Abstract

Mr M. F. Osmaston congratulated Dr Price on the clarity of his exposition of the development of schuppen structure at the toe of an allochthon. His argument for the development of above-unity values of λ (now widely called the geostatic ratio) as a means of initiating décollement in the absence of tectonic compression is, however, erroneous. He appears to have misunderstood the innumerable reports of high (but subunity) geostatic ratios found at great depth in the course of deep drilling, for he invokes a high geostatic ratio at great depth, capped by a seal at much shallower depth. In fact, as reference to any well log will show, the highest geostatic ratio always occurs within and just below the seal, decreasing steadily below it and sharply above it. Fluid pressure below the seal is essentially unrelated to that prevailing above it, as many a driller knows to his cost. In the common circumstance that a series of seals has developed at different levels, peak geostatic ratios are present at each seal but the peak values do not necessarily increase with depth, although the fluid pressures, of course, do. Reports of geostatic ratios marginally exceeding unity have always been unsubstantiated when the actual rock column densities were used in the calculation.

The condition for décollement could arise, however, from the tilting of an over-pressured (but λ < 1) stratum, due to the pressure of the upslope water column within the stratum. In that case, failure would first occur near the foot

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