CORDILLERAN DETACHMENTS AS DYNAMIC INTERFACES BETWEEN METEORIC AND DEEP FLUID SYSTEMS

North American Cordillera detachments bound metamorphic core complexes that comprise upper greenschist to amphibolite facies metamorphic rocks (including migmatite) and granite bodies that crystallized during detachment activity. Metamorphic and magmatic processes generate deep-seated fluids, which, in some cases, reach the base of the detachment zone, particularly where magma crystallized. In addition, detachment zones are invariably permeated by meteoric fluids as indicated by very negative hydrogen isotope ratios that are obtained from fabric-forming minerals such as white mica and biotite. The detachments for which these data exist include Thor-Odin, Valhalla, Kettle, Bitterroot, Pioneers, Raft River, Ruby Mountains and Snake Range. These data show that large-scale convective flow takes surface fluids to detachment depths. This requires a basal heat source, which is provided by advection of hot material by ductile footwall thinning and/or emplacement of magma and partially molten rocks.

Dating of synkinematic minerals in detachments from Thor-Odin (Eocene) and Raft River and Snake Range (Miocene) suggests downward propagation of ductile deformation and progressive incorporation of detachment rocks into the hanging wall during rapid cooling. Collision of the two fundamental fluid flow systems is tied to the dynamics of detachments. Deep-seated fluids localize strain at the brittle-ductile transition while the circulation of surface fluids refrigerates the detachment system, influences the orogenic geotherm, and favors a high thermal gradient across the detachment zone. The detachment is therefore a thermal and mechanical boundary layer. The interaction of deformation, heat, and fluid-related processes explains why strain localizes over a relatively thin zone (100-1000 m) from the detachment inception to its exhumation.