sedimentary challenge to snowball earth evidence from the magnetic field fossilized in sedimentary rocks suggests that more than 600 million years ago ice occupied tropical latitudes a popular explanation for these findings the snowball earth concept envisages a fully frozen earth for millions of years caused by a runaway ice albedo feedback a rapid catastrophic meltback at very high levels of atmospheric carbon dioxide is thought to have ended this extreme climatic state however sedimentary rocks deposited during these cold intervals indicate that dynamic glaciers and ice streams continued to deliver large amounts of sediment to open oceans throughout the glacial cycle the sedimentary evidence therefore indicates that despite the severity of glaciation some oceans must have remained ice free significant areas of open ocean have important implications for the survival and diversification of life and for the workings of the global carbon cycle the snowball earth concept envisages a fully frozen earth for millions of years several times during the neoproterzoic era between 1 000 and 542 million years ago however the sedimentary evidence suggests that despite the severity of glaciation some oceans must have remained ice free

multiple foliation development during thrusting and synchronous formation of vertical shear zones abstract three orthogonal foliations developed during large scale n s thrusting in the central portion of the mount isa block australia two of these foliations which are well preserved in their original orientations in porphyroblasts of pelitic schists but considerably affected and commonly destroyed in the schist matrix due to the effects of subsequent deformation and metamorphism formed synchronously and have a common stretching lineation which is near horizontal and trends n s one foliation formed near horizontal and the other near vertical striking n s both developed during n s directed thrusting with the n s vertical foliation accommodating sinistral differential displacement between the thrust belt to the west and that to the east the third foliation which dips steeply to the north and strikes e w formed during n s shortening of the thrust belt and tends to overprint the other foliations formed in the same orogeny the n s vertical foliation is well preserved within a narrow belt of gneissic rocks called the wonga duchess belt that also locally contains the subsequently folded remains of the foliation that formed near horizontal this belt separates multiply thrust repeated beds to the west from large scale apparently less repetitive thrusts to the east the direction of thrusting appears to change across this belt from n to s on the west to s to n on the east the differential deformation to either side of the belt was taken up by sinistral shear along this belt numerous related but smaller scale vertical shear zones occur up to 3 km away to either side of this narrow zone the primary vertical foliation within the wonga duchess belt appears to have formed during transform like faulting by progressive inhomogeneous simple shear with no component of orthogonal shortening

role of plutonism in low pressure metamorphic belt formation wickham and oxburgh1 recently proposed that low pressure high temperature low p high t metamorphism in the eastern pyrenees and possibly all low p high t metamorphic belts resulted from anomalously high mantle heat flow brought about by rifting their model is largely constrained by the presence of nearby synmetamorphic rift related sedimentary rocks and the interpretation that the migmatites and granites are the product of in situ melting in the presence of an anomalously steep geotherm here we present an alternative model in which low p high t metamorphism pro grade reactions at pressures near or below the al2sio5 triple point results from contact effects near sill like igneous intrusions at intermediate crustal levels low p high t conditions can be achieved through this process in regions of continent continent collision with normal mantle heat flux as well as in zones of extension our model is based on studies of the low p high t metamorphic terrane in the new england appalachians

assessing the origin of old apparent ages derived by pb stepwise leaching of vein hosted epidote from mount isa northwest queensland australia epidote metasomatism affected large areas of tholeiitic metabasalts of the 1 780 ma eastern creek volcanics in the western fold belt of the proterozoic mount isa inlier hydrothermal epidote generally occurs in quartz veins parallel to or boudinaged within the dominant s2 fabrics which formed during the regional metamorphic peak at 1 570 ma associated with the isan orogeny previously published stable isotopic and halogen data suggest that the fluids responsible for epidote formation are metamorphic in origin with an evaporitic component application of the pb stepwise leaching technique to the epidote does not separate radiogenic pb4 and common pb2 generating little spread in 206pb 204pb between 16 0 and 30 5 the causes for this relatively low range are twofold there is little radiogenic pb in the epidotes the most radiogenic steps account for 1 of pb released and both pb2 and uranogenic pb4 substitute into the same site in the epidote crystal lattice consequently age regressions using the pb stepwise leaching data give ages between 150 and 1 500 myrs older than the host rocks and over 450 myrs older than the thermal metamorphic peak these old ages are attributed to chemical inheritance from the host metabasalts via radiogenic pb release by breakdown of phases such as zircon monazite titanomagnetite and ilmenite during metamorphism this idea is supported by trace element data and chrondrite normalized rare earth element patterns that are similar to both the metabasalts and epidotes except for a variable eu anomaly in the latter relatively high fo2 during vein formation fe3 dominates in the epidote crystal lattice would allow the incorporation of th4 and exclusion of u6 and would explain elevated th u ratios up to 12 in epidote compared with the host metabasalts non incorporation of u would explain the relatively low u pb ratios and non radiogenic character of the epidote this process may provide a source of metal for the small u deposits around mount isa and may also suggest a relationship between u mineralization and regional cu mobilization during the isan orogeny our work suggests that non conventional geochronometers should be used only if additional geological information and geochemical data e g mineral chemistry trace elements are available to evaluate any resulting age calculations

carbonate diagenesis in ice the fine particles created by glacial comminution of carbonate minerals are susceptible to interaction with meltwater and the isotopic composition of fine carbonate in glacial deposits has the potential of recording this interaction here we focus on the carbonate fraction of glacial sediment and debris within basal ice at the tsanfleuron glacier switzerland size separates of basal ice debris and melt out tills show a collinear isotopic trend interpreted as a mixing line between detritus 18 o pdb 5 and 13 o pdb 0 6 and authigenic calcite which in one case had 18 o values as light as 18 7 and 13 c values as light as 5 7 in most cases the finer fractions with calcite crystals as small as 0 1 m are enriched in the isotopically light component which is interpreted to form largely by freezing in basal ice the isotopic compositions of ice and included gases could be influenced by this process melt out tills inherit the authigenic calcite thus carbonate bearing till should no longer be regarded simply as clastic sediment

the ca 1740 1710 ma leichhardt event inversion of a continental rift and revision of the tectonic evolution of the north australian craton abstract interpretation and modelling of high resolution regional geophysical data of the central leichhardt river fault trough in the mount isa inlier are used to determine the timing of a major basin inversion event following the development of the ca 1780 1740 ma leichhardt superbasin inversion of the leichhardt superbasin formed the regional north south trending leichhardt anticline during east west shortening the limbs of the anticline are overprinted by several east west trending wedge shaped non magnetic sub basins filled with ca 1710 ma calvert and isa superbasin successions these relationships suggest inversion of the leichhardt superbasin occurred between ca 1740 and 1710 ma the event is also known to have affected the northern and eastern north australian craton the scale of the inversion suggests it was a significant event that we have defined as the leichhardt event this event requires a major tectonic driver to the east of the north australian craton possibly the accretion of a micro continental ribbon to the east of the mount isa inlier the results of this study have implication for paleogeographic reconstruction of the australian continent during the formation of nuna because eastern north australian craton faced an ocean at ca 1740 1720 ma the results also challenge the significance and intensity of crustal shortening associated with the ca 1600 1500 ma isan orogeny throughout the western mount isa inlier

a critical assessment of the tectono thermal memory of rocks and definition of tectono metamorphic units evidence from fabric and degree of metamorphic transformations

evidence from microstructures of deformable bed conditions within drumlins chimney bluffs new york state abstract discussion on the process of drumlin formation has been ongoing for over a century at present two dominant schools of thought prevail viz formation by a subglacial hydraulic mechanisms and b due to subglacial sediment deformation the latter mechanism depends on the existence of a subglacial deformable bed evidence from part of the large central new york drumlin field reveals evidence of syn depositional deformation during the emplacement of diamictons that constitute the major portion of drumlins exposed along shore bluffs in chimney bluffs state park these diamictons cannot be lodgement till facies members but appear to comprise facies indicative of melange style deposition under subglacial deformable bed conditions the evidence obtained from microstructures and plasma fabric within these diamictons supports the existence of a deformable bed during diamicton emplacement it is likely that formation of the drumlins occurred under these same subglacial bed conditions

rhenium osmium systematics of the mount isa copper orebody and the eastern creek volcanics queensland australia implications for ore genesis the syn tectonic breccia hosted mount isa cu deposit in northwest queensland is the largest sediment hosted cu deposit in australia whole rock samples of chalcopyrite rich cu ore form an isochron with a re os age of 1 372 41 ma this age is more than 100 ma younger than the previously accepted age of cu ore formation an ar ar mineral age for biotite separated from the host rocks within the alteration envelope to the cu orebody this discrepancy cannot be unequivocally resolved due to a lack of other absolute geochronological constraints for cu mineralisation or the deformation event associated with cu emplacement the 1 372 41 ma date may reflect a the time of cu deposition b the time of a hydrothermal event that reset the re os signature of the cu ore or c mixing of the re os isotope systematics between the host rocks and cu bearing fluids however a range of published ar ar and rb sr dates for potassic alteration associated with cu mineralisation also records an event between 1 350 and 1 400 ma and these are consistent with the 1 372 ma re os age the 1 8 ga eastern creek volcanics are a series of tholeiitic basalts with a primary magmatic cu enrichment which occur adjacent to the mount isa cu deposit the whole rock os isotopic signature of the eastern creek volcanics ranges from mantle like values for the upper pickwick member to more radiogenic crustal values for the lower cromwell member the re os isotope signature of the cu ores overlaps with those calculated for the two volcanic members at 1 372 ma hence the os isotope data are supportive of the concept that the os in the cu ores was sourced from the eastern creek volcanics by inference it is therefore postulated that the eastern creek volcanics are the source of cu in the mount isa deposit as both os and cu are readily transported by oxidised hydrothermal fluids such as those that are thought to have formed the cu orebody the pickwick member yields a re os isochron age of 1 833 51 ma which is within error of previously reported age constraints the initial 187os 188os isotopic ratio of 0 114 0 067 os 0 7 is slightly subchondritic and together with other trace element geochemical constraints is consistent with a subcontinental lithospheric mantle source the pickwick member records a minimum age of ca 1 95 ga for melt depletion in the subcontinental lithospheric mantle beneath the mount isa inlier prior to the extraction of the magmas which formed the eastern creek volcanics this corresponds with the end of subduction related magmatism along the eastern margin of the northern australian craton which included the mount isa inlier

progressive deformation structures associated with ductile thrusts in the moine nappe sutherland n scotland in the deeper parts of mountain belts polyphase structural sequences observed at outcrop scale can arise due either to overprinting of regionally distinct deformation phases e g reworking changes in orogenic boundary conditions or to localized controls that bring about transient changes in the patterns of ductile flow these are unlikely to be mutually exclusive processes and examples from scotland demonstrate that once regionally separate events are delimited using radiometric evidence it is possible to isolate complex deformation sequences arising due to local controls in the western moine nappe of sutherland the dominant structures were formed during caledonian ductile thrusting towards the wnw whilst earlier precambrian phases are relatively minor in importance two groups of caledonian folds and fabrics are recognized in many exposures main phase d2 structures which are broadly contemporaneous with ductile thrust fabrics and later secondary phase f3 folds the latter can be divided into two geometric groups sheath fold types which formed initially as wnw overturned buckles subsequently modified by ductile shearing and asymmetric types which are commonly open folds apparently formed with axes close or sub parallel to the thrust transport direction secondary structures show a close spatial association with high strain zones along ductile thrusts and can be shown to have formed during the later stages of thrusting in certain critical exposures i propose that they may form due to strain perturbations resulting from variations in the relative rates of ductile flow within the mylonites where differential shearing occurs due to flow normal perturbations wrench related asymmetric fold types may form in contrast secondary sheath fold structures may result from localized compression phases caused by flow parallel perturbations local flow perturbation models may be appropriate in situations where the distribution of later structures is apparently related to spatial variations in strain intensity and or there is a strong geometric and kinematic similarity between sets of folds and fabrics if such features are not observed a regional polyphase sequence is more likely and it is therefore necessary to obtain radiometric dates or some other independent source of data to demonstrate a time separation

structural evolution across the eastern ghats mobile belt bastar craton boundary india hot over cold thrusting in an ancient collision zone along the western margin of the eastern ghats mobile belt egmb ultrahigh temperature granulites are thrust westward over hornblende granites and sedimentary rocks of the bastar craton multiply deformed migmatitic gneisses and porphyritic charnockite of the egmb unit preserve thrust related shear fabrics s3m in the cratonic foreland undeformed granites show a progressive decrease in grain size and increase in penetrative foliation to the east evolving into orthogneiss near the mylonitized contact zone with charnockite foreland fabrics with a consistent top to the west shear sense are conformable with s3m in charnockite and correlate with s3b in polydeformed orthogneisses of cratonic windows s3b in the windows parallels s3m in overlying egmb migmatitic gneisses while s3m accompanied granulite metamorphism in the egmb s3b temperature estimates vary from t 700 c in the windows to t 550 c in the west decreasing temperature and later fabric formation in the west are explained by an evolving thermal profile in the cold craton which is caused by thrusting against hot lower crustal egmb rocks based on lithologic structural and metamorphic variations across the contact and resemblances between the egmb and rayner complex the craton mobile belt boundary is considered a result of indo antarctic collision leading to the formation of an ancient supercontinent

the role of thrusting in the structural development of the mount isa mine and its relevance to exploration in the surrounding region resolution and prediction of the structure underground at mount isa from the surface geology is now possible using evidence for north to south movement on large scale thrusts during the first deformation at 1610 ma this evidence for the direction of movement during thrusting is preserved 1 macroscopically within three successive thrust imbricates as the north south alignment of respective truncations of older stratigraphic units by the angular unconformity at the base of the mount isa group 2 macroscopically and mesoscopically as a zone of vertical north south striking foliation with a horizontal stretching lineation that formed on a sinistral transform fault during dalong the wonga duchess belt 3 mesoscop ically as a regionally distributed north south oriented stretching lineation li that is locally well preserved on s0 surfaces in the hinges of folds with north south axial planes that formed during the second regional deformation and 4 microscopically as a horizontal north south aligned mineral elongation lineation defined by inclusion trails preserved in porphyroblasts that grew early during the second deformation adjacent to the wonga duchess belt the swing in strike of a large scale 70o west dipping lateral ramp on the western margin of the large scale kokkalukkanurker duplex from north south parallel to the direction of movement during thrusting to north northwest south southeast caused a portion of the meerenurker roof thrust lying on this lateral ramp to be overturned for at least 50 km near mount isa this overturning resulted from tilting the 70o west dipping lateral ramp through another 70 o as it rode obliquely north south over the north northwest south southeast striking portion of the same lateral ramp on the next imbricate to the south this generated the gross structural relationships seen in three dimensions at mount isa whereby the greenstone basement that truncates mount isa group rocks below the mine overlies them to the west the second deformation involved intense east west shortening and formed macroscopic folds that conveniently expose varying levels of the thrust pile generated during the first deformation in higher grade rocks west of mount isa and to either side of the wonga belt axial plane schistosity development and or reactivation of bedding during this deformation was quite intense and tended to obliterate and or rotate foliations and lineations formed during the first deformation only in the hinges of the folds formed during the second deformation where the deformation was less intense or silicification had occurred are earlier lineations preserved relatively unaffected by this event the geology of the mount isa valley and the rocks to the west of the mine is controlled by a series of north south striking high angle reverse faults that slice through the meerenurker thrust as it crosses the above mentioned lateral ramp on the western edge of the large scale duplex two of the faults mount gordon and mount isa dip steeply west with similar dis placements of 4 to 6 km west side up the other two faults holly and tastoka dip steeply east with the latter fault having a more significant displacement of 2 5 to 3 km east side up that greatly affects the geology on the eastern side of the mount isa valley the effects of these faults on the geometry produced by thrusting during the first deformation and folding during the second allow accurate prediction of the third dimension anywhere in the mount isa valley and the region immediately to the west consequently they can be used to predict the location of structurally controlled sites of potential copper mineralization similar to those found in the mount isa mine where mineralization formed during the third deformation

comment on evolution of co 2 so 2 hcl and hno 3 in the volcanic plumes from etna by voigt et al geophys res lett 41 doi 10 1002 2013gl058974 editor s note the following comment and reply arise from an article published in geophysical research letters by voigt et al 2014 the article addresses a volcanology topic and the commenters take issue with some conclusions and offer an analysis of their own voigt and co authors have responded why is this comment and reply being published in the bulletin it is because geophysical research letters is one of a number of journals that do not offer any published forum for discussion of the papers they publish this is a matter of editorial policy and a decision for each journal the bulletin of volcanology does provide a forum for discussion of articles published when contacted by marcello liotta with the request that the bulletin consider hosting a discussion of the voigt et al volcanology article in grl i agreed to do so if the grl authors were willing to engagewith the comment voigt and co authors were willing to do so and have been allowed a small amount of additional space to summarize for bulletin readers the key points of the grl paper under discussion before responding directly to the comment from liotta and rizzo i hope that bulletin readers find the discussion and reply of interest james d l white executive editor bulletin of volcanology

progressive folding and foliation development in a sheared coticule bearing phyllite abstract an evolutionary sequence of fold and foliation development interpreted to be the result of a single progressive deformation is seen in exposures of strongly sheared phyllite and coticule 10 km south of the e w trending chedabucto fault in the meguma terrane of nova scotia canada transposition sensu sander at mesoscopic and microscopic scales is fundamental to this development as are fold hinge rotation and migration the transposition process is cyclic with folds of layering and foliation developing rotating and being refolded fold hinges and axial surfaces repeatedly tend towards stable end orientations but do not reach them due to the cyclic transposition process the foliation is a steady state feature continually reconstituted by microscale transposition and dynamic recrystallization several important consequences of these contemporaneous processes are 1 geometrical relationships between folds and foliation are variable in the same outcrop even though both developed synchronously and many folds have the geometrical relationships of cleavage transected folds 2 the foliation is transient in orientation and therefore cannot track the xy principal plane of total finite strain and 3 fold hinge migration may be a common process especially in deformations involving a large component of simple shearing the observed relationships may be applicable to study of fold and foliation development in orogenic belts in general

geochronological constraints on tourmaline formation in the western fold belt of the mount isa inlier australia evidence for large scale metamorphism at 1 57 ga boron metasomatism during two deformational events resulted in tourmaline formation in parts of late palaeoproterozoic eastern creek volcanics of the western fold belt of the mount isa inlier despite low u pb ratios 1 as measured by laser ablation icp ms when the step leaching process was applied to the tourmaline it process extracted pb fractions with 206 pb 204 pb up to 76 illustrating the method s ability to efficiently separate radiogenic and common pb that are hosted in different crystal lattice sites tourmaline from two syn d2 metamorphic hydrothermal veins at anderson s lode and eldorado yield similar pb step leaching ages of 1573 12 and 1577 48 ma respectively tourmaline from a post d2 but pre d3a vein at king was dated at 1528 51 ma although there is evidence to suggest some contribution from u rich inclusions i e zircon and monazite to some leach steps the isochrons are thought to provide robust ages for hydrothermal tourmaline formation during two discrete thermal events this is supported by estimates of pb closure temperatures in tourmaline derived from ionic porosity models the 1575 ma ages for d2 also reflect the main metamorphic event in the western fold belt but are 40 myr older than accepted age estimates for this episode the revised timing for peak metamorphism tectonism in the western fold belt corresponds with the age of metamorphism in the eastern fold belt suggesting broadly coeval tectonic histories across the entire inlier from at least 1600 ma

geomorphological criteria for identifying pleistocene ice streams ice streams are critical regulatory mechanisms in contemporary ice sheets it has been inferred that they also had a significant effect on the dynamics of former ice sheets subsequently many people have invoked their widespread occurrence from a variety of formerly glaciated areas hypothesised locations however have often outweighed meaningful evidence this paper addresses the problem using the characteristics of contemporary ice streams as a basis for their identification from former ice sheet beds a convergence of knowledge gained from contemporary ice stream research coupled with theories of glacial geomorphology allows several geomorphological criteria to be identified as suggestive signatures of ice stream activity it is envisaged that the geomorphological criteria developed here will introduce a more objective approach to the study of former ice streams the criteria are used to construct conceptual land system models of the beds of former ice streams and it is hoped such models can provide an observational template upon which hypotheses of former ice streams can be better based

chemical and stable isotopic characteristics of syn tectonic tourmaline from the western fold belt mount isa inlier queensland australia abstract understanding hydrothermal fluid flow events related to orogenic events is important because they provide large scale redistribution of metals which may become concentrated in ore deposits in this study we determine the origin of tourmaline bearing quartz veins and pegmatites in the western fold belt of the proterozoic mount isa inlier a region which host numerous syn orogenic cu deposits this paper uses tourmaline mineral chemistry and isotopic analyses in situ b and bulk o and h isotopes to fingerprint the fluid reservoirs that contributed to the fluid budget during the isan orogeny all the tourmaline samples have schorl dravite solid solution compositions quartz tourmaline 18 o 11 0 12 6 and 7 7 9 4 v smow respectively pairs yield equilibrium formation temperatures of 305 to 575 c with the higher temperatures related to pegmatitic tourmaline tourmaline d values have a relatively narrow range from 68 to 56 v smow in most cases calculated 18 o fluid and d fluid values are compatible with primary magmatic water with a small component of metamorphic 18 o enriched formation waters however there are large variations in tourmaline 11 b values which range from 10 7 to 17 0 with the majority of values between 10 and 15 variations in 11 b tur in the pegmatites suggest that boron degassing generated relative 11 b depletion during crystallization similar 11 b tur values in quartz tourmaline veins spatially associated with the pegmatites in cordierite and sillimanite bearing metamorphic rocks west of the mount isa fault demonstrate a similar origin for b in this tourmaline in contrast quartz tourmaline veins of similar timing hosted by greenschist grade metamorphic rocks east of the mount isa fault have much higher 11 b tur values 6 2 to 10 7 which reflect the involvement of a distinct b isotopic reservoir mostly likely a near surface derived evaporitic brine thus the b isotopic data reveal important differences in fluid origin that are not reflected in the 18 o values therefore at least two isotopically distinct fluids one sourced from fractionating anatectic pegmatites and the other from external basinal brines circulated contemporaneously in separate areas during the isan orogeny there is no evidence based on the tourmaline isotopic data to suggest that these fluids mixed which demonstrates that the hydrothermal plumbing system in the western fold belt during the isan orogeny was disconnected probably due to differences in thermal structure metamorphic grade across the western fold belt during orogenesis

particle and aggregate mobility in till microscopic evidence of subglacial processes abstract microscopic studies of tills show that the deforming bed of glaciers is made up of minute rotational elements or wheels consisting of a nucleus alone or a nucleus with spiralling arms of fine grains these fine grains can move from one wheel to the next providing a process of high grain mobility within a deforming subglacial till recognition of this process explains a variety of glacial distribution patterns i e the mineralogical reflection of subjacent lithologies in the till matrix the side by side occurrence of far travelled erratics and locally derived matrix and the shape of geochemical anomalies

microstructural evidence for n s shortening in the mount isa inlier nw queensland australia the preservation of early w e trending foliations in porphyroblasts revealed by independent 3d measurement techniques 3d microstructural analyses of porphyroblast inclusion trails using both the asymmetry switch method for determining foliation intersection axes preserved in porphyroblasts fias and the recently developed fitpitch method reveal w e and n s trending fia sets in the white blow formation of the mount isa inlier each method reveals two subsets of fias centered on each of these major trends these were distinguished based on the relative timing trend and orientation of inclusion trail patterns thirty six samples were analyzed using both techniques and produced very similar results pitches of the inclusion trails preserved within the porphyroblasts in vertically oriented thin sections and trends in horizontal sections yield distinct near orthogonal modal orientations from all the analyzed samples this indicates that the porphyroblasts host successive fabrics as crenulation foliations and did not rotate with respect to geographical axes w e and n s trending fias have been obtained from both garnet and staurolite porphyroblasts hosting differentiated crenulation cleavages garnet and staurolite growth during bulk north south shortening recorded the development of multiple foliations and an associated succession of metamorphic events at middle amphibolite facies conditions that predates the metamorphic history generally recognized in this terrain this period of bulk shortening and associated metamorphism formed during a period of orogenesis called o1 w e shortening formed n s striking foliations that preserve a period of orogenesis o2 and another succession of metamorphism involving more phases of porphyroblast growth preserving n s trending fias overprinting of successive fia trends wsw ene wnw ese nnw sse and ssw nne suggests a relative clockwise rotation of the bulk shortening direction through time as it switches from n s to w e overall with a major tectonic break or decompression between o1 and o2 the porphyroblast inclusion trail patterns preserving w e trending fias provide a window into the lengthy period of earlier deformation and metamorphism that is no longer preserved within the matrix foliations

structural evolution of a foreland fold and thrust belt the umbria marche apennines italy abstract outcrop scale structures that record a progressive sequence of deformation can be used to clarify the kinematic evolution of an entire fold and thrust belt the temporal progression of the compressional deformation as reconstructed by means of a mesoscopic structural analysis provides a key for unraveling the kinematic history of the umbria marche apennines central italy overprinting relationships at outcrop scale allow for the recognition of three distinct structural stages which are in sequence a layer parallel shortening b folding c thrusting individual stages are explained in the framework of a progressive deformation model where shortening of the sedimentary cover was continuous and occurred during a single contractional episode as a consequence of tip line folding processes a comparison of this history with those reported from other fold and thrust belts shows that layer parallel shortening folding and thrusting are sequentially dominant processes in areas which have experienced compressional deformation

structural evolution of a 1 6 ga orogeny related to the final assembly of the supercontinent nuna coupling of episodic and progressive deformation

proterozoic low pressure high temperature metamorphism and an anticlockwise p t t path for the hazeldene area mount isa inlier queensland australia in the hazeldene area situated in the mount isa inlier queensland the metamorphic grade changes from chlorite zone through biotite and cordierite zones to sillimanite k feldspar zone microstructural studies of rocks near the sillimanite isograd demonstrate that cordierite grew early during the development of a steep foliation s2 was replaced by biotite andalusite and sillimanite at the metamorphic peak late in s2 and in turn by kyanite chlorite adjacent to localized small post d2 shear zones although the anticlockwise p t t path is well defined the precise p t conditions are uncertain because of problems with experimental and thermodynamic data the best estimate for the metamorphic peak for rocks close to the sillimanite isograd is around 600 c at 4 kbar the metamorphism has been dated at 1544 ma and was synchronous with a major crustal shortening event because proposed extensional events occurred more than 60 ma earlier their contribution to the peak metamorphic thermal perturbation would have been insignificant the syn metamorphic mica creek pegmatites the abundance of high heat producing elements in the nearby pre d2 sybella granite and advective heat by fluids which caused considerable metasomatism in the hazeldene area may have each contributed to the thermal budget however the metamorphic thermal gradient may be 80 c km 1 or higher strongly suggesting a local magmatic control as none are known in the area such syn metamorphic plutons would have to lie beneath the exposed high grade rocks

geometry of a middle proterozoic extensional decollement in northeastern australia abstract an 80 km long belt of gneissic and mylonitic rocks is exposed in the core of a major anticlinorium in the middle proterozoic mt isa inlier of northeastern australia these rocks in the mary kathleen fold belt represent an early originally horizontal mid crustal shear zone which is interpreted as an extensional decollement between a lower ductile sheet and an upper more brittle plate the decollement was subhorizontal over a distance of at least 80 km and its depth was perhaps as shallow as 7 km shearing was accompanied by the syn tectonic intrusion of a bimodal complex of granitic and basic rocks over a period of 70 90 m y moderate to extreme ductile shearing was pervasive throughout the 1 1 5 km thickness of the lower plate that is now exposed massive dilation accompanied shearing and approximately 70 of the volume of the shear zone is occupied by igneous intrusive rocks which have been variably deformed to gneissic granite mylonite and amphibolite strain softening driven by this high thermal input is responsible for the pervasive ductile response at such shallow depths peak metamorphic conditions at the decollement were approximately 625 c to 675 c at 2 kbar intense ductile shearing in the upper plate is restricted to the lower few hundred metres above this zone brittle processes dominate although evidence is presented that the sheet as a whole has undergone a component of ductile shearing probably heterogeneously distributed large sheet like granite and dolerite bodies were introduced into dilation sites associated with synthetic faults in this plate but unlike their synchronous counterparts in the lower plate about 1 km below these are internally undeformed the middle proterozoic history of the area involves a cycle of extension and shortening with the shortening axis perpendicular to the earlier extension axis we speculate that the driving mechanism for both extension and shortening is mantle convection with switches in stress related to mantle upwelling

isotopic and structural responses of granite to successive deformation and metamorphism within the sybella granite west of mount isa in northern australia differential response of u pb and rb sr isotopic systems to deformation metamorphism enables ages of emplacement and three discrete deformation metamorphic events to be determined based on zircon u pb systems two of the plutons were emplaced at 1671 pm 8 ma and 1668 pm 26 21 ma and the third at 1610 pm 10 ma a separate microgranite body gives aberrantly old u pb ages of up to 1780 ma due to partial inheritance of older zircon the three discrete deformation metamorphic events d 1 d 2 and d 3 occurred at 1610 pm 13 ma 1544 pm 12 ma and 1510 pm 13 ma respectively based on whole rock rb sr isotopic systems the youngest body was emplaced syntectonically and has possible significance to the origin of the mount isa ag pb zn orebodies homogenization of rb and sr isotopes via extensive syntectonic fluid migration associated with deformation partitioning and resultant solution transfer appears to have been

progressive refolding in ductile shear zones abstract the foliation formed in a ductile shear zone can become folded by continuing shear in the zone without the foliation having to enter the shortening sector of the flow field if the foliation lies parallel to the shear plane infinitesimal variations in the rate of shear strain cause compensating rotations that may amplify into folds small increases in the rate cause backward rotations counter to the sense of shear these do not amplify small decreases cause forward rotations that do amplify

the proterozoic mount isa fault zone northeastern australia is it really a ca 1 9 ga terrane bounding suture abstract in marked contrast to palaeoproterozoic laurentia the location of sutures and boundaries of discrete crustal fragments amalgamated during palaeoproterozoic formation of the north australian craton remain highly speculative interpretations of suture locations have relied heavily on the analysis of regional geophysical datasets because of sparse exposure of rocks of the appropriate age the mount isa fault zone has been interpreted as one such palaeoproterozoic terrane bounding suture furthermore the coincidence of this fault zone with major shale hosted massive sulphide pb zn ag orebodies has led to speculations that trans lithospheric faults may be an important ingredient for the development of this deposit type this study has integrated geophysical and geochemical data to test the statute of the mount isa fault as a terrane bounding suture forward modelling of gravity data shows that basement rocks on either side of the mount isa fault have similar densities these interpretations are consistent with geochemical observations and sm nd data that suggest that basement lithologies on either side of the mount isa fault are geochemically and isotopically indistinguishable from each other and that the mount isa fault is unlikely to represent a suture zone that separates different palaeoproterozoic terranes our data indicate that the crustal blocks on both sides of the mount isa fault zone must have been in within close proximity of each other since the palaeoproterozoic and that the western fold belt was part of the ancestral north australian craton well before the 1 89 1 87 ga barramundi orogeny it appears that deep crustal variations in density may be related to the boundary between a shallowly west dipping high density mafic to ultramafic plate and low density basement rocks this interpretation in turn impacts on crustal scale models for the development of shale hosted massive sulphide pb zn mineralisation which do not require trans lithospheric faults to tap deep seated metal reservoirs and or mantle plumbing systems the approach applied herein demonstrates the value of multi disciplinary investigations to the critical assessment of long lived proterozoic fault systems which in the absence of methodical analysis are commonly assumed to represent terrane bounding sutures

an interglacial on snowball earth dynamic ice behaviour revealed in the chuos formation namibia the sturtian is the oldest ca 716 ma of three pan global glaciations in the cryogenian at omutirapo in northern namibia a 2 km wide 400 m deep palaeovalley is filled by glaciogenic strata of the chuos formation which represents the sturtian glacial record sedimentary logging of an exceptionally high quality exposure permits detailed stratigraphic descriptions and interpretations allowing two glacial cycles to be identified at the base of the exposed succession strong evidence supporting glaciation includes diamictites ice rafted dropstones and intensely sheared zones of interpreted subglacial origin these facies collectively represent ice proximal to ice rafted deposits upsection dropstone free mudstones in the middle of the succession and the absence of diamictites imply sedimentation free from glacial influence however the reappearance of glacial deposits above indicates a phase of sturtian glacial re advance comparison with age equivalent strata in south australia where evidence for sea ice free sedimentation has been established previously suggests that a sturtian interglacial may have been extensive implying global scale waxing and waning of ice sheets during a cryogenian glacial event

an investigation of subglacial shear zone processes from weybourne norfolk uk abstract the glacial sediments of north norfolk are a type site for subglacial deforming bed sediments this investigation focussed on subglacial shear zone process at the field and thin section scale in order to understand subglacial processes as well as considering the implications for regional stratigraphies the sandy and chalky tills from three sites within 1 km at weybourne norfolk showed evidence for subglacial deformation associated with simple shear producing extension compression and rotation it was demonstrated how these processes interacted to cause chalk fragmentation and predictable fabric strengths dependant on sorting and grain size it is suggested that the marly drift is a diachronous unit and the resultant stratigraphy at weybourne reflects one deformation till resulting from a series of ice advances but with a lithology derived from the local chalk bed rock with some far travelled erratics which have undergone different degrees of chalk fragmentation reflecting the nature and duration of the subglacial deformation

successive development of plane noncylindrical folds in progressive deformation in the history of superposed deformations of the iron formations at the western border of the kolar gold field in s india an important event was the successive growth of broadly coaxial plane noncylindrical folds in course of a progressive deformation concomitant with development of ductile mesoscopic shear zones the noncylindrical folds were initiated as active folds by the creation of a buckling instability at successive stages on newly developed foliation surfaces the nucleation of noncylindrical folds and the subsequent axial plane folding of the tightened mature folds are explained by the mechanical inhomogeneity of the rocks and the heterogeneous character of strain the correlation between increasing tightness and increasing noncylindricity of the folds indicates that the initial curvatures of hinge lines were accentuated by an extension parallel to the subhorizontal stretching lineation from the patterns of deformed lineations over folds of varying tightnesses it is concluded that the passive accentuation of hinge line curvatures was mostly achieved when the folds had already become isoclinal or very tight

heterogeneous exhumation of the mount isa orogen in ne australia after 1 6 ga nuna assembly new high precision 40ar 39ar thermochronological constraints

deformation progressive or multiphase abstract complexly deformed rocks that exhibit refolded folds and overprinting planar fabrics are often interpreted as the result of polyphase deformation and described in a scheme of deformation phases d 1 d 2 d n at the same time a number of models have been presented over the last decades that explain how complex and overprinting structures can form by single phase progressive deformation as strain accumulates strain generally accumulates heterogeneously in a deforming region depending on strain intensity and geometry flow parameters rheology and lithology the result is regions of different structural complexity fabrics and fabric orientations that can be sorted into different structural facies such structural pattern should be mapped and presented before defining a scheme of deformation phases and attempts should be made to explain them by a single structural kinematic model such as partitioned transpression only if this is not possible or if independent geochronologic data reveals a more punctuated history should we define an additional tectonic regime and deformation phase multiphase and progressive deformation histories are not mutually exclusive and it is important that both possibilities are considered when describing the deformation history of any complexly deformed region when the concept of deformation phases is employed the number of phases should be minimized and be related to major tectonic event of external character uncritically defining deformation phases can easily generate a complicated discrete deformation history with no link to tectonic reality

the elatina glaciation late cryogenian marinoan epoch south australia sedimentary facies and palaeoenvironments abstract the late cryogenian elatina glaciation in south australia of marinoan age is named after the elatina formation of the glaciogenic yerelina subgroup which covers 200 000 km2 in the adelaide geosyncline and on the cratonic stuart shelf and is up to 1500 m thick the elatina glaciation is marked by numerous facies that are like those of phanerozoic glaciations basal diamictite displaying glacitectonites with penetrative deformation of preglacial beds indicating grounded ice or scouring by icebergs glaciomarine diamictites containing numerous faceted and striated clasts up to several metres across of intrabasinal and extrabasinal origin laminated siltstone and mudstone containing scattered ice rafted dropstones in outer marine shelf environments sandstones deposited in fluvial deltaic and inner marine shelf settings tidalites deposited in estuaries and on tidal deltas during interstadial rise of sea level with cyclic tidal rhythmites recording the annual oscillation of sea level and displaying wave generated ripple marks which together indicate long lived and extensive open seas siltstone containing acicular crystal pseudomorphs implying the formation of evaporite minerals in littoral deposits permafrost regolith of frost shattered quartzite breccia 20 m deep displaying large scale periglacial structures including primary sand wedges 3 m deep that indicate a frigid strongly seasonal climate near sea level periglacial aeolian sandstone covering 25 000 km2 and containing primary sand wedges near its base these deposits record a spectrum of settings ranging from permafrost regolith and periglacial sand sheet on the stuart shelf in the west through fluvial deltaic and inner marine shelf in the western and central parts of the adelaide geosyncline to outer marine shelf in the north and southeast the widespread and persistent rainout of fine grained sediment and ice rafted debris indicates that the sea was not frozen over during the elatina glaciation no direct age determination is available for the elatina glaciation and only maximum and minimum age limits of 640 and 580 ma respectively can at present be applied high quality palaeomagnetic data for red beds from the elatina formation supported by positive fold tests on soft sediment slump folds that demonstrate the early acquisition of magnetic remanence indicate deposition within 10 of the palaeoequator the yerelina subgroup in the adelaide geosyncline and on the stuart shelf is disconformably to unconformably overlain by the nuccaleena formation cap carbonate that marks the early ediacaran post glacial marine transgression the presence in near equatorial palaeolatitudes of glaciomarine deposition grounded ice and permafrost near sea level a strongly seasonal periglacial climate and widespread open seas implies a paradoxical palaeoclimate and palaeogeographic setting for the elatina glaciation the strong evidence for a non actualistic late cryogenian glacial climate in south australia has been a stimulus to worldwide multidisciplinary research on cryogenian glaciogenic successions

late proterozoic glacial carbonates in northeast spitsbergen new insights into the carbonate tillite association carbonate rich glacial deposits from two discrete vendian glacial periods are described the older is represented by the 24 40 m thick petrovbreen member e2 of the elbobreen formation which contains abundant detrital dolomite clasts in e2 and their possible source rocks have positive 13 c and negative 18 o pdb values in contrast the carbonate mud fraction of e2 sediments has different cathodoluminescence characteristics from clasts slightly negative 13 c values and higher fe and mn concentrations than clasts oxygen isotopes vary from 2 5 to 4 5 pdb thought to be related to various seawater meltwater mixtures in the depositional environment preservation of information about glacial sedimentary environments is attributed to early diagenetic recrystallization forced by excess surface free energy ostwald s ripening and coinciding with sulphate reduction the younger glaciation is represented by the wilsonbreen formation 170 m which has a distinctive glaciolacustrine middle carbonate member w2 w2 contains precipitated periglacial carbonates with high mn fe ratio both limestone rhythmitic and stromatolitic and dolostone rhythmites stromatolites and dolomite rich sandstones which are compared with carbonates in modern antarctic lakes evaporitic environments for dolomitic sandstones and stromatolitic dolostones are indicated by heavy oxygen isotope values up to 10 5 pdb high na concentration and evidence for dissolved evaporites high mn concentration in detrital dolostones in w2 is suggestive of syn sedimentary dolomite recrystallization in freshwater diamictites and haematitic siltstones the carbonate tillite association ultimately arises from the erosion of underlying carbonates which originated in this case under radically different climatic conditions glacial depositional waters then became carbonate saturated as a result of dissolution of detrital carbonate massive recrystallization of glacially transported carbonate is proposed as a geologically significant process with considerable potential for palaeoenvironmental analysis in glacial lakes carbonate precipitated in response to evaporation or photosynthesis carbonate precipitation as the result of seawater freezing or in warm interglacial conditions is not yet established oxygen isotope value are inconsistent too heavy with the presence of high latitude meltwaters implying that glaciation extended to low latitudes as proposed by harland

proterozoic extensional deformation in the mount isa inlier queensland australia the earliest of four distinct phases of deformation recognized in the central part of the proterozoic mount isa inlier involved brittle extensional faulting at shallow crustal levels extensional faulting produced stacks of imbricate fault slices listric normal faults and characteristic tourmalinerich breccias structures belonging to this phase occur over a large part of the inlier and indicate an important phase of basin forming crustal or lithospheric extension at 1750 1730 ma late intense ductile deformation and tight folding of the imbricate systems destroyed part of these older structures and obscures their existence in many parts of the inlier

analysis and interpretation of composite foliations in areas of progressive deformation abstract in areas of progressive deformation where successive structures develop during a relatively continuous deformation within a geologically short time period traditional chronological notations of structural elements e g s 1 s 2 etc can give erroneous impressions of how large rock masses evolve in time and space we demonstrate from field examples that successive structures can develop which a are comparable in morphology and orientation but of different ages b are different in morphology but of comparable age and c show rapid morphological changes over short distances under such conditions correct identification of the relative age of structures is often difficult to impossible we consider the concepts of a composite foliation and of transposition cycles as vehicles to objectively evaluate the significance of different sets of structures in the evolution of larger rock masses we suggest that 1 structural elements be labeled using morphological notation adding numerical subscripts only when independent evidence is available 2 geologists more fully acknowledge and integrate the concept of deformation partitioning into their models and 3 when analyzing areas of multiple deformation more emphasis is placed on the relationship between domains of differing complexity integration of these three perspectives in the analysis will lead to a more realistic basis upon which to model the structural evolution of large rock masses

polyphase deformation in the western mount isa inlier australia episodic or continuous deformation abstract the polyphase deformation history of the mount novit ranges western mount isa inlier australia involved the development of four generations of n s to nw plunging folds development of which alternated with movement on n s striking faults shear zones with west over east displacement the spatial relationships between successive structures and the distinct similarities in the kinematics style and asymmetry are all consistent with continuous deformation during e w to ne sw shortening additional support for this hypothesis is based on the development of the three major fold generations and the intervening faults and shear zones during low p high t metamorphism although the duration of this metamorphic event cannot be constrained recent studies have linked low p metamorphism to heat advection by migrating melts thermal modelling indicates that such metamorphic events will be short lived in comparison to those associated with thermal relaxation of over thickened crust low p metamorphism is unlikely to persist through two or more separate tectonic events therefore syn metamorphic polyphase deformation in the mount novit ranges apparently formed during a single continuous tectonic episode this structural history differs from both the regional scheme and that of previous workers in this area the main differences are i no evidence was found for early thrust faulting prior to n s folding ii there are four generations of n s to nw se folds as opposed to two and iii this study favors a single consistent tectonic regime in contrast to the interpretation of three discrete isolated tectonic events

a new microstructural mapping methodology for the identification analysis and interpretation of polyphase deformation within subglacial sediments although integrated clast macro and microfabric studies have been used to provide evidence of palaeo ice flow directions to discriminate between diamictons deposited in differing sedimentary environments and to model the response of tills to subglacial deformation such studies have typically only examined the bulk fabric developed within the diamicton largely ignoring the potential for several phases of fabric development within these typically polydeformed deposits we present a new graphical method for the identification description and analysis of clast microfabrics in polydeformed tills this method utilises high resolution digital scans of orientated thin sections imported into a standard computer graphics package to measure the orientation of long axes of detrital skeleton grains included within the till these data are then used to identify the main clast microfabrics present within the till allowing the delineation of clast microfabric domains in which elongate grains share a similar preferred orientation this process leads to the identification characterisation and most importantly the interrelationships between the various generations of microfabrics the relationships between these clast microfabrics and other microstructures e g plasmic fabrics turbate structures shears folds faults etc can also be examined providing a detailed microfabric microstructural map of the thin section clast long axis orientation data collected for different parts of a thin section are plotted on a series of rose diagrams allowing the variation in clast microfabric orientation to be displayed graphically a new set of terminology for the description of clast microfabrics in tills which is based upon the system used by metamorphic petrologists is also presented three case studies of the fabrics present within subglacial sediments of pleistocene age as well as the present are used as examples of how to apply this methodology

does porewater or meltwater control tunnel valley genesis case studies from the hirnantian of morocco several ordovician tunnel valleys are exposed in the moroccan anti atlas mountains including the alnif and the foum larjamme tunnel valleys located 150 km away from each other sedimentological and deformational analyses of these two glacial troughs reveal that differing processes lead to their formations the alnif tunnel valley contains numerous deformation structures within sediments both below and above the main glacial erosion contact surface ball structures and clastic dykes occur within preglacial sediments down to 35 m below glacial incisions while overlying glacial sediments contain fluted surfaces clastic dykes dewatering structures folds and radial step normal faults the characteristics of the alnif tunnel valley can be explained by a porewater pressure driven model of formation where the localized increase of basal shear stress and porewater pressure underneath subglacial deforming zones lead to the development of a dense hydrofracture network in the preglacial bed these processes of hydraulic brecciation promoted subglacial remobilization of the preglacial material and contributed to the formation of the tunnel valley the foum larjamme tunnel valley displays undisturbed preglacial sediments and few dewatering structures at the base of the glacial sedimentary infill which suggests relatively low porewater pressures within the tunnel valley during formation this second type of tunnel valley where porewater pressure remained relatively low appears to have been formed by meltwater erosion the undulating base of the foum larjamme tunnel valley implies progressive erosion by a stable subglacial braided network of nye channels or alternatively by channels migrating laterally during episodic minor subglacial outbursts these two tunnel valleys highlight the regional variability of processes involved in the formation of tunnel valleys the distribution of palaeo ice streams in north africa illustrate that morphologies and processes involved in the formation of tunnel valleys vary between ice stream and inter ice stream zones due to variations in meltwater availability the topography and bed lithological properties

giant cross beds in the neoproterozoic port askaig formation scotland implications for snowball earth abstract the neoproterozoic port askaig formation is a thick succession of interbedded diamictite conglomerate sandstone and mudstone that records glacially influenced marine sedimentation in the extensional dalradian basin giant cross beds individual units up to 11 m thick are observed in sandstone at the base of member iii of the port askaig formation the cross bedded sandstones are closely associated with other sandstone diamictite and conglomerate facies and are characterized by both planar and trough shaped foreset laminae with an average maximum dip angle of 14 the cross bedding has compound internal structure and indicates a predominant paleocurrent direction towards the south the sandstone is medium grained well sorted and quartzitic the giant cross beds are interpreted to have formed as a result of the migration of large subaqueous simple and compound dunes in a marine setting based on sedimentological analysis of the sandstone and associated facies as well as the tectonic setting of the port askaig formation the giant dunes are thought to have formed under the influence of tidal currents and indicate open marine ice free depositional conditions the sedimentology of the cross bedded sandstone and its close association with diamictite and conglomerate indicate repeated ice margin advance and withdrawal with periods of both ice free and ice influenced conditions the environmental conditions recorded in these port askaig deposits appear inconsistent with deep freeze or rapid meltback associated with a snowball earth

neglected basement ductile deformation in balanced section restoration an example from the central southern alps northern italy abstract in the construction of balanced sections through thick skinned belts basement bodies are frequently assumed to be rigid and internally undeformed with the exception of regional scale fault bend folding and only backstripping of thrust faults is performed during their retrodeformation possibly leading to underestimations of the regional shortening this simplifying assumption is generally made because reliable information on internal strain in basement is lacking and the sedimentary cover of the internal parts of thrust belts has been removed by erosion in a basement exposure in the eastern orobic alps alpine age ductile structures mainly chevron to sub isoclinal folds rarely associated with cleavage formation were recognized on the basis of overprinting criteria folding style and fabric orientation and their areal distribution was determined by foliation trace mapping although unevenly distributed in different lithologies ductile deformation is considerable average shortening is 47 neglecting such internal strain during construction of balanced sections leads for this area to an underestimation of at least 10 but could be more than 40 if the observed basement shortening is extrapolated to other basement involved thrust bodies across the entire width of the deformed belt it is concluded that detailed structural studies of the basement for example those using a foliation trace mapping technique are necessary to define the internal strain of basement bodies so that the shortening of thick skinned belts can be more accurately calculated

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

thrusting and duplex formation at mount isa queensland australia large scale repetition of mount isa and haslingden group rocks in the anticline north of mount isa is interpreted as the core of a thrust duplex which has been subsequently folded parallel to the direction of thrusting displacement of mount isa group rocks above the duplex is extremely significant to the regional geology and exploration for more mount isa style pb zn and cu orebodies interpretation of the geometry of the thrust belt indicates thrusting on different stratigraphical levels below and above the duplex during its formation horses formed from a heelwards protruding footwall ridge instead of a trough and the complex effects on stratigraphical cross sections through duplexes of sloping lateral footwall ramps on horses

subglacial till formation sedimentary characteristics and classification we review the major subglacial till forming processes as presently understood by glacial researchers and define the parameters within which tills are produced and reconcile them with sedimentary end members processes of deformation flow sliding lodgement and ploughing coexist at the base of temperate glacier ice and act to mobilize and transport sediment and deposit it as various end members ranging from glacitectonically folded and faulted stratified material to texturally homogeneous diamicton the dominance of any one subglacial process varies both spatially and temporally giving rise to the possibility that a till or complex till sequence contains a superimposed signature of former transportation deposition at the ice bed interface we recommend that while glacial geologists and geomorphologists should be able to recognize the sedimentary imprints of various subglacial processes genetic fingerprinting of subglacial tills should be less process specific and till classification must reflect the range of products encompassed by the subglacial till production continuum glacial geologists can presently unequivocally identify a glacitectonite rock or sediment that has been deformed by subglacial shearing deformation but retains some of the structural characteristics of the parent material b subglacial traction till sediment deposited by a glacier sole either sliding over and or deforming its bed the sediment having been released directly from the ice by pressure melting and or liberated from the substrate and then disaggregated and completely or largely homogenised by shearing and support the theoretical case for c melt out till sediment released by the melting of stagnant or slowly moving debris rich glacier ice and directly deposited without subsequent transport or deformation because observations on contemporary glaciers reveal that their beds are most likely to be mosaics of deformation and sliding and warm based and cold based conditions the patterns of which change temporally and spatially it is extremely unlikely that subglacial till end members in the geological record will be anything but hybrids produced by the range of processes operative in the subglacial traction zone

evidence for early extensional tectonics in the proterozoic mount isa inlier australia in the proterozoic mount isa inlier queensland australia structures belonging to an early brittle deformation event are overprinted and strongly disturbed by a phase of ductile east west shortening many of these early structures have been attributed to thrusting but data from the southern part of the inlier provide new evidence for extensional faulting analysis of the internal geometry of imbricates can provide a tool for the reconstruction of these strongly reworked precambrian fault belts

deformation pattern and evolution of the structures in the penganga group the pranhita godavari valley india probable effects of grenvillian movement on a mesoproterozoic basin abstract recent studies reveal imprints of tectonic deformation in the mesoproterozoic rocks of the pranhita godavari p g valley india outcrop scale structures that record a progressive sequence of deformation can thus be used to clarify the kinematic evolution of the penganga group a mesoproterozoic succession in the northern and central parts along the sw marginal proterozoic belt of the pg valley overprinting relationships among the structures in the northern part indicate three distinct structural stages 1 layer parallel shortening 2 thrusting and folding in combination and 3 extensional joint and fracture opening the former two stages are explained in the framework of a progressive deformation model the extensional structures were produced during exhumation similar structures are also developed in the central part of the sw belt of the pg valley though the two areas slightly differ in the overall pattern of deformation the geometry of the progressive deformation structures from both regions of the belt suggests that they were produced under a regime of ne sw ene wsw directed regional compression the deformation in the proterozoic basin is possibly related to the grenvillian orogeny recorded in the eastern ghats mobile belt and may represent a small scale meso neoproterozoic convergence event