Tectonic Synthesis of the Northern Subandean Trend, South America, Based on Landsat Data

T. L. Gubbels, J. R. Everett, R. J. Staskowski, J. F. Amos

The northern Subandean fold-thrust belt and foreland basin lies along the eastern side of the Andes of Colombia, Ecuador and northern Peru. This remote area includes the Andean foothills, part of the jungle-covered western Amazon rain forest, and the Llanos region of Colombia. The highly prospective character of the area is demonstrated by the discoveries of the supergiant Cusiana and Canto Limon fields in the last decade. To guide exploration strategies in this remote and poorly known region, we prepared a regional tectonic synthesis extending from 8°S to 8°N, based on a newly created 43-scene geocoded digital Landsat MSS mosaic. This cloud-minimized synoptic mosaic provides a unique view of the regional geology and structure and serves as the best available ca tographic base. An accompanying regional stratigraphic synthesis allows key paleogeographic information to be tied directly to specific tectonic features expressed on the mosaic.

Within this region, the Subandean fold-thrust belt is segmented into four discrete zones of characteristic structural style. Segment boundaries are related to pre-Cretaceous paleotectonic controls, rather than dynamic controls imposed by the subducting Nazca plate. In the southernmost segment, south of 5°S, active thin-skinned deformation extends over 200 km east of the Andean topographic front and is controlled by the extent of a thick interval of pre-Cretaceous salt. Salt diapirism and thickening along anticlinal axes are characteristic of this segment; the impressive width of this belt is likely related to a low-viscosity, subhorizontal salt decollement. The folded zone narrows considerably to the north; between 5° and 1°S, the fold-thrust belt is less than 100 km wi e and is buried by a thick sequence of slightly deformed Tertiary clastic strata. Between 1°S and 4°N the Subandean zone is basement-involved and the deformation front is emergent. Precambrian basement is thrust over the Tertiary fill of the foreland basin. In the northernmost segment, extending from 4° to 8°N, several thrust-tip thin-skinned folds exceed lengths of 100 km. The Cusiana field is located on the southern nose of one of these folds; similar highly prospective look-alike structures are present in several other areas along trend.

Critical elements of successful petroleum exploration in this region include recognizing: 1) the importance of ancestral structural controls and related along-strike variation in structural style; 2) the presence of Andean (mid to late Tertiary) structures far to the east of the present topographic front; 3) the existence of numerous untested structural look-alikes to known fields; and 4) the possibility of buried prospective Paleozoic basins within the foreland region.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995