Gravity and height changes in the ocean-continent transition zone in western Colombia

Crustal studies in western Colombia, by deep seismic, gravity and geomagnetic surveys, during the last two decades have revealed an extremely anomalous crustal structure as compared to the South American Andes further south. Strong gravity gradients and differences in seismic velocities showed a transition from oceanic to continental character between the Western and Central Andes.

Measured gravity and height variations of opposite sign and lengths of 50 to 100 km on three east-west running profiles correlate surprisingly well with the typical positive Bouguer anomaly of the Western Andes which represents an isostatic instability. A gravity decrease of 0.5–1.0 mGal on two profiles and an increase on an intermediate one and corresponding ratios of gravity to apparent height variations of nearly ?20 mGal/m are interpreted as consequences of deep-seated density variations. They may be related to collision tectonics and recent obduction processes between aseismic ridges riding on the Pacific Nazca plate and the continent.