TECTONICS OF CONCAVE CONTINENTAL MARGINS: A COMPARISON OF THE CASCADIA AND CENTRAL ANDES FOREARCS

Most convergent continental margins are convex towards the subducting oceanic plate but in Cascadia and in the Central Andes of northern Chile and southern Peru, the margin is concave towards the subducted plate. Except for the curvature, there is little in common between these two areas: The Central Andes is an eroding continental margin in a hyperarid environment with a barren trench, dominated by normal faulting. The Cascades are an accreting margin with extensive trench fill and numerous thrusts in the accretionary prism. Convergence rate in the Andean case is currently 6.5 cm but most forearc structures began to develop during considerably faster rates; convergence rates in Cascadia are more variable but slower than in the central Andes. Finally, both the volcanic arc and Wadati-Benioff Zone seismicity are much more robustly developed in the Central Andes than in that Cascades. These differences make it all the more likely that the similarities are due, at least in part, to the curvature. Most significantly, both forearcs show evidence of finite, margin-parallel shortening (e.g., the Seattle and Tacoma faults in Cascadia and the E to ENE striking reverse faults of northern Chile). In Cascadia, where GPS stations are dense enough, instantaneous margin parallel shortening also occurs and the strain is contractional. GPS data from both areas, when inverted for vertical axis rotation rate, show a change from clockwise to counterclockwise rotation across the axis of curvature, a pattern matched by paleomagnetic rotations in the Central Andes. We suggest that margin-parallel shortening and opposing senses of vertical axis rotation are characteristic of concave convergent. Both of these effects are predicted by elastic models of locked, concave convergent plate boundaries.