The fluid-pressure hypothesis of overthrust faulting is extended in terms of stratigraphically controlled clay diapirism in the upper 2 or 3 km of a young, regressive marine sedimentary sequence.
Such a sequence, with thick clay loaded by coarse clastic sediments, is mechanically unstable because the clay beds initially are undercompacted and have abnormal interstitial fluid pressures. This instability leads to clay diapirism in the form of narrow, rather steep anticlines separated by broad, gentle synclines. The anticlinal trends are roughly parallel to the margins of the sedimentary basin or to the depositional strike.
If a slope is induced before the excess pressures dissipate, gravity sliding and overthrust faulting may take place. The direction of such slope will commonly be normal to the margin of the sedimentary basin or to the depositional strike. The maximum length of such thrust sheets (in the direction of transport) would be determined by the distance between diapiric anticlinal trends.
The direction of overthrusting will normally be away from the orogen, in the direction of progradation of the regressive sequence.