EVIDENCE OF MIDDLE TO LATE CRETACEOUS COMPRESSIVE DEFORMATION IN THE HIGH ANDES OF MENDOZA, ARGENTINA

The goal of this contribution is to present new direct evidence of Middle to Late Cretaceous compressive deformation in the High Andes of Mendoza, Argentina. The study area is located in the southern end of the modern Pampean flat slab segment of the central Andes (~33°S).

Syntectonic-strata were observed in the Cretaceous Diamante Formation (DF) of Barremian to Cenomanian age. A series of unconformity-bounded conglomerate packages with different dip domains were observed in a ~50 m outcrop exposure. The geometry of the interpreted synorogenic strata consists in four domains bounded by three syntectonic angular unconformities. It is proposed that this geometry was generated by a series of clockwise and counterclockwise rotations of beds and subsequent unconformable overlap by a new deposit leading to the development of opposing dips in successively younger unconformity-bounded packages.

Other indirect evidence that suggest a deformation event is given by the presence of limestone clasts in the upper sections of the DF that are thought to come from the uplifted early Cretaceous Mendoza Group.

The observed synorogenic deposits are located up-sequence of lake deposits within the same unit which in other sectors of the basin were assigned a Hauterivian - Barremian age, therefore a post-Barremian (post ~118 Ma) age for the synorogenic strata and, consequently, for the deformation event is proposed.

Several lines of evidence help to constraint the age of the deformation event: a 108 ± 12 Ma 40Ar-39Ar age was reported for a regional-scale ductile shear zone in the Coastal Range of Central Chile (32°S), this together with reported fast exhumation rates from fission track ages suggest that high erosion/exhumation rates were coeval with crustal shortening and uplift since ~110 Ma, which is coincident with an increase in the convergence velocity that led to a change on the tectonic regime from extensional to compressional, documented by expansion and eastward arc migration, basin closure, crustal shortening and uplift in the inner-arc and the inception of compressive deformation in the foreland as documented here.

As a conclusion the DF deposits are considered of synorogenic origin related to the inception of compressive deformation since ~110 Ma, indicating the beginning of the Andean uplift at this latitude.