EARTHQUAKES OF MAY 19 60 IN CHILE — SAINT-AMAND 353 



air. It is cut off on the south by a large, northwesterly trending fault 

 that forms the valley m which the town of Capitan Paistene lies. The 

 ulteresting thing is that the batholith appears to be dextrally offset for 

 several tens of kilometers along this fault. More detailed mapping in 

 this area might reveal whether this were a real off'set. 



The central valley at this latitude is narrower than in the region of 

 Chilian, and southward of Victoria the valley is replaced by a series 

 of low hills. The valley is a broad, graben-like depression bounded 

 on the east by the frontal scarp of the Andes. The valley in the region 

 of Chilian is thought to be filled with several thousand meters of sedi- 

 ments of glacial and fluvial origin [15] . 



The frontal scarp of the Andes is markedly straight and abrupt, 

 being bounded by a series of subparallel north-south faults of great 

 vertical displacement. 



The Andes, here, are composed of andesite porphyries and sediments 

 derived therefrom, of probable mid-Cretaceous age, covered in part 

 by effluvia of Recent volcanoes and intruded occasionally by masses 

 of Andean granodiorite and diorite. These are folded and faulted. 

 The mountains have a general level of about 2,000 m., some volcanoes 

 reaching over 3,500 m. The eastern side of the Andes is bounded 

 by a series of north-south faults forming large blockranges with a 

 descent, in some places gradual, in otliers abrupt, to the Argentine 

 pampas. 



At about latitude 39i/^° S. the coast suffers an embay ment. The 

 coast range, here composed entirely of the metamorphic sequence, has 

 degenerated to a series of low hills extending to within a few kilo- 

 meters of the line of lakes. The central valley is absent, except for a 

 narrow strip between the hills and the lakes Rinihue, Pangipulli, and 

 Calafquen (fig. 2). The character of the Andes, here, is different. 

 The average summit level of the hills is about 1,000 m. less than that 

 of those to the north. 



The change begins at about Volcan Llaima (fig. 2) . A line of lakes, 

 apparently bounded on the west by a system of faults and lying in 

 partially glaciated valleys, extends to the south like beads on a string. 

 The next "lake" comprises Seno Reloncavi and the Golfo de Ancud. 

 A line of active and dormant volcanoes rising above the dissected, 

 mesalike surface of the Andes dominates the eastern sky lines. 



The mountains here, as those farther north, are composed of the 

 same volcanic-sedimentary sequence, partially covered with lavas and 

 intruded by granitic rocks. The outstanding difference is that the 

 intrusive rocks form almost the entire range, the batholith now being 

 confined to the Andes rather than to the coastal range. The question 

 arises, has the batholith been offset by some gigantic sinistral-shear 

 system, or is it a different batholith from that of the coastal hills ? 



