the rock elm meteorite impact structure wisconsin geology and shock metamorphic effects in quartz the rock elm structure in southwest wisconsin is an anomalous circular area of highly deformed rocks 6 5 km in diameter located in a region of virtually horizontal undeformed sedimentary rocks shock produced planar microstructures pms have been identified in quartz grains in several lithologies associated with the structure sandstones quartzite pebbles and breccia two distinct types of pms are pres ent p1 features which appear identical to planar fractures pfs or cleavage and p2 features which are interpreted as possible incipient planar deformation features pdfs the latter are uniquely produced by the shock waves associated with meteorite impact events both types of pms are oriented parallel to specific crystallographic planes in the quartz most commonly to c 0001 11 batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document overline 2 end document 2 and r z 10 batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document overline 1 end document 1 the association of unusual structurally deformed strata with distinct shock produced microdeformation features in their quartz bearing rocks establishes rock elm as a meteorite impact structure and supports the view that the presence of multiple parallel cleavages in quartz may be used independently as a criterion for meteorite impact preliminary paleontological studies indicate a minimum age of middle ordovician for the rock elm structure a similar age estimate 450 400 ma is obtained independently by combining the results of studies of the general morphology of complex impact structures with estimated rates of sedimentation for the region such methods may be applicable to dating other old and deeply eroded impact structures formed in sedimentary target rocks

geochemical and sr nd isotopic constraints on the mantle source of neoproterozoic mafic dikes of the rifted eastern laurentian margin north central appalachians usa abstract abundant and widely distributed unmetamorphosed mafic dikes intrude mesoproterozoic rocks of the new jersey highlands the age of the dikes is imprecisely known but interpreted to fall between 615 and 576 ma which is consistent with the range of ages of mafic dikes from labrador and nova scotia south to pennsylvania that were emplaced along the rifted eastern laurentian margin new jersey highlands dikes are a few cm to 18 m wide and have lengths of as much as several km they have sharp largely discordant contacts against enclosing mesoproterozoic rocks and aphanitic chilled margins that grade into coarser grained interiors columnar joints are present locally and suggest emplacement at a shallow crustal level geochemical compositions of the dikes range from alkalic to less common tholeiitic basalt having generally high tio2 p2o5 zr nb y and la yb and low mgo cr and ni tio2 contents define high ti and low ti dikes that differ in high field strength elements hfse and light rare earth elements lree but overlap in abundances of most other elements dike magma evolved in an ascending mantle plume of oib like asthenosphere from enriched higher tio2 compositions to more depleted lower tio2 compositions subtle differences in the dike compositions are due to variations in the amount of partial melting within the plume and the depth of melt segregation sr nd isotope values of both dike compositions overlap and are characterized by end t of 1 5 to 3 8 and initial 87sr 86sr ranging from 0 7032 to 0 7077 higher sr isotope ratios are interpreted as resulting from local interaction of the dike magma with heterogeneous high 87sr 86sr lithospheric mantle having emi or emii like geochemical characteristics dikes form tabular structures that have long segments striking an average of n44 e and short segments striking about east west their regional geometries form right stepping rhomb shaped patterns due to emplacement into rift related dilational fractures likely formed through a combination of southeast directed extension and strike slip shear stresses geochemical compositions of the dikes are the same regardless of their structural trend or location implying they formed during a single magmatic event they along with other mafic dikes in the north central appalachians were emplaced in a within plate tectonic setting along the rifted margin of eastern laurentia prior to opening of the iapetus ocean

a new interpretation of the structure of the sept iles intrusive suite canada the layered mafic intrusion at sept iles canada is one of the largest intrusions in the world a new interpretation of its structure is proposed based on a review of its geology and a comparison with the skaergaard intrusion greenland several different magmatic components are recognized hence the name sept iles intrusive suite siis is proposed emplacement of the suite may have been preceded by eruption of flood basalts the first magmas of the suite rose in the crust to accumulate beneath the density filter afforded by the basalts the largest component is the sept iles mafic intrusion simi the lower series of the simi is dominated by leucotroctolites and leucogabbros above it lie the layered series which is largely comprised of gabbro and troctolite both these units are unchanged from earlier interpretations the anorthosites s l gabbros and monzogabbros formerly called the transitional series are now considered to be the upper border series developed by floatation of plagioclase common autoliths in the layered series are parts of the hydrothermally altered upper border series from towards the interior of the intrusion which have foundered and settled through the magma the contamination of the magma that accompanied this event oxidised iron in the magma and led to the precipitation of magnetite around the periphery of the intrusion the subsequent depletion of fe3 and or increase in sio2 cao and p2o5 may have induced apatite saturation and accumulation to form two layers rich in apatite near the base and at top of the layered series granitic magma was developed by fractional crystallisation and was emplaced along the roof of the chamber where it acquired large quantities of xenoliths these were probably derived from the flood basalts their evolved members and fragments of mafic dykes chilled by the granitic magma accumulations of monzonite pillows in this unit testify to another magmatic event and a floor to the granitic magma chamber indicating lateral transport of magma chemically distinct syenites in the upper part of the intrusion are part of the point du criade intrusion a large late composite sill diabase and leucogabbro components show a close link with the simi and all the acidic magmas may have originally formed by differentiation of the main magma in cupolas towards the centre of the intrusion a series of late gabbro intrusions that cut the simi may represent a rejuvenation of magmatism the border zone is a mass of fine grained rocks that occurs along the border of the simi it may be another magmatic component or just the lateral border series of the simi

the first description and confirmation of the vista alegre impact structure in the parana flood basalts of southern brazil abstract the vista alegre structure centered at 25 57 s and 52 41 w has been recently proposed as a meteorite impact structure the 9 5 km diameter structure is located in the parana state of southern brazil within the parana basin which contains one of the largest and most extensive flood basalt provinces on earth the parana flood basalts belong to the serra geral formation and are temporally related to the opening of the south atlantic ocean having been dated at about 133 132 ma tholeiitic basalts dominate the western portion of parana state with some minor rhyodacites morphologically vista alegre has a prominent circular outline in the form of an incomplete ring of escarpments and an inner depression the presence of a central uplift is not obvious but it is inferred by the occurrence of deformed sandstone blocks near the center of the structure these sandstones are possibly related to the triassic piramboia formation and or to the cretaceous botucatu formation these units are normally at stratigraphic depths of about 700 800 m below the present surface in this portion of the parana basin the structure appears to be in an advanced erosion stage and its interior is occupied by a soil cover several meters thick extensively used for agriculture as a result there are limited outcrops in the interior of the structure all of polymict breccias some of them melt bearing we report the extensive occurrence of shatter cones in the form of fine grained rock clasts within the polymict breccias the shatter cone bearing breccias occur at different locations within the structure separated by several kilometers the nested shatter cones range in size from about 0 5 to 20 cm for individual cones and up to half a meter for complete assemblages the shatter cones formed in fine grained parana flood basalt and might be the first examples of shatter cones in such a rock type in addition planar deformation features pdfs were found in quartz grains within sedimentary rock clasts of the polymict breccia these findings confirm the impact origin of the vista alegre structure

scaling impact melting and crater dimensions implications for the lunar cratering record abstract the dimensions of large craters formed by impact are controlled to a large extent by gravity whereas the volume of impact melt created during the same event is essentially independent of gravity this differential scaling fosters size dependent changes in the dynamics of impact crater and basin formation as well as in the final morphologies of the resulting structures a variety of such effects can be observed in the lunar cratering record and some predictions can be made on the basis of calculations of impact melting and crater dimensions among them are the following 1 as event magnitude increases the volume of melt created relative to that of the crater will grow and more will be retained inside the rim of the crater or basin 2 the depth of melting will exceed the depth of excavation at diameters that essentially coincide with both the inflection in the depth diameter trend and the simple to complex transition 3 the volume of melt will exceed that of the transient cavity at a cavity diameter on the order of the diameter of the moon this would arguably correspond to a moon melting event 4 small lunar craters only rarely display exterior flows of impact melt because the relatively small volumes of melt created can become choked with clasts increasing the melt s viscosity and chilling it rapidly larger craters and basins should suffer little from such a process 5 deep melting near the projectile s axis of penetration during larger events will yield a progression in cen tral structure morphology with growing event magnitude this sequence should range from single peaks through multiple peaks to peak rings 6 the minimum depth of origin of central peak material should co incide with the maximum depth of melting the main central peak in a crater the size of tycho should have had a preimpact depth of close to 15 km

the status of salterella as a lower cambrian index fossil in the north american cordillera there is strong evidence that salterella is restricted to the medial part of the bonnia olenellus zone reported younger and older occurrences are attributed to either stratigraphic error or misidentification an unnamed genus with a superficial resemblance immediately postdates salterella but it lacks an inner laminar cone in the cordillera salterella occurs in inner detrital and middle carbonate belt strata that were deposited in the upper half of grand cycle b and the lower half of grand cycle c in and near the appalachian mountains of north america and in greenland spitsbergen and scotland occurrences of salterella are consistent with a medial bonnia olenellus zone age however this cannot be as rigorously tested as in the cordillera because of fewer taxa associated with the genus and in super and subjacent strata outer detrital basinal strata of the taconic allochthon contain salterella but it is uncertain whether the fossils are of local or derived origin

maximum magnitude mmax in the central and eastern united states for the 2014 u s geological survey hazard model probabilistic seismic hazard assessment psha requires an estimate of m max the moment magnitude m of the largest earthquake that could occur within a specified area sparse seismicity hinders m max estimation in the central and eastern united states ceus and tectonically similar regions worldwide stable continental regions scrs a new global catalog of moderate to large scr earthquakes is analyzed with minimal assumptions about enigmatic geologic controls on scr m max an earlier observation that scr earthquakes of m 7 0 and larger occur in young 250 23 ma passive continental margins and associated rifts but not in cratons is not strongly supported by the new catalog scr earthquakes of m 7 5 and larger are slightly more numerous and reach slightly higher m in young passive margins and rifts than in cratons however overall histograms of m from young margins and rifts and from cratons are statistically indistinguishable this conclusion is robust under uncertainties in m the locations of scr boundaries and which of two available global scr catalogs is used the conclusion stems largely from recent findings that 1 large southeast asian earthquakes once thought to be scr were in actively deforming crust and 2 long escarpments in cratonic australia were formed by prehistoric faulting the 2014 seismic hazard model of the u s geological survey represents ceus m max as four point probability distributions the distributions have weighted averages of m 7 0 in cratons and m 7 4 in passive margins and rifts these weighted averages are consistent with m max estimates of other scr pshas of the ceus southeastern canada australia and india

geochemistry and u pb zircon age of comenditic metafelsites of the tibbit hill formation quebec appalachians metafelsites in waterloo area quebec represent the only known silicic volcanic rocks in the predominantly basaltic tibbit hill formation low grade metamorphism accompanied by hydration and albitization has converted the felsic volcanic rocks mainly to muscovite quartz albite schists the volcanic parent of these metafelsites was formed partly as lava flows and partly as tuffs the principal compositional type was a comendite a component of intermediate rocks is also present but its extent is undetermined and probably minor u pb zircon studies of the metafelsites have yielded a reliable age of this early cambrian age is probably representative of the age of the tibbit hill formation as a whole the tibbit hill formation accumulated at one of the clearest examples of a rrr rift rift rift triple junction the sutton mountains triple junction of the continental rift system formed as a prelude to the opening of the iapetus ocean its volcanic rocks are products of the youngest major episode of rift rela

paleomagnetism of the callander complex and the cambrian apparent polar wander path for north america the 7 km2 circular callander alkaline complex was emplaced into anorthositic and granitic gneisses of the grenville province in the canadian shield about 575 5 ma ago at the start of the cambrian the complex has not been subsequently metamorphosed or tilted detailed alternating field and thermal step demagnetization of 252 specimens from 29 sites led to the identification of a characteristic a magnetization component with a direction of d 82 2 i 82 7 95 3 1 k 83 n 26 sites in 5 sites of mesocratic to leucocratic syenite from the core of the complex in 5 sites of fenitized host rock from its aureole and in 16 sites of lamprophyre from radiating dikes isothermal remanent magnetization tests show that the a component is retained by both magnetite and hematite in a complete spectrum of domain sizes a reversals test suggests and a contact test shows the a component to be primary its pole position at 46 3 s 121 4 e dp 5 9 dm 6 1 does not fall on published but poorly defined

chemical stratigraphy of the parana lavas south america classification of magma types and their spatial distribution a new classification scheme has been developed to assign the lava flows of the parana continental flood basalt province south america into geochemically distinct magma types with six basaltic major and trace element abundances and or ratios by mapping out the spatial distribution of these magma types within the lava sequences it has been possible to determine the internal stratigraphy of the lava pile on a regional scale previous studies on road profiles traversing the well exposed coastal serra geral escarpment of southern brazil are summarised together with results from some new sampled sections more widespread stratigraphical investigations of the parana lavas have been hampered by the lack of sufficient topographic relief and the cover of sedimentary rocks however access to drill core chippings from nine boreholes in the central parana region has provided a unique opportunity to investigate the stratigraphy of the otherwise inaccessible deeper levels of the lava pile and to map out stratigraphic variations in three dimensions the borehole samples have indicated cated a stacking of units of different magma types all overlapping towards the north which suggests that the main locus of magmatism moved northwards with time within the parana basin this migration could be related to the northward propagation of rifting during the initiation of the south atlantic ocean maps of the surface distribution of samples of each magma type show a pattern consistent with the stratigraphy inferred from the boreholes although suggesting that the shift in magmatism may have been towards the northwest on the basis of geochemical similarities between magma types and their inferred stratigraphical relationships it is proposed that the parana can be divided into two principal magmatic centres 1 an older one in the south comprising the gramado esmeralda and urubici magma types and 2 a younger one developed about 750 km to the north formed by the pitanga paranapanema and ribeira magma types

petrology of the grenville swarm of gabbro dikes canadian precambrian shield the grenville tholeiitic dikes of late proterozoic or early paleozoic age cut marbles and gneisses of the grenville province within and adjacent to the ottawa rift structure where traversed the swarm consists of about 40 large dikes up to 100 m wide representing a crustal extension of 1 km the principal minerals are olivine augite pigeonite plagioclase magnetite ilmenite k feldspar and quartz crystals of pigeonite around olivine and complex augite pigeonite composite grains suggest that the reactions olivine melt pigeonite and pigeonite melt augite have taken place conspicuous zoning and grain to grain variation in the composition of augite are consistent with fractional crystallization but crystal melt equilibrium during crystal growth is indicated by a restricted range in pyroxene paleotemperatures of 1180 1060 c rock textures subophitic combined ophitic subophitic and equigranular in dike centres are related to dike width and are determined principally by the influence of

feather features a low shock pressure indicator in quartz 1 feather features are a recently discovered type of microstructure in shocked quartz they occur as short parallel to subparallel lamellae with spacing similar to that of planar deformation features pdfs these lamellae are always found in combination with a planar fracture pf feather features are crystallographically controlled 0001 101 and 112 orientations are common while 103 and 102 orientations are lacking samples from the nordlinger ries and the matt wilson impact structures are compared with plane wave shock recovery experiments 16 gpa microscopic analysis of quartz grains with feather features show that their formation is linked to shearing along the associated pfs during shock deformation the generation of shear fractures with feather features in plane wave shock recovery experiments suggests a pressure range of 7 10 gpa although further constraints are needed for an upper and lower pressure limit the orientation of feather features in rock samples is suggested to be controlled by the direction of the principal axis of stress with sheared pfs at 45 angles to the axis and most feather feature lamellae aligned parallel to the axis 20 the majority of feather feature lamellae emanate from the pfs in the same direction and in experiments point in the direction from which the shock wave came on the basis of their appearance in shocked quartz grains from 26 impact craters and the current lack of reports of these features in endogenically deformed crustal rocks their uniqueness as a new type of impact induced planar microstructure alongside pdfs and pfs is proposed

the extension of grenville basement beneath the northern appalachians results from the quebec maine seismic reflection and refraction surveys from 1983 to 1984 the united states geological survey and the geological survey of canada undertook a series of seismic experiments to determine the crustal structure of the appalachian mountain belt in maine and southeast quebec these experiments included four deep crustal seismic reflection profiles perpendicular to the strike of regional geological trends and several high resolution refraction wide angle reflection profiles both parallel and perpendicular to strike here the data recorded in quebec and the northwest part of maine are interpreted in conjunction with existing reflection profiles from quebec synthetic seismogram calculations in laterally varying media are used to model the refraction wide angle reflection data while reprocessing migration and attribute plotting are used in the interpretation of the reflection data a major zone of reflections extending over a distance exceeding 200 km can be traced from shallow depths beneath the st lawrence lowlands southeast to about 25 km depth beneath the southeast edge of the chain lakes massif this zone of reflections is interpreted as a master decollement separating autochthonous grenville basement from overlying allochthonous rocks of the appalachian orogen including foreshortened miogeoclinal rocks the remnants of one or more magmatic arcs the connecticut valley gaspe synclinorium and the chain lakes massif in the vicinity of the chain lakes massif extension has caused grenville basement to be thinned by as much as 40 reflections from the decollement show strong layering and evidence for basement faulting associated with the guadeloupe and other faults normal incidence and wide angle reflections are observed to be almost continuous beneath the baie verte brompton line and connecticut valley gaspe synclinorium indicating that the baie verte brompton line is confined to the upper crust the connecticut valley gaspe synclinorium is characterized by p wave velocities of about 5 3 km s 1 and is shown to be a shallow structure with a maximum depth of about 3 km rock velocities immediately below the connecticut valley gaspe synclinorium are typical of chain lakes material 6 1 km s 1 a new model for the tectonic development of the region is proposed in which all pre silurian units between the baie verte brompton line and the southeastern edge of the chain lakes massif are allochthonous and confined to the upper crust we infer that the chain lakes massif volcanics of the ascot weedon formation and associated deeper crustal rocks were thrust over grenvillian basement as a composite unit and may now underlie parts of the connecticut valley gaspe synclinorium

gravity and magnetic anomalies of the sutton mountains region quebec and vermont expressions of rift volcanics related to the opening of iapetus prominent nearly coincident positive gravity and magnetic anomalies occur in the sutton mountains region centered about 100 km east of montreal quebec several lines of evidence indicate that t

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

paleomagnetism of the 550 ma skinner cove volcanics of western newfoundland and the opening of the iapetus ocean the skinner cove formation of western newfoundland is an alkali volcanic suite transported in the ordovician onto laurentia s iapetus ocean margin it has previously yielded a u pb zircon age of 550 5 3 2 ma has no penetrative deformation and has only been affected by zeolite facies metamorphism at 10 sites flows and dykes of the skinner cove formation exhibit a stable characteristic a remanence carried by magnetite this a remanence is shown to be primary by an intraformational conglomerate test using 20 trachybasalt clasts whereas 11 alkali basalt clasts were overprinted probably because they originally contained titanomagnetite from which rutile exsolved soon after burial by overlying flows after tilt correction the mean a remanence direction is southeast and down d 144 i 32 95 11 k 21 n 10 sites the paleolatitude calculated from the ten a site virtual geomagnetic poles is 19 s 9 which likely represents the paleolatitude of laurentia s iapetan margin at 550 ma evidence that the skinner cove formation originated at this margin includes its structural position within the humber arm allochthon its within plate trace element geochemistry and its similarity in age to other alkali magmatism that more certainly represents laurentia s iapetan margin comparison with other laurentian paleomagnetic data implies that laurentia drifted very rapidly northward from the south polar region to the equator between 570 and 550 ma we suggest that the start of this rapid northward drift at 570 ma marked the onset of iapetus sea floor spreading between laurentia and west gondwana further well constrained paleomagnetic data of 570 to 550 ma age are needed from laurentia to test its high rate of northward drift and from west gondwanan cratons to test that they remained relatively stationary

lithostratigraphic and allostratigraphic framework of the cambrian ordovician potsdam group and correlations across early paleozoic southern laurentia the potsdam group is a cambrian to lower ordovician siliciclastic unit that crops out along the southeastern margins of the ottawa graben from its base upward the potsdam consists of the ausable hannawa falls and keeseville formations in addition the potsdam is subdivided into three allounits allounit 1 comprises the ausable and hannawa falls and allounits 2 and 3 respectively the lower and upper parts of the keeseville allounit 1 records early to middle cambrian syn rift arkosic fluvial sedimentation ausable formation with interfingering mudstone arkose and dolostone of the marine altona member recording transgression of the easternmost part of the ottawa graben rift sedimentation was followed by a middle cambrian climate change resulting in local quartzose aeolian sedimentation hannawa falls formation allounit 1 sedimentation termination coincided with latest middle cambrian tectonic reactivation of parts of the ottawa graben allounit 2 lower keeseville records mainly upper cambrian quartzose fluvial sedimentation with transgression of the northern ottawa graben resulting in deposition of mixed carbonate siliciclastic strata of the marine riviere aux outardes member sedimentation was then terminated by an earliest ordovician regression and unconformity development allounit 3 upper keeseville records diachronous transgression across the ottawa graben that by the arenigian culminated in mixed carbonate siliciclastic shallow marine sedimentation theresa formation the contact separating the potsdam group and theresa formation is conformable except locally in parts of the northern ottawa graben where the presence of localized islands and or coastal salients resulted in subaerial exposure and erosion of the uppermost potsdam strata and accordingly unconformity development

age of the grenville dyke swarm ontario quebec implications for the timing of lapetan rifting u pb baddeleyite and zircon ages for three diabase dykes from widely spaced localities within the grenville dyke swarm indicate a single age of emplacement at ma the 700 km long grenville dyke swarm located in the southeastern part of the canadian shield was emplaced syntectonically with the development of the ottawa graben this graben may represent a plume generated lapetan failed arm that developed at the onset of the breakup of laurentia other precisely dated lapetan rift related units such as the callander alkaline complex and the tibbit hill formation volcanic rocks indicate a protracted 36 ma period of rifting and magmatism prior to volcanism along this segment of the lapetan margin the age of the grenville dykes is the youngest in a progression of precisely dated mafic magmatic events from the 723 ma franklin dykes and sills to the 615 ma long range dykes along the northern and northeastern margins of laurentia respectively thus the age for these dykes represents a key time marker for

crystallization sequence and magma chamber processes in the ferrobasaltic sept iles layered intrusion canada the sept iles layered intrusion quebec canada 564 ma is a large plutonic body with a diameter of 80 km and a thickness of 6 km made up from its base to top of a layered series with troctolite and gabbro and an upper border series with anorthosite capped by cupolas of a type granite chilled margin compositions suggest a ferrobasaltic parental magma close to that of the skaergaard intrusion but much richer in iron and titanium samples from drill cores and surface sampling of the 4 7 km thick layered series reveal a succession of massive troctolites and layered gabbros that contain 24 fe ti oxide layers cm to m thick and many anorthositic autolithic blocks the sequence of crystallization in the layered series is plagioclase an72 34 and olivine fo75 21 followed by magnetite and ilmenite then ca rich pyroxene and finally apatite an olivine gap is observed between fo66 and fo59 the saturation of fe ti oxides before ca rich pyroxene is interpreted to be the result of the high feot and tio2 contents and the low cao content of the parental magma contamination by old continental crust has occurred during crystallization of the layered series as indicated by sr isotopic compositions 87sr 86sr564 0 70360 0 70497 the differentiation trend of the intrusion is interrupted by two large and many small reversals to more primitive compositions of cumulus phases cr content of magnetite and lower sr isotope ratios these reversals and the intermittent disappearance of some phases are interpreted as resulting from magma chamber replenishments by undifferentiated primitive basaltic magma and mixing with the resident magma the author 2010 published by oxford university press all rights reserved for permissions please e mail journals permissions oxfordjournals org

an early cambrian deltaic fluvial model for an iapetan rift arm drainage system southeastern quebec based on the model that during the rifting stage beginning at ca 590 ma the ottawa graben an iapetan failed arm localized a large river that flowed into the nascent iapetus a search was made for the related fluvial and deltaic deposits the search led to the identification of fluvial deposits as predicted by the model deltaic deposits were also identified despite complications brought about by deformation metamorphism and thrusting although they probably belong to a late phase of delta buildup in the early cambrian older deltaic deposits of the river probably lie buried beneath a volcanic shield that built up at the proximal end of the graben ca 554 ma the successful application of the model supports the paradigm of rift arm failed arm rivers and their deltas as proposed by k burke and j f dewey nearly two decades ago investigations based on this paradigm in appropriate geological situations may provide insights into problems related to continental rifting and breakup and provide inform

chicxulub crater yucatan tektites impact glasses and the geochemistry of target rocks and breccias a comparison of breccias and basement rocks from four drill holes near the chicxulub impact structure yucatan a possible cretaceous tertiary k t boundary crater with black and yellow impact glasses from the haitian k t boundary layer shows that mixtures of andesite like rocks with carbonates and evaporites from chicxulub are compositionally different from the yellow glasses at the time of impact the chicxulub area was covered by evaporitic and carbonate deposits several kilometres thick because impact glasses are generally produced by shock melting of surface rocks a link between the haitian glasses and chicxulub is difficult to understand the 34 s isotopic signature of chicxulub breccias and evaporites is incompatible with that of a continental k t boundary we suggest that the pre impact target stratigraphy at chicxulub as well as the interpretation of the carbonate evaporite rocks as breccias should be revised

40ar 39ar ages for plutons of the monteregian hills quebec evidence for a single episode of cretaceous magmatism 40ar 39ar data for biotite and amphibole from six plutonic complexes of the monteregian petrographic province in quebec canada are presented generally uncomplicated and concordant incremental heating spectra are observed although there is evidence for minor excess 40ar at several localities biotites and amphiboles are concordant consistent with rapid cooling of the high level complexes ages for different phases of individual complexes are analytically indistinguishable and suggest that the various principal lithologies are cogenetic the 40ar 39ar ages of six monteregian complexes st hilaire rougemont johnson shefford brome megantic all fall within the very restricted range of 124 1 m y it is suggested that all of the monteregian plutons with the exception of oka were formed within a short interval 1 2 m y during the cretaceous consistent with paleomagmetic results the new ages along with geochemical characteristics support derivation of the alkaline magmas by a fixed mantle hot spot which also produced broadly similar complexes in new england and the chain of new england seamounts

geochemistry and petrogenesis of the tibbit hill metavolcanic suite of the appalachian fold belt quebec vermont a plume related and fractionated assemblage the late neoproterozoic metavolcanic tibbit hill formation thf covers a large crustal segment exposed along a belt about 250 km long in the appalachian fold belt quebec vermont it is predominantly basaltic in composition but contains a minor component of felsic and intermediate alkaline volcanic lithologies geochemically the tibbit hill formation forms a continuum in composition and exhibits a wide range of sio2 44 76 wt percent covering the entire spectrum from alkali basalt to trachyte and comendite this mildly alkaline suite is relatively enriched in incompatible elements and exhibits a wide range of zr 138 1493 ppm nb 15 139 ppm and y 18 185 ppm concentrations among other elements the concentrations of the hfse and the ree gradually increase toward the more evolved lithologies the chondrite normalized ree patterns are fractionated lree enriched over hree parallel to subparallel and generally uniform but with negative eu anomalies developed in the more felsic varieties these geochemical features underline the comagmatic nature of the entire suite and are consistent with a fractionated basalt to comendite suites the incompat ible element profiles suggest that most of these elements including nb zr ti y and the ree have not been affected by metamorphism as they remained largely intact within the thf rocks chemical features of the mafic rocks are typical of within plate basalts and suggest that their melts were derived from a fertile or plume related mantle source chemical features of the felsic and intermediate rocks are typical of anorogenic a1 type suites related to hotspots mantle plumes or continental rift zones this is consistent with the regional geological context with the volcanism associated with an iapetan rrr triple junction occurring shortly before the onset of seafloor spreading geochemical modelling shows that the thf basaltic magma was produced by a very small degree of batch partial melting f 2 5 percent of a garnet bearing primitive mantle source garnet lherzolite a final basaltic melt segrega tion depth is estimated at 80 to 100 km melting probably occurred within the thermal anomaly of a rising mantle plume beneath the sutton mountains triple junction near the quebec maine border fractionation of thf basaltic magma produced minor trachytic and comenditic magmas the volcanic assemblage of the afar rift for example boina centre appears to represent a modern analogue to the thf volcanic suite

a late precambrian rift related igneous suite in western newfoundland a granite that yields a u pb zircon age of 602 10 ma is associated with mafic and silicic volcanic rocks and metamorphic equivalents near deer lake in western newfoundland the granitic rocks are named the round pond granite and the combined granite volcanic suite is assigned to the hughes lake complex all of the rocks are contained in the hughes lake structural slice that occurs above other allochthonous rocks and the autochthonous cambrian ordovician carbonate sequence of western newfoundland the round pond granite is cut by metadiabase dykes mafic volcanic rocks interpreted as coeval with the dykes occur along the southeast side of the granite a thick sequence of arkosic metagreywackes and psammitic to pelitic schists of the mount musgrave group occurs stratigraphically above the mafic volcanic rocks regional correlations imply that the mount musgrave group is of late precambrian early cambrian age thus setting an upper stratigraphic limit to the age of the hughes lake complex perthitic and

geology of lonar crater india lonar crater india is one of the youngest and best preserved impact structures on earth the 1 88 km diameter simple crater formed entirely within the deccan traps making it a useful analogue for small craters on the basaltic surfaces of the other terrestrial planets and the moon in this study we present a meter scale resolution digital elevation model geological map of lonar crater and the surrounding area and radiocarbon ages for histosols beneath the distal ejecta impact related deformation of the target rock consists of upturned basalt fl ows in the upper crater walls and recumbent folding around rim concentric subhorizontal noncylindrical fold axes at the crater rim the rim fold hinge is preserved around 10 15 of the crater although tearing in the rim fold is inferred from fi eld and paleomagnetic observations no tear faults are identifi ed indicating that large displacements in the crater walls are not characteristic of small craters in basalt one signifi cant normal fault structure is observed in the crater wall that offsets slightly older layer parallel slip faults there is little fl uvial erosion of the continuous ejecta blanket portions of the ejecta blanket are overlain by aerodynamically and rotationally sculpted glassy impact spherules in particular in the eastern and western rim as well as in the depression north of the crater known as little lonar the emplacement of the continuous ejecta blanket can be likened to a radial groundhugging debris fl ow based on the preserved thickness distribution of the ejecta the effi cient exchange of clasts between the ejecta fl ow and the underlying histosol and the lack of sorting and stratifi cation in the bulk of the ejecta the ejecta profi le is thickened at the distal edge and similar to fl ejecta structures observed on mars

a model for iapetan rifting of laurentia based on neoproterozoic dikes and related rocks geologic evidence of the neoproterozoic rifting of laurentia during breakup of rodinia is recorded in basement massifs of the cratonic margin by dike swarms volcanic and plutonic rocks and rift related clastic sedimentary sequences the spatial and temporal distribution of these geologic features varies both within and between the massifs but preserves evidence concerning the timing and nature of rifting the most salient features include 1 a rift related magmatic event recorded in the french broad massif and the southern and central shenandoah massif that is distinctly older than that recorded in the northern shenandoah massif and northward 2 felsic volcanic centers at the north ends of both french broad and shenandoah massifs accompanied by dike swarms 3 differences in volume between massifs of cover sequence volcanic rocks and rift related clastic rocks and 4 wnw orientation of the grenville dike swarm in contrast to the predominately ne orientation of other neo proterozoic dikes previously proposed rifting mechanisms to explain these features include rift transform and plume triple junction systems the rift transform system best explains features 1 2 and 3 listed here and we propose that it represents the dominant rifting mechanism for most of the laurentian margin to explain feature 4 as well as magmatic ages and geochemical trends in the northern appalachians we propose that a plume triple junction system evolved into the rift transform system a ca 600 ma mantle plume centered east of the sutton mountains generated the radial dike swarm of the adirondack massif and the grenville dike swarm and a collocated triple junction generated the northern part of the rift transform system an eastern branch of this system produced the long range dike swarm in newfoundland and a subsequent western branch produced the ca 554 ma tibbit hill volcanics and the ca 550 ma rift related magmatism of newfoundland mwr206 20 page 1

geology and impact features of vargeao dome southern brazil abstract vargeao dome southern brazil is a circular feature formed in lava flows of the lower cretaceous serra geral formation and in sandstones of the parana basin even though its impact origin was already proposed in the 1980s little information about its geological and impact features is available in the literature the structure has a rim rim diameter of approximately 12 km and comprises several ring like concentric features with multiple concentric lineaments the presence of a central uplift is suggested by the occurrence of deformed sandstone strata of the botucatu and piramboia formations we present the morphological structural characteristics of vargeao dome characterize the different rock types that occur in its interior mainly brecciated volcanic rocks bvr of the serra geral formation and discuss the deformation and shock features in the volcanic rocks and in sandstones these features comprise shatter cones in sandstone and basalt as well as planar microstructures in quartz a geochemical comparison of the target rock equivalents from outside the structure with the shocked rocks from its interior shows that both the bvrs and the brecciated sandstone have a composition largely similar to that of the corresponding unshocked lithologies no traces of meteoritic material have been found so far the results confirm the impact origin of vargeao dome making it one of the largest among the rare impact craters in basaltic targets known on earth

u pb baddeleyite ages distribution and geochemistry of 925 ma mafic dykes and 900 ma sills in the north china craton evidence for a neoproterozoic mantle plume numerous neoproterozoic mafic dykes referred to as the dashigou swarm are identified in the central and southeastern parts of the north china craton ncc they are 305 010 degrees trending dykes with widths of similar to 10 100 m and exposed lengths of several to 10 km precise u pb isotope dilution thermal ionization mass spectrometry id tims measurements on baddeleyite grains separated from three dykes yield 207 pb 206 pb average ages of 924 0 3 7 ma dashigou dyke 921 8 2 6 ma yangjiaogou dyke and 925 8 1 7 ma taohuagou dyke baddeleyite grains from a late stage pegmatite vein in the dashigou dyke were analyzed by secondary ion mass spectrometry sims methods these yield a 207 pb 206 pb average age of 920 4 5 7 ma the dashigou dyke swarm exhibits an overall radiating geometry the overall fan angle is about 60 degrees with a focal point located along the southern margin of the eastern ncc where a large ca 900 ma sill swarm was previously recognized the rift system hosting these slightly younger sills named the xu huai rift system could represent two breakup parallel arms of a rift rift rift triple junction related to the initiation of the magma center that produced the dashigou dykes the sills have similar characteristics and could be cogenetic with the dashigou dykes the dashigou dykes are coarse grained composed mainly of clinopyroxene and plagioclase feldspar with or without olivine one of the most primitive dykes has 47 79 wt sio 2 6 41 wt mgo 1 38 wt tio 2 17 77 wt al 2 o 3 10 47 wt cao and 0 62 wt k 2 o it shows slight enrichment in light rare earth elements and a slightly positive eu eu anomaly 1 1 and is slightly depleted in high field strength elements compared to neighboring elements on a primitive mantle normalized spidergram the dashigou dykes show some similarities with enriched mid ocean ridge basalts e morb or ocean island basalts oib and have epsilon nd t values of 1 8 to 3 1 and 87 sr 86 sr t values of 0 7019 0 7047 t 920 ma all these characteristics indicate that they are not likely derived from the ancient lithospheric mantle under the ncc but rather from a mantle source below in the asthenosphere this 925900 ma magmatism represents a second sub lithospheric mantle upwelling event following the 1780 1730 ma event that occurred shortly similar to 70 ma after the formation of the subcontinental lithospheric mantle sclm therefore the sclm of the ncc was multiply metasomatized during asthenospheric upwellings at least at 1780 1730 and 925 900 ma before most of its eastern part was finally removed during the mesozoic collectively these similar to 925 ma dykes and similar to 900 ma sills constitute a large igneous province lip with an areal extent of about 0 5 mkm 2 and a diameter of about 1000 km this lip probably resulted from a neoproterozoic mantle plume centered along the present southern margin of the eastern ncc and probably resulted in the break off and rifting away of a separate crustal block we speculate that this conjugate block could have been the combined sao francisco congo craton on the basis of precisely matched ages for the bahia dykes sao francisco craton and gangila mayumbian volcanic associations western part of the congo craton c 2011 elsevier b v all rights reserved less

geochemistry of the grenville dyke swarm role of plume source mantle in magma genesis major and trace element data of samples from the approximately 590 ma grenville dyke swarm in the southeastern parts of the canadian shield were used to evaluate the extent of crustal contamination of the dyke magma and to model its probable mantle source the dyke rocks are mostly saturated quartz tholeiites a few samples represent transitional basalts their overall chemistries are similar to those of continental flood basalts their compositions resemble those of liquids initially produced in the garnet peridotite mantle zone and then subjected to polybaric fractionation however fractionation models fail to mass balance ti fe k and p geochemical and isotopic criteria indicate that crustal contamination was minimal the incompatible elements zr y k rb nb v ta hf and th show hyperbolic elemental ratio ratio distributions indicative of mixing between two end members one end member has elemental ratios similar to those of a depleted n morb source and the second of a p morb i e ertile or plume morb source the more enriched dyke compositions can be successfully reproduced by mixing models in the ratio of 6 1 between the most depleted morb like dyke magma and a liquid composition similar to metasomatic glass veinlets coexisting with mica peridotite mantle xenoliths and subsequent crystal fractionation the grenville dykes are probably a synrift swarm that was emplaced along a rift arm related to an iapetan rrr triple junction that is thought to have formed over a mantle plume the chemistry of the dyke swarm is consistent with its derivation from a plume head in that the minor enriched source can be identified with plume source mantle and the major depleted source with entrained mantle the magma probably originated in the upper cooler parts of the plume head

magmatism and continental break up in the south atlantic high precision40ar 39ar geochronology abstract a detailed ar ar study of the parana etendeka continental flood basalts cfb has been undertaken using laser spot analysis data provide information not only on the age of the samples but also on the variability of the non radiogenic ar component and state of alteration the results indicate the parana etendeka cfb were erupted over 10 million years between 137 and 127 ma providing a minimum overall eruption rate of 0 1 km3 yr 1 this is an order of magnitude less than that previously proposed for this the deccan and siberian cfb provinces but it is similar to estimates for hawaii iceland and the columbia river cfb in detail the new ar ar analyses indicate that magmatism within the parana etendeka province migrated from nw to se prior to and during the opening of the south atlantic providing an explanation for the asymmetry of the cfb lavas about the south atlantic moreover chemically defined magma types were erupted at different times in different places and so within the paranathey may not be used as reliable chronostratigraphic units rather such magmatic units may reflect the extent of compositionally distinct source regions in the uppermost mantle and indicate that partial melting took place over a wide area under the paranabasin the nw se migration of the onset of magmatism might be interpreted as a plume trace but the inferred rate of movement is 3 times faster than that inferred from subsequent magmatism on the rio grande rise it is argued that the onset of magmatism reflected extensional tectonics normal to the ponta grossa dyke swarm given that magmatism preceded the main phase of rifting and that extension across the ponta grossa was moderate current plume models have difficulty in predicting sufficient melting within the asthenosphere

puncoviscana folded belt in northwestern argentina testimony of late proterozoic rodinia fragmentation and pre gondwana collisional episodes stratigraphic correlations and tectonic analysis suggest that the puncoviscana fold belt of northwestern argentina was an intracontinental basin with bimodal igneous suites that formed in connection with the breakup of the rodinia supercontinent at 800 ma several lines of evidences point to an initial lithosphere rupture possibly induced by a rising mantle plume the earliest synrift igneous products are represented by ultra potassic dykes and alkaline lava flows of high lree hree and low zr nb y nb ratios the dyke emplacements and the initiation of rifting were probably synchronous they pass laterally and upwards middle part of the puncoviscana succession into basalts of alkaline transitional character oib like source the distinctive chemical feature of these lavas are very similar to the source of oceanic island basalts thus they are thought to represent a magmatism associated with the rift and rift drift transition stage during this stage of rifting probably true oceanic crust was formed the upper part of the puncoviscana sequence late precambrian lower cambrian in age comprises a thick and monotonous sequence of pillow lavas massive basaltic flows and minor volcanic breccias and hyaloclastites these lavas exhibit morb trace element characteristics with high feot and tio2 low k2o and p2o5 flat light ree spectra little or no depletion in nb and ta this volcanism consists of the major and latest effusive episode from the puncoviscana basin which was slightly modified by subduction processes the geodynamical model proposed for the generation of these volcanic rocks could have been developed in two stages in the first stage the volcanic event is compatible with a progressive opening of a continental rift leading to formation of a mature oceanic basin in contrast the second stage shows the effects of a completed wilson cycle including a primitive volcanic arc which continued until the accreted cuyania arequipa belen antofalla caba terrane against the proto gondwana western borderland of the amazonian shield 535 ma

transport of sporosarcina pasteurii in sandstone and its significance for subsurface engineering technologies the development of microbially mediated technologies for subsurface remediation and rock engineering is steadily increasing however we are lacking experimental data and models to predict bacterial movement through rock matrices here breakthrough curves btcs were obtained to quantify the transport of the ureolytic bacterium sporosarcina pasteurii through sandstone cores as a function of core length 1 8 7 5 cm bacterial density 4 10 6 to 9 10 7 cells ml and flow rate 5 8 17 5 m s s pasteurii was easily immobilised within the homogeneous sandstone matrix 80 in comparison to a packed sand column 20 under similar experimental conditions and percentage recovery decreased almost linearly with increasing rock core length moreover a decrease in bacterial density or flow rate enhanced bacterial retention a numerical model based on 1d advection dispersion models used for unconsolidated sand was fitted to the btc data obtained here for sandstone good agreement between data and model was obtained at shorter rock core lengths 4 cm suggesting that physicochemical filtration processes are similar in homogeneous packed sand and sandstones at these lengths discrepancies were however observed at longer core lengths and with varying flow rates indicating that the attributes of consolidated rock might impact bacterial transport progressively more with increasing core length implications of these results on microbial mineralisation technologies currently being developed for sealing fluid paths in subsurface environment is discussed

low angle collision with earth the elliptical impact crater matt wilson northern territory australia nearly all meteorite impact craters on earth are circular however 4 of craters should be formed by impacts at angles lower than 12 from the horizontal which should result in elongated crater structures the crater forming process that produces elliptical shapes is poorly understood we document the first elliptical crater on earth that contains a central uplift and that provides insights into the mechanisms of crater formation at a critical threshold angle of 10 15 the dimensions of the proterozoic matt wilson impact structure northern territory australia are 7 5 by 6 3 km corresponding to an aspect ratio of 1 2 with its long axis trending northeast southwest the exposed crater floor shows a preferred stacking of thrust sheets within the central uplift and in the surrounding syncline indicating northeast southwest shortening and a material transport top to the sw this is consistent with an up range to down range motion of rock caused by remnant horizontal momentum transferred from the impacting projectile to the target this preferential deformation interferes with a radially oriented convergent material flow characteristic for crater collapse the matt wilson crater provides evidence for the usefulness of structural asymmetries as a diagnostic tool to infer impact vectors the new impact crater is confirmed by the presence of planar deformation features planar fractures in quartz grains and its structural inventory

target rocks impact glasses and melt rocks from the lonar impact crater india petrography and geochemistry the lonar crater india is the only well preserved simple crater on earth in continental flood basalts it is excavated in the deccan trap basalts of cretaceous tertiary age a representative set of target basalts including the basalt flows excavated by the crater and a variety of impact breccias and impact glasses were analyzed for their major and trace element compositions impact glasses and breccias were found inside and outside the crater rim in a variety of morphological forms and shapes comparable geochemical patterns of immobile elements e g rees for glass melt rock and basalt indicates minimal fractionation between the target rocks and the impactites we found only little indication of post impact hydrothermal alteration in terms of volatile trace element changes no clear indication of an extraterrestrial component was found in any of our breccias and impact glasses indicating either a low level of contamination or a non chondritic or otherwise iridium poor impactor

3 d 40ar39ar geochronology in the parana continental flood basalt province new 40ar39ar analyses of borehole samples from the parana etendeka continental flood basalt province provide critical evidence of eruption rates in unexposed regions when combined with surface samples the new data clarify the duration of this major volcanic province new ages from both surface and borehole samples confirm a 10 12 million year duration of magmatism which post dates the tithonian mass extinction and provide a unique 3 dimensional picture of the spatial and temporal eruption of the lava pile chemically defined magma types are diachronous and the onset of magmatism occurred 500 1000 km inland migrating southeast towards the incipient south atlantic ocean calculated eruption rates increased from 0 03 km3 yr 1 between 138 and 135 ma through 0 13 km3 yr 1 between 135 and 133 ma to 0 21 km3 yr 1 between 133 and 131 ma supporting the notion that the rate of magmatism increased with time and proximity to the developing south atlantic after 131 ma the continental eruption rates dropped dramatically to 0 01 km3 yr 1 presumably because rifting had occurred and eruption centred in the oceanic rio grande rise with eruption rates of around 0 3 0 5 km3 yr 1 given the available constraints on the potential temperature of the plume thickness of the continental lithosphere and degrees of extension through time predicted melt production rates within the plume seriously underestimate the calculated eruption rates on the continent however the eruption rates can be reconciled if melting occurred by conductive heating of volatile enriched mantle the diachronous nature of the magma types supports the idea that melting occurred over a wide area and that the different magma types reflect different source regions rather than the temporal evolution of magmas from a single source given that 10 million years is appropriate to the timescales for conductive heating above the mantle plume it is suggested that these distinct source regions were located within the lithospheric mantle consistent with geochemical studies

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

a plume generated segment of the rifted margin of laurentia southern canadian appalachians seen through a completed wilson cycle the southern part of the canadian appalachians and the adjacent parts of the craton show a range of distinctive features pertaining to the evolution of the rift and rift drift transition stages of a segment of the continental margin of laurentia despite the effects of a completed wilson cycle several lines of evidence point to an initial lithospheric rupture in the form of a three pronged rift possibly induced by a rising mantle plume the earliest synrift igneous products to penetrate the crust in the area are represented by a swarm of tholeiitic diabase dykes intruded at ca 590 ma the swarm is more than 700 km long and radiates from the inferred location of the rrr triple junction dyke emplacement and the initiation of rifting were probably synchronous approximately 35 m y later a rapid outburst of volcanism occurred at the triple junction probably as a harbinger of sea floor spreading the volcanic products are predominantly basalts of mildly alkaline to transitional character but with a minor comenditic component the voluminous nature of the volcanic outburst and the compositional characteristics of its products indicate higher than normal asthenospheric temperatures coupled with thinning of the lithosphere suggesting continued mantle plume activity the volcanic sequence is overlain by rift facies elastics that are locally interrupted and augmented by a deltaic deposit representing the delta of a river that drained the failed arm into the nascent lapetus ocean the rift facies elastics and the associated deltaic deposits are overlain by rocks of marine origin signalling the beginning of the drifting stage possibly at ca 550 ma