21st Bullerwell Lecture

When: Tuesday evening, 27 March 2001
Where: XXVI General Assembly of the European Geophysical Society, Acropolis,
Nice
(See http://www.copernicus.org/EGS/egsga/nice01/nice01.htm)

Interrogating Sedimentary Basins

Dr  Nicky White, Department of Earth Sciences, Cambridge University

70% of the Earth's surface is covered in >2 km of sedimentary rock and the
largest accumulations occur on rifted continental margins.  Sedimentary
basins are of obvious economic significance but they also contain a wealth
of information about the temporal and spatial changes caused by a variety of
geological processes.  Superb three-dimensional seismic images of the
structure and fill of basins such as the North Sea are now available.  As
geophysicists, we are interested in extracting quantitative information
about Earth processes, preferably by inverse modelling, from these
spectacular data.

How do basins form?  Although it is generally accepted that many basins are
generated by thinning of the lithospheric plate, the way in which basins
grow through time and space has received little attention.  It is now
opportune to address this particular problem  by analysing three-dimensional
seismic datasets.    Here, I describe and apply an inverse algorithm which
extracts  the spatial and temporal pattern of strain rate from sedimentary
basins.   At present, only the two-dimensional problem is tractable but  my
approach can be regarded as a stepping stone towards a generalised
three-dimensional inversion.

My starting point is a simple forward model which allows  basin stratigraphy
to be calculated from any given strain rate distribution.  This forward
model includes potentially important effects such as the lithosphere's
elastic thickness and the  two-dimensional
conduction/advection of heat.  Conversely,  inverse modelling determines
strain rate variation by minimising the misfit between predicted and
observed patterns of basin subsidence.  No prior  assumptions about the
number, duration or intensity of rifting episodes are necessary. Instead,
strain rate is allowed to vary smoothly throughout time and space. I have
successfully inverted different synthetic  sedimentary basin geometries
which were generated by forward modelling.   Sensitivity analysis
demonstrates that strain rate patterns can be recovered with confidence. The
relationship between stratigraphic misfit and elastic thickness shows that
usually  an upper limit, but not a lower limit,  for elastic thickness can
be retrieved.

This inversion algorithm has been applied to Phanerozoic sedimentary basins
located worldwide. In each example,  observed subsidence profiles can be
automatically and accurately fitted.  The calculated distributions of strain
rate are corroborated by independent information about the number and
duration of rifting episodes.   Peak strain rate estimates are comparable to
present-day measurements in  actively extending basins such as the Aegean
Sea and the Basin and Range Province. These estimates  will help to
constrain the dynamical evolution of thinning continental lithosphere.
Strain rate patterns also govern the heatflow history and can be used to
construct accurate three-dimensional maturation models. Our results indicate
that  acceptable misfits are achieved when the elastic thickness of the
lithosphere  is less than 2-4 km.  This surprising observation suggests that
the lithosphere beneath sedimentary basins remains weak both during and
after rifting.   It also underscores the importance of inverting datasets so
that the existence, uniqueness, and resolution of solutions can be analysed.

In summary, three-dimensional seismic imaging of sedimentary basins has
generated a rich variety of sub-surface  data whose full potential has yet
to be exploited by geophysicists. There are many important and tractable
problems which can be addressed using these data. Here, I propose one line
of enquiry which  uses an inverse approach to extract information about
lithospheric thinning.