SUBANDEAN THRUST AND FOLD BELT OF NORTHWESTERN ARGENTINA. GEOMETRY AND TIMING OF THE ANDEAN EVOLUTION

The Subandean belt of the central Andes is one of the premier thin-skinned thrust belt belts of the world, as well as the prime example of such a belt in a retroarc, non-collisional setting. Understanding of the Subandean, however, has long been hampered by the lack of detailed knowledge of the ages of motion of individual thrust plates. Such information, however, is critical for hydrocarbon exploration as well as for comparison with modern geodetic GPS data, and evaluation of seismic hazard. New field, seismic reflection, and geochronological data from NW Argentina put this part of the Subandean belt in the same league as the very best dated thrust belts of the world.

The decollement for the southern Subandean belt dips 2-3°W and lies mostly within Silurian shales. However, a unique aspect of the belt is an intermediate detachment level in thick Devonian shales. Below this horizon structures are relatively simple fault-bend folds whereas above, vertical limbed lift-off structures are decoupled from the underlying sequence. At 22°-23°S, we have documented ~60 km of shortening; about 65% occurred in sequence from west to east, whereas the remaining 35% is out-of-sequence.

Subandean belt timing has been determined by magnetic reversal stratigraphy, tephra chronology, unconformities and growth strata geometries in foreland basin strata. An increase in the accumulation rate in the western part of the foreland basin at about 8.5-9 Ma marks the onset of Subandean deformation when a crystalline basement thrust plate to the west ramped up to the current decollement level. Using vertical separation diagrams, we pinpoint the initial uplift of the Pintascayo Range at 7.6 Ma and the Baja Oran Range at ~6.9 Ma; both were active simultaneously until at least 4.7 Ma. To the east initial deformation is younger: in the San Antonio Range fault movement began at 4.4 Ma, and the Aguarague uplift started at ~2.8 Ma. Since 4 Ma, new and reactivated thrust faulting characterizes almost all the Subandean Ranges, especially after 2 Ma. In concert with the balanced sections, these results suggest shortening rates varying from less than 5 to as great as about 10-12 mm/a over the last 10 Ma.