Kevin C. Brown

M. Sc. Thesis

Extensional Models Applied to the North American Midcontinent Rift Beneath Lake Superior

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The 1.1 Ga Midcontinent Rift (MCR) of North America is a major feature of the continental interior, characterized by pronounced geophysical anomalies along its 2000 km length. Deep seismic reflection profiles of the MCR, collected in Lake Superior as part of the Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE) experiment, show a 30 km deep basin with dominantly volcanic infill.

Kinematic models constrained by GLIMPCE data indicate that the rift basin size and shape, and the dip of syn-rift fill, were produced by extension along detachments with a ramp-flat geometry. The basin depth and shape depended on the detachment shape, the amount of extension, and depended strongly on the basin fill density in these local isostatic models. Crustal extension must be matched at depth by mantle lithosphere extension. Constraints were placed on the mantle lithosphere thinning geometry by assuming that rifting was passive and that the basalt volume inferred from GLIMPCE data to fill the rift was produced by decompression melting. Calculations using simple and pure shear models indicated that only a narrow zone of pure shear can produce sufficient melt.

Insight gained from kinematic models was used to develop dynamic, finite element models of MCR evolution beneath Lake Superior. The dynamic models differed from the kinematic models in that: 1) deformation was not prescribed, but evolved according to the lithospheric rheology in response to an applied extensional stress; 2) flexural effects were included in the resulting topography; and 3) two-dimensional thermal calculations were included. The nucleate extension, the finite element models each had a crustal weak zone and a zone of mantle weakness. Elastic, plastic, or temperature-dependent creep behaviour was assumed, depending on the state of stress and temperature. The dynamic models supported the conclusions from the kinematic models. A narrow lithospheric weak zone was required to initiate extension that resulted in the MCR. This narrow weak zone may be developed by shearing related to compression during the Grenville Orogeny.

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Supervisors: Chris beaumont