Strain partitioning in accretionary orogens, and its effects on orogenic collapse: Insights from western North America

Changes in relative plate motions during the construction of accretionary orogens generally result in varying structural styles along the length of the orogen. These disparate structural styles can be interpreted as having been formed by different tectonic regimes along the orogenic axis that formed at the same time. If the orogen is considered at the large scale, the differences in the way in which the crust responds during accretion can be explained by large-scale strain partitioning within the same overall tectonic environment. The westernmost Canadian Cordillera records the transition from Late Cretaceous dextral strike-slip faulting to near-orthogonal compression along the orogenic axis. We postulate that the transition between strike-slip-dominated to compression-dominated tectonics represents a Late Cretaceous partitioning of strain that resulted in a significant difference in crustal rheology along strike of the orogeny. This had a dramatic effect on subsequent Tertiary orogen-scale extension. We propose that plate readjustments in the Tertiary led to orogen-perpendicular collapse in portions of the orogen, facilitated by decoupling between the middle and lower crusts along thermally weakened layers. In contrast, localized, orogen-parallel extension occurred in other portions of the orogen, along kinematically linked, large dextral strike-slip faults where the upper crust remained coupled to the middle and lower crust. New data indicate that partitioning of strain occurs across very large regions within any orogenic system, and that the way in which strain is partitioned can lead to dramatic differences in future orogenic processes. It becomes apparent from these data that orogens must be examined as a whole and that differing structural styles of similar ages are likely responses to the same overall tectonic regime.