from quantitative 3d seismic stratigraphy to sequence stratigraphy insights into the vertical and lateral variability of shelf margin depositional systems at different stratigraphic orders abstract a major challenge in sequence stratigraphy is objectively identifying stratigraphic surfaces and sequences across multiple scales of observation identification is commonly dependent on the resolution of the data used i e seismic vs well data its dimension i e 1d vs 2d vs 3d or the criteria chosen to select sequence boundaries through shelf edge trajectory analysis the clinothem i e highest order seismic sequence identified on seismic data can constitute the elementary building block of an observation based and data driven quantitative workflow to develop sequence stratigraphic frameworks across different orders and ranks of hierarchy here we use high quality 3d seismic data to interpret a late tithonian early cretaceous shelf margin the lower barrow group lbg developed in the northern carnarvon basin on the north west shelf of australia based on full volume seismic interpretation techniques that integrate the 3d variability of the data when identifying seismic unconformities a high resolution seismic stratigraphic framework was built 73 interpreted clinothems with an average time duration of 63 000 yrs the computation of high frequency shelf edge trajectory angle tse curves on selected seismic cross sections is used to objectively pick sequence stratigraphic surfaces based on the accommodation succession method thereby highlighting small changes in trajectory and proposing a method reproducible by interpreters based on the same quantitative data within the d lobispinosum interval 142 3 140 9 ma the definition of stratigraphic sequences and composite stratigraphic sequences through this workflow is used to discriminate the controls at high and low temporal frequency on the vertical and lateral variability which is here quantified of this shelf slope basin system the results show that the high frequency interplay between short term glacio eustasy i e milankovitch eccentricity cycles of 100 000 yrs and sediment supply locus of fluvial input along the margin impacted the three dimensional stratigraphic architecture of the lbg in contrast tectonic subsidence had a significant impact on the stratigraphic architecture of the lbg within the main depocentre at lower temporal frequency by overprinting the eustatic signal and accelerating decelerating the rates of accommodation creation however identification of long term glacio eustatic milankovitch cycles 400 000 yrs outside the main depocentre where the rates of accommodation creation due to rift related subsidence are moderate also suggests low frequency eustatic control therefore the vertical and lateral variability of the lbg results from variations in sediment supply and subsidence regime under local i e process regime currents regional i e tectonics and global i e eustasy climate forcing parameters interplaying across timescales in contrast to standard sequence stratigraphic workflows that are based on model dependent choices to select sequence boundaries quantitative 3d seismic stratigraphy constitutes an improved method to interpret 3d seismic data in shelf margin depositional systems within a sequence stratigraphic framework which provides an observation based and model independent tool allowing the definition of stratigraphic sequences with results that are reproducible across multiple stratigraphers this work highlights the need for developing new sequence stratigraphic tools and methods that integrate the 4d variability of depositional systems and moves beyond the two dimensionality inherent to current sequence stratigraphic methods quantitative 3d seismic stratigraphy represents a first step towards the creation of 3d sequence stratigraphic workflows that could improve the prediction of stratigraphic patterns and facies relationships source reservoir seal distribution across shelf margins

clinoforms and clinoform systems review and dynamic classification scheme for shorelines subaqueous deltas shelf edges and continental margins abstract clinoforms are inclined and normally basinward dipping horizons developed over a range of spatial and temporal scales in both siliciclastic and carbonatic systems the study of clinoform successions underpins sequence stratigraphy and all efforts to reconstruct the relative partitioning of reservoir seal and source rocks along shoreline to basin floor profiles here we review clinoform research and propose a more systematic description and classification of clinoforms this is a crucial step to improve predictions of facies and lithology distribution within shoreline to continental shelf and abyssal plain successions together with the genesis drivers and dynamics of their constituent sedimentary units four basic clinoform types are here distinguished in delta shorelines lacustrine and marine environments on the basis of their overall spatial and temporal scale morphology outbuilding dynamic and geodynamic and depositional setting 1 2 delta scale clinoforms which in turns are sub divided into shoreline and delta scale subaqueous clinoforms 3 shelf edge clinoforms and 4 continental margin clinoforms delta scale clinoform sets are tens of metres high and typically represent 1 103 kyr with progradation rates ranging from 1 000 100 000 m kyr for shorelines and subaerial deltas to 100 20 000 m kyr for subaqueous deltas shelf edge clinoform sets are hundreds of metres high and are nucleated and accreted in 0 1 20 myr usual progradation rates of 1 100 m kyr by successive cross shelf transits of delta scale clinoforms continental margin clinoform sets are thousands of metres high hallmark key geodynamic crustal boundaries e g continent ocean transition and slowly prograde basinwards in ca 5 100 myr with typical rates of 0 1 10 m kyr as a consequence of the very different progradation rates and of the difficulty of large scale clinothems to backstep during transgressions shorelines are the most dynamic clinoforms with regards to position continental margins the least and shelf edges are intermediate shortly after a transgression therefore the four clinoform types may prograde synchronously along shoreline to abyssal plain transects forming compound clinoform systems during the subsequent regressive cycle however due to the dissimilarity in progradation rates different clinoform types will normally merge progressively with each other giving rise to hybrid clinoforms e g shelf edge deltas and fewer depositional breaks in slope are distinguished along a single shoreline to abyssal plain transect overall all clinoform systems are the result of the dynamic evolution of compound and hybrid clinoforms along a temporal and spatial continuum regulated by the cyclical backstepping of the smaller scale system within natural progradational retrogradational cycles of larger scale clinothem outbuilding all clinothem types may show either an accretionary active or draping passive style depending on the proximity to the sediment source draping clinothems are nearly always composed of condensed fine grained sediments while actively accreting clinothems can be composed of predominantly coarse grained i e reservoir forming or predominantly fine grained i e non reservoir lithotypes a novel hierarchical classification scheme for both recent and ancient clinoforms is here proposed consisting of 12 classes the four basic clinoform types delta scale shoreline delta scale subaqueous shelf edge and continental margin are sub divided into eight accretionary active and draping passive sub types 8 division each accretionary sub type is then sub divided into a sandstone prone and mudstone prone variant 12 division which can be at least tentatively predicted on the basis of the clinoform morphology even in the absence of direct stratigraphic logs

in situ u pb sr nd and hf isotopic analysis of eudialyte by la mc icp ms abstract eudialytes are a group of complex na ca zirconosilicate minerals that generally occur in peralkaline agpaitic syenites given that eudialytes are easily altered and commonly contain inclusions of earlier crystallized minerals in situ laser ablation is considered as the most suitable method to determine their u pb ages and sr nd and hf isotopic compositions electron microprobe and la icp ms analysis of eudialytes from nepheline syenites from ilimaussaq greenland khibiny and lovozero russia saima china tamazeght morocco kipawa and mont saint hilaire canada pocos de caldas brazil and langesund fjord norway indicate that this mineral typically has high contents of u pb nb ta zr hf and rare earth elements ree analysis of an in house standard eudialyte by both solution and laser ablation methods demonstrates that precise and accurate u pb ages can be obtained from eudialyte after correction for the common pb content the high sr nd and hf contents in eudialyte coupled with the generally low rb sr and lu hf ratios also permit the precise determination of in situ sr nd and hf isotopic ratios by la mc icp ms methods however some eudialytes with rb sr ratio of 0 02 and yb sr 0 1 and lu hf 0 08 or yb hf 0 4 0 5 cannot be used for in situ sr and hf isotopic measurement eudialyte is the only mineral investigated to date for which it is possible to determine simultaneously u pb ages and sr nd and hf isotopic compositions

a physical explanation of the relation between flank uplifts and the breakup unconformity at rifted continental margins rift flank uplifts and the breakup or postrift unconformity are characteristic features of many rifted passive or atlantic type continental margins but are not predicted as primary features of simple lithospheric stretching models the explanations that have been proposed for their origin remain controversial either because they call upon special circumstances or because they are difficult to test plane strain finite element models are used in this paper to explore the dynamics of lithospheric necking during rifting and rupture the results agree with the conceptual interpretation that uplift of the rift boundaries to form flank mountains and uplift of the basin responsible for the breakup unconformity are related consequences of regional isostatic compensation of mass that is redistributed during the necking and rupture phases although the amplitudes of these uplifts depend on the model parameter values the relations between and relative signs of these two effects appear to be fundamental the explanation we propose may have been missed in recent studies because there has been a tendency to concentrate on kinematic stretching models which assume local isostatic equilibrium through out the rifting process such an approach is predicated on the assumption that the lithosphere has an insignificant strength or flexural rigidity during extension which is not true if our explanation is correct

mechanics of fold and thrust belts and accretionary wedges cohesive coulomb theory a critically tapered fold and thrust belt or submarine accretionary wedge is one that is on the verge of coulomb failure everywhere including its base where frictional sliding along a decollement is assumed to be occurring cohesion within a wedge can add significantly to the overall strength near the toe the effect of this is to decrease the near toe taper leading to a critical topographic profile that is concave upward if the decollement is planar we obtain an approximate self consistent solution for the state of stress within a thin skinned cohesive critical coulomb wedge and determine the relationship between the wedge taper and its strength and basal friction the theory is then applied to the presently deforming fold and thrust belt of western taiwan fitting of theoretical critical wedge shapes to topographic profiles and measurements of the step up angles of thrust faults from the basal decollement are used to constrain the taiwan wedge strength parameters an attractive assertion fully consistent with all the observations is that the mechanics of fold and thrust belts and accretionary wedges is governed by normal frictional and fracture strengths of rocks measured in the laboratory in particular if byerlee s law b 0 85 is adopted as the coefficient of sliding friction on the base we find a coefficient of internal friction 0 9 1 0 in the wedge and a wedge cohesion so 5 20 mpa other solutions having strengths and ambient stresses up to 4 times lower than this can also however satisfy the data

the stratigraphic record of prebreakup geodynamics evidence from the barrow delta offshore northwest australia the structural and stratigraphic evolution of rift basins and passive margins has been widely studied with many analyses demonstrating that delta systems can provide important records of postrift geodynamic processes however the apparent lack of ancient synbreakup delta systems and the paucity of seismic imaging across continent ocean boundaries mean that the transition from continental rifting to oceanic spreading remains poorly understood the early cretaceous barrow group of the north carnarvon basin offshore nw australia was a major deltaic system that formed during the latter stages of continental rifting and represents a rich sedimentary archive documenting uplift subsidence and erosion of the margin we use a regional database of 2 d and 3 d seismic and well data to constrain the internal architecture of the barrow group our results highlight three major depocenters the exmouth and barrow subbasins and southern exmouth plateau overcompaction of pre cretaceous sedimentary rocks in the south carnarvon basin and pervasive reworking of permian and triassic palynomorphs in the offshore barrow group suggests that the onshore south carnarvon basin originally contained a thicker sedimentary succession which was uplifted and eroded prior to breakup backstripping of sedimentary successions encountered in wells in the exmouth plateau depocenter indicates that anomalously rapid tectonic subsidence 0 24 mm yr 1 accommodated barrow group deposition despite evidence for minimal contemporaneous upper crustal extension our results suggest that classic models of uniform extension cannot account for the observations of uplift and subsidence in the north carnarvon basin and may indicate a period of depth dependent extension or dynamic topography preceding breakup

precambrian tectonic evolution of part of gondwana southwestern australia southwestern australia has a long dynamic precambrian history of crustal fragmentation and aggregation by continental collision and accretion this interpretation contrasts with the widely held belief that most of australia was part of a proterozoic supercontinent in which all tectonic magmatic and metamorphic activity was intracontinental the pilbara and yilgarn cratons are two quite different fragments of archean continental crust that collided to form the capricorn orogen between 2000 and 1600 ma the albany fraser orogen formed along the southern margin of the yilgarn craton during another major continental collision at 1300 to 1100 ma rifting removed the northeastern part of these combined cratons and orogens between 1100 and 700 ma subsequent continental collision along this rifted margin formed the paterson orogen at 700 to 600 ma therefore the assembly of this considerable part of gondwana may not have been completed until about 600 ma

the geodynamics of lithospheric extension lithospheric extension is a fundamental plate tectonic process controlling the collapse of mountain belts the break up of continents and the formation of new oceanic basins this review summarises major advances on understanding a the nature of the ocean continent transition b the origin and evolution of detachment faults c the dynamic strength of continental lithosphere during extension and d the role of magmatism during extension major steps have been made in mapping out the complexities of the ocean continent transition zone particularly in magma poor rifted margins where extensional structures are not masked by voluminous magmatism the role of detachment faulting during rifting seems to be crucial and explains observations on depth dependent stretching of the lithosphere despite new insights into the dynamic behaviour of the lithosphere its rheological response remains a major uncertainty in modelling the evolution of extensional systems likewise the multiple roles that basaltic magmatism may have in modifying extension remain a key question future insights into the global behaviour of extensional systems will be gained by constraining lithospheric rheology through integrating structural evolution and magmatism in the field geodetic measurements of active extension thermochronology seismic surveys and drilling programs 2008 elsevier b v all rights reserved

the many surface expressions of mantle dynamics plate tectonic theory suggests that present day topography can be explained by the repeated interactions between the tectonic plates moving along earth s surface however mounting evidence indicates that a significant proportion of earth s topography results from the viscous stresses created by flow within the underlying mantle rather than by the moving plates this dynamic topography is transient varying as mantle flow changes and is characterized by small amplitudes and long wavelengths it is therefore often hidden by or confused with the more obvious topographic anomalies resulting from horizontal tectonic movements however dynamic topography can influence surface processes and thus enter the geological record it has for example played a role in the establishment of amazon drainage patterns in turn surface processes such as the erosion of topographical anomalies could affect mantle flow this emerging view of dynamic topography suggests that the concept of plate tectonics as the driver of surface deformation needs to be extended to include the vertical coupling between the mantle and the surface unravelling this coupling back in time with the help of models and the geological record can potentially provide unprecedented insights into past mantle dynamics

proterozoic anorthosite granite nain batholith structure and intrusion processes in an active lithosphere scale fault zone northern labrador we present field evidence of the structure and evolution of a typical proterozoic massif type anorthosite the anorthosite forms a batholith composed of numerous plutons and ring complexes intruded episodically in two main sequences each producing anorthositic rocks and monzonite ferrodiorite followed by granite the batholith is located where a lithosphere scale fault zone 50 km wide and extending over 1000 km from greenland to labrador intersected an older continental suture intermittent episodes of mainly sinistral transtension along this gardar voisey s bay fault zone were associated with the generation of a number of pulses of magmatism and controlled the rise of magmas to middle and upper crustal levels between 1363 and 1289 ma both the large and small scale structures and magma intrusion processes are similar to those of many cordilleran batholiths with tabular plutons associated with ring dykes emplaced by cauldron subsidence and basin shaped plutons associated with cone sheets these

magmatism and rift margin evolution evidence from northwest australia deep crustal seismic observations from northwest australia demonstrate that the cuvier margin is a volcanic passive margin that formed as greater india rifted away from australia in the early cretaceous formation of the cuvier basin and initial sea floor spreading resulted in the emplacement of exceptionally thick oceanic crust while contemporaneous spreading off the adjacent exmouth plateau formed normal thickness oceanic crust crust of normal thickness was generated in the cuvier basin within 3 4 m y after spreading began there is no evidence for hot spot activity with the appropriate timing or in the appropriate spatial pattern to account for the difference in volcanism along the margin we believe that rapid rifting of the cuvier margin created strong lateral temperature gradients in the subjacent upper mantle at the time of breakup this induced small scale convection in the upper mantle that increased the rate of upwelling and thus the volume of melt generated by decompression partial melting in turn this resulted in a larger volume of magma at the initiation of sea floor spreading and thicker oceanic crust was initially emplaced rifting on the exmouth plateau happened over a much longer period of time and no such convective partial melting occurred the volume of melt associated with rifting was therefore entirely the result of passive upwelling and a normal thickness of oceanic crust was emplaced immediately adjacent to the exmouth plateau

raft tectonics the effects of basal slope angle and sedimentation rate on progressive extension abstract in the gulf of guinea the upper cretaceous tertiary sedimentary cover has undergone gravity gliding above an aptian salt layer for various basal slope angles from 0 to 4 gliding started during albian time and evolved up to the present with variable sedimentation rates faulting patterns are seen to vary in particular as a function of the basal slope angle and the syn kinematic sedimentation rates a series of laboratory experiments on small scale models is used to study the effects of the two parameters mentioned above models are composed of two layer slabs with newtonian silicone putty at the base to represent a basal salt decollement and dry sand on top to represent the sedimentary overburden models are allowed to deform under their own weight for various basal slope angles ranging from 0 to 5 with two different syn kinematic sedimentation rates it is shown that the basal slope angle controls the location of faulting in the overburden within a single downslope deformation domain for values smaller than 2 and within two domains lying downslope and upslope for higher values of the width of the deformation domains also varies as a function of overburden faulting always starts with syn kinematic grabens which evolve into symmetric or asymmetric grabens or into tilted blocks bounded by listric normal faults while synthetic listric normal faults characterize the downslope deformation domains both synthetic and antithetic listric normal faults can occur in the upslope deformation domain ductile deformation within the basal decollement layers results from variable combinations of pure shear and simple shear interaction of ductile deformation in the ductile decollement layer with faulting in the overburden locally produces complex and heterogeneous strain patterns notably within salt rollers finally increasing rates of syn kinematic sedimentation are seen to increase the rate of downslope displacement and to enhance and favour the development of listric normal faults

australia s southern margin a product of oblique extension abstract recently developed detachment models of continental margin formation interpret the southern margin of australia to have formed when the lower plate australian margin was pulled out from beneath the upper plate antarctic margin data now available and summarised in this paper point very strongly to a generally nw se direction of initial continental extension for the southern margin in contrast to the widely held picture of simple n s rifting the evidence for this extension direction comes from the analysis of deep seismic data acquired by the bureau of mineral resources in 1986 in the central great australian bight gab the gravity field of the gab seismic and magnetic basement structures in the eyre sub basin polda trough ceduna depocentre and duntroon basin and from the analysis of the magnetic seafloor spreading anomalies produced during the slow first phase of drifting between australia and antarctica further it is now believed that the formation of the southern margin of australia can be described in terms of three phases of continental extension el to e3 and two phases of drifting d1 and d2 in summary these phases were as follows 1 e1 approximately 300 km of late jurassic or older to early cretaceous nw se oriented extension in the gab with strike slip motion in the nascent otway basin and along the tasmanian margin 2 e2 120 km of early cretaceous nne ssw oriented extension which formed the basins of southeastern australia otway bass gippsland and which probably produced a structural overprinting in the gab basin 3 e3 d1 minor continental extension and the first 500 km of slow drift between australia and antarctica on an azimuth of 165 wrenching on the tasmanian margin 4 d2 2600 km of fast drifting between australia and antarctica on a n s azimuth

a nd and o isotope study of the ree rich peralkaline strange lake granite implications for mesoproterozoic a type magmatism in the core zone ne canada it is well established that a type granites enriched in high field strength elements such as zr nb and the ree form in anorogenic tectonic settings the sources of these elements and the processes controlling their unusual enrichment however are still debated they are addressed here using neodymium and oxygen isotope analyses of samples from the 1 24 ga strange lake pluton in the paleoproterozoic core zone of quebec labrador an a type granitic body characterized by hyper enrichment in the ree zr and nb age corrected end values for bulk rock samples and sodic amphiboles mainly arfvedsonite from the pluton range from 0 6 to 5 7 and 0 3 to 5 3 respectively the end values for the napeu kainiut quartz monzonite which hosts the pluton range from 4 8 to 8 1 the 147sm 144nd ratios of the suite and the host quartz monzonite range from 0 0967 to 0 1659 large variations that can be explained by in situ fractionation of early lree minerals strange lake and late hydrothermal hree remobilization oxygen isotope analyses of quartz of both strange lake and the host yielded 18o values between 8 2 and 9 1 which are considerably higher than the mantle value of 5 7 0 2 bulk rock oxygen isotope analyses of biotite gneisses in the vicinity of the strange lake pluton yielded 18o values of 6 3 8 6 and 9 6 the negative end values and positive 18o values of the strange lake and napeu kainiut samples indicate that both magmas experienced considerable crustal contamination the extent of this contamination was estimated assuming that the contaminants were sedimentary derived rocks from the underlying archean mistinibi para gneiss complex which is characterized by low end and high 18o values mixing of 5 15 of a gneiss having an end value of 15 and a 18o value of 11 with a moderately enriched mantle source end 0 9 18o 6 3 would produce values similar to those obtained for the strange lake granites based on analogies between the nain plutonic suite and the gardar alkaline igneous province sw greenland we conclude that the strange lake pluton and associated ree mineralized anorogenic bodies formed from a combination of subduction induced fertilization of the sublithospheric mantle crustal extension and in situ magma evolution

depth dependent stretching a different approach uplift of the crust during rifting is a major problem that has been left unresolved by uniform stretching models various depth dependent stretching models have been proposed to account for this but these require decoupling of the crust and mantle and result in space problems within the mantle a significant modification of these discontinuous nonuniform or depth dependent stretching models is proposed here that accounts for synrift uplift and doming but does not result in these problems the model involves continuous nonuniform stretching within a polygonal region bounded by symmetrical outward sloping boundaries within the mantle this geometry results in thinning of the lithosphere below crustal regions that are less stretched or unstretched crustal uplift resulting from asthenospheric upwelling below these regions this in turn results in stretching factors for the mantle that are less than those for the crust a relationship opposite to relationships derived by discontinuous nonuniform stretching models corollary predictions for mantle derived heat flow within and outside regions of stretching of these two different depth dependent models may allow them to be distinguished from each other and from regions where uniform stretching predominates

structural evolution of shale diapirs from reactive rise to mud volcanism 3d seismic data from the baram delta offshore brunei darussalam two areas of shale diapirism in a deltaic setting were examined using 3d seismic data from offshore brunei to determine the origin evolution and geometry of shale diapirs the study areas cover parts of the egret and ampa fields within the baram delta province the ampa diapir consists of a small bulge of the mobile setap shale around which lies a zone of chaotically disrupted country rock reflections at the crest of this chaotic zone vertical cylindrical shale pipes arise that end in a wider head the diapir heads wear halos of high amplitude reflections probably due to the presence of methane gas the egret data show the same features except that the bulge of the source layer is not detected due to low reflectivity and chaotic reflections at the base of the overburden the bulging of the setap shale on the ampa data is interpreted as modified reactive diapirism in response to differential loading the initial reactive diapir shape was modified by lateral or oblique upward migration of fluidized shales into the hanging wall of the fault controlling the reactive diapir this probably occurred as a network of dykes and sills the presence of some preserved bedding is indicated by weak but coherent reflectivity from the area initially defined as chaotic shale the spreading of overpressure and the injection of shale into the overburden is thought to be a late phenomenon possibly late pliocene or early pleistocene and the mobility of the shale is attributed to overpressure increase due to gas generation within the setap shale and maybe the lower part of the overburden gas migration perhaps ahead of the diapir intrusion probably facilitated the mud injection the ampa and egret diapirs formed in two diapiric phases an early reactive diapirism and a later active diapirism but the phases have little in common they are seperated by about 9 million years of inactivity and differ in process and probably differ in degree of overpressure in overpressure generating mechanism and in properties of the diapiric mud

anomalous subsidence history of the west siberian basin as an indicator for episodes of mantle induced dynamic topography abstract vertical intraplate motions are not easily related to plate tectonics understanding their underlying mechanism is further complicated by poor constraints on the magnitude and timing of these motions western siberia is a prominent intraplate area affected by vertical tectonics shortly after the assemblage of pangaea the region experienced substantial uplift followed by a long period of subsidence that led to formation of the world s largest sedimentary basin the west siberian basin its sedimentary cover has been extensively explored and described in the soviet literature here we digitise compilations of stratigraphic data to calculate the basin s subsidence history by backstripping analysis our results confirm the prolonged and high amplitude regional subsidence that has been noted before and constitutes the basin s first order vertical motion however our backstripping results also reveal a secondary mode of shorter spatial and temporal scales induced by the migration of the maximum subsidence across the basin importantly the generally slow subsidence of the basin was interrupted by uplift events in the early and late cretaceous and in the middle oligocene while the secondary mode of vertical motion is not easily understood in terms of traditional subsidence models restricted to lithospheric cooling after stretching it is consistent with rapid uplift events that have been reported for other locations such as elevated passive continental margins we discuss the geodynamic implications and conclude that the basin may hold important constraints on dynamic topography induced by sublithospheric mantle flow processes

thermal uplift and erosion across the continent ocean transform boundary of the southern exmouth plateau thermal evolution of the continental lithosphere at a continent ocean transform margin is examined using a two dimensional heat conduction model all heating is assumed to result from the emplacement of new oceanic ridge against the continent the assumed initial continental temperature gradient is probably best applied to regions having experienced finite duration periods of rifting lasting tens of millions of years a deep seismic reflection profile across the southern paleo transform margin of the exmouth plateau northwest australia tests the predictions of the model using this reflection profile it is estimated that up to 3 5 km of sediments have been eroded from the continental rim diminishing to almost no erosion at 60 km from the continent ocean transform boundary this trend and these values for erosion can be matched approximately using the model under the condition of local isostasy a finite difference scheme is employed where the surface elevation is the result of the competing 1 thermal uplift up 2 surficial erosion down and 3 local isostatic rebound up in response to the erosion the model predicts that most of the erosion ceases by 40 ma after ridge emplacement and that of the order of 1000 km3 eroded sediments are shed for every 10 km of transform length

a 40ar 39ar and u pb isotopic study of the ilimaussaq complex south greenland implications for the 40k decay constant and for the duration of magmatic activity in a peralkaline complex abstract magmatism in the gardar province south greenland is related to two main rifting events at 1280 ma and 1180 to 1140 ma little is known about the duration of the magmatic activity in a specific complex the ilimaussaq intrusion belongs to the second period of rifting and comprises an extraordinary diversity of granitic and syenitic rock types which intruded and fractionated in three successive magmatic events as the intrusion contains some of the most evolved incompatible element rich rocks on earth it was chosen for a detailed geochronological study to quantify the duration of melt production intrusion fractionation and cooling amphiboles which are abundant in all rock types including pegmatites and late magmatic veins were dated using the 40 ar 39 ar technique since the solidus temperature of the most evolved melts is below the closure temperature of amphibole and no later heating event occurred the 40 ar 39 ar ages reflect the magmatic crystallisation and can be used to determine the duration of igneous differentiation the 40 ar 39 ar plateau ages range between 1142 6 2 2 ma and 1152 3 3 7 ma using the steiger and jager steiger r h and jager e 1977 subcommision on geochronology convention on the use of decay constants in geo and cosmochronology earth planet sci lett 36 359 362 40 k decay constant these ages are younger than an u pb age of 1160 5 ma on baddeleyite from the first magma batch our results indicate that the 40 k decay constant of steiger and jager may be too high a lambda similar to that proposed by kwon et al kwon j min k bickel p and renne p r 2002 statistical methods for jointly estimating decay constant of 40 k and age of a dating standard math geol 34 457 474 is required to make the 40 ar 39 ar match the u pb age

sedimentation and shale tectonics of the northwestern niger delta front abstract the modern day slope and outer shelf of the niger delta are characterized by fault bounded sedimentary depocentres and intervening shale structures some of which are diapiric using two dimensional seismic reflection data and stratigraphic information from five wells the neogene history of a 70 30 km portion of the northwestern niger delta shelf and slope has been examined the shale structures were established by late miocene time probably in response to lateral shale withdrawal from beneath the advancing deltaic load combined with compressional uplift and folding of pro delta strata during pliocene and pleistocene time these structures were buried by the prograding delta and extensional growth faulting commenced stacked onlap relationships and stacked erosional channels indicate that the shale structures continued to influence sedimentation patterns throughout this period throughout the pliocene extension and subsidence were concentrated in a graben like depocentre bounded by both regional and counter regional growth faults during the early pleistocene subsidence on the regional growth fault ceased and growth was transferred to the counter regional growth fault set the geometry and history of the depocentre is analogous to salt related depocentres typical of the northern gulf of mexico and atlantic margins of brazil and angola to explain the history of the niger delta study area a model is proposed in which the miocene pleistocene sedimentary package has slid basinward breaking up during extension thereby creating seaward stepping depocentres examples from analogue modelling studies are used to suggest possible origins for the large counter regional growth faults

metallogeny of south greenland a review of geological evolution mineral occurrences and geochemical exploration data abstract south greenland has been the site of historic mining of cryolite copper graphite and gold hosts mineral deposits with gold uranium zinc niobium tantalum zirconium hafnium ree iron titanium vanadium fluorite and graphite and has additional potential for lithium beryllium phosphorus gallium and thorium data from stream sediment geochemical surveys document that south greenland is enriched in a range of these elements relative to the rest of greenland and to estimates of the upper crust composition distribution patterns for individual elements within south greenland exhibit enriched regions that are spatially related to lithological units crustal structure and known mineralisation the northern domain of south greenland includes the southernmost part of the orthogneiss dominated north atlantic craton orogenic gold mineralisation is hosted by quartz veins and hydrothermally altered rocks associated with shear zones intersecting the mesoarchaean tartoq group of mafic metavolcanic rocks geochemical exploration indicates that additional potential for gold mineralisation exists within palaeoproterozoic supracrustal rocks overlying the archaean basement rocks formed during the palaeoproterozoic ketilidian orogeny occupy a major part of south greenland and has been divided into two domains the central domain is underlain by the julianehab igneous complex forming a 100 km wide ene wsw zone centrally across south greenland intrusive and extrusive mostly felsic magmatic rocks were emplaced in two main stages 1850 1830 and 1800 1780 ma in a continental arc setting positive anomalies in aeromagnetic data indicate that mafic plutons are common in the late igneous complex intra arc mafic metavolcanic rocks contain syngenetic stratabound copper sulphide and epigenetic shear zone hosted copper silver gold mineralisation at kobberminebugt and kangerluluk whereas metasedimentary and metapyroclastic rocks contain stratabound uraninite mineralisation orthomagmatic iron titanium vanadium mineralisation is hosted by a gabbro a potential for porphyry type mineralisation related to the late intrusive stages of the julianehab igneous complex is suggested by showings with copper molybdenum and gold together with stream sediment anomalies for these elements vein type uranium mineralisation occurs in fault zones in the julianehab igneous complex related to mesoproterozoic rifting the southern domain contains an assemblage of palaeoproterozoic metasedimentary and metavolcanic rocks that underwent moderate to strong deformation peak ht lp metamorphism and partial melting with subsequent retrograde exhumation at 1790 1765 ma the supracrustal rocks contain syngenetic au as sb u and zn mineralisation in volcanic or graphite and sulphide rich sedimentary environments graphite was mined historically at two sites many stream sediment gold anomalies are located in a ne trending belt along the boundary between the early julianehab complex and the supracrustal rocks to the south they reflect a number of auriferous quartz vein occurrences including the nalunaq gold deposit hosted in a system of shear zones and probably generated as orogenic gold during ketilidian accretion the 1755 1730 ma a type ilua plutonic suite is the latest magmatic event in the ketilidian orogen the 1300 1140 ma gardar period involved continental rifting sedimentation and alkaline magmatism numerous dykes and 10 ring shaped intrusion complexes were formed across south greenland an orthomagmatic iron titanium vanadium deposit is hosted by troctolitic gabbro residual magmas and fluids resulting from extreme magmatic differentiation possibly combined with assimilation of older crust created mineral deposits including cryolite that was mined at ivigtut large low grade deposits of uranium rare earth elements zinc at kvanefjeld and tantalum niobium rare earth element zirconium at kringlerne in the ilimaussaq complex as well as tantalum niobium rare earth elements at motzfeldt so in the igaliko complex the south greenland crustal evolution records effects of mantle processes such as lithospheric extension subduction and underplating which resulted in recurrent magma emplacement in tectonically active environments as such the geology of south greenland reflects events and circumstances that are favourable to the generation and preservation of hydrothermal ore forming fluid systems during the ketilidian orogeny as well as to the development of extreme rock compositions within the gardar alkaline igneous province

genetic stratigraphic sequences in basin analysis ii application to northwest gulf of mexico cenozoic basin the northwest gulf of mexico cenozoic sedimentary wedge illustrates the application of genetic stratigraphic sequence analysis and documents several general conclusions 1 sequences defined by regional marine flooding are the principal genetic stratigraphic units of the basin fill continental margins are characterized by repetitive episodes of basin margin offlap punctuated by periods of transgression and marine flooding of the depositional platform 2 continental margin outbuilding is concentrated at one or more shelf edge deltaic depocenters separated by interdeltaic bights depocenters remain fixed during a depositional episode but commonly relocate during transgression and flooding 3 a distinct syndepositional structural style in prograding continental margins results in sporadic uplift of a basin fringing peripheral bulge and accentuates preservation of shelf margin facies along zones of extensional normal faulting and enhanced subsidence 4 genetic stratigraphic sequences commonly reflect an evolving interplay among two or even three variables for example early cenozoic gulf sequences are most closely related to tectonic events of the intraplate source terrane which in turn affect rate and location of sediment supply and basin margin response to loading late cenozoic sequences more closely reflect proposed eustatic cycles

volcanic margin formation and mesozoic rift propagators in the cuvier abyssal plain off western australia the western margin of australia is characterized by synrift and postrift magmatism which is not well understood a joint interpretation of magnetic anomaly satellite gravity anomaly and seismic data from the cuvier abyssal plain and margin shows that the breakup between india and australia started circa 136 ma m14 and was followed by two rift propagation events which transferred portions of the indian plate to the australian plate post breakup magmatism continued with the emplacement of the wallaby and zenith plateaus 17 18 km thick at their centers along a transform margin two narrow magmatic edifices adjacent to the wallaby plateau sonne and sonja ridges represent an extinct ridge and a pseudofault respectively they formed by excess volcanism probably by lateral migration of buoyant melt along upside down crustal drainage channels from the melt source underneath the wallaby plateau in a mantle plume scenario a small plume 400 km diameter located underneath the rift could have locally uplifted the bernier platform and exmouth sub basin in the early cretaceous and left a track consistent with the azimuth of the wallaby and zenith plateaus in this case ridge plume interaction would have caused two consecutive ridge propagation events towards the plume while the ridge moved away from the hotspot the abrupt end of the hotspot track west of the zenith plateau would be a consequence of the accelerating south eastward motion of the spreading ridge relative to the mantle after 120 ma leaving the mantle plume underneath the indian plate an alternative nonmantle plume scenario is based on the observation that between breakup and chron m0 120 ma the ocean crust in the southern cuvier abyssal plain was formed while the spreading ridge abutted indian continental crust small scale convection may have been initiated during rifting in the early cretaceous and maintained until the wallaby zenith ridge transform intersection passed by the eastern edge of indian continental crust at chron m0

eustatic curve for the middle jurassic cretaceous based on russian platform and siberian stratigraphy zonal resolution we have used the stratigraphy of the central part of the russian platform and surrounding regions to construct a calibrated eustatic curve for the bajocian through the santonian the study area is centrally located in the large eurasian continental craton and was covered by shallow seas during much of the jurassic and cretaceous the geographic setting was a very low gradient ramp that was repeatedly flooded and exposed analysis of stratal geometry of the region suggests tectonic stability throughout most of mesozoic marine deposition the paleogeography of the region led to extremely low rates of sediment influx as a result accommodation potential was limited and is interpreted to have been determined primarily by eustatic variations the central part of the russian latform thus provides a useful frame of reference for the quantification of eustatic variations throughout the jurassic and cretaceous the biostratigraphy of the russian platform provides the basis for reliably determining age and eustatic events the mesozoic section of the central part of the russian platform is characterized by numerous hiatuses in this study we filled the sediment gaps left by unconformities in the central part of the russian platform with data from stratigraphic information from the more continuous stratigraphy of the neighboring subsiding regions such as northern siberia although these sections reflect subsidence the time scale of variations in subsidence rate is probably long relative to the duration of the stratigraphic gaps to be filled so the subsidence rate can be calculated and filtered from the stratigraphic data we thus have compiled a more complete eustatic curve than would be p ssible on the basis of russian platform stratigraphy alone relative sea level curves were generated by backstripping stratigraphic data from well and outcrop sections distributed throughout the central part of the russian platform for determining paleowater depth we developed a model specifically designed for this region based on paleoecology sedimentology geochemistry and paleogeography the curve describes a series of high frequency eustatic events superimposed on longer term trends many of the events identified from our study can be correlated to those found by haq et al 1988 and other sea level studies from other parts of the world but there are significant differences in the relative magnitudes of events because the eustatic curve resulting from this study is based on a stable reference frame the curve can be used in sedimentary basin modeling and as a tool for quantifying subsidence history from the stratigraphy of passive margins basins and other active regions

extensional tectonics in the jeanne d arc basin offshore newfoundland implications for the timing of break up between grand banks and iberia

nature of the continent ocean transition zone along the southern australian continental margin a comparison of the naturaliste plateau sw australia and the central great australian bight sectors we document the interpretation of three crustal sections from coincident deep seismic reflection gravity and magnetic data acquired on australia s southern margin one section from the naturaliste plateau and the diamantina zone and two in the great australian bight gab interpretations are based on an integrated study of deep multichannel seismic gravity and magnetic data together with sparse sonobuoy and dredging information all interpreted sections of the margin show a transition from thinned continental crust through a wide continent ocean transition zone cotz in the gab the transition is to slow sea floor spreading oceanic crust that dates from breakup in the campanian c 83 ma in the naturaliste diamantina margin the earliest oceanic crust is undated the cotz on these margins is geologically and geophysically complex but interpretation of all data including dredge hauls is consistent with the presence of a mixture of modified continental lower crust breakup related volcanics and exhumed continental mantle serpentinized detachment faults are not well imaged but have been inferred from high amplitude magnetic signatures interpreted to arise from magnetite associated with the hydration of peridotites alternative models for the structure of the cotz involving either mafic underplating or aborted sea floor spreading have been explored but are considered unlikely on this margin similarity in the final architecture of these margins has major implications for the nature of rifting in the southern rift system and may point to the entire 4000 km long system being non volcanic in character second order differences in geometry and morphology of the two areas studied are unlikely to be a function of strain rate instead they probably reflect complexities owing to the multiple tectonic events that occurred during final gondwanide fragmentation the most dramatic of these is the impact of hotspot activity in the kerguelen plateau which commenced some 50 ma prior to final breakup in that sector

estimation of snow water equivalence using sir c x sar ii inferring snow depth and particle size for pt i see ibid vol 38 no 6 p 2465 74 2000 the relationship between snow water equivalence swe and sar backscattering coefficients at c and x band 5 5 and 9 6 ghz can be either positive or negative therefore discovery of the relationship with an empirical approach is unrealistic instead the authors estimate snow depth and particle size using sir c x sar imagery from a physically based first order backscattering model through analyses of the importance of each scattering term and its sensitivity to snow properties using numerically simulated backscattering values the authors develop semi empirical models for characterizing the snow ground interaction terms the relationships between the ground surface backscattering components and the snowpack extinction properties at c band and x band with these relationships snow depth and optical equivalent grain size can be estimated from sir c x sar measurements validation using three sir c x sar images shows that the algorithm performs usefully for incidence angles greater than 300 with root mean square errors rmses of 34 cm and 0 27 mm for estimating snow depth and ice optical equivalent particle radius respectively

from mantle to motzfeldt a genetic model for syenite hosted ta nb mineralisation abstract a genetic model for the motzfeldt tantalum niobium rich syenite in south west greenland considered to be one of the world s largest ta prospects is presented the motzfeldt primary magma formed early in regional gardar 1273 6 ma rifting isotope signatures indicate that the hf had multiple sources involving juvenile gardar hf mixed with older palaeoproterozoic or archaean hf we infer that other high field strength elements hfse similarly had multiple sources the magma differentiated in the crust and ascended before emplacement at the regional unconformity between ketilidian basement and eriksfjord supracrustals the hfse rich magmas crystallised ta rich pyrochlore which formed pyrochlore rich crystal mushes and it is these pyrochlore rich horizons rich in ta and nb that are the focus of exploration the roof zone chilled and repeated sheeting at the roof provided a complex suite of cross cutting syenite variants including pyrochlore microsyenite in a hot sheeted roof model the area was subject to hydrothermal alteration which recrystallized alkali feldspar to coarse perthite and modified the mafic minerals to hematite creating the friable and striking pink nature of the motzfeldt so centre carbon and oxygen isotope investigation of carbonate constrains fluid evolution and shows that carbonate is primarily mantle derived but late stage hydrothermal alteration moved the oxygen isotopes towards more positive values up to 21 the hydrothermal fluid was exceptionally fluorine rich and mobilised many elements including u and pb but did not transport hfse such as ta hf and nb although the u and pb content of the pyrochlore was enhanced by the fluid the hfse contents remained unchanged and therefore hf isotopes were unaffected by fluid interaction while the effect on hydrothermal alteration on the visual appearance of the rock is striking magmatic processes concentrated hfse including ta and the hydrothermal phase has not altered the grade exploration for hfse mineralisation commonly relies on airborne radiometric surveying which is particularly sensitive to the presence of u th a crucial lesson from motzfeldt is that the best target is unaltered pyrochlore which was identified less easily by radiometric survey careful petrological mineral studies are necessary before airborne survey data can be fully interpreted

lower crustal extension across the northern carnarvon basin australia evidence for an eastward dipping detachment asymmetry and strain partitioning along conjugate margins are often explained in terms of detachment faults nevertheless cogent evidence for their existence remains limited furthermore even when inferred detachment faults are imaged seismically it is difficult to demonstrate that differential displacement has occurred across these surfaces we present tectonic and stratigraphic evidence from the northern carnarvon basin northwest australia that documents the existence of an intracrustal eastward dipping detachment by integrating sequence stratigraphy with kinematic and isostatic models of basin development we conclude that the northern carnarvon basin was formed as a consequence of four extension events 1 a broadly distributed late permian event 2 a predominantly localized rhaetian event responsible for the inception of the barrow and dampier subbasins 3 a localized callovian fault reactivation within the barrow dampier subbasins and 4 a tithonian valanginian event that generated large post valanginian regional subsidence across the northern carnarvon basin with only minor accompanying brittle deformation and erosional truncation the regional distribution and amplitude of the post valanginian subsidence are not consistent with the minor amounts of tithonian valanginian brittle upper crustal extension observed on the margin large portions of the platform were emergent or at very shallow water depths prior to the tithonian valanginian extension to match the distribution and magnitude of the post valanginian thermal type subsidence requires significant lower crustal and mantle extension across the northern carnarvon basin such a distribution of extension implies the existence of an eastward dipping intracrustal detachment having a ramp flat ramp geometry that effectively thinned the lower crust and lithospheric mantle the detachment breached the surface close to the position of the continent ocean boundary west of the exmouth plateau the flat component of the detachment occurred at midcrustal depths 15 km across the plateau and ramped beneath the australian continent lower crustal ductile extension provides a viable mechanism to generate large regional subsidence with little attendant brittle deformation which may explain the paradox that both sides of many conjugate margins appear to be the upper plate

crustal stretching style variations in the northern margin of the south china sea linking deep seismic profiles with regional scale gravity inversion is a powerful tool to deduce the architecture of rifted margins and their structural evolution here we map upper and lower crustal thicknesses of the northern south china sea scs margin in order to investigate the occurrence of depth dependent crustal extension from the proximal to the distal margin by comparing upper and lower crustal stretching factors we find that the northern margin of the scs is segmented in three parts 1 sedimentary basins where upper crust is stretched more than lower crust 2 distal margin where lower crust is stretched more than upper crust 3 mostly proximal margin regions where the two layers have similar stretching factors our results suggest that sedimentary basins and distal margin prominently feature depth dependent extension however accommodated by different processes while differential thinning within sedimentary basins appears to be governed by lateral pressure variations inducing lower crustal flow we suggest the distal margin to be affected by a combination of mantle flow induced lower crustal shearing and sequential fault activity during crustal hyper extension

interactions between continental breakup dynamics and large scale delta system evolution insights from the cretaceous ceduna delta system bight basin southern australian margin 1 the interpretation of two regional seismic reflection profiles and the construction of a balanced cross section through the southern australian margin bight basin are designed to analyze the influence of the australia antarctica continental breakup process on the kinematic evolution of the cretaceous ceduna delta system the data show that the structural architecture of this delta system consists of two stacked delta systems the lower white pointer delta system late albian santonian is an unstable tectonic wedge regionally detached seaward above late albian ductile shales sequential restoration suggests that the overall gravitational sliding behavior of the white pointer delta wedge 45 km of seaward extension i e 27 is partially balanced by the tectonic denudation of the subcontinental mantle we are able to estimate the horizontal stretching rate of the mantle exhumation between 2 and 5 km ma 1 the associated uplift of the distal part of the margin and associated flexural subsidence in the proximal part of the basin are partially responsible for the decrease of the gravitational sliding of the white pointer delta system lithospheric failure occurs at 84 ma through the rapid exhumation of the mantle the upper hammerhead delta system late santonian maastrichtian forms a stable tectonic wedge developed during initial slow seafloor spreading and sag basin evolution of the australian side margin lateral variation of basin slope related to the geometry of the underlying white pointer delta wedge is associated with distal raft tectonic structures sustained by high sedimentation rates finally we propose a conceptual low angle detachment fault model for the evolution of the australian antarctic conjugate margins in which the antarctic margin corresponds to the upper plate and the australian margin to the lower plate

rifted margin architecture and crustal rheology reviewing iberia newfoundland central south atlantic and south china sea abstract crustal rheology controls the style of rifting and ultimately the architecture of rifted margins here we review the formation of three magma poor margin pairs iberia newfoundland the central segment of the south atlantic rift and the south china sea by integrating observational data into a numerical forward modelling framework we utilise a 2d version of the finite element code slim3d which includes nonlinear temperature and stress dependent elasto visco plastic rheology and is able to reproduce a wide range of rift related deformation processes such as flexure lower crustal flow and faulting extension in cold strong or thin crust is accommodated by brittle faults and ductile shear zones that facilitate narrow rifts with asymmetric fault geometries hot weak or thick continental crust is dominated by ductile deformation and often extends symmetrically into a wide rift system this simple recipe provides the standard framework to understand initial rift geometry however it is insufficient to account for the dynamics of intermediate and late rift stages that shape the final margin architecture asymmetric conjugate margins where one side is wide and the other narrow can be formed via both wide and narrow rift styles which we reproduce with weak and strong crustal rheologies respectively exemplified by the iberia newfoundland conjugates and the central south atlantic we define three characteristic rift phases an initial phase of simultaneous faulting an intermediate phase of rift migration that involves sequential fault activity and finally the breakup phase crustal rheology plays an overarching role in governing the dynamics of these asymmetric margins we illustrate that weak rheologies generally prolong the phase of simultaneous faulting while rift migration is enabled by initial fault asymmetry as well as relatively weak crust formation of the predominantly symmetric and wide margins of the south china sea was controlled by extraordinarily weak crust that extended the phase of simultaneous faulting until breakup the weak crustal rheology of this region relates to the south china sea s pre rift history where plate convergence lead to crustal thickening and magmatic additions in a back arc regime shortly before the onset of rifting

an analysis of geomagnetic field reversals supports the validity of the geocentric axial dipole gad hypothesis in the precambrian abstract the geocentric axial dipole gad hypothesis has been regarded as the cornerstone of paleomagnetism for decades and disputes on its applicability have mostly dealt with the precambrian data one way to analyze the zonal harmonics of the geomagnetic field and the validity of gad hypothesis is based on the angular difference between the normal and reversed polarities also referred to as reversal asymmetry for this purpose a comprehensive compilation of the reversals of archaean and proterozoic geomagnetic field has been done using a novel paleomagnetic database with 3246 entries globally given the uncertainties prevailing in this type of analysis results from 106 reversal records show no dependence of the reversal asymmetry on paleolatitude thus supporting the outcome of our inclination frequency analysis of the precambrian field in addition to the inclination asymmetry the non antiparallelity between n and r also involves a declination asymmetry which is more difficult to determine and deals with the non zonal features of the field based on our dual polarity data we propose that the geocentric axial dipole gad is a viable approximation of the geomagnetic field throughout the precambrian

paleomagnetism and geochemistry of 1144 ma lamprophyre dikes northwestern ontario implications for the north american polar wander and plate velocities

deformation style and basin fill architecture of the offshore limon back arc basin costa rica abstract the limon back arc basin located at the eastern coast of costa rica is part of the southern central american arc trench system basin evolution started in late cretaceous times as response to the onset of the subduction of the farallon plate below the caribbean plate the limon basin can be subdivided into a northern and a southern sub basin separated by the e w trending trans isthmic fault system a regional grid of offshore seismic lines allows the comparative analysis of the basin fill architecture and the deformation style in both sub basins the northern sub basin shows exclusively normal faults the basin fill architecture resembles a passive continental margin setting showing a distinct wedge shaped geometry and seaward propagating depositional units the area of the rio san juan delta is characterized by gravity driven deltaic deformation exhibiting a series of listric growth faults both on the shelf and in slope areas in contrast to the northern basin the southern sub basin is characterized by the development of thin skinned fold and thrust belt tectonics best recorded in concentric or asymmetric hangingwall anticlines separated by listric or planar southwestward dipping thrust faults due to the pronounced ne propagating of folding the shelf is broader and the slopes are steeper in the southern sub basin compared with the northern sub basin thus both sub basins show a very different tectonic style in the north limon basin an extensional regime established whereas in the south limon basin compression dominated however in both sub basins the basal detachment is probably controlled by a lithological change from limestone to shale

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

seasonal and short term variability of multifrequency polarimetric radar backscatter of alpine terrain from sir c x sar and airsar data signatures of glaciated and ice free areas were analyzed from polarimetric sar data at c l and p band and single polarization x band data the data base includes an airsar scene from june 25 1991 and sir c x sar images from april and october 1994 srl 1 and srl 2 acquired over the high alpine test site o spl uml tztal in austria the environmental conditions were different at the time of the three experiments ground measurements meteorological observations and backscattering modeling are the basis for interpreting the backscattering signatures seasonal differences are due mainly to the presence or absence of snow and due to changes of its properties short term variations of snow conditions can be monitored at c and x band for unglaciated areas the surface roughness has a dominant influence on backscattering in all seasons the dependence of the mean backscattering and correlation coefficients on the incidence angle was analyzed spectral and depolarization ratios and the magnitude of the hhvv correlation coefficient were selected as components of the multidimensional feature vector for studying the target separability good separability was found between the accumulation and ablation areas on the glaciers whereas on ice free areas the dominance of surface roughness limits the discrimination of different surface types short term variations of backscattering have significant impact for the classification of accumulation and ablation areas on glaciers as verified by comparisons with field data

asymmetric lithospheric extension the role of frictional plastic strain softening inferred from numerical experiments plane strain thermomechanical finite element model experiments of lithospheric extension are used to investigate the effects of strain softening in the frictional plastic regime on the asymmetry of extension strain softening is considered in cases where the crust is either strongly or weakly coupled to the mantle and as the extension velocity varies from 0 3 to 30 cm yr in the absence of strain softening extension is symmetric ss mode when strain softening takes the form of a reduction in the internal angle of friction with increasing strain lithospheric extension may be asymmetric at a lithospheric scale aa mode or exhibit crustal asymmetry concomitant with mantle symmetry as mode the different styles depend on the relative control of the system by the frictional plastic and ductile layers which promote asymmetry and symmetry respectively high extension velocities and weak ductile crust mantle coupling tend to suppress the fundamental asymmetry induced by frictional strain softening this is because they respectively increase the effective strength of the lower lithosphere and decrease the control by frictional plasticity

climate changes control offshore crustal structure at south china sea continental margin a b s t r a c t rifted continental lithosphere subsides as a consequence of combined crustal thinning and mantle lithosphere cooling yet basins on some continental margins experience anomalous subsidence events that postdate active extension deep basins on the northern margin of the south china sea notably the baiyun sag show basement subsidence accelerating after 21 ma postdating extension by several million years we combine geophysical observations and numerical forward modeling to show that loading of the offshore basins by increased sediment flux caused by faster onshore erosion following early miocene monsoon intensification is a viable trigger for ductile flow after the cessation of active extension this illustrates that offshore basin dynamics at continental margins with weak crust can be controlled by onshore surface processes in a newly recognized form of climate tectonic coupling

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

low angle detachment origin for the red sea rift system abstract the tectonic and magmatic history of the jizan coastal plain tihama asir southwest arabia suggests a two stage evolution a first stage of extension began during the oligocene and ended with uplift of the arabian graben shoulder which began about 14 ma ago it was followed by a period of approximately 10 ma characterized by magmatic and tectonic quiescence a second stage of extension began roughly contemporaneously with the onset of seafloor spreading in the southern red sea some 4 5 ma ago and is still active today the geometry of faulting in the jizan area supports a wernicke model of simple shear for the development of the southern red sea regional asymmetries of the red sea area such as the distribution of volcanism the marginal topography and asymmetries in the geophysical signatures are consistent with such a model available seismic profiles allow a rough estimate for values of the arabian red sea margin and were used to simulate subsidence history and heat flow of the red sea for classical two layer stretching models neither finite uniform nor finite non uniform stretching models can account for observed subsidence and heat flow data thus two model scenarios of whole lithosphere normal simple shear are presented for the geological history of the southwestern arabian margin of the red sea these models are limited because of the serravallian rearrangement in the kinematics of the red sea

non pegmatitic deposits of beryllium mineralogy geology phase equilibria and origin non pegmatitic occurrences of be minerals constitute a diverse set of geologic environments of considerable mineralogical and petrological interest they currently provide the majority of the world s be ore and emeralds and they contain the greatest resource of these commodities of the approximately 100 be minerals known see chapter 1 by grew appendix a most occur in hydrothermal deposits or non pegmatitic igneous rocks where their distribution varies systematically with the setting and origin table 1 fig 1 view this table table 1 beryllium minerals in non pegmatitic deposits formulas use and occurrence 1 figure 1 chemography of the principal solid phases in the beo al2o3 sio2 h2o f2o 1 bash system with the projected positions of helvite group and alkali be silicates also shown are generalized fields for some of the major types natural of occurrences cf table 1 fig 4 see text for discussion beryllium minerals are best known from geologic systems associated with felsic magmatism they also occur in a variety of settings that lack evident igneous associations environments range from the surface to the deep crust and host rocks range from feldspathic to carbonate to ultramafic in composition genetically related igneous rocks are felsic and share low calcium and high f contents but are diverse in composition setting and origin compositions range from strongly peraluminous to peralkaline and can be silica undersaturated beryllium minerals also occur in metamorphic and basinal environments and are redistributed by surface processes table 2 summarizes the types and significance of major groups of occurrences by their lithologic associations figure 2 shows the global distribution of some important examples and regional belts for most types at least one example has been described in some detail and can be used to help evaluate general patterns however

revisiting the paleomagnetism of the 1 476 ga st francois mountains igneous province missouri 1 a paleomagnetic investigation of the st francois mountains igneous province in southeastern missouri provides a key 1476 16 ma paleomagnetic pole for laurentia the pole 13 2 s 219 0 e dp 4 7 dm 8 0 is considered primary on the basis of positive conglomerate inverse baked contact and fold tests an analysis of 1470 1430 ma poles from laurentia highlights key differences between poles obtained from the belt supergroup electra lake gabbro and cratonic north america paleolatitudes based on the lower belt supergroup poles are enigmatic as two previous studies yielded a difference of 10 our new pole combined with an analysis of previous results favors the higher latitude interpretation for the lower belt supergroup paleolatitudes from the younger belt rocks indicate lower latitudes than coeval rocks from elsewhere in laurentia for which there has been no adequate explanation a comparison of the st francois mountain pole with similar age poles from baltica siberia and australia allow first order tests of proposed continental configurations paleomagnetic data from australia are compatible with proposed rodinia reconstructions whereas paleomagnetic data from baltica are not we are unable to rigorously test the alternative suggestion that places siberia against the western margin of laurentia due in part to large errors associated with siberian paleomagnetic data

estimating palaeo water depth from the physical rock record abstract palaeo water depth and its change with time is a key factor in environmental analysis and in sedimentology in general the literature published on this topic and on related subjects however is substantial albeit scattered within and between different communities geologists oceanographers engineers physicists etc here i focus on proxies for palaeo bathymetry as obtained from the physical rock record floral faunal and chemical evidence for palaeo water depth being equally important must be dealt with elsewhere an assessment of the main depth related parameters in actualistic settings is presented and their applicability to the fossil sedimentary record discussed the outcome reveals the full complexity of this topic there is for example no such thing as an average wave base depth applicable to a wide range of fossil case settings this because wave and current climates in general and particularly so in neritic and coastal settings are intriguingly complex furthermore observations from modern oceans are not a priori applicable to fossil ones acknowledging these problems and limitations critically evaluated quantitative proposals for depth indicative facies and features in the sedimentary record are presented and error bars discussed

quartz cementation history of sandstones revealed by high resolution sims oxygen isotope analysis abstract evaluating the timing and origin of quartz cement is central to understanding how porosity is lost in sandstones during burial kinetic models of quartz cementation have been calibrated using large scale datasets but have never been tested at the microscopic level at which cement forms here we use high precision in situ oxygen isotope analyses on sandstone from the jurassic ness formation from the north sea to reveal the growth history of single quartz overgrowths to a resolution of 2 m measured 18o cement range from 28 to 20 v smow in early to late cement and are consistent with quartz cementation models that propose the bulk of quartz precipitates as a continuous process beginning at 60 70 c quantitative x ray diffraction analyses and clay mineralogical analysis of interbedded shales are inconsistent with a silica source from shale implying that silica for the cement is sourced internally to the sand these isotope data are broadly consistent with predictive conceptual quartz cementation models and provide a critical link from micron scale measurements to basin scale predictions and observations

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

models of strain variation in nappes and thrust sheets a review abstract strain within nappes and thrust sheets can be modelled using simple boundary conditions an original rectangular prism of material resting on a thrust plane is considered to be subject to various pure and simple shear components operating parallel to the thrust deformation gradient tensors are determined for the models and solutions found for the principal strains a two dimensional model considers heterogeneous simple shear combined with a stretch in the direction of shear simple shear zones arise as a special case where 1 recumbent fold nappes characteristic of gravitational collapse can be modelled with 1 and thrust sheets with layer parallel shortening and folding with differential transport of nappes gives rise to additional wrench type shears solutions for the principal strains in this type of model are found and used to interpret side wall ramps and steep zones parallel to nappe movements by including a stretch in the transport direction these differential transport models produce prolate strains 1 and oblate strains finally a brief consideration of bending strains produced in a sheet passing over a ramp indicates reversals in the shear direction during the strain history these predict possible crenulation of early thrust fabrics at ramps

holocene evolution of the western orinoco delta venezuela the pristine nature of the orinoco delta of eastern venezuela provides unique opportunities to study the geologic processes and environments of a major tropical delta remote sensing images shallow cores and radiocarbon dating of organic remains form the basis for describing deltaic environments and interpreting the holocene history of the delta the orinoco delta can be subdivided into two major sectors the southeast sector is dominated by the rio grande the principal distributary and complex networks of anastomosing fluvial and tidal channels the abundance of siliciclastic deposits suggests that fluvial processes such as overbank flooding strongly influence this part of the delta in contrast the northwest sector is represented by few major distributaries and overbank sedimentation is less widespread relative to the southeast sector peat is abundant and occurs in herbaceous and forested swamps that are individually up to 200 km 2 in area northwest directed littoral currents transport large volumes of suspended sediment and produce prominent mudcapes along the northwest coast mapping of surface sediments vegetation and major landforms identified four principal geomorphic systems within the western delta plain 1 distributary channels 2 interdistributary flood basins 3 fluvial marine transitional environments and 4 marine influenced coastal environments coring and radiocarbon dating of deltaic deposits show that the northern delta shoreline has prograded 20 30 km during the late holocene sea level highstand progradation has been accomplished by a combination of distributary avulsion and mudcape progradation this style of deltaic progradation differs markedly from other deltas such as the mississippi where distributary avulsion leads to coastal land loss rather than shoreline progradation the key difference is that the orinoco delta coastal zone receives prodigious amounts of sediment from northwest moving littoral currents that transport sediment from as far away as the amazon system 1600 km late holocene progradation of the delta has decreased delta plain gradients increased water levels and minimized overbank flooding and siliciclastic sedimentation in the northwest sector these conditions coupled with large amounts of direct precipitation have led to widespread peat accumulation in interdistributary basins because peat forming environments cover up to 5000 km 2 of the delta plain the orinoco may be an excellent analogue for interpreting ancient deltaic peat deposits

mechanism of thin skinned detachment in the amazon fan assessing the importance of fluid overpressure and hydrocarbon generation abstract since about 10 ma a wedge of clastic sediment up to 10 km in thickness has accumulated near the mouth of the amazon river forming the amazon fan thin skinned extensional and contractional structures root into a common basal detachment hundreds of kilometers long extensional faults have formed landward of the prograding delta slope and contractional folds and faults have formed seaward of it by plan view restoration we show that extension and contraction balance we therefore infer that slope instability has provided the driving force for thin skinned deformation on seismic data the detachment appears as a sharp surface within late cretaceous strata however there is no evidence at that level for soft layers such as evaporites or muds abnormal fluid pressure provides a more reasonable mechanism for the detachment at depths of several thousand meters deep wells have encountered fluid overpressures amongst the possible causes of abnormal fluid pressures are compaction and hydrocarbon generation although compaction may have contributed to fluid overpressures encountered in wells we argue that hydrocarbon generation provides the best explanation for detachment in cretaceous rocks which were old and lithified when it occurred in particular the broadly triangular area of detachment coincides with an inferred pod of cenomanian turonian source rock which is interpreted to have been in the gas window during detached deformation we have developed new techniques of sandbox modelling to investigate the geometries of fault patterns produced by fluid flow through porous media for the amazon fan we have been able to reproduce the structural pattern on the assumption that fluid overpressures existed over a broadly triangular area beneath the detachment

an accumulation map for the greenland dry snow facies derived from spaceborne radar 1 an important component of ice sheet mass balance calculations is the snow accumulation rate traditionally determined across the ice sheet from interpolated ice core data we derive an accumulation rate map within the greenland dry snow facies using c band radar backscatter and the mean annual temperature values for the backscatter were obtained from the ers 1 synthetic aperture radar mosaic acquired during september november 1992 an empirical equation based on elevation and latitude is used to determine the mean annual temperature we examine the influence of accumulation rate and mean annual temperature on c band 5 3 ghz radar backscatter using a forward model which incorporates snow metamorphosis and radar backscatter components our model is run over a range of accumulation and temperature conditions consistent with the greenland dry snow facies on the basis of our modeled results a look up table is generated which uniquely maps the measured radar backscatter and mean annual temperature to accumulation rate our results compare favorably 15 4 average difference with 124 in situ accumulation rate measurements falling within our study area we attribute some of the differences to 1 unreliable in situ data points which were based on a single year s accumulation and 2 a systematic underestimate of accumulation rate by our forward model in the low accumulation regions associated with the northern portion of our map with questionable data points and those above 77 n removed the average differences were less than 10 3 as determined from 73 ice cores

impacts of an accumulation hiatus on the physical properties of firn at a low accumulation polar site 1 recent field investigations of a megadune region of east antarctica provide evidence that differences in grain size thermal conductivity and permeability across a megadune profile are due to spatial accumulation variability in the absence of significant microclimate variations the megadunes are low amplitude 2 8 m long wavelength 2 5 km bands with perceptible but low accumulation less than 40 mm water equivalent weq yr 1 and accumulation hiatus within several kilometers proximity as determined by remote sensing surface feature classification and ground penetrating radar profiling our hypothesis that accumulation rate impacts the extent of temperature gradient driven metamorphic growth in low accumulation rate sites is supported by measurements of various firn physical properties relatively small differences in accumulation rate less than 40 mm weq yr 1 result in large differences in physical properties including grain size thermal conductivity and permeability which are apparent in satellite based microwave data from both passive and active sensors the differences in physical snow structure between low accumulation areas and accumulation hiatus areas in the near surface are sufficiently distinct that evidence of past accumulation hiatus should be observable in the physical and chemical properties of an ice core record

tectonosedimentary evolution of the deep iberia newfoundland margins evidence for a complex breakup history most of the conceptual ideas concerning sedimentary architecture and tectonic evolution of deep rifted margins are based on either intracontinental rift basins or proximal margins both of which underwent only small amounts of crustal thinning in this paper we investigate the tectonosedimentary and morphotectonic evolution related to continental breakup of the highly extended deep iberia newfoundland margins our results show that continental breakup is a complex process distributed in time and space on the basis of mapping of dated seismic units and borehole data we are able to identify two major phases of extension during a first phase dated as tithonian to barremian 145 128 ma deformation is related to exhumation of mantle rocks basins become younger oceanward and fault geometry changes from upward to downward concave resulting in complex sedimentary structures and basin geometries a second phase dated as latest aptian 112 ma overprints previously exhumed mantle and accreted juvenile oceanic crust over more than 200 km leading to the formation of basement highs the observed complex breakup history challenges classical concepts of rifting and leads to new interpretations for the tectonosedimentary evolution of deep rifted margins

the development of late stage continental breakup seismic reflection and borehole evidence from the danakil depression ethiopia during continental breakup the locus of strain shifts from a broad region of border faulting and ductile plate stretching to a narrow zone of magma intrusion in a young ocean basin recent studies of volcanic rifts and margins worldwide suggest this shift occurs subaerially before the onset of seafloor spreading we test this hypothesis using recently acquired seismic reflection and borehole data from the danakil depression ethiopia a unique region of transition between continental rifting and seafloor spreading our data located near dallol 30 km northwest of the erta ale volcanic segment reveal a remarkably thick 1 km sequence of young 100 ka evaporites in a basin bound by a major 400 m throw east dipping normal fault to generate such a large amount of subsidence in such a relatively short time we propose that upper crustal extension in danakil is currently dominated by faulting not magmatic intrusion given the region s markedly thinned crust 15 km thick relative to elsewhere in afar where magma assisted rifting dominates and maintains crustal thickness at 25 km mechanical extension in danakil is likely coupled with ductile extension of the lower crust and mantle lithosphere despite proximity to the voluminous lavas of the active erta ale volcanic segment evidence for igneous material in the upper 2 km of the 6 to 10 km wide basin is limited late stage stretching was likely aided by thermal strain induced lithospheric weakening following protracted magma assisted rifting basin formation immediately prior to the onset of seafloor spreading may also explain the accumulation of thick marine seepage fed evaporite sequences akin to those observed for example along the south atlantic rifted margins

na and k distribution in agpaitic pegmatites abstract composition and zoning of amphibole in agpaitic pegmatites of the 1 16 ga ilimaussaq complex south greenland record the chemical evolution of the final stages of an already extremely fractionated melt our results show that the general differentiation trends found in the earlier rocks of the complex are continued in the pegmatites albeit with some significant modifications the dominating exchange mechanism of na si ca al in the amphiboles of the magmatic stage changes to k si ca al and k na in some pegmatitic samples na k ratios in amphiboles which generally increase in the course of the ilimaussaq fractionation partly display a reversal during the crystallization of the most differentiated amphiboles the alkali trends are probably related to the buffering of na and k activity by the co crystallization of albite and microcline this buffering favors na in cooling fluids this mechanism is lost when analcime replaces feldspar as a stable phase in the late stages of crystallization e g due to locally elevated h2o activity analcime does not incorporate significant amounts of k and accordingly amphibole incorporates more k in analcime bearing assemblages the na k variation in amphiboles in the ilimaussaq pegmatites allow a detailed view into the late stage evolutionary trends of a textbook agpaitic complex the transition from silicate melt to aqueous fluid is recorded by the change of the dominant alkali ion in the pegmatitic amphiboles from na to k only in the very latest stage virtually k free mineral assemblages in analcime aegirine veins support the existence of a na dominated aqueous fluid

the nile deep sea fan preliminary results from a swath bathymetry survey we present and discuss a set of data mainly swath bathymetry backscatter images and a few seismic data that have been recently recorded over large areas of the nile deep sea fan between water depths of 1200 and 3000 m these data demonstrate the presence of at least three distinct morphostructural provinces where interacting sedimentary tectonic and salt tectonic processes control the present day morphology and the recent evolution of the major mediterranean deep sea fan the role of the ongoing collision presently occurring between the continental eratosthenes seamount and the cyprus arc may have been and still is a determining parameter for the development of the nile deep sea fan

depositional wedge cycles on evolving plate margin western and northwestern australia the barrow province lies at the transition between the western and northwestern rift domains of the australian continental margin as these domains evolved during the mesozoic five depositional wedge sequences developed in the eastern half of the province each sequence possesses distinct stratigraphic and sedimentological characteristics which differs from the others as a function of proximity direction and style of rift related diastrophism along the evolving margin the first depositional sequence developed during the triassic as part of a regional tethyan facing passive margin clastic wedge this event was followed during the early and middle jurassic by a second more provincial sequence in response to early syn rift fault related subsidence by the callovian a deep anastomosing rift complex had formed and deposition of the third late jurassic cycle was initially very localized confined by the new graben topography however stratigraphic continuity returned during post breakup subsidence of the northwestern margin as subregional depositional systems began to bury the fragmented borderland this cycle was terminated at the close of the jurassic in response to renewed rifting between western australia and greater india sediment stripped from t e uplifted rift shoulders fed the northerly prograding barrow delta system to form a fourth depositional sequence during the late jurassic early neocomian the final cycle commenced as greater india uncoupled from australia during the middle neocomian post breakup subsidence is reflected by a major regional transgression and the development of a second margin wide depositional wedge during the later cretaceous the close temporal and spatial relationship that exists between these cycles and the evolution of the western and northwestern australian continental margin emphasizes the importance of tectonic control upon sedimentation

crust mantle interaction in the evolution of the ilimaussaq complex south greenland nd isotopic studies sm nd isotopic compositions were determined for the peralkaline ilimaussaq complex of the gardar province of southern greenland the majority of the samples in the agpaitic and augite syenitic units have near chondritic initial nd 0 whereas a few samples trend towards nd values as low as 6 at the time of intrusion 1143 ma this latter value from a sample taken from the margin of the complex lying on the evolutionary trend for ketilidian country rock granitoids suggests that large scale contamination took place only at the margins of the complex the similarity of the nd isotopic compositions of the augite syenite and agpaitic units suggests that their parental magmas were derived from the same reservoir a comparison of the nd with existing sr and hf isotopic data for the complex suggests an origin by combined assimilation fractionation processes assimilation fractional crystallization modeling of the isotopic compositions indicates that the ilimaussaq magmas could have formed through fractional crystallization of a basaltic melt while assimilating granitic crust the model requires initially higher assimilation rates from basalt to augite syenite composition with subsequent decreasing assimilation rates from augite syenite to agpaitic compositions alkali granites which formed after the intrusion of the augite syenites have isotopic compositions intermediate between those of the augite syenites and the surrounding ketilidian basement this implies even greater amounts of assimilation and is interpreted as evidence for an origin through fractionation of a basaltic or augite syenite magma with concurrent assimilation of ketilidian crust

timing of proterozoic magmatism in the gardar province southern greenland the gardar province in southern greenland is believed to have experienced three episodes of rifting all associated with the extrusion of large volumes of alkalic magma a nepheline syenite of the motzfeldt center yields a u pb apatite age of 1 35 0 01 ga an augite syenite from the ilimaussaq complex yields a sm nd pyroxene alkali feldspar magnetite age of 1 13 0 05 ga on the basis of field relationships the eriksfjord formation lavas have been interpreted as the oldest manifestation of gardar magmatism sm nd plagioclase pyroxene wholerock ages of 1 17 0 03 ga and 1 20 0 03 ga for the middle member of the eriksfjord formation however indicate that these lavas may be part of the youngest magmatic activity the available chronological data indicate that the gardar province was active for at least 200 m y from 1 35 to 1 12 ga it is difficult to reconcile this protracted history with plume models for the genesis of such alkaline flood basalt provinces

fe isotope systematics of coexisting amphibole and pyroxene in the alkaline igneous rock suite of the ilimaussaq complex south greenland abstract the ilimaussaq intrusion south greenland provides an exceptional test case for investigating the changes of stable fe isotope fractionation of solidus phases with changes in the fe 3 fe ratio of an evolving melt the intrusion comprises a sequence of four melt batches that were fed from the same parental alkali basaltic magma differentiation produced cumulate rocks that range from augite syenite phase i over peralkaline granite phase ii to agpaitic syenites phases iiia and iiib fe 3 fe ratios in amphiboles increase substantially from phase i to phase ii and iii rocks and mark a major change in the parental magma composition from augite syenites to peralkaline granites and agpaitic syenites before this transition olivine clinopyroxene and amphibole in augite syenite the most primitive rock type in the ilimaussaq complex have a uniform fe isotope composition that is identical to that of the bulk of igneous crustal rocks and approximated by the average isotopic composition of basalts 56 54 fe irmm 014 0 072 0 046 after the transition amphiboles in the peralkaline granites and agpaitic syenites yield significantly heavier fe isotope compositions with 56 54 fe irmm 014 values ranging from 0 123 to 0 237 contamination of the ilimaussaq magma by ongoing crustal assimilation as cause for this increase can be excluded on the grounds of nd isotope data large scale metasomatic overprint with an external fluid can also be dismissed based on amphibole o and li isotope systematics rather the increase towards heavy fe isotope compositions most likely reflects the change in chemical compositions of amphiboles calcic in augite syenite to sodic in the agpaitic syenites and their fe 3 fe ratios that mirror changes in the chemical composition of the melt and its oxygen fugacity a sensitive adjustment of equilibrium fe isotope fractionation factors to amphibole ferric ferrous ratios is also supported by beta factors calculated from mossbauer spetroscopy data comparison of the measured isotope fractionation between clinopyroxene and amphibole with that predicted from mossbauer data reveal fe isotope systematics close to equilibrium in augite syenites but fe isotopic disequilibrium between these two phases in phase iiia agpaitic syenites these results are in agreement with o and li isotope systematics while amphiboles in all ilimaussaq lithologies crystallized at temperatures between 650 and 850 c textural evidence reveals later clinopyroxene crystallization at temperatures as low as 300 400 c therefore isotopic equilibrium at crystallization conditions between these two phases can not be expected but importantly subsolidus reequilibration can also be dismissed

how continents break up insights from papua new guinea the woodlark basin in the western pacific forms a continuous system of active continental rifting evolving to well developed seafloor spreading thin sediment cover in the basin and a dominantly nonvolcanic rift phase permit basement fabric and structures to be imaged by swath mapping and seismic reflection data in the continental and oceanic parts of the basin magnetic isochrons indicate a single euler pole of opening for most of the basin history and allow us to infer the opening kinematics along the rifted margins in agreement with rigid plate tectonic models continental rifting initiated geologically synchronously at 6 ma along the length of the protomargins within a deforming plate boundary zone strain localization and seafloor spreading however developed in a time transgressive fashion from east to west within this zone of deformation spreading centers formed within the rheologically weaker protocontinental margins surrounded by stronger oceanic lithosphere in the solomon and coral seas the transition to spreading occurred after a rather uniform degree of continental extension 200 40 km both early and late stage rifting involved high and low angle normal faults we identify distinct styles in the transition from rifting to spreading which we refer to as nucleation propagation and stalling these breakup styles impart varyingly concordant to discordant relationships between the adjacent oceanic and continental rift structures continental transform margins which are or were juxtaposed against the ends of spreading centers show no evidence for thermal uplift or igneous underplating the initial spreading segments achieved much of their length at nucleation within rift basins separated along strike by accommodation zones with subsequent lengthening by spreading propagation into rifting continental crust this early propagation and the subsequent development of transform faults between initially nontransform spreading segment offsets produced rift and spreading segmentation boundaries that are not simply correlated the spreading centers nucleated approximately orthogonal in strike to the opening direction but as the protomargins were oblique to this direction nucleation jumps occurred in order to maintain the new spreading centers within the protomargins thus stepwise spreading nucleation in order to remain within a rheologically weak zone rather than rupturing of the lithosphere by stress concentration at the tip of a propagating ridge axis is the dominant form of the rifting to spreading transition in the woodlark basin