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

Topography in fold-thrust belts over geologic time reflects the development of an orogenic Coulomb wedge that represents a balance of tectonic and erosional forcings. The establishment of critically tapered topography is generally viewed under two contrasting mechanical frameworks: (i) shortening and rock uplift are occurring everywhere suggesting an orogenic wedge under mechanical failure everywhere; and (ii) rock displacement takes place along discrete fault planes, including the translation of uplifted topography laterally. Here we investigate whether the topography in central Nepal is maintained by a combination of rock uplift during sequential fault activity, the lateral translation of topography over ramp-flat subsurface geometries, and alternating phases of hinterland incision during out-of-sequence faulting and deformation front activity; if this is the case, then erosional efficacy dynamically varies along the orogenic wedge, in contrast to a wedge under mechanical failure everywhere. We test this hypothesis by employing a structural-kinematic model of the Neogene fold-thrust belt evolution of central Nepal and integrate this into a surface processes model applying end-member climatic scenarios, i.e., uniform precipitation and climatic change over geologic time. Model output is validated by comparing predicted geomorphic metrics with observed ones. Our results indicate a dynamic variability of erosional efficacy that promotes the interplay of two modes of orogenic wedge behaviour: phases of lateral translation of uplifted topography and in-sequence propagation of deformation fronts, and phases of hinterland incision during out-of-sequence fault activity.