strain state and kinematic evolution of a fold nappe beneath the samail ophiolite oman abstract the saih hatat domal culmination of the oman mountains arabian peninsula contains a major refolded ne facing recumbent anticlinal fold nappe within pre ordovician sedimentary cover autochthon and mesozoic platform carbonates that underlie the samail and hawasina nappes tracts of overturned stratigraphy covering over a 1000 km 2 are associated with a marked strain gradient towards sheath like a type folds that define the fold nappe lower limb transitional into an elongated sw facing recumbent syncline and the underlying upper plate lower plate shear zone directions of stretching recorded by fibrous pressure shadow fringes on pyrite show a 20 30 counterclockwise rotation in the foliation plane strain and kinematic constraints require the fold nappe to form progressively during top to the ne shearing of the arabian margin with 1 development of a high strain zone and 2 subsequent folding of the strongly deformed to transposed stratigraphy in the hanging wall to the shear zone rolling of the autochthonous stratigraphic layering of the upper limb through the hinge accompanied by differential shear strain produced the regional scale highly attenuated a type folds at the structurally lowest levels this occurred by simple underthrusting of the footwall lower plate as the upper limb of the fold nappe has to be pinned to the autochthon

melange genesis and ophiolite emplacement related to subduction of the northern margin of the tauride anatolide continent central and western turkey the tauride anatolide continent stretching for c 900 km across western and central turkey is one of the world s best example of a subducted exhumed passive margin within a collisional orogen twelve widely separated areas were studied and correlated to develop a new plate tectonic model a metamorphosed rifted continental margin of triassic lower cretaceous age tauride anatolide platform is overlain by upper cretaceous cenomanian lower maastrichtian pelagic sediments and then by both tectonic melange subduction complexes and sedimentary melange foredeep gravity complexes the melanges are overthrust by unmetamorphosed ophiolitic rocks commonly peridotites with swarms of diabase gabbro dykes and are underlain by metamorphic soles new geochemical evidence from basaltic blocks in the melange indicates predominantly subduction influenced within plate and mid ocean ridge type settings the dykes cutting the ophiolites were probably intruded during early stage intra oceanic arc genesis the metamorphosed continental margin melanges and ophiolites in the north anatolides are correlated with unmetamorphosed equivalents in the taurides further south e g beysehir and lycian nappes oceanic crust of triassic late cretaceous age formed between the gondwana related tauride anatolide continent in the south and the eurasia related sakarya microcontinent in the north following late triassic early cretaceous passive margin subsidence the continental margin was covered by cenomanian turonian pelagic carbonates c 98 90 ma ophiolites formed in an intra oceanic subduction zone setting in response to northward subduction probably within a two stranded ocean with the inner tauride ocean in the se and the izmir ankara erzincan ocean in the north nw metamorphic soles relate to intra oceanic subduction c 95 90 ma oceanic sedimentary igneous rocks accreted to the advancing supra subduction oceanic slab the tauride anatolide continental margin then underwent diachronous collision with the trench c 85 ma deeply subducted and metamorphosed at hp lt c 80 ma accretionary ophiolitic and exhumed hp lt rocks were gravity reworked into a southward migrating flexural foredeep and progressively overridden c 70 63 ma slices of the upper part of the platform and its margin detached and were thrust southwards as the tauride lycian and beysehir nappes together with regional scale ophiolites the continental margin and melange were simultaneously exhumed during maastrichtian early paleocene 70 63 ma and transgressed by shallow water sediments beginning in the late maastrichtian in the east c 64 ma and the mid late paleocene c 60 ma further west remnant oceanic crust was consumed during early cenozoic time followed by mid eocene 45 40 ma diachronous continental collision and a second phase of regional deformation rather than being progressive there were two stages of collision first upper cretaceous ophiolite emplacement driven by continental margin subduction trench collision and secondly eocene collision of the tauride and sakarya eurasian continents

birth death and resurrection the life cycle of suprasubduction zone ophiolites 1 suprasubduction zone ssz ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites this sequence includes the following 1 birth which entails the formation of the ophiolite above a nascent or reconfigured subduction zone this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex 2 youth during which is continued melting of refractory asthenosphere depleted during birth occurs in response to fluid flux from the subducting slab with extensional deformation of the older plutonic suite eruption of refractory lavas and the intrusion of wehrlite pyroxenite 3 maturity with the onset of semistable arc volcanism typically calc alkaline as the subduction zone matures and stabilizes and the intrusion of quartz diorite and eruption of silicic lavas and 4 death which is the sudden demise of active spreading and ophiolite related volcanism which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites 5 resurrection with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction the early stages 1 3 may be diachronous and each stage may overlap in both time and space the existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the ssz tectonic setting

alpine and pacific styles of phanerozoic mountain building subduction zone petrogenesis of continental crust a broad continuum exists between two distinct end member types of mountain building alpine type orogenic belts develop during subduction of an ocean basin between two continental blocks resulting in collision they are characterized by an imbricate sequence of oceanward verging nappes some alpine belts exhibit superimposed late stage backthrusting sediments are chiefly platform carbonates and siliciclastics in some cases associated with minor amounts of bimodal volcanics pre existing granitic gneisses and related continental rocks constitute an autochthonous parautochthonous basement metamorphism of deeply subducted portions of the orogen ranges from relatively high pressure hp to ultrahigh pressure uhp calcalkaline volcanic plutonic rocks are rare and have peraluminous s type bulk compositions in contrast pacific type orogens develop within and landward from long sustained oceanic subduction zones they consist of an outboard oceanic trench accretionary prism and an inboard continental margin island arc the oceanic assemblage consists of first cycle in part melanged volcaniclastics and minor but widespread cherts deep water carbonates intimately mixed with disaggregated ophiolites the section recrystallized under hp conditions recumbent fold vergence is oceanward a massive slightly older to coeval calcalkaline arc is sited landward from the trench complex on the stable non subducted plate it consists of abundant dominantly intermediate metaluminous i type volcanics resting on old crust both assemblages are thrown into open folds intruded by comagmatic i type granitoids and metamorphosed locally to regionally under high t low p conditions in the subduction channel of collisional and outboard circumpacific terranes combined extension above and subduction below allows buoyancy driven ascent of ductile thin aspect ratio slices of hp uhp complexes to midcrustal levels where most closely approached neutral buoyancy exposure of rising sheets caused by erosion and gravitational collapse results in moderate amounts of sedimentary debris because exhumed sialic slivers are of modest volume at massive sialic buildups associated with convergent plate cusps syntaxes tectonic aneurysms may help transport hp uhp complexes from mid to upper crustal levels the closure of relatively small ocean basins that typify many intracratonic suture zones provides only limited production of intermediate and silicic melts so volcanic plutonic belts are poorly developed in alpine orogens compared with circumpacific convergent plate junctions generation of a calcalkaline arc mainly depends on volatile evolution at the depth of magma generation phase equilibrium studies show that under typical subduction zone p t trajectories clinoamphibole ca al hydrous silicates constitute the major hydroxyl bearing phases in deep seated metamorphic rocks of morb composition other hydrous minerals are of minor abundance ca and na clinoamphiboles dehydrate at pressures of above approximately 2 gpa but low temperature devolatilization may be delayed by pressure overstepping thus metabasaltic blueschists and amphibolites expel h2o at melt generation depths and commonly achieve stable eclogitic assemblages partly serpentinized mantle beneath the oceanic crust dehydrates at roughly comparable conditions for reasonable subduction zone geothermal gradients however white micas biotites remain stable to pressures 3 gpa accordingly attending descent to depths of 100 km mica rich quartzofeldspathic lithologies that constitute much of the continental crust fail to evolve substantial amounts of h2o and transform incompletely to stable eclogite facies assemblages underflow of amphibolitized oceanic lithosphere thus generates most of the deep seated volatile flux and the consequent partial melting to produce the calcalkaline suite along and above a subduction zone where large volumes of micaceous intermediate and felsic crustal materials are carried down to great depths volatile flux severely diminishes thus continental collision in general does not produce a volcanic plutonic arc whereas in contrast the long continued contemporaneous underflow of oceanic lithosphere does

paleoecology of a 90 000 year lacustrine sequence from fog lake baffin island arctic canada a 137 cm sediment core from fog lake a small upland basin that was not glaciated during the late wisconsinan reveals the following stratigraphic succession i basal diamicton ii laminated silts iii compacted dewatered gyttja iv stratified minerogenic sediments and v overlying gyttja units i iii and most of unit iv were deposited prior to the holocene unit v represents approximately the last 8000 14c years alternate luminescence dating techniques tl and irsl place unit iii in latter oxygen isotope stage ois 5 74 000 years bp a suite of 20 ams 14c dates suggests that much of the post ois 5 paleoenvironmental record is characterized by either extremely low continuous sediment accumulation or by one or more episodes of more rapid sedimentation diverse pollen and spore assemblages have concentrations that are unexpectedly high throughout the pre holocene sediments 10 000 grains cm 3 by comparison to the holocene 5000 grains cm 3 exceedingly low sediment accumulation rates appear to have resulted in the concentration of atmospherically derived pollen and spores diatom assemblages record the evolution of the lake s chemistry and past algal production several abrupt changes occurred synchronously in both the terrestrial and aquatic components of the system and these appear to represent ecological responses to pre holocene climatic variability the sediments of fog lake provide the first integrated paleoecological record older than holocene for a vast region of the canadian arctic thereby refining our understanding of long term arctic ecosystem ontogeny 2000 elsevier science ltd all rights reserved

instability of amuay cliffside this paper presents a case study of cliffside instability geotechnical analyses employing field measurements and laboratory test results obtained over a 30 yr period indicate that rising pore pressures trigger slides along the cliff visual observation surveys and field exploration show that most of the landslides have wedge type failure geometry shear occurs through a nearly horizontal stratum of weak clay brown fat clay the strength of the fat clay decreases with deformation fluids generated by domestic and industrial activities have resulted in a perched water layer above the stratum of fat clay for more than two decades the pore pressure on top of the fat clay has risen a wedge type of stability analysis can give the correct factor of safety i e 1 0 for the slides which have occured asce

analysis of a sample containing a shear surface from a recent landslip south cotswolds uk the paper reports analysis of a block sample taken from a trial pit hand dug across the shear surface of an active landslide in the south cotswolds the slide is located on the upper fuller s earth some 50m downslope of the overlying great oolite limestone the weathering of which has produced ooliths and gravel sized fragments which are incorporated within the slipped mass sampling at 5 mm intervals confirmed the inverse relationship between the natural moisture content and calcite content in order to assess the significance of changes in moisture content in the fine fraction the influence of the non sorptive calcite was removed and a normalized moisture content profile established examination of the lower shear surface using scanning electron microscopy confirmed the parallel orientation of the clay minerals for a thickness of some 1 m this thin veneer extended over areas of granular material highlighting the importance of obtaining representative samples when shear strength assessment is under

eye to eye with a mega sheath fold a case study from wadi mayh northern oman mountains sheath folds are highly curvilinear folds typically considered to develop by hinge rotation toward the transport direction during intense deformation most sheath folds are recognized on the centi meter to meter scale with very few detailed studies of larger kilometer scale structures we here present evidence for a megascale sheath fold superbly exposed in the northern oman mountains the saih hatat culmination consists of an exhumed highpressure subduction zone with deep level eclogites and garnet blueschists as sifah unit epidote blueschists hulw unit and carpholite bearing metasediments part of the upper plate sheath folds are present at a variety of scales throughout the as sifah hulw and upper plate tectonic packages curvilinear fold hinges along a 15 km long profi le outline the wadi mayh mega sheath fold which closes to the ssw and faces to the nne it consists of at least four secondary sheath folds with cross sectional y z eye folds measur ing 200 m stacked on top of one another within permian and triassic shelf car bonates these eye folds display highly elliptical geometries ryz 5 and cat s eye fold patterns consistent with simple general shear the sheath fold is enveloped by ordovician and older rocks that do not show equal amounts of internal deformation the upper bounding envelope is a detachment that has been subsequently sheath folded the lower boundary of the sheath fold is the upper plate lower plate detachment which forms a major high strain shear zone showing little vertical displacement but signifi cant horizontal displacement the sheath fold displays a ssw elongation x axis of at least 15 km and possibly up to 25 km coupled with signifi cant amounts of vertical fl attening and wnw ese shortening

partial melting and deformation the physical properties of partially molten rocks are directly coupled to the grain scale as well as the broader scale distribution of the melt phase at the grain scale if melt forms isolated pockets in a silicate matrix its influence on plastic flow is generally relatively minor in contrast if the melt wets all of the grain grain interfaces it may dramatically lower the viscosity of the rock at the broader scale deformation will localize to form shear zones in melt rich regions that commonly develop in partially molten rocks in turn the melt distribution evolves during plastic deformation of a partially molten rock melt located in triple junctions takes on a pronounced preferred orientation in addition melt often segregates during plastic deformation to form melt enriched bands separated by melt depleted regions this chapter develops two interrelated themes with emphasis on the relationship between melt distribution structure and the plastic flow property of partially molten rocks the first section concentrates on thermodynamic constraints and experimental observations on the distribution of melt in a non deforming rock that is in a partially molten rock exposed simply to a hydrostatic state of stress the second section then builds on these boundary conditions to introduce theoretically predicted and experimentally determined flow laws describing plastic deformation of partially molten rocks finally the third section examines the influence of deformation i e a non hydrostatic state of stress on melt distribution and the associated implications for the rheological properties of partially molten crustal and mantle rocks under a hydrostatic state of stress the melt distribution in a partially molten rock is governed by the relative values of the solid melt and solid solid interfacial energies sm and ss nb although the term melt is used throughout this paper the designation fluid or liquid could equally well be substituted the melt distribution

rational design treatment of slides in overconsolidated clays and clay shales during the early part of the past century engineers were repeatedly surprised by unforeseen movements of valley slopes on overconsolidated flat bedded clays and clay shales movements have occurr

tectonics of the arabian margin associated with the formation and exhumation of high pressure rocks sultanate of oman continental crustal rocks now structurally beneath the allochthonous samail ophiolite underwent blueschist to eclogite facies metamorphism prior to the emplacement of the oman ophiolite onto the arabian margin the recognition of a major low angle fault within this polydeformed and polymeta morphosed sequence of metacarbonates metabasites quartzites and quartz mica schists greatly simplifies the interpretation of the structure and metamorphic zonation within the saih hatat window ne oman carpholite bearing upper plate rocks consist of pre permian to jurassic rocks that have been folded into large recumbent nappe structures which exhibit a marked increase in deformation intensity toward the boundary with the lower plate the lower plate rocks have higher peak pressure and temperature assemblages they are exposed in two windows separated by jabal abu daud although more intensely deformed the lower plate rocks are recognizable as metamorphosed continental platform sediments fold axes are parallel to the regional nne ssw lineation sense of shear indicators yield a transport direction of south over north in the lower plate opposite to the sense of motion inferred for the emplacement of the ophiolite lower plate eclogite facies metabasalts are only preserved in kilometer scale megaboudins found in the easternmost window at as sifah these metamorphic assemblages along with their remnant east west fabrics define the existence of a short lived arabian platform directed nascent subduction zone the stretching lineation elsewhere within the upper and lower plates the hatat schist the basement and the metamorphic sole of the ophiolite is consistently nne ssw suggesting that the exhumation of high pressure metamorphic rocks of saih hatat is related to the ophiolite obduction exhumation of the high pressure rocks was accompanied by intense deformation involving regional scale fold nappes in a convergent margin setting the geotherm remained suppressed for a period 30 myr greater than the thermal relaxation time of the crust the geometric and thermal constraints from oman may be applicable to the general problem of the formation and preservation of high pressure low temperature rocks

formation and exhumation of blueschists and eclogites from ne oman new perspectives from rb sr and 40ar 39ar dating seven eclogite facies samples from lithologically different units which structurally underlie the semail ophiolite were dated by the 40 ar 39 ar and rb sr methods despite extensive efforts phengite dated by the 40 ar 39 ar method yielded saddle hump or irregularly shaped spectra with uninterpretable isochrons the total gas ages for the phengite ranged from 136 to 85 ma clinopyroxene phengite epidote phengite and whole rock phengite rb sr isochrons for the same samples yielded ages of 78i2 ma we therefore conclude that the eclogite facies rocks cooled through 500 u ca tc 7 8i2 ma and that the 40 ar 39 ar dates can only constrain maximum ages due to the occurrence of excess ar inhomogeneously distributed in different sites our new results lead us to conclude that high pressure metamorphism of the oman margin took place in the late cretaceous contemporaneous with ophiolite emplacement previously published structural and petrological data lead us to suggest that this metamorphism resulted from intracontinental subduction and crustal thickening along a ne dipping zone choking of this subduction zone followed by ductile thinning of a crustal mass wedged between deeply subducted continental material and overthrust shelf and slope units facilitated the exhumation of the eclogite facies rocks from depths of c 50 km to 10 15 km within c 10 ma and led to their juxtaposition against overlying lower grade rocks final exhumation of all high pressure rocks was driven primarily by erosion and assisted by normal faulting in the upper plate

fourth canadian geotechnical colloquium strength and slope stability in canadian soft clay deposits it is generally admitted that the long term stability of clay slopes should be analysed in terms of effective stresses which necessitates the knowledge of the pore pressure conditions and of the relationship between the shear strength and the effective stresses this paper deals with the determination of shear strength for stability analysis of clay slopes in eastern canada first the various concepts pertinent to slope stability in soft clay are examined and a certain justification of the use of the post peak or large strain strength is demonstrated in a second part 14 case records of natural slopes or permanent excavation slopes are presented and analysed in terms of the post peak strength finally values of post peak strength determined in the laboratory for more than 30 sites are presented and it is attempted to derive correlations by which the shear strength parameters could be readily approximated from other properties such as the apparent preconsolidation pressure or the vane strength

subduction zone polarity in the oman mountains implications for ophiolite emplacement two end member models have been proposed to account for the structure and metamorphism of rocks beneath the semail ophiolite in the oman mountains model a involves a single continuous ne directed subduction away from the continental margin during the late cretaceous the ophiolite and underlying thrust sheets of distal to proximal oceanic sediments were emplaced a minimum of 250 km sw onto the continental margin subduction of triassic jurassic oceanic basalts to c 10 kbar c 39 km depth led to the accretion of amphibolite facies rocks to the base of the ophiolite thrusting propagated towards the continental margin and ended with subduction of the thinned continental crust to c 20 kbar c 78 km depth choking the subduction zone buoyancy forces caused the rapid exhumation of eclogites blueschists and carpholite grade hp rocks along the ne margin of the continental plate during the later phase of foreland propagating thin skinned thrusting in the sw ne facing backfolding and backthrusting occurred in the hinterland with the final exhumation of the hp rocks model b follows recent suggestions that a nascent sw dipping subduction zone dipping beneath the continental margin existed between 130 and 95 ma prior to formation and emplacement of the ophiolite a major ne facing fold nappe structure in the pre permian basement rocks of saih hatat is interpreted as reflecting subduction beneath the margin two high pressure metamorphic events have been suggested the first predating ophiolite emplacement the second caused by ophiolite loading this model is untenable being based on a misinterpretation of the ne facing structures in northern saih hatat and on some dubious older 40ar 39ar cooling ages from the eclogite facies rocks of as sifah we conclude that all structures in northern oman and all the reliable geochronology point to a single emplacement obduction event lasting from cenomanian turonian time c 95 ma when amphibolites were accreted along the metamorphic sole of the ophiolite to campanian time when the continental margin was subducted to the ne producing blueschists and eclogites to the final thin skinned emplacement of all thrust sheets which ended before the late maastrichtian at c 68 ma

delayed collapse of cut slopes in stiff clay delayed failure of railway cutting slopes in stiff clays excavated in the nineteenth and early twentieth centuries has been studied for some forty years known failures have generally been deep seated more recently failures have been a problem in the slopes of motorway cuttings and embankments although generally these have been shallow the average operational strength at failure in these slides has been significantly less than the peak strength and progressive failure has been postulated as the probable cause of this progressive failure can now be analysed using advanced numerical techniques a series of coupled finite element analyses have been conducted assuming strain softening soil with properties based on the brown london clay and the results are reported they show that progressive failure is considerable and fully explain the observed field behaviour the delays experienced in the field are also recovered by the analyses progressive failure is generated primarily by the high lateral stresse

comment on structural evolution metamorphism and restoration of the arabian continental margin saih hatat region oman mountains by m p searle et al

structural and stratigraphic controls on the origin and tectonic history of a subducted continental margin oman eclogites and blueschists exposed in saih hatat oman record the subduction and exhumation of continental crustal material beneath the cretaceous semail ophiolite during ophiolite obduction the eclogite bearing lower plate originally part of oman s distal continental margin is exposed in two tectonic windows through the less metamorphosed upper plate by a previously mapped low angle high strain decollement structure a major tectonic break currently poorly exposed records the juxtaposition of the highest pressure eclogites and garnet blueschists against lower pressure epidote blueschists the subsequent exhumation of the entire lower plate to mid crustal levels is marked by a pervasive shearing event associated with a regional greenschist facies overprint the decollement truncates structures and the metamorphic field gradient in the lower plate but does not significantly truncate structures or stratigraphy in the upper plate it is not responsible for the exhumation of the high pressure rocks to mid crustal levels most of the displacement across this structure was accommodated during continuing convergence after the subduction system had ceased to be active and post ophiolite emplacement onto the platform carbonate sequences a revised tectonic model is presented which accounts for the structural geochronological and metamorphic observations

exhumation of high pressure rocks in northeastern oman the exhumation of high p rocks in the northeast saih hatat window oman mountains formed regional nappes associated with greenschist facies metamorphism exhumation resulted in the juxtaposition of two plates with different structural style stratigraphy and metamorphic history a major crustal discontinuity separates the two plates with lower metamorphic grade rocks in the hanging wall lower plate units were exhumed during strong top to the northeast shearing resulting in the formation of regional closures that evolved by folding and transposition of the high pressure fabrics while the lithostatic pressure was decreasing upper plate units were later exhumed with the juxtaposition of the two plates truncation of lower plate closures and the formation of upper plate nappes that are attenuated and sheathlike toward the upper and lower plate boundary in some high p terranes apparent increases in metamorphic grade down structural section and low grade c type shear bands are often attributed to extensional exhumation in oman however these exhumation features formed during crustal scale contraction at a convergent margin the low geothermal gradient regimes afforded by convergent margins are critical for the generation and preservation of high p low t rocks

slope stability analyses in stiff fissured clays results of torsional ring shear direct shear and triaxial compression tests on cohesive soils reveal that the fully softened shear strength is stress dependent and related to the type of clay mineral and quantity of clay size particles an empirical relationship for the fully softened friction angle is presented that is a function of liquid limit clay size fraction and effective normal stress studies of first time slides i e slopes that have not undergone previous sliding in stiff fissured clay with a liquid limit between 41 and 130 suggest that the mobilized shear strength along the failure surface can be lower than the fully softened shear strength recommendations are presented for estimating the mobilized shear strength in first time slides based on soil plasticity soils with a liquid limit greater than 30 exhibit a large difference between the fully softened and residual friction angles in these soils the presence or absence of a pre existing shear surface should be clarified

transition from subduction to exhumation related fabrics in glaucophane bearing eclogites oman evidence from relative fabric chronology and 40ar 39ar ages abstract controversy over the age of peak metamorphism and therefore the tectonic evolution of the arabian margin relates to the polydeformed and polymetamorphosed nature of glaucophane bearing eclogites from the saih hatat window beneath the allochthonous samail ophiolite in ne oman these eclogites contain relicts of earlier fabrics structures and metamorphic assemblages and provide a record of change from subduction to exhumation the eclogites are part of a mafic layer that was disrupted into boudins up to 0 5 km in length within a lower plate shear zone as sifah shear zone the megaboudins not only preserve the relicts of the highest grade of metamorphism but also an early ene trending lineation and sheathlike isoclines enveloped by the flat lying schistosity the boudin bearing layer is isoclinally folded with calc schist mafic schist and quartz mica schist where the regional folds have axes parallel to the ne trending stretching lineation a type folds textural evidence suggests multiple growth events for garnet and clinopyroxene requiring polymetamorphism of the mafic layers that formed the eclogite megaboudins the surrounding calc schist and quartz mica schist are both intensely deformed with transposition foliation containing an ne trending lineation in phengite and asymmetric shear indicators such as c type shear bands and asymmetric pressure shadows around garnets that give top to the ne sense of shear although consistent ene trending lineations in all the boudins suggest that they have largely acted as passive nonrotating rigid bodies the presence of ne vergent asymmetric mesofolds extensive dynamic recrystallisation multiple generations of phengites and a range of 40ar 39ar apparent ages within the megaboudins suggest however that they have not acted entirely passively during the later deformation phengites isolated from the high p low t fabrics show groupings in 40ar 39ar apparent ages interpreted as distinct metamorphic cooling intervals at 140 135 120 98 and 92 80 ma microstructural relations suggest that age groupings younger than 100 ma reflect phengite growth during exhumation with the top to the ne shearing the older ages 120 110 ma from fabrics that give top to the s shear sense may reflect growth during the subduction phase the combination of groupings of apparent argon ages older than the crystallisation age of the samail ophiolite the suggestion of different geothermal gradients and superposed metamorphism suggest that the eclogites and garnet blueschists formed as a result of underthrusting along a break that was not directly related to the metamorphic sole of the ophiolite the glaucophane eclogites are interpreted as having formed at different times under varying pressure temperature conditions during underthrusting with variations in the rate of underthrusting allowing thermal equilibration and or rapid cooling at different crustal levels

the jabal akhdar dome in the oman mountains evolution of a dynamic fracture system the mesozoic succession of the jabal akhdar dome in the oman mountains hosts complex networks of fractures and veins in carbonates which are a clear example of dynamic fracture opening and sealing in a highly overpressured system the area underwent several tectonic events during the late cretaceous and cenozoic including the obduction of the samail ophiolite and hawasina nappes followed by uplift and compression due to the arabia eurasia convergence this study presents the results of an extensive tectonic survey and correlates subseismic scale structures in jabal akhdar faults fractures veins and stylolites with the main tectonic events in the northeastern arabian plate as some of the studied formations host large oil reserves in neighboring areas determining the relative timing of these events in the exhumed rocks is important to understand hydrocarbon distribution and fracture patterns in these reservoirs the formation of early veins and stylolites in the oman mountains is followed by top to the south layer parallel shearing that may be associated with the obduction of the samail and hawasina nappes this compressional tectonic event is followed by normal dip slip to oblique slip faults and veins top to the northeast layer parallel shearing which corresponds to the first stage of exhumation of the autochthonous rocks offsets these structures our new data indicate that this first phase of events is overprinted by complex strike slip networks of veins and fractures as well as by the reactivation and onset of seismic scale faults strike slip structures belong to three distinct events the first one nw se oriented compression is probably associated with the oblique collision of the indian plate against the arabian platform during the late campanian to the mid eocene the second event e w oriented compression is likely to have been formed during the late oligocene middle miocene during uplift the last event ne sw oriented compression probably took place during the miocene pliocene structures of the first two strike slip events have the same orientation as seismic scale faults observed in the subsurface of oman and abu dhabi in addition increasing vein intensity towards the top of the autochthonous formations in the oman mountains as well as the small angle between conjugate vein sets indicate that high fluid pressures that are thought to be present during strike slip deformation

new high precision p t estimates for oman blueschists implications for obduction nappe stacking and exhumation processes oman blueschists and eclogites lie below the obduction nappe of the semail ophiolite in one of the key areas on earth for the study of plate convergence here new metamorphic and tectonic constraints are provided for the central yet poorly constrained hulw unit sandwiched between the low grade units 10 kbar 300 c and the as sifah eclogites pmax 23 kbar tmax 600 c tweequ multi equilibrium thermobarometry using both compositional mapping and spot analyses and raman spectroscopy of carbonaceous material yield a high precision p t path for the hulw and as sheikh units and reveal that they shared a common p t history in four stages i a pressure decrease from 10 12 kbar 250 300 c to 7 9 kbar 300 350 c ii almost isobaric heating at 8 10 kbar from 300 350 c to 450 500 c iii a pressure decrease at moderate temperatures 450 500 c and iv isobaric cooling at 5 6 kbar from 450 500 to 300 c no significant pressure or temperature gap is observed across the upper plate lower plate discontinuity to the north and west of the hulw unit the combination of tectonic and p t data constrains the stacking chronology of the three main metamorphic units comprising the saih hatat window i e the ruwi quryat the hulw as sheikh and the diqdah as sifah units these results strengthen the view that the tectonic and metamorphic data are conveniently accounted for by a simple n vergent continental subduction of the passive arabian margin below the obduction nappe along a cold p t gradient

stress induced response from both natural and construction related processes in the deepening of the peace river valley b c the peace river rises in the rocky mountain trench and flows easterly through the rocky mountains before flowing onto the plains in northeastern british columbia near the alberta border stress measurements conducted at two of three damsites near the boundary of the foothills and the plains reveal high remnant lateral stresses and a k0 of about 2 in proceeding 90 km downriver from the w a c bennett dam past the peace canyon dam to the proposed site c dam near fort st john the stratigraphic section changes from competent massive sandstones of lower cretaceous age to soft weak shales at the top of the lower cretaceous section valley downcutting by the peace river through these highly stressed rocks has led to valley rebound features that have both similarities and differences where observed at each of the three damsites differences in rock strength at each site contribute to the variations in features noted these valley rebound features include vertical relief joints horizontal bedding plane join

multiphase structural evolution of a continental margin during obduction orogeny insights from the jebel akhdar dome oman mountains the structural evolution of the carbonate platform in the footwall of the semail ophiolite emplaced onto the passive continental margin of arabia helps to better understand the early stages of obduction related orogens these early stages are rarely observable in other orogens as they are mostly overprinted by later mountain building phases we present an extensive structural analysis of the jebel akhdar anticline the largest tectonic window of the oman mountains and integrate it on different scales outcrop observations can be linked to plate motion data providing an absolute timeframe for structural generations consistent with radiometric dating of veins top to s overthrusting of the semail ophiolite and hawasina nappes onto the carbonate platform during high plate convergence rates between arabia and eurasia caused rapid burial and overpressure generation and migration of hydrocarbons and bedding confined veins but no major deformation in the carbonate platform at reduced convergence rates subsequent tectonic thinning of the ophiolite took place above a top to nne crustal scale ductile shear zone deforming existing veins and forming a cleavage in clay rich layers in early campanian times ongoing extension occurred along normal to oblique slip faults forming horst graben structures and a precursor of the jebel akhdar dome campanian to maastrichtian this was followed by ne sw oriented ductile shortening and the formation of the jebel akhdar dome deforming the earlier structures thereafter exhumation was associated with low angle normal faults on the northern flank of the anticline we correlate the top to nne crustal scale shear zone with a similar structure in the saih hatat window to develop a unified model of the tectonic evolution of the oman mountains

thermal regime of continental subduction the record from exhumed hp lt terranes new caledonia oman corsica abstract unlike the thermal structures of present day subduction or collision zones the variety of possible thermal evolutions or thermal regimes accompanying the transient stage of continental subduction i e the shift from oceanic subduction to continental collision remains poorly known we herein show that the thermal regime of continental subduction can be confidently retrieved from three well documented fossil settings i e from high pressure low temperature continental material from oman new caledonia corsica that were not modified by later collision or a later metamorphic imprint based on recently published data we show the striking similarity of the overall thermal and tectonic patterns of these three hp belts derived from the subduction of a continental margin these natural examples suggest that the thermal regime of continental subduction is largely invariant through time hence suggesting a steady state thermal structure and independent from the initial geodynamic setting i e from the initial thermal structure of oceanic subduction the nature of the incoming plate or of the upper plate we propose that continental cover units subducted over a short time period i e 10 my represent cold underplated material that buffers the subduction thermal regime whatever the exact structure nature or thermal structure of incoming material similarities in the type size and p t conditions of the various tectonic units and in the overall tectonic organization point to specific accretionary type subduction dynamics yet to differences in long term mechanical coupling between the three case studies our study thereby provides constraints on exhumation dynamics and models of continental subduction

structural evolution metamorphism and restoration of the arabian continental margin saih hatat region oman mountains abstract 170 m y of relatively stable passive margin sedimentation along the northern continental margin of arabia was abruptly terminated during the cenomanian turonian 95 ma when the oman continental margin collapsed and subsided rapidly aruma basin to accommodate obduction of the semail ophiolite complex and underlying thrust sheets haybi and hawasina complexes in the oman mountains the ophiolite was emplaced at least 200 km over the passive margin probably over 450 km in total from ne to sw during the later stages of obduction the leading edge of the continental margin was subducted to depths where carpholite bearing rocks 6 8 kbar blueschist 12 15 kbar and eclogite ca 20 kbar facies metamorphism formed in a ductile deforming ne dipping subduction zone along the northeastern margin of the saih hatat culmination in ne oman the mesozoic shelf carbonates are deformed by recumbent folding on all scales the present day structure is dominated by four sw vergent thrust slices in carpholite grade rocks at higher structural levels overlying lower units of crossite bearing blueschist hulw unit garnet glaucophane chloritoid blueschists and garnet clinopyroxene phengite glaucophane eclogites 15 20 kbar the largest shear zone separates 7 8 kbar hulw unit blueschists above from 12 15 kbar retrogressed eclogites and ca 20 kbar eclogites of the as sifah unit below further inboard sw a major shear zone separates the lower plate hulw blueschist facies rocks from the upper plate sedimentary rocks upper plate lower plate discontinuity which also show carpholite grade hp metamorphism towards the ne restoration of the entire saih hatat continental margin shows that the upper plate lower plate discontinuity cuts down section to the sw as far as the proterozoic hatat schists ne directed extensional crenulation schistosity and nne oriented stretching lineations in the eclogite and blueschist facies rocks are consistent with sw directed exhumation of footwall hp rocks ne facing folds and spectacular sheath folds with greatly attenuated limbs in the upper plate sediments are interpreted as antithetic backfolds with shortening in the upper plate balanced by the subduction of the lower plate consistent with a ne directed subduction of the continental margin rocks beneath the sw obducting ophiolite haybi and hawasina thrust sheets recent suggestions of a nascent sw directed subduction beneath the oman margin are not consistent with the sedimentary evolution of the shelf and slope carbonates or the geological structure of saih hatat two main phases of tectonic uplift of saih hatat and jebel akhdar are firstly the latest cretaceous ramp culmination associated with foreland propagating thrusts in the sw and backfolds in the ne and secondly post late eocene oligocene uplift by ca 2000 m

ductile thrusting recorded by the garnet isograd from blueschist facies metapelites of the ile de groix armorican massif france

effects of freeze thaw and softening on a natural clay at low stresses weathering processes such as softening and freeze thaw cycling affect the properties of clays care must therefore be taken when selecting strength and compressibility parameters for analysis of natural slopes compacted clay embankments and trench excavations in which significant proportions of the cross section can be affected by climatic weathering samples of plastic lake agassiz clay from winnipeg were consolidated anisotropically in the laboratory to axial stresses less than or equal to the in situ effective overburden pressure they were therefore all overconsolidated with respect to the field preconsolidation pressure the samples were then loaded under drained or undrained conditions along steeply rising stress paths in p q stress space one group of samples was tested immediately to identify the undisturbed behavior a second group was subjected to freeze thaw cycles and a third group allowed to swell freely before testing the freeze thaw cycling produced increased compressibility and pore

tectonic sedimentary evolution of the western margin of the mesozoic vardar ocean evidence from the pelagonian and almopias zones northern greece

3d geometry and kinematic evolution of the wadi mayh sheath fold oman using detailed mapping from high resolution photography abstract the wadi mayh sheath fold in north eastern oman is one of the largest and best exposed sheath folds known and presents a unique opportunity to better understand this somewhat enigmatic style of deformation we undertook high resolution photographic surveying along wadi mayh to document the sheath fold in 61 georeferenced panoramic photomerges here we present ten such images that provide a structural interpretation of the sheath fold and surrounding structure we resolve this structure in a simplified three dimensional model and in two orthogonal cross sections and propose a kinematic evolution to explain the geometry the wadi mayh sheath fold is the most prominent example within what we suggest is a composite sequence of sheath folds which is itself enclosed within a ssw closing recumbent syncline at the base of the major saih hatat nappe sheath folding is accommodated within permian saiq formation limestones showing carpholite assemblages 6 8 kbar 275 375 c a major discontinuity separates this sequence from enveloping older rock units the sequence formed during progressive top to north ductile shearing as the overlying nappe migrated northwards with respect to the underthrusting hulw unit this process occurred during ssw directed exhumation of partially subducted continental crust in ne oman approximately 15 ma after obduction of the oman ophiolite initiated

geometry and significance of internal windows and regional isoclinal folds in northeast saih hatat sultanate of oman saih hatat contains tiered sets of low angle structural breaks that reflect a complex stacking of sheets which requires multiple upward movement of units within an unstable orogenic wedge in a convergent setting early recumbent folds within the saih hatat window are subparallel to but cut by a major low angle contractional fault decollement that separates lower plate high pressure rocks some eclogites exposed in two windows hulw and as sifah windows from high pressure upper plate rocks containing carpholite and lawsonite parts of this decollement had been mapped or speculated upon to explain the variation in metamorphic grade across saih hatat upper plate fold nappes are controlled by the mapped decollement surface below the decollement sheath like lower plate closures formed independently in an intense zone of top to the northeast shearing eclogites occur within mafic boudins at the structurally lowest level of the retrograde lower plate shear zone this zone was responsible for the partial exhumation of the lower plate very early in the observed structural history and has a normal movement sense the two lower plate windows are essentially foliation domes reflecting fold interference between regional scale sheath like recumbent isoclines and more open upright northwest north and northeast trending fold sets the early upper plate fold nappes reflect a major top to the north or northeast movement of the rocks away from the oman margin at the same time the ophiolite was being emplaced over the top in the opposite direction top to the southwest or south south directed faulting and shearing is superimposed on the early recumbent folds in the mountain range mapped relationships and structural observations agree with bureau de recherches geologiques et minieres mapping that recognised fold nappes in this part of the oman mountains all of the mapped structures are associated with exhumation of the high pressure rocks making reconstruction of the margin during high pressure metamorphism problematic

the selborne cutting stability experiment the paper describes a field experiment in which a 9 m deep cut slope in gault clay was brought to failure by pore pressure recharge the geology of the site is described and the procedures used to

creep and failure of slopes in clays the accumulated experience on slope stability is reviewed with due consideration to the time dependent behaviour of natural clays prior to failure a consistent picture of the behaviour of both man

the failure of carsington dam the upstream slope of carsington dam failed just before its completion in 1984 the slip started on the right abutment and spread across the valley to a length of nearly 500m extensive investigations were conducted and further information was obtained when the dam was reconstructed the initial slip sheared through the core which contained shear surfaces due to rutting and along a layer of yellow clay which contained solifluction shears both materials were brittle with low residual strengths the factor of safety based on measured peak strengths was about 1 4 with reductions to allow for shears it was 1 2 it had been reduced to 1 0 by progressive failure finite element fe analyses in which strain softening was reproduced confirmed this it was concluded that the slide spread by lateral load transfer through sections of the dam with a safety factor greater than 1 in the vailey centre the slide passed through the base of the mudstone fill a combination of fe and limit equilibrium analysis ind

residual strength of clays in landslides folded strata and the laboratory the post peak drop in drained shear strength of an overconsolidated clay may be considered as taking place in two stages first at relatively small displacements the strength decreases to the fully softened or critical state value owing to an increase in water content dilatancy second after much larger displacements the strength falls to the residual value owing to reorientation of platy clay minerals parallel to the direction of shearing if the clay fraction is less than about 25 the second stage scarcely comes into operation the clay behaves much like a sand or silt with angles of residual shearing resistance typically greater than 20 degrees conversely when the clay fraction is about 50 residual strength is controlled almost entirely by sliding friction of the clay minerals and further increase in clay fraction has little effect the angles of residual shearing resistance of the three most commonly occurring clay minerals are approximately 15 degrees for kaolinite 10 degrees for illite or clay mica and 5 degrees for montmorillonite when the clay fraction lies between 25 and 50 there is a transitional type of behaviour residual strength being dependent on the percentage of clay particles as well as on their nature the post peak drop in strength of a normally consolidated clay is due only to particle reorientation measurements of strength on natural shear surfaces agree within practical limits of variation with values derived from back analysis of reactivated landslides this field residual strength can be recovered by multiple reversal shear box tests on cut plane samples but in high clay fraction materials it is typically somewhat higher than the strength measured in ring shear tests residual strength is little affected by variation in the slow rates of displacement encountered in reactivated landslides and in the usual laboratory tests but at rates faster than about 100 mm min qualitative changes take place in the pattern of behaviour a substantial gain in strength is followed with increasing displacement by a fall to a minimum value in clays and low clay fraction silts this minimum is not less than the slow or static residual but in clayey silts with clay fractions around 15 25 according to tests currently in progress the minimum can be as low as one half of the static value author trrl

residual shear strength mobilized in first time slope failures this paper presents a review of long term stability of stiff clay and clay shale slopes and detailed reanalyses of 99 case histories of slope failures in 36 soft clays to stiff clays and clay shales we analyzed 107 sections using the observed actual slip surface in a first time slope failure in clay or shale part or all of the slip surface is unsheared prior to the occurrence of the landslide most stiff clays and clay shales contain stratigraphic discontinuities such as bedding planes and laminations the fully softened shear strength is shown to be the lower bound for mobilized shear strength in first time slope failures in homogeneous soft to stiff clays and on the slip surfaces cutting across bedding planes and laminations for many of the first time slope failures it appears that part of the slip surface is at the residual condition for excavated slopes the residual condition could be present before the final slope is formed or it may develop in response to excavation by progressive deformation along nearly horizontal surfaces including bedding planes or laminations in addition to the permeability dependent rise in porewater pressure and softening delayed first time failure of slopes in stiff clays and clay shales is caused by propagation of the residual condition into the slope on horizontal or subhorizontal surfaces including stratigraphic discontinuities the residual condition is present on the entire surface of reactivated landslides

lias clay slope sections and their implications for the prediction of limiting or threshold slope angles a series of previously published engineering studies of lias clay slopes in the east midlands is summarized along with the description of some further slope sections all the slopes studied show evidence of landsliding though some of it may be of periglacial origin the very disparate nature of the different landslides suggest that various different erosion regimes were responsible for the original slope formation it is believed that the smaller landslides are more typical of the area piezometric observations show that at the present day maximum pore pressures are proportionately less in the larger landslides the average value of the pore pressure parameter ru varying between 0 48 for shallow landslides to 0 32 in the largest stability analyses show that the residual strength is strongly stress dependent r c r 0 varying from 16 in shallow landslides to 10 in the case of the largest these and other factors have important implications for the often used practice of combining laboratory measurements of residual strength with the infinite slope method of stability analysis to predict threshold slope inclinations for landsliding