strain heating as a mechanism for partial melting and ultrahigh temperature metamorphism in convergent orogens implications of temperature dependent thermal diffusivity and rheology 1 we explore the importance of strain heating in ductile shear zones for production of leucogranites and high temperature metamorphism in collisional orogens with temperature dependent thermal diffusivity d and rock rheology new measurements on metamorphic rocks from the bohemian massif show that d is 1 3 mm2 s at 25 c and decreases exponentially to as low as 0 4 mm2 s at 600 c this temperature dependence causes model lithospheric geotherms to be straighter compared to geotherms calculated with a constant d using power law parameters for rheology we show that deformation of quartzite at strain rate of 3 10 13 s 1 produces 100 w m3 at 550 c and 7 w m3 at 800 c deformation of stronger clinopyroxenite at this strain rate produces 8 w m3 even at 1050 c when strain heating is introduced by a 3 km thick ductile shear zone with quartz rheology at the depth of 35 km in a lithosphere that has 70 km thick crust and 60 km thick lithospheric mantle the schist solidus is reached by 8 myr in the vicinity of the shear zone deforming at the strain rate of 3 10 13 s 1 for clinopyroxenite rheology ultrahigh temperature uht metamorphic conditions are reached in 40 myr after initiation of strain heating two dimensional models that replicate the exhumation of the greater himalaya crystalline rocks above the main central thrust produce extensive partial melting and an inverted metamorphic field gradient occurrences of leucogranites within crustal shear zone systems may be evidence of the coupling between deformation and heat production in the crust

geobarometry geothermometry and late archean geotherms from the granulite facies terrain of south india geobarometry and geothermometry of garnet bearing chamockitic and metapelitic assemblages from the high grade terrain from south india indicate three fields of equilibration high pressure granulites p 8 3 pm 1 0 kb t 760 pm 40 circ c are restricted to the charnockites and mafic granulites of the north slopes of the nilgiris these place a minimum constraint of 30 km on the maximum thickness of late archean crust medium pressure granulites p 6 4 pm 1 0 kb t 735 pm 40 circ c characterize the charnockites of the central nilgiris and the madras granulites to the east low pressure granulites p 5 0 pm 1 0 kb t 700 pm 20 circ c characterize the charnockites and metapelites south of the bhavani shear zone although migmatites from the kodaikanal massif may reflect somewhat higher temperatures all three granulite fields lie on a metamorphic geotherm piezothermic array which is strongly convex to the temperature axis and characteristic of convective heat transfer

the age and origin of younger granitic plutons of the shaw batholith in the archaean pilbara block western australia the whole rock pb pb method has been used to date four of the younger mainly adamellite late tectonic plutonic phases within the ca 3 5 ga shaw batholith of the archaean east pilbara block three suites give ages within error of 2966 ma porphyritic granites at 2948 50 ma leuco adamellites at 2943 46 ma and garden creek adamellite at 3007 48 ma the post tectonic cooglegong adamellite gives an age of 2847 34 ma the sm nd model isotopic systematics of all four suites indicate derivation from crust ranging between ca 3200 and 3600 ma in age the sources for these four younger plutonic phases were heterogeneous and although exhibiting some isotopic characteristics of the older 3 5 3 3 ga calc alkali plutonic suites were more depleted in the lil elements rb u and th in addition the garden creek adamellite and the cooglegong adamellite lack the very fractionated and hree depleted ree patterns characteristic of both the older calc alkali plutonic rocks and the porphyritic granites and leuco adamellites the crust underlying the shaw batholith at ca 2950 ma must have been both markedly heterogeneous and variably depleted a conclusion consistent with the complex tectonic and plutonic evolution of this region

geophysics hot blanket in earth s deep crust studies of rocks from earth s crust suggest that the lower crust is a good thermal insulator the knock on effects of this finding are many one being the crust s increased potential to generate more magma the rate of heat transfer by conduction is the dominant factor that determines the thermal evolution of planetary crust and lithosphere most thermal models of the earth s crust assume constant values for thermal diffusivity owing to large experimental uncertainties in measuring this property of rocks at high temperature whittington et al have used recent advances in laser flash analysis on three different crustal rocks types to show that thermal diffusivity strongly decreases with increasing temperature they find thermal diffusivity to be about half that commonly assumed at mid crustal temperatures and therefore conclude that the hot middle and lower crust is a much more effective thermal insulator than previously thought they also present models of lithospheric thermal evolution during continental collision and demonstrate that the temperature dependence of rock properties leads to a positive feedback between strain heating in shear zones and more efficient thermal insulation removing the requirement for unusually high radiogenic heat production to achieve crustal melting temperatures

a model for the analysis of rapid landslide motion across three dimensional terrain a new numerical model for the dynamic analysis of rapid flow slides debris flows and avalanches has been developed the model is an extension of an earlier algorithm and is implemented using a nu

thermal diffusivity of rhyolitic glasses and melts effects of temperature crystals and dissolved water thermal diffusivity d was measured using laser flash analysis on pristine and remelted obsidian sam ples from mono craters california these high silica rhyo lites contain between 0 013 and 1 10 wt h2o and 0 to 2 vol crystallites at room temperature dglass varies from 0 63 to 0 68 mm 2 s 1 with more crystalline samples having higher d a st increases dglass decreases approaching a constant value of 0 55 mm 2 s 1 near 700 k the glass data are fit with a simple model as an exponential function of temperature and a linear function of crystallinity dissolved water contents up to 1 1 wt have no statistically significant effect on the thermal diffusivity of the glass upon crossing the glass transition d decreases rapidly near 1 000 k for the hydrous melts and 1 200 k for anhydrous melts rhy olitic melts have a dmelt of 0 51 mm 2 s 1 thermal conduc tivity k0d cp of rhyolitic glass and melt increases slightly with t because heat capacity cp increases with t more strongly than density and d decrease the thermal conductivity of rhyolitic melts is 1 5 wm 1 k 1 a nd should vary little over the likely range of magmatic temperatures and water contents these values of d and k are similar to those of major crustal rock types and granitic protoliths at magmatic temperatures suggesting that changes in thermal properties accompanying partial melting of the crust should be relatively minor numerical models of shallow rhyolite intrusions indicate that the key difference in thermal history between bodies that quench to obsidian and those that crystallize results from the release of latent heat of crystallization latent heat release enables bodies that crystallize to remain at high temperatures for much longer times and cool more slowly than glassy bodies the time to solidification is similar in both cases however because solidification requires cooling through the glass transition in the first case and cooling only to the solidus in the second

heat flow gravity and structure of the abitibi belt superior province canada implications for mantle heat flow the results of new heat flow measurements are presented for eleven sites located in the abitibi greenstone belt of the archean superior province these new and previously published heat flow data are used with gravity and seismic data to constrain the crustal structure and composition of the abitibi subprovince and to determine the mantle heat flow beneath the canadian shield in order to analyze the long wavelength variations in heat flow and bouguer gravity anomaly the crust was divided into three layers with distinctive composition and physical properties average density heat production and thickness for each layer were varied within a range compatible with measurements and yielded crustal models consistent with gravity and heat flow data the dominant contribution to crustal heat production is that of a tonalitic type layer which has the lowest density the models require mantle heat flow to range between 10 and 14 mw m 2 the westward increase in heat flow requires the tonalitic layer to thicken from the grenville front toward the kapuskasing uplift at the expense of the greenstone supracrustals long wavelength gravity data are more affected by changes in crustal thickness than by composition and the low bouguer anomaly near the eastern grenville front is caused by 5 6 km of crustal thickening in greenstone terranes the long wavelength variations in heat flow are more useful for determining average crustal composition and the thickness of supracrustals than the gravity data

in situ u pb granulite hosted zircon dates kapuskasing structural zone ontario a late archean large igneous province lip as a substrate for juvenile crust abstract a geochronological study of the kapuskasing structural zone ksz a section of late archean lower crust within the southern superior province canadian shield was undertaken to test a plate tectonic model for the late archean abitibi opatica terrane as a modified large igneous province lip new geochronological data allow us to interpret 1 construction of the lip in an environment where inherited zircons were derived from an older sialic crust and preserved in the granulite grade lower crust and 2 high grade metamorphism in the lower crust related to crustal refinement of the lip u pb and pb pb dates were obtained by la icp ms with beams of 8 30 m the study yielded 21 interpretable zircon dates spanning 500 m yr from one mafic granulite and two garnetiferous metatonalites nine of the dates are significantly older than the oldest known magmatic event recorded in the upper crust of the abitibi opatica terrane and indicate 2760 ma proto crustal material in the ksz lower crust the rema

at what stress level is the central indian ocean lithosphere buckling internal contrasts in strength are responsible for lithospheric buckling these are quantified by comparing the indian ocean data to two dimensional visco elasto plastic numerical models where the material properties depend on temperature and pressure the central indian basin is known for its intraplate seismicity and long wavelength undulations of the sea floor and associated gravity signatures to simulate the amplitudes of undulations that reach 1 km within about 11 ma of compression and 60 km of shortening the required mean yield strength of the lithosphere is 400 mpa if either a hydrostatic fluid pressure is considered in the crust or a mechanical decoupling at the depth of the moho small crustal wavelengths are superimposed on the long wavelength deformation in agreement with observations it is then possible to match the alternative indication that buckling commenced only 4 ma ago with a total amount of shortening of 30 km and with a required yield strength of 200 mpa about 10 of homogeneous thickening accompanies buckling taking into account variable thermal conductivity demonstrates that a change in the geotherm is sufficient to increase buckling amplitudes by 10 within 5 ma fl 2000 elsevier science b v all rights reserved

do cenozoic analogues support a plate tectonic origin for earth s earliest continental crust archean continental crust largely comprises the trondhjemite tonalite and granodiorite dacite ttg d suite of igneous rocks formation of the earliest archean 3 5 ga ttg ds is controversial being attributed to either subduction zone processes with active plate tectonics or thermochemical mantle convection with no plate tectonic processes a suite of cenozoic adakite like lavas in jamaica has geochemical compositions comparable to early archean ttg d the data indicate that the adakites were generated by underthrusting or subducting and partial melting of oceanic plateau crust beneath jamaica this setting is analogous to proposed plate tectonic processes in the early archean where hot thick and more buoyant archean oceanic crust underthrusts adjacent plates the new adakite data imply that earliest archean ttg d continental crust could have formed above primitive subduction zones

thermal conductivity of mgo periclase at high pressure implications for the d region abstract the thermal conductivity of the lowermost mantle determines the rate at which heat flows across the core mantle boundary and consequently plays a central role in the thermal evolution of both the mantle as well as the core in an effort to test and improve model estimates of thermal conductivity in the deep mantle we perform first principles computations of the lattice conductivity for mgo periclase the method uses the peierls boltzmann transport equation via a combination of first principles molecular dynamics and first principles lattice dynamics phonon lifetimes are found to be inversely proportional to temperature and increases 3 fold as the density is increased from 3 37 to 5 49 g cm 3 thermal conductivities increase nearly 6 fold over the same density interval and show excellent agreement with the available experimental measurements we use these results to assess how conductivity measurements of mantle minerals can be reliably extrapolated to conditions characteristic of the lowermost mantle and find debye theory to be superior for this task we then apply this insight to estimate the conductivity of mgsio 3 perovskite in the lowermost mantle and combine this with our computational results for mgo periclase to construct a model estimate for a representative lower mantle assemblage we find the mantle conductivity to be 5 9 0 6 w m k at the top of the thermal boundary layer and 4 0 0 5 w m k at its base implying core mantle boundary heat flux values at the lower end of geophysical estimates

archean crust as revealed in the kapuskasing uplift superior province canada in the central superior province of the canadian shield a 120 km wide transition from the low grade michipicoten greenstone belt to the high grade kapuskasing structural zone represents an oblique section through some 20 km of archean crust uplifted along a northwest dipping thrust fault the restored vertical section through the upper and middle crust consists of three megalayers with undulating boundaries 1 0 to 25 km a high grade heterogeneous gneissic assemblage in part older than the upper supracrustal succession

transport properties of low sanidine single crystals glasses and melts at high temperature thermal diffusivity d was measured using laser flash analysis from oriented single crystal low sani dine k0 92na0 08al0 99fe 3 si2 95o8 and three glasses near kalsi3o8 viscosity measurements of the three super cooled liquids in the range 10 6 8 to 10 12 3 pa s confirm near arrhenian behavior varying subtly with composition for crystal and glass d decreases with t approaching a constant near 1 000 k dsat 0 65 0 3 mm 2 s 1 for bulk crystal and 0 53 0 03 mm 2 s 1 for the glass a rapid decrease near 1 400 k is consistent with crossing the glass transition melt behavior is approximated by d 0 475 0 01 mm 2 s 1 thermal conductivity klat of glass calculated using previous heat capacity cp and new density data increases with t because cp strongly increases with t for melt klat reaches a plateau near 1 45 w m 1 k 1 and is always below klat of the crystal melting of potassium feldspars impedes heat transport providing positive thermal feedback that may promote further melting in continental crust

transport properties of glassy and molten lavas as a function of temperature and composition abstract we provide measurements of thermal diffusivity d heat capacity cp and viscosity for 12 remelted natural lavas and 4 synthetic glasses and melts ranging in composition from leucogranite to low silica basalt and calculate their thermal conductivity both viscosity and the glass transition temperature decrease with decreasing melt polymerization for basaltic glasses d is low 0 5 mm2 s 1 at room temperature decreases slightly with increasing temperature and then drops upon melting to 0 25 to 0 35 mm2 s 1 other samples behave similarly despite scatter clear correlations exist between d of glass or melt with si content density nbo t and most strongly with fragility m glass thermal diffusivity is represented by d ft g ht where f g and h are fitting parameters for melts d t was resolved only for dacite andesite and morb a positive slope is consistent with other iron bearing samples glass and liquid cp depend on density and other physical properties but not exactly in the same manner as d we calculate thermal conductivity k from these data and demonstrate that k for glasses is described by a maier kelly formula large scatter exists for k at 298 k but silicic to intermediate melts have k between 1 8 and 1 3 wm 1 k 1 whereas basaltic melts are constrained to 1 4 0 1 wm 1 k 1 low values for thermal diffusivity and viscosity for basaltic melts suggests that basalts transfer heat much more efficiently by advection than by conduction alone and that partially molten zones in the mantle quickly become more thermally insulating than non molten zones potentially contributing to melt localization during decompression melting

the composite archaean grey gneisses petrological significance and evidence for a non unique tectonic setting for archaean crustal growth abstract the geodynamic context of formation of the archaean continental crust is a matter of debate the crust is largely made of grey gneiss complexes a composite rock assemblage dominated by granitoids that are generally regarded as belonging to the ttg tonalite trondhjemite granodiorite series using a large database of published ttg and grey gneiss compositions it is possible to show that the granitoids forming grey gneiss complexes actually belong to at least four main geochemical groups i a potassic component made of granodiorites and formed by melting of existing crustal lithologies and ii three sodic groups ttg proper that comprise low medium and high pressure groups the geochemistry of the low pressure group is consistent with derivation from a plagioclase and garnet amphibolite the medium pressure group was formed in equilibrium with a garnet rich plagioclase poor amphibolite whereas the high pressure group derived from a rutile bearing eclogite as the temperature of melting of metamafic rocks is largely independent from pressure this corresponds to melting along a range of contrasting geothermal gradients in turn reflecting a range of tectonic sites for the formation of the archaean continental crust

thermal diffusivity of olivine group minerals at high temperature thermal diffusivity d data from 12 oriented single crystals and seven polycrystalline samples of olivine group minerals were acquired with the laser flash method at temperatures t of up to 1500 c samples included forsterite fe mg binary olivines sinhalite and chrysoberyl specimens were characterized using infrared spectroscopy and electron microprobe analysis crystal orientation and chemistry both affect d for our single crystals d 100 d 001 d 010 at all temperatures thermal diffusivity decreases with increasing t and becomes virtually constant at high temperatures at room temperature d 001 of pure forsterite has the highest observed values but substitution of a small amount of co in forsterite 0 3 wt coo lowers d by 20 substitution of 10 fe for mg in forsterite as in typical mantle olivine lowers d by 50 at room temperature mantle olivine has d 3 25 1 66 and 2 59 mm2 s for the 100 010 and 001 orientations respectively the values decrease to 0 93 0 87 mm2 s at 790 985 c for 100 0 54 51 mm2 s at 590 740 c for 010 and 0 83 0 79 mm2 s at 740 890 c for 001 two dunite samples have d of 0 55 0 56 mm2 s at 890 1080 c showing the effect of preferred orientation of grains dominated by 010 thermal diffusivity of polycrystalline samples is controlled by the large amounts of olivine present minor phases offset the curves for d t from the value of the olivine mineral our laser flash measurements isolate the phonon component of heat transfer from radiative transfer and show that the phonon contribution becomes nearly constant for the high temperatures expected in the mantle the other microscopic component diffusive radiative transfer depends strongly on temperature and this temperature dependence likely exerts greater control on mantle convection

detrital zircon constraints on grenville sedimentation at the margin of laurentia abstract the southern grenville province is made up of metaigneous and metasedimentary rocks that were formed and metamorphosed as part of the mesoproterozoic active margin of laurentia grenville supergroup quartzites from the adirondack highlands new york the morin terrane quebec and new jersey highlands were analyzed to determine detrital and metamorphic zircon u pb ages and to help constrain mesoproterozoic tectonics these grenville quartzites have broadly similar detrital age populations as each other suggesting similar sedimentary source regions and depositional settings in back arc basin environments at the laurentian margin during geon 12 all samples contain detrital zircon with 1 45 1 25 ga ages consistent with proximal grenville source regions they also contain detrital ages that point to derivation from the mazatzal 1 7 1 6 ga yavapai 1 8 1 7 ga and penokean 1 9 1 8ga orogens of the north american midcontinent and have a diverse population of archean ages these results and published data from adirondack lowlands metasediments chiarenzelli et al 2015 2017 show distinctly laurentian sources and the related and likely connected nature of the sedimentary histories of the adirondack highlands and lowlands morin terrane and new jersey highlands metamorphic zircon formation is common in high grade quartzites and ages are variable between samples depending on the metamorphic history of each terrane and the melt productivity of different rocks peck et al 2010a adirondack highlands samples experienced metamorphic zircon grown during the shawinigan orogeny and anorthosite suite emplacement 1 19 1 14 ga morin terrane samples are from the morin shear zone which was active and caused metamorphic zircon growth during the ottawan phase of the grenvillian orogeny 1 09 1 02 ga the new jersey highlands sample has metamorphic zircon formed during the rigolet phase of the grenvillian orogeny 1010 980 ma

thermal transport properties of major archean rock types to high temperature and implications for cratonic geotherms abstract the thermal structure of archean terranes depends in part on the thermal transport properties of their distinctive rock types we determined thermal diffusivity d of a suite of 14 greenstone mafic granulite and tonalite trondhjemite granodiorite ttg rocks at temperatures up to 1000 c at atmospheric pressure using laser flash analysis which lacks systematic errors associated with conventional contact techniques at room temperature d ranges from 3 8 mm 2 s 1 for banded iron formation to 0 8 mm 2 s 1 for a quartz poor trondjhemite for all samples thermal diffusivity decreases with increasing temperature such that d for the suite converges around 0 7 0 1 mm 2 s 1 at 700 c combining these results with measured density and heat capacity calculated from modal mineralogy provides thermal conductivity as a function of temperature for each rock suite whereas near surface thermal conductivity varies from 2 7 to 4 2 w m 1 k 1 depending on rock type thermal conductivity of the lower crust is 2 w m 1 k 1 and only weakly dependent on lithology which is more thermally resistive than the uppermost mantle 3 5 w m 1 k 1 use of temperature dependent thermal transport properties in numerical models reveals that surface geotherms can be relatively insensitive to crustal heat production because high heat producing ttg rocks typically have high thermal diffusivity and conductivity at low temperatures and consequently are more efficient conductors of the heat they produce this finding implies that calculations of crustal heat production from surface borehole measurements may entail significant uncertainties

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

seismic reflection profiles across deep continental crust exposed in the kapuskasing uplift structure seismic imaging of precambrian cratons holds the promise of deep structural mapping and interpretation of fundamental crustal construction processes deep structures may be identified by tracing reflections to the surface1 particularly in exposed crustal cross sections2 elevated along deeply penetrating faults but many such cross sections have structural orientations too steep for successful reflection imaging like the ivrea zone3 or occur in plate boundary settings not representative of continental interiors by contrast the archaean greenstone terrain exposed in the kapus kasing uplift of canada4 5 has gently dipping structures and an intracratonic setting within the superior province thus providing an opportunity to examine the third dimension of continental crust down to 25 30 km palaeodepth6 here we present new seismic reflection data which although generally supportive of structural models based on surface geology4 gravity modeling5 and seismic refraction studies7 8 also indicate that the kapuskasing structure is a relatively thin thrust sheet containing seismically reflective sequences of high grade rock which may be analogues of seismi cally reflective in situ lower crust

new heat flow data from the immediate vicinity of the kola super deep borehole vertical variation in heat flow confirmed and attributed to advection abstract we present new heat flow values and other geothermal data in the upper crystalline crust in the immediate vicinity of the 12 4 km deep kola super deep borehole nw russia our results show a systematic vertical increase in geothermal gradient and heat flow density as deep as we could measure 1 6 km our results confirm earlier results on vertical heat flow trends of in the uppermost part of the kola super deep hole and imply that the thermal regime is not in steady state conductive conditions in an area of 3 km 5 km measurements were performed in 1 2 km deep boreholes surrounding the kola super deep hole and on core samples from these holes temperature logs are available from 36 holes core data exists from 23 boreholes with a total length of 11 5 km at a vertical resolution of 10 m we carried out a very detailed study on thermal conductivity with regard to anisotropy inhomogeneity and temperature dependence tensor components of thermal conductivity were determined on 1375 core samples from 21 boreholes in 3400 measurements additionally we measured specific heat capacity heat generation rate density porosity and permeability on selected subsets of core samples heat flow from 19 boreholes varies between 31 and 45 mw m 2 with an average value of 38 mw m 2 in most boreholes the vertical heat flow profiles show a considerable variation with depth this is consistent with observations in the upper part of the kola super deep borehole we conclude that this variation is not caused by technical operations but reflects a natural process it is considered to be due to a combination of advective structural and paleoclimatic effects preliminary 3 d numerical modeling of heat and flow in the study area provides an indication of relative contributions of each of these factors advective heat transfer turns out to have a major influence on the vertical variation of heat flow although transient changes in surface temperature may also cause a significant variation heterogeneity of the rocks in the study area is less important

thermal diffusivity of igneous rocks at elevated pressure and temperature thermal diffusivity measurements of seven igneous rocks were made to temperatures of 400 sup 0 c and pressures of 200 mpa the measuring method was based on the concept of cylindrical symmetry and periodic heat pulses the seven rocks measured were westerly rhode island granite climax stock nevada quartz monzonite pomona washington basalt atikokan ontario canada granite creighton ontario canada gabbro east bull lake ontario canada gabbro and stripa sweden granite the diffusivity of all the rocks showed a positive linear dependence on inverse temperature and excluding the east bull lake gabbro showed a linear dependence on quartz content quartz content varied from 0 to 31 by volume diffusivity in all cases rose or remained steady with increasing confining pressure the pressure effect was strongest at lowest pressures and vanished by levels between 10 and 100 mpa depending on rock type the pressure effect measured as a percentage change in diffusivity is stronger in the four rocks of granite composition than in the three of basaltic composition our results agree well with existing thermal diffusivity measurements at atmospheric pressure

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

heat production in an archean crustal profile and implications for heat flow and mobilization of heat producing elements we have measured concentrations of heat producing elements th u and k in 58 samples representative of the main lithologies in a 100 km transect of the superior province of the canadian shield from the michipicoten wawa greenstone belt near wawa ontario through a domal gneiss terrane of amphibolite grade to the granulite belt of the kapuskasing structural zone near foleyet this transect has been interpreted as an oblique cross section through some 25 km of crust uplifted along a major thrust fault and thus provides an opportunity to examine in detail a continuous profile into deep continental crust of archean age mean heat production values for these terranes based on aereal distribution of major rock types and calculated from their th u and k concentrations are michipicoten greenstone belt 0 72 w m 3 wawa domal gneiss terrane amphibolite grade 1 37 w m 3 kapuskasing granulites 0 44 w m 3 among the silicic plutonic rocks tonalites granites and their derivative gneisses the relatively large variation in heat production correlates with modal abundances of accessory minerals including allanite sphene zircon and apatite we interpret these variations as primary pre metamorphic the relatively high weighted mean heat production of the domal gneiss terrane can be accounted for by the larger proportion there of late stage th u and k rich granitoid plutons these may have been derived from the underlying kapuskasing granulite terrane leaving it slightly depleted in heat producing elements transport of th u and k therefore could have taken place in silicate melts rather than in aqueous or carbonic metamorphic fluids this conclusion is supported by the lack of a statistically significant difference in heat production between tonalites tonalite gneisses and mafic rocks of amphibolite versus granulite grade the pre metamorphic radioactivity profile for this crustal section is likely to have been uniformly low with a mean heat production value less than 1 w m 3 this result is distinctly different from measured profiles in more silicic terranes which show decreasing heat production with depth this implies fundamental differences in crustal radioactivity distributions between granitic and more mafic terranes and may be an important factor in selective reactivation of lithologically different terranes possibly resulting in preferential stabilization of basic terranes in the geological record our results indicate that a previously determined apparently linear heat flow heat production relationship for the kapuskasing area does not relate to the distribution of heat production with depth low but significant heat production 0 4 0 5 w m 3 continues to lower crustal depths with no correlation to the depth parameter from the linear relationship this low heat production may be a minimum average granulite heat production and suggests that in general heat flow through the moho is 8 10 mw m 2 lower than the reduced heat flow calculated from the heat flow heat production regression