Geology of the Imperial Valley California

The Imperial Valley is located about 150 miles southeast of Los Angeles. It is a section of a much larger geologic structure -- the Salton Trough -- which is about 1,000 miles in length. The structure extends from San Gorgonio Pass southeast to the Mexican border, including the Gulf of California and beyond the tip of the Baja California Peninsula. The surrounding mountains are largely faulted blocks of the Southern California batholith of Mesozoic age, overlain by fragments of an earlier metamorphic complex. The valley basin consists of a sedimentary fill of sands and gravels ranging up to 15,000 feet in thickness. The layers slope gently down-valley, and contain several important aquifers. The valley is laced with major members of the San Andreas Fault system. Minor to moderate earthquake events are common, but severe shocks have not been experienced in recorded history. The entire trough, including the Gulf is an extension of the East Pacific Rise, a zone of separation in Earth's crust. Deep sea submergence instruments have observed many phenomena of crustal formation. The axis of the Rise, hence of the Salton Valley as well, is a great transform fault that is having the effect of separating an enormous slab of North America, consisting of the Baja Peninsula and coastal California away from the mainland, with movement to the northwest and out to sea as a terranne. Table of Contents Chapter 1 The San Jacinto and Santa Rosa Mountains Chapter 2 The Eastern Mountains Chapter 3 San Gorgonio Pass Chapter 4 The Hills Chapter 5 Desert Sand Chapter 6 The Palm Oases Chapter 7 Ancient Lake Cahuilla Chapter 8 The Salton Sea Chapter 9 The Gulf of California Chapter 10 Volcanic Activity Chapter 11 Water: A Desert Imperative Chapter 12 Water Supplies for the Thirsty Desert Chapter 13 The Colorado River Chapter 14 The Coachella Aquifer Chapter 15 Water Rights Chapter 16 The Dynamics of an Earthquake Chapter 17 The Richter Scale Chapter 18 The San Andreas Fault System Chapter 19 A Role Call of Earthquakes Chapter 20 Summary CHAPTER 1 THE SAN JACINTO AND THE SANTA ROSA MOUNTAINS FROM THE SIERRA NEVADA MOUNTAINS of Central California to the southern tip of Baja California, a single massive block of granitic rocks extends over a distance of 1,500 miles. This block represents two huge magma bodies that were intruded simultaneously during Mesozoic time, about 100 million years ago. Known as the Southern California batholith and the Sierra Nevada batholith,[1] these granitic rocks consitute the core of many mountain ranges in western North America. The combined bodies underlie an area exceeding 40,000 square miles, with the southern California batholith being the larger of the two masses. Even so, these two batholiths are members of a greater batholithic system that comprise the core rocks of mountain ranges that extend more than 4,000 miles along the coastal margins of North and South America from Alaska to Chile. Radiometric dating has assigned an estimated date of origin at 90 to 105 million years ago, or middle to late Cretaceous time. This places the intrusion as associated with a major mountain-building episode when North and South America were pushed westward on their plates. The Genesis of the Mountains Formation of a mass of molten rock as large as these batholiths is not a random event. It is a process associated with subduction zones and plate movement. The two Mesozoic batholiths of California are a case in point. In Late Jurassic time, about 150 million years ago, as part of a vast crustal disturbance, the westward-moving American plates began to override the Pacific plate, forcing the oceanic plate downward into the crust as a subduction zone. Under increasing depth and pressure, the descending crustal materials were converted into a molten magma. Before the end of the Jurassic, about 100 million years ago, the magma began to move slowly upward into the crust just beneath the surface. In so doing, it absorbed and converted the sedimentary rock overburden by heat and pressure into a complex new series of metamorphic rocks. The mahogany-colored rocks visible today on the lower mountain slopes from Palm Springs to Palm Desert are metamorphic rock remnants of that time and process. During mid-Cenozoic time, some 50 million years ago, under continued pressure of the westward moving plates, the leading edge began to crumple, initiating a mountain- building episode. The crust was under enormous stress, and the rocks ruptured along vertical faults. Mountain blocks began to rise in a series of steps, with long periods of erosion between episodes of uplift. This period of mountain building reached its climax within the past few million years. Today, like the tips of an iceberg, the mountain ranges of southern California and Baja California are the exposed protrusions of this one mass. The largest of these are the Peninsular Ranges, which extend from the Santa Ana Mountains, near Riverside, to the tip of Baja California, a distance of nearly 1,000 miles. The west coast of mainland Mexico from the Gulf of Tehuantepec to the Los Angeles basin also includes numerous extensive mountain ranges of the same rock types. Erosion kept pace with uplift, and most of the overlying metamorphic layer was stripped away. Remnants of these very old rocks remain today in exposures in the upper elevations of the Santa Rosa Mountains and along the base of the San Jacinto Mountains. The Southern California Batholith The batholith is a massive intrusion of several types of granitic rock, successively injected into the crust as a series of plutons.[2] The plutons represent multiple injections over a period that may have been as long as 10 million years. The batholithic mass contains many high-silica related rock types, but more than 90 percent of the mass consists of five igneous rocks: diorite, quartz diorite, granodiorite, quartz monzonite and granite . All occur as large bodies, derived from the separate plutons. Of these, light-colored diorite and granodiorite are most important to the Salton Valley, comprising the core rocks of both the Santa Rosa and San Jacinto Mountains. In contrast, the granitic rocks in the desert mountains to the east, e.g. the Little San Bernardino Mountains are chiefly granodiorite and quartz monzonite which are higher in feldspar and alumina and lower in silica than the rocks of the main body as represented by the San Jacinto Mountaisn. Notwithstanding these local variations, the batholith as a whole is remarkably homogeneous, consisting almost entirely of the lighter igneous rock types. The darker ultra-mafic rock series are rare, and alkaline types such as syenite are almost non-existent. The Peninsular Ranges of California The geomorphic unit, nearly 1000 miles in length, is known collectively as the Peninsular Ranges. The San Jacinto and Santa Rosa Mountains are ranges within this major mountain system. The individual ranges of the Peninsular Ranges are gigantic fault blocks, most of which are tilted to the west, toward the sea. The bedrock core in all ranges of the system is the Mesozoic granitic rock complex of the southern California batholith. The blocks are discrete mountain ranges, separated by valleys, all defined by faults which lie generally parallel to the boundary system, and all are oriented northwest-southeast. Each of the blocks has had its own distinctive landscape evolution depending on the nature and magnitude of the fault movements that created it. In their uplift, the mountain blocks tilted west. With few exceptions, the highest elevations in the individual ranges are located close to the eastern scarp, while the western side slopes gradually. Hence, the common geomorphic feature of most of the ranges is a western subdued upland surface with gentle relief, and a steep eroded eastern scarp descending abruptly to the valley floor. This describes the mountains which form the western margin of the Salton Valley the Santa Rosa Mountains and the San Jacinto Mountains. The mechanism for this is not completely understood. The most current perception is that the entire region is underlain by a massive detachment fault.[3] Under persistent stress, the separated mass breaks into blocks which then rotate along normal faults to accommodate the tensional strain. The tilt of the mountain blocks is dramatically evident to travelers crossing the mountains by the east-west highways. The approaches to the mountains from the east entails slower travel over steep mountain grades. On the other hand, travelers entering the mountains from the west drive up gradual slopes that are hardly noticeable. There are several nearby examples of this, such as the spectacular climb by Highway 74, from Palm Desert. The highway climbs Seven Level Grade up the face of the Santa Rosa Mountains, then descends over a gradual slope to Anza Valley. Another example is Highway 243 from Banning to Idyllwild. As a topographic unit the western margin of the Peninsular Ranges is inland several miles from the sea. The main structure continues beneath a sloping seacoast plain and under the sea as a broad submerged continental borderland whose geology closely resembles the landward portion. The islands of Catalina, San Nicholas and Santa Barbara are the exposed tops of elevated blocks of this same structure. The blocks are separated by deep troughs aligned northwest-southeast. There are many submerged near-vertical fault scarps that define the margins of these blocks. As a result, the submarine topography off the coast of Southern California consists of elevated blocks exposed as islands separated by downthrown blocks of deep-water canyons. The submarine geology of the Gulf of California appears to be very similar, and even on a grander scale with extremely deep basins within the narrow Gulf.

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