VARIATIONS IN BOTH STRUCTURAL STYLE AND STRATIGRAPHY IN THE EASTERN FOLD AND THRUST BELT OF THE PERUVIAN ANDES ACROSS THE NORTHERN BOUNDARY OF THE ANDEAN PLATEAU

New detailed mapping was conducted along two transects in the Andean fold-thrust belt of the Peruvian Andes in order to investigate how structural differences might explain an abrupt change in orogen width. Significant differences exist in both structural style and stratigraphy between the two transects. Across the narrow northern section (12-11°S), shortening is accommodated through closely spaced (~5km), dominantly east vergent thrust faults and west vergent frontal thrusts. The southern transect extends across the northern margin of the Andean plateau (14-13°S). Deformation is accomplished through predominately west verging folds and thrusts with two frontal east verging thrust faults. Along this transect faults are widely spaced (~15km) with stratigraphic separations up to 1.5km. The predominant style of deformation is folding at several scales. Regional folds have wavelengths of 2-10km and amplitudes up to 2km.

A representative stratigraphic column along the northern transect includes Precambrian (schist, gneiss), Devonian (shales, quartzite ~700m), Carboniferous (quartzite ~600m), Permian (volcanics, sandstone, ~2000m), Triassic (sandstone ~600m), Jurassic (limestone, shale, ~1200m), and Cretaceous (limestone, sandstone, ~3000m). Tertiary rocks are preserved in limited areas west of the volcanic arc and in ~8km wide synclines at the eastern edge of the deformation front. Cretaceous rocks, dominate the front of the northern fold-thrust belt. The lack of a foreland basin may be due to large, long wavelength low amplitude basement uplifts proposed for the area. The rocks exposed in the fold-thrust belt of the southern transect are Ordovician (~3000m), Silurian-Devonian (~1500m), Carboniferous (~1000m), Permian (~1000m), Triassic (~600m), Jurassic (~300m), Cretaceous (~300m). Tertiary rocks reach thicknesses of 10km in the western Altiplano basin and up to 3km in the eastern foreland basin. Both sections have a total pre-Andean stratigraphic section of ~8km.

We feel that the differences in structural style and stratigraphy observed in map pattern is significant enough to play an important part in the abrupt change in orogen width through this region. This hypothesis will be tested through a series of orogen scale balanced cross sections across the northern termination of the Andean Plateau.