GRAVITY-BASED MASS-BALANCE OF AN ALLUVIAL-FAN GIANT - THE ARCAS FAN, PAMPA DEL TAMARUGAL, NORTHERN CHILE

fans and their catchments provide joint erosion and sedimentation regi ons, where exogenic mass transfer can be handled as a finite volume pr oblem. The Areas Fan in the Andean forearc of Northern Chile, one of t he world's largest alluvial fans, has been mapped by a high-resolution gravity survey to calculate sedimentary mass by means of interactive 3D modeling. Erosive mass is estimated by a digital 3D volume model of the catchment. The fan volume of 110 km(3) (Delta V = +/-6%) with a m ean sediment density of 1.80 g cm(-3) (Delta rho = +/-2%) yields a mas s of 198x10(9) ton (Delta m = +/-8%). The catchment loss of 115 km(3) (Delta V = +/-6%) of rock with a mean density of 2,36 g cm(-3) Delta r ho = +/-2%) points to a total erosion of 271x10(9) ton (Delta m = +/-8 %), about 73x10(9) ton (Delta m = +/-8%) more than calculated in the f an. The mass difference of 27% is related to subsurface dissolution, w ind deflation, and surface denudation post-dating deposition. K-Ar age s of ignimbrites topping a peneplain (7.3+/-0.2 Ma), which covered par ts of the catchment, an ignimbrite intercalated in the oldest fan sedi ments (7.2+/-0.2 Ma) and tuff layers near the distal fan surface (6.8/-0.2 Ma) suggest a short duration (0.5 my) of the exogenic mass trans fer. The erosion rate of 0.318 m ka(-1) and the sedimentation rate of 0.296 m ka(-1) are interpreted as a climatic signal for more humid con ditions in Late Miocene times. The erosion rate is nearly equal to the erosion rates of about 0.300 m ka(-1) proposed for the Bolivian Andes in the Late Miocene. Area and extent of the mass turnover were too in significant to cause catchment uplift due to erosion and fan area defl ection due to sedimentation.