The Importance of Wide-Angle Seismic Data for Models of Extensional Basins

T.A. Minshull¹, D.J. Shillington¹, C.L. Scott¹, N.J. White², and R.A. Edwards^1
1National Oceanography Centre, University of Southampton, UK
²University of Cambridge, UK

A key ingredient in the understanding of extensional basin evolution is determining the distribution of strain in space and time. This strain evolution largely controls the thermal evolution, may be predicted by dynamic models of varying complexity. During extension, strain is distributed in complex patterns between the crust and the mantle lithosphere, in a manner that depends strongly on the supposed rheology. Unfortunately, there is currently no clear agreement on the rheology of the continental lithosphere. Kinematic models side-step this issue by avoiding the need to specify rheology, and can be used to infer the strain evolution that is required to match observed subsidence or stratigraphic data.

For a given pre-extensional crustal geometry, models of real basins make specific predictions about the variation of crustal thickness through the basin. Crustal thickness estimates from deep seismic reflection data are susceptible to errors and uncertainties in velocities both within the crystalline crust and in the basin fill, as well as ambiguities in reflector identification. Estimates from gravity data are fundamentally non-unique and require important assumptions about unknown densities. Currently the only reliable means to determine crustal thickness variations accurately is with wide-angle seismic data. Moho depth may be determined typically with an uncertainty of 1 km or less. Seismic velocity is closely correlated with density, so wide-angle data also allow accurate measurement of loads on the lithosphere. In the case of highly stretched basins, wide-angle data place strong constraints on the presence and, if present, the location of oceanic crust. Such data may also provide detailed information on the basin fill, for example distinguishing siliclastic from carbonate rocks beyond the reach of drilling, and mapping the lateral extent of deep overpressured formations.

We illustrate the importance of wide-angle seismic data with an example from the eastern Black Sea basin, where we have integrated results from a densely sampled wide-angle survey conducted in 2005 with those from a kinematic inversion of subsidence data for strain rate history. This basin is thought to have formed through back-arc extension, but estimates of the timing of the major extension range from Jurassic to Eocene, and the degree of extension is poorly known, as is the nature of the crust in the centre of the basin. Large uncertainties in paleo-water depth mean that a variety of strain rate histories are compatible with subsidence data; the wide-angle data allow us to discard some of these models and provide additional indirect constraints on the history of the basin fill and hence the likely subsidence history. We present a model for evolution of the basin that predicts successfully both stratigraphic and crustal thickness observations, and argue that accurate knowledge of the latter is critical to all basin models.

 

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands