Richard Styron

In many locations in the Himalaya and Tibet, extensional stepovers on strike-slip faults occur beneath pre-existing topographic highs. An influential physical model of orogens, explaining contemporaneous high-elevation normal faulting and low-elevation reverse faulting, holds that horizontal tectonic compression is invariant across the orogen while vertical stress varies with topography, changing the balance of stresses. This model is two-dimensional and requires topography to be supported isostatically, and therefore cannot fully describe strike-slip to normal fault transitions beneath mountains to small mountain ranges, as this is a three-dimensional deformation field and topography of this wavelength is supported isostatically. I introduce a 3D elastic model describing the modulation of fault kinematics by shorter-wavelength topographic stress, and show how the model can place constraints on the tectonic stress field. I then calculate the topographic stress field on the Western Nepal Fault System, and use topographic stresses and observed fault kinematics to invert for the tectonic stress field. The results yield a maximum tectonic compression of 0–0.2 rho gz and minimum tectonic compression of -0.1–0.1 rho gz, and reproduce kinematics from normal, strike-slip and thrust faults and earthquakes in and around western Nepal, including the 2015 Gorkha earthquake. This demonstrates that where vertical and a horizontal principal stress are near equal, 1-10 km scale variations in topography can change fault kinematics, and that pre-existing topography can influence the location of subsequent faults and stepovers.