Straight Creek Fault
Encyclopedia
The Straight Creek Fault (SCF) is the principal north-south strike-slip 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-Tertiary
igneous and metamorphic rocks of the North Cascades on the east from the younger accreted terranes 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, where it joins the Fraser River Fault system; the combined system (over 570 km [340 miles] long) 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 to 41 million years ago (in the Eocene
epoch), just after the wedge of crust now carrying the Olympic Mountains
pushed into the continental margin. It has been suggested that initiation of the SCF may be due to events on the Kula Plate
.
The SCF seems to be related to the Darrington—Devils Mountain Fault (DDMF), which runs due east from the southern end of Vancouver Island
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.
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 Canada as the Cascade Mountains...
, where it separates the pre-Tertiary
Tertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...
igneous and metamorphic rocks of the North Cascades on the east from the younger accreted terranes on the west. The SCF can be traced from its junction with the Olympic-Wallowa Lineament
Olympic-Wallowa Lineament
The Olympic-Wallowa lineament – first reported by cartographer Erwin Raisz in 1945 on a relief map of the continental United States – is a physiographic feature of unknown origin in the state of Washington running approximately from the town of Port Angeles, on the Olympic Peninsula...
(OWL) near the town of Easton northward into British Columbia, where it joins the Fraser River Fault system; the combined system (over 570 km [340 miles] long) 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 to 41 million years ago (in the Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
epoch), just after the wedge of crust now carrying the Olympic Mountains
Olympic Mountains
The Olympic Mountains is a mountain range on the Olympic Peninsula of western Washington in the United States. The mountains, part of the Pacific Coast Ranges, are not especially high - Mount Olympus is the highest at - but the western slopes of the Olympics rise directly out of the Pacific...
pushed into the continental margin. It has been suggested that initiation of the SCF may be due to events on the Kula Plate
Kula Plate
The Kula Plate is an oceanic tectonic plate under the northern Pacific Ocean south of the Near Islands segment of the Aleutian Islands. It is subducting under the North American Plate at the Aleutian Trench and is surrounded by the Pacific Plate...
.
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
Vancouver Island is a large island in British Columbia, Canada. It is one of several North American locations named after George Vancouver, the British Royal Navy officer who explored the Pacific Northwest coast of North America between 1791 and 1794...
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
- Geology of the Pacific NorthwestGeology of the Pacific NorthwestThe geology of the Pacific Northwest refers to the study of the composition , structure, physical properties and the processes that shape the Pacific Northwest region of the United States and Canada...
- North CascadesNorth CascadesThe 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 Canada as the Cascade Mountains...
- Olympic-Wallowa LineamentOlympic-Wallowa LineamentThe Olympic-Wallowa lineament – first reported by cartographer Erwin Raisz in 1945 on a relief map of the continental United States – is a physiographic feature of unknown origin in the state of Washington running approximately from the town of Port Angeles, on the Olympic Peninsula...
External links
- Burke Museum web site Geologic history of Washington.
- Evolution of the Pacific Northwest Good text on the geology of Cascadia.
- Snoqualmie Pass quadrangle USGS map showing the Straight Creek Fault.