AN: T71E-003
TI: Terranes and Terrane Boundaries Along the West Flank of the Coast Mountains Near Portland Canal, SE Alaska
AU: George Gehrels
AF: Dept. of Geosciences, University of Arizona, AZ 85721; 520-621-6026
EM: ggehrels@geo.arizona.edu
AU: BJ Darby
AF: Dept. of Geosciences, University of Arizona, AZ 85721; 520-621-6026

AB:
Terranes recognized along the west flank of the Coast Mountains in the Portland Canalarea include, from west to east: (1) Alexander terrane (AT) = late Proterozoic to Triassic arc-type, oceanic plateau, and rift assemblages, (2) Gravina belt (GB) = Middle Jurassic-Lower Cretaceous basinal strata and felsic volcanics, (3) a sequence of Jurassic and perhaps older felsic and mafic volcanic rocks of uncertain terrane affinity, and (4) Yukon-Tanana terrane (YTT) = upper Proterozoic-lower Paleozoic metapelite, quartzite, and marble, mid-Paleozoic felsic to mafic metavolcanic and metaplutonic rocks, and upper Paleozoic metapelite with subordinate marble and volcanic rocks. In contrast to regions to the northwest and southeast where these rocks are imbricated along obvious mid-Cretaceous thrust faults, such structures have not been recognized in the Portland Canal area. Rather, the first-order mid-Cretaceous structure is a southwest-vergent, southeast-plunging syncline that is tight (and probably faulted) to the northwest and open to the southeast. The southwestern (upright) limb consists of east-facing Triassic and older Alexander terrane rocks overlain by Jura-Cretaceous Gravina strata. The northeastern limb, which is inverted in the north, consists of southwest-facing Gravina strata that grade into the Jurassic and perhaps older volcanic rocks, which in turn overlie a southwest-facing panel of YTT rocks. These relations indicate that the Alexander terrane was in close proximity to YTT rocks prior to the deposition of Gravina strata, which form an overlap assemblage across the older terranes. The fundamental accretionary structure in the area is therefore pre-Late Jurassic in age and covered in many areas by the Gravina belt.