FLOW AND DEFORMATION IN DEEP-ARC CRUST

Studies of exhumed orogenic crust and models for lateral and vertical crustal flow have commonly been formulated on the basis of collisional orogens, whereas much less is known about such processes in the deep crust of Cordilleran-type magmatic arcs. Recent studies, however, of Mesozoic arcs with crustal thicknesses of >45 km, such as Fiordland (NZ), Sierra Nevada, and North Cascades, provide new insights into tectonic processes in deep arc crust. The volume of metamorphosed sedimentary and volcanic rocks varies significantly in these arcs, as they are minor (<10%) components of the Sierra Nevada (e.g., Saleeby and colleagues) and Fiordland (e.g., Klepeis and colleagues) arcs, but probably form roughly 50% of the deep (ca. 25-40 km) crust of the Cascades. There, they include metamorphosed oceanic sediments and basalts, an older Triassic island arc sequence, back-arc clastic rocks, and potentially forearc clastic rocks, all buried to at least 25 km. The introduction of weak metasedimentary rocks into the deep Cascades crust, where they are intruded by tonalitic plutons, resulted in complex lateral variations in rheology that strongly influenced the behavior of the arc. The deep Cascades crust is marked by gently dipping folded contacts and foliation, whereas Fiordland has linked subhorizontal and steep contractional shear zones, and the Sierra Nevada records downward flow and marked vertical anisotropy defined by steep tabular plutons, metamorphic screens, and foliations. Exhumation during orogenic collapse in the Cascades and Fiordland occurred in part by shear on detachments in which mid-crustal rocks form the hanging wall; shear was accompanied by vertical thinning, horizontal stretching, and a significant component of pure shear, particularly in stronger deformed plutonic rocks. Transtensional flow in the Cascades occurred during migmatization of mainly the weak metasedimentary rocks, and was oblique to parallel, rather than normal, to the arc, in part reflecting dextral shear along the plate margin and along-strike gradients in crustal thickness and thermally controlled rheology. Overall, the differences in deep-crustal flow between these arcs probably reflect variations in the volume of metasedimentary rocks, geothermal gradients, and influence of oblique convergence along the plate margin.