Rekonstruktion der Subduktion des Yakutat-Terranes anhand der tektonischen Entwicklung des Kodiak- und Middleton-Schelfs im nördlichen Golf von Alaska : eine regionale Studie über den Einfluß plattentektonischer Ereignisse auf das strukturelle Bild einer konvergenten Plattengrenze

A model describing the Cenozoic migration of the Yakutat Terrane, a composite oceanic and continental tectonostratigraphic terrane of the Gulf of Alaska, and its entry into the Alaskan subduction zone is developed here. The model includes new data and results acquired during the cruise SO 96-2 KODIAKSEIS with the German R/V SONNE in 1994. A review of various geophysical data sets (seismic reflection and refraction sections, free-air gravity) in combination with the results from geologic analyses (basin analysis techniques, plate tectonic modeling, geomorphologic interpretations) provides constraints on the collision of the Yakutat Terrane with the convergent margin of the eastern Gulf of Alaska. The correspondence between regional tectonism of the Kodiak and Middleton shelves and the subduction of the Yakutat Terrane suggests a cause-and-effect relationship that is constrained by the timing of tectonic events and the uplift / subsidence history along the fore-arc. The basaltic basement of the Yakutat Terrane originated on seamounts near the Kula-Farallon spreading center during the early to middle Eocene. Coeval and geochemically similar basalts, occuring in a linear accreted belt from southern Vancouver Island to the southern Oregon Coast Range, are correlative with the Yakutat Terrane basalts. It is likely that the northward migration of the Yakutat Terrane started from a position seaward of N-Washington and Vancouver Island sometime between the middle to late Eocene. During its northward movement close to the coast of British Columbia and southeast Alaska, voluminous elastic sediments that originated from the uplifted regions of the coastal batholithic complex were shed on its composite basement and the adjacent abyssal plain of the Kula plate. Simultaneously, the development of the Zodiac fan located southwest Yakutat Terrane started about 42 Ma and from the late Oligocene (24 Ma) the western part of the Surveyor fan began to form. The now-subducted western portion of the Yakutat Terrane bridged the gap between the deep-sea fans and the continental source terrains of North America. The huge sediment masses that probably accumulated on the abyssal plain west of the Yakutat Terrane were partly accreted along the convergent margin (Sitkalidak Fm.). The subduction and temporary underplating of these sediments along the Aleutian subduction zone led to the emergence of the Kodiak Island group during the Eocene and widespread subaerial erosion of the upper continental slope and seaward edge of the shelf during the Oligocene and lower Miocene. Meanwhile the terrestrially influenced sediments of the Sitkinak Fm. were deposited in a few landward depressions. After this period of intensive erosion the whole shelf area was inundated and subsided locally (Tugidak basin, Stevenson basin). Equidimensional fore-arc basins were filled with as much as 5 to 7 km of little-deformed Neogene sediments since the middle Miocene (Narrow Cape Fm. & Tugidak Fm.). The process that created the initial depocenters of this sedimentary basins remains enigmatic. About 10 Ma the hypothesized leading edge of the Yakutat Terrane collided with the Middleton shelf and eastern Kodiak shelf and was subsequently subducted in a north-northwesterly direction. The collision caused widespread tectonic erosion of the lower continental slope and affected the structural evolution of the Middleton and northeastern Kodiak shelves beginning at the southwest and progressing to the northeast. The trailing edge of the Yakutat Terrane was subducted along the northeastern Kodiak shelf about 6.4 Ma and left an indentation in the slope as an erosional scar. The scar was subsequently filled with younger accreted sediments (mainly younger than about 3 Ma) but has not been healed completely. Likewise, the Portlock anticline and escarpment were built during this event and the depressions along the shelf became closed basins as ridges developed at the edge of the shelf. The basins were separated by broad basement highs trending transverse to the regional trend of the margin. It appears that the collision and subduction of the Yakutat Terrane caused the successive uplift of the St. Elias, Chugach and Kenai mountain ranges. Increased rates of erosion and ediment accumulation provided a huge glacially-derived sediment supply that comprise the upper sequence of the Surveyor fan, the Baranof fan and the upper sequences in the fore-arc basins (Tugidak Fm.). Today the southwestern portion of the presently subducted Yakutat Terrane extends below the Kenai Peninsula as far as Augustine Island. Its subducted northern edge, if not integrated into the mantle, might extend to the Alaska Range region and runs from beneath the western Wrangell volcanic field to the Yakutat Bay region. Two main conclusions are infered from this model: the Yakutat Terrane was attached to, and migrated with the Pacific plate ("coupling model") during its northward migration and extends farther west than thought from previous studies. The interpretations of seismic refraction profiles (Bracher et al., 1994) suggest such a westward extension of the Yakutat Terrane. The model presented is in good agreement with palaeomagnetic and geologic data, integrates the evolution of the deep-sea fans and explains the migration process of the Yakutat Terrane with a simple mechanism