proterozoic mafic ultramafic intrusions in the arunta region central australia part 2 event chronology and regional correlations abstract the arunta region in central australia records multiple proterozoic crustal processes over a 1500 million year period the timing of mafic ultramafic magmatism in this evolution is constrained by shrimp u pb dating of zircon which is a primary phase in mafic ultramafic intrusions across the region the earliest mafic magmatism was part of the 1810 1800 ma stafford event and a second episode is recognised during the 1790 1770 ma yambah event zircon overgrowths in some of these early intrusions record metamorphism during the strangways event a regionally pervasive metamorphic episode whose termination at ca 1690 ma coincided with intrusion of dolerite dykes gabbro intrusion during the ca 1640 ma liebig event is related to the geologically distinct warumpi province in the south of the region there is no record of mafic magmatism during the ca 1590 ma chewings event later systems include the ca 1130 ma alkaline ultramafic mordor complex and basaltic bodies are components of the irindina province which experienced high grade metamorphism during the palaeozoic the dating associates mafic ultramafic magmatism with each of these events which suggests that repeated extensional systems are an important aspect of the tectonic evolution the example of routinely obtaining zircon for dating from mafic and even ultramafic intrusions using an integrated approach of field and geochemical criteria to guide sampling opens the possiblility of dating mafic ultramafic intrusions elsewhere direct dating of mafic ultramafic magmatism permits its significance in proterozoic event systems to be evaluated directly free of assumed or conjectural timing correlations with the felsic plutons that are more often studied for age control

textural map units in quartzo feldspathic mylonitic rocks the classical macrotextural subdivision of quartzo feldspathic mylonitic rocks yields only three rock types protomylonite mylonite and ultramylonite this restriction impedes detailed mapping of the internal textural transitions common in wide deep seated crustal scale shear zones where such transitions may occur over kilometres and involve several clearly mappable textural types the introduction of two objectively defined field mapping terms homoclastic and heteroclastic describing the macroscopic grain size distribution within the porphyroclast population provides descriptive flexibility without changing the matrix porphyroclast basis of the established classification this allows the description of textural paths other than protomylonite mylonite ultramylonite and facilitates the consideration of textural paths in terms of strain partitioning between the constituent grains of the deforming aggregate rather than as a simple function of finite strain

flow laws for rocks containing two non linear viscous phases a phenomenological approach abstract mylonitic rocks and rock analogue materials reveal two basic types of structure 1 a load bearing framework lbf of strong phase contains isolated pockets of weak phase 2 interconnected layers of weak phase iwl separate boudins and clasts of strong phase aggregates with the lbf microstructure are characterized by nearly uniform strain rate stress is concentrated in the load bearing framework in aggregates with an iwl microstructure strain rate and sometimes also stress are higher in the interconnected weak phase than in the boudins and clasts of strong phase the degree of stress and strain partitioning depends strongly on the viscous strength contrast and on the relative amounts of the constituent mineral phases based on these observations the rheology of two phase rock is modelled with separate functions for lbf and iwl microstructures a new flow law is derived for rock with iwl structure in which two phases undergo dislocation creep the flow law expresses composite creep strength in terms of temperature bulk strain rate and the volume proportions and creep parameters of the minerals in the rock strain rate and stress are averaged in the constituent phases and slip along phase boundaries maintains strain compatibility within the aggregate composite strengths predicted with the iwl flow law fall well within the uniform stress and uniform strain rate bounds and are generally consistent with the viscous strengths of experimentally deformed bimineralic aggregates a hypothesis of viscous strain energy minimization is used to determine the relative stability of the lbf and iwl microstructures during steady state creep the iwl microstructure is predicted to be stable over a broad range of two phase compositions and mineral strength contrasts whereas the lbf microstructure is stable only in rocks with low volume proportions of weak phase and low to moderate mineral strength contrasts the iwl flow law indicates that rheological stratification in the lithosphere depends strongly on rock composition especially in rocks with low volume proportions of a weak phase and high mineral strength contrasts

striding athabasca mylonite zone complex archean deep crustal deformation in the east athabasca mylonite triangle northern saskatchewan the geophysically defined snowbird tectonic zone is manifested in northernmost saskatchewan as a deep crustal multistage mylonitic structure the east athabasca mylonite triangle the triangle located at the northeastern apex of a stiff crustal scale lozenge is composed of mid archean annealed mylonites and late archean ribbon mylonites formed during two granulite facies events 850 1000 c 1 0 gpa the flow pattern in the mylonites is geometrically and kinematically complex and corresponds to that expected adjacent to the apex of a stiff elliptical volume subjected to subhorizontal regional extension parallel to its principal axis the late archean mylonites are divided into an upper structural deck entirely occupied by a dip slip shear zone and an underlying lower deck the latter is divided into two upright conjugate strike slip shear zones separated by a low strain septum which deformed by progressive coaxial flow the flow pattern in the mid archean mylonites is compatible with that of

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

a numerical study of the stratiform region of a fast moving squall line part ii relationship between mass pressure and momentum fields abstract in a companion paper a two dimensional simulation of a fast moving tropical squall line was successfully compared to observations performed during the copt81 experiment over west africa the full ice phase parameterization is shown to be crucial in the simulation of trailing anvil precipitation different diagnostic tools are applied to the simulated fields to further our understanding of the scale interactions within a squall line type mesoscale convective system the pressure organization is characterized by two marked features important for explaining the inner circulation first a front to rear midlevel pressure gradient and second the surface pressure mesohigh extending from the gust front to the rear of the most active part of the trailing stratiform region based on the hydrostatic approximation an original method of decomposition of the pressure field is proposed whereby dynamical and buoyant contributions depend only on the horizontal and vertical respectively the mean pressure i

the snake range decollement an exhumed mid tertiary ductile brittle transition the snake range decollement srd in east central nevada separates supracrustal rocks extended by normal faulting from ductilely deformed igneous and metamorphic rocks a well known stratigraphy unaffected by earlier faulting permits analysis of both upper and lower plate strain leading to a better understanding of how vastly different rock types and deformational styles are juxtaposed along low angle faults in metamorphic core complexes middle cambrian to permian upper plate rocks are cut by two generations of ne trending east directed normal faults both generations were initiated as high angle 60 planar faults that flattened abruptly into the srd and rotated domino style to low angles yielding a total rotation of bedding of about 80 90 faulting is tertiary in age as 35 m y old volcanic rocks are involved and resulted in about 450 500 extension in a n55w s55e direction the srd developed as a subhorizontal surface 6 7 km deep at the top of the cambrian pioche shale lower plate granitic rocks and their late precambrian cambrian metamorphic country rocks were involved in progressive ductile to brittle extension at low greenschist grade forming a penetrative subhorizontal foliation and n55 70w lineation that increases in intensity eastward and upward toward the srd stretching and thinning in the lower plate is coaxial and comparable in magnitude to upper plate extension and is interpreted as synchronous k ar ages ranging from 20 to 40 m y in the lower plate suggests the n snake range represents a tertiary thermal anomaly we conclude that the srd developed as a ductile brittle transition zone at 6 7 km depth gravity data suggests that the gently domed srd is cut by younger basin and range faults but the geology of adjacent ranges suggests that the srd does not continue more than 60 km in any given direction the lack of stratigraphic omission across the srd rules out large amounts of movement on a surface that originally cut downsection and we suggest that extensional detachment faults such as the srd can be developed locally as boundaries between brittlely extended rocks and underlying ductile extension and intrusion

progressive development of structures in a ductile shear zone in the singhbhum shear zone of eastern india successive generations of folds grew in response to a progressive ductile shearing during this deformation a mylonitic foliation was initiated and was repeatedly transposed the majority of fold hinges were formed in an arcuate manner at low angles to the y axis in an e w trending subhorizontal position and major segments of the fold hinges were then rotated towards the down dip northerly plunging x axis the striping and intersection lineations were rotated in the same manner the down dip mylonitic lineation is a composite structure represented by rotated early lineations and newly superimposed stretching lineations the consistent asymmetry of the folds the angular relations between c and s surfaces and the evidence of two dimensional boudinage indicate that the deformation was non coaxial but with a flattening type of strain with 1 2 the degree of non coaxiality varied both in space and time from the progressive development of mesoscopic structures it is concluded that the 2 3 km wide belt of ductile shear gave rise to successive anastomosing shear zones of mesoscopic scale when a new set of shear lenses was superimposed on already sheared rocks the preexisting foliation generally lay at a low angle to the lenses no new folds developed where the acute angle was sympathetic to the sense of shear displacements where the acute angle was counter to the sense of shear the pre existing foliation lying in the instantaneous shortening field was deformed into a set of asymmetric folds

strain geometry microstructure and mineral chemistry in metagabbro shear zones a study of softening mechanisms during progressive mylonitization abstract the strain geometry microstructure and metamorphism is described from two minor shear zones from the chatelaudren metagabbro n brittany a serially slabbed shear zone reveals a strain geometry consistent with simple shear deformation strain calculations based on x trajectory angles coincide with those obtained from elliptical mineral clusters at high values of strain only strain profiles typically show a broad low strain region with a narrow high strain peak at the centre of the zones microstructures typically show distinct grain size reduction in both amphibole and feldspar towards the high strain region of shear zones and this is discussed in terms of deformation mechanisms related to strain softening a palaeostress estimate based on recrystallized feldspar grain sizes gives a differential stress of 32 mpa for the low strain region and 119 mpa in the shear zone centre electron probe analyses reveal chemical and mineralogical changes accompanying metamorphism within the shear zones this suggests local conditions favourable for ionic diffusion and the activity of fluids is implied

microstructural and crystal fabric evolution during shear zone formation abstract the microstructures and crystal fabrics associated with the development of an amphibolite facies quartzo feldspathic mylonitic shear zone torridon nw scotland have been investigated using sem electron channelling our results illustrate a variety of microstructures and fabrics which attest to a complex shear zone deformation history microstructural variation is particularly pronounced at low shear strains significant intragranular deformation occurs via a domino faulting style process whilst mechanical incompatibilities between individual grains result in characteristic grain boundary deformation accommodation microstructures a sudden reduction in grain size defines the transition to medium shear strains but many of the boundaries inherited from the original and low shear strain regions can still be recognized and define distinctive bands oriented at low angles to the shear zone margin grains within these bands have somewhat steeper preferred dimensional orientations these domains persist into the high shear strain mylonitic region where they are oriented subparallel to the shear zone margin and consist of sub 20 m grains the microstructures suggest that the principal deformation mechanism was intracrystalline plasticity with contributions from grain size reduction via dynamic recrystallization grain boundary migration and grain boundary sliding crystal fabrics measured from the shear zone vary with position depending on the shear strain involved and are consistent with the operation of several crystal slip systems e g prism basal rhomb and acute rhomb planes in a consistent direction probably parallel to a and or m they also reveal the presence of dauphine twinning and suggest that this may be a significant process in quartz deformation a single crystal fabric evolution path linking the shear zone margin fabric with the mylonitic fabric was not observed rather the mylonitic fabric reflects the instantaneous fabric which developed at a particular location for a particular shear strain and original parental grain orientation the mature shear zone therefore consists of a series of deformed original grains stacked on top of each other in a manner which preserves original grain boundaries and intragranular features which develop during shear zone evolution the stability of some microstructures to higher shear strains the exploitation of others at lower shear strains and a continuously evolving crystal fabric mean that the strain gradient observed across many shear zones is unlikely to be equivalent to a time gradient

the structure and classification of numerically simulated convective storms in directionally varying wind shears abstract using a three dimensional numerical cloud model we investigate the effects of directionally varying wind shear on convective storm structure and evolution over a wide range of shear magnitudes as with a previous series of experiments using unidirectional wind shear profiles weisman and klemp the current results evince a spectrum of storm types ranging from short lived single cells at low shears multicells at intermediate shears to supercells at high shears with a clockwise curved hodograph the supercellular growth is confined to the right flank of the storm system while multicellular growth is favored on the left flank an analysis of the dynamic structure of the various cells reveals that the quasi steady supercell updrafts are strongly enhanced by dynamically induced pressure gradients on the right flank of the storm system we use this feature along with other related storm characteristics such as updraft rotation to propose a dynamically based storm classification scheme following

planar non planar and refolded sheath folds in the phulad shear zone rajasthan india abstract progressive ductile shearing in the phulad shear zone of rajasthan india has produced a complex history of folding with development of planar non planar and refolded sheath folds there are three generations of reclined folds f 1 f 2 and f 3 with a striping lineation l 1 parallel to the hinge lines of f 1 the planar sheath folds of f 1 have long subparallel hinge lines at the flanks joining up in hairpin curves at relatively small apices l 1 swerves harmoniously with the curving of f 1 hinge line there is a strong down dip mineral lineation parallel to the striping lineation in most places but intersecting it at apices of first generation sheath folds both the striping and the mineral lineation are deformed in u patterns over the hinges of reclined f 2 and f 3 folding of axial surfaces and hinge lines of earlier reclined folds by later folds was accompanied by very large stretching and led to the development of non planar sheaths the reclined folds of all the three generations were deformed by a group of subhorizontal folds each generation of fold initially grew with the hinge line at a very low angle with the y axis of bulk non coaxial strain and was subsequently rotated towards the down dip direction of maximum stretching the patterns of deformed lineations indicate that the stretching along the x direction was extremely large much in excess of 6000 percent

a numerical investigation of the organization and interaction of the convective and stratiform regions of tropical squall lines abstract a set of 13 two dimensional numerical simulations based on the 22 and 23 june soundings from the convectionprofonde tropicale in 1981 copt81 experiment in west africa is used to study the organization and interactionof the convective and stratiform regions of squall line type convective systems the initial wind profiles arecharacterized by the african easterly jet aej and the tropical easterly jet tej located at about 3 5 km and14 km respectively the physical processes that generate and maintain the mesoscale inflow at the rear of squall line type mesoscaleconvective systems are thereby examined horizontal potential temperature gradients generated by a combinationof latent heat release in the convective region and unsaturated mesoscale descent both modulated by evaporation cause a horizontal pressure gradient and generate horizontal line parallel vorticity the rear inflow is a consequenceof these processes the convective activity induces a significant upscale influence ahead of the

deformation and recrystallization of plagioclase along a temperature gradient an example from the bergell tonalite abstract syn to post mylonitic tilting of the bergell tonalite allows investigation of the deformation and recrystallization of plagioclase grains along a temperature gradient from the lower to the upper amphibolite facies at the lowest temperatures recrystallization occurs by nucleation and growth of new grains having a different composition from the old grains in contrast at the highest temperatures recrystallization occurs by subgrain rotation associated with grain boundary migration the temperature increase is inferred to induce this transition in two ways first at higher temperatures the anorthite content of the new grains is higher and thus the comositional difference between old and new grains is lower hence the chemical driving potential is small compared with recrystallization at lower temperatures second higher temperatures facilitate climb of dislocations and thus subgrain formation as well as grain boundary mobility the temperature increase is also associated with a transition in the dominant deformation mechanism intracrystalline plasticity dominates deformation at lower temperatures whereas at higher temperatures deformation occurs primarily by diffusion accommodated grain boundary sliding as indicated by the weakening of the indicatrix preferred orientation and the formation of a mixed plagioclase biotite matrix with increasing deformation therefore the strength of the mid to lower crust may be overestimated by dislocation creep flow laws for plagioclase lower crust rheology for polyphase rocks is better approximated by constitutive relationships involving diffusion accommodated grain boundary sliding

deep crustal shear zones high grade tectonites and associated metasomatic alteration in the limpopo belt south africa implications for deep crustal processes large thrust sheets of complexly folded d 1 migmatitic granulites bounded by a regional system of anastomosing ductile shear zones d 2 characterize the southern marginal zone of the limpopo belt tectonites within these shear zones are subdivided into primary straight gneisses with preserved textures that can unambiguously be referred to the d 2 exhumation stage annealed straight gneisses that recrystallized during post d 2 cooling and classic mylonites that post date the limpopo orogeny primary straight gneisses within these shear zones contain textural and mineralogical features that suggest they developed by a shear related process of grain size reduction and fabric formation due to decreasing pressure and temperature under granulite facies conditions this process is evident from the synkinematic replacement of garnet by cordierite and hypersthene symplectites the resultant asymmetric sigmoidal corona textures enhance the shear fabric in contrast pelitic granulite

the dependence of numerically simulated convective storms on vertical wind shear and buoyancy abstract the effects of vertical wind shear and buoyancy on convective storm structure and evolution are investigated with the use of a three dimensional numerical cloud model by varying the magnitude of buoyant energy and one directional vertical shear over a wide range of environmental conditions associated with severe storms the model is able to produce a spectrum of storm types qualitatively similar to those observed in nature these include short lived single cells certain types of multicells and rotating supercells the relationship between wind shear and buoyancy is expressed in terms of a nondimensional convective parameter which delineates various regimes of storm structure and in particular suggests optimal conditions for the development of supercell type storms applications of this parameter to well documented severe storm cases agree favorably with the model results suggesting both the value of the model in studying these modes of convection as well as the value of this representation i

microfabric study on the deformational and thermal history of the alpi apuane marbles carrara marbles italy marbles from different geometrical and structural positions within the alpi apuane metamorphic complex show a large variability in microfabric types as indicated by microstructure c axis orientation and temperature analysis statically recrystallized samples showing a granoblastic microstructure and polygonal grain boundaries are characterized by a grain size variation from east to west from 80 100 m to 250 300 m this is correlated with an equilibration calcite dolomite temperature from 360 380 c to 420 430 c two kinds of dynamically recrystallized microstructures have been investigated a first one exhibiting coarse grains 150 200 m with lobate grain boundaries and a strong shape preferred orientation and a second one characterized by a smaller grain size 40 50 m and predominantly straight grain boundaries these microstructural types associated with localized post thermal peak shear zones and meter to kilometer scale folds are interpreted as related to high strain and high temperature crystal plastic deformation mechanisms dislocation creep associated with predominant grain boundary migration type b1 or subgrain rotation recrystallization type b2 these differences in dynamically recrystallized microstructures are related to equilibration temperatures higher in type b1 390 c than in type b2 370 340 c we have been able to relate the development of the different microfabric types to the successive stages of deformation of the alpi apuane metamorphic complex

estimation of site response in kachchh gujarat india region using h v spectral ratios of aftershocks of the 2001 mw 7 7 bhuj earthquake site response in the aftershock zone of 2001 bhuj mw 7 7 earthquake has been studied using the h v spectral ratio method using 454 aftershocks mw 2 5 4 7 recorded at twelve three component digital strong motion and eight three component digital seismograph sites the mean amplification factor obtained for soft sediment sites quaternary tertiary varies from 0 75 6 03 times for 1 3 hz and 0 49 3 27 times for 3 10 hz the mean amplification factors obtained for hard sediment sites hard jurassic mesozoic sediments range from 0 32 3 24 times for 1 3 hz and 0 37 2 18 times for 310 hz the upper bounds of the larger mean amplification factors for 1 3 hz are found to be of the order of 3 13 6 03 at chopadwa vadawa kavada vondh adhoi jahwarnagar and gadhada whereas the upper bounds of the higher mean amplification factors at 3 10 hz are estimated to be of the order of 2 00 3 27 at tapar chopadwa adhoi jahwarnagar gandhidham and khingarpur the site response estimated at bhuj suggests a typical hard rock site behavior preliminary site response maps for 1 3 hz and 310 hz frequency ranges have been prepared for the area extending from 23 23 85 n and 69 65 70 85 e these frequency ranges are considered on the basis of the fact that the natural frequencies of multi story buildings 3 to 10 floor range between 1 3 hz while the natural frequencies for 1 to 3 story buildings vary from 3 10 hz the 1 3 hz map delineates two distinct zones of maximum site amplification 3 times one lying in the nw quadrant of the study area covering jahwarnagar kavada and gadadha and the other in the se quadrant of the study area with a peak of 6 03 at chopadwa covering an area of 70 km 50 km while the 3 10 hz map shows more than 2 times site amplification value over the entire study area except ne quadrant two patches in the southwest corner covering bhuj and anjar and one patch at the center covering vondh manfara and sikara the zones for large site amplification values 3 times are found at tapar chopadwa adhoi and chobari the estimated site response values show a good correlation with the distribution of geological formations as well as observed ground deformation in the epicentral zone

life cycle of a mesoscale circular gust front observed by a c band doppler radar in west africa abstracton 10 july 2006 during the special observation period sop of the african monsoon multidisciplinary analysis amma campaign a small convective system initiated over niamey and propagated westward in the vicinity of several instruments activated in the area including the massachusetts institute of technology mit c band doppler radar and the atmospheric radiation measurement arm mobile facility the system started after a typical convective development of the planetary boundary layer it grew and propagated within the scope of the radar range so that its entire life cycle is documented from the precluding shallow convection to its traveling gust front the analysis of the observations during the transitions from organized dry convection to shallow convection and from shallow convection to deep convection lends support to the significant role played by surface temperature heterogeneities and boundary layer processes in the initiation of deep convection in semiarid conditions the analysis

a simple model of convective drafts accounting for the perturbation pressure term abstracta simple anelastic two column model of convective updraft accounting explicitly for the perturbation pressure term is developed there is no vertical wind shear in the environment and two

development of the late proterozoic to mid paleozoic intracratonic amadeus basin in central australia a key to understanding tectonic forces in plate interiors the intracratonic amadeus basin in central australia is a complex composite basin covering 17 000 km with at least nine distinct episodes of evolution between 900 and 300 ma these have been identified in neighboring central australian basins and are characterized by intervals of renewed subsidence followed by intervals of erosion and changes in basin shape several unconformities separating the tectonostratigraphic sequences represent periods of mild regional uplifts and correlate with major episodes of compressive tectonism at the evolving margin of the early australian plate in the absence of direct geological evidence for thermal events in the amadeus basin after the initial period of subsidence ended possibly at about 800 ma the correlations found between events in the amadeus basin those in the neighboring interconnected basins and tectonic events at the continental or plate margins emphasize that regional horizontal stress fields contribute significantly to primary basin forming mechanisms this suggests that stresses generated at continental or plate margins may propagate to the plate interior with the lithosphere acting as a stress guide throughout the history of the amadeus basin both periods when compressional stresses were dominant and when extensional tectonics controlled subsidence can be identified there were also transitional periods when neither compression nor extension dominated so that other processes masked the effects of weak horizontal stresses the most recent compressional thrust belt concentrated at the northern margin between 300 and 400 ma and has partly obscured the record of earlier basin shape and size

perturbation pressure at the base of cumulus clouds in low shear abstract we examine the pressure fields wound the cloud base updraft of three cumulus clouds observed in environments with low vertical shear of the horizontal wind near cloud base these fields are compared to the corresponding pressure fields beneath convective clouds embedded in moderate to large shear all of the pressure fields are derived from aircraft measurements taken during the 1981 cooperative convective experiment ccope the pressure fields associated with these low shear clouds are weaker than those for the clouds in higher shear furthermore the low shear fields are not consistently dominated by the dynamic pressure created by the interaction of the cloud base updraft with the vertical shear of the horizontal wind the weaker dynamic pressure is due to the smaller size and intensity of the cloud base updraft as well as the smaller vertical shear of the horizontal wind the reduction of the dynamic pressure allows buoyancy effects on the pressure field to become more apparent

shrimp u_pb zircon geochronology of granites in the arunta inlier central australia implications for proterozoic crustal evolution abstract new shrimp u pb zircon isotopic ages are reported for ten granitic suites in the eastern arunta inlier these data combined with previous u pb zircon studies in the region constrain the timing of major palaeoproterozoic igneous events within the arunta inlier and establish a geochronological framework for understanding lithostratigraphic relationships tectonic events and geochemical and isotopic evolution of the granites in the region most granites in the arunta inlier including both syn tectonic and post tectonic types yield zircon ages in the range of 1770 1710 ma which are significantly younger than the 1880 1850 ma orogenic barramundi igneous association recognised throughout many other proterozoic terrains in northern australia no rocks of the barramundi association have been identified in this study the previously proposed barramundi type suites including the jervois and dneiper granites yield ages of 1771 6 and 1771 15 ma respectively many of the granite suites contain inherited zircons with ages in the range of 1890 1830 ma similar to the age of the barramundi association this and the age of the atnarpa igneous complex zhao and cooper 1992 which is coeval with but geochemically distinct from the barramundi association suggest that the 1880 1850 ma orogenic event affected the arunta inlier but has been obscured by the younger tectonic events the youth of tectonic events in the arunta inlier as compared with other proterozoic terrains of northern australia may have resulted from the inlier being located on the southern margin of the northern australian orogenic province during the palaeoproterozoic with the generation of granites in the arunta inlier possibly being related to subduction the combined data indicate that major granitic activity occurred at 1880 1850 1820 1770 1750 1730 1710 1660 1650 and 1615 1590 ma this result is broadly consistent with earlier summaries of the other proterozoic terrains in northern australia although the intensity of each episode varies in different regions

shear localisation in upper mantle peridotites upper mantle peridotite bodies at the earth s surface contain relict structures and microstructures which provide direct information on the role and the mechanisms of shear localisation in the upper mantle deformation which occurred at high temperatures t 950 50 c is relatively homogeneous within domains ranging in scale from a few kilometres to a few tens of kilometres below 950 50 c strain is localised into centimetre to several hundred metre wide shear zones which commonly contain hydrated mylonitic peridotites the microstructures developed in the peridotites suggest there is a correlation between the occurrence of shear localisation and the occurrence of strain softening and brittle deformation processes the most important strain softening processes are inferred to be structural and reaction induced softening structural softening processes include dynamic recrystallisation and strain induced transitions from dislocation creep to some form of grain size sensitive gss creep reaction induced softening is related to the formation of fine grained polyphase reaction products which deform by gss creep and the formation of weak sheet silicates such as phlogopite chlorite talc and antigorite from experimental studies these softening processes and brittle deformation processes are inferred to occur mainly at temperatures less than about 910 160 c this temperature range is inferred to be a significant rheological transition in the upper mantle below 910 160 c deformation during orogenesis may be accommodated by an anastomosing network of hydrated mylonitic shear zones with a distinct perhaps weak rheology at higher temperatures strain is accommodated in much wider deformation zones

tectonic evolution of proterozoic australia proterozoic australia has long been interpreted as a single intact continent in which all tectonic and magmatic activity was intracratonic this paper proposes an alternative hypothesis in which numerous fragments of continental crust were assembled by plate tectonic processes the assembly was completed between 1300 and 1100 ma when the crustal fragments were combined as an early component of the rodinian supercontinent rifting and fragmentation of archaean continents began in the late archaean and continued into the proterozoic passive margin deposits such as those of the hamersley basin accumulated on isolated fragments of archaean crust these numerous fragments were subsequently assembled into three cratons by 1830 ma a west australian craton was established by collision of the archaean pilbara and yilgarn cratons which were joined along the capricorn orogen similarly a south australian craton developed by amalgamation of the proto gawler and proto curnamona cratons along the kimban orogen a north australian craton appears to have formed by accretion of numerous crustal fragments including the kimberley pine creek lucas and altjawarra cratons with sutures marked by the king leopold halls creek tennant creek and proto isan orogens the southern margin of the north australian craton was the site of repeated terrane accretion and orogenic activity between 1880 ma and 1400 ma this included an orogenic event at 1880 1850 ma the strangways 1780 1730 ma argilke 1680 1650 ma and chewings 1620 1580 ma orogenies and the intracratonic anmatjira uplift 1500 1400 ma intracratonic rifting at 1750 to 1710 ma and 1640 to 1600 ma produced the mcarthur basin and related minor basins parts of which were deformed by the isan orogeny at 1600 and 1530 ma rifting along the line of the capricorn orogen led to deposition in the overlying intracratonic bangemall basin between 1630 and 1300 ma along the eastern margin of the south australian craton the 1670 to 1600 ma olarian orogeny marks interaction with now obscured continental crust to the east tectonic activity between 1300 and 1100 ma led to the assembly of proterozoic australia as an early component of the supercontinent of rodinia this first involved the amalgamation of the west australian and north australian cratons followed by collision with the south australian craton the centralian superbasin developed over the junction of the north south and west australian cratons between 830 and 750 ma rifting to the east formed the adelaide geosyncline at 830 ma this was followed by the breakup of rodinia with the rifting apart of laurentia and gondwanaland along the eastern margin of proterozoic australia at 750 ma and the subsequent formation of the palaeo pacific ocean after the breakup of rodinia a series of northeast southwest compressional events followed by periods of relaxation reflect the assembly of a new supercontinent old lines of weakness were reactivated culminating in the intracratonic king leopold paterson petermann ranges and pinjarra orogenies between 620 and 540 ma subsequent reactivation continued into the phanerozoic with the widespread eruption of continental flood basalts and the formation of intracratonic basins 540 530 ma

on the relationship between the formation of shear zones and the form of the flow law for rocks undergoing dynamic recrystallization abstract the formation and maintenance of shear zones in rocks at high temperatures when viscous flow takes place at constant volume requires strain weakening several processes can lead to a strain weakening characteristic in the flow of such rocks to high strains but conventional experimental rock mechanics techniques cannot normally provide high quality mechanical data to high strains recent experimental data for calcite marble have demonstrated strain weakening accompanied by dynamic recrystallization a power law with strain exponent r can be used to describe the dependence of the flow stress on strain together with the conventional description of the dependence of strain rate on stress described by a stress exponent n the stable localization of strain into shear zones is favoured by more negative values of r e g r 0 8 and more positive values of n e g n 5 to 7 a further important factor is the critical strain e0 for the onset of recrystallization and hence strain weakening shear zone formation is favoured by smaller values for e0 shear zone formation weakens the crustal section in which they occur but the amount of weakening may not be great perhaps no more than 50 of the previous steady state flow stress certain rock types such as mafic igneous rocks may through their mechanical properties be more susceptible to shear zone formation than others such as mica schists it is emphasized that the analysis presented is based upon a strain dependent flow law that describes single phase dynamic recrystallization by grain boundary migration that leads to enhanced recovery without any change in deformation mechanism no consideration is given to possibly important effects arising from the activation of grain size sensitive flow or to potential weakening effects arising from syntectonic metamorphic reactions

ductile structures and instabilities their implication for variscan tectonics in the ardennes abstract increasing consideration of instabilities may explain the development of structures at all scales in naturally deformed rocks this address is a review of geometric scale free concepts applied to the tectonic study of ductile structures the notion of instabilities is first given to introduce non equilibrium dynamics such as strain localisation and single deformation events producing superimposed structures these concepts lead to the idea that orogens are regions of accumulated strain in zones dominated by non coaxial deformation consequently stretching lineations depend on finite strain and do not represent the plate motions repeated phases of deformation represent local perturbations during a kinematic and dynamic event rather than being correlatable as regional phases the fold and thrust belt of the northern variscan belt in the ardennes is taken as a natural example where superposed structures represent the instable response to forces among which plate tectonic forces play a subsidiary role

sticky thermals evidence for a dominant balance between buoyancy and drag in cloud updrafts abstractthe vertical velocities of convective clouds are of great practical interest because of their influence on many phenomena including severe weather and stratospheric moistening however the magnitudes of forces giving rise to these vertical velocities are poorly understood and the dominant balance is in dispute here an algorithm is used to extract thousands of cloud thermals from a large eddy simulation of deep and tropical maritime convection using a streamfunction to define natural boundaries for these thermals the dominant balance in the vertical momentum equation is revealed cloud thermals rise with a nearly constant speed determined by their buoyancy and the standard drag law with a drag coefficient of 0 6 contrary to suggestions that cloud thermals might be slippery with a dominant balance between buoyancy and acceleration cloud thermals are found here to be sticky with a dominant balance between buoyancy and drag

the influence of the shear induced pressure gradient on thunderstorm motion abstract in the present investigation we propose a simple theory to explain how a veering environmental wind shear vector can cause an initially symmetric updraft to grow preferentially to the right of the shear vector and acquire cyclonic rotation the explanation offered is based on linear theory which predicts that interaction of the mean shear with the updraft produces favorable vertical pressure gradients along its right flank to asses the validity of linear theory for large amplitude updrafts the three dimensional shallow anelastic equations are numerically integrated using a simple parameterization for latent heating within a cloud and the linear and nonlinear forcing terms are separately analyzed these results suggest that although the nonlinear effects strongly promote splitting of the updraft the linear forcing remains the dominant factor in preferentially enhancing updraft growth on the right flank we believe this differential forcing is a major contributor to the observed predominance o

characteristics of a kilometer scale high strain zone in the lower continental crust mt hay block central australia abstract a granulite facies high strain zone was studied on capricorn ridge part of the mt hay block granulites in the arunta inlier central australia deformation occurred in these primarily mafic composition granulites at lower crustal conditions 8 kb 800 c at 1740 1690 ma pervasive and consistently oriented foliation striking ssw and dipping steeply and lineations pitching steeply eastward occur within the shear zone the high strain zone is recognized by the presence of sl tectonites parallelism of grain shape fabrics and lithologic boundaries transposition of lithologic boundaries into parallelism with the foliation and the local occurrence of sheath like folds field based fabric mapping uses compositional foliation intensity as a proxy for finite strain to document strain variations within the capricorn ridge high strain zone there is no observed microstructural grain size reduction that corresponds to these strain variations fabric intensity increases from the centers of the major lithologic units towards the contacts 100 m scale strain localization within capricorn ridge occurs adjacent to major lithologic contacts indicating competency contrast during lower crustal deformation

striding athabasca mylonite zone implications for the archean and early proterozoic tectonics of the western canadian shield study of the northern saskatchewan district of mackenzie segment of the snowbird tectonic zone suggests that fragments of relatively stiff mid archean crust possibly arc related have controlled the localization shape and complex kinematics of the multistage striding athabasca mylonite zone during the archean as well as the geometry of the early proterozoic rifted margin of the western churchill continent by the late archean the striding athabasca mylonite zone was located in the interior of the western churchill continent well removed from the contemporaneous plate margins except for the alberta segment the snowbird tectonic zone was not the site of an early proterozoic plate margin we suggest that the geometry of the archean early proterozoic boundary in the western canadian shield represents a jagged continental margin composed of a pair of reentrants defined by rifted and transform segments these segments were inherited from early proterozoic breakup and controlled by the archean structure

thermo tectonic evolution of the basement complex around okene nigeria with special reference to deformation mechanism abstract at least three successive thermo tectonic episodes the last of which involved a largescale heterogeneous simple shear deformation can be structurally distinguished in the okene area of the basement complex of nigeria evidence for the first episode is only observed in a suite of banded gneisses early gneiss the earliest metamorphic fabric in the banded gneisses predates the fabric in a swarm of minor metamorphosed intermediate basic dykes the second episode is distinguished by the earliest fabric in a suite of metasediments and meta intrusives main phase gneiss which is co eval with the fabric of the intermediate basic dykes the early gneiss occurs as competent relics interleaved between the foliation planes of the main phase gneiss both suites constituting a large scale sheeted rock body the gneiss complex sigmoidal gently plunging asymmetric folds of the gneiss complex and intrusives of pan african age late intrusive suite which cross cut the gneiss complex distinguish the third episode because a sub horizontal stretching lineation superimposed on the gneiss complex is sub parallel to the axes of the folds and the late intrusive suite is tectonised only in the steep limb and tight hinge regions of the folds the last episode is inferred to have involved a lateral heterogeneous simple shear of an originally flat lying gneiss complex

the importance of incompetence metabasaltic dikes that are known to be competent and to deform more slowly than granitoid country rocks in the upper crust are likely to be incompetent and deform faster than granitoid hosts in the higher temperatures of the lower crust layers that deform as competent strain markers develop inward pointing cusps these deformation structures are called folds in shortened layers and necks in extended layers and are reasonably well understood by contrast layers that deform as incompetent markers develop strong internal foliations and outward pointing cusps these structures are currently known as mullions in shortened contacts and inverse folds in extended contacts but are comparatively poorly understood here we emphasize the importance of the deformation structures in the incompetent hydrated metabasaltic intrusions and enclaves that must be among the structures that seismic reflection profiles attempt to image deep in the continental crust

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

a theory of finite strain variation through contrasting layers and its bearing on cleavage refraction abstract a theory of finite strain variation in contrasting viscous layers is presented the theory is applicable to layers which are oblique to two principal strains but parallel to the third and is not restricted to plane strain results may be obtained algebraically or by use of the mohr diagram for strain solutions are given for various examples of layering attitude and viscosity ratio measured with respect to a reference layer it is shown that the finite strain ellipsoid changes in shape and orientation across contrasting layers and in some cases the principal axes may be exchanged low strain is indicated in relatively more viscous layers and high strain with extension at a smaller angle to the layering in less viscous layers the latter may approximate simple shear parallel to layering if the strain is sufficiently high the geological implications of the theory are strong strain variations in layered rock sequences it is suggested that in general strain will be inhomogeneous from layer to layer both in orientation and amount the relationship of strain and cleavage are reviewed in the introduction to establish the validity of predicting cleavage patterns from strain data qualitative comparisons between the theoretical results of strain variation and natural cleavage refraction in layered rocks would appear to justify the assumption that cleavages of varied morphologies are sub parallel to the xy planes of strain with this assumption some three dimensional features of planar and linear fabric refraction in contrasting rocks are predicted

implications for mechanical properties of brittle faults from observations of the punchbowl fault zone california field observations of the punchbowl fault zone an inactive trace of the san andreas are integrated with results from experimental deformation of naturally deformed punchbowl fault rocks for a qualitative description of the mechanical properties of the fault and additional information for conceptual models of crustal faulting the punchbowl fault zone consists of a single continuous gouge layer bounded by zones of extensively damaged host rock fault displacements were not only localized to the gouge layer but also to discrete shear surfaces within the gouge deformation in the exposure studied probably occurred at depths of 2 to 4 km and was dominated by cataclastic mechanisms textural data also suggest that significant amounts of pore fluids were present during faulting and that fluid assisted mechanisms such as dissolution diffusion and precipitation were operative the experimental data on specimens collected from the fault zone suggest that there is a gradual decrease in strength and elastic modulus and an increase in relative ductility and permeability toward the main gouge zone the gouge layer has fairly uniform mechanical properites and it has significantly lower strength elastic modulus and permeability than both the damaged and the undeformed host rock for the punchbowl fault and possibly other brittle faults the variations in loading of the gouge zone with time are primarily governed by the morphology of the fault and the mechanical properties of the damaged host rock in addition the damaged zone acts as the permeable unit of the fault zone and surrounding rock it appears that the gouge primarily governs whether displacements are localized and it therefore may have a significant influence on the mode of slip

gregarious tropical convection abstract a beat source with a vertical profile like that of observed tropical mesoscale convective systems mcss is shown to cause through inviscid gravity wave dynamics upward displacement at low levels in a mesoscale region surrounding the heating typical values are 10 30 area contraction at the surface everywhere within 270 km of the heating 6 h after it starts as a result conditions near an existing mcs but beyond the area of mcs outflow become more favorable for the development of additional convection this theory predicts that cloud clusters should be gregarious infrared satellite imagery confirms that almost half of the cold cloudiness observed in a month over the oceanic warm pool region was contributed by just 14 objectively defined multiday supercluters

formation of the maturin foreland basin eastern venezuela thrust sheet loading or subduction dynamic topography 1 the maturin basin in eastern venezuela is considered a good example of a peripheral foreland basin earthquake and tomographic data indicate that eastern venezuela is affected by the oblique subduction of the south american plate underneath the caribbean plate new forward flexural isostatic modeling of eastern venezuela has been carried out in order to determine whether the maturin basin was generated purely by thrust sheet loading from the serrania and monagas foreland thrust belts a sequence of forward models from middle miocene to present was generated for 3 profiles across the serrania del interior thrust belt the monagas foreland thrust belt and the maturin foreland basin the predictions of these models are constrained using seismic reflection and well data the flexural isostatic modeling shows that thrust sheet loading associated with the serrania del interior and monagas foreland thrust belts is insufficient to generate the observed subsidence within the maturin basin dynamic fluid flow modeling of subduction related dynamic topography of eastern venezuela has been used to investigate the influence of south american plate subduction on the generation of the accommodation space observed in the maturin basin fluid flow modeling of subduction related dynamic topography suggests that the subduction of the south american lithospheric mantle caused downward deflection of the south american crust affecting the maturin basin and the serrania thrust belt this modeling suggests that the maturin basin subsidence has two components 55 related to thrust sheet loading and 45 driven by continental subduction