formation and evolution of the tancheng lujiang wrench fault system a major shear system to the northwest of the pacific ocean abstract in the northwestern peri pacific tectonic belt on the continental margin of east asia there is a series of major faults trending ne nne i e the tancheng lujiang wrench fault system this fault system extends more than 5000 km with a width of 800 1000 km in the continent and a general strike of n30 e on the basis of the results of special research on the major faults and a lot of new information about the fault system the authors discuss early mesozoic triassic jurassic period sinistral transcurrent faulting in the different regions it is suggested that in the eastern continent of east asia the e w trending tectonic pattern of the paleozoic and beginning of the mesozoic appear in the nuclei of the sino korean continent massif and the yangtze continent massif these ancient continent massifs accreted both southwards and northwards which finally led to their collision and suturing to the angura continent massif later this tectonic framework was changed greatly by the tancheng lujiang wrench fault system along which left lateral horizontal displacement of 1000 1500 km took place since the mesozoic the dislocation and deformation of horizontal displacement of the tancheng lujiang fault system produced a series of structures trending ne nne in the continental margin which have greatly transformed the ancient structures of the preceding mesozoic this is an important diktyogenese in the continent of east asia during the horizontal displacement motion of these faults the folds nappes and decollement structures pull apart basins and subductions within the continent occurred in the continent at the same time some large sinistral ductile shear zones of horizontal displacement appeared the depth of the deformed horizon of ductile shear which belongs to simple shear was controlled by the tectonic element and distance away from the continental margin whilst the ductile shear deformation occurred the fault dynamo thermal metamorphism and the fault remelting took place and magma activity appeared within the continent in the mesozoic the fault zones used to be high energy zones and recently became the seismic belts within the continent formation of the tan lu fault system was due to the collision between the southern and northern blocks its large sinistral horizontal displacement in the mesozoic was closely related to the kula plate moving northwards therefore the continent disintegrated by shearing which produced some new terranes the shear edge belt was formed between the continent and ocean the change of relative motion between the continent and pacific ocean plate transformed the tan lu fault system into first extension and then compression clarifying the characteristics of the tan lu fault system is an important key in understanding the evolution of the east asia continent since the mesozoic

provenance of glacial marine conglomerates in the permian lagar formation of southern tibet evidence for affinity of the lhasa terrane with australia abstract paleogeographic reconstructions of the lhasa terrane of the southern tibetan plateau are uncertain in paleozoic and investigating the paleogeographic location of the terrane via the current geographical configuration is difficult however gravels contained within glacial sediments offer an opportunity to solve this problem the lagar formation in the central lhasa sub terrane contains glacial marine deposits that incorporate gravel clasts composed of granitoids sandstone quartzite limestone and rhyolite these diamictites were derived from continental glaciation and were deposited mainly near a grounding line or ice front in littoral neritic to neritic environments we present new zircon u pb isotopic and whole rock geochemical data for the tonalitic conglomerates of the lagar formation the results indicate that the tonalites crystallized during the paleoproterozoic 1 76 ga and show a geochemical affinity with i type granite the tonalites typically have negative eu anomalies eu 0 31 0 37 high concentrations of rare earth elements depletion in high field strength elements enrichment in th marked positive zircon ehf t values 6 46 14 68 end t values of 1 24 to 1 76 and tdm2 ages of 2 37 2 13 ga all of these data indicate that the tonalites were produced by the partial melting of modified mafic crust or underplate in a subduction related environment on the basis of a comprehensive analysis of all 1 8 1 7 ga magmatism in east gondwana we propose that the most likely provenance of the tonalitic conglomerates is southern arunta and mt isa in northern australia therefore the central lhasa sub terrane may have been located near australia during the paleozoic

lithium isotope fractionation by diffusion in minerals part 2 olivine abstract recent experiments have shown that lithium isotopes can be significantly fractionated by diffusion in silicate liquids and in augite here we report new laboratory experiments that document similarly large lithium isotopic fractionation by diffusion in olivine two types of experiments were used a powder source method where lithium from finely ground spodumene lialsi2o6 diffused into oriented san carlos olivine and piston cylinder annealing experiments where kunlun clinopyroxene 30 ppm lithium and oriented san carlos olivine 2 ppm lithium were juxtaposed the lithium concentration along traverses across the run products was measured using both laser ablation as a source for a varian 820 ms quadrupole mass spectrometer and a cameca 1270 secondary ion mass spectrometer the cameca 1270 was also used to measure the lithium isotopic fractionation across olivine grains recovered from the experiments the lithium isotopes were found to be fractionationed by many tens of permil in the diffusion boundary layer at the grain edges as a result of 6li diffusing significantly faster than 7li the lithium concentration and isotopic fractionation data across the olivine recovered from the different experiments were modeled using calculations in which lithium was assumed to be of two distinct types one being fast diffusing interstitial lithium the other much less mobile lithium on a metal site the two site diffusion model involves a large number of independent parameters and we found that different choices of the parameters can produce very comparable fits to the lithium concentration profiles and associated isotopic fractionation because of this nonuniqueness we are able to determine only a range for the relative diffusivity of 6li compared to 7li when the mass dependence of lithium diffusion is parameterized as d 6 li d 7 li 7 6 the isotope fractionation for diffusion along the a and c crystallographic direction of olivine can be fit by 0 4 0 1 while the fractionation in the b direction appears to be somewhat lower model calculations were also used to fit the lithium concentration and isotopic fractionation across a natural olivine grain from a peridotite xenolith from the eastern north china craton the isotopic data were fit using values 0 3 0 36 similar to that of the laboratory experiments this along with the fact that the isotopic fractionation is restricted to that part of the mineral with a gradient in lithium concentration is strong evidence that the lithium zoning of this mineral grain is the result of lithium loss by diffusion and thus that it can be used as illustrated to constrain the cooling history

compression of oceanic lithosphere an analysis of intraplate deformation in the central indian basin intraplate compressional deformation in the central indian basin is expressed as linear e w trending topographic undulations and geoid anomlies with characteristic spacings of approximately 200 km to elucidate the nature of this deformation we develop models of an oceanic lithosphere in a state of horizontal compression the lithosphere is treated as a viscous or plastic layer of uniform strength which overlies a viscous half space in which strength decreases exponentially with depth we compare models in which deformation with a dominant wavelength develops in response to flexural buckling and the hydrodynamic growth of instabilities for the viscous flexural buckling model deformation is constrained to occur in the antisymmetric mode by flexural folding which is characterized by uniform bending of the layer in the hydrodynamic flow model a range of deformational styles are possible in a strong viscous lithosphere deformation of the layer occurs by flexural folding at a wavelength which agrees with that predicted from flexural buckling theory in a lithosphere of intermediate strength the layer deforms by folding characterized by thickening which localizes beneath topographic highs in a relatively weak lithosphere the layer incurs an even greater amount of localized thickening and deforms in the symmetric or pinch and swell mode by inverse boudinage if the layer has a plastic rheology deformation occurs by inverse boudinage independent of the yield stress results from models in which the layer folds either flexurally or with periodic thickening are consistent with the observed depth distribution of seismicity in the central indian basin and with experimental rock rheological data because of the trade off between buoyancy forces and layer strength in determining the dominant wavelength a lithosphere which was overlain by weak sediments when intraplate deformation initiated can be characterized by a compressive layer strength almost a factor of 3 less than a lithosphere which was initially sediment free for a range of plausible models which consider both a sediment covered and sediment free lithosphere the observed wavelength requires deviatoric stresses in the indian plate of several hundreds of mpa

the scourie dyke suite petrogenesis and geochemical nature of the proterozoic sub continental mantle the petrogenesis of bronzite picrite olivine gabbro norite and quartz tholeiite dykes which make up the 2 39 b y scourie dyke swarm cutting the archaean lewisian gneisses of n w scotland is interpreted on the basis of their major and trace element geochemistry most of the dykes bear primary amphibole and or phlogopite and with one exception are all hypersthene or quartz normative apart from one tholeiite dyke which shows relative light rare earth element depletion all the dykes show enrichment in light rare earths and large ion lithophile elements they do not however show an equivalent enrichment in other incompatible high field strength ions such as nb and ta and in this respect resemble island arc and calc alkaline basalts the different dyke types have distinctive rare earth patterns and other trace element ratios which are maintained over a range of major element compositions

shrimp u pb zircon ages of granitoid rocks in the luliang complex implications for the accretion and evolution of the trans north china orogen abstract the luliang complex is situated in the central segment of the trans north china orogen tnco a continent continent collisional belt along which the discrete eastern and western blocks amalgamated to form the basement of the north china craton the complex consists of supracrustal and granitoid rocks of which the latter can be subdivided into pre tectonic ttg gneisses including the yunzhongshan guojiazhuang and chijianling guandishan gneisses syn tectonic gneissic granites represented by the huijiazhuang gneissic granite and post tectonic granites including the luyashan charnockite luchaogou porphyritic granite and tangershang guandishan massive granite the pre tectonic ttg gneisses are mostly calc alkaline and considered to have formed in a magmatic arc environment shrimp u pb analyses reveal that the yunzhongshan gneisses were emplaced at 2499 9 ma representing the earliest arc related magmatic event in the luliang complex this was followed by the intrusion of the guojiazhuang gneisses at 2375 10 ma the most widespread arc related magmatic event in the region was the emplacement of the chijianling guandishan granitoid gneisses of which the tonalitic granodioritic and monzogranitic gneisses were emplaced at 2199 11 ma 2180 7 ma and 2173 7 ma respectively metamorphic zircon overgrowth rims from a tonalitic gneiss yielded a weighted mean 207 pb 206 pb age of 1872 7 ma consistent with the metamorphic age range of 1880 1820 ma defined by metamorphic zircons from various high grade rocks in the tnco thus the evolution of the luliang complex involved emplacement of the yunzhongshan granitoids at 2499 ma the guajiazhuang granitoids at 2375 ma and the chijianling guandishan granitoids at 2199 2173 ma with the final collision between the eastern and western blocks in this area occurring at 1872 7 ma the whole series of magmatic events lasting nearly 650 ma this suggests that the trans north china orogen represents a long lived magmatic arc the new shrimp zircon data combined with structural and petrological studies also reveal the existence of both syn and post collisional granites in the luliang complex the former is represented by the huijiazhuang gneissic granite that was emplaced at 1832 11 ma whereas the latter include the 1815 5 ma luyashan charnockite 1807 10 ma luchaogou porphyritic granite 1798 11 ma guandishan massive granite and 1790 14 ma tangershan massive granite

tectonic history of the tharsis region mars the faults of the tharsis region of mars have been mapped and delineated on the basis of age the orientation of each group of faults was analyzed to determine if they were radial to a point the results indicate that there are at least four discrete centers of faulting within the tharsis area which are of significantly different age the four centers proposed are from oldest to youngest 1 in the thaumasia highlands 2 in northern syria planum and finally 3 and 4 near pavonis mons while episodes 3 and 4 occupy the same geographic position they are separated in time by the development of the tharsis lava plains and shields each episode of tectonism was separated from the next by a period of major basaltic volcanism the tectonic features of the tharsis area are thus viewed as being the cumulative result of the superposition of several discrete episodes of faulting rather than the result of one episode about a single point

rates of hydrothermal cooling of new oceanic upper crust derived from lithium geospeedometry episodic emplacement and cooling of lavas and dikes at mid ocean ridges leads to large fluctuations in hydrothermal fluxes and biological activity however the processes operating beneath the seafloor during these transient events such as permeability creation and dike cooling are poorly understood we have developed a new approach to determine the cooling rate of the sheeted dike complex based on the extent of diffusion of lithium from plagioclase into clinopyroxene during cooling we have calibrated this li geospeedometer using new high temperature experiments to determine both the temperature dependence of the partitioning of li between plagioclase and clinopyroxene and the diffusion coefficient for li in clinopyroxene application of this method to lavas and dikes from odp hole 504b shows that cooling rates vary dramatically with depth in the upper oceanic crust extremely rapid cooling rates 450 c hr 1 in the upper part of the sheeted dike complex are sufficient to power hydrothermal megaplume formation within the overlying water column

strike slip faulting of ridged plains near valles marineris mars the surface of mars shows abundant evidence of extensional and contractional deformation in the form of normal faults grabens and wrinkle ridges 1 3 in contrast strike slip faults have been considered to be extremely rare or absent on mars4 5 this view is based on the lack of significant lateral offset of pre existing markers such as craters across suspected faults thus mars is believed to have evolved without plate tectonics or other large scale lateral motions of the lithosphere6 however careful study of viking orbiter images is revealing that strike slip faults may be present on mars7 here i identify and document several well preserved examples of martian strike slip faults and examine their relationships to wrinkle ridges the strike slip faulting predates or overlaps periods of wrinkle ridge growth south east of valles marineris and some wrinkle ridges may have nucleated and grown as a result of strike slip displacements along the echelon fault arrays lateral displacements of several kilometres inferred along these arrays may be related to tectonism in tharsis

the basil cu co deposit eastern arunta region northern territory australia a metamorphosed volcanic hosted massive sulphide deposit abstract the basil cu co deposit harts range central australia is hosted by the riddock amphibolite a sequence that has been metamorphosed at upper amphibolite to granulite facies conditions at 480 460 ma larapinta event and subsequently reworked at amphibolite facies conditions 450 300 ma as a result many of the primary mineralization textures and other features that could characterise ore genesis have been obliterated however preserved textures and mineral relationships in the mineralized zone allow some constraints to be placed on the genetic history of the deposit using mineralogical petrographic and geochemical studies of host rocks and sulphides results of this study permit at least two genetic models to be ruled out firstly whole rock geochemistry and garnet compositions suggest that the deposit is not a skarn system secondly the lack of any significant ni signature and the presence of abundant zircons in the host amphibolite indicating that not all host rocks are mafic in composition and or magmatic in character make an orthomagmatic ni cu pge system unlikely alternatively basil is assigned to a volcanic hosted massive sulphide vhms style of mineralization formed on the seafloor within basaltic and sedimentary host rocks typical of deposits occurring in such settings the lack of a recognisable hydrothermal alteration zone is consistent with either destruction of the alteration zone during metamorphism or detachment of the ore from alteration during later deformation the occurrence of sulphide inclusions within garnet and amphibole indicates that the sulphides must be syn metamorphic or earlier partitioning of trace elements between pyrite and co existing pyrrhotite suggests that re crystallization occurred under equilibrium conditions the composition of sphalerite coexisting with pyrite and pyrrhotite indicates crystallization at pressures of at least 10 kbar consistent with peak metamorphism during the early ordovician larapinta event zr in titanite geothermometry indicates peak temperatures of 730 745 c

banded amphibolites of the harts range meta igneous complex central australia an early proterozoic basalt tonalite suite abstract a multistage petrogenesis is proposed for banded metabasaltic amphibolite and tonalite tectonically interlayered on a fine centimetre scale within upper and lower amphibolite sequences of the harts range meta igneous complex eastern arunta block central australia the basaltic amphibolites exhibit only slight rare earth element ree fractionation ce n yb n 2 5 and can be related by plagioclase clinopyroxene dominated low pressure fractional crystallisation in the tholeiitic trend early olivine removal from a moderately aluminous trace element impoverished tholeiitic picrite parent magma caused significant ni depletion in the evolved residual liquids that gave rise to the basaltic amphibolites the primary picritic melt was generated by high degrees 30 of hydrous partial melting of a depleted mantle source selectively re enriched in large ion lithophile lil elements chondrite normalised ree patterns for low al 2 o 3 tonalitic layers within the basaltic amphibolite units are moderately fractionated ce n yb n 3 3 7 3 and show significant negative eu anomalies ree modelling indicates that the precursors to the tonalitic rocks could have been generated by small degrees 10 of partial melting of a light rare earth element lree enriched basaltic parent with plagioclase clinopyroxene and minor hornblende comprising coexisting residual phases melting at relatively low p h 2 o the amphibolite sequence is interpreted as a complexly folded and metamorphosed bimodal basalt tonalite suite formed at an early proterozoic rifted continental margin or intra cratonic rift

lithium isotopes in global mid ocean ridge basalts abstract the lithium isotope compositions of 30 well characterized samples of glassy lavas from the three major mid ocean ridge segments of the world spanning a wide range in radiogenic isotope and elemental content and sea floor physical parameters have been measured the overall data set shows a significant range in 7li 1 6 to 5 6 with no global correlation between li isotopes and other geochemical or tectonic parameters the samples with the greatest lithophile element depletion n morb k2o tio2

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

behavior of li and its isotopes during serpentinization of oceanic peridotites analyses of li and li isotopes in serpentinized peridotites have been performed using thermo ionisation mass spectrometry tims and secondary ion mass spectrometry sims techniques on samples collected from the southwest indian ridge swir in the bulk samples li concentrations range from 0 6 to 8 2 ppm while whole rock 6li values range from 2 9 to 14 in situ analyses display a greater range in both li concentration 0 1 19 5 ppm and li isotopic composition 27 to 19 with the serpentinized portions having higher li concentrations than the associated relict phases these variations may reflect changes in li partitioning and isotopic fractionation between serpentine and fluid with temperature and water rock ratio they may also be explained by changes in the composition of the serpentinizing fluid over the course of serpentinization as the serpentine forms by interaction with a circulating fluid it preferentially removes 6li causing the li in the fluid to become isotopically heavier the isotopic composition of the initial hydrothermal fluid is dominated by basalt derived li which easily overwhelms the very low li content originally present in seawater as this fluid circulates through ultramafic rocks it induces the formation of serpentine that incorporates this mantle derived li hence li in serpentine is mainly derived from oceanic crust rather than from seawater and serpentinization involves li recycling within this crust consequently li isotopes are good tracers of the hydrothermal contribution in serpentinizing fluid these results imply that serpentinized peridotites are probably only a minor sink of oceanic li

arrested kinetic li isotope fractionation at the margin of the ilimaussaq complex south greenland evidence for open system processes during final cooling of peralkaline igneous rocks li contents li and isotopic composition 7 li of mafic minerals mainly amphibole and clinopyroxene from the alkaline to peralkaline ilimaussaq plutonic complex south greenland track the behavior of li and its isotopes during magmatic differentiation and final cooling of an alkaline igneous system li in amphibole increase from b10 ppm in caamphiboles of the least differentiated unit to n3000 ppm in na amphiboles of the highly evolved units in contrast li in clinopyroxene are comparatively low b85 ppm and do not vary systematically with differentiation the distribution of li between amphibole and pyroxene is controlled by the major element composition of the minerals ca rich and na rich respectively and changes in oxygen fugacity due to li incorporation via coupled substitution with ferric iron during magmatic differentiation 7 li values of all minerals span a wide range from 17 to 8 with the different intrusive units of the complex having distinct li isotopic systematics amphiboles which dominate the li budget of whole rocks from the inner part of the complex have constant 7 li of 1 8 2 2 2 n 15 this value reflects a homogeneous melt reservoir and is consistent with their mantle derivation in agreement with published o and nd isotopic data clinopyroxenes of these samples are consistently lighter with 7 liamph cpx as large as 8 and are thus not in li isotope equilibrium these low values probably reflect late stage diffusion of li into clinopyroxene during final cooling of the rocks thus enriching the clinopyroxene in 6 li at the margin of the complex 7 li in the syenites increases systematically from 2 to high values of 14 this coupled with the observed li isotope systematics of the granitic country rocks reflects post magmatic open system processes occurring during final cooling of the intrusion although the shape and magnitude of the li isotope and elemental profiles through syenite and country rock are suggestive of diffusion driven isotope fractionation they cannot be modeled by one dimensional diffusive transport and point to circulation of a fluid having a high 7 li value possibly seawater along the chilled contact in all this study

geochemistry and petrogenesis of the fiskenaesset anorthosite complex southern west greenland nature of the parent magma abstract the petrogenesis of the fiskenaesset anorthosite body has been investigated using major and trace element data for a large range of rock types from each zone of the complex the chemistry of these ultramafic to anorthositic cumulates is interpreted in terms of crystal fractionation of a parental trace element impoverished tholeiitic magma involving crystallisation of the cumulus phases olivine orthopyroxene clinopyroxene and dominant plagioclase feldspar amphibole appears not to have been a significant cumulus phase at any stage of crystallisation of the body the abundant amphibole found in the rocks of the complex being produced by primary intercumulus crystallisation supplemented by secondary metamorphic recrystallisation similarly magnetite is unlikely to have been a significant early cumulus phase although together with chromite it crystallised as a cumulus phase at high stratigraphic levels in the complex the metamorphism appears to be largely isochemical although sub solidus metamorphic re equilibration of the ree can be demonstrated on a grain size scale the spatial and temporal association between the anorthosite complex and the bordering metavolcanic amphibolites is matched by a strong similarity between the observed trace element chemistry of the amphibolites and the trace element chemistry of calculated successive liquids for the complex this is taken to suggest a genetic relationship between the evolution of the anorthosite complex and enclosing amphibolites the presence of trace element impoverished amphibolites which are not cumulates with trace element abundances comparable to those of the suggested parental liquid to the anorthosite complex is used to derive a major element composition for the primary fiskenasset magma this composition approximates a moderately aluminous tholeiitic basalt which may have been generated by hydrous fusion of previously depleted mantle this primary magma underwent crystal fractionation under low pressure conditions allowing the development of extensive plagioclase cumulates the fiskenaesset anorthosite and similar bodies cannot represent a cumulate residue complementary to the enclosing voluminous tonalitic gneisses which have a calc alkaline chemistry controlled by high pressure crystal liquid fractionation rather the association between the cumulate layered complex and bordering supracrustal sequence may imply an ancient ocean crust analogue for the development of this component of archaean high grade terrains it is suggested that slices of such archaean ocean floor may be emplaced laterally into the base of the continental crust during subduction of oceanic lithosphere at cordilleran type continental margins

metamorphic events in the eastern arunta inlier part 2 nd_sr_ar isotopic constraints in the harts ranges the central province of the eastern arunta inlier is characterised by upper amphibolite facies ortho and paragneisses previously published u pb dating of zircons as well as new sm nd and rb sr data indicate that the intrusive ages of both mafic and felsic orthogneisses are in the range 1780 1740 ma strangways orogeny these rocks had initial nd values of 0 to 3 and depleted mantle model ages of 2 5 2 1 ga and they intruded supracrustal rocks which are now preserved as paragneisses with initial nd of about 15 sm nd and rb sr whole rock isotope data indicate that the hornblende rich amphibolitic gneisses and mylonites in the harts range detachment zone ding and james 1989 were equilibrated at 1473 ma 133 when the rocks experienced hydration isotopic homogenisation and thrust faulting this is the anmatjira event described elsewhere in the arunta inlier and is a predominantly structural influence with thrust faults and hydrous fluid flux accordingly the record of this event by radiogenic geochronometers has been diffuse a problem also exacerbated by the probable influence on these rocks of least one more subsequent lower temperature amphibolite facies metamorphic event in spite of the indications of 1470 ma isotopic homogenisation of amphibolites in the harts ranges rb sr isotopic analyses of all minerals and 40ar 39ar from hornblendes yield ages in the range 450 300 ma using the sm nd system a few garnet hornblende pairs give 1400 1250 ma ages but several of these pairs also yield early palaeozoic ages these data suggest that the central province of the eastern arunta inlier also experienced an early palaeozoic thermal event having cooled from temperatures 550 c during and immediately before the carboniferous alice springs orogeny in the southern province south of the illogwa shear zone micas record mesoproterozoic ages 1500 1250 ma for the rb sr k ar and ar ar systems equivalent to the whole rock ages obtained from the central province to the north of the illogwa shear zone clearly the exposed southern basement cooled to 350 c presumably as a result of uplift and exhumation during the anmatjira event when basement now exposed in the central province remained at depth undergoing one or more additional amphibolite facies events the spectacular contrast in cooling histories across the illogwa shear zone emphasises that this is an important crustal scale tectonic structure

origin of periodically spaced wrinkle ridges on the tharsis plateau of mars ridged plains are a major geologic unit on mars and are a predominant unit on the tharsis plateau a region that has undergone uplift extension and extensive shield and flood volcanism these units probably flood volcanic in origin are characterized by landforms classed as wrinkle ridges wrinkle ridges are interpreted to be folds resulting from buckling followed by reverse to thrust faulting flexure fracture a prominent characteristic of many of the wrinkle ridges on mars particularly those in the tharsis ridge system is the periodic nature of their spacing the periodic spacing has been evaluated in six major provinces on the tharsis plateau with regions further divided into domains based largely on variation in ridge orientation the average spacing of the tharsis ridges based on 2934 measurements is 30 km in an effort to account for the periodic nature of the wrinkle ridges the ridged plains material is modeled as both a single member and a multilayer with frictionless contacts that has buckled at a critical wavelength of folding free slip between layers is assumed based on the possible existence of mechanically weak interbeds in the ridged plains sequence separating groups of flows the near surface mechanical structure in the area of ridged plains is approximated by a strong layer or layers overlying a weak megaregolith of finite thickness overlying a strong basement buckling of the ridged plains is assumed to be decoupled from the basement by the weak megaregolith decoupling is enhanced if the megaregolith were water or ice rich at the time of deformation the rheologic behavior of the ridged plains and megaregolith is approximated by a linear elastic and linear viscous material the models are examined for a range in 1 the strength contrast between the ridged plains material and the underlying megaregolith 2 thickness of the ridged plains material 3 thickness of the megaregolith and 4 number of layers the elastic model can explain the observed ridge spacing only if the ridged plains are relatively thick 2400 m with multiple layers n 1 and relatively high contrasts in young s modulus e e0 100 the high e e0 required is possible only if the megaregolith was water rich at the time of deformation over the same range in values of the parameters viscous buckling is much less restricted the viscous model can explain the observed ridge spacing over a wide range in ridged plains thickness 250 m to several kilometers megaregolith thickness number of layers n 1 to 8 and contrasts in viscosity 0 10 in addition viscous buckling is viable if the megaregolith were dry water rich or ice rich at the time of deformation the absence of folds with a cross sectional geometry in the shape of a sinusoid anticline syncline pairs may be the result of initial deformation of the ridged plains material into low amplitude folds in infinitesimal strain followed by plastic yielding in the cores of the anticlines in finite strain initial elastic or viscous buckling coupled with plastic yielding confined to the hinge area followed by the development of reverse to thrust faulting could account for the asymmetric fold geometry of many of the wrinkle ridges

developments in the understanding and application of lithium isotopes in the earth and planetary sciences the significant relative mass difference c 16 between the two stable isotopes of li approximately 6li 7 5 7li 92 5 coupled with broad elemental dispersion in earth and planetary materials makes this a system of considerable interest in fingerprinting geochemical processes determining mass balances and in thermometry natural mass fractionation in this system is responsible for c 6 variation among materials examined to date fig 1 although the modern era of li isotope quantification has begun there are still many questions about the li isotopic compositions of fundamental materials and the nature of fractionation by important mechanisms that are unanswered e g hoefs 1997 figure 1 summary of lithium isotopic compositions of earth and planetary materials filled bars are solid samples open bars are liquids see text for references and details the purpose of this chapter is to summarize the current understanding of li isotopes in geo and cosmochemical systems and to indicate 1 where li isotopes have a high probability of adding new understanding of these systems 2 where some of the more significant deficits in knowledge exist the small but burgeoning li isotope community has not yet compiled the volume of peer reviewed literature needed to adequately assess even that which has been studied to date as a result significant portions of this chapter are based on data reported in abstracts and as such are more than normally subject to revisions over time this chapter is anticipated to serve as a starting point for those interested in research incorporating li isotope geochemistry or in understanding the state of extant research experiments in li isotope fractionation knowledge of significant li isotopic fractionation during basic chemical processes is long standing the early experiments by taylor and urey 1938 in which li isotopes were fractionated by incomplete extraction of an aqueous solution from a zeolite exchange

geochemistry of metatholeiites from the harts range central australia implications for mantle source heterogeneity in a proterozoic mobile belt metatholeiite series from early middle proterozoic basement and cover sequences in the harts range central australia display marked differences in trace element abundances and ratios that reflect the chemical heterogeneity of their mantle sources the trace element data reveal a multistage geochemical evolution for the attendant proterozoic sub continental mantle early formed supracrustal tholeiites from the oonagalabi and entia basement gneiss complexes display slight to moderate light rare earth element lree enrichment ce yb n 1 8 3 9 unfractionated heavy ree distributions and chondritic zr nb and zr y ratios they were generated by variable degrees 15 30 of partial melting of a relatively undepleted mantle source in an extensional ensialic rift environment later formed metabasites comprise numerous sills that intrude the supracrustal sequence within the entia gneiss complex primary basaltic liquids for these rocks possessed more strongly fractionated ree levels ce yb n 3 0 12 3 large ion lithophile lil element enrichment and pronounced depletions in high field strength elements their high la nb ba zr and cr y ratios resemble those of convergent plate margin basaltic magmas derived from mantle sources that have been metasomatised as a result of subduction related processes metatholeiites from the harts range meta igneous complex in the cover sequence display high zr nb ratios reflecting a mantle source that was strongly depleted in incompatible elements during a previous melting event tholeiites from the lower part of the cover sequence show distinct lil element and lree depletion ce yb n 0 8 1 0 features resembling depleted n type mid ocean ridge basalts and some continental tholeiites tholeiites from the upper part of the cover sequence show enrichment of highly incompatible lil elements especially ba and k relative to ree and pronounced decoupling between high field strength elements and ree resulting in high la nb ratios they represent melts derived from a mantle source which has been depleted in a basaltic component and subsequently re enriched in incompatible elements e g via introduction of a subduction related component the inferred mantle evolution is compatible with a tectonic model that involves successive extensional and compressional episodes during repeated ensialic orogenesis as proposed for the harts range basement and cover sequences

seawater intrusion through the oceanic crust and carbonate sediment in the equatorial pacific lithium abundance and isotopic evidence 1 large scale lateral advection of seawater within the oceanic crust is thought to be the cause of low heat flow in the equatorial pacific until now supportive evidence is limited to reversals of b and 11b ca mg so4 2 sr and 87sr 86sr in sediment pore waters here we report new evidence from a detailed study of li and its isotopes in sediment cores from odp sites 844 and 851 carbonates at these sites were significantly recrystallized leading to large variations in li and 7li in the sediments and associated pore waters in addition to diagenetic effects distinct reversals in lithium concentration and isotopic ratio toward modern seawater composition are observed in waters near the basaltic basement lending further support to the seawater intrusion hypothesis three endmembers are identified in the pore waters seawater a diagenetically altered component and evolved crustal fluid the new lithium isotopic results underscore the importance of diagenetic artifacts in carbonate sediments

strain and displacement in the harts range detachment zone a structural study of the bruna gneiss from the western margin of the entia dome central australia abstract a detailed structural and strain study has been carried out on the bruna granitic gneiss and its bounding thrusts from the western margin of the entia dome in the eastern harts ranges the zone containing the bruna gneiss has been described recently as comprising the harts range detachment zone hrdz a major low angle lithospheric detachment along which both major extensional and contractional movements have occurred the bruna gneiss contains a ductile fabric which varies from an augen streaky foliation through a ribbon gneiss to protomylonite and mylonite in what is predominantly a granitic gneiss conformably overlying the bruna gneiss is a distinctive dark coloured megacrystic gneiss layer which intruded independently from the bulk of the bruna gneiss and followed a different subsequent strain history principal stretch lineations are consistently north plunging or horizontal n s except where folded the bruna gneiss fabrics are complexly folded about axes in an asymmetric manner which suggests interruption of ductile slip and crumpling of the fabrics followed by continuing slip on other discrete surfaces and shear zones strain measurements of elongated feldspars and aggregates indicate moderate to high strains with predominantly plane strain symmetries strain is partitioned into planar parallel zones and suggests dominant heterogeneous simple shear as the main non coaxial flow mechanism flattening strains occur near the bases of two major zones and constrictional strains occur within some zones a strain integration profile across the detachment zone produces total displacements of only a few kilometres suggesting that much of the overall indicated displacement is partitioned into the discrete bounding thrusts asymmetric augen pressure shadows and shear bands provide kinematic indicators in the bruna gneiss most show reverse sense displacement of the overlying harts range cover in a southerly direction over the entia gneiss basement opposite extensional sense movements occur in some zones and especially at the margins of the detachment zone

detrital zircon provenance constraints on the evolution of the harts range metamorphic complex central australia links to the centralian superbasin until recently it has been widely accepted that protoliths to metasediments of the harts range metamorphic complex central australia were deposited prior to c 1 75 ga and form part of the palaeoproterozoic arunta inlier however new sensitive high resolution ion microprobe u pb analyses of detrital zircon together with recently published data suggest that they were deposited coeval with c 545 520 ma sediments from the adjacent little metamorphosed neoproterozoic to palaeozoic centralian superbasin protoliths of the harts range metamorphic complex were deposited in the irindina sub basin an early to mid cambrian rift located between the present day amadeus and georgina basin remnants of the centralian superbasin deposition occurred during a widespread and long lived interval of extension in parts of central australia associated with eruption of the voluminous kalkarinji continental flood basalts the harts range metamorphic complex was metamorphosed to upper amphibolite to granulite facies conditions within c 40 ma of deposition of its sedimentary protoliths

stress in the indo australian plate cloetingh s and wortel r 1986 stress in the ind austr ian plate in b johnson and a w bally editors intraplate deformation characteristics processes and causes tectonophysics 132 49 67 we modelled the state of stress in the indo australian plate in order to investigate quantitatively variations observed in tectonic style the numerical procedure incorporates the dependence of slab pull and ridge push on the age of the oceanic lithosphere estimates are presented for the average net resistive forces at the himalayan collision zone the suction force acting on the overriding indo australian plate segment at the tonga kermadec trench and the drag at the base of the lithosphere our modelhng shows a concen ation of compressive stresses of the order of 3 5 lcbar in the ninetyeast ridge area the effects of the compressive resistance associated with himalayan collision and subduction of young lithosphere off the northern part of the sunda arc are focused in this region the stress field as calculated gives a consistent explanation for the observed concentration of seismic activity stein and okal 1978 and significant deformation in the oceanic crust weissel et al 1980 mcadoo and sandwell 1985 in the area the calculated stress field in the area adjacent to the southeast and central india ridges is characterized by tension parallel to the spreading axis this explains the concentration of near ridge normal faulting seismicity with t axes subparaile1 to the spreading ridge in the indian ocean as recently observed by bergman et al 1984 and wiens and stein 1984 the regional stress field along the strike of the sunda arc varies from compression seaward of and parallel to the sumatra trench segment to tension perpendicular to the java flores segment this explains the selective occurrence of well developed grabens seaward off the java flores segment of the trench observed by hilde 1983 our modelling shows that the observed rotation of the stress field denham et al 1979 in the australian continent is mainly the consequence of its geographic position relative to the surrounding trench segments and the variations of the forces acting on the down going slab in each of these the state of compression in west and central australia is induced by the action of resistive forces at the himalayan and banda arc collision zones the joint occurrence of high levels of compression in the plate s interior and normal faulting seismicity in the near ridge areas is a transient feature unique to the present dynamic situation of the indo australian plate

amphibolites from the entia gneiss complex eastern arunta inlier geochemical evidence for a proterozoic transition from extensional to compressional tectonics geochemically distinct metabasic amphibolite suites within the dominantly quartzo feldspathic entia basement gneiss complex egc harts range central australia imply two separate magmatic episodes of contrasting tectonic affinity that contributed to the early mid proterozoic crustal evolution of the eastern arunta inlier the metabasic suites directly reflect a distinctive style of proterozoic tectonics involving sequential mantle plume generated extension crustal thinning and subsequent compression crustal thickening that took place on a lithospheric detachment zone during ensialic orogenesis metabasic rocks from an early formed supracrustal association were generated in an intra continental rift environment they have the geochemical characteristics of continental tholeiites similar to metabasites in other eastern arunta basement gneiss terranes younger basic to ultrabasic amphibolites comprise thick massive deformed and metamorphosed sills and small layered igneous bodies that intruded the earlier formed supracrustal sequence trace element characteristics of these rocks e g high la nb and zr nb typify subduction related magmas the data imply an arc or collisional type environment this regime post dating the tensional i e rifting stage a metacumulus origin is proposed for many of these rocks their continuous geochemical trends imply extensive pyroxene dominated fractionation of mantle derived parental magmas in differentiating sill complexes emplaced at depths in the range 20 25 km incompatible element distributions and continuous overlapping major element trends indicate that the cumulate metapyroxenite bodies are related to the early fractionation of voluminous tonalite granodiorite orthogneiss intrusives that comprise a significant portion of the entia dome and which closely resemble intrusive suites commonly observed in cordilleran settings anomalously high k zr rb zr and ba zr ratios for these entia amphibolites together with variations in sio2 ce yb n and k rb ratios indicate contamination of some of the entia magmas by continental crust most likely by small degrees of melting of easily fusible felsic gneiss component aided by high surface area to volume ratio of the sills

using in situ trace element determinations to monitor partial melting processes in metabasites peak metamorphism 800 850 c 8 10 kbar in the harts range meta igneous complex harts range central australia was associated with localized partial melting by the reaction hornblende plagioclase quartz h2o1 4 garnet clinopyroxene titanite melt in situ trace element determinations of prograde peak and retrograde minerals in migmatitic metabasites and associated tonalitic melts using laser ablation icp ms has allowed monitoring of a range of partial melting processes melting melt segregation and back reaction between crystallizing melt and restitic minerals mass balance calculations indicate that titanite is a major carrier of trace elements such as ti nb ta sm u and th and therefore may be an important accessory phase to control the redistribution of these elements during the partial melting of amphibolites titanite preferentially incorporates ta over nb and hence residual titanite might assist in the formation of melts with high nb ta the fact that single minerals record different rare earth element ree patterns from prograde to peak to retrograde conditions demonstrates that ree diffusion is not significant up to 800 c therefore trace element analysis in minerals can be a powerful tool to investigate high grade metamorphic processes beyond the limits given by major elements

diffusion driven extreme lithium isotopic fractionation in country rocks of the tin mountain pegmatite abstract lithium concentrations and isotopic compositions in the country rocks amphibolites and schists of the tin mountain pegmatite show systematic changes with distance to the contact both li and 7 li decrease dramatically along a 10 m traverse from the pegmatite into amphibolite with li concentration decreasing from 471 to 68 ppm and 7 li decreasing from 7 6 to 19 9 rubidium and cs also decrease from the pegmatite contact into the country rock but only within the first 2 m of the contact after which their concentrations remain constant neither mixing between pegmatite fluids and amphibolite nor li isotope fractionation by rayleigh distillation during fluid infiltration is a likely explanation of these observations due to the extremely light isotopic composition required for the amphibolite end member in the mixing model 7 li 20 and the similarly extreme isotopic fractionation required in a rayleigh distillation model rather these variations are likely due to isotopic fractionation accompanying li diffusion from the li rich pegmatite li 450 to 730 ppm into amphibolites li 20 ppm the fact that other alkali element concentrations vary only within 2 m of the contact reflects the orders of magnitude faster diffusion of li relative to heavier elements quartz mica schists in contact with the pegmatite also show large variations in both li and 7 li as a function of distance from contact 1 wt to 70 ppm and 10 8 to 18 6 respectively but over a longer distance of 30 m lithium concentrations of the schist decrease from 1 wt adjacent to the contact to 70 ppm 300 m from the contact the latter is a typical concentration in metapelites the nature of the 7 li variations in the schists is different than in the amphibolites schists within the first 2 m of the contact have nearly identical 7 li of 10 which mimics that of the estimated bulk pegmatite 8 to 11 at a distance of 30 m the 7 li reaches the lowest value in the schists of 18 6 similar to the lowest amphibolite measured at a distance of 300 m the 7 li climbs back to 2 5 which is within the range of 7 li of other schists in the region and metapelites worldwide the behavior of li in the schists can also be modeled by li diffusion with the effective diffusion coefficient in the schist being 10 times greater than that in the amphibolite the effective diffusion coefficients of li in the amphibolite and schist are 2 orders of magnitude greater that those in minerals which implicates the importance of fluid assisted grain boundary diffusion over solid state diffusion in transporting li through these rocks

geochemistry and petrogenesis of archaean metavolcanic amphibolites from fiskenaesset s w greenland abstract the meta volcanic amphibolites closely associated with the fiskenaesset anorthosite complex can be subdivided on the basis of trace element patterns or mineral chemistry by far the most abundant type has light rare earth element lree depleted ree patterns and displays a wide range in trace element abundances chemically comparable amphibolites can be recognised throughout the ca 2800 m yr west greenland terrain the geochemistry of the basaltic amphibolites is dominantly controlled by fractional crystallisation processes although variable degrees of partial melting may also be important the required crystal extract plagioclase dominated in the proposed fractionation scheme is very similar to the primary mineralogy of cumulates of the fiskenaesset complex and trace element models support a genetic relationship between the anorthosite complex and enclosing host amphibolites the application of trace element discrimination to assign tectonic environment in the archaean is arguable however details of the trace element chemistry especially chondritic la ta ratios are taken to suggest out of a range of likely tectonic environments an ocean floor rather than island arc affinity for the fiskenaesset amphibolites the large ion lithophile lil elements display erratic distribution patterns but are generally enriched relative to the ree this appears not to be related to high grade metamorphism but may be a relict feature of seafloor alteration the association of the cumulate sequence with meta volcanic amphibolites and metasediments probably represents an ocean floor assemblage emplaced into the lower crust during crustal accretion

lithospheric scale buckling and thrust structures on mars the coprates rise and south tharsis ridge belt detailed photogeologic mapping documents for the first time the deformation sequence of the coprates rise materials of middle noachian through perhaps early hesperian age were involved in the horizontal shortening of crustal and lithospheric rocks that produced uplift and asymmetric east vergent folding of these layers during the late noachian and perhaps early hesperian previous suggestions of normal faulting as the deforming mechanism are not supported by the new mapping or by dislocation models of rise topography the most likely mechanism for producing the coprates rise is folding due to buckling instability and perhaps thrust faulting an extensive ridge belt concentric to south tharsis is defined by a newly recognized system of approximately coeval long wavelength topographic ridges and swales that include the coprates rise horizontal shortening of crust and lithosphere is manifested by periodically spaced structures at two different scales wrinkle ridges spaced tens of kilometers apart and the south tharsis system of ridges spaced hundreds of kilometers apart both types of structures may be associated with early volcanotectonic activity and crustal thickening in south tharsis these and other large ridges indicate that lithospheric buckling and thrust faulting may have been common on early mars

lithium isotope analysis of olivine by sims calibration of a matrix effect and application to magmatic phenocrysts high spatial resolution analysis of light element isotope variations by secondary ion mass spectrometry sims has numerous applications in geochemistry and cosmochemistry recent attention has focused on 7li 6li variations in magmatic phenocrysts to infer the volatile degassing history of their parent magmas and on minerals from mantle samples to determine source region processes and the recycling history of mantle reservoirs in these studies the effect of mineral composition on the 7li 6li ratio measured by sims has been considered secondary and generally disregarded we show using a suite of nine olivines analyzed by mc icp ms or tims that there is a substantial effect of composition on the 7li 6li ratio of olivine measured by sims for magnesian olivine 74 fo 94 the effect is a linear function of composition with 7li increasing by 1 3 for each mole percent decrease in forsterite component at higher fe contents the relationship ceases to be linear the composition range over which linear behavior is exhibited appears to depend on instrumental conditions a calibration of this matrix effect over the linear range is presented assuming the measurement of 7li 6li relative to an olivine standard of known composition application of this calibration to a suite of olivines separated from basaltic lavas from ko olau hawai i demonstrates that the matrix effect is responsible for a geologically spurious correlation between 7li and mg however after correction the olivines retain evidence of li isotope heterogeneity the degree and nature of which differs in each of the four separates studied these results emphasize the importance of compositional correction for sims measurement of 7li in olivine particularly in zoned crystals and support previous conclusions that li isotope variability in igneous materials is subject to late stage disturbance the significant matrix effect demonstrated for olivine suggests that matrix effects in other minerals require further evaluation

tectonic history of the syria planum province of mars we attribute most of the development of extensive fractures in the tharsis region to discrete tectonic provinces within the region rather than to tharsis as a single entity one of these provinces is in syria planum faults and collapse structures in the syria planum tectonic province on mars are grouped into 13 sets based on relative age areal distribution and morphology according to superposition and fault crosscutting relations and crater counts we designate six distinct episodes of tectonic activity in the following chronologic order stage i is an early to late noachian deformation forming mostly east west structures fault set ia some large volcanoes also formed faults were produced possibly by flexural uplift also arcuate north trending grabens set ib indicate that faulting is transitional to the next stage stage ii is late noachian to early hesperian radial faulting centered in syria planum possibly due to isostatic uplift in late noachian set iia to early hesperian set iib time stage iii is early to late hesperian faulting tangential to syria planum that was related to local centers of uplift sets iii1 iii3 on the periphery of syria planum stage iv is a late hesperian graben formation that was circumferential to syria planum set iva caused either by collapse associated with eruption of magma or by flexure of the lithosphere due to volcanic loading in association with volcanism minor faulting occurred producing an oval pattern of faults in southwest syria planum set ivbl and a radial pattern south of the planum set ivb2 that apparently rejuvenated buried stage ii faults stage v is a late hesperian to early amazonian development of grabens and troughs of noctis labyrinthus and western valles marineris set v that was probably instigated by local uplift exposure of groundwater or ground ice zones may have produced further collapse and trough enlargement stage vi is early amazonian northwest trending faulting in noctis fossae set vil perhaps due to tharsis monies centered tectonism and north northwest normal faulting along the eastern side of the claritas rise set vi2 that was due to tectonic subsidence the duration of tectonic activity in the syria planum province was perhaps 2 3 b y photoclinometric topographic profiles across 132 grabens and fault scarps show that syria planum grabens have widths average of 2 5 km and most range from 1 to 6 km similar to lunar grabens but the martian grabens have slightly higher side walls average about 132 m and gentler wall slopes average of 9 and range of 2 25 than lunar grabens 93 m high and 18 slopes scarp degradation on mars has progressed through quakes impact shaking and dry slope processes the lower slopes may be due to mars higher gravity estimates of the amount of extension for individual grabens range from 20 to 350 m most estimates of the thickness of the faulted layer range from 0 5 to 4 5 km average is 1 5 km this thickness range corresponds closely to the 0 8 to 3 6 km range in depth for pits troughs and canyons in noctis labyrinthus and along the walls of valles marineris we propose that the predominant 1 to 1 5 km values obtained for both the thickness of the faulted layer and the depths of the pits troughs and theater heads of the canyons reflect the initial depth to the water table in this region as governed by the depth to the base of ground ice maximum depths for these features may indicate lowered groundwater table depths and the base of ejecta material

the petrology and geochemistry of granitic gneisses from the east arunta inlier central australia implications for proterozoic crustal development the entia gneiss complex is an early to middle proterozoic inlier composed of upper amphibolite facies orthogneisses and paragneisses in the eastern end of the arunta block in central australia it is separated from overlying structurally distinct compositionally different and probably younger gneisses of the harts range complex by a subhorizontal detachment zone this paper discusses the petrology of four major granitic gneiss bodies one of these the bruna gneiss has intruded this detachment zone the other three are components of the structurally underlying entia gneiss complex the entia gneiss complex has a distinctive subhorizontally layered character with recumbent folds and flat lying schistosity the granitic gneisses that are components of this complex appear to have intruded an actively deforming terrain and are folded sheet like bodies the orthogneisses of the entia complex range from gabbro to true granite in composition and the granites are i type on both geochemical and petrographic criteria they are significantly different from the relatively alkali rich granites of equivalent age which occur in some of the other northern australian proterozoic blocks and are most like the granites of the modern cordilleran belts particularly in the association of diorite and gabbro with granodiorite it is suggested they are formed in narrow collisional belts formed as a result of closure of intracratonic rift basins the high surface area to volume ratio of these granites which is a consequence of the deformational environment into which they were intruded has resulted in considerable wall rock exchange in the more differentiated end members of the suites this has led to contamination by a net gain of k2o rb and ba and loss of relatively compatible elements including the ree y nb and tio2 compared with the syn tectonic entia granites the bruna granite gneiss is post tectonic and shows geochemical affinities with a type or anorogenic palaeozoic granites most of this granite is confined to the detachment zone but the very latest phases also intrude the harts range cover sequence an important conclusion of this study is that detachment zones of the type described from this area of the eastern arunta block exercise strong control on plutonism into the early to middle proterozoic crust as long as the very mobile style of early proterozoic tectonics continues the intracrustal dislocation zone confines syn tectonic granitic magmatism to deeper levels

structural evolution of the harts range area and its implication for the development of the arunta block central australia abstract a division of the high grade metamorphics of the arunta block in the harts range area into two main groups on the basis of lithology structure metamorphism and igneous activity is suggested as a result of detailed mapping the lower group is regarded as a basement terrain and comprises major layered felsic or granitic gneiss minor amphibolite and mafic granulite while the structurally overlying group is regarded as a cover sequence irindina supracrustal assemblage and comprises predominantly metasediments pelitic and semi pelitic gneiss calc silicate and quartzite and amphibolite anorthosite and ultramafic rock of the harts range meta igneous complex the cover sequence might have been deposited in an aulacogen or adjacent to a continental margin which developed by rifting of a pre existing basement the two groups both show early major ductile tectonothermal stages with four major fold generations and a later discrete shear event in the basement and two major fold generations and a later series of stacked nappes developing in the cover the basement s early orogenic evolution with associated granulite facies metamorphism is typical of intensely reworked early precambrian terrains and developed spatially and or temporally separated from the cover cover deformation closely resembles continental margin cordillera type overthrusting and thus thin skinned tectonics associated with large scale crustal shortening during the later harts range remobilization the basement and cover were juxtaposed by displacement on a fundamental crustal dislocation recumbent contraction fault mylonites repeatedly developed along the dislocation which was also isoclinally folded intruded by a major megacrystic granitoid bruna gneiss sheet and repeatedly rejuvenated the history of development of the dislocation is documented and described as a process of underthrusting of basement beneath cover which led to the final termination of the orogenic activity

critical taper wedge mechanics of fold and thrust belts on venus initial results from magellan fold and thrust belts exist on venus at the margins of crystal blocks such as plateaus tesserae and coronae and as ridge belts within the plains these fold belts display a number of key features that are consistent with their formation by critical taper wedge mechanics a mechanics that is well known for fold and thrust belts and accretionary wedges on earth for example an analysis of fold geometry at the toe of the artemis chasma fold belt indicates fault bend folding above a regionally extensive decollement horizon at a depth of about 1 5 km near surface deformation on venus is interpreted to be brittle and is anticipated to be dominated by cohesive strength in the upper 1 2 km critical taper wedge mechanics under anticipated venus conditions suggests that brittle wedges should have maximum surface slopes in the range 10 20 which is similar to some estimated slopes in the steep parts of the fold belts the low taper toes of fold belts may be cohesion dominated toes on either brittle or plastic decollement horizons once the base of the wedge undergoes the brittle plastic transition the surface slope is expected to flatten to near horizontal in qualitative agreement with many topographic profiles of fold and thrust belts on venus the estimated depth of the brittle plastic transition is uncertain based on rock mechanics data but is expected to be close enough to the surface to be affected by the atmospheric temperature gradient the relief of fold belts measured between the toe of the wedge and the flat crest displays a remarkable linear dependence on absolute elevation figure 15 ranging from 6 km for maxwell montes at an elevation of 10 km to a few hundred meters at the lowest planetary elevations 0 to 2 km this remarkable phenomenon appears to reflect an absolute elevation dependence of the depth of the brittle plastic transition possibly controlled by an isostatic coupling of elevation lithospheric thickness and geothermal gradient

large lithium isotopic variations in minerals from peridotite xenoliths from the eastern north china craton abstractto investigate the effects of melt rock interaction on li isotope fractionation we report in situ li concentrations and 7li of olivine ol orthopyroxene opx and clinopyroxene cpx for six peridotite xenoliths from the eastern north china craton these xenoliths contain two lherzolites two cpx rich lherzolites and two wehrlites and are variably metasomatized lithium isotope zonation is observed in most peridotite minerals the majority of cpx grains display isotopically light cores with lower li concentrations than the heavy rims however the opx grains show a different style of zonation from cpx where higher li concentrations in the cores are associated with much lighter 7li olivines in most peridotites have a restricted range of li concentration and 7li within individual grains whereas the olivines in a lherzolite show isotopically light cores 10 3 with high li concentrations 2 3 ppm and heavy rims 5 5 with low li concentrations 1 7 ppm these li isotopic variations in m

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

upb and rbsr geochronology and geological evolution of the harts range ruby mine area of the arunta inlier central australia abstract supracrustal and meta igneous rock units from the ruby mine area of the harts range eastern arunta inlier central australia have been dated by the zircon u pb and rb sr total rock tr and mica methods a well defined zircon discordia for a weakly deformed specimen of the bruna granite gneiss yields an age of emplacement of 1748 4 5 ma thereby constraining the minimum age of the irindina supracrustal assemblage metapelitic gneiss within the supracrustals and a meta igneous ultramafic boudin from the associated harts range meta igneous complex yield highly discordant zircon data revealing a strong early palaeozoic overprint rb sr tr data from anorthositic gneisses associated with the ultramafic boudin are highly disturbed also apparently during the lower palaeozoic however rb sr model age calculations and the zircon u pb data suggest a maximum age of about 2000 ma for the supracrustal and meta igneous rocks and argue for new proterozoic crust formation zircon u pb data from a deformed pegmatite emplaced in the meta igneous complex yield an emplacement age of 520 4 5 ma further pointing to lower palaeozoic magmatism and deformation correlations of u content and calculated 206 pb 238 u age for the ultramafic boudin zircons suggest that new growth of low u zircons occurred during retrogression associated with this event the sr isotope systematics of the anorthositic gneisses can also be interpreted in terms of introduction of palaeozoic sr our data suggest lower palaeozoic possibly delamerian tectonothermal activity to be more important in the evolution of the harts range area than previously recognised on the other hand rb sr mica ages for deformed and undeformed pegmatites and tr isochrons for the latter show that pervasive tectonothernal activity had ceased by about 315 ma and that regional cooling occurred between about 345 and 325 ma local shear zone biotite resetting may have persisted to about 300 ma consistent with the previously recognised alice springs orogeny possible dilational pb loss in the bruna zircons occurred at about 103 ma

evidence for an ancient impact basin in daedalia planum mars geologic mapping has provided observations that some features on mars are morphologically similar to lunar imbrium sculpture materials these grooved features occur in clustered sets of a dozen or more are spaced 1 to 10 km apart and are unlike other linear martian features such as yardangs faults ancient valley networks and mass wasting features we propose that they represent impact materials from an ancient basin the orientations of these features were plotted as lines on a schmidt net and the density of the resulting intersections suggests a likely source area at approximately 26 0 125 0 or roughly the center of daedalia planum numerous features geologic materials and earth based radar data indicate that a basin with a diameter of approximately 4500 km with an early noachian age is centered at this location four interior rings with diameters of 1100 1500 2200 and 3200 km and an exterior ring with a diameter of approximately 6400 km were found the rings are spaced with a ratio of 2 0 1 which is similar to the basin ring spacing on the other terrestrial planets both concentric and infrequently radial basin related faults were recognized including claritas fossae gordii dorsum and the thaumasia graben faults or zones of weakness related to the basin rings may have influenced the position of the mangala valles source area and arsia mons based on the location and morphology of early hesperian to early amazonian volcanics composing daedalia planum the daedalia basin may also have extensive deep seated interior faults capable of extruding mare materials to the surface

orientation and origin of ridges in the lunae palus coprates region of mars numerous wrinkle ridges similar in appearance to lunar mare ridges occur on volcanic plains units surrounding the tharsis region of mars where they are oriented circumferential to the center of the plateau by analogy with lunar mare ridges and from morphologic evidence such as the smooth slopes of arches these features are interpreted to result primarily from compressional folding of the volcanic plains although the uppermost ridge may represent faulting of the near surface materials east of the tharsis region ridges are best developed in the lunae palus and coprates quadrangles where they can be used to indicate the degree of compressional stress occurring there comparison of ridge orientations in these quadrangles to a tharsis center at 1 n 122 w indicates that more than 60 of the total length of ridges is orthogonal to a normal from this center non orthogonal ridges are seen to have general northerly orientations after removal of the tharsis orthogonal population trends of graben in surrounding terrain indicate that non orthogonal ridges are not greatly affected by regional scale structures comparison of the frequency of orthogonal and non orthogonal ridges further suggests that loading due to the tharsis plateau was effective in producing compressional features as far as 4000 km from the center of the load additional sources for compressional stress are necessary to account for ridge systems at greater distances from the tharsis plateau

coldspots and hotspots global tectonics and mantle dynamics of venus magellan radar images and altimetry data and pioneer venus gravity data of major highlands and lowlands on venus are examined with the objective of relating these tectonic and volcanic landforms to convection within venus mantle two roughly circular lowlands atalanta and lavinia planitiae are bowl shaped depressions which contain compressional tectonic features and lack hotspot related features such as coronae and shield volcanoes they are identified as surface expressions of young mantle coldspots or regions of mantle downwelling highlands can be divided into two distinct groups volcanic rises and plateau shaped highlands volcanic rises are domical circular to elongate regions characterized by volcanic construction extensional tectonism large positive gravity and geoid anomalies centered on the highlands and large apparent depths of compensation volcanic rises include beta bell atla and western eistla regiones and are identified as surface expressions of large mantle hotspots or plumes plateau shaped highlands are steep sided elevated regions that range from circular alpha regio to elongate or irregular ovda regio tellus regio in shape their surfaces are dominated by complex ridged terrain rather than the shield volcanoes and flows that dominate volcanic rises most plateau shaped highlands contain compressional structures some of which commonly lie along their margins and have small gravity and geoid anomalies and small apparent depths of compensation compared to volcanic rises plateau shaped highlands which include those mentioned above and western ishtar terra thetis regio and phoebe regio are identified as surface expressions of mantle coldspots or regions of downwelling the highland coldspot features are elevated as a consequence of crustal thickening crustal thickening is absent or minor at lowland coldspots the importance of mantle downwelling in the tectonic deformation of venus suggests that its mantle like the mantle of earth is strongly heated from within the major venusian hotspots attest to the existence of large mantle plumes carrying heat upward from the planet s core

diffusion induced li isotopic fractionation during the cooling of magmatic rocks the case of pyroxene phenocrysts from nakhlite meteorites ion microprobe was used to measure li abundances and isotopic compositions in pyroxenes from three martian meteorites belonging to the nakhlite family the profiles performed across augite crystals from northwest africa 817 show a large isotopic zoning from crystal cores 7li not vert similar 0 to rims 7li not vert similar 20 while li abundances are almost constant not vert similar9 2 g g unlike nwa 817 the pyroxene studied in the miller range 03346 nakhlite shows a zoning in li abundance with concentrations increasing from not vert similar2 5 g g in the core to not vert similar9 g g in the rim the augite rim 7li 7 is slightly enriched in 7li with regard to the core 7li 4 but most of the isotopic variations observed occur at an intermediate position along the profile where 7li falls down to not vert similar 11 in the case of nakhla li concentrations in augite increase from cores not vert similar3 5 g g to rims not vert similar6 5 g g while the 7li variation is restricted i e between 7li 6 0 and 12 6 for the three meteorites the li abundances were also measured in the groundmass which was found to be enriched in lithium not vert similar10 g g conventional magmatic and post magmatic processes such as alteration and fractional crystallization fail to explain the dataset obtained on nakhlites degassing processes which were previously proposed to explain the li distribution in shergottite crystals cannot result in the strong decoupling between li abundances and isotopic composition observed in nakhlites we suggest that the original magmatic li distributions concentrations and isotopic compositions in nakhlites have been modified by diffusion of li from the li rich groundmass towards the pyroxene crystals during sub solidus cooling diffusion appears to have been efficient for nwa 817 and mil 03346 but apparently did not produce a significant migration of li in nakhla possibly because of the lower abundance of groundmass in the latter diffusion induced li redistributions may also affect terrestrial porphyric rocks but very specific cooling rates are required to quench the diffusion profiles as observed in two of the present nakhlites

the origin of large local uplift in extensional regions large localized uplift is commonly observed in continental regions undergoing extension these observations can be modelled by planar high angle normal faulting of an elastic upper crust overlying an inviscid lower crust isostasy provides the necessary driving force the model quantifies the role of flexural rigidity density variations in the crust and erosion and deposition of sediment