Temperature gradients within and adjacent to the Taupo Volcanic Zone

Thompson, G. E.

doi:10.1080/00288306.1980.10424149

Cited by 6 publications

(3 citation statements)

References 2 publications

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“…On the lower boundary there is no fluid flow, and the adopted thermal conductivity of 2.5 W/m/K and the assumption of a steadystate conductive geothermal gradient of 20 • C/km for tectonically stable continental crust (e.g., Ranalli 1986) is met by imposing a laterally constant basal heat flux of 50 mW/m 2 . For the TVZ region 50 mW/m 2 is a low heat flux, but falls within the range of measured heat flows (e.g., Studt and Thompson 1969;Thompson 1977Thompson , 1980. This heat flux is augmented within the rift in later models by addition of magmatic heat sources, as described in Sect.…”

Section: Maximum Shear Stress (Mpa)mentioning

confidence: 95%

Convective Flows in a TVZ-like Setting with a Brittle/Ductile Transition

Kissling¹,

Ellis

Charpentier

et al. 2009

Transp Porous Med

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This article presents a simple method for coupling the equations for fluid and heat flow in a porous medium with a rock mechanics model based on a simplified Mohr-Coulomb yield criterion. The aim is to investigate the effects of introducing a brittle-ductile transition zone (BDTZ) into models of the large scale hydrology of systems like the Taupo Volcanic Zone (TVZ) in New Zealand. The coupling between fluid/heat flow and rock mechanics is only partial in that it assumes that the local strain rate and lithostatic pressure are known a priori. A simple empirical relationship between the permeabilities of rock in its brittle and ductile states is also assumed, which mimics the effect of slow thermal creep in closing fluid pathways by reducing the effective permeability. This model is first applied to a number of simple situations where large scale convection of groundwater occurs above a depth of 20-km. These examples demonstrate the formation of a horizontal brittle-ductile transition zone under conductive conditions with uniform strain, and how this responds to local changes in pressure, temperature and strain rate. The presence of low permeability below the BDTZ effectively defines the greatest depth to which groundwater can steadily penetrate, providing a feedback to these models which influences the transport of heat and mass on all the length scales. The TVZ provides inspiration for the second model setting. Localised sources of water and heat, modelled on magmatic sill-like structures at a depth of 10-km, induce a BDTZ which shallows from 14-km to less than 10-km, consistent with geophysical estimates in the TVZ. A common feature of the models is that strong downflows of surface water occur in permeable regions adjacent to the heat sources, which depress the BDTZ by several kilometres between the geothermal areas. Water with near-surface temperatures can thus exist at great depths in these regions. Lastly, the models imply that the geothermal areas 123 336 W. Kissling et al. in the TVZ probably do not occur directly above any localised heat source, but are instead displaced towards the centre of the TVZ rift.

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Section: Maximum Shear Stress (Mpa)mentioning

confidence: 95%

Convective Flows in a TVZ-like Setting with a Brittle/Ductile Transition

Kissling¹,

Ellis

Charpentier

et al. 2009

Transp Porous Med

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“…Hornblende barometry (Anderson & Smith, ) from the Banská Hodruša granodiorites brought comparative results with pressure determination of 110–200 MPa (P. Konečný, ), corresponding to the granodiorite emplacement depths of 3.3–5.5 km and extremely high geothermal gradient of the order of 200 °C/km that is entirely plausible estimate for active volcanic regions (e.g. Thompson, ). Our (U–Th)/He data document a rapid cooling to near‐surface temperatures that followed in less than 1 Myr after the emplacement.…”

Section: Interpretation and Dicussionmentioning

confidence: 96%

Rapid cooling and geospeedometry of granitic rocks exhumation within a volcanic arc: A case study from the Central Slovakian Neovolcanic Field (Western Carpathians)

Kohút

Danišík

2017

Island Arc

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U–Pb Sensitive High‐Resolution Ion MicroProbe (SHRIMP) dating of zircon in combination with (U–Th)/He dating of zircon and apatite is applied to constrain the emplacement and exhumation history of the youngest granitic rocks in the Western Carpathians collected in the Central Slovakian Neovolcanic Field. Two samples of diorite from the locality Banky, and granodiorite from Banská Hodruša yield the U–Pb zircon concordia ages of 15.21 ±0.19 Ma and 12.92 ±0.27 Ma, respectively, recording the time of zircon crystallization and the intrusions’ emplacement. Zircon (U–Th)/He ages of 14.70 ±0.94 (Banky) and 12.65 ±0.61 Ma (Banská Hodruša), and apatite (U–Th)/He ages of 14.45 ±0.70 Ma (diorite) and 12.26 ±0.77 Ma (granodiorite) are less than 1 Myr younger than the corresponding zircon U–Pb ages. For both diorite and granodiorite rocks their chronological data thus document a simple cooling process from magmatic crystallization/solidification temperatures to near‐surface temperatures in the Middle Miocene, without subsequent reheating. Geospeedometry data suggest for rapid cooling at an average rate of 678 ±158 °C/Myr, and the exhumation rate of 5 mm/year corresponding to active tectonic‐forced exhumation. The quick cooling is interpreted to record the exhumation of the studied granitic rocks complex that closely followed its emplacement, and was likely accompanied by a drop in the paleo‐geothermal gradient due to cessation of volcanic activity in the area.

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“…The three geographic zones of the TVZ are delineated based on eruptive rates, manifestation and composition. The central TVZ is characterized by a heat output of ∼4.2GW (concentrated almost exclusively at the geothermal fields; Thompson, 1980;Bibby et al, 1995) and >6000 km 3 of Quaternary, rhyolitic volcanism (Wilson et al, 2009, calderas, Figure 1.4). The northern and southern zones (northeast of Kawerau and southwest of Tokaanu, not shown in Figure 1.4) are characterized by andesite-dacite composite cones (larger in the south) and very little rhyolitic or basaltic products.…”

Section: • Stimulation Testingmentioning

confidence: 99%

Characterizing microseismicity at the Rotokawa and Ngatamariki geothermal fields

Hopp¹

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<p>In this thesis, we construct a four-year (2012–2015) catalog of microearthquakes for the Ngatamariki and Rotokawa geothermal fields in the Taupō Volcanic Zone of New Zealand, and use these data to improve the knowledge of reservoir behavior. These microearthquakes occur frequently, often every few seconds, and therefore provide a tool that we use to assess reservoir properties with dense spatial and temporal resolution as well as to illuminate the underlying processes of seismogenesis. Using a matched-filter detection technique we detect and precisely relocate nearly 9000 events, from which we calculate 982 focal mechanisms. At Ngatamariki, these results constitute the first detailed analysis of seismicity at a newly-developed resource. It has been commonly assumed that induced shear on fractures increases reservoir permeability by offsetting asperities on either fracture wall, thereby propping the fracture open. During stimulation treatments of two boreholes (NM08 and NM09), borehole permeability experiences logarithmic growth. At NM08, this growth occurs for eight days in the absence of seismicity, while at NM09 only nine microearthquakes are observed during the one-month treatment. This suggests that hydro-shear, the process of inducing seismicity through increased pore pressure at critically-stressed fractures, is not the dominant mechanism of permeability increase at many geothermal wells. Instead, aseismic processes, likely thermal and overpressure induced fracture opening, dominate well stimulation in high-temperature geothermal settings. At Rotokawa, the earthquake frequency-magnitude distribution (b-value) is positively correlated with both proximity to major injection wells and depth. In an inferred pressure compartment near injection well RK23, b is ~1.18, but is <1.0 elsewhere, suggesting a connection between increased pore-fluid pressure and small-magnitude events. In addition, throughout the reservoir b increases from a value of ~1.0 at injection depth to almost 1.5 two kilometers below the reservoir, consistent with observations at volcanic areas elsewhere, but opposing the conventional wisdom that b-value is inversely proportional to differential stress. Finally, the 982 focal mechanism observations that we invert for stress show a normal faulting regime throughout both reservoirs. At Rotokawa, a lowering stress ratio, v, after reintroduction of injection well RK23 (v drops from 0.9 to 0.2 over six months) indicates that anisotropic reservoir cooling affects the reservoir stress state through a process of preferential stress reduction.</p>

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Temperature gradients within and adjacent to the Taupo Volcanic Zone

Cited by 6 publications

References 2 publications

Convective Flows in a TVZ-like Setting with a Brittle/Ductile Transition

Convective Flows in a TVZ-like Setting with a Brittle/Ductile Transition

Rapid cooling and geospeedometry of granitic rocks exhumation within a volcanic arc: A case study from the Central Slovakian Neovolcanic Field (Western Carpathians)

Characterizing microseismicity at the Rotokawa and Ngatamariki geothermal fields

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