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Abstract: Physical Models of Normal-Fault Relays Between Variably Offset Grabens

B. C. Vendeville, J. Le Calvez

We conducted a series of scaled models in which two grabens initially offset along the direction of regional extension subsequently propagated freely along strike while synkinematic sediments were added. Models comprised a thick layer of dry sand overlying a thin lubricating layer of viscous silicone. The experiments are applicable to both crustal scale and to the scale of a brittle sedimentary cover above a salt or shale decollement.

Results show that fault orientation and linkage vary with graben offset (^DgrL) and model thickness (h). Grabens with small offset (0 < ^DgrL < <fr>h</>2</fr>) propagated along strike while remaining perpendicular to the direction of regional extension; the grabens eventually linked. Fault traces were initially jagged but straightened after further extension and sedimentation. Grabens with moderate offsets (h/2 < ^DgrL < 2h) widened as they propagated toward each other. Soft fault linkage created ramps of strained overburden blocks between faults. Later, ramps were cut by hard-linked faults. Hour-glass structures formed where faults of opposite vergence intersected. Fault slip progressively decreased along strike. Where graben offset was large (^DgrL > 2h), a transtensional graben was generated oblique to the direction of regional extension, providing hard linkage with each initial graben. Fault-slip gradients along fault strike remained low. We infer that formation of a new oblique graben reflects a size threshold above which the rock volume between the grabens (proportional to graben offset) cannot rotate or deform by soft linkage to accommodate deformation.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France