late pleistocene glaciation of the kosciuszko massif snowy mountains australia abstract late pleistocene glaciation of the australian mainland was restricted to a small area of the southeastern highlands geomorphic mapping of the area and exposure dating using the in situ produced cosmogenic isotope 10be provides evidence for at least two distinct glaciations the early kosciuszko glaciation consisted of a single glacier advance before 59 300 5400 years ago snowy river advance the late kosciuszko glaciation comprised three glacier advances 32 000 2500 headley tarn advance 19 100 1600 blue lake advance and 16 800 1400 years ago mt twynam advance the early kosciuszko glaciation was the most extensive and the late kosciuszko advances were progressively less extensive these periods of glaciation in the highlands correspond to episodes of periglacial activity and peaks in lake levels and river discharge at lower elevations in southeastern australia glacier advances on the kosciuszko massif correlate with advances in tasmania south america and new zealand and are broadly representative of hemispheric climate changes during the last glacial cycle

direct correlation of terrestrial and marine paleoclimatic records from four glacial interglacial cycles dsdp site 594 southwest pacific abstract over the last 350 ka changes in the composition of vegetation on new zealand inferred from pollen analysis of the upper 40 m of dsdp site 594 at 2 4 ka sample intervals reflect regional climatic variations which appear synchronous with implied variations in glacier fluctuation and in global climatostratigraphy described from sedimentary and oxygen isotope records from the same samples nelson et al 1985 pollen assemblages from isotope stages 1 5e 7a 7b and 9 are distinguished by conifer and broadleaf forest taxa which vary in composition between the last four interglacials suggesting significant differences in precipitation temperature and or migration rates glacial pollen assemblages which imply the expansion of herbland and decline in forest components show less variation and generally indicate comparatively cool conditions on the east coast of south island interstadial vegetation is composed of a mosaic of shrubland herbland vegetation the close correspondence between variations in the amplitude and timing of these continuous records of forest development in the changing vegetation of south island new zealand and oxygen isotope climatostratigraphy supports previous suggestions that late quaternary southern and northern hemisphere climatic fluctuations were essentially synchronous

lithophile elements in huronian low ti continental tholeiites from canada and evolution of the precambrian mantle abstract huronian basalts from central ontario canada dated at about 2450 ma and associated with an early rifting episode are classified as siliceous low ti nb tholeiites they display strong enrichment in large ion lithophile lile and light rare earth lree elements compared to modern oceanic lavas the tectonic setting and geochemistry resemble mesozoic rift related low ti flood basalts including the ferrar group of antarctica and the parana and equivalent etendeka volcanics of south brazil and namibia respectively high lile lree ratios are also similar to subduction related island arc tholeiites and it is suggested that enrichment of the huronian lithospheric mantle source occurred through ancient subduction of crustal material probably during formation and consolidation of the archean continental crust melting models suggest that huronian subcontinental mantle source compositions derived from least contaminated aphyric mafic end members had already undergone a complex evolution including withdrawal of archean basalts and hydrous enrichment in incompatible components despite several subsequent melting episodes and a second probably magmatic enrichment event however many aspects of the huronian source signature were preserved and appeared in later basaltic products of this mantle mass keweenawan volcanics for example dated at about 1100 ma preserve low p zr ti and hree abundances

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

nd and sr isotopic systematics of central australian granulites chronology of crustal development and constraints on the evolution of lower continental crust nd and sr isotopic data are reported for a granulite terrain in the proterozoic arunta block of central australia sm nd data from a wide range of rock types define a crust formation age of 2 070 125 ma and provide further evidence for voluminous crustal growth in the proterozoic an nd value of 1 5 0 8 indicates a depleted mantle source for this crustal segment and there is no evidence for a large component of significantly older sialic crust field relationships geochemistry and rb sr data for mafic and felsic granulites indicate that intracrustal differentiation and polyphase deformation were followed by granulite facies metamorphism rb depletion at 1 800 ma rb sr data for strongly retrogressed granulites define an age of 1 700 ma which is interpreted as the time of retrograde biotite growth partial melting at the presently exposed crustal level and anatexis at deeper crustal levels were broadly coeval with the retrograde metamorphism sm nd and rb sr isotopic systematics of minerals indicate that the terrain cooled slowly did not experience significant uplift until 1 000 ma and remained at temperatures above 320 c until the late palaeozoic the mineral data are consistent with geological relationships and petrological evidence for a prolonged period of isobaric cooling followed by uplift late in the metamorphic evolution of the terrain the granulite protoliths appear to have formed in a rift which closed within 280 ma of initial separation deformation and granulite facies metamorphism at 1 800 ma are interpreted to be a consequence of collision between the continental blocks which defined the rift regional retrogression and granitoid magmatism at 1 700 ma are attributed to underthrusting of the granulites by lower grade rocks in the final stages of collision subsequent events in the cooling and uplift history appear to have been controlled by the presence of long lived major faults in the crust and a prolonged history of episodic compression in the continental lithosphere the results of this study suggest that granulite terrains in general cannot be equated with lower continental crust but instead represent assemblages of mainly supracrustal rocks which in some instances have been involved in major collision events

collision tectonics in the nw himalayas west himalayan tectonics involve the collision of microplates between the indian and asian plates the kohistan complex consists largely of tightly folded basic volcanics and sediments generated as late jurassic to late cretaceous island arcs these were intruded by post folding mid cretaceous eocene plutonics produced from continued subduction of the indian plate after closure of a suture between kohistan and the karakorum the himalayan structures show major thrust sheets and the kohistan arc is essentially a crustal pop up with southward upright and northward verging structures developed above a thick ductile decoupling zone the indus suture which can be traced for 100 km beneath kohistan on large reentrants this pop up formed by a two stage process closure of the northern suture followed by closure of the southern indus suture granitic rocks of the kohis tan ladakh batholith dated at 100 40 ma post date most of the structures related to the northern suture but were deformed and carried southwards on shear structures related to the indus suture postcollisional deformation carried this kohistan complex on deep decoupling zones over the indian plate on a series of imbricated gneiss sheets the thrusts climbing up section in the movement direction so that in the far s some override their own molasse debris folds above these deep decoupling zones deformed their overlying thrust sheets into large antiforms i e the nanga parbat and hazara syntaxes the nanga parbat syntaxis probably formed due to a shear couple near a branch line where one of the main himalayan thrusts joined the indus suture beneath kohistan crustal delamination to produce the imbricated gneiss sheets could not account for all the displacement of india into asia suggested by palaeomagnetic data there must also have been lateral displacement as demonstrated by the large oblique slip shear zone in the hunza valley n of kohistan

stratigraphic and structural constraints on the proterozoic tectonic history of the olary block south australia abstract the lower proterozoic of the olary block is divided into five rock suites defining a conformable stratigraphic sequence correlated with the willyama supergroup of the broken hill block which has a probable depositional age no greater than 1800 ma the sequence was deformed and metamorphosed during the olarian orogeny at 1580 ma definition of the stratigraphy permits regional structural analysis and the resolution of two folding events an initial recumbent fold terrain redeformed by upright folds with a near vertical axial surface the olarian orogeny comprises a single regional compression axis with both folding events indicating a se direction of transport the described tectonic fabrics are consistent with simple shear deformation generating the early recumbent terrain

the evolution of weathering profiles through time new insights from uranium series isotopes the sustainability of soil is a major issue for society in principle the evolution of soil resources can be constrained by comparing the rates of soil erosion and production cosmogenic isotopes provide one measurement of soil erosion rates they can also be used to estimate soil production rates but only if erosion is assumed to be balanced by production this implies that the evolution of soil resources thinning thickening or constant since soil thickness is assumed to be constant with time here we utilise an independent method to estimate soil production rates using uranium series u series isotopes the study of a site in temperate australia undisturbed by human activity shows that soil production rates inferred from u series isotopes are similar to erosion rates derived from beryllium 10 10be measurements implying that at this site there is no net accumulation or loss of soil saprolite production rates the migration rate of the weathering front into the bedrock are also similar to erosion rates so the thickness of the entire weathering profile is effectively in steady state this study demonstrates that the combination of u series and cosmogenic isotopes can be used to quantitatively assess soil evolution and the development of weathering profiles preliminary observations suggest that the rate of bedrock weathering i e saprolite production in temperate australia is of the same order of magnitude as that inferred for laterites in tropical climates this may suggest that for thick weathering profiles although the extent of weathering strongly differs between temperate and tropical climates the migration of the weathering front into the bedrock occurs at a relatively uniform rate regardless of present day climatic conditions

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

inversion of field data in fault tectonics to obtain the regional stress i single phase fault populations a new method of computing the stress tensor summary we attempt a general definition of the inverse problem of computing the components of the regional stress tensor from a set of field data including the measurements of the strike and dip of several faults and the directions and senses of relative motion along these faults as indicated by slickenslides in previous treatments of this problem no experimental errors could be taken into account except those in measuring the pitch of slickenslides thus errors in the orientation of the fault strike and dip which have considerable practical importance were neglected in our work all experimental errors in the field measurements are taken into account so that the agreement between the computed stress tensor and the set of field measurements can be rigorously checked

development of conjugate shear bands during bulk simple shearing abstract in rocks possessing a strong planar fabric shear bands of constant shear sense and oriented at an oblique angle to the foliation are considered by many authors to be characteristic of a non coaxial bulk deformation history whereas conjugate shear bands are considered to indicate coaxial shortening however in two areas where bulk deformation history appears to be non coaxial cap corse corsica and ile de groix brittany conjugate shear bands are observed in order to investigate this problem experiments were performed by bulk simple shearing using plasticine as a rock analogue when slip between layers of the model is permitted shear bands of normal fault geometry form with both the same and opposite shear sense as the bulk simple shearing at approximately the same angle with the layering 40 irrespective of layer orientation in the undeformed state for initial orientations of 50 30 and 15 shear bands are initially formed within individual layers and may propagate across layer interfaces when further movement along these is inhibited the existence of conjugate shear bands in corsica and ile de groix is therefore not incompatible with a model of bulk simple shearing for these two regions in field studies one should perhaps exercise care in using shear bands to determine the kind of motion or the sense of bulk shearing

proterozoic evolution and tectonic setting of the northwest paterson orogen western australia abstract the proterozoic igneous deformation and metamorphic histories of the palaeoproterozoic rudall complex in the northwestern paterson orogen can be linked to those of the arunta inlier in central australia and in part with the capricorn orogen in central western australia the similarities in deformation and metamorphic histories for these widely separated regions indicate a palaeoproterozoic continent continent collisional event between the palaeoproterozoic west australian and north australian cratons between c 1830 and 1765 ma in the paterson orogen this palaeoproterozoic collisional event resulted in the yapungku orogeny which included thrust stacking of clastic sedimentary and volcanic rocks deposition of the protoliths for the c 1790 ma siliciclastic paragneiss succession contemporaneous with granitic intrusion and metamorphism up to granulite facies during this 65 million year period the arunta inlier and capricorn orogen were deformed metamorphosed at medium to high grades and intruded by granitoids during the strangways orogeny in the arunta inlier and the capricorn orogeny in the capricorn orogen the neoproterozoic tarcunyah throssell and lamil groups are clastic sedimentary sequences that were deposited after 1070 ma in the northwestern paterson orogen and deformed by the miles orogeny before 678 ma the miles orogeny produced a northwesterly trending fold and fault system of tight to isoclinal upright and overturned folds and thrust faults the orogeny may have been coincident with the c 750 720 ma areyonga tectonic movement affecting the arunta inlier and the lower neoproterozoic part of the amadeus basin in central australia at c 550 ma the paterson orogeny which is most likely equivalent to the petermann orogeny in the musgrave complex of central australia deformed the northwestern paterson orogen and was preceded by local intrusion of granites the similarities of styles and timing of deformation in the northwestern paterson orogen arunta inlier and capricorn orogen indicate that these three regions were probably linked during most of the proterozoic

temporal and lateral variation in the development of growth faults and growth strata in western niger delta nigeria this study examines eight syndepositional faults and syntectonic sediments in five major fault blocks in western niger delta offshore nigeria on three dimensional seismic data the initiation lateral growth and retreat periods of activity and quiescence and decay of faulting around these blocks can be ascertained by analyzing a series of time structure and isopach maps the study area can be subdivided into three structural zones the first structural zone is a northwestern zone characterized by a major counterregional growth fault in the deep subsurface this deep seated structure is superposed by an array of younger regional growth faults displacing a kilometer thick sedimentary overburden that accumulated on the former footwall the second structural zone is a central to eastern zone that seems mostly unaffected by young deltaic faulting this zone is characterized by the thinnest sedimentary record of the study area the third structural zone is a southeastern zone that is dominated by a large listric backstepping master fault zone associated with a kilometer scale rollover system regional structural and stratigraphic analyses document an apparently strong relationship between syntectonic sedimentation and syndepositional fault activity in that phases of significant fault activity lateral fault growth and fault migration concur with major depositional phases in turn areas and intervals characterized by the least sediment accumulation also record the lowest fault activity however one particularity of the studied system is that it underwent at least one period of seaward fault progression that coincided with a backstepping of faulting on the landward side although the forward stepping of faulting near the delta front can be interpreted as the consequence of the progressive loading during delta progradation the contemporaneous backstepping of faulting further inboard likely reflects the sustained lateral growth of mature deltaic faults into previously undeformed proximal parts of the depocenter the results of this study thus document that although an apparent correlation with the superimposed depositional system exists on a regional scale inboard deltaic faults may persist to grow irrespective of sedimentary loading the recognition of such fault trends is particularly important for estimating the influence of late stage fault movement on hydrocarbon migration or the discovery of subtle fault controlled hanging wall reservoirs

a dynamical model of lithosphere extension and sedimentary basin formation convection in the upper mantle can give rise to normal stresses on the base of the continental lithosphere that may cause localized elevation of its surface and extensional deviatoric stresses this possibility is investigated assuming homogeneous biaxial strain in a continental lithosphere whose strength is governed by a combination of brittle failure and power law creep in the crust and by brittle power law or dorn law behavior in the mantle for a given geotherm and for rheologies based on current laboratory data the lithosphere responds to convection induced uplift in one of three ways depending primarily on the magnitude of the driving stresses these regimes are as follows 1 when the driving stress exceeds a threshold value the resulting strain rates are high enough greater than about 3 10 16 s 1 that the lithosphere strains almost isothermally its effective rheology is strain thinning and it undergoes accelerating extension the magnitude of the dynamic uplift required to produce the threshold value of the driving stress depends on the geotherm characterized here by the moho temperature and varies from about 1 km for a moho temperature of 700 c to around 3 km for a moho temperature of 550 c an uncertainty of at least 100 c is attached to these temperatures owing to the inaccuracies in extrapolating laboratory data to geological strain rates 2 at stress levels due to less than about half of the critical uplift extensional strain is negligible 3 at intermediate stress levels moderate uplift produces extension that is bounded by strength increases due to cooling of the ductile portion of the lithosphere during extension

evolution of the hourglass structures in the laminaria high timor sea implications for hydrocarbon traps abstract in cross section an hourglass structure can be visualized as an older horst block and superimposed younger graben bounding faults of the horst and graben blocks represent separate conjugate fault systems formed by two distinct episodes of extension in the timor sea during late jurassic early cretaceous 1st phase and middle miocene pliocene 2nd phase with an 120 my hiatus of limited or no fault activity in between horst blocks were formed by the 1st phase of extension and buried post deformation with the onset of the 2nd phase of extension the hourglass geometry began to form by nucleation of the graben bounding faults in the shallow sedimentary section in isolation from the horst bounding faults location of the graben is biased by the buried horst block and graben bounding faults grew down dip from the shallow locus of nucleation toward the underlying horst block on which only minor reactivation occurred detachment of the two systems in this way was predominantly controlled by the first order mechanical layering a thick shale rich ductile layer separates the horst and graben bounding fault systems and acts as a barrier to vertical fault propagation confinement of the graben bounding faults into the shallow section was also facilitated by outer arc style extension due to lithospheric flexure controlling the 2nd phase strain in the region the complex evolution history and the composite nature of the hourglass structures resulted in systematic along dip variation of displacement this variation predominantly relates to syn kinematic deposition and location of fault tips that are controlled by the ductile layer the presented evolution model of the hourglass structures concentrates fault tips and related stress perturbation onto the top seal and is likely to be detrimental to top seal integrity

geology structure and lithostratigraphic framework of the rakhine coastal ranges in western myanmar implications for the collision of the india plate and west myanmar block abstract termination of the eastward subduction of the indian plate is inferred to be latest eocene in age in the north easternmost assam and myanmar regions followed by the collision of the indian continent with the asia plate this event is typically represented by the underthrusting of the middle and upper eocene sylhet formation which was deposited on the western margin of india plate beneath the eocene disang formation while the northern margin of india collided against eurasia and tibet along a zone which lies south of the triassic jurassic deep marine sediments and yarlung ophiolite in the latest eocene later movement of the naga and disang thrusts to the east suggests convergence between the burma and indian plates as both moved northwards and india rotated anti clockwise after the collision the stratigraphic records indicate that an early oligocene emplacement of the deep marine sedimentary sequence on the india plate probably within 10 ma interval age during the india asia collision and moved northwards oblique convergence could also have resulted from possible clockwise rotation of se asia relative to the indian plate accompanying the eastward translation of south china along strike slip faults the deformation of western and southern myanmar is still active today with under thrusting related to subduction the basement of the indian plate below the indo myanmar ranges is moving towards the north with respect to the rest of asia this study provides the syn and post deformation history of neogene siliciclastic sequences of the rakhine coastal ranges in western myanmar allowing structural and kinematic interpretation of the oblique subduction and collision history the growth of these folds and faults and associated uplift were resulted from the tectonic processes and evolution in which the burma plate was wedged in the subduction zone and dragged towards the north with the indian plate between internal n s trending right lateral shearing and external e w shortening

cretaceous cenozoic tectonic events in the western myanmar burma assam region correlation of late mesozoic mafic and ophiolitic rocks suggests that in the late lower cretaceous a northeasterly facing mafic arc was emplaced onto southeast borneo western sumatra western myanmar burma then 1100 km south of its present position the mogok belt of myanmar and the denqen bangong co ophiolite zone in tibet all then situated on the southwestern margin of asia reversal in tectonic polarity and eastward subduction generated a late cretaceous magmatic arc in western myanmar and sumatra and its continuation through the mogok belt into the gandise batholith of tibet magmatism interrupted in the latest cretaceous probably resumed in the western myanmar tibet and sumatra arc segments in the early palaeogene it was again interrupted in the mid eocene when an east vergent ophiolite nappe overrode the indo burman ranges deforming lower eocene turbidites derived from the magmatic arc to the east following renewed northward and eastward subduction of ocean floor in the latest eocene india collided with northern myanmar and with tibet along a suture south of the jurassic triassic flysch and adjacent yarlung ophiolite in the mogok segment of the late cretaceous arc crust thickened in the early cretaceous orogeny underwent extension to form a metamorphic core complex as myanmar west of the sagaing fault moved northward more than half of this northward movement preceded the well documented 450 km of post early miocene dextral displacement on the sagaing fault and related spreading in the andaman sea generation of late cretaceous to early eocene i and s type granites in and east of the mogok belt was related either to crustal shortening or more probably to subsequent extension in the mogok belt

high grade reworking of central australian granulites part 1 structural evolution abstract four distinct deformational events d2 d5 accompanying a granulite facies metamorphic cycle m2 m5 are shown to structurally overprint rework pre existing granulites m1 of the northeast strangways range in the central arunta block the first metamorphic cycle m1 at 1800 ma involved an anticlockwise p t path peaking at 850 950 c at 8 9 kbar m1 metamorphism involved widespread partial melting that produced stromitic migmatites and map scale concordant granitic gneisses isobaric cooling after the peak and accompanying hydration gave rise to a wide variety of coronitic and symplectitic reaction textures that enclose and replace m1 mineral parageneses no kinematic structuring is associated with m1 and the period encompassing the thermal peak and immediately subsequent are thought to have been absent of deviatoric stress the second metamorphic cycle m2 m5 possibly at 1400 1500 ma involved a clockwise p t path with a maximum p of 9 10 kbar and a thermal peak of approximately 800 c this metamorphic cycle accompanied the major ductile deformations recognized d2 d5 and has been labelled the proterozoic reworking the proterozoic reworking has been divided into two thermo barometric and structurally distinct periods d2 d3 involved regionally inclined ene wsw non coaxial shear of high bulk shear strains giving rise to an intense often mylonitic pervasive fabric s2 l2 and isoclinal and sheath folds on all scales f2 f3 s2 l2 is the first recognized kinematic fabric and does enclose and partially recrystallize m1 mineral parageneses including late m1 metamorphic reaction textures d2 d3 deformation was due to crustal shortening and gave rise to crustal over thickening loading accompanying prograde metamorphism m2 d4 d5 involved upright open and asymmetrical folding f4 and e w trending shear zone development s5 within a regionally extensive system of inclined oblique sinistral transpression both d2 d3 and d4 d5 episodes occurred under the same approximately e w directed compressive stress and are considered sequential episodes in the one tectonothermal cycle f4 folding accompanied significant melt formation and the peak of metamorphism of the m2 m5 metamorphic cycle s5 shear zones accompanied 3 4 kbar of decompression with cooling presumably during uplift in isostatic response to crustal over thickening in d2 d3 progression from inclined non coaxial shear to transpression occurred in response to an increase in the relative buoyancy forces limiting crustal thickening as is typical of many compressional orogens however the proterozoic reworking is atypical of simple linear or arcuate mountain belts because late stage shearing s5 is aligned sub parallel not at a high angle to the transport vector of the earlier ductile deformations d2 d3 consequently this region is modelled in terms of crustal shortening directed along the length of an e w trending orogen confined to the north and south by relatively stable crustal blocks when e w shortening could no longer be accommodated by crustal thickening strain was partitioned by sinistral transpression between the north and south bounding crustal blocks

metamorphism and crustal considerations in the harts range and neighbouring regions arunta inlier central australia abstract the harts range region consists of several segments or terrains which have behaved essentially independently though some overlap of history permitting correlation is apparent some analogy with the terrane concept is envisaged the harts range terrains are 1 the strangways orogenic belt sob south of the muller thrust 2 the sob of the oonagalabi tongue 3 the sob of the entia dome together comprising a basement and 4 the harts range cover hrc all separated from each other by thrusts or faults the terrains consist of supracrustal sequences comprising metasediments and some metabasites thought to have been deposited in extensional rift depressions plus varying amounts of intrusive granitoids both granulite facies and amphibolite facies mineralogies characterise the basement terrain of the oonagalabi tongue and that south of the muller thrust the entia dome and cover terrains have only amphibolite facies mineralogy a combination of thermobarometry and petrogenetic data suggests equilibration of the sob south of the muller thrust at 700 800 c and 5 7 kbar and of the hrc in the ruby mine area at 680 780 c and 6 8 kbar some thickening of the supracrustal accumulations owing to compression is considered necessary for the development of these pressures comparison of the harts range terrains with the strangways terrain shows some similarities and some differences

fluids and melting under upper amphibolite facies conditions internal buffering paths on temperature water activity diagrams for the pelitic model system naaio 2 kaio 2 caai 2 o 4 al 2 sio 5 sio 2 h 2 o give rise to sequences of assemblages which correspond to those observed in upper amphibolite facies terranes in particular the persistence of the assemblage quartz alkali feldspar plagioclase muscovite sillimanite across a zone and the rapidly changing composition of plagioclase across this zone can be accounted for in a natural way

development of crestal normal faults associated with deepwater fold growth deepwater anticlines during their early stages of development approximately between interlimb angles of 180 and 140 grow and accumulate weak fine grained and poorly lithified sediment on their crests growth of the anticlines induced gravitational instability within the syn kinematic sediments and development of a range of shallow gravity driven failure mechanisms in addition to shallow tens of metres depth to detachment rotational landslides deeper seated normal fault swarms also affect the fold crests and upper fold limbs these faults tend to penetrate to depths between 200 and 800 m are 100s of metres to a few kilometres long planar to weakly curved in cross section and display throws in the order of tens of metres the faults do not sole out into a detachment but die out downwards they can occur as single sets or tiers of faults that have developed at different times in map view the fault patterns range from straight to anastamosing to highly curved with complex cross cutting relationships the number and orientation of faults corresponds to the maximum slope dip and direction and consequently the faults are inferred to be mainly the product of gravity induced deformation although tangential stresses arising from outer arc bending of a fold or crestal collapse in response to a mobile sedimentary unit may contribute to some extent

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

vergence an evaluation abstract fold asymmetry is an independent concept distinct from facing fold vergence is a direction relating to the sense of asymmetry of folds the complementary concept of cleavage vergence is defined allowing asymmetry to be used to locate major fold traces where no minor folds exist cleavage vergence is a valuable tool in complex structural terrains both concepts can be used singly or in combination to identify refolding vergence can define structural way up where sedimentological or stratigraphic way up cannot be recognised

tectono thermal evolution of the palaeoproterozoic granites tanami orogen north australian craton implications from hornblende and biotite 40ar 39ar geochronology abstract the palaeoproterozoic granites tanami orogen gto hosts a number of gold deposits located in the southern margin of the north australian craton the major stratigraphic succession is the palaeoproterozoic tanami group which is subdivided into the dead bullock formation and conformably overlying killi killi formation new geochemical data for the ca 1864 coora and groundrush dolerite sills in the dead bullock formation suggests that they have the same characteristics with the enriched back arc basin basaltic rocks from the former stubbins formation such as tholeiitic affinity high tio 2 contents 0 94 to 1 24 wt and low mg 41 45 slightly enriched lile rb th u and k weakly depleted hfse nb ta and relatively flat ree patterns their magma was generated by high degree decompressional melting 5 15 of the asthenosphere source with an input of 3 4 subduction related material the petrological and geochemical similarities of igneous rocks provide new evidence for the assignment of the ca 1864 ma former stubbins formation and the mount charles formations in the dead bullock formation of the tanami group these conclusions confirmed that the extensive palaeoproterozoic tanami group was deposited in a back arc basin environment hornblende and biotite 40 ar 39 ar geochronological study identified three major tectono thermal events in the gto since the deposition of the dead bullock formation the ca 1840 ma 40 ar 39 ar cooling age of metamorphic hornblende from the coora and groundrush dolerite sills in the dead bullock formation provided precise age constraint for the first palaeoproterozoic tectono thermal event during the evolution of the granites tanami back arc basin this age is highly consistent with the ca 1850 1840 ma subduction and peak metamorphism events in the north australian craton nac associated with the halls creek orogeny in the halls creek orogen and the tennant orogeny in the tennant creek inlier the 40 ar 39 ar age of 1753 8 ma for the early biotite bi b from the dead bullock formation is a cooling age record of the peak greenschist facies metamorphism during the ca 1795 ma tanami orogeny which is interpreted as the continental continental collisional event between the gto and the arunta orogen in the central australian craton to the south the approximately 40 million year slow cooling is a combined result of exhumation heat released from the upwelling asthenospheric mantle regional magmatism and long term crustal residence this tectono thermal event was associated with gold mineralisation in the gto and the proterozoic nac was finally cratonised during this period the late biotite bi g is a recrystallization product of hydrothermal alteration its 40 ar 39 ar age of 1718 8 ma revealed the last tectono thermal overprint to the gto and arunta orogen this age reflected the distal thermal influence by the ca 1723 ma strangways event in the arunta orogen

deep crystal response to continental collision the limpopo belt of southern africa the limpopo belt is an example of an archean 2700 ma continental collisional zone in which the thickened crust 70 km has virtually attained equilibrium with the adjacent non thickened plates by a process of upward movement and lateral spreading of the underthrust high grade metamorphosed plates

palaeovolcanology and tectonic setting of a proterozoic metatholeiitic sequence near the baltic shield margin northern norway abstract the kvenvik greenstone formation 1500 m thick is a very well preserved 2 0 1 8 ga old sequence of morb type tholeiitic metabasaltic lavas and volcaniclastic rocks they were deposited as cyclically repeated couplets in a continental shallow water to terrestrial environment near the margin of the baltic shield the effusive facies comprise massive and amygdaloidal lava pillow lava pillow breccia and hyaloclastite volcaniclastic facies are ash and accretionary lapilli tuff lapilli tuff and tuffaceous sedimentary rocks lithofacies associations and primary structures in the tuffs indicate air fall base surge and pyroclastic flow deposits with little input of epiclastic material the prevalence of shallow water to subaerial emplacement throughout the kvenvik greenstone formation and correlative sequences to the south indicates deposition in a spasmodically subsiding basin almost certainly formed by taphrogenic tectonism the rifting may have been incidental to coeval plate convergence tectonism evidenced by calc alkalic magmatism in the repparfjord komagfjord area 50 100 km northeast of the kvenvik area

three dimensional geometry and growth of conjugate normal faults abstract good quality 2 d and 3 d seismic reflection data from the timor sea are used to determine the threedimensional geometry displacement patterns and development of intersecting conjugate normal faults these data are supplemented by data from previous physical modelling studies conjugate structures which comprise two intersecting opposed dipping normal faults or fault sets form synchronously on a geological time scale and develop due to the incidental intersection of the faults factors which affect both the formation and imaging of these structures include the fault density the spatial distribution of opposed dipping faults the seismic resolution and the vertical extent of the imaged fault data large conjugate structures grow from smaller ones larger conjugates are associated with more numerous and larger faults than small structures on the scale of the seismic data fault throws range from ca 10 400 m synchronous fault movements are accommodated by a reduction of displacements on discrete fault surfaces towards the fault intersection zone and a corresponding increase in ductile strain of this region high strains in the volume proximal to the fault intersection zone are expressed as thinning of stratigraphic units between the conjugate faults and are believed to be accommodated by numerous small sub seismic faults intersection of two opposed dipping faults does not prevent their continued synchronous movement and does not result in mechanical locking of the system

collisional emplacement history of the naga andaman ophiolites and the position of the eastern indian suture abstract dismembered late mesozoic ophiolites occur in two parallel belts along the eastern margin of the indian plate the eastern belt closely following the magmatic arc of the central burma basin coincides with a zone of high gravity it is considered to mark a zone of steeply dipping mafic ultramafic rocks and continental metamorphic rocks which are the locus of two closely juxtaposed sutures in contrast the western belt which follows the eastern margin of the indo burma range and the andaman outer island arc broadly follows a zone of negative gravity anomalies here the ophiolites occur mainly as rootless subhorizontal bodies overlying eocene oligocene flyschoid sediments two sets of ophiolites that were accreted during the early cretaceous and mid eocene are juxtaposed in this belt these are inferred to be westward propagated nappes from the eastern belt emplaced during the late oligocene collision between the burmese and indo burma andaman microcontinents ophiolite occurrences in the andaman islands belong to the western belt and are generally interpreted as upthrust oceanic crust accreted due to prolonged subduction activity to the west of the island arc this phase of subduction began only in the late miocene and thus could not have produced the ophiolitic rocks which were accreted in the late early eocene

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

complex strain patterns developed at the frontal and lateral tips to shear zones and thrust zones abstract many of the complex strain patterns seen in shear zones and thrust zones such as variable fabric orientations refolded folds and fabrics together with folds with hinges almost parallel to the main transport direction can be explained in terms of differential movement within the shear zones these strains are developed at the frontal and lateral tips of the zones as they propagate examples are taken from the moine thrust zone of scotland which show variations in strains particularly at the lateral tips the form of differential movement described here may lead to complex strain paths and non plane strain ellipsoids and the spatial variations in finite strain may be used to delineate zones of extensional and compressional flow and differential movement in the shear zones or thrusts

silica undersaturated sapphirine spinel and kornerupine granulite facies rocks ne strangways range central australia small pods of silica undersaturated al rich and mg rich granulite facies rocks containing sapphirine pleonastic spinel kornerupine cordierite orthopyroxene corundum sillimanite and gedrite are scattered throughout the ne strangways range central australia these are divided into four distinct rock types namely orthopyroxene rich aluminous granofels and metapelitic gneisses containing sapphirine spinel or kornerupine two granulite facies metamorphic events are recognized of which only the first m1 is considered in this paper peak metamorphic mineral parageneses indicate that the m1 thermal maximum occurred at approximately 900 950 c and 8 9 kbar all samples are characterized by profuse and diverse coronitic and symplectic reaction textures these are interpreted as evidence for the sequential crossing of the following reactions in the system fmas cordierite spinel corundum sapphirine sillimanite cordierite spinel orthopyroxene sapphirine sillimanite sapphirine spinel sillimanite orthopyroxene corundum sapphirine sillimanite cordierite orthopyroxene corundum phase stability relationships in fmas and mash indicate an anticlockwise p t path terminated by isobaric cooling such a path is exemplified by early low p mineral parageneses containing spinel corundum and gedrite and the occurrence of both prograde and retrograde corundum reaction textures preserve evidence for an increase in ah2o and ab2o3 with progressive isobaric cooling this hydrous retrogression resulted from crystallization of intimately associated m1 partial melt segregations there is no evidence for voluminous magmatic accretion giving rise to the high m1 thermal gradient the m1 p t path may be the result of either lithospheric thinning after both crustal thickening and burial of the supracrustal terrane or concomitant crustal thickening and mantle lithosphere thinning

kilometre scale sheath fold at mattmark and implications for transport direction in the alps folds were the first tectonic features of rocks to be studied and as such are taken as a reference for the interpretation of other structures intuitively fold axes were assumed to be subperpendicular to the direction of transport nevertheless many studies of metamorphic belts showing intense ductile deformation describe folds with axes close to the transport direction in particular sheath folds1 9 are highly curvilinear folds elongated parallel to the direction of transport we discuss here a spectacular large sheath fold situated in a crustal shear zone in the monte rosa nappe swiss alps our analysis implies that both the geometry and significance of folds in the internal alps must be re examined we further demonstrate the existence of crustal ductile shear zones in an area of the alps where previously emphasis was generally placed on fold nappes the movements on these shear zones is often tens of kilometres

combined effects of eurasia sunda oblique convergence and east tibetan crustal flow on the active tectonics of burma it is widely accepted that deformation of the india sunda plate is the result of partitioned hyper oblique convergence presently sub meridian dextral strike slip faulting accommodates this india sunda motion in a buffer zone the burma platelet this wide dextral strike slip shear zone is complicated by the side effect of the tibet plateau collapse that can be described in term of crustal flow and gravity tectonics the loss of potential energy related to this plateau collapse affects most of the burmese platelet particularly in its northernmost part interaction of these two distinct geodynamic processes is recorded in the gps based regional strain field the analysis of seismic focal mechanism but also from direct geologic observations both onshore and offshore myanmar and bangladesh we propose the apparent e w shortening component of this so called partitioned hyper oblique subduction is only the effect of regional gravitational forces related to the tibet plateau collapse whereas the ns strike slip faulting accommodates the india sunda motion

a tectonic model for the emplacement of the main donegal granite nw ireland the main donegal granite is shown to lie in a major late stage ne sw trending transcurrent sinistral shear zone in the caledonides of nw ireland the zone which is bent both laterally and upwards around the pluton has displacements which diminish sw along its length from a maximum of approximately 20 km at the ne end of the pluton the granite has also been deformed by this sinistral shear but it has a strain component of its own the nature and development of which indicate that the pluton was accommodated rather than forcefully emplaced in the shear zone strain variations between granite and wall rock and an early plagioclase fabric associated with the petrographic banding of the granite suggest that intrusion and deformation were more or less synchronous at around 400 ma it is suggested that unconsolidated granite in the shear zone locally created high strain rates and instability which caused the zone to bend split lengthwise and progressively create an internal low pressure zone into which the main granite was emplaced the wide variety of emplacement styles of the other radiometrically synchronous plutons of the batholith can be attributed to the progressive development of the shear zone thorr and fanad in tension between the zone and a complimentary structure to the n ardara and toories in local compression at the shear zone termination rosses in tension on the outer arc of the bending zone and trawenagh bay as displaced main granite magma in a similar tensional setting during the final stages of deformation

the emplacement of the wuluma granite melt generation and migration along steeply dipping extensional fractures at the close of the late strangways orogenic event arunta block central australia during the paleoproterozoic the strangways metamorphic complex in the arunta block central australia underwent two major deformation events under granulite facies conditions namely the early strangways event 1800 1780 ma and the late strangways event the early strangways event d1m1 produced a regional granulite facies gneissic foliation which outlines structural trends in the strangways metamorphic complex the late strangways event d2m2 is a compressional event which produced spectacular regional sheath folds and high temperature shear zones melt segregations bearing orthopyroxene garnet cordierite k feldspar quartz overprint these structures and mark the end of the second granulite facies event m2 in the wuluma hills sw strangways metamorphic complex pressures of 5 6 kbar and temperatures greater than 750 c were reached during this event the wuluma granite consists of steeply dipping sheets of granites which overprint the late strangways structures and are parallel to the regional gneissic foliation the rotation and steepening of the regional gneissic foliation occurred during the late strangways event the wuluma granite formed at the close of the late strangways event as extensional fractures opened parallel to the steeply dipping gneissic foliation fluids migrated into these extensional cracks causing in situ partial melting of the granulites the introduction of hydrous fluids and their subsequent release during the crystallization of the granitic melts generated a pervasive metasomatic event m3 which is characterized by the retrogression of cordierite to sillimanite biotite titanomagnetite a u pb ion probe zircon age of 1728 3 ma for the wuluma granite constrains the timing of m3 thus the wuluma granite is part of an orogenic cycle which began 1745 ma ago anamarra granite with regional folding under granulite facies conditions m2 and ended 1728 ma ago with the formation of the wuluma granite and pervasive regional retrogression m3

nd isotopic systematics and chemistry of central australian sapphirine granulites an example of rare earth element mobility abstract lower proterozoic sapphirine bearing and associated granulites from central australia exhibit the greatest range of present day 143 nd 144 nd ratios nd o 26 5 to 112 3 yet reported for rocks believed to be cogenetic the nd isotopic data and ree abundances of these rocks demonstrate extreme fractionation of the rare earths during the formation of stratiform cu pb zn sulfide deposits with which they are closely associated field relationships petrography and chemistry of the sapphirine granulites suggest that their protoliths comprised chlorite rich rocks which were generated by hydrothermal alteration of a range of rock types prior to metamorphism calculations employing ree abundances of the sapphirine granulites and associated rocks combined with bulk solid fluid distribution coefficient data yield high fluid rock ratios consistent with a pre metamorphic hydrothermal origin for the unusual ree patterns the sm nd data for these rocks define an age of 1760 75ma which is significantly younger than the crust formation age of the terrain 2070 125ma but indistinguishable from the rb sr whole rock age for granulite facies metamorphism 1790 35ma these data are interpreted in terms of major hydrothermal fractionation of the rare earths shortly perhaps tens of millions of years before granulite facies metamorphism followed by redistribution of nd isotopes or small fractionations of the sm nd ratio during the granulite facies event and possibly also during intense retrogression which reset rb sr whole rock and u pb zircon and monazite systematics at about 1700 ma

effect of bacteria on the elemental composition of early diagenetic siderite implications for palaeoenvironmental interpretations many ancient early diagenetic siderite concretions are thought to have been precipitated by microbially mediated reactions and their trace element chemistry has been used to infer palaeoenvironments of formation e g marine versus freshwater in this study pure cultures of the microorganism geobacter metallireducens were used to precipitate siderite in the laboratory at a range of temperatures 18 40 c magnesium and calcium in three different ratios 3 1 as in seawater 1 1 and 1 3 as in freshwater or manganese were added to some cultures at each incubation temperature to study trace element incorporation the siderite produced exhibited the rhombohedral crystal form typical of concretionary siderite however this microbial siderite did not simply retain the trace element chemistry of the water from which it precipitated as is assumed in palaeoenvironmental interpretations instead manganese and calcium incorporation were found to be inversely proportional to the rate of microbial activity which itself is dependent upon the nutritional status of the microorganisms magnesium incorporation was found to be rate independent and at high mg ca ratios it caused inhibition of calcium incorporation in short the influence of microorganisms on the trace element composition of early diagenetic siderite must be taken into account in order to produce valid palaeoenvironmental interpretations

the thermotectonic evolution of a proterozoic low pressure granulite dome west uusimaa sw finland three fold generations have been recognized in svecofennian rocks 1 800 ma from west uusimaa sw finland the first one f1 might be related to thrusting and imbrication tectonics at plate collision contacts the main generation f2 is due to a n s horizontal crustal shortening which created at first e w trending upright folds in the whole region and later tightened these f2 folds in the western part of the belt whereas conjugate shear zones and tectonic lenses of competent rock bodies developed in the eastern part simultaneously the metamorphic conditions rose from amphibolite to granulite facies in this eastern part which is known as the west uusimaa complex the amphibolite to granulite facies transition zone along the western boundary of the granulite facies complex is studied in detail a number of prograde mineral reactions are telescoped in this transition zone the breakdown of biotite and amphibole to ortho clino pyroxene in metaigneous rocks the appearance of garnet in cordierite bearing metapelites and the appearance of scapolite in calcareous rocks distinct mineralogical changes also occur in this zone which cross cuts all major structures and rock units and are only affected by late f3 folding open disharmonic folds with approximately n s trending axial planes and young shear zones associated with pseudotachylite generation the absence of any evidence of block faulting and tilting of the crust that could be associated with the granulite complex suggests that the whole region represents one crustal level

metamorphic events in the eastern arunta inlier part 1 metamorphic petrology abstract in the entia dome of the eastern arunta inlier kyanite facies metamorphism is recorded over a substantial area at least 160 km 2 and thus contrasts with the regionally extensive belts of high temperature low pressure andalusite sillimanite facies metamorphism documented elsewhere in the arunta inlier this amphibolite facies event appears to postdate regional granulite facies metamorphism evidence for which is almost totally obliterated in the entia dome prograde kyanite occurs in meta basites in comparatively rare associations involving hornblende staurolite and anorthite and in other rocks in association with gedrite and hornblende or garnet and biotite these kyanite bearing assemblages equilibrated at intermediate temperatures 570 710 c at peak pressures of 7 kbar and are overprinted by lower pressure assemblages including hornblende gedrite cordierite and sillimanite while this sequence of reactions may be simply interpreted in terms of a single clockwise orogenic cycle textural evidence and the available mineral isotopic data suggest a more complex history involving multiple heating and cooling episodes during the mesoproterozoic and possibly the palaeozoic the occurrence of regional retrogressive kyanite facies assemblages in the eastern arunta inlier provides evidence that the widely held perception that proterozoic crustal evolution in australia involved a distinct ensialic orogenic style characterised by high geothermal gradients etheridge et al 1987 loosveld and etheridge 1990 sandiford and powell 1991 is not universally true rather petrological evidence presented here with isotopic constraints described by foden et al 1995 suggests that the harts range region has experienced substantial crustal reworking at elevated temperatures during both the proterozoic and palaeozoic under thermal regimes more familiar to modern continental collision zones

climatic and vegetation control on sediment dynamics during the last glacial cycle as climate is changing rapidly there is an increasing need to understand how water and soil resources respond to climate change soil and sediment dynamics are sensitive to several external factors such as climate vegetation type and distribution human activity and tectonic activity however the relationship between erosion and changes in these factors is difficult to constrain with current available approaches here we show that uranium isotopes in sediments from river paleochannels can be used to reconstruct variations in the residence time of sediments in a catchment over the past 100 k y we find that sediment residence times increase by an order of magnitude during interglacials compared to glacial periods this is interpreted as a change in sediment stores in the landscape that are tapped by catchment erosion young upland soils during glacial periods reworking of old alluvial sediments during interglacials a direct correlation is found between the sediment residence time and climatic parameters sea surface temperature atmospheric carbon dioxide content and paleorainfall estimates suggesting that during a glacial cycle sediment dynamics closely follow variations in climate however this relationship is not simple because there is no correlation between sediment residence time and paleodischarge estimates because sediment residence time variations correlate with changes in vegetation inferred from pollen data it is hypothesized that the influence of climate on erosion over a glacial cycle may be indirect and operates via the influence of climate on the type of plant ecosystems within a catchment if verified elsewhere this conclusion would emphasize the important role of biology in the physical evolution of earth s surface here observed over a 100 k y time scale

the mortagne granite pluton france emplaced by pull apart along a shear zone structural and gravimetric arguments and regional implication the emplacement mechanism and kinematics and the 3 d geometry of the variscan granite pluton of mortagne are interpreted using systematic measurements of the granite structure allied with micro structural observations from a detailed gravimetric study it is proposed that magma infilled a pull apart structure which formed along a jog of a northwest southeast trending sinistral zone of shear a second episode of shearing under a dextral regime and independent of emplacement was responsible for perigranitic tectonites these successive transcurrent opposite movements already documented for variscan times in western france are discussed concerning the age of the mortagne pluton 313 15 m y

structure and kinematics of the indo burmese wedge recent and fast growth of the outer wedge 1 the northern sunda subduction zone offshore burma and the associated indo burmese wedge mark the active eastern boundary of the burma platelet jammed between the india plate and the sunda plate the aim of this paper is to provide a structural and kinematics analysis of the indo burmese wedge based on seismic reflection geodetic and geological field data we found out that the indo burmese wedge is the place of diffuse strain partitioning right lateral shearing in the innermost part and e w shortening in the outermost part in the outer wedge thick skinned deformation overprints thin skinned deformation it can be explained by the required preservation of the critical taper after the fast westward propagation of the outer wedge above a very efficient clayey decollement layer the thick skinned deformation is here characterized by major right lateral strike slip faults the kaladan fault and the chittagong coastal fault and yet a typical internal deformation of the indo burmese wedge we suggest that the westward migration of internal right lateral shear deformation allows preservation of the strain partitioning ratios between internal n s trending right lateral shearing and external e w shortening some seismic lines show that the outer wedge deformation is not older than 2 ma its propagation could have been enhanced by the large amount of sediments filling up the sylhet flexural basin formed after the pliocene tectonic uplift of the shillong plateau

an 40ar 39ar study of a polymetamorphic complex in the arunta block central australia a polymetamorphic complex in the arunta block central australia which has undergone metasomatism involving introduction of rb and loss of sr has been investigated by 40ar 39ar step and total degassing techniques in an attempt to date five metamorphic events m1 m5 identified microstructurally the complex isotopic data and apparent age spectra obtained have been interpreted in the light of constraints provided by an associated tectonometamorphic study together with independent geochronological controls from the immediate study area and elsewhere in the arunta block this has led to the delineation of ages of 1 780 1 820 m y for m1 1 690 1 720 m y for m2 1 450 1 500 m y for m3 1 030 1 060 m y for m4 and 370 510 for m5 the polyphase m5 event is correlated with the alice springs orogeny previously accorded younger age limits of 330 370 m y the latter are interpreted to reflect closure of mica k ar systems as uplift continued subsequent to the close of the event possibly culminating in minor high level brittle deformation and the development of pseudotachylytes in the study area

an assessment based on u_pb zircon data of rb_sr dating in the arunta inlier central australia abstract the chronology of the arunta inlier was originally based mostly on rb sr dating although some of the ages derived from that technique are now shown to be erroneous those which were constrained by structural observations and which relied only on whole rock and not mineral analyses have proven to be remarkably realistic ion microprobe data show that even conventional multigrain u pb zircon analysis is not a particularly appropriate dating technique in a region of this complexity three new u pb zircon ages and others previously derived from the same isotopic system date three of the main tectonic events that were imposed on the arunta inlier the widespread strangways orogeny of the northern and central provinces occurred at 1760 1750 ma the argilke event is not older than 1680 ma demonstrating that the southern province is the youngest in the inlier the dominantly thermal ormiston event is dated at 1140 ma

the search for a topographic signature of life landscapes are shaped by the uplift deformation and breakdown of bedrock and the erosion transport and deposition of sediment life is important in all of these processes over short timescales the impact of life is quite apparent rock weathering soil formation and erosion slope stability and river dynamics are directly influenced by biotic processes that mediate chemical reactions dilate soil disrupt the ground surface and add strength with a weave of roots over geologic time biotic effects are less obvious but equally important biota affect climate and climatic conditions dictate the mechanisms and rates of erosion that control topographic evolution apart from the obvious influence of humans does the resulting landscape bear an unmistakable stamp of life the influence of life on topography is a topic that has remained largely unexplored erosion laws that explicitly include biotic effects are needed to explore how intrinsically small scale biotic processes can influence the form of entire landscapes and to determine whether these processes create a distinctive topography

structural constraints on the proterozoic reworking of archaean crust in the rayner complex macrobertson and kemp land coast east antarctica abstract the high grade metamorphic rocks exposed along the macrobertson and east kemp land coasts consist of in the west archaean gneisses reworked during a ductile mid proterozoic deformation and in the east foliated proterozoic charnockite gneiss containing large remnants of gneiss of probable archaean age evidence for three folding events is recognized in the reworked gneisses an early recumbent terrain is deformed by two upright events possessing near vertical axial surfaces the charnockite gneiss preserves evidence of only the final event these three events comprise the rayner structural episode which was a consequence of the ductile thrusting of archaean gneiss westward toward a stable archaean block the napier complex kinematic indicators are consistent with formation of the proterozoic recumbent terrain by dominantly shear deformation dissection of both the rayner and napier complexes by retrograde shear zones and mylonite pseudotachylite zones was a consequence of the northward thrusting of the two complexes during the cambrian

late quaternary aeolian dunes on the presently humid blue mountains eastern australia abstract sand dunes on the newnes plateau 1000 m a s l west of sydney were active during the last glacial maximum the scattered sand dunes are forested under the modern humid temperate climate regime dune types range from parabolic to transverse lee dunes and sand sheets or patches all point to the presence of conditions marginal for aeolian activity made possible through wind acceleration on windward slopes ready sand supply from the weathered sandstone of the plateau and sparse vegetation cover modern climate envelopes of sand dune activity in australia predict that unrealistically drier conditions are necessary to allow wind transport at this site only additional impediments to plant growth such as lower temperature and lower atmospheric carbon dioxide concentrations appear to allow the necessary conditions for dune formation these observations and conclusions extend our understanding of the extremes of the lgm climate in humid eastern australia confirming that the widespread treeless vegetation was also sparse even in areas that today have annual rainfall above 1000 mm

borrego springs santa rosa mylonite zone a late cretaceous west directed thrust in southern california the borrego springs shear zone section of the eastern peninsular ranges mylonite zone eprmz contains unequivocal evidence for ductile shearing with an east over west sense of movement field evidence includes the presence of s and c planes and shear bands all lines of microstructural evidence are internally consistent and support the field observations it appears that the data are compatible only with post mid cretaceous west directed thrusting on the eprmz

a barometric response to late compression in the strangways metamorphic complex arunta block central australia evidence for four major deformation events d1 d4 and two metamorphic events m1 m2 is shown by granulite and amphibolite facies rocks of the strangways metamorphic complex central australia which is divided into two domains the ongeva granulites to the east and the anamarra granite domain to the west d1 and d2 produced layer parallel foliations with many of the effects of d1 overprinted by intense non coaxial shear during d2 low pressure granulite facies metamorphism m1 accompanied d1 2 at conditions of p 5 3 1 2 kbar and t 750 c the effects of m1 are characterized by s1 spinel quartz bearing assemblages with overprinting s2b sillimanite biotite magnetite assemblages implying approximately isobaric cooling effects of m2 are characterized by s1 2 cordierite pseudomorphed by orthopyroxene sillimanite biotite magnetite symplectites at conditions of p 7 5 0 8 kbar and t 800 c this retrograde increase in pressure was probably the result of crustal thickening during d3 which produced ne plunging f3 and f4 folds in the ongeva granulites and a widespread shear foliation in the anamarra granite domain the increase in pressure with m2 may reflect the beginning of a clockwise p t t path typical of collisional style tectonics sillimanite and orthopyroxene bearing ultramylonites were produced during d4 an extension event parallel to the d3 transport axis and probably related to crustal collapse after thickening during d3 wide n dipping shear zones d5 containing sillimanite kyanite staurolite and overprinting greenschist facies assemblages bound the strangways metamorphic complex these zones indicate a s directed sense of shear and were probably responsible for bringing the strangways metamorphic complex to higher crustal levels during both a probable late proterozoic event and the mid carboniferous alice springs orogeny

tectonic analysis of fault slip data sets using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio 2 3 1 3 between principal stress values 1 2 3 compression being positive no assumption is made concerning the orientation of fault planes relative to stress axes so that reactived faults are taken into account as well as newly created ones qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years the practical limitation of the methods and the necessity for critical field observations are emphasized these methods can be applied to focal mechanisms of earthquakes a more complex analysis of heterogeneous data sets involving an iterative separation of different stress systems is also discussed this analysis enables one to distinguish successive faulting events careful qualitative study in the field is in all cases essential

sediment transport time measured with u series isotopes results from odp north atlantic drift site 984 abstract high precision uranium isotope measurements of marine clastic sediments are used to measure the transport and storage time of sediment from source to site of deposition the approach is demonstrated on fine grained late pleistocene deep sea sediments from ocean drilling program site 984a on the bjorn drift in the north atlantic the sediments are siliciclastic with up to 30 carbonate and dated by 18o of benthic foraminifera nd and sr isotopes indicate that provenance has oscillated between a proximal source during the last three interglacial periods volcanic rocks from iceland and a distal continental source during glacial periods an unexpected finding is that the 234u 238u ratios of the silicate portion of the sediment isolated by leaching with hydrochloric acid are significantly less than the secular equilibrium value and show large and systematic variations that are correlated with glacial cycles and sediment provenance the 234u depletions are inferred to be due to recoil loss of 234th and are used to calculate comminution ages of the sediment the time elapsed between the generation of the small 50 m sediment grains in the source areas by comminution of bedrock and the time of deposition on the seafloor transport times the difference between comminution ages and depositional ages vary from less than 10 ky to about 300 to 400 ky for the site 984a sediments long transport times may reflect prior storage in soils on continental shelves or elsewhere on the seafloor transport time may also be a measure of bottom current strength during the most recent interglacial periods the detritus from distal continental sources is diluted with sediment from iceland that is rapidly transported to the site of deposition the comminution age approach could be used to date quaternary non marine sediments soils and atmospheric dust and may be enhanced by concomitant measurement of 226ra 230th 230th 234u and cosmogenic nuclides

tectonic diapiric and sedimentary chaotic rocks of the rakhine coast western myanmar abstract the western margin of myanmar is the northern extension the active sunda india eurasia subduction zone coastal regions and offshore islands have remarkable exposures of chaotic rock terranes along wave cut terraces that allow characteristics of tectonic sedimentary and diapiric melanges to be recognized tectonic shear zones tectonic melanges contain fragments of cretaceous ophiolites chrome spinel bearing peridotites and radiolarian cherts that are in contact with thrust packets of eocene turbidite units broken formations the turbidites contain shale rich beds that have been sheared during soft sediment deformation sedimentary broken formations and are sandwiched between undeformed thick sandy beds these are mass transport deposits mtds that most likely formed during deposition of the initial detritus of the himalayan orogenic zone probably trench slope basins on the accretionary prism the ophiolitic and turbiditic thrust slices have been exhumed and are currently being intruded by active mud volcanoes that bring fragments of units up from depth to the surface forming diapiric melanges these diapiric melange bodies contain only small fragments

outcrop examples of soft sediment deformation associated with normal fault terminations in deepwater eocene turbidites a previously undescribed conjugate fault termination style abstract a set of small scale layer bound faults developed in a package of three eocene turbidite sandstone beds in a wave cut platform thandwe beach myanmar display unusual downward terminations between converging conjugate faults the faults were initially triggered by loading during deposition of the uppermost turbidite sandstone in the fault affected stratigraphic package the growth of a small gravity driven anticline and subsequent loading by a post deformation turbidite modified the initial fault geometry and re mobilized part of the layer bound sequence causing thinning of the sequence to zero in the vicinity of the fold crest typically conjugate faults display an x shaped pattern of intersection while the observed faults before reaching the point of intersection die out in a drastically thinned basal sandstone located in a keystone block which requires either an underlying detachment or that a volume of sand and fluid was lost from the basal layer probably a combination of gravity driven detachment primary movement and volume loss secondary permitted development of the faults the structures described represent yet another aspect of the wide variety of gravity driven features that form in the deepwater turbidite fan settings it is thought that such features have not been previously described in the literature and represent a new style variant within the general theme of conjugate fault development

determining sources and transit times of suspended sediment in the murrumbidgee river new south wales australia using fallout 137cs and 210pb sediment budgets typically require an estimate of the proportional yield from erosion sources to sediments in transport and storage this becomes increasingly difficult as catchments become larger and erosion storage and deposition processes become more complex we demonstrate how fallout radionuclides can be used to estimate the proportional contributions to sediment load from a tripartite classification of erosion sources in a large catchment the mid murrumbidgee 13 500 km2 the three major potential sources of sediment within this catchment are cultivated lands 22 of the surface area uncultivated pastoral lands 78 and the numerous channels and gullies found in this region concentrations of the fallout radionuclides 210pbex and 137cs in representative samples from each of these three sources are significantly different employing these values in a simple mixing model shows that the largest contribution of material is currently derived from subsoil channel gully sources alterations to the suspended sediment 210pbex signature by in situ labeling and decay are also considered applying the model with different concentrations of 210pbex decayed as a function of residence time in channels suggests that the mean residence time of fine grained material within this system is 10 5 years however differences in 137cs concentrations observed between flood and low flow sediments and the presence of the short lived cosmogenic 7be suggest that residence time of some of this fine grained material may be of the order of only weeks to months

geochronological constraints on orogenic events in the arunta inlier a review early workers subdivided the arunta inlier central australia into three major tectonic provinces classified the principal tectonostratigraphic units into three divisions and separated the tectonomagmatic history into five discrete events ranging in age from 1800 ma to 300 ma a review of the evidence for tectonic boundaries within the arunta inlier suggests that no major structural discontinuity exists between the northern and central tectonic provinces but that the redbank thrust zone is at least in part a province boundary the tectonostratigraphic division concept also requires revision as division 3 rocks of the northern and southern arunta cannot be time equivalents division 1 rocks grade into division 2 in the northern region and division 3 of the northern region may be equivalent to division 1 in the central region rocks originally assigned to divisions are better grouped into a larger number of lithological assemblages until better stratigraphic and isotopic correlations can be made compilation of rb sr and 40ar39ar age determinations obtained before 1984 indicate an almost complete age spectrum from 1800 ma to 1000 ma but age clusters exist at 1400 ma 1100 ma and 400 300 ma which have geological significance subdivision of older ages into discrete tectonic events was not justified from this data alone but more recent u pb isotopic analysis of zircons in rocks designated to particular structural metamorphic events has revealed a complex tectonic history in the 1880 1600 ma interval the northern and central provinces underwent major tectonism between 1780 1730 ma but much of the southern province was not deformed until 1600 ma the revised tectonic history nomenclature is proposed for the arunta inlier following deposition of a widespread turbiditic unit at or before 1880 ma possibly on thinned archaean crust the northern and central provinces underwent localized tectonism magmatism at 1880 ma yuendumu tectonic event and 1820 ma stafford tectonic event prior to the widespread strangways orogeny which is subdivided into an early phase at 1780 1770 ma and late phase at 1745 1730 ma it is not clear whether the strangways orogeny is a single arunta wide event or a series of separate short lived smaller scale events although the major deformation in all areas appears to have involved w to sw directed thrusting in the southern province the major events occurred at 1680 ma argilke tectonic event and 1600 ma chewings orogeny the latter involving n directed thrusting that was penecontemporaneous with post tectonic pegmatite intrusion in the northern province suggesting linkage of both regions at this stage the younger history of the inlier is characterised by several stages of uplift and isolated thermal magmatic events major mesoproterozoic crustal exhumation anmatjira uplift phase has been interpreted from widespread 1500 1400 ma ages including regional scale closure of 40ar 39ar systems in hornblende from mafic granulite outcrop scale rb sr total rock isotopic systems in mylonitized granitoids and sm nd garnet hornblende pairs in mylonitized amphibolites a 1150 ma thermal event in the southern part of the inlier teapot magmatic event is recorded by intrusion of granite pegmatites and the alkaline mordor igneous complex and by isotopic resetting of rb sr mineral systems dolerite dyke injection at 1050 ma reflects limited extensional tectonism that may have related to formation of the amadeus basin the second major uplift event was the alice springs orogeny at 400 300 ma an unusual intracratonic thick and thin skinned thrust event that involved limited retrograde amphibolite facies but extensive greenschist facies metamorphism and restricted pegmatite intrusion

stratigraphic architecture resulting from late quaternary evolution of the riverine plain south eastern australia the riverine plain of south eastern australia is the result of prolonged cenozoic fluvial activity single thread anabranching and distributary channels and floodplains and associated aeolian dunes characterize the uppermost sequences based on detailed interpretations of late quaternary fluvial sedimentation and surficial stratigraphy for this 77 000 km2 basin earlier prior stream and ancestral stream models of fluvial deposition deduced from limited stratigraphic and chronological evidence are replaced with aggradational palaeochannel and migrational palaeochannel models thermoluminescence dating reveals four distinct phases of palaeochannel activity between 105 and 12 ka the first coleambally phase late in oxygen isotope stage 5 the second kerarbury phase in stage 3 the third gum creek phase before and the fourth yanco phase after the last glacial maximum lgm in stage 2 the first three of these phases were characterized by mixed load laterally migrating sinuous palaeochannels with occasional transitions to a straighter bedload dominated mode and vice versa the first two phases concluded with a bedload dominated episode resulting in aggradational palaeochannels on the surface of the plain and the third phase prior to the lgm did also in its downstream reaches the phase following the lgm was characterized entirely by large mixed load sinuous migrational palaeochannels these exhibited no terminating bedload episode because the onset of holocene climates reduced the size of the flood peaks greatly diminished the supply of bedload from the upper catchments and resulted in streams evolving to their present highly sinuous suspended load form the result is a complex stratigraphic architecture consisting of vertically and laterally accreted units extending over hundreds of kilometres in the form of channel sand stringers sand sheets and derivative aeolian dunes partially or wholly encased in overbank fines

caterpillar tectonics in the harts range area a kinship between two sequential proterozoic extension collision orogenic belts within the eastern arunta inlier of central australia abstract we propose that in the harts range area of the arunta inlier of central australia a basement and cover relationship previously described by us represents a thick complexly deformed contact zone between an early middle proterozoic fold belt the strangways orogenic belt sob and a younger middle proterozoic orogenic belt the harts range orogenic belt both of which contain their own supracrustal sequences rocks of the sob were deposited and possibly metamorphosed to a maximum grade of granulite facies at 1800 ma during an early phase of crustal extension they were subsequently intruded by a significant proportion of acidic magmas at 1767 ma and compressed to form a distinctively nne ssw trending orogenic belt a wnw ese trending shallowly north dipping zone of intense and repeated deformation termed the harts range detachment zone hrdz formed initially during a period of nne ssw crustal extension as a major low angle normal shear detachment zone within and crosses at a high angle to the trend of the basement rocks of the sob the hrdz has been intermittently active as a zone of crustal weakness since its initiation the first extensional movement on this zone began the formation of the harts range mobile belt hrmb which displays many of the characteristics of modern tectonic zones activated by plate movement a supracrustal cover sequence the harts range cover hrc formed in the major geosynclinal basin which developed on the subsiding upper plate of the hrdz continued crustal extension juxtaposed the cover sequence which was then metamorphosed to a maximum grade of amphibolite facies and granulite facies basement of the hrdz before a fundamental change in movement sense led to crustal contraction and a collision related thrusting phase the hrmb which includes the hrdz and the hrc was then internally deformed over a relatively very short time span 15 20 ma before voluminous granitic magma intruded mainly along the hrdz during the harts range orogeny hro the intrusion of this major granitoid at 1747 ma precisely fixes the time scale of evolution for the earlier phases of the hro the hro is documented in detail as a peculiar orogenic style of crustal contraction which was largely taken up by reverse sense discrete thrusting and shearing on faults and shear zones associated with the hrdz displacement on the hrdz was a stick slip process with repeated punctuation of thrust slip movement by pinning at points located progressively to the south each period of cessation of thrust slip movement was followed by folding of discrete thrusts of captured basement lobes within the hrdz and of the overlying supracrustal sequence the individual discrete movement phases and folding episodes are each named and described as is a model of this unusual type of proterozoic tectonic activity

volcanogenic cu pb zn bodies in granulites of the central arunta block central australia small unexploited copper lead zinc deposits characterized by a distinctive wall rock association of cordierite quartzite silica undersaturated rocks calc silicate rocks and impure marbles occur in quartzofeldspathic gneisses and mafic granulites of the strangways metamorphic complex central arunta block central australia available data support the hypothesis that these are metamorphosed volcanogenic ore bodies the chemical compositions of the quartzofeldspathic gneisses are comparable with those of less metamorphosed felsic igneous rocks particularly the felsic igneous rocks emplaced in the north australian orogenic province in the interval 1880 1800 ma and the mafic granulites are chemically similar to basalts olivine normative tholeiites the wall rock suite can be correlated from chemistry and lithological association with the suites of wall rocks found in unmetamorphosed volcanogenic ore deposits that the protolith of the cordierite quartzites may well have been leached tuff similar to the illite chlorite quartz tuff found in volcanogenic ore deposits is also shown by retrogression of the granulitefacies assemblage cordierite garnet ortho pyroxene biotite quartz in the cordierite quartzites to cordierite anthophyllite bearing assemblages and thence to chlorite muscovite quartz assemblages lenses of silica undersaturated rocks with spinel and less commonly sapphirine are interpreted as the metamorphosed equivalents of chlorite rich pods found within leached tuffs in volcanogenic ore deposits the wall rocks form sheet like bodies this suggests that they were deposited in relatively shallow water thus precluding the formation of massive sulphides

metamorphic and tectonic evolution of granulites arunta block central australia petrographical and chemical evidence presented here indicate a prolonged metamorphic evolution of the granulite terrain in the northern strangways range arunta block central australia nsr in fig 1 beginning with migmatization followed by dry granulite metamorphism at high temperatures t and moderate pressures p estimated at 850 920 c 8 1 kbar1 this occurred at 1 800 myr refs 2 3 very shortly after deposition of the protolith4 a bimodal volcanic suite5 and associated sediments granulite metamorphism was followed by isobaric cooling during which there was an episode of pervasive but partial hydration later retrogression and hydration localized along shear zones occurred at the normal geothermal gradient in the kyanite stability field a tensional rift like regime best describes the geological environment in which the protolith formed was deeply buried metamorphosed with high heat flow and then remained in isostatic equilibrium metamorphic parageneses recording uplift may be matched with mid palaeozoic tectonic events in central australia if this correlation is correct the rocks of the northern strangways range may have remained deeply buried for 1 400 myr during which the shear zones were reactivated several times fig 2

the duration of the strangways event in central australia evidence for prolonged deep crust processes abstract protoliths of the strangways metamorphic complex smc in the arunta region of central australia are dated to the 1810 1800 ma stafford magmatic event by shrimp u pb dating of relict igneous zircon within granulite facies metamorphic rocks detrital zircons preserved in associated metasediment have a provenance age spectrum similar to that in the lander package sedimentary rocks which are widespread in the region the onset and duration of granulite facies metamorphism is recorded differentially across the smc depending on lithology varying mechanisms of metamorphic zircon crystallisation and the specific p t t paths of individual rocks in different sectors of the complex the earliest recorded thermal process is partial melting in the kanandra granulite at 1734 3 ma zircon ages indicate that melts at edwards creek 1716 3 ma and coles hill 1708 6 ma formed later overgrowths indicate renewed metamorphic zircon precipitation in the kanandra granulite probably from fluids as late as ca 1690 ma a 50 myr continuum of metamorphic zircon ages from ca 1740 to 1690 ma is recorded in a single rock at utnalanama and attributed to prolonged solid state metamorphic zircon formation the strangways event was not a transient and discrete process with a single age there is evidence for prolonged residence at granulite facies conditions with both discrete crystallisation events and prolonged crystallisation responses in different locations termination of the high grade metamorphism coincided with local extension indicated by the intrusion of a dolerite dyke swarm at 1689 8 ma the strangways event coincides with the segment between prominent bends b0 and b1 of the australian palaeomagnetic polar wander path relating the thermal process to major direction changes in the movement of the north australian plate the geographic distribution of high grade metamorphism within the smc shallow crust emplacement of magmas in the tennant region and a low temperature ar isotope record in the tanami region is consistent with effects propagating northwards from a plate boundary south of the smc

intense non coaxial shear and the development of mega scale sheath folds in the arunta block central australia abstract the strangways orogenic belt of 1800 ma granulites in the central arunta block was completely reworked at granulite facies grades during the middle proterozoic 1400 1500 ma the bulk strain regime of the initial episodes of this reworking is presented ductile reworking involved non coaxial deformation on a regional scale and produced map scale sheath folds high bulk shear strains are inferred on microscopic and macroscopic scales and the spatial variation in bulk shear strain throughout the region is discussed the high bulk shear strains were accommodated by east over west ductile ramping and fold repetition resulting in easterly inclined structures crustal shortening and crustal over thickening

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press