Abstract: Mesozoic Magmatism and Accretion, Southwest Alaska Margin

J. Casey Moore, William Connelly,

The Alaska-Aleutian Range and the Kodiak-Kenai Shelf expose evidence for at least two phases of magmatism and accretion during the Mesozoic. In the Alaska-Aleutian Range, the initial phase of Mesozoic magmatism is documented by volcanic and volcaniclastic rocks of Norian to Callovian age (ca. 210 to 165 m.y.B.P.) coupled with party cognate quartz dioritic and granodioritic plutonic rocks with concordant K-Ar ages of 176 to 154 m.y.B.P. No evidence for magmatism is known for the later Jurassic and Early Cretaceous; however, thick and extensive arkosic deposits of these ages record the unroofing of the preexisting Jurassic plutonic arc. In the Late Cretaceous, magmatism resumed in the Alaska-Aleutian Range as documented by plutonic intrusions with concordant K-Ar ages of 83 to 58 m.y.B.P.

During the Mesozoic three lithologically distinct tectonic belts were accreted along the Kodiak-Kenai Shelf. A blueschist and greenschist terrane comprises the oldest and landward-most belt and yields K-Ar ages of about 190 m.y.B.P. This schist terrane is succeeded on its seaward side by a tectonic melange composed of blocks of ultramafic and gabbroic rocks, greenstone, sandstone, pre-Mesozoic and Cretaceous radiolarian chert, Permian and Upper Triassic limestone, all enclosed in a cherty argillite matrix. These rocks were underthrust to N38 ± 11°W with concurrent prehnite-pumpellyite facies metamorphism. This tectonic melange is succeeded in turn on its seaward margin by an Upper Cretaceous (Maestrichtian) turbidite sequence, which initially was underthrust to N28 ± 8°W and subsequently intruded by Paleocene granitic rocks.

The metamorphism of the blueschist and greenschist terrane in the Kodiak Islands and Kenai Peninsula occurred during the first phase of magmatism in the Alaska-Aleutian Range. The time of accretion of the succeeding tectonic melange may be correlated with either the Late Triassic to early Late Jurassic or Late Cretaceous to Paleocene period of Alaska-Aleutian Range magmatism, depending on the structural interpretation of the fossiliferous tectonic blocks. For either time period the direction of underthrusting is strongly oblique to the trend of the respective magmatic arc. The Upper Cretaceous turbidite sequence was accreted during Late Cretaceous to Paleocene magmatism, and its structurally determined direction of underthrusting is consistent with the Kula-North America motion predicted by a purely plate-tectonic reconstruction.

AAPG Search and Discovery Article #90966©1977 Alaska Geological Society 1977 Symposium, Anchorage, Alaska