14.2: Bedrock Geology of the Transverse Ranges Province

A Review of the Geologic Map of this Region

The excerpt of the state geologic map shown in Figure \(\PageIndex{1}\) highlights major geologic features in the Transverse Ranges. The San Andreas fault is a prominent feature, trending northwest and forming the southern boundary of the western portion of this province, including the lozenge-shaped San Bernadino Mountains. As the San Andreas Fault trends northwestward, it crosses to the northern boundary of the San Gabriel Mountains. The southern boundary of the San Gabriel Mountains is the San Gabriel Fault. Other major fault systems in this region include a number of east-west striking faults which are truncated by the southern San Gabriel Fault on the east. These faults extend offshore to the west.

Precambrian and Mesozoic metamorphic and igneous intrusive rocks occur in the eastern region, corresponding to the San Bernadino and San Gabriel Mountains. Patches of Paleozoic sedimentary rocks occur in the north-central San Bernadino Mountains, and patches of Paleozoic sedimentary rocks are found in the western San Gabriel Mountains. The western portion of the Transverse Ranges Province, including the Channel Islands, is dominated by younger (Neogene and Paleogene) sedimentary rocks and volcanics. The southern edges of the province and patches within the western part of the province include Quaternary surficial deposits.

Proterozoic Metamorphic and Igneous Basement Rocks

The geologic map of the region helpful for understanding the distribution of bedrock geology. Elongate east-west mountainous regions generally demarcate the location of the oldest rocks, which have been brought to the surface via faulting Figure \(\PageIndex{1}\).

Figure \(\PageIndex{1}\): Geology of the Transverse Ranges Province of California. Excerpt from the Geologic Map of California with the Transverse Ranges Province highlighted. "Excerpt from the Geologic Map of California" by E. Wright, is licensed under CC BY-NC / a derivative of the original work. Access a detailed description of this image

The oldest rocks in this region are the Proterozoic metamorphic and igneous rocks exposed throughout the San Gabriel Mountains and the extreme eastern and western San Bernardino Mountains (Figure \(\PageIndex{2}\)). These rocks are complexly deformed metasedimentary rocks that include pelitic schists, calcareous gneisses, amphibolites, and igneous gneisses of various protolith compositions that range in age from 1.71 to 1.6 Ga.

Figure \(\PageIndex{2}\): Foliated Mendenhall Gneiss. Boulder of alternating black and white foliated layers is approximately 33 cm (13 in) in length. "Mendenhall Gneiss' by M.A. House, is licensed under CC BY-NC-SA.

A suite of younger plutonic rocks dated at approximately 1.3 Ga intrudes the older units. An interesting component of this phase of plutonism is an anorthosite complex. Unlike the other units, this complex of plagioclase-rich crystalline rocks (Figure \(\PageIndex{3}\) is less highly deformed and some of the original cumulate textures are preserved.

Figure \(\PageIndex{3}\): The San Gabriel Anorthosite outcrops adjacent to CA 2 in the central San Gabriel Mountains. The white rocks of the anorthosite are distinctive, as are associated mafic dikes which are seen in the left portion of this image. "Anorthosite Road Cut" by M.A. House, is licensed under CC BY-NC-SA.

Late Proterozoic-Early Paleozoic rocks are also found in the San Bernardino Mountains. These rocks include meta-sedimentary rocks that are thought to represent ancient passive margin facies that can be correlated to those in the Death Valley and White Mountains regions (see Basin and Range).

Box \(\PageIndex{1}\): More About Anorthosites

Anorthosites are interesting and relatively unusual plutonic rocks that are more than 90% Ca-plagioclase (Inset Figure \(\PageIndex{1}\)). They also have some Fe- or Mg-bearing minerals such as pyroxenes and olivines, and Fe/Ti oxides. Anorthosites make up much of the highlands of our moon, but they are relatively rare on Earth.

Inset Figure \(\PageIndex{1}\): A sample of San Gabriel Anorthosite from Soledad Canyon, Los Angeles County. It is composed almost entirely of Ca-plagioclase. "San Gabriel Anorthosite (Mesoproterozoic, ~1.2 Ga; Soledad Canyon, Los Angeles County, California, USA) 1" by James St. John via Flickr is licensed under CC BY.

The formation of anorthosites has been enigmatic for petrologists for several reasons. The main problem is that in many areas where these large igneous bodies have been described, it’s difficult to explain the massive accumulations of concentrated Ca-rich plagioclase crystals in these massifs. Fractional crystallization is a mechanism that has been proposed for the formation of these plutons. In this process, low-density plagioclase crystals are buoyantly separated from denser Fe-bearing minerals within a cooling magma chamber (Inset Figure \(\PageIndex{2}\)). As shown in this image, as denser materials sink within the chamber, cumulate layers form within the magma chamber as it cools. These layers concentrate denser minerals within distinct layers at the base, and less dense minerals like Ca-Plagioclase, concentrate at upper levels in the chamber. These denser layers are effectively separated from the remaining magma, which evolves to a more felsic composition than that with which it began.

Inset Figure \(\PageIndex{2}\): Formation of a cumulate during partial crystallization. "Evolved Magma" by D. Perkins, is licensed under CC BY-NC-SA.

When such layers are well-developed, these geologic structures are called layered mafic intrusions. The concentrated mafic and ultramafic layers in these structures may include economically valuable concentrations of platinum, palladium, nickel, titanium, and chromium. These features have been exploited by mining operations in the Bushveld Anorthosite Complex in South Africa and the Stillwater Complex in Montana, USA.

Geologic evidence for fractional crystallization and density stratification is preserved in the San Gabriel Mountains anorthosite complex, although there are no economically distinct layers like those in larger layered intrusions. Instead, the anorthosite is associated with low-quartz rock types such as syenite, and mafic rocks such as gabbros.

Despite this seemingly simple explanation for the formation of many of these plutonic complexes, clear evidence for fractional crystallization is sometimes missing, leaving petrologists stumped as to their origin. This is because our understanding of the formation of such Ca-Plagioclase rich rocks requires the accumulation of very mafic components as well, and they are sometimes absent. Further, many of the massif-style anorthosites like that in the San Gabriel Mountains range in age from 1.9-1.2 Ga. As a result, the scientific community continues to debate the composition of the original magma that leads to the formation of these structures. One possibility is that conditions of the crust and mantle at that time of their formation were different in such a way that anorthositic complexes could form via fractional crystallization and allow for the rapid and complete removal of any cumulate layers that might have formed. An interesting petrologic problem for sure!

The Mesozoic Pelona Schist and the Vincent Thrust

A prominent basement unit within the Transverse Ranges is the Pelona Schist. This well-foliated unit is mostly found as a gray schist, but contains distinctive lenses of greenschist facies metabasalts, meta-sandstones and calcareous units. This unit, which may represent a metamorphosed turbidite sequence, is thought to underlie the entire Transverse Ranges region since it appears as clasts within younger sedimentary units. Similar rocks have been found in eastern California and into Arizona, where this unit is variably called the Rand or Orocopia schist, and to the north in the Coast ranges, where it is thought to be equivalent to the Franciscan complex (see Coast Ranges).

Mesozoic Plutonism

Like much of California, Mesozoic plutonism associated with arc volcanism impacted the Transverse Ranges. Within the San Gabriel and San Bernardino Mountains, granitic and dioritic plutonic bodies were emplaced in three stages at 207-241 Ma, at 165-150 Ma and at 85-120 Ma. The compositions of these units are comparable to those in the Sierra Nevada Range (see Sierra Nevada) and indicate that the ancient Mesozoic arc extended to this region at that time.

References

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