Tuapeka fault zone : brittle faulting and sedimentation, Otago, New Zealand

Walrond, Mark Tapio, 1970-

The Tuapeka Fault Zone (TFZ) is a complex NW-SE zone of brittle deformation, extending from Waitahtu1a Gully to Beaumont, in southern Otago. Good exposures of individual faults occur within this zone, the best being at Gabriel's Gully Historic Reserve, and at Waitahuna Gully. Spectacular fault planes, exposed by the removal of the hanging wall Blue Spur Conglomerates (BSC) (late Cretaceous) during former mining operations can be seen at these two localities. The fault plane exposed at Gabriel's Gully (Gabriel's Gully No 1 Fault) extends down-dip for up to 200m, along a strike length of 400m. TI1is offers excellent exposure of a variety of brittle fault rocks. The fault core is dominantly composed of an extremely fine grained "brown ultracataclasite" unit, 20 cm thick with minor intercalated clay rich (green ultracataclasite) and carbonate cemented ultracataclasites (white ultracataclasite). In the footwall adjacent to this ultracataclasite "pavement", is a damaged zone (approx. Sm thick) comprising variably deformed and chloritically altered schists, calcite cemented fault breccias, derived from the schist and minor cataclasites. Surprisingly little deformation has occurred in ilie hanging wall BSC which shows no evidence for significant cataclasis, although numerous faults sub-parallel to the fault plane do occur within it. Porphyroclast mineralogy of t~e ultracataclasites (quartz-albite-epidote-titanite) largely mirrors that of the adjacent protolith1 which comprises Caples Terrane basement schists of pumpellyiteactinolite facies. These make up both the footwall and hanging wall lithologies, with additional minor BSC occurring in the hanging wall. In addition, several secondary minerals, useful for temperature estimation, occur in the fault rocks. These are interpreted to have been formed both by direct fluid introduction (calcite, quartz, chalcedony, stilbite, pyrite) and by alteration reactions of protolith minerals (smectite derived from albite alteration), in a low temperature ( <100"C), fault related hydrothermal system. Secondary minerals occur as veins (calcite, stilbite, pyrite, chalcedony) as well as within the matrix of the fault rocks (smectite, pyrite), attesting to significant levels of fluid flow. Microstructural shear sense indicators within the ultracataclasites indicate that the fault has experienced two separate phases of movement, an earlier normal dip-slip phase (late Cretaceous), followed more recently by a reverse dip-slip phase (Tertiary). Slickensides are rare, but occasional frictional wear striae on the ultracataclasite surface, as well as distributed shears with quartz fibre growths in the footwall schists, suggest predominantly dip-slip motion. Microscopic shear sense indicators are best developed in clay rich lensoidal domains (green ultracataclasites) which occur within the more common quartzofeldspathic brown ultracataclasites. The most reliable of these indicators are arrays of R1 Riedel shears with associated oblique P-foliations, similar to structures commonly observed in clay gouge shearing experiments. Additional microstucturnl shear sense indicators include fault parallel Y-shears, rotated porphyroclasts and vein microfaults. Outcrop scale field evidence from around the study area also suggests that the TFZ has experienced reversal. The Blue Spur Conglomerate makes up the hanging wall lithology at several localities (Gabriel's Gully, Waitahuna Gully, Wetherstones, Forsyth), strongly suggesting that the fault was ex tensional during the deposition of the conglomerates (late Cretaceous). Conjugate normal faults at Big Hill provide further evidence for extensional faulting. Field evidence for the reverse phase of movement is common. At Wetherstones, a well exposed NE dipping reverse fault has emplaced basement schists over ilie BSC. In addition, spectacularly developed tight macroscopic folds with steeply dipping axial planes occur at Wetherstones dump. These strongly suggest a compressional regime. At Gabriel's Gully Historic Reserve the BSC dips into the fault plane at an angle of 30". This is interpreted to b.e a drag effect resulting from reverse faulting. Mesoscopic kink folds, developed in the chlorite altered schists of the·Gabriel's Gully No 1 Fault, also suggest compression, as do patterns of calcite veining. The BSC (late Cretaceous) consists mainly of quartz-greywacke-schist conglomerates in a silly, smectite-rich, blue-green matrix which oxidises to a brown colour upon weathering. Subordinate horizons of sands, silts and coal horizons also occur. The deposit is interpreted to have been deposited by a major river system operating in late Cretaceous times. Paleocurrent data indicates a source from ilie SW, which is consistent with clast provenance. An additional minor source from the NE is also inferred. Occurrences of BSC are typically found preserved only in fault angle depressions of the TFZ, but the deposit was presumably once far more extensive. The BSC is auriferous and was the subject of intensive gold mining operations last century. Gold samples obtained from the deposit display spectacular evidence for secondary precipitation, and are closely associated with pyrite and smectite.

v, 197 p. : ill. (some col.), maps (some col.) ; 30 cm.

1996Walrond

POLYGON ((169.74489821639952 -46.033782700301579,169.657971508603964 -45.927319143786093,169.424764290524678 -45.91989527551047,169.433624002631973 -45.837580823681222,169.435190363778247 -45.770946238166516,169.583100522547767 -45.778566865628299,169.581464683087376 -45.835317682977085,169.771643343905538 -45.902417347313374,169.859477104906347 -46.014177408168649,169.872643170602089 -46.035257488378171,169.74489821639952 -46.033782700301579))