| 40Ar/39Ar age and 39Ar kinetic studies performed with K-feldspars sampled from above the Vincent Thrust (VT) allow reconstruction of its slip history during Late Cretaceous-Early Tertiary shallow subduction. Variational methods were applied to the multiple diffusion domain model to produce best fit thermal histories. The K-feldspar T-t results indicate that a temperature difference of ~150—C was maintained from > 60 Ma to < 55 Ma between positions that are presently at nearly the same elevation and are located 5 and 15 km west of the VT. Using a geothermal gradient and dip angle estimated from geologic constraints, simple numerical heat-flow models were used to determine the slip velocity and relative vertical separation of the samples during thrusting by requiring calculated T-t results to fit the K-feldspar thermal histories. For models in which cooling was due solely to subduction of colder rocks (hanging wall stationary), solutions most compatible with the K-feldspar results were yielded by underthrusting rates of ~1.4 cm/yr. and a vertical separation during the Late Cretaceous/Early Tertiary of 8.5 km. Allowing denudation of the hanging wall during thrusting (footwall stationary) provides somewhat more satisfactory fits to the data. For these models, ~0.2-0.4 cm/yr. displacement occurs along the VT from 65 Ma to 50 Ma along a fault plane inclined at 15—. Because only net vertical displacement can be constrained by K-feldspar data, our calculated slip rate is only a relative value that is inversely proportional to the dip angle at the time of thrusting. |