Active Arc-Continent Collision - Earthquakes, Gravity-Anomalies, and Fault Kinematics in the Huon-Finisterre Collision Zone, Papua-New-Guinea

The Huon-Finisterre island are terrane is actively colliding with the north edge of the Australikan contient. The collision provides a rare opportunity to study continental accretion while it occurs. We examine the geometry and kinematics lof the collision by comparing earthquake source parameters to surface fault geometries and plate motions, and we constrain the forces active in the collision by comparing topographic loads to gravity anomalies. Waveform inversion is used to constrain focal mechanism for 21 shallow earthquakes the occurred between 1966 and 1992 (seismic moment 10(17) to 3 x 10(20) N m). Twelve earthquake show thrust faulting at 22-37 km depth. The largest thrust events are on the north side of the Huon Peninsula and are consistent with slip on the Ramu Markham thrust fault zone, the northeast dipping thrust fault system that bounds the Huon-Finisterre terrane. Thus much of the terrane's crust but little of its mantle is presently beihng added to the Australian continent. The large thrust earthquakes also reveal a plausible mechanism for the uplift of Pleistocene coral terrances on the north side of the Huon Peninsula. Bouguer gravity anomalies are too negative to allow simple regional compensation of topography and require large additional downward forces to depress the lower plate beneath the Huon Peninsula. With such forces, plate configurations are found that are consistent with observed gravity and basin geometry. Ohter earthquakes give evidence of deformation above and below the Ramu-Markham thrust system. Four thrust events, 22-27 km depth directly below the Ramu-Markham fault outcrop, are too deep to be part of a planar Ramu-Markham thrust system and may connect to the north dipping Highlands thrust system farther south. Two large strike-slip faulting earthquakes and their aftershockds, in 1970 and 1987, show faulting within the upper plate of the thrust system. The inferred fault planes show slip vectors parallel to those on nearby thrust faults, and may represent small offsets in the overriding plate. These faults, along with small normal-faulting earthquakes beneath the Huon-Finisterre ranges and a 25-degrees along-strike rotation of slip vectors, demonstrate the presence of along-strike extension of the accreting terrane and along-strike compression of the lower plate.