Scenes from San Juan and Mendoza Provinces 
in the Flat Subduction Region

All photos
Jim Reynolds

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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Last updated October 4, 2005