GEOLOGIC STRUCTURE 



93 



tainable. About half a mile northwest of the Calero 

 Dam the Calero fault sends off a branch that trends 

 east-southeast along the northern edge of Calero Res- 

 ervoir to the east edge of the district. The evidence 

 for this fault is not strong, but part of its course is 

 marked by straight canyons and part by a difference 

 in the color of the residual soil on opposite sides. 

 Still another branch goes northwestward from the 

 Calero fault at Llagas Canyon, and may reach Al- 

 maden Canyon at the sharp bend 1 mile below the 

 Hacienda. It is marked by lenticular bodies of ser- 

 pentine and a little silica-carbonate rock. 



The age of the strike-slip faults in the Los Capi- 

 tancillos block is probably about the same as that of 

 the shear zones and is definitely earlier than middle 

 Miocene. The Enriquita fault and several parallel 

 shorter faults appear to merge into the Ben Trovato 

 fault zone, as though they were contemporaneous with 

 movements along it, but they also seem to displace 

 the northern margin of the shear zone. Some of the 

 strike-slip faults cut serpentine, but others seem to 

 have been intruded by serpentine. Locally, the ap- 

 parent intrusion of serpentine into them can be inter- 

 preted as due to drag, but elsewhere, as in the south- 

 eastern part of the New Almaden mine area, this ex- 

 planation seems less probable than simple intrusion. 

 These bits of evidence, though inconclusive, suggest 

 that the strike-slip faults of the Los Capitancillos 

 block were formed before the end of the period of 

 normal intrusion of serpentine, and soon after the ini- 

 tial development of the major shear zones, in Late 

 Cretaceous time. 



DIP-SIJP FAULTS 



Faults with dip-slip movement are widely distrib- 

 uted through the district, and unlike the strike-slip 

 faults, they cut all the rocks older than the Quater- 

 nary alluvium. These faults differ widely in amount 

 and direction of movement; a few are reverse faults, 

 but most of them are normal. The available evidence 

 for dating them places the age of faulting only within 

 broad limits; most of them, however, are later than 

 late Miocene, and on a few there was recurrent move- 

 ment in Pliocene time. 



A post-Cretaceous reverse (?) fault near the south 

 boundary of the district forms the contact between the 

 Upper Cretaceuos rock of the Sierra Azul and the 

 Franciscan group and serpentine. This fault, which 

 has been named the Sierra Azul fault, extends for a 

 distance of about 21/2 miles from the slope east of the 

 easternmost tributary of Almaden Canyon, and reaches 

 the main crest of the Sierra Azul about 3,800 feet 

 south of Mount Umunhum. Its general course is 

 about west -northwest ; its dip is to the northeast. The 



fault cuts off structures in the two formations sepa- 

 rated by it, but has little topographic expression. 

 From the relative ages of the rocks that have been 

 brought together by the fault, one can tell that the 

 southwest block has been depressed relatively to the 

 northeast block, which suggests it is a reverse fault, 

 but the horiozntal component of movement is not 

 known. 



The most extensive post-Cretaceous fault in the dis- 

 trict is the Shannon- fault, which trends a little north 

 of west across the entire district and separates the 

 Los Capitancillos block from the Santa Teresa block. 

 This fault is covered at the east edge of the district 

 by alluvium in a small embayment from the Santa 

 Clara Valley ; to the west it is exposed south of Coyote 

 Peak, but again becomes buried under the alluvium of 

 Alamitos Creek north of the Calero Dam; it emerges 

 east of the Senator mine, crosses the hills north of the 

 mine, and follows valleys down to Guadalupe Canyon ; 

 from Guadalupe Canyon it continues westward be- 

 yond the west boundary of the district. Several other 

 faults diverge northward from the Shannon fault in 

 the west half of the district, and west of the Guada- 

 lupe mine there is a mile-long interval wherein the 

 Shannon fault gives way to a complex braid of faults. 



The dip of the main Shannon fault, and that of 

 most of its branches, is apparently steep to the north, 

 and the movement is believed to be largely down the 

 dip. Some of its smaller branches, however, seem to 

 be reverse faults. The net downthrow to the north 

 cannot be determined accurately, but in places it cei: 

 tainly amounts to several hundred feet. The faulting 

 is believed to have occurred in the early or middle 

 Pliocene time. 



The Shannon fault and its branches are represented 

 in the area west of Los Gatos Creek by five parallel 

 faults of comparatively small displacement. These 

 faults were mapped largely on the evidence afforded 

 by straight-line stream valleys and other topographic 

 anomalies, developed on a surface underlain by the 

 gravels of the Santa Clara formation. They are ob- 

 viously younger than the Pliocene and Pleistocene 

 Santa Clara formation, which they offset, and older 

 than the Quaternary alluvium filling the Santa Clara 

 Valley. They strike about N. 50 W., and the one 

 farthest north dips northeastward and is normal. The 

 dips of the others were not observed, but on all of 

 them the downthrow appears to be to the north, and 

 they probably are all normal. Because of the relative 

 recency of these faults and their small offsets, they are 

 believed to represent minor recurrent movement along 

 branches of the older Shannon fault. 



