abstract

Two seismic refraction-wide angle reflection profiles made in the Western Cordillera of the Colombian Andes show high, upper crustal (O to 15 km) velocities: the compressional velocity increases from 4.5 km/sec at the surface to 6.7 km/sec at 6 km depth and to 7.0 km/sec at 11 km depth; the shear velocity increases from 2.5 km/sec at the surface to 4.0 km/sec at 11 km depth. Travel-time and amplitude modeling of first arrivals has resulted in an upper crustal velocity-depth function which is characterized by gradients, and modeling of reflected arrivals has resulted in two alternative lower crustal (15 to 29 km) models, one with and one without velocity inversions. An intra-crustal boundary occurs at 19 to 21 km depth, and the moho at 26.5 to 29 km depth.

These results, which are based on 1976 recordings to both the north and west of a quarry located near 4°N, confirm indications from 1973 reconnaissance data of high-seismic velocities at shallow depths at both ∼4°N and ∼1°N. A new, laterally inhomogeneous upper-crustal model for the 1973 seismic data at 1°N has been made which indicates 6.7 km/sec material at a depth of 3.0 km in the Cordillera.

The upper crustal velocities from the Cordillera are similar to those from well-controlled determinations for ocean crust. These seismic results, combined with the positive Bouguer gravity anomalies and the geological evidence, suggest that the upper crust of the Western Cordillera consists of igneous oceanic materials of Cretaceous age.

The velocity structure of the lower crust cannot be accurately determined with the present data; the Cordillera may consist entirely of oceanic rocks, or of ∼15 km of oceanic rocks overlying continental rocks.

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