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This institution reserves the right to refuse to accept a copying order if, in its judgment, fulfillment of the order would involve violation of copyright law. Field Trip No. 2 STRUCTURAL GEOLOGY AND TECTONICS OF THE SALTON TROUGH, SOUTHERN CALIFORNIA by S. Biehler and R.W. Rex Department of Geological Sciences University of California Riverside, California 92502 March 1971 30 CONTENTS Introduction........ ....32 .32 Regional Geologic Setting Physiography ..... .32 .35 Regional Tectonics . Geology ....... 37 Road Log ...'..... 37 Stop 1 ........ 38 Stop 2 ........ 38 Stop 3 ........ 39 Stop 4 ........ 39 Stop 5 ........ 39 Stop 6 ........ 40 Stop 7 ......,. 40 Stop 8 ........ 40 Stop 9 ........ 41 Bibliography ....... 41 List of I Ilustrations Figure 1. Index map showing route of travel. F igure 2. Generalized Geologic Map of the Salton Trough. Figure 3. Columnar Section for the Northeastern Coachella Valley. 31 Introduction One of the most striking topographic and tectonic features of southern California is the Salton trough. This structural depression is the northward continuation of the Gulf of California and extends from the Mexican tidelands at the head of the Gulf to San Gorgonio Pass, 80 miles east of Los Angeles. The Salton trough is an area of diverse geological complexity. Its major faulting, thick sedimentary deposits, recent volcanism and potential geothermal reservoirs, all have interesting geophysical expressions of local and regional importance. REGIONAL GEOLOGIC SETTING Physiography The Gulf of California is a relatively narrow structural depression less than 100 miles wide and over 700 miles long. Although the Gulf terminates 60 miles south of the international border the characteristic structural depression continues for 240 miles northwest from the Gulf to San Gorgonio Pass. This northern segment of the Gulf of California structural province, called the Salton trough, is marked by a broad flat alluviated valley with an area of 10,000 square miles, of which 2,000 square miles lie below sea level. The Colorado river delta south of the international border rises to a height of 40 to 50 feet above sea level, forming a natural dam which prevents this sink from being inundated by sea water from the Gulf of California. Geophysically and physiographically the Salton trough can be divided into four units: the Coachella Valley, Salton Sea, Imperial Valley and the Colorado River delta proper. The Coachella Valley extends southeast from San Gorgonio Pass to the north end of the Salton Sea. It is the narrowest segment of the Salton trough with an average width of less than 15 miles. The borders of the Coachella Valley are well defined on the southwest by the San Jacinto and Santa Rosa mountains of the Peninsular Range province and on the northeast by the Little San Bernardino and Orocopia mountains of the Transverse Range province. The mountains which border the Coachella Valley have an average elevation which is approximately 1500 feet higher than their counterparts to the southeast which form the borders of the Imperial Valley. As the Coachella Valley narrows in the northwest the average elevation of the bordering ranges increases. At San Gorgonio Pass where the two highest peaks of southern California, San Gorgonio and San Jacinto, form the borders of the pass, there is 10,000 feet of relief in a few miles. The Salton Sea is located in the transition zone between the narrow Coachella Valley in the north and the broad Imperial Valley in the south. This topographic center of the Salton trough is covered by a large man-made lake approximately 35 miles long and 10 to 15 miles , wide, which forms a sink for all of the drainage in the area. The surface of the sea is presently 234 feet below sea level with a maximum measured depth of 44 feet. Prior to the flooding of the sink, this area was a broad flat sandy playa which at one time formed the bottom of an ancient fresh-water lake. Before the disastrous inundation of the area in 1905-1907 by the accidental diversion of the Colorado River, the lowest exposed land area 32 ->'• RIVERSIDE RI»\ A Stop I 996070 0 110 20 30 PALM SPRINGS 1 I 1 1 INDIO MILES N Stop9 195 Stop 8 Stop7 Stop 2 01.-P' 04 e #e Q. -7 0 Stop 0•e 6 0 56' 99 % WESTMORLAND -'FA f< Stop•5 GLAMIS BRAWLEY Stop•3 IMPERIAL HOLTVILLE 8 .Stop4 SAN DIEGO EL CENTRO 80 98 8 CALEXICO -- -,-1- ------ --.-i ....,---- -- U....----S.A.-- - -- I- --- ---I. -MEXICO Figure 1. Index Map Showing Route of Travel 1 111-v. ' 1 7 9 I< Fig.2. 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