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STRUCTURAL GEOLOGY OF NORTH AMERICA 



believed to be the offset of the other (Hill and Dibblee, 1953). The 

 shift to the northwest has been about 5 miles. See A-A', Fig. 29-13. If 

 such is true, and if the horizontal displacement along the San Andreas has 

 been in the order of 300 miles, then the San Andreas is much older and 

 had been active a long time before the Big Pine-Garlock fault came into 

 existence. The 5 mile offset would indicate that the age is Pleistocene. 

 There is no question about the recency of activity along the Big Pine and 

 Garlock faults, but the time of beginning may be suspect. It could be that 

 the Big Pine fault originated many miles to the south and by coincidence 

 is now about opposite the Garlock. 



Hill and Dibblee believe the strain system of the Big Pine-Garlock 

 shear and the San Andreas shear is a conjugate or complementary one 

 with the south wedge moving against the north wedge. Moody and Hill 

 (1956) elaborate on the stress-strain relations of the San Andreas system 

 in which they call the strike-slip faults of large displacement "wrench 

 faults." They develop second and third order effects and believe they 

 demonstrate at least eight directions of wrench faulting and four direc- 

 tions of folding or thrusting possible. They conclude that dynamically 

 the orientation of the Garlock is not correct for the primary left lateral 

 direction, and it would more nearly fit a theoretical position for a second- 

 order left lateral fault, assuming north-south compression. The Trans- 

 verse Range may represent the primary fold direction, consequently 

 shortening the crust in this area and altering the San Andreas direction 

 (Moody and Hill, 1956). 



Relation to Pacific Fracture Zones 



Great fracture zones trend generally westward from the United States, 

 Mexico, and Central America across the Pacific. These are depicted in 

 Chapter 32, and their relation to the fault system of California is shown 

 in Fig. 32.15. The relation of the two systems is an enigma. 



Origin of Gulf of California 



Recent seismic work in the Gulf of California has shown the crust 

 there to be oceanic (H. W. Menard, personal communication), and conse- 



quently one's impulse is to postulate drift of the peninsula away from the 

 mainland. Not only westward but northwestward drift compatible with 

 movement along the San Andreas fault must be postulated. If the Coast 

 Ranges oceanward of the San Andreas fault and the Peninsular Ranges 

 with Baja California are moved as a unit southeastward in the amount of 

 movement proposed by Hill and Dibblee from the Cretaceous to the 

 present, the Baja California is brought into a likely former position with 

 the mainland. The Nevadan belt of the Sierra Madre del Sur would con- 

 tinue in this arrangement without break into Baja California, as postulated 

 in Chapter 38. The restored relations are shown in Fig. 29.13. 



Two difficulties appear; the long unit has to be bent slightly to make 

 the fit, and it has to snake around the major bend of the San Andreas 

 fault east of the Los Angeles Basin in making its way to the northwest. 

 The passage is accomplished in a more straight-away course if a good 

 deal of the movement occurred along the Elsinor, San Jacinto, and San 

 Gabriel faults. Hill and Dibblee have commented that the San Gabriel 

 fault may have been principally active in the past. It seems possible that 

 the segment of the strip now making up southern California has been 

 pressed somewhat against the continent since late Miocene time, and 

 although right lateral movement has continued along the San Andreas 

 fault that the folds and thrusts of the Transverse Ranges were thereby 

 formed. If a subcrustal convection current is carrying the strip north- 

 westward, then the current might have become a little deflected toward 

 the continent in the southern California region and the unusual complex 

 of structures formed there. We could imagine that the Elsinor fault first 

 carried the brunt of the dislocation, then the Jacinto, and finally the San 

 Andreas through the Salton Sea area, as the compressive component of 

 the carrying force increased against the continent. All these faults are 

 still comparatively active. 



Seismicity in the Coast Ranges 



Figure 29.14 shows the general seismicity of the California region. The 

 epicenters are scattered through the San Andreas fault zone more widely 

 than might be expected, yet there is a general clustering along the great 

 fault. The Agua Blanka fault of northern Baja California is believed to 



