VARIATIONS IN EXHUMATION LEVEL AND UPLIFT-RATE RELATED TO OBLIQUE-SLIP RAMP GEOMETRY, ALPINE FAULT, CENTRAL SOUTHERN ALPS, NEW ZEALAND

We assemble geophysical and geological data from the Southern Alps, New Zealand to explore the effect of increasing local convergence rate on a major oblique-slip fault on the topography, exhumation level, structure, and ramp behavior of rocks uplifted at the inboard side of a two-sided orogen. A central segment of the Southern Alps has a higher uplift rate, more relief, and a narrower orogenic width than surrounding regions. There, the east-tilted crust of the Pacific Plate exposes a narrower crustal section in plan view, and exhumes rocks that yield late Cenozoic (“reset”) thermochronometric ages that are younger than those in surrounding regions. Eleven new ⁴⁰Ar/³⁹Ar samples of hornblende from the hangingwall of the Alpine fault indicate that lower crustal rocks exhumed in the late Cenozoic from temperatures of >550°C are confined to an elongate, 20 x 5 km-wide region in the central Southern Alps. This culmination of deeply exhumed rocks is the only part of the present-day range that may have achieved exhumational steady-state. Elsewhere, the hangingwall of the Alpine fault retains mid Cenozoic and older (“inherited”) Ar/Ar and K-Ar ages for micas and hornblende. Subhorizontally layered, remnant plugs of the original crustal hangingwall ramp are apparently still preserved in the northern and southern Alps (the Alpine fault must have <70 km of dip-slip there). At 40-50° mean dip, the central Alpine fault is inferred to dip ~15-20° more steeply than a contiguous reach of the fault to the south. We infer that these two segments are connected by a lateral ramp striking parallel to the plate motion. The delaminated Pacific Plate has been “torn” above this lateral ramp, as is expressed at the surface by a diffuse zone of enhanced seismic activity and oblique-slip deformation. To the north, rocks are sharply flexed across the frontal ramp of the Alpine fault, resulting in the failure of steeply dipping shears. To the south, ramp deformation is more broadly distributed and does not result in this style of failure. The Ar/Ar age structure adjacent to the Alpine fault in the central region suggests that the basal ~3 km of the Pacific Plate has been “dragged” by reverse-slip ductile shearing. Apparently, the lower crust has experienced viscous coupling across a wide zone rather than focused slip in a narrow fault-weakened zone.