AN: T71E-10TI: Structural History of the Coast Shear Zone Near     Portland Inlet, AK/BCAU: KA KlepeisEM: keith@es.su.oz.au AU: ML CrawfordEM: mcrawfor@brynmawr.eduAU: GE GehrelsEM: ggehrels@geo.arizona.eduAB:             The Coast shear zone forms a crustal-scale boundary     between segments of the Coast Ranges that record     different structural, metamorphic and intrusive histories     during emplacement of $\sim72$ Ma to $\sim55$ Ma plutons     of the Coast orogen batholith.  In the area of our study the     ACCRETE seismic profile records a $\sim4$ km east-down     step in the Moho across the shear zone.  Near vertical,     high-strain zones along the western side of the Coast     shear zone parallel the strike of the orogen for >800 km.      We have determined the geometry, kinematic significance     and timing of fabric elements within the Coast shear     where a minimum of coeval plutonic activity results in a     well preserved history of deformation. Two phases of     deformation ($D_{2}$ followed by $D_{3}$) comprise the     shear zone. $D_{2}$ deforms $\sim65$ Ma plutons and     involved dominantly coaxial shear strains with northeast-    southwest flattening and  two directions of extension.  A     down-dip, sillimanite mineral lineation ($L_{2}$) generally     plunges east.  Despite a lack of evidence for bulk non-    coaxial shear strains during $D_{2}$, metamorphic data     suggest that high T/P rocks east of the Coast shear zone     moved up relative to the metamorphic rocks on the west at     this time.  By $\sim55$ Ma, $D_{3}$ produced steep to sub-    vertical fabrics, a kilometer-scale dextral deflection of     older fabrics, minor shear zones, $F_{3}$ folds and an     $S_{3}$ cleavage.  $D_{3}$ fabric elements suggest bulk non-    coaxial shear strains.  $F_{3}$ fold kinematics and $S_{3}$     structures suggest the maximum principal $D_{3}$ stress     direction $\sigma_{1}$ was oblique to the orogen, moderately     to steeply inclined to the southwest.  The deflection of     older fabrics during $D_{3}$ is consistent with crustal     scale transpressional deformation within the shear zone     involving a minimum of 10's of km of oblique dextral     displacement; there is no evidence for $\sim2500$ km     dextral offset on Coast shear zone (Baja BC model). The     east-side-down component of the oblique-dextral shear     couple model for $D_{3}$ deformation is consistent with the     east-side-down Moho discontinuity imaged by the     ACCRETE seismic data. We interpret $D_{3}$ to result     from crustal relaxation as the the region changed from a     dominantly convergent to a dominantly strike-slip regime.      SC: TDE: DE: DE: MN: Fall Meeting 1996