Geology of the Fiordland Area - GNS Science

a zone of amphibolite facies gneiss up to 500 m thick.Movement on these extensional shear zones contributedto the partial exhumation of western and central Fiordlandbetween ca. 111 and 100 a ( radshaw 1989b; ibson& Ireland 1995). Extension had largely ceased by ca.108 Ma at Mt Irene (Scott & Cooper 2006) and ca. 88 Maat Doubtful Sound ( ing et al. 2008), although regionalexhumation continued into the latest Cretaceous (Flowerset al. 2005).The coincidence of extensional ductile shears with someof the contacts between Western Fiordland Orthogneissand country rock, and apparent differences in metamorphichistories across these shears, ha e led to interpretations ofthe Western Fiordland Orthogneiss as the lower plate of ametamorphic core complex ( ibson et al. 1988; ibson& Ireland 1995; Hill 1995a,b). However, the widespreadpreser ation of intrusi e contacts and high-pressurecontact metamorphic aureoles in rocks adjacent to WesternFiordland Orthogneiss plutons indicates only minor offseton the extensional shears (Bradshaw 1985, 1990; Daczko etal. 2002a; Allibone et al. 2009b; Clarke et al. 2009). Onlylocally do these juxtapose rocks with different metamorphichistories (Scott & Cooper 2006).The Late Cretaceous, transpressional Straight Ri er andAnita shear zones (Fig. 67) may be more significant andlonger li ed than other Cretaceous transpressional shearzones in Fiordland. These two shear zones are imposedo er older structures, and include the youngest fabricsof any of the intra-batholithic shears within Fiordland.They are characterised by subvertical foliation(s), oftenmylonitic, which in turn are cut by sub- ertical shearzones with greenschist facies alteration and/or crenulationcleavage and brittle faults (Fig. 69). The mylonites maybe Late Cretaceous in age, and the youngest Anita Shearone fabrics are probably associated with mo ementon the Alpine Fault (Hill 1995a,b; lepeis et al. 1999;Sutherland et al. 2000; ing et al. 2008). The older AnitaShear one fabrics probably formed in the deep crust,during Early Cretaceous transpression, or may be inheritedfrom a Paleozoic structure. The Straight River ShearZone (including the Straight River Fault of Oliver 1980)is not a terrane boundary ( ing et al. 2008), but severalfeatures within the Anita Shear one suggest it may be amajor Paleozoic boundary within the Western Province.Possibly allochthonous ultramafic rocks are imbricatedbetween Arthur Ri er Complex (and Western FiordlandOrthogneiss) to the east, and probable Buller terrane rocksto the west of the shear zone. Contrasting Paleozoic andMesozoic plutonic rocks in Westland and Fiordland arejuxtaposed across the Anita Shear one when Alpine FaultCenozoic offset is restored.LATE MESO OIC TO CENO OIC TECTONICSExtension continued into the Late Cretaceous, e entuallyresulting in the splitting of ealandia from Australia andthe opening of the Tasman Sea. In Fiordland, this riftrelatedextension at around 85 a was oriented northeastsouthwest(Tulloch et al. 2009b). Extension-related uplifton higher le el brittle faults brought plutonic rocks insouthern Fiordland to the surface by ca. 100 a, and formedfault-controlled basins that were filled by Puysegur Groupsediments. Other brittle and brittle-ductile faults acti e orreacti ated in the Late Cretaceous include the Lake Fraser,Dusky, Wilmot and Spey-Mica Burn faults (Fig. 67).Terrane boundary faults in northeastern Fiordland were alsoreacti ated ( ortimer et al. 1999a; Turnbull 2000). Thesefaults generally have greenschist facies alteration zones,and cut earlier extensional ductile shear fabrics. Pegmatiticdikes, and the Post Office and Five Fingers dike swarms,may be coe al with extension.The Fiordland area then became relati ely quiescent untilthe Middle Eocene, when seafloor spreading propagatedfrom the Emerald asin, far to the southwest, into westernSouthland (Sutherland 1995a; King 2000). By the LateEocene, regional extension had created fault-controlled andmostly non-marine sedimentary basins along the oonlight70Figure 69 Steeply dippingfoliation in orsley Plutondioritic orthogneiss (highlightedby a dark mafic band), on thesoutheastern side of the AnitaShear Zone, north of CatseyeBay. The foliation is folded andthe left limb (by the hammer)is completely strained out intothe younger mylonitic shearzone fabric. here the myloniticfoliation completely overprintsthe orsley Pluton protolith, therocks are mapped as JaggedGneiss. The mafic band is offsetby a minor young epidotisedbrittle fault following the folda ial plane.