geochronology of early to middle paleozoic tectonic development in the southwest newfoundland gander zone southwest newfoundland s gander zone was once considered part of the collided eastern margin of an early paleozoic iapetus ocean distinguished from the rest of the gander zone by the extensive exposure of a polydeformed amphibolite to greenschist facies sequence considered devonian in age overlying presumed precambrian basement it was later found that truly devonian strata were confined to the cape ray fault system the local northwestern boundary of the zone u pb zircon isotopic ages of 452 13 51 ma and 458 22 29 ma on metavolcanic rocks in the upper part of the sequence southeast of the fault system indicate that ordovician and possibly older rocks underlie the gander zone itself a temporal framework for the three main stages of regional deformation corresponding to pre metamorphic recumbent folding d 1 and transpression d 2 and d 3 is presented using u pb and 40 ar 39 ar dates of granitoid and metamorphic rocks d 1 is bracketed as late o

hydrothermal petroleum from lacustrine sedimentary organic matter in the east african rift cape kalamba oil seeps occur at the south end of the ubwari peninsula at the intersection of faults controlling the morphology of the northern basin of the tanganyika rift east africa oil samples collected at the surface of the lake 3 4 km offshore from cape kalamba have been studied the aliphatic hydrocarbon and biomarker compositions with the absence of the typical suite of polynuclear aromatic hydrocarbons indicate an origin from hydrothermal alteration of immature microbial biomass in the sediments these data show a similarity between a tar sample from the beach and the petroleum from the oil seeps and confirm that the source of these oils is from organic matter consisting mainly of bacterial and degraded algal biomass altered by hydrothermal activity the compositions also demonstrate a 200 degrees c temperature for formation generation of this hydrothermal petroleum similar to the fluid temperature identified for the pemba hydrothermal site located 150 km north of cape kalamba the 14c age of 25 6 ka b p obtained for the tar ball suggests that pleistocene lake sediments could be the source rock hydrothermal generation may have occurred slightly before 25 ka b p during a dry climatic environment when the lake level was lower than today these results also suggest that the cape kalamba hydrothermal activity did not occur in connection with an increased flux of meteoric water higher water tables and lake levels as demonstrated in the kenya rift and for the pemba site hydrothermal petroleum formation is a facile process also in continental rift systems and should be considered in exploration for energy resources in such locales

strain trajectories above the main central thrust himalaya in southern tibet as a result of the collision of india with asia1 3 a thick pile of precambrian crystalline rocks the main central sheet was transported southwards over the indian continent a marked linear fabric a mineral stretching lineation within the main central sheet varies only slightly from a north south trend which is interpreted as paralleling the direction of transport of the nappe3 6 our study sought to extend the previous observations to the hitherto poorly known part of the main central sheet in southern tibet china the stretching lineation is dominantly north south in the lower part of the sheet but exhibits continuous and discontinuous variations from north south to east west in the upper part7 these variations of lineation trajectory indicate two shear components one subparallel to and one at a high angle with the himalayan belt the recognition of westward wrench shearing in the main central sheet implies a component of relative displacement parallel to the himalayan belt and not only a radial one as generally assumed in previous models this suggests that the kinematic story of the intracontinental deformation following the continental collision is probably not one of simple north south convergence

deformation resulting from regional extension during pluton ascent and emplacement central sierra nevada california solid state foliation lineation small scale folds and domainal shear zones have developed in preexisting granitic and minor metasedimentary wallrock during a combined deformation involving a regional extensional strain and ascent and emplacement of the mt givens pluton mgp mylonite is common throughout much of the 1 2 km wide 10 km long shear zone with ultramylonite best developed near the contact with the mgp which itself lacks significant solid state deformation migmatization accompanies ultramylonite formation in the northern half of the zone but both these features are poorly developed or absent in its southern half where the shear zone is distributed over a wider area strain estimates across the shear zone using microgranitic enclaves as markers show a positive gradient and an increasing ratio of simple shear pure shear towards the mgp microprobe analyses on hornblende and plagioclase yield pressure and temperature estimates of 3 5 kb and 680 c respectively during shear zone formation at least at its late stages of development zircon pb u and 40 ar 39 ar ages constrain timing of the high temperature movement on the shear zone to 90 ma essentially the age of the mgp although movement immediately prior to that time appears likely we speculate that a regional extensional shear zone was developing prior to the emplacement of the mgp which as it ascended heated the wallrock facilitating both further strain in the zone as well as buoyant rise of the pluton along the zone the mgp was near its critical melt fraction during the last several kilometers of its ascent and could have possessed sufficient viscosity strength to impose a weak shear strain on the shear zone rocks although most of the foliation and extensional features in the zone are probably related to the regional strain field late stage folding of the foliation is attributed to shouldering aside of the wallrock by the mgp during the last increment of its ascent and final emplacement

textural and compositional variability across littoral segments of lake tanganyika the effect of asymmetric basin structure on sedimentation in large rift lakes lake tanganyika part of the east african rift system represents one of the most widely cited modern analogs for interpreting ancient rift lakes to date few published detailed sedimentologic studies of the modern sediments allow for comparisons to outcrop and well bore scale observations within ancient strata four recurrent structural margin types exist along the alternating half graben structure of the lake hinged margins axial margins accommodation zone margins and escarpment margins the hinged margin consists of a series of structurally controlled benches over which long continuous tracts of bioclastic lag deposits predominate clastic sands are limited to moderate size silty deltas and long narrow shoreface sands the axial margin is dominated by a wave dominated silt rich delta system accommodation zone margins consist of bioclastic lag deposits atop structural highs whereas carbonate and clastic mud accumulates farther offshore escarpment margins contain small fan delta deposits alternating along shore with talus deposits offshore carbonate and clastic mud is present away from active gravity flow deposition total organic carbon toc and pyrolysis data from fine grained samples subtly reflect the contrasts in margin types but these values are controlled more directly by water depth although facies are similar among all margin types their spatial distribution in particular the degree to which facies tracts trend parallel to shore best discriminates among the different margin types these data suggest that unique but predictable associations of reservoir seal and source facies exist along each of the different margin types

hydrothermal activity in the northern tanganyika rift east africa abstract hydrothermal fluids from sub lacustrine and sub aerial sites were sampled in the north tanganyika rift during the 1991 tanganydro expedition two types of hydrothermal end member fluids a na hco 3 type and a na cl type have been identified within the sample set the na hco 3 hydrothermal fluids are emitted at the level of the sub lacustrine pemba and sub aerial kabezi and mashuzi sites which are located in the northernmost bujumbura sub basin the na cl hydrothermal fluid vents at the level of the sub lacustrine cape banza field on the southeastern edge of the ubwari ridge the two types of hydrothermal fluids display isotopic and chemical evidence of interaction with magmatic volatiles the magmatic effects are strongest in the fluid discharging on the pemba hydrothermal field where abundant co 2 bubbling accompanies fluid emission the 13 c values of the co 2 gas sampled at pemba and at cape banza indicate a magmatic origin the pemba fluid shows the highest alkalinity and boron content and the lowest ph of the hydrothermal end member fluids and it has equilibrated at depth at the highest temperature 214 7 c in addition the sulfide mineralizations associated with the pemba hydrothermal vents are enriched in as and tl which similarly to b can be transported as volatiles in magmatic fluids the northern tanganyika na hco 3 fluids are similar to the na hco 3 fluid of the geothermal system of lake bogoria in the eastern branch of the east african rift and they can be compared with the na hco 3 hydrothermal water that is inferred to flow into lake kivu the cape banza na cl fluids may derive from the mixing between meteoric water and a deep seated basement brine similar to the ones found in old continental shields such as the canadian shield the high equilibrium temperatures and the geochemistry of the north tanganyika rift hydrothermal fluids suggest that degassing magmatic bodies are present at depth in the area these magmatic bodies may be either the southern expression of the south kivu magmatism or a more recent activity not yet expressed as surface volcanism the hydrothermal sites are located along major active rift bounding faults which can be associated with deep and strong seismicity as in the case of the pemba field such deep fractures may have favoured the uprise of the magmatic bodies that are the source of heat and volatiles to the hydrothermal systems the circulation of hydrothermal fluids within the tanganyika lacustrine sedimentary pile could play an important role in the maturation of the organic matter enclosed in the lake sediments and the subsequent oil generation and migration

structural evolution of a major fault zone the cape ray fault zone southwestern newfoundland canada the cape ray fault zone is a major paleozoic structure in southwestern newfoundland and occurs at or close to the boundary between two major continental blocks laurentia and avalonia a detailed structural analysis demonstrates that the fault records early reverse sinistral thrusting of the grand bay complex at amphibolite grade d2 followed by a protracted event d3 characterized by reverse dextral thrusting of the grand bay complex rocks on top of the supracrustal rocks of the windsor point group and retrogression to greenschist facies as well as a pre 384 ma orogen parallel dextral transcurrent mylonite d4 during the later stages of the collision regional scale strain partitioning induced heterogeneity of strain both along and across the strike of the cape ray fault zone the east west oriented segment of the cape ray fault zone is a tear fault that accommodated differential displacement along the length of the fault later stages of the deformation include post 384 ma sinistral transcurrent r

a 1 3 million year record of synchronous faulting in the hangingwall and border fault of a half graben in the malawi nyasa rift abstract this paper analyzes throw depth t z profiles from a high resolution 2d reflection seismic grid in the central basin of lake malawi to investigate whether evidence exists 1 for migration of faulting away from the border fault of the half graben and 2 that faults in the hangingwall lengthened over the last 1 3 million years we use the high precision age model from a 2005 scientific drilling project in our study area to constrain the ages of our seismic horizons and examine a fault array and two individual faults within the hangingwall of the central basin border fault we account for climatic and sedimentological controls on stratal growth with a lake level curve that accompanies the age model a comparison of our hangingwall t z profiles with published throw distance t x profiles for the border fault shows synchronous faulting over the last 1 3 m y rather than basinward migration of faulting furthermore we find no evidence for significant propagation of the tips of the hangingwall faults in the last 1 3 m y and conclude that the lack of basinward migration of faulting is a consequence of strain localization on faults established at an early stage in basin development

episodic reactivation of a late precambrian mylonite zone on the gondwanan margin of the appalachians southern newfoundland the grand bruit fault zone of southern newfoundland is a fundamental structure within late precambrian basement on the gondwanan margin of the appalachian orogen within the fault zone a sequence of structures documents changes in the sense of ductile displacement from 1 reverse dip slip to 2 dextral strike slip to 3 sinistral oblique slip and finally to 4 dextral lateral offsets fault movements along this structure were punctuated by emplacement of a variety of plutons and minor intrusions which when precisely dated allow these movements to be bracketed at between 571 ma and 564 ma 497 ma and 427 ma 424 ma and 420 ma and 421 ma and 387 ma respectively the tectonic evolution of the gondwanan inlier of southern newfoundland is mirrored in large part by the record of mylonite development within the grand bruit fault zone these tectonic events are attributable to well constrained regional orogenic events of both the pan african and appalachian cycles newly formed shear zones in the fault zone reactivate parts of much older faults of similar regional orientation and are in some cases kinematically indistinguishable from the ancestral structures integration of precise geochronological data with the sequence of overprinted fault structures demonstrates that although the role of progressive deformation in shear zone development was important the observed disposition of structures and rock units is primarily a function of polyorogenic accretion as a multiple reactivated structural lineament in a gondwanan basement inlier the fault zone exerted fundamental control over the tectonic development of the leading edge of the convergent southeast margin of the orogen

internal structure of the cadillac tectonic zone southeast of val d or abitibi greenstone belt quebec the kirkland lake larder lake cadillac break is a major fault zone of the abitibi greenstone belt well known for its spatially associated gold camps a detailed structural analysis of this fault zone in the val d or area has shown the presence of a 200 750 m wide zone of high strain defined here as the cadillac tectonic zone ctz which includes the narrower schist zone generally regarded as the cadillac break the boundaries of the ctz which coincide with major lithologic contacts strike approximately east west and dip 80 to the north the internal structure of the ctz is best analysed in terms of two increments of the same progressive deformation the d1 strain increment is characterized by an east west striking subvertical s1 foliation at low angle in strike to the bound aries of the ctz and containing subvertical elongation lineation primary lithological contacts have been transposed into s1 and folded by f1a folds f1b intrafolial folds of s1 ranging from noncylindrical subhorizontal f

stratigraphic architecture and evolution of a barrier seagrass bank in the mid late holocene shark bay australia abstract within the faure sill complex shark bay western australia a combination of remote sensing analysis seismic stratigraphy and cores to ground truth together with radiocarbon dating demonstrate the interconnection between sediment body morphologies seagrass related substrates and pre existing topography and reveal the system as a channel bank complex sea level fluctuations appear to have largely controlled the hydrodynamic conditions of the bank contributing to each stage of its evolution 1 not earlier than 8 5 8 0 ka bp in a lowstand period after an erosive event of underlying palaeosurfaces seagrass establishment progressively contributed to initiating bank growth 2 around 6800 years bp bank accumulation reached its apex in conjunction with a rapid sea transgression 3 during the late holocene succeeding a slow decline to present sea level bank growth continued to fill available accommodation space and a number of hiatuses indicating temporal and spatial discontinuities within the process of bank building are recognised average depositional rates of bank building 1 3 m ka conform to previous estimates derived for seagrass banks but rates are strongly facies dependent attesting to the dynamic nature of this channel bank complex the extensive seagrass meadows are essential for a wide range of aspects of the environment of the shark bay area not only are they particularly important for the entire shallow benthic ecosystem but they also had a major role in the partial closure of the southern basins and hence determining the development of hypersaline conditions and associated oolitic microbial and evaporitic facies in hamelin pool and l haridon bight moreover this system has a critical role in producing sequestering and storing organic carbon

crenarchaeotal membrane lipids in lake sediments a new paleotemperature proxy for continental paleoclimate reconstruction paleoclimate studies of continental environments have been hampered by the lack of an independent paleotemperature proxy a novel sea surface temperature proxy has been proposed for marine systems based on membrane lipids of marine crenarchaeota this proxy will provide an independent continental paleotemperature tool that will allow paleoclimatologists to address fundamental questions about temperature variability in continental environments and its relationship to climate change

progressive deformation structures associated with ductile thrusts in the moine nappe sutherland n scotland in the deeper parts of mountain belts polyphase structural sequences observed at outcrop scale can arise due either to overprinting of regionally distinct deformation phases e g reworking changes in orogenic boundary conditions or to localized controls that bring about transient changes in the patterns of ductile flow these are unlikely to be mutually exclusive processes and examples from scotland demonstrate that once regionally separate events are delimited using radiometric evidence it is possible to isolate complex deformation sequences arising due to local controls in the western moine nappe of sutherland the dominant structures were formed during caledonian ductile thrusting towards the wnw whilst earlier precambrian phases are relatively minor in importance two groups of caledonian folds and fabrics are recognized in many exposures main phase d2 structures which are broadly contemporaneous with ductile thrust fabrics and later secondary phase f3 folds the latter can be divided into two geometric groups sheath fold types which formed initially as wnw overturned buckles subsequently modified by ductile shearing and asymmetric types which are commonly open folds apparently formed with axes close or sub parallel to the thrust transport direction secondary structures show a close spatial association with high strain zones along ductile thrusts and can be shown to have formed during the later stages of thrusting in certain critical exposures i propose that they may form due to strain perturbations resulting from variations in the relative rates of ductile flow within the mylonites where differential shearing occurs due to flow normal perturbations wrench related asymmetric fold types may form in contrast secondary sheath fold structures may result from localized compression phases caused by flow parallel perturbations local flow perturbation models may be appropriate in situations where the distribution of later structures is apparently related to spatial variations in strain intensity and or there is a strong geometric and kinematic similarity between sets of folds and fabrics if such features are not observed a regional polyphase sequence is more likely and it is therefore necessary to obtain radiometric dates or some other independent source of data to demonstrate a time separation

quaternary sedimentation and subsidence history of lake baikal siberia based on seismic stratigraphy and coring the long continuous high latitude stratigraphic record of lake baikal was deposited in three broad sedimentary environ ments defined by high resolution seismic reflection and coring meth ods 1 turbidite depositional systems by far the most widespread characterizing most of the margins and floors of the main basins of the lake 2 large deltas of major drainages and 3 tectonically or topographically isolated ridges and banks holocene sedimentation rates based on radiocarbon ages vary by more than an order of mag nitude among these environments from less than about 0 03 mm yr on ridges and banks to more than about 0 3 mm yr on basin floors ex trapolating these rates with a correction for compaction yields ten tative estimates of about 25 and 11 ma for the inception of rifting in the central and north basins respectively and less than 6 ma for the 200 m sediment depth on academician ridge the selenga delta has the distinctive form of a classic prograding gilbert type delta but its history appears to represent a complex com bination of tectonism and sedimentation the central part of the delta is underlain by prograding shallow water sequences now several hun dred meters below the lake surface these deposits and much of the delta slope are mantled by fine grained deep water hemipelagic de posits whose base is estimated to be about 650 000 years old modern coarse grained sediment bypasses the delta slope through fault con trolled canyons that feed large subaqueous fans at the ends of the south and central basins these relations along with abundant other evidence of recent faulting and the great depths of the central and south basins suggest that these two rift basins have experienced a period of unusually rapid subsidence over the last 650 000 years dur ing at least part of which sedimentation has failed to keep pace

glacier dynamics and paleoclimatic change during the last glaciation of eastern ellesmere island canada a 300 km transect along the east coast of ellesmere island fills a major gap in the late quaternary data base of the canadian high arctic the last glacial maximum lgm is marked by prominent moraines and meltwater channels that terminate within 30 km of modern ice margins shells in glaciomarine deposits collected beyond the lgm indicate deglaciation by more extensive ice prior to 35 ka bp more than 60 14c dates from glaciomarine sediments provide a chronology for past ice dynamics during the lgm to the north while many areas remained ice free due to severe aridity several glaciers remained in contact with the sea until 7 1 ka bp farther south most glaciers reached the coast and significantly infilled several fiords this southward increase in glacier extent is due to larger glacial catchment basins and increased precipitation towards storm tracks in northern baffin bay the earliest dates on deglaciation along the transect range from 8 1 to 7 7 ka bp initial retreat was controlled by the exten

conservation implications of the provenance of modern sediment on a shell rich platform of lake tanganyika kigoma tz shell rich substrate shell beds in the littoral reaches of lake tanganyika africa form a unique substrate that host a number of endemic organisms including sponges and shell dwelling cichlid fish the predominant gastropod neothauma tanganyensis whose shells comprise the majority of the shell beds are themselves endemic to the lake sedimentologic and observational data suggest that one of these shell beds located between the towns of kigoma and ujiji tanzania on the western side of central lake tanganyika is impacted by modern sedimentation in the northern portion of the study discrete patches of the shell beds between 6 30 m water depth are blanketed by 20 to 50 wt mud in these areas there are few to no sponges nor shell dwelling cichlid fish as evidenced by lack of occurrences of fish nests with shells however in the southern part of the study area within the same depth range the shell beds contain less mud and sponges and fish nests with shells are more common mineralogic and geochemical data suggest the patterns and composition of the sediment are consistent with two sources the sediment accumulating on the shell beds in the northern portion of the study area matches the chemistry of the weathered precambrian metasedimentary rocks in hillslopes adjacent to the lake within a region undergoing severe land use changes the sediment accumulating along the southern portion of the study site particularly in deeper water matches the chemistry of sediment from the luiche river which forms a delta at the southern end of the study site this delta has prograded up to 1 km within the last 40 years however in this setting the bulk of deltaic sediment is shunted into deeper water and so appears to represent a less severe threat to the shallow water shell bed habitat than the local hillslopes this is one of a few detailed studies of the sedimentology of a shell bed habitat in lake tanganyika and the first to tie the sediment to its source this has important implications for conservation of these habitats as methods and practices of conservations vary based on the types of disturbance the study also has implications for biologists interested in community structure as certain species of cichlids appear to be efficient engineers of the substrate forming shelly nests but in areas of high mud content are absent

crustal structure at a young continental rift a receiver function study from the tanganyika rift the southern tanganyika rift within the western rift africa has earthquakes to depths of 37 km yet few constraints exist on crustal thickness or of early stage rifting processes in apparently amagmatic rift sectors the aim of the tanga14 experiment was to constrain bulk crustal properties to test whether magmatic processes modify the lithosphere in areas of deep seismicity and the degree of lithospheric thinning we use eleven broadband seismometers to implement receiver function analysis using h stacking a method sensitive to crustal thickness and vp vs ratio to determine bulk crustal properties analyses include extensive error analysis through bootstrap variance and phase weighted stacking results show the archean tanzanian craton and bangweulu block are characterized by vp vs ratios of 1 75 1 77 implying a felsic bulk composition crust beneath the fault bounded basins has high vp vs 1 9 anorthosite bodies and surface sediments within the region may contribute to localized high vp vs however elevated vp vs values within fault bounded extensional basins where elevated heat flow hydrothermal vent sites and deep earthquakes are observed suggest that magma may be intruding the lower crust beneath the southern tanganyika rift crustal thicknesses on near the relatively un extended tanzanian craton and bangweulu block are 41 6 42 0 km this contrasts with the tanganyika rift where crustal thicknesses are 31 6 km to 39 km from north to south our results provide evidence for 20 crustal thinning localized to fault bounded basins taken together they suggest a previously unrecognized role of magma intrusion in early stage continental rifting in the western rift africa

origin of kinkbands and shear band cleavage in shear zones an experimental study specimens of artificial kcl mica schist have been deformed in simple shear with different initial orientations 10 intervals between 0 and 170 of the mica schistosity s1 relative to the imposed simple shear plane for most experiments the angular shear strain was 30 the strain rate 4 8 10 4s 1 and the stress normal to the imposed shear plane 20 mpa all experiments were conducted at room temperature deformation of the specimens is achieved largely by slip parallel to the s1 mica foliation however two new types of slip surfaces also develop as either a single orientation set or as a pair of conjugate sets these new slip surfaces are kinkbands and a shear band cleavage the type and number of sets of new slip surfaces which develop depends on the orientation of s1 relative to the shortening and extension fields of the instantaneous strain ellipsoid our experiments indicate that kinkbands or shear band cleavage form in shear zones in which s1 is inclined to the margin of the zone which structure forms and whether it occurs as a single set of structures or as a conjugate pair depends on the initial orientation of s1 in situations where s1 is parallel to the margins of the zone neither structure develops if the deformation is a simple shear the presence of kinkbands or shear band cleavage in such zones where s1 is parallel to the margins is an indication that the zones were transtensional or transpressional respectively we conclude that shear band cleavage is not only an excellent indicator of shear sense but also a qualitative indicator of the ratio of s1 normal shortening to s1 parallel shear conjugate shear bands indicate that s1 normal shortening was relatively strong and single shear band sets indicate that s1 parallel shear was relatively strong

latest pleistocene and holocene glaciation of baffin island arctic canada key patterns and chronologies abstract melting glaciers and ice caps on baffin island contribute roughly half of the sea level rise from all ice in arctic canada although they comprise only one fourth of the total ice in the region the uncertain future response of arctic glaciers and ice caps to climate change motivates the use of paleodata to evaluate the sensitivity of glaciers to past warm intervals and to constrain mechanisms that drive glacier change we review the key patterns and chronologies of latest pleistocene and holocene glaciation on baffin island the deglaciation by the laurentide ice sheet occurred generally slowly and steadily throughout the holocene to its present margin barnes ice cap except for two periods of rapid retreat an early interval 12 to 10 ka when outlet glaciers retreated rapidly through deep fiords and sounds and a later interval 7 ka when ice over foxe basin collapsed in coastal settings alpine glaciers were smaller during the younger dryas period than during the little ice age at least some alpine glaciers apparently survived the early holocene thermal maximum which was several degrees warmer than today although data on glacier extent during the early holocene is extremely sparse following the early holocene thermal maximum glaciers advanced during neoglaciation beginning in some places as early as 6 ka although most sites do not record near little ice age positions until 3 5 to 2 5 ka alpine glaciers reached their largest holocene extents during the little ice age when temperatures were 1 1 5 c cooler than during the late 20th century synchronous advances across baffin island throughout neoglaciation indicate sub milankovitch controls on glaciation that could involve major volcanic eruptions and solar variability future work should further elucidate the state of glaciers and ice caps during the early holocene thermal maximum and glacier response to climate forcing mechanisms

the geometrical relationship between the stretching lineation and the movement direction of shear zones abstract the geometrical relationship between foliations and lineations in shear zones is considered quantitatively assuming that the internal structures of shear zones have monoclinic symmetry the common practice of directly correlating the pitch or plunge of the stretching lineation with the obliquity of the shear direction is challenged it is the orthogonal projection of the stretching lineation on the shear zone boundary that is parallel to the shear direction a shear zone is parallel to a plane containing both the pole to the symmetry plane and the strike of the shear zone and the shear direction is parallel to the intersection of the symmetry plane and the shear zone boundary the symmetry plane is determined by the stretching lineation and the poles to the s foliation the implications of this study for the interpretation of shear zones are discussed with reference to an actual example

geological interpretation of a high resolution reflection seismic survey at the buchans mine newfoundland sixteen kilometres of high resolution vibroseis reflection seismic data have been acquired in the vicinity of the former buchans mine direct identification of the cause of several reflectors is possible because the geology is tightly constrained by underground workings and drill holes both of which locally exceed 1 km depth many of the mine scale thrust faults are imaged as reflectors but conformable and intrusive contacts generally responded poorly a significant shallow dipping thrust the powerline fault is recognized below the orebodies and traced throughout the buchans area primarily as a result of the seismic survey it truncates ore stratigraphy and forms the floor thrust of a large duplex stack which hosts all the orebodies its presence has negative implications for exploration in the immediate mine area several lines of evidence suggest that this fault has a significant component of out of sequence movement a strong reflector 4 5 km below buchans is correlated with the surface expression

the role of pre existing precambrian structures in rift evolution the albertine and rhino grabens uganda abstract we integrated shuttle radar topography mission srtm digital elevation models dem airborne magnetic radiometric and three dimensional full tensor gravity gradiometry 3d ftg data to investigate the role of precambrian structures in the evolution of the largely amagmatic miocene recent aged albertine and rhino grabens in uganda these grabens represent the northern segment of the western branch of the east african rift system ears the two ne trending grabens are connected by a right stepping transfer zone and they extend within the archean paleoproterozoic northeast congo block which represents the northeastern extension of the congo craton our results show the following and highlight the importance of pre existing structures in the evolution of continental rift systems 1 the ne extent of the albertine full graben is controlled by ne trending precambrian fabric and the graben terminates at its northeastern end when it encounters a multiply folded precambrian basement terrain with poorly developed nw trending structural grain additionally the northeastern termination of the albertine graben coincides with the presence of nw trending right stepping high density bodies within the precambrian terrain 2 the transfer zone between the albertine and rhino grabens is controlled by ne trending precambrian structures which might have facilitated the development of relay ramp faults 3 strain localization within the better developed southeastern border fault of the rhino half graben is facilitated by the presence of precambrian structures better aligned in a ne direction in the southeastern part of the basin compared to its northwestern part 4 further to the northeast the rhino graben is segmented and transitions into a narrower ene trending half graben with a better developed border fault on its northwestern side this segmentation coincides with the presence of n trending precambrian structures 5 the rhino graben terminates farther northeast against the nw trending precambrian aswa shear zone a prominent structure with complex but generally nw trending fabric

tectonic uplift and eastern africa aridification the history of eastern african hominids has been linked to a progressive increase of open grassland during the past 8 million years this trend was explained by global climatic processes which do not account for the massive uplift of eastern african topography that occurred during this period atmosphere and biosphere simulations quantify the role played by these tectonic events the reduced topographic barrier before 8 million years ago permitted a zonal circulation with associated moisture transport and strong precipitation our results suggest that the uplift itself led to a drastic reorganization of atmospheric circulation engendering the strong aridification and paleoenvironmental changes suggested by the data

a new lower ordovician arenig faunule from the coy pond complex central newfoundland and a refined understanding of the closure of the iapetus ocean a newly discovered fossiliferous horizon within sediments belonging to the coy pond complex of the exploits subzone in central newfoundland yields the graptolite undulograptus austrodentatus s l and cyclopygid trilobite cyclopyge grandis brevirhachis this late arenig faunule constrains the upper age limit of the ophiolite complex and is the first record of an early ordovician cyclopygid trilobite in north america this is consistent with a paleogeographic affinity for south central newfoundland with the northern oceanic margin of avalonia in a peri gondwanan position during the early ordovician and contrasts with coeval shelly and graptolitic faunas from the notre dame subzone of central newfoundland which show marked laurentian affinities the exploits subzone is generally considered equivalent to the region of scotland lying south of the southern upland fault the newfoundland discovery which is supported by faunal data from elsewhere in newfoundland and in ireland suggests that the region around t

40 ar 39 ar amphibole ages from the kingston complex new brunswick evidence for silurian devonian tectonothermal activity and implications for the accretion of the avalon composite terrane the kingston complex of southern new brunswick canada is a regional ne trending belt of mafic and felsic dikes bordered by dextral ductile shear zones the complex separates two contrasting late precambrian early paleozoic tectonostratigraphic assemblages near the nw margin of the avalon composite terrane rhyolitic dikes within the complex record an early silurian crystallization age dike orientations suggest that emplacement of the complex accompanied sinistral movement interpreted to reflect accretion of the avalon composite terrane to cratonic north america mafic dikes within the complex display greenschist to epidote amphibolite facies metamorphic assemblages that developed during dextral transpression amphibole concentrates from seven metamorphosed mafic dikes record late silurian to early devonian ca 416 390 ma 36 ar 40 ar vs 39 ar 40 ar isotope correlation ages interpreted to date closely the dextral transpressive deformation together with other recent age data the 40 a

granite magma generation ascent and emplacement within a transpressional orogen crustal thickening during transpressive orogenesis may produce anatectic granites which may then localize deformation leading to transcurrent movement granites may be transported from sites of generation through the mid crust in dyke like channelways within relatively narrow strike slip shear zones which link to major fault zones in the upper crust extensional jogs within fault systems provide developing sites for the assembly of plutons from magma arriving from below the model is based upon observations from the cadomian belt of nw france which exposes sections through middle and upper levels of the late precambrian crust within different elements of the orogen the mechanism provides a favourable alternative to diapirism and explains the common collocation of granites and shear zone fault systems within orogenic belts

the zambezi delta mozambique channel east africa high resolution dating combining bio orbital and seismic stratigraphies to determine climate palaeoprecipitation and tectonic controls on a passive margin abstract the zambezi delta draining the southern african plateau and the southern part of the east african rift is one of a the largest delta of africa with a long lasting history starting during early cretaceous with more than 12 km of sediments deposited the zambezi delta is therefore a unique archive of the past topographic evolution of southern and eastern africa and their related deformations but also of the climate changes global and regional consequences of local topographic growths understanding this archive supposes to get a high resolution dating of the sediments our two objectives are here 1 to construct an age model of the zambezi cenozoic delta using a combination of biostratigraphy orbital stratigraphy and sequence stratigraphy and 2 to determine the palaeoprecipitation variations of the zambezi catchment from the oligocene to present day in a known tectonic framework the neogene sequences were dated at high resolution assuming that the third order sequences are of eustatic origin and record long term eccentricity cycles the sequences were correlated in ages on the calculated earth orbital solutions of laskar for the time intervals provided by the biostratigraphy nannofossils planktonic foraminifers the palaeoprecipitation record was based on the definition of a humidity index based on pollen analysis and associated botanical associations the late oligocene was a quite wet period getting dryer in the uppermost chattian the base tortonian 11 ma was a humid period the messinian was a dry period with a slight increase of the humidity during the zanclean and a sharp increase around the zanclean piacenzian boundary the zambezi delta has recorded the uplifts of the southern african plateau around 85 ma and around 25 ma and those of the southward migration of the east african rift since 5 5 ma

comparing species diversity in the modern and fossil record of lakes abstract lake faunal diversity has been the focus of many modern studies however there have been few studies comparing the lacustrine diversity in the modern and fossil record whether or not this comparison is possible is dependent on preservation by comparing the exceptional preservation of the konservat lagerstatten of the eocene green river formation with its various modern analogs in east africa unique factors controlling diversity within a lake system can be determined such factors would include lake surface area lake longevity productivity and lake type in order to effectively compare lake diversity in the modern and fossil record we first estimated the potential preservational bias of the fauna from modern lake tanganyika and determined that at maximum approximately 43 8 of species 59 3 of genera and 65 8 of families would be identifiable with this taphonomic filter in place we compared fish species diversity versus lake area no strong relationship was found faunal species div

late mid holocene sea level oscillation a possible cause abstract sea level oscillated between 5500 and 3500 years ago at murrells inlet south carolina chezzetcook and baie verte nova scotia and montmagny quebec the oscillation is well constrained by foraminiferal marsh zonations in three locations and by diatoms in the fourth one the implications are 1 there was a eustatic sea level oscillation of about 2 10 m in the late mid holocene on the southeast coast of north america south carolina to quebec that is not predicted by present geophysical models of relative sea level change 2 this oscillation coincides with oceanographic cooling on the east coast of canada that we associate with melting ice and 3 this sea level oscillation climatic event coincides exactly with the end of pyramid building in egypt which is suggested to have resulted from a climate change i e drought cooling this sea level climatic change is a prime example of feedback where climatic warming in the mid holocene promoted ice melt in the arctic which subsequently caused climatic cooling by opening up arctic channels releasing cold water into the inner labrador current that continued to intensify until 4000 years ago this sea level event may also be the best way of measuring when the final ice melted since most estimates of the ages of the last melting are based on end moraine dates in the arctic which may not coincide with when the last ice actually melted out since there is no way of dating the final ice positions

comparative sequence stratigraphy of low latitude versus high latitude lacustrine rift basins seismic data examples from the east african and baikal rifts abstract lakes baikal malawi and tanganyika are the world s three largest rift valley lakes and are the classic modern examples of lacustrine rift basins all the rift lakes are segmented into half graben basins and seismic reflection datasets reveal how this segmentation controls the filling of the rift basins through time in the early stages of rifting basins are fed primarily by flexural margin and axial margin drainage systems at the climax of syn rift sedimentation however when the basins are deeply subsided almost all the margins are walled off by rift shoulder uplifts and sediment flux into the basins is concentrated at accommodation zone and axial margin river deltas flexural margin unconformities are commonplace in the tropical lakes but less so in high latitude lake baikal lake levels are extremely dynamic in the tropical lakes and in low latitude systems in general because of the predominance of evaporation in the hydrologic cycle in those systems evaporation is minimized in relation to inflow in the high latitude lake baikal and in most high latitude systems and consequently major sequence boundaries tend to be tectonically controlled in that type of system the acoustic stratigraphies of the tropical lakes are dominated by high frequency and high amplitude lake level shifts whereas in high latitude lake baikal stratigraphic cycles are dominated by tectonism and sediment supply variations

extensional and contractional deformation in the blue ridge province virginia basement rocks of the blue ridge anticlinorium in central and northern virginia were involved in two temporally distinct deformation episodes the older event is characterized by a locally pervasive low strain fabric and high strain shear zones a preferred grain shape orientation in recovered quartz feldspar and biotite combined with a poorly developed symmetric quartz c axis pattern defines the low strain fabric kinematic indicators are typically symmetric but where asymmetric they consistently indicate top down to the southeast extensional motion discrete shear zones are within and close to late proterozoic plutons kinematic evidence fron these zones records large noncoaxial strains also with a down to the southeast extensional shear sense feldspar microstructures range from cataclastic fractures to complete recovery suggesting that deformation occurred at greenschist to amphibolite grade both low strain foliations and high strain shear zones are cut by paleozoic thrust related tectonites on the basis of kinematic evidence and spatial association with late proterozoic plutons cross cutting relations with thrust related structures and regional geologic relations we suggest that extensional deformation is related to late proterozoic rifting of the laurentian margin the kinematic histories of the rockfish valley fault zone and related shear zones appear to be relatively simple and consistent with paleozoic west to northwest directed thrusting the rockfish valley fault zone does not separate distinctly different basement terranes and thus its displacement is thought to be small mylonitic rocks from this fault zone display crystal plastic deformation in quartz and cataclastic deformation in feldspars under lower to middle greenschist facies conditions solution transfer processes were an important deformation mechanism in these tectonites thrust related mylonitic rocks are overprinted by related cataclasite and semibrittle shear bands suggestive of a transition toward more brittle deformation as the thrust system cut higher in the crust the rockfish valley fault zone appears to predate the formation of the south mountain cleavage in the valley and ridge province but it may be part of a lengthy progressive thrust related deformation

pronounced central uplift identified in the bosumtwi impact structure ghana using multichannel seismic reflection data the lake bosumtwi impact structure is the youngest and best preserved complex terrestrial impact crater and serves as an important reference site for the study of cratering processes because the impacting body struck continental crystalline target rocks and not a submerged sedimentary platform no significant backwash processes have modified the crater morphology not only may bosumtwi contain the best preserved central uplift structure on earth but it is the most accessible relatively large young crater in the solar system generated in a large gravity field there is a well established link between the lake bosumtwi impact structure and the ivory coast tektite field and the lacustrine sediments within the crater contain a unique 1 m y record of paleoclimate in the continental tropics south of the sahel eight profiles of marine type multichannel seismic reflection mcs data were acquired from the 8 km diameter similar to75 m deep lake that fills much of the crater these were augmented by wide angle seismic data acquired with ocean bottom hydrophones mcs data reveal a well defined central uplift near the northwest central part of the lake and a maximum postimpact lacustrine sediment thickness of similar to310 m the central uplift structure has a diameter of 1 9 km and a maximum height of 130 m above the annular moat inside the crater an intermediate velocity layer 3200 m s beneath the lacustrine sediment is interpreted as fallback breccia or a breccia melt horizon the measured apparent depth of the crater d a is 500 m implying a slightly higher aspect ratio for the structure than predicted from published empirical relationships the bosumtwi structure is a small complex crater that deviates slightly from trends predicted from classical scaling laws perhaps because of the effects of a large gravity field

promontory promontory collision in the canadian appalachians in cape breton island and southwestern newfoundland the appalachian orogen is extremely narrow the rocks were strongly deformed and underwent high grade barrovian type metamorphism in silurian time and the orogen parallel silurian thrusts are dominantly west vergent antithetic to the westward subduction this area is bounded by two transverse dextral wrench faults the canso fault to the southwest and the gunflap hills fault to the northeast these tectono stratigraphic structural and metamorphic features are explained by silurian collision between the st lawrence promontory on the laurentian margin and the cabot promontory on the avalon margin during the final closure of the iapetus ocean the west vergent antithetic thrusting is interpreted to be related to tectonic wedging while the two transverse wrench faults accommodated differential movement from promontories to reentrants during the collision promontory promontory collision should be a common process and probably played a significant role in shaping the tectonic framework of many orogenic belts

comparative petrology of gneissic rocks in southwestern newfoundland in southwestern newfoundland pelitic migmatites of the meelpaeg subzone of the gander zone are separated by faults and plutons from metasedimentary rocks of the port aux basques gneiss complex pb

quaternary ostracodes and molluscs from the rukwa basin tanzania and their evolutionary and paleobiogeographic implications abstract much of the spectacular biodiversity of the african great lakes is endemic to single lake basins so that the margins of these basins or their lakes coincide with biogeographic boundaries longstanding debate surrounds the evolution of these endemic species the stability of bioprovinces and the exchange of faunas between them over geologic time as the rift developed because these debates are currently unsettled we are uncertain of how much existing distribution patterns are determined by modern hydrological barriers versus reflecting past history this study reports on late quaternary fossils from the rukwa basin and integrates geological and paleoecological data to explore faunal exchange between freshwater bioprovinces in particular with lake tanganyika lake rukwa s water level showed large fluctuations over the last 25 ky and for most of this period the lake contained large habitat diversity with different species assemblages and taphonomic controls along its northern and southern shores comparison of fossil and modern invertebrate assemblages suggests faunal persistence through the last glacial maximum but with an extirpation event that occurred in the last 5 ky some of the molluscs and ostracodes studied here are closely related to taxa or part of clades that are currently endemic to lake tanganyika but others testify to wider and perhaps older faunal exchanges between the rukwa bioprovince and those of lake malawi and the upper congo in particular lake mweru the rukwa basin has a long history of rifting and lacustrine conditions and at least temporarily its ecosystems appear to have functioned as satellites to lake tanganyika in which intralacustrine speciation occurred paleontological studies of the rukwa faunas are particularly relevant because of the basin s important role in the late cenozoic biogeography of tropical africa and because many of the molecular traces potentially revealing this history would have been erased in the late holocene extirpation

estimating the age of formation of lakes an example from lake tanganyika east african rift system age estimates for ancient lakes are important for determining their histories and their rates of biotic and tectonic evolution in the absence of dated core material from the lake s sedimentary basement several techniques have been used to generate such age estimates the most common of these herein called the reflection seismic radiocarbon method rsrm combines estimates of short term sediment accumulation rates derived from radiocarbon dated cores and depth to basement estimates derived from reflection seismic data at or near the same locality to estimate an age to basement age estimates form the rsrm suggest that the structural basins of central lake tanganyika began to form between 9 and 12 ma estimates for the northern and southern basins are younger 7 to 8 ma and 2 to 4 ma respectively the diachroneity of estimates for different segments of the lake is equivocal and may be due to erosional loss of record in the northern and southern structural basins or to progressive opening of the rift the rsrm age estimates for lake tanganyika are considerably younger than most prior estimates and clarify the extensional history of the western branch of the east african rift system 31 refs 3 figs 1 tab

tectonic setting and regional significance of the port aux basques gneiss sw newfoundland the port aux basques gneiss of southwest newfoundland comprises three paragneiss and schist assemblages the grand bay and port aux basques complexes and the harbour le cou group they are intruded by large volumes of basic sheets and coeval middle ordovician granitoids the margaree and kelby cove orthogneiss which also cut a tectonic slice of earlier ultrabasic to basic rocks within the grand bay complex the big barachois assemblage the grand bay and port aux basques complexes are juxtaposed by the late silurian isle aux morts fault zone against the harbour le cou group to the east which dominantly comprises sillimanite and garnet bearing pyritiferous metasediments with amphibolitic intrusions and meta pillow basalts in its lower stratigraphic units geochemical studies have shown that metabasites of the grand bay complex port aux basques complex margaree and kelby cove orthogneiss comprise fractionated and crustally contaminated morb like and within plate tholeiites suggestive of an ensialic okinawa type back arc basin these have comparable characteristics to metabasites in the harbour le cou group and suggest that these units comprise remnants of an extensive mid ordovician marginal basin developed within a promontory on the peri gondwanan continental margin in contrast the big barachois assemblage in part comprises a distinct suite of pre mid ordovician island arc tholeiites

the last glaciation of east central ellesmere island nunavut ice dynamics deglacial chronology and sea level change during the last glacial maximum of east central ellesmere island trunk glaciers inundated the landscape entering the smith sound ice stream accelerator mass spectrometry ams dates on individual shell fragments in till indicate that the ice advanced after 19 ka bp the geomorphic and sedimentary signatures left by the trunk glaciers indicate that the glaciers were polythermal the configuration and chronology of this ice is relevant to the reconstruction of ice core records from northwestern greenland the history of iceberg rafting of clastic sediments to northern baffin bay the reopening of the seaway between the arctic ocean and baffin bay and the regional variability of arctic paleoenvironments deglaciation began with the separation of ellesmere island and greenland ice at fiord mouths 8 8 5 ka bp ice reached fiord heads between 6 5 and 4 4 ka bp trunk glacier retreat from the fiords of east central ellesmere island occurred up to 3000 years later than in west coast fiords this later retrea

new insights into the appalachian orogen from geology and geochronology along the cape ray fault zone southwest newfoundland reverse oblique overthrusting of high grade port aux basques gneiss over the lower grade supracrustal windsor point group along the cape ray fault zone occurred during late silurian to early devonian time and is attributed to major oblique collision between gondwanan and laurentian continental blocks new u pb zircon 40 ar 39 ar and k ar data limit the timing of this orogenic event the megacrystic granitic phase of the cape ray igneous complex and the cape ray tonalite have been dated at 488 3 ma and 469 2 ma respectively a black rhyolite from the windsor point group yielded an ordovician age 453 5 4 ma a pretectonic gabbro sill within the windsor point group dated at 424 4 3 ma is cut by the windowglass hill granite dated at 424 2 ma synkinematic hornblende and biotite from the grand bay complex mylonites of the port aux basques gneiss have 40 ar 39 ar ages of 407 4 ma and 403 4 ma respectively which is indicative of a late silurian early devonian age for the amphibolite grade deformation these ages are compatible with a previously determined 415 ma monazite age interpreted as the time of peak metamorphism in the port aux basques gneiss and suggests rapid exhumation the east west flexure of the cape ray fault zone represents a tear fault accommodating differential shortening between the cape ray fault zone and the correlative fault system in central newfoundland mid devonian granites 386 3 384 2 ma intervene between the thrusting events and a younger sinistral transcurrent movement this deformation and magmatic history can be correlated throughout the northern appalachians along other major fault boundaries

ice keel seabed features in marine channels of the central canadian arctic archipelago evidence for former ice streams and iceberg scouring abstract the study area lies within the central part of the canadian arctic archipelago a region that was covered by the laurentide ice sheet during the late wisconsinan glaciation and earlier multibeam imagery from widely spaced transects indicates the presence of linear groove and ridge features on the seabed at several localities in peel sound franklin strait northern larsen sound and within m clintock channel these lineations resemble features in antarctica and in several formerly glaciated regions that have been interpreted to be sole marks emplaced beneath fast flowing ice streams based on these analogies a similar origin is inferred for the lineations on the channel floors within the study area the lineations are oriented parallel to the channel axes and margins they occur on all transects within the bathymetrically deeper area at the junction of franklin strait and peel sound northward in peel sound they occur extensively on the western and central transects and more locally on the eastern transect their north south orientation is normal to that of glacial flow features on somerset island and most of eastern prince of wales island which border peel sound to the east and west respectively the trend of the lineations is northeasterly parallel to the channel axis in franklin strait and mainly northerly in larsen sound and m clintock channel elsewhere the seabed imagery commonly displays scours of various sizes and orientations created by the keels of icebergs seabed sediments revealed by 3 5 khz sub bottom profiles are interpreted to consist primarily of ice contact sediments that in part are thinly mantled by draped water lain sediments the age of the lineations has not been established possibly their formation was coincident with an ice stream in the m clintock channel eastern victoria island region which formed an ice shelf in viscount melville sound that grounded on southern melville and byam martin islands at ca 10 4 9 6 14 c ka bp alternatively they could result from later glacial events

strain refraction in layered systems abstract strain refraction across competence contrasts is presented as a simple model consisting of two components a homogeneous strain and a heterogeneous simple shear for newtonian materials the ratio of the layer parallel simple shear component in adjacent layers is the inverse of their viscosity ratio strong changes in ellipsoid size shape and orientation are predicted across viscosity contrasts the geological implications of strain refraction theory are considered within the context of the cleavage strain debate the particular relationships of relative competence and strain revealed by the refraction model may contribute to the problem of why cleavages of different morphologies in rocks of different lithologies and kinematic histories should appear to be subparallel to the xy planes of measured strain ellipsoids competent rocks should develop dominantly layer orthogonal strain and incompetent layers shear dominated deformation a variety of structural features ranging from cleavage refraction changing lineation orientations folds transected by cleavage changes from coaxial to non coaxial deformation and ramp flat fault geometry may be the result of stress and strain refraction in rocks

uplift of africa as a potential cause for neogene intensification of the benguela upwelling system the benguela current cooled over the past 12 million years numerical modelling suggests that uplift of parts of africa during this time enhanced coastal low level winds and promoted greater upwelling of cool subsurface waters

experimental insights into the partitioning of motion within zones of oblique subduction abstract in zones of oblique convergence the motion is often partitioned into subduction and an associated strike slip component fitch 1972 in this paper we first describe the geodynamic setting of the philippine region as a case study of deformation by strike slip faulting behind parallel subduction zones then we present six small scale laboratory experiments with analogue materials to investigate how partitioning may be controlled by internal boundaries between oceanic lithosphere and continental or island arc lithosphere during the early stages of a typical experiment deformation is taken up on oblique thrusts subduction zones which root into internal boundaries paired strike slip faults develop at later stages thereafter subduction zones and paired strike slip faults operate simultaneously the partitioning occurs without any need for complex constraints on external boundaries

the partitioning of deformation in flowing rock masses abstract most geological structures owe their development to heterogeneous and or non steady flow flow partitioning refers to the division of the instantaneous velocity field into components related to translation strain and rotation flow partitioning during the development of a geological structure usually changes with time and different histories of flow are involved at different points in the developing structure heterogeneity leads to the development of flow domains with different characteristics notably the degree of non coaxiality of deformation and different partitionings of the rotational component of flow into shear induced vorticity and spin during non steady flow there is often continual repartitioning of vorticity between the spin and shear induced components spin is an important aspect of flow leading to the development of structure this paper discusses the partitioning of deformation in a flowing rock mass coaxial flow tends to occur in isotropic materials in regions near weakly constrained boundaries non coaxial flow is favoured by suitably oriented material anisotropy and or the presence of well defined boundary constraints involving parallel relative displacements a developing anisotropy e g a crystallographic fabric in a shear zone can induce a change from coaxial to non coaxial flow other effects are related to the scale of approximation for example bulk shear induced vorticity can be locally converted to spin flow in the crust and mantle of the earth produces geological structures on all scales and involves many different types of rheological response the factors that affect flow partitioning e g spin versus non coaxial deformation are therefore of interest

interference between pluton expansion and non coaxial tectonic deformation three dimensional computer model and field implications abstract structures in orogenic belts are greatly controlled by interactions between pluton related and tectonic related strain fields however many aspects of the geometry and evolution of these structures are poorly known for example 1 what is the geometry of these structures if regional deformation is coaxial 2 how does this geometry change with progressive deformation this paper describes a three dimensional simulation of strain shape and magnitude as well as foliation and lineation patterns formed by interactions between pluton expansion and homogeneous coaxial deformation this simulation suggests that interactions between pluton expansion and regional uniaxial shortening generate patterns that are distinct from the patterns produced by regional uniaxial extension or regional non coaxial deformation for example shortening produces doughnut shaped constrictional rings about the pluton whereas extension produces apple shaped flattening regions around the pluton in addition shortening produces one continuous doughnut shaped low strain region around the pluton on the other hand extension creates two separate ellipsoidal low strain regions at the ends of the pluton furthermore shortening forms foliation rings that have triangular two dimensional sections and lineation is concentric around the axis of maximum shortening in contrast extension produces two separate foliation and lineation cones at the ends of the pluton all these structural and strain elements migrate away from the pluton with increasing expansion rates and towards the pluton with increasing tectonic deformation rates although structural studies around plutons should always integrate microstructural radiometric and field measurements simulation generated patterns can be used to evaluate 1 origin geometry and evolution of structures around plutons 2 mechanisms that explain inclusion trails within porphyroblasts 3 contribution of pluton related strains to strains measured in the country rock and 4 type and three dimensional orientation of regional kinematic schemes

linking watershed disturbance with nearshore sedimentation and the shell beds of lake tanganyika mahale mountains tanzania extensive deposits of shell rich sediments in the nearshore environment of lake tanganyika africa form a unique habitat for a diverse group of endemic gastropods crustaceans fish and sponges among other organisms anthropogenic alteration of the lake s hinterland from deforestation fires and agriculture threaten this crucial habitat through sediment pollution of the littoral environment this study examines the sedimentology of these nearshore shell beds along a study area north of the mahale mountains tanzania to test whether varying levels of watershed disturbance determined by analysis of satellite imagery within three moderately sized watersheds 100 km2 results in sedimentological differences offshore the results show that shell beds adjacent to disturbed watersheds lagosa and rukoma contain more fine grained sediment compared to shell beds offshore of the undisturbed watershed katumbi widespread burning clearance and land cultivation and rains of the wet season are causal factors that result in significant differences in fine grained sedimentation larger sediment plumes offshore and increased shoreline progradation in areas most affected by sedimentation sponges are largely absent and shell coverage which is a measure of fish nesting capabilities is lower the discovery of a small population of live neothauma tanganyicense offshore of the katumbi river suggests that the presence of shell bed forming gastropods in front of a relatively low disturbance watershed and their conspicuous absence offshore of the other two watersheds could be related to recent watershed disturbance and increased sedimentation the increased fine grained sedimentation and limited shell production by the reduced modern gastropod populations may signal that the shell bed ecosystem of lake tanganyika is at risk

the geology and structural history of the cape ray fault zone in southwestern newfoundland structural studies within the cape ray fault zone southwestern newfoundland indicate that three tectonic events d1 to d3 have deformed all regional lithologic units these lithologies are distributed as a tonalitic terrain intrusive into ophiolitic remnants to the northwest a central volcanic sedimentary terrain and a staurolite kyanite gneiss terrain to the southeast d1 produced the dominant northeasterly striking planar fabrics in the host rocks that are axial planar to isoclinal folds the fold axes parallel a synkinematic mineral elongation lineation d2 formed open asymmetrical folds with curvilinear fold axes and local crenulation cleavage the d3 event produced local conjugate kink folds radiometric dates on deformed plutons indicate these deformational events were post 369 12 ma post middle devonian and pre 352 6 ma pre late carboniferous the fault was previously interpreted as a cryptic suture along which opposed margins of the iapetus ocean were juxtaposed supposedly this sut

pluton emplacement along an active ductile thrust zone piute mountains southeastern california interaction between deformational and solidification processes plutons that are emplaced synchronously with regional deformation offer the opportunity to understand the partitioning of deformation and metamorphism in time and space during orogeny the 85 ma east piute pluton of the eastern mojave desert is an unusually clear example of a syntectonic pluton it was emplaced during thrusting in the southern cordillera combined petrologic and structutal studies lead to empirical models for the dynamic interaction of magmatic and deformational processes during granitoid emplacement and final crystallization trace and major element and isotopic data suggest that closed system differentiation of a single magma batch was responsible for compositional variation granodiorite to leucocratic granite correlation between degree of chemical evolution of compositional units and intrusive sequence suggests that discrete melt segregation events punctuated fractional crystallization modeling of trace element data suggests segregation of monzogranite melts after 50 70 crystallization of the magma and segregation of leucocratic granite after about 80 crystallization deformation promoted segregation of melt fractions at discrete times during crystallization during final ascent and after the magma had crystallized sufficiently 50 70 to attain shear strength monzogranite liquid was incompletely segregated along the active thrust zone at the southern margin of the pluton at essentially the present crustal level and after about 80 crystallization the remaining leucocratic granite melts were more effectively segregated and migrated into thrust related extensional openings including tension gashes undeformed pegmatite and aplite dike arrays indicate that final crystallization coincided with final increments of shear zone movement solid state deformational fabrics and brittle synthetic thrusts that offset the pluton margin record sharp temperature decline during and after crystallization because plutons crystallize rapidly they can be used to estimate strain rates during orogeny thermal modeling suggests that the transition from rheological liquid to solid in the east piute pluton occurred during an interval of about 10 4 yr as final increments of ductile deformation coincided with final crystallization of evolved compositional units total strain recorded by fabric in the pluton apparently accumulated during this interval shear strains estimated from the s c fabric in the granodiorite unit and rotation of pegmatite filled vein arrays are on the order of 1 thus strain rates in the shear zone were on the order of 10 12 sec during crystallization of the pluton this corresponds to displacements of tens of centimeters per year these high strain rates suggest punctuated melt enhanced movement episodes during contractional orogeny

lake level and paleoenvironmental history of lake tanganyika africa as inferred from late holocene and modern stromatolites fossil and living stromatolites are abundant around the margins of lake tanganyika africa and provide a wealth of paleolimnologic and paleoclimatic information for the late holocene six lines of evidence show that stromatolites and cements are precipitating in the lake today 1 carbonate saturation state calculations 2 documentation of living stromatolites and their depth distribution 3 new stable isotope data showing the lake s present mixing state and ancient evaporation and inflow balance 4 new radiocarbon data and a reevaluation of apparent 14 c ages derived from lake tanganyika carbonates 5 the presence of modern mg calcite cements derived from lake waters and 6 the presence of modern biologically mediated mg calcite precipitates in the lake lake tanganyika s lake levels have been remarkably stable over the past 2800 yr fluctuating around the marginally open to marginally closed level through most of this time period lake lowstands and high 18 o values from the ninth century b c to the early fifth century a d indicate that the lake basin was comparatively dry during this time however the period prior to the most recent opening of lake kivu into the lake tanganyika basin ca a d 550 was not marked by major lake lowstands nor was this opening accompanied by a dramatic lakelevel rise the kivu opening was roughly coincident with a significant shift toward isotopically lighter 18 o and 13 c lake water which persists today the lake remained close to its outlet level between the sixth and thirteenth centuries a d lake levels rose between the fourteenth and sixteenth centuries at some time between the late sixteenth and early nineteenth centuries lake level fell to perhaps its lowest level in the past 2800 yr by the early nineteenth century lake level had begun to rise to the overflow level apparently the result of a regional increase in precipitation evaporation ratios weak 18 o 13 c covariance for late holocene carbonates suggests that the surface elevation of the lake has remained near the outlet level with only occasional periods of closure however there is no simple relationship between solute input from lake kivu isotope input from lake kivu and lake levels in lake tanganyika lake kivu waters are the primary source of major ions in lake tanganyika but are much less important in controlling the 18 o and the lake level of lake tanganyika because the ruzizi river s discharge into lake tanganyika is largely derived from sources other than lake kivu the overflow events in the two lakes have been uncoupled during the late holocene

depth extent and kinematics of faulting in the southern tanganyika rift africa unusually deep earthquakes occur beneath rift segments with and without surface expressions of magmatism in the east african rift system ears the tanganyika rift is part of the western rift and has no surface evidence of magmatism the tang14 array was deployed in the southern tanganyika rift where earthquakes of magnitude up to 7 4 have occurred to probe crust and upper mantle structure and evaluate fault kinematics 474 earthquakes detected between june 2014 and september 2015 are located using a new regional velocity model the precise locations magnitudes and source mechanisms of local and teleseismic earthquakes are used to determine seismogenic layer thickness delineate active faults evaluate regional extension direction and evaluate kinematics of border faults the active faults span more than 350 km with deep normal faults transecting the thick bangweulu craton indicating a wide plate boundary zone the seismogenic layer thickness is 42 km spanning the entire crust beneath the rift basins and their uplifted flanks earthquakes in the upper mantle are also detected deep earthquakes with steep nodal planes occur along subsurface projections of tanganyika and rukwa border faults indicating that large offset 5 km faults penetrate to the base of the crust and are the current locus of strain the focal mechanisms continuous depth distribution and correlation with mapped structures indicate that steep deep border faults maintain a half graben morphology over at least 12 my of basin evolution fault scaling based on our results suggests m 7 earthquakes along tanganyika border faults are possible

seepage rate of hydrothermally generated petroleum in east african rift lakes an example from lake tanganyika acknowledgements we thank michael king and cgg npa satellite mapping for providing us with the gosd dataset of tanzania and the permission to publish the sar images of cape kalumba oil seeps funding this research did not receive any specific grant from funding agencies in the public commercial or not for profit sectors

postglacial emergence in the canadian high arctic integrating glacioisostasy eustasy and late deglaciation throughout the last glaciation the innuitian sea rather than glaciers occupied many fiords and channels of the queen elizabeth islands two alternative hypotheses which constitute end members are presented to account for the transgression of the innuitian sea between 18 and 8 8 ka at which time it reached marine limit hypothesis a proposes that the last ice load was fully established by 18 ka and that it remained stable while sea level rose eustatically from approximately 60 m asl to marine limit by 8 8 ka hypothesis b proposes an advance of glaciers from present day positions after 14 ka when increased precipitation allowed rapid glacial loading to accompany the eustatic sea level rise by the early holocene when glaciers stood at the last ice limit evidence suggests maximum warming and a shift to a negative mass balance it is now recognized that the stable relative sea level at marine limit must record the balance between the rate of eustatic rise and the rate of uplift due to glacial unloadi

the chaparral shear zone deformation partitioning and heterogeneous bulk crustal shortening during proterozoic orogeny in central arizona the proterozoic chaparral shear zone of central arizona is one of a network of subvertical northeast striking shear zones that divide the proterozoic orogenic belt of arizona into tectonic blocks the zone is several kilometers wide and contains variably developed mylonitic foliation that is subparallel to the regional subvertical foliation stretching lineations plunge shallowly northeast and displacement across the zone was dominantly right lateral several lines of evidence constrain displacement across the zone to be greater than 5 km but probably less than tens of kilometers 1 stratigraphic and plutonic rocks can be correlated across the zone 2 structural and metamorphic histories of tectonic blocks on opposite sides of the zone are similar and 3 integration of shear strain estimated by deflection of earlier fabric suggests greater than 5 km of strike slip more important than scale of movement across the zone structural studies have clarified several aspects of the assembly history of the orogen early north to northwest striking s 1 foliation on both sides of the chaparral shear zone records one or more deformational events that took place between 1 75 and 1 7 ga early fabrics record northeast southwest or east west shortening and west verging thrusting that we interpret to have formed during development of a primitive arc complex in contrast intense deformation in the chaparral shear zone took place during regional northwest southeast shortening d 2 that produced the dominant subvertical northeast striking foliation and the present block architecture of the orogen d 2 is interpreted to have been in part synchronous with emplacement of 1 70 ga granitoids and to record assembly of volcanic belts to north america of general interest is the character of d 2 deformation partitioning in and across the shear zone two types of high strain domains were generated during shortening one type represented by rocks southeast of the shear zone involved extreme shortening and transposition by folding the other type represented by the shear zone involved simple shear deformation in a complicated array of anastomosing and conjugate shear zones overprinting relationships suggest that the second type may have nucleated on the first implying an important component of deformation partitioning in time as well as space mylonitic fabric in the shear zone developed by progressive heterogeneous simple shear followed by brittle fracturing conjugate shear bands suggest a shallow southeast plunging 1 late during strike slip deformation this is consistent with regional d 2 shortening but not with the incremental shortening direction expected during right lateral simple shear this supports a regional kinematic model in which the chaparral shear zone right lateral and a temporally related left lateral shear zone were regional scale conjugate shear zones that accommodated heterogeneous d 2 northwest southeast orogenic shortening via escape block tectonics thus orogen parallel strike slip displacements in the chaparral shear zone were a response to partitioned shortening in the late stages of accretion rather than transpressional accretion during oblique subduction

palaeoceanography tectonically driven upwelling upwelling within the highly productive benguela current off the namibian coast began in and intensified throughout the neogene epoch model simulations indicate its development was intimately connected to evolving topography and mountain uplift in africa