92 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



A very long strike-slip fault, named the Rincon 

 fault, branches eastward from the Soda Spring fault 

 on the north slope of Soda Spring Canyon about \\/<* 

 miles upstream from the mouth. It trends about east- 

 southeast to the saddle south of El Sombroso, and 

 from there it extends more nearly due east for about 

 3 miles. It merges into the Berrocal fault zone about 

 half a mile northeast of Bald Mountain. A branch of 

 this fault continues on the east-southeast course from 

 the ridge south of El Sombroso for more than 2 miles, 

 and apparently dies out in a fold in the thick body of 

 clastic sedimentary rocks underlying the upper course 

 of Guadalupe Creek. The Rincon fault offsets basaltic 

 greenstone at a very low angle to the strike, and both 

 the main and the branch faults seem to cut off several 

 folds. Both faults have rather striking topographic 

 expression in some places, the best examples being 

 unusual right-angle turns and abrupt changes in gra- 

 dient in the upper tributaries of Soda Spring Canyon. 

 The chief evidence for it being a right-lateral fault is 

 the 1-mile offset of the greenstone northwest of Bald 

 Mountain and its relation to other right-lateral faults. 



Another major fault within the El Sombroso block, 

 named the Limekiln fault, is roughly parallel to the 

 Rincon fault and about half a mile north of it. Evi- 

 dence for its existence is largely based on a straight - 

 line offset of the limestone and tachylite horizons to 

 the west, a straight-line contact between greenstone 

 and clastic sedimentary rocks along its eastern part. 

 and anomalies in the attitude of adjacent beds. Only 

 here and there does this fault have any topographic 

 expression. The displacement along the Limekiln 

 fault is problematical. Because of its relation to the 

 Ben Trovato shear zone and its near parallelism with 

 the strike-slip Rincon fault, one is tempted to assume 

 it has similar movement along it. If it has chiefly 

 strike-slip movement, however, the displacement of the 

 limestone and greenstone near the west edge of the 

 district would indicate left-lateral movement, which 

 is contrary to the movement along the other strike-slip 

 faults. This apparent conflict may be the result of 

 sizable dip-slip movement more than compensating the 

 displacement due to strike-slip movement, for this 

 fault does parallel others in the Blossom Hill area 

 which have chiefly dip-slip movement. No uniform 

 dip-slip movement, however, will account for the ap- 

 parent offsets, and we are inclined to believe it really 

 has a large component of left-lateral displacement. 



The three strike-slip faults that cut the Los Capi- 

 tancillos block strike more nearly northwest than 

 those in the El Sombroso block and cut across the 

 main folds at wider angles. The most westerly of 

 these is the Enriquita fault, which trends about N. 



30 W., on the average, from the Guadalupe Reser- 

 voir across Los Capitancillos Ridge to the canyon be- 

 low the Senator mine. The north end of the En- 

 riquita fault is complicated by a structural "knot" 

 of serpentine and other rocks, but it is evidently cut 

 off by the post-Miocene Shannon fault. Along much 

 of its length the trace of the Enriquita is marked by 

 slivers of serpentine believed to have been dragged 

 along the fault, or squeezed into it, and judging by the 

 dip of these masses the fault dips eastward at a mod- 

 erately steep angle. The strike-slip offset along this 

 fault is believed to be about 9,000 feet, the western 

 side having moved northward. This estimate is based 

 on the offset of the serpentine sills, of the greenstone 

 body marking the south limb of the anticline, and of 

 the northern limit of the Ben Trovato shear zone. The 

 amount of dip-slip movement cannot be determined, 

 but is believed to be small. 



Parallel to the Enriquita fault and about li/ 2 miles 

 east of it is another strike-slip fault, the Almaden 

 fault, which extends southward from the Shannon 

 fault to the Mine Hill area. The apparent offset of 

 this fault is best indicated just south of where it 

 crosses a small tongue of alluvium on the border of 

 the Santa Clara Valley; a greenstone band on the 

 west side of the fault is here shifted 1,800 feet north- 

 ward from its counterpart on the east side. Within 

 the New Almaden mine area mapped in detail the 

 fault could not be followed on the surface: however, 

 it seems to have been exposed in some of the mine 

 workings that are now inaccessible. On the ridge 

 north of Deep Gulch, thin beds of chert on the east 

 side of the fault as projected are folded in such a 

 manner as to take up most of the strike-slip offset 

 indicated along the traceable part of the fault. 



Three branching strike-slip faults occur near the 

 east edge of the district in the Los Capitancillos block. 

 The largest of these, the Calero fault, is exposed in 

 the south wall of Llagas Canyon at the edge of the 

 district and trends N. 40 W. to and beyond Calero 

 Dam, where it disappears beneath the valley alluvium. 

 This fault is marked by a straight -line contact be- 

 tween tachylitic greenstone and graywacke a little 

 southeast of the Calero Reservoir: it also appears to 

 offset the limestone, and it has striking topographic 

 expression in the upland area north of Llagas Creek. 

 where it follows straight stream courses and passes 

 through marked saddles in two ridge tops. The hori- 

 zontal component of movement along it, as indicated 

 by the offset of the limestone beds, is about. 3,500 feet, 

 with the southwest block shifted to the northwest rela- 

 tively to the northeast block. No definite data on the 

 vertical component of movement on the fault are ob- 



