the late cretaceous to recent tectonic history of the pacific ocean basin abstract a vast ocean basin has spanned the region between the americas asia and australasia for well over 100 myr represented today by the pacific ocean its evolution includes a number of plate fragmentation and plate capture events such as the formation of the vancouver nazca and cocos plates from the break up of the farallon plate and the incorporation of the bellingshausen kula and aluk phoenix plates which have been studied individually but never been synthesised into one coherent model of ocean basin evolution previous regional tectonic models of the pacific typically restrict their scope to either the north or south pacific and global kinematic models fail to incorporate some of the complexities in the pacific plate evolution e g the independent motion of the bellingshausen and aluk plates thereby limiting their usefulness for understanding tectonic events and processes occurring in the pacific ocean perimeter we derive relative plate motions with 95 uncertainties for the pacific farallon vancouver kula pacific bellingshausen pacific and early pacific west antarctic spreading systems based on recent data including marine gravity anomalies well constrained fracture zone traces and a large compilation of magnetic anomaly identifications we find our well constrained relative plate motions result in a good match to the fracture zone traces and magnetic anomaly identifications in both the north and south pacific in conjunction with recently published and well constrained relative plate motions for other pacific spreading systems e g aluk west antarctic pacific cocos recent pacific west antarctic spreading we explore variations in the age of the oceanic crust seafloor spreading rates and crustal accretion and find considerable refinements have been made in the central and southern pacific asymmetries in crustal accretion within the overall pacific basin where both flanks of the spreading system are preserved have typically deviated less than 5 from symmetry and large variations in crustal accretion along the southern east pacific rise i e pacific nazca farallon spreading appear to be unique to this spreading corridor through a relative plate motion circuit we explore the implied convergence history along the north and south america s where we find that the inclusion of small tectonic plate fragments such as the aluk plate are critical for reconciling the history of convergence with onshore geological evidence

normal fault development during two phases of non coaxial extension an experimental study we use scaled experimental analog models to study the effect of a pre existing fault fabric on fault development during extension in our models a homogeneous layer of wet clay undergoes two noncoaxial phases of extension whose directions differ by up to 45 the normal faults that develop during the first phase create a pronounced fault fabric that influences normal fault development during the second phase in all models the pre existing faults are reactivated during the second phase of extension their sense of slip depends on the angle between the first and second phase extension directions specifically the component of dip slip relative to strike slip decreases as the angle between the first and second phase extension directions increases new normal faults also form during the second phase of extension in all models the number of new fault segments increases as the angle between the first and second phase extension directions increases the orientations of the new normal fault segments are both orthogonal and oblique to the second phase extension direction indicating that both the secondphase extension direction and the pre existing fault fabric control the orientation of new fault segments some of the new normal faults cut and offset the pre existing faults whereas others terminate against them producing complex fault patterns and interactions the modeling results explain fault interactions observed in the jeanne d arc rift basin of offshore newfoundland canada and the reactivation of abyssal hill normal faults at outer highs near subduction zones

contourite drifts on early passive margins as an indicator of established lithospheric breakup the albian cenomanian breakup sequence bs offshore northwest iberia is mapped described and characterised for the first time in terms of its seismic and depositional facies the interpreted dataset comprises a large grid of regional 2d seismic reflection profiles complemented by industry and odp dsdp borehole data within the bs are observed distinct seismic facies that reflect the presence of a black shales and fine grained turbidites b mass transport deposits mtds and coarse grained turbidites and c contourite drifts borehole data show that these depositional systems developed as mixed carbonate siliciclastic sediments proximally and as organic carbon rich mudstones black shales distally on the northwest iberia margin mtds and turbidites tend to occur on the continental slope frequently in association with large scale olistostromes distally these change into interbedded fine grained turbidites and black shales showing widespread evidence of deep water current activity towards the top of the bs current activity is expressed by intra bs erosional surfaces and sediment drifts the results in this paper are important as they demonstrate that contourite drifts are ubiquitous features in the study area after aptian albian lithospheric breakup therefore we interpret the recognition of contourite drifts in northwest iberia as having significant palaeogeographic implications contourite drifts materialise the onset of important deep water circulation marking the establishment of oceanic gateways between two fully separated continental margins as a corollary we postulate the generation of deep water geostrophic currents to have had significant impact on north atlantic climate and ocean circulation during the albian cenomanian with the record of such impacts being preserved in the contourite drifts analysed in this work

seismic stratigraphic record of transition from mesozoic subduction to continental breakup in the zealandia sector of eastern gondwana abstract the southwest pacific between australia new zealand and new caledonia is a block of continental crust zealandia that moved away from australia and antarctica after a long period of subduction beneath eastern gondwana we use 100 000 line km of seismic reflection profiles to identify intra continental basins related to the gondwana active margin overlain with erosional unconformity by retrogradational strata we interpret this regional scale first order unconformity the eastern gondwana composite surface and seismic stratigraphic megasequence pattern to represent the transition from subduction to continental breakup and separation of eastern gondwana rocks that make up the lower seismic stratigraphic megasequence zealandia 3 permian to early cretaceous have been drilled near new zealand and can be correlated with murihiku supergroup rocks farther north on the lord howe rise we correlate this megasequence with triassic to jurassic rocks of the clarence moreton basin of eastern australia we reinterpret the fairway aotea basin to be underlain by a wedge of zealandia 3 deposits thrust in a retro arc like foreland basin that was active immediately prior to breakup the lower rift and retrogradational megasequence zealandia 2 late cretaceous to eocene we correlate with the pakawau and kapuni groups of taranaki basin new zealand and the formation a charbon and phtanites of new caledonia the boundary with the shallowest megasequence zealandia 1 late eocene to present is the eocene oligocene unconformity which is primarily overlain by pelagic carbonate rocks this regional unconformity likely represents the onset of the modern tonga kermadec subduction system

perspectives on cretaceous gondwana break up from detrital zircon provenance of southern zealandia sandstones detrital zircon u pb ages in 37 sandstones from late early late cretaceous marine and non marine successions across southern zealandia indicate a provenance from local basement within present day zealandia samples from taranaki basin were derived from median and karamea batholith granitoids with transport directions from west to east samples from west coast western southland and great south basins contain components derived more locally and more variably from median batholith and rahu suite granitoids and or the palaeozoic buller terrane west coast basin samples have more plutonic contributions and great south basin localities have more albian aged c 110 100 ma zircons samples from canterbury basin were sourced from torlesse composite terrane basement the provenance variations are present in both marine and non marine sandstones and suggest localized watersheds this fits an interpretation of late cretaceous deposition in rift controlled basins across southern zealandia during pre gondwana break up regional extension more speculatively some additional source areas may have been created at the rifted margins of zealandia during this break up

redox state of the archean atmosphere evidence from detrital heavy minerals in ca 3250 2750 ma sandstones from the pilbara craton australia the presence of detrital uraninite and pyrite in fluvial placers of the witwatersrand basin south africa has been used to infer low levels of atmospheric oxygen during the archean 2500 ma however recent studies advocate a hydrothermal origin for these minerals thereby limiting their value as constraints on the composition of the early atmosphere in contrast ca 3250 2750 ma fluvial siliciclastic sediments from the pilbara craton in australia have never undergone significant hydrothermal alteration and their heavy minerals are of unequivocal detrital origin these heavy minerals include the redox sensitive phases pyrite uraninite and gersdorffite along with more inert zircon rutile chromite and monazite locally siderite is a major constituent to 90 of the heavy mineral population with grains displaying evidence for several episodes of erosion rounding and subsequent authigenic overgrowth detrital siderite is very rare in post archean sandstones largely due to its instability in oxidizing environments however its frequent survival of prolonged transport in well mixed and therefore well aerated archean river waters that contained little organic matter strongly implies that the contemporary atmosphere was indeed much less oxidizing than at present moreover concentrations of reduced sulfur species must have been very low in surface fluids for siderite to survive repeated transportation events without pyritization

siderite cannot be used as co2 sensor for archaean atmospheres abstract it was proposed to utilize siderite feco3 in mid to late archaean superior type banded as a proxy to constrain the co2 partial pressure of archaean atmospheres implicit in this proposition is that siderite was a primary carbonate mineral that crystallized directly from fe2 enriched archaean seawater in equilibrium with atmospheric co2 to our knowledge that proposition has not been demonstrated to be valid we test with water gas exchange experiments under controlled co2 partial pressures if siderite can be stabilized as a primary mineral in fe2 bearing seawater reduced seawater proxies enriched in fe2 and mn2 are equilibrated with reduced n2 ch4 co2 h2 gas phases with variable co2 the solid phases stabilized in fe2 enriched water compositions are amorphous ferrous iron hydroxy carbonates crystalline siderite feco3 is not found to be a stable phase the phases precipitating from mn2 enriched water include crystalline rhodochrosite mnco3 and possibly amorphous mn enriched phases based on these results we advise against using siderite in banded iron formations as a co2 sensor for the archaean atmosphere

nonlinear internal waves in the south china sea observation of the conversion of depression internal waves to elevation internal waves 1 the conversion of depression nonlinear internal solitons to elevation internal waves has been observed twice at the south china sea continental shelf break shipboard x band radar tow yo ctd adcp and high frequency acoustic flow visualization of the process are presented the data focuses on up slope propagation of depression internal solitons from a water depth of 264 m to a water depth of 110 m dissipation by large amplitude shear instabilities was observed the kinetic energy density was found to be a decreasing monotonic function of range with an energy dissipation rate coefficient of 0 063 km 1 the rate of energy dissipation over the 16 5 km soliton propagation path was 0 17 w m m simple numerical models of the waveform evolution are qualitatively compared to the observations the observations extracted scale parameters and dissipation rates should be useful for testing a variety of evolution equation representations of internal soliton propagation

three dimensional dynamical model of continental rift propagation and margin plateau formation we investigate the dynamics of rift propagation through continental lithosphere and the resulting differential extension along passive margins using a full three dimensional finite element implementation of the yield stress envelope model in its initial configuration the three dimensional model represents a 1000 km by 1000 km area of continental lithosphere an initial weak trend representing a suture zone or other preexisting weakness within the continental lithosphere extends 500 km from one model edge to the center of the model and terminates the model region beyond the weak trend is uniform model conditions are varied by changing the thickness and composition of the crust the geotherm and extension rate the strength of the initial weakness and the dimensions of the preweakened and uniform regions for each model breakup nucleates at a point on the weak trend in response to an applied constant extension rate the break propagates rapidly along the weak trend and then slows as it crosses the uniform region this produces a locked zone in which breakup is delayed resulting in an increase in the width of the model margin analogous to offsets associated with margin plateaus the total extension required to tear across the 1000 km length of the model is 210 km for 40 km thick crust 400 km for 45 km thick crust and 710 km for 50 km thick crust smaller increases in total extension occur for elevated geotherms and reduced extension rates this indicates that prior extension estimates based on subsidence which exceed 600 km for margin plateaus can be explained by rift propagation in three dimensions differential extension along the model margin is linearly related to the length of the locked zone the slope of the line which we call is a function of the rheological properties of the locked zone and reflects the extent to which continental lithosphere within the locked zone resists rift propagation model calculations for crustal thickness from 35 km to 50 km and compositions from quartz diorite to granite predict a range in from 0 2 to 1 3 we compare values computed from our model to those estimated from the subsidence of bonavista platform and rockall plateau conjugate margins in the north atlantic basin and georges bank and blake plateau and their conjugate margins in the central atlantic basin these results indicate that the differential extension associated with these locked zones which ranges from 170 km to 550 km can be predicted in terms of their initial crustal thickness and their length along the margins

new insights into the tectonic inversion of north canterbury and the regional structural context of the 2010 2011 canterbury earthquake sequence new zealand the 2010 2011 canterbury earthquake sequence highlighted the existence of previously unknown active faults beneath the north canterbury plains and pegasus bay south island new zealand we provide new insights into the geometry and kinematics of ongoing deformation by analyzing marine seismic data to produce new maps of regional faults and cross sectional reconstructions of deformation history active faulting and folding extends up to 30 km offshore and involves reactivation of sets of late cretaceous paleogene normal faults under nw se tectonic compression the active faults consist predominantly of ne sw striking se dipping reverse faults and less commonly e w to nw se faults suitably oriented for strike slip reactivation additionally newly developing reverse faults obliquely segment and overprint the inherited basement fabric and impose geometric and kinematic complexities revealed by mapping and reverse displacement profiles of markers the quaternary reverse slip rates decrease from 0 1 0 3 mm yr beneath northern pegasus bay to 0 05 mm yr approaching banks peninsula fault growth modeling involving trishear fault propagation folding mechanisms successfully restores an evolutionary sequence of progressive fault inversion revealing a history of reactivated individual faults tectonic inversion and overprinting processes beneath pegasus bay are immature and 1 2 0 4 ma old with no evidence of systematic spatial migration of deformation our marine data analyses give insights into the structural context of the 2010 2011 canterbury earthquake sequence while the combined onshore to offshore data provide an excellent illustration of fault growth associated with immature inversion tectonics in which selective fault reactivation results from compressive stress imposed across a complex network of inherited faults

fault relays bends and branch lines abstract branch lines between normal faults and their sub parallel splays mapped from 3d seismic reflection data show a range of forms from straight lines to closed loops the different geometries are interpreted as representing stages in the failure of relay zones and in the progressive replacement of fault tip lines with fault branch lines the geometries of these normal fault branch lines are similar to those for thrusts previously inferred from limited two dimensional data the orientation of the axis of a relay and its associated bends relative to a fault slip direction is identified as an important control on the structures developed within the relay neutral restraining and releasing bends can each occur on any fault type normal reverse and strike slip but data bias is a major factor in determining which bend geometry is most often observed with each fault type on normal faults the initial relay zone geometry controls the dominant branch line orientation and the same control is likely on branch lines associated with the other modes of faulting a review of the relay geometries and strains occurring with all three modes of faulting highlights the role of the orientation of the mechanical anisotropy of a bedded sequence relative to the orientations of fault surface and slip directions this relative orientation determines how the relay strain is accommodated and hence the degree of hard linkage and development of branch lines

prolonged post rift magmatism on highly extended crust of divergent continental margins baiyun sag south china sea three dimensional 3d seismic borehole and geochemical data reveal a prolonged phase of post rift magmatism on highly extended crust of the baiyun sag south china sea two volcanic complexes are identified and described in the context of continental rifting and diachronous continental breakup of the south china sea biostratigraphic data from exploration wells by7 1 and by2 complemented by k ar datings from core samples confirm that magmatic activity in the baiyun sag occurred in two main stages 1 a first episode at the base of the miocene 23 8 ma and 2 a second episode occurring at the end of the early miocene 17 6 ma the relative location of volcanic complexes in the baiyun sag and their stratigraphic position reveals prolonged magmatism inboard of the ocean continent transition zone during continental breakup we suggest that magmatism in the baiyun sag reflects progressive continental breakup in the south china sea with the last volcanic episode marking the end of a breakup sequence representing the early post rift tectonic events associated with the continental breakup process seismic and borehole data from this breakup sequence records diachronous magma emplacement and complex changes in depositional environments during continental breakup c 2016 the authors published by elsevier b v

low pco2 in the pore water not in the archean air arising from m t rosing d k bird n h sleep c j bjerrum 744 747 2010 10 1038 nature08955 the solar luminosity during the archean 3 8 to 2 5 billion years ago was 20 25 lower than at present and was probably compensated at least in part by a stronger greenhouse effect1 2 3 rosing et al 4 estimate the archean partial pressure of carbon dioxide to have been about 10 3 bar on the basis of the simultaneous occurrence of magnetite fe3o4 and siderite feco3 in banded iron formations bifs a type of chemical sediment here we question a central assumption by rosing et al 4 that the mineralogy of bifs reflects near thermodynamic equilibrium with the atmosphere ocean system just as the presence of authigenic pyrite in modern sediments does not imply that the modern atmosphere is anoxic the mineralogy of bifs cannot be used to argue for a low archean atmosphere carbon dioxide is therefore still a viable greenhouse gas candidate with which to explain the warm archean climate

progressive localisation of strain during the evolution of a normal fault population abstract the evolution of a syn sedimentary normal fault population has been established from the analysis of a high quality 3d seismic reflection survey from the timor sea offshore nw australia growth of the fault population which initiated ca 6 ma ago occurred in three distinct but temporally overlapping stages the initial stage of fault population evolution was characterised by rapid growth of fault length in the first 1 2 ma of extension displacement rates of individual faults were established during this stage and were constant throughout subsequent fault growth the second stage involved amplification of fault displacements on existing fault traces with minimal fault propagation during the third stage shortening of the traces of active faults and high mortality rates for small faults resulted in a net decrease in fault trace length on successively younger horizons this stage was accompanied by a declining extension rate fault throw populations have constant slopes on each syn faulting horizon however an up sequence shallowing of the slope of length and geometric moment populations demonstrates a progressive concentration of strain onto fewer and larger faults through time if extension were to continue beyond the present value of 4 5 this strain localisation might culminate in the formation of a single active through going fault

rotation and offset of the gondwana convergent margin in the new zealand region following cretaceous jamming of hikurangi plateau large igneous province subduction jamming of the hikurangi plateau large igneous province subduction within the chatham rise convergent margin of gondwana at circa 105 ma led to offset and rotation of the convergent margin before subduction ceased in the new zealand region at circa 100 ma the southern limit of the plateau following leading slab break off is highlighted by a lineament of prominent horst blocks in the southern bounty trough subduction jamming of the gondwana margin and accompanying compression of the onshore margin and or extension of the offshore margin has led to two 60 km left lateral sse offsets of the chatham rise convergent margin at the coast and in inland canterbury recognition of the onshore chatham rise using the gravity data also highlights the correlation of the inland chatham rise and central south island seismicity in a similar manner to the rotation of cretaceous spreading ridge and transform fault fabric adjacent to the osbourn trough spreading ridge the convergence direction at the gondwana margin was rotated anticlockwise to n s between 105 and 100 ma most of this rotation has been accommodated by offshore extension and margin offset the divergence between the anticlockwise rotation of offshore crustal structure and the jammed onshore margin led to the development of the great south basin at 105 100 ma further offshore in the bounty trough extensional zones formed between crustal blocks rotated to adjust to a changed cretaceous direction of subduction are evident in gravity and seismic profiles

herringbone calcite petrography and environmental significance abstract herringbone calcite is a previously undescribed carbonate cement and sea floor precipitate that is common in archean carbonates but rare in proterozoic and phanerozoic rocks it is abundant in the 2520 ma campbellrand malmani platform south africa where field relationships such as erosional truncation of layers of herringbone calcite and interbedding of herringbone calcite with grainstones demonstrate that it precipitated from ambient marine water this interpretation is supported by depositional relationships in the 2 6 ga huntsman limestone of the bulawayo greenstone belt zimbabwe the 2 6 ga carawine dolomite australia the 1 90 ga rocknest formation and the 1 8 1 2 ga dismal lakes group canada the ordovician porterfi ld carbonate buildup virginia and various silurian carbonate buildups in the midcontinent united states each of these occurrences is associated with anaerobic depositional environments or organic rich sediments herringbone calcite consists of alternating light and dark crenulated bands each light dark pair is 0 5 1 0 mm thick microscopically each pair of bands consists of a row of elongate crystals with their long axes aligned perpendicular to banding and along the growth direction of the cement the bases of the crystals are optically unoriented but upwards in each crystal the optical c axis rotates until it is perpendicular to crystal elongation the tops of the elongate crystal are thus optically aligned and length slow the light bands of herringbone calcite correspond to the optically oriented parts of the elongate crystals whereas the dark bands correspond to the optically unoriented lower parts of the elongate crystals microspar crystals are also present in some dark ba ds a mg calcite precursor for herringbone calcite now preserved as low mg calcite or dolomite is supported by the presence of micro dolomite inclusions and textural differences between herringbone calcite and textures interpreted as neomorphosed former aragonite or low mg calcite precipitation of herringbone calcite may be consistent with a diffusionally controlled growth model involving branching growth of fibrous crystals and diffusion of a precipitation inhibitor away from the crystallization surface since herringbone calcite is associated with anaerobic depositional environments the inhibitor promoting precipitation of herringbone calcite may be present only in poorly oxygenated sea water thus the stratigraphic distribution of herringbone calcite may be an important indicator of the abundance of oxygen in carbonate depositional environments through time

slip re orientation in oblique rifts oblique extension is expected to result in a combination of dip slip and strike slip displacement along faults with strike orthogonal and oblique to the extension direction respectively this general concept is in disagreement with observations from natural oblique rifts where faults show dip slip kinematics indicating pure extension irrespective of the fault strike with respect to the regional extension direction consequently along oblique structures slip is re oriented and oblique to the applied extension direction besides at fault scale slip is re oriented along strike such that it is dip slip at the fault center and becomes highly oblique slip toward the fault tips here we use analogue experiments to show that this discrepancy can be resolved when a preexisting weak zone wz is present in the crust at the onset of oblique extension the wz is implemented within the lower crust and strikes oblique to the extension direction our experimental results show that an inherited wz within the ductile crust favors the re orientation of slip such that oblique extension results in pure dip slip displacement on faults that strike oblique with respect to the extension direction furthermore we show that slip is re oriented along strike of major faults such that the fault center shows dip slip kinematics whereas its tips display strike slip kinematics these findings call into question the use of paleostress reconstructions to constrain plate kinematics in oblique extensional tectonic settings

iron isotopes constrain biologic and abiologic processes in banded iron formation genesis abstract the voluminous 2 5 ga banded iron formations bifs from the hamersley basin australia and transvaal craton south africa record an extensive period of fe redox cycling the major fe bearing minerals in the hamersley transvaal bifs magnetite and siderite did not form in fe isotope equilibrium but instead reflect distinct formation pathways the near zero average 56fe values for magnetite record a strong inheritance from fe3 oxide hydroxide precursors that formed in the upper water column through complete or near complete oxidation transformation of the fe3 oxide hydroxide precursors to magnetite occurred through several diagenetic processes that produced a range of 56fe values 1 addition of marine hydrothermal fe 2 aq 2 complete reduction by bacterial dissimilatory iron reduction dir and 3 interaction with excess fe 2 aq that had low 56fe values and was produced by dir most siderite has slightly negative 56fe values of 0 5 that indicate equilibrium with late archean seawater although some very negative 56fe values may record dir support for an important role of dir in siderite formation in bifs comes from previously published c isotope data on siderite which may be explained as a mixture of c from bacterial and seawater sources several factors likely contributed to the important role that dir played in bif formation including high rates of ferric oxide hydroxide formation in the upper water column delivery of organic carbon produced by photosynthesis and low clastic input we infer that dir driven fe redox cycling was much more important at this time than in modern marine systems the low pyrite contents of magnetite and siderite facies bifs suggests that bacterial sulfate reduction was minor at least in the environments of bif formation and the absence of sulfide was important in preserving magnetite and siderite in the bifs minerals that are poorly preserved in the modern marine record the paucity of negative 56fe values in older early archean and younger early proterozoic bifs suggests that the extensive 2 5 ga hamersley transvaal bifs may record a period of maximum expansion of dir in earth s history

primary and diagenetic controls of isotopic compositions of iron formation carbonates mineralogic chemical and isotopic compositions have been determined for 97 carbonate microbands in five core segments from the early proterozoic 2 5 ga dales gorge member of the brockman iron formation hamersley basin western australia samples were obtained both from banded iron formation bif macrobands 9 12 at paraburdoo on the southern margin of the basin and from bif macroband 13 at wittenoom 130 km nw at paraburdoo oxygen isotopic compositions of coexisting chert and magnetite microbands were measured and indicated final equilibration temperatures ranging from 60 160 degrees c this range is consistent with observed mineral assemblages and indicates a considerable temperature gradient across the basin cf t approximately 300 degrees c at wittenoom becker and clayton 1976 carbon isotopic compositions of carbonates are near 7 vs pdb at paraburdoo and 10 5 at wittenoom but the greater isotopic depletion at wittenoom appears related to primary or diagenetic processes not metamorphism contents of organic carbon are consistently low isotopic depletion is roughly correlated with iron abundance and together with petrographic observations and chemical balances is consistent with the model of bif deposition introduced by beukes et al 1990 primary siderite delta approximately 5 precipitated from an anoxic water column depleted in 13c additional depletion of 13c is associated with coprecipitation of iron oxides and organic carbon oxygen isotopic abundances of microbanded carbonates are similar to those under and overlying massive marine carbonates ranging from 17 6 to 21 0 vs smow 9 6 to 12 9 vs pdb millimeter scale variations in abundances of 13c and 18o are associated with diagenetic replacement of primary siderite by secondary ankerite and or magnetite it is shown that these isotopic variations cannot result from mineral dependent fractionations metamorphism or the influence of large volumes of water in an open system

late cretaceous coeval multi directional extension in south zealandia implications for eastern gondwana breakup abstract gondwana continental crust in south zealandia was deformed by cretaceous rift faults with multiple orientations our interpretation of 2d and 3d seismic reflection lines tied to wells in the canterbury basin of south zealandia shows three coeval sets of faults with trends of ne sw e w and nw se geometric and displacement analyses of segmented faults from each set suggests that they mainly accommodated normal dip slip producing multi directional stretching growth strata indicate that the three fault sets were active synchronously over 20 myr from 105 to 85 ma the fault sets are present throughout zealandia and roughly parallel to spreading centres produced during gondwana breakup primarily defining the present day margins of zealandia faults trending ne sw and e w are parallel to the mid ocean ridge separating south zealandia and western antarctica 800 1000 km from the canterbury basin while nw se rift faults are sub parallel to the spreading centre between australia and north zealandia sub parallelism between spreading centres and rift faulting is consistent with a model in which the multiple directions of extension produced mild 500 km from these centres ceased or continued at much diminished rates

reservoir characterisation of syn rift and post rift sandstones in frontier basins an example from the cretaceous of canterbury and great south basins new zealand abstract a paucity of well data in frontier basins presents serious challenges for reservoir characterisation studies around the world in this paper we provide the first regional study addressing reservoir distribution composition and quality in the frontier canterbury basin cb and great south basin gsb of new zealand and show how the results can be used to help reduce uncertainties associated with reservoir presence and effectiveness cretaceous strata in canterbury and great south basins represent two stages of basin history the oldest cenomanian coniacian mid cretaceous strata are syn rift deposits that developed in response to the breakup of the eastern margin of gondwana while overlying santonian maastrichtian late cretaceous strata are post rift deposits that signify the start of rift drift transgression these cretaceous deposits are characterised by a change in facies from predominantly alluvial fluvial to marginal shallow marine that is recognisable in outcrop and wells and from seismic response based on our petrographic examination of 99 cretaceous samples from well and outcrop locations we conclude that there are significant differences in sandstone texture and mineralogy that are related to differences in depositional facies basin stratigraphy geographic location and degree of diagenetic alteration sandstones from the earliest part of the syn rift succession largely reflect the composition of the underlying basement geology typically granitoid or quartzose western province rocks in gsb and lithic rich eastern province metasedimentary rocks in cb younger strata representing part of the post rift succession are generally better sorted with detrital compositions reflecting sediment mixing of several basement source areas based on available data reservoir quality is directly related to sandstone texture and detrital composition factors of provenance and facies compaction a factor of burial depth temperature and degree or type of diagenetic alteration best quality sandstones are predicted to occur in the late cretaceous post rift succession of gsb and locally in cb either where sediment is derived from local granitoids or where high energy depositional processes have resulted in quartz enriched compositions

sedimentology of the paleoproterozoic kungarra formation turee creek group western australia a conformable record of the transition from early to modern earth abstract this paper presents the first detailed sedimentological analysis of the paleoproterozoic kungarra formation the lowermost of three formations comprising the turee creek group in western australia which was deposited across the rise in atmospheric oxygen the great oxidation event or goe and the transition from early to modern earth the data show that the kungarra formation has a gradational conformable lower contact with underlying banded iron formation of the hamersley group and predominantly comprises an upward shallowing succession from deepwater shales and siltstones through rippled fine grained sandstones and stromatolitic carbonates to tidal flat deposits that immediately underlie coastal fluvial deposits of the overlying koolbye formation at the base of the kungarra formation is a gradual transition from alternating units of magnetic green shale and thin units of banded iron formation that pass upsection to units of non magnetic shale and ferruginous chert and grey chert reflecting a gradual loss of iron from the world s oceans accompanying the rise of atmospheric oxygen a falling stage systems tract is recognised above this transition in the hardey syncline area capped by stromatolitic carbonates and a period of exposure marked by an erosional unconformity and carbonate beachrock two glacio eustatic cycles are recognised within the middle to upper parts of the kungarra formation each of which is marked by the rapid onset of falling systems tracts and characterised by falling systems tracts during and following diamictite deposition stratigraphic data are used to infer a depobasin filled by a sediment wedge prograding from southeast to northwest in contrast to previous models of a north northeastward deepening foreland basin the lack of seismites or internal unconformities within the formation precludes a foredeep setting rather deposition is interpreted as having occurred within an intracratonic basin with detritus sourced from erosion of uplifted bedrock to the southeast

geochemistry of hydrothermal fluids from the pacmanus northeast pual and vienna woods hydrothermal fields manus basin papua new guinea abstract processes controlling the composition of seafloor hydrothermal fluids in silicic back arc or near arc crustal settings remain poorly constrained despite growing evidence for extensive magmatic hydrothermal activity in such environments we conducted a survey of vent fluid compositions from two contrasting sites in the manus back arc basin papua new guinea to examine the influence of variations in host rock composition and magmatic inputs both a function of arc proximity on hydrothermal fluid chemistry fluid samples were collected from felsic hosted hydrothermal vent fields located on pual ridge pacmanus and northeast ne pual near the active new britain arc and a basalt hosted vent field vienna woods located farther from the arc on the manus spreading center vienna woods fluids were characterized by relatively uniform endmember temperatures 273 285 c and major element compositions low dissolved co 2 concentrations 4 4 mmol kg and high measured ph 4 2 4 9 at 25 c temperatures and compositions were highly variable at pacmanus ne pual and a large newly discovered vent area fenway was observed to be vigorously venting boiling 358 c fluid all pacmanus fluids are characterized by negative d h 2 o values in contrast to positive values at vienna woods suggesting substantial magmatic water input to circulating fluids at pual ridge low measured ph 25 c values 2 6 2 7 high endmember co 2 up to 274 mmol kg and negative 34 s h 2 s values down to 2 7 in some vent fluids are also consistent with degassing of acid volatile species from evolved magma dissolved co 2 at pacmanus is more enriched in 13 c 4 1 to 2 3 than vienna woods 5 2 to 5 7 suggesting a contribution of slab derived carbon the mobile elements e g li k rb cs and b are also greatly enriched in pacmanus fluids reflecting increased abundances in the crust there relative to the manus spreading center variations in alkali and dissolved gas abundances with cl at pacmanus and ne pual suggest that phase separation has affected fluid chemistry despite the low temperatures of many vents in further contrast to vienna woods substantial modification of pacmanus ne pual fluids has taken place as a result of seawater ingress into the upflow zone consistently high measured mg concentrations as well as trends of increasingly non conservative so 4 behavior decreasing endmember ca cl and sr cl ratios with increased mg indicate extensive subsurface anhydrite deposition is occurring as a result of subsurface seawater entrainment decreased ph and endmember fe mn ratios in higher mg fluids indicate that the associated mixing cooling gives rise to sulfide deposition and secondary acidity production several low temperature 80 c fluids at pacmanus ne pual also show evidence for anhydrite dissolution and water rock interaction fixation of b subsequent to seawater entrainment hence the evolution of fluid compositions at pual ridge reflects the cumulative effects of water rock interaction admixing and reaction of fluids exsolved from silicic magma phase separation segregation and seawater ingress into upflow zones

the generation and trapping of solitary waves over topography observations of tidal flow over a sill show that upstream solitary wave generation occurred in conjunction with the downstream movement of a streamline bifurcation over the obstacle crest solitary waves appeared upstream of the bifurcation riding on the surface stratification shear flow instabilities on the plunging interface were also evident and may contribute to wave generation through subharmonic interaction nonlinear waves generated downstream of an internal control became trapped in the supercritical flow

displacement loss on growth faults due to sediment compaction compaction models of normal growth faults combined with displacement data from a natural growth fault cape egmont fault new zealand are used to quantify the loss of displacement arising from compaction in circumstances where growth faults are blanketed and buried by compacting sediment in both footwall and hangingwall comparison of imposed tectonic displacements with the preserved displacements indicates that although the thicknesses of sand shale sequences may decrease by up to c 55 due to compaction associated losses in displacement on growth faults are typically 70 and post faulting burial is large i e km scale

terrane boundary reactivation barriers to lateral fault propagation and reactivated fabrics rifting across the median batholith zone great south basin new zealand prominent pre existing structural heterogeneities within the lithosphere may localise or partition deformation during tectonic events the ne trending great south basin offshore new zealand formed perpendicular to a series of underlying crustal terranes including the dominantly granitic median batholith zone which along with the boundaries between individual terranes exert a strong control on rift physiography and kinematics we find that the crustal to lithospheric scale southern terrane boundary of the median batholith zone is associated with a crustal scale shear zone that was reactivated during late cretaceous extension between zealandia and australia this reactivated terrane boundary is oriented at a high angle to the faults defining the great south basin we identify a large granitic laccolith along the southern margin of the median batholith expressed as sub horizontal packages of reflectivity and acoustically transparent areas on seismic reflection data the presence of this strong granitic body inhibits the lateral south westward propagation of ne trending faults which segment into a series of splays that rotate to align along the margin as they approach further we also identify two e w and ne sw oriented basement fabrics likely corresponding to prominent foliations which are exploited by small scale faults across the basin we show that different mechanisms of structural inheritance are able to operate simultaneously and somewhat independently within rift systems at different scales of observation the presence of structural heterogeneities across all scales need to be incorporated into our understanding of the structural evolution of complex rift systems

the breakup sequence and associated lithospheric breakup surface their significance in the context of rifted continental margins west iberia and newfoundland margins north atlantic regional 2d seismic reflection profiles and borehole data are used to characterise the syn to post rift transition in the shallow offshore porto basin and in deep offshore regions of west iberia and newfoundland east canada the interpreted data highlight the development of a regional stratigraphic surface at the time of complete lithospheric breakup between west iberia and newfoundland this surface usually called breakup unconformity is renamed in this work as lithospheric breakup surface lbs on the basis that 1 it is not always developed as an unconformity and 2 all lithosphere is involved on the breakup process not only the continental crust depositional changes occur across the lbs in association with late aptian lithospheric breakup which is marked by the deposition of a breakup sequence bs rather than a single stratigraphic surface stratigraphic correlations between strata in shallow and deeper parts of the two margins lead us to propose the breakup sequence bs as representing the transitional period between lithospheric breakup and the establishment of thermal relaxation as the main process controlling subsidence on divergent continental margins the results in this work are important for other continental margins as they demonstrate that during lithospheric breakup significant quantities of sediment bypassed the inner proximal margins of west iberia and newfoundland on their way to the outer proximal margin in addition the interpreted data show that complete lithospheric breakup between conjugate margins is recorded by similar tectono stratigraphic events in iberia and newfoundland these events are associated with reservoir successions in sediment overfilled basins and with carbon rich strata black shales in sediment starved basins

mid cretaceous oblique rifting of west antarctica emplacement and rapid cooling of the fosdick mountains migmatite cored gneiss dome abstract in marie byrd land west antarctica the fosdick mountains migmatite cored gneiss dome was exhumed from mid to lower middle crustal depths during the incipient stage of the west antarctic rift system in the mid cretaceous prior to and during exhumation major crustal melting and deformation included transfer and emplacement of voluminous granitic material and numerous intrusions of mantle derived diorite in dikes a succession of melt and magma related structures formed at temperatures in excess of 665 50 c based on ti in zircon thermometry these record a transition from wrench to oblique extensional deformation that culminated in the development of the oblique south fosdick detachment zone solid state fabrics within the detachment zone and overprinting brittle structures record translation of the detachment zone and dome to shallow levels to determine the duration of exhumation and cooling we sampled granite and gneisses at high spatial resolution for u pb zircon geochronology and 40 ar 39 ar hornblende and biotite thermochronology u pb zircon crystallization ages for the youngest granites are 102 ma three hornblende ages are 103 to 100 ma and 12 biotite ages are 101 to 99 ma all overlap within uncertainty the coincidence of zircon crystallization ages with 40 ar 39 ar cooling ages indicates cooling rates 100 c m y that when considered together with overprinting structures indicates rapid exhumation of granite and migmatite from deep to shallow crustal levels within a transcurrent setting orientations of structures and age constrained crosscutting relationships indicate counterclockwise rotation of stretching axes from oblique extension into nearly orthogonal extension with respect to the marie byrd land margin the rotation may be a result of localized extension arising from unroofing and arching of the fosdick dome extensional opening within a pull apart zone or changes in plate boundary configuration the rapid tectonic and temperature evolution of the fosdick mountains dome lends support to recently developed numerical models of crustal flow and cooling in orogenic crust undergoing extension transtension and accords with numerous studies of migmatite cored gneiss domes in transcurrent settings

a phase of transient subsidence sediment bypass and deposition of regressive transgressive cycles during the breakup of iberia and newfoundland abstract seismic outcrop and well data from west iberia and newfoundland are used to investigate sediment stacking patterns during continental breakup as a function of tectonic subsidence in west iberia two breakup sequences are revealed on seismic data by marked strata offlap oceanwards from the present day continental shelf this character is similar to newfoundland where correlative strata comprise lower cretaceous cenomanian coarse grained siliciclastics accumulated around local sediment source areas the interpreted data reveal that the two breakup sequences 1 materialise sediment bypass onto continental slope depocentres that experienced important tectonic subsidence during continental breakup but without showing typical syn rift growth packages 2 generate specific forced regressive stratigraphic intervals that relate to uplift and exhumation of the proximal margin subsidence and sediment stacking patterns in both west iberia and newfoundland reflect similar continental breakup processes as they evolved from the upper lithosphere to their mantle breakup stages on both margins coarse grained siliciclastic units on the proximal margin give rise to thick shaley successions in deep water basins this work also confirms that in a setting dominated by a significant sediment influx yet lacking the burial rates of continental slope basins in newfoundland west iberia comprised accommodation driven basins during continental breakup not necessarily sediment starved as a corollary of our analysis we classify breakup sequences around the world based on the characteristic lithologies of their regressive transgressive depositional cycles

metamorphic discontinuities in orogenic belts example of the garnet biotite albite zone in the otago schist new zealand a regional petrographic reconnaissance of psammitic and pelitic rocks in the otago schist new zealand has revealed the presence of garnet grossalspite with typical rim composition almandine41 spessartine25 grossular33 pyrope1 and biotite in 37 new samples more than doubling the previously known number a new garnet biotite albite zone can now be defined in the greenschist facies otago schist that is distinct from the better known biotite garnet and oligoclase zones in the along strike alpine schist the garnet biotite albite zone is in part metamorphically discontinuous with adjacent schists and does not support models of simple continuous progressive jurassic regional metamorphism in otago the structurally higher lower grade boundary of the zone coincides in at least three places with previously mapped regional shear zones the structurally lower expected higher grade boundary of the zone appears to be obliterated by a chlorite zone overprint which can be spatially related to alpine schist recrystallisation of cretaceous age the otago situation serves as an example of the subtle metamorphic discontinuities that probably pervade many orogenic belts

shoaling of nonlinear internal waves in massachusetts bay 1 the shoaling of the nonlinear internal tide in massachusetts bay is studied with a fully nonlinear and nonhydrostatic model the results are compared with current and temperature observations obtained during the august 1998 massachusetts bay internal wave experiment and observations from a shorter experiment which took place in september 2001 the model shows how the approaching nonlinear undular bore interacts strongly with a shoaling bottom offshore of where kdv theory predicts polarity switching should occur it is shown that the shoaling process is dominated by nonlinearity and the model results are interpreted with the aid of a two layer nonlinear but hydrostatic model after interacting with the shoaling bottom the undular bore emerges on the shallow shelf inshore of the 30 m isobath as a nonlinear internal tide with a range of possible shapes all of which are found in the available observational record

a nonhydrostatic version of fvcom 2 mechanistic study of tidally generated nonlinear internal waves in massachusetts bay 1 the generation propagation and dissipation processes of large amplitude nonlinear internal waves in massachusetts bay during the stratified season were examined using the nonhydrostatic finite volume coastal ocean model fvcom nh the model reproduced well the characteristics of the high frequency internal waves observed in massachusetts bay in august 1998 the model experiments suggested that internal waves over stellwagen bank are generated by the interaction of tidal currents with steep bottom topography through a process of forming a large density front on the western slope of the bank by the release of an initial density perturbation near ebb flood transition nonlinear steepening of the density front into a deep density depression and disintegrating of the density depression into a wave train earth s rotation tends to transfer the cross bank tidal kinetic energy into the along bank direction and thus reduces the intensity of the density perturbation at ebb flood transition and density depression in the flood period the internal wave packet propagates as a leading edge feature of the internal tidal wave and the faster propagation speed of the high frequency internal waves in massachusetts bay is caused by earth s rotation the model experiments suggested that bottom friction can significantly influence the cross bank scale of the density perturbation and thus the density depression during wave generation and the dissipation during the wave s shoaling inclusion of vertical mixing using the mellor yamada level 2 5 turbulence closure model had only a marginal effect on wave evolution the model results support the internal wave theory proposed by lee and beardsley 1974 but are in disagreement with the lee wave mechanism proposed by maxworthy 1979

mineralogical constraints on precambrian pco2 arising from m t rosing d k bird n h sleep c j bjerrum 744 747 2010 10 1038 nature08955 rosing and others1 recently proposed a new model for the early earth s atmospheric composition they suggest on the basis of mineral assemblages in sedimentary rocks that atmospheric co2 and ch4 concentrations have been consistently overestimated this proposal is intriguing given that high concentrations of these gases are traditionally considered to be the solution to the faint young sun paradox of why the earth did not freeze over in our sun s youth when solar luminosity was greatly decreased rosing and others1 instead invoke a decreased planetary albedo during the earth s early history new insights into this problem which has long captivated those interested in the earth s early climate and biological history are welcome however we believe that the arguments presented by rosing and others1 for roughly modern values of the partial pressure of co2 are not robust

reflecting tidal wave beams and local generation of solitary waves in the ocean thermocline it is generally accepted that ocean internal solitary waves can arise from the interaction of the barotropic tide with the continental shelf which generates an internal tide that in turn steepens and forms solitary waves as it propagates shorewards some field observations however reveal large amplitude internal solitary waves in deep water hundreds of kilometres away from the continental shelf suggesting an alternative generation mechanism tidal flow over steep topography forces a propagating beam of internal tidal wave energy which impacts the thermocline at a considerable distance from the forcing site and gives rise to internal solitary waves there motivated by this possibility a simple nonlinear long wave model is proposed for the interaction of a tidal wave beam with the thermocline and the ensuing local generation of solitary waves the thermocline is modelled as a density jump across the interface of a shallow homogeneous fluid layer on top of a deep uniformly stratified fluid and a finite amplitude propagating internal wave beam of tidal frequency in the lower fluid is assumed to be incident and reflected at the interface the induced weakly nonlinear long wave disturbance on the interface is governed in the far field by an integral differential equation which accounts for nonlinear and dispersive effects as well as energy loss owing to radiation into the lower fluid depending on the strength of the thermocline and the intensity of the incident beam nonlinear wave steepening can overcome radiation damping so a series of solitary waves may arise in the thermocline sample numerical solutions of the governing evolution equation suggest that this mechanism is quite robust for typical oceanic conditions

upstream internal jumps in stratified sill flow observations of formation evolution and release abstract the time dependent response of upstream undular bores and internal hydraulic jumps from initial formation to eventual release is documented two events characterized by qualitatively different responses are discussed in the first case an undular bore develops upstream of the sill crest this disturbance remains upstream through the ebb tidal flow but is transformed to a hydraulic jump as its amplitude increases toward the end of ebb tide it is released and subsequently disperses into a group of solitary like waves during the second event an upstream jump also develops at an early stage of the tide however it is subsequently swept downstream by the tidal flow such that the upstream region then appears featureless approaching slack tide as an exchange flow becomes established a large bore or gravity current is emitted the different responses seen in these two events are interpreted in terms of the froude number associated with the near surface stratification

tectonic controls on atmospheric climatic and biological evolution 3 5 2 4ga abstract this paper presents a speculative scenario for the evolution of the earth s surface environment and biological community 3 5 2 4 ga based on the geologic record and its interpretation available geologic evidence suggests that the earth s climate before 3 2 ga was hot probably 60 73 c as a result of a co 2 and ch 4 greenhouse the early biological community was probably dominated by anoxygenic photosynthetic thermophiles cyanobacteria if they evolved before 3 1 ga would have struggled to survive at such high temperatures as a result the atmosphere contained extremely low levels of o 2 and 33 s shows wide departures from 0 indicating mass independent fractionation of s isotopes in the upper atmosphere this hot early climate collapsed 3 1 3 0 ga due to growth of large new blocks of continental crust cratons the weathering of which resulted in the depletion of atmospheric co 2 and eventual drawdown of ch 4 due to formation and rainout of methane aerosols cooling may have culminated in glaciation about 3 0 2 9 ga this climatic catastrophe and attendant changes in atmospheric composition drove a major biological revolution 3 0 2 7 ga characterized by the emergence of new low temperature taxa including cyanobacteria and their spread throughout surface environments at the expense of extreme thermophiles including methanogens during a period of general tectonic stability 3 0 2 7 ga the earth s surface was characterized by overall clement conditions with temperatures 2 after 2 75 ga and by 2 7 ga surface temperatures had returned to 60 c cyanobacteria were again suppressed o 2 production waned and atmospheric mass independent fractionation is again indicated by extreme variations in 33 s these conditions persisted perhaps intermittently until about 2 5 2 4 ga when weathering and erosion of vast new blocks of continental crust formed about 2 65 2 6 ga caused the second and probably last collapse of a 60 c surface climate broad tectonic climatic and biological events 3 5 2 9 ga are remarkably parallel to those of the late archean and paleoproterozoic 2 75 2 2 ga and neoproterozoic 1 0 0 50 ga these 500 myr long cycles of greenstone volcanism and crustal generation climatic and atmospheric instability and biological innovation reflect the long term interaction of the tectonic atmospheric climatic and biological components of the earth s surface system they suggest that the evolution of the earth s interior expressed through its control on the formation of large blocks of continental crust has influenced atmospheric composition and climate which in turn have provided a fundamental control on the timing and directions of biological evolution throughout earth history

source and propagation of internal solitary waves in the northeastern south china sea 1 large amplitude internal solitary waves isws observed near dongsha island in the south china sea originate in tide topography interactions at luzon strait their arrival times at two moorings s7 at 117 17 e 21 37 n and y at 117 13 2 e 21 2 8 n are investigated with respect to model predicted barotropic tidal currents over lan yu ridge at luzon strait each isw packet can be associated with a westward tidal current peak the time lags between the isws and the barotropic tidal currents are 57 6 0 9 hours at s7 and 55 1 1 0 hours at y consistent with the mode one internal waves propagating nondispersively through the region s bathymetry and climatological stratification larger isws usually arrive earlier than smaller ones consistent with the theoretical relation between nonlinear wave speed and wave amplitude the observation that the isws are associated with westward tidal currents with without the presence of earlier eastward tidal currents suggests that they are generated by nonlinear steepening of internal tides rather than by the lee wave mechanism an idealized nonlinearization distance over which the isws are generated in internal tide troughs is estimated to be 260 40 km from luzon strait

on the generation and evolution of nonlinear internal waves in the south china sea the nonhydrostatic regional ocean modeling system is applied to the nonlinear internal waves or solitons that are generated at the luzon ridge in the south china sea the luzon ridge near the batan islands is represented by an idealized ridge with a height of 2 6 km on a flat bottom model runs are performed for various ridge shapes and a symmetric tidal forcings the model is in the mixed tidal lee wave regime the barotropic tide over the ridge generates first mode waves through the internal tide release mechanism westward traveling solitons emerge from these first mode waves through nonlinear steepening in the internal tide release mechanism asymmetric tides with strong eastward currents can generate strong westward solitons the eastward current creates an elevation wave with a higher energy density west of the ridge and as soon as the current slackens the wave is released westward on its backslope strong solitons develop the energy density is further enhanced by nonlinearities such as differences in phase speeds and energy fluxes related to lee waves a modal and harmonic decomposition shows the generation of vertical modes and higher temporal harmonics and indicates significant wave wave interaction e g triads in the mixed tidal lee wave regime more energy is contained in the first mode compared to the higher modes hence linear internal tide beams are less well defined and strong solitons develop

rare earth element systematics of the chemically precipitated component in early precambrian iron formations and the evolution of the terrestrial atmosphere hydrosphere lithosphere system irrespective of provenance age and metamorphic grade the chemically precipitated component in early precambrian 2 3 ga iron formations ifs displays smyb cn 1 which reflects the corresponding ratios of contemporaneous seawater in conjunction with end if end shale jacobsen and pimentel klose 1988a b this rare earth element ree signature reveals that the ree distribution in early precambrian ifs must be explained by mixing between a marine bottom and a surface water component and that the rees and by analogy the fe cannot be derived from weathering of a continental source eusm n ratios of detritus free ifs are controlled by the marine bottom water component where eusm cn 1 results from ree input from high temperature high t hydrothermal fluids which received their ree signature during high t alteration of oceanic crust at spreading centres decreasing eusm cn of ifs with decreasing depositional age reflect the vaning importance of high t compared to low t 200 c hydrothermal fluids hence oceanic crust alteration for the ree budget of marine bottom waters and are related to decreasing upper mantle temperatures smyb n ratios of detritus free ifs are controlled by the marine surface water component whose ree distribution is affected by the same processes which operate today in the entire ocean mixing calculations reveal that smyb cn in early precambrian marine surface waters was significantly lower than it is today to explain this difference two mechanisms are discussed on the basis of higher pco2 and lower po2 levels of the precambrian atmosphere 1 1 higher co32 in precambrian river and coastal waters and 2 2 redox cycling of the rees at an oceanwide chemocline which separated anoxic and oxic water masses however these processes are synergetic and could have operated in unison the ree distribution in precambrian ifs is described as a result of mixing in a multicomponent system where high t and low t hydrothermal fluids contributed to the marine bottom water and ree input from the dissolved ree pool in river waters after some modification in estuaries dominated the ree distribution in marine surface waters after mixing of bottom and surface waters their combined ree load was quantitatively scavenged onto precipitating iron oxihydroxides or iron carbonates and deposited with the chemically precipitated component after addition of variable amounts of clastic detritus this eventually resulted in the ree distribution observed in precambrian ifs

abrupt plate accelerations shape rifted continental margins rifted margins are formed by persistent stretching of continental lithosphere until breakup is achieved it is well known that strain rate dependent processes control rift evolution yet quantified extension histories of earth s major passive margins have become available only recently here we investigate rift kinematics globally by applying a new geotectonic analysis technique to revised global plate reconstructions we find that rifted margins feature an initial slow rift phase less than ten millimetres per year full rate and that an abrupt increase of plate divergence introduces a fast rift phase plate acceleration takes place before continental rupture and considerable margin area is created during each phase we reproduce the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions the extension models suggest that the two phase velocity behaviour is caused by a rift intrinsic strength velocity feedback which can be robustly inferred for diverse lithosphere configurations and rheologies our results explain differences between proximal and distal margin areas and demonstrate that abrupt plate acceleration during continental rifting is controlled by the nonlinear decay of the resistive rift strength force this mechanism provides an explanation for several previously unexplained rapid absolute plate motion changes offering new insights into the balance of plate driving forces through time

hydrothermal fe fluxes during the precambrian effect of low oceanic sulfate concentrations and low hydrostatic pressure on the composition of black smokers abstract modern mid ocean ridge hydrothermal systems typically release vent fluids to the ocean with dissolved h2s in excess of fe these fluids are the consequence of high temperature interactions between sulfate rich seawater and mid ocean ridge basalt at conditions near the critical point for seawater the precipitation of fes and fes2 during venting titrates most of the fe from the fluid significantly reducing the net flux of fe to the open ocean here we suggest that hydrothermal fluids emanating from precambrian seafloor systems older than 1 8 ga had fe h2s ratios 1 and with fh2 higher than today because seawater lacked its primary oxidant dissolved sulfate ion this predominance of fe over h2s would have promoted the establishment of an iron rich deep ocean and the deposition of banded iron formations bif accordingly the end of bif deposition at 1 8 ga was the result of the buildup of sulfate in seawater from oxidative weathering and its return at 750 ma the result of reductions in seawater sulfate concentrations during snowball earth episodes enhanced by elevated fe concentrations during depressurization of hydrothermal systems by large eustatic sea level falls moreover precambrian chemosynthetic vent communities may have been based on h2 synthesis rather than on h2s oxidation as they largely are today

the generation of internal waves by tidal flow over stellwagen bank the generation mechanism of internal waves by tidal flow over stellwagen bank in massachusetts bay is qualitatively clarified first in order to demonstrate the continuous generation process of internal waves a numerical simulation is carried out for the hydrographic condition during a field study by chereskin 1983 the resulting internal waveforms agree well with the acoustic images obtained at several stages of the tidal flow next the generation mechanism for these internal waves is analyzed via a physical interpretation of the calculated result this analysis is made by use of characteristics which describe the propagation of the first and second mode internal waves it is shown that each mode internal wave with an upstream phase propagation is efficiently amplified while being carried downstream toward the location where the maximum tidal flow becomes critical with the decrease of the tidal flow the internal wave thus amplified begins to propagate upstream in previous studies these internal waves were interpreted as quasi steady lee waves the present analysis however provides a satisfactory explanation for the time dependent features that are shown in numerical simulation this indicates that the internal waves over the bank are transient waves formed in response to a time varying tidal flow and not quasi steady lee waves

upstream internal hydraulic jumps abstract in stratified tidal flow over a sill the character of the upstream response is determined by a froude number fs based on the stratification near the surface this is distinguished from the froude number governing the response in the neighborhood of the sill crest which is based on the weak density step associated with a flow bifurcation for moderate values of fs the upstream response consists of nonlinear waves or a weak undular bore for larger values of fs a strong quasi stationary internal hydraulic jump dominates the upstream response at sufficiently large values of fs the upstream bore is swept downstream and lost acoustic backscatter and velocity data are presented for the case of a strong internal bore or gravity current in a tidally modulated sill flow numerical simulations with varying near surface stratification are presented to illustrate the upstream responses at different values of fs the theory of two layer hydraulic flows is invoked to account for the development of the

late eocene early miocene facies and stratigraphic development taranaki basin new zealand the transition to plate boundary tectonics during regional transgression eight latest eocene to earliest miocene stratigraphic surfaces have been identified in petroleum well data from the taranaki basin new zealand these surfaces define seven regional sedimentary packages of variable thickness and lithofacies forming a mixed siliciclastic carbonate system the evolving tectonic setting particularly the initial development of the australian pacific convergent margin controlled geographic stratigraphic and facies variability this tectonic signal overprinted a regional transgressive trend that culminated in latest oligocene times the earliest influence of active compressional tectonics is reflected in the preservation of latest eocene early oligocene deepwater sediments in the northern taranaki basin thickness patterns for all mid oligocene units onwards show a shift in sedimentation to the eastern taranaki basin controlled by reverse movement on the taranaki fault system this resulted in the deposition of a thick sedimentary wedge initially of coarse clastic sediments later carbonate dominated in the foredeep close to the fault in contrast oligocene active normal faulting in a small sub basin in the south may represent the most northerly evidence for rifting in southern zealandia related to emerald basin formation the early miocene period saw a return to clastic dominated deposition the onset of regional regression and the southward propagation of compressional tectonics

in situ observations of dissolved iron and manganese in hydrothermal vent plumes juan de fuca ridge in situ measurements of dissolved manganese and total dissolved iron were conducted in hydrothermal plumes over the juan de fuca ridge using a submersible chemical analyzer scanner the scanner was deployed as part of a ctd transmissometer rosette instrument package on both tow yos and vertical casts during the vents leg i cruise in 1989 dissolved manganese and total dissolved iron concentrations along with temperature and light attenuation anomalies were determined over the ridge crest every 5 s discrete samples for laboratory analyses of dissolved iron ii total dissolved iron ii iii and manganese were also collected metal to heat ratios me q measured in situ were extremely variable in one steady state plume while an event plume had constant me q uniform values of mn q in the event plume demonstrate that mn behaves conservatively in the near field plume variability in the mn q ratios in a steady state plume indicated the presence of at least two hydrothermal sources with distinct me q values a simple mixing model shows that the contribution of mn from high me q sources with a composition characteristic of black smoker vents varies between 1 and 99 within the core of the steady state plume with an average value of 55 on average over 50 of the excess heat within the plume originates from low me q ratio sources with a composition characteristic of low temperature diffuse flow vent fluids less than 4 of the volume of hydrothermal fluids in the plume originates from black smokers the fe ii concentrations were used to provide an estimate of plume age on a transect across the ridge axis plume ages were about 2 5 days on axis and 12 days off axis these plume ages were modeled to provide estimates of plume transport and horizontal diffusion and show excellent agreement with ages determined using 222rn

cretaceous termination of subduction at the zealandia margin of gondwana the view from the paleo trench abstract during the permian to cretaceous zealandia occupied a position on the proto pacific facing convergent margin of gondwana subduction on this margin ceased somewhere between 105 ma and perhaps 70 ma but the timing of this tectonic transition remains controversial resolution of this uncertainty is important for tectonic reconstructions of the southwest pacific and for global plate tectonic models here we revisit the problem by reference to new stratigraphic and geochemical data from the east coast basin of new zealand which occupied a position adjacent to and possibly superimposed on the relict cretaceous subduction trench at the time subduction ceased this basin is expected to preserve direct structural and stratigraphic evidence for or against late cretaceous subduction in western parts of the east coast basin the western sub belt a conspicuous unconformity separates undoubted accretionary prism rocks of the torlesse composite terrane from younger cretaceous cover units strata beneath and overlying this unconformity vary in age from place to place but abundant paleontological data and published ages of detrital zircons some reinterpreted herein indicate that exposed torlesse rocks are nowhere younger than 100 ma overlying zealandia megasequence cover strata are mostly younger than 110 ma between 110 and 85 ma the entire length of the western sub belt reveals complex stratigraphic architecture of relatively small scale depocentres subjected to alternating episodes of subsidence and local uplift and erosion there is widespread evidence for compression on the western sub belt over this period although the overall amount of shortening is likely to be relatively modest in contrast the eastern sub belt preserves a record of near continuous and apparently simple deposition over the same interval of time we see little and somewhat equivocal evidence for extension in either sub belt through the late cretaceous superimposed on this general pattern we observe discrete widespread or basin wide tectonic events at 95 ma and within the intervals 86 83 ma and 83 81 ma indicated by the presence of unconformities and sedimentary facies changes importantly all these events can be correlated with coeval events recorded elsewhere in zealandia suggesting that the east coast basin was subject to the same tectonic regime as the rest of zealandia and shared a common history during the mid to late cretaceous integrating these findings with data from elsewhere in zealandia we argue that there is diverse evidence to indicate that subduction finished along the new zealand segment of the gondwana margin by 100 ma that said the causes of on going late cretaceous compression in the east coast basin and in other parts of zealandia are not well constrained although several possible explanations are plausible there are few analogue abandoned subduction margins described in the literature however the situation in the south shetland trench shows strong similarities with patterns and complexities observed in the east coast basin

the role of long term rifting history on modes of continental lithosphere extension continental lithosphere extension results in complex basin types with differing structural styles subsidence thermal histories and melt production many studies have examined the role of initial rheological layering geothermal gradients and extension rates during a single rifting event this approach neglects the tectonic history of many basins that are marked by multiple rifting events here we address the role of repeated extension on long term lithospheric strain modes and the resulting basins highlighting cases most affected by previous rifting events we use numerical models of a lithosphere undergoing two rifting events of differing extension rates and separated by cooling to show the effect of early events on subsequent evolution the combination of boundary displacement velocity in both events leads to the formation of various rift basin types ranging from narrow to wide to hyperextended and with variation of subsidence patterns degrees of symmetry and melt yield we show that basin type subsidence and melt production might be strongly affected by previous rifting events illustrating cases in which the previous rifting history cannot be neglected our models reproduce the first order features of earth s sedimentary basins and propose a classification to guide the interpretation of extensional basins and their evolution

normal vs strike slip faulting during rift development in east africa the malawi rift kinematic analysis of neogene and quaternary faults demonstrates that the direction of extension in the malawi rift rotated from east northeast to southeast rift development commenced with the formation of half grabens bounded by northwest north and northeast striking normal faults owing to slightly oblique rifting the northwest striking faults in the northernmost rift segment show a small dextral oblique slip component whereas north and northeast oriented faults in the central part of the rift display a sinistral oblique slip component this first event resulted in block faulting and basin subsidence which is largely responsible for the present day basin morphology of lake malawi a major change in fault kinematics occurred because of rotation of the extension direction and permutation of the principal compressive and intermediate axes the structural pattern inherited from the first rifting phase was no longer suitably oriented to accommodate extensional deformation and strike slip faulting assumed a major role the strike slip regime amplified uplift of basement ridges within the rift in regions of local transpression but it also created alluvial basins because of local transtension this new kinematic style is compatible with the recent seismicity older faults that show mainly the first deformational increment are restricted to the outermost parts of the rift toward the center the faults depict an increase in strike slip components of movement suggesting deformation propagation toward the rift center which results in a narrowing of the active rift environments with time

scaling systematics of fault sizes on a large scale range fault map abstract a large scale high quality fault map was produced for an intensively mined area of the u k south yorkshire coalfield this database has been used to investigate both the scaling properties of fault systems and the robustness of techniques for the prediction of sub resolution faulting the mapped area is 87 km 2 and contains 2257 individual fault traces with on average one throw reading available for every 82 m of fault trace recorded throw values range from 10 cm to 180 m and fault trace lengths from 10 m to 12 km two sets of normal faults nw and ne striking are present together with local wnw striking dextral strike slip zones size populations are power law over at least 2 5 orders of magnitude and the exponents of the throw maximum throw and fault trace length population curves are interrelated and taking into account the effects of resolution on trace length populations are internally consistent derivatives of the primary map reproduce resolutions typical of offshore seismics and illustrate the limitations of seismic data power law fault size distributions provide a basis for sub seismic fault prediction for this data set a fault throw resolution of better than 10 m would be required to provide an accurate prediction of the throw population down to ca 0 6m

the generation and evolution of nonlinear internal waves in the deep basin of the south china sea abstracttime series observations of nonlinear internal waves in the deep basin of the south china sea are used to evaluate mechanisms for their generation and evolution internal tides are generated by tidal currents over ridges in luzon strait and steepen as they travel west subsequently generating high frequency nonlinear waves although nonlinear internal waves appear repeatedly on the western slopes of the south china sea their appearance in the deep basin is intermittent and more closely related to the amplitude of the semidiurnal than the predominant diurnal tidal current in luzon strait as the internal tide propagates westward it evolves under the influence of nonlinearity rotation and nonhydrostatic dispersion the interaction between nonlinearity and rotation transforms the internal tide into a parabolic or corner shape a fully nonlinear two layer internal wave model explains the observed characteristics of internal tide evolution in the deep basin for different representative forcing cond

generation and propagation of nonlinear internal waves in massachusetts bay 1 during the summer nonlinear internal waves nliws are commonly observed propagating in massachusetts bay the topography of the area is unique in the sense that the generation area over stellwagen bank is only 25 km away from the shoaling area and thus it represents an excellent natural laboratory to study the life cycle of nliws to assist in the interpretation of the data collected during the 1998 massachusetts bay internal wave experiment mbiwe98 a fully nonlinear and nonhydrostatic model covering the generation shoaling region was developed to investigate the response of the system to the range of background and driving conditions observed simplified models were also used to elucidate the role of nonlinearity and dispersion in shaping the nliw field this paper concentrates on the generation process and the subsequent evolution inthe basin themodel was foundtoreproducewell the range of propagation characteristics observed arrival time propagation speed amplitude and provided a coherent framework to interpret the observations comparison with a fully nonlinear hydrostatic model shows that during the generation and initial evolution of the waves as they move away from stellwagen bank dispersive effects play a negligible role thus the problem can be well understood considering the geometry of the characteristics along which the riemann invariants of the hydrostatic problem propagate dispersion plays a role only during the evolution of the undular bore in the middle of stellwagen basin the consequences for modeling nliws within hydrostatic models are briefly discussed at the end

microbiological processes in banded iron formation deposition banded iron formations have been studied for decades particularly regarding their potential as archives of the precambrian environment in spite of this effort the mechanism of their deposition and specifically the role that microbes played in the precipitation of banded iron formation minerals remains unresolved evidence of an anoxic earth with only localized oxic areas until the great oxidation event ca 2 45 to 2 32 ga makes the investigation of o2 independent mechanisms for banded iron formation deposition relevant recent studies have explored the long standing proposition that archean banded iron formations may have been formed and diagenetically modified by anaerobic microbial metabolisms these efforts encompass a wide array of approaches including isotope ecophysiological and phylogeny studies molecular and mineral marker analysis and sedimentological reconstructions herein the current theories of microbial processes in banded iron formation mineral deposition with particular regard to the mechanisms of chemical sedimentation and post depositional alteration are described the main findings of recent years are summarized and compared here and suggestions are made regarding cross disciplinary information still required to constrain the role of the biosphere in banded iron formation deposition

evolution of nonlinear internal waves in the east and south china seas synthetic aperture radar sar images from ers i have been used to study the characteristics of internal waves northeast and south of taiwan in the east china sea and east of hainan island in the south china sea rank ordered packets of internal solitons propagating shoreward from the edge of the continental shelf were observed in the sar images on the basis of the assumption of a semidiurnal tidal origin the wave speed can be estimated and is consistent with the internal wave theory by using the sar images and hydrographic data internal waves of elevation have been identified in shallow water by a thicker mixed layer as compared with the bottom layer on the continental shelf the generation mechanism includes the influences of the tide and the kuroshio intrusion across the continental shelf for the formations of elevation internal waves the effects of water depth on the evolution of solitons and wave packets are modeled by the nonlinear kortweg devries kdv type equation and linked to satellite image observations the numerical calculations of internal wave evolution on the continental shelf have been performed and compared with the sar observations for a case of depression waves in deep water the solitons first disintegrate into dispersive wave trains and then evolve to a packet of elevation waves in the shallow water area after they pass through a turning point of approximately equal layer depths that has been observed in the sar image and simulated by the numerical model the importance of the dissipation effect in the coastal area is also discussed and demonstrated

early oligocene marine canyon channel systems implications for regional paleogeography in the canterbury basin new zealand abstract the oligocene is a time of maximum submersion of the zealandia landmass it coincides with changes in oceanographic currents as well as with the inception of the cenozoic plate boundary through new zealand our analysis of offshore wells and seismic reflection lines in the canterbury basin reveals condensed oligocene strata

depositional architecture and structural evolution of a region immediately inboard of the locus of continental breakup liwan sub basin south china sea new 3d seismic data and regional 2d seismic profiles from the northern south china sea the most extensive dataset imaging a distal rifted margin in the world are used to characterize a region located immediately inboard of the locus of cenozoic continental breakup the interpreted dataset images a 6 km thick continental crust in which the moho and the base of syn rift sediment are observed as clear well resolved seismic reflections this extremely thinned continental crust was offset at its base by a complex detachment fault system from which oceanward dipping listric faults propagated vertically to bound six separate tilted blocks in a style akin to tectonic rafts the seismic reflection data in this work allowed us to investigate the thickness of synand post rift strata above tilt blocks to reveal that the early middle eocene syn rift topography was gradually blanketed in the late eocene 38 ma after 33 ma earliest oligocene the main depocenter on the margin migrated to the south of the liwan sub basin i e oceanward as recorded by the thickening of strata within a breakup sequence this work is important as it demonstrates how closely structures and sedimentation within the liwan sub basin were controlled by a basal rift related detachment system which is imaged in detail by 3d seismic data for the first time on a rifted continental margin continental breakup was marked by a shift in the locus of subsidence and crustal stretching toward ocean crust within a time period spanning 16 m y we extrapolate our findings from the south china sea to the development of asymmetric passive margins across the world

focusing of relative plate motion at a continental transform fault cenozoic dextral displacement 700 km on new zealand s alpine fault reversing 225 km of late cretaceous sinistral motion the widely accepted 450 km cenozoic dextral strike slip displacement on new zealand s alpine fault is large for continental strike slip faults but it is still less than 60 of the cenozoic relative plate motion between the australian and pacific plates through zealandia with the remaining motion assumed to be taken up by rotation and displacement on other faults in a zone up to 300 km wide we show here that the 450 km total displacement across the alpine fault is an artifact of assumptions about the geometry of new zealand s basement terranes in the eocene and the actual cenozoic dextral displacement across the active trace is greater than 665 km with more than 700 km and 94 of the relative plate motion in the last 25 ma at an average rate in excess of 28 mm yr it reverses more than 225 km and 300 km of sinistral shear through zealandia in the late cretaceous when zealandia lay on the margin of gondwana providing a direct constraint on the kinematics of extension between east and west antarctica at this time