Pliocene Quaternary sedimentation and Alpine Fault related tectonics in the lower Cascade valley, South Westland, New Zealand

Study of Pliocene and Quaternary sediments west of the Alpine Fault in the Cascade valley, South Westland, New Zealand, has allowed determination of Alpine Fault displacement rate and coastal uplift rate over the last 3.5 m.y. Exposures of the Pliocene Halfway Formation (latest Opoitian-Waipipian) are composed of marine sand and conglomerate deposited in c. 200-1000 m water depth. Beds are gently dipping and weakly deformed, with the direction of principal shortening oriented at a high angle to the Alpine Fault and plunging gently northwest (c. 10 degrees/340 degrees). Fiordland-derived clasts indicate a minimum of 95-100 km of lateral offset on the Alpine Fault since deposition of Halfway Formation. Paleogeographic evidence suggests that first-order features of the drainage were similar in Pliocene time to those of the present day. Quaternary moraines and fluvioglacial sediments are subdivided on the basis of composition and morphology into five groups: C1 (oldest) to C5 (youngest). The C1 deposits have a provenance consistent with a glacier flowing from the Pyke valley. The C2 and C3 deposits are c. 95% ultramafic and 5% Haast Schist derived, whereas the C4 and C5 deposits are c. 75% ultramafic and 25% Haast Schist derived. The increasing component of Haast Schist in younger moraines probably reflects the changing headwater configuration of the Cascade glacier as new tributaries were progressively captured as a result of Alpine Fault displacement. Teer Formation (new) is composed of shallow-marine fossiliferous glacial silts, which lie unconformably above Halfway Formation and beneath C3 and C4 moraines. Clast compositions indicate that source regions to the east of the Alpine Fault were in the vicinity of the Pyke valley and that Teer Formation may be a fiord correlative of the C1 morainic deposits. Based on an offset of c. 18-32 km and an Alpine Fault displacement rate of 27 +/- 6 mm/yr, an age for the Teer Formation and C1 deposits of 0.9 +/- 0.4 Ma is interpolated. Uplift rate estimates based on Halfway and Teer Formation coastal outcrops are in the range 0.1-0.5 mm/yr.