some forms of mountain peaks in the canadian rockies controlled by their rock structure abstract different mountain shapes in sedimentary sequences in the canadian rockies were enhanced by glacial erosion and have been modified post glacially by gravity driven slope processes slope modification by both glacial erosion and post glaciation landslides is related to slope geometry and rock structure particularly bedding dip five mountain peak shapes in monoclinal sequences each fall into different ranges of bedding dips 1 castellate and 2 matterhorn mountains occur in sub horizontal beds and their slopes are generally 35 65 and oblique to both bedding and joints 3 cuestas develop in gently to moderately dipping beds dip slopes and steeper normal escarpments form their cataclinal and anaclinal sides respectively with the dihedral angle between them about 90 4 hogbacks in moderately to steeply dipping beds have similar slope angles on both cataclinal and anaclinal slopes cataclinal slopes are either dip slopes or underdip slopes but anaclinal slopes are often steepened escarpments the dihedral angle between the slopes is usually slope gradients on a sample of 34 peaks are closely related to landslides particularly rock slides and rock topples these landslide modes are controlled by bedding dips

constraints on central uplift structure from the manicouagan impact crater recent drilling operations at the 90 km diameter late triassic manicouagan impact crater of quebec canada have provided new insight into the internal structure of a complex craters central region previous work had indicated that the impact event generated a 55 km diameter sheet of molten rock of relatively consistent originally 400 m thickness floran et al 1978 the drilling data reveals melt sheet thicknesses of up to 1500 m with kilometer scale lateral and substantial vertical variations in the geometry of the crater floor beneath the melt sheet the thickest melt section occurs in a 1500 m deep central trough encircled by a horseshoe shaped uplift of precambrian basement the uplift constitutes a modified central peak structure at least part of which breached the melt sheet mineralogical and compositional segregation differentiation of the thicker melt sheet section coupled with a lack of fractionation in the thinner units shows that the footwall geometry and associated trough structure were in place prior to melt sheet solidification marked lateral changes in sub melt sheet basement relief support the existence of a castellated footwall that was created by high angle impact related offsets of 100s to 1000s of meters this indicates that deformation during the modification stage of the cratering process was primarily facilitated by large displacement fault systems this work suggests that manicouagan is a central peak basin with rings which does not appear to fit with current complex crater classification schemes

the manicouagan impact structure an analysis of its original dimensions and form manicouagan quebec 51 23 n 68 42 w is the most intensively studied large complex terrestrial impact structure in a predominantly crystalline target the ground truth data available from manicouagan have considerable potential for interpreting the subsurface characteristics of lunar impact structures of comparable morphology and size two contrasting hypotheses however exist for the preerosional form of manicouagan 1 a multi ring basin with a final diameter of 75 km and a transient cavity diameter of 30 45 km floran and dence 1976 and 2 an endogenically modified peak ring basin with a final diameter of 100 km and a transient cavity diameter of 80 km orphal and schultz 1978 both hypotheses are based largely on topographic data and interpret the prominent 65 km diameter annular moat as a graben like feature formed either 1 by collapse of the outer slope of the transient cavity rim or 2 by uplift of the final crater floor through post impact intrusive activity the present analysis suggests that this annular moat is primarily an erosional feature and that the original form of the impact structure cannot be determined unequivocally on the basis of present topography if all the available topographic geologic and geophysical data are considered however then an internally consistent interpretation of original dimensions is possible the distribution of shock deformation effects in the basement rocks of the present crater floor suggests an original transient cavity diameter of 60 km the 55 km diameter impact melt covered inner plateau provides a minimum estimate for the diameter of the final floor of the crater and the annular moat with its outliers of downdropped ordovician limestone is interpreted as marking the contact between the floor and the innermost slump blocks of the final rim there is no compelling evidence to regard the annular moat as a graben by analogy with fresh lunar and mercurian structures a floor width of 55 km suggests a final rim diameter of 86 95 km this is consistent with the topographic data from outside the annular moat and with the residual peripheral gravity low which suggests a final rim diameter of 85 95 km due to erosion it is difficult to assign manicouagan to a particular morphological class of impact structure but on the basis of the available data it is most likely that the preerosional form was that of a central peak crater or possibly a peak ring basin at the time of its formation manicouagan may have been dimensionally and possibly morphologically similar to the 96 km diameter lunar crater copernicus

a diode laser system for heating minerals for u th he chronometry we have developed a diode laser 25 w 808 nm system for he extraction from minerals for u th he chronometry the laser beam is delivered via a 600 m fiber cable and focused using a binocular microscope temperatures necessary for he release from apatite 500 600 c and zircon 1100 1300 c encapsulated in pt foil tubes are attained by heating to 0 5 w for 30 s and 1 25 2 5 w for 20 min respectively using a defocused beam heating at these powers does not result in measurable u and or th loss from apatite as noted by the preservation of the distinct th u in multiple splits of two different durango apatite crystals analyses of durango and the california institute of technology internal standard apatite 97mr22 yield u th he ages of 32 8 1 8 ma 1 n 11 and 4 6 0 5 1 n 5 respectively well within accepted ages the u th he age and th u of five fish canyon tuff zircon aliquots yield 29 3 2 2 ma 1 and 0 6 0 03 respectively and are indistinguishable from ages produced by resistance furnace he extraction heating of unencapsulated minerals shows that the diode laser couples well with optically opaque minerals e g hornblende biotite muscovite garnet and basalt groundmass suggesting that diode lasers offer a cheap small low maintenance alternative to nd yag and ar ion lasers for 40ar 39ar cosmogenic noble gas and stable isotope studies

fission track dating of haughton astrobleme and included biota devon island canada haughton astrobleme is a major extraterrestrial impact structure located on devon island in the canadian arctic archipelago northwest territories apatite grains separated from shocked precambrian gneiss contained in a polymict breccia from the center of the astrobleme yielded a fission track date of 22 4 million 1 4 million years before the present or early miocene aquitanian this provides a date for the impact event and an upper limit on the age of crater filling lake sediments and a flora and vertebrate fauna occurring in them a geologically precise date for these fossils provides an important biostratigraphic reference point for interpreting the biotic evolution of the arctic

lithoprobe line 55 integration of out of plane seismic results with surface structure metamorphism and geochronology and the tectonic evolution of the eastern grenville province lithoprobe line 55 in the grenville province of eastern quebec provides unusually good control on the three dimensional 3 d geometry and structural relationships among the major lithological units there archean basement underlies the exposed proterozoic rocks along the entire seismic line and there is a lateral ramp in this basement immediately behind a lobate stack of thrust slices of high pressure metamorphic rocks comprising the manicouagan imbricate zone miz integration of the 3 d geometry with p t and geochronological data allows derivation of a tectonic model for the region the miz was buried to depths 60 km at 1050 ma preservation of its high pressure assemblages and the absence of metamorphism at 990 ma which is characteristic of lower pressure metamorphic rocks that tectonically overlie them indicates the miz rocks were rapidly unroofed early in the tectonic history there were two discrete pulses of crustal thickening during the grenvillian orogeny in this region the first inv

chronology of the central atlantic magmatic province implications for the central atlantic rifting processes and the triassic jurassic biotic crisis abstract the central atlantic magmatic province camp is among the largest igneous provinces on earth emplaced synchronously with or just prior to the triassic jurassic t j boundary ca 200 ma in great part due to the controversial connection between the occurrence of camp and the events of the t j boundary the demand for better constraints on the duration and eruptive chronology of this province has increased more than 100 new 40 ar 39 ar ages have been published in the last 15 years with more than half of these in the last 3 years a careful review and selection of available ages as well as the publication of 16 new ages from the carolinas newark basin usa french guyana and morocco are presented judicious selection yields a total of 58 accepted age determinations for camp magmatism ranging from 202 to 190 ma covering every part of the camp a more complete picture develops with intrusive camp magmatism commencing as early as 202 ma extrusive activity initiated abruptly 200 ma reaching peak volume and intensity around 199 ma on the african margin the main period of camp magmatism is confirmed as brief but is suggested to consist of at least two phases over 1 5 ma with magmatism commencing along the africa north american margins and slightly later along the south american margin two volumetrically minor but distinctive magmatic peaks centered at 195 and 192 ma are mirrored in data from all three continents and highlighted by our statistical approach models describing rifting and thermal input and magma production on these timescales are explored despite significant advances in our understanding of the chronology of camp more data of better quality and broader geographical coverage are needed to completely characterize the evolution of the camp and infer its geodynamic origin in addition lack of a well defined t j boundary age as well as the absence of a relevant basis for comparison between u pb and 40 ar 39 ar data for this time period remain limiting factors to unambiguously linking camp in time with the events of the t j boundary

shelf edge tide correlated eddies along the southeastern united states 1 high frequency radar observations in the southeastern united states have revealed sequences of small short lived cyclonic eddies along the shoreward edge of the gulf stream that spin up as the local tide turns alongshelf antiparallel to the stream eddies propagate equatorward along the shelf edge sometimes progressing shoreward before dissipating one to three hours later they are distinctly different from gulf stream meander eddies which propagate poleward in this article radar and mooring data are used to establish three important aspects of these new eddies they represent an instability process operating at a previously unidentified frequency scale and cross stream position they contribute to shoreward momentum fluxes defining a link between gulf stream and outer shelf subtidal variability and illustrating a mechanism to justify locally large horizontal eddy viscosity estimates and they transport properties across the shelf edge importing nutrients onto the shelf and transferring heat between the gulf stream and the coastal ocean

an u th he age for the shallow marine wetumpka impact structure alabama usa single crystal u th he dating was applied to 24 apatite and 23 zircon grains from the wetumpka impact structure alabama usa this small approximately 5 7 6 km impact crater was formed in a shallow marine environment with no known preserved impact melt thus offering a challenge to common geochronological techniques a mean u th he apatite and zircon age of 84 4 1 4 ma 2 was obtained which is within error of the previously estimated late cretaceous impact age of approximately 83 5 ma in addition helium diffusion modeling of apatite and zircon grains during fireball contact shock metamorphism and hydrothermal events was undertaken to show the influence of these individual thermal processes on resetting u th he ages in the wetumpka samples this study has shown that the u th he geochronological technique has real potential for dating impact structures especially smaller and eroded impact structures that lack impact melt lithologies

pressure induced amorphization of albitic plagioclase in an externally heated diamond anvil cell 1 high pressure and high temperature experiments of albitic plagioclase up to 41 gpa and 270 c were carried out using an externally heated diamond anvil cell raman spectroscopy and transmission electron microscopy of the recovered samples revealed that the amorphization of albite was complete at 37 gpa and room temperature the amorphization pressure at 170 c was nearly the same as that at room temperature in contrast the pressure largely decreased to 31 gpa at 270 c in comparison with the amorphization pressure of albite in laboratory shock experiments that in the present static compression experiments is significantly lower 10 gpa even at room temperature this suggests that shorter pressure duration results in a lower degree of amorphization of plagioclase the formation of maskelynite in shocked meteorites does not necessarily require the very high shock pressure 30 90 gpa that was previously estimated on the basis of shock recovery experiments

contrasting tectonic styles in the northern grenville province implications for the dynamics of orogenic fronts in western labrador tectonic elements of the parautochthonous belt of the northern grenville orogen include a regional mid crustal thin skinned metamorphic fold thrust belt structurally overlain by a thick skinned thrust sheet with a high pressure metamorphic signature in central ontario the parautochthonous belt is composed of a major reverse sense shear zone along which deep crustal rocks have been uplifted and exhumed tectonic models for both areas involve crustal shortening by delamination in the lower crust and tectonically driven exhumation up a crustal scale ramp the fold thrust belt northwest of the ramp in labrador is a result of the development of critical topography in the thrust wedge at the top of the ramp and the accretion of thrust slices to the base of the wedge during its propagation onto the foreland the fold thrust belt is absent in ontario because syntectonic erosion of the thrust wedge prevented the development of critical topography

martian surface paleotemperatures from thermochronology of meteorites the temporal evolution of past martian surface temperatures is poorly known we used thermochronology and published noble gas and petrographic data to constrain the temperature histories of the nakhlites and martian meteorite alh84001 we found that the nakhlites have not been heated to more than 350 c since they formed our calculations also suggest that for most of the past 4 billion years ambient near surface temperatures on mars are unlikely to have been much higher than the present cold 0 c state

transient and steady state creep response of ice i and magnesium sulfate hydrate eutectic aggregates 1 using uniaxial compression creep experiments we characterized the transient and steady state deformation behaviors of eutectic aggregates of system ice i and mgso4 11h2o ms11 meridianiite which has significance because of its likely presence on moons of the outer solar system synthetic samples of eutectic liquid bulk composition which produce eutectic colonies containing 0 35 0 50 volume fraction ms11 were tested as functions of colony size and lamellar spacing temperature 230 250 k and confining pressure 0 1 and 50 mpa to strains 0 2 up to a differential stress of 6 mpa the ice i ms11 aggregates display an order of magnitude higher effective viscosity and higher stress sensitivity than do aggregates of pure polycrystalline ice at the same conditions the creep data and associated microstructural observations demonstrate however that the aggregates are additionally more brittle than pure ice approaching rate independent plasticity that includes rupture of the hydrate phase at 6 8 mpa depending on the scale of the microstructure microstructures of deformed samples reveal forms of semibrittle flow in which the hydrate phase fractures while the ice phase deforms plastically semibrittle flow in the icy shell of a planetary body would truncate the lithospheric strength envelope and thereby decrease the depth to the brittle ductile transition by 55 and reduce the failure limit for compressional surface features from 10 to 6 mpa a constitutive equation that includes eutectic colony boundary sliding and intracolony flow is used to describe the steady state rheology of the eutectic aggregates

oligocene miocene spreading history of the northern south fiji basin and implications for the evolution of the new zealand plate boundary a tectonic model of the evolution of the northern half of the south fiji basin including the minerva triple junction and cook fracture zone is developed from regional gravity multibeam bathymetry and a new interpretation of magnetic anomalies pinned to radiometric dates of oceanic crust in the basin the geometry and age of a portion of the minerva triple junction and the cook minerva spreading center the connection from the triple junction to the cook fracture zone which accommodated coeval opening of the norfolk basin are resolved with multibeam bathymetry and magnetics the south fiji basin opened from about 34 to 15 ma in an anticlockwise sweep about an euler pole located at the northern end of the present lau ridge this rotation and a rigidly straight southeastward motion of the three kings ridge were accommodated by the configuration of the triple junction changing from ridge fault fault to ridge ridge fault to ridge ridge ridge during this evolution the southeastern arm of the system the julia fracture zone underwent several transformations and the cook minerva spreading center experienced repeated ridge jumps the kinematics of the northern south fiji basin dictate to a large extent the evolution of the southern south fiji basin and the norfolk basin this in turn leads to the interpretation of a complex trench trench double transform fault framework at the northern new zealand margin which explains most aspects of the geology structure and arc volcanic history of the margin and provides a radical new setting for the origin of the northland allochthon

dating the cooling of exhumed central uplifts of impact structures by the u th he method a case study at manicouagan abstract forty titanite grain fragments from 9 central uplift samples of metamorphosed anorthosite from the manicouagan impact structure were dated by the u th he technique a u th he central age of 207 1 6 4 ma 2 standard error se n 40 has been determined with 4 outlier ages removed the central age is refined to 208 9 5 1 ma 2 se both of these ages are within error of the previously determined u pb zircon age of 214 1 ma 2 derived from the impact melt manicouagan s central uplift formed due to rapid elevation from 7 10 km depth as part of the modification stage of the impact process which has facilitated the dating of its emplacement due to resulting rapid exhumation and cooling and closure of the u th he system in titanite correlation with the previous u pb zircon 214 1 ma impact melt crystallization age indicates that the u th he titanite dating technique offers a new approach to dating complex impact structures in the absence of viable melt sheets or other melt products the youngest ca 195 ma u th he dates preserved in some titanite fragments are synchronous with early jurassic rift induced lithospheric thinning and associated igneous activity that defines the central atlantic magmatic province camp the u th he titanite data from manicouagan indicate that the influence of this regional event may extend west of the previously proposed limit of camp activity

the manicouagan impact melt sheet evidence for isotopic homogenization with limited assimilation abstract the rb sr pb pb and sm nd isotope chemistry of impact generated melt and target rocks has been determined from the 214 ma 90 km rim diameter manicouagan impact structure of canada fifty seven samples were obtained from 8 field sites and 11 drill core sites across the impact melt sheet located on the 55 km diameter central island ile rene levasseur the results reveal that the impact melt though locally differentiated via fractional crystallization was isotopically homogenized during its formation following its derivation primarily from charnockites and mesocratic gneisses the isotopic signatures of the target rocks indicate that the protolith for the melt was the proterozoic manicouagan imbricate zone and that the underlying archean gagnon terrane was not involved in melt production or its subsequent modification via assimilation despite impact melt resting on archean lithologies in the southwest sector of the melt sheet limited assimilation of the footwall in this sector is attributed to the relatively rapid cooling of a thinner peripheral melt body

heat flow and structure of the lithosphere in the eastern canadian shield twenty two new determinations of heat flow and radiogenic heat production in the superior and grenville provinces of the canadian shield are presented the new data and previously published measurements strongly constrain the thermal structure of the eastern canadian shield in the abitibi greenstone belt heat flow gradually increases from 29 mw m 2 near the grenville front to 44 mw m 2 east of the kapuskasing uplift this heat flow variation is interpreted in terms of crustal thickening and increased thickness of a tonalitic layer with average heat production of about 1 1 w m 3 this interpretation based on estimated heat production of major rock types in the region is consistent with crustal models derived from recent seismic reflection and refraction studies it also leads to an estimate of about 12 mw m 2 for the mantle heat flow beneath the area the average heat flow in the grenville province 41 10 mw m 2 is the same as that of the superior province this similarity and the lack of significant variation of heat flow across the grenville front indicate that the crust on both sides of the front has similar heat production and thus composition in the western part of the grenville province heat flow reaches high values in the vicinity of the boundary between the allochtonous polycyclic and monocyclic belts where enriched granitic plutons are found in the crystalline terranes in the central part of the grenville province heat flow and heat production are related to each other the parameters of the linear heat flow heat production relationship qr 30 2 in mw m 2 and d 7 1 1 7 km are close to those of the much younger appalachian province implying that the higher appalachian heat flow is due solely to higher heat production in the upper crust the data provide no evidence for variation of mantle heat flow between the superior grenville and appalachian provinces whose tectonic ages range between 2700 and 400 ma the small value of the mantle heat flow about 12 mw m 2 implies that the depth to the 450 c isotherm which controls the effective elastic thickness of the lithosphere is very sensitive to crustal heat production

impact thermochronology and the age of haughton impact structure canada 1 most successful efforts to determine the ages of impact events are based on the isotope geochronology of crystalline or glassy impact melts studies of impact sites on earth show that many form without significant melt production meaning that traditional geochronologic approaches can yield unsatisfying results we describe here an alternative approach based on theoretical calculations that even brief thermal events related to impact can reset the isotopic systematics of unmelted target rocks thermochronometers based on the production of radiogenic 4he in accessory minerals are particularly amenable to complete resetting by impact and should yield robust impact ages if helium systematics are not further disturbed during post impact thermal events we illustrate the utility of this method through a presentation of a new zircon u th he date for the haughton impact structure canada of 23 5 2 0 ma

helium irradiation study on zircon synthetic zrsio4 and mildly to strongly radiation damaged natural zircon samples were irradiated with 8 8 mev 4he2 ions fluences in the range 1 1013 5 1016 ions cm2 for comparison an additional irradiation experiment was done with 30 mev 16o6 ions fluence 1 1015 ions cm2 the light ion irradiation resulted in the generation of new synthetic zrsio4 or additional mildly to strongly metamict natural samples damage the maximum extent of the damage is observed in a shallow depth range approximately 32 33 m 8 8 mev he and 12 m 30 mev o below the sample surface i e near the end of the ion trajectories these depth values and the observed damage distribution correspond well to defect distribution patterns as predicted by monte carlo simulations the irradiation damage is recognised from the notable broadening of raman active vibrational modes lowered interference colours i e decreased birefringence and changes in the optical activity i e luminescence emission at very low damage levels a broad band yellow emission centre is generated whereas at elevated damage levels this centre is suppressed and samples experience a general decrease in their emission intensity most remarkably there is no indication of notable structural recovery in pre damaged natural zircon as induced by the light ion irradiation which questions the relevance of alpha assisted annealing of radiation damage in natural zircon

conjugate divergent margins an introduction the main objective of this book is to provide a global overview of divergent margins based on geological and geophysical interpretation of sedimentary basins along the south central and north atlantic conjugate margins from plate tectonics and crustal scales to a more detailed description of stratigraphical and structural elements that are responsible for petroleum plays these themes are complemented by geodynamic concepts based on physical and numerical models and by comparisons with present day embryonic margins which are succinctly discussed in some papers

helium loss from martian meteorites mainly induced by shock metamorphism evidence from new data and a literature compilation noble gas data from martian meteorites have provided key constraints about their origin and evolution and their parent body these meteorites have witnessed varying shock metamorphic overprinting at least 5 to 14 gpa for the nakhlites and up to 45 55 gpa e g the lherzolitic shergottite allan hills alh a77005 solar heating cosmic ray exposure and weathering both on mars and earth influences on the helium budgets of martian meteorites were evaluated by using a new data set and literature data concentrations of 3he 4he u and th are measured and shock pressures for same sample aliquots of 13 martian meteorites were determined to asses a possible relationship between shock pressure and helium concentration partitioning of 4he into cosmogenic and radiogenic components was performed using the lowest 4he 3he ratio we measured on mineral separates 4he 3he 4 1 pyroxene of alha77005 our study revealed significant losses of radiogenic 4he systematics of cosmogenic 3he and neon led to the conclusion that solar radiation heating during transfer from mars to earth and terrestrial weathering can be ruled out as major causes of the observed losses of radiogenic helium in bulk meteorites for bulk rock we observed a correlation of shock pressure and radiogenic 4he loss ranging between 20 for chassigny and other moderately shocked martian meteorites up to total loss for meteorites shocked above 40 gpa a steep increase of loss occurs around 30 gpa the pressure at which plagioclase transforms to maskelynite this correlation suggests significant 4he loss induced by shock metamorphism noble gas loss in rocks is seen as diffusion due to 1 the temperature increase during shock loading shock temperature and 2 the remaining waste heat after adiabatic unloading post shock temperature modeling of 4he diffusion in the main u th carrier phase apatite showed that post shock temperatures of 300 c are necessary to explain observed losses this temperature corresponds to the post shock temperature calculated for bulk rocks shocked at about 40 gpa from our investigation data survey and modeling we conclude that the shock event during launch of the meteorites is the principal cause for 4he loss

shock veins in the central uplift of the manicouagan impact structure context and genesis we describe the development of shock veins that penetrate the anorthositic central uplift of the manicouagan impact structure they occur as thin 2 5 mm wide linear micro fault systems that can be traced for several meters in length and which predominantly trend radially from the point of impact the shock veins are distinguished by the development of maskelynite along vein margins and stishovite in vein matrices which are otherwise absent in non veined regions these phases define a shock excursion of up to 30 gpa in contrast to bulk shock effects of 12 gpa defined by development of shatter cones planar fractures and planar deformation features in various minerals the shock veins at manicouagan share many similarities with vein systems developed in meteorites they also provide an in situ context with which to better understand meteoroid source and lofting conditions in addition to containing high pressure phases the shock veins exhibit evidence for high temperature partial melting of host silicate clasts with the generation of flow textured fragments and glasses the formation of microcrystallites and dendrites from some melts indicates rapid cooling we propose a two stage generation mechanism comprising an initial high pressure shock excursion estimated to last 0 5 s based on projectile size considerations followed by a longer high temperature pulse of a few seconds duration we suggest that the shock excursion is initiated by target heterogeneities that cause distortions in the hemispherically propagating shock front this results in radially oriented tearing and vein formation with shock amplification occurring via intra vein shock reverberation high speed displacement along the veins is driven by stress release on rarefaction which results in frictional melting via adiabatic heating

high pressure high temperature rocks from the base of thick continental crust geology and age constraints from the manicouagan imbricate zone eastern grenville province the grenville province of the canadian shield is composed of crustal material with laurentian affinities involved in continental collision during the grenvillian orogeny between 1 19 and 0 98 ga in the eastern grenville province the manicouagan imbricate zone miz is composed of paleoproterozoic labradorian 1 6 ga and mesoproterozoic pinwarian 1 4 ga crustal segments that were metamorphosed under high pressure high temperature high p t conditions p 1800 mpa and 800 t 900 c and intruded by synmetamorphic gabbroic stocks and dykes and postmetamorphic granite during a 1 0 ga crustal shortening event in the grenvillian orogeny the gabbroic dykes display high p mineral assemblages and a withinplate tholeiite signature attesting to the intrusion of asthenospheric melts during the high p metamorphism the structural configuration of miz is that of a thrust stack lelukuau terrane overlain by an extensional assembly of slices tshenukutish terrane to the north lelukuau terrane overlies archean basement and paleoproterozoic supracrustal rocks of the gagnon terrane along a thrust contact tshenukutish terrane is tectonically overlain to the south by crustal slices composed of pre grenvillian and early grenvillian lithologic units that are correlative with those in miz but which were at midcrustal levels during the grenvillian orogeny low p segment of the eastern grenville province and were intruded by anorthosite and granitoids farther south a likely tectonothermal evolution of miz at 1 0 ga involves crustal shortening by imbrication in a postsubduction phase of continental collision followed by convective removal of the underlying thickened lithospheric mantle and the rise of hot asthenospheric material close to the base of the crust exhumation of miz likely occurred from the basal levels of the crust and was achieved by 1 nw directed thrusting over a crustal scale ramp archean basement of the gagnon terrane with coeval extension at the higher levels of the pile stage 1 extension and 2 a second extension event stage 2 extension coeval with emplacement of postmetamorphic granite at the boundary between miz and the low p segment

u th he dating of terrestrial impact structures the manicouagan example the accurate dating of meteorite impact structures on earth has proven to be challenging melt sheets are amenable to high precision dating by the u pb and 40ar 39ar methods but many impact events do not produce them or they are not preserved in cases where high temperature shock metamorphism of the target materials has occurred without widespread melting these isotopic chronometers may be partially reset and yield dates that are difficult to interpret unambiguously as the age of impact however the u th he chronometer is sensitive to thermal resetting and can provide a powerful new tool for dating impactites we report u th he dates for accessory minerals from the manicouagan impact structure in quebec canada nine zircons from a melt sheet sample yield a weighted mean age of 213 2 5 4 ma 2se indistinguishable from the published 214 1 ma 2 u pb zircon age for the impact in contrast five apatites from this sample yield dates between 205 9 6 5 and 162 0 5 3 ma 2 indicating variable postimpact helium loss due to low temperature thermal disturbance preimpact titanite crystals from a shocked meta anorthosite sample yield two dates consistent with the impact age at 212 27 and 214 13 ma 2 and two younger dates of 189 6 6 9 and 192 2 9 8 ma 2 suggestive of postimpact helium loss these results indicate that u th he chronometry is a suitable method for dating impact events although interpretation of the results requires recognition of possible 4he loss related to reheating subsequent to impact

shock metamorphic features and their zoning and orientation in the precambrian rocks of the manicouagan structure quebec canada abstract the manicouagan structure of impact origin is located in eastern quebec about 300 km north of the st lawrence river the complex is of triassic age the shock deformation of the more important rock forming minerals of the autochthonous or at least parautochthonous rocks within the structure is described four zones of shock metamorphism are delineated based on the deformation features of plagioclase 1 plagioclase without planar features 2 plagioclase with planar features 3 first occurrence of plagioclase with partial isotropization and 4 first occurrence of plagioclase with total isotropization these four zones are more or less concentric and indicate a general increase in the grade of shock metamorphism towards the centre of the structure however several small areas probably underlain by allochthonous rocks appear not to fit this general picture biotite and garnet show evidence of a shock orientation of their deformation features an impact origin for the structure is favoured the impact possibly triggering volcanism

diagenesis in neogene coal sequence a study on soluble organic matter abstract sedimentary materials for the 2 km neogene quaternary section in the tiszapalkonya i corehole north eastern hungary pannonian basin have been used to study the processes of diagenesis the age of lignite coal and sediment samples penetrated ranged between 6 and 9 million years with a corresponding temperature increases from 18 to 105 c over the section studied the soluble organic matter from 65 coal strata was analyzed by different methods in some cases the insoluble organic matter was also characterized by rock eval and vitrinite reflectance measurements the values of bitumen corg and s1 in the top samples have obviously demonstrated that the late diagenetic processes have not taken place yet the insolubilization was observed in the case of the long chain aliphatic esters by their decreasing proportion in bitumens the variation of certain indices pr phr pr n c17 n c22 n c23 and cpi also showed a distinctive depth interval 800 1200 m for insolubilization during late diagenesis the epimerization of 16 h phyllocladane to 16 h phyllocladane has encountered below 1300 m and was still incomplete at about 1600 m depth the traces of the onset of early oil genesis was recognized at about 1500 m depth

tectono thermal evolution of deep crust in a mesoproterozoic continental collision setting the manicouagan example high pressure metamorphic rocks in continental collision belts provide important information on the orogenic evolution of deep crust in the grenville province a prime example is the manicouagan imbricate zone miz a mesoproterozoic crustal section that was metamorphosed up to 1800 mpa and 850 950 c at 1050 ma high tempera tures and the presence of synmetamorphic within plate tholeiitic gabbro intrusion in the miz attest to thermal perturba tion during burial and emplacement of mantle derived melts consistent with thinning of lithospheric mantle subsequently the miz was extruded by a combination of northwest directed thrusting over a crustal scale ramp and ex tension at higher crustal levels in the middle miz extension was coeval with thrusting but perpendicular to the thrust direction and was apparently controlled by ductility contrasts between slices in contrast the highest levels show evi dence of southeast directed extension i e of opposite sense to the thrust direction that postdated thrusting within the miz this episode was followed by renewed northwest directed thrusting of medium pressure mesoproterozoic units over the miz and then by final southeast directed extension overall configuration of the miz was achieved between 1040 990 ma and is consistent with outward propagation of the orogen alternating periods of thrusting and exten sion displacement are a likely result of adjustments permitting maintenance of dynamic equilibrium within the advanc ing stack the tectonothermal characteristics of the miz allow comparison between the grenville province and younger thermally perturbed orogens i e variscan belt and may be the final product of a tibetan plateau style evo lution

the microstructural response of quartz and feldspar under shock loading at variable temperatures shock recovery experiments were carried out on westerly granite and hospital hill quartzite targets in the peak pressure range 8 to 25 gpa preshock temperatures of 25 450 and 750 c and pulse durations of 2 to 7 s using internally heated momentum traps and explosive plane wave generators optical and transmission electron microscopy analyses of quartz and feldspar shocked at 25 c revealed the previously documented progression with increasing pressure 1 fracturing 2 planar fractures and shock mosaicism 3 shock mosaicism and planar deformation features pdfs and 4 isotropization this same sequence is observed for experiments at elevated preshock temperature but with specific microstructures occurring at lower pressures than those in previous experiments at room temperature at 750 c strong shock mosaicism partially thermally recovered is characteristic of feldspar shocked to 8 gpa whereas 15 gpa is required for its development in quartz and for the generation of pdfs in both minerals the results suggest that threshold pressures for formation of the various microstructures and phases are expected to vary systematically as a function of the preshock temperature of the target material we suggest that pdfs are generated in the shock transition by progressive heterogeneous phase transformation of the crystal structure to form dense glass or high pressure polymorphs the onset pressures for pdfs in specific crystallographic orientations is not influenced strongly by temperature but the character of the pdfs does change as preshock temperature is increased at the same peak shock stress the change from multiple sets of thin pdfs at low temperature to thicker single sets of pdfs at moderate temperature and finally to complete isotropization at high temperatures reflects a change in the phase transformation mechanism as a function of temperature in contrast development of shock mosaicism in quartz and feldspar occurs throughout the duration of shock loading and is better developed at elevated temperatures where the kinetics are enhanced by the additional thermal energy in the target

magma flow regimes in sills deduced from ar isotope systematics of host rocks the best place to seek evidence of the style of past magma flow through a conduit is in the country rock heat flow has been studied in country rock adjacent to two tertiary dolerite sills intruding the caledonian schists and quartzites on the isle of mull scotland radiogenic 40ar loss within mica grains in the thermal aureoles of the intrusions has been measured at high spatial resolution using the ultra violet laser ablation micro probe to discriminate between a history of prolonged magma flow a history of conductive cooling following laminar flow and instantaneous emplacement of the intrusions the 40ar 39ar mica data and thermal modeling suggest that a prolonged period of magma flow of 3 5 months resulted in extensive argon loss from the micas country rock melting and mineral breakdown adjacent to a 6 m sill these features were absent from the wall rocks of a smaller 2 7 m thick sill which exhibited even less argon loss than might have been predicted for an instantaneous intrusion if the heat loss from the 6 m sill observed in one locality had been repeated along its length it would have formed an important magma conduit to the mull volcano but dolerite is not a common flow composition on mull if on the other hand the heat loss from the sill varies along strike it constitutes strong evidence for channelling and heterogeneous flow within the sill

helium diffusion and u th he thermochronometry of titanite we have explored the diffusivity characteristics of radiogenic he in titanite sphene and have developed analytical techniques for u th he dating of this mineral results of incremental he outgassing experiments performed on titanites from a variety of geological environments suggest a thermally activated volume diffusion mechanism with an activation energy of 44 6 3 4 2 kcal mol and a frequency factor of 60 cm 2 s diffusivity is highly linearly correlated with the inverse square of the grain size indicating that the he diffusion domain in titanite is the crystal itself for typical titanite grains of 200 to 800 m minimum dimension the he closure temperature is in the range 191 to 218 c for a cooling rate of 10 c myr there is no indication in the titanites we studied that radiation damage plays a major role in he diffusion u th he ages of titanites from quickly cooled rocks yield ages with 5 8 1 reproducibility that are concordant with known ages and u th he ages of titanites from slowly cooled rocks are consistent with independently established cooling paths supporting the closure temperature estimates these experiments suggest that titanite u th he ages may be useful for constraining cooling histories at temperatures near the lower limit of those accessed by feldspar 40 ar 39 ar dating but higher than apatite fission track or u th he dating

the influence of natural radiation damage on helium diffusion kinetics in apatite abstract stepwise degassing diffusion experiments on 39 different apatite samples using radiogenic 4he and proton induced 3he reveal a range in closure temperature tc from 50 to 115 c for a cooling rate of 10 c myr there is no correlation between helium diffusion and apatite chemistry including f cl ratio but the closure temperature is positively correlated with the radiogenic 4he concentration 4he in each sample we argue that 4he is a proxy for a sample s natural exposure to actinide radioactivity below the closure temperature and that helium diffusion in apatite is impeded by radiation induced damage to the apatite structure the kinetics must therefore be an evolving function of time measured diffusivities thus reflect a snapshot in time and cannot alone be applied to the thermochronometric interpretation of a given sample the effect of radiation damage on helium diffusion appears to far exceed other known controls on helium diffusivity including grain size our diffusion data are well described by a previously proposed quantitative model that consists of two arrhenius relations one for volume diffusion through undamaged mineral structure and one for release of helium from radiation damage traps the unknown parameters in this trapping model were determined from the diffusion experiments and allow us to develop a tentative mathematical function that relates diffusivity to temperature and 4he by inserting this function into a 4he production diffusion model we have explored how these results affect the interpretation of apatite u th he thermochronometry the model predicts that the effective 4he closure temperature of apatite will vary with cooling rate and effective u concentration eu and may differ from the commonly assumed tc of 70 c by up to 15 c the 4he partial retention zone will look similar to previous expectations but its depth will depend on accumulation time and on eu most notably samples subjected to reheating after accumulation of substantial radiation damage will be more retentive than previously expected these predictions are consistent with recent observations of unexpected apatite u th he ages in some settings

structural evidence from shock metamorphism in simple and complex impact craters linking observations to theory the structure of canadian impact craters formed in crystalline rocks is analyzed using shock metamorphism and evidence for movement along shear zones the analysis is based on an interpretation that beyond the near field region shock pressure attenuates down axis as p r 2 in agreement with nuclear test and computed results and as p r 3 near the surface in both simple and complex craters the transient cavity is defined by the limit of fragmentation due to direct and reflected shock waves the intersection of the transient cavity with hemispheric shock isobars indicates that the transient cavity has a parabolic form weakening by dilation during early uplift allows late stage slumping of the walls of simple craters this is controlled by a spheroidal primary shear of radius rs 2dt where dt is the depth of the transient crater due to excavation and initial compression with increasing crater diameter the size of the transient cavity decreases relative to the shock imprint suggesting that fragmentation and excavation is limited by progressively earlier collapse of the margins under gravity central peak formation in complex craters may be initiated by relaxation of the shock compressed central parautochthone so the primary shear lubricated by friction melting meets below the crater floor and drives the continuing upward motion