Evidence for Block Rotations and Basal Shear in the World’s Fastest Slipping Continental Shear Zone in NW New Guinea

Colleen W. Stevens¹, Robert McCaffrey¹, Yehuda Bock², Joachim F. Genrich², Manuel Pubellier³, and Cecep Subarya⁴

¹Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180 USA

²Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, La Jolla, CA 92093 USA

³Laboratoire de Géologie, Ecole Normale Supérieure, 24, Rue Lhomond, F-75231Paris, Cedex 05 France

⁴BAKOSURTANAL, Jl. Raya Jakarta Bogor, km 46, Cibinong, Jawa Barat, Indonesia

Global Positioning System measurements from 1991 to 1997 reveal that the Bird’s Head region of eastern Indonesia moves at 75 to 80 mm/a relative to northern Australia along a continental shear zone that may be as wide as 300 kilometers. The estimated slip rate across the shear zone is twice as fast as any other found in a continent yet the shear zone contributes little to the seismic moment release in the region. Movements of points outside the deforming zone suggest simple shear across the zone while some points within the shear zone show significant velocities normal to the edges, consistent with deformation accommodated at least in part by rotating blocks rather than solely by sub-parallel strike-slip faults. A simple analysis of the possible forces acting on the Bird’s Head continental block suggests that it may be driven by basal drag from the Pacific plate sliding beneath it.