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Variations in exhumation level and uplift rate along the obliqu-slip Alpine fault, central Southern Alps, New Zealand

Timothy A. Little^,1, Simon Cox^,2, Julie K. Vry^,3 and Geoffrey Batt^#,4

¹ School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand
² Institute of Geological and Nuclear Sciences, Private Bag 1930, Dunedin, New Zealand
³ School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand
⁴ Geology Department, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK

We use geophysical and geological data from the Southern Alps to explore the relationship between plate motions and crustal structure on the geomorphology, exhumation state, and deformation style of rocks uplifted along a major obliqu-slip fault. A 50-km-long segment of the Southern Alps has a higher uplift rate, more relief, deeper exhumation, and a narrower width than surrounding regions. There, the delaminated, east-tilted crust of the Pacific Plate yields the youngest, late Cenozoic thermochronometric ages. Contours for fission-track, Ar/Ar and K-Ar ages on several different minerals define an asymmetrically nested pattern of ages that increase away from the western side of the central Southern Alps. Eleven new ⁴⁰Ar/³⁹Ar samples of hornblende from the hanging wall of the Alpine fault indicate that lower crustal rocks exhumed from >500 °C in the late Cenozoic are confined to a 20-km-long culmination at the southern end of the central Southern Alps. Ages as low as 3–5 Ma imply tim-integrated vertical exhumation rates as high as 6–9 mm/yr. This is the only part of this 5–8 Ma range that may have achieved exhumational steady state. Remnant plugs of the original crustal hanging wall ramp are apparently preserved outside the central Alps, implying <70 km of fault convergence there. ⁴⁰Ar/³⁹Ar age trends for hornblende near the Alpine fault suggest that horizontal surfaces in the lower crust in the Pacific Plate have been overturned by revers-slip ductile shearing across a zone of distributed deformation that extends 2 km beyond the 1-km-thick, basal mylonite zone. At the broadest, orogen scale, higher uplift rates throughout the central Southern Alps may be related to a rheologically controlled increase in the convergent velocity of points to the east of the Alpine fault, associated with a strengthening of the Pacific crust. At a more local scale, maximum rates of uplift are inferred to occur near Franz-Josef and Fox Glaciers because the Alpine fault steepens at depth. Structural data suggests that its footwall ramp is curved, and that the fault's dip steepens by 15–20° relative to its attitude farther to the south. This 10–20-km-long restraining bend may enhance local rates of rock uplift near Franz Josef and Fox Glaciers. Contemporary normal stress and shear resistance may also be increased on this part of the Alpine fault, helping to explain the central region's quiet historic seismicity and apparently strongly locked nature.

Key Words: oblique convergence • oblique ramping • exhumation • thermochronology • Southern Alps • Alpine fault

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