Combining Tectonics and Thermal Fields in Taranaki Basin, New Zealand

Abstract

The Taranaki Basin, New Zealand, is used to illustrate the relationships between tectonics and thermal fields in an active sedimentary basin. The Cenozoic basin lies 400 km west of the Pacific-Australian subduction plate boundary and 200 km above the subducting Pacific plate. The western half of the basin (Western Platform) has not been affected by Miocene to Recent plate boundary deformation whereas the eastern half (Eastern Mobile Belt) has experienced at various times rapid burial, uplift and denudation, structural inversion, and igneous intrusion. This regional separation permits calibration of the various tectonic and thermal interplays, a feature not afforded most tectonically active sedimentary basins. Present-day surface heat-flow values average 57 mWnr −2 in both the western and eastern regions. Surface heat flow is remarkably constant in the Western Platform, but varies by more than 20 mWm −2 in the Eastern Mobile Belt. Much of this surface heat-flow variation is reconciled with detailed basin analysis and thermal modeling that accounts for burial and exhumation, both of which locally exceed 2 km in time spans of a few million years. The largest lateral surface heat-flow variation in the Eastern Mobile Belt is a south to north increase from 50 to 75 mWm −2 for a lateral distance of less than 50 km. The high heat flow is spatially coincident with the onshore Taranaki volcanoes suggesting a causal relationship. Apatite fission track and vitrinite reflectance data and modeling, in combination with detailed basin analysis, show that the heat flow has been elevated for <1 My and probably is associated with upper crustal intrusion. Even though we can account for most of the obvious anomalies in the surface heat-flow distribution, hidden thermal anomalies exist that are not noticeable at the surface. After accounting for the obvious burial, exhumation, igneous, and thrust-related transient thermal effects, analysis of modeled basal heat flow (applied at 40 km depth) reveals significant anomalies in the thermal budget. Basal heat flow varies by more than a factor of two between wells in the Eastern Mobile Belt and Western Platform regions, even where surface heat flow is the same. Our modeling shows that a combination of low heat-producing crust and the heat sink effects of crustal thickening below the basin in the Eastern Mobile Belt can account for the thermal budget anomalies, which is consistent with gravity and magnetic data. A significant aspect of this study is that some thermal anomalies may be revealed only after combining geothermics and detailed basin analysis.