DENUDATION, FISSION-TRACK ANALYSIS AND THE LONG-TERM EVOLUTION OF PASSIVE MARGIN TOPOGRAPHY - APPLICATION TO THE SOUTHEAST BRAZILIAN MARGIN

A detailed transect of fission track data is combined with present day elevation to constrain the evolution of topography across the continental margin of southeast Brazil since break-up. The fission track data show clearly that rocks on the low-lying coastal plain have been exhumed from temperatures of >110-120 degrees C over the last 100 Ma, while the basalt capping the escarpment has not been at temperatures >50 degrees C since its eruption similar to 130 Ma. The fission track data are broadly consistent with a model of scarp retreat. Modelling the denudation history and associated isostatic rebound highlights the sensitivity of the predictions to the assumed effective flexural rigidity of the continental lithosphere. Estimates of denudation based on modelling the topography alone rang from 2 to 6 km and the amount of denudation possible before elevation reaches sea-level is reduced for higher flexural rigidities (10(23-25) Nm). Consequently high geothermal gradients (45-60 degrees C/km) are required to predict the fission track data satisfactorily. The greater inferred levels of denudation for flexural rigidities of 10(19-21) Nm allow the fission track data to be predicted adequately with gradients of 15-30 degrees C, values similar to present day estimates. An appropriate value for the flexural rigidity needs to be determined independently before we can confidently constrain the contributions of long term tectonic uplift and erosional rebound to the present day elevation.