NUMERICAL AND ANALOG STUDY OF THE ST. ELIAS OROGENY

Convergence of the Yakutat terrane into southern Alaska is a unique example of active terrane accretion. In this region, the Yakutat terrane accretes in a transition area from transform faulting along the west coast of North America to subduction under southern Alaska. In this transition area, the terrane has accreted into and partially subducted under Alaska. To better understand the dynamics of terrane accretion in this setting, we use both 3-D numerical modeling techniques and analog models. Numerical modeling allows multiple hypotheses to be tested and compared with field and analog observations. Analog sand modeling simulates horizontal and lateral accretion as a sand terrane collides into an L-shaped backstop of the same material composition. Surficial displacement of chalk particles has been tracked and analyzed to characterize horizontal strain rates at different points during accretion. Shear strain along the lateral accretion front is strongly by simple shear with some contraction. Rotation along this front is clockwise. The frontal accretion zone is dominated by material contraction near the active thrust faults. We compare the analog results with numerical patterns in 3-D and with the natural analog. The lateral accretion zone is analogous to the Fairweather-Queen Charlotte transform fault along western Canada. The frontal accretion zone is analogous to the Chugach-St. Elias fault along southern Alaska.