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Title: The mechanics of active faulting and mountain building
Author: Wimpenny, Samuel Edward
ISNI:       0000 0004 7972 8714
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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The aims of this thesis are to place new constraints on the rheological properties of active faults, and to investigate the influence of fault rheology on mountain building. The first chapter studies the postseismic deformation following the 2003 Bam earthquake in south-eastern Iran, with the intention of probing the material properties of the fault zone. Measurements of ground deformation following the earthquake made using InSAR suggest there was an unusually small amount of postseismic afterslip at Bam compared to similar earthquakes. I use numerical calculations of stress-driven afterslip to show that, in order to account for the limited postseismic afterslip at Bam, most of the fault zone must remain frictionally locked. I also find that, to explain the postseismic InSAR measurements and the long-term growth of topography at Bam, there must be either an E-W deviatoric stress of 2-10 MPa acting across the fault zone, or some component of fabric on the fault zone controlling the rake of fault slip. The second chapter expands in scope, investigating the orogen-scale deformation of the high Andes in south Peru. First I present a source model for a Mw 6.1 normal-faulting earthquake that occurred in the shallow crust of the south Peruvian Altiplano, which indicates that the Andes are extending parallel to the direction of shortening in the adjacent sub-Andean forelands. I then discuss a compilation of earthquake source models from across the Andes, using the earthquake depth distribution and slip vectors to infer that buoyancy forces are important in controlling the pattern of deformation. Calculations of the buoyancy forces indicate that faults cutting through the foreland support forces ~4-8 TN/m and have effective coefficients of static friction < 0.2. Finally, I speculate that the recent normal faulting in the Andes is a result of a reduction in the shear stresses transmitted across faults on the eastern margin of the range, causing the high plateau to extend and thrust eastwards over the adjacent South American foreland. The final chapter remains focused on the normal faulting in south Peru. I use field observations and remote sensing to show that a 20 km-wide band of normal faults between Cusco and Lake Titicaca are extending NNE-SSW to NE-SW at ~1-3 mm/yr. The normal faults became active in the past ~5 Myrs and re-activate pre-existing reverse faults. To account for the extension rates across the normal faults, I calculate that the average shear stresses transmitted across the sub-Andean detachment may have decreased by 0.02-3 MPa, which is 0.1-30\% of their absolute value. I conclude that within most mountain belts the rate and style of faulting is sensitive to small spatial and temporal variations in the material properties of faults in their forelands.
Supervisor: Copley, Alex Sponsor: British Geological Survey ; Centre for the Observation and Modelling of Volcanoes ; Tectonics and Earthquakes (COMET)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Earthquakes ; Faults ; Mountains