RECONSTRUCTION OF TOPOGRAPHY AND RELATED DEPOSITIONAL SYSTEMS DURING ACTIVE THRUSTING

Reliable reconstruction of former topography in deformed regions is co mmonly difficult, due to degradation of former erosional and depositio nal surfaces. In contrast to most modem landscapes, however, ancient l ocalities can sometimes provide dearer insights on subsurface geometri es of deposition, deformation, and erosion and on their variations thr ough time. In some exceptional circumstances, ancient depositional seq uences are preserved in direct juxtaposition with the structures that controlled their geometrical and sedimentological character. We descri be here the evolving topography and depositional responses caused by t he late Eocene growth of a detachment fold and related thrusts in the southern Pyrenees. Topography within these deforming systems can be re constructed on the basis of (1) relief associated with paleovalleys, ( 2) geometric relationships of syntectonic strata with adjacent structu res, and (3) relief of hanging walls above depositional or erosional s urfaces of the same age. Onlapping, offlapping, and overlapping strati graphic relationships are interpreted in the context of the relative r ate of sediment accumulation versus the rate of uplift of the crest of the fold. In the study area, two contrasting fluvial systems provided sediment to the deforming area: a large longitudinal system, flowing parallel to the fold axes and carrying detritus from the distant hinte rland, and a smaller transverse system that carried locally derived cl asts. During fold growth, syntectonic sedimentary beds (growth strata) were progressively rotated in the forelimb of the fold. Proximal unco nformities developed in the forelimb growth strata, when accumulation rates were low. Topographic relief on the backlimb of the growing fold caused transverse paleovalleys (>150 m deep) to be incised at high an gles to the fold axis. A switch from incision to infilling of the pale ovalleys appears to be controlled by relative rates of subsidence, sed iment supply and accumulation, and uplift. During an interval of rapid accumulation and low rates of subsidence and uplift, the effects of r ising local base levels propagated up the transverse valleys, where th ey initiated backfilling of the paleovalleys. As deformation began on an adjacent, more hinterlandward thrust, waning growth of the detachme nt fold permitted depositional overlap of its crest, as sedimentation shifted toward the hinterland. Subsequently, as the new footwall was f olded, longitudinal rivers filled the space formerly occupied by trans verse rivers, and a new detachment fold grew in the very shallow (<25 m) subsurface. Although similar examples are scarce in the geological record, the synthesis from this Pyrenean locale illustrates how strata l geometries, reconstructed river patterns, precise stratigraphic ages , and preserved erosional surfaces can be combined to reconstruct evol ving topography during active folding and faulting in terrestrial envi ronments.