crustal structure of the northern nova scotia rifted continental margin eastern canada 1 the nova scotia continental margin off eastern canada marks a transition from a volcanic to a nonvolcanic style of rifting the northern nonvolcanic segment of the margin was studied by a 490 km long refraction seismic line with dense air gun shots coincident with previous deep reflection profiles a p wave velocity model was developed from forward and inverse modeling of the wide angle data from 19 ocean bottom seismometers and coincident normal incidence reflection profiles the continental crust has a maximum thickness of 36 km and is divided into three layers with velocities of 5 7 6 9 km s crustal thinning down to 3 km occurs in a 180 km wide zone and the sediment cover in this area is up to 15 km thick farther seaward a 150 km wide transition zone is observed with a 5 km thick lower layer 7 2 7 6 km s interpreted as partially serpentinized mantle at the landward end this layer is overlain by highly altered continental crust 5 4 km s extending up to the seaward limit of the jurassic salt province farther seaward the upper layer is interpreted as exhumed and highly serpentinized mantle 5 1 km s separated from the lower layer by subhorizontal reflectivity which probably represents a serpentinization front oceanic crustal thickness is 4 km with layer 2 velocities of 4 6 5 0 km s layer 3 velocities of 6 4 6 55 km s are lower than typical lower oceanic crust velocities but consistent with a low magma supply and increased tectonism as observed on the reflection profile this reduced magma production might be related to the proximity of the newfoundland transform margin

processes of forearc and accretionary complex formation during arc continent collision in the southern ural mountains the southern ural mountains of russia contain a well preserved well exposed paleozoic accretionary wedge and forearc that can be readily compared to those in active arc continent collision zones the early convergent history in the southern ural mountains is marked by the generation of boninite bearing arc tholeiites in the magnitogorsk forearc followed by arc tholeiite to calc alkaline volcanism with the entry of the east european craton continental crust in the subduction zone volcanism waned and stopped and high pressure metamorphism of its leading edge took place the arrival of the full thickness of the continental crust at the subduction zone is marked by increased sedimentation in the forearc basin and deposition of arc derived volcaniclastic turbidites across the subducting slab these together with offscraped continental material the exhumed high pressure rocks and a lherzolite massif formed an accretionary wedge a broad melange zone containing ultramafic fragments separates the forearc basement from the accretionary wedge and marks the damage zone that developed along the backstop region shallow water carbonates deposited unconformably on top of the mildly deformed arc record the end of the collision and the collapse of the arc

structural architecture of the southern and middle urals foreland from reflection seismic profiles structural architecture of the southern and middle urals foreland from reflection seismic profiles

nature of the continent ocean transition on the non volcanic rifted margin of the central great australian bight a region of 50 120km width defines the continent ocean transition cot in the central great australian bight it is characterized by a thin apron of post break up sediments overlying complexly deformed sediments and intruded crust bounded landward by a basement ridge complex and oceanward by rough oceanic basement recently acquired deep reflection and refraction seismic data have significantly enhanced understanding of the cot and basement ridge modelled gravity and magnetic data and features interpreted from seismic data are consistent with aspects of extensional and break up models proposed for the west iberia margin many of the features and relationships observed beneath the outer margin of the central great australian bight can be explained by extension within a lithosphere scale pure shear environment involving four layers brittle upper crust and upper mantle and ductile lower crust and lower lithospheric mantle the cot is interpreted to be underlain by extended continental lithosphere thus the continentocean boundary is unequivocally defined between oceanic crust and the cot and appears to be associated with sea floor spreading magnetic anomaly 33 indicating that break up and sea floor spreading did not commence until c 83 ma early carnpanian time later than the currently accepted 95ma age the major part of the basement ridge complex is probably a combination of serpentinized peridotites and mafic intrusions or extrusions derived by mantle upwelling and limited partial melting the magmatic products of this process probably cooled during chron 34 producing a distinctive magnetic anomaly but one that does not relate to break up and sea floor spreading

modal mineralogy and chemical composition of the uralide lower crust determined from physical properties data abstract modeling of rock physical properties of the uralide lower crust is carried out using the physprops excel worksheet which calculates its p wave vp s wave vs poisson s ratio and density the mineral modal abundance and composition of mafic granulite mafic garnet granulite amphibolite and gabbronorite is modeled and the composition of each is determined by converting the aggregate mineralogy into wt oxides the modeled wt oxide composition of mafic garnet granulite fit well with those estimated for post archean terrains and all are within 1 of the average of all lower crustal xenoliths the modeled gabbronorite also shows a reasonable fit with these estimates the modeled amphibolite compositions show variations in the wt sio2 feo and mgo with respect to the estimates of mafic granulite and amphibolite the differences in the composition across the uralide lower crust are largely due to the presence or absence of garnet in the mafic granulite which may be a result of higher temperatures having been reached in the interior part of the orogen

relict subduction zones in canada 1 relict subduction zones that are interpreted on 10 lithoprobe deep seismic reflection profiles in canada provide key observational evidence that lithosphere plate interactions have operated with essentially the same resultant geometry for at least 2 7 gyr subduction zone reflections are generally found beneath known collisional belts and their orientations are often consistent with the locations of exposed arcs sutures and subduction complexes they project from the lower crust into the upper mantle and provide key geometric relationships between the crust moho and upper mantle the interpretation of subduction zone reflections as being syntectonic establishes a minimum age for the overlying lower crust and mantle wedge and precludes inferred formation of the moho and upper mantle by postorogenic delamination in these areas the orientation of subduction zone reflections may be synthetic or antithetic to exposed sutures indicating juxtaposition by subcretion or by delamination respectively the mantle wedge above the subduction zone reflections may exhibit sparse reflectivity subduction zone reflections commonly dip toward the craton indicating a preferred dip of subduction during the last stages of convergence within an orogen or else a higher likelihood of preservation beneath older domains interpreted ages of the canadian examples range from recent to neoarchean 2 7 ga providing observational evidence that plate tectonics or similar processes have been active since at least the neoarchean the locations and geometries of these relict subduction zones provide a snapshot of accretion boundaries in the mantle and establish a framework for the neoarchean recent accretionary architecture of the north american lithosphere

arc continent collisions sediment recycling and the maintenance of the continental crust author posting geological society of london 2009 this is the author s version of the work it is posted here by permission of geological society of london for personal use not for redistribution the definitive version was published in geological society london special publications 318 2009 75 103 doi 10 1144 sp318 3

geochemical evolution of arc magmatism during arc continent collision south mayo ireland the lough nafooey arc in the western irish caledonides collided with laurentia during the early ordovician this event is recorded in the stratigraphy of the south mayo trough the preserved forearc basin of this system lavas at the base of the oldest lough nafooey group show intraoceanic arc composition end t decreases and light rare earth elements rees become more enriched upsection in the lough nafooey group ca 495 481 ma reflecting early collision with laurentia the subsequent tourmakeady group ca 481 470 ma is rhyolitic light ree enriched and has strongly negative end t these rocks were erupted during the grampian orogeny the rosroe and mweelrea formations 470 ma show wide scatter of la sm and nb zr values suggesting mixed mantle sources this correlates with rapid exhumation of the adjacent connemara metamorphic terrane the chemical evolution of the arc supports models of collision orogeny and tectonic unroofing within 15 m y and shows that genesis of magmas more enriched than continental crust can occur during arc continent collision clarifying the potential role of arc volcanism in continental crust formation

precise eclogitization ages deduced from rb sr mineral systematics the maksyutov complex southern urals russia abstract the maksyutov complex southern urals russia is a well preserved example of subduction related high pressure metamorphism one of its two litho tectonic units consists of rocks that experienced eclogite facies conditions published 40ar 39ar data on phengite u pb data on rutile and sm nd mineral data define a cluster of ages around 370 to 380 ma nevertheless no consensus exists as to the detailed interpretation of data and the exact age of eclogitization we present new high precision internal mineral rb sr isochrons for eclogite facies metabasites felsic eclogites and eclogite facies quartz veins nine isochrons mainly controlled by omphacite and white mica phases give concordant ages with an average value of 375 2 ma 2 microtextural features such as prograde growth zoning in eclogite facies phases suggest that the assemblages dated formed at a stage of prograde metamorphism sr isotopic equilibria among eclogite facies phases and among eclogite facies fluid veins and the host rocks indicate that our ages reflect crystallization ages related to the prograde metamorphic probably fluid mediated eclogitization reactions this interpretation is reinforced by data from fluid precipitated quartzitic eclogites whose modal composition together with intergrowth relationships conclusively imply closed system behavior after crystallization the possible occurrence of a pre 375 ma event of ultra high pressure metamorphism uhpm in the maksyutov complex is disproved by isotope systematics microtextures and mineral zoning patterns

the ocean continent boundary off the western continental margin of iberia crustal structure west of galicia bank a seismic refraction transect across the galicia bank continental margin shows that the original continental crust thins westward from 17 to 2 km immediately east of a margin parallel peridotite ridge pr immediately west of the pr oceanic crust is only 2 5 3 5 km thick but farther west oceanward it thickens to 7 km the pr caps a similar to 60 km wide lens shaped serpentinized peridotite body underlying both thinned continental and thin oceanic crust when superimposed on a reflection time version of the velocity model the s reflector is clearly intracrustal at its east end westward s cuts down to lower crustal levels eventually coinciding with the top of the serpentinized peridotite lens original crust mantle boundary these observations render almost impossible the seafloor exposure of the pr by s acting as a top to the west detachment fault numerical models of melting and borehole subsidence information constrain our rifting model the easternmost continental crust experienced a total stretching factor of 4 3 most likely in two stages it probably occurred over similar to 25 m y with the highest fate of stretching at the beginning of the main earlier rift phase valanginian 141 135 ma the 3 4 7 km thick continental crust depending on whether serpentinized peridotite is assigned to crust or mantle which may include melt products requires stretching factors 6 more than 11 7 and a rift duration of more than 25 13 m y the thin oceanic crust immediately west of the pr is explained by conductive cooling of the mantle during the long prebreakup stretching phase which temporarily caused reduced melting immediately after breakup

a cross section through the zilair nappe southern urals a structural transect in the zilair kugarchi area involves the western part of the suvanyak complex the zilair nappe and the eastern part of the foreland thrust and fold belt this section has been analyzed using field microstructural and seismic data the cross section shows the transition from the hinterland to the foreland in the footwall to the suture of the southern urals the rocks involved range from early palaeozoic to permian in age a characteristic of the zilair nappe is the dominance of a succession of volcanic greywackes and mudrocks of late devonian age zilair formation the metamorphic grade decreases from east to west from greenschist facies to diagenetic conditions the structure of the cross section mainly comprises west directed thrusts and thrust related folds with an associated cleavage fold vergence changes along the section depending on of the distance to the associated thrust and its geometry the zilair thrust which separates the zilair nappe from the foreland thrust and fold belt accommodated ca 10 km displacement and the characteristics of the deformation are similar on both sides of it the contact between the zilair nappe and suvanyak complex is a west dipping normal fault that does not represent a major tectonic boundary

slab like high velocity anomaly in the uppermost mantle beneath the dabie sulu orogen seismic tomography is used to determine three dimensional velocity structure of the dabie sulu orogen in eastern china the high resolution image of the p waves velocity structure is obtained for the uppermost mantle to a depth of at least 150 km along the latitudes of 30 38 n and longitudes of 114 122 e results of this inversion reveal a slab like high velocity anomaly under this orogen in the depth from moho discontinuity to at least 110 km it may represent a remnant of the subducted yangtze block during the period of triassic continent continent collision

mantle structure under gibraltar constrained by dispersion of body waves 1 westudytheafrica iberiaplateboundaryinthevicinity of gibraltar numerous models have been proposed for that region throughout the last decades proposing mechanisms that range widely from continental delamination convective removal to subduction of oceanic lithosphere to better constrain upper mantle structure under the region we study waveforms of p waves that traverse the alboran sea region between spain and morocco these show dispersive behavior which together with early arrival times confirms the presence of an anomalous upper mantle structure under the alboran sea the dispersion is consistent with that expected from subducted lithosphere waveforms of body waves therefore provide a way to better constrain the elusive mantle structure and dynamics of the alboran sea region citation bokelmann g and e maufroy 2007 mantle structure under gibraltar constrained by dispersion of body waves geophys res lett 34 l22305 doi 10 1029 2007gl030964

seismic wide angle constraints on the crust of the southern urals a wide angle seismic reflection refraction data set was acquired during spring 1995 across the southern urals to characterize the lithosphere beneath this paleozoic orogen the wide angle reflectivity features a strong frequency dependence while the lower crustal reflectivity is in the range of 6 15 hz the pmp is characterized by frequencies below 6 hz after detailed frequency filtering the seismic phases constrain a new average p wave velocity crustal model that consists of an upper layer of 5 0 6 0 km s which correlates with the surface geology 5 7 km depths at which the velocities increase to 6 2 6 3 km s 10 30 km depths at which on average the crust is characterized by velocities of 6 6 km s and finally the lower crust from 30 35 km down to the moho which has velocities ranging from 6 8 to 7 4 km s two different s wave velocity models one for the n s and one for the e w were derived from the analysis of the horizontal component recordings crustal sections of poisson s ratio and anisotropy were calculated from the velocity models the poisson s ratio increases in the lower crust at both sides of the root zone a localized 2 3 anisotropy zone is imaged within the lower crust beneath the terranes east of the root this feature is supported by time differences in the sms phase and by the particle motion diagrams which reveal two polarized directions of motion velocities are higher in the central part of the orogen than for the siberian and eastern plates these seismic recordings support a 50 56 km crustal thickness beneath the central part of the orogen in contrast to moho depths of 45 km documented at the edges of the transect the lateral variation of the pmp phase in frequency content and in waveform can be taken as evidence of different genetic origins of the moho in the southern urals

geometric and kinematic evolution of the foreland thrust and fold belt in the southern urals the first balanced and restored cross section through the foreland thrust and fold belt of the southern urals along the belaya river of bashkortostan is presented this cross section is used to discuss the paleozoic structure and kinematic evolution of the area and to provide quantitative estimates of fault displacement and shortening the belaya river thrust stack can be divided into a late imbricate thrust system nygush thrust system the burzyan thrust sheet and an early duplex system timirovo thrust system overprinting relationships indicate two deformation events during which the foreland thrust and fold belt evolved by a forward propagating thrust sequence the first deformation event involved emplacement of the low metamorphic grade timirovo thrust system and the overlying zilair nappe during accretion of the magnitogorsk volcanic arc with the east european craton at the end of the devonian a deformational hiatus followed until the end of the carboniferous at which time a second deformation event related to final closure of the uralian ocean resulted in development of the nygush thrust system the calculated shortening in the belaya river area 16 7 km or 17 is significantly less than that in other foreland thrust and fold belts the basal thrust in the belaya river section is located within the basement and cuts continuously up section with a staircase like geometry instead of following an easy glide horizon

seismic reflections from the mantle represent relict subduction zones within the continental lithosphere in many areas of the world the continental lithospheric upper mantle contains bright continuous regionally extensive seismic reflectors despite the increasingly common observation of such mantle reflectors on deep seismic reflection profiles their significance and geologic origin remain obscure we report results from a series of seismic experiments acquired across two of the brightest of these reflectors and provide new constraints upon the composition and thickness of the reflecting region these new seismic data reveal regionally extensive dipping and subhorizonal slabs of high velocity 8 4 km s high density 3500 kg m 3 material several kilometres 2 km thick entrained within otherwise unremarkable upper mantle the geometry physical properties and geologic setting of these mantle reflectors suggest that they represent fragments of eclogitic oceanic crust a relict of pre caledonian oceanic subduction now preserved within the lower continental lithosphere such relict subduction zones appear to be widespread within the continental lithosphere and to exert an important influence upon the location and style of subsequent continental deformation

mapping the moho beneath the southern urals with wide angle reflections a stack of the wide angle reflection refraction component of the urseis 95 experiment provides the first well resolved imaged of the moho beneath the southern urals the processing consisted of low pass filter 0 6 hz cmp sorting and a nmo correction without stretch the pmp phase a very narrow band and low frequency up to 6 hz wavelet changes character from west to east along the transect in the depth converted section the moho reaches a maximum depth of 53 2 km beneath the magnitogorsk arc thickness estimates determined from high amplitudes at near critical distances also support a 53 km thick crust a selective offset stack consisting of traces at 150 250 km offset indicate an undulating irregular moho suggesting either strong lateral velocity variations or high topographic relief beneath the magnitogorsk arc

the crustal structure across the transform continental margin off ghana eastern equatorial atlantic forward modeling of a suite of wide angle seismic lines across the transform continental margin at the eastern end of the romanche fracture zone off ghana has shown the transition from continental to oceanic crust to be confined to a narrow 6 to 11 km wide zone located at the foot of the steep continental slope the structure of the adjacent oceanic and continental crusts has also been resolved these results are confirmed and enhanced by gravity and magnetic models the crust of the ocean continent transition zone is characterized by high velocities 5 8 7 3 km s 1 a high density 3 10 mg m 3 and high magnetizations 1 1 1 25 a m 1 this rules out a purely continental origin for the zone and suggests that it may be formed of basic igneous rocks intruded when the hot oceanic spreading center migrated along the margin the oceanic crust within 70 km of the margin is abnormally thin compared to normal atlantic oceanic crust and shows an average thickness of just 4 4 km we suggest that the region of abnormally thin oceanic crust is the result of a reduced magma supply due to a combination of closely spaced fracture zones and the conductive loss of heat from the upwelling oceanic mantle in small basins surrounded on at least three sides by cold continental lithosphere the preferred seismic model also shows a new fracture zone 30 km southeast of the ocean continent transition which is characterized by low velocities in the upper crust a lens shaped layer with velocities of 7 2 7 4 km s 1 at the base of the crust and a crustal thickness of just 3 4 km the continental crust appears largely unaffected by the proximity of the adjacent oceanic crust there is no evidence for underplating of the continental crust adjacent to the ocean continent transition zone

modeling the evolution of continental subduction processes in the pamir hindu kush region several geological and geophysical studies suggest the presence of two converging subduction zones in the western syntaxis of the india eurasia collision zone with steep northward subduction of indian lithosphere beneath the hindu kush and southward subduction of asian lithosphere under the pamir we investigate the geometry and the timing of these subduction processes seismic tomography images are used to constrain the geometry of the two slabs under the pamir and the hindu kush by measuring the tomographically inferred length of the indian slab under the hindu kush region and by comparison with paleomagnetic reconstructions we estimate that the process of indian slab break off most likely occurred at the early stage of the collision at ca 44 48 ma we infer that after slab break off western india continued its northward motion until its northern boundary reached the present hindu kush region where it began to subduct we estimate an age for the initiation of subduction in this region of about 8 ma we apply 2d thermo kinematic and rheological numerical modeling to compute the temperature distribution and brittle field along two vertical sections across the hindu kush and pamir regions for the hindu kush we introduce parameters reproducing fast and near vertical subduction whereas in the pamir region we use the results from published studies to reproduce slower subduction modeling results indicate that faster subduction under the hindu kush results in a deeper brittle region which is consistent with observed deeper seismicity than in the pamir region

basement influence on foreland thrust and fold belt development an example from the southern urals abstract the reactivation of two sets of pre existing basement faults provided strong control on the geometric and kinematic evolution of the paleozoic south urals foreland thrust and fold belt pre existing basement faults oriented subparallel to the uralide structural grain were related to several stages of precambrian deformation reactivation focused the deformation along these earlier faults resulting in the subsequent incorporation of crystalline basement thrust sheets in the thrust belt at an early stage of its evolution pre existing basement faults oriented subperpendicular to the developing thrust belt structural grain were related to aulacogens in the archean crystalline basement that were filled by variable thickness of riphean aged sediments these influenced the localisation and development of numerous structures in the thrust belt that caused along strike structural changes related to lateral ramps recognition of these features and their influence on the geometric and kinematic evolution of the south urals foreland thrust and fold belt are important for understanding seismic reflection fabrics in the south urals thrust belt and for potential field 3 d geological and kinematic modelling

precambrian moho offset and tectonic stability of the east european platform from the urseis deep seismic profile deep seismic reflection data from the 1995 urals seismic experiment and integrated studies urseis transect of the southern urals document a distinct 5 km vertical offset of the moho beneath the uralian foreland here named the makarovo fault zone this offset does not disrupt the overlying riphean sedimentary section nor does it correlate with any major surface lineaments or known phanerozoic subduction features it is thus interpreted to be an archean or early proterozoic fault zone possibly a terrane boundary that disrupts an even older moho most important is the implication that this structure and the moho it offsets were not significantly modified by two subsequent cycles of continental collision that affected this region the baikalian 700 570 ma and the late paleozoic uralian 350 230 ma orogenic phases similarly the pronounced reflectivity of the moho beneath the uralian foreland must also be a precambrian signature not a young feature as would be implied by certain recent thermo rheological and reflectivity models that associate a reflective lower crust and moho with areas that underwent pronounced synorogenic or postorogenic collapse the makarovo fault zone provides yet another demonstration that relicts of ancient tectonic processes can survive for long periods of time in the lower crust and that the moho can be a key marker horizon in delineating the tectonic evolution of the lithosphere

a model for continental crust genesis by arc accretion rare earth element evidence from the irish caledonides abstract the formation of continental crust is a complex problem with a paradox at its center continental material is believed to form by arc magmatism a model that does not reconcile the bulk mafic and light rare earth element lree depleted composition of intra oceanic arcs with the andesitic lree enriched composition of continents we present evidence supporting an arc origin for continental crust by demonstrating significant changes in magmatic chemistry during arc continent collision we use as an example the connemara region of the western irish caledonides where metamorphism and orogenesis of dalradian sedimentary sequences record the collision of the oceanic lough nafooey arc with the passive margin of laurentia at 475 ma arenig this arc was mafic and lree depleted during its oceanic activity during collision rhyolites more lree enriched than the surrounding continental crust were erupted while the mid crust was intruded by similarly lree enriched gabbro and diorite crystal fractionation caused lree enrichment to exceed that expected by assimilation of continental crust alone we propose that subsequent loss of the corresponding lower crustal cumulates into the mantle removed this lree depleted material from the net crust added to the continent the combined processes of crystal fractionation and lower crustal loss during arc continent collision drive the average bulk arc composition toward that of average continental crust

preliminary fission track results from the southern urals sterlitamak to magnitogorsk abstract results are reported from a pilot fission track study of a west east traverse across the southern urals from sterlitamak through to the dzhabyk granite east of magnitogorsk which was undertaken in order to study the low temperature 300 c thermotectonic post uralian history of the mountain belt the transect crosses the major tectonic units including the main uralian fault the suture zone which separates the east european craton from palaeozoic oceanic island arc and continental terranes of asia zircon fission track ages range from 226 ma to 604 ma in the foreland thrust and fold belt with a single age of 262 ma in the magnitogorsk zone an age grouping of about 250 ma for zircons on either side of the main uralian fault suggests that there has been no differential movement identifiable through fission track analysis along this line since the triassic the similarity of apatite ages ranging from 180 to 210 ma further verifies little differential movement within the footwall to the suture since the jurassic in the central part of the magnitogorsk zone apatite ages are older they are slightly annealed crystallisation cooling ages but further confirm that there has been only little tectonic activity since the triassic except in the east uralian zone apatite fission track analysis reveals variable denudation rates the highest rates occurred during the permo triassic ending at about 210 ma just prior to the final development of the jurassic peneplain this was followed possibly by a period of burial before final cooling exhumation from approximately middle cretaceous onwards

seismic reflection coefficients from mantle fault zones summary several bright reflections from structures within the mantle can be seen on birps deep seismic reflection profiles we have calculated apparent reflection coefficients for the brightest of these events and obtain values around 0 1 it is not possible to produce such large reflections by either compositional layering or seismic anisotropy if olivine and pyroxene are the only significant minerals in the mantle these large reflections can be produced by a mafic layer or a partially hydrated layer within normal peridotite the brightest reflections seem to be best explained as major faults or shear zones within the mantle

crustal recycling by slab failure models for crustal recycling are based upon estimates of the amount of material being returned to the mantle today thus a better understanding of those processes will help to constrain the models we suggest through the use of an early proterozoic example in the wopmay orogen that failure of the subducting slab during collision leads to large quantities of crustal material being recycled into the mantle that this process has been active since the early proterozoic and that it may explain the paucity of rift deposits in other orogens

crustal architecture of the southern uralides from true amplitude processing of the urals seismic experiment and integrated studies urseis vibroseis profile true amplitude processing of the urals seismic experiment and integrated studies urseis vibroseis deep reflection seismic data acquired by the urseis consortium shows the southern uralide crust to be composed of four major blocks with distinctive reflection characteristics these blocks are juxtaposed along crustal scale boundaries the foreland thrust and fold belt developed from the east european craton crust is imaged as subhorizontal to east dipping reflectivity that can be related to its paleozoic and older tectonic history the moho beneath the foreland thrust and fold belt is not imaged in the vibroseis data set the main uralian fault the major arc continent suture is unreflective but its subsurface location can be inferred by the truncation of the reflection pattern of the east european craton and its contrast with that of the magnitogorsk arc the magnitogorsk arc reflectivity is characterized by patchy noncoherent to coherent reflections in the upper 10 15 km that are interpreted to be related to the arc volcanic rocks below this reflectivity is diffuse or the arc crust is transparent and the moho is not imaged the east magnitogorsk fault zone which juxtaposes the arc against the east uralian zone is not imaged by the data the upper 5 to 6 km of the east uralian zone corresponding to the dzhabyk granite is transparent below the granite the crust is characterised by east dipping patches of moderately coherent high amplitude reflections that in the east become shallowly west dipping a 10 km thick west dipping band of coherent high amplitude reflections between 12 and 35 km depth corresponding to the kartaly reflection sequence extends beneath almost the entire east uralian zone the crust beneath the easternmost east uralian zone reaches 53 km in thickness the upper and middle crust of the trans uralian zone is characterized by a series of east and west dipping concave upward moderately coherent high amplitude reflections the lowermost middle and lower crust displays thin bands of west dipping coherent low amplitude reflectivity the moho is imaged as a sharp transition from reflective lower crust to transparent upper mantle at 49 km depth and the lower crustal reflections appear to merge with it the urseis vibroseis data are integrated with results from the explosion source reflection data the wide angle reflection data and the surface geology to place constraints on the crustal architecture of the orogen and on the timing of its assemblage

metamorphic evolution of eclogite and associated garnet mica schist in the high pressure metamorphic maksyutov complex ural russia metasedimentary garnet mica schists are interlayered with metabasic garnet omphacite schists and enclose eclogite boudins in the high pressure metamorphic maksyutov complex in the southern urals russia these three rock types were investigated in one outcrop and compared chemographically and thermobarometrically the fe mg distributions between garnet rim omphacite and garnet rim phengite pairs indicate different equilibration temperatures for the three samples with the lowest temperature 500 c 1 5 gpa for the eclogite boudin an intermediate temperature 630 c 1 7 gpa for the foliated eclogite and the highest temperature 650 c 1 7 gpa for the garnet mica schist the garnets in garnet mica schist enclose abundant chloritoid relics and the fe mg distribution between chloritoid and garnet records an earlier high temperature stage 650 c 2 0 gpa before the garnet rim phengite temperatures were reached together with some minimum and maximum pressure estimates three different prograde pressure temperature paths and a common retrograde metamorphic evolution are interpreted from the chemographic and thermobarometric data the different early metamorphic evolutions and conditions confirm the variability of protoliths which are also indicated by different u pb zircon and rutile ages

the growth and destruction of continental crust during arc continent collision in the southern urals abstract the southern urals of russia contain a well preserved example of a paleozoic arc continent collision in which the laurussia margin was subducted beneath the magnitogorsk island arc in the devonian providing an ideal field area for studying the possible growth and destruction of the continental crust during this process high pressure rocks derived from the leading edge of the continental margin indicate that it was subducted to a depth of between 70 km eclogite assemblages and 120 km micro diamonds the vast majority of the high pressure rocks have a sedimentary protolith with mafic eclogite having been derived from dikes intruding into the sediments high pressure rocks derived from the mafic ganulite that currently makes up the middle and lower crust of the southern urals do not occur in outcrop suggesting that much of the subducted margin remained in the upper mantle however extensive geological geochemical and geophysical data do not unequivocally clarify its subsequent fate crustal delamination and foundering into the mantle can be ruled out and recycling through the volcanic arc system appears to have been minor while the metamorphic products of subducted continental crust can remain in the mantle for long periods of time in the southern urals it has not been imaged by the available geophysical data sets a possible explanation for this is that the progressive metamorphism of mafic granulite to eclogite assemblages would have altered its physical properties to those of typical mantle lithologies making them difficult to detect by geophysical methods it is estimated that the volume of continental crust that was subducted and lost to the mantle was approximately one third of the volume that was added to the laurussia margin by the accretion of the magnitogorsk arc

crustal root beneath the urals wide angle seismic evidence wide angle reflection and refraction data acquired as part of the urseis 995 geophysical experiment across the southern uralide orogen provide evidence for a 12 to 15 kilometer thick crustal root yielding a total crustal thickness of 55 to 58 kilometers strong reflections from the mohorovicic discontinuity moho at relatively small precritical distances suggest that the crust mantle transition beneath the crustal root is a sharp feature the derived p and s wave velocity models constrain key physical properties of the crust including the depth of the mafic rocks of the magnitogorsk volcanic arc and the existence of a lower crustal zone of possible basic rock enrichment beneath the east uralian zone

structure and evolution of the magnitogorsk forearc basin identifying upper crustal processes during arc continent collision in the southern urals the southern urals of russia contain a well preserved example of a paleozoic arc continent collision in which the intraoceanic magnitogorsk volcanic arc and its forearc basin sediments accreted to the east european craton during the devonian the magnitogorsk arc records the evolution from incipient intraoceanic subduction to a mature arc and by comparing its surface geological features with those in active arc continent collision settings it is possible to identify upper crustal processes that were active in the southern urals the arc edifice can be divided into western and eastern volcanic fronts that were active during different stages of arc evolution and for which two distinct phases of forearc basin development can be recognized the late lower to middle devonian aktau formation represents a remnant of the intraoceanic to collisional forearc basin to the irendyk volcanic front whereas the middle devonian to lower carboniferous ulutau koltubanian and zilair formations were deposited in a suture forearc basin to the east magnitogorsk volcanic front it was not until the late devonian that these two basins were joined structural mapping combined with reflection seismic profiling shows these basins to be affected by open nonlinear volcanic basement cored synsedimentary folds the karamalytash anticline appears to have the geometry of a growth fold that formed during deposition of sediments in the suture forearc basin the forearc region is affected by minor thrusting that involves the volcanic basement although it is not clear if these thrusts reactivate preexisting trench parallel faults synsedimentary deformation slumping and olistostrome development were common throughout the suture forearc basin history but were especially widespread during the late devonian when the full thickness of the continental crust is interpreted as having arrived at the subduction zone

the main tectonic events depositional history and the palaeogeography of the southern urals during the riphean early palaeozoic abstract investigation of former and new sections in the classic riphean region of the bashkirian anticlinorium ba revealed that this palaeozoic segment of the passive margin of the east european platform eep during its proterozoic history was initially formed as an intracratonic aulacogen basin which developed as an inter cratonic half structure only after the early ordovician opening of the uralian palaeo ocean the originally opposite half structure of this riphean basin may have been located either on the afro european part of gondwana or along the margins of the siberian craton or elsewhere but definitely not along the conjugate permian collision suture with the kazakh block or the mugodzhary microcontinent the ca 1150 ma 1635 to 488 ma depositional history of the bashkirian basin was punctuated several times due to major tectonic events folding downfaulting uplift magmatic intrusions metamorphism which may have caused extended periods of erosion and non deposition furthermore numerous cyclothems of unknown regional extent are incorporated into the enormous time span involved in the evolution of the ca 12 15 km thick riphean sequence the riphean vendian sedimentary assemblages reflect alternating shallow marine to peritidal and continental fluviatile environments the regional distribution of which is illustrated in sets of palaeogeographic maps angular unconformities are seen at the base of the lower riphean ai formation dated at 1635 ma at the base of the middle riphean mashak formation dated at 1340 ma at the base of the upper riphean zilmerdak formation approx 1000 ma and below the middle ordovician quartzites containing shelly fossils which overlie steeply folded and slightly metamorphosed proterozoic suites along the southeastern margin of the bashkirian anticlinorium during the vendian to ordovician interval major eastern parts of the bashkirian anticlinorium as well as regions further north along the european margin of the urals were subjected to strong diastrophism probably related to the cadomian or baikalian orogeny later on during the ordovician the western half part of the bashkirian basin turned into a marginal basin of the east european platform and thus was modified by the early stages of the uralian oceanic development

deep structure of the continental lithosphere in an unextended orogen an explosive source seismic reflection profile in the urals urals seismic experiment and integrated studies urseis 1995 explosive source near vertical seismic reflection data from the urals seismic experiment and integrated studies urseis profile displays an image of orogenic lithosphere to depths of 225 km the reflective crustal section is characterized by dipping crustal scale shear zones and a pronounced crustal root that show no evidence of overprinting by extension and are preserved since late paleozoic collision toward the east the uralian crustal root is composed of a partially subducted volcanic arc while to the west it has affinity with the east european platform the reflection character of the moho varies across tectonic strike and implies that the moho 1 acted as a structural detachment beneath the trans uralian zone 2 forms a tectonic boundary between terranes beneath the east uralian zone 3 consists of a transitional zone from eclogitized lower crust to peridotitic upper mantle in the crustal root 4 is a 3 9 km thick reflective zone beneath the fold and thrust belt and 5 is a boundary 200 m thick in the west mantle reflections at 80 100 km depth 22 24 s may represent a continuation of the main uralian fault into the subcrustal lithosphere alternatively these reflectors may signify paleozoic or younger mantle shear zones or the top of a zone of partial melting in the upper mantle deeper mantle reflections at 140 160 km 35 45 s and 225 km 55 s may image mafic intrusions at the base of the lithosphere or along localized shear zones in the upper mantle

longitudinal deformation of a lava flow the influence of bingham rheology abstract the behaviour of a lava flow is reproduced by a two dimensional model of a bingham liquid flowing down a slope the liquid is described by two parameters viscosity and yield stress both strongly temperature dependent assuming liquidus temperature at the eruption vent the temperature decrease due to the heat loss by radiation produces changes in the rheological parameters and consequently in velocity strain and strain rate along the flow velocity compressive strain strain rate and stress along the flow direction are computed as functions of the distance from the vent and of time for different kinds of lava flows basic and acidic and are compared with corresponding results for a newtonian liquid the model shows that in connection with the pronounced velocity decrease occurring at a certain distance from the vent the compressive strain strain rate and stress also show a strong variation a greater compressive strain is in fact induced in a bingham flow cooling by radiation than in a newtonian flow this behaviour may explain the presence of folds which are commonly observed at the surface of cooled lava flows

crustal scale structure and evolution of an arc continent collision zone in the southern urals russia the outcropping geology of the southern urals contains a well preserved accretionary complex related to the paleozoic collision that took place between the magnitogorsk arc and the former east european craton the crustal scale structure of the accretionary complex has been determined from outcropping field geology that is integrated with three reflection seismic profiles the reflection profiles show the accretionary complex to be highly reflective allowing direct comparison of many reflections with surface geological features we interpret the accretionary complex to be a thrust stack that is composed of shallowly subducted continental shelf and rise material syncollisional sediments derived from the arc deeply subducted high pressure gneisses that are intercalated with eclogites and blueschist and at the highest structural level ophiolite complexes it is bound at the base by a thrust and at the rear by a highly deformed zone the main uralian fault adjacent to the backstop the magnitogorsk arc deposition of the late devonian volcaniclastic sediments of the zilair formation appears to be related to collision uplift and erosion of the arc possibly following the arrival of the full thickness of the east european craton continental crust at the subduction zone with the arrival of the continental crust at the subduction zone offscraping and underplating of paleozoic slope and platform material took place at the base of the accretionary complex uplift of the arc was followed by its collapse and the unconformable deposition of lower carboniferous shallow water carbonates on top of it a time lag of 10 15 myr occurred between the high pressure metamorphism and the subsequent arrival of the east european craton at the subduction zone

adjustment of the ocean under buoyancy forces part ii the role of planetary waves abstract a numerical ocean general circulation model is used to investigate the early stages in the adjustment to equilibrium of an ocean initially at rest with imposed uniform meridional potential temperature gradients which yield density gradients representative of those observed in the north atlantic the main feature of the adjustment during the early stages the first year is the formation and decay of a subtropical warm core and a subpolar cold core density gyre the gyres are formed by the irreversible winding up of the initially zonal isotherms by first baroclinic mode coastally trapped dissipative kelvin waves this phase and the north south asymmetry arising from the variation in viscous diffusive kelvin wave properties with latitude were discussed in part i of this series within a month effects become significant especially in the evolution of the southern gyre which develops a distinct east west asymmetry through western intensification and long planetary wave propagation of b

40ar 39ar u pb and sm nd constraints on the timing of metamorphic events in the maksyutov complex southern ural mountains new 40 ar 39 ar u pb and sm nd data from 13 metamorphic samples constrain the formation and exhumation of the high pressure maksyutov complex in the southern uralide orogen the maksyutov complex records the highest pressure metamorphism in the southern uralides and is part of the thrust stack that composes the footwall beneath the main ural fault the new isotopic data from eclogite in the lower unit of the maksyutov complex include 40 ar 39 ar data from phengite u pb data from rutile and apatite and sm nd data from garnet clinopyroxene rutile and apatite these data indicate that eclogite facies metamorphism occurred in the mid devonian epoch c 380 ma corresponding with eastward subduction of the east european craton beneath the magnitogorsk island arc after partial exhumation the lower unit was juxtaposed c 360 ma based on 40 ar 39 ar white mica data with the middle and upper units at mid crustal levels during the development of a d 3 shear zone within the subduction zone by the late carboniferous epoch the maksyutov complex was exhumed to upper crustal levels compatible with sedimentologic evidence that at this time the foreland thrust and fold belt began to form and material was shed off it into the foreland basin

composition of the uralide crust from seismic velocity v p v s heat flow gravity and magnetic data abstract p wave velocity vp s wave velocity vs poisson s ratio heat flow potential field and surface geological data are integrated to constrain a model for the composition of the uralide crust along the urseis transect the model is constructed using published laboratory measurements of vp vs and density for a variety of crustal rock types these laboratory data have been corrected for depth pressure and the uralides temperature depth function the model shows clear differences between the composition of the old continental crustal nucleus of the east european craton and the newly added crust of the accreted arc terranes to the east the crust of the east european craton is more felsic than that of the magnitogorsk and east uralian zones and the latter two have a lowermost crust whose characteristics indicate a high garnet content mafic garnet granulite and or the presence of hornblendite the overall composition of the arc terranes is basaltic physical properties suggest that eclogite is not present in the arc terranes or if present it exists in small amounts that are below the resolution of the data set the lack of eclogite in the lower crust favours an intracrustal differentiation model for the evolution of the bulk composition of the continental crust nevertheless the absence of surface uplift the lack of metamorphism and late orogenic mantle melts and the current crustal thickness indicate that crustal thinning did not affect the bulk composition of the uralide crust

archaean subduction inferred from seismic images of a mantle suture in the superior province plate tectonics provides the basis for the interpretation of most current terrestrial tectonic activity and is widely accepted as having been active over much of the earth s history1 yet the timing of initiation of this process is subject to debate2 9 so far the earliest seismic evidence for plate tectonics has come from a fossil mantle suture in the svecofennian orogen 1 89 gyr ago 10 and from inferred plate convergence subduction and accretion in the trans hudson orogen 1 91 1 79 gyr ago 11 as yet seismic data from archaean areas have been able to demonstrate only the importance of compression in the construction of the continental crust12 15 here we present seismic data from a collision zone in the superior province of canada involving the abitibi granite greenstone subprovince and the plutonic arc related opatica belt we interpret dipping seismic reflections that extend 30km into the mantle as representing a relict 2 69 gyr old suture associated with subduction although crustal structure lithospheric thicknesses and convergence rates may have differed from those seen today these seismic data provide direct evidence that plate tectonics was active in late archaean times 800 myr earlier than indicated by previous seismic reflection surveys

the structural architecture of the footwall to the main uralian fault southern urals abstract the uralide orogen is a linear collisional belt formed during the upper paleozoic as a result of convergence between the east european craton and outboard terranes and accretion of intervening island arcs and obduction of oceanic crust onto the east european craton this paper presents a new structural subdivision of the southern urals to the west of the main suture zone that puts this part of the mountain belt in a context similar to that of other collisional orogenic belts this subdivision consists of a west vergent thrust stack that is made up of an autochthonous to parautochthonous foreland basin and a parautochthonous foreland thrust and fold belt flanked to the east by a metamorphic thrust stack the latter units are structurally overlain by allochthonous rocks of an accretionary complex slope sediments oceanic crust and high grade eclogite blueschist bearing gneiss where possible these units are defined on the basis of their internal stratigraphy and structure and on the nature of their bounding faults

tomographic evidence for the eurasian lithosphere subducting beneath south taiwan 1 a tomographic image of the crust and upper mantle beneath the taiwan island as derived from local regional and teleseismic data simultaneously revealed a high velocity zone from the surface down to a depth of 300 km locating beneath south taiwan to the south of latitude 24 n we interpret this high velocity zone to be the subducted eurasian lithosphere our result is different from a global tomography study that shows the eurasian plate subducting beneath most part of the taiwan island down to 670 km depth this down going model of the eurasian lithosphere implies that the tectonic framework of taiwan has been changing from subduction in the south to collision in the north which provides geophysical evidence supporting the previous proposed models the subducted eurasian lithosphere colliding with the subducted philippine sea plate would have contributed to the mountain building active seismicity and crustal deformation in the study region due to the lower part of the eurasian continental plate subducted beneath south taiwan

mantle unrooting in collisional settings abstract we present a two dimensional numerical model to study the thermo mechanical evolution of the lithosphere under a convergence regime in order to define the conditions that lead to lithospheric mantle break up and consequent unrooting a newtonian rheology with a temperature dependent viscosity is considered the system is not closed and horizontal flow through lateral boundaries is permitted a horizontal velocity is imposed at the top of the model to simulate compression whereas velocity vanishes at the bottom of the model the initial conditions correspond to a homogeneous lithosphere with a constant heat production in the crust the analysis of variations of maximum shear stress strain rate and total kinetic energy allowed us to define four major stages during the mantle unrooting process orogenic growth initiation of gravitational instability until lithospheric failure sinking of the detached lithosphere and relaxation of the system numerical results also show that the conditions for lithospheric unrooting strongly depend on the convergence velocity the wideness of the deformation zone and the imposed rheology

lithosphere scale seismic image of the southern urals from explosion source reflection profiling explosive source deep seismic reflection data from the southern ural mountains of central russia provided a lithosphere scale image of the central eurasian plate that reveals deep reflections 35 to 45 seconds in travel time 130 to 170 kilometers deep from the mantle the data display laterally variable reflectivity at the base of the crust that deepens beneath the central part of the profile documenting a crustal thickness of 55 to 60 kilometers beneath the axis of the orogen these data provide an image of the structure of the crust and underlying mantle lithosphere in a preserved collisional orogen perhaps to the base of the lithosphere

into the forbidden zone very low temperatures at high pressures have long been believed to constitute a forbidden zone not realized in earth however recent studies of ultrahigh pressure uhp terranes indicate that these rocks have experienced such forbidden zone conditions liou et al discuss the implications of these observations for plate tectonics

along strike structural variations in the foreland thrust and fold belt of the southern urals abstract the foreland thrust and fold belt of the southern urals has a number of along strike structural changes in which regional scale structures terminate abruptly one important along strike structural change is the southern termination of the precambrian cored bashkirian anticlinorium which plunges southward beneath the synformal zilair nappe balanced and restored dip cross sections a n s oriented strike section together with a geological interpretation of a cdp reflection seismic profile have been used to determine the 3d structure and the amount of shortening in the foreland thrust and fold belt in the area of the bashkirian anticlinorium and the zilair nappe the geometry of the foreland thrust and fold belt is that of a west vergent basement involved thrust stack with shortening on the order of 13 to 17 approximately 17 km the small amount of shortening allows a correlation to be made between the along strike structural changes and pre existing basement structures in particular the southern termination of the bashkirian anticlinorium coincides with a precambrian age aulacogen the sernovodsk abdulino aulacogen developed in the basement an e w oriented tear fault the belaya tear fault appears to have developed in the basement along the northern margin of the sernovodsk abdulino aulacogen the belaya tear fault was active during much of the history of the foreland thrust and fold belt and exerted strong control on its structural and tectonosedimentary development because of the small amount of shortening involved the foreland thrust and fold belt of the southern urals provides an important example for understanding the effects of pre existing basement structures on the development of foreland thrust and fold belt s

uralian magmatism an overview abstract this paper is an attempt to summarize current knowledge of uralian magmatism focusing on those aspects relevant for understanding its geodynamic evolution the urals consist of three tectonomagmatic domains a suture sector in the west and two n s imbricated island arc continental sectors in the east the suture sector comprises lower palaeozoic mafic ultramafic complexes which show eastward impoverishment in lile thus reflecting the transition of the subcontinental lithospheric mantle of the russian plate to the suboceanic lithospheric mantle of the subducted uralian palaeo ocean the two island arc continental sectors represent the transition from oceanic to continental environments in the middle and south urals collisional magmatism started in the silurian and persisted till the permian migrating progressively eastward and increasing in abundance of lile and 87 sr 86 sr initial magmatic polarity is very similar to that of modern subduction zones and indicates that the subducted slab was dipping eastward during that period the northern and southern island arc continental sectors show many similarities regarding the nature and spatial temporal distribution of magmatism but there are also some important differences which probably indicate somewhat different geodynamic regimes in the northern sector carboniferous tonalite granodiorite batholiths have features compatible with an origin by melting of the oceanic crust in the subducted slab in the southern sector however carboniferous tonalite granodiorite batholiths have features more consistent with a melting event within the lower continental crust above the subduction zone than with melting within the subducted slab upper carboniferous permian granites have high 87 sr 86 sr initial in the north e g 0 7120 in the murzinka batholith but very low 87 sr 86 sr initial in the south e g 0 7045 in the dzhabyk batholith in spite of rocks from both batholiths being equally peraluminous and showing evidence of derivation by anatexis of metasediments the low 87 sr 86 sr initial at dzhabyk might indicate that in the southern sector the accretionary prism grew very rapidly thus allowing involvement in upper palaeozoic melting events

images of a lower crustal oceanic slab direct evidence for tectonic accretion in the archean western superior province the archean western superior province in canada is the type area for proposed archean plate tectonics seismic images from this region provide direct evidence for assembly of the craton by terrane accretion and for a large slab of remnant oceanic crust preserved at the base of the crust this slab with inferred garnet amphibolite composition adds a critical piece of evidence to previous suggestions that archean subduction was at a shallow angle and that some neoarchean tonalite trondhjemite granodiorite suites distinct from most modern day suprasubduction magmas are melts primarily derived directly from subducted slabs

on the nature of mantle heterogeneities and discontinuities evidence from a very dense wide angle shot record abstract a seismic survey with a receiver spacing of 50 m provided one of the most densely sampled wide angle seismic reflection images of the lithosphere this unique data set recorded by an 18 km long spread reveals that at wide angles the shallow subcrustal mantle features high amplitude reflectivity which contrasts with a lack of reflectivity at latter travel times this change in the seismic signature is located at approximately 120 150 km depth which correlates with the depth estimates of the lithosphere asthenosphere boundary lab of previous dss studies this seismic signature can be simulated by two layer mantle model both layers with similar average velocities differ in their degree of heterogeneity the shallow heterogeneous layer and the deeper and more homogeneous one correlate with the lithosphere and the asthenosphere respectively studies involving surface outcrops of ultramafic massifs and mantle xenoliths infer that the upper mantle is a heterogeneous mixture of ultramafic rocks lherzolites harzburgites pyroxenites peridotites dunites and small amounts of eclogites laboratory measurements of physical properties of these mantle rocks indicate that compositional variations alone can account for the wide angle reflectivity a temperature increase would homogenize the mixture decreasing the seismic reflection properties due to melting processes it is proposed that this would take place below 120 150 km 1200 c the lab

preserved collisional crustal structure of the southern urals revealed by vibroseis profiling the vibroseis reflection profiling component of the urseis 95 experiment provides a high resolution crustal scale image of the unextended southern uralide orogen a marked lateral and vertical variation of reflectivity throughout the entire crust differentiates the former margin of the east european craton to the west from accreted terranes to the east between these regions is a less reflective zone corresponding to the magnitogorsk volcanic arc and the crustal root zone continuous reflections and reflective domains that correlate at the surface with major tectonic features define a bivergent orogen in which the paleozoic collisional structure has been largely preserved