Structure and fluid migration in a late Cenozoic duplex system forming the Main Divide in the central Southern Alps, New Zealand

Abstract

The Alpine Schist immediately west of the Main Divide of the Southern Alps is a west-dipping duplex system consisting of an imbricated stack of rock slabs, each c. 250-1000 m thick. The imbricated stack has low grade, little deformed pumpellyite-actinolite facies semischists at the base, overlain by progressively higher grade and more deformed schists. The structurally highest slab mapped consists of multiply-deformed biotite zone schist. The duplex lies on the hanging wall of the northwest-dipping Main Divide Fault Zone, which separates semischist from structurally underlying greywacke. Rock slabs are internally disrupted by faults subparallel to layering, and consist of lozenge-shaped blocks of 10-100 m. Fault zones separating rock slabs consist of <50 cm thick zones of weakly lithified cataclasite and soft gouge. Slickensides on fault surfaces plunge northwest. The duplex formed during late Cenozoic rise of the Southern Alps.Four generations of postmetamorphic veins cut rock slabs of the duplex system. The earliest veins contain quartz, chlorite, calcite, and sulphides and lie subparallel to foliation in the higher grade slabs only. A distinctive set of openspace-filling fissure veins in a lower greenschist facies slab contains euhedral quartz, adularia, bladed calcite, and chlorite. The whole duplex system is cut by steeply dipping shears and fractures containing quartz and ankerite infilling and breccia cementation. Late stage iron oxy-hydroxidecoated fractures, generally steeply dipping, cut all rock slabs. Pyrite and, locally, arsenopyrite accompanies quartz and ankerite veins in extensional sites of steeply dipping late stage fractures, and gold contents up to 850 ppb occur. This G96031Received 2 September 1996; accepted 9 May 1997 gold mineralisation occurred within c. 1 km of the surface and had epithermal affinities.Hydrothermal fluids mineralising the various fractures range in temperature between 350°C and near-ambient temperatures. The quartz-adularia-calcite veins formed at 0.5-2.5 km depth before duplex formation, and later veins formed at shallower levels after duplex formation. Vein carbonates have δ 18 O(SMOW) between 11.4 and26.1%o, and Δ 13 C(PDB) between -0.6 and -11.4%o. Calculated isotopic ratios for the mineralising fluids show little evidence for meteoric origin. The fluid may be meteoric water which has undergone isotopic exchange with host rocks, metamorphic water, or a mixture of these, and mineralisation occurred during rise of the fluids from depth.

Structure and neotectonics on the Main Divide, upper Wilberforce valley, mid Canterbury, New Zealand

“…Hence, our observations show that, in the upper Wilberforce valley, most of the structures are due to dip-slip motion on a southeast-directed reverse fault system. This sense of movement is similar to that observed across the MDFZ in the Mt Cook area (Findlay 1980;Cox & Findlay 1995;Cox et al 1997).…”

“…The MDFZ is a late Cenozoic structure (Cox & Findlay 1995), which is an important structural boundary between tectonically active inboard (near Alpine Fault) and outboard (east of the Main Divide) structural zones. The inboard zone is dominated by west-dipping slabs of schistose metasediments stacked up on oblique thrusts, which strike subparallel to the Alpine Fault and the MDFZ (Cox et al 1997). The outboard zone is dominated by steeply dipping faults striking oblique to the MDFZ, many of which are predominantly oblique strikeslip faults (Koons 1994;Templeton et al 1999).…”

Upper crustal structure beneath the eastern Southern Alps and the Mackenzie Basin, New Zealand, derived from seismic reflection data

“…The metamorphic peak in adjacent Alpine Schist, and corresponding development of pervasive foliation and isograds, also occurred before the modern phase of oblique convergence and uplift (Little et al 2002 and references therein). Although the Alpine Schist was tilted and structurally reorganised during the late Cenozoic (Cox et al 1997), schist fabrics contain only a minor penetrative neotectonic overprint (Little et al 2002).…”

Lead isotope constraints on the origin of Cenozoic orogenic gold systems in the Southern Alps and northwestern Otago, South Island, New Zealand

“…Metamorphic grade ranges from pumpellyite-actinolite near the Main Divide of the Southern Alps to amphibolite facies adjacent to the Alpine Fault. Metamorphic zones are steeply dipping and metamorphic boundaries are folded and faulted (Cox et al 1997;Craw 1998). A prominent fault zone, the Main Divide Fault Zone, generally forms the boundary between low grade Alpine Schist and non-schistose metagreywacke of the Torlesse composite terrane (Cox & Findlay 1995).…”

“…6 km. Deep-sourced crustally exchanged fluids are an important component of gold-bearing fluids in this hydrothermal system (Cox et al 1997;Craw et al 2002;Campbell et al 2004;Craw 2006).…”