GSA Data Repository Item DR Age of Displaced Submarine Glacial Geomorphology: the Last Glacier Retreat from the Fiordland Coast

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1 Philip M. Barnes: Post-glacial (<20 ka) dextral slip rate of the offshore Alpine Fault, New Zealand. GSA Data Repository Item DR Age of Displaced Submarine Glacial Geomorphology: the Last Glacier Retreat from the Fiordland Coast The dextrally displaced lateral moraines on the shelf south of Milford Sound have not been directly dated. However, as part of this study the coeval surface of the displaced glacial outwash fans has been dated at six sites over a 120 km lateral extent along the Fiordland margin (Table DR1). Shallow-water mollusc shell fragments entrained in the deep-water gravel debris flow deposits on the relict surface of the fans were recovered in sediment core and dredge samples, and dated by radiocarbon methods. Excluding a clearly reworked sample (Tan0405#20), the age of five samples cluster within 1500 years at the 95% confidence level, with a mean age of 16.7 cal. ka. If it is assumed that the shells were entrained into the debris flows shortly after death of the animals, and that the deposits were emplaced at or immediately after the final stages of direct outwash from the tidewater glaciers grounded at fiord entrance sills, then the mean age for these deposits is a good representation for the age of glacier retreat from the coast, and essentially the abrupt abandonment of the moraines and outwash fans. Considering the similarity in alpine altitude, it is unsurprising that the age of glacier retreat appears to be approximately coeval along the 120 km lateral extent of the northern and central Fiordland coast.

2 The mean surface outwash gravel age of 16.7 cal. ka off the Fiordland coast is generally consistent with other published glaciation records from Westland and eastern Fiordland. The youngest moraines near the coast at Cascade Plateau in south Westland, some 80 km north of Milford Sound, are undated but have been inferred to be < 19 cal. ka on the basis of cosmogenic exposure dates of ka from moraine boulders on higher and older moraines (Sutherland et al., 2007). This is consistent with dated glacial lake silts from Lake McKerrow (Fig. 1A) that indicate that the former McKerrow glacier had retreated back across the Alpine Fault in that valley by 15.6 ± 0.8 cal. ka (Sutherland and Norris, 1995). In Westland three major glacier advances to at or near the maximum, that is to the coastal edge of the lowland coastal plain, culminated at about 28, 21 and 19 cal. ka (Suggate and Almond, 2005). Multiple younger recessional moraines on the coastal lowlands, however, are not well dated but have been assumed to have formed largely by about 18 ka. Minor late glacial advances occurred in high catchment areas of Westland between 13.5 and 11.0 cal. ka (Alloway et al., 2007). Speleothem records from eastern Fiordland indicate deglaciation after 19.1 cal. ka and before 16.6 cal. ka (Williams, 1996). Considering these published glaciation records from Westland and eastern Fiordland, and the possibility that the dated coeval (~16.7 cal. ka) surface deposits on the submarine outwash fans could have been emplaced after glacier retreat from the coast, and therefore after development of the primary fan and moraine geomorphology, an age range of cal. ka is adopted for the age of the glacial features displaced by the Alpine Fault west of Fiordland, with a best estimate of 17 cal. ka. This age differs slightly from the 18 +/- 1

3 ka age adopted by Sutherland et al. (2006a) for glacial features near the Alpine Fault in south Westland. Dextral Displacements of Submarine Glacial Geomorphology Site 1: Transit moraine North (Lat ; Long ) The Alpine Fault displaces laterally a submerged glacial moraine ridge on the continental shelf between Milford Sound and Transit Valley (Fig. DR1; Fig.3B) (Barnes et al., 2005). The moraine ridge is about 1 km wide, 15 m high, is mantled by large glacial boulders, and is highly reflective in multibeam backscatter sonar imagery. The inner edge of the moraine is partly buried by mobile sediment near the rocky coast. The moraine is displaced by two traces of the Alpine Fault, which steps over across the ridge. The stepover width is about 500 m, and is associated with a component of normal displacement on the fault, leading to fault scarps on the moraine up to 20 m high. Onshore lateral moraine lines the northeast wall of Transit Valley, and forms a sharp moraine ridge that is about 1500 m long. It is truncated by the modern sea-cliff, but can be extrapolated over the ~1000 m to the Alpine Fault. The combination of the step over in the fault trace, partial burial of the submarine moraine by mobile sediment, and onshore to offshore projection uncertainties makes estimation of the dextral displacement difficult at this site. It is inferred that the crest of the moraine is displaced by a total of about 470 ± 120 m across both fault traces (Fig. DR1; Table DR2). Site 2: Transit moraine South (Lat ; Long ) A submerged glacial moraine on the middle shelf off the southern side of Transit Valley, some 4.5 km southwest of site 1, is also displaced by the Alpine Fault (Fig. DR2). The displacement is essentially purely dextral. The moraine is arcuate, about 300 m wide, has

4 about 5 m of relief, and is highly reflective in multibeam backscatter sonar imagery. Part of the moraine may be buried by modern mobile sediment on the inner shelf, masking its original geometry, however the general outline and crest of the moraine allows estimation of 440 ± 65 m of dextral displacement by the Alpine Fault. Site 3: Poison Bay moraine North (Lat ; Long ) At site 3, an extremely well preserved glacial moraine on the middle shelf off the mouth of Poison Bay is dextrally displaced with no significant vertical displacement (Fig. DR2 and Fig. 2A). The main part of the moraine is about 450 m wide, 12 m high, and is highly reflective in multibeam backscatter sonar imagery, suggesting it is also covered by large boulders (e.g., Fig. 2C). The southern part of the moraine curves southwestward toward the centre of Poison Bay and is irregular in form, masking identification of the dextral displacement. The northeastern edge of the moraine however, is a very sharp escarpment that makes an excellent piercing point where it is displaced by the fault. The displacement of the northern edge is estimated to be 465 ± 15 m (Fig. DR3, Table DR2). Site 4: Sutherland channels (Lat ; Long ) The Alpine Fault crosses the edge of the continental shelf and enters the northern end of George Basin off the mouth of Sutherland Sound (Fig. 2B; Barnes et al., 2005). The shelf in this area is only about 1 km wide. At site 4, a series of ridges and gullies in the head of a canyon system that once supplied sediment to Sutherland Fan are dextrally displaced by the Alpine Fault (Fig. DR4). The fault forms a prominent landward-facing scarp. The axis of a conspicuous gully is displaced by 460 m, with about 100 m of uncertainty derived from projection of the axis over m to the fault trace in about 340 m water depth (Fig. DR4B; Table DR2).

5 Site 5: George South channels (Lat ; Long ) At site 5, about 7 km south of George Sound, the Alpine Fault crosses the upper continental slope about 2.5 km offshore (Fig. DR5). The fault forms a landward-facing scarp about 200 m high. At the head of a prominent slope canyon, in 300 m water depth, two well defined channels are dextrally displaced by the fault. The displacement of each channel axis can be estimated to be 460 ± 40 m. Site 6: Caswell Fan North (Lat ; Long ) The Alpine Fault crosses Caswell Fan in about m water depth off the mouth of Caswell Sound (Fig. 3A). At site 6, the fault displaces a series of exceptionally well preserved, linear aggradational ridges and gullies with about 20 m of relief on the relict glacial outwash surface (Fig. DR6; see also Fig. 5). At this site the vertical displacement is not significant and varies along strike. Seismic profiles show the fault dips steeply towards the coast (Barnes et al., 2005). The relict submarine geomorphology can be perfectly reconstructed after recovery of 540 ± 25 m of dextral displacement (Table DR2). Site 7: Caswell Fan South (Lat ; Long ) About 3-4 km southwest of site 6, the fault crosses similar ridges and gullies on the southern side of Caswell Fan (Fig. DR7). At two separate sites, collectively referred to here as site 7, the fault displaces prominent linear ridges and gullies dextrally by 530 ± 40 m (Table DR2). The ridges and gullies have about m of relief, and can be reconstructed to their original geometry (not shown here). The displacement is constant over the 4 km between the sites. The uncertainty in the displacement is derived primarily from projection of prominent escarpments and gully axes over about 250 m to the fault trace.

6 Site 8: Doubtful Fan Central (Lat ; Long ) The Alpine Fault crosses Doubtful Fan about 10 km off the mouth of Doubtful Sound (Fig. 1A). The fan is about 10 km wide (Barnes et al., 2005). On the centre of the fan at site 8 (Fig. DR8) a series of aggradational ridges and gullies with relief of about 5-15 m are dextrally displaced by 540 ± 30 m (Table DR2). The fault locally forms a landwardfacing scarp as a result of the dextral displacement. There is negligible vertical displacement of the outwash surface, which is convex at the scale of the fan. The estimate of uncertainty in the displacement derives from projection of ridge slopes and gully axes to the fault trace. Site 9: Doubtful Fan South (Lat ; Long ) At site 9 the fault crosses the southern edge of the last glacial surface of Doubtful Fan (Fig. DR9A). The fault trace continues southwest for several kilometers across an elevated and dissected older fan surface, and then along the northwestern margin of Dagg Basin where a releasing bend on the fault results in localized extension and increased vertical displacement (Barnes et al., 2005). The southern edge of Doubtful Fan is characterized by a prominent escarpment up to 70 m high marked by the white dashed line in Fig. DR9A. This escarpment can be reconstructed to a single linear feature after recovery of 520 ± 30 m of dextral displacement (Fig. DR9B). The older dissected fan surface and a prominent ridge on the western side of Doubtful Fan can be reconstructed after recovery of 2225 ± 75 m of dextral displacement on a slip azimuth of 039 (Fig. DR9C). The reconstruction accounts for substantial subsidence in the northern end of Dagg Basin, and realigns elevated features of the old fan surface. Based on the total displacement of 2225 ± 75 m, and the post 17 ka displacement rate

7 (Table DR2), the age of the older fan surface is estimated to be 71 ± 11 kyrs old. This age is consistent with a major glacial advance documented in south Westland by Sutherland et al. (2007). Comparison of Onshore-to-offshore Alpine Fault Slip Rates along Strike between South Westland and Central Fiordland Post-glacial (<20 ka) dextral displacements on the southern, offshore part of the Alpine Fault can be compared with similar aged features along an adjacent 80 km section of the fault on land, south of Jacksons Bay in south Westland (Sutherland et al., 2006a) (Fig. DR1). Whilst there is significant overlap between the ranges of uncertainty from individual sites, a regression line based on the individual displacements, and differences in the weighted mean displacements of three separate groups (south Westland, North group, and South group), indicate an increase in post-glacial displacement to the south. Using the absolute offsets an unpaired Students t-test assuming equal variances indicates a probability (P one tail) of 0.05 (t statistic -1.8, 11 degrees of freedom (df)), indicating that the southward increase in the weighted mean post-glacial displacement, from onshore in South Westland (416 m) to the Milford to George sound shelf (463 m) is significant at the 5 % and 1 % levels. It follows from a similar comparison of the displacements in south Westland and offshore between Caswell and Doubtful sounds (South group) that the latter weighted mean (534 m) is significantly larger (t -7.0, df 10, P one tail 0) at the 1 % level. If we assume all these glacial features are coeval, the displacements indicate that the fault slip rate increases to the south over the km length of the fault between south Westland and the central Fiordland margin.

8 Tables and Figure Captions Table DR1. Radiocarbon dates of submarine glacial outwash fans on the Fiordland continental margin. The samples were recovered from the surface of the fans in seabed cores and dredge samples acquired in 2004 on Tangaroa voyage Tan0405. Shell fragments were identified by Bruce Marshall at Te Papa Tongarewa Museum of New Zealand, and dated by the Institute of Geological & Nuclear Sciences. Table DR2. Post-glacial dextral displacements and slip rate on the southern Alpine Fault. Figure DR1. Site 1. Bathymetric map of a submerged glacial moraine displaced by the Alpine Fault on the inner continental shelf off the northern side of Transit Valley. See Fig. 2B for location of site 1. The bathymetry is developed from 5 m binned, R.V. Tangaroa SIMRAD EM300 multibeam data. Bathymetric contours are at a 2 m interval. Note the extrapolation uncertainties in projecting the crest of the onshore lateral moraine offshore (see also Barnes et al., 2005), and the 500 m wide step-over in the Alpine Fault trace (bold white arrow) which complicates identification of the dextral displacement at this site. Figure DR2. Site 2. A. Bathymetric map of a submerged arcuate glacial moraine displaced by the Alpine Fault on the inner continental shelf off the southern side of Transit Valley. See Fig. 2B for location of site 2. Bathymetry data as in Fig. DR1. The crest and general outline of the moraine are marked by the bold blue and white dashed lines, respectively. The uncertainties in the displacement are indicated by the fine blue

9 dashed lines each side of the crest. B. SIMRAD EM300 multibeam backscatter sonar image revealing the highly reflective nature of the boulder moraine. Figure DR3. Site 3. A. Bathymetric map of a submerged glacial moraine displaced by the Alpine Fault on the inner continental shelf off the northern side of Poison Bay. See Fig. 2B for location of site 3. Bathymetry data as in Fig. DR1. The general outline of the moraine is marked by the bold white dashed line. B. Enlargement showing the displacement and uncertainties of the sharp northern edge of the moraine (see also Fig. 2A). Figure DR4. Site 4. A. Bathymetric map of ridges and gullies at the shelf break, displaced by the Alpine Fault, off the northern side of Sutherland Sound. See Fig. 2B for location of site 4. The bathymetry is developed from 5 m and 25 m binned, R.V. Tangaroa SIMRAD EM300 multibeam data. Bathymetric contours are at a 10 m interval. B. Enlargement showing the displacement and uncertainties of a prominent gully axis. Figure DR5. Site 5. Bathymetric map of the Alpine Fault crossing the upper continental slope about 7 km south of George Sound. The bathymetry is developed from 25 m binned, R.V. Tangaroa SIMRAD EM300 multibeam data. Bathymetric contours are at a 10 m interval. The displacement and uncertainties of two prominent channels are highlighted.

10 Figure DR6. Site 6. A. Bathymetric map of the Alpine Fault crossing the northern part of Caswell Fan. See Figs. 1A and 3A for location. (See also Fig. 4). Bathymetry data as in Fig. DR5. B. Enlargement of geomorphic features on the outwash surface numbered 1 to 5 perfectly reconstructed after recovery of 540 m of dextral displacement. Figure DR7. Site 7. Bathymetric map of the Alpine Fault crossing the southern part of Caswell Fan between Caswell and Charles sounds. Bathymetry data as in Fig. DR5. The displacement and uncertainties of 530 ± 40 m of two sets of ridges and channels about 4 km apart are highlighted. Figure DR8. Site 8. A. Bathymetric map of the Alpine Fault crossing Doubtful Fan off Doubtful Sound. Bathymetry data as in Fig. DR5. The location of Doubtful Sound is shown on Fig. 1A. White arrows indicate the dextral displacement of prominent geomorphology on the outwash surface. B. Reconstruction of the ridges and gullies on the fan surface after recovery of 540 m of dextral displacement. The red dashed and yellow lines show the present day and restored positions of the geomorphic features, respectively, relative to the fan surface east of the fault. Figure DR9. Site 9. A. Bathymetric map of the middle continental slope off Doubtful and Dagg sounds, where the Alpine Fault (red line) crosses the southern edge of last glacial Doubtful Fan and an elevated and dissected older fan surface, and bounds the northwestern margin of Dagg Basin. Bathymetry data as in Fig. DR5. White dashed lines mark the southern edge of the last glacial Doubtful Fan, which is displaced by 520 ± 30

11 m on a slip azimuth of 039. Yellow lines mark an older geomorphic feature on the fan surface displaced by 2225 ± 75 m. B. Reconstruction of the southern edge of Doubtful Fan after recovery of 520 m of dextral displacement. C. Reconstruction of an older fan surface south of the last Glacial Doubtful Fan after recovery of 2225 m of dextral displacement on a slip azimuth of 039.

12 TABLE DR1. RADIOCARBON DATES OF SUBMARINE GLACIAL OUTWASH FANS Sample Water Deposit Location Dated shell Radiocarbon age Calibrated radiocarbon age Number Depth (yr BP) ( yr BP) (m) Lat ( S) Long ( E) Tan0405#41 * Gravel beneath 0.1 m post-glacial sediment Cardita aoteana ± ± 430 (0.39 M) NZA Tan0405#8 Graded gravel beneath 0.9 m postglacial mud (0.95 m) Mytilidae ± ± 510 NZA Tan0405#12 Gravel beneath ± ± 350 (1.1 m) reworked postglacial sediments Maoricolpus roseus NZA Tan0405#53 Mud immediately ± ± 560 (0.43 m) overlying youngest Plantic forams gravel NZA Tan0405#38 Gravel beneath ± ± 500 (0.24 m) reworked postglacial sediments Tawera spissa NZA Tan0405#20 (1.07 m) Surface graded gravel Mytilus galloprovincialis ± ± 200 NZA * National Institute of Water & Atmospheric Research station number. Bracketed number, e.g., (0.39 M), is the sample depth below seabed in sediment core. Rafter Radiocarbon Laboratory code, Institute of Geological & Nuclear Sciences. Calibrated age expressed at 2 sigma confidence level.

14 TABLE DR2. DEXTRAL DISPLACEMENTS AND SLIP RATE ON THE SOUTHERN, OFFSHORE ALPINE FAULT Site Displaced markers Location Lat Long ( S) ( E) Age (cal. ka) # C t +ve (cal. ka) C t -ve (cal. ka) Offset (m) C x Slip rate (mm/yr) (m) SR alt 1 SR alt 2 North group (Milford to George sounds) Mean Transit 1 moraine Nth Transit 2 moraine Sth Poison Bay 3 moraine Nth Sutherland 4 channels George Sth 5 channels (-3.9, +2.7) (-2.5, +2.4) (-2.5, +2.9) Standard deviation (MC) Weighted mean (-3.0, +1.8) (±1.0) (-1.5, +1.8) South group (Caswell to Doubtful sound) Caswell Fan Nth ridges & 6 gullies Mean Caswell Fan Sth ridges & gullies Doubtful fan central ridges & gullies Doubtful fan Sth ridge (-3.5, +2.1) (-1.1, +1.2) (-1.9, +2.1) Standard deviation MC Weighted mean (-3.5, +2.1) (±1.1) (-1.9, +2.1) # Adopted age for glacial features considering dated samples in Table DR1 and published regional data on glacial advances. C t +ve Upper confidence interval in adopted glacial age; C t -ve Lower confidence interval in adopted glacial age. C x Offset confidence interval half-width. All slip rate uncertainties are derived from combined Monte-Carlo simulations of normal distributions of the individual offsets and the respective age models (with 1500 random simulations in each distribution), and are expressed at the 95% confidence level. SR alt 1 Alternative estimate of slip rate using the first five actual shell ages (mean age 16.7 cal. ka) in Table DR1, as indicative of the glacial geomorphic age. SR alt 2 Alternative estimate of slip rate for 18 ± 1 cal. ka glacial age model (Sutherland et al. 2006). Standard deviation (MC) derived from the Monte-Carlo simulations of the offsets. Weighted mean follows method of Sutherland et al. (2006), whereby weighting contributions account for level of measurement precision, i.e., displacement measurements with lower uncertainty carry more weight in the mean.

Sutherland et al: Glacial chronology, NZ

Sutherland et al: Glacial chronology, NZ Orbital forcing of mid-latitude southern hemisphere glaciation since 100 ka, inferred from cosmogenic nuclide ages of moraine boulders from the Cascade Plateau, southwest New Zealand Rupert Sutherland