Magma emplacement during arc‐perpendicular shortening: An example from the Cascades crystalline core, Washington

It is increasingly inferred by others that active arcs at all crustal levels are undergoing arc‐perpendicular extension and that magma utilizes this extension or extension along arc‐parallel, strike‐slip faults during emplacement. However, a review of stress fields in modern arcs and deformation which occurred during magmatism in ancient arcs and our own emplacement studies of individual plutons indicate that magma emplacement typically occurred during arc‐perpendicular shortening without the aid of strike‐slip faulting. This conclusion raises the question of how host rock is displaced during magma emplacement in shortening and thickening orogens. Evaluation of host rock material transfer processes that operated during emplacement of the 93 Ma, 10‐ to 14‐km‐deep Mount Stuart batholith, Washington, provides one example and includes the following: (1) ductile downward flow in a narrow (< 0.2 body radii) structural aureole, (2) stoping, and, of lesser importance, (3) regional folding, thrusting, and subhorizontal arc‐parallel extension. We see no evidence that arc‐perpendicular extension or local extension along strike‐slip faults made space for this pluton nor for most other plutons in contractional arcs. Instead we argue that both localized and rapid (around plutons) and regional and slower downward flow during growth of crustal roots is a common process in such arcs.