PROGRESSIVE DEFORMATION AND THE FORMATION OF AN INVERTED METAMORPHIC BELT, COAST MOUNTAINS, ALASKA

Despite years of debate, the tectonic boundary conditions and physical mechanisms responsible for forming inverted metamorphic field gradients remain

contentious. Models include post-metamorphic structural intercalation of various crustal levels, frictional or advective heating during imbrication, and transposition of a normal geothermal gradient during syn-metamorphic non-coaxial flow. In SE Alaska, inverted Barrovian metamorphism locally occupies the Insular Superterrane and North American affinity rock boundary. Previous work attributed the inverted mineral isograds to the intrusion of overlying tonalitic sills at ca. 70 Ma or a ca. 90 Ma regional metamorphic origin. We present new structural, thermobarometric, and petrochronologic analysis from a traverse through the sequence evaluating the timing of inverted metamorphism and constraining the physical and thermal mechanisms responsible for its formation.

At Blackerby Ridge, near Juneau, AK, inverted metamorphic isograds are preserved in a NW striking, 5.5 km thick tectonostratigraphic package bound to the W by the Gastineau Channel fault. From structurally lowest to highest, the sequence includes Gravina overlap assemblage, Taku terrane, and Yukon Tanana Terrane, overlain to the E by the Great Tonalite Sill and pluton-gneiss belt. The Sumdum and Fanshaw Faults are discrete thrusts inferred between metamorphic packages. GARB-GASP thermobarometry agrees with previous work indicating that the structurally highest rocks experienced up to 650-700C and 9 Kbar (amphibolite-grade metamorphism), with metamorphic grade progressively decreasing structurally downward to sub-greenschist facies. Tectonites preserve protomylonitic to mylonitic textures. New in-situ U-Pb monazite-xenotime petrochronology reveals deformation and metamorphism decrease in age with structural depth from highest ~67 Ma, progressively younging downward to the structurally lowest ~56 Ma. Monazite preserve two age/compositional populations, downward younging low-Y domains inferred to have grown during prograde metamorphism, and a later generation of high-Y domains inferred to form during garnet breakdown. Integrated structural and petrochronologic results indicate that inverted metamorphism at Blackerby Ridge formed during progressive deformation within a thick (>5 km) ductile shear zone from 67-56 Ma.