Lower crustal flow: The origin of Late Cenozoic extension north of the eastern Snake River Plain

Recent work has shown that the initiation of late Cenozoic faulting and concomitant footwall uplift north of the eastern Snake River Plain (eSRP) are much younger than previously thought. Examples of these young ages include the Centennial Range (< 2.0 Ma), Gravely Range (< 2.0 Ma), Lemhi Range (< 6.6 Ma), Beaverhead Mts. (< 6.6 Ma), Tendoy Mts. (< 6.6 Ma). Basins south of the eSRP exhibit a bi-modal distribution of growth ages during the Neogene. Seismic moment tensor and earthquake rupture data define extension directions that are both oblique to and symmetric about the axis of the eSRP. However, extension directions on the eSRP itself are parallel to the axis. The authors propose that the orientations of extension are a response to lower crustal flow in a conduit formed between the mid-crust and the upper mantle. Estimates of the lower crustal pressure gradients, geothermal gradient, and channel dimensions are used calculate a lower crustal flux between the extending regions north of the eSRP and the eSRP. This value is three orders of magnitude greater than the estimated flux based on geologically determined strain rates. These calculations suggest that lower crustal flow is a viable mechanism to explain extension northmore » of the eSRP as well as to explain the origin of the extension throughout the Intermountain seismic belt. The advantage of this model is that upper crustal extension does not have to couple with upper mantle extension and thereby it is not necessary to invoke far field stress changes to explain changes in the local stress field.« less