observations and geologic significance of marine snow in a shallow water partially enclosed marine embayment because of their fragile nature marine snow sized aggregates aggregates 0 5 mm in longest dimension have been generally overlooked in shallow water environments we report in situ observations and measurements in cape lookout bight north carolina that indicate that marine snow is probably always present and usually abundant underwater photographs show that over a period of hours marine snow varies in size from 8 to 71 10 5 ml and in abundance from 291 to 489 l 1 but that such variations in size and abundance are not obviously related to tide total suspended sediment concentration or current speeds in the 10 to 20 cm s 1 range photographs and concurrent transmissometer records suggest that variations in size shape and abundance of marine snow may be sufficiently great that particulates do not remain optically constant and thus affect beam transmission irrespective of total suspended sediment concentration levels marine snow is significant geologically in that it has high settling rates 50 to 200 m per day and may be the package in which much of the vertical mass flux occurs if so it offers an explanation for the high accumulation rates observed in coastal and estuarine fluid mud deposits

lead 210 sediment geochronology in a changing coastal environment abstract sediment accumulation rate studies utilizing excess 210 pb and 137 cs were conducted as part of recent investigations of biogeochemical cycling at a single site in cape lookout bight a rapidly changing coastal basin on the outer banks of north carolina u s a cores three meters in length reveal a depositional history for the bight interior characterized by a gradual transition in texture from coarse grained to fine grained material over the period 1946 1979 this transition is controlled by progressive enclosure of the bight by an active northerly migrating recurved spit the textural gradation is periodically interrupted by layers of well sorted sand associated with major storm events lead 210 data indicate that the upper meter of the sediment has accumulated at a rate of 3 35 to 4 71 g cm 2 yr 1 or approximately 8 4 to 11 8 cm yr 1 at o 0 84 below 120 cm depth dilution of clay and silt by low activity sand necessitates correction of the 210 pb profile in order to establish a geochronology grain size 210 pb distribution measurements at three depths reveal that the specific activity dpm g 1 of clay is 3 2 times that of silt and 24 7 times that of sand corrections of bulk sediment excess 210 pb activities based on these measurements lead to dates for textural changes which are consistent with charted changes in basin morphology and major storm events sixteen 137 cs measurements between 33 241 cm depth reveal a peak activity at 105 115 cm and indicate a minimum sedimentation rate of approximately 2 7 g cm 2 yr 1

1 8 1 5 ga links between the north and south australian cratons and the early middle proterozoic configuration of australia abstract the archaean and early middle proterozoic 1 8 1 5 ga terranes of the north australian craton and the south australian craton are separated by 400 km of ca 1 33 1 10 ga orogenic belts and phanerozoic sediments however there is a diverse range of geological phenomena that correlate between the component terranes of the two cratons and provide evidence for a shared tectonic evolution between approximately 1 8 and 1 5 ga in order to honour these correlations we propose a reconstruction in which the south australian craton is rotated 52 counterclockwise about a pole located at 136 e and 25 s present day coordinates relative to its current position this reconstruction aligns the ca 1 8 1 6 ga orogenic belts preserved in the arunta inlier and the gawler craton and the ca 1 6 1 5 ga orogenic belts preserved in the mount isa block and the curnamona province before 1 5 ga the south australian craton was not a separate entity but part of a greater proto australian continent which was characterised by accretion along a southward migrating convergent margin ca 1 8 1 6 ga followed by convergence along the eastern margin ca 1 6 1 5 ga after 1 5 ga the south australian craton broke away from the north australian craton only to be reattached in its current position during the ca 1 33 1 10 ga albany fraser and musgrave orogenies

lithospheric structures and possible processes in phanerozoic eastern australia from deep seismic investigations abstract the eromanga basin in eastern australia developed during four major episodes on the rifted eastern margin of the australian precambrian craton seismic reflection profiling together with coincident wide angle reflection refraction profiling indicates structures and compositions in the crust which are dominant in various depth ranges some structural features seem to be limited to the upper crust other deep reflection events and high crustal velocities predominate in the lower crust with mafic intrusion from upper mantle sources being one preferred interpretation these features emphasize the difference in structures and composition between the upper and lower parts of the crust yet other structural and deep reflection features are interpreted as being truly lithospheric in scale because they seem to have seismic expression right from the surface to the upper mantle similar interpretations are made for other continental regions indicating that lithospheric processes affect different depth ranges of the crust and upper mantle in a variety of ways any modelling of such processes in the lithosphere must lead to these quite different end results at appropriate depth ranges

the tectonostratigraphic history of the proterozoic northern lawn hill platform australia an integrated intracontinental basin analysis abstract the northern lawn hill platform nlhp comprises an area of approximately 16 000 km2 in north central australia thick packages of proterozoic strata outcrop in the region although large areas are also covered by younger strata the packages contain basin sediments and igneous rocks that span 400 m y we present a regional synthesis of approximately 150 m y of the tectonostratigraphic history of the area which hosts several world class base metals deposits the interpretation is based primarily on a sequence stratigraphic and structural interpretation of seismic data a common approach in the petroleum resource sector but rare in analyses of mineral resources models of subsurface geology and a basement template are provided using observed geopotential data that are iteratively calibrated with other data geochronological palaeomagnetic geochemical and sequence stratigraphic investigations on outcrop and drill core also are integrated into the history the seismic data allow separated outcrop belts to be confidently correlated for the first time geochronological data control the basin history delineated by this interpretation palaeomagnetic data allow the results presented here to be extrapolated regionally and globally although considered to be deposited on entirely intracontinental lithosphere the basin phases record significant fluctuations in the volume and geometry of accommodation space for sedimentation multiple pulses of deformation are recorded by changing depositional geometries we propose a history of re activation of a basement template and speculate on geodynamical mechanisms for accommodation space fluctuations and geometric variability the detailed understanding of the basin evolution of the nlhp gained by integrated basin analysis provides a powerful tool for predictive fluid flow models as an aid to resource exploration it should also provide a useful analogy in understanding tectonic processes and basin evolution of other similar aged intracontinental basins

dynamics of continental accretion three dimensional dynamic computer models show how accretionary tectonic margins evolve from the initial plate collision state through a period of plate margin instability and then re establish a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region subduction zones where one plate dives under another become congested when they try to accommodate buoyant exotic crust louis moresi et al describe new numerical models of continental accretion that follow the entire process from the initial collision state through a period of plate margin instability to the re establishment of a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region subduction zones become congested when they try to consume buoyant exotic crust the accretionary mountain belts orogens that form at these convergent plate margins have been the principal sites of lateral continental growth through earth s history modern examples of accretionary margins are the north american cordilleras and southwest pacific subduction zones the geologic record contains abundant accretionary orogens such as the tasmanides1 along the eastern margin of the supercontinent gondwana and the altaides which formed on the southern margin of laurasia2 in modern and ancient examples of long lived accretionary orogens the overriding plate is subjected to episodes of crustal extension and back arc basin development often related to subduction rollback3 and transient episodes of orogenesis and crustal shortening4 5 6 7 coincident with accretion of exotic crust here we present three dimensional dynamic models that show how accretionary margins evolve from the initial collision through a period of plate margin instability to re establishment of a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region the complexity of the morphology and the evolution of the system are caused by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonally to the convergence direction we find geological and geophysical evidence for this process in the tasmanides of eastern australia and infer that this is a recurrent and global phenomenon

measurement of size and settling velocity of suspended aggregates on the northern california continental shelf abstract a high resolution video camera attached to a miniature sediment trap was used to make time lapse images of suspended aggregates settling through the water column the video trap system was placed 2 m above the seabed in the strataform study area off eureka california and was programmed to take a 7 second time sequence of settling particles every 6 h over a 4 month period the camera field of view within the trap is 15 mm and the minimum particle resolution is approximately 130 m data analysis was carried out with a video frame grabber and digital imaging software to determine the particle size shape and settling velocity results of the first deployment of this system september 1995 show that particle sizes elliptical nominal diameter determined from the video images ranged from 130 to 740 m with a modal size of 300 m the average form factor ratio of short to long semi axis of the aggregates was 0 71 measured settling velocities varied from 0 09 to 8 13 mm s in terms of weight percent or mass the median size of the aggregate distribution is 600 m analysis of settled particles collected from the sediment trap shows disaggregated or component grains ranging from 1 0 to 63 m with a modal size of 26 m and a median size of 11 m additionally an analysis was carried out to separate the relative importance of particle shape and excess density on settling velocity and these results were compared to observations using the equations for settling velocity of spheres and prolate spheroids and substituting published relationships for size versus excess density into these equations the settling velocity for combinations of shape and excess density has been calculated results show that differences between sphere and prolate spheroid with major axis both parallel and perpendicular to flow approximations caused less than a 13 difference in computed settling velocity in contrast the various size excess density relationships used to compute settling velocity caused order of magnitude deviations from measured values in all cases however the approximations of settling velocity for aggregates provide a significantly better fit to data than approximations based on disaggregated sediment size distribution

seismic imaging of major tectonic features in the crust of phanerozoic eastern australia abstract crustal architecture can be recognised from seismic fabric characteristics and velocity information across phanerozoic eastern australia three major crustal sub divisions are identified from data in southern queensland they occur under west to east the nebine ridge taroom trough and new england fold belt with each crustal sub division having its own internal architecture but separated by major bounding structures geosutures these features are assessed against evolutionary models for the region the significant conclusions are as follows 1 1 the nebine ridge forms the southeastern part of the thomson fold belt but has a crustal architecture distinctly different from that under the central part of the fold belt tentatively supporting an evolutionary model involving fragmentation and eastward stretching and thinning of a precambrian australian craton the ridge crust has possibly been considerably altered by carboniferous plutonism and compressional events 2 2 between the nebine ridge and the crust under the taroom trough of the bowen basin there is a major low angle 5 10 geosuture the foyleview geosuture which extends from upper to lower crustal levels along a series of prominent horizons it was probably active during the major carboniferous tectonism which severely structured devonian basins west of the nebine ridge the geosuture forms the southeastern boundary of the thomson fold belt 3 3 the crustal architecture under the taroom trough has little obvious structure above an outstanding moho transition zone however the prominent meandarra gravity ridge striking along the trough axis highlights the possibility of high density mafic volcanics within the crust and the probability of a rift model being appropriate early in the trough s history the sedimentary basin architecture supports the concept of an early extensional history for the crust within a poorly defined n s trending strike slip regime 4 4 between the taroom trough and the new england fold belt we interpret a high angle geosuture extending through the crust the burunga mooki geosuture reactivation during late palaeozoic early mesozoic times produced a series of reverse faults along the uplifted eastern margin of the trough some of these structures are tentatively considered to be positive flower structures above deeper faults with strike slip movement the geosuture separates regions with distinctly different seismic fabrics and played an important role in the formation of the bowen gunne dah sydney basin system 5 5 within the new england fold belt crustal domains can be recognized associated with late palaeozoic oroclinal bending and early mesozoic trans tensional basins major oroclinal bending of the upper crust in the west of the fold belt is interpreted above a mid crustal detachment the texas detachment in the central part of the fold belt the seismic data support a trans tensional mechanism for the formation of triassic basins with steeply dipping bounding faults under the east of the fold belt an imbricate thrust stack accretionary wedge is clearly imaged above a mid crustal horizon the brisbane mid crustal detachment upper crustal deformation appears to have occurred above this detachment 6 6 from the nebine ridge to the coast the moho is clearly defined below a 3 km thick moho transition zone hast of the thomson fold belt the moho level is identified as a gently undulating feature at 36 38 km depth and probably re established after the major late palaeozoic tectonic events which formed the crust under the taroom trough and new england fold belt this contrasts with the middle palaeozoic lower crustal moho features which appear to be preserved under the thomson fold belt 7 7 the thickest crust under the nebine ridge about 44 km appears to be associated with lower crustal wedging from the east there is an apparent thinning of the non sedimentary crust under the deepest basin taroom trough of about 30 compared with the crust to the west and east no crustal thickening is evident in the region of oroclinal bending within the new england fold belt

the 1800 1610 ma stratigraphic and magmatic history of the eastern succession mount isa inlier and correlations with adjacent paleoproterozoic terranes the chronostratigraphic and magmatic framework for the proterozoic mount isa inlier northwest queensland has been substantially revised to match insights provided by isotopic age constraints accumulated over recent years this material and existing chronometric constraints is used to construct a new time space plot for the inlier and to compare eastern succession volcanosedimentary accumulations with temporally equivalent units in the western fold belt the accumulation of volcanosedimentary sequences in the eastern succession occurred in four main periods namely 1890 1850 ma 1800 1750 ma 1710 1690 ma and 1680 1610 ma temporally equivalent units can be identified in the western fold belt periods of sedimentary accumulation were immediately preceded by bimodal igneous activity across the inlier the locus of bimodal volcanism migrates from west to east with time intracontinental rifting is the most likely tectonic setting for the sedimentary basins the majority of the soldiers cap group was deposited between ca 1675 and 1650 ma with the exception of units in the boomarra horst now known to belong to older sequences deposited ca 1775 1750 ma and the llewellyn creek formation at snake creek which may be as old as ca 1690 ma upper amphibolite facies rock units in the southwest e g the gandry dam gneiss are likely to be higher metamorphic grade temporal equivalents of the soldiers cap group five rock units previously assigned to the cover sequence 2 mary kathleen group namely the answer slate kuridala formation staveley formation marimo slate and agate downs siltstone are now known to have been deposited between ca 1675 and 1610 ma these units bar the kuridala formation have been removed from the cover sequence 2 mary kathleen group and given the informal name young australia group the kuridala formation has been included in the soldiers cap group the rock units within the young australia group are likely to be lateral facies equivalents of the younger units within the soldiers cap group rock units within the mount albert group although undated are considered likely to be temporal equivalents of the soldiers cap group and the young australia group the roxmere quartzite is included here in the young australia group that it tectonically overlays the ballara quartzite and mitakoodi quartzite represent a single regionally extensive sequence deposited at ca 1755 ma the double crossing metamorphics previously mapped as basement are likely to correlate with the argylla formation and mitakoodi quartzite most of the eastern succession to the west of the pilgrim fault is likely to have been a topographic high between ca 1780 and 1755 ma prior to the ballara quartzite unconformably overlying the ca 1780 ma argylla formation detrital zircons of argylla and wonga ages indicate that the mary kathleen belt is likely to have been a topographic high at least periodically during accumulation of the soldiers cap and young australia groups soldiers cap group units in the southeast contain numerous detrital zircons giving ages between ca 1730 and 1710 indicating a possible siliciclastic source to the east the host rocks to the pb zn ag deposits are restricted to cover sequence 3 units deposited between ca 1680 and 1590 ma and with the exception of century young from the southeast towards the northwest cu au deposits and prospects are hosted in units from all the major periods of volcanosedimentary accumulation however many of the most significant deposits are found within the younger sequences

basal slip and mechanical anisotropy of biotite the basal slip systems of biotite and their mechanical expressions have been investigated by shortening single crystals oriented to maximize and minimize shear stresses on 001 samples loaded at 45 to 001 exhibit gentle external rotations associated with dislocation glide high angle kink bands in these samples unlike those developed in micas loaded parallel to 001 are limited to sample corners samples shortened perpendicular to 001 show no evidence of nonbasal slip and fail by fracture over all conditions tested the mechanical response of biotite shortened at 45 to 001 is nearly perfectly elastic plastic stress strain curves are characterized by a steep elastic slope a sharply defined yield point and continued deformation at low mostly 1 stresses measured beyond the yield point are insensitive to confining pressure over the range 200 to 500 mpa and exhibit weak dependencies upon strain rate and temperature assuming an exponential relationship between differential stress d and strain rate e of the form e cexp d exp q rt the data collected over strain rates and temperatures of 10 7 to 10 4 s 1 and 20 to 400 c respectively are best fit by an exponential constant of 0 41 0 08 mpa 1 and an activation energy q of 82 13 kj mol a power law fits the data equally well with n 18 4 and q 51 9 kj mol samples oriented favorably for slip in directions 100 and 110 are measurably weaker than those shortened at 45 to 010 and 310 consistent with the reported burgers vectors 100 1 2 110 and 1 2 110 the anisotropy of biotite is further revealed by contrasting these plastic strengths with results of samples deformed parallel and perpendicular to 001 previous studies have shown that biotite loaded in the 001 plane is strong prior to the nucleation of kink bands the strength of biotite shortened perpendicular to 001 exceeds that measured parallel to 001 and is pressure dependent application of the results to deformation within the continental crust suggests that biotite oriented favorably for slip is much weaker than most other silicates over a wide range of geologic conditions its presence within foliated rocks and shear zones may limit locally the stresses that can be supported

aeromagnetic patterns of half graben and basin inversion implications for sediment hosted massive sulfide pb zn ag exploration the inverted extensional basins of the palaeoproterozoic north australian craton are amongst the most metallogenically endowed on earth for sediment hosted massive sulphide shms pb zn ag deposits faults that evolved during the evolution of these basins are an important exploration vector as they have behaved as conduits for mineralizing fluids controlled facies architecture and impacted on hydrothermal convection cells we attempt to map out ancient normal faults using integrated structural and aeromagnetic analysis to identify faults that are either buried beneath younger cover or have been inverted during later orogenic events this technique can be used for targeting potential conduits for mineralising fluids during shms pb zn ag exploration as well as a tool to map the architecture of extensional basins we have constructed simplified cross sections of half graben and inverted half graben and have forward modelled their magnetic response the aeromagnetic signature of a pre inverted half graben shows that the tilt block is characterised by a shallow magnetic gradient related to the shallowing of the magnetic marker beneath the half graben normal offset of the marker unit across a normal fault is characterised by a steeper magnetic gradient as the depth to the marker horizon increases simplified cross sections of isolated growth anticlines and the hanging wall buttressing show that the general shape of the magnetic profiles are maintained until the half graben has been completely inverted and the magnetic marker horizon displays reverse offset along the reactivated normal fault

sedimentation in the albemarle pamlico lagoonal system synthesis and hypotheses abstract relative to its size and in terms of its sedimentary processes the albemarle pamlico lagoonal system is one of the least studied coastal lagoons in the united states a synthesis of the current knowledge about sedimentation in the albemarle pamlico system is presented based on archived data published over the last 30 years and new unpublished data collected over the past two years the literature reflects considerable past research effort on sediment grain size and mineralogy but very little on sediment dynamics or flux of suspended particulate material the physiographic complexity of the system which includes barrier islands a deep central basin four river systems and extensive fringing embayments does not appear to be reflected in the form of sedimentologic complexity in general the surficial cover of sediments ranges from medium sands in the inlets and on the shoals of the outer banks to fine silts and organic rich clays in the central basin and embayed river mouths and channels the transition zones from sands to muds are typically sharp and are usually related to bathymetry it is concluded that 1 parts of the albemarle pamlico lagoonal system may have high sedimentation rates relative to rates of sediment input and that rapid vertical flux by large aggregates offers an explanation for the fluid mud deposits that would otherwise be absent 2 fine grained sediments that escape the estuarine sediment trap are confined to pamlico sound by the outer banks barrier islands and are simply recycled until finally coming to rest in the deep basin 3 short term advective processes are secondary to longer term processes such as sea level rise and barrier island migration in the net advection of albemarle pamlico sediments and 4 at the present rate of terrigenous sediment input and relative sea level rise the albemarle pamlico lagoonal system will never reach a sediment filled state

determination of elastic constants of a single crystal topaz and their temperature dependence via sphere resonance method abstract water and halogens in ocean floor sediments transported by a descending slab might play important roles in geodynamic processes imaging subducted sediments through seismological observations requires a thorough understanding of elastic properties of sediment origin hydrous minerals topaz is a sediment origin hydrous mineral which is formed at the depth of 250 350 km on a cold subducting slab we determined elastic constants and their temperature derivatives of a natural single crystal of topaz al 1 97 sio 4 f 1 56 oh 0 42 at the temperature from 271 5 to 312 7 k by using the sphere resonance method elastic constants at an ambient temperature t 291 9 k are c 11 281 21 1 gpa c 22 346 23 9 gpa c 33 294 99 9 gpa c 44 108 49 1 gpa c 55 132 47 1 gpa c 66 130 32 1 gpa c 12 121 48 3 gpa c 13 80 94 3 gpa and c 23 81 77 2 gpa since our sample al 2 sio 4 f 1 56 oh 0 42 was relatively rich in fluorine only small differences in elastic constants can be seen between our sample and fluorine end member elastic constants of oh rich topaz should be experimentally investigated to understand the influence of f oh substitution on elasticity of topaz all the elastic constants decrease linearly with increasing temperature the temperature derivatives are dc 11 dt 0 014 3 gpa k dc 22 dt 0 010 7 gpa k dc 33 dt 0 021 5 gpa k dc 44 dt 0 011 1 gpa k dc 55 dt 0 016 2 gpa k dc 66 dt 0 0101 2 gpa k dc 12 dt 0 0041 6 gpa k dc 13 dt 0 001 2 gpa k and dc 23 dt 0 002 1 gpa k the isotropic seismic velocities in topaz are distinctly higher than those in olivine at 10 gpa and 300 1400 k there should be a strong velocity contrast between the overlying mantle and the thin sediment origin layer at the depth around 300 km a seismological technique like the receiver function technique should be applied to detect a thin layer of topaz in a cold subduction zone

friction on subduction megathrust faults beyond the illite muscovite transition abstract previous experimental studies addressing subduction megathrust friction have focused on shallow fault gouges dominated by smectite illite and quartz here we aim to determine the effect of the transition to muscovite rich fault rock at depths beyond the illite dominated region of the seismogenic zone to achieve this rotary friction experiments were performed on simulated muscovite quartz 65 35 gouge at 100 600 c an effective normal stress of 170 mpa pore pressures of 100 and 200 mpa and sliding velocities of 1 100 m s we isolated the effects of muscovite by shearing pure muscovite gouge at 200 600 c under otherwise identical conditions at low shear strains 1 muscovite quartz gouge showed a friction coefficient of 0 4 0 5 while pure muscovite gouge exhibited 0 3 both gouges showed an increase in with and temperature the muscovite quartz gouge was characterised by velocity strengthening or neutral behaviour at 100 350 c velocity weakening at 350 500 c and velocity strengthening at 500 600 c in contrast pure muscovite gouge showed predominantly velocity strengthening or neutral behaviour demonstrating a key role of quartz in producing velocity weakening the velocity weakening regime in the mixed gouge extends that of 250 400 c recently reported for illite quartz gouge up to 500 c taking into account the observed effect of sliding velocity on a b the data for illite quartz and muscovite quartz together help explain the depth extent of seismogenesis on subduction megathrusts interestingly comparison with previous microphysical models suggests that the mechanism causing velocity weakening in phyllosilicate quartz gouges is one of granular flow involving competition between shear induced dilation and compaction through thermally activated quartz deformation

three dimensional structure along the inverted palaeoproterozoic fiery creek fault system mount isa terrane australia abstract the nw dipping fiery creek fault system located in the northern mount isa terrane comprises numerous sub parallel faults that record multiple episodes of palaeo to mesoproterozoic movement hanging wall wedge shaped stratal geometries and marked stratal thickness variation across the fault system indicate that the earliest movement occurred during episodic intracontinental extension mount isa rift event ca 1710 1655 ma reactivation of the fault system during regional shortening and basin inversion associated with the mesoproterozoic isan orogeny ca 1590 1500 ma resulted in complex three dimensional hanging wall geometries and highly variable strain in the hanging wall strata along the fault system this has resulted in the development of discrete hanging wall deformation compartments that are characterised by different structural styles high strain compartments are characterised by relatively intense folding and the development of break back thrusts whereas low strain compartments are only weakly folded variations in hanging wall strain are attributed to selective reactivation of normal fault segments controlled by the pre inversion fault dip and lithological contrasts across the faults variation of the pre inversion fault dip is interpreted to have been caused by episodic tilt block rotation during crustal extension moderately dipping faults active early in the mount isa rift event show the greatest degree of reactivation whereas younger and steeper normal faults have behaved as buttresses during inversion with strain focussed in zones of upright folding in the hanging wall

crustal architecture and geodynamics of north queensland australia insights from deep seismic reflection profiling abstract a deep crustal seismic reflection and magnetotelluric survey conducted in 2007 established the architecture and geodynamic framework of north queensland australia results based on the interpretation of the deep seismic data include the discovery of a major west dipping paleoproterozoic or older crustal boundary considered to be an ancient suture zone separating relatively nonreflective thick crust of the mount isa province from thinner two layered crust to the east farther to the east a second major crustal boundary also dips west or southwest offsetting the moho and extending below it and is interpreted as a fossil subduction zone across the region the lower crust is mostly highly reflective and is subdivided into three mappable seismic provinces but they have not been tracked to the surface in the east the greenvale and charters towers provinces part of the thomson orogen have been mapped on the surface as two discrete provinces but the seismic interpretation raises the possibility that these two provinces are continuous in the subsurface and also extend northwards to beneath the hodgkinson province originally forming part of an extensive neoproterozoic cambrian passive margin continuation of the thomson orogen at depth beneath the hodgkinson and broken river provinces suggests that these provinces which formed in an oceanic environment possibly as an accretionary wedge at a convergent margin have been thrust westwards onto the older continental passive margin the tasman line originally defined to represent the eastern limit of precambrian rocks in australia has a complicated geometry in three dimensions which is related to regional deformational events during the paleozoic overall the seismic data show evidence for a continental margin with a long history paleoproterozoic to early mesozoic but showing only limited outward growth by crustal accretion because of a repeated history of overthrust shortening during repeated phases of orogenesis

role of large particles in the transport of elements and organic compounds through the oceanic water column abstract during the past decade data from a variety of sources have been obtained which show conclusively that the relatively rare large particles sinking through the water column are responsible for the majority of the downward vertical mass flux in the sea this finding has important implications for understanding the transfer distribution and fate of elements and organic compounds in marine waters the large 100 m detrital particles responsible for vertical flux are primarily biogenic and range in size and composition from small discrete fecal pellets and plankton hard parts to large amorphous aggregates or snow which contain both organic and inorganic constituents depending on size shape and density these particles sink at rates ranging from 1000 m day 1 several methods have been developed for sampling these particles of which in situ sediment trapping has probably furnished the most comprehensive qualitative and quantitative information on the role large particles play in material transport flux studies have highlighted the importance of marine heterotrophs in packaging fine suspended particulate matter into large rapidly sinking particles which accelerate the movement of incorporated materials to depth large particle production via biological packaging is not restricted to the euphotic zone but can occur at all depths and information is now accruing on rates of production of large particles in the water column chemical analyses of sedimenting particles collected in sediment traps and those sampled by other means have allowed quantifying vertical fluxes and residence times of elements radionuclides and organic compounds natural and anthropogenic in various oceanic regimes pertinent studies dealing with the above aspects are reviewed and several areas for future research are suggested

timing of superposed volcanism in the proterozoic mount isa inlier australia metamorphosed proterozoic successions in the mount isa inlier contain a variety of volcanic suites interspersed at several intervals throughout the 30 km thick stratigraphic pile the u pb isotopic systematics of zircons crystallised in superposed felsic volcanic units allow detailed definition of the chronology of this volcanism and therefore of the major depositional cycles this supracrustal development spanned a 270 ma interval in the early to mid proterozoic new u pb zircon data from the earliest recognised volcanic cycle leichhardt metamorphics yield a pooled age for this event at 1867 5 ma a far northern segment of the same basement succession contains a geochemically similar felsic sequence 1852 7 ma old these results confirm the coevality between this felsic volcanism and nearby intrusive bodies of the same age the second major cycle of volcanism is 60 80 ma younger than the first this interval representing the maximum duration of the intervening regional unconformity dacitic and mafic volcanic rocks bottletree formation which unconformably overlie the western flank of the 1870 ma basement are dated in two areas at 1790 810 and 1808 1722 rhyodacites argylla formation on the eastern side of the basement have a similar age 1783 5 ma so for purposes of correlation and crustal evolution these two sequences flanking either side of the basement can be considered to comprise a single volcanic cycle subsequent cycles of felsic volcanism are recognised in stratigraphically younger sequences which include rhyolites with well resolved u pb zircon discordance patterns indicating ages of 1720 7 ma doherty formation and 1678 3 ma carters bore rhyolite the youngest volcanic cycle 1603 6 ma is evidenced by rhyolitic lavas in a part of the sequence previously considered to be much older at least 1740 ma recognition of such a young volcanic episode has an important economic bearing for it implies that rocks equivalent in age to the prospective pb zn rich mount isa group in the west ca 1670 ma could well exist in the eastern succession of the inlier

a shortened intraplate rift system in the proterozoic mount isa terrane nw queensland australia the leichhardt river fault trough of the mount isa terrane developed a complex extensional architecture between approximately 1800 and 1600 ma forming the underlying template upon which compressional structures were superimposed during the 1590 to 1500 ma isan orogeny basin fill material accumulated during at least five multiphase periods of rifting and associated postrift subsidence forming a stacked succession of unconformity bounded sequences initial e w extension was associated with a massive magmatic event half graben greater than 50 km in width and of alternating asymmetry localized the extrusion of up to 4 km of continental tholeiites thereafter a period of n s extension resulted in southward tapering north tilted half graben in which synrift basaltic and siliciclastic strata accumulated n s extension was followed by regional postrift subsidence and the deposition of a laterally continuous quartzite carbonate package a multiphase period of e w to nw se extension ensued during which time two unconformity bounded sequences accumulated the stratal architectures of these sequences are strongly asymmetric in cross section exhibiting a pronounced rotational thickening toward the east consistent with their deposition in the hanging walls of east dipping tilt blocks between 15 and 40 km in width finally a period of n s extension resulted in the development of e w trending f1 drag synclines in the highest level cover rocks the association of angular unconformities and block bounding faults e w trending synclines and e w striking faults and the unique internal fold geometries of fault blocks suggest that many fault bounded blocks originated as coherent structural entities during rifting and continued to act as such during subsequent shortening

post orogenic 1500 ma thermal history of the proterozoic eastern fold belt mount isa inlier australia the paucity of sedimentary rocks younger than 1500 ma within the mt isa inlier renders it difficult to determine its post orogenic exhumation and tectonic history except by inference from the stratigraphy of the surrounding areas we present hornblende white mica biotite and alkali feldspar 40ar 39ar data from the eastern fold belt of the inlier which reveal a previously unrecognised complex post orogenic non linear cooling history plateau and total fusion 40ar 39ar ages range between 1500 and 1130 ma and record increases in regional cooling rates of up to 18 c my during 1460 1410 and 1280 1050 ma forward modelling of the alkali feldspar 40ar 39ar arrhenius parameters reveals subsequent increases in cooling rates during 740 640 and 600 500 ma the cooling episodes were driven by both erosional exhumation at maximum rates of 0 9 km my and thermal relaxation following magmatic events early mesoproterozoic cooling is synchronous with exhumation and shearing during the anmatjira event in the arunta block and late mesoproterozoic cooling occurred at the same time as magmatic events in the arunta and musgrave blocks as well as grenville aged orogenesis in the albany fraser belt more localised cooling during the middle neoproterozoic may be a far field response to the separation of australia and laurentia whereas latest neoproterozoic cambrian cooling and exhumation was probably driven by collisional tectonics associated with the petermann ranges and delamerian orogenic events major changes in the stress field of the australian plate that are synchronous with the assembly and break up of rodinia and gondwana resulted in the repeated brittle reactivation of the eastern fold belt probably via the displacement of long lived basement faults

kinks in mica role of dislocations and 001 cleavage abstract kinking in natural and experimentally deformed biotite and muscovite has been investigated by transmission electron microscopy tem it is found that complex microstructures exist in the vicinity of kink band boundaries kbbs gentle bending of 001 planes plays an important role in the kinking process where 001 planes are bent several degrees edge dislocations are observed in complex walls between active slip planes together with broad bands of dislocations parallel to 001 microfracturing and dilation occur parallel to 001 planes and the kbb a model is presented to explain the origin of observed kbbs which may be applicable to other crystalline materials with only one active slip plane

geochronology of the larderello geothermal field new data and the closure temperature issue the larderello geothermal field is generally accepted to have been produced by a granite intrusion at 4 9 km depth hydrothermal parageneses and fluid inclusions always formed at temperatures greater than or equal to the current ones which implies that the field has always undergone a roughly monotonic cooling history fluctuations 40 k since intrusion of the granite at 4 ma the heat required to maintain the thermal anomaly over such a long period is supplied by a seismically anomalous body of 32000 km3 rooted in the mantle borehole minerals from larderello are thus a unique well calibrated natural example of thermally induced ar and sr loss under geological conditions and time spans the observations biotites retain ar above 450 c agree well with other albeit less precise geological determinations but contrast with laboratory determinations of diffusivity from the literature we therefore performed a hydrothermal experiment on two larderello biotites and derived a diffusivity dlab 370 c 5 3 10 18 cm2s 1 in agreement with published estimates of diffusivity in annite from dlab and the rejuvenation of the k ar ages we calculate maximum survival times at the present in hole temperatures they trend smoothly over almost two orders of magnitude from 352 ka to 5 3 ka anticorrelating with depth laboratory diffusivities are inconsistent not only with geological facts but also among themselves from the geologically constrained lifetime of the thermal anomaly we derive a diffusivity dg 370 c 3 81 1021 cm2s 1 3 1 orders of magnitude lower than dlab the cause of these discrepancies must be sought among various laboratory artefacts overstepping of a critical temperature t enhanced diffusivities in wet experiments presence of fast pathway dislocation and pipe diffusion and of dissolution reprecipitation reactions which we imaged by scanning electron microscopy these phenomena are minor in geological settings in the absence of mineral transformation reactions complete or near complete resetting is achieved only by volume diffusion therefore laboratory determinations will necessarily result in apparent diffusivities that are too high compared to those actually effecting the resetting of natural geochronometers

characterization of a proposed palaeoproterozoic suture in the crust beneath the curnamona province australia abstract we use constrained forward and inverse modeling of bouguer gravity data to investigate a regional positive gravity anomaly that defines but is not confined to the eastern margin of the curnamona province best fit results imply that the positive gravity values are sourced from a relatively dense terrane 0 04 mg m 3 with respect to surrounding crust located at middle crustal levels 16 27 km depth which is likely dismembered and offset into discrete slivers by later tectonism consideration of pre 1500 ma plate reconstructions of the australian continent correlates this terrane with an interpreted buried suture in the mount isa inlier of northern australia where allochthonous crust was amalgamated to the australian margin at ca 2 20 1 85 ga bouguer gravity forward and inverse modeling also delineates a shallow west dipping crustal scale structure that possibly represents a primary boundary between precambrian basement of australia and the eastern fold belts accreted to the precambrian margin during the palaeozoic

a crystal chemical basis for pb retention and fission track annealing systematics in u bearing minerals with implications for geochronology this study develops an empirical crystal chemical framework for systematizing the kinetics of pb loss and fission track annealing in u bearing minerals ionic porosity z the fraction of a mineral s unit cell volume not occupied by ions potentially accounts for kinetic behavior by monitoring mean metal oxygen bond length strength various tests of a general kinetics porosity relationship are presented based upon diverse mineral data including 1 pb diffusion parameters 2 measured closure temperatures tc for fission track annealing and 3 retentivities of both pb and fission tracks from apparent age data every kinetic parameter including tc and mineral age for both the u pb and fission track systems is inversely correlated with z within the sub assemblage zircon z 29 titanite 34 and apatite 38 assuming a diffusional closure model pb isotopic transport phenomena are described by a tc z scale calibrated with field based tc data for titanite 680 20 c and apatite 500 c extrapolation of this scale yields tc estimates for the following minerals staurolite tc 1060 c z 25 garnet 1010 c 26 5 zircon 900 c monazite xenotime and epidote 750 c 32 and ca clinopyroxene 670 30 c 34 1 depending on composition these empirical results imply that a u pb pb date for staurolite or garnet records the time of mineral growth not post growth isotopic closure as also concluded in recent field studies because z systematizes fission track annealing this recrystallization process like volume diffusion must also be rate limited by the strength of chemical bonds the extent to which other recrystallization processes are likewise rate limited is important to u pb geochronology because they potentially compete with diffusion as mechanisms for pb isotopic resetting in nature

crustal architecture of basin inversion during the proterozoic isan orogeny eastern mount isa inlier australia abstract the folded and metamorphosed supracrustal rocks of the eastern margin of the mount isa inlier nw queensland australia provide a record of basin inversion processes at high temperatures from the foreland toward the core of a developing orogen deep water turbiditic metasedimentary rocks of the lower maronan supergroup ca 1 71 1 66 ga occupy the hanging wall of a crustal scale thrust the cloncurry overthrust which separates them from calc silicate and meta evaporitic rocks of the doherty formation ca 1 74 1 72 ga in the footwall during the early stages of the isan orogeny ca 1 60 1 58 ga the rocks of the maronan supergroup were expelled from their basin and thrust to the north northwest shortening was restricted to the hanging wall and produced a 10 km scale north to northwest verging nappe with a crumpled frontal zone of upright east west trending folds this morphology is consistent with a push from the rear mechanism of nappe emplacement with the simple shear component of deformation increasingly accommodated within the hanging wall down dip toward the core of the orogen metamorphism associated with this folding reached upper amphibolite facies temperatures at moderate to low pressures 4 6 kbar metamorphic zones in the hanging wall of the cloncurry overthrust were antiformally folded with continuing deformation the relationship between faulting folding and metamorphic zoning across the cloncurry overthrust are consistent with a thrust system despite that fact that younger rocks are juxtaposed over older basin inversion provides the best explanation for this phenomenon within this context metamorphism during the early stages of orogenesis can be explained by the inheritance of elevated geothermal gradients produced by lithospheric thinning and sediment loading during formation of the basin the overall orogenic architecture was controlled by a combination of thermal softening which focussed early deformation within the basin and weak zones at the base of the basin and the base of the crust this resulted in crustal scale thrusts of opposite vergence that partitioned upper crustal and lower crustal deformation

the crystal chemical basis for ar retention in micas inferences from interlayer partitioning and implications for geochronology cation partitioning data for coexisting muscovite and biotite are shown to be useful indicators of relative interlayer bond length strength in these minerals these data therefore provide a useful crystal chemical perspective on relative mass transfer kinetics of radiogenic isotopes and account for the observation that biotite is generally less retentive of 40ar and 87sr than coexisting muscovite partitioning behavior of trace elements underscores three reasons why overall interlayer bonding in biotite is weaker than in muscovite first the preferences of large rb cs in biotite and of small la3 and na in muscovite indicate a relatively spacious interlayer volume in biotite suggesting a longer mean k o bond second the preference of interlayer vacancies in biotite with some all possibly h2o h3o filled suggests that its adjacent 2 1 sheets are connected by fewer interlayer bonds per unit cell than those of muscovite third the relative exclusion of large ba2 from biotite despite its large interlayer sites is attributed to o h bonds pointing into the interlayer cavity sub normal to 001 k ba2 h repulsion thereby induced by the bare proton both destabilizes ba2 and weakens k o bonds in contrast muscovite offers a more favorable electrostatic environment for ba2 substitution since its o h bonds are directed into the vacant m1 octahedral site sub parallel to 001 this hypothesis is supported by the observation that progressive f oh 1 exchange enhances ba2 partitioning into biotite phlogopite relative to coexisting muscovite these crystal chemical differences between biotite and muscovite are mirrored in calculated values of ionic porosity z i defined here as the percentage of their interlayer unit cell volume not occupied by ions a monitor of ionic packing density and geometry z i is inversely correlated with k o bond strength which appears to be the rate determining kinetic common denominator for a variety of processes affecting micas including those responsible for loss of radiogenic isotopes in biotite and muscovite accordingly the relatively longer weaker k o bonds of biotite are envisioned as being more easily stretched during volume diffusion or broken during recrystallization or retrograde alteration this in turn accounts for common observations of enhanced radiogenic ar sr loss and younger 40ar 39ar and rb sr ages in natural biotite high z i relative to coexisting muscovite lower z i significantly this pattern may arise irrespective of isotopic loss mechanism diffusion or recrystallization etc and it follows that any age discordance observed between muscovite and biotite cannot be ascribed uniquely to one mechanism or the other without appropriate field petrographic and petrologic constraints extension of this partitioning porosity based synthesis leads to prediction of corollary age retentivity composition effects among chemically diverse trioctahedral and dioctahedral micas which are best field tested in terranes that cooled slowly under dry static conditions pressure effects on argon retention are also inferred from the porosity model

40ar 39ar age gradients in micas from a high temperature low pressure metamorphic terrain evidence for very slow cooling and implications for the interpretation of age spectra estimates of the time temperature history of geologic samples are often based on 40 ar 39 ar ages obtained through incremental heating experiments on minerals with different ar retentivities laser incre mental heating of single crystals of muscovite and biotite from an 1700 ma monzogranite from a low pressure high temperature metamorphic terrain in central arizona suggested an essentially uniform distribution of radiogenic 40 ar in each crystal and yielded ages of 1412 5 ma for the muscovite and 1410 10 ma for the biotite the similarity in these ages implies rapid 25 k m y cooling of the monzogranite after a reheating episode slightly before 1412 ma in contrast detailed age mapping of single crystals from the same samples with the laser microprobe revealed large radiogenic 40 ar gradients indicative of very slow 400 m y evidence for such low cooling rates has important implications for the longevity of high thermal gradients in the early middle proterozoic lithosphere in addition inconsistency between the laser mapping and incremental heating results for these micas suggests caution when interpreting incremental heating data for slowly cooled samples

the australian central eromanga basin project an introduction abstract the australian bureau of mineral resources is carrying out a major multidisciplinary program of geological and geophysical studies in southwestern queensland in cooperation with the geological survey of queensland the project is aimed at providing information on the regional structure and depositional history of the central eromanga basin and the underlying adavale cooper and galilee basins the information being obtained is particularly relevant to a better understanding of the petroleum prospectivity of the area the program includes geophysical surveys involving 1400 km of new six fold cdp seismic reflection coverage on regional traverses up to 400 km long crossing the main structural elements of the area gravity measurements along all new seismic reflection traverses refraction surveys along two major east west and north south traverses and magnetotelluric soundings along the same major east west traverse landsat imagery studies are providing new perspective on many regional structures when used in conjunction with seismic and gravity information wireline logs and synthetic seismograms are being used with the new seismic data to re examine stratigraphic correlations palynologic and lithologic studies are underway to assist in determining depositional environments source rock maturation hydrological and geochemical studies are providing information on the generation and migration of hydrocarbons a significant feature of the program is the extension of the recording time of all new reflection data to 20 s to obtain good quality deep crustal reflection information comparable to that obtained on cocorp programs in the united states the reflection data is being interpreted with the refraction gravity and magnetotelluric data to investigate the relationship of deep crustal and upper mantle features to the sedimentary basins in the central eromanga basin area

the mount wright arc a cambrian subduction system developed on the continental margin of east gondwana koonenberry belt eastern australia abstract the mount wright arc in the koonenberry belt in eastern australia is associated with two early to middle cambrian lithostratigraphic groups developed onto the late neoproterozoic volcanic passive margin of east gondwana the gnalta group includes a calc alkaline basalt andesite dacite suite mount wright volcanics interpreted to represent the volcanic component of the arc volcaniclastic gnalta group rocks now buried in the bancannia trough represent the continental back arc developed immediately behind the arc in a manner analogous to the modern taupo volcanic zone of new zealand east of the gnalta group is the ponto group a deep marine sedimentary package that includes tholeiitic lavas bittles tank volcanics and felsic tuffs interpreted as part of a fore arc sequence the configuration of these units suggests the mount wright arc developed on continental crust in response to west dipping subduction along the east gondwana margin in contrast with some models for cambrian convergence on other sections of the delamerian orogen which invoke east dipping subduction and arc accretion by arc continent collision this convergent margin was deformed by the middle cambrian delamerian orogeny which involved initial co axial shortening followed by sinistral transpression and oroclinal folding around the edge of the curnamona province

a framework of overprinting orogens based on interpretation of the mount isa deep seismic transect a deep seismic reflection transect and coincident refraction study reveal the crustal structure developed during mesoproterozoic orogenesis preserved in the mount isa inlier of northwest queensland australia geologic interpretation of the reflection data indicates a two stage evolution for east west shortening the first stage involved west directed folding and thrusting above a decollement in the eastern part of the terrane this decollement system was the forehand of an orogenic system rooted east of the present day inlier the second stage is characterized by a network of basement cutting reverse and strike slip faults that dissected the decollement this overprinting system was linked to crustal thickening within the mount isa terrane the refraction profile indicates the crust beneath the inlier is currently approximately 55 km thick rocks with transitional velocities 7 km s in the lower 15 km are interpreted as mafic underplate formed during the pre orogenic rift history of the inlier the refraction profile also reveals two west dipping high velocity layers within the crust evaluating the origin of these layers in light of the kinematics of crustal thickening suggests they are portions of the underplated material thrust to midcrustal levels during the second stage of shortening the overprinting orogenic scenario has implications for the geodynamic evolution of the inlier that should have a direct bearing on mineralizing systems localized by reverse faults developed during the second stage of shortening this basement cutting stage established a linked fault network able to tap deeper crustal circulation during the intrusion of late to postorogenic granitoids

contrasts in lithospheric structure within the australian craton insights from surface wave tomography abstract contrasts in the seismic structure of the lithosphere within and between elements of the australian craton are imaged using surface wave tomography new data from the wacraton and tigger experiments are integrated with re processed data from previous temporary deployments of broad band seismometers and permanent seismic stations the much improved path coverage in critical regions allows an interpretation of structures in the west of australia and a detailed comparison between different cratonic regions improvements to the waveform inversion procedure and a new multi scale tomographic method increase the reliability of the tomographic images in the shallowest part of the model 75 km a region of lowered velocity is imaged beneath central australia and confirmed by the delayed arrival times of body waves for short paths within the cratonic lithosphere there is clearly structure at scale lengths of a few hundred kilometres resolution tests indicate that path coverage within the continent is sufficient to reveal features of this size in the upper part of our model in western australia differences are seen beneath and within the archaean cratons at depths greater than 150 km faster velocities are imaged beneath the yilgarn craton than beneath the pilbara craton in the complex north australian craton a fast wavespeed anomaly continuing to at least 250 km is observed below parts of the craton suggesting the possibility of archaean lithosphere underlying areas of dominantly proterozoic surface geology

correlation between velocity dependence of friction and strain localization in large displacement experiments on feldspar muscovite and biotite gouge abstract room temperature rotary shear experiments on rock powders of albite feldspar and muscovite and biotite micas show that the degree of localization of shear strain in these materials correlates with the frictional behavior in both feldspar and mica experiments increasing localization correlates with a lower coefficient of friction and a more negative velocity dependence of friction feldspar gouges tend to have localized deformation along one or more boundary parallel shears and a negative velocity dependence more localized deformation correlates with a lower coefficient of friction and a more negative velocity dependence in general micas deform throughout a broader zone than feldspar and have a lower coefficient of friction compared to muscovite biotite is weaker has a negative rather than a positive velocity dependence and shows more localized deformation for materials as diverse as feldspars and micas localization along boundary parallel shears correlates with negative velocity dependence along compliant natural faults a negative velocity dependence would result in a greater tendency for unstable sliding thus localized boundary parallel shears may indicate negative velocity dependence and slip by earthquakes on natural faults

fossil whale preservation implies high diatom accumulation rate in the miocene pliocene pisco formation of peru diatomaceous deposits in the miocene pliocene pisco for mation contain abundant whales preserved in pristine condition bones articulated or at least closely associated in some cases in cluding preserved baleen the well preserved whales indicate rapid burial the 346 whales within 1 5 km 2 of surveyed surface were not buried as an event but were distributed uninterrupted through an 80 m thick sedimentary section the diatomaceous sediment lacks repeating primary laminations but instead is mostly massive with irregular laminations and speckles there is no evidence for bioturbation by invertebrates in the whale bearing sediment cur rent depositional models do not account for the volume of diato maceous sediments or the taphonomic features of the whales these taphonomic and sedimentary features suggest that rapid burial due to high diatom accumulation in part by lateral advection into pro tected shallow embayments is responsible for the superb preser vation of these whales leading to a higher upper limit on phyto plankton accumulation rates than previously documented

late paleo mesoproterozoic plate margin deformation in the southern gawler craton insights from structural and aeromagnetic analysis abstract the integration of structural analysis of outcrop and aeromagnetic interpretation has revealed a complex tectonic evolution for the southern gawler craton that reflects the regional tectonic signal during the late paleo to early mesoproterozoic results suggest that exposures at yarlbrinda hill comprise the margin of a highly deformed st peter suite pluton emplaced within the southwestern gawler craton at ca 1611 ma d 1 at outcrop records east to northeast directed ductile thrusting regionally this deformation overprints north south trending dextral yarlbrinda shear zone fabrics within ca 1622 ma st peter suite granite and ca 1690 1670 ma tunkillia suite granite progressive east to northeast directed thrust movements led to exhumation of the deformed ca 1611 ma granite d 2 deformation is recorded by the brittle reactivation of first generation mylonitic fabrics accompanied by cataclasis alteration and pseudotachylite development d 3 records a switch to northwest southeast shortening which led to the folding and tear faulting of the d 1 2 shear zone a switch to extension and subsequent rotations of the principle stress axes led to hiltaba suite granite emplacement and contemporaneous faulting under east west extension during d 4 and west northwest east southeast extension during d 5 deformation of ca 1622 ma st peter suite granite is consistent with movement on the yarlbrinda shear zone and requires the st peter suite to have been emplaced into or adjacent to the ca 1690 1670 ma tunkillia suite prior to shear zone activity and on the overriding plate during subduction d 1 d 3 deformation is constrained by the emplacement of ca 1580 ma hiltaba suite granite and correlates with the wartakan orogeny in the southeastern gawler craton and the olarian orogeny in the curnamona province a 52 disparity between the shortening directions in these respective areas suggests different degrees of mesoproterozoic rotation which likely aided the formation of an intervening basin province the remnants of which are presently preserved in the mount painter inlier the record of convergent deformation is geochronologically constrained to a 30 myr period and places this region of the central gawler craton near a late paleoproterozoic convergent plate margin prior to a period of transient extension beginning at ca 1580 ma the continental scale signal of tectonic events can be recognized at the outcrop scale highlighting the importance of detailed outcrop studies in sparsely exposed areas

age of the barramundi orogeny in northern australia by means of ion microprobe and conventional u pb zircon studies abstract u pb zircon geochronological studies are used to quantify the tectonostratigraphic history of a major crust forming event in early proterozoic terranes of northern australia magmatism and tectonic processes associated with this event the barramundi orogeny appear similar over very wide areas about one third of the australian continent the barramundi event marks the deformation metamorphism and cratonization of the earliest proterozoic 2 0 1 9 ga basinal successions and it was followed by and closely linked to extensive orogenic felsic magmatism the yaringa metamorphics are an inlier of paragneiss and schist that pre dates 1865 3 ma leichhardt felsic magmatism and younger basin formation in the mount isa inlier from an ion microprobe u pb zircon study of massive and migmatitic gneiss the time of yaringa metamorphism is established at 1890 8 ma recognized younger proterozoic deformations in the interval 1610 1550 ma are generally not evident in zircon u pb systems but have totally reset local rb sr whole rock values conventional u pb work on multi grain zircon samples of yaringa gneiss gives a geologically meaningless age 2090 40 ma that is an artifact of complex zircon growth part of which was initially inherited from a 2 55 to 2 2 ga provenance and the other major part of which was formed in situ during the 1890 ma high grade metamorphism predictably for simple zircon systems in low grade metavolcanic terranes conventional and ion microprobe zircon ages are in close agreement dating of volcaniclastic sequences above and below the barramundi tectonic break in the pine creek inlier brackets the orogeny in this terrane to a similar time frame 1885 1870 ma some zircons which crystallized at this time form selvedges around older cores whose age of 2100 2000 ma may be relics of an earlier deep crustal underplating the uniform chemistry of post barramundi felsic magmatism and the similar tectonic setting of the areally extensive barramundi event are mirrored by their temporal coherency cycles of geosynclinal deposition burial metamorphism and uplift took place during intervals of several million years or less implying comparable rates for early proterozoic orogenesis to those established for tertiary and quaternary settings

early tectonic evolution of the thomson orogen in queensland inferred from constrained magnetic and gravity data abstract the crustal architecture as well as the kinematic evolution of the thomson orogen in queensland is poorly resolved because the region is concealed under thick phanerozoic sedimentary basins and the basement geology is known from limited drill holes combined potential field and seismic interpretation indicates that the thomson orogen is characterized by prominent regional ne and nw trending structural grain defined by long wavelength and low amplitude geophysical anomalies the smooth magnetic signature is interpreted to reflect deeply buried source bodies in the mid to lower crust short wavelength positive magnetic features that correlate with negative gravity anomalies are interpreted to represent shallower granitic intrusions they appear to be focused along major fault zones that might have controlled the locus for magmatism the eastern thomson orogen is characterized by a prominent ne structural grain and orthogonal faults and fold interference patterns resulting in a series of troughs and highs the western thomson orogen consists of a series of nw trending structures interpreted to reflect reverse faults sedimentation and basin development are interpreted to have initiated in the neoproterozoic to early cambrian during e w to ene wsw extension possibly related to the rodinia break up this extensional event was followed by late cambrian shortening recorded in the maneroo platform and the diamantina river domain which possibly correlates with the delamerian orogeny renewed deposition and volcanism occurred during the ordovician and may have continued until late silurian resulting in thinned proterozoic basement crust and extensive basin systems that formed in a distal continental back arc environment our interpretation places the thomson orogen to the west of the neoproterozoic passive margin preserved in the anakie inlier the region is likely to represent the internal extensional architecture during the rodinia break up that has been subsequently extensively modified by multiple extensional basin forming events and transient episodes of crustal shortening and basin inversion

experimental deformation of muscovite abstract the strength and mechanical anisotropy of muscovite have been investigated by shortening single crystals at 45 0 and 90 to the basal plane 001 at temperatures t from 20 to 400 c confining pressures pc from 10 to 400 mpa and strain rates e from 2 5 10 7 to 2 3 10 4 s 1 samples deformed as 45 to 001 exhibit smooth undulatory extinction in thin section associated with dislocation glide with few well developed low angle kink boundaries basal shear strengths depend on confining pressure at pc e a exp a exp q rt results from temperature and strain rate stepping experiments performed at pc 200 mpa yield an activation energy q of 47 19 kj mol 1 and an exponential constant a of 0 5 0 2 mpa 1 samples loaded parallel to the basal plane shorten by the development of sharply defined kink bands strengths associated with kink band formation are higher than those associated with glide in samples compressed at 45 to 001 and exhibit significant pressure dependence 0 8 up to confining pressures of 200 mpa kink bands are generated at the ends of samples where contact is made with the pistons samples loaded at 90 to 001 are much stronger than samples of all other orientations and deformation is accomplished by fracture fracture strengths exhibit a strong dependence on confining pressure 0 9 similar in magnitude to other silicates that are brittle at the same conditions by comparison with biotite muscovite is weaker in shear on 001 at the experimental conditions tested with an activation energy for glide q that is significantly smaller than that for glide in biotite

evolution of the mount woods inlier northern gawler craton southern australia an integrated structural and aeromagnetic analysis structural mapping integrated with interpretation and forward modelling of aeromagnetic data form complimentary and powerful tools for regional structural analysis because both techniques focus on architecture and overprinting relationships this approach is used to constrain the geometry and evolution of the sparsely exposed mount woods inlier in the northern gawler craton the mount woods inlier records a history of poly phase deformation high temperature metamorphism and syn and post orogenic magmatism between ca 1736 and 1584 ma the earliest deformation involved isoclinal folding and the development of bedding parallel and axial planar gneissic foliation s1 this was accompanied by high temperature upper amphibolite to granulite facies metamorphism at ca 1736 ma during subsequent north south shortening d2 open to isoclinal south southeast oriented f2 folds developed as the palaeoproterozoic successions of the inlier were thrust over the archaean nuclei of the gawler craton the syn d2 engenina adamellite was emplaced at ca 1692 ma the post d2 history involved shear zone development and localised folding exhumation of metamorphic rocks and deposition of clastic sediments prior to the emplacement of the ca 1584 ma granite balta suite the mount woods inlier is interpreted as the northern continuation of the kimban orogen

imaging the basement architecture across the cork fault in queensland using magnetic and gravity data abstract the basement rocks in central queensland are largely obscured by the phanerozoic sedimentary succession and direct information comes only from sparse geological data 2 5d forward modelling of regional aeromagnetic and bouguer gravity data has been undertaken to unveil the crustal architecture at the junction of the proterozoic mount isa terrane and the phanerozoic thomson orogen in queensland the most prominent geophysical character is represented by the abrupt termination of positive nnw trending geophysical anomalies of the mount isa terrane against ne trending low amplitude gravity and magnetic anomalies of the diamantina river domain of the thomson orogen potential field forward modelling indicates that the lower basement crust of the thomson orogen is petrophysically indistinguishable from the crust of the proterozoic mount isa terrane we interpret that the prominent gradients associated with the cork fault represent the displacement and burial of the mount isa terrane crust under the thomson orogen in this context the cork fault is inferred to represent a fundamental crustal discontinuity but does not represent the eastern margin of rodinia because the thomson orogen is floored by precambrian continental crust initiation of the cork fault might have occurred as early as the mesoproterozoic at that time the cork fault was a major structure that developed during the separation of the mount isa terrane and the curnamona province it is inferred that this break up occurred during initial n s to nnw sse extension the cork fault formed part of a network of major north and south dipping normal faults active at that time the cork fault was repeatedly reactivated during the neoproterozoic rodinia break up and during the phanerozoic evolution of the tasmanides we suggest that the segment of the inferred tasman line along the southern termination of the mount isa terrane should be regarded as a set of structures offsetting the proterozoic crust rather than a simple lineament dividing the proterozoic and the phanerozoic australian continent

multi scale integrated structural and aeromagnetic analysis to guide tectonic models an example from the eastern musgrave province central australia abstract in many polydeformed precambrian provinces the understanding of their tectonic history is limited by insufficient exposure which leads to difficulty in developing links between local and regional architecture and results in poorly constrained models of their regional scale architecture high resolution aeromagnetic data allows interpretation and modelling at a scale comparable to structural mapping and by integrating these methods these links can be developed with high confidence as an example we integrate structural mapping aeromagnetic data interpretation and 3d magnetic inversions in a multi scale structural analysis of the polydeformed eastern musgrave province in central australia deformation during the ca 1200 ma musgravian orogeny is characterised by an originally northeast trending structural grain defined by shear zones and tight to isoclinal folds at all scales 3d magnetic inversion models of syn to post musgravian granites indicate that this structural grain is predominantly west dipping this architecture suggests that a northeast oriented fold and thrust belt developed as a result of northwest southeast compression in central australia during the musgravian orogeny the next deformation event is characterised by an array of originally east southeast trending shear zones that truncate and offset the musgravian structural grain with apparent dextral displacements of up to 1 km these shear zones are commonly co located with mafic and granitic dykes suggesting a dilational origin and they are interpreted to have initiated under northeast southwest extension during the ca 1080 ma giles event the architecture of these two mesoproterozoic events was later extensively overprinted by two deformation events during the pan african aged ca 550 ma petermann orogeny the first is characterised by the widespread reactivation of mesoproterozoic structures resulting in a network of shear zones of variable orientation and the second is characterised by the development of crustal scale transpressional shear zones motion on which has caused the rotation of lithospheric blocks and the reorientation of prior structural trends by up to 90 collectively the architecture of these two events suggests broadly north south crustal shortening in central australia as an intraplate response to gondwana assembly this example demonstrates that the integration of structural geology with aeromagnetic interpretation and modelling can be used to develop high confidence links between local and regional structures these links permit the derivation of better constrained models of major tectonic events particularly in regions where outcrop is poor and the structural architecture is complex

sedimentology and high frequency sequence stratigraphy of a forearc extensional basin the miocene caleta herradura formation mejillones peninsula northern chile abstract facies and sequence stratigraphic interpretation for the 380 m thick caleta herradura formation miocene are presented based on detailed and comprehensive outcrop data from the caleta herradura half graben mejillones peninsula northern chile the caleta herradura formation contains an array of lithofacies comprising sandstones sandy mudstones diatomites and breccio conglomerates that are interpreted as the products of inner shelf to non marine depositional settings complete exposure allows for recognition of a number of distinct and laterally persistent key stratal surfaces that permitted the identification of twenty five high frequency meter to tens of meters scale sequences based on their internal organization two main motifs of such unconformity bounded depositional packages can be distinguished deepening and deepening to shallowing upwards transgressive and highstand systems tracts were resolved based on such facies stacking patterns the origin of sequences is difficult to decipher and remains somewhat uncertain however several lines of direct and indirect evidence constrain interpretations for both the sequence architecture and bounding unconformities and help to discriminate between the various mechanisms that may have driven their development in essence these demonstrate that neither tectonically driven sea level oscillations nor climatically induced changes in sediment supply can adequately explain the distinctive features of unconformities and the facies architecture of sediments they delimit instead these high frequency changes in relative sea level are best explained as a consequence of glacio eustatic oscillations the contemporaneous deep ocean proxy records appear to support a causal link between the observed stratigraphic cyclicity and glacio eustatic changes in sea level due to modulation of short term milankovitch scale events by longer period astronomical variations however the type of astronomical forcing remains elusive hindered by lack of adequate age controls and a poor understanding of pacing mechanisms of high frequency climate changes during the middle to late miocene

inssev an instrument to measure the size and settling velocity of flocs in situ abstract an instrument has been developed to observe the settling of individual flocs in turbid water in order to measure size and settling velocity spectra of estuarine cohesive suspended sediments inssev in situ settling velocity instrument is bed mounted and comprises a computer controlled decelerator chamber that collects a sample of water from which some of the suspended matter is allowed to enter the top of a settling column the settling flocs are viewed using a miniature video system subsequent analysis of video tapes can provide direct measurements of size and settling velocity of individual flocs down to 20 m from this information floc density can be estimated the main feature of the instrument is its ability to video flocs in situ irrespective of the concentration in the estuary with as little disturbance to their hydrodynamic environment as possible preliminary field testing in the tamar estuary has given useful results in flow velocities up to 0 4 m s 1 and in concentrations up to 200 mg 1 1 the video recordings show large numbers of low density flocs with multiple structures linked by fine organic fibres

detrital zircon ages in neoproterozoic to ordovician siliciclastic rocks northeastern australia implications for the tectonic history of the east gondwana continental margin u pb detrital zircon ages in variably metamorphosed dominantly fine grained clastic successions are used in northeastern australia to identify two major successions along the east gondwana margin the older succession is of probable late neoproterozoic age and is considered part of a passive margin associated with rifting at c 600 ma most detrital zircons have ages in the range 1000 1300 ma and were probably derived from an extension of a late mesoproterozoic 1050 1200 ma orogenic belt from the central australian musgrave complex located 1500 km to the west no evidence has been found for 600 800 ma rifting of a rodinian supercontinent and therefore it is suggested that breakup must have occurred well outboard of the present early palaeozoic east gondwana margin the younger succession is of early palaeozoic age and contains the distinctive 500 600 ma detrital zircon signature that is widespread in east gondwana in addition to some samples with ages in the range 460 510 ma consistent with local igneous sources the younger succession is related to the active margin of gondwana that developed on the former passive margin in a back arc setting and the source of 510 600 ma zircons is considered to be a composite of rift related and back arc volcanic sources

deep crustal reflection results from the central eromanga basin australia abstract from 1980 to 1982 deep seismic reflection profiles were recorded across the central eromanga basin in eastern australia to study the regional structure stratigraphy and geological history of the eromanga basin and infra basins the reflection data were recorded to 20 s to obtain additional information on the nature and structure of the crust below the sediments and their relationship to the development of the basins the seismic sections show good quality reflections from the deep crust as well as from the sedimentary layers based on the character strength coherence continuity and spatial distribution of the reflections the sections can be divided into four zones the tope zone between 0 and 2 5 s shows fairly uniform coherent and continuous events which correlate with the mesozoic and late palaeozoic sediments the zone from 2 5 to 8 s 4 to 22 km does not show any primary reflections and is interpreted as the highly deformed metasedimentary and metavolcanic rocks of the early palaeozoic thomson orogen underlying the sediments without any recognisable reflection or diffraction patterns in this zone it is difficult to say whether the faulting and folding observed in the sediments extend into the upper crustal basement the deeper zone of numerous reflection segments between 8 and 12 5 s 22 to 36 km is interpreted as thin laminae of alternating low and high velocity intermediate and basic rocks and correlates with the lower crust bounded by refraction velocity discontinuities the lowest zone of no reflections below 12 5 s corresponds with the upper mantle the reflection character and thickness as well as the refraction velocity structure of the crust under the central eromanga basin area are significantly different from those of the precambrian crust under the georgina basin to the northwest it is proposed that the crust under the eromanga basin is extensionally attenuated crust which had been intruded by sills of basaltic melt from the underlying asthenosphere

mid crustal detachments controlling basin deformation ramp synforms in southwestern queensland abstract deep seismic data from the adavale basin region of southwestern queensland suggest that westward carboniferous movement of the upper crust over mid crustal ramps was responsible for the present day geometry of two synforms containing devonian sediments we interpret the troughs as hanging wall synforms formed by sub vertical simple shear within the upper crust during down dip movement over the ramp one of these structures the quilpie trough in the southern adavale basin is an asymmetric trough interpreted to be an extensional ramp syncline it overlies a west dipping ramp in the boundary between the seismically non reflective upper crust and the seismically reflective lower crust formation of this basin by the relative movement of the upper crustal thomson fold belt over the mid crustal ramp is postulated the significant asymmetry of the trough suggests that lower crustal extension by pure shear may have occurred during upper crustal translation the other structure the westgate trough contains two palaeozoic sequences we propose that the devonian lower sequence is in a ramp syncline formed by folding of the sediments in a conveyor belt style mechanism during a translation of the upper crust the upper sequence is interpreted as a ramp basin in which sediments were deposited in the synform as it developed deep seismic reflection data from the central eromanga basin region indicate that relative horizontal translation can occur between the upper and lower crust on mid crustal detachments the geometry of the synforms suggests that above these detachments crustal deformation has a sub vertical simple shear style whereas the lower crust below the detachments seems to deform in a pure shear style recognition of relative movement of tens of kilometres on mid crustal detachments is important for our understanding of basin geometry and development in the central eromanga basin

tectonic evolution of proterozoic australia proterozoic australia has long been interpreted as a single intact continent in which all tectonic and magmatic activity was intracratonic this paper proposes an alternative hypothesis in which numerous fragments of continental crust were assembled by plate tectonic processes the assembly was completed between 1300 and 1100 ma when the crustal fragments were combined as an early component of the rodinian supercontinent rifting and fragmentation of archaean continents began in the late archaean and continued into the proterozoic passive margin deposits such as those of the hamersley basin accumulated on isolated fragments of archaean crust these numerous fragments were subsequently assembled into three cratons by 1830 ma a west australian craton was established by collision of the archaean pilbara and yilgarn cratons which were joined along the capricorn orogen similarly a south australian craton developed by amalgamation of the proto gawler and proto curnamona cratons along the kimban orogen a north australian craton appears to have formed by accretion of numerous crustal fragments including the kimberley pine creek lucas and altjawarra cratons with sutures marked by the king leopold halls creek tennant creek and proto isan orogens the southern margin of the north australian craton was the site of repeated terrane accretion and orogenic activity between 1880 ma and 1400 ma this included an orogenic event at 1880 1850 ma the strangways 1780 1730 ma argilke 1680 1650 ma and chewings 1620 1580 ma orogenies and the intracratonic anmatjira uplift 1500 1400 ma intracratonic rifting at 1750 to 1710 ma and 1640 to 1600 ma produced the mcarthur basin and related minor basins parts of which were deformed by the isan orogeny at 1600 and 1530 ma rifting along the line of the capricorn orogen led to deposition in the overlying intracratonic bangemall basin between 1630 and 1300 ma along the eastern margin of the south australian craton the 1670 to 1600 ma olarian orogeny marks interaction with now obscured continental crust to the east tectonic activity between 1300 and 1100 ma led to the assembly of proterozoic australia as an early component of the supercontinent of rodinia this first involved the amalgamation of the west australian and north australian cratons followed by collision with the south australian craton the centralian superbasin developed over the junction of the north south and west australian cratons between 830 and 750 ma rifting to the east formed the adelaide geosyncline at 830 ma this was followed by the breakup of rodinia with the rifting apart of laurentia and gondwanaland along the eastern margin of proterozoic australia at 750 ma and the subsequent formation of the palaeo pacific ocean after the breakup of rodinia a series of northeast southwest compressional events followed by periods of relaxation reflect the assembly of a new supercontinent old lines of weakness were reactivated culminating in the intracratonic king leopold paterson petermann ranges and pinjarra orogenies between 620 and 540 ma subsequent reactivation continued into the phanerozoic with the widespread eruption of continental flood basalts and the formation of intracratonic basins 540 530 ma

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

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

volume self diffusion of oxygen in biotite muscovite and phlogopite micas the kinetics of oxygen self diffusion have been measured in natural samples of biotite muscovite and phlogopite micas under hydrothermal conditions using both bulk exchange and profiling analyses all experiments were run at 1000 bars 100 mpa water pressure except those designed to measure water pressure dependence which ranged from 200 to 2000 bars the arrhenius relations obtained for layer parallel diffusive transport in the three micas by bulk exchange experiments fit to an infinite cylinder model are mineral biotite muscovite phlogopite 34 2 39 5 42 3 9 1 10 6 7 7 10 5 1 4 10 4 t c 500 800 512 700 600 900 the results of ion microprobe sims depth profiling perpendicular to the layers parallel to c axis yield diffusion coefficients 3 to 4 orders of magnitude lower than for transport parallel to the layers a single layer parallel step profiling experiment yields a diffusion coefficient identical within uncertainty to those obtained from bulk exchange experiments a slight dependence of diffusion rate on water pressure may obtain in phlogopite however the difference is approximately within the uncertainty of the measurement diffusion rates are similar within about a half an order of magnitude for a range of biotite composition from annite4 to annite63 as well as for muscovite comparison of oxygen diffusion rates in biotite from this study with ar k and rb diffusion from published data supports the idea that water is the oxygen bearing species in hydrothermal diffusion experiments the implication of these results for natural systems is that the micas especially biotite will continue to exchange oxygen isotopes via volume diffusion down to temperatures as low as 300 c the rapid diffusion kinetics may explain biotite oxygen isotope disequilibrium which is commonly observed even in rocks where other mineral phases preserve equilibrium compositions full size table table options view in workspace download as csv

high resolution aeromagnetic data over central australia assist grenville era 1300 1100 ma rodinia reconstructions 1 high resolution aeromagnetic data covering the polydeformed and poorly exposed musgrave province of australia reveals the grenville aged crustal architecture a combination of upward continuation and the potential field tilt filter out magnetic anomalies relating to a late neoproterozoic orogenic event and emphasize the more subtle magnetic structural grain formed during the ca 1320 1150 ma musgravian orogeny the resulting images indicate that crustal architecture is defined by the distribution of ca 1150 ma magnetic granitoids within less magnetic basement and is dominantly northeast trending continuous beneath the amadeus and officer basins and defines an orogenic belt that connects mesoproterozoic provinces the orogenic belt is truncated within australia and precludes a direct east trending connection between the musgrave province and contemporaneous orogens in laurentia clockwise rotation of the south australian craton and subsequent collision with the north and west australian cratons is our preferred model to explain the grenville aged architecture observed in australia

comparing 1800 1600 ma accretionary and basin processes in australia and laurentia possible geographic connections in columbia abstract we focus on potential pinning points on the southern and eastern margins of australia and the southern and western margins of laurentia and interpret a convergent plate boundary along the southern margin of the proto australian continent during the palaeo to mesoproterozoic 1800 1600 ma specific geological events within the southern australian margin can be correlated with events preserved along the medicine bow yavapai mazatzal belt in southwestern laurentia these belts were characterised by southward continental growth with varying degrees of crustal reworking and accretion of juvenile material in australia crustal reworking appears to have been the dominant process from 1800 to 1670 ma with more juvenile material accreted at 1620 ma in contrast the southern laurentian terranes were dominated by accretion of juvenile material from 1800 to 1620 ma although reworked continental crust was also accreted to the margin in the interior of both continents basins developed with comparable histories from 1800 to 1670 ma thereafter the evolution of the interior basins diverged with sag phase processes dominant in northern australia and a protracted depositional hiatus in laurentia the geologic evolution is consistent with palaeomagnetic data and provides further evidence for geographic links between eastern australia and western laurentia from 1800 to 1600 ma

the low velocity layer in subduction zone structure and elasticity of glaucophane at high pressures abstract we investigated the structure equation of state and elasticity of glaucophane na2mg3al2si8o22 oh 2 up to 9 gpa which encompasses its experimentally observed stability field we find that the pressure volume results for glaucophane are well represented by a third order birch murnaghan formulation with k 0 81 gpa k 0 4 5 and v 0 899 4 a3 the full elastic constant tensor reveals significantly larger stiffness along the 1 0 0 plane the 1 0 0 direction is the relatively softer this could be rationalized in terms of the stacking of the stiffer tetrahedral units along 0 1 0 and 0 0 1 directions within the crystal structure glaucophane is a dominant mineral constituent of blueschist facies rock and has significantly lower velocities compared to garnet bearing eclogites in addition glaucophane is anisotropic and could account for the observed low velocity layer in the subducting slabs at depth range within the thermodynamic stability of glaucophane at high pressures beyond stability of glaucophane hydrous phase such as lawsonite could account for the observed low velocity layers in certain subduction zones

lap shear testing of biotite and phlogopite crystals and the application of interferometric strain displacement gages to mineralogy crystals of biotite and phlogopite were deformed in simple shear to determine the influence of compositional and structural variables on behavior during mechanical deformation the use of an interferometric strain displacement gage isdg with modified sample grips allowed the testing of small samples that did not contain significant partings along cleavage planes these grips also permitted the use of very small compressive forces resolved normal to the basal planes overall differences in strength between samples from different localities could not be simply related to compositional differences such as f content or with the density of growth spirals perpendicular to the basal planes which might impede the motion of basal dislocations differences in strength among samples from the same series were related to the orientation of the applied shear stress and these strength differences correlated with f content these represent the first known experiments with isdgs in mineralogy and demonstrate that there is significant potential for their application to geological problems

deep seismic reflection data suggest major compressional deformation of the crust in the central eromanga basin area during the mid carboniferous abstract scismic reflection data recorded to 20 s in the central eromanga basin area show good quality reflections from the deep crust as well as the shallow sedimentary layers the data along a 270 km long east west traverse indicate warping and low angle faulting in the lower crust which is compatible with asymmetric folding in the devonian sediments this interpretation is consistent with a major compressional deformation of the crust in this area during the mid carboniferous kanimblan orogeny

intracratonic strike slip partitioned transpression and the formation and exhumation of eclogite facies rocks an example from the musgrave block central australia mountain belts developed in an intracratonic setting during the neoproterozoic in the center of australia mesoproterozoic granulite and amphibolite facies gneisses of the musgrave block were overprinted during the petermann orogeny 550 ma with zones of high strain concentrated along broadly east west trending shear zones these zones formed under eclogite facies t 650 c and p 12 kbars to greenschist facies conditions and may form part of a strike slip related crustal scale flower type structure granulite facies gneisses preserve ages older than 700 ma suggesting that these rocks did not experience temperatures greater than 350 c at 550 ma for these rocks not to have been thermally equilibrated to temperatures of 350 c at a depth of 12 kbars requires either an extraordinarily low geothermal gradient 9 c km 1 or if the geothermal gradient was more typical 20 c km 1 then transport to that depth and subsequent exhumation occurring in a short time interval 40 myr

implications for proterozoic plate margin evolution from geophysical analysis and crustal scale modeling within the western gawler craton australia abstract complexly deformed late archean to mesoproterozoic rocks of the western gawler craton are poorly exposed thus reducing the ability to extract meaningful geological structural and tectonic information this study focuses on shear zone analysis via a constrained integration of qualitative aeromagnetic and bouguer gravity data interpretation with quantitative two dimensional forward modeling this integration provides a means to interpret the crustal architecture as well as constrain a sequence of events which aid the interpretation of a tectonic history for the western gawler craton results indicate a polyphase shearing history dominated by a crustal scale array of predominantly west dipping shear zones the initiation of east west trending sz1 structures was likely coincident with ca 1750 1720 ma crustal extension high heat flow and deposition on thinned areas of archean crust with the development of a dense possibly underplated lower crustal component sz1 r2 dextral transpressional movements reactivated sz1 during the ca 1720 1670 ma kimban orogeny which is interpreted to record collision between the gawler craton and the north australian craton during this event basins marginal to and in the interior of the archean crustal block of the western gawler craton were inverted and the dense lower crust was vertically offset causing some of the long wavelength 30 50 km bouguer gravity anomalies presently observed shear zone activity is constrained until ca 1680 ma during which time subduction rollback established a new arc position at the southern margin of the archean crust with magmatism persisting until ca 1670 ma sz3 structures overprint and offset the interpreted axis of the kimban orogeny within the archean crust these structures record a major phase of crustal scale sinistral strike slip movement associated with north south shortening between ca 1630 and 1540 ma prior to their reactivation at ca 1450 ma this sz3 r4 reactivation is interpreted to cause southwest directed crustal transport which was focused as west side up movements internal to the archean continent whereas dextral strike slip was focused at a major rheological boundary between thick archean crust to the south and thick paleoproterozoic basins to the north

structural styles and basin evolution in eromanga region eastern australia the eromanga region of eastern australia has undergone at least four major tectonic cycles the results of which are recognized in the structures seen on seismic sections the late proterozoic early phanerozoic cycle ended with the formation of the thomson foldbelt which forms basement in the region the subsequent reactivations of the fault systems within this basement and deeper lithosphere have produced the structural styles now important as exploration targets most reactivations are interpreted as resulting from crustal shortening events affecting sedimentary strata deposited in an epicontinental shallow water environment with small strike slip adjustments affecting some structures the dominant style of deformation evident from the seismic sections is that seen across high angle thrust faults in other parts of the world with the amount of overthrusting being comparatively small generally the maximum throw on faults in the devonian strata can be measured in thousands of meters whereas those in the permian triassic and jurassic cretaceous strata are a few hundred meters at most the devonian sequences of the adavale basin and associated troughs were deformed during two carboniferous events the first with crustal shortening in a north south direction and the second in an east west direction associated small strike slip movements were accommodated along a number of faults including the warrego grenfield warbreccan and canaway faults southwest of our study area in the permian triassic cooper basin other workers have interpreted a northwest southeast crustal shortening event during the late permian and another wrench induced northeast southwest crustal shortening during the triassic during the middle tertiary a crustal shortening event regionally affected the jurassic cretaceous eromanga basin sequence and older sedimentary rocks this event is seen pro inently in a deformation zone between the canaway and cunnavalla faults with the direction of crustal shortening being northeast southwest small strike slip movements are interpreted as partially decoupling events from neighboring provinces the adavale basin and cheepie shelf to the east and southeast and the cooper basin and warbreccan dome to the southwest and northwest respectively in this partial decoupling the canaway cunnavalla warrego and warbreccan faults are interpreted as playing an important role the basin modifying tectonic episodes within the lithosphere control structural style these relatively small scale events have resulted in structures that provide attractive petroleum prospects

influence of f oh 1 substitution on the relative mechanical strength of rock forming micas microtextural and experimental studies have yielded conflicting data on the relative mechanical strengths of muscovite and biotite wilson and bell 1979 kronenberg et al 1990 mares and kronenberg 1993 we propose a crystal chemical resolution to this conflict namely that 001 dislocation glide in biotite is rate limited by its fluorine content significant f oh 1 substitution and concomitant removal of hydroxyl h directed into the interlayer cavity potentially increases mechanical strength of biotite in two ways 1 it eliminates k h repulsion thereby strengthening the interlayer bonds and 2 it allows k to sink deeper into the interlayer cavity the resultant geometry being less favorable to basal slip in testing this hypothesis we analyzed the naturally deformed biotite studied by wilson and bell 1979 and documented its very low f content xf 0 02 compared to that of the biotite experimentally deformed by kronenberg et al 1990 our model and the comparative xf data explain why the biotite of wilson and bell 1979 deformed more easily in nature than its coexisting muscovite whereas the biotite of kronenberg et al 1990 was mechanically stronger than muscovite similarly deformed by mares and kronenberg 1993 our reconciliation of these otherwise conflicting results provides a framework for predicting mechanical strength of natural micas based upon the extent of their f oh 1 substitution our synthesis highlights the potential role of crystal chemistry in determining mechanical behavior in multicomponent mineral families further testing of crystal chemical effects on rheology will require mineral specimens of both appropriate composition and sufficient size

an assessment of a time of transition laser sizer in measuring suspended particles in the ocean abstract an assessment of the performance of the laser sizer cis 1 by galai for the size range of 0 5 150 m was conducted in a cruise across the continental shelf north of taiwan two types of measurements for the laser sizer were compared with the sizes analyzed by microscopic examination the measurements of cis 1 were performed with 1 a liquid flow pump system and 2 a small volume cuvette system the results of this study showed that although the liquid flow system analyzed a larger volume of water sample its measurements compared badly with that of microscopic examination but occasionally accurate results did appear when the particles were less fragile the measurements with the cuvette system generally agreed well with that of microscopy the mean value of the average deviation between the measurements of microscopy and the cuvette system was 33 it was also found that the narrower the particle size spectrum the more accurate the cis 1 s measurements were

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

crustal architecture across phanerozoic australia along the eromanga brisbane geoscience transect evolution and analogues abstract the crustal architecture across phanerozoic australia has been established from seismic data along an 1100 km long east west transect the eromanga brisbane geoscience transect this has enabled a better understanding of deep structures and processes that have controlled the development of major sedimentary basin systems it has shown that crustal dynamics throughout geological history have played an important role in the development of these sedimentary basins and that structures developed during the early palaeozoic have influenced and continue to influence basin systems the transect crosses three major basement provinces of the tasman orogenic system in eastern australia the thomson northern lachlan and new england orogens the basement geology in the transect region has until now been only poorly understood because it is largely obscured by the mesozoic cover rocks of the eromanga surat and clarence moreton basins the transect interpretation has firmly identified crustal scale ramp structures multiple intra crustal detachment surfaces strike slip fault architecture lower crustal magmatism underplating mono remobilisation and intra crustal terranes in the geological reconstructions of southern queensland the boundaries between the orogens the foyleview and burunga mooki geosutures have been identified as lithospheric scale structures that have influenced the evolution of the tasman orogenic system as a whole

stratigraphy and depositional setting of the upper mcnamara group lawn hills region northwest queensland the mesoproterozoic upper mcnamara group forms the uppermost division of the lawn hill platform in the western mount isa block several zinc and lead occurrences including the century deposit are hosted by clastic and carbonate sediments within this interval the upper mcnamara group is subdivided into four formations the shady bore quartzite riversleigh siltstone termite range formation and lawn hill formation the shady bore quartzite comprises up to 460 m of sandstone and dolomitic siltstone deposited in a marginal to shallow subaqueous environment the riversleigh siltstone is up to 2 900 m thick in the southern lawn hill region where four members r 1 to r 4 are recognized member r 1 to r 3 sandstone carbonate and shale were mostly deposited in mid and outer shelf environments indicating a regional transgression above the shady bore quartzite sediment transport during the r 1 to r 3 interval was mostly from the south member r 4 shale and sandstone represent deeper subaqueous deposits riversleigh siltstone sedimentation focused on a depocenter in the southern lawn hill region all members display an abrupt decrease in thickness and facies changes to the north of the termite range and barramundi faults which are interpreted as active structures during deposition the termite range formation is up to 1 300 m thick and comprises two fining upward sequences t 1 and t 2 both dominated by sandstone in the lower part and by shale in the upper part the termite range formation is sandiest in the north and thins to the south and east sandstone beds are interpreted as high concentration turbidites deposited in a deep subaqueous environment t 1 and t 2 are interpreted as two major episodes of turbidite fan sedimentation sourced from the northwest fault activity to the west of the lawn hill region at about 1640 ma is inferred to account for deposition of sandstone and the major shift in provenance from the south to the northwest the lawn hill formation is 2 200 m thick in the southern lawn hill region where six members h 1 to h 6 are recognized member h 1 shale and sandstone were deposited in outer shelf and deeper environments member h 2 and h 3 tuffaceous sediments were deposited in a mid shelf environment indicating a regional regression member h 1 to h 3 sediments were sourced from the west member h 4 shale and siltstone were deposited at outer shelf and greater depths indicating a regional transgression above member h 3 member h 5 sandstone beds are interpreted as high concentration turbidites deposited in prograding turbidite fans member h 1 to h 3 sedimentation occurred throughout the lawn hill region but a major depocenter was located in the northeast at this time member h 1 h 2 and h 3 sediments rapidly thicken to the north of the termite range fault which is interpreted as an active structure during deposition of this interval the northward shift of the depocenter with respect to the riversleigh siltstone can be attributed to a reversal in movement along the termite range fault member h 5 sandstone beds are widespread throughout the northern lawn hill platform and their deposition indicates another significant change in the distribution and style of sedimentation deposition in response to uplift in the south can be interpreted by the age of the sandstone 1595 ma which may coincide with the onset of the d 1 phase of the isan orogeny this study provides a framework for the identification of prospective areas for base metal mineralization within the upper mcnamara group based on a knowledge of sediment architectures and their controls

sedimentary processes of fine grained material and the effect of seawall construction in the daeho macrotidal flat nearshore area northern west coast of korea abstract sedimentary processes on the lower tidal flat of the reclaimed seosan bay daeho tidal flat at the west coast of korea have undergone significant changes since 1984 when seawalls over 8 km long were constructed at the bay mouth comparison of pre and post construction tidal flow models indicates that in general tidal currents have increased considerably in front of the daeho tidal flat with tidal flow directions having changed from shore normal to shore parallel the 12 h measurements of currents and suspended sediment concentrations at four anchored hydrographic stations show nearshore suspended sediments drifting southward throughout the year bypassing the daeho tidal flat cyclic seasonal variations occur prominently in the grain texture of the tidal flat sediments and associated sedimentation rates in summer fine grained sediments deposit at a rate of 2 6 mm month particularly in sheltered places during the winter monsoon erosional processes are greatly enhanced by the interplay of tidal currents and waves up to 30 mm month this sedimentary seasonality is reflected in the variations in the concentration of nearshore suspended matter the concentration of suspended matter is much higher in winter 60 80 mg l because of vigorous resuspension activities compared with the summer concentration 10 30 mg l the direct effect of seawall construction coupled with strong natural seasonality can be seen from the continuous monitoring of sedimentation rates on the tidal flat the 2 year 1993 1995 measurements indicate that the entire daeho tidal flat was eroded during that period at the abnormally high average rate of 10 cm year the causal blocking of bayward sediment and water movements by any artificial structure might lead to similar erosional conditions to many other bays facing the yellow sea on the both chinese and korean coastlines subjected to energetic winter wave attack

timing and exhumation of eclogite facies shear zones musgrave block central australia timing constraints on shear zones can provide an insight into the kinematic and exhumation evolution of metamorphic belts in the musgrave block central australia granulite facies gneisses have been affected to varying degrees by mylonitic deformation some of which attained eclogite facies the davenport shear zone is a dominant strike slip system that formed at eclogite facies conditions t 650 c and p 12 0 kbar sm nd mineral isochrons obtained from equilibrated high pressure assemblages as well as 40ar 39ar data show that the eclogite and greenschist facies high strain overprints were coeval at c 550 ma mylonitic processes do not appear to have reset the u pb system in zircon but may have partially disturbed it the thermal gradient in the musgrave block crust at c 550 ma was c 16 c km 1 and at c 535 ma was c 18 c km 1 based on p t estimates of eclogite and greenschist facies shear zones respectively these estimates are similar to present day geothermal gradients in many stable continental shield areas suggesting that the region did not undergo a significant transient perturbation of the geotherm therefore in the musgrave block cooling subsequent to eclogite facies metamorphism appears to have been controlled by exhumation rather than by the removal of a heat source estimated exhumation rates in the range 0 2 to 1 5 mm year 1 are comparable with other orogenic belts rather than cratonic areas elsewhere

new seismic reflection results in the central eromanga basin queensland australia the key to understanding its tectonic evolution abstract regional seismic traverses were recorded from 1980 to 1982 by the bureau of mineral resources geology and geophysics as part of a major multidisciplinary program aimed at studying the geological evolution of the central eromanga basin area and its petroleum potential six fold cdp reflection data were obtained to 20 s reflection time on 1400 km of traverses up to 400 km long crossing the main structural features of the area additional seismic reflection information to complement the study was obtained from 2300 km of older mainly single coverage analogue data which was transcribed to digital format and reprocessed the central eromanga basin area has a complex history and contains four phanerozoic potentially hydrocarbon bearing basins each separated by unconformities i e the devonian adavale basin the late carboniferous to triassic cooper and galilee basins and the jurassic cretaceous eromanga basin the seismic results are being interpreted with the aid of synthetic seismograms from key wells to provide information on the extent nature and relationship of these basins the quality of the reflection data from the devonian sequence is generally very good and these data are proving to be most useful in the study thick devonian sediments were deposited over a wide area well beyond the present confines of the adavale basin and its associated troughs intensive folding and faulting accompanied by major basement uplift including the canaway ridge in the centre of the area took place during the mid carboniferous kanimblan orogeny this was followed by a period of erosion which truncated the devonian sequence and resulted in separation of the adavale basin from the other devonian troughs in the area the preliminary seismic results give some indication also of the extent and nature of the cooper and galilee basins sediments these are generally thin in the central area and thicken to the west and east respectively where coal measures produce good lithologic and seismic markers gentle folding and minor faulting apparent in the seismic reflection sections from the overlying eromanga basin sequence have resulted mainly from basement uplift and minor movements along pre existing faults continuing into the late mid tertiary and possibly from some compaction of the thick sedimentary section the seismic information is also assisting in studies of the petroleum potential of the area providing information particularly on structuring timing and depth of burial of the prospective sediments

tectonic framework and evolution of the gawler craton southern australia the gawler craton is the major crustal province in the southern australian proterozoic and is pivotal in models seeking to describe the evolution of proterozoic australia the craton is host to the olympic dam iron oxide cu au u ree deposit as well as a number of other iron oxide copper gold iocg au and iron ore deposits the evolution of the gawler craton is dominated by two major phases of tectonic activity late archean and late paleoproterozoic early mesoproterozoic in total spanning ca 1 billion years the late archean 2560 2500 ma basin development was coeval with arclike felsic magmatism and mafic ultramafic magmas including komatiites collisional deformation between 2480 and 2420 ma led to ca 400 m y of tectonic quiescence conceivably within an early paleoproterozoic continental interior the second major phase of tectonic activity was in the middle to late paleoproterozoic and early mesoproterozoic 2000 1450 ma paleoproterozoic tectonism was initially dominated by rift related events which produced a series of basins over the interval ca 1900 to 1730 ma some of which contain significant iron ore reserves transient contractional deformation at ca 1850 ma led to batholith scale granitic magmatism in part derived from melting of the late archean basement major basin development was finally terminated by the 1730 to 1690 ma kimban orogeny the effects of which are widespread and include the formation of crustal scale shear systems granitic magmatism and low to high grade metamorphism the kimban orogeny was followed by a renewed period of extension between ca 1680 and 1640 ma leading to local magmatism and sedimentation which appears in part to have been coeval with the high grade ca 1650 ma ooldean event in the western gawler craton at ca 1620 to 1615 ma the generation of the arc related st peter suite in the southern gawler craton implies that craton components were located at an active plate margin the arc environment was superseded by a transition to a continental interior regime which coincided with the voluminous gawler range volcanics 1595 1590 ma and hiltaba suite granitoids 1595 1575 ma the hiltaba suite is dominated by high t fractionated felsic rocks with coeval mafic magmatism confirming a mantle involvement emplacement of the hiltaba suite coincided with the major mineralizing interval in the gawler craton regionally two distinct mineral systems have been recognized the olympic iocg province in the eastern part of the craton and the gold dominated systems within the central gawler gold province the spatial distribution of iocg versus au dominated mineral systems appears to reflect regional variations in crustal composition and hiltaba suite petrogeneses hiltaba aged granites in the olympic iocg province are isotopically more evolved richer in u and th and oxidized compared to similar aged granites in the au dominated province modern day heat flow in the iocg province is significantly higher 90 10 mwm 2 compared to the au dominated province 54 5 mwm 2 suggesting there are important lithospheric compositional differences between the two metallogenic provinces which may reflect an older phase of craton assembly widespread northwest southeast contractional deformation coeval with the emplacement of the hiltaba suite is expressed by the formation and or reactivation of shear zones ranging up to crustal scale within this regime northwest trending structures such as those in the vicinity of the olympic dam deposit are likely to have accommodated dilation associated with strike slip movements intersection between these structures and northeast trending contractional faults may have formed suitable structural traps for 1590 to 1580 ma mineralization the timing of syn hiltaba deformation overlaps with transpression and medium to high grade metamorphism associated with the ca 1570 to 1540 ma kararan orogeny which dominates the geophysical architecture of the central northern part of the craton the youngest phase of deformation in the craton is expressed by reactivation of shear zones between ca 1470 and 1450 ma and regional cooling

interlayering and interlayer slip in biotite as seen by hrtem partial interlayers of brucite like sheets which form with the adjacent talc like layers of the mica single chlorite layers interstratified in the mica structure brucite interlayering is interpreted as a brucitization of an interlayer level of the mica where partial slip or cleavage has previously occurred example of a chemical process at the atomic scale favored by deformation microstructures modified journal abstract

the crustal scale architecture of the eastern succession mount isa the influence of inversion the three dimensional crustal architecture of the eastern part of the mount isa inlier is investigated from serial cross sections constructed using geological map data revised chronostratigraphy gravity magnetics worms multiscale wavelet edges of potential field data and seismic data the top part of the crust consists of rift and platform type metasediments that were deposited in three cover sequences from 1850 to 1610 ma these rocks constitute the mount isa eastern succession and they were intruded by mafic felsic plutons dykes and sills of various ages before and during the isan orogeny ca 1 6 1 5 ga the eastern succession overlies a felsic metamorphic basement which in turn sits on a tonalitic gabbroic lower crust the depositional basin architecture for the eastern succession was controlled by major n s trending structures that penetrated the lower crust and accommodated e w extension these structures also underlie major upper crustal structures such as the mitakoodi culmination and snake creek anticline that were formed by contraction in the isan orogeny positive inversion may therefore have been a key process in the evolution of the eastern part of the inlier and governs its architecture at the crustal scale inversion involved reactivation of basement penetrating structures which localised contractional structures in the cover sequences above as well as influencing pluton emplacement the felsic metamorphic basement may have been penetratively deformed during inversion the spatial association between the basin controlling and contractional structures suggests that either early extensional displacements were completely reversed by later contraction or that much of the eastern succession has remained essentially parauthochthonous relative to the basement

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

three dimensional inversion modelling of a neoproterozoic basin in the southern prince charles mountains east antarctica abstract three dimensional inversion modelling of airborne magnetic data acquired in the prince charles mountains east antarctica provides an insight into the sub ice distribution and three dimensional geometry of a neoproterozoic sedimentary basin a three dimensional starting model was created from two two dimensional gm sys modelling and our current geological understanding of the study area three dimensional vpmg inversion modelling was performed on the aeromagnetic data to obtain an acceptable fit between the observed response collected in the field and the calculated response of the three dimensional model modelling suggests that the base of the basin undulates and is relatively unstructured however the margins of the basin thicken from north 4 km to south 11 km the volume of the sodruzhestvo group that in fills the sedimentary basin has been calculated at approximately 35 000 km3 modelling of banded iron formations at the southern margin of the basin and a sharp magnetic contrast in the north reveal that both contacts dip toward the south we interpret this asymmetric geometry of the sedimentary basin as having began as a set of half grabens bounded by an underlying listric fault that flattened at depth below the sedimentary basin subsequent early palaeozoic inversion of this structure resulted in reactivation along the low angle basal detachment but rather than taking its original course underneath the sedimentary basin the fault ramped up along the southern margin this process caused exhumation of the underlying banded iron formations which are now juxtaposed at similar crustal levels to the neoproterozoic cover rocks

estuarine flocs their size settling velocity and density the size and settling velocity of flocs of suspended sediments from the chesapeake bay were measured in laboratory experiments and in the field the curves between floc diameter and settling velocity showed a non stoke s relationship that was best described by the following settling velocity cm s 1 73 floc diameter cm 0 78 furthermore the relationship between floc diameter and settling velocity indicated that the floe density decreased as the floes increased in size in addition to changing density a cylinder shape was found to be the best shape factor