Rappahannock Group: Late Cenozoic sedimentation and tectonics contemporaneous with Alpine Fault movement

Abstract

A Late Miocene to Pliocene/Lower Pleistocene sedimentary sequence is located 3–15 km west of the New Zealand Alpine Fault and north-west of Lewis Pass. Rappahannock Group is non-marine, and formations are based on the pebble content of conglomerates which are interbedded with sandstones and carbonaceous siltstones. Evidence for the timing of transcurrent movement on the Alpine Fault is provided by provenance studies of pebbles in these conglomerates. The basal Frog Flat Formation and overlying Spargo Formation contain conglomerates with 58-87% arenite pebbles. These arenite pebbles are characterised mainly by a quartz modal count of generally < 10 and are thought to be of Caples/Pelorus origin. Pelorus rocks on the western side of the Alpine Fault, north of Rappahannock Group, are rejected as the arenite source area as there is no evidence of Pelorus pebble transportation south to the Rappahannock Group. The Caples terrane is a more probable source area and is thought to have been located across the Alpine Fault adjacent, and to the east of, the depositional basin during the Late Miocene. Approximately 420 km of subsequent dextral shift on the Alpine Fault has resulted in the locating of the Caples terrane in its present position in west Otago/Southland. Supporting evidence for large scale dextral shear on the Alpine Fault in the Late Cenozoic is provided by the deformation of Rappahannock Group into a zigzag en echelon fold pattern. A progressive change in the trend of the northern fold axis, from the base to the top of the sequence, suggests folding was synchronous with deposition. Schist pebbles in Devils Knob Formation, uppermost in Rappahannock Group, are derived from both the Alpine Schist Belt and the Otago Schist. Schist pebble content rises from 5% at its base to 91% at the highest existing level. Metamorphic grade of the pebbles rises from chlorite and biotite schists in the loiwer part of the sequence to garnet schist at the top. These trends reflect southward movement of the Otago schists and continued uplift of the Alpine Schist Belt east of the Alpine Fault supplying increasing quantities of schist and exposing schist of higher grade. The depositional and deformational record of Rappahannock Group supports both extensive uplift and 420 km lateral shift on the New Zealand Alpine Fault since the Late Miocene.