Kinematics of oblique collision and ramping inferred from microstructures and strain in middle crustal rocks, central Southern Alps, New Zealand

The hanging wall of the Alpine Fault near Franz Josef Glacier has been exhu med during the past similar to2-3 m.y. providing a sample of the ductilely deformed middle crust of a modem obliquely convergent orogen. Presently exp osed rocks of the Pacific Plate are inferred to have undergone several phas es of ductile deformation as they moved westward above a mid-crustal detach ment. Initially they were transpressed across the outboard part of the orog en, resulting in oblate fabrics with a down-dip stretch. Later, they encoun tered the Alpine Fault, experiencing an oblique-slip backshearing on vertic al planes. This escalator-like deformation tilted and thinned the incoming crust onto that crustal-scale oblique ramp. This style of hanging wall defo rmation may affect only the most rapidly uplifting, central part of the Sou thern Alps because of the low flexural rigidity of the crust in that region and its displacement over a relatively sharp ramp-angle at depth. A 3D tra nspressive flow affected mylonites locally near the fault, but their shear direction remained parallel to plate motion, ruling out ductile 'extrusion' as an important process in this orogen. Outside the mylonite zone, late Ce nozoic shortening is inferred to be modest (30-40%), as measured from defor mation of younger biotite grains. Oblique collision is dominated by transla tion on the Alpine Fault, and rocks migrate rapidly through the deforming z one, preventing the accumulation of large finite strains. Transpression may play a minor role in oblique collision. (C) 2001 Elsevier Science Ltd. All rights reserved.