COAST RANGES OF THE PACIFIC AND THE SAN ANDREAS FAULT SYSTEM 



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Rockies. The Coast Ranges have a more heterogeneous basement, which 

 has served to localize the thrusts; the strata in them are generally less 

 indurated; the scale is somewhat smaller; and the movement along the 

 thrust surfaces is generally less. 



In addition to the thrusts, there are transverse faults, some of which 

 cut almost completely across a range. They relate to the uplift of the Santa 

 Lucia Range, because in its southern part, each transverse fault is down- 

 thrown on the south, and the range becomes progressively lower in eleva- 

 tion in that direction ( Taliaferro, 1943b ) . 



There was little volcanism in the central Coast Ranges during the 

 Pleistocene as compared with the extensive and important volcanism in 

 the Sierra Nevada Range and in the Cascades. Olivine basalt flows and 

 agglomerates occur in the Santa Lucia and Diablo ranges and along the 

 east side of Santa Clara Valley. 



The Tertiary structural history was much like that of the Late Jurassic 

 and Cretaceous in the following respects. The Orogeny was generally 

 severest westward, because the unconformities are more angular and 

 bring rocks of greater age differences together the farther west from the 

 Great Valley they are observed; and volcanism continued, with tuffs and 

 flows a characteristic part of middle Eocene, upper Eocene, lower Mio- 

 cene, middle and upper Miocene, lower and middle Pliocene, and Pleisto- 

 cene formations. 



The mid-Pleistocene orogeny occurred farther inland (eastward) than 

 the late Pliocene orogeny and is a contrary note to the generalization of 

 increasing intensity westward. However, the two disturbances are closely 

 connected in time and may be part of a general wave of deformation 

 originating in the west and progressing eastward. 



The mid-Pleistocene disturbance is associated with the final disappear- 

 ance of the Tertiary troughs of deposition and the foundering of consider- 

 able segments of the Coast Ranges into the Pacific. It is evident from 

 inspection of the tectonic and geologic maps that the sea has transgressed 

 part of the Coast Range orogenic belt; the structures are discontinuous 

 at the present shore line. Also, the reconstructed Tertiary uplifts and 

 troughs head out to sea, as if only half exposed in the Coast Ranges. 

 Recent detailed mapping of the ocean floor off California has revealed a 



topography much like that in the Coast Ranges, and it can best be ex- 

 plained as the surficial expression of the long-evolving volcanic archipel- 

 ago of Paleozoic, Mesozoic, and Cenozoic time, with particular respect to 

 the late Pliocene and Pleistocene deformations. The interpretation of the 

 topography of the sea floor will be taken up later in a separate chapter. 

 The San Andreas fault, that stretches through the three divisions of the 

 Coast Ranges of California will be considered later. 



Erosion Following Main Orogeny 



Following the late Pliocene and mid-Pleistocene orogeny, which re- 

 sulted in rapid uplift and oversteepening of the mountain fronts, vigorous 

 erosion reduced the escarpments and ranges until now there is little 

 physiographic evidence left of individual faults, although some of them 

 were of several thousand feet displacement. Conspicuous features of the 

 rapid erosion are the landslides from the oversteepened mountain fronts. 

 Some were gigantic in size and took place coincident with the thrusting 

 and uplift of the ranges; others have occurred since. In places, there is a 

 definite sequence of slides observable, detected by different amounts of 

 dissection. They obscure the true structure of the mountain front in many 

 places. 



Late Pleistocene and Recent Gentle Folding 



In the Los Angeles, Ventura, and San Joaquin basins, gentle folds have 

 developed so recently that they have been little modified by erosion, and 

 precise elevation surveys show that movement is still going on vigorously. 

 The subject will be taken up at greater length under the next major head- 

 ing, "Southern Coast or Transverse Ranges of California." 



Terraces 



Terraces are numerous and well developed along the shore and in in- 

 terior valleys of the Coast Ranges. The marine terraces are found at 

 elevations up to 1500 feet, and attest the rise of the Coast Ranges in very 

 recent times. They are cut on the beveled edges of the folded Plio- 

 Pleistocene sediments, and therefore are very young. Individual terraces 

 are difficult if not impossible to follow from one region to another, and 



