FAULT FEATURES OF S ALTON BASIN, CALIFORNIA 223 



FAULTS SOUTHWEST OF SALTON BASIN 



The faulting along the southwestern side of the basin is appar- 

 ently of two ages, and took place in somewhat different directions, 

 the intersection of the two fault systems probably accounting for 

 the present irregular outline of the basin. Thus great mountain 

 salients, such as the Santa Rosa Mountains, the Vallecito-Fish 

 mountain spur, and the projection of the Peninsular Mountains 

 along the Mexican Boundary, are separated by big re-entrant valleys, 

 such as San Felipe Valley and the Carrizo Creek Valley. 



The oldest of these fault systems, if this inference is correct, has 

 a strike about N. 10 W. and is represented by three notable escarp- 

 ments. One lies along the east base of the San Jacinto Mountains, 

 passing up Palm Canyon. Another is at the west side of Borego 

 Valley. The third extends from Agua Cahente Springs southward 

 up Carrizo Gorge, along the east face of the Laguna Mountains. 



Evidence of a fault along the northeast and east face of Mount 

 San Jacinto was obtained near Whitewater. Just west of White- 

 water Point the mountain face is composed of pink and gray granite, 

 and of a grayish marble. The marble and granite are arranged 

 in layers turned on edge with a strike about N. 20 W., and a dip of 

 75° or more to the northeast. Although the quantity of marble is 

 much less than that of the granite it constitutes a considerable part 

 of the mountain mass, the layers ranging from a few inches to 50 

 feet in thickness. The alternation of rocks is well exhibited in a 

 prospect tunnel in Sec. 23, T. 3 S., R. 3 E., where the material pene- 

 trated has the appearance of a gigantic fault breccia, and the 

 contact surfaces are abundantly slickensided. This intermixture 

 of material probably resulted from step-faulting, the successive 

 breaks along many parallel lines causing an intimate mixture of the 

 different rocks. Associated with this prominent fault line is the 

 warm spring at Palm Springs, whose water is believed to be derived 

 from granitic rocks^ 



West of Borego Valley no observations were made to confirm 

 the existence of a fault except to note that the mountain front in 

 that region is a very steep and straight escarpment from 2,000 to 

 4,000 feet in height. 



' The inference is based on unpublished analytical data. 



