|Scenes from San Juan and
in the Flat Subduction Region
||Triassic redbeds exposed at Valle de la Luna Provincial Park: Two of
the oldest known dinosaurs, Eoraptor and Hererrasaurus, were
discovered in these strata by Argentine and American paleontologists. The
park is located on the eastern flank of the Sierra de Valle Fertil in the
western Sierras Pampeanas. It is renowned for its lunar-like landscape
and sculpted rock exposures.
|Middle Triassic lacustrine strata comprise the
bulk of the Ischigualasto Formation where the early dinosaurs were
discovered in Valle de la Luna Provincial Park.
||This balanced rock is one of many fascinating,
wind-sculpted features in the park.
|Complicated structure is displayed in these Mesozoic
and Cenozoic strata exposed in the Cuesta de Huaco in the Eastern Precordillera.
The Eastern Precordillera actually has a closer affinity with the thick-skinned
Sierras Pampeanas immediately to the east than it does with the thin-skinned
Precordillera immediately to the west. The range is interpreted to to be
a thick-skinned structure that has yet to be unroofed due to its very young
age (It's still rising!).
||The Río Jáchal is an antecedent
stream sourced in the Cordillera Frontal (distant range) that cuts through
the Precordillera. Paleozoic strata with North American affinities are
exposed in the sides of the gorge.
|Ordovician pillow lavas are exposed in a quarry
and in roadcuts on the western side of the Precordillera Occidental near
Rodeo, San Juan. They are variously interpreted to represent the top of
an ophiolite complex and deposits within a rift basin.
||Middle Miocene volcaniclastic strata are exposed
on the western flank of the Precordillera Occidental (right background)
near Rodeo. The Precordillera is separated from the base of the Cordillera
Frontal (left background) by the Iglesia Basin--a classic piggyback basin.
This photo was taken in 1984. A reservoir now occupies the area in
the middle distance.
|Neogene volcaniclastic strata (light-colored)
unconformably overlie the Ordoviciam pillow lavas (dark) on the western
flank of the Precordillera Occidental near Rodeo.
||The Cordillera Frontal as seen from the western
base of the Precordillera Occidental near Rodeo.
|The highest peaks in the western Hemisphere are found in the Cordillera
Frontal along the west side of the Uspallata-Calingasta- Iglesia Basin
in San Juan and Mendoza provinces. There are no Quaternary volcanoes in
the range because of the subhorizontal subduction angle between the Nazca
and South American plates at this latitude.
||The Río San Juan is another large antecedent
river that cuts through the Precordillera between Calingasta and Ullum.
The road through the gorge provides spectacular views throughout its 80
km length. A series of dams that will flood the gorge is presently under
construction. In 2001, the one-lane gravel road was only accessible to the
public at night.
|A winter sunset in the Precordillera Central
along the Río San Juan Gorge.
||One of two middle Miocene, dacitic, subvolcanic
edifices that border the Río San Juan at the eastern entrance to
the gorge near Ullum. Neogene volcanism was extremely rare in the flat
subduction area and appears to have been restricted to dacitic dome complexes
in the west and undersaturated lavas farther to the east. No active volcanoes
are present in the region today.
|Eucalyptus trees border the roads through the
vineyards near Ullum, just west of the city of San Juan.
||Cerro Aconcagua is the highest mountain outside
of Asia. This view, taken near the Río Mendoza, about 30 km south
of Mendoza, shows nearly 20,000 feet of relief.
|A view of the high Andes from west of Mendoza. The Uspallata piggyback
basin is the southernmost of this type of basin along the eastern base
of the Cordillera Frontal.
||A river terrace cut by the Río Mendoza developed in the alluvial
fan at the eastern base of the Andes. The fan provides a smooth ascent
to a pass across the mountains into Chile. Charles Darwin followed this
route on his return to H. M. S. Beagle in Valparaiso, Chile after
leaving the ship to cross the Andes to visit Mendoza. This route also holds
an important place in Argentine and Chilean history. Argentine Liberator
San Martín led his Army of the Andes across the Andes to join with Chilean
forces to defeat the Spanish in two decisive battles.
|The summit of Aconcagua (left-center) as seen
from the entrance to Aconcagua National Park near the Chilean border. The
top is often enveloped in clouds.
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October 4, 2005