Fault-related Structures Research Articles

Computer modelling and visualisation of the structural deformation caused by movement along geological faults

The computer modelling and visualisation of deformation caused by movement along faults has enhanced our understanding of the evolution of fault-related structures in the geological record. In particular, the development of computer software to carry out structural restoration and section balancing has provided earth scientists with an effective tool for validating structural interpretations constructed from geological and geophysical data. This paper describes both two- and three-dimensional geometric methods for modelling hanging wall deformation in response to fault movement. Equations are presented for the definition of two-dimensional fault geometries and for the determination of hanging wall geometry following movement over these faults. The Chevron and inclined shear constructions and fault-bend fold theory are described in a format to enable easy conversion into computer algorithms. The modelling of fault movement in three-dimensions is also considered in the context of the Chevron construction. Schematic models are presented which show hanging wall deformation caused by extensional, compressional and, most importantly, strike-slip movement over a complex fault surface. In addition, a new geometric technique for the restoration of deformed hanging wall surfaces is described. This technique has been called flexural flattening and involves flattening a surface represented as a mesh of triangles, back to horizontal. It has the advantages of maintaining the area of the surface before and after restoration and is relatively simple to apply in comparison to three-dimensional implementations of existing geometric methods.

Two Important Kinds of Fault-Related Structures and Their Significance for Hydrocarbon Accumulation in the Tarim Basin, China

The Tarim Basin is a giant hydrocarbon-bearing basin with a low degree of exploration. Most of the hydrocarbons discovered in the basin are related to strike-slip faults and to thrusts. Strike-slip-fault related structures with en echelon geometry are developed mainly on uplift zones and on slopes in the basin. Thrust-related structures with a linear geometry are developed mainly in depressions parallel to mountain fronts. Fault-related structures are the main structural hydrocarbon pools in the basin, of which base-thrusting structures and thrust nappe structures are the two most favorable.

Conical drag folds as kinematic indicators for strike-slip fault motion

Conical drag folds are developed in the vicinity of faults where rock sequences with significantly different mechanical properties are subjected to oblique shear. The drag folds formed by passive rotation of a competent bed surrounded by flowing incompetent media. Mesoscale conical drag folds can serve as kinematic indicators. The conical fold axis coincides with the rotation axis of slip, and the angle θ between the small circle of poles to bedding and the conical axis is equal to the angle between the pole to tilted bedding and the rotation axis of slip. The sense of asymmetry of the drag fold reflects the sense of shear. The relationship between conical folding and sense of shear established for the mesoscale can be applied to larger fault zones. For example, the geometry of conical drag folds along with other fault-related structures reveal sinistral strike-slip motion in the Mount Arif area of the Negev.

Strains and structures associated with a terrane bounding stretching fault: the Melones fault zone, central Sierra Nevada, California

An examination of the structures, total strains, and rock types across the Melones fault zone indicates that in the central Sierra Nevada this fault separates and locally imbricates two distinct terranes. East of the fault, a widespread early Jurassic cleavage displays west-northwest to northwest strikes and is associated with moderate to high strain intensities. These structures form in Jura-Triassic metasedimentary rocks in part derived from a continental source. A weak northwest striking cleavage and low strain intensities occur west of the fault in lower Jurassic volcanic rocks and upper Jurassic slates and sandstones. The fault zone itself consists of steep east-dipping ductile-brittle shear zones or stretching faults along which the early Jurassic structures are transposed parallel to the Jura-Cretaceous and fault-related structures. Large flattening type strains are common in the fault zone and may be the result of heterogeneous strains developed when the shear zones bend around resistant blocks or by volume losses. Total strains in the eastern hanging wall reflect the superposition of fault-related on older strains; therefore, stretching lineations in this hanging wall do not reflect directions of fault motion. Stretching lineations in the footwall and fault zone, kinematic indicators in the fault zone, and a small eastward jump in metamorphic grade suggest an east-over-west or reverse sense of movement; no evidence for significant strike-slip or oblique-slip motion was found. Removal of footwall strains indicates that the Jura-Cretaceous shortening across the fault was responsible for both the transposition of older structures east of the fault and the rotation of the fault to its present steep dips. Thus, the Melones fault is a large ductile-brittle shear zone along which the eastern hanging wall (Merced River Terrane) moved to the southwest over the western footwall (eastern Foothills Terrane).

Enigmatic boulders and syn-sedimentary faulting in the Carboniferous Limestone of the Isle of Man

SUMMARY A small area of Lower Carboniferous (early Brigantian) Limestone in a coastal inlet in the south of the Isle of Man contains boulders of pale bioclastic limestone lying upon and overlain by thin undulating beds of dark calcareous mudstone. It is shown that during deposition of this facies the sea bed dipped towards a syn-sedimentary fault on the north side of the inlet where approximately 3m of normal movement occurred prior to its deposition. 50m to the south, a limebank developed above the crest of a small rollover anticline in the hanging wall of the fault. Instability of this structure led to an episode of sediment collapse. A density flow, containing blocks and boulders of pale limestone, slid northwards over black muddy sediments that were accumulating in the deeper water close to the fault. Both lithologies also show evidence of soft sediment deformation. This study shows that carbonate boulder beds and other fault-related structures can be extremely local in nature.