Quaternary structural and paleo-environmental evolution of the Alpine Fault near Haast, New Zealand, from 2D seismic reflection and gravity data

ABSTRACTThe geometry of the upper-crustal portion of the Alpine Fault plate boundary on New Zealand’s South Island, is poorly constrained where it cuts through the sedimentary units of the Westland coastal plain. Two seismic reflection and gravity lines are presented here to better image the upper km of the fault beneath the most recent fault rupture scarps south of the Haast River. Analysis of the seismic lines suggests that multiple fault strands have cut across post-glacial sedimentary strata that overlie the glacially eroded basement. Total combined fault offsets of 39 m at the Haast River and 33 m at the Turnbull River over <17 ka is consistent with previously determined uplift rates. Dip on the fault strands is interpreted to be between ∼53 and ∼76° SE. 2D gravity profiles constrain the interpretation of the interface between sedimentary units and basement, which has considerable 3D variability resulting from glacial erosion processes.