Extraction, Storage and Eruption of Multiple Isolated Magma Batches in the Paired Mamaku and Ohakuri Eruption, Taupo Volcanic Zone, New Zealand

The Taupo Volcanic Zone (TVZ) is well known for its extraordinary rate of rhyolitic magma generation and caldera-forming eruptions. Less is known about how large volumes of rhyolitic magma are extracted and stored prior to eruption, and the role tectonics might play in the process of melt extraction and control of caldera eruption(s). Here we present a new model for the extraction, storage and simultaneous eruption of the >245 km3 paired Mamaku and Ohakuri magmas sourced from calderas centred ∼30 km apart (the Rotorua and Ohakuri calderas, respectively) in the central TVZ. The Mamaku and Ohakuri ignimbrites share a similar bulk pumice composition and the same phenocryst assemblage; however, bulk-rock compositions suggest several poorly mixed magma types in each erupted volume, which are randomly distributed throughout the eruptive deposits. To refine models of the pre-eruptive geometry of the magmatic system and discuss a possible origin for triggering of each eruption, we present an expanded database of matrix glass and quartz-hosted melt inclusion compositions along with the existing bulk-rock and mineral compositions. Major and trace element compositions show that the region produced five different magma batches, extracted from the same source region, and a continuous intermediate mush zone beneath the Mamaku-Ohakuri region is suggested here. These magma batches were most probably juxtaposed but isolated from each other in the upper crust, and evolved separately until eruption. The observed geochemical differences between the batches are likely to be generated by different extraction conditions of the rhyolitic melt from a slightly heterogeneous mush. The lack of evidence for more mafic recharge prior to eruption (for example, there are no bright cathodoluminescence rims on quartz crystals) suggests that a magmatic input is unlikely to be an eruption trigger. However, tectonic activity could be an efficient way to trigger the eruption of isolated magma batches, with the evacuation of one magma batch causing a disturbance to the local stress field and activating regionally linked faults, which then lead to the eruption of additional magma batches and associated caldera subsidence. In addition, the extensional tectonic regime coupled with a high heat flux could be the controlling factor in the emplacement of some of the shallowest and most SiO2-rich magmas on Eart

A Combined Field and Numerical Approach to Understanding Dilute Pyroclastic Density Current Dynamics and Hazard Potential: Auckland Volcanic Field, New Zealand

The most dangerous and deadly hazards associated with phreatomagmatic eruption in the Auckland Volcanic Field (AVF; Auckland, New Zealand) are those related to volcanic base surges - dilute, ground-hugging, particle laden currents with dynamic pressures capable of severe to complete structural damage. We use the well-exposed base surge deposits of the Maungataketake tuff ring, (Manukau coast, Auckland) to reconstruct flow dynamics and destructive potential of base surges produced during the eruption. The initial base surge(s) snapped trees up to 0.5 m in diameter near their base as far as 0.7-0.9 km from the vent. Beyond this distance the trees were encapsulated and buried by the surge in growth position. Using the tree diameter and yield strength of the wood we calculate that dynamic pressures (Pdyn) in excess of 12–35 kPa are necessary to cause the observed damage. Next we develop a quantitative model for flow of and sedimentation from a radiallyspreading, dilute pyroclastic density currents (PDCs) to determine the damage potential of the base surges produced during the early phases of the eruption and explore the implications of this potential on future eruptions in the region. We find that initial conditions with velocities on the order of 65 m s- 1, bulk density of 38 kg m-3 and initial, near-vent current thicknesses of 60 m reproduce the fieldbased Pdyn estimates and runout distances. A sensitivity analysis revealed that lower initial bulk densities result in shorter run-out distances, more rapid deceleration of the current and lower dynamic pressures. Initial velocity does not have a strong influence on run-out distance, although higher initial velocity and slope slightly decrease runout distance due to higher rates of atmospheric entrainment. Using this model we determine that for base surges with runout distances of up to 4 km, complete destruction can be expected within 0.5 km from the vent, moderate destruction can be expected up to 2 km, but much less damage is expected up to the final runout distance of 4 km. For larger eruptions (base surge runout distance 4–6 km), Pdyn of \u3e 35 kPa can be expected up to 2.5 km from source, ensuring complete destruction within this area. Moderate damage to reinforced structures and damage to weaker structures can be expected up to 6 km from source. In both cases hot ash may still cause damage due to igniting flammable materials in the distal-most regions of a base surge. This work illustrates our ability to combine field observations and numerical models to explore the depositional mechanisms, macroscale current dynamics, and potential impact of dilute PDCs. Thus, this approach may serve as a tool to understand the damage potential and extent of previous and potential future eruptions in the AVF

Climbing the crustal ladder: Magma storage-depth evolution during a volcanic flare-up

© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Science Advances 4 (2018): eaap7567, doi:10.1126/sciadv.aap7567.Very large eruptions (>50 km3) and supereruptions (>450 km3) reveal Earth’s capacity to produce and store enormous quantities (>1000 km3) of crystal-poor, eruptible magma in the shallow crust. We explore the interplay between crustal evolution and volcanism during a volcanic flare-up in the Taupo Volcanic Zone (TVZ, New Zealand) using a combination of quartz-feldspar-melt equilibration pressures and time scales of quartz crystallization. Over the course of the flare-up, crystallization depths became progressively shallower, showing the gradual conditioning of the crust. Yet, quartz crystallization times were invariably very short (<100 years), demonstrating that very large reservoirs of eruptible magma were transient crustal features. We conclude that the dynamic nature of the TVZ crust favored magma eruption over storage. Episodic tapping of eruptible magmas likely prevented a supereruption. Instead, multiple very large bodies of eruptible magma were assembled and erupted in decadal time scales.This work was supported by the NSF (EAR-1151337) and by two Vanderbilt University Discovery Grants

Assessing debris flows using LIDAR differencing: 18 May 2005 Matata event, New Zealand

The town of Matata in the Eastern Bay of Plenty (New Zealand) experienced an extreme rainfall event on the 18 May 2005. This event triggered widespread landslips and large debris flows in the Awatarariki and Waitepuru catchments behind Matata. The Light Detection and Ranging technology (LIDAR) data sets flown prior to and following this event have been differenced and used in conjunction with a detailed field study to identify the distribution of debris and major sediment pathways which, from the Awatarariki catchment, transported at least 350,000 ± 50,000 m3 of debris. Debris flows were initially confined to stream valleys and controlled by the density and hydraulic thrust of the currents, before emerging onto the Awatarariki debris fan where a complex system of unconfined sediment pathways developed. Here, large boulders, clasts, logs and entire homes were deposited as the flows decelerated. Downstream from the debris fan, the pre-existing coastal foredune topography played a significant role in deflecting the more dilute currents that in filled lagoonal swale systems in both directions. The differenced LIDAR data have revealed several sectors characterised by significant variation in clast size, thickness and volume of debris as well as areas where post-debris flow cleanup and grading operations have resulted in man-made levees, sediment dumps, scoured channels and substantial graded areas. The application of differenced LIDAR data to a debris flow event demonstrates the techniques potential as a precise and powerful tool for hazard mapping and assessment

Experimental Thermal Stimulation of the Rotokawa Andesite

Thermal cycling of rock by heating and rapid quenching in water significantly affects its physical, mechanical and elastic properties. In this study we present a novel technique where specially designed equipment simulates the cyclic thermal stimulation processes employed by the conventional geothermal industry. To enhance productivity and injectivity of geothermal wells, geothermal operators commonly inject fluids cooler than reservoirs into wells at pressures less than natural fracture gradients which can result in enhanced fluid handling capacities. In an attempt to better understand this process, the investigation of thermal stimulation at a laboratory scale has been conceived and implemented. We have designed and built an apparatus that allows the heating and quenching of representative samples by thermal stimulation in a pressure vessel capable of attaining 350°C and 24 MPa and sustaining pressure during quenching cycles. Core sourced from production wells in an active commercial geothermal field has been tested in the apparatus. Our studies have characterized specimens prior to and subsequent to thermal stimulation for density, porosity, permeability, micro-structural texture, mineralogical fabrics, acoustic velocities, dynamic and static elastic moduli. Our results indicate that our stimulation apparatus is capable of enhancing both microscopic and macroscopic permeability, increasing porosity, reducing bulk density and attenuating seismic velocities. We have enhanced porosity in our specimens by up to 1.0 (vol% ) over original values, attenuated compressional wave velocities by up to 15% and enhanced permeabilities by nearly an order of magnitude over initially observed values. We utilize scanning electron microscopy to evaluate the microstructural change to samples, supplementing physical property investigations. The results imply that thermal stimulation can be successfully replicated in the laboratory and is coupled with both thermal and chemical components. The implications of this study for future laboratory and field scale stimulation testing are then considered

Ignimbrite flare-ups and their drivers: A New Zealand perspective

© 2016 Elsevier B.V. Ignimbrite flare-ups are periods of intense silicic volcanism characterized by multiple caldera-forming eruptions that together evacuate 103 to 104 km3 of magma. Ignimbrite flare-ups of different ages and from different tectonic settings have been well documented; however, in the literature, the distinction between an ‘ignimbrite flare-up’ and a ‘magmatic flare-up’ is not always obvious. We argue that the distinction is an important one as magmatic flare-ups do not always necessitate an ignimbrite flare-up, and thus the drivers for both require more investigation. Here we focus our review on the North Island continental arc of New Zealand, which is rarely included in published comparative studies of arc magmatism. Yet, it is well known for its extraordinary production of high-silica rhyolite and intensity of caldera-forming eruptions relative to other active arc systems, and a highly resolved understanding of the subduction plate boundary characterized by a rapidly migrating arc. Much of the present-day geologic footprint of the active part of the arc, the Taupo Volcanic Zone (TVZ), was established in a remarkable ignimbrite flare-up event between ~ 350 and ~ 280 ka. During this time, eight ignimbrite-forming eruptions occurred, evacuating ~ 3000 km3 of magma, and formed calderas that pepper a 90 × 40 km area. We divide the flare-up into 3 pulses of caldera activity, and track the magmatic input from the build-up to the first pulse, through to the final caldera forming eruption of the last pulse. Based on a comparison between New Zealand and other documented examples worldwide, we propose three ignimbrite flare-up categories based on their longevity and intensity. Most ignimbrite flare-ups last 106 to 107 years (categories 2 and 1, respectively), erupt magma volumes exceeding 104 km3 and are interpreted to be driven from depth by high mantle flux (i.e. magmatic flare-ups). Here, we draw attention to a new, much shorter timescale flare-up (104 to 105 years; category 3) as exemplified by the ~ 350 to ~ 280 TVZ example. Using the erupted volumes for several documented flare-ups at the three category timescales, we calculate a mantle input based on published silicic intrusive:extrusive ratios and isotopically derived ratios for basaltic input. Our results show that category 3 ignimbrite flare-up eruption rates typically exceed 10 km3 kyr− 1, have multiple caldera-forming eruptions, and are defined by mantle input rates that can be sustained through the duration of the ignimbrite flare-up and are at least an order of magnitude above the longer timescale category 1 and 2 ignimbrite flare-ups. For the TVZ, such high ignimbrite productivity over just tens of thousands of years, is related to an efficient feedback loop between high mantle flux and accelerated rifting that together are responsible for a remarkably thin and extended continental crust. We argue, that despite its unique nature relative to other continental arcs, there may be important magmatic-tectonic feedbacks that can be gleaned from the highly resolved record of caldera and ignimbrite volcanism in the TVZ, and applied to studying short timescale ignimbrite flare-ups elsewhere

U-Pb dating of zircon in hydrothermally altered rocks as a correlation tool: application to the Mangakino geothermal field, New Zealand

Recognition and correlation of rock units within geothermal fields is often hampered by high degrees of alteration that obscure primary mineralogies and lithological boundaries and preclude direct dating by radiometric techniques. Magmatic zircons are commonly present in silicic volcanic rocks, where zircon saturation was achieved and zircons crystallized up to the point of eruption. Young zircons are highly resistant to hydrothermal alteration and can yield a record of their crystallization ages in otherwise heavily altered rocks. Zircon crystallization age spectra have been obtained from three samples of cuttings and a core sample from ignimbrite penetrated in 3 drillholes up to ~3.2 km deep at the Mangakino geothermal field in New Zealand. The crystallization ages are similar between the drillcore and cuttings samples, indicating that downhole mixing of cuttings has not been important, and showing collectively that volcanic units of closely similar ages are represented between ~1.4 and ~3.2 km depth. This is despite apparent changes in the inferred primary volcanic lithology that had led to earlier inferences that multiple ignimbrites of contrasting age were present in this depth interval. Comparisons of zircon crystallization age spectra and inferred primary mineralogical characteristics from the drillhole samples with surficial ignimbrites that crop out west of Mangakino suggest that the boreholes have entered a >1.8-km-thick intracaldera fill of ignimbrite generated in the closely-spaced Kidnappers and Rocky Hill eruptions at ~1 Ma

Quantifying the stress distribution at the Rotokawa Geothermal Field, New Zealand

Knowledge of the orientation and magnitude of the principal stresses can be used to model the behavior of faults and fractures, and determine how they may influence fracture hosted permeability in geothermal reservoirs. The permeability of the Rotokawa geothermal reservoir is dominantly fracture hosted and tectonic stresses are largely responsible for maintaining fluid flow in the reservoir. Reactivation of a fault or fracture depends on its orientation relative to the orientation of the stress field and the magnitude of the principle stresses. The purpose of this study is to determine the magnitude of the three principal stress axes at Rotokawa, and how they vary spatially. This will help our understanding of the distribution of fracture-hosted permeability in the reservoir. In the extensional tectonic settings, such as the Taupo Volcanic Zone, the magnitude of the vertical stress is dominated by the weight of the overburden. Previous rock density studies on core from Rotokawa wells and on rock from other geothermal fields are used here, along with variable thicknesses of different geologic units, to model the vertical stress. Leak-off tests and acoustic images that contain stress induced features are used to quantify aspects of the minimum and maximum horizontal stresses. We show that the differential stress between the vertical and minimum horizontal is near the threshold for frictional failure. More importantly, preliminary results of our study indicate that spatial variation in the vertical stress magnitude may be an important factor in fracture permeability. This study highlights some of the difficulties faced when attempting to estimate stress magnitudes in a geothermal reservoir hosted in a complex volcanic terrain

Spatial distribution, and origin of paleo-rockfalls in the Rapaki area of the Port Hills, Christchuch, New Zealand

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