The perturbation of the thermal equilibrium, produced by subsidence and sedimentation in the earth's outer layers is investigated by means of two models. In one the heat reaching the surface is assumed to come from the deep interior, in the other, to be generated in the crust.
The models consist of three horizontal layers, the top one representing the sedimentary layers, the middle one representing the crystalline part of the continental crust, and the lowest one representing the plastic substratum. It is assumed that at a. particular instant the thickness of the top layer is increased. A theory is developed to determine the subsequent temperature variations. Also, asymptotic values are found for large times.
A finite subsidence velocity introduces an uncertainty in the time of origin of the rapid-subsidence model. For events which occur long after the subsidence, the instantaneous assumption is adequate. Moreover, it is adequate without this restriction in the lower part of the crust and in the substratum.
For large times, the deviation from the final steady state is a linear function of the depth. When the heat comes from the interior, the temperature deviation from the final steady state varies as t−3/2, whereas when the heat is generated in the crust, it varies as t−1/2.
The temperature variations are evaluated for subsidences of 6 and 13 km. The sedimentary layer is assumed to be 2 km thick initially, and the crystalline crust is assumed to be 30 km thick. The time variation of the different components of the temperature variations gives an insight into the propagation of thermal disturbances in the earth. Thermal adjustment requires millions of years.
After subsidence, the temperature increases rapidly during the first 20 million years or so. Thereafter, the rate of increase is much smaller. The increase is much more rapid in the sediments than in the lower part of the crust. At the base of the crust, the increase is about 22°–23° C per km of subsidence, if the thermal conductivity is 0.006 cgs. However, if the conductivity decrease with temperature in the range of crustal temperatures was taken into consideration, much larger increases would be obtained.
The times required for thermal adjustment are large enough to be significant in certain geological processes. The temperature increase after subsidence should affect the rate of lithification of sediments and the strength of the crust.
Both the stresses and the temperature of a certain portion of sediment increase with subsidence. At depths greater than a few hundred feet, the confining stress on the grain matrix corresponds, practically without any time lag, to the current depth of burial. On the other hand, the adjustment of the temperature lags appreciably. Thus, lithification may be incomplete at certain depths because the temperature is still far below its final value. Lithification is likely to be completed within the first 20 million years after the subsidence.
During the folding stage of a geosyncline, the shearing rate of the crustal rocks increases appreciably. The crustal “solid viscosity” may decrease markedly when the temperature reaches the value it has at the base of an undisturbed crust. Thus, the crustal strength decreases relatively quickly up to about 20 million years after the subsidence and thereafter much more slowly. Because of this decrease, the folding stage of a geosyncline would occur 15–20 million years after the subsidence.
If heat comes from the deep interior, there is, for a certain time, a zone of cooling in the substratum. The corresponding increase in the solid viscosity of the substratum may effectively lock, for a certain time, the subsequent uplift of the crust.