DEVELOPMENT OF INVERTED METAMORPHIC ISOGRADS DURING PROGRESSIVE NON-COAXIAL FLOW AND CONTRACTIONAL UPLIFT IN COAST MOUNTAINS, ALASKA

Inverted metamorphic sequences are prevalent in major contractional orogens worldwide, but models for their formation remain debated. Models range from bulk sub-simple shear transposing paleoisotherms to discrete zones of simple shear juxtaposing disparate crustal levels. In the northern Coast Mountains of southeastern Alaska, an inverted metamorphic sequence occupies the Insular-Intermontane suture zone for 1000 km along strike, representing a major orogen-scale contractional structure. We conducted a detailed traverse of the sequence on Blackerby Ridge in Juneau, AK to better constrain a thermomechanical model of the region during inversion of mineral isograds.

Metapelitic LS tectonites record an inverted series of mineral isograds ranging from chlorite to garnet, kyanite and sillimanite with minor metapsammitic rock and marble interleaved. Textures range from protomylonitic to mylonitic. Foliation in the metapelites is defined by the alignment of platy minerals and compositional banding. Limbs of isoclinal folds are parallel to the foliation, implying a prior foliation forming event. Throughout the sequence, foliations strike 290-358º NW and dip from 35-65º NE (n=240) without systematic variation. Both stretching (n=134) and intersection lineations (n=32) plunge moderately ~30-33º NE. Kinematic indicators indicate east-side-up, west-vergent, reverse sense deformation throughout the transect. Progressive ductile deformation is recorded by the presence of LS fabrics, sheath folds, and folded aplite veins with axial planes parallel with the regional foliation. In-situ monazite-xenotime petrochronology, phase equilibrium modeling, and thermobarometry are currently being performed on a spatially dense sample set throughout the traverse. Preliminary monazite petrochronology indicates a structural downward-younging trend from 67-56 Ma with low Y cores and high Y rims, consistent with regional uplift during contractional deformation. With this robust petrochronologic data set to accompany detailed structural field data, we interpret the inverted metamorphic sequence in the northern Coast Mountains to have formed via non-coaxial shear of melt-weakened mid to lower crust during terminal suturing of the Insular terrane to North America.