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

Taranaki Basin, New Zealand is located adjacent to the Australian-Pacific P late 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 bo th 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 We stern Platform outside the plate boundary deformation zone where no uplift nor exhumation has occurred and (2) the Eastern Mobile Belt, which lies ins ide the deformation zone and has been variably uplifted and exhumed. Exhuma tion magnitudes for sedimentary deposits of the basin are estimated from of fset 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 su rface 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 i n 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 paralle l 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 u nconformity, range from 850 to 3000 m; most of this exhumation occurred on discrete contractional structures in late Miocene to early Pliocene time, p robably 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 S W structural tilt of onshore strata. The regional and consistent pattern of exhumation in the eastern region contrasts with the discrete structure-rel ated exhumation in the southern region and is consistent with models of the rmal and flexural uplift.