Читать книгу Continental Rifted Margins 2 - Gwenn Peron-Pinvidic - Страница 19

Beneath rift basins such as the North Sea, the crust thins from ~30 km to ~20 km (e.g. Wood and Barton 1983). Together with the presence of normal faults and the subsidence history, the thinning indicates that the basin formed by lithospheric extension (McKenzie 1978) with normal faults accommodating extension of the upper crust, ductile flow extension in the lower crust and lower lithosphere, and possible shear zones (Reston 1993) accommodated the extension of the uppermost mantle. However, early studies suggested that the amount of fault-controlled extension was far less than that required to thin the crust (Ziegler 1983): there is an extension discrepancy (Figure 1.5). Improvement in imaging has reduced the discrepancy under such basins to amounts expected in the presence of subseismic deformation (10%–50% of the fault-controlled extension, Marrett and Allmendinger 1992), that is the deformation occurring between faults and accommodating the distortion of faulted blocks, but as extension increases, especially at rifted margins, this discrepancy increases. This is the case at the WIM, where the amount of extension above S that can be deduced from the geometry of the faulted blocks (Figure 1.5) is considerably less than that implied by the subsidence rate (e.g. Sibuet 1992) and by measured crustal thinning (e.g. Reston 2005). Typically, the faults observed at the WIM indicate about 50%–60% extension (stretching factors of 1.5–1.6, that is thinning factors of 0.4 or lower, Figure 1.5), which is too low for the hyper-thinned crust observed at the WIM. The amount of thinning either requires more extension, or the crust to have been thinned by some other method. This is the extension discrepancy which provides fundamental constraints on the process of crustal thinning towards continental breakup.

Model M1 implies that all the brittle faulting is imaged (Figure 1.7); thus, top basement should, except where subject to erosion, be capped by prerift and early synrift rocks.

Model M2 infers that top basement is locally the exhumed slip surface of an earlier fault (Figure 1.7), cut across and displaced by the more recent faults.

In Model M3, the timing of the fault movement should systematically get younger oceanwards (Figure 1.7). Only minor extension discrepancy is excepted towards the deep margin (Ranero and Pérez-Gussinyé 2010), where the crust cut by later faults has locally been prethinned by movement on preceding faults.