source spectra and spectral attenuation of seismic waves from mexican earthquakes and evidence of amplification in the hill zone of mexico city it is well known that seismic waves are enormously amplified at lake bed sites with respect to hill zone sites in mexico city it has been suggested that even hill zone sites suffer amplification in this paper we use eight recent well recorded earthquakes to determine spectral attenuation of seismic waves and to quantify the suggested amplification in the hill zone results show that indeed spectral amplitudes at these sites are roughly 10 times greater than expected for sites outside mexico city at similar hypocentral distances this implies that lake bed sites are amplified by a factor of 100 to 500 at some frequencies between 0 2 to 0 7 hz the source spectra retrieved from the regression analysis agree well with those obtained from epicentral recordings alone for two of the largest earthquakes ms 8 1 and 7 6 whose seismic moments are well determined and whose source spectra can be patched in a very broad band of frequencies 0 015 to 20 hz we observe clear deviation from an 2 model the spectra at intermediate frequencies from about 0 05 to 2 hz show a sag whether the observed spectra deviate from an 2 model for events with ms 6 depends on the reliability of the reported long period seismic moments if they are considered reliable then these events also deviate from the model if however the reported seismic moments of smaller events are ignored then the observed spectra can be fitted well with the 2 model

variations of apparent stresses and stress drops prior to the earthquake of 6 may 1984 mb 5 8 in the adak seismic zone abstract on 6 may 1984 a m b 5 8 earthquake occurred within a subregion of the adak seismic zone that had shown clustering of m b 4 5 earthquakes during the last 3 yr this region has been examined for premonitory variations in stress drop and apparent stresses calculated from the s wave spectra of 69 microearthquakes m d 13 nm decrease with decreasing moment

constraints on upper mantle viscosity from the flow induced pressure gradient across the australian continental keel 1 the thickness of continental lithosphere varies considerably from tectonically active to cratonic regions where it can be as thick as 250 300 km embedded in the upper mantle like a ship when driven to move by a velocity imposed at the surface a continental keel is expected to induce a pressure gradient in the mantle we hypothesize that the viscosity of the asthenosphere or the shear coupling between lower lithosphere and asthenosphere should control this pressure effect and thus the resulting dynamic topography we perform three dimensional finite element calculations to examine the effects of forcing a continental keel by an imposed surface velocity with the australian region as a case study when the upper mantle is strong but still weaker than the lower mantle positive dynamic topography is created around the leading edge and negative dynamic topography is created around the trailing edge of the keel which is measurable by positive and negative geoid anomalies respectively for a weak upper mantle the effect is much reduced we analyze geoidal and gravity anomalies in the australian region by spatiospectral localization using slepian functions the method allows us to remove a best fit estimate of the geographically localized low spherical harmonic degree contributions regional geoid anomalies thus filtered are on the order of 10 m across the australian continent with a spatial pattern similar to that predicted by the models the comparison of modeled and observed geoid anomalies places constraints on mantle viscosity structure models with a two layer mantle cannot sufficiently constrain the ratio of viscosity between the upper and lower mantle the addition of a third weak upper mantle layer an asthenosphere amplifies the effects of keels our three layer models with lower mantle viscosity of 3 1022 pa s suggest that the upper mantle asthenosphere is 300 times weaker than the lower mantle while the transition zone 400 670 km depths has a viscosity varying between 1021 and 1022 pa s

the role of radiogenic heat production in the thermal evolution of a proterozoic granulite facies orogenic belt eastern ghats indian shield abstract the eastern ghats belt egb is a deeply eroded proterozoic orogenic belt juxtaposed against the archaean dharwar bastar singhbhum cratons in the indian shield the tectono metamorphic history of this belt is broadly similar to the grenvillian orogenic belts the egb exposes dominantly the granulite facies rocks such as charnockites khondalites and migmatitic gneisses with less abundant intermediate to mafic granulites the granulites are intruded by small plutons of granites syenites and anorthosites the northern and southern parts of the belt negb and segb have distinct crustal histories but 1600 1000 550 ma old tectono metamorphic events are common to both the belts in this research we attempt to understand the present day and past thermal state of this orogen on the basis of our radioelement k u and th measurements at 562 sites using in situ gamma ray spectrometry with other geological and geophysical constraints the dominant rock types of the negb and segb have similar radioelement abundances 3 k 3 ppm u and 30 ppm th and heat production heat production of charnockites gneisses khondalites and intermediate granulites of the negb is 2 9 2 8 2 9 and 1 1 w m 3 respectively in the segb they are 2 7 2 5 2 8 and 0 6 w m 3 respectively and the mafic granulites are the lowest in heat production 0 3 w m 3 on the basis of the heat production data and crustal petrologic models we show that the crustal contribution of the negb and segb is 54 and 45 mw m 2 respectively which are broadly in good agreement with the available surface heat flow data thermal models of the present day egb crust indicate that the temperature at the moho is 550 c radioelements and heat production of the eastern ghats granulites are higher than the other granulite belts in the indian shield the crustal radiogenic heat contribution and moho temperatures of the egb are higher than the adjoining archaean cratons this could also be responsible for the development of inverted metamorphic isograds in the tectonic boundary between the egb and the adjoining cratons during a collisional orogenic event at about 550 ma ago

terrestrial heat flow in botswana and namibia we report terrestrial heat flow measurements from 25 new sites in botswana and namibia twelve sites from the archean kaapvaal zimbabwe craton and the limpopo belt have a mean of 47 4 mw m 2 standard error of the mean but show a trend of increasing heat flow from less than 40 mw m 2 within the cratonic nucleus to about 60 mw m 2 at its edge thirteen sites in the proterozoic and pan african mobile belts that surround the craton have a mean of 66 3 mw m 2 increasing from about 60 mw m 2 at the cratonic margin to more than 70 mw m 2 several hundred kilometers away radiogenic heat production in near surface basement rocks was measured for four sites on the craton and seven sites in the mobile belts the limited heat flow heat production data from the craton are insufficient to establish a correlation heat flow and heat production data from the mobile belts when combined with published data from six sites in zambia exhibit a linear trend with an intercept of 50 3 mw m 2 and a slope of 5 5 1 1 km some but not all of the observed difference in surface heat flow between the mobile belts and the cratonic nucleus may be due to differences in crustal heat production that part of the regional variation in surface heat flow which originates at greater depths implies a fundamental difference in thermal structure of the lithosphere between the craton and the mobile belts in southern africa

the specific case of the mid proterozoic rapakivi granites and associated suite within the context of the columbia supercontinent abstract rapakivi granites and the associated suite anorthosite mangerite and charnockite that developed during the mid proterozoic escape the global rules that control usual granitic magma formation about 40 points that characterise the proterozoic magmatism confirm the originality of the magmatism rapakivi granites developed without a global orogenic context within the supercontinent columbia the intrusions extend from the east european shield to the western us through fennoscandia greenland and labrador other occurrences in the amazonian shield australia or south china are less well documented they show no definite trend in age or chemistry that would explain large scale mantle plume effects a mantle upwelling of material is contradictory with the re assembly of a supercontinent because it occurred before this magmatic episode 1 9 1 8 ga at 1 7 ga the supercontinent columbia amalgamated with the collision of the yapavai and mazatzal provinces a model is suggested that takes into account the supercontinent re assembly defining a downwelling flow in the mantle that anchors the continent above it in contrast to a material flow heat is still delivered to the base of the continental lithosphere and is focused toward the juvenile suture zone the base of the crust must reach 1200 1300 c before producing anorthosite magmas under such a high temperature gradient lasting over a long time magmas are transferred toward the upper crust giving the thin 5 km and square shape 100 km that the intrusions presently have heat delivery is essentially conductive leading to long time spans for intrusions the presence of the supercontinent immobile over a descending cell poloidal mode of convection developed a tangential force toroidal mode of convection that partly split the continent through strike slip deformation due to plate rotation it developed progressively between 1 57 and 1 3 ga starting from fennoscandia and then passing to amazonia western us and labrador in a clockwise sense the associated rotation induced sinistral shear manifested by small scale shear zones and the orientation of late magmatic facies topaz bearing granites in each province the proterozoic magmatism appears to be unique because it requires a supercontinent with a zone of juvenile crust surrounded by older cratons the present moho still shows remnants of this process having bumpy undulations that may reach 22 km in amplitude over a distance of 200 km

ground motion prediction model from local earthquakes of the mexico basin at the hill zone of mexico city abstract we developed ground motion prediction equations gmpes based on bayesian regressions to predict response spectra at sites located in the hill zone of mexico city for local earthquakes of the mexico basin the coefficients of the gmpes are built as functions of magnitude 2 3 m w 3 8 and hypocentral distance 8 r 60 km using 15 local earthquakes we compare our results with five gmpes an attenuation model obtained in the northern region of the subduction zone in the pacific coast of mexico from small earthquakes and four other existing gmpes for shallow crustal earthquakes two of the gmpes although not the most recent ones are still used to perform probabilistic seismic hazard analysis for the region under study

geophysics of proposed proterozoic sutures in canada abstract sutures have been identified or proposed at several sites in the canadian shield although many authors have suggested that some form of primitive plate tectonic processes have played a major role in the formation of the archean crust of the superior structural province proterozoic sutures resembling phanerozoic examples have been proposed only within the younger churchill and grenville provinces and at or near the boundaries between the older superior and slave provinces and the younger provinces gravity magnetic seismic and paleomagnetic data related to three proposed sutures in the latter category have been interpreted and generally support the view that the shield comprises a mosaic of once separated but now joined cratonic blocks the sutures vary in complexity from wide zones containing distinctive rocks formed at ancient accreting and consuming plate margins to cryptic sutures marked only by abrupt changes in metamorphism and zones of cataclasis paired negative and positive gravity anomalies associated with these sutures have been interpreted in terms of juxtaposed crustal blocks of different density and thickness magnetic anomaly patterns change abruptly at structural province boundaries available seismic results support the concept of thickened crust in the younger province as derived from gravity modelling most authors prefer interpretation of proterozoic pole positions in terms of one plate models but in some cases the interpretation of paleomagnetic data is equivocal and two plate models have been proposed

lithospheric composition and thermal structure of the arabian shield in jordan abstract in this paper a unique set of samples from the uppermost crust down to the lithospheric mantle of jordan is analyzed for composition and petrophysical properties density thermal conductivity radiogenic heat production these data covering a vertical section of almost 65 km are used in conjunction with surface heat flow to generate a detailed and comprehensive lithospheric thermal model that reflects the conditions of the arabian shield as prior to the post oligocene onset of lithosphere thinning and voluminous basaltic volcanism the pre miocene model geotherms based on conductive surface heat flows of 55 and 60 mw m 2 a meet the range of lithosphere asthenosphere boundary depths of 110 160 km known from seismology b conform to results of thermomechanical models on the origin of the dead sea basin that started in miocene time and c are consistent with typical xenolith derived geotherms for terranes of similar age and lithospheric thickness moho temperatures at depths between 35 and 40 km of the as in pre miocene times were most likely in the order of 530 650 c with mantle heat flows averaging between 24 and 29 mw m 2 results contradict former views of the late proterozoic early cambrian stabilized as being an anomalously cold terrane a cold thermal structure inferred from previously measured low surface heat flows generally 45 mw m 2 is inconsistent with the thickness composition and petrophysical properties of the stable lithosphere of the shield

landscape evolution and agro sylvo pastoral activities on the gorgan plain ne iran in the last 6000 years the gorgan plain ne iran is characterized by fertile soils formed on a loess plateau and is at present primarily exploited for intensive agriculture however the timing and intensity of the human impact on the landscape in the past are still unclear a sediment core taken from the centre of the eastern gorgan plain in the kongor lake covering the major part of the holocene from 6 1 to 0 8 ka all ages are calibrated before present has been studied for pollen non pollen palynomorphs botanical macroremains insects charcoal geochemistry biomarkers and magnetism in order to provide new insights into the evolution of the landscape and to estimate the intensity of human activities the data obtained suggest a dry period between 5 9 and 3 9 ka and an increase in regional humidity afterwards with a maximum between 2 7 and 0 7 ka during the period of the persian empires achaemenid through sasanian and the islamic era the eastern part of the gorgan plain was characterized by open steppe landscapes d

estimation of ground motion in mexico city from a repeat of the m 7 0 acambay earthquake of 1912 abstract the acambay earthquake of 1912 m 7 0 which occurred in the central trans mexican volcanic belt ctmvb about 100 km west northwest of mexico city has been thought to be a critical scenario event in the estimation of seismic hazard of the city we use seismograms of two small earthquakes located near actopan 15 december 2003 18 may 2010 and recorded at station cuig a hill zone site in mexico city as empirical green s functions egfs for the acambay region because actopan like acambay is situated in the ctmvb and both are located at about the same distance from cuig the use of the recordings of the actopan earthquakes as egfs is partly justified we first analyze data of the two small earthquakes at a local station dhig to estimate their seismic moment and stress drop as there is considerable uncertainty in the estimated stress drops of the two events we choose a reasonable range of values for them and apply a technique of random summation of egfs to simulate ground motion at cuig from a postulated m w 7 earthquake the estimated geometric mean horizontal peak ground acceleration pga and peak ground velocity pgv at cuig range from 2 7 to 9 7 cm s 2 and from 1 1 to 3 0 cm s respectively ground motion maps for the entire city are presented using a simulated trace at cuig and the known transfer functions of many sites within the city the results are consistent with reported seismic intensities in mexico city and pga and peak ground displacement pgd at a 2 s period at the seismic station of tacubaya during the acambay earthquake estimated ground motions suggest that a repeat of the event does not present significant hazard to mexico city

source spectral properties of small earthquakes in the northern north sea abstract source parameters of 40 small shallow earthquakes m 2 5 3 5 in the northern north sea region were calculated using p wave spectra of digital records from the western norway network station hya the results show a difference in the source spectral properties of earthquakes generated in the north sea rift zone and the uplift zone of the norwegian coastal margins corner frequencies fo of p wave spectra are constant for the margin zone events for the seismic moment interval e12 e14 nm there is a tendency of decreasing fo with the seismic moment for rift zone events rift zone events have larger stress drops than margin zone events with the same magnitude increase of stress drop with moment was observed for both zones

mesozoic and cenozoic tectono thermal reconstruction of the western bohai bay basin east china with implications for hydrocarbon generation and migration abstract studies on the tectono thermal evolution of the western bohai bay basin wbbb including the jizhong sub basin and cangxian uplift are important for further petroleum exploration and exploitation this study aims to reconstruct the tectono thermal history of the wbbb based on a combination of 125 vitrinite reflectance ro data and seven apatite fission track aft and nine apatite u th he ahe ages the effective aft ages range from 60 3 to 97 0 ma with mean track lengths of 10 86 12 64 m the variable single grain aft ages are explained to be due to the parameter dpar the effective ahe ages range from 21 1 ma to 75 0 ma two rapid cooling events revealed by the thermal modeling results occurred during the early late cretaceous 130 90 ma and late eocene early miocene 40 18 ma respectively the fuping complex which is located in the western margin of the wbbb and mainly composed of the paleo proterozoic high grade gneisses experienced the intensive extension detachment during the early late cretaceous along with the subduction of the izanagi plate that was probably responsible for the first cooling event the second cooling event was caused by the subduction of the pacific plate during the middle eocene early miocene 43 23 ma as well as the collision of the indian and eurasian plates and the rollback of the subducting pacific plate during the early miocene 23 18 ma the wbbb experienced two heat flow peaks of 75 and 80 mw m2 during the early cretaceous and the late eocene respectively during the late eocene early miocene rapid subsidence in the baxian depression one secondary tectonic unit of the jizhong sub basin coupled with the rapid uplift in the cangxian uplift caused the shahejie formation of the middle upper eocene to generate gas during the early miocene meanwhile tectonic movement during the early miocene also caused faults to become migration paths as revealed by the homogenization temperature of the fluid inclusion this study provides new insights regarding the tectonic and hydrocarbon evolution of the wbbb

long wavelength geoid the effect of continental roots and lithosphere thickness variations summary the existence of continental roots and the gradual thickening of the cooling oceanic lithosphere give rise to large scale rheological heterogeneities in the uppermost mantle the effect of these heterogeneities on the long wavelength geoid is investigated using a 3 d mantle flow model involving a low viscosity asthenosphere beneath the oceanic lithosphere and the tectonically active continental regions and a thick highly viscous lithosphere beneath ancient continents below 400 km the mantle viscosity is laterally homogeneous with a lower mantle more viscous than the overlying layer the mantle circulation is driven by imposed surface velocities nuvel 1 hs2 gripp gordon 1990 and by the density anomalies inferred from the tomographic models p16b30 and s16b30 masters et al 1996 the geoid heights both due to plate motion and due to internal loading differ by as much as 30 per cent between the models with and without lateral viscosity variations in contrast to what was suggested in previous studies spherical harmonics 2 and 3 are strongly affected by the lateral viscosity variations these differences in the forward problem suggest that the response to the inverse problem that consists of finding the profile of viscosity as a function of depth providing the best fit to the geoid should be considerably affected by the lateral viscosity variations the shear stresses at the base of the plates induced by the imposed surface velocities and those induced by the internal loading are sensitive to the lateral viscosity variations this suggests that the lateral viscosity variations are very important for understanding the stress field or the forces acting on the plates the direction of these shear stresses which should be linked to the anisotropy direction is very different from the plate motion direction

dynamic cause of marginal lithospheric thinning and implications for craton destruction a comparison of the north china superior and yilgarn cratons we present a comparative tectonic analysis of the north china craton ncc which has lost parts of its root with the yilgarn and superior cratons which preserve their roots we compare the geophysical structure and tectonic histories of these cratons to search for reasons why some cratons lose their roots while others retain them based on the comparison and analysis of geological geophysical and geochemical data it is clear that the lithospheric thinning beneath craton margins is a common phenomenon which may be caused by convergence between plates however craton destruction is not always accompanied by lithospheric thinning except for cratons that suffered subduction and collision from multiple sides the western block also known as the ordos block of the ncc yilgarn and superior cratons have not experienced craton destruction the common ground among them is that they are surrounded by weak zones e g mobile belts or orogens that sheltered the cratons from deformation which contribute

the heat flow from the continents the terrestrial heat flow is defined as the quantity of heat escaping per unit time from the earth s interior across each unit area of the earth s solid surface the quantity varies from place to place over the surface of the earth as well as with time throughout earth history the total heat being lost from the earth at a given time is the integral of the heat flow taken over the entire surface of earth the heat flow from the continents is therefore but a part of the larger picture of global heat loss indeed the lesser part when compared to the heat loss through the floor of the ocean basins pollack 1980 sclater et al 1980 davies 1980 this is so not only because of the lesser continental area but also because of the generally smaller heat flow found on the continents than in the ocean basins thus a full discussion of the thermal regime of the earth and the mechanisms by which it exhausts heat from the interior must center on the heat loss through the oceans a topic which lies beyond the scope of this review nonetheless because of the geological antiquity of at least some of their constituent terranes the continents provide the only direct evidence about the thermal tectonic and petrologic processes of the earth at times before the birth of any of the modem ocean basins some two hundred million years ago because inferences about earlier times are drawn wholly from materials now preserved in continents one must be cautious not to overgeneralize about global tectonothermal processes from observations confined to continents

the dynamic history of the trans mexican volcanic belt and the mexico subduction zone abstract the trans mexican volcanic belt tmvb is a 1000 km long neogene continental arc showing a large variation in composition and volcanic style and an intra arc extensional tectonics it overlies the rivera and cocos slabs which display marked changes in geometry geophysical studies indicate that lithospheric mantle is very thin or absent beneath the forearc and arc the fluids from the slab are released in a 40 to 100 km wide belt beneath the frontal part of the arc and the lower crust beneath the arc is partially molten east of 101 w the tmvb is built on a precambrian to paleozoic crust with thickness of 50 55 km west of 101 w the tmvb is underlain by jurassic to cenozoic marine and continental arcs with a 35 40 km thick crust the evolution of the tmvb occurred in four stages 1 from 20 to 10 ma the initial andesitic arc moved inland showing progressively drier melting and eventually slab melting suggesting flattening of the subducted slab 2 since 11 ma a pulse of mafic volcanism migrated from west to east reaching the gulf of mexico by 7 ma this mafic lavas marks the lateral propagation of a slab tear triggered by cessation of subduction beneath baja california 3 thereafter the volcanic front started moving trenchward with a marked phase of silicic volcanism between 7 5 and 3 ma local emplacement of small volume intraplate like basalts since 5 ma and development of extensional faulting these features are related to slab rollback enhancing asthenophere flux into the mantle wedge and promoting partial melting of the crust 4 the modern arc consists of a frontal belt dominated by flux and slab melting and a rear belt characterized by more differentiated rocks or by mafic lavas with little or no evidence of subduction fluids but higher asthenosphere fingerprint

crustal radiogenic heat production and the selective survival of ancient continental crust if the oldest terrestrial rocks are to be used as a representative sample of the early terrestrial crust it is important to consider the possibility that they may represent a biased sample chemical data indicate that archean crust is on average depleted in potassium uranium and thorium relative to younger crust radiogenic isotopes of these elements produce a significant portion of surface heat flow from continents and low mean heat flow in archean terrains relative to younger terrains also indicates that these elements are relatively depleted in archean crust in stable continental lithosphere the primary variable controlling variations in the geotherm is crustal radiogenic heat production qualitative evidence suggests that terrains with low radiogenic heat production and thus a relatively cool geotherm are less susceptible to magmatic and tectonic reactivation than terrains with higher heat production and a hotter geotherm since crustal radiogenic isotope abundances must have been greater in the past if the isotopes were immobile in stable crust the relative effect of crustal heat production must have been greater and thus the range in geotherms must have been greater for the same mantle heat flow it is suggested that crust with low heat production is less susceptible to orogenic reworking than high heat production crust and thus there may have been selective stabilization of low heat production low potassium uranium and thorium crust early in earth history either archean crustal generation was by a mechanism that perhaps fortuitously produced only low heat production crust or all high heat production crust was reworked by orogenesis during the archean and did not stabilize in significant volumes until the earliest proterozoic perhaps in association with a reduction in mantle temperatures and mean heat flow if selective crustal reworking did operate during the archean surviving archean terrains may represent a biased sample of the early terrestrial continental crust

conductive incubation and the origin of dome and keel structure in archean granite greenstone terrains a model based on the eastern pilbara craton western australia 1 the archean east pilbara granite greenstone terrane epggt of the pilbara craton in western australia preserves a prolonged record of voluminous mafic dominated volcanism and felsic plutonism commencing at circa 3515 ma and ultimately resulting in the development of spectacular dome and keel structures early crustal growth was dominated by basaltic magmatism and the regional development of a 12 18 km thick greenstone sequence by circa 3335 ma overlying widespread 3470 3430 ma tonalite trondhjemite granodiorite ttg suites emplaced into the middle crust stratigraphic and structural relationships imply that voluminous granitoid plutonism and crustal scale dome and keel formation in the southeastern epggt was not initiated until circa 3325 ma but proceeded rapidly over the interval 3325 3308 ma this scenario is consistent with a conductive incubation period of 10 100 myr duration following the burial of radiogenic granitic crust beneath the accumulated greenstone pile for heat production parameters appropriate to the 3515 3308 ma epggt we show that the burial of 3470 3430 ma granitoid rich crust beneath the 3335 ma euro basalt and its precursors led to long term crustal temperature increases of 170 234 c at 35 km depth with a corresponding reduction in effective viscosity of 2 5 orders of magnitude in combination with the greenstone over granitoid density inversion the changes in effective viscosity of the mid to deep crust due to the burial of heat sources may have provided the crucial trigger for the initiation of dome and keel formation similarly cooling and strengthening of the crust during syndoming exhumation of the heat production potentially terminated large scale dome and keel amplification

long lasting epeirogenic uplift from mantle plumes and the origin of the southern african plateau 1 we investigate under what conditions the present day long wavelength anomalous elevation 500 m of the southern african plateau can be attributed to mesozoic plume events the anomalous long wavelength topography of the southern african plateau cannot be easily explained by recent heating of the upper mantle as opposed to other areas of the african superswell and geomorphological studies provide few hard constraints on the timing of the uplift a mesozoic origin for the plateau uplift is attractive in that southern africa experienced several large magmatic events in the mesozoic we formulate numerical and scaling models to investigate if plume material ponded beneath cratonic lithosphere in the mesozoic could produce 500 m of present day elevation we find that starting plume heads are ineffective at producing much long lasting uplift because the material spreads into a thin layer tens of km thick the ponded plume material also suppresses secondary convection by having a stable boundary at its base over time heat continues to flow out of the top of the lithosphere and a net imbalance of heat flow in the lithosphere develops resulting in subsidence even after the plume material has cooled to the temperature of the mantle adiabat secondary convection supplies less heat to the lithosphere than is lost upward by conduction the cratonic lithosphere returns to its original thermal structure on a timescale of less than 200 m y even two mantle starting plume heads one at the time of karoo volcanism ca 183 ma and the other at the time of kimberlite eruption ca 80 90 ma in our models cannot produce much 200 m of the present day anomalous elevation however significant uplift can be generated by plume tails provided they linger beneath the lithosphere for 25 30 m y and if the uplift effects of mesozoic plume heads and tails are considered together then it is possible to account for 500 m of present day elevation consequently a mesozoic plume model for plateau uplift in southern africa appears to be a viable model if plume tails delivered heat to the southern african lithosphere over a period of 25 m y or more

upper mantle structure beneath the arabian peninsula and northern red sea from teleseismic body wave tomography implications for the origin of cenozoic uplift and volcanism in the arabian shield upper mantle structure between 150 and 400 km depth is imaged beneath the arabian shield and northern red sea by modeling p and s traveltime residuals from teleseismic events recorded on the saudi arabia national digital seismic network the 1995 1997 saudi arabian passcal experiment and three permanent stations rayn eil and mrni relative traveltime residuals were obtained using a multichannel cross correlation method and inverted for upper mantle structure using vandecar s inversion method the resulting images reveal a low velocity region 1 5 for the p model and 3 for the s model trending nw se along the western side of the arabian shield and broadening to the northeast beneath the makkah madinah nafud volcanic line we attribute the low velocities to a mantle thermal anomaly that could be as large as 330 k and that is associated with the cenozoic uplift of and volcanic centers on the shield our tomographic images are not consistent with models invoking separate mantle upwellings beneath the northern and southern regions of the shield and instead favor single plume or superplume models we also find little evidence for low velocities beneath the northern red sea suggesting that there might not be a geodynamic link between rifting in the red sea and plateau uplift and volcanism in the shield

precambrian continental crust of india and its evolution the indian precambrian continental crust exhibits a variety of geological features fashioned at different times by different geotectonic processes the bulk of this crust was formed prior to 2600 m y ago and remobilized at least twice between 2600 2000 m y ago early proterozoic mobile belt epmb and 2000 1500 m y ago middle proterozoic mobile belt mpmb three early precambrian nucleii karnataka kn jeypore bastar jbn and singhbhum sn appear to have survived in the craton and are characterized by low grade supracrustals and tonalitic trondhjemite gneisses formed 3800 2600 m y ago the epmb event involved sedimentation amphibolite granulite facies metamorphism and co 2 k metasomatism and produced amphibolite facies rocks and k granites in the north and charnockite and other granulite facies rocks in the south gold sporadically distributed in the supracrustal rocks of the craton was remobilized during the epmb event k granites form a garland around the central dharwar craton sugg

low mantle heat flow at the edge of the north american continent voisey bay labrador heat flow measurements in 4 deep drillholes near voisey bay labrador have yielded the lowest value ever reported in the canadian shield 22 mw m 2 this very reliable estimate is also one of the lowest continental heat flow values world wide it requires the crust to be very poor in radioelements in this part of the archean nain province it also strongly supports the view that mantle heat flow is low 15 mw m 2 throughout the canadian shield with no trend of increasing mantle heat flow near the edges of the continent it also raises questions about the controlling mechanism for rifting and the opening of the labrador sea at 100 ma

lateral variation of lg wave propagation in southern mexico 1 in this study we investigated lateral variation of lg wave propagation in southern mexico from recordings of 92 crustal earthquakes along 591 travel paths the efficiency of lg propagation was measured in terms of lgto pn spectral ratio it was found that lgpropagation is inefficient for travel paths through the gulf of mexico coastal plains and the gulf of tehuantepec areas with thick layers of sediments an average lg quality factor qlg as a function of frequency for southern mexico was estimated for the efficient lg travel paths the relation obtained for qlg in the frequency range 1 6 8 hz is qlg f 204 f0 85 the lateral variation of qlg 1was solved as a mixed determined inverse tomography problem separately for each frequency in which a spatial smoothness constraint was imposed and a priori information was added in poorly covered regions the spatial resolution obtained was about 200 km it was found that the trans mexican volcanic belt the gulf of mexico coastal plains and the area east of 94 w are characterized by lower than average qlgvalues i e higher attenuation high qlg values were obtained for the mixteco oaxaca terranes while for the guerrero terrane values similar to the average were obtained the results show a correlation between qlg and crustal structure and provide valuable information on lateral variation of qlg which is needed for reliable prediction of ground motion during future earthquakes

crustal structure of the grenville front and adjacent terranes under the auspices of the great lakes international multidisciplinary program on crustal evolution approximately 320 km of deep seismic reflection data were collected in lake huron along a profile that extends east from the manitoulin terrane across the grenville front to the interior of the grenville orogen the manitoulin terrane is characterized by a series of gently east dipping reflections at about 20 km depth that separate a highly reflective lower crustal layer from a markedly less reflective upper layer imaged by strong reflections at the western end of a spectacular band of moderately east dipping reflections the grenville front clearly truncates manitoulin terrane structures to the west these data are interpreted in terms of a speculative two stage model involving 1 creation of a major decollement during northward collision of an allochthonous terrane with the southern superior cratonic margin 1 83 1 89 ga penokean orogeny and 2 northwest directed stacking of microterranes at the southeastern margin of the craton followed by crust penetrating ductile imbrication under high pressure high temperature conditions leading to the ramping of deeply buried rocks to the near surface 1 0 1 3 ga grenvillian orogeny

the seismic alert system for mexico city an evaluation of its performance and a strategy for its improvement the seismic alert system sas for mexico city has now been in operation for about 15 years the sas takes advantage of the fact that the city is located more than 300 km from the foci of many of the potentially damaging earthquakes the system consists of 15 accelerometers located along the coast of the state of guerrero above a segment of subduction plate boundary that is a mature seismic gap an algorithm estimates the magnitudes of earthquakes from the near source accelerograms and issues public and restricted alerts for earthquakes with m 6 and 5 m 6 respectively an evaluation of the sas s performance during 1991 2004 reveals a surprisingly high rate of failure and false alerts this poor performance results from an inadequate detection algorithm and a limited areal coverage by the sas this renders the alert system of limited use in this article we propose an alternative strategy for detecting earthquakes potentially damaging to mexico city that differs substantially from the one presently implemented by the sas although our analysis is based on close to source accelerograms of 45 mexican earthquakes and the corresponding peak accelerations recorded at a reference site in mexico city cu there is no restriction on the distance to the field station except that its location should provide sufficient alert time to mexico city based on these data an attenuation relation is derived to compute expected peak acceleration at cu a red from root mean square acceleration a rms at a field station the relation permits specification of an a red threshold given the peak acceleration at cu a cu for which an alert is desired along with prescribed probabilities of failure and false alert we find that the use of bandpass filtered 0 2 1 0 hz accelerograms leads to an improved performance of the sas the choice of the filter is guided by the frequency band of amplification of seismic waves in the lake bed zone of mexico city we think that a single level of general public alert may be the best option a good choice appears to be an alert for a cu 2 gal for 0 2 1 0 hz bandpass filtered accelerograms with 1 probability of failure to accomplish this we must set a red 0 8 gal the data since 1985 suggest that such an alert would occur about once or twice a year and the event will be felt by most persons in the lake bed zone the proposed algorithm along with an array of sensors located 30 to 40 km apart and distributed in a roughly semicircular arc of 310 km radius centered at mexico city should considerably improve the areal coverage and performance of the sas and potentially save thousands of lives

geothermal modelling of the lithosphere in the central baltic shield and its southern slope lithospheric temperature and heat flow density hfd were studied in the central baltic fennoscandian shield and its subsurface continuation to the south along a transect trending from eastern finland to southern estonia the transect represents an example of a low hfd 30 mw m 2 archaean craton on a thick 150 190 km lithosphere surrounded by early and middle proterozoic mobile belts on a thinner 110 150 km lithosphere with slightly elevated hfd 35 55 mw m 2 numerical 2 d conductive models were constructed in which peridotite solidus temperatures were assigned to those depths which correspond to the seismically determined lithosphere asthenosphere boundary this technique was found to reduce the effect of uncertainties in heat production and thermal conductivity values on the simulation results upper crustal heat production values for the finnish terrain were taken from published geochemical analyses of outcropping rocks for the estonian terrain new heat production values were measured from core samples representing nineteen deep boreholes middle and lower crustal lithologies were estimated with the aid of the deep seismic vpvs data and corresponding heat production values were adapted from global xenolith averages and from data for granulites cropping out in other precambrian areas the results of the modelling suggest that the lithosphere and moho depth variations are only weakly reflected in the measured surface heat flow data which are mainly controlled by heat sources in the upper crust the simulated heat flow densities at 50 km depth approximately at the moho are relatively low and range from 12 mw m 2 at the archaean northeastern end to 19 mw m 2 on the proterozoic southwestern end of the transect simulated temperatures at 50 km depth increase from northeast to southwest ranging from 450 550 c in eastern finland to about 650 c in estonia sensitivity of the simulations to parameter changes was studied by varying the heat production and thermal conductivity values the extreme values for the moho temperature estimates thus obtained may be about 50 k lower or 100 k higher than the values above the corresponding sensitivity of the moho hfd is about 6 mw m 2 and of the surface hfd 5 20 mw m 2 respectively

a review of crust upper mantle structure in the precambrian areas of australia and implications for precambrian crustal evolution the western two thirds of the australian continent are underlain by precambrian rocks seismically this region is characterised by negative teleseismic travel time residuals indicating a thick cold tectosphere despite extensive platform cover and phanerozoic crustal reworking in a few intracratonic sedimentary basins the precambrian crust can be classified broadly into archaean and proterozoic cratons and proterozoic mobile belts the archaean cratons have a two layered crust 25 35 km thick with observed velocities in the lowermost crust in the range 6 8 7 2 km s 1 the moho is a distinct feature with a velocity contrast of 1 0 km s 1 vertical reflection probes in the southern yilgarn block yielded record sections that have numerous laterally discontinuous but distinct reflections especially in the lower crustal layer that are especially concentrated around the boundary between the two crustal layers and the moho the seismic reflections and velocities are consistent with a predominantly felsic crust with mafic lenses constituting 20 35 of the lowermost crust and with the metamorphic grade ranging from granulite in the upper to middle crust to garnet granulite and even eclogite at the base of the crust the proterozoic mobile belts and cratons a have multi layered crust 45 50 km thick velocities below 25 km depth are almost always 7 0 km s 1 and reach 7 4 7 6 km s 1 in the lowermost crust velocities such as these are characteristic of gabbroic basaltic underplate the velocity contrast is 1 0 km s 1 at the moho which is less distinct than in archaean terranes record sections from vertical reflection probes show a highly reflective crust with no correlation between seismic layers in the refraction models and the distribution of vertical reflections the contrasting character of the crust in archaean and proterozoic provinces favours a tectonic model for the evolution of precambrian crust in which predominantly felsic archaean protocrust was tectonically reworked and underplated to produce proterozoic crust the simple two layered protocrust became more complicated in the process the introduction of vast amounts of underplate especially near the crust mantle boundary thickened the crust and destroyed the character of the moho it overprinted the reflection character of the crust so that the bands of reflections now do not correlate with the boundaries between the crustal layers the velocities in the lower crust are high because of the underplate

lateral variations of lg coda q in southern mexico broad band digital three component data recorded at unm a geoscope station were used to estimate lg coda q for 34 medium size 3 9 m b 6 3 earthquakes with travel paths laying in different geological provinces of southern mexico in an effort to establish the possible existence of geological structures acting as wave guides and or travel paths of low attenuation between the pacific coast and the valley of mexico the stacked spectral ratio method proposed by xie and nuttli 1988 was chosen for computing the coda q the variation range of q 0 q at 1 hz and the frequency dependence parameter estimates averaged on the frequency interval of 0 5 to 2 hz for the regions and the three components considered are i guerrero region 173 q 0 182 and 0 6 q 0 0 7 ii oaxaca region 183 q 0 198 and 0 6 q 0 0 8 iii michoacan jalisco region 187 q 0 204 and 0 7 q 0 0 8 and iv eastern portion of the transmexican volcanic belt tmvb 313 q 0 335 and 0 9 the results show a very high coda q for the tmvb as compared to other regions of southern mexico this unexpected result is difficult to reconcile with the geophysical characteristics of the tmvb e g low seismicity high volcanic activity and high heat flow typical of a highly attenuating low q region visual inspection of seismograms indicates that for earthquakes with seismic waves traveling along the tmvb the amplitude decay of lg coda is anomalously slow as compared to other earthquakes in southern mexico thus it seems that the high q value found does not entirely reflect the attenuation characteristics of the tmvb but it is probably contaminated by a wave guide effect this phenomenon produces an enhancement in the time duration of the lg wave trains travelling along this geological structure this result is important to establish the role played by the transmission medium in the extremely long duration of ground motion observed during the september 19 1985 michoacan earthquake the overall spatial distribution of coda q values indicates that events with focus in the michoacan jalisco and oaxaca regions yield slightly higher values than those from guerrero this feature is more pronounced for the horizontal component of coda q a slight dependence of average coda q 1 on earthquake focal depth is observed in the frequency range of 0 2 to 1 0 hz approximately on the horizontal component deeper h 50 km events yield lower values of q 1 than shallower events for frequencies higher than 1 0 hz no clear dependence of q 1 on focal depth is observed however due to the estimates uncertainties this result is not clearly established

a comparison of site response techniques using earthquake data and ambient seismic noise analysis in the large urban areas of santiago de chile summary situated in an active tectonic region santiago de chile the country s capital with more than six million inhabitants faces tremendous earthquake risk macroseismic data for the 1985 valparaiso event show large variations in the distribution of damage to buildings within short distances indicating strong effects of local sediments on ground motion therefore a temporary seismic network was installed in the urban area for recording earthquake activity and a study was carried out aiming to estimate site amplification derived from horizontal tovertical h v spectral ratios from earthquake data ehv and ambient noise nhv as well as using the standard spectral ratio ssr technique with a nearby reference station located on igneous rock the results lead to the following conclusions 1 the analysis of earthquake data shows significant dependence on the local geological structure with respect to amplitude and duration 2 an amplification of ground motion at frequencies higher than the fundamental one can be found this amplification would not be found when looking at nhv ratios alone 3 the analysis of nhv spectral ratios shows that they can only provide a lower bound in amplitude for site amplification 4 p wave site responses always show lower amplitudes than those derived by s waves and sometimes even fail to provide some frequencies of amplification 5 no variability in terms of time and amplitude is observed in the analysis of the h v ratio of noise 6 due to the geological conditions in some parts of the investigated area the fundamental resonance frequency of a site is difficult to estimate following standard criteria proposed by the sesame consortium suggesting that these are too restrictive under certain circumstances

terrestrial heat flow in the sirt basin libya and the pattern of heat flow across northern africa we report 66 new heat flow and 24 new heat production measurements from the sirt basin a late jurassic miocene sedimentary depression in north central libya underlain by late proterozoic basement heat flow determinations were made using bottom hole temperatures from oil wells and thermal conductivity measurements from drill core and cuttings heat production measurements come from core samples of basement rock heat flow is fairly uniform throughout the basin with a mean of 72 9 s d mw m 2 it is not clear if heat flow from the sirt basin is elevated as a consequence of its origin as a late mesozoic rift the difference between the mean basin heat flow and the global mean heat flow from tectonically undisturbed late proterozoic terrains 55 17 mw m 2 is 17 mw m 2 but this difference lies within the uncertainties associated with these mean heat flow estimates if heat flow from the sirt basin is elevated it could be caused by enhanced crustal heat production and need not be attributed to thermal alteration of the lithosphere related to basin formation mean crustal heat production is 3 9 2 1 w m 3 1 2 to 3 times greater than surface heat production in other proterozoic terrains in africa from west to east the pattern of heat flow across northern africa is characterized by high 80 110 mw m 2 heat flow throughout most of northwestern africa normal to perhaps slightly elevated heat flow in the sirt basin low to normal 35 55 mw m 2 heat flow in egypt inboard of the red sea and high heat 75 100 mw m 2 flow along the red sea high heat flow near the red sea and in northwestern africa along the mediterranean coast can be readily attributed to cenozoic tectonic activity but high heat flow in the paleozoic sahara basins of southern algeria is harder to understand within the tectonic framework of northern africa a possible explanation advanced previously is that elevated heat flow in the sahara basins arises from a regional thermal anomaly within the north african lithosphere if that explanation is correct then the heat flow distribution in the sirt basin and in egypt away from the red sea suggests that the postulated lithospheric thermal anomaly does not extend beyond the sahara basins to the east

heat flow and crustal thermal structure in the late archaean closepet granite batholith south india the late archaean closepet granite batholith in south india is exposed at different crustal levels grading from greenschist facies in the north through amphibolite and granulite facies in the south along a 400 km long segment in the dharwar craton two areas pavagada and magadi located in the main mass of the batholith best represent the granitoid of the greenschist and amphibolite facies crustal levels respectively heat flow estimates of 38 mw m 2 from pavagada and 25 mw m 2 from magadi have been obtained through measurements in deep 430 and 445 m and carefully sited boreholes measurements made in four boreholes of opportunity in pavagada area yield a mean heat flow of 39 4 s d mw m 2 which is in good agreement with the estimate from deep borehole the study therefore demonstrates a clear cut heat flow variation concomitant with the crustal levels exposed in the two areas the mean heat production estimates for the greenschist facies and amphibolite facies layers constituting the main mass of the batholith are 2 9 and 1 8 w m 3 respectively the enhanced heat flow in the pavagada area is consistent with the occurrence of a radioelement enriched 2 km thick greenschist facies layer granitoid overlying the granitoid of the amphibolite facies layer which is twice as thick as represented in the magadi area the crustal heat production models indicate similar mantle heat flow estimates in the range 12 14 mw m 2 consistent with the other parts of the greenstone granite gneiss terrain of the dharwar craton

heat flow in the trans hudson orogen of the canadian shield implications for proterozoic continental growth fourteen new heat flow and radiogenic heat production measurements have been obtained in the paleo proterozoic trans hudson orogen of the canadian shield this orogen which consists of several distinctive belts corresponds to a pulse of crustal growth through island arc magmatism between 1 9 and 1 8 ga the data now available include 17 previously published measurements heat flow variations that are related to the history of magmatism and internal differentiation of the belts provide constraints on the crustal assemblages in the different belts of the orogen the average and standard deviation of heat flow values for the entire orogen 425 11 mw m 2 are identical to those of the older superior province and of the younger grenville province for the orogen as a whole heat flow is weakly correlated to the heat production of surface rocks high heat flow values are found in the thompson belt consisting of metasedimentary rocks deposited on the ancient continental margin of the superior craton there the accumulation of sediments derived from older and differentiated continental upper crust has resulted in significant concentrations of rdioelements in large volumes of rocks the heat flow is low in the belts that expose juvenile proterozoic crust consisting mostly of arc related volcanic rocks in the flin flon snow lake belt the average heat flow is the same as the average of the orogen the low heat production and the lack of correlation between heat flow and heat production suggest that the supracrustal volcanics exposed a t the surface are thin and rest on a basement richer in radioelements in the lynn lake belt the heat flow is significantly lower than the average for the orogen although the surface heat production is not low the heat flow data require a thin 10 kin surface layer overlying the mid and lower crust depleted in ra dioelements around the town of lynn lake heat flow is consistently low over a distance of m40 kin the coincidence between this cold spot and anomalously thick crust suggests that deep crustal roots may be preserved because of the stronger rheology implied by the low temperatures the evolution of the trans hudson orogen exemplifies the interplay between the processes generating rocks of evolved composition which require crustal thickening and those forming normal continental crust with average thickness which require crustal flow and soft crustal rheology

electrical conductivity structure of north west fennoscandia from three dimensional inversion of magnetotelluric data abstract new magnetotelluric mt data in north west fennoscandia were acquired within the framework of the project magnetotellurics in the scandes masca the project focuses on the investigation of the crustal and upper mantle lithospheric structure in the transition zone from stable precambrian cratonic interior to passive continental margin beneath the caledonian orogen and the scandinavian mountains in western fennoscandia an array of 59 simultaneous long period and 220 broad band mt sites were occupied in the summers of 2011 to 2013 the 3 d inversion of the masca data was obtained using the modem 3 d code the full impedance and tipper data were used for the inversion the rocks of archaean and proterozoic basement towards east and the caledonian nappes towards west are modelled as resistive structures in the central and southern parts the whole crust is resistive and reflects the trans scandinavian igneous belt granitoids the middle to lower crust of the svecofennian province is conductive an uppermost crustal conductor is revealed in the skelleftea ore district the south end of the kittila greenstone belt is seen in the models as a strong upper to middle crustal conductor in the caledonides the highly conductive alum shales are observed along the caledonian thrust front a map of the crustal conductance for the north west fennoscandian shield is presented

the grenville front tectonic zone results from the 1986 great lakes onshore seismic wide angle reflection and refraction experiment the grenville front which marks the orogenic boundary between the archean superior structural province and the much younger grenville province to the southeast is one of the major tectonic features of the canadian shield within canada it is approximately 1900 km in length extending from the north shore of lake huron across ontario and quebec to labrador in 1986 a major coincident onship near vertical reflection and onshore wide angle reflection refraction experiment glimpce great lakes international multidisciplinary program on crustal evolution was conducted along a series of lines across the great lakes one of the lines line j ran across georgian bay and lake huron for a distance of 350 km and crossed the grenville front tectonic zone gftz the seismic signals from the air gun array source were well recorded by the onshore stations up to distances of 250 km with a seismic trace spacing of 50 62 5 m the gftz had a profound effect on the nature of the reflector patterns observed on the onshore seismic sections data recorded by the stations on the east end of the line indicate that the crustal p phases are very complex and form a shinglelike pattern of reflected waves data recorded by stations at the center and at the western end of the line show that the pg phases are normal and lack the shinglelike appearance this character of arrivals was also observed on the corresponding s wave sections a combined p and s wave forward modeling analysis shows that the gftz is composed of bands of reflectors dipping at angles of 20 35 extending to the lower crust these reflectors were also well imaged on the coincident near vertical reflection data reflectors under the britt domain to the east of the gftz have a shallower dip than those along the zone the structure of the crust under the manitoulin terrane to the west of the gftz is laterally homogeneous with a major intracrustal reflector at a depth of 17 20 km below the surface poisson s ratio is slightly higher to the east of the grenville front compared to the region to the west the travel times of the pmp signals indicate that the moho may be deeper under the gftz than under the surrounding regions our results give added support to tectonic theories that the grenville front owes its origin to a continental collision process

continental dynamics of eastern china insights from tectonic history and receiver function analysis east asia records a complex sequence of tectonic processes with the mosaic of cratonic blocks and orogenic belts in china preserving evidence for multiple episodes of amalgamation and reworking from the archean to cenozoic the prolonged subduction of the pacific plate from the east has been invoked to explain the volcanic activities in the ne china region and in the songliao basin as well as the extensive destruction of the sub continental lithospheric mantle of the north china craton ncc the collision between the ncc and the yangtze craton involved deep subduction and exhumation of ultrahigh pressure metamorphic orogen in the south china craton the yangtze and cathaysia blocks collided along the shaoxing jiangshan pingxiang suture here we present an overview of the results from multidisciplinary studies on the major tectonic events in the eastern china region and evaluate the geodynamics of these in the light of our recent work based on receiver function analysis the compression induced by the westward subduction of the pacific plate in the ncc and south china appears to have triggered lower crustal and lithospheric delamination we also identify mantle upwelling as a dominant trigger for the major tectonic process in this region

are the norwegian mountains compensated by a mantle thermal anomaly at depth article i nfo article history the norwegian mountains or scandes represent a long mountain range stretching from southern norway to the arctic and is characterised by a rugged topography and peaks up to 2 5 km high the origin of this mountain chain far away from any plate boundary remains a matter of passionate debates inside the geoscientific community hot mantle fingers originating from the iceland plume impacting the base of the scandinavian lithosphere and creating asthenospheric diapirs is one of the most accepted hypotheses for explaining cenozoic uplift in norway in order to test this hypothesis we conducted integrated gravity and thermal modelling we used the dense ngu gravity grid i e one measurement every 3 km and modelled the depth extent and the mass deficit associated to the compensating loads located below the southern scandes assuming that the density deficit below the scandes is purely thermal in origin thermal modelling allowed for testing the magnitude of the potentially associated temperature anomaly and its impact on surface heat flow recently acquired heat flow data were used in order to constrain the results from the thermal modelling the results of our integrated geophysical modelling rule out the possibility that the present day topography of the southern scandes is compensated by a deep seated asthenospheric diapir however our modelling does not exclude that thermal processes in the deep mantle could have initiated or assisted recent uplift of the southern scandes

abrupt climate variability since the last deglaciation based on a high resolution multi proxy peat record from nw iran the hand that rocked the cradle of civilization abstract we present a high resolution sub decadal to centennial multi proxy reconstruction of aeolian input and changes in palaeohydrological conditions based on a 13000 yr record from neor lake s peripheral peat in nw iran variations in relative abundances of refractory al zr ti and si redox sensitive fe and mobile k and rb elements total organic carbon toc 13 c toc compound specific leaf wax hydrogen isotopes d carbon accumulation rates and dust fluxes presented here fill a large gap in the existing terrestrial paleoclimate records from the interior of west asia our results suggest that a transition occurred from dry and dusty conditions during the younger dryas yd to a relatively wetter period with higher carbon accumulation rates and low aeolian input during the early holocene 9000 6000 yr bp this period was followed by relatively drier and dustier conditions during middle to late holocene which is consistent with orbital changes in insolation that affected much of the northern hemisphere numerous episodes of high aeolian input spanning a few decades to millennia are prevalent during the middle to late holocene wavelet analysis of variations in ti abundances as a proxy for aeolian input revealed notable periodicities at 230 320 and 470 years with significant periodicities centered around 820 1550 and 3110 years over the last 13000 years comparison with palaeoclimate archives from west asia the north atlantic and african lakes point to a teleconnection between north atlantic climate and the interior of west asia during the last glacial termination and the holocene epoch we further assess the potential role of abrupt climate change on early human societies by comparing our record of palaeoclimate variability with historical geological and archaeological archives from this region the terrestrial record from this study confirms previous evidence from marine sediments of the arabian sea that suggested climate change influenced the termination of the akkadian empire in addition nearly all observed episodes of enhanced dust deposition during the middle to late holocene coincided with times of drought famine and power transitions across the iranian plateau mesopotamia and the eastern mediterranean region these findings indicate that while socio economic factors are traditionally considered to shape ancient human societies in this region the influence of abrupt climate change should not be underestimated

crustal heat production and style of metamorphism a comparison between two archean high grade provinces in the limpopo belt southern africa abstract different characteristics of late archean metamorphism in the northern and southern marginal zones of the limpopo belt in southern africa are examined in the light of differences between these provinces in contents of heat generating elements in the northern marginal zone intrusion of charnoenderbitic magmas lasted from 2 75 to 2 58 ga accompanied by repeated low to medium pressure granulite facies metamorphism with some anticlockwise p t paths in contrast the medium pressure granulites of the southern marginal zone experienced peak metamorphism at 2 69 ga without much evidence of plutonism at that time followed by near isobaric cooling at approximately 6 5 kbar as a basis for our approach we used steady state geotherms calculated for the deeply eroded marginal zones for late archean times on the basis of their average k th and u concentrations and assuming low basal heat flow values of 20 mw m 2 the southern marginal zone and the adjacent northern kaapvaal craton are both characterised by low average th and u concentrations th 1 1 and 1 5 ppm u 0 3 and 0 65 ppm which are not a consequence of high grade metamorphism the average th and u concentrations in the zimbabwe craton 8 8 and 2 2 ppm are much higher than those in the kaapvaal craton and those found in the northern marginal zone are even higher 13 8 and 2 6 ppm in the southern marginal zone peak metamorphic temperatures are much above reasonable steady state geotherms and peak conditions could only have been attained through transient very high basal heat flux which in the context of a subduction zone might have resulted from slab breakoff or slab windows the northern marginal zone in contrast has such a large internal heat generation that high grade metamorphic conditions plot close to steady state geotherms even with basal heat flux values of 20 mw m 2 and minor crustal thickening or basal heat flux increase would have sufficed to exceed fluid absent tonalite solidus temperatures in the lower crust the repeated high grade metamorphism and charnoenderbite intrusions over a period of 1 7 108 years in the province can be explained by this observation both marginal zones of the limpopo belt can be understood as archean active continental margins of the kaapvaal and zimbabwe craton respectively while the problems of explaining the metamorphism in the southern marginal zone i e the source of the required heat are similar to those faced in many present day margins or orogens the northern marginal zone is non uniformitarian in the sense that its extreme heat generation rate could not be a property of any geological province today

shape of the subducted rivera and cocos plates in southern mexico seismic and tectonic implications the geometry of the subducted rivera and cocos plates beneath the north american plate in southern mexico was determined based on the accurately located hypocenters of local and teleseismic earthquakes the hypocenters of the teleseisms were relocated and the focal depths of 21 events were constrained using a body wave inversion scheme the suduction in southern mexico may be approximated as a subhorizontal slab bounded at the edges by the steep subduction geometry of the cocos plate beneath the caribbean plate to the east and of the rivera plate beneath north america to the west the dip of the interplate contact geometry is constant to a depth of 30 km and lateral changes in the dip of the subducted plate are only observed once it is decoupled from the overriding plate on the basis of the seismicity the focal mechanisms and the geometry of the downgoing slab southern mexico may be segmented into four regions 1 the jalisco region to the west where the rivera plate subducts at a steep angle that resembles the geometry of the cocos plate beneath the caribbean plate in central america 2 the michoacan region where the dip angle of the cocos plate decreases gradually toward the southeast 3 the guerrero oaxaca region bounded approximately by the onshore projection of the orozco and o gorman fracture zones where the subducted slab is almost subhorizontal and underplates the upper continental plate for about 250 km and 4 the southern oaxaca and chiapas region in southeastern mexico where the dip of the subduction gradually increases to a steeper subduction in central america these drastic changes in dip do not appear to take place on tear faults suggesting that smooth contortions accommodate these changes in geometry the inferred 80 and 100 km depth contours of the subducted slab lie beneath the southern front of the trans mexican volcanic belt suggesting it is directly related to the subduction thus the observed nonparallelism with the middle american trench is apparently due to the changing geometry of the rivera and cocos plates beneath the north american plate in southern mexico and not to zones of weakness in the crust of the north american plate as some authors have suggested

q of lg waves in the central mexican volcanic belt from seismograms of nine shallow coastal earthquakes recorded at a pair of broadband stations we estimate q of lg waves in the part of central mexican volcanic belt mvb that includes the valley of mexico the two stations straddle the central mvb and are located on cretaceous limestone a weighted least square fit to the q 1 f data in the frequency range 0 25 to 8 hz yields q f 98 f 0 72 this estimate of q is lower than the corresponding q in the forearc region that is given by q f 273 f 0 66 note that our estimate of q f corresponds to a 200 km wide zone of the mvb the result of this study sheds light on the characteristics of seismic waves as they traverse through the mvb where they undergo dramatic amplification in the valley of mexico it also provides one of the critical elements needed in the estimation of expected ground motions at sites to the north of the mvb from future coastal earthquakes the lower q of lg waves in the mvb as compared with the forearc region seems correlated with lower resistivity reported in the mvb relative to the forearc region

radiogenic heat production in the continental crust abstract the thermal structure and evolution of continents depend strongly on the amount and distribution of radioactive heat sources in the crust determining the contribution of crustal rocks beneath a superficial layer is a major challenge because heat production depends weakly on major element composition and physical properties such as seismic wavespeed and density enriched granitic intrusives that lie at the current erosion level have a large impact on the surface heat flux but little influence on temperatures in the deep crust many lower crustal rocks that are poor in heat producing elements are restites from ancient orogenic events implying that enrichment of the upper crust was achieved at the expense of deeper crustal levels for the same total heat production concentrating heat sources in an upper layer acts to reduce temperatures in the lower crust thereby allowing stabilization of the crust the present day structure of the crust is a consequence of orogeny and should not be adopted for thermal models of the orogenic event itself this review summarizes information extracted from large data sets on heat flow and heat production and provides estimates of crustal stratification and heat production in several geological provinces analysis of global and regional data sets reveals the absence of a positive correlation between surface heat flow and crustal thickness showing that the average crustal heat production is not constant differences of heat flow between geological provinces are due in large part to changes of crustal structure and bulk composition collating values of the bulk crustal heat production in a few age intervals reveals a clear trend of decrease with increasing age this trend can be accounted for by radioactive decay indicating that thermal conditions at the time of crustal stabilization have not changed significantly for the average crustal thickness of 40 km moho temperatures are near solidus values at the time of stabilization suggesting an intrinsic thermal control on crustal thickness and heat production distribution crustal thickening by more than about 10 km above this mean value induces changes of gravitational potential energy that exceed the strength of the lithosphere for several provinces where strong constraints on heat production are available it is shown that prior to intracrustal fractionation only modest amounts of thickening were needed to generate the conditions of ultra high temperature metamorphism the tell tale signature of crustal heat production is anatectic and metamorphic events that lag the cessation of orogenic activity by several tens of million years crustal heat production is decreasing with time because it is due to the radioactive decay of uranium thorium and potassium its rundown is responsible for the secular cooling of lithospheric roots at a typical rate of about 100 k gy 1 implying complex thermal interactions with a convecting mantle that is not cooling at the same rate

stress drop slip type earthquake magnitude and seismic hazard care must be taken to provide reliable antiseismic protection in earthquake prone areas where the impact of a large earthquake in megacities with industrial facilities as well as in ordinary buildings is liable to cause massive loss of human life and to cripple the nation s economy this protection needs to take into account not only vibratory ground motion but also permanent ground failure and notably surface faulting hazard a fact tragically illustrated during the recent events of turkey and taiwan the purpose of this contribution is to conduct a survey of our current state of knowledge concerning theoretical relationships between earthquake source parameters such as moment magnitude m m w surface wave magnitude m s seismic moment m o stress drop rupture length l and displacement on the fault d a relationship is proposed that links m s to l and m s 2 log l 1 33 log 1 66 using a simple rupture model earthquake data from all over the world for which the parameters of rupture length fault width w fault displacement and surface wave magnitude are available have enabled stress drop values to be computed for each event using both geological observation 1 and the previously proposed equation 2 the results obtained tend to indicate that stress drop values increase versus fault width depth up to approximately 15 km corresponding perhaps to the brittle ductile boundary this increase is more pronounced in the case of reverse faults than it is for those with strike slip or normal mechanisms an equation of the type kw n has been used to fit the data and preliminary values for k and n have been supplied for the three slip types for depths in excess of 15 km the data do not display significant variation 100 bars these results are in agreement with certain laboratory models of the continental lithosphere although more data particularly in the large magnitude range are needed to ascertain whether it is the w or the l model that better describes earthquake scaling laws stress drop does not appear to be fault length dependent thus being supportive of the l model the risk of surface faulting is dependent on the dynamic environment of the fault that is the stress drop the rupture length and the fault width although statistics show that surface faulting appears in most instances at magnitudes of at least 6 1 data from certain regions indicate that seismicity at superficial depths is under certain conditions accompanied by significant surface faulting even for magnitudes as small as 5 5 suggesting a change in scaling law the threshold magnitude for surface faulting is accordingly seen to depend on the rheology of materials in the fault area and on the stress environment manuscript received 31 may 2000

high resolution heat flow density in the lower congo basin 1 heat flow density hfd has been derived in the lower congo basin from 293 conventional probe measurements 424 oil exploration data and about 1 000 000 bottom simulating reflector bsr values each set of data provides independent estimates of temperature gradients associated with specific uncertainties and geographic distributions but a statistical comparison in overlapping domains shows a relatively good consistency in situ conductivity has been measured in conventional surveys and estimated from rock composition and porosity in oil exploration boreholes in the deep offshore measurements and estimates are consistently low and increase significantly toward the shelf domain thermal conductivity does not change significantly in the interval between the water bottom and the bsr and can be estimated from the surface measurements to provide an inference of hfd the combined sets of hfd in the lower congo basin cover the overall domain of the continental margin and a part of the oceanic domain high resolution imaging of the bsr by three dimensional 3 d seismic studies provides in addition good quality nominal resolution of 500 m in the deep offshore at short length scales on the order of kilometers large variations of hfd from 30 to 140 mwm 2 exist above salt diapirs and canyons related to heat refraction on the other hand no small scale effect related to fluid flow has been detected although active venting is known at larger length scale hfd in the deep offshore is different on both sides of the congo submarine channel 54 8 mwm 2 to the north and 48 12 mwm 2 to the south which are also characterized by different basin tectonic styles at the margin scale hfd is significantly different in the oceanic domain 42 3 mwm 2 the deep continental offshore 52 10 mwm 2 and the continental shelf 65 15 mwm 2 a progressive reduction of the radiogenic heat contribution in the extended continental crust is the more likely the cause of these differences mantle heat flow below the continental margin is consequently high compared to that expected 15 mwm 2 for old shields such as congo

alkalic ocean island basalt type and calc alkalic volcanism in the mexican volcanic belt a case for plume related magmatism and propagating rifting at an active margin the mexican volcanic belt has been traditionally regarded as a classic case of subduction related calc alkalic volcanism however a series of geologic geophysical and petrological arguments makes this simple relationship doubtful a seismic gap beneath the belt a large scale mantle anomaly a graben triple junction domain and the presence of volumetrically important oceanic island basalt oib volcanism throughout the belt suggest a more complex tectonic scenario involving plume and subduction related processes we here propose a model involving the development of a propagating rift opening from west to east in response to plume activity the process started in miocene time within the western sector of the belt guadalajara and gave rise to a graben triple junction and oib type and calc alkalic volcanism extension and volcanism proceeded to the east giving rise to progressively younger ages for the initiation of oib type volcanism 1 miocene in the west e g guadalajara 2 pliocene in the central zone e g michoacan guanajuato and 3 quaternary farther east e g chichinautzin geochemical evidence suggests that part of the modern calc alkalic volcanism e g chichinautzin may be derived from magma mixing between the oib mafic magmas and silicic crust derived magmas however we do not preclude some influence of the subducting slab in the generation of other e g jorullo calc alkalic volcanic rocks our model suggests a currently unrooted upper plume attached to the subcontinental lithosphere which defines a hot zone beneath the mexican volcanic belt

thermal evolution of the upper yangtze craton secular cooling and short lived thermal perturbations abstract the upper yangtze craton uyc has stayed in a long term stabilization state its thermal history can be portioned into two parts secular evolutionary trend and short lived thermal perturbations the former is simulated by a two dimensional forward transient thermal model and the latter is discussed by reviewing previous studies the numerical modeling indicates the uyc tended to be cooling as it adjusted to the asynchronous rates of change in the internal radiogenic heat production and in the convecting mantle temperature below the cratonic root the average cooling rate of the uyc has been 58 c ga since the end of archean faster than that of the convecting mantle the surface heat flow presents a decreasing trend from 82 mw m2 at 2 5 ga to 53 mw m2 at present day which constructs the long term evolutionary path the thermal perturbations derived from the occasional tectono thermal events such as mantle plume activity and regional lithosphere extension serves as paroxysmal factors the thermal effects of the emeishan mantle plume were very strong yielding to the maximum heat flow anomaly of 100 mw m2 at the late permian in contrast the influences of regional lithosphere extension during both the early paleozoic period and the early permian middle triassic period were rather weak resulting in a surface heat flow increase of

hot rocks in a cold place high sub glacial heat flow in east antarctica numerical models are the primary predictive tools for understanding the dynamic behaviour of the antarctic ice sheet however a key boundary parameter sub glacial heat flow remains poorly constrained we show that variations in abundance and distribution of heat producing elements within the antarctic continental crust result in greater and more variable regional sub glacial heat flows than currently assumed in ice modelling studies such elevated heat flows would have a fundamental effect on ice sheet behaviour and highlight that geological controls on heat flow must be considered to obtain more accurate and refined predictions of ice mass balance and sea level change supplementary materials heat flow and heat production definitions heat production data and details of the 2d model of the gross geometry of the continental lithosphere for a section through prydz bay are available at www geolsoc org uk sup18690

thermal evolution of cratons in china abstract craton s thermal state is one of the main gateways to understand the craton dynamics three cratons in china the tarim north china and yangtze cratons are in different thermal states and differ in their stabilities low heat flow usually corresponds to stable craton but not vice versa the tarim craton is cold 44 mw m2 and still keeps its stability now the western north china craton and the upper yangtze craton are not too cold 50 60 mw m2 but also remain stable the eastern north china craton and the middle lower yangtze craton are relatively hot with heat flow of 60 70 mw m2 both have been heated and destroyed in the mesozoic all these cratons have experienced complex tectonothermal events since their formation some tectonothermal events only thin and heat craton but others can destroy craton several sedimentary basins are developed in these cratons which recorded individual thermal histories these thermal records could help to understand the thermal evolution of these cratons and provide constraints on craton dynamics including stability or destruction the thermal results indicate parts of craton in western china tarim and sichuan have secular cooling histories which were locally disturbed by mantle plume activities in the permian the middle lower yangtze craton has undergone similar thermal evolution as the eastern north china craton all parts of cratons in eastern china might all have been destroyed due to subduction and dehydration of the pacific plate during the mesozoic

crustal heterogeneities and the thermal structure of the continental crust the observed linear relationship between surface heat flow q0 and the radiogenic heat production of near surface rocks a0 discovered almost two decades ago has been a primary tool used to infer the distribution of heat production in the continental crust recent evidence on the nature of compositional heterogeneities in the continental crust has led us to reexamine the q0 a0 relationship and the likely implications for the thermal structure of the lower continental crust numerical simulations of crustal thermal models indicate that some interpretations derived from this relation may be flawed in particular the length scale determined from the q0 a0 function normally thought to describe the depth distribution of crustal heat sources is significantly affected by the horizontal scale of crustal heterogeneity as a consequence most estimates of lower crustal heat flow and lower crustal temperatures obtained using heat flow data with the q0 a0 relationship are overestimates this overestimation is proportionally largest in areas of low mantle heat flux such as cratons where basal heat flux may be overestimated by more than a factor of two

a global analysis of heat flow from precambrian terrains implications for the thermal structure of archean and proterozoic lithosphere previous studies of heat flow from precambrian terrains have yielded two different relationships a global temporal relationship between heat flow and tectonic age and a regional spatial relationship between heat flow and the proximity of archean cratons we analyze heat flow from tectonically stable precambrian terrains worldwide to address questions associated with these two heat flow patterns 1 is the spatial relationship a global pattern 2 do the two heat flow relationships have a common underpinning in answer to the first question our data analysis reveals a widespread spatial pattern between heat flow and the proximity of archean cratons which is characterized by low heat flow in archean cratons and proterozoic terrains adjacent to cratonic margins and higher heat flow in proterozoic terrains that are more than a few hundred kilometers from a craton to address the second question we examine three previous interpretative models of precambrian heat flow 1 simple cooling of a thermal boundary layer 2 thicker lithosphere in archean terrains than in proterozoic and 3 greater heat production in proterozoic crust than in archean model 1 predicts essentially no change in heat flow in terrains older than 1 5 ga and therefore does not likely provide a common underpinning for the precambrian heat flow patterns models 2 and 3 when combined with the special structural configuration of sutures can independently yield both the spatial and temporal heat flow distributions and thus alone or together may be considered candidates to explain the precambrian heat flow patterns however thermal models of the lithosphere in which the heat flow patterns are explained entirely by variations in crustal heat production cannot satisfy constraints on upper mantle temperatures beneath archean cratons derived from xenolith thermobarometry the thermobarometry constraints can be satisfied by models in which differences in lithospheric thickness are the principal factor controlling the surface heat flow distributions if the heat flow patterns result primarily from variations in lithospheric thickness then a different temporal heat flow relationship for precambrian terrains can be proposed one in which heat flow varies inversely with the age of stabilization of the lithosphere

high t low p metamorphism in the palaeoproterozoic halls creek orogen northern australia the middle crustal response to a mantle related transient thermal pulse high t low p metamorphic rocks of the palaeoproterozoic central halls creek orogen in northern australia are characterised by low radiogenic heat production high upper crustal thermal gradients locally exceeding 40 c km 1 sustained for over 30 myr and a large number of layered mafic ultramafic intrusions with mantle related geochemical signatures in order to account for this combination of geological and thermal characteristics we model the middle crustal response to a transient mantle related heat pulse resulting from a temporary reduction in the thickness of the mantle lithosphere this mechanism has the potential to raise mid crustal temperatures by 150 400 c within 10 20 myr following initiation of the mantle temperature anomaly via conductive dissipation through the crust the magnitude and timing of maximum temperatures attained depend strongly on the proximity duration and lateral extent of the thermal anomaly in the mantle lithosphere and decrease sharply in response to anomalies that are seated deeper than 50 60 km maintained for 5 myr in duration and or have half widths 100 km maximum temperatures are also intimately linked to the thermal properties of the model crust primarily due to their influence on the steady state background thermal gradient the amplitudes of temperature increases in the crust are principally a function of depth and are broadly independent of crustal thermal parameters mid crustal felsic and mafic plutonism is a predictable consequence of perturbed thermal regimes in the mantle and the lowermost crust and the advection of voluminous magmas has the potential to raise temperatures in the middle crust very quickly although pluton related thermal signatures significantly dissipate within 10 myr even for very large high temperature intrusive bodies the interaction of pluton and mantle related thermal effects has the potential to maintain host rock temperatures in excess of 400 450 c for up to 30 myr in some parts of the mid crust the numerical models presented here support the notion that transient mantle related heat sources have the capacity to contribute significantly to the thermal budget of metamorphism in high t low p metamorphic belts especially in those characterised by low surface heat flow very high peak metamorphic geothermal gradients and abundant mafic intrusions

ghosts of lithospheres past imaging an evolving lithospheric mantle in southern africa group ii 143 117 ma and group i kimberlites 108 74 ma intrude across the southwest boundary of the kaapvaal craton sampling the same volume of the subcontinental lithospheric mantle sclm in two time slices major and trace element analyses of 3699 peridotitic garnet xenocrysts were used to construct paleogeotherms for 17 kimberlite localities and to place each garnet at its depth of origin the ti contents of each garnet and the calculated x mg of its coexisting olivine were projected onto a southwest northeast section across the craton boundary and splines were used to interpolate between the virtual boreholes in each age group the sections show that the cratonic sclm extends at least 75 km southwest of the mapped craton boundary suggesting a dipping contact marked differences between the time slice sections show that between 117 and 108 ma the sclm on both sides of the craton boundary was heated and chemically refertilized by infiltrating asthenosphere derived melts thinning the depleted layer by 40 km the thermal and geochemical changes record a significant tectonothermal event that may be related to changes in the stress field associated with opening of the south atlantic or with mantle upwelling

heat production and thermal conductivity of rocks from the pikwitonei sachigo continental cross section central manitoba implications for the thermal structure of archean crust the pikwitonei and sachigo subprovinces of central manitoba provide a cross sectional view of the superior province crust in cross section the upper to mid level crust is composed of synformal greenstone belts surrounded by tonalitic gneisses both of which are intruded by granitoid plutons this crustal structure persists downward into the granulite facies where keels of the greenstone belts can be found to constrain thermal models of the crust we measured heat production and thermal conductivity in 60 rocks from this terrain using standard gamma ray spectrometry and divided bar techniques large vertical and lateral heterogeneities in heat production in the upper crust are evident heat production is high in granites and metasedimentary rocks intermediate in tonalite gneisses and low in the portions of greenstone belts dominated by mafic meta igneous rocks in the deeper granulite facies rocks heat production decreases by a factor of two in the tonalitic gneisses and remains low in the high grad

effects of random horizontal variations in radiogenic heat source distribution on its relationship with heat flow lateral conduction resulting from variations in the radiogenic heat generation is known to modify the heat flow heat generation relation and to bias the evaluation of thickness of radiogenic heat sources in this paper a statistical approach is used to account for a random horizontal variation in radiogenic heat sources whereas their vertical variation is simulated by simple models relationships between spectral densities and autocorrelation functions of heat generation and heat flow are obtained when the horizontal scale length of heat generation distribution is not much larger than the vertical one lateral heat conduction modifies drastically the statistical properties of heat flow distribution and the apparent thickness obtained from the heat flow heat generation plot is highly reduced the procedure is applied to a set of actual heat flow and heat generation data recorded over new hampshire u s a heat production data and especially heat flow data are affected by different types of noise and errors which can be analyzed through their autocorrelation functions the depth scale of heat generation deduced empirically does not seem to have much relation with any geophysical dimension

comment on dynamic surface topography a new interpretation based on mantle flow models derived from seismic tomography by a m forte w r peltier a m dziewonski and r l woodward

the major precambrian terranes of eastern south america a study of their regional and chronological evolution abstract the archaean of south america comprises two types of domain the low grade granite greenstone assemblages of cratonic type and the high grade granulite gneiss terranes of mobile zones related to the 2 7 ga old jequie event equivalent to the limpopo liberia event in africa the geological evolution of the proterozoic terranes of eastern south america is intimately linked to three major tectogenetic cycles and their related tectono thermal events i e the transamazonian the uruscuano and the brasiliano cycles of early middle and late proterozoic age respectively the first equivalent to the eburnian of africa is characterized by the reworking of archaean crust and accompanied by intense magmatism several major geotectonic provinces such as the amazonian province the sao luis province the paramirim province and the rio de la plata province were stabilized after this event the uruacuano belt seems to have been the only mobile zone active during the middle proterozoic and consequently the uruacuano cycle equivalent to the kibaran of africa has a relatively restricted regional imprint over the south american platform conversely the late precambrian brasiliano cycle equivalent to the pan african affected extensive areas adjacent to the atlantic coast and in central brazil the development of all these major mobile zones appears to be related to ensialic intracratonic processes including intense reworking of pre existing crust and within them there seems to be an almost complete absence of oceanic volcanism except for minor basaltic to andesitic volcanics found in the ribeira belt of late precambrian age

on the effect of continents on mantle convection at the earth s surface continents and oceans impose different thermal boundary conditions at the top of the mantle laboratory experiments are used to investigate the consequences of this for mantle convection the upper boundary of the experimental tank was made of copper plates enforcing a fixed temperature and had a conductive lid of finite width in the middle beneath this lid the thermal boundary condition was of the mixed type with a biot number depending on the dimensions and thermal conductivity of the lid experimental values of the biot number were scaled to earth values experiments were run for a large range of rayleigh numbers from 104 to 107 and for several lid widths the effects of temperature dependent viscosity and of the shape of the lid were investigated at steady state in all cases there is an upwelling beneath the conductive lid which feeds two symmetrical and elongated convective cells three different dynamic regimes were identified as a function of rayleigh number independently of the lid width at rayleigh numbers lower than 1 2 105 the upwelling is steady both in geometry and temperature structure at rayleigh numbers between 1 2 105 and 2 106 this central upwelling is intermittent at larger values of the rayleigh number there is no longer a simple upwelling structure but a set of small plumes rising together and distorted by a cellular circulation of large horizontal extent thus the conductive lid always imposes a large scale flow pattern the length of these convective cells is a function of lid width it is equal to the lid width at large values and decreases to the rayleigh benard value as the lid width decreases to zero a fluid loop model explains the most important features of this form of convection the cell length is such that the upwelling temperature is minimized for a given rayleigh number and lid width and is an increasing function of lid width and a decreasing function of rayleigh number transient experiments demonstrate that the large scale flow structure develops rapidly with even small horizontal temperature differences implications for the earth are that large scale convection cells exist in conditions which in the absence of continents would probably lead to a chaotic convection pattern dominated by plumes at high rayleigh number continental breakup is effected by a large scale line upwelling structure which includes a number of individual plumes

cenozoic tectonic and depth age evolution of the indonesian gateway and associated back arc basins abstract we reconstruct the tectonic and paleo age and depth history of the indonesian seaway and associated southeast asian back arc basins to create a comprehensive paleogeographic framework for the region during the cenozoic we combine published tectonic models with revised models of the evolution of back arc basins based on available marine magnetic anomaly data and a recently published high resolution 1 min marine gravity anomaly grid which reveals many fine scale tectonic features previously not resolved reconstructions of the now subducted ceno tethys ocean floor are integrated with a revised absolute plate motion reference frame for the indian ocean based on moving hotspots all marginal seas north of australia formed in a back arc setting with the caroline and celebes seas opening north of a northward dipping subduction zone and the solomon sea south of a southward dipping subduction zone north of australia we suggest that the evolution of the two sub basins of the caroline sea from 36 to 25 ma followed different seafloor spreading patterns with the western sub basin being influenced by the manus hotspot our model suggests that collision between the melanesian and the caroline arcs led to cessation of oceanic crustal production in the caroline basin shortly before 25 ma combined geophysical and geological observations suggest that several major tectonic events occurred north of australia at around 45 ma roughly at the time when australia started moving northwards at fast rates similar to the present we suggest that this event is related to a relocation of the subduction zone nw of australia under the philippine sea plate due to a collision and subsequent accretion of old pacific plate material to the northward subducting australian plate our gridded oceanic paleo depth maps capture the history of marginal seas north of australia and the stepwise destruction of the ceno tethys ocean floor in the northern wharton basin and the west pacific providing boundary conditions for both paleoceanograpic and geodynamic models a regional analysis of basement depth anomalies reveals that median negative anomalous depth of several back arc basins is of the order of 650 800 m with a range of 300 1100 m accompanied by negative regional heatflow anomalies suggesting that mantle driven dynamic topography plays a larger role in modulating regional basement depth than previously suggested regional shear wave mantle tomography cross sections highlight that all marginal basins that exhibit large negative dynamic topography are underlain by massive buried slab material in the lower portion of the upper mantle and or below the transition zone supporting the notion that the observed paired negative dynamic topography and heatflow anomalies are due to basin formation above slab burial grounds

the surface heat flow of the arabian shield in jordan abstract surface heat flow in southern jordan western part of the arabian plate was determined in a dense cluster of five up to 900 m deep boreholes that have encountered sedimentary rocks of paleozoic ordovician and silurian age these rocks are underlain by an igneous and metamorphic basement which has been studied for its radiogenic heat production along the eastern margin of the dead sea transform dst fault system the heat flow calculated from continuous temperature logs and laboratory measured thermal conductivity of drillcores and surface samples averages to 60 3 3 4 mw m 2 and contrasts the common view of the late proterozoic consolidated arabian shield constituting a low heat flow province of 45 mw m 2 although only characterizing an area of about 300 km 2 this average is unlikely representing a positive local anomaly caused by voluminous hhp granites rhyolites at shallow depths instead a heat flow of 60 mw m 2 is considered a robust estimate of the phanerozoic conductive surface heat flow not only for jordan but for the arabian shield in areas unaffected by younger reactivation the large variation in conductive heat flow 36 88 mw m 2 previously observed in jordan southern syria and saudi arabia is irreconcilable with their broad similarity in lithosphere structure and composition and rather reflects a combination of factors including low quality temperature data and insufficient knowledge on thermal rock properties

local earthquakes of the mexico basin in mexico city κ q source spectra and stress drop abstract we analyzed 14 local earthquakes of the mexico basin 2 3 m w 3 8 recorded on hill sites at local distances 8 0 r 60 5 km to study the spectral attenuation of seismic waves spectral decay parameter kappa quality factor q source spectra and brune stress drop we found an average 0 value of 0 0457 s in hill sites which is similar to values reported in other regions assuming a 1 r geometrical spreading we obtain a quality factor given by q f 72 f 0 83 source spectra of all studied events are in good agreement with the 2 model from the synthetic modeling of ground motions at local distances we explain an anomalous behavior of the corner frequency versus seismic moment and also explain the relatively low values observed for stress drop 5 mpa the resulting high spectral attenuation values obtained may be responsible for the low acceleration values of ground motions observed for crustal earthquakes occurring inside and near to the mexico basin

age dependence of continental heat flow fantasy and facts abstract extensive use of empirical heat flow age relations in the field of thermal studies indicates that the basic concept of a relation between heat flow and age is deeply entrenched the idea of a thermal time constant and thermal thickness for the lithosphere depends on the validity of a heat flow age relation several thermal and thermo mechanical lithospheric models are constrained by a heat flow age relation a corrected heat flow age relation or a reduced heat flow age relation the theory of plate tectonics provides a physical basis for a heat flow age relation for the oceans reliable heat flow measurements from well sedimented areas of the oceans are known to support it on the contrary the theory of plate tectonics does not support an age relation for the continents which is not well realized it is pointed out that according to the theory of plate tectonics a distinct heat flow profile should characterize a cenozoic mesozoic orogenic belt over the continents and a variety of profiles could be expected on account of several possible interactions at convergent plate boundaries a specific characteristic heat flow value corresponding to a particular age loses meaning and therefore a heat flow age relation or curve becomes conceptually invalid presently available continental heat flow data statistically analysed in a proper manner do not as they should not support an age relation attempts at correcting age dependent heat flow means for crustal radiogenic heat to obtain an age relation for mantle heat flow do not yield meaningful results

heat flow measurements in late palaeoproterozoic to permian geological provinces in south and central norway and a new heat flow map of fennoscandia and the norwegian greenland sea abstract new heat flow determinations from 9 sites in south and central norway situated in late palaeoproterozoic to permian geological provinces show that topographically and palaeoclimatically corrected surface heat flow generally varies between 50 and 60 mw m 2 the palaeoclimatic corrections are significant typically constituting 10 20 of the final heat flow values higher heat flow was encountered at 3 sites however only at the fredrikstad site situated in the high heat producing iddefjord granite could this elevated heat flow be related to high heat production a compilation of new and previously published heat flow data from norway show that on the scale of major geological provinces heat flow varies significantly and is generally overlapping the lowest heat flow values ca 30 mw m 2 come from the early palaeoproterozoic karasjok kautokeino greenstone belt and the early neoproterozoic egersund anorthosite complex both of which are dominated by low heat producing rocks the highest heat flow values ca 60 80 mw m 2 come from early neoproterozoic high heat producing post orogenic granites in south norway these geological provinces display relatively little variation in lithology and heat production which allows such relationships to be made in general however significant lithological variation within geological provinces makes relationships between heat flow and geological factors such as tectonometamorphic age lithology and heat production on a country scale obscure at best in contrast a compilation of almost 1000 heat flow measurements from fennoscandia and the norwegian greenland sea shows a distinct correlation between heat flow and tectonic age the archaean provinces in ne fennoscandia karelia display low heat flow of ca 35 40 mw m 2 increasing gradually to ca 50 55 mw m 2 southwestwards in palaeoproterozoic and mesoproterozoic provinces the transscandinavian igneous belt which consists largely of relatively high heat producing granites breaks this trend with a heat flow of ca 60 65 mw m 2 on the norwegian continental margin the heat flow is rather constant at ca 60 mw m 2 and the oceanic crust shows an exponential increase in heat flow from ca 60 mw m 2 near the continent ocean boundary oceanic crust ca 55 million years old to 200 mw m 2 near the mid atlantic spreading ridge similar to heat flow determined from oceanic crust elsewhere as well as to theoretical models of oceanic heat flow although a number of factors may render individual heat flow measurements uncertain the abundance of available data obtained by different methods and the clear correlation between heat flow and geology provide confidence as to the significance of these heat flow values

influence of depth focal mechanism and tectonic setting on the shape and duration of earthquake source time functions source time functions of 255 moderate to great earthquakes obtained from inversions of teleseismic body waves by tanioka and ruff 1997 and coworkers were compared in a systematic way they were scaled to remove the effect of moment and to allow the direct comparison and averaging of time function shape as well as duration time function durations picked by tanioka and ruff 1997 are proportional to the cube root of seismic moment if moments from the harvard centroid moment tensor catalog are used the average duration of scaled time functions is shorter and the average shape has a more abrupt termination for deeper events than shallower ones with a distinct change occurring at 40 km depth the complexity of the time functions as quantified by the number of subevents appears to decrease below 40 km depth furthermore among events shallower than 40 km the average duration of scaled time functions is shorter and their average shape has a more abrupt termination 1 for events with strike slip focal mechanisms compared to thrust events and 2 for the few thrust events associated with an intraplate setting compared to the majority associated with an interplate subduction boundary in each of these cases events in more technically and seismically active settings have a longer duration and a more gradual termination this can be interpreted in terms of lower stress drops and or slower rupture velocities at active plate boundaries suggesting that fault rheology depends on slip rate and may evolve as total fault slip accumulates furthermore differences in average time function shape and duration associated with different subduction zones suggest that differences exist in the rheology on the plate boundaries at the various subduction zones supporting data table is available via web browser or via anonymous ftp from ftp kosmos agu org directory append username anonymous password guest subdirectories in the ftp site are arranged by paper number information on seraching and submitting electronic supplements is found at http www agu org pubs esupp about html