First-order foreland cycles: Interplay of flexural tectonics, dynamic loading, and sedimentation

Foreland systems on overriding plates (i.e., retroarc settings) form through the deflection of the lithosphere in response to a combination of supra- and sublithospheric loads. Supracrustal loading by orogens leads to the partitioning of foreland systems into foredeep, forebulge, and back-bulge flexural provinces. Thrusting in the orogenic belt results in foredeep subsidence and forebulge uplift, and the reverse occurs as orogenic load is removed by erosion or extension. This pattern generates out-of-phase ('reciprocal') stratigraphies across the flexural hingeline that separates the foredeep from the forebulge. Coupled with flexural tectonics, accommodation is also modified by dynamic (sublithospheric) loading. The latter mechanism operates at larger, continental scales, with rates controlled by the velocity and the angle of subduction. The interplay of accommodation and sediment supply during the lifespan of a foreland system controls the long-term shift from underfilled to overfilled accommodation conditions, which defines the first-order foreland cycle. Predictable shifts in the balance between flexural tectonics and dynamic loading during the evolution of a foreland system allow the subdivision of the first-order foreland cycle into early and late stages dominated by flexural tectonics, separated by an intermediate stage dominated by system-wide dynamic subsidence. The early stage is defined by an underfilled foredeep that hosts a deep-water environment, and an exposed forebulge whose progradation and erosion generates the 'forebulge unconformity' at the base of the first-order foreland sequence. Submergence of the forebulge during the intermediate stage establishes an interior seaway across the entire foreland system. The late stage marks a shift to a system-wide continental setting, which defines the overfilled phase of the foreland cycle.