Paleogeography and Andean structural geometry, northwest Argentina

Abstract

Neogene tectonic segmentation of the Andean Cordillera in northwestern Argentina is a function of both the dip of the subducted plate and of major paleogeographic structures and sedimentary basins. Vergence, fault dip, and lateral continuity of Neogene structures can be related to the character and pre‐existing geometries and structures of the materials in which they form. South of 27°S the thick‐skinned Sierras Pampeanas, which occur mostly over a flat segment of the subducted Nazca Plate, formed where little sedimentary cover overlay crystalline basement. North of 24°S, the thin‐skinned Subandean belt, overlying an inclined slab segment, formed within a thick, elongate, eastward‐tapering trough of Paleozoic and Mesozoic sediments that may represent a complex of pre‐Andean foreland basin deposits. The transitional region between 24°–27°S (the Santa Barbara System and Cordillera Oriental) is characterized paleogeographically by complex cross‐trending structures and irregular basin geometries, and the Neogene structures are equally complex and variable. The paleogeographic features of NW Argentina are a product of compressional tectonism during much of the Paleozoic that resulted in an orogenic belt oriented obliquely to the modern Andes. This earlier orogen was disrupted by several zones of transverse structures and by Cretaceous extensional or pull‐apart basins. These features clearly influence the geometry of the superposed Andean structures. More intriguing, however, is the possibility that preexisting structures of the continental lithosphere may in some way help to modify the geometry of the subducted plate.