GEOLOGIC STRUCTURE 



79 



axial planes ,or plunges of folds. As an additional 

 structural complication, most of the large faults that 

 cut the rocks form wide shear zones that are not obvi- 

 ous in the field, and most of them strike nearly paral- 

 lel to the bedding. Many of these faults are believed 

 to have a large component of strike-slip displacement. 

 Some of the difficulties can be minimized by the 

 tedious and time-consuming process of following out 

 in detail all contacts between major types of rocks, 

 and that, was done in mapping the mineralized north- 

 east half of the district; but even after the distribu- 

 tion of the greenstones and sediments was known, 

 many important structures were not obvious. In at- 

 tempting to further unravel the structure of the New 

 Almaden district an additional refinement was made 

 tentatively by distinguishing varieties of greenstone, 

 such as tuff, breccia, pillow lava, and tachylitic rock, 

 and also by trying to draw fine distinctions in the 

 clastic sediments. We found that the greenstones 

 could be fairly well divided into three groups tachy- 

 litic rocks, mafic tuffs, and more massive lavas but 

 we were unable to make subdivisions of the clastic 

 rocks that were persistent enough to aid in correla- 

 tion. By means of this additional refinement two 

 partial stratigraphic sections were recognized in the 

 area. Only one of these was valid over a sufficiently 

 large extent to be of much use. It consisted of feld- 

 spathic graywacke, tachylitic rocks, and thin discon- 

 tinuous lenses of foraminiferal limestone, which, as 

 they generally lay close to the tachylitic rocks, were 

 believed to be at a single horizon rather than at sev- 

 eral horizons. The distribution of these units which 

 aided in understanding the structure is shown in fig- 

 ure 56. The other sequence, which consisted of tuff, 

 graywacke, and massive lavas, was recognized only on 

 Mine Hill, where it was useful in determining local 

 structures of economic importance. The mapping of 

 the distribution of these partial sections led to the 

 delineation of some folds, several faults, and the major 

 shear zone of the district, and when this had been 

 done the structural pattern of the area was outlined. 

 Other faults were then added on the basis of exposures 

 of abnormally sheared rock, topographic expression, 

 apparent linear features in the pattern of rock distri- 

 bution or in the topography, and the alinement of ser- 

 pentine bodies that appeared by their shapes to have 

 been controlled by faults. The resultant map of the 

 district we believe to be as reliable as the exposures 

 allow, and although it may omit faults that strike 

 virtually parallel to the bedding of the rocks of the 

 Franciscan group, it probably depicts all the dominant 

 structures of the district. 



PRELIMINARY OUTLINE OF THE GEOLOGIC 

 STRUCTURE 



The geologic structure of the older rocks of the 

 New Almaden district is so complex in detail that 

 most exposures exhibit at least minor folds or faults. 

 Despite this complexity, the larger lithologic units 

 have retained considerable continuity, and the major 

 structures are reasonably simple ones, which can be 

 worked out with a fair degree of assurance. (See fig. 

 58.) All major structures have trends falling between 

 west and north-northwest. The rocks of the Francis- 

 can group in general strike about west -north west in 

 the southern two-thirds of the district and nearly east- 

 west in the northern third ; in much of the district 

 they dip to the north. A major anticlinal fold trends 

 west-northwest along Los Capitancillos Ridge and ex- 

 tends southwards to a point near the junction of Long- 

 wall Canyon and Llagas Creek; however, it is offset by 

 several faults and so poorly represented by the atti- 

 tudes of the rocks exposed at the surface that it might 

 have remained undiscovered had we not had the bene- 

 fit of the many exposures provided by the quicksilver 

 mines. 



The larger faults that traverse the Franciscan strata 

 are nearly parallel to the strike of the strata but 

 somewhat steeper in dip. The most remarkable of 

 these is the Ben Trovato shear zone, which strikes 

 west-northwest through the center of the district, at- 

 tains a maximum width of 4,000 feet, and has an 

 apparent offset of about 10 miles. From this zone, 

 faults with thousands of feet of offset diverge north- 

 ward, and others with large, but less positively known, 

 offset diverge westward. Other faults that parallel 

 the Ben Trovato zone, both to the north and south of 

 it, have a strike-slip component of at least several 

 hundred feet. In addition, there are many discon- 

 tinuous and generally unmappable faults resulting 

 from folding, crushing, or the intrusion of serpentine. 

 All these structures are believed to have originated 

 shortly after the deposition of the Franciscan rocks, 

 but many have been modified by later movement. 

 Probably all the larger faults originated in response 

 to the same forces that produce right lateral displace- 

 ment along the San Andreas fault, which is only a 

 few miles southwest of the district. (See fig. 57.) 



The serpentine associated with the Franciscan group 

 is partly in sills and partly in steep tabular bodies 

 along faults. The time of the intrusion of the ser- 

 pentine to its present position is probably not the 

 same for all of the masses. The sill-like bodies ante- 

 date most of the faulting, whereas those lying along 

 faults were dragged or squeezed into their present 



