identifying granite sources by shrimp u pb zircon geochronology an application to the lachlan foldbelt the potential genetic link between granites and their host sediments can be assessed using zircon age inheritance patterns in the lachlan fold belt southeastern australia granites and associated high grade metasedimentary rocks intrude low grade ordovician country rock this relationship is well exposed in the tallangatta region northeast victoria part of the wagga omeo metamorphic complex in this region granites two i types and two s types have intruded during the mid late silurian between approximately 410 430 ma based on the ages of magmatic zircons the age spectra for inherited zircons from the granites have been compared with those of detrital zircons from the enclosing low and high grade metasediments in broad terms both for detrital zircons in all four sediments and for inherited zircons in three of the four granites the dominant ages are early paleozoic and late precambrian with sporadic older precambrian ages extending up to 3 5 ga the ages of the youngest detrital zircons from the low grade lockhart and talgarno terranes limit the time of sedimentation to ca 466 ma or younger the youngest detrital zircons from two samples of the high grade gundowring terrane are 473 ma making these sediments ordovician or younger not cambrian as originally suggested however the individual age spectra for the four selected metasediments are not well matched when closely examined the age spectra of the inherited zircons in the granites also do not adequately match those in any of the metasediments thus the metasediments might not be representative of the actual source rocks of the granites while the exact source of the granites cannot be identified from the analysed samples the existence of a large population of ca 495 ma inherited zircon grains in the s type granites requires that the granite source contains a significant proportion of cambrian or younger material this does not preclude the existence of a precambrian basement to the lachlan fold belt but indicates that at the level of s type magma generation a cambrian and or younger protolith is required

regional structural controls of gold mineralisation bendigo and castlemaine goldfields central victoria australia the bendigo and castlemaine goldfields are classic examples of structurally controlled orogenic gold deposits in the bendigo zone of central victoria se australia detailed mapping and biostratigraphic interpretation has led to a better understanding of the regional structural controls of this type of gold quartz mineralisation mineralised quartz veins are hosted by the castlemaine group an early to middle ordovician turbidite succession at least 3 000 m thick gold deposits are controlled by low displacement faults that are clustered into several belts the goldfields indicating a regional structural control the timing of mineralisation overlapped with that of the major period of deformation including folding cleavage development and regional faulting the bendigo and castlemaine goldfields are located in an area termed the whitelaw thrust sheet bounded by two unmineralised high displacement regional scale faults mapping has revealed an interrelationship between the regional scale faults regional structural style and goldfield location the goldfields lie immediately west of the boundary between the upper and lower portions of the thrust sheet and are characterised by symmetric folds with sub horizontal to synclinal enveloping surfaces relatively low co axial strains and moderate cleavage development the non gold bearing areas immediately east of each goldfield correspond with the lower part of the whitelaw thrust sheet and are characterised by higher non coaxial strains stronger cleavage and folds with wide west dipping limbs giving rise to easterly vergent sections and steeply west dipping enveloping surfaces that mineralisation was an integral part of the thin skinned style of deformation in the central bendigo zone is indicated by timing relationships and the interrelationship between local scale mineralised structures and regional scale features such as large displacement unmineralised faults regional variations in fold style and overall thrust sheet geometry the work supports previous models that suggest mineralised fluids were focussed along a linked system of deep seated faults the primary conduits may have been major regional scale intrazone faults which are inferred to sole into detachments near the base of the castlemaine group it is proposed that these structures linked with minor intrazone faults and then with networks of low displacement mineralised faults that were strongly controlled by folds the location of minor intrazone faults was probably controlled by internal thrust sheet geometry the distribution of gold deposits and of gold production suggests that maximum fluid flow was concentrated along the eastern margins of networks of low displacement faults

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

the ocean continent boundary off the western continental margin of iberia i crustal structure at 40 30 n summary the western continental margin of the iberian peninsula has the characteristics of a rifted non volcanic margin with half graben and tilted fault blocks seen in several places the ocean continent boundary ocb is therefore expected to be where thinned continental crust and oceanic crust are juxtaposed as at many similar margins worldwide it is particularly useful to locate the ocb off western iberia in order to constrain the pre rift fit of north america to iberia and by implication the shape of the proto bay of biscay the fit is only marginally constrained by sea floor spreading magnetic anomalies because anomaly 34 is believed to be far to the west of the ocb and it is even possible that all older oceanic crust was created during the cretaceous constant polarity interval the best way to distinguish oceanic crust from thinned continental crust appears to be the crustal seismic velocity structure therefore in 1986 a series of seismic refraction lines was shot parallel to and normal to the continental margin these lines enabled us to bracket the location of the ocb a further constraint on the location was obtained by modelling an east west magnetic profile which included the enigmatic j anomaly this anomaly can be explained as either just pre anomaly mo or as part of the cretaceous constant polarity interval depending on whether spreading began about 127 or after 118 myr ago respectively the evidence favours the former explanation lastly the depth to acoustic basement was contoured from a compilation of seismic reflection profiles this indicated a new fracture zone at 41 15 n which offsets the ocb a few key reflection profiles also suggest that the ocb can be identified by an abrupt landward step down in acoustic basement we conclude that the ocb in the eastern iberia abyssal plain lies between 12 10 and 12 30 w and has a trend just east of north this westerly location is consistent with recent estimates of the location of the ocb off the grand banks but brings into question the proposed location at about 11 w of the ocb in the tagus abyssal plain

a low velocity zone within the layer 3 region of 118 myr old oceanic crust in the western north atlantic summary during the north atlantic transect nat experiment 11 expanding spread profiles esps were acquired using a 3300 cubic inch 30 element tuned airgun array s v prospekta and a 48 channel seismic streamer r v moore extremely high density seismograms were obtained that have allowed us to transform the observed data to the domain of intercept time and ray parameter tau p with high precision velocity structures have been obtained by tau sum inversion ray tracing and synthetic wkbj and reflectivity seismogram methods both in tau p and x t domains a detailed analysis of esp 5 suggests the presence of a low velocity zone within the lower oceanic crust at the location of magnetic anomaly mo 118 ma it is apparently 2 86km thick has a p wave velocity of 6 5 km s 1 s wave velocity of 3 6 km s 1 and occupies much of the depth interval normally associated with oceanic layer 3 converted shear wave arrivals have been used to obtain the shear velocity structure we find that the vp vs ratio in the uppermost part of the crust is about 1 9 and much of the crustal section has a vp vsratio greater than 3 attenuation of p and s waves has been studied by amplitude analysis and we find that in most of the lower crust qs is equal to qp comparison with solutions from esps in younger and older parts of the basin suggests that esp 5 is unique in displaying a low velocity zone despite this the total thickness at esp 5 is not significantly different than at other locations we suggest that this provides strong evidence that the low velocity zone occurs within the lower crust and does not result from serpentinization of the upper mantle beneath thin oceanic crust

deep crustal structure and magmatic processes the inception of seafloor spreading in the norwegian greenland sea summary an extensive investigation into the deep crustal structure of the conjugate norwegian and e greenland margins utilizing two ship expanded spread and wide aperture cdp profiling has led to the identification of a unique suite of crustal structures developed during the onset of seafloor spreading on both margins a region of low velocity crust was found proximal to marginal escarpments immediately seaward of the low velocity crustal block seaward dipping reflectors occur within thick crust that is interpreted to be wholly igneous a product of voluminous melt production enhanced by convective partial melting processes this earliest formed oceanic crust is actually several kilometres thicker than the low velocity crust or the adjacent continental crust normal oceanic crustal thicknesses are not produced until about 5 my after the initiation of spreading when convective partial melting has abated and the magma budget has reduced the extrusive unit reduces to normal thicknesses long before the intrusive and plutonic section indicating that the thickening or thinning of oceanic crust is accomplished primarily by variations in the latter

crustal structure of the sheared southwestern barents sea continental margin abstract eighteen expanding spread profiles from the southwestern barents sea continental margin were studied with a variety of seismic interpretation techniques and in conjunction with seismic reflection data the velocity depth profiles from the slope intercept herglotz wiechert and sum methods were compared and contrasted to the velocity depth curve calculated from the iterative forward modelling of the amplitudes the velocity depth profiles derived from the sum and forward modelling of the amplitudes were used to produce synthetics in both the x t and p domains to choose the technique that most accurately represented the data the results showed the oceanic crust at the senja fracture zone to be different from standard oceanic crust with a higher than average velocity for layer 2 of 5 7 6 4 km s 1 and slightly smaller crystalline crustal thickness the continent ocean transition at crustal depths was shown to be narrow in the range of 10 20 km our results were consistent with two different crustal provinces east of the senja fracture zone the tertiary marginal basin and the cretaceous basin region the velocity depth curves enabled us in a gross sense to suggest the depths to deep sedimentary interfaces and the top of the crystalline crust that were normally not observed in the reflection profiles depths down to 20 km were inferred for the base of the sedimentary section in the southwestern barents sea the crust thinned gently towards the continent ocean boundary at the senja fracture zone whereas the crustal thickness changed abruptly below the hornsund fault zone at the margin farther north we associated the change in crustal structure with cenozoic in the thick crust beneath the svalbard platform in the north the depths to mantle determined on the shelf were consistent with that observed on a deep multichannel seismic line that crosses the area

crustal structure of the french guiana margin west equatorial atlantic summary geophysical data from the amazon cone experiment are used to determine the structure and evolution of the french guiana and northeast brazil continental margin and to better understand the origin and development of along margin segmentation a 427 km long combined multichannel reflection and wide angle refraction seismic profile acquired across the southern french guiana margin is interpreted where plate reconstructions suggest a rift type setting the resulting model shows a crustal structure in which 35 37 km thick pre rift continental crust is thinned by a factor of 6 4 over a distance of 70 km associated with continental breakup and the initiation and establishment of seafloor spreading the ocean continent boundary is a transition zone up to 45 km in width in which the two layered oceanic type crustal structure develops although relatively thin at 3 5 5 0 km such thin oceanic crust appears characteristic of the margin as a whole there is no evidence of rift related magmatism either as seaward dipping sequences in the reflection data or as a high velocity region in the lower crust in the p wave velocity model and as a such the margin is identified as non volcanic in type however there is also no evidence of the rotated fault block and graben structures characteristic of rifted margins consequently the thin oceanic crust the rapidity of continental crustal thinning and the absence of characteristic riftrelated structures leads to the conclusion that the southern french guiana margin has instead developed in an oblique rift setting in which transform motion also played a significant role in the evolution of the resulting crustal structure and along margin segmentation in structural style

neodymium isotopic composition of ordovician conodonts as a seawater proxy testing paleogeography 1 to evaluate recent ordovician paleogeographic reconstructions end values were determined in conodonts as a proxy for the isotopic variations in ancient seawater using samples from the major cratons and microplates isotopic variations reveal the existence of distinct oceanic masses and epeiric seas that constrain the position of plates and terranes relative to these water masses and hence constrain models of regional and global paleogeography the isotopic patterns show a consistent picture of the changes in the ordovician oceans the end values for water masses associated with laurentia are strongly negative 28 to 18 in the early ordovician evolving over time to higher values in the range 13 to 5 the early ordovician signature of laurentia is in marked contrast to other cratons and microplates which have a range of values from 10 to 5 samples from south china show interesting signals that reflect a greater similarity to laurentia than other peri gondwana terranes the isotopic variations are a function of both regional geology and global tectonic processes the most obvious being the taconic orogeny and onset of the closure of the iapetus ocean regional and global models of paleogeography are considered in light of these proxy signals this study also reveals that conodonts are powerful geochemical tools for obtaining information on ancient water masses the use of the nd isotopic signatures from conodonts provides a method independent of paleomagnetism and biogeography to test paleogeographic models by integrating such information a more multidisciplinary approach is possible

high seismic velocities associated with reflections from within the lower oceanic crust near the continental margin of eastern canada abstract a deep seismic reflection profile on the continental rise off eastern canada shows strong dipping reflections apparently from within lower oceanic crust over a region of limited lateral extent the origin of these and similar reflections described elsewhere is uncertain this paper reports new seismic refraction results from the region of these reflections they appear to be associated with a layer of p wave velocity of about 7 4 km s 1 or higher while the surrounding crust has a velocity 7 km s 1 the moho with velocity 8 1 km s 1 lies just below the reflectors and the total crustal thickness is around 8 km the crust is interpreted to be oceanic rather than continental and the high velocities of the dipping reflectors are comparable to those of the mafic ultramafic crust mantle transition zone in the bay of islands ophiolite complex

oroclines in the tasmanides abstract oroclines are features of orogenic belts that exhibit curvature in plan on a map scale with the additional requirement that this curvature resulted from relative rotation of the limbs around a vertical or near vertical axis much debate has centred on contrasts between thin skinned and thick skinned systems and on the question of whether the structural fabric was imposed prior to or synchronous with the development of curvature leading to a variety of inconsistent terminology a simple hierarchy in which all map scale curved orogenic features which pass a test for rotation are termed oroclines as distinct from primary arcs and in which these are then subdivided by timing into progressive synchronous fabric and curvature and secondary pre existing fabric is then rotated and by mechanism into thin or thick skinned is applied to a suite of curved orogenic features in the tasmanides of eastern australia this analysis includes the first formal application of the orocline test palaeomagnetic or palaeocurrent directions plotted against structural strike to the tasmanides the neoproterozoic to early mesozoic tasmanides comprise four orogens each of which contains curved orogenic features which are highlighted by extensive aeromagnetic datasets the delamerian orogen developed on the gondwana margin exhibits two curved structures in the neoproterozoic to cambrian adelaide fold belt termed the fleurieu and nackara arcs previous structural interpretations have indicated that the nackara arc is non rotational and hence not oroclinal re examination of existing palaeomagnetic data including a positive palaeomagnetic orocline test shows the nackara arc to in fact be a hinge of the adelaide orocline curvature in the adelaide orocline was dominantly progressive and thin skinned but may have been influenced by secondary rotation of the pre existing partly underlying curnamona craton mega kinks in a buried delamerian arc volcanic chain revealed in aeromagnetic images follow the curvature of the nackara arc suggesting an accompanying thick skinned secondary oroclinal deformation further from the continental margin orogenic curvature in the palaeozoic lachlan orogen has only been widely recognised since the advent of tilt filtered aeromagnetic images and a model of oroclinal folding with two hinges riverina and tambo and strike slip offset during the silurian bindian orogeny has recently been proposed to explain perplexing repetitions of sequence and inversions of structural vergence and directions of sediment provenance palaeomagnetic data indicates rotation of elements within the proposed orocline and a preliminary study indicates a positive palaeomagnetic orocline test for the riverina hinge palaeocurrent data around the tambo hinge provide a positive if scattered orocline test an independently conceived numeric model of the roll back response to congestion of a subduction zone has generated an oroclinal structure which shares many features with the proposed lachlan orocline and deep crustal ambient noise seismic data confirms that the major curved and strike slip features persist throughout the crust the lachlan orocline appears to be secondary and thick skinned orogenic curvature in the largely covered thomson orogen occurs along its southern border with the lachlan orogen the putative olepoloko orocline and around its geometrically complex boundary with the mossman orogen in northern queensland the charters towers orocline rotation of the olepoloko structure cannot be confirmed and competing models for its formation leave open the question of whether this is a primary arc a thin skinned progressive orocline or a thick skinned secondary orocline palaeomagnetic data from the charters towers orocline are limited to two poles but a change in declination between the silurian pole and the devonian pole matches the apparent rotation of the orocline which appears to have been thick skinned and secondary in contrast to the other orogens consensus that at least part of the curvature in the palaeozoic to triassic new england orogen is oroclinal has been broad although not universal there remains substantial debate over the number of hinges from two to four and mechanism palaeomagnetic poles have previously been cited as evidence of rotation of blocks within the orocline but this paper presents the first formal palaeomagnetic orocline test which is positive for the manning and texas hinges a palaeocurrent orocline test of the manning hinge in younger rocks than the palaeomagnetic sample is negative constraining rotation in the southern manning hinge to the carboniferous before 322 ma while rotation in the northern texas hinge appears to be latest carboniferous or permian the existence of a hastings hinge is questionable but if real its rotation also appears to be younger than that of the manning hinge thick skinned secondary rotation of the hinges of the new england orocline appears to have been diachronous

silurian to mid devonian basin development of the melbourne zone lachlan fold belt southeastern australia abstract the melbourne zone comprises early ordovician to early devonian marine turbidites which pass conformably upward into a mid devonian fluviatile succession there are four pulses of silurian to mid devonian deep marine sandstone dominated sedimentation early silurian late llandovery late silurian ludlow earliest devonian lochkovian and late early devonian emsian two dispersal patterns have been defined using more than 1100 palaeocurrent measurements mainly from sole marks and cross laminations in graded beds together with sandstone compositions the older pattern of silurian to earliest devonian age contains the lowest three sandstone pulses palaeocurrents and provenance define a wedge of southwesterly derived sediment of largely cratonic provenance thinning eastward this older dispersal pattern is part of an early ordovician to earliest devonian east facing passive continental margin succession palaeocurrents and provenance in the emsian sandstone pulse comprise three patterns 1 west to southwesterly directed palaeocurrents associated with fine to coarse grained locally conglomeratic lithic sandstones containing a high proportion of volcanic detritus 2 east to northeasterly directed palaeocurrents associated with fine to medium grained quartz lithic sandstones 3 north to northwesterly and south to southeasterly directed palaeocurrents associated with fine to medium grained sandstones of variable lithic composition the palaeocurrent and provenance pattern defines a nnw elongate basin with a tectonically active eastern margin and is similar to the coeval mathinna basin of northeastern tasmania both basins are part of the same system of wrench basins which developed along the western side of the wagga omeo metamorphic belt during the earliest devonian to middle devonian the change in tectonic setting in the earliest devonian appears to have occurred during an interval of significant dextral translation of the eastern lachlan fold belt towards the sse along the governor and associated fault zones

extensional model for the subsidence of the northern united states atlantic continental margin the simple extensional model for the subsidence of the northern united states atlantic continental margin attributes subsidence which results in sedimentary basin formation to extension and thinning of continental lithosphere the model is used to predict the tectonic subsidence histories that would be associated with various amounts of extension actual tectonic subsidence history curves are calculated from data collected in the continental offshore stratigraphic test cost wells b 2 b 3 g 1 and g 2 and are found to be consistent with the predictions

the seismic velocity structure of northern appalachian crust around western newfoundland the crustal velocity structure beneath western newfoundland is constrained by a reanalysis of older regional refraction profiles together with an analysis of one new profile initial interpretation of the older data gave inconsistent and discontinuous structures that are difficult to reconcile with more recent deep reflection profiles we also show that traveltimes predicted by the earlier models often do not yield acceptable fits to the original observations our reinterpretation reveals a simpler pattern in which the crust is characterized by a persistent high velocity lower crustal hvlc layer this layer has velocities of 7 0 7 9 kmls and thicknesses of 5 23 km it is thickest beneath the grenville crustal block east of the appalachian structural front and thins or is possibly absent within the central block analysis of the new much higher resolution profile off western newfoundland confirms the existence of the hvlc layer with a velocity of 7 2 kmls and thicknesses of 11 19 km increasing to

physical modelling of deformation in the tasman orogenic zone abstract structural vergence within the western subprovince of the lachlan fold belt is towards the hinterland rather than the foreland in contrast to many well known orogenic belts high angle reverse faults and upright folds verge eastwards away from the australian craton towards the inferred centre of orogenic and magmatic activity we designed a series of analogue models to test the anomalous vergence in the western lachlan fold belt particularly the interaction of a stable australian craton with tasman line geometry interacting with weaker oceanic or transitional lithospheric material we found consistently that vergence direction in the models was towards the hinterland not the foreland as in the western lachlan fold belt irrespective of the way the model was deformed strength gradients between the oceanic and cratonic lithosphere control the deformation patterns an important result of the models is that they demonstrate that fold belts with different vergences can be generated without the requirement of subducting oceanic lithosphere

evolution of proterozoic terrains beneath the williston basin a new tectonic map of the precambrian basement beneath the phanerozoic williston basin of central north america shows the archean superior craton separated from the archean churchill and wyoming cratons by a broad region of mostly proterozoic origin included in the proterozoic terrain are the vestiges of two or more volcanic island arcs and associated fore arc and back arc basins the north american central plains electrical conductor and related seismic low velocity zones are explained by the presence of hydrated oceanic type crustal material within the remnant back arc basins and adjoining terrains using geological geophysical and geochronological data from the williston basin and exposed canadian shield we derive a plausible plate tectonic model for the proterozoic evolution of this region

crustal structure of a nonvolcanic rifted margin east of newfoundland a detailed seismic refraction study was undertaken of the rifted continental margin southeast of the grand banks of newfoundland along a preexisting deep multichannel seismic reflection profile the primary objectives were to determine the topography of moho across the margin to detect any evidence for crustal underplating and to determine the position of the continent ocean boundary data were obtained from nine ocean bottom seismometers deployed along the profile with an array of six 1000 inch3 air guns as the source two additional refraction lines were shot parallel to structure over the continental slope the resultant velocity model is well constrained and consistent with the observed gravity the crust thins abruptly beneath the continental slope to a major structural transition identified as the continent ocean boundary there is no significant magmatic underplating of the thinned continental crust seaward of the transition the seismic velocity structure is characterized by a 50 km wide zone of intermediate velocity 7 2 7 6 km s beyond this is a thin 3 5 km crust with low velocity 4 5 5 km s and high gradient over mantle with a velocity of 7 7 km s increasing to 8 0 km s the zone of intermediate velocity is interpreted to be highly serpentinized mantle material beneath a very thin crust suggesting little or no magmatic activity during and immediately after rifting this interpretation is consistent with results from adjacent and conjugate margins

ordovician marginal basin evolution near the palaeo pacific east gondwana margin australia although the ordovician macquarie arc part of the eastern lachlan orogen of southeastern australia has long been considered to be an intra oceanic arc within an accretionary orogen key characteristics contrast with more typical examples of accreted arcs significantly multiple stacked phases of mafic to intermediate volcanic rocks with suprasubduction zone chemistry are flanked to the east and west by extensive coeval continental margin turbidite chert and black shale sequences by analysing stratigraphic and contact relationships within and between the volcanic rocks ophiolitic components sensu lato and turbidite sequences constrained by precise biostratigraphy we document a repeated cycle of uplift upper plate extension and collapse common to all sequences this cycle is interpreted as resulting from localized extension rifting or hyper extension and lower crustal delamination within a continent margin sequence developed upon an already established marginal or back arc basin of probable middle to late cambrian age our interpretation provides a counter example to prevailing arc accretion models by inferring extensional tectonics at the palaeo pacific east gondwana margin during the ordovician with development of alkalic and calc alkalic cu au porphyry deposits away from an active arc system

gravity anomalies crustal structure and flexure of the lithosphere at the baltimore canyon trough seismic and gravity anomaly data have been used to examine the long term mechanical properties of the lithosphere beneath the baltimore canyon trough a 15 km deep sedimentary basin in the rifted u s atlantic margin seismic data constrain the crustal structure at the time of rifting as well as the amount of sedimentation and erosion that has occurred the gravity effect of the initial crustal structure sedimentary loading and erosional unloading have been computed and compared to the observed free air gravity anomaly the best fitting model is one in which the elastic thickness of the lithosphere te changes at the east coast magnetic anomaly landward of the anomaly te is significantly smaller than previously reported values for continental or oceanic lithosphere of the same thermal age seaward of the anomaly however te follows the depth to the 450 c oceanic isotherm which also describes the response of oceanic lithosphere to seamount and oceanic island loads these results are interpreted as indicating that the shelf part of the trough is underlain by continental lithosphere which following rifting did not acquire any significant long term strength

a rift drift delamination model of continental evolution palaeozoic tectonic development of eastern australia the adelaide lachlan and new england fold belts of eastern australia record the continental growth of eastern gondwanaland during the palaeozoic the new england fold belt nefb represents a tectonic collage formed by subduction accretion during the late paleozoic but contains remnants of subduction related rocks that date from the cambrian the lachlan fold belt lfb developed inboard from the nefb initially as an amalgam of proterozoic continental and cambrian oceanic fragments that formed by rift and drift at the leading edge of eastern gondwanaland convergent tectonism at 500 ma welded the fragments to the craton and formed the adelaide fold belt oceanward dispersal of detritus across the lfb produced an overlap assemblage of quartzose ordovician turbidites and a new subduction zone developed at the eastern margin the tectonic setting was similar to the modern philippines plate of the western pacific behind the arc silurian devonian convergent tectonics converted the 1700 2000 km wide continental margin of the proto lfb into a 750 km wide thin skinned fold magmatic belt within 60 ma the driving force was delamination which produced a ubiquitous basaltic underplate that generated 1 regional low p metamorphism 2 local anticlockwise p t t paths and crustal scale isobaric cooling 3 voluminous syn to late tectonic granitoids emplaced at rates approximately twice that of modern arcs and 4 normal thickness crust despite an average of 60 shortening delamination occurred in two separate areas originally 1000 km apart first in the eastern then in the western part of the lfb the lfb and its northern counterpart the thomson fold belt represent approximately one fifth of the australian continent and evolved from dispersed fragments into stable continental crust in 300 ma such rapid growth rates are typical of the 2 7 2 5 ga and 1 9 1 7 ga periods when voluminous widespread granitoids and large turbidite dominated low p metamorphic belts were produced in settings that are not obviously subduction related therefore a similar process of rift drift delamination riddel tectonics may have periodically operated throughout earth history

middle palaeozoic thrusting in the eastern lachlan fold belt southeastern australia abstract early to middle palaeozoic clastic and volcanic strata in the goulburn bungonia region 150 km southwest of sydney are divided by the n trending yarralaw fault into two domains in the western domain major e w shortening formed folds axial planar cleavage and w dipping contraction faults in all pre upper devonian units this deformation formed an anticlinorium cored by a w dipping thrust system with an overturned e younging limb in the eastern domain upright flattened chevron folds occur in ordovician strata with stratal repetition along steep contraction faults spaced at intervals of 1 km or less zones of tectonic melange occur in the hangingwalls of these faults and have a well developed scaly fabric with a steeply plunging striation overprinting a bedding parallel cleavage in mudstone in interbedded silurian shale and limestone a classical stair step trajectory is recognized for the frome hill fault which duplicates the succession and has undergone back rotation to its present steep dip overall thrusting progressed from east to west with deformation in the eastern domain pre dating intrusion of the early devonian marulan batholith whereas farther west deformation continued into the middle devonian the folds and thrusts in the goulburn bungonia region are part of a major fold thrust zone that extends throughout the eastern lachlan fold belt and which was formed in the middle silurian to middle devonian during a period of plate convergence between gondwana and the palaeo pacific ocean the fold thrust zone is inferred to have an arc frontal arc setting above a w dipping subduction zone and the deformation relates to underthrusting of an allochthonous terrane with major shortening in the hangingwall block

crustal structure of the sw moroccan margin from wide angle and reflection seismic data the dakhla experiment part b the tectonic heritage abstract the dakhla 2002 experiment on the northwest african margin obtained deep penetration multichannel reflection mcs and wide angle seismic obs data these results were combined with total magnetic intensity data kinematic reconstructions and geological field studies to decipher the early tectonic and sedimentary history of the south moroccan margin adjacent to the reguibat shield the structural study of this margin shows that the crust thins from about 28 km beneath the un thinned continental domain to less than 10 km over a lateral distance of about 120 km the dakhla margin differs from the structural scheme of the adjacent african margin segments there the moho rise generally observed just above the deepening of the top of the basement is achieved with an offset of around 50 km with respect to the latter in early jurassic time the break up of continental crust in what is now the central atlantic led to the formation of two basins one on either side of the reguibat which correspond to the older parts of segments of the late palaeozoic hercynian orogen the dakhla segment appears as a precambrian cratonic zone squeezed between two orogenic segments which have remained unaffected by break up processes and the lower crust of this particular domain has behaved differently from all other neighbouring appalachian north america and mauritanide domains this observation points to the important role of tectonic inheritance in the structural development of passive continental margins

subsidence history crustal structure and thermal evolution of the valencia trough a young extensional basin in the western mediterranean the valencia trough is an extensional basin that began rifting during the late oligoceneearly miocene yet it is located within a region of compression between the european and african plates seismic reflection seismic refraction well gravity and geoid data have been used to determine the relative importance of extensional and compressional processes in controlling the structure and evolution of the trough although the trough appears symmetric on bathymetric maps seismic and gravity and geoid data indicate a fundamental asymmetry in its deep crustal and upper mantle structure two dimensional flexural backstripping techniques have been used to estimate the total amount of crustal stretching along a transect of the trough between the catalan coastal ranges and mallorca there is good agreement between the predicted depth to the mono based on a stretching model and the seismically constrained depth beneath the catalan margin but poor agreement beneath mallorca the seismic moho beneath mallorca can be explained by a model in which the crust was modified by the flexural effects of thrust fold loading geoid data suggest that the differences in the crustal structure between the two margins extend into the upper mantle the catalan margin is characterized by a thermal and mechanical boundary which marks the transition between cold unstretched lithosphere and hot thin lithosphere while mallorca is located above a broad region of lithospheric thinning the subsidence history and amount of crustal thinning in the catalan margin resemble that of typical atlantic type nonvolcanic margin however a narrow transition zone 60 km the lack of any evidence for a conjugate margin and the absence of oceanic crust suggest fundamental differences in the way that extensional processes may occur in convergent plate settings

a cocorp deep reflection profile across the buried reelfoot rift south central united states abstract a cocorp deep reflection survey across the northern mississippi embayment reveals essential features of the late precambrian early paleozoic reelfoot rift stratified reflections delineating the sedimentary fill of the rift basin are of variable thickness the base ranging from 2 0 to a maximum of 3 5 s twt 4 to 8 km depth in contrast the base of phanerozoic the sedimentary section immediately east of the rift is at 1 4 s 2 8 km depth correlation with a basement penetrating well indicates that the base of the upper cretaceous holocene mississippi embayment supergroup base of the cambrian ordovician knox group and the lower upper cambrian bonneterre formation are basin wide seismic stratigraphic markers clastic sedimentary strata of the lamotte formation which immediately overlie precambrian basement show clear angular discordance with the overlying subhorizontal bonneterre formation indicating that the base of the bonneterre marks the transition from active extension to passive subsidence within the reelfoot rift palinspastic restoration of the basement suggests relatively modest extension associated with rift formation 17 the blytheville arch an axial antiformal feature as well as a number of lesser structures indicative of multiple episodes of fault reactivation are also evident on the cocorp profile the crystalline basement beneath the reelfoot rift is relatively devoid of reflections down to 5 0 s 12 15 km depth beyond which it passes gradationally into a complexly reflective diffractive middle and lower crust the lower crust however does not exhibit the distinctly laminated character commonly observed in mesozoic cenozoic extensional provinces the crust mantle transition beneath the rift is characterized by a gradational downward cessation of crustal reflectivity at 12 13 s 40 km depth followed by an unreflective zone which in turn is followed by a distinct horizontal reflection at 14 5 s the latter is not observed on existing profiles outside the rift at present it is not possible to determine strictly whether the deep horizon represents moho or alternatively a discrete feature sill within the uppermost mantle however its spatial association with the reelfoot rift suggests that the two are genetically related regional relations together with the existence of dipping fabric in the deep crust beneath the rift suggest the possibility that the reelfoot rift formed along the southern extension of the circa 1 ga grenville front

structure of the earth oceanic crust and uppermost mantle the past four years have witnessed the introduction of a variety of new instruments and methods for the study of the seismic structure of the oceanic crust and lithosphere the application of these and existing tools has led to the discovery of a number of new phenomena and to a fuller understanding of the genesis and evolution of the oceanic ithosphere borehole seismic instrumentation has become more widely employed ocean bottom seismographs while generally decreasing in number have become significantly more reliable and useful and multichannel seismic systems have been employed in innovative experiments ranging from studies of fracture zones to the regular detection of magma chambers beneath rise axes the techniques available for the analysis of seismic data have become more sophisticated waveforms collected in seismic experiments can now be used directly in constructing and evaluating seismic velocity models and travel time data are regularly inverted directly for structure trial and error modeling has become increasingly unimportant marine seismologists are becoming increasingly involved in understanding the coupling between the ocean and the underlying oceanic lithosphere this has led to a more complete understanding of seafloor noise processes and the partitioning of energy between acoustic and elastic waves

paleozoic intraplate escape tectonics in gondwanaland and major strike slip duplication in the lachlan orogen of southeastern australia intraplate tectonic escape along the eastern margin of gondwanaland in late silurian to middle devonian time was caused by the eastward movement of antarctica along a major intraplate tear resulting collisions caused a crustal block from the southern part of the lachlan orogen of southeastern australia to become detached from its lower lithosphere and escape to the northwest as a crustal flake this escaping block then became incorporated into the western part of the lachlan orogen as a separate terrane

dynamic modeling of divergent margin formation application to the u s atlantic margin abstract a finite element model of extension in the continental lithosphere shows that on the u s atlantic margin the presence of pre existing weaknesses within the lithosphere can account for variations in subsidence and extension hinge zone development and some aspects of magmatic activity during rifting the lithosphere is treated as a layered viscoplastic material consisting of a quartz diorite crust and dunite mantle the crustal weakness may results from thrust faulting during the paleozoic applachian orogeny and is treated as a region within the crust with a weaker rheology the mantle weakness results from thickening of the crust below the pre rift appalachian orogen the model indicates that subsidence in the early stages of rifting is focused in the region encompassing in crustal weakness at the location where the triassic rift basins formed at about 225 195 ma extension within the mantle during this stage of rifting is diffuse and a shear zone develops within the lower crust connecting the crustal weakness to the mantle weakness in a manner akin to simple shear as rifting continues extension within the mantle becomes focused in a narrow region under the mantle weakness the crust above the mantle weakness is thermally weakened and the locus of subsidence and crustal extension shifts to the region overlying the mantle weakness the style of extension from this time onward is similar to pure shear seafloor spreading ultimately develops over the position of the original mantle weakness at about 175 ma the model accounts for a number of hitherto unexplained features of the u s atlantic margin including the shifting of the locus of extension from the triassic basins to the deep offshore basins hinge zone development timing and position of magmatic activity the location of the initiation of seafloor spreading and the asymmetry of the conjugate margins of the rift the predicted shift from simple shear to pure shear rifting style may be a characteristics of other continental rifts and passive margins

on the evolution of rifted continental margins comparison of models and observations for the nova scotian margin summary a model for the formation and evolution of rifted continental margins based on lithospheric extension during rifting and its thermal and mechanical consequences is proposed model predictions are then compared with geological and geophysical observations from a transect of the 185 ma old rifted margin off nova scotia through the scotian basin three kinematic models of the rifting process are discussed these are 1 the uniform extension model in which the amount of extension is uniform with depth but varies with position across the margin 2 the uniform extension and melt segregation model which has similar properties but also provides an explanation for the properties of the extended continental crust and its transition to oceanic crust by postulating that basaltic melt segregates from the asthenosphere and migrates to the crust and 3 the depth dependent extension model in which the first order consequences of rapidly changing rheological properties with depth in the lithosphere are included by decoupling the lithosphere into two zones with depth each of which undergoes differing amounts of extension these rift models predict the form of crustal and lithospheric thinning subsidence and temperature change once the amount of extension has been determined this is estimated from seismic measurements of present crustal thickness on the assumption that the crust had a uniform thickness equal to that currently measured in the adjacent continental region before rifting occurred rifting is also considered to occur instantaneously a time stepping thermo mechanical model is used to predict the cooling of rifted margin additional thermal contraction subsidence and its amplification by water and sediment loading thermal aspects are calculated using a finite difference model of time dependent conductive heat transport whereas regional isostatic response to loading is calculated by a finite element model the models are coupled because the temperature distribution is used to define a rheological lithosphere an elastic region with thickness that varies in time and space as the model evolves for flexural calculations in the mechanical model secondary coupling occurs through perturbations to the temperature field by sedimentary thermal blanketing and advection of heat during isostatic adjustment the model predictions of 1 sedimentary basin stratigraphy 2 moho position 3 free air gravity anomaly 4 age depth relations for deep exploratory wells and 5 subsidence and temperature histories agree well with observations from the scotian basin additional effects due to sedimentary and crustal radiogenic heat production lateral heat transport and the possible existence of a near surface brittle listric faulted region created during rifting are also considered it is concluded that a model in which rifting occurred by depth dependent extension and which includes a finite thickness for the rheological lithosphere radiogenic heat production in the sediments and crust and a brittle listric faulted crustal layer affords an accurate description of the first order processes that occur during rifting and evolution of this margin

structural signature of sediment accretion in a palaeozoic accretionary complex southeastern australia abstract chaotic block in matrix melange broken formation along high strain zones associated with large scale imbrication early bedding parallel cleavage recumbent folds in turbidites and structural complexity in cherts are key elements of a mid palaeozoic subduction complex in the eastern part of the lachlan fold belt southeastern australia the complex consists of an imbricated turbidite chert and basalt sequence of mid cambrian to late ordovician age structural and biostratigraphic controls require a complexly imbricated sequence with reversals in younging seawards towards the inferred former trench subduction zone as in young subduction accretionary complexes such as the kodiak accretionary complex in the aleutians and the shimanto complex of japan subduction accretion or underplating in the narooma accretionary complex is not typified by duplexing but shows a strain dependent transition from an inland belt of chevron folding cut by reverse faults to a coastal belt with an early bedding parallel fabric isoclinal recumbent folding poly deformation and differentiated layering in quartzrich turbiditic greywackes greywacke units show multiple cleavage fabrics formed in one folding event cherts below the turbidites show multiple folding events and zones of broken formation with intense veining and stylolitisation the structurally lowest units including deformed pillow basalts and block in matrix melange show strong planar linear fabrics in the block in matrix melange prolate shaped pods of greywacke and chert are aligned sub parallel to the bulk extension direction defined by mica pull aparts and pressure shadows on pyrite within the mudstone matrix mica neocrystallisation and pressure solution are the dominant deformation mechanisms at the base of the complex

deep velocity structure of rifted continental crust u s mid atlantic margin from wide angle reflection refraction data we present new ocean bottom wide angle seismic data collected during the 1990 edge mid atlantic multichannel seismic experiment which provide a measurement of the deep velocity structure of rifted appalachian continental crust beneath the u s east coast continental margin reflections from the entire crust and moho are visible from offsets of zero to 100 km one dimensional inverse traveltime modeling reveals a 34 km thick crust consisting of four layers beneath the post rift sediments with velocities of 5 9 6 3 6 4 and 6 85 km s this velocity structure is indistinguishable from that found beneath several appalachian terranes in the northern appalachians of new england the average velocity of 6 85 km s in the lower crust limits the landward extent of the high velocity 7 2 7 4 km s lower crust found farther seaward and supports the interpretation of that layer as the result of rift related magmatic underplating rather than as thinned pre existing lower continental crust

thin skinned tectonics in part of the lachlan fold belt near delegate southeastern australia a middle silurian thin skinned style of deformation is documented in the southeastern corner of the lachlan fold belt thrusts are south to southeast verging and are mainly localized within or at the base of an upper ordovician incompetent black shale and also in inferred underlying lower ordovician turbidites the eastern margin of the thrust belt is a later regional high angle reverse fault that corresponds to the i s line we suggest that the thin skinned structures developed on a cryptic terrane of old rigid continental crust identified by previous granitoid source data surrounded by less rigid crust such areally restricted thrust belts may occur in the inboard parts of other fold belts and may form at different times in their evolution

the geology and gold deposits of the victorian gold province abstract the palaeozoic succession of victoria represents a major world gold province with a total production of 2500 t of gold i e 78 million oz on a global scale central victoria represents one of the most gold mineralized areas outside the witwatersrand of south africa and remains the prime example of a slate belt gold province also known as turbidite hosted or shale greywacke gold province production came from quartz veins 1000 t modern placers 1200 t and palaeoplacers 300 t up to 7000 mines produced gold from quartz veins however the 168 mines of this type which produced over 1 t au contributed 68 of the primary gold production twelve goldfields have each produced 30 t au approximately one million ounces from all sources with bendigo 697 t ballarat 408 t and castlemaine 173 t being the largest these twelve goldfields have contributed 71 of the combined primary and secondary production gold mining commenced in 1851 declined dramatically around 1914 and has not increased significantly since the palaeozoic succession of victoria is part of an extensive fold belt along much of eastern australia i e the tasman orogenic belt in victoria the succession is dominated by cambrian to early devonian clastic metasedimentary rocks that have undergone major upright deformation and low grade metamorphism culminating in the middle devonian tabberabberan orogeny north south inliers of cambrian metavolcanic and metasedimentary rocks associated with regional fault zones divide the province into several geologically distinct zones in central victoria there are particularly well developed acid volcanic complexes of late devonian age and synchronous peraluminous granites which represent high temperature crustal melting tertiary to quaternary basalts have covered some of the palaeozoic gold deposits and have also covered several rich gold placer deposits of cainozoic age larger goldfields including bendigo and ballarat occur in ordovician slate and greywacke sequences but a wide range of metamorphosed sedimentary and igneous rock types are host to goldfields most if not all primary deposits demonstrate structural control although the controlling structural features differ between deposits wallrock alteration is strongly influenced by host rock in metasedimentary rocks it is limited and subtle but it is more pervasive in mafic and felsic igneous rocks carbonates muscovite and pyrite are the most widespread alteration minerals in these rocks representing co 2 k and s addition enrichment of as within gold deposits is common with sb important in some small deposits whereas base metals are only abundant locally and bi w mo and te show strong spatial association with granites and are rarely associated with economic gold deposits granites are an important component of the palaeozoic succession in victoria no large deposits are within granites but a few goldfields including maldon 65 t au are within contact aureoles of either s or i type granites of differing degrees of fractionation and silica content a single stage but protracted and possibly diachronous period of gold introduction near the time of the middle devonian tabberabberan deformation can explain geological relationships at many gold deposits the possibility that this event was slightly older i e silurian or early devonian in the west of the state cannot be precluded from the available evidence the mineralizing event appears linked to the same thermal event as diorite lamprophyre dykes and graite intrusions acidic and basaltic volcanism deformation and regional metamorphism the limited amount of fluid chemistry data from victorian gold deposits suggests low salinity fluids which contained co 2 and variable methane with homogenization temperatures from 150 to 300 c the sulphur isotopic compositions of pyrite are mostly 1 to 5 with some values slightly above 10 mineralizing temperatures are inferred to be near 300 c and pressure around 1 2 kbar sulphide rich gold deposits in the east of victoria many of which contain significant base metals contrast with the rest of the province which contains mostly gold deposits with lower sulphide content and this eastern area should possibly be viewed as separate from the victorian gold province per se the benefits to victorian gold mining coming from the 1980 s global gold boom have been insignificant compared to the historical importance of the victorian gold province there seems little justification for assuming that all the large deposits have been discovered and there is reason to believe new deposit styles may emerge to complement the limited current exploration activity which is focussed on the search for extensions and repeats of known deposit types a major advance in scientific understanding of these oft neglected deposits could kindle a major exploration and production revival

structure of the lower crust beneath the carolina trough u s atlantic continental margin data from three large offset seismic profiles provide information on the crustal structure beneath the carolina trough the profiles obtained by the u s geological survey the naval oceanographic research development agency and the scripps institution of oceanography in 1985 were oriented parallel to the trough and were located 1 seaward of the east coast magnetic anomaly ecma which is generally thought to represent the boundary between oceanic and continental crust 2 along the axis of the trough between the ecma and the hinge zone which is thought to reflect the landward limit of highly stretched and altered transitional crust and 3 along the carolina platform landward of the basement hinge zone on crust thought to have been thinned only slightly during rifting these data constrain the velocity structure of the lower crust and provide evidence for a thick lens of high velocity 7 1 km s lower crustal material that extends beneath the carolina trough and the adjacent ocean basin this lens reaches a maximum thickness of about 13 km beneath the deepest part of the trough thins to about 5 km seaward of the ecma and is either very thin or absent landward of the hinge zone it is interpreted to represent material that was underplated beneath and or intruded into the crust during the late stage of continental rifting and that led to an anomalously thick plutonic layer during the early seafloor spreading phase these data thus support the recent conclusions of white et al 1987b and mutter et al 1988 that the initiation of seafloor spreading is attended in many if not most cases by the generation of an anomalously large volume of melt

crustal structure of the continent ocean transition off the cote d ivoire ghana transform margin implications for thermal exchanges across the palaeotransform boundary summary crustal structure of the continent ocean transition off the cote d ivoire ghana transform margin is determined by 2 d modelling of wide angle seismic data and gravity data the resulting models together with geological data recently collected by drilling on the margin are compared with available thermomechanical models of transform margins in particular we test the predicted effects on the transform margin crust of continental heating by the adjacent oceanic lithosphere these effects include i ductile flow of the lower continental crust either parallel to the transform direction or perpendicular to it from the continent towards the ocean and ii thermal uplift of the transform margin in the region of the cote d ivoire ghana transform margin the seismic and gravity data indicate that the continental crust remains uniform in thickness as the continent ocean boundary is approached the interpreted models lack a transitional domain between continental and oceanic crusts and abnormally thin oceanic crust is found along the margin these results exclude large scale ductile flow of lower continental crust during the margin formation the geological data are moreover not consistent with thermal uplift of the margin during the period of contact between the transform margin and the spreading centre our geophysical and geological data thus infer only moderate heating of the transform continental margin by the adjacent oceanic lithosphere moderate heating of the cote d ivoire ghana transform margin during margin formation implies that the thermal exchange between the transform margin and the adjacent spreading centre were lower than predicted by available thermal models two main factors may have contributed to reduce thermal exchanges across the continent ocean boundary the first factor is that the oceanic edge may have been colder than assumed by thermal models an abnormally cold spreading axis along the transform margin when the oceanic crust was emplaced is supported by the atypical structure of the oceanic crust along the cote d ivoire ghana transform margin the oceanic crustal structure is similar to that found along oceanic fracture zones where oceanic crust is also emplaced near a ridge transform intersection in both cases abnormally low thermal gradients at the spreading centre along the transform margin may be explained by the cumulative effect of i a cold edge effect of the adjacent continental lithosphere ii accretion at the end of a ridge segment away from the main magma supply point along the spreading axis and iii an efficient upward cooling of the oceanic lithosphere owing to deep hydrothermal circulation within the faulted lithosphere the second factor causing moderate thermal exchanges is the possibility of large amounts of water in oceanic lithospheric faults which reduces the lateral thermal conduction in the lithosphere since water is much less conductive than rock during the margin formation the oceanic edge thus seems to have acted as a thermal buffer between the cold continental margin and the hot spreading centre we propose that this thermal buffer hypothesis may be generalized to most transform margins implying a lower amount of continental heating than assumed by the available thermomechanical models

a proposed super thick sedimentary basin bay of bengal a super thick 22km sedimentary basin under the northern bay of bengal is proposed the hypothesis is based on data from surface wave dispersion seismic refraction sn attenuation and geology we present new high frequency sn data which indicate a cold upper mantle beneath the bay of bengal we propose that the oceanic crust in this region is in fact nearly normal and that the sedimentary section is at least 6 km thicker than previously thought with velocities at the base of the sediments having been increased to near 6 5 km sec by high pressure metamorphism reinterpretation of the refraction data indicate a post india asia collision thickness of over 16 km

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

geology and taphonomy of the l espinau dinosaur bonebed a singular lagoonal site from the maastrichtian of south central pyrenees abstract the l espinau site is a dinosaur bonebed from the upper cretaceous of the south central pyrenees north eastern spain that have provided hundreds of bone remains attributed to hadrosauroids together with a rich assemblage of herpetofauna fish and microflora magnetostratigraphy calibrated the site with the early late maastrichtian and the combined sedimentology stable isotope geochemistry and palaeoecology revealed that this fossil site formed in a lagoon in which a mixed freshwater brackish palaeoenvironment was developed this setting displays a south north charophyte zonation from freshwater clavator brachycerus dominated assemblage to brackish or eurihaline conditions feistiella malladae dominated assemblage revealing a palaeoenvironment change towards the coast sedimentology and taphonomy bidirectional arrangement of long bones abrasion and disarticulation indicate that the l espinau site is the result of a cohesive mass flow event originated very close to the sea this process entrained and mixed fauna from both the terrestrial and the brackish marine environment of a lagoon an increasing of the water runoff e g by intense rainfall reworking poorly consolidated sediments is considered here as the most probable triggering mechanism mass flow hosted bonebeds are commonly linked to fluvial palaeoenvironments so our study case is a rare example of bones accumulating near the sea this study adds evidence that hadrosauroids inhabited littoral environments during the maastrichtian in the southern pyrenean area

diapirism and structural thickening in an early palaeozoic subduction complex southeastern new south wales australia abstract a major problem in the study of subduction complexes has been to distinguish between processes of mud diapirism deformation of unlithified sediments frontal accretion and underplating in the batemans bay area cambro ordovician rocks of the south coast of new south wales eastern australia contain common mud rich melanges that occur amongst an imbricated succession of early to middle ordovician quartz turbidites late ordovician black mudstone and chert cambrian limestone and mafic volcanics an early set of fluidal obliquely trending folds are restricted to the late ordovician black mudstone and chert and probably formed by gravitational down slope movement on a lower trench slope features indicative of deformation of unlithified sediments are still preserved and include mud injections delicate small scale faulting and the absence of grain scale deformation tight to isoclinal north south folds with axial planar west dipping cleavage overprint all units and formed during the younger main phase deformation that post dated early deformation in the prism the melange has been strongly overprinted by this main phase deformation early melange formation is attributed to mud diapirism involving intrusion along early accretionary faults bedding planes and in pipe like features resulting in chaotic outcrop and map patterns

chronology and tectonic framework of turbidite hosted gold deposits in the western lachlan fold belt victoria results abstract 40 ar 39 ar data constrain the history of metamorphism deformation and mineralization in the western subprovince of the lachlan fold belt rocks in the mount stavely volcanic complex and the bushy creek pluton give 40 ar 39 ar ages of about 500 ma for hornblende and biotite the dates from the mount stavely volcanic complex limit the age of mineralization in the metavolcanic rocks of the stavely belt whereas the date for the bushy creek pluton extends the area of cambrian ordovician plutons of the delamerian orogeny to east of the grampians mountains and helps define the surface expression of the western border of the paleozoic lachlan fold belt 40 ar 39 ar dating of metamorphic mica growth phengite in cleaved slates and phyllites within the landsborough fault avoca fault zone heathcote fault zone and mount wellington fault zone gives ages of 453 2 440 2 426 4 and 410 390 ma respectively these data indicate a progression of deformation from west to east starting in late ordovician early silurian time in the stawell and bendigo ballarat structural zones the major phase of deformation in the western subprovince of the lachlan fold belt is therefore silurian and not devonian sericites from major gold deposits in the stawell zone and bendigo ballarat zone give 40 ar 39 ar dates of about 440 ma that are coincident with folding and thrusting a second episode of mineralization and fault reactivation at about 420 to 410 ma is found in some deposits in these zones based on data from a previous study in the melbourne zone including the woods point dike swarm and locally in the bendigo ballarat zone some gold deposits give sericite dates of about 380 360 ma these ages are consistent with the close association of many melbourne zone gold deposits with devonian dikes and plutons the results indicate that gold mineralization in the stawell zone bendigo ballarat zone and melbourne zone was episodic and both temporally and spatially associated with crustal heating linked to regional metamorphism and plutonism initial stages of mineralization in the province comprising many of the large deposits in western and central victoria e g bendigo ballarat stawell wattle gully tarnagulla accompanied metamorphism folding and thrusting subsequent mineralization is widespread but produced mainly smaller deposits

array length magnification through the use of multiple sources and receiving arrays for wide aperture seismic experiments very long effective arrays can be synthesized by using multiple sources and receiving arrays in marine seismic work where a source is coincident with the end of each array the total length of the arrays can be multiplied by a factor between n 1 and n where n is the number of sources three ships each with a source and 3 6 km array can be positioned so that the offset range 0 to 25 2 km can be continuously sampled with duplication only in the 0 to 3 6 km range with 100 m group spacing these data can be gathered into 252 fold common depth point cdp data with a cdp spacing of 150 m while it is not likely that we will ever see five or six ships steaming in line two and three ships will be used in the near future for detailed sedimentary and deep crustal studies

provenance of early palaeozoic sandstones southeastern australia part 1 vertical changes through the bengal fan type deposit abstract ordovician strata of the lachlan fold belt in southeastern australia are dominated by a turbidite succession that in its undeformed state was comparable in size to the modern day bengal fan the ancient fan was derived from the northern part of the cambrian ross delamerian orogenic belt that formed at a convergent boundary between plates containing gondwanaland and part of the palaeo pacific ocean respectively substrate to the submarine fan consisted of mafic volcanic rocks with an immature island arc affinity similar to the eocene izu mariana bonin system sandstone compositions determined from several stratigraphically constrained sections demonstrate a uniform framework composition dominated by monocrystalline quartz with less abundant feldspar polycrystalline quartz metamorphic rock fragments sedimentary rock fragments and mica these indicate derivation from a low grade metamorphic terrain up sequence variation is limited to decreasing feldspar and lithic fragments and increasing quartz indicating increased weathering and lowering of topography in the source terrain over time 25 of counted sandstones in the late ordovician interval contain a small component of mafic intermediate volcanic rock fragments that accompanied a change in palaeocurrent direction from eastwards to northwards in the early silurian the ordovician sequence was disrupted by the formation of several major deformation zones between these zones developed several deep marine sedimentary basins that contain a continuous succession from the ordovician into the early silurian early silurian sandstone is similar in composition to underlying early ordovician rocks except that the feldspar component is generally absent early silurian sandstone was derived from uplifted portions of the ordovician sequence and includes texturally inverted types with sandstone containing a bimodal mixture of coarse well rounded quartz and fine angular quartz

early palaeozoic terranes in new zealand and their relationship to the lachlan fold belt abstract two tectonostratigraphic terranes of palaeozoic age are recognised in the western province of new zealand separated by the north trending anatoki thrust the buller western terrane is composed largely of a relatively uniform suite of quartz rich elastics mainly turbiditic sandstone siltstone and black shale and ranges from basal ordovician to late ordovician in age it is an equivalent of the ballarat bendigo belt of victoria the takaka eastern terrane contains a varied assemblage of middle and late cambrian volcanics volcaniclastics carbonates and turbidites ordovician carbonate shale and sandstone and silurian quartzite the closest equivalent in australia is early palaeozoic rocks of west tasmania the main deformational events in the latest ordovician or silurian buller terrane latest silurian takaka terrane and middle devonian both terranes can be equated with the benambran bowning and tabberabberan orogenies of eastern australia respectively palaeozoic granites are restricted to buller terrane and are spread over a belt 300 450 km wide age range on sparse isotopic data is 370 310 ma late devonian carboniferous with the bulk in the early carboniferous s type granite predominates with several a type granites and a single i type the similarity in the nature and timing of the main events in the sedimentary tectonic and volcanic history of new zealand clearly link it with the lachlan fold belt on the other hand the age and chemistry of the s type new zealand palaeozoic granites make it unlikely that they are the direct equivalents of granite terranes of the lachlan fold belt or antarctica

a correction for sediment load upon the ocean floor uniform versus varying sediment density estimations implications for isostatic correction abstract the isostatic correction for the sediment load on oceanic basement has often been calculated using a uniform sediment density irrespective of either water depth or total sediment thickness an alternative method is proposed here in which the sediment density is calculated using the density depth equations of hamilton 1976 for calcareous clay and terrigenous sediment sequences this alternative method takes into account the water depth and total sediment thickness when estimating the sediment density for example deep water 4000 m sediment sequences are likely to be clay rich thick sediment sequences 2000 m usually have a high terrigenous component and shallow sequences are likely to be calcareous rich sediment thicknesses in excess of 1500 m are generally close to a continental margin these density estimations are compared with measurements from 9 ocean drilling program drill sites 1 deep sea drilling project drill site and from densities derived from seismic interval velocities the most commonly used estimates of uniform sediment density were used to calculate isostatic corrections for a 12 km thick sediment sequence these results were then compared with the results determined using the proposed alternative method of estimating sediment densities the proposed method produces isostatic corrections similar to those derived by published methods for sediment sequences

the east coast magnetic anomaly a model is proposed for the east coast magnetic anomaly that for the first time incorporates an observed basement feature a normal fault that consistently lies below the landward flank of the anomaly the east coast magnetic anomaly is similar to slope anomalies found over passive continental margins in many parts of the world thus the proposed model can be used to interpret slope anomalies in general

shallow water turbidites an example from the mallacoota beds australia abstract a late ordovician turbidite sequence the mallacoota beds exhibits vertical lithofacies associations that suggest deposition in a fan like environment however there are also features indicating occasional episodic reworking by water currents reworking features include both wave current and unidirectional ripple pavements sharp topped turbidites a difference in current direction between superimposed current rippling and underlying bouma c division ripples lensiodal quartzites and occasional hcs beds comparisons between the mallacoota beds and the depositional products of a variety of open ocean currents would indicate that storm related currents were the most likely reworking mechanism this would indicate that the sequence was deposited within storm wave base depositional depths for part of the sequence are estimated at 90 200 m based on a wave generated ripple model therefore these turbidites may have been produced by storm surge with channeling within the turbidites due to the localisation of rip currents during hurricane conditions a generalised model for the characteristic features of turbidites deposited between fair weather wave base and storm wave base is presented it is suggested that such turbidites will contain only occasional examples of reworking and that the frequency of such reworking events per stratigraphic interval might be used as a relative measure of water depth throughout a sequence moreover those turbidite sequences that have sharp contacts between the sandstones and the overlying mudstones may have been modified by storms and therefore deposited in shallower water than previously thought

deposition of ungraded muds from high density non turbulent turbidity currents turbidity current deposits of mud have recently been found up to 10m in thickness resulting from single depositional events1 2 turbidites 1 5 m thick from the madeira abyssal plain3 4 display long segments with little vertical change of grain size5 6 this suggests that the currents froze in some way making them unable to sort sediment here we suggest that a decline in flow speed of these currents led to a concentration gradient in silt close to the bed that was sufficiently steep to stratify the flow a concentration gradient of 18 kg m 3 over 1 5 m height gives a layer richardson number above the critical value of 0 25 for damping of turbulence we propose that with turbulence severely damped a dense suspension of fine material consolidates like a static suspension while continuing to flow downslope the current changes from a fully turbulent flow to a decelerating flow with suppressed turbulence and increasing density in the latter state interparticle forces prevent differential settling of the coarser grains

crustal underplating and evolution in the red sea rift uplifted gabbro gneiss crustal complexes on zabargad and brothers islands the nature of the crust of the red sea outside the oceanic axial trough segments can be inferred from uplifted crustal sections exposed on the islands of zabargad and the brothers in the central and northern red sea zabargad exposes in addition to mantle derived peridotites a crustal section consisting of metagabbros and metapyroxenites associated with silicic gneisses and cut by doleritic dikes seismic velocities of samples from this section are compatible with crustal velocities obtained by seismic refraction experiments in the central and northern red sea the zabargad peridotitesilicic gneiss gabbroic association provides a sample of upper mantle lower crust in a continent before the inception of a major rifting event the petrology and mineral chemistry of the zabargad metagabbros suggest that they were originally part of a basic layered complex which crystallized at relatively high pressure 900 1000 mpa or 9 10 kbar i e over 30 km deep the gneisses have tonalitic trondhjemitic affinity and were originally equilibrated in the garnet granulite facies implying also high pressure and derivation from the lower continental crust this suggests that after the subcrustal continental lithosphere along the proto red sea rift had been consumed possibly by convective thinning but before any significant thinning of the crust basaltic melts intruded the base of the continental crust underplating it with gabbroic complexes thus crustal rifting was probably preceded by a thermal anomaly in the mantle which supports an active rifting hypothesis the gabbro pyroxenite complex and the gneisses underwent subsequently retrometamorphic events under decreasing pt conditions from granulite to amphibolite to greenschist facies implying decompression and uplift probably related to the gradual thinning of the continental crust during rifting during this stage additional gabbroic intrusions and basaltic dikes exemplified by the gabbros and dolerites from the brothers islands were emplaced at decreasing depths beneath the thinned continental crust the northern red sea crust is presently in this stage it is neither oceanic nor continental it lacks a continuous thick basaltic upper crustal layer and is made prevalently of gabbroic and doleritic intrusives with relicts of sialic continental crust subsequently basaltic injections became progressively focused in a narrow axial zone and seafloor spreading initiated at discrete equidistant hot points as observed presently in the central red sea which by axial propagation evolved into nearly continuous zones of oceanic crust accretion as observed presently in the southern red sea processes similar to those proposed here for the evolution of the red sea rift can be documented in passive margins of the atlantic and in some alpine ophiolite complexes

provenance of early palaeozoic sandstones southeastern australia part 2 cratonic to arc switching abstract early palaeozoic strata of the northeastern lachlan fold belt australia consist of an early ordovician sequence adaminaby group of quartz rich turbidites and chert and the conformably overlying late ordovician early silurian coomber formation a mafic volcaniclastic sequence of lithic sandstone and mudstone with mafic volcanic and intrusive rocks analysis of detrital modes from the adaminaby group indicate a uniform quartzose petrofacies q 91 f 2 l 7 composed of deep water quartz and sublithic sandstones derived from the west by contrast the lithic sandstone of the coomber formation represents a volcanolithic petrofacies q 1 f 34 l 65 of mainly intermediate volcanic detritus with a tendency to become more silicic up section these sandstones are considered to be derived from a calc alkaline andesitic basaltic volcanic centre in the eastern lachlan fold belt the change from a quartz rich petrofacies to a volcanolithic petrofacies was sharp with no evidence of mixing this change primarily reflects a rapid outpouring of mafic volcanic material in the late ordovician together with an approximately concurrent eustatic rise in sea level

deep structure of the west african continental margin congo zaire angola between 5 s and 8 s from reflection refraction seismics and gravity data summary the deep structure of the west african continental margin between 5 s and 8 s was investigated using vertical reflection and wide angle reflection refraction techniques during the zaiango project a joint programme conducted in 2000 april by ifremer and totalfinaelf to penetrate below the salt layer a non conventional low frequency seismic source was used in the single bubble mode together with ocean bottom instruments hydrophones and seismometers and a 4 5 km long streamer that recorded multichannel seismic reflection mcs the data show that the continental crust thins abruptly over a lateral distance of less than 50 km from 30 km thick below the continental platform based on gravity data to less than 4 km thick below the lower congo basin that formed prior to the aptian salt deposition this subsalt sedimentary basin 180 km wide 4 km thick with velocities varying from 4 7 km s 1 to 5 8 km s 1 at the bottom is located between the foot of the continental slope and the oceanic domain it is underlain by crust of an intermediary or transitional type between continental crust and what can be recognized as oceanic crust in the transitional zone a crustal upper layer is present below the pre salt sedimentary basin 3 to 7 km thick with velocities increasing from 5 8 km s 1 at the top to 6 8 km s 1 at the bottom of the layer this layer appears to thin regularly from 6 7 km thick below the depocentre of the pre salt sedimentary basin to 3 4 km thick below the western termination of the basin below this upper crustal layer an anomalous velocity layer 7 2 to 7 8 km s 1 is documented below the eastern side of the basin where the crustal thinning is at a maximum the origin of this layer is unknown several arguments like rifting duration between 15 ma and 30 ma or the absence of seaward dipping reflectors precludes the hypothesis of underplated mantle material but other hypotheses such as serpentinized material or high grade metamorphic crustal rocks or a mixture of mafic and ultramafic crustal rocks are plausible near the ocean termination of the basin the transitional zone is bounded to the west by a basement ridge that is clearly documented on two profiles 7 11 and 14 having a dense ocean bottom seismometer hydrophone obs obh spacing on these profiles an anomalous velocity layer is present in the westernmost part of the transitional zone below the basement ridge and in the oceanic domain this layer absent on profile 3 may be related either to oceanization and slow seafloor spreading processes or to a consequence of the rifting process

how preexisting weaknesses control the style of continental breakup continental breakup is a highly variable process differences occur in the relative timing and extent of volcanism uplift and graben formation as well as in the mode and amount of continental extension before breakup we propose a model which reconciles this variability with the previously recognized tendency for breakup to occur along preexisting weak trends continental lithosphere is viewed as a composite material composed of two strong layers one in the upper mantle and one in the middle crust finite element simulation indicates that extensional failure at weaknesses in the mantle causes concentrated extension in the mantle and diffuse extension in the crust this leads to early melt segregation and volcanism margin uplift during the late stages of the extension process and relatively narrow symmetrical extended margins in contrast failure at weaknesses in the crustal strong zone causes focused extension in the crust and diffuse extension in the mantle this produces initial graben formation cooling in the lower crust and upper mantle and broad asymmetrical extended margins volcanism only occurs late in the process failure at laterally offset weaknesses within both strong layers leads to a deformation pattern dominated simple shear thus differences in the prerift configuration of the continental lithosphere can control the overall style of continental breakup

deep penetration seismic soundings across the northern margin of the south china sea twenty reversed two ship expanding spread profiles esps with maximum source receiver offsets of 100 km were collected in three transects across the rifted northern margin of the south china sea source receiver offset versus two way travel time x t data were mapped into the intercept time versus ray parameter p domain and velocity depth solutions were obtained by a combination of sum inversion in the p domain and ray tracing in both the p and x t domains arrivals from the moho were detected on 17 of the esps the depths to moho determined for individual esp interpretations have reproducibilities of 0 1 km to 3 km in most cases the moho depth has been determined to within 1 5 km moho depths determined in this investigation represent a significant improvement over previous estimates of moho along the margin from gravity data variations in present day crustal thickness measured from top of prerift basement to moho are one measure of the amount and nature of the crustal thinning associated with the rifting of continental crust preceding the formation of the adjacent south china sea basin the esp interpretations reveal that across the eastern portion of the south china margin the crust appears to thin more or less continuously toward the continent ocean boundary in the west esp interpretations also show a general trend of seaward crustal thinning but in addition indicate at least two instances of focused localized crustal thinning crustal velocities and the relative proportion of upper crust vp 6 4 km s are used to identify areas of the south china margin with similar and contrasting crustal structures variations in these properties are believed to result primarily from contrasting prerift crustal structure across the margin however magmatic underplating during rifting depth dependent extension and pleistocene igneous intrusions may also have contributed to the variations in present crustal structure reliable information about variations in crustal thickness and velocity structure across and along the south china margin is an important prerequisite to understanding better the nature of the spatially variable rifting processes which dominated the formation of this margin

convective partial melting 1 a model for the formation of thick basaltic sequences during the initiation of spreading stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure temperature environment where partial melting occurs the amount of melt liberated is however insufficient to account for the thickness of igneous crust emplaced near the continent ocean boundary at some passive margins these igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide seismic studies also suggest that the transition from continental to oceanic crust on these volcanic margins is relatively abrupt their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading to account for these observations we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle deep hot mantle material is transported upward by the convection and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling numerical calculations show that convective partial melting can provide substantial additions to melting due to passive upwelling alone using this model we propose an evolutionary scheme for volcanic passive margins emphasize the contrast with nonvolcanic margins and describe conditions which might lead to the development of both types

multichannel seismic images of the oceanic crust s internal structure evidence for a magma chamber beneath the mesozoic mid atlantic ridge near the base of the mesozoic oceanic crust of the western north atlantic we recognize a discontinuous diffractive layer horizon r moho is a distinct uniform and more continuous horizon beneath variations in seismic thickness of the crust occur primarily through changes in the r to moho layer the maximum thickness occurring midway between fracture zones we propose that horizon r may mark the top of a layered cumulate sequence that formed on the floor of a relatively persistent crustal magma chamber the crust having minimum thickness is spatially restricted to a small strip near fracture zones and comprises only the super r unit we further propose that this implies the sustained accretion of normal oceanic crust at the mesozoic mid atlantic ridge required the presence of a magma chamber

the tectonic significance of ordovician magmatism in the eastern lachlan fold belt abstract the shoshonitic rock series is a suite of igneous rocks derived from the mantle and containing high potassium plus low abundances of high field strength elements it is commonly associated with calcalkaline igneous rocks but in most modern examples a contemporary active subduction zone is not present mantle derived shoshonitic magmatism in the lachlan fold belt commenced in the early ordovician and terminated at the end of the ordovician when it was replaced by tholeiitic magmatism and the onset of wholesale melting of the lower to middle crust associated calcalkaline igneous rocks are insignificant the shoshonites are widely distributed across the eastern part of the fold belt in a series of discontinuous belts and volcanic centres the largest of which is the orange wellington belt extending from east of cowra to dubbo the broad geographical distribution is not typical of the curvilinear belts of volcanics associated with subduction zones and would be even greater if the intervening basins or troughs were unfolded all rocks analyzed to date are either clearly shoshonitic or show chemical features that indicate shoshonitic affinities prior to alteration and metamorphism in some areas lavas rich in olivine and clinopyroxene potassic trachybasalts dominate in others the more evolved plagioclase rich shoshonites are most abundant it is suggested that the ordovician magmatism was not accompanied by coeval subduction as has been generally assumed more likely the lithospheric mantle beneath the lachlan fold belt was modified during an earlier subduction event associated with the assembling of the basement blocks recognized from granite studies the cause of melting in the ordovician may be related to lithospheric heating or overturning prior to the upwelling of unmodified asthenosphere the hot 1200 c asthenosphere was the source for the silurian tholeiites and provided conductive heat for the melting of the lower to middle crust

origin of seaward dipping reflectors in oceanic crust off the norwegian margin by subaerial sea floor spreading a remarkable layered acoustic stratification is observed in the upper oceanic crust adjacent to the norwegian continental margin it comprises a seaward dipping complex of reflectors in the form of a wedge we suggest that it is a layered igneous sequence that results when crustal accretion occurs at a subaerial spreading axis and that this phenomenon may commonly occur during the earliest phase of ocean basin genesis

multichannel seismic evidence for anomalously thin crust at blake spur fracture zone a two ship wide aperture multichannel seismic reflection profile obtained across a small western north atlantic fracture zone provides convincing new evidence for unusually thin crust at oceanic fracture zones reflections from the crust mantle boundary typically observed at about 2 s below the basement reflection rise to only 0 8 s beneath the fracture zone trough this corresponds to crustal thicknesses of perhaps as little as 2 to 2 4 km in the fracture zone less than half the thickness of normal oceanic crust this is the first time that moho reflections have been observed across an oceanic fracture zone and the first convincing evidence that anomalously thin crust may be associated even with fracture zones of relatively small offset the existence of thin crust at both large and small offset fracture zones suggests that the magnitude of the thermal edge effect at ridge transform intersections is not the only factor responsible for the accretion of unusually thin crust in fracture zones

basin forming processes and the deep structure of the campos basin offshore brazil abstract this paper discusses a worldwide dataset of deep seismic reflection profiles that help the understanding of crustal behaviour and the genetic processes that are involved in the formation of sedimentary basins the events observed in seismic reflection data at lower crustal depths correspond to impedance contrasts associated with one or a combination of the following factors shear zones thin layers of intrusive rocks underplated material gneissic banding fluids and heterogeneities in the crust and mantle several types of sedimentary basins have been recognized with the aid of deep seismic reflection data basins formed along active master faults basins formed along inactive faults basins with no fault control basins formed by low angle detachment faults and basins formed by pervasive pure shear or an approximately pure shear where the lower crust has been locally extended by a different amount than the upper crust the mechanisms of basin forming processes recognized in deep seismic profiles are a simple shear along low angle detachment faults extending to the lower crust and maybe the upper mantle b thinning of the lower crust and mantle by ductile processes convective mechanisms or flow in the mantle and the upper crust by faulting resulting in an overall pure shear and c thinning of the lower crust associated with phase changes in the mantle or non conservation of mass two end member models of basin formation involving lithospheric stretching may be recognized in the south atlantic simple shear stretching where only the upper crust is involved in the process and mantle uplift is offset relative to the basin and pure shear stretching where uplift of the mantle vertically balances the basin fill the first mechanism has been suggested for the onshore tucano basin the data obtained for the offshore campos basin suggests regional lithospheric stretching and crustal thinning with moho uplift compensating for sediment accumulation in the basin depocentre complex relationships between sediment accumulation and crustal thinning are obtained in the western limit of the campos basin

ordovician terranes of the southeastern lachlan fold belt stratigraphy structure and palaeogeographic reconstruction abstract the ordovician of the southeastern lachlan fold belt lfb is represented by two contrasting provinces the molong volcanic province with shoshonitic basalts and shallow marine sediments in the north and a very extensive but relatively thin quartzose flysch chert and black shale sequence elsewhere this paper concentrates on the latter and reviews its stratigraphy correlation sedimentary facies palaeocurrent directions and structure the molong volcanic province contributed insignificant sediment to the surrounding turbidites and there is no evidence that the rise was a fragmented arc as is postulated by most previous workers the rise may have been a hot spot volcano and the southeastern lfb appears to have been a passive margin in the ordovician within the flysch province two terranes are recognized the melbourne terrane in the west and benambra terrane in the east both showing overall upward and eastward fining in the ordovician the boundary is the mount wellington fault zone along which a post ordovician dextral strike slip of up to 400 km is postulated

formation and evolution of the nova scotian rifted margin evidence from deep seismic reflection data a deep marine seismic reflection profile was obtained across the mesozoic rifted continental margin off nova scotia eastern canada this profile crosses the seotian basin one of the deepest basins on the margin of eastern north america and it complements other deep crustal seismic data on this margin the seismic data have been interpreted in conjunction with gravity anomaly and subsidence data they show significant thinning of the continental crust over a zone about 200 km wide the mode of extensional deformation is probably a combination of pure and simple shear there is evidence for simple shear in the crust the continent ocean boundary lies near the seaward edge of synrift salt below the continental rise a 100 km wide zone of very thin approximately 9 km or less continental or transitional crust extends seaward from the outer shelf to this boundary reflectivity of the oceanic crust adjacent to the margin shows evidence of progressive igneous construction perhaps modified by extensional faulting this margin is nonvolcanic and the transition to the volcanic margin off the eastern united states occurs about 500 km southwest of the seismic line the width of the zone of crustal extension is much greater on this nonvolcanic margin segment than it is on the volcanic margin to the south it seems likely that the prerift fabric of the continental lithosphere controls this width a narrow rift may be prone to vigorous asthenospheric convection and therefore to more voluminous volcanism however significantly narrower zones of crustal extension occur on other nonvolcanic margins so factors in addition to rift width such as the rate of rifting may also be important

ordovician contourites in the lachlan fold belt eastern australia abstract the ordovician sequence in the northeastern lachlan fold belt consists of three lithologic units representing an overall fining upward succession from turbidites to black shale the lowest unit dominated by turbidite beds represents a small segment of an extensive ordovician submarine fan sequence that covered much of southeastern australia the overlying unit consists predominantly of ripple bedded sandstone with beds 3 5 cm thick these beds are texturally mature containing heavy minerals mica and silt concentrated on the cross laminae the ripple bedded unit has a mean palaeocurrent trend to 321 which is at 60 to sole marks and almost normal to the trend of ripple cross bedding in the underlying and associated turbidite beds this unit is here interpreted as a sandy contourite sequence that can be attributed to contour currents moving northwards along the eastern margin of gondwana from areas of subpolar downwelling off southern africa and america the two major fining upward successions in the ordovician sequence in southeastern australia may reflect eustatic sea level rises during the middle and late ordovician and thus provide evidence of interglacial periods in the darrawilian and late gisbornian to bolindian the contourites would have accumulated in the latter stages of the glacial periods after sea level had risen sufficiently to cut off major sediment supply to the submarine fan but before the temperature had risen to the extent of preventing deep thermohaline circulation

geochemistry and diagenesis of stratabound calcite cement layers within the rannoch formation of the brent group murchison field north viking graben northern north sea abstract up to four calcite cemented horizons doggers form impermeable barriers to fluid flow within the middle jurassic rannoch formation and are correlatable across the murchison field calcite precipitated during early diagenesis within high porosity permeability sandstones at the top of coarsening shoaling upward shoreface cycles calcite 13c and 18o compositions range from 4 1 to 13 4 pdb and 6 6 to 16 7 pdb respectively sr concentrations of up to 1334 ppm are consistent with marine carbonate sources probably shell fragments but no viable intraformational carbonate source has been identified in the murchison field area initial 87sr 86sr compositions 0 71109 0 71266 are higher than middle jurassic seawater 0 7073 and consistent with precipitation from modified porewaters containing significant proportions of continentally derived meteoric fluids enriched in 87sr as a result of basement weathering or percolation through hinterland soils unconsolidated detritus an internal source of 87sr is not considered viable in view of the high proportion up to 25 clastic constituents of unaltered detrital alkali feldspar and mica within the rannoch formation geochemical and isotope data indicate correlations between increasing 18o composition and increasing iron and magnesium content within calcite calcium concentrations decrease with increasing 18o for calcites geochemical data trends can be interpreted differently in terms of either static or evolving 18o porewater models static 18o porewater models using jurassic early cretaceous meteoric water 18 o 6 smow predict cement precipitation temperatures of 17 77 c however 13c compositions are more depleted than those typical of carbon derived from shell debris and correlation between decreasing 18o and decreasing 13c suggests modification of a meteoric derived porewater system during burial via contribution of low 13c carbon derived from a deep basinal source however it is difficult to constrain accurately the degree to which porewater 18o may have evolved during burial the degree to which porewater 18o could have evolved from that of a jurassic early cretaceous meteoric water can be loosely constrained as from 18 o 6 smow to 18 o 2 smow if the temperature at which the most 18o depleted cements precipitated is limited by present day bottom hole temperatures 110 c for wells currently at maximum burial depth

moho and lower crustal reflectivity beneath a young rift basin results from a two ship wide aperture seismic reflection experiment in the valencia trough western mediterranean summary we present new images of the lower crust and moho beneath the valencia trough a young rift basin in the western mediterranean these images were obtained from a two ship wide aperture reflection experiment and show several features not distinguishable on previously available conventional single ship reflection profiles the moho which was previously only seen intermittently can now be confidently traced throughout the basin we have constructed a present day depth to moho map and estimated the degree of crustal thinning for the whole basin crustal thinning is at a maximum in the centre of the basin where values reach 3 15 0 25 at the margins of the basin the value decreases to 1 5 0 1 the reflective character of the lower crust and moho is different beneath different parts of the basin we have been able to correlate these differences with the amount of stretching we therefore interpret the variations of the observed lower crustal reflectivity as having been caused by the most recent neogene stretching event that opened the valencia trough along the iberian margin there is well developed lower crustal reflectivity consisting of 1 2s two way time twt of 1 4 km long near horizontal reflectors underlain by a more continuous although not significantly stronger reflector interpreted to be the reflection moho offshore this lower crustal reflective unit thins rapidly such that it is undetectable 40 50 km from the coastline where the crust has been stretched by a factor of 1 7 0 1 as the lower crustal reflectivity becomes undetectable the reflection moho becomes a robust continuous event where exceeds 2 4 0 2 however the moho is a weak event and difficult to trace we infer that either the extension itself or associated melting significantly weakened or even destroyed the lower crustal reflectivity in the centre of the basin and enhanced the moho where extension was moderate the balearic margin is somewhat anomalous in that there appears to have been flexural loading of the crust due to thrusting and folding that occurred at the same time as extension in the valencia trough the lower crust shows evidence of weak but locally variable lower crustal reflectivity it is possible that the lower crustal reflectivity was preserved simply because the moho was flexed downward and so decompression and hence melting of the upper mantle was restricted this suggests that the melting itself rather than the extension is the primary mechanism of lower crustal modification

ordovician continental margin terranes in the lachlan orogen australia implications for tectonics in an accretionary orogen along the east gondwana margin 1 four continental margin turbidite black shale terranes of the lachlan orogen in the southern tasmanides of eastern australia formed in two major systems along the east gondwana margin and constrain the ordovician assembly of this accretionary orogen key features are the dissimilar stratigraphies of the adjacent bendigo and melbourne terranes in the western system the dissimilar stratigraphies of the adjacent melbourne and albury bega terranes that reflect juxtaposition of the two systems during the middle devonian and the presence of the albury bega terrane both west and east of the macquarie arc in the eastern system that also includes the ocean floor narooma terrane and igneous ocean crust terrane s repetition of the albury bega terrane either side of the arc requires either rifting or orogen parallel strike slip duplication of a once contiguous package terrane interactions began in the earliest gisbornian with early docking uplift deformation and exchange of detritus amalgamation occurred in the earliest silurian benambran orogeny with accretion in the middle devonian over 40 myr discrete turbidite terranes aligned along the gondwana margin in two systems were converted into a very wide orogen characterized by the along strike juxtaposition of superficially similar terranes

metallogeny and tectonic development of the tasman fold belt system in victoria current evidence suggests that most of victoria is underlain by a relatively thick 20 km basement of sialic composition of assumed proterozoic age this basement is nowhere exposed and its structural relationship with exposed palaeozoic rocks is conjectural this uncertainty has resulted in both ensimatic and ensialic tectonic models being proposed for victoria during the cambrian mineralization associated with cambrian igneous activity shows a variety of styles from minor orthomagmatic chromite deposits through au and cu deposits of syngenetic or epigenetic origin to fe mn ba occurrences of exhalative volcanogenic affiliation cambrian volcanism and associated sedimentation was followed by the deposition of dominantly quartz rich turbidites with interbedded shale and siliceous units subsequent to the epi ordovician benambran orogeny late silurian crustal extension caused several rifts to open along roughly orthogonal nw and ne aligned fractures within these fault bounded depressions thick acid volcanic sequences were deposited in close association with shallow marine sediments mineralization in these upper silurian rocks comprises polymetallic base metal sulphide lenses and minor disseminations at least some of which are of exhalative volcanogenic affiliation the silurian rifts were obliterated and their rocks strongly deformed during the bindian bowning deformation during late silurian to early devonian time this in turn was followed by another episode of crustal extension and rifting during which the formation of a broad meridional trough marks the buchan rift a very thick sequence of largely subaerial bimodal volcanics is overlain by shelf limestone and mudstone a variety of minor base metal barite manganese and iron mineralization is hosted by these volcanics and shelf sediments the mid devonian tabberabberan orogeny was followed in the late devonian by bimodal volcanism and granite intrusion and red bed type non marine sedimentation in central victoria thick bimodal volcanics were erupted into a series of cauldron subsidences and intruded by comagmatic granites bimodal volcanism also occurred in the mount howitt province farther east but was followed by deposition of extensive fluviatile and lacustrine sediments mainly mudstone sandstone and minor conglomerate in the mansfield basin these contain minor sedimentary copper occurrences there are four distinct episodes of granite emplacement in victoria namely late cambrian early ordovician delamerian in the glenelg zone early silurian benambran in the highlands zone early devonian bindian in the grampians ararat bendigo highlands and mallacoota zones and middle devonian carboniferous post tabberabberan in the ararat bendigo melbourne howqua and highlands zones data for the delamerian granitoids are sketchy but in the remaining groups s type granitoids predominate with the exception of eastern victoria east of the yalmy fault i s line where only i and a type granitoids occur a variety of sn mo w deposits and prospects are associated with the benambran and younger intrusive phases victoria is a major gold province which has produced nearly 2 5 106 kg gold primary gold occurs in a number of geological settings including veins and disseminations spatially associated with mafic cambrian volcanism vein deposits in turbiditic sequences of central and eastern victoria veins associated with mafic and intermediate intrusives of mid to late devonian age and minor amounts associated with a variety of granitoids and porphyry dykes

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

overthrust terranes in the lachlan fold belt southeastern australia the paleozoic history of the lachlan fold belt involves terrane translation and accretion of discrete allochthonous fragments four major terranes developed adjacent to the gondwana continental margin and were overthrust in the middle devonian to form a partly composite crust for southeastern australia