Retroarc foreland systems - evolution through time

Retroarc foreland systems form through the flexural 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 flexural provinces, i.e. the foredeep, forebulge, and back-bulge. Renewed 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 of opposite vertical tectonics modifies the relative amounts of available accommodation in the two flexural provinces, and may generate out of phase (reciprocal) proximal to distal stratigraphies. Coupled with flexural tectonics, additional accommodation may be created or destroyed by the superimposed effects of eustasy and dynamic (sublithospheric) loading. The latter mechanism operates at regional scales, and depends on the dynamics and geometry of the subduction processes underneath the basin. The eustatic and tectonic controls on accommodation may generate sequences and unconformities over a wide range of time scales, both > and <10(6) yr. The interplay of base level changes and sediment supply controls the degree to which the available accommodation is consumed by sedimentation. This defines the underfilled, filled, and overfilled stages in the evolution of a foreland system, in which depositional processes relate to sedimentation in deep marine, shallow marine, and fluvial environments respectively. Each stage results in typical stratigraphic patterns in the rock record, reflecting the unique nature of flexural and longer-wavelength controls on accommodation. Predictable shifts in the balance between flexural tectonics and dynamic loading allow subdivision of the first-order foreland cycle into early and late phases of evolution dominated by flexural tectonics, and a middle phase dominated by system-wide dynamic subsidence. The early phase dominated by flexural tectonics corresponds to an early underfilled foredeep and a forebulge elevated above base level, whose erosion and rapid progradation results in the formation of the forebulge (basal) unconformity. The middle phase dominated by dynamic subsidence corresponds to a stage of system-wide sedimentation, when the forebulge subsides below the base level and the foredeep goes from a late underfilled to a filled state. The late stage dominated by flexural tectonics corresponds to the first-order overfilled stage of foreland evolution, when fluvial sedimentation is out of phase across the flexural hingeline of the foreland system. (C) 2004 Elsevier Ltd. All rights reserved.