CONCAVE-OUTBOARD MARGIN GEOMETRY AND ONGOING OROCLINAL BENDING AT CASCADIA (Invited Presentation)

The concave-inboard geometry of most convergent margins is often described as resulting from the depression of the edge of a thin spherical cap. However, the Cascadia subduction trench displays a concave outboard geometry for a distance greater than 500 km. Using paleomagnetic, structural and GNSS data, we suggest that the Cascadia upper plate has been folded from the Miocene to Recent into an orocline with an axial trace that bisects the Olympic Peninsula at the apex of the trench concavity. We argue that oroclinal bending at Cascadia occurs by: 1) folding by flexural slip on the orocline limbs; and 2) shortening, uplift, and escape within the core of the fold at the Olympic Mountains. These processes result in relative motion of the forearc towards the arc at the core of the orocline, and sustained opposing rotations of the upper plate on the orocline limbs. We propose that oroclinal bending is promoted and maintained by along-strike variations in plate-boundary tractions resulting from the geometry of the plate interface at depth and suggest that these processes can contribute to the development of long-wavelength concave-outboard margins, such as the Arica Bend near Bolivia, South America.