CONTINENTAL DECRETION DURING TERRANE ACCRETION IN THE CANADIAN CORDILLERA

The Cordillera in western Canada has long been considered a type locality for the ‘growth’ and outward expansion of continental lithosphere by accretion of terranes. However, ever since the earliest days of the Canadian Lithoprobe project, it has been apparent that terranes delineated on the surface do not project to substantial depths. Rather, they are detached and rest above deep crustal and upper mantle rocks to which they bear little or no genetic relationship. As a result, continental growth in this region can not be assumed to be proportional to the apparent area increase by terrane accretion. Challenges for interpreting the lithospheric evolution of the Cordillera in Canada have been to determine the nature and origin of the deep crust and lithosphere underlying the regions of accreted terranes, and to determine the tectonic relationships between the lithosphere, the terranes, and, of course, ancient western North America. Results from stratigraphic relationships, geochronology of upper mantle xenoliths, and structural cross sections have been combined with regional geophysical surveys to provide a coherent and consistent interpretation that much of the crust and lithosphere in western Canada has been part of western North America since Precambrian. Cross sections of the lithosphere illustrate that lower crustal rocks as far west as the Okanagan region in the south and as far west as the Alaska panhandle in the north can be stratigraphically and seismically correlated to the ancient margin. Furthermore, retrodeformation of rocks that were deposited on or adjacent to the margin leads to a conclusion that, prior to the onset of terrane accretion, the North American margin and associated rocks projected at least as far west (today’s coordinates) as the modern margin. Thus, as flakes of terranes were added to the surface in the western regions of the Cordillera, the North American lithosphere may have been tectonically eroded from below. As a result, the process of terrane accretion in western Canada apparently resulted in a net decretion (Dewey and Windley, Phil. Trans. R. Lond, 1981) of continental lithosphere below.