8.7: Chapter Summary
- Page ID
- 21505
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8.1 Regional Extent and Overview of the Basin and Range
- The Basin and Range of California is part of a much larger physiographic province that extends to Utah and south to Mexico.
- The region is known for its linear fault bounded ranges and basins.
- This arid region lies in the rain-shadow of the Sierra Nevada mountains to the west.
- The Basin and Range Province is a large continental rift zone.
8.2 Rifting in the Basin and Range
- Continental rifting can be caused by heat buildup beneath the continent or by changes in plate boundary configurations.
- The onset of rifting in the Basin and Range Province was approximately 17 Ma.
- Extension associated with the Basin and Range Province doubled the width of the crust!
- Development of this continental rift zone is associated with plate boundary changes to the west.
- Continental rifts are associated with distinctive patterns of sedimentation, faulting and volcanism.
8.3 Rift-Related Faulting in Eastern California
- Normal fault structures found in this region are horst-graben or half-grabens.
- Brittle normal faults are steeply dipping.
- Ongoing seismic activity associated with the Walker Lane is responsible for earthquakes as large as Mw 7.1 that are felt throughout southern California.
- Pull-apart basins form in some areas, produced by a combination of strike-slip and normal faulting.
- Metamorphic core complexes exposed in footwall rocks record evidence of earlier crustal extension.
8.4 Sedimentary Products of Rifting in the Basin and Range
- Erosion of uplifted fault blocks forms large alluvial fans along range fronts.
- Finer sediments fill the centers of rift basins.
- Sediment thicknesses can be quite large in the basins.
- Shallow seasonal lakes occupy the centers of many rift basins.
- The modern system of basin lakes are dry lakes that were once much larger, but have been reduced due to climate change and diversion by the City of Los Angeles.
- Tufa towers and strand lines record the maximum heights of ancient basin lakes.
- Desiccation of the Owens Lake in the southern Owens Valley is a major source of air pollution in the region.
8.5 Rift-Related Volcanism in Eastern California
- Crustal thinning leads to decompression melting of mantle rocks.
- Volcanic activity produced by decompression melting produces a range of volcanic products (domes, cinder cones, lava flows and pyroclastic flows) and compositions.
- Volcanism in the Basin and Range Province is young, generally co-eval with faulting.
- The Bishop Tuff was produced by a large pyroclastic eruption of the Long Valley Caldera less than 1 m.y. ago.
- The Long Valley Caldera is part of a chain of young (<1 m.y.) volcanic features that extends to Mono lake
- Continued volcanic activity in the Basin and Range Province presents a risk of seismic activity, volcanic flows, pyroclastic eruptions and gas emissions.
- Geothermal activity associated with current volcanic heating is an important source of energy.
8.6 The Oldest Rocks in the Basin and Range
- The oldest rocks in the Basin and Range Province are exposed in ranges bounded by normal faults.
- The oldest rocks date back to 1.7 billion years ago, and record the rifting of Rodinia, as well as evidence for the “snowball earth”.
- Most of the Paleozoic rocks in the region are associated with the development of a passive margin following rifting.
- The Snowball Earth refers to global glaciation during the Neoproterozoic era.
- Evidence for rifting and glaciation are found in the oldest sedimentary rocks of this region.