Tectonostratigraphic evolution of the Sinu accretionary prism and Lower Magdalena forearc basin formed above a shallow-dipping subduction zone, Caribbean margin of Colombia
Bernal-Olaya, Rocio 1985-
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This dissertation is an integrated geologic and geophysical study of the slowly converging (20 mm/yr), amagmatic, and shallowly subducting (4-8˚) system of the Caribbean margin of Colombia that includes both its offshore accretionary prism (Sinu belt) and its overlying deep (8-9 km) and wide (150-200 km) forearc basin (Lower Magdalena basin, LMB). Based on 29000 earthquake relocations, interpretations of ~1200 km of offshore deep-penetration seismic reflection lines recorded to depths of 20 km along with coincident gravity models, and 7000 km2 of onshore seismic reflection lines tied to 33 wells, I define an east-southeastward (110˚) dipping zone of shallow subduction beneath northwestern Colombia, an actively deforming accretionary prism, and an Oligocene to Recent forearc basin. Low angle subduction occurs as a 13-16 km thick Late Cretaceous oceanic plateau subducts beneath South America. Tomographic data reveal that the subducted slab exhibits an unusual negative velocity anomaly at depths greater than 80 km that I interpret as an incipient slab break off related to the clogging of the trench by the 30-km-thick Panama arc. In the accretionary prism, Pliocene-Pleistocene deformation developed on an imbricated backward breaking imbricated fan of thrust faults with out-of-sequence thrust faulting (OOST) within inner and outer parts of the prism. Inactive strike-slip faults were identified in a trench-parallel orientation where the subduction convergence direction is more oblique (>13mm/yr). In the forearc basin, I describe initial Oligocene subsidence of the forearc basin along 70˚-110˚striking normal faults that remained active until Early Miocene. An angular unconformity spanning the interval of 11 to 7 Ma seems to evidence the initiation of low angle subduction. The Lower Magdalena basin (LMB) overfills with sediments of shallow marine- transitional facies and spills offshore along the proto-delta of the Magdalena fan where up to 5 km deposited during the Pleistocene to Recent. I propose that this change from the steeper to shallowing dipping slab in the Middle Miocene led to increasing elevation of the forearc high and the western part of LMB where a clear unconformity is subsequently deformed.