ANDEAN INVERSION OF A PERMO-TRIASSIC RIFT SYSTEM IN SOUTHERN PERU: IMPLICATIONS FOR CENOZOIC SHORTENING, FLEXURE, AND DEFORMATION ADVANCE

Pre-Andean extensional systems influenced subsequent shortening patterns during Cenozoic construction of the central Andes. However, the style and timing of earlier rifting and later inversion remain poorly defined. New field mapping, structural transects, and thermochronology from the Eastern Cordillera in southern Peru provide insights into how preexisting Permo-Triassic rift structures influenced the geometry, kinematics and magnitude of Andean shortening and foreland basin partitioning. (1) A new subcrop map from recent structural transects shows that the Permo-Triassic Mitu Group was variably deposited on lower and upper Paleozoic units. Schematic cross sections show that the subcrop relationships coincide with major structural boundaries, and suggest extensional block faulting during Mitu deposition. (2) Field relationships show angular unconformities between the Mitu Gp. and underlying pre-rift stratigraphy. However, (3) newly updated geologic maps show that normal fault relationships generally are not preserved, suggesting that major pre-Andean structures were reactivated as fold-thrust structures during Cenozoic shortening. These features are observed at oblique, non-coaxial orientations relative to regional Andean trends. The presence of an Eastern Cordillera Permo-Triassic rift system confirms previous hypotheses, enlarging the regional footprint of seismically imaged extensional structures in the Subandes and Amazon foreland. Earlier K-Ar and ⁴⁰Ar/³⁹Ar and our new apatite (U-Th)/He data from the Permo-Triassic Coasa pluton show ~38 Ma and ~16 Ma cooling ages, respectively. These data suggest a complex >20 Myr cooling history during Andean shortening. We suggest that this zone was possibly reactivated several times by inversion of favorably oriented structures. Paired with new age control for other fold-thrust structures, we suggest that the northern Altiplano and Eastern Cordillera in southern Peru may have experienced out-of-sequence deformation, synchronous with spatially distributed shortening over a broad region. This evidence for reactivation and heterogeneous shortening has implications for central Andean crustal thickening style, controls on flexural subsidence, basin partitioning, and uplift during shortening and deformation propagation.