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

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

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

Contact email: j.elliott@leeds.ac.uk

This exciting 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 et al., 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 and Houseman, 2013). This project will address these major questions by combining new observations from satellite geodesy (InSAR and GPS) with rheological constraints from lower-crustal rocks exhumed in the Southern Alps (Figure 1a). The outcome of this interdisciplinary research will be to put better constraints on the fundamental processes of the tectonic growth of mountains and the distribution of seismic hazard along a major continental fault zone.

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 width of the South Island of New Zealand, revealing the geometry of faulting and the rate and distribution of interseismic strain. This can be used to constrain rheological models of mountain building.

 The South Island of New Zealand is being deformed rapidly by the oblique collision of two regions of continental crust. This has created the dramatic Southern Alps and the Alpine Fault mountain range. Measuring the present-day rates of vertical motion across the Southern Alps is key to unravelling how they are formed, but existing measurements from GPS are widely distributed (Beavan et al. 2010, Figure 1b). The recent launch of the Sentinel-1 radar constellation provides an exciting opportunity and the technical ability to map vertical motions over the entire South Island for the first time (Elliott et al. 2016). By combining these observations with geological constraints on the rheology of rocks exhumed from the middle and lower crust  the student will be uniquely placed to understand 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.

The Alpine fault is unusual for a strike-slip fault in that it has a relatively shallow dip south-eastwards beneath the Southern Alps, whereas similar faults elsewhere are nearly vertical. As a result of the oblique convergence on this dipping structure, there is a significant vertical component of tectonic deformation (Lamb et al., 2015), leading to crustal thickening and mountain growth. However, in addition to the external tectonic driving forces, there are internal gravitational forces and surface processes from erosional and glacial unloading to be considered. Measuring rates of deformation can help resolve the various contributions these forces make to mountain building. Additionally, as this major fault is anticipated to rupture in a big earthquake this century, determining the rate of interseismic strain accumulation and constraining any changes in slip rate behaviour relative to measured background rates are important for determining seismic hazard for the region (Stirling et al., 2012).

As a consequence of the rapid vertical uplift along the Alpine fault, we have the unique opportunity to sample in the field, rocks that have undergone at mid to lower crustal levels the deformation that still governs deformation in the present day as recorded by geophysical techniques. Therefore, we have the unique opportunity to link the rock evidence directly to the recent deformation behaviour. To constrain the mechanisms of deformation, such as rheological behaviour and the interplay between ductile and brittle deformation at depth, the student will use macro- to microscale deformation features (Fig. 2 & Fig. 3)

Figure 2: Field examples of pinch and swell structure shapes from New Zealand used as viscosity indicators in lower continental crust (Gardner et al. 2016).

Figure 3: Photomicrograph of sample from the Alpine fault showing brittle and ductile structures; Cal (Calcite), Qz (Quartz), Fsp (Feldspar), Bt (biotite) 

Objectives

In this project, the student will work with leading scientists at Leeds (John Elliott, Tim Wright & Sandra Piazolo) and GNS New Zealand (Ian Hamling) to apply the latest techniques in measuring active tectonics, faulting and continental deformation to the Southern Alps and Alpine Fault of New Zealand. 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 motion using InSAR in the South Island of New Zealand. These data can then be used to constrain a number of hypotheses concerning the geometry and slip rate of the Alpine fault along strike, the extent of off-fault distributed deformation and potentially the width and behaviour of the shear zone at depth.
  2. In collaboration with our 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.

Potential for high impact outcome

Active tectonics and earthquake hazard is a pressing issue facing many countries. We are in a unique position at Leeds to bring together a range of observational, modelling and field approaches to answer important unresolved questions about the relative activity of faulting around population centres distributed in major orogenic zones. The research topic has immediate relevance to improving our understanding of the link between faulting and mountain growth, and has the potential to better inform seismic hazard in the region of New Zealand. We anticipate the project generating several papers. There will be ample opportunities to deliver the results of the project at international conferences in addition to UK meetings. Through in-country collaborators, there will be the opportunity to communicate the earthquake hazard to local authorities and civil protection planners.

Training

The student will work under the supervision of Dr. John Elliott, Prof. Tim Wright and Dr Sandra Piazolo within the Tectonics group of the Institute of Geophysics & Tectonics in the School of Earth & Environment at Leeds.  The Institute also hosts the Centre for the Observation and Modelling of Earthquakes, Volcanoes and Tectonics (COMET http://comet.nerc.ac.uk/) which provides a large group of researchers engaged in active tectonics research with whom the student can interact. The project is a CASE studentship with our partner GNS New Zealand. As such the student will spend 3 months working with co-supervisor Dr Ian Hamling in Lower Hutt (Wellington, New Zealand). Dr Hamling has worked on a number of important tectonic and volcanic subjects in New Zealand, including the most recent Kaikoura earthquake (Hamling et al., 2017). This project provides a high level of specialist scientific training in: (i) Satellite geodesy and remote sensing, (ii) Geological Fieldskills, (iii) Laboratory analysis including state-of-the-art microstructural and –chemical analysis, (iv) Data processing and interpretation and (v) Deformation Modelling. The successful PhD student will have access to a broad spectrum of training workshops put on by the Faculty that include an extensive range from scientific computing through to managing your degree, to preparing for your viva (http://www.emeskillstraining.leeds.ac.uk/). The student will also have the opportunity to engage with a wider range of scientists within COMET at a number of other UK institutions who have a broad interest in problems of active tectonics and earthquakes.

Student profile

The student should have a strong interest in active tectonics problems, a desire to undertake laboratory and fieldwork overseas, and a strong background in a quantitative science (earth sciences, geophysics, geology, physics, natural sciences),

References

  • 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(16), doi:10.1029/2010GL044165.
  • Burov, E. B. (2010)
  • The equivalent elastic thickness (T e), seismicity and the long-term rheology of continental lithosphere: Time to burn-out “crème brûlée”?: Insights from large-scale geodynamic modelling, Tectonophysics, 484(1), 4-26, doi:10.1016/j.tecto.2009.06.013.
  • Elliott, J. R., R. J. Walters & T. J. Wright (2016)
  • The role of space-based observation in understanding and responding to active tectonics and earthquakes, Nature Communications, 7, doi:10.1038/ncomms13844.
  • Jackson, J., McKenzie, D. A. N., Priestley, K., & Emmerson, B. (2008)
  • New views on the structure and rheology of the lithosphere, Journal of the Geological Society, 165(2), 453-465 doi:10.1144/0016-76492007-109.
  • Gardner, R. L., Piazolo, S., & Daczko, N. R. (2016) 
  • Shape of pinch and swell structures as a viscosity indicator: Application to lower crustal polyphase rock. Journal of Structural Geology, 88, 32-45, doi:10.1016/j.jsg.2016.04.012.
  • Hamling, I. J., S. Hreinsdottir, K. Clark, J. R. Elliott, C. Liang, E. Fielding, N. Litchfield, P. Villamor, L. Wallace, T. J. Wright, et al. (2017)
  • Complex multifault rupture during the 2016 Mw 7.8 Kaikoura earthquake, New Zealand, Science, 356, 154, doi:10.1126/science.aam7194.
  • Lamb, S., Smith, E., Stern, T. & Warren‐Smith, E. (2015)
  • Continent‐scale strike‐slip on a low‐angle fault beneath New Zealand's Southern Alps: Implications for crustal thickening in oblique collision zones, Geochemistry, Geophysics, Geosystems, 16(9), 3076-3096, doi:10.1002/2015GC005990.
  • 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(5), 2544-2557, doi:10.1002/jgrb.50203.
  • Stirling, M., McVerry, G., Gerstenberger, M., Litchfield, N., Van Dissen, R., Berryman, K., & Lamarche, G. (2012). National seismic hazard model for New Zealand: 2010 update, Bulletin of the Seismological Society of America, 102(4), 1514-1542, doi:10.1785/0120110170.
  • Walters, R. J., J. R. Elliott, Z. Li, & B. Parsons (2013)
  • Rapid strain accumulation on the Ashkabad fault (Turkmenistan): a robust slip rate estimate from MERIS-corrected InSAR data, Journal Geophysical Research, 118, 3674-3690, doi:10.1002/jgrb.50236.

Related undergraduate subjects:

  • Earth science
  • Geology
  • Geophysics
  • Natural sciences
  • Physics
  • Remote sensing