high archean climatic temperature inferred from oxygen isotope geochemistry of cherts in the 3 5 ga swaziland supergroup south africa new and compiled oxygen isotope data combined with the results of geological and sedimentological studies demonstrate that enclaves of synsedimentary to very early diagenetic cherts are widely preserved in the 3 5 3 2 ga swaziland supergroup barberton greenstone belt south africa the low 1 8 o values of these cherts indicate extremely high ocean temperatures of 55 85 c previously the large depletion in 1 8 o shown by all barberton cherts relative to their phanerozoic counterparts has been attributed to low 1 8 o in archean oceans chert formation during late diagenesis wholesale loss of 1 8 o during alteration and or regional silicification of sediments around hot springs these alternative explanations are not compatible with the new results cherts in the onverwacht group display an isotopic stratigraphy that is inversely repeated in conglomerates in the overlying fig tree and moodies groups demonstrating that the chert 8 0 o values were fixed prior to archean uplift and erosion which started at 3 26 ga the maximum 1 8 o value in barberton cherts 22 is lower than the minimum values 23 in phanerozoic bedded cherts precluding late diagenesis as the explanation of the overall low 1 8 o values regional metamorphic hydrothermal or long term resetting of original 1 8 o values is also precluded by preservation of 1 8 o values across different metamorphic grades and by systematic 1 8 o differences among interbedded chert types stratigraphic units and conglomerate clasts the 7 1 8 o variation of these archean cherts is similar to that of phanerozoic deep sea cherts formed when opal converted to microquartz during burial but the actual archean values are 10 lower marine opal was apparently converted to microquartz during burial to depths of 1 km cherts with 1 8 o 15 reflect conversion during deepest burial or in local areas of enhanced geothermal gradient and or hydrothermal activity cherts with higher 1 8 o values formed during early diagenesis and indicate an extremely hot archean ocean and surface environment

kinematics of primary contacts between low and relatively high pressure rocks in orogens abstract primary contacts among low and relatively high pressure rocks in convergent orogens have been mainly interpreted as a result of extensional tectonics or reimbrication of high pressure thrust units already resting on low pressure ones and by consequent inversion of their stacking order our work in northern calabria allows us to ascertain the nature and kinematics of several of these contacts and to work out a general model that considers primary pressure break contacts as resulting from the growth of a deep passive roof duplex within a subduction accretion system estimates of pressure differences across these contacts may be obtained for a simple passive roof duplex geometry as a function of the aspect ratio of the structure and of the angle of subduction our results suggest therefore that there may be a further possibility in addition to extension or reimbrication which explains pressure break contacts in orogens

abyssal sediment waves in the gulf of mexico an enigma a sediment wave field in the western gulf of mexico is similar to many other fields that are located on continental rises and related to thermohaline contour currents especially western boundary undercurrents such currents should not occur in the gulf because it is silled at present the only postulated source of deep currents in the gulf is the loop current in the eastern half eddies detached from it and rossby waves but no thermohaline currents if any of the postulated processes cause the mud waves it is a new mode of formation if they do not much more needs to be learned about deep gulf circulation these questions are important because the sediment waves represent paleoceanographic conditions that have persisted for over 5 million years

alpine orogenic p t t deformation history of the catena costiera area and surrounding regions calabrian arc southern italy the nappe edifice of north calabria revised with insights on the tyrrhenian apennine system formation 1 the nappe structured belt of calabria constitutes the eastward termination of the southern branch of the alpine mediterranean belt that delimits the northern edge of the africa plate contrasting hypotheses for the origin and tectonic significance of the north calabrian nappe edifice have been proposed and kinematic data from north calabria have been used to support different interpretations of the alps apennines linkage and the polarity of the tethyan subduction in the apennine region we reconstruct the architecture of the north calabria nappe edifice through a multidisciplinary approach which integrates structural investigations with metamorphic thermobarometry and 40ar 39ar geochronology results from this study indicate that north calabria consists of a tertiary nappe stack resulting from superimposed top to the west extensional shearing late oligocene to middle miocene in age onto a previously structured top to the east compressional belt eocene to oligocene in age this study also documents that the top to the west extensional tectonics was achieved by means of regionally sized extensional detachment fault systems stretching apart and translating as allochthonous fragments the previously accreted units thinning operated by top to the west extensional detachment tectonics also resulted in the direct juxtaposition of non alpine or slightly alpine metamorphosed units upper plate complex onto the previously exhumed deep seated portions of the orogenic wedge metamorphosed under blueschist facies metamorphic conditions lower plate complex these findings support a new tectonic scenario for the orogenic history of north calabria which may be adequately framed within the tertiary apennine tyrrhenian system evolution

tectonic role of ophiolite bearing terranes in the development of the southern apennines orogenic belt new data on ophiolite bearing terranes of the liguride complex together with some information on the terranes of the sicilide complex result in a better understanding of the role and tectonic significance of these units in the construction of the southern apennines orogenic belt the liguride complex is composed of two main tectonic units overlain by a thick turbiditic sequence of late oligocene middle miocene age the uppermost one frido unit is a polydeformed and polymetamorphosed sequence composed of two tectonic subunits of shales and calc schists respectively containing blocks of ophiolite garnet gneiss amphibolites and granitoids this unit is thrust over the un metamorphosed terranes calabro lucano flysch unit consisting of a broken formation with blocks of late jurassic ophiolite and their sedimentary cover cretaceous eocene pelagic sediments and late oligocene volcaniclastic deposits the frido unit underwent hp lt metamorphism p 8 10 kb t 400 500 c resulting in glaucophane and lawsonite assemblages in the ophiolitic rocks and aragonite in the meta limestones and calc schists followed by greenschist fades metamorphism p 4 kb t 300 350 c from a structural point of view units of the liguride complex comprise structures developed at different structural levels indicating progressive non coaxial deformation in response to tectonic transport towards the n ne the ophiolite bearing terranes of the liguride complex can be considered as a remnant of an accretionary complex in which the calabro lucano flysch unit represents the toe of the wedge where frontal accretion processes occur and the frido unit is a deeper portion emplacement of the frido unit is explained as being due to formation of a deep duplex structure during the early stage of continental collision processes the polarity of tectonic transport provides new evidence that the liguride complex represents a suture zone between the apulian and the calabrian blocks the age of collision appears to be not older than late oligocene the allochtonous terranes of the liguride and sicilide complexes therefore represent a complete accretionary wedge which records first subduction of the neotethys ocean beneath the calabrian europe continental margin and later continental collision with the african block

sand wave fields beneath the loop current gulf of mexico reworking of fan sands extensive fields of large barchan like sand waves and longitudinal sand ribbons have been mapped by deep towed seamarc ia sidescan sonar on part of the middle and lower mississippi fan that lies in about 3200 m of water the area is beneath the strongly flowing loop current the bedforms have not been adequately sampled but probably consist of winnowed siliciclastic foraminiferal sands the size about 200 m from wingtip to wingtip and shape of the large barchans is consistent with a previously observed peak current speed of 30 cm s measured 25 m above the seabed the types of small scale bedforms and the scoured surfaces of chemical crusts seen on nearby bottom photographs indicate that near bed currents in excess of 30 cm s may sometimes occur at the time of the survey the sand transport direction was to the northwest in the opposite direction to the loop current but consistent with there being a deep boundary current along the foot of the florida escarpment some reworking of the underlying sandy turbidites and debris flow deposits is apparent on the sidescan sonar records reworking by deep sea currents resulting in erosion and in deposits characterised by coarsening upwards structures and cross bedding is a process that has been proposed for sand found in cores in shallower parts of the gulf of mexico this process is more widespread than hitherto supposed

structure kinematics and metamorphism in the liguride complex southern apennines italy abstract the liguride complex of south italy is the remains of a cretaceous paleogene accretionary wedge which was emplaced onto the african margin during early miocene time transport directions of thrust sheets and nappes in the internal zone are roughly towards the present day northeast transport directions match well with the corresponding slip vectors derived from relative motion of the calabria sardinia corsica block with respect to africa including adria during late eocene to oligocene time deformation of the internal zone included extensive underthrusting followed by underplating of already lithified sediments ophiolitic rocks and continental basement on to the base of an accretionary wedge down to depths in places exceeding 30 km overthickening by massive underplating caused late paleogene gravity driven extension within the accretionary wedge the contact between the lower ophiolitic nappe and the upper ophiolitic nappe is an extensional fault zone of this age transport directions during the gravity driven deformation do not match plate derived slip vectors it is inferred that the gravitational potential of the wedge dominated the kinematics of the late palaeogene extensional deformation

silica diagenesis of neogene diatomaceous and volcaniclastic sediments in northern japan marine diatomaceous siliceous sediments in neogene sections of northern japan contrast with the monterey shale of california in containing many intercalations of acidic volcaniclastic sediments diagenesis of these sediments from deep boreholes and surface sections was investigated three diagenetic zones biogenic opal opal ct and quartz zones are recognized in siliceous sediments corresponding roughly to amorphous silica low cristobalite and quartz zones in acidic vitric volcaniclastic sediments opal ct consists almost exclusively of silica and water while low cristobalite contains appreciable amounts of a1 ca na and k in subsurface sections values of d 101 spacing of opal ct decrease progressively with increasing burial depth the progressive ordering is not associated with additional silica cementation in surface sections the behaviour of d 101 spacing is complicated owing to the modification of the progressive ordering developed during burial diagenesis by later silica cementation during uplift the cementing opal ct is probably precipitated from percolating groundwater which dissolves siliceous skeletons in porous diatomaceous mudstones overlying the opal ct porcellanite opaline cherts that form during burial diagenesis are designated as early opaline chert while those which form during uplift are later opaline chert the later opaline chert contains two groups of opal ct one is progressively ordered opal ct and the other is additionally cemented opal ct with higher d 101 spacing than that in the host porcellanite in diatomaceous siliceous sediments early opaline chert is scarce most if not all opaline cherts in surface sections are of later origin

deformation partitioning during transpressional emplacement of a mantle extrusion wedge the ronda peridotites western betic cordillera spain abstract composite tectonic emplacement of the world9s largest outcropping body of subcontinental mantle rocks the ronda peridotites occurred during early miocene continental subduction and as a result of oblique plate convergence partitioning of transpressional deformation between coeval orogen parallel wrenching and orogen perpendicular pure thrusting components is recorded by shear zone kinematics and dynamothermal metamorphism in the footwall to the ultramafic rocks left lateral shear characterizing the deeper high pressure eclogitic portions of the continental subduction system propagated through the mantle into the overlying continental crust of the overriding plate whereas top to the foreland frontal thrusting dominated at the leading edge of the hot peridotite body top to the hinterland shear along the upper contact of the peridotites is consistent with the kinematics expected for an extrusion wedge consisting of subcontinental mantle rocks on the other hand coeval strike parallel extension and thinning of the crustal rocks overlying the peridotites confirms that similarly to other alpine mediterranean examples partitioned transpression resulted in the development of a complex deformation pattern with kinematically linked shear zones aiding exhumation the proposed model provides a new coherent scenario for syn convergence peridotite emplacement and exhumation in the western betic cordillera

neogene depositional processes on the texas louisiana continental slope revealed by high resolution seismic studies systematic interpretation of high resolution seismic profiles 3 5 10 khz echograms and 100 hz air gun from the continental slope and rise of offshore texas and louisiana from the shelf break across the highly deformed zone of salt diapirs folds and intraslope basins to water depths greater than 3 500 m seaward of the sigsbee escarpment reveals the geometries scales distributions and depositional processes of deep water deposits in this region the 3 5 10 khz data indicate that redistribution of sediment by mass transport processes a ubiquitous and a wide spectrum of slumps slide blocks and debris flows are observed from small isolated deposits in intraslope basins to large complex deposits covering hundreds of kilometers the reflection character echo character of many intraslope basin and canyon floors indicates sand rich deposits however aggradational leveed channel systems characteristic of deep sea fans are not observed in the steep walled intraslope basins in contrast some large submarine fan channel levee complexes do extend seaward from the mouths of these large canyons and basins at the base of the sigsbee escarpment to form one or more large deep sea fans on the continental rise apparently large quantities of terrigenous sediments can be transported by turbidity flows through the intricate network of rugged intraslope basins and canyonsmore to feed these fans evidence of sediment redistribution by bottom currents contour currents is also observed in the form of sediment waves and drift deposits across a large area of the continental rise seaward of the southeastern base of the sigsbee escarpment less

criteria for identifying structures related to true crustal extension in orogens faults and shear zones in orogens may be related to crustal shortening or extension or both true extensional structures should ultimately intersect the earth s surface when traced out in a direction opposite to that of hangingwall transport the present dip of a fault in an orogen and its relation to local layering are unreliable guides the observation of low pressure metamorphic rocks structurally above high pressure metamorphic rocks is not sufficient to diagnose crustal extension a valid metamorphic criterion is the difference in pressure time histories between footwall and hangingwall if the footwall was decompressing faster than the hangingwall during shear zone movement then the shear zone was truly extending the crust the pressure time history can be constrained via the temperature time and pressure temperature histories ideally crustal extension inferred from structural data should be confirmed using diagnostic ptt information

oxygen and hydrogen isotope evidence for a temperate climate 3 42 billion years ago the widely held view that the archaean climate around 3 5 billion years ago was remarkably warm with ocean temperatures perhaps as high as 80 c has been questioned on the grounds that the established method of estimating ancient ocean temperature from the oxygen isotope ratios of sedimentary deposits is subject to significant uncertainty michael hren et al have adopted a different approach to estimating ocean temperature based on an analysis of both oxygen and hydrogen isotopes of 3 4 billion year old buck reef chert sediments from south africa the isotopes sampled are consistent with formation in waters no warmer than about 40 c suggesting that earth s early oceans may have been far cooler than previously thought the study of stable oxygen isotope ratios 18o of precambrian cherts suggests that ocean temperatures during the archaean era about 3 5 billion years ago were between 55 c and 85 c but uncertainty about the 18o of the primitive ocean has led to considerable debate regarding this conclusion here a combined analysis of oxygen and hydrogen istopes sampled from 3 42 billion year old buck reef chert rocks in south africa indicates that the ancient ocean was much cooler than previously thought stable oxygen isotope ratios 18o of precambrian cherts have been used to establish much of our understanding of the early climate history of earth1 2 3 and suggest that ocean temperatures during the archaean era 3 5 billion years ago were between 55 c and 85 c ref 2 but because of uncertainty in the 18o of the primitive ocean there is considerable debate regarding this conclusion examination of modern and ancient cherts indicates that another approach using a combined analysis of 18o and hydrogen isotopes d rather than 18o alone can provide a firmer constraint on formational temperatures without independent knowledge of the isotopic composition of ambient waters4 5 here we show that 18o and d sampled from 3 42 billion year old buck reef chert rocks in south africa are consistent with formation from waters at varied low temperatures the most 18o enriched buck reef chert rocks record the lowest diagenetic temperatures and were formed in equilibrium with waters below 40 c geochemical and sedimentary evidence suggests that the buck reef chert was formed in shallow to deep marine conditions so our results indicate that the palaeoarchaean ocean was isotopically depleted relative to the modern ocean and far cooler 40 c than previously thought2

early diagenesis of biogenic silica in the amazon delta alteration authigenic clay formation and storage deltaic environments are commonly assumed to be relatively minor sites of biogenic silica burial because of the small quantities of opaline silica detected by most operational analytical techniques rapid conversion of biogenic silica into authigenic silicates is also often discounted as a significant control on oceanic silica budgets a variety of evidence for extensive early diagenetic alteration of biogenic silica in rapidly accumulating amazon delta sediments indicates that both of these general assumptions are unjustified apparent lack of significant biogenic silica storage in deltaic environments particularly in the tropics may be largely an artifact of operational definitions that do not include early diagenetic products of biogenic silica biogenic silica particles buried in suboxic amazon delta deposits can be unaltered partially dissolved covered with aluminosilicate or metal rich coatings or completely reconstituted into authigenic k fe rich aluminosilicate minerals pore water k mg f si and solid phase distributions direct observations of particles laboratory experiments and depositional context indicate that authigenic clays form rapidly 1 yr in the seasonally reworked surface layer 0 5 2 m of the delta topset and are disseminated during sediment remobilization fe al oxide rich debris derived from the tropical drainage basin is an abundant reactant and thus the supply of biogenic silica is a major control on the amount of clay formed the mild 1 na2co3 alkaline leach procedure commonly used to estimate biogenic silica was modified to include an initial mild leach step with 0 1n hcl to remove metal oxide coatings and to activate poorly crystalline authigenic phases for alkaline dissolution well crystallized clays are not significantly affected by this modification nor is bulk amazon river bed sediment the two step procedure indicates that 90 of the biogenic silica originally present in deposits is converted to clay or otherwise altered raising the effective quantity of biogenic silica stored from 33 to 296 mol si g 1 1 8 sio2 biogenic si stored in the delta increases away from the river mouth across shelf and along the dispersal system where primary production is highest the k si ratio of labile authigenic material is 0 19 mol mol 1 far higher than amazon river suspended matter 0 07 mol mol 1 diagenetic models indicate formation rates in the mobile sediment layer of 2 8 mol k g 1 yr 1 16 mol si g 1 yr 1 inclusion of authigenic alteration products of biogenic silica in estimates of reactive si burial increases the deltaic storage of riverine si to 22 of the amazon river input the rapid formation of aluminosilicates from biogenic sio2 seawater solutes and remobilized fe al oxides represents a form of reverse weathering rapid reverse weathering reactions in tropical muds and deltaic deposits the largest sediment depocenters on earth confirms the general importance of these processes in oceanic elemental cycles

the cretan ophiolite bearing melange greece a remnant of alpine accretionary wedge abstract new data collected in central crete on the ophiolite bearing units result in a better understanding of the role and tectonic significance of these units in the construction of this segment of the hellenides these nappes are composed of three main tectonic units that characterized by different metamorphic facies conditions are represented by an un metamorphosed lower unit a greenschist to hp greenschist facies intermediate unit and a blueschist facies upper unit these chaotic thrust nappes include blocks of oceanic and continental deriving rocks and can be considered as a remnant of an accretionary complex the lower unit represents the toe of the wedge whereas the intermediate and upper units refer to the innermost and deeper subducted portions exhumed and superimposed on top of each other during the early stages of continental collision the structural evolution of the accretionary wedge was controlled by four main contractional deformation events that including distinct groups of structures developed at different crustal levels were driven by sse and ssw directions of tectonic transport our data strongly suggest that the ophiolite bearing tectonic wedge was accreted during the paleogene subduction of a late jurassic cretaceous oceanic realm beneath the continental margin of the pelagonian domain and it was successively involved in the continental collision with the adria block the greenschist to blueschist facies metamorphism and the subsequent exhumation and emplacement of the intermediate and upper units above the frontal portion of the wedge may mainly be due to deep duplexing marking the onset of continental collision we thus suggest that the ophiolite bearing units of crete represent a single suture zone related to the closure of a unique oceanic domain pindos cycladic ocean subducted beneath the internal hellenides platform continental domains thus assuming the significance of a southern oceanic seaway of the largest eastern neotethys developed since the triassic between eurasia and africa

rapid post mortem maturation of diatom silica oxygen isotope values 1 oxygen isotope values of diatom silica 18odiatom are increasingly used as paleoclimate proxies however the magnitude and timing of post mortem alteration of 18odiatom values has been unclear in freshwater diatom silica from a human made pond in northern new mexico post mortem alteration of 18odiatom values occurs within one year of sediment burial diatom silica collected antemortem has an average 18odiatom value of 21 5 vsmow 1 3 n 23 whereas diatom silica from two sediment cores from the same freshwater environment records significantly higher 18odiatom values 28 9 0 8 n 13 the difference in ante and post mortem 18odiatom values indicate post mortem alteration of diatom silica oxygen that results in a 7 increase in 18o values this study demonstrates that 18odiatom values reach mature values within 0 5 years of frustule death initial diatom 18o values that record growing conditions are rapidly overwritten during silica maturation and the mature diatom 18o values approach isotopic equilibrium for quartz water the rapid post mortem alteration of diatom 18o values explains much of the disparate data regarding silica water fractionation for diatom silica and has a profound effect on the use of diatom silica 18o values as a paleoclimate proxy in lacustrine and marine environments

holocene glacial discharge fluctuations and recent instability in east antarctica article history antarctica holds the largest ice sheet in the world the east antarctic ice sheet eais and plays a significant role in both local and global climate through the interactions between ice sheets ocean sea ice and atmosphere our understanding of east antarctica holocene climate variability relies mainly on ice cores that however do not document glacial discharge history here we present the first high resolution 18 odiatom record derived from two marine sediment cores retrieved on the east antarctic continental shelf to reconstruct glacial discharge off adelie land and george v land al gvl over the last 11 000 years from decadal to centennial resolution our results suggest multi centennial glacier advances and retreats until 2000 cal yr bp followed by a period of relative instability marked by two major glacial retreats centered at 1700 cal yr bp and 1980 ce we suggest that the multi centennial oscillations during the early mid holocene reflect glacier fluctuations in response to long term local seasonal insolation and short term solar variability we also propose that 18

sense of overthrust shear in the alpine nappes of calabria southern italy abstract southern italy consists tectonically of ophiolite and basement nappes thrust over the apenninic sedimentary nappes whilst all more recent authors agree that the sediments of the apenninic nappes were deposited on apulian basement i e on african continental crust and that the ophiolites were associated with the oceanic basement of the mesozoic tethys the provenance of the basement nappes is still debated new data based on microstructural criteria have shown that the main shear sense of the ophiolite nappes and of the overlying basement nappes in northern calabria is from west to east in today s co ordinate system the basement nappes might not therefore be of austroalpine african provenance but could be of european origin

from diatom opal a δ18o to chert δ18o in deep sea sediments abstract 18o signature of marine deep sea cherts was previously used to reconstruct past ocean temperature and oceanic bottom water 18o through the cenozoic and mesozoic periods oxygen isotopes in deep sea cherts which were never exposed to meteoric water exhibit a wide range of values indicating that the evolution and maturation of biogenic amorphous opal opal a to opal ct and microquartz chert is accompanied by isotopic changes we measured 18o of diatom opal a opal ct and microquartz chert in downcore profiles of sediment cores odp 795 and 799 from the sea of japan diatom opal a opal ct and microquartz chert were separated and purified from the bulk sediment this was followed by xrd sem eds and xps to identify and quantify the mineral phases the qualitative elemental concentrations and the amounts of authigenic clays diatom opal a transforms to opal ct and microquartz chert when it is deposited in sediment originally rich in diatoms relative to other lithologic constituents such as clay diatom opal a 18o is in the range of 34 38 18o of opal ct is 25 at the first phase transformation at 330 mbsf in odp 795 and 420 mbsf in odp 799 this phase transformation occurs at a temperature of about 40 c 18o of opal ct remains essentially unchanged until it undergoes the second phase transformation to microquartz chert at 470 mbsf in odp 795 and at 580 mbsf in odp 799 at which point its 18o decreases abruptly to 20 this phase transformation occurs at a temperature of about 60 c the 18o values of opal ct and microquartz chert appear to reflect equilibrium formation temperatures of silica corresponding to the geothermal gradient and the local porewater 18o the 18o of opal ct and microquartz chert are controlled by the geothermal temperatures and compositions of pore waters during polymorphic transformations deep within the sediment indicating that the 18o of these phases cannot be used to determine temperature or composition of seawater during diatom growth

structural signature of tectonic processes in the calabrian arc southern italy evidence from the oceanic derived diamante terranova unit the diamante terranova unit is an oceanic derived element within the thrust and nappe structure of the northern sector of the calabrian arc its metamorphic and structural signature reveals a long history of se dipping subduction and collision related deformation of an oceanic lithospheric section underlying the narrow diamante terranova basin the deformation history of the diamante terranova unit may be related to three main tectonic phases the first one is characterized by a structural association which includes features that developed during a progressive deformation related to the evolution of a deep seated shear zone kinematic indicators characterizing subduction related structures clearly show a top to the northwest sense of overthrust shear the second tectonic phase that can be recognized in the area is characterized by structures that may be related to the onset of shear deformation induced by a postcollisional northeastward motion of the overriding plate the third tectonic phase is recorded by brittle features related to the latest tectonic events affecting the calabrian arc during the opening of the tyrrhenian sea shear sense indicators and available radiometric and stratigraphic information together with plate kinematic data allowed us to infer the time evolution of this sector of the calabrian arc and to assess that the diamante terranova unit cannot be considered as an eoalpine element since its deformation related to subduction collision processes occurred prior to the end of the cretaceous

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

passive roof duplex geometry in the frontal structures of the kirthar and sulaiman mountain belts pakistan abstract exploration for hydrocarbons over the past few years has greatly improved our understanding of the geometry of frontal mountain belt structures in this study we introduce and discuss the concept of the passive roof duplex using as the main example the kirthar and sulaiman ranges in the baluchistan province of pakistan structures similar to those described here have been recognized previously in other mountain belts and they appear to exist as a common feature in many more frontal regions of mountain belts our example of a passive roof duplex which we describe from pakistan is compared briefly with similar structures reported by others the passive roof duplex is here defined as a duplex whose roof thrust has backthrust sense passive roof thrust and whose roof sequence those rocks lying above the roof thrust remains relatively stationary during foreland directed piggy back style propagation of horses within the duplex