structural geology Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover informati ...

, strain partitioning is the distribution of the total strain experienced on a rock, area, or region, in terms of different strain intensity and strain type (i.e. pure shear,

simple shear Simple shear is a deformation in which parallel planes in a material remain parallel and maintain a constant distance, while translating relative to each other. In fluid mechanics In fluid mechanics, simple shear is a special case of deforma ...

, dilatation). This process is observed on a range of scales spanning from the grain –

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

scale to the plate – lithospheric scale, and occurs in both the brittle and plastic deformation regimes. The manner and intensity by which strain is distributed are controlled by a number of factors listed below.

Influencing factors

All four of these factors below may individually or in combination contribute toward the distribution of strain. Therefore, each of these factors must be taken into consideration when analyzing how and why strain is partitioned: :

anisotropy Anisotropy () is the structural property of non-uniformity in different directions, as opposed to isotropy. An anisotropic object or pattern has properties that differ according to direction of measurement. For example, many materials exhibit ve ...

such as preexisting structures, compositional layering, or cleavage planes. Isotropic lines "separate mutually orthogonal principle trajectories on each side. In a plane-strain field, the strain is zero at isotropic points and lines, and they can be termed neutral points and neutral lines."Jean-Pierre Brun (1983) "Isotropic points and lines in strain fields", Journal of Structural Geology 5(3):321–7 :

rheology Rheology (; ) is the study of the flow of matter, primarily in a fluid (liquid or gas) state but also as "soft solids" or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applie ...

: boundary conditions – the geometrical and mechanical properties and : stress orientation – critical angles by which stress is applied.

Subdivisions

Strain partitioning across the literature is diverse and has been divided into three subdivisions according to the American Geological Institute: : superposition of individual strain components that produce the finite strain : the accumulation of strain influenced by constituent rock materials and : individual deformation mechanisms that contribute toward producing the finite strain.

Superposition of individual strain components

The superposition of individual strain components can be expressed at the tectonic scale involving oblique convergent margins and transpression / transtension tectonic regimes.

Oblique convergent margins

Strain partitioning at an oblique convergent margin

Convergent margins where the angle of subduction is oblique will often result in the partitioning of strain into an arc parallel component (accommodated by strike slip faults or shear zones) and an arc normal component (accommodated through

thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. I ...

s). This occurs as a response to

shear stress Shear stress (often denoted by , Greek alphabet, Greek: tau) is the component of stress (physics), stress coplanar with a material cross section. It arises from the shear force, the component of force vector parallel to the material cross secti ...

exerted at the base of the overriding plate that is not perpendicular to the plate margin.

Fundamental factors which control strain partitioning within oblique orogens

*Stress orientation: Increased

subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...

angle increases the arc parallel component. *Rheology and anisotropy: Mechanical properties of the wedge: (coulomb vs plastic) influence the wedge geometry. *Boundary conditions: The friction and geometry between the backstop and the wedge constitute the boundary conditions.

Example: Himalayan Orogen

The

Himalaya The Himalayas, or Himalaya ( ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the Earth's highest peaks, including the highest, Mount Everest. More than 100 pea ...

is a strain partitioned

orogen An orogenic belt, orogen, or mobile belt, is a zone of Earth's crust affected by orogeny. An orogenic belt develops when a continental plate crumples and is uplifted to form one or more mountain ranges; this involves a series of geological proc ...

which resulted from the oblique convergence between India and Asia. Convergence between the two landmasses persists today at a rate of 2 cm/yr. The obliquity of plate convergence increases toward the western portion of the orogen, thus inducing a greater magnitude of strain partitioning within the western Himalaya than in the central. The table below shows relative velocities of India's convergence with Asia. The lateral variability in velocity between the central and marginal regions of the orogen suggest strain is partitioned due to oblique convergence.

Transpression and transtension

Strain partitioning is common within transpressive and transtensive tectonic domains. Both regimes involve a component of pure shear (transpression – compressive, transtension – extensive) and a component of simple shear. Strain may be partitioned by the development of a

strike slip fault In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

shear zone In geology, a shear zone is a thin zone within the Earth's crust or upper mantle that has been strongly deformed, due to the walls of rock on either side of the zone slipping past each other. In the upper crust, where rock is brittle, the shear ...

across the actively deforming region.

Example: Coast Mountains British Columbia

The

Coast Mountains The Coast Mountains () are a major mountain range in the Pacific Coast Ranges of western North America, extending from southwestern Yukon through the Alaska Panhandle and virtually all of the British Columbia Coast, Coast of British Columbia sout ...

of British Columbia are interpreted as a transpressive orogen which formed during the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

. Oblique subduction induced the development of several shear zones which strike parallel to the orogen. The presence of these shear zones suggest that strain is partitioned within the Coast Orogen which resulted in horizontal translation of terranes for several hundred kilometers parallel to the orogen. Homogeneous and partitioned strain within transpressive and transtensive tectonic regimes

Homogeneous and partitioned strain within transpressive and transtensive tectonic regimes

Strain factorization

Strain factorization is a mathematical approach to quantify and characterize the variation of strain components in terms of the intensity and distribution that produces the finite strain throughout a deformed region. This effort is achieved through matrix multiplication. Refer to the figure below to conceptually visualize what is obtained through strain factorization. Conceptual illustration of strain factorization

Influence of rock material rheology

At the grain and crystal scale, strain partitioning may occur between minerals (or clasts and matrix) governed by their rheological contrasts. Constituent minerals of differing rheological properties in a rock will accumulate strain differently, thus inducing mechanically preferable structures and fabrics.

Example

Rocks that contain incompetent (mechanically weak) minerals such as

mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into fragile elastic plates. This characteristic is described as ''perfect basal cleavage''. Mica is co ...

s and more competent (mechanically stronger) minerals such as

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

feldspar Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagiocl ...

s, may develop a shear band fabric. The incompetent minerals will preferentially form the C-surfaces and competent minerals will form along the S-surfaces.

Individual deformation mechanisms

Strain partitioning is also known as a procedure for decomposing the overall strain into individual deformation mechanisms which allowed for strain to be accommodated. This approach is performed from geometrical analysis of rocks on the grain – crystal scale. Strain partitioning of deformation mechanisms incorporates those mechanisms which occur both simultaneously and/or subsequently as tectonic conditions evolve, as deformation mechanisms are a function of strain rate and pressure-temperature conditions. Performing such a procedure is important for structural and tectonic analysis as it provides parameters and constraints for constructing deformation models.

References

{{Structural geology Structural geology