GEOLOGIC STRUCTURE 



87 



FIGURE 60. Interbedded graywacke and eiltstone of the Franciscan 

 group, showing typical irregularities due to rock flowage and minor 

 faulting. 



FIGURE 61. Plastically deformed and faulted layer of greenstone (gr) 

 in graywacke and siltstone. Photograph taken in the Cora Blanca 

 part of the New Almaden mine. Note banded dolomite vein in lower 

 right. 



is traceable for several miles along the crest of the 

 ridge. The southwest limb is clearly defined near the 

 Guadalupe mine, where it dips 45 SW., but near the 

 New Almaden mine, where it is closest to the Ben 

 Trovato shear zone, this limb is partly obliterated by 

 the development of parallel folds and faults. The 

 northeast Hank of the anticline dips about 50 NE. 

 near the New Almaden mine, but is faulted off west 

 of the Senator mine. The band of greenstone extend- 



ing from the New Almaden Dam to the junction of 

 the Enriquita and Shannon faults serves admirably 

 to show both the general continuity and the detailed 

 complexity of the fold along Los Capitancillos Ridge. 

 The anticline probably also extends southwest of Alma- 

 den Canyon at least to Llagas Creek, for northwest of 

 Llagas Creek the arcuate pattern of serpentine sug- 

 gests a southeast plunging anticline. If this area is 

 in gross structure an anticline, it must be highly de- 

 formed because neither the observed attitudes nor the 

 pattern of rock distribution can be made to fit any rea- 

 sonably simple structure. 



In parts of the district where one does not have the 

 advantage of three-dimensional observation afforded 

 by mine workings, the character of folds too large to 

 see in a single exposure is somewhat conjectural. Un- 

 less they involve distinctive lithologic units, such folds 

 must be reconstructed from the attitudes plotted on 

 the map, and attitudes observed locally in the rocks 

 of the Franciscan group may or may not be truly 

 representative. Therefore, the nature of the larger 

 structures within the fault-bounded segments of the 

 El Sombroso block, for example, could be variously 

 interpreted. Most of the measured attitudes show 

 northwesterly strikes and northeasterly dips, from 

 which it may fairly be concluded that the entire block 

 is tilted to the northeast. This general tilt, however, 

 has minor folds superimposed upon it, as is shown by 

 the regular divergence of closely spaced strike sym- 

 bols in parts of the area. A few folds that show op- 

 posing dips, and are thus anticlines or synclines in 

 the usual sense of the terms, may be demonstrated and 

 are shown in figure 58. The best examples of these 

 are a parallel anticline and syncline in the northwest- 

 ern part of El Sombroso block. The axes of both, 

 although sinuous, trend in an easterly direction. In 

 the area to the south and east of these folds, the beds 

 have a consistent east-west trend and northerly dip, 

 but between the Guadalupe mine and El Sombroso 

 the strike swings through northwest to north, and 

 locally exhibit a northeast strike and southeast dip. 

 Other folds of the same order of size may be inferred 

 from available data near the southwest corner of the 

 district in the rocks underlying a ridge to the north 

 of Soda Spring Canyon about 2i/ 2 or 3 miles upstream 

 from Los Gatos Creek and also in the area north and 

 northwest of Mount Umunhum. The axes of all these 

 folds have a general easterly trend. 



Smaller folds also due to regional forces are difficult 

 to distinguish from structures that have resulted from 

 the large-scale flowage of the rocks that is common in 

 the district, particularly in the Los Capitancillos block. 

 Some local undulatory folds measuring a few hun- 



