Engineering behaviour and mechanical - empirical relationships for a problematic New Zealand tropical residual soil

Unlike sedimentary clays, many residual soils do not exhibit clear mechanical-empirical relationships to assist in their engineering characterisation. In contrast, this paper discusses one residual clay in which such relationships may be determined, and examines whether the effects of structure in this clay may be assessed using a framework previously developed for sedimentary clays. The Northland Allochthon residual clay of New Zealand is a problematic soil of the fersiallitic type, prone to slope instability. Atterberg limit tests on soils from five field sites in the same geological unit show considerable variation, but that they are mechanically related. Triaxial tests were performed on reconstituted and intact soil specimens from one field site. Normalization of the strength envelope using the equivalent stress on the intrinsic compression line suggests that soil structure, destroyed in reconstituted specimens, plays a role in the shear strength of this soil in its intact state. Overconsolidated behaviour, in the absence of geological preloading, suggests the existence of a pseudo-preconsolidation pressure associated with weathering processes. The results show that the saturated mechanical behaviour of this residual soil is in line with that of sedimentary clays and that mechanical-empirical relationships developed for such clays may be applied in this case

Giant rafted pumice blocks from the most recent eruption of Taupo volcano, New Zealand: Insights from palaeomagnetic and textural data

Giant blocks of pumice lie strewn along a former shoreline of intracaldera Lake Taupo, New Zealand, and are the sole subaerial evidence of the most recent volcanism at the Taupo supervolcano. Geochemically they are identical to material erupted during the complex and multiphase 1.8 ka Taupo eruption, which they post-date by one to two decades. The blocks, some of which are >10 m long, show complex jointing patterns indicative of both surface chilling and continued interior expansion, as well as heterogeneous vesicularity, with dense rims (mean density 917 kg/m3) grading via an intervening transition zone (mean density 844 kg/m3) into a more highly vesicular interior (mean density 815 kg/m3). Analysis of thermal demagnetisation data indicates significant reorientation of the blocks as they cooled through a series of blocking temperatures. Some parts of block rims cooled to below 580 °C well before emplacement on the shore, whereas other parts in the interior and transition zones, which cooled more slowly, acquired different orientations before stranding. Some block interiors cooled after blocks were finally deposited, and record the direction of the 1.8 ka field. The blocks are believed to be derived from one or both of a pair of rhyolitic lava domes that developed on the bed of Lake Taupo several decades after the climactic Taupo eruption over the inferred vent area.These, and similar giant rafted pumice blocks in other marine and lacustrine settings raise a number of questions about how volatile-rich felsic magma can be erupted underwater with only limited thermal fragmentation. Furthermore, the prolonged flotation of out-sized fragments of vesiculated magma formed during subaqueous dome-growth contrasts with the rapid sinking of smaller pieces of hot plinian pumice under laboratory conditions. The genesis of pumice forming the blocks is not entirely clear. Most simply the blocks may represent part of a vesiculated carapace of a growing lava dome, broken loose as the dome grew and deformed then rising buoyantly to the surface. Parts of the carapace could also be released by local magma-water explosions. Some textures of the pumice, however, suggest fresher magma released from beneath the carapace. This may suggest that silicic dikes and pillows/pods intruded into a growing mound of silicic hyaloclastite, itself formed by quench fragmentation and thermal granulation of the dike margins. This fragmental cover would have inhibited cooling of a still-hot and actively vesiculating interior, which was then released to float to the surface by gravitational destabilisation and collapse of the growing pile. Following their formation, the large fragments of pumice floated to the lake's surface, where they were blown ashore to become embedded in accumulating transgressive shoreface sediments and continue cooling

The million-year evolution of the glacial trimline in the southernmost Ellsworth Mountains, Antarctica

An elevated erosional trimline in the heart of West Antarctica in the Ellsworth Mountains tells of thicker ice in the past and represents an important yet ambiguous stage in the evolution of the Antarctic Ice Sheet. Here we analyse the geomorphology of massifs in the southernmost Heritage Range where the surfaces associated with the trimline are overlain by surficial deposits that have the potential to be dated through cosmogenic nuclide analysis. Analysis of 100 rock samples reveals that some clasts have been exposed on glacially moulded surfaces for 1.4 Ma and perhaps more than 3.5 Ma, while others reflect fluctuations in thickness during Quaternary glacial cycles. Modelling the age of the glacially moulded bedrock surface based on cosmogenic 10Be, 26Al and 21Ne concentrations from a depth-profile indicates a minimum exposure age of 2.1–2.6 Ma. We conclude that the glacially eroded surfaces adjacent to the trimline predate the Last Glacial Maximum and indeed the Quaternary. Since erosion was by warm-based ice near an ice-sheet upper margin, we suggest it first occurred during the early glaciations of Antarctica before the stepped cooling of the mid-Miocene at ∼14 Ma. This was a time when the interior Antarctic continent had summers warm enough for tundra vegetation to grow and for mountain glaciers to consist of ice at the pressure melting point. During these milder conditions, and subsequently, erosion of glacial troughs is likely to have lowered the ice-sheet surface in relation to the mountains. This means that the range of orbitally induced cyclic fluctuations in ice thickness have progressively been confined to lower elevations

Tectonic significance of Late Cretaceous Radiolaria from the obducted Matakaoa Volcanics, East Cape, North Island, New Zealand(MEMORIAL VOLUME TO THE LATE PROFESSOR TERUHIKO SAMESHIMA)

Radiolarian faunas from several localities in the Matakaoa Volcanics of the Mangaroa Range, North Island, New Zealand indicate Late Cretaceous (Campanian to Maastrichtian) ages. This contrasts with Late Paleocene/Early Eocene ages obtained from foraminifera approximately 3 km higher in the section. The radiolarian faunas are cosmopolitan and may have been deposited at mid to high latitude. A new structural analysis indicates that the fossils occur in a sequence of intercalated pillow lavas, sediments and dolerites about 10 km thick equivalent of layer 2B of the oceanic crust. This great thickness implies that there has been much repetition by shortening deformation. Layering dips steeply; younging is generally to the north and the sequence is affected by km-scale steeply plunging folds associated with decollements. Structural development involved first formation of melange, and subsequent refolding of these structures to give the present dextral, steeply plunging folds. It is not clear yet how another phase of deformation, steepening and overturning of layering in the Matakaoa Volcanics, relates to these two tectonic events.publishe

Vein deflections and thickness variations of epithermal quartz veins as indicators of fracture coalescence

Epithermal quartz veins at the Broken Hills gold deposit, New Zealand, strike N-S and dip steeply westward. Small changes in the orientations of the three main lodes and associated mesoscopic veins define deflection lines that rake steeply in the vein plane. A method of constructing the opening vector from the three-dimensional geometry of a vein deflection is presented. The resulting vectors plunge steeply and are at low angles to the main veins and the deflection lines, indicating a large component of normal dip-slip shear during the opening of these veins. Vein thickness distributions are power-law, with similar fractal dimensions to previously reported values for non-stratabound vein arrays. The vein system at Broken Hills developed by linking of isolated extension-dominated shear veins with shear-dominated veins, generating sub-vertical and sub-horizontal fluid flow pathways