Leech River Fault
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The Leech River Fault extends across the southern tip of
Vancouver Island Vancouver Island is an island in the northeastern Pacific Ocean and part of the Canadian province of British Columbia. The island is in length, in width at its widest point, and in total area, while are of land. The island is the largest ...
in
British Columbia British Columbia is the westernmost Provinces and territories of Canada, province of Canada. Situated in the Pacific Northwest between the Pacific Ocean and the Rocky Mountains, the province has a diverse geography, with rugged landscapes that ...
, Canada, creating the distinctively straight, narrow, and steep-sided valley, occupied by Loss Creek and two reservoirs, that runs from Sombrio Point (southeast of
Port Renfrew Port Renfrew is a small unincorporated community located on the south shore of Port San Juan, an inlet on the west coast of Vancouver Island in British Columbia, Canada. Port Renfrew has a population of 262 (as of the 2021 Canadian census) and ...
and Sombrio Beach) due east to the Leech River, and then turns southeast to run past
Victoria Victoria most commonly refers to: * Queen Victoria (1819–1901), Queen of the United Kingdom and Empress of India * Victoria (state), a state of Australia * Victoria, British Columbia, Canada, a provincial capital * Victoria, Seychelles, the capi ...
. It is a
thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. I ...
that marks the northernmost exposure of the Crescent Terrane (part of
Siletzia Siletzia is a massive formation of early to middle Eocene epoch marine basalts and interbedded sediments in the forearc of the Cascadia subduction zone, on the west coast of North America. It forms the basement rock under western Oregon an ...
), where basalt of the
Metchosin Igneous Complex The Metchosin Igneous Complex is an ophiolite sequence in southwestern British Columbia, Canada, located at the southern end of Vancouver Island. It is the northernmost expression of the Siletzia terrane and consists of three rock units, including ...
(correlative with the Crescent Formation on the
Olympic Peninsula The Olympic Peninsula is a large peninsula in Western Washington that lies across Puget Sound from Seattle, and contains Olympic National Park. It is bounded on the west by the Pacific Ocean, the north by the Strait of Juan de Fuca, and the ...
) is dragged under Vancouver Island by the subducting Juan de Fuca plate. About ten kilometres north the nearly parallel ''San Juan Fault'' marks the southern limit of rock of the Wrangellia terrane, which underlies most of Vancouver Island. Between these two northeast-dipping thrust faults are the ''Leech River Complex'' and (near
Port Renfrew Port Renfrew is a small unincorporated community located on the south shore of Port San Juan, an inlet on the west coast of Vancouver Island in British Columbia, Canada. Port Renfrew has a population of 262 (as of the 2021 Canadian census) and ...
, but also outcrops near
Victoria Victoria most commonly refers to: * Queen Victoria (1819–1901), Queen of the United Kingdom and Empress of India * Victoria (state), a state of Australia * Victoria, British Columbia, Canada, a provincial capital * Victoria, Seychelles, the capi ...
) the ''Pandora Peak Unit''. These, along with the ''Pacific Rim Complex'' further up the coast, are remnants of the Pacific Rim Terrane which was crushed between Wrangellia and Siletzia. The contact between the bottom of Wrangellia and the top of the subducted PRT continues northwest along the coast as the ''West Coast Fault'', and southeast towards Victoria as the ''Survey Mountain Fault''. The Leech River Fault (LRF) extends off-shore towards
Cape Flattery Cape Flattery () is the northwesternmost point of the contiguous United States. It is in Clallam County, Washington on the Olympic Peninsula, where the Strait of Juan de Fuca joins the Pacific Ocean. It is also part of the Makah Reservation, a ...
, where the Crescent—Pacific Rim contact continues northwest as the ''Tofino Fault'' (TF). The bottom contact of the Crescent Terrane has been uplifted and exposed along the edge of the Olympic Peninsula, where it is known as the ''Hurricane Ridge Fault'' (HRF). West of
Cape Flattery Cape Flattery () is the northwesternmost point of the contiguous United States. It is in Clallam County, Washington on the Olympic Peninsula, where the Strait of Juan de Fuca joins the Pacific Ocean. It is also part of the Makah Reservation, a ...
the southernmost edge of the Crescent Terrane appears to be an extension of the LRF, suggesting left-lateral
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 ...
along the Leech River Fault that has offset the Crescent Terrane nearly its whole width. The Leech River Fault (LRF) is a key element in understanding regional tectonic history as it and the San Juan Fault mark a change from the relatively simply subduction zone structure to the northwest, to the complex structures of the San Juan Islands and the Puget Lowland to the east and southeast. The straightness of its surface trace suggests the LRF has been a strike-slip fault. In these respects the LRF is very similar to the Devils Mountain Fault, which can be traced due west from near
Mount Vernon Mount Vernon is the former residence and plantation of George Washington, a Founding Father, commander of the Continental Army in the Revolutionary War, and the first president of the United States, and his wife, Martha. An American landmar ...
to a point just south of Victoria. Near the Leech River, where the ''Survey Mountain fault'' marks the eastern limit of the Leech River Complex, the LRF and the lineament it follows make a sharp turn to the south (heading S70°E) to run down the
Goldstream River The Goldstream River is a tributary of the Columbia River, joining that stream via the Lake Revelstoke reservoir after running largely west from the heart of the northern Selkirk Mountains. The river's name derives from the Big Bend Gold Rush o ...
past
Victoria Victoria most commonly refers to: * Queen Victoria (1819–1901), Queen of the United Kingdom and Empress of India * Victoria (state), a state of Australia * Victoria, British Columbia, Canada, a provincial capital * Victoria, Seychelles, the capi ...
and into the
Juan de Fuca Strait The Strait of Juan de Fuca (officially named Juan de Fuca Strait in Canada) is a body of water about long that is the Salish Sea's main outlet to the Pacific Ocean. The international boundary between Canada and the United States runs down the ce ...
. Such sharp turns are geometrically impossible for individual strike-slip faults, but the southeast-striking Goldstream arm of the LRF parallels the cross-cutting Survey Mountain fault. Strong aeromagnetic anomalies in the Strait that curve from Victoria to
Discovery Bay Discovery Bay is a picturesque residential community located on Lantau Island. The 2021 census recorded a population of 19,336 residents in DB, with 55% of them being non-Chinese. DB is home to a significant community compared of expatriates ...
(west of
Port Townsend Port Townsend is a city on the Quimper Peninsula in Jefferson County, Washington, United States. The population was 10,148 at the 2020 United States Census. It is the county seat and only incorporated city of Jefferson County. In addition ...
) led to early speculation that the LRF, and the eastern limit of the Crescent Formation, ran down Discovery Bay and the west side of
Puget Sound Puget Sound ( ; ) is a complex estuary, estuarine system of interconnected Marine habitat, marine waterways and basins located on the northwest coast of the U.S. state of Washington (state), Washington. As a part of the Salish Sea, the sound ...
, following the edge of a relict
continental margin A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental marg ...
just as the Tofino Fault follows the present margin. More recent interpretations of marine seismic reflection studies align the Goldstream arm of the LRF with the Southern Whidbey Island Fault (SWIF), with splays branching towards Discovery Bay. Discovery of Crescent Formation basalt in an exploration well on
Whidbey Island Whidbey Island (historical spellings Whidby, Whitbey, or Whitby) is the largest of the islands composing Island County, Washington, Island County, Washington (state), Washington, in the United States, and the largest island in Washington stat ...
(just east of Port Townsend) has led some writers to locate this extension of the LRF onto the southern part of Whidbey Island, just where the SWIF is found. While the eastern contact of the Crescent Formation is believed to not extend east of Whidbey Island, and may double-back westward under
Seattle Seattle ( ) is the most populous city in the U.S. state of Washington and in the Pacific Northwest region of North America. With a population of 780,995 in 2024, it is the 18th-most populous city in the United States. The city is the cou ...
, active faulting on the SWIF extends southeast to where it connects with the Rattlesnake Mountain Fault Zone. Thus it appears that the Leech River and Devils Mountain faults were once a single left-lateral oblique fault (having both horizontal
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 ...
and vertical
dip-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 f ...
motion) that has been offset by right-lateral motion along the extended Survey Mountain—Southern Whidbey Island Fault. The earlier history of the LRF is revealed by certain metamorphic rocks of the Pacific Rim terrane with a very distinctive mix of minerals. These are also found in the
San Juan Islands The San Juan Islands is an archipelago in the Pacific Northwest of the United States between the U.S. state of Washington and Vancouver Island, British Columbia, Canada. The San Juan Islands are part of Washington state, and form the core of ...
and in isolated outcrops spread across the Cascades. They formed between 100 and 84 Ma (millions of years ago) during the
Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cre ...
when the Pacific Rim terrane was crushed between Wrangellia and the North American continental plate, dismembered, and the pieces smeared along what was then the edge of the continent. Continued right-lateral transpression carried outlying portions of Wrangellia and likely some odd pieces of the Pacific Rim terrane northwest to Gulf of Alaska. Note that the ancient continental margin was not along the modern coast line (that runs due south from Vancouver Island), but turned in at the modern day Juan de Fuca Strait and followed the Olympic–Wallowa Lineament (OWL) towards southern Idaho, paralleling the general trend of faulting in British Columbia and the
North Cascades The North Cascades are a section of the Cascade Range of western North America. They span the border between the Provinces and territories of Canada, Canadian province of British Columbia and the U.S. state of Washington (state), Washington and ...
. About 50 million years ago the
Siletzia Siletzia is a massive formation of early to middle Eocene epoch marine basalts and interbedded sediments in the forearc of the Cascadia subduction zone, on the west coast of North America. It forms the basement rock under western Oregon an ...
terrane, being borne to the northeast by the subducting plate, refused to be subducted. It ran it into the edge of the continent and embayed the overlying crust, bending the section of the Wrangellia—Pacific Rim contact now known as the San Juan fault to its current easterly orientation. This also initiated the oblique left-lateral Devils Mountain fault, including the section now known as the Leech River fault, and its right-lateral extension, the Darrington fault, that strikes southward from the town of Darrington to converge with the right-lateral strike-slip Straight Creek Fault at the OWL (see map). About 42 million years ago this northeastward force rotated to a northerly direction which, striking the SWIF more obliquely, caused the strike-slip movement that offset the LRF past Victoria. It was previously believed that apparently undisturbed glacial deposits lying across the fault showed it had been inactive since the last glaciation, and that displacements on the Devils Mountain fault were accommodated through other faults in the San Juan Islands. However, a 2017 study reported that the Leech River fault has experienced at least two, and possibly more, large, surface-rupturing earthquakes since the last ice age, and that earthquakes on these faults should be expected as a result of displacements on other faults such as the DDMF and SWIF.


See also

*
San Juan Valley San Juan Valley, sometimes called San Benito Valley formerly Canada de San Benito or Llano De San Juan is a valley that has its head near the Gabilan Range. Bounded on the north by the Lomerias Muertas and the Flint Hills and south and east by t ...


External links

* The Leech River and San Juan faults are very prominent in a Landsat 7 image featured on the cover o
this issue
of ''GSA Today''.
Geoscape Victoria
Poster with maps and a physiographic view of the fault..


Notes


References

*. *. *. *. *. *. *. * *. *. *. *. *. * * *. *. *. {{Authority control Juan de Fuca region Seismic faults of British Columbia