Spatial variations of pressure-temperature-timing-deformation conditions of ductile thrusts: Examples from the Himalayan and Variscan orogens
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Orogen-scale thrust-sense shear zones evolve dynamically through time and space as they accommodate crustal shortening during continental collision. When these shear zones activate in different parts of an orogen is central to the understanding of how orogenic systems evolve. These shear zones carry rocks from the deep metamorphic core (hinterland) towards the toe of the orogenic wedge (foreland). Rocks therefore record different pressure (P), temperature (T), timing and deformation conditions as they move through segments of a shear zone. These recorded conditions are used here to track deformation on crustal-scale shear zones in the direction of tectonic transport, from the hinterland towards the foreland, of two orogenic systems, the Himalaya and the Variscan orogen. In the western Nepal Himalaya, a major shear zone, the Main Central thrust (MCT), is exposed multiple times in the transport direction. U-Th/Pb monazite petrochronology, U/Pb zircon geochronology, 40Ar/39Ar muscovite thermochronology, metamorphic P-T analyses, and quartz microstructures are used to complement field mapping and determine the recorded conditions of metamorphism and deformation in the MCT zone. Each of the three studied MCT transects reached peak P-T conditions and cooled through 450-350°C at different times. These data indicate that deformation on the MCT was diachronous in the transport direction and that a single shear zone transect is not representative of the timing and duration of deformation on the MCT. Field mapping, microstructures and both in situ and step-heating 40Ar/39Ar white mica geochronology are used to examine a series of successive thrust-sense shear zones exposed on the island of Sardinia, Italy. These shear zones are located intermediate to the hinterland and foreland of the Variscan orogen and record a common timing of mylonite formation and ductile deformation from 325-318 Ma on all three thrusts. The results from this thesis indicate that P-T, timing and deformation conditions can be used effectively to track ductile deformation of rocks as they move through a shear zone that evolves through different kinematic regimes. The timing and duration of ductile deformation may be diachronous in the transport direction and may not evolve unidirectionally from hinterland to foreland.
URI for this recordhttp://hdl.handle.net/1974/25852
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