AN: T72A-01TI: Slab Rollback and Decoupling, a Testable Tectonic     Model for the Evolution of the Paleogene Northern Coast     Mountains Orogen, BC and AlaskaAU: ML CrawfordAU: WA CrawfordEM: mcrawfor@brynmawr.eduAB:             The model we propose for the tectonic evolution of the     Coast Mountains orogen north of 54 N, is based on     evidence recorded in the mid-crustal crystalline rocks of     the Prince Rupert and Ketchikan quadrangles.     Convergent tectonism accompanied by igneous activity     related to eastward-dipping subduction began prior to     100 Ma.  During the mid- to late-Cretaceous a decrease     in igneous activity along the western side of the orogen     and a widening of the magmatic arc in two bulges into     the intermontane belt suggest a decrease in subduction     angle. Formation of the Skeena fold and thrust belt east     of the arc may also reflect changing plate subduction     angle.  We suggest that at about 70-65 Ma plate motion     slowed, the angle of subduction increased, resulting in     a 65-50 Ma Paleogene batholith emplaced into the older     Cretaceous arc. The subducting plate then detached and     mantle upwelling into the slab gap generated mafic     melts that triggered tonalitic-gabbroic magmatism along     the western side of the batholith.  Preliminary data on     plutons of the Paleogene batholith that show that the     older plutons in this batholith are primarily granodiorite     emplaced at moderate depths. The younger steep-sided     or gently dipping plutons along the western side of the     batholith are mafic to intermediate foliated tonalite     associated with basaltic synplutonic dikes and gabbro     bodies. The steep sided tabular plutons extend in a     nearly continuous 5-15 km wide band (the 'foliated     tonalite sill') at least 700 km from Douglas Channel to     north of Juneau. We suggest slab detachment, which     may have occurred earlier north along the orogen, was     also accompanied by a relaxation of regional     compressional stresses that led: (1) to orogen-wide rapid     uplift after ~55 Ma; (2) to Eocene igneous activity     along the eastern side of the crystalline core of the     orogen manifested as irregular massive shallow level     granite plutons; (3) to shear zones within the batholith     that exhibit small amounts of orogen-parallel offset; (4)     to west-side-up displacement within the Coast shear     zone; and finally (5) to an end of deformation and a     hiatus in igneous activity that lasted until the Miocene.    SC: TDE: DE: DE: MN: Fall Meeting 1996