Sunday, October 26, 2014

Orcas Island Tectonic Zone on East Sound

Mount Constitution and East Sound on Orcas Island 

East Sound and fishing boat

East Sound is a fjord on the south side of Orcas Island. It consists of a deep bay with Mount Constitution rising to over 2,400 feet to the east as well as other steep slopes and cliffs much of it on private land. The small town of Eastsound (note the spelling) is located at the north, upper end of the inlet.
Several major tectonic thrust fault zone structures cut across the Sound and are exposed along the shoreline rocks. These faults juxtapose a wide variety of accreted terranes. Near the upper end of the Sound is a tectonic zone where Jurassic to Tiassic Orcas Chert, Triassic to Permian Deadman Bay Volcanics and pre-Devonian Turtleback Complex are interleaved and mixed up within a tectonic zone and so mangled to be often indistinguishable. Thinking through where these terranes originated and how they ended up here requires a broad understanding of global plate tectonics over deep geologic time as well as correlating slivers of rocks spread out over huge distances by plate tectonics all done while pulling out chemical and mineral data the rocks involved. Brown, Housen and Schermer provide an overview of the San Juan Island thrusts here: gsa-cord-07-san-juans.pdf

Slickenslides on mafic surface

Highly sheared meta volcanics

One slope is so sheared that the rock is highly eroded and susceptible to slope failures and is breaking down into clay

Fault breccia with intact cobble sounded by highly sheared rock  

Fault breccia

Shear zones with a fluid-like fabric around fragments of in tact breccia fragments

Highly sheared rock can be a bit a of problem for slope stability. There are several spots around Orcas Island that this is a problem. In this case the angle of the shearing and orientation with the shore was not problematic.

Within the very mafic pulverized rock, a stringy barked tree was hanging onto the poor soil - Juniperus maritima (catching-up-with-juniperus-maritima).

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