Flat Moho beneath orogens and extensional regions: What controls it?

Flat Moho is a characteristic feature beneath extended continental lithosphere and orogenic plateaus; however, the physical processes that govern their formation remain poorly understood. In particular, the mechanical conditions required for lower crustal flow to effectively suppress Moho deflection are still debated. It has been proposed that lower crustal flow may facilitate lateral mass redistribution, thereby limiting Moho deflection and Moho relief during extension. Here, we compare seismological (receiver function) and gravity data and geodynamic models to identify the controls of Moho variation across various tectonic regions. Namely, we perform two suites of two-dimensional visco-plastic numerical models using the finite element code ASPECT with systematically vary (1) the minimum effective viscosity of the lower crust, and (2) its brittle strength, represented by cohesion. Each model simulates the extension of a 50 km-thick crust overlying a previously thinned lithospheric mantle, allowing us to isolate the rheological controls on Moho geometry and crustal deformation. Our results show that the primary factor governing Moho topography is the viscosity of the lower crust. When the lower crust is weak (≤ 10¹⁸ Pa·s), the viscous flow efficiently redistributes the material, leading to diffuse deformation and flat Moho (ΔMoho < 5 km). In contrast, high-viscosity models (≥ 10²¹ Pa·s) exhibit localized crustal thinning and pronounced Moho deflection, with relief up to 50 km and slopes exceeding 0.04 km/km. Varying the cohesion of the upper crust influences the distribution of brittle strain, but has a limited effect on Moho morphology. We conclude that flat Moho geometries arise from the integrated mechanical response of the crustal column where a sufficiently weak lower crust accommodates crust-mantle decoupling. These findings provide a quantitative framework to interpret observed flat Moho patterns in extensional settings such as the western Anatolia, the Basin and Range Province, and Tibetan Plateau.