The Brothers Fault Zone (BFZ) is the most notable of a set of northwest-trending fault zones including the Eugene–Denio, McLoughlin, and Vale zones that dominate the geological structure of most 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 Idaho. T ...

. These are also representative of a regional pattern of generally northwest-striking geological features ranging from

Walker Lane The Walker Lane is a geologic trough roughly aligned with the California/Nevada border southward to where Death Valley intersects the Garlock Fault, a major left lateral, or sinistral, strike-slip fault. The north-northwest end of the Walker L ...

on the California–Nevada border to the Olympic–Wallowa Lineament in Washington; these are generally associated with the regional extension and faulting of the

Basin and Range Province The Basin and Range Province is a vast physiographic region covering much of the inland Western United States and northwestern Mexico. It is defined by unique basin and range topography, characterized by abrupt changes in elevation, alternating ...

, of which the BFZ is considered the northern boundary. The relationships with other features is complex. At the BFZ's eastern end, near the

Steens Mountain Steens Mountain is in the southeastern part of the U.S. state of Oregon, and is a large fault-block mountain. Located in Harney County, it stretches some north to south, and rises from the west side the Alvord Desert at elevation of about t ...

fault, the zone of surface faulting turns slightly to the south-southeast (possibly due to rotation of Nevada), then follows the Northern Nevada Rift to form the Oregon–Nevada lineament, with a total length of over . Lavas associated with the Nevada Rift have been dated to 16.3 Ma (millions of years ago), close to the inception of basin-and-range faulting, but there is a suspicion that the rifting developed on an older strike-slip fault, possibly connected with the

East Pacific Rise The East Pacific Rise is a mid-ocean rise (termed an oceanic rise and not a mid-ocean ridge due to its higher rate of spreading that results in less elevation increase and more regular terrain), a divergent tectonic plate boundary located alon ...

. At its west end, just past the

Newberry Volcano Newberry Volcano is a large active shield-shaped stratovolcano located about south of Bend, Oregon, United States, east of the major crest of the Cascade Range, within the Newberry National Volcanic Monument. Its highest point is Paulina Peak ...

, the BFZ terminates at the north-striking Sisters Fault, part of the High Cascades geological province. But further south the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Tertiary Tertiary ( ) is a widely used but obsolete term for the geologic period from 66 million to 2.6 million years ago. The period began with the demise of the non-avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start ...

bedrock. Aside from aligning with the northwestern edge of several

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 ...

that have accreted to the North American continent (such as the

Klamath Mountains The Klamath Mountains are a rugged and lightly populated mountain range in northwestern California and southwestern Oregon in the western United States. As a mountain system within both the greater Pacific Coast Ranges and the California Coast ...

on the southern coast of Oregon, and Blue Mountains Province just north of the BFZ), the KBML has no apparent surface manifestation; it is believed to reflect a deeper structure, possible a pre-Tertiary continental margin. The BFZ is the northern edge of the Basin and Range Province, a region of northwest directed extension, wherefore the BFZ also accommodates the offset with the Blue Mountains Province. It does this not by through-going strike-slip faulting (such as with the

San Andreas fault The San Andreas Fault is a continental transform fault that extends roughly through California. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizonta ...

to the south) but by a series of hundreds of ''en echelon'' normal faults somewhat crosswise to the zone itself. Such faulting is believed to be the surface manifestation of a deeper shear zone. Newberry-Yellowstone tracks

The Brothers Fault Zone is also the locus of the High Lava Plains (HLP) volcanism of central and southeastern Oregon. HLP volcanism is notable for showing an age progression from 16 Ma at its eastern end (near the Oregon–Nevada–Idaho corner) to the active Newberry Volcano at its western end. This age progression mirrors the very similar progression along the track of the

Yellowstone Hotspot The Yellowstone hotspot is a volcanic hotspot in the United States responsible for large scale volcanism in Idaho, Montana, Nevada, Oregon, and Wyoming, formed as the North American tectonic plate moved over it. It formed the eastern Snake Riv ...

, which appears to have a common origin. Also closely related in space and time is the eruption of the 16.6 Ma

Steens Basalts Steens may refer to: *Steens, Mississippi, an unincorporated community *Steens Mountain, a fault-block mountain in southeast Oregon state See also *Ron Steens Ronaldus "Ron" Franciscus Johannes Steens (born 18 June 1952) is a former field hock ...

, the initial and most voluminous phase of the

Columbia River Basalt Group The Columbia River Basalt Group is the youngest, smallest and one of the best-preserved continental flood basalt province on Earth, covering over mainly eastern Oregon and Washington, western Idaho, and part of northern Nevada. The basalt grou ...

lava flows that blanket eastern Oregon and the entire southeastern quarter of Washington. These also show a similar age progression to the north; there is a great deal of debate on how all of these are related, and how they initiated. The track of the Yellowstone Hotspot is neatly explained by motion of the North American

craton A craton (, , or ; from grc-gre, κράτος "strength") is an old and stable part of the continental lithosphere, which consists of Earth's two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging an ...

over a plume ( hotspot) of material rising from the mantle. But this explanation fails for the so-called "Newberry hotspot" track, which lies oblique to the motion of the craton. One suggestion is that the large blob of molten rock at the head of the plume was sheared off by the advancing edge of the craton, and remained entrained in front of the craton to melt the lithosphere in a broad region, while the tail of the plume was overrun to make the Yellowstone Hotspot track. Age progressive volcanism along the Brothers Fault Zone (as well as the Steens Fault and North Nevada Rift) results from the expanding lithospheric melt opportunistically exploiting existing crustal faults.; ; . See als
www.MantlePlumes.org
In summary, the Brothers Fault Zone, and the related Eugene–Denio, McLoughlin, and Vale zones, appear to be deep-seated structures in accreted terranes bounded by the Klamath—Blue Mountains Lineament, which has been reactivated by Basin and Range extension, and exploited by mid-Miocene volcanism associated with the Yellowstone Hotspot.

Notes

References

* * * * * * * * * {{Faults Geology of Oregon Seismic faults of Oregon