new york alabama lineament a buried right slip fault bordering the appalachians and mid continent north america the new york alabama ny al lineament recognized in 1978 is a magnetic anomaly that delineates a fundamental though historically enigmatic crustal boundary in eastern north america that is deeply buried beneath the appalachian basin data not in the original aeromagnetic data set particularly the lack of any information available at the time to constrain the southern continuation of the anomaly southwest of tennessee left the source of the lineament open to conjecture we use modern digital aeromagnetic maps to fill in these data gaps and for the first time constrain the southern termination of the ny al lineament our analysis indicates that the lineament reflects a crustal scale right lateral strike slip fault that has displaced anomalies attributed to grenville orogenesis by 220 km palinspastic restoration of this displacement rearranges the trace of the grenville belt in southern rodinia and implies only passive influence on later formed appalachian structures the precise timing of dextral movement on the ny al structure is not resolvable from the existing data set but it must have occurred during one of or combinations of the following events 1 a late postcontractional post ottawan stage of the grenville orogeny 2 late neoproterozoic to cambrian rifting of laurentia or 3 right slip reactivation during the late neoproterozoic cambrian rifting of laurentia or during appalachian movements our palinspastic reconstruction also implies that the host rocks for modern earthquakes in the eastern tennessee seismic zone are metasedimentary gneisses and it provides an explanation for the spatial location and size of the seismic zone

age and sr isotopic signature of the catoctin volcanic province implications for subcrustal mantle evolution variations in chemical and sr isotopic signatures of a series of basaltic flows have been correlated with newly mapped stratigraphic sections in the upper precambrian catoctin flood basalts in the central appalachians element mobility through fluid interaction is commonly observed in flows with a well developed fabric but some samples with more massive textures commonly in the cores of thicker flows still retain relict clinopyroxene and igneous textures and show only minor element redistribution five whole rock samples with relict igneous textures and a clinopyroxene mineral separate yield an isochron with an age of 570 36 ma 2sigma and a strontium initial ratio of 0 7035 1 this age is consistent with recent paleontological evidence for a latest precambrian time for catoctin volcanism and along with isotopic age determinations of 680 710 ma for other rift related magmatism suggests either a two stage or a very long protracted episode of continental rifting paleogeographic reconstruction indicates that the original site of eruption for the lavas must have been at least 56 km to the east in the same geographic location from which the mesozoic appalachian tholeiite terrane evolved variations in sr isotopic signatures and differences in major element abundances between these two sequences suggest mantle heterogeneitiesmore that must have developed beneath the eastern north american craton during the interim 400 m y less

700 ma rift event in the blue ridge province of virginia a unique time constraint on pre iapetan rifting of laurentia discovery of compositionally distinctive rhyolites produced from a peralkaline volcanic center within the late neoproterozoic robertson river batholith and which are intercalated with terrestrial deposits of the previously undated mechum river formation indicates that rift related sedimentation and magmatism were ongoing at 702 705 ma about 130 m y before the breakup of laurentia recognition of late neoproterozoic age rhyolites in central virginia indicates that rift related magmatism was more widespread along the incipient eastern present coordinates margin of laurentia than previously thought neoproterozoic rifting in the blue ridge province of virginia appears to have been part of a broad scale process that affected most if not all of the preserved margins of laurentia the association of similar extension related terrestrial deposits and compositionally distinctive felsic rocks elsewhere may provide a unique marker for testing proposed predrift configurations of the late neoproterozoic supercontinent rodinia

geochronology of the mesoproterozoic state farm gneiss and associated neoproterozoic granitoids goochland terrane virginia the goochland terrane is an isolated block of mesoproterozoic crust in the piedmont province of central virginia we report u pb zircon dates and whole rock major and trace element data for the state farm gneiss one of the main units in the terrane and additional results for a newly recognized suite of neoproterozoic granitoids that intrude the gneiss the state farm gneiss ranges in bulk composition from quartz monzodiorite to granite and three samples yield u pb zircon dates of 1046 7 6 ma 1039 18 11 ma and 1023 10 ma we interpret these dates as igneous crystallization ages which indicate a maximum emplacement interval of ca 1057 1013 ma for these samples neoproterozoic granitoids including the fine creek mills and flat rock granites are more alkaline than the state farm gneiss and display all compositional characteristics of a type granites e g high fe mg ga ga al nb zn y zr u pb zircon analyses from five separate bodies indicate crystallization ages from ca 654 to 588 ma but all results are complicated by mesoproterozoic ca 1 ga inheritance coupled in some cases with secondary pb loss we interpret the ages and compositions of the younger rocks to reflect neoproterozoic rifting of the goochland terrane but its location during rifting is uncertain the terrane may have been separated from laurentia during the neoproterozoic breakup of rodinia and later reattached

a comparative analysis of pre silurian crustal building blocks of the northern and the southern appalachian orogen the new york promontory serves as the divide between the northern and southern segments of the appalachian orogen antiquated subdivisions distinct for each segment implied that they had lithotectonic histories that were independent of each other using new lithotectonic subdivisions we compare first order features of the pre silurian orogenic building blocks in order to test the validity of the implication of independent lithotectonic histories for the two segments three lithotectonic divisions termed here the laurentian iapetan and the peri gondwanan realms characterize the entire orogen the laurentian realm composed of native north american rocks is remarkably uniform for the length of the orogen it records the multistage neoproterozoic early paleozoic rift drift history of the appalachian passive margin formation of a taconic seaway and the ultimate demise of both in the middle ordovician the iapetan realm encompasses mainly oceanic and magmatic arc tracts that once lay within the iapetus ocean between laurentia and gondwana in the northern segment the realm is divisible on the basis of stratigraphy and faunal provinciality into peri laurentian and peri gondwanan tracts that were amalgamated in the late ordovician south of new york stratigraphic and faunal controls decrease markedly rock associations are not inconsistent with those of the northern appalachians although second order differences exist exposed exotic crustal blocks of the peri gondwanan realm include ganderia avalonia and meguma in the north and carolinia in the south carolinia most closely resembles ganderia both in early evolution and late ordovician silurian docking to laurentia our comparison indicates that to a first order the pre silurian appalachian orogen developed uniformly starting with complex rifting and a subsequent drift phase to form the appalachian margin followed by the consolidation of iapetan components and ending with accretion of the peri gonwanan ganderia and carolinia this deduction implies that any first order differences between northern and southern segments post date late ordovician consolidation of a large portion of the orogen

alleghanian tectono thermal evolution of the dextral transcurrent hylas zone virginia piedmont u s a abstract the hylas zone is the northernmost segment of the eastern piedmont fault system and lies in the grenville goochland terrane virginia like the other major segments the nutbush creek zone the hollister zone and the modoc irmo zone the hylas zone experienced late paleozoic dextral transcurrent shearing early deformation in the zone was ductile and produced type i and type ii s c mylonites under amphibolite grade metamorphic conditions later deformation was at the brittle ductile transition at which time feldspar underwent microfaulting and cataclasis while quartz formed ribbons the feldspar exhibits a strain dependent sequence of microstructures including kink bands antithetic extensional microfaults pull apart bends in the microfaults by development of transverse fractures and microboudinage correlation of mineral mechanical response with a thermal decay curve based on isotopic mineral ages yields a temporally constrained deformation history ductile dextral shearing occurred subsequent to the intrusion of the 330 ma petersburg granite and passed into the brittle ductile transition by approximately 260 ma dextral faulting terminated before 240 ma the appalachian dextral transcurrent faulting event therefore continued through permian in some areas

tonian fe ti p ferronorite and alkali anorthosite in the northern appalachian orogen southern new brunswick canada amazonian basement in ganderia abstract the lower coverdale plutonic suite is located in the ganderian brookville terrane of southern new brunswick canada it does not outcrop at surface but has been intersected in 11 drill holes at depths of 100 200 m below unconformably overlying carboniferous sedimentary rocks and extends to a depth of at least 1100 m no host rocks are present in the drill core which consists entirely of plutonic and meta plutonic rocks the dominant rock types are anorthosite mela anorthosite and ferronorite with layers of ilmenite and ilmenite apatite rocks they are intruded by abundant undated mafic dykes and minor ca 390 ma felsic dykes a ferronorite sample yielded a tonian age of 975 5 2 3 ma interpreted to be the igneous crystallization age of the plutonic suite a mela anorthosite sample yielded a similar but less precise age of 979 16 ma likely also the igneous crystallization age the anorthosite has characteristics of alkali anorthosite massifs including andesine an30 35 commonly with exsolution lamellae of k feldspar the lower coverdale suite is interpreted to have been emplaced during orogen scale extension following crustal thickening associated with the grenvillian orogeny the lower coverdale anorthosite and ferronorite have more isotopically evolved signatures higher 206pb 204pb and 207pb 204pb and lower nd compared to typical laurentian grenville age plutons we infer from these data that the lower coverdale plutonic suite originated in the amazonian part of the grenville orogen during the early stages of tonian breakup of rodinia it was then incorporated into ganderia before accreting to laurentia as part of the brookville terrane during the pangean phase of supercontinent assembly

ion microprobe 232th 208pb ages from high common pb monazite morefield mine amelia county virginia implications for alleghanian tectonics monazite ce th po4 from a pegmatite in the morefield mine of the eastern piedmont of central virginia has unusually high and variable amounts of common pb leading to problematic interpretations of its u th pb ages and how the monazite relates to nearby granite intrusions and faults to address these issues we analyze a single large monazite grain from the pegmatite using electron microprobe analysis empa n 64 laser ablation inductively coupled plasma mass spectrometry la icp ms n 58 and secondary ion mass spectrometry sims n 59 the monazite study grain exhibits compositional variations in proximity to microcracks consistent regions of secondary alteration and recrystallization although the compositions of these regions fit the ideal stoichiometry of monazite they have lower si th u and y and higher p rare earth element ree and ca concentrations compared to visibly unaltered portions of the grain la icp ms and sims analyses demonstrate that common pb as proxied by 204pb is enriched in proximity to microcrack regions and correlates with 137ba sims 232th 208pb analysis from grain regions with lowest contents of common pb 208pb comprises 99 of pb isotopes yields two sets of ages 263 5 3 0 ma 1 mswd 1 7 n 11 and 234 1 3 3 ma 1 mswd 0 4 n 13 regionally the ages are similar to the youngest appalachian pegmatite bodies emplaced during the terminal alleghanian laurentia africa collision however the monazite ages are younger than locally surrounding intrusions the closest intrusive in distance 30 km and age is the petersburg granite 296 33 0 11 ma zircon 238u 206pb the emplacement of which coincided with activity along the hylas fault because the fault experienced a brittle ductile transition in the late permian 262 ma coeval with the older 232th 208pb monazite age group we postulate that the hylas fault may have operated as a fluid migration system between the petersburg granite and morefield mine pegmatite the younger monazite age coincides with triassic normal and or sinistral faulting linked to the development and deformation of local rift basins

middle proterozoic age for the montpelier anorthosite goochland terrane eastern piedmont virginia uranium lead dating of zircons from the montpelier anorthosite confirms previous interpretations based on equivocal evidence that the goochland terrane in the eastern piedmont of virginia contains grenvillian basement rocks of middle proterozoic age a very few prismatic elongate euhedral zircons which contain 12 29 ppm uranium are interpreted to be igneous in origin the vast majority of zircons are more equant subangular to anhedral contain 38 52 ppm uranium and are interpreted to be metamorphic in origin one fraction of elongate zircon and four fragments of a very large zircon occurring in a nelsonite segregation yield an upper intercept age of 1045 10 ma interpreted as the time of anorthosite crystallization irregularly shaped metamorphic zircons are dated at 1011 2 ma weighted average of the 207pb 206pb ages the u pb isotopic systematics of metamorphic titanite were reset during the alleghanian orogeny at 297 5 ma these data provide a minimum age for gneisses of the goochland terrane that are intruded by the anorthosite middle proterozoic basement rocks of the goochland terrane may be correlative with those in the shenandoah massif of the blue ridge tectonic province as suggested by similarities between the montpelier anorthosite and the roseland anorthosite although the areal extent of middle proterozoic basement and basement cover relations in the eastern piedmont remain unresolved results of this investigation indicate that the goochland terrane is an internal massif of laurentian crust rather than an exotic accreted terrane

biosignatures link microorganisms to iron mineralization in a paleoaquifer concretions preferentially cemented masses within sediments and sedimentary rocks are records of sediment diagenesis and tracers of pore water chemistry for over a century rinded spheroidal structures that exhibit an fe iii oxide rich exterior and fe poor core have been described as oxidation products of fe ii carbonate concretions however mechanisms governing fe iii oxide precipitation within these structures remain an enigma here we present chemical and morphological evidence of microbial biosignatures in association with fe iii oxides in the fe iii oxide rich rind of spheroidal concretions collected from the jurassic navajo sandstone southwest united states implicating a microbial role in fe biomineralization the amount of total organic carbon in the exterior fe iii oxides exceeded measured values in the friable interior the mean 13 c value of organic carbon from the fe iii oxide cemented exterior 13 c of 20 55 is consistent with a biogenic signature from autotrophic bacteria scanning electron micrographs reveal microstructures consistent with bacterial size and morphology including a twisted stalk morphotype that resembled an fe ii oxidizing microorganism gallionella sp nanoscale associations of fe o c and n with bacterial morphotypes demonstrate microorganisms associated with fe iii oxides together these results indicate that autotrophic microorganisms were present during fe iii oxide precipitation and present microbial catalysis as a mechanism of fe iii oxide concretion formation microbial biosignatures in rinded fe iii oxide rich concretions within an exhumed quaternary aquifer has broad implications for detection of life within the geological record on earth as well as other fe rich rocky planets such as mars where both fe ii carbonate and fe iii oxide rich concretions have been identified

40ar 39ar dating of silurian and late devonian cleavages in lower greenschist facies rocks in the westminster terrane maryland usa 40 ar 39 ar dating of muscovite biotite and k feldspar combined with microstructural analysis of lower greenschist facies polymetamorphic phyllitic rocks and marbles were successfully used to decipher the thermal and tectonic histories of the westminster and adjacent terranes in western maryland the presence of unreset detrital muscovite in some samples demonstrates that temperatures in these rocks never exceeded the closure temperature for argon diffusion in muscovite 350 50 c minor biotite in some arkoses constrains the minimum metamorphic temperatures to 320 c these data show an early silurian ca 430 ma cleavage in the western part of the westminster terrane and a late devonian event ca 370 ma in the eastern westminster and adjacent potomac terranes these two cleavage domains are separated by the ne trending newly identified parrs ridge fault zone we propose that the sinistral transpressive collision of the carolina terrane with laurentia emplaced the western portion of the westminster terrane in the pennsylvania embayment along the martic fault where it was folded and cleaved at ca 430 ma but otherwise largely sheltered from later deformation the later late devonian dextral transpressive accretion of the outboard potomac terrane thrust rocks of the eastern westminster and potomac terranes to the west causing late devonian 360 370 ma s 2 cleavage in these rocks but only minimal discrete overprinting s 3 cleavages in rocks farther west final juxtaposition and thermal convergence of these terranes occurred along reactivated dextral strike slip faults in the alleghanian at ca 300 ma

the bakersville dike swarm geochronology and petrogenesis of late proterozoic basaltic magmatism in the southern appalachian blue ridge

early jointing in coal and black shale evidence for an appalachian wide stress field as a prelude to the alleghanian orogeny early ene striking joints present coordinates within both pennsylvanian coal and devonian black shale of the central and southern appalachians reflect an approximately rectilinear stress field with a dimension 1500 km this appalachian wide stress field awsf dates from the time of joint propagation when both the coal and shale were buried to the oil window during the 10 15 m y period straddling the pennsylvanian permian boundary the awsf was generated during the final assembly of pangea as a consequence of plate boundary tractions arising from late stage oblique convergence where maximum horizontal stress s h of the awsf was parallel to the direction of closure between gondwana and laurentia after closure the awsf persisted during dextral slip of peri gondwanan microcontinents when sh appears to have crosscut plate scale trans current faults at around 30 sup o following 10 m y of dextral slip during tightening of gondwana against laurentia the awsf was disrupted by local stress fields associated with thrusting on master basement decollements to produce the local orocline shaped alleghanian map pattern seen today

introduction to atlantic hydrothermal activity seafloor hydrothermal research has advanced rapidly from local to global scope through a sequence of discoveries hydrothermal research at seafloor spreading centers began in the mid 1960s with the discovery of hot metalliferous brines and sediments ponded in deeps along the slow spreading half rate 1 cm yr 1 axis of the red sea chamock 1964 miller 1964 swallow and crease 1965 miller et al 1966 hunt et al 1967 bischoff 1969 at the same time a hydrothermal metalliferous component was identified in sediments of the east pacific rise skomyakova 1965 arrhenins and bonatti 1965 bostrom and peterson 1966 geophysicists recognized that heat flow measurements at spreading centers could only be explained by convective cooling of the crust with circulating seawater elder 1967 lister 1972

extensional and contractional deformation in the blue ridge province virginia basement rocks of the blue ridge anticlinorium in central and northern virginia were involved in two temporally distinct deformation episodes the older event is characterized by a locally pervasive low strain fabric and high strain shear zones a preferred grain shape orientation in recovered quartz feldspar and biotite combined with a poorly developed symmetric quartz c axis pattern defines the low strain fabric kinematic indicators are typically symmetric but where asymmetric they consistently indicate top down to the southeast extensional motion discrete shear zones are within and close to late proterozoic plutons kinematic evidence fron these zones records large noncoaxial strains also with a down to the southeast extensional shear sense feldspar microstructures range from cataclastic fractures to complete recovery suggesting that deformation occurred at greenschist to amphibolite grade both low strain foliations and high strain shear zones are cut by paleozoic thrust related tectonites on the basis of kinematic evidence and spatial association with late proterozoic plutons cross cutting relations with thrust related structures and regional geologic relations we suggest that extensional deformation is related to late proterozoic rifting of the laurentian margin the kinematic histories of the rockfish valley fault zone and related shear zones appear to be relatively simple and consistent with paleozoic west to northwest directed thrusting the rockfish valley fault zone does not separate distinctly different basement terranes and thus its displacement is thought to be small mylonitic rocks from this fault zone display crystal plastic deformation in quartz and cataclastic deformation in feldspars under lower to middle greenschist facies conditions solution transfer processes were an important deformation mechanism in these tectonites thrust related mylonitic rocks are overprinted by related cataclasite and semibrittle shear bands suggestive of a transition toward more brittle deformation as the thrust system cut higher in the crust the rockfish valley fault zone appears to predate the formation of the south mountain cleavage in the valley and ridge province but it may be part of a lengthy progressive thrust related deformation

tectonic evolution of the southeastern margin of the yangtze block constraints from shrimp u pb and la icp ms hf isotopic studies of zircon from the eastern jiangnan orogenic belt and implications for the tectonic interpretation of south china abstract the jiangnan orogenic belt is a ne sw trending fold belt in the southeastern yangtze block of south china the eastern segment of the belt located in northeastern jiangxi and southern anhui provinces is predominated by greenschist facies metamorphosed volcanic sedimentary strata with minor ophiolite and their protoliths are considered to have formed in a back arc basin zircon shrimp u pb dating reveals that these rocks were deposited emplaced in the period 842 825 ma zircon la icp ms hf analyses show that most igneous zircons from the volcanic sedimentary rocks possess positive hf t values of 0 09 12 74 corresponding to depleted source model ages of 0 9 1 4 ga this suggests a significant neoproterozoic juvenile crustal growth event in the southeastern part of the yangtze block which is also documented by gabbro 833 ma from ophiolite with positive hf t values of 5 24 13 25 and hafnium model ages of 0 87 1 20 ga these new data combined with previous results indicate the presence of an arc and back arc basin system along the southeastern margin of the yangtze block at 842 825 ma collision of the arc with the proposed shuangxiwu composite terrane which had previously formed away from the southeastern margin of the yangtze block through possible subduction and collision led to the closure of the back arc basin and the formation of the jiangnan belt in the period 825 815 ma we suggest that south china is an accretionary orogen and that orogen parallel motion along terrane bounding shear zones may have played a significant role in its amalgamation

northern ancestry for the goochland terrane as a displaced fragment of laurentia the ancestral position of the goochland terrane an isolated block containing mesoproterozoic crust in the appalachian piedmont of eastern virginia during grenvillian orogenesis has direct bearing on the breakup of rodinia ages lithology geochemistry and pb and nd isotope compositions of the montpelier anorthosite and state farm gneiss of the goochland terrane allow correlation with the grenvillian roseland anorthosite and associated igneous suites of the blue ridge farther west in virginia in contrast extensive metapelites which underwent devonian metamorphism and intrusion and amphibolites of the goochland terrane lack equivalents in the blue ridge dextral slip faults which bound the goochland terrane as well as dextral faults farther north suggest large scale translation prior to late paleozoic reaccretion ages of extension related a type granitoids within the virginia blue ridge 735 680 ma suggest physical separation from compositionally similar granitoids in the goochland terrane 630 588 ma reading prong 602 ma and manhattan prong 563 ma pre iapetan restoration of the goochland terrane northeastward of the blue ridge and outboard of the reading prong accounts for its affinities to the blue ridge unique anorthosites manhattan prong abundant amphibolites and reading prong ca 600 ma magmatism thus similar to the translation of madagascar out of africa the goochland terrane was rifted from laurentia marooned in oceanic crust and then dextrally translated southward 500 km prior to late paleozoic reaccretion documentation of large scale dextral translation of eastern laurentia relative to its fragments and other cratons during the neoproterozoic and paleozoic may assist in locating far traveled pieces of the grenville orogen for reconstruction of rodinia

northern ancestry for the goochland terrane as a displaced fragment of laurentia comment and reply reply we welcome the opportunity to discuss further our ideas regarding translational tectonics along the iapetan margin of laurentia we look forward to seeing published papers on the alleghanian history of the goochland terrane based on recent abstracts of our colleagues from william and mary but

age and origin of anorthosites charnockites and granulites in the central virginia blue ridge nd and sr isotopic evidence rb sr isotopic data for anorthosites charnockites ferrodioritic to quartz monzonitic plutons and high grade gneisses of the blue ridge of central virginia show evidence of post emplacement metamorphism but in some cases retain grenville ages the pedlar river charnockite suite yields an isochron age of 1021 36 ma initial 87sr 86sr ratio of 0 7047 6 which agrees with published u pb zircon ages five samples of that unit which contain paleozoic mylonitic fabrics define a regression line of 683 ma interpreted as a mixing line with no age significance samples of the roseland anorthosite complex show excessive scatter on a rb sr evolution diagram probably due to paleozoic 475 m y metamorphism data from the ferrodioritic to quartz monzonitic plutons of the area yield an age of 1009 26 ma inital ratio 0 7058 4 which is in the range of the u pb zircon ages of 1000 1100 ma the stage road layered gneiss yields an age of 1147 34 ma initial ratio of 0 7047 5 sm nd data for the pedlar river charnockite suite reflect a pre grenville age of 1489 118 ma nd 6 7 1 2 data for the roseland anorthosite complex and the ferrodioritic to quartz monzonitic plutons yield grenville isochron ages of 1045 44 ma nd 1 0 0 3 and 1027 101 ma nd 1 4 1 0 respectively two roseland anorthosite samples plot far above the isochron demonstrating the effects of post emplacement disturbance of sm nd systematics while mylonitized pedlar river charnockite suite samples show no evidence of sm nd redistribution the disparity of the sm nd age and other isotopic ages for the pedlar river charnockite suite probably reflects a sm nd source age suggesting the presence of an older crust within this portion of the ca 1 ga old basement

northward extension of carolina slate belt stratigraphy and structure south central virginia results from geologic mapping geologic mapping in south central virginia demonstrates that the stratigraphy and structure of the carolina slate belt extend northward across a steep thermal gradient into upper amphibolite facies correlative gneiss and schist the neoproterozoic greenschist facies hyco aaron and virgilina formations were traced northward from their type localities near virgilina virginia along a simple upright northeast trending isoclinal syncline this syncline is called the dryburg syncline and is a northern extension of the more complex virgilina synclinorium progressively higher grade equivalents of the hyco and aaron formations were mapped northward along the axial trace of the refolded and westwardly overturned dryburg syncline through the keysville and green bay 7 5 minute quadrangles and across the northern end of the carolina slate belt as interpreted on previous geologic maps hyco rocks including felsic metatuff metawacke and amphibolite become gneisses upgrade with areas of local anatexis and the segregation of granitic melt into leucosomes with biotite selvages phyllite of the aaron formation becomes garnet bearing mica schist aaron formation rocks disconformably overlie the primarily felsic volcanic and volcaniclastic rocks of the hyco formation as evidenced by repeated truncation of internal contacts within the hyco on both limbs of the dryburg syncline at the aaron hyco contact east northeast trending isograds defined successively by the first appearance of garnet then kyanite staurolite in sufficiently aluminous rocks are superposed on the stratigraphic units and synclinal structure at moderate to high angles to strike the textural distinction between gneisses and identifiable sedimentary structures occurs near the kyanite staurolite in isograd development of the steep thermal gradient and regional penetrative fabric is interpreted to result from emplacement of the goochland terrane adjacent to the northern end of the slate belt during alleghanian orogenesis this mapping study indicates that the carolina slate belt does not terminate on the north against through going faults or rest on higher grade basement as previously suggested

circum iapetus paleogeography of the precambrian cambrian transition with a new paleomagnetic constraint from laurentia abstract the paleogeography of the precambrian cambrian transition is still poorly known but is fundamental for understanding late neoproterozoic climate extremes and the spatial associations of the rapidly evolving ediacaran and cambrian fauna this period has also been proposed to host unusual geodynamic events such as rapid plate motions and or the bulk tumbling of the earth with respect to its spin axis called true polar wander tpw new paleomagnetic and 40ar 39ar geochronologic results have been obtained from a pair of shallowly emplaced syenitic intrusions in western quebec the mont rigaud and the chatham grenville stocks are related to a failed rift arm of the iapetus ocean both intrusions bear hornblendes which provide overlapping 40ar 39ar plateau ages of 533 2 1 1 ma mont rigaud and 531 4 3 4 ma chatham grenville corresponding to early cambrian nemakit daldynian time characteristic paleomagnetic reversed directions from both intrusions are easterly and shallow d 89 3 i 30 1 95 6 2 k 38 8 n 15 sites and the mont rigaud intrusion also carries antipodal normal polarity directions the combined result yields a north paleopole at location 11 9 s 184 6 e dp 3 8 dm 6 9 placing the montreal region at 16 4 s paleolatitude in the early cambrian in good agreement with late ediacaran and middle to late cambrian paleomagnetic results from laurentia laurentia likely resided at low southerly paleolatitudes throughout the cambrian ruling out a major inertial interchange tpw event for early to middle cambrian time a comparison of late ediacaran to middle cambrian paleomagnetic results from laurentia baltica siberia and elements of west gondwana implies that by early cambrian time a large paleolatitudinal gap existed between laurentia and its presumed conjugate rift margins thus placing a wide iapetus ocean between them the position of baltica within the circum iapetus paleogeography is contentious due to differing interpretations of the few paleomagnetic results during ediacaran time laurentia underwent large apparent polar wander that is manifested as motion from low latitude at 615 ma to the south polar region at 590 575 ma and then rapidly back to low latitude by 565 550 ma the large and perhaps rapid apparent polar wander of laurentia and other continents in the ediacaran remains enigmatic and controversial

northern ancestry for the goochland terrane as a displaced fragment of laurentia comment and reply comment the recent article by bartholomew and tollo 2004 1 concludes that the enigmatic goochland terrane is a displaced laurentian fragment in their model the goochland terrane originated in the new york promontory rifted away from laurentia during the formation of iapetus and was subsequently

petrology and u pb geochronology of the robertson river igneous suite blue ridge province virginia evidence for multistage magmatism associated with an early episode of laurentian rifting

neoproterozoic a type magmatism in the western sierras pampeanas argentina evidence for rodinia break up along a proto iapetus rift a type orthogneisses of mid neoproterozoic age 774 6 ma u pb shrimp zircon age are reported for the first time from the grenvillian basement of the western sierras pampeanas in argentina these anorogenic meta igneous rocks represent the latest event of rodinia break up so far recognized in grenvillian basement exposures across andean south america moreover they compare well with a type granitoids and volcanic rocks along the appalachian margin of laurentia blue ridge thus adding to former evidence that the western sierras pampeanas grenvillian basement was left on the conjugate rifted margin of eastern laurentia during rodinia break up and the consequent opening of the iapetus ocean

comparative evolution of the iapetus and rheic oceans a north america perspective abstract differences in the origin and evolution of the iapetan and rheic oceans profoundly affected the development of the ouachitan appalachian variscan orogen the iapetus ocean was initiated in the late neoproterozoic early cambrian by the separation of large continental landmasses laurentia baltica gondwana whereas the rheic ocean originated in the late cambrian early ordovician as the result of the separation of several ribbon continents collectively termed peri gondwanan terranes from the northern margin of gondwana compared to the classic passive margin developed along the laurentian margin of iapetus passive margin successions to the rheic ocean are more limited in extent during convergence both margins of the iapetus ocean developed western pacific type arc back arc systems whereas subduction of the rheic ocean was primarily directed beneath laurentia and was andean in style closure of iapetus involved a number of arc back arc accretionary events and had occurred by the middle silurian as a result of laurentia baltica collision to form laurussia and the accretion of the peri gondwanan terranes rheic ocean closure occurred in the carboniferous by way of continent continent collision between laurussia and gondwana with gondwana on the lower plate within iapetus the formation of ophiolites occurred early in the orogenic cycle and was followed soon after by obduction hp lt metamorphism and foreland directed thrusting that is intimately related to accretionary events these features dominate the northern appalachian orogen where ht lp metamorphism is attributed to post accretionary slab break off events in contrast hp lt metamorphism within the rheic ocean reflects a type subduction during laurussia gondwana collision and is rapidly succeeded by ht lp metamorphism attributed to orogenic collapse ophiolites are rare and were formed either very early early ordovician or very late devonian carboniferous in the evolution of the ocean the early ordovician ophiolites were not obducted until the carboniferous at least 100 million years after their protoliths were formed rheic ocean closure resulted in the formation of large scale bivergent structures within the orogen with symmetry changing on either side of the rheic suture comparison of the evolution of the iapetus and rheic oceans suggests that differences in rift configuration passive margin development and subduction zone geometry within the two oceans profoundly influenced the subsequent style of orogenesis within the ouachita appalachian variscan orogen