COAST RANGES OF THE PACIFIC AND THE SAN ANDREAS FAULT SYSTEM 



45'; 



Tms 



Tinm- 



Tmm -^2 



Fig. 29.7. Evolution of the Castle Mountain Range. Ideal sections showing late Upper Miocene 

 folding (lowest section), erosion and deposition of McLure shale (middle section, latest Upper 

 Miocene), and thrusting and folding in late Pliocene (upper section). The marginal thrusts of 

 the Castle Mountain Range developed in the sites of the Upper Miocene anticlines. More 

 thrusting occurred in mid-Pleistocene which is not represented. After Reed and Hollister, 1936. 

 Section H-H', Fig. 29.1. Tms, Santa Margarita sandstone; Tmm, MsLure shale; Tp, Purisma fm. 

 ] 



"Margarita sands 100 to 300 feet thick of late Miocene age were deposited 

 lover a fairly even-floored basement complex consisting chiefly of Fran- 

 jciscan, but with remnants of Knoxville, Shasta, and Upper Cretaceous 

 .sediments. After, or perhaps even during the deposition of these sands, 

 igentle anticlinal folding occurred along two subparallel lines 6 to 8 miles 

 apart, which correspond approximately to the present margins of the 

 range. The maximum observed dip of the flanks is 11 degrees. The two 

 (anticlinal ridges were planed off, perhaps as rapidly as they rose, and the 

 McLure shale was then deposited over the region. Where it crosses the 

 ;two anticlines, it lies unconformably on the Santa Margarita sands and on 

 the Franciscan. Elsewhere, the Santa Margarita and McLure are conform- 

 able, and in places they appear to grade into one another. In the later 



Pliocene and Pleistocene deformation, thrusts developed approximately 

 in the sites of the anticlines. 



Late Pliocene and Mid-Pleistocene Phases 



The thick accumulation of Miocene sediments was accentuated, in gen- 

 eral, by further deposition in the same troughs in early and middle Plio- 

 cene time. The gentle compressive movements, which started in the 

 Miocene and then relaxed for a while, surged to a peak in the late Pliocene 

 and again to another peak in the mid-Pleistocene. The last surge is prob- 

 ably still climactic. 



The folds and thrust faults that are the conspicuous features of cross 

 sections and field observation are largely the result of these two move- 

 ments. Cross sections D-D', F-F', G-C, and I-I' of Fig. 29.8 are espe- 

 cially illustrative of the compressional deformation to which the rocks of 

 the Diablo uplift and the San Joaquin embayment in the central Coast 

 Ranges were subjected. 



Opinions differ as to the relative importance of the two phases. In some 

 places, only one has been recognized. According to Taliaferro ( 1943b ) , the 

 geologists in general who have worked in the western part of the Coast 

 Ranges have emphasized the importance of the late Pliocene disturbance 

 there, and those who have worked chiefly in the eastern part have stressed 

 the mid-Pleistocene compression. 



The regions underlain at comparatively shallow depths by crystalline 

 rocks, or those where the crystalline rocks were exposed, yielded by fault- 

 ing; and those underlain by thick sections of strata (8000-20,000 feet) 

 yielded by folding and thrusting ( Taliaferro, 1943b ) , except for the eastern 

 part of the San Joaquin embayment which was left little deformed and is 

 now the Great Valley. The ranges are generally bordered by thrusts, but 

 the individual thrusts can be traced only 20 to 25 miles. As a thrust dies 

 out, its place is commonly taken by one or more en echelon faults. The 

 thrusting is both westward and eastward, with some structural units 

 (ranges) being bordered by complementary inward-dipping faults. The 

 thrusts marginal to the structural units generally have shallower dips than 

 those within. The structure of the Central Coast Ranges as interpreted in 

 the cross sections is rather similar to that of the Montana and Alberta 



