structural history and granite emplacement in the rottenstone domain during closure of the trans hudson orogen davin lake northern saskatchewan the rottenstone domain at davin lake northern saskatchewan exhibits structural and granite emplacement evidence for crustal thickening and possible himalayan style extrusion overprinted by transpressional strain increasing toward the contact with the wathaman batholith three discrete rottenstone subdomains parallel the regional strike of the trans hudson orogen i the southeast rottenstone subdomain sersd with gently northwest dipping migmatitic straight gneiss s1 and white granitoid rocks with pinch and swell structures parallel to the straight gneissosity ii the central rottenstone subdomain crsd which partly preserves the same nw dipping fabric s1 but is overprinted at its margins by tight upright f2 folds and includes a stockwork of pink monzogranitic aplites and pegmatites and iii the northwest rottenstone subdomain nwrsd in which the f2 folds are generally tighter and penetrative and its network of narrow white granitoid rocks is deformed and transposed by the f2 folds but in

a newly recognized type of slickenside striation abstract this note summarizes current views on the origin of slickenside striations and points to the existence of a type of striation not readily explained by established genetic ideas such striations have been seen on fault surfaces in naturally deformed shale and in experimentally deformed paraffin wax they are defined by arrays of shallow slip parallel ridges and grooves of millimetric width and below on any given slip surface the ridges are as common as the grooves and display similar dimensions and profile shapes across any pair of slip surfaces in contact at the end of faulting ridges protruding from one surface fit into or nest with grooves on the other surface the origin of these features is not yet understood

relationship between non cylindrical fold geometry and the shear direction in monoclinic and triclinic shear zones we investigate the relationship between non cylindrical fold geometry and the shear direction of the hosting high strain zone by numerical modeling and show that apical axes of non cylindrical folds may develop into directions highly oblique to the shear direction if the zone has a pure shear component the common practice of using well developed sheath folds as indicators for the shear direction is not reliable and the use of immature non cylindrical or sheath folds appears more reliable hinge lines of mature sheath folds approach parallelism with the fabric attractor to which all material lines rotate which can have variable angles with respect to the shear direction in thinning zones the fabric attractor is the direction of the maximum principal strain rate of the pure shear component and is parallel to the shear zone boundary in thickening zones it lies somewhere in the quadrant between the direction of the maximum principal strain rate of the pure shear component perpendicular to the shear zone boundary and the simple shear direction and is generally oblique to the shear zone boundary the exact location depends on the flow geometry of the shear zone 2007 elsevier ltd all rights reserved

reply to discussion by g i alsop and r e holdsworth of sheath fold development with viscosity contrast analogue experiments in bulk simple shear by marques et al journal of structural geology 30 1348 1353

sheath fold development with viscosity contrast analogue experiments in bulk simple shear it has been shown experimentally that sheath folds can develop passively by kinematic amplification of precursor non cylindrical deflections however sheath folds are most commonly multilayers where rheological contrast can be expected non passive sheath folding therefore we studied a natural occurrence of sheath folds where layers show rheological contrast and used analogue modelling to investigate sheath fold development with viscosity contrast the initial deflection in the experiments was always hemispherical and planar layering parallel to the shear plane under the chosen experimental conditions the results show that 1 the viscosity contrast between layers should be lower than one order of magnitude 10 to generate a sheath fold in both higher and lower viscosity layers otherwise the higher viscosity layer behaves as effectively rigid 2 the lower viscosity layer develops into a tubular fold dragged by the higher viscosity layer that undergoes mixed strain low and shear plane parallel translation high 3 the ratio between ellipticities of external and internal ellipses r re ri significantly departs from 1 as the viscosity contrast increases e g r 0 6 for 7 5 in the case where the higher viscosity layer caps the lower viscosity layer this is in great contrast with passively developed sheath folds for which r 1 when the precursor deflection is hemispherical from the present experimental study and previous work investigating other variables we conclude that r should be cautiously used and that extraction of kinematical information from sheath fold geometry alone can be erroneous

mapping the u s west coast surface circulation a multiyear analysis of high frequency radar observations signals with phase speeds of o 10 and o 100 to 300 km day 1 and time scales of 2 to 3 weeks the signals with slow phase speed are only observed in southern california it is hypothesized that they are scattered and reflected by shoreline curvature and bathymetry change and do not penetrate north of point conception the seasonal transition of alongshore surfacecirculationforcedbyupwelling favorablewindsandtheirrelaxationiscapturedinfine detail submesoscaleeddies identifiedusingflowgeometry haverossbynumbersof0 1to3 diameters in the range of 10 to 60 km and persistence for 2 to 12 days the hfr surface currents resolve coastal surface ocean variability continuously across scales from

patterns of upwelling and relaxation around monterey bay based on long term observations of surface currents from high frequency radar abstract a five year record of surface current observations from hf radar in the region around monterey bay are used to describe the dominant circulation patterns under upwelling and downwelling favorable wind forcing conditional averages based on local wind observations reveal more structure and intensity in the circulation patterns than do traditional temporal averages the predominant upwelling circulation pattern in the region is found to reach full strength after about three days of persistent upwelling favorable wind forcing in the opposite phase under downwelling favorable winds the circulation patterns differ between summer and winter with evidence for strong connectivity between the offshore waters and the coastline around monterey bay in summer that is not present in winter

preparing to predict the second autonomous ocean sampling network aosn ii experiment in the monterey bay abstract the autonomous ocean sampling network phase two aosn ii experiment was conducted in and offshore from the monterey bay on the central california coast during july 23 september 6 2003 the objective of the experiment was to learn how to apply new tools technologies and analysis techniques to adaptively sample the coastal ocean in a manner demonstrably superior to traditional methodologies and to use the information gathered to improve predictive skill for quantities of interest to end users the scientific goal was to study the upwelling relaxation cycle near an open coastal bay in an eastern boundary current region particularly as it developed and spread from a coastal headland the suite of observational tools used included a low flying aircraft a fleet of underwater gliders including several under adaptive autonomous control and propeller driven auvs in addition to moorings ships and other more traditional hardware the data were delivered in real time and assimilated into the harvard ocean prediction system hops the navy coastal ocean model ncom and the jet propulsion laboratory implementation of the regional ocean modeling system jpl roms two upwelling events and one relaxation event were sampled during the experiment the upwelling in both cases began when a pool of cold water less than 13 c appeared near cape ano nuevo and subsequently spread offshore and southward across the bay as the equatorward wind stress continued the primary difference between the events was that the first event spread offshore and southward while the second event spread only southward and not offshore the difference is attributed to the position and strength of meanders and eddies of the california current system offshore which blocked or steered the cold upwelled water the space and time scales of the mesoscale variability were much shorter than have been previously observed in deep water eddies offshore additional process studies are needed to elucidate the dynamics of the flow

structural analysis of sheath folds with horizontal x axes northeast canada abstract early proterozoic supracrustal rocks occur below a thick nappe of archaean basement gneiss in the melville peninsula where sheath folds are exposed in a wide zone of middle proterozoic dynamothermal metamorphism outcrop patterns of truncated isoclinal sheath folds resemble cylindrical folds except in relatively small areas around the paraboloidal caps bulk extension axes are parallel to strike in the belt as shown by isoclinal sheath folds with horizontal central axes x axes as well as similarly aligned mullion structure and rotated scapolite prisms extension axes converge from northeast to southwest in the apparent flow direction

northern monterey bay upwelling shadow front observations of a coastally and surface trapped buoyant plume 1 during the upwelling season in central california northwesterly winds along the coast produce a strong upwelling jet that originates at point ano nuevo and flows southward across the mouth of monterey bay a convergent front with a mean temperature change of 3 77 0 29c develops between the warm interior waters and the cold offshore upwelling jet to examine the forcing mechanisms driving the location and movement of the upwelling shadow front and its effects on biological communities in northern monterey bay oceanographic conditions were monitored using cross shelf mooring arrays drifters and hydrographic surveys along a 20 km stretch of coast extending northwestward from santa cruz california during the upwelling season of 2007 may september the alongshore location of the upwelling shadow front at the northern edge of the bay was driven by regional wind forcing through an alongshore pressure gradient buoyancy forces due to the temperature change across the front and local wind forcing the diurnal sea breeze the upwelling shadow front behaved as a surface trapped buoyant current which is superimposed on a poleward barotropic current moving up and down the coast up to several kilometers each day we surmise that the front is advected poleward by a preexisting northward barotropic current of 0 10 m s 1 that arises due to an alongshore pressure gradient caused by focused upwelling at point ano nuevo the frontal circulation onshore surface currents breaks the typical two dimensional wind driven cross shelf circulation offshore surface currents and introduces another way for water and the material it contains e g pollutants larvae to go across the shelf toward shore

the geometry and kinematics of flow perturbation folds abstract minor folds formed synchronous with ductile deformation in high strain zones can preserve a record of the scale and kinematics of heterogeneous flow using structures associated with wnw directed caledonian thrusting in n scotland we show that localised perturbations in flow resulted in the generation of predominantly cylindrical minor folds with hinges lying at low angles to the transport direction these define a series of larger scale fold culminations reflecting surging flow or depressions reflecting slackening flow that are bisected by transport parallel culmination and depression surfaces the fold patterns suggest a dominance of layer normal differential shearing due to gradients in shear strain normal to transport culmination surfaces are marked by along strike reversals in the polarity of structural facing and vergence of minor folds which contrary to classic fold patterns define reverse asymmetric relationships culmination surfaces separate folding in to clockwise z folds and anticlockwise s folds domains relative to the transport lineation the dip of fold axial planes systematically increases as their strike becomes sub parallel to transport resulting in a 3d statistical fanning arrangement centred about the transport direction thus mean s and z fold axial planes intersect precisely parallel to the transport lineation and potentially provide a means of determining transport directions in cases where lineations are poorly preserved culminations display convergent fold patterns with fold hinges becoming sub parallel to transport towards the culmination surface and underlying detachment whilst axial planes define overall concave up listric geometries which are bisected by the culmination surface thus around culminations and depressions there are ordered scale independent relationships between transport direction shear sense fold facing vergence and hinge axial plane orientations the techniques described here can be applied and used predictively within any kinematically coherent system of ductile flow

a view of the 1993 1994 california current based on surface drifters floats and remotely sensed data near surface drifters were deployed in the california current system during the period 1993 1994 in order to study mean and eddy currents the deployment scheme allowed a 2 year period where data density was high enough to make meaningful estimates of eddy statistics and of seasonal mean currents and variances on a 5 grid eddy variability which tended to be isotropic was higher near the coast standard deviations around 0 2 m s 1 and decreased offshore down to 0 1 m s 1 seasonal mean surface flow west of california was southward during all seasons if a davidson current existed during 1993 1995 it was probably either short lived or only found closer to the coast than our study area i e within 100 150 km of shore in some cases drifter populations within a discrete feature were high enough to allow some detailed resolution of its behavior we note a tendency for individual eddies which can last for up to 3 5 months to drift westward at rates of o 5 km day 1 not all eddies in the region are surface intensified there is clear evidence for at least the occasional presence of a thin meandering southward jet in the region that can be 1000 km or more long this jet can be identified with that found in the coastal transition zone program in association with cool filaments the jet appears to become broader and slower south of point conception we hypothesize on the basis of others results as well as ours that the eddies that are strongest at the surface result primarily from instabilities on a southward alongshore mean current and that eddies that are maximal at depth are spawned by the northward undercurrent

observations of upwelling and relaxation events in the northern monterey bay during august 2000 1 the interplay between the point ano nuevo upwelling center an offshore anticyclonic mesoscale eddy and the waters of the monterey bay was studied during a series of up and downwelling favorable wind events during august 2000 the upwelling events were characterized by the appearance of cold salty water at point ano nuevo at the north end of the bay that subsequently spread southward across the mouth of the bay as the winds continued during the downwelling relaxation events the surface current and temperature response was dominated by the onshore translation of the offshore eddy and by local surface heating in the bay itself the circulation within the bay was cyclonic during both wind regimes but slightly more barotropic under poleward forcing the icon model a nested data assimilating sigma coordinate model was used to simulate the upwelling and relaxation events and calculate the subsurface current and density fields the model reproduced the dominant current and temperature patterns outside the bay including the southward flowing upwelling filament the movement of the offshore eddy the poleward flow off point sur and the circulation within the bay the model salinity fields at the surface and 100 m levels show that during upwelling the bay was filled with higher salinity water stemming from the point ano nuevo upwelling center to the north during downwelling the source water for both the surface and 100 m levels was the colder fresher california current water offshore which had advected southward well past point pinos during the previous upwelling event

recurrent frontal slicks of a coastal ocean upwelling shadow 1 marine ecosystems are greatly influenced by the structure and dynamics of fronts in coastal upwelling systems frontogenesis occurs frequently by upwelling and transport of cold water and warming in sheltered upwelling shadow retention sites monterey bay in the california current upwelling system hosts a dynamic upwelling shadow environment using a decade of satellite synthetic aperture radar sar observations with ancillary remote sensing and in situ data we describe recurrent surface slicks that develop along the seaward periphery of the monterey bay upwelling shadow and we examine their relationships with fronts slick median dimensions 17 5 km long and 0 8 km wide describe their elongated structure although the typical pattern is a single slick multiple slicks may concurrently develop in association with different types of fronts repeated volume surveys through a front underlying a slick revealed lateral mixing and interleaving of regional water types velocity fields from coastal hf radar show that slicks may coincide with a variety of surface circulation patterns that they may extend contiguously across regions having very different surface velocity and that they may be separated from the shear front of upwelling filaments by 5 10 km slicks occur in all seasons and may coincide with both upwelling and downwelling wind forcing surfactant accumulation in small scale convergence zones is indicated as the primary mechanism of slick formation not ocean current shear or small scale air sea coupling the results of this study emphasize the role of upwelling system fronts in creating small scale structure and dynamics that influence plankton ecology

using along strike variation in strain and kinematics to define the movement direction of curved transpressional shear zones an example from northwestern superior province manitoba a review of current understanding of structures in shear zones reveals that correlating the orientation of stretching lineations with movement direction once a common practice based on the simple shear model is not generally applicable to transpressional zones it is difficult to determine the movement direction of a transpressional zone based on internal structures we show that the movement direction can be better defined by considering the variation in strain geometry and kinematics of differently oriented and kinematically related shear zones or different segments of a curved shear zone application of this method to a shear zone in the northwestern superior province of manitoba shows that the movement direction there is approximately north south in contrast to a commonly held view for similarly oriented shear zones in the superior province that the movement direction is approximately east west

remotely sensed surface currents in monterey bay from shore based hf radar coastal ocean dynamics application radar near surface currents in monterey bay derived from a network of shore based hf radars are presented for august december 1994 and compared with those from april to september 1992 focus is placed on the low frequency 2 to 30 day period motions in the remotely sensed data and on comparison of radar derived currents with moored current and wind observations ship based acoustic doppler current profiler observations satellite based surface temperature imagery and surface drifter velocities the radar derived picture of the late summer mean flow is very similar in the two realizations and is consistent with historical data flow is equatorward in the outer part of the bay poleward in a narrow band nearshore and very sluggish in the middle of the bay low pass filtered time series of radar derived currents are highly correlated with moored current observations and with winds in the outer part of the bay the vector time series are also coherent across a broad frequency band with currents typically in phase between 1 and 9 m depths and with 1 m currents typically 40 60 to the right of the wind overall these results confirm the utility of coastal ocean dynamics applications radar codar type hf radars for the study of coastal surface currents out to ranges 50 km from shore particularly for highly averaged fields data variability and comparison with in situ observations for high frequency 1 to 48 hour period motions point to the need to better characterize and minimize sources of error in the radar observations

progressive folding and foliation development in a sheared coticule bearing phyllite abstract an evolutionary sequence of fold and foliation development interpreted to be the result of a single progressive deformation is seen in exposures of strongly sheared phyllite and coticule 10 km south of the e w trending chedabucto fault in the meguma terrane of nova scotia canada transposition sensu sander at mesoscopic and microscopic scales is fundamental to this development as are fold hinge rotation and migration the transposition process is cyclic with folds of layering and foliation developing rotating and being refolded fold hinges and axial surfaces repeatedly tend towards stable end orientations but do not reach them due to the cyclic transposition process the foliation is a steady state feature continually reconstituted by microscale transposition and dynamic recrystallization several important consequences of these contemporaneous processes are 1 geometrical relationships between folds and foliation are variable in the same outcrop even though both developed synchronously and many folds have the geometrical relationships of cleavage transected folds 2 the foliation is transient in orientation and therefore cannot track the xy principal plane of total finite strain and 3 fold hinge migration may be a common process especially in deformations involving a large component of simple shearing the observed relationships may be applicable to study of fold and foliation development in orogenic belts in general

contrasting cw and ccw tectono metamorphic belts in the eastern himalayan syntaxis quantification of p t t paths and tectonic interpretation abstract in the eastern himalayan syntaxis the indus tsangpo suture zone separates the himalaya to the south from the southern lhasa terrane to the north in this study we combine petrology zircon u pb geochronology and phase equilibrium modeling to quantify the p t t paths of metapelites from the namche barwa complex part of the himalaya and the nyingchi complex part of the southern lhasa terrane the namche barwa complex is interpreted to record slow cooling across a hp granulite facies p t field at 12 kbar from 790 c to the solidus at 750 c between c 50 ma and c 17 ma followed by decompression with slight heating to a p t field at 5 5 8 kbar and 790 825 c at c 3 ma consistent with the post peak segment of a cw p t t path by contrast the nyingchi complex records a ccw p t t path characterized by compression through peak granulite facies p t conditions of 7 5 9 kbar at 760 780 c before re crossing the solidus at p of 11 kbar at c 35 31 ma based on the youngest published spot date from a detrital core in zircon from metapelite the maximum depositional age for the protoliths of the nyingchi complex is c 53 ma we propose a tectonic scenario in which the nyingchi complex ccw p t t path was buried in the interval c 53 35 ma synchronous with heating then compression during ongoing convergence related to the india asia collision by contrast the namche barwa complex cw p t t path which represents the subducted indian continental lithosphere records relatively fast exhumation from c 17 to c 3 ma that was initiated by crustal scale folding linked to orogen scale gravitational spreading and thrusting enhanced by surface processes

the structural evolution of sheath folds a case study from cap de creus abstract it has long been recognised that within zones of intense non coaxial deformation fold hinges may rotate progressively towards the transport direction ultimately resulting in highly curvilinear sheath folds however there is a surprising lack of detailed and systematic field analysis of such evolving sheath folds this case study therefore focuses on the sequential development of cm scale curvilinear folds in the greenschist facies el llimac shear zone cap de creus spain this simple shear dominated dextral shear zone displays superb three dimensional exposures of sheath folds defined by mylonitic quartz bands within phyllonite increasing amounts of fold hinge curvature are marked by hinge segments rotating into sub parallelism with the mineral lineation lm whilst the acute angle between the axial planar hinge girdle and foliation also displays a sequential reduction although lm bisects the noses of sheath folds it is also clearly folded and wrapped around the sheath hinges lm typically preserves a larger angle with the fold hinge on the lower limb l compared to the upper u limb l u suggesting that lm failed to achieve a steady orientation on the lower limb adjacent sheath fold hinges forming fold pairs may display the same sense of hinge arcing to define synthetic curvature or alternatively opposing directions of antithetic curvature such patterns reflect original buckle fold geometries coupled with the direction of shearing the ratio of long short fold limbs decreases with increasing hinge curvilinearity indicating sheath folds developed via stretching of the short limb rather than migrating or rolling hinge models this study unequivocally demonstrates that both hinges of fold pairs become curvilinear with sheaths closing in the transport direction recording greater hinge line curvilinearity compared to adjacent return hinges this may provide a useful guide to bulk shear sense

mesoscopic structure of the punchbowl fault southern california and the geologic and geophysical structure of active strike slip faults we examine the distribution density and orientation of outcrop scale structures related to the punchbowl fault an exhumed ancient trace of the san andreas fault southern california in order to determine the structure of the fault zone the punchbowl fault has 44 km of right lateral slip and cuts the cretaceous pelona schist in the study area the mesoscopic structures examined include fractures small faults and veins they were inventoried using scan lines at closely spaced stations along three strike perpendicular traverses 200 250 m long across the fault the fault zone thickness is a function of the type of structure measured slip along narrow 2 m wide ultracataclasite cores of the faults results in foliation reorientation over a distance of 50 m from the cores fracture and fault densities appear to increase 50 80 m from the fault cores and vein densities are highly variable across the fault zone fractures and faults in the damaged zone have a variety of orientations but most are at high angles to the main fault zone when coupled with previous geochemical and microstructural data these data show that large displacement faults of the san andreas system are up to 200 250 m thick and enclose zones of mineralogic and geochemical alteration that are 20 30 m thick extreme slip localization occurs over zones 1 5 m thick when reconciled with geophysical imaging our data suggest that trapped headwaves travel in the damaged zone and that some aftershock events produce slip on faults and fractures which often have orientations very diaerent from the principal slip surfaces 7 2000 elsevier science ltd all rights reserved

satellite observations and modeling of meanders in the california current system off oregon and northern california abstract infrared satellite images from the west coasts of oregon and northern california are used to identify meander patterns in sea surface temperature which appear as large cold tongues extending offshore two relatively long series of images from 1982 and a few examples from 1980 and 1983 demonstrate the evolution of the cold tongues from an initial variety of scales 60 200 km to the fastest growing waves 110 130 km and then finally to tongues with longer wavelengths 400 km this is observed to occur over periods of about three months in summer and fall when the coastal circulation is composed of a southward surface current over a northward undercurrent the initial shorter scale features are believed to be excited by the interaction between the mean current and the coastal topography baroclinic instability associated with the vertical shear between the surface current and the undercurrent is found to be responsible for the growth of the features observed in the satellite imagery a nonlinear

kinematics of rock flow and the interpretation of geological structures with particular reference to shear zones abstract current interpretations of structures are generally based on homogeneous and steady deformation models despite the fact that both the heterogeneity of rocks materially rheologically and geometrically and the time dependence of imposed geological conditions give rise to significant heterogeneous and non steady flow in concert with field observations we emphasize that the expectation of heterogeneity and non steadiness is the key to understanding natural deformation and that in order to carry out successful structural analysis and tectonic interpretation it is necessary to recognize the first order distinction between imposed boundary conditions typically used to define the tectonic regime e g transcurrent transpression and the response recorded by rocks within the zone structures and fabrics using s c fabric as an example it is demonstrated how flow with a non zero spinning component resulting from the rheological contrasts and or geologically realistic time dependent boundary displacement can drastically change the ideal geometric and kinematic relations between the fabric and the host zone in agreement with both theoretical analysis and field observation it is shown that natural flow regimes range from pure shear to pure rotation including super simple shear in consideration of the heterogeneity and non steadiness of natural deformation kinematic analysis is justifiable only within a homogeneous domain and steady period flow kinematics and mechanisms are interrelated in that firstly mechanisms provide internal constraints on kinematics ensuring that only certain flows are possible and secondly flow kinematics will favour development of certain mechanisms

deformation path in high strain zones with reference to slip partitioning in transpressional plate boundary regions abstract the current status of the kinematics and strain geometry of high strain zone studies is briefly summarized a general high strain zone has a triclinic deformation path and monoclinic shear zones are special end member cases fabrics observed in natural shear zones and theoretical considerations based on continuum mechanics are compatible with this conclusion non steady deformation paths remain difficult to deal with and may ultimately rely on a realistic mechanical treatment of high strain zones which may be possible when our knowledge of the mechanical behavior of rocks under natural deformation conditions is improved an examination of the phenomenon of slip partitioning in transpressional plate boundary regions shows that the bulk deformation path in the forearc area trench parallel high strain zone is generally triclinic the alpine fault in the south island of new zealand provides an example of a currently active triclinic shear zone the southern knee lake shear zone of manitoba canada provides an archean example of a triclinic shear zone

structural reconstruction of the flin flon volcanogenic massive sulfide mining district saskatchewan and manitoba canada the flin flon mining district is part of a greenstone belt the flin flon glennie complex in the paleoproterozoic trans hudson orogen its tectonic history began prior to 1872 ma with the development of regional folds the faulted f1 burley lake syncline and f2 hidden lake syncline during d1 and d2 intraoceanic accretion of the 1888 ma flin flon arc to other volcanic terranes amalgamation of the flin flon to glennie terrane possibly during d3 produced a w propagating thrust fold belt and basins in which fluvial sedimentary rocks were deposited between 1847 ma and 1842 ma as the fold thrust belt migrated westward these rocks were incorporated into a stack of e dipping thrust sheets bounded by nnw striking thrust faults 1920 fault and internally folded by w verging folds pipeline mud lake and grant lake synclines subsequent d4 collision of the flin flon glennie complex with the archean sask microcontinent was broadly coeval with but outlasted the emplacement of 1840 ma phantom lake dikes d4 produced a second truncating fold thrust system characterized by n directed thrust faults club lake and railway faults and e trending folds flin flon creek syncline these folds were overprinted by two regional cleavages and the thrust faults were reactivated as oblique slip shear zones either late during the same collisional event d5 or during terminal collision d6 of the sask craton and flin flon glennie complex with the superior craton at 1 83 to 1 79 ga the flin flon volcanogenic massive sulfide ore system was thrust imbricated during d3 and d4 and ore lenses were stretched parallel to a regional se plunging stretching lineation that formed during d4 and was later modified during d5

the origin of ridge in groove slickenside striae and associated steps in an s c mylonite abstract a three dimensional study of an s c mylonite reveals that the s surfaces anastomose around porphyro clasts and c surfaces are not truly planar but curvi planar with the linear generator parallel to the movement direction it is suggested that it may be common for shear surfaces at all scales to take this form in the material studied surfaces exposed by parting primarily along c surfaces which are a product of ductile deformation and are penetrative have all the characteristics of classic slickensides on the slickensides striae of the ridge in groove type and steps of both congruous and incongruous types are developed the ridge in groove shape of the striae is directly related to the curvi planar shape of the c surfaces incongruous steps are smooth they are the result of parting along s surfaces congruous steps are rough first order congruous steps are formed by fracturing between individual c surfaces whereas second order congruous steps form by breaking off the tips of the dihedra between s and c surfaces thus the smoothness roughness technique can be used to determine the sense of shear in strongly deformed rocks where the angle between s and c surfaces is too small to be recognized in the field this may prove to be a useful shear sense indicator

comment on eye and sheath folds in turbidite convolute lamination aberystwyth grits group wales by h l o mcclelland n h woodcock c gladstone journal of structural geology 33 2011 1140 1147

sustainable transpression an examination of strain and kinematics in deforming zones with migrating boundaries most orogenic belts owe their development to oblique convergence and commonly have many orogen parallel transpressional high strain zones to constrain the tectonic history of orogenic belts by structural and fabric analysis of rocks it is desirable to understand quantitatively the relationship between the boundary conditions and the resulting strain distribution and kinematics in these zones most current models for transpression assume homogeneous deformation confined by boundaries that are fixed to material planes this creates a strain compatibility problem at the margins of the active deforming zone and also requires that the strain rate normal to the zone boundaries increase to implausibly high values soon after the onset of oblique convergence transpressional motion the latter contradicts with the observation that transpressional motion can be sustained throughout an orogeny the assumption that zone boundaries are fixed to material planes is unrealistic the outstanding problems of current transpressional models are resolved in this paper by allowing the zone boundaries to migrate through the rock material the consequence of zone boundary migration for the strain field and kinematics within a transpressional high strain zone is investigated mathematically the implications of the modeling for fabric interpretation are discussed the modeling makes general predictions consistent with observed planar and linear fabric patterns in natural transpressional high strain zones it predicts that foliations in transpressional high strain zones are subparallel to the zone boundaries regardless of variation in the imposed boundary conditions lineations cluster along the great circle girdle subparallel to the average foliation the spread of the lineations may vary from point maxima to complete girdles

when do dragfolds not develop into sheath folds in shear zones abstract using a unified model for high strain zones rotation paths of material lines in different flow types are investigated and presented as stereographic projections a material fabric element such as a mineral lineation or a fold axis can be plotted on such stereograms and the rotation path revealed using this method we examine the rotation of folds and the development of sheath folds in general three dimensional zonal deformation we show that the evolution of dragfold geometry in a high strain zone depends on the flow type and the initial perturbation of the fold axis in simple shear zones and thickening zones monoclinic or triclinic dragfolds will evolve into sheath folds in monoclinic thinning zones with biaxially stretching boundaries if the shear direction is parallel to the maximum stretching direction of the boundary sheath folds will develop if the shear direction is parallel to the minimum stretching direction of the boundary dragfolds become more cylindrical as deformation advances in thinning zones with boundaries equally stretched in all directions the likelihood of sheath fold development depends on the ratio of the shear strain rate to the thinning rate in triclinic thinning zones fold evolution depends on the details of the flow and the value of initial fold axis perturbation

initiation of folding and boudinage in wrench shear and transpression abstract a linear stability analysis is performed on a deforming layered three dimensional linear viscous system the system consists of a single layer of viscosity embedded in a medium of viscosity the layer is oriented normal to the z axis and both the layer and medium are subjected to uniform simple shearing parallel to the y axis wrench shear with superposed uniform shortening parallel to x and extension parallel to z transpression the stability of cylindrical perturbations of the form a t cos ax y is examined it is found that fold type perturbations are unstable and pinch and swell disturbances are unstable in some cases for the case of wrench shear alone the fastest growing buckling disturbances are oriented at 45 to the positive y axis while the fastest growing pinch and swell disturbances have positive growth rates and are oriented 90 to the fold axes additional shortening parallel to the x axis transpression causes the fold axes to initiate at lower angles to the y axis pinch and swell disturbances may or may not be unstable in transpression depending on the magnitude of stretching parallel to the z axis

recumbent folds key structural elements in orogenic belts abstract this review has two main parts the first of them presents existing ideas and data related to recumbent folds reviewing aspects such as the physical conditions of the development of these folds the strain inside the folded layers the kinematic mechanisms of their formation the role of gravitational forces the tectonic context of their development and the structures associated with them in the second part the above ideas are discussed and possible mechanisms for the development of these folds are presented it is proposed that initial perturbations of the layers are essential to give rise to the asymmetry of recumbent folds these perturbations may be non planarities of the layering or may be linked to the existence of a core or basement of competent rock that hinders the normal propagation of the deformation this could explain why many large recumbent folds have a root zone deformation with an important component of simple shear is a general condition for the formation of recumbent folds in areas with very low grade metamorphism competent layers often play an active role during the deformation and undergo buckling with the development of an overturned fold limb which can be stretched and thinned to finally produce a pair of recumbent folds separated by a thrust in areas with low or medium metamorphism buckling under a simple shear regime is probably the most important mechanism for producing large folds with gentle or moderately dipping axial surfaces subsequent kinematic amplification by coaxial strain components with vertical maximum shortening is important for the formation of recumbent folds these components involve a sub horizontal stretching that can cause a problem of strain compatibility and give rise to a basal thrust in areas deformed under high p and t conditions recumbent folds can develop by flow perturbations and kinematic amplification of folds this is probably a common mechanism in ductile shear zones

vorticity decomposition and its application to sectional flow characterization abstract great insights have been gained by applying the concept of vorticity to the study of the natural flow of rocks geologists have used various schemes of vorticity decomposition depending on the purpose of their studies the fundamental decomposition divides the overall vorticity into that of the material with respect to the instantaneous stretching axes and the spin of these axes in an external reference frame in which the flow is defined this represents a new contribution to continuum mechanics material science and structural geology its implications for the kinematic analysis of geological structures and fabrics are more significant than have been realized in virtually every geological situation the flow has a spin component and therefore vorticity decomposition needs to be considered the decomposition of vorticity is applied to the characterization of the two dimensional flow on a material plane sectional flow in a general three dimensional flow field the analysis of the sectional flow and sectional non coaxiality is a precondition for a determination of the initial orientation of structural elements such as the orientation of a fold developed in a competent layer that is embedded in a high strain zone and their temporal evolution theoretically it may also be possible to derive the symmetry of a natural deformation path by examining fabric symmetries on differently oriented sections from deformed rock samples

shear zones and rock history abstract shear zones that have widened with time can display margin to interior microstructural profiles unlike the real microstructural history of the interior rocks shear zones that have not widened with time can provide more reliable views of microstructural history and therefore of deformation processes even the latter zones can mislead however where there has been an annealing overprint during or following the shearing interval these sources of uncertainty need to be acknowledged more regularly and if possible eliminated by workers interpreting deformation processes from shear zone strain gradients

transected folds and transpression how are they associated abstract folds transected by cleavage are a distinctive structural feature in the caledonian appalachian orogenic belt with the advent of a tectonic model of transpression for this belt there has been an assumption that transected folds are a symptom of transpressive deformation in particular that clockwise transection signifies sinistral transpression we examine this tenet and conclude that there is no unique explanation for folds transected by a synchronous cleavage explanations can be found for either clockwise or anticlockwise transection for sinistral or dextral transpression each requiring assumptions about the inclination of bedding to the transpression vector or requiring that cleavage does not track the finite xy plane a further explanation which may fit the regional data best requires a non constant transpressive deformation with initial pure shear dominated deformation giving way to increasing transcurrent movement in time none of these models can be assumed to be right without supporting data on the incremental and finite strain histories of such regions

sheath folds as discriminators of bulk strain type abstract nested elliptical closures defining eye folds represent classic y z cross sections through highly curvilinear sheath folds generated during intense ductile deformation in metamorphic rocks systematic analysis of 1425 such eye folds based on our own field observations and examples from the published literature reveals distinct and consistent differences in ellipticites measured from the outer r yz to the inner most r y z elliptical rings of individual sheaths the variation in overall aspect ratios from outer to inner rings is defined as r where r r yz r y z and may display a relative increase or decrease in ellipticity to define cats eye r bulls eye r 1 fold patterns respectively layer thicknesses may also be measured along the y axis parallel to the axial surface t y and at 90 to this along the z axis t z to define the ratio of t yz sheath folds generated during broadly simple shear deformation k 1 display mean r yz 4 61 t yz 3 31 and cats eye folds r 0 69 sheath folds generated during general shear also display cats eye folds with identical mean r 0 69 values but greater thickness variations and elliptical ratios t yz 4 35 r yz 5 76 thus within both simple and general shear dominated deformations the overall variation in layer thickness t yz and ellipticity of eye folds r yz increases with increasing deformation and a greater component of pure shear whilst the r value remains constant and reflects original fold patterns sheath folds formed during constrictional k 1 deformation display markedly lower aspect ratios r yz 2 42 and thickness variations t yz 2 94 together with distinctive bulls eye folds r 1 23 these empirical relationships suggest fundamental and universal constraints on curvilinear fold generation across this broad spectrum of deformation types and allow sheath folds to act as both effective 95 consistent and robust discriminators of bulk strain type

the deformation matrix for simultaneous simple shearing pure shearing and volume change and its application to transpression transtension tectonics simultaneous simple shearing and pure shearing with or without additional volume change can be combined into a single upper triangular deformation matrix the off diagonal term is named the effective shear strain and is a simple function of the pure shearing and simple shearing components a three dimensional deformation matrix for the simultaneous combination of coaxial deformation with or without additional volume change and up to three simple shearing systems with mutually orthogonal shear planes is also presented by using this matrix one can easily extract the various properties of incremental as well as finite strain and the progressive as well as finite rotation of passive markers during deformation the case of transpression transtension is revised using the unified deformation matrix the orientation of the major axis of the strain ellipsoid 1 is always horizontal if the deformation is transtensional switches from horizontal to vertical during transpressional wrenching 1 wk 0 81 for constant vorticity deformations and is always vertical for highly transpressional deformations wk 0 81 for transpression material lines initially rotate towards the horizontal shearing direction but generally turn to rotate towards the vertical axis after a certain strain for transtension all material lines rotate towards a direction in the horizontal plane which is oblique to the shearing direction

interpretation of deformation fabrics of infrastructure zone rocks in the context of channel flow and other tectonic models infrastructure zones are essentially horizontal to shallowly dipping crustal scale zones of non coaxial flow with two possible interpretations a a crustal scale shear zone transporting upper crust over lower crust and or mantle transport flow or b a zone of channel flow in which there is a flux of weak crust between relatively strong upper and lower crust and or mantle away from the centre of the orogen transport flow has a constant shear sense across the zone whereas in channel flow the sense of shear reverses across the zone channel flow may be driven by extrusion extrusive channel flow due to the two channel walls approaching one another or by a pressure gradient along the channd normal channel flow with no convergence of the walls necessary arguments based on strain compatibility and mechanics suggest that extrusive channel flow is unlikely kinematic vorticity numbers have been used to show that infrastructure and other shear zones have undergone flattening strains but we show that the numbers are incompatible with extrusion we also show that in addition to the problems inherent in determining kinematic vorticity numbers from fabric the numbers cannot be related to bulk flow in mechanically heterogeneous zones because of flow partitioning drag folds are a better indication albeit qualitative of whether a zone is thinning or not they also give a conservative estimate of the minimum accumulated shear strains and may inhibit strain localization like snowball garnets they indicate shear strains that are so large that the pure shear thinning component for a steady shear zone has to be small

cape st marys shear zone and the halifax group rockville notch group disconformity southwestern nova scotia structural development and tectonic significance the cape st marys shear zone situated in a corridor of alleghanian reworking in the southwestern meguma terrane contains the deformed discordant contact of lower ordovician slate of the halifax group with the silurian white rock formation close to the contact the alleghanian cleavage s2 is parallel to the contact in both units with s0 in the white rock formation metavolcanic rocks and halifax slate parallel to and discordant to the contact the geometry of deformed neoacadian minor folds quartz fringes on sulphide grains and micro porphyroclasts demonstrate thrust sense shear white rock formation over halifax slate pure shear and volume loss are inferred as components of the strain path from s2 microstructure and estimates of strain in the halifax slate estimates of shear strain imply moderate displacements within the cape st marys shear zone during deformation of the northwestern limb of the cape st marys syncline the discordant contact of the halifax slate with the white rock formation

structural style and tectonic significance of the jianglang dome in the eastern margin of the tibetan plateau china abstract the jianglang tectonic dome jtd lies on the eastern margin of the tibetan plateau and consists of three tectono stratigraphic units separated by two extensional detachment faults a schist complex a middle ductile slab and an overlying sedimentary unit the schist complex is composed of quartz mica schists and amphibolites of possible mesoproterozoic age and exhibits two early foliations s 1 and s 2 overprinted by two later foliations s 3 and s 4 the ductilely deformed middle slab consists of paleozoic quartzite phyllite marble amphibolite and meta basalt it is characterized by pervasive intrafolial and intraformational recumbent folds with a penetrative axial foliation s 1 an equivalent of s 3 in the basement rocks the cover unit the triassic xikang group of flysch sedimentary rocks experienced shallow level buckling and flattening deformation with an axial cleavage defining s 1 an equivalent of s 4 in the middle slab and basement rocks the jtd appears to have experienced three stages of extension immediately after orogeny the earliest d 1 recorded by an axial surface s 1 of s 0 intrafolial folds and sub parallel to s 0 in the schist complex probably occurred prior to the collision between the south china and north china blocks this extensional event was followed by subduction of the yangtze block to the west towards the eastern margin of the tibetan plateau the two detachment faults are characterized by structures formed by ductile brittle to brittle shear deformation which are sub parallel to the layer of s 2 in the lower detachment and to s 0 in the upper detachment these faults mark the second stage of extension and may have caused thinning and selective removal of the sinian and paleozoic strata as well as exhumation of the schist complex and the middle slab in the middle jurassic 177 ma the final stage of extension d 5 shown by brittle deformation and thermo metamorphism from the lower detachment to the xikang group was associated with the indian eurasia collision

the monashee decollement at cariboo alp southern flank of the monashee complex southern british columbia canada abstract the monashee decollement is a crustal scale compressional shear zone in the southern canadian cordillera it is well exposed in alpine terrane and is imaged in the subsurface by lithoprobe seismic reflection profiles the decollement separates middle to upper crustal allochthonous assemblages from underlying middle to lower crustal rocks of the orogen development of the exposed part of the decollement at cariboo alp in the southern thor odin culmination has included formation of major isoclinal folds superposition of smaller scale folds during progressive deformation and transposition of early formed fabrics and stratigraphic boundaries into the dominant sw dipping mylonitic planar fabric hinge lines of early folds have been rotated toward the strong consistent sw plunging mineral stretching lineation as a result of ne directed shearing of fold limbs and thrust stacking of these folded sheets between a basal shear zone and overlying allochthonous assemblages an imbricate zone developed within the middle crust during peak metamorphic conditions

tubular folds and sheath folds definitions and conceptual models for their development with examples from the grapesvare area northern sweden abstract two parameters are used in the classification of highly non cylindrical folds the hinge line angle and the ratio of the length of the cone axis to that of the long axis of the cross section of the structure at the position where the hinge angle was measured if in a non cylindrical fold a cross section can be selected so that the hinge angle w measured where the cross section cuts the cone is less than 90 and the length of the cone axis x is more than a quarter of the length of the long axis y of the cross section the fold is a sheath fold a tight sheath fold with w x y 1 can be termed a tubular fold a tubular fold is thus an extreme type of sheath fold while sheath folds with w 20 but x y 0 25 but tubular folds from the grapesvare area comply to the definition given above and they occur in a structural setting that is consistent with the models described

the hongliuhe fold and thrust belt evidence of terminal collision and suture reactivation after the early permian in the beishan orogenic collage northwest china abstract the early permian strata of the hongliuhe formation are located in the beishan orogenic collage a tectonically significant region between the tianshan orogen and the southern mongolian accretionary system of the central asian orogenic belt also called the altaids stratigraphic reconstruction of the formation shows a general fining upwards sequence of rhythmically interbedded sandstones and siltstones with thick basal successions of conglomerates all unconformably overlying an earlier mid paleozoic volcanic assemblage the clast lithotypes of conglomerates successively change from being polymictic metamorphic tectonites at the base to a nearly monomictic granitoids mid section illustrating the unroofing sequence of an arc related provenance the intra oceanic gongpoquan composite arc that comprises most of the central beishan fits the clast lithotype profile of the conglomerates and is the most probable provenance the hongliuhe formation stratigraphy experienced a thin skinned fold and thrust belt style of deformation that has a consistent southward vergence deformation in the lower stratigraphic levels created thrust imbrication of formation scale blocks each separated by steep north over south brittle ductile shear zones the blocks are internally deformed with large scale folding with one significant block containing a sheath fold 15 km in breadth the upper stratigraphic levels accommodated strain through low angle thrust ramping shear folding imbricate fanning and duplexing this indicates that the hongliuhe formation was syn orogenically deposited in a progressively deepening terrestrial to nearshore environment within the gongpoquan composite arc our observations and data indicate that the basin overlies an earlier paleozoic suture zone this implies that the northern half of the gongpoquan composite arc overrode the southern half in a fault system reactivation centered about this zone of weakness and created a stratigraphic and structural response consistent with a subduction or collision driven fold and thrust belt its southward structural vergence is supporting evidence of a north dipping subduction related fault system under the northern gongpoquan the timing of both the genesis of the hongliuhe formation and this deformation event coincides with the subduction of the liuyuan ocean or docking of the dunhuang block further south at the southern margin of the gongpoquan composite arc

the partitioning of deformation in flowing rock masses abstract most geological structures owe their development to heterogeneous and or non steady flow flow partitioning refers to the division of the instantaneous velocity field into components related to translation strain and rotation flow partitioning during the development of a geological structure usually changes with time and different histories of flow are involved at different points in the developing structure heterogeneity leads to the development of flow domains with different characteristics notably the degree of non coaxiality of deformation and different partitionings of the rotational component of flow into shear induced vorticity and spin during non steady flow there is often continual repartitioning of vorticity between the spin and shear induced components spin is an important aspect of flow leading to the development of structure this paper discusses the partitioning of deformation in a flowing rock mass coaxial flow tends to occur in isotropic materials in regions near weakly constrained boundaries non coaxial flow is favoured by suitably oriented material anisotropy and or the presence of well defined boundary constraints involving parallel relative displacements a developing anisotropy e g a crystallographic fabric in a shear zone can induce a change from coaxial to non coaxial flow other effects are related to the scale of approximation for example bulk shear induced vorticity can be locally converted to spin flow in the crust and mantle of the earth produces geological structures on all scales and involves many different types of rheological response the factors that affect flow partitioning e g spin versus non coaxial deformation are therefore of interest

strain and kinematic analysis in general shear zones we present a unifying theory for the development and distribution of strain markers and kinematic indicators in zones of general shear and thus provide a framework in which data previously considered contradictory may be understood rigid and deformable porphyroclast systems including and complex grains are potential indicators of both strain and flow the shapes and distribution of such porphyroclast systems may be used to distinguish among different tectonic regimes general shear is divided into two fields sub simple shear in which the rotational component of the strain is less than that for simple shear and super simple shear in which the rotational component is greater than for simple shear sub simple shear may involve narrowing or broadening of shear zones super simple shear regimes are possible in local regions such as the vicinities of deformable porphyroclasts but must be enclosed by regimes of sub simple shear the polar mohr constructions for finite deformation and flow are useful to analyze general shear in theory the hyperbolic net is employed for practical plotting of real data and derivation of the kinematic vorticity number wn this number represents the relative contributions of pure and simple shear in steady flow in nature deformation is thought to build up and decay by processes that may invalidate the assumption of constant flow regime we therefore introduce the concept of accelerating deformation and analyze the implications of non steady flow for the shearing histories of deformed objects

constrictional strain in a non coaxial shear zone implications for fold and rock fabric development central mojave metamorphic core complex california finite strain and fold analyses of footwall mylonites in the central mojave metamorphic core complex cmmcc reveal two distinct deformation paths that generated non coaxial constrictional strain the first path recorded in the waterman hills accomplishes constrictional strain through the formation of l tectonites at the grain scale the second path recorded in the mitchel range involves a combination of plane strain at the grain scale and y axis shortening through syn mylonitic folding the ductile shear zone in the waterman hills is approximately 500 m thick and is developed entirely within a miocene syn kinematic granodiorite strain magnitude decreases structurally downward the mylonites contain abundant ne directed non coaxial microstructures yet are l tectonites rf o analysis of quartz ribbon grain shapes indicate prolate strains k 6 and bulk chemical data suggest that mylonitization was isochemical or involved volume loss therefore ductile shearing in the waterman hills involved true constrictional strain at the grain scale immediately to the southeast in the mitchel range the lateral continuation of the shear zone is more than 1000 m thick and primarily involves lithologically heterogeneous pre tertiary basement the mylonitic fabric comprises a well developed foliation and ne trending stretching lineation rf o strain analysis was applied to grain shapes of cataclasized and strongly altered garnet porphyroclasts in a peraluminous granite in contrast to the waterman hills finite strains in these l s tectonites approximate plane strain k 1 1 however the mylonitic fabric is strongly folded about axes uniformly oriented subparallel to the stretching lineation the coaxial folds range from open to isoclinal we interpret the folds to have nucleated with axes parallel to the stretching lineation during mylonitization and to have accomplished y axis shortening in the shear zone macroscopic folds of the brittle detachment spatially correspond to open folds in the ductile shear zone suggesting that y axis shortening continued after the fault zone had reached the brittle regime corrugations and folds oriented subparallel to the transport direction are characteristic features of fault zones in cordilleran metamorphic core complexes therefore constrictional strain may be common in core complexes strain compatibility with upper plate rocks in core complexes may be maintained by y axis shortening along conjugate strike slip faults a nearly uniaxial stress field 3 2 1 is consistent with most of the structures formed in basin and range extension and could generate the observed constrictional strain

large magnitude continental extension an example from the central mojave metamorphic core complex the central mojave metamorphic core complex is defined by a belt of miocene brittle ductile extension and coeval magmatism the brittle ductile fault zone defines a basin and dome geometry that results from the interference of two orthogonal fold sets that we infer to have formed by mechanically independent processes one fold set contains axes that lie parallel to the extension direction of the shear zone and has a maximum characteristic wavelength of about 10 km the axial surfaces of these folds can be traced from the footwall mylonites through the brittle detachment and into hanging wall strata however folds of mylonitic layering have smaller interlimb angles than those of the brittle detachment suggesting that the folds began to form during ductile shearing and continued to amplify in the brittle regime possibly after movement across the fault zone ceased mesoscopic fabrics related to the transport parallel fold set indicate that the folds record true crustal shortening perpendicular to the extension direction we interpret these folds to form in response to elevated horizontal compressive stress perpendicular to the extension direction and suggest that this stress regime may be a natural consequence of large magnitude extension the other fold set has axes perpendicular to the extension direction and a characteristic maximum wavelength of about 50 km mesoscopic fabrics related to these folds include northwest striking joints kink bands and en echelon tension gash arrays these fabrics formed after mylonitization and record both layer parallel extension and northeast side up subvertical shear the postmylonitic fabrics are kinematically compatible with rolling hinge style isostatic rebound of the footwall following tectonic denudation the relative timing of extension related magma intrusion and ductile deformation varies through the central mojave metamorphic core complex on the scale of the small mountain ranges that make up the central mojave metamorphic core complex no correlation was observed between either shear zone thickness or intensity of ductile deformation and either the proximity or relative volume of extension related igneous rocks this suggests that models that invoke a single upper crustal genetic relationship such as magmatism triggering extension or vice versa do not apply to the central mojave metamorphic core complex systematic variation in the relative timing of dike emplacement and mylonitization suggests that at the time of dike emplacement rocks in the mitchel range were below the brittle ductile transition while those in the hinkley hills were above it the hinkley hills and mitchel range are separated by 2 km in the dip direction of the fault zone which suggests that the vertical thickness of the brittle ductile transition probably was between 100 and 950 m

high strain zones a unified model abstract where a shear zone is bounded by undeformed wall rocks and there is no volume change the deformation path within the zone on the bulk scale approximates a simple shear where the zone boundaries are stretched and the shear direction between the two boundaries is parallel to one of the principal stretching directions of the boundaries the deformation path within the zone is not a simple shear but still has monoclinic symmetry both simple shear and general monoclinic deformation paths should be regarded as special cases in nature because generally not only are the zone boundaries stretched but also the shear is oblique to the principal stretching directions of the boundaries in this general case the deformation path is triclinic we establish a model which includes this general case also taking into consideration volume change five independent parameters are required to characterize the rate of deformation of the flow four are sufficient to characterize the flow kinematics the model is generally triclinic and orthorhombic and monoclinic deformations are subgroups by varying the values of the characterizing parameters the various types of high strain zones can be represented the kinematics of flow within such zones and the accumulated finite deformation geometry are investigated many field observations such as for example lineations in transcurrent shear zones that vary between vertical and horizontal cleavage transected folds and the absence of sheath folds in some shear zones where folds have been significantly rotated can sometimes be better interpreted in the context of our general model

folds and the strain ellipsoid a general model abstract a new general model is proposed to account for non orthogonal relationships of fold axes and axial surfaces to the axes of the finite strain ellipsoid descriptions in the literature of such relationships e g plunge variations and transecting cleavage are reviewed and the development of the model is outlined the model is described in which plane strain k 1 or slight flattening strain k 0 71 or slight constrictional strain k 1 85 are imposed on a suite of layers with oblique orientations to the bulk strain axes x y z folds will initiate perpendicular to the direction of shortening in the layer and develop as non material lines moving through the layer by angular migration at geologically realistic finite strains the folds will be transected by xy cleavage in slight flattening strain folds are likely to be cylindroidal with axes not measurably oblique to xy in slight constrictional strain en echelon periclines are predicted with marked transection by cleavage in some examples the interference of two directions of folding will give rise to severe plunge variations and even downward facing although strain markers frequently indicate finite strains in the flattening field this may result from the superimposition of slightly constrictional tectonic strain on compactional flattening

a micromechanical approach for simulating multiscale fabrics in large scale high strain zones theory and application 1 deformation fabrics in earth s crust and mantle are commonly used to constrain the tectonic history deformation mechanisms and rheological properties of the lithosphere their formation involves heterogeneous and multiscale deformation processes that current single scale models cannot capture here we present a micromechanics based multiorder power law approach mopla for the simulation of multiscale fabrics in crustal scale high strain zones we consider the progressive deformation in a crustal high strain zone on three different scales on the macroscopic scale representing the average assemblage of rock units at a point we regard the rock mass as a continuum made of many first order elements the progressive deformation of first order elements in the macroscopic flow field simulates tectonic transposition on the scale of an individual first order element we regard it as an eshelby inhomogeneity embedded in a poly element continuum we apply eshelby s inhomogeneity formalism for power law materials to relate the flow field inside a first order element to the macroscopic flow field on the scale pertinent to structures observed on the outcrop or smaller scale the partitioned flow fields inside individual first order elements are used to examine the fabric development we implement mopla in mathcad apply the approach to a natural example of the cascade lake shear zone and discuss the implications of multiscale deformation our model predicts lineation patterns observed in natural high strain zones that have remained unexplained by single scale models

a fundamental problem with the kinematic interpretation of geological structures abstract non steady flows are ubiquitous in nature not only do imposed boundary conditions vary with time and rock rheology change during the course of deformation but also deformation is generally heterogeneous and all of these conditions lead to non steady flow histories in modern kinematic analysis flow apophyses the instantaneous stretching axes and the vorticity vector collectively referred to as the eigen directions of deformation are used in place of bruno sander s kinematic axes for deformation with a steady flow history this practice is well justified and has led to great advances in structural geology but for non steady flow histories the geometrical relationships among eigen directions flow pattern vary with time this makes it inappropriate to correlate an association of structures and fabrics with certain time invariant flow patterns and hence raises the question how should we interpret geological structures and fabrics kinematically without the assumption of homogeneous and steady deformation we suggest that the answer lies primarily in forward modeling of deformation based on a knowledge of rock properties

shear zone related folds folds in ductile shear zones are common structures that have a variety of origins these can be pre existing folds that become modified or folds developed during the shearing event among the syn shearing folds a second subdivision is based on the relative age of the folded surface which can be pre existing or newly formed during the shearing event in each of the three categories final fold geometry and orientation show complex relationships with the kinematic frame the final fold geometry depends on the vorticity within the shear zone the rheology and the initial orientation of the folded surface relative to the kinematic framework it follows that folds are complex structures difficult to use as kinematic indicators however in shear zones where undeformed wall rocks with pre shear structures are accessible and where kinematics can be well established folds can provide a valuable natural means to understand the initiation and evolution of structures under non coaxial regimes we point to the need of discriminating among different plausible categories based on the nature of the folded surface and on the inherent structural features of each category q 2004 elsevier ltd all rights reserved