50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...

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Terrace Tunnel South Portal to the Basin Reserve, in Wellington City. The bypass is predominately at street level, but a section of the northbound lanes from Willis Street / Abel Smith Street intersection to Ghuznee Street involves construction of a depressed two-lane road. Opus International Consultants is Transit's consultants for this project from its inception to detailed design and currently construction management, and has developed innovative approaches to resolve the geotechnical challenges. The depressed section will pass under a new bridge at Vivian Street and will require extensive retaining structures. The innovative trench design incorporates a propped reinforced trough structure and 200 m long soil nail retaining walls between the Terrace Tunnel portal and Abel Smith Street. The up to 7 m high soil nail walls are supported by a total length of about 7000 m of soil nails. This will be the largest soil nail wall in New Zealand. Extensive geotechnical site investigations have been carried out to determine ground and groundwater conditions in the vicinity of the bypass route, particularly where the proposed road will be depressed below existing ground level, and adjacent to existing buildings. In the propped wall section, groundwater levels will be temporarily lowered during construction, then allowed to recharge. In the soil nail sections, groundwater levels will be permanently lowered. During construction a limited amount of excavation will be opened at any one time to minimise the amount of drawdown in groundwater levels. Artesian pressure relief wells are being drilled into bedrock to relieve groundwater pressures are underway. Building condition surveys, before during and after the construction are being undertaken. A programme was developed to monitor the groundwater, ground deformation, and subsidence. Instrumentation comprises of piezometers, inclinometers and settlement stations which were installed prior to commencement of excavation. Multiple piezometers were installed in many of the boreholes to record and monitor the groundwater levels in different aquifers. Over 90 vibrating wire and standpipe piezometer combinations were installed in 41 different boreholes throughout the Wellington Inner City Bypass area, measuring a number of different aquifers. Vibrating wire piezometers were used at locations important for construction monitoring, as they respond more quickly to groundwater level changes that may occur during construction, especially in the bedrock aquifer that will be drawn down during construction of the walls. The piezometers were installed in a number of site investigation stages. All 41 piezometer sets will be monitored regularly throughout the Wellington Inner City Bypass construction, a difficult undertaking in a dynamic urban environment, with traffic, vandalism, road maintenance, and urban development making continued monitoring challenging work. POSTER TRANSITION FROM SOFT-SEDIMENT TO WEAK ROCK DEFORMATION IN TURBIDITES OF THE MIOCENE WAITEMATA PIGGY-BACK BASIN, NORTHERN NEW ZEALAND K. B. Spörli &J.V.Rowland Department of Geology, University of Auckland, Private Bag 92019, Auckland. (kb.sporli*auckland.ac.nz) The well known, spectacular shore platform and cliff exposures of the Miocene Waitemata Group on eastern Whangaparaoa Peninsula 50 km north of Auckland provide a world-class example of weak rock deformation, the neglected domain between soft-sediment and hard rock deformation. As a new convergent plate boundary propagated southwards through northern New Zealand in the Late Oligocene, the Northland Allochthon was emplaced. The allochthon subsequently formed a mobile base for the Waitemata piggy-back basin, resulting in a complex interaction between tectonic and gravity-driven shallow deformation. Quartzpoor clastic sequences of the Waitemata Group at Whangaparaoa display a protracted sequence of deformations: D1, syn-sedimentary slumping at the top of the sedimentary sequence and including seismites; D2, large scale deeper-seated sliding and extensional low-angle shearing, associated with generation of boudinage and broken formation; D3, contractional shearing at low angle to bedding associated with thrust faults and inclined folds, indicating transport mostly to the SE; D4, thrusting and folding in the opposite direction; D5, further folding including sinistral shear; D6, steep faults. The deformation sequence suggests continuous or intermittent southeast-ward transport of units with increasing sedimentary and structural burial. Phase D3 boudinage and broken formation are localised and, with a few exceptions, occurred by brittle deformation. Subsequent folds show little variation in bedding thickness around their hinges. By phase D3, the rocks had therefore passed from the softsediment state to low levels of consolidation. However, with a compressive strength of ~5 MPa they are weak rocks even today. These deformations, although more intensive than elsewhere, are typical for the entire Waitemata basin. Such weak-rock deformation must be important in other sedimentary basins, especially in those associated with active convergent plate 50 th Kaikoura05 -82- Kaikoura 2005
oundaries and with immature source areas for their sediments. ORAL GEOPHYSICAL EXPLORATION OF THE ALPINE FAULT ZONE AND EVIDENCE FOR HIGH FLUID PRESSURE Tim Stern 1 , David Okaya 2 & Stuart Henrys 3 School of Earth Sciences, Victoria University Dept. of Geol. Sci., Uni. of Southern California, Los Angeles, USA IGNS, Lower Hutt. (tim.stern*vuw.ac.nz) A fresh look at the problem of sustaining fluid pressures in active fault zones can be obtained by studying a major continental transform other than the San Andreas Fault. The Alpine Fault (AF) of central South Island is one such example. From a global perspective the AF is unusual as it is a continental transform where both strike-slip (~ 35 mm/y) and dip slip (~10 mm/y) movement take place on the same fault plane. Moreover, geophysical evidence shows this fault plane dips at 40 to 60 o from the Earth’s surface down to a depth of at least 30 km where it soles out into what we interpret to be a decollement surface. Detailed analysis of seismic travel time anomalies and seismic reflection amplitudes show that above the dipping fault-plane is a banana-shaped region where seismic P-wave velocities are reduced by 6- 10%. This low-velocity region has dimensions of roughly 20 x 40 km with the shallowest and deepest points being at ~ 8 and 35 km, respectively. A magnetotelluric study shows a low electrical resistivity zone in the crust that correlates closely with the region of low seismic velocities. Interconnected water at, or close to, lithostatic pressure is the physical condition that would give rise to both the low resistivity and low P-wave velocities. Corroborative geological evidence for high fluid pressures can be seen in the form of young, undeformed, quartz veins that are within freshly exhumed crust east of the surface exposure of the Alpine fault. The question then arises: how are high fluid pressures contained in the fault zone? A simple answer is that fluid is supplied to the root of the fault zone at a rate that is higher than it can be removed via porous flow through the crust. Greywacke-schist rocks that are transported into the fault zone are thickened as they travel down a decollement surface; this surface has been partially imaged with seismic reflection methods. As the crustal rocks are thickened water will be released (zeolite � amphibolite prograde metamorphism) at an estimated rate of 0.15 % wt per km of burial. On the basis of our seismic crustal structure models, and assuming a regular continental permeability of 10 -17 m 2 , we calculate that water is supplied to the root of the Alpine fault zone at a rate that is about 40 times greater than it can bleed off through crust of regular permeability. For the majority of plate motion to be carried on a single inclined fault plane the fault itself is likely to be weak or of low friction. Most continental transforms with a component of convergence partition motion into a vertical strike slip fault and distributed thrust faults. Southern California is an example. We therefore argue that it is high fluid pressure, due largely to fluid-release from prograde metamorphism that reduces effective normal stresses in the Alpine fault zone. And it is possibly this reduction in normal stresses that allows plate motion to focus on a single inclined plane. In light of recently published findings of periodic slip and “slow” seismic tremor associated with fluidsaturated parts of other plate boundaries, it seems important to undertake long-term seismic and GPS monitoring of the central section of the Alpine Fault. ORAL SEISMIC HAZARD ANALYSIS IN NEW ZEALAND DURING THE LIFETIME OF THE GEOLOGICAL SOCIETY M.W. Stirling, G.H. McVerry, K.R. Berryman, &W.D.Smith Institute of Geological & Nuclear Sciences, PO Box 30368, Lower Hutt (m.stirling*gns.cri.nz) New Zealand sits astride the boundary of the Australian and Pacific Plates, where relative plate motion is expressed by frequent, and often damaging earthquakes. The Geological Society of New Zealand has been in existence throughout the evolution of earthquake hazard analysis in New Zealand, and many Society members have contributed to the associated advances in the science. The 1960s saw major contributions to understanding the patterns of seismicity in New Zealand, and the relationships between active faults and earthquakes. Prominent scientists such as the late Frank Evison and late Gerald Lensen were leaders in these efforts, along with Robin Adams, who showed the importance of parameters like tectonic setting, fault slip type and soil properties in the assessment of earthquake hazard. The USAbased formulation of probabilistic seismic hazard (PSH) methodology by Allin Cornell in the late 1960s led to the development of PSH models for New Zealand in the 1970s and 80s. These PSH models were often the combined products of engineering seismologists, seismologists and geologists, but largely limited to the historical record of seismicity. The well-known “Smith and 50 th Kaikoura05 -83- Kaikoura 2005
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CHARACTERISATION OF NEW ZEALAND NEP
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myrtaceous. One 20 mm diameter flow
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ange of lower resolution or fragmen
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CRETACEOUS-EOCENE TRANSGRESSION MAR
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consists of a well-developed canyon
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model. The relative position of the
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converts to plate theory. Thus, the
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and on the following morning the mi
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the LGM grass peak. This period of
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offshore west coast to offshore eas
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4 Institut de Recherche pour le Dé
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VEGETATION AND CLIMATE CHANGE AT TH
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wider range. The youngest layer has
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few normal faults visible especiall
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TRACKING CRUSTAL DIFFERENTIATION AN
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Page 45 and 46: Tephra beds are commonly used to ai
Page 47 and 48: The MCC is bounded by deep-seated t
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