debris flows and debris torrents in the southern canadian cordillera in canada debris torrents also referred to as channelized debris flows occur in parts of british columbia alberta and the yukon at least 17 deaths and an estimated 100 million of damage to bridges and property can be attributed to this natural hazard the debris mainly comprises large boulders rock fragments gravel to clay sized material tree and wood mulch materials that accumulate in the mountain creeks to be susceptible to a debris torrent a creek must have a drainage area within a critical range a profile that is sufficiently steep an accumulation of debris and some form of triggering mechanism the most common triggering mechanism is an extreme water discharge which may result from a very intense rainfall or a temporary damming of the creek in western canada the resulting torrents involve masses of debris typically less than 50 000 m3 that travel down creeks at speeds between 3 and 12 m s several passive and active forms of mitigation can be used to reduce the impact of debris to

geomorphic effects of large debris flows on channel morphology at north fork mountain eastern west virginia usa extreme rainfall in june 1949 and november 1985 triggered numerous large debris flows on the steep slopes of north fork mountain eastern west virginia detailed mapping at four sites and field observations of several others indicate that the debris flows began in steep hillslope hollows propagated downslope through the channel system eroded channel sediment produced complex distributions of deposits in lower gradient channels and delivered sediment to floodwaters beyond the debris flow termini based on the distribution of deposits and eroded surfaces up to four zones were identified with each debris flow an upper failure zone a middle transport erosion zone a lower deposition zone and a sediment laden floodwater zone immediately downstream from the debris flow terminus geomorphic effects of the debris flows in these zones are spatially variable the initiation of debris flows in the failure zones and passage through the transport erosion zones are characterized by degradation 2300 to 17 000 m3 of sediment was eroded from these zones the total volume of channel erosion in the transport erosion zones was 1 3 to 1 5 times greater than the total volume of sediment that initially failed indicating that the debris flows were effective erosion agents as they travelled through the transport erosion zones the overall response in the deposition zones was aggradation however up to 43 per cent of the sediment delivered to these zones was eroded by floodwaters from joining tributaries immediately after debris flow deposition this sediment was incorporated into floodwaters downstream from the debris flow termini causing considerable erosion and deposition in these channels 1998 john wiley sons ltd

tracing archaean terranes under greenland s icecap u th pb hf isotopic study of zircons from melt water rivers in the isua area abstract the archaean gneisses of the nuuk area southern west greenland are partitioned into tectonostratigraphic terranes blocks of arc like crust that evolved independently until they coalesced by collisional orogeny to map terranes to the east under the inland ice sand samples were taken from rivers issuing from the edge of the icecap three from the isua area and one from 20 km to the south bedrock along this part of the ice front consists of 3820 3600 ma amphibolite facies rocks 40 km south of isua kapisilik terrane in the ivisaartoq area and also from 10 km to the north there are mesoarchaean amphibolite facies gneisses 3070 2950 ma exposed at the ice front in the moraine fields in the isua area there are erratic blocks of granulite facies gneisses these were sourced from a hidden terrane to the east under the ice because no such rocks are exposed in the isua area the majority of the zircons from the sand samples yielded close to concordant u pb ages apart from one 2414 ma grain all are archaean the isua sands show 2695 2710 and 2730 ma then 2790 2805 and 2840 ma clusters with a few grains back to 2950 ma and then a complex 3440 3960 ma spectrum less than 1 of the grains have ages between 3000 and 3400 ma in the isua sands neoarchaean and late mesoarchaean zircons form 50 of the population the sand 20 km to the south shows a similar span of zircon ages but there is a 2960 ma peak not seen in the isua sands as the eoarchaean and late mesoarchaean neoarchaean cycles progress the spread of th u and hf values seen in each zircon age population increases this is interpreted as repeated addition of juvenile material to the crust but also with increasing amounts of high temperature tectonothermal recycling of older materials within a package however there is no isotopic evidence for any contribution of eoarchaean crust to the later 2750 ma archaean cycle s showing evolution in disparate terranes furthermore no evidence for hadean material beneath the ice is seen in either the u pb ages or hf isotopic compositions of the detrital zircons on the other hand 2750 2500 ma metamorphic zircons show a spread of initial hf values of zero to 25 with the most negative indicating zircon growth in eoarchaean rocks overall these results closely mirror those obtained from direct sampling of rocks of the archaean terranes exposed beyond the icecap the zircon data indicate that mesoarchaean rocks equivalent to the kapisilik terrane are absent under the ice instead zircons were sourced from a late mesoarchaean to neoarchaean terrane affected by high grade metamorphism whose age spectra perfectly matches the tasiusarsuaq terrane to the southeast of nuuk guided by the zircon results and aeromagnetic signatures a terrane map is presented that extrapolates the tasiusarsuaq terrane northwards under the inland ice towards the tuno terrane in the north of the craton implications for different syntheses on the evolution of archaean crust in southern west greenland are discussed

enhanced sediment delivery in a changing climate in semi arid mountain basins implications for water resource management and aquatic habitat in the northern rocky mountains the delivery and transport of sediment through mountain rivers affects aquatic habitat and water resource infrastructure while climate change is widely expected to produce significant changes in hydrology and stream temperature the effects of climate change on sediment yield have received less attention in the northern rocky mountains we expect climate change to increase sediment yield primarily through changes in temperature and hydrology that promote vegetation disturbances i e wildfire insect pathogen outbreak drought related die off here we synthesize existing data from central idaho to explore 1 how sediment yields are likely to respond to climate change in semi arid basins influenced by wildfire 2 the potential consequences for aquatic habitat and water resource infrastructure and 3 prospects for mitigating sediment yields in forest basins recent climate driven increases in the severity and extent of wildfire suggest that basin scale sediment yields within the next few years to decades could be greater than the long term average rate of 146 t km 2 year 1 observed for central idaho these elevated sediment yields will likely impact downstream reservoirs which were designed under conditions of historically lower sediment yield episodic erosional events massive debris flows that dominate post fire sediment yields are impractical to mitigate leaving road restoration as the most viable management opportunity for offsetting climate related increases in sediment yield however short term sediment yields from experimental basins with roads are three orders of magnitude smaller than those from individual fire related events on the order of 101 t km 2 year 1 compared to 104 t km 2 year 1 respectively for similar contributing areas suggesting that road restoration would provide a relatively minor reduction in sediment loads at the basin scale nevertheless the ecologically damaging effects of fine sediment material

dating the oldest terrestrial rocks fact and fiction abstract this paper is a review of well documented geochronological and related isotopic evidence for the age and total crustal residence time of the oldest known terrestrial rocks mainly using rb 1bsr sm 1bnd and u 1bpb decay schemes the oldest known stable sialic crust of true continental character predominantly composed of calc alkaline orthogneisses was produced during major mantle differentiation some 3 7 3 5 ga ago and is represented on several continents enclaves of older supracrustal rocks as well as a major remnant of a volcano sedimentary sequence of greenstone belt affinity dated at 3 8 ga are preserved within the surrounding gneisses in southern west greenland recent ionmicroprobe evidence for 4 1 ga old detrital zircon grains in a younger early archaean metasedimentary sequence in western australia is also discussed several cases are reviewed where claims for early archaean 3 4 ga dates are believed to be erroneous and to result from oversimplistic and incorrect interpretation of isotopic data some of the geological and geochemical mechanisms which have given rise to claims for spuriously old sm 1bnd u 1bpb and pb pb dates are critically discussed to supplement published data we present several new sets of whole rock sm 1bnd and pb pb isotopic data from west greenland labrador zimbabwe and eastern india relevant to the overall topic of this review

late archaean terrane accretion in the godthab region southern west greenland tectono stratigraphic terranes fault bounded blocks of the earth s crust characterized by a geological history distinct from that of adjacent terranes1 are now widely recognized in phanerozoic and proterozoic orogenic belts and their present configurations are thought to result from plate tectonic processes similar to those in operation today2 4 there has been much debate about whether such processes operated in archaean times5 6 here we present new data from the archaean high grade gneiss complex of the godthab now nuuk region of southern west greenland which show that discrete tectonic units recognized in the south of the region7 can be more widely identified at least four terranes are recognized each of which evolved separately until their juxtaposition in the late archaean this suggests that at least some archaean high grade gneiss complexes may resemble more recent orogenic belts formed by plate tectonic processes

stream power framework for predicting geomorphic change the 2013 colorado front range flood abstract the colorado front range flood of september 2013 induced a diverse range of geomorphic changes along numerous stream corridors providing an opportunity to assess responses to a large flood in a semiarid landscape we defined six classes of geomorphic change related to peak unit stream power and valley confinement for 531 stream reaches over 226 km spanning a gradient of channel scales and slope geomorphic change was generally driven by erosion of channel margins in confined reaches and by a combination of deposition and erosion in unconfined reaches the magnitude of geomorphic change typically increased with unit stream power with greater responses observed in unconfined channels cumulative logit modeling indicated that total stream power or unit stream power unit stream power gradient and valley confinement are significant predictors of geomorphic response for this flood event based on this dataset thresholds for geomorphic adjustment were defined for channel slopes 230 w m 2 16 lb ft s at least 10 of the investigated sites experienced substantial channel widening and a credible potential for avulsions braiding and loss of adjacent road embankments associated with 480 w m 2 33 lb ft s at least 10 of the investigated sites experienced such geomorphic change infrequent to numerous eroded banks were very likely with 700 w m 2 48 lb ft s with substantial channel widening or major geomorphic change shifting from credible to likely importantly in reaches where there were large reductions in as the valley form shifted from confined to relatively unconfined large amounts of deposition induced reach scale geomorphic change occurred in some locations at relatively low additionally alluvial channels with slopes 3 had greater resistance to geomorphic change likely caused by armoring by larger bed material and increased flow resistance from enhanced bedforms finally we describe how these results can potentially be used by practitioners for assessing the risk of geomorphic change when evaluating current or planned conditions

application of lidar and discriminant analysis to determine landscape characteristics for different types of slope failures in heavily vegetated steep terrain horseshoe run watershed west virginia abstract a landslide inventory map was created for horseshoe run watershed west virginia using high resolution topographic data obtained from airborne lidar a total of 152 landslides were remotely mapped and classified as planar slides rotational slumps debris flows debris fans debris slides and active slopes based on morphologic characteristics interpreted from lidar derived shaded relief maps with field verification at several locations seven landscape variables were calculated for the watershed from the high resolution 0 5 m topographic data elevation slope angle slope aspect distance from roads distance from streams plan curvature and profile curvature statistical analyses were conducted to determine the mean and standard deviations for each variable and corresponding slope failure type discriminant analyses were conducted on the different types of failed slopes with a maximum 86 accuracy level to determine which landscape characteristics contribute to each type of landslide results from the discriminant analyses indicate that statistically significant differences exist between the failure classes mapped within the watershed and that slope angle slope aspect and landscape curvature are important variables influencing the type of process the results from this study have important implications for predicting where certain types of slope failures may occur based on landscape characteristics

refolded nappes formed during late archaean terrane assembly godthabsfjord southern west greenland recent work has provided a new tectono metamorphic framework for the archaean evolution of the godthabsfjord region which can be divided into discrete tectonic units each characterized by different magmatic structural and metamorphic histories these tectonic units were assembled in the late archaean post 2750 ma the present structural patterns seen in the region date from the period 2750 2600 ma and do not reflect earlier structural events as was previously considered late archaean structural evolution began with a period of interthrusting of early archaean sialic and late archaean oceanic crustal segments coeval with or followed by isoclinal nappe formation under amphibolite facies conditions linear fabrics in early archaean rocks were rotated into parallelism with those newly formed parallel to f1 hinge lines in the late archaean rocks a second major thrusting event emplaced granulite facies rocks structurally on top of the amphibolite facies rocks subsequent to this event a series of f2 upright or steeply inclined basin and dome structures formed some of which have sheath morphology this second folding episode acting upon the isoclinal nappes produced the regional scale interference patterns observed the early linear fabrics were re oriented around these second generation structures data are produced from selected areas to illustrate the fabric patterns and structural continuity that exists throughout the region

magma generated structures and their subsequent development in the late archaean evolution of northern buksefjorden southern west greenland the inugssugssuaq nappe is made up of a membrane of anorthositic rocks sheathed by grey gneisses of nuk age 2 800 ma and with a core of highly xenolithic nuk grey gneiss the structure was generated by magmatic diapirism followed by tectonic activity but both stages probably resulted from thermally induced gravitational instability relations with geometrically similar structures in the area are discussed

chemical assembly of archean anorthosites from amphibolite and granulite facies terranes sw greenland we report whole rock major and trace element compositions obtained by xrf and ina methods for the amphibolite facies buksefjorden and granulite facies nordland anorthosites sw greenland in a previous petrologic study on the same sample suite we documented differences in texture mineralogy and mineral compositions between these two anorthosite bodies chemical analyses confirm differences in composition between the two bodies but these differences cannot be explained by variations in metamorphic conditions and point towards differences in the nature of their protoliths analyzed nordland samples are anorthosites and leucogabbros with 88 98 normative plagioclase whereas those from buksefjorden include anorthosite leucogabbro and gabbro with 55 95 normative plagioclase two or more compositional groupings can be recognized at each site which correspond to differences in color and mineralogy of the hand samples samples from buksefjorden are mainly quartz normative whereas those from nordland are olivine nepheline normative other differences include higher ni co ratio and ree contents in the granulite facies anorthosites from nordland ree pattern shapes are similar however being moderately fractionated at 0 5 10 chondrites with positive eu anomalies calculated equilibrium melt patterns are similar for both anorthosites being relatively flat at 50 150 chondrites suggesting unfractionated but evolved parental magmas olivine must have been present in the protoliths of the nordland rocks compared with buksefjorden otherwise the protoliths contained plagioclase with variable an content an62 an92 and a mafic component with variable fe mg mg 0 3 0 8 this mafic component was either hornblende or a combination of ortho and clinopyroxene in fixed proportions plus a small amount of magnetite mixing calculations demonstrate that some buksefjorden anorthosites contain two varieties of plagioclase a calcic type that may correspond to cumulus crystals and a sodic type that may correspond to a trapped melt component on plots of normative whole rock an versus mg compositions of the buksefjorden and nordland anorthosites form crude negative arrays that differ from the generally positive trends of mafic layered intrusions kiglapait skaergaard and from the generally flats trends of plagioclase rich cumulate rocks st urbain and stillwater anorthosites this difference provides further evidence for the distinctive nature of archean calcic anorthosite complexes compared with other types of mafic intrusions moreover this distribution of data points is consistent with the assembly of the protolith of the sw greenland anorthosites mainly as mixtures of plagioclase and hornblende finally the field for the buksefjorden and nordland anorthosites overlaps only slightly with that for the fiskenaesset complex thus extending the known range of compositions for archean anorthosites in west greenland

origin of granitic magma by crustal remobilisation rb sr and pb pb geochronology and isotope geochemistry of the late archaean qorqut granite complex of southern west greenland abstract rb sr and pb pb whole rock isochrons on the qorqut granite complex yield ages of 2530 30 myr initial 87 sr 86 sr 0 7081 0 0008 and 2580 80 myr respectively a model relating initial sr and pb isotopic compositions of the qorqut granites to the sr and pb isotopic compositions of the amitsoq gneisses ca 3700 myr and nuk gneisses ca 2900 myr at 2550 myr ago as well as sr and pb contents of the gneiss units suggests that between 40 and 50 of the qorqut granite magma was generated by partial melting of amitsoq gneisses and the remainder by partial melting of nuk gneisses

comparative geomorphic analysis of surficial deposits at three central appalachian watersheds implications for controls on sediment transport efficiency abstract factors that control the routing and storage of sediments in the appalachian region are poorly understood this study involves a comparative geomorphic analysis of three watersheds underlain by sandstones and shales of the acadian clastic wedge these areas include the fernow experimental forest tucker county west virginia the north fork basin pocahontas county west virginia and the little river basin augusta county virginia gis based analyses of surficial map units allow first order approximation of sediment storage volumes in valley bottoms estimates of volumes are examined in tandem with morphometric analyses and the distribution of bedrock channels to make inferences regarding controls on sediment transport efficiency in the central appalachians the fernow and north fork areas are characterized by v shaped valleys with mixed reaches of alluvial bedrock channels distributed throughout the drainage network in contrast the little river valley is notably wider and gravelly alluvial fill is abundant comparator watershed parameters for the fernow north fork and little river areas include respectively 1 basin area 15 2 km 2 49 3 km 2 41 5 km 2 2 basin relief 0 586 km 0 533 km 0 828 km 3 drainage density 4 2 km 1 3 3 km 1 4 7 km 1 4 ruggedness 2 5 1 7 3 9 5 shreve magnitude 139 287 380 6 total valley bottom area km 2 0 76 km 2 1 86 km 2 3 09 km 2 7 average hillslope gradients 17 2 18 4 22 1 8 total debris fan surface area 0 113 km 2 0 165 km 2 0 486 km 2 and 9 debris fan frequency 2 0 km 2 1 0 km 2 2 8 km 2 the storage volumes in valley bottoms were estimated using map polygon areas and surface heights above channel grade the little river contains significantly higher sediment volumes in floodplain terrace and fan storage compartments total volumes of the valley bottoms are approximately twice that of the fernow and north fork areas combined unit storage volumes for the fernow north fork and little river are 5 2 10 4 m 3 km 2 5 5 10 4 m 3 km 2 and 1 6 10 5 m 3 km 2 respectively a conceptual model postulates that valley width morphometry and style of delivery from hillslopes are the primary factors controlling the efficiency of sediment transport steep debris flow prone hillslopes at the little river deliver high volumes of gravelly sediment at magnitudes greater than transport capacity of the channel patterns of stream power are complex as low order tributaries are under capacity and high order tributaries over capacity with respect to sediment load aggraded alluvial fill insulates valley floor bedrock from vertical erosion and valley widening dominates expansion of the valley width creates a positive response via increased storage capacity and lower unit stream power conversely the fernow and north fork are characterized by diffusive mass movement on hillslopes with incremental bedload transport to higher order tributaries rates of hillslope delivery are balanced by the rate of channel export mixed alluvial bedrock reaches provide the optimal channel configuration for active incision of the valley floor low expansion of valley width promotes high unit stream power and processes of vertical erosion the model implies that the fernow and north fork have been more effective at sediment transport during the late quaternary given similar climatic and tectonic settings variation in bedrock lithofacies is likely the primary factor modulating the efficiency of sediment transport

establishment of a 3 83 ga magmatic age for the akilia tonalite southern west greenland a recurring challenge to ion microprobe u pb zircon geochronology has been to discriminate between preservation of original igneous zircon populations and inherited grains this has proved particularly problematic in studying the polyphase metamorphic rocks that dominate early archean gneissic terranes for example differing interpretations exist for the origin of complex zircon populations investigated by u pb ion microprobe zircon geochronology from a granitoid orthogneiss body previously used to establish a minimum age for a supracrustal enclave on akilia island in southern west greenland we describe a method whereby the geochemistry of the akilia orthogneiss coupled with a u th pb vs age depth profile in a zircon from the same rock permits direct assessment of zircon inheritance results reveal evidence for three phases of concentric zircon growth at 3 83 ga v3 6 ga and 2 7 2 5 ga zircon growth at both v3 6 ga and 2 7 2 5 ga is consistent with precipitation from a metamorphic fluid depth profile u th pb data demonstrate that only the s 3 8 ga zircon core could have crystallized from the host rock we conclude that the magmatic age of this rock is 3 83 7 0 01 ga not v3 65 ga as has been previously proposed the u th pb zircon depth profile technique has wider applications to resolving other geochronological debates in highgrade metamorphic terranes where zircons populations are diverse and individual grains record complex overgrowth and dissolution re precipitation features 2002 elsevier science b v all rights reserved

new age ca 2970 ma mantle source composition and geodynamic constraints on the archean fiskenaesset anorthosite complex sw greenland the archean fiskenaesset complex sw greenland consists of an association of ca 550 meter thick layered anorthosite leucogabbro gabbro and ultramafic rocks peridotite pyroxenite dunite hornblendite the complex was intruded by tonalite trondhjemite and granodiorite ttg sheets now orthogneisses during thrusting that was followed by several phases of isoclinal folding the trace element systematics of the fiskenaesset complex and associated volcanic rocks are consistent with a supra subduction zone geodynamic setting the fiskenaesset anorthosites leucogabbros gabbros and ultramafic rocks collectively yield an sm nd errorchron age of 2973 28 ma mswd 33 with an average initial epsilon nd 3 3 0 7 consistent with a long term depleted mantle source regression of pb isotope data define an age of 2945 36 ma mswd 44 and the regression line intersects the average growth curve at 3036 ma slightly lower pb pb errorchron age is interpreted as reflecting partial disturbance of the u pb system in gabbros leucogabbros and ultramafic rocks during intrusion of ttgs complex internal structures in zircons from orthogneisses reveal several episodes of zircon growth and recrystallization taking place between ca 3200 and 2650 ma zircon ages peak at about 3200 3100 3000 2950 2820 and 2750 ma the 3200 3000 ma zircon cores are interpreted as inherited xenocrysts from older reworked crustal rocks 2950 ma is considered as an approximate intrusion age of sampled ttgs the 29402650 ma ages are attributed to metamorphic overgrowth and recrystallization in response to multiple tectonothermal events that affected the fiskenaesset region on the basis of recently published trace element data and new nd and pb isotope and u pb zircon age data a three stage geodynamic model is proposed to explain the evolution of the fiskenaesset complex stage 1 represents the formation of depleted shallow mantle source 3000 ma epsilon nd 3 3 0 7 for the complex stage 2 corresponds to the development of an intra oceanic island arc between 3000 2950 ma stage 3 is characterized by the collision of the island arc with either a passive continental margin or with an older arc between 2950 2940 ma c 2010 elsevier b v all rights reserved less

the early archaean itsaq gneiss complex of southern west greenland the importance of field observations in interpreting age and isotopic constraints for early terrestrial evolution geochemical and isotopic studies of small volumes of variably preserved 3600 ma rocks in gneiss complexes are crucial for documenting early earth history in the itsaq gneiss complex of the nuuk region west greenland there is dispute whether the granitic sensu lato orthogneisses dominating it are mainly products of a single ca 3650 ma crust formation super event or whether they formed in several unrelated events between ca 3850 and 3560 ma which of these interpretations of the dates is correct has major implications regarding what the whole rock radiogenic isotopic record pb pb sm nd rb sr reveals about continental crust formation and early terrestrial differentiation there is also debate whether some west greenland metasedimentary rocks with 12c 13c data interpreted as evidence for life are 3850 ma or only 3650 ma old establishing the correct age for these rocks is important for debates concerning early surficial environments and origin of life controversies have arisen because of different approaches taken by different workers specifically with respect to how much emphasis is placed on field geology in interpreting dates and isotopic data in this paper field observations and sampling from low strain zones where the origin and geological context of the rocks are best preserved and understood are closely integrated with u pb zircon dates and cathodoluminescence cl imagery of the zircons this approach shows that most single phase well preserved meta granitoid samples have simple zircon populations dominated by oscillatory zoned prismatic grains formed when their host magmas crystallized on the other hand migmatites and some strongly deformed banded gneisses have much more complex zircon populations the combined field evidence and zircon geochronology on the itsaq gneiss complex demonstrate that 1 some areas contain exposed orthogneisses formed during multiple magmatic thermal events between ca 3850 and 3560 ma and are not as suggested by kamber and moorbath 1998 dominated by ca 3650 ma granitoids containing abundant 3650 ma zircons inherited from cryptic unexposed older rocks 2 abundant 3750 ma granitoids are present which are locally well preserved 3 some water lain sediments reported as showing c isotope evidence for life were deposited as early as 3850 ma 4 the whole rock sm nd isochron approach fails to distinguish with any confidence 3650 ma from 3800 ma rocks 5 however it reinforces previous indications for markedly depleted 2 5 nd domains in the pre 3750 ma mantle

new pieces to the archaean terrane jigsaw puzzle in the nuuk region southern west greenland steps in transforming a simple insight into a complex regional tectonothermal model in the south of the nuuk region of west greenland our 1980s mapping recognized four archaean gneiss terranes f ringehavn tre brodre tasiusarsuaq and akia terranes with different protolith ages and separate early tectonothermal histories later in the archaean these were juxtaposed and then experienced the same 2700 2500 ma tectonothermal events here we abandon extrapolation of only these four terranes across the whole region and distinguish two new terranes in the ne the northernmost isukasia terrane previously regarded as the northernmost exposure of the f ringehavn terrane consists of palaeoarchaean rocks 3600 ma tectonically bounded on its south by the 3075 2960 ma kapisilik terrane these were juxtaposed and metamorphosed together by 2950 ma the previously recognized f ringehavn terrane to the sw is another separate entity of palaeoarchaean rocks that was juxtaposed with adjacent terranes only after c 2800 ma hence in an increasingly complex regional model there were several mid to neoarchaean terrane assembly events with superimposed orogenies from c 2950 ma until after 2700 ma although the f ringehavn and isukasia terranes were incorporated into the later archaean terrane collage at different times they might be fragments from a larger palaeoarchaean complex rifted apart from c 3500 ma onwards

late archaean tectonics in the faeringehavn tre brodre area south of buksefjorden southern west greenland the faeringehavn tre brodre area consists of three distinct terranes tectonically jux taposed by a previously unrecognized event contacts between the terranes are mylonitic shear zones truncating lithological units in adjacent terranes the terrenes are 1 faeringehavn terrane largely composed of early archaean amitsoq gneiss cut by younger archaean granitic gneiss defined here as the satut gneiss 2 tre brodre terrane comprising mid archaean malene supracrustal rocks anorthosite complex and polyphase nuk gneisses 3 tasiusarsuaq terrane largely comprising mid archaean nuk gneisses affected by c 2800 ma granulite facies metamorphism the tasiusarsuaq terrane is structurally above both the faeringehavn and tre brodre terranes which at c 2800 ma experienced a lower grade of metamorphism juxtaposition of the terranes took place between 2800 and 2500 ma and involved thrusting and crustal thickening subsequent re equilibration involved folding steeply inclined shear belts intrusion of synkinematic granitoids under amphibolite facies conditions and retrogression of granulite facies assemblages this thrusting post dates the 2800 ma granulite facies metamorphism it is younger and distinct from the thrusting postulated to explain the intercalation of the early archaean amitsoq gneisses and the mid archaean malene supracrustal rocks associated with intrusion of the nuk gneiss precursors described from godthabsfjord the tectonic breaks provide for the first time marker horizons which can be used to assess the amount and type of late archaean deformation in the southern godthabsfjord region

morphology and sedimentology of a complex debris flow in a clay shale basin coupling morphological sedimentological and rheological studies to numerical simulations is of primary interest in defining debris flow hazard on alluvial fans in particular numerical runout models must be carefully calibrated by morphological observations this is particularly true in clay shale basins where hillslopes can provide a large quantity of poorly sorted solid materials to the torrent and thus change both the mechanics of the debris flow and its runout distance in this context a study has been completed on the faucon stream southeastern french alps with the objectives of 1 defining morphological and sedimentological characteristics of torrential watersheds located in clay shales and 2 evaluating through a case study the scouring potential of debris flows affecting a clay shale basin morphological surveys grain size distributions and petrographic analyses of the debris flow deposits demonstrate the granular character of the flow during the first hectometre and its muddy character from there to its terminus on the debris fan these observations and laboratory tests suggest that the contributing areas along the channel have supplied the bulk of the flow material copyright 2005 john wiley sons ltd

geochronology and isotopic variation of the early archaean amitsoq gneisses of the isukasia area southern west greenland abstract relatively unaltered samples of the amitsoq gneiss complex in the isukasia area were identified and analysed despite the effects of metamorphism and local proterozoic hydrothermal alteration and contamination the parents of the three lithologies that make up the amitsoq gneisses of the area were emplaced over about 320 ma taking a weighted average of dates from u pb on zircons rb sr on wr sm nd on wr and pb pb on wr the most probable ages of intrusion are grey gneisses at 3690 50 ma white gneisses at 3590 40 ma and pegmatitic gneisses at 3370 60 ma the rb sr isochron initial 87 sr 86 sr ratios for the grey white and pegmatitic gneisses are 0 7006 1 0 7016 1 and 0 8248 3 respectively the corresponding average nd t values are 1 0 0 7 and 0 2 since the ree chemistry of the rocks that were the direct parents of the grey gneisses is not known the 1 0 average nd t of the grey gneisses need not imply generation from a depleted mantle the sr initial ratio and nd t values for the white and pegmatitic gneisses are compatible with consecutive derivation at two discrete intervals by partial melting of crust mainly comprised of rocks similar to the grey gneisses the progressive evolution of nd with t for the amitsoq gneisses in the early archaean shows that values of nd t near zero in granitoids do not necessarily indicate derivation from the undepleted mantle chur

secular changes in tonalite trondhjemite granodiorite composition as markers of the progressive cooling of earth archean tonalite trondhjemite granodiorite associations ttg are classically thought to generate through partial melting of hydrous metabasalts however the chemical composition of the least differentiated ttg parental magmas evolved from 4 0 to 2 5 ga during this interval the mg as well as the ni and cr contents increased which is interpreted as reflecting increased interactions between felsic melts generated by metabasalt melting and mantle peridotite similarly cao na2o and sr also increased over time thus reflecting an increase in the abundance of plagioclase in the melt residue the presence or absence of residual plagioclase is interpreted in terms of melting depth the demonstrated interaction between ttg parental magmas and the mantle rules out their genesis by fusion of previously underplated metabasalt and favors the melting of subducted slab material at 4 0 ga earth s geothermal gradient was sufficiently high to allow slab melting at shallow depths where plagioclase was stable consequently due to the small thickness of the overlying mantle wedge felsic magmas interacted little with the mantle at 2 5 ga however owing to lower geothermal gradients the melting depth was greater and plagioclase became no longer stable in the thick mantle wedge overlying the subducted slab as a result felsic magmas reacted strongly with the mantle peridotite the changes of ttg composition during archean time can be thus interpreted as reflecting the progressive cooling of earth

the influence of debris flow rheology on fan morphology owens valley california we have investigated factors controlling the surface morphology of debris flow fans comprising the bajada along the western slope of owens valley california these fans have average slopes of 4 an extensive network of abandoned boulder lined channels rough undulatory surfaces near the range front and smooth distal surfaces field relationships and mechanical considerations indicate that the channels of the bajada are products of fluvial incision and not debris flow scour this is significant because detailed geomorphic maps indicate that the channels strongly influence the pattern of debris flow deposition the locus of debris flow deposition on a channelized fan surface is set by the interaction of debris flows with the channel system and is controlled by channel size channel gradient flow volume flow hydrograph and flow rheology debris flow behavior is most directly controlled by variations in bulk sediment concentration and its influence on flow rheology whereas low sediment concentration debris flows tend to smooth the surface of the lower fan spreading into thin sheets and filling channels and surface undulations repeated deposition of high sediment concentration debris flows produces the rugged topography of the upper fan the texture of the fan surface rough or smooth is determined by the relative volumetric importance of these two types of debris flow in addition channel avulsions and the associated long term shifting of depositional loci are driven by in channel deposition of debris flows with the highest sediment concentrations these debris flows therefore play a critical role in determining both the structure of the channel network and the long term pattern of deposition on the fan surface as a whole we infer that the frequency distribution of debris flow rheologies set by source terrain geology and hydrology is an important control on fan morphology

hafnium isotope evidence for a transition in the dynamics of continental growth 3 2 gyr ago earth s lithosphere probably experienced an evolution towards the modern plate tectonic regime owing to secular changes in mantle temperature radiogenic isotope variations are interpreted as evidence for the declining rates of continental crustal growth over time with some estimates suggesting that over 70 of the present continental crustal reservoir was extracted by the end of the archaean eon patterns of crustal growth and reworking in rocks younger than three billion years gyr are thought to reflect the assembly and break up of supercontinents by wilson cycle processes and mark an important change in lithosphere dynamics in southern west greenland numerous studies have however argued for subduction settings and crust growth by arc accretion back to 3 8 gyr ago suggesting that modern day tectonic regimes operated during the formation of the earliest crustal rock record here we report in situ uranium lead hafnium and oxygen isotope data from zircons of basement rocks in southern west greenland across the critical time period during which modern like tectonic regimes could have initiated our data show pronounced differences in the hafnium isotope time patterns across this interval requiring changes in the characteristics of the magmatic protolith the observations suggest that 3 9 3 5 gyr old rocks differentiated from a 3 9 gyr old source reservoir with a chondritic to slightly depleted hafnium isotope composition in contrast rocks formed after 3 2 gyr ago register the first additions of juvenile depleted material that is new mantle derived crust since 3 9 gyr ago and are characterized by striking shifts in hafnium isotope ratios similar to those shown by phanerozoic subduction related orogens these data suggest a transitional period 3 5 3 2 gyr ago from an ancient 3 9 3 5 gyr old crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3 2 gyr ago that involved juvenile crust generation by plate tectonic processes

testing the model of late archean terrane accretion in southern west greenland a comparison of the timing of geological events across the qarliit nunaat fault buksefjorden region abstract the prevailing tectonic model for the archean gneiss complex of southern west greenland holds that terranes with different metamorphic histories and protolith ages were juxtaposed between 2 80 and 2 72 ga this model was tested by using detailed mapping and u pb geochronology to compare the timing of geological events across the qarliit nunaat fault the boundary between terranes in the buksefjorden region the fault was interpreted as a terrane boundary mostly because it is a narrow zone of mylonitic rocks that separates granulite or retrograde amphibolite facies gneisses of the tasiusarsuaq terrane tt from prograde amphibolite facies gneisses of the akulleq terrane at and it separates tonalitic gneisses of different age 3 87 3 60 and 2 80 ga in the at versus 2 92 2 86 ga in the tt dating of similar homogeneous gneisses in each terrane yielded ages of 2843 3 ma in the tt tonalite and 2824 2 ma in the at granodiorite ikkattoq gneiss granulite facies metamorphism in the tt is thought to have coincided with intrusion of a 2805 2 ma orthopyroxene pegmatite and zircon and titanite from nearby amphibolitic gneiss yielded ages of 2779 2 and 2660 10 ma respectively metamorphic minerals in the at are younger zircon and titanite from amphibolitic gneiss are 2717 3 and 2510 10 ma respectively and monazite from pelitic schist is 2732 5 and 2708 4 ma a concordant pegmatite layer that underwent a substantial amount of shearing along the qarliit nunaat fault was dated at 2725 2 other pegmatites that intruded relatively late in the structural history crystallized at 2710 2 in the tt and 2541 2 ma in the at these results support the prevailing tectonic model by showing that i similar homogeneous gneisses in each terrane are different age ii 2805 ma granulite facies metamorphism in the tt predated 2740 2700 ma amphibolite facies metamorphism in the at and iii shearing along the terrane boundary occurred after 2725 ma

role of debris flows in long term landscape denudation in the central appalachians of virginia four major storms that triggered debris flows in the virginia west virginia appalachians provide new insights into the role of high magnitude low frequency floods in longterm denudation and landscape evolution in mountainous terrain storm denudation in the blue ridge mountain drainage basins is approximately an order of magnitude greater compared to basins located in the mountains of the valley and ridge province this difference is probably the result of higher storm rainfall from the blue ridge storms radiocarbon dating of debris flow deposits in the blue ridge indicates a debris flow return interval of not more than 2 4 k y in mountainous river basins this finding combined with measurements of basin denudation suggests that approximately half of the long term denudation from mechanical load occurs episodically by debris flow processes although floods of moderate magnitude are largely responsible for mobilizing sediment in lowgradient streams our data suggest that high magnitude low frequency events are the most significant component in delivering coarse grained regolith from mountainous hollows and channels to the lowland floodplains

evolution and assembly of archean gneiss terranes in the godthabsfjord region southern west greenland structural metamorphic and isotopic evidence the godthabsfjord region of southern west greenland comprises several terranes that were assembled between 2750 and 2550 ma and folded during amphibolite facies metamorphism the terranes which are dominated by gneisses of different ages and show different preassembly metamorphic and structural histories are 1 the faeringehavn terrane containing the 3820 3600 ma amitsoq gneisses with granulite facies metamorphism at circa 3600 ma 2 the akia terrane containing the 3070 2940 ma nuk gneisses with granulite amphibolite facies metamorphism at circa 2980 ma 3 the tasiusarsuaq terrane containing circa 2900 ma gneisses with granulite facies metamorphism at circa 2800 ma and 4 the tre brodre terrane containing the 2800 2750 ma ikkattoq gneisses with amphibolite facies metamorphism at 2800 2750 ma metamorphic assemblages and structures formed prior to terrane assembly were variably overprinted during amphibolite facies metamorphism and heterogeneous strain associated with assembly recognition of the region as consisting of several terranes indicates that the anatomy of some archean high grade gneiss complexes may resemble that of orogens of proterozoic and phanerozoic age that formed as a consequence of plate tectonic processes

formation of earth s early archaean continental crust abstract subduction of oceanic crust at an unusually low angle has been proposed as a model for the growth of continental crust older than about 2 5 ga at modern zones of low angle or flat subduction magmatic additions to new crust come from partial melting of both the subducting oceanic crust slab and the thin wedge of mantle above the slab evidence for both a slab and wedge source is preserved in most late archaean 3 0 2 5 ga terrains but we find little evidence that a mantle wedge contributed to crustal growth prior to 3 1 ga this lack of evidence in part reflects a dearth of exposed crust aged between 3 0 and 3 3 ga but also suggests that subduction enriched mantle source regions did not develop before 3 3 ga and possibly not before 3 1 ga in contrast to most modern terrains and some late archaean terrains early archaean 3 3 ga continental crust evolved through direct melting of thick mafic crust we invoke a process of subduction that does not include the development of a mantle wedge and call this process archaean flat subduction to distinguish it from modern low angle subduction

archaean fluid assisted crustal cannibalism recorded by low δ 18 o and negative e hf t isotopic signatures of west greenland granite zircon the role of fluids during archaean intra crustal magmatism has been investigated via integrated shrimp u pb 18o and la mc icpms 176hf isotopic zircon analysis six rock samples studied are all from the nuuk region southern west greenland including two 3 69 ga granitic and trondhjemitic gneisses a 3 64 ga granitic augen gneiss a 2 82 ga granodioritic ikkattoq gneiss a migmatite with late neoarchaean neosome and a homogeneous granite of the 2 56 ga qorqut granite complex qgc all zircon grains were thoroughly imaged to facilitate analysis of magmatic growth domains within the zircon analysed there is no evidence for metamictization initial ehf zircon values n 63 are largely sub chondritic indicating the granitic host magmas were generated by the remelting of older un radiogenic crustal components zircon from some granite samples displays more than one 207pb 206pb age and correlated with 176hf 177hf compositions can trace multiple phases of remelting or recrystallization during the archaean model ages calculated using lu hf arrays for each sample indicate that the crustal parental rocks to the granites granodiorites and trondhjemites segregated from a chondrite like reservoir at an earlier time during the archaean corresponding to known formation periods of more primitive tonalite trondhjemite granodiorite ttg gneisses zircon from the 3 69 ga granite the migmatite and qgc granite contains eoarchaean cores with chondritic 176hf 177hf and mantle like 18o compositions the age and geochemical signatures from these inherited components are identical to those of surrounding tonalitic gneisses further suggesting genesis of these granites by remelting of broadly tonalitic protoliths zircon oxygen isotopic compositions n 62 over nine age populations six igneous and three inherited have weighted mean or mean 18o values ranging from 5 8 0 6 to 3 7 0 5 the 3 64 ga granitic augen gneiss sample displays the highest 18o with a mildly supra mantle composition of 5 8 0 6 inherited eoarchaean ttg derived zircon shows mantle like values igneous zircon from all other samples spanning more than a billion years of archaean time record low 18o sub mantle compositions these are the first low 18o signatures reported from archaean zircon and represent low 18o magmas formed by the remelting and metamorphism of older crustal rocks following high temperature hydrothermal alteration by meteoric water meteoric fluid ingress coupled with crustal extension associated high heat flow and intra crustal melting are a viable mechanism for the production of the low 18o granites granodiorites and trondhjemites reported here both high and low 18o magmas may have been generated in extensional environments and are distinct in composition from phanerozoic i type granitic plutonic systems which are typified by increasing 18o during intra crustal reworking this suggests that archaean magmatic processes studied here were subtly different from those operating on the modern earth and involved extensional tectonic regimes and the predominance of remelting of hydrothermally altered crystalline basement

adjacent terranes with ca 2715 and 2650 ma high pressure metamorphic assemblages in the nuuk region of the north atlantic craton southern west greenland complexities of neoarchaean collisional orogeny abstract in the gneiss complex of the nuuk region of the north atlantic craton in southern west greenland terrane juxtaposition was followed by neoarchaean folding under amphibolite facies conditions with widespread low pressure recrystallisation 5 kbar and 550 700 c the complex metamorphic overprinting requires that the p t history related to actual terrane assembly has to be extracted from very small relicts of older metamorphic assemblages combined with the u pb dating petrology and geochemistry of metamorphic zircons in the south of the region the faeringehavn terrane eoarchaean orthogneisses is tectonically overlain by a supracrustal package of amphibolites and paragneisses ca 2840 ma felsic volcano sedimentary protoliths this package is juxtaposed against a higher tectonic level represented by the tre brodre terrane 2825 ma orthogneisses and the tasiusarsuaq terrane 2920 2810 ma orthogneisses the terranes were assembled by 2710 2720 ma as shown by dating of granitic sheets intruded along the terrane boundary mylonites in the faeringehavn terrane and in the overlying 2840 ma supracrustal package relict early high pressure assemblages 12 8 kbar 700 750 c are clinopyroxene garnet plagioclase quartz hornblende in mafic rocks and garnet kyanite rutile bearing assemblages in paragneisses these are commonly replaced by lower pressure assemblages 7 5 kbar such as cordierite sillimanite garnet in paragneisses and hornblende plagioclase quartz garnet in mafic rocks in situ partial melting took place during both low and high pressure regimes metamorphic zircon in the high and low pressure assemblages yields dates of ca 2715 ma mostly with errors of the kapisilik terrane 3050 2960 ma orthogneisses and another supracrustal assemblage of amphibolites and ca 2800 ma quartzo feldspathic metasedimentary rocks are exposed north of the faeringehavn terrane and within fold cores along its western margin are bounded by folded neoarchaean mylonites the kapisilik terrane and this supracrustal assemblage include high pressure metamorphic remnants in amphibolites and metasediments metamorphic segregations with garnet clinopyroxene and kyanite respectively that formed at ca 2650 ma these remnants are overprinted by high temperature but lower pressure metamorphic events at ca 2630 2610 and 2580 ma thus remnants of early metamorphism reveal mutually exclusive ca 2715 and 2650 ma high pressure events in adjacent tectonically juxtaposed terranes repeated high pressure metamorphism in adjacent terranes is observed in younger complex collisional orogens

u pb zircon study of tectonically bounded blocks of 2940 2840 ma crust with different metamorphic histories paamiut region south west greenland implications for the tectonic assembly of the north atlantic craton abstract new fieldwork map interpretation petrography and single zircon u pb geochronology has allowed the identification of different crustal blocks in the paamiut region in the southern portion of the west greenland archaean craton changes of metamorphic grade from only amphibolite facies to granulite facies some subsequently retrogressed corresponds with zones of archaean high strain ductile deformation mylonites u pb zircon dates are presented for the ttg tonalite trondhjemite granodiorite protoliths from each block in the paamiut region and the southern portion of the previously identified tasiusarsuaq terrane lying to the north the southern part of the tasiusarsuaq terrane contains 2880 2860 ma ttg rocks and underwent amphibolite facies metamorphism structurally underneath the tasiusarsuaq terrane to the south is the sioraq block containing 2870 2830 ma ttg rocks partly retrogressed from granulite facies structurally underneath and to the south is the paamiut block dominated by 2850 2770 ma granodioritic rocks that have only undergone amphibolite facies metamorphism also structurally overlying the paamiut block but cropping out separately from the sioraq block is the neria block this appears to be dominated by 2940 2920 ma gneisses that have been totally retrogressed from granulite facies and strongly deformed in the southernmost part of the region the neria block overlies the greenschist to lowermost amphibolite facies sermiligaarsuk block that contains the 2945 ma tartoq group rocks from all the blocks record ancient loss of pb from zircons and some new zircon growth at 2820 ma interpreted to indicate a high grade metamorphic event at that time including granulite facies metamorphism in the sioraq and neria blocks the blocks of different metamorphic grade are interpreted to have moved to their current positions after the 2820 ma metamorphism explaining the change in metamorphic history across some mylonites and ductile shear zones which deform and retrogress granulite facies textures the juxtaposed blocks and their contacts were subsequently folded under amphibolite facies conditions the contacts are cut by undeformed palaeoproterozoic dolerite dykes which post date amphibolite facies metamorphism these results together with previously published data from the godthabsfjord region north of paamiut shows that the north atlantic craton in west greenland from ivittuut in the south to maniitsoq in the north 550 km consists of a mosaic of ductile fault bounded packages that attained their present relative positions in the late archaean

two metamorphic gold mineralization events confirmed by lu hf isotope dating of garnet in the late archean storo au deposit nuuk region of sw greenland abstract we report lu hf isotope ages for garnet from the late archean storo supracrustal belt ssb ranging from 2697 8 ma to 2646 13 ma these ages essentially overlap with two previously reported re os isotope ages for arsenopyrite associated with au mineralization within the ssb which implies that both au forming events took place while garnet was stable and thus occurred during amphibolite facies metamorphic conditions furthermore we report bulk rock lu hf isotope data for amphibolite and aluminous gneiss from the ssb the latter rock type has previously been suggested to represent the pre metamorphic hydrothermal alteration product associated with low temperature epithermal au formation however the initial hf isotope compositions of these two rock types are significantly different and therefore demonstrate that they do not share a common origin because the lu hf isotope system is essentially fluid immobile instead the aluminous gneiss represents a pelitic rock with mixed mafic and felsic sources which is in agreement with the formation of both au mineralization events during orogenic amphibolite facies metamorphic processes which were therefore structurally controlled a too young bulk rock errorchron age of 2553 72 ma for the amphibolite unit 2720 ma is interpreted to reflect resetting of the system during the intrusion and melt infiltration of the ca 2560 ma qorqut granite complex

re os and u pb constraints on gold mineralisation events in the meso to neoarchaean storo greenstone belt storo southern west greenland the storo greenstone belt in godthabsfjord southern west greenland consists of juxtaposed rock units of different age and origin and hosts gold mineralisation that is associated with arsenopyrite along a contact between lithological units and along the axial plane of a large fold core the age and origin of the gold is debatable but in this paper we present new arsenopyrite re os and zircon u pb data to constrain the age of the storo gold deposit a 2 71 0 05 ga arsenopyrite isochron and 2 707 0 008 ga highly radiogenic arsenopyrite from a mineralisation along a rock contact together with a 2 64 0 02 ga arsenopyrite isochron along the axial plane of the fold core indicate a two stage mineralisation process while the 2 707 0 008 ga highly radiogenic arsenopyrite provides firm support for an early mineralisation event a mixing origin cannot yet be excluded for the 2 71 0 05 ga isochron the 2 64 0 02 ga isochron is in perfect agreement with recent u pb zircon data nutman et al 2007 precambrian research 159 19 32 and these data are best explained by orogenic mineralisation during amphibolite facies metamorphism along structural weak planes the initial os 187 os 188 value of 0 56 0 16 for the 2 64 0 02 ga isochron indicates a crustal source for the metals whereas the initial os 187 os 188 0 1 0 6 for the 2 71 0 05 ga isochron remains unconstrained re os data are best explained by relatively short crustal residence times of less than 0 1 ga wherein the os and associated metals were extracted from the mantle at a time younger than 2 8 ga and in which the 2 64 0 02 ga stage formed by mobilisation of an earlier mineralisation detrital zircon constraints imply volcanism and sediment deposition for parts of the belt at 2 84 ga and that these units were tectonically juxtaposed to similar to 3 05 ga rocks within the belt the units that were deposited 2 84 ga record metamorphic zircon u pb ages of similar to 2 63 ga but evidence for 2 72 ga metamorphism which is ubiquitous in adjacent terranes is lacking c 2012 elsevier b v all rights reserved

debris flows as geomorphic agents in the huachuca mountains of southeastern arizona abstract numerous debris flows occurred in the huachuca mountains of southeastern arizona during the summer rainy season of 1988 in areas that were burned by a forest fire earlier in the summer debris flows occurred following a major forest fire in 1977 as well suggesting a causal link between fires and debris flows abundant evidence of older debris flows preserved along channels and in mountain front fans indicates that debris flows have occurred repeteadly during the late quaternary in this environment soil development in sequences of debris flow deposits indicates that debris flows probably recur over time intervals of several hundred to a thousand years in individual drainage basins in the study area surface runoff in the steep drainage basins of the huachuca mountains is greatly enhanced following forest fires as the hillslopes are denuded of their vegetative cover water and sediment eroded from the hillslope regolith are rapidly introduced into the upper reaches of tributary channels by widespread rilling and slope wash during rainfall events this influx of water and sediment destabilizes regolith previously accumulated in the channel triggering debris flows that scour the channel to bedrock in the upper reaches following a debris flow the scoured trapezoidally shaped channel gradually assumes a swale shape and the percentage of exposed bedrock declines as material is introduced from the slopes debris flows do a tremendous amount of work in a very short time however and are the major channel forming events where the tributary channels enter larger trunk channels the debris flows serve as the main source of very coarse sediment the local slope and coarse particle distribution of the trunk channel depend on the competence of water flows in the channel to transport the material introduced by debris flows where the smaller channels drain directly to the mountain front debris flows create extensive alluvial fans which dominate the morphology of the basin range boundary time intervals between debris flows in the drainage basins of the huachuca mountains are probably controlled by complex interactions among climate forest fires and slope processes fires destroy the protective vegetation that stabilizes the upper catchment slopes and inhibits erosion however not every fire that burns a catchment causes debris flows because sufficient weathered material must accumulate in the upper channel reaches to initiate a large debris flow if such accumulation has not occurred the material introduced to a channel following a forest fire will move only a short distance down the channel thus the episodic nature of debris flows probably depends on rates of slope weathering and erosion which are in turn controlled by climate both directly and through vegetation and forest fires

the early archaean nulliak supracrustal assemblage northern labrador the nulliak supracrustal assemblage the remains of ca 3800 ma succession of volcanic and sedimentary rocks was broken up by intrusion of the protoliths of the early archaean uivak orthogneisses and then deformed metamorphosed and variably metasomatised several times under upper amphibolite to granulite facies conditions in the archaean amphibolites of komatiitic basalt and tholeiitic chemical affinity are the most important nulliak assemblage lithologies high al2o3 metagabbroic rocks and anorthosites also occur interlayered with the amphibolites are marbles calc silicate rocks and banded iron formation interpreted as chemical sediments that were probably laid down in a shallow water environment also found are felsic rocks probably derived by reworking of penecontemporaneous felsic volcanic rocks and garnet and sillimanite bearing paragneisses derived from pelites all these lithologies are randomly interlayered on a scale down to 1 m or less the occurrence of 3850 3900 ma cores for zi

coarse grained nonmarine volcaniclastic sediment terminology and depositional process modern and ancient volcaniclastic sedimentary sequences contain depositional units whose features cannot be attributed to fully turbulent dilute stream flow or viscous debris flow the characteristics of these poorly sorted sediments suggest rapid deposition from high concentration dispersions but not en masse sedimentation thus appears related to high discharge flows intermediate in sediment water ratio between stream flow and debris flow the term hyperconcentrated flood flow is proposed for describing this intermediate condition hyperconcentrated flood flow deposits are distinguished from debris flow deposits by lack of matrix support or reverse grading and instead exhibit distribution normal grading and horizontal stratification these deposits are distinguished from normal dilute stream flow deposits by lack of cross stratification in sand facies and by very poor sorting poor imbrication and numerous clasts with long axes oriented parallel to flow direction in gravel facies the horizontal bedding that dominates sandy hyperconcentrated flood flow deposits consists of sediment too coarse grained and strata too thick to have been produced in the boundary layer of the upper flow regime and should not be confused with the more familiar thin graded laminae of fine to medium grained sand often associated with parting lineation hyperconcentrated flood flow deposits are not unique to volcanic settings they also occur in arid alluvial fan sequences debris flow and hyperconcentrated flood flow deposits however are much thicker and more extensive in volcanic regions than on alluvial fans because explosive volcanism leads to rapid mobilization of large volumes of sediment and water on a scale unparalleled in nonvolcanic settings in volcanic regions therefore these deposits have greater preservation potential show greater lateral variability and are more voluminous transformation of channelized debris flow to hyperconcentrated flood flow by dilution with stream water recently observed at mount st helens is recorded in ancient volcaniclastic sequences and may serve as the primary mechanism for generating hyperconcentrated flood flow

setting of the 2560 ma qorqut granite complex in the archean crustal evolution of southern west greenland the archean gneiss complex of west greenland contains packages of unrelated rocks created during relatively short periods of time in arc like magmatic environments and having similarities to rocks formed at phanerozoic convergent plate boundaries the terranes of new archean crust were amalgamated by collisional orogeny and then partitioned by post assembly tectonic processes having summarized the origin of west greenland archean crust in arc like environments this paper then focuses on new data concerning the latest neoarchean post terrane assembly intra continental tectonic and magmatic evolution of the region following the youngest documented high pressure metamorphism in a clockwise p t t loop at 2650 ma attributed to tectonic thickening of the crust there is in west greenland a 150 million year record of intermittent production of crustally derived granite shearing and folding under amphibolite facies conditions this is exemplified by the ssw nne orientated neoarchean qorqut granite complex qgc which forms a myriad of closely spaced coeval sheets ne of nuuk town shrimp u pb zircon dating of a homogeneous gray granite sheet gives a magmatic age of 2561 11 ma with 3800 to 3600 and 3070 to 2970 ma zircon xenocrysts the 40 km long fjringehavn straight belt a lower amphibolite facies vertical shear zone runs from the qgc s se margin and contains strongly deformed granite sheets with a u pb zircon age of 2565 8 ma but is cut by undeformed granite sheets dated at 2555 12 ma the 60 km long ivisaartoq fault consisting of lowermost amphibolite facies mylonite runs from the northwestern end of the main mass of granite it formed post 2630 ma because granites of that age are truncated by it near the qgc s northeastern extent 2559 3 ma granitic lithons in a folded mylonite are cut by 2521 72 ma granite sheets at a deep structural level at the northern end of the qgc deformed granitic neosomes give ages of 2567 9 and 2563 5 ma therefore at 2560 ma the ages within error for strongly deformed to non deformed granite bodies shows that the qgc is not a largely post kinematic intrusion as previously thought but was coeval with lowermost amphibolite facies metamorphism and shear zones with an important strike slip component late in the development of regional non cylindrical upright folds the main body of the qgc appears to be essentially post kinematic only because it was emplaced in a node of dilation during the heterogeneous predominantly strike slip deformation melting at this node may have been triggered by meteoric water percolating down dilational fractures causing metasomatism melting of these altered rocks gave rise to the low 18o signature of qgc igneous zircons due to the hydrous nature of the melting event the qgc was emplaced immediately above its migmatitic generation zone these late neoarchean shear zones of the nuuk region partition and disrupt the earlier formed mosaic of amalgamated terranes of unrelated rocks such tectonic patterns are seen more recently for example in holocene asian intra continental tectonics along the north side of the himalayas

can metamorphic reactions proceed faster than bulk strain available constraints on metamorphic reaction rates derived from the study of natural systems are similar to or slightly lower than the bulk strain rates measured in the same rocks here we explore whether this apparent relationship is merely coincidence or due to a more fundamental mechanistic link between reaction and strain grain boundary migration accommodated dislocation creep gbmdc or grain boundary diffusion creep gbdc i e pressure solution both of which involve dissolution precipitation as we define it will occur simultaneously with mineral reactions involving dissolution precipitation in the presence of a non zero deviatoric stress the exact relationships between reaction and strain are different depending on whether gbmdc or gbdc is controlling strain but the mechanistic link exists in both cases we present theoretical arguments which show that bulk strain by gbmdc or gbdc which may additionally be accommodated by processes not involving dissolution precipitation such as dislocation glide and climb or grain boundary sliding should in most cases be somewhat faster than the bulk reaction rates as observed with few exceptions for natural metamorphic systems undergoing plastic deformation strain rates provide an upper limit for bulk reaction rates occurring simultaneously in the same rocks the data suggest that mineral reaction rates may typically be within one order of magnitude of the strain rate

the geodynamic evolution of mesoarchean anorthosite complexes inferred from the naajat kuuat complex southern west greenland layered anorthosite complexes are typical components of archean crustal domains however the geodynamic settings in which they were emplaced are still discussed as geological relationships are oft

response of zircon u pb isotopes and whole rock geochemistry to co2 fluid induced granulite facies metamorphism kabbaldurga karnataka south india the arrested prograde amphibolite to granulite facies transition at kabbaldurga south india overprints archacan amphibolite facies nebulitic gneisses and the late archaean closepet granite previous studies have shown that this facies transition was controlled by a channelled influx of a dehydrating fluid assumed to be co2 at 750 c and 5 5 kbar confining pressure the effect of this type of prograde transition on zircon u pb isotopic systematics and whole rock geochemistry has been studied using 1 kg amphibolite facies transitional and granulite facies domains from a single block of gneiss the zircon populations from all three domains have essentially similar morphology and u pb systematics this similarity shows that at the conditions under which the prograde granulite facies transition took place via fluid influx the zircon u pb systematics were not disturbed by the process using the pooled data from all three domains it is concluded that the protolith of the gneiss formed at 2965 4 ma 2 and that zircons also grew during an anatectic event common to all domains at 2528 5 ma the granulite facies metamorphism has not been dated directly due to the lack of response to the zircon u pb isotopic systematies to it however field and petrographic criteria dictate that its maximum age is 2528 5 ma the age of the anatectic event common to each domain in the gneiss block which was overprinted during the granulite facies event for most major and trace elements consistent enrichment or depletion trends associated with the transition to granulite facies cannot be identified with confidence however the granulite facies portion is lree light rare earth element enriched and h heavy ree depleted compared with the amphibolite facies domain and the transitional domain is at intermediate values the isotopic and geochemical evidence presented supports the conclusion that the granulite facies charnockitic rocks at kabbaldurga were not formed by removal of an anatectic melt but that they formed later by simple metamorphic overprint of amphibolite facies rocks

initial pb of the amitsoq gneiss revisited implications for the timing of early archaean crustal evolution in west greenland reply abstract the rapid change in 207 pb 206 pb ratio during early earth history provides a powerful constraint for temporal resolution of archaean crustal evolution new and published data for amitsoq gneiss feldspars and whole rocks define the earth s least radiogenic silicate pb known until now the most important feature of the data is that independent of the exact slope of the regression in a common pb diagram a very well defined intercept with plausible terrestrial pb evolution lines is obtained between 3 63 and 3 66 ga identical to a combined pb pb regression age of 3654 73 ma mswd 17 6 we conclude that the magmatic precursors of the amitsoq orthogneisses separated from a depleted mantle like source at ca 3 65 ga this contrasts sharply with claims based on ion probe u pb zircon dating that the amitsoq gneisses had a prolonged episodic magmatic evolution extending from ca 3 87 to 3 60 ga pb isotopes offer no evidence that any amitsoq gneiss analysed until now began its evolution in the crust as long ago as 3 85 ga we propose that zircons significantly older than 3 65 ga were inherited from chemically evolved rocks as old as ca 3 85 ga geochemical arguments are presented to support this proposal new feldspar and whole rock pb isotope data are also reported for sheets of quartz dioritic gneiss on akilia island that yield the oldest zircon u pb age clusters in the range 3 84 3 87 ga these sheets are discordant to a metasedimentary enclave containing accessory apatite with graphite inclusions whose c isotope ratios are regarded as biogenic in origin as above pb isotopes in the sheets are consistent with a magmatic age of 3 65 ga which we regard as the true minimum age for these waterlain sediments and their possibly biogenic markers additional pb isotope data as well as limited published sm nd and u pb zircon data on the akilia enclaves themselves suggest an age between ca 3 65 and 3 70 ga for their original deposition some 150 200 ma younger than proposed previously thus the question of overlap of earliest life with a bolide impact scenario terminating 3 80 ga as on the moon becomes irrelevant

timing of late archaean terrane assembly crustal thickening and granite emplacement in the nuuk region southern west greenland abstract a breakthrough in understanding the structural and metamorphic evolution of archaean gneiss complexes occurred with the recognition that the nuuk region of southern west greenland comprised separate terranes assembled in the late archaean from northwest to southeast these are the akia 3220 2970 ma akulleq 3870 3600 and 2820 ma and tasiusarsuaq 2920 2860 ma terranes the minimum time of assembly is recorded by the first event common to all component terranes using shrimp u pb zircon geochronology the oldest events common to all terranes including emplacement of crustally derived granites contemporaneous metamorphism and anatexis have been dated at 2710 2725 ma in the akulleq terrane areas where in situ diatexite formed abundant granitoid sheets were intruded and common growth of metamorphic zircon mostly low th u in most lithologies occurred in the akia and tasiusarsuaq terranes there was only intrusion of a lesser number of 2720 ma granitoid sheets because metamorphic zircons of that age have not been found the 2720 ma event is interpreted as marking or shortly following terrane assembly when the diverse components of the akulleq terrane were tectonically juxtaposed with the other two this new documentation of metamorphic and associated igneous events within an accreting cratonic region is an illustration that the stabilisation of extensive areas of archaean gneisses can be due to accretionary tectonics long after the individual components were first formed

origin of archean tonalite trondhjemite granodiorite ttg suites and granites in the fiskenaesset region southern west greenland implications for continental growth abstract mesoarchean to neoarchean orthogneisses 2 95 2 79 ga in the fiskenaesset region southern west greenland are composed of an older suite of metamorphosed tonalites trondhjemites and granodiorites ttgs and a younger suite of high k granites the ttgs are characterized by high al 2 o 3 14 2 18 6 wt na 2 o 3 4 5 13 wt and sr 205 777 ppm and low y 0 7 17 4 ppm contents on chondrite and n morb normalized trace element diagrams the ttgs have the following geochemical characteristics 1 highly fractionated ree patterns la yb cn 14 664 la sm cn 4 3 11 0 gd yb cn 1 5 19 7 2 strong positive anomalies of sr sr sr 1 0 15 9 and pb pb pb 1 4 34 9 and 3 large negative anomalies of nb nb nb 0 01 0 34 and ti ti ti 0 1 0 6 the geochemical characteristics of the ttgs and trace element modeling suggest that they were generated by partial melting of hydrous basalts amphibolites at the base of a thickened magmatic arc leaving a rutile bearing eclogite residue field observations suggest that spatially and temporarily associated tholeiitic basalts now amphibolites in the fiskenaesset region might have been the sources of ttg melts the high k granites have steep ree patterns la yb cn 3 8 506 la sm cn 2 7 18 9 gd yb cn 0 92 12 1 and display variably negative eu anomalies eu eu 0 37 0 96 and moderate sr 84 539 ppm contents four outlier granite samples have variably positive eu eu eu 1 0 12 anomalies given that the granodiorites have higher k 2 o na 2 o than the tonalites and trondhjemites it is suggested that the granites were derived from partial melting of the granodiorites it is speculated that the dense eclogitic residues left after ttg melt extraction were foundered into the sub arc mantle leading to basaltic underplating beneath the lower rust melting of the granodiorites in response to the basaltic underplating resulted in the production of high k granitic melts formation of the fiskenaesset ttgs the foundering of the eclogitic residues into the mantle and the emplacement of the high k granites led to the growth of archean continental crust in the fiskenaesset region

initiation and flow behavior of the 1980 pine creek and muddy river lahars mount st helens washington two large high velocity lahars volcanic debris flows were triggered by a pyroclastic surge during the first few minutes of the may 18 1980 eruption of mount st helens the initial surge cloud evolved progressively by gravity segregation from a gas mobilized highly inflated density flow to a dense water mobilized basal debris flow lahar and accompanying ash cloud as it flowed down the east flank of the volcano the main source of the water for the lahars was probably from eroded snow and ice incorporated into the flow by turbulent mixing but ground water expelled together with the rock debris by the initiating volcanic explosions also may have contributed peak lahar discharge from the pine muddy fan upper smith creek and ape canyon probably exceeded 250 000 m 3 s initially but decreased exponentially in the downstream direction total volume of the lahars was in excess of 1 4 10 7 m 3 initial peak flow velocities in excess of 30 m s also decreased markedly downstream where flow was not impeded velocity was strongly related to the depth slope term r 2 3 s 1 2 from the manning uniform flow equation as a power law function during much of the route traveled lahar flow appears to have been supercritical deposits left in channels were generally thin relative to flow depth 0 to 2 5 m particles up to small boulder size were randomly distributed in the poorly sorted nonstratified matrix indicating complete suspension in a fully developed debris flow slurry however much larger clasts were transported as bed load computed sediment concentrations of matrix slurry samples ranged from 84 to 91 solids by weight and were similar for the two lahars two indirect methods for computing peak flow velocity previously only tentatively applied to debris flows were tested for accuracy by comparing computed lahar arrival times with recorded arrival times at swift reservoir the computed velocities appear to be 15 slower than the recorded velocities which is consistent with the restriction that the velocity formulas produce only minimum values

geochemistry of ultramafic rocks and hornblendite veins in the fiskenaesset layered anorthosite complex sw greenland evidence for hydrous upper mantle in the archean abstract the fiskenaesset complex sw greenland contains the world s best preserved archean 2970 ma layered anorthosite leucogabbro gabbro and ultramafic association the complex was emplaced into archean oceanic crust distal from continental lithosphere and later intruded by tonalites trondhjemites and granodiorites ttg constituting archean continental crust the complex and bordering ttg intrusions were variably affected by granulite facies metamorphism and retrogressed under amphibolite facies conditions this study presents new whole rock major and trace element petrographic and sem scanning electron microscope bse backscatter electron image data for a 45 60 m thick ultramafic sill and cross cutting hornblendite veins the ultramafic sill is composed of 75 olivine pyroxene hornblendite 15 pyroxene hornblendite and 10 hornblende pyroxenite despite granulite facies metamorphism and multiple phases of deformation the fiskenaesset ultramafic sill shares many petrographic characteristics of unmetamorphosed mafic to ultramafic layered intrusions and alaskan type ultramafic complexes hornblende appears to have originated as an igneous mineral but underwent extensive recrystallization during granulite facies metamorphism formation of orthopyroxene magnetite symplectitic vermicular intergrowths mainly at the expense of olivine is attributed to chemical reactions between late stage residual hydrous melts and olivine field geochemical and petrographic observations suggest that the ultramafic sill did not undergo significant fractional crystallization following its intrusion high mgo 16 31 wt contents suggest that the sill was emplaced as a crystal mush rather than as melt only petrographic observations and bse images indicate that olivine orthopyroxene and clinopyroxene were the major crystal phases whereas hornblende represents the main interstitial liquid phase in the crystal mush the sill exhibits three different ree patterns corresponding to three cryptic layers although the origin of these ree patterns is not fully understood they may reflect the intrusion of three batches of magma containing different hornblende forming melt crystal ratios geochemical data indicate that the cross cutting hornblendite veins likely originated from evolved late stage melts but were not related to the melts of the ultramafic sill large negative nb anomalies nb nb 0 04 0 66 suggest that the fiskenaesset hornblendites hornblende pyroxenites and hornblendite veins were derived from a hydrous sub arc mantle peridotite phanerozoic hornblendites are typically associated with supra subduction zone ophiolites and magmatic arcs accordingly it is suggested that water was recycled to the source of the fiskenaesset hornblendites through subduction of altered oceanic crust recycling of water to the upper mantle via subduction not only resulted in the generation of hornblendites but also played an important role in the formation of ttg dominated archean continental crust therefore the field petrographic and geochemical data presented in this study suggest that the origin of the mesoarchean to neoarchean terrane in the fiskenaesset region sw greenland is consistent with subduction zone geodynamic processes rather than non uniformitarian density driven vertical crustal overturn and diapiric processes

the influence of debris flows on channels and valley floors in the oregon coast range u s a debris flows are one of the most important processes which influence the morphology of channels and valley floors in the oregon coast range debris flows that initiate in bedrock hollows at heads of first order basins erode the long accumulated sediment and organic debris from the floors of headwater first and second order channels this material is deposited on valley floors in the form of fans levees and terraces in channels deposits of debris flows control the distribution of boulders the stochastic nature of sediment supply to alluvial channels by debris flows promotes cycling between channel aggradation which results in a gravel bed morphology and channel degradation which results in a mixed bedrock and boulder bed morphology temporal and spatial variability of channel bed morphology is expected in other landscapes where debris flows are an important process

sedimentologic and geomorphic variations in storm generated alluvial fans howgill fells northwest england in june 1982 a storm with a return period greater than 100 yr but lasting less than 2 5 hr destabilized hillslopes and produced a suite of geomorphologically and sedimentologically diverse alluvial fans thirteen major fans were deposited at the tributary junctions between small 2 catchments and two north flowing headwater streams of the river lune northwest england storm generated fans spread over or became inset into older stable fans and produced localized vertical accretion of as much as 3 m and lateral accretion of as much as 100 m sedimentary processes operating during deposition involved debris flow transitional flow and streamflow six facies types are recognized on the basis of depositional topography sedimentary texture and fabric and matrix content viscous debris flow d1 dilute debris flow d2 transitional flow t1 fluvial bars and lobes s1 s2 and fluvial sheet gravels s3 regionally streamflow deposition prevails over debris flow deposition and type s3 facies has the greatest areal extent temporal and spatial variations in facies deposition during the storm however resulted from water sediment ratio variations fan deposition involved an early phase of debris flow to transitional flow due to large inputs of sediment from hillslope failures this was followed by a systematic change to more dilute conditions resulting in streamflow deposition and eventually in channel incision a significant amount of geomorphic work and complex variations in sedimentary processes during the storm resulted in part from extensive overland flow and hillslope destabilization discriminant analyses indicate that catchment size channel gradient and percentage of area eroded during the storm controlled whether debris flow or streamflow facies dominated a fan sequence smaller steeper catchments had a greater percentage of the area yielding sediment and are dominanted by debris flows whereas larger catchments produced more runoff resulting in dilution and streamflow the results indicate that the facies sequences and fan entrenchment in the howgill fells which are typically considered products of longer term climatic change or tectonics in other localities are here primarily affected by thresholds related to catchment geomorphology by the type of sediment available and by the position within the storm cell

building mesoarchaean crust upon eoarchaean roots the akia terrane west greenland constraining the source genesis and evolution of archaean felsic crust is key to understanding the growth and stabilization of cratons the akia terrane part of the north atlantic craton west greenland is comprised of meso to neoarchaean orthogneiss with associated supracrustal rocks we report zircon u pb and lu hf isotope data and whole rock geochemistry from samples of gneiss and supracrustals from the northern akia terrane including from the finnefjeld orthogneiss complex which has recently been interpreted as an impact structure isotope data record two major episodes of continental crust production at ca 3 2 and 3 0 ga minor ca 2 7 and 2 5 ga magmatic events have more evolved ehf interpreted as reworking of existing crust perhaps linked to terrane assembly felsic rocks from the finnefjeld orthogneiss complex were derived from the same source at the same time as the surrounding tonalites but from shallower melting requiring any bolide driven melting event to have occurred almost simultaneously alongside the production of the surrounding crust a simpler alternative has the finnefjeld complex and surrounding tonalite representing the coeval genesis of evolved crust over a substantial lithospheric depth hafnium isotope data from the two major mesoarchaean crust forming episodes record a contribution from older mafic eoarchaean crust invoking the involvement of an eoarchaean root in the growth of younger mesoarchaean crust puts important constraints on geodynamic models of the formation of the discrete terranes that ultimately assembled to form earth s cratons

the whole rock sm nd age for the 2825 ma ikkattoq gneisses greenland is 800 ma too young insights into archaean ttg petrogenesis abstract the ikkattoq gneisses of the archaean gneiss complex in the nuuk region southern west greenland are the orthogneiss component within the amphibolite facies tre brodre terrane they have mostly granodioritic compositions with a small amount of quartz diorite sm nd isotopic data for a quartz diorite and five granodiorite ikkattoq gneiss samples from within 5 km of the ikkattoq fjord type locality yielded a regression with a slope equivalent to 2005 52 ma mswd 0 72 regardless of the low mswd this cannot be the true age of the ikkattoq gneisses because all ikkattoq gneisses yield u pb zircon dates of c 2825 ma and they are cut by the undeformed 2560 ma qorqut granite complex this anomalously young regression age resulted instead from mixing of different nd components indicating that the ikkattoq gneisses are derived from mixed source materials taking the true age of the ikkattoq gneisses as 2825 ma from u pb zircon dating the range of initial e nd in the ikkattoq gneisses is 7 1 to 1 8 the negative initial e nd values mean that older light rare earth enriched sialic crust contributed to the igneous precursors of the ikkattoq gneisses this nd evidence for contribution of older sialic crust is supported by positive e sr values for the ikkattoq gneisses with e nd values as low as 7 1 this older crustal component has to be eoarchaean the presence of scarce quartz diorites low sio 2 high mgo suggests that ultramafic rocks upper mantle metasomatised by the passage of fluids or silicic melts were another contributing source the ikkattoq gneisses are proposed as a complex suite incorporating material derived from melting of much older sialic crust and probably upper mantle the intercalation of tectonostratigraphic terranes during collisional orogeny at c 2720 ma destroyed the architecture of this 2825 ma magmatic system and the ikkattoq gneisses now form a slice tectonically isolated from their source region in terms of trace element parameters the ikkattoq gneisses resemble phanerozoic volcanic arc granites thus an andean style arc setting for the generation of the ikkattoq gneiss precursors is possible other archaean ttg suites of the nuuk region are generally thought to represent predominantly juvenile additions to the crust in the broadest sense they do because isotopic work over the past 30 years has demonstrated that they do not represent wholesale recycling of considerably older crust however in detail within these broadly juvenile suites a contribution from older crust can be detected thus c 3000 ma type nuk gneisses from around nuuk town show a spread in e nd values down to 1 7 in this case the likely older crustal component was 3230 ma quartz diorite that occurs as enclaves in the c 3000 ma suite thus to a lesser or greater degree some meso to neoarchaean ttg suites in the nuuk region display the same internal complexities and evidence for mixed sources as modern arc suites developed near the margins of older crust

geochemistry of the mesoarchean fiskenaesset complex at majorqap qâva sw greenland evidence for two different magma compositions abstract the mesoarchean fiskenaesset complex was emplaced as multiple sills and dykes of magma and crystal mush into oceanic crust forming an association of ca 550 meter thick layered anorthosites leucogabbros gabbros hornblendites pyroxenites dunites and peridotites the complex was intruded by tonalites trondhjemites and granodiorites ttg and underwent multiple phases of deformation the fiskenaesset complex and bordering ttg intrusions were variably affected by granulite facies metamorphism and retrogressed under amphibolite facies conditions this study presents new field petrographic and major and trace element geochemical data for the six major stratigraphic units of the fiskenaesset complex at majorqap q va sw greenland in addition new high precision trace element data are reported for hornblendes from three stratigraphic units the sampled major lithological units from bottom to top are lower gabbro ultramafic lower leucogabbro middle gabbro upper leucogabbro and anorthosite both whole rock and hornblende trace element data are consistent with crystallization of the complex from two different parental magmas originating from different mantle sources subsequent differentiation of these parental magmas in the oceanic arc crust resulted in two evolved geochemical rock suites suite a and suite b which are both characterized by depletions of hfse nb ta zr and hf relative to th u lree and lile rb ba and enrichments of sr and pb relative to mree these data are consistent with a magmatic arc geodynamic setting

the efficacy of stream power and flow duration on geomorphic responses to catastrophic flooding abstract geomorphologists have long studied the impacts of extreme floods yet the association between the magnitude of flow parameters discharge velocity shear stress or stream power and resulting geomorphic effectiveness remains vague and non deterministic attempts have been made to include flow duration and total expenditure of stream power in combination with peak unit stream power as important variables but there has been minimal exploration of this hydraulic combination taking advantage of tropical storm irene s rapid track through eastern vermont usa in late summer 2011 this paper presents the array of geomorphic responses to a short duration time to peak of

petrogenesis of archaean trondhjemites tonalites and granodiorites from eastern finland major and trace element geochemistry

geochemistry of anorthositic differentiated sills in the archean 2970 ma fiskenaesset complex sw greenland implications for parental magma compositions geodynamic setting and secular heat flow in arcs abstract the fiskenaesset complex sw greenland is one of the best preserved layered archean intrusions in the world consisting of an association of ca 550 meter thick anorthosite leucogabbro gabbro and ultramafic rocks dunite peridotite pyroxenite and hornblendite despite poly phase deformation and amphibolite to granulite facies metamorphism primary cumulate textures and igneous layering are well preserved in the complex this study reports new major and trace element data for three variably thick 1 to 5 m differentiated dunite through peridotite pyroxenite gabbro leucogabbro to anorthosite sequences sequences 1 2 and 3 in the sinarssuk area of the fiskenaesset region on several variation diagrams samples from these sequences plot along a well defined liquid line of descent consistent with in situ fractional crystallization the average chemical compositions of these sequences are used to constrain their approximate parental magma compositions petrographic observations and geochemical data suggest that sequences 2 and 3 solidified from evolved magmas that underwent olivine fractionation prior to their intrusion in contrast sequence 1 appears to have been derived from a near primary parental magma sio2 43 wt mgo 20 wt al2o3 16 wt cao 9 3 wt ni 840 ppm mg number 80 the trace element patterns of this parental magma are comparable to those of phanerozoic boninites consistent with a supra subduction zone geodynamic setting if the relative thickness of ultramafic layers the sum of dunite peridotite and pyroxenite layers in differentiated sequences is taken as an analog for the original complex emplaced into archean oceanic crust the fiskenaesset complex might have had a minimum thickness of 1000 m with a 500 m thick ultramafic unit at the bottom the thickness of the ultramafic unit in the preserved complex is less than 50 m suggesting that more than 90 of the original ultramafic unit was either delaminated and recycled back into the mantle as a residual cumulate or was destroyed during thrusting and ttg intrusion both the fiskenaesset complex and associated tholeiitic basalts display similar th nb lree patterns and plot along the same differentiation trend on zr versus incompatible trace element diagrams suggesting a possible petrogenetic link between the two suites of rocks however basalts do not display the same differentiation trend as the complex on several major and trace element diagrams in addition basalts and parental magma to the complex do not plot along the same fractionation line on al2o3 tio2 versus incompatible trace element diagrams implying that the fiskenaesset layered intrusive rocks were not derived from tholeiitic basalts through fractional crystallization as previously thought accordingly we infer that the fiskenaesset complex and spatially associated basalts were derived from different mantle sources parental magmas to the fiskenaesset complex originated from a mantle source that was metasomatized by highly aluminous slab derived melts layered anorthosite complexes are mostly restricted to the archean and typically associated with tonalite trondhjemite granodiorite ttg gneisses the petrogenesis of both suites appears to have been controlled mainly by slab melting endorsing independent evidence for a secular change from slab dominated melting to wedge dominated melting in arc magmatism at the end of the archean reflecting higher geothermal gradients in archean arcs than post archean counterparts

the u th pb systematics of zircons from the type nuk gneisses godthabsfjord west greenland abstract u pb analyses were made on zircons separated from different lithologies within the nuk gneisses taken from godthab nuuk townsite and from the south west shore of bjorneoen size fractions of the zircons from each rock yield linear discordia which give apparent crystallisation ages for the parents of the nuk gneisses of 2890 3065 myr this spread of ages is not a result of multiple intrusion but more likely results from different degrees of diffusion and metamorphic episodic lead loss by the zircons since u pb and th pb discordance are similar loss of radiogenic lead and not u or th loss gain by radiation damage diffusion and through metamorphism is the main factor causing discordance of the u pb dates the results show that the discordia line given by the analyses of fractions of a single zircon population may not give a true crystallisation age when the rock has been strongly metamorphosed this interpretation is supported by rb sr whole rock analyses of the same rocks in which the nuk townsite rocks scatter about a reference isochron of 2840 myr while the bjorneoen samples give an isochron mswd 0 55 of 3076 27 myr and the initial ratio at 0 70205 0 00009 the zircon results do not rule out the possibility that more than one metamorphic episode affected the original nuk gneiss parents prior to 2520 myr qorqut granite intrusion a date of 2660 myr was obtained from an aplite intruded during a period of plutonic activity that included the formation of a linear belt of intense ductile deformation that extends through south western bjorneoen and godthab town