Reconstruction of topography and related depositional systems during active thrusting

Abstract

Reliable reconstruction of former topography in deformed regions is commonly difficult, due to degradation of former erosional and depositional surfaces. In contrast to most modem landscapes, however, ancient localities can sometimes provide clearer insights on subsurface geometries of deposition, deformation, and erosion and on their variations through time. In some exceptional circumstances, ancient depositional sequences are preserved in direct juxtaposition with the structures that controlled their geometrical and sedimentological character. We describe here the evolving topography and depositional responses caused by the late Eocene growth of a detachment fold and related thrusts in the southern Pyrenees. Topography within these deforming systems can be reconstructed on the basis of (1) relief associated with paleovalleys, (2) geometric relationships of syntectonic strata with adjacent structures, and (3) relief of hanging walls above depositional or erosional surfaces of the same age. Onlapping, offlapping, and overlapping stratigraphic relationships are interpreted in the context of the relative rate 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 hinterland, and a smaller transverse system that carried locally derived clasts. During fold growth, syntectonic sedimentary beds (growth strata) were progressively rotated in the forelimb of the fold. Proximal unconformities 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 angles to the fold axis. A switch from incision to infilling of the paleovalleys appears to be controlled by relative rates of subsidence, sediment supply and accumulation, and uplift. During an interval of rapid accumulation and low rates of subsidence and uplift, the effects of rising local base levels propagated up the transverse valleys, where they initiated backfilling of the paleovalleys. As deformation began on an adjacent, more hinterlandward thrust, waning growth of the detachment fold permitted depositional overlap of its crest, as sedimentation shifted toward the hinterland. Subsequently, as the new footwall was folded, longitudinal rivers filled the space formerly occupied by transverse 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 stratal geometries, reconstructed river patterns, precise stratigraphic ages, and preserved erosional surfaces can be combined to reconstruct evolving topography during active folding and faulting in terrestrial environments.