geochemistry and zircon geochronology of the permian a type hasanrobat granite sanandaj sirjan belt a new record of the gondwana break up in iran the sanandaj sirjan metamorphic plutonic belt ssb in west central iran is a polyphase metamorphic terrain composed of dominantly greenschist grade metasedimentary and metavolcanic rocks and felsic to mafic plutons of neoproterozoic phanerozoic ages the hasanrobat granite in central ssb occurs as a single pluton 20 km 2 surface area with relatively consistent mineralogy and chemistry quartz alkali feldspars microcline and perthite sodic plagioclases and biotite are the main constituents commonly associated with minor amphibole accessory phases include zircon allanite apatite and magnetite the country rocks are upper carboniferous lower permian sandstones and dolomitic limestones scattered patches of skarn type assemblages dominated by tremolite and talc occur in the dolomitic limestones and sandstones are recrystallized to a coarse grained quartzite at contact with the granite the granite is metaluminous to slightly peraluminous and is distinguished by high feo t mgo ratios typical of ferroan a type granites the a type affinity is also reflected by high na 2o k 2o high ga al ratios high contents of large ion lithophile elements lile high field strength elements hfse and rare earth elements ree as well as low contents of sr and distinct negative eu anomalies the biotites are aluminous fe rich and plot near the siderophyllite corner in the quadrilateral biotite diagram they are further distinguished by high fluorine contents 0 61 to 1 33 wt amphibole is ferrohastingsite in composition ion microprobe analyses of zircon grains separated from a representative granite sample yielded concordant u pb ages with weighted mean 206 pb 238 u age of 288 3 3 6 ma the granite and the country rocks are cut by a set of mafic dykes with asthenosphere like geochemical signatures such association suggests anorogenic intraplate magmatism in lower permian in the region this provides further evidence for and significantly constrains timing of a major extension in upper paleozoic in iran previously inferred from the rock record the extension led to the gondwana break up and the opening of neotethys ocean between sanandaj sirjan and zagros in permian

residence of ree y th and u in granites and crustal protoliths implications for the chemistry of crustal melts a systematic study with laser ablation icp ms scanning electron microscopy and electron microprobe revealed that 70 95 wt of ree except eu y th and u in granite rocks and crustal protoliths reside within reeythu rich accessories whose nature composition and associations change with the rock aluminosity the accessory assemblage of peraluminous granites migmatites and high grade rocks is composed of monazite xenotime in low ca varieties apatite zircon thorthosilicate uraninite and betafite pyrochlore metaluminous granites have allanite sphene apatite zircon monazite and thorthosilicaie peralkaline granites have aeschinite fergusonite samarskite bastnaesite fluocerite allanite sphene zircon monazite xenotime and th orthosilicate granulite grade garnets are enriched in nd and sm by no less than one order of magnitude with respect to amphibolite grade garnets granulitegrade feldspars are also enriched in lree with respect to amphibolite grade feldspars accessories cause non henrian behaviour of ree y th and u during melt solid partitioning because elevated fractions of monazite xenotime and zircon in common migmatites are included within major minerals their behaviour during anatexis is controlled by that of their host settling curves calculated for a convecting magma show that accessories are too small to settle appreciably being separated from the melt as inclusions within larger minerals biotite has the greatest tendency to include accessories thereby indirectly controlling the geochemistry of ree y th and u we conclude that ree y th and u are unsuitable for petrogenetical modelling of granitoids through equilibrium based trace element fractionation equations

the southern caspian a compressional depression floored by a trapped modified oceanic crust the south caspian intracontinental depression floored by oceanic basement is a relatively stable block with minor deformation surrounded by active fold thrust belts of arcuate form talesh alborz and kopeh dagh mountains which have undergone intense late cainozoic crustal shortening the basin is interpreted as a neogene quaternary compressional depression bounded by multi role mountain bordering reverse faults and apparently floored by a late paleozoic triassic or late mesozoic early tertiary modified oceanic crust trapped along an old geosuture it may be a relic of an old paleozoic triassic ocean or else a marginal sea developed behind a mesozoic paleogene ocean and analysis of geological and geophysical data enables a scheme to be suggested the general arcuate shape of the alborz and the talesh bordering mountain belts follows the pattern of the supposed rigid and thickened ocean crust of south caspian depression a tectono sedimentary study of the south caspian region coupled w

petrogenesis of post collisional a type granitoids from the urumieh dokhtar magmatic assemblage southwestern kerman iran constraints on the arabian eurasian continental collision abstract three plutons deh siahan bande bagh and baghe khoshk sharghi collectively referred to as the dbb hereafter in southwestern kerman in the southeastern part of the urumieh dokhtar magmatic assemblage udma of the zagros orogenic belt differ from the typical calc alkaline metaluminous i type intrusions of the region the dbb intrusions have a distinct lithological assemblage varying from diorite through monzogranite and monzonite to alkali feldspar syenite and alkali granite the dbb granitoids are metaluminous to slightly peraluminous alkaline to shoshonitic in composition and have high total alkali contents with k2o na2o high feot mgo values and low cao and mgo contents they are enriched in some liles such as rb and th and hfses such as zr y and rees except eu and depleted in sr and ba relative to primordial mantle and have low concentrations of transitional metals these features along with various geochemical discriminant diagrams suggest that the dbb granitoids are post collisional a type granitoids which had not been recognized previously in the udma the chondrite normalized ree patterns of the dbb granitoids show slightly enriched light rees la sm n 2 26 4 13 negative eu anomalies eu eu n 0 19 0 74 and flat heavy ree patterns gd yb n 0 80 1 87 the negative eu anomaly indicates an important role for plagioclase and or k feldspar during fractional crystallization whole rock rb sr isotope analysis yields an isochron age of 33 1 ma with an initial 87sr 86sr value of 0 7049 0 0001 whole rock sm nd isotope analysis gives endt values from 2 56 to 3 62 at 33 ma the positive endt and low isr values of the dbb granitoids together with their tdm of 0 6 0 7 ga suggest their formation from partial melting of a lithospheric mantle source modified by fluids or melts from earlier subduction processes melting of lithospheric mantle occurred via a dehydration melting process at pressures below the garnet stability field as a consequence of lithospheric mantle delamination or break off of a subducted slab and melting of the lithospheric mantle by upwelling of hot asthenosphere on the basis of rb sr age dating and the post collisional geochemical signatures of the dbb granitoids along with extensive pre collisional volcanic eruptions in middle eocene we suggest late eocene for the time of collision between the arabian and central iranian plates this also implies that the calc alkaline i type intrusions in the southwestern kerman and in other parts of the udma may have formed in a post collisional context

voyages of seaweeds the role of macroalgae in sediment transport abstract at least 26 genera of macroalgae have been observed attached to and carrying sedimentary materials rock veneer or carbonate to beaches and the sea floor most studies have been descriptive with few quantifying flux rates this study reviews and quantifies the mainly biological literature on macroalgae mediated sediment transport in coastal and marine environments dislodgement of macroalgae from a hard substrate can result in erosion up to 0 01 mm y 1 the proportion of buoyant macroalgae carrying rock material usually a thin veneer but sometimes boulders up to 365 kg varies by location but can reach as high as 35 macroalgae carrying attached carbonate material e g crustose coralline algae shells are more common up to 85 of holdfasts on a beach may contain up to 116 g caco 3 each sand and carbonate trapped in haptera along with encrusting carbonate producing organisms have a mass of up to tens of grammes while most dislodged macroalgae land close to where they came from others may be stranded many thousands of km from their place of origin sediment deposited by macroalgal transport depends largely on the characteristics of local source rocks while hard rocks may only provide a few g m 2 y 1 highly fractured basalt has been shown to provide up to 78 g m 2 y 1 rock transport by macroalgae is less than carbonate flux which may reach over 8000 g m 2 y 1 based on the projected abundance of macroalgal rafts some 350 tonnes of sediment both carbonate and clastic may be afloat in the southern ocean at any one time macroalgal transport may explain the presence of exotic rocks and shells found far from their source environment while cold climate rafting of sediment is dominated by glacial processes in warm periods rafting may be limited to biological transport overall the transport of rock and carbonate by macroalgae has the potential in some environments of being a significant source of sediment

magnetotelluric analysis using mohr circles the mohr circle most commonly met in the analysis of mechanical stress is used to depict magnetotelluric impedance information taking the real and quadrature parts of magnetotelluric tensors separately the magnetotelluric concepts of two dimensionality three dimensionality skew and anisotropy are then all given quantitative expression on a diagram as are various magnetotelluric invariants in particular a new invariant the central impedance becomes evident in a discussion of effective impedances some insight is gained into impedance rotations and an anisotropy angle is defined analogous to skew angle mohr circles are also tested to depict the effects of the shear and twist operations on a regionally two dimensional structure generally the application of shear or twist results in an impedance tensor with a mohr circle of typical three dimensional form

petrogenesis and tectonic implications of late carboniferous a type granites and gabbronorites in nw iran geochronological and geochemical constraints abstract carboniferous igneous rocks constitute volumetrically minor components of iranian crust but preserve important information about the magmatic and tectonic history of sw asia ghushchi granites and gabbronorites in nw iran comprise a bimodal magmatic suite that intruded ediacaran cambrian gneiss and are good representatives of carboniferous igneous activity precise sims u pb zircon ages indicate that the gabbronorites and granites were emplaced synchronously at 320 ma ghushchi granites show a type magmatic affinities with typical enrichments in alkalis ga zr nb and y depletion in sr and p and fractionated ree patterns showing strong negative eu anomalies the gabbronorites are enriched in lrees nb ta and other incompatible trace elements and are similar in geochemistry to oib type rocks granites and gabbronorites have similar e nd t 1 3 to 3 4 and 0 1 to 4 4 respectively and zircon e hf t 1 7 to 6 2 and 0 94 to 6 5 respectively the similar variation in bulk rock e nd t and zircon e hf t values and radiometric ages for the granites and gabbronorites indicate a genetic relationship between mafic and felsic magmas either a crystal fractionation or silicate liquid immiscibility process further work is needed to resolve petrogenetic details the compositional characteristics of the bimodal ghushchi complex are most consistent with magmatic activity in an extensional tectonic environment this extension may have occurred during rifting of cadomian fragments away from northern gondwana during early phases of neotethys opening

sr nd isotope geochemistry and tectonomagmatic setting of the dehsalm cu mo porphyry mineralizing intrusives from lut block eastern iran the dehsalm cu mo bearing porphyritic granitoids belong to the lut block volcanic plutonic belt central eastern iran these rocks range in composition from gabbro diorite to granite with dominance of monzonites and quartz monzonites and have geochemical features of high k calc alkaline to shoshonitic volcanic arc suites primitive mantle normalized trace element spider diagrams display strong enrichment in large ion lithophile elements such as rb ba and cs and depletions in some high field strength elements e g nb ti y and hree chondrite normalized plots display significant lree enrichments high lan ybn and a lack of eu anomaly high sr y and la yb ratios of dehsalm intrusives reveal that despite their k rich composition these granitoids show some resemblances with adakitic rocks a rb sr whole rock feldspar biotite age of 33 1 ma was obtained in a quartz monzonite sample and coincides within error with a previous geochronological result in chah shaljami granitoids further northwest within the lut block 87sr 86sr i and endi isotopic ratios range from 0 70481 to 0 70508 and from 1 5 to 2 5 respectively which fits into a supra subduction mantle wedge source for the parental melts and indicates that crustal contribution for magma diversification was of limited importance sr and nd isotopic compositions together with major and trace element geochemistry point to an origin of the parental magmas by melting of a metasomatized mantle source with phlogopite breakdown playing a significant role in the geochemical fingerprints of the parental magmas small amounts of residual garnet in the mantle source also help to explain some trace element patterns geochemical features of dehsalm porphyries and its association with cu mo mineralization agree with a mature continental arc setting related to the convergence of afghan and lut plates during oligocene

resolving miss conceptions and misconceptions a geological approach to sedimentary surface textures generated by microbial and abiotic processes the rock record contains a rich variety of sedimentary surface textures on siliciclastic sandstone siltstone and mudstone bedding planes in recent years an increasing number of these textures have been attributed to surficial microbial mats at the time of deposition resulting in their classification as microbially induced sedimentary structures or miss research into miss has developed at a rapid rate resulting in a number of misconceptions in the literature here we attempt to rectify these miss misunderstandings the first part of this paper surveys the stratigraphic and environmental range of reported miss revealing that contrary to popular belief there are more reported miss bearing rock units of phanerozoic than precambrian age furthermore miss exhibit a pan environmental and almost continuous record since the archean claims for the stratigraphic restriction of miss to intervals prior to the evolution of grazing organisms or after mass extinction events as well as claims for the environmental restriction of miss appear to result from sampling bias in the second part of the paper we suggest that raised awareness of miss has come at the cost of a decreasing appreciation of abiotic processes that may create morphologically similar features by introducing the umbrella term sedimentary surface textures of which miss are one subset we suggest a practical methodology for classifying such structures in the geological record we illustrate how elucidating the formative mechanisms of ancient sedimentary surface textures usually requires consideration of a suite of sedimentological evidence from surrounding strata resultant interpretations microbial or non microbial should be couched within a reasonable degree of uncertainty this approach recognizes that morphological similarity alone does not constitute scientific proof of a common origin and reinstates a passive descriptive terminology for sedimentary surface textures that cannot be achieved with the current miss lexicon it is hoped that this new terminology will reduce the number of overly sensational and misleading claims of miss occurrence and permit the means to practically separate initial observation from interpretation furthermore this methodology offers a scientific approach that appreciates the low likelihood of conclusively identifying microbial structures from visual appearance alone informing the search for true miss in earth s geological record and potentially on other planetary bodies such as mars

sedimentology and paleontology of a carboniferous log jam a localized sandstone split in the mary lee coal early pennsylvanian langsettian of the black warrior basin alabama has a channel form geometry and preserves a concentrated log and gravel pebbles cobbles and boulders assemblage at the top of the fill sequence gravel lithotypes within and amongst rooting structures of lycopsid cordaitean and calamitean trees are indicative of an appalachian orogenic provenance and support an allochthonous origin for some of the logs in addition a skolithos ichnological assemblage within the channel is indicative of opportunist colonization during channel fill a low sinuosity geometry characterizes the overall channel form belt exhibiting a general northwesterly trend paleocurrent measurements from the cross bedded sandstone at the margin of the channel belt indicate flow was to the northwest log orientations at these sites are subperpendicular to perpendicular to the overall sediment transport direction and are interpreted to represent an ancient log jam the genesis of the fluvial channel the introduction and emplacement of the concentrated log assemblage the effects of this floating log accumulation on sedimentation within the channel and the relationship between paleocurrent and log orientation are discussed the red river located in the southern united states is used as a modern analog for comparative purposes

mapping geoelectric fields during magnetic storms synthetic analysis of empirical united states impedances empirical impedance tensors obtained from earthscope magnetotelluric data at sites distributed across the midwestern united states are used to examine the feasibility of mapping magnetic storm induction of geoelectric fields with these tensors in order to isolate the effects of earth conductivity structure we perform a synthetic analysis calculating geoelectric field variations induced by a geomagnetic field that is geographically uniform but varying sinusoidally with a chosen set of oscillation frequencies that are characteristic of magnetic storm variations for north south oriented geomagnetic oscillations at a period of t0 100 s induced geoelectric field vectors show substantial geographically distributed differences in amplitude approximately a factor of 100 direction up to 130 and phase over a quarter wavelength these differences are the result of three dimensional earth conductivity structure and they highlight a shortcoming of one dimensional conductivity models and other synthetic models not derived from direct geophysical measurement that are used in the evaluation of storm time geoelectric hazards for the electric power grid industry a hypothetical extremely intense magnetic storm having 500 nt amplitude at t0 100 s would induce geoelectric fields with an average amplitude across the midwestern united states of about 2 71 v km but with a representative site to site range of 0 15 v km to 16 77 v km significant improvement in the evaluation of such hazards will require detailed knowledge of the earth s interior three dimensional conductivity structure

methods used in the u s s r to reduce near surface inhomogeneity effects on deep magnetotelluric sounding abstract the effects of near surface inhomogeneities on deep magnetotelluric sounding are considered methods are suggested allowing for and ruling out the near surface effects examples for practical application of these methods are given

implications for the lithospheric geometry of the iapetus suture beneath ireland based on electrical resistivity models from deep probing magnetotellurics

a hybrid origin for the qianshan a type granite northeast china geochemical and sr nd hf isotopic evidence abstract major and trace element whole rock sr and nd isotope and zircon hf isotope data are reported for a suite of a type granites and mafic microgranular enclaves from the early cretaceous 126 2 ma qianshan pluton liaodong peninsula northeast china with the aim of investigating the sources and petrogenesis of a type granites the qianshan pluton includes hornblende alkali feldspar granite graphic biotite granite and mafic microgranular enclaves the hornblende alkali feldspar granites have high sio 2 fe 2 o 3t mgo k 2 o na 2 o rb zr and lree contents and low ba and sr concentrations with strongly negative eu anomalies their high rb sr 87 rb 86 sr 16 76 24 15 and initial 87 sr 86 sr ratios 0 7215 to 0 7283 negative e nd t values 14 1 to 16 5 and zircon e hf t values 18 9 to 11 5 indicate they were mainly derived from a crustal source but with involvement of high e nd t and e hf t materials graphic biotite granites have similar geochemical features and sr nd hf isotopic compositions to enclaves indicating they were the result of crystal fractionation of evolved mafic magmas but with involvement of low e nd t and e hf t materials the mafic enclaves have an igneous texture and contain acicular apatite suggesting quenching of mafic magmas that have co mingled with the host granites they have low initial 87 sr 86 sr ratios 0 7097 0 7148 negative e nd t 14 5 to 11 9 and zircon e hf t 17 1 to 6 9 values and are enriched in liles and lrees and depleted in hfses when coupled with the high mgo mg up to 54 this indicates derivation from an enriched lithospheric mantle source but contaminated by crustal materials geochemical and sr nd and zircon hf isotopic compositions rule out simple crystal liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks instead magma mixing of mantle derived mafic and crustal derived magmas coupled with crystal fractionation is compatible with the data this example shows that at least some a type granites formed through a complex process involving mantle and crustal derived magma mixing crystal fractionation and infracrustal melting

should the electric line be straight in magnetotelluric surveys the usual description of electric field measurements in terms of potential differences is not entirely adequate at high frequencies in general the telluric electric field is non conservative and voltage measurements depend on the contour described by the cable a simple error analysis helps to recognize those situations where systematic errors may be significant for present day standards

a review of the relationships between granitoid types their origins and their geodynamic environments abstract granitoids are divided into several types according to their mineral assemblages their field and petrographical features and their chemical and isotopic characteristics this typology complements most of the recent classifications because it is not based solely on chemical and isotopic criteria but also on the field petrographical and mineralogical criteria it thus has the advantage of distinguishing the various granitoid types in the field in most cases the proposed classification shares many similarities with the twenty most used genetic classifications of granitoids both types of peraluminous granitoids are of crustal origin the tholeiitic alkaline and peralkaline granitoids are of mantle origin and both types of calc alkaline granitoids are of mixed origin and involve both crustal and mantle materials each granitoid type is generated and emplaced in a very specific tectonic setting each stage of the wilson cycle is characterised by typical associations of granitoids well typed and precisely dated granitoids can then complement structural approaches and indicate on the geodynamic environment with reference to some case studies the use of granitoids as tracers of the geodynamic evolution is also proposed and discussed

the simulation of magnetic variation anomalies using single station data abstract hypothetical event maps of the vertical field derived from single station magnetic variation transfer functions are biased by the anomalous horizontal fields an iterative scheme proposed earlier is used to estimate and correct for the anomalous fields the scheme here is applied to single station transfer functions from 90 sites in southern scotland and northern england it converges to a stable solution in four iterations and the results compare favourably with those predicted in the southern part of the area using measured inter station transfer functions the resultant hypothetical event maps of the anomalous horizontal fields show that the northumberland trough and southern uplands conductivity anomalies are both strikingly linear features which correlate more closely than previously supposed with other geological and geophysical observations

deep crustal geoelectric structure beneath the northumberland basin in terms of lateral variations in conductivity structure the southern southern uplands and northumberland basin are characterized by a region of attenuated vertical magnetic fields with small spatial gradients reflecting the presence of a substantial conducting zone five magnetotelluric data sets from the region have been analysed to provide accurate and unbiased estimates of the impedance tensor the response data are used to investigate the deep geoelectric crustal structure of the region three appropriate sets of response data have been subjected to two construction algorithms for 1 d inversion the geoelectric profiles recovered identify a deep crustal conducting zone underlying the northumberland basin the zone modelled as a layered structure dips steeply from mid crustal depths underneath the northumberland basin to lower crustal depths to the nw the structure thus correlates in location and geometry with a deep crustal reflecting wedge detected offshore by a deep seismic reflection profile

petrogenesis and tectonic setting of the tuyeh darvar granitoid northern iran constraints from zircon u pb geochronology and sr nd isotope geochemistry abstract tuyeh darvar granitoid which outcrop ca 45 km sw of damghan city in the eastern alborz zone comprise mainly the pluton emplaced into the barut formation of lower cambrian ages zircon u pb ages show carboniferous ages 325 3 ma for the formation of this granitoid the granitoid is mostly metaluminous ferroan and alkalic monzonite to monzodiorite these rocks have high values of feot mgo and ga al high concentrations of k2o na2o low abundances of mgo and transitional elements plots normalized to chondrite and primitive mantle compositions show strong enrichments of lree relative to hree and of lile relative to hfse accompannied by negative anomalies of nb and sr they contain fe rich hydrous mafic minerals and magnetite these features are typical of a type granites sr nd isotopic geochemistry with initial nd values from 1 1 to 1 5 and initial 87sr 86sr ratios between 0 70562 and 0 70678 are consistent with magmatic differentiation from mafic melts produced from an enriched mantle source however other models such as melting of mafic crust or mixing of components from depleted mantle and continental crust cannot be discarded on the basis of the u pb zircon age 325 3 ma and the known magmatic tectonic regime in iran during the paleozoic it is suggested that the pluton formed in a rift environment related to extensional structures of the alborz block in early carboniferous time

two dimensional interpretation of three dimensional magnetotelluric data an example of limitations and resolution summary interpretation of magnetotelluric mt data for three dimensional 3 d regional conductivity structures remains uncommon and two dimensional 2 d models are often considered an adequate approach in this paper we examine 2 d interpretation of 3 d data by considering the synthetic responses of a 3 d structure chosen specifically to highlight the advantages and limitations of 2 d interpretation 2 d models were obtained from inversion of the synthetic 3 d data set with different conditions noise and distortion applied to the data we demonstrate the importance of understanding galvanic distortion of the data and show how 2 d inversion is improved when the regional data are corrected prior to modelling when the 3 d conductive structure is located below the profile the models obtained suggest that the effects of finite strike are not significant if the structure has a strike extent greater than about one half of a skin depth in this case the use of only tm mode data determined better the horizontal extent of the 3 d anomaly when the profiles are located away from the 3 d conductive structure the use of only tm mode data can imagine phantom conductive structures below the profile in this case the use of both polarizations produced a better determination of the subsurface structures it is important to note that the main structures are identified in all the cases considered here although in some cases the large data misfit would cause scepticism about features of the models

structures of the zagros fold thrust belt in iran the zagros fold thrust belt in iran forms the external part of the zagros active orogenic wedge it includes a sequence of heterogeneous latest neoproterozoic phanerozoic sedimentary cover strata 7 to 12 km thick and composed of alternating incompetent and competent layers overlying precambrian crystalline basement with a complex pre zagros structural fabric balancing structures of the cover statigraphic units in the zagros fold thrust belt requires in sequence and out of sequence involvement of the precambrian basement in the deformation six detailed balanced and retrodeformable cross sections which are constructed based on geological and geophysical data across various sectors of the belt show fault bend and fault propagation folds interpreted to have formed by slip on their subjacent thrusts out of sequence basement rooted thrusts as the interpretations in the constructed cross sections suggest have breached the cover basement interface and using incompetent cover strata for propagation cut across the cover structures and have created associated new folds superimposed upon the pre existing structures this style of deformation which has resulted in structural complexity of the belt characterizes a 200 to 300 km wide zone of distributed partly synchronous deformation with along strike and across strike variations it also implies that the zagros fold thrust belt as the external part of the orogenic wedge is still in its subcritical condition with internal deformation to achieve a critical taper in creating structural complexity in addition to out of sequence thrusting mechanically weak layers evaporites and mudstones of the cover strata have played a significant role by providing several detachment horizons shortening estimates across the belt are variable based on the constructed cross sections and their restorations minimum shortening estimates range from 16 percent to 30 percent in different sectors of the belt

separation of local and regional information in distorted gds response functions by hypothetical event analysis summary electromagnetic investigations are usually intended to examine regional structures where induction takes place at a given period range however the regional information is often distorted by galvanic eiects at local conductivity boundaries bahr 1985 and groom bailey 1989 developed a physical distortion model for decomposing the mt impedance tensor based upon local galvanic distortion of a regional 2 d electromagnetic eld we have extended their method to predict the magnetic variation elds created at an array of sites the magnetic response functions at periods around 1000 s may be distorted by large scale inhomogeneities in the upper or middle crust in this period range the data measured by a magnetometer array contain common information that can be extracted if the data set is treated as a unit for example by using hypothetical event analysis with this technique it is always possible to recover the regional strike direction from distorted data even if a strong spatially varying regional vertical eld component is present in the data set the determination of the regional impedance phases on the other hand is far more sensitive to deviations from the physical distortion model the approach has been used to investigate the iapetus data set for the array which covers an area of 200 km 300 km in northern england southern scotland the technique revealed a common regional strike azimuth of ca n125 0 e in the period range 500 2000 s this direction diiers from the strike indicated by the induction arrows which seem in uenced mainly by local current concentrations along the east weststriking northumberland trough and a ne sw striking mid crustal conductor both impedance phases are positive and diier by ca 1 0 0 which supports the assumptions of distortion elds in the data set and that the regional structure is 2 d

causes of high crustal conductivity beneath the iapetus suture zone in great britain summary two magnetic variation anomalies in southern scotland and northern england have been linked to the position of the iapetus suture previous magnetotelluric measurements along a 140 km profile that crosses both anomalies have been supplemented by new high quality broad band observations groom bailey decomposition of the impedance and hypothetical event analysis of the magnetic variation data agree in defining different strike directions for the two halves of the mt profile n50 e for the northern part over the southern uplands and n90 e for the southern part over the northumberland trough te and tm mode apparent resistivity and phase data at representative frequencies have been inverted for the two sub profiles separately using the smooth inverse of smith booker 1990 the uppermost layer has a low resistivity is variable in thickness and correlates well with the known position and thickness of carboniferous sedimentary rocks the second layer has a very high resistivity thousands of m and reaches the surface where the lower palaeozoic metamorphic rocks of the southern uplands crop out a relatively rapid transition to low resistivities occurs at depths of between 8 and 16 km the conducting layer appears to be quasi continuous but where the profile crosses the anomalies identified by magnetic variation measurements the conductance increases and the upper surface is shallower the spatial coverage of the magnetic variation data has enabled us to extrapolate conductive features away from the line of section and project the electrical image onto the nec vertical incidence seismic reflection profile in the north sea there is excellent agreement between a number of features in the acoustical and electrical images the shallowing of the low resistivity layer to form a narrow wedge like feature corresponds to the offshore position of the stublick fault while to the north of the fault the top of the layer coincides with a south dipping reflector thought to be a thrust however the zones of high conductance and high lower crustal reflectivity do not in general correlate the good conductor beneath he northumberland trough spans two zones which were differentiated on ihe nec profile in terms of their reflectivity the shape of the conductor s upper surface in the vicinity of the stublick fault agrees well with the model of chadwick holliday 1991 who proposed that the iapetus suture was a whole crustal shear with a gently dipping central ramp the coincidence between the present day low resistivity layer and a surface of weakness that was active 300 400 myr ago is much more readily explained in terms of mineralogy the presence of graphite than the presence of fluids there is also strong support for interpreting the northern conductor in the same way its upper surface is relatively flat and occupies the position predicted as the horizontal detachment surface over which wedges of the southern uplands rocks were thrust its northeastward extension is sampled by a group of carboniferous vents from which xenolith suites of crust and upper mantle origin have been obtained the middle to upper crust is believed to be represented by quartzo feldspathic gneisses that contain abundant graphite

characteristics and provenance of log transported gravels in a carboniferous channel deposit abstract gravel ranging from pebble to boulder size is preserved inside or dispersed among vitrinized logs within a coastal channel sandstone sequence of the upper pottsville formation black warrior basin northwestern alabama the gravel was transported from extrabasinal areas to the coastal regime in log rafts composed of hollow trees pebble accumulations represent a partial infilling of the logs while they lay on a gravel covered stream bed at extrabasinal sites cobbles and boulders were transported entrapped within root structures of trees introduced into the river or may have been trapped on or within log rafts only a limited number of clasts were available for study because the mining bench from which they were collected was exposed for a limited time and is now reclaimed a suite of 300 clasts consists of metamorphic rocks phyllite quartzite schist and gneiss igneous rocks granodiorite and pegmatitic quartz and sedimentary rocks coarse to fine grained subarkose the dominance of low and high grade metamorphic plutonic igneous and sedimentary rocks and the absence of volcanic or volcaniclastic rocks indicates an intensely deformed and uplifted source area when this suite of rocks is compared with the two possible orogenic sources ouachitas or southern appalachian the southern appalachian orogen conforms best this interpretation modifies presently accepted models and indicates an eastern expansion of the source area from the ouachitas to the adjacent part of the appalachians during the early pennsylvanian

the electric field in northern england and southern scotland implications for geomagnetically induced currents summary magnetotelluric mt data in the form of mt tensors are used to estimate directly the size and spatial distribution of the electric field in northern england and southern scotland with the aim of predicting the flow of geomagnetically induced currents gic in power networks in the region mt and geomagnetic deep sounding data from a number of different field campaigns at a period of 750 s are employed the mt data are cast in the form of telluric vectors which allow a joint hypothetical event analysis hea of both geomagnetic deep sounding and mt data this analysis reveals qualitatively the pervasive effects of electric field distortion in the region two approaches are taken to understand how the spatial structure of the regional electromagnetic field is affected by local distortions and what the origin of these distortions might be the dimensionality and form of electric field distortion of the mt tensors is investigated using the weaver et al and bahr classification schemes and by examining the misfit of a galvanic distortion model as a function of rotation angle at sites where the galvanic distortion model is found to be appropriate the regional mt tensors are recovered using tensor decomposition techniques it is found that recovering the regional mt response reconciles the geometry of induced currents implied by the mt data with that of the magnetic variation anomalies lilley s central impedances are used to calculate rotationally invariant effective telluric responses in the southern uplands the magnitude of the effective telluric response is approximately 0 25 0 5 mv km 1 nt 1 but as the southern uplands fault is approached it rises steadily to 3 mv km 1 nt 1 in the midland valley the effective telluric response is approximately 0 5 mv km 1 nt 1 which rises steadily to 2 5 mv km 1 nt 1 as the southern uplands and highland boundary faults are approached to the southeast and northwest respectively therefore the increase in the magnitude of the effective telluric response correlates with the approach of a major tectonic boundary such as the southern uplands fault these results show that the induced electric field strength varies considerably throughout the central scotland region in addition the hea indicates that due to lateral changes in conductivity structure the direction of the electric field deviates significantly from the regional direction implied by the polarization azimuth of the primary geomagnetic induction therefore any attempts to model the flow of gic in the region need to account for the spatial variation of both the magnitude and azimuth of the electric field

application of sequence and genetic stratigraphic concepts to carboniferous coal bearing strata an example from the black warrior basin usa carboniferous strata provide an excellent example on which to test the application of genetic stratigraphic and sequence stratigraphic concepts both approaches are employed in the evaluation of the coal bearing strata of the black warrior basin south eastern usa bounding hiatal surfaces have been recognized in the succession of rock that includes the mary lee coal zone within the framework of genetic stratigraphy one genetic sequence has been identified comprised of offlap progradational and aggradational facies and onlap aggradational and transgressive facies components seven parasequence sets have been delimited according to the concepts of sequence stratigraphy these have been ascribed to transgressive highstand and shelf margin systems tracts the identification of components of these contrasting frameworks provide the basis for evaluating other carboniferous strata

detrital history of the lower pennsylvanian pottsville formation in the cahaba synclinorium of alabama u s a abstract the greater black warrior basin gbwb is an important late paleozoic depocenter of the southern appalachian foreland basin containing a thick 2 5 km synorogenic clastic wedge belonging to the lower pennsylvanian pottsville formation pottsville clastics are cyclothemic consisting of interbedded sandstone siltstone claystone shale and numerous coal beds with orthoquartzite sandstone and conglomerate at the base basal pottsville is dominated by quartz rich sediments feldspars increase upsection presence of heavy minerals including garnets and rutile suggest exhumation of deeper sections of orogenic belts for pottsville sandstones compositions of these sandstones qt 60 f 7 l 33 reflect a recycled orogenic provenance 40 ar 39 ar analysis of detrital muscovite from the pottsville formation yields ages as young as early pennsylvanian that appear to reflect a very brief lag time for the deposition of some of the pottsville sediment prominent detrital muscovite age modes of ca 460 450 ma 380 370 ma and 330 320 ma are evident among these samples and are interpreted to represent sediment from catchments containing taconian acadian and alleghanian lithologies we interpret the gbwb orogenic repository to have received detritus via lower pennsylvanian drainages of the southern appalachians analogous to the modern ganges brahmaputra drainages of the himalayas there are dramatic differences in single crystal 40 ar 39 ar age modes for muscovite among these samples that likely reflect combinations of changes in transverse and longitudinal sediment supply alleghanian tectonics and progressive development of appalachian catchments

a geophysical approach to the granite batholith under the eastern southern uplands scotland from our interpretation of the bouguer gravity and aeromagnetic anomalies in south east scotland we conclude that a massive granite batholith underlies the greater part of the eastern southern uplands the granite model which we computed earlier from gravity anomalies in the tweeddale area fits the observed magnetic anomalies closely if a normal magnetization of 0 095 a m 1 is assigned similar to values found for exposed local granites further gravity modelling shows that apart from the tweeddale boss the granite shallows to less than 1 km near lammer law in east lothian and extends north of the lammermuir fault a model for the east lothian volcanics was computed from their aeromagnetic anomalies then their gravitational effect was combined with that estimated for the devonian and carboniferous sediments and the result stripped off the observed gravity field the residual gravity anomalies were used to generate a two dimensional model for the granite north of the lammermuir fault the expected tectonic consequences of a massive granite batholith in the eastern southern uplands are compared with the known development of faults and sedimentary basins around its margins

rare earth element distributions among minerals in a granodiorite and their petrogenetic implications a study of the distribution of lanthanide rare earths in a granodiorite from the eastern peninsular ranges batholith southern california reveals that a large fraction of the ree in this rock resides in the accessory phases sphene and allanite the minerals plagioclase alkali feldspar biotite epidote and apatite each contribute approximately 1 or less of each ree to the whole rock with the exception of eu for which plagioclase contributes 7 sphene and allanite together contain 80 to 95 of each ree each of these phases is zoned in ree concentration with substantial decreases from core to margin textural observations argue for relatively early saturation and precipitation of sphene and allanite in the magma ree zoning trends in sphene and allanite and unexpectedly low ree concentrations in largely later crystallizing minerals such as feldspar indicate that the precipitation of sphene and allanite significantly reduced ree concentrations in residual melts these results illustrate the potential that sphene and allanite have for controlling the behavior of ree in granitic magmas available information collectively suggest that the sampled granodiorite existed as a complete melt that the ree contained in the assemblage of phases were derived by direct crystallization from the melt and that the melt behaved essentially as a closed system once crystallization of the phases now present began close correspondences between the major and trace element chemistries of the granodiorite and phenocryst poor lavas from similar tectonic settings support these conclusions the ree pattern of the granodiorite melt appears to have originated at depth and is characteristic of its source regions and derivation mechanism the high liquidus temperature of a granodiorite melt 1000 c indicates the importance of mantle derived components within the sources of batholithic magmas in the peninsular ranges

geophysical images of the deep crust the iapetus suture the iapetus suture arguably the most fundamental lineament of british and irish structure has been previously identified on the birps winch deep seismic reflection profiles offshore as a nw dipping feature new depth migrated interpretations of these and other reflection data show good correlation of structure for 100 km along strike in the northern irish sea towards magnetotelluric stations onshore in the north of england and southern uplands of scotland three different methods of inverting data from these sites all reveal a thin nw dipping slab of high conductivity a contour map of the suture trace dipping at 15 25 nw can be constructed down to and through the moho at 28 km depth this map suggests crustal stretching below the solway basin the mylonitic suture zone is characterized by good seismic reflection layering high conductivity presumably fluid produced and a steep increase of p wave velocity over the few kilometres of its width crustal refraction experiments lispb and cssp provide useful velocities but locally misleading structure so that the combination of the seismic reflection and geoelectric methods may be the most powerful means of determining deep crustal structure with a good degree of both vertical and lateral resolution

upland ecology of some late carboniferous cordaitalean trees from nova scotia and england abstract permineralised logs and charcoal fragments composed of dadoxylon type wood and derived from cordaitalean trees are described from the upper carboniferous westphalian of nova scotia and england they exclusively occur in sandstone bodies interpreted as the deposits of braided meandering and anastomosing river channels the architecture and ecology of this type of cordaitalean has not yet been satisfactorily described estimates of tree height using an allometric approach suggest that the largest trees attained an approximate height of ca 45 m the absence of true growth rings in the logs indicate that they grew under non seasonal humid tropical conditions however the presence of charred wood fragments implies that droughts of sufficient intensity to permit wildfire did occasionally occur the charcoal fragments are usually associated with incised poorly sorted chaotically bedded channel facies interpreted as flood deposits dramatic increases in runoff and sediment discharge are known to follow the destruction of vegetation in modern wildfire events and the high energy charcoal bearing channel facies described may represent a similar geomorphic response to cordaitalean vegetation fires geomorphic sensitivity to fire is greatest for steeply sloping terrains and it is therefore probable that the cordaitaleans described in this paper occupied upland settings this palaeoecological interpretation is supported by occurrence of the cordaitalean remains in river channel deposits immediately downstream of regions of palaeo upland and by the palynological record the presence of upland forests in late carboniferous times has important implications for numerical modelling of global geochemical cycles

magnetic variation anomalies in northern england and southern scotland single station transfer functions linking the time variations of the vertical and horizontal components of the magnetic field at stations in northern england and southern scotland have been compiled into a uniform data set from hypothetical event analysis we show here that there are two distinct anomalies in the borders region one runs south west to north east immediately to the south east of the southern uplands fault the second follows the northumberland basin and seems to exist because the conductive sedimentary rocks filling the basin create a link between the irish and north seas if the lapetus suture is marked by a conductivity anomaly as has been suggested1 2 these results place it beneath the southern uplands unless it is masked by the surface conductor in the northumberland basin

a high resolution magnetotelluric survey of the iapetus suture zone in southwest scotland summary magnetic variation mv surveys show a 40 km wide zone of high electrical conductivity in the middle crust extending across the southern uplands of scotland from the north sea to galloway a new magnetotelluric mt survey across the conductor in galloway was designed to improve the resolution of structure in the depth range 1 15 km and at the same time assess the improvement in resolution that could be achieved in a practical situation by optimizing data acquisition processing analysis and modelling a 40 km long profile was measured doubled for part of its length at a separation of 1 km with a typical site spacing of 2 km or less it was supplemented by shorter parallel and orthogonal lines giving a total of 40 sites galvanic distortion models involving regional 2 d structures gave poor fits to the impedance tensors both for the array as a whole and for individual sites confirming the picture from induction arrow maps that many sites were influenced by 3 d structures on scales of a few kilometres and upwards other indicators of the electrical strike regional and local mv measurements the spatial structure of the groom bailey regional impedance phase and of the rotated off diagonal phase gave values more in agreement with the geological strike of n52 e which was adopted as the most appropriate coordinate system into which to resolve the data the mt impedances were accordingly rotated into directions n38 w tm and n52 e te and inverted using the 2 d code of rodi mackie the investigation explored the sensitivity of the outcome to a wide range of starting model resistivities and roughness parameters and to different data subsets the te mode data were insensitive to structural detail and the best fit achieved was unacceptable suggesting the influence of structure outside the plane of the section the tm mode data could be fitted satisfactorily by varying the model roughness and gave the best control on the geometry of the conductive bodies forward modelling showed that the model generated from the tm mode also fitted the mv data adequately the inversion resolved the conductive zone into distinct blocks with edges matching faults mapped in the surface geology the leadhills fardingmullach and orlock bridge faults and the moffat valley lineament in the resistivity image the faults are vertical structures disrupting other features to a depth of 15 km which agrees with the view that although they originated as thrusts they were reactivated by strike slip motion during oblique closure of the iapetus ocean within some of the blocks are highly conductive regions no more than 4 km wide the most likely explanation is graphite or other metallic mineralization localized in shear zones

inversion tectonics in central alborz iran abstract structural analyses carried out in the southern central alborz iran have shown that the evolution of this belt has been strongly conditioned by the inversion of pre existent extensional faults inversion tectonics has been identified especially in the southern part of the belt and it is related to the reactivation of grabens formed in the foreland of the late triassic eo cimmerian orogen resulting from the accretion of the iranian block to eurasia three main associations of structures active during the late cenozoic have been distinguished 1 e w thrust faults and folds followed by the activation of 2 e w right lateral strike slip faults associated to large ene wsw trending en echelon folds and finally 3 ese wnw to se nw thrust faults and left lateral strike slip faults inverting in some cases previous e w right lateral faults central alborz is strongly controlled by the geometry of pre existing tectonic discontinuities which are responsible for strain partitioning between strike slip and reverse faults during convergence these results provide new insights on the style and evolution of this complex intracontinental belt which can be considered a significant example of transpressive tectonics

local and regional induction in the british isles electric currents induced in the shallow seas and deep ocean around the british isles have a profound effect upon the electromagnetic fields observed at stations on the land the configuration of anomalous currents changes with frequency and causes corresponding changes in the geomagnetic transfer functions magnetic variations have been recorded at a dense network of sites in southern scotland and northern england single station transfer functions have been used to generate hypothetical event maps of the anomalous vertical field and hence to infer the configuration of the anomalous internal currents at periods exceeding 2000 s the vertical field is dominated by the effects of electric currents to the west presumably in the atlantic ocean in the period range 400 2000 s anomalous currents are concentrated in a thin sheet comprising the shallow seas the t hick sequences of post caledonian sedimentary rock which underlie them and the extensions of the sedimentary basins into the land the response of the individual basin is determined not only by its local conductivity structure but also by the extent of its connection to the shallow seas i e its regional importance within the conductive sheet at periods less than 200 s on the other hand the anomalous fields at inland sites are principally determined by the local geological structure these results confirm conclusions reached from theoretical studies of electromagnetic induction in a heterogeneous surface layer park orange madden that electromagnetic response data measured in a 3 d environment such as the northumberland basin must be interpreted using 3 d models if one or 2 d models are used the data must be corrected on the basis of regional measurements

a type granites in northeastern china age and geochemical constraints on their petrogenesis abstract a type granites are widely distributed in northeastern china ne china they were emplaced during three major episodes the permian late triassic to early jurassic and early cretaceous and evolved in different tectonic regimes according to their mineralogical and geochemical characteristics two subgroups of a type granites aluminous and peralkaline can be recognized the peralkaline subgroup contains alkali mafic minerals such as riebeckite arfvedsonite and sodic pyroxene while the aluminous subgroup contains annite and fe rich calcic or sodic calcic amphibole with respect to the aluminous subgroup the peralkaline granites contain higher rb ga and total rare earth elements ree but lower mgo cao al 2 o 3 ba and sr based on the discrimination criteria of eby geology 20 1992 641 the permian and late triassic to early jurassic a type granites belong to the a 2 post orogenic type whereas the early cretaceous granites are of the a 1 anorogenic type nd isotopic compositions of these a type granites indicate their derivation from a dominantly juvenile crustal source their origin is thought to have involved partial melting of an underplated lower crustal source because the generation of a type granites requires high melting temperature we propose models involving slab break off lithospheric delamination and extension the permian a type granites have the same age range as those in eastern junggar southern mongolia and central inner mongolia they occur along a major suture and form a narrow belt between the north china and siberian cratons we suggest that their formation was associated with post collisional slab break off the late triassic to early jurassic a type granites are likely to be the product of lithospheric delamination after the final collision of the major crustal blocks in the late paleozoic to early triassic the early cretaceous a type granites have an anorogenic affinity and were possibly associated with rifting in eastern china at this time associated with the onset of paleo pacific subduction consequently we conclude that the a type granites in ne china were generated at three different times involving multiple processes operative in different tectonic environments

inversion of electromagnetic induction data for the iapetus suture zone in the uk abstract the iapetus suture zone isz corresponding to the join between the european crust of northern england and the north american derived crust of scotland has been examined by numerous seismic and induction experiments over a 20 year period the university of edinburgh electromagnetic em induction group is in the process of building a database comprising magnetotelluric and magnetic variation stations from this region in this paper we present 2d inversions of selected induction sites along roughly north south traverses across the isz in northern england and southern scotland the first inversion involves both the transverse electric te and transverse magnetic tm modes for 10 audio magnetotelluric approximately 100 0 1 hz sites care has been taken to remove the effects of static shifts this inversion resolves a conductive feature beginning under the southern northumberland trough at depths of 10 km dipping to the south below the alston block in the second inversion 12 longer period magnetic variation sites were added to the original data set improving the resolution of this feature which correlates well with a dipping bright reflector detected by the north east coast nec seismic reflection experiment in addition the southern uplands anomaly is imaged no north dipping mid to deep crustal conductor is observed which would correlate to north dipping palaeozoic subduction under laurentia