TOPOGRAPHIC ACCESSIBILITY AND THE TECTONIC INTERPRETATION OF GRAVITY-DATA

During recent gravity surveys in Patagonia, we were unable to acquire data on the North Patagonian Icecap, which is completely inaccessible for land surveying. The Icecap is clearly of tectonic significance, bu t, because of its tectonic history, it is uplifted and rugged, and con sequently inaccessible. Therefore the distribution of geophysical obse rvations which can be acquired in a rugged field area is dependent on the tectonic history. This suggests that a tectonic history which is r econstructed from geophysical data may be systematically biased. We us e models of local and flexural isostasy to estimate the gravity errors associated with interpolation across inaccessible topography The grav ity error is largest for Pratt isostasy, where the mass deficit which supports the topography is at relatively shallow depths. The gravity e rror is least for flexural isostasy, because in this case the inaccess ible topography is supported regionally by a mass deficit which extend s beyond the inaccessible region. An analytical flexural interpolation scheme is proposed for interpolation across data gaps associated with inaccessible topography. Flexural theory and Gauss's theorem are used to predict the gravity anomaly due to the mass excess of the inaccess ible topography. We apply this scheme to the North Patagonian Icecap, to demonstrate that flexural interpolation predicts a relative gravity low at the site of the Icecap, which would not be predicted by purely geometrical interpolation schemes.