The underestimated Proterozoic component of the Canadian Cordillera accretionary margin
Published:January 01, 2009
D. B. Snyder, M. Pilkington, R. M. Clowes, F. A. Cook, 2009. "The underestimated Proterozoic component of the Canadian Cordillera accretionary margin", Earth Accretionary Systems in Space and Time, P. A. Cawood, A. Kröner
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Analysis of several types of seismic and potential field geophysical data consistently indicate that the majority of the crust underlying the Canadian Cordillera and much of western Canada was originally Proterozoic sedimentary rocks shed off the Canadian Shield into rift or basin structures between 1.84 and 0.54 Ga. These variably metamorphosed strata were primarily quartz- and limestone-rich sediments and thus have distinctive geophysical signatures because of their lower density, lower magnetization, and lower Poisson’s ratio compared with more mafic rocks. The sediments formed a prograding wedge that has a distinctive, internally reflective, seismic stratigraphy. In the east, these Proterozoic sedimentary rocks thicken at a ‘hinge line’ defined by the margin of the pre-1.84 Ga crystalline basement of the Canadian Shield; previous work mapped this hinge line locally using deep reflection profiles and regionally using distinctive gravity gradients. Here we assemble previously published results of several geophysical methods to define the overall shape of the wedge along the margin and westward to where it pinches out at the modern Moho beneath the crustal collage of exotic and suspect terranes accreted onto North America during the Mesozoic. The volume of crust occupied by this wedge limits the thickness of most accreted terranes to several kilometres and suggests that deeper portions of the accreted blocks detached or underthrust the wedge during accretion and are no longer contiguous to crust exposed at the surface. This type Cenozoic accretionary orogen thus spent most of its prior geological history as a passive or extensional margin punctuated by only a few, brief convergent or accretionary events.
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Earth Accretionary Systems in Space and Time
Accretionary orogens form at convergent plate boundaries and include the supra-subduction zone forearc, magmatic arc and backarc components. They can be broken into retreating and advancing types, based on their kinematic framework and resulting geological character.
Accretionary systems have been active throughout Earth history, extending back until at least 3.2 Ga, and provide an important constraint on the initiation of horizontal motion of lithospheric plates on Earth.
Accretionary orogens have been responsible for major growth of the continental lithosphere, through the addition of juvenile magmatic products, but are also major sites of consumption and reworking of continental crust through time.
The aim of this volume is to provide a better understanding of accretionary processes and their role in the formation and evolution of the continental crust. Fourteen papers deal with general aspects of accretion and metamorphism and discuss examples of accretionary orogens and crustal growth through Earth history, from the Archaean to the Cenozoic.