Geology of Southern California.pdf - Grossmont College

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4 Geology of Southern California � Figure SC.2 In normal faults, the hanging wall (HW) is displaced downward relative to the foot wall (FW). � Figure SC.3 A normal fault in the Mojave Desert. The block on the left is the hanging wall and has moved down relative to the block on the right. � Figure SC.4 Downward folds are called synclines and upward folds are known as anticlines. Axial plane Axis Syncline Anticline Axis Limb Frank DeCourten � Figure SC.5 Rock layers folded by compression helped to elevate the Calico Mountains in the Mojave Desert province. � Figure SC.6 Reverse faults are those in which the hanging wall is displaced upward. Such faults have contributed to mountainbuilding in many southern California locations. past several hundred million years have, at different times and in different places, produced compression, shear, and tension. These fundamental forces have deformed rock sequences to produce many different kinds of faults and folds that accompanied the uplift of mountains in southern California. Normal faults (Figures SC.2 and SC.3) are related to tensional forces that are especially prominent in the Basin and Range and Mojave Desert provinces. In these regions, linear mountains bounded by normal faults and separated from each other by down-faulted basins dominate the landscape. Elsewhere, compression generated by the convergence of plates and other rock masses has led to the development of anticlines and synclines (Figures SC.4 and SC.5), along with reverse faults (Figure SC.6). The shortening of the earth’s crust caused by folding and reverse faulting is linked to the uplift of prominent mountains in the Transverse Ranges and southern Coast Ranges provinces. Shear forces are associated with strike-slip faults, such as those in the famous San Andreas fault zone Reed Wicander
in southern California (Figure SC.7). Only minor vertical offset occurs along a strike-slip fault across which the lateral motion of rock masses is perfectly parallel. However, many strike-slip faults in southern California involve blocks moving in opposite directions at a slight angle. This geometry induces a component of either tension or compression that results in vertical motion along the strike-slip fault. Such faults are known as oblique-slip faults and are involved in mountain-building in several places along the San Andreas fault system. Most of southern California’s mountains are very young, having developed over just the past 20 to 25 million years. In fact, none of the mountains we will discuss in the following sections can be regarded as fully developed. They are all still evolving as the plate tectonics of southern California changes in response to the motions of the Pacific and North American plates. We will return to the changing arrangement of tectonic plates in later sections. Section Southern California.1 Summary ● The unique landscape of southern California is extremely varied, with high mountains, expansive deserts, sandy and rocky beaches, brushy hills, and broad river valleys in close proximity to each other. ● Eight physiographic provinces are represented in the varied terrain of southern California: the southern Sierra Nevada, the southern Coast Ranges, the southern Great Valley, the Transverse Ranges, the Peninsular Ranges, the Colorado Desert, the Mojave Desert, and the main part of the Basin and Range. ● A geologic map portrays the distribution of rock types and ages at the surface along with information about the orientation of rock bodies and the location and extent of geological structures such as faults and folds. ● The rocks of southern California are extremely varied. Igneous rocks are exposed in many places and include Mesozoic plutonic rocks similar to granite and Cenozoic volcanic rocks such as basalt, rhyolite, and tuff. Sedimentary rocks are also widespread including Mesozoic and Cenozoic shale and sandstone of marine origin along with thick sequences of Cenozoic terrestrial sedimentary rocks. Metamorphic rocks are represented primarily by schist and gneiss of either Precambrian or Mesozoic age. ● The mountains of southern California are relatively young and have formed through normal faulting under tension in the Basin and Range, folding and reverse faulting generated by compression in the Transverse Range, and through oblique faulting related to shear stress in several places along the San Andreas fault zone. P A C I F I C O C E A N P ACIFIC PLA TE Southern California.1 Introduction 5 � Figure SC.7 The San Andreas fault zone marks the transform boundary between the Pacific and North American plates. Note that Los Angeles is moving to the northwest relative to North America. Oceanic ridge Juan de Fuca Ridge JUAN DE FUCA PLA TE San Francisco Zone of subduction CALIFORNIA San Andreas fault Los Angeles OREGON BRITISH COLUMBIA Seattle WASHINGTON NOR TH AMERICAN PLA TE Transform faults NEVADA
Page 1 and 2: Southern California Geology of Sout
Page 3 and 4: ESSENTIAL QUESTIONS TO ASK Southern
Page 5: southern portion. For the purposes
Page 9 and 10: Frank DeCourten directly from the w
Page 11 and 12: The collision between North America
Page 13 and 14: Frank DeCourten � Figure SC.15 Th
Page 15 and 16: state since that time. Much of this
Page 17 and 18: � Figure SC.21 Location of the th
Page 19 and 20: � Figure SC.24 Groundwater flow i
Page 21 and 22: Courtesy of the U.S. Geological Sur
Page 23 and 24: � Figure SC.32 The Salinian block
Page 25 and 26: Courtesy of the U.S. Geological Sur
Page 27 and 28: Dick Hilton � Figure SC.39 Precam
Page 29 and 30: Sue Monroe Image courtesy of NASA/E
Page 31 and 32: Dick Hilton � Figure SC.51 Light-
Page 33 and 34: contain many elements such as boron
Page 35 and 36: Reed Wicander Dick Hilton � Figur
Page 37 and 38: Dick Hilton Marli Bryant Miller �
Page 39 and 40: Ted Soqui/Sygma/Corbis historic eve
Page 41 and 42: John S. Shelton Nick Ut/AP/Wide Wor
Page 43 and 44: closer to the edge of the retreatin
Page 45 and 46: Over-pumping of groundwater in the
Page 47 and 48: cinder cone - a small steep-sided v

Geology of Southern California.pdf - Grossmont College

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