Orientation Of Finite Strain Research Articles

Kinematic vorticity flow analysis and 40Ar/ 39Ar geochronology related to inclined extrusion of the HP–LT metamorphic rocks along the Zagros accretionary prism, Iran

The Zagros accretionary prism in southwestern Iran is exposed along the NW–SE trending of the Zagros Thrust System and inclined Zagros transpression zone. This accretionary prism consists of two units: the upper sedimentary mélange unit on top and the high-pressure metamorphic mélange unit at the bottom. Both units show characteristics of a tectonic wedge. The upper unit consists of type-I, II and III mélanges which display S-, C- and C′-type shear-band cleavages, quartz ribbons and rectangular or fish-head boudins. The lower unit fabrics display σ-, and δ-type porphyroclasts and quartz ribbon mylonites. These fabrics formed from a combination of 60.5% simple shear and 39.5% pure shear. Both components were involved in a lateral exhumation of the high-pressure/low-temperature metamorphic rocks in an inclined transpression wedge-shaped geometry. The estimated kinematic vorticity number ( W k ) was calculated from quartz c-axis patterns, rotation of porphyroclasts and orientation of finite strain with respect to shear zone boundaries. Using the mean estimated W m value of 0.84, the inclination angle for the thrust wedge on top of the NE-subducting Neo-Tethyan oceanic lithosphere is 18°. The 40Ar/ 39Ar plateau ages of early generations of biotite from the lower metamorphic mélange are 119.95 ± 0.88 Ma and 112.58 ± 0.66 Ma. This late Aptian age is related to early thrusting and formation of HP–LT metamorphic rocks. The dating of two amphibole samples from the amphibolite yields a weighted mean age of 91.23 ± 0.89 Ma. This Turonian–Cenomanian age suggests a later metamorphic event associated with subduction and obduction of the Neyriz ophiolite and later lateral extrusion of HP–LT metamorphic rocks along the inclined Zagros accretionary prism.

The influence of plate motions on three‐dimensional back arc mantle flow and shear wave splitting

Both the polarization direction of the fast shear waves and the types of deformation within overriding plates vary between the back arc basins of western Pacific subduction zones. The goal of this study is to test the possibility that motions of the overriding plates may control the patterns of seismic anisotropy and therefore the observed shear wave splitting. We calculated three‐dimensional models of viscous asthenospheric flow driven by the motions of the subducting slab and overriding plates. Shear wave splitting was calculated for polymineralic anisotropy within the back arc mantle wedge assuming that the anisotropy was created by flow‐induced strain. Predicted splitting may differ substantially depending on whether anisotropy is computed directly using polycrystalline plasticity models or is based on the orientation of finite strain. A parameter study shows that both finite strain and textural anisotropy developed within three‐dimensional, plate‐coupled asthenospheric flow models are very heterogeneous when back arc shearing occurs within the overriding plate. Therefore predicted shear wave splitting varies strongly as a function of plate motion boundary conditions and with ray path through the back arc asthenosphere. Flow models incorporating plate motion boundary conditions for the Tonga, southern Kuril, and eastern Aleutian subduction zones produce splitting parameters consistent with observations from each region. Trench‐parallel flow generated by small variations in the dip of the subducting plate may be important in explaining observed fast directions of anisotropy sampled within the innermost corner of the mantle wedge.

Analysis and structural implications of a kinematic model of similar folding

An analysis of a current kinematic model of similar folding is developed with the purpose of establishing theoretical criteria suitable for comparison with structural data obtained from naturally occurring similar folds. Equations are derived to relate the deformation parameters of the fold-model to the shape and orientation of finite ellipsoids for elemental units of homogeneous deformation within a similar fold. These equations provide a graphical strain-chart for the fold-model, and it is shown that the related deformation and geometric parameters of the elemental strain ellipsoids defining the overall inhomogeneous pattern of strain in any theoretical similar fold, will plot on the strain-chart as points on a straight-line locus. This simple diagnostic criterion of the fold-model is of practical interest where quantitative estimates of the magnitude and orientation of finite strain can be obtained from suitable dimensional markers in different parts of natural similar folds. The strain analysis is used to deduce quantitative information concerning the geometry of slaty cleavage (schistosity or foliation) in theoretical similar folds using the widely-accepted hypothesis that this type of penetrative structure is formed normal to the maximum compressive strain in naturally deformed rocks. The patterns of cleavage defined in this way by the fold-model are related to the geometry of folds derived from layers of any initial orientation to the tectonic axes. It is shown that the relative orientation of cleavage and layering at different points in the ac-tectonic plane of theoretical similar folds can be represented on the strain-chart for the fold-model, and give rise to distinctive patterns which can be regarded as diagnostic criteria for the fold-model and the associated theory of cleavage. Attention is also directed to the changing pattern of strain in response to a progressive development of deformation by the postulated mechanism of similar folding.