three dimensional splay fault geometry and implications for tsunami generation megasplay faults very long thrust faults that rise from the subduction plate boundary megathrust and intersect the sea floor at the landward edge of the accretionary prism are thought to play a role in tsunami genesis we imaged a megasplay thrust system along the nankai trough in three dimensions which allowed us to map the splay fault geometry and its lateral continuity the megasplay is continuous from the main plate interface fault upwards to the sea floor where it cuts older thrust slices of the frontal accretionary prism the thrust geometry and evidence of large scale slumping of surficial sediments show that the fault is active and that the activity has evolved toward the landward direction with time contrary to the usual seaward progression of accretionary thrusts the megasplay fault has progressively steepened substantially increasing the potential for vertical uplift of the sea floor with slip we conclude that slip on the megasplay fault most likely contributed to generating devastating historic tsunamis such as the 1944 moment magnitude 8 1 tonankai event and it is this geometry that makes this margin and others like it particularly prone to tsunami genesis

new insights into deformation and fluid flow processes in the nankai trough accretionary prism results of ocean drilling program leg 190 moore g f taira a klaus a becker l boeckel b cragg b a dean a fergusson c l henry p hirano s hisamitsu t et al 2001 new insights into deformation and fluid flow processes in the nankai trough accretionary prism results of ocean drilling program leg 190 geochemistry geophysics geosystems 2 article no 2001gc000166

minas fault zone late paleozoic history of an intra continental orogenic transform fault in the canadian appalachians abstract the minas fault zone mfz defines the boundary between the avalon and meguma terranes in the canadian appalachians and is exposed in mainland nova scotia and southern new brunswick these terranes originated along the gondwanan margin but had accreted to laurentia by the middle devonian the surface trace of the mfz is adjacent to the southern margin of the latedevonian permian maritimes basin the late devonian late carboniferous evolution of the mfz involves several episodes of oblique dextral shear that resulted in basin formation and inversion and at various times the zone was the focus of magmatism regional fluid flow and mineralization in the late devonian early carboniferous asymmetric rifting accompanied by dextral shear produced two coeval sequences the horton group which is dominated by continental clastic strata and the fountain lake group which consists predominantly of bimodal volcanic rocks that overlie high level plutons emplaced along active shear zones the overall tectonic environment may have been dominated by dextral transtension along the southern margin of laurentia which corresponded with the northern flank of the rheic ocean a major change in the evolution of the minas fault zone occurred in the late mississippian early pennsylvanian and produced the e w chedabucto fault clockwise rotation of pre existing structures local zones of transtension and transpression as well as regional fluid flow and extensive mineralization this major change may reflect the onset of laurentia gondwana oblique collision the effects of which continued into the latest carboniferous with coeval development of flower structures and pull apart basins in zones of local transpression and transtension

petrology chronology and sequence of vein systems systematic magmatic and hydrothermal history of a major intracontinental shear zone canadian appalachians abstract intra continental shear zones developed during continental collision may experience prolonged magmatism and mineralization the cobequid shear zone formed part of a ne sw trending orogen parallel shear system in the late devonian early carboniferous where syn tectonic granite gabbro plutons and volcanic rocks 4 km thick were progressively deformed in late carboniferous to permian alleghanian collision of africa with laurentia formed the e w trending minas fault zone reactivating parts of the cobequid shear zone the 50 ma history of hydrothermal mineralization following pluton emplacement is difficult to resolve from field relationships of veins but sem study of thin sections provides clear detail on the sequence of mineralization the general paragenesis is albite quartz chlorite monazite biotite calcite allanite pyrite fe carbonates fe oxides minor sulfides calcite and synchysite chronology was determined from literature reports and new u pb la icpms dating of monazite and allanite in veins vein mineralization was closely linked to magmatic events vein emplacement occurred preferentially during fault movement recognised from basin margin inversion as a result of fractures opening in the damage zone of master faults the sequence of mineralization from ca 355 ma riebeckite and albite veins to ca 327 305 ma siderite magnetite and sulfide mineralization resembles precambrian iron oxide copper gold iocg systems in the literature the abundant magmatic na halogens and co2 in veins and some magmatic bodies characteristic of iocg systems were derived from the deeply subducted rheic ocean slab with little terrigenous sediment regional extension of the magdalen basin caused asthenospheric upwelling and melting of the previously metasomatized sub continental lithospheric mantle crustal scale strike slip faulting facilitated the rise of magmas resulting in high heat flow driving an active hydrothermal system

temporal variation of titanite morphology and chemistry in a long lived shear zone the clarke head syenite in the minas fault zone nova scotia abstract the mineral chemistry of titanite has been studied in hydrothermally altered syenite preserved as blocks in a salt wall within a regional intracontinental shear zone the study focusses on the controls on temporal variation in rees zr nb u and th several generations of titanite are recognised on the basis of morphology chemistry and relationship to other secondary minerals magmatic euhedral to subhedral titanite has ti and zr contents characteristic of magmatic titanite in other studies but does not have a magmatic abundance of ree rather it is 102 104 times depleted in lree and only slightly depleted in hree the leaching of lree began during a regionally recognised phase of potassic alteration as amphibole and biotite in gabbro and diorite recrystallized along shear zones and released k ca and f later some 15 ma after emplacement the alkali feldspar in the syenite was largely altered to scapolite and analcime by interaction with adjacent halite and gypsum deposits titanite formed at that time from dissolution reprecipitation reactions in large rutile crystals is less depleted in lree as a result of the dominance of cl in hydrothermal waters following that event f dominated hydrothermal waters prevailed again resulting in more lree depletion in previously formed titanite and in neoformed hydrothermal titanite at that time there was also depletion in zr and u the trace elements in hydrothermal titanite are thus very informative on the hydrothermal history of the rocks a combination of morphology and trace element signature allows different hydrothermal stages to be distinguished

late archean regional deformation and structural controls on gold quartz vein mineralization in the northwestern slave province n w t canada late archean crustal accretion in the northwestern slave province is suggested to have involved approximately west northwest east southeast directed horizontal compression that produced three episodes of deformation recognizable in the northwestern anialik river igneous complex aric and anialik river greenstone belt argb observations show that the aric was probably emplaced as a series of synvolcanic sills prior to the earliest deformational event regional shortening produced a pervasive foliation downdip lineations and folding in the argb and an early subsequently folded foliation in the northwestern aric postfold ductile and brittle ductile deformation produced regional and outcrop scale shear zones including the sheeted zone which defines the aric argb contact younger rocks of the northwestern argb appear to have been tectonically juxtaposed along the sheeted zone against the older rocks of the northwestern aric greater brittle response enhanced permeability and cyclical increases in f

titanium mobilization by hydrocarbon fluids related to sill intrusion in a sedimentary sequence scotland abstract an occurrence of vein hosted bitumen in carboniferous rocks central scotland exhibits unusual enrichments in titanium zirconium and chromium detected by electron microprobe these elements occur as mineral inclusions of titanium and titanium iron oxides a close association between titanium rich tuffs and a quartz dolerite sill suggests that metals were leached from igneous rocks by a hydrocarbon rich fluid generated by the heat of the intrusion into organic rich shales this occurrence supports other evidence for widespread titanium mobility under low temperature conditions inclusions of nickel arsenide may reflect regionally important mineralization including former ore deposits related to sills and dykes as sills are extensively intruded into petroleum prospective basins offshore they may have a significant influence upon sediment geochemistry

when seamounts subduct data from an array of seismometers on the sea floor show the complex pattern of earthquakes around subducted seamounts

quantitative quality of distributed acoustic sensing vertical seismic profile data great advances have been made in distributed acoustic sensing das vertical seismic profile vsp data acquisition hardware and software here we capture a quantitative assessment of the quality of das data at a single point in time we apply comprehensive testing methods to determine the reliability of the data and its suitability as a supplement to geophone data or to gain access to wells where it would be difficult to deploy geophones the test measurements are made on das and geophone data which were collected at the same time and in the same well we analyze the first breaks for waveform consistency signal to noise s n ratio and slowness then we examine the corridor stacks for waveform consistency and s n ratio finally we test the properties of the measurement including linearity repeatability reliability and response as a function of the angle of incidence of a seismic wave to the fiber the results show that the das vsp data provide accurate formation slowness logs and reliable ampl

zr nb ree mineralization in peralkaline granites from the amis complex brandberg namibia evidence for magmatic pre enrichment from melt inclusions the amis complex is a late intrusive phase associated with the brandberg anorogenic granite intrusion it consists of peralkaline arfvedsonite bearing granitic and pegmatitic dikes and sills and is characterized by locally extreme enrichments in ree and rare metals with high charge ionic radius ratios such as zr and nb the highest concentrations e g 1 7 wt zr 0 3 wt nb 0 5 wt total ree are found in aegirine albite aplites that formed around arfvedsonite pegmatite cores quartz hosted melt inclusions in coarse grained arfvedsonite granite and pegmatite give unequivocal evidence that the peralkaline composition and rare metal enrichments are primary magmatic features electron microprobe and sims analyses of rehomogenized inclusion glasses reveal their peralkaline high si rhyolite compositions avg molar na k al 1 1 and very high contents of h2o max 10 6 wt and f 2 2 wt inclusion glasses also have high concentrations of incompatible trace elements including nb min 803 ppm zr min 3 187 ppm and ree y min 1 271 ppm the ree zr and nb compositions of the aegirine albite aplites plot along the same linear enrichment trends as the melt inclusion glasses and y ho ratios mostly display unfractionated near chondritic values the chemical and textural features of the aegirine albite aplites are compatible with an origin by rapid crystallization compositional quenching after volatile loss from a residual peralkaline granitic melt of a composition like that of the melt inclusions interaction of postmagmatic fluorine rich aqueous fluids with the arfvedsonite granites caused local remobilization of ree and secondary mineralization of fluorocarbonate phases such as bastnaesite and fractionation of y and ho

early carboniferous deformation and mineralization in the cobequid shear zone nova scotia an 40ar 39ar geochronology study latest devonian to early carboniferous igneous rocks along the cobequid shear zone of central nova scotia record a complex history of intrusion volcanism deformation and hydrothermal alteration twenty new 40ar 39ar age determinations have been made on biotite and hornblende separates from rocks that constrain these events the mafic plutons of the cobequid shear zone are synchronous with the previously dated granitoid plutons 363 355 4 ma with the exception of the younger folly lake gabbro 355 350 4 ma high temperature shear deformation continued along the cobequid shear zone during this entire period of pluton emplacement several samples from mylonitic and hydrothermally altered rocks with dates ca 340 ma indicate renewed movement along the cobequid shear zone at this time accompanied by minor magmatism younger 330 320 ma ages reflect final movement along the rockland brook fault resulting in mid namurian uplift of the cobequid highlands following this regional alleghenian event ther

lamellar magnetism and exchange bias in billion year old titanohematite with nanoscale ilmenite exsolution lamellae i mineral and magnetic characterization

ion microprobe 232th 208pb ages from high common pb monazite morefield mine amelia county virginia implications for alleghanian tectonics monazite ce th po4 from a pegmatite in the morefield mine of the eastern piedmont of central virginia has unusually high and variable amounts of common pb leading to problematic interpretations of its u th pb ages and how the monazite relates to nearby granite intrusions and faults to address these issues we analyze a single large monazite grain from the pegmatite using electron microprobe analysis empa n 64 laser ablation inductively coupled plasma mass spectrometry la icp ms n 58 and secondary ion mass spectrometry sims n 59 the monazite study grain exhibits compositional variations in proximity to microcracks consistent regions of secondary alteration and recrystallization although the compositions of these regions fit the ideal stoichiometry of monazite they have lower si th u and y and higher p rare earth element ree and ca concentrations compared to visibly unaltered portions of the grain la icp ms and sims analyses demonstrate that common pb as proxied by 204pb is enriched in proximity to microcrack regions and correlates with 137ba sims 232th 208pb analysis from grain regions with lowest contents of common pb 208pb comprises 99 of pb isotopes yields two sets of ages 263 5 3 0 ma 1 mswd 1 7 n 11 and 234 1 3 3 ma 1 mswd 0 4 n 13 regionally the ages are similar to the youngest appalachian pegmatite bodies emplaced during the terminal alleghanian laurentia africa collision however the monazite ages are younger than locally surrounding intrusions the closest intrusive in distance 30 km and age is the petersburg granite 296 33 0 11 ma zircon 238u 206pb the emplacement of which coincided with activity along the hylas fault because the fault experienced a brittle ductile transition in the late permian 262 ma coeval with the older 232th 208pb monazite age group we postulate that the hylas fault may have operated as a fluid migration system between the petersburg granite and morefield mine pegmatite the younger monazite age coincides with triassic normal and or sinistral faulting linked to the development and deformation of local rift basins

hydrothermal alteration of chevkinite ce in the shuiquangou syenitic intrusion northern china abstract a rare earth iron titanosilicate mineral closely associated with amphibole was found in two altered syenite samples from the shuiquangou syenitic complex based on electron microprobe analyses the mineral is chevkinite ce the mineral is altered and mantled by a corona of allanite ilmenite titanite epidote quartz mass balance calculations reveal that the former chevkinite was replaced directly by an allanite and ilmenite association allanite ilmenite zone in the inner part of the corona texture it appears that during the replacement si and ti were immobile whereas al fe mn mg and ca were externally supplied and a proportion of ree and th were transferred out of the former chevkinite the released ree and th become incorporated in allanite and titanite in the allanite ilmenite titanite mantle where allanite contains higher nd sm and y but lower la and ce than that in the allanite ilmenite zone suggesting that redistribution of these elements took place and the mobility of nd sm and y is enhanced during hydrothermal alteration

the mobility of zirconium and other immobile elements during hydrothermal alteration abstract development of zircon and other zr phases in hydrothermal deposits indicates that zr can be highly mobile in these systems mobility is most common in but not restricted to f rich hydrothermal systems related to alkalic f rich igneous suites these suites can range from peralkaline through metaluminous to peraluminous a few examples are neither alkalic nor f rich three locations in the trans pecos magmatic province texas u s a demonstrate this hydrothermal zr mobility all three igneous systems are alkalic and f rich but vary in alkali al ratios peralkaline rhyolites and trachytes in the christmas mountains contain as much as 2100 ppm zr mostly in aegirine or arfvedsonite zircon is rare or absent fluorspar replacement deposits in limestone at contacts with the rhyolites contain as much as 38 000 ppm zr occurring as small disseminated zircons the deposits also are enriched in a variety of incompatible elements including be rare earth elements ree y nb mo hf pb th and u the sierra blanca intrusions a series of mildly peraluminous f rich rhyolite laccoliths contain as much as 1000 ppm zr mostly as zircon hydrothermal zircon occurs as overgrowths on magmatic grains as veinlets connected to overgrowths and in fluorspar replacement bodies in adjacent limestone the highest zr concentrations in fluorspar are 200 ppm metaluminous quartz monzonite from the infiernito caldera contains 400 600 ppm zr mostly as zircon euhedral zircon in quartz fluorite veins in the quartz monzonite indicates mobility of zr zirconium concentrations in the veins are unknown but the paucity of zircon suggests little zr enrichment relative to the host zircon and more rarely zirconolite occur in skarn in the ertsberg district of irian jaya indonesia unlike in texas related igneous rocks are metaluminous and the hydrothermal system was f poor worldwide hydrothermal zircon other zr phases and ti and al bearing phases occur in skarn epithermal precious metal veins volcanogenic massive sulfide deposits and mylonites we propose that differences in zr mineralogy of igneous source rocks is an important factor in determining the availability of zr to hydrothermal fluids although zr concentrations in the sierra blanca and christmas mountains rhyolites are similar zr enrichment in fluorspar was much greater in the christmas mountains we suggest that hydrothermal solutions could easily break down aegirine and arfvedsonite to release zr but that zircon was only moderately attacked therefore far more zr was available for transport and subsequent deposition in the christmas mountains than at sierra blanca availability of other trace elements probably is also governed by their mineral host although zr mobility is most common in f rich hydrothermal systems related to alkalic and f rich igneous systems mobility at ertsberg may have been promoted by sulfate complexing

rare earth and trace element mobility in mid crustal shear zones insights from the mont blanc massif western alps the behaviour of rare earth elements ree during fluid rock interaction in mid crustal shear zones has received little attention despite their potential for mass balance calculation and isotopic tracing during deformation in this study several cases of large ree mobility during alpine fluid driven shear zone development in the pre alpine granitic basement of the mont blanc massif are considered on a regional scale the undeformed granite compositions range within 5 wt sio2 70 5 75 3 wt and magmatic chemical variations are of the order of 10 20 ascribed to minor effects of crystal fractionation major and trace element mobility observed in shear zones largely exceeds these initial variations shear zones developed a range of mineral assemblages as a result of shearing at mid crustal depths at not vert similar0 5 gpa 400 c five main shear zone assemblages involve muscovite chlorite epidote actinolite and calcite respectively as major phases in most cases selective enrichments of light or heavy ree and y ta hf are observed ree mobility is unrelated to deformation style cataclastic mylonitic the intensity of strain and to the shear zone s major metamorphic mineral assemblages instead the changes in ree concentrations are ascribed to the alteration of pre existing magmatic ree bearing minerals during deformation related fluid rock interaction and to the syntectonic precipitation of metamorphic ree bearing minerals mainly monazite bastnasite aeschynite and tombarthite minor proportions 2 of these accessory phases with grain sizes mostly 20 m account for enrichments of up to 5 1 compared to the initial granite whole rock ree budget the stability of the ree phases appears to be largely dependent on the altering fluid composition ree mobility is ascribed to changes in ph and to the availability of co32 po42 and so42 ligands in the fluid such processes are likely to influence the mobility of ree y hf and ta in shear zones

chronology of devonian to early carboniferous rifting and igneous activity in southern magdalen basin based on u pb zircon dating fifteen u pb zircon radiometric age determinations have been made on igneous rocks of middle devonian to early carboniferous age from the southern margin of the magdalen basin in cape breton island and northern mainland nova scotia volcanic rocks interbed with early rift basin sedimentary rocks with some palynological biostratigraphy dated intrusive rocks cut these sedimentary units our biostratigraphically constrained ages are in close agreement with the current devonian time scale combined with previously published data the age determinations show that igneous activity occurred in four pulses middle devonian 390 385 ma early late devonian 375 370 ma latest devonian to early tournaisian 365 354 ma and late tournaisian to early visean ca 339 ma middle devonian 385 389 ma volcanic rocks are confined to the guysborough group the fisset brook formation basalt and minor rhyolite in the type area and elsewhere in cape breton island and northern mainland nova scotia is late devonian c

porosity corrections for smectite rich sediments impact on studies of compaction fluid generation and tectonic history porosity is a fundamental parameter that must be correctly determined in order to relate physical property hydrologic and chemical flux studies to natural systems traditional porosity determinations generated from physical property or seismic data can greatly overestimate the true porosity of sediments in which hydrated minerals such as smectite are abundant to produce a true porosity distribution such data must be corrected to account for the h 2 o residing in smectite interlayers which can make up to 25 of the total hydrated mineral mass such h 2 o is easily removed from the mineral by oven drying and or exposure to low humidities standard physical property measurements can be corrected provided the weight percent smectite in the sediment is known and the interlayer h 2 o content of the mineral can be estimated we illustrate the significant consequences of this correction by comparing profiles of reported and corrected porosities for smectite rich barbados abyssal plain and accretionary wedge sediments from ocean drilling program leg 110 sites 671 and 672

deformation of accretionary wedges in response to seamount subduction insights from sandbox experiments sandbox experiments using a two dimensional and a three dimensional approach are used to study the deformation of margins in response to seamount subduction successive mechanisms of deformation are activated during the subduction of conical seamounts first reactivation of the frontal thrusts and compaction of the accretionary wedge is observed then back thrusting and conjugate strike slip faulting develops above the leading slope of the subducted seamount the basal d6collement is deflected upward in the wake of the subducting high and a large shadow zone develops behind the seamount trailing slope consequently frontal accretion is inhibited and part of the frontal margin is dragged into the subduction zone when the main d6collement returns to its basal level in the wake of the seamount the margin records a rapid subsidence and a new accretionary wedge develops closing the margin reentrant the sediments underthrusted in the wake of the seamount into the shadow zone are underplated beneath the rear part of the accretionary wedge substantial shortening and thickening of the deformable seaward termination of the upper plate basement associated with basal erosion is observed seamount subduction induces significant material transfer within the accretionary wedge favors large tectonic erosion of the frontal margin and thickening of the rear part of the margin the subduction and underplating of relatively undeformed water ladden sediments associated with fluid expulsion along the fractures affecting the margin could modify the fluid pressure along the basal ddcollement consequently significant variations of the effective basal friction and local mechanical coupling between the two plates could be expected around the subducting seamount

the provenance of jurassic and lower cretaceous clastic sediments offshore southwestern nova scotia jurassic and cretaceous sandstones in the shelburne subbasin and fundy basin offshore nova scotia are poorly known but are of current interest for petroleum exploration the goal of this study is to determine the provenance of sandstones and shales which will contribute to a better understanding of regional tectonics and paleogeography in the study area mineral and lithic clast chemistry was determined from samples from conventional cores and cuttings from exploration wells using scanning electron microscopy and an electron microprobe whole rock geochemical composition of shales was used to test the hypotheses regarding provenance of mesozoic clastic sedimentary rocks in the sw scotian basin lower jurassic clastic sedimentary rocks in the fundy basin contain magnetite biotite and chlorite suggesting local supply from the north mountain basalt and meguma terrane whereas pyrope and anthophyllite suggest small supply from distant sources in the sw scotian basin detrital minerals lithic clasts and shale geochemistry from middle jurassic to early cretaceous indicate a predominant meguma terrane source and transport by local rivers rare spinel and garnet grains of meta ultramafic rocks only in the middle jurassic at the mohawk b 93 well suggest minor supply from the rising labrador rift via the same river that transported distant sediments to the fundy basin lower cretaceous sandstones from the mohican i 100 well contain minor garnet spinel and tourmaline from meta ultramafic rocks characteristic of sediment supplied to the central scotian basin at that time the dominant meguma terrane provenance precludes thick deep water sandstones in the eastern part of the shelburne subbasin but the evidence of middle jurassic distant river supply through the fundy basin is encouraging for deep water reservoir quality in the western part

the effect of seamount subduction on seismic coupling it has long been speculated that the subduction of seamounts may have profound mechanical consequences particularly on the generation of large subduction zone earthquakes we consider this problem as a special case of the seismic coupling model of scholz and campos the model predicts that the subduction of a large seamount will increase the normal stress across the subduction interface and hence will enhance seismic coupling in the case of coupled arcs this increased coupling will result in a large increase in the recurrence intervals of earthquakes such events will therefore appear rare in the instrumental record as is observed for decoupled arcs the subduction of a large seamount should cause them to become locally coupled three of the latter localities have been found with satellite altimetry data two on the tonga kermadec trench and one on the izu bonin trench the diagnostic feature is a shallowing or obstruction of the trench axis adjacent to a chain or group of large seamounts on the subducting plate in each case the small to moderate plate interface seismicity typical of decoupled seismic zones is almost completely absent in a 100 200 km arc segment indicating that the subduction zone is locally locked as predicted by the model the occurrence of large interplate earthquakes at both tonga sites confirms that they are seismically coupled

y ree nb ta ti oxide ab2o6 minerals from rel ree euxenite subtype pegmatites of the třebic pluton czech republic substitutions and fractionation trends this paper describes subtitutions and fractionation trends in the y ree nb ta ti oxides aeschynite and euxenite group minerals from nyf euxenite subtype pegmatites of t ebic pluton

magnetic mean field modelling of solid solutions theoretical foundations and application to the hematite ilmenite system k fabian 1 v p shcherbakov 2 3 s a mcenroe 4 p robinson1 and b p burton5 1geological survey of norway n 7491 trondheim norway e mail karl fabian ngu no 2geophysical observatory borok of russian academy of sciences borok yaroslvaskaya oblast 152742 russia 3institute of geology and petroleum technologies kazan volga region federal university russia 4norwegian university of science and technology n 7491 trondheim norway 5nist 100 bureau drive gaithersburg md 20878 usa

geology and mineralogy of the ashram zone carbonatite eldor complex quebec abstract the ashram zone which is host to the ashram rare earth element ree deposit occurs within the eldor carbonatite complex quebec canada the complex is located within the paleoproterozoic new quebec orogen labrador trough and has been subjected to greenschist metamorphism and folding during the hudsonian orogeny at 1 75 ga to date consanguineous undersaturated alkaline rocks have not been recognized within or adjacent to the complex it is evident that the bulk compositions of the rocks essentially magnesiocarbonatites and ferrocarbonatites do not represent those of liquid compositions as many are complex breccias which have been subjected to later hydrothermal activity the ashram zone is dominated by diverse textural varieties of carbonatite which include fluorite rich schlieren carbonatites coarse to medium grained granular carbonatites fine grained commonly mosaic textured quartz bearing carbonatites and colloform carbonatites compositional and textural data are provided for the minerals present in the carbonatites the major rock forming minerals are diverse ca mg fe carbonates fluorite and quartz the carbonates range in their compositional evolution from rare dolomite through ferrodolomite and magnesian siderite to siderite the principal ree bearing minerals of the ashram deposit are monazite ce and monazite nd with lesser amounts of bastnaesite ce and bastnaesite nd the minor and accessory mineral suite is characterized by the presence of apatite phlogopite xenotime diverse sc and sn bearing nb ti minerals niobian rutile nioboaeschynite samarskite barite sphalerite several uncommon but here relatively abundant ba and ba be minerals bafertisite magbasite barylite betrandite sanbornite cebaite yangzhumingite cassiterite galena pyrite and rare magnetite and potassium feldspar pyrochlore is absent and the nb ti oxide assemblage is similar to that found in nyf pegmatites associated with f rich a type granitoids the mineralogy of the ashram deposit compared to that of other carbonatites associated with undersaturated silicate rocks is unique especially with respect to the abundance of fluorite and monazite commonly with nd enrichment ba be enrichment the nyf type nb ti oxide assemblage especially xenotime y nb aeschynite samarskite phlogopite potassium feldspar quartz rich residua with granitoid characteristics paucity of magnetite pyrochlore and sr bearing carbonates the ashram deposit is considered to be a late magmatic to hydrothermal f ree magnesio to ferrocarbonatite derived from as yet unknown consanguineous antecedents

the ree ti mineral chevkinite in comenditic magmas from gran canaria spain a syxrf probe study the ree ti silicate chevkinite has been recognised previously in miocene ignimbrites from gran canaria and in correlative offshore syn ignimbrite turbidites we have estimated the partition coefficients of ree y zr and nb for chevkinite and co existing peralkaline rhyolitic comendite glass using synchrotron xrf probe analyses syxrf in order to evaluate the role of this mineral in the ree budget of felsic peralkaline magmas the zr nb ratio of the chevkinite is 1 55 1 7 strongly contrasting with zr nb of 6 5 in the associated glass zr shows a three fold enrichment in chevkinite relative to the residual melt whereas nb is enriched by a factor 10 the enrichment of ce and la in chevkinite is even more significant namely 19 wt ce and 12 wt la compared to 236 ppm ce and 119 ppm la in the glass chevkinite glass ratios are 988 30 for la 806 30 for ce 626 30 for pr 615 40 for nd 392 50 for sm 225 30 for eu 142 25 for gd 72 20 for dy for trace elements we derived kdte of 74 25 for y 8 for hf 50 for th 15 5 for nb and 3 55 0 4 for zr mineral glass ratios for co existing titanite are 28 10 for la 86 20 for ce 98 30 for pr 134 35 for nd 240 50 for sm 50 20 for eu 96 25 for gd 82 25 for dy 99 30 for y 45 10 for nb and 3 0 5 for zr based on these data the removal of only 0 05 wt of chevkinite from a magma with initially 300 ppm ce would deplete the melt by 93 ppm to yield 207 ppm ce in the residual liquid chevkinite thus appears when present to be the controlling mineral within the lree budget of evolved peralkaline magmas

the role of dextral transpressional faulting in the evolution of an early carboniferous mafic felsic plutonic and volcanic complex cobequid highlands nova scotia canada abstract the wentworth plutonic complex consisting of gabbro and granite was emplaced in the earliest carboniferous in the cobequid shear zone of the northern appalachians the plutonic complex is coeval with a 5 km thick pile of volcanic rocks early alkalic a type granite correlates with thick felsic pyroclastics and minor basalt which are overlain by 1 5 km thick basalts that correlate with a large gabbro pluton that is intruded in turn by late granites the basalt and gabbro are fe rich tholeiites the geochemistry of the late granites suggests that they formed by differentiation of a granodioritic magma resulting from assimilation of early granite by the gabbroic magma the wentworth plutonic complex lies on the north side of the dextral rockland brook fault near the western tip of wedge shaped basement block of the avalon terrane field observations of mesoscopic structures and map contacts show that the plutonic bodies at all structural levels are related to transpressive strike slip faults dykes parallel to the mylonitic foliation in the rockland brook fault zone and at the contacts between igneous phases suggest that the plutons developed largely through dyke to pluton construction the plutonism was initiated by dyking related to major faults under transpression that was partitioned into shear zone bounded blocks while the sinking of those blocks finally provided the space for mafic magma emplacement dyking was active over at least a 10 ma time period the overall location of plutonism in the cobequid shear zone appears related to its position at the intersection of the shear zone bounding the southwestern margin of the magdalen basin and the e w transpressional contact of the avalon and meguma terranes magmatism enabled thermomechanical softening of the crust and the vertical and lateral extrusion of the wedge shaped basement blocks whose movement controlled the localisation of the voluminous magmatic activity

structure of the nankai trough accretionary zone from multichannel seismic reflection data new multichannel seismic reflection data collected over the nankai trough image the accretionary complex in two areas the international program of ocean drilling leg 87 transect area western area and the region of upcoming ocean drilling program leg 131 eastern area the incoming shikoku basin sedimentary section consists of hemipelagic clays and thin terrigenous turbidites the basin section is overlain by a trench wedge that is 12 16 km wide and 350 750 m thick at the thrust front accretionary deformation begins in a protothrust zone that is characterized by thickening and seaward tilting of the trench wedge the zone in the western area is 4 5 km wide and is characterized by kink folds the zone in the eastern area is only 2 5 km wide and does not exhibit such folds the frontal thrusts in each area are imaged as fault plane reflections and ramp upward from within the basin hemipelagic section the overthrusting sediments form fault bend folds over these ramps thrust spacing at the toe of the slope is 1 5 2 5 km the second thrust cuts up from an inferred decollement within the shikoku basin sedimentary section in the eastern area a reflection marking the top of the basin pelagic sediment section changes from normal to reversed polarity about 6 3 km seaward of the thrust front and underlies the entire protothrust zone this reflector continues with reversed polarity under the accretionary complex and is at the level of the basal decollement the underlying basin pelagic section is apparently thrust undisturbed beneath the accretionary prism the reversal of polarity indicates a change in reflection coefficient that is due to a combination of decreasing seismic velocity and density across the interface this decrease in velocity and density may indicate that the decollement is a zone of high porosity due to fluid expulsion from deeper within the accretionary prism the reflections from the first and second thrusts are also reversed polarity possibly indicating that they also are pathways of fluid expulsion the critical wedge taper of the western area is greater than that of the eastern area an observation that is consistent with the existence of an overpressured decollement in the eastern area

monitoring horizontal well hydraulic stimulations and geomechanical deformation processes in the unconventional shales of the midland basin using fiber based time lapse vsps microseismic and strain data

distributed acoustic sensing for reservoir monitoring with vertical seismic profiling distributed acoustic sensing is a novel technology for seismic data acquisition particularly suitable for vertical seismic profiling it is a break through for low cost on demand seismic monitoring of reservoirs both onshore and offshore in this article we explain how distributed acoustic sensing works and demonstrate its usability for typical vertical seismic profiling applications such as checkshots imaging and time lapse monitoring we show numerous data examples and discuss distributed acoustic sensing as an enabler of seismic monitoring with 3d vertical seismic profiling key words borehole geophysics acquisition seismics time lapse monitoring

excess pore pressures within subducting sediments does the proportion of accreted versus subducted sediments matter 1 although previous numerical modeling studies have examined pore pressures in both accretionary and non accretionary subduction zones they have not addressed the question of whether the proportion of accreted versus subducted sediments significantly affects pore pressures this modeling study used consistent parameters to compare excess pore pressures directly below the decollement for varying proportions of accreted sediment within the initial 5 to 12 kilometers arcward of the deformation front subduction zones with lesser accretion thickness show lower simulated excess pore pressures than those with greater accretion thickness due to the ease of fluid escape when overlying wedge sediments are thin however at greater distances arcward of the deformation front the magnitude of excess pore pressures significantly increases and the excess pore pressures peak further arcward as the proportion of underthrust sediments increases this difference suggests that the proportion of accreted versus subducted sediment could potentially affect seismogenesis

the relationship between ree y nb th minerals and the evolution of an a type granite wentworth pluton nova scotia the wentworth pluton in the eastern cobequid highlands consists principally of metaluminous to peralkaline a type granite 362 ma a large part of which was remelted by a major gabbro intrusion 357 ma magmatic minerals like allanite ce chevkinite ce zircon and hingganite y and post magmatic mineral phases such as ree bearing epidote samarskite y aeschynite y fersmite thorite like phases and hydroxylbastnasite ce were identified the presence of fluorine in the parental magma indicated by whole rock geochemical data and the presence of fluorite increased the solubility of monazite and xenotime and thus facilitated retainment of rare metals in the magmatic system fractionation of allanite ce and chevkinite ce led to a melt enriched in heavy rare earth elements hree from which hingganite y crystallized during late magmatic stages the remelting of the early granite led to fluorine and sulfur release in volatile phases which circulated with hydrothermal fluids thus mobilizing the ree and other rare metals reduction of fluorine activity during the late to post solidus crystallization resulted in the precipitation of hree and rare metals in samarskite y thereby enriching the residual hydrothermal fluids in light rare earth elements lree post magmatic lree minerals such as hydroxylbastnasite ce either replaced earlier minerals or precipitated from these hydrothermal fluids carbonate fluids involved in a late regional hydrothermal circulation event along the cobequid chedabucto fault 320 315 ma promoted ti mobility and the formation of tio 2 minerals and probably of aeschynite y this mineralogical diversity in addition to the complex geological history of the pluton provides a unique opportunity to correlate the formation of individual rare metal minerals to different stages of pluton evolution and thus provide an insight to the formation conditions of these minerals

transition of accretionary wedge structures around the up dip limit of the seismogenic subduction zone the nankai accretionary prism is divided into three segments outer and inner wedges and their transition zone these wedges reflect different aspects of wedge taper internal deformation and basal plate boundary fault the outer wedge is characterized by narrow critical taper internal deformation by in sequence fold and thrust and aseismic decollement the inner wedge represents a stable narrow taper weakly deformed internal structure and seismogenic plate boundary fault along its base the transition zone between the two wedges shows large critical taper with steep surface slope internal structure of out of sequence thrust and step down of decollement onto the sediment oceanic basement interface the trench slope break and oceanward margin of forearc basin is located around the landward edge of this transition zone these common aspects might be related to the lithification of both accreted and underthrust sediments and the resultant switch of the plate boundary fault deformation and lithification process recorded in exhumed on land melange of accretionary complexes suggest that the step down of the plate boundary decollement occurs around the up dip limit of seismogenic subduction zone

zircons from syros cyclades greece recrystallization and mobilization of zircon during high pressure metamorphism zircons were studied from high pressure low temperature metamorphosed meta igneous lithologies from syros these rocks carry several zircon generations related to each other by dissolution reprecipitation processes one generation is pristine zircon that shows growth zoning relatively elevated contents of trivalent cations and high th u ratios the other end member is a skeletal zircon generation with negligible trivalent cation contents and low th u ratios 0 1 texturally between these two there is a range of zircon crystals with complex inclusion populations of y hree th phases and fluid inclusions showing variable progress of replacement recrystallization both end members yield distinct sensitive high resolution ion microprobe shrimp u pb ages the pristine generation has an age of 80 2 1 6ma from a metagabbro and 76 4 2 1ma from a meta plagiogranite dyke the skeletal low th u zircon generation yields an age of 52 4 0 8ma the older late cretaceous zircons are interpreted to date emplacement of the magmatic protoliths in a small segment of oceanic crust the younger eocene age however dates a zircon recrystallization event which possibly coincides with high solubility and mobility of high field strength elements in a high pressure aqueous fluid phase intergrowth relations between zircon and peak metamorphic garnet and excellent agreement of the u pb ages with white mica ar ar ages for the same samples support the conclusion that eocene is the true age of high pressure metamorphism on syros

along strike variations in underthrust sediment dewatering on the nicoya margin costa rica related to the updip limit of seismicity 1 along the costa rican subduction zone offshore the nicoya peninsula an offset in the updip limit of seismicity coincides with a transition between subduction of warm crust generated at the cocos nazca spreading center and cool crust formed at the east pacific rise we evaluate whether the observed difference in thermal state of incoming crust would result in significant differences in sediment dehydration reaction progress along strike thought to be a control on the updip limit of seismicity we combine thermal models with models of dehydration reaction kinetics for opal and smectite to estimate the distribution of diagenetic fluid sources the modeled distribution of diagenetic fluid sources mimics the pattern of the updip limit of seismicity seismicity begins 15 20 km landward of most of the smectite to illite transition this suggests that the location of the updip limit of seismicity may be influenced by the dissipation of fluid overpressures landward of smectite to illite dehydration

dissolution reprecipitation of zircon at low temperature high pressure conditions lanzo massif italy abst r a c t an eclogite facies meta plagiogranite from the lanzo massif western alps italy contains crystals of zircon intimately associated with allanite zircon displays different microtextures ranging from pristine euhedral and magmatic to fractured porous varieties with mosaic zoning and pervasive recrystallization into euhedral microcrystals fractures and voids in the recrystallized zircon microcrystals are mainly filled by high pressure na rich pyroxene electron backscattered diffraction analysis revealed a similar crystallographic orientation for primary magmatic zircon crystals and microcrystals with less than 2 misorientation among neighboring microdomains the textural change is coupled with chemical and isotopic modifications recrystallized zircon domains contain significantly less th and light to mid ree but are richer in sr than magmatic zircon crystals magmatic zircon preserves the protolith u pb age of 163 5 1 7 ma whereas zircon microcrystals have a mean age of 55 1 ma the coexisting allanite also contains inclusions of na rich pyroxene and has chemical features elevated sr and ni contents and lack of eu anomaly indicating formation at high pressure despite being associated texturally with zircon allanite yields a younger th pb age of 46 5 3 0 ma suggesting that the lanzo unit remained at relatively high pressure conditions for 8 m y zircon recrystallization proceeded with volume reduction and loss of material to an alkaline metamorphic fluid that acted as the agent for a coupled dissolution reprecipitation process recrystallization occurred with minimum transport in a low strain environment and was not significantly enhanced by metamictization the source of the fluid for zircon recrystallization is most probably related to prograde devolatilization reactions in the surrounding serpentinite

fractured latest devonian granites of the west moose river pluton along the cobequid shear zone nova scotia implications for regional mineralization latest devonian 365 358 ma a type granites in the cobequid highlands host complex sequences of rare earth element ree and other hydrothermal minerals the west moose river pluton is the only pluton truncated and brittly deformed by the mid carboniferous 327 ma strike slip minas fault zone during the alleghanian orogeny fractures in the granite provide a record of several deformational and hydrothermal events with distinct mineral assemblages early sodic alteration produced albitisation of feldspar and riebeckite and tourmaline veins the 18 o of albite and albitised granite 5 6 is similar to other regional granites suggesting a mantle source of albitising fluids nearby halite deposits are younger and thus not a source of na early chlorite veins were followed by potassic alteration and hydrothermal biotite and by diabase and lamprophyre dyke emplacement euhedral magnetite occupies new cross cutting fractures and vugs correlated with regional iron oxide carbonate sulphide

crustal velocity field of southwest japan subduction and arc arc collision we investigate crustal deformation in southwest japan over a 3 year period revealed by a permanent dense global positioning system gps array southwest japan is a part of the amurian plate a microplate moving about 10 mm yr toward the east with respect to the eurasian plate it overrides the philippine sea plate at the nankai trough and collides with the northeast japan arc in the central part of japan in this paper we first derive gps site velocities relative to the stable part of the amurian plate in order to isolate signals of crustal deformation caused by the subduction and or the collision the velocity field has a conspicuous feature indicating the interseismic elastic loading by the philippine sea plate slab at the nankai trough characterized by the northwestward movements of points throughout the studied area their amplitudes are the largest at the pacific coast and decay toward the japan sea coast with a subtle systematic shift of azimuths a model assuming an elastic half space the convergence rate at the nankai trough based on a refined euler vector and the strength of the coupling inferred from a thermal model could explain the velocity field in the western part of the studied area to a large extent those in the eastern part systematically deviate from them and the residual components there show east west shortening and north south extension this may represent crustal thickening and trenchward extrusion of crustal blocks caused by the collision between southwest and northeast japan this suggests that the collision between southwest and northeast japan gives rise to not only crustal thickening but also trenchward extrusion of crustal block a velocity contrast was found across the median tectonic line the largest inland active fault in japan but the current permanent gps network is not dense enough for us to discuss its coupling depth

compressional and shear wave studies of distributed acoustic sensing acquired vertical seismic profile data abstract understanding the strengths and limitations of rapidly advancing distributed acoustic sensing das technology used for recording vertical seismic profile vsp data is achieved by comparing das and geophone data sets using both compressional wave p wave and shear wave s wave vsp data and their corresponding geophysical answer products we validate the kinematics time and dynamics amplitude of das vsp data by examining the extracted slowness values response to incident angles corridor stacks and common depth point cdp transforms for kinematics validation the slowness values computed from p and s wave components of das vsp data agree with the geophone slowness values for dynamics validation we confirm the cos2 response of the fiber to the incident angle of the seismic wavefield for p waves and sin 2 for s waves the amplitudes of the p wave corridor stacks are comparable the s wave corridor stacks are similar for shallow events and differ for later events due to the limited r

a simplified analysis of parameters controlling dewatering in accretionary prisms abstract one of the many dynamic geologic processes taking place at subduction zones is the compaction driven dewatering of seafloor sediments as they are accreted to the overriding plate the rate of dewatering is equal to the divergence of the sediment matrix velocity field this rate can be estimated analytically if simplifying assumptions are made about the geometry of the prism and motion of the sediments the analytical expression depends only on the sediment accretion velocity thickness of the accreted section prism taper angle and sediment porosity distribution a sensitivity analysis of the solution shows that the fluid production distribution is relatively insensitive to the sediment porosity distribution however the solution is very sensitive to the taper angle of the wedge high angle wedges expel almost all of the incoming water within 20 km while low angle wedges may retain a significant fraction of the incoming water for 50 or more kilometers thickness of the incoming section is also important dewatering of thin accreted sections is more concentrated near the toe of the wedge analyses of transects through the northern barbados makran vancouver and nankai accretionary prisms illustrate a range of dewatering rates and spatial distributions of dewatering

a field experiment of walkaway distributed acoustic sensing vertical seismic profile in a deep and deviated onshore well in japan using a fibre optic cable deployed inside coiled tubing abstract a two dimensional walkaway vertical seismic profiling survey using distributed acoustic sensing was conducted at an onshore site in japan the maximum depth and the deviation of the observation well were more than 4 000 m and 81 degrees respectively among the several methods for installing fibre optic cables we adopted the inside coiled tubing method in which coiled tubing containing a fibre optic cable is deployed the signal to noise ratio of the raw shot gather was low possibly due to poor coupling between the fibre optic cable and the subsurface formation resulting from the fibre optic cable deployment method and the existence of considerable tubewave noise nevertheless direct p wave arrivals p p reflections and p s converted waves exhibited acceptable signal to noise ratios after careful optimization of gauge length for distributed acoustic sensing optical processing and the application of carefully parameterized tubewave noise suppression one of the challenges in current distributed acoustic sensing vertical seismic profile data processing is the separation of p and s waves using only one component measurements hence we applied moveout correction using two dimensional ray tracing this process effectively highlights only reflected p waves which are used in subsequent subsurface imaging comparison with synthetic well seismograms and two dimensional surface seismic data confirms that the final imaging result has a sufficiently high quality for subsurface monitoring we acquired distributed acoustic sensing vertical seismic profile data under both flowing conditions and closed conditions in which the well was shut off and no fluid flow was allowed the two imaging results are comparable and suggest the possibility of subsurface imaging and time lapse monitoring using data acquired under flowing conditions the results of this study suggest that by adopting the inside coiled tubing method without drilling a new observation well more affordable distributed acoustic sensing vertical seismic profile monitoring can be achieved in fields such as co2 capture and storage and unconventional shale projects where monitoring costs have to be minimized

geochemical evolution of earliest carboniferous continental tholeiitic basalts along a crustal scale shear zone southwestern maritimes basin eastern canada abstract continental tholeiites were extruded locally within the extensional wrench maritimes basin from about 375 to 330 ma late devonian early carboniferous basalts of the early carboniferous diamond brook formation are up to 1 5 km thick and overlie a succession of predominantly felsic volcanic rocks reaching several kilometres in thickness the volcanic rocks and co genetic plutons occur along a crustal scale strike slip fault that facilitated the transport of relatively primitive magma resulting in basalts with high mg number ni and cr end 4 and relatively low tio2 v y and zr fractional crystallization of plagioclase and lesser clinopyroxene in the absence of iron oxide fractionation resulted in repetitive extrusion cycles hundreds of metres in thickness that show an upward increase in plagioclase phenocrysts or megacrysts and in tio2 and other high field strength elements hfses rare andesites are interpreted as products of partial melting of underplated gabbro the relatively primitive basalts resemble low ti basalts found in other continental flood basalt provinces and have trace element and isotopic characteristics that have been ascribed in the literature either to partial melting of or contamination of asthenospheric melts by subcontinental lithospheric mantle in the maritimes basin similar continental tholeiites were extruded intermittently over a 45 ma period high feot 12 normalised to 8 mgo suggests high potential temperature in the source mantle presumably within a mantle plume the youngest eruption products geochemically and isotopically resemble himu oib plume activity may have gradually migrated eastward from late devonian to permian from the maritimes basin through scotland to norway and north germany

interactions between deformation and fluids in the frontal thrust region of the nantroseize transect offshore the kii peninsula japan results from iodp expedition 316 sites c0006 and c0007 integrated ocean drilling program iodp expedition 316 sites c0006 and c0007 examined the deformation front of the nankai accretionary prism offshore the kii peninsula japan in the drilling area the frontal thrust shows unusual behavior as compared to other regions of the nankai trough drilling results integrated with observations from seismic reflection profiles suggest that the frontal thrust has been active since 0 78 0 436 ma and accommodated 13 to 34 of the estimated plate convergence during that time the remainder has likely been distributed among out of sequence thrusts further landward and or accommodated through diffuse shortening unlike results of previous drilling on the nankai margin porosity data provide no indication of undercompaction beneath thrust faults furthermore pore water geochemistry data lack clear indicators of fluid flow from depth these differences may be related to coarser material with higher permeability or more complex patterns of faulting that could potentially provide more avenues for fluid escape in turn fluid pressures may affect deformation well drained sand rich material under the frontal thrust could have increased fault strength and helped to maintain a large taper angle near the toe recent resumption of normal frontal imbrication is inferred from seismic reflection data associated decollement propagation into weaker sediments at depth may help explain evidence for recent slope failures within the frontal thrust region this evidence consists of seafloor bathymetry normal faults documented in cores and low porosities in near surface sediments that suggest removal of overlying material overall results provide insight into the complex interactions between incoming materials deformation and fluids in the frontal thrust region

modeling the seismic response of individual hydraulic fracturing stages observed in a time lapse distributed acoustic sensing vertical seismic profiling surveyseismic response of fracturing stages

fluid rock interaction in autoliths of agpaitic nepheline syenites in the ilimaussaq intrusion south greenland abstract within the 1 16 ga old ilimaussaq intrusion up to 700 m large autoliths occur in one stratigraphic unit of the layered floor series of agpaitic nepheline syenites kakortokites these autoliths consist of two different rock types augite syenite and naujaite agpaitic nepheline syenite all three rock types show a number of alteration features related to the entrapment of the autoliths in the kakortokite magma caused by the interaction with a fluid phase in the kakortokites the oxidation of primary arfvedsonite to aegirine and fluorite is restricted to the close proximity to the autoliths close to the surrounding kakortokite the primary mafic phases of the augite syenites augite fayalite fe ti oxides are completely replaced by arfvedsonite aenigmatite biotite aegirine and fluorite the decomposition of primary hastingsite to spectacular aegirine augite nepheline aenigmatite symplectites can be observed up to several meters inside the autoliths additionally fluorite formed at grain boundaries of primary nepheline in the naujaite autoliths primary arfvedsonite is replaced by aegirine biotite intergrowths and abundant aenigmatite is occasionally replaced by ti rich aegirine and fe ti oxides the mineral reactions in the autoliths are used to decipher details of the late to post magmatic processes in a peralkaline syenitic intrusion mineral equilibria record an evolution governed by falling temperature 620 to ca 500 c and increasing relative oxygen fugacity from fmq 1 to above fmq 4 quantification of the observed mineral reactions reveals the infiltration of the autoliths with an oxidizing fluid phase rich in na and f and minor addition of k volatiles h and f and in some cases also fe ti and ca mg released from primary autolith phases were mainly just relocated within the autoliths

seamount subduction erosion in the nankai trough and its potential impact on the seismogenic zone seamount subduction along subduction zone plate boundary thrusts has long been implicated as a mechanism for abrasion and tectonic erosion of the base of the overriding plate however tectonic erosion processes have not been examined in detail with high quality three dimensional 3 d seismic reflection imaging in 1999 we acquired 3 d seismic reflection data from the nankai trough subduction zone to image the plate boundary fault and the overlying accretionary wedge structure fortuitously these data reveal a small to 1 km high basement ridge that has subducted to 7 km subseafloor updip from the basement ridge a 1 km thick sequence of sediment from the base of the accretionary wedge appears to be missing we interpret these data as evidence for tectonic erosion of the base of the accretionary wedge following the basement ridge subduction tectonic erosion has removed more than 25 km 3 from the updip edge of the seismogenic zone and carried it down into the seismogenic zone the tectonically eroded sediments are presumed to enhance fault zone fluid content potentially reducing fault zone effective stress and may temporarily inhibit earthquake rupture potential after the passage of the ridge the boundary fault returns to its former position in a period of enhanced underplating

nature and origin of lamellar magnetism in the hematite ilmenite series grains consisting of finely exsolved members of the hematite ilmenite solid solution series such as are present in some slowly cooled middle proterozoic igneous and metamorphic rocks impart unusually strong and stable remanent magnetization tem analysis shows multiple generations of ilmenite and hematite exsolution lamellae with lamellar widths ranging from millimeters to nanometers rock magnetic experiments suggest remanence is thermally locked to the antiferromagnetism of the hematite component of the intergrowths yet is stronger than can be explained by canted antiferromagnetic caf hematite or coexisting paramagnetic pm fe ti ordered r 3 ilmenite alone in alternating field demagnetization to 100 mt many samples lose little remanence indicating that the nrm is stable over billions of years this feature has implications for understanding magnetism of deep rocks on earth or on planets like mars that no longer have a magnetic field atomic scale simulations of an r 3 ilmenite lamella in a caf hematite host based on empirical cation cation and spin spin pair interaction parameters show that contacts of the lamella are occupied by contact layers with a hybrid composition of fe ions intermediate between fe2 rich layers in ilmenite and fe3 rich layers in hematite structural configurations dictate that each lamella has two contact layers magnetically in phase with each other and out of phase with the magnetic moment of an odd non self canceling fe3 rich layer in the hematite host the two contact layers and the odd hematite layer form a magnetic substructure with opposite but unequal magnetic moments a lamellar ferrimagnetism made possible by the exsolution because it is confined to magnetic interaction involving the moments of just three ionic layers associated with each individual exsolution lamella lamellar magnetism is unique and quite distinct from conventional ferrimagnetism simulation cells indicate that the magnetic moments of contact layers are locked to the magnetic moments of adjacent af hematite layers and are parallel to the basal plane 001 thus lamellar magnetism is created at the temperature of chemical exsolution and is a chemical remanent rather than thermal remanent magnetization however in thermal demagnetization experiments too short for lamellar resorption demagnetization temperatures are those of the caf hematite considerably higher than temperatures of original lamellae formation internal crystal structure cannot dictate that the contact layers of different lamellae will form magnetically in phase with each other to give the highest net magnetic moment but magnetic moments of lamellae can be made to form in phase by the external force of the magnetizing field at the time of exsolution a thesis of this paper is that an external magnetic field can dictate the magnetic moments and hence the chemical location of ilmenite lamellae in a hematite host and that once in place neither the location nor the magnetic moment will be easily disturbed in an ilmenite host the external magnetic field cannot control the chemical location of a hematite lamella which is dictated by the enclosing ilmenite but once lamellae have formed the field can dictate their magnetic moments these moments however are not locked chemically to the host resulting in lower coercivity the effectiveness of the external force in single crystals is dictated by their orientation with respect to the magnetizing field in grains with 001 oriented parallel to the field it would be effective in producing in phase magnetic moments and very strong remanence in grains with 001 normal to the field the field would be less effective in producing in phase magnetic moments hence producing weak remanence the most intense lamellar magnetism per formula unit occurs with in phase magnetization high lamellar yields and the largest number of lamellae per unit volume i e smallest lamellar size compared to the magnetic moment per formula unit m pfu and magnetic moment per unit volume m v of end member magnetite m pfu 4 b m v 480 ka m and hematite m pfu 0 0115 b m v 2 1 ka m results for some atomic models reasonably tied to natural conditions are m pfu 0 46 1 36 b and m v 84 250 ka m

mass and fluid flux during accretion at the alaskan margin prestack depth migration of a seismic reflection transect across the alaskan convergent margin clarified structure and provided an undistorted tectonic cross section between the trench floor and a backstop of eroded eocene miocene rock the accretionary prism displays three tectonic styles seaward of the deformation front trench sediment is cut by incipient faults confined to the subsurface and distributed deformation in the 10 km landward of the deformation front thrust faults develop into imbricate structures above a lower detachment in the remaining 20 km longer and more steeply dipping thrust units form a classical imbricate prism above an upper detachment a velocity model at the scale of the thrust slices was derived from depth processing and seismic refraction data the resulting true scale tectonic cross section was balanced to estimate horizontal contraction a porosity field was derived through the velocity porosity relation in five nearby drill holes it indicates porosity reduction patterns characteristic of each tectonic style seaward of the deformation front the strain rate is high and pore fluid pressure elevates but porosity reduction is minimal in the 10 km landward of the deformation front tectonism and dewatering are most rapid sea floor venting was observed only here in the classic accretionary zone strain rates decrease and dewatering is moderate here an upper detachment propagates across the lower parts of imbricate slices and doubles the underthrust mass one third of the trench sediment section is underthrust at the deformation front whereas two thirds of the sediment section is subducted beneath the backstop

allanite and chevkinite in a type granites and syenites of the graciosa province southern brazil abstract allanite and chevkinite perrierite are important accessory phases in the granitic and syenitic a type rocks of the graciosa province southern brazil allanite ce is the main ree phase in the granites of the aluminous association which includes metaluminous to weakly peraluminous granites while chevkinite ce is the typical ree phase in the metaluminous syenites and alkali feldspar syenites and the peralkaline alkali feldspar granites of the alkaline association allanite shows complex zoning patterns but is always rich in the ferriallanite molecule and in tio 2 a compositional trait that distinguishes allanite in a type aluminous associations from allanite in calcic alkali calcic and calc alkaline granitoids normalized ree patterns show strong fractionation of heavy over light ree with la n nd n ratios between 3 and 5 and la n y n ratios between 200 and 400 primary allanite is variably affected by post magmatic transformations due to the interaction with a fluid phase and metamictization caused by radioactive decay chevkinite ce is always primary and is only present in the alkaline association the early crystallization of chevkinite indicates that the parental magmas were probably close to ree saturation the ree y abundance in chevkinite 43 49 wt is almost twice that observed in primary allanite yet the normalized patterns are similar in terms of fractionation between light and heavy ree as indicated by similar la n nd n ratios in chevkinite and allanite la n y n ratios are on average lower in chevkinite 90 300 chevkinite compositions were altered by interaction with post magmatic fluids which led to marked increase in tio 2 and a reduction in ree and fe t post magmatic allanite is present in both the alkaline and aluminous associations and corresponds to ferriallanite ce in the alkaline association and allanite y in the aluminous association compared to primary allanite post magmatic allanite ce shows smaller ree fractionation 2 n nd n 5 times chondrite the mineralogy of the rocks indicates crystallization conditions close to the titanite magnetite quartz amphibole ilmenite tmqai buffer for the aluminous association and close to the fayalite quartz magnetite fqm buffer for the alkaline association and temperatures between 800 and 900 c liquidus and ca 750 c solidus the experimentally determined stability fields for chevkinite and allanite are in agreement with the textural evidence for early chevkinite crystallization and late allanite crystallization even though allanite and chevkinite only rarely coexist in the alkaline association wherever allanite and chevkinite coexist they reveal reaction textures such that it is possible that only allanite or chevkinite is stable at any given condition the distribution of allanite and chevkinite in the alkaline association suggests that the composition of the magmas is an important control on the stability fields of these minerals a fact that is supported by the presence of primary allanite in rocks formed by processes of mixing and mingling of magmas in this sense the importance of composition on the stability of allanite and chevkinite may be larger than previously suggested

fluorine and chlorine behaviour during progressive dehydration melting consequences for granite geochemistry and metallogeny dehydration melting of biotite is the main control on crustal differentiation in the mid to lower continental crust because this reaction produces the most voluminous and most mobile granitic melts biotite breaks down over a broad temperature interval so the partitioning behaviour of elements between biotite and melt is likely to vary it has been hypothesized that fluorine may stabilize biotite to higher melting temperatures because biotite is typically f rich in ultra high temperature uht metamorphic rocks if true f would be an important influence on crustal differentiation because not only would it broaden the temperature range of melting but also elevated f concentration decreases melt viscosity furthermore ligand partitioning between biotite and melt may be an important influence on the metallogeny of magmas this study used electron microprobe analysis of biotite in rocks from the ballachulish and rogaland metamorphic aureoles to investigate the concentration of f and cl in biotite heated to 600 1 000 c results show a broad increase in biotite f content up to 5 04 f with temperature until 850 920 c beyond which f content decreases 2 5 f chlorine concentrations in biotite are consistently lower 1 cl and show a progressive decrease after the onset of partial melting it was found that mg content and the processes that control mg distribution are most strongly correlated with f and cl concentration in biotite calculations based on these results indicate that f enriched biotite could be a significant source of f for continental crust derived melts generation of a hot f rich melt at uht conditions could be important for transporting lower crustal metals to the upper crust