90 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



on its limbs do not exceed 60, and are little steeper 

 than dips near the trough. Other folds mapped in 

 several of the smaller Miocene fault blocks of the dis- 

 trict are similar, but some in the Berrocal fault zone 

 are tighter. (See section B-B', pi. 1.) 



Mini i it Clara formation 



No folds, either small or large, have been observed 

 in the Santa Clara formation, but at several places 

 near faults these relatively young gravels appear to 

 have been tilted. The greatest inclination was seen 

 in the high terraces just south of Los Gatos, where 

 dips as great as 18 were recorded. 



FAULTS 



The faults in the New Almaden district, like the 

 folds, vary considerably in size and character; they 

 include major and minor shear zones, strike-slip faults, 

 normal and reverse faults, minor faults due to intru- 

 sions, and widely distributed but uniformly oriented 

 tension fractures. Of these, only the shear zones, 

 strike-slip faults, and tension fractures appear to have 

 any relation to the development of the quicksilver ore 

 deposits, and only the tension fractures are known to 

 cut the ore bodies. 



Only three periods of extensive faulting in the area 

 can be demonstrated, but there may have been others. 

 The oldest faulting is believed to have closely followed 

 the deposition of the lower Upper Cretaceous (Ceno- 

 manian) Franciscan rocks, for it has affected the rocks 

 of this group but not the Upper Cretaceous rocks of 

 the Sierra Azul, which are believed to be somewhat 

 younger. This faulting is characterized by the devel- 

 opment of shear zones and faults with strike-slip move- 

 ment. The youngest faulting, the normal faulting in 

 the Santa Teresa block, offset the Santa Clara forma- 

 tion and therefore must be as recent as Pleistocene. 

 The offsets of the Santa Clara formation, however, 

 are apparently not so great as the offsets of the Mio- 

 cene rocks in the Blossom Hills along the same and 

 related faults, and this difference indicates an exten- 

 sive period of faulting in early or middle Pliocene 

 time. On some of the older faults also recurrent 

 movement has obviously taken place, and has served 

 to confuse the record of faulting in the area. 



In the following paragraphs each type of fault is 

 described, the available data on direction and amount 

 of movement are recorded, and the evidence for the 

 dating of each type is considered. 



SHEAR ZONES 



Two extensive zones of sheared, squeezed, and broken 

 rocks are among the largest, yet least obvious, struc- 

 tural features in the district. Similar shear zones are 



rather characteristic of the Franciscan group through- 

 out the Coast Ranges of California, and have been 

 described (Eckel and others, 1941, p. 533-535; and 

 Bailey, 1946. p. 209-210) as occurring in several other 

 widely separated quicksilver districts. In the New 

 Almaden district the larger of these zones, named the 

 Ben Trovato fault zone, extends from the extreme 

 southeast corner of the district with a general trend 

 of N. 60 W. through the heart of the area to Shan- 

 non Road, where it is cut off by a younger fault 

 a total distance of more than 11 miles. Its maximum 

 width, as exposed in the southeastern part of the dis- 

 trict, is about 4,500 feet, but to the northwest it is 

 appreciably narrower. A second shear zone, named 

 the Coyote Peak fault zone, extends X. 75 W. from 

 the vicinity of Coyote Peak at the eastern edge of the 

 district, for more than 4 miles: its west end is buried 

 under the alluvium of Alamitos Creek. It attains its 

 maximum width of about 2,000 feet just south of 

 Coyote Peak. The dip of neither zone can be deter- 

 mined with certainty, but both appear to be nearly 

 vertical. 



The shear zones consist chiefly of a sheared matrix 

 of the weaker clastic sedimentary rocks surrounding 

 irregular inclusions of hard graywacke, greenstone, 

 limestone, chert, and glaucophane-bearing metamor- 

 phic rocks (pi. 1). The inclusions are a few feet to 

 2,000 feet in greatest diameter, and in shape they range 

 from equidimensional blocks to thin sinuous lenses. 

 They commonly have slickensided surfaces, although 

 internally many are little sheared or otherwise de- 

 formed. Outcrops in these zones consist largely of the 

 isolated blocks, but in a few places the matrix is well 

 exposed, notably along the sides of the Almaden Res- 

 servoir; in these exposures it is extremely sheared, 

 plastically deformed, and tightly folded. The borders 

 of the shear zones must necessarily be placed some- 

 what arbitrarily, as the highly deformed rocks grade 

 imperceptibly into less deformed rocks of the Fran- 

 ciscan group. In many places the apparent margin 

 is highly irregular, but the southwestern boundary of 

 the Ben Trovato zone is sharp in part where formed 

 by a post-Miocene fault. Serpentine has been in- 

 truded into the western part of the shear zone in the 

 Santa Teresa Hills, but it is strangely scarce in the 

 Ben Trovato zone. 



The shear zones are marked by rather distinct vege 

 tation and topography: in most places they underlie 

 grasslands that extend over ridges and valleys, cut- 

 ting a wide swath through the generally brushy ter- 

 rain. In part of the area, as, for example, south of 

 the Ix)s Capitancillos Ridge, the path of the Ben Tro- 

 vato zone is marked by longitudinal canyons, but else- 



