15.6: Chapter Summary

15.1 Geomorphology of the Peninsular Ranges

• The Peninsular Ranges extend from southern California into Mexico and in Mexico form the spine of the Baja California Peninsula.
• The Peninsular Ranges are the remains of part of the batholith that was beneath the volcanic arc that dominated the geology of western North America during the Mesozoic.
• The uplift and subsequent erosion of the Mesozoic volcanic arc exposed the batholith and created monadnocks, some of which are large enough to be small mountain ranges.
• The Peninsular Ranges form a drainage divide and create the rain shadow effect for the Colorado Desert province.
• The eastern two of the three major fault zones within the Peninsular Ranges are considered part of the larger San Andreas fault system, even though the San Andreas fault zone is outside the Peninsular Ranges province.
• All of the major fault zones within the Peninsular Ranges are right-lateral strike-slip fault zones and align with the orientation of the San Andreas fault zone.
• The coastal plain is an erosional coast dominated by marine terraces.
• Estuaries and lagoons are common along the coast, as well as mass wasting.

15.2 Tectonics and Geologic History of the Peninsular Ranges

• Before or during the early Paleozoic what will become California is offshore of a passive margin.
• During the Permian-Triassic subduction begins offshore of the passive margin and an island arc forms as a batholith is created below.
• During the early Cretaceous another island arc accretes to form the Cuyamaca-Laguna Mountain shear zone, part of the boundary between the western and eastern batholiths of today. Pluton formation and volcanism are in what is the western part of today’s batholith.
• Near the end of the early Cretaceous, at about 105 million years ago, the angle of subduction became shallower and pluton formation and volcanism moves further from the trench and from what is now the western part of today’s batholith to the eastern part.
• During the late Cretaceous, at about 85 million years ago, the angle of subduction changes again and becomes even more shallow; pluton formation and volcanism move even further from the trench and completely out of the Peninsular Ranges province. Subduction continues offshore, but in the Peninsular Ranges there is no volcanism, uplift slows and erosion becomes the dominant surface process.
• During the Paleogene, 30-25 million years ago, the plate boundary changes from subduction to transform as part of the mid-ocean ridge system reaches the subduction zone.
• During the early Neogene, 20 million years ago, extension begins. East of the Peninsular Ranges the Salton Trough develops; west of the Peninsular ranges in the California borderlands, a valley and ridge topography forms. The coastal areas now have sediment sources from both the Peninsular Ranges to the east and the borderlands to the west.
• During the late Neogene, about 7 to 5 million years ago, the extension to the east in the Salton Trough develops into a divergent plate boundary and the Gulf of California opens. The plate boundary is transferred and the Peninsular Ranges are now part of the Pacific plate.
• During the early Pleistocene, approximately 2 million years ago, the transform part of the plate boundary jumps to the San Andreas fault zone.

15.3 Rocks of the Peninsular Ranges

• Most of the rock in the Peninsular Ranges formed as plutonic igneous rock in a large Mesozoic batholith. The igneous rock of the Peninsular Ranges batholith includes all three major types of plutonic igneous rock – felsic (granitic), intermediate (dioritic), and mafic (gabbroic); with the most common rock being tonalite, a felsic rock related to granite.
• The oldest rock in the Peninsular Ranges province is the metamorphic rock of the roof pendants above the batholith.
• The Peninsular Ranges batholith is divided into a western and an eastern part based upon changes in chemistry, mineralogy, and geophysical properties of the rock. The rocks of the western batholith are older and more varied than are the rocks of the eastern batholith. The rocks in the eastern batholith are also more likely to be felsic.
• The rocks in the eastern Peninsular Ranges batholith are similar to the rocks of the Sierra Nevada batholith in age, chemistry, and mineralogy.
• The Peninsular Ranges are famous for pegmatitic minerals and gemstones.
• The volcanoes of the Peninsular Ranges eroded away long ago and the volcanic rocks remaining are mostly basalts and andesites on the western slopes of the mountains and clasts in sedimentary rock of the coastal plain.
• Most of the sedimentary rock of the Peninsular Ranges is on the coastal plain. Before the Neogene, sediment sources were to the east in the mountains. During the Miocene, sediment also came from the north and the west.
• The distinctive Poway Group cobbles were used to help reconstruct the geography of the Paleogene in southern California.
• The sedimentary rock of the coastal plain records a long history of repeating cycles of the rise and fall of sea level.
• Dinosaur fossils have been found in the Cretaceous sedimentary rocks of the coastal plain.

15.4 Geologic Hazards of the Peninsular Ranges

• The major geologic hazard faced by the residents of the Peninsular Ranges province is a large earthquake on one the major fault zones within the province or on the San Andreas fault zone, just outside the province.
• The three major fault zones within the Peninsular Ranges province are the San Jacinto, the Elsinore, and the Newport-Inglewood-Rose Canyon fault zones. All of these fault zones have a history of moderate to large earthquakes in historic times.
• The Peninsular Ranges province has a history of major floods. After the flood of 1938, dams were built to help with both flood control and water storage.
• Saltwater intrusion of the coastal aquifers is a major challenge to having potable groundwater.
• Injection wells and fluid barriers are the most common method used to try and combat saltwater intrusion.
• Mass wasting is a common problem on the sea cliffs and bluffs of the marine terraces along the coast.
• Climate change will exacerbate the current geologic hazards in the Peninsular Ranges and probably make them occur more frequently as it also floods coastal areas.

15.5 Natural Resources of the Peninsular Ranges

• Potable water is a major resource that is lacking in the Peninsular Ranges province.
• Most of the potable water in the Peninsular Ranges is purchased and imported from either the Colorado River or the rivers of northern California and must be transported across the San Andreas fault zone.
• Efforts to help with the water challenges in the Peninsular Ranges include conservation, recycling, and desalination.
• While gold mining occurred and was successful in the Peninsular Ranges batholith in the past; the gold deposits were never as plentiful and as rich as in the Sierra Nevada batholith.
• Gemstone mining still occurs in the Peninsular Ranges.
• Most mining for precious metals ended in the Peninsular Ranges after WWII.
• Oil and gas production occurs in the northernmost part of the Peninsular Ranges province in the southern part of the Los Angeles basin.
• Most of what has been mined in the Peninsular Ranges and is still mined today are building commodities such as sand and gravel, limestone and marble, building stone, clay, and specialty sand.
• Previously both coal and salt have been mined in the Peninsular Ranges.