an inverted double seismic zone in chile evidence of phase transformation in the subducted slab data from two microseismic field experiments in northern chile revealed an elongated cluster of earthquakes in the subducted nazca plate at a depth of about 100 kilometers in which down dip tensional events were consistently shallower than a family of compressional earthquakes this double seismic zone shows a distribution of stresses of opposite polarity relative to that observed in other double seismic zones in the world the distribution of stresses in northern chile supports the notion that at depths of between 90 to 150 kilometers the basalt to eclogite transformation of the subducting oceanic crust induces tensional deformation in the upper part of the subducted slab and compressional deformation in the underlying mantle

stable isotope evidence for limited fluid infiltration of deep crustal rocks from the ivrea zone italy isotopic and petrologic studies of the ivrea zone a segment of deep crustal high grade rocks suggest that metamorphism did not involve the transfer of large quantities of co sub 2 from mantle to crust high grade ivrea zone calcites may retain high delta sup 18 o up to 24 per thousand smow indicating little interaction with externally derived fluid graphite isotopic compositions delta sup 13 c 10 per thousand to 25 per thousand pdb that do not vary with grade are attributed to mixing between carbonate carbon and biogenic noncarbonate carbon calcites from high grade carbonate poor amphibolites have delta sup 13 c of about 1 per thousand pdb and sedimentary not infiltrative origins the general lack of carbon and oxygen isotopic homogenization suggests that fluid interactions may be explained in terms of fluid generated internally to the ivrea zone metasedimentary rocks

rejuvenation of continental lithosphere beneath an intracratonic basin abstract the effective elastic thickness of the lithosphere in the williston basin region has been determined for several time intervals by fitting an elastic flexure equation to the shape of the basin over time the elastic thickness increases from about 40 km just after the basin began subsiding 450 m y b p to about 80 km at the present these results agree well with the increasing elastic thickness with age predicted for a cooling 250 km thick plate taking elastic thickness to be depth to the 450 c isotherm if we assume a thermal age of the lithosphere of about 500 m y radiometric dates of basement in the area however generally yield ages of 1 7 b y churchill province and 2 5 b y superior province because basement ages are generally much greater than the inferred lithospheric thermal age in the williston basin region we suggest that lithosphere in this area was thermally rejuvenated about 500 m y b p by the same process es responsible for the thermal subsidence of the basin temperatures were not high enough in the upper crust to reset most radiometric clocks additional support for a thermal event 500 m y b p comes from fission track ages and a single rb sr age of basement rocks of about 500 m y lithospheric rejuvenation has been modelled using a one dimensional finite difference thermal model the base of a 250 km thick plate is heated from 1333 c to 1850 c approximately the solidus of basalt at that depth for approximately 100 m y with conduction only the depth to the 450 c isotherm slowly decreases then slowly increases after heating ceases rapid thinning and slow thickening of the lithosphere can be achieved only if convective thinning is simulated in the model the model with convection yields results consistent with the observed increase in lithospheric thickness as well as the observed subsidence of the basin over time

mechanisms of uplift preceding rifting abstract mareschal j c 1983 mechanisms of uplift preceding rifting in p morgan and b h baker editors processes of continental rifting tectonophysics 94 51 66 the feasibility of three mechanisms that have been proposed to account for the uplifts and heat flow anomalies which might precede rifting is examined the conductive heating following an increase in the heat flux at the base of the lithosphere could result in large scale uplifts 1000 m for 15 mw m2 change in heat flux on a time scale of 108 years a pulse of additional heat flow could induce a more rapid uplift but not the surface heat flow anomaly which is always delayed the advection of heat by magma intrusion into the lithosphere could also produce the uplift and heat flow anomaly if magmas are injected at a steady rate an uplift of the order of 1000 m can be induced in about 50 106 years but it requires the rate of magma injection to be equivalent to 0 3 of the total volume of the lithosphere per 106 years a short intense episode of magma invasion could explain a rapid uplift and change in surface heat flow but it requires the replacement of about 30 of the total volume of the lithosphere the complete replacement of the lithosphere by the rise of a diapir of lighter and hotter asthenospheric material has been modelled numerically the model indicates that the rise of the asthenospheric diapir can take place in a very short time 3 106 years and produce the surface uplift and the crustal thinning observed in rift systems provided that following heating the lower lithosphere viscosity is reduced to 1021 pa s

dynamic link between the level of ductile crustal flow and style of normal faulting of brittle crust abstract in a rheologically layered crust compositional layers have an upper elasto plastic part and a lower viscous one when broken the upper elastic part undergoes flexure which is upward for the foot wall and downward for the hanging wall as a consequence of bending stresses will develop locally that can overcome the strength of the plate and therefore impose the migration of active fault in the lower viscous part of each compositional layer rocks can potentially flow numerical modelling of the behaviour of a crust made up of two compositional layers during and following extension shows that flow can take place not only in the lower crust but also and more importantly in the lower part of the upper crust the ability of crustal rocks to flow influences the style and kinematics of rifted regions when no flow occurs subsidence will affect the extending areas both hanging wall and foot wall will subside with respect to an absolute reference frame such as sea level and there will be a strict proportionality between extension and thinning in addition the downward movement of the fault blocks will decrease the local stresses created in the foot wall and increase those of the hanging wall thereby imposing a migration of fault towards the hanging wall this is the behaviour of extensional settings developed on stabilised crust and which evolved in a passive margin when flow does take place middle crustal rocks will move towards the rifting zone causing isostatically driven upward movements that will be superimposed on movements associated with crustal and lithospheric thinning consequently fault blocks will move upwards and the crust will show more extension than thinning the upward movements will decrease the stresses developed in the hanging walls and increase those of the foot wall faults will then migrate towards the foot wall such a mode of deformation is expected in regions with thickened crust and has its most apparent expression in core complexes

role of crustal stretching on subsidence of the continental crust abstract the principal areas of subsidence of the continental crust are wide intracontinental basins and passive continental margins in many intracontinental basins crustal extension amounts to only a few percent and is unable to explain their subsidence as shown for the timan pechora pre caspian and west texas basins the deep black sea and south caspian basins are underlain by highly attenuated continental crust having velocities in the 7 km s range that shows no evidence for significant extension in passive margins upper crustal extension ranges between 10 and 40 whereas the lower crust is attenuated by 2 3 times e g campos basin north biscay margin large discrepancy between upper crustal stretching and lower crustal attenuation is seen in the crustal configuration of many intracontinental grabens e g baikal and dniepr prypyat basins only in highly extended areas such as the basin and range province and the east china rift system does crustal stretching account for the observed crustal thinning the observed discrepancy between upper extension and crustal thinning across intracontinental rifts and passive margins and particularly in some wide intracontinental basins suggests that crustal stretching alone cannot account for the subsidence of these basins phase transformations in the lower crust probably facilitated by the presence of hydrous fluids can cause rapid 1 ma as well as long term 200 1500 ma upward displacement of the moho discontinuity resulting in high amplitude subsidence of the crust in areas of crustal extension this mechanism can overprint subsidence induced by thermal relaxation of the lithosphere the case of the black sea and south caspian basins shows that density increase of the lithosphere can also occur independently of crustal stretching

crustal scale ductile fault systems in the arunta inlier central australia detailed geologic observations along three 100 km long traverses across the early proterozoic arunta inlier central australia indicate that paleozoic ductile shear zones are organized into crustal scale structural systems which control the map pattern and distribution of metamorphic zones two provinces can be distinguished 1 1 a southern one where shear zones form a series of hinterland dipping and south verging imbricate reverse faults in the west redbank area grading eastward into a foreland dipping duplex in the arltunga nappe complex directly north of this complex flat lying paleozoic south directed ductile shear affects a large portion of the crust a point which has been underestimated so far 2 2 the northern province is characterized by both south and north dipping shear zones showing reverse motion to produce a crustal scale pop up structure presumably resulting from a more coaxial deformation history strike slip shear zones become predominant in the eastern part of this province stratigraphic and cross cutting relationships indicate that the southern systems developed first the northern systems probably developed in response to the lock up stages of overthrusting in the southern province one important result of this work is that the distribution of granulite facies rocks and major geophysical anomalies throughout the arunta inlier is best explained by their position relative to the paleozoic ductile fault systems described above there is no need to invoke extensive proterozoic uplift of these lower crustal materials

numerical modeling on the relationship between thermal uplift and subsequent rapid subsidence discussions on the evolution of the tainan basin existing evidence shows that an oligocene erosion event occurred on the northern continental margin of the south china sea and the tainan basin area might be at the center of this event followed by a rapid tectonic subsidence in the late oligocene and early miocene period the rapid tectonic subsidence is mainly thermal controlled and the effect of the yichu fault on the tainan basin is limited to the basin s eastern part we develop a 2 d thermal mechanical kinematic numerical model to explore the relationship between thermal uplift and subsequent rapid subsidence in the tainan basin our modeling indicates that the oligocene uplift erosion and subsequent rapid subsidence could be caused by a thermal event and the differential subsidence of the basement caused by thermal contraction can initiate the development of small faults however it also suggests that other mechanisms might be needed to jointly account for the observed erosions citation shi x h xu x qiu k xia x yang and y li 2008 numerical modeling on the relationship between thermal uplift and subsequent rapid subsidence discussions on the evolution of the tainan basin tectonics 27 tc6003 doi 10 1029 2007tc002163

phase change and thermal subsidence of the williston basin summary a mechanism of subsidence for the williston basin is proposed and tested the mechanism combines cooling and thermal contraction of the lithosphere with a phase transformation in the lower crust the cooling and the phase boundary movement following a sudden change in the thermal boundary condition at the base of the lithosphere are calculated isostatic adjustments and sea level variations are included to determine subsidence and sediment accumulation at the basin s centre which are compared with the data the subsidence record could not be explained by temperature boundary conditions the calculations show that 1 the heat flow boundary condition explains the long duration more than 350 myrs of subsidence of the williston basin and 2 the delay of the phase change subsidence about 40myrs explains an acceleration of subsidence in the early stages of the basin s evolution when isostatic adjustments and the effect of sea level variations are included the calculated sediment accumulation history fits well the sediment record of the williston basin

the effect of lithospheric phase transitions on subsidence of extending continental lithosphere abstract a new simple model for sedimentary basin formation which combines both stretching and phase transitions occurring together in the lithosphere is proposed and two important aspects in sedimentary basin formation are addressed 1 the explanation of the domal uplift preceding rifting and 2 the ability of the model to explain the contradiction between a small amount of crustal stretching and the large amount of thermal subsidence without the need to invoke differential stretching between the crust and mantle relationships for synrift initial and thermal post rift subsidence are derived the model predicts that extension of continental lithosphere could produce synrifting uplift up to a threshold amount of extension and then subsidence for greater extension three dimensionless parameters quantify the contribution of the phase transition to the subsidence history 1 the clapeyron slope normalized to the initial geothermal gradient 2 the ratio of depth of phase transition to the thickness of the lithosphere and 3 the ratio of density effect due to phase transition to the change in density due to thermal expansion an evaluation of these parameters has been carried out by using thermodynamical data for the garnet spinel transition for upper mantle rocks our simple model shows that by including the effects of lithospheric phase transition in existing stretching models of sedimentary basin formation we can obtain an extra significant parameter controlling the subsidence and the entire extensional tectonics of such basins

from high plateaux to marginal basins the active role of the mantle current geodynamic models pose many serious problems for the description of continental tectonic processes serious difficulties arise also in the description of some of the ocean basins deformation of the earth s crust appears to be concentrated along several mobile megashear belts it is proposed that these could be the surficial expression of mantle chimneys which extend to great depths within these structures post orogenic high plateaux form and as time passes these develop a central depression enclosed within arcs or indeed full scale rings of mountains the enlargement and deepening over time of the back arc depression occurs in concert with the radial migration of a topographic wave in this respect the following structures represent different phases in an evolutionary sequence a the tibetan plateau b the north american cordillera and c the tyrrhenian basin and calabrian arc the development of this succession reflects a particularly high degree of activity in the mantle in which diapiric rising of asthenospheric matter elevates the land and causes tensional rifting the resulting centrifugal movement of the mantle melt results in crustal foundering and engulfment of the area behind the arc which thus becomes susceptible to the subsequent processes of com pressional deformation

a review of the in ouzzal granulitic terrane tuareg shield algeria its significance within the pan african trans saharan belt the in ouzzal terrane iot or in ouzzal granulite unit iogu is an elongated palaeoproterozoic block within the neoproterozoic pan african belt of north west africa the granulites derive from archaean protoliths that include a large volume of metasediments which were deposited on a 3 2 ga gneissic basement near peak granulite facies conditions between 2 17 and 2 ga were estimated at p 10 kbar and t rising from 800 to 1000 c premetamorphic orthogneisses were intruded at 2 5 ga and followed by the emplacement of syn to late kinematic charnockites syenites and carbonatites at around 2 ga cooling of the granulites occurred till 1800 ma shortly after its exhumation coeval with crustal extension and related alkaline magmatism in adjacent areas the iot was buried beneath late palaeoproterozoic and neoproterozoic cover sequences and then behaved as a rigid block both margins are lithospheric faults as evidenced by the occurrence of shear zone related mafic and felsic plutons pan african tectonothermal events were negligible in the north but granulites in the south were significantly reworked under lower greenschist facies conditions during the northern motion of the block with respect to both the western and the eastern pan african terranes the cambrian molasse associated with widespread alkaline volcanism and subvolcanic granites is horizontal in the north the iot which was part of a larger continental mass including its counterpart in northern mali is interpreted as an exotic terrane which may have docked during pan african plate convergence and lateral collision the unchanged pediplain since c 1 7 ga in the north suggests that the iot is underlain by thick palaeoproterozoic lithospheric mantle whereas its southern part is probably allochthonous and overlies pan african structural units

lithospheric shear zones and mantle crust connections abstract a crustal scale ductile shear zone network in the precambrian granulite facies crust of madagascar is examined to determine the nature of the connections between the mantle and lower crust based on three independent data sets field and satellite mapping c and o isotope geochemistry and gravimetry this crust is divided into three zones 1 outside of shear zones 2 minor shear zones that are 350 km long up to 1000 km and 20 35 km wide the mantle is uplifted by about 10 km beneath the major shear zones the major shear zones are rooted in and are inferred to be controlled by the mantle they directly tapped mantle derived co 2 the small scale minor shear zones were controlled by crustal processes and focused crustally derived h 2 o rich co 2 fluids the regular distribution of the shear zones on a crustal scale is in agreement with models of buckling of the continental lithosphere in a compressional context the propagation of these mechanical instabilities promoted and channelled fluid flow these major pan african shear zones thinned the crust and were reactivated during the subsequent drifting of madagascar and opening of the indian ocean during jurassic to cretaceous times they also controlled many of the brittle fault zones in the overlying sedimentary basins mantle rooted large scale shear zones are inferred to be a general feature of cratonic areas reactivated by shear zone systems

geodynamics of rifting and implications for hydrocarbon habitat abstract rifting activity is probably mainly governed by frictional forces exerted on the base of the lithosphere by the convecting sub lithospheric upper mantle combined with deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system and by far filed stresses controlling the interaction of lithospheric plates moving relative to each other mantle plumes rising from the deep mantle do not appear to be a major driving force of rifting therefore it is questioned whether a distinction between active and passive rifts is still justified lithospheric extension triggers by decompression partial melting of the lower lithosphere and upper asthenosphere magmatic processes contribute materially to thinning of the subcrustal lithosphere and probably also of the lower crust this could explain the frequently observed discrepancy between upper crustal extension by faulting and lower crustal attenuation as defined by refraction and deep reflection seismic data the duration of the rifting stage of palaeorifts and passive margins is highly variable depending on plate interaction crustal separation can be achieved after as little as 7 ma and as much as 280 ma of rifting activity the availability of crustal discontinuities does not appear to play a significant role in the time required for crustal separation to be achieved the structural style of rifts is controlled by the physical state of the lithosphere the mode and amount of its extension and the lithological composition of pre and syn rift sediments subsidence of rift basins is controlled by the balance between isostatic adjustment of the crust to lithospheric stretching and uplift induced by mechanical and thermal attenuation of the subcrustal lithosphere post rift basin subsidence is mainly governed by the decay of rift induced thermal anomalies densification of the lower crust and intra plate stresses provide overprinting effects in view of this it is unlikely that quantitative analyses of post rift basin subsidence give a direct measure of lithospheric extension tectonically active rifts palaeorifts and passive margin basins contain major hydrocarbon provinces their hydrocarbon charge can rely exclusively on pre rift syn rift or post rift sedimentary sequences or a combination thereof maturation of source rocks can be achieved during the syn and or post rift stage of basin evolution during rifting conductive and convective heat transfer accounts for elevated geothermal gradients these play an important role in the maturation of pre and syn rift source rocks thermal gradients decrease asymptotically during the post rift stage

stretching factor estimation for the long duration and multi stage continental extensional tectonics application to the baiyun sag in the northern margin of the south china sea abstract conventional subsidence analysis methods are generally based on the assumption of instantaneous stretching which totally ignores the heat loss during continental rifting for long duration or multi episodic extensional tectonics such assumption is no longer valid and could cause significant deviation from real subsidence here i extend original finite stretching model to a multi stage scenario in terms of geothermal and structural successions between two adjacent rifting events a procedure is designed to estimate stretching factor by fitting the whole tectonic subsidence history in a self consistent manner on the basis of the extended model i apply this procedure to the baiyun sag in the continent slope of the south china sea subsidence analysis shows that the sag is characterized by two episodes of rifting the first stage 49 to 23 8 ma resulted in stretching factors in the range of 1 3 to 4 0 with the greatest stretching at the sag center while the second one 17 5 to 10 5 ma yielded minor stretching factors of 1 1 to 1 2 these estimates agree well with that determined from crustal thinning for the northern and central potions of the sag but significant discrepancy occurs between the southern edge of the sag and the present continent ocean boundary where the crustal thickness is less than 20 km and potentially extensive magmatic intrusion was well imaged in the seismic profiles i suggest that the magma assisted rheologically weakening and possible lower crust ductile flow contributed to the extension discrepancy the margin north of the baiyun sag having a relatively thick crust and vacant of magmatic activity was basically stretched in a depth uniform style it is inferred that preexisting weakness in the lithosphere and abundant sediment supply were responsible for the extremely thick cenozoic sediment developed in the baiyun sag

magma assisted strain localization in an orogen parallel transcurrent shear zone of southern brazil in a lithospheric scale orogen parallel transcurrent shear zone of the pan african dom feliciano belt of southern brazil two successive generations of magmas an early calc alkaline and a late peraluminous have been emplaced during deformation microstructures show that these granitoids experienced a progressive deformation from magmatic to solid state under decreasing temperature conditions magmatic deformation is indicated by the coexistence of aligned k feldspar plagioclase micas and or tourmaline with undeformed quartz submagmatic deformation is characterized by strain features such as fractures lattice bending or replacement reactions affecting only the early crystallized phases high temperature solid state deformation is characterized by extensive grain boundary migration in quartz myrmekitic k feldspar replacement and dynamic recrystallization of both k feldspar and plagioclase decreasing temperature during solid state deformation is inferred from changes in quartz crystallographic fabrics decrease in grain size of recrystallized feldspars and lower ti amount in recrystallized biotites final low temperature deformation is characterized by feldspar replacement by micas the geochemical evolution of the synkinematic magmatism from calc alkaline metaluminous granodiorites with intermediate 87sr 86sr initial ratio to peraluminous granites with very high 87sr 86sr initial ratio suggests an early lower crustal source or a mixed mantle crustal source followed by a middle to upper crustal source for the melts shearing in lithospheric faults may induce partial melting in the lower crust by shear heating in the upper mantle but whatever the process initiating partial melting lithospheric transcurrent shear zones may collect melt at different depths because they enhance the vertical permeability of the crust these zones may then act as heat conductors by advection promoting an upward propagation of partial melting in the crust synkinematic granitoids localize most if not all deformation in the studied shear zone the regional continuity and the pervasive character of the magmatic fabric in the various synkinematic granitic bodies consistently displaying similar plane and direction of flow argue for accommodation of large amounts of orogen parallel movement by viscous deformation of these magmas moreover activation of high temperature deformation mechanisms probably allowed a much easier deformation of the hot synkinematic granites than of the colder country rock and consequently contributed significantly to the localization of deformation finally the small extent of the low temperature deformation suggests that the strike slip deformation ended approximately synchronously with the final cooling of the peraluminous granites the evolution of the deformation reflects the strong influence of synkinematic magma emplacement and subsequent cooling on the thermomechanical evolution of the shear zone magma intrusion in an orogen scale transcurrent shear zone deeply modifies the rheological behavior of the continental crust it triggers an efficient thermomechanical softening localized within the fault that may subsist long enough for large displacements to be accommodated therefore the close association of deformation and synkinematic magmatism probably represents an important factor controlling the mechanical response of continental plates in collisional environments

physical properties of crustal and upper mantle rocks with regards to lithosphere dynamics and high pressure mineralogy this paper focusses on the physical properties of rocks and property changes occurring during geodynamic processes it addresses a number of outstanding petrophysical problems in the framework of crustal and mantle deformation the major themes to be discussed will be the seismic signature of crustal and mantle shear zones the nature of crustal and mantle anisotropy and the contribution of anisotropic rocks to seismic reflectivity in particular seismic shear waves traversing anisotropic strata can reveal important information on the spatial orientation of deformed structures because the measured seismic anisotropy has a structural origin emphasis will be placed on the relations between shear wave splitting and fabric anisotropy fabric anisotropy and strain and strain and geologic tectonic processes the second part of this paper discusses the effect that metamorphic phase transitions in crustal and mantle rocks may have on rock physical properties velocity density specifically the possible role that the gabbro eclogite transition could have in crustal delamination and recycling of material into the mantle will be considered

tectonics and global change inferences from the late cenozoic subsidence and uplift patterns in the atlantic mediterranean region summary quantitative subsidence analysis for a number of rifted basins in the northern atlantic mediterranean region provides evidence for rapid phases of plioquaternary subsidence the observed acceleration in tectonic subsidence occurs after a phase of general quiescence in subsidence in these basins and deviates from predictions of stretching models the latter indicate a decay of subsidence with time after mesozoic tertiary basin formatioh and a slow tectonic subsidence in plio quaternary times a possible explanation for the observed patterns of anomalous subsidence could be an increase in the level of intraplate compression in the northern atlantic region intraplate stress changes in the plio quaternary are related to the dynamics of african eurasian collision processes and a reorganization of spreading directions in the atlantic possibly reflecting a plate reorganization of global nature it seems that the plio quaternary record reflects a period of increased levels of neotectonic activity interplaying with periods of de glaciation stress induced topography in the onshore parts of continental margins coupled with the stress induced subsidence in the offshore deeper parts of the basins could have contributed to recent phases of uplift in fennoscandia augmenting the uplift induced by glacial unloading estimates of ice thicknesses are directly inferred from the observed uplift ignoring other driving mechanisms whereas topography plays a crucial role in the dynamics of glaciation it is therefore important to quantdy the interplay of rapid tectonic uplift and subsidence phases with climatic effects during the plio quaternary

fluid flow vs scale of shear zones in the lower continental crust and the granulite paradox the external or internal origin of co2 and h2o in marbles and metabasites from a large scale granulite section is related to their structural setting outside shear zones within major shear zones 25 km wide by 340 km long or within minor shear zones 10 km wide by 150 km long outside shear zones marbles have isotopic compositions similar to their protholiths values and metabasites record mantle 13c signatures marbles from major shear zones show little variation of 18o but are depleted in 13c down to 3 due to exchange with mantle derived carbon in addition to an earlier input of mantle carbon related to mafic magmatism major shear zones act as conduits for mantle carbon influx they are probably mantle rooted oxygen was buffered by the crust marbles and their related skarns from minor shear zones were subjected to devolatilization reactions and water rich infiltration from crustal sources with no evidence for a mantle carbon contribution large isotopic variations at the metre scale in the shear zones reflect the heterogeneous distribution of fluid flow associated with heterogeneous deformation and contrasting petrologic features and permeabilities these geochemical structural relationships reconcile the conflicting arguments on the origin of fluids during granulite genesis

hydrogen isotope systematics of submarine basalts the d h ratios and water contents in fresh submarine basalts from the mid atlantic ridge the east pacific rise and hawaii indicate that the primary d h ratios of many submarine lavas have been altered by processes including 1 outgassing 2 addition of seawater at magmatic temperature and 3 low temperature hydration of glass decreases in d and h2o from exteriors to interiors of pillows are explained by outgassing of water whereas inverse relations between d and h2o in basalts from the galapagos rise and the famous area are attributed to outgassing of ch4 and h2 a good correlation between d values and h2o is observed in a suite of submarine tholeiites dredged from the kilauea east rift zone where seawater added directly to the magma affected only the isotopic compositions of hydrogen and argon analyses of some glassy rims indicate that the outer millimeter of the glass can undergo lowtemperature hydration by hydroxyl groups having d values as low as 100 d values vary with h2o contents of subaerial transitional basalts from molokai hawaii and subaerial alkali basalts from the society islands indicating that the primary d values were similar to those of submarine lavas extrapolations to possible unaltered d values and h2o contents indicate that the primary d values of most thoteiite and alkali basalts are near 80 5 the weight percentages of water are variable 0 15 0 35 for mor tholeiites about 0 25 for hawaiian tholeiites and up to 1 1 for alkali basalts the primary d values of 80 for most basalts are comparable to those measured for deep seated phlogopites these results indicate that hydrogen in marked contrast to other elements such as sr nd pb and o has a uniform isotopic composition in the mantle this uniformity is best explained by the presence of a homogeneous reservoir of hydrogen that has existed in the mantle since the very early history of the earth

a mechanism for crustal thinning without lateral extension in some continental regions thin crust is found without observing significant indications of extensional tectonics we argue that in such regions upwelling mantle flows may give rise to sublithospheric loading stresses which may be able to squeeze out the ductile lower crust but leave the upper crust and sub crustal lithosphere essentially unstretched this model allows us to constrain the viscosity of the lower crust if the amount and time scale of crustal thinning is observed in any subsidence basin we applied our model to the pannonian basin hungary and found lower crustal viscosities between 1020 and 1021 pa s comparing this viscosity with laboratory data of crustal rocks results in lower crustal temperatures of about 600 to 800 c which are in good agreement with geothermal models our model might also help to understand the appearance of a very thick crust and lithosphere without any pronounced topography as it is found in fennoscandia

the gabbro eclogite phase transition and the elevation of mountain belts on venus maxwell montes standing up to 7 km above the adjacent highland plateaus constitute the highest mountain belt on venus because the thickness of the crust is likely to be limited by the gabbro garnet granulite eclogite phase transitions this relief is difficult to reconcile with the assumption of thermodynamic equilibrium and a standard airy isostatic model we explore the hypothesis that the crust mantle boundary is not in phase equilibrium but rather is rate limited by the temperature dependent volume diffusion of the slowest ionic species under the simplifying assumption that the mountains formed by uniform horizontal shortening of the crust and lithospheric mantle at a constant rate we solve the one dimensional thermal evolution problem the time dependent density structure and surface elevation are calculated by assuming a temperature dependent reaction rate and local airy isostatic compensation for a rate of horizontal strain of 10 15 s 1 or greater the rise in temperature at the base of the crust during mountain formation is modest to negligible the deepening lower crust is metastable and surface elevation increases as the crust is thickened for strain rates less than 10 16 s 1 in contrast crustal temperature increases with time because of internal heat production and the lower crust is more readily transformed to the dense eclogite assemblage for such models a maximum elevation is reached during crustal shortening while this maximum relief is 7 km or more for some models a smaller density contrast between crust and mantle than assumed here 500 kg m 3 and incorporation of horizontal heat transport would lessen this value we therefore favor formation of the mountain belt at a strain rate at least of order 10 15 s 1 by this reasoning maxwell montes must be comparatively young of order 50 ma

limited fluid rock interaction at marble gneiss contacts during cretaceous granulite facies metamorphism seward peninsula alaska stable isotope profiles show that flat lying marble units acted as impermeable barriers to upward fluid flow in transitional amphibolite granulite grade rocks of the kigluaik mountains seward peninsula alaska the degree of permeability is related to the composition of the marble the margin of a thick pure dolomite marble chemically reacted with underlying metasyenite ah2o 0 2 to form a 2 cm boundary layer of calcite forsterite by introduction of sio2 no fluid penetrated past this reaction front although the high temperature of metamorphism 800 c allowed transport of carbon and oxygen isotopes for an additional 2 cm by diffusion through the solid dolomite a second marble with a higher silica content underwent more decarbonation which enhanced porosity and lead to a greater extent of isotope transport 2 3 m in contact with quartzo feld spathic gneiss below an estimate of total fluid flux across the bottom of this marble layer based on the shape of the isotope profile is 1 cm3 cm2 directed down out of the marble at two other marble gneiss contacts steep isotopic gradients coincide with lithologic contacts indicating very little cross lithology fluid flow the extent of diffusional transport of isotopes in the marbles is limited and interpreted as indicating the transient presence of a pore fluid generated by thermally driven devolatilization reactions no wholesale pervasive advection of c o h fluid occurred across the thick continuous marble units near the exposed base of the kigluaik group section during the entire regional metamorphic cycle activities of pore fluid species were controlled by internal processes movement of volatiles and stable isotopes between contrasting rock types was dominantly diffusive channelized fluid pathways through the marble units developed during uplift and cooling but were not present during peak metamorphism heating of the section occurred by conduction probably from an underlying magma source and not by advection of a c o h fluid

two dimensional inverse modeling of sedimentary basin subsidence recently developed mathematical models of subsidence in extensional basins provide a good basis for forward modeling however the large number of parameters to be specified by the user makes it difficult not only to do the modeling itself but also to judge the meaning of the results we present a new method for automatic searching of the best fitting parameter set of a 2d basin formation model weighted goal functions are used in the minimization process by the inverse problem solver algorithm the nominal parameter set in the present case includes profiles of the crustal and subcrustal thinning factors and the level of lithosphere necking the method was tested on synthetic data with parameters that were perturbed beforehand with some restrictions the algorithms are capable of resolving such perturbations the inversion technique has been applied to two profiles crossing the dnieper donets basin ukraine the crustal thinning factors obtained argue for a scissors like style of basin opening high subcrustal thinning values are necessary in order to explain the abnormally thick carboniferous section in the basin these values should be treated as cumulative ones due to the currently unresolvable influence of other rifting related processes particularly phase transitions

a phase of transient subsidence sediment bypass and deposition of regressive transgressive cycles during the breakup of iberia and newfoundland abstract seismic outcrop and well data from west iberia and newfoundland are used to investigate sediment stacking patterns during continental breakup as a function of tectonic subsidence in west iberia two breakup sequences are revealed on seismic data by marked strata offlap oceanwards from the present day continental shelf this character is similar to newfoundland where correlative strata comprise lower cretaceous cenomanian coarse grained siliciclastics accumulated around local sediment source areas the interpreted data reveal that the two breakup sequences 1 materialise sediment bypass onto continental slope depocentres that experienced important tectonic subsidence during continental breakup but without showing typical syn rift growth packages 2 generate specific forced regressive stratigraphic intervals that relate to uplift and exhumation of the proximal margin subsidence and sediment stacking patterns in both west iberia and newfoundland reflect similar continental breakup processes as they evolved from the upper lithosphere to their mantle breakup stages on both margins coarse grained siliciclastic units on the proximal margin give rise to thick shaley successions in deep water basins this work also confirms that in a setting dominated by a significant sediment influx yet lacking the burial rates of continental slope basins in newfoundland west iberia comprised accommodation driven basins during continental breakup not necessarily sediment starved as a corollary of our analysis we classify breakup sequences around the world based on the characteristic lithologies of their regressive transgressive depositional cycles

the origin of a mississippi valley type deposit in the viburnum trend southeast missouri petrologic and isotopic studies of the base metal sulfide mineralization in the buick mine have established the following paragenesis an early stage of fine grained pyrite and marcasite replaced the dolostone host rock the iron disulfides and dolostone were replaced by sphalerite during the main stage of sphalerite precipitation delta 34 s sp 16 5 to 19 0ppm following the sphalerite two main stages of galena were precipitated a cuboctahedral stage delta 34 s gn 21 3 to 10 0ppm during which galena replaced dolostone and sphalerite and grew in vugs in the dolostone and in the sphalerite and a cubic stage delta 34 s gn 10 5 to 0 9ppm the two stages of galena precipitation were separated by a period during which the main stage of sphalerite and the cuboctahedral galena were partially dissolved and bladed marcasite delta 34 s mc 2 3 to 4 1ppm was precipitated and followed by a period of mild etching of the cubic galena minor amounts of pale sphalerite delta 34 s sp 14 0 to 16 5ppm were then precipitated white and colorless rhombs of dolomite were precipitated at the same time as some of the main stage of sphalerite the cubic galena and the pale sphalerite after the pale sphalerite was precipitated chalcopyrite quartz and siegenite were precipitated followed by calcite and finally kaolinite the paragenetic and textural data for both galena and sphalerite in the buick mine strongly suggest that these minerals were precipitated and dissolved repetitively during ore formation similar textural evidence for galena reported by other workers for different mines in the viburnum trend implies that repetitive precipitation and dissolution of galena was probably an important process during the formation of the entire viburnum trend fluid inclusions in late quartz crystals coprecipitated with chalcopyrite have filling and freezing temperatures within the ranges reported for sphalerite from other parts of the viburnum trend demonstrating that the ore forming solutions in the viburnum trend were probably at temperatures in the range 105 degrees to 141 degrees c assuming a maximum pressure correction of about 11 degrees c and were extremely saline containing about 4 0 to 5 0 molal chloride consideration of typical chemical analyses of fluid inclusions from other mississippi valley type deposits suggests that the maximum likely concentration of dissolved co 2 in the ore forming solutions was about 0 05 molal at these concentrations the ore forming solutions that travelled to the site of deposition could have had ph9s about two units less than neutral and still have been in equilibrium with dolomite or calcite the ranges of sulfur and lead isotopic compositions of galena from the buick mine are very similar to those established by earlier studies of the old lead belt and adjacent subdistricts however the correlation of isotopically lighter sulfur with more radiogenic lead is much stronger for the viburnum trend suggesting that galena was precipitated from many solutions each of which carried lead and sulfur together and had different lead and overall sulfur isotopic compositions the textural paragenetic and isotopic data for the buick mine cannot be easily explained either by a sulfate reduction model in which sulfate was reduced by organic matter or methane or mixing models in which base metals and reduced sulfur were transported separately to the site of deposition instead the data strongly support a reduced sulfur model for ore formation according to which the base metals were transported together with reduced sulfur as h 2 s in the same solutions to the sites of deposition the reduced sulfur model is also supported by a theoretical calculation of the solubilities of galena and sphalerite which demonstrate that low but economically significant concentrations of lead or zinc could have been transported as chloride complexes together with equivalent concentrations of reduced sulfur at temperatures of 100 degrees to 150 degrees c and ph9s of one to two units less than neutral and could have been precipitated by ph increases cooling or dilution at the sites of deposition

contemporary state of stress and neotectonic deformation in the carpathian pannonian region the carpathian pannonian region has been characterized by a two stage tectonic evolution since the beginning of the neogene during early and mid miocene the lateral eastward escape of the pannonian fragment caused thrusting and folding in the outer carpathian mountains south and westward directed subduction of the eurasian lithosphere calc alkaline volcanism along the inner carpathian bend and localized fault controlled subsidence of basins in the pannonian region this style of tectonic deformation ceased by the end of the mid miocene the neotectonic stage began in the late miocene it is characterized by differential regional subsidence with maximum rates in the west and east and minimum rates in the centre of the pannonian basin further characteristics of the central pannonian basin are the anomalous high heat flow values the thin crust and lithosphere the neotectonic stress field has been determined by in situ stress measurements the analyses of borehole breakouts and fault plane solutions of earthquakes in situ stress measurements by the doorstopper and triaxial strain cell methods indicate high compressional stresses in the western pannonian basin with its maximum in wnw ese to nw se direction in contrast to its western part the central pannonian basin shows tensional stresses near the earth s surface with maximum tension in the same wnw ese direction borehole breakout data indicate a general wnw ese orientation of maximum horizontal stress in the western part of the pannonian basin and in the eastern part as well whereas in the central pannonian basin this direction is the preferred azimuth of minimum horizontal stress it is suggested that the neotectonic deformations and stresses in the carpathian pannonian region have a sublithospheric origin asthenospheric convection with an upwelling mantle flow below the centre of the pannonian basin and downwelling flows along the cold lithospheric roots below the eastern carpathians and the alps seems to be most plausible accordingly the relative uplift of the central pannonian basin the high heat flow and also the tensional stresses are explained as the surface expression of an upstreaming branch of a localized convection cell below the pannonian basin

the effects of temperature dependent viscosity on mantle convection with the two major phase transitions abstract we have studied the consequences of including temperature dependent viscosity in mantle convection with both of the major phase transitions included we have employed a two dimensional cartesian model in a box of aspect ratio four the time dependent equations have been cast within an extended boussinesq framework in which each of the phase changes has been simulated by an enhanced thermal expansivity both latent heat release and viscous dissipation have been considered two different forms of temperature dependent viscosity the arrhenius and the frank kamenetzky forms have been employed we have considered both models with a constant temperature equilibrium boundary condition at the core mantle boundary and models with a time varying non equilibrium temperature boundary condition from cooling of the core the effects of temperature dependent viscosity are to enhance the tendency of the convective system to be layered because of the influences of the latent heat release and the mechanical heating on creating a sharp viscosity drop across the transition zone we have found that there is a non linear coupling between latent heat release from the interaction of the hot plume with the endothermic phase transition and the temperature dependent viscosity this positive feedback process can give rise to the production of very high temperatures in the transition zone which may cause melting in the region between the two major phase transitions flushing events produced by the non equilibrium boundary condition are much more dramatic than those generated by the equilibrium boundary condition the non linear coupling between latent heat release and temperature dependent viscosity is also stronger in the non equilibrium models comparison with models without the presence of latent heat release and viscous dissipation shows that the models without the heating terms produce a greater degree of mass flux across the endothermic phase boundary because of the lack of the viscosity stratification in the interior of the convective flow results of this moded can be applied to other types of endothermic phase transition found in the lithosphere or to the spinel perovskite transition in the deep portion of the martian mantle

fluid rock interaction in the granulites of madagascar and lithospheric scale transfer of fluids abstract the nature of synmetamorphic fluids and their flow is examined in the granulitic lower crust of madagascar part of a precambrian crustal scale network of vertical ductile shear zones based on three independent data sets field and satellite mapping c o and h isotope geochemistry and gravimetry this crust is divided into three zones outside of shear zones minor shear zones 350 km long and 20 35 km wide the major shear zones are rooted in and are controlled by the mantle they tapped mantle derived co2 with carbon fluxes of the same order of magnitude as oceanic ridge degassing one major shear zone shows abundant phlogopite diopside apatite calcite mineralizations a well known paragenesis in mantle metasomatism due to mantle fluid infiltration and their interaction with the crust carbonatitic magmas possibly collected in the major shear zones at the base of the crust and may be the source for co2 upwellings as well as other metasomatic agents small scale minor shear zones were controlled by crustal deformation processes and focused crustally derived h2o rich fluids pervasive fluid circulation was restricted to the vicinity

the role of lithospheric phase transitions on seafloor flattening at old ages abstract uplift of old ocean seafloor relative to the familiar t 1 2 curve is generally presumed to be a consequence of an additional heat source at the base of the oceanic lithosphere several geodynamical mechanisms have been invoked to explain this behavior we have taken a petrological rather than geodynamical approach and considered whether the causes of the flattening can be found within the conductive portion of the lithosphere where viscous creep is insignificant accordingly self consistent calculations of the phase assemblages of several candidate mantle compositions have been performed subject to the constraint of experimentally determined calorimetric measurements we have found that in general for temperatures below around 800 c and pressures greater than 10 kbar the phase change from spinel peridotite to garnet peridotite has a significantly negative clapeyron slope which could cause some amount of uplift of old seafloor the transition for representative thermal profiles should be depressed from about 35 km at 40 ma to 45 km at 160 ma since spinel peridotite is of lower density the net effect is to raise the seafloor topography the extent of the uplift which depends primarily on the al 2 o 3 content of the peridotite and on the effective thermal diffusivity of the lithosphere should be on the order of 140 220 m

carbon isotope stratigraphy a tool for basin to carbonate platform correlation the early cretaceous carbonate carbon isotope stratigraphy established in pelagic limestones is marked by several pronounced excursions towards positive 13c values we investigated a biostratigraphically and palaeomagnetically calibrated aptian section to see whether c isotope stratigraphy could be recognized in shallow water carbonates the chosen carbonate platform sections are located in southern italy and have been dated by biostratigraphy bulk samples chosen from the barremian albian part of the sequence were analysed for their o and c isotope compositions the c isotope curve established shows two major positive excursions which can be correlated with the synchronous and globally recognized aptian c isotope events the data provide evidence that c isotope stratigraphy can be used as a powerful correlation tool between pelagic and shallow water limestone sequences

phase changes and thermal subsidence in intracontinental sedimentary basins summary a model of tectonic subsidence is developed to explain the late acceleration of subsidence observed in some intracratonic sedimentary basins the proposed mechanism combines the effect of thermal contraction of an initially hot lithosphere with the effect of a subcrustal phase transformation that moves under changing pressure and temperature conditions the subsidence following a sudden change in temperature at the base of the lithosphere is calculated the calculations show that 1 phase changes if present and activated contribute substantially to the subsidence of sedimentary basins 2 because the effect of phase change is delayed subsidence accelerates after a time on the order of 20 myr and 3 the duration of the subsidence is on the order of 100 to 150 myr during the late stages of subsidence the phase change is the dominating mechanism an application to the michigan basin is presented the calculated sediment accumulation history fits the record well when the effect of sea level changes is included in the model

the effects of the spinel garnet phase transition on the formation of rifted sedimentary basins summary we have examined the effects of the spinel garnet phase transition on subsidence of extensional sedimentary basins for a constant positive clapeyron slope dp dt the phase boundary moves downwards in the syn rift and upwards in the post rift phase for a non linear clapeyron curve dp dt 0 above 900 c and dp dt 0 below 900 c theory predicts for the reaction of the spinel garnet phase transition the direction of phase boundary movement is dependent on the stretching factor the position of the clapeyron curve and the lithospheric thickness a smaller syn rift and larger post rift subsidence are predicted for a deeper phase boundary and a thicker lithosphere the model with a non linear clapeyron curve is applied to the subsidence histories of a young extensional basin gulf of lion and an old continental margin eastern canada the observed syn rift uplift and the larger post rift subsidence can be reasonably explained by this model where the optimum depth of the phase boundary for eastern canada 90 km is consistent with the estimate from seismic observations and is larger than that for the gulf of lion 50 km the depth of the spinel garnet phase boundary is sensitive to the composition of mantle rocks and increases with the extraction of basaltic components from the lithosphere compatible with our result that the phase boundary is deeper for an older and thicker lithosphere thus the surface movement associated with the rifting for these areas may reflect the chemical evolution of the continental lithosphere

subsidence of the paris basin and its bearing on the late variscan lithosphere evolution a comparison between plate and chablis models abstract in this study two different models for the lithospheric thermal evolution are used to predict the long term subsidence of the paris basin the plate or chablis models have a constant temperature or constant heat flow at the base of the lithosphere these boundary conditions yield a different thermal relaxation time for a given equilibrium lithospheric thickness the final thermal steady state does not depend on the boundary condition fixed at the base of the lithosphere the thermal evolution of the lithosphere is computed taking into account temperature and pressure dependent thermal characteristics metamorphism in the crust top crustal erosion and phase transition in the mantle spinel garnet the long term tectonic subsidence of the paris basin constrains the mesozoic thermal evolution of the lithosphere and the amplitude of the initial thermal anomaly the chablis model including the effects of spinel garnet phase transition predicts present day lithospheric thickness in better agreement with observation than the plate model the chablis model involves a basal heat transfer of 37 mw m2 similar to the value beneath oceanic plates the long term subsidence of the paris basin obviously results from the decay of a thermal anomaly initiated during late variscan times we first assume that at 300 ma the lithosphere was less than 150 km thick and the thickened crust started returning to its present thickness 35 km by uniform lithospheric extension the subsidence data can be explained by short stephano autunian as well as long stephano triassic lasting extension these hypotheses both implicitly refer to extensional collapse of the variscan belt however the evolution of the variscan belt from 350 to 300 ma may have involved almost no crustal thickening but an anomalously thin lithosphere ca 55 km the corresponding thermal evolution of the lithosphere is computed using the chablis model with an initially high heat transfer of 61 mw m2 at the base of the base of the lithosphere at 280 ma this heat transfer is tuned to 40 mw m2 leading to lithospheric cooling from 280 to 30 ma which can also explain the paris basin subsidence for the short lasting extension hypothesis the late variscan lithospheric evolution predicted with the chablis model yields paleogeography magmatism and retrograde p t t paths in good agreement with those reported from areas around the paris basin conversely the plate model does not allow mantle melting during the end of carboniferous and permian times the long lasting extension hypothesis does not explain the paleogeographic evolution the timing of magmatism and retrograde p t paths showing isothermal decompression the hypothesis with a strong lithospheric delamination cannot be ruled out however it is less plausible than the extensional collapse hypothesis

evidence for co2 infiltration in granulite facies marbles from lofoten vesteralen norway abstract lofoten vesteralen comprises an amphibolite to granulite facies terrain metamorphosed in the granulite facies at 1 8 ga amphibolite facies marbles retain close to premetamorphic carbon and oxygen isotopic signatures 13 c 10 pdb 18 o 26 smow the margins of marbles near the amphibolite granulite transition and granulite facies marbles show large shifts to much lighter carbon and oxygen to 13 c 7 pdb and 18 o 10 smow these shifts imply exchange with a co 2 bearing fluid large co 2 rock weight ratios were achieved in granulite facies marbles and this could reflect large scale infiltration of co 2 into the granulite facies crust the occurrence of carbonic metamorphism in at least some granulite terrains could be a reflection of high mantle co 2 degassing rates in the precambrian or the recycling of carbon in a subduction related environment

petrologic phase equilibria and stable isotope fractionations of carbonate silicate parageneses from granulite grade rocks of sri lanka abstract the calc silicate rocks and marbles of central and southern sri lanka can be characterized by three groups of parageneses cc dol phl fo spl di acc in the northern and eastern part of the highland complex di scp spn cc acc following to the west and wo di scp spn cc acc in the most southwestern part this distribution mirrors the compositional variation of the sedimentary precursors and the regional pressure gradient during prograde metamorphism decarbonation with internal fluid buffering increased xco2 to 0 75 graphite precipitation at near peak metamorphic conditions at constant oxygen fugacity reduced xco2 to 0 5 the variation of 18o 26 to 9 and 13c 4 to 5 of carbonates can largely be explained by primary differences in the isotopic composition and in the silicate carbonate contents of the protoliths the particular light isotopic compositions are due to fractionation processes during decarbonation of the rocks the dolomite free scapolite bearing rocks in general are isotopically lighter than dolomite bearing parageneses calcite graphite temperatures in the range of 750 900 c were calculated using the experimentally derived thermometer of scheele 1991 calcite silicate temperatures 150 900 c were calculated with the increment method hoffbauer et al 1994 these temperatures reflect near peak equilibrium as well as different degrees of retrograde isotope resetting

structural modelling of regional depth profiles in the voring basin implications for the structural and stratigraphic development of the norwegian passive margin interpretation of key regional seismic lines across the voring basin has identified a number of seismic megasequences that have provided a framework for further basin analysis these megasequences have been classified as pre syn and post rift with respect to the late cretaceous early tertiary phase of extension forward and reverse structural and stratigraphic modelling of two of these regional lines has been performed in an attempt to test and validate the structural and stratigraphic interpretations of the syn and post rift megasequences reverse thermal subsidence modelling flexural backstripping of these lines to the base tertiary was performed using a laterally varying beta factor across the sections a number of features on each of the depth profiles are considered to be regional subaerial datums for the backstripped sections the backstripped templates were then forward modelled using the flexural cantilever model the forward models calculate a laterally varying beta profile along their length which can be compared to the reverse models thereby ensuring an internally consistent structural model between both the reverse and forward modelling techniques the modelling work has highlighted the existence of a significant discrepancy between the extension measured from the forward models fault heaves and the extension needed to produce geologically acceptable backstripped profiles this observation is of particular importance as it may place a significant constraint on the mode of lithospheric extension that can be used to explain the evolution of the norwegian continental margin

initiation and propagation of shear zones in a heterogeneous continental lithosphere continental plates represent rheologically heterogeneous media in which complex finite strain fields may develop due to interaction of plate tectonic processes with intraplate heterogeneities such a deformation pattern is displayed by the borborema shear zone system in northeastern brazil it involves continental scale curvilinear e w trending right lateral transcurrent shear zones that branch off from a major ne trending right lateral strike slip deformation zone formed at the northern termination of the sao francisco craton and that finally terminate in transpressive metasedimentary belts we suggest that this strain field results from the compressional deformation of a highly heterogeneous continental lithosphere composed of a stiff domain craton and rheologically weaker domains basins the effect of a low viscosity domain on the deformation of a continental plate and in particular the perturbation induced by this domain on the development of a shear zone formed at the termination of a stiff block were investigated using numerical modeling the low strength domain induces an enhanced strain localization and the geometry of the shear zone is significantly modified it is either split forming a branched shear zone system in which one branch maintains its original orientation while the other rotates toward the low viscosity domain or completely rotated toward the weak block the perturbation of the finite strain field depends on the ability of the weak domain to accommodate deformation which is controlled by its initial viscosity contrast relative to the surrounding lithosphere its orientation relative to the convergence direction and its distance from the shear zone initiated at the termination of the stiff block the interaction between imposed boundary conditions tectonic forces and plate geometry and the internal structure of the plate may give rise to highly heterogeneous strain fields as exhibited by the borborema shear zone system in which intraplate rheological heterogeneities induce the development of branched or sinuous shear zones a heterogeneous continental plate subjected to a normal convergence may therefore display significant lateral variations in strain intensity with shear zones bordering nearly undeformed blocks and in deformation regimes and vertical strains that would result in contrasting metamorphic and uplift histories

tectonic subsidence of the lomonosov ridge the cenozoic sedimentary record revealed by the integrated ocean drilling program s arctic coring expedition acex to the lomonosov ridge microcontinent in 2004 is characterized by an unconformity attributed to the period 44 18 ma according to conventional thermal kinematic models the microcontinent should have subsided to 1 km depth owing to rifting and subsequent separation from the barents kara sea margin at 56 ma we propose an alternative model incorporating a simple pressure temperature p t relation for mantle density using this model we can explain the missing stratigraphic section by post breakup uplift and erosion the pattern of linear magnetic anomalies and the spreading geometry imply that the generation of oceanic crust in the central eurasia basin could have been restricted and confined by non volcanic thinning of the mantle lithosphere at an early stage ca 56 40 ma in response to a rise in temperature the mantle mineral composition may have changed through breakdown of spinel peridotite and formation of less dense plagioclase peridotite the consequence of lithosphere heating and related mineral phase transitions would be post breakup uplift followed by rapid subsidence to the deep water environment observed on the lomonosov ridge today

back arc rifting initiated with a hot and wet continental lithosphere abstract how back arc rifting is initiated is studied by inferring the tectonic force and strain rate regimes from published subsidence data for nine back arc basins in the european tethyan belt and in the western pacific the results show that rifting only begins after some particular magnitude of tectonic force is reached this implies that subduction has already progressed to a point such that the total negative buoyancy of the slab becomes so large that the relevant tectonic force exceeds its bending strength once back arc rifting has been initiated the force required to maintain it gradually decays this indicates that the processes governing back arc extension are fast enough that lithosphere weakening due to an increase in geothermal gradient exceeds strengthening due to the thermal relaxation and crustal thinning this is achieved given currently accepted estimates for the maximum force 7 tn m if the thickness of the thermal lithosphere is significantly less than 125 km and if the lithosphere has a wet rheology meaning that a weak continental lithosphere is required for the initiation of back arc rifting the presence of significant strength in the uppermost mantle is not precluded but it is not required

stable isotopic signatures of superposed fluid events in granulite facies marbles of the rauer group east antarctica the role of volatiles in the stabilization of the lower granulite facies crust is contentious opposing models invoke infiltration of co2 rich fluids or generally vapour absent conditions during granulite facies metamorphism stable isotope and petrological studies of granulite facies metacarbonates can provide constraints on these models in this study data are presented from metre scale forsteritic marble boudins within archaean intermediate to felsic orthogneisses from the rauer group east antarctica forsteritic marble layers and associated calcsilicates preserve a range of 13c and 18o depleted calcite isotope values 13c 9 9 to 3 0 pdb 18o 4 0 to 12 1 smow a coupled trend of 13c and 18o depletion 2 5 respectively from core to rim across one marble layer is inconsistent with pervasive co2 infiltration during granulite facies metamorphism but does indicate localized fluid rock interaction at another locality more pervasive fluid infiltration has resulted in calcite having uniformly low carbonatite like 18o and 13c values a favoured mechanism for the low 18o and 13c values of the marbles is infiltration by fluids that were derived from or equilibrated with a magmatic source it is likely that this fluid rock interaction occurred prior to high grade metamorphism other fluid rock histories are not however ruled out by the available data coupled trends of 13c and 18o depletion are modified to even lower values by the superposed development of small scale metasomatic reaction zones between marbles and internally folded mafic interlayers the timing of development of these layers is uncertain but may be related to archaean high temperature 1000dc granulite facies metamorphism