kilometre scale folding in the teslin zone northern canadian cordillera and its tectonic implications for the accretion of the yukon tanana terrane to north america reply the teslin zone in south central yukon has previously been described as a discrete zone with a steep foliation unique to the zone it includes the anvil assemblage and the narrowest portion of the

linked thrust and strike slip faulting during late cretaceous terrane accretion in the san juan thrust system northwest cascades orogen washington the san juan thrust system makes up the western elements of the late cretaceous northwest cascades orogen of washington a stack of metamorphic nappes with terranes ranging in age from early paleozoic to middle cretaceous the tectonic development of the nappes is an unresolved issue important to understanding plate convergent processes operating in the north american cordillera in the creta ceous as well as at other times two terrane bounding fault zones were studied in this project the lopez and the rosario fault zones the fault zone rocks were affected by two cataclastic plastic late cretaceous deformational events d1 and d2 that included accretion and lateral motion during blueschist facies metamorphic conditions d1 produced upright folds and a subvertical axial planar s1 foliation fabric in the lopez fault zone and southwest vergent folds and gently northeast dipping axial planar s1 foliation in the rosario fault zone in the lopez fault zone d2 produced northwest striking and subvertical strike slip shear zones s2 with dominant sinistral displacements whereas in the rosario fault zone d2 generated low angle s2 shear zones and various slip crenulation fabrics formed by northwest directed thrust motion the d1 fabrics are inferred to be related to initiation of subduction related accretion of the nappes by orogen normal southwest northeast shortening and accretionary tectonics the subsequent d2 fabrics developed as temporally overlapping subduction related low angle thrusts in the rosario fault zone and high angle sinistral strike slip faults in the lopez fault zone in an overall transpressive setting due to oblique convergence this model suggests that the high angle lopez fault zone d2 was located above a gently northeast dipping thrust detachment s2 expressed by the rosario fault zone that allowed the accreted nappes to move parallel to the orogen the kinematic model cannot validate regional scale dextral terrane translation of the san juan thrust system as inferred by paleomagnetic data elsewhere in the western united states but addresses important tectonic processes caused by oblique subduction and changing plate conditions

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

volcanics and structure of the famous narrowgate rift evidence for cyclic evolution amar 1 a near bottom photographic survey on the mid atlantic ridge from the famous region south to transform b reveals structural and volcanologic evidence for an alternating widening and narrowing rift valley extension wedges appear to be propagating south from transform a and north from transform b these merge at mt mars where the rift valley is narrowest and shallowest narrowgate giving the valley a symmetrical hourglass shape three major volcanic en echelon lineaments trend n10 e 10 oblique to the strike of the inner walls faulting and fissuring are antisymmetrical across the rift valley in contrast to the major extension this contrasting antisymmetry reflects the surficial distribution of stress across crust of varying strengths whereas the large scale wedge phenomenon determines the periodicity of transition from a narrow valley to a wide valley using vector analysis the extension wedges are propagating at about 9 cm yr toward each other at this rate a narrow rift valley will be fully transformed into a wide rift in 0 64 m y

formerly emerging crustal blocks in the equatorial atlantic abstract anomalous crustal topographic highs exceeding the level predicted by the thermal contraction model by up to 2 3 km are observed along the romanche transform zone in the equatorial atlantic previous studies of shallow water reef limestones recovered from one of the shallowest sites on these crustal highs indicated that this site was at or above sea level 5 million years ago and subsided since at an average rate one order of magnitude faster than the subsidence estimated by thermal contraction of the crust seismic reflection profiles obtained across the romanche transform zone suggest that the anomalous highs are capped by reef limestones not only where limestones were actually sampled but also at other locations these findings support the idea that long segments of crust reached close to sea level in the past along the romanche transform zone the vertical crustal motions are probably caused by tectonism typical of long offset transforms inasmuch as the romanche has been a ridge ridge transform since the earliest stages of the opening of the equatorial atlantic it is likely that intense vertical tectonic motions occurred along it throughout the evolution of the atlantic support for this hypothesis is provided by the recovery during dsdp leg 4 of shallow water reef limestone of the eocene age from the summit of the north brazilian ridge along the western extension of the romanche fracture zone the presence of shallow or emergent crust across the equatorial zone during the early stages of opening had probably important consequences upon the water circulation between the north and the south atlantic and may even have provided land bridges for faunal migrations between africa and south america in early cenozoic times after the two continents had already separated

bowser basin northern british columbia constraints on the timing of initial subsidence and stikinia north america terrane interactions clastic strata composing the northern bowser basin record the accretion of stikinia to the composite western edge of the north american plate cache creek quesnellia slide mountain kootenay north america in early middle jurassic time and the concomitant demise of the intervening cache creek ocean initial flexural subsidence of the northern bowser basin resulting from thrust loading of cache creek terrane on stikinia is represented by an organic rich shale of aalenian age the abou formation of the spatsizi group coarse grained sediment first appeared in early bajocian time following uplift and subaerial exposure of cache creek rocks in the upper plate thus the inception of the bowser basin was aalenian rather than bajocian as believed by earlier workers aalenian southwest vergent thrusting at the composite western edge of north america is also known from southern british columbia a coincidence that implicates collision with stikinia in the south as a cause of that deformation

bending anisotropy a mechanical constraint on the orientation of fold axes in an anisotropic medium abstract bending anisotropy is the property of a layer of material whereby it bends more easily in some directions than it does in others in macroscopically homogeneous layers bending anisotropy results from the material itself being rheologically anisotropic in the plane of the layering in this paper we investigate bending anisotropy in materials with orthorhombic symmetry and linear elastic or viscous behaviour although such models are rheologically simple we believe they may provide a first approximation to the behaviour of some tectonites with linear and planar fabrics thus we predict that in rocks with strong linear fabrics folds will form with axes nearly parallel to the linear fabrics for a variety of different stress fields this has important geological consequences

tectonic affinity of nisutlin and anvil assemblage strata from the teslin tectonic zone northern canadian cordillera constraints from neodymium isotope and geochemical evidence we present geochemical and isotopic data for nisutlin assemblage metasedimentary rocks and anvil assemblage greenstones from the teslin tectonic zone of the northern canadian cordillera this study aims to establish the tectonic setting of formation for the sedimentary and basaltic protoliths of these highly deformed and metamorphosed rocks and thereby place constraints on the origin of these enigmatic rocks for which differing tectonic models have been proposed for the nisutlin assemblage metasedimentary rocks the geochemical and isotopic data show that two widely different source regions contributed detritus to the original sediments one source region was felsic upper crustal material with nd isotopic compositions compatible with ultimate derivation from the north american continent depleted mantle model age tdm 2 5 2 8 ga the second source region is deduced to be chemically primitive crust basaltic andesitic with a short crustal residence history 0 9 ga for the anvil assemblage greenstones immobile trace element abundances are dissimilar to within plate oceanic arc and normal mid ocean ridge basalts and similar to calc alkaline basalts in active continental margin settings we interpret the paleosetting for the nisutlin assemblage to be at the outermost margin of the ancestral north american continent in an area which received detrital input from the distal north american craton in the early paleozoic however this area also received detritus from a chemically and isotopically juvenile magmatic arc source a source type not known from paleozoic metasedimentary rocks from the miogeoclinal sequence on the basis of similar nd isotopic relationships recorded elsewhere in deformed paleozoic rocks of the orogen we infer that these geochemical signals reflect tectonic processes of regional extent the trace element geochemistry of the anvil assemblage greenstones does not support a correclation with known paleozoic greenstones of the slide mountain terrane which some tectonic models have advocated

backflow and margin parallel shear within an ancient subduction complex lineated and foliated tectonites of the teslin suture zone yukon represent the deep seated part of a west dipping subduction complex that formed seaward of western north america in permian triassic time the tectonites were progressively underplated onto the hanging wall plate during b type subduction with continued subduction and underplating the boundary between the hanging wall and subduction channel migrated oceanward resulting in growth of the hanging wail plate at depth these tectonites preserve a progressive record of backflow dextral margin parallel shear and downflow within the subduction channel early jurassic a type subduction of the leading edge of western north america resulted in eastward overthrusting of the tectonites as a coherent block onto north american strata these tectonites provide empirical evidence that backflow margin parallel displacement and downflow can all occur within the deeper parts of subduction zones

evidence for large scale rotations at spreading centers from the josephine ophiolite abstract repetition of the josephine ophiolite by large scale open folding and imbricate faulting provides an excellent opportunity to study the geometric relationships between sheeted dikes igneous layering and bedding measurements of over 600 dike margins from the sheeted dike complex and high level gabbro indicate that dikes are oriented at 20 40 to bedding in metasedimentary rocks overlying the ophiolite in addition igneous layering is discordant to gross lithologic layering and is oriented at a large angle to both sheeted dikes and bedding similar relationships may occur in several other ophiolites the inclined sheeted dikes and discordant igneous layering are best explained by a 45 rotation of new oceanic crust ophiolite immediately following formation at a spreading center rotation could have been accomplished by either subsidence or faulting further evidence for large scale rotations of oceanic crust include frequent anomalous magnetic inclinations in drill cores of layer 2 from the atlantic

geochemical effects of primary migration of petroleum in kimmeridge source rocks from brae field area north sea ii molecular composition of alkylated naphthalenes phenanthrenes benzo and dibenzothiophenes abstract variations with depth in the quantitative distributions of di and tricyclic alkylaromatic hydrocarbons and sulfur heterocycles determined for a total of thirty two core samples from two thick shale intervals and thirteen thin interbedded shale layers of the upper jurassic age kimmeridge clay formation brae field area north sea reveal in part the effects of depletion with hydrocarbon expulsion trends of increasing depletion with approach to the nearest shale sandstone contacts as documented by a decrease in carbon normalized yields are more distinctive for the alkylnaphthalenes as compared to the alkylphenanthrenes and alkyldibenzothiophenes but less drastic than previously observed for long chain alkanes cf part i leythaeuser et al 1988 lowest yields consistently were encountered at the outermost 1 2 metres of the thick shale intervals and for the thin shale layers where the relative expulsion efficiencies were much higher for c15 to c30n alkanes 80 95 than for c2 and c3 alkylnaphthalenes 30 40 in contrast c2 and c3 alkylbenzothiophene yields were higher in these shale portions than further inside the thick shale which is interpreted to result from a post expulsion selective replenishment from the adjacent oil accumulation expulsion was not associated with any detectable fractionation among c1 and c3 alkylnaphthalenes or c1 and c2 alkylphenanthrenes as evidenced by a mass balance approach and concentration ratios of individual alkylaromatic isomers commonly used as maturity indicators based on a reconstruction of the quantitative distributions of aromatic hydrocarbons and sulfur aromatics expelled from thin shale layers from early mature through to mature stages of thermal evolution it was verified that the methylphenanthrene index mpi 1 and methyldibenzothiophene ratio mdr of the expelled oil reflects the average of the maturity levels during which the source rock has expelled petroleum

episodicity of mesozoic terrane accretion along the pacific margin of gondwana implications for superplume plate interactions a review of evidence for deformation and terrane accretion on the late triassic early jurassic margins of pangea and the mid cretaceous margins of the palaeo pacific ocean shows that deformation was global and synchronous with probable superplume events late triassic early jurassic deformation appears to be concentrated in the period 202 197 ma and was coeval with eruption of the central atlantic magmatic province onset of pangaea break up a period of extended normal magnetic polarity and a major mass extinction event all possible expressions of a superplume event mid cretaceous deformation occurred in two brief periods the first from approximately 116 to 110 ma in the west palaeo pacific and the second from roughly 105 to 99 ma in the east palaeo pacific with both events possibly represented in northeast siberia this deformation was coeval with eruption of major oceanic plateaus core complex formation and rifting of new zealand from gondwana the cretaceous normal polarity epoch and a major radiation of flowering plants and several animal groups all linked with the mid cretaceous superplume event a simple unifying mechanism is presented suggesting that large continental or oceanic plates when impacted by a superplume tend to break up reorganize associated with gravitational spreading away from a broad thermally generated topographic high and with a resulting short lived pulse of plate marginal deformation and terrane accretion

mississippian assembly of the nisutlin assemblage evidence from primary contact relationships and mississippian magmatism in the teslin tectonic zone part of the yukon tanana terrane of south central yukon metamorphosed and ductilely deformed sedimentary plutonic and volcanic rocks of the nisutlin and anvil assemblages make up the yukon tanana terrane in the teslin tectonic zone study area the nisutlin assemblage consists of siliceous schist quartzite and graphitic phyllite that share a primary depositional contact and early mississippian tonalite to quartz diorite that intrudes the siliceous schist quartzite and possibly the graphitic phyllite the anvil assemblage includes metagabbro and mafic schist greenstone that share an intrusive contact relationship tonalite to quartz diorite of the nisutlin assemblage is characterized by minor zircon inheritance with an average proterozoic age end 350 ma values of 2 5 to 6 2 and nd model ages of 1 50 1 79 ga these data suggest that the magmatic bodies have inherited a component of continentally derived material primary contact relationships and age data indicate that the nisutlin assemblage had formed by mississippian time and regional correlations sho

form and dimensions of dykes in eastern iceland abstract a study was made of about 700 dykes in eastern iceland the majority of these belong to three swarms about 73 dip within 10 of the vertical most strike between 10 and 40 ne the strike of the dykes within the southernmost swarm alftafjordur changes along its trace from almost n at the north end to nne ssw southward along the swarm the average thickness of the dykes is about 4 1 m and the thickness does not change notably along the alftafjordur swarm the thinner dykes tend to have smaller grains than the thicker dykes of five dykes followed laterally the longest is over 22 km the thickness of individual dykes changes irregularly along their length and the dyke is often offset where its thickness changes many dykes appear to be completely discontinuous but some parts are connected by veins where the dykes end in a vertical section most of them simply taper away only about 10 5 of the dykes occupy faults the mechanical and thermal effects of the dykes on the country rock are small many of the dykes appear to be non feeders i e dykes that never reached the surface to feed lava flows using the length width ratio the depth of origin of three dykes has been estimated the maximum depth of origin of these three dykes is 7 5 km 9 km and 10 km below the original surface

lithospheric structure of the yukon northern canadian cordillera obtained from magnetotelluric data 1 two goals of lithoprobe s geoscientific studies in the phanerozoic accretionary cordillera of western north america were to define the subsurface geometries of the terranes and to infer the physical conditions of the crust these questions were addressed in canada s southern cordillera a decade ago and have more recently been addressed in the northern cordillera of which one component of the new studies is magnetotelluric mt profiling from ancestral north american rocks to the coast we present a resistivity cross section and its interpretation of the northern cordillera derived from modeling data from 42 mt sites along a 470 km long ne sw profile beneath the coast belt southwestern end of the profile a deep crustal low resistivity layer dips inland we interpret the crustal part of this conductor as being due to metasedimentary rocks emplaced and metamorphosed during paleocene kula plate subduction a strong lateral transition in lithospheric mantle resistivity exists below the intermontane belt that is spatially coincident with changes in chemical and isotopic characteristics of tertiary to recent alkaline lavas suggesting that isotopically enriched lithosphere related to the coast belt basalts extends partly beneath the intermontane belt the unusually high lower crustal resistivity in the intermontane and omineca belts similar in value to the resistivity found in the unextended part of central british columbia excludes the presence of fluids or conducting metasediments finally our resistivity model displays strong lateral variation of the middle and lower crust between different terranes within the same belt as a result of the complex structural evolution of the lithosphere index terms 0925 exploration geophysics magnetic and electrical methods 1515 geomagnetism and paleomagnetism geomagnetic induction 8110 tectonophysics continental tectonics general 0905 8120 tectonophysics dynamics of lithosphere and mantle general keywords magnetotelluric accreted terranes northern canadian cordillera

patterns of folding and strain influenced by linearly anisotropic bands are there characteristic patterns of deformation in rocks dominated by linear fabrics detailed examination of deformed calcite fibres shows strain partitioning into shortening normal to the fibres achieved by pressure solution by buckling of inclusion bands within the fibres and shear parallel to the fibre long axes folding of rocks with opposed directions of anisotropy may lead to discontinuities due to the incompatibility of the folding directions

eclogitic rocks in the st cyr klippe yukon and their tectonic significance until recently the nisutlin allochthonous assemblage a part of the yukon tanana composite terrane interpreted as trench melange from a late paleozoic mesozoic arc system was the only tectonic

p t evolution of the teslin suture zone and cassiar tectonites yukon canada evidence for a and b type subduction mineral composition and quantitative thermobarometric studies indicate that the teslin taylor mountain and nisutlin terranes within the teslin suture zone tsz yukon record widespread high p t metamorphic conditions consistent with subduction zone dynamothermal metamorphism the highest p t conditions 575 750 c and 9 17 kbar are preserved in tectonites formed during normal dip slip ductile shear dextral strike slip tectonites record lower p t conditions 400 550 c and 5 8 kbar and tectonites which show reverse shear have peak temperatures of c 420 c and a minimum peak pressure of 3 kbar dynamothermal metamorphism took place in a west dipping b type subduction zone outboard of western north america in permo triassic time tsz tectonites were underplated against the hangingwall plate of the subduction zone following subduction of the ocean basin which separated north america from the hangingwall plate tsz tectonites were overthrust eastward as a coherent structural package as a result of a type subduction of cassiar strata in early jurassic time par autochthonous cassiar tectonites which comprised the leading edge of the western north american margin record prograde moderate p high t metamorphism 550 750 c and 7 13 kbar synchronous with top to the east ductile shear metamorphism occurred as a result of subduction of the north american margin into the tsz subduction zone in early jurassic time following metamorphism cassiar tectonites cooled slowly from 500 to 300 c during the period middle jurassic to middle cretaceous tsz and cassiar tectonites were deformed during changing p t conditions data from each of these tectonite packages indicate that grain scale strain partitioning may have allowed local recrystallization of individual minerals by the addition of mechanical energy the composition of the new grains reflects the p t conditions under which that particular grain was deformed

gondwana derived terranes in the northern hellenides the hellenides constitute an integral part of the alpine orogenic system in southeast europe despite the recognition of several subparallel zones which are interpreted as terranes separated by ophiolitic sutures e g pindos and vardar sutures the classical view of an orogen with a foreland fold and thrust belt a central crystalline zone and a rather undeformed hinterland is still under discussion this paper concentrates on basement terranes of exotic provenance in two of the internal zones of the hellenides that support the interpretation of the hellenides as an accretionary orogen formed by amalgamation of crustal segments during the subduction of tethyan oceanic basins the oldest of these terranes the florina terrane in the pelagonian zone is composed of neoproterozoic arc related orthogneisses two other exotic terranes occur east of the vardar zone within the serbo macedonian massif the pirgadikia terrane is a microterrane in the southern chalkidiki peninsula that consists of panafrican mylonitic orthogneisses with volcanic arc related trace element geochemistry and sr isotopic composition the vertiskos terrane occupies the northwestern part of the serbo macedonian massif and is primarily composed of coarse grained volcanic arc related peraluminous orthogneisses of silurian age these terranes are exotic in relation to the other parts of the hellenides the provenance of the late proterozoic pan african florina and pirgadikia terranes is assumed to be gondwanan whereas the silurian vertiskos terrane may have been part of the so called hun terrane which formed at the northern active continental margin of gondwana in the early paleozoic

late proterozoic and silurian basement units within the serbo macedonian massif northern greece the significance of terrane accretion in the hellenides

the movement of hydrocarbons in shales oil and gas hydrocarbons are formed at temperatures above 100 c by the chemical breakdown of the organic remains of dead organisms preserved in fine grained rocks called source rocks the organic remains are known as kerogen if enough hydrocarbons are formed some are expelled from the source rocks we have analysed kimmeridge clay upper jurassic shales from the north sea which are currently expelling hydrocarbons into adjacent sandstones the results could be explained if most expulsion occurs by the pressure driven flow of a discrete hydrocarbon phase through the pores of shales and into the pores of the sandstones as suggested previously1 in addition capillary forces seem to increase the efficiency of expulsion close to the interfaces between the shales and interbedded sandstones it appears that molecular diffusion of hydrocarbons with less than about ten carbon atoms alters the composition of the hydrocarbon fluids remaining in and expelled from the shales

seismic characteristics and identification of negative flower structures positive flower structures and positive structural inversion negative and positive flower structures and positive inverted structures imply specific modes of formation and their distinctive characteristics make them important criteria for the identification of certain structural styles a negative flower structure from the andaman sea consists of a shallow synform bounded by upward spreading strands of a wrench fault that have mostly normal separations paralleling monoclines and oblique en echelon normal faults flank the divergent wrench fault a positive flower structure from the ardmore basin oklahoma consists of a shallow antiform displaced by the upward diverging strands of a wrench fault that have mostly reverse separations en echelon folds are present on either side of this convergent wrench fault flower structures may resemble extensional or contractional fault blocks on individual seismic profiles flower structures are identified by their unique internal fault and fold architecture by their association with a straight and throughgoing zone of deformation and by their lack of a consistent sense of separation or regional upthrown side positive structural inversion at the rambutan oil field south sumatra basin has formed a shallow anticlinorium and has partly uplifted the underlying graben deeper fault segments bounding the graben have retained their normal fault profiles but at shallow levels some of these faults have reverse separations similar changes of separation with depth may be produced by strike slip but sets of faults with listric profiles and nearby blocklike trend patterns identify the inversion as a dip slip event where correlation of reflections is uncertain or deep control is unobtainable positive inverted structures may also resemble contrational fault blocks detached normal fault assemblages and certain negative inverted structures

an examination of the cataclastic fabrics and structures of parts of nisutlin anvil and simpson allochthons central yukon test of the arc continent collision model abstract collision of an arc complex against the yukon part of the north american craton during the mesozoic resulted in northeastward transport of arc rocks onto the craton the arc rocks comprise three distinguishable tectonostratigraphic assemblages called nisutlin allochthon quartz muscovite and chlorite schist derived from sedimentary and intermediate volcanic protoliths anvil allochthon amphibolite gabbro ultramafic rocks and sepentinite and simpson allochthon granitic and granodioritic rocks and schist derived from them deformation recovery and recrystallization structures show that parts of the allochthons are blastomylonite formed at temperatures of 350 700 c and at depths between 15 and 40 km the stacking order of the allochthons is inconsistent and complex and locally the cataclastic rocks are overthrust by autochthonous strata the allochthons are truncated by late steep faults that have mainly strike slip displacement and may be contemporaneous with tintina fault the proposed tectonic history of the region is supported by detailed results of the study

major dextral transcurrent displacements along the northern rocky mountain trench and related lineaments in north central british columbia the northern rocky mountain trench and a number of other prominent lineaments along and east of the eastern margin of the intermontane belt mark faults along which dextral transcurrent movements have been dominant offsets of shelf to off shelf facies boundaries in lower paleozoic rocks indicate a cumulative displacement of at least 750 km and probably 900 km within the system of faults related to those in the northern rocky mountain and the tintina trenches farther west another system of faults appears to offset plutons and stratigraphic assemblages along the eastern margin of the intermontane belt by as much as 300 km these faults including the kutcho and the pinchi connect in part with the teslin suture zone in yukon territory and probably with the fraser river straight creek fault zone in southern british columbia although dextral transcurrent faulting may have taken place between the middle jurassic and early cenozoic the most convincing evidence points to middle cretaceous and particularly to early cenozoic eocene displacements the eocene movements were temporally related to plutonism volcanism lamprophyre dike emplacement high heat flow sedimentation in grabens and rapid uplift of northwesterly trending elongate ranges climactic episodes of granite emplacement particularly in and near the northern omineca crystalline belt at 100 m y 70 m y and 50 m y ago may have been facilitated by changes from dominantly compressional to dominantly transcurrent and related tensional strain

contrasting seismic characteristics of three major faults in northwestern canada the lithoprobe slave northern cordillera lithospheric evolution snorcle profiles crossed three major tectonic zones of the northwestern canadian shield and northern canadian cordillera that are diverse in age and in depth of penetration the oldest 2630 2590 ma the yellowknife river fault zone formed as a strike slip fault in a tensional strain regime reflector attenuation or truncations align vertically beneath the fault trace through much of the crust implying a near vertical fault plane the youngest 60 10 ma the tintina fault zone produced cumulative dextral strike slip displacements of 425 km perhaps 800 km tomographic velocity and ray trace models of reflection data indicate that several fault splays form a tectonic zone 30 km wide at the surface but truncations of deeper crustal reflections suggest that the zone thins in the mid crust and widens near the moho this apparent variable width versus depth of the tintina fault is atypical of major strike slip faults worldwide the teslin fault was an active terrane boundary during accretion of terranes onto north america observed reflection geometries indicate that the juxtapositions of highly contrasting metamorphic grades across the teslin fault are confined to the upper crust along snorcle line 3 implying that the fault soles eastward into a mid crustal detachment at the interpreted top of north american crust the limited depth extent of the teslin fault zone is similar to some models of the san andreas fault and may result from their similar histories as convergent margin structures

structure of the sheeted dike complex of the samail ophiolite near ibra oman the sheeted dike complex is a regionally mappable geologic unit with a consistent position in the samail ophiolite stratigraphy discontinuous exposures of the complex in the ibra area of the southeast oman mountains provide structural data that allow reconstruction of a paleo spreading ridge axis of 347 for the region and suggests a maximum spreading width of 275 km for the ophiolite contact relations within the well exposed al ahmadi section indicate that the complex was formed by multiple parallel intrusion over a narrow zone at the paleo ridge axis split dikes are abundant facing directions indicate a spreading ridge southwest of the present outcrop area in conflict with other spreading axis direction indicators

analysis of petroleum fractions by tlc fid applications to petroleum reservoir description abstract thin layer chromatography and flame ionization detection iatroscan tlc fid offers a rapid and accurate method for quantifying saturated and aromatic hydrocarbons and resin asphaltene fractions in solvent extracts of petroleum source rocks reservoir rocks and crude oils while the instrument is superficially simple special precautions need to be taken in calibration and parameter setting due to complexities arising from the operational characteristics of the detector system nevertheless the technique offers superior speed and precision to conventional mplc analysis of petroleum fractions this paper describes the choice of operational parameters provides comparison with the traditional group type analysis technique mplc and presents some geochemical applications

geochemical evaluation of niger delta sedimentary organic rocks a new insight a geochemical evaluation of niger delta organic matter was carried out using supercritical fluid extraction sfe sample preparation procedure comparison of geochemical significance of gas chromatographic data of rock extracts of sfe with those of soxhlet extraction method from previous studies was made in order to establish the usefulness of sfe in geochemical exploration the assessment of geochemical character of the rock samples from the comparison and interpretation of other geochemical parameters were used to give more insights into understanding the source rocks characteristics of onshore and shelf portions of the niger delta basin the results of the gas chromatographic gc analysis of the rock extracts across the lithostratigraphic units show that pr ph pr nc17 pr nc18 cpi and odd even preference ranged from 0 07 to 12 39 0 04 to 6 66 0 05 to 13 80 0 12 to 8 4 and 0 06 to 8 12 respectively the rock eval pyrolysis data and geochemical ratios and parameters calculated from the gc data showed that most of the samples are mature and have strong terrestrial provenance while a few samples have strong marine provenance the few marine source rocks are located in the deeper depth horizon pr ph and standard geochemical plots indicate that most of samples were derived from organic matter deposited in less reducing conditions i e more of oxidizing conditions while a few samples have predominantly influence of reducing conditions the results of trace metal analysis of older samples from agbada formation also indicate marine and mixed organic matter input deposited in less reducing conditions the results obtained in this study are comparable with those obtained from previous studies when soxhlet extraction method was used and also indicated the presence of more than one petroleum systems in the niger delta

evolution of the yukon tanana terrane evidence from southeastern yukon territory the yukon tanana terrane ytt in southeastern yukon consists predominantly of a mid paleozoic volcanic plutonic arc assemblage built on continental crust the ytt had experienced strong deformation and metamorphism by late triassic time by mid cretaceous the metamorphic complex and late triassic clastic rocks derived from it were imbricated with middle and upper paleozoic ophiolitic sheets the ytt differs markedly from adjacent parts of the north american continental margin in both its depositional and tectonic histories the composition and tectonic history of the ytt is comparable to that of the kootenay and barkerville terranes of southern and central british columbia from which the ytt may have been displaced

effect of co2 flood on geochemistry of mcelroy oil abstract geochemical characterization has been completed on a series of oil samples collected over time from the mcelroy field texas where a pilot co 2 flood project was initiated a few years ago a non waxy 31 gravity o api oil is produced from carbonate reservoirs within the permian grayburg formation the samples were taken from various wells over a time span of one and one half years for monitoring changes in oil composition induced by co 2 injection over time the oil samples taken from about 40 wells shortly after the start of co 2 injection showed a marked reduction 50 in the asphaltene content indicating co 2 induced asphaltene deposition had occurred mapping asphaltene reduction could provide information on the flow direction of co 2 in the reservoir hydrocarbon profiles of the oils from some wells also showed a significant reduction in the heavy paraffin content i e c 25 suggesting deposition of heavy hydrocarbons within the reservoir the observation of a slight increase in oil api gravity is also consistent with precipitation of heavy components from the oil phase due to co 2 injection the organic deposits or flocs which can cause restriction of fluid flow in the reservoir by reducing porosity and permeability have probably contributed to reduction in injectivity experienced by some injection wells shortly after the start of co 2 injection field observations have been duplicated in laboratory core flood experiments to mimic the field recovery process the experiments show that most of heavy organics were deposited near the co 2 injection port suggesting organic deposition in the field occurs near the injection wells rather than producing wells despite the continued co 2 flood some molecular properties such as sterane and hopane distributions remain relatively unchanged these geochemical parameters are commonly derived from ratios of compounds with similar molecular properties and are generally not affected by the co 2 flood as precipitation of heavy hydrocarbons causes little fractionation among these compounds thus oil samples from fields under the gas flood are still viable for correlation source evaluation and maturity determination

geochemical effects of primary migration of petroleum in kimmeridge source rocks from brae field area north sea i gross composition of c15 soluble organic matter and molecular composition of c15 saturated hydrocarbons in mature oil prone source rocks of the upper jurassic kimmeridge clay in the brae field area north sea the yield and the gross composition of solvent extractabte pctroleum likc compounds as well as the molecular proportions of c15 saturated hydrocarbons are primarily controlled by the effects of hydrocarbon expulsion during primary migration within sample series extending from shale centres towards adjacent reservoir sandstones regular and pronounced yield decreases have been documented as well as marked compositional changes for c15 soluble organic matter and its compound class fractions saturated hydrocarbons aromatic hydrocarbons nso compounds and for normal and isoprenoid alkanes relative migration losses indicating relative expulsion efficiencies were determined for the c15 alkanes throughout the outer five meters of thick shales relative expulsion efficiencies increase regularly and drastically for all compounds considered e g exceeding 90 for the c15 n alkanes in the outermost shale samples samples from thin 5 50 cm shale layers interbedded between reservoir sandstones reveal similarly high expulsion efficiencies for n alkanes the degree of depletion remains uniform throughout the molecular range c15 to c30i e no compositional fractionation occurred during primary migration which is interpreted to reflect expulsion of oil as a single phase fluid a simple conceptual model is proposed to explain the mechanisms and relationships of generation and expulsion processes in time and space a geochemically relevant conclusion is that the c15 soluble organic matter recovered by solvent extraction of mature source rocks represents only a portion of the total non volatile petroleum like material originally generated

dsdp hole 504b the first reference section over 1 km through layer 2 of the oceanic crust hole 504b provides a reference section for correlation of in situ petrological and geophysical studies with regional crustal models based on dredged samples and remote surveys and at least for the medium spreading rate costa rica rift hole 504b confirms that the main features of the upper part of an idealized ophiolite sequence occurs in the oceanic crust

blueschist and eclogite in mylonitic allochthons ross river and watson lake areas southeastern yukon two new occurrences of eclogite associated with mylonitic rocks are documented the first near ross river is in a quartz rich metasedimentary host that includes glaucophane schist the host forms part of a belt of high pressure rocks more than 50 km long and several kilometres wide between ross river and faro in the second occurrence north of watson lake eclogite is associated with mafic and ultramafic rocks occurring in a klippe more than 100 km2 in area that rests on unmetamorphosed triassic rocks of the north american miogeocline geothermobarometry shows that the eclogites were metamorphosed at 10 15 kb 1 kb 100 mpa between 470 and 750 c these peak conditions are comparable to those for other eclogites in the yukon and adjacent alaska the age of metamorphism and mylonitization is triassic or older the high pressure rocks occur at the leading edge of the most inboard accreted terrane along its boundary with north american miogeoclinal strata over a length of several hundred kilometres t

indosinian high strain deformation for the yunkaidashan tectonic belt south china kinematics and 40ar 39ar geochronological constraints 1 structural and 40ar 39ar data from the yunkaidashan belt document kinematic and tectonothermal characteristics of early mesozoic indosinian orogenesis in the southern part of the south china block the yunkaidashan belt is tectonically divided from east to west into the wuchuang sihui shear zone xinyi gaozhou block and the fengshan qinxi shear zone indosinian structural elements ascribed to the indosinian orogeny include d2 and d3 deformation the early d2 phase is characterized by folding and thrusting with associated foliation and lineation development related to nw se transpression under amphibolite to greenschist facies conditions this event is heterogeneously overprinted by d3 deformation characterized by a gentle dipping s3 foliation subhorizontally to shallowly plunging l3 lineation some reactived d2 folds and low angle normal faults the d3 fabrics suggest a sinistral transtensional regime under greenschist facies metamorphism the timing of the d2 and d3 events have been constrained to the early to middle triassic 248 220 ma and late triassic 220 200 ma respectively on the basis of 40ar 39ar geochronology and regional geological relations the change from oblique thrusting d2 to sinistral transtension d3 may reflect oblique convergence and crustal thickening followed by relaxation of the overthickened crust in combination with the regional relations from xuefengshan to yunkaidashan and on to wuyishan the early phase of the indosinian orogeny constituted a large scale positive flower structure and is related to the intracontinental convergence during the assembly of pangea in which the less competent south china orogen was squeezed between the more competent north china and indosinian blocks

timing of high pressure metamorphism in the yukon tanana terrane canadian cordillera constraints from u pb zircon dating of eclogite from the teslin tectonic zone zircon from eclogite near last peak in the teslin tectonic zone yielded a u pb isotopic age of 269 2 ma 2 the first precise age for such a rock in the yukon tanana terrane of the canadian cordillera both the morphology and geochemistry of the eclogitic zircons indicate a metamorphic origin and the u pb age therefore constrains the timing of peak high pressure metamorphism in this rock the u pb age demonstrates for the first time that an early permian high pressure metamorphic event occurred in rocks now making up the teslin tectonic zone and possibly elsewhere in the yukon tanana terrane this u pb age provides a new geochronologic pin in the evolution of the yukon tanana terrane prior to its mesozoic accretion to the north american continental margin and combined with recent 40ar 39ar muscovite data indicates that high pressure metamorphism at this time was a relatively short lived event

the slide mountain terrane and the structural evolution of the finlayson lake fault zone southeastern yukon the finlayson lake fault zone forms a fundamental but little studied tectonic boundary between strata of autochthonous north america and the accreted slide mountain and yukon tanana terranes in southeastern yukon a structural and petrologic study was undertaken to examine the depositional environment of the slide mountain terrane its tectono thermal evolution in the fault zone and its relationship with the yukon tanana terrane the slide mountain and yukon tanana terranes are divisible into units dominated by metavolcanic and metasedimentary rocks field observations and whole rock geochemistry indicate that slide mountain greenstone is ocean floor basalt deposited in a deep submarine basin with a proximal terrigenous sediment influx either a marginal or ocean basin setting is supported by the data slide mountain greenstone is thrust northeastward over metasedimentary rocks of slide mountain terrane and southwestward over rocks of the yukon tanana terrane regional metamorphic grade ranges from su

axial intrusion zone beneath the median valley of the mid atlantic ridge at 37 n detected by explosion seismology summary four seismic refraction lines three of which had shots every 250 m were shot across along and parallel to the median valley of the mid atlantic ridge at 37 n a method has been developed for calculating the effect on the travel times of the rough sea floor relief beneath the profiles and has been used to correct all the travel times for this effect most arrivals were from a main refractor of apparent velocity 5 4 to 6 3 km s 1 only beyond 35 km were faster arrivals observed from an 8 09 36 km s 1 refractor the main refractor corresponds in depth at least approximately to the top of layer 3 of the ocean basins but its velocity is significantly less than normal for layer 3 perhaps due to dip a study of time residuals along two profiles across the median valley indicates the presence of a 2 to 3 km wide low velocity zone about 3 2 km s 1 beneath the median valley floor this zone extends over the upper 2 5 km of the crust and is believed to represent a zone of intrusion through which magma passes on its way to the sea floor

ultramafic intrusions in the lewis hills massif bay of islands ophiolite complex newfoundland implications for igneous processes at oceanic fracture zones the coastal complex of western newfoundland has been interpreted as oceanic crust and upper mantle which formed in an oceanic fracture zone within the coastal complex a suite of peridotite bodies and associated basaltic dikes occur the least deformed peridotites in these bodies have cumulate textures and have apparently been separated from any gabbroic cumulates which formed in association with them intrusive relationships with surrounding metamorphic and cumulate rocks are well preserved and suggest that the peridotite bodies are crystal mush intrusions derived from mobilized ultramafic cumulates which were originally deposited within relatively small isolated magma chambers these processes are believed to be characteristic of ridge transform intersections or leaky transform regions two petrologically distinct types of peridotite have been identified one type has lithologies and mineral chemistry similar to those of many oceanic plutonic rocks and these lithologies may be interpreted as differentiates produced in the evolution of typical mid ocean ridge basalts morbs the second type of peridotite although lithologically similar to the first has relatively high cao al 2 o 3 ratios and relatively low tio 2 and na 2 o this second type of peridotite cannot be simply related to the genesis of typical morbs chilled margins and fine grained dikes along the margins of some of these peridotite bodies are high magnesium basalts

structural divergence and transpression in the teslin tectonic zone southern yukon territory the structural and tectonic evolution of the teslin tectonic zone a complex belt of ductilely deformed rocks forming the southern extension of the yukon tanana terrane in the northern canadian cordillera is elucidated with new field data and by the reinterpretation of existing data the zone includes greenschist to amphibolite facies metasedimentary and metaplutonic rocks of the nisutlin and anvil assemblages characterized by s and l s tectonite fabrics primary contact relationships and ages show that most rocks in the zone were contiguous by mississippian time mapping identified a number of strain domains which preserve s fabrics dipping northeasterly or southwesterly variable mineral lineation orientations and a variety of shear directions including easterly and westerly vergent thrust shear down to the east and down to the west normal shear and dextral strike slip shear regional constraints and microstructures suggest that latest ductile deformation was late triassic to early jurassic in age structural characteristics are explained most effectively by convergent dominated transpression in combination with tectonic wedging and associated back thrusting above a low angle west dipping detachment oblique eastward and upward movement of the rocks over the detachment which may have coincided with the top of thinned north american continental crust produced easterly vergent shear and localized or widespread tectonic wedging and back thrusting produced westerly vergent shear the transpressive deformation postdates formation of the nisutlin assemblage by mississippian time and subduction of the slide mountain ocean beneath the nisutlin assemblage in permian to early triassic time the western margin of the teslin tectonic zone was truncated by cretaceous strike slip faults and translated northward rocks of the peraluminous orthogneiss assemblage of the yukon tanana terrane in central yukon acted as the tectonic wedge and were overthrust by rocks of the nisutlin and anvil assemblages in early jurassic time before unroofing by local extension in cretaceous time

the inception of the maliac ocean regional geological constraints on the western circum rhodope belt northern greece abstract considerable debate surrounds the early alpine paleogeographic significance of the circum rhodope belt crb greece in particular whether it was a rifted section of the southern euroasian foreland or of the late mesozoic back arc system of the vardar ocean or of both in the attempt to clarify the complex nature of the crb nine new lithostratigraphic formations with its members in consideration of the spatiotemporal and regional geological context are constrained by tracing the changes in regional plate interactions and linked magmato sedimentary record the analysis of the stratigraphic data in combination with the highly complex yet poorly constrained regional geology is employed to reconstruct the late permian triassic and latest jurassic developments of this part of the southern european foreland a continuum between the rifting and triassic opening of the maliac ocean the carnian eo cimmerian docking is supported by the consolidated new data embedded into a collection of the nine distinct lithostratigraphic formations the terminal vardar plate approach during the latest jurassic earliest cretaceous and the convergence with the investigated portion positioned to the south of the balkan hellenic foreland thrust belt configured this regional neotethyan flysch trough whereas the paleocene eocene east west shortening and nappe stacking ejected the incoherent early alpine system referred to as the crb the ejected permo triassic including the early jurassic ophiolite bearing formations of the crb are matched to be in line with those of the regional southeast european importance in fact the results place the western crb or eastern maliac ocean to lie at the distal southern margin promontory of the eurasian craton aligned with the early and late paleozoic crystalline terranes paleozoic to lower triassic variscan basement segment of the pelagonian unit veles series and lower complex of the serbo macedonian unit vertiskos unit

interplay between geochemistry and magma dynamics during magma interaction an example from the sithonia plutonic complex ne greece orogenic granitoids often display mineralogical and geochemical features suggesting that open system magmatic processes played a key role in their evolution this is testified by the presence of enclaves of more mafic magmas dispersed into the granitoid mass the occurrence of strong disequilibrium textures in mineralogical phases and or extreme geochemical and isotopic variability in this contribution intrusive rocks constituting the sithonia plutonic complex northern greece are studied on the basis of mineral chemistry whole rock major trace element geochemistry and sr and nd isotopic composition sithonia rocks can be divided into a basic group bearing macroscopic mafic enclaves microscopic disequilibrium textures geochemical and isotopic evidence of magma interaction and an acid group in which most geochemical and isotopic features are consistent with a magma mixing process but macroscopic and microscopic features are lacking a two step mixing plus fractional crystallization mfc process is considered responsible for the evolution of the basic group the firststepexplainsthechemicalvariationinthemaficenclavegroup abasicmagma representedbytheleastevolvedenclaves interacted

multistage supercritical fluid extraction of petroleum source rocks application to samples from kimmeridge clay and posidonia shale formations abstract a study to determine whether supercritical fluid extraction can replace liquid extraction of source rocks has been initiated the extractions have been performed with plain carbon dioxide with small amounts of 2 propanol mixed with carbon dioxide and with carbon disulfide and tetrahydrofuran added to carbon dioxide to increase extraction yields multistage extractions were performed in order to obtain a compromise between rapid extraction and selective extraction the extracts were collected either in a solvent or on a sorbent cartridge in a stream of water the yields were similar to or higher than those obtained by liquid extraction with a 3 stage extraction the first extract was dominated by saturates the second by aromatics and the third by nso compounds in preliminary results obtained with 1 2 g samples of posidonia shale 0 8 soluble organic matter was extracted with multistage sfe and each stage was completed in 30 min the extracts could be used for the determination of biomakers

serbo macedonian revisited a silurian basement terrane from northern gondwana in the internal hellenides greece abstract new geochronological and geochemical data on basement orthogneisses from the vertiskos unit of the serbo macedonian massif smm internal hellenides northern greece are used in order to constrain the pre alpine tectonic history of the basement units in the metamorphic hinterland of the hellenides the prevailing rock types in the vertiskos crystalline basement are coarse grained biotite augengneisses with subordinate leucocratic muscovite gneisses and two mica gneisses zircon pb pb and u pb ages on 20 samples range from 425 9 4 2 ma to 443 4 5 5 ma with a mean of 432 2 3 2 ma and are interpreted as primary crystallisation ages of the basement granites on the basis of the magmatic internal structure of the zircon grains trace element and isotope geochemistry of the gneisses show that they originated in a magmatic arc setting but also contain material from pre existing continental crust the rocks are slightly peraluminous and some leucocratic gneisses are strongly depleted in incompatible trace elements the zircon ages document an early silurian magmatic phase in the internal hellenides the association of lithologies and ages is distinctly different from that of the adjacent basement massifs this difference in basement provenance and the fact that the vertiskos unit is bordered by ophiolitic material both to the west and to the east leads to the conclusion that this part of the smm is an exotic terrane of northern gondwanan origin which was finally accreted to its present position during the alpine orogeny the vertiskos terrane may have been part of the hun superterrane which evolved at the northern active continental margin of gondwana in the early palaeozoic and rifted away from it during the opening of the rheic ocean in the cambrian to silurian parts of this superterrane such as the vertiskos terrane are present throughout the variscan and alpine orogens

geochemical and nd isotopic constraints for the origin of eclogite protoliths northern cordillera implications for the paleozoic tectonic evolution of the yukon tanana terrane on the basis of trace element data basaltic protoliths for paleozoic eclogites from the yukon tanana terrane ytt have diverse origins eclogites from stewart lake and the simpson range have char

structural signatures and 40ar 39ar geochronology of the indosinian xuefengshan tectonic belt south china block abstract to advance our understanding of the kinematics of the xuefengshan tectonic belt xfstb and the mesozoic tectonics of the south china block scb this paper presents new data and interpretations from our structural studies and thermochronological dating the xfstb is characterized by greenschist facies metamorphism and development of s and s l tectonic fabrics an asymmetric positive flower structural pattern is composed of nw wnw and se ese dipping cleavages faults and shear zones kinematic indicators indicate a dominant top to wnw thrusting with a sinistral strike slip component on the ese dipping shear zones and top to ese back thrusting on the wnw dipping shear zones the quartz c axis orientations of mylonitic rocks exhibit monoclinic point maximum asymmetry indicative of a sinistral shear sense under the a basal gliding condition the timing of the major deformation event has been constrained to the middle triassic to early jurassic 244 195 ma on the basis of 40ar 39ar geochronology and other geological observations the structural characteristics of the xfstb are probably related to an indosinian oblique convergent zone in combination with tectonic wedging and associated back thrusting all above a low angle se dipping basal detachment oblique northwestward and upward movement along the basal detachment was partitioned into the nw wnw directed thrusting se ese directed back thrusting and subsidiary strike slip movement along the ne and nne trending faults and shear zones the xfstb may represent part of a huge structural fan between the yangtze and cathaysian blocks and can be interpreted as a product of the indosinian intracontinental collision involving a weak zone

kilometre scale folding in the teslin zone northern canadian cordillera and its tectonic implications for the accretion of the yukon tanana terrane to north america discussion despite the often divergent opinions regarding the evolution of the teslin suture zone tsz previous tectonic models broadly agree that the tsz comprises a ductile shear zone that constitutes a fundamental crustal boundary e g tempelman kluit 1979 erdmer 1985 hansen 1989 stevens and erdmer 1996 hansen and dusel bacon 1998 in contrast de keijzer et al 1999 offer a radical model in which the tsz is a rootless polydeformed nappe obducted onto north america according to their interpretation the tsz does not represent a shear zone although the influx of new ideas in science is always commendable the methodology by which de keijzer et al 1999 arrive at their interpretation is scientifically flawed they ignore or arbitrarily dismiss published tsz data and they fail to present evidence in support of key elements of their model although their data may be valid we question 1 their interpretation of linear fabric elements 2 the existence of kilometre scale folding and 3 evidence for north american rocks in the western tsz de keijzer et al 1999 discard the interpretation of the tsz as a complex ductile shear zone and assert that apart from the western tsz most lineations represent crenulations and bedding cleavage intersections they state that they did not observe stretching lineations or fabric asymmetries in contrast previous workers erdmer 1985 hansen 1989 hansen et al 1989 1991 stevens and erdmer 1996 oliver 1996 hansen and dusel bacon 1998 describe numerous structural domains within the tsz and correlative l s tectonites in east central alaska based on elongation lineation orientation and microstructural asymmetry which they attribute to widespread ductile shear across the entire tsz thus knowledge of whether or not the tsz represents a zone of ductile shear deformation is critical to models of tsz evolution the presence and nature of quartz

the morton black hypothesis for the thinning of continental crust revisited in western afar abstract geological observations along the western margin of afar show that the morton black model in which thinning of upper continental crust at a nascent continental margin is accomplished through block faulting and tilting is not wholly applicable rather dikes are concentrated into swarms suggestive of important direct dilatation faults are concentrated into sets within which no block tilting steeper than 45g is observed therefore attenuation of the afar margins requires a significant role for processes other than block tilting and in some margin sectors the transition from continental to neo oceanic crust may be relatively abrupt

u pb rb sr and k ar isotopic constraints for ductile deformation and related metamorphism in the teslin suture zone yukon tanana terrane south central yukon the teslin suture zone tsz comprises a portion of the yukon tanana terrane yt in the yukon formed by steeply dipping layering and l s tectonite foliation the tsz forms the fundamental tectonic boundary between rocks deposited along the ancient margin of north america and allochthonous terranes to the west tsz tectonites evolved during initial penetrative dip slip deformation dds and later dextral strike slip shear dss along steep 1 3 km wide shear zones several workers have speculated that the tsz and related yt heterogeneous ductile deformation and associated metamorphism are devonian to mississippian in age and related to the intrusion of a similar age orthogneiss throughout the yukon and alaska however recent structural and metamorphic studies of the tsz provide evidence contradicting this view new isotopic evidence presented herein indicates that tsz dynamothermal metamorphism was cooled by early jurassic time that it cannot be related to devonian mississippian and permian granitic

the chemistry of detrital chromian spinels and its implications for the geodynamic evolution of the eastern alps detrital chromian spinel is a key mineral in paleogeographic reconstructions the chemical compositions of the spinels provide even more specific information about their host ophiolites because their chemistry is related to the degree of melting of the mantle peridotite and thus to the geotectonic environment of ocean crust formation

geologie und bau des sithonia ophioliths chalkidiki ne griechenland anmerkungen zur bildung ozeanischer krusten der oberjurassische sithonia ophiolith der chalkidiki ne griechenland bietet die moglichkeit die an der bildung fossiler ozeanischer krusten beteiligten prozesse ausfuhrlich zu untersuchen

the structures of two ophiolite massifs bay of islands newfoundland a model for the oceanic crust and upper mantle abstract for a detailed analysis of their main structures accumulation planes foliation planes and lineations the peridotite gabbro units of table mountain and blow me down mountain bay of islands newfoundland have been divided into several structural units each one being characterized by specific microstructures of deformation or accumulation a fishbone pattern with the accumulation plane in the mafic cumulates dipping toward the ridge and the foliation plane in the ultramafic tectonites dipping away from it is postulated for the structure of the lithosphere formed at a spreading center the tectonic structures in the peridotites are related to asthenospheric flow in the uppermost mantle and the direction of the ridge is inferred from the lineation trend in the peridotites the structures in the cumulates are related to subsidence of the magma chamber floor during spreading at the base of the massifs the tectonic structures in the mafic metamorphic rocks are parallel to those observed the latter have been deformed at moderate temperatures under high stresses and display mylonitic textures at their contact with the mafic tectonites from these superimposed textures we infer a kinematic model for the oceanic thrusting of the lithosphere the direction of thrusting is given by the lineation trend and its sense by the analysis of shear all these data are combined into a paleogeographic scheme for the newfoundland caledonides

the josephine ophiolite northwestern california the 157 m y old josephine ophiolite is a complete ophiolite consisting of the following units harzburgite tectonite cumulate ultramafics cumulate gabbro interlayered with cumulate ultramafics cumulate gabbro high level gabbro and diorite a 1 5 km thick sheeted dike complex and 0 4 km of pillow lava and breccia the ophiolite is conformably overlain by the galice formation consisting of a thin siliceous pelagic sequence overlain by a thick sequence of turbidites having both volcanic arc and continental margin sources the ophiolite and overlying galice formation were intruded by numerous 150 to 151 m y old calc alkaline dikes and sills followed shortly by deformation and regional metamorphism under conditions of prehnite pumpellyite to lower greenschist facies the josephine peridotite makes up the base of the ophiolite and covers 800 km2 it is an alpine type peridotite that represents upper mantle material and the residue of partial melting ultramafic rocks overlying the josephine peridotite are olivine spinel clinopyroxene cumulates that grade upward into clinopyroxene and orthopyroxene olivine plagioclase cumulates and finally to clinopyroxene plagioclase cumulates high level gabbros and diorites are characterized by highly variable textures complex intrusive relationships strongly zoned plagioclase and abundant mafic dikes plagiogranites occur at this level as dikes small intrusions and screens between mafic dikes but they are rare the sheeted dike complex consists of mafic dikes averaging 0 6 m in thickness chilled margins are locally well preserved and many of the dikes have been split and intruded by younger dikes resulting in half and marginless dikes the dike complex is composed of 100 sheeted dikes near the center but contains gabbro screens near the base and pillow lava and breccia screens in its upper parts the dikes are inclined at 40 to bedding after correcting for folding and are inferred to have been rotated shortly after formation the extrusive sequence consists predominantly of pillow lavas but contains abundant pillow breccias and some massive lavas some of the breccias formed as talus deposits whereas others are graded and matrix supported indicating transport and redeposition possibly in an area of rugged submarine topography the dikes and lavas have geochemical affinities to island arc tholeiites but are in some aspects transitional to mid ocean ridge basalts the dikes and lavas are cogenetic with the underlying plutonic sequence as indicated by similarity in crystallization sequence contradictory intrusive relationships between sheeted dikes and high level plutonic rocks and lack of diabase dikes in the lowermost cumulates and harzburgite the upper portions of the ophiolite were hydrothermally metamorphosed at the spreading center resulting in downward changes in mineralogy texture and 18o in addition metalliferous sediments and massive sulfides that occur overlying and within the pillow lavas probably were formed by precipitation from thermal springs a geothermal gradient of 250 350 c km is inferred from mineral assemblages on the basis primarily of its regional setting the ophiolite is inferred to have formed in a back arc basin in particular it is within a thrust sheet sandwiched between a coeval magmatic arc complex and a slightly older arc complex remnant arc this interpretation is consistent with abundant volcanic arc detritus in graywackes overlying the ophiolite the island arc magmatic affinities of the lavas and dikes may be the result of formation of the ophiolite during the earliest stages of back arc spreading

structural and kinematic evolution of the teslin suture zone yukon record of an ancient transpressional margin abstract the teslin suture zone tsz yukon which forms the fundamental boundary between rocks deposited along the ancient margin of north america and allochthonous terranes to the west preserves a complex history of pre mid jurassic convergence and dextral strike slip translation which is overprinted locally by late cretaceous dextral strike slip shear the tsz comprised of a 15 20 km thick structural sequence of ductilely deformed sedimentary and volcanic strata basalt peridotite and granitoids is divisible into distinct elongate structural domains based on the distribution of two well defined but differently oriented stretching lineations lds and lss lds and lss both formed during non coaxial ductile deformation lds trends e w and plunges down dip whereas lss trends nnw sse and plunges shallowly two 1 2 km wide nnw trending anastomosing shear zones of lss cross cut lds structures indicating lss is younger other field relations indicate further that lds began forming earlier than lss followed by a period of coeval lds and lss formation latest motion was dominantly parallel to lss synkinematic mineral assemblages associated with both lds and lss related structures record greenschist to albite epidote amphibolite facies conditions lds tectonites record a complex movement history to the west textural asymmetries indicate west side down or normal shear whereas in eastern domains top to the east thrust style asymmetries dominate lss tectonites consistently record dextral or top to the north shear parallel to lss tectonic motion began as chiefly penetrative dip slip shear lds at a high angle to the present trend of the tsz and evolved to dominantly dextral strike slip shear lss parallel to the trend of the zone the geometry kinematics and sequence of deformation in tsz tectonites evoke a general model of terrane accretion during oblique plate convergence involving initial shortening at a high angle to the convergent margin and progression to margin parallel translation

from an ocean floor wrench zone origin to transpressional tectonic emplacement of the sithonia ophiolite eastern vardar suture zone northern greece in the hellenides of northern greece the sithonia back arc ophiolite constitute an element of the vardar suture zone against the chortiatis island arc magmatic suite the melissochori formation and the serbo macedonian massif further north at the mesozoic continental margin of eurasia a granodiorite from the chortiatis island arc magmatic suite crystallized at 160 ma as derived from new u pb zircon geochronology and confirms the end of arc magmatic activity that started at around 173 ma located southerly of the chortiatis island arc magmatic suite the sithonia ophiolite had igneous life from 159 to 149 ma and the ophiolite interfinger with clastic carbonate kimmeridgian sediments magmatic structures i e sheeted dykes in the ophiolite witness for ne trending rift axis while the transform faults and fracture zones sketch nw se transcurrent transtension like propagation of the rift spreading center at sithonia that is consistent with a dextral wrench corridor already proposed for the ophiolite origin in the eastern vardar zone the tectonic emplacement of the sithonia ophiolite involved dextral ene to se strike slip sense of shear and sw and ne reverse thrust sense of shear on mostly steep foliation s1 subhorizontal lineation l1 and associated variably inclined f1 fold axes this structural grain and kinematics are shared by adjacent chortiatis island arc magmatic suite and the melissochori formation the coexistence of strike parallel and thrust components of displacement along discrete dextral strike slip shear zones and internal deformation of the mentioned units is interpreted to result from a bulk dextral transpressive deformation regime developed in greenschist facies metamorphic conditions the back arc ocean floor previous structural architecture with faults and fracture zones where kimmeridgian sediments deposited in troughs was used by discrete strike slip shear zones in which these sediments involved and the shear zones become the sites for strain partitioning of transpressional deformation available biostratigraphic and radiometric age constraints define latest jurassic earliest cretaceous tithonian berriasian to early valanginian time frame for the sithonia ophiolite northeastward tectonic emplacement accomodated by dextral transpression that led to the ophiolite accretion to the chortiatis island arc magmatic suite and its trench fill exposed in the melissochori formation and further north toward the serbo macedonian margin of eurasia

mantle flow at oceanic spreading centers models derived from ophiolites abstract the internal structures of eleven ophiolite complexes have been structurally investigated the attitude of the foliation and the lineation in the peridotites is related to asthenospheric flow in the uppermost mantle at an oceanic spreading center from the orientation of these structures with respect to the paleohorizontal plane two oceanic situations are distinguished the horizontally spreading asthenosphere illustrated by the table mountain massif newfoundland is characterized by 1 a homogeneous regular and sub horizontal plastic flow pattern over large distances 2 b flat paleo mono and absence of large scale folding 3 c rare economical chromite deposits and depleted mantle the diapirically spreading asthenosphere illustrated by the acoje type philippines displays 1 a inhomogeneous and sub vertical plastic flow pattern 2 b deformed paleo moho and large scale folds 3 c important economical chromite deposits and less depleted mantle presumably due to secondary magmatic impregnation this leads to vogt s model of asthenospheric flow beneath an oceanic ridge the direction of which being equated with the dike swarm in this model the horizontally spreading asthenosphere is derived from the diapirically spreading one in the following way the vertical mantle flow at the diapir center diverges horizontally in every direction including that parallel to the ridge at about 50 km from the diapir center the mantle flow becomes progressively perpendicular to the ridge due to the damming effect of transform faults an important economical consequence of this model is that large chromite deposits are expected preferentially in the diapiric type of ophiolite because chromite is preferentially precipitated at the diapir center from pristine magma