14.4: Tectonic and Structural Evolution of the Transverse Ranges

A Changing Plate Boundary

The Transverse Ranges Province has evolved over time from an active Mesozoic subduction zone to Early-Middle Cenozoic transtension produced by simultaneous extension, shearing and rotation, and finally to a modern transpressional (compression and shear) setting beginning approximately 5 Ma. These dramatic changes can be linked to specific stages of the development of the San Andreas system and its fault system precursors beginning at approximately 20 Ma (Figure \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): Sequential diagrams showing plate-tectonic evolution of the San Andreas transform fault system. "Formation of the San Andreas" by USGS/W.P. Irwin, is in the public domain. Access a detailed description of this image.

Geologic relationships described in the previous section indicate that the future Transverse Ranges region was offshore or adjacent to an active subduction zone and volcanic arc through the Early Cenozoic. This convergent plate boundary separated the Farallon and North American Plates and, as shown in Figure \(\PageIndex{1}\), persisted until approximately 30 Ma. However, as the Pacific Plate approached this plate boundary, a triple junction developed that launched the development of the ancestor to our modern transform boundary. As the single triple junction evolved into the current two separate triple junctions (the Mendocino and Rivera Triple Junctions) that migrated away from one another, a transform boundary evolved in between them as shown in Figure \(\PageIndex{1}\) in the panel labeled "20 Ma".

This initial boundary was not the San Andreas Fault System that we think of today. Rather, it was located farther to the west and offset was accommodated by different faults. The diagrams in Figure \(\PageIndex{1}\) illustrate the change in position of the transform boundary at 10 Ma, 5 Ma and today. Over this time, the boundary migrated inland. For a broader review of these changes, see A Brief Geologic History of California.

Rotation of the Western Transverse Ranges

At about 20 Ma, large portions of the Transverse Ranges Province were farther south than today. Over time, they migrated northward and began to rotate clockwise relative to North America and one another as shown in Figure \(\PageIndex{2}\). In this image, the Mendocino Triple Junction is north of the future location of San Francisco, and separates a transform boundary, a convergent boundary to the north, and a developing transform boundary to the southeast. At the same time, the Rivera Triple Junction is located offshore (west) of what will become Santa Barbara, and both are almost directly west of the future San Diego.

Figure \(\PageIndex{2}\): Sequential diagrams showing the rotation of the Western Transverse Ranges. Time panels are shown for 20, 12, and 4 Ma, and today. Labels are Santa Barbara (SB), San Francisco (SF), and San Diego (SD). The white arrow aims northwest, indicating the relative motion of the Pacific Plate relative to the North American Plate. The red lines are the plate boundaries at different times. Dashed lines indicate future crustal blocks that will be rotated via plate interactions. "Rotation of the Western Transverse Ranges" by M. A. House, a derivative of the original, is in the public domain. Access a detailed description of this image.

This rotation was accompanied by crustal thinning and renewed basin subsidence as indicated in the portions of Figure \(\PageIndex{2}\) labeled 12 and 4 Ma. Areas that were particularly impacted in the western Transverse Ranges Province include the Santa Monica Mountains, the Santa Ynez Range, and the northern Channel Islands (Santa Rosa, Santa Cruz, Santa Barbara, San Miguel, and Anacapa Island). Paleomagnetic data and stratigraphic studies suggest that the sedimentary units preserved in these regions were deposited offshore of modern-day San Diego and were carried northward into their current position. They have also been significantly reoriented through more than 75 degrees of clockwise rotation beginning in Miocene times.

Along with clockwise rotation, local extension produced deep pull-apart basins that evolved into the major depocenters (such as the Ventura Basin) across the Western Transverse Ranges (see the chapter on the Basin and Range Province for a discussion of pull-apart basins). Normal faulting also produced non-marine basins (such as the Soledad Basin) found in the western San Gabriel Mountains, and the interaction of these normal faults with the evolving transform boundary produced additional transtensional pull-apart basins such as the Ridge Basin, which provide important information about the history of faulting in this region.

The influx of Miocene volcanic units discussed in the section on basin stratigraphy accompanied this rotation, presumably as volcanic eruptions produced magma that was created via decompression melting in response to crustal thinning (see the Chapter on the Basin and Range).

As rotation progressed and the developing San Andreas Fault System continued to interact with the North American Plate, the future plate boundary adjacent to the Transverse Ranges region was no longer parallel to the relative plate motion between the Pacific and North American plates. This so-called “Big Bend” in the San Andreas system caused the transtensional setting to evolve into one of transpression across the region beginning at approximately 5 Ma.

This can be seen in the panel labeled 4 Ma in Figure \(\PageIndex{2}\), in which the elongated Santa Barbara Block is oriented almost east-west, and the triangular basin that had opened south of it is larger. The elongate crustal block to the south has continued to migrate northward and the area that will become San Diego is now separated from the rest of North America by a transform boundary that extends northward from the proto-Gulf of California.

Transpression

Transpression results when the forces of compression and shear are distributed across this zone due to the geometry of the plate boundary here (Figure \(\PageIndex{3}\)).

Figure \(\PageIndex{3}\): Diagram of a restraining bend geometry similar to that of the Transverse Ranges Province. A northwest striking right-lateral strike-slip is offset from a similar fault by a zone of reverse faulting such that there is a left step in the plate boundary from south to north. The restraining bend is bounded by two north-dipping reverse faults that cause mountains to form in this region. "Restraining Bend" by GeoAsh is licensed under CC BY-SA 4.0. Access a detailed description of this image.

Consider that we typically discuss the San Andreas fault as a “right-lateral strike-slip fault”. This is true along much of its length where it is oriented parallel to the overall relative plate motion between the North American and Pacific Plates that it separates. However, the situation is somewhat more complex in this region where the San Andreas fault system strikes roughly northwest-southeast. Here, the plate motion has a component that is perpendicular to the orientation of the boundary, creating a situation of compression along with shear that is reflected in the geologic structures that form. While the San Andreas Fault System itself is certainly dominated by right-lateral shear motion, many of the faults across the Transverse Ranges Province are reverse faults or even left-lateral!

Transpression has dramatically modified this region over a relatively brief period and continues to be active today. Across the Transverse Ranges Province today, the Earth’s crust is compressed and shortens by about 10–15 mm/yr (0.4-0.6 in/yr). Figure \(\PageIndex{4}\) shows GPS (Global Positioning System) vectors for sites across this region. The length of the vectors reflects the rate of movement; the angle reflects the direction. The figure shows that throughout the Transverse Ranges Province, motion is to the northwest, but regions east of the San Andreas Fault System have little or no motion. In the area where the San Andreas Fault System bends eastward through the Transverse Ranges Province, vectors are no longer parallel to the fault boundary, which is the cause of transpression here.

Figure \(\PageIndex{4}\): Current GPS (global positioning system) permanent station network in Southern California, showing movement of locations throughout the region because of the large-scale movement of the Pacific Plate relative to the North American Plate. Vectors generally indicate the region is moving to the north west. Vector lengths indicate a range of rates that are approximately 40 mm/yr. To the east of the San Andreas fault, the vectors are much shorter. "SCIGN map vectors" by the Southern California Earthquake Center, USC is in the public domain. Access a detailed description of this image.

This compression also results in the uplift of the western Transverse Ranges Province; ranges here have been uplifted rapidly during the Quaternary, with rates of uplift estimated at 4—8 mm/yr (0.16-0.32 in/yr). These rapid rates of uplift are averages; uplift associated with individual earthquakes can be quite significant. For example, one event in 1971 (the San Fernando Earthquake) produced 2 m (6.6 ft) of uplift in the nearby mountains!

Additional evidence of uplift is seen along coastal regions and in the northern Channel Islands. In these coastal areas, numerous marine terraces can be used to track the rate and timing of uplift (Figure \(\PageIndex{5}\)). A marine terrace is formed by an uplifted wave-cut platform (see Chapter 16) that often contains ancient beach deposits and shells. These deposits were actively formed when the surface was at sea level; waves erode the bedrock to a flat surface. Over time, some of these surfaces are tectonically uplifted (they can also be stranded above sea level if sea level drops, but the situation in the Transverse Ranges is one of tectonic uplift due to transpression). These uplifted shorelines have been used to constrain rates and episodes of coastal uplift over thousands of years that date to hundreds of thousands of years.

Figure \(\PageIndex{5}\): "Stair-step" landscape of uplifted marine terraces on western San Clemente Island are similar to those found throughout the Transverse Ranges Province coastal region. The lowest terrace, where the road near the shore is visible, has been dated to 120,000 years by U-series analysis of corals (Muhs and others, 2002). "Marine terraces" by Daniel Muhs/USGS is in the public domain.

Box \(\PageIndex{1}\): Denudation Rates and Topography

The following text is adapted from "Denudation rate chronologies and the topographic development of the San Bernardino Mountains, California". This work, by Steve Binnie and William M. Phillips is licensed under CC BY-NC-SA 3.0 / a derivative from the original work.

How does the topography of mountains develop? What roles do crustal processes such as faulting and surface processes such as erosion play? These questions have long been a focus of geomorphic research. In the past decade, improved techniques for measuring rates of denudation (removal of material leading to a reduction in the relief and elevation of a landscape) have sparked new insights into these topics. The following is a case study for a landscape astride the San Andreas fault zone that uses denudation rates measured over different time scales to track topographic development.

The following study constructs a denudation rate chronology that compares rates averaged over millions of years with rates averaged over thousands to hundreds of thousands to years.

The San Bernardino Mountains in southern California (Inset Figure \(\PageIndex{1}\)) is an ideal location for examining how topography develops. This range retains remnants of topography formed prior to the beginning of mountain building in the late Miocene, about 6 Ma. The relict topography consists of a gently undulating plateau in the north and central portions of the mountains. Much of this low-relief landscape is mantled by deeply weathered granite originally contiguous with a similar granitic weathering mantle found in the lower lying Mojave Desert to the north. The weathering of the granite formed under a more humid climate prior to uplift of the San Bernardino Mountains. The southern third of the range comprises a series of east-west–trending ridges of high, rugged topography separated by major faults located in the intervening valleys. Here the relic landscape has been erased by vigorous erosion. These characteristics make it possible to track topographic development from early block uplift to later slicing up of the landscape by faulting.

Inset Figure \(\PageIndex{1}\): Location map of the San Bernardino Mountains. The north and central portions of the mountains exhibit a broad, gently undulating plateau that contrasts sharply with the steep narrow ridges in the south. Pink shaded areas are the basins sampled for cosmogenic nuclide analysis (modified from Binnie et al., 2008). "Location map of the San Bernardino Mountains" by Steve Binnie and William M. Phillips is licensed under CC BY-NC-SA 3.0.

Denudation rates (reported in units of millimeters per thousand years or mm/ka) were measured in three different ways and over three different timescales in the San Bernardino Mountains. First, by dating and reconstructing the pre-uplift topography, rates of denudation since uplift began can be determined. These rates are computed by dividing the amount of incision of the relic topography by the age of uplift initiation. Second, in the southern part of the mountains, thermochronology was used.

Thermochronology uses radiometric dating of minerals in rocks together with information about the thermal structure of the earth's crust. Each type of mineral has a closure temperature corresponding to the temperature at which the mineral begins to retain the products of radioactive decay. The depth below the earth's surface at which closure begins can be determined from geothermal gradients (rate at which temperature increases with depth). Dividing the closure depth by the radiometric age gives a denudation rate (some geologists term this rate "exhumation" rather than denudation). Thermochronologic denudation rates in the San Bernardino Mountains are also averaged over millions of years. The third method uses terrestrial cosmogenic nuclides. Terrestrial cosmogenic nuclides are generated by cosmic rays penetrating the top few meters of the Earth's surface. The concentration of these nuclides reflects a balance between the rate of production and the rate at which they are lost due to denudation and radioactive decay. By measuring the concentration of nuclides in surface rocks, calculating a value for the nuclide production rate at the sampling site, and using known values for radioactive decay, a denudation rate can be determined. As this denudation rate is averaged over the length of time the nuclides have resided in the near-surface, the period over which the rate is applicable is itself a function of the denudation rate, but will typically be on the order of thousands to hundreds of thousands of years. These principles also apply to spatially averaged denudation rates for whole river basins determined by sampling alluvial (stream) sediments; this is the procedure used in this study.

Results (Inset Figure \(\PageIndex{2}\)) show that denudation rates measured with all techniques are much lower in the northern part of the San Bernardino Mountains (70-100 mm/ka) where relic topography is preserved than in the southern part (180-1500 mm/ka). For example, in the northern margin of the plateau, average long term denudation rates are 70 mm/yr and cosmogenic rates are 95 mm/yr, with errors of 40 and 10 mm/yr, respectively. To the south, at the plateau, average long term denudation rates are 50 mm/yr and cosmogenic rates are 100 mm/yr, with errors of 30 and 20 mm/yr, respectively. At the southern maring of the plateau, average long term denudation rates are 180 mm/yr and cosmogenic rates are 670 mm/yr, with errors of 90 and 170 mm/yr, respectively. Immediately north of the San Adnreas fault, the highest rates are found: average long term denudation rates are 1200 mm/yr and cosmogenic rates are 1500 mm/yr, with errors of 400 and 290 mm/yr, respectively. When incision and thermochronologic rates averaged over million year timescales are compared with cosmogenic rates averaged over thousands to hundreds of thousands of years, we see that rates in the south are similar despite the very different averaging periods. This suggests that denudation rates of the narrow, rapidly uplifting ridges in the south have been relatively consistent over the last million years or so.

Inset Figure \(\PageIndex{2}\): A north-south transect through the San Bernardino Mountains giving mean denudation rates for the plateau, its northern and southern margins, and one of the steep southern ridges. Denudation rates (in mm/ka) derived over million year timescales using incision and thermochronology techniques are shown in red; rates measured over millennial timescales by cosmogenic nuclide analysis are shown in blue. Dashed gray lines indicate major faults. Rates in the south are similar over different timescales; rates on the plateau and its southern margin vary over different timescales (modified from Binnie et al., 2008). "A north-south transect through the San Bernardino Mountains" by Steve Binnie and William M. Phillips is licensed under CC BY-NC-SA 3.0.

Denudation rates in the north and central parts, on and around the edges of the plateau, are more rapid over shorter timescales, with the increase being more pronounced around the southern margin of the plateau than on the top. One explanation for this faster, more recent denudation is that the topography of the plateau is evolving, and doing so by the headward retreat of the steep drainage basins around the southern plateau periphery cutting back and gradually removing the plateau and the pre-uplift topography. Comparing the cosmogenic nuclide-derived denudation rates with average hillslope gradients (measured in angles of the steepness of slopes) of the San Bernardino Mountains illustrates another important result (Inset Figure \(\PageIndex{3}\)). Here, average hill-slope gradients of 10-30° corrsponds to low denudation rates (less than 100 mm/ka), while denudation rates increase linearly at hill slopes of more than 30° . A broadly linear relationship exists between denudation rates and hillslope gradients; that is, denudation increases as hillslopes steepen. This relationship decouples when hillslopes are steeper than around 30°, whereupon denudation rates no longer correspond to hillslope angles. This approximately 30° value delineates a change in how slopes erode.

Inset Figure \(\PageIndex{3}\): Denudation rates for the basins plotted against their respective average hillslope gradients. A broadly linear trend between the two is apparent where hillslope gradients are low (on the plateau). This trend breaks down for hillslopes of approximately 30° (found mostly in the southern regions of the mountains). Using space-for-time substitution, the left to right trajectory of this plot represents the progressive development of threshold hillslopes during mountain building."Denudation rates for the basins" by Steve Binnie and William M. Phillips is licensed under CC BY-NC-SA 3.0.

The survival of pre-uplift topography on the plateau, coupled with the denudation rate chronologies, suggests a sequence for the landscape evolution of the San Bernardino Mountains. The left hand side of the denudation rate-hillslope gradient plot (Inset Figure \(\PageIndex{3}\)) shows data mostly from the top of the plateau, where pre-uplift surfaces indicate that topography has been relatively unaffected by orogenesis. The right hand side of the plot contains data mostly from the southern parts of the mountains, where hillslopes are at threshold. Different parts of the San Bernardino Mountains are, therefore, experiencing different stages of the topographic response to orogenesis. As mountains begin to develop, their slopes steepen, denudation rates increase and, if crustal uplift is rapid enough, topography achieves threshold hillslopes. This notion of how threshold topography develops may be applicable to other ranges similar to the San Bernardino Mountains.

References

1. Bierman, P., & Steig, E. J. (1996). Estimating rates of denudation using cosmogenic isotope abundances in sediment. Earth Surface Processes and Landforms, 21(2), 125-140.
2. Binnie, S. A., Phillip, W. M., Summerfield, M. A., Fifield, L. K., & Spotila, J. A. (2008). Patterns of denudation through time in the San Bernardino Mountains, California: Implications for early-stage orogenesis. Earth and Planetary Science Letters, 276(1-2), 62-72.
3. Binnie, S. A., Phillips, W. M., Summerfield, M. A., & Fifield, L. K. (2007). Tectonic uplift, threshold hillslopes, and denudation rates in a developing mountain range. Geology, 35(8), 743-746.
4. Burbank, D. W. (1996). Bedrock incision, rock uplift and threshold hillslopes in the northwestern Himalayas. Nature, 279 (6565), 505-510.
5. Ehlig, P. L. (1981). Origin and tectonic history of the basement terrane of the San Gabriel Mountains, Central Transverse Ranges. In The Geotectonic Development of California (Vol. 1, pp. 253-283). Prentice-Hall.
6. Granger, D. E., Kirchner, J. W., & Finkel, R. (1996). Spatially Averaged Long-Term Erosion Rates Measured from In Situ-Produced Cosmogenic Nuclides in Alluvial Sediment. Journal of Geology, 104 (3), 249-258.
7. Hall, C. A. (1981). Evolution of the western Transverse Ranges microplate: Late Cenozoic faulting and basinal development. In The Geotectonic Development of California (pp. 559-582). Prentice-Hall.
8. Harden, D. R. (2004). California Geology. Pearson Prentice Hall.
9. Ingersoll, R. V., & Ernst, W. G. (1987). Cenozoic Basin Development of Coastal California (R. V. Ingersoll & W. G. Ernst, Eds.). Prentice-Hall.
10. Lal, D. (1991). Cosmic ray labeling of erosion surfaces; in situ nuclide production rates and erosion models. Earth and Planetary Science Letters, 104(2-4), 424-439.
11. Meisling, K. E., & Weldon, R. J. (1989). Late Cenozoic tectonics of the northwestern San Bernardino Mountains, Southern California. Geological Society of America Bulletin, 101(1), 106-128.
12. Montgomery, D. R., & Brandon, M. T. (2002). Topographic controls on erosion rates in tectonically active mountain ranges. Earth and Planetary Science Letters, 201(3-4), 481-489.
13. Muhs, D. R. (2022). MIS 5e sea-level history along the Pacific coast of North America. Earth System Science Data, 14(3), 1271-1330. https://doi.org/10.5194/essd-14-1271-2022
14. Muhs, D. R., Simmons, K. R., Kennedy, G. L., & Rockwell, T. K. (2002). The last interglacial period on the Pacific Coast of North America: Timing and paleoclimate. GSA Bulletin, 114(5), 569–592. https://doi.org/10.1130/0016-7606(2002)114<0569:TLIPOT>2.0.CO;2
15. Norris, R. M., & Webb, R. W. (1976). Geology of California. Wiley.
16. Oberlander, T. M. (1972). Morphogenesis of granitic boulder slopes in the Mojave Desert, California. Journal of Geology, 80(1), 1-20.
17. Proctor, R. J., & Crook, Jr., R. (1987). Sierra Madre thrust fault, Arcadia, California. In Cordilleran Section of the Geological Society of America Centennial Field Guide (218th ed., Vol. 1, p. 217). Geological Society of America.
18. Sidder, A. (2020, 10 9). Southern California’s Crustal Motion Tells of Earthquake Hazards. EOS, 101. https://doi.org/10.1029/2020EO150199.
19. Spotila, J. A., Farley, K. A., Douglas, Y. J., & Reiners, P. W. (2001). Near-field transpressive deformation along the San Andreas fault zone in Southern California, based on exhumation constrained by (U-Th)/He dating. Journal of Geophysical Research, 106(B12), 30,909-30,922.
20. Spotila, J. A., House, M. A., Blythe, A. E., Niemi, N. A., & Bank, G. C. (2002). Controls on the erosion and geomorphic evolution of the San Bernardino and San Gabriel Mountains, Southern California. Contributions to crustal evolution of the Southwestern United States. Special Paper - Geological Society of America, 365, 205-230.
21. Wallace, R. E. (1990). The San Andreas Fault System, California" US Geological Survey Professional Paper 1515. USGS.
22. Yerkes, R. F., McCulloh, T. H., Schoellhamer, J. E., & Vedder, J. G. (1965). Geology of the Los Angeles Basin, California-an Introduction. USGS.