fractal nature and scaling of normal faults in the espanola basin rio grande rift new mexico implications for fault growth and brittle strain abstract quaternary faults in the western espanola basin of the rio grande rift show a power law size displacement distribution suggesting that faulting in this region is scale invariant and that faults are self similar the power law or fractal distribution is characterized by fractal dimension of 0 66 to 0 79 and represents a young immature active fault population in a continental extensional regime based on this distribution it is estimated that unobserved faults with very small displacements account for up to 6 of the total strain since 1 2 ma total extension in this part of the basin has been at least 5 a direct correlation exists between maximum displacement and length of faults in this area suggesting that they obey a scaling relationship in which the ratio of log d max log l is 5 10 3 this ratio is nearly constant for faults whose lengths span three orders of magnitude indicating that there is no difference in the scaling relationship of displacement and length between faults of all sizes considering previous models these fault characteristics suggest that in the western espanola basin 1 host rock shear strengths are low 2 remote shear stresses were probably high and 3 most faults do not extend throughout the brittle crust finally displacement profiles on five of the largest faults arc asymmetric and show a rapid decrease in displacement from the point of maximum displacement toward the fault tip the fractal nature scaling relationship and distribution of displacement on faults are used to suggest that faults grew by nearly proportional increases in displacement and length perhaps by mechanisms dominated by propagating shear fractures rather than by linkage of pre existing joints or faults

the importance of small scale faulting in regional extension a recurring observation in many studies of extensional basins has been that the amount of extension visible on normal faults for example on seismic reflection profiles is significantly less than the amount of extension indicated by crustal thickness and thermal subsidence1 5 one mechanism suggested to account for this discrepancy is small scale faulting with offsets too small to be resolved seismically6 7 but earthquake studies8 10 indicate that small faults are responsible for only a small fraction of the total seismic moment in an active area scholz and cowie11 have recently attempted to extend this approach to the total strain at the end of a finite deformation interval by combining scaling laws describing the distributions of fault lengths and displacements here we present fault displacement data that directly conflict with scholz and cowie s conclusions and imply that up to 40 of the extension may be missed by summing fault offsets on basin profiles the fault population at the end of a long deformation interval may differ substantially from that responsible for the earthquake population at any one time

age origin and significance of the wugang bif in the taihua complex southern north china craton abstract the wugang bif hosted in a neoarchean taihua complex th in the wugang area at the southern margin of the north china craton ncc is one of the important iron ore in central plain in this study we report new data on whole rock compositions of iron ore sample u pb isotopic compositions of detrital zircon from metasedimentary rocks one bif sample and two associated garnet paragneisses in combination with literature data they yielded a minimum age peak of 2 60 ga for 110 detrital zircons of magmatic origin from the tieshanmiao formation dominantly bif bearing strata that we interpret as an upper limit age for the wugang bif depositional age thirty six detrital zircons of metamorphic origin gave a maximum metamorphic age peak of 2 45 ga reflecting the lower limit age thus the depositional ages of wugang bif can constrained at 2 60 2 45 ga the presence of proximal detrital zircons in bif samples river flux contributions and some bif samples have true negative ce anomalies suggest that they were deposited in near shore continental shelf or back arc basin environments coinciding with that they are typically interbedded with marble quartz arenite and other metasedimentary rocks but lack of volcanic material in particular both hanging wall and footwall for tiegukeng bif orebody from tieshanmiao formation are marble except for several diopside dykes 1943 ma orebody is dominantly composed of pyroxene rich bif quartz rich bif and other metasedimentary rocks the wugang bif displayed the weak positive eu eu sn anomalies average 1 74 associated with the low cr 10 ppm and v

u pb zircon chronology geochemistry and isotopes of the changyi banded iron formation in the eastern shandong province constraints on bif genesis and implications for paleoproterozoic tectonic evolution of the north china craton abstract the changyi banded iron formation bif in the eastern north china craton ncc occurs within the paleoproterozoic fenzishan group the bif shows alternating quartz rich light and magnetite rich dark bands with magnetite 15 65 vol quartz 25 65 vol and amphibole 15 30 vol constituting the major minerals minor garnet epidote chlorite calcite biotite and pyrite occur locally the bif bands are interlayered with amphibolite hornblende gneiss biotite quartz schist garnet biotite schist biotite gneiss and leptynite and are intruded by granites la icp ms u pb dating on zircons separated from the bif bands and the wallrocks constrains the depositional age as 2240 2193 ma and metamorphic age as 1864 ma the dominant composition of sio 2 fe 2 o 3 t average value of 92 3 wt of the bif bands suggests their formation mainly through chemical precipitation however the widely varying contents of major elements such as al 2 o 3 0 58 6 99 wt mgo 1 00 3 86 wt cao 0 22 4 19 wt and trace elements such as rb 2 06 40 4 ppm sr 9 36 42 5 ppm zr 0 91 23 6 ppm hf 0 04 0 75 ppm cr 89 1 341 ppm co 2 94 30 4 ppm and ni 1 43 52 0 ppm clearly indicate the incorporation of clastics especially continental felsic clastics as also confirmed by the presence of ancient detrital zircons in the bif bands when normalized against post archean average shale paas the seawater like signatures of ree distribution patterns such as lree depletion positive la and y anomalies and superchondritic y ho ratios average value of 36 3 support the deposition in seawater strong positive eu anomalies eu eu paas 1 14 2 86 also suggest the participation of hydrothermal fluids in addition the sympathetic correlation between cr co and ni as well as the co ni cu vs ree and the al 2 o 3 vs sio 2 relations further indicates that the iron and silica mainly originated from hydrothermal fluids combined with regional geological investigation and protolith restoration of the wallrocks a continental rift environment is suggested for the changyi bif deposition the appearance of negative ce paas anomalies might suggest the influence of the great oxidation event at the time of deposition the changyi bif witnessed the major paleoproterozoic rifting collision events in the ncc and their unique distribution in the ncc contrasts with other examples elsewhere in the world

earthquake faulting as a structural process abstract structural geology is concerned with the history of movement in the earth s crust and the processes by which displacements occur in the upper one third to one half of deforming continental crust displacement is accommodated largely by seismic slip increments on existing faults it follows that earthquakes and related processes are an integral part of structural geology traditionally structural geologists have been preoccupied with the complexity of the finite deformation within fault zones and with the stress states prevailing at the initiation of faults in intact crust future structural work should be directed more towards understanding the dynamic character of fault reactivation during incremental slip and related effects questions of interest include rheological and geometrical controls on the initiation perturbation and termination of ruptures directivity effects associated with rupture propagation the recognition of structures resulting from repeated stress cycling within seismogenic crust and identification of structural features diagnostic of shear stress levels during faulting structures arising from the inter relationships between slip episodes and induced fluid flow are of special importance because these dynamic fault processes appear influential in the development of much fault hosted mineralization mesothermal gold quartz lodes hosted in high angle reverse shear zones of mixed brittle ductile character form illustrative examples of structures that arguably can only be interpreted by seismo structural analysis embodying the concepts listed above

strike slip duplexes abstract strike slip fault systems often contain zones of steep imbricate faults geometrically similar to imbricate fans and duplexes in dip slip thrust and normal fault systems they are evident in map view rather than in vertical sections examples of duplexes are cited from both active and ancient systems and from theoretical and physical models duplexes may form at bends on strike slip faults by a process kinematically analogous to the sequential imbrication of ramps on dip slip faults however some may form and many may initiate as non sequential riedel fractures at fault offsets or on straight fault segments this process is more marked than in dip slip systems where primary anisotropy such as bedding exerts more control on fault geometry strike slip duplexes may be shunted along the fault system parallel to the regional slip vector however duplexes or individual horses will usually also move up or down perpendicular to the slip vector because of the unconstrained upper surface to the fault system these factors mean that no section through a strike slip system should be expected to area balance the faults of strike slip duplexes and imbricate fans may root in kinematically necessary low dip faults or may converge downwards and appear in vertical sections as flower structures

scaling properties of normal fault populations in the western corinth graben greece implications for fault growth in large strain settings abstract a variety of one dimensional and two dimensional map techniques of fractal analysis have been employed to investigate the scaling properties of the dominant wnw trending normal fault population of the western corinth graben in central greece to assess possible constraints on fault geometry during fault growth the data have been compiled separately from the northern and southern domains of the graben which have experienced differing amounts of finite strain based on power law distributions of the fault throw the applied extension is estimated at 30 and 22 for the northern and southern domains respectively however the fault spacing and fault length populations in both domains obey similar power law relationships these conclusions are also supported by maximum throw vs trace length data which demonstrate that the faults in the northern domain have systematically higher values for the slip to length ratios than those in the southern domain it is proposed that the corinth graben has reached fracture saturation that is the fault spacing stops evolving and displacement increases rather than faults lengthening to accommodate the increasing extension this growth process consistent with throw distributions along individual faults may be explained either in terms of a stress feedback mechanism or by the mechanical interaction between the wnw trending normal faults with a set of nne trending transverse faults

seismic strain rates in regions of distributed continental deformation we compare the rates and configurations of seismic deformation determined from moment tensor mechanisms of earthquakes in five continental regions with the rates and configurations expected if the seismicity reflected the relative plate motions in the regions the five regions the aegean iran california western new guinea and central asia are characterized by high levels of seismicity distributed over hundreds to thousands of kilometers from the accepted plate boundaries the earthquake mechanisms are compiled primarily from the catalog of centroid moment tensor solutions which is nearly complete over the interval 1977 1987 for earthquakes greater than ms 5 the thickness of the seismogenic layer in each region is constrained by the results of modeling broadband body wave phases and of previous earthquake studies in the aegean california and western new guinea the observed configurations of deformation agree with a simple model of distributed accommodation of relative plate motion in iran and central asia the moment tensors indicate that there is appreciable strain parallel to the strikes of the belts we augment our 10 year estimates of seismic deformation with the available historical moment and magnitude estimates for the five regions and find that moderate and large earthquakes account for between 25 and 70 of the deformation of the seismogenic layer

geochemistry of ttg and ttg like gneisses from lushan taihua complex in the southern north china craton implications for late archean crustal accretion the late archean taihua complex mainly composed of amphibolite and ttg and ttg like gneisses is volumetrically most important metamorphic rock suites scattered along the southern margin of the north china craton ncc zircon shrimp u pb dating shows two episodes 2765 13 and 2723 9 ma of archean magmatism in the lushan area with distinct geochemical features the early 2765 13 ma suite ttg like gneisses has low sio2 52 5 66 1 wt high mg 0 47 0 68 low hree ybn 3 0 5 4 and y 8 07 13 9 ppm with low to moderate la yb n 6 7 37 1 and sr y 25 9 119 3 the younger 2723 9 ma suite ttg gneisses has high sio2 63 5 74 3 wt low mg 0 13 0 52 very low ree ybn 1 1 20 2 43 both suites show pronounced negative nb ta and ti anomalies on the primitive mantle normalized spidergram the ttg like gneiss suite has similar bulk rock nd and zircon hf model ages 3 0 ga with end t 0 0 26 1 46 and is interpreted as resulting from melt of mantle interactions with the melts derived from partial melting of subducted ocean crust with a residual assemblage of garnet clinopyroxene rutile amphibole which favors subducted slab model for the late archean ttg the ttg gneiss suite has abundant relic zircons 2 95 2 80 ga with end t 0 1 31 to 0 23 which is best interpreted as derived from partial melting of thickened lower continental crust with a garnet amphibolite residue garnet amphibole rutile significant high pressure fractional crystallization garnet amphibole and accumulation plagioclase are also required in the petrogenesis the lushan amphibolite with nearly flat primitive mantle normalized trace element pattern is interpreted to represent early ocean crust metamorphism these observations suggest a possible model of late archean crustal accretion from ocean crust to continental terrain in the southern north china craton

young rift kinematics in the tadjoura rift western gulf of aden republic of djibouti the tadjoura rift forms the westernmost edge of the westerly propagating sheba ridge between arabia a nd somalia as it enters into the afar depression fro m structural and remote sensing dataset the tadjou ra rift is interpreted as an asymmetrical south facing half gr aben about 40 km wide dominated by a large boundary fault zone to the north it is partially filled up by the 1 3 ma old gulf basalts which onlapped the older s omali basalts along its shallower southern flexural margin the m ajor and trace element analysis of 78 young onshore lavas allows us to distinguish and map four distinct basa ltic types namely the gulf somali goumarre and h ayyabley basalts these results together with radiometric a ge data lead us to propose a revised volcano strat igraphic sketch of the two exposed tadjoura rift margins an d to discriminate and date several distinct fault n etworks of this oblique rift morphological and statistical an alyses of onshore extensional fault populations sho w marked changes in structural styles along strike in a dir ection parallel to the rift axis these major fault disturbances are assigned to the arrest of axial fault tip propagati on against pre existing discontinuities in the ns o riented arta transverse zone according to our model the sinist ral jump of rifting into the asal ghoubbet rift seg ment results from structural inheritance in contrast with the en echelon or transform mechanism of propagation that prevail ed along the entire length of the gulf of aden extensi onal system

fault spacing in the el teniente mine central chile evidence for nonfractal fault geometry we utilize the excellent rock exposure provided by the network of mining tunnels in el teniente mine central chile to test the applicability of fractal fault spacing laws to a 2 km 3 km 1 3 km body of crystalline rock in an andean shear zone fractal spacing laws can be particularly useful because they allow prediction of fault spacing at unsampled scales because of the restrictions of tunnel geometry in the mine we use only scanline one dimensional analyses the large number of parallel tunnels allows us to assess the consistency of one dimensional scanline samples over a two dimensional area we use two methods 1 an interval counting technique and 2 standard cumulative statistics to test the hypothesis of fractal fault geometry in the shear zone the interval counting method produces deceptively good fits to a fractal distribution in fact by analyzing the residuals and differential slopes of interval counting and cumulative statistical plots and by comparison with synthetic fractal negative exponential and log normal distribution data we conclude that a negative exponential distribution best describes fault spatial distribution at the mine we present three possible explanations for the negative exponential spacing distribution 1 the rock body is at the earliest stage in a deformational path that will produce varying spacing distributions as deformation proceeds 2 the spacing distribution is inherited through joint reactivation and 3 the present spacing distribution is the result of a superposition of two spacing distributions each of which is not necessarily negative exponential additionally we attempt to use the deviation of mean spacings in parallel scanlines after wu and pollard 1995 to further classify deformational maturity we show that this method cannot be used to compare population development in faults which follow a negative exponential spacing distribution because of the negative exponential s equivalence of mean and standard deviation instead we use a relative measure of parallel scanline deviation to compare regions the distinctly smaller standard deviation of mean spacings in one region the t3 isla region may be a signature of increased local finite strain accumulation

iron isotopic fractionation during continental weathering abstract the effect of continental weathering on the iron isotope compositions of natural materials is investigated unweathered igneous rocks pelagic clay and dust fall within the range 56 fe 0 0 3 rivers with large suspended loads also have 56 fe values near zero dilute streams have 56 fe values that trend towards lower 56 fe 1 suggesting that dissolved riverine iron is isotopically light relative to igneous rocks bulk soil and soil leaches display systematically different 56 fe profiles indicating that isotopically distinct fe pools are generated during pedogenesis nannofossil ooze which contains fe scavenged from the ocean water column has 56 fe 0 but is consistent with seawater dissolved fe having negative 56 fe it is inferred that continental weathering under modern oxidizing earth surface conditions preferentially releases dissolved fe with negative 56 fe which is transported in rivers to the ocean a preliminary analysis of the marine fe budget suggests that riverine fe has a substantial role in determining the 56 fe of both the modern and ancient oceans but other inputs particularly that from diagenesis of marine sediments may also be important since the chemical pathways of fe processing during weathering are dependent on oxidation state and biological activity fe isotopes may prove useful for detecting changes in these parameters in the geologic past

the effect of water on the viscosity of a haplogranitic melt under p t x conditions relevant to silicic volcanism the viscosities of hydrous haplogranitic melts synthesized by hydrothermal fusion at 2 kbar pressure and 800 to 1040 c have been measured at temperatures just above the glass transition and at a pressure of 1 bar using micropenetration techniques the micropenetration viscometry has been performed in the viscosity range of 109 pa s to 1012 pa s the samples ranged in water content from 0 4 to 3 5 wt for samples with up to 2 5 wt h2o the water contents have been determined using infrared spectroscopy obtained before and after each viscometry experiment to be constant over the duration of the measurements above this water content a measurable loss of water occurs during the viscometry the viscosity data illustrate an extremely nonlinear decrease in viscosity with added water the viscosity drops drastically with the addition of 0 5 wt of water and then shallows out at water contents of 2 wt an additional viscosity datum point obtained from the analysis of fluid inclusions in a water saturated hpg8 confirms a near invariance of the viscosity with the addition of water between 2 and 6 wt these measurements may be compared directly with the data of hess et al 1995 in press for the effects of excess alkali and alkaline earth oxides on the viscosity of hpg8 also obtained at 1 bar the viscosity of the melts compared on an equivalent molar basis increases in the order h2o li2o na2o k2o rb2o cs2o bao sro cao mgo beo the extraordinary decrease in melt viscosity with added water is poorly reproduced by the calculation scheme of shaw 1972 for the range of water contents investigated here the speciation of water in the quenched glasses can be used to quantify the dependence of the viscosity on hydroxyl content considering only the hydroxyl groups as active fluidizers in the hydrous melts the nonlinearity of the viscosity decrease and the difference with the effects of the alkali oxides becomes larger consequences for degassing calcalkaline rhyolite are discussed

mesoarchean to paleoproterozoic crustal evolution of the taihua complex in the southern north china craton abstract the taihua complex is exposed mostly in the central western part along the southern margin of the north china craton ncc in this study field geological geochemical la icp ms zircon u pb age dating and lu hf isotopic analysis are applied to the orthorgneisses felsic leucosomes and granites of the taihua complex in order to provide constraints on its crustal evolution from the mesoarchean to paleoproterozoic ca 2 9 1 8 ga the ca 2 90 2 73 ga magmatism was an important episode of contemporaneous crustal growth and crustal reworking among them the ca 2 90 ga granodioritic enclaves have features similar to those of the archean sanukitoids and were generated by the direct partial melting of peridotitic mantle wedge in the subduction processes representing the early continental nucleus the ca 2 84 ga high mg ttgs were derived from the partial melting of subducted oceanic crust after interaction with mantle wedge whereas the ca 2 82 2 77 ga low mg ttgs were originated from the partial melting of underplated basaltic crust pointing to a series of continuous arc magmatic activities the formation of the ca 2 75 2 73 ga crust derived granites and felsic leucosomes in association with the contemporaneous metamorphism indicate the presence of a crustal anatexis which probably was the result of an important magmatic underplating event the magmatism between 2 9 ga and 2 73 ga indicates that the increased crust mantle interaction and crustal differentiation stimulated the crust in the taihua complex to become more matural the ca 2 57 2 43 ga magmatism represents another prominent magmatism in the taihua complex the ttg gneisses and dioritic gneisses aged at ca 2 57 2 50 ga were derived from partial melting of the hydrated basalts under eclogite to garnet amphibolite facies conditions with interaction between initial melts and mantle wedge indicating that incessant subduction led to the amalgamation of micro blocks the intrusion of granitic rocks felsic leucosomes ttg like gneisses accompanied by metamorphism during ca 2 50 2 43 ga indicate that a dominant crustal anatexis event occurred in association with the cratonic stabilization processes the ca 2 36 2 24 ga magmatism was also characterized by reworking of the basement rocks which are represented by the intrusion of calc alkaline monzogneisses a type granitic gneisses and mafic dykes this scenario was coeval with the formation of volcanic sedimentary sequences indicating the presence of an extension event in the taihua complex the ca 1 96 1 85 ga tectono thermal event represented the last phase of compressional deformation and consequent metamorphism leading to reworking of the pre existed basement rocks this event formed a wnw ese trending metamorphic belt that extends over 1000 km along the southern margin of the ncc and was well concordant with the world wide orogeny associated with the amalgamation of nuna supercontinent subsequently widespread a type granites mafic dykes and volcanic rocks aged of ca 1 84 1 74 ga formed marking the final consolidation of the crystalline basement at the taihua complex since then the southern ncc became stable and entered into the stage of development of platform

evaluation of a relaxation geospeedometer for volcanic glasses abstract the cooling of silicate melts influences the character of volcanic processes primarily through the effect of temperature on melt viscosity but also through the derivative thermal properties expansivity and heat capacity if the cooling rates of volcanic facies can be established then the undoubtedly complex processes of erupting cooling and degassing magmas can be more readily understood the frozen structure of every glass volcanic or synthetic is influenced by the previous cooling history moreover the relaxation of the glass structure back to equilibrium during reheating across the glass transition is also dependent on the quenched in structure and thus can be used to determine the cooling rate history in this study differential scanning calorimetry dsc measurements have been used to determine the enthalpic relaxation of four volcanic glasses each glass has been heated at known heating rates in the calorimeter to temperatures above the glass transition interval the first heating of the glass involves the relaxation of the structure frozen in during the volcanic cooling subsequent controlled heating and cooling in the calorimeter enables the determination of a set of kinetic parameters which can be used to model the heat capacity curves by modeling the transient values of heat capacity across the glass transition interval the relaxation of enthalpy and the volcanic cooling rate across the glass transition interval can be obtained from our study we can demonstrate four different cooling rates for volcanic glasses over a range of three orders of magnitude from 4 c min 1 to 2 c day 1 we propose that the enthalpic relaxation of silicate melts can be applied as a geospeedometer to the study of volcanic rocks from a variety of occurrences

deformation within foreland thrust sheets by populations of minor faults the dominant deformation mechanism within foreland thrust sheets is the displacement of blocks of weakly deformed rock on discrete mineral coated minor faults in highly deformed layers at the bases of three southern appalachian foreland sheets such minor faults are members of families or populations of consistently oriented structures the deformation style is similar in all three sheets rock nearest each thrust was shortened in the direction of sheet transport and thickened by a family of faults oriented at low angles to the thrusts rock further above each thrust experienced only the first of these two deformation stages statistical analyses of fault populations from these three sheets demonstrate that their deformation is quantitatively as well as qualitatively similar deformation due to movement on minor faults is statistically homogeneous at the scale of an outcrop in one of these sheets strain due to minor faults can be measured in that sheet a strain profile across this sheet is similar to strain profiles across continuously deformed sheets from internal portions of mountain belts suggesting that their responses to emplacement are similar

displacement and deformation associated with a lateral thrust termination southern golden gate range southern nevada u s a abstract the golden gate thrust offers an excellent opportunity to study displacement and deformation at a lateral thrust tip slip across this e vergent thrust is uniformly about 2 4 km for the southern 2 km of exposed strike length and dies out to zero in the northern 5 km constant displacement and uniform hangingwall structure along the southern one third of exposed strike length suggest that along strike displacement variation and consequent deformation are localized near the lateral tip structures located at and north of the lateral tip are consistent with transpression and later uplift as slip accumulated on the thrust fold and fault orientations and calcite twinning strain record possible transpression north of the lateral tip e w striking normal faults at the thrust tip record extension that is interpreted to be the result of the uplift and translation of a structurally continuous rigid block located north of the normal faults this block is connected to and was uplifted with the crest of the hangingwall anticline and was separated by the normal faults from thrust related folding to the south the main hangingwall structure is an anticline that in southern exposures trends n s parallel to the thrust but bends westward and opens into a box fold to the north the hangingwall anticline superficially resembles a classic fault propagation fold formed by a migrating ductile bead however we are forced to reject the ductile bead hypothesis for the fold because 1 the geometry of the hangingwall anticline far from the thrust tip cannot have evolved from the geometry at the thrust tip and 2 twinned calcite strain data from along the thrust suggest that strain was homogeneous and coaxial rather than inhomogeneous and progressively non coaxial as predicted for a migrating ductile bead the three dimensional structural and kinematic relations at the lateral tip of the golden gate thrust appear to be the result of deformation around a pinned tip although the reason that the tip was pinned at its present location is unclear we interpret the thrust to have propagated relatively quickly to its ultimate extent with little initial displacement indicating that it was easier to fault the rocks than to fold them

alternative model of thrust fault propagation a widely accepted explanation for the geometry of thrust faults is that initial failures occur on deeply buried planes of weak rock and that thrust faults propagate toward the surface along a staircase trajectory we propose an alternative model that applies gretener s beam failure mechanism to a multilayered sequence invoking compatibility conditions which demand that a thrust propagate both upsection and downsection we suggest that ramps form first at shallow levels and are subsequently connected by flat faults this hypothesis also explains the formation of many minor structures associated with thrusts such as backthrusts wedge structures pop ups and duplexes and provides a unified conceptual framework in which to evaluate field observations

stratigraphic changes of ge si ree y and silicon isotopes as insights into the deposition of a mesoarchaean banded iron formation in order to determine the origin of silicon si in banded iron formation bif we have undertaken a multi tracer study combining ree y data ge si ratios and si isotopes d 30 si on stratigraphically resolved layers from a 2 95 ga bif from the pongola supergroup south africa si in both si rich and fe rich layers has a common origin represented by a seawater reservoir strongly influenced by continent derived freshwaters 10 and very limited o 0 1 high t hydrothermal fluids as indicated by eu anomalies and y ho ratios the coevolution of d 30 si signatures of si and fe rich

petrology and geochemistry of the 2 9ga itilliarsuk banded iron formation and associated supracrustal rocks west greenland source characteristics and depositional environment abstract here we present new field petrographic and geochemical data from the 2 9 ga itilliarsuk banded iron formation bif and associated lithologies within the itilliarsuk supracrustal belt south eastern nussuuaq west greenland the supracrustals represent a volcanic sedimentary sequence which rests unconformably on a basement of tonalite trondhjemite granodiorite ttg lithologies felsic metagreywackes meta semipelites and thinly bedded ferruginous shales were identified intercalated with the itilliarsuk bif other associated rocks include metapelites acidic metavolcanics and metagabbroic sills the supracrustals have experienced amphibolite facies metamorphism which has resulted in complete resetting of the u pb system with an apparent age of 1895 48 ma this tectono metamorphic event corresponds well with the paleoproterozoic rinkian orogeny known from this region the itilliarsuk oxide facies bif has been divided into two segments on the basis of major and trace elements chemistry a shaley bif with a strong clastic component and a more chemically pure bif the shaley bif contains high terrigenous influx as reflected by elevated al 2 o 3 up to 12 wt tio 2 high field strength elements hfse and transition metals the chemically pure bif is characterised by alternating high iron 68 wt and high silica 64 wt bands with low total rare earths and yttrium rey al 2 o 3 tio 2 and hfse contents suggesting a low detrital component the least altered bands of the bif record diagnostic archaean seawater features with post archaean average shale paas normalised positive la and eu anomalies enrichment in heavy rare earth elements hree relative to light rare earth elements lree pr yb paas t dm model ages of 3 23 2 85 ga and initial nd nd i values in the range of 0 43 to 4 05 average of 1 35 in contrast the chemically pure fe rich bands yield t dm model ages of 3 61 3 22 ga and nd i values from 2 87 to 0 09 average of 1 29 the associated supracrustal rocks in the study area have significantly higher positive nd i values the 143 nd 144 nd in the itilliarsuk bif therefore contrasts world bifs by exhibiting radiogenic positive nd i values in shallow seawaters where the rey s were controlled by a local depleted continental crust whereas the negative nd i values found in the iron rich layers suggest that the submarine hydrothermal source was influenced by an enriched mantle possibly an older subcontinental lithospheric segment the felsic metagreywackes are immature first cycle sio 2 al 2 o 3 4 4 la yb 1 chon metasediments with affinities to ttg suites primarily extrusives whereas the meta semipelites and metapelites contain a larger mafic contribution with higher content of fe 2 o 3 mgo cr ni and hrees this suggests that the bif was deposited in a highly unstable basin presumably in a palaeo continental slope or outer shelf environment with frequent fluctuations of epiclastic and volcanogenic sediments derived from adjacent bimodal sources the t dm model ages and the use of th sc zr and la th sc tectonic discrimination plots indicate that the metasediments were sourced from a juvenile ocean island arc setting

small earthquakes tectonic forces earthquake scaling and frequency of occurrence relations require that small earthquakes be just as important as larger ones in redistributing the forces that drive relative displacements across active faults of any dimension including plate boundaries

archean crustal evolution in the southeastern north china craton new data from the huoqiu complex abstract the southeastern margin of the north china craton ncc is characterized by zircon ages of dominantly 2 6 ga and differs significantly from most other regions in the ncc with mostly 2 5 ga zircons in order to better understand the precambrian crustal evolution of the ncc we conducted a study of in situ zircon u pb dating trace element and hf isotope analyses as well as whole rock geochemistry on a comprehensive set of core samples from three drilling holes intersecting the basement rocks of the huoqiu area at the southeastern margin of the ncc our data reveal three magmatic events at 3 02 ga 2 77 ga and 2 71 ga in addition seven analyses of possibly inherited zircon cores plot along a discordant line with an upper intercept age of 3262 35 ma indicating possible existence of a paleoarchean crystalline basement the 3 02 ga zircons have positive hf t values close to the depleted mantle growth curve and their u pb ages are only slightly younger than the tdm1 model ages hence recording a mesoarchean crust forming event the 2 77 ga zircons also register a period of crustal growth but probably with some involvement of older crust the 2 71 ga zircons have lower hf t values and older tdm1 model ages than the 2 77 ga zircons suggesting that the former were mainly derived from older continental crust in contrast to the traditional idea that 2 7 ga was a major period of crustal growth in the ncc

scaling properties within the gulf of corinth greece comparison between offshore and onshore active faults abstract the gulf of corinth greece is a 110 km long by 30 km wide active graben displaying strong seismicity hosted both on north and south dipping normal faults this complex fault pattern consists of two fault populations offshore and onshore the offshore fault population is investigated by densely arranged seismic reflection profiles during the last 20 years whereas the onshore fault population displays spectacular and well exposed faults delineated by high accuracy mapping we analyzed fault length and throw in order to study the scaling properties of 136 well determined offshore and onshore faults and the comparison between the two datasets we examined the statistical properties on both fault populations in order to describe the role of segmentation in the growth of faults and the different stages of the evolution of the fault networks our results on power law relationships associated with the scaling properties of the fault zones in the gulf of corinth suggest that both fault populations are bi fractal providing the initiation of a sature state in deformation in addition the vertical throw of faults shows that both fault populations have similar properties but different distributions below and above 5 km respectively displacement length ratios show that faults larger than 9 km appear to accumulate throw without any dramatic change to their length these observations combined with other geophysical studies within the gulf suggest that the characteristic fault lengths of 5 km and 9 km can be correlated to the crustal mechanical structure and the seismicity of the gulf

revealing hidden structures the application of cathodoluminescence and back scattered electron imaging to dating zircons from lower crustal xenoliths abstract the use of cathodoluminescence cl and or back scattered electron bse imaging techniques with in situ ion probe analyses of zircons can help unravel complex crustal histories of metamorphic rocks that otherwise might remain elusive using these techniques we have imaged zircons from three lower crustal xenolith suites that have previously been dated by shrimp sensitive high resolution ion microprobe in all three cases the zircons are featureless in transmitted light but cl and bse reveal internal structures that correlate with distinct growth events generally cl and bse images reveal similar structures with cl showing finer detail neither imaging technique is capable of delineating all growth features in every sample and the best results are obtained using a combination of the two techniques igneous cores in zircons commonly emit a different color cl emission in many cases zoning in the cores is truncated indicating that the zircons either spent time in the supracrustal environment after their initial crystallization and prior to the granulite facies event s or that part of the core zircon was resorbed during the subsequent metamorphic event metamorphic rims when present are commonly 10 to 30 m thick and are nearly always unzoned and featureless igneous cores and metamorphic overgrowths commonly have distinctive cl emission spectra and trace element concentrations however the cl spectra can only be used to qualify chemical differences as a linear relationship has not been shown to exist between cl intensity and trace element concentration in natural zircons in many cases the hf y p u and the heavy rare earth elements hrees concentrations can be correlated to igneous and metamorphic growth using a combination of cl and bse imaging techniques and in situ trace element analyses with either the electron microprobe or pixe particle induced x ray emission

mineralogical and microfabric characteristics of magnetite in the wuyang precambrian bifs southern north china craton implications for genesis and depositional processes of the associated bifs abstract precambrian banded iron formations bifs are widely distributed in the north china craton ncc among them the wuyang bifs located in the southern margin of ncc occur in the late archaean tieshanmiao formation and can be subdivided in two different sub types i quartz magnetite bifs qmb consisting of magnetite fine microcrystalline quartz and minor calcite and ii pyroxene magnetite bifs pmb composed of pyroxene fine microcrystalline quartz and subordinate feldspars both sub types display apparent discrepancies in terms of petrography and mineral composition as shown in electron backscattered diffraction ebsd images and micrographs magnetite grains from the qmb range in size from tens up to hundreds of m whereas magnetite crystals from the pmb can be up to a few tens of m across the x ray diffraction xrd structural data indicate that magnetite from both bif sub types is equiaxed cubic and was generated by sedimentary metamorphic processes the cell parameters of magnetite in the qmb are a b c 8 396 a and z 8 which deviate slightly from these of magnetite in the pmb a b c 8 394 a and z 8 the analytical results of raman spectroscopy analysis revealed micro structural signatures of both magnetite raman shifts near 552 cm 1 and 673 cm 1 and hematite raman shifts near 227 cm 1 295 cm 1 and 413 cm 1 in magnetite from both qmb and pmb the crystallinity degree is similar for magnetite micro structures but varies significantly for hematite micro structures oxygen fugacity fo2 conditions fluctuated during the recrystallization of magnetite in the qmb whereas no evident variation of fo2 occurred during the formation of magnetite in the pmb analytical results of laser ablation inductively coupled plasma mass spectrometry la icp ms show that the si al and mg abundances are higher in magnetite from the qmb whereas the ti and mn contents are more elevated in magnetite from the pmb magnetite composition also denotes that both bif sub types are sedimentary metamorphic origin whereas the deposition of pmb was also affected by volcanic activities overall data indicate that the differences in the depositional environment of each bif sub type are due to the involvement of volcanic eruption processes in the genesis of the pmb thus this paper indicated that the qmb was deposited by chemical deposition at the long term interval of volcanic eruptions and the pmb were the product of chemical deposition affected by the volcanic eruption

changes in fault displacement populations correlated to linkage between faults in carboniferous strata exposed at the western margin of the southern appalachian u s a fold thrust belt mesoscopic faults accommodated strains as the cumberland plateau thrust sheet moved 1 km wnw on a sub horizontal detachment these faults are confined to a layer 75 m thick above the detachment in the upper part of this 75 m thick layer a laterally persistent coal seam separates two stratigraphic units subjected to different amounts of sub horizontal elongation distinct sets of mesoscopic faults that cut bedding at high angles and have normal offsets accommodate the strains in each unit below the coal seam a sample line 500 m long intersects 504 faults a plot of log n size rank vs log d dip separation in mm for these faults is linear suggesting that a scaling law n ad c holds for this population least squares regression gives c 0 8 and log10 a 3 1 above the coal seam a sample line 250 m long intersects 745 faults a plot of log n vs log d for these faults has two distinct linear trends faults with separations 1 25 m yield a scaling law n ad c with c 0 5 and log10 a 2 9 seventeen faults with separations d 1 25 m define a second linear segment n a d c with c 1 8 and log10 a 6 9 the break in slope at d 1 25 m corresponds to faults with heights of 30 m roughly equal to the thickness of the faulted strata faults here with d 1 25 m and d 1 25 m which exhibit different scaling characteristics are respectively analogous to faults within and those that extend across the brittle crust in regional studies comparing the two displacement populations described here small offset faults make a proportionally smaller contribution to the total strain in the larger strain setting this pattern which is observed elsewhere suggests that fault systems within individual layers exhibit different scaling characteristics at different stages in their history this contribution argues that fault linkage is the key to such changes in fault scaling behavior it is a mesoscopic to macroscopic structural change that alters the subsequent development of faults fault linkage like fault size or sampling dimension should be considered in examinations of fault scaling laws

critical displacement for normal fault nucleation from en echelon vein arrays in limestones a case study from the southern apennines italy abstract the process of fault initiation by the coalescence of en echelon arrays of tensile cracks has been discussed by several workers and is now well established however not many studies have tried to quantitatively describe this process in terms of displacement and shear strain that control the early stages of fault nucleation in this study a large number of conjugate en echelon vein arrays showing extensional offsets have been analysed these structures are exposed in well bedded micritic limestones deformed at very low grade conditions in the apennine mountain belt of southern italy three different types of structures can be distinguished i brittle ductile shear zones characterised by arrays of en echelon calcite filled tension gashes ii shear zones showing incipient development of discontinuous shear parallel fractures cutting through the vein array and iii faulted shear zones in which a continuous discrete normal fault breaks through and clearly developed from an original en echelon vein array critical values of displacement and shear strain have been determined for the onset of normal fault nucleation from brittle ductile shear zones therefore suggesting that these parameters exert a major control on fault initiation a diagram of shear zone population vs displacement also shows that this parameter obeys a power law of the type generally followed by normal faults in the frictional regime our results suggest that during the early stages of fault nucleation displacement and shear strain control the switch from dominant ductile viscous deformation mechanisms accompanied by brittle fracturing responsible for vein formation to the frictional regime

faulted joints kinematics displacement length scaling relations and criteria for their identification structural geometries and kinematics based on two sets of joints pinnate joints and fault striations reveal that some mesoscale faults at split mountain utah originated as joints unlike many other types of faults displacements d across faulted joints do not scale with lengths l and therefore do not adhere to published fault scaling laws rather fault size corresponds initially to original joint length which in turn is controlled by bed thickness for bed confined joints although faulted joints will grow in length with increasing slip the total change in length is negligible compared to the original length leading to an independence of d from l during early stages of joint reactivation therefore attempts to predict fault length gouge thickness or hydrologic properties based solely upon d lscaling laws could yield misleading results for faulted joints pinnate joints distinguishable from wing cracks developed within the dilational quadrants along faulted joints and help to constrain the kinematics of joint reactivation q 2001 elsevier science ltd all rights reserved

fractal analysis of a quaternary fault array in the central apennines italy abstract the monte vettore fault array in the central apennines fold and thrust belt is part of a major quaternary fault system displaying the main features of a regional negative flower structure associated with a roughly n s trending left lateral strike slip fault zone fractal analysis of the fault trace pattern of the area has shown that the box counting curve derived for the monte vettore array is defined by two straight line segments each showing a characteristic slope value the first segment which refers to a spatial distribution extending over about one order of magnitude is characterized by a fractal dimension of about 1 6 whereas the second one fits a euclidean i e a non fractal distribution the box counting analysis emphasizes that faults with variable sizes exhibit different scaling relations hence suggesting that fault linkage played a major role in the development and evolution of the monte vettore fault array the split point between the two segments falls at a value of log l s corresponding to a box size of 635 m hence suggesting that self similarity among faults within the array breaks down below this value fractal analysis of the monte vettore array also provides statistically based segmentation criteria for analysing a young immature fault structure capable of generating medium size earthquakes in a continental geotectonic setting

geochemistry u pb zircon geochronology and sm nd isotopes of the xincai banded iron formation in the southern margin of the north china craton implications on neoarchean seawater compositions and solute sources abstract banded iron formations bifs that are distal from submarine volcanic activity and generally lack direct relationships with volcanic rocks have been extensively used as proxies to infer the chemical composition of ancient seawater and trace the effects of continental weathering at the time of their deposition here we discuss the major and trace elements and sm nd isotopic geochemistry of the poorly studied xincai bif located west of the superior type huoqiu bif along a nw se trending supracrustal belt in the southern margin of the north china craton to understand the compositions and solute sources of seawater during the xincai bif precipitation u pb zircon dating of granitic intrusive rock and biotite schist interlayer of the bif constrains the depositional age of the xincai bif to 2 5 2 7 ga for the first time indicating that the xincai bif could be part of the huoqiu bif or consist of contemporaneous sediments formed in a similar tectonic depositional environment shale normalized rare earth element and yttrium rey patterns of the bulk and individual band samples display features that are characteristic of other archean bifs with light ree depletion relative to heavy ree positive lasn gdsn and ysn anomalies indicative of pure chemical sedimentation the marine origin of these samples is further supported by their super chondritic y ho ratios average y ho 47 the very low al ti and high field strength element contents of the xincai bif indicate an essentially detritus free component the bulk bif samples show a narrow range of end t values from 1 5 to 1 0 and most of the samples have negative end t values 0 3 on average which is different from archean bifs with mantle derived depleted nd isotopes deviations toward negative end t values of the iron bands 0 5 to 0 1 and the silica bands 4 7 to 0 5 further indicate a continental nd contribution in addition positive eusn anomalies that were found in all of the bulk and individual band samples indicate that a high t hydrothermal input also contributed to their rey signatures it is considered that both high t hydrothermal fluids and ambient seawater with solutes from erosion and weathering of a nearby landmass can provide their rey signatures

geometry segmentation pattern and displacement variations along a major apennine thrust zone central italy abstract hundreds of kilometres long arc shaped continuous thrust faults commonly imply very large displacements by detachment dominated thrusting therefore their occurrence has a large impact on fold thrust belt structure one of the major thrust faults of the central northern apennines the umbria marche sabina thrust zone is traditionally believed to represent one such feature characterised by displacements of several tens of kilometres however detailed studies of this structure revealed that it is actually composed of a series of partially overlapping fault segments rather than consisting of a single continuous thrust cross section balancing and restoration carried out by the integration of surface geological data with available sub surface information points out relatively limited amounts of thrust displacement

petrogenesis of the archean tonalite trondhjemite granodiorite ttg and granites in the lushan area southern margin of the north china craton implications for crustal accretion and transformation abstract tonalite trondhjemite granodiorite ttg assemblages and abundant high k calc alkaline magmatism are considered to be closely related to craton stabilization and usually record a tectonic transition three episodes of granitic magmatism are identified in the lushan area at the southern margin of the north china craton ncc on the basis of new la icp ms zircon u pb ages and whole rock geochemical data they are ttg gneisses sodic potassic granite spg suite and potassic granite dated at 2840 ma 2760 ma and 2570 ma respectively the ttg gneisses have high sio 2 variable mg cr and ni contents with high la yb n and sr y ratios suggesting that the parental magmas were derived by partial melting of subducted oceanic crust they show no obvious correlations between sr y and dy yb or la yb n and dy yb and all have negative nb and ta but positive zr and hf anomalies and variable dy yb nb ta zr hf and zr sm ratios suggesting melting of a deep source 45 km with rutile eclogite and garnet amphibolite in the residue the ttg samples have positive hf t values 2 6 to 7 0 with the youngest t dm c 2896 ma close to the crystallization age indicating that they were produced by partial melting of juvenile crustal materials in an arc setting rocks of the spg suite have high sio 2 contents variable na 2 o k 2 o ratios 0 41 10 91 and low y and yb contents with high sr y ratios indicating an affinity to i type granites they display depleted and concave upward ree patterns between middle and heavy rees and higher zr sm than ttg gneisses possibly resulting from amphibole removal by fractional crystallization or as a residual phase during melting meanwhile the rocks have a large variation of dy yb without obvious correlations between sr y and dy yb and la yb n and dy yb in combination with variable but uniformly low y contents therefore we suggest that the spg samples were derived from partial melting with garnet in the residue magmatic zircons have hf t values from 4 3 to 4 2 with t dm c ages from 3200 ma to 2877 ma indicative of older mafic lower crustal sources the rocks formed within a short time interval with the ttg and their emplacement was accompanied by regional granulite and amphibolite metamorphism implying that they may also have formed in an arc setting the potassic granite has high sio 2 74 79 wt and k 2 o na 2 o 9 22 wt contents with a low 10 000ga al 1 89 ratio and t zr 670 c magmatic zircons have a narrow range of hf t values 2 0 to 4 1 with t dm c ages 2942 2692 ma older than the formation age but consistent with the formation age ca 2760 ma of the lushan spg suite the potassic granite has a hf isotope composition identical to that of the spg suite and is enriched in lree and depleted in y with a positive eu anomaly suggesting that the spg suite is the main source one zircon yielded an inherited age of 2837 36 ma consistent with the age of the ttg gneisses indicating that the gneisses could also be the source of the potassic granite the two types of source rocks indicate a shallow crust depth hence the heat required for partial melting of granodioritic rocks to form the potassic granite could have been provided by the upwelling of mantle or mantle derived melts in an extensional setting therefore the ttg gneisses spg suite and potassic granite in the lushan area record a tectonic transition from a compressive to an extensional setting in the southern margin of the ncc between 2 85 and 2 50 ga

origin of the disseminated magnetite pyroxenite in the tieshanmiao type iron deposits in the wuyang region of henan province china abstract the tieshanmiao type iron deposits in the southern north china craton comprise two types of ores banded pyroxene magnetite quartzite bmq and disseminated magnetite pyroxenite dmp whether the quartz poor dmp represents metamorphosed iron bearing ultramafic rocks or chemical sedimentary rocks is still unclear pyroxene compositions in the dmp are low in al 2 o 3 and tio 2 which are similar to those from the bmq and altered marble and pyroxenite however the compositions are different from those in the metamorphosed ultramafic rocks the dmp and bmq also show similar major element contents with dominant sio 2 total fe 2 o 3 cao mgo but low contents of al 2 o 3 tio 2 mno na 2 o k 2 o indicating a similar source through submarine chemical precipitation with little input from terrestrial or volcanic materials the bmq dmp and magnetite separates from these rocks exhibit seawater like signatures of ree patterns with lree depletion positive la gd and y anomalies and high y ho ratios indicating that seawater participated in the formation of the iron ores combined with strong positive eu anomalies we infer that the deposition of the bmq and dmp was mainly controlled by the mixing of seawater with hydrothermal fluids the lack of negative ce anomalies of the dmp bmq and magnetite separates indicate an anoxic marine environment the dmp is rich in carbonate but relatively poor in silica and the bmq is rich in silica but poor in carbonate the protoliths of the dmp and bmq in the tieshanmiao type iron deposits are inferred to be quartz carbonate iron bearing formations which underwent subsequent metamorphism

brittle to ductile transition due to large strains along the white rock thrust wind river mountains wyoming abstract the deformed foreland of the rocky mountain cordilleras is characterized by major reverse faults that bring precambrian basement rocks to the surface the white rock thrust in the wind river mountains of wyoming is one such fault that developed at fairly shallow levels in the crust detailed study of grain size variations and textures within the fault zone provide a framework for discussion of the mechanics of movement along the fault textural criteria are suggested for distinguishing between dominantly brittle and dominantly ductile deformation a transition from brittle fracturing to ductile deformation is seen with increasing strain and resultant decrease in grain size along the fault zone diffusional creep in the final stages of deformation leads to strain softening and may allow large strains to take place along narrow zones at low deviatoric stresses this may be one way of obtaining large displacements along major faults at shallow levels in the crust

age and genesis of the neoarchean algoma type banded iron formations from the dengfeng greenstone belt southern north china craton geochronological geochemical and sm nd isotopic constraints abstract banded iron formations bifs are important for understanding the composition and redox condition of paleo oceans as well as the evolution of the precambrian crust the neoarchean dengfeng greenstone belt ca 2 54 2 50 ga in the southern north china craton ncc consists of metamorphosed igneous and sedimentary assemblages including some bifs here we present mineralogical geochronological geochemical and sm nd isotopic data of the dengfeng bifs for the first time to constrain their age origins and source characteristics and then discuss their paleo environmental implications the bifs are mainly associated with metabasaltic rocks and quartz rich schists and can be classified as algoma type bifs zircon u pb dating of quartz rich schists interbedded with the bifs coupled with ages of wall rock metabasites demonstrate that the dengfeng bifs formed at 2 53 2 51 ga most bif samples have high contents of sio2 and fe2o3t and extremely low contents of al2o3 tio2 and high field strength elements e g zr hf th u indicating that they are detrital free deposits in the post archean australia shale normalized multi element diagram the detritus free bif samples are characterized by depleted light rare earth elements positive la and y anomalies and suprachondritic y ho ratios resembling geochemical features of modern seawater the absence of negative ce anomaly indicates an anoxic condition positive eu anomalies and relatively high concentrations of transition metal elements suggest that high temperature hydrothermal fluids contributed to the source of bifs isotopically all bif samples have positive end t values 0 12 to 4 53 consistent with the range of those 0 10 to 4 31 of ca 2 66 2 50 ga dengfeng metabasites and ttg gneisses suggesting input of possible juvenile crustal fluxes the elemental and isotopic characteristics of the dengfeng bifs collectively suggest that they precipitated from anoxic seawater with various input of hydrothermal fluids and oceanic crust derived fluxes

aggregate properties of fracture populations abstract empirical studies indicate that the individual attributes of both faults and extension fractures follow power law scaling aggregate properties of fracture populations are important in a variety of problems and can be specified in terms of the scaling parameters of individual fracture attributes development of an expression for an aggregate property requires consideration of a number of independent factors including the topologic dimension of the aggregate property the topologic dimension of sampling and the possibility of scaling changes for fractures that span some mechanically significant layer the riemann zeta function provides an alternative to integration for the analytical and numerical solution of aggregate problems previous work regarding aggregate properties of fracture populations has focused on the strain due to faulting new expressions are developed here for other aggregate properties of interest fracture surface area fracture porosity fracture permeability and shear wave anisotropy a general characteristic of these aggregate properties is that for most values of scaling exponents the aggregate properties are dependent on the size of the sampling domain this implies that the aggregate properties are scale dependent additionally it appears that fracture surface area is concentrated in the smallest fractures of many populations fracture porosity is concentrated in the largest fractures of most populations but not as strongly as fracture permeability which probably derives almost entirely from the largest fractures in populations

deformation of a foreland carbonate thrust system sawtooth range montana an analysis of mesoscale faulting associated with thrust belt development was completed along a transect crossing several foreland thrust sheets of the central sawtooth range montana at a regional scale a balanced cross section of the central sawtooth range constrained by well seismic and surface data indicates a minimum of 60 horizontal shortening of the upper paleozoic carbonate bank sequence accommodated by a forward developing thrust system subsurface data delineates a 4 westward dipping precambrian basement and westward thickening of the precambrian belt supergroup major decollements within the central sawtooth range are present at the base of the devonian jefferson formation in the lower mississippian allan mountain limestone and in the cretaceous colorado group within the diversion french norwegian and beaver thrust sheets of the central sawtooth range deformation has been partitioned into arrays of meter to decimetersized faults that constitute brittle deformation zones bdzs bdz thickness is directly related to fault displacement but is not related to fault trajectories or subsequent foreland imbrication the ratio of bdz thickness to displacement is nearly constant with bdz thicknesses ranging from 65 m to 200 m for faults whose displacements range from 2 km to 5 km the bdzs of each thrust sheet can be separated into three regions defined by differences in fault diversity fault intensity and fault kinematics proportionally the locations of boundaries between regions within the bdzs are remarkably constant for each of the four thrust sheets studied indicating that the bdzs widen uniformly during thrust sheet emplacement bdz bases are characterized by an interlocking network of faults that exhibit great diversity and accommodate mesoscale cataclastic flow bdz bases are dominated by oblique slip faults whereas the middle and upper regions of bdzs are dominantly dip or strike slip faults in all regions of a bdz localized zones of contractional extensional or more commonly transport parallel strike slip faults bounded by bedding plane detachments are present fault length m2 of outcrop decreases logarithmically to zero from a maximum of 4 m m2 across the french and norwegian bdzs zones of intense faulting alter this general pattern of decreasing fault intensity in the diversion and beaver thrust sheets

metamorphic p t t paths retrieved from the amphibolites lushan terrane henan province and reappraisal of the paleoproterozoic tectonic evolution of the trans north china orogen abstract garnetiferous amphibolites preserving prograde inclusion assemblages m1 within garnet porphyroblasts metamorphic peak assemblages m2 and post peak symplectitic assemblages m3 have been found in the lushan terrane henan province taihua metamorphic complex southernmost section of the ns striking palaeoproterozoic trans north china orogen tnco geothermobarometric computation shows that the m1 m2 and m3 assemblages were formed under the p t conditions of 650 730 c 4 9 8 1 kbar 740 810 c 9 10 8 kbar and 750 760 c 5 0 7 7 kbar respectively suggesting that these amphibolites record clockwise p t paths including nearly isothermal decompression itd segments similar to the west alpine type p t paths indicative of being resulted from an orogenic process high resolution sims u pb dating of metamorphic zircons suggests that the metamorphism took place at ca 1 95 1 75 ga these data imply that the palaeoproterozoic collision between the eastern and western blocks of the north china craton along the southernmost terminal of the tnco not only started earlier but also continued longer than the middle and north sections of the tnco

geochronology and geochemistry of 2 5 to 2 4 ga granitic plutons from the southern margin of the north china craton implications for a tectonic transition from arc to post collisional setting abstract the xutai and lujiagou plutons exposed in the songshan jishan area in the southern margin of the north china craton ncc are studied to constrain the tectonic regime of the ncc between 2 5 and 2 4 ga the xutai pluton dated at 2509 33 ma mainly consists of tonalite and monzogranite with a calc alkaline affinity rittmann index

the partitioning of trace elements between ilmenite ulvospinel armalcolite and silicate melts with implications for the early differentiation of the moon mineral melt trace element partition coefficients for a large number of trace elements zr hf ta nb v co cu zn sr ree sb sn mo cr w u th were determined experimentally for ilmenite fetio3 ulvospinel fe2tio4 minerals of the armalcolite solid solution fe mg ti2o5 the measured partition coefficients indicate that the high field strength elements zr hf nb and ta and some transition metals v cr co sn are moderately compatible in iron titanium oxides whereas other elements ree y sr u th mo sb and w are strongly incompatible the partition coefficients for u are always significantly higher than the th partition coefficients which makes fe ti oxides efficient in fractionating u from th our data also indicate that ilmenite and other iron titanium oxides are able to fractionate w from hf the latter of which is much more compatible this makes the iron titanium oxides very effective agents to influence hf w isotopic ratios during the evolution of the moon in addition we also present some trace element partition coefficients for iron and titanium bearing orthopyroxene

biologically recycled continental iron is a major component in banded iron formations banded iron formations bifs record a time of extensive fe deposition in the precambrian oceans but the sources and pathways for metals in bifs remain controversial here we present fe and nd isotope data that indicate two sources of fe for the large bif units deposited 2 5 billion y ago high end and 56fe signatures in some bif samples record a hydrothermal component but correlated decreases in end and 56fe values reflect contributions from a continental component the continental fe source is best explained by fe mobilization on the continental margin by microbial dissimilatory iron reduction dir and confirms for the first time to our knowledge a microbially driven fe shuttle for the largest bifs on earth detailed sampling at various scales shows that the proportions of hydrothermal and continental fe sources were invariant over periods of 100 103 y indicating that there was no seasonal control although fe sources varied on longer timescales of 105 106 y suggesting a control by marine basin circulation these results show that fe sources and pathways for bifs reflect the interplay between abiologic hydrothermal and biologic processes where the latter reflects dir that operated on a basin wide scale in the archean

estimates of strain due to brittle faulting sampling of fault populations abstract the geometry of sampling domains is a first order consideration in the characterization of brittle fault populations in most cases descriptions of fault size distributions based on map cross section traverse or borehole data systematically underestimate the number of small faults present in a volume the geometry of sampling domains may be accounted for using an empirical proportionality between fault displacement and trace length estimates of strain in which the sampling geometry is considered suggest that small faults accommodate a significant portion of the total strain due to the brittle faulting process

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

biogeochemical cycling of iron in the archean paleoproterozoic earth constraints from iron isotope variations in sedimentary rocks from the kaapvaal and pilbara cratons abstract iron isotope compositions of low metamorphic grade samples of archean paleoproterozoic sedimentary rocks obtained from fresh drill core from the kaapvaal craton in south africa and from the pilbara craton in australia vary by 3 in 56 fe 54 fe ratios reflecting a variety of weathering and diagenetic processes depositional ages for the 120 samples studied range from 3 3 to 2 2 ga and fe c and s contents define several compositional groups including samples rich in fe organic carbon carbonate and sulfide the 56 fe values for low c org low c carb and low s sedimentary rocks are close to 0 the average of igneous rocks this range is essentially the same as that of c org poor late cenozoic loess aerosol river loads and marine sediments and those of c org poor phanerozoic proterozoic shales that these 56 fe values are the same as those of igneous rocks suggests that fe has behaved conservatively in bulk sediments during sedimentary transport diagenesis and lithification since the archean these observations indicate that if atmospheric o 2 contents rose dramatically between 2 4 and 2 2 ga as proposed by many workers such a rise did not produce a significant change in the bulk fe budget of the terrestrial sedimentary system if the archean atmosphere was anoxic and fe was lost from bedrock during soil formation any isotopic fractionation between aqueous ferrous fe fe aq 2 and fe bearing minerals must have been negligible in contrast if the archean atmosphere was oxic fe would have been retained as fe 3 hydroxides during weathering as it is today which would produce minimal net isotopic fractionation in bulk detrital sediments siderite rich samples have 56 fe values of 0 5 0 5 and experimentally determined fe aq 2 siderite fractionation factors suggest that these rocks formed from fe aq 2 that had similar or slightly higher 56 fe values the 56 fe values calculated for fe aq 2 overlaps those of modern submarine hydrothermal fluids but it is also possible that fe aq 2 had 56 fe values higher than those of modern hydrothermal fluids depending upon the fe aq 2 fe carbonate fractionation factor that is used in contrast c org rich samples and magnetite rich samples have strongly negative 56 fe values generally between 2 3 and 1 0 and available fluid mineral fractionation factors suggest that the fe bearing minerals siderite and magnetite in these rocks formed in the presence of fe aq 2 that had very low 56 fe values between 3 and 1 reduction of fe 3 hydroxide by sulfide precipitation of sulfide minerals or incongruent dissolution of silicate minerals are considered unlikely means to produce significant quantities of low 56 fe fe aq 2 we interpret microbial dissimilatory fe 3 reduction dir as the best explanation for producing such low 56 fe values for fe aq 2 and our results suggest that dir was a significant form of respiration since at least 2 9 ga

petrogenesis of the 2 1 ga lushan garnet bearing quartz monzonite on the southern margin of the north china craton and its tectonic implications abstract the lushan garnet bearing quartz monzonite was emplaced into the western taihua complex at 2134 17 ma on the southern margin of the north china craton ncc the rocks contain sio 2 contents of 56 98 59 05 wt with high k 2 o na 2 o 7 46 9 14 wt and al 2 o 3 15 87 17 58 wt and low cao 2 38 2 77 wt and mgo 1 18 1 50 wt contents and belong to an intermediate shoshonitic series magmatic garnet grains in the rocks are chemically homogeneous with relatively high cao 6 36 7 92 wt but low mno 1 45 1 94 wt and mgo 2 47 3 45 wt contents and high fe mn ratios they contain 61 94 66 39 mol almandine 18 60 23 40 mol grossular 10 06 15 11 mol pyrope and 1 09 4 32 mol spessartine similar to high pressure garnets formed from m type magma but different from those found in s type granites and metapelites phase equilibrium diagrams in combination with trace element distribution coefficients between zircon and garnet suggest whole rock zircon saturation temperatures varying from 891 c to 951 c and the mineral assemblage of garnet plagioclase amphibole or biotite in these rocks is stable at 15 kbar and 890 950 c with h 2 o t 800 c all of the rocks are enriched in lile and lree with low nb la ratios 0 24 0 61 similar to those of lithospheric mantle which has a nb la ratio of 0 5 therefore the garnet and whole rock chemical compositions indicate that the lushan quartz monzonite possibly formed by small degrees of partial melting of a phlogopite bearing lithospheric mantle the variable zircon hf t values 3 5 to 4 1 with t dm c from 2 34 ga to 2 51 ga suggest involvement of crustal materials the rocks have slightly negative nb and ta anomalies characteristics of island arc basalts iab however their high zr 598 926 ppm and zr y ratios 17 1 21 8 suggest an affinity with ocean island basalt oib hence the mixed iab and oib like geochemical signatures are indicative of continental intra plate rifting possibly related to extension and thinning of the lithosphere and upwelling of asthenosphere our study proposes that an intra continental rifting event occurred at 2 2 2 0 ga on the southern margin of the ncc after the final amalgamation of ncc

changes of ge si ree y and smnd isotopes in alternating fe and si rich mesobands reveal source heterogeneity of the 2 54 ga sijiaying banded iron formation in eastern hebei china abstract the north china craton ncc is one of the most important regions hosting abundant banded iron formations bifs the 2 54 ga sijiaying bif the best preserved and most extensive deposit in eastern hebei is intercalated and closely associated with meta volcanic rocks of the luanxian group in this context major and trace element and sm nd isotopic analyses of individual mesobands of a sijiaying bif specimen were conducted to characterize the source and depositional environment over a transient period low al 2 o 3 tio 2 and high field strength elements hfses concentrations show that the bif is relatively detritus free shale normalized rare earth and yttrium distributions ree y of individual bif mesobands exhibit positive la anomalies enrichment in hree relative to lree and mree and suprachondritic y ho ratios which are typical features of marine waters throughout the archean and proterozoic the presence of consistently positive eu anomalies indicates a significant high t hydrothermal input while the absence of true ce anomalies suggests deposition from an anoxic water column by comparison with other typical bifs e g the isua bif from greenland the kuruman bif from south africa the sijiaying bif is depleted in hree and appears to record variations in solute fluxes related to changing intensities of hydrothermal activity these features coupled with sm nd isotopic relations and ge si distributional patterns point to two decoupled sources controlling the depositional environment of the bif and thus reveal source heterogeneity for silica and iron of the sijiaying bif high fe fluxes were associated with seafloor vented hydrothermal fluids which received their sm nd isotopic signature from alteration of depleted oceanic crust whereas significant amounts of silica were associated with ambient seawater whose ree signature was controlled by solutes derived from weathering of nearby mesoarchean continental landmasses the old up to 3 0 ga nd t dm model ages of si rich mesobands of the bif support such a scenario

predicting reservoir scale faults with area balance application to growth stratigraphy a reservoir that appears to lack faults at one scale of resolution or at one sampling density may nevertheless contain faults that are below the resolution of the observations the area depth relationship from a balanced cross section is shown to contain the necessary information for predicting the sub resolution fault heave existing area depth theory is extended to include growth units allowing structural length and thickness changes to be separated from the depositional changes the technique is validated with a numerical model of a growth full graben and a sand box model of a half graben then field tested in the gilbertown graben a growth structure within the regional peripheral fault trend along the northern margin of the gulf of mexico a cross section developed from wells alone is used to infer the abundance of sub resolution faults by the area balance technique a small but significant amount of sub resolution extension is predicted and then confirmed with a highresolution seismic line q 2003 elsevier science ltd all rights reserved

u pb age and hf isotopes of detrital zircons from the southeastern north china craton meso to neoarchean episodic crustal growth in a shifting tectonic regime abstract the north china craton ncc represents one of only a few cratonic nuclei on the globe with a geological history extending back to the eoarchean however extensive ca 2 5 ga crustal reworking has destroyed a considerable portion of the pre existing crustal record hindering the investigation of tectonothermal evolution prior to 2 5 ga the huoqiu complex hqc located at the southeastern margin of the ncc preserves the vestiges of crustal components that survived the ca 2 5 ga tectonothermal events which provide the opportunity to investigate the meso to neoarchean episodic crustal evolution of the ncc here we present results from in situ detrital zircon u pb dating and hf isotope analyses on zircons from three paragneisses in three drill cores that cut through the basement of the hqc in combination with published data the concordant age spectra of the detrital zircons in the paragneisses yield 207 pb 206 pb ages of 2343 3997 ma that cluster into two principal age populations with peaks at 3015 and 2755 ma one zircon grain dated at 3997 8 ma with 98 concordance provides new evidence for 4 0 ga components in the ncc the e hf t values of all zircons range from 5 2 to 6 5 with most of the spots n 31 of 47 showing positive values indicating at least two episodes of juvenile continental crustal growth at 3 01 ga and 2 75 ga the older episode is recorded only in few ancient cratons suggesting limited crustal accretion occurred globally at a time of subdued mantle derived magmatism in contrast the younger episode is coincident with a global rise in magmatic activity in the early neoarchean the geochemical and geochronological data suggest that the 3 01 ga juvenile crust was likely generated in an island arc subduction system whereas the 2 75 ga crustal rocks were probably formed during magmatic underplating and subsequent partial melting of lower crustal mafic rocks consequently a tectonic transition is suggested from a compressive to an extensional setting along the southeastern margin of the ncc between 3 01 and 2 75 ga this sequence of events heralds a shift from a mixture of net crustal growth and crustal reworking during multiple short lived magmatic pulses to fragmentation and dispersal of the early continental nucleus within 260 ma

controls on strain localization in a two dimensional elastoplastic layer insights into size frequency scaling of extensional fault populations 1 we use a two dimensional 2 d finite element code to investigate how mechanical properties and boundary conditions influence progressive strain localization during extension of a 2 d elastoplastic layer the deforming medium is modeled using a strain softening von mises rheology with a gaussian heterogeneity in yield strength distributed randomly in space specifically we examine the effects of changing 1 the thickness of the deforming layer 2 the basal boundary condition 3 the range of strengths in the deforming layer 4 the strength loss on failure and 5 the total accommodated strain discrete zones of plastic shear strain are observed to nucleate and grow progressively during each experiment in order to quantify and thus discriminate between the deformation patterns produced under differing conditions we calculate the size frequency distributions of the shear band populations size is defined as the total plastic strain represented by each discrete structure a wide range of size frequency distributions is observed including both power law and exponential as well as distributions that show breaks in scaling with a transition from power law at small sizes to exponential at large sizes we show that these variations in population statistics are directly reflecting a change in strain localization in space and time caused by changing model parameters furthermore by making an analogy between the model shear bands and tectonic faults we provide insights into key features of fault size frequency distributions that have been measured in continental and oceanic extensional settings and also observed in analogue experiments

episodic negative anomalies of cerium at the depositional onset of the 2 65 ga itabira iron formation quadrilatero ferrifero of minas gerais brazil abstract magnetite bands in sericitic phyllite of the batatal formation mark the transition to the overlying 2 65 ga itabira iron formation or caue itabirite in the quadrilatero ferrifero of minas gerais brazil the magnetite bands have low non crustal th u ratios suprachondritic y ho ratios and sporadic anomalies of ce that are characterised as truly negative the data point to mild oxygenation of sea water at the depositional onset of the itabira iron formation some 200 ma before the great oxidation event

neodymium isotopes in archean seawater and implications for the marine nd cycle in earth s early oceans abstract published neodymium nd isotopic data for archean iron formations if suggest that overall seawater throughout the archean typically displayed 143nd 144nd close to bulk earth values with nd t between 1 5 and 2 5 neodymium isotopic ratios in seawater during deposition of the 3 8 isua greenland if likely displayed positive nd 3 8 ga of 2 5 as suggested by if g an isua reference if that is considered the best archive for early archean seawater seawater 143nd 144nd ratios dominated by radiogenic nd positive nd t seem to have persisted for much of the archean as if from the pietersburg greenstone belt south africa suggest seawater nd 2 95 ga 1 however similarly aged 2 9 ga ifs from south africa indicate that significant variations in seawater 143nd 144nd occurred and clearly show influences from isotopically distinct crustal sources these variations are apparently related to depositional environment with cratonic margin shallow water ifs possessing a continental nd t of 3 while ifs associated with sub aqueous mafic volcanics display more radiogenic positive nd t values such variation in seawater 143nd 144nd is not possible in an isotopically well mixed ocean and similar to today it appears that marine nd cycling in the archean produced water masses with distinct nd isotopic ratios since the presence of banded iron formations requires a reducing archean ocean capable of transporting fe metal oxide precipitation and scavenging processes near deep sea hydrothermal vent systems would not have scavenged mantle nd i e nd sourced from alteration of oceanic crust we propose that bulk anoxic seawater prior to 2 7 ga possessed relatively constant positive nd t of 1 to 2 whereas local shallow water masses associated with exposed evolved crust could possess distinctly different lower nd t

partitioning of deformation within several external thrust zones of the appalachian orogen mesoscopic contractional and extensional faults dominate many external thrust zones the fault arrays occur in the hanging walls of some thrusts and in the footwalls of others in a thrust zone from the little mountains fold thrust belt of new york state mesoscopic faults cut and offset a major thrust contact and involved rocks from the thrust s hanging wall and its footwall simultaneously the little mountains exposure indicates that minor faults were active during thrust slip and are not early formed features which were passively translated with the remainder of a sheet when both contractional and extensional mesoscopic faults occur in thrust zones foreland dipping extensional faults overprint contractional faults contractional and extensional microfaults with r 1 and r 2 riedel shear geometries are common in external thrust zone cataclasites and resemble the mesoscopic fault patterns though the evolution of cataclasite layers differs from that of mesoscopic fault arrays contraction and ext

fractal geometry in the san andreas fault system it has been noted that the spatial distribution of earthquakes and the mode of strain release in the san andreas fault system is related to the complexity of fault geometry because of their rough appearance over many length scales faults can be regarded as fractal surfaces direct estimates of fractal dimension d of portions of the san andreas fault system between the northern gabilan range and the salton sea including the postulated extent of the great 1857 fort tejon earthquake are obtained from measured fault lengths analogous to the lengths of coastlines as discussed by mandelbrot regions characterized by complicated fault geometry are associated with larger values of d based on fault traces mapped at a scale of 1 750 000 d is 1 3 for this reach of the fault defined as a 30 km wide band about a main fault trace for that part near parkfield which could be associated with the nucleation of the 1857 earthquake d is 1 1 at this same scale d is 1 4 for the san andreas and related faults near san bernardino where the 1857 rupture stopped compared to 1 2 for the san andreas san juan fault segments near the point of arrest of the 1966 parkfield earthquake at finer map scales 1 24 000 and 1 62 500 critical lengths of 500 m and 1 km are identified which might relate to the extent of off san andreas fault offsets the critical lengths also suggest that fault geometry is not self similar if this fractal geometry persists through the seismic cycle it may be possible to use a quantitative measure of complexity to explain the occurrence of great and characteristic earthquakes along a given reach of fault

age and tectonic setting of late archean greenstone gneiss terrain in henan province china as revealed by single grain zircon dating the authors report precise u pb zircon ages for single grains of a metarhyodacite from the late archean dengfeng greenstone belt in henan province china near the southern margin of the north china craton most grains belong to an igneous population whose u pb isotopic systematics define a straight line intersecting concordia at 2512 12 ma and this is interpreted as the time of crystallization of the original greenstone volcanics several grains are distinctly older between 2576 9 and 2945 44 ma and the authors interpret the older grains as xenocrysts of pre greenstone continental crust that provide evidence for crustal derivation or crustal contamination of the original rhyodacitic lava the xenocrysts suggest evolution of the dengfeng greenstone belt in a continental environment that may be represented by the taihua high grade gneisses bordering the dengfeng greenstones and for which the authors obtained ages of 2806 7 and 2841 6 ma the data add evidence to the now widely held concept that most archean greenstones developed on or near older continental crust and were therefore prone to crustal contamination in such cases conventional zircon dating may not always record the precise age of rock formation

combined u pb hafnium and oxygen isotope analysis of zircons from meta igneous rocks in the southern north china craton reveal multiple events in the late mesoarchean early neoarchean abstract we report the results of a comprehensive isotopic investigation of zircons from metamorphosed tonalites and amphibolites from lushan in the far south of the trans north china orogen of the north china craton that reveal the oldest rocks yet dated from the orogen a detailed investigation of the internal structures of zircons combined with targeted sims u pb and oxygen isotope analyses and icp ms hf zircon analyses has resulted in the discovery of previously unknown geological events in the north china craton magmatic zircon from the tonalite and amphibolite was found to be essentially coeval with a small spread of ages from 2829 18 ma to 2832 11 ma in the tonalites and from 2838 35 ma to 2845 23 ma in amphibolite hafnium isotope data suggest derivation of both tonalite and amphibolite from a depleted mantle source with minor crustal contamination oxygen isotope ratios in zircons range from 5 1 to 7 3 vsmow with high 18 o results recorded only from metamorphic domains the earliest stage of metamorphic zircon growth present in all four samples analyzed in this study occurred between 2772 17 22 in the tonalites and 2776 20 and 2792 12 ma in the amphibolites a younger metamorphic event at 2638 61 in the tonalite and between 2671 13 and 2651 13 ma in the amphibolites is recognized in zircons from three of the four samples investigated this is the first time that metamorphic events at either of these times have been recorded in the north china craton importantly there is no evidence for the 1 8 ga high grade metamorphic event that is manifest in the northern and central parts of the trans north china orogen

fault size distributions are they really power law abstract power law fault size throw distributions spanning ca seven orders of magnitude are analysed using sample lines from seismic coal mine plan and outcrop data sets outcrop data sets generally have lower power law exponent values than seismic data sets consistent with a general decrease in exponent value with decrease in fault size if such a relationship does exist then probably it cannot be accounted for by sampling bias alone we suggest that either i fault size populations are not power law over a large scale range or ii they are power law but at smaller scales show a greater range of exponent values due to spatial clustering

changes in frequency size relationship from small to large earthquakes the constant b value observed in frequency magnitude distributions of earthquakes has been taken as an indication of self similarity at all magnitudes hence earthquake properties should scale uniformly in the same way from small to large earthquakes it has often been observed however that the seismic moment released in small earthquakes scales differently with rupture length than it does for large events1 2 where the crossover between small and large events is defined at a rupture dimension equal to the down dip width of the seismogenic zone a possible explanation for this contradiction is that frequency size distributions are biased by small earthquakes small events dominate most global earthquake catalogues because of the short time period covered another source of bias in size distributions is the saturation of earthquake magnitudes for large events here we correct biases in calculations of b values and present evidence for a change in b value in frequency size distributions we find that a break in self similarity from small to large earthquakes occurs at a point where the dimension of the event equals the down dip width of the seismogenic layer

long term seismogenic process for major earthquakes in subduction zones abstract a qualitative physical process for the long term seismogenesis of major earthquakes in subduction zones is proposed on the basis of quantitative empirical evidence that swarms mainshocks and aftershocks are closely related phenomena the relations which have been identified in the comprehensive long term catalogues of new zealand and japan represent swarms as predictors of mainshocks with respect to location time and magnitude clustering of swarms and of mainshock aftershock events is allowed for with a database of 15 sequences of swarms mainshocks and aftershocks tests are being conducted with the object of refining the relations and evaluating them as a possible means of practical synoptic forecasting three sequences have culminated in major earthquakes since the tests began and the systematic study now relates a total of 36 swarms with 29 mainshock aftershock events these empirical results strengthen and quantify the connection between swarms and major earthquakes which several authors have demonstrated by means of numerical physical modelling the proposed seismogenic process includes swarms mainshocks and aftershocks as separate event stages which are related by predictability interevent conditions are specified according to the mogi criteria for the medium cracks at which fractures subsequently occur constitute nonuniformity in the mogi sense and post earthquake healing restores uniformity where the gutenberg richter relation occurs it is accepted as possible evidence of deterministic chaos and unpredictability as a corollary the process is noncyclical the principle of scaling is held to apply except when modified by large scale boundaries in the medium subduction zones and some other localities where water is abundant are indicated by the main empirical studies as favourable to the occurrence of swarms fluid overpressuring is therefore proposed as a mechanism for the self triggering of swarms and this is supported by additional examples of the predictive relations occurring in conditions of high fluid pressure including the vicinity of large man made reservoirs the process can be tested by systematic studies in other subduction regions given adequate catalogues for quantifying the algorithm it also has implications for other tectonic environments with swarms replaced by cognate more protracted seismicity precursors

extensional duplexing in the york cliffs strike slip fault system southern coastal maine abstract a 150 m long sinistral strike slip fault system consisting of left stepping en echelon fault segments cuts deformed paleozoic metasediments and early mesozoic dike intrusions in coastal terrace outcrops in southern maine the longer fault segments follow the vertical bedding in the metasediments and accommodate approximately 1 m of displacement detailed mapping at scales down to 1 6 indicates that overlap zones between offset layer parallel fault segments consist of high dilation angular breccias rounded attrition breccias and extensional duplexes where the offset segments straddle upright quartzite layers in one 10 m long duplex r and r shears cut the layering in the quartzite at 28 and 69 respectively relative to the dominant layer parallel fault segments creating a doubly tapered imbricate r shear lens segmentation structure individual imbricate lenses accommodate additional layer parallel extension through conjugate x x shear fractures with average orientations of 109 and 60 respectively relative to the dominant fault segments extensional fractures oblique to the dominant fault segments are rare within this duplex but commonly form distinctive horsetail arrays at fault terminations that average 44 to the layer parallel faults strain analysis shows that approximately 60 80 cm of measured strike slip displacement is accommodated internally within this duplex as layer parallel extensional strain using the r shear and x x shear fracture mechanisms the strain distribution and observed cross cutting relations suggest an outward symmetric growth of the duplex structure from left stepping sinistral fault segments that developed sequences of tip to plane r shear overlap linkages during continued displacements cyclic r r to x x shear fracturing may relate to stress reorientation following r shear ruptures during episodic growth of the duplex and incremental slip of the fault system

varves in marine sediments a review the global compilation of reported marine varved sedimentary records throughout the quaternary contains 52 sites marine varve deposition and preservation typically depend on environmental and sedimentological conditions such as a sufficiently high sedimentation rate severe depletion of dissolved oxygen in bottom water to exclude bioturbation by macrobenthos and a seasonally varying sedimentary input to yield a recognizable rhythmic varve pattern additional oceanographic factors include the strength and depth range of the oxygen minimum zone omz and regional anthropogenic eutrophication from point sources such as large polluted rivers quaternary marine varves are not only found in those parts of the open ocean that comply with these conditions but also in fjords embayments and estuaries with thermohaline density stratification and nearshore marine lakes with strong hydrologic connections to ocean water this review critically describes settings and sedimentological characteristics of marine sites where varves have been reported broader applications of marine varve records are discussed for example in terms of radiocarbon calibration on high resolution time scales constraining paleoceanographic variability and global change teleconnections diagnosing factors affecting modern fish population dynamics based on past performance detailing biogeochemical cycles and deciphering complex factors influencing marine productivity varves in saline water bodies without hydrologic connection to the global ocean are not included in this review and instead pertain to the companion review on lacustrine varves where also general and fundamental principles of varve deposition classification and dating are covered zolitschka et al 2015 the postulated occurrence of varves in pre quaternary rocks is critically discussed in this review with representative examples in the case of non evaporitic laminations in fine grained ancient marine rocks such as banded iron formations and black shales laminations may not be varves but instead may have multiple alternative origins such as event beds or formation via bottom currents that transport and sort silt sized particles clay floccules and organic mineral aggregates in the form of migrating bedload ripples

fault growth by linkage observations and implications from analogue models abstract using time sequence analyses of extensional fault models we demonstrate the pivotal role played by fault segmentation in the accumulation of displacement and length during the growth of faults experiments are described in which incremental steps during the development of individual faults have been reconstructed from time lapse photographs taken during model deformation these records confirm the composite segment hierarchy of fault structure a pattern that is frequently recognised in many natural arrays they reveal the progressive enlargement of individual faults to be the product of a repetitive cycle of tip line propagation overlap and linkage between nearest neighbours by contrasting the displacement patterns of successive increments during growth convincing evidence is also presented to suggest that individual segments of faults may remain kinematically independent once they are physically linked this behaviour is shown to be responsible for the characteristic saw tooth patterns often recognised in strike parallel fault displacement profiles such patterns are believed to arise where relict segment boundaries remain preserved as asperities to slip so that displacement is confined to discrete parts of a fault plane surface growth in this way also causes the maximum displacement d and surface length l of faults to continually change by different proportions incremental displacement records presented here corroborate field evidence which shows that linkage between fault segments during growth is responsible for a significant component of the spread of values often recorded in d versus l compilations finally we speculate that linkage between fault segments also accounts for transient irregularities recorded in the frequency distribution of the fault length populations of each model

strain hardening and strain softening in fault zones from foreland thrusts the movement of thrust sheets is accomplished in part by deforming the rocks adjacent to thrust faults in foreland fold and thrust belts emplacement generates an array of mesoscopic faults in a thick layer along the base of a sheet ductile deformation of rocks in the layer occurs by displacing fault bounded blocks relative to one another in a kind of mesoscopic grain boundary sliding analyses of data from three southern appalachian thrust sheets indicate that this block sliding is first accompanied by strain hardening then by strain softening and it is accommodated by intra block fractures small faults and veins the density of accommodation features increases near thrusts and where these features become sufficiently numerous a layer of cataclastic rocks forms along the thrust we describe foliated cataclasites from two thrusts the microstructures of which indicate that the cataclasites exhibited extreme ductility interrupted occasionally by brittle processes deformation of the cataclasites was accompanied by strain softening strain softening deformation in fault arrays near thrusts is genetically related to strain softening deformation in cataclasites during the formation of the cataclasite rocks in the sheet are impelled to cross a strain energy hump after which they deform at either higher rates or lower stresses

metallogeny of the north china craton link with secular changes in the evolving earth abstract the north china craton ncc has experienced a complex geological evolution since the early precambrian and carries important records of secular changes in tectonics and metallogeny here we synthesize the salient geological and tectonic features of the evolution and destruction of the ncc vis a vis major metallogenic events and the formation of potential ore deposits we identify a close relationship between the major geological events in the ncc and those reported elsewhere on the globe we trace the records of a regular change in the pattern of metallogeny mineral deposit character spatial distribution and genetic mechanisms which closely match the timing and styles of the major geological and tectonic events in this craton the ncc went through five major tectonic cycles 1 neoarchean crustal growth and stabilization 2 paleoproterozoic rifting subduction accretion collision with imprints of the great oxidation event goe 3 late paleoproterozoic neoproterozoic multi stage rifting 4 paleozoic orogenesis at the margins of the craton and 5 mesozoic extensional tectonics associated with lithospheric thinning and decratonization coinciding with these major geological events are five major metallogenic systems identified as follows 1 an archean bif system 2 paleoproterozoic cu pb zn and mg b systems 3 a mesoproterozoic ree fe pb zn system 4 a paleozoic orogenic cu mo system and 5 mesozoic intracontinental au and ag pb zn and mo systems the ore deposit types in each of these metallogenic systems show distinct characteristics and tectonic affinities from early precambrian through late precambrian to paleozoic and mesozoic the ncc records a transition from primitive to modern style plate tectonics evidence for imbricated oceanic plate stratigraphy in a subduction accretion setting and collisional orogenesis along at least three major zones of ocean closure are documented major transitions in tectonic style and surface environmental changes recorded in other parts of the world are also reflected in the geological history and metallogenic events in the ncc large scale gold deposits formed through intraplate tectonics during the mesozoic provide important insights into mantle dynamics and crust mantle interaction associated with lithospheric thinning and craton destruction the ncc provides one of the best examples for documenting secular changes in the geological history and metallogenic epochs of an evolving earth

fault length statistics and implications of graben sets at candor mensa mars abstract the lengths of individual martian grabens from three spatially and temporally distinct regional sets are analyzed by a two dimensional window sampling method length distributions are influenced by fault growth and coalescence processes image resolution window size and the post faulting geologic history of the region cumulative length frequency distributions can be linear for graben lengths between 30 and 100 km with slopes on the area plot of 2 suggesting self similar growth of the graben array post faulting geologic processes contribute to concave cumulative length distributions on the log log plot with curtailment and truncation biases producing strongly non linear distributions for smaller and longer lengths respectively recent evidence for a positive correlation between stratigraphic throw and graben length from analogous terrestrial grabens implies that the magnitudes of throw and extensional strain may also be dependent on relative position along the martian grabens

analysis of the relationships between strain polarity and population slope for normal fault systems abstract the evolution of normal fault populations and their synthetic and antithetic sub populations has been studied for a number of fault systems imaged from seismic reflection data relationships between fault strain and polarity and the slope of fault throw populations have been investigated by backstripping a fault system in the inner moray firth north sea and by comparison between throw populations for pre faulting horizons from an additional 11 fault systems with variable extensional strains fault population slopes decrease with increasing strain reflecting strain localisation onto progressively fewer and larger faults synthetic and antithetic sub populations also show an inverse relationship between slope and the strain they accommodate but the slopes of antithetic sub populations are independent of the total strain on the fault system the slopes of antithetic sub populations are higher than those of synthetic sub populations from the same area and the difference in slope correlates with the polarity of the fault system i e the proportion of the total strain accommodated by the two sub populations the data are consistent with progressive decrease in the slopes of sub populations until the antithetic fault sets become inactive the absence of a correlation between strain and polarity is attributed to the effects of fault system reactivation

prediction of macrofracture properties using microfracture information mesaverde group sandstones san juan basin new mexico abstract a better understanding of relationships between microfractures and macrofractures is prerequisite to the extrapolation of observed microfracture properties as a basis for inferring properties of associated macrofractures the hypothesis that micro and macrofractures represent different size fractions of the same fracture sets and consequently have linked attributes was tested on sandstones of the mesaverde group san juan basin new mexico the use of a cathodoluminescence detector attached to a scanning electron microscope sem allowed observation of microfractures not readily visible using conventional microscopic techniques the orientations of microfractures reproduce the orientations recorded at the macroscopic scale from cores and outcrops fracture size distributions are best modeled using power laws and extrapolation of microfracture frequencies to the scale of macrofractures successfully predicts the macrofracture frequencies fracture size distributions change at the length scale of the thickness of the mechanical layer typically by increasing the exponent of their power law distribution this change is independent of censoring biases and reflects changes in sampling topology and or fracture growth mechanisms

hadean to neoarchean episodic crustal growth detrital zircon records in paleoproterozoic quartzites from the southern north china craton abstract the archean terranes exposed in several regions of the north china craton ncc provide important windows to evaluate the hadean archean continental crustal evolution history here we report results from sims zircon geochronology and oxygen isotopes as well as la icpms zircon hf isotopic analyses on detrital zircons from paleoproterozoic quartzites to probe the early crustal evolution in the southern ncc our data show episodic magmatism during the eoarchean paleoarchean mesoarchean neoarchean and paleoproterozoic as inferred from the concordant ages of 3 6 ga and 2 2 ga and the well defined upper intercept ages of 3404 30 ma 2919 31 ma 2772 9 5 ma 2698 4 7 ma 2652 6 7 ma and 2532 5 2 ma the hf and oxygen isotopic compositions of eoarchean paleoarchean zircons demonstrate that the cratonic nucleus of the ncc was built in the hadean 4 0 ga similar to the timing of formation of the nuclei of old cratons elsewhere on the globe such as the yilgarn craton western australia and the slave craton canada with the eoarchean paleoarchean 3 8 ga 3 6 ga and 3 4 ga marking the first phase of crustal reworking of the hadean crust the salient aspect of our data is that the meoarchean to early neoarchean 3 0 2 6 ga with a peak at 2 7 ga was a significant period for crustal growth from the mantle throughout the ncc comparable with the event of major archean crustal growth worldwide the latest neoarchean 2 5 ga marks a period of significant crustal reworking that led to remelting of the newly formed juvenile mafic crust 2 7 ga integration with the data obtained from the ttg tonalite trondhjemite granodiorite and granitoid rocks of the archean terranes and the lower crustal granulite xenoliths from various parts of the ncc suggests that the episodic eoarchean via paleoarchean and mesoarchean to neoarchean crustal growth as well as crustal reworking intensely destroyed the ancient cratonic nucleus resulting in the scarce preservation of the hadean and eoarchean remnants in both the upper and lower crust of the ncc following the paleoproterozoic sedimentation of the archean zircons apart from pb loss during the subsequent thermal events there was no major alteration in the hf and oxygen isotopic compositions thus our detrital zircon data provide convincing evidence for the repeated crustal growth and reworking of the ncc in the archean

palaeoproterozoic metamorphic evolution and geochronology of the wugang block southeastern terminal of the trans north china orogen abstract amphibolites of the wugang block in the taihua metamorphic complex which is located in the southeastern terminal of the palaeoproterozoic trans north china orogen tnco preserve three generations of metamorphic mineral assemblages i e the prograde m 1 peak m 2 and retrograde m 3 assemblages forming during the palaeoproterozoic tectono metamorphic event these amphibolites record metamorphic p t conditions of 4 8 6 7 kbar 720 750 c m 1 9 0 10 6 kbar 710 780 c m 2 and 7 1 8 5 kbar 700 780 c m 3 which define clockwise p t paths involving isothermal decompression itd the metamorphic peak reached upper amphibolite facies and high resolution in situ sims u pb dating of metamorphic zircons suggests that the peak metamorphism took place at 1 96 1 92 ga in the wugang block the present new data suggest that the wugang block was also involved in the continental collision between the eastern and western blocks of the north china craton ncc in the palaeoproterozoic combining metamorphic and geochronological data concerning the tnco obtained by previous scholars it may be anticipated that the middle and southern sections of the tnco were involved in a long lived continental collision and the collision event can be possibly traced back to as early as 1 96 ga and lasted to as late as 1 80 ga

lateral variations in mylonite zone thickness as influenced by fluid rock interactions linville falls fault north carolina abstract over a distance of approximately 20 km along strike the linville falls mylonite varies in thickness from 1 m at linville falls to 60 m at banner elk along strike pressure temperature and displacement variations are minimized allowing this study to focus on the influences of fluid behavior and protolith mineralogy on fault zone development the protolith at linville falls contains mainly k feldspar perthite and quartz while at banner elk the protolith contains plagioclase and quartz at linville falls quartz deformed by dynamic recrystallization feldspar by intragranular fracturing and alteration to quartz and mica and mica by sliding along cleavage planes modal mineralogies change from the protolith to the mylonite with quartz decreasing from 39 to 19 and feldspar from 59 to 1 5 muscovite increases from mass balance calculations based on major and trace element geochemistry indicate approximately 75 volume loss at linville falls and 20 at banner elk fluid rock ratios estimated from the calculated depletions of si are an order of magnitude higher at linville falls than at banner elk fluids infiltrated the fault zone over a thicker zone at banner elk than at linville falls because the plagioclase altered more readily than k feldspar creating new pathways for fluids fluids migrated preferentially through channels along the fault zone creating a three dimensional network of higher fluid flow

fractal analysis applied to characteristic segments of the san andreas fault fractal theory is applied in a quantitative analysis of the san andreas fault saf geometry the method which directly measures the increase in total fault length with a decrease in ruler size gives the fractal dimension d and scaling properties for the chosen length band 0 5 1000 km a physical interpretation of d is that it measures the irregularity of the fault trace in the selected band the fault is subdivided into six segments of distinctive seismic behavior a main fault trace which shows either maximum coseismic slip or creeping was selected for analysis with three alternative branches examined for the saf system south of san bernardino branches of the fault trace are not considered in this work fractal dimensions calculated for the different segments range from 1 0008 to 1 0191 and are different from 1 0 to the 95 confidence interval these small changes reflect the overall smoothness of the main fault trace significant variations in d among segments indicate heterogeneities in the fault smoothness along strike d also changes significantly from the short length band to the long length band where the demarcation point ranges from 1 to 2 km the short length band has larger d values a slight correlation is obtained between the fractal dimension of the main trace and the extent of subparallel faulting this indicates some correspondence between the main fault trace irregularity and the complexity of subsidiary fault traces in plan view

chemistry source and behavior of fluids involved in alpine thrusting of the central pyrenees quartz veins occupying extensional fractures that formed relatively late in the alpine thrusting history of the pyrenees contain abundant inclusions of hypersaline brines small amounts of bitumen also occur a range of brine compositions is apparent from the freezing behavior of fluid inclusions from different samples with large variations in na ca ratio but many individual samples contain only one brine composition all brine types display a bimodal distribution of homogenization temperature which is interpreted as the result of fluctuation in fluid pressure from a maximum value equal to lithostatic at 5 7 km depth and around 270 c to a value up to 700 bars lower this is ascribed to dilatancy pumping chemical analyses of brines leached from two samples yields balanced analyses for 14 elements both analyses appear to represent sedimentary formation waters and are closely analogous to oil field brines a basement origin is precluded by the presence of bitumens and the low k ca ratios differences between the analyses probably reflect different source bed mineralogy including the presence or absence of dolomite but triassic red beds are likely to be a major source the occurrence of these brines in the fault planes provides constraints on both the sources and movement of the fluids involved in thrusting significant infiltration of either surface waters or metamorphic water of dehydration can be ruled out because of the high salinities typically 20 25 wt dissolved salts hence although permeability was locally high due to fracturing around the faults fluids were pumped into these fractures from sedimentary formations adjacent to the fault individual fluid pathways were short lived relative to the duration of the deformation event so that different fluids were pumped into the faults at different times in response to local fault propagation

fault scaling laws and the temporal evolution of fault systems abstract through an analysis of the temporal evolution of duplex fault systems this contribution shows that it is unlikely that all faults in a deformed area will conform to a single frequency size scaling relationship the development of a duplex leads to different size frequency relationships for the faults that compose the duplex and those confined to individual horses the faults that compose the duplex define segments with relatively steep stopes on a log n number of faults i e frequency vs log d displacement magnitude i e size plot faults within individual horses define segments with relatively shallow slopes on a log n vs log d plot the distinction between these two types of faults in a duplex is akin to the distinction between large active faults which cut the entire seismogenic layer and small active faults which do not extend across the seismogenic layer if as is often the case the faults that compose a duplex do not extend across the seismogenic layer the stepped nature of the resulting log n vs log d plot may make it particularly difficult to assess the contribution of these small faults to regional deformation since duplex geometries result in part from anisotropies present in deforming rocks the anisotropy present in nearly all crustal rocks will affect the size frequency relationship observed for systems of faults different parts of a deforming rock mass are likely to have different initial anisotropies combining data on fault systems from markedly different portions of a deforming region may then obscure the unique characteristics of the size frequency relationship in either area and may lead to inaccurate assessments of the relative contributions of small and large faults to regional fault accommodated strains

continentally derived solutes in shallow archean seawater rare earth element and nd isotope evidence in iron formation from the 2 9 ga pongola supergroup south africa abstract the chemical composition of surface water in the photic zone of the precambrian ocean is almost exclusively known from studies of stromatolitic carbonates while banded iron formations ifs have provided information on the composition of deeper waters here we discuss the trace element and nd isotope geochemistry of very shallow water if from the pongola supergroup south africa to gain a better understanding of solute sources to mesoarchean shallow coastal seawater the pongola supergroup formed on the stable margin of the kaapvaal craton 2 9 ga ago and contains banded iron formations ifs that represent the oldest documented superior type iron formations the ifs are near shore pure chemical sediments and shale normalized rare earth and yttrium distributions reysn exhibit positive lasn gdsn and ysn anomalies which are typical features of marine waters throughout the archean and proterozoic the marine origin of these samples is further supported by super chondritic y ho ratios average y ho 42 relative to older isua ifs 3 7 ga from greenland and younger kuruman ifs 2 5 ga also from south africa the pongola ifs are depleted in heavy rare earth elements hree and appear to record variations in solute fluxes related to sea level rise and fall sm nd isotopes were used to identify potential sediment and solute sources within pongola shales and ifs the nd t for pongola shales ranges from 2 7 to 4 2 and nd t values for the coeval iron formation samples range 1 9 to 4 3 are generally indistinguishable from those of the shales although two if samples display nd t as low as 8 1 and 10 9 the similarity in nd isotope signatures between the shale and iron formation suggests that mantle derived rey were not a significant nd source within the pongola depositional environment though the presence of positive eu anomalies in the if samples indicates that high t hydrothermal input did contribute to their rey signature isotopic mass balance calculations indicate that most 72 of the nd in these seawater precipitates was derived from continental sources if previous models of fe nd distributions in archean ifs are applied then the pongola ifs suggest that continental fluxes of fe to archean seawater were significantly greater than are generally considered

further evidence for 1 85 ga metamorphism in the central zone of the north china craton shrimp u pb dating of zircon from metamorphic rocks in the lushan area henan province abstract this paper reports shrimp zircon u pb dating of precambrian supracrustal and granitic rocks from the lushan area henan province in the southern portion of the central zone also referred to as the trans north china orogen of the north china craton a graphite garnet sillimanite gneiss sample tw0006 1 of the shangtaihua group gives a range of inherited zircon ages from 2 73 to 2 26 ga and a metamorphic zircon age of 1 84 0 07 ga a garnet bearing gneissic granitoid sample twj358 1 which is considered to intrude the shangtaihua group gives a magmatic zircon age of 2 14 0 02 ga and a metamorphic zircon age of 1 87 ga the metamorphic zircon ages of 1 87 1 84 ga obtained in this study indicate that an important tectonothermal event occurred at the end of the paleoproterozoic in the lushan area this supports the southern continuation of a central zone in the north china craton that workers have recently considered to result from continent continent collision it is also evident that the shangtaihua group was formed during the paleoproterozoic between 2 26 and 2 14 ga and not during the archean as previously considered

morphology and geometry of duplexes formed during flexural slip folding abstract bedding parallel slip on the limbs of chevron style folds during flexural slip folding results occasionally in the formation of small scale duplexes the duplexes described here from upper carboniferous turbidites in sw england generally range in thickness from 0 3 to 30 cm and are up to 2 4 m long they have smooth nearly flat roofs and both the internal thrusts and the slickenfibres on them are commonly oblique to the mean transport direction as indicated by slickenfibres on the floor and roof thrusts the morphology of the duplexes suggests that they developed between active floor and roof thrusts and they show a characteristic lack of hangingwall anticlines on the link thrusts their shear sense always corresponds to that required by flexural slip and the total bedding parallel displacement across each is commensurate with that predicted by the flexural slip model all thrust surfaces in the duplexes are marked by quartz fibre veins a feature which distinguishes them from soft sediment and other pre folding duplexes and carry slickenfibres whose mean orientation and complex variations in slip direction on different fibre sheets on the same slip surface are identical to those on flexural slip movement horizons from the same fold limb the duplexes form either as a result of resistance to thrust propagation by local facies or bed thickness changes or develop as transfer structures between the tips of movement horizons propagating along adjacent slip surfaces late stage duplexes develop from imbricated fibre veins and also form on slip surfaces oblique to the axial planes of major folds