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Abstract: Erosion Influence on Basin Inversion (Southern North Sea Upper Cretaceous Example)

Thierry Nalpas, Jean-Pierre Brun, Francois Guillocheau

Basin inversion generally results from lithosphere compression. Within a typical example of extensional basin, such as a graben bounded by steeply dipping normal faults, inversion proceeds by reactivation of normal faults in strike-slip reverse faults, but newly reverse faults can develop. Within a regional stress field with a constant ^sgr1 value, the uplift of the graben fill increases the value of ^sgr3 on reactivated normal faults, leading to a decrease of the differential stress ^sgr1-^sgr3, which can therefore pass beyond the critical level of reactivation, and inversion can stop. If erosion is active during inversion and uplift, the ablation of sediments, on the contrary, decreases the ^sgr3 value allowing inversion to continue.

In compression, the factors which control the amount of graben inversion are mainly: the obliquity angle between graben and ^sgr1, the value of ^sgr1, the age of the graben, and the rate of erosion.

The example of the Broad Fourteens basin (Netherlands offshore) in the Southern Nord Sea, which is inverted during the Upper Cretaceous Alpine compression, illustrates the influence of erosion during inversion. Without erosion, the graben fill would have been uplifted to 5,000 m above the level of graben platforms. This never occurred, and, for the above reasons, is mechanically impossible. The total amount of inversion results from a sequence of uplift steps, controlled by the variations of ^sgr3. To each uplift step corresponds a surface of unconformity. Sequential stratigraphy of the synchronous chalk deposits is used to calibrate the sequence of events.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France