A “Glimpce“ of the Deep Crust Beneath the Great Lakes
- Robert F. Mereu,
- Stephan Mueller and
- David M. Fountain
Published Online: 9 APR 2013
Copyright 1989 by the American Geophysical Union
Properties and Processes of Earth's Lower Crust
How to Cite
Green, A. G., Cannon, W. F., Milkereit, B., Hutchinson, D. R., Davidson, A., Behrendt, J. C., Spencer, C., Lee, M. W., Morel-á-LáHuissier, P. and Agena, W. F. (1989) A “Glimpce“ of the Deep Crust Beneath the Great Lakes, in Properties and Processes of Earth's Lower Crust (eds R. F. Mereu, S. Mueller and D. M. Fountain), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM051p0065
- Published Online: 9 APR 2013
- Published Print: 1 JAN 1989
Print ISBN: 9780875904566
Online ISBN: 9781118666388
Approximately 1350 km of multichannel seismic reflection data and an equivalent amount of seismic refraction data have been collected across the North American Great Lakes. the seismic surveys, sponsored by the Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE), were designed to resolve the deep crustal structure of the 1.89–1.82 Ga Penokean orogen, the 1.3–1.0 Ga Grenville orogen and the 1.11–1.09 Ga Mid-continent rift system. In north-central Lake Huron a band of gently east-dipping reflections at about 18–20 km depth separates a complex and highly reflective lower crustal layer from a markedly less reflective upper crustal layer. the lower layer is interpreted as attenuated crust of the Superior cratonic margin and the upper layer is assumed to be composed of continental margin deposits of the Huron supergroup, an exotic mass that collided with the cratonic margin during the Penokean orogeny, and younger granites and rhyolites. the intervening band of gently east-dipping reflections may delineate a master decollement zone, active during the Penokean orogeny. All structures in north-central Lake Huron are truncated abruptly at the western end of Georgian Bay by the Grenville Front, represented by the westernmost event of a spectacular series of moderately east-dipping reflections. These reflections, which extend from the surface to about 55 km depth, are interpreted as discontinuities between gneissic and migmatitic rocks of varying lithology. Where exposed in the Grenville front tectonic zone, the different rock units are commonly juxtaposed along mylonite zones with a northwest thrust sense. Moho may deepen by 5-10 km along a narrow region just southeast of the Grenville front.
Seismic reflection data from Lake Superior reveal an extraordinary thickness of Keweenawan mafic lavas and sedimentary rocks deposited in a number of discrete grabens/half-grabens of the Midcontinent (Keweenawan) rift system. Total vertical thickness of layered Keweenawan strata exceeds 30 km beneath some parts of the lake. the geometry of the central basin changes significantly along the axis of the Midcontinent rift system. In western Lake Superior the Keweenawan lavas thicken northward toward a south-dipping growth fault, in the center of the lake they thicken southward toward a northdipping growth fault, and in the eastern part of the lake the structures are relatively symmetric. These mega-grabens and mega-half-grabens with differing asymmetries are separated by transfer faults or accommodation zones recognized on potential field maps. the segmented character of the MRS is similar to that observed along some passive continental margins and the East African rift system.