toward an understanding of disequilibrium dihedral angles in mafic rocks the median dihedral angle at clinopyroxene plagioclase plagioclase junctions in mafic rocks cpp is generally lower than equilibrium 109 plus minus 2 observation of a wide range of mafic bodies demonstrates that previous work on systematic variations of cpp is incorrect in several important respects firstly the spatial distribution of plagioclase compositional zoning demonstrates that the final geometry of three grain junctions and hence cpp is formed during solidification the igneous process sub solidus textural modification in most dolerites and gabbros previously thought to be the dominant control on cpp is insignificant cpp is governed by mass transport constraints the inhibiting effects of small pore size on crystallization and variation in relative growth rates of pyroxene and plagioclase during rapid cooling pyroxene preferentially fills wider pores while the narrower pores remain melt filled resulting in an initial value of cpp of 78 rather than 60 which would be expected if all melt filled pores were filled with pyroxene lower cooling rates create a higher initial cpp due to changes in relative growth rates of the two minerals at the nascent three grain junction low cpp associated with cuspate clinopyroxene grains at triple junctions can also be diagnostic of infiltration of previously melt free rocks by late stage evolved liquids the metasomatic process modification of cpp by sub solidus textural equilibration the metamorphic process is only important for fine grained mafic rocks such as chilled margins and intra plutonic chill zones in coarse grained gabbros from shallow crustal intrusions the metamorphic process occurs only in the centres of oikocrysts associated with rounding of chadacrysts

crustal seismic velocity structure near faial and pico islands azores from local earthquake tomography the azores archipelago portugal is located on an oceanic plateau in a geodynamic environment prone to intense seismic and volcanic activity in order to investigate the crustal structure in this region we have conducted a local earthquake tomography study in the area of the islands of faial pico and s jorge using data recorded in july 1998 the july 9th 1998 earthquake near faial island triggered an aftershock sequence of thousands of events that lasted for several months and were recorded by a total of 14 stations located on the three islands surrounding the epicentral area in the upper crustal layers consistency is seen between the tomographic results and the islands surface volcanic units beneath the faial central volcano a low vp b6 0 km s anomaly roughly located at 3 7 km depth suggests a connection to the plumbing system possibly the presence of a magma chamber in ne faial a high vp n6 3 km s body was found located at mid lower crust most likely an intrusion of gabbroic composition that is bordered by the registered seismic activity its shape suggesting a tectonic controlled mechanism the relocated hypocenters together with the overall analysis of the tomographic model suggest a tectonic segmentation of faial island the crustal thickness under the islands volcanic buildings of the faial pico area was estimated at around 14 km 2007 elsevier b v all rights reserved

a quantification of the glacial imprint on relief development in the french western alps abstract the morphology of the western alps has been strongly influenced by quaternary glaciations on the basis of observations of glacial morphology in the belledonne grandes rousses taillefer and pelvoux ecrins massifs south eastern france we reconstitute the glacial trimline and equilibrium line altitude ela during the most extensive glaciation meg our best estimate of the meg ela is 1800 100m using digital elevation models we compare our glacial reconstruction with the relief structure of nine major catchments draining the massifs modal elevations of the largest catchments occur at 2000 2500m and coincide with minima in plots of mean slope angles as a function of elevation modal elevations and slope minima occur between the modern and meg elas confirming a strong glacial imprint on relief in order to quantify glacial valley carving in the massifs we isolated high elevation low relief surfaces that form rock shoulders adjacent to the glacial valleys from a digital elevation model and constructed an interpolated surface passing through these subtracting the present day topography from this surface allows us to quantify the maximum glacial valley depths maximum valley depths determined in this manner are typically 1000m with spatial maxima occurring around the location of the meg ela in most valleys these numbers do not take into account glacial valley widening and local glacial overdeepenings the also neglect however potential pre glacial fluvial valley incision which could account for 20 50 of the measured valley depths in spite of these problems inferred valley depths are reasonably well correlated with the mean reconstructed ice thickness and constitute about half of the sub ridgeline relief of the studied catchments these results lead us to propose a significant quaternary increase in the relief of the french western alps controlled by climate and associated with the initiation of alpine glaciations for reasonable values of the effective elastic thickness of the lithosphere the isostatic response to glacial valley carving reaches values of 300m across the massifs this number is insufficient to substantially offset topographic lowering due to regional denudation and we conclude that the isostatic response to glacial valley carving has not increased peak elevations significantly

eoalpine and mesoalpine tectonics in the southern alps according to structural and stratigraphic data eoalpine and mesoalpine tectonics can be clearly recognized in the southern alps and distinguished from the better known neoalpine deformation

progressive metamorphism of the chert bearing durness limestone in the beinn an dubhaich aureole isle of skye scotland a reexamination

the rare earth elements and uranium in garnets from the beinn an dubhaich aureole skye scotland uk constraints on processes in a dynamic hydrothermal system garnets from skarns in the beinn an dubhaich granite aureole isle of skye scotland have a large range of concentrations of uranium 0 2 358 ppm and the rare earth elements ree 23 4724 ppm variations in these concentrations correlate with major element zonation within the garnets and with changes in the shape of ree patterns typical patterns in most garnets display light ree lree enrichment flat heavy ree hree distribution and a negative eu anomaly these patterns are interpreted to represent equilibrium trace element exchange between pre existing pyroxene hydrothermal fluid and calcic garnets iron rich zones are characterized by positive eu anomalies and an increase in the abundance of the lree relative to the hree these patterns are interpreted as resulting from changes in ree speciation related to the introduction of externally buffered fluid to the skarn system relatively fe poor zones show strongly hree enriched patterns with negative eu anomalies and in some instances depletions in y relative to ho and dy which are interpreted as resulting from surface sorption of the ree during rapid disequilibrium garnet growth strong correlations between u abundance and the ree patterns indicate that the same processes have affected u distribution both types of pattern can be modified by the effects of closed system crystallization on ree abundance in the fluid and changes in fluid major element chemistry

the recent fault scarps of the western alps france tectonic surface ruptures or gravitational sackung scarps a combined mapping geomorphic levelling and 10be dating approach abstract in the western alps some recent scarps were previously interpreted as surface ruptures of tectonic reverse and normal faults that agree with microseismicity and gps measurements our analysis shows that in fact there are hundreds of recent scarps up to 30 m high and 2 1 km long with only pure normal motions they share the same characteristics as typical sackung scarps the scarps are mainly uphill facing parallel to the ridge crests and the contour lines they are relatively short less than 2 1 km with respect to tectonic fault ruptures and organized in swarms they cut screes and relict rock glaciers with a slow commonly 1 mm year average slip rate in the aiguilles grives massif these sackung scarps clearly express the gravitational toppling of sub vertical bedding planes in hard rocks in contrast the belledonne outer crystalline massif exhibits scarps that stem from the gravitational reactivation of conjugate tectonic faults the recent faults extend to about 1600 m beneath the rognier ridge crest but are always above the valley floor the main scarp swarm is 9 2 km long and constitutes the largest sackung ever described in the western alps 10 be dating of a scarp and offset surfaces shows that 4 m slip may have occurred rapidly in less than 3800 years sometimes between the end of the glaciation and 8800 1900 years ago this dating together with the location of some faults far from the deep glacial valleys suggests that sagging might have been triggered by strong earthquakes during a post glacial period of probably enhanced seismicity the belledonne and synclinal median faults just beneath the rognier sackung could have been the sources of this seismicity

steep oxygen isotope gradients at marble metagranite contacts in the northwest adirondack mountains new york usa products of fluid hosted diffusion steep gradients in oxygen isotope ratios are recorded from two contacts between grenville marbles calcite 18o 21 24 and metamorphosed granitic intrusives whole rock 18o 11 12 in the northwest adirondacks oxygen and carbon isotope fractionations between coexisting minerals in the marbles and the metagranites are consistent with isotopic equilibration at peak metamorphic temperatures the 18o profiles are sigmoidal with inflection points at the contacts and are developed over 5 6 m quantitative fluid infiltration diffusion modeling suggests that the isotopic gradients are best explained as the result of fluid hosted diffusion for porosities of 10 3 to 10 6 the profiles could have formed in 103 107 years via an interconnected grain boundary fluid the granites were probably emplaced at shallow crustal levels prior to the regional grenville metamorphism and isotopic exchange is most likely to have occurred at that time contact metamorphism at these localities involved devolatilisation of the marbles and the time estimates may reflect the periods over which fluid producing reactions took place and an enhanced porosity was maintained in the marbles there is a progressive increase in the percentage of diopside and k feldspar at the expense of phlogopite and tremolite towards the metagranites these mineralogical changes probably occurred due to the temperature increase associated with granite intrusion and diffusion of h2o from the granite the two localities lie on either side of the regional metamorphic amphibolite to granulite facies transition the preservation of isotopic profiles formed during contact metamorphism suggests that there was little pervasive fluid flow at this crustal level during the regional metamorphism or at any time thereafter steep isotopic gradients are present at orthogneiss marble contacts elsewhere in the adirondacks suggesting that this conclusion may be generally applicable to this terrain

evidence for dendritic crystallization of forsterite olivine during contact metamorphism of siliceous dolostones alta stock aureole utah forsterite crystals produced during metamorphism of siliceous dolomites in the alta utah contact aureole were investigated in situ and with crystal separates to constrain their aureole scale textural and morphological variability as the contact is approached forsterite crystals increase in abundance but decrease in average size suggesting greater kinetic influence on forsterite nucleation and growth in the inner aureole crystal morphologies vary widely from dendritic to skeletal to euhedral shapes across both periclase and forsterite zones highly skeletal forms of forsterite preserve crystallographically controlled internal scaffolding reentrants and internal void boundaries scanning electron microscope imaging of texturally more mature forsterite crystals reveals the common occurrence of surface features such as hopper structures ridges and terraces features that have been linked to dendritic crystallization of olivine in igneous rocks near end member forsterite crystals fo99 from the middle to outer forsterite zone exhibit complex cathodoluminescence cl patterns which can be linked to grain scale heterogeneities in the trace elements li b p and ti these grain scale cl zoning patterns can be correlated to crystallographic forms typical of forsterite the textural trends morphological characteristics surface features and cl patterns suggest that euhedral forsterite crystals grew by a process of early dendritic crystallization hopper development and infilling of hopper reentrants these results caution against interpreting all grain scale zoning profiles preserved in metamorphic forsterites as simple core to rim time sequences consideration of the possible role of dendritic crystallization in the development of forsterite morphologies is required to interpret 2 d element or isotope zoning profiles preserved in individual forsterite grains

short paper textures of forsterite calcite marbles from the beinn an dubhaich aureole skye and implications for the structure of metamorphic porosity in the aureole of the beinn an dubhaich granite forsterite grew with dihedral angles of less than 110 against calcite but was progressively and variably annealed towards the equilibrium dihedral angle of 165 new experimental estimates of calcite calcite fluid dihedral angles are presented ranging from 50 90 the early low angles may be due to rapid olivine growth into a fluid filled porosity the preservation of a range of textural states in the beinn an dubhaich aureole has the potential to provide information on the kinetics of grain boundary readjustments in metamorphic rocks which control pore geometry and permeability

quality of ptrm acquisition in pyrrhotite bearing contact metamorphic limestones possibility of a continuous record of earth magnetic field variations abstract the rockmagnetic and palaeomagnetic signal in pyrrhotite bearing limestones of different contact metamorphic settings were investigated related to intrusions ranging from small sills to large magmatic complexes magnetic susceptibility the pyrrhotite magnetite ratio and thermal modelling serve as an investigative tool to define three metamorphic zonations a contact zone of a mixed magnetic assemblage and low susceptibilities a pyrrhotite bearing transitional zone where full thermoremanent magnetisations trms are acquired due to temperatures above the curie temperature of pyrrhotite t c po and a marginal zone containing pyrrhotite and magnetite generated at temperatures below t c po the fact that trms can consist of independent ptrms is successfully tested by modified thellier experiments it is shown that a metamorphic environment with low fluid circulation provides a scenario for the recording of independent ptrms multicomponent behaviour of the nrm residing in samples from the transitional zone can therefore be interpreted as a continuous record of earth magnetic field variations

interplay between erosion and tectonics in the western alps bedrock fission track analysis high resolution petrography and heavy mineral analyses of sediments are used to investigate the relationships between erosion and tectonics in the western alps along the aosta valley cross section exhumation rates based on fission track data are higher in the fault bounded western block than in the eastern block 0 4 1 5 vs 0 1 0 3 mm yr 1 erosion rates based on the analysis of bed load in the dora baltea drainage display the same pattern and have similar magnitudes in the relative sub basins 0 4 0 7 vs 0 04 0 08 mm yr 1 results highlight that climate relief and lithology are not the controlling factors of erosion in the western alps the main driving force behind erosion is instead tectonics that causes the differential upward motion of crustal blocks

tectonic geomorphology of the triglav lakes valley easternmost southern alps nw slovenia abstract we present a study on the impact of litho structural setting and neotectonic activity on meso and macro scale relief production in alpine areas the topography of the high alpine triglav lakes valley nw slovenia was studied by means of detailed mapping and stratigraphic study of the valley the triglav lakes valley is characterised by a generally asymmetric transverse e w profile a very steep eastern slope a relatively flat valley and a relatively gentle western slope on the transverse profile the valley floor is essentially flat gently dipping towards the east in the longitudinal cross section however the valley floor is marked by sharply defined fault blocks extending in a w e to nw se direction additionally the highest block elevations 2100 m is in the northern part of the valley the lowest elevations 1600 m in the southern part of the valley our research shows that the triglav lakes valley directly represents the topographic expression of paleogene neogene thrusting and faulting having recorded the following geomorphologic evolutionary stages 1 an oligocene to early miocene w vergent thrusting phase with steep w facing slopes of the eastern part of the valley directly representing the thrusting front and 2 a neogene to present strike slip faulting in nne ssw direction with two bifurcating right lateral strike slip systems we show that the triglav lakes valley almost perfectly mimics the wedge shaped damage zone located between these faults

evidence of recent surface faulting and surface rupture in the fore alps of veneto and trentino ne italy abstract the fore alps of western veneto and trentino regions belong to the central southern alps ne italy in which there is little evidence of very fresh surface ruptures or surface faulting this does not seem to match historical data about earthquakes some of which have been very intense the strong influence of the inherited structures makes it difficult to detect a direct link between morphotectonic features and present day stress fields in the present study four areas orsara scandole naole and soran with surface faulting and surface rupture features were examined and models of morphotectonic evolution are discussed in the lessini mountains the orsara graben and scandole ridge show examples of surface faulting and surface rupture respectively reactivating paleogene normal faults and fractures within the orsara graben rocky bluffs displace the previous morphological features the bluffs are some decimetres to some metres high and are practically devoid of evidence of either physical or chemical weathering on the slopes above them are steep areas which may be interpreted as the remnants of previous strongly weathered bluffs the scandole ridge has many trenches some with rocky walls which may be the result of several episodes of morphotectonic rupture in the giudicarie belt the naole and mt soran surface faulting landforms are details within large frontal culmination walls of neogene thrusts the naole ridge corresponds to the southeastern sector of monte baldo here inside a fault angle valley a sinuous scarp originating from surface faulting marks the base of the fault scarp slope ridge splitting is the expression of the backward migration of separation niches due to slope tectonics also evidenced on the slope by several terrace like features and by a lower belt of very thick slope breccias on mt soran in the gruppo di brenta massif the surface faulting scarp faces uphill giving rise to a trench like feature downvalley of the scarp there is the niche of a large landslide dated to 3 kyears b p all these landforms are consistent with slope tectonic movements caused by intense earthquakes whereas the morphostructures in the lessini mountains are the result of responses by sensitive structures the naole and mt soran features express the evolution of frontal culmination walls of thrusts with clear evidence of present day tectonic activity on the basis of the weathering of the scarps and associated features the relative seismotectonic episodes probably occurred between the bronze and middle ages

litho structural control morphotectonics and deep seated gravitational deformations in the evolution of alpine relief a case study in the lower susa valley italian western alps abstract an integrated study of quaternary geology and geomorphology structural geology and morphotectonic lineament analysis has been performed for the lower susa valley in order to evaluate the relationships between neotectonics of the central internal side of the italian western alps and the morphodynamics of the mountain relief a regional nne ssw fault system the colle delle finestre fault zone cffz separates two sectors the middle and the lower susa valleys characterized by different fault patterns ene wsw regional faults represent the main brittle structures in the middle susa valley while e w faults prevail in the lower susa valley geomorphic anomalies plio quaternary deformed deposits and distribution of glacial deposits suggest differential uplift of fault bounded sectors distribution of deep seated gravitational slope deformations and landslides depends on lithological and structural controls suggesting a close relationship between post glacial morphodynamic evolution and quaternary fault activity this activity is also indicated by instrumental data and historical earthquakes ms ranging from 4 to 5 that struck the lower susa valley

equilibrium dihedral angles in the system h2o co2 nacl calcite and implications for fluid flow during metamorphism fluid calcite calcite dihedral angles have been measured for fluids in the system h2o co2 nacl between 1 and 2 kbar and 550 750 c it is found that the calcite calcite h2o dihedral angle decreases steadily with addition of nacl from a value of about 80 pure water to 44 60 wt nacl the co2 h2o system displays a well defined minimum at x co 2 0 5 with a dihedral angle of 50 in contrast to those of pure co2 and h2o which are 90 and 80 respectively experiments containing fluids which are immiscible at run conditions showed a bimodal distribution of dihedral angles in the co2 h2o nacl system which can be approximately correlated with the compositions of the two fluid phases such bimodality was only observed for immiscible fluids in the h2o nacl system if the quench rate exceeded about 200 c per min this is probably due to the extremely rapid establishment of the single phase dihedral angle on quenching the fluid phase topology in devolatilising marbles will only be a connected network for very saline brines and fluids with x co 2 close to 0 5 fluids trapped in fluid inclusions in calcite grains in marbles may be predominantly h2o rich or co2 rich and of low salinity all other fluid compositions in the h2o co2 nacl calcite system will occupy isolated pores the largest of which will grow at the expense of the smallest escape of fluid produced during devolatilisation reactions under such conditions will occur by fluid overpressuring and hydrofracture in contrast previous experimental studies of quartz fluid dihedral angles between 950 and 1100 c watson and brenan 1987 predict that quartz dominated lithologies will permit pervasive flow of h2o nacl fluids but not of h2o co2 fluids documented geological examples of differences in permeability and fluid flow mechanism between metamorphic argillites psammites and limestones which support the results of the experimental studies are discussed

the relief of the swiss alps and adjacent areas and its relation to lithology and structure topographic analysis from a 250 m dem abstract in this paper we discuss the large scale geomorphological characteristics of the swiss alps based on numerical analysis of a digital elevation model and compare these to an erodibility map constructed from a geotechnical map of switzerland and regional geomorphological studies comparing the erodibility map with the large scale morphometry shows an intimate relationship between mountain scale erodibility and topography on average higher mean elevations and steeper mean slopes correlate with regions where rocks of low erodibility prevail areas with high peaks as well as the main water divides are controlled by the presence of bedrock with low to very low detachability the drainage network of the swiss alps shows a close relationship to the lithological differences as well major longitudinal valleys follow easily erodible units in the eastern and western part of the swiss alps the highest values of local relief are located to the south of the main water divide whereas in the central part local relief is higher to the north of the main water divide the large scale geomorphic characteristics regarded in the framework of the geological history of uplift and denudation suggest that low and very low erodibilities lead to the development of areas of high elevations which are likely to persist over periods of 10 15 ma as the analysis of the lepontine area shows 20 ma after cessation of exhumation such high elevations are likely to be worn down and to manifest themselves as high relief only

scismotectonics of the northwest external dinarides abstract the region of the tectonic contact between the alps and the dinarides is studied comparing all the available geological geophysical and seismological data the peculiar crustal structures in particular the difference in thickness of the alps dinarides and pannonian basin can be correlated to the seismically active zones the highest seismicity is related to the adria microplate underthrusting the southern alps beneath the friulian piedmont arc this movement produces a composite lateral slip and an underthrust which is subparallel to the dinarides along the yugoslav coast the seismicity in western yugoslavia is more diffuse than in friuli although it is possible to pick out three narrow parallel bands with dextral shear subparallel to the periadriatic vardar fault system which separate the apulia from the pannonian area the interaction of this system with the sinistral strike slip faults of the western carpathians belt enhances the seismicity of the easternmost sector of the alps analogies and differences in the seismotectonic behaviour between central friuli and western yugoslavia are studied and a unique model with a vertical mechanical disengagement at the rear of the compressive fronts of the southern alps and external dinarides is proposed

neo alpine structural evolution and present day tectonic activity of the eastern southern alps the case of the ravne fault nw slovenia abstract the ravne fault is an actively propagating nw se trending dextral strike slip fault in the julian alps of nw slovenia which has been responsible for two moderate sized earthquakes in the last decade strike slip displacements on moderate steep fault planes are responsible for the recent seismic activity that is confined to shallow crustal levels the fault is growing by interaction of individual right stepping fault segments and breaching of local transtensional step over zones the fault geometry is controlled by the original geometry of the nw se trending thrust zone modified by successive faulting within the fault zone in the modern n directed maximum horizontal stress regime the segmented fault is lengthening by active growth at the fault s nw end the spatial distribution of earthquake clusters shows that activity on strike slip segments and thrust faults is contemporaneous detailed analysis of the spatial pattern of earthquake events and surface fault geometries suggests that for earthquakes of similar magnitudes and similar fault kinematics the deciding factor for whether an earthquake rupture will breach a step over zone is the relationship between the lengths of individual neighbouring fault segments and fault separation distances in the step over zones the ravne fault represents an example of a tectonic structure that lies in an area subjected to multiple tectonic events under different regional stress conditions at epicentral depths the fault system is accommodating recent strain along newly formed fault planes whereas in upper parts of the crust the activity is distributed over a wider deformation zone that includes reactivated brittle thrust faults

dehydration reaction and isotope front transport induced by co2infiltration at nuliyam south india the field relations from a quarry at nuliyam south india illustrate dehydration of an amphibolite facies gneiss to granulite facies charnockite by co2 influx over a scale of 30 m both the calc silicate source of the fluids and the full extent of their penetration into the gneiss are preserved in a continuous section fluid flow is by a hydraulic fracture mechanism but is thought to be pervasive the sharp reaction front predicted by the continuum mechanical theory for advective fluid transport is not observed the front spreading is on too large a scale for either diffusive or dispersive control and is due to local kinetic disequilibrium between the fluid and rock although the divariant nature of the reaction may also have a limited effect the time integrated fluid flux varies from the instantaneous porosity at the fluid front to 20 vol adjacent to the calc silicate carbon isotope budgets suggest that decarbonation of the calc silicate by a rayleigh fractionation process provides a sufficient source for the co2 influxing into the gneiss graphite abundances vary from 0 01 to 0 1 by weight it is principally derived by precipitation from the fluid and may be modelled from phase equilibria carbon isotope fronts coincide with the reaction front on the scale of sampling although isotopic disequilibrium between graphite and inclusion co2 also implies local fluid rock disequilibrium

present state of tectonic stress of the friuli area eastern southern alps stress orientations are determined for the friuli area from focal mechanisms using the inversion technique of gephart and forsyth 1984 the data consist of 244 fault plane solutions selected for the period 1988 1995 where the data coverage was poor this period was extended to 1978 1996 local magnitude ranges from 2 0 to 4 3 and the depths extend from 4 to 16 km the friuli area is subdivided into five seismotectonic zones on the basis of tectonic characteristics seismic activity and focal mechanisms and for each zone a stress tensor is determined the results show that the active tectonic stress in the friuli area is heterogeneous and conditioned by the complex structural pattern which is characterized by two indented tectonic wedges the western and central parts of the area are mainly affected by a compressional state of stress while in the eastern part a strike slip state of stress prevails the orientation of the principal stresses varies from the western to the eastern part of the area and differences are also noticeable with depth in the central part of the friuli area the maximum compressional stress is n s trending at shallow depths 10 km changing to nw se at deeper levels the maximum compressional stress is nw se oriented in the western part while in the eastern part it is n s oriented

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