Faults Shear Zones

As simple shear displacement increases, the principal finite elongation and the finite plane of flattening make a progressively smaller angle with the shear zone walls.

Progressive shearing leads to a progressive intensification of these planar and linear fabrics and a change in their orientation. Because ductile shear zones generally show a maximum displacement gradient in the zone center, it follows that the tectonically induced planar fabrics of shear zones generally show a characteristic sigmoidal form.

Where a set of pre-existing sub-parallel planes cross a brittle-ductile fault shear zone with varying displacement gradient, variations in local shear sets up a similar folding style where their axial planes are parallel to the zone.

Pre-existing fold axes take up new orientations in the  fault shear zone. These lines come to lie closer to the direction of the shear, and are often very striking at high shear strains.

Theory predicts that only three possible orientations of principal stress axes can occur, and therefore only three types of faults are found near the surface: normal to the earth’s surface giving rise to high angle normal faults, normal to vertical strike-slip faults, and normal to low angle.

Of particular interest to exploration geologists is the behavior of basement faults shear zones that pass from ductile at deep levels to brittle behavior at higher crustal levels. This problem is of major importance in exploration plays that involve basement-cover relations in orogenic belts.

Ductile basement faults shear zones must initiate and propagate at some point in the rock. The spacing of adjacent shear zones depends upon the numbers of such initiation points.

Ductile basement faults shear zones are characteristically devoid of discrete rupture surfaces that weaken the shear zone. Instead, later ductile displacements in the zone appear to continue to act along the zone of previously deformed rock.

The rocks of basement faults shear zones commonly have a smaller grain size than the parent undeformed material outside the shear zone. It is well established that the deformability of crystalline aggregate increases as the grain size decreases.

The strained rock becomes progressively easier to deform, a feature termed strain softening (Ramsay and Graham, 1970).

Once a critical level of finite strain has been reached, further displacement would be most easily accomplished by continued deformation of the less deformed marginal parts of the shear zone, and so widening the zone by deformation of the undeformed walls.

Some basement faults shear zones develop with accompanying volume change. In deep seated ductile shear zones in high metamorphic environments the extent of volume change is probably <1 per cent. However, in low-grade metamorphic environments volume change can be 20-50 percent.

Where basement faults shear zones are exposed in the orogenic zone it is usually seen to be accompanied by shortenings of the order of hundreds of kilometers are common. Many of the folds and overall geometry of the nappes are related to the movement of the sheet over step-like ramps in the underlying irregular sub-thrust surface.

In the more internal parts of the orogenic belt the basement faults shear zones pass from basement to cover so that folds of similar style found in the basement penetrate into the cover strata. The ductile shearing is often so intense that the original basement-cover unconformity is often completely obliterated.

At deeper levels the rheological contrasts of the cover layers become less we pass through a lower angle brittle -ductile shear zone transition into conjugate low angle ductile shear zones. These basement faults shear zones enable the deep crustal levels to extend: they can give rise to boudinage or to conjugate folding of the layering in the basement upon the orientation of this layering. Conjugate pairing of the shear zones is necessary for a complete graben tectonic pattern at higher levels.

Sets of basement faults shear zones lineaments in the rock section typically occur along shear zones in the basement that are periodically reactivated by tectonics or gravitational loading. Consequently, they have influenced depositional patterns and locations of structures throughout geological time.

Structural interpretations that detect sets of faults shear zones lineaments seek feature evidence in discontinuities such as edges and ridges identified in multi-scale derivative data sets

Our interpretation methodology is based on quantitative phase congruency and symmetry images of features derived from multi-scale and multi-source gradients of the gravity and magnetic fields in a realistic structural context.

Phase preserving mapping of high dynamic range field images use edge strength (phase congruency), and ridge strength (+/- phase symmetries) in order to improve assessment of structural interpretation of gravity and magnetic fields related to faults shear zones.

Phase congruency is a robust measure of feature significance detecting faults shear zones features at any phase angle. Phase symmetry is a similarly invariant measure of symmetry in an image.

Adding important detail to the packets of faults shear zones lineaments that control graben geometries, transcurrent faults, depth-to-basement.

A suite of carefully chosen derivatives can shed light upon the following:

• Structural trends from faults shear zones and fractures
• Ductile shear zones in basement
• Ductile-brittle faults shear zones
• Brittle faults shear zones
• Kinematics & stress regimes of faults shear zones

Sets of basement faults shear zones lineaments control fluid migration and transport, including magma, hydrothermal fluids and hydrocarbons of economic significance. Once basement faults shear zones are established, they are often long-lived features prone to multiple reactivation over large time-scales.

Metamorphosed rocks can be far more complicated than the stratified rocks. We provide significantly more detailed analyses of basement faults shear zones and lineaments such as region stress regimes, strain meta-data including volume changes that indicate controls on seismic fluid pumping that can provide new insights into mineralization.

Eslan Labs provide structure-focused assessments of faults shear zones lineaments to the exploration risks and opportunities in mature, frontier and under-explored basins enabling you to identify the geological context of any region.

Our geophysicists and structural geologists work to provide improved solutions to better understand the geophysical effects of basement faults shear zones lineaments and the tectonic evolution of the crust. Our experts have many years experience integrating disciplines including fault kinematics and structural analysis; geodynamics, and plate modeling.

We create a set of structural and crustal architecture databases, including stress regime interpretations of basement faults shear zones lineaments and depth-to-basement studies in intracratonic basins with plate model analyses that underpin our petroleum system assessments. An important objective is assessment of the type of deformation, the geometries of basement faults shear zones lineaments, crustal types, and basin fill.

Mapping basement faults shear zones lineaments focus on transformations of the magnetic and gravity fields that suppress the regional field and emphasize details without the need to apply often quite arbitrary regional-residual filtering techniques, these include:

• Reduction to pole or reduction to equator of dipolar magnetic fields must be performed before any processing can be performed.
• Horizontal-Vertical derivatives accentuate short-wavelength components
• Analytic signals highlight  subtle structures and discontinuities
• Tilt Derivatives
• Shaded relief transforms reveal basement faults shear zones and lineaments

Multi-scale analytics based on the enhanced horizontal derivative (EHD ) of Fedi and Florio, 2001 –  a high resolution boundary estimator for detecting basement faults shear zones and lineaments based on the weighted sum of vertical derivatives.

Multi-source data fusion finds coherent basement faults shear zones and lineament structures based on fusing complementary multiple data types and by suppressing the effects of noise and processing artifacts.

Multi-level modeling where large-scale basement faults shear zones and lineament anomalies are revealed and thus can be inferred as deep sources.

Fusion of multi-source, multi-scale and multi-level features generate our integrated final interpretation. This avoids the “stack-of-sticks” maps produced by too-zealous interpretation and overly enthusiastic lineament hunting.