Examples of ice pack rigidity and mobility characteristics determined from ice motion

A method has been developed to determine ice pack rigidity and mobility using observed ice motion. Using this method, one may determine how solidly the ice pack is frozen in near real-time. In addition, spatial and temporal variations in the freezing and thawing of the ice pack can be studied. Various degrees of ice rigidity were considered using remotely-sensed ice motion off the N coast of Alaska during 1975 and 1979. Summer-time ice rigidities were detected first in late June 1975 and lasted through September 1975. However, in 1979 considerably higher rigidities were found in August while summer-like rigidities were detected into late November. Analyses of atmospheric pressure distributions suggest that less mechanical breakup occurred in the summer of 1979, resulting in the greater rigidities during August of that year. In addition, minimum ice coverage was 21% less in the Beaufort Sea in 1979 than in 1975. The result was a relatively large percent of thinner ice for November of 1979 than for 1975, the likely cause of the less rigid conditions detected during the fall of 1979.Nomenclature D deviation in height (m) of a pressure level from the standard atmosphere (Huschke 1959). - e_T time lag (h) at which the autocorrelation of ice speed drops to e^–1 - SL the large-scale disturbance or longwave component of a scalar field; in this study, the 500 mb circulation (Holl 1963). - U mean ice speed over a given time interval - V variance of ice speed over a given time interval     

Origin of fault domains and fault-domain boundaries (transfer zones and accommodation zones) in extensional provinces: Result of random nucleation and self-organized fault growth

In many extensional provinces, large normal faults dip in the same direction forming fault domains. Features variously named transfer faults, transfer zones, and accommodation zones (hereafter non-genetically referred to as fault-domain boundaries) separate adjacent fault domains. Experimental modeling of distributed extension provides insights on the origin, geometry, and evolution of these fault domains and fault-domain boundaries. In our scaled models, a homogeneous layer of wet clay or dry sand overlies a latex sheet that is stretched orthogonally or obliquely between two rigid sheets. Fault domains and fault-domain boundaries develop in all models in both map view and cross-section. The number, size, and arrangement of fault domains as well as the number and orientation of fault-domain boundaries are variable, even for models with identical boundary conditions. The fault-domain boundaries in our models differ profoundly from those in many published conceptual models of transfer/accommodation zones. In our models, fault-domain boundaries are broad zones of deformation (not discrete strike-slip or oblique-slip faults), their orientations are not systematically related to the extension direction, and they can form spontaneously without any prescribed pre-existing zones of weakness. We propose that fault domains develop because early-formed faults perturb the stress field, causing new nearby faults to dip in the same direction (self-organized growth). As extension continues, faults from adjacent fault domains propagate toward each another. Because opposite-dipping faults interfere with one another in the zone of overlap, the faults stop propagating. In this case, the geometry of the domain boundaries depends on the spatial arrangement of the earliest formed faults, a result of the random distribution of the largest flaws at which the faults nucleate.     

Streamlined bedrock terrain and fast ice flow, Jakobshavns Isbrae, West Greenland: implications for ice stream and ice sheet dynamics

This study investigates the marginal subglacial bedrock bedforms of Jakobshavns Isbrae, West Greenland, in order to examine the processes governing bedform evolution in ice stream and ice sheet areas, and to reconstruct the interplay between ice stream and ice sheet dynamics. Differences in bedform morphology (roche moutonnee or whaleback) are used to explore contrasts in basal conditions between fast and slow ice flow. Bedform density is higher in ice stream areas and whalebacks are common. We interpret that this is related to higher ice velocities and thicker ice which suppress bed separation. However, modification of whalebacks by plucking occurs during deglaciation due to ice thinning, flow deceleration, crevassing and fluctuations in basal water pressure. The bedform evidence points to widespread basal sliding during past advances of Jakobshavns Isbrae. This was encouraged by increased basal temperatures and melting at depth, as well as the steep marginal gradients of Jakobshavns Isfjord which allowed rapid downslope evacuation of meltwater leading to strong ice/bedrock coupling and scouring. In contrast to soft-bedded ice stream bedforms, the occurrence of fixed basal perturbations and higher bed roughness in rigid bed settings prevents the basal ice subsole from maintaining a stable form which, coupled with secondary plucking, counteracts the development of bedforms with high elongation ratios. Cross-cutting striae and double-plucked, rectilinear bedforms suggest that Jakobshavns Isbrae became partially unconfined during growth phases, causing localised diffluent flow and changes in ice sheet dynamics around Disko Bugt. It is likely that Disko Bugt harboured a convergent ice flow system during repeated glacial cycles, resulting in the formation of a large coalesced ice stream which reached the continental shelf edge.     

Numerical modelling of concrete flow: homogeneous approach

The aim of this paper is to model numerically concrete flow inside formworks like the Lbox. For this purpose, we use a finite element method with Lagrangian integration points (FEMLIP). We are able to follow in time and space material motion with any type of material behaviour, including non‐linear and time‐dependent ones. We also can deal with free surfaces or material interfaces. Bingham's rheology is used for fresh concrete behaviour. In order to compare with experiments, we have considered three concretes (OC, HPC and SCC) with contrasted rheologies. Their yield stress is identified by experimental slump tests and also compared with the value given by a formulation concrete software. Experimental data are found to be quite close to numerical predictions. We have also made some experimental flow tests in a LBOX. We measured the flow speed and the flow shape in the final stage. The numerical modelling of these experiments is very encouraging and shows the capability of the FEMLIP using the Bingham's law to model concrete flow and filling properties. Copyright © 2005 John Wiley & Sons, Ltd.     

The Zymoetz River landslide, British Columbia, Canada: description and dynamic analysis of a rock slide–debris flow

The Zymoetz River landslide is a recent example of an extremely mobile type of landslide known as a rock slide–debris flow. It began as a failure of 900,000 m³ of bedrock, which mobilized an additional 500,000 m³ of surficial material in its path, transforming into a large debris flow that traveled over 4 km from its source. Seasonal snow and meltwater in the proximal part of the path were important factors. A recently developed dynamic model that accounts for material entrainment, DAN3D, was used to back-analyze this event. The two distinct phases of motion were modeled using different basal rheologies: a frictional model in the proximal path and a Voellmy model in the distal path, following the initiation of significant entrainment. Very good agreement between the observed and simulated results was achieved, suggesting that entrainment capabilities are essential for the successful simulation of this type of landslide.     

Plastic compaction of cemented granular materials

We analytically relate hydrostatic stress to strain in a random dense pack of identical spheres cemented at their contacts. The spheres are elastic and the cement is perfectly plastic. This solution for the sphere pack is based on a solution for the normal interaction of two cemented spheres. Initially, the two spheres touch each other at a point. We show that, as loading increases and cement becomes plastic, a finite (Hertzian) direct-contact area between the spheres necessarily has to develop and progress. The stress-strain behavior of the pack depends on the cement's yield limit and on the amount of cement. At the same hydrostatic stress, the deformation of the cemented aggregate is smaller than that of the uncemented one. This difference becomes large as the yield limit increases. We calculate the bulk modulus of an aggregate from the stress-strain curve. In the plasticity domain, the bulk modulus of the cemented aggregate is smaller than that of the uncemented one. The difference between the two may easily reach 50%. Of course, as the cement's yield limit decreases, the aggregate's stress-strain curve and the bulk modulus approach those of the uncemented sphere pack. This theoretical conclusion is qualitatively supported by experiments on epoxy-cemented glass beads. The maximum contact stress in the cemented aggregate may be less than a half of that in the uncemented one. This result explains an experiment where an uncemented glass bead sample failed at a hydrostatic stress of 50 MPa, whereas an epoxy-cemented sample stayed intact.     

Damage formation associated with bending under a constant moment

Deformation within the Earth's lithosphere is largely controlled by the rheology of the rock. Fracture and faulting are characterized by elastic rheologies with brittle mechanisms, while folding and flow are characterized by plastic and/or viscous rheologies due to ductile mechanisms. However, it has been recognized that deformation that resembles ductile behavior can be produced within the confines of the brittle lithosphere. Specific examples are folds that form in the shallow crust, steep hinges at subduction zones that are accompanied by seismicity, and large-scale deformation at plate boundaries. In these cases, the brittle lithosphere behaves elastically with fracture and faulting yet produces ductile behavior. In this paper, we attempt to simulate such ductile behavior in elastic materials using continuum damage mechanics. Engineers utilize damage mechanics to model the continuum deformation of brittle materials. We utilize a modified form of damage mechanics that represents a reduction in frictional strength of preexisting fractures and faults. We use this empirical approach to simulate the bending of the lithosphere under the application of a constant moment.We use numerical simulations to obtain elastostatic solutions for plate bending and where the longitudinal stress at a particular node exceeds a yield stress, we apply damage to reduce Young's modulus at the node. Damage is calculated at each time step by a power-law relationship of the ratio of the yield stress to the longitudinal stress and the yield strain to the longitudinal strain. This results in the relaxation of the material due to increasing damage. To test our method, we apply our damage rheology to an infinite plate deforming under a constant bending moment. We simulate a wide range of behaviors from slow relaxation to instantaneous failure, over timescales that span six orders of magnitude. Using this method, stress relaxation produces elastic-perfectly plastic behavior in cases where failure does not occur. For cases of failure, we observe a rapid increase in damage leading to failure, analogous to the acceleration of microcrack formation and acoustic emissions prior to failure. The changes in the rate of damage accumulation in failure cases are similar to the changes in b-values of acoustic emissions observed in triaxial compression tests of fractured rock and b-value changes prior to some large earthquakes. Thus continuum damage mechanics can simulate the phenomenon of ductile behavior due to brittle mechanisms as well as observations of laboratory experiments and seismicity.     

亚洲东部“大三角”地震构造区的周边和深部动力环境

亚洲东部存在一个巨大的三角形地震构造区域,大体上,喜马拉雅山脉、帕米尔—天山—阿尔泰山—贝加尔和东经105°线是它的3个边界,主要覆盖中国和蒙古国西部众多高原、山脉及山间盆地。三角区内现今构造活动和地震广泛强烈,地壳破碎,显示不均匀的块体边界和块内变形;区外基本上是稳定的刚性陆块,地震很少,变形较弱,处于整体缓慢运动之中。这个宽阔的板内变形区起源于印度、菲律宾海—西太平洋和欧亚三大板块之间的动力作用以及深部地幔流的影响。向北快速运动的印度次大陆已近水平地插入到西藏板块下,沿喜马拉雅弧产生多种运动和变形,并向亚洲内部远距离地扩散。沿东经95°～100°,向北的地壳运动向东和东南方向偏转,阻截了喜马拉雅弧东端的北向运动;而在喜马拉雅弧西端,帕米尔继续向北挤进中亚,受天山—阿尔泰山—贝加尔一线西北側稳定地壳的限制,扩散的变形被中国、蒙古、俄罗斯边境地区一系列EW向和NW向的老断层吸收并在它们的西端终止。菲律宾海—西太平洋向欧亚大陆的消减-俯冲导致沿海沟-岛弧的漫长而狭窄的地震带,但对亚洲大陆的水平挤压较小,未能阻挡亚洲大陆东部向东移动。其部分原因可能是俯冲板片受到来自欧亚大陆下的ES向地幔流的推挤,这个ES向地幔流与来自印度下面的N向地幔流在西藏中部汇合并向东偏转,在大尺度上与GPS观测到的地表移动图像一致。     

Crystal structural features of the olivine → spinel transition

Comparisons of structural features of olivine (α phase), spinel (γ phase), and the modified spinel (β phase) lead to predictions of possible mechanisms for the olivine → spinel transitions. In the olivine structure, rigid tetrahedral edges and shared octahedral edges form columns of corner-sharing trigonal dipyramids parallel to the a axis. These rigid columns are separated by weaker, unshared octahedral edges which may be stretched to reduce cation-cation repulsion. As a result, olivine has a relatively loose structure and is stable at low pressure. At elevated pressure, olivine transforms to the more compact spinel structure, in which the rigid tetrahedral edges and shared octahedral edges form a three dimensional network instead of aligned columns. These structural differences explain how compressibility and thermal expansion may be taken up mainly by octahedral sites in olivine, but are evenly distributed over both octahedral and tetrahedral sites in spinel. Because the closest packings of oxygens and interstitial cation distributions differ between olivine (h.c.p.) and spinel (c.c.p.), the olivine structure may have to disintegrate during its transformation to spinel, so that the olivine → spinel transition involves processes of nucleation and growth. The migration of atoms across the olivine-spinel interface is thus a complicated process of random walk without a definite path. In the β phase → spinel transition, however, the diffusion of cations may follow a definite path in restricted regions because oxygen closest packings and cation distributions are similar in the two structures. If the oxygen packing remains intact during the β → γ transition, the transformation will be an intracrystalline process leading to domain structure in the spinel product.     

A non-hydrostatic model for the numerical study of landslide-generated waves

This paper proposes and demonstrates a two-layer depth-averaged model with non-hydrostatic pressure correction to simulate landslide-generated waves. Landslide (lower layer) and water (upper layer) motions are governed by the general shallow water equations derived from mass and momentum conservation laws. The landslide motion and wave generation/propagation are separately formulated, but they form a coupled system. Our model combines some features of the landslide analysis model DAN3D and the tsunami analysis model COMCOT and adds a non-hydrostatic pressure correction. We use the new model to simulate a 2007 rock avalanche-generated wave event at Chehalis Lake, British Columbia, Canada. The model results match both the observed distribution of the rock avalanche deposit in the lake and the wave run-up trimline along the shoreline. Sensitivity analyses demonstrate the importance of accounting for the non-hydrostatic dynamic pressure at the landslide-water interface, as well as the influence of the internal strength of the landslide on the size of the generated waves. Finally, we compare the numerical results of landslide-generated waves simulated with frictional and Voellmy rheologies. Similar maximum wave run-ups can be obtained using the two different rheologies, but the frictional model better reproduces the known limit of the rock avalanche deposit and is thus considered to yield the best overall results in this particular case.     

An anisotropic elastic‐decohesive constitutive relation for sea ice

Thermodynamic growth or melt and mechanical redistribution due to lead opening or ridge formation shape the thickness distribution of the Arctic ice cover and impact the overall strength of pack ice. Specifically, the deformation and strength of ice are not isotropic but vary with the thickness and lead orientation. To reflect these facts, we develop an anisotropic, elastic‐decohesive constitutive model for sea ice together with a model to describe an oriented, ice thickness distribution. The tight connection between the mechanical response and the thickness distribution is an improvement over a previous model that only depended on the average ice thickness. The model describes mechanical responses anisotropically in both the elastic and failure regimes. In the elastic regime, the constitutive relation implicitly reflects strong and weak directions of the pack ice depending on the distribution of thin ice (including open water) and thicker ice (e.g., multi‐year ice or ridges). In the failure regime, the model predicts both failure initiation and the lead orientation. Evolution from initial failure to complete failure when traction‐free crack surfaces are formed is also modeled. Crack or lead width is determined during the evolution. Various examples of failure surfaces are presented to describe the behavior of modeled ice when the thickness distribution varies. The model predictions are also illustrated and compared with previous modeling efforts by examining regions of ice under idealized loading. Copyright © 2015 John Wiley & Sons, Ltd.     

Stress and strain during fault-controlled lithospheric extension—insights from numerical experiments

Two-dimensional finite element techniques are used to study the temporal evolution and spatial distribution of stress and strain during lithospheric extension. The thermomechanical model includes a pre-existing fault in the upper crust to account for the reactivation of older tectonic elements. The fault is described using contact elements which allow for independent meshing of hanging wall and foot wall as well as simulation of large differential displacements between the fault blocks. Numerical models are run for three different initial temperature distributions representing extension of weak, moderately strong and strong lithosphere and three different extension velocities. In spite of the simple geodynamic boundary conditions selected, i.e., wholesale extension at a constant rate, stress and strain vary substantially throughout the lithosphere. In particular, in case of the weak lithosphere model, lower crustal flow towards the locus of maximum upper crustal extension results in the formation of a lower crustal dome while maintaining a subhorizontal Moho relief. The core of the dome experiences hardly any internal deformation, although it is the part of the lower crust which is exhumed the most. Stress fields in the lower crustal dome vary significantly from the regional trend underlining mechanical decoupling of the lower crust from the rest of the lithosphere. These differences diminish if cooler temperatures and, hence, stronger rheologies are considered. Lithospheric strength also exerts a profound control on the basin architecture and the surface expressions of extension, i.e., rift flank uplift and basin subsidence. If the lower crust is sufficiently weak, its flow towards the region of extended upper crust can provide a threshold value for the maximum subsidence which can be achieved during the syn-rift stage. In spite of continuous regional extension, corresponding burial history plots show exponentially decreasing subsidence rates which would traditionally be interpreted in terms of lithospheric cooling during the post-rift stage. The models provide templates to genetically link the surface and sub-surface expressions of lithospheric extension, for which usually no contemporaneous observations are possible. In particular, they help to decipher the information on the physical state of the lithosphere at the time of extension which is stored in the architecture and subsidence record of sedimentary basins.     

The nature of ordering and ordering defects in dolomite

The apparent lack of transformation-induced domains and stacking disorder defects in natural dolomites is considered in light of the different types of order in the dolomite structure. Experimentally produced twin domain boundaries and basal stacking defects are documented in a dolomite using high-resolution electron microscopy and electron diffraction techniques. Results reveal that upon cation ordering to form the dolomite structure, a twin domain is favored over an antiphase domain. The twin domain boundaries closely resemble antiphase boundaries (APB's) and are in contrast for superlattice reflections. However, background contrast within domains is shown to be different when imaged using certain fundamental reflections. The results allow speculation about the nature of ordering at low temperatures. Observations of twin domain boundaries in samples annealed at different temperatures allow estimation of the critical ordering temperature at 1,100°–1,150° C in stoichiometric dolomite.     

基于物理模型试验的碎屑流流态化运动特征分析

流态化运动是高速远程滑坡的主要运动形式,是揭示高速远程滑坡运动机理的重要基础。基于粒子图像测速（PIV）分析方法,采用物理模型试验对不同粒径组成条件下的颗粒流内部的速度分布、剪切变形及流态特征进行了研究,并对高速远程滑坡流态化运动特征进行了讨论分析。结果表明:碎屑流流态化运动特征与颗粒粒径呈显著的相关性,随着粒径的减小或细颗粒含量的增加,颗粒流底部相对于边界的滑动速度以及整体的运动速度均呈逐渐减小的趋势,颗粒流内部剪切变形程度增加,颗粒的运动形式由“滑动”向“流动”转变;当颗粒粒径较小或细颗粒含量较高时,颗粒流内部剪切速率增大的趋势在颗粒流底部更加显著,反映了粒径减小有助于促进颗粒流内部剪切向底部的集中;在同一颗粒流的不同运动阶段及不同纵向深度,其流态特征具有显著差别,颗粒流前缘及尾部主要呈惯性态,颗粒间以碰撞作用为主,而主体部分则主要呈密集态,颗粒间以摩擦接触作用为主;在颗粒流表面及底部,颗粒间相互作用方式主要是碰撞作用,中间部分则以摩擦作用为主;对于不同粒径的颗粒流,随着粒径的增大或粗颗粒含量的增加,颗粒流内部颗粒的碰撞作用加强,颗粒流整体趋于向惯性态转变。     

微粒对多晶冰流变行为的影响(Ⅰ):应力下的流变行为

研究了应力下微粒对多晶冰流变行为的影响.结果表明:微粒分布在晶界或同时分布在晶界和晶内都将增加多晶冰的流变速率.当微粒只分布在晶界时,含微粒浓度为1 wt.%的多晶冰有最高的最小流变速率;当微粒同时分布在晶内与晶界时,多晶冰的最小流变速率基本与微粒浓度无关.微粒提高流变速率的原因来源于提高了流变过程中的位错密度,分布在晶界和晶内的微粒可以分别通过阻碍晶界滑移,发展内应力及Frank-Read位错增值机制提高流变过程中的位错密度.     

The subglacial thermal organisation (STO) of ice sheets

This paper examines ice-sheet wide variations in subglacial thermal regime and ice dynamics using the landform record exposed on the beds of former mid-latitude ice sheets (the Laurentide, Cordilleran, Fennoscandian and British-Irish Ice Sheets). We compare the landform patterns beneath these former ice sheets to the flow organisation beneath parts of the contemporary Antarctic Ice Sheet inferred from RADARSAT-1 Antarctic Mapping Project (RAMP) data. The evidence preserved in the landform record and observed on contemporary ice masses can be grouped into four major ice-dynamical components that collectively define the subglacial thermal organisation (STO) of ice sheets. These ice-dynamical components are frozen-bed patches, ice streams, ice-stream tributaries and lateral shear zones. Frozen-bed patches appear at a wide range of spatial scales, spanning four orders of magnitude. In some areas, frozen-bed zones comprise large proportions of the bed (e.g. near the ice divide in continental areas), whilst in other areas they constitute isolated “islands” in areas dominated by thawed-bed conditions. Ice streams, narrow zones of fast flow in ice sheets that are otherwise dominated by slow sheet flow, are also common features of Quaternary ice sheets. Tributaries to ice streams flow at velocities intermediate between full ice-stream and sheet flow, and may divert ice drainage from one primary ice-stream corridor to an adjacent one. Sharp lateral boundaries between landforms indicate sliding and non-sliding conditions, respectively. These lateral boundaries represent important discontinuities in the glacial landscape and mark the location of shear zones between thawed-bed ice streams and intervening frozen-bed areas. We use the landform evidence in the area around Great Bear Lake, Canada to trace the evolution of an ice-stream web through time, demonstrating that frozen-bed patches are integral components of this complex system. We conclude that frozen-bed patches are important for the stability of ice sheets because they laterally constrain and isolate peripheral drainage basins and their ice streams.     

http://www.sciencedirect.com/science/article/pii/S1674987111000594

A new model is suggested for the history of the Baikal Rift,in deviation from the classic two-stage evolution scenario,based on a synthesis of the available data from the Baikal Basin and revised correlation between tectonic-lithological-stratigraphic complexes(TLSC) in sedimentary sections around Lake Baikal and seismic stratigraphic sequences(SSS) in the lake sediments.Unlike the previous models,the revised model places the onset of rifting during Late Cretaceous and comprises three major stages which are subdivided into several substages.The stages and the substages are separated by events of tectonic activity and stress reversal when additional compression produced folds and shear structures.The events that mark the stage boundaries show up as gaps,unconformities,and deformation features in the deposition patterns. The earliest Late Cretaceous-Oligocene stage began long before the India-Eurasia collision in a setting of diffuse extension that acted over a large territory of Asia.The NW-SE far-field pure extension produced an NE-striking half-graben oriented along an old zone of weakness at the edge of the Siberian craton.That was already the onset of rift evolution recorded in weathered lacustrine deposits on the Baikal shore and in a wedge-shaped acoustically transparent seismic unit in the lake sediments.The second stage spanning Late Oligocene-Early Pliocene time began with a stress change when the effect from the Eocene India-Eurasia collision had reached the region and became a major control of its geodynamics.The EW and NE transpression and shear from the collisional front transformed the Late Cretaceous half-graben into a U-shaped one which accumulated a deformed layered sequence of sediments.Rifting at the latest stage was driven by extension from a local source associated with hot mantle material rising to the base of the rifted crust.The asthenospheric upwarp first induced the growth of the Baikal dome and the related change from finer to coarser molasse deposition.With time,the upwarp became a more powerful stress source than the collision,and the stress vector returned to the previous NW-SE extension that changed the rift geometry back to a half-graben. The layered Late Pliocene-Quaternary subaerial tectonic-lithological-stratigraphic and the Quaternary submarine seismic stratigraphic units filling the latest half-graben remained almost undeformed.The rifting mechanisms were thus passive during two earlier stages and active during the third stage. The three-stage model of the rift history does not rule out the previous division into two major stages but rather extends its limits back into time as far as the Maastrichtian.Our model is consistent with geological, stratigraphic,structural,and geophysical data and provides further insights into the understanding of rifting in the Baikal region in particular and continental rifting in general.     

Seven types of subgrain boundaries in octachloropropane

Subgrain boundaries in thin sheets of octachloropropane deformed at 0.7–0.8 T_M on the stage of a microscope are seen to appear in the material in seven different ways. Type I boundaries show the classical evolution by polygonization of bent crystals. Type II are essentially kink boundaries, which migrate sideways during deformation to reach their present positions in the crystals. Type III develop at the sites of former grain boundaries, by reduction of misorientation of adjacent grains. Type IV and V originate by impingement of migrating grain boundaries or subgrain boundaries, respectively. Type VI propagate in their own planes behind migrating grain boundaries to which they are attached. Type VII develop statically from optically strain-free grains by a process probably otherwise similar to the Type I process. Two thirds of the boundaries are Types I or II. In view of the variety of subgrain boundary histories in OCP, interpretation of similar features in minerals ought to be undertaken cautiously. Criteria are needed for telling the different types of subgrain boundaries apart in situations where only a final view of the structure is available, as in optical and electron micrographs of rocks.     

P-T path development derived from shearband boudin microstructure

This work focuses on the development of a regional P-T-path from the Malpica-Lamego Ductile Shear Zone, NW Portugal, based on the microstructures of shearband boudins evolved during progressive simple shear. The combination of microstructural analysis, fluid inclusion studies, crystallographic preffered orientation and fractal geometry analyses, allows to link several stages in the internal evolution of the boudin to regional P-T conditions. The boudinage process is initiated under differential stress after the original layer achieved sufficient viscosity contrast relative to the surrounding matrix. Two main transformations occur simultaneously: i) change in the external shape with continuous evolution from tabular rigid body to sigmoidal asymmetric morphology (shearband boudin) and ii) localized dynamic recrystallization in the sharp-tips of the structure (acute edge of shearband boudin), and along the boudin's margin and grain boundaries. Smaller recrystallized grains, particularly in the sharp-tip domains, accommodate most of the external strain, and larger relict grains are preserved in the centre. Dynamic recrystallization under constant strain rates and strain partitioning inside the boudins is indicated by fractal geometry based on grain boundary and grain area analysis. Progressive deformation leads to the generation of structural and textural heterogeneous domains inside the boudins, and is recorded by quartz c-axis orientation analysis and fluid inclusion studies. The last deformation episode shows the final formation of the blunt-tip domain and internal secondary shear planes. The regional P-T path begins with the crystallization of andalusite after an internal shearband boudin dilation event and ends with quartz dynamic recrystallization on boudin tips. The main deformation stage (310/315 Ma) led to reactivation of internal secondary shear zones with sillimanite crystallization.     

Structural softening of the lithosphere

Structural softening is a decrease in the amount of stress needed to deform the lithosphere at a particular rate because of its structural reorganization while all true rheological properties remain constant. Structural softening is fundamentally different than material softening, where the decrease in stress is generated by a change in rheological properties with progressive deformation, such as grain size reduction resulting from large shearing strain. We study structural softening generated by folding of the crust-mantle boundary, which is a structural instability that inevitably develops during compression of the mechanically layered lithosphere. For ductile rheologies, the stress decrease represents a decrease of the effective lithospheric viscosity, which is proportional to the ratio of stress to lithospheric shortening strain rate. We present analytical and numerical results quantifying the decrease in stress and effective viscosity that occur during shortening at a constant rate. The decrease in effective viscosity can be up to 10-fold.