1 10 1 https://theses.otagogeology.org.nz/files/original/f85520e0eb43266068d4a9c41e908b60.pdf 594ecbd8b0b41a855daaf3a1db6508d4 Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Geology theses OU Geology thesis Thesis or dissertation completed by University of Otago Geology students Location WKT (WGS84) The location stored in WKT (WGS84) format POLYGON ((176.333460732912329 -37.663987423543418,177.005428982437024 -37.86470324877925,175.703755616851822 -39.367886940547812,175.514151721838346 -39.347112241207078,175.431716503745633 -39.227295538289717,176.333460732912329 -37.663987423543418)) Author last name Last name of the Author Rowland Project type Is it an MSc, PhD, BSc(Hons) or PGDipSci? PhD Advisers Who supervised/advised this student Sibson, R. Abstract The Abstract for this thesis The Taupo Volcanic Zone (TVZ) is a young (~ 2 . Ma) arc-related region of continental · extension at the southern end of the Havre Trough, characterised by intense volcanism, and geothermal activity. Extension (predominantly orthogonal) is accommodated by a structurally and magmatically segmented translithospheric rift structure: the Ruaumoko Rift System, (RRS). The varying trends of extensional structures lie perpendicular to least principal str·ess trajectories in a locally heterogeneous stress field with normal dip-slip as the predominant fault mechanism. Within the onshore RRS, three 1"' order rift segments from SW to NE (S1, S2, S3) with characteristic length scale - 65 km are recognised, one of which (S2), is further partitioned into 2"d order segments of length ~ 20 km. Major accommodation zones coincide with active rhyolitic calderas (Taupo Caldera, Okataina Caldera) between the 1 '' order rift segments. Although the TVZ is currently the most active rhyolitic centre on Earth, producing c. 0.3 m3/s magma since 340 ka, convective heat transfer by aqueous fluid transport, (4200 ± 500 MW channelled through over 20 geothermal fields), is thought to be four times that transferred through volcanic eruption. Significant volumes of solutes (silica dominant) are therefore likely to be transported and redeposited within the seismically defined 6 - 8 km thick ~onvection zone, affecting permeability, rock strength and hydromechanical behaviour. Hydrological modelling within the TVZ has generally assumed flow distributed throughout a uniform rock mass or isotropic permeability within different rock layers, with topographically driven flow dominant in the near surface. This, despite the evidence from borehole data and from fossil hydrothermal systems (e.g., greywacke-hosted vein system at Kuaotunu, Coromandel) for concentrated large-volume channel flow along faults and extension fractures. Hydrothermal fluid focussing is of particular importance to the exploration industry in its search for · epithermal mineralisation, while the presence of structurally controlled directional permeability has important implications for the exploration for, and extraction of, geothermal fluids. This dissertation therefore considers structural and physical controls on fluid redistribution. It is argued that stress-controlled directional permeability characterises TVZ hydrology, with fluid focussing important near the base of the convection system and locally within cover sequences. The rifting process, in concert with large-volume volcanic·· eruptions and magrnatism, has generated, and continues to modify, a heterogeneous crustal assemblage and permeability structure. Two simple rift-related hydrological regimes are recognised: (1) Horizontal hydraulic connectivity in rift segments. Juxtaposed assemblages of highporosity pyroclastic rocks and lake sediments, low-porosity lavas, low-porosity greywacke basement, and most likely dikes in various stages of cooling, form variably linked cr2-elongate fluid compartments because of static fault-fl1,1id and dike-fluid interactions, enhancing flow along strike. Dynamic fault-fluid interactions and dikefluid interactions also drive along strike flow (e.g., 1922 Taupo swarm, 1987 Edgecumbe earthquake). Aftershock migration in the 1987 Edgecumbe sequence suggests that transient post-rupture permeabilities could exceed the typical bulk permeability in geothermal systems by several orders of magnitude. (2) Vertical hydraulic connectivity in accommodation zones. Interaction between offset rift segments induces tensile stress concentrations in accommodation zones, generating and maintaining a localised dilation zone. Additionally, damage zones at the lateral tips of seismogenic scale normal faults and interaction between fault splays should enhance vertical structural permeability in these regions. Accommodation zones may have developed above reactivated NW -trending basement structures; in which case, formation of vertical pipe-like conduits is probable under the current stress regime. Porosity destruction through mineral deposition occurs within boiling zones in the geothermal systems (e.g., Ohakuri) and is inferred near the base of the convective zone throughout the rift system. This, coupled with the expected increase in low-porosity rock types with depth (greywacke, intrusions, unfractured andesite), suggests that focussed flow exploiting structural 111 ,I ,,1 permeability will predominate at depths approaching the base of the convection system. Localised overpressured fluid compartments are likely to develop under these conditions, leading to episodic fault-valving through distributed fault-fracture meshes, with subsequent self-sealing. The peak in seismic swarm events at commensurate depths throughout the rift system (4 - 6 km) may relate to this process. The spatial distribution of geothermal plumes within the RRS has previously been described as , 'uniform' and similar to the pattern produced by the heating of a fluid saturated isotropic porous medium from below. However, pattern inspection demonstrates that the plumes are not uniformly distributed, notwithstanding a local area of regular spacing coincident with the highheat flux low-fault density Taupo-Reporoa Basin. Departures from uniform spacing can be explained by rift-related fluid redirection and, to a lesser extent, exploitation of caldera-related structural permeability. Moreover, the Taupo-Reporoa Basin, bounded by seismogenic-scale normal faults, contains a thick sequence of low-density low-porosity pyroclastic rocks, beneath which basalt sill formation seems likely. The regular spacing of plumes in this region may therefore reflect the presence of a localised sill at depth, which acts as a hotplate heating the overlying fluid saturated porous medium. Department The department where the student is studying primarily. Geology Named locality Named locality describing the field area location. Taupo Volcanic Zone Thesis description Number of pages, maps, CDs, etc. xv, 262 p. : ill. (some col.), maps (some col., some folded) ; 30 cm. Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Identifier An unambiguous reference to the resource within a given context 2001Rowland Creator An entity primarily responsible for making the resource Rowland, Julie V. (Julie Varina) Date A point or period of time associated with an event in the lifecycle of the resource 2001 Title A name given to the resource Hydrothermal fluid redistribution in a magmatic continental rift : the Ruaumoko rift system, Taupo volcanic zone, New Zealand Subject The topic of the resource Structural geology Igneous geology Volcanology continental rift fluid movement hydrothermal fluids magmatic continental rift magmatism rifts Ruaomoko Rift System