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Continent-continent collisions in the Paleoproterozoic : exploring the effects of convergence obliquity and temperature on orogenesis

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Title: Continent-continent collisions in the Paleoproterozoic : exploring the effects of convergence obliquity and temperature on orogenesis
Author(s): Tuikka, Leevi
Contributor: University of Helsinki, Faculty of Science
Degree program: Master's Programme in Geology and Geophysics
Specialisation: Solid Earth Geophysics
Language: English
Acceptance year: 2023
On the hotter Paleoproterozoic Earth, the regime of plate tectonics was likely in transition, including features from the early Earth but having e.g. subduction already established. The different mode of tectonics affected also on the deformation on plate boundaries, on orogenesis, for example. Due to the increased lithospheric temperatures, the Paleoproterozoic orogens were hotter and lower. Such conditions made also HP-LT metamorphism rare. In addition to the different temperature conditions, the past tectonics pose another challenge. Largely the Paleoproterozoic rocks that can be accessed today, are the remnants of ancient middle or lower crustal layers, which have exhumed due to deep levels of erosion. Hence, a great amount of evidence of past orogens is erased. To overcome this issue, geodynamic modeling is used to build set of 19 continent-continent collision models, with the temperature conditions ranging from the Paleoproterozoic to the Phanerozoic. Additionally, P-T-t paths are recorded in the models for comparison of pressure-temperature conditions using pseudosection diagrams. Another significant quantity governing the deformation on collisional continental margins, is the angle of convergence obliquity. With roughly 60° obliquity, full strain partitioning should be triggered in a orogen, forming a strike-slip fault. In the models, different temperature cases with the obliquity angle are varied. The geodynamic modeling software to produce the models was DOUAR, a 3D thermo-mechanical code coupled with erosion model FastScape. The initial models with 35 km thick crust were not able to produce proper strain partitioning due to low resolution, so another set of models with 45 km thick crust was run. On top of that, the lower crustal strength was varied. Outcome of the thicker crust was wider orogens, and hence more space for strain partitioning to develop. However, strain partitioning was not able to be preserved that well for the whole 40 Ma, which was the runtime of the models. Though in terms of strain partitioning, the results were not ideal, the angle of obliquity affected on the crustal shear zones as well, in a interesting way.
Keyword(s): geodynamic modeling continental collision the Paleoproterozoic era high-performance computing

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