Deeper Part of Taupo Volcanic Zone as Revealed by Geophysics - Essay Example

Running head: TAUPO VOLCANIC ZONE Deeper Part of Taupo Volcanic Zone as Revealed by Geophysics Name Course Tutor Date Introduction The Taupo Volcanic Zone (TVZ) is an extensive ‘inverse rhyolitic volcano’ found on the North Islands of New Zealand. With a stretch of caldera, the Taupo Volcanic Zone is the most active volcano and is therefore held accountable for most volcanic activities in the New Zealand. These activities are usually aggravated by the back arc basin principle which explains on how the magmatic substances found in the mantle well up to the crust. Geophysics studies carried out in the past and present indicate that this basin is found above the subduction zone found to the East of the extended volcanic crust. The basaltic and rhyolite calderas are nonetheless held answerable for the nature of activities found in this area. According to experts, the explosions experienced by the Taupo Volcanic Zone result to sticky magma with high silica content leading to Geophysics and other studies with a bid of establishing the internal features. It is evident from history that while silica rich magma is rising, it is contaminated by the continental crust and gas thereby resulting to an explosive mix (Explorevolcanoes.com, 2013). The Taupo Volcanic Zone is characterised by variable components due to longitudinal segmentation. These include features such as andesite composites and rhyolitic structures. The study on remote sensing and general structural data has been used to come up with the relationship between the internal struture and evolution of the Taupo complex (Bryan, Riley, Jerram, Stephens, & Leat, 2002). On the other hand, scientists have been able to come up with ideas that correspond to the dextral transtension of stratovolcanic features which are less active by nature. The interior geophysics is purely based on the past and modern developments on physical formation as most of the areas of study such as calderas are laced with evidences of younger volcamism. Another course of study is the faulting element which is also said to be active within the Taupo Volcanic vicinity. Phrases such as ‘sub-circular monogenetic’ and ‘post-collapse volcanism’ have emerged due to the progressive property of evolution a study on the Taupo Volcanic Zone (Spinks, Acocella, Cole, & Bassett, 2005). Finally, economic activities such as geothermal energy drilling have enhanced further studies on the geophysics of the Taupo Volcanic Zone aimed at maximizing the output from this expansive natural resource. Deeper part of the TVZ as revealed by geophysics It is evident that caldera and basement collapse are common occurrences in the Taupo Volcanic Zone with an exception of the Rotura. This is accompanied by the extension of the superficial regions which now remain undefined. According to Wilson (2012), there is enough information to support the fact that Maroa, Okataina and Taupo vent sites possess deep seated lineation and faults. In addition, the fused ignimbrites are protuberant due to the consequences of surface water on the eruptive behaviour of the calderas (Wilson, Rogan, Smith, J., A. , & Houghton, 2012). Onshore faulting is accounted for by geodetic triangulation, surface mapping and micro earthquake recording. In this case combinations of the above geophysical investigations have been used to make findings on how tectonic deformations are associated to late quaternary faulting. High resolution seismic profiling demonstrates that late quaternary tectonics are characterised by continuous normal faults of up to width. Faults extending from Mount Ruapehu to the Bay of Plenty have been discovered in the mid-twentieth century with the main course projected by the earthquakes which are concentrated at Lake Taupo. Faults along the north western margin are displaced by the Matahina ignimbrite that extends to the Matata Fault Zone (Wright, 1990). Seismic data has been used to illustrate that offshore faulting is outspread up to 2km deep into the isobaths on the continental slope. Long range sonar scans data shows shore parallel faults in areas surrounding the Motuhora and Rurima Islands. Bathymetry studies have also specified the morphology of the White Island Fault as comprising of a single scarp and a normal number of faults. The profiles represent glacial transgression as the sediments are displaced vertically across the faults. The Rurima faults are also said to jut seaward but cannot be traced on upper continental gradient. Seismic reflectors have further been used to collect data to prove that actually the vertical fault displacement phenomenon is present among the faults. The Pukehoko fault juts landward and has been a subject of study to the point that geophysicists have been able to come up with confirmations of vertically displaced tectonic structures. Still on the faults, the Nukuhou fault demonstrates repetitive state of vertical postglacial displacement. Another feature which stands out is the Ohiwa fault that extends deep into the White Island Canyon. This fault is in no way special thus the conclusion that most of the deep Taupo Volcanic Zone features tend to have taken a common trend of vertical postglacial displacement (Alistair & Browne, 1987). The calc-alkaline and the intra-arc nature of Taupo Volcanic Zone have attracted a lot of investigations on the tectonic formations. The Taupo–Hikurangi arc-trench system is found underneath the Australian plate and it is undeniably contiguous at the Kermadec Ridge– Havre towards the north. Underground intra-arc rifting in the North Island is associated with all forms of crustal thinning that extends up to . The calderas found in the central part have been found possess high plutonic and rhyolite substances at the bottom hence the inference that the region is highly volcanic. Studies have it that the caldera complexes found on Taupo Volcanic Zone are very fertile particularly Okataina, Reporoa, Rotorua and Taupo. The eruption rates range from to in the last making them conducive for geothermal studies (Spinks K. , Acocella, Cole, & Bassett, 2005). Structural analysis through remote sensing carried out over years shows that the modern TVZ comprises of five sections. These sections are purely due to dextral shear along the Taupo Volcanic Zone; estimated at approximately displacement. The extension index is projected at approximately andfor the dextral shear coefficient. These figures have been ascertained based on the internal features at the Whakatane and Ruapehu caldera complexes. Geophysical data is enough prove that major calderas portray the major activities in the Taupo volcanic locality (Spinks K. , Acocella, Cole, & Bassett, 2005). Located at around North West of the Okataina rift is the Rotorua caldera. This caldera is ignimbrite by nature meaning that it possesses a high eruption volume. Topographical investigations indicate that it is actually a single event caldera which shall not be erupting any time soon. The magma system is associated to Mamaku thus the geochemical formation it possesses. Gravity anomaly is consistently negative with a North South elongate as indicated by the magnetic studies. The internal collapse structure shows a fill up of volcaniclastic materials without restricting faults despite the active tectonics. The caldera dome complex consists of youthful morphologies that render the caldera’s topography partially continuous. Older faults are however found on the south east boundary of the caldera adjacent to the Okataina region (Cole, Milner, & Spinks, 2005). The Reporoa caldera is characterised by the lithic components which are due to asymmetrical collapse coupled by the radial pyroclastic flows. Its magma system is however declared geochemically distinct as the existence of older deposits on the caldera rims is evident. The caldera’s base portrays negative gravity and low caldera fill as the outstanding characteristics. Seismic data has recognized that the caldera slopes on either sides of the eastern and western margin possess a good correspondence towards the topographic expressions set to analyse the gravity behaviour of this feature. The interior magnetic studies show that the Reporoa caldera has consistent asymmetric collapse and a buried dome complex. The caldera contains old faults and there is no evidence of younger faults having formed over time thus it remains unaffected by further geographical activities in the area (Cole, Milner, & Spinks, 2005). The Okataina Caldera Complex is traversed with post-caldera rhyolite collapse structures which date back to. The magmatic volume is estimated at for both Matahina and Rotoiti ignimbrite which extend to the north and south of the caldera. From geographical data it is observed that the lavas underneath the caldera are petrogenetically distinct. There are two forms of pyroclastic magma found in this caldera, all due to small batches of eruptive activities. The internal residual gravity anomaly is negative although the north to south elongation is consistently mapped along the boundaries. The volcaniclastic sediments have been found to possess low densities for inner landscapes of up to deep as per gravity and magnetic data extracted from this zone. The collapse structure of the Okataina Caldera complex is estimated at a depth of with a clear low (Explorevolcanoes.com, 2013). The ignimbrites and lava are morphologically delineated between the youthful features and old ones forming an outstanding margin. At the floor of the caldera there are rhyolite massifs which dictate its composite nature. The topography extends from east south east towards the west south west faulting of the caldera. Deep inside the caldera, the rhyolite dome transects the linear vents that trend alongside the faults at the west just adjacent to the Kapenga and Whakatane segments (Explorevolcanoes.com, 2013). The Taupo caldera complex is the most active of all calderas on the Taupo Volcanic Zone. History indicates that the latest eruption occurred at around resulting to an estimated emission of magma and related substances. The petrological studies carried out in the mid-twentieth century showed that the magmatic system of Taupo caldera complex is distinctive. The caldera’s topography indicates that the larger homogeneous bodies such as Oruanui and Taupo resulted from distinct magma batches. However lithic componentry research indicates that Oruanui and Taupo lithic complements are purely made up of collapse ignimbrite structures. Data obtained through various geophysical methods has been used to document the fact that the gravity anomaly is negative at the northern part and perpendicular to the eastern boundary of the Taupo Volcanic Complex. The materials found in this region are of low density which demonstrates a discrepancy in structural boundary subsidence at the south of the Caldera (Cole, Milner, & Spinks, 2005). Digital elevation models illustrate the exact location of Taupo to be in the axial rift zone. Deep down the caldera topography, geophysicists have indicated regional faults and a series of other secondary features such as the bays and peninsulas. Below the north east depression is a fault that separates the Tongariro Volcanic center and the southern part of the Taupo Complex. Vents occurring at the eastern edge of the complex cannot go unmentioned since they also feature prominently (Institute of Geological and Nuclear Sciences Ltd, 2010). Geothermal research has associated the Taupo Volcanic Zone with subducted oceanic crust below the Indian and Australian plate. Melting occurs as the depth increases while the molten material is ejected to the crust due to buoyancy. This suggests the rapid extension of up to in terms of area with the ascending magma rate having been established as . It is this magma that supplies the heat for both geothermal and volcanic activities on the Taupo Volcanic Zone (Kissling & Weir, 2005). The geological composition of the rocks found on this volcanic zone is largely affected by the high chloride percentages of hot pressurized geothermal fluids emitted. As they pass through the rocks they dissolve in the silica rocks causing a major dominance in composition thus steam heated water ejected to the ground contains high amounts of hydrogen sulfides and oxides dissolved from the rocks. Another feature that reflects the acidic nature of the topography found on this geographical location is the fumarole which emerge from silica sinter depositions (Cody, 2007). The Taupo fault belt formed 2Ma ago is the main source of heat in this volcanic zone. This belt is therefore considered to be the main channel for flow of ground water into the deep crust which in turn heats the water by hydrothermal convection. The temperature gradients shift within the Taupo Volcanic Zone, an explanation fit for the history of volcanism. It is also understood that the components from the deep magma are transported to the surface due to permeability properties of the topography and the nature of the heating materials. Isotopical studies have been used to portray that the underground water sources are linked to rainfall in the area (Cody, 2007). Edgecumbe and other aftershock features resulting from earthquake swarms have been detected as a result of geothermal studies directed at establishing their distribution up to depth. A sharp drop in distribution among these features is observed for depth while the rocks become more brittle and ductile. Hydrothermal fluid is highly circulated in deeper areas of the Taupo Volcanic Zone the reason being that ductile rocks found in the lower limits cannot hold the convectional heat for a long period. The heat generation models are used as an alibi for the consistent permeable heat sources that infiltrate into the pores created during the tectonic extension. Magnetotelluric data shows that the Taupo fault belt below is caused by magma reactions with the permeable rock. It has also been established that the hydrothermal activities towards the eastern Taupo Volcanic zone are highest at depth (Cody, 2007). Deeper down the Taupo Volcanic Zone there are narrow plumes of water that are propagated by the convectional currents to the geothermal wells that have been used to tap the energy potential in the area. This fluid basically captures the heat through hydrothermal alteration to achieve superheated temperatures beyond (Williams, 2008 ). The resistivity models illustrate that these reaction occur in low resistivity areas of depths approximately from deep. These experiments also suggest that brittle features occur in high temperatures and great depths. Approximately deep in the resistive zone there is fracture permeability features fed with hot fluids extending into the rocky fractures. Magnetotelluric and seismic surveys further give an indication of fluid ejection at the Rotokawa geothermal fields. The local intrusions observed at the Whakamaru ignimbrite are accountable of providing heat to the zone for up to (Bertrand, et al., 2011). Some of the zones such as the Ngatamariki diorite are concluded to have cooled down due to lack of geothermal heat source. The intrusive sources of heat are likely to distribute heat through the convectional process to the whole geothermal system. The ductile rocks ranging from tend to bulge upwards at the foot due to continued exposure to heat. It is also projected that the hydrostatic pressure tends to be high between depths hence the elevated fracture permeability of the hot fluids that are found below the earth crust (Bertrand, et al., 2011). Close to twenty hydrothermal activities are purported to be scattered in the Taupo locality. Geophysicists demonstrate that these points are evenly spaced with their points of recharge as their spacers. These activities are considered to have been active between to with a high heat flow of. This heat is due to the arc volcanism that is caused by asthenosphere upwelling and plastic deformation of the crust and the boundaries. Researchers have used Ohaaki Broadlands situated at the eastern boundary of Taupo Volcanic zone to illustrate the intensity of hydrothermal activities. The physical composition of the Ohaaki consists of young quaternary rhyolites, ignimbrites, water tuffs, argillite, andesite, dacites, greywacke, and lacustrine rocks. According to the stratigraphy, basement fractures are the main conduits through which fluid flow. Temperature inversion is a common occurrence which occurs at superficial depths; an explanation for the up flow mechanism ( Simmons, Browne, & Scott, 2004). The highest heat levels registered at the Taupo Volcanic Zone’s geothermal wells are well beyond. These have been recorded in the geothermal fields such as Kawerau and Mokai at the east, a complete opposite of those found in the west of Taupo. At the temperature is expected to be around resulting to a temperature gradient of depth. These temperatures are however based on data obtained up to deep leaving a room for future further studies. Magma temperatures are also predicted to be approximately although this may be lower at points experiencing ground water flow (Bignall, 2011). Magnetotelluric measurements carried out to investigate the deep electric structure of the Taupo Volcanic Zone have made it clear it transects with the south eastern central volcanic region. The three dimensional phase tensor method applied indicates low resistivity below a depth of up to. This phenomenon is however consistent and low velocity transition boundary zones such those at the brittle and ductile rocks. The natural geothermal heat output of this area is approximated at. The electro-resistivity of the background geothermal fields amounts to as shown by the bipole-dipole resistivity surveys. This decreases at a tune of per kilometre of depth despite an inversion after the mark. This is due to combined low resistivity at shallow depths and an elevated temperature as one goes deeper. The low resistivity is responsible for heat transfer in the deeper Taupo Zone as is observed through geophysical measurements. From the geochemical data gathered it is true to state that the partial melting process occurs for up to an approximated of the solid rock. Inferences have been drawn that such liquid rock has a bulk resistivity ranging from (Alistair & Browne, 1987). The major shift in electrical resistivity is brought about by the altered mineral composition of the rocks found in the Taupo Volcanic Zone (Alcaraz, Rattenbury, Soengkono, Bignall, & Lane, 2012). Petrographic studies carried out on samples extracted deep indicate the existence of basalts with varied phenocryst, xenocrysts and xenoliths contents especially from the Okataina caldera. Phenocrysts found have as low as and for Okataina and Kapenga segments respectively despite the low percentages in xenocrysts and xenoliths. The extracts also show the presence of the rare compounds such as clinopyroxene, olivine and orthopyroxene. Plagioclase is the major ingredient of phenocryst at the euhedral-subhedral phase while olivine or augite comprise of the minor anhedral-subhedral phase. These patterns are however not consistent at some points of the Taupo topography as they seem to be concentrated with melted feldspar phenocrysts (Leonard, Cole, Nairn, & Self , 2002). Irregular fracturing is often observed at the liquidus stage when most rocks are still rich with the fluorine isotopes. Grey black glassy rocks have also been mined hence proving a point that the interstitial matrix still exists for the early stages of eruption. Also found in the rocks is the plume-like textured silicic glass that occurs in the thin vesicle regions especially below the base rims of the calderas. Accelerated crystallization at the boundaries is evident as plagioclase phenocrysts and cryptocrystalline aggregate are the major observable constituents. On the other hand aphyric basalts are concentrated at the vesicle axes that are found on the round trachytic surroundings. The basaltic magma conduits are characterized by the presence of xenoliths and xenocrysts which occur in form granite rocks. Extracts of plagioclase xenocrysts show that the process of solidification is stabilized due to fractionation in deeper zones (Hiess, Cole, & Spinks, 2007). Geophysicists have found evidences of mixing and mingling in the deep parts of Taupo Volcanic Zone. Rock types such as feldspar, orthopyroxene and quartz xenocrysts indicate that this phenomenon is actually rampant than in areas with more pronounced rhyolite and granite xenoliths in their topographies. The phenocrysts texture equilibrium is so imbalanced in contrast to the temperatures and composition; a major attribute to the mingling activities. The rock types are defined by contrasting compositions due to exposure to various heating and cooling temperatures. The glassy interface also illustrates the high probability that the calcic plagioclase laths that lace the edges of the topographic boundaries are as a result of mixing. The rhyolite eruptive deposits all over the Taupo Volcanic Zone serve as cheap evidence for this occurrence. The patch groundmass is also a clever indicator on the look of things as this describes the rapid disturbed cooling. This is due to differences in compositions, while mingling is caused by the conditions harsh temperature differences in magma (Murphy, Sparks, Barclay, Carroll, & Brewer, 2000). Mineralogy on compounds found in greater depths show that phenocrysts and microphenocrysts are the major components of feldspars, a major basalt in the Taupo zone. The compositional analysis shows inclusion of elements such as plagioclase microphenocrysts and silicic substances due to normal fractionation. Olivines have a variation in subhedral-anhedral basalts and microphenocrysts which are normally found along the topography boundaries. On the contrary, the clinopyroxenes have been found to consist of aphyric and porphyritic basalts particularly in the reaction centers (Graham, Cole, Briggs, Gamble, & Smith, 1995). Petrogenetic models in a bid to investigate the activities occurring deep down in the mantle indicate partial melting at the wedge between the subducting plate and the lithosphere. McCulloch and Gamble (1991) demonstrate that slab components due to this process have resulted to chemically enriched lithosphere. Differential melting is also evident due to activities such as crystal fractionation coupled with crustal contamination. The ratio is stipulated as beneath the Taupo Volcanic Zone, while the upwelling is active on the back-arc region. The source pressure is estimated at a maximum high of whereas the relationship between the samples varies according to depth (McCulloch & Gamble, Geochemical and Geodynamical Constraints on Subduction Zone Magmatism, 1991). Seismic studies have confirmed that beyond of depth, the velocities are uncontrolled thus brittle fractures are likely contrasting the nature of activities on the crust topography. Velocities of up to have been recorded for material movement and heat exchange in this layer. Inferences have been made that such areas are typically the upper mantle which indicates how superficial hot or molten materials are in the Taupo Volcanic Zone. The low velocity in the upper mantle cannot be demystified through studies relating to earthquake tomography according to Sherburn (2003) in Hiess (2007). This phenomenon is however explained by the mafic underplating revealing on how buoyancy subsides thereby setting the low velocity. Low residual levels due to evolved melts are accountable for low diffusion reactions and high alumina contents for substances found in this zone. A velocity of is projected for materials of up to a depth of all attributed to volatility of the partially melted magma. The Poisson ratio for magma accompaniments is actually a high of (Harrison & White, 2004). Crustal contamination is quite obvious from as per geochemical and petrographic studies carried out through final component identification for bulky deposits. Processes in the lower depths are identified through highly volatile compounds of elements such as sulfur and lithium. According to Gamble (1990), extracts infer a complex evolution in the lithospheric magma chambers and their respective plumbing systems. Morphology of this nature largely attribute to the anomalous heat transfer observed in the asthenosphere. Another factor attributing to such activities is the crustal thinning (Hiess, Cole, & Spinks, 2007). Conclusion The deeper part of Taupo Volcanic Zone as revealed by geophysics consists of numerous features worth further research. Intensified activities in the area due to better and advanced forms of data collection are likely to put most of the geographical doubts to rest. The major features discussed in this paper include the outstanding faults and the widespread calderas of the Taupo Volcanic Zone. As observed, the major methods used by geophysicists include geothermal and hydrothermal researches which seek to establish the heat exchange activities deep down the volcanoes. Through this ventures it has been established that the Taupo area consist vast amounts of heat energy that vary from. Magnetotelluric and internal residual gravity studies indicate a negative anomaly on most of the Taupo calderas. Petrogenesis studies have also been used to check on the ignimbrite nature of the materials found in the volcanic zone thus the ability to judge correctly on the fertility. Additionally three dimensional tools of data collection have been utilized in collection of scientifically viable data used to establish the topographical composition of the Taupo irrespective of depth. Seismic studies have been employed to come up with inferences on brittle materials and across the boundaries study. On the other hand mineralogy studies have been used to establish the basaltic compounds and the mixing and mingling phenomenon. References Alcaraz, S. A., Rattenbury, M. S., Soengkono, S., Bignall, G., & Lane, R. (2012). A 3D Multi-disciplinary Interpretation of the Basement of the Taupo Volcanic Zone, Newzealand. Thirty-Seventh Workshop on Geothermal Reservoir Engineering. Alistair , I. N., & Browne, P. (1987). Active Volcanoes and Geothermal Systems, Taupo volcanic zone: International Volcanological Congress. New Zealand Geological Survey. Bertrand, E. A., Caldwell, G., Hill, G. J., Wallin, E. L., Bennie, S. L., Cozens, N., et al. (2011). Magnetotelluric Imaging of Upper-crustal Convection Plumes Beneath the Taupo Volcanic Zone, New Zealand. Geophysical Research Letters, Vol. 39, L02304. Bignall, G. (2011). Taupo Volcanic Zone Deep Geothermal Drilling Project. Taupo: New Zealand Geological Survey. Bryan, S. E., Riley, T. R., Jerram, D. A., Stephens, C. J., & Leat, P. T. (2002). Silicic Volcanism: An Undervalued Component of Large Igneous Provinces and Volcanic Rifted Margins. Geological Society of America Special Paper 362, 99-120. Cody, A. D. (2007). Geodiversity of Geothermal Fields in the Taupo Volcanic Zone. Wellington: New Zealand Department of Conservation. Cole, J. W., Milner, D. M., & Spinks, K. D. (2005). Calderas and caldera structures: a review. Earth-Science Reviews 69, 1 – 26. Explorevolcanoes.com. (2013). Taupo Volcanic Zone:New Zealand . Retrieved January 10, 2013, from Explorevolcanoes.com: http://www.explorevolcanoes.com/Taupo-volcanic-zone-new-zealand.html Graham, I. J., Cole, J. W., Briggs, R. M., Gamble, J. A., & Smith, I. A. (1995). Petrology and Petrogenesis of Volcanic Rcks from the Taupo Volcanic Zone: A Review. Journal of Volcanologyand Geothermal Research 68, 59-87. Harrison, A. J., & White, R. S. (2004). Crustal Structure of the Taupo Volcanic Zone, New Zealand Stretching and Igneous Intrusion. Geophysical Research Letters 31, L13615, doi:10.1029/2004GL019885. Hiess, J., Cole, J. W., & Spinks, K. D. (2007). High-Alumina Basalts of the Taupo Volcanic Zone, New Zealand: Influence of the Crust and Crustal Structure. Ingham, M. (2005). Deep electrical structure of the Central Volcanic Region and Taupo Volcanic Zone, New Zealand. Earth Planets Space Vol. 57, 591–603. Institute of Geological and Nuclear Sciences Ltd. (2010, September 12). Taupo Volcano. Retrieved January 10, 2013, from Institute of Geological and Nuclear Sciences Ltd: http://www.gns.cri.nz/Home/Learning/Science-Topics/Volcanoes/New-Zealand-Volcanoes/Taupo-Volcano Kissling, W. M., & Weir, G. J. (2005). The spatial distribution of the geothermal fields in the Taupo Volcanic Zone, New Zealand. Journal of Volcanology and Geothermal Research 145, 136 – 150. Leonard, G. S., Cole, J. W., Nairn, I. A., & Self , S. (2002). Basalt Triggering of the C. AD 1305 Kaharoa Rhyolite Eruption, Tarawera Volcanic Complex, New Zealand. Journal of Volcanology and Geothermal Research 115, 461-486. McCulloch , M. T., & Gamble, J. A. (1991). Geochemical and Geodynamical Constraints on Subduction Zone Magmatism. Earth and Planetary Science Letters 102, 358-374. McCulloch, M. T., & Gamble, J. A. (1991). Geochemical and Geodynamical Constraints on Subduction Zone Magmatism. Earth and Planetary Science Letters 102, 358-574. Murphy, M. D., Sparks, R. S., Barclay, J., Carroll, M. R., & Brewer, T. S. (2000). Remobilisation of Andesite Magma by Intrusion of Mafic Magma at the Soufriere Hills Volcano Monserrat, West Indies. Journal of Petrology 41, 21-42. Nairn, I. A. (2010, January 26). Okataina Volcanic Centre Geology. Retrieved January 10, 2013, from Institute of Geological and Nuclear Sciences Ltd: http://www.gns.cri.nz/Home/Learning/Science-Topics/Volcanoes/New-Zealand-Volcanoes/Volcano-Geology-and-Hazards/Okataina-Volcanic-Centre-Geology Sherburn, S., Bannister, S., & Bibby, H. (2003). Seismic Velocity Structure of the Central Taupo Volcanic Zone, New Zealand, from Local Earthquake Tomography. Journal of Volcanology and Geothermal Research 122, 69-88. Simmons, S. F., Browne, P., & Scott, B. (2004). Geothermal Systems. Field Trip Guides, 13-40. Spinks, K. D., Acocella, V., Cole, J. W., & Bassett, K. N. (2005). Structural control of volcanism and caldera development in the transtensional Taupo Volcanic Zone, New Zealand. Journal of Volcanology and Geothermal Research 144, 7– 22. Spinks, K., Acocella, V., Cole, J., & Bassett, K. (2005). Structural control of volcanism and caldera development in the transtensional Taupo Volcanic Zone, New Zealand. Journal of Volcanology and Geothermal Research 144, 7-22. Williams, I. (2008 ). Geothermal Fields, Taupo Volcanic Zone, north Island, New Zealand. Retrieved January 11, 2013, from http://www.anaspides.net/earth_life_sciences/geothermal_fields_taupo_volcanic_zone_nz.html Wilson, N., Rogan, A. M., Smith, I., J., N. D., A. , N. I., & Houghton, B. F. (2012). Caldera volcanoes of the Taupo Volcanic Zone, New Zealand. Journal of Geophysical Research: Solid Earth (1978–2012) Volume 89, Issue B10, 8463–8484. Wright, C. (1990). Late Quaternary Faulting of the Offshore Whakatane Graben, Taupo Volcanic Zone, Newzealand. New Zealand Journal of Geology and Geophysics, 245-256. Read More

Seismic data has been used to illustrate that offshore faulting is outspread up to 2km deep into the isobaths on the continental slope. Long range sonar scans data shows shore parallel faults in areas surrounding the Motuhora and Rurima Islands. Bathymetry studies have also specified the morphology of the White Island Fault as comprising of a single scarp and a normal number of faults. The profiles represent glacial transgression as the sediments are displaced vertically across the faults. The Rurima faults are also said to jut seaward but cannot be traced on upper continental gradient.

Seismic reflectors have further been used to collect data to prove that actually the vertical fault displacement phenomenon is present among the faults. The Pukehoko fault juts landward and has been a subject of study to the point that geophysicists have been able to come up with confirmations of vertically displaced tectonic structures. Still on the faults, the Nukuhou fault demonstrates repetitive state of vertical postglacial displacement. Another feature which stands out is the Ohiwa fault that extends deep into the White Island Canyon.

This fault is in no way special thus the conclusion that most of the deep Taupo Volcanic Zone features tend to have taken a common trend of vertical postglacial displacement (Alistair & Browne, 1987). The calc-alkaline and the intra-arc nature of Taupo Volcanic Zone have attracted a lot of investigations on the tectonic formations. The Taupo–Hikurangi arc-trench system is found underneath the Australian plate and it is undeniably contiguous at the Kermadec Ridge– Havre towards the north.

Underground intra-arc rifting in the North Island is associated with all forms of crustal thinning that extends up to . The calderas found in the central part have been found possess high plutonic and rhyolite substances at the bottom hence the inference that the region is highly volcanic. Studies have it that the caldera complexes found on Taupo Volcanic Zone are very fertile particularly Okataina, Reporoa, Rotorua and Taupo. The eruption rates range from to in the last making them conducive for geothermal studies (Spinks K.

, Acocella, Cole, & Bassett, 2005). Structural analysis through remote sensing carried out over years shows that the modern TVZ comprises of five sections. These sections are purely due to dextral shear along the Taupo Volcanic Zone; estimated at approximately displacement. The extension index is projected at approximately andfor the dextral shear coefficient. These figures have been ascertained based on the internal features at the Whakatane and Ruapehu caldera complexes. Geophysical data is enough prove that major calderas portray the major activities in the Taupo volcanic locality (Spinks K.

, Acocella, Cole, & Bassett, 2005). Located at around North West of the Okataina rift is the Rotorua caldera. This caldera is ignimbrite by nature meaning that it possesses a high eruption volume. Topographical investigations indicate that it is actually a single event caldera which shall not be erupting any time soon. The magma system is associated to Mamaku thus the geochemical formation it possesses. Gravity anomaly is consistently negative with a North South elongate as indicated by the magnetic studies.

The internal collapse structure shows a fill up of volcaniclastic materials without restricting faults despite the active tectonics. The caldera dome complex consists of youthful morphologies that render the caldera’s topography partially continuous. Older faults are however found on the south east boundary of the caldera adjacent to the Okataina region (Cole, Milner, & Spinks, 2005). The Reporoa caldera is characterised by the lithic components which are due to asymmetrical collapse coupled by the radial pyroclastic flows.

Its magma system is however declared geochemically distinct as the existence of older deposits on the caldera rims is evident. The caldera’s base portrays negative gravity and low caldera fill as the outstanding characteristics.

Read More