Opposite Shear Sense Research Articles

MANTLE EXHUMATION PROCESSES IN THE OCEAN CONTINENT TRANSITION OF MAGMA-POOR RIFTED MARGINS

MANTLE EXHUMATION PROCESSES IN THE OCEAN CONTINENT TRANSITION OF MAGMA-POOR RIFTED MARGINS

Exhumation process of the Nago subduction-related metamorphic rocks, Okinawa, Ryukyu island arc

The Kunigami zone in Okinawa is an extension of the Shimanto zone, Japan. The rocks make up the main part of the Nago metamorphic rocks, and such metamorphic rocks are exceptional in the Shimanto zone. The Anne complex, in the older Motobu zone, is also metamorphosed. The reason for why and how this kind of the metamorphism occurred, and especially why and how the metamorphic rocks were exhumed, is yet uncertain and unresolved. To understand the metamorphic and exhumation process in Okinawa, a structural study is undertaken, and its relation to the Eocene ridge subduction is discussed. We believe exhumation was performed by formation of a D2 extrusion wedge, made up of the Nago metamorphic rocks. The base for this wedge is a subduction thrust, and the roof is a detachment fault. Internally, there exists another Kijoka detachment fault, which is a brittle low-angle fault with top to the northwest shear sense, and the D2 major recumbent folds and thrusts show top to the southeast opposite shear sense in the Kunigami zone. This is the first report that finds detachment faults from the typical and ancient accretionary complex. M2 is mostly retrograde related to exhumation, but its medium P/T-type prograde metamorphism, abnormal at subduction zones, represents a high thermal gradient during ridge subduction. As a result, this ridge subduction is responsible for exhumation. At the time of accretion of the Kunigami zone, D1 ductile contraction and constriction exhibited top to the southeast shear sense, but an opposite and extensional shear sense is recognized in the proto-wedge. During D1, the wedge had already been active and begun to exhume. M1 of the Miyagi complex is accretion related and also of medium P/T-type metamorphism, and is a consequence of Cretaceous ridge subduction without any ability to cause much exhumation.

The strain path and kinematics of lava domes: An example from Lipari (Aeolian Islands, Southern Tyrrhenian Sea, Italy)

A circular dome from Lipari Island consists of latitic enclaves hosted in a rhyolitic magma. A strain analysis of the enclaves has been carried out and the pure shear (α) and simple shear (γ) deformation, the vorticity number Wk and flow kinematics are determined. The axial ratio Rf of the enclaves and the angle ϕ between the long axis of the enclaves and the transport direction are measured in 131 sites concentrated in the axial zone (z1), upper‐distal zone (z2) and basal zone (z3) of the flow. The ϕ values depict a pattern suggesting endogenous growth. In z1, α/γ > 1. In z2, 1 < α < 1.35 and −1.8 < γ < −0.5. In z3, 1 < α < 1.5 and 0.3 < γ < 2.8. In z1, Wk < 0.5. In z2 and z3, Wk > 0.8. Lateral stretching due to the upward motion of magma from the conduit prevails in z1. Here the increase of pure shear strain from the bottom to the top reflects strain accumulation due to endogenous growth. z2 and z3 suffer simple shear deformation. In z3, the sense of shear is consistent with the transport direction. An opposite sense of shear characterizes z2. This is due to velocity gradients located in the lower and upper portions of the flow. The kinematics is viscous spreading in z1 and viscous gliding (hyperbolic flow) in z2–z3. Possible rupturing of the exterior may be important in z1, where lateral stretching occurs, and in z3, where γ is at a maximum. The effusion rate is ∼1.93 m3/s. Strain rates calculated using structural data span a range from 1.9 to 5.8.10−6 s−1.

Sheeted intrusion of the synkinematic McDoogle pluton, Sierra Nevada, California

Field, microstructural, and geochronologic evidence indicates that the Late Cretaceous McDoogle pluton, located near the eastern margin of the Sierra Nevada batholith, was emplaced as a subvertically sheeted complex into a steep reverse-sense shear zone. Evidence of internal subvertical sheeting includes abundant concordant wall-rock inclusions and screens that separate the pluton from adjacent Jurassic plutons, preservation of a ghost tectonostratigraphy in the distribution of the inclusions, and rare interphase contacts. The solid-state tectonic fabric in the wall rocks and the magmatic, submagmatic, and weak solid-state fabrics in the McDoogle pluton all are concordant and record northeast-southwest horizontal shortening, subvertical extension, and a significant component of northeast-side-up simple shear. Intrusive contacts generally are concordant with the fabrics but, where discordances occur, the wall-rock fabric invariably is truncated by the contact. However, late synkinematic emplacement of the McDoogle pluton is indicated by synintrusive boudinage of apophyses of the pluton and by the overall concordance of pluton fabrics including magmatic lineation. Zircon U-Pb isotope dates were obtained from the quartz monzodiorite central phase of the McDoogle pluton (94 ± 4 Ma), the mafic granodiorite border phase (94.8 ± 0.6 Ma), and an older hornblende granodiorite included in the central phase (97.6 ± 0.4 Ma). Granodiorite orthogneiss in the wall rock yielded a U-Pb zircon date of 164 +2.1/–6.8 Ma, which we interpret to be the crystallization age and which is indistinguishable from a new age of 164.5 ± 2.3 Ma for the Twin Lakes pluton. A U-Pb titanite date of 94.4 ± 0.9 Ma from the 164 Ma orthogneiss sample may reflect thermal effects of the intruding McDoogle pluton, synkinematic growth of titanite in the shear zone, or perhaps both. The age of the Sawmill Lake shear zone that hosts the McDoogle pluton is bracketed between 148 Ma, the age of the Independence dike swarm, and 92 Ma, the age of the Lamarck pluton that crosscuts both the McDoogle pluton and the shear-zone fabrics. Regional evidence suggests a shear-zone age younger than 110 Ma, but older deformation is possible. The predominance of horizontal shortening over transcurrent motion and an age older than 90 Ma exclude a direct relationship to the dextral Sierra Crest shear system, which has been proposed to pass near the study area. The location, age constraints, and kinematics of the shear zone are consistent with both the later stages of movement in the East Sierran thrust system and isostatic sinking of the arc magmatic complex into its substrate.

Crystal fabric evolution in lava flows: results from numerical simulations

The analysis of the preferred orientation of crystals (crystal fabric) in magmatic rocks has become a widely used technique for the reconstruction of the flow history. However, little is known about the evolution of the fabric during flow. Here, numerical simulations are used to study the fabric evolution of low-concentration, laminar magmatic flows (e.g. lava flows). The fabric evolution of (a) particle populations with a specified shape (cube, tablet, prism, and transitional shape) and (b) crystal populations from a lava flow is analyzed in different flow geometries (simple shear, hyperbolic and pure shear flows) assuming plane strain. Results show that fabric analysis of the whole crystal population gives little information about flow kinematics, whereas the comparative analyses of crystal sub-populations with different shapes allow us to recognize the flow geometry. Simple shear flow produces oscillating to pseudo-stable fabrics. The fabric strength is lower with respect to that of hyperbolic and pure shear flows and the preferred orientation of crystals does not coincide with the flow direction, except for large strain and specified shapes. Sub-fabrics with opposite sense of shear may also develop, depending on the crystal shape and finite strain. Pure shear and hyperbolic flows show stable to pseudo-stable fabrics. The preferred crystal orientation may or may not coincide with the flow direction according to whether flow is in pure or hyperbolic shear. Results from numerical simulations are comparable with those from experimental models and natural examples. The fabric strength depends on the number of crystals and caution must be used in extrapolating the results beyond the scale of observation. The finite strain in a sample from a lava flow from the Aeolian Islands is determined by the comparative analysis of the calculated and measured fabric parameters (fabric intensity and crystal preferred orientation). Criteria to discriminate among fabrics produced by different flow types are also provided.

Mid- to Late Cretaceous ductile deformation and thermal evolution of the crust in the northern Dome Rock Mountains, Arizona

Mid- to Late Cretaceous basement-involved deformation in the Maria Tectonic Belt of southeastern California and west-central Arizona varies in style from low-angle overthrusts to km-scale fold nappes. To establish the relationship between tectonite fabrics and thermal evolution of the rocks, structural and petrographic analysis was conducted in the northern Dome Rock Mountains, where the two different styles converge. A penetrative northeast-dipping foliation forms the most conspicuous fabric in an ∼5 km wide zone of distributed ductile deformation. Within the broader zone of ductile deformation, northeast-dipping shear zones contain evidence for both southwest- and northeast-directed deformation. Textural analysis of mylonitic rocks reveals that southwest-directed deformation occurred under prograde conditions, whereas northeast-directed deformation occurred under retrograde conditions. Southwest-directed deformation resulted in tubular (extreme sheath) fold development on a kilometer scale in the footwall of a reverse-sense shear zone. Northeast-directed deformation cuts the map-scale tubular fold and is inferred to have developed as a manifestation of gravitational collapse of rocks mechanically weakened by fluid infiltration and upper greenschist- to lower amphibolite-facies metamorphism. We propose that the mode and degree of fluid infiltration, rather than temperature or structural level alone, controlled the style of deformation in the Maria Belt.

Syn-convergent exhumation implied by progressive deformation and metamorphism in the Valle dell’Orco transect, NW Italian Alps

Structural, metamorphic and geochronological data from the Valle del'Orco are best explained by an orogenic wedge model in which crustal slices from continent and ocean acted as passive blocks at different times in the Alpine orogeny. Deformation generally migrated westwards in this section, and metamorphic recrystallization at any given point was largely controlled by the pervasiveness of strain. The data suggest a dynamic crustal stacking system lasting from 70 to 35 Ma, rather than continental collision following long-lived B-type subduction, and offer a relevant and simple model for eclogite exhumation that requires exhumation of ductile shear-bounded slices within the orogenic wedge. The easternmost, internal unit of the Sesia-Lanzo Zone, the Eclogitic Micaschist Complex, was subducted and pervasively recrystallized under eclogite facies conditions during the early stages of convergence in the late Cretaceous. During the development of the major preserved structures, the eastern Eclogitic Micaschist Complex was undergoing general non-coaxial shear with an overall top-down to the east vorticity, suggesting a potential mechanism of exhuming an eclogitic 'pip'. Slices of pre-Alpine basement, the Vasario-Sparone Subunit, were incorporated into the lower (western) parts of this unit during the latest part of this phase, but do not show penetrative effects of the early Alpine deformation. They were passively folded during the early decompressional part of the P—T history, prior to the unit being juxtaposed against the underlying Gneiss Minuti Complex under blueschist-greenschist facies conditions. The Gneiss Minuti Complex itself was then recrystallized at high strain in the greenschist-facies in the latest Eocene. Fabrics developed at this stage indicate top-down-to-east extension which may have accommodated the exhumation of eclogitized Piemont oceanic material and the European continental material of the Gran Paradiso massif. Evidence of northwest-directed thrusting within the Gran Paradiso unit at the same time suggests again that the crustal block was transported on a deeper reverse-sense shear zone that presumably links to the more external thrusts now exposed to the west. High-pressure mineral lineations within the Piemont zone cannot be unequivocally assigned to either the burial or exhumation paths, but are consistent with the generally east-west transport directions observed in the structurally higher units.

Enéchelon sigmoidal vein arrays hosted by faults

The morphology of enéchelon sigmoidal veins generally supports an extension fracture origin for host fractures. Nevertheless, a shear fracture origin has been proposed in some cases, mainly on geometric arguments, often without morphological support. This study describes a system of sigmoidal enéchelon quartz veins in a deformed greywacke which displays morphological evidence that the veins are hosted by faults. The veins occur in the hinge zone of an upright horizontal antiform and their configuration is consistent with both faulting and vein opening occurring during folding. The formation of close-spaced vertical dextral faults transformed thick massive greywacke beds into a discontinuous multilayer structure of rock bridges between faults. As the length to thickness ratio of the multilayer structure increased, the deformation mechanism changed to one of rotation and bending of rock bridges along sinistral shear displacement zones resulting in sigmoidal openings between rock bridges. Morphological features which indicate that the host fractures are faults include: (1) continuity of host fractures well past the tips of the sigmoidal veins and shear displacement of markers by unopened host fractures, (2) pinnate extension veins emanating from host fractures particularly near terminations and (3) shear quartz fibres on host fractures. Basic kinematic analysis shows that shear displacement (with opposite shear sense to the array) must occur before sigmoidal forms develop as the curvature produces contractional jogs which are incompatible with vein opening. Recognition of these features may assist in determining the role of shear displacement on host fractures of sigmoidal enéchelon vein arrays.

True and apparent geometric variability of en-échelon vein arrays

The wide range of vein-array angles reported for en-échelon vein arrays can be attributed to both true and apparent geometric effects. A survey of published figures supports a continuum of orientations rather than distinct populations related to different fracture mechanisms. The results of the survey indicate a modal veinarray angle of less than 45 ° which indicates area increase as described by a displacement vector oriented outward from the array. A stereometric analysis of the effects of exposure-plane orientation on the apparent vein-array angle is developed. Examples of vein arrays, from the Nambucca Slate Belt of eastern Australia, exposed on two differently-oriented surfaces are shown. These examples include non-profile exposures with apparent vein-array angles which are (1) less than, (2) greater than, (3) approximately equal to, and (4) of opposite shear sense to the true vein-array angle.

The magmatic, metamorphic and tectonic evolution of the Northern Marginal Zone of the Limpopo Belt in Zimbabwe

The Northern Marginal Zone of the Limpopo Belt in southern Africa comprises a Plutonic Assemblage of granitoids including a distinctive suite of porphyroclastic granites. and a much less abundant Supracrustal Assemblage of metabasites and iron formations. These rocks are at granulite facies above a normal thickness of continental crust. Most of the Plutonic Assemblage are intrusive rocks that crystallized from dry melts from 2800 to 2600 Ma, with a relatively simple thermal history. They may have been derived from partial melting of a mafic source. Some supracrustal rocks have experienced two thermal events at granulite facies. A reverse-sense shear zone forms the boundary of the Northern Marginal Zone with the Zimbabwe craton. The southern boundary is the Triangle shear zone, which is proven as a continuous structure along a much greater strike length than previously documented. A widespread sub-vertical foliation in the Northern Marginal Zone and the reverse shear zone formed during progressive NNW-SSE shortening. Crustal thickening occurred both magmatically and tectonically in the late Archaean, and was accompanied by synchronous uplift. Protracted magmatism provides a mechanism to incorporate supracrustal rocks into the lower crust, and can explain the occurrence of more than a single thermal event.

Ductile crust exhumation and extensional detachments in the central Aegean (Cyclades and Evvia Islands).

The Aegean continental domain is known to be the site of widespread “back-arc” extension since at least 13 Ma, on the basis of seismotectonic, stratigraphic and fault analysis studies. This extension is documented to overprint structures related to the Mesozoic-Cenozoic Hellenic orogeny. Features attributed to early thrusting include the overall ductile deformation within two broad belts that have suffered HP/LT metamorphism across the Aegean. This study presents a structural analysis of the central Aegean area (Cyclades and Evvia Islands), examining in particular the relationship between ductile and brittle deformation, both in the field and on a regional scale. Extension appears to be responsible for most of the ductile deformation within HP rock units that have experienced penetrative greenschist facies and higher grade metamorphic over-printing. On each studied island, progressive extensional deformation has occurred through the development of a major normal-sense detachment zone down to depths of about 18-25 km. Large displacement along the detachment zone accounts for rapid cooling and exhumation of ductile lower crust to form a local metamorphic dome or core complex. Structural and stratigraphic features support a progressive migration of normal faulting away from the dome axis, and a rotation of previously active faults toward low dips, as in kinematic models recently suggested for the development of extensional detachment systems. All the studied domes, except that seen on los Island, show a dominant top-to-the north or north-east sense of shear, while on the southern flank of many of them, an opposite sense of shear is observed, displaying the same progressive evolution from ductile to brittle rock behaviour. This opposite sense of shear is thought not to result from shearing along a major conjugate detachment zone, as in some recent models, but from the accommodation in the ductile crust of upward bending of the brittle upper crust in the footwall of the north-dipping detachment. Available radiometric and stratigraphie data indicate an early minimum age (22-19 Ma) for the onset of extension. The relationship between early metamorphic domes and shallow-dipping detachments, on one hand, and Messinian-Quaternary steep normal faults and grabens, on the other hand, is best explained with the progressive and continuous development of new normal faults away from the domes axes, rather than with a two-stage evolutionary model (core-complex stage, then Basin-and-Range stage) of the type invoked for the North American Cordillera.

Contrasting tectonic styles in the northern Grenville province: Implications for the dynamics of orogenic fronts

Research Article| December 01, 1993 Contrasting tectonic styles in the northern Grenville province: Implications for the dynamics of orogenic fronts T. Rivers; T. Rivers 1Department of Earth Sciences, Memorial University, St. John's, Newfoundland A1B 3X5, Canada Search for other works by this author on: GSW Google Scholar J.A.M. van Gool; J.A.M. van Gool 1Department of Earth Sciences, Memorial University, St. John's, Newfoundland A1B 3X5, Canada Search for other works by this author on: GSW Google Scholar J. N. Connelly J. N. Connelly 1Department of Earth Sciences, Memorial University, St. John's, Newfoundland A1B 3X5, Canada Search for other works by this author on: GSW Google Scholar Geology (1993) 21 (12): 1127–1130. https://doi.org/10.1130/0091-7613(1993)021<1127:CTSITN>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation T. Rivers, J.A.M. van Gool, J. N. Connelly; Contrasting tectonic styles in the northern Grenville province: Implications for the dynamics of orogenic fronts. Geology 1993;; 21 (12): 1127–1130. doi: https://doi.org/10.1130/0091-7613(1993)021<1127:CTSITN>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract In western Labrador, tectonic elements of the parautochthonous belt of the northern Grenville orogen include a regional, mid-crustal, thin-skinned, metamorphic fold-thrust belt structurally overlain by a thick-skinned thrust sheet with a high-pressure metamorphic signature. In central Ontario, the parautochthonous belt is composed of a major reverse-sense shear zone, along which deep crustal rocks have been uplifted and exhumed. Tectonic models for both areas involve crustal shortening by delamination in the lower crust and tectonically driven exhumation up a crustal-scale ramp. The fold-thrust belt northwest of the ramp in Labrador is a result of the development of critical topography in the thrust wedge at the top of the ramp and the accretion of thrust slices to the base of the wedge during its propagation onto the foreland. The fold-thrust belt is absent in Ontario because syntectonic erosion of the thrust wedge prevented the development of critical topography. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Mechanics of extensional wedges

The extensional wedges that overlie low‐angle basal detachment faults are characterized by a tapered cross section similar to that exhibited by a fold‐and‐thrust belt, but with an opposite sense of shear on the basal fault. Such wedges are common in zones of crustal extension, such as the Basin and Range province of the North American Cordillera, the North Sea, and the U.S. Gulf Coast. This paper examines the mechanics of these upper crustal extensional wedges, using a modification of the compressional Coulomb critical taper model developed for fold‐and‐thrust belts and accretionary wedges. If a compressional wedge is mechanically analogous to a wedge of soil being pushed up an incline by a moving bulldozer, then an extensional wedge is analogous to the same wedge with the bulldozer moving in reverse gear down the incline. A critically tapered extensional wedge is one that is on the verge of Coulomb failure everywhere. A wedge whose taper is narrower may be slid stably down the same incline without internal deformation; a wedge whose taper is greater will fail by normal faulting and reduce its taper until it is critical. After testing the quantitative predictions of this extensional critical taper theory in a series of laboratory experiments on dry sand wedges, we apply the theory to a pair of active extensional wedges in the Brazos area, offshore Texas. Both wedges are underlain by well‐imaged basal detachment faults dipping at 17° down to depths of 10 km; one wedge exhibits relatively little internal deformation, whereas the other is pervasively faulted along a series of moderately dipping synthetic normal faults. Direct and indirect estimates of the pore fluid pressure distribution within the two wedges suggest that the undeformed wedge is stable whereas the pervasively faulted wedge is unstable but nearly critical. Regional values for the coefficients of basal and internal friction in excess of 0.3 together with vertical cohesion gradients in excess of 1 kPa/m are consistent with the observed pore pressures and the geometry of the wedges, as well as with in situ stress data acquired during drilling and the dips of active synthetic normal faults. Such values are comparable to values inferred from mechanical and thermal models of compressional wedges, and they are consistent with laboratory rock mechanics data.

Deformation partitioning, shear zone development and the role of undeformable objects

The formation of a mylonite zone by homogeneous progressive simple shear is highly unlikely because heterogeneous strain of grains with different compositions causes deformation partitioning and results in progressive inhomogeneous simple shear or non-coaxial progressive bulk inhomogeneous shortening. However the presence of rigid bodies in a zone of rock undergoing bulk simple shear spreads and homogenizes the stress field such that the instantaneous displacement field is essentially homogeneous at a scale up to at least four times the diameter of the rigid object. That is, the portion of rock surrounding the rigid body deforms by homogeneous progressive simple shear and the rigid object is forced to rotate. This has considerable implications for the rotation or lack thereof of unstrained porphyroclasts and porphyroblasts, and hence the information such grains preserve about the sense of shear on their margins or through inclusion trails. Geometric relationships between porphyroclasts or porphyroblasts and the surrounding matrix that are superficially similar can indicate exactly the opposite sense of shear depending on whether the porphyroclasts or porphyroblasts have or have not rotated leading to considerable problems in interpretation of the bulk movement in a shear zone. Resolution of these relationships enables ready distinction of whether the deformation history involved progressive bulk inhomogeneous shortening or progressive simple shear. In particular these results have considerable implications for the origin of blueschist minerals preserved in garnet porphyroblasts via the roles of strain softening and hardening during mylonitization due to the destruction of feldspar porphyroclasts and growth of garnet porphyroblasts respectively.

Development of conjugate shear bands during bulk simple shearing

In rocks possessing a strong planar fabric, shear bands of constant shear sense and oriented at an oblique angle to the foliation are considered by many authors to be characteristic of a non-coaxial bulk deformation history, whereas conjugate shear bands are considered to indicate coaxial shortening. However, in two areas where bulk deformation history appears to be non-coaxial (Cap Corse, Corsica and Ile de Groix, Brittany), conjugate shear bands are observed. In order to investigate this problem, experiments were performed by bulk simple shearing using Plasticine as a rock analogue. When slip between layers of the model is permitted, shear bands of normal-fault geometry form with both the same and opposite shear sense as the bulk simple shearing at approximately the same angle with the layering (40°) irrespective of layer orientation in the undeformed state (for initial orientations of 50, 30 and 15°). Shear bands are initially formed within individual layers and may propagate across layer interfaces when further movement along these is inhibited. The existence of conjugate shear bands in Corsica and Ile de Groix is therefore not incompatible with a model of bulk simple shearing for these two regions. In field studies, one should perhaps exercise care in using shear bands to determine the kind of motion or the sense of bulk shearing.

Subduction, back-arc spreading and global mantle flow

The shapes and orientations of Benioff zones beneath island arcs, interpreted as marking the location of subducted lithosphere, provide the best presently available constraints on the global convective flow pattern associated with plate motions. This global flow influences the dynamics of subduction. Subduction zone phenomena therefore provide powerful tests for models of mantle flow. We compute global flow models which, while simple, include those features which are best constrained, namely the observed plate velocities, applied as boundary conditions, and the density contrasts given by thermal models of the lithosphere and subducted slabs. Two viscosity structures are used; for one, flow is confined to the upper mantle, while for the other, flow extends throughout the mantle. Instantaneous flow velocity vectors match observed Benioff zone dips and shapes for the model which allows mantle-wide flow but not for the upper mantle model, which has a highly contorted flow pattern. The effect of trench migration on particle trajectories is calculated; it is not important if subduction velocities are greater than migration rates. Two-dimensional finite element models show that including a coherent high viscosity slab does not change these conclusions. A coherent high viscosity slab extending deep into the upper mantle would significantly slow subduction if flow were confined to the upper mantle. The maximum earthquake magnitude, M_w, for island arcs correlates well with the age of the subducted slab and pressure gradient between the trench and back-arc region for the whole mantle, but not the upper mantle, flow model. The correlations with orientations of Benioff zones and seismic coupling strongly suggest that the global return flow associated with plate motions extends below 700 km. For both models, regions of back-arc spreading have asthenospheric shear pulling the back-arc toward the trench; regions without back-arc spreading have the opposite sense of shear, suggesting global flow strongly influences back-arc spreading.

Slip planes and asymmetric slip in fatigue of iron single crystals

Abstract Slip planes in iron single crystals are identified and the relative critical resolved shear stresses for slip on those planes are estimated from the results of torsional fatigue experiments. The torsional mode of fatigue is uniquely suited for these purposes because, generally, two different slip systems operate in a given region of maximum resolved shear stress. One slip system can be identified with one sense of shear and the other slip system with the opposite sense of shear. This effect is due to asymmetry of slip on certain planes. Slip is found to occur on {110}, {112} and {123} planes and to be asymmetric on {112} and {123} planes. The relative critical resolved shear stresses are ranked from tests on a number of different orientations.