Abstract: Indicators of Plate Tectonics in Southern Alaska and Adjacent Deep-Sea Area, Late Paleozoic Through Cenozoic Time

David W. Scholl, Alan A. Cooper, Michael S. Marlow

Indicators of plate tectonics are the evidence for the movement and generation of large segments of crustal rocks. In southern Alaska the indicators of major plate activity are terranes of (1) magmatic rock, (2) sedimentary rock that accumulated depositionally or tectonically on an active margin (i.e., the so-called subduction complex of an underthrust margin), (3) ophiolite sequences(?), and (4) allochthonous terranes. Indicators in the north Pacific and Bering Sea regions are terranes of (1) linear magnetic anomalies, (2) linear seamount groups, and (3) Benioff seismic zones.

Late Paleozoic through Triassic:
In oceanic areas of the north Pacific there are no indicators remaining for pre-Mesozoic plate motion. However, on land, an extensive belt of intermediate magmatic rocks of Pennsylvanian and Permian age stretching discontinuously from southern Alaska (Taku-Skolai terrane) to British Columbia (Sicker volcanic rocks) is good evidence that, in fact, Pacific crust underthrust much of its northeastern perimeter 250 to 300 m.y. ago. Subsequently, in Middle and Late Triassic time, great thicknesses of basic igneous rocks associated with shallow-water deposits formed along parts of this same belt (Nikoli Greenstone; Karmutsen Basalt). These rocks may be indicators of rifting caused by oblique underthrusting or even relative strike-dip movement between Alaskan and Pacific crust. Significantly, scraps of an ophiolite(?) sequence and associated glaucophane-bearing metamorphic rocks and volcanic and sedimentary units in southwestern Alaska suggest that a convergent margin (magmatic arc or a subduction complex) formed here in the Late Triassic. Astonishingly, paleomagnetic data imply that these Triassic rocks, and those associated with the late Paleozoic magmatic arc on the north, may be part of an allochthonous terrane of pre-Cretaceous rocks that formed thousands of kilometers south of nuclear (Yukon terrane) Alaska. Jurassic and Early Cretaceous:
The eruptive and plutonic rocks of the Talkeetna magmatic arc attest that the southwestern margin of Alaska (probably including the Kuskokwim area) was underthrust in Early and Middle Jurassic time possibly as a continuation of Triassic activity. Sedimentary rocks of the flanking and more seaward Chugach terrane (McHugh-basal Valdez Complex) may represent the deposits of the associated active margin. Whether underthrusting of Pacific lithosphere continued into Late Jurassic and Early Cretaceous time is problematical owing to the uncertain age and formative history of the Mesozoic rocks of the Chugach terrane. Possible evidence exists in Lower and Middle Jurassic plutonic rocks that Pacific crust also underthrust the present southeastern margin of Alaska. However, the igneous rocks of the interior Gravina-Nutzotin belt and the active margin deposits of the coastal Yakutat Group-Sitka graywacke rocks attest that this margin was underthrust during Late Jurassic and Early Cretaceous time. The existence of the intervening rocks of the Alexander terrane, presumably an allochthonous tract of Paleozoic rocks derived from a distant southerly source, implies that the direction of underthrusting was northward and oblique to the trend of the margin. Farther west, a fragmented ophiolite sequence in the Chulitna area may be evidence of a coeval closing of a marginal sea that formerly occupied much of south-central Alaska. Closure presumably was forged by the northward migration of the allochthonous terrane of pre-Cretaceous rocks against nuclear Alaska.

Late Cretaceous:
North-striking magnetic anomalies of probable Early Cretaceous age recorded in the Bering Sea are evidence that Pacific crust, possibly the Kula plate, underthrust Alaska from the south during most of Late Cretaceous time. Additional indicators for underthrusting are the belts of Upper Cretaceous magmatic and deformed sedimentary rocks that stretch from Siberia via the Bering Shelf to southern Alaska (Shumigan-Kodiak belt). Similar rocks (Yakutat Group) are present in southeastern Alaska, which, presumably, also was underthrust by Pacific lithosphere (Farallon plate?).

Cenozoic:
Magmatic growth of the Aleutian Ridge, plutonism in southwestern Alaska, age progression of magnetic lineations in the north Pacific, and southward lengthening of the Emperor Seamount chain are indicators that northward underthrusting of Alaska (by the Pacific plate) continued into the early Tertiary. However, terrigenous sediment masses in the north Pacific imply that during the past 50 m.y. the amount of underthrusting has not exceeded about 500 km, although the lengths of seamount chains in the Gulf of Alaska suggest nearly twice this amount. The modern Benioff zone, coastal orogenic activity, and the outbreak of late Cenozoic volcanism are indicators of strong and sustained Pacific-American plate interaction since the late Miocene.

In summary, indicator terranes in southern Alaska provide evidence for almost continuous interaction between Pacific and Alaskan crust during the past 300 m.y. Models of global plate motion imply that since earliest Jurassic time as much as 15,000 km of northward-moving Pacific crust have been subducted beneath Alaska, an amount perhaps not equaled elsewhere in the world. Nonetheless, resolution of the history of plate tectonics in southern Alaska is complicated by uncertainties concerning the significance of (1) paleomagnetic data attesting to either enormous northward transport of allochthonous terranes to reach Alaska, or clockwise rotation of southwestern Alaska from a more southeasterly position; (2) geologic information indicating the opposite rotation to form the so-called Alas a orocline; and (3) the correct age and formational history of the Jurassic and Early Cretaceous rock complexes of coastal southern Alaska. There is measurable truth, therefore, in the assertion that our understanding of Alaskan tectonics has reached the stage of unabashed surprises.

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