himalayan type escape tectonics along the superior boundary zone in manitoba canada abstract the superior boundary zone separates the neoarchean superior province to the southeast from the paleoproterozoic reindeer zone of the trans hudson orogen to the northwest in northern manitoba late convergence along the zone in the paleoproterozoic has previously been interpreted in terms of a promontory model where the superior province indents the reindeer zone towards the northwest we present a modified promontory model that includes a wnw transport direction of the superior province and lateral escape of a segment of the superior province the split lake block along the northern margin of the promontory the model is based on the kinematics of an array of shear zones along the superior boundary zone that are interpreted as having been active concurrently tectonic escape of the split lake block occurred in a manner similar to the extrusion and rotation of the indochina block along the eastern margin of the himalayas but at a much smaller scale our model explains shear zone kinematics but also the curvilinear nature of some of the shear zones and a high component of along strike stretch along one of the shear zones

precise u pb zircon ages for the molson dyke swarm and the fox river sill constraints for early proterozoic crustal evolution in northeastern manitoba canada precise u pb zircon ages have been obtained for samples from the molson dyke swarm and the fox river sill in ne manitoba canada the ages determined for the cross lake and cuthbert lake dykes are 1 883 7 1 5 1 7and 1 883 2 ma respectively and are in excellent agreement with the 1 882 9 1 4 1 5ma age obtained for the fox river sill these results support the contention that the emplacement of the fox river sill and the molson dyke swarm was contemporaneous and also demonstrate the potential for correlating mafic igneous activity in widely spaced localities the timing of early proterozoic mafic magmatism in the western superior province appears to be synchronous with igneous activity in other parts of the circum superior belt and in the trans hudson orogen to the west the emplacement of the molson dyke swarm at 1 883 ma indicates a 700 ma interval of quiescence between the final igneous activity that is recorded in the archean basement and dyke intrusion the presence of deformed equivalents of molson dykes in the thompson nickel belt indicates that the intense deformation in this belt occurred sometime after 1 883 ma

neoproterozoic granitoid in northwest sulu and its bearing on the north china south china blocks boundary in east china 1 middle neoproterozoic granitoid rocks are found in the wulian region that is located in the northwestern part of the sulu orogen and is approximately bounded by the wulian yantai fault in southeast zircon u pb dating yields concordant ages of 738 10 to 758 5 ma for three granites and one gabbro hornblende ar ar dating on the gabbro gives consistent plateau and isochron ages of 719 1 3 7 and 717 9 7 2 ma respectively this suggests that the wulian granitoids correspond to bimodal magmatism during the mid neoproterozoic in the northern margin of the south china block in response to the breakup of rodinia supercontinent they were tectonic slices scraped off from the upper part of the south china block during the triassic continental subduction this demonstrates that the suture location between the north china block and the south china block lies north of the wulian and penglai groups rather than along the wulian yantai fault that marks the northern margin of ultrahigh pressure metamorphic belt

relative contribution of magmatic and post magmatic processes in the genesis of the thompson mine ni co sulfide ores manitoba canada abstract the ni co pge sulfide deposits of the thompson nickel belt tnb in northern manitoba canada are part of the fifth largest nickel camp in the world based on contained nickel past production from the tnb deposits is 2500 kt ni the thompson deposit is located on the eastern and southern flanks of the thompson dome structure which is a re folded nappe structure formed during collision of the trans hudson orogen with the canadian shield at 1 9 1 7 ga the thompson deposit is almost entirely hosted by p2 member sulfidic metasedimentary rocks of the paleoproterozoic ospwagan group variably serpentinised and altered dunites peridotites and pyroxenites contain disseminated sulfides and have a spatial association with sediment hosted ni sulfides which comprise the bulk of the ore types these rocks formed from rift related komatiitic magmas that were emplaced at 1 88 ga and subsequently deformed by boudinage thinning folding and stacking disseminated sulfide mineralization in the large serpentinised peridotite and dunite intrusions that host the birchtree and pipe ni co sulfide deposits typically has 4 6 wt ni in 100 sulfide the disseminated sulfides in the less abundant and much smaller boudinaged serpentinised peridotite and dunite bodies associated with the thompson deposit have 7 10 wt ni in 100 sulfide the majority of thompson mine sulfides are hosted in the p2 member of the pipe formation which is a sulfidic schist developed from a shale prololith the mineralization in the schist includes both low ni tenor 13 wt ni in sulfide define a systematic trend that mirrors the configuration of the second order doubly plunging f3 structures on the flanks of the dome although moderate to high ni tenor mineralization is sometimes localized in fold hinges more typically the highest ni tenor mineralization is located on the flanks of the fold structures there is no indication of the mineralogical and geochemical signatures of sedimentary exhalative or hydrothermal processes in the genesis of the thompson ores the primary origin of the mineralization is undoubtedly magmatic and this was a critical stage in the development of economic mineralization variations in metal tenor in disseminated sulfides contained in ultramafic rock indicate a higher magma sulfide ratio in the thompson parental magma relative to birchtree and pipe the variation in ni tenor of the semi massive and massive sulfide broadly supports this conclusion but the variations in metal tenor in the thompson ores was likely created partly during deformation the sequence of rocks was modified by burial and loading of the crust d2 events to a peak temperature of 750 c and pressure of 7 5 kbar the third major phase of deformation d3 was a sinistral transpression d3 event which generated the dome and basin configuration of the tnb these conditions allowed for progressive deformation and reformation of pyrrhotite and pentlandite into monosulfide solid solution as pressure and temperature increased this process is termed sulfide kinesis separation of the ductile monosulfide solid solution from granular pentlandite would result in an effective separation of ni during metamorphism and the monosulfide solid solution would likely be spread out in the stratigraphy to form a broad halo around the main deposit to produce the low ni tenor sulfide reformation of pentlandite and pyrrhotite after the peak d2 event would explain the broad footprint of the mineral system the effect of the d3 event at lower pressure and temperature would have been to locally redistribute deform and repeat the lenses of sulfide the understanding of the relationships between petrology stratigraphy structure and geochemistry has assisted in formulating a predictive exploration model that has triggered new discoveries to the north and south of the mine and provides a framework for understanding ore genesis in deformed terrains and the future exploration of the thompson nickel belt

mesozoic sequence stratigraphy and its response to tectonic evolution of the eastern depression north yellow sea basin north china abstract the north yellow sea basin is a meso cenozoic continental sedimentary basin located between the tan lu fault and the imjingang orogenic belt the mesozoic strata of the eastern depression in the north yellow sea basin consist of lower lower part of the upper jurassic lower cretaceous middle upper part of the upper jurassic lower cretaceous and upper lower cretaceous supersequences comprising six third order sequences as inferred from seismic core and well log data there are five lithology lithofacies architectures summarized in the third order sequences of wells a f d e j g and h and they are the interbedded coarse and fine asymmetric coarse fine fine coarse fine coarse fine coarse and asymmetric fine coarse the basin controlling faults and their assembly configurations during each tectonic stage were inherited and differential in the depression the basin controlling faults were dominated by extension and strike slip movement increased gradually during the early stage of the late jurassic early cretaceous the strike slip faults were dominant at the late stage of the late jurassic early cretaceous right lateral trans extensional faulting created the uplift depression structural framework of the eastern depression there was another trans extension in the depression during the early cretaceous wells a f and d record subsidence at three different structural locations in a trans extensional tectonic setting from the late jurassic to the early cretaceous the fault movement changed from transtension to transpression and the eastern uplift extended from the basin edge into the depression center now the center turned into a structural high the present day central structural belt is a potential area for hydrocarbon accumulation and its extended or down dip area may become prospective areas for oil hydrocarbon traps analogous to the buried hill or igneous type oil reservoirs may be present in the up dip area of the eastern uplift

glacial sediments and erosional pavements near the cambrian precambrian boundary in western henan province china exceptionally well preserved glaciated pavements and a variety of glacigenic sediments comprising the luoquan formation in henan province central china are believed to have formed close to the cambrian precambrian boundary they may therefore be younger than the better known sinian tillites representing the changan and nantuo ice ages they perhaps belong to a distinct glacial epoch that is recorded elsewhere in china and in other continents but for which there is less evidence than for earlier late proterozoic glacial periods lithofacies represented in the luoquan formation are massive diamictite signifying deposition as lodgement or melt out till bedded diamictite waterlain till or glaciomarine glaciolacustrine sediments with dropstones rhythmites with dropstones glaciolacustrine or fjord deposits possibly varves tabular conglomerates and sandstones outwash deltaic or sub aqueous mass flow deposits lens shaped breccia deposits sub aqueous mass flow deposits and gravel sand wedges frost contraction crack infillings the glaciated pavements reveal well preserved striations grooves often intersecting 9plastically moulded9 forms crescentic gouges and steps on a regional scale the linear features are of variable orientation but they indicate a dominant ice flow direction towards the southeast all this evidence points to upland glaciation with erosion followed by deposition in a terrestrial glacial and glaciolacustrine environment

the distinctive characteristics of the sino korean and the yangtze plates abstract the sino korean and yangtze plates in china have been distinguished by comparing their geological geochemical and geophysical characteristics and by recognizing the distinctive tectonic role of these plates during their geological evolution many lines of evidence suggest that they have always been separate plates the time of formation of the crystalline basement the occurrence of particular elements feo mgo mo li rb sc cu and the ratio of elements zr hf nb ta in both crust and mantle whether middle proterozoic sequences are metamorphosed or un metamorphosed whether or not the rocks were affected by the jinning event about 850 ma the phosphate and black shale systems of the early cambrian the disconformities in the sequence at about 400 ma the development of coal systems in the late paleozoic evidence for movements of the plates during the paleozoic the thickness of lithosphere variation in the average p and shear wave velocities the presence or absence of a low resistance layer in the crust all these characteristics distinguish the sino korean plate from the yangtze plate and show that gross differences can be recognized in the geochemistry geophysics and geological evolution of the two plates since the archaean there is no evidence that they once formed part of single plate

north american margin origin of quesnel terrane strata in the southern canadian cordillera inferences from geochemical and nd isotopic characteristics of triassic metasedimentary rocks new geochemical and nd isotopic data from triassic ca 220 ma clastic metasedimentary rocks assigned to the quesnel terrane quesnellia in southeastern british columbia show that detritus was derived from both primitive and evolved sources twenty four samples yield end 2 2 0 values between 9 1 and 5 6 independent of sample location these samples have eu anomalies between 0 6 and 1 0 la n yb n ratios between 3 8 and 11 4 la sc ratios ranging from 0 38 to 4 76 and th sc ratios between 0 09 and 1 62 this range of isotopic and geochemical values overlaps that of samples from upper paleozoic to mesozoic rocks of the north american miogeocline and upper triassic nicola group volcanic rocks of the quesnel terrane the range in nd isotopic and geochemical values supports the interpretation that detritus was derived from both continental north america evolved cratonic source and a volcanic arc primitive source developing on the continental margin these data do not support previous interpretations of the quesnel terrane as an intraoceanic arc the inferred depositional link between north america and the quesnel arc in triassic time is ca 40 ma older than the hypothesized obduction of the quesnel terrane onto the north american continental margin and when combined with existing geological information strongly supports a pericratonic nonexotic origin for the quesnel terrane in southeastern british columbia

drumlins of estonia abstract drumlins megaflutings and other subglacial streamlined forms highly variable in shape and size are widely distributed in estonia as well as on the bottom of the gulf of finland which was also affected by continental ice their total number amounts to 1000 the main drumlin fields are most common in central estonia on lateral slopes of bedrock uplands and or in depressions between them and they serve as a transitional landscape type between predominantly erosional in northern estonia and depositional zones in southern estonia of the scandinavian ice sheet nevertheless the local details of bedrock topography and glacial and deglacial history have led to some deviations e g the unique saadjarv field of big drumlins and megaflutings is situated in a glacier shadow on the lee side of a bedrock upland being of varied composition and age drumlins and related streamlined forms represent complex interactions between erosional transporting and depositional processes although the dynamics of their formation are still somewhat enigmatic it seems that in estonia drumlins were formed both during ice advances and retreats in some cases there are moraine hillocks kames and eskers and other ice disintegration features superimposed on drumlins

the planar shape of drumlins abstract the asymmetry of the planar shape of drumlins is an established paradigm in the literature and characterizes drumlins as resembling tear drops with a blunt bullet shaped stoss end and a tapering pointed lee end it is widely cited and never been seriously questioned in this paper the planar shape of 44 500 drumlins mapped in various regional settings from drumlin fields in north america and northern europe were objectively analysed by means of geographic information system tools two parameters were considered the first denoted here as as pl focuses on the relative position of the point of intersection between the axes of the maximum length and the maximum width it is defined as the distance between the upstream i e beginning of the drumlin and the intersection point measured along the longitudinal axis divided by the entire length of the long axis results indicate that the intersection point of the majority of drumlins 64 is very close to the longitudinal midpoint 0 33 as pl as pl a is defined as the ratio between the area of the upstream half of the drumlin to that of the entire drumlin results show that for most drumlins 81 the upper half area is almost as large as the down half 0 45 as pl a

the composition and internal structure of drumlins complexity commonality and implications for a unifying theory of their formation investigation of drumlins is significant to both glaciology and palaeoglaciology but the sheer diversity of their composition and internal structure is often cited as a major obstacle towards a satisfactory unifying explanation of their formation this paper presents the first systematic survey of the vast literature on this subject with the aim of concisely summarising observations and identifying any emergent patterns or commonality that theories of drumlin formation should be able to explain results confirm that investigations are often limited by availability of suitable sediment exposures 40 of studies report data from 5 drumlins and 44 do not specify sample size although borehole data and geophysical techniques can alleviate this problem however it is clear that the constituents of drumlins are incredibly diverse in terms of their composition e g a range of lithologies clast shapes sizes and fabrics structure e g sediments that are sorted homogeneous surface conformable unconformable and evidence of deformation e g ranging from pervasive to non pervasive limited to absent despite this diversity our review leads us to suggest that drumlin composition can be simplified to five basic types i mainly bedrock ii part bedrock part till iii mainly till iv part till part sorted sediments and v mainly sorted sediments this is a potentially significant step in that it reduces the oft cited complexity of drumlin composition and provides a more realistic goal for theories or numerical models of drumlin formation to target the various types can occur within the same drumlin field which leaves us with two possible implications for drumlin formation 1 different types of drumlin are formed seeded by different processes despite being morphologically similar equifinality investigation of drumlin composition may therefore reveal diagnostic processes explanations for these different types of drumlin and we argue that bedrock drumlins are an example 2 a single process occurs across large parts of the ice bed interface to create drumlinised terrain in a variety of sediments investigation of drumlin composition may in this case simply reflect pre existing sediments but importantly the way in which the drumlin forming mechanism modifies is modified by them we argue that the latter simpler explanation applies to all other types of drumlin excluding purely bedrock forms and conclude that the diversity in drumlin composition is no obstacle to a single unifying theory

geochemical correlation in deltaic successions a reality check whole rock geochemical analysis can aid the stratigraphic correlation of faunally barren successions compared to petrographic techniques or heavy mineral separations such analysis is quick and objective yet rarely has the reliability of the method been tested against unequivocal correlations we have applied it to the kenilworth member of the upper cretaceous blackhawk formation a well exposed and easily correlated 100 m thick unit in the deltaic successions of the book cliffs utah united states this is the first evaluation of geochemical analysis as an aid to sequence stratigraphic interpretation at the parasequence scale from the geochemistry alone it is not possible with statistical confidence to separate individual parasequences or systems tracts nor the fluvial incised valley from the marine shoreface units the magnitude of geochemical variation in a single sandstone bed over a 30 m lateral traverse is as great as vertically through an entire parasequence y nb rb and ti abundances are inversely correlated with grain size thus geochemical signatures follow sedimentological trends and distal parts of a parasequence appear distinct from proximal parts y ratios compensate for this grain size effect but introduce confusing distortions for elements such as zr that are not correlated with grain size contrary to some predictions zr is as common in the finest grained units as the coarsest and sr is not correlated with ca in some sections sr is also inversely correlated to grain size and cannot be used as an indicator for cement abundance

nature of assean lake ancient crust manitoba a combined shrimp id tims u pb geochronology and sm nd isotope study abstract the assean lake crustal complex alcc located at the northwest periphery of the superior province in northern manitoba canada hosts an ancient assembly of rocks that record a complex and prolonged crustal history spanning more than two billion years from 3 9 to 1 8 ga a supracrustal assemblage consisting of amphibolite grade greywacke quartz arenite arkose mafic to intermediate volcanics and to a lesser extent iron formation dominates the alcc and likely formed in a shallow marine arc back arc setting the supracrustal rocks are intruded by amphibolite grade calc alkaline tonalite to granite orthogneiss nd model ages for felsic para and orthogneisses range from 3 5 to 4 2 ga and suggest that paleoarchean basement may be preserved at assean lake shrimp u pb zircon ages and id tims conventional u pb zircon and monazite ages show that felsic magmatism occurred at 3 1 3 2 ga whereas a record extending back to at least 3 9 ga is preserved in detritus in paragneiss that may have a depositional age of 3 2 ga orthogneiss was metamorphosed at 2 68 ga whereas local migmatisation of paragneiss occurred at 2 61 ga monazite growth events at 2 6 2 45 and 1 8 ga might represent episodes of accretion of archean crustal segments including the alcc and possibly the sask craton trans hudson basement along the northwest superior province margin at present matching segments of the exotic alcc are unknown but remnants may exist in parts of the sask craton

lithostratigraphy and geochemistry of the high grade metasedimentary rocks in the thompson nickel belt and adjacent kisseynew domain manitoba implications for nickel exploration exploration for nickel in the thompson nickel belt is focused on specific paragneiss units within the paleoproterozoic sedimentary and volcanic cover succession ospwagan group on the northwestern margin of the archean superior craton these units host ultramafic bodies and all significant nickel deposits in the thompson nickel belt knowledge of the lithostratigraphy is therefore essential for finding the units with the highest potential for the discovery of new nickel deposits this is made practical by a distinctive stratigraphic succession with numerous marker units including metamorphosed quartzite carbonate pelite iron formation and graywacke as summarized in this paper however the sedimentary rocks are generally highly deformed and metamorphosed making the recognition of some units difficult and their sedimentary origin obscure to better recognize the sedimentary protoliths of orehosting and related units a simple plot of sio 2 versus al 2 o 3 is used to estimate the approximate proportions of siliciclastic and chemical sedimentary components this demonstrates that much of the succession which was previously considered to be derived from calcareous and ferruginous chemical sediments is a hybrid containing a substantial siliciclastic component thus explaining the present mineralogy and geochemistry and assisting in the recognition of units at various metamorphic grades extended element plots using the most abundant metapelite unit in the ospwagan group as the normalizing factor show a common provenance for the entire group they also illustrate a clear difference between the ospwagan group and the underlying basements gneiss and allow the ospwagan group to be distinguished from petrographically similar but economically barren younger paragneiss units of the burntwood and grass river groups unlike the ospwagan group the younger units which predominate in the kisseynew domain adjoining the thompson nickel belt were derived mainly from an exotic paleoproterozoic arc related source and therefore have a different geochemical signature this geochemical difference can be utilized in areas of sparse geologic information such as beneath phanerozoic or glacial cover where initial exploration drilling tests new geophysical targets combined litho and chemostratigraphy improve insight into basin evolution which probably occurred on a ca 2 ga passive continental margin or shelf the ca 1880 ma maficultramafic magmatism related to the nickel deposits may have formed in a different tectonic environment possibly in a continental back arc associated with a recently proposed episode of felsic calc alkaline plutonism

sequence stratigraphy of the middle cambrian daegi formation korea and its bearing on the regional stratigraphic correlation abstract the sediments that comprise the middle cambrian daegi formation in eastern central korea are interpreted to have been deposited in a carbonate ramp setting the daegi formation consists of nine lithofacies argillaceous rock ribbon rock oolitic limestone skeletal limestone microbial limestone intraclastic limestone micritic limestone bioturbated limestone and crystalline limestone which are grouped into three facies associations presenting from base up outer ramp grain shoal and inner ramp facies associations two depositional sequences can be distinguished in the daegi formation sequence 1 in the lower part of the daegi formation comprises a lower transgressive systems tract tst and an upper highstand systems tract hst the tst consists of terrigenous deposits with subordinate limestones while the hst contains various types of limestone sequence 2 comprises the upper half of the daegi formation and was deposited during a tst the sequence stratigraphic analysis of the daegi formation suggests that the sediments were deposited during one 3rd order sea level change during the middle to late middle cambrian the sequence stratigraphic framework of the daegi formation can be excellently correlated with coeval deposits in north china the zhangxia and the lower part of the overlying gushan formations which belong to the same tectonic block as the korean peninsula the sino korean block skb the almost identical sequence stratigraphic frameworks in two separate basins suggest that the sequence stratigraphic development on the skb was mainly influenced by eustatic changes during the middle cambrian

indentation induced tearing of a subducting continent evidence from the tan lu fault zone east china abstract vertical tearing of oceanic slabs has been well documented at subduction zones it remains unclear whether a subducting continent can tear vertically origin of the continental scale tan lu fault zone tlfz in east china provides an example of the vertical tearing of the subducting continent at the convergent stage the dabie and sulu orogens between the north china plate ncp and the yangtze plate yzp are left laterally offset about 400 km along the ne striking tlfz but the fault zone terminates abruptly at the southeastern corner of the dabie orogen suggesting unique origin of the fault zone we present structural evidence to show that the tlfz initiated as a sinistral fault zone that is exposed in ductile shear belts just to the east of the dabie orogen our muscovite 40ar 39ar dating results coupled with existing age data indicate that the tlfz formed at 240 230 ma middle triassic when the yzp continental crust was subducted beneath the ncp along both the dabie and sulu sutures we also show that se directed extrusion of the subducted crust in the dabie orogen occurred during 230 209 ma late triassic and overprinted contractional deformation in the tlfz to the southeast of the dabie orogen syn collisional folds and thrusts in the yzp to the east of the tlfz exhibit evidence for large scale dragging by the sinistral fault zone whereas those in the ncp to the west are perpendicular to the tlfz without obvious evidence for drag combining these lines of evidence with published data on the tectonic evolution of the two orogenic belts we propose an indentation induced continent tearing model for the origin of the tlfz we suggest that the present southern boundary of the ncp represents its original shape with a promontory in front of the dabie orogen at the oceanic subduction stage the overriding ncp promontory led to vertical tearing of the subducting oceanic plate along the tlfz which formed the eastern boundary of the promontory once the rigid ncp indenter collided with the passive yzp along the dabie orogen the oceanic slab tear propagated into the yzp lithosphere resulting in long distance low angle subduction of the yzp underneath the ncp indenter and nne ward horizontal motion of the torn yzp east of the tlfz until the final collision at the sulu orogen thus the indentation induced the vertical tearing of the subducting yzp along the tlfz

crustal suture preserved in the paleoproterozoic trans hudson orogen canada lithoprobe seismic reflection data acquired in 1994 within the trans hudson orogen include a 180 km profile that crosses the boundary between a collapsed marginal basin kisseynew domain and an older volcanic plutonic arc terrane lynn lake belt the seismic data image a prominent band of northeast dipping anastomosing reflections that extend to 10 s two way traveltime from the surface location of a geological boundary where interpreted oceanic crust is preserved and separate thick crust 16 s of the kisseynew domain from the abruptly thinner crust 12 s of the lynn lake belt highly reflective lower crust 10 16 s of the kisseynew domain dips gently northward and extends laterally for at least 40 km beneath the adjacent lynn lake belt in a manner reminiscent of the western alps complementary gravity modeling shows that the thickness of metasedimentary rocks within the kisseynew domain is 20 3 km the seismic image is interpreted as portraying a crustal scale collisional suture that formed as the end result of northward subduction of backarc basin crust ancestral kisseynew basin floor and subsequent contraction and thrusting of the remnant kisseynew basin fill under the margin of the lynn lake belt

segmentation and inversion of the hangjinqi fault zone the northern ordos basin north china abstract this paper deals with the segmentation and inversion of the hangjinqi fault zone hfz which is the dominant structure in the northern part of the ordos basin in north china hfz was reactivated during the late triassic and obliquely inverted during the middle jurassic shortening subsurface geological mapping and structural analysis were carried out to determine the segmentation and kinematic history of the deformation the hfz was a left stepping fault zone and was made up of three segments the porjianghaizi fault pf wulanjilinmiao fault wf and sanyanjing fault sf which are separated by two relay ramps two distinct phases can be identified in its structural evolution 1 during the late triassic compressional deformation the hfz was characterized by shortening and thrusting to the north and 2 during the middle jurassic phase the hfz was oblique to the extensional fault trends the reverse faults were reactivated as dextral strike slip faults as a result of transtensional inversion the inversion ratio of the hfz indicates an increase in deformational degree from east to west over the whole region the first deformation stage resulted from the n s compression between the south china and north china plates during the late triassic the second deformation stage of compression was related to the west northwestward subduction of the paleo pacific plate during the middle jurassic in the jurassic deformation framework the hfz may be interpreted as an accommodation structure parallel to the yanshan yinshan orogenic belt developed in the northern ordos area

u pb zircon and re os isotope geochronology of mineralized ultramafic intrusions and associated nickel ores from the thompson nickel belt manitoba canada a u pb zircon age 1880 5 ma has been obtained from a mineralized ultramafic body in the setting lake area of the thompson nickel belt this crucial new age for ultramafic magmatism in the thompson belt in conjunction with the precise age determinations established for the molson dike swarm cross lake dike 1883 7 1 7 1 5 ma cuthbert lake dike 1883 2 ma and the fox river sill 1882 9 1 5 1 4 ma by heaman et al 1986 provides the first direct evidence for the contemporaneous nature of mafic ultramafic magmatism along the northwest margin of the superior craton this magmatic event also appears to be synchronous with mafic magmatism in other parts of the circum superior belt and the trans hudson orogen re os isotope systematics of mineralized samples from seven deposits that span the length of the belt produce an age similar to the u pb zircon age but with a much larger error 1885 49 ma with an initial os of 4 6 the os isotope data demonstrate that the source of the thompson belt ores including those orebodies hosted in ultramafic rocks and those hosted in pelitic rocks was dominantly mantle derived establishing that thompson belt magmatism is synchronous with the molson dike swarm and the extensive fox river sill has important new exploration implications and requires modification of existing concepts which have presumed that thompson belt magmatism is significantly older than these other intrusive suites also it would appear that other important ni and ni cu deposits elsewhere in the circum superior belt and in the trans hudson orogen formed at or near 1 88 ga and thus this age could be considered an important and widespread nickel mineralizing metallogenic interval during the early proterozoic

sm nd isotope technique as an exploration tool delineating the northern extension of the thompson nickel belt manitoba canada application of nd isotope mapping to aid mineral exploration has been tested successfully in areas north of the well prospected productive portion of the thompson nickel belt in manitoba canada economic nickel deposits in this main part of the belt are restricted to the ospwagan group cover sequence and ultramafic sills overlying archean basement gneisses distinguishing some units in the ospwagan group from its basement and less prospective paleoproterozoic paragneiss of the burntwood group in the area is difficult based on lithology stratigraphy geochemistry and structure alone however the ospwagan group has a unique nd isotope signature that allows it to be identified with certainty a compilation of nd isotope data from the main portion of the thompson nickel belt and a new dataset from drill core and reconnaissance mapping clearly support early exploration indications that basement gneiss and narrow belts of the ospwagan group typical of the belt extend northeast of the main part of belt for at least 100 km this confirms the presence of a large new area for nickel exploration the limits of which are still unknown in this poorly exposed northern area crustal residence nd model ages t cr are instrumental in clearly separating 1 the ospwagan group supracrustal rocks t cr 2 8 3 2 ga that host the nickel deposits from 2 neoarchean basement gneiss t cr 3 2 3 5 ga 3 juvenile overthrust burntwood t cr 2 2 2 6 ga grass river and sickle t cr 2 0 2 4 ga groups paragneisses and 4 fragments of a mesoarchean gneiss complex t cr 3 5 4 1 ga the latter being the host of precious metal prospects the variation in nd model ages not only distinguishes the different supracrustal successions but the variable inheritance of ancient crustal material in units of archean gneiss reflected in nd model ages serves as a tool to unravelling the complexly interleaved structure of this crust and aids in tracing the potentially nickel hosting ospwagan group in narrow structural keels the robustness of the nd isotope system under the conditions of high grade metamorphism and migmatization attained in the thompson nickel belt makes this an ideal tool for distinguishing between units of economically prospective and barren gneiss in frontier areas where other methods have failed are difficult to apply or have led to erroneous interpretations

pb sr nd isotopes in surficial materials at the pebble porphyry cu au mo deposit southwestern alaska can the mineralizing fingerprint be detected through cover the cretaceous pebble porphyry cu au mo deposit is covered by tundra and glacigenic sediments pb sr nd measurements were done on sediments and soils to establish baseline conditions prior to the onset of mining operations and contribute to the development of exploration methods for concealed base metal deposits of this type pebble rocks have a moderate range for 206pb 204pb 18 574 to 18 874 207pb 204pb 15 484 to 15 526 and 208pb 204pb 38 053 to 38 266 mineralized granodiorite shows a modest spread in 87sr 86sr 0 704354 0 707621 and 143nd 144nd 0 512639 0 512750 age corrected 89 ma values for the granodiorite yield relatively unradiogenic pb e g 207pb 204pb 15 52 low values of 87sr 86sr and positive values of nd 1 00 4 52 that attest to a major contribution of mantle derived source rocks pond sediments and soils have similar pb isotope signatures and 87sr 86sr and 143nd 144nd values that resemble the mineralized granodiorites glacial events have obscured the recognition of isotope signatures of mineralized rocks in the sediments and soils baseline radiogenic isotope compositions prior to the onset of mining operations reflect natural erosion transport and deposition of heterogeneous till sheets that included debris from barren rocks mineralized granodiorite and sulfides from the pebble deposit and other country rocks that pre and postdate the mineralization events isotopic variations suggest that natural weathering of the deposit is generally reflected in these surficial materials the isotope data provide geochemical constraints to glimpse through the extensive cover and together with other geochemical observations provide a vector to concealed mineralized rocks genetically linked with the pebble deposit

physical environment of drumlin formation review of published descriptions of drumlin fields suggests that the following conditions are important to drumlin growth 1 compressive longitudinal and possibly extending transverse strain rates in the ice 2 thin ice such as occurs near the glacier margin and 3 high pore water pressure in the subglacial sediments most drumlin fields display all of these and no fields of well developed drumlins were found that did not on the oilier hand the lithology of drumlin forming sediment appears not to be important in promoting drumlin growth since it varied widely nor are the lithology and large scale topography of the bed

evolution of the snow lake portion of the palaeoproterozoic flin flon and kisseynew belts trans hudson orogen manitoba canada abstract understanding growth of early precambrian crust requires knowledge of the evolution of granite greenstone and metasedimentary belts two of the fundamental building blocks of early precambrian cratons u pb geochronologic data from the snow lake area which straddles the flin flon granite greenstone and kisseynew metasedimentary belts of the trans hudson orogen documents a 60 70 ma period of palaeoproterozoic crust formation from inception of these belts in ocean basins through their eventual structural telescoping during ocean basin consumption and collision with bounding archaean cratons earliest volcanic and plutonic rocks of the flin flon granite greenstone terrane at snow lake formed 1 89 ga mainly in a juvenile oceanic arc tectonic setting sedimentary rocks of the kisseynew belt previously thought to be contemporaneous with snow lake area arc volcanism are up to 30 50 ma younger 1 86 1 84 ga and postdate deformation associated with 1 88 1 87 ga arc arc collisions and 1 87 1 84 ga calc alkaline plutonism that affected much of the western flin flon belt deformation at snow lake occurred during two widely spaced episodes both of which effect the post 1 86 1 84 ga turbidite sequence and thus postdate 1 88 1 87 ga intraoceanic accretion in the western flin flon belt the older deformational episode involved isoclinal folding formed during 1 84 ga convergence of the kisseynew and flin flon belts and thrusting of kisseynew sedimentary lithologies southwards onto the flin flon volcanic rocks the younger deformational episode occurred 15 25 ma later and involved further folding and southwest thrusting accompanied by a 1 81 ga high grade thermal metamorphic episode centred in the kisseynew belt only minor increase of crustal material in the form of late intrusions were added following this episode

reactivation of the archean lower crust implications for zircon geochronology elemental and sr nd hf isotopic geochemistry of late mesozoic granitoids from northwestern jiaodong terrane the north china craton the late mesozoic granitoids widely distributed in the northwestern jiaodong terrane are important markers of cratonic destruction and lithospheric thinning of the eastern north china craton ncc here we investigate the late jurassic linglong and luanjiahe granites and report zircon u pb emplacement ages of 157 159 ma these rocks also contain abundant late archean paleoproterozoic neoproterozoic early paleozoic and triassic inherited zircons suggesting the involvement of continental crustal materials from both the ncc and yangtze craton in magma tectonics the rocks investigated in this study show high na2o k2o and low mgo and are peraluminous with enrichment in lrees and liles rb ba u and sr and depletion in hfses nb ta p and ti they also display low ehf t values and high sr y ratios comparable to adakitic rocks suggesting that the linglong and luanjiahe granitoids formed under relatively high pressure conditions and were likely derived from the partial melting of the thickened lower crust of the ncc the guojialing granodiorites were emplaced in the early cretaceous 129 ma and also contain abundant late archean and paleoproterozoic inherited zircons the rocks possess high cao tfe2o3 and mgo and are metaluminous with enrichment in lrees and liles and depletion in hfses they are also characterized by high sr y ratios and have higher end t and ehf t values than the late jurassic granitoids suggesting the involvement of mantle components in the magmatic source we correlate the magma tectonics with the processes accompanying the subduction of the pacific plate beneath the ncc and the associated asthenospheric upwelling

geochronology and geochemistry of metamorphic rocks in the jiaobei terrane constraints on its tectonic affinity in the sulu orogen abstract tectonic affinity of tectono lithological units close to ultrahigh pressure metamorphic belt is a key issue for understanding the geodynamics of continental collision this is particularly so for the jiaobei terrane northeast of the dabie sulu orogenic belt in china new data from la icpms zircon u pb dating whole rock elements and nd sr isotopes and mineral o isotopes are presented for metamorphic rocks from this terrane the results place geochronological and geochemical constraints on their protolith nature and metamorphic timing and thus on its tectonic affinity to one of the two triassic collided continents the north and the south china blocks protolith ages for ttg gneiss amphibolite and mafic granulite are 2 7 2 5 and 2 4 ga respectively regional metamorphism took place extensively at 1 76 ga protolith of the ttg gneiss was generated by partial melting of mantle derived rocks at the root of a thickened crust protolith of the amphibolite was probably a product of arc like magmatism protolith of the mafic granulite was derived from a depleted mantle source both of them were locally contaminated by supracrustal materials protoliths of paragneiss and schist in the fenzishan group were mostly derived from supracrustal sources but protolith of amphibolite in the fenzishan group is of mantle derived signature unlike the uhp metaigneous rocks in the dabie sulu orogenic belt that show unusual 18 o depletion the jiaobei metamorphic rocks have basically preserved their original mantle like o isotope compositions in general the nature and timing of geological events recorded in the metamorphic rocks from the jiaobei terrane are comparable with those from the north china block rather than the south china block thus the jiaobei terrane is concluded to have tectonic affinity to the former but behave like a micro continent during the triassic continental collision the 1 76 ga regional metamorphism in the jiaobei terrane is likely related to reworking of the arc continent collisional orogen in the periphery of the north china block rather than the 1 85 ga collision event between the eastern and western north china blocks the present study lends support to the common assumption that the suture boundary between the north and south china blocks in the sulu orogen is located along the wulian yantai fault tectonic mingling along the wulian yantai fault is probably related to subduction erosion during the continental collision

long term exhumation history of the inner mongolian plateau constrained by apatite fission track analysis the inner mongolian plateau along the southeastern flank of the wider mongolian plateau is a vast undulating surface ranging in elevation between 900 and 1500 m above sea level the peculiar topography of this area is assumed to be closely related to its complex tectono thermal evolution since late paleozoic the lithospheric structure of the plateau includes three continental blocks the mandula and the bart obo blocks form the southern margin of the central asian orogenic belt in that area and to the south the plateau includes the northern margin of the north china craton apatite fission track aft ages and track length distributions from 13 basement outcrops situated in the main tectonic blocks forming the inner mongolian plateau were determined in order to reconstruct its denudation history the thermal histories inferred from these data imply multi phased differential exhumation burying processes from the late paleozoic to the early cretaceous this complex thermal history is largely related to the early middle triassic closure of the paleo asian ocean the jurassic closure of the mongol okhotsk ocean and the early cretaceous orogenic collapse of the mongol okhotsk belt finally since late cretaceous no further major tectonic movement occurred and the inner mongolian plateau has been largely peneplained

crustal structure of china from deep seismic sounding profiles abstract more than 36 000 km of deep seismic sounding dss profiles have been collected in china since 1958 however the results of these profiles are not well known in the west due to the language barrier in this paper we summarize the crustal structure of china with a new contour map of crustal thickness nine representative crustal columns and maps showing profile locations average crustal velocity and pn velocity the most remarkable aspect of the crustal structure of china is the well known 70 km thickness of the crust of the tibetan plateau the thick 45 70 km crust of western china is separated from the thinner 30 45 km crust of eastern china by the north south trending seismic belt 105 e the average crustal velocity of china ranges from 6 15 to 6 45 km s indicating a felsic to intermediate bulk crustal composition upper mantle pn velocities are 8 0 0 2 km s equal to the global continental average we interpret these results in terms of the most recent thermo tectonic events that have modified the crust in much of eastern china cenozoic crustal extension has produced a thin crust with a low average crustal velocity similar to western europe and the basin and range province western usa in western china mesozoic and cenozoic arc continent and continent continent collisions have led to crustal growth and thickening inferences on the process of crustal thickening are provided by the deep crustal velocity structure as determined by dss profiles and other seismological studies a high velocity 7 0 7 4 km s lower crustal layer has been reported in western china only beneath the southernmost tibetan plateau we identify this high velocity layer as the cold lower crust of the subducting indian plate as the indian crust is injected northward into the tibetan lower crust it heats and assimilates by partial melting a process that results in a reduction in the seismic velocity of the lower crust in the central and northern tibetan plateau

in situ zircon u pb oxygen and hafnium isotopic compositions of jurassic granites from the north china craton evidence for triassic subduction of continental crust and subsequent metamorphism related 18o depletion abstract six samples of the linglong granitic pluton from the southeastern margin of the north china craton yield similar jurassic magmatic zircon u pb ages and evolved whole rock sr nd isotopic compositions they are weakly peraluminous to strongly peraluminous with a cnk molar al 2 o 3 cao na 2 o k 2 o ratios of 1 05 to 1 3 except one sample with an a cnk of 0 98 however they have contrasting inherited zircon ages and magmatic zircon hf and o isotopic compositions two samples have 2 5 ga inherited zircons and high 18 o magmatic zircons and are interpreted to be derived dominantly from the lower crust of the north china craton another two samples with dominantly triassic inherited zircons and low 18 o magmatic zircons are considered to be derived mainly from the subducted continental crust of the south china craton and the source was considered to be significantly depleted in 18 o during triassic metamorphism the rest two samples have dominantly neoproterozoic inherited zircons and intermediate 18 o magmatic zircons and are also considered to be derived mainly from the subducted continental crust of the south china craton but the source was not significantly depleted in 18 o during triassic metamorphism the different lithologies whole rock e nd t and zircon e hf t between the samples also strongly support that they are derived from different sources although magmatic zircons from the six samples have variable 18 o from 0 7 to 8 5 and e hf t from 27 6 to 11 8 individual samples have more limited e hf t and 18 o the 18 o and e hf t of magmatic zircons from the samples indicate no contribution of mantle derived magmas to the linglong granites this is supported by the fact of no coeval mantle derived rocks in the region the results suggest that large variations of e hf t and 18 o do not necessarily indicate mixing between mantle and crustal derived magmas instead they may reflect heterogeneity in the source the granites are interpreted to result from partial melting of the thickened crust caused by triassic subduction of the south china craton continental crust beneath the north china craton subsequent lithospheric extension and orogenic collapse are the most likely mechanism for the granites

u pb age and nd isotopic evidence for archean terrane development and crustal recycling in the south central wabigoon subprovince canada u pb geochronology is presented for 19 volcanic and associated plutonic rocks from nine greenstone belts within the southern part of the central wabigoon subprovince cws western superior province data indicate that early felsic volcanism 3 014 2 999 ma was coeval with emplacement of the marmion batholith at 3 002 ma this was followed by periods of mafic volcanism with minor felsic and komatiitic compositions between 2 963 and 2 898 ma across the south cws plume driven mafic and komatiitic volcanism occurred in the lumby lake and lac des mille lacs belts at ca 2 828 ma eruption of komatiitic tuffs in the steep rock belt at 2 780 ma was proceeded by more widespread mafic and minor intermediate to felsic volcanism at 2 729 2 726 ma zircon inheritance and nd isotope data indicate that the cws south of the garden lake belt comprises a juvenile 3 0 ga terrane the marmion terrane subsequent magmatism was ensialic occurring in a continental although submerged setting recycling of the 3 0 ga marmion basement during successive volcanic events although somewhat cryptic was widespread and represents a significant archean process 2 726 ma rhyolite in the garden lake belt has a 3 22 ga nd model age and lies at the southern boundary of the distinct north cws which has a 3 3 3 2 ga crustal prehistory the north cws may represent a heavily reworked extension of the adjacent 3 3 ga winnipeg river subprovince and was probably separate from the marmion terrane at 3 0 ga these two recycled terranes differ significantly from the juvenile neoarchean volcanic dominated western wabigoon although the boundaries between these terranes are in most places cryptic they may indeed represent suture zones which were fundamental to the development of the wabigoon subprovince

structure geochronology and petrogenesis of the late triassic puziba granitoid dikes in the mianlue suture zone qinling orogen china we present structural field observations in situ zircon u pb ages hf isotopic compositions geochemistry and sr nd isotope data from the puziba granitoid intrusions in the qinling orogenic belt china these intrusions occur as ene wsw oriented dike swarms in a highly deformed phyllite matrix melange along several major fault zones within the mianlue suture zone granitoid rocks have a relatively wide range of sio 2 69 77 wt volatile free and total alkali contents na 2 o k 2 o 2 76 4 83 wt and k 2 o na 2 o ratios 1 and they mostly fall in the granite granodiorite fields in the total alkalis versus silica diagram they display relative enrichments of light rare earth elements lrees and relatively flat heavy rare earth elements patterns hrees with weak negative eu anomalies eu 0 56 0 95 on chondrite normalized rare earth element ree diagrams they show enrichment in th and zr and depletion in high field strength elements hfses e g nb p and ti they are enriched in large ion lithophile elements lile and lrees and have low zircon e hf t 2 8 to 4 2 values relatively high initial 87 sr 86 sr ratios 0 7107 0 7188 and variable mg 22 46 their u pb zircon crystallization age is ca 215 ma but they also show inherited zircon age clusters in the paleoproterozoic mesoproterozoic neoproterozoic and paleozoic with two prominent peaks at ca 753 ma and ca 805 ma the high rb and ta elemental abundances smooth chondrite normalized ree patterns and e nd t values 5 to 3 of the puziba dike rocks characterize them as k rich calc alkaline granites their magmas were derived from partial melting of garnet amphibole bearing lower crust that was triggered by slab breakoff induced asthenospheric upwelling in the aftermath of the oblique collision of the north and south china blocks during the late triassic emplacement of the ene wsw oriented puziba dike swarms was associated with a transtensional stress regime along across the collisional mianlue suture zone

kisseynew metasedimentary gneiss belt trans hudson orogen canada back arc origin and collisional inversion the kisseynew metasedimentary gneiss belt initially developed in a back arc setting the ages of detrital zircons and crosscutting plutons constrain turbidite sedimentation to 1 855 1 841 ga postdating early deformation and metamorphism associated with arc arc and arc continent collisions affecting much of the trans hudson orogen these turbidites are interpreted to have been deposited in a back arc basin behind a retreating subduction boundary the kisseynew domain and its southern flank are marked by 1 84 1 83 ga magmatic rocks that may be related to subduction of back arc basin oceanic crust collapse of the kisseynew basin by ductile fold and thrust deformation began during 1 84 1 83 ga magmatism and continued through peak metamorphism at ca 1 820 1 805 ga from authors

nd isotope mapping of a cryptic continental suture grenville province of ontario abstract nd isotope data are presented for 70 archean and paleoproterozoic grey gneisses from the grenville parautochthonous belt in ontario these data constrain an age boundary which separates partially reworked archean basement to the northwest from a paleoproterozoic arc terrane to the southeast this boundary is interpreted as a crustal scale continental suture but its exposure is cryptic due to metamorphic recrystallisation during the grenville orogeny mesoproterozoic stitching plutons injected across the boundary suggest that it has a minimum age of 1 5 ga and is probably a paleoproterozoic structure archean basement northwest of the suture was intensely plutonised by the 1 75 1 7 ga killarnean ensialic arc juvenile paleoproterozoic basement southeast of the suture has a maximum crustal formation age of less than 2 0 ga it is probably an accreted penokean terrane which was reworked by the same killarnean ensialic arc

thermal evolution of the kisseynew sedimentary gneiss belt manitoba metamorphism at an early proterozoic accretionary margin

drumlin location as a response to bedrock topography on the southeastern slope of the fennoscandian shield abstract there are several thousand drumlins in estonia northern latvia and on the bottom of the gulf of finland drumlins occur most frequently behind bedrock elevations on the slopes of which they form huge crag and tail formations and in marginal areas of depressions where the monolithic glacier disintegrated into tongues which moved at different velocities a dense net of boreholes and detailed geophysical measurements showed that even small irregularities of the bedrock topography and slight variations in the resistance of bedrock to glacial erosion exerted remarkable influence upon the location shape and internal structure of drumlins and drumlin like forms this accounts for the occurrence of drumlins with distinctly different structures within the same drumlin field however the drumlins were formed in a variety of ways and at different times under the effect of glaciers with different thickness in most cases they seem to have formed near the ice margin under rather thin glacier ice during its rapid advance when the dynamics of the ice responded to topographical peculiarities

deep seismic profile across a proterozoic collision zone surprises at depth collisions between continents generate the world s most striking mountains the deep structure of these collision zones inferred from seismic reflection profiles1 3 can be equally dramatic if less well understood improved knowledge of the structure of ancient collisional belts now known to have developed through plate tectonic processes4 5 can provide important new insights into the mechanisms of crustal growth although many such belts have been partly destroyed by severe post collisional deformation6 7 or magmatism8 some have been preserved in the stable interiors of continents9 one example is the early proterozoic trans hudson orogen9 10 in western canada here we present seismic reflection images across the entire orogen which reveal a broadly symmetric structure with reflections linked to island arc rocks dipping beneath both bounding archaean cratons the observed reflection geometries imply a doubling of crustal thickness during the collision along crust penetrating faults the presence of a significant amount of archaean crust at depth the extent of which could not have been determined from surface mapping suggests the need for caution in inferring crustal growth rate from the surface area of juvenile crust11 12

geochemistry of 1 9 ga morb and oib like basalts from the amisk collage flin flon belt canada evidence for an intra oceanic origin subaqueously erupted basalts that occur in kilometre scale allochthons within the 1 9 ga flin flon belt canada appear to have been generated at oceanic ridges and possibly oceanic plateaus remote from archean cratons the ocean floor basalts fall into two categories 1 n type resembling n morbs and mariana type back arc basin basalts depleted to flat ree patterns high zrnb variable thnb and initial nd 3 3 to 5 4 2 e type resembling transitional and plume morbs slightly enriched ree patterns lower zrnb initial nd 3 1 to 4 5 in the largest and best studied allochthon the elbow athapapuskow assemblage mixing between depleted n morb and enriched oib sources or melts coupled with variable addition of a subduction lile component can explain the chemical variations in the basalts zircon u pb dates of 1904 ma for a syn volcanic diabase sill and 1901 6 5 ma for a gabbro peridotite cumulate complex demonstrate that crystallization of the ocean floor basalts overlapped with in part eruption of the tectonically juxtaposed 1 90 1 88 ga arc volcanic rocks the elbow athapapuskow allochthon is interpreted as back arc basin crust that developed simultaneously with flin flon arc magmatism subaerially erupted basalts that chemically resemble tholeiitic oibs 8 14 mgo relative hree depletion initial nd 2 2 to 3 4 occur in tectonic contact with the elbow athapapuskow assemblage the oibs may have been generated by deeper garnet residue melting of enriched mantle tapped during extension in the elbow athapapuskow back arc basin and were possibly erupted onto a remnant arc deeper mantle melting is also indicated by the presence of the lree enriched oceanic plateau like basalts of the sandy bay assemblage the back arc 01b and plateau volcanic assemblages were jux taposed against ca 1 9 ga arc assemblages in a philippines like intraoceanic accretionary complex by 1 87 ga

glacial geomorphology of the northwestern weddell sea eastern antarctic peninsula continental shelf shifting ice flow patterns during deglaciation abstract during the last glacial maximum grounded ice from the expanded antarctic peninsula ice sheet extended across the continental shelf grounded and flowing ice created a distinctive array of glacial geomorphic features on the sea floor which were then exposed as the ice sheet retreated the recent disintegration of the northern parts of the larsen ice shelf larsen a and b have permitted acquisition of marine geophysical data in previously inaccessible and unmapped areas we present a reconstruction of the evolving ice flow path and ice sheet geometry of the eastern antarctic peninsula with particular focus paid to newly surveyed areas that shed light on the dynamics of a marine terminating glacial geomorphic environment where ice shelves play a major role in grounding line stability shifting flow directions were mapped in several areas including across the seal nunataks which divide larsen a and b and offshore of larsen c indicating flow reorientation that reflects the changing ice sheet geometry as retreat neared the modern coastline the measured flow indicators in this area reveal comparatively high elongation ratios 20 indicating rapid ice flow evidence of possible previous ice shelf collapses are noted near the shelf break further illustrating the critical protective effect that ice shelves impart to marine terminating glacial environments modern ice retreat is governed in part by reorganization of flow patterns accompanying grounding line movement such reorganizations happened in the past and can aid understanding of modern processes

direct dating of ultramafic sills and mafic intrusions associated with ni sulfide mineralization in the thompson nickel belt manitoba canada the paleoproterozoic thompson nickel belt a segment of the northwest margin of the archean superior craton in manitoba is one of the largest nickel producing regions in the world and the second largest ni cu pge platinum group elements mining camp in canada after sudbury the nickel sulfide ores are hosted in or associated with ultramafic sills of komatiitic affinity that were emplaced into sulfur rich metasedimentary rocks of the ospwagan group and subsequently strongly deformed and metamorphosed amphibolite to granulite facies during the trans hudson orogeny a u pb zircon age of 1880 2 1 4 ma for a metaperidotite olivine orthopyroxenite from the upper part of a boudinaged ultramafic sill at the pipe 2 deposit is interpreted as the age of crystallization of the intrusion the first age from an ultramafic sill containing mined nickel sulfide in the thompson nickel belt this age confirms a direct petrogenetic relationship between ni cu pge mineralization and widespread mafic ultramafic magmatic activity molson igneous events at ca 1 88 ga along the superior province craton margin e g thompson nickel belt fox river belt cape smith belt new quebec orogen at the south pit of the thompson mine a u pb zircon age of 1860 7 0 7 ma for a composite mafic intrusion garnet amphibolite to layered metagabbro identifies a new age of mafic magmatic activity in the thompson nickel belt that is unrelated to nickel sulfide mineralization the thermal effects of the trans hudson orogeny are recorded by a range of zircon dates and ca 1760 ma metamorphic titanite from the south pit intrusion and are consistent with interpretations that regional metamorphism occurred as a single progressive event the results of this study open up the possibility of using the high precision u pb geochronology of mineralized ultramafic rocks to precisely constrain the duration of mafic ultramafic magmatism in the thompson nickel belt and assess potential age differences between ultramafic sills across and along strike of this world class nickel metallotect

nd isotope and geochemical constraints on the depositional setting of paleoproterozoic metasedimentary rocks along the margin of the archean hearne craton saskatchewan canada abstract the cree lake zone in the southwestern part of the hearne craton reworked during the paleoproterozoic trans hudson orogeny includes the wollaston group a sequence of paleoproterozoic metasedimentary rocks as these rocks have been multiply deformed and metamorphosed to high grade a geochemical and isotopic study was initiated to see through the overprinting events to better constrain the provenance and evolution of the wollaston group the wollaston group consists of thick sedimentary successions representing different depositional and tectonic environments that can be subdivided into two subgroups the lower subgroup is represented by deep water turbidite to shallow water marine deposits the upper subgroup separated from the lower one by a regional unconformity comprises molasse type assemblages ranging from talus to shallow marine lacustrine with subordinate volcanogenic rocks major and trace element data obtained from 31 psammopelitic rocks indicate that the clastic material was derived from sources consisting of both felsic and mafic material although felsic sources are more important in the samples from the lower subgroup sedimentary tectonic discrimination diagrams demonstrate that most of the wollaston group was deposited on an active continental margin rather than a passive margin the e nd values for the wollaston group psammopelites calculated at 1 92 ga the approximate age of the boundary between the lower and upper subgroups vary from 3 4 to 6 8 which yields an approximate 70 30 mixture between juvenile paleoproterozoic detritus and archean detritus from the hearne craton a secondary source of detritus may be 1 95 2 0 ga rocks of the taltson orogen which is exposed to the west of the archean rae hearne craton the wollaston group records the complete evolution of the western margin of the trans hudson orogen during the paleoproterozoic the rifting of an archean continent at ca 2 1 ga led to the deposition of the passive margin siliciclastic sediments comprising the lowermost but poorly preserved part of wollaston group the formation of a continental arc rottenstone along the hearne margin at ca 1 92 ga resulted in a change in tectonic environment from a passive margin to a back arc basin and detritus in most of the lower subgroup was then derived from both basement and newly formed magmatic arc collision with the juvenile la ronge arc led to uplift along the hearne margin which transformed the back arc basin into a retroarc foreland basin at ca 1 88 ga sedimentary starvation and shallowing of the basin led to widespread accumulation of carbonate and evaporite deposits in the west whereas rapidly uplifted orogenic belts in the east provided sources of coarse grained clastic detritus forming the wollaston upper subgroup closure and inversion of the sedimentary basin occurred by ca 1 86 ga coincident with the emplacement of mafic to felsic intrusive rocks in the cree lake zone and the emplacement of the voluminous ca 1860 ma andean type wathaman batholith into the continental margin closure was manifested by the onset of foreland directed thrusting of the wollaston group

ice flow pattern and extent of the last scandinavian ice sheet southeast of the baltic sea a review of scientific literature geological maps and available previous digital data on ice marginal positions glacial landforms and sediment distribution are utilised to reconstruct ice streams and lobes and the maximum lgm and subsequent recessional ice marginal positions of the scandinavian ice sheet sis southeast of the baltic sea this paper presents preliminary results of the ongoing research that aims to build a geographic information system gis based model on the extent and timing of the last sis in the area between its maximum extent and the baltic sea digitized subglacial bedforms ice marginal and other glacial landforms and features from published sources are compared and validated against digital elevation model dem this has allowed specifying revising and questioning the location of the lgm and to interpret and correlate the post lgm ice streams with marginal positions morphological evidence demonstrates that large ice streams of between 100 and 300 km lengths were in operation ca 2 3 ka after the lgm and formed the middle and north lithuanian end moraines ice streams from the onega and white sea basins had high lateral slopes and clearly channelled flow while the ice streams that drained the ice sheet through the ladoga ilmen depression and in the eastern baltic usually had a fan shaped flow pattern and morphologically unclear lateral slopes in elevated areas and highlands that are located between ice streams a crossing of lineated bedforms is common one set of the crossing lineations on zemaitia eastern kursa vidzeme haanja latgale and sudoma highlands that are 200 350 km inside of the lgm margin has northwest southeast orientation that in general conforms to the direction of ice flow during the lgm and baltija phases it is expected that the presented compilation will stimulate discussions and scrutiny of earlier published data and generate ideas for future investigations

suture zone geometry along an irregular paleoproterozoic margin the superior boundary zone manitoba canada lithoprobe acquired a 190 km long deep seismic reflection profile across the superior boundary zone at the eastern margin of the paleoproterozoic trans hudson orogen the profile is located 250 km south of a major promontory along the ancient margin of the superior craton and complements earlier profiles closer to the promontory the new data image the remnants of a craton verging thrust belt younger than 1864 ma within the superior boundary zone extending to a maximum depth of 15 km above a shallowly west dipping basal decollement across the surface suture zone a dip reversal to east dipping reflections occurs within the adjacent reindeer zone where a crustal scale imbricate stack is imaged reindeer zone lower crust and upper mantle extend eastward beneath the superior craton margin for 40 100 km preservation of a foreland thrust belt within the superior boundary zone is explained as a result of reduced collisional convergence and subsequent exhumation within a reentrant flanking the thompson promontory