15.2: Tectonics and Geologic History of the Peninsular Ranges Province

Given a Few 100 Million Years, What Could Possibly Happen?

In some parts of the world, the rocks of today reveal a history of continuous change occurring in an area as plate tectonic forces slowly move that location over Earth’s surface. That is not the geologic history of California (see A Brief Geologic History of California) nor of the Peninsular Ranges. The rocks and landforms seen in the Peninsular Ranges of today have a long history with three distinct phases. First, the region was a passive margin far removed from a plate boundary, then part of a subduction complex at a convergent plate boundary, where terranes were accreted from areas to the west. Finally it became a transform plate boundary, which also brought in terranes, but now from locations to the south. Each phase overprinted the earlier ones, and was then also overprinted by ongoing erosion, deposition, and changes in sea level.

Before the Late Paleozoic/Mesozoic - A Passive Margin

Before and during much of the Paleozoic, the west coast of North America and what would become California was a passive margin or an area offshore of a passive margin. The supercontinent prior to Pangea, Rodinia, had started to break up during the Precambrian, and as part of that break-up, a Proto-Pacific Oceanic basin formed as the Laurentia plate (most of today’s North America) and Antarctica split apart (Figure \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): Possible reconstruction of the supercontinent of Rodinia starting to break apart at 1.1 Ga or 1100 Ma. The 1.1 Ga belts are fold mountain belts created by the continental collisions that built Rodinia. Laurentia will eventually become most of what is now North America. East Antarctica is starting to separate from it and the proto-Pacific Ocean basin is starting to form. “Rodinia Breakup” by Steven Newton, a derivative of "Rodinia Reconstruction", is licensed under CC BY-NC 4.0. Access a detailed description of this figure.

A modern example of a passive margin is the current east coast of the United States, with the nearest plate boundary, the Mid-Atlantic ridge, half an ocean away. Along such a passive margin, the dominant geologic processes are erosion and deposition in coastal, nearshore, and continuingly deeper water environments. Each depositional environment leaves its signature in the rock and these signatures are found in the Paleozoic rock of southern California.

While evidence of these depositional environments remains in the rocks in other parts of southern California, little evidence remains in the Peninsular Ranges province. Most of the evidence for this passive margin is found northeast of the Peninsular Ranges in Death Valley in the Mojave province (see Geologic Overview and Evolution of the Mojave and Colorado Desert Provinces) and in Nevada. The Paleozoic rocks of these areas are interpreted as a nearshore, carbonate platform with water depths increasing to the west.

Remnants of the presumed equivalent of these carbonate rocks are found in the eastern and northern parts of the Peninsular Ranges as metamorphic rock. They are presumed equivalent, because the intrusion of the batholithic rocks from below metamorphosed and essentially destroyed the fossils in them that could have been used to definitively correlate them. What remains are roof pendants, septa, and inclusions of the now-metamorphosed rock. Roof pendants are rock that has not yet eroded off the top of the batholith. Septa occur when the plutons making up the batholith while rising sometimes leave part of the metamorphosed rock trapped between; septa therefore separate different plutons or different parts of the same pluton. Inclusions are pieces of rock that were torn away as the pluton was rising, fell into the magma, but didn’t melt.

Most of these rocks were subsequently eroded away as the Peninsular Ranges uplifted and the batholith was exposed. However, some of these rocks can still be found today in the San Jacinto and the Santa Rosa Mountains, and near the city of Riverside. The marble quarries in and near the Jurupa Mountains near Riverside (Figure \(\PageIndex{2}\)) are a major source for the carbonate rock used to make cement and concrete throughout southern California since the 1890s.

Figure \(\PageIndex{2}\): The white rock of Jensen’s Quarry at the base of the hills in the middle distance is part of a roof pendant and is also an abandoned marble quarry in the Jurupa Mountains, northwest of Riverside, California. The San Gabriel Mountains of the Transverse Ranges are in the background. “Jensen’s Quarry in the Distance” by Debra Brooks is a derivative of "Riverside Panorama" by Staticfish via Wikimedia Commoms and is licensed under CC BY-NC 4.0.

Mesozoic and Early Cenozoic - Subduction, Batholiths, and Volcanism

Early in the Mesozoic, the west coast of North America began to change dramatically as the existing passive margin became an active margin. A subduction zone formed as the North American plate began to override the oceanic, Farallon plate to the west; this correlates with the break-up of Pangea and the eventual opening of the Proto-Atlantic Ocean. From Canada to Mexico a convergent plate boundary formed, with subduction (see Tectonic Plates, Plate Motions, and Plate Boundaries) and uplift occurring near the coast, batholiths forming at depth, and eventually volcanic island chains forming along the eastern shore of the Pacific Ocean. A modern example of California-then would be the Indonesian archipelago of today (Figure \(\PageIndex{3}\)). While the volcanoes have long since eroded away completely, the magma chambers that fed the volcanoes have become batholiths (see The Sierra Nevada Batholith).

Figure \(\PageIndex{3}\): Satellite image of the Indonesian Archipelago. Astronaut Photo ISS056-E-098152 by NASA is in the public domain.

Subduction began during the Permian-Triassic and continues through to today in northernmost California, whereas it ended in southern California approximately 30 to 25 million years ago. Subduction can be relatively simple and uniform, with oceanic lithosphere being overridden by continental lithosphere, or it can be quite complex, which is what happened in the Peninsular Ranges province. One major complication is that the subducting oceanic lithosphere is not always uniform and may include oceanic plateaus, island arcs, or even small microplates of continental lithosphere. When such irregularities are part of the subducting plate either they may be accreted to the overriding plate (see Assembling a Continent), or they may change the angle of the subducting plate, which influences whether or not molten rock forms at depth. Also, if molten rock is forming at depth, it may melt at a different depth than it did previously. If the depth at which rock melts changes, the chemistry of the magma changes because it moves upward through either a greater or lesser amount of upper mantle and/or crustal rock as it rises (see The Cascadia Subduction Zone and the Cascade Continental Volcanic Arc).

During the mid-Jurassic, approximately 170 to 160 million years ago, the oldest rocks of the batholith formed under an offshore island arc. Batholiths usually are an amalgamation of many smaller intrusions, or plutons, that intrude near each other or into each other, forming the larger structure. Later in the Cretaceous, approximately 120 million years ago, part of an island arc system accreted to the North American plate forming the Cuyamaca-Laguna Mountain shear zone. It is one of the features, along with age, chemistry, mineralogy, and geophysical properties that define the separation between the eastern and western parts of today’s batholith. Subduction continued through this collision and magma production was concentrated in what is now the western areas of today’s batholith.

Query \(\PageIndex{1}\)

The earliest deposits of sedimentary rock in the Peninsular Ranges province are dated by marine fossils to be Jurassic. This is a time of uplift and therefore erosion, but little sediment accumulated in the province except along the western seacoast, and then unconformably on metasedimentary rock of marine origin. The inference is that the pre-existing sedimentary rock was metamorphosed by the intrusion of the plutons below and that the continuing uplift made for steeper slopes and fewer onshore depressions in which sediment could accumulate, so much, if not most of the sediment being eroded was washed out to sea. Also, much of the sediment that was deposited is of volcanic origin with deposition more extensive to the north and less extensive to the south. The protolith for much of the metamorphic rock found in the interior of the mountains is also inferred to be sedimentary rock of this age.

At approximately 105 million years ago, the angle of the subducting plate became shallower, and magma generation moved east farther from the location of the trench, concentrating under what is now the eastern part of today’s batholith. The geological conditions changed and therefore magma generation changed, becoming more voluminous, chemically homogenous, and felsic. The resulting plutons and their rise towards the surface helped uplift the eastern parts of the batholith, which is why today’s mountains are taller in the east than in the west. The magmatic stoping through preexisting plutons helped create the dike complexes that lead to both the development of gold and other metal deposits in what would become the mining districts of northern San Diego and southern Riverside counties.

Around 85 million years ago, subduction shallowed even further, and magma generation moved even further east and out of the Peninsular Ranges province altogether. With the subsequent uplift of the Peninsular Ranges, the volcanoes that formed in those island arcs and onshore areas have long since died and been eroded away. What remains of them in the rock record is some of their lava flows preserved within the stratigraphy of the western Peninsular Ranges, their eroded clasts in the sedimentary rocks of the coastal plain and offshore, and the plutons of the Peninsular Ranges batholith that were the magma scources for their lava flows.

Sediment deposition during the Cretaceous varies from north to south. To the north, sediment deposition is dominated by a sequence of forearc basin sediments unconformably overlying volcanic rock, while to the south, there is also conglomeritic sediment deposited unconformably on both volcanic rock and the exposed rocks of the batholith. The movement of magma generation to the east, meant that while erosion continued, uplift in this area slowed, as cooling and contraction of the plutons accelerated because of the lack of new magma rising from below. The Cretaceous is also a time of globally high sea level and the rise of sea level meant marine sediment was deposited on the older non-marine sedimentary rock.

Query \(\PageIndex{2}\)

Query \(\PageIndex{3}\)

Query \(\PageIndex{4}\)

Late Cenozoic - From Subduction to Transform Faulting

Subduction of the Farallon plate under the North American plate continued through the Mesozoic and into the Cenozoic. For the earliest Cenozoic, one of the largest differences that occurred is the absence of igneous activity. No new igneous intrusions, and no new volcanic activity but lots of erosion. Especially to the south in San Diego county, erosion there continued all the way down to the plutonic rocks of the batholith. This continued through the early and middle Paleogene with relatively few sedimentary rocks deposited during the Paleocene, but sedimentation increasing during the Eocene. This is also a time of globally high sea level, so while the older rock is non-marine, most of the later rock is marine.

Everything changed dramatically during the late Paleogene (Figure \(\PageIndex{4}\) – 30 Ma). In southern California, somewhere between 30-25 million years ago part of the mid-ocean ridge system in the Pacific Ocean intersected with the subduction zone and two triple-junctions formed. This would start the evolution of the plate boundary into the transform plate boundary of today.

Figure \(\PageIndex{4}\): A sequence of maps showing the evolution of the plate boundary in California over the last 30 million years from a subduction zone to a transform plate boundary. This series of maps are all oriented with east at the top rather than the convention of north at the top. "San Andreas Fault Sequential Diagrams" by W.P. Irwin in USGS Professional Paper 1515 is in the public domain. Access a detailed description of this figure.

Prior to 30 million years ago, the entire coast of California and Mexico was a subduction zone, with the Farallon plate subducting beneath North America. The Pacific plate was separated from the Farallon plate by a mid-ocean ridge or spreading center. Around 30 million years ago, a corner of the Pacific plate began to approach the subduction zone. As the Pacific plate intersected with the subduction zone, the plate boundary reconfigured as two triple junctions separated by a transform fault (Figure \(\PageIndex{4}\) – 20 Ma).This new transform fault divided the Farallon plate into two plates. The Juan de Fuca plate (the northern remnant of the Farallon plate) continued subducting under California north of the Mendocino triple junction, while the Cocos plate (the southern remnant of the Farallon plate) continued subducting under Mexico. An offshore transform fault (the proto-San Andreas fault) began to form between the Mendocino and the Rivera triple junctions. The Baja California Peninsula did not yet exist and what would become the Peninsular Ranges of today were on the North American plate.

Extension began and greatly affected the areas both east and west of the Peninsular Ranges about 20 million years ago. To the west it would form the offshore, southern California Borderlands and to the east it would form the Salton Trough. The extension in the offshore borderlands is dominated by a series of east-dipping normal faults and the upper corners of the fault blocks form offshore islands with deep basins between (Figure \(\PageIndex{5}\) and also Rift-Related Faulting in Eastern California). The islands form a sediment source area to the west, and sedimentation on the western side of the ranges is sourced from both the islands to the west and the mountains to the east.

Figure \(\PageIndex{5}\): Formation of tilted fault blocks. "Tilted blocks" by Aturn4000 via Wikimedia Commons is licensed under CC BY-SA 3.0. Access a detailed description of this figure.

Complications have increased over the last 20 million years as the plate boundary for California and Mexico transitioned from a pattern of subduction and transform faulting to today’s configuration of subduction, transform faulting, and divergence. Video 15.2.1 shows the movement that occurred as faulted blocks moved north over time.

Video \(\PageIndex{1}\): Development of the Plate Boundary of southern California and northern Mexico over the last 20 Ma

It may be helpful to preview the rest of the discussion in the text as a short video. This video has no narration, but is described, step-by-step, in the text. Details on the rotation of the Transverse Ranges to the north of the Peninsular Ranges are covered in the chapter on the Transverse Ranges (see Tectonic and Structural Evolution of the Transverse Ranges).

By 10 million years ago (Figure \(\PageIndex{4}\) – 10 Ma), the transform fault that forms the plate boundary had grown longer, as the triple junctions further separated. The Peninsular Ranges were still on the North American plate.

Moving forward to about 5 million years ago (Figure \(\PageIndex{4}\) – 5 Ma), the active trace of the transform fault in the north jumped to the shoreline and included the San Gregorio-Hosgri fault system of central California, while the active trace of the transform fault system to the south was still offshore. However, the region that would become the Gulf of California is starting to slowly extend. The Peninsular Ranges were not yet part of a peninsula at this time but would be soon; estimates of the opening of the Gulf of California range from approximately 7 to 4 million years ago.

During the last 5 million years (Figure \(\PageIndex{4}\) - present), the Gulf of California has continued to open, as the extension that was occurring in the proto-Gulf of California shifts to transform faults connecting small spreading centers, and the plate boundary has shifted or jumped to the east and has become a divergent plate boundary. With the opening of the Gulf of California and the start of a divergent plate boundary, the Peninsular Ranges are now on the Pacific plate. Approximately 2 million years ago, the active transform boundary jumped to its current location as the San Andreas fault zone.

The plate boundary of today is convergent in the north, a transform fault for most of the rest of California, and divergent in the south-easternmost part of California. Volcanism has resumed in the divergent Salton Trough to the east and tectonic activity continues as the Gulf of California opens. In the northern coastal plain of the Peninsular Ranges, broad, shallow anticlines and synclines have formed because of the resulting transpression. The earthquake activity along with global sea level changes caused by the Pleistocene glaciations created multiple episodes of marine terracing on the coastal plain.

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