proterozoic accretionary tectonics at the southern margin of the archean wyoming craton the proterozoic cheyenne belt at the southern margin of the archean wyoming craton consists of strongly deformed lithotectonic blocks bound by major mylonite zones from north to south and structurally lowest to highest these blocks include 1 archean crystalline basement and associated early proterozoic miogeoclinal rocks 2 an amphibolite grade orthogneiss terrane of unknown age containing probable rift related mafic intrusive rocks 3 a 1750 1790 ma marginal basin succession consisting of upper amphibolite grade pelitic and volcano genie schist associated peraluminous granite and minor ultramafic rocks and 4 a 1750 1790 ma intermediate to mafic plutonic metamorphic complex interpreted as the deep roots of an island arc system the earliest deformation d1 produced a synmetamorphic penetrative horizontal transposition foliation and associated recumbent folds tectonic blocks were juxtaposed along major mylonite zones during d2 macroscopic anil microscopic structures associated with d2 indicate northward thrusting along low angle mylonite zones of successively deeper crustal blocks over supracrustal rocks of the wyoming craton at about 1750 ma thrusting occurred at minimum temperatures of 475 c and produced an inverted metamorphic gradient in pelitic rocks north of the belt mylonite zones were subsequently steepened and reactivated under greenschist facies conditions during a period of dextral strike slip d3 between 1750 and 1400 ma cataclasis d4 and brecciation associated with the laramide orogeny d5 locally overprint earlier structures oblique convergence between an island arc and the archean continental nucleus may explain accretion crustal shortening and subsequent strike slip the phanerozoic accretionary events of eastern and western north america provide an analog for this model presence of similar lithologies and shear zones south of the cheyenne belt suggest that the southern margin of the wyoming craton may have been a long lived zone of crustal accretion

dynamical mechanisms for the south china sea seasonal circulation and thermohaline variabilities abstract the seasonal ocean circulation and the seasonal thermal structure in the south china sea scs were studied numerically using the princeton ocean model pom with 20 km horizontal resolution and 23 sigma levels conforming to a realistic bottom topography a 16 month control run was performed using climatological monthly mean wind stresses restoring type surface salt and heat and observational oceanic inflow outflow at the open boundaries the seasonally averaged effects of isolated forcing terms are presented and analyzed from the following experiments 1 nonlinear dynamic effects removed 2 wind effects removed and 3 open boundary inflow outflow set to zero this procedure allowed analysis of the contribution of individual parameters to the general hydrology and specific features of the scs for example coastal jets mesoscale topographic gyres and countercurrents the results show that the pom model has the capability of simulating seasonal variations of the scs circulation and thermoha

current observations on the development of the yellow sea warm current historically the climatological mean northward flow in the yellow sea trough has been referred to as the yellow sea warm current yswc since it was thought to originate as a branch of the tsushima current which transports warm kuroshio waters however the tsushima current does not penetrate into the yellow sea interior during winter the onset of what is referred to as the yswc during winter is directly observed using measurements of current profiles pressure and temperature and a wind climatology for the time period july 1995 through january 1996 the yswc is a result of weak southerly winds during summer during fall transition and winter monsoon periods strong northerly wind bursts drive a north to south rise in pressure extending from the trough over to the korean coast and force a northward flow in the yellow sea trough the rise in pressure and corresponding north wind forced yswc begin at the end of october for this measurement period and are observed into january these measurements show that the yswc has different seasonal forcing mechanisms direct current measurements are also presented which suggest a current flow north of cheju sometimes referred to as the cheju warm current

evolution of the early proterozoic colorado province constraints from u pb geochronology the colorado province represents an addition of a belt of rocks more than 500 km wide to the southern margin of the archean wyoming craton during the early proterozoic between about 1790 and 1660 ma correspondence in ages between metamorphism deformation and plutonism association of volcanic rocks with comagmatic calc alkalic plutons and lack of older basement are all consistent with the interpretation that the rocks of the province are products of arc magmatism and cannibalistic sedimentation along a convergent margin at the southern edge of the craton

40ar 39ar thermochronology of mesoproterozoic metamorphism in the colorado front range abstract a low pressure metamorphic episode in the colorado front range has been identified by the presence of staurolite andalusite cordierite and garnet porphyroblasts overprinting earlier assemblages the overprinting assemblages and reaction textures are most consistent with porphyroblast growth on a prograde metamorphic path with peak temperatures exceeding 525 c twenty eight 40ar 39ar dates on hornblende muscovite biotite and microcline were used to infer the age and thermal conditions of metamorphism muscovite and biotite 40ar 39ar ages fall mainly in the interval 1400 1340 ma consistent with cooling through the closure temperature interval of micas 400 300 c after about 1400 ma in contrast hornblende apparent ages documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fon

metamorphic petrology of the northeast front range colorado the pingree park area the pingree park area in the northeast front range colorado is underlain by precambrian pelitic and semipelitic gneiss and schist that were metamorphosed about 1 75 b y ago forming a distinct zonal pattern of minerals indicating an increase in grade of metamorphism to the west and north three isograds have been identified the andalusite sillimanite isograd is defined by the first occurrence of sillimanite in the prograde direction and is interpreted to mark the reaction of andalusite to sillimanite a band as much as 1 km across contains both andalusite and sillimanite and represents a divariant band or reaction overstepping of 20 c or less the k feldspar sillimanite isograd separates microcline sillimanite from prograde muscovite quartz assemblages and is interpreted to represent the reaction k na muscovite quartz k na microcline sillimanite h 2 o despite the presence of significant amounts of sodium in both muscovite and microcline the reaction shows no perceptible divariant band there has been extensive reaction of microcline sillimanite to retrograde muscovite quartz the migmatite in isograd is defined by the first occurrence of migmatitic rocks in the prograde direction and is interpreted to represent the inception of partial melting the isograds form an eastward plunging set of synformal surface the south limb of which has a gentle dip and the north limb of which apparently has a much steeper dip based on the chemical composition of the minerals and comparison with experimentally calibrated equilibria the conditions of metamorphism at the k feldspar sillimanite isograd are estimated to have been p h 2 o p total 3 to 4 kbar t 650 30 c f o 2 10 12 to 10 15 bars and f hf 1 4 to 1 bar the geothermal gradient at the peak of metamorphism was 60 c km

spreading evolution of the troodos ophiolite cyprus some recent developments particularly the recognition of transform tectonized and nontransform tectonized fabrics close to the southern troodos transform fault provide a framework for a new evolutionary model of the troodos ophiolite this model proposes that most of the ophiolite formed at the solea axis against a dextrally slipping southern troodos transform spreading was largely accommodated by magmatic accretion the ridge then jumped to the east a last phase of amagmatic stretching driving normal faulting above a detachment horizon in the solea graben a ridge jump boundary separates the old crust formed at the solea axis from the new crust generated at the outside corner of the new ridge with a dextrally slipping continuation of the southern troodos transform the final episode involved extension of the crust generated at the new ridge to form the larnaca graben in the eastern part of the ophiolite

wind driven effects on the yellow sea warm current the yellow sea is a shallow basin writh an average depth of 44 m located between china and the korean peninsula one of the dominant ocean circulation features of the yellow sea is a warm wrater intrusion known as the yellow sea warm current this feature is present throughout the year but reaches its farthest northward extension in winter the circulation of the yellow east china and bohai seas was modeled using the princeton ocean model to better understand the dynamics of the yellow sea warm current the horizontal resolution of the model varies from 8 km in the yellow sea to 25 km in the east china sea twenty four sigma levels are used to define the vertical structure the model uses daily atmospheric forcing from the navy operational global atmospheric prediction system for 1993 open boundary conditions are applied at the taiwan strait the tsushima korea strait an area south of taiwan and the tokara strait with a closed boundary south of the ryukyu islands the model results are examined to determine the effect of the wind on the northward extension of the warm water intrusion using both water mass characteristics and northward velocity components sensitivity tests and spectrum analyses performed to study the influence of the wind on the yellow sea warm current show that winds modify the pathway and extent of the yellow sea warm current the current s origin however appears to be due to external forcing from the current systems developed in the east china sea

initiation of subduction zones along transform and accreting plate boundaries triple junction evolution and forearc spreading centres implications for ophiolitic geology and obduction

electron microprobe monazite dating of ca 1 71 1 63 ga and ca 1 45 1 38 ga deformation in the homestake shear zone colorado origin and early evolution of a persistent intracontinental tectonic zone the homestake shear zone one of the principal precambrian structures within the colorado mineral belt has a history of tectonism that extends from the proterozoic to the tertiary new field mapping microstructural analysis and electron microprobe u th pb monazite dates define the style kinematics and timing of multistage proterozoic ductile deformation early subhorizontal shearing d1 progressed to northwest southeast crustal shortening d2 during a protracted ca 1710 1630 ma orogeny involving high temperature metamorphism partial melting and emplacement of the ca 1 7 ga cross creek batholith the subvertical northeast trend of the shear zone was established by d2 shortening a major episode of simple shear displacement occurred ca 1 4 ga during emplacement of the st kevin batholith complex kinematics are recorded by 1380 ma southeast side down dextral mylonite zones d3 and post 1380 ma southeast side up dextral ultramylonite pseudotachylyte zones d4 that were both localized in d2 foliation domains the succession of ductile tectonites in the homestake records 300 m y of episodic deformation at progressively shallower crustal levels as colorado evolved from an active accretionary margin 1 8 1 7 ga through crustal assembly 1 7 1 6 ga to a tectonically active continental interior post 1 45 ga the homestake shear zone constitutes a classic example of ductile shear zone reactivation and the development of persistent tectonic zones in the continental lithosphere

proterozoic farwell mountain lester mountain suture zone northern colorado subduction flip and progressive assembly of arcs this paper considers the amalgamation of arc and oceanic terranes to be the main mechanism of ca 1 8 1 6 ga continental crustal growth in southwestern laurentia on the basis of geologic and seismic reflection data and teleseismic images we propose the farwell mountain lester mountain suture zone as the northern most paleoproterozoic arc arc suture north dipping farwell mountain seismic reflections project from 18 km depths to the surface and are interpreted to represent conjugate thrusting as the 1 79 1 77 ga green mountain arc was partially underthrust beneath the archean craton we speculate that a north dipping high velocity mantle slab in the teleseismic image is a continuation of this thrust zone south dipping lester mountain reflections project from 22 km depths to the surface and are interpreted to be a thrust zone between the green mountain arc and the 1 76 1 72 ga rawah block surface features of the farwell mountain lester mountain suture zone are 1 marble chert rock with sillimanite pods ultramafic rocks sulfide deposits and pillow basalts which we interpret to be a dismembered accretionary complex 2 an axial planar fabric to north verging isoclinal folds f 2 which we interpret to be part of a north vergent fold and thrust system and 3 a metamorphic break between 500 c rocks to the north and 610 c rocks to the south which we attribute to reactivation of the zone movement on the farwell mountain backthrust is inferred to relate to cheyenne belt suturing at 1 78 1 75 ga we correlate suturing in the farwell mountain lester mountain suture zone to s 1 d 1 1 746 1 74 ga in the soda creek fish creek shear zone we attribute the complexity of this broad suture zone to initial conjugate thrusting plus overprinting and steepening of accretionary structures by subsequent tectonism

tectonic setting of the bela ophiolites southern pakistan abstract the bela ophiolites consist of a 3 5 km thick sequence of basaltic pillow lava inter flow sedimentary rocks chert argillite limestone and diabase gabbro sills debris of serpentinite up to kilometer sized blocks serpentinite carbonate breccia and less commonly of basalt gabbro peridotite pelagic limestone and their metamorphic equivalents occur throughout the ophiolite sequence and may form extensive horizons the age of the ophiolites as indicated by pelagic microfauna is aptian early maestrichtian the lava flows belong to a spilite keratophyre basaltic andesite suite of tholeiites enriched in fe tio 2 and other trace elements the lava flows although similar to the e type morb differ in details such as la ta ratio ranges 9 62 18 04 for the bela lavas versus 10 15 for the morb the diabase gabbro sills are similar to the associated lava flows however due to their relative enrichment in certain elements e g fe ti hf th and ta etc they may represent more fractionated late melts a comparison of the stratigraphic and structural features of the bela ophiolites with modern oceanic environments reveals that these ophiolites probably originated in a large oceanic fracture zone leaky transform this is based on lithotectonic features such as the common occurrence of debris of foliated serpentinite and serpentinite carbonate breccia bearing rocks in the ophiolite sequence and on geodynamic considerations the fracture zone ancestral to the bela ophiolites probably acted as a boundary between the indian and the neo tethys plates during the cretaceous analogous to the present day owen fracture zone which separates the indian and the arabian plates this ancestral fracture zone was destroyed when transform movement gave way to oblique convergence culminating in the ophiolite obduction in paleocene early eocene time the ophiolite obduction on the west indian continental margin was preceded by alkaline wpb type volcanism which is represented by the porali basalts the bibai lavas and the deccan basalts

crustal structure transition from oceanic arc to continental arc eastern aleutian islands and alaska peninsula abstract the aleutian island arc crosses from the pacific ocean to the north american continent at the island of unimak 3 d finite difference traveltime inversion of our onshore offshore seismic reflection refraction data gives a velocity model of the crust and uppermost mantle the arc crust is on average 30 km thick but thickens to almost 40 km under the western alaska peninsula the transition from oceanic arc to continental arc is characterised by a decrease in average velocity in the upper crust from about 6 5 km s to less than 6 0 km s with no systematic change in the velocity of the lower crust throughout our study area in the upper 15 km of the crust the highest velocities are observed in the fore arc just south of the volcanic line in the lower crust the lowest velocities of just 6 2 km s are found close to the volcanic line the uppermost mantle is quite heterogeneous with velocities ranging from 7 6 to 8 2 km s in part due to the thermal gradient from cold fore arc to hot back arc whereas the aleutian oceanic fore arc has higher seismic velocities than average continental crust throughout the crust the peninsula section is close to the continental average in the upper c 20 km of the crust we infer that repeated episodes of arc magmatism can produce a felsic to intermediate upper crust as is observed in the continents but arc magmatism produces a thicker mafic lower crust than the average continent retains some of the excess mafic material in the island arc crust can be attributed to pre existing oceanic crust which is less evident or absent in a continental arc

west florida shelf mean circulation observed with long term moorings 1 the mean circulation on the west florida continental shelf is described using long term current measurements bounded by the florida peninsula to the east and the gulf of mexico to the west the west florida continental shelf mean flow is oriented approximately along isobath and southward the mean velocity vectors veer systematically with depth shoreward over shallow water and seaward over deeper water this polarization change implies that the mean flow is upwelling over shallow water and downwelling seaward from the inner shelf such a well organized three dimensional coastal ocean circulation pattern revealed by an unprecedented set of observations and explained on the basis of wind forcing and density field adjustment has important implications for both fisheries and red tide occurrences

early proterozoic metasediments from north central colorado metamorphism provenance and tectonic setting early proterozoic metasediments from the big thompson canyon area in north central colorado increase in metamorphic grade from the biotite zone to the sillimanite zone representing a low pressure facies series that probably developed in response to a steep geotherm over a thermal high high grade rocks are enriched in k 2 o al 2 o 3 fe 2 o 3 mgo tio 2 cr sc cs rb ba and light ree and are depleted in sio 2 cao and zr relative to low grade rocks these differences chiefly reflect differences in the original sandstone shale ratio detrital grains in medium and coarse metasediments are quartz quartzite and feldspar and they suggest a recycled orogen provenance in which granitic rocks and quartzites dominate in the source the abundance of shale in the original succession also attests to the importance of granite in the source major and trace element contents of the rocks constrain the input of mafic to dacitic detritus to less than a few percent ree patterns in all rocks are similar to phanerozoic shales and cratonic sandstones the big thompson canyon metasediments may have been derived from an uplifted collisional orogen in southern wyoming in which unroofed proterozoic granitic rocks and quartzites of the snowy pass supergroup served as the principal sources

stratigraphy and depositional setting of the proterozoic snowy pass supergroup southeastern wyoming record of an early proterozoic atlantic type cratonic margin metasedimentary rocks in the medicine bow mountains and sierra madre are divided into four groups the 3 km thick phantom lake metamorphic suite contains strongly deformed metavolcanic and metasedimentary rocks that are crosscut by late archean granites the 2 5 km thick deep lake group unconformably overlies the phantom lake suite and late archean granites and contains fluvial sediments including radioactive quartz pebble conglomerates and glaciomarine deposits both successions are intruded by large sills of tholeiitic gabbro the 4 5 km thick lower libby creek group is inferred to be in thrust fault contact with older units and contains sediments recording transgressions and regressions across a macrotidal delta this succession is intruded by the 2 000 m y old gaps intrusion and comagmatic tholeiitic to weakly alkalic dikes the 3 km thick upper libby creek group is bounded by a thrust fault below and by the cheyenne belt above and contains carbonates and marine slates the early proterozoic deep lake lower libby creek and upper libby creek groups collectively are named the snowy pass supergroup lithologies and stratification sequence in the well preserved medicine bow mountain section suggest transgressive miogeoclinal sedimentation during the early proterozoic paleocurrent data indicate that fluvial then deltaic sedimentation of the deep lake and lower libby creek groups took place on a southwest dipping paleoslope parallel to the inferred south cratonic boundary of the wyoming province this and a few west directed paleocurrents suggest a continental or microcontinental block to the south bounding sedimentation in a northeast elongated basin a rift setting for deposition of these units explains the transgressional character of the sediments the deltaic sedimentation with paleocurrents parallel to the cratonic boundary and the 120 bend in the cheyenne belt between the medicine bow mountains and the sierra madre the upper libby creek group is interpreted to represent open marine conditions following separation of the two continental blocks tholeiitic sills in the deep lake group and tholeiitic to weakly alkalic dikes in the libby creek group are thought to be related to basaltic igneous activity associated with compound early proterozoic rifting between 2 300 and 2 000 m y ago

lead isotopic evidence for the origin of paleo and mesoproterozoic rocks of the colorado province u s a abstract lead isotopic ratios of k feldspars and whole rocks from 1 7 and 1 4 ga plutonic rocks of the colorado province are relatively non radiogenic for 207 pb 204 pb plotting below the average crust model curve of stacey and kramers 1975 indicating that the terrane was derived primarily from juvenile mantle material slightly more radiogenic ratios in the northern part of the terrane near the archean wyoming province suggest minor inclusion of an older component the data from 1 7 ga plutons plot in a broad field suggesting two episodes of re equilibration with whole rock pb probably related to heating events in the mesoproterozoic 1 4 ga and cretaceous 70 ma possible differences in calculated whole rock th u coupled with slight pb isotopic variations along the north south transect suggest either a terrane boundary through central colorado near salida and gunnison or fundamental differences in source rocks metasedimentary vs metavolcanic u pb analyses of multigrain splits of detrital zircons from quartzites throughout the colorado province have failed to identify archean detritus the oldest 207 pb 206 pb ages found in two samples of quartzite from northern colorado are about 2 0 ga perhaps derived from rocks of the trans hudson orogen in contrast to 2 75 ga detrital zircon in a paleoproterozoic quartzite from the southern part of the wyoming province while we are not yet able to discern if these ages are true provenance ages or mixtures of archean and paleoproterozoic components the absence of easily recognizable archean zircons supports other isotopic data and a conclusion that most of the paleoproterozoic crust of the colorado province was ultimately derived from a juvenile at 1 8 ga mantle reservoir

a parametric model for the yellow sea thermal variability a thermal parametric model has been developed for analyzing observed regional sea temperature profiles based on a layered structure of temperature fields mixed layer thermocline and deep layers it contains three major components 1 a first guess parametric model 2 high resolution profiles interpolated from observed profiles and 3 fitting of high resolution profiles to the parametric model the output of this parametric model is a set of major characteristics of each profile sea surface temperature mixed layer depth thermocline depth thermocline temperature gradient and deep layer stratification analyzing nearly 15 000 yellow sea historical 1950 1988 temperature profiles conductivity temperature depth station 4825 expendable bathythermograph 3213 bathythermograph 6965 from the naval oceanographic office s master oceanographic observation data set by this parametric model the yellow sea thermal field reveals dual structure one layer vertically uniform during winter and multilayer mixed layer thermocline sublayer during summer strong seasonal variations were also found in mixed layer depth thermocline depth and thermocline strength

an early proterozoic volcanic arc succession in southeastern wyoming the green mountain formation of early proterozoic age in the sierra madre range of southeastern wyoming comprises a bimodal mafic and felsic volcanic assemblage the rocks which are chiefly breccias agglomerates flows and volcaniclastic sediments represent both subaerial and submarine eruptions and in part were redeposited in fluvial and nearshore marine environments volcanic rocks are clearly calc alkaline in character and share a large number of geochemical features in common with continental margin arcs or evolved oceanic arc systems the low mg numbers and ni contents of the basalts require 30 40 olivine fractional crystallization and the high contents of the most incompatible elements high la sm n ratios and low zr nb ratios require an undepleted or enriched mantle source geochemical data are consistent with an origin for the felsic volcanics and associated encampment river granodiorite by shallow fractional crystallization of calc alkaline basalt in a tectonic setting similar to modern ar

evidence of tide storm and wave interaction on a precambrian siliciclastic shelf the 1 700 m y ortega group new mexico abstract the 1 700 m y ortega group in northern new mexico is a sequence of quartz arenites and subordinate mudstones in excess of 1 000 m sedimentation took place in diverse shallow shelf environments under the influence of tidal wave and storm processes time velocity asymmetry of tidal flow on the inner shelf produced large scale trough cross beds and tabular planar cross beds with multiple reactivation surfaces in contrast tabular cosets structured internally by small scale trough cross beds and herringbone cross beds resulted from symmetrical tidal flow on the inner shelf fair weather waves reworked the upper surface of tidal sand bodies on the inner shelf during quiet periods within the tidal rhythm during storms sand was entrained into the water column on the proximal inner she f leaving winnowed pebble lags at the top of tidal sand bodies sand transported by storm induced gradient currents was deposited on the outer shelf as discrete lobes progradation of sand lobes produced 1 11 m thick genetic packages which display an upward thickening of sandstone beds and are capped by cosets of tabular planar laminated sandstones storm sands were reworked by wave processes on the proximal outer shelf whereas storm deposited sands on the distal outer shelf remained unmodified no evidence of tidal processes exists on the outer shelf distribution of facies and paleocurrent patterns indicates that the shelf sloped to the south and experienced an overall transgression which culminated in the drowning of the outer shelf with onlap of black basinal muds absence of shallow water turbidites below storm wave base reflects low gradients on the shelf vertical transition from tide to storm dominated sedimentation in the ortega group does not represent a change in depositional style with time on the shelf rather all facies coexisted across the shelf with tidal processes prevailing on the shallow inner shelf and storm processes on the outer shelf the german bight in the north sea and parts of the bering sea off alaska are holocene counterparts for the depositional framework envisaged for the ortega group in both pro imal tide dominated environments pass distally into deeper water storm dominated environments

tectonic setting for the genesis of oceanic plagiogranites evidence from a paleo spreading structure in the oman ophiolite abstract in the oman ophiolite detailed structural and lithological mapping of the 60 45 km maqsad area has revealed several plagiogranite intrusions ranging from metre scale dykes to hectometre scale bodies these plagiogranites are spatially related to kilometre scale mafic plutons displaying generally pegmatitic textures and locally deformed in amphibolite facies conditions along shear zones up to a few hundred metres thick isotope data demonstrate the mantle origin of these pegmatites and plagiogranites nd ranges from 6 3 to 9 9 and has an average value 26 samples of 8 1 within the field of present day morbs and similar to the average nd 7 8 of the oman ophiolite primary igneous sequence these intrusions are not restricted to a given structural level their paleovertical extent exceeds 5 6 km from 2 km below the moho up to the base of the sheeted dyke complex their modal composition evolves up section from predominantly pyroxenites and gabbro norites in the mantle harzburgites to gabbro norites diorites and quartz diorites at crustal level petrography whole rock and mineral chemistry show that this vertical succession of lithologies can be simply explained by low pressure fractional crystallization of a hydrated basaltic melt plagiogranites although more abundant at upper crustal level are not restricted to this horizon plagioclase composition in plagiogranites presents a remarkable evolution with paleodepth from highly calcic 70 95 an in the mantle section intermediate and highly scattered in the lower crust 10 85 an to quite sodic at upper crustal level 5 20 an highly calcic plagioclases in quartz bearing rocks is a puzzling character inconsistent with fractional crystallization even if high degrees of fractional crystallization of a hydrated basaltic melt remain the simplest way to account for the chemistry of the upper crustal plagiogranites processes such as fluid induced remelting or assimilation of country rocks must be invoked to account for the chemistry and field characteristics of the deeper plagiogranites mantle and crustal structures in the maqsad area indicate that the pegmatites and plagiogranites emplaced in an ocean ridge setting when a mantle diapir soaked with basaltic melt intruded a cool hydrothermally altered lithosphere they were eventually transposed in an off axis setting as the ascension of the diapir progressed up to moho level it is concluded that the formation of large volumes of buoyant leucocratic rocks is possible at ocean ridges provided periods of amagmatic spreading are long enough to induce the growth and alteration of an axial lithospheric lid this is likely more frequent at slow spreading centres where mantle upwelling is discontinuous in space and time

xenolithic evidence for proterozoic crustal evolution beneath the colorado plateau tertiary diatremes of the navajo volcanic field brought a wide variety of proterozoic xenoliths to the surface of the colorado plateau examination of crustal xenoliths from the navajo volcanic field diatremes permits reconstruction of proterozoic pressure temperature p t histories beneath the colorado plateau diatremes from the northwest part of the navajo volcanic field carry the greatest variety of xenoliths including metasedimentary rocks amphibolites felsic gneisses mafic granulites and crustally derived eclogites these rock types show variable degrees of hydrous alteration and evidence for complex reaction histories in contrast diatremes from the southeast part of the navajo volcanic field contain primarily mafic and felsic granulites that show fewer reaction textures and less alteration metasedimentary and eclogitic xenoliths are absent in these diatremes the p t paths from the northwest xenoliths are counterclockwise and reach temperatures as high as 850 c and pressures equal to or greater than 10 kbar later hydrous alteration occurred around 500 c and 8 12 kbar xenoliths from the southeast diatremes preserve little evidence of their p t evolution but paths involve heating and or decompression the differing rock types p t paths and alteration histories of the northwest and southeast populations suggest that proterozoic tectonism juxtaposed two distinct crustal blocks beneath the colorado plateau previous workers have postulated that the boundary between the proterozoic yavapai and mazatzal provinces occurs in the region our data support the existence of a northeast trending boundary beneath the four corners area if the eclogitic metamorphism and hydrous alteration are proterozoic in age their restricted occurrence beneath the northwest part of the plateau suggests that subduction was northwest dipping during proterozoic accretion of the mazatzal province onto north america

early proterozoic oceanic crust in the northern colorado front range implications for crustal growth and initiation of basement faults 1 early proterozoic oceanic crust exposed in a shear zone in the northern colorado front range provides insight into the tectonic evolution of the colorado province tectonically intermingled mylonitic amphibolite locally pillowed metagabbro clinopyroxenite metachert and melange are preserved within the buckhorn creek shear zone west of fort collins whole rock geochemical data support formation of the igneous protoliths in an oceanic environment major and trace element data overlap samples from modern oceanic ridges and are distinct from modern arc trends metagabbros and clinopyroxenites preserve a subhorizontal lineation defined by igneous pyroxenes core and mantle microstructures indicate initial deformation at temperatures 900 c secondary amphiboles define both subhorizontal and subvertical lineations kinematic analysis indicates that the rocks experienced e w strike slip deformation prior to and during amphibolite facies metamorphism followed by n s shortening 40ar 39ar data from secondary hornblende yield age steps as old as 1 9 ga based on the chemical data and kinematic evidence for strike slip deformation we propose that the buckhorn creek rocks originated at an oceanic ridge transform intersection the high t pyroxene fabrics formed during ocean floor deformation prior to 1 7 ga regional deformation of the colorado province the buckhorn creek fragment likely represents original oceanic basement for 1 8 1 7 ga arc magmatism and sedimentation that ultimately formed the colorado province continental crust reactivation of original oceanic fracture zones may be responsible for the network of regularly spaced northeast trending shear zones in colorado once formed subvertical transform faults are likely to survive the transition from oceanic to continental crust and persist as long lived crustal discontinuities

examination of the deep levels of an island arc evidence from the tonsina ultramafic mafic assemblage tonsina alaska the tonsina ultramafic and mafic assemblage located in the northern chugach mountains along the trans alaska crustal transect tact is interpreted to be the remains of a magma chamber that crystallized at the base of a mature interoceanic island arc it is one of a number of isolated masses of peridotite and gabbro that comprise the basal part of the peninsular terrane along the northern fringe of the chugach mountains in southern alaska these masses the roots of a mid jurassic island arc have been uplifted along the north dipping border ranges fault system and crop out sporadically for 700 km from kodiak island to eastern alaska the tonsina ultramafic mafic assemblage is a layered sequence whose basal rocks are dunite and harzburgite 1 km thick overlain by a narrow zone of websterite and clinopyroxenite above these ultramafic rocks is a 5 to 6 km thick layered gabbro that is garnet bearing at its base textural relationships and chemical trends throughout the series of dunite pyroxenite garnet bearing gabbro and spinel gabbronorite point to an origin by igneous accumulation from a hydrous tholeiitic parent magma at deep levels i e at the base of a mature island arc before being emplaced along the border ranges fault the rocks were crystallized from a melt at pressures between 9 5 and 11 kbar 28 to 33 km depth and equilibrated at temperatures of 800 to 90o c based on phase relationships in the cumulus assemblages pyroxene thermobarometry and olivine spinel thermometry transitional sequences from ultramafic to mafic rocks are characterized by increasingly al rich pyroxenes and spinels or garnet and the absence of olivine plagioclase assemblages deep level fractionation of al poor phases such as olivine and pyroxene at moderately high pressures and temperatures resulted in concentration of al in the evolving magma as crystallization continued the phases recorded this alumina concentration and the resultant gabbros are al rich this observed trend supports the theory of high alumina basalt generation by fractional crystallization of olivine and pyroxene comparison of the tonsina rocks with aleutian lower crustal xenoliths as well as other exposures of arc related lower crustal magma chambers in japan pakistan and western north america indicates that this process is a common mechanism for island arc magma evolution at depth

a proterozoic volcano plutonic terrane gunnison and salida areas colorado early proterozoic supracrustal rocks near gunnison and salida colorado include tholeiitic basalt dacite to rhyolite and intercalated sedimentary rocks these were intruded by essentially synchronous gabbroic sheets folded and intruded by major plutons ranging from quartz diorite to granite preservation of primary volcanic and sedimentary features is good to excellent sedimentary rocks in the gunnison area are mostly turbidite wackes whereas near salida they are mostly near source volcaniclastic rocks u pb age measurements on zircons indicate that the earliest bimodal volcanic rocks erupted between 1770 and 1760 m y ago and were intruded by granitic plutons between 1760 and 1750 m y ago renewed bimodal volcanism occurred between 1740 and 1730 m y ago and was followed by a period of deformation producing two fold sets before 1725 m y ago when the first of a series of younger plutons was emplaced the compositions and features of the sedimentary rocks and their association with volcanics are con

fracture zone subduction and reactivation across the puysegur ridge trench system southern new zealand a two dimentional kinematic model for lithospheric thrusting that considers the flexural interaction between the underriding and overriding plates was used to assess the mechanical implications of subducting trench parallel fracture zones on the topography and free air gravity anomalies of the puysegur ridge trench system fracture zones in the underriding plate are simulated by vertical discontinuities across which bending and shearing stresses cannot be supported the implication of reactivating fracture zones on the outer trench slope is to suppress the flexural bulge creating topographic relief between the fracture zone and the trench and reducing the trench gravity anomaly in the studied region the trench subparallel l atalante fracture zone accounts for the lack of a flexural bulge strong outer trench slope and gravity gradients the southward decrease of the trench fracture zone distance results in a decrease of the trench gravity anomaly the effect of subducting and vertically reactivating fracture zones within the underriding plate is to segment the load of the overriding plate creating a trough located above the underthrust fracture zone the vertical offset and gravity modifications are functions of the fracture zone position relative to the trench this model explains the 3500 m deep troughs and the 1700 m subsidence observed at the puysegur ridge we speculate that reactivating underthrust fracture zones can facilitate strike slip faults development within the upper plate in case of oblique subduction and are conducive to the formation and dispersion of terranes at convergent margins this process can account for basal tectonic erosion

terrane accretion production and continental growth a perspective based on the origin and tectonic fate of the aleutian bering sea region orogenesis in the aleutian bering sea region would create an expansive new area of pacific rim mountain belts the region itself formed about 55 ma as a consequence of the suturing of a single exotic fragment of oceanic crust aleutia to the pacific s alaskan siberian margin a massive overlap assemblage of the igneous crust of the aleutian arc and the thick sedimentary masses of the aleutian basin have since accumulated above the captured basement terrane of aleutia future closure of the aleutian bering sea region either northward toward the continent or southward toward the aleutian arc would structurally mold new continental crust to the north american plate the resulting beringian orogen would be constructed of a collage of suspect terranes although some terranes would include exotic crustal rocks formed as far as 5000 km away most terranes would be kindred or cotetonic blocks composed of the overlap assemblage and of relatively local 100 1000 km derivation the aleutian bering sea perspective bolsters the common supposition that although disrupted and smeared by transcurrent faulting examples of kindred assemblages should exist and perhaps commonly in ocean rim mountain belts

co existing aluminum silicates in quartz veins a quantitative approach for determining andalusite sillimanite equilibrium in natural samples using oxygen isotopes abs t rac t massive quartz veins with large andalusite and sillimanite crystals in textural equilibrium were found in a proterozoic province in the northern colorado front range the 1 8 o values of the andalusite and sillimanite are identical supporting the idea that these aluminum silicate as polymorphs crystallized at the same time these data are consistent with the findings of sharp 1995 who reported no fractionation associated with polymorphism of the aluminum silicates quartz aluminum silicate fractionations from veins in textural equilibrium and disequilibrium are 2 63 2 93 and 2 20 2 25 respectively temperature estimates from quartz aluminum silicate oxygen isotope fractionations range from 603 652 c for equilibrium veins and geologically unreasonable temperatures of 728 788 c from veins in textural disequilibrium formation temperatures determined from isotope thermometry constrain the location of the veins on the andalusite sillimanite univariant curve thus defining a pressure range of 1 2 2 6 kbar for vein formation the vein p t estimates from this study agree well with previous host rock p t estimates from nearby localities and in conjunction with available ar thermochronology indicate that the veins likely formed during a mesoproterozoic magmatic event in the colorado front range

early proterozoic bimodal volcanic rocks in central colorado u s a part i petrography stratigraphy and depositional history abstract early proterozoic metamorphic rocks of the salida area include two exceptionally well preserved sections of undeformed volcanogenic rocks a northern section nearly 4000 m thick and a southern section 1700 m thick the stratigraphic and lithologic character of the sections are the same indicating that they were formed contemporaneously in the same subaqueous environment the suite is strongly bimodal consisting of felsic volcaniclastic rocks 50 mafic volcanic and volcaniclastic rocks 22 and gabbro diabase sheets 23 the remaining rocks are mostly pelitic metasediments the felsic rocks which dominate the upper portions of both sections comprise four distinct facies crystal lithic tuff breccia crystal lithic lapilli breccia tuff bedded tuff and massive tuff lithic fragments are typically foreign dominated by pelitic material crystal clasts are feldspar mainly plagioclase and quartz sequences of these facies representing cycles of eruptive activity are uniform in thickness and extend along strike for the entire width of precambrian exposure up to 4 km but individual facies units especially the coarser ones are limited in their lateral extent individual sequences average about 100 m thick but continuous exposures of felsic rocks locally exceed 600 m primary sedimentary structures and vertical and lateral variations indicate deposition typically following remobilization as debris flows and turbidites in a shallow marine environment of low relief mafic volcanic and volcaniclastic rocks form accumulations up to 600 m thick and extend uniformly for several kilometers along strike although individual units are more restricted laterally the principal rock types are basalt flows basaltic lithic breccia pillow lava pillow breccia bedded basaltic lapillistone and laminated basaltic tuff all deposited in a submarine setting accumulations typically including several types are interbedded with felsic rocks but mechanical mixing of mafic and felsic volcaniclastics is rare suggesting separate eruptive centers for the mafic and felsic volcanics the northern section may be more proximal than the southern section as indicated by the greater abundance of flows the thicker sheets of gabbro and the greater abundance of mafic rocks relative to felsic rocks gabbro diabase sheets up to 350 m thick and dominantly concordant intrude throughout both sections they are characterized by lack of internal differentiation simple mineralogy consisting of plagioclase and hornblende pseudomorphous after pyroxene and sub ophitic texture the minor pelitic material indicates that the area was accessible to continental sediment sources although their location is not known the salida bimodal volcanic suite is part of a more extensive accumulation of similar 1700 1800 ma supracrustal rocks that extends west to the gunnison area and perhaps south into northern new mexico

1 700 myr greenstone volcanic successions in southwestern north america and isotopic evolution of proterozoic mantle 1 700 myr greenstone volcanic successions in southwestern north america and isotopic evolution of proterozoic mantle

petrology isotope geochemistry and chemical budgets of oceanic gabbros seawater interactions in the equatorial atlantic petrological and chemical variations as well as oxygen and strontium isotopic data are presented for metagabbros from the romanche and vema fracture zones these rocks were affected by several types and degrees of alterations ranging from slight hydrothermal alteration to complete amphibolitization five major kinds of alteration processes ranging from late magmatic deuteric alteration stage i to low temperature 150 c alteration stage v were identified water rock interactions between 300 and 650 c are the most dominant interactions resulting in the most prevailing secondary mineralogical assemblages which characterize the amphibolite and or greenschist facies amphibole plagioclase epidote titanite chlorite prehnite hydrothermal alteration of these gabbroic rocks results in isotopic exchanges between rocks and seawater derived fluids these exchanges lead to decrease of gabbroic 18o toward values as low as 3 9 and larger sr isotopic variations than other oceanic gabbroic rocks 87sr 86sr ratios shift to 0 7029 0 7051 calculation of a chemical budget indicates that metagabbros are hydrated and enriched in fe and probably in mg and cl while si ca and ti are released to the hydrothermal fluids in addition to metamorphic recrystallization and geochemical transformation hydrothermal alteration of oceanic gabbros contributes to the control of the global ocean geochemistry

seasonal thermocline in the china seas and northwestern pacific ocean climatological seasonal variations of the thermocline in the china seas and northwestern pacific ocean were studied using historical data from 1930 through 2001 707 624 profiles the quantitative roles of surface thermal buoyancy b q haline buoyancy flux b p and total buoyancy flux b against the wind induced mixing tau in different seasons and regions were also explored using the buoyancy ratio r b q b p and the monin obukhov depth ratio delta respectively the thermocline has obvious seasonal variations in the study area north of 20 degrees n there is no thermocline along the west coast of the bohai sea bs yellow sea ys and northern east china sea from december to march resulting from surface cooling and wind mixing the significantly different variation of the thermocline strength on and off the chinese shelf is mainly caused by the fact that the thermal stratification is enhanced by bottom tidal mixing on the shelf the delta indicates that the thermocline depth on the chinese shelf is mainly dominated by b in summer while it is dominated by tau in winter it reveals an opposite feature in the kuroshio region the dominating factor is b in winter associated with the large heat buoyancy loss there south of 20 degrees n the dominating factor is similar to that on the shelf with the more obvious b dominant characteristic during the monsoon transition periods the r demonstrates that b is mainly controlled by b q all year round with some sporadically b p dominated regions in the tropical area in winter and in the bs and eastern ys in september

a tectonic model for ridge transform ridge plate boundaries implications for the structure of oceanic lithosphere abstract the first order geologic and morphologic relationships at along and proximal to ridge transform ridge plate boundaries are used to construct an empirical and speculative tectonic model the distinctive but variable morphotectonic fabric and crustal structure of the transform domain are the product of a tectonic continuum that ranges from very slow rates of strain associated with very thick edges of lithosphere slowly slipping ridge transfrom ridge plate boundaries to extremely high rates of strain associated with very thin edges of lithosphere fast slipping ridge transform ridge plate boundaries the geometry of a ridge transform intersection necessitates the juxtaposition of a relatively cold thick edge of lithosphere against the truncated end of an accreting plate boundary the cold face of lithosphere cools the adjacent wedge of asthenosphere rising beneath the axis of accretion and restricts the amount of partial melting thus attenuating the volume of basaltic melt segregated from the asthenosphere per unit time the shallow level manifestation of this cold edge effect is a thinner oceanic crust at depth the evolving lithosphere thickens rapidly as the cold edge is approached creating changes in the properties of the young lithosphere and forming a weld of upper mantle material against the cold edge of truncating lithosphere the ridge transform weld of upper mantle material creates a shear couple in the lithosphere underlying the intersection resulting in the progressive reorientation of the maximum tensile stress from normal to the ridge axis at some distance from the ridge transform intersection to an oblique angle near the boundary the brittle carapace of oceanic crust that overlies the mantle weld will deform accordingly with the development of oblique dip slip faults the model predicts that the geologic expressions of this cold boundary effect will become more dramatic with increasing thickness of the truncating edge at very slow rates of accretion 100 km juxtapose thick 30 50 km edges of lithosphere against the accreting plate boundary all but nullifying the processes that lead to the emplacement of normal oceanic lithosphere in this environment a relatively strong mantle weld will form at the ridge transform intersection and the thick edges of opposing lithosphere along the length of the transform will tend to confine the strike slip tectonism to a relatively narrow and temporally stable principal transform displacement zone in contrast at very fast rates of accretion large offset transforms 100 km place relatively thin edges of lithosphere

does the arc accretion model adequately explain the paleoproterozoic evolution of southern laurentia an expanded interpretation comment and reply comment assembly of lithosphere in southern laurentia is best interpreted to have taken place along a long lived 1 8 1 0 ga active margin via subduction accretion processes broadly analogous to present day convergence between australia and indonesia karlstrom and bowring 1988 1 karlstrom et al

paleoproterozoic rocks of central colorado accreted arcs or extended older crust paleoproterozoic 1770 1735 ma bimodal volcanic rocks in central colorado have been considered to represent southward growth of laurentia by arc accretion although the bimodality of these rocks suggests an extensional continental setting rather than continental or oceanic arcs there has been little evidence for pre 1800 ma crust south of the wyoming craton other than the 1840 1 ma elves chasm pluton in the upper granite gorge of the grand canyon arizona we report shrimp u pb ages of inherited zircons from metarhyolites and plutons in central colorado that are latest archean earliest proterozoic 2520 2000 ma and trans hudson penokean 1878 1814 ma associated quartzites contain detrital zircons with mean ages of 1735 ma indicating only local derivation a meta arkose however contains detrital zircons of trans hudson penokean and archean ages we believe it likely that the 1900 1800 ma trans hudson penokean orogens including archean enclaves extended farther south and west than is currently thought and were the source of the bimodal volcanic rocks and associated plutons during the period 1770 1700 ma

modeling temperature inversion in southeastern yellow sea during winter 2016 a significant temperature inversion with temperature differences larger than 3 c was observed in the southeastern yellow sea ys during february 2016 by analyzing in situ hydrographic profiles and results from a regional ocean model for the ys this study examines the spatiotemporal evolution of the temperature inversion and its connection with wind induced currents in winter observations reveal that in winter when the northwesterly wind prevails over the ys the temperature inversion occurs largely at the frontal zone southwest of korea where warm saline water of a kuroshio origin meets cold fresh coastal water our model successfully captures the temperature inversion observed in the winter of 2016 and suggests a close relation between northwesterly wind bursts and the occurrence of the large inversion in this respect the strong northwesterly wind drove cold coastal water southward in the upper layer via ekman transport which pushed the water mass southward and increased the sea level slope in the frontal zone in southeastern ys the intensified sea level slope propagated northward away from the frontal zone as a shelf wave causing a northward upwind flow response along the ys trough in the lower layer thereby resulting in the large temperature inversion diagnostic analysis of the momentum balance shows that the westward pressure gradient which developed with shelf wave propagation along the ys trough was balanced with the coriolis force in accordance with the northward upwind current in and around the inversion area

internal wave interactions with equatorial deep jets part ii acceleration of the jets abstract what drives the equatorial deep jets is a puzzle because of their isolation from surface forcing by the intervening main pycnocline and the equatorial undercurrent and from lateral boundaries by distances of tens of thousands of kilometers it would take decades for energy to propagate to the jets midbasin location from boundary sources their persistence points to some mechanism maintaining them in situ the authors hypothesize that the ambient internal wave field deposits momentum fluxes at critical layers within the deep jets and using calculated momentum and energy flux divergences as forcing estimate acceleration of the mean zonal flow in the deep jets internal wave momentum flux divergences are more than sufficient to sustain the jets acting to sharpen the shear between the jets on timescales of months to years predicted energy flux divergences produce turbulent dissipation rates compatible with those observed

palaeozoic arc growth deformation and migration across the lachlan fold belt southeastern australia abstract the palaeozoic lachlan fold belt lfb is a low pressure metamorphic belt characterized by greenschist facies rocks and infolded volcanic sequences which indicate that the belt has not undergone substantial uplift at any stage porphyroblast matrix relationships in one of the highest grade metamorphic zones of the lfb the omeo complex indicate that the thermal peak was reached before or was synchronous with the earliest deformation similar to that in high grade metamorphic zones of the northern arunta inlier central australia in both areas the metamorphism is of low p high t type centred on major granite intrusions as regional aureoles and characterized by anti clockwise p t t paths however in the outer lower grade parts of the aureoles peak metamorphic conditions post date the earliest deformation this diachronous relationship indicates that 1 outward migration of a thermal front was centred on major granitoid intrusions batholiths 2 deformation propagated outward more rapidly than migration of the thermal front and 3 plastic and ductile deformation began only after heating around the batholiths thus orogeny was initiated after and localized by heat focusing in the mid crust associated with batholith emplacement therefore the early deformation is a result of thermal softening it is typically subhorizontal ductile shear at mid crustal levels but is characterized by upright to inclined folds at upper crustal levels in the latter environment the typically discordant contact aureole plutons are often interpreted as post tectonic granitoids but they were part of the ongoing mid crustal thermal perturbation that induced regional greenschist facies metamorphism which overprinted the early formed upright folds as it migrated to upper crustal levels orogeny migrated generally eastward through the lfb it began with the early silurian benambran orogeny centred on the wagga metamorphic belt wmb and terminated with the carboniferous kanimblan orogeny which was most intense in the hill end trough eastward migration of orogeny across central victoria part of the west lfb began in the ballarat bendigo zone during the early devonian and terminated in the melbourne zone in the middle devonian orogeny was centred on the meridional batholiths of the lfb each of which are considered to represent the transitory axis of an ancient magmatic arc stepwise but generally eastward arc migration caused the eastward migration of orogeny a tectonic model based on the modern southwest pacific arc system can be applied to the lfb following cambrian intra oceanic arc growth associated with west dipping subduction an ordovician marginal sea developed partly on stretched cambrian crust and was flooded by an extensive turbiditic wedge closure of the eastern part the east lfb began in the early silurian after a magmatic arc developed over the wmb possibly associated with an east dipping subduction zone after re establishment of west dipping subduction in the middle silurian absolute motion retreat of the upper australian plate caused by oblique plate convergence resulted in dextral transtension and the generation of siluro devonian inter arc and back arc basins transient compression resulted in deformation along the magmatic arc and induced formation of a new outboard eastward arc system periodic compression within the evolving arc and back arc system resulted in a series of east younging east verging linear siluro devonian fold and thrust belts which were localized adjacent to elongate batholiths that represent the relict arc systems local westward jumps in deformation and plutonism occurred the most significant is the 600 km westward jump into the west lfb caused by closure of the remainder of the passive ordovician back arc basin melbourne and bendigo ballarat zones west dipping subduction beneath the basin produced a low pressure fold thrust belt similar to the east lfb though the granitoids have a more primitive isotopic character indicating closure of thin dominantly oceanic crust in the early to middle devonian localized deformation induced by thermal softening of the crust within magmatic arcs that have developed on earlier passive margins is likely to produce anticlockwise p 7 t paths and may be a typical feature of southwest pacific style tectonism

early proterozoic assembly of tectonostratigraphic terranes in southwestern north america a 500 km wide early proterozoic orogenic belt in arizona and adjacent areas is divided into lithotectonic blocks by northeast and north trending shear zones structural and u pb zircon studies suggest that these blocks may have experienced different tectonic histories prior to their juxtaposition by thrust and strike slip movements on shear zones a northwestern province here called the yavapai province is composed of at least five tectonic blocks and was assembled at about 1700 ma a southeastern province here called the mazatzal province is composed of at least three blocks and was assembled and juxtaposed with the northwestern province during the mazatzal orogeny between 1695 and 1630 ma the diversity of seemingly incompatible 1700 ma tectonic regimes now in close proximity to each other and the absence of systematic cross strike changes in age and character of deformation plutonism and metamorphism suggest a model whereby the orogenic belt developed and north america grew by assembly of d

early proterozoic bimodal volcanic rocks in central colorado u s a part ii geochemistry petrogenesis and tectonic setting abstract the early proterozoic supracrustal rocks of the salida area of central colorado consist of strongly bimodal sequences of volcanogenic rocks the mafic rocks basalts basaltic volcaniclastics and related gabbro sheets are distinctly tholeiitic display a strong iron enrichment trend and typically contain less than 50 sio 2 the felsites are rhyolites to dacites and contain more than 70 sio 2 major and trace element modeling show that the mafic rocks underwent two stages of crystal fractionation the first involving olivine and plagioclase the second involving plagioclase and clinopyroxene fractionation occurred within individual injections as they rose toward the surface rather than in a single magma chamber at depth field relations and major element data support the derivation of the felsic rocks from a magma generated by anatexis of sialic crust however the low sr and high heavy ree concentrations in these rocks are not compatible with a partial melting model and suggest that the felsic volcanic rocks could have been derived by extensive fractional crystallization of the mafic magma normalized trace element abundances and trace element ratios of the mafic rocks are most like continental flood basalts such as the columbia river basalts they also display some similarity to immature back arc basin tholeiites developed on continental crust such as those of the sarmiento complex the felsic rocks have strong chemical affinities to within plate rhyolites rather than calc alkaline rhyolites from orogenic areas the chemical data as well as the petrographic stratigraphic and regional field data all indicate that the early proterozoic supracrustal rocks of the salida area developed along a continental margin probably within an immature back arc basin underlain by sialic crust remnants of the arc system of similar age may lie to the south in northern new mexico and southwestern colorado

origin and emplacement of ophiolites ever since their recognition ophiolites have been a source of controversy although all workers have agreed upon their importance a number of conflicting schools of thought have become unified in the interpretation of ophiolite complexes as fragments of oceanic crust and mantle formed at spreading centers an expanded version of the penrose ophiolite definition is proposed herein called the ophiolite association taking into account rocks which bear upon the origin and significance of ophiolite complexes an ideal ophiolite association includes the following units from bottom to top 1 a crystalline basement and shallow water sedimentary sequence 2 a tectonic unit of thrust slices of continental margin and rise and abyssal sediments and or melange 3 a metamorphic unit as much as a few hundred meters thick generally with higher grade rocks over lower grade ones 4 ultramafic tectonite unit composed dominantly of multiply deformed peridotite dunite and minor chromitite 5 a cumulate complex ultramafic at the base grading to mafic or intermediate at the top 6 a noncumulus unit of varitextured gabbro and minor trondhjemite 7 a sheeted dike complex 8 an extrusive section of massive and pillowed flows and intercalated sediments 9 an abyssal or bathyal sediment sequence which may include radiolarian chert red pelagic limestone metalliferous sediments breccias and or pyroclastic deposits and 10 postemplacement deposits of laterite reef limestone or shallow marine or subaerial sediments this expanded ophiolite association allows greater precision in inferring original origin and emplacement some ophiolite complexes particularly in the tethys herein designated tethyan type exhibit units 1 2 3 and 4 whereas others particularly in the cordilleran belt herein termed cordilleran type lack units 1 2 3 and in places 4 the emplacement of the former probably occurred during collision of a passive margin with a subduction zone or incipient subduction zone emplacement of the cordilleran type generally is not clear cut the nature of the spreading center which produced a given ophiolite is indicated more clearly by the presence or absence of units 5 9 than by chemistry or mineralogy some ophiolite complexes lack complete sections and may have formed along transform faults others contain volcanic pyroclastic deposits instead of pelagic sedimentary sequence and probably formed within or near an arc several ophiolite complexes may have originated by conversion of a transform fault into a subduction zone such conversion may result from normal plate evolution or from major plate reorganization major times of ophiolite emplacements appear to correspond with global plate reorganizations

anomalous oceanic lithosphere formed in a leaky transform fault evidence from the western limassol forest complex cyprus detailed studies in the western limassol forest complex cyprus have revealed plutonic and structural histories that differ markedly from the penrose type ophiolite of the troodos massif to the north a tectonized harzburgite of upper mantle origin acts as host to multiple mafic and ultramafic intrusions forming a plutonic core complex mafic and picrite dykes cut and are cut by e w trending shear zones demonstrating syn tectonic magmatism a retrogressive metamorphic and progressive cooling history of the intrusive suite indicates uplift of the host mantle harzburgite while in a sea floor setting it is suggested here that the geological evolution of the western limassol forest complex indicates formation of anomalous oceanic lithosphere in the active domain of an upper cretaceous leaky transform fault

dynamics of subduction initiation at preexisting fault zones we show that a new subduction zone can initiate at a preexisting dipping fault zone with reasonable plate forces consistent with what is observed in the western pacific the dynamics of subduction initiation within a viscoelastic medium has been systematically explored with the finite element method we investigate the compression of oceanic lithosphere with both force and velocity boundary conditions and track the thermal structure in which heat is transported by both advection and diffusion the viscosity of the medium is non newtonian and temperature dependent we also examine the influence of a perfectly plastic yield stress a new method to model the preexisting weak zone as a graded fault with remeshing is introduced the first 400 km of plate convergence is examined during which time there are profound changes in plate boundary dynamics initially topography across the plate boundary is characterized by over 1000 m of uplift close to the trench subsequently a bathymetric depression develops on the overriding plate with an initial subsidence rate of 125 m myr for a slab subducting at 2 cm yr tectonic subsidence of the overriding plate is primarily dependent upon the depth of slab penetration for a plate force of 4 10 12 n m subduction initiates even with a fault shear stress of 3 mpa the resistance most consistent with seismological observations however subduction will not initiate when this fault shear stress is 5 mpa our results are consistent with plate reconstructions which predict the initiation of subduction across preexisting weak zones within oceanic basins

microstructural features of high temperature shear zones in gabbros of the northern apennine ophiolites abstract this paper presents preliminary data on the microstructural evolution of upper amphibolite facies mylonitic metagabbros from the bracco ophiolite complex northern apennines italy high temperature deformation possibly related to the initial stages of breakup and oceanization in the ligurian tethys led to the development of three different types of fabrics in the coarse grained mg gabbros the most widespread rock type in this area with progressive deformations coarse grained porphyroclastic mylonites fabric type 1 banded fabric type 2 and flinty ultramylonites fabric type 3 were formed in plagioclase and clinopyroxene the two main minerals in the studied rocks optical microstructures and scanning electron microscopy analyses indicate crystal plasticity as the main deformation mechanism a possible change from rotation recrystallization regime through grain boundary migration to diffusion assisted high temperature grain boundary sliding is suggested for the plagioclase grains in high temperature shear zones

amphibolitized sheared gabbros from ophiolites as indicators of the evolution of the oceanic crust bay of islands newfoundland abstract in the bay of islands ophiolite complex both the basal part of the cumulate gabbro sequence and the underlying peridotites have been plastically deformed higher in the series the cumulate gabbros are undeformed except in localized shear zones slightly to strongly oblique to the layering the resulting shear bands are narrow and composed either of mylonitic amphibole bearing gabbros or of highly foliated amphibolites the amphibole composition and nature and the chemistry of coexisting minerals combined with structural evidence imply that a first episode of shearing subhorizontal in the crust occurred at decreasing temperatures ranging from 750 to 450 c this shearing probably represent the response of the lithosphere to stresses also responsible of asthenospheric flow in the upper mantle a second episode of shearing oblique to the first one and probably related to cooling of the crust away from the ridge occurred at lower temperatures ranging from 500 to 300 c the surrounding gabbros have undergone static hydrothermal metamorphism at temperatures decreasing from 650 to 450 c all metamorphic events occurred in hydrous conditions in the vicinity of the ridge axis

the cheyenne belt analysis of a proterozoic suture in southern wyoming abstract the cheyenne belt of southeastern wyoming is a major shear zone which separates archean rocks of the wyoming province to the north from 1800 1600 ma old eugeoclinal gneisses to the south miogeoclinal rocks 2500 2000 ma old unconformably overlie archean basement immediately north of the shear zone and were deposited under transgressive conditions along a rift formed continental margin intrusive tholeiitic sills and dikes are interpreted as rift related intrusions and a date of 2000 ma on a felsic differentiate of these intrusions gives the approximate age of rifting there are no known post 2000 ma felsic intrusions north of the cheyenne belt volcanogenic gneisses and abundant syntectonic calc alkaline plutons of the southern terrane are interpreted as island are volcanic and plutonic rocks the volcanics are a bimodal basalt rhyolite assemblage plutons include large gabbroic complexes and quartz diorite 1780 ma syntectonic granitoids 1730 1630 ma and post tectonic anorthosite and granite 1400 ma there is no evidence for archean crust south of the cheyenne belt structural data thrusts in the miogeoclinal rocks vertical stretching lineations and the same fold geometries north and south of the shear zone suggest that juxtaposition of the two terranes took place by thrusting of the southern terrane island arc over the northern terrane craton and miogeocline probably as a continuation of the south dipping subduction which generated calc alkaline plutons of the southern terrane a metamorphic discontinuity across the shear zone with greenschist facies rocks to the north and upper amphibolite facies rocks and migmatites to the south also suggests thrusting of the southern terrane deeper crustal levels over the northern terrane shallower levels the cheyenne belt may be a deeply eroded master decollement perhaps analogous to a ramp in the master decollement in the southern appalachians this interpretation of the cheyenne belt as a proterozoic suture zone provides an explanation for the geologic geochronologic geophysical metallogenic and metamorphic discontinuities across the shear zone

microstructures and lattice preferred orientations in experimentally deformed clinopyroxene aggregates microstructures and lattice preferred orientations lpo were analysed on experimentally deformed natural and hot pressed clinopyroxene aggregates in order to understand the relationship between deformation processes and evolving microstructures the lpo was measured using electron backscatter diffraction techniques in the scanning electron microscope sem microstructures were observed by polarized light microscopy and by orientation contrast in the sem natural samples sleaford bay pyroxenite were deformed in axial compression stepping tests up to 16 shortening these samples deformed mainly by twinning and dislocation glide with very little recrystallization no clear lpo evolution apart from the initial lpo could be attributed to deformation synthetic clinopyroxenite samples were hot pressed from powders of the same material with three different grain size ranges and deformed in compression stepping experiments up to 28 shortening in the samples with coarse 30 m and intermediate 20 m grain sizes deformation was dominated by dislocation creep accommodated by subgrain rotation recrystallization the recrystallized grains have sizes up to 8 m and represent 15 25 of the sampled area the sample with the finest initial grain size 5 m deformed dominantly by diffusion creep accompanied by grain boundary migration all of the hot pressed samples deformed in compression have a similar texture consisting of a girdle of c 001 axes normal to compression and a point maximum of b 010 axes and a 100 poles parallel to the compression direction the lpo of recrystallized grains was separated on spatially resolved orientation maps and shows an s type fabric with b 010 parallel to the compression axis and a random a 100 distribution the bulk fabric was largely present in the hot pressed starting material and is interpreted as the result of compaction during cold pressing at moderate strains in axial compression the initial compaction fabric has been weakly overprinted by the recrystallization fabric one hot pressed sample coarse grain size was deformed in torsion to a shear strain of 0 1 the microstructure indicates dominant dislocation creep and contains about 20 recrystallized grains the texture evolved from the rotationally symmetric compaction lpo to an oblique fabric

the rattlesnake creek terrane klamath mountains california an early mesozoic volcanic arc and its basement of tectonically disrupted oceanic crust the rattlesnake creek terrane in the south western klamath mountains consists of a serpentinite matrix melange basement unconformably overlain by a coherent cover sequence of upper triassic and lower jurassic volcanic hemipelagic and clastic sedimentary rocks both the meelange and cover sequence are intruded by a suite of ca 193 207 ma u pb zircon gabbroic to quartz dioritic plutons these rocks collectively record a complex multistage evolution through several distinct tectonic environments blocks in the meelange basement include peridotite greenstone amphibolite pillow basalt various generally mafic plutonic rocks metachert and limestone all suspended in a matrix of sheared serpentinite trace element chemistry indicates that basalt amphibolite and greenstone blocks were derived from normal to enriched mid oceanic ridge basalt n morb to e morb and within plate basalt wpb protoliths there is no geochemical evidence that any blocks were derived from a magmatic arc nor do any of the sedimentary rocks contained within the meelange reflect deposition near a terrigenous sediment source collectively these relations indicate that the basement meelange was derived by disruption of oceanic crust and upper mantle in a setting far removed from either an arc or a terrigenous landmass and are most consistent with an interpretation that meelange formation oc curred in an oceanic fracture zone the overlying cover sequence is divisible into a stratigraphically lower salt creek assemblage and a stratigraphically higher dubakella mountain assemblage both deposited in a marine setting volcanic rocks in the lower assemblage are basaltic pillow lava and pillow breccia with the trace element characteristics of a primitive island arc tholeiite suite whereas those in the upper assemblage are predominantly chnopyroxene phyric basalt to basaltic andesite and compositionally similar volcaniclastic rocks with the geochemical characteristics of a more evolved cale alkaline to shoshonitic volcanic arc suite the geochemical character of these volcanic rocks is indistinguishable from that of the coeval early mesozoic plutons leading to the conclusion that these plutons represent the intrusive roots of the volcanic succession collectively these rocks reflect construction of a late triassic to early jurassic island arc and indicate that subduction had initiated within or near the under lying fracture zone assemblage by at least the late triassic the latest stages of crustal disruption in the basement thus probably occurred in a subduction zone environment both cover sequence units also contain inter layered chert and argillite as well as coarse epiclastic rocks with an abundance of quartzose sedimentary and metasedimentary grains these elastic rocks provide the first evidence for terrigenous input into the rattle snake creek terrane and indicate that the early mesozoic arc developed in proximity to a terrigenous sediment source probably paleozoic rocks of the western north american cordillera several lines of evidence suggest that early mesozoic arc magmatism was accompanied by an episode of extensional tectonism these include structural evidence for an early mesozoic phase of high angle faulting that appears to have localized cover sequence deposition in fault bounded grabens sedimentologic evidence indicating that both the basement and terrigenous sediment sources experienced episodic pulses of uplift and erosion during cover sequence deposition and widespread intrusion of arc plutons into the upper crustal levels of the are carapace both the basement complex and the early mesozoic arc assemblage of the rattlesnake creek terrane were then deformed by thrust faulting folding and penetrative foliation development during over thrusting from the east by the western hayfork terrane in the middle jurassic regional de formation was in response to northeast south west directed shortening and was accompanied by metamorphism to subgreenschist grade insofar as the western hayfork terrane represents part of a middle jurassic arc whose plutonic roots intrude most klamath mountain terranes east of and including the rattle snake creek terrane it is clear that the rattlesnake creek terrane formed part of the klamath mountain province by middle jurassic time the paleogeographic and tectonic relation of the rattlesnake creek terrane to the rest of the klamath mountains prior to the middle jurassic however is unclear

crustal evolution of southern laurentia during the paleoproterozoic insights from zircon hf isotopic studies of ca 1 75 ga rocks in central colorado lu hf depleted mantle model tdm ages obtained by analysis of zircons previously dated by u pb methods demonstrate that ca 1 75 ga bimodal paleoproterozoic rocks in the gunnison salida region of central colorado and by extension in much of the southwestern u s were formed by partial melting of preexisting crustal rocks the ages of which mostly ages greater than 2 0 ga indicate that range from 1 83 to 1 87 ga some calculated lu hf tdm even older crust was probably involved consistent with the limited presence of ca 2 5 ga xenocrystic zircons in some rocks these results suggest that rocks related to the trans hudson and penokean orogens are cryptically present much farther to the south than previously believed coupled with the bimodality of the volcanic suite in central colorado and indeed in much of the southwestern u s these results indicate that in contrast to current juvenile arc accretion models melting of older crust related to extensional tectonics played an important role in the genesis of many magmatic rocks between 1 6 and 1 8 ga

squeezing water from a stone h2o in nominally anhydrous minerals from granulite xenoliths and deep hydrous fractional crystallization although 10 of earth s water resides within continents ho distribution throughout the continental lithosphere and partitioning of ho in nominally anhydrous minerals nams remain poorly co

plate tectonics model for proterozoic continental accretion in the southwestern united states proterozoic crustal provinces in the southwestern united states decrease in age south from the archean wyoming province supracrustal rocks define three major age provinces from north to south 1 72 to 1 80 1 65 to 1 72 and 1 1 to 1 2 b y successions within each province are similar and define two assemblages a bimodal volcanic assemblage commonly overlain by a quartzite shale assemblage precambrian granitic plutons intrude all three provinces geochemical and sr isotope data indicate a variably depleted upper mantle source for basalts and a short lived these observations can be explained by a plate tectonics model involving successive marginal basin closures and andean type orogenies associated with southward migrating arcs the net result is to accrete 1 300 km of continental crust to the southwestern margin of north america between 1 8 and 1 1 b y ago

neotethyan closure history of western anatolia a geodynamic discussion this paper addresses the lithosphere scale subduction collision history of the eastern termination of the aegean retreating subduction system i e western anatolia although there is some general consensus on the protracted subduction evolution of the aegean since the early cenozoic at least correlation with western anatolia has been widely debated for more than several decades in western anatolia three main tectonic configurations have been envisaged in the past years to reconstruct slab dynamics during the closure of the neotethyan oceanic realm since the late cretaceous some authors have suggested an aegean type scenario with the continuous subduction of a single lithospheric slab punctuated by episodic slab roll back and trench retreat whereas others assumed a discontinuous subduction history marked by intermittent slab break off during either the campanian ca 75 ma or the early eocene ca 55 50 ma the third view implies three partly contemporaneous subduction zones our review of these models points to key debated aspects that can be re evaluated in the light of multidisciplinary constraints from the literature our discussion leads us to address the timing of subduction initiation the existence of hypothetical ocean basins the number of intervening subduction zones between the taurides and the pontides the palaeogeographic origin of tectonic units and the possibility for slab break off during either the campanian or the early eocene thence we put forward a favoured tectonic scenario featuring two successive phases of subduction of a single lithospheric slab and episodic accretion of two continental domains separated by a continental trough representing the eastern end of the cycladic ocean of the aegean the lack of univocal evidence for slab break off in western anatolia and southward younging hp lt metamorphism in continental tectonic units from 85 70 to 50 ma in the late cretaceous palaeogene period suggests continuous subduction since 110 ma marked by roll back episodes in the palaeocene and the oligo miocene and slab tearing below western anatolia during the miocene

does the arc accretion model adequately explain the paleoproterozoic evolution of southern laurentia an expanded interpretation a common model for the paleoproterozoic crustal evolution of southern laurentia involves southerly accretion of successively younger island arcs from 1780 to 1650 ma this model may be oversimplified however for although andesite and basaltic andesite major components of modern island arcs occur they are relatively rare and large volumes of rocks are dominantly bimodal metavolcanic assemblages and related granitoids ophiolites occur but they are also rare and only one accretionary melange has been described inherited zircons and common pb and nd isotopic data indicate involvement of older crust in ca 1750 ma rocks of the mojave province of southeastern california and western arizona and the bimodal assemblages of central colorado these data suggest that the bimodal volcanic rocks were derived from pre existing continental crust likely of ca 1850 ma trans hudson penokean age during extension and partial melting associated with the development of transpressional crustal scale shear zones the occurrence of the 1840 1 ma elves chasm pluton in the grand canyon demonstrates that rocks of trans hudson penokean age are present in southwestern laurentia

geochemistry of the dubois greenstone succession an early proterozoic bimodal volcanic association in west central colorado abstract the dubois greenstone succession an early proterozoic supracrustal succession in west central colorado is composed of fine grained felsic volcaniclastic sediments ash flow tuffs and tholeiitic flows sills and dikes the volcanic rocks comprise a bimodal suite of tholeiite and rhyolite the tholeiites are divided into two groups group i exhibiting nearly flat ree patterns 20 30 chondrites and group ii with light ree enriched patterns 40 70 chondrites these groups can be produced by two stage melting of an undepleted or enriched garnet lherzolite source in which garnet remains in the residue of only the first melting stage which produces group ii magmas the felsic magmas can be produced by partial melting of a high grade gneiss in the lower crust in which zircon is entrained in the magma the dubois greenstone succession appears to have formed by felsic and mafic subaqueous eruptions in which some ash flows upon entering water underwent phreatomagmatic explosions giving rise to detritus which formed the volcaniclastic sediments although tectonic setting cannot be assigned to the succession with a high degree of confidence an immature back arc basin developed on sialic crust in consistent with most data

reduced rapakivi type granites the tholeiite connection reduced rapakivi type granites are the most iron enriched and reduced i e least oxidized of the anorogenic granite association the low oxygen fugacity and chemical composition of these granites severely limit their sources in this paper we argue that reduced rapakivi type granites and their eruptive equivalents high potassium fayalite rhyolites are derived from mafic sources because tholeiitic magmas and their derivatives have the required low oxygen fugacity reduced rapakivi type granites are produced either by extreme differentiation of basaltic melts or by partial melting of underplated basalts and their differentiated equivalents they form in extensional environments where the asthenosphere is present at shallow depths we envision three stages in the origin of these rocks 1 tholeiitic melts are emplaced at the base of the crust 2 continued introduction of heat partially remelts these tholeiitic rocks and 3 the hot dry melts so produced migrate into the middle crust to produce rapakivi batholiths or erupt as rhyolites partial melting of felsic continental crust may accompany the intrusion of rapakivi type magmas thereby producing the other metaluminous and peraluminous granite compositions of the anorogenic suite

seasonal variations of the yellow river plume in the bohai sea a model study 1 we investigated the behavior of the yellow river plume in the bohai sea through a year and its controlling mechanisms using a numerical model driven by monthly averaged wind stress heat flux and river discharge the model produced a clear seasonal behavior of the yellow river plume in summer the yellow river plume extends northeastward into the central bohai sea in autumn it turns southward and flows into laizhou bay along the coast in winter most of the diluted water is found around southeastern corner of laizhou bay and in spring a low salinity area appears in southwest laizhou bay with the same model several well designed numerical experiments were performed to investigate the effect on the behavior of the yellow river plume of tidal current river discharge wind stress and thermal stratification the tidal currents promote the offshore spread of diluted water while the tide induced residual currents prevent the plume from extending further downstream in the direction of kelvin wave propagation wind stresses lead to pronounced changes in the path and shape of the plume in autumn and winter northwesterly or northerly winds drive the buoyant water into laizhou bay along the coast in summer the combination of southeasterly winds and thermal stratification drives the plume extend northeastward into the central bohai sea by changing only the magnitude of wind stress or river discharge in the experiments we examined the sensitivity of summertime plume to the magnitude of wind stress and river discharge

hot orogens tectonic switching and creation of continental crust many granulite terrains were too hot to have formed during continental collision rather along with many high grade metamorphic terrains that typify continental crust most formed in accretionary orogens during tectonic switching when prolonged lithospheric extension was interrupted by intermittent transient contraction based on modern and ancient examples tectonic switching occurs when slab retreat induces upper plate extension causing arc splitting formation of microcontinent slivers and backarc basins then intermittent arrival of buoyant oceanic plateaus induces transient flat subduction or slab flip and crustal thickening during slab retreat basaltic magmas produced from decompressed asthenosphere advect into the extending orogen causing granulite facies metamorphism and granite generation but subsequent thickening during flat subduction cools the region thickening is focused in the thermally softened backarc region and if sediment filled a hot short lived 10 m y narrow 50 100 km orogenic belt forms such thickening is often misleadingly ascribed to arc or microcontinent collision once slab retreat mode is reestablished lithospheric extension recommences and a new arc backarc system forms generally outboard arrival of another plateau will reverse the procedure and another short lived hot orogen will form within the orogenic system cycles of tectonic switching efficiently produce continental crust

petrogenesis of the fiambala gabbroic intrusion northwestern argentina a deep crustal syntectonic pluton in a continental magmatic arc

temperature inversion in china seas temperature inversion was reported as a common phenomenon in the areas near the southeastern chinese coast region a west and south of the korean peninsula region b and north and east of the shandong peninsula region c during october may in the present study based on hydrographic data archived from 1930 through 2001 319 029 profiles the inversion was found to be remarkable with obvious temporal and spatial variabilities in both magnitude and coverage with higher probabilities in region a up to about 60 and region c 40 50 than in region b 15 20 the analysis shows that seasonal variation of the net air sea heat flux is closely related to the occurrence time of the inversion in the three areas while the yangtze and yellow river freshwater plumes in the surface layer and ocean origin saline water in the subsurface layer maintain stable stratification it seems that the evaporation excessive precipitation flux makes little contribution to maintaining the stable inversion advection of surface fresh water by the wind driven coastal currents results in the expansion of inversion in regions a and c the inversion lasts for the longest period in region a october may sustained by the taiwan warm current carrying the subsurface saline water while evolution of the inversion in region b is mainly controlled by the yellow sea warm current

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

phase equilibria in the system k 2 o feo mgo al 2 o 3 sio 2 h 2 o co 2 and the stability limit of stilpnomelane in metamorphosed precambrian iron formations the phase relations of al and fe bearing silicates in the system k2o feo mgo al2o3 sio2 h2o co2 in the presence of quartz and magnetite are discussed on the basis of mineralogic and petrologic data from precambrian iron formations and blueschist facies meta ironstone from the franciscan formation california these relations allow an estimation of the physiochemical conditions during low grade metamorphism of iron formations petrologic data together with available experimental and predicted thermodynamic data on the associated minerals place the upper stability limit of stilpnomelane in iron formations at about 430 470 c and 5 6 kilobars fe end member stilpnomelane can persist to a maximum temperature of 500 c and pressures up to 6 7 kilobars although it is unlikely to occur in metamorphosed iron formations in iron formation occurrences the stilpnomelane stability field is bordered by four equilibrium reactions with the assemblages stilpnomelane zussmanite chlorite minnesotaite stilpnomelane zussmanite chlorite grunerite stilpnomelane biotite chlorite grunerite and stilpnomelane biotite almandine grunerite the stability field is reduced by increasing x co2 and xmgstil and is also a function of a k a h in the metamorphic fluid if the value of a k a h is smaller than that defined by the above assemblages stilpnomelane decomposes to chlorite but if larger it is replaced by biotite at pressures less than 4 kilobars the zussmanite field is restricted to a very high value of a k a h 5 0 in log units at 1 0 kilobar where iron formation assemblages are not stable

geochemistry and petrogenesis of early proterozoic amphibolites west central colorado u s a abstract early proterozoic supracrustal rocks exposed in west central colorado include amphibolites intimately associated with felsic volcanics and associated volcaniclastic sediments three successions are recognized the dubois succession 1770 1760 ma is comprised chiefly of amphibolites and felsic volcanics the amphibolites exhibit low concentrations of light ree and hfse and are broadly similar in composition to tholeiites from immature island arcs and in some respects to nmorb the cochetopa succession 1745 1730 ma includes large amounts of felsic volcanics and volcaniclastic sediments together with amphibolites that exhibit variable but slight incompatible element enrichment similar to basalts from evolved island arc systems and associated back arc basins the black canyon succession is coeval with the cochetopa succession and is comprised chiefly of volcaniclastic sediments with minor amphibolite these amphibolites exhibit significant incompatible element enrichment similar to basalts from continental margin arc systems geochemical modeling indicates that the amphibolites evolved by closed system fractional crystallization of depleted for the dubois succession to variably enriched parental basalts for the cochetopa and black canyon successions chiefly at shallow depths incompatible element distributions in the amphibolites indicate that mantle sources changed with time from relatively depleted 1770 1760 ma to variable but slightly enriched 1745 1730 ma a provisional model calls upon the dubois succession forming in an island arc system followed by collision with the archean wyoming province at 1750 ma the cochetopa succession was deposited in an intra arc basin developed within a new continental margin arc and the black canyon succession formed in a back arc basin associated with this arc

crustal structure of north atlantic fracture zones seismic studies have established that large offset transforms along the slow spreading mid atlantic ridge exhibit anomalous crustal structures that fall well outside the range typically associated with oceanic crust seismically fracture zone crust in the north atlantic is extremely heterogeneous in both thickness and internal structure it is frequently quite thin 1 2 km thick and is characterized by low compressional wave velocities and the absence of a normal seismic layer 3 a more gradual crustal thinning can extend up to several tens of kilometers from these fracture zones anomalously thin crust has also been inferred from both seismic and gravity studies at smaller ridge axis discontinuities along the mid atlantic ridge the geological nature of the seismically anomalous crust found within atlantic fracture zones and how this crust forms are still controversial one interpretation consistent with available seismic observations is that the crust within north atlantic fracture zones consists of a thin intensely fractured and hydrothermally altered basaltic section overlying ultramafics that are extensively serpentinized in places variations in apparent seismic crustal thickness along fracture zones may reflect different degrees of serpentinization of the upper mantle section or changes in the thickness of the igneous crust the existence of a thinner crustal section in fracture zones can be explained by a reduced magma supply within a broad region near ridge offsets due to the three dimensional nature of upwelling beneath a segmented spreading center and by tectonic dismemberment of the crust by large scale detachment faults that form preferentially in the cold brittle lithosphere near the ends of segments