Brothers Fault Zone

Brothers Fault Zone
Major geological structures in Washington and Oregon:  BFZ – Brothers Fault Zone; EDFZ – Eugene–Denio Zone; MFZ – McLoughlin Zone; NR – Northern Nevada Rift; SM – Steens Mountain and fault; OIG – Oregon–Idaho Grabens; HB – Harney Basin; NC – Newberry Caldera; KBML – Klamath – Blue Mountain Lineament. Light green indicates extent of Columbia River Basalt Group flows; orange line indicates juncture of North American craton and accreted terranes. The Vale fault zone (not shown) is in the northeast corner of Oregon, near the Olympic–Wallowa Lineament (OWL).
Country	United States
State	Oregon
Location	Between Steens Mountain (east) and Newberry Volcano (west), approximately
- coordinates	43°48′49″N 120°36′12″W / 43.81361°N 120.60333°W / 43.81361; -120.60333Coordinates: 43°48′49″N 120°36′12″W / 43.81361°N 120.60333°W / 43.81361; -120.60333
Length	160 mi (257 km), SE–NW
Width	50 mi (80 km), SW–NE

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. These are also representative of a regional pattern of generally northwest-striking geological features ranging from Walker Lane 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, 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 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 75 km (47 mi). 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.

At its west end, just past the Newberry Volcano, the BFZ terminates at the north-striking Sisters Fault, part of the High Cascades geological province. But further south the Pliocene (<5 Ma) High Cascades volcanic trend is offset right-laterally about 10 to 20 km (6.2 to 12.4 mi) by the Eugene–Denio fault zone, and another 15 to 20 km (9.3 to 12.4 mi) by the McLoughlin zone.

At a possibly more fundamental level, the Brothers, Eugene–Denio, and McLoughlin zones, and possibly the Vale zone, all terminate near the Klamath – Blue Mountains Lineament (KBML; shortened and slightly misplaced on the map at right). The KBML is a prominent, 700 km (430 mi) long southwest-to-northeast-striking gravitational anomaly that crosses all of Oregon. It coincides with the southern boundary of the Columbia Embayment (the Olympic–Wallowa Lineament in Washington being the northern boundary), a region noticeably lacking in pre-Tertiary bedrock. Aside from aligning with the northwestern edge of several terranes that have accreted to the North American continent (such as the Klamath Mountains 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.