The kinematics and dynamics of active mountain ranges
This thesis explores the controls on tectonic deformation in and around active continental mountain belts, and comprises three interconnected studies. The first chapter investigates lateral variations in the foreland of the New Guinea Highlands, a young mountain range which sits on the northern edge of the Australian plate. I construct new earthquake source models across central and western New Guinea, and combine these data with thermal and elastic modelling. I find that pre-existing structural contrasts in the Australian lithosphere control along-strike variations in the temperature structure, seismogenic thickness, and strength of the New Guinea foreland. The underthrusting foreland supports the elevation of the overriding mountain belt, yet the elevation of the Highlands is not closely correlated with the foreland seismogenic thickness; this is likely due to the time taken to thicken the crust following changes in the rheology and strength of the foreland over time. Using force-balance arguments, I estimate the static coefficient of friction on foreland faults to be between 0.01–0.28. The second study expands in scope to examine the forces governing deformation along mountain range fronts, by analysing slip vectors from reverse-faulting earthquakes in 15 active mountain belts. I find that, along most range margins, slip vectors are better aligned with topographic gradients than with the direction of convergent plate motion. This implies that the gravitational buoyancy force acting between a mountain and its foreland exerts a strong control on the direction of slip. This effect is observed most clearly in high-elevation ranges like the Himalayas. By considering the force balance in these deformation belts, I suggest that, in most mountain ranges, the underthrusting foreland must act as a rigid base to the overlying range front, allowing it to act like a gravity current. Local factors, such as weak sediments in the foreland basin, can allow the gravitational force to be dominant even in low elevation ranges like the Zagros. The final chapter builds on these results by modelling a viscous flow over a rigid base, to examine the underlying dynamics. I explore how varying a range of geometric and rheological parameters can influence the instantaneous velocity field within the model mountain range. I find that the gravitational buoyancy force has a strong influence on the resulting velocities along the range front, supporting the observations made in the previous chapter. The velocity field is also significantly affected by the viscosity contrast between the upper and lower crust, and by variations in viscosity (within one order of magnitude) along-strike of the mountain range.
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