Poster presented at American Geophysical Union 2002 Fall Meeting, San Francisco
Across the highest uplift region of the central Southern Alps
(Figure 1, Figure
2, Figure 3),
New Zealand, four zones with characteristic mechanical and fluid flow properties
have been predicted by identifying the topographic, thermal and deformation
forces that drive fluid flow (Figure 1, Koons
et al. 1998).
1 -the inboard region, dominated by high geothermal gradients and topographically
driven meteoric fluid.
2 -the Main Divide, a region of minor extension within the orogen.
3 - the crustal root where strain-induced metamorphism generates fluid.
4 - the outboard region of oblique thrusting in which surface flow dominates.
However, field and geochemical observations combined with magnetotelluric measurements
from across the Southern Alps (Figure 4,
Wannamaker et al. in press, Upton et al. in review-a) produce observations and
interpretations that are apparently at variance with predictions.
At the plate boundary, fluids and veins are dominated by a meteoric fluid signal, overprinting a deeper rock-exchanged signature, but the concentration of veins, hot spring activity and a near vertical conductive signal occur not on the Alpine Fault, but rather to the east of it. The Main Divide region, which shows evidence of deep fluid egress, is relatively non-conductive. East of the Main Divide, a near vertical conductor reaches the surface coincident with a major shallow-dipping backthrust, the Forest Creek Fault. Stable isotopic analyses of fault-hosted calcites range from 10‰ to 25‰ d18O and 0‰ to -28‰ d13C, reflecting mixing of three parent fluids: meteoric, basinal and minor deeper rock-exchanged fluids at temperatures of between 10°C and 90°C in the upper 3-4 km of the crust (Upton et al. in review-a).
Active faulting in the region is dominated by NE- and N-striking,
oppositely- dipping thrust fault pairs (Figure 5).
The thrust faults bound the series of ranges and intermontane basins that make
up the outboard (Upton et al. in review-b).
Extensive calcite veining, hematite alteration (Figure
5) and clay-rich fault gouge all point to fluid flow through these fault
zones.
Fault calcites have a predominately near-surface isotopic signature made up of meteoric and basinal fluids (Figure 6) with a small component of deeper fluid. These results suggest mixing of the surface fluids in the top 3-4 km, with limited imput from deep-sourced fluids travelling up the fault zone (Figure. 7).
The vertical conductor reaches the surface coincident with the Forest Creek Fault system (Figure 4).
The Main Divide region is cut by steeply dipping shears and
fractures which are mineralised in extensional sites (Cox et al. 1997) (Figure
8).
Despite high rainfall and very steep topographic gradients, there is no evidence
for high fluxes of meteoric water being as important in the Main Divide region
as it is in the outboard or inboard regions. The isotopic signal of fault rocks
is dominated by a rock-exchanged signature (Cox et al. 1997, Becker et al. 2000).
Widespread hydrothermal activity including Au veining has occurred along many structures in the Main Divide region (Cox et al. 1997, Koons et al. 1998, Becker et al. 2000).
Elevated arsenic levels in some host greywackes and argillites suggest that hydrothermal activity pervaded the host rock as well as forming veins (Becker et al. 2000).
The inboard region is dominated by the Alpine Fault, the boundary between material of the Australian and the Pacific plates.
A series of warm springs are located from 1 to 20 km distance from the trace of the Alpine Fault (Gair 1967) (Figure 9). These have a meteoric isotopic signature. Along the MT transect, warm springs are found at c. 10 km from the Alpine Fault trace.
Post-metamorphic veins have a rock-exchanged isotopic signature overprinted by a meteoric signature. Near-surface veins have a strong meteoric signature.
In order to identify regions undergoing volume change during deformation, we modelled the three-dimensional mechanics of a deforming orogen, representing Australian/Pacific plate collision, in which a two layer crust collides obliquely with an elastic block (Figure 10). The imposed relative motion has a ratio of boundary-parallel velocity to boundary-normal velocity of 4:1 in a dextral sense. The Pacific block is viscously coupled at the base to the underlying layer. We monitor the strain through components of contraction and rotation where contraction is accommodated by reverse movement (Figure 11).
Evidence from warm springs, veining and the MT interpretation suggest that upper crustal fluid flow is bouyancy-driven and occurs along smaller structures, rather than along the Alpine Fault, the major structure in the inboard region. The MT interpretation suggests some connectivity between upper and lower crustal fluid. However, upper crustal fluid is dominated by a meteoric signature.
Above 8 km the region is resistive and therefore thought to be non-conductive or contain poorly connected fluid, or both. Flow within this region may be episodic, with fluid being driven upward during short-term structural events. Such short-term events are unlikely to be detected by a single MT survey. Alternatively, interconnected fluid within the upper crust may not be continuous along strike of the mountains and the two-dimensional MT did not coincide spatially with a region of interconnected fluid.
Isotopically, fluid in this region reflects dominance of shallow
fluids with minor deep fluid input. The MT signal reaches the surface coincident
with an intersection zone of several active faults, which dip moderately (30°
to 60°) and/or strike perpendicular to the interpreted strike of the high-conductivity
signal. The vertical MT signal cannot be related to any one of these structures
below 3-4 kms depth. A secondary conductor dips west to a depth of c. 4 km and
probably relates to the west Forest Creek Fault. We suggest that the near vertical
conductor located beneath Forest Creek in the outboard region represents fluid
presence resulting from continuous leakage from beneath the brittle-ductile
transition. Rates of leakage are so slow that the resultant small volumes of
deep-sourced fluid reaching the surface are diluted by more rapid and voluminous
near-surface fluid flow.
Phil Wannamaker and other members of the magnetotelluric working group are thanked for permission to use their MT results in figure 4. Many conversations with Grant Caldwell have helped to fine tune the ideas presented here. This research was supported financially by the University of Otago, the NZ Public Good Science Fund (Contract UO0813) and the Foundation of Research, Science and Technology (Contract UO0818). The Department of Conservation and local landowners kindly gave permission to conduct research in the area. Sonia Mellish, Brad Eales, Eric Nelson, and Barbara Nevins ably provided field assistance. GeographX is acknowledged for the shaded relief map used in figure 1.
BECKER, J.A., CRAW, D., HORTON, T., & CHAMBERLAIN, C.P. (2000) Gold mineralisation near the Main Divide, upper Wilberforce valley, Southern Alps, New Zealand. New Zealand Journal of Geology and Geophysics, 43, 199-215.
COX, S.C., CRAW, D., & CHAMBERLAIN, C.P. (1997) Structure and fluid migration in a late Cenozoic duplex system forming the Main Divide in the central Southern Alps, New Zealand. New Zealand Journal of Geology and Geophysics, 40, 359-373.
CRAW, D. (1997) Fluid inclusion evidence for geothermal structure beneath the Southern Alps, New Zealand. New Zealand Journal of Geology and Geophysics, 40, 43-52.
GAIR, H.S., 1967, Geological map of New Zealand, 1:250000 scale, Sheet 20-Mt Cook: New Zealand Geological Survey.
KOONS, P.O. (1989) The topographic evolution of collision mountain belts: A
numerical look at the Southern Alps, New Zealand. American Journal of Science,
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UPTON, P., CRAW, D., CALDWELL, G., KOONS, P.O., JAMES, Z., WANNAMAKER, P., JIRACEK, G.R., & CHAMBERLAIN, C.P. (in review - a) Upper crustal fluid flow in the Outboard Region of the Southern Alps, New Zealand. Geofluids.
UPTON, P., CRAW, D., JAMES, Z., & KOONS, P.O. (in review - b) Structure and neotectonics of the Southern Two Thumb Range, mid-Canterbury, New Zealand. New Zealand Journal of Geology and Geophysics.
WANNAMAKER, P.E., JIRACEK, G.R., STODT, J.A., CALDWELL, T.G., GONZALEZ, V.M., MCKNIGHT, J.D., & PORTER, A.D. (in press) Fluid Generation and Pathways Beneath an Active Compressional Orogen, the New Zealand Southern Alps, Inferred from Magnetotelluric Data. Journal of Geophysical Research.
