Tectonic Drivers of Foreland Basin Unconformities

Unconformities within foreland basins reflect tectonic, climatic, eustatic, and internally driven (autogenic) processes along convergent plate boundaries. These processes determine zones of erosion, sediment bypass, and deposition in the thrust belt, foreland basin, and distal craton over various time frames. Recognition of long-lived (>1–20 Myr) stratigraphic hiatuses defined by regional disconformities (or highly condensed sections) with little to no angular discordance presents a challenge to steady-state models of foreland basin evolution. In retroarc systems, potential mechanisms of protracted accommodation reduction and unconformity generation include: (1) shortening in the frontal thrust belt and proximal foreland; (2) growth and migration of a broad low-relief forebulge; (3) uplift of intraforeland basement blocks; (4) isostatic rebound during a pause in shortening; (5) oblique convergence with accompanying cessation of flexural subsidence; (6) reduced accommodation due to a change in sea level, climate, or erosion; (7) regional uplift during flat-slab subduction; and (8) dynamic uplift associated with a slab window, slab breakoff, in-plane stresses, or other mantle process. These contrasting mechanisms are observed in Andean-type systems, with notable cases involving increased interplate coupling during active flat slab subduction and slab window generation associated with subduction of an oceanic spreading ridge. In the ancient record, the drivers of unconformity development can be distinguished on the basis of the spatial distribution, stratigraphic position, paleoenvironmental context, and duration of foreland hiatuses within the geodynamic framework of the orogenic system. Such stratigraphic studies benefit from detailed assessments of the geochronological, geochemical, provenance, and subcrop relationships across unconformity surfaces.