Landslides DOI 10.1007/s10346-009-0156-5 Received: 19 November 2008 Accepted: 8 May 2009 © Springer-Verlag 2009 Guillaume Chevalier . Tim Davies . Mauri McSaveney The prehistoric Mt Wilberg rock avalanche, Westland, New Zealand Abstract The Mt Wilberg rock avalanche in Westland, New Zealand occurred before 1300 AD and may have occurred as a consequence of an Alpine fault earthquake in ca. 1220 AD or earlier. Its ∼40×10 6 m 3 deposit may have briefly obstructed the Wanganui River, but only about 25% of its surface morphology still survives, on terraces isolated from river erosion. The landslide appears to have moved initially as a block, in a direction controlled by a strong rock mass at the base of the source area, before disintegrating and spreading across terraces, fans, and floodplains. Rock avalanche deposits in Westland have relatively short expected lifetimes in the rugged terrain and high rainfall of the area; hence, the hazard from such events is under-represented by their current remnants. Keywords Rock avalanche . Earthquake trigger . Emplacement mechanics . Dating . Coseismic landslide probability . New Zealand . Westland Introduction Major earthquakes in mountainous landscapes are expected to cause large coseismic rock avalanches (e.g., Keefer 1984, 1994) because the intense ground accelerations can initiate deep-seated failure surfaces, or exceed the shear resistance on existing ones. Hazards to human facilities and infrastructure from large rock avalanches are potentially severe, due to the large mass, high velocity and long runout of such events; they can also block rivers causing consequential dambreak and sedimentation hazards. The western range front of the Southern Alps, New Zealand (Fig. 1) is known to have been affected by several great (M∼ 8) earthquakes on the interplate Alpine fault in the last millennium (approximately 1717 AD, 1620 AD, 1420 AD, and 1200 AD: Yetton 1998; Sutherland et al. 2006); however, previously, only one substantial landslide along the range front had been studied—the 40×10 6 m 3 Round Top deposit (Wright 1998). We here report on another deposit of similar size at the range front, the Mt Wilberg rock avalanche beside the Wanganui River (Figs. 1, 2, and 3), mentioned but not studied in Wright (1998) and Korup (2004). Like the Round Top deposit, this appears to pre-date the last two Alpine fault earthquakes, in ∼1720 AD and ∼1620 AD, leaving the conundrum that no large landslide deposit has yet been found dating from the last Alpine Fault earthquake even though it has been estimated to have been a Great Earthquake (Yetton 1998). The degrees to which the Mt Wilberg deposit has been eroded by Wanganui River and the 1999 Mt Adams deposit has been eroded by Poerua River help explain the apparent paucity of rock avalanche deposits in Westland. Regional setting The geological and geomorphological setting of the western Southern Alps is well described by, for example, Korup (2004, 2005) and Cox and Barrell (2007). Briefly, 60–80% of the transpressional motion between the Pacific and Australian Plates localizes on the Alpine fault, with strike-slip motion of 23±2 mm/ year and dip slip of 12±4 mm/year normal to the strike direction (Norris and Cooper 2001, De Mets et al. 1994; Yetton 1998; Sutherland et al. 2006). The plate boundary is locked above 11 km depth, except for fault ruptures that cause major to great earthquakes, for which there is evidence of about eight in the last two millennia (P. Almond, Lincoln University, New Zealand, pers comm 2007), each causing up to 8 m of horizontal and 3 m of vertical offset on the fault trace. High grade (k-feldspar amphibolite) schists are exhumed near the fault zone, with a narrow zone of mylonite and curly schist immediately southeast of the fault. The mylonite locally contains pseudotachylite. The degree of P–T metamorphism reduces to the south-east, towards the main divide of the Southern Alps where weakly metamor- phosed greywacke sandstones (zeolite-facies metamorphism) are exposed (Fig. 4; Cox and Barrell 2007). West of the fault are basement rocks of the Australasian Plate, comprising Greenland Group sandstone, granitic hills and ranges, and other lithologies of Cretaceous and Tertiary ages, overlain in many places by massive Pleistocene moraines. At intervals of 10–20 km, large rivers exit NW-trending mountain valleys to flow on extensive braidplains, 10–20 km across the foreland between deeply eroded moraine ridges to the sea. There is soil sequence evidence that episodic, widespread aggradation of the braid and flood plains has occurred, at least in the last 1,400 years (Berryman et al. 2001; Davies and Korup 2007), possibly in response to major earthquakes. Rainfall varies between ∼5 ma -1 at the range front to ca. 10–15 ma -1 about 10 km west of the main divide (Henderson and Thompson 1999); snow rarely falls on the foreland but can fall at any time of year above ∼1,500 m, and significant glaciers remain above ∼2,500 m. There is some evidence that the mountain hypsometry is in dynamic equilibrium with tectonic uplift; this is not so clear west of the fault (Davies and Korup 2007), but since sea level stabilized about 6,000 years ago, sufficient fluvially transported sediment has crossed the fault and traversed the foreland that foreland storage has reached long-term dynamic equilibrium. Local setting Mt Wilberg forms a NE-trending ridge on the west side of the Wanganui River at the range front of the Southern Alps, Westland (Figs. 1 and 5). To the WNW, this ridge is bounded by the valley of Harald Creek, which has formed along a trace of the Alpine fault. To the south-east, the ridge is flanked by the Wanganui River. The Mt Wilberg rock avalanche fell from the north side of the ridge from elevations between 150 and 500 m asl. Harald Creek flows partly along the trace of the Alpine fault and has formed a large alluvial/debris flow fan which abuts against the Original Paper Landslides