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Late Holocene surface ruptures on the southern Wairarapa fault, New Zealand: Link between earthquakes and the uplifting of beach ridges on a rocky coast

T.A. Little^1,*, R. Van Dissen^2,, E. Schermer^3, and R. Carne^1,#

¹SCHOOL OF GEOGRAPHY, ENVIRONMENT AND EARTH SCIENCES, VICTORIA UNIVERSITY OF WELLINGTON, P.O. BOX 600, WELLINGTON 6140, NEW ZEALAND
²GNS SCIENCE, P.O. BOX 30368, LOWER HUTT 5010, NEW ZEALAND
³GEOLOGY DEPARTMENT, WESTERN WASHINGTON UNIVERSITY, MS9080, BELLINGHAM, WASHINGTON 98225, USA

View larger version (194K): [in this window] [in a new window]   Figure 1. (A) Tectonic index map showing major active faults and other structures of the southern North Island, New Zealand (largely after Barnes, 2005; Begg and Johnston, 2000; Lee and Begg, 2002), and location of sites along the Wairarapa fault where paleoseismic data have been collected previously and during this study (large open circles). Smaller rectangles show location of detailed Wairarapa fault maps of Figure A and Figure 2. Cross section X–X' is presented in Figure 11B. Inset on upper left shows plate-tectonic setting of New Zealand (plate motions taken from DeMets et al., 1990, 1994). NIDFB—North Island Dextral Fault Belt. (B) Oblique aerial photograph of uplifted beach ridges and Holocene wave-cut platform at Turakirae Head, view looking NW (photo by Lloyd Homer, GNS Science as annotated in McSaveney et al., 2006). The two uplifted Pleistocene marine terraces in the background were studied and assigned provisional ages by Ota et al. (1981). Distance between the two headlands is about 4 kilometers.

View larger version (68K): [in this window] [in a new window]   Figure 2. Active fault traces and folds along the Wharekauhau thrust along the southern section of the Wairarapa fault zone (after Begg and Johnston, 2000; revisions by Little et al., 2008). Grid marks (in meters) refer to the New Zealand Map Grid Coordinate System.

View larger version (45K): [in this window] [in a new window]   Figure 3. Microtopographic map of the Pigeon Bush stream offset site showing location of beheaded stream channels and the Pigeon Bush 1 and 2 trenches (PB-1, PB-2, respectively). Base map is from Rodgers and Little (2006) and was surveyed using a combination of real-time kinematic differential global positioning system (GPS) methods employing a differential correction that was tied to a base station at benchmark LUCENA (NZMS270-S27A) and 3-D laser-ranging techniques. Trench corners were located by real-time kinematic GPS survey tied to the same base station. Grid marks (in meters) refer to the New Zealand Map Grid Coordinate System. M.S.L.—mean sea level; O.S.L.—optically stimulated luminescence.

View larger version (42K): [in this window] [in a new window]   Figure 4. Logs of the Pigeon Bush trenches. (A) NE wall of Pigeon Bush 1 (PB-1) trench across the younger of the two abandoned stream channels; (B) SW wall of the same trench; (C) SE wall of the Pigeon Bush 2 (PB-2) trench across the older of the two abandoned stream channels. See Figure 3 for location of the trenches. See Table 1 for 14C analytical results of the numbered samples. The unit labels and brief unit descriptions for each trench pertain only to that particular trench wall and are not meant to apply necessarily to any other trench log. See Appendix B for detailed stratigraphic descriptions of the logged units.

View larger version (23K): [in this window] [in a new window]   Figure 5. Microtopographical map of the Riverslea fault trenching locality. Map is based on 1020 points that were surveyed using real-time kinematic differential global positioning system (GPS) methods. GPS survey was tied to an arbitrary local base station and was not corrected against a gazetted geodetic benchmark. Grid marks (in meters) refer to the New Zealand Map Grid Coordinate System. M.S.L.—mean sea level.

View larger version (53K): [in this window] [in a new window]   Figure 6. (A) Log of SW wall of Riverslea 1 trench (RVL-1) across folded scarp of Wharekauhau thrust near Manganui Stream. (B) Photograph of inclined (probably tectonically tilted) laminae in fine- to coarse-grained sand layer (unit ss-5 in RVL-1 trench). (C) Log of part of the NE wall of the nearby Riverslea 2 trench (RVL-2). (D) Photograph of infilled fault fissure in RVL-2 trench. See Figures 2 and 5 for trench locations. See Table 1 for 14C analytical results of the numbered samples. See Appendix C for detailed stratigraphic description of the logged units. M.S.L.—mean sea level.

View larger version (56K): [in this window] [in a new window]   Figure 7. (A) Vertical aerial photograph of Cross Creek pull-apart graben showing trench sites and fault-trace locations. The photograph by Lloyd Homer (GNS Science) predates track construction that has mostly destroyed the scarplets on the NW side of the graben. (B) Microtopographical map of the NE end of the pull-apart graben showing trench locations. Map is based on 1505 points that were surveyed using real-time kinematic differential global positioning system (GPS) methods. GPS survey was tied to a base station at benchmark LUCENA (NZMS270-S27A). Grid marks (in meters) refer to the New Zealand Map Grid Coordinate System. (C) Auger transect and topographic profile across Cross Creek pull-apart graben showing depth to gravel and location of the 14C sample Auger-3. See part B for transect location. Note that scale of the profile is slightly enlarged relative to B, and that it is vertically exaggerated by 2:1. M.S.L.—mean sea level.

View larger version (103K): [in this window] [in a new window]   Figure 8. Logs of trenches CC-1 and CC-4 across the southern bounding fault of the Cross Creek pull-apart graben. See Figures A, 2, 5, and 7B for location of trenches, Figure B-i for a photograph of the trench sites, Appendix D for detailed stratigraphic descriptions, and Table 1 for 14C analytical results of the numbered samples: (A) Log of Cross Creek 1 trench (CC-1). (B) Log of the nearby Cross Creek 4 trench (CC-4). View has been inverted for ease of comparison with Figure 8A. (C) Photograph of unlogged NE wall of trench CC-4 just prior to its collapse, showing exposure of fault-draping intrapeat unconformity and location of 14C samples CC-4-2 and CC-4-3. An enlarged version of this figure is presented as Figure C.

View larger version (74K): [in this window] [in a new window]   Figure 9. Logs of trenches CC-2 and CC-3 across the northern bounding fault of the Cross Creek pull-apart graben. See Figures A, 1, 5, and 7B for location of trenches, Figure B-ii for a photograph of the trench sites, Appendix F for detailed stratigraphic descriptions, and Table 1 for 14C analytical results of the numbered samples. (A) Log of NE wall Cross Creek 2 trench (CC-2). An enlarged version of the log is presented as Figure D. (B) Log of NE wall of Cross Creek 3 (CC-3). (C) Log of SW wall of CC-3. View is inverted for ease of comparison with parts A and B. An enlarged version of the CC-3 logs are presented in Figure E.

View larger version (43K): [in this window] [in a new window]   Figure 10. Time-space plot of surface-rupturing Wairarapa fault earthquake events inferred from all available types of paleoseismological data (Tables 1, 2, and 3). Data for Tea Creek trench (events labeled, T) are from Van Dissen and Berryman (1996). Diatom-based paleoenvironmental data for coastal uplift events at Lake Kohangapiripiri (events, K) are from Cochran et al. (2007). Uplift events near Turakirae Head on Palliser Bay (events, Tk) inferred from raised beach ridges (BR-1 to BR-5) are taken from McSaveney et al. (2006). Small numbers quoted in meters denote inferred single-event uplift magnitudes for a given event. For Palliser Bay data, these refer to the maximum uplifts near Turakirae Head as inferred by McSaveney et al. (2006). Name of key 14C samples that bracket the timing of rupture events in trenches (this study) are identified at maximum and minimum limits of error bars (95% confidence) and refer to the field sample names listed in Table 1. Colored horizontal bands depict the maximum and minimum age range (at 95% confidence) for each earthquake event as derived from an analysis of the composite set of trench data and 14C results.

View larger version (30K): [in this window] [in a new window]   Figure 11. (A) Beach-ridge topographic profiles drawn orthogonal to the Cape Palliser–Turakirae Head coastline on either side of the crest of the Rimutaka anticline (from McSaveney et al., 2006). (B) Schematic cross-section X–X' across southern North Island, New Zealand, showing major faults in upper plate of the Hikurangi subduction margin, including strike-slip and contractional faults at the southern end of the Wairarapa fault zone. Currently inactive faults are dashed. Position of the currently locked part of the subduction interface is taken from Wallace et al.'s (2004) modeling of global positioning system (GPS) data. For location of cross section, see Figure 1. M.S.L.—mean sea level.