Thermal cycles and horizontal tectonics

Increasing the temperature through the Earth’s crust will mean that the rock wants to expand laterally. However, the crust is restrained from expanding by its interaction with the relatively stiff inner mantle and core. The result will be the development of massive in-plane compressive stresses. Taking the rock of the crust to have an average coefficient of thermal expansion =12x10^-6m/m/^oC, then an increase in average temperature of say 20^oC would, if unrestrained, induce a tensile strain of 240x10^-6. This would be typical of the changes occurring near the surface during glacial and interglacial periods. Over a continental landmass of roughly circular shape, having an in-plane radius of say a=3000 km, this unrestrained expansion would give rise to an outward, in-plane, radial movement of 720 m. This would represent the maximum level of in-plane displacement available to fold, shear, or otherwise distort the Earth’s crust when these upper strata are restrained by the underlying deep crust and adjacent plate during the heating or compression cycle. Such distortions would undoubtedly occur selectively as will be discussed later. They could certainly account for a lot of folding, shear faulting, fracture and associated seismic and volcanic activity within the crustal layers during the warming phase near the surface of the crust.

The level of compressive stress developed during the warming cycles will of course depend upon the average temperature and its gradient into the crust. The levels actually reached would in turn depend upon the forces needed to fail the particular area of crust, whether by folding, shearing, or whatever. But with no failure to relieve the compressive strain of 240x10^-6, a relatively hard rock having an elastic modulus of say E = 30x10⁺³ MPa, will develop a compressive stress of 7.2 MPa (720 Tonnes for every 1 square metre of rock). While this would be lower than the stress expected, on average, to be needed to crush the rock it could, when integrated over significant crustal depth, certainly be enough to induce various forms of geometric failure such as folding and faulting. And because the Earth’s crust is highly non-homogeneous local stress concentrations may well be enough to induce local crushing failures in the rock.

Over the longer period adjustments of geothermal gradient that might be expected during a warm age the depth of penetration of the surficial thermal waves will be that much greater. An idealised form of the thermal equilibrium reached over the full thickness of the crust during a typical warm age, will develop considerably greater levels of compressive restraint, as is suggested in Figure 5. Compressive stresses would be anticipated to show gradients through the crust thickness similar to the changes in geothermal temperature gradients – at least until depths where temperature starts to change the rheological properties so that there is a reduction in the effective stress changes. But these stress changes could reach levels of considerable significance to tectonic processes. If for example a 200^oC temperature change occurred at a particular level within the crust the horizontal compressive stresses if horizontal expansion is fully restrained could reach values of around 72 MPa, or if unrestrained could over the area of a typical continental landmass produce an outward motion of over 7 km. At these levels considerable tectonic activity becomes possible. 

During the warm-up phases at various temporal scales alluded to above, the kinematic adjustments within the crust involving sudden releases of stored energy would be expected to occur with compressive related failure modes. These failure modes would occur when the strain build-up reaches the level required for this or that particular failure mode to be induced; they could be expected to be progressive and accumulative, and to occur at different locations at different times over the entire period of the crustal warming. At the end of the warm-up period it might be anticipated that the greater part of the compressive energy will have been transferred into the distortions characterising the various failure modes, whether they be mountain building, crustal over-riding, shear faulting, strata folding or metamorphic adjustments. So by the start of the next cooling period the crust would consequently contain very little residual horizontal compressive stress.

As the crust cools over an ice age it will want to contract. Being again prevented from doing so by the effectively rigid inner core and mantle, tensile stresses will be developed. Over such a long period, adjustment of geothermal gradient might be expected to produce substantial levels of tensile restraint, as is suggested in Figure 6. Tensile stresses would again be anticipated to show gradients through the crust thickness similar to the changes in geothermal temperature gradients. Such a cooling period could be termed the tension cycle.

Reversing the above scoping calculations, a drop in average temperature of 200^oC at a given level within the crust will produce tensile stresses of around 72 MPa, more than sufficient to cause tensile cracking of the rock, opening-up fissures and rifts into which high pressure magma could be extruded. Adequate also to account for considerable shear distortion and slip on fault planes, often in a slip-stick fashion typical of that occurring in earthquakes. It is likely that the tensile fractures would be concentrated in those areas where the crust is at its weakest. With oceanic crust being apparently so much thinner than that of continental crust, at least in the present phase of the dynamic tectonic cycle to be elaborated later, it would be expected that most, but by no means all, of these fractures would be located on the ocean floors. While the dominant fractures would be anticipated to be concentric with the geographic centre of the continental “plate”, the restrained contractions could also produce radial fractures and transform faults.

 

Much of the above post has been taken from the paper "On the Causes of Vertical Motions of Lithosphere", James G A Croll, Frontiers meeting, Geological Society of London, November, 2011.