--> Abstract: Normal Fault Inversion by Orthogonal Compression, by C. R. Nogueira and F. O. Marques; #90072 (2007)

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Normal Fault Inversion by Orthogonal Compression

C. R. Nogueira and F. O. Marques
Universidade de Lisboa, Lisbon, Portugal

Theoretical and experimental studies have shown that, under Coulomb conditions, normal faults are not reactivated when compression is applied horizontal and normal to fault strike. However, in nature inversion has been observed. Our aim is therefore to study the particular conditions under which early normal faults are later inverted as pure reverse faults. The very low stress levels under which many faults are activated lead to the conclusion that weakening mechanisms must be active. To weaken our model faults we used silicone putty.
We used a parallelepiped box with four fixed walls and one mobile that worked as a vertical piston drove by a computer controlled stepping motor. Fine dry sand was used as the analogue of brittle rocks and silicone putty with Newtonian behaviour as analogue of weakening mechanism. A model planar normal fault dipping 60º toward piston existed in the undeformed state, striking orthogonal to piston movement. To obtain insight of inversion processes on weakened pre-existing normal faults, three configurations were used: a) one weak fault; b) two widely spaced parallel weak faults; and c) weak base together with weak fault.
Results of the experiments show that: 1) inversion of the normal faults occurred for all three configurations; 2) inversion was long living and created a very high critical taper angle before the first thrust formed ahead of the inverted normal fault; 3) vertical sand thickening in the hanging block also added to the taper angle; 4) basal weak layer also favours normal fault inversion.

 

AAPG Search and Discovery Article #90072 © 2007 AAPG and AAPG European Region Conference, Athens, Greece