--> Abstract: Superposed Extensional and Contractional Structures, by J. A. Helwig and R. Wiener; #90942 (1997).

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Abstract: Superposed extensional and contractional structures

HELWIG, JAMES A., and RICHARD WIENER

The geometry of superposed structures reflects the mismatch of extensional and contractional deformation templates. A simple expression of this mismatch is the fact that they are non-commutative; in fact, inversion is a misnomer.

At shallow depths in sedimentary basins, simple Coulomb-Mohr shear failure of granular cohesionless layers produces normal faults dipping 60 degrees or reverse faults dipping 30 degrees Superposition of extension and contraction produces simple cross-cutting faults. Unusual rigid or ductile layers cause deflection of fault trajectories.

At greater depths, compaction and cementation increase the strength and anisotropy of multilayers and create high fluid pressures. Such horizons of high or low strength control the mechanical response to stress. Thus normal and reverse faults develop ramp and flat trajectories that lead to fold geometries controlled by fault shape. However, the deformation mechanism in extension is across-layer shear and in compression is layer parallel shear. Therefore the geometry of rollover anticlines and thrust-belt anticlines differs dramatically.

Therefore the mechanical basis of geological structures demands dramatic differences in superposed extension-contraction. Hence fault reactivation is often local and limited to anomalously steep or flat portions of early structures, and complemented by short-cut faults. Deformed bedding planes and early detachment horizons suitably oriented in a later stress field are reactivated. On a large scale, earlier structures act only as material flaws to deflect later structures. So normal faults tend to buttress bedding slip during later contraction, determining the location of ramps of fault-bend/propagation folds. If cover rocks are not detached, major crustal faults cutting basement are reactivated in a manner strongly affected by the anisotropy of the basement rocks and stress orientation.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria