In addition to topographic uplift, orogenesis commonly results in a region of subsidence called a foreland basin or foredeep (Dickinson, 1974). The foreland lies at the external edge of the orogen toward the undeformed continental interior (e.g. Fig. 10.7). If a volcanic arc is present, it coincides with the backarc region of the margin. Its counterpart, the hinterland, corresponds to the internal zone of the orogen where the mountains are highest and rocks the most intensely deformed.
Foreland basins form where crustal thickening and topographic uplift create a mass of crust that is large enough to cause flexure (Section 2.11.4) of the continental craton. This flexure creates a depression that extends much farther into the surrounding craton than the margin of the thickened crust. It is bounded on one side by the advancing thrust front and on the other by a small flexural uplift called a forebulge (e.g. Fig. 10.18). The basin collects sedimentary material (molasse) that pours off the uplifting mountains as they experience erosion and as thrust sheets transport material onto the craton. Its stratigraphy provides an important record of the timing, paleogeography, and progressive evolution of orogenic events.
The shape of a foreland basin is controlled by the strength and rheology of the lithosphere. A low flexural rigidity, which characterizes young, hot and weak lithosphere, results in a narrow, deep basin. A high flexural rigidity, which characterizes old, cool and strong lithosphere, produces a wide basin with a better-developed forebulge (Flemings & Jordan, 1990; Jordan & Watts, 2005). Variations in the strength and temperature of the lithosphere can thus cause the character of the foreland basin to change along the strike of the orogen. Other factors such as inherited stratigraphic and structural inhomogeneities also influence basin geometry. In the Andes, an along-strike segmentation of the foreland partly coincides with variations in these properties and with the segmented geometry of the subducted Nazca plate (Section 10.2.3).
As a result of lithospheric flexure, the sediment thickness in a foreland basin decreases away from the mountain front to a feather edge on the forebulge (Flemings & Jordan, 1990; Gomez et al., 2005). Close to the mountain range the sediments are coarse grained and deposited in a shallow water or continental environment; at the feather edge they are fine grained and often turbid-itic. The sediments thus form a characteristic wedge-shaped sequence in profile whose stratigraphy reflects the subsidence history of the basin as it grows and migrates outwards during convergence. The stratigraphy is thus characterized by units that thin laterally, overstep older members, or may be truncated by erosion.
Belts of deformed sedimentary rock in which the layers are folded and duplicated by thrust faults are common in foreland basins. Like their counterparts in accretionary prisms (Section 9.7, Fig. 9.20) and in zones of transpression (Section 8.2, Fig. 8.8b), foreland fold and thrust belts form as the crust is shortened in a regime of compression (Fig. 10.5). During shortening, small sedimentary basins called piggyback basins may form on the top of moving thrust sheets.
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