Geometry and kinematics of shear zones formed during continental extension in eastern Papua New Guinea

Abstract

The D'Entrecasteaux Islands, eastern Papua New Guinea, lie in an area of continental extension that has been active since the mid-Miocene. During the last 4 Ma the metamorphic basement rocks composing most of the islands have been uplifted and tectonically exhumed from depths of approximately 35 km. Tectonic exhumation appears to have been controlled by deformation in broad (kilometre-scale) mylonitic shear zones. A progressive evolution is documented in the shear zones, characterized by: increasing localized deformation; a change from dominantly ductile to dominantly brittle processes; and decreasing metamorphic grade (i.e. retrograde metamorphism). The evolution is believed to be the result of uplift and cooling of the basement accompanying shear zone movement. The kinematic history of the shear zones is complex. A number of sinistrally offset extensional detachment zones separate a multiply deformed, high-grade metamorphic basement from a cover of largely undeformed ultramafic and mafic cover rocks. The detachment zones are connected by transverse shear zones. These transverse zones appear to act as sinistral strike-slip zones which also accommodate transfer motion between the detachment zones.