Geological formations in Tegucigalpa from Bouguer anomalies using Talwani polygons

Abstract

Two 20-to-25-km-long gravity transects oriented W-E and NE-SW across Tegucigalpa were analyzed for cross-sectional 2D geological structure down to depths of 1,800 meters, by means of forward modelling of residual gravity using Talwani’s gravity calculations for arbitrarily shaped polygons. For this purpose, we produced tables of Bouguer anomalies for the Tegucigalpa quadrangle by refining values from a previous study with improved terrain and normal gravity corrections. Both polygon geometries and apparent densities were adjusted within the constrains provided by surface geology, stratigraphy and cross sections available for the study area. Positive residual gravity found above both profile’s midsection suggests either an intrusive pluton or mass of altered lutite and sandstone (Jkhg formation), up to 1.5km thick, underlying a 600-m-thick bed of sedimentary deposits (Kvn). Gravity lows are interpreted as the thickening of Tertiary tuff deposits (Tpm) over the edges of the profiles, to a depth of about 500 to 1100 m. Notwithstanding the non uniqueness and uncertainties inherent to sub-surface models derived from residual Bouguer gravity alone, this study represents a first attempt at exploring the complex subsurface geology below Tegucigalpa down to 1900m over lengths up to 25 km, using the gravity method.