The dynamics of flowing, subsurface salt sheets: observational constraints and theoretical models

The dynamics of flowing, subsurface salt sheets: observational constraints and theoretical models

What is the viscosity of salt as it deforms at depth in the crust? Is it Newtonian? What are the effects of the layer geometry? How do fluids move through the salt and what are their effects?

Thick deposits of crystalline salt form when saline waters evaporate at the surface of the Earth. These deposits can be buried by sediment to depths of tens of kilometres. The low density and low viscosity of solid salt makes these layers unstable and easily deformable. They are inferred to become detachment surfaces that cause large-scale motion of the overlying sediments. However, in such systems the kinematics and dynamics of the salt layer cannot be observed directly. Seismic surveying is useful, but it is difficult to image salt bodies because of their low seismic velocity. Recently, seismic images of the Levant Basin salt formation have been shown to contain kinematic indicators of the salt deformation. These observations provide a rare opportunity to constrain detailed dynamical models.

Analysis of a 3D seismic survey by Cartwright et al. (2018, https://doi.org/10.1130/G40219.1) revealed a sequence of fluid escape structures that traverse a 1500m salt layer. These pipe-like features were emplaced vertically from a common injection point. They were then sheared with the salt, leading to replacement by a new, vertical pipe. The emplacement of the 21 distinct pipes is constrained by the sedimentary sequence above to have occured over a timescale of 1.7 Ma. Hence these features represent a unique, in situ kinematic marker of salt deformation. The leading-order deformation of the salt is Couette flow (simple shear driven by lateral displacement of the sediments above). However, a detailed examination of the structure of the displaced pipes suggests a more complex pattern of flow. This may arise from non-Newtonian viscosity or from a growing undulation of the basal surface where the pipes originate.

Aims of the Project

The first aim of the project is to develop a theoretical/computational model of the salt layer and bounding sediments in two dimensions, along the direction of flow.  This will be used to interpret the detailed measurements of the fluid escape pipes in terms of dynamics and salt rheology.

A second aim of the project will be to understand and model the fluid pipes.  How do they traverse the nominally impermeable salt layer?  Are the pipes open continuously or episodically? What determines the time over which a pipe remains active as it deforms? This is an open-ended set of research questions related to the physics of reactive fluid flow and coupled mechanical and fluid dynamical phenomena.  There may be applications of this approach to a broad range of  questions relating to fluid transport across low permeability media.

Methods to be used

Mathematical modelling; partial differential equations for fluid dynamics; code development for numerical solutions; statistical analysis

Any specialised skills the student will need

The prospective student should have a training in geodynamics, applied mathematics or physics.

If interested please contact Richard Katz richard.katz@earth.ox.ac.uk , Joe Cartwright joe.cartwright@earth.ox.ac.uk and Christopher MacMinn christopher.macminn@eng.ox.ac.uk

 

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