Geology https://doi.org/jzj (2012)

During subduction, sediment is scraped off the down-going oceanic plate and accreted to the overriding plate; changes in the rate of accretion are commonly thought to arise from variations in tectonic motions at the plate boundary. An analysis of the sediment attached to the North American Plate identified a batch of material that was suddenly accreted during a tectonic event within the continent.

Trevor Dumitru at Stanford University, USA, and colleagues use U-Pb dating to assess the timing of sediment accretion at the Franciscan subduction zone, located in the northwestern US, as well as the source of the sediments. They find that the subduction zone received a sudden surge of sediments about 53 to 49 million years ago. The sediments originally came from Idaho, several hundred kilometres away from the subduction zone, and were delivered to the Pacific Ocean and the subducting plate by rivers draining the continent. The timing of the accretionary surge coincides with a period of tectonic extension, uplift and erosion in Idaho.

The researchers therefore suggest that the increase in sediment accreted to the edge of the North American Plate about 50 million years ago was caused by tectonic activity inside the interior of the continent, far away from the plate boundary.