GEOMETRY AND KINEMATICS OF SHEAR ZONES FORMED DURING CONTINENTAL EXTENSION IN EASTERN PAPUA-NEW-GUINEA

The D'Entrecasteaux Islands, eastern Papua New Guinea. lie in an area of continental extension that has been active since the mid-Miocene. D uring the last 4 Ma the metamorphic basement rocks composing most of t he islands have been uplifted and tectonically exhumed from depths of approximately 35 km. Tectonic exhumation appears to have been controll ed 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 resul t of uplift and cooling of the basement accompanying shear zone moveme nt. The kinematic history of the shear zones is complex. A number of s inistrally offset extensional detachment zones separate a multiply def ormed, high-grade metamorphic basement from a cover of largely undefor med ultramafic and mafic cover rocks. The detachment zones are connect ed by transverse shear zones. These transverse zones appear to act as sinistral strike-slip zones which also accommodate transfer motion bet ween the detachment zones.