This change in kinematics and timing is consistent with previous ideas on the formation age of the Nankai forearc sliver (~0.8 Ma) based primarily on geologic and geomorphic data from Shikoku Island (Ohmori, 1978; Saito, 1999; Okamura, 1990). Further seaward in the modern accretionary prism, Gulick et al. (2010) and Sacks et al. (2013) show a transition from shortening perpendicular to the plate boundary to extension perpendicular to the boundary <1.0 Ma. Although these authors propose that this change reflects subduction zone dynamics, a change in plate kinematics is also possible. In fact, in the eastern part of the forearc, Yamaji (2000) mapped a similar change in fault kinematics and attributed the change to a counterclockwise rotation of the relative motion of the Philippine Sea Plate at ~1.0 Ma.
The contrasting deformation histories along two plate boundaries with different regional strikes argue for a regional-scale change in plate motion of the PSP wrt Eurasia. Along the N-S trending Taiwan margin, the structural regime changed from strike-slip to orthogonal shortening. In contrast, along the ENE-trending margin in SW Japan, the structural regime changed from orthogonal shortening to strike-slip or extension. These contrasting deformation histories from different margins suggest a significant and relatively recent (<1.0 Ma) CCW rotation in the motion of the Philippine Sea Plate relative to Eurasia. This conclusion is consistent with plate reconstructions (Wu et al., 2016).
In Taiwan, before ~1.0 Ma, deformation in the orogenic core was partitioned into LL strike-slip shear and west-directed thrusting along a generally N-S trending plate boundary. Ho et al. (2022) have proposed that this orogen-scale strain partitioning reflected highly oblique plate convergence. Recent mapping also identified a subhorizontal fabric, “S3,” that developed relatively late during oblique plate convergence. More importantly, these metamorphic fabrics have been folded and rotated in the northern Backbone Range as spreading in the Okinawa Trough propagated into the Ilan Plain ~ 1.0 Ma, signaling an end to highly oblique and partitioned plate convergence and the start of more orthogonal convergence. Structural and sedimentological data also suggest rapid uplift and erosion ~1 Ma (Hsieh et al., 2020; Lai et al., 2021; 2022). Additionally, limited low-temperature geochronologic data suggest an acceleration in exhumation cooling <1 Ma (Hsu et al., 2016).
In SW Japan, geological and geophysical studies suggest a change in kinematic regimes from nearly orthogonal plate convergence to RL oblique convergence ~1.0 Ma. For example, geologic data from the Median Tectonic Line south of Osaka document a change from thrusting to right-lateral strike-slip faulting at about ~0.8 Ma (Sato et al., 2015). This change in kinematics and timing is consistent with previous ideas on the formation age of the Nankai forearc sliver (~0.8 Ma) based primarily on geologic and geomorphic data from Shikoku Island (Ohmori, 1978; Saito, 1999; Okamura, 1990). Further seaward in the modern accretionary prism, Gulick et al. (2010) and Sacks et al. (2013) show a transition from shortening perpendicular to the plate boundary to extension perpendicular to the boundary <1.0 Ma. Although these authors propose that this change reflects subduction zone dynamics, a change in plate kinematics is also possible. In fact, in the eastern part of the forearc, Yamaji (2000) mapped a similar change in fault kinematics and attributed the change to a counterclockwise rotation of the relative motion of the Philippine Sea Plate at ~1.0 Ma.
The contrasting deformation histories along two plate boundaries with different regional strikes argue for a regional-scale change in plate motion of the PSP wrt Eurasia. Along the N-S trending Taiwan margin, the structural regime changed from strike-slip to orthogonal shortening. In contrast, along the ENE-trending margin in SW Japan, the structural regime changed from orthogonal shortening to strike-slip or extension. These contrasting deformation histories from different margins suggest a significant and relatively recent (<1.0 Ma) CCW rotation in the motion of the Philippine Sea Plate relative to Eurasia. This conclusion is consistent with plate reconstructions (Wu et al., 2016).