Abstract: Overview of Alaska Plate Tectonics

J. Kaspar Arbenz

The general features of the present-day plate-tectonics setting of Alaska are understood fairly well; however, many of the details remain unresolved. All the land areas of Alaska as well as the adjoining oceanic regions of the Bering Sea and Canadian basins of the Arctic Ocean are part of the North American plate that extends well into Asiatic USSR. The southern border of the North American plate abuts against the Pacific plate and consists, from east to west, of a complex ridge-trench-transform, a convergent subduction segment, and a trench-trench-transform in the western Aleutians. The western border of the North American plate lies in Soviet Siberia where a poorly defined group of subplates separates it from the Eurasian plate.

The actively convergent segment of the Aleutian arc is characterized by a Benioff zone (B subduction) that dips northwest to about 200 km. A notable decrease of dip of this Benioff zone is present from southwest to northeast as the convergent boundary changes from an intraoceanic island arc to an Andean-type active margin.

The present ridge-trench-transform fault zone of southern and southeastern Alaska consists of a complex westward-spreading horsetail of right-lateral strike-slip faults that merge eastward in the Queen Charlotte fault. As these faults diverge from the truly strike-slip segment off the Queen Charlotte Islands, they pick up increasing components of oblique underthrusting until, in the crotch of the Gulf of Alaska, they pass into or abut against numerous thrusts of the convergent-subduction zone.

The trench-trench-transform of the western Aleutians is, in its regional aspects, more simple in that the movement appears to be concentrated more clearly in one master fault.

North of the plate boundary the Alaskan part of the North American plate is divisible into oceanic and continental segments by two now passive continental margins, the Beaufort and the Beringian margins. Actively subsiding as well as tectonically passive back-arc basins form the containers of Quaternary depocenters both on land and in the sea. On the Pacific side of the presently active volcanic arc one can recognize a complex suite of fore-arc and transform basins that receive sediment in continental, shallow-marine, and deep-marine environments. The geometry, character, and displacements of Alaskan plate boundaries in the geologic past are much more difficult to recognize and reconstruct, and interpretations become increasingly more tenuous the farther back one looks. Particularly Mesozoic and pre-Mesozoic geometries have been altered profoundly, fragmented, and obliterated by subduction, compressional telescoping, strike-slip faulting metamorphic overprinting, and erosion during the late Mesozoic-early Tertiary orogenic events that have affected most of Alaska.

Through much of the Tertiary a setting similar to the present was in operation. The southern plate boundary of the North American plate was near its present position but the interior branches of the transform system (Denali-Totschunda-Castle Mountain) were intermittently much more active than today. The sum of Tertiary strike-slip displacements along all branches of this transform system between the main body of the North American and Pacific plates must be in the order of several hundred kilometers. This Tertiary through recent plate-boundary configuration probably originated in the Paleocene when some profound change in the relative plate movements caused a shift of the active convergence zone away from the Bering Shelf margin to the Aleutian Trench, thereby coupling a former piece of the northern Pacific Ocean (Kula plate?) with the North American plate.

From late Paleozoic through Early Cretaceous the Cordilleran convergent-plate boundary of southern Alaska was in the vicinity of the Border Ranges fault, resulting in a highly telescoped subduction complex (Uyak-McHugh-Strelna). The western continuation of this subduction and/or transform boundary was probably near the Beringian shelf break and in the southern Koryak Mountains of the eastern USSR. In the Late Cretaceous the plate boundary migrated south, incorporating in the subduction complex the massive deep-water fan assemblies of the Kodiak-Valdez Formations. Mesozoic back-arc successor basins occupied large parts of interior Alaska and the Bering Shelf. Their contents (mostly clastic and volcanic rocks) became severely deformed and foreshortened during the late Mesozoic orogeny. In the north, along the Kobuk-Tintina fault zones a south-dipping subduction zone (A subduction) was active during much of the Mesozoic and into the Tertiary resulting in an orogenic welt (the ancestral Brooks Range) that supplied clastic materials north and northeastward into a typical foredeep (the Colville geosyncline) and across the Barrow-Prudhoe arch in the Beaufort Sea. The end product of this subduction was the overthrust and foldbelt of the Brooks Range accompanied by strike-slip displacements along the "root zone.

The late Paleozoic (Permian-Pennsylvanian) was a period of major tectonic rearrangements. The major Cordilleran subduction zone shifted from near the present Denali line to the region of the Border Ranges fault. During this process a volcanic arc was formed well south of the Denali line and a series of back-arc basins was established or initiated. The southern tier of these back-arc basins (e.g., Goodnews, Nushagak-Alaska Range-Nutotzin-Dezadeash-Seymour) was probably, at least in part, underlain by oceanic crust. The northern group of back-arc basins (e.g., Kobuk, Tofty, Kuskokwim) resulted from a breakup by rifting and/or attenuation of an early to middle Paleozoic platform which occupied much of central and western Alaska. It is possible that during this period a major segment of the southern margin of this early Paleozoic platform became separated by rifting and left-lateral strike-slip movement and lodged in the late Paleozoic-early Mesozoic subduction complex on the southeast. This block, the Alexander terrane of southeastern Alaska, may have been derived from a present location near the central Alaska Range.

The early to middle Paleozoic record of central, western, and northern Alaska indicates stable-platform conditions. The platform extended westward into Soviet Asia including rocks exposed in the Chukotsk Peninsula, the Kolyimski massif, and the Cherskiy Ranges. To the northeast this platform probably linked up with the Canadian shield across the abortive aulacogen of the Richardson Trough and to the southeast it passed into the unstable, eugeosynclinal Cordilleran embayment of the heavily metamorphosed Fairbanks-Yukon terrane. A very incomplete record shows that both the northern and the southern edge of this Proterozoic to early Paleozoic continental platform were active subduction margins. The northern active margin (with a south dipping Benioff zone?) followed along the norther rim of the North American continent, its activity culminating in the Innuitian orogeny (in northern Alaska in late Middle Devonian) with the formation of an orogenic land source, angular unconformity, and a well-developed marginal foredeep (Kanayut-Huntfork). The northern land source remained active into the Jurassic; thereafter, the margin collapsed in Alaska and was overrun by southern-source sediment. Whereas much of the evidence of this active margin disappeared in Alaska and neighboring areas, a more complete record is preserved in the northeast in the Arctic Islands and in northern Greenland. Furthermore, the southern part of the Innuitian active belt of Alaska became involved in the north-yielding late Mesozoic deformation. Thus the Late Devonian foredeep sequence today is preserved at most exclusively in allochthonous thrust sheets of the Brooks Range. The southern, Cordilleran margin of the early Paleozoic platform was probably also an active convergent or transform margin, as indicated by abundant volcanic rocks in the Alexander terrane. Whether this margin underwent an Antler-related orogeny prior to the late Paleozoic breakup is uncertain but not implausible.

Much of the plate-tectonic evolution of Alaska remains obscure. Unquestionably, detailed mapping with reveal new evidence confirming or refuting the presently postulated events but large elements of the record have been irretrievably lost by subduction, erosion, and later massive burial.

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