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Mass and heat transfer processes in magmatic orogens driven by magmatism, tectonic deformation, and surface erosion
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Magmatic orogens are continental arcs and collisional belts that are associated with syn-tectonic magmatism. They record heat and mass transfer processes acting from the mantle to the surface. While magmatism, deformation, and surface erosion each take place at various depth levels, they are dynamically linked processes through interaction in the lithosphere. This dissertation presents an investigation of how heat and mass transfer processes in the lithosphere affect the evolution of magmatic orogens. I first present an evaluation of magma genesis resulting from partial melting of underthrusted lower crust. The causes of episodic magmatism in the Mesozoic Sierra Nevada continental magmatic arc and the sources of high-magma flux (flare-ups) are under debate. Here, I use the results of numerical modeling and scaling analysis to assess the mass balance and thermodynamic feasibility of generating arc magma as a result of partial melting of underthrusted lower crust. I show with a constant underthrusting rate of 5 km/Myr, the magmatic thickening rate is 0.1-0.3 km/Myr, accounting for 10-30% of the magmatic thickening rate during a flare-up. The cumulative volume of magma generated from the partial melting of a 20-km-thick underthrusted lower crust is on the order of 105 km3, ~10-40% of the estimated magma volume generated during a flare-up. Therefore, the results show that partial melting of underthrusted lower crust plays a subsidiary role in driving a magmatic flare-up event. Additional magma derived from the mantle and/or other crustal sources are needed to achieve the observed magmatic output during flare-ups. However, the arc root developed by partial melting of the underthrusted crust reduces the time needed to obtain the critical thickness for root foundering, thus influencing the tempo of arc magmatism. As magma ascends into the crust, its interactions with the deforming crust are recorded in the exhumation history of a tilted crustal section and fabrics in plutons. The Gangdese Batholith is well exposed in southern Tibet and presents a unique opportunity to investigate the evolution of a continental arc from subduction to collision. I applied Al-in-hornblende barometry across the eastern Gangdese Batholith to obtain pluton emplacement pressures to identify potential spatial trends in bedrock pressure. The results reveal a regional paleo-depth pattern with plutons emplaced at 1-2 kbar in the west near Lhasa that deepens to 6-12 kbars in the east, near Nyingchi. By coupling the pressure data with U-Pb zircon ages, I estimate the exhumation history of the Gangdese Batholith since 100 Ma and show a sequence of exhumation and burial phases as well as the expected changes in crustal thickness, reflecting major tectonic events including the development of a continental arc and the India-Asia continent-continent collision. I hypothesize that the Gangdese Batholith was tilted due to differential exhumation along the E-W direction since ~10 Ma, associated with the formation of the eastern Himalayan syntaxis. Along with the exhumation history, I also studied fabrics recorded by the Gangdese plutons and report magmatic fabric measurements from the eastern Gangdese Batholith, aimed to decipher the crustal response to changes in India-Asia convergence style from subduction to collision. Results show magmatic fabric orientations are variable through time and represent: (1) a pre-collision Late Cretaceous subduction phase of orogen-perpendicular contraction and crustal thickening. (2) Transitional Paleocene-Eocene crustal thinning and a change in crustal contraction stress from ~N-S to ~E-W. (3) Post-Eocene crustal thickening without a clear, dominant principal stress direction. The pre-collision Late Cretaceous fabrics are interpreted to record approximate head-on subduction of the Neo-Tethyan oceanic plate beneath the Asian continent, while the post-collision fabrics reflect the enigmatic nature of the India-Asia collision, as well as the complexity of the post-collisional processes. Motivated by the recognition of the tilted Gangdese crust, I explored the role of surface erosion in driving solid earth processes. Particularly, whether erosion-driven rock uplift is responsible for the rapid exhumation of the eastern Himalayan syntaxis (EHS). Results of numerical simulations show that localized surface erosion (5 km/Myr) is able to exhume lower crust from depths of >40 km on timescale of ~10 Myr, produce high topography, and generate partial melt in the lower crust. Erosion-driven advection elevates the local geothermal gradient and reduces crustal viscosity, promoting deformation. Exhumation is sustained by isostatic flow resulting from lithostatic pressure difference and amplified by crustal diapirism, associated with the presence of hot and buoyant molten rocks in the weakened advection channel. Such diapiric upwellings trigger a rapid acceleration in rock uplift rates to values greater than the driving erosional forcing and cause localized surface uplift, resulting in topography higher than surrounding regions. The erosion-driven exhumation model demonstrates the intricate coupling between surface erosion and rock uplift, as well as the active role of surface erosion in driving orogenic evolution.