Late Neogene exhumation patterns in Taranaki Basin (New Zealand): Evidence from offset porosity-depth trends

Taranaki Basin, New Zealand is located adjacent to the Australian-Pacific Plate boundary where the tectonic regime changes from dominantly subduction-related to the north to transpression-related along the Alpine Fault to the south. During the Neogene, burial and exhumation varied extensively, in both time and space, in response to subsidence and uplift along this evolving plate boundary zone. The basin can be divided into two regions: (1) the Western Platform outside the plate boundary deformation zone where no uplift nor exhumation has occurred and (2) the Eastern Mobile Belt, which lies inside the deformation zone and has been variably uplifted and exhumed. Exhumation magnitudes for sedimentary deposits of the basin are estimated from offset porosity-depth trends. The analysis is based on correlating sonic log travel times with compensated density logs, permitting sonic travel time to be used as a proxy for porosity. Twelve Western Platform wells are used to define a standard exponential porosity-depth trend with an extrapolated surface porosity of 50% and an exponential decay factor of 2265 m, valid for mudstone/shale sections spanning a depth range of 300-3000 m. This curve is a calibration curve against which offset porosity-depth Vends from wells in exhumed regions can be compared to determine apparent exhumation amounts. Porosity-depth trends for 43 Eastern Mobile Belt wells are offset 0-2800 m shallower relative to the Western Platform trend but generally are parallel to it. In the southern region of the Eastern Mobile Belt, net exhumation amounts, which are the sum of the porosity-depth Vend offset and depth to unconformity, range from 850 to 3000 m; most of this exhumation occurred on discrete contractional structures in late Miocene to early Pliocene time, probably associated with an increased rate of convergence across the Alpine Fault system to the south. In the eastern region of the Eastern Mobile Belt, net exhumation progressively increases from 0 m in the south to 1800 m in the northeast part of the basin, consistent with the 2 degrees-4 degrees SW structural tilt of onshore strata. The regional and consistent pattern of exhumation in the eastern region contrasts with the discrete structure-related exhumation in the southern region and is consistent with models of thermal and flexural uplift.