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

a mid cretaceous age for the palmer land event antarctic peninsula implications for terrane accretion timing and gondwana palaeolatitudes new structural and age data suggest that west gondwana may have been at lower palaeolatitudes than previously interpreted from albian sequences in gondwana marginal suspect terranes the palmer land event which juxtaposed mesozoic terranes on the gondwana margin deformed granitoids in the southern antarctic peninsula u pb shrimp dating of zircons from a microgranite dyke yields a crystallization age of 106 9 1 1 ma this result and re interpretation of the structural position of another granite pluton date the palmer land event and probable terrane collision as late early cretaceous and not latest jurassic as formerly interpreted

interpretation of new aeromagnetic anomaly data from the central antarctic peninsula new high resolution aeromagnetic anomaly data from the central antarctic peninsula reveal a magnetic signature typical of a magmatic arc system analysis of the new aeromagnetic map together with susceptibility data and 2 5 dimensional modelling support the interpretation of the pacific margin anomaly as reflecting a mid crustal batholithic province consisting of magnetite rich plutons examination and analysis of the magnetic anomaly map shows variations in magnetic character along the antarctic peninsula these variations in conjunction with geological and other geophysical data sets are interpreted as indicating distinct segments of continental crust of particular importance is the identification of three distinct areas of basement on the eastern side of the antarctic peninsula of which the central area 66 s to 68 appears to be gneissic basement at a relatively high crustal level to the north of 66 s the basement is formed of late palaeozoic triassic accretionary material and to the south of 68 s the basement is unknown segmentation is a common feature of arcs worldwide and previous studies of the antarctic peninsula have linked the continental segmentation to the arrival of a series of ridge crests at the subducting pacific margin this study indicates that the major segmentation of the continental crust is linked to the pre cretaceous crustal structure of the peninsula which was subsequently modified by extension during the cenozoic it is suggested that this cenozoic extension was caused by a reduction in plate convergence rates at two distinct times rather than the arrival of ridge segments at the trench as previously proposed

joint geophysical and petrological models for the lithosphere structure of the antarctic peninsula continental margin summary the antarctic peninsula ap is a composite magmatic arc terrane formed at the pacific margin of gondwana through the late mesozoic and cenozoic subduction has stopped progressively from southwest to northeast as a result of a series of ridge trench collisions subduction may be active today in the northern part of the ap adjacent to the south shetland islands the subduction system is confined by the shackleton and hero fracture zones the magmatic arc of the ap continental margin is marked by high amplitude gravity and magnetic anomaly belts reaching highest amplitudes in the region of the south shetland islands and trench the sources for these anomalies are highly magnetic and dense batholiths of mafic bulk composition which were intruded in the cretaceous due to partial melting of upper mantle and lower crustal rocks 2 d gravity and magnetic models provide new insights into crustal and upper mantle structure of the active and passive margin segments of the northern ap our models incorporate seismic refraction constraints and physical property data this enables us to better constrain both moho geometry and petrological interpretations in the crust and upper mantle model along the dss 12 profile crosses the ap margin near the anvers island and shows typical features of a passive continental margin the second model along the dss 17 profile extends from the drake passage through the south shetland trench islands system and bransfield strait to the ap and indicates an active continental margin linked to slow subduction and on going continental rifting in the backarc region continental rifting beneath the bransfield strait is associated with an upward of hot upper mantle rocks and with extensive magmatic underplating

magnetic anomalies in bahia esperanza a window of magmatic arc intrusions and glacier erosion over the northeastern antarctic peninsula abstract bahia esperanza constituting the ne tip of the antarctic peninsula is made up of paleozoic clastic sedimentary rocks overlain by a jurassic volcano sedimentary series and intruded by cretaceous gabbros and diorites the area is located along the southern part of the pacific margin magnetic anomaly belt field magnetic researches during february 2010 contribute to determining the deep geometry of the intermediate and basic intrusive rocks moreover the new field data help constrain the regional pacific margin anomaly characterized up to now only by aeromagnetic and marine data field magnetic susceptibility measurements of intrusive intermediate and basic rocks responsible for magnetic anomalies ranges from 0 5 10 3 si in diorites to values between 0 75 10 3 si and 1 3 10 3 si in gabbros in addition a significant remanent magnetism should also have contributed to the anomalies the regional magnetic anomaly is characterized by a westward increase from 100 nt up to 750 nt associated with large intrusive diorite bodies they probably underlie most of the western slopes of mount flora gabbros in the nobby nunatak determine local residual rough anomalies that extend northwards and westwards pointing to the irregular geometry of the top of the basic rocks bodies below the piramide peak glacier however the southern and eastern boundaries with the buenos aires glacier are sharp related to deep glacier incision as a result of the glacier dynamics magnetic anomalies are also detected north of the nobby nunatak due to the extension of the anomalous body and the presence of gabbro blocks in the moraines the bahia esperanza region is a key area where onshore field geological and magnetic research allows us to constrain the shape of the crustal igneous intrusions and the basement glacier geometry providing accurate data that complete regional aeromagnetic research

provenance of triassic cretaceous sandstones in the antarctic peninsula implications for terrane models during gondwana breakup abstract the antarctic peninsula is the largest of twelve micro continental fragments between southern south america east antarctica and new zealand their original positions and movement history following gondwana breakup are poorly constrained recent work on the antarctic peninsula fore arc arc back arc suggests it consists of two suspect terranes that originated elsewhere and collided with the pacific margin of gondwana in the cretaceous this study reviews provenance models for three laterally extensive kilometers thick sedimentary units in the antarctic peninsula using previous studies and new sandstone detrital modes and chemistry 1 the trinity peninsula group accretionary complex triassic was possibly derived from a glaciated continental margin and may consist of several accreted tracts the deformed quartzose view point and legoupil formations were eroded from a deeply eroded stable continental margin containing an old arc and mature quartzose sandstones with provenance ages suggesting sources in north patagonia and local antarctic peninsula basement the weakly deformed and more feldspathic hope bay formation was derived from an eroded arc possibly the early permian arc of the north patagonian massif 2 the lemay group accretionary complex middle triassic middle cretaceous is dominated by arkoses derived from an eroded continental margin of carboniferous and permian age possibly in the marie byrd land sector with several belts of volcanilithic sandstones representing episodes of arc magmatism the youngest part of the lemay group contains abundant metamorphic detritus derived from basement uplifted during mid cretaceous terrane docking 3 the latady formation in the southern antarctic peninsula has a completely different petrology comprising mature sublitharenites 190 ma of gondwanan provenance deposited in restricted rift basins younger volcanic litharenites 180 ma are dominated by ree enriched low eu eu rhyolitic material eroded from silicic calderas associated with gondwana breakup abundant dacitic to andesitic detritus in the youngest lf 160 145 ma was derived from a contemporaneous arc possibly on the thurston island block as the latady evolved into a back arc basin

autochthonous v accreted terrane development of continental margins a revised in situ tectonic history of the antarctic peninsula the allochthonous terrane accretion model previously proposed for the geological development of the antarctic peninsula continental margin arc is reviewed in light of recent data and the geology is reinterpreted as having evolved as an in situ continental arc this is based upon the following factors 1 the presence of early palaeozoic basement and stratigraphic correlation of sequences between the autochthonous and previously proposed allochthonous terranes 2 isotopic evidence for similar deep crustal structure across the different terranes 3 ocean island basalt magmas and deep marine sedimentary rocks formed during continental margin extension within the previously proposed accretionary wedge sequence i e not formed against an active oceanic arc 4 the distribution of magnetic susceptibility measurements and aeromagnetic data locating the palaeo subduction zone along the west of the peninsula 5 a lack of clear palaeomagnetic distinction between the terranes the following alternative tectonic history is proposed 1 amalgamation and persistence of gondwana 2 subsequent silicic large igneous province magmatism and extension 3 development and history of andean subduction until its cessation in the cenozoic a number of features in the antarctic peninsula correlate with those of other circum pacific margins supporting a global evaluation of allochthonous v autochthonous margin development to aid our understanding of crustal growth mechanisms

large igneous province on the us atlantic margin and implications for magmatism during continental breakup rifted continental margins commonly include sections of igneous rock more than twice as thick as normal oceanic crust explanations for this voluminous magmatic accretion during rifting include plume models1 3 which require a deep seated thermal or chemical anomaly in upwelling mantle and non plume models4 7 which call on broad shallow thermal anomalies and or rapid upwelling of mantle through the melting zone new seismic models from two transects across the continent ocean transition on the us atlantic margin8 10 confirm the presence of a 20 25 km thick igneous section here we argue that the similarity of the crustal structure on these and two previous transects spanning 1 000 km of the margin and the association of thick igneous crust with the east coast magnetic anomaly11 imply that the thick igneous section extends along the entire margin and may have a volume of as much as 3 2 106 km3 the distribution of volcanic and plutonic rocks details of the seismic structure and lack of independent evidence for a hotspot are difficult to reconcile with plume models and suggest that non plume processes created the thick igneous crust

postseismic stress and pore pressure readjustment and aftershock distributions abstract the time and spatial readjustment of stress and pore pressure due to a strike slip rupture are investigated by means of a model which treats the earth s crust as a linear poro elastic material infiltrated by fluids the analysis is based on a fundamental solution obtained by rice and cleary who treated the solid and fluid constituents as separately compressible the main shock is modeled as a sudden uniform stress drop and the resulting slip dislocation is maintained in time this suddenly introduced dislocation distribution sets up stress and pore pressure fields diffusion of fluid takes place from regions undergoing compression to regions undergoing dilation hence the stress and pore pressure fields evolve over time with time scales which depend on the flow parameters of the rock medium it is assumed that aftershocks occur when the coulomb stress defined as c n p where n and are the increase in normal stress and shear stress across a potential slip surface is a friction coefficient and p is the pore pressure increase exceeds the shear strength of the slip surface the coseismic elastic solution confirms the suggestion of spatial distribution of aftershocks by stein and lisowski the transient solution predicts a small rotation of the aftershock zone from the compression into the dilation region at the two ends of the main rupture whereas the aftershock zones for off fault aftershocks appear to expand and shift into the dilating regions a new plot is developed which provides a means to study the association between high coulomb stress increase with off fault aftershock locations the aftershock distributions of three earthquakes pasinler 1983 borrego mountain 1968 and haicheng 1975 are compared to the zones predicted by the model the earthquake aftershocks are skewed towards the dilatant regions and in some cases form distinct off fault clusters in the areas of high coulomb stress the ability of the model to provide a primary aftershock mechanism and the implications of this mechanism for material properties are discussed in the contexts of these sample earthquakes the model predicts the generally observed spatial expansion of the aftershock zones and suggests an explanation for the presence or absence of off fault aftershock clusters however the inaccuracy in location of aftershocks and the small magnitude change in predicted coulomb stress makes it difficult to confirm or disprove the theory that aftershock distributions are controlled by fluid flow

the generation of sodic granite magmas western palmer land antarctic peninsula subduction related mesozoic to cainozoic granites s l in western palmer land antarctic peninsula have similar chemical compositions to archean tonalite trondhjemite granodiorite ttg suites phanerozoic slab melts adakites and to experimental partial melts of basaltic material in equilibrium with amphibole pyroxene garnet they are predominantly sodic metaluminous and most have al2o3 15 wt and y 36 km between triassic and tertiary times the initial nd t of the magmatism increased and sr t decreased suggesting that new continental crust was produced during this period underplating by mafic magma was an important crustal growth mechanism in the arc the generation of abnormally thick crust and its subse quent fusion is considered to be a consequence of ca 180 ma of subduction and associated magmatism in the region an implication of the model is that dense garnet amphibolite and eclogite residues from partial melting of the lower crust will accumulate in theory the setting was appropriate for such residues to detach from the base of the crust and to sink into the convecting mantle such a process would leave the rest of the crust enriched in large ion lithophile elements lree but depleted in hree y

the cenozoic subduction history of the pacific margin of the antarctic peninsula ridge crest trench interactions new magnetic anomaly identifications w of the shackleton fracture zone show 5 spreading sections separated by fracture zones in the 2 most southerly the ridge crest collided with a trench at the margin of the antarctic peninsula only 6 5 and 4 ma ago the latest of a series of collisions starting at the base of the peninsula so ma ago following each collision spreading and subduction both stopped opposite the south shetland trench and actively extending bransfield strait spreading also stopped 4 ma ago but before the spreading centre reached the trench the tendency of the subducting plate to continue sinking probably initiated the opening of the bransfield strait the parallelism between fracture zone orientation on the descending plate and convergence direction previously thought responsible for the tectonic segmentation of the peninsula was effective for only 10 30 ma before collision before that either the fracture zones did not extend to the trench or the subducting ocean floor formed at the farallon phoenix nazca aluk boundary with fracture zones oblique to the subduction direction the aleutian arc kula ridge model in which arc magmatism virtually ceases when ocean floor younger than 25 30 ma occurs beneath the arc fits the distribution of antarctic peninsula radiometric dates explaining a 50 60 ma gap between collisions and youngest ages in the s and possibly the migration of the youngest activity towards the trench thus gaps in the geologic record of arc magmatism need not imply cessation of subduction the progressive steepening of the peninsular margin towards the 4 ma collision site suggests tectonic erosion of the fore arc as the ridge crest approached

short paper the significance of mid palaeozoic basement in graham land antarctic peninsula a mid palaeozoic basement to the antarctic peninsula is confirmed by a geochronological study of the gneissose country rock of the mesozoic magmatic arc of eastern graham land rb sr whole rock ages of 410 15 and 426 12 ma from samples of orthogneiss indicate a previously unrecognized episode of granitic magmatism in the antarctic peninsula during the silurian sm nd data on garnet whole rock pairs indicate that subsequent amphibolite facies metamorphism occurred in late carboniferous times probably as a result of plutonism

continental crust crustal underplating and low q upper mantle beneath an oceanic island arc a detailed structural model of the crust subducting slab and underlying upper mantle across the northern izu ogasawara bonin island arc system is derived from a marine seismic reflection and ocean bottom seismographic refraction survey and subsequent forward modeling combined with tomographic inversion the model indicates that the crust is thickest beneath the presently active rift zone and a granitic crust may have formed in the mid crust a highly attenuative mantle that is one with low quality q seems to be confined mainly beneath the presently active rift zone in contrast high p wave velocity persists in the lower crust between the forearc and eastern margin of the back arc basin suggesting a large scale magma input responsible for the arc formation

an experimental study of the influence of oxygen fugacity on fe ti oxide stability phase relations and mineral melt equilibria in ferro basaltic systems

the geochemical and mineralogical fingerprint of west antarctica s weak underbelly pine island and thwaites glaciers abstract the marine based west antarctic ice sheet wais is considered the most unstable part of the antarctic ice sheet with particular vulnerability in the amundsen sea sector where glaciers are melting at an alarming rate far field sea level data and ice sheet models have pointed towards at least one major wais disintegration during the late quaternary but direct evidence for past collapse s from ice proximal geological archives remains elusive in order to facilitate geochemical and mineralogical tracing of the two most important glaciers draining into the amundsen sea i e pine island glacier pig and thwaites glacier tg we here provide the first multi proxy provenance analysis of 26 seafloor surface sediment samples from pine island bay our data show that the fingerprints of detritus delivered by pig and tg are clearly distinct near the ice shelf fronts of both ice stream systems for all grain sizes and proxies investigated glacial detritus delivered by pig is characterised by low end values 9 high 87sr 86sr ratios 0 728 low smectite content the geochemical and mineralogical fingerprints for pig and tg reported here provide novel insights into sub ice geology and allow us to trace both drainage systems in the geological past under environmental conditions more similar to those envisioned in the next 50 to 100 years

multicomponent vsp imaging of tight gas sands low porosity bossier and cotton valley sands of the east texas basin u s have only a small acoustic impedance contrast with the encasing shales but a greater relative contrast in shear wave impedance vertical seismic profile vsp data acquired with both a near offset and far offset p wave source clearly demonstrate the p p reflectivity and p s mode conversions within the bossier section we designate conventional p wave reflectivity as p p shear wave reflectivity as s s and p wave shear wave mode conversion data as p s while bossier p p reflectivity is low it appears to be adequate for mapping thick sandbodies such as the york sandstone the main exploration target in this area however p p reflectivity is even lower and is inadequate for imaging the overlying cotton valley sands in contrast the far offset vsp data acquired with a p wave source demonstrate a high level of p s mode conversion which is used to image this interval with definition that is not provided by p p reflectivity this provides strong support for the use of p s mode conversion imaging for seismic characterization of tight sand reservoirs near offset shear wave vsp data acquired with a shearwave source show low s n ratio and limited bandwidth for the downgoing waveform because of the depth of the target shear wave energy appears to have a more limited range of propagation than p waves such effects may also have a strong negative impact on multicomponent imaging of these sands using surface seismic techniques multicomponent 3d vsp imaging provides a superior solution by placing the geophones closer to the subsurface zone of interest

structure and evolution of cenozoic arc magmatism on the antarctic peninsula a high resolution aeromagnetic perspective structure and evolution of cenozoic arc magmatism on the antarctic peninsula a high resolution aeromagnetic perspective

new aerogeophysical view of the antarctic peninsula more pieces less puzzle new airborne geophysical data reveal subglacial imprints of crustal growth of the antarctic peninsula by mesozoic arc magmatism and terrane accretion along the paleo pacific margin of gondwana potential field signatures indicate that the antarctic peninsula batholith is a composite magmatic arc terrane comprising two distinct arcs separated by a 1500 km long suture zone similar to the peninsular ranges batholith in southern and baja california aeromagnetic aerogravity and geological data suggest that a mafic early cretaceous western arc was juxtaposed against a more felsic eastern arc which in mid cretaceous times was intruded by highly magnetic tonalitic granodioritic plutons of island arc affinity suturing of the two arcs against the gondwana margin caused the mid cretaceous palmer land orogenic event convergence and suturing may have been driven by two subduction zones or alternatively by a decrease in slab dip leading to an inboard migration of the arc as in california

the auriga nunataks shear zone mesozoic transfer faulting and arc deformation in northwest palmer land antarctica the auriga nunataks shear zone places new tectonic and temporal constraints on the mesozoic evolution of west antarctica the shear zone is a long lived arc orthogonal ductile transfer fault that preserves a history of regional mesozoic compressional basement deformation and extensional arc pluton emplacement in the antarctic peninsula magmatic arc it forms an east west trending positive flower structure 2 4 km wide exposed along 5 km of strike marble graphite bearing pyroxene granulite granodiorite diorite amphibolite and gabbro are deformed to mylonite and marble hosted tectonic breccia the mesozoic history of the shear zone is interpreted as follows emplacement of granodiorite diorite at circa 206 ma during early jurassic dextral transtension and metamorphism that peaked at circa 188 ma d1 brecciation of marble and mylonitization of gneiss by late jurassic to early cretaceous sinistral shear during the peninsula wide palmer land orogeny d2 early cretaceous dextral transtension with emplacement of gabbro and garnet leucogranite between 140 and 135 ma d3 and mid cretaceous ocean vergent thrusting and sinistral transpression between 125 ma and 80 ma with a peak at circa 110 ma that folded mylonitized and brecciated preexisting plutonic and metamorphic rocks d4 this is responsible for the current geometry of the shear zone the auriga nunataks shear zone transferred motion between arc parallel compressional and extensional structural elements and by hosting plutons appears to have acted like a leaky transform fault during episodes of regional extension

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

aeromagnetic evidence for a buried early cretaceous magmatic arc northeast japan positive aeromagnetic anomalies recent drilling and models constructed from these data delineate the plutonic roots of the early cretaceous kitakami magmatic arc in northeast japan buried plutons mostly offshore produce belts of positive magnetic anomalies these anomalies and magnetotelluric data suggest that the plutons form a batholith 70 120 km wide nearly 800 km long and 10 15 km thick the batholith may mark the location of the main kitakami arc most of the exposed kitakami plutons are 2 20 km in diameter some are 3 km thick the small plutons line up along nw trending faults some may have been satellite vents that tapped into the magma supply of the main arc the batholithic roots of the main arc now compose almost half of the modern japan forearc basement steep magnetic gradients offset anomalies and basin stratigraphy portray extensive faulting of the kitakami batholith during oblique subduction in the late cretaceous and rifting in the miocene the eastern boundary of the kitakami batholith lies between 90 and 140 km west of the modern trench much closer than the 300 km distance between the active arc and trench the early cretaceous forearc basin and accretionary prism may underlie the modern forearc basin east of the batholith but clear evidence is lacking much of the early cretaceous margin including most of its forearc therefore is missing how the material was removed is unknown it could have been strike slip faulted eroded by subduction related processes or both

experimental study of liquid evolution in an fe rich layered mafic intrusion constraints of fe ti oxide precipitation on the t f o 2 and t ϱ paths of tholeiitic magmas the newark island layered intrusion a composite intrusion displaying a similar fractionation sequence to the skaergaard has both dikes which preserved liquids fed into the intrusion and chilled pillows of liquids resident in the chamber this study reports experimentally determined one atmosphere liquid lines of descent of these compositions as a function of oxygen fugacity which varies from qfm quartz fayalite magnetite to 0 5 log10 units above iw iron wustite these experiments reveal a strong oxygen fugacity dependence on the order of appearance and relative abundances of the fe ti oxide minerals titanomagnetite saturates prior to ilmenite at qfm but the order is reversed at lower oxygen fugacities in the layered series of the newark island intrusion ilmenite arrives shortly before titanomagnetite and the titanomagnetite ilmenite ratio decreases monotonically after the cumulus appearance of titanomagnetite comparison of the crystallization sequence in the intrusion with that of the experiments requires that the oxygen fugacity in the intrusion increased relative to qfm before titanomagnetite saturation and decreased afterward but always remained between the qfm and iw buffers similar trends in the modes of the fe ti oxides ilmenite and titanomagnetite in the skaergaard kiglapait and somerset dam intrusions along with fe2o3 feo ratios in morbs suggest that such a temperature oxygen fugacity path may be typical of tholeiitic magma differentiation calculations of the temperature density paths of the experimental liquids indicate that at all possible oxygen fugacities the density must have decreased abruptly after fe ti oxide saturation accordingly liquids replenishing the intrusion after fe ti oxide saturation should pond at the bottom of the chamber quenching against older cumulates field observation at the newark island intrusion confirm this prediction the similarities in the fractionation paths of several other layered intrusions to that of the newark island intrusion suggest that the density of the liquids in these intrusions also decreased after fe ti oxide saturation experiments on a suggested initial skaergaard liquid are consistent with this model

asperities barriers characteristic earthquakes and strong motion prediction recent results from paleoseismology suggest that earthquakes recurrent on a given fault may often have the same characteristic length and amount of slip stable asperities and barriers which survive many earthquakes can explain these results the activity of volcano usu japan since 1977 which produced a dacite dome with a total upheaval of 180 m offered a great opportunity to study the recurrence behavior of a fault seismologists at hokkaido university revealed a family of repeated earthquakes with nearly the same amount of slip generated from the same fault plane on the crater wall along which the dome moved upward this family can be explained by the existence of a stable asperity on the crater wall and is called the asperity type family they found however another type of family of earthquakes which share the same fault plane but show a great range of variation in the amount of slip the behavior of the latter family can be explained by the presence of weak barriers over a fault plane defined by strong stable barriers and this family is referred to as the barrier type family this type of earthquake model has been used successfully in the interpretation of observed strong motion acceleration spectra from california and the results reveal the stability of local stress drop among different earthquakes the agreement found between local stress drops estimated from strong motion data and those inferred from geological observation supports the possibility for predicting earthquake strong motion directly from geological observation of the causative fault

rb sr geochronology of graham land antarctica thirty five new rb sr whole rock and mineral isochrons are reported for igneous rocks from graham land magmatism was essentially continuous but peaks of activity occurred in early jurassic early cretaceous late cretaceous and eocene times the oldest dated granite is triassic 209 3 ma volcanic rocks were erupted throughout this history and some previously supposed upper jurassic volcanic rocks must be mid cretaceous or younger a change in magma type from uniformly acid to predominantly basic or intermediate combined with westward migration of the arc may be partly responsible for previously reported transverse geochemical variations there is a marked trend of decreasing initial 87 sr 86 sr ratio of the plutonic rocks with time from c 0 707 to 0 704 although volcanic rocks tend to have higher values this trend is ascribed to the waning influence of an as yet unidentified pre magmatic crustal basement due to increasing depth of magma generation close parallels to the observed age pattern imply continuity of the andean belt through southern south america into graham land back to early mesozoic times and are not compatible with reconstructions of gondwana in which the antarctic peninsula is located w of south america

granitoid pluton formation by spreading of continental crust the wiley glacier complex northwest palmer land antarctica abstract the emplacement mechanism geometry and isotope geochemistry of plutons of the wiley glacier complex suggest that new continental crust grew by multiple injection of tonalitic dykes during dextral transtension in the antarctic peninsula magmatic arc in early cretaceous times the suggested mechanism is analogous to basalt dyke injection during sea floor spreading during normal dextral shear the burns bluff pluton a sheeted moderately east dipping syn magmatically sheared tonalite granodiorite intruded syn magmatically sheared quartz diorite of the creswick gap pluton and 140 5 ma hornblende gabbro u pb dating of zircon and ar ar dating of hornblende and biotite suggest that both granite s l plutons were emplaced between 145 and 140 ma but that extensional shearing was active from the time of emplacement until ca 127 ma the burns bluff pluton is chilled at its margin and grades through mylonitised porphyritic tonalite granodiorite sheets and tonalite granodiorite sheets with minor chilling to a kilometre scale body of coarse grained hypidiomorphic tonalite granodiorite co magmatic microdiorite forms dykes and abundant synplutonic mafic enclaves these dykes opened as echelon veins during episodic dextral shear and were deformed to trains of enclaves during continued normal dextral shear pluton marginal porphyritic and hypidiomorphic tonalite granodiorite forms large fault hosted sheets emplaced progressively under extension with minor dextral shear kinematic indicators from pluton marginal granite s l dykes suggest that early in pluton accretion intrusive sheets cooled rapidly with simple shear prior to full crystallisation changing to ductile simple shear during cooling kinematic indicators towards the pluton core suggest that as the pluton grew and cooled more slowly emplacement switched from sheeting to in situ inflation with simple shear distributed across a broad zone prior to full crystallisation of magma cross cutting relationships with the coeval syn extensional creswick gap pluton suggest that the burns bluff pluton was emplaced in a steeper second generation shear structure like those in normal fault systems this suggests that the wiley glacier complex was emplaced above the base of the brittle ductile transition zone 15 18 km depth the burns bluff pluton has nd and sr isotope values that range from mantle dominated nd141 3 8 87 sr 86 sr 141 0 70468 to more crustally influenced nd141 1 7 87 sr 86 sr 141 0 70652 this range probably represents different degrees of mixing between mantle derived magma and lower crustal partial melts generated in the garnet stability zone 40 km depth addition of new crustal material by mafic underplating at the base of the crust and by redistribution of granitic s l and mafic modified underplated magma to mid crustal levels along extensional shear zones as the arc spread were the primary mechanisms of crustal growth

interpretation of reconnaissance gravity and aeromagnetic surveys of the antarctic peninsula two dimensional models are presented for the crustal structure of the antarctic peninsula a mesozoic cenozoic magmatic arc on the southeast pacific margin the models are constrained by measured rock properties the west coast magnetic anomaly wcma is caused by a zone of mafic plutons which is 70 150 km wide and over 1500 km long in modelling of bouguer anomalies isostatic compensation is maintained by crustal thickness variations the model moho is deepest 35 km beneath the mid cretaceous axis of the arc and shallowest 20 km beneath a late cenozoic marginal basin positive isostatic residual gravity anomalies are associated with the wcma and a mafic terrane in the mesozoic accretionary complex three broad areas of consistent crustal structure are identified the junctions of these areas are sinistral fault zones which displace the curvilinear magnetic physiographic and gravity patterns by several tens of kilometers

basement chronology of the antarctic peninsula recurrent magmatism and anatexis in the palaeozoic gondwana margin abstract a revised pre jurassic chronology is presented for the antarctic peninsula on the basis of new u pb zircon dating using both conventional 11 samples and shrimp microprobe data 10 samples the age range for plutonism and high grade metamorphism is from 435 8 ma silurian to c 206 ma approximately the triassic jurassic boundary with peaks of activity in devonian carboniferous permian and mid triassic times although some are represented by rocks with limited outcrops the new data confirm the importance of mid and latest triassic magmatic events previously identified using rb sr geochronology the adie inlet gneiss previously thought to be neoproterozoic is recognized as a permian migmatite derived from paragneiss with a provenance dominated by cambrian granitoids granite gneiss from nw palmer land previously dated as cambrian is shown to be triassic detrital zircons in metasedimentary rocks and inherited zircons in granitoids are dominated by mesoproterozoic to cambrian components with sparse palaeoproterozoic to archaean grains suggesting sources within gondwana at times these sources were nearby as shown by an archaean cobble sized granite clast from a permo triassic conglomerate another clast in the same conglomerate previously thought to be devonian in age has an ordovician crystallization age

arc evolution a magnetic perspective from the antarctic peninsula the antarctic peninsula is a mesozoic cenozoic magmatic arc built on palaeozoic and younger basement it was formed by processes related to the subduction of pacific ocean floor at its western margin although subduction has now ceased along most of its length the peninsula features all the tectonic components commonly associated with a developing arc system basement accretionary complex magmatic arc arc related basins intra arc extension and post subduction volcanism seventeen thousand kilometres of high resolution aeromagnetic data have recently been collected in a transect across part of the arc covering an area 290 by 230 km and incorporating examples of most of the above tectonic components the new map reveals distinct magnetic signatures which can now be related to each of these components in a way that was not possible with reconnaissance data sets a characteristic magnetic anomaly pattern for each component is described and comparisons drawn with magnetic studies of other arc regions

a 9 c 2 d land seismic experiment for lithology estimation of a permian clastic reservoir the past 15 years have seen several successful applications of multicomponent seismic technology in hydrocarbon exploration many successes have been reported in the marine environment where statics and surface wave noise are less serious than on land nevertheless research into land multicomponent methods continues with many encouraging results

geodynamic evolution of the antarctic peninsula during mesozoic times and its bearing on weddell sea history this review of the tectonic evolution of the antarctic peninsula during mesozoic times highlights four main events 1 late triassic late jurassic extension 2 late jurassic early cretaceous dextral transpression 3 early cretaceous extension and 4 mid cretaceous compression magmatism was virtually continuous during much of this period with the exception of possible breaks in the known record in early jurassic and late jurassic early cretaceous times the second of these breaks corresponded to the first compressional event there was no apparent hiatus in the magmatic record during the mid cretaceous compressional event although there was a significant change in the pattern of sedimentation in the larsen basin on the eastern margin of the weddell sea at about that time the tectonic evolution of the peninsula is compared to and puts some constraints on existing weddell sea models the late triassic late jurassic arc extension correlates with initial rifting in the weddell sea region during sinistral motion between east and west gondwana however there is no known record of large scale pre mid jurassic transcurrent deformation in the antarctic peninsula that would have been consistent with rotation of west antarctic crustal blocks in the initial rifting period the peninsula wide late jurassic early cretaceous compressional event may correlate with the major change in gondwana plate motions from e w to n s african reference frame separation of east and west gondwana this change probably resulted in formation of an ocean continent boundary along the northern margin of the weddell sea embayment and initial seafloor spreading geological data do not seem to support subduction of southwestern proto weddell sea oceanic lithosphere beneath the eastern margin of the peninsula at that time cretaceous arc extension was coeval with the initial seafloor spreading phase in the weddell sea mid cretaceous arc compression linked to a global increase in ocean floor spreading rates and a superplume event correlates with a change from ne sw to nw se spreading in the weddell sea

observation of diffusion processes in earthquake populations and implications for the predictability of seismicity systems scale invariance either in space or in time has been shown in many papers to characterize earthquake distributions unfortunately little work has been dedicated to looking at the general space time scaling invariance of seismicity systems even though a better understanding of how the two domains spatial and temporal link together could help the development of the stochastic dynamical modeling of earthquake populations in this paper we report the observation of diffusion processes of temporally correlated seismic activity for three different data sets a mine creighton mine canada the long valley caldera in eastern california and a 7 year period of recorded seismic activity in southern california the observed subdiffusion processes are indicative of the general space time scaling of the system taking the form of a slow power law growth r t similar to t h of the mean distance r t between the main event arid the temporally correlated afterevents occuring after a delay t h is found on average to be small 0 1 for creighton mine 0 22 for the long valley caldera and 0 22 for the southern california main events with magnitude greater than or equal to 1 5 but fluctuates significantly from one main event to the other the diffusion is found to be intermittent non gaussian and multiscaling and except for the long valley caldera a systematic correlation between the sizes of the main event and subsequent afterevents and the growth exponent h is observed while classical viscous relaxation models e g elastic listhosphere plastic asthenosphere coupling or fluid flow triggered by sudden changes in pore pressure have been proposed to characterize this relaxation by homogeneous i e nonintermittent normal h 0 5 diffusion processes the direct implication of the reported results is that seismicity systems at spatial scales from meters to hundreds of kilometers and small microearthquakes in a mine to intermediate magnitudes relax spatiotemporally in a nonelastic way revealing the stochastic space time scale invariant nature of such systems since these diffusion processes correspond to a loss of information with time on the location of the main event they can be used to investigate the limits of predictability at all spatial scales of seismicity systems in terms of the spatiotemporal clustering of temporally correlated earthquakes

crustal growth of the antarctic peninsula by accretion magmatism and extension a subduction accretion model incorporating new geophysical data is presented to explain the geology of the antarctic peninsula from late palaeozoic to cenozoic time according to the model the peninsula consists of overlapping accretionary magmatic and extensional regimes that were diachronous across the peninsula and have built the crust to its present form the crust which contains a small proportion of sialic basement was mainly formed by accretionary and magmatic processes and modified to its present shape by extension the gondwanide orogeny for the antarctic peninsula is interpreted in terms of the accretionary processes

a lower cretaceous syn extensional magmatic source for a linear belt of positive magnetic anomalies the pacific margin anomaly pma western palmer land antarctica ar ar laserprobe dating suggests that in western palmer land plutons associated with a curvilinear belt of positive magnetic anomalies along the pacific margin of the antarctic peninsula the pacific margin anomaly pma are early cretaceous in age the new ages combined with published structural and geochemical studies suggest that highly magnetically susceptible gabbroic to tonalitic granodioritic rocks the probable source of the palmer land segment of the pma were generated during early cretaceous extension when mantle derived basaltic magma intruded mafic lower to middle crust continued extension uplifted newly generated lower to middle crust through the curie isotherm ca 600 c forming the magnetic anomaly the pma broadly tracks an arc parallel band in western palmer land where crustal extension and uplift of lower crust were greatest the close spatial relationship between the pma and early cretaceous syn extensional plutons suggests that anomaly area can be used as a crude proxy for the volume of a related plutonic complex the areal extent of the pma indicates that a significant proportion of the arc crust was newly generated during the early cretaceous in western palmer land

crustal structure of the antarctic peninsula sector of the gondwana margin around anvers island from geophysical data abstract we investigated the crustal structure of the northern antarctic peninsula ap a composite magmatic arc terrane at the pacific margin of gondwana it was formed in mesozoic cenozoic by collision accretion of allochthonous terranes to the gondwana margin during subduction and progressive ridge collision of former phoenix plate under the ap the ap is distinguished by belts of high amplitude gravity and magnetic anomalies inferred to indicate the presence of plutons of mafic composition comprising the huge ap batholith for a better understanding of the lithosphere structure and tectonic processes of the ap continental margin we performed 2d gravity and magnetic modeling along line iii iii which crosses the margin south of anvers island our model incorporates data on seismic refraction dss 13 and reflection line 8545 03 profiles and physical property data our modeling shows that the crust of the ap margin near anvers island is of continental type and comprises two crustal domains which were welded together during mid cretaceous collision on the east there is a thick 40 km high velocity density continental crust of the gondwana margin which occupies the ap inner shelf and mainland and larsen ice shelf the space between the gondwanian block and oceanic crust of the bellingshausen sea is occupied by thinner 26 km crust of lower densities of supposed accreted terrane characterized by gravity anomalies caused probably by accretional tectonics as a result of cretaceous subduction two belts of magnetic anomalies forming the pma pacific margin anomaly probably belong to different crustal domains the western belt of the pma relates to accretional block while the eastern belt of the pma corresponds to the gondwana margin block this may represent an evidence that the pma relates to a tectono magmatic event occurred after collision of gondwana margin and accreted terrane in mid cretaceous and had been imprinted on already formed continental margin

the eastern palmer land shear zone a new terrane accretion model for the mesozoic development of the antarctic peninsula a major ductile fault zone the eastern palmer land shear zone has been identified east of the spine of the southern antarctic peninsula this shear zone separates newly identified geological domains and indicates that during late jurassic terrane accretion and collision two and possibly three separate terranes collided resulting in the palmer land orogeny the orogeny is best developed in eastern palmer land and eastern ellsworth land there shallow marine sedimentary rocks of the latady formation and a metamorphic and igneous basement complex of possible lower palaeozoic to pre early jurassic age are thrust and folded this forms an arcuate east directed foreland fold and thrust belt up to 100 km wide and 750 km long parallel to the axis of the antarctic peninsula the newly identified antarctic peninsula domains include 1 a parautochthonous eastern domain that represents part of the margin of the gondwana continent comparable to the western province of new zealand the ross province superterrane of marie byrd land the eastern series of south central chile the pampa de agnia and tepuel rocks of north patagonia and the cordillera darwin rocks of tierra del fuego 2 a suspect central domain that represents an allochthonous microcontinental magmatic arc terrane comparable to the median tectonic zone of new zealand the amundsen province superterrane of marie byrd land and coastal cordillera of north chile and 3 a suspect western domain with strong similarities to the eastern province of new zealand western series of south central chile and chonos metamorphic complex of north patagonia that represents either a subduction accretion complex to the central domain or another separate crustal fragment although an allochthonous terrane hypothesis for the antarctic peninsula remains to be fully tested this has much in common with models for the new zealand and south american parts of the pacific margin of gondwana the identification of a potential allochthonous terrane continent collision zone allows us to define the edge of the gondwana continent in the antarctic peninsula sector of the supercontinent margin which has implications for mesozoic reconstructions of gondwana

magnetic susceptibility and relation to initial 87sr 86sr for granitoids of the central sierra nevada california measurement of the magnetic susceptibility of more than 6000 samples of granitic rock from the mariposa 1 by 2 quadrangle which crosses the central part of the sierra nevada batholith between 37 and 38 n latitude shows that magnetic susceptibility values are above 10 2 si units in the east and central parts of the batholith and drop abruptly to less than 10 3 si units in the western foothills in a narrow transitional zone intermediate values 10 3 to 10 2 prevail magnetic susceptibility appears to decrease slightly westward within the zones of both high and low values magnetic susceptibility in plutonic rocks is chiefly a function of the abundance of magnetite which depends in turn on the total iron content of the rocks and their oxidation ratio lower magnetic susceptibilities of felsic members of sierran intrusive suites and of some felsic rock units relative to adjacent mafic rock units commonly reflect differences in total iron content but the differences of magnetic susceptibility that define the regional pattern generally are much larger and are determined chiefly by the oxidation ratios of the rocks the relatively unaltered condition of the samples and restriction of 18o to the range of 7 to 10 3 indicate that neither hydrothermal fluids nor subsolidus alteration were important in modifying oxidation ratios correlations of magnetic susceptibility with initial 87sr 86sr suggest that oxidation ratios have been inherited from the source regions for the magmas from which the rocks crystallized reduction of fe3 to fe2 by organic carbon or other reducing substances may also have affected magnetic susceptibility

yardsticks for industrial tomography seismic tomography has been developed and applied for decades in seismological applications and for basic research purposes during the last decade large scale applications in the oil and gas industry became standard as tomostatics and velocity modelling for pre stack depth migration in this paper i take a snapshot of some current industrial applications quantifying practical aspects by yardsticks such as data and model size and i try to draw a road map for the current decade

magnetic susceptibilities of rocks of the antarctic peninsula implications for the redox state of the batholith and the extent of metamorphic zones a combination of 1555 magnetic susceptibility measurements from the antarctic peninsula between 65 s to 72 s for which the raw data are made available and petrological structural and metamorphic data has been used to improve understanding of 1 magma sources for the antarctic peninsula batholith 2 genetic relationships between granitoids and metamorphic rocks and 3 to provide a semi quantitative measure of the extent and grade of metamorphic zones the redox state of jurassic plutonic rocks indicates that these had continental magma sources in the tectonostratigraphic terranes of the central and eastern domains triassic and cretaceous plutons on the other hand had subduction related sources in the central domain of the western antarctic peninsula and continental sources in the gondwana margin eastern domain circum pacific comparisons of magnetite ilmenite belt geometries identify the antarctic peninsula belt as a natural continuation of comparable south american belts susceptibility trends are generally retained by orthogneisses suggesting that they formed by deformation of plutonic rocks mostly at or below upper amphibolite facies conditions metasedimentary rocks are weakly magnetic across the peninsula and are spatially associated with continentally derived or crustally contaminated plutons ilmenite series orthogneisses and metabasites from strongly magnetic plutonic environments show areas of very low magnetic susceptibilities that may be linked to fluid enhanced tectonometamorphism induced reduction of magnetic susceptibility the distribution of reduced magnetic susceptibility areas follows the surface traces of major shear and fault zones in palmer land our results are consistent with tectonic interpretations of oblique compression and uplift of an arc block on the pacific margin of gondwana during the palmer land event