Piccolo, Andrea (2015) "Exhumation Mechanisms in Hot Orogens: insight from numerical modelling". [Magistrali biennali]
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Hot orogens are characterised by high temperatures and large dimensions. The temperature is related to the crustal thickness achieved during the evolution of the orogenetic system. The high temperature achieved by this kind of orogens is high enough to induce a pervasive and massive partial melting of the middleupper crust. The partially melted rocks have a lower density and viscosity than the unmelted counterpart. The partially melted region that forms in the interior of the orogens affects the post collisional evolution of the orogen and affects the way in which the rocks exhume at the surface. The partially melted rocks self-organize in a hot channel that flows underneath the orogen, and affects large portions of the orogenetic system. Since this low viscosity channel has high mobility and it is highly sensitive to pressure changes, one of the most intriguing issues is to analyse if the changes of the topographic relief, due to the surface processes, affects the advection of the hot channel and therefore the exhumation patterns of the rocks. The best example of hot orogens is the Himalayan range in which there is abundant geological and geophysical evidence supporting widespread partial melting. The exhumation mechanism of the Great Himalayan Sequence (GHS) was one of the most important arguments of the Himalayan research in the last decades, and the models proposed have been based on the coupling between tectonics and climate. The most popular and predictive of these models was the channel flow model proposed by Beaumont [Beaumont et al., 2001] in which the GHS is interpreted as the coherent slice of Indian partially melted crust which has migrated from the Tibet, in the North, to the southern flank of the Himalayan range due to the established pressure gradient between the Tibetan plateu and the orogeny front. According to this model, the exhumation of the GHS is caused by the high and focused erosion in the souther flank. Recent researches have pointed out that the structural complexity of GHS is incompatible with the channel flow model. The aim of this thesis work is to reproduce all the principal features of hot orogens by using numerical modelling in order to i) test the influence of the surface processes on the partially melted crust and on the exhumation patterns of the rocks, and ii) compare the simulation results with the observations from the Himalayan range in order to verify if the exhumation patterns predicted by the channel flow model is a inherent feature of hot orogens. Results show that partial melting in the mid-upper crust always forms, owing to the accreted highly radiogenic sediments and the thick orogenic crust. Different patterns of exhumation and metamorphism are reproduced, most of which are consistent with different exhumation models proposed. Additionally, the results of the simulations give other important insights: i) the focused erosion is not the necessary condition to the exhumation of the partially melted rocks and of the channel flow; ii) the shape of the channel flow is affected by the timing of the erosion; iii) the sedimentation rate have first order implication on the development of the orogens.
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