a transitional alkalic dolerite dike suite of mesozoic age in southeastern new england dike rocks from the new england platform of rhode island and adjacent massachusetts consist of premetamorphic and post metamorphic suites the older group includes metamorphosed dolerite minette and schistose dioritic rocks post metamorphic dikes consist of dolerite and sparse monchiquite the post metamorphic dolerites are of comparable age to the eastern north american dolerite suite associated with the mesozoic basins along the eastern seaboard of north america however the southeastern new england dolerites exhibit mineralogy and chemistry more typical of a transitional alkalic suite compared to the more subalkalic tholeiitic dolerites of the eastern north american suite both suites are compatible with a rift tectonic setting but the more alkalic dolerites may represent a deeper source of small volume melts compared to the eastern north american dolerites these more alkaline melts may have concentrated at local centers or they may be typical of flank dolerites as opposed to the less alkalic varieties that occur within the central axial rift

development of the palaeogeography of pangaea from late carboniferous to early permian abstract old and new well and poorly known lithofacies data of moscovian and artinskian age in the area limited by the arctic sea and central africa the atlantic ocean and the ural mountains have been plotted on present day maps the facies mapped and related environments allow the recognition of a series of different palaeogeographic palaeotectonic units including the late carboniferous to early permian deep sea to palaeo tethyan ocean west siberia kazakhstan continent and uralian kazakhstan tienshan arc uralian foredeep up to the closing arms of the uralian ocean precaspian basin russian platform donets rift basin caucasian moesian dobrogean polish oslo basinal belt syn tectonic hercynian foreland basin intramontane post hercynian basins iran anatolian hellenic dinaric carnic basin branching into the hungarian seaway apennine basin branching into the cantabrian and south portuguese basins oman iraq levantine sicily deep basin and inferred oceanic sea way south peri tethyan platform basins east arabian cratonic basin and north african intracratonic basins the subsidence sedimentation trends of these units were correlated and compared through lithostratigraphic logs bathymetric curves and uncorrected cumulative stratigraphic curves the sets of original and processed data were used to test two different palaeodynamic models a pangaea a model static from late carboniferous to triassic and a mobile pangaea b model with different dextral displacements between laurussia and gondwanaland in the same time interval the best fit for our data requires a strike slip offset of about 800 km from moscovian to artinskian time this model implies a first quasi pangaea or pangaea b assembly at the carboniferous permian transition an ephemeral pangaea b break up driven by an early permian oblique rift across the mediterranean to caribbean areas and a final pangaea a assembly in the mid to late permian the two palinspastic maps describing the model have been cross checked by comparison with an independent set of biogeographic features of late carboniferous and early permian overall floral reptile and marine benthic organism distribution is consistent with the early permian trans pangaea seaway inferred from facies and palaeodynamic analyses

kinematic history of the narragansett basin massachusetts and rhode island constraints on late paleozoic plate reconstructions the formation of the narragansett basin of rhode island and massachusetts and the polyphase deformation geometry of the pennsylvanian age basin sediments can be explained by collision and strike slip interactions between an avalonian microcontinent the european north american plate and the african plate during the late paleozoic sinistral movement between an avalonian microcontinent and the european north american plate resulted in formation of pull apart basins subsequently africa moved toward north america along an east west trending dextral megashear zone and collided with the eastern margin of southern north america the narragansett basin is located on the avalonian microcontinent near the suture with the north american continent and in the transition zone between the megashear and the collision tectonic regimes the basin shape stratigraphy and sedimentary relationships can be explained by the coalescence of two composite grabens which formed at different times the thicker northern graben is only affected by one deformation whereas the southern graben shows evidence of four distinct phases of postrifting deformation the first two coaxial folding deformations of the basin sediments are attributed to the closure of the basin complex during collision of africa and north america the third and fourth deformations are interpreted as sinistral r riedal and dextral shears respectively associated with the east west trending megashear system a prograde barrovian metamorphic front started in the western portion of the southern graben slightly after the first deformation and migrated northward and eastward where it is synchronous with the second deformation

40ar 39ar ages from the narragansett basin and southern rhode island basement terrane their bearing on the extent and timing of alleghenian tectonothermal events in new england pennsylvanian sedimentary rocks record the effects of progressive barrovian metamorphism and polydeformation in the narragansett basin of rhode island whole rock phyllite samples from low grade portions of this terrane display internally concordant 40 ar 39 ar age spectra with plateau ages of 248 to 258 m y four samples biotite from higher grade rocks records plateau ages of 240 to 248 m y eight samples similar 40 ar 39 ar biotite ages are displayed westward across the southern rhode island basement terrane 239 to 250 m y nine samples although coexisting hornblende records slightly older plateau ages of 255 m y two samples the 40 ar 39 ar plateau ages are interpreted to date times of post metamorphic cooling through temperatures required for argon retention in the various mineral phases examined the ages suggest that a distinct late paleozoic alleghenian thermal event affected much of southern rhode island together with the depositional ages of the variably metamorphosed pennsylvanian rocks westphalian b stephanian a or b the 40 ar 39 ar data permit calibration of the sequence of alleghenian tectonothermal events recorded in the narragansett basin the controls suggest a permian event lasting about 40 to 50 m y between initial barrovian regional metamorphism and synchronous d 1 d 2 ductile deformation between 295 and 275 m y and post d 3 cooling below argon retention temperatures between 250 and 240 m y

deformation induced reconstitution and local resetting of mineral equilibria in polymetamorphic gneisses tectonic and metamorphic implications a sequence of at least three al2sio5 bearing mineral assemblages are preserved in successively overprinted ductile shear zones in the willimantic window connecticut the ductile deformation localized at and near the boundary between the putnam nashoba terrane and underlying avalon terrane is characterized by a network of anastomozing shear zones that outline metre scale tectonic blocks of migmatitic kfs sil gt bi pg qtz ilm ru gneiss these assemblages record acadian or older metamorphic conditions of 6 kbar 700d c mylonitic gneisses in shear zones that define block margins were formed by reconstitution and recrystallization of the migmatitic gneiss the reconstituted rocks exhibit relict ky st grt pl bt qtz rt ilm assemblages and require a minimum pressure for the ky str grade metamorphism of 8 5 kbar kyanite in block margins is widely replaced by sillimanite and locally by andalusite during a period of post alleghanian ductile deformation the interiors of blocks do not record this sequence of polymorphs the pattern of reconstitution is accounted for by localization of strain along block margins within a regionally extensive terrane bounding fault zone strain provided the activation energy for recrystallization and retrograde mineral reactions the p t conditions of post alleghanian ductile deformation evolved from 600d c and 6 kbar to 550d c and 3 kbar the occurrence of ky str bearing assemblages overprinting acadian kfs sil bearing assemblages and subsequently overprinted by alleghanian sillimanite and andalusite bearing assemblages along with reset hornblende 40ar 39 ar mineral ages from mississippian to permian requires a prograde alleghanian metamorphism of rocks previously metamorphosed during the acadian thus mineral assemblages from gneisses in the willimantic fault zone retain evidence of a protracted tectonothermal evolution that included high grade acadian orogenesis tectonic loading resulting from alleghanian collision of avalon with north america and tectonic exhumation in permo triassic time the c 3 kbar pressure decrease between prograde and retrograde alleghanian metamorphic conditions corresponds to 10 km of crust that must have been tectonically excised from the base of the putnam nashoba terrane cover sequence following alleghanian orogenesis in south eastern new england

permian bulk shortening in the narragansett basin of southeastern new england usa pennsylvanian age sediments within the narragansett basin allow the effects of alleghanian deformation to be distinguished from the taconic and acadian orogenies that affected the rocks to the west three dimensional microstructural analyses of both matrix foliations and structures preserved in porphyroblasts define two extended periods of deformation and metamorphism about two differently trending foliation intersection inflection axes in porphyroblasts fias ssw nne trending fias set 1 formed first followed by wsw ene trending fias set 2 the ssw nne fia trends from the south central zone lie parallel to regional folds in the entire southern graben the changes in fia trends reveal that the direction of maximum bulk shortening changed from wnw ese to nnw sse during amphibolite facies metamorphism within the central zone of the basin they reveal a more varied history than the simple kinematic framework of solely w e directed bulk shortening that was previously suggested

early rifting in the narragansett basin massachusetts rhode island evidence from late devonian bimodal volcanic rocks abstract the basal wamsutta formation of the narragansett basin massachusetts rhode island contains documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 373 pm 2 end document ma rhyolite that is unexpectedly 60 m yr older than sedimentary strata containing late pennsylvanian floras higher in the basin fill the transitional alkalic geochemistry of the rhyolite and associated basalt signifies late devonian rifting pre dating major basin subsidence surface rifting was accompanied by voluminous alkalic intrusions of the 380 370 m

differential response of zircon u pb isotopic systematics to metamorphism across a lithologic boundary an example from the hope valley shear zone southeastern massachusetts usa a detailed morphological chemical and isotopic study of zircons from a single outcrop of two mineralogically and chemically distinct units of the late precambrian ponaganset gneiss was undertaken to investigate the effects of mylonitization and metamorphism on u pb isotopic systematics late paleozoic amphibolite grade approx 600 c mylonitization of the ponaganset gneiss at this locality is associated with movement along the hope valley shear zone the response of zircon to metamorphism in each gneiss unit is distinct zircons in gray augen gneiss are uncorroded and not overgrown whereas zircons from fluorite bearing pink granitic gneiss are variably corroded and over 50 bear opaque overgrowths the zircon overgrowths are chemically distinct from the primary cores and contain high conentrations of hf u hree and th mylonite derived from the gray gneiss contains only a small population of hf u rich metamorphic zircon but zircons in the pink gneiss derived mylonite are dominated by the hf u rich metamorphic component in terms of their u pb isotopic systematics overgrowth free zircons from both units are markedly discordant gray 10 20 pink 35 but overgrown zircons from the pink gneiss are up to 70 discordant zircons from the mylonites yield younger pb pb and u pb ages than those of the protolith gneisses and isotopic data from each gneiss mylonite pair define a linear array on concordia plots upper intercept ages of the gray gneiss 621 27 ma and the pink gneiss 635 50 ma indicate that the crystallization of both units was coeval and the lower intercept ages gray 270 92 ma pink 285 26 ma fall within the range of other published age estimates for alleghanian metamorphism in southeastern new england e g zartman et al 1988 new growth of zircon suggests that zr was mobile during metamorphism the presence of fluorite in the pink gneiss and a discontinuity in log left f text h text 2 text o mathord left vphantom f text h text 2 text o f text hf right kern nulldelimiterspace f text hf right values obtained from biotite across the pink gneiss gray gneiss contact indicates that dissolution and reprecipitation of zircon may be related to local variations in hf fugacity zircon dissolution reprecipitation in the pink gneiss and the lack of similar features in the contiguous gray gneiss suggests that the degree of isotopic perturbation of zircon during metamorphism is related to bulk chemistry fluid chemistry and or the degree of fluid rock interaction

late paleozoic to cenozoic history of the offshore sydney basin atlantic canada the sydney basin covers a large offshore area south of newfoundland with a well exposed outcrop belt on cape breton island the geological history of the poorly known offshore area is interpreted using an industry seismic grid and lithoprobe line 86 5 tied to outcrops and two wells the mid devonian to upper carboniferous permian basin fill is 6 7 km thick and represents three extensional phases with intervening and succeeding compressive phases the mid devonian mcadams lake formation was deposited in a local half graben during early post acadian extension following deformation a suite of early carboniferous extensional basins filled mainly with horton group conglomerates developed on northeast trending and southeast dipping master faults some faults developed along acadian terrane boundaries the windsor group extends over the master faults to onlap basement as a result of thermal sag and visean eustatic rise mid carboniferous deformation linked to the alleghanian orogeny reactivated faults an

crustal thickening by tectonic wedging of the ganderian rocks southern new england usa evidence from cataclastic zircon microstructures and u pb ages abstract zircon grains in granites from avalonian and ganderian rocks in southern new england host brittle internal structures that are interpreted here to show the host rocks to be allochthonous significant among these structures are inherited cores that are commonly fractured and even dismembered this intense fracturing is interpreted as reflecting an important cataclastic event the maximum age of fracturing is constrained to be younger than the age of the youngest broken magmatic core 360 4 ma the minimum age must be older than the oldest zircon rim that heals these fractures that is early permian the occurrence of cataclastic zircon requires that these rocks were relatively cold i e in the upper crust during the carboniferous and therefore were not involved in the pervasive high grade acadian metamorphism the present map pattern of inter tonguing high grade and low grade slices of peri gondwanan rocks shows that they were assembled by crustal scale tectonic wedging the consequent thickening ultimately led to anatexis in the lower slices during the permian thus evidence for cataclasis of zircon grains from the avalon and gander terranes of southern new england suggests that these peri gondwanan terranes remained outboard of laurentia during the well documented early devonian acadian orogeny and were first fully accreted to north america during the alleghanian permian orogeny

the distribution style and intensity of alleghanian metamorphism in south central new england petrologic evidence from the pelham and willimantic domes abstract alleghanian metamorphism in south central new england is characterized by late pennsylvanian documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape mathrm kyanite mathrm muscovite mathrm staurolite end document grade heterogeneous reconstitution of migmatitic documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepack

40 ar 39 ar dating of cleavage formation in tuffs during anchizonal metamorphism 40ar 39ar incremental release analyses were carried out on whole rock and constituent white mica illite rich size fractions 0 63 1 to 6 3 20 m within two very low grade penetratively cleaved metatuffs of contrasting anchizonal metamorphic grade northeastern rheinisches schiefergebirge federal republic of germany one sample from the upper anchizone displays internally concordant 40ar 39ar spectra with plateau ages ranging between ca 316 and 325 ma these are similar to conventional k ar ages determined for the whole rock and size fractions together the isotopic results suggest that cleavage formed at ca 320 ma during a concomitant very low grade metamorphism this is consistent with biostratigraphic controls which suggest that metamorphism and cleavage formation occurred during the westphalian

deformation of upper carboniferous coal measures in the sydney basin evidence for late alleghanian tectonism in atlantic canada upper carboniferous stephanian coal measures of the sydney basin are deformed by reverse faults and folds with dips of up to 50 on the limbs these structures represent compressional or transpressional events donkin episode of unknown sequence and timing offshore swath bathymetry data indicate that northwest striking reverse faults are folded by prominent northeast trending folds vitrinite reflectance patterns of oriented coal samples from the fold limbs are uniaxial negative with minimum reflectance axes rmin near normal to bedding and fanning across fold axes this pattern implies that the bulk of coalification took place through burial compaction without directed horizontal stress prior to the deformation that produced the folds slight divergence between rmin axes and poles to bedding may reflect slight tilting associated with differential subsidence or early reverse faulting before or during coalification or may lie within analytical error one moderately biaxial sample suggests that some deformation accompanied coalification locally available apatite fission track data for the sydney basin indicate that cooling of the coal measures to below 100 20 c was underway by the middle to late triassic or earlier because cooling implies that coalification was largely complete the fission track results are compatible with latest paleozoic or early mesozoic deformation we infer that the donkin episode represents late alleghanian permian compression within the appalachian orogen if so the sydney structures are manifestations of a little known tectonic episode in atlantic canada however mesozoic cenozoic deformation cannot be ruled out resume les series houilleres du bassin de sydney carbonifere superieur stephanien sont deformees par des failles inverses et des plis deverses dont les flancs atteignent des pendages de 50o ces structures representent des evenements de compression ou de transpression episode donkin dont la sequence et la synchronisation sont inconnues les donnees de bathymetrie par secteurs au large indiquent que les failles inverses a direction nord ouest sont pliees par des plis proeminents a tendance nord est les patrons du facteur de reflexion de la vitrinite provenant d echantillons de charbon orientes preleves sur les flancs des plis sont uniaxes negatifs avec des axes de reflectance minimum rmin presque perpendiculaires au litage et se dispersant en travers des axes de plis ce patron implique que la plus grande part de la carbonisation a eu lieu par compaction d enfouissement sans contrainte horizontale directe avant la deformation qui a produit les plis la legere divergence entre les axes rmin et les poles du litage pourrait refleter un leger basculement associe a une subsidence differentielle ou a des failles inverses precoces avant ou durant la carbonisation ou encore cette divergence pourrait etre a l interieur de la fourchette de l erreur analytique notre echantillon moderement biaxe suggere qu une certaine deformation ait localement accompagne la carbonisation pour le bassin de sydney les donnees disponibles de datation par traces de fission sur l apatite indiquent que le refroidissement des series houilleres a des temperatures inferieures a 100 20 oc etait deja en cours au trias moyen a tardif ou plus tot puisque le refroidissement implique que la carbonisation etait presque terminee les resultats de datation par traces de fission concordent avec la deformation au paleozoique terminal ou au mesozoique precoce nous concluons que l episode donkin represente une compression a l alleghanien tardif permien a l interieur de l orogene des appalaches si tel est le cas les structures de sydney sont des evidences d un episode tectonique peu connu au canada atlantique cependant une deformation au mesozoique cenozoique ne peut etre rejetee d emblee

alleghanian polyphase deformation of the hope valley shear zone southeastern new england structural petrographical and rb sr isotopic studies in the highly deformed basement rocks of southern rhode island have allowed the identification of the southern continuation of the hope valley shear zone hvsz previous work recognized the hvsz as an alleghanian tectonic boundary separating the distinct late precambrian esmond dedham and hope valley terranes in southeastern new england two important components of the esmond dedham terrane the late precambrian ponaganset gneiss and the devonian scituate granite gneiss now have been traced with structural continuity into southern rhode island the distribution of these gneissic units and their structures shows that the southern segment of the hvsz has been strongly deformed abruptly deflecting it from a nw se orientation to a n s trend the deflection of the hvsz marks an abrupt structural boundary between two distinct regionally developed mineral lineation domains a northwestern domain with north to nnw trends characteristic of most of the hvsz and a southeastern domain with ne trends structural relations suggest that all mineral lineations were originally parallel along the hvsz with a north to nnw trend the deflection formed during a subsequent phase of ductile right lateral shear producing a clockwise rotation of both lineations and the hvsz in southern rhode island thus basement gneisses in southeastern new england have suffered polyphase alleghanian deformation at high metamorphic grades it has been argued that during the late paleozoic southern new england was located in a structural transition zone between convergent tectonics in the central and southern appalachians and wrench tectonics in the northern appalachians the structural link between these two distinct tectonic regimes is an e w right lateral transcurrent fault the late dextral shear deformation described in southern rhode island basement rocks appears to have resulted from right lateral movement at depth within this transcurrent fault zone

fluid inclusion evidence for basement decompression during permo triassic extension se new england usa co2 bearing fluid inclusions in strongly lineated but weakly foliated late precambrian gneisses within the hope valley shear zone of connecticut and rhode island are of mixed composition xco2 0 1 7 wt nacl equivalent and variable density 0 59 0 86 g ml and occur mainly as isolated inclusions also present are dilute 3 wt nacl equivalent aqueous inclusions which occur on healed fractures related to greenschist facies retrograde metamorphism isochores for dense isolated co2 bearing inclusions indicate pressures of 7 5 9 kbar at 500 600 c the estimated temperature conditions of peak metamorphism published 40ar 39ar hornblende plateau age spectra indicate cooling through about 500 c at 265 5 ma isochores for low density co2 bearing inclusions and aqueous inclusions intersect at the conditions of retrograde metamorphism 325 400 c and indicate pressures of 3 4 kbar published 40ar 39ar biotite plateau ages indicate cooling through about 300 c at 250 5 ma these data define a p t uplift curve for the region which is convex towards the temperature axis and indicate uplift rates between 0 4 and 3 3 mm year in permian time exhumation of basement gneisses was coeval with normal west down motion along the regional basement cover contact honey hill lake char willimantic fault system and is interpreted as due to post orogenic extensional collapse of the alleghanian orogeny

a shear zone origin for alleghanian permian multiple deformation in eastern massachusetts the area around worcester massachusetts has been used to determine which deformational and metamorphic features are due to the alleghanian permian orogeny and which are pre alleghanian isolated fault bounded inliers of carboniferous rocks display evidence of a single metamorphism and the formation of two prominent cleavages the first cleavage formed synchronously with metamorphism pre carboniferous metasedimentary rocks have been affected by two metamorphisms and contain three prominent cleavages the initial cleavage formed during the first metamorphism and the second cleavage formed during the second retrogressive metamorphism thus the second two cleavages and the second metamorphism in pre carboniferous rocks are interpreted as alleghanian normal displacements on two distinct faults are bracketed by the formation of the first and second alleghanian cleavages the initial faulting along the newly defined wachusett mylonite zone wmz formed a wide zone of ductile mylonites which dips moderately to the northwest and contains elongation lineations which trend northwest the second faulting along the clinton newbury fault cnf occurred along a steeply inclined plane which cuts the wmz mylonites and contains a thin zone of phyllonites and mylonites which have elongation lineations which trend west alleghanian cleavages and metamorphism are confined to a mappable zone which is approximately 15 km wide where well defined this zone is interpreted as a ductile shear zone which moved twice forming the first and second alleghanian cleavages early formed pre alleghanian metamorphic minerals are retrograded to hydrous phyllosilicates and new hydrous minerals formed in the shear zone indicating that it was a pathway for fluid flow the initial motion was left lateral with a thrust component the second cleavage formed during top to the northwest normal displacements thus the history of alleghanian tectonism in this area began with sinistral faulting and then included three displacements along normal faults or shear zones this suggests that the strain history included two distinct episodes with the second experiencing slightly different strain orientations at different times resulting in first wmz normal motion then cnf normal motion and finally normal displacements related to the second alleghanian cleavages this history agrees well with other work on the nature of the alleghanian orogeny in the northeast united states

theoretical derivation of a brie like fluid mixing law prediction of the velocity of acoustic waves in partially saturated rocks is very important in geophysical applications the need to accurately predict acoustic velocities has resulted in a widespread popularity of brie s effective fluid mixing law this empirical model together with gassmann s formula are used routinely in fluid substitution problems in petroleum geophysics and seismic monitoring of carbon capture and storage most attempts to justify brie s model have been focused on interpretation in terms of patchy saturation models and attaching meaning to the brie parameter in terms of the patch size in this paper using a microstructural description of the rock and a parameter relating to capillary pressure we calculate an effective fluid modulus that is very similar to brie s law the fluid mixing law we propose is independent of frequency and has a solid theoretical foundation this proposed law produces analytically harmonic and arithmetic averaging at the endpoints our results indicate that brie like behaviour may not necessarily be related to frequency and patch size dependent phenomena

p t t paths and differential alleghanian loading and uplift of the bronson hill terrane south central new england late paleozoic u pb ages of sphene and 4 0 ar 3 9 ar cooling ages of amphibole and muscovite from rocks of the bronson hill terrane in connecticut and central massachusetts reflect a late paleozoic alleghanian overprinton acadian metamorphic rocks prograde alleghanian sphenes crystallized during the late pennsylvanian and eliminate the possibility that amphibole ages reflect delayed permian cooling from devonian acadian metamorphism fourteen new amphibole ages from connecticut form a north to south trend of decreasing age from 294 to 245 ma while in massachusetts four new amphibole ages together with three others from the literature produce a random carboniferous pattern seven new muscovite ages support existing data indicating uniform cooling throughout the bronson hill terrane through 350 c in the early triassic the rate of permian cooling defined by amphibole muscovite pairs increases from 4 c my in northern connecticut to 50 c my near long island sound hinged loading and hinged but delayed exhumation in the southern part of the bronson hill terrane with the hinge in central connecticut explain these ages and cooling rates as well as a southerly increasing metamorphic field gradient one dimensional thermal modeling indicates that loading of bronson hill rocks must have begun by the late mississippian the time of peak alleghanian metamorphic temperature decreases southward from early permian in northern connecticut to late permian to the south these results demonstrate that the metamorphic effects of the alleghanian orogeny are not restricted to the avalon terrane of southeastern new england on the contrary the alleghanian orogeny reset 4 0 ar 3 9 ar mineral ages recrystallized minerals partially melted felsic rocks and transposed fabrics at least as far west as the bronson hill terrane in south central new england

40ar 39ar ages of detrital muscovite and whole rock slate phyllite narragansett basin ri ma usa implications for rejuvenation during very low grade metamorphism late pennsylvanian sedimentary rocks in the narragansett basin were metamorphosed lower anchizone to sillimanite grade during late paleozoic regional metamorphism at ca 275 280 ma twenty five variably sized concentrates of detrital muscovite were prepared from samples collected within contrasting low grade areas diagenesis lower greenschist facies microprobe analyses suggest that the constituent detrital grains are not chemically internally zoned however some grains within several concentrates display very narrow 25 m compositionally distinct low grade epitaxial peripheral overgrowths detrital muscovite concentrates from the lower anchizone are characterized by internally concordant 40ar 39ar age spectra which define plateau ages of ca 350 360 ma these are interpreted to date post devonian acadian cooling within proximal source areas concentrates from lower grade sectors of the middle anchizone display slightly discordant spectra in which apparent ages systematically increase from ca 250 275 ma to define intermediate and high temperature plateaus of ca 360 400 ma detrital muscovite within samples from higher grade sectors of the middle anchizone and the upper anchizone are characterized by systematic low age discordance throughout both low and intermediate temperature increments high temperature ages only range up to ca 330 ma six size fractions of detrital muscovite from a sample collected within the lower greenschist facies have similarly discordant spectra in which apparent ages increase slightly throughout the analyses from ca 250 ma to 275 ma the detrital muscovite results are interpreted to reflect variable affects of late paleozoic regional metamorphism however it is uncertain to what extent the systematic low age spectra discordance reflects intracrystalline gradients in the concentration of 40ar and or experimental evolution of gas from relatively non retentive epitaxial overgrowths however low age discordance occurs regardless of the extent of epitaxial overgrowth intermediate temperature increments evolved during 40ar 39ar whole rock analyses of five slate phyllite samples are characterized by internally consistent apparent k ca ratios these are attributed to gas evolved from constituent very fine grained white mica samples from lower grade portions of the middle anchizone are characterized by intermediate temperature apparent ages which systematically increase from ca 275 300 ma to ca 360 375 ma before evolution of a high temperature contribution from detrital plagioclase feldspar this age variation may reflect partial late paleozoic rejuvenation of very fine grained detrital material with a source age similar to that for the detrital muscovites slate phyllite samples from upper sectors of the middle anchizone and from the upper anchizone were completely rejuvenated during late paleozoic metamorphism and record intermediate and high temperature plateau ages of ca 270 290 ma these data document that metamorphic conditions of the lower to middle biotite zone ca 325 350 c are required to completely rejuvenate intracrystalline argon systems of detrital muscovite therefore the 40ar 39ar dating method may be useful in determination of detrital muscovite provenance and in resolution of the metamorphic evolution of low grade terranes

contrasting p t t paths thermochronologic evidence for a late paleozoic final assembly of the avalon composite terrane in the new england appalachians strongly contrasting pressure temperature time paths for the avalon composite terrane and the structurally overlying putnam nashoba zone in eastern new england obtained from thermochronologic and thermobarometric data are best explained by a late paleozoic underthrusting of cover rocks by the avalon composite terrane we present new ar and u pb thermochronologic data that show that in the southern hope valley zone permian 280 ma anatectic metamorphic conditions of 700 c and 6 kbar were quenched by relatively rapid cooling 12 c m y and exhumation 0 5 km m y for 40 m y in contrast peak metamorphic conditions in the putnam nashoba zone predate silurian intrusions and slower cooling 3 5 c m y began at about 400 ma one dimensional thermal modeling suggests that these two belts were not in thermal equilibrium during the permian metamorphism of the avalon composite terrane because of the absence of high grade alleghanian metamorphism in rocks overlying the avalon terrane we conclude that high grade alleghanian metamorphism in the avalon terrane occurred east of rocks now overlying it and that significant motion between avalon and this cover occurred after peak alleghanian metamorphism similarly contrasting metamorphic histories between avalon inliers willimantic window massabesic complex gneiss pelham dome and their cover rocks reveals the regional significance of this boundary the core rocks all show permian cooling but the cover rocks show post acadian cooling ages decreasing from east to west to the pelham area where hornblende ages in avalon and cover differ by only 35 rather than 80 m y model calculations show that thermal equilibrium between instantaneously thrusted blocks of rocks is generally obtained in tens of millions of years consequently underthrusting of avalon is constrained to be middle mississippian or younger because the leading edge of the underthrusting block would have been heated the longest and would have most closely approached thermal equilibrium with its cover core rocks of the pelham dome must have been relatively close to this leading edge thus carboniferous to permian underplating from a generally eastward direction best explains these thermochronologic relationships

a tectonic model for the late paleozoic of southeastern new england hornblende and biotite sup 40 ar sup 39 ar age spectra from rocks in south central connecticut help define a permian triassic cooling curve for the area together with petrologic and structural information a time temperature pressure strain path is established similar data for the narragansett basin in rhode island and massachusetts allow correlation of the late paleozoic histories of the two areas together these data suggest that in the late paleozoic south central new england was part of a fold thrust belt and the narragansett basin was a retroarc foreland basin nw se compression during the final assembly of pangaea resulted in se directed thrusting causing the development of clastic wedges in adjacent rhode island and massachusetts in the late carboniferous early permian a clockwise rotation of this deformation from nw to nne led to northward underthrusting and concomitant uplift of both eastern connecticut and rhode island in the permian and triassic

alleghanian paleostress reconstruction in the northern appalachians intraplate deformation between laurentia and gondwana a numerical paleostress tensor analysis using striated fault planes has been conducted in the quebec reentrant of the northern appalachians to characterize the stress field of late paleozoic deformations three directions of maximum compressional stress axes s1 have been found and correlated to 1 an early north northwest south southeast compression 2 a north northeast south southwest compression and 3 a late west northwest east southeast compression fault populations associated with these stress regimes are present in all tectonic zones of the quebec and northern new brunswick appalachians directions of 1 axes determined in the northern appalachians resemble in orientation and in relative chronology layer parallel shortening fabrics and joint patterns found in the appalachian foreland of the central appalachians the paleostress regimes are interpreted as the record of intraplate deformation related to the indentation of gondwana into laurentia during the late paleozoic alleghanian orogeny