Tectonic and structural evolution of the Ghost Rocks formation, Kodiak Island, Alaska
University of California, Santa Cruz
174 p., Illus., Maps
Alaska Resources Library & Information Services: QE602 B9; Rasmuson Library: ALASKA QE601.2 B97 1981a
The Kodiak Islands constitute an uplifted portion of the Alaska continental shelf and expose one of the most complete sequences of accreted deep sea rocks preserved anywhere in the world. The Paleocene Ghost Rocks Formation exposed along the southeast side of Kodiak Island is the next youngest sequence of deep-sea rocks. This formation is composed predominantly of intensely disrupted and deformed turbidite deposits. Conspicuous belts of interbedded volcanic and associated plutonic rocks also occur locally. I have completed a detailed study of the structural evolution of the Ghost Rocks Formation in order to better understand the accretionary process. In addition, I compiled the Late Cretaceous to early Tertiary magnetic anomaly data from the northeast Pacific and reinterpreted the plate history of this region. The magnetic anomaly patterns indicate that the west-trending limb of the Kula-Pacific-Farallon triple junction ceased spreading about 52-53 m.y. ago. Following this ridge demise the northeast and southeast trending limbs fragmented and realigned, forming several shorter northtrending ridge fragments. This time of ridge deactivation coincides with the temporal boundary between the Ghost Rocks Formation and a younger sequence of rocks exposed on Kodiak Island. I interpret the igneous rocks in the Ghost Rocks Formation to be the consequence of an early Tertiary encounter between the Kodiak margin and the northeast trending limb of the Kula-Pacific-Farallon triple junction. My detailed study of the Ghost Rocks Formation revealed two contrasting structural styles: melange and relatively coherent terranes of folded turbidite deposits. The coherent terranes are interpreted as slope basin deposits. The deformation axes recorded by folds and cleavage indicate that these terranes were multiply deformed in the Paleocene by subhorizontal shortening. These deformation axes are still in their subhorizontal orientation indicating that these terranes and the Ghost Rocks Formation have not been tilted landward by the accretion and imbrication of younger, more seaward belts. The melange terranes have been multiply and complexly deformed by progressive layer parallel shear. The structures and deformation history of these terranes contrast markedly with the coherent terranes. I interpret the melange terranes to represent accreted trench deposits because of their unique deformation history.
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