The influence of the properties of the lower crust on the architecture of rifts
Abstract
Rift systems provide the best natural laboratories for studying continental deformation because the sediments which accumulate and are preserved in them provide a sensitive record of the vertical and horizontal strains accumulated over protracted periods of time. Major, strike parallel, changes in the structural architecture of the North Sea rift system are revealed by deep seismic reflection profiling. Where the rift is crossed orthogonally by a pre-rift structural high the rift is characterised by large normal faults. Either side of the high the extension is accommodated by a broad zone of smaller faults characterised by small displacements. A recent compilation of wide-angle refraction data for the same region reveals the changes in rift structure are mirrored by changes in the physical properties of the lower crust. Distinct thick high-velocity lower crust lies beneath the pre-rift structural high. This observation is consistent with observations that extensional deformation of old thick and cold crust is characterised by large deeply penetrating normal faults. The size of these faults has been related to the thickness of the seismogenic layer, but the control on the thickness of the seismogenic layer is poorly constrained. We make two key observations from the North Sea rift system. First, the variation in style of faulting is confined to the syn-rift section of the basin. Second, the thickness of the sediments deposited during the post-rift phase, when the mantle lithosphere is thermally recovering from the syn-rift deformation, is smoothly varying and there are no variations in this section which correlate with those in the underlying syn-rift section. Consequently, we conclude that the post rift section is a proxy for smoothly varying mantle deformation. Therefore, the variation in style of faulting in the syn-rift section must be attributed to variations in properties of the crust. We conclude that large scale continental deformation is dominantly controlled by the strength of the sub-continental mantle lithosphere, but the near surface architecture of the zone of deformation is a function of the properties of the crust.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....1852K