structural anatomy of the kushtagi schist belt dharwar craton south india an example of archaean transpression abstract the nw se trending kushtagi schist belt in the eastern dharwar craton records the tectonic consequences of a transpression regime it was formed due to late archaean oblique convergence in the dharwar craton this relatively unknown schist belt is characterized by two major episodes of deformation the dominant d 1 deformation forming tight to isoclinal non cylindrical folds and a regional axial planar schistosity is a consequence of transpression the transpression related sinistral shear regime is followed by development of a conjugate set of f 2 folds and s 2 cleavages that are superimposed on the d 1 structures the maximum shortening direction associated with the f 2 folds is nw se which suggests longitudinal compression subsequent to transpression riedel shear fractures were developed in the sinistral shear regime and dolerite dykes were emplaced along the conjugate riedel shear fractures intrusive granite bodies within and outside the schist belt were emplaced in the late phase of d 1 transpression

experimental calibration of the aluminum in hornblende geobarometer with application to long valley caldera california volcanic rocks a new geobarometer based on the al content of igneous hornblendes in equilibrium with melt fluid biotite quartz sanidine plagioclase sphene and magnetite or ilmenite has been calibrated experimentally the calibration was performed by equilibrating the required phase assemblage over the pressure range 2 8 kbar at 740 780c and then analyzing euhedral hornblendes in equilibrium with glass melt experiments were performed on natural samples of both volcanic and plutonic rocks earlier empirical calibrations of this geobarometer relied on analyzing natural hornblendes from plutons with the required phase assemblage and inferring pressure from nearby metamorphic country rocks the experimental calibration differs from the empirical calibrations especially above 5 kbar and shows that the al content of hornblendes in equilibrium with the required phase assemblage is greater for a given total pressure than previously thought the geobarometers uncertainty is dramatically reduced the derived equation is p plus minus 0 5 kbar 3 minus 3 46 plus minus 0 24 4 23 plus minus 0 13 al sup t the geobarometer is applied to post bishop tuff volcanic rocks from long valley caldera california and reveals that most rhyodacites in this complex erupted from depths of about 6 km these eruptions occurred over 500 000 yr suggesting that the rhyodacitic magma reservoir beneath longmore valley had reached a steady p depth t state less

insights from quartz cathodoluminescence zoning into crystallization of the vinalhaven granite coastal maine the vinalhaven intrusive complex provides field and petrographic evidence for multiple replenishments of mafic and silicic magmas mingling and limited mixing and rejuvenation of granite quartz in granitic rocks preserves a record of those processes in the form of cathodoluminescence cl zoning which is related to concentration of titanium and to temperature of crystallization using the new titaniq ti in quartz geothermometer injection of mafic melts into partly crystalline vinalhaven granite resulted in partial quartz resorption followed by higher temperature growth from h2o undersaturated melt this is shown by steep rimward increases in cl intensity and ti content across discordant boundaries that truncate older growth zones quartz zoning in granite affected by mafic magmas displays large rimward jumps in ti content whereas quartz in granitic feeders and in granite far from mafic rocks typically displays broad rims with decreasing ti contents consistent with slow cooling without thermal disruptions due to mafic recharge

development of asymmetric shale pull aparts in evaporite shear zones abstract antithetically rotated lozenge shaped shale pull aparts are quite common in the shallow domains of the evaporite shear zones of the jura in northern switzerland the pull aparts are formed by synthetic shear fractures related to a transpressive strain field that also induces the antithetic rotation of the pull aparts immediately after their formation a simple analytical model shows that the acute angle of fracture and the antithetic rotation are determined by structural and rheological parameters and the direction of maximum compressive stress the pull aparts are commonly separated by sulphate veins as the elongation resulting from antithetic rotation does not compensate for the bulk elongation despite this some pull aparts are bordered by synthetic drag zones antithetically rotated pull aparts are indicative either of compression normal to shear zone boundaries in decollement type shear zones or in shear zones deformed by simple shear of an initial obliquity of the dismembered layers relative to the shear zone boundary resemblance of the shale pull aparts to type 1 asymmetric pull aparts and book shelf structures both of which rotate synthetically demonstrates that rotated and asymmetric pull aparts may be ambiguous kinematic indicators

paleozoic laurentia gondwana interaction and the origin of the appalachian andean mountain system laurentia the rift bounded precambrian nucleus of north america may have broken out from a neoproterozoic supercontinent between east and west gondwana several lines of evidence suggest that the appalachian margin of laurentia subsequently collided with the proto andean margin of the amalgamated gondwana supercontinent in different relative positions during early and mid paleozoic time in route to final docking against northwest africa to complete the assembly of pangea hence the appalachian and andean orogens may have originated as a single mountain system the overall hypothesis retains the same paleomagnetic and paleobiogeographic controls as previous global reconstructions for the paleozoic era laurentia gondwana collisions may help to explain contemporaneous unconformities in the paleozoic sedimentary cover of the laurentian gondwanan and baltic cratons

systematic changes in orientation of linear mylonitic fabrics an example of strain partitioning during transpressional deformation in north golpaygan sanandaj sirjan zone iran abstract kilometer scale shallowly dipping nw striking top to the ne reverse and dextral strike slip shear zones occur in metamorphic rocks of north golpaygan these metamorphic rocks are exposed at the ne margin of the central part of the sanandaj sirjan zone in the hinterland of the zagros orogen nw striking top to the ne normal shear zones were also found in a small part of the study area structural evidence of three deformation stages were found pre mylonitization metamorphic mineral growth happened during d1 the main mylonitization event was during the d2 deformational event following coaxial refolding synchronous to retrograde metamorphism of amphibolite to greenschist facies in the late cretaceous paleocene and before d3 folding and related mylonitization we documented the systematic changes in the orientations of d2 linear fabrics especially stretching lineations and superimposition relations of structures it is concluded that the dextral strike slip and dip slip shear zones were coeval kinematic domains of partitioned dextral transpression the shallowly dipping reverse and strike slip shear zones are compatible with partitioning in a very inclined transpressional model fabric relations reflect that the top to the ne normal shear zones were not produced during deformation partitioning of inclined dextral transpression the late cretaceous paleocene strain partitioning was followed by later n s shortening and ne extension in the north golpaygan area

bulk rock geochemistry and plagioclase zoning in lavas exposed along the northern flank of the western blanco depression northeast pacific insight into open system magma chamber processes abstract the present study is based on a set of lavas and crosscutting dikes collected by dives along detailed vertical transects on the northern flank of the western part of the blanco transform fault northeast pacific the studied area consists of a small basin the western blanco depression wbd extending from the southern end of the juan de fuca ridge to a pseudofault trace 60 km eastward the northern scarp of the wbd comprises a volcanic unit overlying a sheeted dike complex major and trace element data coupled with sr nd isotope ratios reveal a two component mantle source composed by an isotopically depleted matrix variably veined by more enriched material one chemical group ns2 indistinguishable from the other northern scarp samples on the basis of trace element data has an unusually depleted isotopic composition typical of a nearly pure mantle end member some cogenetic samples of the northern scarp have been used to constrain the differentiation modalities anorthite and mgo content profiles in plagioclase xenocrysts and phenocrysts reveal i the existence of h2o bearing evolved melts in the mushy zones and ii the occurrence of mixing process between these melts and anhydrous mafic liquids the hydration is supported by other petrographic features such as high magmatic fo2 values calculated from fe ti oxide pairs and the presence of pyroxene inclusions in plagioclase phenocrysts mixing consistent with the existence of ni rich ferrobasalts is interpreted to be the consequence of the reservoir refilling by mafic liquids mg 70 these petrological and geochemical evidences are combined with the evolution of mg with depth to suggest a periodic open system magma chamber evolution beneath the southern end of the juan de fuca ridge

deformation path partitioning within a transpressive shear zone marble cove newfoundland detailed study of the structures in marble cove newfoundland indicates that they formed during a single transpressional event during which deformation was partitioned between dominantly strike slip and dominantly dip slip domains marble cove exhibits steeply dipping foliations steeply to shallowly plunging stretching lineations greenschist facies mineral assemblages and abundant quartz veins within the broad domain where stretching lineations plunge steeply sense of shear indicators and asymmetry of quartz c axis distributions in mylonitized quartz veins indicate dominantly reverse motion with a component of dextral shear adjacent greenschists exhibit orthorhombic symmetry and apparently accommodated pure shear the abrupt transition to a narrow high strain domain of shallowly plunging stretching lineations is accommodated by a rheologically distinct serpentinite body shear bands in this domain indicate dextral strike slip movement with a reverse component smaller conjugate shear bands are evident in thin section indicating a component of pure shear sense of shear indicators and asymmetry of c axis distributions in mylonitized quartz veins also record dextral strike slip and reverse shear with a component of pure shear crenulations and asymmetric folds in foliation overprint all other fabrics in marble cove and record a final phase of reverse shear with a dextral component with partitioning of this kind structures recording strike slip movement may be limited in areal extent and therefore less likely to be exposed than structures recording dip slip motion such partitioning within transpressional shear zones may therefore impede recognition of strike parallel motions within orogenic belts

stretching lineations in transpressional shear zones an example from the sierra nevada batholith california abstract ductile shear zones and associated stretching lineations are generally considered to be the result of two dimensional simple shear deformations with stretching lineations interpreted to rotate into parallelism with the direction of tectonic transport with increasing deformation however stretching lineations perpendicular to the inferred tectonic transport direction are displayed by some shear zones field studies in the sierra nevada batholith have revealed a single shear zone the rosy finch gem lake shear zone that contains both steeply plunging stretching lineations in older metamorphosed sedimentary and granitic rocks and shallowly plunging stretching lineations in syntectonic granitoids dextral sense of shear indicators are found in all these units and deformation occurred simultaneously along the length of the shear zone theoretical strain modeling indicates that stretching lineations can be either horizontal or vertical within transpressional shear zones to investigate the kinematics of three dimensional deformation we examine the role of stretching lineations which form as a result of finite strain including the possibility of partitioning a component of the strike slip movement onto shear bands comparing the field data with the strain modeling we interpret the along strike variation in lineation behavior within the rosy finch gem lake shear zone to be a result of the differences in finite strain recorded by different units the older units showing steeply plunging lineation have recorded more finite strain across a narrower shear zone than the syntectonic granites which record shallowly plunging lineations we conclude that the orientation of the lineation within transpressional shear zones does not necessarily correlate with the transport direction but may reflect along strike variations in finite strain and or strain partitioning

strain partitioning in transpression zones abstract transprcssional strain acting upon structurally anisotropic rocks can be partitioned into separate deformational domains of pure shear and simple shear this contrasts with homogeneous transpression in which both the pure shear and the simple shear strain components are uniformly distributed across the zone of deformation structural weaknesses capable of partially or fully accommodating one component of deformation include lithological contacts rheological heterogeneities and faults or shear zones situated within the deformation zone or lying along its boundaries partitioning of transprcssional strain can occur when stress is applied oblique to pre existing structural weaknesses or can occur during later stages of progressive strain when the early deformation of isotropic rocks imparts sufficient anisotropy to allow subsequent strain to be partitioned partitioning of transpressional strain into domains lying parallel to the deformation zone boundaries can be distinguished from fault stepped transpression in which strain is partitioned along the length of a segmented fault zone mesofracture analysis of rocks affected by mid devonian deformation on both sides of the highland boundary fault zone hbfz in central scotland shows that strain was not homogeneous the mesofracture data suggest that regional north south compression orientated oblique to the pre existing ne sw trending hbfz was partitioned into separate deformational domains the hbfz accommodated most of the simple shear component whilst the rocks flanking the zone were deformed predominantly by pure shear a contemporary example is the san andreas fault in central california where analyses of neo tectonic stresses show that the direction of principal compression is perpendicular to the fault zone many examples of the partitioning of transpressional strain have also been recognised at destructive plate margins where the direction of plate motion is oblique to the edge of the over riding plate

late gondwanide tectonic rotations within gondwanaland geologic and paleomagnetic evidence from the ellsworth whitmore mountains crustal block of west antarctica and from the falkland islands of south america indicates that tectonic displacement of major portions of the samfrau geosyncline occurred after the early mesozoic gondwanide folding but prior to seafloor spreading comparison with continental deformation in the collisional alpine himalayan belt offers a mechanism for the displacements while supporting the hypothesis that the gondwanian orogeny resulted from collision of a fore arc and magmatic arc terrane with the panthalassic margin of the gondwana craton during closure of a marginal basin the displacements had major consequences for the ensuing evolution of the south atlantic weddell sea ross sea region

stratigraphic evidence for the ross orogeny in the ellsworth mountains west antarctica implication for the evolution of the paleo pacific margin of gondwana early paleozoic crustal deformation has been recognized along the paleo pacific margin of gondwana including southern africa saldanian tectonism and the transantarctic mountains of east antarctica ross orogeny previously no stratigraphic evidence for the ross orogeny had been recognized within the ellsworth mountains of west antarctica believed to have been contiguous with the transantarctic mountains prior to the mesozoic breakup of gondwana a disconformity between middle to upper cambrian heritage group and upper cambrian to devonian crashsite group strata indicates that the ross orogeny did affect the ellsworth mountains providing additional evidence that these rocks were once part of east antarctica fluvial conglomerates overlying platform carbonates of the upper heritage group were derived in part from plutonic volcanic and metamorphic terranes in east antarctica elsewhere in the ellsworth mountains the disconformity is expressed by an abrupt change from shallow carbonate platform deposits to siliciclastic shelf deposits similarities in lower paleozoic stratigraphy between the ellsworth mountains parts of the pensacola mountains east antarctica and eastern south africa imply that these regions were inboard of the zone of deformation that affected the proto pacific margin of gondwana during the ross orogeny and saldanian tectonism

chapter 8 late dresbachian idamean and other trilobite faunas from the heritage range ellsworth mountains west antarctica

crystal mush compaction and the origin of pegmatitic segregation sheets in a thick flood basalt flow in the mesozoic hartford basin connecticut

late acadian alleghenian transpressional deformation evidence from asymmetric boudinage in the casco bay area coastal maine abstract asymmetric competent layer and foliation boudinage in the casco bay area of coastal maine formed during regional f 2 hinge parallel elongation and distributed dextral strike slip movement it consists of backward rotated oblique shear fracture boudinage and forward rotated extension fracture boudinage initially symmetric quartz filled extension fractures were intermittently developed as boudin partings orthogonal to the prominent ductile stretching lineation due to fault related fluctuations in fluid pressure and tectonic stresses modification by concurrent non coaxial deformation related to dextral strike slip movement resulted in the clockwise rotation sinistral slip and vein parallel elongation reflected in the forward rotated asymmetric boudinage discordant planar pegmatite veins and variably oriented boudinaged pegmatite sheets common in these exposures underwent a similar evolution the development of this asymmetric boudinage is attributed to progressive brittle ductile non coaxial deformation related to orogen parallel dextral transpression along the norumbega nonesuch river strike slip fault system during the later stages of the acadian orogeny and subsequent alleghenian deformation

assimilation of ocean crust by hawaiitic and mugearitic magmas an example from eiao marquesas abstract three holes were drilled in the northern dominique central sophie and southern naore parts of the volcanic shield of eiao island in the marquesas archipelago french polynesia these were continuously cored with an excellent recovery and reached depths of 500 m sophie and 800 m dominique and naore k ar dating indicates that the volcanic sequence penetrated was emplaced between 5 52 0 05 ma and 4 95 0 04 ma petrographic types include tholeiites alkali basalts picrobasalts hawaiites mugearites and trachytes intermediate lavas hawaiites and mugearites are fairly common they occur within stratigraphic units denoted d1 d2 d3 s1 s2 s3 n1 and n2 respectively d dominique s sophie n naore the d3 s1 and n1 hawaiites and mugearites appear to be derived from fractional crystallization of parental alkali basalt magmas the other groups are characterized by variable enrichments in sr d1 d2 and s3 p2o5 d2 th s2 and n2 and light rare earth elements d2 or by depletion in heavy rare earth elements s2 and n2 we suggest that these trace element characteristics reflect open system differentiation processes which occurred during the building of eiao volcanic shield two stages of assimilation coupled with fractional crystallization afc are proposed the calculated compositions of the two contaminants suggest that they were leucocratic materials located at different levels of the oceanic crust finally a water induced assimilation process is invoked to explain the origin of the s3 hawaiites these lavas are enriched in al2o3 na2o and sr inferred to reflect plagioclase breakdown within the eiao plumbing system

tectonic significance and consequences of the gondwanide orogeny in northern patagonia argentina abstract the tectonic configuration of northern patagonia is recognized through geological and geophysical studies which define several areas characterized by positive and negative gravity anomalies with steep gravity gradients positive areas are interpreted as continental crust with grenvillian and pampean basement the pampean basement occurs to the north and south of the interpreted suture between patagonia and gondwana south america most negative anomalies are interpreted to represent gondwana igneous and sedimentary rocks areas characterized by steep gravity gradients are coincident with shear zones and mylonitic belts that were active during gondwana times a period of great magmatic and tectonic activity in northern patagonia dextral movement along the e w trending huincul fault zone resulted in block collage west northwest crustal translations and counterclockwise rotation of large crustal blocks belonging to the north patagonia massif the indentation of blocks within the north patagonia massif resulted in tectonic escape of the surrounding blocks each with a different trajectory escape tectonics may explain the diversity of stress directions during gondwana orogeny

factors affecting the kinematic interpretation of asymmetric boudinage in shear zones abstract based on work along a major mylonite zone in the northern appalachians and scale model studies a new mechanism for the origin of asymmetric boudins in shear zones is proposed along the honey hill fault in southern connecticut granitic sills intruded into calc silicate gneisses and schists were oblique to the boundaries of the mylonite zone and experienced the following sequence in the production of sigmoidal boudins 1 as the calc silicate schists experienced mylonitization and flow the more competent coarse grained granitic sills deformed by extensional fracturing and quartz veining 2 continued extension of granitic sills was accommodated by normal shear on early formed quartz veins 3 continuing extension of sills and ductile modification of the corners of boudins resulted in granitic fish with tails which stream from the top of the boudin in the down dip direction and from the bottom in the up dip direction based on a variety of kinematic indicators the sense of asymmetry of the tails is identical to that expected for recrystallization tails on sheared augen structure models composed of silicone putty and plasticine were created to investigate the effect of pre shearing geometry on boudin evolution and were deformed in a simple shear device the models reproduce the kinematics deduced from field relations and suggest that one of the primary factors in controlling the amount of extension is the angle which early veins make with the shear zone boundary varying the angular relationships in the models suggests that other pre shearing geometric factors may affect the geometry of boudins formed in this way boudins formed through this mechanism appear very similar to hanmer s type ii asymmetric boudins because the pre shearing geometry can exert a control on boudin asymmetry caution should be used when attempting to deduce shear sense or shear strain values from asymmetric boudinage

tracing solute infiltration using a combined method of dye tracer test and electrical resistivity tomography in an undisturbed forest soil profile an accurate prediction of solute infiltration in a soil profile is important in the area of environmental science groundwater and civil engineering we examined the infiltration pattern and monitored the infiltration process using a combined method of dye tracer test and electrical resistivity tomography ert in an undisturbed field soil 1 m 1 m a homogeneous matrix flow was observed in the surface soil a horizon but a preferential flow along macropores and residual rock structure was the dominant infiltration pattern in the subsurface soil saturated interflow along the slopping boundaries of a and c1 horizons and of an upper sandy layer and a lower thin clay layer in the c horizon was also observed the result of ert showed that matrix flow started first in a horizon and then the infiltration was followed by the preferential flows along the sloping interfaces and macropores the ert did not show as much detail as the dye stained image for the preferential flow however the area with the higher staining density where preferential flow was dominant showed a relatively lower electrical resistivity the result of this study indicates that ert can be applied for the monitoring of solute transportation in the vadose zone copyright 2010 john wiley sons ltd

cambrian palaeomagnetic data confirm a natal embayment location for the ellsworth whitmore mountains antarctica in gondwana reconstructions summary the ellsworth whitmore mountains ewm are one of five terranes that form west antarctica constraining the positions of these terranes in pre break up gondwana is crucial to understanding the history of the palaeo pacific gondwana margin we report the results of a detailed palaeomagnetic investigation of the ewm which comprises some 150 sites in six formations ranging in age from cambrian to permian five of the studied units yield only viscous remnant magnetizations of recent age or unstable natural remanent magnetizations the remaining unit the mid late cambrian frazier ridge formation yielded stable magnetizations at 16 of 35 sites this component passes a fold test at the 95 per cent confidence level indicating that it pre dates permian deformation and we argue that it is of primary origin the resulting palaeopole 9 n 293 e a95 5 1 is in good agreement with two previously published palaeopoles from similarly aged rocks in the ewm collectively these data indicate that the ewm were located in the natal embayment prior to gondwana break up and underwent 90 of anticlockwise rotation during break up all three studies however yield inclinations

vertical and fold axis parallel extension within a slate belt in a transpressive setting northern appalachians abstract an incremental and finite strain study of a mid paleozoic slate belt in a transpressive setting in gaspe canadian appalachians has been undertaken using syntectonic fibers from pyrite type pressure shadows regionally strain estimates are quite consistent and reflect only slightly higher strain in the central part of the anticlinorial structure defining the slate belt extensions of up to 160 vertically and 110 parallel to the fold axis were recorded in the slate belt combined with an estimated overall regional shortening of 80 the fibers first rotated toward the final position of the cleavage within a vertical section normal to the cleavage and then rotated from a vertical to a horizontal orientation this change in fiber orientation observed in cleavage parallel sections indicates an abrupt change in the incremental stretching direction from sub vertical to sub horizontal fold axis parallel the vertical extension is related to folding and cleavage development during coaxial flattening the horizontal extension reflects the strain imposed on the rocks during regional simple shear which produced strike slip faulting and further tightening of the folds folding cleavage development fold axis parallel extension and faulting are all related to oblique convergence during the middle devonian acadian orogeny in this part of the canadian appalachians and clearly indicate an overall transition from a pure shortening deformation to a simple shear dominant deformation in an overall transpressive setting

the mafic silicic layered intrusions of saint jean du doigt france and north guernsey channel islands armorican massif gabbro diorite layering and mafic cumulate pegmatoid association abstract the saint jean du doigt france and north guernsey channel islands intrusive complexes hereafter referred to as sjic and ngic respectively are examples of mafic silicic layered intrusions in the armorican massif both are characterized by the occurrence of 1 a basal peripheral gabbroic unit interlayered with sheets generally dioritic in composition occasionally gabbroic and crossed by leucocratic diapirs and pipes from monzodioritic to q monzonitic in composition 2 peripheral pegmatoids associated with mafic cumulates and 3 coeval granitoids beside these main similarities some contrasted features lead us to propose two distinct models of formation the variscan sjic includes tholeiitic mafic rocks monzogabbro that locally mingle and mix with leucocratic components monzonite or q monzonite the cadomian ngic is calc alkaline the sjic sheet bearing gabbro is homogeneous from a petrologic point of view whereas the ngic exhibits gabbroic macrorhythmic sequences with mineral layering the sr nd isotopic compositions of the sjic gabbros are significantly different from those of the associated dioritic layers this is not the case in the ngic where the magmas could be cogenetic we argue that the sjic gabbro was a liquid that crystallized in situ without significant crystal settling by contrast the rhythmic sequences of the ngic are consistent with crystal accumulation subsequently both can be seen as mafic reservoirs which were repeatedly invaded by magmas of intermediate composition we interpret the sheets in the sjic as the result of horizontal spreading of dioritic metastable magmas into a gabbroic reservoir crystallizing from below at levels of neutral buoyancy injections and convection in the central part of the reservoir possibly resulted in spectacular mixing mingling structures in the ngic the emplacement of the dioritic sheets was rather controlled by pre existing rhythmic cumulative structures in both intrusions late differentiated diapirs were extracted from the dioritic sheets associated peripheral pegmatoids are thought to result from the crystallization of liquids issued from a mafic intercumulus melt in the presence of a fluid phase this extraction might have been enhanced by the disruption of the peripheral cumulate stack perhaps following pressure drops

partitioning of strike slip and convergent motion between eurasia and arabia in eastern turkey and the caucasus the plate motion model nuvel 1 de mets et al 1990 predicts a strongly oblique component to the convergence between arabia and eurasia at the longitude of the caucasus earthquake focal mechanisms suggest that this oblique convergence is partitioned into pure right lateral strike slip motion in eastern turkey and pure shortening by thrusting in the greater caucasus such partitioning offers an explanation for the apparent eastward continuation of the right lateral faulting of the north anatolian fault from turkey into iran from a summation of seismic moment tensors between 1911 and 1991 it appears that most of the right lateral strike slip motion in eastern turkey is accommodated by faulting in large earthquakes while most of the shortening in the caucasus appears to occur aseismically

development of kinematic partitioning within a pure shear dominated dextral transpression zone the southern ellsworth mountains antarctica the heritage range of the southern ellsworth mountains west antarctica is composed of cambrian to permian sedimentary and volcanic rocks which were deformed during the permo triassic gondwanian orogeny the structural grain of the heritage range exhibits a previously unrecognized 18 swing from 333 153 in the north to 315 135 in the south a change in structural and kinematic style accompanies this strike swing with a structure consistent with near orthogonal shortening present in the south and a structural style consistent with dextral transpression within the central and northern heritage range kinematic partitioning is present within the central heritage range where strikeparallel contemporaneous domains of dextral and reverse shear have developed simultaneously with the regional cleavage comparison of the structure and kinematics within both structural domains suggests that the central and northern heritage range experienced pure shear dominated dextral transpression with an approximate angle of relative shortening of 65 70 results derived by integrating field data into a published kinematic partitioning model suggest relatively efficient kinematic partitioning has occurred however such efficient partitioning cannot be explained by strain partitioning models based purely on plate boundary conditions therefore it is proposed that pre existing weak structures were present within the unexposed basement facilitating the apparent high percentage of kinematic partitioning

a complete arc trench system recognized in gondwana sequences of the antarctic peninsula region prior to late triassic early jurassic times the geological history of the antarctic peninsula region was dominated almost entirely by gondwana sequences that together comprised a major arc trench system subduction complex trench slope break and fore arc basin sedimentation can all be recognized and deposition was at least partly on early palaeozoic or older continental crust the only evidence for a contemporaneous magmatic arc situated in the antarctic peninsula at this time consists of patchy occurrences of metavolcanic rocks possibly representing the frontal edge of the arc and the major outcrop area of these rocks is believed to lie under the broad shallow continental shelf e of the antarctic peninsula this is contrary to most current hypotheses in which a marginal basin presumably floored by oceanic crust is thought to crop out close to the e coast of the antarctic peninsula however the complete absence of substantial outcrops of pre jurassic volcanic rocks anywhere in eastern antarctica and south africa which are the closest and most likely places in which these should exist supports the new proposal moreover the identification of back arc elements of the arc trench system foreland fold thrust belt and retro arc foreland basin in eastern antarctica and south africa greatly strengthens the likelihood of the arc cropping out in the area suggested towards the end of the triassic and during the early jurassic periods an intense diastrophic event or culmination of events of orogenic magnitude occurred gondwanian orogeny causing substantial redistribution of the pre existing elements of the arc trench system in particular all the fore arc sequences were strongly deformed some possibly for the first time e g in the fore arc basin and became firmly accreted to the continental margin moreover the magmatic foci migrated trenchwards to intrude the deformed rocks in the antarctic peninsula because many of the plutons were emplaced synkinematically they often closely resemble basement gneisses the distinction between these rocks remains a serious problem in the antarctic peninsula and it can be solved only partially by radiometric dating

creation and destruction of weddell sea floor in the jurassic new paleomagnetic data invite reassessment of the breakup history of parts of the pacific margin of gondwana due to the mesozoic opening of the southern ocean basins these data indicate clockwise rotation of the antarctic peninsula relative to east antarctica between 175 and 155 ma that would have created as much as 1000 km of pre anomaly m25 ocean floor in the weddell sea basin the paleomagnetic data also suggest that between 155 and 130 ma the antarctic peninsula rotated counterclockwise relative to east antarctica rotation that along with the southward motion of east antarctica would have caused subduction of the newly created weddell sea ocean floor beneath the southern antarctic peninsula east vergent folding and thrusting of jurassic back arc basin rocks along the southeastern antarctic peninsula known as the palmer land event probably resulted from deformation along this newly created convergent margin the presence of jurassic weddell sea floor would have isolated eastern gondwana east antarctica australia and india from the antarctic peninsula and western gondwana africa and south america encouraging increased marine faunal connections between the pacific and weddell sea mozambique somali ocean basins during the 175 to 130 ma interval after 130 ma the antarctic peninsula would have been connected to east antarctica via the other west antarctic terranes thereby facilitating land migrations between eastern and western gondwana

west antarctica in gondwanaland crustal blocks reconstruction and breakup processes abstract a combined bas usarp west antarctic tectonic project has provided new constraints on the crustal structure and geological evolution of west antarctica and its relationship to the rest of gondwanaland rb sr age dating has confirmed the presence of 1200 1000 m y old precambrian gneisses within west antarctica and aeromagnetic data have defined the extent of this precambrian crustal block paleomagnetic data have constrained the position of four of the five geologically distinctive west antarctic crustal blocks within gondwanaland they can be restored to their original position prior to breakup by 15 25 counter clockwise rotation with little or no relative displacement between the blocks a major period of within plate middle jurassic magmatism associated with an important crustal melting event that is clearly related to the breakup of the supercontinent has also been identified based on new isotopic and geochemical data

inclined transpression in the neka valley eastern alborz iran three major nappes in the neka valley in the eastern alborz mountains of iran allow the cimmerian to present convergence following the oblique collision between iran and the southern margin of eurasia this work reports the identification of an inclined transpression zone recognized by field investigations and strain analyses of the geometries of formations and detailed mesoscopic structural analyses of multiple faults folds and a cleavage the main structures encountered include refolded recumbent asymmetric fold nappes highly curved fold hinges in a transpression zone that dips 37 to the nw between boundaries thrusts striking from n050 to n060 the angle the angle between the zone boundary and direction of horizontal far field shortening is about 80 the north west and south east boundaries of this zone coincide with the haji abad thrust and the shah kuh thrust respectively fold axes generally trend ne sw and step to both right and left as a result of strike slip components of fault displacements strain analyses using fry s method on macroscopic ooids and fusulina deformed into oblate ellipsoids indicate that the natural strain varies between 2 1 and 3 14 the estimated angle between the maximum instantaneous strain axis isamax and the transpression zone boundary is between 6 and 20 the estimated oblique convergence angle therefore ranges between 31 and 43 the average kinematic vorticity number wk is 0 6 in a zone of sinistral pure shear dominated inclined triclinic transpression these results support the applicability of kinematic models of triclinic transpression to natural brittle ductile shear zones

mesozoic evolution of west antarctica and the weddell sea basin new paleomagnetic constraints paleomagnetic data from the antarctic peninsula and our recent results from the ellsworth whitmore mountains block suggest that since the middle jurassic these two west antarctic blocks have undergone little relative movement and together have rotated relative to the east antarctic craton new data from lower cretaceous rocks from the thurston island region of west antarctica suggest that on the basis of paleomagnetic constraints the antarctic peninsula ellsworth whitmore mountains and thurston island blocks define a single entity which we call weddellia some motion between these blocks is possible within the limits of the paleomagnetic data between the middle jurassic and early cretaceous weddellia remained attached to west gondwanaland while east antarctica moved southward dextrally relative to weddellia from the early cretaceous to mid cretaceous weddellia rotated clockwise 30 and moved sinistrally approximately 2500 km relative to east antarctica to its present day position we suggest the early to mid cretaceous to be the time of the main if not initial opening of the weddell sea

oblique plate motion and continental tectonics three dimensional deformation is necessarily associated with oblique plate convergence and commonly results in partitioning of deformation between contractional and transcurrent components along plate margins kinematic models of strike slip partitioning for transpression and transtension allow the exact relation among three critical parameters plate motion instantaneous strain axes and degree of strike slip partitioning to be calculated application to two end member tectonic environments characterized by a low south island new zealand and high central california degree of strike slip partitioning demonstrates a remarkable consistency among the three parameters suggesting that strike slip partitioned transpression is a valid model for deformation in these regions the extreme degree of strike slip partitioning in wrench dominated systems such as central california is tentatively associated with a fundamental misorientation of finite and instantaneous strain axes

monzonitic series from the variscan tormes dome central iberian zone petrogenetic evolution from monzogabbro to granite magmas abstract in the iberian massif rocks of the k rich plutonic series are not abundant but towards internal parts of the belt represented by the central iberian zone there are some sectors where shoshonitic plutonism occurs over broad areas one of these areas is the anatectic tormes dome encompassing two similar studied plutons pereruela and vitigudino a monzonitic association has been defined ranging from monzogabbros to quartz monzonites or scarce monzogranites enrichment in lree p sr and ba a high water content up to 5 5 and a high degree of oxidation ni nio buffer are the main features of magmas parental to these monzonitic rocks these petrographic and geochemical features allow these granitoids to be ascribed to the shoshonitic type sh type rather than to the i type the minimum emplacement pressure range is 410 230 mpa whereas the estimated solidus temperature range is 940 765 c i e above water saturated solidus o sr and nd isotopes point to open system processes apatite cathodoluminescence suggests that magma mixing was unlikely to have occurred for the most enriched rocks assimilation fractional crystallisation afc modelling was performed for both plutons permitting assimilation crystallization rates to be estimated between 0 16 and 0 25 different contaminants have been inferred a metapelite at upper crustal level for the vitigudino pluton and a granulitic orthogneiss for the pereruela pluton a liquid line of descent linking monzogabbroic members to quartz monzonites monzogranites can be reconstructed some loss of water and oxygen can be inferred although water remained in the system below the solidus giving rise to auto metamorphism at ca 500 c on the other hand afc processes are unlikely to have been the main factor in controlling the characteristic enrichment of the monzonitic series since the least contaminated samples are the most lile and lree enriched for both plutons thus a source controlled chemical signature can be inferred the experimental data indicate that a hybrid protholith a mixture of peridotite amphibolite and metapelite can account for the enrichment and hydrated nature of the least evolved monzogabbroic magmas late orogenic slab break off and subsequent post collisional extensional events appear to be a suitable scenario to provide the heat that triggered partial melting close to the crust mantle boundary

neogene oblique convergence and strain partitioning along the north tabriz fault nw iran abstract the north tabriz fault is one of the main active geological structures in nw iran it is reactivated under a pure shear dominated transpressional tectonic regime induced by high angle 50 oblique convergence between the arabia and eurasia plates the first stage structures in the study area are folds and thrust faults that are mostly observed in miocene sedimentary rocks and are cut by right lateral strike slip faults that are part of the north tabriz fault structural analysis of the north tabriz fault revealed flower structure geometry with thrust faults and related folds on both sides of the fault and strain partitioning from gps data 7 3 mm year of right lateral plate motion has been calculated for nw iran with 57 72 strike slip partitioning resulting in a maximum right lateral slip rate for the north tabriz fault of 4 2 5 3 mm year since the total strike slip displacement along the ntf has been estimated at 17 3 km with the slip rate of 4 2 5 3 mm year the maximum age of inception of the ntf strike slip movement was at 3 2 4 ma upper pliocene

isotopic evidence for multiple contributions to felsic magma chambers gouldsboro granite coastal maine abstract the gouldsboro granite forms part of the coastal maine magmatic province a region characterized by granitic plutons that are intimately linked temporally and petrogenetically with abundant co existing mafic magmas the pluton is complex and preserves a felsic magma chamber underlain by contemporaneous mafic magmas the transition between the two now preserved as a zone of chilled mafic sheets and pillows in granite mafic components have highly variably isotopic compositions as a result of contamination either at depth or following injection into the magma chamber intermediate dikes with identical isotopic compositions to more mafic dikes suggest that closed system fractionation may be occurring in deeper level chambers prior to injection to shallower levels the granitic portion of the pluton has the highest nd isotopic composition e nd 3 0 of plutons in the region whereas the mafic lithologies have nd isotopic compositions e nd 3 5 that are the lowest in the region and similar to the granite and suggestive of prolonged interactions and homogenization of the two components sr and nd isotopic data for felsic enclaves are inconsistent with previously suggested models of diffusional exchange between the contemporaneous mafic magmas and the host granite to explain highly variable alkali contents the felsic enclaves have relatively low nd isotopic compositions e nd 2 1 indicative of the involvement of a third lower e nd melt during granite petrogenesis perhaps represented by pristine granitic dikes contemporaneous with the nearby pleasant bay layered intrusion the dikes at pleasant bay and the felsic enclaves at gouldsboro likely represent remnants of the silicic magmas that originally fed and replenished the overlying granitic magma chambers the large isotopic and chemical contrasts between the enclaves and granitic dikes and granitic magmas may be in part a consequence of extended interactions between the granitic magmas and co existing mafic magmas by mixing mingling and diffusion alternatively the granitic magmas may represent an additional crustal source using granitic rocks such as these with abundant evidence for interactions with mafic magmas complicate their use in constraining crustal sources and tectonic settings fine grained dike rocks may provide more meaningful information but must be used with caution as these may also have experienced compositional changes during mafic felsic interactions

gondwanian age dextral transpression and spatial kinematic partitioning within the heritage range ellsworth mountains west antarctica the ellsworth mountains west antarctica consist of two mountain ranges the sentinel and heritage ranges the more southerly heritage range is composed of a lower paleozoic sedimentary and volcanic rock sequence deformed during a single major deformation event in the early mesozoic this gondwanian orogeny possibly resulted from andean style convergence along the southern margin of gondwana prior to break up of the super continent and the subsequent translation of the ellsworth mountains from a position close to the natal embayment of southern africa to that of the present day rocks of the heritage range are intensely folded with close to tight upright to inclined folds plunging gently about a horizontal axis trending nnw sse locally folds plunge moderately to subvertical toward the nnw possessing asymmetries consistent with a dextral sense of shear cleavage is generally axial planar displaying downdip and strike parallel stretching lineations that are frequently associated with domains of reverse and dextral shear striking parallel to the regional structural grain the spatial and temporal relationship of fractures developed within these domains as a result of noncoaxial shear in addition to the progressive incremental strain histories derived from mineral fibres in strain shadows indicate the contemporaneous nature of these shear domains strain analysis of deformed tuffaceous diamictites and oncolithic limestones reveal k values 1 mean 0 59 throughout the heritage range the coexistence of strike parallel dextral oblique and reverse shear domains abrupt reorientation of progressive strain axes steep cleavage dips and k values 1 are all consistent with a dextral transpressive deformation regime previously unrecognized in the ellsworth mountains a model of highly oblique pure shear dominated transpression associated with efficient spatial partitioning of the strike slip component of shear is proposed to describe the structural relationships observed

fault mechanics and the kinematics of block rotations in many strike slip tectonic settings large rotations up to 100 of crustal blocks have been inferred from paleomagnetic data these blocks are bounded by sets of parallel faults which accommodate the relative motion between the blocks as regional deformation progresses simple geometrical considerations require that the faults must also rotate in this paper we show that on the basis of mechanical considerations the amount of fault rotation permissible under a stationary stress field is limited to 20 to 45 consequently block rotations that are larger than 40 or 45 require more than one set of accompanying faults to accommodate the block rotation examples of such multiple sets with 40 to 45 between them as predicted by the model were recognized in sistan iran in yerington the lake mead area nevada and in southern california

bone shaped boudins in progressive shearing abstract some peculiar bone shaped boudins are observed in two areas of progressive shearing they are characterized by large quartz filled gashes that separate thinner boudins these gashes often have a consistent obliquity suggesting a rotation synthetic to the shear sense these may be formed by rigid material that crystallizes in tension gashes between the boudins during further ongoing deformation these behaved as rigid inclusions in a more ductile matrix and show systematic rotations the sense of rotation of the gashes constitutes a shear criterion

deformation mechanics in inclined brittle ductile transpression zones insights from 3d finite element modelling abstract most natural examples of transpression zones developed at oblique convergence regime are inherently 3d and have inclined boundaries a 3d finite element model with an elasto plastic rheology is used to investigate the structural and mechanical evolution of inclined transpression zones in a rock sequence above a frictional basal detachment inelastic constitutive relationships allow permanent strains to develop in response to the applied loads fe modelling results show that oblique convergence is accommodated by discrete deformation at the main pre existing inclined faults 70 and by distributed brittle and ductile deformation at active blocks oblique contraction at the active blocks resulted mainly in layer parallel shortening orthogonal to the model outer boundaries whereas thickening in the horizontal and vertical directions was accommodated via layer parallel fault strike parallel extension and up dip extrusion i e inclined extrusion lateral extrusion should have compensated the rest and or volume loss took place folding and thickening of the mobile backstop produced a non cylindrical asymmetric bi vergent anticline where permanent strains developed principally in the steep forelimb secondary conjugate fault zones also accommodate oblique slip and contribute to uplift displacement vectors within the transpression zone are rotated counter clockwise ca 20 30 with respect to vectors in the fixed backstop areas with higher rotation values seem to correlate with those showing higher ellipticity values the presence of pre existing faults favored strain partitioning from the onset of deformation fe modelling results compared with analytical natural example and analogue modelling results show that our mechanical modelling can overall match inclined transpression zones geometry that present different modes of strain partitioning and localisation

basaltic pillow mounds in the vinalhaven intrusion maine the vinalhaven intrusion is a dominantly granitic pluton of probable devonian age located on vinalhaven island and adjacent islands maine it consists of four main units coarse grained granite fine grained granite a gabbro diorite unit consisting of interlayered mafic hybrid and granitic rocks and a heterogeneous granitic unit the gabbro diorite unit occurs along the south and east coast of the island as a sheet like body hundreds of meters to more than 1 km thick that dips beneath the central granitic units and rests on heterogeneous granitic rocks that form the base of the intrusion and are exposed on islands to the southeast load cast and pipe structures at the bases of mafic sheets indicate that the gabbro diorite unit represents a sequence of basaltic injections that ponded on crystal rich mush at the base of a silicic magma chamber and variably interacted with overlying crystal poor granitic magma the pluton therefore represents a fossilized silicic magma chamber that was periodically replenished by basaltic magma near the base of the gabbro diorite unit some basaltic injections produced large mounds up to more than 10 m high and 100 m wide of tightly packed meter scale chilled basaltic pillows tubes and sheets in a granitic matrix the mounds appear to represent flow fronts of basaltic injections that entered and ponded on the floor of a silicic magma chamber although physical conditions differ significantly these plutonic pillow mounds appear to share many characteristics with submarine pillow basalts and lava flows

palaeomagnetic evidence for a displaced terrain in western antartica an outstanding problem in antarctic tectonics is the relationship between the east antarctic craton and the late palaeozoic cenozoic orogenic belts that comprise western antarctica1 antarctic peninsula ellsworth land eights coast and marie byrd land see fig 1a related problems are the place of western antarctica in reconstructions of gondwanaland and the evolution of the pacific margin of gondwanaland we present here palaeomagnetic data derived from folded cambrian argillites that crop out in the ellsworth mountains which were collected during the 1979 80 austral field season although the early palaeozoic palaeomagnetic field of gondwanaland is characterized by rapid apparent polar wander at the present level of coverage the data are consistent with rotation of the ellsworth mountain block with respect to the east antarctic craton as postulated by schopf2 this result supports a microplate nature of western antarctica