GRAVITY INTERPRETATION OF THE COMAYAGUA VALLEY, HONDURAS

The Comayagua Valley in central Honduras is part of a series of discontinuous topographic lows that some researchers have informally termed the Honduras depression. Because 75 percent of the Republic of Honduras is extremely rugged and mountainous, travel is difficult and few geophysical investigations have been conducted in the region. We reprocessed gravity data collected by geothermal consultants during the 1980s and performed over 450 terrain corrections to calculate the complete Bouguer gravity anomaly for the Comayagua Valley. The regional trend was estimated (using gravity stations outside the valley) and removed, revealing a residual gravity anomaly exceeding –30 milligals. We used vertical prisms on a 1-km grid and a Bott-like recursion procedure to calculate the thickness of sediment fill throughout the valley. Assuming a density contrast of -400 kg-m^-3 between country rock and sedimentary fill, results reveal over 2,600 m of sediment fill 7 km west-southwest of Comayagua, where the surface elevation is less than 600 m. This places the base of fill over 2 km below sea level. The area below sea level resembles two elliptical basins, filled with more than 400 cubic kilometers of sediment, with major axes trending N 20^O to 25^O W. The deeper northwestern basin is 17 km long and 8 km wide. Steep gravity gradients suggest that faults form the northeast and west sides of this deeper basin, and that these faults converge northwest of Comayagua. Sedimentary fill in the northeast and southwest corners of the Comayagua Valley is relatively thin, suggesting that post-tectonic mass wasting has significantly modified the visible expression of basin-forming tectonics. These concealed basins resemble subsidence calderas.