Arkell.1956.Jurassic..

CHAPTER 25
THE ANDES OF SOUTH AMERICA
For 5000 miles, from the Caribbean Sea to Cape Horn, the Andes build
an unbroken rampart along the coast of South America. On the east lie
the plains of the South American or Brazilian shield, with its continuations
in Guiana and Patagonia, subdivided by forested downwarps forming the
Amazon and Parana basins. On the west lies the Pacific. Between these
two level and rigid tracts, the one sial, the other sima, rise the Andes,
bounded by belts of major faults. The range consists essentially of a high
plateau, up to 10,000 ft. above the sea, formed largely of folded Mesozoic
sediments covered with Mesozoic, Tertiary and Recent volcanic rocks,
from which rise two lines of great Tertiary to Recent volcanoes. The
central Andes, the widest and highest part, opposite the re-entrant in the
middle of the west coast of the continent, reach a width of 300-400 miles.
Northward and southward the ranges narrow to less than 100 miles and
finally break down. At Cape Horn the trend-lines swing round eastwards
into the South Antilles loop, largely submerged beneath the sea; at the
north end they fan out in the Columbian virgation.
This great meridional line of weakness differs fundamentally from the
Alpine-Himalayan ranges of southern Eurasia, although broadly contemporaneous.
The Andes, though by no means simple in structure,
show no recumbent folds or major low-angle overthrusts : nappe structures
are absent, and inverted folds exceptional (Douglas, 1920, p. 57; Heim,
1949). Instead of a broad mobile belt like the Tethys geosyncline, becoming
crushed and underthrust by the approach of two sialic continental shields,
the Andes have arisen mainly through magmatic activity along the continental
slope. The Mesozoic sediments from end to end are due to marginal
encroachments of the Pacific Ocean over the South American shield.
Along this continental slope in Triassic times a submarine swell began
to be formed off-shore by uprise of a batholith elongated parallel to the
coast. As it rose it enclosed a trough in which the Jurassic sea laid down
sediments ranging from Hettangian to Tithonian, varying in completeness
from place to place as movements of the crust caused transgressions at
one time or place and regressions at another. Volcanoes rose along the
swell, which at this stage probably resembled the island arcs of the presentday
Pacific and Indonesia (Gerth, 1939, p. 11). Volcanic activity, at least
in the central region, reached a climax in the Kimeridgian, with the outpouring
of enormous sheets of basic lavas (the 'porphyrite formation')
and explosion-pyroclastics. Finally, in the Upper Cretaceous the
batholith underlying the island arc underwent major upheaval, folding
the Jurassic rocks to the east of it, and giving birth to the coastal cordillera.
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