Magnetic stratigraphy of the Bucaramanga alluvial fan: Evidence for a ≤3 mm/yr slip rate for the Bucaramanga-Santa Marta Fault, Colombia

The 550 km long Bucaramanga-Santa Marta Fault is one of the main active tectonic features of NW South America. It is a left-lateral strike-slip fault bounding the Maracaibo block, and straddling northern Colombia from the Caribbean Sea to the Eastern Cordillera. Variable total displacement values (from 40 to 110 km), and present-day slip rates (from 0.01 to 10 mm/yr) have been proposed so far for the Bucaramanga Fault. Here we report on the paleomagnetic investigation of a Plio-Pleistocene (?) continental alluvial fan juxtaposed to the Bucaramanga Fault, and horizontally displaced by 2.5 km with respect to its feeding river. Nine (out of fourteen) reliable paleomagnetic directions define a succession of six different magnetic polarity zones that, lacking additional age constraints, can be correlated with several tracks of the Plio-Pleistocene magnetic polarity time scale. If the youngest age model is considered, most recent sediments of the fan can be reasonably dated at 0.8 Ma (Brunhes-Matuyama chron transition), translating into a maximum 3 mm/yr slip rate for the Bucaramanga Fault. Older age models would obviously yield smaller slip rates. Our paleomagnetic sites, located at 4–10 km from the fault, do not show significant rotations, implying weak fault coupling and/or ductile upper crust behavior adjacent to the Bucaramanga Fault.