/o: 



K. F. MATHER FRONT RANGES OF THE ANDES 



r»« 



ANTICLINE / . 

 DIP »«d STTtl^C •/ 



Figure 19. — Tectonic Map 

 of the Area shown in Fig- 

 ure 1 



cline and is situated a short distance east of 

 the crest of each fold (see figures 18 and 19). 

 These faults are thrusts with the overhanging 

 wall on the west and with the fault-plane in- 

 clined at a high angle from the horizon. Dis- 

 placement along each fault-plane ranges from 

 3,000 to 10,000 feet, or even more. In many 

 places the fault movement is more significant 

 than the fold, but elsewhere the fold is the more 

 prominent of the two. 



With the possible exception of the Cuestas de 

 Ipahuazo, the structure of which has not been 

 satisfactorily determined, each of the cuestas, 

 or groups of hogback hills, described above is a 

 tilted fault-block with monoclinal dip toward 

 the west. These blocks are scores of miles in 

 length from north to south and have a width 

 from east to west of 5 to 15 miles. Each is ter- 

 minated on the east by a major fault-plane, 

 vertical or nearly so, with the upthrow side on 

 the west, along which the displacement amounts 

 to several thousand feet. The strata of which 

 these fault-blocks are composed dip toward the 

 west at angles varying between 20 degrees and 

 70 degrees or even more. In general, the dip is 

 greatest at the east margin of the block, where, 

 indeed, the strata are sometimes vertical, and 

 decreases in amount toward the west. 



Each of the intermontane lowlands is a com- 

 paratively broad, shallow syncline which extends 

 for scores of miles in a north-south direction 

 and is between 5 and 20 miles in width. On 

 either side of the trough the strata rise with 

 increasing dip until they merge into the flanks 

 of the adjacent anticline or the westward por- 

 tion of the adjacent monocline. Beneath the 

 western margin of the Chaco plains the strata 

 dip downward toward the east at angles which 

 decrease with distance from the mountain front. 

 Apparently, the Chaco is structurally an ex- 



