The Blanco Fracture Zone or Blanco Transform Fault Zone (BTFZ) is a right lateral

transform fault A transform fault or transform boundary, is a fault along a plate boundary where the motion is predominantly horizontal. It ends abruptly where it connects to another plate boundary, either another transform, a spreading ridge, or a subducti ...

zone, which runs northwest off the coast of

Oregon Oregon () is a state in the Pacific Northwest region of the Western United States. The Columbia River delineates much of Oregon's northern boundary with Washington, while the Snake River delineates much of its eastern boundary with Idah ...

in the

Pacific Northwest The Pacific Northwest (sometimes Cascadia, or simply abbreviated as PNW) is a geographic region in western North America bounded by its coastal waters of the Pacific Ocean to the west and, loosely, by the Rocky Mountains to the east. Though ...

of the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., federal district, five ma ...

, extending from the Gorda Ridge in the south to the

Juan de Fuca Ridge The Juan de Fuca Ridge is a mid-ocean spreading center and divergent plate boundary located off the coast of the Pacific Northwest region of North America. The ridge separates the Pacific Plate to the west and the Juan de Fuca Plate to the eas ...

in the north.

Morphology

The Blanco Transform Fault Zone is an approximately 350 km long zone that varies in width between 20 and 75 km. The Blanco

Fracture Zone A fracture zone is a linear feature on the ocean floor—often hundreds, even thousands of kilometers long—resulting from the action of offset mid-ocean ridge axis segments. They are a consequence of plate tectonics. Lithospheric plates on ei ...

starts about 150 km off Cape Blanco, and extends northwest to about 500 km off of

Newport Newport most commonly refers to: *Newport, Wales *Newport, Rhode Island, US Newport or New Port may also refer to: Places Asia *Newport City, Metro Manila, a Philippine district in Pasay Europe Ireland *Newport, County Mayo, a town on the ...

. It consists of a series of deep basins interrupted by transform faults. The western part of the fracture zone, from the Cascadia Depression to the Juan de Fuca Ridge, moves at 1.4 cm/a; the eastern segment, from the Cascadia Depression to the Gorda Ridge moves at 3.9 cm/a. The whole zone averages a slip rate of 2.0 cm/a. Through it,

Cascadia Channel Cascadia Channel is the most extensive deep-sea channel currently known (as of 1969) of the Pacific Ocean. It extends across Cascadia Abyssal Plain, through the Blanco Fracture Zone, and into Tufts Abyssal Plain. Notably, Cascadia Channel has t ...

passes.

Eastern segment

The principal feature of the eastern portion of the zone is the Blanco Ridge, a 150 km right lateral-moving fault that is responsible for the largest earthquakes in the region. The ridge itself varies between 3.5 and 7 km wide, and peaks between 600 and 1000 m above the seafloor. The ridge likely formed through extensive shearing and subsequent

serpentinization Serpentinization is a hydration and metamorphic transformation of ferromagnesian minerals, such as olivine and pyroxene, in mafic and ultramafic rock to produce serpentinite. Minerals formed by serpentinization include the serpentine group miner ...

from the intrusion of seawater. The Gorda Depression, a 10 km wide extensional basin, connects the eastern end of the Blanco Ridge to the Gorda Ridge. This basin is around 4400 m deep in the center, 11 km across in the NE-SW direction, and 18 km wide in the NW-SE direction.

Center

The Cascadia Depression connects the two halves of the Blanco Transform Fault Zone. The depression is elongated, around 20 km in the NE-SW direction, but only about 8 km in the NW-SE direction. Roughly 500 m of sediments line the floor of the depression, mostly

turbidite A turbidite is the geologic deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean. Sequencing Turbidites w ...

s. Channels cut through the southern end of the depression are remnants of turbidity flows originating from the

Missoula floods The Missoula floods (also known as the Spokane floods or the Bretz floods or Bretz's floods) were cataclysmic glacial lake outburst floods that swept periodically across eastern Washington and down the Columbia River Gorge at the end of the la ...

, but there are also signs of more recent, local turbidity currents. There is also evidence of active

seafloor spreading Seafloor spreading or Seafloor spread is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. History of study Earlier theories by Alfred Wegene ...

in this zone, indicating that the Cascadia Depression likely became separated from one of the spreading ridges that border the Blanco Transform Fault Zone.

Western segment

The western part of the zone is made up of a series of depressions separated by transform fault ridges. The Surveyor Depression is the shallowest and smallest of these depressions, reaching a maximum depth of 3300 m and only measuring at 5 km across. It is connected through the Parks Plateau fault to the Cascadia Depression. A small

sill Sill may refer to: * Sill (dock), a weir at the low water mark retaining water within a dock * Sill (geology), a subhorizontal sheet intrusion of molten or solidified magma * Sill (geostatistics) * Sill (river), a river in Austria * Sill plate, a ...

connects the Surveyor Depression to the East Blanco Depression, a broad basin which is composed of a series of small deeps ranging from 3600 m to 3700m. Small intermediate ridges rise up a few hundred meters to separate these local ravines. A larger sill rises up to 2900 m depth to separate the East Blanco Depression from the final major depression, the West Blanco Depression. The West Blanco Depression is shallow in the south, reaching only a depth of 3400 m, but quickly reaches depth near the Juan de Fuca Ridge, dropping down to 4800 m. This basin is at a slightly different angle than the other western depressions, likely due to interference from the Juan de Fuca Ridge. All of these depressions are likely pull-apart basins, the result of extensional stresses from the two nearby spreading centers.

Earthquakes

General

Most large events in this zone occur on the Blanco Ridge, as the motion on this fault accounts for the majority of the plate movement. Strike slip faulting occurs in this region; motion on the fault is parallel to the motion of the plate. Tectonic activity in the central part of the zone is weaker and deeper than on the Blanco Ridge. Typically this activity is consistent with

normal fault 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 tectoni ...

ing, although interference with the Cascadia Depression spreading center changes the motion on some of the faults. Shallow earthquakes consistent with strike-slip faulting occur on the western side of the zone, but their large distribution indicates that multiple faults exist. Generally these faults run along the walls of the bathymetric basins, but near the Juan de Fuca ridge they become distorted, and rotate towards the ridge axis instead.

Swarms

On January 9, 1994, a large series or

swarm Swarm behaviour, or swarming, is a collective behaviour exhibited by entities, particularly animals, of similar size which aggregate together, perhaps milling about the same spot or perhaps moving ''en masse'' or migrating in some direction. ...

of earthquakes occurred in the East Blanco Depression. Acoustic signals recorded during these events indicated that an eruption occurred in this zone. Further investigation revealed an active hydrothermal vent, the first of its kind to be discovered in a transform fault zone. In March and April 2008, a swarm of moderate earthquakes occurred both near and within the Blanco zone. The swarm began on March 30 when over 600 measurable tremors began occurring north of the zone within the Juan de Fuca Plate. On April 23, activity moved to the Blanco fault zone itself, near its junction with the Gorda Ridge. Another series of earthquakes occurred in June 2015. Spread out over a period of a few days, some reached magnitudes of 5.8. In December of 2021, a swarm of earthquakes occurred on the northwest portion of the Blanco Fracture Zone, with multiple being stronger than magnitude 5. On June 15, 2022, a swarm of nine earthquakes took place, the maximum magnitude being 5.6.

References

{{reflist

External links

Oregon State University – Earthquake Swarm
Fracture zones Geology of the Pacific Ocean