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The Evolution of Craton Margin Geometry Through Time
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- Author / Creator
- Mallyon, Deirdre A
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In western Canada, the Archean-aged cratonic core of the North American continent is flanked on its western edge by the Cordillera, which forms the back arc region of the Cascadia subduction zone. The boundary between these two geologic provinces is expressed at the surface by a system of NNW-SSE trending strike-slip faults, known as the Rocky Mountain Trench and Tintina Fault. Striking contrasts are observed between the craton in the east and Cordillera in the west; surface heat flow increases from an average of ~45 mW/m2 to ~75 mW/m2, lithosphere thickness decreases from ~60 km to ~250 km and decreases in seismic shear wave velocities of ~80 m/s are observed. Interestingly, the lateral transition between these contrasts in the lithosphere is estimated to occur over lateral distances of less than ~100 km. In addition to these observations, new geophysical data collected in western Canada suggests that the geometry of the craton margin shows significant along-strike (i.e. north-south) variation; transitioning from dipping eastward below the continental interior, to dipping westward below the back arc, with a vertical transition occurring between ~52 and 53°N. Motivated by these new observations, the present study uses thermally-mechanically coupled geodynamic models in order to: 1) determine the necessary rheological and density structure of the craton lithosphere that facilitates/maintains vertical and west dipping craton margin geometries; 2) investigate the influence of geodynamic processes such as gravitational instability, large-scale mantle flow and crustal shortening on the evolution of craton margin geometry; and 3) determine whether vertical and westward dipping craton margin geometries are transient over times scales of tens of millions of years, or long-lived features that persist hundreds of millions of years. iii The results of the numerical modelling experiments show that vertical and west-dipping craton margin geometries can be both produced and maintained; however, the craton mantle lithosphere must be strong and buoyant relative to the surrounding mantle. Vertical structures are observed when the relative effective viscosity is increased by a factor of 15 or more and the compositional density is reduced by ~10-20 kg/m3, whereas west-dipping geometries are observed when relative effective viscosity is increased by a factor of 5 or more, and when compositional density is reduced by ~30-40 kg/m3. The influence of large-scale mantle flow on the craton margin is strongly dependent on the direction of flow compared to the influence of flow velocity. Vertical and west-dipping craton margin geometries are enhanced when flow is directed from the interior of the craton toward its outer margin. Conversely, flow that is directed from the back arc region toward the craton enhances erosion of the craton margin, resulting in craton margins that dip toward the interior of the continent. Vertical craton margins are readily maintained in crustal shortening experiments; however, west-dipping craton margins are largely absent. Overall, the results of this study suggest that vertical and west-dipping craton margins are both transient and long-lived features.
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- Subjects / Keywords
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- Graduation date
- Spring 2017
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- Type of Item
- Thesis
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- Degree
- Master of Science
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- License
- This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.