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Mechanics, Dynamics and Geometry of Basin Inversion |
Department of Geology, University of Otago, PO Box 56, Dunedin, New Zealand
Inversion structures, associated with the compressional reactivation of moderate to steeply dipping normal faults inherited from earlier crustal extension, form important structural traps for hydrocarbons. Migration into these traps must occur syn- or post-inversion. Seismic reflection profiles show that inversion is frequently highly selective, with only some of an existing normal fault set being reactivated. Neglecting marked stress-field heterogeneity, frictional mechanics suggests three possible explanations for this selective reactivation: (1) preferential reactivation of shallowest-dipping normal faults in a region that previously underwent the greatest extensional ‘dominoing’ of fault blocks; (2) the presence of anomalously low friction material along particular faults; and (3) a heterogeneous distribution of fluid overpressures (Pf > hydrostatic) with preferential reactivation occurring in the area of most intense overpressuring. The last possibility is favoured by the likelihood that fluid overpressures develop during inversion as a consequence of the dramatic increase in mean stress that accompanies the transition from an extensional stress regime, with high capacity to store fluids, to a compressional regime with comparatively low storage capacity.
Compressional reactivation of moderately to steeply dipping faults likely requires significant overpressures; in the case of faults with dips in excess of the frictional ‘lock-up’ angle (typically 50–60°), supralithostatic fluid pressures (Pf >
3 = v) are a necessary prefailure condition in rupture nucleation sites. Extreme fault-valve action then becomes possible, with postseismic flushing of fluids upwards along faults from overpressured compartments. Evidence for such activity comes from mesothermal Au-quartz veins hosted in steep reverse faults, where repeated attainment of supralithostatic pressures alternated with discharge episodes along the faults. Episodes of vertical hydrocarbon migration along reverse faults are therefore a likely accompaniment to basin inversion; there are, for instance, historical records of postseismic discharge of aqueous and hydrocarbon fluids from the overpressured basins of the Western Transverse Ranges, California, where steep reverse faults remain active today. Careful assessment of the relative timing of fault reactivation during inversion and hydrocarbon migration is needed to evaluate the hypothesis for ancient inverted basins.
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