Abstract
Situated ~20 km northeast of Haast in a remote part of South Westland, New Zealand, the ~65 km2 Bald Hill-Maori Saddle region had not been mapped in detail prior to this research. This study provides a detailed account of the Bald Hill-Maori Saddle region’s geology and shows that it is dominated by metasedimentary and igneous basement lithologies, but also comprises a minor component of the Cretaceous-Recent South Westland stratigraphic sequence. It is also shown that the Bald Hill-Maori Saddle region has a complex structural history, with evidence for at least two separate major episodes of tectonism: Cretaceous extension and Cenozoic transpression.
Greenland Group metasediments are the dominant lithology exposed within the Bald Hill-Maori Saddle region and consist of a tightly-folded and indurated metaturbidite sequence composed of interbedded, quartz-dominated, immature poly-cyclic psammites and pelites that have been regionally metamorphosed to lower-greenschist facies. Due to emplacement of the Whakapohai Granite ± additional concealed plutons to within 2 km of the current level of exposure, much of the Greenland Group within the Bald Hill-Maori Saddle region has been contact metamorphosed to biotite ± andalusite ± cordierite hornfelses. At the southeastern extremity of the Bald Hill-Maori Saddle region, situated between the Alpine and Wrong Faults and juxtaposed against Greenland Group metasediments across an intensely mylonitized décollement zone, the newly defined Cattle Track Gneiss consists of a poorly-exposed, amphibolite facies, mylonitic paragneiss equivalent to the Greenland Group, which was uplifted from the mid-crust during formation of the Cretaceous aged Maori Saddle complex. Hosted within Greenland Group metasediments, the Maori Saddle Granite, the Whakapohai Granite, and lamprophyre dykes constitute igneous basement rocks in the Bald Hill-Maori Saddle region. Incorporated as part of the Karamea Granitoids, yet separate from the sub-suites, the 384 ± 3.5 Ma, weakly fractionated, calc-alkaline, peraluminous, S-type Maori Saddle Granite exposed at the southern extremity of the Bald Hill-Maori Saddle region, was generated at the obliquely-convergent paleo-Pacific margin of Gondwana in response to crustal thickening associated with the Tabberabbean Orogeny and is most likely affiliated with similar plutons in Eastern Tasmania, Australia. Exposed as a stock midway up the Whakapohai River and as several pegmatite dykes between the Diamond and Wrong Faults, the newly discovered, highly-fractionated, calc-alkaline, peraluminous, S-type Whakapohai Granite is geochemically comparable with Karamea Suite plutons of the Karamea Granitoids. Thus, like Karamea Suite plutons elsewhere in New Zealand, it too was likely generated in an extensional environment associated with slab-rollback/abandonment or a one-off delamination event at the obliquely-convergent paleo-Pacific margin of Gondwana, during Mid-Paleozoic time. Two sets of lamprophyre dykes crop out in the Bald Hill-Maori Saddle region: alkaline lamprophyres in the Maori Saddle area; and calc-alkaline lamprophyres near the Munro Mistake-Whakapohai River confluence and in Mistake Creek. Alkaline lamprophyre dykes are Mid- to Late-Cretaceous in age, crosscut all basement lithologies except an intensely mylonitized décollement zone, and are affiliated with swarms of alkaline lamprophyre dykes that intruded Western Province rocks elsewhere in New Zealand during Cretaceous region-wide extension. Calc-alkaline lamprophyre dykes in the Bald Hill-Maori Saddle region form a north-south trending en echelon array and have a strong subduction-related geochemical signature, which implies an emplacement age coincident with when the Bald Hill-Maori Saddle region was last affected by subduction: during the Mid-Paleozoic. Consequently, these lamprophyre dykes are separate from all other known lamprophyre dyke swarms in New Zealand. Capping the summit of Bald Hill, the lower-member of the Otumotu Formation, the oldest unit of the Cretaceous-Recent South Westland stratigraphic sequence, consists of a flat lying/gently dipping fanglomerate deposit dominated by locally-derived quartz and Greenland Group detritus. The Otumotu Formation is Early- to Late-Cretaceous in age and infills extensional half grabens that formed within the detached upper-plate of the Maori Saddle Complex. Additional exposures of units constituting the Cretaceous-Recent South Westland stratigraphic sequence consist of: an Arnott Basalt feeder dyke that crosscuts the Otumotu Formation near the summit of Bald Hill; an internally faulted fault slice comprising Tauperikaka Coal Measures; Tokakoriri Formation Porphyry Point Member, and Tititira Formation; internally structurally complex Tititira Formation and Tauperikaka Coal Measures that constitute part of the vertical limb of the Coastal Monocline; and a thin veneer and a series of terraces composed of Quaternary aged alluvium, colluvium, paleo-river/glacial outwash, shallow-marine, and glacial moraine/till deposits.
In Cretaceous time, following the Rangitata Orogeny, the Bald Hill-Maori Saddle region underwent region-wide extension and the Maori Saddle Complex formed. The Maori Saddle Complex is an extensional complex with metamorphic core complex affinities, which, like similar complexes elsewhere in New Zealand, comprises a ductilely deformed, high-grade, lower-plate (Cattle Track Gneiss) that has been exhumed and juxtaposed against a brittlely deformed, low-grade, upper-plate (Greenland Group) as a consequence of mylonitic shearing along a low-angle décollement zone. Due to Cenozoic tectonism, the Maori Saddle Complex has since been overprinted with Cenozoic structures, which add complexity. Cenozoic tectonism associated with the Early-Miocene propagation of the Alpine Fault Australia-Pacific plate boundary through New Zealand and the Late-Miocene to present transpression across it, caused uplift of a 10-20 km wide belt of Western Province rocks between the Alpine Fault and a steeply dipping reverse fault in basement rocks beneath the present-day coastline. As a result, the overlying Cretaceous-Paleogene-Neogene sedimentary and volcanic deposits were folded into the Coastal Monocline; of which, the flat lying/gently dipping Otumotu Formation at the summit of Bald Hill forms the upper-limb and the steeply dipping deposits along the adjacent coastal section form the vertical limb. Such Cenozoic tectonism also produced a series of northeast-southwest trending fault zones that crosscut the region, including the newly defined Mathias, Diamond, and Wrong Faults. Along each of these newly defined faults, the fault block to the southeast has been uplifted relative to the northwestern fault block and rocks from progressively deeper in the crust have been exposed. Ongoing transpression across the Alpine Fault Australian-Pacific plate boundary has uplifted ~740 ka paleo-river/glacial outwash terraces, deposited by the Haast River, by ~400-500 m. Likewise, 123 ± 7 ka paleo-shallow-marine terraces at Knights Point have been uplifted 113 m. Based on the ages and elevations of these terraces, average uplift rates in the Bald Hill-Maori Saddle region have accelerated from ~0.55 mm/yr (~740 ka to 123 ± 7 ka) to 0.86 mm/yr (123 ± 7 ka to present) during Mid- to Late-Quaternary time.