chronology of deformation within the turbidite dominated lachlan orogen implications for the tectonic evolution of eastern australia and gondwana ar ar data from fabric forming white mica in slates syntectonic quartz veins and granitic mylonites constrain the timing of metamorphism deformation and exhumation in the lachlan orogen australia these data also help define the tectonic evolution of the tasmanides during paleozoic time the lachlan orogen formed by the progressive accretion of a thick turbidite fan sequence and volcanic terrains to gondwana during the closing of a small marginal ocean or back arc basin this tectonic setting was similar to the present western and southwestern pacific region accretion of the lachlan orogen to gondwana occurred by closing of the basin system by subduction accretion processes and some translation the process is typified in the western lachlan orogen by a major eastward migrating deformation involving chevron folding and faulting over an eastward propagating decollement melange zone and is recorded by ar ar mica ages ranging from 455 ma in the western part to 390 ma in the eastern part in the central lachlan orogen deformation migrated southwestward from 440 430 to 405 ma away from the high temperature wagga omeo metamorphic complex where deformation metamorphism occurred between 440 and 400 ma in the north 400 ma mica ages record deformation and inversion of structures in the cobar basin in the eastern lachlan orogen ar ar mica dates range from 450 to 340 ma ages of 455 445 ma are yielded by the narooma accretionary complex 405 390 ma ages are found along the major thrust faults bounding high grade metamorphic complexes and 360 340 ma cooling ages are found in the inverted extensional basins e g hill end and related structural zones the ar ar results also document periods of reactivation on early formed structures during later deformation elsewhere in the orogen

cooling and uplift patterns in the lepontine alps south central switzerland and an age of vertical movement on the insubric fault line 96 new fission track ft apatite and zircon k ar and rb sr biotite and muscovite ages are presented for 19 samples mainly acid gneisses from a 40 km traverse through the lepontine alps in the maggia valley south central switzerland plotting measured mineral ages against assumed system closure temperatures yields cooling rates for each sample the entire profile shows a fairly uniform late neogene recent mean uplift rate of 0 5 mm a confirmed by a gradient of ft apatite age with elevation cooling from higher temperatures occurred earlier in the south where uplift rates of 2 2 mm a in the steep belt root zone indicate 9 km early miocene uplift of the northern pennine block this uplift started before 23 ma and is interpreted as resulting from a major phase of backthrusting along the insubric line and as dating the formation of the mylonite belt estimated cooling rates constrain the timing of lepontine mid tertiary metamorphism 3 schematic models are proposed which also consider published rb sr k ar mica and hornblende and u pb monazite ages slow cooling differential initial heating and subsequent cooling of different parts of the central alps and post 38 ma cooling with syntectonic metamorphism at 27 ma are postulated as alternative interpretations of isotopic data and geologic evidence from extrapolation between k ar and rb sr mica ages and apatite ft ages 240 50 c is proposed as the closure temperature for the retention of fission tracks in zircon

40ar 39ar geochronology of the eocene green river formation wyoming the deposits of eocene lake gosiute that constitute the green river formation of wyoming contain numerous tuff beds that represent isochronous correlatable stratigraphic markers tuff beds selected for 40ar 39ar analysis occur within laminated mudstone are matrix supported and lack evidence of reworking these tuffs contain 2 15 euhedral phenocrysts of quartz plagioclase sanidine biotite and minor amphibole pyroxene and zircon encased in a matrix of altered glassy ash air abrasion and handpicking under refractive index oils were required to obtain clean unaltered phenocrysts of sanidine 40ar 39ar age determinations from single crystal and 1 mg multigrain aliquots of sanidine and biotite allowed the identification and exclusion of xenocrystic contamination laser fusion experiments on phenocrysts from the rife firehole c bed grey main sixth and analcite tuff beds from the tipton wilkins peak and laney members yielded weighted mean ages 2 analytical uncertainties of 51 25 0 31 ma 50 70 0 14 ma 50 56 0 26 ma 50 39 0 13 ma 49 96 0 08 ma 49 70 0 10 ma and 48 94 0 12 ma respectively ages for sanidine and biotite from the main tuff are indistinguishable when presumably xenocrystic contaminants are excluded from the age calculation moreover the 40ar 39ar ages are consistent with the stratigraphic order of the tuff beds and with provenance in the absaroka and challis volcanic fields our 40ar 39ar based age model indicates that sediment accumulated three times more rapidly 327 86 m yr during the evaporative wilkins peak phase than the freshwater to saline tipton 88 34 m yr and laney 104 18 m yr phases the much lower accumulation rates for the tipton and laney members are permissive of an annual origin for 1 mm thick laminae and precessional forcing of 1 3 m thick depositional cycles in these units however previously described cycles in the wilkins peak member have average durations that are significantly shorter than the 19 23 k y precessional modes the green river formation encompasses an 5 m y period between ca 53 5 and 48 5 ma spanning magnetic chrons 24n through 21r the green river formation was therefore deposited during the warmest period of the cenozoic corresponding to the early eocene climatic optimum as defined by the global marine o isotope record deposition of bedded evaporites trona of the wilkins peak member began at ca 51 ma or 2 m y after the onset of the highest inferred temperatures of the climatic optimum the bridgerian wasatchian faunal turnover occurred subsequently during wilkins peak time at ca 50 6 ma thus our 40ar 39ar ages strongly suggest that wilkins peak evaporite deposition and the turnover from wasatchian to bridgerian fauna were not directly caused by the initiation of maximum greenhouse conditions

petrologic evidence for k feldspar metasomatism in granulite facies rocks abstract we present evidence for k feldspar metasomatism in charnockitic granulites from two well known terranes the shevaroy hills massif s india 750 c 8 kbars and the bamble sector s e norway 790 c 7 5 kbars in the form of k feldspar veins principally along plagioclase and quartz grain boundaries and in the form of highly variable antiperthitic patches of k feldspar in an uneven scattering of plagioclase grains with one exception orthopyroxene or amphibole grains in contact with these k feldspar veins show no alteration to secondary biotite indicating that the h 2 o activity of the fluids responsible for these veins must have been relatively low a high ba concentration in these veins also suggests a metasomatic origin point counted back scattered electron photomicrographs along with microprobe analyses provide reintegrated k feldspar and plagioclase compositions for 8 to 12 predetermined random areas per thin section for three samples from bamble and three samples from the shevaroy hills these reintegrated feldspar compositions plot over a range of temperatures on the feldspar ternary for each sample and indicate saturation temperatures above the mean temperature for either region with a few reintegrated compositions above the 1000 c isotherm and with the lower cut off temperatures at 700 c and 600 c for the shevaroy and bamble samples respectively these patterns suggest that exsolution alone could not have been responsible for the formation of the k feldspar veins and patches in these rocks we suggest that these veins are due to the influx of complex supercritical low h 2 o activity brines shortly after peak metamorphic conditions that this influx continued during the initial phases of post peak metamorphic uplift and that these fluids represent the first stage in a series of fluid influxes in which the h 2 o activity increased as uplift continued reaction of these brines with potassium undersaturated plagioclase grains formed k feldspar veins along grain boundaries and fractures as well as diffusing into the plagioclase which became supersaturated and exsolved k feldspar as antiperthite patches during uplift and cooling formation of secondary biotite halos around orthopyroxene in one sample associated with these k feldspar veins can be explained by heightened h 2 o activity in the brines due to enrichment in h 2 o emplacement of the veins at lower pressure during uplift or both compared to the other shevaroy and bamble samples

ree geochemistry of early precambrian charnockites and tonalitic granodioritic gneisses of the qianan region eastern hebei north china abstract the charnockites and gneisses from the qianan area eastern hebei province china were investigated for their isotope and trace element abundances the majority of the charnockites are acid in composition only a few samples have a composition corresponding to intermediate to basic rocks a rb sr whole rock isochron of the charnockites gives an age of 2 65 0 05 ga the highly fractionated ree patterns and the variable ce yb n ratios in these rocks are interpreted as being a result of igneous processes it is believed that the charnockitic rocks are related to each other by fractional crystallization under high temperatures and low p h 2 o conditions the major precipitating phases are thought to be orthopyroxene and plagioclase by contrast the qianan gneisses are mainly of tonalitic and granodioritic composition however a few samples have composition corresponding to granite s s probably produced by metasomatic processes a rb sr whole rock isochron of the qianan gneisses yields an age of 2 13 0 06 ga and this age probably represents a remobilization event the qianan gneisses do not contain hypersthene they have strongly fractionated ree patterns similar to those found in many archaean tonalitic rocks of the world in addition the total ree abundances and the magnitudes of the eu anomaly show regular variation with the sio 2 content this may suggest that the qianan gneisses might have evolved under hydrous conditions with hornblende exerting a major control on their trace element chemistry

post orogenic thermotectonic history of the mount buffalo region lachlan fold belt australia evidence for mesozoic to cenozoic wrench fault reactivation abstract the early devonian mount buffalo granite is exposed today as an inselberg at the dissected western margin of the southeastern highlands of victoria australia it dominates a block of deeply incised ordovician turbidites that appears structurally isolated from the surrounding highlands by treating these highlands as an elastic plate subject to flexure or warping previous workers have proposed that this region of the lachlan fold belt has experienced a relatively simple thermotectonic and landscape evolution history following shallow level intrusion of the granite however results of apatite and zircon fission track analyses from the granite indicate the occurrence of major previously unrecognized episodes of 1 heating burial prior to 246 ma early triassic and prior to 70 ma late cretaceous and 2 rapid cooling denudation at some time between 246 and 220 ma early to late triassic and between 70 and 50 ma late cretaceous to early tertiary while previous interpretations of the landscape history of the region suggest the prolonged subaerial exposure of the granite following shallow crustal residence our results clearly indicate this region of the lachlan fold belt has experienced a dynamic post paleozoic history thus this new dataset forces a radical re interpretation of the landscape evolution of the mount buffalo structural block which has significant implications for our understanding of the mesozoic and cenozoic geomorphic and structural development of southeastern australia we therefore propose a model incorporating reactivation of wrench faults that may explain the complex thermotectonic history while accommodating the detailed observations of previous workers in this area

near field response to transpression along the southern san andreas fault based on exhumation of the northern san gabriel mountains southern california 1 near field mountain building is common along transpressive faults but how vertical deformation relates to oblique plate motion versus local structural complexity has not been universally established we have investigated the mechanisms of exhumation along the southern san andreas fault in the northern san gabriel mountains to examine the effects of pure shear dominated transpression obliquity 20 although rugged topography suggests near field uplift low temperature thermochronometry indicates minimal exhumation apatite u th he ages are 10 ma along narrow ridges within the fault zone implying near field rock uplift is not pervasive along this oblique plate boundary ages farther from the fault are younger 5 ma and suggest a deformation event occurred during the transition of activity from the san gabriel fault to the modern san andreas strand these results reveal a heterogeneous crustal response to transpression and imply that secondary structures or geometrical complexities play a primary role in near field bedrock uplift along transpressive faults

a magnetostratigraphic study of the sediments of the ridge basin southern california and its tectonic and sedimentologic implications a magnetic polarity zonation has been established for the upper castaic formation and lower ridge route and peace valley formations of the ridge basin of the central transverse ranges of southern california the zonation is based on the analysis of over 1900 samples from 669 sites in the 5800 m thick sedimentary sequence rock magnetic studies provide evidence that the carrier of the primary remanence is magnetite and that the magnetization was acquired at or shortly after deposition of the sediment although many samples contain a secondary normal overprint systematic analysis of the changes in direction upon demagnetization leads to an accurate determination of the original polarity thirteen polarity intervals are recognized on the basis of biostratigraphic constraints imposed by a marine invertebrate faunaat the base of the sequence and a terrestrial vertebrate fauna at the top the magnetic polarity zonation can be correlated to the magnetic polarity time scale the studied interval extends from the epoch 5 epoch 6 boundary to the middle of epoch 8 polarity changes within the studied interval represent chronostratigraphic horizons which can be used to analyze tectonic and sedimentologic processes which are recorded in the sediments of the ridge basin the locus of the tectonic activity within the basin has been dated as occurring on the clearwater fault from 8 1 to 7 8 m y ago on the liebre faultfrom 7 3 to 6 1 m y ago and on the north liebre fault from 6 0 to 5 0 m y ago during this entire period of time the san gabriel fault on the western margin of the basin was continuously active and a period of particularly vigorous activity has been dated at 7 3 to 7 0 m y the mean declinations of samples from the normal magnetozones provide no evidence for the existence of a major rotation of the ridge basin subsequent to about 8 5 m y ago sedimentation rates for interbedded sandstone and shale units within the basin have been separately determined as 0 2 m 1000 years and 3 0 m 1000 years respectively the water content of certain deformed structures within the sequence has been determined to have been 35 and 40 at the time of deformation

fission track dating of phosphate minerals and the thermochronology of apatite several phosphate minerals have been investigated for their usefulness in geochronology and thermochronology by the fission track method of these apatite ca5 po4 3 f cl oh has proved pre eminently suitable for this purpose for reasons discussed below apatite was one of the first minerals amongst many others to be investigated for fission track dating by fleischer and price 1964 and has subsequently become by far the most important of all the minerals used for dating by this method the usefulness of apatites for fission track dating arises from their near universal tendency to concentrate uranium within their structure at the time of crystallization and their widespread occurrence in all of the major rock groups price and walker 1963 first recognized the possibility that the accumulation of radiation damage tracks in natural minerals from the spontaneous nuclear fission of 238u within their lattices could be used for geological dating they also demonstrated that a simple chemical etching procedure served to enlarge these fission tracks to optical dimensions so that they could be observed and measured under an ordinary optical microscope price and walker 1962b 1963 this simple procedure quickly opened the way for a wide range of nuclear particle track studies in natural minerals and glasses e g fleischer et al 1975 the most important of which was fission track dating fleischer et al 1964 also showed that spontaneous or fossil fission tracks provided an explanation for various anomalous etch pits in apatite which had puzzled crystallographers over many years the essence of a fission track age determination involves measuring the number of tracks that have accumulated over the lifetime of the mineral along with an estimate of the amount of uranium that is present knowing the rate of spontaneous fission decay a geological age can be calculated detailed

low pressure regional metamorphism in the omeo metamorphic complex victoria australia the omeo metamorphic complex forms the southern end of the wagga metamorphic belt which is the main locus of palaeozoic low pressure metamorphism in the lachlan fold belt south eastern australia it comprises metamorphosed ordovician quartz rich turbidites originally derived from precambrian cratonic rocks prograde regional metamorphism occurred in the early silurian very soon after sedimentation had ceased the sequence of metamorphic zones with increasing grade is chlorite biotite cordierite andalusite k feldspar and sillimanite k feldspar migmatites occur in the sillimanite k feldspar zone but large bodies of s type granite were derived from rocks underlying the exposed ordovician sequence p and t estimates for the highest grade rocks are t 700 c and p 3 5 kbar indicating a very high p t gradient of 65 c km the high heat flow during prograde metamorphism probably resulted from a combination of a thermal anomaly persisting from a pre metamorphic back arc basin environment and intrusion of hot mantle derived magmas into the lower and middle crust regional retrograde metamorphism coincided with a general reheating of the crust in the siluro devonian accompanied by intrusion of many i type plutons and resetting of the k ar dates of some earlier plutons the omeo metamorphic complex was exposed to erosion at this time

high pressure granulites at the dawn of the proterozoic abstracthigh pressure metamorphism is uncommon in the ancient geo logical record kanja malai in the salem crustal block southern india contains high pressure kyanite garnet bearing felsic granu lites that equilibrated at 14 16 kbar and 820 860 c laser abla tion inductively coupled plasma mass spectrometry u pb zircon and in situ monazite geochronology indicate that these assemblages grew ca 2490 ma these pressure temperature time constraints pro vide a rare record indicating that thickened crust and low apparent thermal gradient conditions existed during the archean proterozoic transition a period of earth history for which the rock record com monly preserves evidence for comparatively high apparent thermal gradients the thermal regimes required to generate these metamor phic conditions are typical of collisional orogenesis and suggest that the continental lithosphere was capable of supporting crustal thick ening to 45 50 km such crustal thickening provides supporting evi dence that tectonic regimes similar to modern earth style tectonics were in operation at the archean proterozoic transition introduction

magnetostratigraphy in pursuit of missing links since the last quadrennial report magneto stratigraphy has continued to flourish the primary issues have been 1 refinements to the geomagnetic reversal time scale grts including recalibrations correlation of biostratigraphic data to the crts and revision of the terminology 2 use of magnetostratigraphic data for estimating rates of such processes as sediment accumulation evolution and faunal migration and 3 magnetic mineralogy and the origin of remanence for convenience we have divided the publications into studies of continental and marine sediments and highlight the most exciting issues within each of these sub headings in the first section of this review we will concentrate on the development of the geomagnetic reversal time scale grts with particular emphasis on refinements from the last few years we then turn our attention to applications of magnetostratigraphy in marine and continental sequences

neoarchean to paleoproterozoic continental evolution and tectonic history of the north china craton a review rocks of archean age are widely distributed in the north china craton ncc and can be divided into high grade regions and granite greenstone belts even though most underwent amphibolite to granulite facies metamorphism quartzo feldspathic gneisses occupy 70 to 85 of the total exposure of archean rocks in the craton supracrustal rocks commonly consist of intercalated volcano sedimentary rocks including metabasites meta intermediate to acid rocks banded iron formations metapelites and minor marbles the oldest rocks outcrop over a small area of 4 km2 and are a quartzite amphibolite sequence that yield 3 8 3 5 ga isotopic ages chemical and geochronological data constrain the earliest main crust forming episode at ca 2 9 2 7 ga when most rocks were metamorphosed to upper amphibolite or granulite facies at moderate pressures mafic granulites can be classified into two textural types referred to as near isobaric cooling ibc and near isothermal decompression itd types high pressure granulites and retrograded eclogites occur mainly near the junction of the provinces of hebei shanxi and inner mongolia in the central ncc and in eastern shandong province of the eastern ncc as well as locally in henan province in the southern ncc these high pressure rocks show clockwise p t paths indicating decompression from 14 kb to 5 6 kb an exposed section interpreted as representing precambrian continental lower crust has also been identified in the northern part of the craton although some mafic ultramafic metamorphic rocks have been interpreted as remnants of ancient oceanic crust and mantle by a number of geologists geochemical and petrological evidence demonstrates that many supracrustal orthogneiss associations have island arc affinities therefore several tectonic models have been proposed including continent continent collision and island arc terrain accretion achieved by arc arc or arc microcontinent collision the ncc underwent multiple high grade metamorphic events of which the two most important took place during the intervals 2600 2450 ma and 1950 1750 ma which are considered to be related to early precambrian supercontinental cycles

petrogenesis and geochronology of precambrian granitoid gneisses in western liaoning province constraints on neoarchean to early paleoproterozoic crustal evolution of the north china craton abstract the north chaoyang fuxin yixian granite greenstone belt ncfy ggb at the northern margin of the eastern continental block of the north china craton is composed chiefly of metamorphic volcanic and volcano sedimentary rocks and granitoid gneisses the magmatic precursors of these granitoid gneisses are diorites tonalites trondhjemites granodiorites and monzogranites these granitoid gneisses can be divided into two groups a high mg group hmg and a low mg group lmg according to their contrasting geochemical characteristics petrogenetic studies indicate that the hmg magmas were derived from partial melting of a subducted oceanic slab which was contaminated by mantle peridotites during their ascent whereas the lmg magmas were generated from the partial melting of lower continental crust composed chiefly of metamorphosed basaltic and pelitic rocks la icpms zircon u pb dating reveals that granitoid gneisses in the hmg and low k granitoid gneisses in the lmg were emplaced at 2511 2521 ma but that the high k granitoid gneisses in the lmg formed later at ca 2495 ma these neoarchean to early paleoproterozoic granitoid gneisses intrusive into the metamorphic volcanic assemblages of the yixian fuxin greenstone belt indicate that the ncfy ggb was developed at a neoarchean to early paleoproterozoic andean type active continental margin

assembling north china craton within the columbia supercontinent the role of double sided subduction abstract an evaluation of recent s wave receiver functions s wave velocities and two versions of p wave tomographic images along various transects in the north china craton provides some clues on the subduction collision history of the different crustal blocks and their final amalgamation within the paleoproterozoic columbia supercontinent interpretation of a n s seismic section of the craton suggests thick slab debris sinking to various depths in the mantle the w e seismic corridors show the preservation of a thick 200 km lithospheric root tectosphere beneath the ordos block and its variable and extensive erosion towards the yanliao block eastern block this zone is characterized by layers with marked velocity contrast and suggests repeated stacking of the remnants of underplated and accreted paleoproterozoic oceanic lithosphere the present day lithosphere asthenosphere boundary beneath this region probably marks the erosional plane along which decratonization occurred through subduction erosion from the east and thermal and material erosion by upwelling asthenosphere from below resulting in the partial destruction of the tectosphere and its thinning towards the east within the asthenosphere below the yanliao block younger and thinner slabs predominate in the absence of any prominent thick high velocity layers these younger slabs define a westward polarity and constitute a mega scale duplex formed by underplating through phanerozoic subduction process particularly the pacific plate subduction from the east the lithologic associations within the inner mongolia suture zone dividing the yinshan block to the north and ordos block to the south correspond to an accreted ocean plate stratigraphic sequence with the tonalite trondhjemite granodiorite ttg gneisses charnockites and calc alkaline granites representing a continental arc built up through subduction from the north the seismic transects bring out a contrasting polarity in the subduction regime with an oblique east to southward subduction of the yinshan block and a westward subduction of the yanliao block here i propose a double sided subduction history for the ncc similar to the ongoing subduction process in the western pacific such double sided subduction is considered to promote rapid amalgamation of continental fragments within supercontinents and the subduction polarities and mantle dynamics of ncc are therefore considered to be critical in evaluating the final assembly of the paleoproterozoic supercontinent columbia

tectonic history from vitrinite reflectance summary the degree of organic metamorphism of carbonaceous matter is expressed in terms of the reflectance of vitrinite vitrinite reflectance is a function of temperature and time a model relating vitrinite reflectance to temperature and time is proposed from published vitrinite reflectance temperature time nomograms a plot of the logarithm of vitrinite reflectance versus depth gives a linear curve for vitrinite reflectance greater than 0 3 per cent in a sedimentary basin which has had a constant geothermal gradient throughout its tectonic history the geothermal gradient c km 1 is equal to the slope of the curve multiplied by 194 8 the model is also applied to sedimentary basins with decreasing geothermal gradient with time i e cooling continental margin basins the vitrinite reflectance model is demonstrated to be consistent with observed reflectance data and tectonic information from the sydney bowen surat and gippsland basins in eastern australia

multiple postorogenic denudation events an example from the eastern lachlan fold belt australia fission track results from 40 surface samples from the eastern part of the lachlan fold belt australia suggest that two distinct episodes of rapid kilometre scale denudation have occurred since the middle carboniferous when deformation within the fold belt is believed to have ceased the first episode during the early triassic was possibly in response to the hunter bowen orogeny which affected the new england fold belt and the sydney bowen basin but the effects of which have previously not been recognized within the lachlan fold belt the second episode occurred during the middle cretaceous possibly in response to the onset of continental extension in the tasman sea ca 96 ma uplift at this time resulted in kilometre scale denudation over much of the southeastern highlands of australia and may have been caused by underplating inward of the rift these results indicate that the lachlan fold belt has remained tectonically active long after the last recognized deformational event in the region and highlight the importance of fission track data in elucidating the posttectonic histories of orogens previously undetected due to a lack of stratigraphic and structural crosscutting relationships

convolute laminations a theoretical analysis example of a pennsylvanian sandstone abstract data from an outcropping laminated interval were collected and analyzed to test the applicability of a theoretical model describing instability of layered systems rayleigh taylor wave perturbations result at the interface between fluids of contrasting density viscosity and thickness in the special case where reverse density and viscosity interlaminations are developed the deformation response produces a single wave with predictable amplitudes wavelengths and amplification rates physical measurements from both the outcropping section and modern sediments suggest the usefulness of the model for the interpretation of convolute laminations internal characteristics of the stratigraphic interval and the developmental sequence of convoluted beds are used to document the developmental history of these structures

a morpho statistical classification of mountain stream reach types in southeastern australia abstract the concepts of sediment transport capacity q c and sediment supply q s have shown promise in broadly differentiating mountain streams the important role of lithology in determining reach characteristics is also noted but as yet not fully included in existing process domain frameworks this study uses topographic and grain size surveys undertaken over 42 channel reaches with slopes between 0 01 and 0 20 m m to describe mountain stream morphology in southeastern australia reaches with two different lithologies are surveyed to specifically address the role of lithology in resultant channel morphology spatial autocorrelation analysis based on moran s i is used to detect the frequency and occurrence of in stream features such as bars and pools these data are combined with the grain size data to describe eight dominant reach morphologies most channel morphologies display broad characteristics similar to previous accounts of mountain streams although a number of intermediate morphologies affecting subtle variations in form and process where evident there was no significant longitudinal arrangement of the observed morphologies particularly with respect to catchment area and slope this is related to the strongly segmented longitudinal profiles characteristic of these streams where increasing catchment area does not result in a commensurate decrease in slope five of the eight channel morphologies identified are lithology dependent with respect to grain size and shape regime diagrams applied to quantify the physical controls on the different reach morphologies identified different q c q s domains for most morphologies however some lithology dependent morphologies could not be differentiated supporting the need to incorporate the effects of dominant grain shape or sphericity in assigning dimensionless critical shear stress values in determining q s overall the differences in mountain stream reach morphologies described in this study reflect the characteristic segmentation of the longitudinal profiles of these streams and contrasts in the supply and breakdown of parent material

character and kinematics of faults within the turbidite dominated lachlan orogen implications for tectonic evolution of eastern australia reply fault zones within turbidite dominated orogenic systems typified by the lachlan orogen of eastern australia are characterised by higher than average strain and intense mica fabrics transposition foliation and isoclinal folds poly deformation with overprinting crenulation cleavages and steeply to moderately plunging meso and micro folds they have a different character compared to the brittle ductile fault zones of classic foreland fold and thrust belts such as the appalachians and the canadian rocky mountains multiple cleavages and transposition layering record a progressive shear related deformation history an intense mica fabric evolves initially during shortening of the overlying sedimentary wedge but is progressively modified during rotation and emplacement to higher structural levels along the steep parts of inferred listric faults the deformed wedge outside the fault zones generally undergoes one phase of deformation shown by a weak to moderately developed slaty cleavage which is parallel to the axial surface of upright subhorizontally plunging chevron folds other faults within the turbidites of the lachlan orogen include the steep zones of ductile strike slip deformation that bound a centrally located high t low p metamorphic complex characterised by s c mylonites these ductile shear zones indicate a southward passage of the metamorphic complex as a crustal wedge with emplacement to higher structural levels along a leading edge ductile thrust fault ar ar dating constrains the timing of regional deformation to be mostly late ordovician through silurian across the lachlan orogen faults in the low grade turbidite sequences record the kinematic evolution of the developing lachlan orogen and indicate progressive deformation associated with simultaneous eastward propagating and migrating deformation fronts in both the western and eastern parts of the fold belt these deformation fronts are related to accretionary style deformation at the leading edges of overriding plates in an inferred southwest pacific type subduction setting from the late ordovician to the mid devonian along the former gondwana margin the fault zones effectively accommodate and preserve movements within the structurally thickening migrating and prograding accretionary wedge

new late eocene pole for australia time averaging of remanence directions and palaeogeographic reference systems summary a new late eocene 36 5 ma south palaeomagnetic pole based on 33 sampling levels in the browns creek formation of the otway basin south eastern australia lies at 65 5 s 112 5 e with ag5 2 5 this pole fills a 30 myr gap in the palaeogene part of the australian apparent polar wander pole path and appears well suited for calibrating the path because of a well defined age and evidence that the sequence was magnetized during a depositional period long enough to thoroughly average out secular variations the remanence seems to have been acquired during postdepositional re working of the sediment the pole agrees with one biostratigraphically dated and three undated poles that constrain the palaeogene trajectory all palaeogene poles on the path lie west of an alternative apparent polar wander path the latter is modelled by low order curve fitting of much more widely scattered poles from basalts in which the scatter appears to be due to incomplete time averaging of remanence directions the new pole also reinforces a previously observed systematic westward displacement by a few degrees of the early tertiary palaeomagnetic poles relative to hotspot derived poles this displacement may be caused by true polar wander and or departures from the geocentric axial dipole model it again highlights the approximateness of the available palaeogeographic reference systems

apatite fission track and u th he data from fennoscandia an example of underestimation of fission track annealing in apatite abstract apatite fission track aft data from cratons have commonly been interpreted in terms of major episodes of sedimentary burial and subsequent denudation often in the absence of any remnant sedimentary rocks or other supporting geological evidence reconciling these interpretations with geologically documented denudation events or outcropping regional stratigraphy can be problematic also u th he ages from cratonic igneous rocks that are older than paired aft data are commonly discarded as too old even when the former are repeatable at the 2 confidence level for example aft data from central fennoscandia have been interpreted to reflect first burial by a caledonian foreland basin several kilometres in thickness and subsequently its denudation however throughout fennoscandia there is no direct evidence that such a deep extensive foreland basin ever existed and indeed some geological evidence points to the contrary additionally a significant number of u th he ages from fennoscandia are older than their aft pairs but have been interpreted by their authors as too old relative to the aft ages and not geologically meaningful despite repeatable results here we propose a new interpretation of aft data from fennoscandia that closely reflects the observed data yet is compatible with the generally accepted geological history discarding the hypothesis of a thick caledonian foreland basin we suggest that 1 aft ages in finland are too young rather than that the u th he ages are too old 2 repeatable sequences of u th he ages are more likely to record the true denudation history of cratons 3 the importance of radiation enhanced annealing at low temperatures has been overlooked and 4 fennoscandian aft data may be divided into an eastern domain strongly affected by radiation enhanced annealing where current aft annealing models are invalid and a western domain much less affected by radiation enhanced annealing where current modelling methods may appropriately be used to extract thermal histories in principle radiation enhanced annealing is also pertinent to the interpretation of young aft ages from apatites rich in emitter actinides the construction of a properly calibrated annealing model that fully incorporates the effects of radiation enhanced annealing is fundamentally important

paleoproterozoic arc magmatism in the north china craton no siderian global plate tectonic shutdown abstract arc magmatism in convergent plate margins has been a major contributor to continental growth following arc arc and arc continent collisions in the archean leading to the amalgamation of micro blocks the north china craton ncc witnessed major pulses of continental arc magmatism during the paleoproterozoic in this study we present geochemistry zircon u pb geochronology and lu hf isotope data from a suite of magmatic rocks sampled from the region of confluence of two major paleoproterozoic suture zones in the ncc the inner mongolia suture zone imsz and the trans north china orogen tnco our zircon u pb geochronological data indicate new zircon growth during multiple tectonothermal events as displayed in the 207pb 206pb weighted mean ages of 2410 41 ma for metagranite 2480 12 ma 2125 18 ma 1946 8 ma 1900 15 ma and 1879 12 ma from metagabbros 2446 11 ma from charnockite and 1904 6 ma and 1901 9 ma from metatuffs the 207pb 206pb upper intercept age of zircons in the khondalite shows 2102 76 ma which is identical to the age obtained from the magmatic zircons in one of the metagabbros the khondalites also carry a group of concordant metamorphic zircons with 207pb 206pb mean age of 1881 20 ma metamorphic zircons in the gabbros and charnockites also yield similar ages of 1890 14 ma and 1852 19 ma respectively the age data suggest prolonged arc magmatism in a convergent margin setting during ca 2 48 to 1 9 ga followed by metamorphism at ca 1 89 1 85 ga associated with the final collision lu hf analyses reveal that the dominant populations of zircons from all the rock types are characterized by positive ehf values 1 9 to 6 8 mean 1 8 the ehf and tdmc data suggest that the magmas were mostly derived from neoarchean and paleoproterozoic juvenile components the salient geochemical features of these rocks attest to magma generation from heterogeneous sources involving subduction derived arc components with minor input from continental crust the results presented in this study together with those from previous investigations in different domains of the imsz and tnco suggest major paleoproterozoic arc magmatic events in the ncc lasting for nearly 600 million years associated with the final assembly of the crustal blocks into a coherent craton construction of the final cratonic architecture of the ncc thus witnessed not only the arc continent amalgamations at 2 7 2 5 ga but also major crust building events in the paleoproterozoic through melts generated from juvenile and recycled components in continental magmatic arc systems along an active convergent margin followed by intense deformation and metamorphism during the final collision at 1 85 1 80 ga the prominent paleoproterozoic magmatic records in the ncc do not support the proposal of global plate tectonic shutdown in the siderian and confirm vigorous convergent margin magmatism and crust building processes throughout the paleoproterozoic

the importance of apatite composition and single grain ages when interpreting fission track data from plutonic rocks a case study from the coast ranges british columbia abstract fission track results determined from granitic rocks exposed within the coast ranges of british columbia indicate that significant variations in apatite chemistry may occur in small plutonic rock samples to test ages determined earlier by parrish using the population method of fission track dating new apparent apatite ages were completed using the external detector method so that single grain ages could be generated in most cases the new apatite ages were similar to the earlier ones within 2 however in many cases it was found that individual samples contained significant variations in single grain apatite ages microprobe analyses showed the spread in ages were directly related to variations in apatite chemistry such that chlorine rich apatites retained older ages than fluorine rich grains examination of the single grain age data suggested that the spread in single grain ages provided information which constrained the thermal history of the sample and that distributions of single grain ages reflected the temperatures at which they resided during cooling these results suggest that it is important to generate more than a few single grain fission track ages when dating apatite from granitic rocks since variations in the chlorine concentration in apatite grains may exist in kg sized granitic rock samples therefore it is no longer a valid assumption that each apatite grain separated from a small granite sample will have a similar chlorine concentration and in future apatite fission track studies of granitic terrains it is important to recognize that the apatite composition might be heterogeneous when interpreting results

paleomagnetism and the motion of large and small plates the focus of paleomagnetic research during past decades has varied greatly the first generation in paleomagnetism might be defined as beginning with the pioneering studies aimed at discovering whether the geomagnetic field has been a reliable reference frame for measuring and timing continental displacements an enormous effort followed in the form of individual paleomagnetic studies of cratonic rocks culminating with the construction of apw apparent polar wander paths for the continents which could be used to test paleogeographic reconstructions coupled with parallel research in marine geophysics and geology the refined apwp which result from this effort have helped to develop a reasonable first order history of cenozoic mesozoic paleogeography for the major plates

geodynamic evolution of the antarctic peninsula during mesozoic times and its bearing on weddell sea history this review of the tectonic evolution of the antarctic peninsula during mesozoic times highlights four main events 1 late triassic late jurassic extension 2 late jurassic early cretaceous dextral transpression 3 early cretaceous extension and 4 mid cretaceous compression magmatism was virtually continuous during much of this period with the exception of possible breaks in the known record in early jurassic and late jurassic early cretaceous times the second of these breaks corresponded to the first compressional event there was no apparent hiatus in the magmatic record during the mid cretaceous compressional event although there was a significant change in the pattern of sedimentation in the larsen basin on the eastern margin of the weddell sea at about that time the tectonic evolution of the peninsula is compared to and puts some constraints on existing weddell sea models the late triassic late jurassic arc extension correlates with initial rifting in the weddell sea region during sinistral motion between east and west gondwana however there is no known record of large scale pre mid jurassic transcurrent deformation in the antarctic peninsula that would have been consistent with rotation of west antarctic crustal blocks in the initial rifting period the peninsula wide late jurassic early cretaceous compressional event may correlate with the major change in gondwana plate motions from e w to n s african reference frame separation of east and west gondwana this change probably resulted in formation of an ocean continent boundary along the northern margin of the weddell sea embayment and initial seafloor spreading geological data do not seem to support subduction of southwestern proto weddell sea oceanic lithosphere beneath the eastern margin of the peninsula at that time cretaceous arc extension was coeval with the initial seafloor spreading phase in the weddell sea mid cretaceous arc compression linked to a global increase in ocean floor spreading rates and a superplume event correlates with a change from ne sw to nw se spreading in the weddell sea

magnetostratigraphy of the late neogene purisima formation santa cruz county california abstract the magnetic polarity zonation of a late neogene sedimentary sequence in santa cruz county california has provided a chronologic framework for studies of the sedimentologic and tectonic processes involved in an episode of basin formation in the vicinity of the san andreas fault system in central coastal california the zonation is based on the analysis of samples from 79 horizons in a 300 m thick section of the purisima formation and a portion of the overlying aromas formation although rock magnetic studies support the hypothesis that the primary carrier of the remanence is magnetite many samples contain a secondary overprint which cannot be completely removed by alternating field demagnetization nevertheless systematic analysis of the behavior of the samples during demagnetization has led to an unambiguous determination of the polarity of each horizon and to the development of a magnetic polarity zonation containing thirteen magnetozones these magnetozones can be correlated to the magnetic polarity time scale using biostratigraphic constraints provided by diatoms in the lower portion of the section and invertebrates and vertebrates in the upper portion the studied section is found to span the interval from the epoch 5 epoch 6 boundary 6 07 mya to the matuyama gauss boundary 2 47 mya with a hiatus corresponding to the upper part of the gilbert epoch 4 5 mya to 3 5 mya this hiatus does not coincide with major regressions in the global sea level curve and is interpreted as a period of tectonic uplift the compression which generated this uplift was probably caused by interplay between the san andreas fault east of the study area and the san gregorio hosgri fault west of it

thermal evolution of rifted continental margins new evidence from fission tracks in basement apatites from southeastern australia the palaeozoic granitic basement of southeastern australia shows a regular pattern of apatite fission track ages which is dominated by a rapid decrease in apparent age towards the rifted continental margin apparent ages away from the margin vary between about 230 and 360 myr but drop sharply to minimum values of around 80 myr near the coast just south of the sydney basin and 150 175 myr elsewhere along the coast fission track length information allows the age pattern to be interpreted as the result of partial to complete resetting of the older ages by a widespread thermal event at about 80 100 myr this event accompanied the rift phase of passive margin development and peaked just before the onset of sea floor spreading in the adjacent tasman sea the zone of reduced apatite ages is deeply embayed around the gippsland basin supporting the suggestion that it originated as a cretaceous failed arm of the tasman rift system the thermal event has affected apatites up to about 130 km from the margin and has produced a characteristic set of bimodal track length distributions in apatites with partially reset ages the bimodality gradually disappears towards the oldest apatite ages which have negatively skewed unimodal distributions the youngest apatites also tend towards unimodal length distributions indicating that thermal annealing of pre existing tracks in these samples went almost to completion maximum temperatures reached in samples presently exposed in the coastal area were thus probably about 110 120 c thermal activity was accompanied by considerable uplift across the region followed by gradual erosion to expose the present land surface post breakup erosion was generally greater towards the new coastline with near coastal samples being previously buried a probable 1 5 2 5 km

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

archean blocks and their boundaries in the north china craton lithological geochemical structural and p t path constraints and tectonic evolution an examination of lithological geochemical geochronological structural and metamorphic p t path data suggests that the basement of the north china craton can be divided into eastern and western blocks separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone called the trans north china orogen the eastern block consists predominantly of late archean domiform tonalitic trondhjemitic granodioritic ttg batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at 2 5 ga with anticlockwise p t paths some early to middle archean rocks are locally present in the eastern block but their tectonic history is unclear due to reworking by the 2 5 ga tectonothermal event the western block has a late archean assemblage structural style and metamorphic history similar to that of the eastern block but it differs in the absence of early to middle archean assemblages and in being overlain by and interleaved with paleoproterozoic khondalites which were affected by a 1 8 ga metamorphic event involving clockwise p t paths a mantle plume model is proposed for the formation and evolution of late archean basement rocks in the eastern and western blocks based on a combination of extensive exposure of ttg gneisses affinities of mafic rocks to continental tholeiitic basalts presence of voluminous komatiitic rocks dominant diaprism related domiform structures anticlockwise p t paths and a short time span from the primary emplacement of ttg and ultramafic to mafic rocks until the onset of regional metamorphism between the two blocks is the trans north china orogen which is bounded by two major fault systems and is composed of late archean to paleoproterozoic ttg gneisses and granitoids interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra arc basin environments these rocks underwent multiple phases of compressional deformation and peak high pressure metamorphism followed by rapid exhumation during the late paleoproterozoic at 1 8 ga as a result of collision between the eastern and western blocks resulting in the amalgamation of the north china craton

archean crustal evolution in china u pb geochronology of the qianxi complex abstract the qianxi complex formerly the qianxi group in eastern hebei province is a key terrain for studying the archean crustal evolution of the sino korean craton in china the evolution of the complex involved several stages of supracrustal deposition and magmatic intrusion except for a few post tectonic intrusives all rock assemblages have been metamorphosed to high grade granulite and amphibolite facies the evolution of the complex has spanned more than 1 ga since about 3 5 ga ago in the present work two areas in the qianan region qianan county have been investigated for u pb age study 1 the caozhuang area in the south and 2 the shuichang yangyashan area in the north important tectonothermal events and magmatic episodes of the qianxi complex were identified by the u pb isotopic analyses of zircons extracted from various rock types the results of age determination and the most likely interpretation in terms of temporal evolution of the complex are summarized schematically the oldest zircons 3 65 3 67 ga of china have been found in a fuchsite bearing quartzite in association with the 3 5 ga sm nd isochron age amphibolites in the caozhuang area this emphasizes the existence of early archean sialic crust in the qianxi complex however no ages older than 2 9 ga have been revealed for grey gneisses enclosing the ca 3 5 ga amphibolite enclaves in the caozhuang area but a leptynite sample of dacitic composition gave a minimum 207pb 206pb age of 3 3 ga obtained by a single zircon stepwise evaporation analysis this age and the 3 5 ga sm nd isochron age for amphibolites suggest that the earliest deposition of supracrustal rocks took place about 3 5 3 3 ga ago the deposition of fuchsite quartzite could have taken place contemporaneously or in an earlier period up to 3 6 ga but its precise stratigraphic positions remains to be determined nevertheless the presence of very ancient zircons clearly advocate for the existence of sialic crust at least 3 6 ga in china in the shuichang yangyashan area a supracrustal sequence containing abundant banded iron formations was intruded by a granitic gneiss dated at 2 96 ga by the conventional multigrain zircon method sm nd model ages of about 3 3 ga were obtained for two amphibolite enclaves in a charnockitic gneiss it appears that supracrustal deposition of the qianxi complex might have migrated to the north and took place in the period of 3 3 3 0 ga in addition to supracrustal rocks the qianxi complex is characterized by polyphase granitoid intrusions except for the contaminated grey gneisses sm nd tdm 3 2 3 3 ga in the caozhuang area and the ca 3 ga granitic gneiss in the yangyashan area most granitoid intrusions are of late archean ages the period of 2 7 2 5 ga has undoubtedly recorded the most important tectonothermal events in the evolution of the sino korean craton the present zircon age study and available age data in the literature demonstrate that the archean crustal development in china culminated towards the end of the archean

phanerozoic reactivation along a fundamental proterozoic crustal fault the darling river lineament australia constraints from apatite fission track thermochronology abstract the darling river of eastern australia flows along a relatively straight course for much of a 2000 km linear feature extending from south australia to the offshore continental margin of queensland this crustal scale feature the darling river lineament drl is considered to have originally been a part of the tasman line a major late proterozoic fracture zone which was active during the breakup of the rodinia supercontinent even though extensive phanerozoic orogenic contraction has since affected the darling river region the present surface expression of the drl indicates relatively recent reactivation time temperature modelling of apatite fission track data from rocks straddling the southwestern portion of the drl suggests that phanerozoic cooling along this part of the lineament principally occurred during three major episodes each of these can be tentatively linked to tectonism at the time late silurian to middle devonian 420 380 ma deformation during either the lachlan orogeny or the alice springs orogeny late permian to early triassic 260 240 ma deformation during the hunter bowen orogeny and early to middle tertiary 60 40 ma deformation in response to pacific plate rearrangement and north australian collision in papua new guinea moreover the results reveal that the magnitude of cooling differed significantly across the lineament during each episode indicating that this major late proterozoic feature has been reactivated several times during the phanerozoic

precambrian superplumes and supercontinents a record in black shales carbon isotopes and paleoclimates prominent maxima in black shale abundance and in black shale total shale ratio occur at 2 0 1 7 ga with less prominent peaks in the late neoproterozoic 800 600 ma and in the late archean 2 7 2 5 ga peaks in chemical index of alteration cia of shales at the same times suggest corresponding warm paleoclimates the peaks in cia and black shale abundance are correlated in time at a 94 confidence level the black shale and cia peaks may reflect the combined effects of mantle superplume events and supercontinent formation at 2 7 and 1 9 ga mantle superplume events may have introduced large amounts of co2 into the atmosphere ocean system increasing depositional rates of carbon and increasing global warming increased black shale deposition may reflect some combination of 1 increased oceanic hydrothermal fluxes introducing nutrients 2 anoxia on continental shelves and 3 disrupted ocean currents the apparent absence of carbon isotope anomalies at these times reflects an increase in the deposition and burial rate of both reduced and oxidized carbon peaks in black shale abundance at 2 1 ga and 800 600 ma correlate with peaks in 13 c in marine carbonates increases in atmospheric oxygen and with high cia values in shales these are all consistent with higher rates of organic carbon burial in black shales at these times these peaks may record the breakup of supercontinents at 2 2 2 0 ga and again at 800 600 ma which resulted in increased numbers of partially closed marine basins disruption of ocean currents and increased hydrothermal vents at ocean ridges all of which led to widespread anoxia 2001 elsevier science b v all rights reserved

thermotectonic history of the bassian rise australia implications for the breakup of eastern gondwana along australia s southeastern margins abstract during the mesozoic breakup of eastern gondwana rifting occurred along both the southern ocean rift sor between australia and antarctica and the tasman sea rift tsr between australia and the lord howe rise as a consequence of breakup tasmania located at the southeastern tip of australia is now surrounded by rifted margins including a failed branch of the sor within bass strait to the north apatite fission track aft results from the furneaux islands located along the bassian rise at the eastern edge of bass strait record two major episodes of rapid cooling denudation since breakup began the first occurred during the middle cretaceous at 94 2 ma and documents the timing of formation of the bassian rise this was followed during the early tertiary by the second episode at some time between 65 45 ma possibly related to pacific plate rearrangement and north australian collision in new guinea importantly the aft results indicate that formation of the bassian rise occurred during the middle cretaceous at the same time as 1 rifting along the tsr began and 2 rifting along the sor within bass strait ceased this suggests a link between the timing of formation of the bassian rise and the initiation and cessation of major rifting events along the adjacent margins

growth and reworking of the early precambrian continental crust in the north china craton constraints from zircon hf isotopes abstract we synthesize more than 2600 hf isotope data on the archean paleoproterozoic zircons from the north china craton ncc recalculation of the data based on single stage and two stage hf model ages of the eastern block of the ncc shows peak ages of 3902 13 ma and 3978 18 ma respectively and also small peaks at 3 5 4 0 ga the majority of zircon ehf t values are positive suggesting the possibility of the crust and the mantle differentiation at ca 3 9 4 0 ga in the eastern block of the ncc most magmatic zircons from the whole of ncc have their hf model age range of 2 4 2 9 ga and the single stage model ages is cluster at 2698 4 ma whereas the two stage model ages concentrate at 2714 5 ma implying that the protoliths were juvenile crustal rocks the most prominent peak at 2 7 ga indicates that this period marks the most important stage of the crust mantle differentiation and crust formation of the ncc the widespread 2 5 ga rocks in the ncc and the absence of the 2 5 ga peaks in hf model ages are consistent with the partial melting and reworking of the juvenile rocks at 2 5 ga furthermore the 2 5 1 7 ga zircon hf isotope features are also related to the reworking of the crustal rocks our results from the integration of a large database suggest that the eastern block and the trans north china orogen have undergone similar crust mantle differentiation and magmatism leading to the conclusion that the essential cratonization of the north china took place at the end of neoarchean

high grade metamorphism during archean paleoproterozoic transition associated with microblock amalgamation in the north china craton mineral phase equilibria and zircon geochronology abstract metamorphic regimes in archean terranes provide important keys to the plate tectonic processes in early earth the north china craton ncc is one of the ancient continental nuclei in asia and recent models propose that the cratonic architecture was built through the assembly of several archean microcontinental blocks into larger crustal blocks here we investigate garnet and pyroxene bearing granulite facies rocks along the periphery of the jiaoliao microcontinental block in the ncc the garnet bearing granulites contain peak mineral assemblage of garnet clinopyroxene orthopyroxene magnetite plagioclase quartz biotite ilmenite mineral phase equilibria computations using pseudosection and geothermobarometry suggest peak p t condition of 800 830 c and 7 8 kbar for metamorphism isopleths using xmg of orthopyroxene and xca of garnet in another sample containing the peak mineral assemblage of garnet orthopyroxene quartz magnetite fluid yield peak p t conditions of 860 920 c and 11 14 kbar geochemical data show tonalitic to granodioritic composition and arc related tectonic setting for the magmatic protoliths of these rocks zircon la icp ms analyses yield well defined discordia with upper intercept ages of 2562 20 ma mswd 0 94 and 2539 21 ma mswd 0 59 which is correlated with the timing of emplacement of the magmatic protolith a younger group of zircons with upper intercept ages of 2449 41 ma mswd 0 83 n 6 as 2449 41 ma mswd 0 83 n 6 and 2480 44 ma mswd 1 2 n 9 constrains the timing of metamorphism zircon lu hf data show dominantly positive ehf t values up to 8 5 and yield crustal residence ages tdmc in the range of 2529 to 2884 ma suggesting magma sources from meso neoarchean juvenile components the high temperature and medium to high pressure metamorphism is considered to have resulted from the subduction collision tectonics associated with microblock amalgamation in the ncc at the end of archean together with the evidence for high pressure metamorphism from an adjacent locality our results correlate with models that predict paired metamorphism at the archean proterozoic transition with the onset of modern style plate tectonics

charnockite geobarometers based on coexisting garnet pyroxene plagioclase quartz numerous geothermometers based on compositions of coexisting minerals have been calibrated experimentally or theoretically during the past 10 yr in contrast few continuous reaction geobarometers exist which have had wide application one based on the reaction of cordierite to garnet sillimanite and quartz is limited by a relatively narrow pressure range of application and also by conflicting calibrations1 2 because the mixing relations of mg and fe and the role of h2o in cordierite3 are not understood another successful barometer uses the assemblage garnet plagioclase al2sio5 and quartz4 5 continuous geobarometers based on the reaction of orthofer rosilite to fayalite plus quartz6 and on the aluminium content of enstatite in equilibrium with garnet7 have been used to a lesser extent the former barometer is limited by rarity of the requisite assemblage and the latter by uncertain experimental calibration for crustal rocks we calibrate here two mineralogical geobarometers from measured thermodynamic data the participating minerals pyroxenes garnet plagioclase and quartz are frequently associated in high grade metamorphic rocks calculation of pressures for several terrains show that the geobarometers yield reasonable and consistent results for the entire range of crustal pressures

microblock amalgamation in the north china craton evidence from neoarchaean magmatic suite in the western margin of the jiaoliao block abstract the archaean earth is considered to have been characterized by microcontinents that formed dominantly through the accretion of oceanic arcs and plateaus the north china craton ncc provides a typical case where at least seven ancient microcontinental nuclei with distinct lithological features and independent tectonic histories were amalgamated into the cratonic framework at the end of the archaean here we investigate a suite of magmatic rocks developed at the periphery of one of these microblocks the jiaoliao block that forms part of the composite eastern block of the ncc we present petrological geochemical and zircon u pb geochronological data from the taipingzhai charnockite suite and associated amphibolites metagabbros and orthogneisses from the qianxi complex geochemically the rocks show a wide range of sio 2 charnockite suite 52 57 75 50 wt metagabbro 43 71 wt amphibolite 50 24 wt garnet bearing biotite 63 73 wt and mgo charnockite suite 0 89 5 01 wt metagabbro 3 99 wt amphibolite 6 23 wt garnet bearing biotite 2 08 wt the composition of the felsic units straddle from diorite through syeno diorite to granite with both alkalic and subalkalic affinity with dominantly magnesian composition and arc related features their immobile trace element relationships suggest calc alkaline affinity they show positive pb ba la nd and gd and negative nb ta sr th and ti anomalies with slightly negative anomalies of ce and y attesting to arc related features in tectonic classification diagrams the rocks plot in the vag syn colg field or the vag area suggesting subduction related origin the dominant population of zircons in all these rocks displays magmatic crystallization features including high th u values with core rims textures indicating subsequent thermal events the zircon u pb data yield upper intercept ages of 2587 10 ma to 2543 17 ma and 207 pb 206 pb mean ages of 2578 7 3 ma to 2536 8 ma for the charnockite suite marking the timing of emplacement of the arc magmas the overgrowth rims as well as discrete neoformed grains are interpreted as dating subsequent metamorphism and yield 207 pb 206 pb ages between 2533 ma to 2490 ma zircons in the metagabbro preserve upper intercept ages of 2556 20 ma representing the crystallization age of this rock the younger ages of 2449 58 ma upper intercept age and 1845 25 ma 207 pb 206 pb spot age are interpreted to represent subsequent multiple thermal events in this area zircons in the amphibolite preserve the 207 pb 206 pb mean age of 2539 9 ma representing the crystallization age of this rock the garnet bearing biotite gneiss shows an upper intercept age of 2562 10 ma mswd 0 66 n 36 and the 207 pb 206 pb mean age of 2561 9 ma mswd 0 63 n 33 which is taken to represent the crystallization age of this rock some inherited zircons are also identified with 207 pb 206 pb ages of 2664 26 ma and 2628 26 ma zircon lu hf data show dominantly positive ehf t values and combined with crustal residence ages the results suggest mesoarchean to neoarchean juvenile crust formation in the ncc we interpret the data presented here to represent a phase of major late neoarchaean arc magmatism along the western margin of the jiaoliao block related to the birth of microcontinental nuclei within the ncc our data suggest that the western and eastern blocks might not have existed as discrete crustal blocks and that the construction of the ncc is a result of the assembly of several microblocks or terranes at the end of archaean similar archean cratonic nuclei in other regions of the world might have formed part of a primitive supercontinent in the early earth

calculation and application of clinopyroxene garnet plagioclase quartz geobarometers recently published thermodynamic and experimental data in a variety of chemical systems have been evaluated to derive gibbs free energies for hedenbergite and pyrope these were used to calculate the geobarometric equilibria

contrasting styles of phanerozoic and precambrian continental collision abstract there are differences in the style of collisional orogens between the phanerozoic and the precambrian most notably the appearance of blueschists and ultrahigh pressure metamorphic uhpm rocks in the geological record since the late neoproterozoic whereas these rocks are absent from older orogens understanding collisional orogenesis in the context of present day values for ambient upper mantle temperature and radiogenic heat production provides a reference from which to extrapolate back to conditions in the precambrian to evaluate differences in the way phanerozoic and precambrian collisional orogens develop a series of experiments was run using a 2 d petrological thermomechanical numerical model in which the collision of spontaneously moving continental plates was simulated for values of ambient upper mantle temperature and radiogenic heat production increasing from those appropriate to the present day thus models of modern collisional orogens involving different modes of exhumation of uhpm rocks were extrapolated back to conditions appropriate for the precambrian based on these experiments an increase of the ambient upper mantle temperature to 80 100 k above the present day value leads to two distinct modes of collision that are different from the modern collision regime and for which the terms truncated hot collision regime strong mafic lower continental crust and two sided hot collision regime weak felsic lower continental crust are proposed some proterozoic orogens record post extension thickening to generate counter clockwise metamorphic p t paths followed by slow close to isobaric retrograde cooling such as occurred in the paleoproterozoic khondalite belt in the north china craton and the late mesoproterozoic early neoproterozoic eastern ghats province part of the eastern ghats belt of peninsular india these orogens have similarities with the truncated hot collision regime in the numerical models assuming subsequent shortening and thickening of the resulting hot lithosphere other proterozoic orogens are characterized by clockwise looping metamorphic p t paths and extensive granite magmatism derived from diverse crustal and subcontinental lithospheric mantle sources these orogens have similarities with the two sided hot collision regime in the numerical models both regimes are associated with shallow slab breakoff that precludes the formation of uhpm rocks the temperature of the ambient upper mantle where this transition in geodynamic regimes occurs corresponds broadly to the neoproterozoic era

the origins of the intraplate stress field in continental australia the ridge push force acting on the indo australian plate exerts a significant torque 8 5 1025n m about a pole at 30 3 n 34 5 e the angular difference between this torque pole and the observed pole of rotation for the plate 19 2 n 35 6 e is less than 12 and suggests that the ridge push force plays an important role in the dynamics of the indo australian plate we have used an elastic finite element analysis to study the predicted intraplate stress field in continental australia for four models which employ different boundary conditions to balance the ridge push torque acting on the plate the modeling indicates that a number of important features of the observed stress field within the australian continent can be explained in terms of balancing the ridge push torque with resistance imposed along the himalaya papua new guinea and new zealand collisional boundaries segments these features include n s to ne sw oriented compression in the northern australia and e w oriented compression in southern australia our analysis also shows that subduction processes along the northern and eastern boundaries provide only second order controls on the intraplate stress field in continental australia

fluid assisted granulite metamorphism a continental journey abstract lower crustal granulites which constitute the base of all continents belong to two series high pressure granulites generated by crustal thickening subduction and ultra high temperature granulites associated with crustal extension fluid inclusions and metasomatic features indicate that the latter were metamorphosed in the presence of low water activity fluids high density co2 and brines which have invaded the lower crust at peak metamorphic conditions fluid assisted granulite metamorphism high pressure and ultra high temperature granulites commonly occur along elongated paired belts they were formed from the early proterozoic onwards during a small number of active periods lasting a few hundreds of m y these periods were separated from each other by longer periods of stability each period ended with the formation of a supercontinent whose amalgamation coincided with low to medium pressure ultra high temperature granulite metamorphism immediately before continental break up it is proposed that large quantities of mantle derived co2 stored in the lower crust at the final stage of supercontinent amalgamation are released into the hydro and atmosphere during breakup of the supercontinent fluid assisted granulite metamorphism therefore appears to be an important mechanism for transferring deep mantle fluids towards the earth s surface possible consequences were for example the sudden end of proterozoic glaciations as well as the post cambrian explosion of life

magnetic polarity stratigraphy a series of previous review articles on paleomagnetic polarity stratigraphy has documented the growth of this field and its passage through various stages of development in which work was concentrated on young lavas young deep sea sediments from piston cores and red beds cox 1969 larson and helsley 1975 opdyke 1972 harrison 1974 irving and pulliah 1976 butler and opdyke 1979 the closely related development of the geomagnetic reversal time scale has been reviewed by ness et al 1980 at the time of the last quadrennial u s national report butler and opdyke 1979 noted that the recent development of digital spinner and cryogenic magnetometers was making it possible to measure large numbers of very weakly magnetized samples in the last four years cryogenic magnetometers have been acquired by many laboratories and studies of magnetic stratigraphy involving stepwise demagnetization of hundreds of samples have become fairly common in view of the time and effort that go into a careful study of paleomagnetic stratigraphy it seems unlikely that this technique will ever become as routine a correlation tool as biostratigraphy

convergence and intraplate deformation in the lachlan fold belt of southeastern australia abstract the lachlan fold belt of southeastern australia is dominated by a widespread oceanic association including cambrian submarine mafic volcanics and an overlying widespread ordovician quartz rich turbidite and black shale succession with scattered mafic to andesitic volcanic centres these rocks are overlain by or in fault contact with deep marine to continental silurian to early carboniferous successions with mafic to silicic volcanics and abundant plutonic rocks major deformation achieved by thrusting and folding affected the fold belt during the silurian to middle devonian and the early carboniferous shortening estimates vary throughout the belt but the widespread ordovician quartz turbidite succession usually has at least 60 shortening which implies an original width of the fold belt of about 1700 km strike slip faulting was important on a local to regional scale but large scale strike slip displacements appear unlikely the large values of shortening are consistent with development of the belt in a convergent margin setting at least for the silurian to carboniferous the nature of the lower crust of the lachlan fold belt has always presented a problem of interpretation but we favour a late proterozoic quasi continental lower crustal layer that has deformed independently of the upper crust by either homogeneous flattening or crustal duplexing during silurian carboniferous convergence and provided a source for silurian devonian granites

a users guide to fission track dating calibration abstract misconceptions regarding the fission track dating method prompt the description of five handling procedures and the derivation of commonly used system calibrations problems of registration geometry inhomogeneous uranium distribution accumulated radiation damage and anisotropic etching largely invalidate all but the population and external detector methods and impose strict limitations upon even their routine application complexities of unique absolute thermal neutron dosimetry are illustrated by the calibration and use of the nbs pre irradiated glass srm 962 the 20 disparity in measured f decay constant values cannot be isolated from neutron fluence measurement and calibration frequently a calibration ratio f is evaluated against an age standard and then split into its component parts to illustrate this interdependence of f and zircon fission track ages in agreement with independent k ar ages are obtained by calculating the same track count data with each of the preferred values of f f 7 03 10 17 yr 1 and8 46 10 17 yr 1 together with appropriate selected neutron dosimetry schemes an alternative approach is presented formally relating unknown ages of samples to known ages of standards either by direct comparison of standard and sample track densities or by the repeated calibration of a glass against age standards practical recommendations are given for the reporting and critical assessment of fission track data

charnockite magmatism during a transitional phase implications for late paleoproterozoic ridge subduction in the north china craton abstract the trans north china orogen tnco is a major paleoproterozoic collisional belt along which the western and eastern blocks of the north china craton were amalgamated the luliang complex occurs along the western margin of this belt and incorporates several arc magmatic suites supracrustal sequences granitoids and charnockites here we investigate the luyashan charnockite suite as well as the surrounding granitoids from this complex we present results from petrology fluid inclusions mineral chemistry whole rock geochemistry and zircon u pb and lu hf isotopic studies the charnockites are dominantly anhydrous with the typical presence of orthopyroxene microthermometric and laser raman spectroscopic data on fluid inclusions in the charnockite characterize the trapped fluids as dominantly co2 with traces of n2 and ch4 geochemically the charnockite suite shows compositional variation from monzogabbro through gabbroic diorite and diorite to monzodiorite they are dominantly calc alkaline and ferroan type and display distinct enrichment in large ion lithophile elements with depletion in hree they also show prominent negative anomalies of nb ta sr and ti and positive anomalies of rb pb zr and hf indicating crustal components in the magma source the majority of the granitoid suite shows 207pb 206pb mean ages in the range of 2172 2032 ma correlating with similar ages reported from various groups in the luliang complex the formation of which is correlated with subduction along an active convergent margin however the charnockites which intrude the granitoids and their dioritic enclaves are clearly younger and show tightly constrained emplacement ages in the range of 1853 1872 ma the recrystallized zircons in the older granitoids also show similar ages suggesting a prominent thermal event at this time these ages are close to the timing of final collision between the western and eastern blocks of the north china craton and associated high grade metamorphism at ca 1 85 ga the zircon lu hf isotope data reveal dominantly negative hf t values 8 7 to 0 1 and their tdmc 2477 3042 ma suggest that the magma sources mainly involved meso to neoarchean reworked crustal components with limited input of paleoproterozoic juvenile components our study reveals that charnockite magmatism and regional metamorphism in the western margin of the tnco broadly overlap in space and time the geochemical features mineral assemblages and fluid inclusion characteristics of the charnockites suggest that the rocks were derived from high temperature 900 1000 c co2 enriched and anhydrous less than 3 h2o magmas the magma evolved into more hydrous conditions during cooling marked by the crystallization of amphiboles at 800 850 c the geochemical features of the charnockite suite also suggest that these rocks were formed during a transitional phase from syn subduction to collision such a scenario is compatible with a tectonic setting involving the subduction of an oceanic ridge when flushing of heat and co2 rich fluids from asthenosphere into the overriding plate might have occurred through the slab window generating hot and anhydrous magmas through melting of the basement rocks the slab window mechanism would also account for broadly coeval high temperature and high pressure granulites reported from the tnco we therefore speculate that the formation of the luyashan charnockite suite might have been associated with ridge subduction at the terminal stage of the convergence