Drivers of Topography in Fold-thrust Belts: A Perspective from Central Nepal

Topography in compressional mountain ranges represents an interface at which tectonic and climatic forces interact. Understanding the relative contribution of these two components to mountain formation has been at the forefront of research over the last two decades. The theory underlying the mechanics that govern these interactions has been built on Coulomb wedge mechanics, i.e., mechanical failure and rock uplift occur everywhere along the wedge and the orogen. Observed rock displacement along single, discrete fault planes, including the translation of uplifted topography laterally, appears to be counter to such mechanics. However, a critically tapered topography across fold-thrust belts still emerges. If a critically tapered topography along an orogenic wedge can be produced by the sequential evolution of the subsurface fault geometry and the associated motion of bedrock over discrete fault planes, then a mechanical failure everywhere is not required. Here, the geomorphic evolution of the fold-thrust belt in central Nepal since the Miocene is investigated using a numerical surface processes model whereby the structural geometry, location and magnitude of fault motion are prescribed and based on observations. In addition, end-member climatic scenarios are adopted, i.e., uniform precipitation and climatic change over geologic time as predicted by atmospheric general circulation models. The experiments reproduce the first-order topography of central Nepal. Our modelling results indicate a dynamic variability of erosional efficacy that promotes the interplay of two modes of orogenic wedge behaviour and are contrary to a mechanical failure everywhere along the wedge: (mode 1) phases of lateral translation of uplifted topography and in-sequence propagation of deformation fronts, and (mode 2) phases of hinterland incision during out-of-sequence fault activity. The successful replication of first-order geomorphic indices in central Nepal in our experiments confirms an unusually long-lasting Miocene to Pliocene activity of the Main Boundary Thrust in central Nepal. This period is followed by Late Pleistocene hinterland incision coeval with out-of-sequence fault activity prior to the onset of rock displacement along the Main Frontal Thrust during a time of increased precipitation relative to today.