From rift basin to mountain range
The mountains of Salta Province are the product of a dramatic geological reversal. During the Cretaceous Period, as the supercontinent of Gondwana was breaking apart, this region of northwestern Argentina was stretched and thinned. This process created a series of deep valleys, or rift basins, known collectively as the Salta Rift Basin. For millions of years, these basins collected enormous quantities of sediment, with deposits reaching thicknesses of up to 5,000 meters. The initial "syn-rift" phase filled the actively sinking valleys with continental red beds—conglomerates and sandstones from alluvial fans and river systems. This sedimentary sequence is known as the Salta Group.
Beginning in the Eocene, around 40-30 million years ago, the tectonic regime shifted entirely. The ongoing subduction of the Nazca Plate under the South American Plate initiated the Andean orogeny, a mountain-building event that continues to this day. The immense east-west compression reactivated the old Cretaceous faults that had originally formed the rift basins. But instead of pulling the crust apart, the compression forced these faults to move in the opposite direction. Normal faults became high-angle reverse and thrust faults. This process, known as tectonic inversion, effectively squeezed the basins shut, hoisting the thousands of meters of sedimentary rock that had filled the valleys upwards. The former basins were turned "inside-out," creating the striking mountain ranges of the Cordillera Oriental and other regional uplifts.
A landscape turned upside down
The stunning landscapes of Salta, particularly in the Quebrada de las Conchas ("Gorge of the Shells"), record this tectonic inversion. Here, layers of sedimentary rock that were once flat on the basin floor are now tilted, folded, and spectacularly exposed. The colorful strata of the Late Cretaceous-Paleocene Yacoraite Formation, for instance, are famous for their red, green, and yellow. These carbonate-rich layers, deposited in a shallow sea that flooded the basin between roughly 72 and 62 million years ago, are now folded into anticlines and synclines by the Andean compression.
Geologists can see direct evidence of the inversion where older, syn-rift rocks of the Pirgua Subgroup have been thrust directly on top of younger rocks from later stages of basin development. This structural arrangement is also important for economic geology. The folding and faulting associated with tectonic inversion created structural traps where hydrocarbons (oil and natural gas) could accumulate, making the remnants of the Salta Rift an area of exploration interest. The entire region shows how ancient geological structures can be reactivated and repurposed by later tectonic events, creating mountain scenery.