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<br />. <br /> <br />. <br /> <br />as the oceanic plate moves northwest. relative to the continental piette. Crustal separation <br />in the Salton Trough is occurring at the rate of I to 2 centimeters per year as Baja <br />California and the Peninsular Ranges of southern California (the parts of the oceanic plate <br />that border the Salton Trough on the southwest) move westward. away from the <br />continental piette. Cumulative gravity movement along the faults is sufficient to <br />accommodate a maximum of about 20,000 feet of Colorado River sedimentation as <br />spreading and subsidence proceed. <br />Colorado River deposits form virtually all of the surface a'ld shallow subsurface <br />geology in the delta region. They are underlain by prc-Tertiary metamorphic, plutonic. <br />and dike rocks, collectively called the basement complex; these rocks also crop out in <br />bordering mountain ranges. Tertiary volca"ic rocks are present in some of the bordering <br />ranges and underlie the sediments in part. Locally, very young volcanic rocks are present <br />in the delta region. Eruptions have been recorded in historic times in at least three places <br />within or bordering the Salton Trough. One, in 1934, was in the Pinacates volcanic field <br />in SOnOTJ, Mexico, about 90 miles southe:.lst of Yuma; another, in 1877, W3S in tht' <br />Chocolate Mountains east of the Salton Sea; and two, in 1852 and 1927, were at Cerra <br />Prieto, about 20 miles southeast of Mexicali, Baja California. <br />The thickness of recent alluvium in the Yuma area ranges from a few hundred <br />feet at the apex of the present day delta to more than 2,000 feet at the Southerly <br />International Boundary near San Luis. Arizona. Dune sand overlies the alluvium in local <br />areas of the delta region (Map No. 1292-303-1011). <br />The alluviulll consists largely of waterhorne clay, silt, sand, and gravel. It is <br />exposed on the mesas (older stream terraces) and in the present river valleys. Most was <br />deposited by the Colorado and Gila Rivers, but some was deposited ':ly ephemeral streams <br />along the mountain fronts. The river deposits are better sorted and hetter rounded than <br />the local lieposits and contain more fine-grained materials. Sanli is the principal constituent, <br />but thick deposits of gravel and clay occur at various levels. <br />A.4.b. Seismicity <br />The San Andreas Fault system is the locus of the most intense seismic activity <br />in the United States (exclusive of Alaska) and the northern Pacific Coast Region of Mexico. <br /> <br />. <br /> <br />16 <br />