development of p t prograde and p retrograde t prograde isogradic surfaces during blueschist to eclogite regional deformation metamorphism in new caledonia as indicated by progressively developed porphyroblast microstructures the mid tertiary blueschists eclogites and eclogitic gneisses of northern new caledonia are the products of four phases of regional meta morphism and deformation d d4 omphacite lawsonite and mn rich garnet isogradic surfaces were developed during the second deformation d2 under prograde pressure and temperature conditions subsequent deformations d3 d folded these d isogradic surfaces however within the p retrograde t prograde metamor phic environment of the d phase omphacite altered to albite and chlorite as a result a late stage sub horizontal isogradic surface developed for omphacite out where this mineral is preserved as relics within syn d albite porphyroblasts other minerals that crystallized for the first time epidote or had rim additions almandine phengite during d also form nearly horizontal isogradic surfaces porphyroblastic garnet and albite contain inclusion trails which allow their microstructural development and crystallization of the matrix to be traced from d to d late syn d the temperature increased mark edly in association with an extensive exothermic decarbonation even though the rocks were in a state of pressure retrogression this caused considerable neocrystallization recrystallization and growth of matrix and porphyroblasts such that although s foliation crenulated by d and d is readily observable almost all signs of stored strain due to d and d have been removed and the deeper schists and eclogitic gneisses superficially appear to have undergone a drastic annealing recrystallization post dating deformation

strain path partitioning within thrust sheets microstructural and petrofabric evidence from the moine thrust zone at loch eriboll northwest scotland abstract quartz c axis fabrics and microstructures have been investigated within a suite of quartzites collected from the loch eriboll area of the moine thrust zone and are used to interpret the detailed processes involved in fabric evolution the intensity of quartz c axis fabrics is directly proportional to the calculated strain magnitude a correlation is also established between the pattern of c axis fabrics and the calculated strain symmetry two kinematic domains are recognized within one of the studied thrust sheets which outcrops immediately beneath the moine thrust within the upper and central levels of the thrust sheet coaxial deformation is indicated by conjugate mutually interfering shear bands globular low strain detrital quartz grains whose c axes are aligned sub parallel to the principal finite shortening direction z and quartz c axis fabrics which are symmetric both in terms of skeletal outline and intensity distribution with respect to mylonitic foliation and lineation non coaxial deformation is indicated within the more intensely deformed and recrystallized quartzites located near the base of the thrust sheet by single sets of shear bands and c axis fabrics which are asymmetric with respect to foliation and lineation tectonic models offering possible explanations for the presence of kinematic strain path domains within thrust sheets are considered

superposed deformations in the eastern alps strain analysis and microfabrics abstract superposition mainly of two eoalpine cretaceous deformation events can be observed in vertical sections and horizontal traverses throughout a decollement zone in the eastern alps consisting of crystalline basement and autochthonous and allochthonous cover stacked in several nappes the first event is an episode with a non coaxial strain history caused by w directed thrusting the second a n directed shortening by folding and fold imbrication strain analysis based both on deformed pebbles and on the preferred orientation of phyllosilicate grains march theory microfabric analysis and observations of the relative timing of deformation and metamorphism together indicate that the superposition of structures caused by the two events amounts to one continuous progressive act of deformation we attribute gradual and consistent variations in intensity and axial ratio of the tectonic strain to a history influenced by a the position of samples in the pile b the relative importance of the strain increments caused by the two events and c rock ductility we interpret variations of the c axis fabrics of quartz in the same way and draw tentative kinematic conclusions for the eoalpine orogeny in the eastern alps

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

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

comment on a numerical model of the cloud topped planetary boundary layer radiation turbulence and spectral microphysics in marine status by a bott t trautmann and w zdunkowski april a 1996 122 635 667 comment on a numerical model of the cloud topped planetary boundary layer radiation turbulence and spectral microphysics in marine status authors reply

reconstructing middle horizon mobility patterns on the coast of peru through strontium isotope analysis the pre columbian coastal site of ancon peru frequently has been interpreted as an important outpost of the highland wari empire inhabited by individuals from the sierra and the coast in order to test this hypothesis bone tooth pairs from 35 middle horizon 550 ad 1000 ad ancon skeletons were analyzed for strontium isotopes which vary by geologic provenance results indicate that 34 of the 35 individuals have 87 sr 86 sr enamel and bone values higher than ancon s biologically available strontium isotope range nitrogen and carbon isotope data from a subset of the ancon skeletons suggest that the higher than expected 87 sr 86 sr values among the ancon sample likely reflects a diet rich in marine resources rather than migratory activity and highlight the need to use multiple lines of evidence to track residence change at sites where individuals relied on resources other than locally grown terrestrial foods the one remaining individual a young female of elite status has an enamel strontium isotope signature much lower than the available data for local fauna and soil indicating that she was not raised locally her enamel 87 sr 86 sr ratio fits well within the range of strontium isotope values established for the inland wari site of conchopata tung t a knudson k j social identities and geographical origins of wari trophy heads from conchopata peru current anthropology in press and suggests that highland wari immigrants may have been present at ancon during the middle horizon

flow in natural shear zones the consequences of spinning flow regimes abstract the ancient flow regime in natural shear zones is often considered to have followed a deformation path comparable to that in theoretical shear zones i e progressive simple shear between rigid wall rocks with a persistent flow plane orientation parallel to the edges of the zone this is often based on the presence of monoclinic fabric elements in the zones which indicate a dominantly non coaxial flow regime though not necessarily persistent simple shear the deformation fabric of a shear zone from the pyrenees illustrates that even at a kinematic vorticity number of w 1 simple shear along the entire deformation path obliqueness of the flow planes with the edges of the zone is possible for some time if incremental stretching axes were rotating with respect to zone edges this implies that ductile deformation must have taken place in the wall rock of the zone such a flow regime may be difficult to recognize with the fabric criteria used at present but leads to an unusual kinematic significance of the shear zone involved in extreme cases the zone may have acted as a passive marker in a ductilely deforming medium it also means that not only w must be known to reconstruct the ancient flow regime in natural shear zones but at least also the time dependence of the orientation of the incremental stretching axes

deformation partitioning and porphyroblast rotation in meta morphic rocks a radical reinterpretation abstract most porphyroblasts never rotate during ductile deformation provided they do not intern ally deform during subsequent events with the exception of relatively uncommon but spectacular examples of spiralling garnets instead the sur rounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast consequently porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails even where these structures have been rotated or obliterated in the matrix due to subsequent deformation these relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subse quent deformation they can also be used to determine the deformation history as porphyro blasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component

foliation development and refraction in metamorphic rocks reactivation of earlier foliations and decrenulation due to shifting patterns of deformation partitioning reactivation of early foliations accounts for much of the progressive strain at more advanced stages of deformation its role has generally been insufficiently emphasized because evidence is best preserved where por phyroblasts which contain inclusion trails are present reactivation occurs when progressive shearing operating in a synthetic anastomosing fashion parallel to the axial planes of folds changes to a combination of coarse and fine scale zones of progressive shearing some of which operate antithetically relative to the bulk shear on a fold limb reactivation of earlier foliations occurs in these latter zones reactivation decrenulates pre existing or just formed crenulations generating shearing along the decrenulated or rotated pre existing foliation planes partitioning of deformation within these foliation planes such that phyl losilicates and or graphite take up progressive shearing strain and other minerals accom modate progressive shortening strain causes dissolution of these other minerals this results in concentration of the phyllosilicates in a simi lar but more penetrative manner to the for mation of a differentiated crenulation cleavage except that the foliation can form or intensify on a fold limb at a considerable angle to the axial plane of synchronous macroscopic folds reactivation can generate bedding parallel schistosity in multideformed and meta morphosed terrains without associated folds heterogeneous reactivation of bedding gen erates rootless intrafolial folds with sigmoidal axial planes from formerly through going struc tures reactivation causes rotation or refrac tion of axial plane foliations forming in the same deformation event causing reactivation in those beds or zones in which an earlier foli ation has been reactivated and results in destruction of the originally axial plane foli ation at high strains reactivation also provides a simple explanation for the apparently wrong sense but normally observed rotation of gar net porphyroblasts whereby the external foli ation has undergone rotation due to antithetic shear on the reactivated foliation alter natively the rotation of the external foliation can be due to its reactivation in a subsequent deformation event porphyroblasts with inclusion trails com monly preserve evidence of reactivation of earl ier foliations and therefore can be used to ident ify the presence of a deformation that has not been recognized by normal geometric methods because of penetrative reactivation reac tivation often reverses the asymmetry between pre existing foliations and bedding on one limb of a later fold leading to problems in the geo metric analysis of an area when the location of early fold hinges is essential the stretching lineation in a reactivated foliation can be rad ically reoriented potentially causing major errors in determining movement directions in mylonitic schistosities in folded thrusts geometric relationships which result from reactivation of foliations around porphyroblasts can be used to aid determination of the timing of the growth of porphyroblasts relative to deformation events other aspects of reac tivation however can lead to complications in timing of porphyroblast growth if the presence of this phenomenon is not recognized for example d grown porphyroblasts may be dis solved against reactivated s and hence appear to have grown syn d

ductile thrusting within subduction complex rocks on signy island south orkney islands abstract a model involving repeated nnw directed ductile thrusting in subduction complex rocks on signy island south orkney islands accounts for the consistent nnw trending alignment of fold axes stretching and shearing directions in the absence of major discrete thrust planes stratigraphic control or younging criteria the evidence for consistent ductile thrusting is more cryptic and is found in folding stretching relations porphyroblast rotations and mylonite fabrics early fold axes parallel to stretching syn tectonic garnet rotations augen lineations and sheath folds all support nnw directed thrusting as do a common late shear band foliation and minor shear zones development of late open folds with axes parallel to stretching may be due to constrictional deformation a lateral ramp or differential movement during thrusting the structures described from signy island may characterize the type of deformation in deep level ductile thrust zones at the base of an accretionary wedge or in a zone of subcretion the thrusting direction on signy island is inferred to be parallel to the direction of subduction in the south orkney islands in the early jurassic plate reconstructions suggest that this direction was at a high angle to the arcuate trend of the antarctic peninsula continental margin

porphyroblast inclusion trails the key to orogenesis detailed microstructural analysis of inclusion trails in hundreds of garnet porphyroblasts from rocks where spiral shaped inclusion trails are common indicates that spiral shaped trails did not form by rotation of the growing porphyroblasts relative to geographic coordinates they formed instead by progressive growth by porphyroblasts over several sets of near orthogonal foliations that successively overprint one another the orientations of these near orthogonal foliations are alternately near vertical and near horizontal in all porphyroblasts examined this provides very strong evidence for lack of porphyroblast rotation the deformation path recorded by these porphyroblasts indicates that the process of orogenesis involves a multiply repeated two stage cycle of 1 crustal shortening and thickening with the development of a near vertical foliation with a steep stretching lineation followed by 2 gravitational instability and collapse of this uplifted pile with the development of a near horizontal foliation gravitational spreading near coaxial vertical shortening and consequent thrusting on the orogen margins correlation of inclusion trail overprinting relationships and asymmetry in porphyroblasts with foliation overprinting relationships observed in the field allows determination of where the rocks studied lie and have moved within an orogen this information combined with information about chemical zoning in porphyroblasts provides details about the structural metamorphic p t t paths the rocks have followed the ductile deformation environment in which a porphyroblast can rotate relative to geographic coordinates during orogenesis is spatially restricted in continental crust to vertical ductile tear transcurrent faults across which there is no component of bulk shortening or transpression

porphyroblast nucleation growth and dissolution in regional metamorphic rocks as a function of deformation partitioning during foliation development in regional metamorphic rocks the partitioning of deformation into progressive shearing and progressive shortening components results in strain and strain rate gradients across the boundaries between the partitioned zones these generate dislocation density gradients and hence chemical potential gradients that drive dissolution and solution transfer phyllosilicates and graphite are well adapted to accommodating progressive shearing without necessarily building up large dislocation density gradients within a grain because of their uniquely layered crystal structure however most silicates and oxides cannot accommodate strain transitions within grains without associated dislocation density gradients and hence are susceptible to dissolution and solution transfer as a consequence zones of progressive shearing become zones of dissolution of most minerals and of concentration of phyllosilicates and graphite exceptions are mylonites where strain rates are commonly high enough for plastic deformation to dominate over diffusion rates and therefore over dissolution and solution transfer porphyroblastic minerals cannot nucleate and grow in zones of active progressive shearing as they would be dissolved by the effects of shearing strain on their boundaries however they can nucleate and grow in zones of progressive shortening and this is aided by the propensity for microfracturing in these zones which allows rapid access of fluids carrying the material presumed to be necessary for nucleation and growth zones of progessive shortening also have a number of characteristics that help to lower the activation energy barrier for nucleation this includes a build up of stored strain energy relative to zones of progressive shearing in which dissolution is occuring porphyroblast growth is generally syndeformational and previously accepted criteria for static growth are not valid when the role of deformation partitioning is taken into account porphyroblasts in a contact aureole do not grow statically either as microfracturing associated with emplacement allows access of fluids in a fashion that is similar to microfracturing in zones of progressive shortening the criteria used for porphyroblast timing can be readily accommodated in terms of deformation partitioning reactivation of deforming foliations and a general lack of rotation of porphyroblasts with the spectacular exception of genuinely spiralling garnet porphyroblasts

solution cleavage and its relationship to major structures in the idaho utah wyoming thrust belt micrites of the jurassic twin creek formation in the idaho utah wyoming thrust belt show spaced cleavage developed during folding and thrusting cleavage in each thrust sheet forms mainly by pressure solution aided by elevated temperatures and pressures caused by increased overburden as a result of emplacement of earlier overriding thrust sheets within each thrust sheet cleavage develops early and is contemporaneous with microscopic to mesoscopic buckle folds and faults large scale ramp folds formed during major motion on the thrust postdate the cleavage and passively rotate it into major fans

sequences of mesostructures related to the development of alpine thrusts in the eastern pyrenees abstract the mesostructures formed during the development of an antiformal stack in the eastern pyrenees are studied the following structural evolution is proposed a pressure solution cleavage develops by layer parallel shortening at initial stages of the thrusting the cleavage is present in all units but mainly in the marls and marly limestones in the siliceous layers a weak cleavage and small scale thrust structures develop the cleavage and the bedding are deformed during the thrusting different structures form in various places of the antiformal stack where the bedding is flat lying southward directed layer parallel shears predominate in the hangingwall ramps kink bands disjunctive stylolitic cleavage and layer parallel shear opposite to the sense of thrust movement develop the described structures constitute a kinematical model for mesostructures developed during the evolution and ramping of thrusts

structures and fabrics in a crustal scale shear zone betic cordillera se spain abstract a broad zone of dominantly ductile high strain deformation lies beneath the aguilon nappe in the sierra alhamilla southern spain it forms part of a crustal scale movement zone traceable through much of the betic cordillera which separates the higher betic nappes from the underlying nevado filabride complex the zone is characterized in outcrop by a distinctive platy foliation and a strong nne trending stretching lineation microstructural characteristics include quartz ribbons mica fish augen of feldspar and other minerals in a matrix of dynamically recrystallized quartz and extensional crenulation cleavages narrow bands of ultramylonite and cataclasite occur within and on the margins of the movement zone deformation occurred under lower greenschist facies conditions and was accompanied by retrogression of earlier higher grade mineral assemblages structures in the movement zone developed in a temporal sequence beginning with isoclinal folding and transposition of older foliations this was followed by the formation of extensional crenulation cleavages and the progressive localization of strain into the ultramylonite bands mylonitic foliation in these bands is deformed by syn mylonite folds restricted to the bands all these structures were then deformed by s to se vergent small scale folds restricted to the movement zone as a whole cataclasis associated with alteration is localized along the ultramylonite bands and indicates a transition to late stage brittle deformation the lower boundary of the movement zone is gradational strain decreases recrystallized grainsize and the degree of recrystallization of quartz increases and pressure solution becomes the dominant deformation mechanism in mica schist asymmetric quartz fabrics in the movement zone indicate a nne sense of shear but variations in the degree of asymmetry suggest that flow was partitioned with the ultramylonite bands taking up much of the shear strain and the intervening rocks deforming more slowly and with a lower degree of non coaxiality diffuse fabrics in the fine grained ultramylonite bands may indicate a switch to a grainsize sensitive deformation mechanism and an overall downward increase in the opening angle of crossed girdle fabrics may reflect increased water activity at depth

the structure and tectonic evolution of the aguilon fold nappe sierra alhamilla betic cordilleras se spain abstract detailed structural work in the sierra alhamilla se spain shows that the aguilon nappe comprising triassic and older metasedimentary rocks is a fold nappe the most prominent set of small scale folds changes from dominantly n vergent in the upper right way up limb to s vergent in the greatly thinned lower limb the nappe closes to the north and must have been emplaced in this direction nappe formation was accompanied by small scale folding and extensive solution transfer producing a pronounced differentiated crenulation cleavage these structures overprint an earlier set of folds and cleavage and are overprinted in turn by late n vergent structures the lower limb of the nappe was thinned and disrupted during continued nappe transport pre triassic schist in the core of the nappe was affected by medium grade metamorphism of probable post triassic age the contact with low grade permo triassic sediments above and below the schist coincides with a distinct change in metamorphic grade this contact may be a post metamorphic extensional fault that is now folded around the nappe which suggests that nappe formation was preceded by extensional faulting this is consistent with gravity spreading as the driving process for nappe emplacement

p t paths from garnet zoning a new technique for deciphering tectonic processes in crystalline terranes a procedure to calculate quantitative rock pressure temperature p t paths based on chemical zoning profiles in garnet combined with mineral chemistry from the other minerals in the assemblage has been applied to samples from diverse tectonic settings with the result that the calculated p t path is shown to be a sensitive monitor of tectonic processes terranes where metamorphic recrystallization paths are controlled by uplift and erosion show heating during decompression in the prograde p t path and cooling during decompression in the retrograde path in nappe terranes where hot rocks are emplaced over cool rocks the upper plate shows cooling during decompression whereas the lower plate shows heating during compression in nappe terranes where cool rocks are emplaced over hot the lower plate shows cooling during compression calculation of p t paths from mineral zoning should provide a powerful new tool for deciphering tectonic processes in crystalline terranes

secondary cleavages in ductile shear zones abstract secondary cleavages developed at late stages in ductile shear zones show several features that are inconsistent with progressive simple shear in the zone these are the orientation of a single secondary cleavage oblique to the shear zone boundaries conjugate sets with opposite senses of shear and multiple sets with the same sense of shear these features can be explained if the bulk flow is partitioned into slip along discrete failure planes parallel to the primary foliation s coaxial stretching along the foliation and spin

progressive refolding in ductile shear zones abstract the foliation formed in a ductile shear zone can become folded by continuing shear in the zone without the foliation having to enter the shortening sector of the flow field if the foliation lies parallel to the shear plane infinitesimal variations in the rate of shear strain cause compensating rotations that may amplify into folds small increases in the rate cause backward rotations counter to the sense of shear these do not amplify small decreases cause forward rotations that do amplify

horizontal cleavage in southeastern sinai the case for a coaxial strain history abstract a horizontal cleavage and associated lineation are developed in a low pressure igneous related metamorphic terrain in southestern sinai the cleavage is axial planar to recumbent folds which are never large and varies from a smooth slaty cleavage to a discrete crenulation cleavage structural evidence on the macro meso and microscales suggests that cleavage and lineation were formed during irrotational extension and not by simple shear estimates of strain using the march method indicate 50 70 vertical shortening this structural evidence when combined with metamorphic and age data strongly suggests that the cleavage was formed by the forceful emplacement of a pluton at depth

collision tectonics in the cenozoic orogenic zone bordering china india and burma abstract the southeast region of the qinghai tibetan plateau is the region where the indian subcontinent collided with eurasia to form arcuate mountain chains along what is now the border area of china india and burma on the basis of cenozoic deformation metamorphism and magmatic activity we can divide this orogenic zone into three tectonic sections the external medial and internal arcuate belts these in turn are divided into seventeen major nappes delineated in a sw to ne cross section through the orogenic zone the external belt composed of the western nappes contains traces of late mesozoic and cenozoic sutures which have undergone significant disruption due to cenozoic orogenesis and its paleozoic strata show affinities to the gondwanaland facies the medial belt including most notably the nappes of the jinsha river region contains the remains of an ancient ocean very probably the paleo tethys which separated the gondwana and yangzi paleolandmasses in late paleozoic early triassic time the internal belt composed of the eastern nappes consists mainly of flysch from china s largest triassic sedimentary basin and its paleozoic strata belong to the yangzi plate

fluid heterogeneities and hornblende stability in interlayered graphitic and nongraphitic schists tauern window eastern alps the assemblage hornblende white mica occurs in graphite free schists at two localities in the southwest corner of the tauern window eastern alps in interbedded graphitic layers 1 mm to 1 m thick however hornblende is typically replaced by pseudomorphs of biotite plagioclase epidote chlorite staurolite in the presence of white mica garnets adjacent to these pseudomorphs have pronounced growth discontinuities near their rims in contrast to the continuously zoned garnets in nongraphitic layers these observations imply that reactions of the type hbl white mica gar bio plag epid chl staur h2o occurred in the graphitic samples but that hbl white mica remained stable in graphite free layers calculation of the equilibrium constants for solid phases in five dehydration equilibria at locality 1 indicates thata h2o in the nongraphitic layers was 6 to 11 times greater thana h2o in the graphitic layers similar calculations involving six dehydration equilibria at locality 2 show no difference ina h2o between layers at the conditions of final equilibration initial differences in fluid composition maintained between the graphitic and nongraphitic layers caused the hbl white mica reaction to occur at differentp t conditions in different horizons of the schists these data indicate that systematic differences in fluid composition were generated during metamorphism of the interlayered graphitic and non graphitic schists but were subsequently homogenized at locality 2 the heterogeneities could initially have been produced while the rocks were in thep t field of co2 h2o immiscibility development of a penetrative layer parallel shear foliation at this time would have prevented subsequent mixing of the fluids across layers after temperatures exceeded the consolute temperature in the co2 h2o system late stage homogenization of fluids at locality 2 is thought to reflect loss of the buffer capacity of the mineral assemblage in response to total consumption of hornblende

deformation and metamorphism on the east side of the green mountain massif in southern vermont an early deformation in the jamaica vermont area on the east side of the green mountain massif produced kilometre scale north plunging f1 folds overturned to the west with an axial planar foliation s1 motion on two major thrust faults juxtaposed three similar but distinctive cover rock sequences after or late in f1 folding smaller scale f2 folds deformed f1 fold limbs and the thrust faults and have an axial planar crenulation cleavage s2 in the structurally higher central and eastern cover rock sequences an early stage of garnet growth began late in s1 development or after it a retrogression partially resorbed first stage garnet and was followed by a second prograde stage of garnet growth late in or after s2 development this metamorphic history resulted in distinctive textural unconformities and zoning anomalies in garnet the western cover rock sequence lacks evidence for a major retrogression f1 folding and f2 folding approximately bracket both the motion on the thrust faults and the retrogression the second prograde metamorphism and the younger deformation are acadian tentative correlation of the first garnet growth stage with ordovician metamorphism recognized in western new england and of motion on the thrust faults with taconian synmetamorphic thrusts in the region suggests that these early thermal and tectonic events occurred in the ordovician the retrogression recorded in the central and eastern cover sequences furthermore may reflect thrusting of these rocks to a structurally higher colder environment followed by reburial and later prograde metamorphism

kinematics of ductile shearing from outcrop to crustal scale in the monte rosa nappe western alps significant ductile deformation has produced widespread mylonitic structures in the basement orthogneisses and the mesozoic cover quartzites and marbles of the monte rosa nappe in the inner part of the western alps we summarize here the results of a detailed microstructural study at several scales of observation in the northern part of the nappe and discuss the kinematic significance and compatibility of such results most of the observed deformations can be accounted for by progressive wnw vergent shearing during and after the late eocene within a ductile shear zone of crustal scale the displacement of the hanging wall may have been in excess of several tens of kilometers however some ese vergent shear criteria cannot be reconciled with this general picture and suggest that local backthrusting occurred later near the toe of the monte rosa nappe

sequential porphyroblast growth and crenulation cleavage development during progressive deformation abstract six stages of crenulation cleavage development during a single deformation event d2 can be recognised in both matrix and porphyroblast inclusion trails in the robertson river formation n e australia these stages progress from undeformed to crenulation and differentiated crenulation of s1 through development of differentiated crenulation cleavage s2 and differentiated schistosity s2 to homogeneous foliation s2 these rocks underwent prograde metamorphism during d2 and chloritoid garnet staurolite andalusite and sillimanite isograds have been mapped most rocks are rich in porphyroblasts which commonly contain well defined inclusion trails the geometry of these trails varies from one mineral type to the next depending on the timing of porphyroblast growth relative to the stage of crenulation cleavage development in the schistose matrix the deformation history involved progressive bulk inhomogeneous shortening and the strain is very heterogeneous on all scales partly as a consequence of this hence each of the six stages of crenulation cleavage development formed locally at various times during d2 therefore if a porphyroblast grew late in the deformation it could overgrow all stages of crenulation cleavage development including the very early ones the temperature increased during metamorphism in any one locality and consequently some porphyroblasts overgrew others that were unstable in the new conditions preserving them from destruction hence the precise timing of mineralogical reactions deduced from isograds and mineral chemistry relative to the stages in schistosity development can also be determined in the andalusite zone the sequence of porphyroblasts in decreasing order of age is biotite garnet biotite staurolite andalusite dissolution of garnet occurred during growth of staurolite and especially andalusite and dissolution of staurolite accompanied andalusite growth

on the internal geometry of mylonite zones mylonite zones are generally characterized by abrupt and very large strain transitions which commonly result in excessively anastomosing schistosities on a wide range of scales when compared with nonmylonitic foliations this geometry is very susceptible to remodification during progressive mylonitization resulting in unusual and complex fold lineation and foliation geometries and interrelationships open folds of the mylonitic foliation with axes parallel to the stretching lineation in the surrounding mylonite cannot have formed by the rotation of fold axes through a large angle within their axial planes as has been usually proposed for isoclinal and sheath folds in mylonitic zones open folds initiate with axes parallel or close to the stretching lineation due to the geometric effects of folding a mylonitic foliation which anastomoses around an ellipsoidal pod of less deformed material this initial geometry also allows the generation of fold axes curved within their axial plane through 1800 about the stretching lineation at the time of nucleation successive mylonitic foliations develop during this folding and refolding process with boundaries that truncate and isolate earlier fold hinges and portions of fold limbs as a consequence stretching and intersection lineations can vary from plane to plane through the mylonite zone although careful examination often reveals a weak overprinting stretching lineation parallel to the bulk movement direction for the whole zone fold asymmetry in mylonite zones is a potential indicator of shear sense across a zone if the fold axes lie at an angle to the bulk stretching lineation direction in such circumstances however a single asymmetry projected onto a plane perpendicular to the mylonitic foliation and containing the bulk stretching lineation can indicate either sense of shear depending on a variety of factors these include whether the foliation folded is primary or mylonitic and in the latter case whether the mylonite zone formed with a steep dip and horizontal stretching lineation or in some other orientation the most satisfactory sense of shear indicator is the asymmetry of s and c planes

lack of porphyroblast rotation in noncoaxially deformed schists from petrel cove south australia and its implications abstract inclusion trails representing an s 1 cleavage demonstrate the lack of porphyroblast rotation during subsequent highly non coaxial deformations the pelitic schists of the kanmantoo group at petrel cove south australia contain two generations of porphyroblasts the first one consists of cordierite porphyroblasts that formed early in d2 and contain straight to slightly sigmoidal inclusion trails of s 1 the second generation consists of andalusite porphyroblasts that overgrew crenulated s 2 late during d3 several hundred inclusion trail traces from cordierites measured from oriented specimens taken throughout a strongly folded area show a horizontal great circle distribution when plotted and contoured on a stereographic projection hence s 1 was planar and horizontal prior to d2 s 1 measurements on limbs and hinges of a mesoscale d2 foldpair show that folding had little effect on porphyroblast orientation as the s 1 orientation remained constant and subhorizontal around the fold hence porphyroblasts have not rotated during any of the non coaxial deformations accompanying and following their growth this is interpreted as a result of the partitioning of the deformation around them s 1 in the matrix has been totally destroyed by the formation of s 2 as a fully differentiated crenulation cleavage what has previously been regarded as inconsistent senses of shear recorded by porphyroblasts around folds is resolved by the fact that the matrix foliation rotated rather than the porphyroblasts due to the effects of deformation partitioning the presence of a subhorizontal s 1 foliation suggests horizontal movements e g thrusting or detachment faulting during the earliest phase of the adelaidian orogeny

zongo san gaban zone eocene foreland boundary of the central andean orogen northwest bolivia and southeast peru discordant muscovite and biotite k ar dates of samples taken in six transects through the cordillera oriental of southeast peru and northwest bolivia combined with sup 40 ar sup 39 ar age spectra reveal a northwest trending structurally cryptic tectonothermal zone of late eocene age ca 38 ma that overprints triassic and older igneous and metamorphic rocks the zone is at least 450 km long but is less than 25 km wide temperatures along its northeast boundary are estimated to have reached 350 sup 0 c no paleogene magmatism has been recognized that could account for the thermal resetting of the k ar isotopic system uplift erosion and southwest verging thrusting accompanied the development of this domain which defined the foreland boundary of the orogen prior to inception of northeast verging andean thrusting in miocene time it is proposed that this tectonothermal zone the zongo san gaban zone zsgz which involves thrusting and basement shortening is the southerly manifestation of the incaic compressional deformation which produced the thin skinned maranon thrust and fold belt mtfb north of the abancay deflection of central peru