Impact of inherited geometries on syn-orogenic foreland basin
Résumé
Foreland basins record the evolution of orogens through sedimentary accumulation and recycling and, as such, represent unique archives of the evolution of orogenic systems. Foreland basins are, however, complex source-to-sink systems responding to the uplift of the mountain range, thrusting, eustasy, climate, and the type of lithologies eroded. The respective contributions of these parameters has been numerically evaluated, the influence of the inherited geometries of the foreland on the sediment routing systems has not yet been investigated.
We use a Landscape Evolution Model (FastScape) to explore the effect of varying foreland paleo-topography on its stratigraphic architecture. Models consist of a half mountain range steadily uplifting (0.5 mm/yr) over 25 Myrs. Eroded material is transported and deposited in a foreland domain and a distal open marine domain. We present 4 setups with varying paleo-topographies in the foreland domain: an initial flat foreland at sea level (M1), an elevated flat continental foreland (+300 m, M2), a pre-existing 1 km-deep and 100 km-wide depression at the foot of the growing orogen with either a flat forebulge at sea level (M3) or an elevated forebulge (+300m; M4).
Our result show that an elevated foreland domain produces, after 25 Myrs, a thinner foreland basin because the faster and efficient sediment export of sediment out of the foreland to the open marine domain reduces the sedimentary load and, in doing so, the flexure. In contrast, a pre-existing depression at the foot of the range, produces a thicker foreland basin because the additional load filling the initial space increases the flexural response. We show that a pre-existing bathymetry is required to preserve marine deposits in the foreland basin. In our experiments, the landscape after 25 Myrs cannot be used to infer the initial foreland geometry as the initial foreland influences is smoothed out after ~10-15 Myrs. The stratigraphic architectures of the foreland basin are nonetheless different. In addition, we show that emplacement of an alluvial fan at the foot of the range results in a transient drop in erosion rate in the range by locally increasing the base-level (autogenetic feedback).
