CONTINENTAL STRUCTURES AND PROCESSES AND SEA-FLOOR S 



corded by the U.S. Geological Survey 

 in California, Nevada, Idaho, Wyo- 

 ming, Utah, and Arizona from 1961 

 to 1963 indicates that crustal thick- 

 ness reaches maxima under the Sierra 

 Nevada Range (42 km.), the Trans- 

 verse Ranges of southern California 

 (37 km.), and in southwestern Nevada 

 (36 km.). The crust is relatively thin 

 under the Coast Ranges of California 

 (24 to 26 km.), the Mojave Desert 

 (28 km.), and parts of the central 

 Basin and Range Province in Nevada 

 and Utah (29 to 30 km.). The base 

 of the crust dips generally from the 

 Basin and Range Province toward 

 greater depths in the Colorado Plateau 

 (43 km.), the middle Rocky Mountains 

 (45 km.), and the Snake River Plain 

 (44 km.). A velocity boundary zone 

 between the upper and lower crust 

 can be well determined only beneath 

 the middle Rocky Mountains, the 

 Snake River Plain, and the northern 

 part of the Basin and Range Province. 

 The average velocity of the western 

 crust is low, typically about 6.1 to 6.2 

 kilometers per second, but signifi- 

 cantly higher in the Colorado Plateau 

 (6.2 to 6.5 km/sec), and the Snake 

 River Plain (6.4 km/sec). Upper- 

 mantle velocity is less than 8.0 kilo- 

 meters per second under the Basin and 

 Range Province, the Sierra Nevada, 

 and the Colorado Plateau and equal 

 to or greater than 8.0 kilometers per 

 second under the Coast Ranges of 

 California, the Mojave Desert, and 

 the middle Rocky Mountains. 



Recent refraction work indicates 

 that the average crustal velocity in the 

 Columbia Plateau is high, as expected, 

 but that the crust is about 10 to 15 

 kilometers thinner than it is in adja- 

 cent areas. Thus, the Columbia Pla- 

 teau has seismic properties similar to 

 those of a somewhat overthickened 

 oceanic crust. So does the Diablo 

 Range of California. The Franciscan 

 formation (a metamorphosed struc- 

 ture) exposed in the Diablo Range, 

 believed by many geologists to have 

 been deposited in a Mesozoic oceanic 

 trench, apparently extends to a depth 

 of 10 to 15 kilometers; it was de- 



posited directly on a basaltic crust 

 that now extends to a total depth of 

 about 25 kilometers. 



Composition of the 

 Continental Crust 



In a rock of a given composition, 

 both metamorphic degree and water 

 content affect seismic velocities at 

 various pressures and temperatures. 

 Recent laboratory research in the 

 United States suggests that pressures 

 and temperatures at most crustal 

 depths would place the rocks within 

 the stability field of eclogite rather 

 than basalt. Seismic velocities in the 

 lower crust formerly interpreted as 

 appropriate for basalt are therefore 

 regarded by many petrologists as 

 more appropriate for more silicic rock. 

 However, the presence of significant 

 amounts of water in the lower crust 

 would produce abundant hydrous 

 minerals in a rock of basaltic com- 

 position; this would result in seismic 

 velocities similar to those in the lower 

 crust. 



Given such uncertainties, it seems 

 that the only positive assertion that 

 can be made about the average com- 

 position of the continental crust is 

 that it is intermediate and probably 

 not too different from monzonite. The 

 lower crust may be basaltic, interme- 

 diate, or even silicic, and the most re- 

 liable guide to its composition is 

 probably geologic association. For ex- 

 ample, in the Snake River Plain, where 

 basalt is exposed at the surface, it 

 seems reasonable to interpret the high 

 velocities of the lower crust as indica- 

 tive of basalt. In other areas, the 

 higher velocities should probably be 

 regarded as indicative of intermediate 

 rock. 



Structure and Composition of the 

 Upper Mantle 



Seismic probing of the upper man- 

 tle has established the existence of 

 two important velocity transition 

 zones: one at a depth of about 400 



kilometers, in which magnesium-rich 

 olivine is transformed with increasing 

 pressure to spinel; and another at a 

 depth of about 650 kilometers, in 

 which spinel is presumed to be trans- 

 formed to compact oxide structures 

 with increasing pressure. Recent esti- 

 mates of the density of the uppermost 

 mantle, based on statistical models 

 using all available evidence, yield 

 densities of 3.5 to 3.6 grams per cm 3 , 

 significantly higher than the densities 

 deduced from the usual velocity- 

 density relations. Rocks of this den- 

 sity and the seismic velocities ob- 

 served just below the Mohorovicic 

 discontinuity could be either eclogite 

 or iron-rich peridotite. Given the 

 lateral heterogeneity of the upper 

 mantle indicated by the variable seis- 

 mic velocities, it seems most reason- 

 able to regard the upper mantle as 

 grossly heterogeneous, consisting pri- 

 marily of peridotite but with large 

 lenses, or blocks, of basaltic, eclogitic, 

 intermediate, and perhaps even silicic 

 material distributed throughout. 



There is much seismic evidence that 

 a low-velocity zone for both P- and 

 S-waves exists in the upper mantle in 

 the western third of the United States, 

 with a velocity minimum at a depth of 

 100 to 150 kilometers. This zone 

 seems to be particularly pronounced 

 in the Basin and Range Province. The 

 low-velocity zone for P-waves is ap- 

 parently absent or greatly subdued in 

 the eastern two-thirds of the United 

 States. The most likely explanation 

 for the low-velocity zone is that the 

 mantle rocks there are partially 

 molten. 



Continental Margin Processes 



The interaction of the laterally 

 spreading sea floors with the con- 

 tinental margins — resulting in the 

 downward plunging of rigid litho- 

 spheric plates beneath the continents, 

 accompanied by shallow- to deep- 

 focus earthquakes and volcanic ac- 

 tivity — has been elucidated by a 

 beautiful synthesis of geological and 



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