forty years of ttg research abstract ttgs tonalite trondhjemite granodiorite are one of the archetypical lithologies of archaean cratons since their original description in the 1970s they have been the subject of many studies and discussions relating to archaean geology in this paper we review the ideas concepts and arguments brought forward in these 40 years and try to address some open questions both old and new the late 1960s and the 1970s mark the appearance of grey gneisses ttg in the scientific literature during this period most work was focused on the identification and description of this suite and the recognition that it is a typical archaean lithology ttgs were already recognised as generated by melting of mafic rocks this was corroborated during the next decade when detailed geochemical ttg studies allowed us to constrain their petrogenesis melting of garnet bearing metamafic rocks and to conclude that they must have been generated by archaean geodynamic processes distinct from their modern counterparts however the geodynamic debate raged for the following 30 years as many distinct tectonic scenarios can be imagined all resulting in the melting of mafic rocks in the garnet stability field the 1990s were dominated by experimental petrology work a wealth of independent studies demonstrated that melting of amphibolites as well as of mafic eclogites can give rise to ttg liquids whether amphibolitic or eclogitic conditions are more likely is still an ongoing debate from 1990s onwards one of the key questions became the comparison with modern adakites as originally defined these arc lavas are reasonably close equivalents to archaean ttgs pending issues largely revolve around definitions as the name ttg has now been applied to most archaean plutonic rocks whether sodic or potassic irrespective of their hree contents this leads to a large range of petrogenetic and tectonic scenarios a fair number of which may well have operated concurrently but are applicable only to some of the rocks lumped together in the ever broadening ttg bin

a model of the western laurentide ice sheet using observations of glacial isostatic adjustment we present the results of a new numerical model of the late glacial western laurentide ice sheet constrained by observations of glacial isostatic adjustment gia including relative sea level ind

ice stream shear margin moraines the distribution and pattern of subglacial bedforms provides valuable insights into the configuration and behaviour of former ice sheets and recent work has begun to address the landform assemblages of palaeo ice streams in this paper we focus on subglacially formed ice stream shear margin moraines these landforms are thought to form in the shear zone between fast flowing stream ice and slow moving sheet ice using satellite imagery and aerial photographs we identify four ice stream shear margin moraines associated with the former m clintock channel palaeo ice stream victoria island arctic canada the moraines range in length from 11 to 22 km maintain fairly constant widths of around 500 m and range in height from 10 to 50 m above the surrounding terrain they are composed of carbonate drift of a similar composition as the ice stream bedforms and have been laid down irrespective of local and regional topography two of the moraines display a lateral offset and are thought to reflect minor migrations of the ice stream margin we explore their possible mode of formation and suggest that they occur when erosion at the ice stream margin provides a surplus of sediment which is smeared out in the downstream direction the identification of other palaeo ice stream shear margin moraines has probably been hampered by their interrupted continuity which can only be appreciated at the large scale it is hoped that the descriptions presented in this paper will help to identify these landforms and increased detection of palaeo ice streams will reveal other ice stream shear margin moraines copyright 2002 john wiley sons ltd

reprocessing and reinterpretation of cocorp southern appalachian profiles abstract reprocessing of cocorp southern appalachian data was focused on basic seismic evidence for continuation of sediments beneath a master decollement the most important evidence is a nearly continuous series of subhorizontal reflections extending from the valley and ridge province into the piedmont province continuity of subhorizontal reflections becomes tenuous in the inner piedmont careful reprocessing has yielded evidence for termination of strong reflections beneath the allochthon and the beginning of a relatively weak and complex series of events termination of sedimentary rocks beneath the piedmont is interpreted from true amplitude seismic data a zone of detachment continues southeast of the sediment termination as far as the master decollement root zone research on stacking velocities has indicated that complex velocity structures could create apparent low stacking velocities this phenomenon may occur in the charlotte belt of georgia bouguer gravity can be modeled as a former craton of normal density with an accreted margin of very slightly higher density variation in crustal thickness also contributes to the bouguer gravity gradient no continuous large scale overthrust is needed southeast of the interpreted master decollement root zone located beneath the kings mountain belt and charlotte belt

postglacial emergence of ungava peninsula and its relationship to glacial history a series of 178 radiocarbon dates of late glacial and postglacial age from raised marine terraces on the hudson strait hudson bay and ungava bay coasts permit a new synthesis of deglaciation h

petrogenesis of plagiogranites in the muslim bagh ophiolite pakistan implications for the generation of archaean continental crust high sio2 rocks referred to as oceanic plagiogranites are common within the crustal sequences of ophiolites however their mode of petrogenesis is controversial with both late stage fractional crystallization and partial melting models being proposed here we present new whole rock data from plagiogranitic dyke like bodies and lenses from the lower and middle sections of the sheeted dyke complex of the cretaceous muslim bagh ophiolite northwestern pakistan the plagiogranites have similar geochemical signatures that are inconsistent with them being the fractionation products of the mafic units of the muslim bagh ophiolite however the plagiogranites all display very low tio2 contents 0 4 wt implying that they formed by partial melting of mafic rocks melt modelling of a crustal gabbro from the muslim bagh ophiolite shows that the trace element signature of the plagiogranites can be replicated by 5 10 melting of a crustal hornblende gabbro with amphibole as a residual phase resulting in a concave up middle rare earth element pattern compositional similarities between the muslim bagh ophiolite plagiogranites and archaean ttg trondhjemite tonalite granodiorite has implications for the generation of juvenile archaean continental crust as the muslim bagh ophiolite was derived in a supra subduction zone it is suggested that some archaean ttg may have been derived from melting of mafic upper crust in early subduction like settings however due to the small volume of muslim bagh ophiolite plagiogranites it is inferred that they can be instructive on the petrogenesis of some but not all archaean ttg

emergence of the canadian rockies and adjacent plains a comparison of physiography between end of laramide time and the present day abstract the landscape of the canadian rockies in southern alberta is not a direct result of constructional processes that is the ridges and peaks have not been pushed into the positions in which we see them today tectonic activity provided original elevation but not mountains at the end of laramide time what are now the front ranges and foothills of the rockies comprised a high elevation upland of relatively low relief the present mountain physiography is the result of 55 60 million years of post orogenic differential erosion in which more resistant rocks have been left at higher elevations than less resistant rocks the canadian rockies and the foothills are developed in a thin skinned thrust and fold belt created during the laramide orogeny the adjacent interior plains cut across foreland basin sediments derived from the mountains the mountains currently consist of large parts of ridges of well indurated paleozoic and locally proterozoic rock alternating with valleys developed in soft mesozoic clastic rock in the foothills where the soft mesozoic rock is at the surface relief is subdued but ridges of more resistant sandstone rise above shaley lowlands the plains are relatively flat but also contain erosional outliers of higher paleo plains surfaces numerous lines of evidence suggest that the mountains and foothills have lost several kilometers of overburden since the end of the laramide orogeny while the western plains have lost at least 2 km requiring that the local relief of the mountains and foothills that we see is erosional in origin local physiography is adjusted to lithology the mountains have high relief because the exposed sub mesozoic rocks can hold up high steep slopes whereas the foothills have low relief because the underlying cretaceous rocks cannot hold up high steep slopes the east facing escarpment at the mountain front is a fault line scarp along a low angle thrust mesozoic rocks involved in the deformation originally extended all the way across the thrust and fold belt and physiography of the belt at the end of laramide time 60 55 ma depended mainly on whether mesozoic or paleozoic proterozoic rocks were exposed at the surface at that time a reconstruction using critical taper theory generally agrees with reconstructions from earlier stratigraphic and paleothermometry studies what are now the front ranges at the eastern edge of the rocky mountains were mostly or perhaps entirely covered with mesozoic rocks and despite that high elevation had a hilly not mountainous character the main ranges in the central rocky mountains were in part stripped of mesozoic cover by then and more mountainous treeline was higher then and the thrust belt may have been largely or entirely vegetated generation of modern relief in the front ranges including the escarpment at the mountain front had to await stripping of mesozoic rocks and incision of rivers into harder substrates in post laramide time the interior plains are an erosional surface that was cut 1 to 3 km below the aggradational top of the foreland basin sediments although some of the present low local relief of the plains results from weakness of underlying cretaceous tertiary rocks the low relief is probably largely related to the process of denudation

proglacial lakes character behaviour and geological importance proglacial lakes are ubiquitous within the quaternary record and can provide exceptional breadth and depth of palaeoenvironmental information present deglaciation is increasing the number and size of proglacial lakes around the world this study provides a synthesis of knowledge on proglacial lake character and behaviour and critically evaluates the importance of proglacial lakes from a geological perspective we show how ice marginal or ice contact lakes and other distal proglacial lakes can be distinguished from each other by geomorphological sedimentological chemical and biological characteristics the key controls on proglacial lake geomorphology and sedimentology are outlined and discussed proglacial lakes can exacerbate mountain glacier and ice sheet margin ablation via mechanical and thermal stresses but very large lakes can moderate summer air temperatures and relatively retard summer ice ablation proglacial lakes interrupt meltwater flux and are very efficient sediment traps hydrological routing and consequent geomorphological activity can be radically modified by sudden drainage of proglacial lakes and resultant glacial lake outburst floods exceptionally large proglacial lake drainages affected global ocean circulation and global climate during the quaternary overall analyses of proglacial lakes can provide a valuable insight into i patterns character and behaviour of mountain glaciers ice sheets and glaciations and ii the impacts of past present and future deglaciation

geochemical and numerical constraints on neoarchean plate tectonics this paper discusses early neoarchean 2 8 2 7 ga plate tectonics by integrating knowledge from new geochemical observations and numerical models based on a geochemical dataset of 295 granitoid samples from the karelian and kola cratons of the fennoscandian shield we divide neoarchean juvenile extracted from oceanic crust or mantle granitoids into three groups 1 low hree heavy rare earth elements ttgs tonalite trondhjemite granodiorite high sio2 low mg low hree higher sr lower ybn and higher nb ta 2 high hree ttgs slightly lower range of sio2 larger range of mgo contents and higher cr and ni contents high hree lower sr higher ybn and lower nb ta and 3 high ba sr sanukitoids medium hree high mg and high k ba sr the main difference between the low and high hree groups lies in their pressure sensitive element contents which indicates high pressure melting conditions for the low hree group and low pressure conditions for the high hree group a possible tectonic scenario for the genesis of the two groups is an incipient hot subduction underneath a thick oceanic plateau protocrust melting in the lower part of thick basaltic oceanic crust could produce ttgs of the low hree type whereas low pressure melting of subducting slab and possible interactions with the mantle wedge at shallow depths would be capable of generating high hree ttgs the third group of archean high ba sr sanukitoids was formed after the ttgs their low sio2 and high mg k ba sr contents suggest an origin by melting in an enriched metasomatized mantle source probably as a result of a slab breakoff following a continental collision or attempted subduction of a thick oceanic plateau ttg protocontinent such hypothesis is supported by numerical modeling results that suggest an increased occurrence of slab breakoff in the archean which locally increased temperatures within the mantle wedge more frequent breakoff resulted because subducting plates were weaker due to rheologically thinner lithosphere and a thicker basaltic crust and tensile stresses within the subducting plate were larger due to a thick crust that transforms from buoyant basalt to dense eclogite

tectonic evolution of the oman mountains an introduction it is now more than 20 years since the geologicalsociety published the geology and tectonics ofthe oman region robertson et al 1990 as oneof its early special publications that volume the blue book as it became fondly known to multiplegenerations of undergraduates has been a majorreference on the geology of oman for many whowork in the region this present volume moremodest in its size compared to its illustrious prede cessor is the fruit of a conference on the geologyof the arabian plate and the oman mountainsheld in january 2012 at sultan qaboos universityin muscat organized by professor sobhi nasirwith colleagues from sultan qaboos university the oman government and the geological societyof oman since the rst comprehensive study ofthe geology of the oman mountains during thelate 1960s and early 1970s by ken glennie and histeam from shell the oman mountains have beenknown to contain some of the most spectacularand best exposed geology not only in the middleeast but across the world a summary of the geo logy and the lithostratigraphy is given in figure 1 the publications of glennie et al 1973 1974 building on many earlier shell geologists internalreports notably d m morton r h tschopp h h wilson b m reinhardt and m w hughes clarke set out the major stratigraphic frameworkfor the whole area de ned the major structuresand critically interpreted the semail ophiolite as athrust sheet of oceanic crust and upper mantleemplaced onto the previously passive continentalmargin of arabia since the glennie et al 1974 memoir was published oil companies have carriedout increasingly more intensive work comparingwell sections in the interior with the permian mesozoic shelf carbonates so beautifully exposedalong numerous wadis cutting through the jebelal akhdar and saih hatat massifs during the late 1970s and 1980s two majorresearchteamsfromtheusgs ledbybobcolemanand cliff hopson and the open university uk led by ian gass john smewing and steve lip pard conducted detailed geological surveys acrossthe ophiolite in the muscat ibra transect usgs and northern ophiolite ou respectively theirresults were published in a special issue of thejournal of geophysical research volume 86 editors coleman hopson 1981 and the geologi cal society of london memoir no 11 lippard etal 1986 major systematic mapping of the omanmountains as well as the interior batain coast anddhofar region has been carried out by the bureaude recherches ge ologiques et minieres brgm france the geological survey of japan and theuniversity of berne switzerland see for examplepeters et al 1991 and the whole uae part of thenorthern oman mountains was recently mapped bythe british geological survey uae ministry ofenergy petroleum mineral resources 2012 during the last 20 years major research groupsfrom france japan usa and the uk have con ducted research projects in the country detailed eld studies of the oman ophiolite have beencombined with geochemical and isotopic studiesto determine the tectonic setting and evolution ofthe late cretaceous oceanic crust and upper mantle boudier juteau 2000 field based studies havealso been linked to ocean drilling sites in particularthe east paci c rise to compare the ophiolite toregions of active fast spreading key to the obduc tion emplacement story are the amphibolite andgreenschist facies rocks in the metamorphic solethat record an inverted metamorphic eld gradientalong the base of the ophiolite the ophiolite hasbeen the source of recent detailed studies involvingthe possible sequestration of co

evolution of late glacial ice marginal lakes on the northwestern canadian shield and their influence on the location of the dubawnt lake palaeo ice stream abstract during deglaciation of the north american laurentide ice sheet large proglacial lakes developed in positions where proglacial drainage was impeded by the ice margin for some of these lakes it is known that subsequent drainage had an abrupt and widespread impact on north atlantic ocean circulation and climate but less is known about the impact that the lakes exerted on ice sheet dynamics this paper reports palaeogeographic reconstructions of the evolution of proglacial lakes during deglaciation across the northwestern canadian shield covering an area in excess of 1 000 000 km2 as the ice sheet retreated some 600 km the interactions between proglacial lakes and ice sheet flow are explored with a particular emphasis on whether the disposition of lakes may have influenced the location of the dubawnt lake ice stream this ice stream falls outside the existing paradigm for ice streams in the laurentide ice sheet because it did not operate over fined grained till or lie in a topographic trough ice margin positions and a digital elevation model are utilised to predict the geometry and depth of proglacial lakes impounded at the margin at 30 km increments during deglaciation palaeogeographic reconstructions match well with previous independent estimates of lake coverage inferred from field evidence and results suggest that the development of a deep lake in the thelon drainage basin may have been influential in initiating the ice stream by inducing calving drawing down ice and triggering fast ice flow this is the only location alongside this sector of the ice sheet where large 3000 km2 deep lakes 120 m are impounded for a significant length of time and exactly matches the location of the ice stream it is speculated that the commencement of calving at the ice sheet margin may have taken the system beyond a threshold and was sufficient to trigger rapid motion but that once initiated calving processes and losses were insignificant to the functioning of the ice stream it is thus concluded that proglacial lakes are likely to have been an important control on ice sheet dynamics during deglaciation of the laurentide ice sheet

moraine formation in northwestern ontario product of subglacial fluvial and glaciolacustrine sedimentation long arcuate stratified end moraines in northwestern ontario may represent major and rapid sedimentation events in glacial lake agassiz rapid lowering of the lake or lift of a marginal ice dam m

tectonic setting for the genesis of oceanic plagiogranites evidence from a paleo spreading structure in the oman ophiolite abstract in the oman ophiolite detailed structural and lithological mapping of the 60 45 km maqsad area has revealed several plagiogranite intrusions ranging from metre scale dykes to hectometre scale bodies these plagiogranites are spatially related to kilometre scale mafic plutons displaying generally pegmatitic textures and locally deformed in amphibolite facies conditions along shear zones up to a few hundred metres thick isotope data demonstrate the mantle origin of these pegmatites and plagiogranites nd ranges from 6 3 to 9 9 and has an average value 26 samples of 8 1 within the field of present day morbs and similar to the average nd 7 8 of the oman ophiolite primary igneous sequence these intrusions are not restricted to a given structural level their paleovertical extent exceeds 5 6 km from 2 km below the moho up to the base of the sheeted dyke complex their modal composition evolves up section from predominantly pyroxenites and gabbro norites in the mantle harzburgites to gabbro norites diorites and quartz diorites at crustal level petrography whole rock and mineral chemistry show that this vertical succession of lithologies can be simply explained by low pressure fractional crystallization of a hydrated basaltic melt plagiogranites although more abundant at upper crustal level are not restricted to this horizon plagioclase composition in plagiogranites presents a remarkable evolution with paleodepth from highly calcic 70 95 an in the mantle section intermediate and highly scattered in the lower crust 10 85 an to quite sodic at upper crustal level 5 20 an highly calcic plagioclases in quartz bearing rocks is a puzzling character inconsistent with fractional crystallization even if high degrees of fractional crystallization of a hydrated basaltic melt remain the simplest way to account for the chemistry of the upper crustal plagiogranites processes such as fluid induced remelting or assimilation of country rocks must be invoked to account for the chemistry and field characteristics of the deeper plagiogranites mantle and crustal structures in the maqsad area indicate that the pegmatites and plagiogranites emplaced in an ocean ridge setting when a mantle diapir soaked with basaltic melt intruded a cool hydrothermally altered lithosphere they were eventually transposed in an off axis setting as the ascension of the diapir progressed up to moho level it is concluded that the formation of large volumes of buoyant leucocratic rocks is possible at ocean ridges provided periods of amagmatic spreading are long enough to induce the growth and alteration of an axial lithospheric lid this is likely more frequent at slow spreading centres where mantle upwelling is discontinuous in space and time

paleomagnetic evidence for the continuity and independent movement of a distinct major crustal block in the southern appalachians the magnetization of 22 granitic and gneissic southern appalachian rock units with estimated cooling ages of 415 250 m y has been determined included are 842 samples from 114 sites within 19 granites 100 sites and 3 gneisses 14 sites located in north carolina south carolina and georgia data for units which cooled to temperatures 20 000 km sup 2 in size a mean paleopole for this group with corrections for maximum tilt estimates exhibits good precision but has a paleosite latitude of approx 10 sup 0 n without tilt correction the paleopole for group b still exhibits an anomalous paleositemore latitude of approx 4 sup 0 n these data indicate that the group b block elberton sparta crustal block lying to the se has an apparent paleosite latitude corresponding to magnetization at a location to the north of the zone containing group a units less

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

thrust surface geometry implications for thrust belt evolution and section balancing techniques abstract the application of modern concepts of thrust tectonics to orogenic belts has yielded many detailed restorable sections thrusts generally restore with a staircase geometry onto the undeformed stratigraphic template since the work of rich 1934 it has been generally assumed that thrusts have propagated with a staircase trajectory and that hangingwall structures result from the movement of the thrust sheet up footwall ramps however thrusts often cut through previously folded strata with a smooth trajectory smooth trajectory thrusts can be identified from footwall geometry and strain state once a smooth trajectory thrust has been identified it is insufficient to produce a balanced deformed section and its restored counterpart sequential balanced sections must be constructed reversing the deformation history the restoration of thrusts to a staircase trajectory has no mechanical significance as the thrust is merely acting as a marker if restored back beyond the time at which it propagated thrust ramps are not necessarily actual steps in the thrust surface they are often just zones where the thrust cuts through bedding in a smoothly curving trajectory ductile deformation in thrust belts can reduce bed length at depth thus removing the need for structures which are necessary to preserve constant bed lengths ductile deformation in the internal zones of orogenic belts renders the construction of balanced sections in these zones highly problematical

laurentide and montane glaciation along the rocky mountain foothills of northeastern british columbia mapping the surficial geology of the trutch map area nts 94g provides new data on the timing of continental and montane glaciations along the foothills of northeastern british columbia striated surfaces on mountain crests were dated to the late wisconsinan substage by cosmogenic dating the striations were produced by eastward flowing ice emanating from the region of the continental divide this ice was thick enough to cross the main ranges and overtop the rocky mountain foothill summits at 2000 m above sea level asl it is argued here that such a flow unhindered by topography could only have been produced by the cordilleran ice sheet and not by local cirque glaciation during this time the cordilleran ice sheet dispersed limestone and schist erratics of western provenance onto the plains beyond the mountain front conversely the laurentide ice sheet did not reach its western limit in the foothills until after cordilleran ice retreated from the area during its maximum the laurentide ice penetra

anisotropy of magnetic susceptibility data indicating remagnetization in diabase dikes induced and remanent magnetic analyses have been performed on drilled samples from 2 generations of unweathered diabase dikes a breccia dike and the host tonalite which make up the basement on which the russell dam foundation was poured the maximum magnetic foliation plane for most samples defined by anisotropy of magnetic susceptibility ams data is vertical and is oriented perpendicular to maximum stresses developed in the region during mid late paleozoic time the excellent ams data consistency between units of different ages is interpreted to be indicative of stress controlled magnetite recrystallization at the site it is inferred that this recrystallization is responsible for the multicomponent remanent magnetization rm in these samples which was probably acquired over long periods of time

deglacial rapid sea level rises caused by ice sheet saddle collapses a simulation of ice sheet retreat reveals two meltwater pulses coinciding with mwp 1a and the 8 200 year event caused by saddle collapses between ice domes during the last deglaciation the retreating ice sheets occasionally released massive amounts of fresh water these meltwater pulses are often implicated as important influences on abrupt climate change but the underlying triggering mechanisms are not clear here lauren gregoire et al show that saddle zones that connect adjacent high domes on ice sheets can be lowered by ongoing warming to an elevation at which they rapidly degrade leading to separate domes and a meltwater pulse that is consistent with geological observations although the mechanism itself is not novel the authors establish that two important sudden cooling events meltwater pulse 1a and the 8 200 year event were probably caused by saddle separation the last deglaciation 21 to 7 thousand years ago was punctuated by several abrupt meltwater pulses which sometimes caused noticeable climate change1 2 around 14 thousand years ago meltwater pulse 1a mwp 1a the largest of these events produced a sea level rise of 14 18 metres over 350 years3 although this enormous surge of water certainly originated from retreating ice sheets there is no consensus on the geographical source or underlying physical mechanisms governing the rapid sea level rise4 5 6 here we present an ice sheet modelling simulation in which the separation of the laurentide and cordilleran ice sheets in north america produces a meltwater pulse corresponding to mwp 1a another meltwater pulse is produced when the labrador and baffin ice domes around hudson bay separate which could be associated with the 8 200 year event the most pronounced abrupt climate event of the past nine thousand years7 for both modelled pulses the saddle between the two ice domes becomes subject to surface melting because of a general surface lowering caused by climate warming the melting then rapidly accelerates as the saddle between the two domes gets lower producing nine metres of sea level rise over 500 years this mechanism of an ice saddle collapse probably explains mwp 1a and the 8 200 year event and sheds light on the consequences of these events on climate

till stratigraphy and sedimentology at the margins of terrestrially terminating ice streams case study of the western canadian prairies and high plains complex glacigenic sediment sequences arranged in large arcuate assemblages and demarcating the lobate termini of fast ice flow corridors in western canada represent the terrestrial equivalents of the trough mouth fans of the marine palaeo ice stream record intensive sedimentological and stratigraphic investigations at a regional scale are employed in the interpretation of thick units or sequences of diamicton and two depositional scenarios are proposed first glacier marginal till thickening and the stacking of till wedges takes place in areas where an ice stream margin was stationary for short periods of time stacks of subglacial traction tills potentially record annual sub marginal incremental thickening at ice stream margins especially in areas where densely spaced recessional push moraines appear to record seasonal climatic forcing second proglacial lake and valley infilling with glacilacustrine rhythmites and mass flow diamictons is associated with the advection of sub marginal till into preglacial bedrock depressions and records ice stream marginal oscillations and debris influx into discontinuous proglacial lakes which act as subaqueous sediment sinks for materials that would otherwise have been used to construct sub marginal till wedges and moraines glacial overriding modifies the deposits produced in both scenarios through deformation and the dislocation and entrainment of bedrock and sediment rafts at the margins of bedrock valley walls resulting in the widespread development of glacitectonites and megablocks the occurrence of the thickest deposits as preglacial valley fills may be significant in groundwater siphoning away from subglacial deforming layers and the promotion of sticky spots

the petrogenesis of leucogranitic dykes intruding the northern semail ophiolite united arab emirates field relationships geochemistry and sr nd isotope systematics the khawr fakkan block of the semail ophiolite united arab emirates exhibits a suite of 10 100 m scale metaluminous to peraluminous granitic intrusions ranging from cordierite andalusite biotite monzogranites to garnet tourmaline leucogranites which intrude mantle sequence harzburgites and lower crustal sequence cumulate gabbros structural constraints suggest that the subduction of continental sedimentary material beneath the hot proto ophiolite in an intra oceanic arc environment led to granulite facies metamorphism at the subduction front and the generation of granitic melts which were emplaced up to the level of the ophiolite moho compositions indicate the analysed granitoids were largely minimum melts that crystallised at variable a h2o and pressures of 3 to 5 kbar the lile sr rb and ba covariation modelling suggests that the granitoids formed largely by the dehydration melting of muscovite rich metasediments initial 87sr 86sr ratios of analysed dykes vary between 0 710 and 0 706 at initial nd values of between 6 3 and 0 5 cogenetic units of a composite sill from ra s dadnah yield a sm nd isochron age of 98 8 9 5 ma mswd 1 18 geochemical and isotopic characteristics of the analysed granitic intrusions indicate that the subducted continental material was derived from oceanic trench fill haybi complex sediments preserved as greenschist asimah area to granulite facies bani hamid area ophiolitic metamorphic sole terranes the sr nd isotope systematics suggest that hybrid granitic melts were derived from pre magmatic mixing of two contrasting subduction zone sources

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

origin of reflections from the brevard fault zone the brevard fault zone is a belt of mylonitic metamorphic rocks that extends northeast over 700 km from alabama into virginia seismic reflection studies show that the fault zone produces excellent reflections which dip gently to the southeast and sole at depth the origin of these reflections has been investigated by measuring compressional wave velocities to 800 mpa confining pressure in 127 samples obtained from drill core which continuously sampled the fault zone to a depth of over 300 m densities are reported as well as chemical analyses at selected depths anisotropy at elevated pressures ranges within the fault zone from less than 5 to over 26 similarly the fault zone rocks show significant chemical variability with sio2 ranging from 58 0 to 70 4 by weight synthetic seismograms generated from the velocity and density measurements show that the reflections originate within the brevard fault zone from the complex interaction of compositional variations and seismic anisotropy resulting from ductile strain essential to this study has been the continuous coring of a major section of the brevard fault zone which has provided the necessary stratigraphic control as well as unweathered samples for laboratory studies

quaternary geology and geomorphology of the dinosaur provincial park area and surrounding plains alberta canada the identification of former glacial lobes drainage diversions and meltwater flood tracks abstract the quaternary geology and geomorphology of the lower red deer river drainage basin centred on the dinosaur provincial park badlands provides information on pre late wisconsinan drainage patterns and the dynamics of former lobes of the laurentide ice sheet in western canada patterns of deglaciation proglacial lake evolution and spillway incision are also reconstructed based upon the distribution of surface materials and glacial glaciofluvial landforms the empress group fluvial and glaciolacustrine sediments which could be as young as 27 ka bp infill the precursor red deer river and its tributaries and document the initial advance of glacier ice into southern alberta glaciotectonic disturbance of older sediments and bedrock the production of deformation tills and the construction of a megafluting complex and cupola hills record the advances of a glacier lobe centred over the study area stratified inter and intra till beds record pulses of subglacial meltwater between phases of subsole deformation the thickening of tills towards the margin of the lobe represents a till wedge an expected product of sediment advection by glaciers moving over deformable beds the eastern margin of the glacier lobe is demarcated by the interlobate suffield hummocky moraine belt which contains overprinted thrust ridges which record diachronous oscillations of neighbouring lobes within the ice sheet proglacial and glaciofluvial sediments were deposited in the area in association with proglacial lake bassano patricia which drained eastwards when the suffield moraine was dissected by spillways changes in the size of glacial lake bassano patricia are clearly documented by a sequence of spillway incisions which culminated in the erosion of scabland topography and the initiation of a new course for the red deer river a 15 km southward diversion of the main channel in distinct contrast to the documented incision histories of other small rivers in alberta one tree creek and little sandhill creek did not start major incisions of the quaternary sediments over buried valley positions until the late holocene when environmental conditions were characterized by higher precipitation

tectonic implications of subthrust structures revealed by seismic profiling of appalachian ouachita orogenic belt a growing body of deep seismic reflection profiles across the appalachian ouachita orogenic belt reveals subthrust structures which have been previously interpreted as products of continental rifting of later thrusting or of some combination of rifting and thrusting the subthrust features can be placed into three general categories 1 small normal faults which offset lower plate shelf sequences and basement in the foreland and have been attributed to thrust loading perhaps reactivating earlier rifting faults 2 moderately dipping seismic events which lie entirely below lower plate shelf sediments in the foreland interpreted in part as strata deposited during the earlier rifting and 3 hinterland dipping reflection sequences beneath more interior portions of the mountain belt previously interpreted as thrust faults which imbricate sediments and basement related to the earlier rifted margin these interpretations are evaluated through a comparison of reflection profiles across the orogenic belt to previously interpreted seismic lines across modern extensional and compressional settings the comparisons suggest that although some normal fault offsets of lower plate shelf sequences may be original or reactivated rifting faults the dipping events entirely beneath the shelf sequence are more diagnostic of the rifting process these dipping sequences are therefore consistent with previous interpretations that large portions of the earlier rifted margin are preserved in an undeformed state beneath thrust sheets in the appalachian foreland the comparisons also suggest an alternative to interpretations of the hinterland dipping sequences as thrust related in that the sequences are similar in seismic appearance to that of volcanic wedges commonly observed in the narrow zone separating continental from oceanic basement on modern passive margins the implication of this new interpretation is that the actual continent ocean boundary related to the earlier rifted margin may be preserved on the lower thrust plate beneath interior portions of the appalachians and ouachitas

appalachian carboniferous dextral strike slip faults an example from brookneal virginia the northern appalachian dextral fault system of late paleozoic age is also present in the central and southern appalachians an example of a dextral strike slip fault is the 4 km wide brookneal shear zone in the southwest virginia piedmont the shear zone is in part superimposed on the melrose granite where an s c mylonite was produced by dynamic recrystallization of all constituent minerals the arvonia metasedimentary and charlotte belt metavolcanic rocks contain spaced dextral shear bands at a consistent 24 3 to the shear zone boundary a minimum displacement estimate of 17 km was obtained from rotated foliation measurements in the melrose granite the age of movement on the brookneal shear zone has been constrained by isotopic dating to between 324 and 300 ma other faults in the southern appalachians including the nutbush creek and modoc zones show similar ages and relative offsets possible plate tectonic models that could account for the late paleozoic dextral fault system throughout the appalachians include 1 tectonic escape resulting from the collision of a plate with north america to the north of the canadian appalachians 2 postcollision interplate readjustments involving counterclockwise rotation of africa relative to north america and 3 oblique convergence of eastern north america with an oceanic plate moving west

peraluminous potassium rich granitoids in the semail ophiolite potassic granites are found as dikes and small stocks in the uppermost part of the mantle sequence of the semail ophiolite predominantly in the northern and less frequently in the central and southern part new chemical analyses show that the biotite granites lie on the same evolutionary trend as the quartz diorites and tonalites occurring as late intrusives in the gabboro and mantle sequence based on mineral chemistry the p t conditions of andalusite cordierite bearing pegmatoidal granites were calculated giving pressures of 1 8 2 2 kbar and a solidus within the stability field of andalusite hydrothermal experiments on five natural granite and tonalite compositions gave minimum melting between 650 and 675 c at 2 kbar at 700 c excess h2o there is already 70 80 melt from the granitic starting compositions potassic granites in the ophiolite can be the result of extreme fractional crystallization in the late intrusive series granitic melts can also be produced by a two stage process in which fluids escaping from the underlying metamorphic sole trigger partial melting of the more felsic members in the ophiolite series two mica granites and pegmatites in the underlying metamorphic sole possibly melting products of amphibolites and biotite gneisses are contemporaneous these escaping fluids could also explain the pb and sr isotopic compositions of the leucocratic dikes indicating variable crustal contamination

formalising an inversion methodology for reconstructing ice sheet retreat patterns from meltwater channels application to the british ice sheet glacial meltwater channels yield important information regarding patterns of palaeo ice sheet retreat however it is important to distinguish between channels formed at different positions in relation to the ice mass e g lateral subglacial in britain whilst meltwater channels have been widely reported in the literature such categorisation is often lacking here we develop an inversion methodology for extracting ice margin retreat patterns from meltwater channels diagnostic geomorphological criteria are presented derived from published descriptions of meltwater channel form by which a channel of unknown origin may be assigned to its appropriate category these criteria are applied to channels recorded in the britice gis database of glacial landforms and we find most channels to be of lateral or subglacial origin the categorisation of channels enables palaeo margin positions and the direction of retreat stage ice flow to be approximated and a preliminary reconstruction is made of fragments of the ice sheet retreat pattern further mapping and integration of melt traces with other geomorphic data will permit a more complete palaeo glaciological reconstruction of the last british ice sheet copyright 2007 john wiley sons ltd

frozen bed fennoscandian and laurentide ice sheets during the last glacial maximum the areal extents of the laurentide and fennoscandian ice sheets during the last glacial maximum about 20 000 years ago are well known1 but thickness estimates range widely from high domed2 to thin3 with large implications for our reconstruction of the climate system regarding for example northern hemisphere atmospheric circulation and global sea levels this uncertainty stems from difficulties in determining the basal temperatures of the ice sheets and the shear strength of subglacial materials4 a knowledge of which would better constrain reconstructions of ice sheet thickness here we show that in the absence of direct data the occurrence of ribbed moraines in modern landscapes can be used to determine the former spatial distribution of frozen and thawed bed conditions we argue that ribbed moraines were formed by brittle fracture of subglacial sediments induced by the excessive stress at the boundary between frozen and thawed bed conditions resulting from the across boundary difference in basal ice velocity maps of glacial landforms from aerial photographs of canada and scandinavia reveal a concentration of ribbed moraines around the ice sheet retreat centres of quebec keewatin newfoundland and west central fennoscandia together with the evidence from relict landscapes that mark glacial areas with frozen bed conditions the distribution of ribbed moraines on both continents suggest that a large area of the laurentide and fennoscandian ice sheets was frozen based and therefore high domed and stable during the last glacial maximum

structure and tectonics of central and southern appalachian valley and ridge and plateau provinces west virginian and virginia the valley and ridge and eastern allegheny plateau from pennsylvania into tennessee is subdivided into distinct allochthonous sheets emplaced over an essentially featureless precambrian basement surface sheet development is related to variations in stratigraphic thickness and lithology as well as regional basement configuration three sheets control central appalachian structural style the massanutten blue ridge sheet is easternmost and is bounded on the west by the little north mountain pulaski fault system continuing westward into the plateau the martinsburg sheet contains anticlinoria and synclinoria uncut by major thrust faults both massanutten blue ridge and martinsburg sheets are seated in upper ordovician martinsburg reedsville shales the underlying waynesboro sheet is rooted in lower cambrian waynesboro rome shales and its imbrication has controlled the locations of major anticlinoria and synclinoria in the upper sheet within the southern appalachians greater imbrication of the lower sheet and increased thrust fault displacement at longitudinal ramps have destroyed the lateral continuity along strike of the central appalachian waynesboro and overlying martinsburg sheets there imbrication to the surface has formed the saltville sheet continuous into alabama as well as the st clair narrows copper creek cove mountain and st paul honaker sheets southwest of the trend change the holston stone mountain sheet adjacent to the blue ridge is rooted primarily in lower and middle cambrian strata and is underlain by the pulaski sheet in the westernmost sector of this region the pine mountain and richlands sheets are seated in devonian shales the pulaski and massanutten blue ridge sheets rooted primarily in upper ordovician shales form a continuous structural unit and include blue ridge and piedmont rocks

evaluation of southern eastern overthrust belt beneath blue ridge piedmont thrust seismic reflection surveys in the southern part of the eastern overthrust reveal that crystalline rocks of the blue ridge and piedmont were thrust westward burying a large segment of sedimentary rocks of the valley and ridge surface and subsurface data suggest that the buried segment includes cambrian and lower ordovician shallow shelf clastic and carbonate rocks overlain by middle ordovician foredeep black shale because eastward increasing regional thermal patterns existed prior to thrusting westward movement of thrust sheets disrupted and telescoped that pattern by placing more thermally mature eastern rocks over less mature western rocks the original thermal pattern can be reconstructed by restoring the thrust sheets to their prethrust position the reconstruction mphasizes that rocks less metamorphosed than greenschist facies are contained in only two thrust sheets all of the saltville and part of the pulaski because subgreenschist rocks of the saltville and pulaski thrust sheets appear to be confined to the subsurface of the blue ridge and westward it is most likely that rocks with commercial gas potential will not be present in the subsurface of the piedmont east of the brevard fault nevertheless the concealed area of the southern eastern overthrust with possible gas potential is about as wide as its exposed part thus the combined surface and subsurface area for future exploration in the southern appalachians is about doubled in size

hummocky moraine sedimentary record of stagnant laurentide ice sheet lobes resting on soft beds abstract over large areas of the western interior plains of north america hummocky moraine hm formed at the margins of laurentide ice sheet lis lobes that flowed upslope against topographic highs current depositional models argue that hm was deposited supraglacially from stagnant debris rich ice disintegration moraine across southern alberta canada map and outcrop data show that hm is composed of fine grained till as much as 25 m thick containing rafts of soft glaciotectonized bedrock and sediment chaotic non oriented hm commonly passes downslope into weakly oriented hummocks washboard moraine that are transitional to drumlins in topographic lows the same subsurface stratigraphy and till facies is present throughout these landforms and others such as doughnut like rim ridges flat topped moraine plateaux and linear disintegration ridges are identified as belonging to subglacially deposited soft bed terrain this terrain is the record of ice lobes moving over deformation till derived from weakly lithified bentonite rich shale drumlins record continued active ice flow in topographic lows during deglaciation whereas hm was produced below the outer stagnant margins of ice lobes by gravitational loading pressing of remnant dead ice blocks into wet plastic till intervening zones of washboard moraine mark the former boundary of active and stagnant ice and show hybrid drumlins whose streamlined form has been altered by subglacial pressing humdrums below dead ice the presence of hummocky moraine over a very large area of interior north america provides additional support for glaciological models of a soft bedded laurentide ice sheet

cosmogenic 36cl dating of the foothills erratics train alberta canada the foothills erratics train of alberta was carried into place during coalescence between montane glaciers and the laurentide ice sheet its age has been controversial late wisconsinan and pre wisconsinan ages have been assigned to it cosmogenic 36 cl exposure dating was carried out on erratics over a 130 km segment of the erratics train seven of eight age determinations support emplacement during the late wisconsinan glaciation these ages eliminate the possibility of an ice free corridor during the last glacial maximum they are also consistent with the hypothesis that the laurentide ice sheet was the most extensive of all pleistocene continental ice sheets in western canada

an updated radiocarbon based ice margin chronology for the last deglaciation of the north american ice sheet complex abstract the north american ice sheet complex naisc consisting of the laurentide cordilleran and innuitian ice sheets was the largest ice mass to repeatedly grow and decay in the northern hemisphere during the quaternary understanding its pattern of retreat following the last glacial maximum is critical for studying many facets of the late quaternary including ice sheet behaviour the evolution of holocene landscapes sea level atmospheric circulation and the peopling of the americas currently the most up to date and authoritative margin chronology for the entire ice sheet complex is featured in two publications geological survey of canada open file 1574 dyke et al 2003 quaternary glaciations extent and chronology part ii dyke 2004 these often cited datasets track ice margin recession in 36 time slices spanning 18 ka to 1 ka all ages in uncalibrated radiocarbon years using a combination of geomorphology stratigraphy and radiocarbon dating however by virtue of being over 15 years old the ice margin chronology requires updating to reflect new work and important revisions this paper updates the aforementioned 36 ice margin maps to reflect new data from regional studies we also update the original radiocarbon dataset from the 2003 2004 papers with 1541 new ages to reflect work up to and including 2018 a major revision is made to the 18 ka ice margin where banks and eglinton islands once considered to be glacial refugia are now shown to be fully glaciated our updated 18 ka ice sheet increased in areal extent from 17 81 to 18 37 million km2 which is an increase of 3 1 in spatial coverage of the naisc at that time elsewhere we also summarize region by region significant changes to the deglaciation sequence this paper integrates new information provided by regional experts and radiocarbon data into the deglaciation sequence while maintaining consistency with the original ice margin positions of dyke et al 2003 and dyke 2004 where new information is lacking this is a pragmatic solution to satisfy the needs of a quaternary research community that requires up to date knowledge of the pattern of ice margin recession of what was once the world s largest ice mass the 36 updated isochrones are available in pdf and shapefile format together with a spreadsheet of the expanded radiocarbon dataset n 5195 ages and estimates of uncertainty for each interval

conodont based thermal maturation of paleozoic rocks in arizona color changes in conodonts are established thermal indices that have been related to the thermal window for hydrocarbon generation and preservation and that also have value for targeting mineralization potential conodont color alteration values from 141 localities in arizona appear to characterize the thermal maturation of the major paleozoic outcrop areas in the state paleozoic rocks in the colorado plateau in part of the belt of faulted plateau like rocks bordering the plateau and in a small area from 31 20 prime to 31 44 primen and from 110 00 prime to 110 40 primew in southeast arizona have thermal potential for oil paleozoic rocks in the same areas as well as in the pedregosa basin southeasternmost arizona have thermal potential for gas f these areas southeast arizona appears to have the best hydrocarbon potential because of the association of possible source beds and favorable thermal maturation levels

the brevard zone in south carolina and adjacent areas an alleghanian orogen scale dextral shear zone reactivated as a thrust fault the displacement history of the brevard zone a linear orogen scale structure in the metamorphic core of the southern appalachians has been controversial geological and seismic data indicate that the brevard zone and the taconic acadian metamorphic rocks in which it resides are contained in a huge crystalline thrust sheet that overlies relatively unmetamorphosed platform sedimentary rocks along a major alleghanian thrust fault along a 100 km segment in south carolina and adjacent states the brevard zone contains three separate structural features all of which dip moderately se 1 the narrow rosman fault which contain tectonic breccia and slices of the buried platform sedimentary rocks 2 the brevard mylonite zone which contains mylonitic rocks that extend about 3 km on either side of the rosman fault and exhibits shear sense criteria which consistently indicate dextral strike slip 3 the brevard fault zone se of the rosman fault which contains stratified metamorphic rocks that are repeated

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

reconstructing the last irish ice sheet 1 changing flow geometries and ice flow dynamics deciphered from the glacial landform record the glacial geomorphological record provides an effective means to reconstruct former ice sheets at ice sheet scale in this paper we document our approach and methods for synthesising and interpre

conflicting mineral and whole rock isochron ages from the late archaean lewisian complex of northwestern scotland implications for geochronology in polymetamorphic high grade terrains we report major and trace element including rare earth element ree geochemistry and whole rock smnd geochronology on two suites of mafic ultramafic inclusions in an agmatite complex within a trondhjemitic host at gruinard bay in the late archaean lewisian complex of northwest scotland a suite of medium grained equigranular amphibolites main amphibolite suite mas has broadly chondritic high field strength element hfse ratios ti zr ca 115 ti y ca 318 y cr ca 0 39 flat heavy ree patterns at five to fifteen times chondrites usually with mild light ree enrichment lan 50 and small eu anomalies this is consistent with an origin as basaltic liquids which may represent suitable mafic precursors for tonalite trondhjemite granodiorite ttg suite magmas as argued previously by rollinson and fowler 1987 a smnd regression line mswd 3 4 obtained from the mas yields an age of 2 943 0 091 ga with an initial end value of 2 69 0 62 a suite of coarser grained hornblendites and metagabbros hornblendite metagabbro suite hms displays significant geochemical differences from the mas exemplified by significantly light ree enriched patterns in which the hornblendites have the highest total ree with lan ndn 1 and large negative eu anomalies and the most felsic metagabbros have low total ree and large positive eu anomalies samples from the hms yield a smnd isochron mswd 0 52 age of 2 846 0 073 ga with an initial end value of 1 10 0 72 the hms geochemistry is consistent with an origin as amphibole and plagioclase cumulates to the local ttg magmas combining the new data with previously published data from the host trondhjemites at gruinard bay whitehouse 1989a yields a smnd isochron mswd 1 09 age of 2 795 0 028 ga with an initial end value of 0 66 0 38 the relatively low initial end values for the hms and combined hms trondhjemite isochrons compared to contemporaneous depleted mantle end 2 8 ca 2 1 to 3 3 is consistent with derivation of a parental magma from light ree enriched components of a mafic basaltic precursor which was itself derived from a depleted mantle source approximately 100 200 ma previously the mas suite at gruinard bay may be representative of such a mafic precursor our new geochronological data differ significantly from the ca 3 3 ga mineral smnd and pbpb regression ages presented recently for two members of the hms by burton et al 1994 which if correct require radical revision of the early geological history of the lewisian complex comparison of the relatively old mineral ages with our new whole rock age contradicts the conventional assumption that mineral isotope systematics are less robust to later disturbance than whole rock systematics however we argue that extensive retrogression evident in all lithologies at gruinard bay makes them less than ideal samples for a mineral isotopic study and it is likely that the ca 3 3 ga ages may be spurious the product of disturbance of isotope systematics during the laxfordian on the basis of the geochronology presented here evolution of the lewisian closely follows generally accepted models for late archaean crust formation

reconstructing the confluence zone between laurentide and cordilleran ice sheets along the rocky mountain foothills south west alberta landform mapping near the athabasca valley was fundamental to determining whether the late wisconsinan laurentide and cordilleran ice sheets lis cis coalesced in this paper we examine the detailed landform record using new lidar digital elevation and hillshade models of the area of confluence and eventual desuturing of these ice sheets this work reveals an earlier more extensive cordilleran advance before coalescence with the lis when the ice sheets coalesced the flow pattern was dominated by ice flow along the mountain front with cordilleran ice flowing out of major trunk valleys but laurentide ice flowing into the foothills up smaller valleys this flow pattern implies that when the ice sheets coalesced the cis was already waning or at least was not at its maximum deglaciation was interrupted by several re advances probably related to the destabilizing effect of proglacial lakes along the ice margin

north american ice sheet build up during the last glacial cycle 115 21 kyr the last glacial maximum lgm outline and subsequent retreat pattern 21e7 kyr of north americanice sheets are reasonably well established however the evolution of the ice sheets during their build upphase towards the lgm between 115 and 21 kyr has remained elusive making it difficult to verifynumerical ice sheet models for this important time interval in this paper we outline the pre lgm icesheet evolution of the laurentide and cordilleran ice sheets by using glacial geological and geomorphologicalrecords to make a first order reconstruction of ice sheet extent and flow pattern we mappedthe entire area covered by the laurentide and cordilleran ice sheets in landsat mss images andapproximately 40 of this area in higher resolution landsat etm images mapping in aerial photographsadded further detail primarily in quebec labrador the cordilleran region and on baffin island our analysis includes the recognition of approximately 500 relative age relationships from crosscuttinglineations together with previously published striae and till fabric data these are used as the basis forrelative age assignments of regional flow patterns for the reconstruction of the most probable ice sheetevolution sequence we employ a stepwise inversion scheme with a clearly defined strategy for delineatingcoherent landforms swarms reflecting flow direction and configuration and linking these topreviously published constraints on relative and absolute chronology our results reveal that icedispersalcentres in keewatin and quebec were dynamically independent for most of pre lgm time andthat a massive quebec dispersal centre rivalling the lgm in extent existed at times when the sw sectorof the ice sheet had not yet developed the oldest flow system in eastern quebec labrador atlanticswarm had an ice divide closer to the labrador coast than later configurations a northern keewatin central arctic ice sheet existed prior to the lgm but is poorly chronologically constrained there is alsoevidence for older and more easterly cordilleran ice sheet divide locations than those that prevailedduring the late wisconsinan in terms of ice sheet build up dynamics it appears that residual ice capsafter warming phases may have played an important role in particular the location and size of remnantice masses at the end of major interstadials i e ois 5c and 5a must have been critical for subsequentbuild up patterns because such remnant uplands may have fostered much more rapid ice sheetgrowth than what would have occurred on a fully deglaciated terrain the ice sheet configuration duringstadials would also be governed largely by the additional topography that such residual ice constitutesbecause of inherent mass balance topography feedbacks

implications of thermal events on thrust emplacement sequence in the appalachian fold and thrust belt some new vitrinite reflectance data interpretation of existing geothermometry data combined with new vitrinite reflectance data within the framework of a detailed composite tectonic setting elucidates the evolution of structural sequencing of thrust sheets during the alleghanian event in the valley and ridge province in virginia that the pulaski thrust sheet preceded the saltville thrust sheet in the emplacement sequence and that both reached thermal maxima prior to or during respective emplacement may be inferred from vitrinite and other geothermometry data in contrast the narrows and st clair thrust sheets probably each attained their thermal maximum after emplacement new vitrinite reflectance data are consistent with cai and other temperature sensitive information heretofore ascertained in the valley and ridge province and support previously established maximum temperatures of ca 200 c for strata of the saltville thrust sheet as young as mississippian r max values from mississippian coals in the price formation of the sal

late wisconsinan cordilleran and laurentide glaciation of the peace river valley east of the rocky mountains british columbia in the past researchers have disagreed over the maximum extent of the cordilleran ice sheet in the peace river valley during the late wisconsinan some workers argued that cordilleran ice reached beyond the rocky mountains and briefly coalesced with the laurentide ice sheet on the westernmost interior plains in contrast others asserted that cordilleran ice did not reach beyond the eastern front of the rocky mountains stratigraphic interpretation of three sections within a middle wisconsinan paleovalley and re examination of a previously published regional stratigraphic framework show that western sourced ice likely the cordilleran ice sheet extended east of the mountain front during the late wisconsinan prior to the incursion of the laurentide ice sheet into the area this conclusion has implications for cordilleran ice sheet reconstruction and modelling and provides insight into the interactions between the cordilleran and laurentide ice sheets during the last glaciation

petrogenesis of oceanic plagiogranites by partial melting of gabbros an experimental study we performed hydrous partial melting experiments at shallow pressures 0 2 gpa under slightly oxidizing conditions nno oxygen buffer on oceanic cumulate gabbros drilled by odp ocean drilling program cruises to evaluate whether the partial melting of oceanic gabbro can generate sio2 rich melts with compositions typical of oceanic plagiogranites the experimental melts of the low temperature runs broadly overlap those of natural plagiogranites at 940 c the normalized sio2 contents of the experimental melts of all systems range between 60 and 61 wt and at 900 c between 63 and 68 wt these liquids are characterized by low tio2 and feotot contents similar to those of natural plagiogranites from the plutonic section of the oceanic crust but in contrast to fe and ti rich low temperature experimental melts obtained in morb systems at 950 c the 1 500 m long drilled gabbroic section of odp hole 735b legs 118 and 176 at the southwest indian ridge contains numerous small plagiogranitic veins often associated with zones which are characterized by high temperature shearing the compositions of the experimental melts obtained at low temperatures match those of the natural plagiogranitic veins while the compositions of the crystals of low temperature runs correspond to those of minerals from high temperature microscopic veins occurring in the gabbroic section of the hole 735b this suggests that the observed plagiogranitic veins are products of a partial melting process triggered by a water rich fluid phase if the temperature estimations for high temperature shear zones are correct up to 1 000 c and a water rich fluid phase is present the formation of plagiogranites by partial melting of gabbros is probably a widespread phenomenon in the genesis of the ocean crust

time perspectives on glacial landscape formation glacial flow chronology at lac aux goelands northeastern quebec canada we used mapping from aerial photographs and field investigations around lac aux goelands whitegull lake northeastern quebec to establish a glacial flow chronology that involves three different glacial landform systems we interpret the youngest of these systems as related to the late wisconsinan deglaciation this system has an up glacier discontinuity that marks a temporal shift from warm based subglacial conditions to non erosive cold based subglacial conditions whereby older glacial landscapes are preserved in the interior of quebec labrador furthermore our results support theories suggesting a northward migrating ice divide to a final position south of ungava bay during the final deglaciation the recognition that a glacial landscape is fragmented and consists of smaller temporally unrelated units is of considerable significance in understanding ice sheet dynamics and patterns of subglacial preservation and destruction this is because a fragmented landscape implies a fragmented formation history where formation of subglacial landforms occurred in restricted subglacial zones during restricted time periods periods of formation must have been separated by periods of preservation and inhibited subglacial reshaping based on estimates of the ice margin retreat velocity and the velocity of the warm based inward migrating zone we suggest that periods of preservation were orders of magnitude longer than periods of subglacial landform formation copyright 2003 john wiley sons ltd

evidence of cordilleran late wisconsinan glaciers in the ice free corridor abstract glaciers flowing down the athabasca river valley during the last glacial period were fed both by local montane glaciers and by cordilleran ice moving easterly across the continental divide from british columbia resulting in a major eastward surge into the ice free corridor the timing of this event has been disputed for many years and has important implications for the study of paleoenvironmental conditions and the north south migration of biota in the corridor chronologic and lithostratigraphic data from the jasper region suggest that this event took place in the late wisconsinan subtill radiocarbon dates on wood in the athabasca river valley indicate that non glacial alluvial sediments began accumulating there sometime before 48 ka a finite date of 29 100 560 bp gsc 3792 on wood under till near jasper provides a maximum age for the onset of the last glaciation in the region although ice from west of the rocky mountain trench probably did not cross the continental divide until after 21 5 ka the occurrence of supraglacial metamorphic erratics as well as the distribution and nature of mapped glacial deposits and the trend of subglacial paleoflow indicators in the region east of the rockies suggests that late wisconsinan glaciers flowing from the mountains coalesced with laurentide glaciers and were deflected southeasterly along the mountain front an even more extensive glacial event may have effected the region in pre late wisconsinan times the model of late wisconsinan glaciation presented here is in agreement with all known radiocarbon data from the area and is especially attractive when supratill dates on non woody lacustrine organics are viewed in light of possible contamination by old carbon

flow pattern evolution of the last british ice sheet we present a 10 stage reconstruction of the evolution in ice flow patterns of the last british ice sheet from build up to demise derived from geomorphological evidence 100 flowsets identified in the subglacial bedform record drumlins mega scale glacial lineations and ribbed moraine are combined with ancillary evidence erratic transport paths absolute dates and a semi independently reconstructed retreat pattern to define flow patterns ice divides and ice sheet margins during build up maximum glaciation and retreat overprinting and cross cutting of landform assemblages are used to define the relative chronology of flow patterns and a tentative absolute chronology is presented based on a collation of available dates for ice advance and retreat the ice flow configuration of the last british ice sheet was not static some ice divides were remarkably stable persisting through multiple stages of the ice sheet evolution whereas others were transient features existing for a short time and or shifting in position 10s km the 10 reconstructed stages of ice sheet geometry capture two main modes of operation first as an integrated ice sheet with a broadly n s orientated ice divide and second as a multi domed ice sheet orientated parallel with the shelf edge a thick integrated ice sheet developed as ice expanded out of source areas in scotland to envelop southerly ice caps in northern england and wales and connect with the irish ice sheet to the west and the scandinavian ice sheet across the north sea following break up of ice over the north sea ice streaming probably drove mass loss and ice sheet thinning to create a more complex divide structure where ice flow patterns were largely controlled by the form of the underlying topography ice surface lowering occurred before separation of and retreat to multiple ice centres centred over high ground we consider this 10 stage reconstruction of the evolution in ice sheet configuration to be the simplest palaeo glaciological interpretation of the flowsets identified from the geomorphological record and their relative timing this empirically based reconstruction of flow pattern geometry provides a framework for more detailed local and regional studies and numerical modelling to provide robust explanations of the observed changes in ice sheet structure in terms of climate and glacial dynamics as a minimum numerical model outputs should be able to reproduce the identified flowset patterns in space and satisfy their chronological order

proglacial lakes and the retreat pattern of the southwest laurentide ice sheet across alberta canada abstract the extent of proglacial lakes following the initial separation of the southwest laurentide ice sheet from the cordilleran ice sheet and its eastward retreat from the canadian rocky mountains has been reconstructed across regions of alberta at a range of scales however to date no studies have integrated all available geological information to produce a province wide deglacial reconstruction that considers the evolution of proglacial lakes as components of the ice marginal system in this paper we utilize a geologically constrained shoreline projection method with a high resolution digital elevation model to reconstruct the evolution of the ice marginal system along the southwest lis during the last deglaciation this method provides new details on the configuration volume drainage history and routing of 240 proglacial lakes as they migrated across alberta and establishes a succession of paleogeographic reconstructions that can place geological evidence of regional and local ice flow reorganizations into a spatiotemporal context these reconstructions demonstrate that although the evolution of proglacial lakes was largely driven by the topography of the emerging landscape and the configuration of the ice margin positive feedbacks in the ice marginal system particularly where margins transitioned from terrestrial to subaqueous settings played a major role in deglacial ice dynamics narrow ribbon shaped lakes that paralleled the ice margin induced relatively minor changes in style and rate of deglaciation whereas the evolution of progressively larger longer lived lakes extending obliquely to the ice margin promoted surging and subsequent rapid retreat

seismic studies of crustal structure during the past four years seismology has continued its role as the key technique for determination of the structure of the continental and oceanic crust by and large however the field and analysis methods used bear little resemblance to those of ten years ago a continued push to increase the number of recording channels has led to the routine use of ocean bottom seismometers sonobuoys and multichannel streamers in oceanic studies and to the use of large sets of portable instruments and multichannel seismic reflection strings on land in studies using earthquakes as sources local and regional arrays provide a capability for obtaining geologically useful two and three dimensional information about the crust

paleomagnetism of quaternary and late tertiary sediments on mokowan butte southwestern alberta mokowan butte in southwestern alberta is capped by 32 m of unconsolidated sediment comprising five tills and their associated paleosols previous stratigraphic work has indicated that the older til

paleozoic and grenvillian structures in the southern appalachians extended interpretation of seismic reflection data interpretive reprocessing of seismic reflection data has elucidated paleozoic and grenvillian structures in the southern appalachians the seismic data include a 7500 km grid of adcoh seisdata and cocorp reflection profiles that traverse the blue ridge and inner picdmont geologic provinces of north carolina south carolina and georgia surface geology and potential field data were used to constrain the interpretation the reprocessed seismic reflection data have delineated the internal and external geometry of the crystalline blue ridge inner piedmont allochthon including the locations of the blue ridge master decollement haycsville fault and brevard fault zone on the basis of the reprocessed data all of the major faults within the allochthonous upper crust sole in the blue ridge master decollement reflections extending to the southeast from beneath the surface location of the hayesville fault to the blue ridge thrust might be the seismic signature of a high strain zone this implies that internal deformation of the blue ridge allochthon associated with the alleghanian orogeny might have occurred farther to the west than has been previously documented from field studies relative amplitude seismic data enabled the discrimination between blue ridge inner piedmont crystalline rocks and underlying lower paleozoic shelf strata thereby delineating the blue ridge thrust the interpreted geometry constrains the top of the shelf sequence beneath the blue ridge to depths of less than 3 km this relatively shallow depth of the shelf strata together with the presence of duplex structures and bright spots that are imaged within the sequence might imply favorable conditions for hydrocarbon exploration beneath the blue ridge midcrustal reflections from within the upper to iower crust are interpreted to originate from preserved grenvillian structures that were reactivated at the basement surface during late proterozoic early cambrian extension reflection continuity is occasionally disrupted by interpreted post grenvillian pre early cambrian low density intrusions topography at the basement surface possibly caused by the intrusions is interpreted to have controlled the formation of some of the structures within the overlying allochthon including blue ridge and brevard fault zone ramps correlation of seismic time structure contour maps with available gravity data and two dimensional gravity modeling suggest that anomalies in the gravity field can be attributed to low density sources within the autochthonous crust discontinuous reflection packages from depths of 36 42 km are interpreted to originate from the mohorovi i discontinuity the reflectors trend about n15 e with a true dip of approximately 15 nw

late wisconsinan deglaciation of alberta processes and paleogeography abstract the scarcity of lake basins in alberta dating earlier than 11 000 bp has been interpreted as indicating the continued presence of active glacial ice because of the related implication that the presence of ice precludes the existence of an ice free corridor it is useful to examine this issue more closely due to the effects of deglaciation many areas of alberta were dominated by chaotic ice stagnation conditions with continual reversal of topography and rapid transformation of the surface the dynamic nature of the environment affects arguments regarding the existence of an ice free corridor as well as having implications for archeological site formation preservation and discovery deglaciation models utilizing paradigms of active ice retreat may result in dichotomous depictions of the land surface as either ice covered or deglaciated due to the insulating properties of supraglacial debris stagnant ice results in a situation that is simultaneously neither and both of the above conditions while the landscape is ice covered and thus not ice free an accessible landscape characterized by glacial karst topography exists on top of the stagnating ice surface general reconstructions of the impact of stagnant ice and glacial karst topography on the paleogeography of alberta are presented for discussion

effects of deformation mechanisms on reservoir potential in central appalachian overthrust belt finite strain associated with various deformation mechanisms can significantly alter the porosity and permeability of reservoir rocks in overthrust belts mechanisms such as pressure solution and cataclasis reduce porosity and permeability whereas extension fracturing and brecciation increase them a theoretical derivation of the relationship between finite strain and original and final porosity indicates that little porosity is preserved in rocks whose strain r exceeds 1 5 studies of variations in the relative importance of these mechanisms are used to define regional and local variations in reservoir properties in the central appalachian overthrust belt regional limits of hydrocarbon potential are defined by combining finite strain data with thermal maturation con dont color alteration index or cai data on the basis of these studies i conclude that the potential for oil production is primarily restricted to the appalachian plateau province the potential for gas production decreases from west to east in the valley and ridge and plateau provinces with its approximate eastern limit defined by the north mountain thrust local variations in reservoir properties depend on original depositional and diagenetic fabric of the reservoir the relative timing of different mechanisms lithologic and textural characteristics and structural and lithotectonic position the possibility of retaining open fractures depends on the relative timing of fracturing and pressure solution fractures formed during the main phase of folding are more likely to be sealed by pressure solved material whereas fractures formed or reactivated late in the deformation are more likely to remain open lithologic units that enhance fracturing and inhibit pressure solution most likely have higher fracture and matrix porosity fine grained argillaceous limestone is more susceptible to pressure solution than orthoquartzite whereas fractu ing is more intense in brittle thin bedded and fine grained units structural positions with high fracture potential include angular fold hinges fault zones and the steep forelimbs of folds the steep forelimbs of folds also are characterized by intense pressure solution and cataclasis which reduce porosity consideration of the above criteria can be useful in evaluating the reservoir potential of prospective structures in the central appalachian and other overthrust belts

glacial geomorphology of the neutral hills uplands southeast alberta canada the process form imprints of dynamic ice streams and surging ice lobes abstract the neutral hills uplands of southern alberta canada is an area of complex and varied glacial landforms dominated by glacitectonic compressional structures but also containing expansive areas of hummocky terrain and kame and kettle topography it lies between the strongly streamlined trunks of the former central alberta cais and maskwa palaeo ice streams of the sw laurentide ice sheet lis and hence comprises an inter ice stream regional moraine zone constructed at around 15 5 cal ka bp this study aimed to compile a regional map of the glacial geomorphology of central southeast alberta in order to decipher the landform sediment signatures of overprinted ice stream margins in terrestrial continental environments and to refine the palaeoglaciological reconstructions for the southwest lis detailed mapping from lidar and aerial imagery identifies distinctive glacial landsystems diagnostic of the partial overprinting of cross cutting ice stream trunks and fast flow lobes widespread evidence of surge diagnostic features indicates that the ice streams experienced repeated flow instabilities consistent with the broader scenario of a highly dynamic and unstable sw lis characterised by markedly transitory and cross cutting palaeo ice streams the inter ice stream moraine zone is characterised by spectacular glacitectonic compression of bedrock cupola hill construction and mega raft displacement but also displays evidence of multi phase stagnant ice melt out where partially overprinted surge lobes advanced into large areas of buried glacier ice contemporaneous ice melting led to the widespread development of glacier karst and the production of eskers at a range of scales the largest of which record deranged drainage patterns indicative of ice walled channel sedimentation controlled by the regional bedrock slope towards the northeast these process form regimes have created a significant local relief that is a product of not only glacitectonic compression of bedrock but also the creation and melting of a melange of ice and bedrock sediment blocks of variable ice volume which are representative of former buried snout ice with a glacier karst system that was repeatedly proglacially thrust due to surging widespread evidence for subglacial channel cutting is likely strongly linked to the transitory surging and cross cutting nature of the palaeo ice streams in the region whereby ice streams switched on and surged in response to the build up migration and marginal outbursts of subglacial water reservoirs in addition to the reduced basal friction caused by the low permeability of the cretaceous bedrock pressurized groundwater and potentially also shallow biogenic gas deposits were likely important to the process form regimes of surging lobes of soft bedded ice streams in a region where ice flow was against an adverse bed slope a scenario that gave rise to a variety of enigmatic landforms such as doughnuts doughnut chains apparent blow out features and possible till eskers as well as glacitectonic mega rafts

laurentide cordilleran ice sheet saddle collapse as a contribution to meltwater pulse 1a the source or sources of meltwater pulse 1a mwp 1a at 14 5 ka recorded at widely distributed sites as a sea level rise of 10 20 m in less than 500 years is uncertain a recent ice modeling study of north america and greenland gregoire et al 2012 has suggested that the collapse of an ice saddle between the laurentide and cordilleran ice sheets with a eustatic sea level equivalent esle of 10 m may have been the dominant contributor to mwp 1a to test this suggestion we predict gravitationally self consistent sea level changes from the last glacial maximum to the present day associated with the gregoire et al 2012 ice model we find that a combination of the saddle collapse scenario and melting outside north america and greenland with an esle of 3 m yields sea level changes across mwp 1a that are consistent with far field sea level records at barbados tahiti and sunda shelf

landform signature of the laurentide and cordilleran ice sheets across alberta during the last glaciation government geological survey maps and research publications have portrayed the distribution of glacial landforms associated with the advance and retreat of the laurentide and cordilleran ice sheets across alberta at a local regional and continental scale to date this information has not been systematically synthesized into a single compilation at a consistent scale although this original work provided valuable information to constrain reconstructions of former ice sheet extent configuration and flow geometry its derivation primarily from the interpretation of aerial photographs and the shuttle radar topography mission 90 m digital elevation model dem may result in methodological inconsistencies and spatial biases these biases together with challenges associated with geomorphic mapping in densely forested areas of western and northern alberta limit the usefulness of previous mapping when applied to inversion based ice sheet reconstructions which have specific input data demands recently ligh

shear wave reflectivity anisotropies poisson s ratios and densities of a southern appalachian paleozoic sedimentary sequence abstract the physical properties of a sequence of paleozoic sedimentary rocks have been examined in detail with an emphasis on laboratory measurements of density shear wave velocity shear wave splitting and vp vs ratios seismic properties of 147 cores from 49 rock samples collected from the thorn hill sedimentary sequence of eastern tennessee are examined in terms of implications for future seismic studies in the southern appalachians the shear wave velocities of these rocks are strongly influenced by the relatively high shear wave velocity of quartz shear wave velocity anisotropy is present in most of the lithologic groups it is highest in the shales while being almost insignificant in the dolostones the related phenomenon of shear wave splitting occurs to some degree in all of the lithologies studied and at high pressures originates from mineral orientation splitting due to preferred mineral orientation is most prominent in the shales but also is significant in some of the carbonates compressional to shear velocity vp vs ratios of approximately 1 82 dolostones and 1 95 limestones effectively characterize the carbonates while other lithologies display wider ranges of vp vs primarily due to the influence of accessory minerals such as quartz densities of the sample suite range from 2 34 g cm3 shale to 2 86 g cm3 dolostone using average data for each lithologic group to calculate normal incidence reflection coefficients significant differences in the shear and compressional reflectivity of hypothetical interfaces are found normal incidence shear and compressional wave synthetic seismograms approximating the subsurface reflectivity of the entire thorn hill section indicate that three zones of high amplitude reflections would be seen on reflection records obtained over this 3327 m thick sequence overall the shear and compressional synthetics are similar in appearance differences are seen at some interfaces in the mississippian devonian interval which are more reflective to shear waves and in the ordovician martinsburg formation which appears more reflective to compressional waves

moist and the continuity of crustal reflector geometry along the caledonian appalachian orogen new aspects of the deep structure of the scottish caledonides are revealed by the moine and outer isles deep seismic reflection traverse moist the caledonian foreland is underlain by an easterly dipping strongly reflecting surface cutting through the moho and traceable to more than 45 km depth thrusts within the foreland basement and caledonian orogen have been reactivated as normal faults bounding half grabens filled with sediments of late palaeozoic to mesozoic age the moine thrust which carries rocks of the orogen over lewisian foreland dips at 20 25 to the e on moist and is either a the westernmost of a series of easterly dipping reflections thrusts which flatten or terminate at 17 20 km depth or b a thrust lying further e which structurally overlies these easterly dipping reflectors by comparison with easterly dipping reflections on cocorp lines in the northern and southern appalachians the latter interpretation is preferred implying that many of the easterly dipping reflections on moist correspond to off shelf metasedimentary rocks with slivers of basement stacked and imbricated against the lewisian basement edge regional seismic refraction and conductivity data from northern scotland suggest that regions of the caledonian orogen have lower mid crustal velocity gradients and higher conductivities than the foreland which might be explained by this interpretation of the easterly dipping reflections despite similarities in crustal reflector geometries there are important differences between the moist and cocorp lines including 1 a remarkably continuous relatively horizontal moho seen on moist data at about 25 km depth and 2 the sedimentary basins offshore from northern scotland which have formed by reactivation under crustal extension of the easterly dipping thrusts furthermore it is probable that the easterly dipping reflectors in these areas of the moist and cocorp lines were formed at different times in the early palaeozoic

melts of sediments in the mantle wedge of the oman ophiolite the rocks in the crustal section of the oman ophiolite show an increasing input of a subduction component with time most likely reflecting the generation of the ophiolite above a subducting slab field relations new geochemical data and nd hf isotope data for felsic to mafic intrusive rocks in the mantle harzburgite from the haylayn block in the oman ophiolite suggest late magmatic events in a mantle wedge shortly before obduction of the ophiolite incompatible element contents and low end and ehf of the felsic rocks exclude differentiation from mafic magmas but are consistent with an origin by partial melting of pelagic sediments similar to leucogranites in continental collision zones these melts apparently mixed with mafic magmas resembling enriched late stage lavas from the ophiolite the leucogranitic intrusions into the mantle wedge confirm the transfer of melts of sediments from the subducted plate into the mantle at subduction zones we suggest that the enrichment of rb k and pb observed in the oman boninites is caused by addition of melts of sediments similar to those from the haylayn block to the boninite source in the mantle wedge

origin and significance of compositional layering in late precambrian sediments blue ridge province north carolina u s a abstract basal arkosic sandstones of the late precambrian grandfather mountain formation contain smallscale deformation zones usually 1 mm thick and spaced 5 to 10 mm apart the deformation zones are composed of green phengitic muscovite which concentrated in the deformation zones during pressure solution removal of quartz assuming plane strain the deformation zones have lost 34 of their area due to solution of quartz and axial ratios of strain ellipses are on the order of 16 1 in the principal plane the deformation zones and associated tight to isoclinal folds in the basal arkoses form a structural style which is transitional between ductile deformation zones in the crystalline basement and kink style folding in the interbedded siltstones and arkoses of the upper grandfather mountain formation since deformation zones may be more common in sedimentary rocks and low grade metasediments than previously realized one should exercise caution when using penetrative fabrics to unravel the structural history of multiply deformed rocks small scale deformation zones can be misinterpreted as a crenulation cleavage cutting an earlier penetrative fabric

early holocene glacial lakes and ice marginal retreat pattern in labrador ungava canada abstract the existence of glacial lakes naskaupi mclean minto melezes and wapussakatoo during the deglaciation of labrador ungava canada was recognized at the end of 1950s however glacial lakes have seldom been included in regional reconstructions to constrain the outline of successive ice margins during the glacial retreat in labrador ungava reconstruction of the ice flow succession in labrador ungava has often primarily focused on till lineation systems these reconstructions often depict a late glacial ice dispersal center situated over central labrador ungava more recent studies on glacial geomorphology including meltwater features however suggest that north central labrador ungava exhibited cold based conditions at least during the latest deglaciation cold based conditions inhibit basal sliding and formation of landforms except for meltwater traces such as meltwater channels glacial lake shorelines and deltas this situation implies that meltwater traces are the main source of information when reconstructing the spatial retreat pattern during a cold based deglaciation evidence presented in this study such as glacial lake shorelines fossil deltas and spillway and drainage channels in north central labrador ungava indicates the existence of numerous previously unmapped glacial lakes the mapped glacial lake features are synthesized to a reconstruction of several glacial lake stages which in turn are used to constrain the late glacial ice margin retreat pattern over the inferred cold based areas of north central labrador ungava a total of 26 glacial lakes 65 sub stages existed during the deglaciation of labrador ungava these lakes were impounded along the southern margin of the shrinking ice sheet the required damming ice margins indicate that the last ice remnant of the laurentide ice sheet in labrador ungava was situated over the southern ungava bay and the adjacent southern shore

large scale reorganization and sedimentation of terrestrial ice streams during late wisconsinan laurentide ice sheet deglaciation glacial geomorphological mapping of shuttle radar topography mission srtm data from the western canadian prairies demonstrates that during the last late wisconsinan deglaciation of the laurentide ice sheet terrestrial ice streams underwent a major reorganization of their flow configuration this reorganization involved a 90 shift in flow direction and was accompanied by a corresponding increase in the influence of topography on streaming flow ice streams included both topographically confined and pure ice streams that flowed independent of topography streaming flow is recorded by suites of highly elongate 60 km long subglacial bedforms bounded sharply at their lateral margins by prominent moraines initial streaming flow was unconfined by topography but was replaced progressively and crosscut by younger topographically confined flows flow reorganization is inferred to have been caused by temporal and spatial variations in the interaction between frozen and thawed bed conditions with thinning and shutdown of one ice stream ultimately triggering initiation of others this highlights the role of internal glaciodynamically driven reorganization in triggering streaming flow within large ice sheets and shows that large scale flow reorganization can occur over the time scale of a single deglaciation in terrestrial ice streams

rare earth element evolution and migration in plagiogranites a record preserved in epidote and allanite of the troodos ophiolite this study was funded by isf research grant 1044 09 to yaron katzir niels jons and wolfgang bach acknowledge support from the dfg research centre excellence cluster the ocean in the earth system

partial melting of metabasic rocks and the generation of tonalitic trondhjemitic granodioritic ttg crust in the archaean constraints from phase equilibrium modelling abstract rocks of tonalitic trondhjemitic granodioritic ttg composition preserved in archaean terranes represent fragments of the earth s earliest formed continental crust and are thought to have formed via partial melting of hydrated metabasalt the geodynamic environments in which such high grade metamorphism and anatexis may have occurred in the early earth is strongly debated constraining the pressure p and temperature t conditions at which melts of appropriate composition can be derived from protoliths containing plausible mineral assemblages is central to addressing this question phase equilibrium modelling has been undertaken for an enriched archaean tholeiite bulk composition a suggested protolith for early earth ttg magmas using newly parameterised thermodynamic models that were specifically developed to evaluate the anatectic behaviour of metabasalt assuming minimal h 2 o saturation at the wet solidus the potential fertility of the studied metabasalt is greatest if the solidus is crossed at a pressure of 11 kbar where the solidus temperature reaches a minimum of 610 c major element compositions and proportions of calculated partial melts show the best correlation with those of natural archaean ttgs when in the p t range 800 950 c and 10 18 kbar which we suggest are optimal conditions for their petrogenesis normative geochemistry suggests that these melts would crystallise to broadly trondhjemitic or tonalitic lithologies calculated modal proportions of garnet plagioclase amphibole and rutile in the residuum which control diagnostic trace element signatures in the melt also show the best agreement with natural and experimental data at these conditions importantly although partial melts calculated outside of this p t range still produce ttg like major element compositions and normative mineralogies they would poorly match the diagnostic trace element signatures of natural archaean examples owing to the absences of key minerals in the residuum this optimal p t range of 800 950 c and 10 18 kbar defined herein is most likely to characterise metamorphism of hydrated basalt at the base of a 40 km thick archaean oceanic plateau overthickened crust or else tectonic underplating via shallow subduction which we therefore suggest are the most likely tectonic settings for the formation of the first voluminous continental crust on the early earth