ANALOGOUS TO ANATOLIA, COLLISIONAL AND NON-COLLISIONAL PLATE BOUNDARY FORCES EXTRUDE WESTERN ALASKA TOWARD OFFSHORE SUBDUCTION ZONES

INTRODUCTION: It is conjectured that since the Eocene, and probably the Late Cretaceous, crustal lithosphere has moved counterclockwise around the NE corner of the Pacific Basin in a 600-800-km-wide belt of laterally translating crust. The moving band defines a plate boundary zone separating Pacific and North American lithosphere that extends ~3500 km from British Colombia (Pacific maritime Canada) northward through Alaska and westward to the Bering Sea. The zone of transiting crust is termed the North Pacific Rim Orogenic Stream or NPRS by Redfield et al. (2007). The NPRS is driven northward by transform coupling along the transform, but highly obliquely underthrust, margin of British Columbia. Pacific rim crust in Alaska is driven by the northward flow through the so-called “oroclinal bend” of south central Alaska to be expulsed west of the orocline toward subduction zones (SZs) residing in the Aleutian-Bering Sea region. This extra-regional tectonic concept is poorly documented by GPS data. Its viability is strongly based on drawing a tectonic analogy with the geologic- and GPS- established pattern of crustal extrusion of Anatolia toward the Hellenic-Cretan SZ system (Fig. 1)

Figure 1: Extra-regional scale pattern of escape tectonism for Anatolia and Alaska

EXTRUSION OF WESTERN ALASKA: Within the NPRS, slip-lines of regional crustal movement are revealed by a family of continental-scale, right-lateral shears that arc around the northeastern rim of the Pacific Basin. Offset evidence establishes that since the Eocene the combined displacement across these faults is ~1000 km or more. West of the orocline, the fault system fans (expands) outward to embrace the whole of Alaska south of the Kobuk fault. The fault pattern strikes SW toward the Aleutian SZ. Because convergence is directed mostly orthogonal to the SW trend of the Alaska SZ and the inboard family of continental shear zones, extrusion or escape tectonism is implied for western Alaska (Fig. 1).

ANATOLIAN AND ALASKAN TECTONISM: Extrusion of Anatolian crust is coupled to the collisional impact of the Arabia plate with Eurasia combined with the co-tectonic roll back of a pre-existing Hellenic-Cretan SZ system. In contrast, in the Late Cretaceous and early Tertiary, extrusion of western Alaska is linked to coupling across a highly oblique plate boundary zone that reached inland across British Columbia at least 600 km (Fig. 1). To the west, co-tectonic extrusion is posited to have forced the Eocene abandonment of the existing, highly oblique SZ of the Beringian margin and the consequent creation of the new offshore Shirshov and Bowers SZs and, possibly slightly earlier, the Aleutian SZ. Strong activation of the NPRS in the Eocene is connected to the subduction of the Kula-Resurrection spreading system beneath the NE periphery of the Pacific Basin and the consequent heating of the lithospheric mantle and crust above a complexity of slab windows (Haeussler et al., 2003).

IMPLICATIONS OF THE NPRS-EXTRUSION CONJECTURE: The NPRS-driven extrusion concept states:

(1) That the Alaska orocline is not the consequence of a physical or static bending of Alaskan crust in the early Tertiary. The NPRS-extrusion concept requires the dynamic movement of crustal material around and through the NE corner of the Pacific rim and expulsion of transiting material to the west.

(2) That until the late Cenozoic the NE corner of the Pacific rim was a broad, inward bowed margin built by terrane accumulation in the Mesozoic and early Tertiary. This rim was strongly pushed in and orogenically thickened by the Miocene entrance of the Yakutat crustal block into the eastern end of the Alaskan SZ.

(3) That new, offshore subduction zones were created by the drive of extruded crustal material toward an unaccommodating structural barrier, in the example here, the transform to highly-oblique underthrust Beringian subduction zone (Fig. 1; Scholl, 2007).

REFERENCES CITED:

Haeussler, P. J., D. C. Bradley, R. E. Wells and M. L. Miller, 2003, Life and death of the Resurrection plate: Evidence for its existence and subduction in the northeastern Pacific in Paleocene–Eocene time: Geological Society of America Bulletin, v. 115, No. 7, p 867-880.

Redfield, T.F., D.W. Scholl, and P.G. Fitzgerald, 2007, Escape tectonics and the extrusion of Alaska: past, present, and future: Geology, v. 35, No. 11, p. doi: 10.1130/G23799A.1

Scholl, D. W., 2007, Viewing the tectonic evolution of the Kamchatka-Aleutian (KAT) connection with an Alaska crustal extrusion perspective, in Volcanism and subduction the Kamchatka region, J. C. Eichelberger , E. Gordeev,P. Izbekov, M. Kasahara, and J. Lees (editors): American Geophysical Union, Monograph 172, p, 3-35.