white mica crystallinity finite strain and cleavage development across a large variscan structure nw spain x ray diffraction analyses were performed on white micas formed under strain and low grade metamorphic conditions from sandstones and siltstones along a transect through the narcea antiform a large structure that separates the external from the internal zones of the variscan belt of nw spain the results indicate a close spatial relationship between the onset of metamorphism and the cleavage front at the cleavage front strain is characterized by oblate ellipsoids that have x y z ratios from 1 to 1 5 the x y z ratio increases westward up to 3 0 as does the grade of metamorphism and is accompanied by an increase in the 9crystallinity9 index of the white mica from very low to low metamorphic grade this increase corresponds qualitatively to an increase in penetrativeness of the axial planar cleavage in the western part of the narcea antiform the major deformation event was the development of kilometre scale reverse shear zones because of polyphase deformation these rocks are not suitable for quantitative strain analysis but qualitatively finite strain can be observed to be prolate and invariably larger in this region than in the eastern sector the metamorphic grade is higher in the western part of the narcea antiform with local maxima occurring next to the base of the shear zones the b 0 cell parameter a semi quantitative geobarometer measured in the white mica is chaotic in the eastern sector showing a clear detrital inheritance in the western sector it is rational due to complete re equilibration during thermal resetting the inverse correlation between finite strain and white mica 9crystallinity9 is close showing that the anchizone epizone limit can be related to the increase of finite strain above values of r s 1 5 the close relation between white mica 9crystallinity9 and strain is suitable for tracking strain variations in extensive fine grained siliciclastic rocks where no appropiate strain markers are found

alleghanian reactivation of the acadian fold belt meguma zone southwest nova scotia shear zones and northwest verging folds define a 30 km wide belt of deformation that overprints the acadian fold belt and telescopes isograds in the meguma zone in southwest nova scotia the shear zones appear to form a linked system that accommodated convergence dominated transpression of the meguma zone against an irregular avalon boundary available geochronological data indicate a mid carboniferous alleghanian variscan age for the overprinting deformation the mid carboniferous basement reactivation in southwest nova scotia is likely coeval with deformation of carboniferous strata and reactivation of basement structures meguma group in the northern meguma zone together these mid carboniferous structures may define a wide belt of alleghanian variscan deformation across the northwest cratonward margin of the meguma zone

minas fault zone late paleozoic history of an intra continental orogenic transform fault in the canadian appalachians abstract the minas fault zone mfz defines the boundary between the avalon and meguma terranes in the canadian appalachians and is exposed in mainland nova scotia and southern new brunswick these terranes originated along the gondwanan margin but had accreted to laurentia by the middle devonian the surface trace of the mfz is adjacent to the southern margin of the latedevonian permian maritimes basin the late devonian late carboniferous evolution of the mfz involves several episodes of oblique dextral shear that resulted in basin formation and inversion and at various times the zone was the focus of magmatism regional fluid flow and mineralization in the late devonian early carboniferous asymmetric rifting accompanied by dextral shear produced two coeval sequences the horton group which is dominated by continental clastic strata and the fountain lake group which consists predominantly of bimodal volcanic rocks that overlie high level plutons emplaced along active shear zones the overall tectonic environment may have been dominated by dextral transtension along the southern margin of laurentia which corresponded with the northern flank of the rheic ocean a major change in the evolution of the minas fault zone occurred in the late mississippian early pennsylvanian and produced the e w chedabucto fault clockwise rotation of pre existing structures local zones of transtension and transpression as well as regional fluid flow and extensive mineralization this major change may reflect the onset of laurentia gondwana oblique collision the effects of which continued into the latest carboniferous with coeval development of flower structures and pull apart basins in zones of local transpression and transtension

orogenesis and basin development u pb detrital zircon age constraints on evolution of the late paleozoic st marys basin central mainland nova scotia abstract the st marys basin along the southern flank of the composite late paleozoic magdalen basin in the canadian appalachians and along the avalon meguma terrane boundary contains late devonian early carboniferous continental clastic rocks of the horton group that were deposited in fluvial and lacustrine environments after the peak of the acadian orogeny shrimp ii geological survey of canada data on approximately 100 detrital zircons from three samples of horton group rocks from the st marys basin show that most of the zircons have been involved in a multistage history recycled from clastic rocks in the adjacent meguma and avalonian terranes although there is a minor contribution from early silurian 411 ma and late devonian suites ca 380 370 ma neoproterozoic ca 700 550 ma and paleoproterozoic ca 2 0 2 2 ga zircon populations predominate with a minor contribution from ca 1 0 1 2 and 1 8 ga zircons published u pb single zircon analyses on clastic sedimentary rocks indicate th

brunswick subduction complex in the canadian appalachians record of the late ordovician to late silurian collision between laurentia and the gander margin of avalon the brunswick subduction complex in the new brunswick part of the canadian appalachians records the late ordovician to late silurian collision between laurentia and the gander margin of avalon the brunswick complex is anomalously well preserved compared with equivalent rocks and structures elsewhere owing to its unique position in the deepest part of the quebec reentrant of the laurentian margin this part of the margin experienced less underthrusting and exhumation and overprinting by orogen parallel faulting than the adjacent promontories where collision started earlier the early southeast to east vergent thrust related structures represent a progressive d1 deformation that formed in response to northwestward subduction of the previously extended gander margin and subsequent tectonic unroofing of the subduction complex the original shallow northwestward dipping envelope to s1 was deformed in the late silurian by d2 upright folds and associated shear zones into a steep belt during terminal collision the d2 structures probably formed in response to sinistral transpression together d1 and d2 indicate that convergence was oblique and sinistral most rocks incorporated in the subduction complex formed in the tetagouche back arc basin that evolved from rifting of an arenig magmatic arc built on the gander margin into a wide marginal basin subduction was initiated in the late ordovician 455 ma in the back arc basin following collision of the middle ordovician popelogan arc with laurentia in the caradoc shortly after its opening in the late arenig 473 ma the closure history of the iapetus ocean involved more than one subduction zone and arc continent collision and rivals the southwestern pacific ocean in its complexities

origin of the rheic ocean rifting along a neoproterozoic suture the rheic ocean is widely believed to have formed in the late cambrian early ordovician as a result of the drift of peri gondwanan terranes such as avalonia and carolina from the northern margin of gondwana and to have been consumed in the devonian carboniferous by continent continent collision during the formation of pangea other peri gondwanan terranes e g armorica ossa morena northwest iberia saxo thuringia moldanubia remained along the gondwanan margin at the time of rheic ocean formation differences in the neoproterozoic histories of these peri gondwanan terranes suggest the location of the rheic ocean rift may have been inherited from neoproterozoic lithospheric structures formed by the accretion and dispersal of peri gondwanan terranes along the northern gondwanan margin prior to rheic ocean opening avalonia and carolina have sm nd isotopic characteristics indicative of recycling of a juvenile ca 1 ga source and they were accreted to the northern gondwanan margin prior to voluminous late neoproterozoic arc magmatism in contrast sm nd isotopic characteristics of most other peri gondwanan terranes closely match those of eburnian basement suggesting they reflect recycling of ancient 2 ga west african crust the basements of terranes initially rifted from gondwana to form the rheic ocean were those that had previously accreted during neoproterozoic orogenesis suggesting the rift was located near the suture between the accreted terranes and cratonic northern gondwana opening of the rheic ocean coincided with the onset of subduction beneath the laurentian margin in its predecessor the iapetus ocean suggesting geodynamic linkages between the destruction of the iapetus ocean and the creation of the rheic ocean

new u pb zircon ages and the duration and division of devonian time newly determined u pb zircon ages of volcanic ashes closely tied to biostratigraphic zones are used to revise the devonian time scale they are 1 417 6 1 0 ma for an ash within the conodont zone of icriodus woschmidti i w hesperius early lochkovian 2 408 3 1 9 ma for an ash of early emsian age correlated with the conodont zones of po dehiscens lower po inversus 3 391 4 1 8 ma for an ash within the po c costatus zone and probably within the upper half of the zone eifelian and 4 381 1 1 3 ma for an ash within the range of the frasnian conodont palmatolepis punctata pa punctata zone to upper pa hassi zone u pb zircon ages for two rhyolites bracketing a palyniferous bed of the pusillites lepidophyta spore zone are dated at 363 8 2 2 ma and 363 4 1 8 ma respectively suggesting an age of363 ma for a level within the late famennian pa g expansa zone these data together with other published zircon ages suggest that the base and top of the devonian lie close to 418 ma and 362 ma respectively thus lengthening the period by20 over current estimates we suggest that the duration of the middle devonian eifelian and givetian is rather brief perhaps no longer than 11 5 myr 394 ma 382 5 ma and that the emsian and famennian are the longest stages in the period with estimated durations of15 5 myr and 14 5 myr respectively 1998 elsevier science b v all rights reserved

estimates of horizontal divergence and vertical velocity in the equatorial pacific abstract horizontal divergence and vertical velocity in the surface mixed layer of the equatorial pacific between 90 and 150 w are estimated from current measurements obtained from trajectories of freely drifting buoys during 1979 1990 the 12 year averaged horizontal divergence is predominantly meridional and has a maximum magnitude of 3 4 10 6 s 1 in a 20 km wide latitude band cantered on the equator using the equation of continuity this divergence corresponds to an upwelling velocity of 1 5 2 10 4 m s 1 at 50 m depth the seasonal variations of equatorial divergence are in good agreement with the local zonal wind stress

constraints to the timing of india eurasia collision a re evaluation of evidence from the indus basin sedimentary rocks of the indus tsangpo suture zone ladakh india deposited within the indus tsangpo suture zone the cenozoic indus basin sedimentary rocks have been interpreted to hold evidence that may constrain the timing of india eurasia collision a conclusion challenged by data presented here the eurasian derived 50 8 51 ma chogdo formation was previously considered to overlie indian plate marine sedimentary rocks in sedimentary contact thus constraining the timing of collision as having occurred by this time using isotopic analysis u pb dating on detrital zircons ar ar dating on detrital white mica sm nd analyses on detrital apatite sandstone and conglomerate petrography mudstone geochemistry facies analysis and geological mapping to characterize and correlate the formations of the indus basin sedimentary rocks we review the nature of these contacts and the identification and correlation of the formations our results reveal that previously interpreted stratigraphic contacts identifying chogdo formation unconformably overlying indian plate sedimentary rocks are incorrect rather we suggest that the inaccuracy of previous interpretations is most likely a result of formation misidentification and thus cannot be used to constrain the timing of india asia collision

40ar 39ar white mica ages reveal neoproterozoic paleozoic provenance and an alleghanian overprint in coeval upper ordovician lower devonian rocks of meguma and avalonia abstract 40ar 39ar analyses ir single grain total fusion of white micas from upper ordovician lower devonian sedimentary strata in the meguma annapolis section and avalon arisaig section terranes of the canadian appalachians complement recent detrital zircon studies by providing constraints on local source areas and identifying the age of low temperature a remarkable outcome of the study is that many muscovite ages are significantly younger than the depositional age of the strata they do not record the middle late devonian age of acadian folding or adjacent intrusions but instead preserve a precise age of 322 6 2 1 ma 16 analyses although the sections sampled display no discernable evidence for carboniferous tectonism these ages are coeval with documented alleghanian deformation in both avalonian and meguma terrane rocks and we interpret them to reflect distributed fluid flow coeval with dextral shear along the avalon meguma terrane boundary

diagenesis to metamorphism transition in an episutural basin the late paleozoic st mary s basin nova scotia canada the late devonian early carboniferous st mary s basin in the canadian appalachians consists of horton group fluviatile and lacustrine clastic rocks the basin occurs along the boundary between the avalon and meguma ter ranes and developed during coeval dextral shear along that boundary x ray diffraction reveals that the rocks contain ubiq uitous quartz k white mica and albite illite smectite mixed layers and chlorite are very common and na k mica kaolinite chlorite smectite mixed layers k feldspar berthierine and rutile occur in some samples crystal chemical pa rameters of white mica indicate the pressure and temperature of mineral growth and discriminate between diagenetic an chizone and low grade metamorphic processes kubler index values measured in the 5 a u peak and the presence of chlorite mica stacks are indicative of high anchizone epizone grades with a crystallinity crystal size and number of de fects that increases towards the chedabucto fault which defines the northern margin of the basin kubler index values measured in the 10 a u peak indicate that a late fluid rich event could have produced the observed illite smectite mixed layers the overall clay mineral content and the b cell dimension of the k white micas are typical of postdepositional evo lution in extensional sedimentary basins with high heat flow 35 8c km taken together our data record two superposed events related to deformation along the basin margins and coeval regional fluid flow in which retrograde reactions at tem perature t 300 8c regional syntheses indicate that this fluid flow may have occurred during epi sodes of late carboniferous dextral shear along the avalon meguma terrane boundary

precollision and postcollision thermal events in the himalaya illite crystallinity and b o spacing analyses of rocks from the lesser himalaya show that the upper paleocene lower eocene subathu formation deposited immediately prior to or early in the india asia collision shows distinctly lower metamorphic grades than the anchiepizone upper precambrian lower paleozoic rocks the latter have been affected by a precollision metamorphic event the probable age of which is paleozoic although a mesozoic age is not ruled out thermal effects of the main central thrust are restricted to a section 1300 m below the thrust therefore the le fort model for metamorphic inversion and granite formation needs modifying because the exposed main central thrust footwall rocks could not have generated the fluids responsible for anatectic melting

post orogenic exhumation of an auriferous terrane the paleoplacer potential of the early carboniferous st marys basin canadian appalachians the st marys basin of mainland nova scotia canada consists of late devonian early carboniferous clastic rocks of the horton group deposited in the waning stages of the ca 420 360 ma acadian orogeny in the canadian appalachians clast lithologies and lithogeochemical analyses indicate that the detritus was predominantly derived from the meguma terrane that occurs to the south of the basin the meguma terrane contains abundant mesothermal gold deposits that are coeval with peak magmatic activity from ca 380 to 370 ma and underwent rapid uplift and erosion between ca 370 and 360 ma within the st marys basin the contact between the lacustrine little stewiacke river formation and the fluviatile barrens hills formation is interpreted to represent a shoreline and a potentially favorable environment for depositing paleoplacer gold geochemical analyses of lithologies adjacent to this contact indicate that the siltstones are predominantly derived from meguma terrane metasedimentary rocks whereas the sandstones and conglomerates are predominantly derived from meguma terrane granitoids geochemical and mineralogical analysis indicate the accumulation of heavy minerals including zircon and gold micron scale 150 m gold identified in the matrix of the conglomerates has a nuggety appearance and flakey microtexture indicative of a detrital origin this observation indicates that the mesothermal deposits were exhumed by the latest devonian consistent with regional tectonic syntheses that invoke rapid uplift of the meguma terrane following peak orogenic activity this study suggests that favorable depositional environments for paleoplacer deposits may occur in horton group rocks that are derived from the meguma terrane

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

mylonite to megabreccia tracking fault events within a transcurrent terrane boundary in nova scotia canada transcurrent terrane boundaries commonly evolve into long lived faults that preserve little evidence for early docking events a remarkable exception is exposed at clarke head along the appalachian meguma terrane boundary in nova scotia canada where a late carboniferous fault megabreccia contains devonian 369 ma u pb zircon granulite grade mylonite fractured by veins filled with visean amphibole ca 335 ma ar ar this fractured mylonite was later mixed with early carboniferous sedimentary rocks during megabrecciation ca 315 310 ma these three fault events are reflected in the tectonostratigraphic record devonian ca 370 360 ma transpressional terrane docking ramped meguma up against avalonia and shed clastic detritus across the fault system the visean brittle deformation recorded by the amphibole veins was coeval with marine regression at surface the late namurian megabrecciation event similarly produced unconformity followed by renewed nonmarine clastic sedimentation the clarke head megabreccia therefore preserves an episodic late paleozoic fault history spanning some 55 m y during convergence between laurentia and gondwana and the assembly of the pangean supercontinent

diastathermal extensional metamorphism at very low grades and possible high grade analogues abstract a genetic link between some areas of very low grade of metamorphism and an extensional setting is argued a model is presented in which the metamorphism develops in response to enhanced thermal gradients generated by extensional processes and the pressure temperature time paths are anticipated as being anticlockwise in form examples of granulite facies metamorphism attributed by sandiford and powell 1 to extensional settings may represent the lower crustal analogue of the very low grade examples examined here extensional processes may thus provide the mechanisms by which metamorphism in a variety of crustal levels can develop the traditional association of regional metamorphism with convergent processes is no longer tenable and the termdiastathermal metamorphism proposed by robinson 2 is appropriate to demonstrate the fundamental difference between these two types of regional metamorphism

40ar 39ar age of shear zones in the southwest meguma zone between yarmouth and meteghan nova scotia the cape st marys cranberry point and chebogue point shear zones lie within a zone of d2 deformation that overprinted d1 fabrics and reactivated f1 folds between the shear zones d2 was accompanied by greenschist facies metamorphism that variably overprinted earlier metamorphism of varying grade a collection of single muscovite grains from pressure shadows defining l2 in a sample from the cape st marys shear zone yields an average apparent age of 316 5 ma the age obtained for these grains from an isotope correlation diagram is 320 5 ma ages for muscovite separates for two samples from the cranberry point shear zone are 325 4 and 319 4 ma an amphibolite from this zone yields a discordant spectrum interpreted to reflect the combined effects of acadian metamorphism and excess argon the muscovite ages approximate the time of syntectonic deformation because muscovite closure temperatures approximate the temperature of metamorphism during d2 thus together with published data from the chebogue

late paleozoic collision delamination short lived magmatism and rapid denudation in the meguma terrane nova scotia canada constraints from 40ar 39ar isotopic data the meguma terrane consists of 8 15 km of early paleozoic stratified rocks deposited on a continental basement that were deformed and metamorphosed during the late paleozoic as a result of lithospheric plate collision the oldest cleavage previously published whole rock 40ar 39ar plateau ages of 415 395 ma dates the onset of crustal thickening which was followed by voluminous but short lived late devonian granitic and minor mafic magmatism 380 370 ma this magmatism may have been the product of delamination of the lower lithosphere and upwelling of asthenosphere which effected melting above the new moho and resulted in intrusion at depths of 5 12 km 40ar 39ar plateau ages of hornblende muscovite and biotite indicate that at the present erosion level most of the meguma terrane cooled through 300 c by 368 360 ma slightly earlier than for the southwestern meguma terrane ca 345 ma the present erosion level was exhumed by the latest devonian early carboniferous visean the age of the

geochemical and isotopic characteristics of early silurian clastic sequences in antigonish highlands nova scotia canada constraints on the accretion of avalonia in the appalachian caledonlde orogen avalonia is a terrane that accreted to laurentia baltica during the development of the appalachian caledonide orogen interpretations of the timing of accretion have been constrained by comparing faunal affinities overstep sequences age and kinematics of inferred accretionary deformational events and controversial paleomagnetic data we show that the time of accretion of avalonia may also be constrained by contrasts in the geochemical and isotopic signatures of its igneous rocks which reflect the characteristics of the underlying continental basement and mantle and sedimentary rocks which reflect provenance early silurian clastic sedimentary rocks of the beechill cove formation antigonish highlands nova scotia were deposited on avalonian crust the formation predominantly consists of approximately 80 m of siltstones and shales deposited in a nearshore environment and derived from the northeast their age is constrained by paleontological data and by directly underlying late ordovician early s

40ar 39ar dating in the lochaber mulgrave area northern mainland nova scotia implications for timing of regional metamorphism and sediment provenance in the late devonian early carboniferous horton group 40ar 39ar dating of whole rock samples and muscovite separates using age spectrum analysis and of single muscovite grains using total fusion analysis yields new insights into the timing of regional metamorphism and sediment provenance in the late devonian early carboniferous horton group in the lochaber mulgrave area of nova scotia the time of regional metamorphism is constrained to ca 340 335 ma by whole rock spectra from well cleaved slate and shale samples from the lowermost clam harbour river and overlying tracadie road formations of the horton group this ca 340 335 ma event may have been the result of burial and deformation of the horton group by older volcanic and sedimentary rocks of the guysborough group which were overthrust from the south as the result of development of a positive flower structure at a restraining bend along the cobequid chedabucto fault system the boundary between the meguma and avalon terranes detrital muscovite ages of ca 410 380 and ca 500 ma were obtained from