Deformation of the Earth's upper crust is localised onto narrow fault zones, which may slip suddenly and catastrophically in earthquakes. Strain in the upper mantle is more broadly distributed and is typically thought to occur by continuous ductile creep. The transition in the lower crust from broad shear zone to a narrow structure in the upper crust is poorly understood but the properties of the lower crust are an important control on the behaviour of the system during the earthquake loading cycle. The properties of lower crustal rocks, and their spatial variation, cannot be measured directly; instead inferences are typically made from seismic observations, exhumed geological analogues, and simple modelling of surface deformation data. Existing seismic experiments have poor resolution in the lower crust; and current geodetic models do not reproduce observations of rapid post-seismic and focussed inter-seismic strain. We propose a multi-disciplinary project with the aim of determining the lower crustal structure of the North Anatolian Fault Zone (NAFZ) in Turkey and explaining the geodetic data. We plan (i) a novel seismic experiment that will provide high-resolution images of lower-crustal structure beneath the NAFZ, (ii) analysis of geodetic measurements of surface displacement, and (iii) petrofabric analysis of an exhumed shear zone that has exposed rocks representative of the mid to lower crust under the fault. We will use these data to constrain geodynamic models of the earthquake cycle using 3D visco-elastic continuum mechanics simulations. Computational experiments simulating the visco-elastic deformation of a faulted block driven by boundary forces will be constrained by geodetic and finite strain observations (seismic images and petrofabric analysis) in order to determine the variation of creep viscosity within and around the fault zone. We aim in this project to explain how the earthquake loading cycle for this major fault system is affected by the lower crustal structure, and ultimately to contribute to better assessment of the seismic hazard associated with it. The resulting synthesis of data and model will guide future investigations for other major strike-slip fault zones.

Origin of Ultra-Low Velocity Zones at the Core-Mantle Boundary

PROJECT funded by NERC



a collaboration with
John Brodholt (UCL)
Liz Vanacore (Leeds)

Using new seismological observations together with mineral physics constraints, we will test hypotheses as to the origin of small scale (10 to 100 kms) heterogeneities called ultra-low velocity zones at the core mantle boundary (CMB). The CMB is Earth’s most import internal boundary. It is a thermal boundary layer(with a temperature contrast in excess of 1500°K), a chemical boundary (being the contact zone between the molten iron of the core and the solid silicate rock of the mantle) and a boundary between two very different convective regimes in the core (convection velocities of the order of 10-4 m/s) and mantle (V~10-10 m/s). The CMB controls the convective patterns of the core, producing Earth’s magnetic field. Heat flow through the CMB affects convection in the mantle, driving plate tectonics. The D" region up to 300 km above the CMB is home to some of the most fascinating structures in the interior of the Earth, ranging from a sharp discontinuity to anisotropy and strong scattering. Perhaps the most intriguing seismic discovery of the last 15 years regarding the lowermost mantle is that of intermittent thin patches of extremely reduced seismic velocities near the CMB dubbed ultra-low velocity zones (ULVZs). ULVZs influence many aspects of mantle dynamics and it has been speculated they are the roots of mantle plumes, areas of core material entering the mantle, remnants of a global magma ocean, an influence on the path of the magnetic poles during polar reversals, and chemically distinct exotic material. Nonetheless, the origin of ULVZs remains unsolved and fundamental questions such as partial melt vs. chemical heterogeneity as source for ULVZs are
debated.

To test hypotheses on the origin of ULVZs we will use a combined mineral-physical and seismological approach. Each of the proposed ULVZ models will lead to specific velocity changes, VP-to-VS ratios and density changes for ULVZs. Currently our knowledge about ULVZ structure and lower mantle material properties is not sufficient to differentiate between the models. We will determine the elastic properties of perovskite and post-perovskite, as function of composition, pressure and temperature to understand the elastic properties of ULVZs. New seismic probes to ULVZs will be employed to determine ULVZ velocities and density, and to specify their lateral extent and thickness. Identification of regions devoid of ULVZs is crucial to understand the connection between mantle flow and ULVZs. We will obtain a map of the CMB indicating ULVZ regions, their seismic velocities and densities. Using forward modeling based on the mineral-physics results we will be able to thoroughly test different models of origin for ULVZs.