geochemistry and provenance of the middle ordovician austin glen member normanskill formation and the taconian orogeny in new england the austin glen member of the upper middle ordovician normanskill formation is a sandstone shale ysch succession deposited in the foreland of the taconian orogen petrographic major and trace element and nd pb isotopic data provide substantial constraints on its provenance lack of k feldspar and paucity of plagioclase in addition to the dominance of sedimentary rock fragments indicate that the source was dominated by recycled sedimentary components major and trace element data support this conclusion and indicate that the provenance of both shales and sandstones was the same no evidence of an ophiolitic or volcanic component was observed interpretation of nd isotopic characteristics are complicated by a partial resetting of the nd isotope system at about the time of sedimentation but indicate that the provenance of the austin glen member had a long term history of light rare earth element lree enrichment average tdm 1a8 ga furthermore nd isotopic compositions are extremely homogeneous end 8a1 0a6 1 s d n 23 at 450 ma the approximate depositional age indicating either a single source or very well mixed sources 207pb 204pb ratios are variable but within the range of pb isotopic compositions typically described as grenvillian the range of 207pb 204pb is greater than expected for the range of 206pb 204pb and suggests an additional component of pb possibly introduced during diagenesis the immediate source of the austin glen member may have been the accretionary prism that developed as older sediments of the laurentian margin were scraped off the basin oor incorporated within the accretionary prism and shed into the basin no evidence indicating the arrival of an undifferentiated island arc or continental fragment during the taconian orogeny has been found the data acquired during this study can be explained almost exclusively by grenville province source components but with possible additional contributions from older laurentian terranes and late proterozoic rift volcanics that are not readily quanti ed but likely to have been minor accordingly we conclude that the taconian orogeny in new england involved either 1 a continental arc that involved exclusively laurentia 2 collision of a continental block with identical geochemical characteristics as laurentia or 3 essentially no detritus from any exotic colliding block island arc or continental fragment reached the foreland basin at the time of austin glen deposition

three dimensional geometry of the reelfoot blind thrust implications for moment release and earthquake magnitude in the new madrid seismic zone mapping of the reelfoot blind thrust using portable array for numerical data acquisition panda seismicity suggests that it is a complex fault that changes its geometry along strike the thrust appears to be bounded to the north by an east trending strike slip fault the southern end of the thrust is defined by seismicity and does not terminate at a known transverse fault the northern portion of the thrust steepens at shallow levels forming a listric concave upward shape in cross section the southern segment of the thrust is interpreted to flatten near the top of the precambrian basement although some segmentation of the blind thrust is observed from the mapping of 3 km wide strips of seismicity oriented perpendicular to the fault it does not appear to be significant enough to prevent rupture along its entire length the area of the blind thrust 1301 km 2 coupled with several estimates of displacement in the 7 february 1812 event is used to determine the moment released during the event the values of m 0 range from 6 8 10 26 to 1 4 10 27 dyne cm with preferred values between 6 8 10 26 and 8 7 10 26 dyne cm similar calculations for an earthquake in a d 1450 yield a moment release of 1 0 10 27 dyne cm computed moment magnitude for the 7 february 1812 event ranges from m w 7 2 to 7 4 with preferred values between m w 7 2 and 7 3 the moment magnitude for the a d 1450 event is computed as m w 7 3 these values for the magnitude of the 1812 earthquake are lower than previous estimates based on historical records of shaking however this does not imply a lowered seismic risk in the new madrid seismic zone as site effects in the mississippi embayment may significantly amplify the ground motion caused by earthquakes of a given magnitude

apatite fission track thermochronology of the southern appalachian basin maryland west virginia and virginia apatite fission track apparent ages 246 pm 37 to 95 pm 18 ma for 26 samples of upper devonian hampshire and chemung formations and middle devonian age tioga ash bed from the southern appalachian basin of maryland virginia and west virginia along with confined track length distributions for 13 of these samples suggest that uplift was contemporaneous with triassic jurassic extension along the atlantic continental margin uplift as measured by apatite fission track analysis began earliest in the northwestern section on the cumberland plateau at sim225 pm 25 ma this area probably required the least amount of erosional unroofing 3 1 km samples from the valley and ridge province of northern west virginia virginia and maryland yield progressively younger apatite fission track apparent ages to the east ranging from 163 pm 10 to 95 pm 18 ma this is consistent with deeper burial in the eastern appalachian basin as indicated by increasing cai indices and geodynamic modeling t

the use of the common pb isotope composition of detrital k feldspar grains as a provenance tool and its application to upper carboniferous paleodrainage northern england k feldspar is a common detrital mineral in sandstones it contains little u or th and hence the common pb isotope composition of unaltered k feldspar is that of the source rock pb isotope variations in igneous and metamorphic crustal rocks define broad spatial patterns that make the pb signature of detrital k feldspar grains a useful provenance tool however it is unclear how robust this signal is and to what extent it can be modified by weathering transport burial diagenesis and or exposure to hydrothermal fluids this study reports an evaluation of the technique using two granite arkose pairs from helmsdale northern scotland and shap northwest england where the composition of the detrital grains can be compared to feldspars in the granite from which they were derived the results of this test have informed a pilot study using the pb isotope composition of detrital k feldspar in upper carboniferous fluvial sedimentary rocks of northern england to constrain the sediment routing pb isotope compositions have been measured using an in situ high resolution single grain technique that can distinguish original composition heterogeneity and subsequent alteration within individual sand grains the results for the granite arkose pairs show that the common pb isotope signature of the detrital k feldspar grains matches the in situ feldspars in the source granites the signal is independent of grain size medium grained sand to granules but alteration along a paleo regolith at helmsdale has resulted in perturbations in pb composition and an increase in the errors on the analyses the application of the technique to arkosic carboniferous serpukhovian bashkirian millstone grit group sandstones in the pennine basin reveals two discrete populations of k feldspars that cannot be distinguished petrographically matching of the compositions to potential sourcelands to the north and northeast of the basin suggest a far traveled 500 km source of k feldspar from the archean paleoproterozoic lewisian rocks of northwest scotland or their extension to the north but also an equally important proximal source emanating from the southern uplands belt of scotland in what would have been the lower part of the hinterland drainage basin of the millstone grit

tracking the sources of icebergs with lead isotopes the provenance of ice rafted debris in heinrich layer 2 the provenance of ice rafted debris ird deposited in the north atlantic before during and after heinrich event 2 has been determined through measuring the lead isotopic composition of single feldspar grains and multiple grain composites from the larger than 150 m size fraction from cores from the eastern and western north atlantic and from the labrador sea single grain analyses are used to identify the specific continental sources of the ird whereas composite samples are used to assess the relative ird contributions from different sources all single grains from heinrich layer 2 h 2 as well as h 2 composites plot along a correlation line on a 207pb 204pb versus 206pb 204pb diagram characteristic of the churchill province of the canadian shield this is yet another strong piece of evidence that this heinrich event was dominated by a massive iceberg discharge of the laurentide ice sheet lobe located over hudson bay in contrast single grains from the ambient glacial sediment above and below h 2 have multiple sources many of them also lie along the correlation line with h 2 grains but many others have pb signatures consistent with derivation from the grenville province and the appalachian range in north america and possibly from scandinavia and greenland composites from the ambient sediment generally lie well to the right of the h 2 reference line in agreement with the results of the single grain analyses the evidence provided by lead isotopes regarding the dominant role played by the hudson bay lobe of the laurentide ice sheet in the development of the heinrich events lends support to the binge purge model advanced by macayeal 1993a b that invokes trapping of geothermal heat by the base of the icecap and subsequent basal melting as the mechanism that triggered the heinrich events

pb isotopes in surficial pelagic sediments from the north atlantic we measured pb isotopic composition and concentration in sediment samples close to the sea water interface in 6 box cores from the ne atlantic 2 box cores from the sargasso sea and one from the us continental shelf the anthropogenic pb input to marine sediments due to the increase of pb contamination over the ocean during the last century can be identified in all these cores in the eastern part of the atlantic i e in regions under aeolian influence from europe pb pollution can be recognized using its distinctive unradiogenic composition clearly different from the upper crustal values commonly found in pre holocene sediments in contrast pb pollution in regions influenced by north american sources can be identified only in detailed concentration profiles because the american pb pollution has an isotopic composition much closer to that of the natural detrital pb input coming from weathering of the continental crust pb excess inventories are in good agreement with fluxes estimated from sedimenttrap data and with the time record of pb contamination increase given by analyses in coral growth bands inventories of pb contamination to the sediments of the mud patch american shelf are tenfold higher 84 g cm2 than those to hatteras and bermuda abyssal plains 4 3 and 2 8 g cm2

40ar 39ar illite dating of late caledonian acadian metamorphism and cooling of k bentonites and slates from the welsh basin u k the vacuum encapsulation laser 40ar39ar technique allows extremely small 10 6 g samples of fine grained materials such as diagenetic clays to be dated here we show that the method can be extended to higher grade clay minerals the integration of transmission electron microscopic tem characterization with 40ar39ar dating of vacuum encapsulated samples permits the resolution of the timing of metamorphic growth cooling from the time of diagenesis we have applied this technique to well characterized lower paleozoic slates and k bentonites from the welsh basin which span the transition from anchizonal to epizonal grade which had been previously studied using rbsr and smnd dating tem observations of epizonal k bentonites and slate showed that illite in these samples is of 2m1 polytype of muscovite like composition and oriented parallel to cleavage suggesting that they are of metamorphic origin total gas ages equivalent to conventional kar ages for encapsulated epizonal k bentonites and slate 340 408 ma are considerably variable the ar retention ages calculated from 39ar and 40ar atoms retained in the sample after irradiation are more consistent 383 411 ma the 39ar recoil losses are minor for illites from whole rock samples of epizonal k bentonites but very significant for clay separates of epizonal slate plateaus in age spectra were observed in epizonal k bentonites and slate the plateau ages 414 421 ma and retention ages 383 411 ma can be correlated with the onset of acadian metamorphism and culmination of uplift and inversion of the welsh basin respectively these ages are significantly younger than the 450 ma ages previously reported for diagenetic clays using the same method suggesting that diagenetic history has been lost in these epizonal k bentonites and slate tem observations of anchizonal slates showed that there are two modes of illite the first mode is similar to that observed in epizonal samples suggesting a metamorphic origin the second mode consists of the 1md polytype has typical diagenetic illite composition and is oriented parallel to bedding suggesting a diagenetic origin total gas ages for encapsulated anchizonal slates vary considerably 361 422 ma the retention ages are more consistent 413 432 ma than the total gas ages the 39ar recoil losses are more significant than those for epizonal k bentonites and slate plateaus in age spectra are generally not observed however the consistent retention ages for the anchizonal slates correspond to the plateau ages for the epizonal samples and are inferred to represent the onset of acadian metamorphism these data when combined with our previously published results for diagenetic shales suggest that thermal conditions near the boundary of anchizonal and epizonal grades are necessary to completely reset ar systems in shales and slates

chronology of paleozoic metamorphism and deformation in the blue ridge thrust complex north carolina and tennessee the blue ridge province in northwestern north carolina and northeastern tennessee records a multiphase collisional and accretionary history from the mesoproterozoic through the paleozoic to constrain the tectonothermal evolution in this region radiometric ages have been determined for 23 regionally metamorphosed amphibolites granitic gneisses and pelitic schists and from mylonites along shear zones that bound thrust sheets and within an internal shear zone the garnet ages from the pumpkin patch a thrust sheet 458 455 and 451 ma are similar to those from the structurally overlying spruce pine thrust sheet 460 456 455 and 450 ma both thrust sheets exhibit similar upper amphibolite facies conditions because of the high closure temperature for garnet the garnet ages are interpreted to date growth at or near the peak of taconic metamorphism devonian metamorphic ages are recognized in the spruce pine thrust sheet where sm nd and rb sr garnet ages of 386 and 393 ma and mineral isochron ages of 397 14 and 375 27 ma are preserved hornblendes record similar sup 40 ar sup 39 ar sm nd and rb sr ages of 398 to 379 ma devonian sup 40 ar sup 39 ar hornblende ages are also recorded in the structurally lower pumpkin patch thrust sheet the devonian mineralmore ages are interpreted to date a discrete tectonothermal event as opposed to uplift and slow cooling from an ordovician metamorphic event the mississippian mylonitization is interpreted to represent thrusting and initial assembly of crystalline sheets associated with the alleghanian orogeny the composite thrust stack of the blue ridge complex was subsequently thrust northwestward along the linville falls fault during middle alleghanian orogeny about 300 ma less

post tectonic magnetizations from the cordova gabbro ontario and palaeozoic reactivation in the grenville province summary the cordova gabbro of southern ontario intrudes 1300 myr old volcanic rocks of the hastings lowlands in the grenville structural province three distinct vector magnetizations a b and c have been isolated using a combination of stable endpoints subtracted vectors from orthogonal vector plots and converging remagnetization circles the a magnetization with mean direction d 294 i 55 5 k 42 ag5 5 5 n 18 sites is a high coercivity high blocking temperature remanence recorded by 49 samples the b magnetization was isolated in 33 samples and has a mean direction d 305 5 1 1 5 k 24 ag5 9 5 n 11 sites b has lower coercivities and blocking temperatures than a where the two are superimposed the a and b palaeopoles 151 e 10 5 s dp 6 dm 8 and 165 5 e 24 n dp so dm 9 5 fall on the grenville track around 900 and 820 ma respectively the a and b magnetizations thus date from uplift and cooling following the grenvillian orogeny the third magnetization the c component has been isolated in 23 samples its mean direction is d 180 i 27 5 k 18 ag5 10 5 n 12 sites the c is a low coercivity low blocking temperature overprint of a and b its palaeopole 102 e 31 n dp 6 5 dm 12 is unlike post 1300 precambrian poles for cratonic north america but matches silurian and late ordovician poles 40ar 39ar plateau ages of 446 and 447 ma determined by lopez martinez and york for plagioclases from one of the cordova samples confirm this age assignment the c magnetization therefore records a previously unrecognized mild thermal or hydrothermal event that occurred in palaeozoic time long after the grenvillian orogeny

lead isotope ratios in niagara escarpment rocks and galenas implications for primary and secondary sulphide deposition galenas occumng in vugs and veins in the dolomites of the eramosa goat island and gasport members of the middle silurian lockport formation in the niagara escarpment have a small but significant range in lead isotopic composition these differences are recorded in specimens from exposures in the niagara gorge in the east to the guelph quarry 100 km to the northwest the galena isotopic data fit linear trends on 207pb 204pb 206pb 204pb and 208pb 204pb 206pb 204pb plots within experimental error the most radiogenic ratios are anomalous giving model ages based on the growth curves of zartman and doe up to 200 ma younger than the most likely age of primary deposition of 300 ma sulphides currently being deposited from springs at the floor of the vinemount quarry have a lead isotopic composition similar to that of some of the galena in the overlying dolomites rocks from several of the formations exposed at this site contain lead only slightly more radiogenic than the most radiogenic galenas these results are consistent with the hypothesis of haynes and mostaghel that the modem sulphides result from local solution and redeposition of older sulphides previously published isotopic ratios for lead in the potassium feldspar constituents of 300 ma plutonic intrusives in the appalachians cluster around the least radiogenic galena ratios and suggest that the lead in the niagara peninsula galenas has been remobilized from rock units in the appalachian basin grenville crystalline basement rocks do not appear to have contributed significantly to this lead a single sample of galena from the eramosa member exposed in the ebel quarry in the michigan basin at the northwestern extension of the escarpment has 207pb 204pb and 206pb 204pb ratios that are substantially more radiogenic than those in any of the galenas from the niagara region of the appalachian basin and a 08pblz04pb ratio that lies significantly below the linear distribution defined by the niagara region data set we tentatively interpret this difference as reflecting derivation of the lead in the two basins from different crustal blocks

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

40ar 39ar thermochronology and alleghanian development of the southernmost appalachian piedmont alabama and southwest georgia 40ar 39ar age spectra of hornblende muscovite and microcline and total fusion ages of biotite from metamorphic rocks of the inner piedmont pine mountain and uchee belts are reported mineral cooling ages from the eastern part of the inner piedmont are as follows hornblende 320 ma muscovite 296 ma biotite 293 ma and microcline diffusional release patterns tmax 267 ma tmin 234 ma a 347 ma hornblende spectrum from the highest inner piedmont structural level sampled is the oldest date determined and implies earlier passage of this level through the 500 c isotherm most release spectra from pine mountain belt units are discordant with little or no apparent geologic meaning modified saddle shaped release patterns for hornblende indicate extraneous argon with a maximum age of 358 ma muscovite from the pine mountain belt cover sequence is 286 ma plateau age and one from the underlying grenville basement is 277 ma correlation age indicating cooling below the 350 c isotherm plateau ages on uchee belt rocks are as follows hornblende from 297 to 288 ma muscovite 285 ma biotite 276 ma and microcline tmax 261 ma tmin 230 ma muscovite fish from a bartletts ferry fault zone phyllonite have a plateau age of 283 ma the 40ar 39ar results combined with other geologic data indicate that 1 a large part of the southern and inner piedmonts of alabama and southwest georgia experienced a late paleozoic amphibolite facies thermal and deformational event contemporaneous with the alleghanian orogeny observed in the foreland 2 the tectonic development of this event characterized by initial crustal thickening followed by right slip and normal slip movements is grossly similar to that described for the amphibolite facies alleghanian belt in the eastern piedmont of south carolina and georgia and 3 extensional movements along the flanks of the pine mountain window occurred between ca 277 ma and the late triassic early jurassic and thus may reflect latest alleghanian extensional collapse or mesozoic rifting

sr o and pb isotope evidence for origin and evolution of etive igneous complex scotland a multi isotope approach has been applied to the origin of one of the largest and latest newer granite complexes of the scottish caledonides contributions from both mantle and unexposed late proterozoic lower continental crust are recognized indicating that granite magmatism is a mechanism for both primary crustal growth and reworking of older crustal rocks

origin of illite in the lower paleozoic of the illinois basin evidence for brine migrations in the lower paleozoic of the illinois basin three illite polytypes are found 2m 1 of detrital origin and 1m d and 1m of diagenetic origin illite polytype quantification of detrital 2m 1 illite and diagenetic 1m d and 1m illite combined with k ar age dating allows extrapolation to apparent detrital and diagenetic illite ages kinetic modeling of smectite illitization combined with the calculated age of illitization can evaluate different origins of illite the diagenetic illite in the lower paleozoic of the illinois basin is interpreted not to have formed solely by burial diagenesis but mainly during multiple brine events the upper ordovician maquoketa group contains diagenetic illite dominantly 1m d with minor 1m with an extrapolated age of 360 m y 356 377 m y and formed from smectite at temperatures of 50 100 c this age falls within the span of dates for illite smectite i s in k bentonites from the upper mississippi valley and is interpreted to be a combined result of illitization by burial diagenesis and either a hydrothermal brine from the southern and deeper part of the basin or a k rich brine from the michigan basin upper mississippi valley area or forest city basin in ordovician and cambrian shale partings and sandstone older than the maquoketa group the diagenetic illite 1m d in shale and 1m in sandstone has an age of 300 m y and formed at temperatures 140 c this late paleozoic age falls within the range of illites from sandstone in the upper mississippi valley and k bentonites of the appalachian basin it coincides with the alleghany orogeny and is interpreted as having formed by gravity driven flow from the uplifted alleghanian ouachita orogenic belt that drove hot 140 c fluids through the illinois basin

evidence for recurrent faulting in the new madrid seismic zone from mini sosie high resolution reflection data a mini sosie high resolution seismic reflection survey was conducted on reelfoot scarp in the northwestern tennessee portion of the new madrid seismic zone interpretation of the mini sosie data revealed the need to reinterpret previously collected reflection data obtained from explosive source and vibroseis surveys interpretation and integration of the three data sets have resulted in a new model for the subsurface of reelfoot scarp and provide evidence for recurrent movement along reelfoot fault the major reverse fault associated with reelfoot scarp estimated displacements on reelfoot fault vary from 60 m 60 ms for late paleozoic rocks to 15 m 20 ms for late eocene sedimentary units no clear offsets are observed on this particular fault for units younger than late eocene age however uplift folding and related structures are observed in younger sediments an observed variation of offset with depth age and the presence of the younger structures are evidence of reactivation of reelfoot faul

an 40ar 39ar geochronological study of the elzevir batholith and its bearing on the tectonothermal history of the southwestern grenville province canada results of 40ar 39ar step heating for muscovite potassium feldspar and plagioclase from the ca 1240 ma elzevir trondhjemite in the central metasedimentary belt of the canadian grenville province have been combined with earlier data and used to deduce a thermal history for the eastern elzevir terrane following the ottawan orogeny 1050 950 ma muscovite yielded precise plateau dates of ca 900 ma whereas the potassium feldspar displayed disturbed age spectra with ca 730 760 ma dates for broad plateau regions this age difference could be explained by slow cooling or by thermal overprinting at 760 800 ma that has updated the microcline the plagioclase spectra provide evidence for a low temperature hydrothermal event ca 380 ma ago that has generated sericite in saussurite and has caused partial argon loss from the microcline this result agrees with the other plagioclase thermochronometry and paleomagnetism on the cordova gabbro 25 km to the west but provides a lower estimate for the upper age brac

mechanisms of argon retention in clays revealed by laser 40ar 39ar dating a method for dating clays is important for studies of weathering diagenesis hydrocarbon migration and the formation of major metalliferous deposits however many attempts have produced imprecise or inaccurate results data from shales show that contrary to expectations the 40ar 39ar dating technique can be successfully used to determine the diagenetic age of ancient sediments because 39ar losses during irradiation are controlled by release from low retentivity sites in illite equivalent to those that have lost radiogenic 40ar in nature rather than by direct recoil as is generally assumed

hydrothermal fluids during mesozoic reactivation of the st lawrence rift system canada c o sr and pb isotopic characterization calcite veins with accessory galena sphalerite barite and fluorite are abundant within the ottawa bonnechere and the charlevoix saguenay failed arms of the st lawrence rift system which developed during early paleozoic times within gneisses of the grenville province of canada the mineral assemblages paragenesis and precipitation temperatures of the veins resemble the mississippi valley deposits crosscutting relationships with carbonate rocks of the st lawrence lowlands and deformed carbonate rock slivers of the charlevoix astrobleme show that the ottawa graben and saguenay graben veins are younger than mid ordovician or post devonian respectively 18o and 13c values for vein calcite are positively correlated and range from 11 to 24 and 9 to 3 respectively 87sr86sr in calcite and barite range from 0 7129 to 0 7081 the fluids that precipitated calcite were part of a hydrothermal system dominated by meteoric water enriched in 18o by exchange with silicate minerals and deep seated carbon either from mantle outgassing deep seated brines of the canadian shield or from oxidation of reduced carbon in high grade metamorphic rocks veins cutting marbles of the grenville supergroup have lower 87sr86sr and higher 18o and 13c values than those hosted in quartzofeldspathic gneisses of the grenville province showing that the fluids were modified by interaction at depth with the marbles stable and sr isotope compositions are also correlated with layb ratios in calcite supporting the fact that the veins formed within similar hydrothermal systems in both the ottawa and saguenay grabens the pb isotopic compositions of vein galena are radiogenic 206pb204pb 18 0 19 7 207pb204pb 15 5 15 7 208pb204pb 37 9 41 0 and positively correlated in conventional pb pb diagrams pb isotope and ree data strongly suggest that the fluids leached selectively at the mineral scale gneisses of the grenville province the basinal fluids within the shelf sequence of the western appalachians had 208pb204pb ratios in the lower paleozoic significantly lower than those of vein galena this excludes the basin brine model of formation for the vein deposits like suggested for the mississippi valley deposits the veins rather formed in the mesozoic in response to the rifting of northeastern north america and the reactivation of the st lawrence rift

development of interlaced mylonites cataclasites and breccias example from the towaliga fault south central appalachians abstract a variety of cataclastic rocks from crush breccias to cataclasites and silicified breccias are associated with retrograde mylonites along the towaliga fault zone of south central appalachians in georgia a zone of alternating breccias and quartz ultra mylonites bordered by quartz mylonites that are roughly laminated with mm scale mica rich bands of cataclasites occur subparallel to the mylonitic foliation elsewhere along the fault zone evidence for plastically deformed microfractures is found in discussing the mechanical evolution of these rocks two possibilities are considered a the cataclasis is an overprint representing deformation in an entirely brittle regime and b cataclasis and mylonitization occurred at t 300 c in a predominantly plastic regime within the framework of the latter model it is suggested that the cataclasite bands are either due to the relaxation of dilational stresses following downward propagation of seismic ruptures or represent strain induced seismic instabilities during plastic shearing

relationships between early paleozoic facies patterns and structural trends in the saltville thrust family tennessee valley and ridge southern appalachians structural style variations across the tennessee segment of the valley and ridge thrust belt are correlated with two facies changes in important structural stratigraphic units the middle and upper cambrian conasauga group and the middle ordovician chickamauga group change their internal stratigraphy regionally at oblique angles to the trend of this segment of the thrust belt these stratigraphic variations correlate geographically with structural style changes within individual thrust sheets whereas other variables such as temperature pressure and magnitude of tectonic transport do not in the conasauga the east to west limestone to shale facies transition coincides with areas where structural styles change along thrusts at the conasauga level the shale dominated and carbonate dominated facies show simple frontal imbricate thrusts whereas in the shale carbonate zone of interlayering duplexes occur in the chickamauga an east west change from basinal shale to shelfal limestone controlled the regional presence of a middle level glide zone in the paths of eastern thrusts and its absence to the west this stratigraphic inhomogeneity also controlled a change in the knoxville area from surficial folding in the northeast to surficial thrusting in the southwest the terminology of structural lithic units and their subsidiary predominant and conforming members is applied in dividing the stratigraphic section into structurally significant packages

timing and deformation conditions of the tallulah falls dome ne georgia implications for the alleghanian orogeny structural domes composed of crystalline rocks metamorphosed to midcrustal conditions are present in every major orogen and record deformation of the deep interior of mountain ranges the tallulah falls dome in northeast georgia is a shear zone bounded body of metasedimentary rocks that is defined by a foliation pattern that crosscuts the regional fabric of the eastern blue ridge crystalline nappe in the southern appalachian mountains previously the tallulah falls dome was interpreted as the expression of duplexed underlying slices of laurentian margin sedimentary rocks imbricated during alleghanian emplacement of the allochthonous blue ridge thrust sheet under greenschist facies conditions we provide new observations of quartz and feldspar recrystallization fabrics and new 40 ar 39 ar dates from muscovite that complement existing thermochronometric ages to determine the timing and conditions of doming within the tallulah falls dome our microstructural observations are consistent with deformation mechanisms active dominantly at amphibolite facies conditions with some samples recording temperatures in excess of 650 c thermochronometric ages define a period of relatively rapid cooling from 550 c to 340 c within and around the tallulah falls dome between 321 and 317 ma with differential cooling across the northwestern dome bounding shear zone lasting until at least 313 ma our observations reveal a period of early alleghanian cooling from amphibolite to greenschist facies conditions associated with displacement along a shear zone bounding the tallulah falls dome across the blue ridge the alleghanian orogeny has long been characterized only by thrusting at greenschist facies conditions similarly dome formation has been interpreted as the result of duplexing within the thin skinned blue ridge crystalline thrust sheet as it was emplaced upon the laurentian margin both of these interpretations are inconsistent with the new data presented here which suggest more complex alleghanian deformation over a broader range in temperatures and perhaps a diverse suite of structural styles our results are best explained by doming of the footwall of a normal displacement shear zone at amphibolite facies conditions during the earliest alleghanian period

overlapping cretaceous and eocene alteration twin creeks carlin type deposit nevada we report here new ar ar dates for adularia illite and muscovite from the twin creeks carlin type deposit all illite samples were obtained from altered intrusive and volcanic rocks that are interlayered with mineralized sedimentary rocks thus largely avoiding complications caused by detrital illite and muscovite in sedimentary rocks interpretation of the illite age spectra was based in part on newly developed models for the behavior of argon in fine grained illite the results indicate that all adularia in the megapit area formed at about 42 ma eocene whereas most illite in the megapit area and muscovite in alteration zones beneath chimney creek formed at about 109 to 103 ma cretaceous additional events involving formation of illite at 200 ma and muscovite at about 310 ma were also observed but were not widespread enough to be considered significant our eocene date for adularia is in complete agreement with previous measurements on adularia from twin creeks but our cretaceous dates are considerably older than dates determined in previous studies of the nearby getchell district particularly the osgood stock which has a date of 98 to 92 ma our results suggest that the osgood stock is part of a more protracted cretaceous igneous hydrothermal event that took place between about 109 and 83 ma the relationship of adularia and illite to gold mineralization is not completely clear making it impossible to use our data to assign a unique age to the twin creeks deposit we favor an eocene age for some gold mineralization because the adularia that we dated is intimately intergrown with auriferous arsenian pyrite typical of carlin type mineralization however adularia has a very restricted distribution in the deposit and temperatures at which it formed were probably not great enough to reset cretaceous and older ages of illite illite is much more widespread and its association with gold is supported by correlation between gold content and k al ratios of mineralized material thus cretaceous illitization was at least an important ground preparing event at twin creeks and it is not possible to show from our data that gold ore or protore were not deposited during this event

the protocontinental nature and regional variability of the central metasedimentary belt of the grenville province lead isotope evidence the lead isotopic compositions of sulphide samples mainly galenas from a representative set of mineral deposits within the central metasedimentary belt of the grenville province have been determi

dating of alteration episodes related to mercury mineralization in the almaden district spain the almadin mercury deposits represent one of the largest geochemical anomalies on earth but the mode and timing of their formation remains a matter of controversy there are two main possibilities hydrothermal solutions associated with alkali basalt volcanism in the silurian and devonian or regional hydrothermal activity during later hercynian metamorphism although these models can be distinguished by determining the age of ore deposition no suitable isotopic method had been applied to the problem prior to this study we report here on vacuum encapsulated and more traditional laser ar ar dating performed on illite concentrates and cr rich micas that are intimately associated with mercury mineraiization in both devonian and silurian host rocks illite is associated with a later stage of hg mineralization cinnabar kaolinite pyrophyllite chlorite which locally replaces an earlier episode associated with carbonates and cr mica the dating results from illite separates suggest that the later stages of hg mineralization or remobilization occurred about 360 ma which is about 20 ma younger than any mercury hosting rocks in the district and coincides with the onset of regional deformation during the hercynian orogeny cr mica ages are variable and range from 365 ma to 427 ma this age range spans the period between the deposition of the criadero quartzite the oldest sedimentary unit hosting mercury and the later episode of mineralization this suggests that mineralization started at least by the mid to lower silurian and the younger argon ages for cr mica could represent partial to nearly total argon loss caused by the hercynian metamorphic event it appears that both postulated styles of mercury mineralization occurred separated in time by up to 80 million years

comparative 40ar 39ar and k ar dating of illite type clay minerals a tentative explanation for age identities and differences abstract the 40 k 40 ar k ar and 40 ar 39 ar dating methods are applied here to the same very small micrometric illite type particles that crystallized under low temperature 5 9 ma the same size fractions dated by the 40 ar 39 ar method give total gas ages ranging from 173 3 2 0 to 228 8 1 6 ma nearly all 40 ar 39 ar total gas ages are slightly older which cannot be explained by the recoil effect only the impact of which being amplified by the inhomogeneous shape of the clay minerals and their crystallographic characteristics with varied crystallinity indices and a particle width about 10 times large than thickness the 40 ar 39 ar data outline some advantages such as the plateaus obtained by incremental step heating of the various size fractions even if not translatable straight as ages of the illite populations they allow identification of two generations of authigenic illite that formed at about 200 and 175 ma and one detrital generation however 40 ar 39 ar dating of clay minerals remains challenging as technical factors such as the non standardized encapsulation may have potential unexpected effects both dating methods have their limitations 1 k ar dating requires relatively large samples ca 10 20 mg incurring potential sample homogeneity problems with two aliquots required for k and ar analysis for an age determination also inducing a higher analytical uncertainty 2 an identified drawback of 40 ar 39 ar dating is ar recoil and therefore potential loss that occurs during neutronic creation of 39 ar from 39 k mostly in the finer mineral particles if all the recoiled 39 ar is redistributed into adjacent grains minerals the final 40 ar 39 ar age of the analyzed size fraction remains theoretically identical but it is not systematic in clay type material the finest grain sizes e g 39 ar recoil and are therefore only datable by the 40 ar 39 ar method using an encapsulation technique that still needs to be technically evaluated in this study 39 ar recoil during irradiation was quantified by encapsulation it ranged from about 20 of the total 39 ar for the 39 ar recoil confirms that grain size increasing minimizes the recoil effect but how it proceeds has still to be explained despite recent developments in 40 ar 39 ar dating the conventional k ar method is still a valuable tool for clay dating due to a convenient and straightforward use supported by a standardized and well controlled technical approach the present comparison of the two ar dating methods as applied to clay material shows that neither method is presently outdated and that they are even of reciprocal use both methods have distinct application fields in clay geochronology and complementary application fields in clay crystallography

40ar39ar dating of very fine grained samples an encapsulated vial procedure to overcome the problem of 39ar recoil loss 40ar39ar dating of very fine grained materials is compromised by the loss of 39ar by recoil and also sometimes other ar isotopes during irradiation an encapsulated sample 40ar39ar procedure is described which overcomes this problem and which produces a k ar equivalent date measurements indicate that the procedure yields reliable and reproducible results it offers the potential for dating small amounts of material of many types for example very fine grained authigenic sheet silicates from rock cores which may suffer ar loss during irradiation

character of the alleghanian orogeny in the southern appalachians part iii regional tectonic relations geological and geochronological studies in the eastern piedmont of the southern appalachians have documented a polyphase late paleozoic deformational chronology d 2 d 4 that is contemporaneous with the alleghanian orogeny recorded in the western appalachian foreland the scale and vergence of d 3 anticlinoria in the eastern piedmont are similar to those of anticlinoria in the western appalachians which are related to ramping of underlying thrust surfaces we suggest that the appalachian decollement extends southeastward beneath the entire piedmont province and that all of the piedmont rocks were displaced at least 175 km northwestward relative to north america during the alleghanian orogeny late paleozoic deformational effects in the eastern piedmont thus are considered cogenetic with alleghanian foreland deformation we interpret the alleghanian orogeny to be the result of oblique dextral convergence and collision of gondwana and laurentia

age and radiogenic isotopic composition of a late to post tectonic anorthosite in the grenville province the labrieville massif quebec abstract the labrieville anorthosite massif lbv is found in the central granulite terrain of the grenville structural province but it displays no evidence of post emplacement deformation or metamorphism implying intrusion following peak grenvillian metamorphic conditions we report u pb zircon dates of 1008 3 4 ma for border leucogabbro and 1010 5 6 ma for a cogenetic jotunite dike intruding anorthosite we interpret these dates as igneous crystallization ages and regard 1010 ma as a reasonable estimate of the emplacement age for lbv lbv is thus the youngest massif anorthosite yet recognized in north america and its age is consistent with late tectonic emplacement relative to the 1 1 1 0 ga grenville orogeny we also report a u pb date of 1015 1 8 ma for metamorphic zircon in a country rock amphibolite this could reflect the age of grenvillian regional metamorphism or perhaps a later heating episode resulting from the intrusion of numerous late felsic plutons in this area rb sr sm nd and u th pb isotopic compositions for four rock types anorthosite jotunite leucogabbro and a plagioclase megacryst span narrow ranges in each case consistent with comagmatism among these units isr t 1010 ma range from 0 7032 0 7034 and are among the lowest yet reported for anorthosite in the grenville province initial nd values are positive 0 8 to 2 5 like other grenville anorthosites pb isotopic compositions lie near the model mantle evolution curve of zartman and doe 1981 implying no involvement of significantly older crust in the petrogenesis of these rocks collectively these data suggest a source for lbv in the mantle or mafic lower crust lbv is a compositionally extreme anorthosite characterized by alkalic plagioclase an32or12 and high levels of sr 2000 ppm and ba 1000 ppm these properties cannot be attributed to simple crustal contamination of mantle derived basalt we suggest alternatively that lbv s compositional features may be linked with its late tectonic character perhaps reflecting partial melting of mafic lower crust brought about by crustal thickening during the grenville orogeny

the chemical character of fluids forming diagenetic illite in the southern appalachian basin diagenetic illite mixed layer illite smectite i s from ordovician and devonian potassium bentonite k bentonite and shales was studied in the proximal southern appalachian basin alabama through virginia to understand in more detail the timing of illite formation as well as attempt to describe the chemistry of the fluids that formed illite during the alleghanian orogeny through combined k rb and oxygen isotope analyses the clay fraction of the k bentonites is composed predominantly of illite smectite i s and chlorite most i s exhibited kalkberg order iisi with 85 illite layers the k ar dates of i s from ordovician k bentonites and devonian k bentonites mostly range from 260 310 ma and indicate i s formed during the alleghanian orogeny internally concordant dates and similar percentages of illite layers of i s are measured from several samples collected within a thick 1 m ordovician k bentonite at fort payne alabama the oxygen isotopic values 1 8 o of i s range from 18 23 smow at lowest levels of thermal maturity and burial as seen by conodont alteration index values cai of 1 5 2 0 the range in 1 8 o values of water in equilibrium with i s is 2 5 3 5 at higher thermal maturity cai 2 5 4 5 the 1 8 o of water in equilibrium with i s is from 4 5 to 12 the 1 8 o analyses are mostly consistent with i s having formed in the presence of orogenic i e saline waters the mean k rb of the appalachian illites from this study is 325 n 20 this mean k rb is also consistent with the formation of diagenetic i s in the presence of a saline fluids having higher k activities than meteoric waters the combined data indicate these diagenetic illites in k bentonites formed from saline orogenic waters during the alleghanian orogeny along the valley and ridge province in the southern appalachian basin although the data rule out formation of diagenetic illite in the presence of meteoric water or long term reaction with connate formation waters the combined data cannot distinguish the source and the mechanism tectonic push or flush from tectonic highlands for the movement of saline fluids in the appalachian basin

deep crustal break back stacking and slow exhumation of the continental footwall beneath a thrusted marginal basin grenville orogen canada abstract granulite to upper amphibolite facies ductile thrusting in the central gneiss belt grenville orogen ontario represents the tectonic shortening of a continental scale footwall beneath the thrust emplaced central metasedimentary belt during the closure of a postulated back arc basin ca 1 19 1 18 ga break back stacking in the footwall occurred at mid to deep crustal depths ca 35 km within tectonically thickened ca 70 km continental crust and culminated with renewed thrusting at the base of the overlying central metasedimentary belt ca 1 08 1 05 ga the individual mylonite belts which constitute the ductile thrust zones and the scale of penetrative deformation of the intervening crystalline thrust sheets are comparable with the largest known examples of high grade thrust belts elsewhere they reflect the large scale thermal and theological boundary conditions of the deformation flow within individual thrust zones may reflect local boundary conditions such as the rheological behaviour of older thrust sheets and the geometry of interfaces within the thrust stack restoration of the thickness of erosionally removed crustal overburden by break back thrusting may retard the rates of exhumation and cooling of a mid to deep crustal thrust stack

lead isotope analyses of leached feldspars constraints on the early crustal history of the grenville orogen abstract lead isotopic data on leached k feldspars plagioclases and muscovite provide constraints on the premetamorphic evolution of the grenville orogen in ontario and new york a boundary between reworked archean crust to the northwest and new mantle derived crust generated during mid proterozoic times to the southeast is identified within the northern portion of the gneiss belt of ontario the identification of juvenile mid proterozoic crust at the southeastern margin of the orogen is consistent with previous investigations based on neodymium model ages however near the grenville front lead isotopes indicate the presence of reworked archean crust rather than primary early proterozoic crust postulated on the basis of neodymium model ages this discrepancy between the lead and neodymium isotopic terranes underscores the necessity of using a multidisciplinary approach to identify possible terrane boundaries only through a combination of the lead with neodymium isotopes is it possible to distinguish between crust that formed during a single major orogenic cycle and crust that represents a mixture of old with new mantle derived material the combined neodymium and lead data imply that the grenville orogen in ontario consists essentially of reworked archean crust and juvenile mantle derived material generated around 1 5 ga the grenville orogen involved archean crust that was depleted in thorium and uranium several hundred ma before high grade metamorphism at ca 1 1 ga such a depletion may have a significant impact on the geotherm achieved during orogeny and may explain the lower metamorphic thermal gradients observed for the gneiss belt

dating of shallow faults new constraints from the alptransit tunnel site switzerland methods for dating shallow faults in the earth9s crust are still evolving and their reliability remains controversial based on results from well preserved fault gouge samples from the alptransit deep tunnel site in switzerland we propose a simplified and readily applicable approach to investigate and date clay rich gouge zones we collected 10 samples covering a north south section of 15 km all sampled faults are developed in orthogneiss host rock have a rather consistent approximately northwest southeast strike are mainly normal and are related to northeast southwest orogen parallel extension during alpine exhumation fine grained clay separates from the fault gouges consist of illite 1m 1m d and 2m 1 polytypes smectite and chlorite with minor amounts of feldspar in some sample fractions the k ar ages for

age source and stratigraphic implications of pb isotope data for conformable sediment hosted base metal deposits in the proterozoic aravalli delhi orogenic belt northwestern india abstract stratiform sediment hosted base metal deposits in the aravalli delhi orogenic belt of northwestern india are hosted by the sediment dominated bhilwara aravalli and delhi sequences of proterozoic age pb isotope data for 36 samples from five ore districts provide useful stratigraphic and approximate age information model pb ages using a newly developed proterozoic model are 1800 ma for the rampura agucha rajpura dariba and saladipura deposits 1700 ma for the zawar deposits and 1100 ma for the ambaji and deri deposits mineralization in this orogenic belt thus occurred at three stages in middle to upper proterozoic times the oldest of these reflected by deposits in the bhilwara sequence was of broad regional extent the rampura agucha deposit is thus hosted by highly metamorphosed rocks that belong to the bhilwara belt and not to an older supracrustal component of the archean banded gneissic complex the host rocks to the saladipura deposit are bhilwara equivalent and are not part of the delhi supergroup as previously mapped and therefore stratigraphic assignments in the area must be re examined model ages of 1100 ma for the ambaji deri deposits are the only evidence obtained in this study regarding the age of the delhi supergroup and are in apparent conflict with much older ages indicated by previous studies rocks of such late upper proterozoic age may be restricted to the southwestern segment of the orogenic belt or alternatively thrust stacking and or interfolding of lithologically similar sequences of a wide range of ages throughout the belt may not yet be evident because of the limited age and isotopic data presently available all the data can be readily explained by the extraction of pb from sediments derived from an upper crustal basement the isotopic compositions of the zawar deposits provide the clearest evidence for an ancient u enriched upper crustal source the ambaji deri pb data suggest a mature status of the delhi are when these deposits were formed

fault dating in the canadian rocky mountains evidence for late cretaceous and early eocene orogenic pulses fault rocks from the classic rocky mountain foreland fold and thrust belt in southwestern canada were dated by ar analysis of clay grain size fractions using x ray diffraction quantification of the detrital and authigenic component of each fraction these determinations give ages for individual faults in the area illite age analysis the resulting ages cluster around 72 and 52 ma here called the rundle and mcconnell pulses respectively challenging the traditional view of gradual forward progression of faulting and thrust belt history of the area the recognition of spatially and temporally restricted deformation episodes offers field support for theoretical models of critically stressed wedges which result in geologically reasonable strain rates for the area in addition to regional considerations this study highlights the potential of direct dating of shallow fault rocks for our understanding of upper crustal kinematics and regional tectonic analysis of ancient orogens

character of the alleghanian orogeny in the southern appalachians part ii geochronological constraints on the tectonothermal evolution of the eastern piedmont in south carolina a nearly concordant u pb zircon age of 550 4 ma is interpreted to closely date crystallization of the epizonal little mountain metatonalite in the southeastern part of the charlotte belt in south carolina this confirms field studies which indicate that the charlotte belt contains a plutonic metaigneous complex that developed as a sub volcanic arc infrastructure contemporaneous with vulcanism manifested in the carolina slate belt both paleontological and geochronological controls indicate that the south carolina slate belt is mostly younger than 570 ma cambrian whereas the slate belt in north carolina and virginia is mostly of late proterozoic age a regionally significant mid paleozoic ca 340 360 ma thermal event is suggested by discordant 40ar 39ar whole rock age spectra of slate phyllite in the northwestern carolina slate belt and from hornblende in the southeastern charlotte belt it is uncertain if this event was associated with deformation in the eastern piedmont however mid paleozoic deformation has been previously documented elsewhere in the western piedmont a slightly disconcordant u pb zircon age of 317 4 ma is interpreted to closely date initial crystallization of the deformed edge field granite and confirms a record of late paleozoic penetrative deformation in the kiokee belt of south carolina u pb isotope data for the lake murray orthogneiss and clouds creek granite are discordant and suggest that the magmas of these plutons were derived by partial melting of a sialic precam brian source and then emplaced in the eastern piedmont at ca 315 ma prior to or during the early stages of the alleghanian orogeny the thermal maximum of alleghanian regional metamorphism amphibolite facies in the kiokee belt greenschist facies in the southeastern part of the carolina slate belt occurred during ca 295 315 ma during the late carboniferous and early permian the eastern piedmont experienced differential uplift erosion and relatively rapid postmeta morphic cooling isothermal surfaces were folded into an antiform synform antiform configuration corresponding to the kiokee carolina slate and charlotte belts respectively the geochronological data provide the following calibration for the late paleozoic deformational chronology recorded in the kiokee and carolina slate belts d2 lake murray deformation ca 295 315 ma d3 clarks hill deformation ca 285 295 ma and d4 irmo deformation ca 268 290 ma

interlaboratory standardization and calibration of day mineral crystallinity and crystallite size data clay mineral crystallinity and crystallite domain size data determined by x ray diffraction xrd are methods extensively used in the characterization of very low grade metamorphic conditions however the lack of sufficient interlaboratory standardization has made comparisons between different research groups unreliable due to significant variations in numerical results obtained a consequence of the different machine conditions measurement methods and sample preparations used during analysis a calibration approach to the standardization of data using rock chip standards is presented which allows data sets produced by different research groups to be directly and quantitatively compared a standardized scale the crystallinity index standard cis is proposed with illite crystallinity anchizonal boundary limits of 0 25d 2 and 0 42d 2 and equivalent illite crystallite sizes of 52 and 23 nm respectively determined by the warren averbach method calibrating both old and new data will enable more reliable comparisons between similar and contrasting geological environments and should improve the accuracy and reliability of correlations made between xrd data and other indicators of very low grade metamorphism hence increasing the value of such clay mineral studies

u pb geochronology of the raglan gabbro belt central metasedimentary belt ontario implications for an ensialic marginal basin in the grenville orogen the raglan gabbro belt of the ontario grenville orogen is coincident with the top of the central metasedimentary belt boundary thrust zone a major mid crustal shear zone separating the central gneiss belt in the footwall from the central metasedimentary belt in the hanging wall it has been suggested that the gabbros making up the belt are coeval that they formed in a marginal basin within the central metasedimentary belt and that they formed a horizon of theologically stiff material that controlled the localization of the top of the boundary thrust zone during its initiation as the marginal basin closed at ca 1190 ma u pb zircon dating of plutons within the raglan gabbro belt was undertaken to test the coeval nature of intrusions in the belt magmatic crystallization ages for three of the gabbros fall in the range 1246 1227 ma and a fourth yields a minimum age of ca 1175 ma the results are permissive of a common origin for the gabbros and allow that the raglan gabbro belt may have been related to

stable lead isotopes contaminant metals and radionuclides in upper hudson river sediment cores implications for improved time stratigraphy and transport processes radionuclide stable lead isotope and trace metal analyses on fine grained sediment cores collected along a 24 mile reach of the upper hudson river were used to establish temporal trends of contaminant loadings to establish stable lead isotopes as an additional stratigraphic tool and as tracers for resolving particle transport fluxes over periods of decades very large contaminant inputs of cd sb pb and cr were evident in the sediment record one potential large source for these metals was from a pigment manufacturing facility in glens falls ny the total range in stable lead isotope ratios observed in well dated cores from about 15 miles downstream of the potential metal inputs was large e g maximum difference in 206 pb 207 pb is 10 and characterized by four major shifts occurring in the 1950s 1960s 1970s and 1980s the temporal trend in 206 pb 207 pb has been used to establish precise dating of a sediment core from 24 miles further downstream the large magnitude and abrupt shifts in stable lead isotope ratios preserved in upper hudson sediment cores provide a way to significantly improve dating models based only on radionuclide analyses cadmium lead and antimony were identified as quite sensitive tracers of upper hudson sediments due to the magnitude of contamination and the lack of significant additional downstream sources of these contaminant metals metal measurements in a pair of sediment cores located 24 miles apart were used to constrain relative fluxes of sediment entering the river between the two coring locations with sediment sections deposited between the early 1960s and the late 1970s in these two cores suggesting that 3 4 times more sediment entered the river between the two coring sites than was transported from upstream these dilution factors agree very well with estimates based on suspended sediment measurements during a flood event in april 1994 and with estimates based on mechanistic model of suspended sediment transport between 1977 and 1992 d 2003 elsevier science b v all rights reserved

thermal 40ar 39ar separation of diagenetic from detrital illitic clays in gulf coast shales the 40ar 39ar age spectra for illite rich mixed layered illite smectite i s from texas gulf coast bentonites contain only a diagenetic component and are characteristically staircase shaped this spectrum shape contrasts with patterns seen for anchizonal and epizonal grade bentonites that are flatter this may be due to the structural and compositional heterogeneity mixed layering typical of low grade clays especially with respect to k on the other hand shales from depths exceeding those of the smectite to illite transformation have both detrital and diagenetic components although diagenetic illite rich i s dominates shale fine fraction detrital mica is observed even in the 0 2 m size fraction age spectra of shales consist of two portions 1 a staircase shaped low temperature section corresponding to degassing of the diagenetic illite rich i s component and 2 a peak in the high temperature section corresponding to degassing of well crystallized detrital mica the data obtained from 40ar 39ar stepwise analysis of these mixtures of diagenetic and detrital components permit at least partial resolution of the diagenetic from the detrital ages we have developed techniques for quantifying the separate diagenetic and detrital components based on subtraction of the detrital contribution from the age spectrum of mixture giving rise to an age corrected for the detrital contamination the resulting corrected 40ar 39ar ages are closer to the diagenetic ages this is in sharp contrast to the old k ar ages of the same samples although still in the developmental state these techniques appear to represent a solution to the problem of contamination of diagenetic illitic clays by detrital illitic phyllosilicates

anorogenic magmatism and the grenvillian orogeny the grenvillian orogeny was preceded by extensive anorogenic volcanism and plutonism in the period 1500 1300 ma in the form of rhyolites epizonal granites anorthosites gabbros alkaline complexes and basic dykes an analogue for the mid proterozoic anorogenic complexes is provided by the 2000 km by 200 km belt of anorogenic complexes in the hoggar niger and nigeria which contain anorthosites gabbros and peralkaline granites and were generated in a cambrian to jurassic rift that farther south led to the formation of the south atlantic an analogue for the 1 106 km2 area of 1500 1350 ma rhyolites and associated epizonal granites that underlie the mid continental united states is provided by the 1 7 106 km2 area of jurassic tobifera rhyolites in argentina which were extruded on the stretched continental margin of south america immediately preceding the opening of the south atlantic the mid proterozoic complexes were intruded close to the continental margin of the grenvillian ocean and were c

xrd based 40ar 39ar age correction for fine grained illite with application to folded carbonates in the monterrey salient northern mexico abstract due to their minute size 40ar 39ar analysis of illite faces significant analytical challenges including mineral characterization and especially effects of grain size and crystallography on 39ar recoil quantifying the effects of 39ar recoil requires the use of sample vacuum encapsulation during irradiation which permits the measurement of the fraction of recoiled 39ar as well as the 39ar and 40ar retained within illite crystals that are released during step heating total gas ages tga are calculated by using both recoiled and retained argon which is functionally equivalent to k ar ages while retention ages ra only involve retained ar in the crystal natural applications have shown that tga fits stratigraphic constraints of geological processes when the average illite crystallite thickness ict is smaller than 10 nm and that ra matches these constraints for icts larger than 50 nm we propose a new age correction method that takes into account the average ict and corresponding recoiled 39ar for a sample with x ray corrected ages xca lying between total gas and retention ages depending on ict this correction is particularly useful in samples containing authigenic illite formed in the anchizone with typical ict values between 10 and 50 nm in three samples containing authigenic illite from cretaceous carbonates in the monterrey salient in northern mexico there is a range in tgas among the different size fractions of 46 49 36 43 and 40 52 ma while ras range from 54 64 47 52 and 53 54 ma respectively xca calculations produce tighter age ranges for these samples of 52 5 56 45 5 48 5 and 49 52 5 ma respectively in an apparent age vs ict or 2m 1illite plot authigenic illite grains show a slope that is in general slightly positive for tga slightly negative for ra but close to zero for xca with thinner crystallites showing more dispersion than thicker ones in order to test if dispersion is due to a different formation history or the result of retention capability degassing spectra were modeled for site xca averages and overall xca average modeling shows that local site age average best match the measured spectra instead of a global average age indicating that illite growth reflects local deformation and is not the result of regional metamorphism modeling also shows that ar degassing spectra are very sensitive to grain size such that age interpretation based on ar plateaus is meaningless for most fine grained clays

clay quantification and ar ar dating of synthetic and natural gouge application to the miocene sierra mazatan detachment fault sonora mexico direct dating of brittle fault rocks has been predicated on the assumption that illite in fault gouge is a mixture of two populations of clays one detrital derived from the wall rock and the other authigenic forming in the brittle fault zone during faulting owing to complex diagenetic histories of wallrock shales in previous fault dating studies this assumption has remained largely untested we demonstrate the validity of our clay quantification technique using calculated wildfire patterns to accurately model both artificial mixtures of 2m1 and 1md illite and natural mixtures in young fault gouges using our quantification approach we obtain well defined detrital and authigenic ages from the sierra mazatan metamorphic core complex in sonora mexico because the age population of the wall rock is well constrained by cooling ages we confirm that the detrital component in the gouge is the age of the footwall granite while the age of the authigenic component is the time at which faulting ceased which is in agreement with field constraints based on the ar ar age and the thermal conditions for the growth of the authigenic illite we also conclude that the sierra mazatan detachment continued slipping in the brittle regime at strain rates similar to those for the plastic regime on the order of 10 14 s 1

dating fault gouges geological faults are fractures in the earth s crust along which there has been some displacement and are thus zones of weakness which are of importance in planning civil engineering projects constructing underground caverns and conducting earthquake research to assess the potential for future displacements and fluid circulation along faults it is helpful to establish the age of the latest episode of mineral growth in fault zones however there have been few attempts to date recrystallized and newly grown minerals developed in fault zones at low temperature1 4 here we report k ar and rb sr ages for clay fractions from two fault zones in the eastern alps and compare them with ages of the same fine fractions from the undeformed host rock the results suggest that the bulk of the clays along these faults were formed during early cretaceous and tertiary times respectively no major displacements seem to have taken place in recent times

fault gouge dating in the southern appalachians usa illite age analysis iaa the method of comparing radiometric ages of successive size fractions with varying percentages of detrital illite has been successfully applied to several rock types including fault gouge shales and argillaceous limestones iaa results are presented for fi ve fault rocks including four clay gouges and one cataclasite from the exhumed southern appalachian foreland fold thrust belt eastern united states determining detrital versus authigenic illite is now an established procedure utilizing x ray analysis to quantify illite polytypes less established however is how to interpret 40 ar 39 ar ages as a function of diagenetic grade both a total gas age incorporating the recoiled argon fraction after irradiation and a retention age omitting the recoiled fraction are obtained for a sample we relate their respective use to diagenetic grade and specifi cally the crystallite thickness of illite when measured crystallite sizes are equal to or smaller than that calculated from ar recoil we use total gas ages here 5 nm while for crystallites greater than 10 nm we use retention ages the four clay gouge ages are all the same within error and much younger than that of the cataclasite we conclude that a major period of frontal appalachian fault activity occurred during the early permian cisuralian 276 280 ma and that these foreland faults were active simultaneously as part of an internally deforming regional thrust wedge

geochronology of shear zones a review abstract shear zones play a major role in the deformation of the crust at a variety of scales as expressions of strain localization during orogeny and rifting and also as reactivated structures they influence the geometry and evolution of orogenic belts and rifts crustal rheology magma ascent and emplacement and fluid flow consequently assessing the timing of shear zone activity is crucial to reconstruct the tectonometamorphic evolution of the lithosphere the interpretation of thermochronologic data from shear zones is however not straightforward in the first place closure temperatures depend on a number of factors grain size cooling rate mineral composition and pressure among others on the other hand deformation related processes such as dynamic recrystallization neocrystallization and fluid circulation seem to be crucial for isotopic systems and thus the obtained ages cannot be solely interpreted as a function of temperature in sheared rocks for this reason geochronologic data from shear zones might not only record cooling below closure temperature conditions but may also be affected by neo or recrystallization fluid assisted deformation and inheritance of the protolith age s in order to robustly reconstruct p t e t paths of long term crustal scale shear zones structural microstructural and petrologic data from mylonites need to be integrated with ages from different thermochronometric systems in addition geochronologic data from associated intrusions and adjacent blocks can provide further irreplaceable constraints on the timing of deformation and its regional implications one of the most challenging aspects that future lines of investigation should analyze is the quantitative evaluation of so far poorly explored aspects of isotopic diffusion particularly the coupling with deformation processes based on natural theoretical and experimental data future works should also investigate the role of strain partitioning and localization processes in order to constrain the timing of deformation in different parts of a shear zone or in different branches of anastomosing shear zone networks

alleghenian regional diagenesis a response to the migration of modified metamorphic fluids derived from beneath the blue ridge piedmont thrust sheet to examine the nature and origin of fluids that caused widespread diagenetic alteration and remagnetization of the appalachian foreland during the late paleozoic we first examined the evidence for metamorphic fluids originating from beneath the blue ridge and piedmont causing extensive alteration along the linville falls fault in the grandfather mountain window north carolina these fluids were hot 310 to 400 c enriched in radiogenic strontium and had delta 18 o values of 3 to 10 0 smow and delta d values of 20 to 10 0 a rb sr isochron from the fault zone rocks yielded an alleghenian age of 301 pm 5 0 ma next we examined the diagenetic fluids that produced alleghenian silicification k feldspar authigenesis and associated dolomitization in remagnetized upper knox group carbonates of the valley and ridge and plateau provinces from tennessee to northern virginia the fluids were hot 140 to 245 c highly saline 14 to 23 wt nacl equivalent enriched in radiogenic str

alleghanian development of the goat rock fault zone southernmost appalachians temporal compatibility with the master decollement the goat rock and associated bartletts ferry fault zones which mark the eastern margin of the pine mountain grenville basement massif are controversial due to the suggestion that they are rare exposed segments of the late paleozoic southern appalachian master decollement the controversy in part stems from reported middle paleozoic acadian radiometric dates postulated as the time of movement along these fault zones ultramylonite samples from the type area at goat rock dam yield a 287 plus minus 15 ma rb sr isochron interpreted as the time of sr isotopic rehomgenization during mylonitization this date is corroborated by late pennsylvanian early permian sup 40 ar sup 39 ar mineral ages on hornblende 297 288 ma and muscovite 285 278 ma from neomineralized and dynamically recrystallized rocks within and straddling the fault zone these late pennsylvanian early permian dates indicate the time of right slip movement alleghenian along the goat rock fault zone which is compatible with the timing suggested by cocorp for thrusting along the southern appalachian master decollement

sw grenville province canada the case against post 1 4 ga accretionary tectonics abstract seven accretionary sutures formed between 1 16 and 1 03 ga have been identified by different authors in the ontario quebec adirondack oqa segment of the mesoproterozoic grenville orogen in canada with one exception the inferred accretionary terrane boundaries lie within or at the margins of the central metasedimentary belt cmb located between the central gneiss belt and the adirondack highlands central granulite terrane however geological geochronological and petrological data suggest that the grenville orogen on both sides of the proposed terrane boundaries sutures preserves a common 1 4 1 03 ga tectonomagmatic history inconsistent with its origin as a post 1 4 ga collage of exotic tectonic blocks features which straddle the proposed 1 16 1 03 ga sutures from the central gneiss belt via the adirondack highlands to the mauricie area include 1 mesoproterozoic continental crust 1 5 1 4 ga forming the host and or basement to younger magmatic and supracrustal suites 2 a 1 35 1 3 ga continental arc remnants of which occur from the cmb boundary zone cmbbz in ontario to the appalachians in the united states built on the 1 5 1 4 ga continental crust 3 intrusions of 1 17 1 13 ga age in the central gneiss belt mafic suite and the adirondack highlands and their quebec extension amcg suite i e anorthosite massifs and related granitoids 4 relics of 1 18 1 14 ga sedimentary basins in the northwestern cmb and the mauricie area we propose that an alternative model can adequately account for the observed geology of this part of the grenville orogen wherein the rocks of the oqa segment were part of an andean type margin between 1 4 and 1 2 ga at 1 35 1 3 ga a continental magmatic arc was built upon the southeastern margin of laurentia represented by the 1 5 1 4 ga mesoproterozoic continental crust the arc split at 1 3 ga forming an ensialic back arc basin relics of which now occur in the northwestern part of the cmb and the back arc basin was flanked to the southeast by an active 1 28 1 25 ga arc collision between the laurentian margin and another continent amazonia occurred at 1 2 ga resulting in closure of the back arc basin and initiation of thrusting along the cmbbz post collisional lithospheric shortening led to convective removal of thickened subcontinental lithosphere upper mantle melting and extension of the overlying crust resulting in widespread magmatic activity at 1 17 1 13 ga including emplacement of the amcg massifs crustal extension generated sedimentary basins now represented by the st boniface sediments in the mauricie area 1 18 to between 1 15 and 1 09 ga and the penecontemporaneous flinton group in the northwestern cmb renewed post collisional granulite facies shortening commenced at 1 12 ga manifested as nappes in the central gneiss belt and thrusting in the mauricie area continued post collisional shortening at 1 08 1 05 ga was more localised resulting in reactivation of thrusting in the cmbbz and initiation of the kinematically compatible tawachiche shear zone along the eastern border of the quebec extension of the adirondack highlands the characteristics of the oqa segment of the grenville orogen can all be accounted for in the context of 1 a 1 4 1 2 ga southeast facing andean type margin to a laurentian upper plate associated with northwest dipping subduction 2 continental collision at 1 2 ga and 3 subsequent continued post collisional shortening without invoking accretion of exotic terranes between 1 4 and 1 0 ga

sr nd and pb isotopes in proterozoic intrusives astride the grenville front in labrador implications for crustal contamination and basement mapping abstract we report sr nd and pb isotopic compositions of mid proterozoic anorthosites and related rocks 1 45 1 65 ga and of younger olivine diabase dikes 1 4 ga from two complexes on either side of the grenville front in labrador anorthositic or diabasic samples from the mealy mountains grenville province and harp lake nain churchill provinces complexes have very similar major minor and trace element compositions but distinctly different isotopic signatures all mealy mountains samples have isr 0 7025 0 7033 end 0 6 to 5 6 and pb isotopic compositions consistent with derivation from a mantle source depleted with respect to nd sm and rb sr pb isotopic compositions for the mealy mountains samples are slightly more radiogenic than model mantle compositions all harp lake samples have isr 0 7032 0 7066 end 0 3 to 4 4 and variable but generally unradiogenic 207pb 204pb and 206pb 204pb compared to model mantle suggesting mixing between a mantle derived component and a u depleted crustal contaminant crustal contaminants are probably a variety of archean high grade quartzofeldspathic gneisses with low u pb ratios and include a component that must be isotopically similar to the early archean 3 6 ga uivak gneisses of labrador or the amitsoq gneisses of west greenland this would imply that the ancient gneiss complex of coastal labrador and greenland is larger than indicated by present surface exposure and may extend in the subsurface as far west as the labrador trough if harp lake and mealy mountains samples were subjected to the same degree of contamination as suggested by their chemical similarities then the mealy contaminants must be much younger probably early or middle proterozoic in age the labrador segment of the grenville front therefore appears to coincide with the southern margin of the archean north atlantic craton and may represent a pre mid proterozoic suture