11:15 〜 11:30
[SIT20-09] Contrasting deformation histories in Taiwan and SW Japan: Evidence for counter-clockwise rotation of the motion vector between the Philippine Sea and Eurasia Plates <1 Ma
キーワード：subduction, collision, accretion, exhumation, orogenesis
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 tails 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 < 1Ma.
In Taiwan, before ~1 Ma, deformation in the orogenic core was characterized by highly oblique plate convergence partitioned into LL strike-slip shear and west-directed thrusting along a generally N-S trending plate boundary. These metamorphic fabrics were folded and rotated in the northern Backbone Range when spreading in the Okinawa Trough propagated into the Ilan Plain ~1 Ma, signaling an end to oblique plate convergence and initiation of WNW shortening. Structural and sedimentological data suggest rapid uplift and erosion starting ~1 Ma and low-temperature geochronologic data from the metamorphic core record acceleration in exhumation cooling <1 Ma, suggesting growth of the orogen at that time.
In SW Japan, before ~1 Ma, geological and geophysical studies suggest a change in kinematic regimes from nearly orthogonal plate convergence to RL oblique-convergence. For example, geologic data from the Median Tectonic Line on Shikoku Island document a change from thrusting to right-lateral strike-slip faulting at about ~0.8 Ma (Mizuno, 1999; Sato et al., 2015). This change in kinematics and timing is consistent with previous ideas on the age of formation 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 Ma.
Taken together, the contrasting deformation histories along two plate boundaries with different strikes argue for a regional-scale change in plate motion of the PSP wrt Eurasia. Along the N-S trending Taiwan margin, the change resulted in a regime change from strike-slip to orthogonal shortening. In contrast, the plate motion change in Japan resulted in a regime change from orthogonal shortening to strike-slip or extension. These contrasting deformation histories from different margins suggest a significant and relatively recent (<1 Ma) CCW rotation in the motion of the Philippine Sea Plate relative to Eurasia. This conclusion is consistent with the plate reconstruction from Wu et al. (2016).
In Taiwan, before ~1 Ma, deformation in the orogenic core was characterized by highly oblique plate convergence partitioned into LL strike-slip shear and west-directed thrusting along a generally N-S trending plate boundary. These metamorphic fabrics were folded and rotated in the northern Backbone Range when spreading in the Okinawa Trough propagated into the Ilan Plain ~1 Ma, signaling an end to oblique plate convergence and initiation of WNW shortening. Structural and sedimentological data suggest rapid uplift and erosion starting ~1 Ma and low-temperature geochronologic data from the metamorphic core record acceleration in exhumation cooling <1 Ma, suggesting growth of the orogen at that time.
In SW Japan, before ~1 Ma, geological and geophysical studies suggest a change in kinematic regimes from nearly orthogonal plate convergence to RL oblique-convergence. For example, geologic data from the Median Tectonic Line on Shikoku Island document a change from thrusting to right-lateral strike-slip faulting at about ~0.8 Ma (Mizuno, 1999; Sato et al., 2015). This change in kinematics and timing is consistent with previous ideas on the age of formation 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 Ma.
Taken together, the contrasting deformation histories along two plate boundaries with different strikes argue for a regional-scale change in plate motion of the PSP wrt Eurasia. Along the N-S trending Taiwan margin, the change resulted in a regime change from strike-slip to orthogonal shortening. In contrast, the plate motion change in Japan resulted in a regime change from orthogonal shortening to strike-slip or extension. These contrasting deformation histories from different margins suggest a significant and relatively recent (<1 Ma) CCW rotation in the motion of the Philippine Sea Plate relative to Eurasia. This conclusion is consistent with the plate reconstruction from Wu et al. (2016).