Contribution of ductile flow to exhumation of low-temperature, high-pressure metamorphic rocks: San Juan-Cascade nappes, NW Washington State


  • Jeffrey G. Feehan,

  • Mark T. Brandon


The San Juan-Cascade (SJC) nappes were subducted to a depth of ∼18 km, metamorphosed under low-temperature, high-pressure conditions, and then exhumed, all within ∼16 m.y. During exhumation, penetrative deformation by solution mass transfer (SMT) resulted in a widespread spaced cleavage. Strain directions were determined for 27 sandstone samples, and absolute strain measurements for a subset of 19 samples. Z directions generally plunge moderately to the NE, and X and Y directions are scattered in the plane perpendicular to Z. SMT deformation is constrictional at the local scale, but the tensor average indicates plane-strain uniaxial shortening at the regional scale, with Sx, Sy, and Sz equaling 1.01, 0.91, and 0.58, respectively. The average flattening plane (XY) dips 30° to the NE, and the average X direction plunges 20° to the north. The large shortening in Z was compensated by a mass-loss volume strain of ∼47%. We present a simple one-dimensional model that illustrates the relationship between finite strain and ductile exhumation for a steady state convergent wedge. Assuming depth-dependent ductile flow and no reversal of principal strain rates with depth, this model indicates that ductile thinning of the SJC nappes accomplished only 13% of the total exhumation, despite a vertical shortening strain of 36%. There is no evidence that normal faulting contributed significantly to exhumation. We conclude that erosion operating at an average rate of ∼1.1 km m.y.−1 was the dominant exhumation process.