deep marine arc apron deposits and syndepositional magmatism in the alisitos group at punta cono baja california mexico rocks exposed at punta cono include very fine grained to coarse grained tuffs lapilli tuffs and tuff breccias deposited in a deep marine environment syndepositional basaltic intrusive activity was common in one locality a hyaloclastite peperite complex formed slumped sections with fluidal basalt clasts derived from intrusions that entered the sediment pile from below are present elsewhere abundant soft sediment folds in fine grained laminated subaqueous fall out tuff suggest steep gradients these are cut by shallow channels filled with coarse grained tuff lapilli tuff and rare tuff breccia the combination of marine fossils extreme textural immaturity abundant slump features and syndepositional magmatism indicates deposition upon the submarine flanks of an active volcano recognition of magma wet sediment interaction is hampered in volcaniclastic rocks because of the similarity between host and intrusive fragments products of magma water sediment interactions at punta maria include 1 jigsaw puzzle hyaloclastite formed by non explosive hydroclastic fragmentation of magma upon contact with water and water bearing sediment 2 peperites produced by mixing of magma with sediment and 3 an unusual tuff breccia unit the result of non explosive mixing of wisps of lava with sediment during remobilization of an unconsolidated section low explosivity magma water sediment interactions are favoured by relatively high hydrostatic pressures in sub wave base settings

paleomagnetism of small basalt exposures in the west puget sound area washington and speculations on the accretionary origin of the olympic mountains twenty two sites from two small exposures of basalt of probable eocene age immediately east of the olympic mountains have the following mean paleomagnetic direction declination 176 5 inclination 66 5 circle of 95 confidence 8 6 this is the only paleomagnetic study of pre miocene rocks anywhere in western washington or oregon that matches the polar wandering curve for north america however a marked fanning of paleomagnetic declinations around the olympic core suggests that concordance in this case is fortuitous and results from oroclinal bending during or immediately after olympic orogenesis creation of the olympic mountains involved accretion of thickened basaltic crust followed by underthrusting of large quantities of oceanic sedimentary debris as the farallon plate pressed northeastward into a recess formed by the thick crystalline blocks of vancouver island and the north cascades

effect of the northward turning of the interplanetary magnetic field on the dipolarization an outstanding question in magnetospheric physics is whether the association between the northward turning of the interplanetary magnetic field imf and the substorm onset is a random relation or a causal relation in this work we investigate the effect of the northward turning of the imf on the dipolarization in the magnetotail by using the global ppmlr mhd simulation model and observations four simulation runs are carried out 1 a dipolarization under steady imf conditions 2 a dipolarization associated with a northward turning of the imf 3 an observed event under steady imf conditions and finally 4 an observed event associated with imf northward turning our results show that 1 the northward turning of the imf can enhance and prolong the dipolarization initiated by internal trigger which is consistent with the statistical studies and 2 the event simulations are similar to the observations all these indicate that the northward turning of the imf could play an important role in substorm triggering however it does not mean that it is necessary

substorms under northward interplanetary magnetic field statistical study it was recently noted that substorms can occur even under prolonged northward interplanetary magnetic field imf conditions based on the substorm list obtained from the image spacecraft we perform a statistical study on the features of substorms during northward imf interval the strength of the substorm is represented by the al index decrease and the total intensity of the auroral bulge four main features have been found as follows 1 most substorms occur soon after a southward imf and intense substorms are more likely to occur for short duration of northward imf period 2t t5th whereas no intense substorms occur after prolonged northward imf condition 2 there is a positive correlation between the strength of the substorm and the two solar wind parameters the imf by and the solar wind dynamic pressure pd 3 the average strength of the substorms during the storm period is much larger than that of the substorms during the period without storm meanwhile nearly all strong storm time substorms occurred either during the intense storm period or during the late main phase or the early recovery phase of the storm 4 about half of substorms either an increase or a decrease in the solar wind dynamic pressure are found within 30tmin preceding each onset time such features indicate that the energy stored in the magnetotail during a previous southward imf period is the main energy source for substorms under northward imf condition especially for intense substorms and both the imf by and the solar wind dynamic pressure play an important role in the energy accumulation during the northward imf period

upper limit of docking time for stikinia and terrane i paleomagnetic evidence from the eocene ootsa lake group british columbia the middle eocene ootsa lake group is exposed in the central portion of the stikine terrane where it was sampled along the shoreline of tahtsa reach and whitesail reach the group consists of dominantly subaerial flows which range in composition from basalt to rhyolite that unconformably overly the jurassic hazelton group detailed alternating field and thermal stepwise demagnetizations were done on all specimens from the 21 sites collected the presence of a normal and reversed polarity remanence a positive fold test and high coercivities and unblocking temperatures indicate that a prefolding primary remanence has been isolated the mean tilt corrected direction of d 002 2 i 69 2 95 7 4 from 13 sites for which paleohorizontal is well known yields a pole position at 354 6 e 88 0 n a95 11 5 which is statistically indistinguishable from published 50 ma reference poles for cratonic north america this evidence demonstrates that the proposed large scale northward displacement of stik

descriptive nomenclature and classification of pyroclastic deposits and fragments recommendations of the iugs subcommission on the systematics of igneous rocks

volcanism associated with extension in an oligocene miocene arc southwestern viti levu fiji the wainimala group rocks of southwestern viti levu fiji represent part of an oligocene miocene island arc some of the early volcanism in this arc took place on a substrate of eocene oligocene frontal arc crust mainly andesitic transitional calcalkaline lavas were erupted from major subaerial volcanic edifices while elsewhere tholeiitic magmas formed low lying basaltic lava fields and small felsic volcanic centres on the seafloor the intrusion of a dense bimodal basalt dacite dyke swarm into the underlying frontal arc crust indicates that significant crustal extension accompanied the eruption of the inter edifice lavas geochemical variations within the wainimala volcanic suite are thought to have been generated through fractional crystallisation processes while relatively slow ascent and mixing of magmas took place beneath the major edifices efficient shallow level fractionation and rapid ascent of magmas occurred in the areas between them the contrasting styles of volcanism may reflect the interaction of ascending magmas with pre existing heterogeneities in the frontal arc substrate

clinoenstatite bearing lava from nepoui new caledonia the lava consists mainly of clinoenstatite phenocrysts 29 sometimes enclosing euhedral chromite a lesser amount of microphenocryst bronzite 20 and a glassy mesostasis clinoenstatite phenocrysts are compositionally homogeneous en 89 fs sub 10 5 wo sub 0 5 with polysynthetic twins on 100 a feature which is believed to indicate inversion from protoenstatite bronzite is variable in composition with en sub 74 84 fs sub 14 22 wo sub 1 5 4 hypersthene en sub 43 61 fs sub 36 52 wo sub 3 4 calcic and subcalcic augite and iron rich hornblende occur as overgrowths on phenocrysts and as quench microlites the new caledonian clinoenstatite boninite is similar chemically mineralogically texturally and in terms of age eocene oligocene and tectonic setting with ophiolite complexes to the three other known clinoenstatite boninite localities cape vogel papua new guinea bonin islands and mariana trench clinoenstatite boninite magmas are believed to result from extensive partial melting of depleted mantle peridotite under hydrous conditions and an abnormally high geothermal gradient modified journal abstract

chemical weathering in antarctica an example of igneous rock particles in big lachman lake sediments james ross island jezerni sedimenty velkeho lachmanova jezera ostrov jamese rosse antarktida dominuji jilovite a prachovite castice r zneho petrografickeho slo eni ty jsou sopecneho p vodu a jsou odvozeni p eva n z mendelova souvrstvi intenzita chemickeho zv travani vulkanick ch castic v sedimentech byla vyjad ena chemick m indexem alterace cia kter popisuje p em nu ivc na jilove mineraly pozorovane mineralni asociace montmorillonitu illitu a kaolinitu pravd podobn pochazela z p em ny sopecneho skla a ivc na jilove mineraly mira zv travani vyjad ena pomoci cia z lakustrialnich sediment je podobna jako u spodnojursk ch na ti bohatach vulkanick ch hornin souvrstvi mount poster a st ednojursk ch na ti bohat ch vulkanick ch hornin ze souvrstvi mapple z antarktickeho poloostrova nami studovane sedimenty ukazuji dobrou shodu se slo enim st ednojursk ch ryolit ze souvrstvi mapple a naopak se castecn lisi od slo eni kenozoick ch bazalt vulkanicke skupiny ostrova jamese rosse zdrojov material z jezernich sediment pochazi z antarktickeho poloostrova a byl pravd podobn transportovan p es kanal prince gustava mal mi ledovci v pr b hu neogenu a kvarteru

igneous activity in the southern andes and northern antarctic peninsula a review the antarctic peninsula and the southern andean continental margin have been the sites of extensive and varied magmatic activity since at least late palaeozoic time the dominant activity has been of calc alkaline character and although andesite and tonalite are the main rock types there are well marked transverse variations in the major and trace element chemistry of the magmas erupted in both areas thus calc alkaline magmas erupted farthest from the ancestral trench are generally more silicic potassic and light ree enriched than magmas erupted closer to the trench in both regions the locus of calc alkaline activity has tended to move trenchwards since the palaeozoic although activity began to wane during the mid cenozoic when subduction of the pacific plate slowed down and ceased a northward cessation of calc alkaline magmatism along the antarctic peninsula can be linked in broad terms to the progressive northwards subduction of sections of the aluk ridge during the cenozoic extensional tectonics within and behind the main igneous belts caused the development of narrow back arc basins in southern chile during the mesozoic and in bransfield strait along the northern antarctic peninsula during recent times tholeiitic magmas with transitional calc alkaline characteristics were erupted in these basins in addition alkalic basalts and related undersaturated magma types were erupted onto the stable continental platform well behind the main calc alkaline and marginal basin magmatic belts during cenozoic and recent times probably in response to extensional tectonics as subduction slowed or ceased

deposition and diagenesis of miocene arc fringing platform and debris apron carbonates southwestern viti levu fiji abstract carbonate deposition was widespread in the eastern melanesian arc during the late early to early middle miocene in southwestern viti levu fiji limestones of this age include a massive to crudely bedded reef or platform facies and a well bedded horizon of redeposited limestone rudites and calcarenite turbidites interbedded with hemipelagic lime mudstones the redeposited limestones are considered to represent a series of carbonate debris aprons deposited in relatively deep water in the early middle miocene the subsurface equivalents of the reef platform limestones are likely to form the best potential hydrocarbon reservoir rocks in fiji carbonate deposition coincided with an increase in epiclastic sediment yield to the forearc basin indicating increased emergence and erosion of the arc it is suggested that by bringing larger submerged areas into the photic zone uplift of the arc was the main factor allowing widespread shallow water carbonate deposition at this time primary pore space in the reef platform carbonates was largely occluded in the marine phreatic zone by microcrystalline cements isopachous fibrous or blocky cement fringes and micritic internal sediments remaining primary pores and secondary biomouldic pores were filled by equant sparry calcite during meteoric phreatic and burial diagenesis reef or platform derived limestone clasts are abundant in the redeposited limestones and there is evidence of an early to middle miocene depositional hiatus accompanied by early subaerial exposure within the largest platform carbonate body it is therefore suggested that the debris apron carbonates were deposited as the result of emergence and erosion of the nearby shallow water limestones which may have been related to sharp but short lived eustatic sea level falls at 16 and 15 ma

sedimentary record of explosive silicic volcanism in a cretaceous deep marine conglomerate succession northern antarctic peninsula lower cretaceous conglomeratic strata exposed on southern sobral peninsula were deposited on a deep marine apron in the back arc larsen basin close to its faulted boundary with the antarctic peninsula magmatic arc the succession is dominated by amalgamated beds of clast supported conglomerate which together with minor intercalated sandstones consist of varied but largely basaltic to andesitic volcanic material and clasts derived from the palaeozoic triassic meta sedimentary basement of the arc most of the volcanic clasts are thought to have been derived from lithified volcanic successions or older synvolcanic deposits rather than from sites of coeval eruption these mixed provenance strata enclose a number of intervals consisting mainly of inverse normally graded conglomerate and graded stratified pebbly sandstone in which the sand fraction is dominated by crystals and vitric grains considered to have been redeposited in the immediate aftermath of explosive silicic arc volcanism like syneruption deposits on non marine volcaniclastic aprons these intervals are more sand prone than the enclosing strata and appear to show evidence of unusually rapid aggradation plagioclase from one such interval has yielded 40ar 39ar ages concordant at 121 ma similar to those obtained from the non marine cerro negro formation deposited within the magmatic arc it is suggested that the two successions can be viewed as counterparts both recording a history of mainly basaltic to andesitic volcanism punctuated by relatively infrequent explosive silicic eruptions whereas the cerro negro formation consists mainly of syneruption deposits most of the volcaniclastic material delivered to the eruption distal deep marine apron appears to have been derived by normal degradation processes only rare silicic eruptions were capable of supplying pyroclastic material rapidly enough and in sufficient quantities to produce compositionally distinct syneruption intervals

unzipping of the volcano arc japan abstract a working hypothesis for the recent evolution of the southern volcano arc japan is presented which calls upon a northward progressing sundering of the arc in response to a northward propagating back arc basin extensional regime this model appears to explain several localized and recent changes in the tectonic and magrnatic evolution of the volcano arc most important among these changes is the unusual composition of iwo jima volcanic rocks this contrasts with normal arc tholeiites typical of the rest of the izu volcano mariana and other primitive arcs in having alkaline tendencies high concentrations of light ree and other incompatible elements and relatively high silica contents in spite of such fractionated characteristics these lavas appear to be very early manifestations of a new volcanic and tectonic cycle in the southern volcano arc these alkaline characteristics and indications of strong regional uplift are consistent with the recent development of an early stage of inter arc basin rifting in the southern volcano arc new bathymetric data are presented in support of this model which indicate 1 1 structural elements of the mariana trough extend north to the southern volcano arc 2 2 both the mariana trough and frontal arc shoal rapidly northwards as the volcano arc is approached 3 3 rugged bathymetry associated with the rifted mariana trough is replaced just south of iwo jima by the development of a huge dome 50 75 km diameter centered around iwo jima such uplifted domes are the immediate precursors of rifts in other environments and it appears that a similar situation may now exist in the southern volcano arc the present distribution of unrifted volcano arc to the north and rifted mariana arc to the south is interpreted not as a stable tectonic configuration but as representing a tectonic snapshot of an arc in the process of being rifted to form a back arc basin

stratigraphy and regional significance of the upper jurassic lower cretaceous byers group livingston island antarctica the byers group exposed on byers peninsula western livingston island antarctica comprises a mudstone dominated sequence at least 1 km thick which accumulated in a marginal fore arc environment the basal 105 m thick anchorage formation consists of radiolarian mudstones and tuff rich interbeds of kimmeridgian tithonian age it correlates with upper jurassic organic rich mudstone units throughout the proto south atlantic region the succeeding 244 m thick devils point formation marks the first major pulse of coarse volcaniclastic material into the basin it is in turn followed by the extensive president beaches formation comprising several hundred metres of finely laminated mudstones with at least two major sandstone intercalations molluscan and dinoflagellate cyst taxa indicate a berriasian age and comparatively nearshore depositional environment for this unit an unconformity of late berriasian or early valanginian age separates the three lowest formations from the chester cone formation the fine grained sealer hill member at the base of the latter is dated as valanginian and grades up into several hundred metres of pebbly sandstones and pebble granule conglomerates these mark the second major volcaniclastic pulse and may be of hauterivian or even younger age definition of this major new group will facilitate more precise upper jurassic lower cretaceous stratigraphical correlations within the southern south america scotia arc antarctic peninsula region it will also aid our understanding of the critical palaeogeographical transition in the northern antarctic peninsula from anoxic basin to active magmatic arc

evolution of intra oceanic arc trench systems abstract intra oceanic island arc trench systems represent a locus of crustal thickening through the addition of mantle derived magmas which form the volcanoes and subjacent stratiform plutons the crust of immature island arcs and adjacent back arc basins is oceanic in chemistry in that it is dominated by mafic e g tholeiitic magmas a magma sequence is recognized in many arc systems which progress from boninite to arc tholeiite to calc alkaline and in some arcs to shoshonites the adjacent back arc basins are formed of morb like tholeiite arc systems may be dominated by extension rather than compression if the subducted and overriding plates are decoupled extension of the arc system may be associated with seaward retreat or roll back of the trench axis alternate cycles of extension and compression of the arc system can lead to both tectonic erosion of the fore arc and accretion of fragments from the incoming sea floor the izu bonin mariana tonga yap and palau trench arc systems are examples of different stages in a dynamic process which starts when subduction begins along old fracture zone traces crustal extension causes both fore arc subsidence and opening of back arc basins island arc volcanic chains and back arc basins are new crust formed largely of fractional melts of the mantle the evolution of an arc system may stop when blocking of the subduction zone or changes in relative plate motion causes overthrusting of back arc lithosphere on to former volcanic arcs many ophiolites preserve remnants of arc system crust and upper mantle

tillamook volcanic series further evidence for tectonic rotation of the oregon coast range paleomagnetic directions from the eocene tillamook volcanic series of the oregon coast range point 46 o clockwise from the expected eocene field direction potassium argon dating of six dikes and flows from this formation yields a mean age of 44 3 0 6 m y these results establish that the oregon coastal block of simpson and cox 1977 extends north to the oregon washington border and that this block has rotated clockwise 46 o 13 o during the past 44 m y the block has undergone no detectable north south translation a two phase model is developed to explain the tectonic history and anomalous magnetic field directions of the pacific northwest region phase i of the rotation active between 50 and 42 m y b p results from fragmentation of the farallon plate with rotation of a fragment during its accretion to north america phase ii of the rotation active between 20 m y b p and the present occurs in association with extension in the basin and range province

jurassic submarine arc apron deposits and associated magma wet sediment interaction northern sierra nevada california abstract jurassic metavolcanic rocks of the northern sierra terrane in northern california are part of an extensive triassic jurassic arc constructed along the western margin of north america in the english mountain area nevada and sierra counties a well exposed volcaniclastic sequence 3 6 km thick records jurassic island arc activity in a submarine environment in the lower part of the sequence thinly bedded andesitic volcanic sandstone turbidites and mudstones of the lower to middle jurassic sailor canyon formation were deposited below storm wave base in a long lived marine basin this unit is overlain conformably by the middle jurassic tuttle lake formation which records rapid accumulation of coarse grained calc alkaline volcanic debris in the proximal parts of a submarine apron around a major volcano that developed within the earlier sailor canyon basin the formation consists primarily of massively bedded matrix supported polymict basaltic to andesitic volcanic breccias deposited from submarine debris flows intercalated pillow hyaloclastite breccias formed during local extrusion of lavas on the seafloor numerous intrusions cut the sequence and are petrographically and chemically identical to clasts in the debris flow deposits peperite occurs along intrusion margins and as abundant isolated pockets indicating intrusion of magma into wet unconsolidated sediments elongate intrusive pods to 13 6 m in length are associated with smaller intrusive pillows and are inferred to represent a network of feeder tubes that extended from larger coherent intrusions and supplied magma to complex zones of magma wet sediment interaction at shallow levels beneath the seafloor syndepositional intrusive activity of this type including formation of abundant peperite probably is a characteristic feature of submarine volcaniclastic sequences developed in proximal island arc settings

island arc carbonates characterization and recognition in the ancient geologic record abstract carbonates of island arc origin that are preserved in paleozoic mesozoic terranes of the north american cordillera exhibit a distinctive suite of paleontologic and lithologic features and share a fundamental similarity with limestones forming in modern volcanic arcs this study provides the first detailed synthesis of carbonate depositional systems in island arcs and documents primary sedimentary constituents based on facies relationships and faunal communities models are developed that show patterns in the long term evolution of shallow marine organisms and the construction evolution and demise of carbonate platforms in island arcs a suite of criteria is identified that may be used to differentiate island arc carbonates from limestones that accumulated in other platform settings biogeographic isolation prolonged subsidence steep submarine slopes and tectonic instability of volcanic edifices contribute to the development of relatively high levels of species endemism impoverished normal marine faunas complex provincial affinities and relict biotas in limestones that are characterized by exceptionally thick platform and periplatform sequences fringing and barrier reefs at the shelf margin extensive lagoonal deposits and rapid lateral and vertical facies changes although destructive tectonic and geologic processes in island arcs may hinder determining the original size and extent of the carbonate platform and particular facies types may not be represented e g fringing and barrier reefs may be replaced by sand shoals at the platform margin many characteristics have potential value for identifying carbonates of island arc origin in the ancient rock record apart from being associated with calc alkaline volcanic and volcaniclastic assemblages the most valuable suite of features for recognizing island arc carbonates is marine biotas that exhibit elevated levels of endemism and mixed paleobiogeographic affinities extraordinary thicknesses of platform shallow marine and periplatform carbonates and rapid facies changes between volcanic and carbonate rocks and between shallow and deep water limestones recognizing that an ensemble of features is distinctive within island arc carbonates considerably enhances identification of volcanic arcs in the ancient geologic record and thus improves the likelihood of successfully unraveling the complex geologic history of ocean basins

modern sand rich and mud rich siliciclastic aprons alternative base of slope turbidite systems to submarine fans base of slope aprons that lack significant channel development and are fed by multiple sediment sources i e numerous slope failures or debris chutes represent an alternative facies organization to submarine fans that possess channels and are fed by canyon point sources of allochthonous sediment two styles of base of slope apron deposition are evident 1 sand rich aprons with rhythmic gradational sand and gravel sheets that evolve to basin plain turbidites as exemplified by crater lake aprons and 2 mud rich aprons composed of chaotic sediment flow lobes without gradation as exemplified by ebro margin aprons

late miocene to pliocene palaeogeography of viti levu fiji islands abstract the island of viti levu is located on the fiji platform in an area that has been tectonically active since at least the late eocene the geological history of viti levu during this time is one of the competing influence of tectonics volcanism and eustatic sea level change in shaping the preserved stratigraphic and therefore palaeogeographic record the focus of this paper is the palaeogeographic evolution of viti levu from the period following the deformation of the tuva group and all older rock units of the vityaz arc around 7 5 ma to the present initial sedimentation during following the deformational event was influenced by the sinistral oblique slip deformation associated with anticlockwise rotation of the platform this tectonically dominated period of island evolution saw the establishment of a number of sedimentary basins many of which continued to receive sediments through until at least the earliest pliocene from 6 5 ma onward volcanic activity became the main influence on the stratigraphic record although evidence for major eustatic events miocene pliocene boundary are recorded in some sequences toward the end of the early pliocene tectonic uplift of viti levu resulted in a poor geological record for this time the quaternary stratigraphic record reflects relatively minor mostly eustatic events in coastal areas around major river systems

volcanism accompanying back arc basin development in the southwest pacific abstract the age and composition of magmatism in fiji and to a lesser extent vanuatu and tonga are evaluated in the light of the fragmentation which has sundered the island arc system they once collectively constituted any of six magmatic phenomena during the past 10 m y may be consequences of adjacent plate reconfigurations trondhjemitic plutonism about 10 m y change from tholeiitic to more calc alkaline volcanism between 10 and 6 m y eruption near the rifted arc margin of voluminous crustally derived high si low k rhyolites 7 m y ago shoshonite dominated basaltic volcanism 5 5 to 3 0 m y change to ocean island like basaltic volcanism at 3 0 0 2 m y and geochemically peculiar as well as tectonically anomalous acid andesites 0 7 3 0 m y ago in kandavu southwestern fiji crustal derivation of the low k rhyolites may reflect maximal heating during initial rifting shoshonites may accompany extreme regional extension above a still arc like source eruption of ocean island like basalts begins once subducted lithosphere no longer contributes to erupted melts or separates fiji from ocean island sources coeval eruption of hybrid andesites may accompany oblique subduction of south fiji basin crust beneath kandavu along the northeast hunter fracture zone

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

depositional features of march 1982 mount st helens sediment flows on 19 march 1982 2 yr after initial eruption mount st helens erupted explosively resulting in the deposition of new sediment flows in the toutle river drainages fig 1 we conducted field observations of these flows during the period 29 march 5 april 1982 by this time the rivers rapidly re establishing pre eruption channel levels had down cut through the new flows allowing us to examine the internal structure of the flows before they could be extensively modified by reconstruction projects the flows were characterized by a distinct three unit sequence a basal massive to graded layer unit 1 consists of large clasts in grain to grain contact it is overlain by a distinctly finer grained stratified unit unit 2 of similar thickness the top unit unit 3 contains very large matrix supported clasts and transported log debris fig 2 this sequence has not previously been documented where its origin could clearly be ascribed to a single catastrophic depositional event the sequence provides a useful model for interpreting similar deposits in the rock record

the northern belt of the longford down inlier ireland and southern uplands scotland an ordovician back arc basin the northern belt of the southern uplands longford down zone contains two compositionally distinct clastic petrofacies suites each ranging from mid to late ordovician age they extend throughout the length of the belt and are generally separated by a strike fault the northwestern metaclast petrofacies suite is characterized by felsic igneous and metamorphic detritus derived from a continental source to the nw whereas the southeastern basic clast petrofacies suite contains abundant calc alkaline detritus derived from an island arc and plutonic metamorphic root zone complex to the s or se a similar source but located to the north is indicated for younger clastic sequences of similar composition in the northern part of the central belt immediately south of a major sinistral shear zone which defines the boundary between the northern and central belts it is concluded in light of other facets of regional geology and possible palaeogeographic configurations that this pattern reflects a back arc basin in the northern belt margined by a volcanic arc massif which once occupied the space now represented by the shear zone and a fore arc basin to the se central and southern belts the northern belt back arc basin is postulated to have been closed and deformed at the end ordovician and subsequently overthrust northwestward by the allochthonous thrust stack imbricated central and southern belts fore arc sequence at the end silurian this in the northern belt resulted in the development of a major recumbent structure downward facing on regional end silurian cleavage

submarine arc volcanism in the southern mariana arc as an ophiolite analogue supra subduction zone ssz ophiolites are being recognized more and more frequently in the geologic record submarine volcanism in intra oceanic arcs is one tectonic setting in which such ophiolites might form the southern seamount province of the mariana arc represents the best documented example of an active intra oceanic submarine arc which can serve as a modern analogue for some ssz ophiolites the southern seamount province comprises nine submarine volcanoes at least seven of which are active or dormant these edifices have erupted plagioclase clinopyroxene olivine phyric basalts andesites dacites and dacitic pumices these lavas are similar to those of the subaerial edifices in the central marianas sio2 49 70 tio2 0 4 1 3 mgo 1 2 5 9 ba 140 450 ppm 87sr 86sr 0 70346 0 70354 but are somewhat more enriched in the light rare earth elements the geochemical characteristics of subaerial mariana arc lavas are preserved in the submarine lavas and the ssp lavas are slightly enriched in incompatible elements relative to those of the subaerial edifices we are ignorant of the composition of any lavas erupted between the principal edifices which may be expected to dominate the volcanic section of ssz ophiolites nevertheless the characteristic high ratio lil hfsc low ti ni and cr and diagnostic mineral compositions calcic plagioclase relatively fe rich olivine are likely to be preserved in unaltered submarine arc lavas the mariana active arc is built on back arc or thinned frontal arc crust and any ophiolite formed from such an arc is likely to exhibit complex intrusive and eruptive relationships of plutonic hypabyssal and volcanic rocks of back arc frontal arc and active arc origin such complicated intrusive relationships if preserved in structurally disrupted ophiolites will make it difficult to reconstruct the geologic history of an ssz ophiolite

tertiary paleomagnetism of the north cascade range washington we have obtained paleomagnetic data for the southern tip of the middle tertiary chilliwack composite batholith located on the canada united states border about 125 km e of vancouver b c thirty four separately oriented samples were collected along a road traverse 1 5 km long located along state highway 22 about 20 km ne of marblemount washington the mean direction after magnetic cleaning is d 182 8 i 65 0 95 1 5 this corresponds to a paleomagnetic pole at 87 5 n 267 5 e close to other poles for tertiary plutons from the north cascades and only slightly displaced from tertiary reference poles from the craton we conclude that the north cascades block has not been rotated or suffered major internal disruption since perhaps late eocene time in marked contrast to all the terranes that surround it

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

arc dynamics and tectonic history of fiji based on stress and kinematic analysis of dikes and faults of the tavua volcano viti levu island fiji 1 stress analysis of dikes and kinematic analysis of faults within the tavua volcano of viti levu fiji indicate an apparent migration of the causative maximum principal stress axis 1 from nw toward north between 5 0 and 3 9 ma the dikes and faults are part of successive structural and magmatic events at the arc scale we present data on the chronological succession of stress indicators in the tavua volcano based on overprinting relationships and published geochronology which also equate with a counterclockwise rotation of fiji assuming an approximately constant n s regional compressive stress orientation during this time the latter interpretation is consistent with paleomagnetism studies that require substantial counterclockwise rotation of the fiji platform since fragmentation of the formerly continuous outer melanesian arc during the latest miocene to earliest pliocene the stress data suggest that the fiji platform has undergone an 50 rotation since 5 ma given that rotation apparently stopped at 3 ma then particularly fast rotation of 25 myr 1 occurred from 5 ma to 3 ma recent seismicity data indicate the platform is currently subject to n s compression the history of late miocene and pliocene events in the region can be related to the consequences arising from collision of an island arc with an oceanic plateau

sedimentation and volcanism in a permian arc related basin southern new zealand an exceptionally well exposed ancient intra arc basin in the permian takitimu group of new zealand contains 14 km of interbedded primary volcanic and marine volcaniclastic rocks of basaltic to rhyodacitic composition these are the products of subaerial and submarine arc volcanism and closely associated turbidite sedimentation the takitimu oceanic arc basin setting formed a dynamic closed sedimentary system in which large volumes of volcaniclastic material generated at the arc was rapidly redeposited in marine basins flanking the eruptive centres volcanism probably included 1 moderate to deep water extrusion of lava and deposition of hyaloclastite 2 extrusive and explosive eruptions from shallow marine to marginally emergent volcanoes in or on the margin of the basin and 3 plinian and phreato plinian eruptions from more distant subaerial vents along the arc much of the newly erupted material was rapidly transported to the adjacent marine basin by debris flows slumping and sliding hemipelagic sedimentation predominated on the outer margin of the basin infrequently interrupted by deposition of ash from the most explosive arc volcanism and the arrival of extremely dilute turbidites turbidite sedimentation prevailed in the remainder of the basin producing a thick prograding volcaniclastic apron adjacent to the arc the volcaniclastic strata closely resemble classic turbidite deposits and show similar lateral facies variations to submarine fan deposits study of such sequences provides insight into poorly understood processes in modern arc related basins

an archean oceanic felsic dyke swarm in a nascent arc the hunter mine group abitibi greenstone belt canada abstract the 2730 ma old hunter mine group hmg a dominantly felsic subaqueous volcanic sequence was formed during early arc construction in the abitibi greenstone belt quebec canada the western part of the hmg contains a felsic dyke swarm up to 1 5 km wide and traceable up section for 2 5 km five distinct generations were identified 1 aphanitic to feldspar phyric dykes 2 quartz feldspar phyric dykes with nd values suggest that an older sialic substrate was not involved in their genesis we infer that the felsic rocks were generated by melting of mafic oceanic crust the swarm was emplaced during nascent oceanic island arc development and was related to rifting of the arc the conformably overlying morb like basalts and basaltic komatiites of the stoughton roquemaure group used the same conduits and further indicate splitting of the arc hmg and associated parts of the abitibi greenstone belts bear a strong resemblance to modern rifted intraoceanic arcs of the western pacific

rapid rotation of fiji paleomagnetic evidence and tectonic implications the timing and magnitude of rotation of fiji in relation to the tectonic evolution of the sw pacific region has been investigated by extensive 126 sites paleomagnetic sampling of late miocene and younger units generally samples yielded well defined characteristic remanence directions which indicate that the main fiji platform rotated by 135 17 5 counterclockwise commencing at or before 10 ma this very large and rapid rotation ceased abruptly at 3 ma coincident with the onset of spreading on the major n s aligned spreading centers in the north fiji basin latest miocene to pliocene paleomagnetic results indicate that viti levu the main island itself has suffered no internal disruption by localized rotations after 5 ma we present a reconstruction for the origin of fiji which differs markedly from previous models such that fiji tonga and lau form a continuous n s aligned arc and that arc reversal was initiated at or near fiji itself

a crustal model for zealandia and fiji summary existing maps of satellite free air gravity and bathymetry and the crust 2 0 global crustal structure model have been used to make new bouguer gravity anomaly and crustal thickness maps of the sw pacific region to analyse the region s crustal structure the new maps clearly outline the limits of the rifted largely submerged continent of zealandia the bouguer gravity anomalies of ocean crust large igneous provinces and backarc basins vary simply and predictably with age the bathymetric fiji platform lies in a water depth of 19 km for the entire fiji platform comparable to stretched and submerged parts of zealandia the question whether the continental crustal thickness of fiji is due to its actually being a rifted part of gondwana zealandia or due to enhanced eocene pliocene magmatism remains open but testable with the latter more likely using the new accurate 3 d crustal thickness and published radiometric dating of lavas we estimate that the magmatic portions of southwest pacific cenozoic subduction related arcs three kings tonga and vanuatu have grown at net volumes which vary from 263 to 1400 km3 km 1 with net addition rates of 17 31 km3 km 1 ma 1

tectonic controls on tholeiitic and calc alkaline magmatism in the aleutian arc the tectonic position of aleutian arc volcanic centers and their magmatic differentiation trends calc alkaline or tholeiitic appear to correlate from 160 w to 175 e the volcanoes form four major arc segments that coincide with earthquake aftershock zones and major geographic features on both the upper and lower plates the tholeiitic volcanoes are large primarily basaltic centers that occur between or at the end of segments where magmas can more easily reach the surface and undergo shallow closed system differentiation the calc alkaline volcanoes are smaller more andesitic centers that occur in the middle of segments where transit through the upper plate is apparently more difficult differentiation is deeper and the intrusive to extrusive ratios are higher than in the tholeiitic centers the chemistry of the least differentiated basalts at both types of centers is similar and suggests a common parent magma probably derived from mantle peridotite conditions for the derivation of the two trends through crystal fractionation support the proposed model the tholeiitic magmas show characteristics i e no hydrous phenocrysts fe enrichment trend parallel ree patterns vitrophyric and esites and dacites consistent with low pressure high temperature crystallization in large shallow magma chambers the calc alkaline magmas show characteristics i e some hydrous phenocrysts no fe enrichment trend nonparallel rare earth patterns porphyritic lavas consistent with higher pressure and lower temperature of crystallization than the tholeiitic series tertiary plutons also show both calc alkaline and tholeiitic trends and appear to be chemically similar to the quaternary volcanoes the larger plutons are calc alkaline and probably represent the extensive 1002 km magma chambers of the small calc alkaline volcanoes the small shallow tholeiitic plutons complement the large tholeiitic volcanoes and reflect the greater percent of extrusive rocks in the tholeiitic series

paleomagnetic results for eocene volcanic rocks from northeastern washington and the tertiary tectonics of the pacific northwest the direction of remanent magnetization for 102 sites in eocene volcanic and volcaniclastic rocks of the o brien creek formation sanpoil volcanics and klondike mountain formation suggests approximately 25 of clockwise rotation of a 100 by 200 km area in northeastern washington the volcanic rocks consist chiefly of rhyodacite and quartz latite flows with intercalated ash flow tuff and volcaniclastic layers these rocks have been sampled at 102 sites distributed among five volcanotectonic depressions the toroda creek republic keller and first thought grabens and the spokane enterprise lineament the volcanic rocks probably range in age from 55 m y to about 48 m y and the 50 to 48 m y old volcanic rocks within this suite appear to be rotated as much as the older rocks previous investigators have shown that 40 m y old and younger plutonic rocks of northwestern washington are not rotated hence we infer that the north central washington rocks were rotated to their present declination between 48 and 40 m y b p during the middle and or late eocene during early eocene time this region was extended in a westward direction through crustal necking gneiss doming diking and graben formation internal deformation of the region related to this crustal extension was extreme but most bedrock units that were formed concurrent with the crustal extension were probably in place prior to the rotation hence we infer that the rotation was chiefly accommodated by movement on faults peripheral to the sampled area faults active during paleogene time appear to define boundaries of a triangular crustal block the sanpoil block encompassing much of northeastern washington northern idaho northwestern montana and adjacent parts of british columbia the faults include the laramide thrusts of the rocky mountain thrust belt the strike slip faults of the lewis and clark line and strike slip faults of the straight creek fraser zone we suggest that during early eocene time the sanpoil block was extended westward through crustal necking and dilation and then during the middle eocene was rotated clockwise and thrust over the craton in a final stage of laramide thrusting the motor driving these deformations presumably was interaction of north america with oceanic lithosphere off its western margin such interaction probably involved right oblique underthrusting and dextral shear

distribution and origin of igneous rocks from the landward slopes of the mariana trench implications for its structure and evolution the landward slope of the mariana trench is composed largely of igneous rocks serpentinites and serpentinized ultramafic rocks occur at nearly all structural levels on the slope from depths of 8000 to 1200 m seamountlike features on the trench slope break are the surface expression of serpentinite diapirs cumulate and massive gabbros are found several varieties of volcanic rocks are common including boninites altered and metamorphosed basalts andesites and dacites the chemical characteristics of the volcanic rocks indicate that nearly all are products of island arc volcanism together with the gabbros these volcanic rocks represent what is probably a late eocene arc complex these rocks were probably the first volcanic products to result from the subduction of the pacific plate beneath the phillippine sea plate their exposure on the trench slope today implies a significant amount of tectonic erosion of the landward slope since eocene time most of this removal of material appears to have occurred during the early stages of subduction there are isolated occurrences on the landward slope of rock assemblages including alkalic basalts chert hyaloclastites upper cretaceous siliceous sediments and shallow water limestones these assemblages are very similar to rocks dredged from seamounts on the offshore flankmore of the trench and their presence on the landward slope suggests that since the cessation of vigorous tectonic erosion there has been episodic accretion of seamount fragments to the landward slope less

radiometric evidence for the age of the subduction complex in the south orkney and south shetland islands west antarctica new k ar and rb sr data are presented for the age of part of the supposed late palaeozoic early mesozoic metamorphic complex in west antarctica here renamed the scotia metamorphic complex these results together with a review of other radiometric and palaeontological evidence fail to confirm a pre mesozoic age for the subduction complex in this region although rocks belonging to the albite epidote amphibolite facies could be as old as permian blueschists and greenschists from the south shetland is appear to be significantly younger and were probably formed in part at least during cretaceous subduction

sedimentation and volcanism in an oligocene miocene intra oceanic are and fore are southwestern viti levu fiji rocks belonging to the lower oligocene to middle miocene wainimala group in southwestern viti levu can be divided into an are assemblage representing the axial part of a volcanic are and a basinal assemblage representing the trenchward flank of the are and the inner part of the fore are basin the are assemblage consists mainly of volcaniclastic rudites and lava flows with minor shallow water limestones and is intruded by mid to late miocene gabbro and tonalite stocks the basinal assemblage to the north is floored by part of an earlier eocene to lower oligocene are the lower part of this assemblage includes volcaniclastic rudites representing the dispersal aprons of major volcanic edifices to the trenchward of the are axis and basaltic lavas and fissure fed rhyolite lava domes and breceia ridges erupted in deeper water away from these centres the rhyolites are directly related to the acidic members of a dense compositionally bimodal dyke swarm that intrudes the underlying earlier are rocks recording substantial crustal extension the basinal volcanic rocks are interbedded with and overlain by a sequence of pelagic carbonates thin bedded epiclastic turbidites and acidic tuffs the basinal succession evolved from an intra are to an inner fore are setting as volcanism within the assemblage died away in the early miocene in the late early miocene increased uplift and emergence of the are massif increased the sediment yield to the fore are basin resulting in the deposition of thicker bedded coarser grained turbidites

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

composition and origin of garnet from the antarctic peninsula volcanic group of trinity peninsula garnet is widely found as a minor constituent in rocks of the antarctic peninsula volcanic group apvg of trinity peninsula it occurs as conspicuous megacrysts or in xenoliths within volcanic rocks of andesitic rhyolitic composition and as detrital grains in the associated terrestrial sediments it is also found as an accessory mineral in many plutonic rocks from the e coast of the antarctic peninsula evidence is presented to show that the garnet can be divided petrographically and chemically into two main groups type a almandine rich primary igneous garnet and type b less almandine and more pyrope rich garnet as xenocrysts or included in xenoliths within the volcanic rocks comparison with published experimental data on garnet occurrence in acidic igneous rocks suggests that high almandine low spessartine garnet in volcanic rocks is a remnant phase of high pressure crystallization from magma at pressures of 7 kbar garnet with a higher pyrope content is regarded as xenocrystal in origin having been derived from garnet bearing country rocks at depth either as accidental inclusions or through direct partial melting restite of the lower crust and implies that a considerable thickness 25 km of crustal material was in existence before the generation of the mesozoic magmatic arc the origin of these calc alkaline magmas may therefore be due at least in part to partial melting of pre existing sialic crustal material

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

x lines in the magnetotail for southward and northward imf conditions utilizing associated observations of geotail and ace satellites from the year of 1998 to 2005 we investigated the x lines in the near earth tail under different interplanetary magnetic field imf conditions the x lines are recognized by the tailward fast flows with negative b z statistically the x lines in the tail can be observed for southward as well as northward imf but more frequently observed for southward imf a typical case on 26 april 2005 showed clear evidence that the x line can occur for northward imf while the geomagnetic activity is particularly quiet further analysis showed that the x line related solar wind has stronger e y and b z components for southward than northward imf in addition the x line related geomagnetic activities are stronger for southward than northward imf

the nadi basin neogene strike slip faulting and sedimentation in a fragmented arc western viti levu fiji the nadi basin in western viti levu is one of a number of sedimentary basins controlled by e w to ne sw faults that developed on the fiji platform in the late miocene the initial basin fill belonging to the upper miocene nadi sedimentary group consists of mainly basement derived breccias and sandstones deposited at bathyal depths by sediment gravity flows on a basin floor dissected by braided e w faults the main source area lay to the south across the actively down faulting basin margin in the latest miocene a change in provenance took place as debris spread southward from a newly developed e w chain of shoshonitic volcanoes koroimavua volcanic group some 12 km north of the southern basin margin continued normal displacement along the southern margin allowed the deposition of a thick sequence of shoshonitic sandstones at bathyal depths on the downthrown side in the earliest pliocene the onset of basin inversion was marked by shallowing to neritic depths and the onlap of sandstones and shoshonitic conglomerates southward onto pre nadi sedimentary group basement as inversion proceeded the basin fill was folded and southward directed reverse displacement took place along the southern basin margin evidence from the nadi and other late miocene basins indicates that they developed as a result of broadly e w directed sinistral strike slip probably related to a shearing collision of the melanesian border plateau with the northern margin of the fiji platform during the breakup of the formerly continuous outer melanesian arc it is suggested that prior to the establishment of the fiji fracture zone at 7 6 ma much of the left lateral displacement currently taking place along that plate boundary was accommodated by strike slip deformation within the fiji platform

subaqueous intermediate to silicic composition explosive volcanism a review abstract mass flow deposits of pyroclastic debris commonly consist of two parts a lower coarse grained basal section and an upper finer grained bedded section subaqueous pyroclastic flow deposits particularly those derived from a vertically directed eruption appear better sorted than their subaerial counterparts because of the efficient sorting characteristics of water compared to air subaqueous deposits frequently are depleted in bubble wall pumice that floats off and vitric ash that remains in suspension hence the deposits show a relative enrichment of crystals especially at the bases from 1 flotation and suspension of the vitric material and 2 ingestion of water at the head of the flow a subaqueous volcaniclastic flow deposit may contain evidence indicative of both turbulent and non turbulent movement the flow may have had a turbulent head and non turbulent body or flow transformations may have occurred from 1 collapse of the eruption column 2 changes in slope and in the velocity of the flow and or 3 the amount of water mixed into the flow retention of heat in the subaqueous environment may be enhanced by lateral eruptions high eruption rates large eruptive volumes and little or no mixing of pyroclastic debris with water the lack of welded subaqueous deposits in the geologic record may be a result of 1 effusive rather than explosive eruptions in a deep water environment 2 explosive phreatic disintegration of pyroclastic flows in a shallow subaqueous environment and or 3 the inability of subaerial pumiceous pyroclastic flows to penetrate the land water interface characteristics of explosive intermediate to silicic composition subaqueous volcanic eruptions also have been summarized in terms of 1 environment of the eruption 2 orientation of the eruption and 3 composition of the juvenile debris these three aspects of explosive subaqueous volcanism are ideal end members and gradations no doubt occur

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

devonian intra oceanic arc rift sedimentation facies development in the gamilaroi terrane new england orogen eastern australia abstract silurian devonian rocks within the gamilaroi terrane part of the new england orogen of eastern australia provide a well preserved on land example of the development of an intra oceanic arc rift ten sedimentary facies represent different phases in the evolution of the gamilaroi rift and are typified by predominantly volcaniclastic conglomerates and mudstones with minor sandstones all of which were deposited by debris flows and turbidity currents subordinate facies include limestones crystal rich volcaniclastic sandstones volcanic breccias and olistostromes the facies are distributed throughout a succession of six lithostratigraphic units each of which represents a distinct phase in the history of the arc rift felsic volcanic and volcaniclastic rocks at the base of the section represent part of the original arc and are overlain by volcaniclastic sandstones and mudstones deposited within an arc basin lower devonian emsian limestones underwent rapid subsidence concurrent with an influx of volcaniclastic sediments indicating initiation of rift related faulting a thick conglomerate succession was deposited rapidly on a debris apron system adjacent to the steep sided rift walls overlying tuffaceous mudstones indicate a decrease in clastic input later in the history of the rift basin thick intervals of massive and pillowed basalts at the top of the succession indicate a major period of volcanism within the rift oceanic island arc rift sequences are characterised by highly variable sedimentary facies distribution facies distribution over the 400 km extent of the gamilaroi terrane is also highly variable modern arc rift zones are divided into a series of smaller separate elongate basins providing an explanation for the high facies variability observed in the field

cretaceous paleomagnetism of the methow pasayten belt washington detailed demagnetization experiments isolated a characteristic remanent magnetization in ten stable sites from the upper cretaceous winthrop and midnight peak formations in the methow pasayten belt of north central washington this remanence agrees best between opposite limbs of a fold the goat peak syncline when corrected for 46 of tilt this is consistent with magnetization acquired during deformation synfolding magnetization may have been facilitated by a thermo chemical event associated with synkinematic intrusions along the axis of folding the mean direction d 12 0 i 61 1 alpha 95 4 8 is highly discordant with respect to the expected direction for north central washington this discordance points to about 1 400 km of poleward transport and 48 of clockwise rotation between 93 and 45 48 ma

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

late mesozoic sedimentation in the northern antarctic peninsula and its relationship to the southern andes two distinct phases of island are evolution are recognized in late mesozoic sedimentary rocks of the northern antarctic peninsula during late jurassic kimmeridgian tithonian times alternating radiolaria rich mudstones and ash fall tuffs were deposited under shallow marine euxinic conditions in association with restricted volcanic centres the name nordenskjold formation is proposed for this lithostratigraphic unit subsequently during the early cretaceous a major episode of volcanism and uplift led to the construction of an emergent are terrane simultaneous development of a retro arc basin resulted in the accumulation of coarse volcaniclastic detritus along the eastern side of the arc retro are sedimentation and intermittent volcanism continued into the late cretaceous and early tertiary although the pattern of sedimentation in the northern antarctic peninsula is broadly comparable to that of the southern andes an active marginal basin has not been recognized in the former area interbedded mudstones and tuffs identical to those of the nordenskjold formation were deposited in the southern andes and south georgia during the late jurassic early cretaceous

on determining paleohorizontal and latitudinal shifts paleomagnetism of spences bridge group british columbia the spences bridge group of late albian age palynology and u pb zircon date of 104 5 ma is situated in the intermontane belt of the southern canadian cordillera zeolite grade andesite lavas and interbedded volcaniclastic rocks have been sampled through a thickness of about 2900 m sampling has been confined to localities whose bedding attitudes are well defined and to rocks that have not undergone internal deformation of the 27 sites studied 25 gave single component magnetizations with well grouped directions high coercivities and high unblocking temperatures applying corrections for geological tilt to all 25 sites yielded minimum dispersion after 65 untilting analysis indicates that this is a consequence of the interaction of two modes of behavior one exhibited by y magnetizations at 8 sites in which minimum dispersion occurs after 35 untilting and one x magnetizations at 17 sites in which minimum dispersion occurs after 100 correction actually the dispersion is essentially constant in the range 88 to 100 correction the y magnetizations are uninterpretable in terms of the paleofield because no estimate of paleohorizontal can be made because of their positive response to the tilt test the x magnetizations were acquired before tilting and it is argued that geologically determined bedding planes are good estimators of paleohorizontal at the time of remanence aquisition there may be errors because of paleoslope but arguments are presented for believing that these are small on this basis the best estimate of the mean direction of the paleofield in spences bridge time is estimated to be d i declination inclination 38 7 63 9 17 sites k 44 compared with k 11 in situ 95 5 4 with a paleopole at 64 4 n 321 0 e k 24 compared with k 5 in situ a95 7 5 when compared with the cratonic reference field for mid cretaceous time this pretilting magnetization indicates a clockwise rotation of the study area of 66 3 11 8 p 0 05 and an apparent displacement from the south of 15 5 7 1 1725 790 km when these and other data previously obtained from cretaceous rocks of the pacific northwest are compiled they show an increase in apparent displacement from east to west across the cordillera this result is discussed in terms of three hypotheses 1 no translation but systematic tilts of about 30 to the southwest 2 translation of a substantial part of the pacific northwest referred to as baja british columbia and is defined more exactly in the text by variable amounts up to more than 2000 km from the south together with clockwise rotations and 3 lesser translation of baja british columbia by about 1500 km combined with variable tilting predominantly to the south it is concluded that paleomagnetic data as they are presently known are consistent with hypotheses 2 and 3 but unless serious sources of error are present are not consistent with hypothesis 1 the most likely candidate for such errors is the uncertainty in estimating initial dip of beds paleoslope in volcanic rock units

submarine volcaniclastic rocks in this chapter we are concerned with underwater volcaniclastic eruptions and products as well as pyroclastic flows and debris generated on land and transported by mass flowage into the sea submarine fallout deposits derived from land based eruptions are treated in chapter 7 clastic materials redistributed as mass flows or turbidity currents originating from the rapid build up of tephra and epiclastic volcanic debris along coastal parts of volcanoes are also discussed in various places throughout the text e g chap 13

lacustrine deltas in a mesozoic alluvial sequence from camp hill antarctica sedimentary rocks of late mesozoic age exposed at camp hill northern antarctic peninsula are associated with calc alkaline volcanic rocks they represent deposition on a fault controlled floodplain with marginal alluvial fans on a volcanic arc finely laminated mudstone and occasional graded laminae were deposited from suspension and by density underflow currents respectively in small shallow lakes thickening and coarsening upward sandstone bodies overlying the lake deposits are interpreted as lacustrine deltas of which two types are preserved 1 gilbert type with steep foresets and 2 mouth bar type which lack steep foresets sections through the latter type reveal the presence of sub environments characteristic of fluvial dominated marine deltas i e prodelta distributary mouth bar and distributary channel abandoned mouth bars resulting from avulsion are recognized it is suggested that the processes which operated during formation of the mouth bar deltas resulted from hyperpycnal flow by contrast the gilbert type delta is thought to be the consequence of a reduced inflow of suspended sediment causing homopycnal flow and thorough mixing of the river and lake waters

the age of the weddell basin the deep oceanic basin occupied by the weddell sea separates the east antarctic craton from the antarctic peninsula a largely post palaeozoic orogenic belt an understanding of the tectonic evolution of the basin would resolve most of the remaining problems of gondwanaland reconstruction which revolve around the relative positions of these two fragments and south america norton and sclater1 conclude that indian ocean marine geophysical data essentially support the smith and hallam fit2 but the unacceptable overlap ofundoubtedly old continental crust3 5 on the antarctic peninsula and falkland plateau implies relative motion between the peninsula and east antarctica such motion is not required by the reconstruction of barron and others7 8 which however can be criticized on other grounds9 10 the marine geophysical observations within the intervening ocean basins have proved useful in formulating plate tectonic reconstructions fracture zone trends seafloor spreading type magnetic anomalies crustal heat flux and crustal depth all provide clues to the timing and direction of the continental motions the acquisition of geophysical data from the weddell sea region has been slow11 13 however the combined interpretation of the two largest data sets extant12 13 now being undertaken seems likely to provide a more tightly constrained gondwanaland solution as a first step we report here an estimated age for the weddell sea floor in the area bounded by 60 and 73 s 5 and 35 w shown in fig 1

paleomagnetism and the motion of large and small plates the focus of paleomagnetic research during past decades has varied greatly the first generation in paleomagnetism might be defined as beginning with the pioneering studies aimed at discovering whether the geomagnetic field has been a reliable reference frame for measuring and timing continental displacements an enormous effort followed in the form of individual paleomagnetic studies of cratonic rocks culminating with the construction of apw apparent polar wander paths for the continents which could be used to test paleogeographic reconstructions coupled with parallel research in marine geophysics and geology the refined apwp which result from this effort have helped to develop a reasonable first order history of cenozoic mesozoic paleogeography for the major plates

paleomagnetism of the eocene kamloops group and the cratonization of terrane i of the canadian cordillera the lower middle eocene 49 4 2 4 ma kamloops group is exposed in the middle of the quesnellia subterrane of terrane i the group consists of the siliciclastic tranquille beds and the overlying dewdrop flats plateau basalts and andesites detailed alternating field af and thermal step demagnetization was carried out on 282 specimens from 26 flow sites and one conglomerate site and saturation isothermal remanent magnetization sirm tests were performed to examine the remanence carriers the petrology of the gently dipping flows the presence of antiparallel normal and reverse remanence the conglomerate test and the fold test all indicate that a primary remanence has been isolated it resides in both magnetite and hematite over a broad range of af coercivities blocking temperatures and domain sizes its mean direction of 355 0 73 4 95 6 9 gives a pole position of 138 4 w 81 4 n dp 11 0 dm 12 3 that is statistically indistinguishable from the 50 ma reference pole for the north a

substorm onset observations by image fuv 1 over the first 2 5 years of operation the fuv instrument on the image spacecraft observed more than 2400 substorm onsets in the northern hemisphere the observations confirm earlier results of statistical studies in terms of a median substorm onset location at 2300 hours mlt and 66 4 degrees magnetic latitude the purpose of this report is to publish the list to allow for further investigation the list can easily be searched for onsets close to certain ground stations or at specific magnetic latitudes or local times as one example of such use we demonstrate how the probability of onset observation was determined for the ground based automatic observatories of the themis time history of events and macroscale interactions during substorms project

magnetic anomalies and tectonic fabric of marginal basins north of new zealand detailed airborne magnetic studies conducted over the region of the s w pacific marginal basins extending from the solomon islands to new zealand suggest that three major phases of basin formation and island arc development have occurred in this region development of the tasman sea took place during the late cretaceous paleocene development of the basins to the east of the tasman sea occurred predominantly during the oligocene as well as during the upper miocene to recent the south fiji basin consisting of the kupe and minerva abyssal plains is marked by the presence of possibly two rrr triple junction spreading centers that were active between the times of anomalies 13 to 7 36 25 5 m y the kupe abyssal plain shows the presence of residual magnetic anomalies 7 to 13 of the eastern limb of the proposed spreading center the western limb appears to have been subducted beneath the present site of the three kings rise this seafloor spreading phase calculated half spreading rate of 35 mm yr was coincident with the overthrusting phase of the new caledonia ultramafic rocks during that period active volcanism along the then continuous solomons new hebrides fiji lau island arc was taking place magnetic anomalies from 1 to 4 0 8 m y b p are seen to extend along a clearly defined lineation pattern over the north fiji basin analysis of the magnetic anomalies suggests that during the upper miocene at about the time of magnetic anomaly 4 8 m y the solomons lau island arc commenced breaking up through the development of an active spreading center calculated half spreading rate of 35 mm yr located between the new hebrides and fiji portions of the island arc portions of the northeastern sector of the minerva abyssal plain were subducted beneath the north fiji basin during this period between 0 8 m y b p at the time of about anomaly 3 to 2 4 5 3 5 m y the development of the north fiji basin extended into the lau basin and havre trough thus separating the tonga and lau ridges as a result a distinct series of active spreading centers developed southward along the axis of the havre trough lau basin the havre trough is marked by present seismic and submarine volcanic activity residual magnetic anomalies 1 2 0 1 8 m y b p form a symmetrical pattern around the axis of the trough extending south from the lau basin to the active taupo white island volcanic zone of new zealand phases of development of the marginal basins described in this paper may be correlated with phases of changes in the direction and rate of pacific plate motion

paleoshoreline record of relative holocene sea levels on pacific islands abstract understanding the history of relative holocene sea levels on pacific islands is important for constraining fundamental geodynamic theories interpreting the environments of early human occupation sites and forecasting future environmental conditions on the islands an observational paleoshoreline record is provided by emergent paleoshoreline indicators formed at higher relative sea levels hence standing at higher elevations than modern counterparts emergent paleoshoreline notches in limestone seacliffs record paleo high tide levels and emergent paleoreef flats record paleo low tide levels whereas emergent paleobeachrock locally records paleo intertidal levels both paleonotches and paleoreefs occur along the coasts of high standing islands exposing volcanic bedrock and uplifted reef complexes but low lying coralline atolls lack sufficient relief to preserve paleonotches controls on relative holocene sea level include global eustatic and regional hydro isostatic changes in ambient sea level relative to island landmasses and shifts in the elevations of islands relative to sea level caused by thermal subsidence of the oceanic lithosphere or thermally rejuvenated loci of hotspot volcanism by flexure of the lithosphere under the load of growing volcanic edifices hawaii samoa society islands by arching of the lithosphere over trench forebulges loyalty islands niue bellona rennell and by tectonism within forearc belts between active volcanic chains and trenches mariana islands tonga vanuatu the dominant pattern of relative sea level change where not overprinted by local tectonism or lithospheric flexure was a uniform early holocene rise in eustatic sea level followed by a regionally variable late holocene hydro isostatic drawdown in sea level the resultant was a mid holocene highstand in relative sea level that affected the development of shoreline morphology throughout the tropical pacific ocean the earliest human migrations into intra oceanic island groups of both the northwestern and southwestern pacific ocean followed close on the heels of the mid holocene sea level highstand and took advantage of newly attractive coastal environments engendered by sea level drawdown the effects of the mid holocene highstand were modified to varying degrees in different island groups by geodynamic uplift or subsidence

shallow marine sedimentation within an active margin basin james ross island antarctica abstract the santa marta formation exposed on northern james ross island antarctica represents shallow marine shelf sedimentation within an active margin basin the formation is approximately 1000 m thick is of uppermost santonian to campanian age and represents part of a 5000 6000 m thick sedimentary sequence forming the larsen basin the larsen basin represents either a back arc basin or a half graben basin developed on the extending margin of the weddell sea twelve sedimentary facies have been recognised which can be subdivided into two groups those which are controlled by shelf processes and those which are related to active arc volcanism background shelf processes include fair weather suspension sedimentation storm sands and rare tidal current reworking active arc processes include direct settling of airfall detritus and rapid resedimentation of volcaniclastic detritus by sandy debris flows and both high and low concentration turbidity currents in addition sheet conglomerates represent debris flows that evolved from syn sedimentary slumps two facies associations representing a mid to outer shelf and an inner shelf depositional setting respectively can be distinguished with an apparent regression in the mid to late campanian

testing terrane transport an inclusive approach to the baja b c controversy the baja british columbia hypothesis holds that a large segment of the western edge of northern north america baja b c was situated alongside california and northern mexico in middle cretaceous time was displaced northward in the late cretaceous and paleocene by north oblique convergence of the kula plate with north america and arrived near its present location by the early eocene a consistent body of paleomagnetic data supports this hypothesis however doubt persists and various crucial tests of a geologic nature have been proposed one such test concerns the provenance of zircons in cretaceous sedimentary basins of baja b c in this paper we use both paleomagnetic data and zircon occurrences to reconfirm the baja b c hypothesis we first argue that the only truly crucial tests yet performed have been paleomagnetic and that all such tests have been positive second we show that detrital zircon data from the upper cretaceous nanaimo group although not a crucial test provide valuable paleogeographic information available data demonstrate a change in detrital zircon provenance in the nanaimo group that closely matches the position of these rocks predicted by the baja b c hypothesis detrital zircons in the nanaimo group suggest a change from a southwestern north american source rich in grenville and 1 4 1 5 ga rocks to an increasing contribution from older parts of the craton such as the wyoming province together detrital zircons and paleomagnetic inclinations allow us to assemble a detailed schedule of northward tectonic transport of the baja b c terranes

can intense substorms occur under northward imf conditions 1 this paper reports on several substorms observed under northward interplanetary magnetic field imf conditions the intensity of which was at least as significant as that of typical substorms under moderately southward imf conditions such northward imf periods were identified during the recovery phase of three strong storms in the case of each storm two or more substorms occurred successively being separated by 1 8 5 h while the imf condition continued persistently northward the substorms are clearly evidenced by auroral and other complementary observations for the most intense substorms the auroral breakup occurred at the magnetic latitude of 58 and for the others it was between 60 and 65 the polar cap size prior to each onset was substantial despite the northward imf conditions the auroral expansion following each onset lasted from a few up to several magnetic local time hours and exhibited a clear poleward expansion feature for most of the events studied geosynchronous magnetic dipolarizations preceded by field stretching and or energetic particle injections occurred the occurrence of such intense substorms implies that a certain large amount of energy remains in the tail even under northward imf conditions the occurrence of two or more successive substorms further implies that even after the release of a certain amount of energy triggered by the substorm the tail can still have a substantial amount of energy left which can be released by a subsequent substorm s we conjecture that an intense substorm during a northward imf period can be expected when such a period belongs to the recovery phase of an intense storm mainly because of large energy loading done by preceding southward imf bz during the storm s main and some early recovery phase in addition we argue that substorm energy can also be supplied by other mechanisms of the solar wind magnetosphere coupling under northward imf conditions such as dayside reconnection in the presence of a substantial imf by component

successive substorm expansions during a period of prolonged northward interplanetary magnetic field 1 we have studied successive substorm expansions that occurred in spite of a period of prolonged northward interplanetary magnetic field imf after the northward imf persisted for almost 19 h a series of 11 very weak to moderate substorm expansions occurred during the following 6 h interval of northward imf on 19 january 1998 separated from each other by a few tens of minutes most of these substorm expansions did not accompany significant changes of the solar wind and the imf as for typical substorms that occur under southward imf conditions spacecraft and ground based instruments observed auroral breakups and enhancements of the westward auroral electrojet and large scale convection in the ionosphere together with plasmoids and dipolarizations in the magnetotail in association with the substorm expansion onsets these signatures suggest that the mechanism of substorm expansion onsets is the same as for typical substorms even under prolonged northward imf conditions we suggest that the large imf by affects large scale convection and energy accumulation in the magnetotail and that the enhanced convection due to the large imf by effect and substorm expansions can be an important factor for the promotion of succeeding substorm expansion onsets

campanian maastrichtian cretaceous stratigraphy of the james ross island area antarctica one of the most important outcrops of uppermost cretaceous campanian maastrichtian sedimentary rocks in the southern high latitudes occurs within the james ross island group northeastern antarctic peninsula these rocks comprise a 1500 2000 m thick sequence of predominantly shallow marine clastic sediments that were deposited within a retro arc basin they are virtually undeformed and have yielded prolific invertebrate and vertebrate faunas and a wide range of plant taxa campanian maastrichtian strata are contained within two component formations of the upper cretaceous lower tertiary marambio group the lower santa marta formation is approximately 1100m thick and has three constituent members ammonites within the santa marta formation indicate an early to late campanian age assignment dinoflagellates suggest that the lower levels of the formation may range into the santonian stage in the northern james ross island to vega island region there is a conformable transition into the overlying lopez de bertodano formation here the basal cape lamb member is unconformably overlain by the sandwich bluff member although there are some lithological and fauna1 discrepancies which are attributed to marked lateral facies changes across the basin it is believed that the cape lamb member can be correlated with the lower to middle regions of the 1200 m thick sequence of undifferentiated lopez de bertodano formation lithologies exposed on seymour island molluscan data indicate a late campanian palaeocene age range for this upper formation dinoflagellates again suggest that the lowest beds may be slightly older the correlation presented provides the basis for a formal subdivision of the campanian and maastrichtian stages in the southern high latitudes it has also revealed two major basin shallowing events in the late campanian early maastrichtian and latest maastrichtian respectively and the existence of a major fault fault zone across southeastern james ross island the improved temporal framework will aid late cretaceous palaeoclimatic and palaeobiological studies in antarctica

early geochemical evolution of an oceanic island arc and backarc fiji and the south fiji basin the initial 25 to 35 ma of oceanic arc volcanism in fiji the early arc stage was dominated by eruption of lree depleted low k tholeiitic basalts and andesites the oldest volcanics include the highest hfse group morb like in composition plus high mg basalts and basic andesites with flat ree patterns as low as 4x chondritic concentrations these erupted either within the volcanic arc during its first 5 ma or in an adjacent backarc during its first ma of formation subsequent volcanics of the early arc stage apparently erupted onto early backarc basin crust most are intermediate between earlier extremes in hfse contents but at least one volcano produced strongly lree depleted island arc tholeiites in contrast medium k tholeiitic and calc alkaline andesites erupted during the mature arc stage parental basalts were enriched in lree but depleted in hree relative to earlier basalts due to a change in source composition counterflowing morb source asthenosphere within the mantle wedge dominated vol

late palaeozoic early mesozoic fore arc basin sedimentary rocks at the pacific margin in western antarctica we infer that the trinity peninsula group of the antarctic peninsula was deposited in a fore arc basin s of composite type located between the gondwanian arc massif consisting of plutonic and older gneissose basement rocks and a subduction complex now represented by greenschists blueschists and albite epidote hornblende bearing rocks of the pre jurassic metamorphic complex

subaqueous sediment gravity flow deposits practical criteria for their field description and classification a new method for the description and classification of subaqueous sediment gravity flow deposits is proposed the classification scheme employs a convenient letter code and divides deposits individual beds into descriptive categories of two hierarchical levels facies and subfacies facies as the higher rank categories are distinguished chiefly on the basis of sediment type i e bed grain size texture a total of 13 facies have been distinguished g gravel gs gravel sand couplet gys gravelly sand s sand sm sand mud couplet ms mud sand couplet tm silt mud couplet mt mud silt couplet m mud mys muddy sand sym sandy mud myg muddy gravel gym gravelly mud subfacies as the lower rank categories are distinguished within the individual facies on the basis of the bed s internal structures the number of subfacies is unlimited and their labelling code includes particular facies symbols see above preceded by lower case letters denoting specific sedimentary structures and their vertical arrangement subfacies thus refer to the bed s intervals or divisions which are labelled as follows m massive unstratified and ungraded g graded unstratified and graded s plane stratified x cross stratified 1 parallel and or cross laminated q liquefied for example subfacies gsg graded to plane stratified gravel are gravel beds that have a lower graded interval and an upper plane stratified interval subfacies xg cross stratified gravel are gravel beds that are cross stratified throughout subfacies sls plane stratified to laminated sand are sand beds that have a lower plane stratified interval and an upper laminated parallel and or cross laminated interval

petrology and geochemistry of the island of sarigan in the mariana arc calc alkaline volcanism in an oceanic setting isotopic studies of rocks from oceanic island arcs such as the marianas indicate that little if any recycling of continental material e g oceanic sediments occurs in these arcs because oceanic arcs are on the average more mafic than the dominantly andesitic continental arcs an important question is whether the andesites of continental arcs are produced by a fundamentally different more complex mechanism than the lavas of oceanic arcs an excellent opportunity for study of this question is provided by the island of sarigan in the mariana active arc on which calc alkaline andesites including hornblende bearing types are exposed along with more mafic lavas

eocene rotation of the coast plutonic complex and intermontane belt paleomagnetism of eocene plutons along the klondike highway paleomagnetic results are reported for the 59 ma skagway 54 ma fraser 53 ma summit lake and 48 ma clifton felsic plutons of the eastern coast plutonic complex cpc that outcrop along the south klondike highway in alaska and british columbia thermal and alternating field step demagnetizing methods yielded stable characteristic remanent magnetization chrm directions for all 29 sites of normal reversed and mixed polarity the chrm resides in single or pseudosingle domain magnetite and or pyrrhotite that is shown to be primary by contact tests with the 47 ma vertical white pass mafic dikes paleopoles from six 56 to 50 ma mean 52 2 ma intermontane belt yukon tanana terrane imb ytt units that cannot be explained by tectonic tilt are compared with nine clustered 59 to 46 ma mean 52 4 ma eastern cpc paleopoles both paleopole populations show nonsignificant poleward northward translation relative to north america imb ytt 3 7 5 3 n cpc 4 3 6 4 s overall 1 2 4 9 s ind

new 42 ma cratonic north american paleomagnetic pole from the yukon underscores another cordilleran paleomagnetism geology conundrum eocene posttectonic plutons of the beaver river alkalic complex in southeastern yukon intruded devonian mississippian and triassic sandstones in the foothills of the canadian cordillera a paleomagnetic collection of 27 sites from three separate plutons produced 326 specimens that were analyzed using alternating field and thermal step demagnetization methods the a component characteristic remanent magnetization chrm resides in magnetite with normal polarity in the 42 6 0 8 ma beaver river pluton reversed polarity in the 42 1 0 7 ma larson creek east pluton and both polarities in the 41 3 0 4 ma larson creek west pluton corresponding with magnetic polarity chrons 20n 19r and the boundary between chron 19r and 18n respectively the chrms of the plutons are indistinguishable 2 with a mean for the 42 0 0 5 ma complex of d 158 8 i 73 1 n 21 sites 95 3 0 k 116 8 a positive paleomagnetic contact test shows the a component to be primary and the poorly isolated b component

geology and geochemistry of early arc volcanic rocks from guam the island of guam is located at the southern end of the mariana fore arc and was the site of early arc volcanism between 43 and 32 m y ago two volcanic units were erupted during the early evolution of guam the late middle eocene facpi formation and the late eocene to early oligocene alutom formation the facpi formation is composed largely of interbedded boninite series pillow lavas pillow breccias and dikes although arc tholeiite series rocks cap the formation in some areas the alutom formation is composed of interbedded volcanic breccias tuffaceous sandstones lava flows and sills calc alkaline arc tholeiite and boninite series compositions are all found in the alutom formation primitive boninite series rocks are relatively high in sio 2 mgo and ni and low in a1 2 o 3 and tio 2 contents compared to basaltic rocks from the arc tholeiite series additionally they have low ti zr ratios and high k zr rb zr and ba zr ratios although no calc alkaline basalts are found in the early arc formations of guam basalts with chemical characteristics transitional between boninite and tholeiite series rocks may be genetically linked to the calc alkaline series among the analyzed samples here are a few with anomalously high y and rare earth element ree contents chemical variation in the boninite series can be explained by early fractionation of olivine and clinopyroxene followed by plagioclase clinopyroxene and orthopyroxene the arc tholeiite series chemical trends can be explained by early olivine fractionation followed by plagioclase clinopyroxene orthopyroxene and magnetite the composition of the least silicic calc alkaline series andesite can be modeled by fractional crystallization of olivine clinopyroxene plagioclase and magnetite from a transitional basalt derivation of more silicic calc alkaline series rocks requires magma mixing or crustal assimilation in addition to crystal fractionation involving hornblende all basalts from the early volcanic series of guam were erupted in an arc setting the boninite series magmas were produced by hydrous partial melting of depleted mantle at relatively shallow levels whereas the arc tholeiite and calc alkaline series magmas were generated by partial melting of less depleted mantle at deeper levels we suggest that because the calc alkaline series parent basalts most likely were richer in incompatible high field strength hfs elements and sio 2 than were the tholeiitic parents they were generated by lower degrees of melting

submarine crystal tuffs their origin using a lower devonian example from southeastern australia summary the submarine crystal rich volcaniclastics of the lower devonian merrions tuff contain concentrations of angular to euhedral volcanic quartz plagioclase and orthoclase lithic fragments largely altered vitriclasts are minor components and the average matrix content is 37 6 associated dacite andesite and rhyodacitic lavas have average groundmass contents of 64 2 rare shards in the graded pelitic tops of the thick volcaniclastic sedimentation units suggest that the mode of fragmentation originally was by explosive magmatic eruptions the sedimentology of the volcaniclastics suggests subsequent redeposition by cold state mass flow processes the volcaniclastics form an isotopically coherent suite and so redeposition must have occurred essentially contemporaneously with eruption the high crystal fragment concentration in these volcaniclastics is higher than lavas and ignimbrites and suggests some process whereby the groundmass fraction of the erupting magma is selectively removed so concentrating the crystal fraction the crystal rich character of the crystal tuffs is not simply due to explosive eruption several primary and secondary factors processes could have interacted to ultimately produce the crystal rich character these being i eruption of highly crystallized magmas 65 phenocrysts ii concentration of crystals in primary eruption columns iii concentration of crystals in any resulting pyroclastic flows fine vitric ashes being elutriated out and being carried away in accompanying trailing ash clouds iv concentration of crystals in secondary eruption columns generated by the flow of hot pyroclastic flows into the ocean and v concentration of crystals by the elutriation of fines into the trailing fine sediment cloud accompanying submarine mass flows resulting from the slumping of volcaniclastic aggregates from shallow marine subaerial settings

paleogeographic reconstruction and origin of the philippine sea abstract our reconstruction of the philippine sea suggests that it formed by two distinct episodes of back arc spreading each of which resulted from seaward retreat of the trench in the first episode the protoizu bonin trench retreated northward and the west philippine basin formed behind the northern half of the palau kyushu ridge in the second episode the izu mariana trench retreated eastward and the shikoku and parece vela basins formed behind it during the last 17 ma the philippine sea basin has been moving northwestward with respect to eurasia shifting the ttt triple junction off central japan westward by about 50 km the motion of the philippine sea with respect to eurasia at the triple junction changed from north northwestward to west northwestward 10 5 ma ago for the period before 17 ma ago we construct two models retreating trench model and anchored slab model the izu bonin trench migrated from south to northeast rotating in a clock wise sense since 48 ma ago in the retreating trench model in the anchored slab model the trench has been fixed with respect to eurasian margin since 43 ma ago we prefer the retreating trench model because the deformation of the plate boundary along the eastern margin of eurasia during 30 17 ma ago is much simpler for this model than for the anchored slab model furthermore rotations of the bonin mariana islands are consistent with those predicted from the retreating trench model the 48 ma ages of the northern part of the palau kyushu ridge and of chichi jima of the bonin islands indicate that there was subduction beneath the northern half of the ridge beginning at least 48 ma ago from this and the subparallelism in trend between the northern part of the palau kyushu ridge and the central basin ridge we propose that the major part of the west philippine basin formed by back arc spreading in a n s direction behind the northern part of the palau kyushu ridge the pacific plate was moving northward with respect to hot spots from 48 to 43 ma ago which implies that the pacific plate is not likely to have been subducting beneath the west philippine basin during this time we speculate that another plate existed south of the pacific plate and thai it was subducting beneath both tile west philippine basin and australia the annihilation of this plate might he a cause for the sudden change of the pacific plate motion at 43 ma ago

paleomagnetism and tectonics of the cretaceous mt stuart batholith of washington translation or tilt abstract the mt stuart batholith is a composite pluton of late cretaceous age that intrudes the crystalline north cascades terrane of northwestern washington its paleomagnetic direction d 10 0 i 45 5 95 4 9 is markedly different from the direction expected for cretaceous rocks from northwestern washington d 330 5 i 73 0 which means that the mt stuart batholith either has moved relative to the north american interior or has been tilted through a substantial angle or both since it acquired its magnetism either tilt or translation are possible geometrically but translation is more likely because 1 local geology apparently does not support tilt in the required direction 2 it probably is not possible for a panel of rock the size of the mt stuart batholith to tilt through the necessary angle ca 35 after its deeper parts have cooled sufficiently to retain remanent magnetization 3 the sense of paleomagnetic discordance found in the mt stuart batholith clockwise rotation of declination flattened inclination is identical to that found in nearly every other allochthonous terrane in the western north american cordillera including every other cretaceous batholith studied except the sierra nevada

paleomagnetism of middle tertiary volcanic rocks from the western cascade series northern california the western cascade series wcs is a 3 5 km thick crudely homoclinal east dipping calcalkaline volcanic sequence of mid oligocene to early miocene age that crops out near the southern tip of the cascade range in northern california the mean direction of remanent magnetization in the wcs is d 4 9 i 57 6 n 53 k 14 4 95 5 3 when compared to a reference direction for the north american craton the wcs direction indicates that the southern cascade range has rotated 14 0 9 0 since the wcs accumulated a difference in mean direction between the lower and upper halves of the wcs suggests that much of this rotation occurred during the late oligocene six other paleomagnetic studies of rock units of an age roughly comparable to the wcs also are available from western oregon northwestern california and southwestern washington all show statistically significant clockwise rotation inclinations of all but one are concordant comparison of directions indicates that the pacific northwest did not rotate as a rigid body but neither did it behave as a collection of small independently rotating domains the amount of rotation found throughout the area increases to the west or northwest suggesting a driving force for rotation operating at the continental margin