MIDDLE AND LATE CENOZOIC SYSTEMS OF THE CENTRAL CORDILLERA 



511 



with a strong northwesterly component in central California keeping the 

 Coast Ranges block snug against the adjacent continental mass. Perhaps 

 the same is true of the Rocky Mountain Trench. Some separation and also 

 horizontal displacement have been postulated for the Rocky Mountain 

 Trench. This movement is possible when the Snake River fault is con- 

 sidered to be one of considerable separation (Chapter 36). 



The drifting away from the continent of Baja California as well as a 

 northwesterly gliding movement seems substantially demonstrated. See 

 Chapter 29. 



A major strike-slip fault is postulated across south-central Arizona at 

 the south margin of the Colorado Plateau. Southern Arizona remained 

 5000-8000 feet below the Colorado Plateau after vertical adjustments 

 occurred in late Cenozoic time, and is generally considered to be a block- 

 faulted region, although not so clearly as the Great Basin in western Utah 

 and Nevada. A few alluvial-filled valleys parallel the grand escarpment 

 and support the concept of down-dropping along major faults. However, 

 a master horizontal couple as indicated on the map of Fig. 31.22 has not 

 been recognized or postulated, as far as the writer knows. This then, is a 

 very speculative element of the framework of movements illustrated on 

 the map. 



The rifting of central New Mexico finds a compatible place in the frame- 

 work. The Sonoran-Chihuahua basin and range region is poorly under- 

 stood, and the illustration of considerable distention there is hardly more 

 than a guess. 



SEISMIC VELOCITY LAYERS IN THE EASTERN GREAT BASIN 

 Seismic Layers 



The recognition of a crustal layer with a velocity of 7.5 ± kilometers per 

 second in several areas of the western United States and Canada comes as 

 a very significant find and perhaps a key to tectonism there. The work of 

 Berg et al. (1960) in the eastern Great Basin, Press (1960) in the Cali- 

 fornia-Nevada region, and the summary article by Diment ( 1961 ) should 

 be referred to. The seismic velocity layers recognized to date are por- 

 trayed in Fig. 31.23. 



Fig. 31.23. Seimic velocity layers in western 

 United States Velocities in kilometers per 

 second (1) Press, 1960; (2) Berg et al., 1960; 

 (3) Meyer ef al., 1960. Refer to Fig. 38.1. 







£20 



u 



° 40 



X 



1- 60 

 a. 

 bj 

 



80 



CALIFORNIA 



NEVADA 



(1) 



EASTERN 



BASIN AND 



RANGE 



(2) 



EASTERN 



MONTANA 



(31 



6 1 



5 7 







6 2 



6.3 







y/ y/ /y 



8 1 



8.1 



8 



Geologic Requirements 



In attempting to interpret the constitution of the seismic layers the 

 following geologic requirements should be kept in mind. 



1. The Great Basin has been distended about 30 miles (50 kilometers) 

 in the last 15 m.y. A strong horizontal coupling along the Pacific- 

 margin is evident, with the Pacific facing blocks moving to the north- 

 west. 



2. The Great Basin has been elevated during the same time 1-1/2 kilo- 

 meters. 



3. The High Plateaus of Utah and the Sierra Nevada have been elevated 

 2—3 kilometers during the same time. 



4. The Colorado Plateau has been elevated 2-2J-2 kilometers during the 

 same time. 



5. Silicic lavas have been poured out over most of the Great Basin in 

 amounts equal to a layer 1-2 kilometers thick since early Oligocene 

 time. This material must have come from the melting of a portion of 

 the silicic crystalline mantle. See Chapter 36. 



6. Equal amounts of basalt (viz., the Columbia basalt field) have flowed 

 to the surface from a source probably immediately below the crystal- 

 line basement, and in the Great Basin the basalt reservoir has been 

 tapped from time to time during the general acidic lava eruptive 

 cycle. 



t 



