structural geology of the penobsquis salt structure late bashkirian inversion tectonics in the moncton basin new brunswick eastern canada this study uses new information acquired from field mapping mineral exploration boreholes and seismic reflection profiles to constrain the structural geology deformation mechanisms and timing of the penobsquis salt structure this information is used to place the development of the salt structure into the context of the tectonic evolution of the moncton basin the penobsquis salt structure is an elongate salt wall separated from underlying rocks by a mylonitic detachment horizon it may be divided into two segments a southwestern asymmetric southeast verging structure and a northeastern strongly asymmetric to overturned southeast verging structure associated with the penobsquis fault an initial low amplitude salt anticline formed during deposition of the mabou group serpukhovian time the salt anticline was modified into a high amplitude salt wall after deposition of cumberland group sediments early bashkirian or later the sense of asymmetry of the penobsquis salt structure the presence of thrust faults within the structure the spatial association of the structure with a major reverse fault in the salt cover and the temporal association of salt movement and compressional tectonic events suggest that the salt structure developed in response to tectonic contraction during inversion of the moncton basin

mesoarchean assembly and stabilization of the eastern kaapvaal craton a structural thermochronological perspective 1 understanding the construction and stabilization of archean continental lithosphere has important implications for models of tectonic and thermal regimes in the early earth as well as for the subsequent evolution of continents we provide new constraints for the amalgamation and stabilization of the eastern kaapvaal craton in the vicinity of the barberton greenstone belt circa 3 3 3 1 ga isotope dilution thermal ionization mass spectrometry u pb geochronology and thermochronology are combined with mapping and structural analysis around the margins of this belt in order to constrain movement on major shear zones that separate high grade orthogneiss terranes from low grade supracrustal rocks and to relate this movement to the timing of differential exhumation new and existing data are consistent with accretion at 3 23 ga within an oblique subduction plate boundary manifested in the bgb as an asymmetric flower like structural geometry for 100 ma following terrane assembly transform boundary tectonics manifested partially as transtension on reactivated faults led to large scale juxtaposition of middle to lower crustal basement orthogneiss complexes against upper crustal greenstone sequences and episodic emplacement of granitic batholiths stabilization of this portion of archean lithosphere involved at least a three stage process 1 creation of a thick and rigid mantle lithosphere during crustal growth subduction and terrane assembly from circa 3 30 to 3 23 ga 2 generation of a rigid crust during strike slip and transtensional tectonics coupled with migration of granites and heat producing elements into the upper crust between circa 3 2 and 3 1 ga and 3 erosion and removal of the uppermost crust during regional peneplanation that lasted until circa 2 9 ga

archaean cratonization and deformation in the northern superior province canada an evaluation of plate tectonic versus vertical tectonic models abstract the archaean minto block northeastern superior province is dominated by tonalite trondhjemite enderbite pyroxene tonalite granodiorite and granite with subordinate mafic rocks and supracrustal belts the plutons have been interpreted as the batholithic roots of andean type plate margins and intra oceanic arcs existing horizontal tectonic models propose that penetrative recrystallization and transposition of older fabrics during terrane assembly at 2 77 and 2 69 ga produced a n nw tectonic grain in the douglas harbour domain northeastern minto block tonalite and trondhjemite dominate the faribault thury complex 2 87 2 73 ga and enderbite constitutes 50 100 km scale ovoid massifs troie and qimussinguat complexes 2 74 2 73 ga magmatic muscovite and epidote in tonalite trondhjemite have corroded edges against quartz plagioclase suggesting resorption during ascent of crystal charged magma foliation maps and air photo interpretation show the common development of 2 10 km scale ovoid structures throughout the douglas harbour domain outcrop and thin section scale structures imply that many plutons experienced a phase of syn magmatic deformation typically followed by high temperature sub magmatic overprints thermobarometric data for plutons indicate near solidus recrystallization at 4 6 kbar pressures the common preservation of syn magmatic fabrics in plutons of different ages seems incompatible with the origin of these fabrics through superimposed regional orogenesis the broad uniformity of intrusion ages and lithologies throughout the minto block and the rarity of shallowly dipping planar fabrics also seem inconsistent with accretion of disparate older terranes each of which should preserve distinct histories a possible alternative explanation for these features is provided by vertical tectonic models whereby buoyant felsic magmas ascended as crystal slurries while dense supracrustal rocks and solidified felsic intrusions emplaced into them subsided as cold fingers 10 20 km scale instabilities shear between upwelling and downwelling limbs would have concentrated in the weak intrusions generating steeply plunging syn magmatic fabrics and producing ductile overprints in solidified rocks

vertical extrusion and horizontal channel flow of orogenic lower crust key exhumation mechanisms in large hot orogens a large database of structural geochronological and petrological data combined with a bouguer anomaly map is used to develop a two stage exhumation model of deep seated rocks in the eastern sector of the variscan belt an early sub vertical fabric developed in the orogenic lower and middle crust during intracrustal folding followed by the vertical extrusion of the lower crustal rocks these events were responsible for exhumation of the orogenic lower crust from depths equivalent to 18 20 kbar to depths equivalent to 8 10 kbar and for coeval burial of upper crustal rocks to depths equivalent to 8 9 kbar following the folding and vertical extrusion event sub horizontal fabrics developed at medium to low pressure in the orogenic lower and middle crust during vertical shortening fabrics that record the early vertical extrusion originated between 350 and 340 ma during building of an orogenic root in response to se directed saxothuringian continental subduction fabrics that record the later sub horizontal exhumation event relate to an eastern promontory of the brunia continent indenting into the rheologically weaker rocks of the orogenic root indentation initiated thrusting or flow of the orogenic crust over the brunia continent in a north directed sub horizontal channel this sub horizontal flow operated between 330 and 325 ma and was responsible for a heterogeneous mixing of blocks and boudins of lower and middle crustal rocks and for their progressive thermal re equilibration the erosion depth as well as the degree of reworking decreases from south to north pointing to an outflow of lower crustal material to the surface which was subsequently eroded and deposited in a foreland basin indentation by the brunia continental promontory was highly noncoaxial with respect to the se oriented saxothuringian continental subduction in the early visean suggesting a major switch of plate configuration during the middle to late visean

models for the archean thermal regime a variety of different regionalized thermal evolution models are presented all models include an oceanic style of heat loss in which mantle convection penetrates to the surface in the form of spreading and subducting plates in continental regions the lithosphere is assumed to act as a conductive lid on mantle convection that can be either layered or mantle wide the continental lithosphere is defined in different ways it can be of fixed thickness insensitive to temperature or extend to some fixed cutoff temperature a thermally controlled rheological lithosphere when a fixed temperature is used widespread melting occurs in the archean and one has the further option of allowing these melts to erupt onto the surface thus providing a new heat transfer mechanism and also a general subsidence of the continental lithosphere due to the continuous volcanic loading the models are first used to determine the concentration of heat producing elements required to have a present day mantle potential temperature of 1350 c starting from a high temperature 2000 c 4 5 billion years ago the results depend primarily on whether the mantle is layered if layered 15 30 ppb uranium normalized to the silicate portion of the earth and withk u 104 th u 3 8 are needed while mantle wide cases need uranium concentrations in excess of 35 ppb the models are also used to calculate the thermal structure of continental lithosphere through time only lithospheres of fixed thickness 175 km can explain the recent discovery of diamonds with model ages of 3 2 3 4 billion years for a thick lithosphere to have existed in the archean it would seem most likely that its thickness be the result of some chemical distinction rather than simply its rheological properties such chemically distinct lithosphere would have greater likelihood of surviving to the present which raises the issue of biased preservation

higher himalayan shear zone sutlej section structural geology and extrusion mechanism by various combinations of simple shear pure shear and channel flow in shifting modes the higher himalayan shear zone hhsz in the sutlej section reveals 1 top to sw ductile shearing 2 top to ne ductile shearing in the upper and the lower strands of the south tibetan detachment system stdsu stdsl and 3 top to sw brittle shearing corroborated by trapezoid shaped minerals in micro scale in the proposed extrusion model of the hhsz the e1 phase during 25 19 ma is marked by simple shearing of the upper sub channel defined by the upper strand of the main central thrust mctu and the top of stdsu as the lower and the upper boundaries respectively subsequently the e2a pulse during 15 14 ma was characterized by simple shear pure shear and channel flow of the entire hhsz finally the e2b pulse during 14 12 ma observed simple shearing and channel flow of the lower sub channel defined by the lower strand of the main central thrust mctl and the top of the stdsl as the lower and the upper boundaries respectively the model explains the constraints of thicknesses of the stdsu and the stdsl along with spatially variable extrusion rate and the inverted metamorphism of the hhsz the model predicts 1 shear strain after ductile extrusion to be maximum at the boundaries of the hhsz which crudely matches with the existing data the other speculations that cannot be checked are 2 uniform shear strain from the mctu to the top of the hhsz in the e1 phase 3 fastest rates of extrusion of the lower boundaries of the stdsu and the stdsl during the e2a and e2b pulses respectively and 4 variable thickness of the stdsl and rare absence of the stdsu non parabolic shear fabrics of the hhsz possibly indicate heterogeneous strain the top to sw brittle shearing around 12 ma augmented the ductile extruded rocks to arrive a shallower depth the brittle ductile extension leading to boudinage possibly did not enhance the extrusion

recent developments in centrifuge modelling of tectonic processes equipment model construction techniques and rheology of model materials abstract the experimental tectonics laboratory at queen s university is equipped with a large capacity centrifuge that is capable of subjecting tectonic models measuring 127 76 mm in plan and up to 51 mm in depth to accelerations as high as 20 000 g this high capacity greatly extends the range of potential model materials and permits the use of relatively stiff and or brittle substances a number of new techniques of model construction have been devised that permit internal and surface strain patterns and kinematic evolution to be monitored in detail one particularly useful technique which will find application in non centrifuged experiments as well allows the preparation of highly uniform anisotropic multilayers composed of alternating layers of plasticine and silicone putty with individual layer thicknesses as low as 20 m and with controllable ratio between thicknesses of the relatively competent and incompetent units examples of models constructed using these new techniques are illustrated one particular type of the commonly used model material silicone putty has been subjected to a series of rheological test the results indicate that at strain rates in the range 10 6 10 3 s 1 applicable to the centrifuge experiments the silicone putty exhibits power law rheology with n 7 2 at higher strain rates the material appears to tend towards linear behaviour available rheological data and dimensional analysis using standard scaling laws and appropriate model ratios suggest that the microlaminated plasticine silicone putty multilayer is a suitable analogue in centrifuged experiments for interbedded sequences of indurate limestone and incompetent shale the excellent degree of dynamic similitude attained is demonstrated by the realistic form of fold and fault structures developed in models constructed of this material

origin of granite greenstone patterns yilgarn block western australia the yilgarn block of western australia consists mainly of archean granite greenstone terrain with oval areas of granitoid rocks fringed by arcuate greenstone belts in a typical part of this terrain the outcrop pattern is shown to result from large scale dome and basin fold interference structures rather than diapiric emplacement of steep sided batholiths the greenstones and underlying granitoid rocks were initially deformed together in a subhorizontal tectonic regime as a result the granitoid rocks recrystallized and a gneissosity developed subparallel to the stratigraphy of the greenstones thick subhorizontal monzogranite sheets were later emplaced into the lower part of the greenstone sequence subsequent deformation generated two successive sets of folds with vertical axial surfaces at a high angle to each other these folds formed dome and basin interference structures ranging from a few centimetres to more than 50 km in diameter

indentation as an extrusion mechanism of lower crustal rocks insight from analogue and numerical modelling application to the eastern bohemian massif abstract recent petrological structural and geochronological studies of the eastern margin of the bohemian massif czech republic suggest a conceptual geodynamical model to explain exhumation of lower crustal 20 kbar 800 c felsic rocks the model involves indentation of a weak orogenic lower crust by an adjacent rigid mantle lithosphere resulting in crustal scale buckling of the weak orogenic lower middle crust interface followed by extrusion of a ductile nappe over the rigid promontory the hypothesis has been investigated using both analogue and numerical models analogue experiments using a three layer sand silicone setup were carried out in rennes laboratory france results show that the most important features of the conceptual model can be reproduced extrusion of lowermost silicone over the indenter and flow of horizontal viscous channel underneath a rigid lid above the actively progressing promontory furthermore experimental results show that a plateau develops above the channelling lower crust two sets of sandbox scale numerical simulations were performed the first set of experiments is designed to study the influence of viscosity stratification within the crust on the extrusion process a second set of experiments were performed in order to quantify the influence of the viscosity and the geometry of the indentor non dimensional scaling laws were derived to predict the maximum extrusion rates associated with the indentation mechanism such laws enable the computation vertical extrusion rates that are in good agreement with natural exhumation rates inferred from petrological data finally we discuss the potential positive feedback of rayleigh taylor instability on vertical extrusion for the case of eastern bohemian massif

weathering and sediment generation in the archean an integrated study of the evolution of siliciclastic sedimentary rocks of the 3 2ga moodies group barberton greenstone belt south africa abstract alluvial and braided fluvial deposits of the 3 2 ga moodies group in the barberton greenstone belt south africa are analyzed petrographically and geochemically to identify their source rocks provenance and assess the degree and type s of weathering required to produce the observed moodies sediment compositions conglomerate sandstone and shale data give somewhat differing pictures of provenance and weathering due to derivation from different components within the source terrane and size and compositional fractionation during transport the results suggest that shale geochemistry provides the most accurate estimate provenance although both rare earth element and trace element th sc zr cr and ti data must be used in combination to give the best results the source area for moodies group sediments was dominated by tonalite felsic volcanic rock komatiite basalt and granite based on mineralogical and major element divergence from estimated source area composition the moodies group sediments are remnants of an aggressive weathering environment labile materials such as komatiite basalt and coarse plagioclase grains decomposed almost entirely to clays and solutes and the chemical index of alteration for moodies shale is well above the global average an aggressive weathering environment in the archean may have been achieved by increased rainfall higher temperatures and or higher atmospheric p c o 2 more likely a combination of these conditions worked to offset the inhibitory weathering effects of a plant free environment

sinking of the dharwar basin south india implications for archaean tectonics abstract archaean tectonic regimes can be constrained by structural analysis of greenstone belts and adjacent granite gneiss masses the late archaean bababudan greenstone belt dharwar craton south india contains a low grade volcanosedimentary basin sequence the dharwar supergroup which unconformably overlies a previously eroded basement made up mainly of granite and gneiss fabric trajectories kinematic indicators and superimposed deformation patterns show that greenstones have undergone a radial converging downward flow with respect to underlying basement rocks as well as strike slip shearing this is attributed to the progressive sagduction gravity sinking of the greenstone belt into its basement along a decollement horizon and to late transcurrent shearing we propose a deformation model for the bababudan belt that may explain common features of many greenstone belts such as decollements and superimposed upright fold patterns which are attributed to the progressive sinking of the supracrustals into their basement the results of our study and other field investigations in the crust of the dharwar craton as well as structural evidence from other cratons zimbabwe and pilbara raise the possibility that a unique continental tectonic regime existed during the archaean controlled by the emplacement of dense volcanic traps mantle underplating and thermal blanketing of the continental crust this tectonic regime differs from that observed within the modern lithosphere as it is characterized by the pervasive development of gravity i e rayleigh taylor instabilities between the volcanic traps and the rest of the continental crust it allowed heat and material transfer through the crust and has to be seen as an efficient recycling mechanism for the young continental crust during periods of mantle underplating the diapiric phenomenon is independent of the boundary forces acting at the limits of the protocontinents and is associated in time and space with large scale accretion events of the continental crust perhaps in hot spot environments

timing of deformation in the norseman wiluna belt yilgarn craton western australia establishing relative and absolute time frameworks for the sedimentary magmatic tectonic and gold mineralisation events in the norseman wiluna belt of the archean yilgarn craton of western australia has long been the main aim of research efforts recently published constraints on the timing of sedimentation and absolute granite ages have emphasized the shortcomings of the established rationale used for interpreting the timing of deformation events in this paper the assumptions underlying this rationale are scrutinized and it is shown that they are the source of significant misinterpretations a revised time chart for the deformation events of the belt is established the first shortening phase to affect the belt d1 was preceded by an extensional event d1e and accompanied by a change from volcanic dominated to plutonic dominated magmatism at approximately 2685 2675 ma later extension d2e controlled deposition of the ca 2655 ma kurrawang sequence and was followed by d2 a major shortening event which folded this sequence d2 must therefore have started after 2655 ma at least 20 ma later than previously thought and after the voluminous 2670 2655 ma high ca granite intrusion younger transcurrent deformation d3 d4 waned at around 2630 ma suggesting that the crustal shortening deformation cycle d2 d4 lasted approximately 20 30 ma contemporaneous with low volume 2650 2630 ma low ca granites and alkaline intrusions time constraints on gold deposits suggest a late mineralisation event between 2640 2630 ma thus d2 d4 deformation cycle and late felsic magmatism define a 20 30 ma long tectonothermal event which culminated with gold mineralisation the finding that d2 folding took place after voluminous high ca granite intrusion led to research into the role of competent bodies during folding by means of numerical models results suggest that buoyancy driven doming of pre tectonic competent bodies trigger growth of antiforms whereas non buoyant competent granite bodies trigger growth of synforms the conspicuous presence of pre folding granites in the cores of anticlines may be a result from active buoyancy doming during folding 2002 elsevier science b v all rights reserved

post collisional upper crustal faulting and deep crustal flow in the eastern wabigoon subprovince of the superior province ontario evidence from structural and 40ar 39ar data from the humboldt bay high strain zone abstract the humboldt bay high strain zone hbhsz is the most prominent archean shear zone within the onaman tashota greenstone belt of the eastern wabigoon subprovince the hbhsz was initiated as a terrane boundary during assembly of the greenstone belt d1 and displays several periods of reactivation d2 d4 during post assembly transpression each deformation phase was restricted to successively narrow zones and transcurrent shear sense changed with each phase the boundary of the north wind pluton adjacent to the shear zone exhibits similar d2 d4 fabric and kinematics to the hbhsz and the relations of plutonic phases to the boundary fabrics demonstrate that it was likely emplaced during d2 transpression the occurrence of plutonism during transpression indicates the lithospheric scale of post assembly phase d2 the best defined 40ar 39ar plateau ages come from texturally simple extensively recrystallized amphibolite that formed locally during d2 in the hbhsz and throughout the margins of the north wind pluton the average of these is ca 2667 ma similar to the maximum mica age and gives a minimum age for d2 texturally complex amphibolite formed by partial recrystallization during d2 contains d2 foliation forming amphibole and porphyroclasts of actinolite these amphibolites yield discordant spectra with age gradients and together with variably lower mica ages may record later reactivation of the shear zone d3 d4 and later events by analogy with the linkage between the late deep crustal flow and upper crustal shear zones in the wawa subprovince the structural and thermal reactivation may be the indicator in a stiff old superstructure of deep crustal flow beneath the eastern wabigoon

ediacaran intracontinental channel flow migmatitic lower crustal rocks in the ediacaran intracontinental petermann orogen central australia are bounded by regional scale low angle shear fabrics that record opposing shear senses new sensitive high resolution ion microprobe shrimp titanite geochronology suggests that the contrasting shear systems developed synchronously we argue that the macro scopic structural and metamorphic architecture satisfies the diagnostic criteria outlined by recently proposed channel flow models and thus the remarkably well preserved petermann orogen is potentially an example of ancient intracontinental channel flow

crustal channel flows 1 numerical models with applications to the tectonics of the himalayan tibetan orogen 1 plane strain thermal mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens in the models radioactive self heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large scale horizontal flow channels can be exhumed and exposed by denudation focused on the high relief transition between plateau and foreland we interpret the himalaya to have evolved in this manner channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws and well developed if there is an additional reduction in viscosity to 1019 pa s this reduction occurs from 700 c to 750 c in the models and is attributed to a small percentage of in situ partial melt melt weakening model ht1 provides an internally consistent explanation for the tectonic evolution of many features of the himalayan tibetan orogenic system erosional exhumation exposes the migmatitic channel equivalent to the greater himalayan sequence ghs between coeval normal and thrust sense ductile shear zones corresponding to the south tibetan detachment and the main central thrust systems outward flow of unstable upper crust rotates these shears to low dip angles in the model both the ghs and the lesser himalayan sequence are derived from indian crust with the latter from much farther south similar models exhibit a range of tectonic styles including the formation of domes resembling north himalayan gneiss domes model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau

regional shortening followed by channel flow induced collapse a new mechanism for dome and keel geometries in neoarchaean granite greenstone terrains the lateral flow and extrusive exhumation of ductile migmatitic gneisses due to a horizontal gradient in lithostatic pressure a process termed channel flow or lateral protrusion has previously been proposed as an important tectonic process in large hot orogens centrifuge simulation of i layer parallel shortening followed by ii collapse of a cover sequence during ductile flow of underlying layers in an analogous manner to channel flow suggests a new mechanism for the development of structures within neoarchaean granite greenstone terrains the centrifuge model incorporates an upper package of silicone modelling clay microlaminates that simulate a greenstone sequence that overlies slightly less dense ductile silicone putties whose rheological properties simulate migmatitic felsic gneiss a low viscosity low density layer along half of the infrastructure superstructure interface simulates the presence of granitoid melt during initial layer parallel shortening upright folds form in the upper greenstone package and the interface with underlying ductile layers is folded in basal silicone layers recumbent to overturned non cylindrical and upright folds form with increasing distance from the moving end wall removal of material parallel to fold hinges at the foreland end of the model i e furthest from the ram used to shorten models in several stages simulates erosion and the difference in gravitational loading thus created induces ductile flow and lateral extrusion of basal silicone layers models aim to reproduce features comparable to those developed during extrusive channel flow and focused exhumation of basement migmatitic gneiss in nature early formed recumbent to inclined folds are then accentuated during simulated channel flow while new recumbent isoclinal folds in basal layers develop broad antiforms and tight synforms similar to the dome and keel geometry that typifies many archaean granite greenstone belts are produced as a late feature in the model channel flow and collapse of a fold thickened crust is therefore proposed as a potential alternative mechanism for the formation of structures in some neoarchaean granite greenstone terrains by analogy with other physical and numerical models channel flow in the neoarchaean may be enhanced by impingement of an upper mantle wedge into the base of the crust our results imply that both lithospheric shortening and non diapiric gravitational instabilities may be responsible for the formation of some archaean dome and keel structures and may account for the juxtaposition of some granite greenstone and high grade gneiss terrains in the archaean model results also show similarities to structures produced during lateral flow and withdrawal of salt in fold belts

experimental insights on the geometry and kinematics of fold and thrust belts above weak viscous evaporitic decollement abstract this paper discusses the differences in mechanical properties and kinematics between fold and thrust belts detaching on evaporitic decollements and those detaching on stronger detachments physical experiments are described that model shortening of a thick brittle cover overlying a weak viscous decollement to gain a better understanding of these differences we tested the influence of 1 the decollement layer thickness and 2 the presence of a deformable backstop on the hinterland side and of a decollement pinch out on the foreland side because of the very low shear strength of the viscous decollement folds and thrusts did not propagate according to a piggy back sequence but by centripetal back and forth propagation additional shortening was accommodated by growth of all existing structures fold symmetry and thrust vergence varied between experiments models confined between two rigid vertical endwalls always deformed by symmetric folding and thrusting in models having a deformable backstop and a foreland decollement pinch out forethrusting initially dominated folds were asymmetric and fault blocks rotated in models having a thick decollement layer folds kept growing asymmetrically diapirism also depended on the initial decollement geometry diapirs formed only in models having a deformable backstop and were restricted to the hanging wall of a fault related fold finally the implications of such kinematics on the thermal history are discussed as well as the likelihood of hydrocarbon maturation and preservation

centrifuge modelling of deformation of a multi layered sequence over a ductile substrate 1 style and 4d geometry of active cover folds during layer parallel shortening centrifuge analogue modelling illustrates the progressive development of active folds in multilayers upon a ductile substrate during layer parallel shortening models simulate folding of a mechanically stratified sedimentary sequence upon migmatitic gneisses in a large hot orogen or upon a thick basal evaporite shale sequence in deeper levels of fold belts the absence of a weak low viscosity and low density layer at the interface promotes infolding of the cover sequence and ductile substrate whereas a planar upper surface to the basal ductile substrate is preserved when it is present whilst fold style wavelength and deformation of the interface with the ductile substrate differ depending on whether a low viscosity and low density layer is present at the base of the cover sequence there is no marked systematic curvature of fold axes as seen in previous sandbox models for fault bend or fault propagation folding during bulk shortening bulk shortening of a layered sequence with relatively thick individual layers above a ductile substrate promotes a regular and upright train of buckle folds whereas thinner layers promote a more irregular distribution of buckle folds with variable vergence style and amplitude buckle folds above a ductile substrate progressively develop during bulk shortening from open and upright to angular and tight and may further develop into cuspate structures above relatively weak horizons relatively thick weak horizons within the layered sequence during bulk shortening interrupt regular fold patterns up structural section and allow out of phase folds to develop above and below the weak horizon

himalayan hinterland verging superstructure folds related to foreland directed infrastructure ductile flow insights from centrifuge analogue modelling abstract the orogenic superstructure ss and infrastructure is constitute two levels of a mountain belt with contrasting structural styles in the nepal himalaya n verging back folds which oppose the orogenic vergence dominate the ss competing explanations for these folds are tested using centrifuge analogue models modelling suggests that ss folding occurs during bulk shortening accompanied by is thickening before is flow focused erosion then instigates is lateral flow and stretching decoupling of the ss and transposition of the lower ss into a detachment zone decoupling at the is ss interface separates an ss dominated by older folds and an is characterised by younger horizontal transposition and stretching of early folds extrusive ductile flow of the is locally modifies fold vergence in the ss the fold asymmetry is thus controlled by the efficiency of coupling between is and ss a low viscosity at the is ss interface favours complete decoupling and hinders modification of fold vergence whereas a higher viscosity is ss interface favours fold vergence modification modelling supports a tectonic scenario in which himalayan hinterland verging folds are the product of early shortening of the ss followed by local modification of fold geometry when the is subsequently stretches and flows during focused erosion and melt enhanced is weakening

gravity driven continental overflow and archaean tectonics whether modern tectonic processes differ substantially from those in archaean times 2 500 myr ago remains controversial one view1 is that archaean tectonic processes were some combination of modern ones occurring faster or more shallowly because of the larger heat output of the early earth but others2 have proposed that significantly different processes operated here i argue that gravitational spreading of archaean continents would have caused them continuously and pervasively to overflow onto adjacent ocean basins and that this process would have naturally ceased at the end of the archaean era because modern continental crust is believed to be ductile rather than brittle below a depth corresponding to a temperature of about 350 400 c ref 3 it seems likely that such a ductile zone was universally present within the hotter archaean continental crust if the mean geothermal gradient of the continents had exceeded 25 30 c km 1 then the resulting ductile zone would have caused continental overflow to occur and such a process can account for many of the distinctive peculiarities observed in the archaean geological record the cessation of continental overflow corresponds naturally to the stabilizing cratonization which marked the end of the archaean era with its timing dependent on the evolution of both the geothermal gradient in the continents and the depth of the ocean basins

deformation style way back when thoughts on the contrasts between archean paleoproterozoic and contemporary orogens abstract in many important ways archean and paleoproterozoic older orogens differ structurally from contemporary examples this essay examines the premise that contrasts between older orogens and contemporary orogens reflect long term changes in the temperature of the continental crust in the density of supracrustal sections and in exhumation rates for example if continental crust were warmer and exhumation rates faster earlier in earth history then higher grade rocks would occur closer to the surface of older orogens and the orogens would be lower and wider this situation might contribute to the formation of wide belts of high grade gneiss found in ancient crust if the high strength layer of the crust were thinner and supracrustal sequences denser earlier in earth history then regional extensional tectonism might lead to crustal scale boudinage and diapirism this situation might explain formation of the extensive dome and keel provinces found in ancient crust testing such speculations through the application of structural analysis coupled with petrologic studies dating and rheological modelling will constrain models of earth s long term physical evolution

centrifuge modelling of folding in high grade rocks during rifting centrifuge modelling is used to simulate the progressive development of structures in a simplified crustal profile during rifting this paper focuses on folding of ductile layers representing the middle to lower crust displacement along a pre existing cut in the layer representing mantle lithosphere creates a broad shear zone in overlying ductile layers and an asymmetric rift in upper layers the footwall of the mantle lithosphere layer and overlying shear zone are rotated to sub horizontal during rise of the basal ductile layer representing asthenosphere as an isostatic response to thinning in the extended model synthetic and antithetic faults develop in models with thicker ductile crustal layers although upper semi brittle layers representing upper crust constitute a simple fault bounded or sag basin the underlying ductile layers are complexly folded open upright folds developed over the crest of the footwall mantle lithosphere layer are tightened and increased in amplitude as they progressively rotate to a recumbent attitude refolding occurs during rise of the basal ductile material representing asthenosphere and from boudinage of overlying competent layers this study suggests that regional scale folds in some high grade terrains previously interpreted as evolving in a convergent margin tectonic setting may instead be produced as a result of flow of the ductile crust during rifting

collision of barchan dunes as a mechanism of size regulation 1 barchans are propagating crescentic dunes that form in arid regions where strong unidirectional winds blow across a firm soil lightly covered with sand recently solitary barchan dunes have been shown to be unstable towards sand flux variation either growing to form mega dunes or shrinking and disappearing when perturbed from equilibrium however observations of large corridors of barchan dunes in the field suggest that these bedforms are stable over long periods of time since dunes migration rates are inversely proportional to their size barchans of different size must collide and these collisions may be crucial in maintaining the stability of dunes in nature here we first explain the unstable behavior of solitary barchans and then illustrate using down scaled physical experiments the qualitative dynamics of binary collisions a stability analysis inspired by these experiments suggests that collisions may indeed regulate the size of barchans migrating in a corridor by redistributing sand from large dunes to smaller ones

implications of channel flow analogue models for extrusion of the higher himalayan shear zone with special reference to the out of sequence thrusting the higher himalayan shear zone hhsz contains a ductile top to n ne shear zone the south tibetan detachment system lower stdsl and an out of sequence thrust oost as well as a top to n ne normal shear at its northern boundary and ubiquitously distributed compressional top to s sw shear throughout the shear zone the oost that was active between 22 ma and the holocene varies in thickness from 50 m to 6 km and in throw from 1 4 to 20 km channel flow analogue models of this structural geology were performed in this work a newtonian viscous polymer pdms was pushed through a horizontal channel leading to an inclined channel with parallel and upward diverging boundaries analogous to the hhsz and allowed to extrude to the free surface a top to n ne shear zone equivalent to the stdsu developed spontaneously this also indirectly connotes an independent flow confined to the southern part of the hhsz gave rise to the stdsl the pdms originally inside the horizontal channel extruded at a faster rate through the upper part of the inclined channel the lower boundary of this faster pdms defined the oost the model oost originated at the corner and reached the vent at positions similar to the natural prototype some time after the channel flow began the genesis of the oost seems to be unrelated to any rheologic contrast or climatic effects profound variations in the flow parameters along the hhsz and the extrusive force probably resulted in variations in the timing location thickness and slip parameters of the oost variation in pressure gradient within the model horizontal channel however could not be matched with the natural prototype channel flow alone presumably did not result in southward propagation of deformation in the himalaya

mechanisms for folding of high grade rocks in extensional tectonic settings abstract this review of structures developed in extensional high grade terrains combined with results of centrifuge analogue modelling illustrates the range of fold styles and mechanisms for folding of amphibolite to granulite facies rocks during rifting or the collapse of a thrust thickened orogen several extensional fold mechanisms such as folding within detachment shear zones are similar to those in contractional settings the metamorphic p t t path and not fold style or mode of formation is therefore required to determine the tectonic setting in which some folds developed other mechanisms such as rollover above and folding between listric normal shear zones and folding due to isostatic adjustments during crustal thinning are unique to extensional tectonic settings several mechanisms for folding during crustal extension produce structures that could easily be misinterpreted as implying regional contraction and hence lead to errors in their tectonic interpretation it is shown that isoclinal recumbent folds refolded by open upright folds may develop during regional extension in the deep crust folds with a thrust sense of asymmetry can develop due to high shear strains within an extensional detachment or from enhanced back rotation of layers between normal shear zones during back rotation folding layers rotated into the shortening field undergo further buckle folding and all may rotate towards orthogonality to the maximum shortening direction this mechanism explains the presence of many transposed folds folds with axial planar pegmatites and folds with opposite vergence in extensional terrains examples of folds in high grade rocks interpreted as forming during regional extension included in this paper are from the grenville province of canada norwegian caledonides albany mobile belt and leeuwin complex of western australia ruby mountains in the basin and range province of nevada the at sund area of greenland the napier complex of enderby land in east antarctica and the kigluaik mountains in western alaska

the orogenic superstructure infrastructure concept revisited quantified and revived the historical superstructure infrastructure concept s i expressed contrasts in structural style and metamorphic grade between shallow and deep orogenic levels two dimensional thermal mechanical models provide a quantitative explanation in terms of progressive crustal shortening and thickening phase 1 thermal relaxation and rheological weakening phase 2 and ductile flow at depth phase 3 results predict an upper crustal superstructure dominated by early steep structures separated across a subhorizontal high strain zone from a ductile infrastructure with late gently dipping structures this is consistent with observations from the western superior province these models can account for contrasts in structural style metamorphic grade seismic reflectivity and age between upper and lower crustal levels in contrast to conventional thrust tectonics models the revived s i model shows how young structures can form beneath older ones during progressive convergence thereby encouraging reassessment of standard seismic reflection interpretations

arc generated blocks with crustal sections in the north atlantic craton of west greenland crustal growth in the archean with modern analogues abstract the ca 700 km long archean craton of west greenland consists of six meso neoarchean ca 3000 2720 ma shear zone bounded crustal blocks that display similar cross sections from south to north ivittuut kvanefjord bjornesund sermilik fiskefjord maniitsoq each block has a southerly upper and a northerly lower zone thus each faces upwards to the south upper zones have prograde amphibolite facies mineralogy and have never been in the granulite facies whereas lower zones reached granulite facies and were partly retrogressed to amphibolite facies upper and lower zones consist predominantly of tonalite trondhjemite granodiorite ttg orthogneisses geochemistry suggests generation by slab melting in subduction settings of island arcs and active continental margins the gneisses contain km thick metavolcanic amphibolite layers typically bordered by km thick layers containing anorthosite and leucogabbro most upper zones contain upper greenschist to amphibolite facies metavolcanic belts including volcaniclastic andesitic rocks the two most prominent metavolcanic belts in the fiskefjord block at qussuk andesitic volcaniclastic rocks garde a a 2007 a mid archaean island arc complex in the eastern akia terrane godthabsfjord southern west greenland journal of the geological society london 164 565 579 and ivisaartoq mafic ultramafic rocks and anorthosite leucogabbro from upper and lower parts of a supra subduction zone system polat a frei r appel p w u dilek y fryer b ordonez calderon j c yang z 2008 the origin and compositions of mesoarchean oceanic crust evidence from the 3075 ma ivisaartoq greenstone belt sw greenland lithos 100 293 321 have island arc geochemical signatures the 2 km thick fiskenaesset complex bjornesund block comprises chromite layered anorthosites leucogabbros and gabbros and local pillow bearing roof pendants from overlying metavolcanic amphibolite the style of deformation changes downwards within crustal blocks upper zones are characterised by linear metavolcanic belts deformed by mostly one major phase of isoclinal folding and lower zones by kilometre scale double triple fold interference patterns everywhere ttg protoliths have intruded anorthositic and volcanic rocks typically along ductile shear zones often so extensively that only anorthositic or amphibolitic lenses are preserved the meso neoarchean crust was thickened by a combination of thrusting isoclinal folding and continued ttg injection dissimilarities in the proportions of anorthositic and metavolcanic rocks in the six blocks suggest that they evolved in several different microcontinents but by similar processes these crustal blocks provide an exceptional example of how continents evolved in the meso neoarchean comparable archean examples in kapuskasing and pikwitonei canada and modern analogues in fiordland new zealand kohistan himalayas southern california batholith peruvian andes and hidaka japan demonstrate that processes of continental growth from island arc to continental arc magmatism and by implication to continental collision were broadly similar throughout most of earth history

shrimp u pb zircon dating of archean core complex formation and pancratonic strike slip deformation in the east pilbara granite greenstone terrain sensitive high resolution ion microprobe shrimp u pb dating of zircons from granitic rocks in the east pilbara granite greenstone terrain has provided time constraints for main tectonic events in the shaw granitoid complex and has shown that deformation was intricately related to granitoid intrusion the earliest recognized deformation activity occurred within the split rock shear zone srsz probably between 3470 and 3420 ma the timing geometry and kinematics of this shear zone suggest that it was a midcrustal detachment in a core complex type extensional setting it was active during the intrusion of synkinematic sheeted granodiorite of the north shaw suite and brittle extensional faulting within the upper crust samples of gray migmatic gneiss and a diorite that occur structurally below the srsz were dated at 3451 1 ma and 3463 2 ma respectively indicating that melting and metamorphism took place in the lower units during extension in the middle to upper crust the 3470 3420 ma extensional event is most consistent with extensional collapse possibly as a result of delamination or convectional thinning of part of the lithosphere activity on the pancratonic strike slip mulgandinnah shear zone msz was constrained by the crystallization age of a synkinematic granitic dyke to have occurred at 2934 2 ma the msz and activity on the sholl shear zone of the western pilbara at 2960 ma smith et al 1998 are consistent with a regional nw se directed compressional stress field at this time steepening of greenstone belt stratigraphy has been attributed to diapirism but probably occurred after a 3300 3200 ma compressional event and following deposition of the youngest clastic sediments in the eastern pilbara stratigraphic sequence diapiric processes did not play a major role during the early evolution of the eastern pilbara craton from 3470 to 3300 ma

a three dimensional perspective on the evolution of archaean crust lithoprobe seismic reflection images in the southwestern superior province in 1990 1991 the lithoprobe project completed 450 km of seismic reflection profiles across the late archaean crust of the southwestern superior province the results define a broad three fold division of crust upper crust in the abitibi greenstone belt is non reflective and is a 6 8 km veneer of volcanic and plutonic supracrustal rocks whereas in the sediment gneiss dominated pontiac subprovince upper crust comprises shallow northwest dipping turbidite sequences mid crust in both the abitibi and the pontiac subprovinces is interpreted as imbricate sequences of metasedimentary and metaplutonic rocks lower crust in both subprovinces has a horizontal layer parallel strycture which may represent interleaved mafic intermediate gneisses the seismic signature of the northern abitibi greenstone belt may be represented in an exposed 25 km crustal section in the kapuskasing stuctural zone preliminary tectonic models based on the seismic data are consistent with a plate tectonic scenario involving oblique subduction and imbrication of sedimentary plutonic and volcanic sequences the northern abitibi supracrustal sequences either represent an allochthon or overlie an allochthonous underthrust metasedimentary and plutonic sequence which may be equivalent to a metasedimentary subprovince such as the pontiac or quetico seismic velocities have yet to be defined however crustal thicknesses are relatively constant at 35 40 km the thinnest crust is adjacent to the grenville front where moho is very well defined

modeling of thrust fronts above ductile and frictional detachments application to structures in the salt range and potwar plateau pakistan series of scaled sandbox models are used to simulate the development of thin skinned simultaneous shortening above adjacent ductile and frictional substrates these models simulate the evolution of the potwar plateau and salt range in pakistan where pale

crustal channel flows 2 numerical models with implications for metamorphism in the himalayan tibetan orogen 1 results from a thermal mechanical model ht1 that includes midcrustal channel flow are compatible with many features of the himalayan tibetan system radioactive self heating and rheological weakening of thickened model orogenic crust lead to the formation of a hot low viscosity midcrustal channel and a broad plateau channel material corresponding to the greater himalayan sequence ghs flows outward from beneath the plateau in response to topographically induced differential pressure at the plateau flank it is exhumed by focused surface denudation and juxtaposed with cooler newly accreted material corresponding to the lesser himalayan sequence lhs the model channel is bounded by coeval thrust and normal sense ductile shear zones interpreted to represent the main central thrust mct zone and south tibetan detachment system respectively inverted metamorphism associated with the model mct zone results from distributed ductile shear along the mct and extrusion of the hot channel a variety of model p t t path styles resembling those observed in the ghs and lhs are produced for points traveling through contrasting tectonic regimes that coexist in different parts of the model predicted times of peak metamorphism cooling and erosion of metamorphic facies are generally compatible with observations although model ghs cooling ages are too young the times of m1 and m2 metamorphic events observed in the ghs correspond to model times of maximum burial and maximum heating respectively the results highlight the need to integrate tectonics and metamorphism in continental collision models and demonstrate the importance of lateral transport of both heat and material in large hot orogens

structural event framework for the eastern yilgarn craton western australia and its implications for orogenic gold abstract the nnw trending tectonic grain of the eastern yilgarn craton eyc was established as a result of predominantly ene wsw directed extension d1 and d3 and e ene w wsw d2 d4 to ne sw directed d5 contraction the result has been a succession of nnw striking temporally discrete fabric elements which can be difficult to interpret reliably at any single location despite this many past workers interpreted the nnw striking fabric as the result of only one regional contractional event and used it as a marker for correlating structural events across the region in order to unravel the complexity this paper presents a new sixfold d1 d6 deformation nomenclature based on 10 000 new mesoscale structural observations including their kinematic analysis and cross cutting relationships these mesoscale data were referenced with regional 3d map patterns stratigraphic magmatic metallogenic considerations and deep seismic reflection images this integrated geodynamic architectural approach is applicable to solving structural event histories in other polydeformed terrains gold mineralisation occurred during the first five events but was particularly concentrated from d3 onwards the d3 event marked the most profound change in the tectonic evolution of the eyc with changes in greenstones granites and tectonic mode lithospheric extension and core complexes with the first significant gold deposited within extensional shear zones that dissect the crust later contraction d4 was imposed at a high angle to the previously established anisotropic architecture the outcome was the creation of a new dynamic permeability framework which resulted in gold mineralisation during nnw striking sinistral strike slip faulting and associated thrusting a further stress switch d5 further modified the architecture resulting in n to nne striking dextral strike slip faulting and the final period of gold mineralisation before late stage extension d6

new u pb and 40ar 39ar ages from the northern margin of the barberton greenstone belt south africa implications for the formation of mesoarchaean gold deposits gold mineralization in the mesoarchaean new consort gold mine in the barberton greenstone belt south africa is situated in the immediate hanging wall of an exhumed composite granitoid gneiss dome known as the stentor pluton the granitoid greenstone contact is characterized by a contact parallel high strain zone that separates the mid crustal gneisses from the greenschist facies greenstone belt in contrast to the lower greenschist facies deposits in the central parts of the greenstone belt the mineralized shear zones at new consort are hosted by distinctly higher grade metamorphic rocks ranging from upper greenschist to upper amphibolite facies conditions a distinct change of ore and alteration assemblages with increasing metamorphic grade points to a temporal and spatial relationship between gold mineralization and the juxtaposition of the stentor gneiss dome against the overlying greenstone belt examination of the structural and petrologic evolution of the mineralized high strain zone combined with new u pb and ar ar data and previously published ages indicates a multistage tectono metamorphic evolution early high t medium p metamorphism ca 600 700 c and 5 1 kbar in the structurally deeper parts of the mine is interpreted to be related to an early period of orogen parallel ne sw extension and associated detachment shearing during the main d2 accretionary event in the centre of the greenstone belt between ca 3260 and 3225 ma recent work has shown that these early high t fabrics were reactivated at ca 3105ma during the subsequent d3 tectonism and associated emplacement of large potassic granite batholiths the gold mineralization was coeval with ne sw extensional exhumation and solid state doming of the stentor gneiss dome during the subsequent d4 deformation this late stage deformation and the gold mineralization postdate the emplacement of the d3 related potassic granites by more than 50 million years as indicated by 1 a u pb age of 3046 27ma for a retrograde titanite generation in a d2 related amphibolite sample 113 and 2 a 3027 7 5ma age for hydrothermal titanite from the so called footwall lens mineralization which temperature has been estimated at 590 40 c this high t event is also recorded by a gcorrected h 3040 4ma hornblende ar ar age from the same sample suggesting that cooling of the stentor gneiss dome shortly followed the exhumation a considerably younger ar ar age 2966 4 ma has been obtained for an amphibolite from the contact parallel high strain zone pointing to non uniform cooling following the mineralization the gold mineralization was largely contemporaneous with the emplacement of nnw trending syn kinematic granitic pegmatites and strain localization along the pegmatite swarm at ca 3030 3040 ma the mineralization is also considerably younger than the gold mineralization in the greenschist facies gold deposits in the barberton greenstone belt which has previously been dated at ca 3080 ma in conjunction with previously published ages the data indicate the au mineralization in the barberton greenstone belt to have occurred during at least two mineralization episodes at ca 3080 and 3040 ma the main phase of au mineralization was introduced during the end of an either protracted or episodic structural and thermal evolution of the granitoid greenstone terrain that may have lasted some 200 ma

geodynamic models of archean continental collision and the formation of mantle lithosphere keels 1 the processes responsible for the formation of thick strong and cold archean sub continental lithospheric mantle mantle keels beneath archean cratons remain elusive here the dynamics of some such processes are studied by forward numerical modeling of the thermo mechanical evolution of continental lithosphere undergoing collision and orogenesis under neoarchean like conditions the numerical experiments illustrate that depending on the composition of the crust and the degree of radioactive heat production rhp in the crust three dominant modes of mantle lithosphere deformation evolve 1 pure shear thickening 2 imbrication 3 and a mode best described as underplating all three modes of deformation result in the thickening and emplacement of plate like mantle lithosphere to depths between 200 km and 350 km the transition from pure shear thickening to imbrication is largely dependent on the degree of rhp in the crust while the transition from the imbrication style to the underplating style is dependent on the composition of the lower crust

neoarchean lithospheric strengthening and the coupling of earth s geochemical reservoirs this paper explores the secular evolution of the height of an isostatically balanced collisional mountain belt in the context of ongoing convergence we show that until the neoarchean continents were unable to sustain topography 2500 m during the neoarchean the continental lithosphere evolved through a rheological threshold allowing for the development of signifi cant topography the consequence of the strengthening of the continental lithosphere is fundamental for the coupling of the earth s geochemical reservoirs the neoarchean was a period of global changes during which exogenic envelopes recorded major shifts in composition toward modern values we propose that during the neoarchean the exogenic envelopes which were until then coupled to the mantle through hydrothermal processes and volcanism also became coupled to the continental crust through reliefgenerating tectonics processes and erosion hence changing the balance between mantle versus crustal interaction with the exogenic earth

book review the techniques of modern structural geology volume 2 folds and fractures john g ramsay and martin i huber academic press london 1987 xi 309 398 pp price stg17 50 us 34 50 paperback

archaean subduction inferred from seismic images of a mantle suture in the superior province plate tectonics provides the basis for the interpretation of most current terrestrial tectonic activity and is widely accepted as having been active over much of the earth s history1 yet the timing of initiation of this process is subject to debate2 9 so far the earliest seismic evidence for plate tectonics has come from a fossil mantle suture in the svecofennian orogen 1 89 gyr ago 10 and from inferred plate convergence subduction and accretion in the trans hudson orogen 1 91 1 79 gyr ago 11 as yet seismic data from archaean areas have been able to demonstrate only the importance of compression in the construction of the continental crust12 15 here we present seismic data from a collision zone in the superior province of canada involving the abitibi granite greenstone subprovince and the plutonic arc related opatica belt we interpret dipping seismic reflections that extend 30km into the mantle as representing a relict 2 69 gyr old suture associated with subduction although crustal structure lithospheric thicknesses and convergence rates may have differed from those seen today these seismic data provide direct evidence that plate tectonics was active in late archaean times 800 myr earlier than indicated by previous seismic reflection surveys

structural evolution of the cross lake greenstone belt in the northwestern superior province manitoba implications for relationship between vertical and horizontal tectonism evidence for both vertical and horizontal movement is well preserved in the cross lake greenstone belt in the northwestern superior province the vertical movement components are concentrated in high strain zones along pluton greenstone contacts and are characterized by pluton side up or greenstone side down movement and the geometry kinematics and strain distribution are consistent with a vertical tectonic model involving diapirism and sagduction the horizontal components are concentrated in major east southeast trending dextral high strain zones and in subordinate northeast trending sinistral antithetic high strain zones and can be readily explained by a horizontal tectonic model involving dextral transpression results of a detailed structural analysis indicate that the vertical and horizontal tectonism were more or less synchronous and there was a transition from dominantly vertical tectonism at the early stages to dominantly horizontal tectonism at the late stages the cross lake group consist

the north pole dome a non diapiric dome in the archaean pilbara craton western australia abstract the northeastern part of the archaean pilbara craton is characterised by an ovoid dome and basin pattern of domal granites separated by synformal greenstones supracrustal rocks this ovoid pattern is variably preserved at various structural and stratigraphic levels with the most deeply eroded domes being granite dominated and the least eroded domes being greenstone dominated the north pole dome npd is an example of a relatively high level dome that has a flanking syncline preserving some of the youngest rocks of the craton fortescue group to be involved in the doming process at the apex of the npd is a small intrusive granite that was considered by some workers to be the top of a large underlying domal granite batholith the npd comprises mostly dense basaltic greenstones that range in age from approximately 3500 to 2700 ma the approximately 3460 ma north pole monzogranite a volumetrically insignificant intrusive granite body intrudes the greenstones in the apex of the dome no marginal shear zones occur around the intrusion a new model based on a detailed gravity traverse across the monzogranite suggests that the granite intrusion is plug like up to 1 5 km thick and does not represent the exposed top of a larger underlying domal batholith results from potential field modelling show that the dome is relatively flat bottomed with a base around 5 5 6 5 km deep the npd has no significant granitic material within the dome but like all greenstones is underlain by felsic crust granite below its base the development of the npd and flanking syncline was a multistage process the first stage of doming involved relatively minor doming tilting possibly associated with the emplacement of the monzogranite because palaeocurrents of synchronous volcanic rocks flowed radially outward from the dome it is likely that this doming was minor as there are no recorded unconformities in the warrawoona group in the npd above these volcanic rocks a major dome forming event tilting 20 occurred in the period between 3240 and 2772 ma and was unrelated to the emplacement of the small granite plug diapirism regional folding and refolding from horizontal compression deformed the area into a domal shape type i fold interference pattern uplift and erosion of the dome was superseded by extension and deposition of flood basalts fortescue group that flowed towards the dome three further stages of shortening folded the regional unconformity and the underlying and overlying units further amplified the underlying dome developed the flanking marble bar syncline as well as fold interference patterns in the fortescue group the npd was developed over a 800 ma time frame ostensibly by a process of fold interference due to multiple stages of horizontal compression this work shows that diapirism was not the cause of the development of the domal geometry of the npd and its flanking syncline rather folding and refolding due to horizontal compression was the principal controlling factor

himalayan metamorphic sequence as an orogenic channel insight from bhutan the bhutan himalayas differ from the rest of the himalayas in two major ways i low grade sedimentary rocks lie above the greater himalayan sequence as klippen i e erosional remains of the south tibetan detachment and ii an out of sequence thrust the kakhtang thrust lies structurally above the klippen and it doubles the exposed thickness of the greater himalayan sequence our field observations and geochronological data constrain the main kinematic events in the bhutan himalayas crystallisation ages of leucogranite dykes deformed by the main central thrust and the south tibetan detachment indicate that these two structures operated together between 16 and 22 ma the out of sequence kakhtang thrust was active at 10 14 ma and was concurrent with reactivation of the south tibetan detachment restoration of the bhutan himalayas prior to the out of sequence thrusting shows that the greater himalayan sequence was the core of a long low viscosity crustal channel extending under the tibetan plateau we propose that the gravity driven southward extrusion of the channel material from underneath the tibetan plateau caused the inverted metamorphic sequence in the lesser himalayan sequence and in the greater himalayan sequence this process also led to occurrences of present day surface rocks that were derived from variable distances down dip but from similar crustal depths such an exhumation pattern can explain the similar peak pressures for the greater himalayan sequence along the length of the himalayas

simultaneous generation of archean crust and subcratonic roots by vertical tectonics archean cratons are characterized by granite greenstone belts by abundant tonalites trondhjemites and granodiorites ttgs and by the presence of strong and buoyant subcratonic lithospheric mantle sclm individually mechanisms for their formation remain controversial and together they provide no clear picture of the tectonic style prevalent during the late hadean and early archean following earlier observation that the dome and keel structure of granite greenstone terranes resembles salt diapirs we present numerical calculations that show that in pre archean to middle archean thermal regimes mafic to ultramafic volcanics overlying a felsic basement will overturn diapirically in as few as 10 ma displacing as much as 60 of the volcanics to the lower crust this suggests that diapirism may have dominated hadean and early archean crustal tectonics furthermore repeated cycles of bimodal volcanism and crustal recycling by diapirism provide a mechanism for making ttgs leaving a restite that contributes to the sclm from above this diapiric tectonism later declined as the earth cooled and plate tectonics became the dominant paradigm

power law viscous materials for analogue experiments new data on the rheology of highly filled silicone polymers abstract the selection of appropriate analogue materials is a central consideration in the design of realistic physical models we investigate the rheology of highly filled silicone polymers in order to find materials with a power law strain rate softening rheology suitable for modelling rock deformation by dislocation creep and report the rheological properties of the materials as functions of the filler content the mixtures exhibit strain rate softening behaviour but with increasing amounts of filler become strain dependent for the strain independent viscous materials flow laws are presented while for strain dependent materials the relative importance of strain and strain rate softening hardening is reported if the stress or strain rate is above a threshold value some highly filled silicone polymers may be considered linear visco elastic strain independent and power law strain rate softening the power law exponent can be raised from 1 to 3 by using mixtures of high viscosity silicone and plasticine however the need for high shear strain rates to obtain the power law rheology imposes some restrictions on the usage of such materials for geodynamic modelling two simple shear experiments are presented that use newtonian and power law strain rate softening materials the results demonstrate how materials with power law rheology result in better strain localization in analogue experiments

mantle shear zones and the evolution of the northern north sea basin mantle reflections observed on deep seismic profiles across the northern north sea may be tentatively interpreted as extensional shear zones modeling them as such explains many of the features of the basin in particular the broad regions of postrift onlap onto the basin flanks consequently it seems likely that the mantle reflections represent mantle shear zones which control the extension of the subcrustal lithosphere

structural evolution of the warrawoona greenstone belt and adjoining granitoid complexes pilbara craton australia implications for archaean tectonic processes abstract the pilbara granite greenstone terrain 3 6 2 8 ga australia is quoted as the key area formed by passive gravity driven solid state diapirism but contrasting views have been reported recently which led us to re examine the key area the warrawoona greenstone belt and the adjoining mt edgar and corunna downs granitoid complexes we found i an asymmetric non synclinal distribution of metamorphic grade and deformation intensity across the warrawoona greenstone belt ii a major shear zone on the margin of the mt edgar granitoid complex with a unidirectional stretching lineation pattern iii a ne trending transcurrent shear zone within the central gneiss complex of the mt edgar granitoid complex iv coeval magmatic activity and doming of the granitoid complexes and v the loss of about 9 km of stratigraphic units in the warrawoona greenstone belt the area has had a prolonged history with at least four deformation phases between ca 3 47 and 2 8 ga the main domal geometry of the mt edgar granitoid complex was obtained during active ne sw extension at 3 31 ga d x 2 which appears to been in response to e w compression possibly the e w compression resulted in crustal thickening and partial melting probably of a 3 46 3 43 ga granitic basement causing crustal weakening collapse and extrusion of the 3 31 ga granitoids extension at 3 31 ga was accommodated by the mt edgar shear zone a mid crustal detachment and by the associated beaton well zone which acted as a transfer zone within the gneissic basement doming of the mt edgar granitoid complex is interpreted to have occurred in an active extensional core complex type mode an indication for deformation prior to 3 32 ga comes from syntectonically intruded gabbro diorite and dolerite sills within the mt edgar shear zone which intruded possibly as early as 3 46 3 45 ga d x 1 ultramafic schistose xenoliths within the gabbro diorite intrusions are the oldest observed deformed rocks formed during d x post doming deformation involved ne sw compression d x 3 followed by dextral transcurrent deformation d x 4 during nw se compression the latter may be related to the regional phase of deformation at 2 93 ga during which the eastern and western pilbara were amalgamated comparison of this deformation sequence to others within the marble bar domain suggests a common tectonic history from 3 46 ga onwards our structural and kinematic data especially the 3 31 ga unidirectional stretching lineation pattern lead us to reject solid state diapirism due to convectional overturn as the main cause for doming of the mt edgar granitoid complex we suggest that the cause lies in a plate tectonic process

thermoluminescence dating tests for lacustrine glaciomarine and floodplain sediments from western washington and british columbia to help further develop reliable procedures for accurate thermoluminescence tl dating of quaternary waterlaid sediments we tested tl dating procedures on sediment types rarely examined six glaciolacustrine samples three samples of glaciomarine drift and eight samples of floodplain deposits we used the partial bleach r beta gamma technique applied to fine silt polymineral grains results from our younger known age glaciolacustrine sediments confirm earlier observations that only the clayey laminae are generally suitable for tl dating a clayey lamina older than ca 140 150 ka produced no age underestimation and this result ca 300 ka suggests that such older lake sediments are probably suitable for tl dating two proximal samples of glaciomarine drift produced large tl age overestimates whereas the single distal glaciomarine drift sample yielded an expected age 177 38 ka suggesting that follow up studies are warranted our results for eight flood plain samples confirm that zeroing of li