THE ACCRETIONARY PROCESSES OF THE PERMIAN AND TRIASSIC SEAMOUNTS, IN THE DEADMAN BAY TERRANE, THE SAN JUAN ISLANDS, WASHINGTON STATE, USA

The Deadman Bay terrane is the Mesozoic accretionary complex in the San Juan Islands, Washington State, USA. The terrane is divided into the Deadman Bay Volcanics and the Orcas Chert. They include oceanic rocks originated in the Permian and Triassic seamounts, which are presumed to have been accreted each by different processes.

The Deadman Bay Volcanics are composed of the Permian shallow-marine limestone, Permian basaltic rocks, Permian limestone-breccia, and Permian to Triassic chert. They are reconstructed as the Permian basaltic seamount and sediments on the top, the upper slope, and the lower flank of the seamount. It is characterized by the stacked, fault-bounded slabs of the oceanic rocks. It is noticed that the primary stratigraphy has been completely preserved in the slabs.

The Orcas Chert consists of the Permian to Lower Jurassic ribbon chert and minor chaotic rocks which overlie on the chert. The thickness of the chaotic rocks is up to a few kilometers. The chaotic rocks contain the isolated blocks of the Triassic shallow-marine limestone and basaltic rocks of several tens centimeters to several hundred meters in size. These blocks are irregularly scattered in the slump-folded chert and siliceous mudstone. The original stratigraphy of the Triassic seamount has been destructed in the chaotic rocks.

The difference in the modes of occurrence of the seamount-related rocks in the Deadman Bay terrane is supposed to be caused by the distinct accretionary processes. The Permian basaltic seamount in the Deadman Bay Volcanics had times enough to been cooled down during the movement toward the convergent plate boundary. The basaltic rocks of the seamount had a high density and were subducted down with the preserved primary stratigraphy. They were scraped off and accreted to the accretionary prism by thrust faults. On the other hand, the seamount of the Orcas Chert reached the convergent plate boundary as soon as it was built up during Triassic time. Therefore the basaltic rocks of the seamount had not sufficiently cooled yet and the density was low. The seamount collided with the accretionary prism and caused the large-scaled collapse of the accretionary prism and the seamount.