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_Journal of Structural Geology 33 (2011) 203e219_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg_ _A critique of vorticity analysis using rigid clasts_ _Changcheng Li*, Dazhi Jiang_ _Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada N6A 5B7_ _article info_ .Article history: Received 15 January 2010 Received in revised form 24 June 2010 Accepted 2 September 2010 Available online 15 September 2010 _Keywords: Monoclinic flow Rigid clast Shear zone Transpression Vorticity number Vorticity analysis_ _abstract_ _Estimating the kinematic vorticity numbers from rock fabrics presents many problems. In this paper we use numerical modeling to investigate the reliability of those widely-used vorticity analysis methods using rigid clasts. We use very simple flows (steady state homogeneous and having monoclinic symmetry) to represent plane-straining zones and transpressional zones and assume that the rotation of rigid clasts perfectly obeys the theory of Jeffery (1922, Proceedings of the Royal Society of London A102, 161e179). These are assumptions made in current vorticity analysis using rigid clasts. Even with these simple assumptions our modeling shows that the current methods have intrinsic uncertainties so large that it is pointless to use the estimated vorticity numbers to constrain shear zone boundary conditions and kinematics. It is perfectly consistent with numerical modeling results if the currently reported vorticity numbers estimated from rigid clasts (in the range of 0.50e0.85) are all interpreted as being from natural shear zones with close-to-simple-shearing flows. The large uncertainties arise because the motion of rigid clasts is intrinsically a three dimensional problem._ _© 2010 Elsevier Ltd. All rights reserved._ Ключевые слова: random number, actual wks, aspect ratio, zhang, signicantly differ, ramsay, nite deformation, kinematic, black dash, kinematics, boundary, polar plot, shear zone, koukouvelas law, nite strain, science, ?ow, rotation behavior, vorticity vector, rotation path, stabilized, transpression, china series, type, sc, symmetry, reference frame, xypolias, zone, plane, elds, shear, rst plot, ratio, journal structural, law, elsevier, jeffery, wallis, simple, rock reviews, initial orientation, number, angular velocity, clast generated, vorticity number, correctly identied, arc method, plotted, mxy, deformation advance, mancktelow, common practice, modeling, range, analytical solution, ildefonse, ?ows, jiang journal, apophysis, ductile, rigid particle, shear plane, structural geology, shape, bouchez, stable position, orientation, theory, london, completely characterize, rigid clast, rotate permanently, shearing, rock, plane straining, truesdell, symmetry axis, schmid, jefferys theory, vorticity analysis, steady, motion, estimated vorticity, stabilized clast, clast plotted, geology, arc, symbols, clast align, rcutoff, analysis method, geophysics, rotating clast, simpson, uncertainty, numerical experiment, method, sullivan, wks, particle, brc method, shape fabric, large uncertainty, constructed, deformation, ?nite, ductile extrusion, analysis, sarkarinejad, rotating, large, acute angle, simple shearing, intrinsic uncertainty, reviews geophysics, eds, cartesian, clast rotation, journal structural geology, vns, reviews, strain, rcrit, rotation sense, rotation, royal society, plot, fabric, simple shear, clast shape, eld, monoclinic ows, solid branch, paper, cartesian coordinate, paul, uncertainty large, plane-straining, passchier, tectonophysics, long axis, original width, ax, rotates permanently, bulk, tectonic evolution, polar coordinate, clast start, rigid inclusion, williams lin, ramberg, numerical modeling, clast rotates, estimated, quartz lpo, porphyroclast, straining, frassi, intermediate, circle, special case, result, williams, interface slip, vector, stable orientation, tullis, ?nite strain, numerical, monoclinic symmetry, axis, estimated vorticity number, hyperbola, clast apophyses, geological, extrusion model, vorticity, wide range, stable, clast, natural shear, journal, natural, pure shear, theoretical rcrit, reach parallelism, carter, marques, li, reach, structural, wenk, kinematic vorticity, modeling result, type clast, fabric evolution, parallel, general, monoclinic, mechanical interaction, jiang, coordinate, case, rigid, open circle, johnson, ramberg passchier, pennacchioni, viscous, cartesian plot