The Valley of Southern California

1. The Valley of Southern California By. Veronica Garcia

2. Map of the valley of southern California

3. Geologic History • First I want to talk a little bit about the geologic history of this area .I'm going to talk about the different plate boundaries , topography, plate tectonics , and the San Andreas Fault zone.

4. Geologic structure •    The geologic structure of Southern California is intricately tied to the convergent boundary between two lithospheric plates. The tectonic collision between the North American and Pacific Plates cause’s large masses of rock to break, bend, and travel in response to the immense forces.

5. The San Andres Fault Zone • The San Andreas Fault Zone is the boundary between these two converging plates. The Pacific Plate and the North American Plate are grinding past one another along a zone of sub parallel faults that is roughly one hundred kilometers wide and has shifted its location several times since the onset of this transform (strike-slip) plate boundary 25-30 million years ago.

6. Topography of Southern Ca. • The rugged topography of southern California is testimony to the long-term tectonic activity associated with movement along the San Andreas Fault Zone. As the plate margins collide, broken blocks between faults are squeezed up or down, producing steep mountains and intervening basins. This exposes a bounty of geologic history, revealing past environments and the geologic activity of ancient Southern California.

7. Plate tectonics • Prior to tectonic plate convergence, the plate boundary along the western edge of North America was quite different. A seafloor spreading center existed off the coast; this massive seafloor fracture allowed magma to form oceanic lithosphere as tectonic plates move apart. One of the plates born at the spreading ridge was the Pacific Plate (moving westward); the other was the Farallon Plate which moved eastward beneath the encroaching North American Plate in a process called subduction. The Farallon Plate was split into the Coco’s and Juan de Fuca Plates when the spreading center was overrun by the North American Plate.

8. Map of the valley of southern California showing the 12 sites.

9. Twelve different significant sites • There are twelve different sites in this area that are significant; they are labeled on the map above. They are Cajon Pass ,Anza Borrego, Salton Trough, Palos Verdes Peninsula, Palmdale Roadcut, Pisgah Volcano, Rainbow Basin, Lake Perris, Dana Point, Red Rock Canyon , Vasquez Rocks , and ST. Francis Dam.

10. Cajon Pass Cajon Pass is located between the San Gabriel and San Bernardino Mountains, the Pass is a Morphological feature attributed to the San Andres Fault motion. One of the features of Cajon Pass is a straight canyon called Lone Pine Canyon. On the south side of the canyon, there are bumps that run parallel to the valley. This is called “bench topography,” and it is one of many indications that there is still tectonism. Another unusual feature is the Inface Bluffs. These bluffs are cliffs made up of alluvial fan deposits. These bluffs have no mountains at the head of the alluvial fan deposits. The activity along the San Andres Fault has moved the source of the alluvial fans

11. Anza Borrego The Anza-Borrego Desert Region has geologic features that have accumulated over the past half billion years. The oldest rocks here were once an ancient seafloor, a great pile of soft sediments that became deeply buried, but it was later invaded by the molten rocks of the Peninsular Ranges Batholith. The geologic record of the rocks found in this area dates back approximately 500,000,000 years. Around 250 million years ago, the passive margin became more violent as subduction activity was initiated between the westward moving of the North American plate and the eastward moving of the Farallon Plate. As time passed the subduction activity caused the Farallon plate to melt in depth.

12. Salton Trough The southern end of the Salton Sea is a region of high heat flow and several geothermal plants that currently take their energy from the hot surface brines. A line of volcanoes rests above the geothermal field and marks the northernmost rift of the spreading center. Salton Trough lies in an extensional boundary between the North American and Pacific tectonic plates. It lies at the northern end of the Sea of Cortez where the spreading –type plate boundary of the east pacific rise transitions to the transform-type plate boundary of the San Andrea's fault system . This picture of Salton Trough shows a geothermal plant that taps into the same heat source that formed this obsidian many years ago.

13. Palos Verdes Peninsula The Palos Verdes Peninsula is a tectonic fault block of seafloor sediments and volcanic rock draped atop a submerged mountain of metamorphic rocks that began rising out of the Pacific Ocean about 1.5 million years ago. The seafloor rocks are made of deep water fine-grained sediments which, in places, consist primarily of diatoms or volcanic ash. Lave flows erupted upon or in the ocean sediments during the early phases of deposition. The eroded terrain that the seafloor sediments settled upon consists of metamorphic rocks created at great depth in or near a subduction zone. These rocks, known as the Catalina Schist, were once part of the subducting Farallon tectonic plate. These rocks were lifted above sea level and eroded before the Monterey Formation was deposited on top. The geologic sequence describes rocks that were first dragged deep into the Earth on a sinking plate, were then uplifted and eroded, dropped down to deep ocean depths to catch the Monterey Formation, and now uplifted again.

14. Palmdale Roadcut This is a popular site in the field of geology because the roadcut slices through the rocks deformed by the powerful and extensive movement of rocks along the San Andreas Fault. the picture I have here shows a series of folds, but no clearly apparent fault. the rocks on the left side of the picture are very different from the rocks on the right side , indicating some sort of geologic boundary. The San Andreas Fault Zone, which runs through the road cut near Palmdale, divides the North American Plate from the Pacific Plate.Almost all geologists agree that the San Andreas Fault Zone separates the North American Plate from the Pacific Plate. Since the fault zone runs through the road cut, it's possible to walk from one continental plate to another in just a few minutes.

15. Pisgah Volcano Pisgah Volcano, commonly designated as "Pisgah Crater" on maps, is a young volcanic cinder cone which last erupted about 2,000 years ago. Pisgah Volcano is readily visible as you drive east of Barstow on Interstate 40, but it no longer looks like a volcano due to artificial and natural erosion. The crater is only about 300 feet high, but the surrounding basalt field extends for miles. Pisgah Volcano is a typical cinder cone. The type of rocks formed by a cinder cone fall into a broad category of rocks called "extrusive igneous", or volcanic rocks. This means that these rocks were formed from lava that cooled at (or very close to) the earth's surface. Pisgah volcano extruded a volcanic rock known as basalt. Basalt is an extrusive igneous rock that is enriched with minerals containing iron and magnesium and it flows easily. Pisgah volcano, in its last stages of activity, threw pyroclasts forcefully from the vent, building a pile of pyroclasts on top of the basalt flows

16. Rainbow Basin Rainbow Basin, is composed of folded and tilted rocks, uncomformities, and sedimentary structures this provides a great display of diverse geologic features. The Rainbow Basin lies within a desert environment that reveals erosive powers of water. A large syncline fold in the rock layers influences the shape of the basin. Originally the sediments in the syncline settled horizontally. The tectonic forces folded the rocks into the shape of a syncline The buried erosional surface is called an unconformity. It represents a gap in the geologic record of the area. Sediments were eroded and carried away before new sediments were deposited. Geologic evidence in sediments inform us about the environment at the time of their deposition. Erosional surfaces, however, represents sediments washed away which then forever hide their secrets.The picture below clearly shows the unconformity at Rainbow Basin.

17. Lake Perris magmas of both felsic and mafic composition were mixed together deep within the earth’s crust to form the plutonic bodies exposed at this lake. These are some of the northernmost exposures of the Peninsular Ranges Batholith, a huge collection of pluton’s known for its wide range of composition. At Lake Perris these igneous rocks were created as magma slowly cooled several kilometers below the earth’s surface. While the magma was still fluid, other magma types mixed into it. The magma chambers of plutonic rock bodies are dynamic mixing zones where evidence of motion within the molten rock may have been preserved during solidification as seen in the rocks at Lake Perris quarry.

18. Dana Point Dana Point, consists of two sedimentary rock sequences brought into juxtaposition with one another by fault activity. The two rock sequences are very different in age: the older beds are Late Oligocene to Early Miocene while the younger were deposited during the Pliocene age

19. Red Rock Canyon Red Rock Canyon State Park , is located north of the nearby Garlock Fault Zone in the western Mojave Desert. It lies where the Sierra Nevada/Tehachapi Mountains meet the high deserts. The canyon is named for its sandstone layers that contain bits of iron which have rusted to color the sedimentary rock. The deposition of the stream carried sand,silt,and gravel layers contain evidence of Miocene life. Several volcanic eruptions added basalt flows and tuffs to the sedimentary sequence. These volcanic units have subsequently become the ridge forming rocks of the area due to the resistance to weathering of the sedimentary layers. These deposits accumulated during the later Miocene Epoch along which there was also occasional volcanic activity. Resistant lava flows and a thick tuff breccia bed are ridge-forming units within the sedimentary sequence. This sedimentary and volcanic rock sequence lies atop a thick accumulation of andesite flows and pyroclastics. Collectively, they are known as the Ricardo Formation. The prominent pink tuff breccia of Red Rock Canyon is about 12-13 million years old. At some point, groundwater saturated in silica and other ions permeated the basalts and deposited various minerals, including fire opal, chalcedony, natrolite, and analcite. Numerous fossils have been recovered from the Ricardo Formation including the remains of camels, rhinoceros, horses, tortoises, skunks, and the extinct and elephant-like Gomphothere.

20. Vasquez Rocks Vasquez Rocks, it is located south of the San Andreas Fault zone along State Highway 14 in the western Transverse Ranges. Vasquez Rocks consists of coarse clastics deposited upon volcanic rocks released as the North American tectonic plate initially collided with the Pacific Plate about 25 million years ago. The sediments at Vasquez Rocks are deposited upon and with numerous basalt flows which constitute a major portion of the lower Vasquez Formation. These basalts were extruded into the rapidly down dropping basin as major faults splintered the crust. For millions of years afterward, repeated episodes of uplift to quiescence produced several distinctive sequences called megacycles. These megacycles are characterized by coarse clastic sand and gravel deposits at the base of the sequence (as uplift became strong) with a fining upward progression (as tectonic activity quiesced) into the siltstones and shale’s of a distal alluvial fan-playa depositional environment.

21. St. Francis Dam On March 12, 1928, one of California's worst catastrophes occurred. The St. Francis Dam broke. Huge torrents of water washed down the San Francesquito Canyon, killing many on the way. The problem was ignorance. The geology of the locality was ignored.The dam was built directly over the San Francisquito Fault. This fault, as with many other faults had "gouge." Fault gouge is broken rocks and debris along the fault. Water can easily seep through the gouge, as it did under the dam. After two years of water slowly eroding away the gouge, a gap large enough for water to flow through formed. Once this gap formed, water could instantly rush through, as it did at midnight on March 12, 1928. The failure was actually two-fold. First, the water flowing through gap formed by the fault gouge broke off the west side of the dam. Second, a huge whirlpool formed in front of the east side of the dam. The whirlpool quickly eroded away the Pelona Schist, which comprised the east side of the San Francesquito Canyon. This formed another gap in the dam. Water poured through this gap, breaking away the east side of the dam.

22. Summary • So basically my power point is an overview of what happens when two different plate boundaries collide . I wanted to show some important geologic features form the state of California . I feel like these 12 sights are very significant to show what happens as the earth below us moves around! I hope it was not to long , and I hope it was interesting for you ! Thanks!

23. Works cited • Color Landforms Atlas of the U.S. ....from John Hopkins University, Applied Physics Lab • | Department of the Interior | U.S. Geological Survey |URL of this page: http://pubs.usgs.gov/pp/pp1687Maintained by: Michael DigglesCreated: March 22, 2005 Last modified: March 22, 2005 (mfd) • URL of this page: http://geopubs.wr.usgs.gov/prof-paper/pp1669/Maintained by: Carolyn DonlinCreated: 3-26-03Last modified: 3-27-03 (cad) • http://ceres.ca.gov/index.html • http://library.caltech.edu/collections/geology/California.htmContent provided by Jim O'Donnell (jimodo AT library.caltech.edu) 1200 E California Blvd, Mail Code 1-32, Pasadena, CA 91125-3200 • http://ngmdb.usgs.gov/In cooperation with the Association of American State Geologists Funded by the USGS National Cooperative Geologic Mapping ProgramMandated by the National Geologic Mapping ActURL http://ngmdb.usgs.gov/ngmdb/ngmdb_home.htmlDatabase project personnelLast Modified: September 17, 2008 • This page is <URL :http://ngmdb.usgs.gov/Other_Resources/rdb_topo.html>Page maintained by Dave Soller (drsoller@usgs.gov)last update: Dec. 26, 2006 • http://sepwww.stanford.edu/oldsep/joe/fault_images/BayAreaSanAndreasFault.html • Scholz, C. H.: The Black Mountain asperity; seismic hazard of the southern San Francisco Peninsula, California, Oct. 1985. (GEOPHYSICAL RESEARCH LETTERS; Vol. 12, No. 10, p. 717-719.) • Ward, P. L., and Page, R. A.: The Loma Prieta Earthquake of October 17, 1989; what happened, what is expected, and what can be done. (United States Government Printing Office, 1989.) • This page: http://walrus.wr.usgs.gov/ask-a-geologist/Maintained by: Rex Sanders, USGSLast changed: 2008 May 19

24. Quiz Time • What two major plates are in Southern California? • What formed the obsidian in the Salton Trough? • What type of rock formed the layers in Red Rock Canyon?

25. Answers • 1. Northern and Pacific Plates • 2. A geothermal Plant • 3. Sandstone layers that contain bits of iron which have rusted to color the sedimentary rock.