GEOLOGIC STRUCTURE 



91 



where it is not followed by the drainage. In detail, 

 the topography along the shear zone tends to be ir- 

 regular, with scattered knobs and knolls, these irregu- 

 larities being largely the result of differences in re- 

 sistance between the included rock masses and the 

 sheared matrix. Landslides develop readily in the 

 parts of shear zones underlain by matrix material con- 

 taining a high proportion of shale. 



The regional effect of the Ben Trovato fault zone is 

 fairly well illustrated by the position of the limestone 

 and tachylite key horizons in the Franciscan group. 

 (See fig. 56.) The discontinuous limestone horizon 

 trends irregularly from the western slope of St. Jo- 

 sephs Hill at the west edge of the district eastward 

 and northeastward to Shannon Road, where it en- 

 counters the shear zone, within which it is repre- 

 sented only by fault-bounded blocks. In the central 

 part of the district the limestone also occurs only in 

 heterogeneously oriented blocks within the shear zone. 

 Near the southeast corner of the district the limestone 

 forms a nearly continuous ledge along the headwaters 

 of Longwall Canyon on the north edge of the shear 

 zone, but isolated boulders are found in the zone even 

 farther to the southeast. Thus, only relatively short 

 segments of the limestone stratum at either end of the 

 district are unaffected by the shear zone. 



To judge from the position of the limestone on 

 opposite sides of the fault zone, the apparent hori- 

 zontal offset is about 10 miles, the southwest side 

 having moved northwest relative to the northeast 

 side. The real offset, however, may not be so great, 

 for the shear zone does not diverge greatly from the 

 regional strike, and the limestone may have been 

 folded into partial parallelism with the shear zone 

 before the shearing took place. The tachylite which 

 underlies the limestone is distributed similarly to the 

 limestone and apparently was affected by the shearing 

 in much the same way. 



The northern shear zone, passing near Coyote Peak, 

 does not cut any good horizon markers, and there 

 seems to be no way of determining the direction and 

 amount of movement along it. 



Both fault zones must be post-Franciscan, for they 

 involve all the rock types of the Franciscan group. 

 The Ben Trovato fault zone is clearly older than 

 middle Miocene, for it is unconformably overlain by 

 rocks of the Monterey at its northwest end. The 

 Coyote Peak fault zone trends west to an area under- 

 lain by the Upper Cretaceous rocks of the Santa 

 Teresa Hill, but these rocks are not sheared and ap- 

 pear to overlie the shear zone unconformably. The 

 shear zones are thus at least as old as the Upper Cre- 

 taceous rocks found in the Santa Teresa Hills, but 



their age in relation to the Upper Cretaceous rocks 

 of the Sierra Azul, which are probably a little older, 

 cannot be determined directly because the shear zones 

 nowhere border these rocks. Chiefly because the Fran- 

 ciscan rocks throughout the area are more sheared 

 than are the Upper Cretaceous rocks of the Sierra 

 Azul, we suspect that the major shear zones were first 

 developed before the deposition of the Upper Cre- 

 taceous rocks of the Sierra Azul. 



STRIKE-SLIP FAULTS 



The New Almaden district contains at least six long 

 faults that are parallel to the general strike of the 

 beds that they traverse or intersect at low angles. 

 The movement along these faults is believed to have 

 been largely horizontal, and they differ from the pre- 

 viously described shear zones mainly in the narrow- 

 ness of the band of rock effected by movement. Three 

 of the faults, and some splits from them, are in the 

 El Sombroso block, and three in the Los Capitancillos 

 block. They involve only rocks of the Franciscan 

 group. 



The Soda Spring fault, in the El Sombroso block, 

 extends for more than 8 miles across the southwest 

 corner of the district. It trends about S. 60 E., 

 nearly parallel with the Ben Trovato shear zone, from 

 a point about 1 mile south of Los Gatos to a point 

 in the middle fork of Almaden Canyon about ! 3 /4 

 miles upstream from Twin Creeks. Its dip, as de- 

 duced from its slight swing toward the northeast in 

 crossing canyons, is believed to be steep to the north- 

 east. The fault does not cut any of the key horizons 

 in Franciscan group, though it does appear to termi- 

 nate several folds. The strongest evidence for its 

 existence is its topographic expression : much of the 

 middle and upper course of Soda Spring Canyon 

 appears to follow the fault line and the saddle be- 

 tween Mount Umunhum and the backbone of Sierra 

 Azul Ridge falls on it; also, the abnormally steep 

 scarps to the northwest of Priest Rock and southeast 

 of Mount Umunhum are apparently due to the pres- 

 ence of this fault. It is believed to be a right lateral 

 fault because of the drag folding expressed by dips in 

 the beds in the upper part of Almaden Canyon and 

 Soda Spring Canyon. 



A short fault branches from the Soda Spring fault 

 1 mile west of Mount Umunhum and extends gener- 

 ally N. 75 W. for about li/ 2 miles along the crest of 

 the Sierra Azul. It separates greenstone from clastic 

 sedimentary rocks, and is marked by minor irregulari- 

 ties in the topography and abrupt changes in the atti- 

 tudes of the beds. 



