Tectonic implications of granite cobbles from the mid-Cretaceous Pororari Group, southwest Nelson, New Zealand

Abstract

Abstract Petrographic and geochemical data confirm the source of granitic detritus in the Hawks Crag Breccia of the lower Buller Gorge to be the Early Cretaceous Buckland, Blackwater, and Steele Plutons, presently disposed to the south and southwest. Detritus in the Hawks Crag Breccia in the Fox River and Bullock Creek may also be derived from the Buckland Pluton, but the provenance of the granitic detritus in this formation in the Pike Stream remains unclear. However, granitic clasts in the Ohika Formation were derived from Paleozoic granites of the Karamea Suite, of which potential source plutons are presently widely distributed to the WNW (Orowaiti River), the north (Britannia Pluton), and to the east (Dunphy Granite). Clasts derived from the deformed basement of the Paparoa Metamorphic Core Complex first appear in the upper part of the Ohika Formation, indicating initial unroofing of the complex at that time. The indicated source directions are consistent with a northwest-southeast trend for the two detachment faults that formed the two Pororari Group basins across the northern and southern ends of the Paparoa Range. The detachment faults formed the footwalls to the asymmetrical basins on the northeast side, and southwest side, of the southern and northern basins, respectively. K-Ar data suggest uplift on the southern Pike Detachment Fault continued after uplift of the northern PaparoaRange on the Ohika Detachment Fault, and that the Pike Fault is more directly associated with the rifting that formed the passive margins to the Tasman Sea. A wide range in Sr-isotopic compositions of granite clasts may reflect primary variations in the source plutons, or Sr exchange between plutonic basement and Greenland Group cover rocks, or increased Rb/Sr, due to alteration along the detachment faults. Muscovite K-Ar ages of 111–113 Ma from clasts collected from the Hawks Crag Breccia in the northern Paparoa Range provide minimum ages for the Buckland and Blackwater Granites, respectively. They indicate minimum uplift rates of 1.1–1.4 mm/a, and suggest that the continental crust in the vicinity of the Paparoa Range was at least 45–55 km thick prior to the onset of Cretaceous extension.