MORPHOTECTONIC EVOLUTION OF RIFTED CONTINENTAL MARGINS - INFERENCES FROM A COUPLED TECTONIC-SURFACE PROCESSES MODEL AND FISSION-TRACK THERMOCHRONOLOGY

We use a numerical model to study the topographic evolution and erosio nal history of rifted continental margins. The model combines a kinema tic description of lithospheric extension (''tectonic model'') with a surface processes model that includes short-range hillslope and long-r ange fluvial transport. The tectonic model predicts the evolution of t he lithospheric temperature and strength (elastic thickness) distribut ion as well as the tectonic uplift through time. This is input into th e surface processes model which calculates the degradation of topograp hy and associated isostatic rebound. Modeled denudation histories acro ss the margin are used to predict apatite fission track age and length patterns. Modeling results indicate that, depending on the adopted pa rameters, an uplifted rift flank should degrade by erosion within 50-1 00 m.y., without significant retreat of the topographic elevation maxi mum. The development of an escarpment system at rifted continental mar gins is in itself not an indication of tectonic rift flank uplift, but results from the existence of a high elevation interior plateau, eros ional base-level lowering as a consequence of rifting and regional iso static response to erosion of the margin. However, apatite fission tra ck thermochronology reveals that the areas seaward of the escarpment a t a number of rifted margins have been exhumed from several kilometers depth. Such amounts of denudation cannot be accommodated with isostat ic rebound alone and require additional tectonic uplift of the rift fl anks. Modeling of apatite fission track patterns suggests that fission track thermochronology dates the onset of rapid erosion which coincid es with the initiation of strong relief (i.e., initiation of rifting). Fission track ages which are younger than the age of rifting thus can not be unambiguously interpreted as excluding prerift uplift. The timi ng of margin uplift can be established only by careful track length an alysis and integration with regional stratigraphic data. The model is applied to the Saudi Arabian Red Sea margin and the southeastern Austr alian highlands, where it is constrained by present-day topography and apatite fission track data, as well as seismic and gravity data. For the Saudi Arabian Red Sea margin, synrift regional plateau uplift with a magnitude of approximately 1 km is inferred, possibly as a result o f asthenospheric upwelling. Flexurally induced tectonic uplift of the rift flank with a magnitude of 2 km is superimposed on this regional u plift. The relatively high elevation of the southeastern Australian hi ghlands, their steep front and the relatively high amounts of erosion suggest that, apart from Mesozoic synrift flexural uplift, Tertiary re juvenation of topography has occurred, possibly as a result of renewed lithospheric thinning and underplating. The low elevation of the Aust ralian interior would inhibit the evolution of a major escarpment in t he absence of such renewed uplift.