Structural and geochronologic history of southern Alaskan blueschists
Detrital zircon data are presented from the Liberty Creek blueschist in the central Chugach Mountains which indicate two distinct periods of the preservation of blueschist facies metamorphism along the southern Alaskan margin. A maximum depositional age of ~136 Ma demonstrates that the Liberty Creek schist was deposited long after the Jurassic cooling ages (196-185 Ma; Lopez-Carmona et al., 2011) recorded in western Alaskan blueschist bodies, revealing two blueschist preservation events. This Early Cretaceous depositional age also indicates that there have been major reorganizations within this subduction complex because the Potter Creek assemblage, directly south of the Liberty Creek schist, is an older (maximum depositional ages of 169 Ma - 156 Ma; Amato et al., 2013) but more shallowly exhumed assemblage. Strike-slip motions have rearranged the accretionary complex by carrying the Potter Creek assemblage outboard and south of the Liberty Creek schist. The predominance of 130-140 Ma zircons in the Liberty Creek schist sample and a population of detrital zircons that is distinct from nearby terranes suggest a sedimentary source different from other related accretionary assemblages. Three suggested Cordilleran source terranes are the Chitina Valley batholiths, ∼200 km east, the Firvale Suite of the Coast Plutonic Complex, ∼1500 km to the southeast near Vancouver, B.C., or the southern Mexican Guerrero terrane, ∼3000 km to the southeast. The detrital zircon signature of the Liberty Creek schist and these distances to potential sources support models suggesting thousands of km of strike-slip movement along the western Cordillera since Cretaceous time (e.g. the Baja-BC hypothesis). Structural data collected from the Iceberg Lake schist, the Liberty Creek schist, and the surrounding Potter Creek assemblage suggest these rocks have recorded a similar strain history. The Iceberg Lake schist, the Liberty Creek schist, and the Potter Creek assemblage have different lithologies, cooling ages, and ages of accretion that span 50 million years. Similar strain patterns include a shallow southerly dipping cleavage (S2) which folds (F2) previous layering, a vertical N-S striking cleavage (S3) which folds F2 folds, and variably cross-cutting low temperature mineral veins that follow these cleavages. Two hypotheses could explain these similar strain patterns. 1) These rock assemblages underwent accretion, metamorphism, and exhumation at the respective times indicated by thermochronologic analyses and observed similarities in recorded strain are caused by a similarly-oriented subduction zone. In this scenario the current geographic locations of these assemblages are a result of latest Cretaceous to Paleogene strike-slip motion on the Border Ranges fault. 2) A regionally complex, time transgressive, three dimensional model of both margin perpendicular and margin parallel motion accounting for accretion, erosion, and dextral transpression has been recorded in this section of the Jurassic-Cretaceous paleo-subduction wedge. After individual earliest histories of accretion, underplating, and metamorphic cooling, central Chugach Mountain assemblages were incrementally moved vertically and horizontally through the subduction wedge at sub-greenschist temperatures. These incremental motions were recorded as shortening perpendicular to the relative direction of motion, S2 and S3 cleavages, and low temperature veins intruding along cleavage planes perpendicular to that shortening.
Day, Erik M, "Structural and geochronologic history of southern Alaskan blueschists" (2014). ETD Collection for University of Texas, El Paso. AAI3682456.