Paleomagnetic rotation pattern of the southern Chile fore‐arc sliver (38°S–42°S): A new tool to evaluate plate locking along subduction zones

Abstract

The Chile fore arc at 37°S–47°S represents the coseismic deformation zone of the 1960 Mw 9.5 Valdivia earthquake. Here we report on the paleomagnetism of 43 Oligocene‐Pleistocene volcanic sites from the fore‐arc sliver between 38°S and 42°S. Sites were gathered west of the 1000 km long Liquiñe‐Ofqui dextral fault zone (LOFZ) that represents the eastern fore‐arc sliver boundary. Nineteen reliable sites reveal that the fore arc is characterized by counterclockwise (CCW) rotations of variable magnitude, except at 40°S–41°S, where ultrafast (>50°/Myr) clockwise (CW) rotations occur within a 30 km wide zone adjacent to the LOFZ. CCW rotation variability (even at close sites) and rapidity (>10°/Myr) suggest that the observed block rotation pattern is related to NW‐SE seismically active sinistral faults crosscutting the whole fore arc. According to previously published data, CW rotations up to 170° also occur east of the LOFZ and have been related to ongoing LOFZ shear. We suggest that the occurrence and width of the eastern fore‐arc sliver undergoing CW rotations is a function of plate coupling along the subduction zone interface. Zones of high coupling enhance stress normal to the LOFZ, induce high LOFZ strength, and yield a wide deformation zone characterized by CW rotations. Conversely, low coupling imply a weak LOFZ, a lack of CW rotations, and a fore arc entirely dominated by CCW rotations related to sinistral fault kinematics. Our locking inferences are in good agreement with those recently derived by GPS analysis and indicate that seismotectonic segment coupling has remained virtually unchanged during the last 5 Ma.

Paleomagnetic Evidence for 25–15 Ma Crust Fragmentation of North Indochina (23–26°N): Consequence of Collision With Greater India NE Corner?

“…The highly rotated sites from the Lanping domain edges suggest that strike‐slip deformation along the ARRSZ and Chongshan shear zones (constrained mainly at 32–15 Ma) resulted in the rotation of small kilometer size blocks up to a distance of at least 12 km from the shear zone (Figure ). Such strike‐slip‐related crustal block rotations have been widely documented elsewhere (e.g., Beck, ; Hernandez‐Moreno et al, , ; Kimura et al, , ; Piper et al, ; Randall et al, ; Ron et al, ). Within north Indochina, Pellegrino et al () showed that crust east of the Gaoligong shear zone was broken in ≥1 km wide crustal blocks that experienced CW rotations of about 180° and that rotations ended at a ~20 km distance from the shear zone.…”

Tectonics and Paleomagnetic Rotation Pattern of Yunnan (24°N–25°N, China): Gaoligong Fault Shear Versus Megablock Drift

“…The described CW paleomagnetic data seem to support a “quasi‐continuous” crust deformation model of strike‐slip fault zones, where the upper brittle crust is broken into small rigid blocks with sizes smaller than the shear zone width, the rotation is CW (CCW) in regions of dextral (sinistral) shear and gradually increases getting closer to the fault, reaching values >90° (Figure c; Hernandez‐Moreno et al, , ; McKenzie & Jackson, ; Nelson & Jones, ; Randall, Lamb, & Mac Niocaill, ; Sonder et al, ).…”

“…To the west of the fault, Pliocene‐Holocene basalts overall show null rotations, consistently with results from the Pliocene whitish silt site Yun07. As discussed above, PSV from basalt sites is likely not averaged out, and we may expect a geomagnetic declination scatter that mostly ranges within ±20° at such latitudes (Hernandez‐Moreno et al, , and discussion therein). Thus, a bias can exist on the average null rotation obtained from volcanic data, but such error is expected to be much smaller than the large CW rotations (up to 176°) observed in Mesozoic red beds.…”

“…Each block rotates rigidly and is bounded by a major shear zone (Gao et al, ; S. Li, Advokaat, et al, ; Tanaka et al, ; Y. B. Tong et al, ; G. Wang et al, ; Zhao et al, ); c) quasi‐continuous block rotation model (e.g., Lamb, ; Randall et al, ; Sonder et al, ; see also Hernandez‐Moreno et al, , ) for the Tengchong and Baoshan blocks at both edges of the Gaoligong shear zone, based on paleomagnetic rotations reported in this work. Rotations of small blocks of Jurassic‐Cretaceous red beds east of the Gaoligong shear zone (Baoshan block) are caused uniquely by dextral shear along the Gaoligong shear zone.…”

Understanding Paleomagnetic Rotations in Sicily: Thrust Versus Strike‐Slip Tectonics

“…On the other hand, strike‐slip faults yield rotations that peak at maximum values along fault trace and decrease—progressively or stepwise—moving away from it (Kimura et al, ; Sonder et al, ). The width of the rotation zone straddling the fault varies from hundreds of meters to few tens of kilometers and is a function of fault length, total displacement, locking of the fault zone, and crust rheology (Hernandez‐Moreno et al, , ; Kimura et al, ; Lamb, ; McKenzie & Jackson, ; Randall et al, ; Sonder et al, ).…”