Abstract: Plate Tectonics in Evolution of Southern Alaska Continental Margin

George Plafker, Terry Bruns, Gary Winkler

Mesozoic and Cenozoic rocks that make up the southern Alaska continental margin record a complex history of crustal-plate mobility and interactions. Southern Alaska began to assume its present form in about middle Cretaceous time by assembly of several northward-moving microplates, including the Alexander terrane and Wrangellia, against a nucleus of Precambrian to middle Paleozoic rocks. During Campanian and Maestrichtian times, a volcanic arc developed along the northwest-trending continental margin as a result of relative northeastward underthrusting by Pacific oceanic crust. A sequence of volcanogenic flysch and melange with subordinate oceanic basalt (Chugach terrane) was accreted along the continental margin northwest of Chatham Strait in a continuous belt up to 100 km wide. A subparallel volcano-plutonic arc was developed on continental crust north of the accretionary belt, and shelf deposits were laid down in arc-trench gap basins. A major shift in plate motions at the beginning of the Tertiary resulted in northwestward movement of Pacific oceanic crust relative to the Alaska continental margin. Also about this time, approximately 95° of counterclockwise oroclinal bending of western Alaska occurred around a main axis through Prince William Sound and a secondary axis through Yakutat Bay. As a consequence of these relative plate movements, ensimatic deep-sea fan deposits and associ ted oceanic tholeiitic basalt of late Paleocene and early Eocene(?) age, that make up the Orca Group, and related rocks were accreted along the western limb of the oroclinal bend to form a belt with an exposed width of 110 km. This belt probably underlies much or all of the continental shelf west of Kayak Island. During middle to late Eocene time, deposition at the continental margin became regressive, with development of a thick coal-bearing lagoon, barrier-beach, and delta complex supplied largely by sediment derived from erosion of the older emergent accretionary sequences and from granitic plutons emplaced in them. Transgression occurred during Oligocene and early Miocene times, and predominantly shaly sediment, locally organic-rich and intercalated with water-laid alkali basaltic tu f, breccia, and pillow lava, accumulated. This unique volcanic episode could be related to subduction of an oceanic spreading center. The present sedimentary and tectonic regimes result from movement of the Pacific plate northwestward relative to the Alaskan continental margin since early Miocene time. This movement caused the Aleutian Trench and volcanic arc to develop by underthrusting or oblique underthrusting of the continental margin in the northern and western Gulf of Alaska, whereas dextral transform faulting prevailed along the eastern margin of the Gulf of Alaska. From the middle Miocene to the present, enormous thicknesses of clastic sediment, including much glacially derived material, were deposited rapidly in a predominantly shallow-shelf environment, while deep-water sedimen s were being deposited in slope basins and in the Aleutian Trench. Simultaneously, deformation related to plate convergence and gravitational sliding has resulted in folding and thrust faulting of the bedded Neogene rocks throughout much of the Gulf of Alaska, together with large-scale strike-slip faulting in the eastern Gulf of Alaska.

AAPG Search and Discovery Article #90962©1978 AAPG 2nd Circum-Pacific Energy and Minerals Resource Conference, Honolulu, Hawaii