MIDDLE AND LATE CENOZOIC SYSTEMS OF THE CENTRAL CORDILLERA 



505 



Fig. 31.14. Cross section of the Tendoy Mountains; Cm, Madison Is.; Ca, Amsden fm.; Cq, 

 Quadrant quartzite; Pmp, Phosphoria fm.; "id, Dinwoody fm.; 'Rw, Woodside fm.; tt, Thaynes fm.; 



Seismicity in the Trench Zone 



After the past pages on the zone of great trenches that extends from 

 Arizona to the Yukon had been written, attention was called to the earth- 

 quake maps of Woolard (see Fig. 31.15). The concentration of major 

 shocks in the zone of trenches is striking. The coincidence not only sup- 

 ports the existence of the zone of faults but also indicates that a number 

 lof them are still active. 



GEOPHYSICAL EVIDENCE 



'Gravity and Seismic Surveys 



The fill of the down-faulted basins in the Basin and Range provinces 

 llends itself to analysis particularly by gravity surveys. Since the alluvium 

 (has lighter density than the lithified bedrock, the magnitude of the gravity 

 anomaly can be related to the depth of fill, and this becomes a measure of 

 the magnitude of faulting. Also, faults concealed beneath the alluvium 

 may be detected, and new light is shed on the fault pattern. The computed 

 cross sections on the basis of gravity surveys have been checked by seismic 

 surveys across the valleys. 



Recent earthquakes in the Great Basin have been studied seismically 

 and the results add to our concepts of Basin and Range structure. 



Jst, Sawtooth fm.; Jr, Rierdon fm.; Kk, Kootenay fm.; Trr, Red Rock conglomerate (Paleocene ?); 

 Tbb, Muddy Creek basin beds. 



Fault Patterns 



Two kinds of patterns appear at present to exist. The one consists of 

 subparallel faults which define graben, horsts, and tilted blocks, and the 

 other of faults in semicircular or polygonal form which bound completely 

 or nearly completely downfaulted blocks. The two are illustrated in 

 Fig. 31.16 of the Owens Lake-Mono Lake region of California. 



Mono Lake Basin 



Mono Lake is in a somewhat triangular-shaped basin about 15 miles in 

 length at the eastern foot of the Sierra Nevada. As a result of gravity and 

 seismic studies Pakiser et al. (1960) conclude that nearly vertical faults 

 bound the triangular-shaped block, and that it has subsided 18,000 ± 5000 

 feet and has received about 300 ^ 100 cubic miles of light clastic sedi- 

 ments and volcanic material of Cenozoic age. The nature of the gravity 

 profile and the interpreted geologic section on the northwest side are 

 shown in Fig. 31.17. A section across the entire basin is given in Fig. 31.18. 

 It will be seen that the basin fill is divided into layered deposits, a lower 

 thick one of relatively high velocity (7800-10,800 feet per second) and 

 an upper thin one ( 2000 feet ) of low velocity ( 5500-6200 feet per second ) . 

 The recent deposits have not been displaced by faulting and conceal the 

 buried faults. The lower deposit is believed to be mostly Tertiary volcanic 



