Print this pagePhysical (Centrifuge) Modelling of the Influence of Early Normal (Growth) Faults on Thrusting, and Implications for Hydrocarbon Trapping
DIXON, John M.
Early normal (growth) faults are shown by scaled physical (centrifuge) modelling to affect the location and timing of nucleation and the displacement of thrust faults. Models are constructed of internally laminated stratigraphic units composed of laminae of plasticine (representing competent rock such as carbonates and coarse clastics) and silicone putty (representing incompetent rock such as shale), and with a primary normal fault (with or without growth-stratal geometry) dipping towards the hinterland at an angle of 20 to 60 degrees. In some models the fault plane is coated with talc to simulate reduced cohesion of a natural fault plane. The model ratio of length is 10{-6} (1mm=1km). The experiments are run at 2500-4000g.
Modelling shows that normal faults with shallow dip and/or low cohesion tend to be reactivated as thrusts at an early stage in the shortening (ahead of the foreland-propagating thrust front). They in effect form "out of sequence". They also accumulate greater displacement than thrusts which nucleate in plane layering in the same region, and they may be identified on this basis as well as on the basis of the growth-stratal thickness change. In contrast, primary faults with steeper dips or high cohesion tend to remain locked or to become locked after limited thrust movement, and to be overprinted by fold-thrust structures. A primary fault which strikes obliquely to the fold-thrust shortening direction is more readily reactivated and accumulates greater displacement where it is closer to the hinterland. Thrusts formed by reactivation of normal faults may be prospective exploration targets because associated structural traps may have formed sufficiently early in the generation-migration sequence to have retained an early hydrocarbon charge.