Upper crustal structure beneath the eastern Southern Alps and the Mackenzie Basin, New Zealand, derived from seismic reflection data

Abstract

A 65 km seismic reflection transect was shot in 1998 across Mackenzie Basin to Mount Cook Village, South Island, New Zealand, to provide a detailed image of the crustal structure in the central Southern Alps. The first 5 s (two-way time (TWT)) of data were processed separately, with a maximum offset of 14 km for each shot, to image the upper 12–15 km of the crust. Data were processed as one continuous section, although the line was physically segmented due to an area of relatively steep topography with no vehicular access. No major, continuous regional-scale features > 10 km are present in the data, but numerous 2–3 km scale reflections and discontinuities occur which are consistent with the known geology of monotonous greywacke sequences overlying schist. Strong, well-defined reflections mark the active Irishman Creek Fault and confirm it to be a southeasterly dipping reverse fault with c. 1300–1700 m of Late Cretaceous-Pleistocene sediments preserved in the footwall and an uplifted greywacke basement “high” in the hanging wall. Some evidence exists for active faults beneath latest Quaternary gravels at the Jollie valley and Tekapo River. Oppositely dipping reflections and discontinuities define a large, c. 15 km wavelength antiform beneath Tasman valley and Mount Cook that is imaged to 10 ± 2 km depths (3.5 s TWT). Two “end-member” interpretations are consistent with the seismic data observations, velocity models, and constraining features of exposed geology, and extend existing geological cross-sections to 10–15 km depth. One interpretation assumes imaged structures are primarily backthrusts developed in response to distributed Cenozoic deformation southeast of the Alpine Fault plate boundary, incorporating features observed in contemporary geodetic strain and numerical plate boundary models. The second interpretation assumes structures are mostly Mesozoic, either reactivated or preserved by late Cenozoic deformation. The main difference between the interpretative cross-sections is the degree to which active structures link into basal detachment and high-strain zones at depth. © 2003 Taylor & Francis Group, LLC.