QUATERNARY GILBERT-STYLE FAN DELTAS AND PODOCARP FORESTS ALONG THE MODERN TRANSPRESSIONAL HOPE FAULT, NEW ZEALAND

Quaternary Gilbert-style fan deltas of the Conway Coast, New Zealand, are exposed as the result of rapid uplift and erosion of the Hawkeswood Range along the strike-slip Hope Fault. Rise of the Hawkeswood Range is due to transpression associated with the modern New Zealand plate boundary in the transition zone between oblique subduction and strike-slip. A fault controlled basin most likely formed at a releasing bend along the obliquely strike-slip Hope fault, similar to the modern Hanmer Basin. The basin bounding fault scarps are inferred to provide the vertical drop for Gilbert-style fan delta deposition.

The fan deltas are exposed in 50-70 m high cliffs and crop out for over 5 km along the coast and up to 2 km inland. Individual foreset beds are normally graded, inversely graded or massive and change rapidly laterally and down transport direction. Beds are interpreted to be deposited by cohesionless debris flows and high-density turbidity currents. In places, the transition from cohesionless debris flow to high-density turbidity current occurs over a very short distance (<5 m); a much shorter distance than described in previous fan delta studies. Clasts within the beds have an unusual horizontal imbrication fabric, which may provide information about flow velocities and fluid escape paths. Mudstone and siltstone deposits of embayment and pro-delta facies interfinger with the foreset and topset conglomerates. These beds often display large-scale hummocky cross-stratification, slumping and huge fluid escape structures.

Laterally at the same stratigraphic horizon as the fan delta deposits are remnants of a c. 8000-year-old drowned Podocarp forest. They are preserved in growth position and are rooted in conglomerate beds within estuary deposits, probably on old levees. Subsidence and drowning of the forest may relate to local collapse of blocks within a developing releasing bend. The forest may have been abruptly dropped below sea level during an earthquake event on the strike-slip fault.

The Conway Coastal region has excellent exposures of Quaternary fan deltas. These young fan delta deposits allow the study of a sedimentary basins evolution in an active fault zone.