Kinking facilitates grain nucleation and modifies crystallographic preferred orientations during high-stress ice deformation

Kinking can accommodate significant amounts of strain during crystal plastic deformation under relatively large stresses and may influence the mechanical properties of cold planetary cryosphere. To better understand the origins, mechanisms, and microstructural effects of kinking, we present detailed microstructural analyses of coarse-grained ice (~1300 µm) deformed under uniaxial compression at -30°C. Microstructural data are generated using cryogenic electron backscattered diffraction (cryo-EBSD). Deformed samples have bimodal grain size distributions, with thin and elongated (aspect ratio ≥ 4) kink domains that develop within, or at the tips of, remnant original grains (≥ 300 µm, aspect ratio < 4). Small, equiaxed subgrains also develop along margins of remnant grains. Moreover, many remnant grains are surrounded by fine-grained mantles of small, recrystallized grains (< 300 µm, aspect ratio < 4). Together, these observations indicate that grain nucleation is facilitated by both kinking and dynamic recrystallization (via subgrain rotation). Low- (< 10°) and high-angle (mostly > 10°, many > 20°) kink bands within remnant grains have misorientation axes that lie predominantly within the basal plane. Moreover, previous studies suggest the kinematics of kinking and subgrain rotation should be fundamentally the same. Therefore, progressive kinking and subgrain rotation should be crystallographically controlled, with rotation around fixed misorientation axes. Furthermore, the c-axes of most kink domains are oriented sub-perpendicular to the sample compression axis, indicating a tight correlation between kinking and the development of crystallographic preferred orientation. Kink band densities are the highest within remnant grains that have basal planes sub-parallel to the compression axis (i.e., c-axes perpendicular to the compression axis)—these data are inconsistent with models suggesting that, if kinking is the only strain-accommodating process, there should be higher kink band densities within grains that have basal planes oblique to the compression axis (for low kink-host misorientation angles, e.g., ≤ 20°, as in this study). One way to rationalize this inconsistency between kink models and experimental observations is that kinking and dynamic recrystallization are both active during deformation, but their relative activities depend on the crystallographic orientations of grains. For grains with basal planes sub-parallel to the compression axis, strain-induced GBM is inhibited, and large intragranular strain incompatibilities can be relaxed via kinking, when other processes such as subgrain rotation recrystallization are insufficient. For grains with basal planes oblique to the compression axis, strain-induced grain boundary migration (GBM) might be efficient enough to relax the strain incompatibility via selective growth of these grains, and kinking is therefore less important. For grains with basal planes sub-perpendicular to the compression axis, kink bands are seldom observed—for these grains, the minimum shear stress required for kinking exceeds the applied compressive stress, such that kinks cannot nucleate.

The influence of backstop geometry in the structural style of the Eastern Cordillera of Colombia: A sandbox modeling approach

&lt;p&gt;Among the foreland belts of the Andean mountain system, the Eastern Cordillera of Colombia (EC) represents a unique example of an isolated, bi-vergent mountain belt. In contrast, to block tectonics of broken foreland basins, it displays a ductile deformation style which involves two mountain fronts with a structural relief of the order of 10 km. Internal parts of the EC have been shortened by buckling at high and a homogeneously strained basement at deeper structural levels. These deformation patterns likely attest to conditions of a thermally weakened backarc setting. Two opposed scenarios have been postulated for its surface uplift and consequent exhumation: 1) an E-migrating deformation front and the formation of progressively forward breaking faults; and 2) the pop-up of a weak crustal welt enclosed by strong foreland blocks. In this latter setting, a synchronous early formation of marginal mountain fronts and a late-stage surface uplift of a central domain may be anticipated. These two constellations compare, in terms of a contrasting model setup, to a foreland migrating orogenic wedge or a relatively stable, doubly vergent wedge formed above a structural discontinuity or rheologic boundaries that acted as sites for the nucleation of the marginal faults.&lt;/p&gt;&lt;p&gt;In this contribution, we opt to examine the &amp;#8220;boundary&amp;#8221; conditions for the development of a doubly vergent wedge formed at the tip line of a rigid tapering backstop, that simulates a rigid foreland block. With respect to the shape of this backstop, we examine the effects of tip angles less than the angle of internal friction (&lt;30&amp;#176;) and find, that at a low tip angle of 10&amp;#176; the pop-up evolves above a forward-breaking principal kink-band with the synchronous formation of a sequence of conjugate back-kinks that cut into the sand pack, as it is pushed toward the backstop. At a moderate tip angle of 20&lt;sup&gt;o &lt;/sup&gt;the forward-breaking kink-band is slightly steeper than the backstop and gives rise to a frontal fold with an overturned limb. This latter geometrical configuration loosely compares to the structural relations of a structural section through the high plains of Bogot&amp;#225;, where the eastern mountain front defines a strongly deformed antiform, that is juxtaposed against an undeformed margin of the adjacent Guyana shield.&lt;/p&gt;

Incremental shortening calculation of the mixed-shear fault-bend folds

&lt;p&gt;Shear fault-bend folds are characterized by a long back-limb that dips more gently than the fault ramp [1]. During the folding growth, the back limb rotates and widens progressively through a combination of limb rotation and kink-band migration. Two end-member models of shear fault-bend folding theories, including simple-shear fault-bend folding (C=0.5) and pure-shear fault-bend folding (C=1), have been developed and widely applied. Mixtures of pure and simple shear (0.5&lt;C&lt;1) are possible and have been found in the natural. Few quantitative methods to limit the shear-index (C) of the shear fault-bend folds so far. The incremental shortening can be calculated based on a simplified equation that assumes the linear relationship between the shortening and the limb rotation angle of the back limb [2]. However, the relationship of these two parameters is nonlinear according to the shear fault-bend folding theory [1]. Calculation results of the linear model have large uncertainty.&lt;/p&gt;&lt;p&gt;Here, we develop a method to calculate the shear-index (C), providing an idea to improve the mixed-shear fault-bend fold models, and establishing a formula to calculate the incremental shortening based on the nonlinear relationship between the back-limb dip angle and the shortening. It is a more general method to calculate the incremental shortening of the shear fault-bend folds.&lt;/p&gt;&lt;p&gt;This model has been applied to the Tugulu anticline in the northern foreland of Chinese Tian Shan, which is a mixed-shear fault-bend fold based on previous studies [3]. Through an analysis of deformed fluvial terraces and growth strata, we develop the shortening history of the Tugulu anticline along the Hutubi River using our developed nonlinear model. Our results show that the Tugulu anticline has a shear-index of ~0.91 and a steady shortening rate of ~1.5mm/yr over the last 500ka.&lt;/p&gt;&lt;p&gt;References:&lt;/p&gt;&lt;ul&gt;&lt;li&gt;[1] Suppe et al. (2004) AAPG Memoir 82: 303-323.&lt;/li&gt; &lt;li&gt;[2] Yue et al. ( 2011) AAPG Memoir 94: 153&amp;#8211;186.&lt;/li&gt; &lt;li&gt;[3] Qiu et al. ( 2019) Tectonophysics 772:228209.&lt;/li&gt; &lt;/ul&gt;

Influence of Temperature and Plastic Strain on Deformation Mechanisms and Kink Band Formation in Homogenized HfNbTaTiZr

Due to its outstanding ductility over a large temperature range, equiatomic HfNbTaTiZr is well-suited for investigating the influence of temperature and plastic strain on deformation mechanisms in concentrated, body centered cubic solid solutions. For this purpose, compression tests in a temperature range from 77 up to 1073 K were performed and terminated at varying plastic strains for comparison of plastic deformation behavior. The microstructure and chemical homogeneity of a homogenized HfNbTaTiZr ingot were evaluated on different length scales. The compression tests reveal that test temperature significantly influences yield strength as well as work hardening behavior. Electron backscatter diffraction aids in shedding light on the acting deformation mechanisms at various temperatures and strains. It is revealed that kink band formation contributes to plastic deformation only in a certain temperature range. Additionally, the kink band misorientation angle distribution significantly differs at varying plastic strains.