Mid-Cenozoic Siberia fault zone (SFZ) in northwest Otago, New Zealand and a study of greenschist facies metamorphism in the Haast schist

White, Stephen, 1942-

The Siberia Fault Zone (SFZ) is a 35-40 km long, NE-striking, steeply NW-dipping structure extending from the head of the Wilkin River to Haast Pass in northwest Otago, New Zealand. Country rocks are mainly Mesozoic greenschist facies Haast Schist of both Torlesse terrane and Aspiring lithological association affinities. Mapping in the schist identified a progressive east to west increase in metamorphic grade, from chlorite zone to a maximum of garnet-peristerite zone immediately SE of the SFZ. Metamorphic grade of rocks on the NW side of the fault is equivalent to biotite zone. Juxtaposition of different metamorphic grade rocks across the fault is first order evidence for a net SE-side-up component of displacement. The SFZ is not a direct continuation of the northerly trending Moonlight Fault (MF), as previously supposed, but it is appropriately regarded as part of the larger Moonlight Fault System (MFS). The MF itself was re-interpreted as contiguous with the post-metamorphic Castalia Antiform. The SFZ consists of two main, subparallel sections linked by a leftstepping jog, within which several shorter fault strands are arranged in a pinnate-like geometry. Splitting and north eastward attenuation of the fault zone near Haast Pass coincides with an increase in the number of subsidiary structures in the area, in particular a suite of E-W striking dextral oblique-slip faults which partly facilitate right-stepping of the SFZ, through the Keystone, to link with the dextral strike-slip Haast River Fault (HRF). Net-slip on the SFZ was dextral oblique. An estimated 3.5 ± 1.5 km component of dextral strike-slip, based on offset of the biotite isograd in the Blue River-Howe Creek area, is a minimum because the total displacement also includes a component of SE-side-up dip-slip displacement. This sense of throw produces a sinistral separation of the isograds which partly counteracts the dextral separation due to dextral strike-slip. Minimum throw on the northeastern section of the fault zone was 1-3 km. Slip on the fault zone was modelled as rotational, with the magnitude of the dip-slip component increasing to the SW at the expense of the strike-slip component. Throw on the SW section of the fault could be > 5 km. Timing of activity on the ~,FZ was constrained, by mutual cross-cutting relationships with the carbonatitic Alpine lamprophyre dyke swarm, to between 28-20 Ma. There are no constraints on whether the fault zone was active before ~30 Ma, but all dykes involved with the SFZ are deformed to a greater or lesser degree indicating that movement probably continued after 20 Ma. Despite the very strong topographic and geomorphologic expression of the SFZ, there is no evidence for Recent faulting, and the fault zone is unlikely to have been active since the late Miocene (i.e. after ~5 Ma). The SFZ and HRF, together with structural lineaments defined by the NE-SW flowing Clarke and Landsborough Rivers, and the Main Divide Fault Zone (MDFZ) near Mt Cook, were interpreted to comprise a related series of right-stepping fault zones which exert a structural control on the position of the Main Divide of the Southern Alps. 11 The overall dextral oblique-slip character of the SFZ means that the central left-stepping section of the fault zone was principally an anti-dilational jog, whereas the northeastern rightstepping section of the fault zone was principally a dilational jog. Fault rocks along selected sections of the fault zone, notably within the left- and right-stepping jogs, have been intensely altered by flux of a C02, Na, Ca, Fe-bearing fluid. Carbon and oxygen isotope ratios, trace element geochemistry, and fault rock replacement mineralogy indicate that the alteration is a form of fenitisation, like that associated with Alpine dyke swarm carbonatites. Unaltered, or only weakly altered, fault rocks preserve microstructural evidence of mixed grain-scale deformation mechanisms. A large proportion of the fault rocks are foliated cataclasites, within which deformation was dominated by cataclastic flow, but textures preserved in quartzose porphyroclasts testify to the early importance of crystal plastic deformation mechanisms. Locally abundant pseudotachylyte veins interlaced with foliated cataclasites occur preferentially on sections of the fault zone where it splits or jogs. Thus, longer-term, macroscopically ductile, aseismic cataclastic flow was episodically punctuated by very short-term, macroscopically brittle seismic events. Microstructures in quartz, feldspar and phyllosilicates, as well as greenschist facies assemblages in the fault rocks, imply temperatures during deformation of 250° -300°C. Assuming a geothermal gradient of 20-25oC/km, this corresponds to a depth of 10-15 km, close to the base of the seismogenic zone. The fault has been exhumed from these depths since c. 20 Ma, with little high-level overprinting of the mid-crustal fault rock fabrics. The timing and kinematics of movement on the SFZ are consistent with this structure playing a significant role in accommodating regional strain during inception and propagation of the Alpine Fault. The SFZ, MF and other Cenozoic structures were integrated with a kinematic model for distributed deformation during development of the mid Cenozoic to present-day plate boundary zone through southern New Zealand. Identifying the structures which were active during development of the Pacific-Australia plate boundary is important because internal deformation of the Pacific and Australian plates is not often allowed for in plate tectonic calculations. The results of this study show that it is both feasible and necessary to allow for distributed strain when making plate reconstructions.

1 v. (various pagings) : ill. (some col.), maps ; 30 cm.

1998White

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