ROLE OF CRUSTAL STRETCHING ON SUBSIDENCE OF THE CONTINENTAL-CRUST

The principal areas of subsidence of the continental crust are wide intracontinental basins and passive continental margins. In many intracontinental basins crustal extension amounts to only a few percent and is unable to explain their subsidence, as shown for the Timan-Pechora, Pre-Caspian and West Texas basins. The deep Black Sea and South Caspian basins are underlain by highly attenuated continental crust, having velocities in the 7 km/s range, that shows no evidence for significant extension. In passive margins, upper crustal extension ranges between 10 and 40%, whereas the lower crust is attenuated by 2-3 times (e.g., Campos Basin, north Biscay margin). Large discrepancy between upper crustal stretching and lower crustal attenuation is seen in the crustal configuration of many intracontinental grabens (e.g., Baikal and Dniepr-Prypyat basins). Only in highly extended areas, such as the Basin and Range Province and the East China rift system, does crustal stretching account for the observed crustal thinning. The observed discrepancy between upper extension and crustal thinning across intracontinental rifts and passive margins, and particularly in some wide intracontinental basins suggests that crustal stretching alone cannot account for the subsidence of these basins. Phase transformations in the lower crust, probably facilitated by the presence of hydrous fluids, can cause rapid (approximately 1 Ma) as well as long-term (200-1500 Ma) upward displacement of the Moho discontinuity, resulting in high amplitude subsidence of the crust. In areas of crustal extension this mechanism can overprint subsidence induced by thermal relaxation of the lithosphere. The case of the Black Sea and South Caspian basins shows that density increase of the lithosphere can also occur independently of crustal stretching.