The Liquiñe-Ofqui fault zone (LOFZ) is a major~1000 km long dextral shear zone of southern Chile, likely related to strain partitioning of Nazca Plate oblique convergence with South America. To understand block rotation pattern along the LOFZ, we paleomagnetically sampled 55 sites (553 samples) between 38°S and 41°S. We gathered Oligocene to Pleistocene volcanics and Miocene granites at a maximum distance of 20 km from the LOFZ, and at both sides of it. Rotations with respect to South America, evaluated for 36 successful sites, show that crust around the LOFZ is fragmented in small blocks,~1 to 10 km in size. While some blocks (at both fault edges) undergo very large 150°-170°rotations, others do not rotate, even adjacent to fault walls. We infer that rotations affected equidimensional blocks, while elongated crust slivers were translated subparallel to the LOFZ, without rotating. Rotation pattern across the LOFZ is markedly asymmetric. East of the fault and adjacent to it, rotations are up to 150°-170°clockwise, and fade out~10 km east of fault. These data support a quasi-continuous crust kinematics, characterized by small rigid blocks drag by the underlying ductile crust flow, and imply 120 km of total fault offset. Conversely, crust west of the LOFZ is cut by seismically active NW-SE sinistral antithetic faults, and yields counterclockwise rotations up to 170°at 8-10 km from LOFZ, besides the unrotated blocks. Further data from the Chile fore arc are needed to understand block rotation kinematics and plate dynamics west of the LOFZ.

A wealth of paleomagnetic data from Yunnan (China) showed in the past a predominant post-Cretaceous clockwise (CW) rotation pattern, mostly explained invoking huge (hundreds of kilometer wide) blocks, laterally escaping (and/or rotating) due to India-Asia collision, separated by major strike-slip shear zones. Here we report on the paleomagnetism of the outcrops close to the Gaoligong dextral shear zone. Fifty paleomagnetic sites (503 samples) were sampled at variable distances (up to~25 km) from mylonites exposed along the fault. Eighteen Jurassic-Cretaceous red bed sites yield systematic CW rotations with respect to Eurasia that peak at maximum (176°) close to the fault, and progressively decrease moving eastward, up to be virtually annulled~20 km east of mylonite contact. West of the fault, 15 Pliocene-Holocene sites from the Tengchong volcanic field do not rotate. Thus, our data show that Gaoligong shear zone activity yielded significant CW rotations that were likely coeval to the main Oligo-Miocene episodes of dextral fault shear. The Gaoligong zone rotation pattern conforms to a quasi-continuous crust kinematic model and shows blocks of ≤1 km size close to the fault that enlarge moving eastward. Rotation values and width of the rotated-deformed zone translate to a 230-290 km Gaoligong shear zone dextral offset. Our work shows that fault shear plays a significant role for Indochina CW rotation occurrence. However, significant rotations at distances >30 km from main faults were also documented, so that additional tectonics-whose relative relevance has not been elucidated yet-must contribute as well to the rotation pattern.Citation:

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.

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