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The Straight Creek Fault (SCF) is the principal north-south
strike-slip
In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...
fault in the state of Washington, with a minimum of 90 kilometers (54 miles) of right-lateral offset, and a major geological structure in the
North Cascade mountains, where it separates the pre-
Cenozoic
The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...
igneous
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...
and
metamorphic
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causin ...
rocks of the North Cascades on the east from the younger accreted
terranes
In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its own ...
on the west. The SCF can be traced from its junction with the
Olympic–Wallowa Lineament (OWL) near the town of
Easton northward into
British Columbia
British Columbia (commonly abbreviated as BC) is the westernmost province of Canada, situated between the Pacific Ocean and the Rocky Mountains. It has a diverse geography, with rugged landscapes that include rocky coastlines, sandy beaches, ...
, where it joins the Fraser River Fault system; the combined system (over 570 km
40 mileslong) is known as the Fraser—Straight Creek Fault system (FSCF).
No trace of the SCF has been found south of the OWL. Some geologists believe it does continue south, with all traces covered by more recent volcanic deposits. Others have speculated that it simply ends, or turns and aligns with the OWL, or has been offset elsewhere. (See discussion at
Olympic–Wallowa Lineament#Straight Creek Fault.)
The SCF offsets the older NNW striking Entiat, Ross Lake, and Chewack-Pasayten faults, but not certain younger features, the period of its strike-slip activity thus being bracketed between 47 and 41 million years ago (in the
Eocene
The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...
epoch), just after the wedge of crust now carrying the
Olympic Mountains
The Olympic Mountains are a mountain range on the Olympic Peninsula of the Pacific Northwest of the United States. The mountains, part of the Pacific Coast Ranges, are not especially high – Mount Olympus is the highest at ; however, the easter ...
pushed into the continental margin. It has been suggested that initiation of the SCF may be due to events on the
Kula Plate
The Kula Plate was an oceanic tectonic plate under the northern Pacific Ocean south of the Near Islands segment of the Aleutian Islands. It has been subducted under the North American Plate at the Aleutian Trench, being replaced by the Pacific Pla ...
.
The SCF seems to be related to the Darrington—Devils Mountain Fault (DDMF), which runs due east from the southern end of
Vancouver Island
Vancouver Island is an island in the northeastern Pacific Ocean and part of the Canadian Provinces and territories of Canada, province of British Columbia. The island is in length, in width at its widest point, and in total area, while are o ...
to the small town of Darrington, then turns nearly south to converge with the SCF near its intersection with the OWL. This bowing appears to be due to the approaching Olympic Mountains. An anomalous rock formation (the Helena—Haystack Melange) just north of Darrington has been correlated with similar rock south of Easton, suggesting considerable right-lateral strike-slip motion on the DDMF. Motion on both faults seems to have been contemporaneous; the relationship between the two is not understood.
[.]
See also
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Geology of the Pacific Northwest
The geology of the Pacific Northwest includes the composition (including rock, minerals, and soils), structure, physical properties and the processes that shape the Pacific Northwest region of North America. The region is part of the Ring of Fir ...
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North Cascades
The North Cascades are a section of the Cascade Range of western North America. They span the border between the Canadian province of British Columbia and the U.S. state of Washington and are officially named in the U.S. and Canada as the Cascad ...
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Olympic–Wallowa Lineament
Notes
References
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External links
Burke Museum web siteGeologic history of Washington.
Evolution of the Pacific NorthwestGood text on the geology of Cascadia.
USGS map showing the Straight Creek Fault.
Geology of Washington (state)
Strike-slip faults