Abstract

The Hubbert-Rubey overthrust hypothesis suggests that overthrusting can occur by gravity sliding and/or a push from the rear if the pore pressure approaches that of the overburden, thus lowering the effective normal stress across the potential failure plane. One suggested means of increasing the fluid pressure above ambient is through thermal decomposition of hydrous minerals in a relatively impermeable rock sequence with progressive burial. Gypsum and/or anhydrite have been commonly observed at the sole of many thrusts; abnormally high fluid pressures have been encountered in drilling such evaporites interbedded with salt and clay. Extreme geothermal gradients from the Gulf Coast geosynclinal area place the calculated gypsum anhydrite plus H2O reaction at depths of 2500–6000 feet, corresponding with estimates of some thrust sheets (e.g., the Jura thrust: 2000–7000 feet).

In a triaxial compression test at 5 kb pressure, we observe a tenfold strength decrease (from 2.7 to 0.25 kb) in sealed polycrystalline gypsum cylinders if the temperature is increased from 100° to 150° C. This marked decrease is interpreted as being due to the dehydration of gypsum with consequent rise in fluid (pore) pressure to a value approaching the confining (overburden) pressure. Microscopic and X-ray observations reveal that gypsum is converted to hemi-hydrate and/or anhydrite plus water at these temperatures, occasionally with rehydration to gypsum. Similar stress-strain data at 2-kb confining pressure show identical results but at slightly lower temperatures. Longer preheating periods and a thousand-fold decrease in strain rate (to 3 · 10−7/sec.) depress this strength-sensitive region further: 80°–130° C. It is expected that over longer equilibration periods (geologic) these gross strength decreases would coincide with the equilibrium gypsum-anhydrite transition.

The effect of dehydration on the strength of gypsum was chosen for investigation partly because of the known occurrence of evaporites along many thrust faults but chiefly because this dehydration was considered to be typical of, but more tractable o t laboratory investigation than, the hydrous-anhydrous reactions of many metamorphic rocks. The recent experimental results of Raleigh and Paterson show a large strength decrease of jacketed specimens of serpentinite when they are heated above the dehydration temperature. Essentially the same dehydration mechanism, with consequent increase of fluid pressure and decrease of rock strength, that is postulated for gypsum may help also to explain the occurrence of ultramafic rocks along tectonic boundaries in many mountain belts and the type of deformation that is widespread in pelitic and carbonate-bearing schists. The fact that at least one wave of metamorphism must be syntectonic in order for the mechanism to operate affords a basis for testing the hypothesis.

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