Revised slip rates for the Alpine fault at Inchbonnie: Implications for plate boundary kinematics of South Island, New Zealand

Abstract

The northeast-striking, dextral-reverse Alpine fault transitions into the Marlborough Fault System near Inchbonnie in the central South Island, New Zealand. New slip-rate estimates for the Alpine fault are presented following a reassessment of the geomorphology and age of displaced late Holocene alluvial surfaces of the Taramakau River at Inchbonnie. Progressive avulsion and abandonment of the Taramakau fl oodplain, aided by fault movements during the late Holocene, have preserved a left-stepping fault scarp that grows in height to the northeast. Surveyed dextral (22.5 ± 2 m) and vertical (4.8 ± 0.5 m) displacements across a left stepover in the fault across an alluvial surface are combined with a precise maximum age from a remnant tree stump (≥1590-1730 yr) to yield dextral, vertical, and reverse-slip rates of 13.6 ± 1.8, 2.9 ± 0.4, and 3.4 ± 0.6 mm/yr, respectively. These values are larger (dextral) and smaller (dip slip) than previous estimates for this site, but they refl ect advances in the local chronology of surfaces and represent improved time-averaged results over 1.7 k.y. A geological kinematic circuit constructed for the central South Island demonstrates that (1) 69%-89% of the Australian-Pacifi c plate motion is accommodated by the major faults (Alpine-Hope-Kakapo) in this transitional area, (2) the 50% drop in slip rate on the Alpine fault between Hokitika and Inchbonnie is taken up by the Hope and Kakapo faults at the southwestern edge of the Marlborough Fault System, and (3) the new slip rates are more compatible with contemporary models of strain partitioning presented from geodesy.

Late Holocene landscape change history related to the Alpine Fault determined from drowned forests in Lake Poerua, Westland, New Zealand

“…2) (Norris and Cooper, 2001;Sutherland et al, 2006). Northeast of Hokitika, the slip rate of the Alpine Fault decreases as plate boundary strain is distributed across a zone of strike-slip faulting called the Marlborough Fault System (Van Dissen and Yeats, 1991;Langridge and Berryman, 2005;Langridge et al, 2010).…”

“…The lake was first described by Europeans from the journeys of Sir Julius von Haast, who explored parts of the West Coast region during the 1860's (von Haast, 1948). Holocene Lake Poerua fills part of a former glacial trough, called the Poerua sub-lobe of the former Taramakau glacier by Langridge et al (2010). The Poerua sublobe was one of three distributary outflow lobes that spread out from the Taramakau glacier on the northwest side of the Alpine Fault during Last Glacial Maximum (LGM; Suggate and Waight, 1999;Suggate, 1965;Alloway et al, 2007;Barrell et al, 2011) (Fig.…”

“…However, unlike the nearby, larger Lake Brunner, which has bathymetry of >100 m water depth and is bounded by end moraines from the LGM, Lake Poerua is not dammed by moraines and is therefore not directly of glacial origin. Instead, Langridge et al (2010) argue that Lake Poerua is very young and has formed since the abandonment of the Poerua sub-lobe (Poerua River valley) by the Taramakau River due to avulsion of that river toward the southwest.…”

“…This tree -probably Matai ( Prumnopitys taxifolia) -colonised a well-drained, abandoned alluvial surface of the former Taramakau River, termed the Inchbonnie-3 (I-3) surface, between ca. 1530-1670 cal yr BP (Langridge et al, 2010).…”

“…The height difference between alluvial surface I-4 and the lake floor (ca. 6.9 m) is equivalent to the vertical deformation across the Alpine Fault since the abandonment of the I-4 surface (see Langridge et al, 2010). and WP119 to better characterise the life history of these trees.…”