Abstract: Structural Inversion: Occurrence, Mechanics, and Implications for Petroleum Exploration
James D. Lowell
Structural inversion, defined as the uplift of previously extended, subsiding regions by later contraction, has been recognized on every continent that has been explored for petroleum. The process can occur at the large scale of deformation in orogenic belts, but this presentation focuses on inversion affecting sedimentary basins and their associated structures. Inversion was probably first described and is widespread in northwestern Europe, including the North Sea; subsequent work has revealed its importance in southeast Asia, Australia and New Zealand, north Africa, and South America. Recognition in North America has lagged, probably because occurrences apparently are not as numerous as in the aforementioned areas, but the mid-continent rift and the Uinta Mountains of northeastern Utah are good examples of basin inversion.
Typically, rift and sag basins can be later inverted. Mainly by reactivation of older normal faults, inversion selects rift basins where, in pure shear, weakening because of necking or thinning of lithosphere has occurred, and where, in simple shear, mechanical detachment surfaces are available for subsequent movement. Sag basins can apparently be inverted in the absence of reactivated normal faults, as in the southern altiplano of Bolivia and offshore Sabah, Borneo. Basins can be inverted by dominant strike slip with some convergent component, e.g., offshore northeast Brazil, and by almost direct compression, e.g., Atlas Mountains, Morocco. Usually, however, inversion is caused by a combination of compression and strike slip (transpression) because the azimuth of maximum principal compressional stress to the direction of original basin trend vectors into an oblique-slip component. This is illustrated in Salta Province of northern Argentina where compression derived from Andean deformation is resolved obliquely against an older rift basin in the foreland nearby. Apparently, compressive forces can be stored for great distances backward from the lead edge of an underthrusted foreland plate to invert rather remote regions and basins that are carried m that plate; this may be a significant part of the mechanism for basin inversion in northwest Europe and the southern North Sea as that foreland region underthrusted along the Alpine system. With respect to the mechanics of structural inversion, the process has been successfully modeled in both clay and sandbox experiments.
Structural inversion has important implications for petroleum exploration. Areas of inversion frequently have tighter porosity for a potential reservoir and faster seismic velocity for a particular stratigraphic interval than would be expected for their present depth of burial. Burial history curves characteristically have an upward inflection at the time of inversion, which can affect the hydrocarbon maturation process. Some source rocks may be overmature for present burial depths. In some presently shallow basins, however, maturation would not have occurred had not source rocks once been buried more deeply. Inversion can cause remigration eft hydrocarbons. Finally, inversion can create the trapping structures.
AAPG Search and Discovery Article #90985©1994-1995 AAPG Distinguished Lecture