Abstract

Geologic evidence shows that the Neoproterozoic rift margin of western North America formed as a set of asymmetric detachments, leading to alternating upper- and lower-plate domains distinguishable by the width and gradient of the continental margin, thickness of overlying strata, uplift-subsidence history, relative structural complexity, and presence or absence of exposed lower-crustal rocks. The gradient of the cratonal margin is a key feature related to the transition between continental and oceanic crust; upper-plate margins are marked by steep gradients, whereas lower-plate margins have gentle gradients. In the Canadian Cordillera, the gradient across this transition changes coherently along the margin, from shallow, to steep, to shallow, in turn from north to south. The sinuosity of the Cretaceous-Tertiary thrust-belt front, the distribution of basement exposures, and patterns of miogeoclinal deposition may therefore be inherited from the original asymmetric Neoproterozoic rift geometry.

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