Crustal structure and thermal anomalies of the Dunedin Region, South Island, New Zealand

Abstract

Abstract. Seven geophysical data sets are used to investigate a transect along the southeast coast of South Island, New Zealand. The specific focus of this study is the Dunedin volcanic center, which last produced volcanics at the surface 13-10 Myr ago. Wide-angle reflection/refraction seismic data along a two-dimensional profile reveal a lowvelocity lower crust and mantle beneath the Dunedin volcanic center. The low-velocity lower crust coincides with a highly reflective region on a nearby multichannel seismic line and may represent a hot, fluid-rich region of the crust. In addition, high mantle helium ratios measured in the Dunedin region suggest a current or recent mantle-melting event. High heat flow recorded in the Dunedin region is consistent with a hot body emplaced in the midcrust --•10 Myr ago (Miocene) whose heat is just reaching the surface today. Uplift of an Oligocene limestone horizon in the Canterbury basin can be explained by a buoyant load beneath the Dunedin volcanic center and low flexural rigidity of the lithosphere beneath the volcanic center during the Miocene. We interpret the data as revealing two separate thermal events beneath the Dunedin volcanic center, one during the Miocene, when volcanism was last occurring at the surface, and the other occurring currently. Active volcanism associated with the current mantle-melting event has yet to reach the surface. Geophysical Data and ModelsData used to develop and constrain models presented here are WAR/R seismic data, MCS data from industry and the SIGHT program, petrophysical measurements, and helium isotope and heat flow data. -1000 '• -, , , , , -, , , , , Figures 3a and 3b). On gather OBH 27, however, the Pn phase, which also passes through relatively fast lower crust (6.5-6.7 km s-•), can be flattened using an apparent velocity of---8.1 km s -• (Figure 3c Figures 5a and 5c) [Holt and Stem, 1994]. As SIGHT lines 4E and 25E approach Dunedin, this reflector is uplifted, and on SIGHT line 25E it is truncated at the surface (Figure 5c). It was presumably deposited horizontally, which makes it a useful marker horizon whose profile can be used to calibrate uplift. The age of uplift can be estimated from the age of the shallowest uplifted horizon on which there are onlapping sediments as seen on SIGHT line 4E (Figure 5a). Industry Well DataTwo industry wells either on, or close to, three of the SIGHT MCS lines (Figure lc) are used to date the horizon that we expect to be the Oligocene limestone and the shallowest uplifted horizon. We mapped both horizons from SIGHT line 4E to SIGHT lines 2E and 25E (Figure 5). On SIGHT lines 25E and 2E the deeper prominent reflector ties with the Oligocene limestone horizon in the well logs of the Galleon-1 well (Figures lc and 5c (Figures 5b and 5c).

Extensional and magmatic nature of the Campbell Plateau and Great South Basin from deep crustal studies

“…The basement surface was interpolated between these constraints. The crustal thickness in the gravity models is only loosely constrained at either end of the model apart from the seismic model (Godfrey et al, 2001;Mortimer et al, 2002). This latter line lies within the near-shore zone of major northwest-southeast crustal thinning (Fig.…”

“…4) limiting its value as a constraint. The velocity model of Godfrey et al (2001) has a crustal thickness of ∼30 km at the intersection with line AWI-20030001. Unfortunately, the gravity model in Mortimer et al (2002), along the SESI line, has omitted the Moho.…”

Geophysical structure of the Southern Alps Orogen, South Island, New Zealand

“…Further south, the sediments thin and the crustal section becomes more complex (Mortimer et al, 2002) with a conspicuous, highly reflective lower crust of restricted lateral extent near Dunedin that is coincident with low lower crustal and upper mantle seismic velocities (Godfrey et al, 2001).…”

“…In 1996, marine MCS reflection data were recorded to 16 s twt in association with the shots for the off-onshore experiments (Greenroyd et al, 2003;Mortimer et al, 2002;Godfrey et al, 2001;Melhuish et al, 2004;Davey, 2005). In 1998 CDP data were recorded overland along the central part of T2 and the west coast part of T1 .…”

“…The parameterization that is adjusted to give the smoothest model with minimum residuals. T2 (Scherwath et al, 2003) and tie lines T3 off the east coast (Godfrey et al, 2001) and Line 3w of the west coast (Melhuish et al, 2004) have been modelled using the forward modelling and inversion code of Zelt and Smith (1992). This method uses sparse parameterization and minimum perturbation from the starting model to minimise residuals.…”