How and why is deformation localised in continental crust?

In this project the student will make and use geological field investigations of the exhumed lower crust to constrain models of strain localisation, and compare these against modern geodetic data from the India-Asia collision.

Over the last few years, geodetic observations of strain from the continents have revealed two modes of behaviour. At fault zones such as the Himalayan Frontal Thrust, Altyn Tagh and Xianshuihe Faults in the India-Eurasia collision zone high geodetic strain rates are observed at the faults themselves. In other regions, including Western Tibet and Mongolia, present-day deformation is diffuse, with little evidence for localisation on the major structures (e.g. Zheng et al., 2017). The underlying reasons for this bi-modal behaviour remain unclear and are highly debated. Understanding processes responsible for the presence or absence of strain localisation in the continents is important for diverse problems ranging from seismic hazard to the distribution of resources.

To address the question of strain localisation, the student will make new observations from exhumed lower-crustal rocks in Greenland, where bi-modal behaviour also appears to be present, and use the geological and geodetic observations to assess proposed mechanisms for strain (de-)localisation in continental crust by running numerical models.

The Nagssugtoqidian Orogen in SW Greenland is characterised by geological "blocks" that differ in whether strain localization is present or absent. These exhibit striking similarities to areas of different behaviour seen in the present day India-Eurasia collision zone. Compilation of structural and metamorphic data from over 20 years of mapping in the area allow identification of areas of interest that represent the areas of strain localization, largely undeformed regions and areas of diffuse strain. Using this unique dataset, freely available to us, as a guide, the student will conduct additional targeted field work to investigate the rheological, chemical and pre-deformation properties of the rocks outcropping today. They will use the results to build models that test whether the metamorphic, deformation and igneous history of areas involved in present day collisions is a deciding factor in the rheological behaviour of the whole orogen.

The latest geodetic strain rate models for the India-Eurasia collision zone are built from more than 2500 individual GPS velocities (Zheng et al., 2017). Work by COMET scientists at Leeds (http://comet.nerc.ac.uk) is using InSAR observations to increase the spatial density of observations (e.g. Wang and Wright, 2012; Wang et al., 2019), but even from the GPS data it is clear that different regimes operate within the India-Eurasia collision zone. The student would refine and improve geodetic models for the India-Eurasia collision zone using the latest observations. We would expect the student to interrogate the data to critically assess the degree of strain localisation that can be observed and how it varies in space (laterally and vertically), potentially also utilizing data derived from ambient noise topography (e.g. Jiang et al. 2014)

For the geological part of the project, the student will interrogate the available data from geological mapping campaigns over the last 20 years available from the Geological Survey of Denmark and Greenland to identify field areas of interest. These areas will then be investigated in detail using latest techniques to infer rheological behaviour of different rock units, field analysis and microstructural work (e.g. Svahnberg & Piazolo, 2010) to derive physico-chemical properties.