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

摘要

The Chile fore arc at 37 degrees S-47 degrees S represents the coseismic deformation zone of the 1960 M-w 9.5 Valdivia earthquake. Here we report on the paleomagnetism of 43 Oligocene-Pleistocene volcanic sites from the fore-arc sliver between 38 degrees S and 42 degrees S. Sites were gathered west of the 1000km long Liquine-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 degrees S-41 degrees S, where ultrafast (>50 degrees/Myr) clockwise (CW) rotations occur within a 30km wide zone adjacent to the LOFZ. CCW rotation variability (even at close sites) and rapidity (>10 degrees/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 degrees 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 5Ma.