inversion tectonics Inversion tectonics describes the process by which formerly extensional, normal faults are reactivated in compression (
positive inversion), and conversely, reverse faults are reactivated in extension (
negative inversion). The effect of positive inversion is to generate uplift, with the result that initially low-lying areas become topographically high. Inversion is a particularly common process at the margins of collisional mountain belts, where passive margins are subsequently incorporated in thrust belts; and along strike-slip fault systems, where rocks can experience repeated episodes of
transpression and transtension.
Inversion can both enhance and reduce the oil and gas prospects for a sedimentary basin. It results in the formation of new structures capable of trapping oil and gas accumulations, many of which are expressed at the present surface as small hills. Surface mapping of these inversion-related anticlines has proved highly successful in finding oil and gas in eastern Sumatra. However, reactivation of faults may breach overlying horizons that seal an oil or gas accumulation, thereby destroying the effectiveness of the trap. Additionally, inversion can mean that reservoir and source rocks have formerly been buried too deeply to have retained significant porosity and organic carbon content. Because it is important to ascertain the maximum depth attained by a succession before its inversion, oil-exploration geologists have evolved specialized techniques to help quantify the magnitude of inversion. Vitrinite reflectance data provide an estimate of maximum burial depth by optical assessment of the thermal alteration of coaly material. Porosity studies compare the actual with the expected exponential decrease in the porosity of sedimentary rocks with depth. Abrupt jumps in this porosity profile correspond to episodes of inversion by an amount proportional to the size of the jump. Fission-track analysis looks at the intensity of spontaneous radiation damage, in the form of fission tracks in common sedimentary minerals such as apatite and zircon. Because the radiation damage occurs at known rates and temperatures, inversion events will be recorded by detectable changes in the rate of formation of fission tracks.
The effects of inversion can be subtle and its recognition requires well-constrained structural cross-sections supplemented, ideally, by seismic reflection profiles. Figure 1 illustrates the two principal observations that allow inversion to be diagnosed. First, thicker packages of
syn-rift strata will occur on the upthrown sides of positively inverted faults. Second, the initially smooth increase in fault displacement, from a fault tip towards its centre, will be disturbed. Inversion is essentially instantaneous, imposing a more-or-less fixed increment of displacement on all parts of the fault plane undergoing inversion. Thus, a positively inverted normal fault will show the greatest reverse displacement where the initial phase of normal displacement was least.
In the Alpine foothills of Provence, Cretaceous syn-rift rocks deposited in the hanging walls of normal growth faults are typically ten times thicker than the equivalent footwall successions. Horizontal compression during the Palaeogene formation of the Alps subsequently inverted the normal faults, thereby placing the thickest units on the upthrown sides of faults whose sense of displacement is manifestly reverse. Seismic reflection profiles from geological settings comparable to the Alps, and analogue models of inversion using accurately scaled sandbox experiments, allow predictions to be made as to the most likely configuration at depth of the Provence examples. Such profiles reveal so-called
null points. A null point is the point along an inverted fault at which the sense of offset of strata each side of the fault changes from normal to reverse (Fig. 1). The position of the null point along a fault provides an indication of the magnitude of the inversion event. With increasing amounts of positive inversion, the null point will migrate progressively deeper, down the fault plane, until the fault is said to have been totally inverted, so that it exhibits a reverse sense of offset along its full length.
Research has tended to concentrate on positive inversion, largely because of its special economic implications. This does not mean, however, that negative inversion is less common, or indeed less important as a geological process. Recent work in regions of lithospheric thickening has demonstrated how, after the relaxation of horizontal compression, the lithosphere may often be too thick to support itself. In north-west Scotland, for example, the lithosphere thickened during Caledonian orogenesis began to collapse under the influence of gravity from Early Devonian times. This collapse was accommodated, at least partly, by negative inversion of Caledonian thrust faults, such as the crustal-scale Moine thrust.
Clearly, not all faults will be equally susceptible to inversion, which may vary over small areas, often within individual sedimentary basins. The main factors governing susceptibility are the steepness of faults prior to their inversion, the orientation of faults relative to the stress doing the inverting, and the frictional resistance to reactivating the faults, as determined by, for example, fluid pressure along the fault plane. Positive inversion may be likened to squeezing a sponge in which the liberation of pore fluid causes local fluid pressure to build up along fault planes, thereby reducing their frictional resistance to reactivation. Faults formed during the initial phase of pre-inversion faulting are sometimes modified in an effort to lessen their steepness, and thereby minimize the resistance to inversion. In south-west Wales, for example, a slice of Precambrian crystalline rock, originally sited in the footwall of a major normal fault, has been sheared off by a relatively low-angle, ‘footwall shortcut fault’, such that it is now incorporated in the hanging wall of an inversion-generated thrust fault.
Jonathan P. Turner
