Present-day foreland development and its relation to plate tectonics in the northern Andes

Abstract

Based on analysis of topography through DEM images, geomorphology, river drainage patterns, and surface geology, the foreland basins of northwestern South America are divided into three areas that coincide with segments of boundaries between the South American, Nazca and Caribbean plates. From 2° 30′ to approximately 4°30′ N, is the southern Llanos or Uplifted foreland that is characterized by uplift and a transpressive environment. Major alluvial fans and terraces representing the wedge top depozone are present on the proximal part, and Mio-Pliocene sediments are exposed on the distal part of the foreland. This foreland is the highest (486 m at the mountain front) and has the steeper slope (0.33°) of the northern Andes. Watersheds in this area exhibit low maturity and elevated heat flow is observed. Associated to this foreland is the Cauca or Nazca Segment that has a steeply dipping slab that originates a chain of recent volcanoes to the west.

The second zone (Main Llanos foreland) is developed north of the Caldas tear zone and extends to the Rio Uribante-Apure. It coincides with a flat slab configuration (Bucaramanga Segment) and is characterized mostly by pure compression. On the foreland this generates deformation of recent terraces and current uplift in the wedge top area that is eroded in several areas. Active sedimentation in an extensive alluvial plain is taking place in the distal foreland. Here the streams are transversal to the Eastern Cordillera, feeding the longitudinal NE-SW trending Meta River. The slope has an average inclination of 0.14°, and an average elevation of 286 m at the mountain front. Watersheds display a high to intermediate maturity, but are less mature to the north, coinciding the Cocuy area of current uplift.

The northernmost zone, or Barinas Foreland, east of the Venezuelan Andes, aligns with the Caribbean Segment. Major subsidence occurred in the Middle-Upper Miocene mainly in the southern basin, migrating north in the Pliocene-Pleistocene. This resulted in a mature foreland to the south with a low regional slope lacking alluvial fans and wedge top facies. In contrast, the northern area has well-developed alluvial fans, a higher regional slope, and displays current deformation at the mountain front.

The northern South American Andes illustrates how plate configuration and the associated subduction of oceanic slabs are linked to distinct foreland morphologies. Nevertheless, other factors, such as tectonic inheritance and climatic conditions, play significant roles in determining the final configuration of a specific foreland.