--> Abstract: Lower Magdalena Basin, Colombia: A Forearc Basin Formed Above a Zone of Active, Shallow Subduction, by Rocio D. Bernal-Olaya, Paul Mann, and Carlos Vargas; #90124 (2011)

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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Lower Magdalena Basin, Colombia: A Forearc Basin Formed Above a Zone of Active, Shallow Subduction

Rocio D. Bernal-Olaya1; Paul Mann1; Carlos Vargas2

(1) Jackson School of Geosciences, The University of Texas at Austin, Austin, TX.

(2) Universidad Nacional de Colombia, Bogota, Colombia.

The Lower Magdalena basin (LMB) is one of the largest (42,000 km2 ) and thickest (~8 km) sedimentary basins in Colombia, yet has yielded only 17 oil discoveries with 77 million barrels of oil and 1112 GCF over the past 40 years of exploration. We integrate seismic tomographic data capable of imaging to depths of 200 km with a compilation of on- and offshore industry seismic profiles recorded to depths of 20 km (16 sec TWT) tied to 30 wells to better understand slab controls on basin geometry. Seismic lines recorded to depths of 20 km show that the Colombian basin crust is subducted eastward at a shallow angle beneath an active submarine accretionary prism (Sinu belt) and an onshore forearc high composed of an older prism of folded and eastwardly-tilted rocks of Eocene to Upper Miocene age (San Jacinto belt). Tomographic data reveal the subducted slab at depths from 17 km dipping at an angle of ~5° to a depth of 30 km beneath the eastern side of the asymmetrical and synclinal LMB. Structural styles affecting the 8 km of basin fill in includes ~2 km-thick Eocene rifts, controlled by ENE-striking normal faults, truncated by a prominent Middle Miocene unconformity that is in turn cut by high-angle faults associated with the Santa Marta strike-slip fault. Based on these observations, we classify the LMB as a forearc basin formed in a zone of shallow, eastward and southeastward subduction. The synclinal LMB is defined on the west by the uplift of an older, accretionary wedge (San Jacinto forearc high) and to the east by sub-vertical strike-slip faults (Santa Marta fault zone). The lack of an active volcanic arc east of the LMB in the Santa Marta massif is explained by the low dip of the subducting Caribbean slab that does not allow the presence of a mantle wedge that can be melted to form volcanoes.