Geology and paleoseismology of the central Alpine Fault, New Zealand

Wright, Craig A. (Craig Andrew)

A 55 kilometre long section of the Alpine Fault was mapped at a 1:25 ,000 scale, from the upper reaches of the Mikonui River in the north, to the northern banlc of the Whataroa River in the south. Recent traces of the Alpine Fault have been identified in 21 locations, some of which yield time-averaged dip-slip and strike-slip rates. The minimum strike-slip rate north of the Waitaha River over the last c.2300 years is 8.4 ± 0.9 rnrn!yr, calculated from the offset of a stream channel across several fault traces. Vertical displacement of the extensive postglacial aggradation surface, across the Alpine Fault zone, was measured in four major river valleys. The age of surface is c.13-14 ka ( calendric ), which provides Holocene-averaged minimum vertical displacement rates of 4.3 - 7.1 mm/yr. Remnants of two uplifted major aggradational fluvial terraces occur on both sides of the Alpine Fault in the Kakapotahi River Valley, the top of the upper terrace being the post-glacial aggradation surface. Assuming a constant vertical displacement rate, the age of the undated lower terrace is 7. 6 ± 1.4 ka. Maximum dextral strike-slip rates calculated from the offset of the terrace risers are 33 ± 8 mm/yr for the upper terrace and 42 ± 9 rnrnlyr for the lower terrace. Substantial small scale variation in the strike and dip of the Alpine Fault plane .is attributed largely to the formation of low angle thrust sheets and the effects of erosion on topography and the resultant stress field. North of the Whataroa River, strike-slip zones linking offset oblique-slip segments are on a scale of 100 m or so rather than the kilometre scale which can be seen further south (c.f. Norris & Cooper 1995). Fault rocks in the footwall are characterised by Fraser Complex gneisses and mylonites, or moraine, overlain by a variable thickness of Quaternary gravel. The hangingwall consists of Haast Schist derived mylonites and cataclasite, with mylonitisation increasing in intensity toward the Alpine Fault. In Kaka Creek, where cataclasite is thrust over river gravel along a fault plane dipping 24° SE. A well exposed section of cataclasite and mylonite in the hangingwall, of at least 350 m true thickness, has foliations typically dipping 45-55° SE. Significant brittle deformation in the hangingwall is limited to within 200 m structurally above the Alpine Fault thrust, and the transition from mylonites to protomylonites occurs between c. 200-250 m from the Alpine Fault. This indicates that the zone of most significant deformation around the Alpine Fault is about 400 m wide at the surface and probably widens out slightly at depth. A band of marble mylonite, 7 m thick in Kaka Creek can be traced discontinuously in the hangingwall for about 23 km along. strike. Extensive carbonate veining of hangingwall rocks occurs in three locations, most notably in a 120 m high exposure in Douglas· Creek. The carbonate is largely dolomitic and contains quartz, pyrite and chalcopyrite. The origin of the carbonate fluid is unknown. North of the Waitaha River, a young fault scarp, cut in schistose alluvial fan gravel and moraine, is exposed in a recently milled podocarp forest at an altitude of 220 m. The scarp uncharacteristically faces southeast with up to 18 m of relative uplift on the north west side. Three sag ponds and a dammed swamp have developed along the downthrown southeast margin of the fault, the largest of which is 70 m long and contains 2.9 m of sediment. Radiocarbon dating of wood from the bottom of sag ponds and from peat horizons from a swamp which has been incrementally dammed by the fault scarp, provide a history of at least five ground rupturing earthquakes on the Alpine Fault in the past 1500 years. Ammal growth vii \ rings in slabs from 25 trees, largely Dacrydium cupressinum (Rimu), which have been selectively, felled along the fault scarp, were analysed for periods of forest-wide reduced radial growth, indicating trauma to the forest by either storms or earthquakes. Dates for four likely periods of strong ground motion since c. 1210 AD were deduced, of which three are consistent with the 14C dates and with dates derived by Yetton et al. (1998). The timing of the last ground-rupturing earthquake at Waitaha was determined as 1720 ± 10 yr AD, consistent with Yetton et al's 1717 AD Toaroha River event. The penultimate ground rupture at Waitaha (here termed the Macgregor Creek event) probably occurred at 1580 ± 10 yr AD, which is. distinct from the 1620 ± 10 yr AD Crane Creek event proposed by Y etton et al. (1998). The Macgregor Creek event, which was accompanied by rockfalls in the central Southern Alps, probably had a rupture length less than c. 100 km. The Alpine Fault has major earthquakes along it every c. 200-300 years and the time elapsed since the last earthquake has been 281 years (to 1998). Displacement on the Alpine Fault probably occurs during single large events. Empirical relationships of earthquake magnitude to fault and rupture parameters restrict the maximum magnitude of an Alpine Fault earthquake to M 7. 8-8 .2, though a series of smaller earthquakes down to c. M 7.1 are possible if segmentation occurs. For a single large earthquake, felt intensities over the South Island would be high, with bedrock intensities in Christchurch being in the region of MM7, and in Dunedin in the region of MM6, depending on rupture directivity and other seismic effects. Local soft-sediment effects would increase the felt intensity by c. 1-3 on the Modified Mercalli intensity scale.

xx, 260 p. : ill. (some col.), maps ; 30 cm.

1998Wright

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