The Phanerozoic vertical motions of the Middle East and North Africa: Indicators of the mechanisms driving undulations of continental platforms
|Category||Paleontology and Stratigraphy|
|Location||International Geological Congress,oslo 2008|
|Holding Date||11 October 2008|
The extensive Phanerozoic sediment cover of the Middle East and North Africa accumulated on a stable continental platform that became stabilized following the Pan African orogeny. It formed during several phases of deposition and erosion, each recording a distinct pattern of vertical motion (e.g. wavelength, geometry, and rate of subsidence/erosion). The long-term accumulation of the sediments records the cumulative results of successive tectonic motions driven by deep seated process, as well as the effects of sea level fluctuations, but in the long-run the tectonic effects dominate. The history of the sedimentary cover can thus help to decipher the geodynamic processes affecting continental platforms.
In the Paleozoic deposition extended >1500 km inland of the continental margin, and was controlled by several basins (depressions) and swells (uplifts), up to 500-1000 km across. The regional pattern of the vertical motions changed in several short periods when the structures were accentuated, some swells were uplifted, and extensive erosion occurred. The new structures were not obviously related to earlier erosion. These structures formed over up to several hundred million years, and persisted after new patterns of undulations were established. Thus the formation of the basin-swell structures seems to express a long-lasting change in the mass distribution (and isostasy) of the lithosphere. A different depositional pattern is recorded by sediments that accumulated along passive continental margin after these formed. They accumulated along elongated belts and wedge out on going inland, often in areas that were previously uplifted and eroded. At the same time oval basins may have formed further inland. Subsidence of areas after erosion suggests that thermal relaxation after heating events had a significant role. However, this affected areas many hundreds of kilometers inland of the regions that were faulted during the rifting event associated with the production of the continental margins. Uplifting of areas hundreds of kilometers across also occurred in association of intraplate volcanism and rifting, which is most obvious in the Cenozoic.
The distinct styles of vertical motions raise the possibility that the different mechanisms operated. Some vertical motions may express disturbances of the thermal regime and subsequent thermal relaxation, or changes in the flow in the mantle. Such processes are expected to have transient effects. Others seem to have resulted from long-lasting changes in the lithosphere. In all cases large areas have been affected and the processes acting on any particular area changed with time.