Print this pageShallow Salt Tectonics above Active Basement Structures
We conducted an analysis of data from the Mediterranean, and experimental modeling to determine how active basement structures affect the supra-salt overburden. First, the overburden can be vertically pinned to the basement: basement subsidence or uplift are transmitted to the overburden. The topography of the overburden mimics that of the basement. Or the overburden may subside or rise independently of the basement. Second, the overburden can be laterally pinned to the basement: basement extension or shortening is transmitted directly to the overburden.
- Early stage - vertical and lateral decoupling: Thick salt acts as an efficient buffer decoupling the overburden from the basement. Any vertical movement of the basement is compensated by lateral salt flow. As a result, the overburden remains horizontal and “oblivious” to the deformation happening at depth. The overburden, though, deforms in response to the regional stress field, extensional or compressional, but the location of the structures does not depend on the location of basement structures.
- Later stage - vertical pinning - lateral decoupling: Thin salt cannot flow fast enough to accommodate the rise or subsidence of the basement. The overburden is vertically pinned to the basement. Changes in topography of the basement are transmitted to the cover. However, even thin salt can act as an efficient detachment surface, and the overburden remains laterally decoupled from the basement. The local surface slope created by basement uplift or subsidence triggers local gravitational gliding away from the rising topographic highs and toward the subsiding topographic lows.
We illustrate these processes using seismic and bathymetric data from the Eastern Mediterranean. First, in an active accretionnary prism, data show extensional structures located in the midst of compressional structures. In physical models, the crests of anticlines initially rise independently of the overburden (vertically decoupled stage). Once salt has thinned above the anticlines, the overburden rises. In response, local gravitational gliding occurs, creating normal faults above the basement anticline’s crests. In the second example, salt and overburden respond to rapid vertical subsidence of crustal basins. Rapid subsidence of the basin (0.2 mm/y) was transmitted to the thin overlying salt and overburden, which glided downslope, triggering thin-skinned extension above the basin’s margins and radial shortening on the basin floor.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009