Cretaceous Stratigraphy and Structure, Western Andes of Peru Between Latitudes 10ø-10ø30^prime

Abstract

Cretaceous tectonics, sedimentation, and volcanism were dominated by oscillatory vertical movements of strips of basement bounded by major shear belts. Two main episodes of movement occurred: a Valanginian to Senonian episode of general subsidence, and a Senonian to Tertiary episode of uplift. In the West Peruvian trough, the products of the first episode comprise a western shale-graywacke-volcanic facies and an eastern sandstone-limestone-shale facies. The facies meet along the Tapacocha axis, a major steep basement shear zone, which allowed the western side to subside faster than the eastern side. Fracturing of the western block in its descent provided channels for volcanic eruptions, whereas, on the eastern intact block, shelf deposits were laid down. During uplift, the sediments on the eastern block were folded as a single unit by decollement on underlying shale; those on the western block were strongly deformed in narrow belts above steep basement fractures. The belt of strongest deformation is along the Tapacocha axis and was accompanied by metamorphism of greenschist facies. The response of the Andes to lateral movement of oceanic crust beneath was to break into ribbonlike strips, parallel with the continental margin, which oscillated in vertical planes.