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 erosional history of rifted continental margins. The model combines a kinematic description of lithospheric extension (''tectonic model'') with a surface processes model that includes short-range hillslope and long-range fluvial transport. The tectonic model predicts the evolution of the lithospheric temperature and strength (elastic thickness) distribution as well as the tectonic uplift through time. This is input into the surface processes model which calculates the degradation of topography and associated isostatic rebound. Modeled denudation histories across the margin are used to predict apatite fission track age and length patterns. Modeling results indicate that, depending on the adopted parameters, an uplifted rift flank should degrade by erosion within 50-100 m.y., without significant retreat of the topographic elevation maximum. The development of an escarpment system at rifted continental margins is in itself not an indication of tectonic rift flank uplift, but results from the existence of a high elevation interior plateau, erosional base-level lowering as a consequence of rifting and regional isostatic response to erosion of the margin. However, apatite fission track thermochronology reveals that the areas seaward of the escarpment at a number of rifted margins have been exhumed from several kilometers depth. Such amounts of denudation cannot be accommodated with isostatic rebound alone and require additional tectonic uplift of the rift flanks. Modeling of apatite fission track patterns suggests that fission track thermochronology dates the onset of rapid erosion which coincides with the initiation of strong relief (i.e., initiation of rifting). Fission track ages which are younger than the age of rifting thus cannot be unambiguously interpreted as excluding prerift uplift. The timing of margin uplift can be established only by careful track length analysis and integration with regional stratigraphic data. The model is applied to the Saudi Arabian Red Sea margin and the southeastern Australian 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 of asthenospheric upwelling. Flexurally induced tectonic uplift of the rift flank with a magnitude of 2 km is superimposed on this regional uplift. The relatively high elevation of the southeastern Australian highlands, their steep front and the relatively high amounts of erosion suggest that, apart from Mesozoic synrift flexural uplift, Tertiary rejuvenation of topography has occurred, possibly as a result of renewed lithospheric thinning and underplating. The low elevation of the Australian interior would inhibit the evolution of a major escarpment in the absence of such renewed uplift.