13.12: Detailed Figure Descriptions

Figure 13.1: Geologic Province Map

A map of California highlights the area that corresponds to the Mojave and Colorado Desert Geomorphic Provinces. The larger Mojave Desert Province is northeast of the is north of the Colorado Desert Province; they are separated by northwest trending San Andreas Fault.  Together, these geomorphic provinces define a triangular region in the southeastern edge of the state. The western boundary of this larger geomorphic province is formed by the states of Nevada and Arizona. The northern boundary is the east-northeast trending Garlock fault, which separates the Mojave and Colorado Desert Provinces to the south from the (from east to west) Basin and Range, Sierra Nevada and Great Valley Province. Along their southwestern boundary, these provinces are bounded by the Transverse Range Province and the Peninsular Ranges Province.  The southern boundary is truncated by the Mexican border.

Figure 13.1.1 

Map of the Mojave and Colorado Desert Provinces from space. The map shows the intersection of the east-northeast trending Garlock fault with the northwest trending San Andreas fault. These faults meet at a point that defines the western limit of the combined Mojave and Colorado Desert Provinces.  To the north of the Garlock Fault is the southern edge of the Basin and Range, Sierra Nevada, and Great Valley Provinces. To the southwest of the San Andreas Fault is the Los Angeles Basin, which lies at the junction of the Transverse Ranges Province to the west, and the Peninsular Ranges Province to the south.  The region encompassed by the Mojave and Colorado Desert Provinces is arid and contains numerous mountain ranges separated by valleys, some of which are very light in color, indicating the presence of seasonal lakes. Major highways cross this region, including US 10, which travels east from Los Angeles; US 15 travels northeast through the center of this province; and US 40 travels east-west across this region. Joshua Tree National Park is situated along the southwest edge of this province, north of US 10 where it meets the San Andreas Fault. The Salton Sea west of the San Andreas Fault, in the adjacent Peninsular Ranges Province.

Figure 13.1.2

Excerpt from the Geologic Map of California with the Mojave and Colorado Desert Provinces highlighted. An excerpt of the larger state map focuses on the combined provinces of the Mojave and Colorado Desert. The regional extent of these provinces is described in Figure 13.1 and Figure 13.2. This map highlights map units according to large ranges of geologic age and lithology. Most of the bedrock geology of this region has been mapped as Quaternary surficial deposits which generally correspond to basin regions. Areas of older rocks are found in uplifted mountains. Isolated regions of pre-Cambrian rocks are located along the boundary between the Mojave and Colorado Desert Provinces, where they form a northwestern linear belt corresponding, and along the eastern edge of the provinces. Smaller occurrences are found throughout the central part of the province. Mesozoic granitic igneous rocks occur throughout the region, as do Cenozoic rocks of volcanic origin. Paleozoic sedimentary and metamorphic rocks are scattered throughout the northeastern portions of the province, as in the central western region. Major fault systems are oriented northwest as well as east west. The northwest trending faults occur in the western part of the province and truncate parallel east-west trending faults in the northern central part of the province.

Figure 13.2.4

Three panels describing the simplified geologic and tectonic history of the Mojave region. Panel A depicts geologic events prior to 250 million years ago when the Mojave region was a passive continental margin. At this time, the ancient Pacific Ocean Basin was situated to the west. A blanket of passive margin marine sediments covered the shallow shelf platform and continental margin on top of very ancient igneous, sedimentary, and metamorphic rocks. Panel B depicts the period of plate convergence along the continental margin roughly 250-60 million years ago. In the region that would become the Sierra Nevada and the Mojave Provinces, a volcanic arc system developed to the west of the active east-directed subduction zone. Panel C depicts the period of extension and faulting of the Great Basin region (including the Mojave Province) that began roughly 30 million years ago. Alluvium-filled basins in down dropped grabens are punctuated by uplifted basement rocks. Younger Strike-slip style fault systems (like the Eastern California Shear Zone and the San Andreas and Garlock Fault System) as well as active erosion are still active in the region today.

Figure 13.5.2

A portion of the Algodones dune field with crescent and linear dunes indicated. A large image of the Algonones dune field is situated between alluvial fans shed from adjacent mountain ranges. The dune field includes both crescent and linear dunes that are emphasized by an inset enlargement. In the enlargement, a zone of arcuate crescent dunes is adjacent to a zone of complex crescent dunes with a more chaotic shape. Long narrow linear dunes that trend northwest-southeast (transverse to the trend of the other dunes in the image) are in the southwest portion of the dune field.  

Figure 13.7.3

Map of the interconnected Pleistocene Lake system of California. A Digital elevation model focuses on the region of the Mojave Desert to the north of the San Gabriel and San Bernadino Mountains and extending to the Colorado River on the east. A network of ancient lakes is developed throughout this region. A central part of this system is the Mojave river, which flows northeastward from the San Gabirle mountains, and connects a number of these lakes including the ancient Lake Manix, Cronese Lakes, and Soda Lake (also known as Lake Mojave). Flow from this system continued northward to Lake Tecopa (near Shoshone, CA), and then into Lake Manley (in the modern Death Valley region). This lake drained into a series of large lakes to the west: Panamint Lake, China Lake and Owens Lake.

Figure 13.9.2

Map showing the relative location of the 1999 Hector Mine. The epicenters of a range of earthquakes in the Mojave Desert Province are plotted atop a digital elevation model. The region depicted is northeast of the San Andreas Fault and includes Joshua Tree National Park in the southeast, the Garlock Fault in the north, and is centered on Barstow, CA. Earthquake epicenters form distinctive clusters: A linear belt of epicenters includes the 1999 Hector Mine Earthquake; this belt trends north-northwest and is located to the east and slightly south of Barstow. A second cluster forms a linear belt that includes the Landers Earthquake epicenter. This belt is parallel to that of the Hector Mine cluster but located slightly west of it. Another large cluster of epicenters is plotted around Big Bear Mountain. The epicenter of the 1999 Hector Mine Earthquake is southeast of Barstow. The epicenter of the Landers Earthquake is south-southeast of Barstow and slightly southwest of the Hector Mine earthquake. Zones of aftershocks are also shown. These form linear belts that trend north-northwest through the Hector Mine epicenter and a second linear belt through the Landers epicenter is parallel to that of the Hector Mine trend but located to the west of it.

Figure 13.9.3

Map of CISN / SCSN Relocations for Brawley seismic zone events. Clusters of epicenters in this map occur in two distinct linear belts. One belt is west-southwest of the Salton Sea, where a cluster of epicenters is aligned with the northwest trending Superstition Hills fault.  The northern edge of this zone is a broad region of epicenters with a northeast trending sense. The second zone falls along the north-northwest trending Imperial Fault to the south and extends northward to the southeastern shores of the Salton Sea, where it merges with a broad northeast trending zone of epicenters and some events offshore in the Salton Sea itself. In this second zone, a recent cluster of events is associated with the northeast trending 2012 Brawley swarm.