Rotations in the actively colliding Finisterre Arc Terrane: paleomagnetic constraints on Plio-Pleistocene evolution of the South Bismarck microplate,northeastern Papua New Guinea

We report paleomagnetic results from 12 Plio-Pleistocene localities in the actively colliding Finisterre Arc Terrane of northeastern Papua New Guinea (PNG). Calcareous, hemipelagic cover rocks possess a stable, syn-collisiona l remagnetization indicating a clockwise rotation of the colliding terrane through about 40 degrees in post-Miocene time. A decrease in paleomagnetic declination anomalies as a function of along-strike distance in the Finiste rre Arc Terrane, analyzed by our preferred model of a linear remagnetizatio n and a migrating Euler pole, suggests an average rotation rate of 8 degree s Ma(-1), in good agreement with the instantaneous rate from global positio ning system geodesy. Thus, we propose that this rotation results from a coh erent, rigid-body rotation of the Finisterre Terrane rather than from seque ntial docking of independently colliding blocks of the terrane. Moreover, w e conclude that these paleomagnetic declinations result mainly from South B ismarck Plate motion, and not decoupled rotation of the crustal terrane ind ependent of the underlying lithosphere. We examine models of a syn-collisio nal remagnetization with both fixed and migrating Euler poles of South Bism arck/Australia plate relative motion, and suggest that the Euler pole descr ibing South Bismarck/Australia Plate motion has migrated southwestward to i ts present location on the collision suture in response to the propagating collision. This plate kinematic model agrees with the variability in depth of the seismogenic slab beneath the collision zone. Our best-fit model of p ole migration describes South Bismarck/Australia relative motion producing a highly oblique collision in its early stages, with the Finisterre Arc Ter rane converging along a left-lateral Ramu-Markham suture, gradually changin g to the nearly orthogonal convergence observed today. (C) 2000 Elsevier Sc ience B.V. All rights reserved.