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-Cenozoic 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).[1]
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,[2] or has been offset elsewhere.[3] (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,[4] the period of its strike-slip activity thus being bracketed between 47 and 41 million years ago (in the Eocene epoch), just after the wedge of crust now carrying the Olympic Mountains pushed into the continental margin.[5] It has been suggested that initiation of the SCF may be due to events on the Kula Plate.[6]
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.[7]
See also edit
Notes edit
- ^ Tabor & others 1984; Tabor 1994; Tabor & others 2000;Cheney & Hayman 2007; Figge 2009 (chs. 3 and 4, "Fraser Fault").
- ^ Tabor & others 1984; Heller & others 1987.
- ^ Cheney 1999, Cheney & Hayman 2007.
- ^ The Snoqualmie batholith, and likely the Barlow Pass Volcanics. Tabor 1994.
- ^ Vance & Miller 1994; Tabor 1994.
- ^ Figge 2009.
- ^ Tabor 1994.
References edit
- Cheney, Eric S. (December 1999), "Geological map of the Easton area, Kittitas County, Washington", Washington Division of Geology and Earth Resources, Open File Report 99-4: 11 p. scale 1:31,680
- Cheney, E. S.; Hayman, N. W. (2007), "Regional tertiary sequence stratigraphy and structure on the eastern flank of the central Cascade Range, Washington", in Stelling, P. L.; Tucker, D. S. (eds.), Floods, faults, and fire: Geological Field Trips in Washington State and Southwest British Columbia, Geological Society of America, pp. 179–208, doi:10.1130/2007.fld009(09)
- Figge, John (2009), Evolution of the Pacific Northwest: An Introduction to the Historical Geology of the Washington State and Southern British Columbia, Northwest Geological Institute, archived from the original on 2009-12-19
- Heller, P. L.; Tabor, R. W.; Suczek, C. A. (1987), "Paleogeographic evolution of the U.S. Pacific Northwest during Paleogene time", Canadian Journal of Earth Sciences, 24: 1652–1667, doi:10.1139/e87-159
- Tabor, J. W. (February 1994), "Late Mesozoic and possible early Tertiary accretion in western Washington State: the Helena—Haystack mélange and the Darrington—Devils Mountain Fault Zone", GSA Bulletin, 106 (2): 217–232, doi:10.1130/0016-7606(1994)106<0217:lmapet>2.3.co;2
- Tabor, R.W.; Frizzell, V.A. Jr.; Vance, J.A.; Naeser, C.W. (January 1984), "Ages and stratigraphy of lower and middle Tertiary sedimentary and volcanic rocks of the central Cascades, Washington: Application to the tectonic history of the Straight Creek fault", GSA Bulletin, 95 (1): 26–44, doi:10.1130/0016-7606(1984)95<26:AASOLA>2.0.CO;2
- Tabor, R.W.; Frizzell, V.A.; Booth, D.B.; Waitt, R.B. (2000), Geologic map of the Snoqualmie Pass 60 minute by 30 minute quadrangle, Washington: U.S. Geological Survey Miscellaneous Investigations Map I-2538, U.S. Geological Survey
- Vance, Joseph A.; Miller, Robert B. (1994), "Another look at the Fraser River-Straight Creek Fault (FRSCF)", GSA Abstracts with Programs, 24: 88
External links edit
- 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.