on the antarctic peninsula batholith the plutonic rocks of the antarctic peninsula magmatic arc form one of the major batholiths of the circum pacific rim the antarctic peninsula batholith is a 1350 km long by 210 km wide structure which was emplaced over the period 240 to 10 ma with a cretaceous peak of activity that started at 142 ma and waned during the late cretaceous early jurassic and late jurassic early cretaceous gaps in intrusive activity probably correspond to episodes of arc compression in a northern zone of the antarctic peninsula the batholith intrudes palaeozoic mesozoic low grade meta sedimentary rocks and in a central zone it intrudes schists and ortho and paragneisses which have late proterozoic nd model ages and were deformed during triassic to early jurassic compression in a southern zone the oldest exposed rocks are permian sedimentary rocks and deformed jurassic volcanic and sedimentary rocks all these pre batholith rocks formed a belt of relatively immature crust along the gondwana margin with few exceptions jurassic plutons crop out only within the central zone many are peraluminous having s like mineralogies and relatively high 87sr 86sri they are considered to consist largely of partial melts of upper crust schists and gneisses and components of mafic magmas that caused the partial fusion by contrast early cretaceous plutons crop out along the length of the batholith few magma compositions appear to have been affected by upper crust the bulk being compositionally independent of the type of country rock they intrude they are dominated by metaluminous calcic si oversaturated 1 type granitoid rocks with relatively low 87sr 86sri intermediate silicic compositions 5 mgo we interpret these to represent partial melts of basic to intermediate igneous locally garnet bearing lower crust contemporaneous mafic magmas e g syn plutonic dykes form a more alkaline si saturated series having higher 143nd 144nd at the same87sr 86sr than the intermediate silicic series to which they are not petrogenetically related the change from limited partial fusion of upper crust in jurassic times to widespread partial fusion of lower crust in early cretaceous times is considered to be a result of an increasing volume of basaltic intrusion into the crust with time

paleomagnetism of coastal california and baja california alternatives to large scale northward transport paleomagnetic data from the santa lucia orocopia sloa and baja borderland bba allochthons of coastal california and baja california have been interpreted to indicate up to 2500 km of post mid cretaceous northward transport of these regions with respect to interior north america however with neogene strike slip offsets taken into account geological interpretations correlate basement rocks of the coastal allochthons with continental basement rocks directly across the san andreas and related fault systems we have examined paleomagnetic data from sloa and bba and conclude that apparent discordances can be explained without large scale pre neogene tectonic transport three major observations are fundamental to this analysis 1 paleolatitudes derived from volcanic rocks of the jurassic eugenia formation of bba and coast range ophiolite of sloa are concordant when compared to revised jurassic reference paleomagnetic poles from interior north america 2 isotopic and paleobarometric data from the peninsula ranges batholith in southern california indicate that the batholith has been tilted northeast side up by an amount that can account for discordant paleomagnetic directions observed in plutonic rocks of the batholith without large scale northward transport 3 literal interpretation of the paleolatitudes determined from paleomagnetic directions in upper cretaceous and paleogene marine sedimentary rocks of sloa and bba requires north then south then north transport and a complex motion history between the two allochthons however concordant paleolatitudes are indicated by some sedimentary rocks while coeval or younger sedimentary rocks of the same allochthon have discordant paleolatitudes coupled with recent documentations of compaction shallowing of paleomagnetic inclination in other marine sedimentary rocks these inconsistencies suggest that paleolatitudes derived from most of the marine sedimentary rocks of sloa and bba are biased towards low paleolatitudes by compaction shallowing

cone penetration test calibration for erksak beaufort sea sand discussion this is a discussion of a paper by the same title which appeared in volume 24 canadian geotechnical journal pp 601 610 the authors of the original paper are k been b e lingnau j h a crooks and b leach

error analysis for the determination of tectonic rotation from paleomagnetic data appendix b is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 001 2 50 payment must accompany order recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25 this has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence 95 an analytical calculation which is valid when error bars are small shows that the correct error bars are 75 to 80 of the values derived in previous studies i have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used the analytical approximation may be used whenever 95 is less than 10 and the mean inclination is not too close to vertical confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20 25 narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data

dyke intrusion into a pre existing joint network the aiguablava lamprophyre dyke swarm catalan coastal ranges abstract a structural analysis has been performed in the upper permian lamprophyric dyke swarm of aiguablava costa brava ne iberia emplacement of sub vertical dykes is related to the presence of a widespread joint network mostly developed during the cooling and decompression of the late variscan leucogranitic host rocks the joint pattern consists on multiple sub vertical joint sets among which two orthogonal sets at n23 and n113 are predominant a sub horizontal set is also prominent the sub vertical dykes have a mean n100 n125 trend which corresponds to the trend of one of the main joint sets however dyke segmentation is noticeable at the dm to cm scale and this is inferred to be related to dyke propagation and emplacement along the variably oriented pre existing joints a mean robust sub horizontal nne ssw net dilation direction was measured from matching dyke jogs markers in the host rock and analysis of maximum dyke thicknesses and this is in line with the minimum principal stress axis 3 derived from a three dimensional paleostress analysis from dyke orientations the inferred maximum principal stress axis 1 is sub vertical indicating that dykes intruded under conditions of tectonic extension furthermore a mohr construction allowed calculation of the stress ratio 0 29 close to a prolate stress ellipsoid and a driving pressure ratio r 0 27 which corresponds to a magmatic pressure almost equal to the intermediate principal stress axis 2 it is inferred that many of the pre existing joint sets were exploited by the magma the wnw ese joint set normal to 3 being the most favorable for dyke emplacement the present study highlights the structural control of a pre existing fracture network on emplacement of the aiguablava lamprophyres in the upper crust during late permian nne ssw brittle extension

the origin of the upper palaeozoic chanaral melange of n chile the fragmented and mixed strata of the chanaral melange extend for some 220 km from n to s in the coastal ranges of n chile 26 20 to 28 20 s the melange was produced in an accretionary wedge resulting from the ne subduction of the ancestral pacific oceanic plate beneath the gondwana land margin during carboniferous times the melange consists essentially of blocks of sandstone in a pelitic matrix these sediments were deposited as deep sea basin plain turbidites prior to their fragmentation other blocks in the melange include mafic volcanic rocks with the geochemical characteristics of oceanic island basalts and rare shallow marine limestones structures in the melange suggest an origin by two distinct deformation processes involving unlithified sediments initial boudinage and break up of the strata was apparently accomplished by intrastratal movement resulting from imbricate thrusting within the accretionary wedge the second process produced cross cutting zones of breccia which are interpreted as fluid escape conduits resulting from the high pore pressures produced during underthrusting

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

spreading structure of the troodos ophiolite cyprus some paleomagnetic constraints it has been suggested that grabens on the northern flank of the troodos ophiolite formed by listric faulting in an oceanic environment and represent axial valleys fossilized by ridge jumping this paleomagnetic survey of the solea graben was carried out to constrain the structural interpretation an analysis was devised which restores both the dikes sampled to vertical and the primary remanent magnetizations to a previously derived structurally corrected remanent magnetization direction typically two solutions for the original dike orientation and the pole and angle of net rotation are obtained for each dike sampled these solutions can be separated if a locally consistent original dike strike is assumed reasons are discussed for the selection of the solution that gives an original northwesterly dike strike results are consistent with a graben rotations are up to 78 at the sites studied rotation vectors are subhorizontal and subparallel to the original dike strike suggestive of an axial process for the development of the structure however on the basis of field observations an alternative mode of origin is proposed invoking ephemeral extreme upper crustal extension by listric faulting on the west flank of an intermediate to fast spreading axis the grabens being formed by antithetic faulting

caledonian plutonism and major lineaments in the sw scottish highlands within the sw grampian highlands several ne sw trending shear zones and faults related to the caledonian transpressional collision are recognized as being distinct from an intersect ing set of nw se trending pre caledonian crustal lineaments which were reactivated during caledonian orogenesis the geochemical and isotopic characteristics of the 425 400 ma granitoids of this region show that they were derived by anatexis of the lower crust associated with a mantle component which provided an influx of mantle heat and or metasomatic fluids their spatial distribution suggests a close genetic relationship with caledonian shear zones having made the fundamental distinction between ascent transport and emplacement arrival configurations possible for granitoid magmas new data are presented for these plutons which show that i a common modal sequence of intrusive phases can be recognized ii these phases are all sited at shear zone or lineament intersections where transtensional zones allowed and facilitated ascent iii emplacement was often by a process of localized in situ ballooning the existence of such lineaments in these orientations in the sw highlands may indicate that the structure of the lower crust can be regarded as a series of blocks bounded by intersecting ductile zones of high strain the establishment of such a pattern of structural interactions within the lower crust could provide a mechanistic framework within which the location of anatectic zones siting and ascent pathways and any subsequent emplacement phenomena can be explained in orogenic belts

structural analysis of dyke emplacement directions as an aid to palaeomagnetic studies an example from northern chile summary a major problem in palaeomagnetic studies of intrusive rocks lies in determining whether or not such rocks have been subjected to post emplacement tilting structural analysis of dyke emplacement directions can be used to show the current attitude of the extension direction for the dyke swarm if the original extension direction at the time of emplacement can be deduced from geological evidence this then provides a field test for post emplacement tilting of the dyke swarm and its host rocks in the example given from northern chile we were able to make a palaeomagnetic study of and structurally analyse three successively younger dyke swarms that intrude similarly younging plutons all three dyke swarms yielded extension directions close to horizontal but with markedly diverent azimuths it is argued that the similarity in the plunge of the extension directions cannot be coincidental and that the dykes and their host plutons have not suvered significant post emplacement tilting this simple technique should be widely applicable in the assessment of post emplacement tilting of dykes in palaeomagnetic studies

the early cenozoic tectonic history of the southeast pacific abstract new marine geophysical data collected in the southeast pacific enable us to define better the early cenozoic tectonic history of the area and to interpret previously unexplained bathymetric features in terms of plate tectonic processes we have identified a major re organization in plate boundaries at anomaly 21 time when a large fragment was broken off the pacific plate and joined to the antarctic plate this interpretation is based on three pieces of evidence 1 a major ese trending feature the humboldt fracture zone marks the southern boundary of the farallon antarctic anomalies identified by weissel et al 1 2 a north south striking trough the hudson trough which represents the scar left by the pacific antarctic ridge when it rifted across old pacific crust 3 a sequence of north south striking magnetic lineations south of the humboldt fracture zone and east of the hudson trough is correlated to anomalies 27 to 22 and identified with pacific aluk spreading

forewarning of static liquefaction landslides abstractstatic liquefaction failure and the ensuing flow slide of slopes have produced significant damages to structures and even loss of lives based on a very large number 1 632 of laboratory s

effects of ridge crest trench interaction on antarctic phoenix spreading forces on a young subducting plate precise measurements of spreading rates on marine magnetic profiles collected to the west of the antarctic peninsula have enabled some consideration of the forces governing plate motion since antarctic phoenix motion has been controlled by the local rather than global force balance over the last 35 m y the total effective driving force per unit length of trench is calculated to have ranged between 2 6 and 3 6 1012 n m which is much less than is commonly thought necessary to support subduction conventional calculations may overestimate slab pull for old slabs because they neglect the effect of extensional disruption in limiting the contribution to the balance of forces at the trench the low estimate of driving forces obtained here implies that resistive forces are also smaller than is generally assumed driving forces show a strong correlation with observed spreading rates which indicates that resistive forces were largely velocity dependent fluid migration up the subduction zone may elevate temperatures in and around the shear zone reducing resistive forces below the levels required by purely conductive models changes in convergence rate may affect the depths of both the brittle ductile deformation boundary and the basalt eclogite phase change causing a negative feedback which would appear as a velocity dependent resistive force the different driving forces acting on the ne and sw parts of the phoenix plate as a consequence of older oceanic lithosphere at the trench in the ne caused antarctic phoenix spreading to take place about a near pole to the sw since 21 m y ago and ultimately resulted in disruption of the phoenix plate about 9 m y ago spreading rates decreased abruptly about 6 m y ago probably because of e w compression across the long transform faults bounding the phoenix plate however spreading on the last three segments of the antarctic phoenix ridge continued at least until 4 m y ago either spreading stopped progressively from sw to ne or the final stage took place about a very near pole to the sw a magnetic quiet zone extends up to 95 km from the margin between the tula fracture zone and the north anvers fracture zone and is thought to indicate that the ridge crest became buried by terrigenous sediment prior to collision the absence of a magnetic quiet zone associated with the most recent ridge crest trench collisions suggests a change in sedimentary regime during the late miocene anomalously fast apparent spreading rates between 23 and 21 m y ago are thought to indicate an error in this part of the magnetic reversal time scale

evidence for a two phase palmer land event from crosscutting structural relationships and emplacement timing of the lassiter coast intrusive suite antarctic peninsula implications for mid cretaceous southern ocean plate configuration 1 new analysis of the relationships between geological structural data and radiometric ages for the lassiter coast intrusive suite indicate that the collisional mid cretaceous palmer land event orogeny in the antarctic peninsula has had two kinematic phases forming an intersection orocline one of which can be related to cretaceous southern ocean plate motions both are compressional phases along the eastern palmer land shear zone phase 1 occurred at 107 ma with a principal paleostrain axis of 341 and is best expressed in southern palmer land although evident elsewhere on the antarctic peninsula phase 2 occurred at 103 ma with a principal paleostrain axis of 259 5 but is confined to between 68 s and 74 s a peak in lassiter coast intrusive suite magma emplacement rate was coeval with phase 1 whereas phase 2 may have coincided with a lull during phase 1 the allochthonous central and western domain terranes may have been transported to the gondwana margin represented by the para autochthonous eastern domain on board the phoenix plate or on board the south american plate the variable provenance indicators from the central and western terranes can be cited to support either or a combination of these scenarios the convergence direction evident from phase 2 structures cannot easily be reproduced in regional plate kinematic models this and the localized evidence for superposition of phase 2 structures on phase 1 structures suggests that phase 2 is unlikely to have occurred solely in response to changes in the kinematics of the large plates considered

syntectonic emplacement of the middle jurassic concon mafic dike swarm coastal range central chile 33 s abstract the concon mafic dike swarm cmds consists of basaltic to andesitic dikes emplaced into deformed late paleozoic granitoids during the development of the jurassic arc of central chile the dikes are divided into an early group of thick dikes 5 12 m and a late group of thin dikes 0 5 3 m two new amphibole 40ar 39ar dates obtained from undeformed and deformed dikes constrain the age of emplacement and deformation of the cmds between 163 and 157 ma based on radiometric ages field observations ams studies and petrographic data we conclude that the emplacement of the cmds was syntectonic with the jurassic arc extension and associated with sinistral displacements along the nw trending structures that host the cmds the common occurrence of already deformed and rotated xenoliths in the dikes indicates that deformation in the granitoids started previously the early thick dikes and country rocks appear to have been remagnetized during the exhumation of deep seated coastal rocks in the early cretaceous around 100 ma the remanent magnetization in late thin dikes is mainly retained by small amounts of low ti magnetite at high temperature and pyrrhotite at low temperature the magnetization in these dikes appears to be primary in origin paleomagnetic results from the thin dikes also indicate that the whole area was tilted 23 to the nnw during cooling of the cmds the nnw sse extension vectors deduced from the paleomagnetic data and internal fabric of dikes are different with respect to extension direction deduced for the middle late jurassic of northern chile pointing to major heterogeneities along the margin of the overriding plate during the mesozoic or differences in the mechanisms driving extension during such period

the dilation direction of intrusive sheets abstract where a dilation dike or sill cuts two non parallel planar features the direction of dilation can be determined by 1 orthographic projection 2 equal area projection schmidt net or 3 analytical geometry

shear wave velocity to evaluate in situ state of cohesionless soils shear wave velocity vs measurements were carried out in a triaxial testing program on three different cohesionless soils the vs was measured using bender elements during consolidation and at ultimate steady state after consolidation the soil samples were loaded in shear under constant strain rate triaxial compression either drained or undrained to determine their ultimate steady or critical state uss at large strains the vs measurements were used to develop relationships between the void ratio e mean normal effective stress p and vs the shear loading results were expressed within the framework of critical state soil mechanics the results of the vs and uss information were combined with the state parameter concept to develop an equation to use field measured vs to estimate the in situ consolidation state within a soil thus the contractive dilative boundary with respect to vertical effective stress for large strain loading can be determined from in situ measurements of vs these can then b

the north chilean coast range an example for the development of an active continental margin analysis of extended plutons in the coast range of north chile between 25 30 and 26 35 led to the recognition of a complex magmatic and structural evolution from the upper paleozoic to the tertiary

a new jurassic recent apparent polar wander path for south america and a review of central andean tectonic models abstract the jurassic to recent palaeomagnetic data from the stable cratonic areas of south america and africa are reviewed and evaluated from the combined data set seven mean reference pole positions are calculated for the early middle and late jurassic early and late cretaceous palaeogene and neogene most of these new palaeopoles are well defined and indicate a complex course of apparent polar wander for the south american plate comparison of the palaeomagnetic data from the andean margin with the reference poles indicates vertical axis rotations defining a broad pattern of anticlockwise rotations north of the arica deflection at approximately 19 s and clockwise rotations to the south further analysis indicates however that the distribution and magnitude of rotations is more complicated than this broad division tectonic models proposed to explain the pattern of rotations and the possible origins of the arica deflection are discussed it is suggested that the rotations probably result from localised in situ rotations controlled by strike slip fault systems or thrust sheets rather than large scale rotation mechanisms the broad rotation pattern may result from the influence of pre existing structures in the margin or variations in the flexural rigidity of the brazilian shield behind the deformation zone

interpretation of in situ test results from the canlex sites one of the primary objectives of the canadian liquefaction experiment canlex project was to evaluate in situ testing techniques and existing interpretation methods as part of the overall goal to

antarctic peninsula continental magnesian andesites indicators of ridge trench interaction during gondwana break up subduction of young hot oceanic lithosphere and a major episode of lithospheric extension have been considered as key tectonomagmatic factors in the genesis of setouchi type continental magnesian andesites we present data on a new discovery of such andesites from the northern part of the antarctic peninsula and propose that subduction of newly created oceanic lithosphere occurred along the proto pacific margin of antarctica during the middle jurassic continental magnesian andesite magmatism was contemporaneous with the early stages of gondwana break up and we suggest that subduction of young hot oceanic lithosphere and ridge trench interaction may have contributed to the disintegration of gondwana by thermally weakening the lithosphere and developing a broad linear extensional zone along the proto pacific margin

paleomagnetic arguments for block rotations along the arakapas fault cyprus paleomagnetic data from the sheeted dike complex north of the arakapas fault reveal significant internal rotations the north south trending dikes have remanent magnetization directions close to the well known westerly paleomagnetic direction of the troodos massif whereas the east west trending dikes near the arakapas fault have northerly remanent magnetization directions the paleomagnetic data are in agreement with the apparent clockwise rotation of the dikes contrary to recent geological analyses which postulate that the pattern of dike trends is due to dike injection into a sigmoidal stress field along a sinistral transform fault the paleomagnetic results indicate clockwise rotations about vertical axes in response to a dextral shear in addition to the rotations around vertical axes small rotations around subhorizontal axes are inferred from the paleomagnetic data for tilted dikes thus these data reinforce the vertical intrusion hypothesis

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

the wiley glacier complex antarctic peninsula pluton growth by pulsing of granitoid magmas early cretaceous gabbro quartz diorite granodiorite and tonalite granodiorite plutons of the wiley glacier complex in the antarctic peninsula magmatic arc were emplaced in a zone of syn magmatic extensional shearing the oldest component pluton is the creswick gap quartz diorite granodiorite pluton and this is cut by slightly younger hornblende gabbro of the moore point pluton these plutons form the wall rock to the burns bluff tonalite granodiorite pluton the youngest component of the complex the burns bluff pluton comprises tonalite granodiorite sheets which at the plutons margins are interleaved with screens of mylonite wall rock the moore point hornblende gabbro has nd141 of ca 4 and sr141 of ca 1 it is predominantly mantle in origin although its parent magma assimilated some continental crust during emplacement the creswick gap pluton has nd141 between ca 4 quartz diorite and ca 1 granodiorite and sr141 from ca 1 quartz diorite to ca 22 granodiorite this pluton contains both mantle and crustal components we suggest that the quartz diorite facies fractionated from a similar parent magma to that of the moore point gabbro whilst the granodiorite fractionated from a gabbro quartz diorite magma during continued crustal assimilation geochemical variations within the creswick gap pluton cannot be generated by afc models some of the granodiorite samples have chemical characteristics which typify partial melts of amphibole garnet bearing crust we conclude that the pluton comprises fractionates of basaltic magma and crustal partial melt the burns bluff tonalite granodiorite sheets have nd141 between ca 4 and ca 2 and sr141 from ca 5 to ca 31 their isotopic and chemical compositions suggest that some are fractionates of basalt gabbro magma others are predominantly partial melts of amphibole garnet bearing crust and some are mixtures of these two magma types the chemical and isotopic compositions of the burns bluff plutons constituent tonalite sheets and its dyke like internal structure suggest that it grew incrementally via the addition of melt batches from a variety of crustal and mantle sources the growth of this pluton via dyking may reflect magma pulsing associated with transtensional movement along the creswick gap shear zone moreover the burns bluff pluton may be a frozen conduit system for structurally higher plutons in the antarctic peninsula batholith

k rich mantle metasomatism control of localization and initiation of lithospheric strike slip faulting a conceptual model is proposed in which bulk transtension or local transtension during bulk simple shear resulting from mantle anisotropy contrasts or lithosphere rheology contrasts of heterogeneously enriched lithospheric mantle triggers localized k rich magmatism which focuses strain and causes nucleation of lithosphere scale transtensional or strike slip shear zones transtension triggered magmatism is most likely to be located at sites of maximum metasomatism of the lithospheric mantle magma generated fractures propagate upwards nucleating zones of lithospheric weakness which focus shear in narrow transcurrent faults or at basin margins in this way magmatism controls fault timing and location although volcanism will be coeval with fault development and volcanoes will appear fault controlled counterintuitively our model suggests that faults are in a sense volcanocontrolled we suggest that this new transtension k rich magmatism transcurrent faulting association represents a hitherto unrecognized genetic relationship as significant as for example the ocean island magma series

mechanisms of continental growth in extensional arcs an example from the andean plate boundary zone during the late triassic jurassic and early cretaceous an extensional magmatic arc was formed in the andean margin of northern chile plutons emplaced at ramps within a hinterland propagating extensional duplex were fed by dikes that transferred magma through the lower crust from a reservoir in the mantle phases of volcanism separated phases of plutonism and our model demonstrates that plutonism was favored only when upper plate extensional fault systems were active when they were not dikes cut across inactive faults and magma rose directly to the surface during volcanic phases of arc growth

breakup of young oceanic lithosphere in the upper part of a subduction zone implications for the emplacement of ophiolites it is investigated by means of numerical modelling whether the interaction between young oceanic lithosphere and a subduction zone can lead to the breakup of a young and thin oceanic plate first a thermal model of the upper part of a subduction zone the region between the trench and the volcanic arc is presented temperatures are modelled for situations in which the age of the subducting oceanic lithosphere gradually decreases culminating in the arrival of a spreading ridge at the trench temperatures are used to infer the strength of material within the subducting plate from a pressure and temperature dependent rheology next it is investigated by means of finite element modelling whether the forces acting upon young subducting oceanic lithosphere recently created at a spreading center in the vicinity of a subduction zone can lead to its breakup our modelling results show that very young oceanic lithosphere may indeed break up during the early phase of its subduction whether breakup occurs depends upon the spreading velocity and the length of the ridge segment that interacts with the trench if breakup occurs some young oceanic lithosphere situated in the upper part of the subduction zone and in the region between the ridge and the trench will be detached from the other parts of the subducting plate the detached sheet of thin oceanic lithosphere will not be subducted to mantle depths instead subduction of the trailing plate at the opposite side of the ridge can lead to the incorporation of this sheet into the forearc region our modelling provides a mechanism to incorporate thin oceanic lithosphere into an arc trench region and eventually upon closure of the ocean basin into an orogenic belt it is shown that a large subset of ophiolites those with a harzburgite dominated mantle section has properties in good agreement with an emplacement history in which the breakup of young oceanic lithosphere in the upper part of a subduction zone is the first phase

the age and stratigraphy of fore arc magmatism on alexander island antarctica fore arc magmatic sequences associated with high mg number andesite lavas unconformably overlie lemay group accretionary complex in alexander island high resolution 40ar 39ar u pb zircon fission track and k ar ages demonstrate that subduction related fore arc magmatism migrated northwards along the length of alexander island between c 80 ma and c 46 ma the magmatic rocks represent a third of the western margin of the antarctic peninsula magmatic arc and are critical to the understanding of the final phase of subduction along the southern antarctic peninsula margin the onset of late cretaceous magmatism is recorded by poorly exposed volcanic rocks on monteverdi peninsula 79 7 2 5 ma in central and northern alexander island the magmatic rocks can be distinguished by the proportion range and types of lithofacies present and by the periods of magmatism represented the volcanic rocks of the colbert mountains range in age from c 69 62 ma and are dominated by large volume dacitic and rhyolitic crystal rich ignimbrites interpreted as caldera fill deposits elgar uplands sequences range in age from c 55 50 ma and contain approximately equal proportions of pyroclastic deposits and less evolved basaltic andesite and andesite lavas including high mg number andesite lavas near the base of three sequences the volcanic rocks of finlandia foothills probably represent the youngest calc alkaline units on alexander island 48 2 ma the sequence is lithologically similar to the elgar uplands and also contains high mg number andesite lavas but it is dominated by polymict conglomerates with minor lavas which were deposited in a graben associated with regional extension plutonic rocks exposed in the rouen mountains adjacent to the elgar uplands yielded a u pb age of 56 3 ma which is in discordance with a previously published rb sr age 46 3 ma probably due to hydrothermal perturbation of the rb sr system northwards migration of magmatism was caused by the progressive collision and subduction of three ridge segments prior to the previously reported ridge crest trench collisions that occurred c 20 30 ma later and following which subduction ceased

integrating s wave seismic reflection data and cone penetration test data using a multiangle multiscale approach a cone penetration test cpt is the most common geotechnical testing method used to estimate in situ the strength properties of soil although cpt provides valuable information this information is restricted to the location of the measurement we propose a new concept to integrate shallow s wave reflection seismic data with cpt data in order to obtain laterally continuous subsoil information in this vein a valid quantitative means to relate seismic reflections to cpt data is a primary requirement the approach proposed here is based on the characterization of the scaling behavior of the local fine scale s wave velocity information extracted from the seismic reflection data and the same behavior of the cpt cone resistance the local velocity contrast information is extracted by linearized zoeppritz inversion of the amplitude preserved prestack reflection data we have formulated a multiscale analysis approach employing the continuous wavelet transform in order to quantitatively characterize the nature

structural setting of the candelaria fe oxide cu au deposit chilean andes 27 30 s the candelaria fe oxide cu au ore deposit lies within the thermal aureole of the lower cretaceous magmatic arc plutonic suite in the candelaria punta del cobre district atacama region northern chile 27 30 s economic mineralization is present within a synplutonic ductile shear zone candelaria shear zone containing foliated and potassically altered volcanic and volcaniclastic rocks of the punta del cobre formation in the candelaria shear zone there was a complex interplay in space and time between ductile and brittle deformation during mineralization at high structural levels brittle moderately to steeply dipping extensional faults curve downward into gently dipping ductilely deformed rocks in the shear zone textural and structural characteristics of the ore and the deformed rocks indicate that the dilation of the sulfide bearing vein systems occurred at the same time as ductile deformation and implies that ductile brittle cycling occurred in a cooling hydrothermal system in the thermal aureole of the arc plutonic rocks the la brea and the san gregorio plutonic complexes are exposed in the plutonic arc immediately adjacent to the candelaria orebody the intrusions are interpreted to be flat lying tabular bodies emplaced by roof uplift floor depression mechanisms during regional extensional deformation an 40 ar 39 ar age of 119 6 1 2 ma and five k ar ages between 123 and 117 ma were obtained for the la brea pluton and an 40 ar 39 ar age of 111 5 0 4 ma was obtained for the san gregorio plutonic complex 40 ar 39 ar ages of 111 0 1 4 and 110 7 1 6 ma obtained from syntectonic biotite from the candelaria shear zone are interpreted to be the age of the mineralization these are indistinguishable from the age of the san gregorio plutonic complex and therefore heat from this pluton could have promoted ductile deformation during emplacement of the orebody this result together with known isotopic similarities between candelaria ore and the plutonic rocks implies that the san gregorio plutonic complex was the most probable source of the mineralizing fluids at candelaria although the critical intrusive unit may not be exposed the ore deposit is located where a roof uplift fold in the side wall of the san gregorio plutonic complex provided a dilatational site which may have drawn hydrothermal fluids out of the adjacent fractionating magmas into a structural trap in the punta del cobre formation below the carbonate rocks of the chanarcillo group

mafic dike swarms in the south shetland islands volcanic arc unravelling multiepisodic magmatism related to subduction and continental rifting eight groups of mafic dikes and related high level stocks cut triassic accretionary complex and mesozoic magmatic arc formations on livingston island some are affected by silicic sericitic alteration related to cretaceous hydrothermal activity and propylitic epidosite alteration analogous to that in ocean floor sheeted dikes alteration was accompanied by major and trace element metasomatism ar ar analysis of the freshest rocks indicates five intrusive events some of which are unexpectedly young groups 1 3 were intruded in the mid to late cretaceous 108 74 ma and were coeval with the calc alkaline arc between 70 and 50 ma relatively rapid and oblique plate convergence led to strike slip tectonism and a pause in magmatism at 52 ma orthogonal slow convergence resulted in extensional faulting and emplacement of calc alkaline group 2 and primitive tholeiitic dikes groups 4 6 between 51 and 45 ma extension of antarctic peninsula southern south american crust culminated in emplacement of mafic to intermediate medium grained plutons and group c porphyries between 44 and 36 ma localized hydrothermal flow along fault zones resulted in partial to complete argon loss from nearby cretaceous lavas and ar ar reset ages of 40 ma in mid cretaceous hydrothermal k feldspar primitive olivine basalts group d and epithermal carbonate veins 31 29 ma were emplaced during along arc extension accompanying the opening of drake passage and powell basin excess argon occurs in two forms strongly held in melt inclusions in the primitive tholeiites and weakly held in some secondary alteration there is no radiometric evidence in the area studied for magmatism related to late cenozoic subduction nor to the pleistocene recent opening of the back arc bransfield rift

a potassic volcanic front in western mexico the lamprophyric and related lavas of san sebastian the superposition of two tectonic regimes rifting and subduction in western mexico has led to the development of a lamprophyric volcanic front 70 km closer to the middle america trench than is the main axis of andesitic volcanism the san sebastian cindercone field represents the northernmost extent of lamprophyric activity in western mexico and lies 40 km trenchward of volcan ceboruco a historically active andesitic stratovolcano the predominant country rocks are cretaceous silicic ash flows although marine sedimentary rocks emplaced between 45 and 115 ma also occur minette absarokite and kersantite lavas were erupted between 0 48 and 0 26 ma in the san sebastian field in close association with flows of basaltic andesite and hornblende andesite there is a gradual transition observed between the two intermediate lava types basaltic andesite and kersantite as plagioclase is suppressed and hornblende and phlogopite are stabilized in melts of intermediate composition this transition reflects the increasing influence of water during crystallization and documents a wide range of hydrous conditions examination of the primary lamprophyric lava types absarokite and minette reveals a remarkable range in their oxidation states if these lavas reflect their mantle source conditions then the redox state of western mexico9s sub arc mantle spans the interval between the ni nio and magnetite hematite buffers a variation in fo 2 of five log units the oxidation and hydrous enrichment of mexico9s sub arc mantle probably formed in response to subduction the eruption of lamprophyric lavas in front of the andesitic suite of the main axis however may have depended upon rifting 1 to trigger melting of the modified mantle and 2 to provide an easy passage to the surface through crustal thinning and block faulting

characterization of unsaturated mine waste a case history it is becoming increasingly common in the mining industry for either crushed ore or filtered mine waste to be stacked to a significant height 100 m in a moist state with little compaction resul

antarctic tectonics constraints from an ers 1 satellite marine gravity field a high resolution gravity field of poorly charted and ice covered ocean near west antarctica from the ross sea east to the weddell sea has been derived with the use of satellite altimetry including ers 1 geodetic phase wave form data this gravity field reveals regional tectonic fabric such as gravity lineations which are the expression of fracture zones left by early 65 to 83 million years ago pacific antarctic sea floor spreading that separated the campbell plateau and new zealand continent from west antarctica these lineations constrain plate motion history and confirm the hypothesis that antarctica behaved as two distinct plates separated from each other by an extensional bellingshausen plate boundary active in the amundsen sea before about 61 million years ago

seismic responses to ridge transform subduction chile triple junction the first local seismic network established in the remote region bordering the chile triple junction monitored earthquakes at a rate of three per day with magnitudes m 0 to m 4 far smaller than any recorded previously from that region focal mechanisms were obtained by the adaptation of the relative amplitude method to local earthquakes epicentral locations define a lineament corresponding to the active segment of a subducted transform fault and events corresponding to the predicted position of a subducted ridge show extensional mechanisms with the exclusion of thrusting and trench parallel mechanisms these data suggest that oceanic spreading and seismic activity continue in the subducted ridge transform system extension in the slab influences the overlying crust giving rise to dextral transtension over the past 10 m y the locus of ridge subduction has migrated northward we postulate that the pattern through time of extension and basin development in the overlying crust has mirrored this

mesozoic magmatic and tectonic events within the andean plate boundary zone 26 27 30 s north chile constraints from 40 ar 39 ar mineral ages in north chile jurassic cretaceous arc magmatism is represented by several narrow belts of plutons and two separate volcanic sequences new 40 ar 39 ar mineral ages confirm five distinct episodes of magma emplacement as plutonic complexes c 202 188 ma c 160 153 ma c 142 138 ma c 130 127 ma and c 106 103 ma conjugate dike sets were emplaced immediately before periods of plutonism and each distinct episode of magmatism prior to c 127 ma was located successively eastward inboard from the subduction boundary features interpreted to result from regional extension within the magmatic arc steeply dipping ductile mylonitic shear zones were developed in wall rocks along western contacts of late jurassic and early cretaceous plutons steeply plunging mineral stretching lineations and a variety of kinematic indicators record an east side down extension along eastern faults of the atacama fault system steeply dipping ductile mylonitic shear zones display subhorizontal mineral stretching lin

tectonic setting of primitive magmas in volcanic arcs an example from the antarctic peninsula abstract primitive magmas representing mantle partial melts minimally affected by fractionation and assimilation are rare in the magmatic arc environment most examples are either associated with high rates of arc parallel extension or occur along faults and dykes perpendicular to the trend of the arc and related to arc compression in two cases the vanuatu and solomon islands arcs such arc compression is being caused by collision of seamounts in the antarctic peninsula primitive mafic dykes were emplaced perpendicular to the continental arc ar ar and k ar data suggest intrusion of the dykes at c 126 106 ma possibly during mid cretaceous regional compression of the arc the dykes form two compositional groups one group has low la n yb n ratios 0 31 0 49 lower nb yb and higher th nb than n morb age corrected nd values of 7 3 to 7 9 and are interpreted as melts of subduction modified sub arc asthenosphere the other has high la n yb n ratios 3 86 8 92 higher nb yb and th nb than n morb age corrected nd values of 2 8 to 3 4 and are interpreted as melts of sub arc lithosphere the absence of dykes compositionally between these groups suggests that the primitive magmas avoided storage and mixing in magma chambers

a tectonomagmatic model for the genesis and emplacement of caledonian calc alkaline lamprophyres petrogenetically related caledonian calc alkaline lamprophyric and shoshonitic dykes in eastern ireland appear coeval with sinistral transtension these dykes are exposed in a 10 km wide zone trending sw from clogher head that strikes anticlockwise of the surface trace of the iapetus suture lamprophyre dykes intruded sinistral riedel shears with a mean strike trend at 040 riedel shears that host lamprophyre dykes strike anticlockwise of primary sinistral shears by 35 consistent with sinistral transtension primary shears strike 070 parallel to the mean strike trend of shoshonitic dykes sinistral primary shears may have controlled the emplacement of caledonian shoshonitic intrusive rocks using lamprophyres from clogher head as examples a new model is proposed for lamprophyre genesis and emplacement related to simple shear in this model transient decompression of metasomatized mantle during simple shear triggers release of volatile rich potassic magma which ascends to crustal levels along steep strike slip faults an association between lamprophyre magmatism and deep faulting suggests that relatively deep 60km seismicity may precede magma ascent this model can account for several paradoxes of caledonian magmatism

role of strike slip faulting in the tectonic evolution of the antarctic peninsula the antarctic peninsula mesozoic magmatic arc has had a long history of dextral strike slip deformation the deformation was initially associated with the development of a wide accretionary complex by the migration of fore arc slivers and the formation and inversion of a thick fore arc basin succession it also formed an important component within major shear zones in the arc and may have controlled the formation of sedimentary basins in the back arc region although some transcurrent motion within the fore arc region was related to a component of oblique subduction the main movement occurred during the breakup of gondwanaland and the formation of a major transtensional rift system a new reconstruction for this part of gondwanaland is presented taking this transcurrent motion into consideration

isotopic and trace element constraints on the origin and evolution of alkaline and calc alkaline magmas in the northwestern mexican volcanic belt a wide variety of rock types has been produced by pliocene to recent volcanism in the northwestern portion of the mexican volcanic belt composite cones erupted rocks typical of calc alkaline volcanic belts associated with subduction monogenetic cinder cones surrounding the composite cones erupted mildly alkaline basalts and related rocks one larger center las navajas produced basalts trachytes and peralkaline rhyolites such alkaline rocks are typically associated with crustal extension related to rifting both subduction and rifting appear to be taking place in western mexico the most basic calc alkaline rocks although somewhat fractionated show depletion in high field strength elements hfse and enrichment in alkaline earth elements relative to other incompatible elements a characteristic typical of magmas in subduction related volcanic arcs the basic alkaline rocks show no depletion in hfse and show higher concentrations of incompatible elements than the basic calc alkaline rocks the calc alkaline rocks show a tight cluster of 87sr 86sr and 143nd 144nd ratios both of which correlate weakly with sio2 basic alkaline rocks have lower sr and higher nd isotopic ratios whereas the more differentiated alkaline rocks have isotopic ratios that overlap with and extend beyond the range of values found in calc alkaline rocks trace element data indicate that calc alkaline magmas could not have been derived by crystal fractionation from the alkaline magmas nor by assimilation of crustal materials by alkaline magmas the evolution of both calc alkaline and alkaline rocks requires a process of crystal fractionation accompanied by assimilation afc of crustal rocks to account for changes in isotopic ratios and trace element concentrations in the more differentiated members of each suite assuming a contaminant located in the lower or middle crust afc modeling shows that the amounts of assimilation required for alkaline magmas are somewhat higher than those for calc alkaline magmas calc alkaline and alkaline rocks appear to have resulted from melting of two distinct sources alkaline from an oib type source and calc alkaline from a mixed mantle and slab derived source

iron oxide copper gold deposits an andean view iron oxide copper gold iocg deposits defined primarily by their elevated magnetite and or hematite contents constitute a broad ill defined clan related to a variety of tectono magmatic settings the youngest and therefore most readily understandable iocg belt is located in the coastal cordillera of northern chile and southern peru where it is part of a volcano plutonic arc of jurassic through early cretaceous age the arc is characterised by voluminous tholeiitic to calc alkaline plutonic complexes of gabbro through granodiorite composition and primitive mantle derived parentage major arc parallel fault systems developed in response to extension and transtension induced by subduction roll back at the retreating convergent margin the arc crust was attenuated and subjected to high heat flow iocg deposits share the arc with massive magnetite deposits the copper deficient end members of the iocg clan as well as with manto type copper and small porphyry copper deposits to create a distinctive metallogenic signature

geochemistry of holocene volcanic rocks associated with ridge subduction in baja california mexico although the geochemistry of magmatism associated with the subduction of oceanic lithosphere is well understood1 6 the geochemical signature of lavas produced shortly after subduction of an oceanic spreading centre has not been characterized the baja california peninsula in mexico provides an ideal scenario to study the latter process geophysical models indicate that in response to oblique collision of the ancestral east pacific rise epr with the western seaboard of north america at 29 myr subduction of the farallon plate was succeeded by the progressive southwards development of a transform fault system7 9 at 12 5 myr a considerable length of the epr was simultaneously subducted beneath baja thus terminating subduction processes along this segment10 by 3 5 myr the locus of transcurrent faulting had switched from the west of baja to within the developing gulf of california coupling baja to the pacific plate11 13 we present here data for holocene volcanic rocks from baja california norte which show that post ridge subduction volcanism has a distinctive geochemistry these results have important consequences for the interpretation of ancient orogenic belts

further evidence of fracture zone induced tectonic segmentation of the antarctic peninsula from detailed aeromagnetic anomalies aeromagnetic anomaly data collected between 67 s and 70 s crossing the antarctic peninsula and adjacent offshore areas show a prominent nw se trend in the magnetic fabric apparent lateral offsets previously recognized in the pacific margin anomaly have been mapped in detail and are shown to be much smaller than previously suggested a 35 km wide zone of subdued magnetic anomalies at the western edge of the pacific margin anomaly bounded by these apparent offsets is interpreted as a downfaulted block of the mafic intermediate batholith thought to be responsible for the pacific margin anomaly the trends of both fracture zones and magnetic lineaments strongly support the link between faulting in the antarctic peninsula magmatic arc and offshore tectonics

relics of a complex triple junction in the weddell sea embayment antarctica abstract interpretation of an airborne magnetic data compilation containing a key new survey together with re tracked satellite gravity data from the weddell sea embayment wse west antarctica suggests rift rift rift triple junction formation at the onset of gondwana breakup in the early middle jurassic a complex system of northwest southeast rifts was active contemporaneously with an east west trending rift this rift activity led to northward separation of the falkland plateau and formation of the weddell sea by sea floor spreading atypically the jurassic passive margin of gondwana shows evidence for coeval extension in two directions and a large volume of interpreted magmatic material this is consistent with initial doming above a mantle plume and we suggest that this resulted in the formation of a triple junction magnetic anomalies indicate a series of faults perpendicular to igneous intrusions and extrusions with outlines that range in shape from lozenges to parallel ridges they show remarkably good spatial correlation with free air gravity anomalies even in areas of sea ice we base a structural elements map and timing sequence for the events in the wse during early gondwana breakup on anomaly cross cutting relationships

ridge trench collision induced switching of arc tectonics and magma sources clues from antarctic peninsula mafic dykes compositions and distributions of mafic dykes in the antarctic peninsula continental arc show that tapping of several mantle sources was tectonically controlled in the cretaceous to tertiary between 135 ma and 55 ma calc alkaline dykes intruded the arc in the late cretaceous however between 95 ma and 65 ma there was a pulse of compositionally diverse magmatism this change resulted from collision of an ocean spreading centre with the trench as a consequence non partitioned dextral transtensional shear in the overriding plate became partitioned into strike slip and extensional domains calc alkaline magmatism was therefore replaced by strike slip related shoshonitic magmatism towards the rear arc and extensionrelated tholeiitic magmatism towards the fore arc oib like dykes were emplaced because of the break in otherwise continuous subduction during the early tertiary subduction continued but ceased after a late tertiary ridge trench collision

influence of placement method on the cone penetration resistance of hydraulically placed sand fills the reclamation for the new airport at chek lap kok in hong kong included the placement of a substantial volume of sand fill by various hydraulic placement techniques which resulted in a wide rang

mesozoic fault systems deformation and fault block rotation in the andean forearc a crustal scale strike slip duplex in the coastal cordillera of northern chile abstract in this paper we discuss the evolution and tectonic significance of the mesozoic trench parallel fault systems which affected the coastal cordillera and their relation to magmatism and crustal rotation the oldest extensional fault system separates basement from rift related late triassic and younger sedimentary units this system i subsequently developed into a wider extensional fault system which acted as the locus of magma ascent and emplacement of the coastal batholith during much of the jurassic to earliest cretaceous period this extensional fault system defined the forearc sliver during this period and was the consequence of a retreating subduction boundary during the early cretaceous c 132 125 ma the kinematics of this fault system changed to transtension ii and accommodated a major component of left lateral strike slip motion the principal fault being the atacama fault zone along which plutons continued to be emplaced the final phase of pluton emplacement within the coastal cordillera appears to be c 106 ma after which this magmatic arc and fault system was abandoned an late cretaceous arc and fault system iii developed some 20 ma later and located some 50 km to the east in what is now the central valley of northern chile this paper seeks to show that the coastal cordillera was deformed as a whole by this late cretaceous fault system iii which formed a crustal scale left lateral transpressional duplex during this deformation the thermally weakened crust was dissected into a series of large scale blocks bounded by nw trending left lateral strike slip faults which merge into a nne ssw fault zone which forms the eastern boundary to the duplex we term this eastern boundary zone the central valley fault zone cvfz and this together with the nw trending faults defines the duplex system which we refer to as a whole as the coastal cordillera fault system ccfs iii we have traced the ccfs duplex between 25 s and 29 s and suspect that it continues northward the timing of the deformation is constrained to be post 106 ma the age of coastal cordillera arc abandonment and pre tertiary based on the deformation and pluton emplacement in and along the central valley fault zone palaeomagnetic data from the fault bound blocks within the ccfs duplex indicate 35 45 of post early cretaceous clockwise rotation with no substantial latitudinal motion we suggest that the observed fault kinematics of the ccfs are consistent with this crustal scale duplex model where rotations would have occurred in response to left lateral transpression

apparent polar wander of the mean lithosphere reference frame summary apparent polar wander in the mean lithosphere no net rotation no net torque uniform drag reference frame is compared with apparent polar wander in the hotspot reference frame over the past 100 myr palaeomagnetic poles and plate rotations previously used to determine an apparent polar wander path for the hotspot reference frame are here used to determine an apparent polar wander path in the mean lithosphere reference frame we find that the two paths are similar especially for late cretaceous time when a 10 20 shift of the pole occurred to first order the hotspots and lithosphere as a whole moved in unison relative to the palaeomagnetic axis during late cretaceous time a non dipole field explanation for the apparent shift can probably be excluded however either motion of the time averaged geomagnetic axis relative to the spin axis or polar wandering could have caused this shift the latter being the more likely explanation

the cone penetration test in sands part ii general inference of state the state parameter concept provides a new method of characterizing sand behaviour but the utility of the concept relies on the ability to measure the state parameter in situ part i of this paper demonstrated a relationship between normalized cone resistance and state parameter for monterey sand in this paper the state parameter approach is applied to data from calibration chamber tests on several sands it is shown that this approach leads to a common relationship for all the data that includes the effects of material type quantified by the slope of the steady state line the state parameter for a sand can therefore be determined in situ with a reasonable precision using a general framework of interpretation which applies to all uncemented sands la notion de parametre d etat fournit une nouvelle methode pour caracteriser le comportement des sables mais son utilite depend de la possibilite de mesurer le parametre d etat in situ la premiere partie de cet article demontre une relation entre la resist

tectonics of the pliocene loreto basin baja california sur mexico and evolution of the gulf of california the pliocene loreto basin is a transtensional basin exposed just north of loreto baja california sur that consists of nonmarine to marine sedimentary rocks and interbedded tuffs on the basis of stratigraphic study and 40ar 39ar dating of tuffs the southern loreto basin began to form at 3 4 ma as an alluvial basin with moderate rates of subsidence 0 4 mm yr between 2 46 and 2 36 ma extremely rapid subsidence 5 10 mm yr of the basin was accompanied by deposition of vertically stacked gilbert type fan deltas during this period the southernmost basin began to be cut by an array of dextral normal faults the basin returned to a moderate rate of subsidence from 2 36 to 2 0 ma when volcanism increased within the northern part of the basin the pacific north america boundary was first fully located within the gulf of california beginning at 3 5 ma at which time the zone of transform related deformation widened to initiate formation of the loreto basin the major change in the loreto basin at 2 46 ma may be coeval with the beginning of faulting in southern california on the elsinore and san jacinto faults these widespread events may indicate a minor change in the plate boundary at 2 5 ma

ground freezing and sampling of foundation soils at duncan dam in situ ground freezing to obtain undisturbed samples of a loose sand beneath the duncan dam in british columbia is described this was the first known use of ground freezing to assist in obtaining samples at depth greater than 10 m in canada once frozen the sand was cored and brought to the surface using a cold regions research and engineering laboratory core barrel which is used extensively in permafrost regions the design installation and performance of the liquid nitrogen freezing systems are described in detail the drilling sampling and preparation of the frozen core for transport to the testing facility are also outlined the quality of the core retrieved during the sampling was judged to be excellent and good agreement was found between the void ratios measured using both the frozen core and high quality gamma gamma density logging techniques which were performed adjacent to ground freezing and sampling locations key words ground freezing cohesionless soils undisturbed sampling

on the origin of fore arc basins new evidence of formation by rifting from the jurassic of alexander island antarctica the middle jurassic lower cretaceous fossil bluff group of alexander island antarctica represents the fill of a fore arc basin unconformably overlying an accretionary complex like most fore arc basins this example had been considered to have a passive origin as a topographic hollow between the arc and the trench slope break recent discoveries of igneous rock coeval with sedimentation have altered this view oxfordian kimmeridgian basaltic and rhyolitic sills and lava flows are found in a restricted area at the north of the basin within a single formation chemically most basalts are high nb types which cannot have originated in a supra subduction zone setting since the age of emplacement of these rocks coincides with a gap in the record of plutonism in the antarctic peninsula volcanic arc it is concluded that a late jurassic pause in subduction led to active rifting to form the fore arc basin

sedimentology and structure of the trench slope to forearc basin transition in the mesozoic of alexander island antarctica the mesozoic forearc of alexander island antarctica is one of the few places in the world where the original stratigraphic relationship between a forearc basin and an accretionary complex is exposed newlydiscovered sedimentary rocks exposed at the western edge of the forearc basin fill the kimmeridgian albian fossil bluff group record the events associated with the basin formation these strata are assigned to the newly defined selene nunatak formation bathonian and atoll nunataks formation bathonian tithonian within the fossil bluff group the selene nunatak formation contains variable thicknesses of conglomeratesand sandstones predominantly derived from the lemay group accretionary complex upon which it is unconformable the formation marks emergence and subsequent erosion of the inner forearc area it is conformably overlain by the1 km thick atoll nunataks formation characterized by thinly bedded mudstones and silty mudstones representing a marine transgression followed by trench slope deposition the atoll nunataks formation marks a phase of subsidence possibly in response to tectonic events in the accretionary prism that are known to have occurred at about the same time the atoll nunataks formation is conformably overlain by the himalia ridge formation a thick sequence of basin wide arc derived conglomerates this transition from fine to coarse grained deposition suggests that a well developed depositional trough and hence trench slope break had formed by that time the atoll nunataks formation therefore spans the formation of the forearc basin and marks the transition from trench slope to forearc basin deposition

tectonic implications of fore arc magmatism and generation of high magnesian andesites alexander island antarctica alexander island situated off the west coast of the antarctic peninsula contains a suite of late cretaceous to early tertiary subduction related magmatic rocks the rocks occupy a fore arc position 100 200 km trenchward of the main arc antarctic peninsula and they become younger northward along the length of the island major and trace element geochemistry for 222 samples shows the suite to be a medium to high k calc alkaline series ranging in composition from picro basalt to rhyolite andesite samples show a large range in mgo and mg with nine samples representing high magnesian andesites sr and nd isotopic data indicate that the andesites range isotopically to more depleted mantle compositions than the associated basalts the dacite rhyolites can be related compositionally to the andesites by assimilation of typical pacific rim accretionary material to produce high magnesian andesite lavas it is necessary to introduce a suitable source of heat into the fore arc thus enabling partial melting of depleted sub fore arc hydrous peridotite a causative link with ridge subduction prior to magmatism is proposed with successive ridge trench collisions producing a temporal migration of the magmatism and high geothermal gradients in an anomalously hot fore arc region

magmatic arc fault systems deformation partitioning and emplacement of granitic complexes in the coastal cordillera north chilean andes 25 30 s to 27 00 s three fault systems were responsible for permian to late cretaceous deformation of the overriding plate of the andean convergent margin in the coastal cordillera of northern chile 25 30 to 27 00 s displacements were linked to crustal growth expressed by the emplacement of a sequence of magmatic arcs the tigrillo fault system active from triassic to early cretaceous time was characterized by arc normal extension with increasing left oblique extension transtension from early jurassic to early cretaceous time stretching of the crust created space for triassic early jurassic and early cretaceous arc basins where epiclastic volcaniclastic and volcanic sequences accumulated in continental to shallow marine environments tabular plutonic complexes were emplaced by roof uplift floor subsidence that allowed a vertical transfer of material in the crust without significant horizontal extension the atacama fault system was initiated at c 132 ma as a mainly left strike slip fault during left oblique extension of the margin elongate tabular plutonic complexes were emplaced within the atacama fault system between c 132 and c 106 ma again by roof uplift floor subsidence mechanisms ductile brittle transitions in synplutonic mylonitic rocks of the atacama fault system provided the setting for kiruna type fe apatite and fe oxide with cu and or au ores the chivato fault system was active as an extensional fault system at the eastern side of the coastal cordillera during displacement on the tigrillo fault system and later between c 125 and c 93 ma as a partitioned left oblique extensional fault system in post early cretaceous time the chivato fault system was inverted by left oblique contraction transpression when nw trending transfer faults some probably reactivated lateral ramps in the tigrillo fault system accommodated clockwise vertical axis rotations of 35 45 contraction inverted the atacama fault system and tigrillo fault system and was responsible for west vergent thin skinned fold thrust deformation in stratified rocks throughout the margin

evaluation of soil state from sbp and cpt a case history the cone pressuremeter test cpt is widely used to determine the in situ state of cohesionless soils however although the cpt is simple inexpensive and accurate the subsequent interpretation

structural geology and geochronology of subduction complexes along the margin of gondwanaland new data from the antarctic peninsula and southernmost andes subduction complexes along the andean margin in central and southern chile yield mid paleozoic to lower mesozoic ages yet they crop out within 100 km of the modern trench that shows evidence of active accretion along much of its length the scarcity of uplifted subduction complex rocks younger than mid mesozoic along the chilean margin and in parts of the scotia arc suggests to us that these old crystalline rocks uplifted in the triassic and jurassic represent a boundary in the forearc beyond which tectonic erosion does not easily occur greenschist blueschist and amphibolite facies subduction complex rocks from the scotia arc were originally thought to be a simple continuation of the subduction complexes in chile based on new 40 ar 39 ar ages the scotia arc subduction complexes reveal a complex history related to distinct local tectonic events and are not a simple continuation of the old accretionary prism in chile structural and metamorphic analysis indicates the earliest and most penetrative deformation in the subduction complexes around the scotia arc occurred at some depth in a subduction zone or zones certainly below the brittle ductile transition and in some cases under blueschist facies conditions we believe that the early subduction related deformation and metamorphism in the greenschist and blueschist facies rocks of the scotia arc to be overprinted by mid cretaceous transpression along the south america antarctica plate boundary in the case of elephant island transpression and subsequent localized transtension in the earliest cenozoic in the case of the darwin complex a mid cenozoic spreading rate change in the case of smith island and early neogene initiation of drake passage opening shackleton fracture zone formation in the case of the gibbs island subgroup

a review of tertiary climate changes in southern south america and the antarctic peninsula part 2 continental conditions abstract climate changes in southern south america and the antarctic peninsula during the tertiary show a strong correlation with ocean warming and cooling events which are in turn related to tectonic processes during periods of accelerated sea floor spreading and mid ocean ridge activity sea levels rose so that parts of the continents were flooded and forests were destroyed however this was balanced by the large scale release of co2 during volcanic outgassing and carbonate precipitation on the continental shelves which caused rising air temperatures and the poleward expansion of sub tropical and temperate forests cooling episodes generally caused an increase in the north south thermal gradient because of an equatorward shift in climate belts so that the westerly winds intensified and brought higher rainfall to the lower latitudes an increase in wind blown dust caused temperatures to drop further by reflecting sunlight back into space the rising andes range had a marked influence on climate patterns up to the middle miocene it was still low enough to allow summer rainfall to reach central and north central chile but after about 14 ma it rose rapidly and effectively blocked the spill over of moisture from the atlantic ocean and amazon basin at this time the cold humboldt current was also established which together with the andes helped to create the arid diagonal of southern south america stretching from the atacama desert to the dry steppes of patagonia this caused the withdrawal of subtropical forests to south central chile and the expansion of sclerophytic vegetation to central chile however at the same time it intercepted more rain from the northeast causing the effect of the south american monsoon to intensify in northwestern argentina and southern bolivia where forest communities presently occur in patagonia glaciation started as early as 10 5 ma but by 7 ma had become a prominent feature of the landscape and continued apparently uninterruptedly into the pleistocene the antarctic peninsula saw its first mountain glaciation between 45 and 41 ma with major ice sheet expansion commencing at about 34 ma isolated stands of nothofagus forests were still present in low lying areas suggesting that the glaciers were initially wet based but dry based glaciers were established at around 8 ma although temperatures rose briefly during the messinian pliocene transition causing sub antarctic flora to retreat to higher elevations of the transantarctic mountains the present cold polar conditions were finally established by about 3 ma

major crustal rotations in the andean margin paleomagnetic results from the coastal cordillera of northern chile paleomagnetic analyses of mesozoic lavas and dike swarms from the northern chilean coastal cordillera between 25 4 s and 26 4 s reveal a clockwise rotation of about 42 magnetizations from lava flows of andesitic basaltic composition of the middle jurassic la negra formation pass both fold and reversal tests and are interpreted as prefolding remanences five dike swarms of middle jurassic to early cretaceous age yield similar directions to that obtained from the la negra formation four of the five swarms have mixed polarity suggesting that they too carry a primary or very early remanence the structural setting of the dikes suggests that they have not suffered any substantial tilting about nonvertical axes since acquisition of the remanence the clockwise rotation of the area is believed to have been the consequence of transpressional deformation of mid late cretaceous age post 100 ma associated with abandonment of the jurassic early cretaceous magmatic arc in this region and its eastward migration to form a new mid late cretaceous magmatic arc in the former back arc region this younger arc is located east of the coastal cordillera and lies in the central valley region the clockwise sense of rotation is consistent with other paleomagnetic data from northern chile and southern bolivia south of the arica deflection in the andean margin although it is the largest yet reported to the north of the arica deflection paleomagnetic studies report counterclockwise rotations and several large scale models have been proposed to explain the overall pattern of rotations models include oroclinal bending of an originally straight margin differential shortening across the margin at a preexisting bend which is subsequently tightened by the passive rotation of the limbs of the bend and distributed shear throughout the margin as a consequence of oblique convergence at a preexisting bend in contrast to these models several workers have argued that rotation is better explained in terms of localized in situ rotations we review these models in light of our results and present a domino type model with blocks bounded by left lateral faults and rotating clockwise in response to mid late cretaceous transpression within a crustal scale shear zone this is consistent with the observed strike slip fault systems identified in the coastal cordillera

tectonic history and new isochron chart of the south pacific we have developed an internally consistent isochron chart and a tectonic history of the south pacific using a combination of new satellite altimeter data and shipboard magnetic and bathymetric data highly accurate vertical deflection profiles 1 2 rad derived from 22 repeat cycles of geosat altimetry reveal subtle lineations in the gravity field associated with the south pacific fracture zones these fracture zone lineations are correlated with sparse shipboard bathymetric identifications of fracture zones and thus can be used to determine paleospreading directions in uncharted areas the high density of geosat altimeter profiles reveals previously unknown details in paleospreading directions on all of the major plates magnetic anomaly identifications and magnetic lineation interpretations from published sources were combined with these fracture zone lineations to produce a tectonic fabric map the tectonic fabric was then used to derive new poles of rotation for 12 selected times in the late cretaceous and cenozoic from our reconstructions we estimated the former location of the spreading centers in order to derive a new set of isochrons interpreted unes of equal age on the ocean floor we believe that the use of new geosat altimeter data in combination with a multi plate reconstruction has led to an improvement in our understanding of south pacific tectonics

tectonic evolution of the antarctic phoenix plate system since 15 ma abstract joint inversion of magnetic isochron and fracture zone data from the extinct antarctic phoenix spreading system in sw drake passage yields seven new finite reconstruction poles the inversion results are very well constrained for such a short length of plate boundary although this is partly because the finite poles are located close to the reconstructed region the optimum use of fracture zone identifications from satellite derived free air gravity data is also important as the stability of stage poles throughout the short intervals in the model affirms the model results describe a well organised spreading system since magnetic anomaly chron c5ad 15 ma in which the phoenix plate rotated about stage poles nearby to the southwest stage pole locations are broadly consistent with a hypothesis of pivoting subduction as the driving force of phoenix plate movement and there is some evidence in the progression of stage poles for late stage movement of the subduction pivot in response to the changing azimuth of the subduction zone at which the phoenix plate was being consumed the model kinematics alone provide no unequivocal support for previous interpretations of disruption of the subducted part of the phoenix plate the very latest stages of spreading saw falling spreading rates between magnetic anomaly chrons c4 8 1 ma and c2a 3 3 ma when the antarctic phoenix ridge became extinct this is consistent with an increase in shear stress across the plate bounding shackleton fracture zone due to a plate reorganisation in the neighbouring scotia sea following the cessation of spreading on the west scotia ridge

a simple method for the determination of the dilation direction of intrusive sheets abstract it is shown that the occurrence of offsets of various orientations along the contacts of an intrusive sheet permits the rapid establishment of the net dilation direction for the sheet the application of the method to some dykes on arran shows that oblique opening of a fissure was sometimes accompanied by a vertical component of opening the technique provides a tool for the analysis of patterns of crustal displacement associated with dyke intrusion

liquefaction and flow failure during earthquakes two aspects of seismically induced liquefaction are discussed which are of vital engineering significance the triggering condition and the consequences of liquefaction the triggering condition is examined with respect to liquefaction analysis note being taken of the onset condition which is governed by cyclic strength consequences of liquefaction are discussed with respect to post seismic stability analysis in which the residual strength plays a major role procedures used for liquefaction analysis based on the results of in situ sounding tests are introduced and the applicability of this method for estimating associated ground settlements is discussed the evaluation of residual strength requires a better understanding of undrained sand behaviour results of extensive laboratory rests on japanese standard sand are examined and new index parameters are proposed to quantify undrained sand behaviour better the results of laboratory tests on silty sands are examined in the same way ah the results are

defining an appropriate steady state line for merriespruit gold tailings failure of the merriespruit gold tailings dam in 1994 in south africa was unusual in that never before had a gold tailings dam in south africa failed in such a catastrophic fashion conventional thinking was that gold tailings would always exhibit dilative characteristics upon loading primarily because of the method of deposition which allows significant consolidation to occur due to sun drying this paper demonstrates that the concept of a steady state line which separates dilative from contractive behaviour upon undrained loading is applicable to merriespruit tailings four particle size distributions of merriespruit tailings were tested to determine the influence of the percent finer than 75 m on the position of the steady state line the tailings with the greater percentage of fines gave a steady state line that plotted above all the others which translated to the requirement that a greater relative density was necessary to produce noncontractive behaviour than for the low fines tailings samples