THE ROLE OF PRE-EXISTING WEAKNESSES IN THE LARAMIDE OROGENY

Stress transmission across plate boundaries during flat-slab subduction and its effect on intraplate pre-existing weaknesses is poorly understood. We investigate inboard deformation of the North American plate 1000 km form the subduction front that occurred during the Upper Cretaceous to Paleogene Laramide orogeny.

Because flat-slab subduction is the most accepted driving mechanism for the Laramide orogeny, we set up a 2D geodynamic model to simulate the formation of a flat-slab. Our model shows how stress is transmitted across the plate boundary via end-loading. Furthermore, we focus our investigation on the relationship between deformation and pre-existing weakness orientation to better understand the timing and magnitude of deformation related to the pre-existing weakness(ess). The results show that pre-existing weakness geometry controls the duration and magnitude of deformation, but not the timing of deformation initiation. Pre-existing weaknesses with shallower dips (30° end-member case) continue to deform throughout the flat-slab event. In contrast, steeply dipping pre-existing weaknesses (60° end-member case) are only active for a short time period that corresponds to the transition from a steeply dipping slab to a flat-slab. In tests with multiple pre-existing weaknesses in the overriding plate, our modeling shows that deformation initiation is not dependent on distance from the trench but on the strength of the weakness.

We propose that timing and location of deformation during the Laramide orogeny were a function of pre-existing weakness strength and orientation.