Dames & Moore, 1999 - USDA Forest Service

analyze the chemistry. The analytical results are presented in Section 5.3. A discussion of the portal
drainage chemistry in terns of mineralization is presented in Section 6.
4.2 GEOLOGY AND SOILS
4.2.1 Regional Geology
4.2.1.1 Tectonic Setting
A discussion of the tectonic setting is presented herein to support the analysis of seismic potential at the
Site. The tectonic setting of the Pacific Northwest for the last 60 million years has been dominated by
collisions between the largest, tectonic elements of the earth, known as plates. Rafted along on the
oceanic plates were island arcs and microcontinents which, during plate collisions, accreted to the North
American Plate. With changing geometry and speed of the plate collisions, the patterns of faulting and
volcanism on the continental plate changed in response. This convergent boundary characterizes and
controls much of the geology and seismicity of the Pacific Northwest to this day.
The Pacific Northwest and adjacent continental margin have been divided into four major tectonic terrains
reflecting the regional tectonic setting of converging plates. These are the continental margin, the fore-arc
terrain, the volcanic arc, and the back-arc terrain. The dynamic interaction between the two major
converging plates (Juan de Fuca and North American) define the characteristic structure and location of
these four terrains with respect to plate geometry and configuration (Atwater, 1970) (Figures 4.2-la and 4.2-
1 b)
The continental margin is the westernmost of the four major tectonic terrains of the North American Plates
and includes the Cascadia Subduction Zone (CSZ), which occurs approximately 60 to 100 miles (1 00 to 175
kilometers) west of the Washington coastline.
The fore-arc terrain is the next terrain inland, and is characterized by deformed and metamorphosed
sedimentary and igneous rocks accreted to the continental plate during the convergence episode.
The volcanic arc terrain lies directly east of the fore-arc terrain, and was caused by the melting of
continental margin rocks during the subduction of the Juan de Fuca Plate beneath the North American Plate,
which has been occumng for the past 38 million years (Vance, 1982).
The back-arc terrain of Washington is located east of the Cascade Mountains, and is underlain primarily by
granitic and metamorphic rocks. Because the back-arc is composed of the accreted terrains of past
collisions, the region east of the Cascade Mountains has complex bedrock geology.
The Site is located in the transition area between the volcanic arc terrain and back-arc terrain, and is situated
approximately 20 miles east to northeast of Glacier Peak, a volcanic cone.
4.2.1.2 Seismicity
A discussion of the regional seismicity is presented herein to support the analysis of seismic potential at
the Site. Earthquakes are the result of sudden releases of built-up stress within the tectonic plates that
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