MID-CRUSTAL TRANSTENSION AND ITS RELATIONSHIP TO ARC MAGMATISM AND RHEOLOGICAL HETEROGENEITY IN AN ANCIENT OROGEN

Structural data and U-Pb zircon geochronology from the central Coast Mountains of southeast Alaska and British Columbia reveal how displacements were partitioned within the middle crust of a continental arc during a period of Paleocene and Eocene (65-50 Ma) transtension. During this interval arc-parallel and oblique (transtensional) displacements preferentially were partitioned inside the batholith, producing networks of moderately and steeply dipping, curved shear zones. These shear zones preferentially occur in areas of Paleocene sill complexes and at the margins of Eocene plutons, and record both strike-slip and extensional displacements parallel to the arc. At the western margin of the Coast Mountains batholith steep shear zones up to 5 km wide record arc-normal extensional displacements that accompanied and outlasted the emplacement of Eocene plutons of mostly tonalitic composition. The relationships in the Coast Mountains also indicate that the style of partitioning and the kinematic history of deformation on arc-parallel shear zones are highly variable over length scales of 50-75 km parallel to the arc. Comparisons among areas located along the NNW strike of the orogen suggest that these variations are controlled by differences in the rheological properties of the middle crust. Highly variable rheologies appear to have resulted from different degrees of partial melting and the heterogeneous emplacement of tonalite plutons within the batholith. This style of displacement partitioning is similar to other obliquely convergent orogens where arc-parallel displacements preferentially occur in zones that are weakened by heat and magma. In addition, zones that record extension and transtension occur in areas of the batholith that lie directly east of the southern end of the thick skinned thrust belt along which the accreted outboard Alexander terrane was thrust under terranes to the east. This relationship leads us to speculate that extension and transtension began in response to changes in plate motions accompanied by terrane collision and indentation.