94 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



The rocks underlying the western part of the Santa 

 Teresa Hills, also, are divided into elongate blocks by 

 parallel and anastomosing faults, which trend about 

 east and have steep dips. The evidence for these 

 faults is stratigraphic, for none of them have more 

 than minor topographic expression. The amount of 

 movement along them, if it is largely vertical, appears 

 to be of the same magnitude as that along the Shannon 

 branch faults of the Blossom Hill area, though the 

 movement on the faults separating rocks of the Fran- 

 ciscan group from younger rocks could be greater. 

 The faults on the Santa Teresa Hills cut no rocks 

 younger than Eocene, and can therefore be dated only 

 as post-Eocene, but their similarity to the Shannon 

 faults makes it likely that they developed in early 

 Pliocene time or shortly thereafter. 



The braid of faults in the area just southwest of 

 the Ben Trovato fault zone is so complex and con- 

 tains so many short faults that they cannot all be de- 

 scribed individually. Each involves rocks of Miocene 

 and Franciscan age, or is traceable into a fault that 

 does involve them, and all are proved by stratigraphic 

 displacement as well as indicated by topographic ex- 

 pression. The faults appear to be steep, and both 

 normal and reverse displacement of several hundred 

 feet have been recognized in this area of elevated, de- 

 pressed, and tilted blocks. The extension of this zone 

 of faults southeast of the area underlain by the for- 

 mations of Miocene age cannot be traced with cer- 

 tainty, because in the absence of younger rocks there 

 is no way of separating post-Miocene faults from older 

 ones. Topographic evidence and the arrangement of 

 geologic contacts suggest, however, that the well- 

 defined post-Miocene fault separating rocks of the 

 Temblor formation from the Ben Trovato fault zone 

 continues along the edge of the shear zone southwest 

 of Mine Hill, and crosses the south boundary of the 

 district in Llagas Canyon. Another fault that may be 

 of about the same age extends southeast from Twin 

 Creeks in Almaden Canyon to the edge of the district, 

 and still another runs parallel to it about 1,000 feet 

 farther southwest. 



FAULTS DUE TO INTRUSION OF SERPENTINE 



The development of the alta along contacts of the 

 serpentine bodies as a result of their emplacement has 

 l>een discussed on pages 18 and 19. The alta has been 

 compared to a fault gouge, and the contacts along 

 which it formed might all be regarded as faults, for 

 they are surfaces dividing two rock masses that have 

 moved relatively to each other (figs. 63. 64). These 

 contacts, however, may also be regarded as intrusive, 

 even though the serpentine at the time of intrusion 



FIGURE 63. Fairly regular contact between serpentine (now altered to 

 silica-carbonate rock) below and alta above. Even though the alta 

 resembles fault gouge, contacts of this type are shown as intrusive 

 contacts on the maps accompanying this report, because the alta IB 

 believed due to Intrusion rather than to later faulting. 



FIGURE 64. Irregular contact between serpentine, now converted to 

 silica-carbonate rock (MCI. and alta exposed In underground workings 

 of the New Almadeu mine. 



was a plastic mass rather than a magma. On the 

 maps and cross sections accompanying this report 

 such contacts are shown as intrusive contacts unless 

 there is evidence of post intrusive faulting along them. 

 Faults that differ in that they cut across the alia 

 along the niarigns of some of the serpentine bodies 

 are also thought to have resulted from forces created 

 by the intrusions. These faults ace largely normal. 

 though a few are reverse, and the displacement along 

 them generally does not exceed 10 feet. Their effect 



