tertiary extensional faulting and evolving ductile brittle transition zones in the northern snake range and vicinity new insights from seismic data new geologic mapping and high resolution seismic data from east central nevada shed light on the evolution of the northern snake range decollement nsrd and its relation to the present basins and ranges we suggest that the nsrd originated in oligocene time as a flat 20 km wide 7 km deep zone of decoupling between localized ductile stretching below and more widely distributed high angle normal faulting above younger east dipping normal faults cut the western part of the nsrd forming the tilted fault blocks and highly asymmetric grabens that constitute the duck creek range duck creek valley and schell creek range spring valley and folded the nsrd to its present antiformal geometry these youngest faults flatten into pervasive subhorizontal reflectors at a depth of 10 to 15 km that we interpret as the youngest ductile brittle transition

geochemistry u pb geochronology and sm nd data from the paleoproterozoic western finland supersuite a key component in the coupled bothnian oroclines abstract the western finland wf supersuite characterized by supracrustal rocks and intruding plutonic rocks forms the core component of the coupled bothnian oroclines in the central part of the svecofennian orogen we present here geochemical isotope data detrital zircon age data on sedimentary rocks and age data on granitoid plutons within the wf supersuite in the pirkanmaa pb and pohjanmaa pob belt representing the southernmost limb of the southern and the hinge zone of the northern bothnian orocline respectively the tampere belt tb in the north and hame belt hb in the south bound the pb basaltic to picritic volcanic rocks in the wf supersuite with a proposed eruption age at ca 1 91 ga 1 92 1 90 ga show locally pillow structures and are typically associated with black shales cherts carbonate bearing rocks based on geochemistry and sm nd data a variable morb wpb type origin is evident and an evolution scenario from earlier wpb type rifting to more morb affinity ocean sea opening is proposed detrital zircon age distributions from sedimentary rocks with 1 91 ga maximum depositional age in the tb pb and hb have been compared and typically the age distributions of

seismic and geoelectric evidence for collisional and extensional events in the fennoscandian shield implications for precambrian crustal evolution extensive deep seismic and electromagnetic sounding programs have resulted in tens of deep refraction seismic profiles normal incidence reflection profiles several hundred magnetotelluric soundings and several magnetovariational array observations results are compiled into four maps 1 moho depth 2 thickness of the lower crustal high velocity layer vp 7 km s 3 depth to the upper interface of the high velocity layer and 4 distribution of upper and middle crustal electrical conductors the moho depth is approximately 40 km in the archaean domain and varies between 40 and 65 km in the svecofennian domain most of the thickness variation 0 24 km is concentrated in the lower crustal high velocity layer electrically the crust is formed of several rather homogeneous either resistive or conductive blocks that are bounded by upper and middle crustal conductive zones the cratonized archaean crust experienced an extensional phase during which a thin mafic lower crustal high velocity layer was produced in association with mafic dykes 2 2 2 1 ga and well conducting volcanogenic and metasedimentary belts 2 5 2 0 ga the moho of the archaean domain was consequently reformed in the early proterozoic major thickening of the crust 50 65 km occurred when a svecofennian island arc and the archaean craton collided the svecofennian orogeny 1 76 1 9 ga resulted in a collage of metasedimentary rocks squeezed between crustal blocks of the island arc the internal terrane boundaries are seen as good reflectors and or inclined or vertical conductors that extend at least to the middle crust the thick high velocity 7 km s lower crust is a combination of mafic lower crust of the island arc crustal blocks and mafic additions from either delamination of the lithosphere after the collision or from a period of late svecofennian subduction the collisional boundary is also preserved as a discontinuous conductive zone between the archaean and svecofennian domains the thickened crust was subsequently thinned to 42 km along e w striking zones during an extensional period characterized by the intrusion of anorogenic rapakivi granitoid batholiths coeval mafic dykes and gabbro anorthosite bodies 1 6 1 5 ga the crust of the central part of the shield has since remained virtually intact

3 d density model of the crust of southern and central finland obtained from joint interpretation of the svekalapko crustal p wave velocity models and gravity data 3 d density model of the crust of southern and central finland obtained from joint interpretation of the svekalapko crustal p wave velocity models and gravity data

thermally softened continental extensional zones arcs and rifts as precursors to thickened orogenic belts abstract intra continental deformation of soft zones during continental collision requires weak continental lithosphere which is able to be shortened across considerable width during later convergence this enables significant thickening with formation of an orogenic root we have examined models with a history of lithospheric thinning by pure shear during an earlier phase of intra continental extension with associated heating geologically this situation is appropriate to intra continental rifts and back arc basins if thinning of elevated thermal structure is decoupled from the thinning of lithology then a weak soft lower crust and sub arc rift mantle result this weak structure has a favoured rheology for subsequent convergent thickening while the lithosphere is still hot these regions are associated with formation of granulites and metamorphic assemblages typical of high temperature low pressure ht lp if convergence starts while the heat input is still active then the failed rifts and arcs are shut off by lateral wedging of the hard lower crust and upper mantle of shoulder regions into the softened arc rift domain such sites are ideal for the formation and for the exhumation of metamorphic core complexes subsequent thickening during convergence leads to ht eclogites when the previous arc rift was hot and to medium t eclogites for a thickened standard geotherm these p t paths are counterclockwise and their shapes are strongly dependent on the amount of previous thinning and type of initial geotherm if the compression starts long after cessation of the extensional event and associated thermal anomaly then the geotherm of the extended area relaxes and the whole region hardens in this case no homogeneous thickening occurs and deep continental roots cannot form

3d diffraction imaging of linear features and its application to seismic monitoring many subsurface features such as faults fractures cracks or fluid content terminations are defined by geological discontinuities the seismic response from such features is encoded in diffractions we develop an algorithm for imaging such discontinuities by detecting edge diffractions the algorithm exploits phase reversal phenomena of edge diffractions and uses them as a criterion to separate these diffractions from specular reflections and diffractions produced by a leaner object the performance of the method is demonstrated on both synthetic and real 3d seismic data the output image focuses the diffracted energy back to its origin and shows high semblance values at the edge of the object the method is applied on conventionally stacked data producing an image that contains only diffraction events called the d volume we also reveal the potential of diffractions to image and track the changes of a co2 plume using time lapse analysis and detect any possible co2 seepage from its primary containment

integrated seismic studies of the baltic shield using data in the gulf of bothnia region in the autumn of 1989 a co operative experiment involving 12 research institutions in northwestern europe collected 2268 km of deep seismic reflection profiles in the gulf of bothnia and the baltic

interpretation of magnetic fabrics in the early proterozoic diabase dykes of keuruu central finland abstract magnetic fabrics petrophysics petrography and chemical compositions are described for 14 diabase dykes located in the keuruu area central finland these early proterozoic 1880 ma dykes sharply cut both low density granodiorites and high density diorites and gabbros the diabase dykes with a nw se strike and an almost vertical dip have a maximum length of a few kilometers and widths of about 1 m strongly developed magnetic fabrics of deformation type anisotropy degree up to 1 4 were observed in six of the dykes and weakly developed fabrics of flow type anisotropy degree below 1 1 in eight dykes no systematic differences in the dyke widths mean densities palaeomagnetic poles mineral compositions degrees of alteration or chemical characteristics were observed between the two types of dykes suggesting that the dykes had a common magma source all the fabrics of deformation type are dominantly ferrimagnetic with tightly aligned horizontal foliations and well grouped lineations which in most cases are parallel to the dyke trends it is suggested that the deformation fabrics were generated as tholeiitic magma was forced to intrude laterally along its level of neutral buoyancy lnb which may have been close to the present erosion level pel in the low density granodiorite environment ferri and paramagnetic fabrics of flow type characterize the dykes located in the denser mafic parts of the crust in these parts the lnb was possibly somewhat higher in the crust so that magma flow passed the pel and formed weakly developed flow fabrics with gently plunging lineations and vertical or variable magnetic foliations

faults and folds fact and fiction abstract after reviewing the microscopic and macroscopic texture of fault zones the localisation of rupture and creep is described and the reduced strength of fault zones is investigated simple strength and viscosity models for the whole lithosphere play a major role for the geometry of fault zones compressional faults thrusts show ramp and flat structures often soling in a weak detachment zone extensional faults prefer a listric shape often turning subhorizontal and being invisible in the seismic lamellae of the ductile lower crust which according to the models is also a broad zone of detachment and decoupling faults in the lower crust are rare and restricted to strain hardened fault rocks or intrinsically rigid rocks like oceanic mafic ultramafic rocks of suture zones transient faults in the lower crust are created by rupture processes of large earthquakes some deep faults moderately dipping or flat are again observed in the rather rigid uppermost mantle the observation of folding inside the crust seems to be connected with an intermediate viscosity range and with a broad vertical transition zone where rigid reactions and ductile processes are mixed various methods of detecting faults by seismic studies are critically reviewed and some sequence of reliability is suggested impedance contrasts polarity dip thickness and multiple fault strings have to be deciphered by wavefront modelling origin and survival of faults are considered to be a consequence of tectonic and thermal evolution under specific stress systems

proterozoic and cenozoic subduction complexes a comparison of geometric features mantle reflections along the lithoprobe slave northern cordillera lithospheric evolution snorcle deep seismic reflection transect in northwestern canada are interpreted as a relict subduction zone geometric features of the data in the vicinity of the interpreted subduction zone are compared with those of the vancouver island profile which images a modern convergent margin comparisons are made at both a regional scale as in the case of the location of the magmatic arc with respect to the subduction zone as well as a local scale such as the geometry of structures within the accretionary wedge the comparison provides a test for the paleosubduction hypothesis and aids the interpretation of the deep reflection profile where it is difficult to trace individual features to outcrop the similarity of geometry and scales of the structures gives support to the interpretation that the lithospheric structures in the wopmay orogen are the products of accretionary processes at 1 88 1 84 ga and that these accretionary processes are similar to those observed today

high resolution body wave tomography beneath the svekalapko array ii anomalous upper mantle structure beneath the central baltic shield high resolution body wave tomography beneath the svekalapko array ii anomalous upper mantle structure beneath the central baltic shield

early proterozoic deposition and deformation at the karelian craton margin in southeastern finland abstract the margin of the karelian craton underwent complex though not chaotic deformation during the svecokarelian orogeny the resultant structural sequence can be subdivided into an early phase that involved translation of thrust nappes and recumbent folds verging towards the craton and a younger phase having more zonal and upright structures these latter include nw trending ductile shear zones associated with widespread pluton emplacement and later n trending faults and shear zones having substantially vertical displacements these features are interpreted in terms of a relative increase in vertical crustal stresses resulting from gravitational and thermal disequilibrium in the overthrust archaean basement despite this structural complexity two distinct depositional provinces are recognized in north karelia separated by a major thrust zone the more easterly hoytiainen province is a narrow essentially intracratonic feature containing locally derived archaean and proterozoic detritus an early phase of chemogenic and pelitic deposition with sporadic tholeiitic volcanism was followed by an influx of coarse clastic detritus largely deposited from turbidity currents within a prograding submarine fan the province apparently records deposition in a transtensional en echelon system of basins initiated near the craton margin at about 2 1 ga in contrast the savo province is largely allochthonous and includes the distinctive outokumpu assemblage commonly interpreted as a remnant of lower proterozoic oceanic crust provenance for the bulk of the overlying metasediments remains problematical although local evidence for deposition on archaean basement exists if an accretionary margin context is considered several potential niches for savo province deposition can be found that appear to satisfy constraints imposed by both isotopic data relating to provenance and the sequence and geometry of svecokarelian deformation

the vaasa migmatitic complex svecofennian orogen finland buildup of a lp ht dome during nuna assembly tectonic evolution of the paleoproterozoic vaasa migmatitic complex vmc in the central part of the svecofennian accretionary orogen is deciphered using aeromagnetic and gravity maps deep seismic

complex lithospheric structure under the central baltic shield from surface wave tomography complex lithospheric structure under the central baltic shield from surface wave tomography

three dimensional crustal structure of the eastern cordillera southwestern canada and northwestern united states lithoprobe and cocorp crustal seismic reflection profiles across the southwestern foreland and omineca belts of southern british columbia and the adjacent united states provide regional three dimensional coverage of orogen parallel structural variations in this area north american cratonic basement and cover rocks are deformed into east verging folds and thrusts which increase in scale and depth to the west this simple pattern may be complicated by out of sequence faulting in the upper and middle crust of the kootenay arc and purcell anticlinorium in southern british columbia the southern omineca belt is the structural and metamorphic hinterland of the foreland belt and contains the monashee priest river crustal scale tectonic ramp which records a protracted history of east directed contraction it is overlain by the eagle river vernon colville crustal scale antiform of probable late cretaceous to paleocene age which locally may have formed above blind thrusts an embayment in the autochthonous craton between the monashee and priest river ramps corresponds spatially with the kootenay arc structural salient and eastward deflection of the lower paleozoic shelf to basin facies transition the south flank of the monashee and valhalla complexes and the north flank of the purcell anticlinorium near the international border correspond with the lateral margins of this east trending embayment on an even larger scale the increasing northward tectonic overlap of accreted terranes onto thick cratonic crust from the northern united states into british columbia and associated basement thickening coincide spatially with the northward increased width and depth of the early tertiary foreland basin in this part of the cordillera eocene crustal extension did not completely mask older contractional fabrics and appears to have been accomplished by two mechanisms 1 in the east by crust penetrating shear zones that separate the omineca and foreland belts and 2 in the west by crustal scale boudinage within and west of the omineca belt

a geodynamic framework for interpreting crustal scale seismic reflectivity patterns in compressional orogens summary 2 d plane strain finite element models are used to calculate crustal deformation patterns within model compressional orogens and the results are compared with natural examples the fundamental driving mechanism for model deformation is provided by asymmetric detachment and underthrusting of mantle lithosphere with deformation being rooted at the stress singularity where the mantle detaches deformation steps up into the overlying crust and spreads laterally according to the rhcological properties assumed for the crust the crust is modelled using plastic frictional and viscous rheologies and incorporates the effects of compositional layering and variable geothermal gradients overall deformation is determined by balancing the internal strength of the crust against the sum of applied boundary stresses plus the gravitational stress induced by mass redistribution within the deforming orogen the amount of convergence in the models is limited to 150 km or less and so the results are not strictly comparable with larger orogens involving greater convergence nevertheless larger orogens seem to evolve through stages that may be understood in terms of the models provided that consideration is confined to smaller scale components that are limited in time and space the continuum deformation calculated by the finite element models is qualitatively converted to equivalent pattcrns of discrete faults and shear zones by associating these discrete features with the areas of highest cumulative strain using the results of sandbox experiments as guides significant styles of deformation include 1 coupling between step up shears and subhorizontal detachments at one or more levels 2 localization of upper crustal deformation by subduction of the lower crust and 3 pinning of deformation by surface denudation a partial catalogue of such model styles is presented in which the strength of basal coupling between crust and mantle the number of rhcological layers within the crust the thickness of subducted lower crust and the extent of surface denudation are varied systematically compressional orogens with a wide range of ages are characterized by crustal scale seismic reflectivity similar to patterns of fault shear zones represented in this catalogue of styles this similarity suggests that crustalscale seismic reflection data can be used to constrain the dynamic processes involved in compressional orogcnesis

1 88 1 87 ga post kinematic intrusions of the central finland granitoid complex a shift from c type to a type magmatism during lithospheric convergence abstract the evolution of the svecofennian orogen spans from 1 95 ga to 1 80 ga with the main crust forming activity having taken place 1 90 1 87 ga ago subsequent deformation was largely concentrated to southern finland where transpressional structures indicate lithospheric convergence until 1 80 ga much of the svecofennian orogen in finland is occupied by the central finland granitoid complex cfgc that mainly consists of i type granodiorite and granite these 1 89 1 88 ga intrusive rocks are typically foliated and considered synkinematic a distinct suite of post kinematic undeformed or slightly foliated plutons crosscut the synkinematic rocks they are predominantly quartz monzonites and monzogranites and have been divided into three types according to their petrographic mineral and chemical characteristics compared to the calc alkaline synkinematic granitoids the post kinematic suite generally has an alkaline affinity with higher fe ti k ba zr and nb and lower mg ca and sr at a given sio 2 content the type 1 post kinematic plutons are peraluminous and contained a prominent sedimentary component in their source the types 2 and 3 plutons are marginally metaluminous to peraluminous the type 2 granites especially those in the western cfgc approach the classic 1 65 1 54 ga rapakivi granites of southern finland in their petrographic characteristics magmatic association tholeiitic mafic rocks and elevated contents of incompatible elements and hence have a type characteristics the type 3 plutons contain a pyroxene bearing margin type 3a or contain pyroxene throughout type 3b the type 3b rocks have c type charnockite magma type affinities and some plutons of this subgroup are alkaline the type 3a plutons appear to be transitional between the type 2 and type 3b plutons the synkinematic magmatic episode in the cfgc area probably involved partial melting of intermediate high k rocks in the lower crust with a magmatic addition from a mafic underplate this magmatic episode left a hornblende and biotite poor granulitic residue in the lower crust heat from the mafic underplate triggered partial melting of the granulite to produce the post kinematic magmatism and also introduced mafic material into the post kinematic magmas the post kinematic magmatism took place over quite a short time period ca 15 ma during overall lithospheric convergence it extended beyond the cfgc area and registers a temporal shift from the northeast toward the west and a general change in the character of magmatism from c type to a type the post kinematic magmatism probably resulted from extensional or transtensional events modifying the tectonically thickened crust partial melting of the mafic lower crust produced c type magmatism soon after the first melting episode the crust was already rather rigid and allowed the mafic magmas to follow a tholeiitic trend when the bimodal a type magmas were formed in the western cfgc

detection of widespread fluids in the tibetan crust by magnetotelluric studies magnetotelluric exploration has shown that the middle and lower crust is anomalously conductive across most of the north to south width of the tibetan plateau the integrated conductivity conductance of the tibetan crust ranges from 3000 to greater than 20 000 siemens in contrast stable continental regions typically exhibit conductances from 20 to 1000 siemens averaging 100 siemens such pervasively high conductance suggests that partial melt and or aqueous fluids are widespread within the tibetan crust in southern tibet the high conductivity layer is at a depth of 15 to 20 kilometers and is probably due to partial melt and aqueous fluids in the crust in northern tibet the conductive layer is at 30 to 40 kilometers and is due to partial melting zones of fluid may represent weaker areas that could accommodate deformation and lower crustal flow

the accretionary svecofennian orogen insight from the babel profiles abstract the babel profiles b c 1 7 form a 1200 km long nearly continuous cross section through the svecofennides the near vertical marine reflection profiles display a wide range of crustal structures that can be associated with both the accretionary svecofennian orogeny 1 96 1 75 ga and the following subjotnian and jotnian rift stages 1 65 1 11 ga the svecofennian accretionary orogeny took place when a number of micro plates with island arc affinities and surface expression of a large archipelago accreted to the continental karelian plate some of the accreting terranes seem to have had older cores that have acted as crustal indentors during the collision babel profiles 3 and 4 image a series of collisional terrane boundaries between karelian continental margin savo arc sa and central finland arc cfa in the north the karelian margin has been both over and underthrust by the savo arc cfa comprises a folded thrust package on top of a continental nucleus and the savo arc the associated subduction zone and accretionary prism are interpreted to lie to the south underneath the bothnian basin area where prominent ne dipping lower to middle crustal reflections are found along babel profiles 1 and 4 an oblique collision of the central finland arc and the continent resulted in the development of the strike slip fault on the young hot savo arc babel profiles 1 6 7 and c image the internal architecture of the southern and central finland arcs the unusually thick crust 55 60 km hosts unreflective high density mafic intrusions acting as a crustal indentor a highly reflective antiform structure developed on the southern side of it southern finland arc complex sfa is an imbrication structure comprising stacked slices of arc related crust on an older continental nucleus bergslagen nucleus prior to the collision the sfa suffered from gravitational collapse during which the crust was thinned profile b images the architecture of the central and southern swedish svecofennides the sormland terrane sob is interpreted as the accretionary prism of the southern finland arc squeezed between the svecofennian collage in the north and the advancing continent to the south the southern continent island arc is characterised by ne dipping crustal reflections and moho offsets as well as step wise increasing thickness of the crust after the final collision large volumes of mantle derived material intruded the crust as large mushroom shaped plutons they are interpreted as the heat source for the tib magmatism in southern and western scandinavia

lithoprobe crustal reflection cross section of the southern canadian cordillera 1 foreland thrust and fold belt to fraser river fault seismic reflection data from the south central canadian cordillera covering the interval from the easternmost metamorphic core complexes near arrow lakes to the fraser river fault system along the fraser river reveal a highly reflective and complex crust the base of the crustal reflectivity interpreted as the reflection moho is clearly delineated by a continuous sharp boundary that is essentially planar and slopes uniformly over a distance of 250 km from about 12 0 s in the east to about 10 5 s in the west this virtual lack of relief at the base of the crust contrasts sharply with surface structures that involve 25 km or more of structural relief some of these surface structures can be readily correlated to structures that are outlined by the reflection data and that can be followed into the middle and lower crust even though part of this area was subjected to large amounts of eocene extension the crust is not divisible into transparent upper and reflective lower layers as it is in parts of the u s cordillera three structural culminations the monashee complex the vernon antiform and the central nicola horst are interpreted on the basis of the reflection configuration and the surface geological relationships to have formed initially during jurassic to eocene compression and then to have been modified and exposed during early and middle eocene extension an example of a compressional structure observed on the profiles is the monashee decollement which can be traced from the surface westward into the lower crust extension is manifested along a variety of normal faults including the regionally extensive low angle okanagan valley eagle river fault system moderately dipping faults such as the columbia river and slocan lake faults and high angle faults such as the quilchena creek and coldwater faults both jurassic to eocene compressional shear zones and early to middle eocene extensional shear zones are listric into the lower crust or moho under the intermontane belt

petrology and geochemistry of mafic granulite xenoliths from the lahtojoki kimberlite pipe eastern finland abstract the lahtojoki kimberlite pipe in kaavi eastern finland contains lower crustal mafic granulite xenoliths with a mineral assemblage cpx amph pl grt opx bt the tweequ thermobarometry indicates crystallization at ca 800 900 c and 0 75 1 25 gpa which corresponds with crustal depths of ca 22 38 km reaction textures and results of thermobarometry in some xenoliths indicate post peak decompression with cooling chemical composition of the xenoliths suggest that their protoliths crystallized from basaltic melts with some k enrichment their ree patterns are slightly lree enriched some xenoliths having small negative eu anomalies the u pb ion probe dating of zircons from two samples yielded variable ages for zircons even in the same xenolith between ca 2 6 and 1 7 ga these ages correspond with main late archaean and palaeoproterozoic orogenic events in the fennoscandian shield the sm nd garnet clinopyroxene isochron age from one dated sample is 1 6 ga which represents either a cooling or reheating of the lower crust by rapakivi magmatism because the petrology geochemistry and ages of xenoliths do not correlate well with the nearest exposed archaean mafic granulites it is evident that the present lower crust underlying the archaean rocks has a considerable palaeoproterozoic component

crustal channel flows 1 numerical models with applications to the tectonics of the himalayan tibetan orogen 1 plane strain thermal mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens in the models radioactive self heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large scale horizontal flow channels can be exhumed and exposed by denudation focused on the high relief transition between plateau and foreland we interpret the himalaya to have evolved in this manner channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws and well developed if there is an additional reduction in viscosity to 1019 pa s this reduction occurs from 700 c to 750 c in the models and is attributed to a small percentage of in situ partial melt melt weakening model ht1 provides an internally consistent explanation for the tectonic evolution of many features of the himalayan tibetan orogenic system erosional exhumation exposes the migmatitic channel equivalent to the greater himalayan sequence ghs between coeval normal and thrust sense ductile shear zones corresponding to the south tibetan detachment and the main central thrust systems outward flow of unstable upper crust rotates these shears to low dip angles in the model both the ghs and the lesser himalayan sequence are derived from indian crust with the latter from much farther south similar models exhibit a range of tectonic styles including the formation of domes resembling north himalayan gneiss domes model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau

receiver function analysis of the crust and upper mantle from the north german basin to the archaean baltic shield summary two passive seismic experiments have been carried out across the trans european suture zone tesz from northern germany to southern sweden tor and across the proterozoicarchaean suture in finland svekalapko to improve our understanding of the processes involved in the creation of the european continent teleseismic earthquakes recorded by the two networks and stations of the grsn and geofon permanent networks have been used for studies of the crust mantle and upper mantle seismic discontinuities with the receiver function method along the tor network the depth to the moho increases from 30 km at the southern edge of the profile to 40 km at the elbe line between the elbe line and tesz the moho branches off and whereas the deeper branch continues at 40 km depth to the tesz a second branch appears at 30 35 km depth the upper branch descends north of the tesz to below 55 km under the northern end of the tor profile the crustal thickening north of the tesz is accompanied by an increase in average vp vs values appearance of intracrustal conversion zones and north dipping features which we interpret as remnants of the subduction and subsequent collision between avalonia and baltica in southern finland beneath the svekalapko network the moho starts in the south at the depth of 40 45 km plunges to about 65 km depth south of the archaean proterozoic suture this deepening of the moho is coincident with a north dipping intracrustal structure apparently related to the subduction and collision and of the proterozoic and archaean provinces in proterozoic north of the line of the suture the moho rises smoothly to 45 50 km depth in the archaean province along the tor profile 410 and 660 discontinuities were hard to detect however manyfold stacking of receiver functions revealed that the conversions from the two discontinuities arrive more or less according to iasp91 predicted time across the svekalapko network 410 and 660 discontinuities arrive markedly earlier than iasp91 theoretical arrival times in particular north of the archaean proterozoic suture in finland the 410 and 660 km conversions arrive about 2 s earlier indicating about 5 per cent higher average upper mantle velocities and lower temperatures than what iasp91 global model predicts

crustal scale rheological transitions during late orogenic collapse abstract orogeny involves crustal thickening followed by thermal relaxation and radiogenic heat production in the thickened crust culminating in crustal melting and magma intrusion which decrease the crustal viscosity by several orders of magnitude and cause late orogenic collapse collapse of the canadian cordillera is expressed in the early tertiary shuswap metamorphic core complex british columbia which displays a three layer crustal section separated by two fundamental rheological discontinuities 1 the brittle ductile transition across which high angle normal faults in the upper crust control basin formation merge into a low angle detachment zone where leucogranite laccoliths ponded and deformed progressively under submagmatic to low temperature conditions and 2 the metatexite diatexite transition across which the rocks lose their solid framework and behave like a viscous magma this transition has the potential to mechanically decouple the upper crust from the rest of the lithosphere during a late orogenic gravitational collapse

isotopic and geochemical constraints on the evolution of the 1 93 1 79 ga svecofennian crust and mantle in finland the central part of the svecofennian domain in finland is characterized by thick lithosphere with crustal thickness up to 60 65 km major collisional events at 1 91 1 90 ga and 1 89 ga and a thrusting event at 1 86 1 84 ga the latter due either to continuation of shortening or to a separate intracrustal collision are considered the major causes of crustal thickening the origin of the svecofennian domain has been formerly attributed to mixing of depleted mantle melts with variable amounts of archaean crustal component via subduction but the new data and reinterpretation of older data imply that besides juvenile crust the svecofennian domain also contains older palaeoproterozoic crustal components no archaean component is found in the 1 93 1 91 ga gneissic tonalites and related felsic volcanics end t 1 4 which occur in the savo belt sb adjacent to the archaean craton the depleted mantle model ages vary from 1 94 ga to 2 33 ga with most values 2 0 ga the analysed syntectonic post thickening granitoids 1 89 ga in the sb exhibit positive end t values ranging from 2 8 to 3 0 pointing to a juvenile source the hypersthene granitoids are similar in age but show epsilon values around 1 which are only slightly lower than found in em derived mafic rocks the isotopic and geochemical data for the central finland granitoid complex and tampere schist belt indicate the occurrence of evolved thick crust 2 0 ga and associated lithospheric mantle already at 1 91 ga the southernmost part of the southern svecofennian in finland also shows evolved type crust 2 0 ga a tentative model for the development of crust and mantle is presented it includes the occurrence of continental nucleus surrounded by more juvenile island arc accretions two successive collisional events thickened both the crust and the subcontinental lithospheric mantle with the greatest relative amount of thickening occurring in the mobile zones convective removal of the lowermost parts of thickened lithosphere caused an upwelling of hot asthenosphere increase in the geotherm and melting of both the enriched subcontinental lithospheric mantle and the convective depleted asthenosphere mafic magmatism during the post thickening stage 1 89 ga thus varied from dominantly enriched to dominantly depleted mantle melts and was associated with variable crustal contamination the thickening of the lithosphere in the central part of the svecofennian domain was not followed by large extension suggesting that the thickness was already great 1 88 1 80 ga ago this together with the thick high velocity lower crust could have provided the isostatic balance needed to stabilize the thick crust

mineral chemistry constraints on the evolution of the 1 88 1 87 ga post kinematic granite plutons in the central finland granitoid complex abstract a suite of post kinematic 1 88 1 87 ga silicic plutons crosscut 1 89 1 88 ga synkinematic granitoids in the central finland granitoid complex cfgc in south central finland the plutons range from biotite hornblende quartz monzonite to syenogranite and include pyroxene and olivine bearing varieties mineral chemical data on feldspars biotite amphibole pyroxenes olivine and oxides of the post kinematic plutons are presented the data are interpreted to show that these plutons register 1 a considerable range in pressure from 2 4 kbar amphibole barometry to 5 7 kbar olivine pyroxene barometry 2 temperatures mostly reflecting resetting during cooling 450 800 c quilf thermometry and 3 low fo2 log fo2 fmq 0 3 to 1 5 quilf equilibria in particular plutons with olivine and pyroxene bearing margins and amphibole dominated central parts record progressive oxidation and hydration upon cooling shifting from the quilf equilibrium toward kuilb the post kinematic granites can be considered post collisional in regard to compressional events in the cfgc and display many of the characteristics of the anorogenic 1 6 ga rapakivi granites further south they were presumably derived from a deep and dry crustal source like the rapakivi granites

seismic reflection response of folded structures and implications for the interpretation of deep seismic reflection profiles seismic reflection characteristics of several common folded structures have been investigated by forward synthetic modeling in order to interpret correctly the seismic data from ancient and modern orogenic belts the studied folds include single order open folds close folds and asymmetric inclined folds as well as multi order folds the results show that complex folded structures and particularly those in deep crust cannot be accurately imaged by conventional 2d reflection profiling it is possible that widely observed subhorizontal discontinuous reflections in the continental crust of compressional folded belts do not necessarily correspond to subhorizontal compositional lamination multi order folded structures may equally enable the formation of such reflections furthermore trains of asymmetric inclined folds produce straight discontinuous dipping reflections that are potentially misinterpreted as thrusting faults or shear zones thus structural geologists must be extremely cautious when trying to convert directly seismic reflection sections to geological profiles for the continental crust

lithological interpretation of crustal composition in the fennoscandian shield with seismic velocity data abstract in this study we report the results of an investigation of lithological interpretation of the crust in the central fennoscandian shield in finland using seismic wide angle velocity models and laboratory measurements on p and s wave velocities of different rock types the velocities adopted from wide angle velocity models were compared with laboratory velocities of different rock types corrected for the crustal pt conditions in the study area the wide angle velocity models indicate that the p wave velocity does not only increase step wise at boundaries of major crustal layers but there is also gradual increase of velocity within the layers on the other hand the laboratory measurements of velocities indicate that no single rock type is able to provide the gradual downward increasing trends thus there must be gradual vertical changes in rock composition the downward increase of velocities indicates that the composition of the crust becomes gradually more mafic with increasing depth we have calculated vertical velocity profiles for a range of possible crustal lithological compositions the finnish crustal velocity profiles require a more mafic composition than an average global continental model would suggest for instance on the sveka 81 transect the calculated models suggest that the crustal velocity profiles can be simulated with rock type mixtures where the upper crust consists of felsic gneisses and granitic granodioritic rocks with a minor contribution of amphibolite and diabase in the middle crust the amphibolite proportion increases the lower crust consists of tonalitic gneiss mafic garnet granulite hornblendite pyroxenite and minor mafic eclogite assuming that these rock types are present in sufficiently extensive and thick layers they would also have sufficiently high acoustic reflection coefficients for generating the generally well developed reflectivity in the crust in the central part of the shield density profiles calculated from the lithological models suggest that there is practically no density contrast at moho in areas of the high velocity lower crust comparison of reflectors from fire 1 and fire 3 transects and the velocity model from sveka 81 wide angle transect indicated that the reflectors correlate with velocity layering but the three dimensional structures of the crust complicate such comparisons

an early proterozoic ophiolite the jormua mafic ultramafic complex northeastern finland abstract the 1 96 ga old jormua complex comprises a fault bounded unit 2 5 20 km in the centre of the kainuu schist belt at the eastern margin of the early proterozoic svecokarelides the complex was dismembered and metamorphosed to lower amphibolite facies during the svecokarelian orogeny in the intact complex the basal unit was formed by serpentinites the middle unit of mafic dykes and the uppermost extrusive unit of basaltic pillow lavas gabbroic rocks are present as intrusive bodies in the serpentinites and narrow interdyke screens within the dyke unit trondhjemitic rocks occur as minor segregations in the gabbroic rocks or as cross cutting narrow dykes in the mafic dyke unit the dyke unit is locally composed exclusively of subparallel metabasalt and metadolerite dykes in places interdyke metagabbro and serpentinite screens are abundant well preserved chilled margins are common and many dykes are split and intruded by younger dykes the chemical composition of the serpentinites indicates derivation from harzburgitic dunitic mantle peridotites the gabbroic rocks which represent obvious tholeiitic cumulates show extreme differentiation similar to gabbros of the high ti ophiolites and gabbros dredged from present midocean ridges in chemistry the trondhjemitic rocks are close to granitic rocks from mid ocean ridges and high ti ophiolites the similar chemical composition of the dykes and lavas of the complex indicates that they are cogenetic both being tholeiitic metabasalts with chondrite normalized ree patterns ranging from slightly lree depleted to slightly lree enriched ones compared with modern rocks the jormua metabasalts most closely resemble transitional midocean ridge basalts it is suggested that the jormua complex and related mafic ultramafic bodies in eastern finland originated as divergent margin ophiolites concomitantly with the break up of the archaean craton in the extensional stage of the svecokarelian orogeny which culminated in the opening of a major ocean after 1 97 ga the tectonic emplacement of these ophiolites into their present locations occurred in connection with thrusting during the compressional stage of the orogeny about 1 9 ga ago

constraints on melting and magma production in the crust major intrusions of granitic rocks are found in several tectonic settings and in all cases crustal melts may contribute to the volumes of granitic magma high grade metamorphism and partial melting of the crust take place predominantly under fluid absent conditions we present a model for calculating the amounts of melt that may be formed by fluid absent breakdown of micas and amphiboles in common crustal rock types pelitic quartzofeldspathic intermediate and mafic melt proportions depend mainly on the kind of source rock the pressure at which melting takes place the temperature and the hydrous mineral h2o content of the source as a consequence of the pressure dependence of water solubility in silicate melts any given source rock will produce more melt by a given fluid absent reaction at lower pressure at a given pressure higher temperature reactions can produce more melt from a given source rock based on a survey of the compositions of common rock types we show that the amounts of melt can vary from 50vol thus crustal rocks vary widely in their fertility as magma sources depending on the types and amounts of hydrous phases they contain in general muscovite breakdown in pelites will yield only small quantities of melt and lead to migmatite formation biotite breakdown in pelites occurs at higher temperature and because most high grade pelites below granulite grade are biotite rich can yield up to about 50 vol of granitoid melt rocks of intermediate composition and hornblende rich mafic rocks are potentially highly fertile magma sources also provided that the high temperatures necessary for biotite and hornblende breakdown are realized pyroxene rich mafic rocks and quartzofeldspathic rocks are much less potentially fertile data suggest that mechanisms exist for the efficient segregation of melt and restite in systems with 30and probably 20vol melt the pressure temperature history of a region can greatly influence crustal source fertility through its control over the occurrence of subsolidus dehydration and melting equilibria

formation and evolution of granite magmas during crustal reworking the significance of diatexites taylor mclennan 1985 the physical processes ina continuous section through reworked archaean crust records the volved particularly how granite magma is formed remain generation of granitic magma and its subsequent development in the controversial miller et al 1988 white chappell opatica subprovince in the canadian shield there the transition 1990 a partially melted source can form a granitic from palaeosome to granite was a closed system process through magma only if large volumes of melt are separated from intermediate stages of patch migmatite and diatexite the average most of their residuum understanding this process is degree of partial melting was less than 30 but the melt crucial to determining how crustal recycling and differfraction was redistributed within individual diatexite layers during entiation occurs for example melt residuum separation deformation regions that lost melt became residual diatexites could be a nearly perfect single stage process at the enriched in tio2 feot mgo cao sc cr co sr rare earth site of partial melting as described for leucosomes in elements ree and high field strength elements hsfe melt metatexite migmatites wickham 1987a sawyer 1991 accumulated to create diatexite magmas enriched in large ion 1994 brown et al 1995 alternatively the separation lithophile elements lile but contaminated with residuum macould be an imperfect multi stage process that yields a terial such diatexite magmas are parental to granites found at magma with a large residual component as is observed higher crustal levels in the terrane flow of the diatexite magma in in diatexite migmatites bea 1991 greenfield et al response to deformation separated some of its residuum into schlieren 1996 sawyer 1996 the metatexite model necessarily parautochthonous plutons were created where ascending granitic produces leucocratic residuum free granites such as magma locally ponded below impermeable layers and structures those described by le fort 1981 and montel et al magma left the anatectic region in dykes and lost its remaining residuum as it crystallized consequently the allochthonous granite 1991 and maximizes the geochemical signature of magmas that rose through 20 km of crust to feed the highest level crustal differentiation in contrast the diatexite model plutons in this region are highly fractionated and essentially free of can produce residuum rich granites and so reduce the residuum geochemical effect of crustal differentiation as chappell 1996 noted how anatectic magmas formed in the deeper crust evolve to the granitic magmas emplaced in the upper crust is also poorly known because few crustal sections

cenozoic and mesozoic structure of the eastern basin and range province utah from cocorp seismic reflection data cocorp seismic reflection data collected from the eastern basin and range in west central utah provide information on cenozoic extensional tectonics mesozoic thrusting and their interrelationships those data show a series of remarkably continuous low angle reflectors that extend more than 120 km perpendicular to strike and can be traced as deep as 15 20 km over that distance none of these events are significantly cut by any high angle normal faults a major detachment beneath the sevier desert can be traced from a surface zone of normal faulting to a depth of 12 15 km with a regional apparent westward dip of 12 tentative correlation of upper and lower plate events suggests 30 60 km of extensional displacement on this detachment whether this structure is a reactivated mesozoic thrust is uncertain west steepening splays off the end of the detachment reach depths of 20 km and may represent a major mesozoic ramp or zones of distributed ductile shearing during extension some events are interpreted to be mesozoic thrusts of which at least one beneath the house range has been reactivated during the cenozoic the snake range decollement dips gently east and has a sense of cenozoic displacement opposite to that of other cenozoic detachments farther east deep events are most numerous beneath the east side of the sevier desert where they occur to depths of 30 km at the top of or perhaps partly within the anomalously low velocity upper mantle

the orogenic superstructure infrastructure concept revisited quantified and revived the historical superstructure infrastructure concept s i expressed contrasts in structural style and metamorphic grade between shallow and deep orogenic levels two dimensional thermal mechanical models provide a quantitative explanation in terms of progressive crustal shortening and thickening phase 1 thermal relaxation and rheological weakening phase 2 and ductile flow at depth phase 3 results predict an upper crustal superstructure dominated by early steep structures separated across a subhorizontal high strain zone from a ductile infrastructure with late gently dipping structures this is consistent with observations from the western superior province these models can account for contrasts in structural style metamorphic grade seismic reflectivity and age between upper and lower crustal levels in contrast to conventional thrust tectonics models the revived s i model shows how young structures can form beneath older ones during progressive convergence thereby encouraging reassessment of standard seismic reflection interpretations

geochemistry and tectonomagmatic affinities of the metavolcanic rocks of the early proterozoic tampere schist belt southern finland abstract the c 1 9 ga old tampere schist belt is part of the svecofennian province of finland and sweden which was formed by extensive mantle derived crustal growth during the early proterozoic the volcanic rocks of the belt range widely in chemical composition part of the variation is due to alteration and reworking but the main control are primary compositional characteristics the rocks range from almost low k tholeiitic units to shoshonitic and trachytic units intermediate rocks of calc alkaline affinities being most common by and large the volcanic rocks resemble those of present day volcanic arcs calc alkaline intermediate rocks dominate and the contents of tio2 in mafic rocks are often low some units are exceptional by exhibiting non arc affinities they are fairly high in ti and may thus mark extensional tectonic events the percentage of andesites dacites rhyolites or percent adr mostly exceeds 50 which suggests that the crust was more than 20 km thick during the eruption of most of the studied rocks the tampere schist belt thus resembles mature island arcs or volcanic arcs at or close to active continental margins rather than immature intra oceanic island arcs archaean crust is absent and it can be inferred that the crust was segregated from the mantle during the proterozoic the svecofennian province as a whole appears to differ from present day convergent plate margins by the rapidity of its crustal evolution

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

unmetamorphosed sedimentary melange with high pressure metamorphic blocks in a nascent forearc basin setting abstract melanges crop out within unmetamorphosed basal great valley group gvg forearc basin strata and between gvg and the underlying coast range ophiolite cro in the san francisco bay region of coastal california these melanges include high pressure hp metamorphic blocks of the franciscan subduction complex that structurally underlies the unmetamorphosed cro as well as blocks of gvg and cro the matrix consists of foliated shale and serpentinite locally interleaved at centimeter scale the melanges strike and dip parallel to bounding gvg sandstones and conglomerates the matrix locally consists of sedimentary breccia and conglomerate made up of clasts of serpentinite and shale gvg sandstones within and bounding the melanges have detrital serpentinite clasts the field relationships indicate a sedimentary origin of the melanges as olistostromal deposits within latest jurassic to earliest cretaceous basal gvg the melanges correlate to units along the eastern margin of the northern coast ranges about 250 km to the north with restoration of post subduction dextral faulting and differ from the latter in the higher proportion of shale stronger matrix foliation and common occurrence of hp blocks similar units may have mistakenly been assigned to the franciscan owing to the foliated nature of the matrix and occurrence of hp blocks this and the broad distribution of localities indicate that these deposits are more widespread than previously believed exhumation rates of coarse grained hp melange blocks may have been 2 to 10 mm yr or higher based on the ages of similar blocks in the coast ranges burial depth of the blocks and depositional age of the enclosing strata exhumation and deposition of franciscan blocks in these gvg melanges predates preserved accretion of similar materials in the adjacent franciscan by at least 30 m y suggesting subduction erosion of previously accreted material or exhumation of the blocks in forearc serpentinite shale diapirs

crustal channel flows 2 numerical models with implications for metamorphism in the himalayan tibetan orogen 1 results from a thermal mechanical model ht1 that includes midcrustal channel flow are compatible with many features of the himalayan tibetan system radioactive self heating and rheological weakening of thickened model orogenic crust lead to the formation of a hot low viscosity midcrustal channel and a broad plateau channel material corresponding to the greater himalayan sequence ghs flows outward from beneath the plateau in response to topographically induced differential pressure at the plateau flank it is exhumed by focused surface denudation and juxtaposed with cooler newly accreted material corresponding to the lesser himalayan sequence lhs the model channel is bounded by coeval thrust and normal sense ductile shear zones interpreted to represent the main central thrust mct zone and south tibetan detachment system respectively inverted metamorphism associated with the model mct zone results from distributed ductile shear along the mct and extrusion of the hot channel a variety of model p t t path styles resembling those observed in the ghs and lhs are produced for points traveling through contrasting tectonic regimes that coexist in different parts of the model predicted times of peak metamorphism cooling and erosion of metamorphic facies are generally compatible with observations although model ghs cooling ages are too young the times of m1 and m2 metamorphic events observed in the ghs correspond to model times of maximum burial and maximum heating respectively the results highlight the need to integrate tectonics and metamorphism in continental collision models and demonstrate the importance of lateral transport of both heat and material in large hot orogens

analog modeling of one way gravitational spreading of hot orogens a case study from the svecofennian orogen fennoscandian shield abstract geodynamic models offer insight into deformation styles of syn and post collisional extensional processes such models often ignore the effect of lateral viscosity contrast between accreted terranes and inherited tectonic boundaries this paper presents a set of crustal scale analog experiments of one way post collisional gravitational spreading testing the effect of crustal heterogeneity by using two mechanically contrasting blocks pre existing cuts with varying dips and three layers with contrasting rheologies two blocks represent strong mature and weak juvenile crustal parts both blocks have three horizontal crustal layers brittle upper weak partially molten middle and dry strong lower layer cuts represent pre existing crustal scale shear zones and tectonic boundaries the experiments indicated that tectonic boundaries may facilitate exhumation and increase extensional strain when the boundaries dip opposite to the flow direction the tectonic boundaries may also split and divide the crust into separate blocks with independent deformation signatures shortening elongation or rotation the tectonic boundaries rotated along vertical axes toward the flow direction the modeling results suggest that in areas with inherited tectonic boundaries and compositional differences the amount of extension is unevenly distributed between the different parts during the widespread unilateral gravitational spreading and that the flow has different effects on the contrasting crustal units in both lateral and vertical directions to validate the modeling the results are compared to geophysical and geological data from the paleoproterozoic svecofennian orogen in the fennoscandian shield the comparison suggests that the orogen underwent 50 extension and was thinned by maximum of 20 via westward gravitational spreading this spreading rotated the crustal scale blocks shear zones and tectonic contacts it also brought exhumed high grade middle crustal blocks into contact with subsided low grade upper crustal blocks we suggest that results of the analog models can be used as proxies in both ancient and modern hot accreted orogens which have undergone post convergence continental mid crustal weakening and have comprised of juxtaposed terranes with varying rheological compositions

three stages to form a large batholith after terrane accretion an example from the svecofennian orogen abstract the magmatic history of batholiths formed as the result of tectonic accretion mirrors the evolution of the underlying crust from island arcs to accretion and collapse chemical composition age and structure give information about the magma source and emplacement depths the target of this study is the central finland batholith cfgc that provides a magmatic record after terrane accretion during the svecofennian orogeny followed by orogenic collapse the crustal source of partial melting has changed during the crustal evolution and differentiation after the terrane accretion and when the stress regime has changed from contraction to extension here we present a three stage magmatic evolution model for accretional batholith in the svecofennian orogen in the first stage the lower crust of the accreted crust melts partially and produces calcic tonalitic melts that are emplaced above the upper middle crustal detachment zone group 1 1887 3 ma in the second stage the middle crust begins to melt producing calc alkalic granodioritic melts that are emplaced close to the surface group 2 1890 1886 ma and at both sides of the upper middle crustal detachment zone group 2 1884 1883 ma in the third stage the lower crust remelts and produces bimodal calc alkalic gabbroic and alkali calcic to alkalic granitic melts group 3 1881 1880 ma which are emplaced above the upper middle crustal detachment zone this kind of crustal differentiation produces a mafic lower an intermediate middle and a felsic upper crust the third stage represents the last magmatic event of the svecofennian accretional orogeny where the high velocity lowermost crust is formed during this stage crustal thickening and widespread mid crustal partial melting have inititated lateral flow after or concurrently with the emplacement of the group 2 granodiorite the lateral flow has reactivated large scale shear zones allowing the transportation of the group 3 rocks from the lower crust to the upper middle crustal detachment zone the change in the degree of deformation at the exposed parts from undeformed through local to pervasive is attributed to differences in the exhumation level

a tomographic crustal velocity model of the central fennoscandian shield summary in this paper local seismic tomographic method is used to find the terrane distribution within the central parts of the accretionary svecofennian orogen this study presents a crustal pwave velocity model and for the first time the s wave velocity and vp vs ratio model of svekalapko area that is 700 800 km 2 in southern and central finland the 3 d model is presented as p and s wave velocity as well as vp vs ratio depth slices and vertical sections the data set comprised of several subsets of crustal pg and sg wave traveltime data from local events recorded by the svekalapko seismic tomography array in 1998 1999 from older controlled source shots recorded at portable stations as well as at permanent stations and from non controlled chemical explosions recorded at permanent seismic stations from 300 local explosions a total of 10 404 pg wave and 9361 sg wave arrival times were inverted to create independent 3 d vp and vs tomographic models from which the vp vs ratio was calculated according to sensitivity tests the optimal horizontal resolution of recognizable velocity structures in the central study area is at least 60 km down to depths of 40 km in the final model the seismic velocities vary smoothly the lateral variations are larger for vp 5 9 6 6 km s 1 than for vs 3 5 3 8 km s 1 in the upper 20 km of the crust at depths of 20 40 km vp varies from 6 5 to 7 2 km s 1 and vs from 3 7 to 4 1 km s 1 the vp vs ratio varies spatially more distinct than p and s wave velocities usually from 1 70 to 1 74 in the upper crust and from 1 72 to 1 78 in the lower crust schist belts and their continuations at depth are associated with lower velocities and lower vp vs ratios vp 6 2 6 8 km s 1 vs 3 6 3 9 km s 1 vp vs 1 68 1 73 than the granitoid areas vp 6 3 7 4 km s 1 vs 3 6 4 2 km s 1 vp vs 1 72 1 78 the svecofennian orogen was accreted from crustal blocks ranging in size from 100 100 km 2 to 200 200 km 2 in cross sectional area the intervening sedimentary belts have ca 0 2 km s 1 lower p and s wave velocities and ca 0 04 lower vp vs ratios tomographic model supports the idea that the thick svecofennian crust was accreted from several crustal terranes some hidden and that the crust was later modified by intra and underplating

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

heterogeneous exhumation of the mount isa orogen in ne australia after 1 6 ga nuna assembly new high precision 40ar 39ar thermochronological constraints

crustal structure across the san andreas fault southern california from teleseismic converted waves crustal structure along a 140 km long profile across the san andreas fault saf in southern california was imaged by stacking teleseismic p s converted phases recorded by a dense short period seismic array the crust mantle discontinuity moho is visible as a continuous feature at a depth around 30 km but is offset 6 to 8 km beneath the saf a small moho disruption can also be seen under the eastern california shear zone ecsz these results suggest that the saf and ecsz extend through the entire crust the moho upwarp under the san gabriel mountains indicates that the mountain ranges were lifted en masse as a result of crustal buckling under horizontal compression

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

alkali calcic and alkaline post orogenic po granite magmatism petrologic constraints and geodynamic settings abstract the end of an orogenic wilson cycle corresponds to amalgamation of terranes into a pangaea and is marked by widespread magmatism dominated by granitoids the post collision event starts with magmatic processes still influenced by subducted crustal materials the dominantly calc alkaline suites show a shift from normal to high k to very high k associations source regions are composed of depleted and later enriched orogenic subcontinental lithospheric mantle affected by dehydration melting and generating more and more k and lile rich magmas in the vicinity of intra crustal magma chambers anatexis by incongruent melting of hydrous minerals may generate peraluminous granitoids bearing mafic enclaves the post collision event ends with emplacement of bimodal post orogenic po suites along transcurrent fault zones two suites are defined i the alkali calcic monzonite monzogranite syenogranite alkali feldspar granite association characterised by biotite plagioclase fractionation and moderate lile hfse enrichments and ii the alkaline monzonite syenite alkali feldspar granite association characterised by amphibole alkali feldspar fractionation and displaying two evolutionary trends one peralkaline with sodic mafic mineralogy and higher enrichments in hfse than in lile and the other aluminous biotite bearing marked by hfse depletion relative to lile due to accessory mineral precipitation alkali calcic and alkaline suites differ essentially in the amounts of water present within intra crustal magma chambers promoting crystallisation of various mineral assemblages the ultimate enriched and not depleted mantle source is identical for the two po suites the more primitive lile and hfse rich source rapidly replaces the older orogenic mantle source during lithosphere delamination and becomes progressively the thermal boundary layer of the new lithosphere present rock compositions are a mixture of major mantle contribution and various crustal components carried by f rich aqueous fluids circulating within convective cells created around magma chambers in favourable areas po suites pre date a new orogenic wilson cycle

paleoproterozoic lateritic paleosol ultra mature mature quartzite meta arkose successions in southern fennoscandia intra orogenic stage during the svecofennian orogeny abstract a paleoproterozoic lateritic paleosol overlain by ultra mature mature quartzite is reported for the first time from three localities in the svecofennian province in southern finland these are important indicators for warm paleoclimate and continental paleoenvironment ultra mature quartzites above the lateritic paleosols are overlain by matrix supported conglomerates followed by mature quartzites these latter rocks are related to incipient rifting followed by main rift stage starting with increasingly more immature meta arkose we present a tectonic model in which exhumation after 1 88 1 87 ga contraction unroofed plutonic rocks volcanism and exhumation related erosion produced deposition of both volcanic and sedimentary material weathering of these rocks and deposition of ultra mature mature quartzites took place between 1 87 and 1 85 1 84 ga followed by rifting at 1 84 ga tectonic slicing during basin inversion thrust the pre rift quartzites and rift related volcanic and sedimentary rocks over older mid crustal rocks and produced a low angle stacking system to accommodate voluminous magmatism at deeper crustal levels and widespread sedimentation at the surface we propose that exhumation due to faulting followed by stabilization of the upper crust 0 15 20 km occurred simultaneously as the detached hot middle and lower crust experienced vertical shortening via ductile flow and extrusion the ca 1 85 ga paleosol quartzite successions in southern fennoscandia have many similarities with the widespread ca 1 7 ga paleosol quartzite successions in the united states both are intra orogenic formed on hot orogens and have simultaneous magmatism at deeper crustal levels and sedimentation at the surface accretion type models presented for southern fennoscandia at 1 87 1 80 ga are not compatible with the available data and either far field setting during outboard subduction or a newly formed continental interior setting are proposed anyhow tectonic models proposed for southern fennoscandia between 1 87 and 1 80 ga should take into account the occurrence of lateritic paleosols and ultra mature quartzites

first deep seismic reflection images of the eastern alps reveal giant crustal wedges and transcrustal ramps 1 the alps are considered as a classical example for an orogen created by continental plate collision in this study we present new images obtained from deep seismic reflection profiling in the eastern alps between munich and venice which give rise to examine and revise existing concepts the seismic sections exhibit a prominent bi verging reflection pattern at crustal scale a major ramp like structure outcropping at the inn valley fault can be traced southward over 80 km into the mountain root where relics of the subducted penninic ocean are expected new models of the evolution of the eastern alps show an upper lower crustal decoupling along transcrustal thrust faults with opposite thrust directions of both the european and the adriatic african continents

exhumation of high pressure rocks a review of concepts and processes the exhumation of high pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures or the transport of the metamorphic rocks through the overburden exhumation cannot be achieved simply by thrusting or strike slip faulting it may be caused by erosion of shortened and thickened crust but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km one or more of the following additional mechanisms may be involved 1 corner flow of low viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock and may explain some occurrences of high pressure tectonic blocks in melange this process does not however appear to be adequate to explain the exhumation of regional high pressure terrains 2 buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings this is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle but cannot explain the emplacement of coherent tracts of high density metamorphic rock into shallow crustal levels some high pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low density mud or serpentinite 3 extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high pressure terrains extension may occur in the upper part of an accretionary wedge thickened by underplating or it may affect the whole lithosphere in a region of intracontinental convergence if surface elevation has been increased by the removal of a lithospheric root in the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain

continental growth by accretion a tectonostratigraphic terrane analysis of the evolution of the western and central baltic shield 2 50 to 1 75 ga tectonostratigraphic terrane analysis of the western and central baltic shield defines a framework for continental growth through the period 2 50 1 75 ga eight discrete terranes can be defined in this part of the svecokarelian orogen the older cratonic kuhmo and iisalmi terranes the hybrid lapland and savo province allochthonous terranes the juvenile island arc like skellefte savonlinna and south finland central sweden terranes the back arc or arc like outokumpu nappe and the ophiolitic oceanic jormua nappe stitching events can also be recognized these include the ca 1 88 1 87 ga svionian magmatic arc the ca 2 44 go koillismaa intrusions the kalevian flysch basin and the bothnian basin constraining the age of these stitching events permits the construction of an accretion history in which the svecokarelian orogeny can be resolved as a number of accretion events deformation episodes related to accretion or magmato tectonic episodes these events defined here include the pre 2 44 go pohjolan accretion of the component parts of the lapland hybrid terrane the ca 1 95 ga kyllikian accretion of the karelian collage the ca 1 90 ga karelian orogeny marked by the development of the savo thrust belt in response to the accretion of the skellefte savonlinna terrane to the karelian collage and the ca 1 88 1 87 ga svionian magmatic arc the whole svecokarelian collage had assembled by ca 1 85 ga an amalgamation succeeded and stitched by a diverse collection of igneous and thermal events through the period 1 85 1 75 ga

deciphering the paleoproterozoic cooling history of the northeastern trans hudson orogen baffin island canada using 40 ar 39 ar step heating and uv laser thermochronology abstract the previously unstudied cooling and exhumation history of mid crustal rocks exposed on southeastern baffin island canada provides new insights into the post orogenic evolution of the paleoproterozoic trans hudson orogen tho new 40 ar 39 ar step heat analyses of biotite muscovite and phlogopite and core to rim intra grain 40 ar 39 ar analyses of muscovite have a range of apparent ages compatible with slow regional cooling following peak metamorphism twenty nine amphibolite to granulite facies rocks were dated using the 40 ar 39 ar step heating laser co 2 method 40 ar 39 ar spot analyses were performed across muscovite grains from three samples using an ultraviolet uv laser to investigate intra grain 40 ar 39 ar age variations step heating apparent ages range from ca 1788 1622 ma for biotite 1720 1630 ma for phlogopite and 1729 1657 ma for muscovite uv spot 40 ar 39 ar analyses in the three muscovite grains range from ca 1661 1640 ma 1675 1645 ma and 1680 1652 ma with core to rim apparent age gradients of 20 30 myr previous studies resolved peak metamorphism in this region to between ca 1860 and 1820 ma and identified late to post tho zircon and monazite populations at ca 1800 1750 ma numerical diffusion models for ar in muscovite were conducted to test different proterozoic cooling and exhumation scenarios comparisons with our 40 ar 39 ar ages attest to cooling rates of 1 2 c myr following peak metamorphism and 1 5 2 5 c myr after ca 1740 ma anomalously old apparent 40 ar 39 ar ages in cases equivalent to u pb zircon rim and monazite ages likely result from incorporation of excess ar the results suggest that mid crustal rocks on southeastern baffin island remained hotter than 420 450 c for 150 200 myr after peak metamorphism with subsequent slow cooling and denudation rates that are typical of proterozoic orogens the apparent absence of orogenic collapse implies that despite high temperatures and estimated maximum crustal thicknesses comparable to those of large hot orogens the tho remained gravitationally stable during its terminal phase

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

terrane accumulation and collapse in central europe seismic and rheological constraints abstract an attempt is made to compare the tectonic units and their evolution in central europe with the deep seismic velocity structure and patterns of reflectivity caledonian and variscan terrane accretion and orogenic collapse dominate the tectonic development in central and western europe and have left their marks in a distinct velocity structure and crustal thickness as well as in the various reflectivity patterns whereas the memory of old collisional structures is still preserved in the rigid upper crust collapse processes have formed and modified the lower crust they have generally created rejuvenated thin crusts with shallow mohos in the variscan internides the center of collision and post orogenic heat pulses the lower crust developed strong and thick seismic lamellae the cooler externides show a thrust and shear pattern in the whole crust and the north german basin experienced large mafic intrusions in the lower crust and developed a high velocity structure with only very thin lamellae on top of the moho the various kinds of reflectivity patterns in the lithosphere can be explained by a thermo rheological model from terrane collision with crustal thickening to collapse in a hot post orogenic setting