Strain partitioning
Instructural geology,strain partitioningis the distribution of the totalstrainexperienced on a rock, area, or region, in terms of different strain intensity and strain type (i.e.pure shear,simple shear,dilatation).[1][2][3]This process is observed on a range of scales spanning from the grain –crystalscale to theplate– lithospheric scale, and occurs in both the brittle and plastic deformation regimes.[1][2]The manner and intensity by which strain is distributed are controlled by a number of factors listed below.[2]
Influencing factors
[edit]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:[2]
- anisotropysuch 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."[4]
- rheology
- boundary conditions – the geometrical and mechanical properties and
- stress orientation – critical angles by which stress is applied.[1][2][3]
Subdivisions
[edit]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.[5]
Superposition of individual strain components
[edit]The superposition of individual strain components can be expressed at the tectonic scale involving oblique convergent margins and transpression / transtension tectonic regimes.[1]
Oblique convergent margins
[edit]![](https://upload.wikimedia.org/wikipedia/commons/thumb/0/03/Strain_partitioning_at_an_oblique_convergent_margin.jpg/440px-Strain_partitioning_at_an_oblique_convergent_margin.jpg)
Convergent margins where the angle ofsubduction is obliquewill 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 throughthrust faults).[6][7]This occurs as a response toshear stressexerted at the base of the overriding plate that is not perpendicular to the plate margin.[6][7][8]
Fundamental factors which control strain partitioning within oblique orogens
[edit]- Stress orientation:Increasedsubductionangle increases the arc parallel component.[6][7]
- Rheology and anisotropy:Mechanical properties of the wedge: (coulomb vs plastic) influence the wedge geometry.[6][7]
- Boundary conditions:The friction and geometry between the backstop and the wedge constitute the boundary conditions.[6][7]
Example: Himalayan Orogen
[edit]TheHimalayais a strain partitionedorogenwhich resulted from the oblique convergence between India and Asia.[9]Convergence between the two landmasses persists today at a rate of 2 cm/yr.[9]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.[9]
The table below[8]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.[8][9]
Location | Arc Normal | Arc Parallel |
---|---|---|
Western | ~10 mm/yr Northward | ~20 mm/yr Westward |
Central | ~30 mm/yr Northward | ~0 mm/yr |
Eastern | ~15 mm/yr Northward | ~20 mm/yr Eastward |
Transpression and transtension
[edit]Strain partitioning is common withintranspressiveandtranstensivetectonic domains.[10][11]Both regimes involve a component of pure shear (transpression – compressive, transtension – extensive) and a component of simple shear.[3][10][11]Strain may be partitioned by the development of astrike slip faultorshear zoneacross the actively deforming region.[10][11]
Example: Coast Mountains British Columbia
[edit]TheCoast Mountainsof British Columbia are interpreted as a transpressive orogen which formed during theCretaceous.[12]Oblique subduction induced the development of several shear zones which strike parallel to the orogen.[12]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.[12]
![](https://upload.wikimedia.org/wikipedia/commons/thumb/4/41/Homogeneous_and_partitioned_strain_within_transpressive_and_transtensive_tectonic_regimes.jpg/600px-Homogeneous_and_partitioned_strain_within_transpressive_and_transtensive_tectonic_regimes.jpg)
Strain factorization
[edit]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.[13][14][15][16]This effort is achieved through matrix multiplication.[14][15]Refer to the figure below to conceptually visualize what is obtained through strain factorization.
![](https://upload.wikimedia.org/wikipedia/commons/thumb/9/9c/Conceptual_illustration_of_strain_factorization.jpg/600px-Conceptual_illustration_of_strain_factorization.jpg)
Influence of rock material rheology
[edit]At the grain and crystal scale, strain partitioning may occur between minerals (or clasts and matrix) governed by theirrheologicalcontrasts.[2][5][17][18]Constituent minerals of differing rheological properties in a rock will accumulate strain differently, thus inducing mechanically preferable structures and fabrics.[17][18]
Example
[edit]![](https://upload.wikimedia.org/wikipedia/commons/thumb/3/33/Deformation_mechanisms_%28Passchier_and_Trouw%29.jpeg/300px-Deformation_mechanisms_%28Passchier_and_Trouw%29.jpeg)
Rocks that contain incompetent (mechanically weak) minerals such asmicasand more competent (mechanically stronger) minerals such asquartzorfeldspars,may develop a shear band fabric.[17][18]The incompetent minerals will preferentially form the C-surfaces and competent minerals will form along the S-surfaces.[17][18]
Individual deformation mechanisms
[edit]Strain partitioning is also known as a procedure for decomposing the overall strain into individualdeformation mechanismswhich allowed for strain to be accommodated.[14]This approach is performed from geometrical analysis of rocks on the grain – crystal scale.[14]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.[14][16]Performing such a procedure is important for structural and tectonic analysis as it provides parameters and constraints for constructing deformation models.[16][20]
See also
[edit]References
[edit]- ^abcdeJones, Richard; Tanner, P.W. Geoff (1995). "Strain partitioning in transpression zones".Journal of Structural Geology.17(6): 793–802.Bibcode:1995JSG....17..793J.doi:10.1016/0191-8141(94)00102-6.
- ^abcdefCarreras, Jordi; Cosgrove, John; Druguet, Elena (2013). "Strain partitioning in banded and/or anisotropic rocks: Implications for inferring tectonic regimes".Journal of Structural Geology.50:7–21.Bibcode:2013JSG....50....7C.doi:10.1016/j.jsg.2012.12.003.
- ^abcdFossen, Haakon (2012).Structural Geology.New York, USA: Cambridge University Press.ISBN978-0-521-51664-8.
- ^Jean-Pierre Brun (1983) "Isotropic points and lines in strain fields",Journal of Structural Geology5(3):321–7
- ^abNeuendorf, Kaus; Mehl, James; Jackson, Julia (2005).Glossary of Geology(5 ed.). Alexandria, VA, United States: American Geological Institute.ISBN978-0-922152-76-6.
- ^abcdefPlatt, J.P. (1993). "Mechanics of Oblique Convergence".Journal of Geophysical Research.98(B9): 16, 239–16, 256.Bibcode:1993JGR....9816239P.doi:10.1029/93JB00888.
- ^abcdeMcCaffrey, Robert (1992). "Oblique Plate Convergence, Slip Vectors, and Forearc Deformation".Journal of Geophysical Research.97(B6): 8905–8915.Bibcode:1992JGR....97.8905M.doi:10.1029/92JB00483.
- ^abcSyron, Richard; Taylor, Michaeal; Murphy, Michael (2011)."Oblique convergence, arc-parallel extension, and the role of strike-slip faulting in the High Himalaya".Geosphere.7(2): 582–596.Bibcode:2011Geosp...7..582S.doi:10.1130/GES00606.1.
- ^abcdMurphy, M.A.; Taylor, M.H.; Gosse, J.; Silver, R.P.; Whipp, D.M.; Beaumont, C. (2014). "Limit of strain partitioning in the Himalaya marked by large earthquakes in western Nepal".Nature Geoscience.7(1): 38–42.Bibcode:2014NatGe...7...38M.doi:10.1038/NGEO2017.
- ^abcdTeyssier, Christian; Tikoff, Basil; Markley, Michelle (1995). "Oblique plate motion and continental tectonics".Geology.23(5): 447.Bibcode:1995Geo....23..447T.doi:10.1130/0091-7613(1995)023<0447:OPMACT>2.3.CO;2.
- ^abcFossen, Haakon; Tikoff, Basil; Teyssier, Christian (1994)."Strain modeling of transpressional and transtensional deformation"(PDF).Norsk Geologisk Tidsskrift.74:134–145.
- ^abcChardon, Dominique; Andronicos, Christopher; Hollister, Lincoln (1999)."Large-scale transpressive shear zone patterns and displacements within magmatic arcs: The Coast Plutonic Complex, British Columbia".Tectonics.18(2): 278–292.Bibcode:1999Tecto..18..278C.doi:10.1029/1998TC900035.
- ^abSanderson, David; Marchini, W.R.D. (1984). "Transpression".Journal of Structural Geology.6(5): 449–458.Bibcode:1984JSG.....6..449S.doi:10.1016/0191-8141(84)90058-0.
- ^abcdefRamsay, John; Huber, Martin (1983).The Techniques of Modern Structural Geology Volume 1: Strain Analysis.London: Academic Press.ISBN978-0-12-576901-3.
- ^abcRamsay, John; Huber, Martin (1987).The Techniques of Modern Structural Geology Volume 2: Folds and Fractures.London: Academic Press.ISBN978-0-12-576902-0.
- ^abcEvans, Mark; Dunne, William (1991). "Strain factorization and partitioning in the North Mountain thrust sheet, central Appalachians, U.S.A".Journal of Structural Geology.13(1): 21–35.Bibcode:1991JSG....13...21E.doi:10.1016/0191-8141(91)90098-4.
- ^abcdGoodwin, Laurel; Tikoff, Basil (2002). "Competency contrast, kinematics, and the development of foliations and lineations in the crust".Journal of Structural Geology.24(6–7): 1065–1085.Bibcode:2002JSG....24.1065G.doi:10.1016/S0191-8141(01)00092-X.
- ^abcdMichibayashi, Katsuyoshi; Murakami, Masami (2007). "Development of a shear band cleavage as a result of strain partitioning".Journal of Structural Geology.29(6): 1070–1082.Bibcode:2007JSG....29.1070M.doi:10.1016/j.jsg.2007.02.003.hdl:10297/508.
- ^Passchier, Cees; Trouw, Rudolph (2005).Micro-tectonics(5th ed.). New York: Springer.ISBN978-3-540-64003-5.
- ^Mitra, Shankar (1976). "A Quantitative Study of Deformation Mechanisms and Finite Strain in Quartzites".Contributions to Mineralogy and Petrology.59(2): 203–226.Bibcode:1976CoMP...59..203M.doi:10.1007/BF00371309.