Physical Modeling of Normal Faults and Graben Relays above Salt: A Qualitative and Quantitative Analysis

By

LE CALVEZ, JOEL H.

Schlumberger Holditch-Reservoir Technologies, College Station, TX,

VENDEVILLE, BRUNO C.

Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX

In salt provinces, thin-skinned extension is typically accommodated by graben faulting in the overburden and viscous flow in the evaporitic layer. As separate grabens propagate along strike, they interact in relay zones that differ according to their across-strike separation. We investigated the pattern of such relay zones in a series of experiments using a new design allowing faults to freely propagate after they have nucleated at predetermined locations. Results allow determining the influence of key geologic parameters on the evolution of graben-relay zones. The main parameter, the across-strike offset between the grabens, controls the size of the relay zone, and its structural style. Where the graben offset is large, faults soft link and their traces curve gently toward one another. Fault throw and amount of diapir rise progressively decrease toward the relay zone. The pattern and timing of fault slip varies along strike. Where the graben offset is smaller, graben faults either intersect one another, forming hourglass structures, or hard link to form a narrow and abrupt relay zone. Experimental results indicate that grabens start to interact only after they have propagated along strike far enough to overlap. Syntectonic sedimentation can prevent the formation of younger faults during extension.