PALEOGENE TECTONISM, MAGMATISM AND CLIMATE IN THE NORTHWESTERN CORNER OF SOUTH AMERICA: UNRAVELLING THE IMPACTS OF THE PRE-ANDES OROGENESIS

Paleogene tectonic activity in the Northern Andes has been investigated using sedimentological, stratigraphic and provenance constrains from rocks preserved in adjacent basins. Cooling events are documented by several thermochronometers that reveal the exhumation patterns from depths corresponding to >250 to 60 ºC. In the northern Andes of Colombia and Venezuela, the Paleogene succession recorded between the Central Cordillera to the west and the easternmost foothills of the Andes to the east records a regional change in sandstone composition, varying from quartzose sandstones at the base to litharenites and felspathic litharenites to the top, and changing back to quartzarenites in lower-middle Eocene strata. This change in composition, in dominantly fine-grained sandstones, cannot be explained by tectonic uplift and unroofing of the western hinterland of the orogen. Zircon and apatite thermochronological studies report a Paleocene or older cooling event in the Central Cordillera, Santa Marta massif and blocks to the west of the Bucaramanga fault; immature sandstone accumulated adjacent to such uplifts. In the Eastern Cordillera, apatite thermochronological studies indicate late Eocene and younger cooling events. However, immature metamorphic lithic fraction does not decrease eastward homogeneously, and those fragments are reported as far as 300 km from proposed uplifted areas to the west. Volcaniclastic rocks and the association of volcanic lithic fragments and volcanic crystal grains (feldspars, zircons) suggest a short volcanic event near the Paleocene- Eocene boundary. Tropical humid conditions in late Paleocene time, as indicated by the large diversity of pollen and leaf prints, make difficult to interpret those volcanic and metamorphic fragments as faraway travelers. Therefore, volcaniclastic material as well as unstable sandstone grains should be derived from closer areas. We hypothesize that such grains came to surface by magmatic activity and associated block tectonics, and these grains dispersed broadly and in a short period of time. Therefore, intra-orogen magmatism and young Laramide-style tectonism should be included as a possible source of unstable basement-derived fragments and as a mechanism to create local uplifts within the basin.