Vertical versus horizontal motions in the Alpine orogenic wedge: stratigraphic response in the foreland basin



The long-term (50 Myr) development of orogenic thrust wedges and their neighbouring foreland basins are inextricably linked. Foreland basins, such as occur nonh of the Alps, flanking the Pyrenees, east of the Apennines and south of the Himalayas, are characterized by an early underfilled (deep-water) phase, followed by a filled (shallow-marine) or overfilled (continental) phase. The extent to which a foreland basin is filled to sea-level can be understood by comparing, over time, the rate at which sediment is delivered to the basin with the rate at which accommodation space is generated within the basin. This approach is applied to the Alpine thrust wedge/North Alpine Foreland Basin (NAFB) system. Using simple geometrical calculations assuming a critically tapered thrust wedge sliding over a foreland plate, preceded by a flexurally induced trough (foreland basin), it is possible to develop some general parameter relationships for the system. The thrust wedge is described firstly in terms of the history of exhumation which is directly linked to denudation and hence sediment generation, and secondly to thrust front advance rates. The accommodation space can be approximated by the product of the thrust front advance rate and the deflection of the basin at the thrust front. The extent to which a basin is, at one instant, being undersupplied or oversupplied (F) can then be described by the ratio of sediment supply to accommodation space generated:

F=exhumation rate. width of thrust wedgeladvance rate. deflection at thrust front

When F> I, the basin is on a trend towards overfilling, and when F< 1 the basin is on a trend towards underfilling. The value for the width of the thrust wedge undergoing exhumation is held as a constant for most of the evolution of the NAFB.

The development of the Alpine thrust wedge/NAFB system is characterized by an initial submarine phase (Cenomanian to mid-Oligocene) of rapid thrust front advance and slow exhumation rates; this resulted in deep-water and shelfal sedimentation in the foreland basin (F< 1), and is described as the occretionary wedge phase. By the mid-Oligocene, exhumation rates were accelerating associated with major backthrusting, and frontal advance rates were slowing down; this resulted in the foreland basin being filled to sea-level (F← 1), accumulating shallow-marine and continental sediments. This period is described as the continental wedge phase. Although F-values probably fell back to close to 1 following the period of major Oligocene exhumation, the basin remained in its filled state for the remainder of its geological history.

Modern studies indicate that rapid exhumation rates lead to increased local relief and denudation. The increased relief causes increased maximum elevations, so enhancing orographic precipitation and glaciation leading to further increases in denudation. This positive feedback loop between denudation, exhumation and climate may have enhanced the rapid inversion of the core of the orogenic wedge during the latter part of its growth.