DOES MAGMATISM FOCUS REGIONAL DEFORMATION: EXAMPLES FROM THE MID-CRUST, NORTH CASCADES, WASHINGTON

Several recent studies have suggested that magmatism can act as a tectonic lubricant and lead to significant 'melt-enhanced' deformation. However, our studies of an exhumed, mid-to-late Cretaceous arc in the Cascades Core suggest that large magmatic bodies do not appear to cause substantial focusing or enhancement of mid and lower crustal deformation. In plutons ranging from 10-30 km emplacement depths in host rocks with ambient temperatures of < 300C to 700C, we typically recognize during cooling and crystallization the following evolution of deformation mechanisms: 1. Early in the crystallization history, neither tectonic nor emplacement-related fabrics are preserved in plutons. 2. As magmas become crystal-rich, pervasive magmatic fabrics are preserved and are often continuous with subsolidus fabrics in host rocks. 3. When a semi-rigid crystal matrix forms, localized magmatic and subsolidus shear zones develop which sometimes yield kinematic information about regional tectonism. 4. When the viscosity of the pluton becomes substantially greater than that of the host rock, subsolidus shear zones sometimes form along one or more boundaries of the pluton. However we typically do not find synmagmatic, regional shear zones or zones of unusually high strains at the ends of plutons. Thus despite the fact that changing magmatic rheology leads to differing deformation mechanisms within plutons, they do not appear to be the sight of substantial focusing of strain. At least in the Cascades, plutons appear to have undergone less than or a similar amount of regional tectonic strain as the surrounding host rocks during crystallization and only focus host rock strain after solidification. We suggest two related impediments to melt enhanced deformation: 1) connectivity of magmatic systems and 2) the shorter time scales of magmatic processes. We suggest that a necessary test of whether or not magmatism enhances deformation in other aureoles is the presence of zones of unusually high displacement or strains in host rocks at the ends of plutons.