P. Upton, Geology Department, University of Otago, P.O. Box 56, Dunedin, phaedra.upton@stonebow.otago.ac.nz
Invited Conference Paper. Geological Society of New Zealand Annual Conference, Hamilton, 27th - 29th November, 2001.
Continental collision creates an orogen consisting of two oppositely facing wedges whose form are a result of rheology, erosion and the relative plate motion. The geometry of the main backbone of the Southern Alps is broadly predictable from the known plate vector, the asymmetric orographic erosion regime of the South Island and crustal rheology. To the east of the main ranges, the structure of the outboard fold and thrust belt is less well understood.
This study focuses on the Two Thumb Range and surrounding basins, the Mackenzie Basin, the Fairlie Basin and the Rangitata/Lake Heron Basin. A NE striking fault system runs for at least 60 km, from Lake Tekapo, through Coal River, Neutral and Forest Creeks to the Rangitata River. The N striking Fox Peak/Butler Creek system in Forest Creek intersects this system. In the central Two Thumb Range, both the NE and N striking systems consist of two closely spaced oppositely dipping thrust faults. In the Fairlie Basin the east-dipping strand of the Fox Peak Fault evolves into a broad antiformal fold in the south.
All faults have been active in the late Cenozoic and thrust older greywacke basement over Tertiary and younger rocks. Uplift was initiated on N to NNE striking faults with the prominent NE striking faults that now cross oblique to the trend of the range forming after the initiation of the N trending range. The intersection of the two sets of faults is currently responsible for the continuing uplift of the Two Thumb Range. Active structures within the northeastern Mackenzie Basin include the Irishman Creek Fault and the Coal River Fault, both of which strike NE-SW. Sub-bottom profiling of Lake Tekapo showed that recent movement on both the Irishman Creek Fault and the Coal River Fault decreases to zero within the lake basin, suggesting the existence of a N-S trending structure within the lake basin.
Fluid flow in an actively deforming system is coupled to the deformation. In the outboard region, we have investigated fluid movement and source using carbon and oxygen stable isotopes. Isotopic analyses of calcites hosted within fault zones reflect mixing of three parent fluids: meteoric, basinal and minor deeper rock-exchanged fluids, in the upper 3-4 km of the crust. These, combined with other results, imply the existence of two fluid regimes within the upper crust of the outboard region. First, leading to the vertical conductor is leakage of rock-exchanged fluid from beneath the brittle-ductile transition. This leakage is slow, continuous and buoyancy-driven. A second flow regime exists in the top few km of the crust. It is episodic, related to faulting, and leads to precipitation of fault-hosted calcite.
Unlike many outboard fold and thrust belts worldwide, which form in sedimentary
strata eroded from the growing main ranges, the eastern Southern Alps consist
of basement rocks, argillites and greywackes which were metamorphosed and then
uplifted and extended in the Mesozoic. Two sets of structures, formed during
Mesozoic extension and oriented NE-SW and NW-SE, create a pre-existing fabric
to the rocks forming the fold and thrust belt. Only the NE-SW set is utilised
by the current tectonic regime, reactivated as thrust faults, but it appears
that they were not favourable oriented until the N-S trending ranges had been
uplifted. Thus form of the outboard fold and thrust belt in the eastern Southern
Alps, and the related fluid flow regime, result from a combination of crustal
rheology, driving forces, and pre-existing structure.