758 K. F. MATHER FRONT RANGES OF THE ANDES 



GUARUI FAULT 



The eastern front of the Sierra de Guarui is the topographical expres- 

 sion of the Guarui fault, a crustal feature closely similar to the Mandi- 

 yuti fault and situated 6 to 8 miles farther westward. The fault trace 

 was mapped for only about 10 miles southward from the point where it 

 crosses Rio Parapiti. In that portion of its extent it curves gently, so 

 that it is convex toward the east. The western side is, as usual, the up- 

 throw side; the displacement is approximately 10,000 feet. Presumably 

 the fault-plane is inclined westward, like the Mandiyuti fault-plane, but 

 the details of its structure were not ascertained. 



Ten miles south of Rio Parapiti the fault -plane is projected into a 

 group of irregular hills which seem to bridge the space between the west- 

 ern outposts of the Sierra de Mandiyuti and the eastern front of the 

 Sierra de Guarui. Here the structure is rather complex, although in 

 general it appears to be an anticlinal fold which is apparently the south- 

 ern extension of the Guarui fault. 



CHIRETI FAULT 



The fault escarpment which forms the eastern front of the Sierra de 

 Chireti and rises boldly above the western outposts of the Sierra de 

 Mandiyuti, 7 miles west of Cuebo, is directly in line with the southward 

 extension of the Guarui fault. The intervening area was not surveyed, 

 and it may be that the Chireti fault is actually the southern continuation 

 of the Guarui fault. In any event, the Sierra de Chireti is a huge block 

 of the earth's crust tilted downward toward the west and elevated along 

 its eastern margin by this fault movement. Apparently, the fault-plane 

 is approximately vertical. The beds immediately east of it dip gently 

 toward the west, and hence there is no suggestion of an anticlinal fold at 

 the place where these observations were made. 



VITIACUA FAULT AND ANTICLINE 



The Vitiacua fault begins close to Ibo and is delineated by the eastern 

 face of the series of low westward-dipping cuestas which are the northern 

 outposts of the Sierra de Vitiacua. These increase in height toward the 

 south, in response to increased displacement along the vertical fault- 

 plane. For several miles the break is a simple one, not complicated by 

 folding, except that involved in the tilting of the upthrow block, but near 

 Cangapi the fault becomes a sheared anticlinal fold. East of the fault- 

 plane there appears a series of hogback foothills, formed of the eastward- 

 dipping strata on the east limb of the broken arch. These increase in 



