MODERN AND PALEO DISPLACEMENT FIELDS IN ARC-CONTINENT COLLISION, PAPUA NEW GUINEA

In understanding the displacement history of an arc system during and after its emplacement onto a continental margin, it is important to understand the sequence of events leading to its present situation. The Finisterre Arc-Continent collision is in the process of suturing an extinct volcanic arc in the New Britain forearc to the northern margin of the island of New Guinea. The kinematics of the collision is constrained by geodetic results from a large network of GPS sites spanning the region of the collision. The velocity field obtained from these data compare well to the paleo-displacement field of the Finisterre volcanic arc as determined from paleomagnetic data spanning the last 1-4 Myrs. The Finisterre arc is on the South Bismarck plate, which has been rotating as a rigid body at 8 degrees/Myr clockwise relative to the rest of PNG from at least 3 Ma to present. We suggest that the Finisterre Arc-Continent collision has had a significant influence on the kinematics of the South Bismarck plate, and has consequently affected the displacement field of active arcs on the South Bismarck plate. The rapid rotation of the South Bismarck plate appears to be the result of resistance to convergence due to arc-continent collision in the northwest, and subduction related forces that drive convergence (such as slab pull) at the New Britain trench to the southeast. This system of forces initiated a rapid clockwise torque on the South Bismarck plate at the onset of the Finisterre collision. The Finisterre collision began about 4 Ma, and paleomagnetic data indicate that rapid rotation of the Finisterre arc has been occurring since this time. The start of spreading at 3.5 Ma in the Manus basin (which is a consequence of South Bismarck plate rotation away from the Pacific plate) originated just following the Finisterre collision, as would be expected from this model.