long lived localized magmatism in central eastern part of the pernambuco alagoas domain borborema province ne brazil implications for tectonic setting heat sources and lithospheric reworking magmatic activity circumscribed to a restricted area lasting for tenths of millions of years allows to link changes in magma chemistry to deformation and thus to determine the tectonic evolution of a region here we describe an area of about 180 40 km in eastern borborema province northeastern brazil where several pulses of magmatism occurred in the time interval 656 562 ma the oldest pulses are represented by orthogneisses bearing a flat lying foliation resulting from deformation and metamorphism of granitoids emplaced in three stages 656 645 ma 632 618 ma and 606 597 ma the oldest and most important unit 656 652 ma old has large length width ratio c 10 alkali calcic signature lacks pronounced negative ta anomalies whereas showing positive hf and zr anomalies in normalized trace element diagrams and yields crystallization age similar to the deposition age of surrounding dominantly clastic metasedimentary units these characteristics point to emplacement in an extensional setting and most probably fit in a rift sag basin model with thermal thinning of the lithosphere generating regional subsidence and magmatism rocks related to the 632 618 ma old stage are represented by orthogneisses with petrographic and geochemical characteristics akin to i type granites indicating a shift to an orogenic setting at around 640 630 ma increased radioactive heating during continued thickening promoted anatexis of metasedimentary rocks at mid crustal levels and segregation of small volumes of peraluminous melts at 606 597 ma the subsequent period of magmatic activity at 587 581 ma witnesses the transition to a transcurrent regime with intrusion of large granitic and syenitic batholiths with high k calc alkalic to shoshonitic affinity intrusion of these plutons added to heat resulting from strain heating to produce peraluminous granites at 573 566 ma through partial melting of metasedimentary sources the final magmatic episode at c 562 ma suggests involvement of the lithospheric mantle and crustal sources perhaps related to localized delamination generally younger orthogneisses and plutons contain zircons inherited from previous magmatic events pointing to important crustal reworking together with the geochemistry of the rocks these results imply only modest addition of juvenile material and thus no net crustal growth in eastern borborema province during the 660 560 ma time interval

thickness and extent of the subglacial till layer beneath an antarctic paleo ice stream fast flowing ice streams and outlet glaciers currently account for as much as 90 of the discharge from the antarctic and greenland ice sheets although the deformation of subglacial material has been proposed as the mechanism for this rapid motion such sediment is usually hidden under several kilometers of ice marine geophysical records have allowed reconstruction of the three dimensional thickness of the sedimentary bed beneath a large antarctic paleo ice stream for the first time fast flow is indicated by streamlined seafloor lineations that form the surface of a layer of low shear strength unsorted sediment averaging 4 6 m thick rapid motion of the paleo ice stream was a result of subglacial deformation within this layer

the internal sediment architecture of a drumlin port byron new york state usa abstract an exposure within the central portion of a large drumlin at port byron new york state usa part of the large new york drumlin field reveals a sequence of steeply dipping cemented sands and gravels of proglacial ice contact deltaic origin overlain by a thin till veneer the sands and gravels appear to have been deposited within the proximal proglacial environment during a late retreat phase of the laurentide ice sheet sometime prior to being overridden by subsequent ice and drumlinized during deposition of the ice contact delta escaping subglacial regelation meltwater permeated the proximal deltaic sediment pile and calcium carbonate was released in a series of pulses to form pore occluding calcite cement within the sand and gravel porespaces the calcium carbonate precipitated into the sands and gravels due to a reduction in hydrostatic pressure and co2 outgassing of the meltwater as it exited from beneath the ice sheet once cemented these deltaic sediments were considerably stronger and acted afterward as an obstacle around which the future ice advance streamed and in turn produced the characteristic drumlin shape in overriding the ice contact deltaic sediments the ice sheet emplaced a thin layer of till which exhibits syndepositional deformation features indicative of being emplaced as a deforming bed layer beneath the advancing ice sheet micromorphological analysis of the overlying till shows that no interstitial or intraclastic calcite occurs within the till

modes of operation of thermo mechanically coupled ice sheets by richard c a hindmarsh geoffrey s boulton grant institute of geology university of edinburgh west mains road edinburgh eh9 3jw scotland u k and kolumban hutter lnstitut fur mechanik technische hochschule oarmstadt hochschulstrasse i 0 6100 oarmstadt federal republic of germany dimensionless lapse rate a dimensionless model of thermo mechanically coupled ice sheets is used to analyse the operation of the system three thermal processes are identified i dissipation having a maximum time scale of thousands of years ii advection having a time scale of tens of thousands of years and iii conduction having a time scale of loo 000 years kinematical processes occur on two time scales i a marginal advective time scale of thousands of years and ii a diffusive time scale of tens of thousands of years dominant in the divide area

ice stream surface texture sticky spots waves and breathers the coupled flow of ice till and water this paper addresses the coupling of flows of ice till and water and the issue of whether such coupling provides mechanisms for meso scale kilometres to tens of kilometres variability in ice sheet flow and texture the question of whether effective pressures at the ice bed interface are statically or hydraulically controlled is examined in this paper the answer is scale dependent and has a significant effect on the relationship between ice surface and basal topography the consequences of these considerations on till flow coupled ice till flow and coupled ice till and water flow are examined an analysis of till flow kinematics and shock formation is carried out the linear stability of coupled long wavelength ice till flow is analysed and regions in parameter space where this flow is unstable with rather small rate constants are found upstream moving ice surface waves are predicted the linear stablity of coupled ice till water flow is examined where water flow is modelled using a basal flow system with effective pressure dependent properties again regions in parameter space where the system is linearly unstable are found this time with relatively rapid rate constants the water pressure exhibits breather modes these analyses assume that there is a substantial basal traction a problem with models of ice streams wholly restrained at the side is identified they seem to predict erosion rates which are unfeasibly large there appears to be sufficient variability in the ice till water system to potentially explain texture in ice stream surfaces variations in ice stream thickness of tens of metres not directly relatable to topography and waves moving upstream or downstream most importantly the ice stream bed system is shown to exhibit meso scale variability simply by coupling ice flow according to the shallow ice approximation till flow according to the hydrostatic thin till approximation and water flow according to an effective pressure dependent hydraulics

microfabric and microshear evolution in deformed till abstract the bed deformation model of glacier movement requires that ice masses shear their beds to high strains for homogeneous basal tills that can result from such deformation however there are no sedimentological criteria for estimating strain magnitude thus a large ring shear device was used to shear two basal tills and their particle fabrics and microshears were studied at various strains up to 108 sand particle fabrics in a longitudinal flow plane became steady at shear strains of 7 39 steady fabrics were moderately strong with s1 eigenvalues of 0 71 0 74 these values are smaller than those from experiments with pebble sized particles perhaps reflecting the greater probability of sand particles colliding with larger grains fabrics parallel to flow strengthened progressively indicating that particles did not reorient with their long axes transverse to flow r1 and r2 microshears developed and persisted to the highest strains attained r1 shears became more pervasive with strain and oriented closer to the shear plane movement along both sets of shears provides an explanation for the plunge of the steady state sand fabric approximately 10 upglacier these experiments provide a basis for estimating till deformation magnitude but results will require cautious application to the geologic record where deformation processes may be ambiguous

the death mask of the antarctic ice sheet comparison of glacial geomorphic features across the continental shelf abstract multibeam swath bathymetry data seismic lines and shallow cores were collected seaward of all of the major drainage outlets of the antarctic ice sheet from the pennell coast of north victoria land to the northwestern weddell sea the results show that during the last glacial maximum the ice sheet extended onto the outer shelf in all areas to the shelf break in many swath bathymetry and deep tow sidescan sonar data from the continental shelf also record the geomorphology left by the retreating ice sheet glacial troughs occur offshore of all major glacial outlets in each drainage area except for parts of the ross sea the inner shelf is characterized by acoustic basement interpreted as crystalline substrate the geomorphology of these inner shelf areas consists of erosional features such as grooves that show that ice flow tended to follow the structural grain of the bedrock outer shelf areas are floored by sedimentary substrate where the direction of ice flow was more directly offshore and depositional features characterize the seafloor in these areas the signature of the grounded ice consists of till deposits and large scale geomorphic features mega scale glacial lineations drumlins occur within the region of contact between crystalline and sedimentary substrates meltwater channels also have been imaged in both inner and outer shelf settings while the presence of meltwater across the shelf has long been suspected we present here the first multibeam image of a meltwater channel on the outer continental shelf of antarctica at the shelf break gullies are in all areas where we know ice reached the shelf break we note the remarkable similarity in form of mega scale glacial lineations observed on the outer shelf over sedimentary substrate across all areas surveyed typically measuring 200 600 m crest to crest this suggests that the shape of these landforms is governed by the same process in each drainage area and that process was occurring under a large area of the ice sheet the erosional and transitional features show much variability in form and distribution

tectono thermal evolution magma emplacement and shear zone development in the caruaru area borborema province ne brazil abstract the neoproterozoic borborema province bp in northeastern brazil is characterized by a regional flat lying foliation and by abundant magmatic rocks commonly spatially associated with large transcurrent shear zones combined field micropetrographic thermobarometric and magmatic fabric studies carried out on plutons country rocks and shear zones in the eastern domain of the province the caruaru area reveal that 1 an early episode of regional deformation with a top to the ne displacement was followed by the development of conjugate strike slip shear zones 2 similar low pressure 650 c metamorphic conditions occurred during the two events 3 plutons intruded the flat lying foliation 4 magma emplacement slightly predated strain localization in transcurrent shear zones but 5 plutons underwent strike slip deformation before complete crystallization therefore regional deformation pluton emplacement and shear zone development were successive events occurring over a relatively short time span 40ar 39ar laser dating of amphibole 584 ma and biotite 545 553 ma single grains supports slow cooling 5 c ma of country rocks through the argon closure temperatures of these minerals complex sometimes discordant amphibole ages bracketed between 552 and 575 ma for shear zones may be attributed to episodic activity and or dike swarm intrusions into them which disturbed the ar system biotite ages for the shear zones 533 545 ma also imply a relatively low cooling rate these data show that the massive injection of magmas in the continental crust of the bp during the brasiliano pan african orogeny produced a long lived thermal anomaly of regional extent the 40ar 39ar ages in the caruaru area are older than in other segments of the bp with similar geological characteristics this indicates a tectono thermal history for the bp that is more complex than previously recognized with either diachronous deformation magmatism and associated low p high t metamorphism or contrasting cooling of the different domains possibly due to differential vertical movements

the signature of strain magnitude in tills associated with the vistula ice stream of the scandinavian ice sheet central poland abstract at oborki in central poland we examined a 3 m thick late weichselian basal till deposited by the vistula ice stream a multi proxy approach involving a study of grain size grain roundness crushing index petrographic composition till fabric and till micromorphology in a closely sampled vertical profile was applied to decipher till formation and ice movement mechanisms the till consists of a succession of four texturally and structurally different units generated by a combination of processes related to effective stresses exerted on the bed by active ice deformation intergranular advection clast ploughing and localized basal decoupling played important roles during till formation micromorphological analysis reveals that the most common s matrix microstructures are grain lineations grain stacks turbate structures till pellets and crushed grains microshears dominate whereas plasmic fabric is not well developed we use grain roundness crushing indices and microshear geometry to determine subglacial strain magnitude in the till matrix microfabric and macrofabric s 1 eigenvalues together with i l indexes indicate low shear strains in the order of 10 10 2 strain estimates vary depending on the particle sizes used suggesting a plastic mode of till deformation all proxies studied suggest complex and temporally variable subglacial conditions under the marginal part of the vistula ice stream probably caused by changes in till rheology related to fluctuating porewater pressures fast ice flow was facilitated by a combination of bed deformation and enhanced basal sliding on a thin water layer our results may also be representative for other terrestrially based pleistocene ice streams moving over soft beds

origin of high potassium talc alkaline i type granitoids many i type granitoid magmas are generated through partial melting of older metaigneous rocks and the compositions of such melts are broadly calc alkaline and metaluminous these melts are granitic to tonalitic and result from thermal extremes in their lower crustal source regions data on the experimental partial melting of common crustal rocks suggest that high k i type granitoid magmas can be derived only from the partial melting of hydrous calc alkaline to high k calc alkaline mafic to intermediate metamorphic rocks in the crust because of their low k2o contents metabasaltic rocks of all kinds are unsuitable as sources and models that propose mixing of mantle derived basaltic magmas and crustal melts are also inadequate there is no requirement that i type calc alkaline magmatism be related in any way to subduction processes

bulk rock and mineral chemistries and ascent rates of high k calc alkalic epidote bearing magmas northeastern brazil abstract a manifestation of the pan african brasiliano orogeny 700 550 ma in northeastern brazil was the emplacement of widespread neoproterozoic granitoids in diverse tectonic terranes among these plutons are the magmatic epidote bearing conceicao das creoulas caldeirao encantado murici and boqueirao plutons located close to the boundary between the alto pajeu and cachoeirinha salgueiro terranes the plutons are high k calc alkalic granodiorites to monzogranites with tabular k feldspar megacrysts pistacite atomic fe 3 fe 3 al in epidote in these granitoids ranges from 21 to 27 high oxygen fugacity log f o 2 19 to 13 and the preservation of epidote suggest that the magma was oxidized al in hornblende barometry indicates hornblende solidification between 6 and 8 kbar at 620 to 780 c according to the hornblende plagioclase thermometer zircon saturation thermometry attests to a near liquidus temperature range from 794 to 853 c partial corrosion of magmatic epidote in these four plutons occurred during an interval of no more than 10 30 years which corresponds to maximum magma ascent rates of 650 1000 m year diking associated with regional shearing probably facilitated rapid transport of granitic magma through hot continental crust at peak metamorphism and permitted survival of epidote that was out of equilibrium at the low pressure of final emplacement similarities between mineralogical composition chemistry and isotopic compositions end 0 60ga between 2 and 5 t dm from 1 2 to 1 3 ga 18 o values 10 v smow of these four plutons and neoproterozoic magmatic epidote bearing plutons elsewhere in northeastern brazil argue for similar metabasaltic mafic sources that had previously experienced low temperature alteration

seafloor geomorphology of the sw barents sea and its glaci dynamic implications abstract three dimensional 3d reflection seismic imaging techniques provide a unique means of identifying large scale submarine geomorphic features produced by former ice sheets we have used such techniques to image the geomorphology of the sw barents sea but using a dense grid of 2d seismic covering a 240 000 2 km large study area supplemented with 3d seismic from selected areas the study area is located at the convergence of ice flow from the former fennoscandian barents sea and svalbard ice sheets the morphology of the major cross shelf troughs is characterized by elongated ridge groove features interpreted to be glacial bedforms produced by fast flowing grounded glaciers mega scale glacial lineations have been grouped into distinct flow sets related to three specific ice flow events these events document the last glacial maximum lgm and two readvances during deglaciation of specific interest is a detailed and well preserved set of seafloor imprints in the bjornoyrenna trough which acted as the main pathway for ice streams draining the surrounding ice sheets the geomorphic features indicate that bjornoyrenna hosted six separate ice stream lobes that diverged fan like at their margins but were not all active simultaneously an end moraine zone 68 km wide up to 85 m high and 300 km long filled the whole width of this major glacial trough as a result of high sediment flux within sub ice stream deformable beds the imprint of this ice stream system can be followed on our seabed images for 500 km however detailed geomorphologic elements demonstrate that these imprints reflect time transgressive behaviour and do not indicate the length or width of single ice streams the seismic derived seafloor geomorphology demonstrates that the barents sea ice behaved in a much more dynamic way during the last glaciation than previously assumed with major shifts between ice originating from the fennoscandian spitsbergen and barents sea ice sheets

flow mechanism of the des moines lobe of the laurentide ice sheet rapid flow of the des moines lobe of the laurentide ice sheet may have been related to its unlithified substrate new reconstructions of the lobe based on moraine elevations sediment subsidence during moraine deposition and flow direction indicators indicate that the lobe may have been 3 times thicker than in previous reconstructions nevertheless implied basal shear stresses are 15 kpa so internal ice deformation was not significant instead the lobe likely moved by a combination of sliding plowing of particles through the bed surface and bed shear consolidation tests on basal till yield preconsolidation stresses of 125 300 kpa so effective normal stresses on the bed were small a model of sliding and plowing indicates that at such stresses most particles gripped by the ice may have plowed easily through the till bed resulting in too small a shear traction on the bed to shear it at depth consistent with this prediction measurements of orientations of clasts in basal till yield a weak fabric implying pervasive bed shear strain less than 2 although some stronger fabrics have been reported by others we infer tentatively that movement was principally at the bed surface by plowing

experimental evidence for high temperature felsic melts formed during basaltic intrusion of the deep crust basaltic intrusion into the continental crust was simulated with melting experiments on layered charges of high alumina olivine tholeiite haot and metapelite at 11 kbar rapid diffusion of cao and na 2 o into the metapelite and h 2 o k 2 o sio 2 and al 2 o 3 into the basalt resulted in spectacular changes in the phase assemblage garnet megacrysts crystallized profusely across the basalt layer cumulate assemblages closely resembled rocks found in underplating localities and glass compositions graded from dacite to rhyolite even at temperatures as high as 1150 c silicic melts can be generated during interaction of basalt with metapelite at temperatures well above the h 2 o saturated granite minimum calculations based on these results and comparison with natural occurrences suggest that high temperature felsic melts with 2 o can form and segregate at rates of 1 cm yr

origin of drumlins on the floor of lake ontario and in upper new york state abstract upper new york state usa contains one of the largest drumlin fields 12 000 km2 on the north america continent consisting of some 10 000 drumlins located between lake ontario in the north and the finger lakes drumlin like bedforms have also recently been discovered in deep water 200 m on the nearby floor of the rochester basin in lake ontario the new york state drumlin field was produced under the laurentide ice sheet 18 000 years ago when it extended southwards to terminate against the allegheny plateau outcrops of drumlin cores exposed by erosion along the lake ontario shoreline provide windows into the stratigraphy and sedimentology of the northern part of the field sedimentological description of 10 coastal drumlin cores combined with quantitative morphometric analysis of 1487 drumlins shows that they are the product of subglacial erosion of till sheets separated by glaciolacustrine deposits a two phase subglacial model for drumlin formation is suggested consisting of initial aggradation of extensive till deposits during ice advance followed by erosion and streamlining possibly accompanying stillstand of the ice margin and building of moraines systematic changes in drumlin morphometry and spacing are evident ranging from small numbers 60 closely spaced ovoid drumlins l w ratios between 1 and 3 drumlin shape appears to be systematically related to sediment thickness available for erosional streamlining elongate spindle forms occur on the crests and lee sides of bedrock escarpments where sediment cover is thin larger ovoid drumlins occur in topographic depressions where sediment thickness is greater offshore streamlined forms on the floor of the deep water rochester basin in lake ontario have long axes at right angles to the trend of onshore drumlins these are composed of rock rock drumlins and have an orientation structurally controlled by a newly identified inlier of proterozoic grenville province metasediments

neoproterozoic high k granites produced by melting of newly formed mafic crust in the huangling region south china abstract the huangling igneous complex in the central yangtze block south china consists of the 850 ma tonalite and trondhjemite which were intruded by the 826 13 ma high k granite the granites are composed of plagioclase 30 60 alkali feldspar 10 30 quartz 20 30 biotite 2 63 1 73 3 wt k 2 o 2 58 4 79 wt and na 2 o 3 90 5 89 wt and low mgo 0 49 1 78 wt and are magnesian alkali calcic in composition their chondrite normalized ree patterns are characterized by enrichment of lree la yb n 14 5 32 8 and mostly negative eu anomalies eu eu 0 5 1 1 they have moderately high sr 202 698 ppm and low y 6 13 18 7 ppm with sr y ratios of 14 64 these large elemental variations probably resulted from fractional crystallization they also have negative whole rock nd 9 75 to 11 73 and zircon hf values 7 1 to 15 8 which are similar to those of the neoproterozoic mafic rocks in the same region suggesting that the granites may have been produced by melting of newly emplaced mafic rocks tonalites and trondhjemites of the huangling complex previously were considered as products by differentiation of water rich basaltic magmas and melting of the thickened archean continental crust in an arc setting respectively therefore the crustal growth and reworking of the yangtze block were probably wholly controlled by the subduction system which resulted in formation of the stable continent during the neoproterozoic

rapid erosion drumlin formation and changing hydrology beneath an antarctic ice stream what happens beneath a glacier affects the way it flows and the landforms left behind when it retreats direct observations from beneath glaciers are however rare and the subglacial environment remains poorly understood we present new repeat observations from west antarctica that show active processes beneath a modern glacier which can normally only be postulated from the geological record we interpret erosion at a rate of 1 m a 1 beneath a fast flowing ice stream followed by cessation of erosion and the formation of a drumlin from mobilized sediment we also interpret both mobilization and increased compaction of basal sediment with associated hydrological changes within the glacier bed all these changes occurred on time scales of a few years or less this variability suggests that an ice stream can reorganize its bed rapidly and that present models of ice dynamics may not simulate all the relevant subglacial processes

evidence from microstructures of deformable bed conditions within drumlins chimney bluffs new york state abstract discussion on the process of drumlin formation has been ongoing for over a century at present two dominant schools of thought prevail viz formation by a subglacial hydraulic mechanisms and b due to subglacial sediment deformation the latter mechanism depends on the existence of a subglacial deformable bed evidence from part of the large central new york drumlin field reveals evidence of syn depositional deformation during the emplacement of diamictons that constitute the major portion of drumlins exposed along shore bluffs in chimney bluffs state park these diamictons cannot be lodgement till facies members but appear to comprise facies indicative of melange style deposition under subglacial deformable bed conditions the evidence obtained from microstructures and plasma fabric within these diamictons supports the existence of a deformable bed during diamicton emplacement it is likely that formation of the drumlins occurred under these same subglacial bed conditions

coupled ice till dynamics and the seeding of drumlins and bedrock forms the geomorphological effects of ice sliding over till internal deformation of till and till sliding over bedrock are considered two questions are examined 1 is the till sheet flow unstable i e is a layer of uniform thickness maintained or not and 2 does the slip of till over bedrock cause amplification of relief of the bedrock such instabilities seem to be necessary to explain such features as drumlins and whaleback forms it is found that the answer to 1 and 2 depends on the position of the system in a parameter space defined by the till rheology and applied shear stress the effective pressure at the ice till interface the thickness of ice and till and the wavelength of the instability two configurations are considered one where the wavelength of the perturbation is much less than the the ice thickness which is related to the classical nye kamb solution for flow over bumps and one where the wavelength is much greater than the ice thickness where the mechanics are described by the shallow ice approximation in both cases substantial areas of parameter space where till sheet and bedrock modes are unstable are found the conceptually related smalley unwin bifurcation is re examined the physical mechanisms by which ice and till flows couple are examined at very short wavelengths 10 m the ice is so rigid that it forces till waves to move at the ice velocity while at long wavelengths 1000 m the flows become essentially uncoupled and till waves move at the kinematic velocity at intermediate wavelengths 100 m high growth rates occur this is postulated to be the scale of drumlin seeding

variable deceleration of whillans ice stream west antarctica the whillans ice stream ice plain wip has been slowing since at least 1963 prior constraints on this slowdown were consistent with a constant long term deceleration rate new observations of ice velocity from 11 continuous and 3 seasonal gps sites indicate the deceleration rate varies through time including on interannual time scales between 2009 and 2012 wip decelerated at a rate 6 1 to 10 9 2 m yr2 that was double the multidecadal average 3 0 to 5 6 2 m yr2 to identify the causes of slowdown we used new and prior velocity estimates to constrain longitudinal and transverse force budget models as well as a higher order inverse model all model results support the conclusion that the observed deceleration of wip is caused by an increase in basal resistance to motion at a rate of 10 to 40 pa yr subglacial processes that may be responsible for strengthening the ice stream bed include basal freeze on changes in subglacial hydrology or increases in the area of resistant basal substrate through differential erosion the observed variability in wip deceleration rate suggests that dynamics in subglacial hydrology plausibly driven by basal freeze on and or activity of subglacial lakes plays a key role in modulating basal resistance to ice motion in the region

size shape and spatial arrangement of mega scale glacial lineations from a large and diverse dataset mega scale glacial lineations msgls are a characteristic landform on ice stream beds solving the puzzle of their formation is key to understanding how ice interacts with its bed and how this in turn influences the dynamics of ice streams however a comprehensive and detailed characterization of this landform s size shape and spatial arrangement which might serve to test and refine formational theories is largely lacking this paper presents a detailed morphometric analysis and comparison of 4043 msgls from eight palaeo ice stream settings three offshore norway and antarctica four onshore canada and one from under a modern ice stream in west antarctica the length of msgls is lower than previously suggested mode 1000 2000 m median 2892 m and they initiate and terminate at various locations on an ice stream bed their spatial arrangement reveals a pattern that is characterized by an exceptional parallel conformity 80 of all mapped msgls have an azimuth within 5 from the mean values and a fairly constant lateral spacing mode 200 300 m median 330 m which we interpret as an indication that msgls are a spatially self organized phenomenon results show that size shape and spatial arrangement of msgls are consistent both within and also generally between different ice stream beds we suggest this results from a common mechanism of formation which is largely insensitive to local factors although the elongation of msgls mode 6 8 median 12 2 is typically higher than features described as drumlins these values and those of their width mode 100 200 m median 268 m overlap which suggests the two landforms are part of a morphological continuum and may share a similar origin we compare their morphometry with explicit predictions made by the groove ploughing and rilling instability theories of msgl formation although the latter was most compatible neither is fully supported by observations 2014 the authors earth surface processes and landforms published by john wiley sons ltd

heat producing elements enriched continental mantle lithosphere and proterozoic intracontinental orogens insights from brasiliano pan african belts abstract collisional and accretionary processes have been recognized in several precambrian orogens but the formation of intracontinental ensialic orogens has remained elusive the recognition that modern orogenic belts as for example the mountain belts of central asia may form in response to far field stresses transmitted to the interior of continental plates removed much of the objection against proterozoic intracontinental deformation however the causes responsible for the localization of contractional deformation are still uncertain due to the strong dependence of lithosphere rheology on temperature lithospheric thermal weakening prior to contraction must be a required condition studies of dioritic rocks from borborema province northeastern brazil and damara belt southern africa indicate that they are derived from ancient incompatible elements enriched subcontinental lithosphere the concentrations of the heat producing elements k 0 6 wt u 0 02 0 2 ppm and th 0 15 1 5 ppm estimated for this source allow calculations of heat production between 0 09 and 0 25 mw m 3 these high values of heat generation may cause increases in temperature of several tens to three hundred degrees celsius with respect to cratonic mantle lithosphere leading to great reductions in lithosphere strength consequently continental domains underlain by hot and weak continental mantle may thicken under the action of relatively feeble tectonic forces high rheological contrasts between these domains and adjacent stiff and cold blocks provide an explanation for formation of intracontinental orogens

till as a glacial tectomict its internal architecture and the development of a typing method for till differentiation abstract till has been studied for over a century yet still a clear understanding of the mechanics of deposition and or emplacement of till eludes us tills in the past have been generally subdivided into sediments formed in various sub environments of the glacial system as such it has been generally agreed that tills can be separated in terms of these types based on various attributes such as clast fabric increasingly such separation into various sub types of till has become however highly debatable and tenuous based upon the use of micromorphology and a greater in depth appreciation of the internal architecture of till it has become apparent that previously established classifications of till are erroneous micromorphology reveals that deposition and or emplacement of tills is a complex tectonic process such that lodgement melt out and flow tills as previously defined do not appear to exist rather a new term better describes tills as glacial tectomicts by using micromorphology in establishing the presence of sets of microstructures a statistical method is introduced that permits differentiation of tills from other diamictons the implications for this new understanding of tills allied to statistical typing is considerable in terms of till formation glacial stratigraphy and understanding glacial environments in terms of stress histories porewater transmission subglacial deformation mechanics till rheological phase changes and the timeline of till formation

geospatial analysis of controls on subglacial bedform morphometry in the new york drumlin field implications for laurentide ice sheet dynamics the outline and trend of 6566 subglacial bedforms in the new york drumlin field have been digitized from digital elevation data a spatial predictive model has been used to extend values of bedform elongation over an area measuring 200 km 110 km the resulting surface is used in conjunction with depth to bedrock data and an assumed duration of ice residence to test three proposed controls on bedform elongation upon comparison the resulting display of morphometry is best explained by differences in ice velocity across the field of study the existence of multiple zones of fast moving ice located along the southern margin of the laurentide ice sheet is implied by the observed patterns of bedform elongation and orientation we present two interpretations that are consistent with the observations first enhanced basal sliding caused by decreasing effective pressure near a calving margin is suggested as a possible mechanism by which localized fast ice flow is initiated and maintained second topographically controlled ice streams likely occupied the fjord like troughs of the appalachian upland northern rim contrary to previous understanding of the laurentide southern margin in new york state the resulting palaeoglaciological reconstruction illustrates a dynamic mosaic of ice stream and or outlet glacier activity copyright 2009 john wiley sons ltd

application of proton microprobe data to trace element partitioning in volcanic rocks abstract in situ proton microprobe analyses are used to calculate partition coefficients d for mn fe cu zr ga rb sr y zr ba and pb and more restricted data for ni ge nb and as in pyroxenes olivines biotites hornblendes feldspars and fe ti oxides from 32 volcanic rocks all with glassy matrices these range from leucitite and picritic basalt to high silica and peralkaline rhyolites data for pigeonite aenigmatite allanite and leucite are included where these occur as additional phases coefficients have also been rayleigh fractionation corrected this correction being significant for those elements with high coefficients in specific mineral phases and where pronounced crystallisation has occurred partition coefficients are presented as plots vs nbo t ratios of coexisting glasses melts negative correlations are observed based on log transformed data for mn zn fe and y pyroxenes mn zn and fe olivines mn and zn fe ti oxides nb y sr and zr hornblendes sr plagioclase and ba cu zn and fe feldspars these element partition coefficients are not correlated with bulk mineral compositions zr and ga are not correlated with either glass or mineral compositions except zr in hornblende the correlations support the role of melt polymerisation in controlling element partitioning at least for those elements listed although possible effects of temperature cannot be uniquely isolated in the present data set the coefficients presented here are generally consistent with published values for equivalent rock compositions one difference being that some of the higher published coefficients e g sr in plagioclase are not matched in the present data set from a practical viewpoint the relative regularity of the coefficient data presented does facilitate choice of relevant coefficient values that are required for petrogenetic modelling procedures

a review of the relationships between granitoid types their origins and their geodynamic environments abstract granitoids are divided into several types according to their mineral assemblages their field and petrographical features and their chemical and isotopic characteristics this typology complements most of the recent classifications because it is not based solely on chemical and isotopic criteria but also on the field petrographical and mineralogical criteria it thus has the advantage of distinguishing the various granitoid types in the field in most cases the proposed classification shares many similarities with the twenty most used genetic classifications of granitoids both types of peraluminous granitoids are of crustal origin the tholeiitic alkaline and peralkaline granitoids are of mantle origin and both types of calc alkaline granitoids are of mixed origin and involve both crustal and mantle materials each granitoid type is generated and emplaced in a very specific tectonic setting each stage of the wilson cycle is characterised by typical associations of granitoids well typed and precisely dated granitoids can then complement structural approaches and indicate on the geodynamic environment with reference to some case studies the use of granitoids as tracers of the geodynamic evolution is also proposed and discussed

close spatial relationship between plutons and shear zones this paper explores the relationship between granitic plutons and transcurrent shear zones using examples from the borborema province northeastern brazil syntectonic plutons of this province generally have en cornue shapes and were emplaced preferentially within or at the margins of wide regional scale transcurrent shear zones particularly where shear zones intersect major lithological boundaries it is argued that the plutons shapes and their closeness to shear zones are indicative of a causal relationship between granite plutons and shear zones with the support of numerical models it is further argued that pluton emplacement in transcurrent terranes is controlled by low mean pressure magma traps at the shoulders of shear zones resulting from strain incompatibility caused by regional variations in rheological properties

a case study in the new york drumlin field an investigation using microsedimentology resulting in the refinement of a theory of drumlin formation abstract the formation of drumlins remains a major enigma it is accepted that drumlins form under active temperate ice most likely within a soft sediment deforming bed at ephemeral sticky points these sticky points likely lead to the causative mechanism around which sediment nucleation occurs the critical question is under what conditions and where and how do sticky spots form a comparative investigation of a drumlin and mega flute in the new york drumlin field as a case study demonstrates that rheological and sedimentological tills from these different forms are similar it is not therefore rheological change alone that must account for drumlin shape and form but likely the advective subglacial basal sediment flux rate at the ice bed the rate of sediment motion between the upper interface at the ice sediment bed boundary and the lower immobile sediment at depth is crucial at the lower decollement between the mobile and immobile sediment units within the deforming sediment package proto drumlin nucleation is likely to occur and develop into a streamlined form the trigger mechanism for such a perturbation is a derivative of sediment rheology and sediment flux rate driven by the overlying ice stresses recent evidence from antarctica lends credence to this new hypothesis that can be related to all drumlins formed under temperate soft sediment deforming bed conditions

emplacement petrological and magnetic susceptibility characteristics of diverse magmatic epidote bearing granitoid rocks in brazil argentina and chile abstract magmatic epidote mep bearing granitoids from five neoproterozoic tectonostratigraphic terranes in northeastern ne brazil early palaeozoic calc alkalic granitoids in northwestern nw argentina and from three batholiths in chile have been studied the elongated shape of some of these plutons suggests that magmas filled fractures and that dyking was probably the major mechanism of emplacement textures reveal that in many cases epidote underwent partial dissolution by host magma and in these cases may have survived dissolution by relatively rapid upward transport by the host magma in plutons where such a mechanism is not evident unevenly distributed epidote at outcrop scale is armoured by biotite or near solidus k feldspar aggregates which probably grew much faster than epidote dissolution preventing complete resorption of epidote by the melt al in hornblende barometry indicates that in most cases amphibole crystallized at p 5 kbar kyanite bearing thermal aureoles surrounding plutons that intruded low grade metamorphic rocks in ne brazil support pluton emplacement at intermediate to high pressure mep show overall chemical variation from 20 to 30 mol mole percent pistacite ps and can be grouped into two compositional ranges ps20 24 and ps27 30 the highest ps contents are in epidotes of plutons in which hornblende solidified under p 3 0 10 3 si typical of magnetite series granitoids crystallized under higher oxygen fugacity in ne brazil argentina and chile it seems that mep is more common in precambrian to palaeozoic ilmenite series granitoids while its occurrence in magnetite series granitoids is more restricted to mesozoic to tertiary granitoids

subglacial hydraulic conditions and their possible impact upon subglacial bed formation abstract in reviewing hypotheses of subglacial bed formation it becomes apparent that a close relationship must exist between the development of these bedforms and subglacial hydraulic conditions a speculative classification of subglacial temperate beds is presented subglacial beds are subdivided into h q and m beds h beds are typically hard beds in which upper interface meltwater activity dominates m beds are those in whichd soft mobile subglacial debris conditions prevail and in which advective pervasive f flow of saturated debris occurs in q beds both of the other bed types occur but are of a spatially and temporally transitory nature thus this latter bed type may be the most common beneath ice masses it is under an m bed state that many drumlin subglacial bedforms and associated forms are thought to develop discussion on the processes involved in mobile debris entrainment and the effect of changing rheological parameters are developed it is argued that such bedform initiation occurs due to anisotropic rheological variations within the mobile debris setting up a feedback effect such conditions are thought to be found most likely at the lower interface of a fast moving ice mass possibly typical of an ice stream in using empirical analyses of mobile debris conditions a limited set of relationships between subglacial meltwater flux rates basal ice velocity debris porosity and bed width are established with reference to mobile debris thicknesses using this relationship a set of possible parameter values that would allow bed formation to be achieved are presented the impact of bed formation under these conditions upon possible sedimentological facies and deformation structures is postulated the wider effect of m bedd conditionsd on subglacial sedimentology is introduced and the need to investigate subglacial hydraulic and bed conditions under past glaciations from the evidence of their glacial sediments and landforms emphasised

ice stream retreat dynamics inferred from an assemblage of landforms in the northern barents sea abstract palaeo records of rapid ice stream retreat are important as they can be used to identify the processes involved and inform investigations of present day ice masses here we document a previously unknown retreat stage in northernmost bjornoyrenna bear island trough in the northern barents sea representing the youngest stage in a stepwise retreat of the bjornoyrenna ice stream we present a descriptive landsystem model for retreat of a marine based ice stream which provides new insights into the glacial dynamics of the episodic retreat stages this model captures the landforms produced during a cycle of 1 fast ice stream flow 2 intense calving of large icebergs locked in a dense matrix of smaller icebergs probably from a collapsed ice shelf 3 ice stream stagnation 4 ice stream floating off forming an ice shelf as the grounding line retreats and eventually 5 ice shelf disintegration we hypothesize that the presented ice stream retreat model reflects glacial surging the formation of corrugated furrows in the study area is consistent with interpretation of similar features in pine island bay west antarctica which are interpreted to have been formed during a massive ice shelf break up and associated grounding line retreat two directions of past ice flow are indicated from streamlined glacial landforms in the study area ice flow from north north east is indicated for the last glacial maximum and an early phase of deglaciation whereas a shift to flow from north north west from an ice dome located over hinlopenstretet svalbard is indicated for the latest deglaciation phases of the bjornoyrenna ice stream

drumlins in ice sheet reconstructions with reference to the western pennines northern england abstract drumlin morphology and distribution can be used to determine the palaeoglaciological conditions under which subglacial bedforms were created by late pleistocene ice sheets detailed field mapping of part of the large drumlin fields in northern england in particular the identification of areas of superimposed drumlins has been shown to be of significance in the delimitation of the ice divide within the area the overall distribution of the subglacial bedforms has also been used to define the extent of ice streams within this part of the late devensian dimlington stadial ice sheet based on the physical principles which underlie the deformational model of drumlin formation and present day glaciological information on subglacial ice sheet conditions variations in drumlin morphology have allowed a first attempt at reconstructing areas of high 100 kpa and low

ice flow of the antarctic ice sheet we present a reference comprehensive high resolution digital mosaic of ice motion in antarctica assembled from multiple satellite interferometric synthetic aperture radar data acquired during the international polar year 2007 to 2009 the data reveal widespread patterned enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent this view of ice sheet motion emphasizes the importance of basal slip dominated tributary flow over deformation dominated ice sheet flow redefines our understanding of ice sheet dynamics and has far reaching implications for the reconstruction and prediction of ice sheet evolution

isotopic evidence for lithospheric thinning during extension southeastern great basin mafic rocks erupted during late cenozoic extension near las vegas nevada show temporal patterns of variation in nd and sr isotope ratios and in bulk chemistry the patterns indicate that depths of magma generation were varying through time and that lithospheric mantle was partially replaced with asthenospheric mantle as extension proceeded in alkalic rocks epsilon sub nd changed through time from minus 9 1 typical of lithospheric mantle in this area before the onset of major 16 ma extension to 6 4 typical of asthenospheric mantle after extension 4 6 ma near the end of the period of major extension 10 6 ma tholeiitic rocks erupted the epsilon sub nd of which ranged from minus 10 1 to minus 7 9 this indicates that the lithosphere had not thinned sufficiently by that time to bring asthenospheric mantle into the depth range of tholeiitic magma generation 33 50 km the lithosphere in the las vegas area appears to have thinned by about 50 less than would be predicted by the magnitude of upper crustal extension a factor of 3 or 4 and indicative of a nonuniform lithospheric response to extension

proterozoic history of the borborema province ne brazil correlations with neighboring cratons and pan african belts and implications for the evolution of western gondwana 1 geological and geochronological correlations between borborema province ne brazil and neighboring cratons and brasiliano pan african belts indicate that the amazonian west african and sao francisco congo cratons and the basement of the araguaia borborema nigerian and cameroon provinces were part of the atlantica supercontinent this continent was established at the end of the transamazonian eburnean cycle 2 0 ga and apart from ubiquitous taphrogenesis in the 1 8 1 7 ga interval remained largely unaffected for the following 1 ga around 1 ga an important magmatic event in borborema province correlates with rifting episodes and anorogenic magmatism in the sao francisco congo and amazonian cratons these events are interpreted as failed attempts to break up atlantica which at this time may have been part of the larger rodinia supercontinent renewed extensional conditions in borborema province during the middle and late neoproterozoic are attributed to far field stresses transmitted to the interior of atlantica by outwardly dipping subduction zones that encircled its northern present day coordinates portion the rarity of petrotectonic assemblages typical of subduction zone environments indicates that extension did not evolve enough to form large oceans basins and thus that the borborema province essentially includes reworked intracontinental domains regional deformation and metamorphism starting at 650 640 ma and shear zone development beginning at 590 595 ma were continuously developed through time and were synchronous throughout most of the borborema araguaia cameroon and nigerian provinces postorogenic conditions were reached 540 530 myr ago while active deformation was still occurring in other belts that accreted around atlantica to form western gondwana

the age distributions of detrital zircons in metasedimentary sequences in eastern borborema province ne brazil evidence for intracontinental sedimentation and orogenesis detrital zircons in metasedimentary rocks attributed to three different complexes in the central and pernambuco alagoas domains of the borborema province northeastern brazil were dated by la icpms two samples from each of the following complexes were analyzed sert nia and surubim in the central domain and rio una in the pernambuco alagoas domain one sample of the sert nia complex yielded only paleoproterozoic zircons whereas neoproterozoic zircons predominate in the other with the youngest zircon indicating deposition after 642 ma one of the samples of the surubim complex yielded again only paleoproterozoic concordant zircon ages in the other sample although paleoproterozoic zircons predominate ages vary from the mesoarchean to the neoproterozoic the age of the youngest zircon in this latter sample indicates deposition after 850 ma and an overgrowth on an archean grain yielded an age of 623 6m a 2 interpreted as the age of peak metamorphism the two samples of the rio una complex yielded predominantly detrital zircons with ages in the ranges 2 2 2 1 ga and 1 7 1 6 ga in this complex the youngest detrital zircon suggests deposition after 917 ma and metamorphic zircons indicate peak metamorphic conditions between 600 ma and 617 ma the results of this study together with available data from the literature allow the following conclusions a the sert nia complex is a neoproterozoic sequence and not paleoproterozoic as previously thought b the similarity in lithological associations detrital zircon populations and c isotope signature between the sert nia and surubim complexes suggest that they belong to the same lithostratigraphic unit c although the rio una complex can be older than the sert nia surubim complex the lack of zircons younger than 900 ma can also result from insufficient sampling or absence of young rocks alongside the drainage system that collected detritus to this complex d the regional metamorphism related to the brasiliano orogeny attained its climax at 630 600 ma e the age spectra of detrital zircons require their derivation from erosion of rocks formed not only during known geological events in the borborema province and in the nearby sao francisco congo craton but also from the amazonian craton suggesting accumulation in an intracratonic setting within a preexisting large continent f the short timespan between deposition and orogenic deformation may explain the overall medium to high temperature metamorphism due to maintenance in the subsequent compressional event of the elevated geothermal gradients produced during broad scale lithosphere extension

basal hydraulic system of a west antarctic ice stream constraints from borehole observations pressure and tracer measurements in boreholes drilled to the bottom of ice stream b west antarctica are used to obtain information about the basal water conduit system in which high water pressures are developed these high pressures presumably make possible the rapid movement of the ice stream pressure in the system is indicated by the borehole water level once connection to the conduit system is made on initial connection here also called breakthrough to the basal water system the water level drops in a few minutes to an initial depth in the range 96 117 m below the surface these water levels are near but mostly somewhat deeper than the flotation level of about 100m depth water level at which basal water pressure and ice overburden pressure are equal which is calculated from depth density profiles and is measured in one borehole the conduit system can be modelled as a continuous or somewhat discontinuous gap between ice and bed the thickness of the gap has to be about 2 mm to account for the water level drop on breakthrough and about 4 mm to fit the results of a salt tracer experiment indicating downstream transport at a speed of 7 5 mm s 1 the above gap conduit model is however ruled out by the way a pressure pulse injected into the basal water system at breakthrough propagates outward from the injection hole and also by the large hole to hole variation in measured basal pressure which if present in a gap conduit system with 2 or 4 mm would result in unacceptably large local water fluxes an alternative model that avoids the e objections called the gap opening model involves opening a gap as injection proceeds starting with a thin film the injection of water under pressure lifts the ice mass a round the borehole creating a gap 3 or 4 mm wide at the ice bed interface evaluated quantitatively the gap opening model accounts for the volume of water that the basal watery system accepts on breakthrough which obviates the gap conduit model in order to transport basal meltwater from upstream it is then necessary for the complete hydraulic model to contain also a network of relatively large conduits of which the most promising type is the canal conduit proposed theoretically by walder and fowler 1994 flat low conduits incised into the till 0 1 m deep and perhaps 1 m wide with a flat ice roof the basal water pressure data suggest that the canals are spaced 50 300 m apart much closer than r tunnel would be the deepest observed water level 117 m i the most likely to reflect the actual water pressure in the canals corresponding to a basal effective pressure of 1 6 bar in this interpretation the shallower water levels are affected by loss of hydraulic head in the narrow passageway s that connect along the bed from borehole to canal s once a borehole has frozen up and any passageways connecting with canals have become closed a pressure sensor in contact with the unfrozen till that underlies the ice will measure the pore pressure in the till given enough time for pressure equilibration this pressure varies considerably with time over the equivalent water level range from 100 to 113 m basal pressure sensors 500 m apart report uncorrelated variations whereas sensors in boreholes 25 m apart report mostly but not entirely well correlated variations of unknown origin in part of the record remarkable anticorrelated variations are interspersed with positively correlated one and there are rare abrupt excursions to extreme water levels as deep as 125m and as shallow as 74 m a diurnal pressure fluctuation intermittently observed may possibly be caused by the ocean tide in the ross sea the lack of any observed variation in ice stream motion when large percentagewise variations in basal effective pressure were occurring according to our data suggests that the observed pressure variations are sufficiently local and so randomly variable from place to place that they are averaged out in the process by which the basal motion of the ice stream is determined by an integration over a large area of the bed

ice stream sticky spots a review of their identification and influence beneath contemporary and palaeo ice streams rapidly flowing ice streams are the arterial drainage routes in continental ice sheets and exert a major influence on ice sheet mass balance recent observations have revealed that ice stream flow exhibits considerable variability with relatively rapid changes taking place in speed and direction this spatial and temporal variability is intimately linked to the conditions at the base of the ice streams and the distribution of localised patches of basal friction known as sticky spots in this paper we provide a detailed review of sticky spot observations from both contemporary and palaeo ice stream beds in order to better understand their nature and influence observations and theoretical considerations reveal four primary causes of stickiness i bedrock bumps ii till free areas iii areas of strong well drained till and iv freeze on of subglacial meltwater these may act together in one location or in isolation and a progressive increase in their distribution could lead to ice stream shut down bedrock bumps are influential under active ice streams where they provide form drag and can create thinner ice which increases the likelihood of basal freeze on increased bed roughness may prevent the lateral migration of some ice streams but bedrock bumps are unlikely to cause ice stream shut down because over long time scales ice stream erosion might be expected to reduce their amplitude the influence of till free areas beneath an ice stream will depend critically on the amount of water that might be drawn out of the surrounding till to lubricate such areas they are likely to be most important in ice stream onset zones but their identification has proved difficult beneath active ice streams if an ice stream operates solely by till deformation it is conceivable that a progressive increase in the exposure of till free areas could lead to shut down through a process of sediment exhaustion areas of strong well drained till have been identified beneath both active and ancient ice streams and are most likely to result from the reorganisation of subglacial meltwater the collapse of an inefficient cannalised system to a more efficient channelised system can occur rapidly and this mechanism has been hypothesised as a candidate for ice stream shut down in both contemporary and palaeo settings basal freeze on has also been observed and inferred from beneath modern and palaeo ice streams and a reduction in basal meltwater supply coupled with ice stream drawdown and the advection of cold ice increases the likelihood of switching off an ice stream a paucity of data from ice stream sticky spots limits a better understanding of their nature distribution and evolution beneath ice streams future technological advances are likely to improve the resolution of the data collected from the beds of modern ice streams but well preserved palaeo ice stream beds also hold potential for investigating their influence on ice stream flow and we present simple landsystems models to aid their identification such data will considerably enhance the basal boundary condition in ice stream models which will ultimately refine our predictions of the response of contemporary ice sheets to future changes in climate

large scale silicate liquid immiscibility a possible example from northeastern brazil abstract ultrapotassic peralkalic silica saturated plutons 580 ma are widespread in the cachoeirinha salgueiro foldbelt northeastern brazil they consist of alkali feldspar syenites with pyroxenite as co magmatic inclusions and syn plutonic or late stage dikes pyroxenite and syenite have the same mineral phases aegirine augite microcline sphene apatite blue amphibole magnetite but only in different proportions rare inclusions of a mixed rock about 60 syenite 40 pyroxenite in an emulsion like texture are also present pyroxenes in the three units are all only slightly zoned silica saturated and extremely low in al2o3 0 2 1 4 amphiboles are mostly k rich richterite characterized by high sio2 low al2o3 and tio2 contents and low mg the three rock types have similar ree chondrite normalized patterns with negative slopes and lack of eu anomaly with the total ree in the pyroxenite greater than that of the syenite trace element patterns for the mixed rock are intermediate between those for the pyroxenite and syenite major element partitioning between pyroxenite and syenite has the same sense as that one observed between immiscible liquids in volcanic lavas and trace element partitioning is similar to the experimentally determined partition of immiscible liquid pairs the rocks have similar high 18o values avg w r 8 smow corrected from pyroxene high initial 87 sr 86 sr ratios about 0 710 and low 143 nd 144 nd avg 0 51104 field and geochemical characteristics indicate chemical equilibrium among the three rock types and suggest liquid immiscibility between syenite and pyroxenite the mixed rock representing the original magma composition

drumlinised glaciofluvial and glaciolacustrine sediments on the smaland peneplain south sweden new information on the growth and decay history of the fennoscandian ice sheets during mis 3 abstract dug trenches in sediment cored drumlins on the smaland peneplain southern sweden occurring intermixed with rock cored drumlins revealed sedimentary successions and facies suggesting that the core sediments were primarily deposited proglacially both in glaciolacustrine settings as delta front and more distal deeper water sediments and as glaciofluvial braidplain sediments from the relationship between the drumlin sorted sediment cores and their surficial drape by a till carapace it is concluded that erosion was the main drumlin forming process here reshaping the original sediment morphology into streamlined landforms after which a more or less thick traction till was deposited over the eroded and or deformed ice bed interface drumlin formation probably took place during the last deglaciation over the area the drumlin core sediments in 14 excavated trenches were dated by optically stimulated luminescence osl most retrieved ages fall into two distinct groups both within marine isotope stage 3 c 60 30 ka when plotted as mean osl ages for each site all but two sites fall either into an older age group c 48 56 ka seven sites or into a younger age group c 28 38 ka five sites these age intervals correlate well with the suggested time spans for the ristinge and klintholm ice advances in denmark by ice flowing from the baltic basin westwards terminating in fan shaped patterns these interstadial ice advances in the southwestern sector of the fennoscandian ice sheet give rise to enigmatic questions on the physics and mechanics of ice sheet build up and decay especially as palaeoenvironmental data for northern fennoscandia suggest widespread ice free conditions within roughly the same time fame as the recorded ice advances in the south

the stability of a viscous till sheet coupled with ice flow considered at wavelengths less than the ice thickness a turbation method is used to analyse the stability of a thin till layer overlain by a deep ice layer ice is modelled as a linearly viscous fluid while the till viscosity has power law dependence on stress and effective pressure a linearized set of equations yields descriptions of the coupling of the ice flow with the sediment flow and reveals parameter ranges where the till perturbation amplitude can grow this shect flow instability is an essential part of any theory of drumlin formation and shows that viscous models of till have the ability to explain typical deforming bed features this is of great significance for large scale ice shect modelling

post collisional two stage magmatism in the east sarmatian orogen east european craton evidence from the olkhovsky ring complex the olkhovsky ring complex orc is composed of a two phase magmatic rock association with quartz diorite quartz monzodiorite granodiorite suite in the outer ring or and trondhjemite as the inner stock is zircon u pb dating reveals that the rocks of the or and is were emplaced at 2070 9 ma and 2044 13 ma respectively the or rocks are intermediate and felsic metaluminous low to moderate alkali both rocks are enriched relative to primitive mantle in rb ba sr k and depleted in nb and ti the or quartz diorite and is trondhjemite are characterized by positive values end t 2 2 and 1 6 respectively the orc belongs to intermediate i type granite with nil or weak crustal contamination whereas the is trondhjemite shows adakite like features that were contaminated with components from earlier magmatic pulses the low degree of contamination of the hypothetical melt its basaltic andesite composition the elevated potassium and magnesium contents and trace element patterns of whole rock and zircon suggest garnet free lower crust or metasomatized mantle wedge as the magma source for the or in a post collisional setting the mechanism for the inner stock trondhjemite was probably similar although the magma source was characterized by the presence of garnet with a moderate degree of melting supplementary material analytical methods and tabulated analytical data are available at https doi org 10 6084 m9 figshare c 3851104

crustal evolution of the pernambuco alagoas complex borborema province ne brazil nd isotopic data from neoproterozoic granitoids the borborema province in northeast brazil forms the western part of a major 600 ma brasiliano pan african mobile belt that extends from brazil through central africa in a pre drift reconstruction of gondwanaland the pernambuco alagoas peal complex is a large area within the borborema province and consists of high grade migmatitic gneisses and granitic batholiths although the peal complex has previously been called a massif or terrane in reality it comprises many different types of protoliths or plutons the protoliths for many of the migmatitic gneisses are difficult to decipher due to the varied deformation and metamorphism during the cariris velhos and brasiliano orogenies nd isotopic data discriminate two major crustal domains in the peal complex the garanhuns and the agua branca domains rocks of the garanhuns domain have nd tdm in the range of 1 8 to 2 4 ga indicating that most of it is underlain by paleoproterozoic crust and does not include significant mesoproterozoic or neoproterozoic crustal additions the marimbondo correntes batholith is inside the garanhuns domain but includes younger tdm ages similar to those found in the agua branca domain this batholith includes contributions from younger sources and may intrude a block of younger allocthonous crust thrust over the older crust of the garanhuns domain rocks of the agua branca domain show tdm within the range of 0 90 to 2 10 ga with most nd model ages between 1 5 and 1 0 ga the pre brasiliano crust in this domain thus includes substantial amounts of mesoproterozoic to neoproterozoic additions the presence of some older model ages suggests that the aguas belas domain may have some vertical crustal zonation with paleoproterozoic and neoproterozoic crustal segments tectonically interleaved during successive accretional or collisional episodes including the ultimate brasiliano collision at 600 ma the mesoproterozoic nd model ages therefore represent mixing of different crustal and mantle sources rather than a discrete episode of mesoproterozoic crustal formation various metaluminous or peraluminous granitoids of brasiliano age from the buique paulo afonso aguas belas caninde and marimbondo correntes batholiths show tdm ranging from 0 90 ga to 1 10 ga suggesting that they either were wholly derived from juvenile material of the 1 0 ga cariris velhos orogeny or contain substantial contributions of juvenile material from the 0 6 ga brasiliano cycle some high k to shoshonitic granitoids from the aguas belas domain show mesoproterozoic 1 4 ga tdm model ages the model ages recorded in these plutons could suggest either that the magma was derived from enriched lithospheric mantle associated with the cariris velhos orogeny or that it was a mixture between enriched lower crust of transamazonian age and juvenile material from the cariris velhos or brasiliano orogenies

theory of glacial erosion transport and deposition as a consequence of subglacial sediment deformation a theory of erosion transport and deposition of unlithified sediments by glaciers is presented it predicts the large scale areal distribution of zones and rates of erosion and deposition in time and space through a complete glacial cycle together with the resultant intensity of large scale lincations drumlins which will be incised in the landscape the theory also predicts the dispersal patterns of subglacial lithologies together with the form of dispersal trains derived from distinctive sources and the vertical and horizontal distribution of lithologies within a till it predicts major erosional discontinuities within tills and the formation of boulder pavements it suggests that the dominant proportion of the lowland tills produced by pleistocene mid latitude ice sheets was generated by subglacial deformation and explains why they are predominantly fine grained the theory is based on an analysis of glacier dynamic processes and therefore can be used to infer the dynamic behaviour of former ice sheets from the distribution of tills and their lithologic composition

intralithospheric differentiation and crustal growth evidence from the borborema province northeastern brazil thousands of cubic kilometers of high k calc alkalic magmas intruded the borborema province northeastern brazil during the neoproterozoic brasiliano orogeny they make up large batholiths in which mantle derived mafic to intermediate rocks coexist with a larger amount of granitoids the relatively low silica contents 61 70 wt sio 2 and moderate to high compatible element concentrations 0 3 3 5 wt mgo 1 5 3 8 wt cao as much as 150 ppm of cr of the granitoids indicate that they contain an appreciable mantle component the similar trace element geochemical high contents of incompatible trace elements and isotopic strongly negative end values signatures of mafic and felsic rocks combined with geochemical modeling suggest that 1 the mafic and felsic rocks are genetically linked 2 the granitic magmas were produced by 20 30 partial melting from a source having geochemical characteristics similar to the mafic rocks and 3 mingling and mixing of felsic magmas with subsequent batches of mafic magmas yielded the silica poor granitoids isotopic data preclude involvement of the asthenosphere in the genesis of the mafic melts and instead indicate their derivation from an old enriched lithospheric mantle therefore addition of mantle material to the crust occurred through internal lithospheric differentiation in contrast with conventional crustal growth models

vapor absent melting at 10 kbar of a biotite and amphibole bearing tonalitic gneiss implications for the generation of a type granites vapor absent melting experiments on a biotite and amphibole bearing archean tonalitic gneiss agc150 at 10 kbar and 875 to 1050 c show that amphibole breaks down from 900 to 950 c producing garnet orthopyroxene and granitic melt biotite dehydration melting produces 10 wt melt up to 950 c via incongruent melting reactions that produce garnet orthopyroxene and titanomagnetite widespread biotite dehydration melting occurs between 950 and 975 c and produces orthopyroxene magnetite titanomagnetite and 20 wt fluorine rich melt up to 0 31 wt f minor f rich 2 7 wt biotite is present even at 1000 c our experiments show that under vapor absent conditions intrusion of hot mantle derived magmas into the lower crust is necessary to initiate widespread biotite dehydration melting in rocks with compositions like agc150 we propose that the high thermal stability of biotite in agc150 suggests that this rock is residual after a previous episode of partial dehydroxylation that left behind somewhat f enriched biotite we show that dehydration melting of such f enriched biotite produces f rich granitic liquids with compositions within the range of a type granite and leaves behind a granulitic residue consisting of orthopyroxene plagioclase quartz titanomagnetite and magnetite

coupling between a glacier and a soft bed i a relation between effective pressure and local shear stress determined from till elasticity to predict the distribution of motion beneath glaciers on soft beds the strength of the coupling between the ice and the bed and its variation with effective pressure must be known a record of shear strain acquired with a tiltmeter emplaced in till beneath storglaciaren sweden indicates that fluctuations in water pressure cause variations in the local shear stress on the bed and that the bed deforms elastically in response to these variations to estimate the shear stress from the elastic component of the total shear strain the shear modulus of the till was measured in relaxation tests conducted in the laboratory with a ring shear device after accounting for the elastic compliance of the device these tests yielded shear moduli of about 1000 and 1800 kpa at confining pressures of 85 and 280 kpa respectively these values are comparable to those of other granular materials undergoing recoverable shear strains of the same magnitude the local shear stress on the till calculated by applying the measured shear moduli to the tilt record scales with p e 1 7 where p e is the effective pressure this relation implies that as p e decreases at the ice till interface shear stresses on the till are reduced and concentrated elsewhere on the bed perhaps where the till is absent or the glacier is frozen to the bed when compared with the strength of the till determined from ring shear tests this relation also accounts for the lack of permanent deformation at depth in the bed during periods of low p e and indicates that most basal motion was by sliding or ploughing

overlapping sr nd hf o isotopic compositions in permian mafic enclaves and host granitoids in alxa block nw china evidence for crust mantle interaction and implications for the generation of silicic igneous provinces abstract in general the mantle provides heat and or material for the generation of the silicic igneous provinces sips the rarity of mafic microgranular enclaves mmes however hampers understanding of the mantle s role in generating sips and the process of crust mantle interaction the widespread distributed mmes in the newly reported alxa sip provide an opportunity to study these processes this study integrates in situ zircon u pb age and hf o isotope analyses whole rock geochemistry and sr nd isotope results for the mmes and host granitoids in the alxa block sims zircon u pb dating reveals that there are two generations of mmes and host granitoids the mmes in the bayannuoergong batholith were formed at ca 278 ma similar to the age 280 ma of host granitoids and the mmes and host granitoids in the yamaitu pluton were formed at ca 272 270 ma all mmes have relatively low sio 2 50 7 61 4 wt and th 0 8 2 8 ppm but relatively high mgo 2 6 4 9 wt cr 23 146 ppm and ni 6 38 ppm contents compared to the host granitoids with sio 2 63 6 77 5 wt th 5 2 41 ppm mgo 0 23 2 1 wt cr 10 38 ppm and ni 5 14 ppm all mmes have whole rock sr nd and zircon hf o isotope compositions similar to their corresponding host granitoids the 280 ma mmes have lower whole rock e nd t 13 5 and higher initial 87 sr 86 sr values 0 7095 and zircon 18 o values 6 3 compared to the e nd t 11 5 initial 87 sr 86 sr values 0 7070 and zircon 18 o values 5 6 of the 270 ma mmes the occurrences of quartz xenocrysts k feldspar megacrysts corroded feldspars and acicular apatites indicate that the mmes are the products of the mixing between mantle and crust derived magmas the striking similarities in the zircon hf o isotopic compositions in both mme host granitoid pairs indicate that the granitoids and mmes have similar sources the granitoids are proposed to be mainly sourced from magmas generated by remelting of newly formed mafic rocks which were generated by earlier basaltic magmas underplating or injected into the mid to lower crust the continuing basaltic magma underplating and remelting of earlier formed mafic rocks in the mid to lower crust indicate that the mantle provided both heat and materials for the generation of the alxa sip the scenario also suggests that basaltic magma underplating may play a more important role in the crustal growth in the alxa sip than it is generally recognized in other sips

reply to comment on compositional and structural control of fission track annealing in apatite by j barbarand a carter i wood and a j hurford chemical geology 198 107 137 2003

formation of mega scale glacial lineations on the dubawnt lake ice stream bed 1 size shape and spacing from a large remote sensing dataset abstract mega scale glacial lineations msgls are the largest flow parallel bedforms produced by ice sheets and are formed beneath rapidly flowing ice streams knowledge of their characteristics and genesis is likely to result in an improved understanding of the rate at which ice and sediment are discharged by ice sheets but there is little consensus as to how they are formed and there are few quantitative datasets of their characteristics with which to formulate or test hypotheses this paper presents the results of a remote sensing survey of 46 000 bedforms on the dubawnt lake palaeo ice stream bed focussing on a central transect of 17 038 that includes highly elongate bedforms previously described as msgls within this transect lineations exceed 10 km in length max 20 km and 23 have elongation ratios 10 1 max 149 1 highly elongate features are interspersed with much shorter drumlin like features but longer bedforms are typically narrower suggesting that their length develops more quickly than or at the expense of their width bedforms are broadly symmetrical in plan form and have a preferred lateral spacing of 50 250 m which implies a regular rather than random pattern of corrugations comparison with drumlins reveals that the more attenuated msgls simply extend the tail of the distribution of data rather than plotting as a separate population taken together this supports the idea of a subglacial bedform continuum primarily controlled by ice velocity but existing hypotheses of msgl formation are either not supported or are insufficiently developed to explain our observations rather we conclude that under conditions of rapid ice flow msgls attain their great length relatively quickly decades through a probable combination of subglacial deformation which attenuates ridges and erosional processes that removes material from between them

subglacial landforms and deposits in central and northern quebec canada with emphasis on rogen moraines abstract the development of the anatomy and the morphology of a relatively simple till sheet deposited over parts of the canadian shield is discussed this is characterized by the common occurrence of well developed streamlined landforms and rogen moraines the sedimentation of the till sheet was an alternating succession of depositional and erosional or landforming events the former leading to the sedimentation of various till lithofacies the latter leading to the development of various morainic terrains this orderly sequence of subglacial events occurred during the deglaciation of central and northern quebec with the controlling factors being bed rock topography and geology ice thickness and basal ice temperatures till lithofacies include in ascending stratigraphic order lodgement till subglacial melt out till immature lodgement till and varieties of supra glacial ablation deposits morainic terrains include drumlinized and fluted ground moraine rogen moraines as well as fluted and unfluted hummocky moraine rogen moraines are formed subglacially a few kilometers behind the glacier margin as a consequence of localized compressive stresses which result in shearing and stacking of slices of near base englacial debris later deposited as a basal melt out till the moraines are believed to be a characteristic landform associated with shield type rock surfaces namely an irregular topography of asymmetric rock basins of all sizes resulting from the areal scouring of hard rocks as well the moraines are believed to be as well a characteristic landform resulting from the flattening profile of the margin of the waning ice sheet the formation of the moraines led to enhanced flow in marginal areas generating late events of streamlining and boulder plucking

influence of subglacial geology on the position of a west antarctic ice stream from seismic observations ice streams drain much of the interior west antarctic ice sheet and buffer the main ice reservoir from oceanic influences1 2 the slow flowing interior feeds the floating ross ice shelf with ice via fast flowing ice streams3 that are believed to modulate sea level change through their control of inland ice storage understanding ice stream behaviour and predicting the response to climate change4 requires a better knowledge of the subglacial geology5 6 it is known that a thawed ice bed and high pressure basal water are necessary but not sufficient conditions to cause ice streaming7 8 moreover it has been hypothesized that a soft sedimentary bed is also required because of its intrinsic low frictional resistance to flow9 and owing to its high erodibility so as to generate till that can deform and lubricate ice motion10 11 or to bury rough features and smooth the bed for sliding here we use seismic observations to provide evidence that one margin of the upglacier part of an ice stream is directly above the boundary of a basin with such sedimentary fill the ice stream is within the basin and the ice outside the basin is slow flowing the basin fill presents an order of magnitude lower frictional resistance to ice flow than the subglacial material outside the basin we conclude that the ice stream position is dependent on subglacial geology

laurentide glacial landscapes the role of ice streams glacial landforms of the north american prairie can be divided into two suites that result from different styles of ice flow 1 a lowland suite of level to streamlined till consistent with formation beneath ice streams and 2 an upland and lobe margin suite of thick hummocky till and glacial thrust blocks consistent with formation at ice stream and ice lobe margins southern laurentide ice lobes hypothetically functioned as outlets of ice streams broad branching lowlands bounded by escarpments mark the stable positions of the ice streams that fed the lobes if the lobes and ice streams were similar to modern ice streams their fast flow was facilitated by high subglacial water pressure favorable geology and topography in the midcontinent encouraged nonuniform ice flow and controlled the location of ice streams and outlet lobes