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The relationship between short-term tectonics and mountain building in New Zealand

Prof Tim Wright (SEE), Dr John Elliott (SEE), Dr Sandra Piazolo (SEE), Dr Ian Hamling (GNS New Zealand)

Project partner(s): GNS New Zealand (CASE)

Contact email: t.j.wright@leeds.ac.uk

The project aims to improve our understanding of how mountains are built by combining high resolution geodetic (InSAR, GPS) measurements of present-day deformation in the South Island of New Zealand with geological data collected from the exhumed roots of the Alpine Fault.

In collisional zones, mountains are made by the interaction of external tectonic driving forces, internal gravitational forces and surface processes (erosion). The response of the crust to these forces is governed by the rheology of the crust. In the continents, one key question is whether crustal strength lies only in the upper seismogenic layer (Jackson, 2008) or is instead found in both the crust and mantle (Burov, 2010). Another key question is the degree to which mountain chains are supported dynamically by flow in the mantle (Molnar, 2013). This project will address these major questions by combining new observations from satellite geodesy (InSAR and GPS) with rheological constrained from lower-crustal rocks exhumed in the Southern Alps of New Zealand, an area of rapid continent-continent collision.

Figure 1: Vertical rates of motion across the Southern Alps in New Zealand, adapted from Beavan et al. (2010). Data are only available for a 1D profile; InSAR observations collected in this project will map vertical rates across the entire South Island of New Zealand.

The South Island of New Zealand is being deformed rapidly by the oblique collision of continental fragments on two tectonic plates (Okaya et al., 2013). This has created the dramatic Southern Alps and the Alpine Fault. Importantly, in the context of this project, high levels of annual rainfall are leading to rapid erosion and exhumation of rocks that had been deformed in the lower crust. Measuring the present-day rates of vertical motion across the Southern Alps is key to unravelling the factors that control the formation of mountains, but existing measurements from GPS are very sparse (Beavan et al., 2010). The recent launch of the Sentinel-1 constellation provides an exciting opportunity to map vertical motions over the entire South Island for the first time (Elliott et al., 2015). By combining these observations with geological constraints on the rheology of rocks exhumed from the lower crust (Gardner et al., 2016), the student will be uniquely placed to test models of how this major mountain chain was formed, and in particular to assess the contribution of strength in the lower crust and dynamic uplift from flow in the mantle.

Objectives

The project will have the following specific objectives:

  1. The student will use data from the new Sentinel-1 radar constellation to measure present-day rates of “line-of-sight” motion using InSAR in the South Island of New Zealand.
  2. In collaboration with CASE partner GNS New Zealand, who are responsible for GPS monitoring of deformation in New Zealand, and COMET partners in the UK, the student will combine the InSAR and GPS observations (e.g. Walters et al., 2014) to estimate a high-resolution 3D velocity field for the South Island of New Zealand. This will be the first time that vertical motions have been mapped across the entire mountain range.
  3. The student will analyse the microstructures and fabrics recorded in geological samples collected from the Southern Alps to place constraints on the rheology of the lower crust during the period of deformation.
  4. The data from the first three components will be used to test simple models of continental collision, assess the strength of the lower crust, and determine the contribution of dynamic topography to the formation of the Southern Alps.

We would expect the balance between these components to vary depending on the specific interests of the student.

Additional Information

The project is a CASE studentship with partner GNS New Zealand. As such the student will spend 3 months working with co-supervisor Dr Ian Hamling in Lower Hutt (Wellington). ). During the visit, it will be possible for the student to take part in a field trip to the Alpine Fault Zone on the South Island to collect sample for subsequent in-depth analysis. The student will have access to the excellent state-of-art analytical equipment available at Leeds. The student will be a member of COMET, the NERC Centre for the Observation and Modelling of Earthquakes, Volcanoes and Tectonics, and will be encouraged to collaborate with COMET partners across the UK.

The project would suit a numerate student with a background in earth sciences, geology, geophysics, physics or similar subjects. They will be provided with training in state of the art geodetic methods, computational techniques, and microstructural analysis.

References/Further Reading

BEAVAN, J., DENYS, P., DENHAM, M., HAGER, B., HERRING, T. & MOLNAR, P. 2010. Distribution of present‐day vertical deformation across the Southern Alps, New Zealand, from 10 years of GPS data. Geophysical Research Letters, 37.

BUROV, E. 2010. The equivalent elastic thickness (Te), seismicity and the long-term rheology of continental lithosphere: Time to burn-out “crème brûlée”?: Insights from large-scale geodynamic modeling. Tectonophysics, 484, 4-26.

ELLIOTT, J., ELLIOTT, A., HOOPER, A., LARSEN, Y., MARINKOVIC, P. & WRIGHT, T. 2015. Earthquake monitoring gets boost from new satellite. Eos, 96.

GARDNER, R. L., PIAZOLO, S. & DACZKO, N. R. 2016. Shape of pinch and swell structures as a viscosity indicator: Application to lower crustal polyphase rocks. Journal of Structural Geology, 88, 32-45.

JACKSON, J., MCKENZIE, D., PRIESTLEY, K. & EMMERSON, B. 2008. New views on the structure and rheology of the lithosphere. Journal of the Geological Society, 165, 453-465.

MOLNAR, P. & HOUSEMAN, G. A. 2013. Rayleigh‐Taylor instability, lithospheric dynamics, surface topography at convergent mountain belts, and gravity anomalies. Journal of Geophysical Research: Solid Earth, 118, 2544-2557.

OKAYA, D., STERN, T. & DAVEY, F. 2013. A Continental Plate Boundary: Tectonics at South Island, New Zealand, John Wiley & Sons.

WALTERS, R., PARSONS, B. & WRIGHT, T. 2014. Constraining crustal velocity fields with InSAR for Eastern Turkey: Limits to the block‐like behaviour of Eastern Anatolia. Journal of Geophysical Research: Solid Earth.

Related undergraduate subjects:

  • Chemistry
  • Earth science
  • Geology
  • Geophysics
  • Mathematics
  • Natural sciences
  • Physics