Department of Engineering Science


Photograph of David Dempsey

David Dempsey

PhD

Thesis

Title: Numerical models of tectonism and geothermal circulation with application to the Taupo Volcanic Zone, New Zealand

Abstract: This thesis discusses the construction and application of a suite of numerical models that describe tectonic and geothermal processes occurring within the Taupo Volcanic Zone (TVZ) of New Zealand's Central North Island. The TVZ is an active rift characterised by a two million year history of rhyolitic volcanism, normal faulting and geothermal circulation. These phenomena operate and interact with one another over a variety of spatial and temporal scales; however, they are ultimately controlled by two processes: tectonic extension, and magmatic intrusion at the base of the crust.

The first part of this thesis discusses generic two-dimensional stress and deformation in an elastic-plastic-viscous crust that hosts a single normal fault. Short episodes of slip punctuate longer periods of quiescence; over many such cycles a critical stress state is evolved and the model demonstrates both coseismic uplift of the footwall scarp and subsidence of the hanging wall. The fault model illustrates non-intuitive energy flows associated with normal earthquakes. During the interseismic period, the crust uses elastic strain energy to cause extension; when an earthquake occurs, work is done by gravity to deform the hanging-wall and replenish some strain energy. Brittle yield of the crust occurs preferentially in the hanging-wall due to the distribution of stress maintained by repeated earthquake activity. This observation, coupled with fault rotation, serves as the basis of a scenario of rift evolution, in which faulting evolves in a basinward direction

The second half of the thesis details numerical models that investigate non-stationary behaviour in natural geothermal systems. First, a large-scale convection model is constructed to examine mass recharge; observations from this model lead to a new method for catchment delineation, which is applied to geothermal fields in the TVZ and Iceland. Second, a dynamic-permeability silica-transport model is used to replicate self-sealing behaviour in geothermal systems over several 100 kyr. Finally, the effects of fault permeability and earthquakes on geothermal upflow are described. Coseismic stress changes from the fault model are juxtaposed as diffusing pressure perturbations upon a fault-controlled inclined upflow zone. This enables an investigation of the short-term disruption to steady-state geothermal activity caused by normal fault earthquakes.”


Selected publications

Dempsey, D. E., Ellis, S. M., Archer, R. A., & Rowland, J. V., (2012), Energetics of normal earthquakes on dip-slip faults, Geology, v. 40, 279-282.

Dempsey, D. E., Rowland, J. V., Zyvoloski, G. A., & Archer, R. A., (2012), Modelling the effects of silica deposition and fault rupture on natural geothermal systems, Journal of Geophysical Research, doi: 10.1029/2012JB009218 .

Dempsey, D. E., Ellis, S. M., Rowland, J. V., & Archer, R. A., (2012), The role of frictional plasticity in the evolution of normal fault systems, Journal of Structural Geology, doi: 10.1016/j.jsg.2012.03.001.

Dempsey, D. E., Langhorne, P. J., Robinson, N. J., Williams, M. J., Haskell, T. H., & Frew, R. D., (2010), Observation and modeling of platelet ice fabric in McMurdo Sound, Antarctica, Journal of Geophysical Research, v. 115, doi:10.1029/2008JC005264.