tectonic evolution of a part of the tethyside orogenic collage the kargi massif northern turkey the central part of the rhodope pontide fragment one of the major tectonic units in turkey provides critical data for evaluating the cimmeride and alpide evolution of the mediterranean tethysides tectonic events that affected the central part of the rhodope pontide fragment since the end of the paleozoic generated east west trending belts with the event of every episode redeforming and partly obliterating the structures of previous episodes this evolution may be conveniently described in terms of three major episodes 1 two different realms of pre dogger oceanic rocks are present in the area the northern realm coincided with main branch of paleo tethys that was being actively destroyed by south dipping subduction the southern realm the karakaya ocean a back arc basin related to this subduction began opening by rifting of a retroarc carbonate platform during the permo triassic to the west a continental domain with sparse magmatism seperated the two oceanic areas toward the east the two oceans become united by the wedging out of the continental domain these two pre dogger oceans closed during the lias and their remnants were emplaced between the southern margin of laurasia and the fragments of the cimmerian continent 2 the second episode partly overlapped the first with rifting south of the cimmerian continent fragment during the lias this rifting was followed by a transgression which covered the ruins of the cimmeride orogenic belt by the malm this rifting concurrently led to the development of the northern branch of the neo tethys and a south facing atlantic type continental margin a southerly thickening sedimentary prism developed on this margin during the lias to early cretaceous interval 3 the floor of the northern branch of neo tethys began to be consumed along the north dipping subduction zone beneath the previosly constructed continental margin this convergent margin generated a magmatic arc to the north and to the south a subduction accretion complex present width of which approaches 80 km to the south of the subduction accretion complex yet another convergent margin is indicated by the presence of the remnants of an ensimatic arc the northern branch of the neo tethys was totally consumed in the late cretaceous to eocene interval

new results and a palaeomagnetic overview of the pontides in northern turkey summary palaeomagnetic results are reported from 16 sites within volcanic and volcano sedimentary rocks of cretaceous and eocene age from the central pontides in northern turkey the sampled area is bounded by the north anatolian fault naf to the south and the black sea to the north the palaeomagnetic data collected demonstrate the presence of both non rotated and anticlockwise rotated regions i declination directions including lower upper cretaceous and eocene sites do not indicate any significant rotations north of where the naf exhibits an arcuate shape ii 20 anticlockwise rotation was observed in the sinop peninsula the northernmost extremity of the central pontides iii in the west of the central pontides two sites with normal and reversed antipodal directions indicate a 20 anticlockwise rotation and iv another region lies to the east of the study area and shows a rotation in an anticlockwise sense of about 5 30 the upper cretaceous and eocene directions reported in this paper and other palaeomagnetic studies show that there is not much difference between the inclination values of the upper cretaceous i 40 1 95 7 2 and the eocene sites i 39 6 95 8 1 furthermore the values of the inclination available from both sedimentary and volcanic upper jurassic lower cretaceous formations are shallower than the upper cretaceous ones this suggests that the entire pontides have undergone a northward drift of over 2000 km since the upper jurassic

propagating rifts and continental breakup a simple quantitative model is proposed whereby continental breakup starts with a phase of continental rifting and continues with the propagation of rifts toward locked zones these zones are deformed in the rifting process while oceanic crust is generated in the wake of the rift tip the deformed zones will cause overlaps when a continental reassembly is attempted the model accounts for the shape and distribution of magnetic quiet zones at ocean continent boundaries and for the detailed geometry of some v shaped zones of oceanic magnetic anomalies the former provide an estimate of the scale at which the hypothesis of rigid plates fails and the latter are used to measure the velocity of rift propagation illustrations are provided for present rifting in the red sea and gulf of aden and for early rifting of the south atlantic and breakup of pangea

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

southward migration of arc magmatism during latest cretaceous associated with slab steepening east pontides n turkey new paleomagnetic data from the amasya region abstract a new paleomagnetic study was carried out on the findikli formation in the amasya region which is to the north of the ankara erzincan neotethyan suture zone in order to better understand the paleogeographic evolution of the pontides tectonic belt during the maastrichtian samples were collected from volcanoclastic sandstones and lavas at 51 sites rock magnetic experiments identified pseudo single domain psd titanomagnetite as the main magnetic carriers at the majority of sites progressive thermal and alternating demagnetization revealed that the characteristic remanent component is removed between 400 and 580 c or 20 100 mt respectively a high temperature component which indicates the presence of titano hematite is found only in some lavas declinations show a systematic dispersion across the study area related to tectonic rotations about vertical axes inclinations are consistent however and yield a mean paleolatitude of 20 0 n 2 5 for the maastrichtian comparison of the results from this study with the reference direction of the east pontides from the turonian campanian is best explained with a model involving slab roll back and marginal basin formation between 90 and 65 ma period

on isostasy at atlantic type continental margins cross spectral techniques have been used to analyze the relationship between free air gravity anomalies and topography of approximately 100 profiles over the continental margins of eastern north america southwest africa and the coral sea lord howe rise the resulting admittance functions for these margins have been interpreted in terms of the flexure model of isostasy the relatively young coral sea lord howe rise margin is associated with the lowest value of the effective elastic thickness te 5 km while the relatively old eastern north american margin is associated with the highest value 10 te 20 km thus te appears to increase with the age of the margin these results which are consistent with flexure studies from the major ocean basins have been interpreted in terms of a simple model in which the flexural strength of the lithosphere increases with age the model suggests that a low rigidity plate or airy type model is most applicable early in rifting history while a high rigidity plate is more applicable later in margin evolution thus sediments at a margin initially load a relatively thin and hot plate while sediments later in margin evolution load a relatively thick and cold plate the model explains a number of features of the tectonic stratigraphic evolution of margins these include a change in flexural style from a narrow and deep basin to a broad and shallow basin coastal plain onlap and uplift in the continental interior furthermore the flexure model of isostasy contributes in a major way to observed free air gravity anomalies at continental margins in particular the model explains the existance of airy isostatic gravity anomalies at margins the amplitude and wavelength of these anomalies are a function of the sediment supply the type of mechanisms controlling sediment deposition up building and or out building and the thermal structure history and age of the margin

evidence of late cretaceous oroclinal bending in north central anatolia palaeomagnetic results from mesozoic and cenozoic rocks along the izmir ankara erzincan suture zone

environments favoring dolomite formation at cold seeps a case study from the gulf of mexico abstract dolomite rarely occurs in modern marine environments while it is ubiquitous in the geological record this apparent contradiction made the formation mechanism of dolomite a long standing problem haunting sedimentologists however dolomite is a constituent of many modern seep carbonates providing an opportunity to investigate the conditions favoring dolomite formation at low temperatures in the shallow subseafloor this study reports the petrology mineralogy carbon and oxygen stable isotopic compositions as well as sulfur isotopic compositions of both carbonate associated sulfate cas and chromium reducible sulfur crs of a set of dolomite bearing carbonates from two seep sites gc140 and gb382 on the northern slope of the gulf of mexico the 13c values of carbonates vary from 50 0 to 3 2 v pdb confirming that carbonate minerals mostly resulted from methane oxidation the corresponding 18o values range from 2 6 to 5 9 v pdb reflecting formation close to equilibrium with seawater composition all carbonate samples with dolomite show similar petrologies and lack skeletal remains of seep dwelling metazoans the homogenous texture of microcrystalline dolomite agrees with its formation in the shallow subseafloor dolomite bearing samples reveal 18ocas 34scas slopes of 0 425 and 0 435 for the two study sites respectively such high slopes are significantly different from the smaller slopes of seep carbonates consisting of calcite from gulf of mexico seeps suggesting low overall sulfate reduction rates during dolomite formation this explanation is also supported by relatively high 34scrs values ranging from 29 4 to 19 0 v cdt and the positive correlation of these values with dolomite contents interestingly an approximately stoichiometric dolomite sample yielded the most positive 34scrs value indicating a formation environment typified by low replenishment of seawater sulfate at a supposedly relatively deep sulfate methane transition zone smtz overall these results confirm that dolomite formation is facilitated where the smtz is situated at relatively great depth where sulfate reduction rates are slowed down due to the buildup of high contents of dissolved sulfide and low seawater sulfate replenishment

the tectonic history of the nigde kirsehir massif and the taurides since the late mesozoic paleomagnetic evidence for two phase orogenic curvature in central anatolia the nigde kirsehir massif known also as the central anatolian block is bordered by the sutures of the neotethys ocean the massif suffered several deformation phases during and after the consumption of the surrounding oceans and the post collisional events of the continental pieces of anatolia in latest cretaceous to miocene previous paleomagnetic studies on the nigde kirsehir massif and its surroundings displayed either insufficient data or have claimed large rotations and or remagnetization in order to understand the tectonic history of the nigde kirsehir massif and its adjacent blocks we have sampled 147 different sites in the age range of upper jurassic to miocene from the nigde kirsehir massif throughout its w sw and e se boundaries and the central southeastern taurides the results display that except the limestones in central taurides all rocks examined carry a primary magnetization among these an important finding is that rotations between the massif and the central eastern taurides indicate an oroclinal bending with counterclockwise rotation of r 41 1 7 6 in the se and clockwise rotation of r 45 9 9 3 in the central taurides from upper cretaceous rocks with respect to the african reference direction paleomagnetic rotations in the se taurides are compatible with the vergent direction of the thrusts generated from consumption of the intra tauride ocean prior to post collisional convergence between taurides and the massif in the central taurides it has been shown that the clockwise rotation of 45 9 9 3 started in middle eocene because of a remagnetization in upper creataceous limestones the deformation was linked to the final closure of the southern neotethys and the collision between the african and eurasian plates in the nigde kirsehir massif counterclockwise rotation up to 25 5 7 3 is recognised during middle eocene and interpreted in terms of block rotation together with the taurides after the miocene a counterclockwise rotation of 16 8 3 9 along the eastern taurides shows that this area was mostly affected by the westward movement of anatolia despite the nigde kirsehir massif and its sw w area the central taurides which is recognised as stable with counterclockwise rotation less than 10

geological evolution of the late mesozoic continental margin of northwestern anatolia abstract the armutlu peninsula is a composite tectonic entity made up of sections of the sakarya continent the rhodope pontide fragment and an ophiolite these were assembled following a continental collision between gondwanaland and laurasia during the late cretaceous the northern margin of the sakarya continent underwent progressively increasing deformation prior to and during the advancing collision due to continued convergence between the two continents initially the leading edge of the continent subsided under the load of an approaching ophiolitic slab following this a north directed thrusting and folding occurred during the turonian progressive elimination and eventual closure of the ocean preceded the thrusting of northerly situated collisiondashinduced nappe packages over the leading edge of the sakarya continent the nappe laden edge of the continental margin then collapsed and steadily subsided under the heavy load of the ophiolitic slab and the northern continental fragment consequently the nappe packages and the ophiolite were collectively metamorphosed during the coniaciandashsantonian interval during the subsidence the main body of the sakarya continent partially detached from its collapsed edge along a fault zone and thus suffered an independent but less severe deformation which lasted until the uplift of the collapsed edge in the campanian from the late campanian onward throughout later orogenic stages the metamorphic and nondashmetamorphic units amalgamated into a single tectonic entity forming a basement for younger cover rocks

cenozoic evolution of the eastern black sea a test of depth dependent stretching models abstract subsidence analysis of the eastern black sea basin suggests that the stratigraphy of this deep extensional basin can be explained by a predominantly pure shear stretching history a strain rate inversion method that assumes pure shear extension obtains good fits between observed and predicted stratigraphy a relatively pure shear strain distribution is also obtained when a strain rate inversion algorithm is applied that allows extension to vary with depth without assuming its existence or form the timing of opening of the eastern black sea which occupied a back arc position during the closure of the tethys ocean has also been a subject of intense debate competing theories called for basin opening during the jurassic cretaceous or paleocene eocene our work suggests that extension likely continued into the early cenozoic in agreement with stratigraphic relationships onshore and with estimates for the timing of arc magmatism further basin deepening also appears to have occurred in the last 20 myr this anomalous subsidence event is focused in the northern part of the basin and reaches its peak at 15 10 ma we suggest that this comparatively localized shortening is associated with the northward movement of the arabian plate we also explore the effects of paleowater depth and elastic thickness on the results these parameters are controversial particularly for deep water basins and margins but their estimation is a necessary step in any analysis of the tectonic subsidence record stored in stratigraphy

continental tectonics selected topics cause and effect relations between plate boundary processes and orogeny were investigated using new and refined plate models for the mesozoic and cenozoic terrane accretion and large components of horizontal shear were shown to be fundamental aspects of orogen evolution deep crustal thrust systems with large lateral transport and vertical stacking of slices of crystalline basement were documented in the cordillera and appalachians new geomechanical models of crystalline thrust sheets and foreland thrust belts emphasized the dominance of tectonic rather than gravitational stresses lithospheric thickening was shown to cause buoyancy forces that control the lateral spread of deformation outward from the interior of an orogen and may modify regional stresses in the vicinity of topographic fronts resulting in extension within the upper part of the lithosphere coeval with shortening at depth

crustal detachment during south atlantic rifting and formation of tucano gabon basin system a series of major sedimentary basins of mesozoic to tertiary age are associated with the brazilian continental margin between 8 and 14 s fig 1 this passive continental margin formed when africa and south america split apart with the initiation of seafloor spreading in the aptian albian3 the offshore basins which border the continental margins on both sides of the atlantic show evidence of both rifting and crustal attenuation before break up and thermal subsidence after it in contrast a combined stratigraphical and gravity study of the brazilian onshore basins which are separated from the offshore basins by a northward widening strip of precambrian suggests that up to 7 km of non marine sediments were deposited during the rifting stage of the south atlantic with no apparent extension of the lower crust beneath these basins and no subsequent thermal subsidence a mechanism for the linked development of these onshore and offshore basins is proposed here along the lines suggested by wernicke4 5 in which upper crustal extension in the onshore region is connected to and balanced against deeper lithospheric extension beneath the incipient passive margin by intracrustal detachments

lead isotope evidence for the structure of the rockall dipping reflector passive margin the passive continental margin south west of rockall plateau is characterized by a thick sequence of oceanward dipping seismic reflectors during leg 81 of the deep sea drilling project these reflectors were sampled at site 553 and proved to consist almost exclusively of basalt here we present lead isotope data which indicate that these basalts may have been contaminated by ancient uranium depleted continental crust or alternatively derived from a sub continental lithospheric mantle source in either case the implications are that the basalts of the south west rockall plateau formed by eruption through and onto continental basement not by subaerial seafloor spreading this conclusion is in accord with gravity models of the area which predict stretched continental crust beneath the dipping reflector sequence

questioning carbonate diagenetic paradigms evidence from the neogene of the bahamas carbonate diagenetic models have been heavily influenced by numerous studies of exposed quaternary limestones as a result meteoric diagenesis is often assumed to be the principle means of altering aragonite rich sediments into calcitic limestones however these models are limited by the scarcity of examples of aragonite rich sediments buried in seawater that have never been influenced by meteoric fluids the bahamas transect cores recovered originally aragonite rich sediments deposited in deep water beyond the easy reach of meteoric waters and provide an opportunity to test current diagenetic paradigms the bahamas transect consists of seven cores drilled in the prograding western margin of great bahama bank the two proximal cores clino and unda were drilled on the platform top and recovered shallow water platform to reef facies overlying deeper margin and proximal slope facies the five distal cores were drilled by odp leg 166 in up to 660 m of water and recovered carbonate slope facies all studied sections are neogene to pleistocene in age diagenetic environments were identified based on petrographic and scanning electron microscopy sem observations xrd mineralogy carbon and oxygen stable isotopic data and trace elements the upper 100 150 m of the two proximal cores were altered in meteoric to mixing zone diagenetic environments but all other intervals were altered exclusively in marine pore fluids during seafloor marineburial and deep burial diagenesis several of the findings of this study question current carbonate diagenetic paradigms these include 1 large scale sea level lowstands may not have chemically active meteoric lenses as we found no meteoric alteration at the 3120 m elevation of the latest pleistocene lowstand rather phreatic meteoric diagenesis appears restricted to within w10 m of the land surface 2 mixing zone diagenesis includes aragonite dissolution and minor lmc cementation but does not show the cavernous porosity or dolomitization predicted by mixing zone diagenetic models current models are based on coastal mixing zones which do not appear to be applicable to these more inland and perhaps more typical locations 3 marine burial diagenesis produces a mature limestone with fabrics formerly considered diagnostic for meteoric diagenesis such as moldic porosity aragonite neomorphism blocky calcite spar and calcite microspar however oxygen stable isotopic data average n 18 o 1x indicate alteration in marine pore fluids only the character of marine burial diagenesis is partially controlled by the nature of the sediment being altered we have identified two end member styles an open system style characterized by dissolution of aragonite without significant cementation and a more closed system style with aragonite dissolution

geomorphology of carbonate platform marginal uppermost slopes insights from a holocene analogue little bahama bank bahamas as the product of a variety of sediment sources and sedimentation and re sedimentation and erosion processes the geomorphology and sedimentology of carbonate slopes are highly variable the purpose of this study is to describe sub bottom profiles and side scan sonar multibeam and optical data acquired by an autonomous underwater vehicle to explore variability in geomorphological and sedimentological character of the present day platform marginal uppermost slope environments 240 m water depth on the north open ocean facing flank of little bahama bank bahamas although at time scales of greater than 100 ka this margin is progradational the data illustrate a complex juxtaposition of erosional and depositional processes and features erosion is evidenced by two prominent escarpments 70 m and 120 m that expose eroded bedded rocky outcrops these escarpments are interpreted to represent relict features related to past sea level positions although they still may be shedding debris aside from erosional remnants sedimentation and active transport is indicated by several features including active bedforms especially above the 70 m escarpment but ripples occur to depths of ca 200 m several mass transport complexes that overlie and cover the lower escarpment gravity flow deposits and rare slump features similarly a thick up to 20 m onlapping sediment wedge interpreted to be holocene in age suggests lateral accretion of the slope by more than 75 m in this period data illustrate that this open ocean margin is distinct from windward margins in the bahamas which typically include near vertical walls of erosion or bypass flanked downdip by rubble and talus and leeward margins which have onlapping muddy wedges but that lack marked terraces or escarpments collectively the results provide perspectives into the nature and controls on complex geomorphological patterns of erosion and deposition in holocene uppermost slope systems concepts potentially applicable to ancient analogues

a type granitoids from the eastern pontides ne turkey records for generation of hybrid a type rocks in a subduction related environment the eastern pontides is characterized by the many of intrusive bodies formed throughout the late mesozoic early cenozoic most of these are i type granitoids but here we present for the first time an a type pluton from the region to assess source characteristics and geodynamic implications the a type pirnalli pluton has a shrimp zircon u pb age of 81 2 1 ma it is composed of granite syenite and quartz monzonite and its enclaves are monzonitic in composition with elevated ga al ratios and low mg 42 the pluton is dominantly metaluminous a cnk 0 88 to 1 00 and belong to shoshonitic and ultra potassic series the samples are highly enriched in lree and show significant negative eu eu eu 0 33 to 0 92 ba nb sr and ti anomalies in the spidergrams the host rocks and their enclaves posses quite similar sr nd isotopic compositions isr 0 70693 to 0 70736 end 81 ma 2 6 to 2 0 tdm 0 94 to 1 12 ga pointing to a lower crustal parental magma with a minor contribution of lithospheric mantle we hypothesize that upwelling of asthenosphere triggered melting of chemically enriched upper mantle and basic magma formed melting of lower crust was provided by the underplating of this basic magma mixing between lower crust and mantle derived melts at depths of lower crust appears to be most reasonable petrogenetic process responsible for generation of the pluton sr nd isotope modeling suggests mixing of 82 90 of the lower crustal derived melt with 10 18 of the mantle derived melt then the hybrid melt ascended to shallower crustal level and underwent a limited fractionation process to generate a variety of rock types our data also suggest that the a type pirnalli pluton likely formed at an extensional environment of active continental margin throughout the late cretaceous ongoing extension then led to opening of black sea as a back arc basin further north of the eastern pontides

diagenetic history and porosity evolution of an early miocene carbonate buildup upper burman limestone yadana gas field offshore myanmar abstract the miocene yadana carbonate platform is a major offshore gas reservoir located in the andaman sea offshore myanmar the integration of petrographic analysis of carbonate rocks stable isotope measurements and seismic interpretation provided new insights into the diagenetic history of the upper burman limestone reservoir the high 28 average porosity of the reservoir is dominantly related to strong development of microporosity moldic and vuggy porosity mud supported coral rich sediments record significant early marine dissolution of aragonite and early lithification of matrix that has led to the development of dense layers stable isotope and petrographic data from the top of the reservoir suggest that the long term depositional hiatus middle to late miocene was related to platform drowning and non deposition in a deep marine setting moderate dolomitization has affected the top and flanks of the platform during the depositional hiatus microporosity development and sparry calcite cementation mainly occurred in marine to marine shallow burial environments a major decrease in porosity averaging 10 below the gas water contact suggests a major phase of porosity evolution during and or after the hydrocarbon emplacement which is mainly related to 1 increased moldic and vuggy porosity development within the gas zone 2 microporosity preservation in loosely packed subrounded micrites within the gas zone and 3 microporosity reduction by calcite precipitation within the water zone leading to the development of densely packed euhedral micrites

the four plutonic belts of the transhimalaya himalaya a chemical mineralogical isotopic and chronological synthesis along a tibet nepal section

origin of the lithospheric tension causing basin formation most fault controlled basin formation within plate interiors occurs by normal faulting in response to horizontal deviatoric tension in the continental crust it is suggested that the tension originates either from the plate boundary forces acting at trenches or as a result of isostatically compensated uplifted regions such as east africa the tension produced by both mechanisms is greatest in high heat flow regions where the upper elastic part of the lithosphere is thinned and weakened particularly widespread tension in the continental lithosphere occurs when subduction takes place on opposite sides of a large continental mass such as pangaea in the early mesozoic where it led to widespread graben formation and in association with hot spot activity to continental splitting

western versus eastern black sea tectonic evolution pre rift lithospheric controls on basin formation abstract we demonstrate the key role of pre rift rheology on the kinematics of basin formation in the western and eastern black sea basins constraints on modelling results are provided by a large data set based on more than 50 000 km of multichannel seismics offshore and onshore wells regional gravity and magnetic surveys refraction seismics and field studies the model supports the presence of important differences in the thickness and in the thermal state of the lithosphere which rifted to form the western middle barremian and the eastern middle paleocene black sea subbasins a 200 km and a 80 km thick pre rift lithosphere appear to have driven the deformation in the western and in the eastern black sea respectively differences in the geometry and in the mechanical properties of the pre rift lithosphere have a strong control on the depth of necking and thus on the basin morphology the model sheds light on palaeotectonic and palaeogeographic reconstructions duration of rifting events location of subsiding areas and erosional surfaces the western and the eastern parts of the black sea appear to be two distinct basins characterised by different evolutionary paths determined by different pre rift conditions

sequence stratigraphy and cyclic development of basal zechstein carbonate evaporite deposits with emphasis on zechstein 2 off platform carbonates upper permian northeast germany abstract although the basal zechstein is classically subdivided into two cycles z1 and z2 lower part it can alternatively be subdivided into three complete third order depositional sequences zs1 zs3 and the associated lowstand systems tract transgressive systems tract of a fourth sequence zs4 third and higher order sea level fluctuations influenced the depositional and diagenetic history of the carbonate evaporite alternations within the basal zechstein the main subject of the present study the zechstein 2 carbonate stassfurt carbonate or ca2 is interpreted to encompass both transgressive predominantly in slope and basinal settings and highstand systems tracts of zechstein sequence zs3 as well as part of the lowstand systems tract of the fourth zechstein sequence zs4 in northeastern germany hydrocarbon reservoirs of the ca2 developed along both northern and southern margins of the southern permian basin on top of the topography of the zechstein 1 cycle z1 this z1 topography resulted from deposition of the werra anhydrite a1 the highstand systems tract of the second zechstein sequence zs2 the northern ca2 carbonate platform is consistently narrow and fringed by coated grain and composite grain bars and shoals it is characterized by a steep fault controlled slope which precluded the formation of in situ shallow water grain dominated carbonates during relative sea level lowstands displaced shallow water coated grain and composite grain packstones wackestones may however occur as slope fan deposits turbidites in contrast the southern carbonate platform is wide tectonically dissected and fringed by grain dominated shallow water carbonates coated grain and composite grain bars and shoals zechstein 2 shallow water carbonates detectable by basal zechstein seismic anomalies also developed on pre existing topographical highs in off platform positions three depositional cycles ca2 cycles i ii and iii most probably reflecting high amplitude fourth order sea level fluctuations can be identified within the stassfurt carbonate our observations suggest that sea level variations possibly reflecting cyclicity in the milankovitch frequency band 0 1 0 4 ma and the pre zechstein topography pronounced during formation of the werra anhydrite were the most important factors in controlling the depositional as well as the diagenetic history of the zechstein 2 carbonate in off platform positions

adakite like granitoid porphyries in the eastern pontides ne turkey potential parental melts and geodynamic implications abstract the tectonic setting of the eastern pontides during the late mesozoic to early cenozoic remains a subject of debate petrogenesis of adakite like granitoid porphyries plays a critical role in determining the nature of the lower continental crust and mantle dynamics during orogenic processes in the region here we describe for the first time the late paleocene to early eocene adakite like granitoid porphyries from the northern part of the eastern pontides although their counterparts in the southern part have recently been found the adakitic porphyries which emplaced into the subduction related turonian santonian volcanics from northern part of the region consist of i type calc alkaline quartz monzonite tonalite sio2 62 89 65 07 wt and high k calc alkaline granodiorite granite associations sio2 69 06 70 43 wt the former displays peraluminous to metaluminous signatures whereas the latter shows peraluminous geochemical character the granite granodiorite porphyries have high k2o 3 32 3 84 wt and low na2o 3 48 4 61 wt and mgo 0 91 1 04 wt relative to the quartz monzonite tonalite association k2o 1 50 1 92 wt na2o 4 08 6 45 wt mgo 1 44 2 07 wt ar ar geochronology studies on the amphibole separates reveal that the adakite like porphyries have a crystallization ages of 51 34 0 27 to 53 55 0 34 ma here we contend that these rocks were formed by partial fusion of a mafic lower continental crust in a collisional phase but not in a subduction setting all the samples exhibit the typical geochemical characteristics of adakite that is high sr 250 1141 ppm high sr y ratios 16 147 low y 6 8 14 8 ppm and low hree concentrations they are similar to adakites formed by slab melting associated with the subduction zone however the rocks exhibit heterogeneity in isotopic composition with isr ranging from 0 70554 to 0 70986 end 51 ma from 8 5 to 0 9 and nd model ages from 0 72 to 1 26 ga the samples are characterized by relatively high th th u and no significant eu anomalies implying that garnet was stable in their source during partial melting the compositional diversity between rock groups is probably related to partial melting of heterogeneous lower crustal source all of the features are inconsistent with a slab melting origin and slab related petrogenetic model but instead they favor an origin by melting of heterogeneous lower continental crust due to a thermal anomaly that was induced by the upwelling of the asthenosphere through a slab break off in a collisional setting these interpretations argue against the evidence for the presence of an early cenozoic arc setting in the eastern pontides instead the early cenozoic time in the region may be attributed to a geodynamic response to a post collisional uplift phase that occurred along the continent continent collision between the pontide and the anatolide tauride blocks

brittle failure in the upper mantle during extension of continental lithosphere i present a model for the stress required to extend or stretch continental lithosphere the lithosphere is considered to have four rheological fields quartz in the brittle field quartz in the ductile field olivine in the brittle field and olivine in the ductile field as continental lithosphere extends the required stress initially decreases due to nearly isothermal thinning of the strong upper mantle olivine near the brittle ductile transition the stress required to continue extending the crust may then increase due to cooling of the upper mantle however the required stress can increase to no more than about 1 3 times the stress required to start extension this contrasts with an earlier study that required exponentially increasing stress to continue extension my model improves on this earlier study by considering the effects of brittle failure in the upper mantle the rheology of the continental lithosphere should be no barrier to its continued extension regardless of the rate of extension

gravity anomalies and flexure of the lithosphere at mountain ranges the bouguer gravity anomaly over the himalayan alpine and appalachian mountains is characterized by a generally asymmetric gravity low which spans the mountains and associated foreland basins the minimum of the gravity low is generally systematically displaced from the region of greatest topographic relief and shows no obvious relationship to surface geology in addition the alps and appalachians are associated with a generally symmetric gravity high that is unrelated to the topographic relief together the gravity low and high form a characteristic positive negative anomaly couple the steep gravity gradient between the positive and negative couple correlates with the insubric line and its equivalents in the alps and the brevard zone in the southern appalachians this gravity anomaly couple is interpreted as evidence for flexure of the continental lithosphere by subsurface buried and surface topographic loading the magnitude of the subsurface load is estimated from the integrated excess mass represented by the positive anomaly and the magnitude of the surface load from the topography by combining these loads the flexure of the lithosphere and the associated bouguer gravity anomaly are calculated for different assumed values of the elastic thickness te of continental lithosphere the best fitting value of te for the himalayas is 80 100 km for the alps 25 50 km and for the appalachians 80 130 km the model calculations suggest that the main contribution to the gravity couple in the alps and appalachians arises not from surface loads such as thrust sheets and nappes but from subsurface loads however in the case of the himalayas the topography appears to be a sufficient load to explain the observed asymmetric gravity low when subsurface loads do exist therefore we would not expect a close correlation between mountain topographic relief and depth to the m discontinuity as required by classical models of isostasy rather the m discontinuity is expected to relate to the center of mass of all the loads acting on the lithosphere the surface and subsurface loads inferred from the gravity anomaly play a major role in the development of mountains and foreland basins the emplacement of large subsurface loads represents the primary event in the alps and appalachians while in the himalayas the emplacement of the surface load is the primary event the emplacement of the subsurface load probably marks the initiation of foreland basin development the regional characteristics of the basin are controlled by the primary load while local variations are controlled by the secondary thrust sheet loads subsequent movement of either load will produce migration of the basin depocenter the origin of the subsurface load is not clear but its association with the insubric line in the alps and the brevard zone in the appalachians suggests that it may be related to the obduction of crustal blocks flakes onto the underlying plate during continental collision or possibly to the preloaded crustal structure of the underlying plate

a new hypothesis for organic preservation of burgess shale taxa in the middle cambrian wheeler formation house range utah cambrian konservat lagerstatten are the most significant fossil deposits for our understanding of the initiation of phanerozoic life although many modes of preservation may occur these deposits most frequently contain nonmineralized fossils preserved in the form of kerogenized carbon films a rare yet important taphonomic pathway that has not previously been explained for any unit by a comprehensive model the middle cambrian wheeler formation of utah one of these lagerstatten contains abundant kerogenized preservation of nonmineralized tissues which occurs within a distinctive taphofacies that accumulated under the following conditions 1 domination of the siliciclastic fraction by clay sized particles 2 close proximity to a carbonate platform which resulted in mixed carbonate clay sediments 3 a well developed oxygen minimum precluding benthic colonization and burrowing and 4 relative proximity to oxic bottom waters facilitating transport of organisms from a habitable environment to one that favored their preservation we propose that preservation of nonmineralized tissues in the wheeler formation may have resulted from a combination of influences that reduced permeability and thus lowered oxidant flux which in turn may have restricted microbial decomposition of some nonmineralized tissues those influences include near bottom anoxia preventing sediment irrigation by restriction of bioturbation reducing conditions near the sediment water interface that may have acted to deflocculate aggregations of clay minerals resulting in low permeability face to face contacts early diagenetic pore occluding carbonate cements and an absence of coarse grains such as silt skeletonized microfossils fecal pellets or bioclasts this model may be applicable to kerogenized preservation of macrofossils in other fossil lagerstatten

on the evolution of rifted continental margins comparison of models and observations for the nova scotian margin summary a model for the formation and evolution of rifted continental margins based on lithospheric extension during rifting and its thermal and mechanical consequences is proposed model predictions are then compared with geological and geophysical observations from a transect of the 185 ma old rifted margin off nova scotia through the scotian basin three kinematic models of the rifting process are discussed these are 1 the uniform extension model in which the amount of extension is uniform with depth but varies with position across the margin 2 the uniform extension and melt segregation model which has similar properties but also provides an explanation for the properties of the extended continental crust and its transition to oceanic crust by postulating that basaltic melt segregates from the asthenosphere and migrates to the crust and 3 the depth dependent extension model in which the first order consequences of rapidly changing rheological properties with depth in the lithosphere are included by decoupling the lithosphere into two zones with depth each of which undergoes differing amounts of extension these rift models predict the form of crustal and lithospheric thinning subsidence and temperature change once the amount of extension has been determined this is estimated from seismic measurements of present crustal thickness on the assumption that the crust had a uniform thickness equal to that currently measured in the adjacent continental region before rifting occurred rifting is also considered to occur instantaneously a time stepping thermo mechanical model is used to predict the cooling of rifted margin additional thermal contraction subsidence and its amplification by water and sediment loading thermal aspects are calculated using a finite difference model of time dependent conductive heat transport whereas regional isostatic response to loading is calculated by a finite element model the models are coupled because the temperature distribution is used to define a rheological lithosphere an elastic region with thickness that varies in time and space as the model evolves for flexural calculations in the mechanical model secondary coupling occurs through perturbations to the temperature field by sedimentary thermal blanketing and advection of heat during isostatic adjustment the model predictions of 1 sedimentary basin stratigraphy 2 moho position 3 free air gravity anomaly 4 age depth relations for deep exploratory wells and 5 subsidence and temperature histories agree well with observations from the scotian basin additional effects due to sedimentary and crustal radiogenic heat production lateral heat transport and the possible existence of a near surface brittle listric faulted region created during rifting are also considered it is concluded that a model in which rifting occurred by depth dependent extension and which includes a finite thickness for the rheological lithosphere radiogenic heat production in the sediments and crust and a brittle listric faulted crustal layer affords an accurate description of the first order processes that occur during rifting and evolution of this margin

continental rifting at pre existing lithospheric weaknesses continental rifting is the poorly understood thermomechanical process by which continents break up and new ocean basins form two dominant styles of continental rifting are recognized1 crevice style rifts are characterized by initial graben formation local uplift of the graben shoulders and little or no volcanism arch volcanic rifts exhibit an initial doming of the crust extensive volcanism and late stage graben formation the prevailing expla nation for the two styles is that crevice rifts form in response to regional horizontal stresses principally derived from the inter action of the lithospheric plates along their edges whereas arch volcanic rifts are thought to be the result of mantle convection currents which upwell directly beneath the rift axis2 we propose an alternative working model of continental rifting in which both styles initiate in response to regional horizontal stresses the different surface expressions of the two styles are explained in terms of differences in the mode of failure at different kinds of pre existing weaknesses in the continental lithosphere rather than differences in the nature of the driving forces involved

a thin viscous sheet model for continental deformation summary for the purposes of describing its large scale and long term deformation the continental lithosphere is regarded as a continuum obeying a newtonian or a power law rheology the flow of a thin sheet of power law material overlying an inviscid substrate is studied under the assumption that vertical gradients of the horizontal velocity are negligible a numerical model is used to investigate the deformation of such a sheet under conditions approximating those of continent continent collision the material flows in response to forces applied to its boundaries for example the indenting of one continent by another and to forces in its interior arising from gradients in crustal thickness the horizontal divergence of the flow produces changes in the crustal thickness and hence a time dependent form to the flow itself for a given set of boundary conditions the flow depends on the stress exponent in the power law rheology n and on the argand number which is a measure of the ratio between the stress arising from crustal thickness contrasts and the stress required to deform the material at the ambient strain rates when the effective viscosity of the medium is very high ar o crustal thickness variations do not influence the flow if the material is newtonian n l the deformation associated with an influx of material approximating an indenter is of much greater lateral dimension than the width of the indenter whereas when material has a power law rheology n 3 5 are used the deformation is confined to a region of lateral extent comparable to that of the indenter as ar increases the forces arising from crustal thickness contrasts exert more influence on the flow and the maximum crustal thickness that can be sustained by a given influx of material is related by a simple expression to the effective viscosity of the medium at the ambient strain rates in the limit of a very weak medium ar 10 the lithosphere is unable to sustain appreciable crustal elevation contrasts the results of these numerical experiments show that systems in which the effective viscosities are such that the maximum deviatoric stresses are between 1 kbar

numerical modeling of intraplate deformation simple mechanical models of continental collision we study a model of continental collision in which one of the continents acts as a rigid die indenting the other plate which flows as an incompressible viscoplastic medium we consider two extreme cases of plane deformation 1 plane strain which corresponds to an infinitely thick lithospheric plate and 2 plane stress corresponding to a very thin plate deformation of the lithosphere a thick plate should be intermediate between those extremes we found that the flow in the plane strain case is quite similar to that obtained by slip line theory the plane stress results are quite different since in this case most of the plate shortening is taken up by the thickening of the lithosphere we also explored the role of boundary conditions on the flow in particular the role of the side walls containing the flow of the lithosphere in the case of a free lateral boundary the main feature is a flow of matter toward this free wall and a s like pattern for the horizontal stress field for a rigid wall on the other hand the plane strain and the plane stress results are quite different in the first case there is a large return flow on the sides of the punch the material being extruded along the only free surface available in the plane stress case the return flow disappears and the material displaced by the penetration of the die tends to thicken the plate the role of a nonlinear constitutive relation is studied for power law creep as the power of the flow limit increases the flow retains its general features but the deformation localizes creating sharper contrasts between high and low strain rate areas in plane stress the effect of nonlinearity is to increase the contrasts in vertical motion available data for asia are discussed in the light of the new results

active continental deformation and regional metamorphism the vertical and horizontal distribution of present day continental deformation is examined to see how tectonic movements may be related to large wavelength perturbations to the temperature and pressure experienced by rocks in the crust earthquakes are generally restricted to the upper part of the continental crust the lower crust is usually aseismic and assumed to be weaker the uppermost mantle beneath continental regions has minor seismic activity that does not account for much deformation but probably indicates an important strength contrast between the lower continental crust and the upper mantle the maximum focal depth of earthquakes in any region appears to be limited by temperature with most restricted to material colder than 350 pm 100 degrees c in the crust and colder than 700 pm 100 degrees c in the mantle at length scales long compared with the thickness of the brittle upper crust the deformation in regions of continental extension or shortening appears to be continuous even though in reality discontinuous movement on faults occurs this probably indicates that the deformation is dominated by distributed flow in the ductile portion of the lithosphere and not by the behaviour of the thin brittle upper crust the distribution of seismicity elevation contrasts and vertical movements at the surface suggests that there is little spatial separation between the brittle deformation in the upper crust and the ductile deformation below on length scales larger than the lithosphere thickness for this reason and because of the short thermal time constant of the crust long wavelength perturbations to the thermal regime are more influenced by the behaviour of the lithosphere as a whole than by the precise geometry of deformation in the crust large scale regional metamorphism in zones of shortening may result from the re establishment of the initial geotherm in thickened crust when the lower part of the lithosphere detaches and falls into the asthenosphere in regions of extension an increased geothermal gradient is unlikely to result in regional metamorphism unless magmatic augmentation to the heat supply is important however if the stretched region is covered by thick sediments the basement may experience a small increase in temperature and remain significantly hotter than it would be if there were no sediment cover while unlikely to account for significant metamorphism this effect may strongly influence the rheological behaviour of the lithosphere in extending regions the rapid vertical movements associated with syn or post orogenic normal faulting in regions of large scale crustal thickening are probably at least as important in exhuming mid crustal metamorphosed rocks and in disrupting patterns of isograds as those associated with erosion

volcanic belts as markers of the mesozoic cenozoic active margin of eurasia abstract from the triassic to the present linear belts of calc alkaline volcanism developed along the southern margin of eurasia in the triassic the calc alkaline belt can be traced in turan fore caucasus crimea and dobrogea its development is related to the closure of the paleo tethys when fragments of gondwana iran afghanistan etc were approaching eurasia since the early jurassic calc alkaline volcanism has been related to subduction of the oceanic crust of the mesozoic tethys five stages of calc alkaline volcanic activity can be distinguished early jurassic middle jurassic early cretaceous late cretaceous paleogene and late miocene quaternary the configuration of the volcanic belts the intensity of the volcanism and the chemistry of the erupted products change considerably from stage to stage mesozoic belts were most pronounced in the pontian transcaucasian segment of the margin starting from the late cretaceous and especially in the cenozoic the volcanic belts can be continuously traced from the balkans to afghanistan most of the belts were actually island arcs formed on the fragments of the hercynian of baikalian basement and in this respect similar to the island arc of japan only the incipient island arcs of senonian age formed close to the passive margin of gondwana are the exceptions they were built on oceanic crust and are geochemically close to the mariana arc at certain times early jurassic eocene parallel chains of island arcs existed in general the situation resembled the present day convergent boundary in the western pacific as closure of the tethys proceeded island arcs gave way to marginal continental volcanic belts with wide spread development of high potassium particularly shoshonitic series peaks of volcanic activity in the bajocian late cretaceous eocene and late miocene present alternated with quiet periods and corresponded to the times when the convergence rate between africa and eurasia increased the opening of the back arc basins is reflected in outbursts of basaltic rift type volcanism these outbursts occurred in the toarcian aalenian bathonian late jurassic aptian turonian and early middle eocene

correction to a thin viscous sheet model for continental deformation summary the governing equation derived by england mckenzie for the deformation of a thin viscous shell contains an error we give the correct derivation here and correct those figures where the change makes a visible difference the correct results differ quantitatively from those of england mckenzie but their conclusions are not affected

the himalayan arc large scale continental subduction oroclinal bending and back arc spreading abstract palaeomagnetic results from the himalayan arc and southern tibet compared with simulated apparent polar wander paths for the indian plate show a consistent pattern of rotations of the himalayan arc relative to the indian shield varying gradually from 45 clockwise in the northwestern himalaya to slightly counterclockwise in the lhasa region this pattern is consistent with continental underthrusting of greater india beneath the tibetan plateau since the early miocene over at least 650 km at the longitude of western nepal and oroclinal bending since the latest miocene available palaeomagnetic observations support the steady state model for the formation of the himalayan arc with refinements as follows 1 collision between greater india s northern boundary and southern asia occurred at equatorial latitudes with progressive suturing from palaeocene in the northwestern himalaya until early eocene in the eastern himalaya 2 continuing convergence and indentation of greater india into southern asia over about 2000 km up to the early miocene resulted in southeastward extrusion of indochina and 3 neogene counterclockwise rotational underthrusting of greater india along the main central thrust with pliocene quaternary oroclinal bending of the himalayan arc

finite strain calculations of continental deformation 2 comparison with the india asia collision zone numerical experiments on a thin viscous sheet model for deformation of continental lithosphere subjected to an indenting boundary condition yield distributions of crustal thickness of stress and strain rate and of latitudinal displacements that may be compared with observations in the india asia collision zone a simple indenting boundary condition applied to initially laterally homogeneous sheets obeying a power law rheology produces results that are in broad agreement with the observations provided that the power law exponent is three or greater and the sheet can support vertically integrated stress differences of 2 1013 5 1012 n m 1 in regions in front of the indenter under these conditions the calculated deformation shows accommodation of convergence primarily by crustal thickening to produce a plateau in front of the indenter palaeomagnetic data from india and tibet and the observed distribution of topography suggest that much of the post eocene convergence of india with asia has been taken up by deformation within asia that involved crustal thickening the principal difference between calculation and observation is the absence from the calculated strain rate fields of east west extension of the plateau in front of the indenting boundary the calculations show that once such a plateau is formed the buoyancy force associated with the crustal thickness contrast inhibits further thickening and the plateau strains at less than half the rate of its immediate surroundings seismically determined regional strain rates exhibit a similar distribution with the tibetan plateau straining at about one quarter the rate of the tien shan and ningxia gansu regions calculated principal compressive stress orientations and regional strain rates agree with the seismically determined quantities in the mongolia baikal tien shan tibet and ningxia gansu regions of asia to within the uncertainty of the latter the vertically integrated stresses that are calculated for the viscous sheet are comparable with those that can be supported by a theologically stratified continental lithosphere obeying laboratory determined flow laws we suggest that the thin viscous sheet model described in this paper and its companion gives a simple and physically plausible description of the observed deformation in central asia in this description the predominant mechanism of accommodation of continental convergence is diffuse crustal thickening with shear on vertical planes playing a subsidiary role once large crustal thickness contrasts have been established

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

gravity anomalies flexure of the indian plate and the structure support and evolution of the himalaya and ganga basin bouguer gravity anomalies along four profiles across the western himalaya and ganga basin show large deviations from local isostatic equilibrium a deficit of mass characterizes the ganga basin and an excess underlies the lesser himalaya both can be understood if the indian plate is flexed down by the distributed load of part of the mountains the cross sectional shape of the ganga basin seems to be controlled by the deflection of the indian plate which we compute assuming the indian plate to overlie an inviscid fluid from the shapes of both bouguer anomaly profiles and the basement topography we place bounds on the flexural rigidity of such a plate if the ganga basin is a steady state feature then the age of the basal sediments in a given locality should be proportional to the distance of that locality from the southern edge of the basin if the rate of convergence of india and the himalaya were constant that rate should equal the distance divided by the corresponding age we find a rate of 10 to 15 mm a for the last 15 to 20 ma which is consistent with a large part of the 50 mm a rate of convergence between india and eurasia being absorbed by the eastward extrusion of parts of tibet profiles of bouguer gravity anomalies show only a small peak or plateau over the southern edge of the lesser himalaya implying that the boundary between the light sediments of the ganga basin and the heavier crustal rocks of the lesser himalaya is not sharp and that there exists some light material beneath the range we infer that some sediment deposited in the ganga basin has been underthrust beneath the lesser himalaya but the quantity is small most of this sediment probably is scraped off the indian plate to make the foothills of the range we find that the load of the high himalaya is too large to be supported solely by elastic stress in the indian plate if the flexural rigidity of the plate is constant and if no other external forces act on the plate the observed gradient in bouguer gravity anomalies increases from an average of about 1 mgal km over the ganga basin and lesser himalaya to about 2 mgal km over the high himalaya this increase in the gravity gradient implies that the moho dips more steeply 10 15 beneath the high himalaya than beneath the lesser himalaya 2 3 we interpret this steepening of the moho to be due to a weakening of the plate which allows it to bend more sharply beneath the high himalaya than farther south with the inclusion of a weak segment of indian plate beneath the high himalaya calculated anomalies show a somewhat increased gradient beneath the high himalaya but when the weight of the entire himalaya is used as a load on the plate calculated anomalies are more negative than observed therefore an external force system is needed to support much of the weight of the high himalaya as well as to bend the plate sufficiently beneath the high himalaya the magnitudes of the bending moment and the force per unit length that must be applied to the end of the plate are compatible with their sources being gravity acting on a part of india s mantle lithosphere stripped of its crust and underthrust beneath southern tibet

a two level concept of plate tectonics application to geodynamics abstract a two level plate tectonics concept is developed on the basis of data on lithosphere rheological stratification this approach differentiates between crust and subcrust plate ensembles separated by a lower crust viscoplastic asthenolayer similarly to classical plate tectonics three types of boundaries are distinguished in the lower layer which do not always coincide with crust plate boundaries especially for continents applications of this concept to geodynamics are considered and a corresponding quantitative analysis for several important processes is carried out a quantitative model of mountain formation and collision plateau origins is proposed also a geodynamic model of the evolution of passive margins taking into account a lower crust viscous flow is considered and its geological consequences are discussed a mechanism of rifting taking into consideration rheological lithospheric layering and its vertical movements caused by extension is developed both a qualitative scheme and quantitative analysis of the slow evolution of intracraton structures of shield basin type taking into account erosion and sedimentation processes are worked through also historical aspects of plate tectonics are discussed from the point of view of the proposed concept

jurassic cretaceous paleomagnetism and paleogeography of the pontides turkey published paleomagnetic data from the pontides indicate anomalously low jurassic and early cretaceous paleolatitudes compatible with the southern neo tethyan african continental margin and significantly different from paleolatitudes predicted for the northern eurasian neo tethyan margin we present a new set of paleomagnetic data from 50 late cretaceous sites mainly from the western pontides and from the eastern pontides and 11 early jurassic sites mainly from the eastern pontides fold tests indicate that the characteristic magnetization components predate eocene folding late cretaceous site mean declinations are northerly in the eastern pontides and rotated to the west by a few tens of degrees in the western pontides late cretaceous site mean declinations are affected by local clockwise rotation in one sampling region close to the north anatolian fault the mean late cretaceous inclinations and resulting paleolatitudes are 41 1 23 5 n and 43 7 25 5 n for the western and eastern pontides respectively for the eastern pontides the early jurassic liassic mean inclination is 60 5 yielding a paleolatitude of 41 4 n considerably further north than previous paleolatitude estimates for the eastern pontides at this time the paleolatitude estimates for the western and eastern pontides are consistent with these units being close to the eurasian continental margin during liassic and late cretaceous time

late pleistocene mixing zone dolomitization southeastern barbados west indies field geochemical and petrographic data for late pleistocene dolomites from southeastern barbados suggest that the dolomite precipitated in the zone of mixing between a coastal meteoric phreatic lens and normal marine waters the dolomite is localized in packstones and wackestones from the algalamphistegina fore reef calcarenite facies stable isotopic evidence suggests that meteoric water dominated the diagenetic fluids responsible for dolomitization carbon isotopes in pure dolomite phases average about 15 0 pdb this light carbon is attributed to the influence of soil gas co2 and precludes substantial mixing with seawater a narrow range of oxygen isotopic compositions coupled with a wide range of carbon compositions attest to the meteoric diagenetic overprint dolomitization likely occurred with as little as a five per cent admixture of seawater strontium compositions of the dolomites indicate probable replacement dolomitization of original unstable mineralogy the dolomite is characterized by low sodium values low concentrations of divalent manganese and iron suggest oxidizing conditions at the time of dolomitization a sequence of petrographic features suggests a progression of diagenetic fluids from more marine to more meteoric early marine diagenesis was followed by replacement dolomitization of skeletal grains and matrix limpid euhedral dolomite cements precipitated in primary intra and interparticle porosity subsequent to replacement dolomitization as waters became progressively less saline dolomite cements alternated with thin bands of syntaxial calcite cement the final diagenetic phase precipitated was a blocky calcite spar cement representing diagenesis in a fresh water lens this sequence of diagenetic features arose as the result of a single fall in eustatic sea level following deposition a stratigraphic eustatic diagenetic model constrains both the timing and rate of dolomitization in southeastern barbados dolomitization initiated as sea level began to fall immediately following the oxygen isotope stage 7 3 high stand some 216 000 yr bp due to the rapidity of late pleistocene glacio eustasy dolomitization locally complete is constrained to have occurred within about 5000 yr

abrupt transition from magma starved to magma rich rifting in the eastern black sea the amount of magmatism that accompanies the extension and rupture of the continental lithosphere varies dramatically at rifts and margins around the world based on widely spaced geophysical transects some margins are known to preserve a transition from magmatically robust to magmatically starved rifting along strike but the nature of the transition is unknown wide angle seismic data from the black sea provide the first direct observations of such a transition and show that it is abrupt occurring over only 20 30 km and coincides with a transform fault this abrupt transition cannot be explained solely by gradual along margin variations in mantle properties since these would be expected to result in a smooth transition from magma poor to magma rich rifting over hundreds of kilometers we suggest that the abruptness of the transition results from the development of three dimensional 3 d melt migration due to along strike variations in extension and thus the thickness of the lithosphere at the time of rifting localized magmatic addition attributed to melt focusing has been observed in modern mid ocean ridges and active rift environments but here we show that such processes can also produce abrupt along strike changes from magma poor to magma rich rifting

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

new structural and u pb zircon data from anafi crystalline basement cyclades greece constraints on the evolution of a late cretaceous magmatic arc in the internal hellenides the asterousia crystalline complex consists of late cretaceous amphibolite facies metamorphic rocks and associated granitoids which can be found in exposures on crete and the cyclades greece it is attributed to the uppermost unit and therefore to the pelagonian domain of the internal hellenides the tectonometamorphic evolution of this unit is still a matter of debate we present new structural and petrological data of asterousia type rocks and greenschist facies metamorphic rocks from the island of anafi in the southern aegean sea as well as u pb zircon ages of granitoids from anafi the crystalline sequence of anafi rests on top of eocene flysch and comprises from bottom to top a anafi greenschist b anafi amphibolite group orthoamphibolite with intercalations of metasedimentary rocks at the base and c chalepa group amphibolite facies metasediments with slices of serpentinite and granitoids la icp ms and id tims 206pb 238u zircon ages of granodiorite from the chalepa group reveal several similar zircon populations suggesting continuous emplacement of granitoids inside a magmatic arc from ca 72 5 to 79 ma the minimum emplacement age of granodioritic magma deduced from the 206pb 238u median age of the youngest zircon population is 72 6 0 1 0 2 ma deformation micro fabrics of granodiorite result from low strain obtained at t 600 c this along with the u pb ages and published k ar ages indicates intrusion of the plutonic rocks at deep structural levels followed by very slow cooling monzogranitic dykes cutting through granodiorite in north eastern anafi are undeformed and yielded a 206pb 238u median age of 69 9 0 7 0 7 ma based on the new and published data the following implications for the tectonometamorphic evolution on anafi can be made 1 obduction and accretion of mantle slices serpentinite to the asterousia type rocks were prior to amphibolite facies metamorphism 2 intrusion of granitoids during the middle to late campanian within a magmatic arc setting and coeval shift of the magmatic arc towards the south 3 maastrichtian intrusion of dykes 4 palaeocene greenschist facies metamorphism and coeval ductile top to the se thrusting of the anafi amphibolite group on top of the anafi greenschist 5 post eocene brittle top to the se thrusting of the anafi greenschist and the anafi amphibolite group on top of flysch sediments 6 clockwise rotation of the system and early oligocene brittle top to the se thrusting of the chalepa group on top of all units mentioned above and 7 ongoing clockwise rotation of the aegean block during the oligocene to early miocene and change in stress field ne sw compression

ophiolitic and metamorphic assemblages of southeast anatolia and their significance in the geological evolution of the orogenic belt the southeast anatolian orogen may be divided into three roughly east west trending structural zones formed as a result of continental collision between the taurus platform and the arabian continent along the orogenic belt metamorphic and ophiolitic rocks occur widely the ophiolites represent remnants of the ocean or oceans which were totally consumed between these converging continental blocks during late cretaceous to miocene period metamorphic rocks formed from the oceanic as well as the continental rocks which were incorporated into a nappe stack during the consumption of the oceanic lithosphere and the progressive southward advance of the nappes toward the arabian continent the metamorphic units together with the ophiolite associations provide stratigraphic and petrologic evidence indicating time place and environment of formation of these units the metamorphic units also provide evidence of nappe transportation stages which are complementary to the data derived from the sedimentary successions in the evaluation of the orogenic evolution of southeast anatolian orogen

faulting and graben formation in western and central europe rifts and rift related basins play a pre eminent role among the sedimentary basins of western and central europe through time grabens developed in a number of different megatectonic settings whereby the principal mechanisms governing their subsidence was crustal stretching and subcrustal erosion the level of volcanic activity associated with rifting is highly variable and can change significantly during the development history of a rift some rifts are totally non volcanic the development of rift domes is generally associated with volcanic activity uplifting of a rift dome can induce a reversal in the subsidence pattern of a rift intracontinental rifts with their thinned crust are prone to inversion when the respective craton is subjected to tangential stresses in the process of inversion the crust of rifts is mechanically thickened again

petrology and geochemistry of post collisional middle eocene volcanic units in north central turkey evidence for magma generation by slab breakoff following the closure of the northern neotethys ocean the eocene volcano sedimentary units of northern anatolia are confined into a narrow zone trending parallel to the intra pontide and izmir ankara erzincan sutures along which the northern branch of the neotethys ocean was closed during a period between late maastrichtian and paleocene the middle eocene formations overlie both the imbricated and highly deformed units of the suture zone which are paleocene or older in age as well as the formations of adjacent continental blocks with a regional disconformity therefore they can be regarded to be post collisional these units are composed of subaerial to shallow marine sedimentary beds i e the orencik formation at the base and a subaerial volcanic unit i e the hamamozu formation in the middle and at the top this sudden facies change from marine to subaerial environment in the middle eocene is a common phenomenon across northern turkey implying that a regional uplift event occurred possibly across the suture zone before the initiation of the volcanism during lutetian the middle eocene lavas span the whole compositional range from basalts to rhyolites and display a calc alkaline character except for alkaline to mildly alkaline lavas from the top of the sequence all lavas display a distinct subduction signature our geochemical data indicate that calc alkaline lavas were derived from a subduction modified source whereas alkaline to mildly alkaline lavas of the late stage were possibly sourced by an enriched mantle domain magmas evolved in magma chambers emplaced possibly at two different crustal levels magmas in deeper 13 km and possibly larger chambers fractionated hydrous mafic minerals e g amphibole and biotite two pyroxenes and plagioclase and assimilated a significant amount of crustal material intermediate to acid calc alkaline lavas and pyroclastics were derived from these chambers magmas in the shallower chambers on the other hand 12 km crystallized anhydrous mineral assemblages assimilated little or no crustal material and fed basic to intermediate lavas in the region both deep and shallow chambers were periodically replenished by mafic magmas we argue that a slab breakoff model explains better than any alternative model i why the volcanism during the middle eocene was confined into a rather narrow belt along the suture zone ii why it initiated almost contemporaneous with a regional uplift after the continental collision event iii why it postdated arc volcanism along the pontides in the north by 15 20 my iv why it assimilated significant amount of crustal material and v why alkalinity of lavas increased in time

authigenic dolomite in bahamian peri platform slope sediment authigenic calcium rich dolomite has been discovered in poorly consolidated lower pliocene upper miocene carbonate slope sediments north of little bahama bank the dolomite was recovered from a 5 6 m long piston core that penetrated a slide scar in 600 m of water below an unconformity at 105 cm which extends from the late pleistocene to about the miocene pliocene boundary x ray diffraction data indicate only calcite 85 99 and dolomite as much as 15 and scanning electron microscopy observations indicate in situ precipitation of dolomite stable carbon isotope data indicate a normal open marine source of carbon and oxygen isotope values are consistent with precipitation of both calcite and dolomite from deep cold open marine waters trace element data reveal that dolomitic sediments are depleted in sr 2 and mg 2 relative to nondolomitic upper pleistocene oozes dolomitization occurred in a deep open marine diagenetic environment via whole scale dissolution of aragonite and magnesian calcite and reprecipitation of calcite and dolomite

convective partial melting 1 a model for the formation of thick basaltic sequences during the initiation of spreading stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure temperature environment where partial melting occurs the amount of melt liberated is however insufficient to account for the thickness of igneous crust emplaced near the continent ocean boundary at some passive margins these igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide seismic studies also suggest that the transition from continental to oceanic crust on these volcanic margins is relatively abrupt their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading to account for these observations we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle deep hot mantle material is transported upward by the convection and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling numerical calculations show that convective partial melting can provide substantial additions to melting due to passive upwelling alone using this model we propose an evolutionary scheme for volcanic passive margins emphasize the contrast with nonvolcanic margins and describe conditions which might lead to the development of both types

a forward approach to the problem of nonlinear viscoelasticity and the thickness of the mechanical lithosphere the physical significance of the mechanical lithosphere is clarified using a nonlinear maxwell flexure model this model reconciles high temperature creep data for rock with the observed long term strength of the mechanical lithosphere an approximate correspondence is established between the nonisothermal maxwell model and an elastic flexure model to define an effective lithosphere thickness he the model calculations are essentially forward since he does not depend in a significant way upon the nominal or prescribed value of beam thickness depending rather upon such variables as temperature and creep activation energy the effective thickness of the maxwell model though a function of time is stable enough over appropriate time scales to explain the long term integrated strength of the lithosphere the strength index is defined as the ratio of axial stress to the axial stress that would be present in a purely elastic fiber suffering the same strain the base of the effective lithosphere is marked by a critical isopleth of strength c 0 3 this critical level though not its position is independent of time since loading the strength index also maps out zones of predominant rheology the narrowness of the transition zone between the elastic lithosphere and the viscous asthenosphere explains the success of the elastic plate in modeling many examples of lithosphere flexure the elastic plate approximation may apply best where the lithosphere is relatively young and hot isolated weak zones in the continental crust may or may not weaken the lithosphere as a whole depending upon the distribution of these zones and the manner in which they deform in general such weakening is likely to be most effective when the base of the crust occurs at midlithospheric depths

new paleomagnetic results from upper cretaceous arc type rocks from the northern and southern branches of the neotethys ocean in anatolia a paleomagnetic study of cretaceous arc type rocks in the central eastern pontides and in the southeastern taurides investigates the tectonic and paleolatitudinal evolution of three volcanic belts in anatolia namely the northern and southern volcanic belts in the pontides and the se taurides volcanic belt the paleomagnetic data indicate that magnetizations were acquired prior to folding at most sampling localities sites except for those in the erzincan area in the eastern pontides the southern volcanic belt was magnetized at a paleolatitude between 23 8 3 8 4 2 n and 20 2 1 2 1 3 n hisarli j geodyn 52 114 128 2011 reported a more northerly paleolatitude 26 6 4 6 5 1 n for the northern volcanic belt the comparison of the new paleomagnetic results with previous ones in anatolia allows to conclude that the southern volcanic belt in the central eastern pontides was emplaced after the northern volcanic belt as a result of slab roll back of the northern neotethys ocean in the late cretaceous in the southeast taurides upper cretaceous arc related sandstones were at a paleolatitude of 16 8 3 8 4 2 the late cretaceous paleomagnetic rotations in the central pontides exhibit a counterclockwise rotation of r pm delta r 37 1 circ pm 5 8 circ group 1 cankiri yaylacayi formation while maastrichtian arc type rocks in the yozgat area group 2 show clockwise rotations r r 33 7 8 4 and r r 29 3 6 0 in the se taurides counterclockwise and clockwise rotations of r pm delta r 48 6 text circ text pm 5 2 text circ text and r pm delta r 34 1 text circ text pm 15 1 text circ text are obtained group 4 elazig magmatic complex the late cretaceous paleomagnetic rotations in the pontides follow a general trend in concordance with the shape of the suture zone after the collision between the pontides and the kirsehir block the affect of the westwards excursion of the anatolian plate and the associated fault bounded block rotations in miocene are observed in the east of the study area and the se taurides

sumeini group oman evolution of a mesozoic carbonate slope on a south tethyan continental margin abstract deposition of the mostly mesozoic sumeini group occurred on the slope between the shallow marine arabian carbonate platform and the deep oceanic hawasina basin or south tethys sea these strata record the evolution of the northeast arabian continental margin from permian and triassic rifting to ocean basin closing with the late cretaceous obduction of the semail ophiolite platform margin type and its evolution can be inferred from exposures of these slope sediments at jebel sumeini in the western oman mountains in the lower permian base of the sequence bedded spicular limestones suggest deep marine conditions this is followed by an enigmatic thick sequence of bedded dolomite in the early to middle triassic small carbonate submarine fans formed along a distally steepened slope a subsequent interval of terrigenous clastic sedimentation was followed by development of a steep escarpment margin during ladinian to norian time with coralgal reefs at the shelf break and a debris apron at the base of the scarp the late triassic is marked by another influx of terrigenous sediment which accompanied widespread emergence of the platform in the jurassic a thick apron of thin bedded limestone formed on the slope this apron was cut by channelized gullies that now include blocks of older reefal material and oolitic sands a well documented submergence of the platform during the tithonian and early cretaceous led to the deposition of radiolarian cherts on the slope and basin upper lower cretaceous megabreccias up to 200 m thick suggest an interval of localized slope instability and or tectonism continued deposition of bedded limestone and marl with lesser calcirudite marks the top of stratigraphic sequence at jebel sumeini the early to middle triassic carbonate submarine fan deposits occur as discrete lenticular sequences of bedded calciturbidite modified grain flow and debris flow deposits which were derived from mostly slope sources the ladinian to norian base of slope debris apron deposits were derived from both slope and shallow marine sources including reefs and appear to form a laterally extensive but narrow belt of thick lenticular to sheet like beds of debris which accumulated over a long interval of time coarse limestones in the jurassic gullied slope deposits are distinct in that they occur randomly as individual channelized beds in a sequence dominated by thin bedded lime mudstones thick lower cretaceous megabreccias are distinguished by their great lateral distribution and general lack of matrix

age dependence of oceanic intraplate seismicity and implications for lithospheric evolution we have determined depths for 16 oceanic intraplate earthquakes using body wave modeling a data set composed of these and 11 other well constrained depths shows that the maximum depth of seismicity deepens with increasing lithospheric age and appears to be bounded by a 700 800 c isotherm the maximum depth to which intraplate earthquakes occur is approximately equal to the flexural elastic thickness but much less than the seismic thickness from surface wave dispersion the maximum faulting depth is consistent with the predictions of rapid weakening at high temperatures for dry olivine rheologies determined by experimental rock mechanics oceanic intraplate earthquakes can occur at greater depths and temperatures than has previously been reported for continental crustal events presumably because oceanic lithosphere is stronger at high temperatures both the number of earthquakes and the cumulative seismic moment decrease with lithospheric age the cause of this trend is unclear possibilities include nonrandom location of preexisting weak zones with respect to the age provinces fault strengthening with age or a decrease in the stress level a decrease in intraplate stress with age is opposite that expected from ridge push stresses which increase with age but may result either from thermoelastic stresses which are greatest in young lithosphere or near ridge stress concentrations due to local heterogeneities in the spreading process

latest cretaceous a 2 type granites in the sakarya zone ne turkey partial melting of mafic lower crust in response to roll back of neo tethyan oceanic lithosphere abstract an integrated study of comprehensive geochronological geochemical and sr nd hf isotopic data was undertaken for the a type topcam pluton that intruded within the sakarya zone ne turkey with the aims of elucidating its origin and tectonic significance and gaining new insights into the generation of aluminous a type granites new la icp ms zircon u pb crystallization ages of 72 and 73 ma indicate emplacement in the late cretaceous time just after extensive metaluminous i type magmatism in the area the pluton consists mainly of alkali feldspar quartz plagioclase amphibole and biotite with accessory minerals such as magnetite apatite and zircon the outcrop is composed of granite syenite monzonite and quartz monzonite and possesses a wide range of sio2 content 57 70 wt with elevated ga al ratios and low mg mostly

evidence of early cretaceous remagnetization in the crimean peninsula a palaeomagnetic study from mesozoic rocks in the crimean and western pontides conjugate margins of the western black sea

propagating rifts crustal extension during continental rifting propagating rifts may not be incompatible with plate tectonic tenets which are considered at the appropriate scale propagation of a rift is a natural consequence of relative motion about a pole of rotation it is not necessarily valid that 1 rifts propagate in one direction or 2 crustal stretching increases with progressive propagation of a rift

one dimensional thermal modeling of the eastern pontides orogenic belt ne turkey the eastern pontides orogenic belt is one of the most complex geodynamic settings in the alpine himalayan belt due to the lack of systematical geological and geophysical data in this study 1d crustal structure and p wave velocity distribution obtained from gravity modeling and seismological data in the area has been used for the development of the thermal model of the eastern pontides orogenic belt the computed temperature depth profiles suggest a temperature of 590 60 c at a moho depth of 35 km indicates the presence of a brittle ductile transition zone this temperature value might be related to water in the subducted crust of the tethys oceanic lithosphere the curie temperature depth value of 29 km which may correspond to the crustal magma chambers is found 5 7 km below the moho depth surface heat flow density values vary from 66 5 and 104 7 mw m 2 high mantle heat flow density value of 48 mw m 2 is obtained for the area should be related to melting of the lithospheric mantle caused by upwelling of asthenosphere

carbonate mineral cycles in ramp slope sediment eastern gulf of mexico carbonate mineral cycles controlled by pleistocene climatic rhythms have been discovered in a 11 6 m long piston core from the west florida carbonate ramp slope at 890 m of water depth in the eastern gulf of mexico relatively high concentrations of aragonite magnesian calcite and dolomite as well as insoluble residues total organics and mud sized sediment all correlate with glacial periods defined by oxygen isotope stratigraphy the aragonite is a low strontium variety derived from pteropods and there is no evidence that concentration variations are controlled by dissolution we propose that these mineral and textural cycles are the product of a combination of increased biological productivity and dilution by fine grained terrigenous mud during glacial episodes aragonite cycles in bahamian periplatform slope sediment have also been correlated with climatic change there the aragonite is a high strontium variety derived from shallow water calcareous algae with relatively high concentrations correlated with high sea levels during interglacial periods the carbonate mineral cycles along the west florida ramp slope are 180 out of phase with bahamian examples and also lack their marked asymmetry it is now apparent that for deepwater continental margin carbonates there are two types of carbonate mineral cyclicity one that is pelagic driven during glacial periods and one that is platform driven during interglacials these contrasts emphasize the differences in deep water cyclic sedimentation on carbonate ramps versus rimmed carbonate platforms that are controlled by physiography sediment source oceanographic setting and proximity to terrigenous input the productivity dilution model proposed here may be a valuable modern analog for ancient carbonate sequences deposited in epicontinental seas or on ramps attached to continental margins

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

platform slope transition during rifting the mid cretaceous succession of the amasya region northern anatolia turkey abstract the mid cretaceous rocks of the amasya region north central anatolia record the geological processes that occurred during the break up of the eastern pontide carbonate platform which existed from mid jurassic to early cretaceous the mid cretaceous sedimentary rocks consists of benthic foraminiferal packstones grainstones of ferhatkaya formation and limestone breccias turbidites pelagic foraminiferal wackestones packstones and radiolarites of sarilar formation corresponding to platform and slope apron environments respectively the lithological development of these units was controlled mainly by active subsidence due to block faulting affecting the eastern pontide carbonate platform due to the break up of the platform during late barremian early aptian times slope and basin sediments were deposited on the deeper parts of the platform re deposited sediments such as limestone breccias and calciturbidites document the infilling of asymmetric grabens during the mid cretaceous rifting phase re deposited sequences which were formed as a fault slope apron sedimentary prism related to this rifting were deposited during thinning and fining upward megacycles a typical megacycle commences with breccias deposited at the base of a fault scarp association in addition the upper part of the platform carbonates ferhatkaya formation includes neptunian dykes composed of red mudstones with pelagic fossils indicating a period of extension

preferential rifting of continents a source of displaced terranes lithospheric rifting while prevalent in the continents rarely occurs in oceanic regions to explain this preferential rifting of continents we compare the total strength of different lithospheres by integrating the limits of lithospheric stress with depth comparisons of total strength indicate that continental lithosphere is weaker than oceanic lithosphere by about a factor of 3 also a thickened crust can halve the total strength of normal continental lithosphere because the weakest area acts as a stress guide any rifting close to an ocean continent boundary would prefer a continental pathway this results in the formation of small continental fragments or microplates that once accreted back to a continent during subduction are seen as displaced terranes in addition the large crustal thicknesses associated with suture zones would ironically make such areas likely locations for future rifting episodes this results in the tendency described as the wilson cycle of new oceans to open along the suture where a former ocean had closed

triassic carbonate submarine fans along the arabian platform margin sumeini group oman the sumeini group formed along the passive continental margin slope that bounded the northeastern edge of the arabian carbonate platform with the initial development of this passive continental margin in oman during early to middle triassic time possibly permian small carbonate submarine fans of the c member of the maqam formation developed along a distally steepened slope the fan deposits occur as several discrete lenticular sequences of genetically related beds of coarsegrained redeposited carbonate calciclastic sediment within a thick interval of basinal lime mudstone and shale repeated pulses of calciclastic sediment were derived from ooid shoals on an adjacent carbonate platform and contain coarser intraclasts eroded from the surrounding slope deposits sediment gravity flows primarily turbidites with lesser debris flows and grain flows transported the coarse sediments to the relatively deep submarine fans channel erosion was a major source of intraformational calcirudite two small submarine fan systems were each recurrently supplied with calciclastic sediment derived from point sources submarine canyons the northern fan system retrogrades and dies out upsection the southern fan system was apparently longer lived calciclastic sediments in it are more prevalent and occur throughout the section the proximal portions of this fan system are dominated by channelized beds of calcirudite which represent inner to mid fan channel complexes the distal portions include mostly lenticular unchannelized beds of calcarenite apparently mid to outer fan lobes carbonate submarine fans appear to be rare in the geological record in comparison with more laterally continuous slope aprons of coarse redeposited sediment the carbonate submarine fans of the c member apparently formed by the funnelling of coarse calciclastic sediment into small submarine canyons which may have developed due to rift and or transform tectonics the alternation of discrete sequences of calciclastic sediment with thick intervals of background sediment resulted from either sea level fluctuations or pulses of tectonic activity

length scales for continental deformation in convergent divergent and strike slip environments analytical and approximate solutions for a thin viscous sheet model the deformation of a thin viscous layer that has a moving boundary is investigated for comparison with zones of deformation in the continental lithosphere exact analytical solutions for the case of a newtonian fluid and approximate solutions for the case of fluid with power law rheology show that when the imposed velocity vector is normal to the boundary congressional or extensional regime the deformation field decays away from the boundary with a characteristic length scale 1 3 to 1 10 the wavelength of the imposed boundary velocity distribution for n between 1 and 10 where n is the stress strain exponent in the rheology in contrast when the imposed velocity vector is parallel to the boundary transcurrent regime the length scale of the deformation field is approximately 4 times smaller in each case these length scales decrease approximately as n the difference in length scales arises even in the absence of any buoyancy forces acting on thickened or thinned crust such forces would modify the ratio of length scales but not sufficiently to affect this result

regional stress field of the indian plate we have calculated the regional stress field in the indian plate implementing dependence of slab pull and ridge push on the age of the oceanic lithosphere in the finite element procedure the high level of the calculated stress field order of a few kbar and the dominance of compression in the plate are consequences of the unique dynamic situation of the present day indian plate the calculated stress field explains the concentration of intraplate deformation in the region of the ninetyeast ridge and yields insight into the variations in stress directions in the australian continent

deep seismic reflection characteristics of the continental crust four types of seismic reflection fabrics appear to characterize the continental crust 1 a cratonic fabric dominated by diffractions and dipping reflections but lacking a pronounced reflection moho 2 regions of thin skinned foreland deformation with abundant shallow but sparse deep reflections 3 a unidirectional fabric that dips toward the hinterland and may represent a crustal scale ramp zone and 4 zones of highly layered lower crust and high amplitude reflection moho the first three types are commonly observed in transects across single mountain belts but the fourth appears to correspond to regions of elevated lower crustal temperatures such as those encountered in rift provinces and perhaps elsewhere

sedimentation cycles and their diagenesis on the slope of a miocene carbonate ramp bahamas odp leg 166 numerous small scale depositional cycles are present in the miocene sediments of seismic sequence m along the margin of great bahama bank odp leg 166 these cycles consist of decimetre to metre scale alternations between light grey and dark grey black wackestones packstones the light grey layers are well cemented and nearly uncompacted they contain planktonic and benthic foraminifera and bioclasts bioturbation in these layers is moderate the dark grey wackestones packstones are uncemented strongly compacted and normally strongly bioturbated the main components are planktonic foraminifera and fine grained bioclasts the dark layers are rich in aragonite and organic carbon and contain around 80 carbonate the light layers show low aragonite and organic carbon contents combined with carbonate values of up to 97 5 light 13c and slightly negative 18o values were observed in the dark uncemented layers while the cemented intervals show heavy 13c values and slightly more positive 18o values the carbon isotope signal between the dark and the light layers shows variations of up to 1 45 both 13c and 18o co vary with carbonate content the formation microscanner images reflect the changes in carbonate mineralogy the natural gamma ray tool shows that variations in siliciclastic content of the examined succession displays a different frequency than the cyclic alternations in carbonate mineralogy the internal stacking pattern of the cycles is closely tied to sea level the dark layers are deposited during rising sea level while the light layers reflect sediment production and export during highstand and falling sea level these cycles thus represent a rather continuous sediment shedding pattern that is clearly related to the ramp morphology and differs from the highstand shedding pattern typical for rimmed flat topped platforms most measured parameters suggest that primary sediment composition played an important role in the cementation process the primary differences in composition were enhanced during shallow burial diagenesis the initially high content of metastable carbonate phases in the periplatform sediments triggered rapid cementation of the primary aragonite rich layers leading to the formation of the light beds dissolution of metastable high mg calcite and aragonite was followed by in situ precipitation of more stable dolomite and calcite diagenesis was enhanced by the coarse grain size of the primary aragonite rich layers in contrast the more pelagic dark calcite rich layers were only slightly affected by diagenesis and could preserve their aragonite content the uncemented layers subsequently were subject to strong mechanical compaction whereas the cemented beds stayed nearly uncompacted

geochemical petrographical and petrophysical evaluations of a heterogeneous stratiform dolomite from a barremian oil field offshore abu dhabi united arab emirates a stratiform dolomite is developed in the barremian carbonates of offshore abu dhabi united arab emirates although the average thickness is only 1 1 m 3 6 ft it is an important drilling target characterized by heterogeneous porosity permeability values in the studied oil field we discuss the origin and development of the dolomite based on geochemical petrographical and petrophysical evaluations using 81 cores recovered from the field the 13c values of the dolomites are relatively high 5 and 87sr 86sr mostly fall in the range of barremian seawater the thin stratiform geometry and the geochemical signatures of the dolomite suggest early dolomitization just below the seafloor that was driven by the diffusion of mg2 from the seawater on top of a shallow platform lateral changes in the degree of dolomitization seem to be controlled by the permeability of the precursors dolomitization progressed further in packstone precursors higher permeability than in wackestone precursors lower permeability which led to greater permeability improvement in the packstone by replacement of fine matrix with larger dolomite crystals subsequently late burial dolomite cementation occurred during oil migration which preferentially affected the higher permeability early dolomite and areally it progressed more in the flank of the field because of the delayed oil charge there dolomite to dolomite recrystallization became dominant after the oil emplacement the recrystallization has been continuing under current burial temperatures 100 c 212 f fully resetting 18o values and modifying the fe and mn concentrations of the dolomites but has not significantly affected their petrography and petrophysics in the oil leg

late proterozoic glacial carbonates in northeast spitsbergen new insights into the carbonate tillite association carbonate rich glacial deposits from two discrete vendian glacial periods are described the older is represented by the 24 40 m thick petrovbreen member e2 of the elbobreen formation which contains abundant detrital dolomite clasts in e2 and their possible source rocks have positive 13 c and negative 18 o pdb values in contrast the carbonate mud fraction of e2 sediments has different cathodoluminescence characteristics from clasts slightly negative 13 c values and higher fe and mn concentrations than clasts oxygen isotopes vary from 2 5 to 4 5 pdb thought to be related to various seawater meltwater mixtures in the depositional environment preservation of information about glacial sedimentary environments is attributed to early diagenetic recrystallization forced by excess surface free energy ostwald s ripening and coinciding with sulphate reduction the younger glaciation is represented by the wilsonbreen formation 170 m which has a distinctive glaciolacustrine middle carbonate member w2 w2 contains precipitated periglacial carbonates with high mn fe ratio both limestone rhythmitic and stromatolitic and dolostone rhythmites stromatolites and dolomite rich sandstones which are compared with carbonates in modern antarctic lakes evaporitic environments for dolomitic sandstones and stromatolitic dolostones are indicated by heavy oxygen isotope values up to 10 5 pdb high na concentration and evidence for dissolved evaporites high mn concentration in detrital dolostones in w2 is suggestive of syn sedimentary dolomite recrystallization in freshwater diamictites and haematitic siltstones the carbonate tillite association ultimately arises from the erosion of underlying carbonates which originated in this case under radically different climatic conditions glacial depositional waters then became carbonate saturated as a result of dissolution of detrital carbonate massive recrystallization of glacially transported carbonate is proposed as a geologically significant process with considerable potential for palaeoenvironmental analysis in glacial lakes carbonate precipitated in response to evaporation or photosynthesis carbonate precipitation as the result of seawater freezing or in warm interglacial conditions is not yet established oxygen isotope value are inconsistent too heavy with the presence of high latitude meltwaters implying that glaciation extended to low latitudes as proposed by harland

continental tectonics in the aftermath of plate tectonics the success of plate tectonics required an acceptance of continental drift and thus a reinterpretation of the large scale geological history of most of the earth but the basic tenet of plate tectonics rigid body movements of large plates of lithosphere fails to apply to continental interiors where buoyant continental crust can detach from the underlying mantle to form mountain ranges and broad zones of diffuse tectonic activity

late cretaceous early eocene tectonic development of the tethyan suture zone in the erzincan area eastern pontides turkey six individual tectonostratigraphic units are identified within the izmir ankara erzincan suture zone in the critical erzincan area of the eastern pontides the ayikayasi formation of campanian maastrichtian age is composed of bedded pelagic limestones intercalated with polymict massive conglomerates the ayikayasi formation conformably overlies the tauride passive margin sequence in the munzur mountains to the south and is interpreted as an underfilled foredeep basin the refahiye complex of possible late cretaceous age is a partial ophiolite composed of 75 by volume serpentinized peridotite mainly harzburgite 20 diabase and minor amounts of gabbro and plagiogranite the complex is interpreted as oceanic lithosphere that formed by spreading above a subduction zone unusual screens of metamorphic rocks e g marble and schist locally occur between sheeted diabase dykes the upper cretaceous karayaprak melange exhibits two lithological associations 1 the basalt radiolarite serpentinite association including depleted arc type basalts 2 the massive neritic limestone lava volcaniclastic association that includes fractionated intermediate composition lavas and is interpreted as accreted neotethyan seamount s the several kilometre thick karadag formation of campanian maastrichtian age is composed of greenschist facies volcanogenic rocks of mainly basaltic to andesitic composition and is interpreted as an emplaced upper cretaceous volcanic arc the campanian early eocene sutpinar formation 1500 m thick is a coarsening upward succession of turbiditic calcarenite sandstone laminated mudrock volcaniclastic sedimentary rocks that includes rare andesitic lava and is interpreted as a regressive forearc basin the late paleocene eocene sipikor formation is a laterally varied succession of shallow marine carbonate and siliciclastic lithofacies that overlies deformed upper cretaceous units with an angular unconformity structural study indicates that the assembled accretionary prism supra subduction zone type oceanic lithosphere and volcanic arc units were emplaced northwards onto the eurasian margin and also southwards onto the tauride gondwana related margin during campanian maastrichtian time further mainly southward thrusting took place during the eocene in this area related to final closure of tethys our preferred tectonic model involves northward subduction supra subduction zone ophiolite genesis and arc magmatism near the northerly eurasian margin of the mesozoic tethys

tectonostratigraphic evolution of the upper cretaceous cenozoic central anatolian basins an integrated study of diachronous ocean basin closure and continental collision

role of a thin komatiite rich oceanic crust in the archean plate tectonic process in the archean tectonic regime the time available for oceanic lithosphere to cool on its journey from ridge to convergent plate boundary was far less than at present under these conditions only an inherently dense lithosphere could have acquired the negative buoyancy necessary for it to subduct at this time plate tectonics could have operated only if 1 the oceanic crust was rich in komatiite lava and 2 the thickness of this crust was not much greater than the thickness of modern oceanic crust

passive margin evolution initiation of subduction and the wilson cycle we have constructed finite element models at various stages of passive margin evolution in which we have incorporated the system of forces acting on the margin depth dependent rheological properties and lateral variations across the margin we have studied the interrelations between age dependent forces geometry and rheology to decipher their net effect on the state of stress at passive margins lithospheric flexure induced by sediment loading dominates the state of stress at passive margins this study has shown that if after a short evolution of the margin time span a few tens of million years subduction has not yet started continued aging of the passive margin alone does not result in conditions more favourable for transformation into an active margin although much geological evidence is available in support of the key role small ocean basins play in orogeny and ophiolite emplacement evolutionary frameworks of the wilson cycle usually are cast in terms of opening and closing of wide ocean basins we propose a more limited role for large oceans in the wilson cycle concept