TIMING OF FORELAND BASIN EVOLUTION AND HINTERLAND EROSION IN THE SOUTH-CENTRAL ANDES OF ARGENTINA, 36˚ S

The Andean orogen is the type-example of an active Cordilleran style margin with a retroarc fold-and-thrust belt and foreland basin. Upper plate shortening, crustal thickening, understhrusting, and arc processes related to subduction of the Nazca plate under South American have led to uplift of the Argentinean Andes. Timing of shortening and foreland basin development seem to be diachrounous along strike, with ages varying from Paleocene initiation in Bolivia and NW Argentina to Jurassic in the Patagonian Andes. Although much of the early stratigraphy has been eroded or covered the Neuquén basin (32° S to 40° S) contains a thick sedimentary sequence ranging in age from late Triassic to Cenozoic. The Neuquén basin initiated as an east-west trending rift basin in the late Paleozoic-early Mesozoic and then evolved into a foreland basin sometime in the late Cretaceous or Cenozoic. Previous workers in the central Neuquén basin (35° S to 39° S) have used changes in detrital zircon signature between the Rayoso Group (~ 120-100 Ma) and the Neuquén Group (~ 95-80 Ma) as evidence for the initiation of foreland basin sedimentation, but have given little discussion to the spatio-temporal evolution of specific foreland basin depozones.

Correlative stratigraphic sections from the Malargüe fold and thrust belt (36˚ S) record a facies change between the marine evaporites of the Huirtin Formation (~125 Ma) and the fluvial sandstones and conglomerates of the Diamante Formation (equivalent to the Neuquén Group). A major, 25-30 myr, unconformity between the Huirtin and Diamante formations potentially represents the passage of the flexural forebulge and preliminary detrital apatite fission track data indicate a change in source exhumation signal at that same transition, which is consistent with an actively eroding and deforming hinterland at ~ 95 Ma. However, there is no significant difference in detrital zircon signature. Instead, a change in provenance and paleocurrent direction occurs in the middle Diamante Formation. This new provenance signature along with a change from west directed to east directed paleocurrents within the Diamante Formation may reflect a late Cretaceous transition from forebulge derived sediment in the distal fordeep to sediment derived from the thrust belt to the west deposited in the proximal foredeep.