structure of the vakhsh fold and thrust belt tadjik ssr geologic mapping on a landsat image base an enhanced landsat image of the northeastern tadjik depression in the soviet union was combined with field data to produce a 1 250 000 geologic map of the vakhsh fold and thrust belt and adjacent autochthon mapping of the relatively continuous geologic structure in this region first required recognizing marker beds and contacts at or near the resolution of the landsat sensor and then defining mappable units satellite image mapping was complemented by field study which was necessary in this recently deformed area both to verify the image mapping and to obtain more accurate structural dips deformation of the cretaceous and tertiary strata of the fold and thrust belt is in the western half of the mapped area conspicuously absent north of a buried basement fault that marks the hinge zone of the late mesozoic passive margin in the eastern half of the mapped area however the major thrusts displacing rocks northward have moved coherent sheets over the block fault these sheets now lie flat atop the autochthon to the north tectonically these observations indicate that 1 the crustal structure inherited from the mesozoic extensional phase has strongly influenced the late cenozoic pattern of deformation 2 the thrust sheets that are displaced over the paleo hinge zone actively load the basement rocks to the north and 3 the development of the fold and thrust belt which is markedly asymmetric has included the progressive overlapping of thrusts the later thrusts apparently formed internal to the older thrusts and subsequently overrode them

a new cretaceous tertiary boundary site at flaxbourne river new zealand biostratigraphy and geochemistry abstract an exceptionally complete rock sequence across the cretaceous tertiary k t boundary has been discovered near the flaxboume river marlborough province south island new zealand the boundary is marked by a large ir anomaly 21 ng g on a decalcified basis with an integrated abundance of 134 ng cm2 after correction for background above the boundary there is a 30 cm transition zone in which a few cretaceous foraminiferal taxa such as hedbergella monmouthensis and guembelitria cretacea survived though with reduced abundance and size only ca 1 2 to 1 4 of normal apparently reflecting environmental stresses inaa and icp analyses show that in addition to ir the boundary clay is also enriched in cr and ni mainly from meteoritic material and as co cu sb and zn from terrestrial sources volcanic sources even when scaled to the 10 7 km 3 volume of the deccan basalts fail by three orders of magnitude to account for the ir and as at the k t boundary and by even larger factors for sb zn cu etc comparison of our data with those from six other k t boundary sites shows that the zn sb as sb and zn as ratios generally fall between crustal and oceanic values suggesting contributions from both sources mass balance calculations show that as and sb could be derived from only 300 500 m of ocean water or also from modest amounts 20 36 g cm 2 of average crustal rock copper and zn on the other hand can only be derived from crustal or mantle rock 5 15 g cm 2 presumably impact ejecta such an amount of ejecta is fairly close to the global fallout of boundary clay 2 5 g cm 2

late cenozoic geodynamic evolution of eastern indonesia this paper presents an internally and globally consistent model of plate evolution in eastern indonesia from middle miocene to present time it is centered on the banda sea region located in the triple junction area between the pacific philippine australia and south east asia plates the geological and geophysical data available from indonesia were until recently insufficient to define a unique plate tectonic model in this paper the new data taken into account clearly restrict the possible interpretations owing to a great number of geological geophysical and geochemical studies the major plate boundaries the sunda banda subduction zone to the south the tarera aiduna fault zone and the seram thrust to the east and the sorong fault zone and molucca sea collision zone to the north are now clearly identified the age of the major tectonic structures is also better known geodetic measurements well constrain the present time plate kinematics we also consider the deformation history within eastern indonesia where numerous short lived microplates and their related microcontinents successively accreted to the asiatic margin moreover magnetic anomalies identification of the north and south banda sea basins allows a precise kinematic reconstruction of the back arc opening we used the plates software to test the coherency of our model presented as a series of 4 plate reconstruction maps from 13 ma to the present finally the origin of oceanic domains restored by our reconstruction is discussed

backthrusting in accretionary wedges backthrusting of the rear of the accretionary wedge arcward over the rocks of the forearc basin is a common aspect of accretionary tectonics an older assumption that backstops of accretionary wedges dip back toward the arc and that accreted material is emplaced below such backstops is contradicted by observations from many active forearcs that indicate 1 backthrusting is common 2 forearc basins are nearly ubiquitous associates of accretionary wedges and 3 forearc basement where imaged appears to diverge from the sedimentary package dipping under the wedge while the overlying sediments are often lifted up against it backthrusting may be favored where relief is high between the crest of the wedge and the surface of the forearc basin because the relief must be supported by shear stress along the backthrust we find ratios of lt lb averaging 2 64 0 8 where lt is the horizontal distance from the crest of the wedge to the front of the toe and lb is the distance from the crest of the wedge to the surface projection of the backthrust this ratio is predicted to increase with total convergence and with sedimentation because the toe undergoes higher deformation rates than the backthrust and sedimentation in the trench favors outbuilding of the toe whereas sedimentation in the forearc basin tends to cancel the gravitational component of the driving stress for the backthrust

a geochemical clock in earliest paleogene pelagic carbonates based on the impact induced os isotope excursion at the cretaceous paleogene boundary 1 an impact induced osmium os isotope excursion provides a unique means of assessing the completeness of marine cretaceous paleogene k pg boundary sections and surmounting challenges associated with constraining the time scale of the earth system recovery from this extreme perturbation a model of the recovery of seawater187os 188os following the impact event allows independent estimates of the time elapsed since the impact which can be directly compared to time estimates derived from biostratigraphy magnetostratigraphy and cyclostratigraphy this approach is tested using data from three deep ocean sites cored by the ocean drilling program odp data from odp 1262b south atlantic and odp 690c southern ocean display the expected 187os 188os minimum very close to the biostratigraphically defined k pg boundary and yield os based accumulation rate estimates similar to those obtained from magnetostratigraphy and orbital tuning in contrast the187os 188os minimum in odp 1209c western pacific occurs 9 cm below the k pg boundary low os concentrations throughout the boundary interval and an implausibly rapid recovery to higher pre impact187os 188os ratios provide strong evidence for a previously unrecognized gap in the k pg interval of site 1209 results presented here provide strong empirical evidence that os isotope data are uniquely valuable in assessing the completeness and accumulation rates of earliest paleogene sediments from the deep sea they are of broad interest because they have implications for astronomical tuning of the geologic time scale and illustrate that whole ocean geochemical perturbations can provide an alternative to biostratigraphy for correlation and timekeeping during abrupt biotic events

the presumed tepla barrandian moldanubian terrane boundary in the orlica mountains sudetes bohemian massif structural and petrological characteristics the sudetes in southwestern poland and northern bohemia expose a palaeozoic collage of the northeastermost extremity of the variscan belt one of presumed terrane boundaries occurs in the orlica mts along the contact between the phyllite amphibolite complex neoproterozoic of the nove m sto unit of probable tepla barrandian affinities in the sw and the orthogneiss early ordovician protolith and mica schist neoproterozoic complexes of the orlica nieznik unit in the ne most likely representing the moldanubian terrane the synmetamorphic structural evolution of both adjacent units comprised four deformation events the nove m sto rocks have recorded an early stage of ductile top to the se thrusting d1 which must have resulted in an early juxtaposition of the both terranes within a nappe pile the original overthrust contact was subsequently modified by a dextral shear dominated event d2 which produced a 1 2 km wide shear zone at the boundary of both units in which earlier fabric elements were overprinted and mostly obliterated this deformation brought the lower grade nove m sto rocks into present day strike slip contact with those of the regionally uplifted higher grade orlica nieznik unit postdating this juxtaposition are e w trending folds f3 which affect both the adjacent units the contact shear zone also contains record of a late top to the sw semi brittle normal slip displacement d4 downthrowing the nove m sto unit the peak amphibolite facies metamorphic conditions were attained in the orlica nieznik unit and in the contact shear zone prior to the d2 event they corresponded to maximum temperatures and pressures of ca 600 c and 10 kbar respectively the d1 thrusting event in the phyllites of the nove m sto unit took place under greenschist facies conditions ca 350 c and 4 5 kbar and its effects mostly survived the later amphibolite facies metamorphism these structural relationships are equivalent to those observed elsewhere in the bohemian massif on the tepla barrandian moldanubian terrane boundary the characteristic features of this boundary are a high metamorphic gradient but lack of metamorphic inversion the occurrence of mid carboniferous stitching plutons and the importance of mid carboniferous deformation that resulted in significant downwards movement of the low grade tepla barrandian terrane relative to the uplifted high grade and hot lower to middle crust of the moldanubian terrane the downthrow of the tepla barrandian occurred on ductile shear zones showing either down dip slip kinematics or that of transfer strike slip as was the case with the orlica mts both kinematics being associated within the same regional scale linked shear zone extensional system the extensional crustal collapse must have been preceded by a se directed crustal stacking achieved through overthrust emplacement of the tepla barrandian on top of the moldanubian terrane

tectonic history of sumba island indonesia since the late cretaceous and its rapid escape into the forearc in the miocene abstract the island of sumba lies within the sunda banda forearc and contains volcanic plutonic and volcaniclastic rocks that record a history of arc volcanism extending from late cretaceous 80 ma or earlier to oligocene 31 ma time during the late cretaceous sumba formed part of a great indonesian volcanic arc system near southeastern eurasia between late cretaceous and early miocene time sumba remained part of that arc system which ceased to be volcanically active by 31 ma during the late miocene 18 ma sumba as part of the relict arc system was situated near the present site of alor and wetar at 16 ma sumba was torn from the relict arc and began to move at a velocity of 50 mm yr in a west south westerly direction volcanism along the modern banda arc soon followed by 7 ma sumba finally came to rest after having moved 450 km into the forearc we interpret the rapid motion of sumba into the forearc as a discrete block to have been a response to the collision between australia and indonesia in the area of timor the savu basin possibly originated as an arc crestal rift which then opened behind sumba as the latter moved westward over the past 16 my more than 3 km of sediment have been deposited in the savu basin by erosion from the surrounding islands sumba stopped moving to the west south west at 7 ma after reaching approximately its present location since then the island has ceased to be a discrete tectonic fragment and has behaved as an integral part of the forearc during the past 7 my compressional tectonics within the forearc has led to elevation of the rocks exposed on sumba by as much as 4 km

proximal cretaceous tertiary boundary impact deposits in the caribbean trace element isotopic and mineralogic studies indicate that the proposed impact at the cretaceous tertiary k t boundary occurred in an ocean basin although a minor component of continental material is required the size and abundance of shocked minerals and the restricted geographic occurrence of the ejecta layer and impact wave deposits suggest an impact between the americas coarse boundary sediments at sites 151 and 153 in the colombian basin and 5 to 450 meter thick boundary sediments in cuba may be deposits of a giant wave produced by a nearby oceanic impact on the southern peninsula of haiti a approximately 50 centimeter thick ejecta layer occurs at the k t boundary this ejecta layer is approximately 25 times as thick as that at any known k t site and suggests an impact site within approximately 1000 kilometers seismic reflection profiles suggest that a buried approximately 300 km diameter candidate structure occurs in the colombian basin

deformation and sedimentation along a developing terrane suture eastern sunda forearc indonesia the collision of the eastern sunda arc with northwest australia has resulted in the development of a suture between the sumba ridge and sawu timor terranes along a zone of intraforearc convergence the developing suture varies from the low angle sawu thrust with attendant mud diapirs in the sumba basin to high angle reverse faults near a basement high of the underthrust sumba ridge terrane bottom currents associated with the flow of pacific ocean deep water into the indian ocean have eroded the terranes and subsequently deposited the detritus in an assemblage of contourites along the suture this study reveals the high structural variability of a terrane suture and the oceanographic influence on the deposition of overlap assemblages

offscraping and underthrusting of sediment at the deformation front of the barbados ridge deep sea drilling project leg 78a on leg 78a we drilled sites 541 and 542 into the seaward edge of the barbados ridge complex and site 543 into the adjacent oceanic crust the calcareous ooze marls and muds at sites 541 and 542 are lithologically and paleontologically similar to the upper strata at site 543 and are apparently offscraped from the down going plate a repetition of miocene over pliocene sediments at site 541 documents major thrust or reverse faulting during offscraping the hemipelagic to pelagic deposits offscraped in the leg 78a area include no terrigenous sand beds but they contain numerous neogene ash layers derived from the lesser antilles arc hence this sequence is quite unlike the siliciclastic dominated terranes on land that are inferred to be accretionary complexes the structural fabric of the offscraped deposits at sites 541 and 542 is disharmonic probably along a decollement with an underlying acoustically layered sequence suggesting selective underthrusting of the latter the acoustically layered sequence correlates seismically with pelagic strata cored at site 543 on the incoming oceanic plate cores recovered from the possible decollement surface at both sites 541 and 542 show scaly foliation and stratal disruption approximately lithostatic fluid pressure measured in the possible decollement zone probably facilitates the underthrusting of the pelagic sediments beneath the offscraped deposits in the incoming section a transition from smectitic to radiolarian mud with associated increases in density and strength probably controls the structural break between offscraped and underthrust strata in the leg 78a area the underthrust pelagic section can be traced seismically at least 30 km arcward of the deformation front beneath the barbados ridge complex

evidence for a submerged and displaced continental borderland north banda sea indonesia the banda sea is separated into two oceanic fragments the south and north banda basins by a large intervening region of ridges and basins called the banda ridges the north and south banda basins appear to be trapped pieces of pre tertiary oceanic crust whereas the banda ridges are a submerged and displaced continental borderland structurally very similar to the southern california offshore borderland analysis of dredged rocks from the banda ridges suggests an origin from western irian jaya

evolution of atauro island temporal constraints on subduction processes beneath the wetar zone banda arc abstract atauro is a key to understanding the late stage volcanic and subduction history of the banda arc to the north of timor a volcanic history of bi modal subaqueous volcanism has been established and new whole rock and trace element geochemical data show two compositional groups basaltic andesite and dacite rhyolite 40ar 39ar geochronology of hornblende from rhyo dacitic lavas confirms that volcanism continued until 3 3 ma following the cessation of volcanism coral reef marine terraces have been uplifted to elevations of 700 m above sea level continuity of the terraces at constant elevations around the island reflects regional scale uplift most likely linked to sublithospheric processes such as slab detachment local scale landscape features of the eastern parts of atauro are strongly controlled by normal faults the continuation of arc related volcanism on atauro until at least 3 3 ma suggests that subduction of australian lithosphere continued until near this time this data is consistent with findings from the earthquake record where the extent of the wetar seismic gap to a depth of 350 km suggests slab breakoff as a result of collision commenced at 4 ma leading to subsequent regional uplift recorded in elevated terraces on atauro and neighbouring islands

seismological evidence for shallow thrusting north of the timor trough summary the eastern sunda arc in indonesia is the site of subduction of the continental crust of australia and its shallow seismicity is manifested by infrequent large ms 7 earthquakes that occur near the volcanic arc within the upper plate rather than as interplate events beneath the forearc in the same region volcanic activity has ceased and small shallow earthquakes are rare we examine two of the larger earthquakes the events of 1962 may 15 ms 7 2 and 1977 august 27 ms 7 0 the 1977 event occurred north of central timor and the inversion of long period p and sh waves indicates a nearly pure thrust mechanism at a depth of 10 km below sea level and a seismic moment of 4 1 1026 dyne cm the p wave nodal planes for this solution strike roughly parallel to the mapped trace of a thrust zone the wetar thrust zone at the southern margin of the backarc basin and the south nodal dipping plane is consistent with the sense of thrusting on this thrust zone a broad zone of compressional deformation is evident in seismic reflection profiles over the epicentral region of the 1977 event and gravity data suggest that a small amount of either isostatically uncompensated crustal thickening or flexure of the underthrusting plate has occurred the 1962 earthquake occurred approximately 300 km north east of the 1977 earthquake and near the eastern end of the volcanically quiet section of the banda arc the similarity of the few available waveforms to those at the same stations for the 1977 event suggests a similar source mechanism modelling of pdif waveforms for the 1962 event indicates a depth of less than 40 km and a preferred centroidal depth of 20 km for which a seismic moment of 5 1026 dyne cm is estimated these fault plane solutions indicate that most of the seismic energy released by the collision between australia and the sunda arc is generated by faulting within the overriding plate and we infer that a major change in the plate geometry is imminent we suggest that most of the relative convergence between the plates is accommodated by thrusting of the marginal basin the banda sea southward beneath the arc and that the present geometry represents the initial stage of the reversal of the arc s polarity

structural styles of an accretionary wedge south of the island of sumba indonesia revealed by seamarc ii side scan sonar the accretionary wedge south of sumba island indonesia is in the early stages of continent island arc collision australian continental shelf sediments are being actively accreted to the sunda arc at the timor trough we have correlated seamarc ii side scan images and bathymetry with 3 5 khz and seismic reflection profiles to construct a structural interpretation of this zone deformation is concentrated on the lower slope of the accretionary wedge within 15 to 25 km of the thrust front above which the strain rate appears to decrease abruptly three distinct structural styles are developed in the survey area the characteristics of the easternmost region are 1 deformation in a fold and thrust geometry 2 presence of mud volcanoes and mud ridges parallel to the thrust front indicating elevated fluid pressure and 3 the development of a long decollement extending 10 to 15 km from the thrust front in contrast the central region shows a more homogeneous thickening resulting mainly from closely spaced faulting seaward verging thrust faults and conjugate strike slip faults are the main structures in this segment of the accretionary wedge mud volcanoes and a very long decollement are absent in the central region we present a hypothesis in which structural style is controlled by the length of the decollement developed in front of the thrust front short thick thrust sheets may favor uniform shortening whereas long thin sheets may be unstable to buckling and deform by folding high fluid pressure may facilitate propagation of the decollement in contrast to the accretion occurring in the other two regions the western part of the accretionary wedge is in the process of being indented and redeformed by a basement ridge

rhodium distribution at the cretaceous tertiary boundary analysed by ultrasensitive laser photoionization the extraordinarily high concentrations of indium and other siderophiles found in cretaceous tertiary k t boundary deposits have been interpreted1 as a result of a large extraterrestrial body falling upon the earth causing mass extinction of the biota dominant in the cretaceous the existing alternative models of the cretaceous terminal event try to explain the anomaly of siderophile elements by their being concentrated through sedimentation or volcanic eruptions2 3 to understand the nature of the k t event it is important to establish the proportions of siderophiles and their isotopes in boundary deposits of that age especially the proportions of platinum group elements their concentrations in extraterrestrial material being high and occurring in ratios differing from those typical of terrestrial material here we present the first data on rhodium concentrations at the k t boundary in the sumbar sm 4 section turkmen ssr obtained by the ultrasensitive laser photoionization spectroscopy lapis technique4 5 the maximum rh concentration in the samples studied is 24 2 ng g 1the rh ir ratio is 0 34 0 06 which is close to the cosmic ratio of these elements the lapis technique is efficient and can be used for determining the concentrations of all siderophile elements and their isotopes in geological materials

terrestrial carbon and nitrogen isotopic ratios from cretaceous tertiary boundary nanodiamonds one hypothesis for the origin of the nanometer size diamonds found at the cretaceous tertiary k t boundary is that they are relict interstellar diamond grains carried by a postulated asteroid the 13 c 12 c and 15 n 14 n ratios of the diamonds from two sites in north america however show that the diamonds are two component mixtures differing in carbon and nitrogen isotopic composition and nitrogen abundance samples from a site from italy show no evidence for either diamond component all the isotopic signatures obtained from the k t boundary are material well distinguished from known meteoritic diamonds particularly the fine grain interstellar diamonds that are abundant in primitive chondrites the k t diamonds were most likely produced during the impact of the asteroid with earth or in a plasma resulting from the associated fireball

constraints on the devonian carboniferous closure of the rheic ocean from a multi method geochronology study of the stare město belt in the sudetes poland and the czech republic abstract this paper attempts to clarify the complex nature of how and when the rheic ocean closed in what is now central europe and with respect to the various terranes that were involved offer a regional chronology for the associated structural metamorphic and igneous processes that accompanied and followed this closure the variscan orogen in europe originated from the multiple collisions of gondwana derived terranes the armorican terrane assemblage with laurussia however many important structural details on the timing of these collision related events remain obscure in the sudetes the stare m sto belt represents a wnw dipping part of the rheic suture that developed from the continental collision of the eastern terranes of the armorican terrane assemblage now in the bohemian massif with the brunovistulian terrane a part of laurussia old red continent in this study the results of monazite th u total pb garnet lu hf and zircon u pb geochronology were integrated into a newly established d1 d3 tectonometamorphic sequence a th u total pb age of 368 ma from a monazite that grew concurrently with d2 metasedimentary garnet as well as lu hf ages of 361 ma and 355 ma obtained from d2 metasedimentary garnets implies that the regional contractional deformation and progressive metamorphism of d2 took place mainly during the famennian late devonian and extended into the visean middle mississippian of the early carboniferous the ion probe u pb zircon ages of 355 ma and 359 ma obtained from leucocratic neosomes in migmatized amphibolites confirmed a lag in the peak temperature that followed crustal thickening during d1 d2 metamorphic monazites dated at 340 ma provide a time for the subsequent d3 dextral transpressional regime the closure of the stare m sto belt segment of the rheic ocean probably resulted from a head on hard collision between the westerly subducting brunovistulian promontory of laurussia and the eastern members of the armorican terrane assemblage thus the rheic ocean closed during the late devonian at 370 360 ma and preceded the collision of the armorican terrane assemblage with east avalonia at the western margin of what is now the bohemian massif following ocean closure the rheic slab may have broken off resulting in the suture zone becoming dominated by lateral tectonic escape movements of the colliding terranes at 340 330 ma visean syntectonic d3 intrusions of granodiorite tonalite magma acted as a hot lubricant and stitched the suture zone together

temporal evolution of mineralization events in the bohemian massif inferred from the re os geochronology of molybdenite molybdenite is a common mineral accompanying sn w au and base metal mineralizations located in different geotectonic units of the bohemian massif but it is also widespread in granitoids and or related quartz veins pegmatites forming disseminated mo mineralization thirty re os ages were obtained for molybdenite samples from the bohemian massif to provide constraints on the timing and duration of mineralization event s within the framework of previously published geochronological data for the host and or associated rocks the obtained data for sn w li deposits in the erzgebirge metallogenetic province indicate the predominance of one and or multiple short time mineralization events taking place between 319 and 323 ma with the exception of the krupka deposit associated with the altenberg teplice caldera where the data may suggest prolonged activity until 315 ma the ages of the pb zn au mo h rky u rakovnika and fe cu as ob i d l mineralizations from the exocontacts of the cista pluton and krkonose jizera plutonic complex respectively provide evidence for synchronous emplacement of the ore and the associated granitic rocks in contrast the padr fe as mo mineralization postdates the age of the associated padr granite disseminated mo mineralization in cadomian and variscan granitoids and or related to quartz veins pegmatites provides re os ages that overlap with the previously published geochronological data for the host rocks suggesting coeval evolution molybdenite samples from the sazava suite granites of the central bohemian plutonic complex cbpc have resolvable younger ages than their host granites but similar to the age of spatially related au mineralization which is associated with the latest evolution of the cbpc

constraining long term denudation and faulting history in intraplate regions by multisystem thermochronology an example of the sudetic marginal fault bohemian massif central europe 1 the rychlebske hory mountain region in the sudetes ne bohemian massif provides a natural laboratory for studies of postorogenic landscape evolution this work reveals both the exhumation history of the region and the paleoactivity along the sudetic marginal fault smf using zircon u th he zhe apatite fission track aft and apatite u th he ahe dating of crystalline basement and postorogenic sedimentary samples most significantly and in direct contradiction of traditional paleogeographic reconstructions this work has found evidence of a large cretaceous sea and regional burial to 6 5 km of the carboniferous permian basement in the late cretaceous 95 80 ma during the burial by sediments of the bohemian cretaceous basin system the smf acted as a normal fault as documented by offset zhe ages across the fault at 85 70 ma the basin was inverted cretaceous strata eroded and basement blocks were exhumed to the near surface at a rate of 300 m ma as evidenced by late cretaceous paleocene aft ages and thermal modeling results there is no appreciable difference in aft and ahe ages across the fault suggesting that the smf acted as a reverse fault during exhumation in the late eocene oligocene the basement was locally heated to 70 c by magmatic activity related to opening of the eger rift system neogene or younger thermal activity was not recorded in the thermochronological data confirming that late cenozoic uplift and erosion of the basement blocks was limited to less than 1 5 km in the study area

pervasive horizontal fabric and rapid vertical extrusion lateral overturning and margin sub parallel flow of deep crustal migmatites northeastern bohemian massif abstract in the west sudetes northeastern bohemia massif geochronometry provides evidence for repeated episodes of rapid cooling that contrasts sharply with an absence of structural evidence for significant tectonic exhumation by crustal extension instead high grade assemblages of the orlica snieznik complex have a regional sub horizontal foliation and sub horizontal lineations that trend parallel to narrow sub vertical shear zones containing exhumed high pressure assemblages mesoscopic petrofabrics combined with anisotropy of magnetic susceptibility ams data from amphibolite facies to migmatitic meta sedimentary and meta igneous rocks reveal remarkably consistent average lineations that plunge shallowly to the ssw on both steep and sub horizontal nne trending planar fabrics the dominant ssw nne fabric orientation is parallel to the bohemia brunia suture which marks a major boundary along the eastern margin of the massif the shape of the ams ellipsoid is predominantly oblate revealing flattened fabrics with only local prolate ellipsoids we envisage that the continental brunian indentor operated as a rigid backstop and allowed the migmatized lower crustal orogenic root to be exhumed along the bohemian margin shortly following terminal arc collision sub vertical extrusion of the orogenic root was arrested in the mid crust where the lower ductile crust was laterally overturned at the base of rigid upper crustal blocks upon reaching the crustal high strength lid the exhumed ductile mass of continental material laterally spread sub parallel to the margin underwent subsequent supra barrovian metamorphism and quickly cooled the application of ams techniques to high grade metamorphic rocks in concert with macroscopic structural observations is a powerful approach for resolving the deformation history of a terrane where visible rock fabrics can be tenuous

exhumation and metamorphism of an ultrahigh grade terrane geochronometric investigations of the sudete mountains bohemia poland and czech republic the sudete mountains ne bohemian massif czech republic and poland preserve abundant eclogitic and granulitic centimetre to decimetre scale boudins enveloped in a predominantly migmatitic matrix published geochronometry and thermobarometry from the uhp and uht rocks broadly constrain the crystallization and initial stage 1 exhumation history for these units however the timing of stage 2 metamorphism and associated unroofing is less well constrained new in situ ion microprobe th pb monazite results together with complementary u pb zircon and electron microprobe analyser total pb monazite results on 11 amphibolite facies gneissic to migmatitic samples place important temporal constraints on the second stage of uhp and uht metamorphism exhumation the orlica snieznik dome records uhp metamorphism occurring at 375 ma and subsequent exhumation to midcrustal levels in supra barrovian conditions at c 345 330 ma in contrast the western gory sowie block preserves evidence of hp granulite conditions at c 400 ma and exhumation to mid crustal levels at 380 370 ma revealing a c 30 million years difference in exhumation events between the neighbouring terranes the eastern gory sowie block preserves ages similar to the orlica sneiznik dome suggesting that different preserved metamorphic cooling histories are juxtaposed across the sudetic marginal fault the bounding niemcza shear zone yields preliminary th pb dates that range from 380 8 ma to 283 2 ma preserving a protracted metamorphic record that spans the exhumation history of the region the distinct collapsed geochronologies of both terranes probably reflect rapid vertical transport of low viscosity crust under supra barrovian conditions near the mid crustal high strength lid during oblique transpressional convergence

the 40ar 39ar ages from the west sudetes ne bohemian massif constraints on the variscan polyphase tectonothermal development

elemental anomalies at the cretaceous tertiary boundary woodside creek new zealand iridium and 26 other elements were determined in shale from the cretaceous tertiary boundary at the locus classicus for iridium anomalies at woodside creek new zealand iridium gold copper cobalt chromium nickel arsenic molybdenum and iron were enriched in the basal 2 millimeters of the 8 millimeter shale parting as compared with the rest of the stratigraphic column no other shale partings in the column had anomalous concentrations of any element when the data were expressed on a carbonate free basis the boundary material showed striking compositional similarities with the stevns klint danish boundary shale elemental concentrations were in general much higher in the new zealand material than in nonboundary shales from elsewhere in the world the high concentration of iridium 153 nanograms per gram in the basal layer of the boundary together with the enrichment of other siderophile elements supports the idea of an extraterrestrial source for much of the material the iridium gold ratio of 2 1 is also in accordance with such a source the iridium content of the basal layer is higher than for any other marine boundary shale obtained on land the integrated iridium value is 187 nanograms per square centimeter of boundary surface

the wetar back arc thrust belt eastern indonesia the effect of accretion against an irregularly shaped arc the 350 km long east west trending wetar back arc thrust belt in eastern indonesia is a result of the collision of australia with the indonesian island arc along the northwest margin of wetar island a short 50 km section of the thrust belt trends northeast coincident with the offset of the indonesian arc by the newly discovered left lateral wetar atauro fault which runs along the shelf region and trends northeast the wetar atauro fault may be viewed as a large scale lateral ramp or wrench fault separating the eastern wetar thrust belt from the western wetar thrust belt the interaction of strike slip faulting and back arc thrust faulting creates several arc blocks whose geometry strongly affects the structure of the deformed wedge of sediment accreting in the wetar back arc thrust belt the varying orientations of the arc backstop make the wetar back arc thrust belt a perfect laboratory for the study of oblique convergence along most of its length the wetar thrust belt parallels the arc slope however along the northeast trending offset in the thrust belt seamarc ii side scan sonar plus seismic reflection data show that the trend of the thrust belt is not parallel to the trend of the arc slope rather it is intermediate between the trend of the arc slope and the perpendicular to the expected convergence direction the side scan images allow us to map the geometry of four main thrust faults the wetar liran atauro and alor faults the wetar liran and atauro faults trend northeast parallel to the thrust front and do not have an en echelon relationship the westernmost fault in the survey area the alor fault trends almost east west the orientation of the wetar atauro fault is consistent with a maximum principal stress within the arc oriented between 8 and 23 west of north depending on the internal coefficient of friction this agrees well with the p axes of nearby earthquake focal mechanisms which trend consistently west of north suggesting that the regional principal tectonic compression is oriented west of north because the thrust belt is oblique to the arc slope we infer the structural directions are influenced by both the arc backstop and by the regional tectonic stress the tectonic compression due to arc continent collision in this region may be modified by the arc geometry in several ways 1 the stress necessary to support the arc topography may be significant close to the arc 2 the change in thickness of the elastic part of the lithosphere may cause a concentration of tectonic stress and 3 the difference in material properties between the relatively rigid arc bulldozer and the weak basin sedimentary fill may be an important factor in producing a thrust belt that conforms roughly to the shape of the arc the structure of the wetar back arc thrust zone demonstrates that the development of small rigid blocks within a major collision zone may produce complex structural patterns and local directions of shortening that are highly oblique to the main direction of plate convergence

new method for the measurement of osmium isotopes applied to a new zealand cretaceous tertiary boundary shale the determination of osmium content and isotopic abundances in geological materials has received increasing attention in recent years following the proposal of alvarez et al 1 that mass extinctions at the end of the cretaceous period were caused by the impact of a large 10km meteorite which left anomalously high iridium levels as a geochemical signature in the boundary shales here we report a new and simple method for measuring osmium in geological materials involving fusion of the sample with sodium peroxide distillation of the osmium as the tetroxide using perchloric acid extraction into chloroform and absorption of the chloroform extract onto graphite powder before instrumental neutron activation analysis in a variant of this technique the chloroform extract is back extracted into an aqueous phase and the osmium isotopes are determined by plasma source mass spectrometry icpms we have used this method on the woodside creek new zealand cretaceous tertiary boundary clay and have obtained the first osmium content 6g ng g 1 for this material the 187os 186os ratio is 1 12 0 16 showing a typical non crustal signature this combined distillation extraction icpms method will prove to be useful for measuring osmium isotopes in other geological materials

cretaceous tertiary boundary event evidence for a short time scale abstract three non meteoritic trace elements sb as and zn are strongly enriched at eleven k t boundary sites along with mainly or partly meteoritic elements ir ni cr fe and co the proportions as sb zn ir are remarkably constant over a 100 fold range in concentration this correlation persists in sub layers of boundary clay and even extends to soot from burned land biomass apparently all the components despite their diverse origins became associated in a single global component prior to deposition no wholly satisfactory source is available for as sb and zn the trace element pattern in volcanic gases does not match that in k t boundary clay with ratios to ir falling short by 1 to 2 orders of magnitude terrestrial rocks do not reach high enough concentrations and modern ocean water contains too little zn but on balance the latter source seems preferable perhaps augmented by volatiles from the impact crater apparently ir bearing ejecta and soot from forest fires coagulated in the stratosphere and then fell out together sweeping out oceanic biomass and anoxically precipiting as sb and zn significantly the amounts of marine and land biomass at the k t boundary correspond to about the steady state global inventory 1 generation and the amounts of as sb and zn are also within a factor of 5 of the global inventory this is expected in a catastrophic but not a gradualist scenario

a model for a continental accretionary wedge developed by oblique collision the ne bohemian massif structural and metamorphic investigations of the northeastern margin of the bohemian massif indicate three main sequential devonian carboniferous tectonic events 1 devonian rifting 2 early carboniferous oblique underthrusting and formation of a continental accretionary wedge 3 eduction of the wedge and late carboniferous transpression devonian rifting of the brunia microcontinent resulted in the formation of two crustal scale boudins associated with the development of two syn rift devonian basins this extensional template strongly influenced the nature of the ensuing variscan contractional deformation early carboniferous 350 330 ma progressive highly oblique underthrusting of the two crustal boudins beneath the lugian terrane to the west generated syn deformational barrovian metamorphism and the formation of a continental accretionary wedge the wedge was further compressed by continued underthrusting of brunia which resulted in the successive vertical extrusion eduction of an upper and lower allochthon derived from the more deeply underthrust crustal boudin the eduction was terminated by a late carboniferous 330 310 ma transpressional event resulting from continued plate convergence release of mantle derived magma during late stage eduction thermally softened the transpressional zones in the more external parts of the wedge the resultant differential displacements gave rise to extensional unroofing of the internal part of the wedge

post variscan thermal history of the moravo silesian lower carboniferous culm basin ne czech republic sw poland abstract apatite fission track analysis aft and zircon u th he thermochronology zhe have been carried out for a lower carboniferous greywacke succession of the moravo silesian culm basin in the nizk jesenik mountains the range of apparent zircon helium ages is 303 233 ma late carboniferous to early triassic in the eastern part of the basin whilst they are significantly younger in the western part ranging from 194 to 163 ma early middle jurassic apatite fission track central ages range from 152 latest jurassic to 44 ma eocene with the majority being grouped between 114 aptian and 57 ma paleocene all samples experienced substantial post depositional thermal reset both the aft ages and the zhe are considerably younger than the depositional ages the mean track length varies in the range between 12 5 and 15 4 m the unimodal track length distribution the relatively short mean track length in most samples and their rather low standard deviation values 1 2 to 2 1 m indicate that their thermal history was determined by variscan and post variscan heating event s followed by a prolonged residence in the apatite partial annealing zone in the mesozoic and finally by cooling in the paleogene geological evidence combined with thermal modeling based on aft and zhe data indicate that the lower carboniferous strata had already reached maximum palaeotemperatures in the late carboniferous however they were presumably later re heated during the permian triassic post variscan extensional tectonics events were responsible for high heat flow that together with carboniferous burial could account for the reset of both thermochronometers a major phase of cooling occurred in the late cretaceous finally exhumation was probably faster in the paleogene causing the present day exposure of the studied rocks

paleomagnetism of younger volcanics from western timor indonesia abstract paleomagnetic research has been carried out on volcanic rocks from two formations exposed in western timor indonesia i e the metan formation of eocene age and the manamas formation of late miocene age because of the chaotic tectonic setting of the rocks at the metan formation it was the inclination data from five lavas only that could be interpreted in the final analysis of the characteristic remanence directions chrm with i 33 6 a 95 4 1 the metan volcanics most probably extruded originally in the northern hemisphere at a latitude of about 17 the volcanics may have extruded on a crustal unit which includes a continental fragment that became detached from gondwana in mesozoic times and subsequently shifted to the southeast asian continent this crustal unit broke away from southeast asia in eocene times moved towards the south and collided 3 ma ago with the frontal part of the australian continental margin the manamas formation of late miocene age includes obducted volcanics consisting of both deep sea pillow lavas and island arc volcanics with chrm directions of d 132 6 i 15 8 no a 95 because 2 sites only and d 295 9 i 26 3 a 95 2 8 respectively the declinations of the chrm directions derived from the rocks from two localities show counterclockwise deviations of approximately 45 and 60 respectively relative to the present earth s field direction the deviating directions can be explained in terms of a counterclockwise rotation of timor which may have begun as early as 3 ma ago and which could be a result of the obliquely northward protruding australian continent where the originally west east directed banda arc rotated in a counterclockwise sense it is also possible that during the obduction the lava piles performed counterclockwise rotations rock magnetic analyses show that magnetite or and titanomagnetite is the main carrier of remanence

the quantitative link between fold geometry mineral fabric and mechanical anisotropy as exemplified by the deformation of amphibolites across a regional metamorphic gradient this work shows lateral variations in fold geometry within an amphibolite unit of constant mineralogical composition under increasing metamorphic grade analysis of the fold geometries indicate 1 medium amplification associated with low post buckle flattening in the garnet zone 2 high amplification coupled with medium post buckle flattening in the staurolite zone and 3 passive amplification dominated by intense post buckle flattening in the sillimanite zone a systematic decrease in the mechanical anisotropy of the folded fabric is observed with increase in metamorphic grade quantitative microstructural study shows contrasting deformation micro mechanisms associated with folding manifested by 1 brittle dominated deformation of amphiboles that form a stress supporting network with a high competence contrast with respect to plagioclase in the garnet zone 2 ductile dominated heterogeneous deformation of an interconnected weak layer structure with low competence contrast in the staurolite zone and 3 homogeneous deformation of a stress supporting framework with low competence contrast in the sillimanite zone the difference in the folding style between the garnet and staurolite zones is associated with the lateral variations in microstructure of the amphibolites inherited from a pre folding metamorphic zonation and with different deformation mechanisms in the hinge zones however the change in fold style observed as one moves into the sillimanite zone is controlled by the recrystallization associated with an important syn folding heat input from an adjacent granite intrusion

structural and anisotropy of magnetic susceptibility records of granitoid sheets emplacement during growth of a continental gneiss dome central sudetes european variscan belt 1 granitoid magmatic sheets emplaced syntectonically during growth of the orlica nieznik mantled gneiss dome central sudetes european variscan belt were examined by means of structural geology quartz and anisotropy of magnetic susceptibility ams fabric studies magmatic emplacement was localized along the eastern transpressive margin of the high metamorphic core and below the low grade detached mantle rimming the southern margin of the dome in the first area the magmatic sheets were emplaced parallel to the dilated subhorizontal foliation of an anisotropic pre orogenic block the resulting melt host rock multilayer localized the transpressive zone along which the bulk of magma was emplaced the ams study shows an across strike fabric zonation underlying a strongly transpressive solid state deformation in the hanging wall magmatic subhorizontal fabric in the foot wall and a transitional fabric in the center of the intrusion in contrast along the southern dome margin magmatic sheets intruded along steep foliations of weakly metamorphic mantle rocks and are affected by recumbent folding and subhorizontal shearing the bulk of the sheets present a shallow dipping magmatic foliation and an along strike magmatic lineation the variations of the quartz and magnetic fabrics are attributed to superimposition of pure shear dominated ductile thinning followed by simple shear dominated detachment onto the original steep fabric ams modeling confirms the role of variation of orientation of pre intrusive anisotropy during progressive deformation on the resulting fabric pattern and helps explaining observed variations in fabric orientations and symmetries this study highlights contrasting mechanical behavior of syntectonically emplaced magmas in different parts of a growing crustal scale mantled gneiss dome

the main ostrava whetstone composition sedimentary processes palaeogeography and geochronology of a major mississippian volcaniclastic unit of the upper silesian basin poland and czech republic the main ostrava whetstone mow is an important lithostratigraphic horizon of the late carboniferous sedimentary fill of the late palaeozoic foreland upper silesian basin it is the largest and best identified volcanogenic horizon in the basin reaching thicknesses of 15 3 m and occupying an area of ca 2 973 km2 and a volume after lithification of 9 24 km3 it consists of volcanic materials transported to the basin probably by an aeolian process just after sedimentation these materials were redeposited a short distance away in a shallow water environment granularity corresponds to a range from argillaceous siltstones to fine grained sandstones the components are dominated by glass shards replaced by clay minerals mixed illite smectite structures in addition to quartz of volcanogenic and terrigenous origins sanidine and a plagioclase close to albite are also present the sedimentary structures micro structures and composition of the mow indicate variable and dynamic hydrodynamic conditions the mow represents a series of flooding events which could be connected with unusual rainfall such major flooding events were most likely induced by volcanic eruptions the available drill core log data were used to construct a digital model of the whetstone which showed an east west zonality in the thicknesses with the majority being synsedimentary ca tims u pb dating the volcanogenic zircons yields an age of 327 35 0 15 ma the source location of the volcanogenic material is not clear however it is presumed to have been located in the west of the upper silesian basin

osmium isotopes in the aerosols of the mantle volcano mauna loa aerosols and reactive gases from the spring 1984 eruption of mauna loa volcano on hawaii were collected and analyzed for osmium and its isotopic composition the measured187os 186os ratio of1 14 0 03 is close to the ratio in matter with solar systemre os abundance this result shows that the aerosols from mauna loa originated in the mantle and that their composition was not or only slightly influenced by their contact with the crust

isotopic constraints on the role of subducted sialic material in indonesian island arc magmatism the island arcs of indonesia exhibit a range of subduction related tectonic environments and consequently are of particular interest in studying the relationship between the nature of the subducted plate and volcanism the sunda arc in java manifests the underthrusting of oceanic lithosphere beneath the newly accreted margin of asia in the banda arc continental material appears to be subducting beneath a volcanic arc that is underlain by oceanic crust 87sr 86sr ratios 0 7040 to 0 7059 in holocene andesitic lavas from java are higher than those from other volcanic arcs except those arcs where sialic contamination has been inferred a group of anomalous calc alkaline lavas characterized by high 87sr 86sr ratios 0 7054 to 0 7059 and relatively high abundances of k and related elements appear to be the result of sialic contamination of a mantle derived melt high 87sr 86sr ratios as large as 0 7095 together with values of 18o as large as 9 2 smow in lavas from the banda arc imply mixing of mantle derived and sialic components because of the tectonic peculiarities of the region the most likely source of contamination is from subduction of terrigenous material derived from the northwest continental shelf of australia lead isotopic ratios in lavas from java are generally transitional to the trends defined by other arc lavas and are characterized by higher 207pb 206pb ratios than oceanic basalts the anomalous calc alkaline lavas in particular are characterized by relatively high 207pb 206pb ratios interpreted to result from subduction and melting of sediments derived from the precambrian shields of australia and india during the opening of the indian ocean neodymium isotope ratios lie within the range defined from other volcanic arcs in terms of 143nd 144nd and 87sr 86sr the banda arc samples lie on a mixing line between compositions typical of the mantle and the upper continental crust that subducted sediments appear as a component in at least some of the holocene indonesian lavas implies melting at the top of the subducted lithospheric slab and provides constraints on the thermal structure of the descending plate

tertiary evolution of the eastern indonesia collision complex abstract eastern indonesia is the zone of interaction between three converging megaplates eurasia the pacific and indo australia the geological basis for interpretations of the tertiary tectonic evolution of eastern indonesia is reviewed and a series of plate tectonic reconstructions for this region at 5 million year intervals covering the last 35 million years is presented the oldest reconstruction predates the onset of regional collisional deformation at this time a simple plate configuration is interpreted consisting of the northward moving australian continent approaching an approximately e w oriented southward facing subduction zone extending from the southern margin of the eurasian continent eastwards into the pacific oceanic domain beginning at about 30 ma the australian continental margin commenced collision with the subduction zone along its entire palinspastically restored northern margin from sulawesi in the west to papua new guinea in the east from this time until ca 24 ma the australian continent indented the former arc trend with the northward convergence of australia absorbed at the palaeo northern boundary of the philippine sea plate the present day palau kyushu ridge at ca 24 ma the present day pattern of oblique convergence between the northern margin of australia and the philippine sea plate began to develop at about this time a large portion of the palaeogene colliding volcanic arc the future eastern philippines began to detach from the northern continental margin by left lateral strike slip from ca 18 ma oblique southward directed subduction commenced at the maramuni arc in northern new guinea at ca 12 ma the sorong fault zone strike slip system developed effectively separating the philippines from the indonesian tectonic domain the sorong fault zone became inactive at ca 6 ma since which time the tectonics of eastern indonesia has been dominated by the anticlockwise rotation of the bird s head structural block by some 30 40 contemporaneously with post 18 ma tectonism the banda arc subduction collision system developed off the northwestern margin of the australian continent convergence between indo australia and eurasia was accommodated initially by northward subduction of the indian ocean and subsequently since ca 8 ma by the development of a second phase of arc continent collision around the former passive continental margin of nw australia

neogene sutures in eastern indonesia five suture zones are described from the zone of collision between the eurasian indian australian and pacific philippine sea plates within the eastern indonesia region these are the molucca sorong sulawesi banda and borneo sutures each of these sutures has a relatively short history compared to most pre neogene orogenic belts but each preserves a record of major changes in tectonics including subduction polarity reversals elimination of volcanic arcs changing plate boundaries and important extension within an overall contractional setting rapid tectonic changes have occurred within periods of less than 5 ma many of these events although important would be overlooked in older orogenic belts because the age resolution required to identify them even when the evidence is preserved is simply not possible q 2000 elsevier science ltd all rights reserved

deformation and exhumation in timor distinct stages of a young orogeny abstract timor island in the outer banda arc bordering the timor sea preserves the orogenic product of an arc continent collision between the australian plate and the banda arc that commenced after 10 9 9 8 ma gts2004 but emerged above sea level only 3 1 ma ago the orogenic pile includes large tracts of material from the australian margin including the permian to middle jurassic gondwana megasequence and the late jurassic to early late miocene australian margin megasequence which occur in thrust slices in addition material from the banda arc side of the plate margin referred to as the banda terrane occurs throughout the island and includes both seafloor metamorphosed igneous material and cover sediments also in thrust sheets however the distribution of thrust slices is unclear in many areas perhaps because only the uppermost nappes of the thrust pile are currently emergent and also because the thrust piles have been disrupted by later high angle faulting evidence from east timor suggests that the major break between deformed pre collisional strata and the relatively undeformed overlying deposits was during the late miocene 9 8 5 5 ma we present evidence for the timing of three distinct phases of orogenic development as determined from east timor including initial collision and emplacement of the early nappes creating loading and diapirism within the 9 8 5 5 ma interval a tectonic quiet interval 5 5 ma 4 5 ma that extended for about a million years during the middle of the collision and may represent the time of locking of the subduction system and a post 4 5 ma phase of uplift unroofing and further diapirism in response to isostatic rebound our conclusions offer an alternative model for the evolution of this part of the banda arc

relationship between median destructive field and remanent coercive forces for dispersed natural magnetite titanomagnetite and hematite summary remanent acquisition curves remanent hysteresis curves and alternating field demagnetization curves were determined for a number of artificial rock specimens containing well defined grain size fractions between 5 and 250 m of natural magnetite titanomagnetite and hematite from these curves the remanent acquisition coercive force h cr the remanent coercive force hcr and the median destructive field of irm h i were determined theoretically these parameters should be the same for an assembly of non interacting homogeneously distributed randomly oriented single domain grains for a given hematite specimen h cr hcr and h i have about the same value in spite of the strong grain size dependence of these parameters for each specimen of magnetite and titanomagnetite the value of h cr is larger than hcr which again is larger than h i however the ratios h cr hcr and h i hcr appear to have a different constant value an interesting relationship which appears to hold for dispersed magnetite titanomagnetite or hematite grains between 5 and 250 m independently of grain size quantity and packing density of the magnetic material is

nature and distribution of deformation across the banda arc australian collision zone at timor recently acquired seismic reflection and seamarc ii side scan and swath bathymetry profiles near timor show that the banda arc australia collision zone has a tectonic framework similar to that of a typical oceanic subduction system deformation is occurring at present most intensely at the foot of the inner slope of the timor trough this deformation front is discontinuously advancing southward as new thrust slices develop within the subducted australian margin strata in contrast present deformation is apparently negligible in the savu basin the complex fore arc basin north of timor a possible significant exception is a postulated right lateral northeast trending fault zone offsetting the outer arc high between savu and roti although back arc thrusting has been documented north of the volcanic arc this component of convergence is minor compared with the scale of ongoing deformation in the timor trough the detailed nature of these surveys has also led to the recognition of along strike variations in deformation in the timor trough and in the savu basin these variations may be related to the variable degree of involvement of the australian continental margin along the arc

multiple factors in the origin of the cretaceous tertiary boundary the role of environmental stress and deccan trap volcanism a review of the scenarios for the cretaceous tertiary k t boundary event is presented and a coherent hypothesis for the origin of the event is formulated many scientists now accept that the event was caused by a meteorite impact at chicxulub in the yucatan peninsula mexico our investigations show that the oceans were already stressed by the end of the late cretaceous as a result of the long term drop in atmospheric co2 the long term drop in sea level and the frequent development of oceanic anoxia extinction of some marine species was already occurring several million years prior to the k t boundary the biota were therefore susceptible to change the eruption of the deccan traps which began at 66 2 ma coincides with the k t boundary events it erupted huge quantities of h2so4 hcl co2 dust and soot into the atmosphere and led to a significant drop in sea level and marked changes in ocean temperature the result was a major reduction in oceanic productivity and the creation of an almost dead ocean the volcanism lasted almost 0 7 m y extinction of biological species was graded and appeared to correlate with the main eruptive events elements such as ir were incorporated into the volcanic ash possibly on soot particles this horizon accumulated under anoxic conditions in local depressions and became the marker horizon for the k t boundary an oxidation front penetrated this horizon leading to the redistribution of elements the eruption of the deccan traps is the largest volcanic event since the permian triassic event at 245 ma it followed a period of 36 m y in which the earth s magnetic field failed to reverse instabilities in the mantle are thought to be responsible for this eruption and therefore for the k t event we therefore believe that the k t event can be explained in terms of the effects of the deccan volcanism on an already stressed biosphere the meteorite impact at chicxulub took place after the onset of deccan volcanism it probably played a regional rather than a global role in the k t extinctions

rifting of the northern margin of the australian continent and the origin of some microcontinents in eastern indonesia abstract continental australia is bounded on the east south and west sides by passive margins and the geological histories of these are well documented falvey and mutter 1981 the northern margin of the australian continent is now an active collision margin its previous history as a passive margin has rarely been examined this paper shows how the late palaeozoic to mesozoic sequence which forms the northern margin of the australian continent in the island of new guinea is readily related to the tectonic stages of a rift drift sequence rifting start of breakup began at about 230 m y ago at the permian triassic boundary the onset of seafloor spreading is marked by a post breakup unconformity and ranges in age along the northern margin of the continent from 185 m y in papua new guinea to 170 m y in irian jaya from there the age of the post breakup unconformity continues to young in a southwesterly direction along the western margin of the australian continent reflecting the opening of the indian ocean off western australia the timing of the onset of spreading in central papua new guinea is consistent with the timing of the initiation of spreading in the proto pacific ocean proposed by nur and ben avraham 1977 by the end of the jurassic the northern margin of the australian continent faced a seaway which linked the proto indian and proto pacific oceans this newly formed ocean was separated from the pre existing oceans of the neo tethys and panthalassa by a screen of continents or microcontinents the identity of this screen is discussed and it is suggested that part of it is preserved in the microcontinents of eastern indonesia

grain size dependence of initial susceptibility and saturation magnetization related parameters of four natural magnetites in the psd md range summary initial susceptibility coercive force and several types of remanent coercive forces were measured on a set of artificial rock specimens containing grain size fractions of a natural magnetite and titanomagnetite both of which are optically homogeneous and a natural titanomagnetite and titanomaghemite both of which show exsolution phenomena it was found subsequently that the optically homogeneous titanomagnetite contains some submicroscopic fe rich inclusions these inclusions are magnetically screened in coarse grains only for the lower grain sizes studied they have an appreciable influence on the data the size of the fractions varied from 5 to 250 m the parameters and several parameter ratios are plotted as a function of grain size several of these parameters can be used as mineralogical and grain size indicators the varying degree of exsolution of the minerals is reflected in the gradients of the plots from plots of initial susceptibility versus grain size it is concluded that the shape of the grains does not play a dominant role in determining the change of this parameter with grain size variation of the various parameters with grain size is explained in terms of a gradual transition from large psd grains to md grains without there being any sharp transition for titanomagnetite x 0 55 the transition takes place at grain sizes that are about 15 m larger than for magnetite

crustal influx indentation ductile thinning and gravity redistribution in a continental wedge building a moldanubian mantled gneiss dome with underthrust saxothuringian material european variscan belt 1 the contribution of lateral forces vertical load gravity redistribution and erosion to the origin of mantled gneiss domes in internal zones of orogens remains debated in the orlica snieznik dome moldanubian zone european variscan belt the polyphase tectono metamorphic history is initially characterized by the development of subhorizontal fabrics associated with medium to high grade metamorphic conditions in different levels of the crust it reflects the eastward influx of a saxothuringian type passive margin sequence below a tepla barrandian upper plate the ongoing influx of continental crust creates a thick felsic orogenic root with hp rocks and migmatitic orthogneiss the orogenic wedge is subsequently indented by the eastern brunia microcontinent producing a multiscale folding of the orogenic infrastructure the resulting kilometre scale folding is associated with the variable burial of the middle crust in synforms and the exhumation of the lower crust in antiforms these localized vertical exchanges of material and heat are coeval with a larger crustal scale folding of the whole infrastructure generating a general uplift of the dome it is exemplified by increasing metamorphic conditions and younging of 40ar 39ar cooling ages toward the extruded migmatitic subdomes cored by hp rocks the vertical growth of the dome induces exhumation by pure shear dominated ductile thinning laterally evolving to non coaxial detachment faulting while erosion feeds the surrounding sedimentary basins modeling of the bouguer anomaly grid is compatible with crustal scale mass transfers between a dense superstructure and a lighter infrastructure the model implies that the moldanubian orlica snieznik mantled gneiss dome derives from polyphase recycling of saxothuringian material

epma and pixe dating of monazite in granulites from stary gieraltow ne bohemian massif poland abstract chemical th u total pb chime dating of monazite by electron probe microanalyzer epma and proton microprobe pixe was carried out on felsic granulites from stary gieraltow poland which represent part of the orlica nieznik dome in the ne bohemian massif analyzed monazite is characterized by mosaic zoning rather than simple core to rim growth and strontium contents of up to 750ppm an isochron age of 347 13ma represents timing of amphibolite facies metamorphism in agreement with previously published estimates

the palaeomagnetism of late permian early triassic and late triassic deposits on timor an australian origin summary studies have been carried out on sediments from two formations in western timor a red crinoid bearing limestones of the maubisse formation of late permian early triassic age and b reddish calcilutites and red shales of the aituto formation of late triassic age the crinoid bearing limestones occur in both a non recrystallized and a recrystallized slightly marmorized facies natural remanent magnetization nrm was analysed by means of progressive demagnetization both using alternating magnetic fields and by heating the determination of acquisition remanence and remanent coercive force demonstrated that in a large number of sites high temperature volcanic haematite is the main carrier of remanence a few sites from the non recrystallized maubisse limestones revealed magnetite as the main remanence carrier the recrystallized maubisse limestones were found to have a secondary remanence with a very high coercivity and a high blocking temperature characteristic remanent magnetization chrm directions could be obtained both from the non recrystallized limestones of the maubisse formation and from the aituto formation for the plate tectonic interpretation of the data only the chrm inclination values have been used because of the complicated often chaotic tectonism on timor palaeolatitudinal positions of 39 and 33 have been derived from the late permian early triassic maubisse formation and the late triassic aituto formation respectively this points to the presence on the island of timor of a displaced terrane of australian origin

multibeam study of the flores backarc thrust belt indonesia using the seamarc ii seafloor mapping tool in conjunction with closely spaced seismic reflection profiles we have mapped a segment of the flores back arc thrust zone structural irregularities along the deformation front of the thrust zone result from changing stratigraphy and basement structure of the lower plate ne trending faults cutting the outer slope of the flores basin are easily mapped because they truncate dense drainage patterns obliquely mud diapirs probably indicating elevated fluid pressures have formed throughout the accretionary wedge but appear to be concentrated as do back arc thrust earthquakes at the ends of thrust faults the overall orientation of the deformation front of the accretionary wedge is 100 suggesting a nne sense of thrust motion and supporting an origin of the thrust zone by collision of the arc with australia rather than by magmatic forcing or gravitational sliding or spreading orientations of faults and folds close to the arc are not consistent with the trends of the frontal thrusts however and the difference may be due to either different initial orientations or to later rotations of structural features as the accretionary wedge grew

the origin of summit basins of the aleutian ridge implications for block rotation of an arc massif it is proposed that many summit basins along the aleutian arc form from the clockwise rotation of blocks of the arc massif summit basins are arc parallel grabens or half grabens formed within the arc massif and are commonly located near or along the axis of late cenozoic volcanism geomorphically the aleutian arc appears to consist of contiguous rhombic blocks of varying size tens to hundreds of kilometers in length the boundaries between adjacent blocks are delineated by fault controlled canyons that cut the southern slope of the arc transverse to its regional trend evidence that these blocks have rotated clockwise is provided by the triangular shaped summit basins bordering the blocks to the north oblique physiographic trends offsets in the summit platform and broad deflections in the southern slope of the arc we present a model for block rotation that involves translation of blocks parallel to an arc it is suggested that block rotation which appears to have accelerated in late cenozoic time is linked to 1 a shift in the euler pole for the pacific plate 2 the consequential start up of late cenozoic volcanism 3 improved interplate coupling instigated by sediment flooding of the aleutian trench and 4 westward subduction of northeast striking segments of the inactive kula pacific ridge

thermo mechanical role of a cambro ordovician paleorift during the variscan collision the ne margin of the bohemian massif abstract the stare m sto sm belt ne margin of the bohemian massif represents a preserved example of an intracontinental cambro ordovician rift that has been shortened during the variscan orogeny the rifted sequence consists of gabbroic intrusions and a leptyno amphibolite complex of cambro ordovician protolith age the latter suffered medium pressure granulite facies cambro ordovician metamorphism associated with extension in a continental protorift the variscan tectonometamorphic event is manifested by convergent orogenesis with syn convergent intrusion of a carboniferous tonalitic sill high temperature medium pressure compressional deformation of gabbros and high temperature metamorphism of peridotites numerical thermal and rheological modelling suggests that the sm paleorift after 140 ma of cooling represented a stiff heterogeneity between an easterly situated cadomian continent and a westerly lying variscan orogenic root zone carboniferous underplating of the paleorift by tonalitic magma was probably responsible for the thermal weakening of both the upper mantle and the mafic lower crust and the subsequent uplift of the remaining rigid part of the cambro ordovician crust all the units were later deformed in transpressional heterogeneous greenschist facies regime at a supracrustal level

tectonics of the hellenic trench a synthesis of sea beam and submersible observations abstract detailed bathymetric investigations were performed in four zones of the hellenic trench system in 1978 using the multi narrow beam echo sounder sea beam on r v jean charcot in 1979 a field study using the 3000 m submersible cyana was conducted in three of the four sites covered by sea beam we summarize here the principal tectonic results of these studies and publish the corresponding sea beam maps it is shown that compressional structures affect the sedimentary cover of the outer wall and trench whereas extensional tectonics is present on the inner wall in addition there is evidence that most of the upper sedimentary cover is not subducted but rather piles up in front of the trench to form the mediterranean ridge we suggest that the surface limit of the zone of mechanical decoupling occurs near the base of the inner wall we show that the gross structure of the trench as well as the strain pattern observed is controlled by the kinematics of subduction and we discuss its evolution since its initiation about 13 m y ago

a multi isotopic and trace element investigation of the cretaceous tertiary boundary layer at stevns klint denmark inferences for the origin and nature of siderophile and lithophile element geochemical anomalies abstract os sr nd and pb isotope data were collected from a profile across the cretaceous tertiary k t boundary layer at stevns klint denmark nd t 65 ma values from within the boundary layer fish clay are lower by 1 unit than those of the underlying maastrichtian limestone and the overlying danian chalk sequences systematic profile upward changes of pb sr and os isotopic compositions and concentrations in the boundary layer cannot be accounted for by in situ growth of daughter products since the sedimentation of the fish clay while os nd and pb isotopes indicate the admixing of less radiogenic components to the fish clay sr isotopes show elevated radiogenic values in the boundary layer relative to the carbonate sequences beneath and above it the sudden change in lithophile e g sr pb and nd isotope compositions at the base of the fish clay and profile upward trends of 87sr 86sr and 206pb 204pb ratios towards those of the overlying danian chalk are interpreted to reflect recovery from enhanced acid rain induced continental local weathering input to the seawater however a continental crustal source is invalid for the siderophile element os in the light of evidence from chromium isotopes for a cosmic origin of the platinum group elements pges and certain moderately siderophile elements cr ni co v in k t boundary sediments including stevns klint shukolyukov and lugmair science 282 1998 927 929 and supported by the finding of projectile debris bauluz et al earth planet sci lett 182 2000 127 136 and the occurrence of abundant ni rich spinel at many k t sites robin et al nature 363 1993 615 617 kyte nature 396 1998 237 239 we favor to explain the sudden drop of 187os 188os ratios from 0 210 to 0 160 at the k t boundary to derive from global fall out of extraterrestrial matter the present 186os 188os ratio of 0 119836 0 000004 measured in the basal layer of the fish clay is within the uncertainty a chondritic value we therefore exclude the possibility of a major contribution of pges to the sediment from iron meteorites chondrite normalized ru ir n ratios of 0 95 0 14 and os ir n ratios of 0 93 0 14 in the fish clay cannot distinguish between abundance ratios of different types of chondrites and strongly sub chondritic pt ir n ratios of 0 62 0 09 2 suggest differential pge remobilization through the sedimentary column and consequently the alteration of inter element ratios pges and the moderately siderophile elements cr ni v and co form an elemental association with systematically upward decreasing concentrations in the fish clay low co ni ratios of 0 12 in the fish clay relative to values of 0 35 in the over and underlying carbonate sequences support mixing of meteorite derived co ni 0 05 and terrestrial upper mantle crustal co ni 0 3 sources while lithophile element isotope data indicate an increased continental crustal input to the fish clay at the k t transition the uncertainty with respect to possible post depositional alteration of abundance patterns of siderophile and moderately siderophile elements though not affecting the chondritic isotopic composition of os does not allow confirmation of indications from chromium isotopes for a carbonaceous cv type meteorite as the preferred k t impactor type by shukolyukov and lugmair science 282 1998 927 929

quaternary hinterland evolution of the active banda arc surface uplift and neotectonic deformation recorded by coral terraces at kisar indonesia abstract coral terrace surveys and u series ages of coral yield a surface uplift rate of 0 5 m ka for kisar island which is an emergent island in the hinterland of the active banda arc continent collision based on this rate kisar first emerged from the ocean as recently as 450 ka these uplifted terraces are gently warped in a pattern of east west striking folds these folds are strike parallel to more developed thrust related folds of similar wavelength imaged by a seismic reflection profile just offshore this deformation shows that the emergence of kisar is influenced by forearc closure along the south dipping kisar thrust however the pinnacle shape of kisar and the protrusion of its metamorphic rocks through the forearc basin sediments also suggest a component of extrusion along shear zones or active doming coral encrusts the island coast in many locations over 100 m above sea level terrace morphology and coral ages are best explained by recognizing major surfaces as mostly growth terraces and minor terraces as mostly erosional into older terraces all reliable and referable coral u series ages determined by mc icp ms correlate with marine isotope stage mis 5e 118 128 ka the only unaltered coral samples are found below 6 m elevation however an unaltered tridacna giant clam shell in growth position at 95 m elevation yields a u series age of 195 31 ka which corresponds to mis 7 this age agrees with the best fit uplift model for the island loose deposits of unaltered coral fragments found at elevations between 8 and 20 m yield u series ages of

multibeam bathymetric survey and high resolution seismic investigations on the barbados ridge complex eastern caribbean a key to the knowledge and interpretation of an accretionary wedge abstract an important problem of active margin processes is the way accretionary wedges form in response to oceanic crust subduction new data have been obtained along the barbados ridge in front of the lesser antilles tertiary island arc they include high resolution multichannel seismic data and accurate sea beam bathymetric maps both have been used to propose a 3 dimensional interpretation of tectonic and sedimentary processes related to subduction such as oceanic off scraping ductile folding relative uplift and subsidence fore arc basin evolution and mud diapirism they all improved our knowledge of the evolution of accretionary prisms and more specifically of the barbados ridge development in plio quaternary time previous tectonic events are deduced from recently published geological data from barbados and the lesser antilles islands

character distribution and tectonic significance of accretionary terranes in the central alaska range the central part of the alaska range near mount mckinley is composed of nine separate tectonostratigraphic terranes that were accreted in southern alaska during late mesozoic time these terranes now form long linear fault bounded belts that are subparallel to the denali fault on the north but oblique to the fault on the south the postaccretion right lateral offset along the denali fault system is about 200 km from north to south the major terranes are 1 yukon tanana terrane metasedimentary and metavolcanic rocks mostly undated but including rocks of known late paleozoic age polymetamorphosed with terminal events in late mesozoic 2 pingston terrane isoclinally folded upper triassic deep water silty limestone quartzite and carbonaceous slate folded with upper paleozoic phyllite chert tuff and minor limestone 3 mckinley terrane upper paleozoic flysch chert and minor limestone intruded by large gabbro sills and dikes and overlain by thick piles of triassic porphyritic pillow lava the top of the section is thick sequence of upper mesozoic conglomerate flysch chert and phyllite 4 dillinger terrane very thick sequence of strongly folded lower paleozoic micaceous sandstone turbidites graptolitic shale and deep water limestone locally overlain unconformably by jurassic fossiliferous sandstone or triassic pillow basalt 5 windy terrane heterogeneous assemblage of serpentinite basalt tuff and chert ophiolite with paleozoic and mesozoic flysch and blocks of mid paleozoic fossiliferous limestone 6 mystic terrane predominantly upper paleozoic flysch and conglomerate but also includes lower paleozoic graptolitic shale pillow basalt and shallow water limestone and upper paleozoic fossiliferous limestone sandstone chert and undated pillow basalt 7 chulitna terrane upper devonian ophiolite overlain by upper paleozoic chert volcanic conglomerate limestone and flysch capped by lower triassic limestone and upper triassic red beds basalt and interbedded basalt and limestone later mesozoic rocks are sandstone chert and argillite 8 westfork terrane jurassic chert sandstone conglomerate and triassic and jurassic crystal tuff 9 broad pass terrane upper paleozoic chert tuff and argillite with blocks of devonian and older limestone locally associated with serpentinite these diverse terranes of mixed oceanic and continental affinities are now juxtaposed to form a complex sequence of folded and faulted rootless nappes major suture zones between them are occupied by intensely deformed upper mesozoic flysch all of the terranes differ markedly from wrangellia the southermost coherent accreted terrane in alaska paleomagnetic measurements from triassic rocks prove that wrangellia originated near the equator probably in the southern heimsphere paleolatitudes for terranes in the central alaska range are not yet determined but other lines of evidence biogeographic and lithologic suggest large scale northern transport for some terranes

the geometry of back arc thrusting along the eastern sunda arc indonesia constraints from earthquake and gravity data the flores earthquake of december 23 1978 represents the first seismological evidence for active back arc thrusting behind the eastern sunda arc this mb 5 8 earthquake occurred north of flores island where seismic reflection profiling has revealed back arc thrusting interpreted to be a reaction to compression of the arc following collision with the australian continent we investigate the source mechanism and depth of the flores earthquake by inversion of long period p waveforms and relocate the earthquake s epicenter incorporating arrival time data from local stations we find that this event occurred beneath the accretionary prism south of the back arc thrust zone at a depth of 11 5 km 7 km below the seafloor the best fitting fault plane solution is consistent with active southward thrusting of the floor of the flores basin beneath the volcanic arc along a 30 dipping fault plane the morphology of the thrust zone and the free air gravity profile over the flores basin in the epicentral region resemble those of oceanic trenches and the observed gravity field is best interpreted if the crust of the flores basin dips smoothly below the accreted wedge at an average angle of about 6 the position of the earthquake hypocenter south of the flores thrust and the fault plane solution suggest that the 1978 earthquake is analogous to thrust events at subduction zones and represents slip between the subducting and overriding plates the flores thrust is thus the surface expression of a deep seated thrust zone and may represent the initial stage of polarity reversal of the eastern sunda arc

microearthquake seismicity and fault plane solutions related to arc continent collision in the eastern sunda arc indonesia the collision of australia with the eastern sunda and banda arcs of indonesia represents a modern example of the early stages of arc continent collision in order to obtain a more detailed view of the tectonics of the collision zone a microearthquake survey was conducted around the savu sea the region that encompasses the transition from subduction of indian ocean lithosphere on the west to collision or subduction of australian continental litho sphere on the east intermediate depth earth quakes were concentrated near pantar island the easternmost active volcano of the sunda arc and outline a northwest dipping zone that strikes n65oe from 70 to 150 km depth the seismic zone probably marks the western edge of the collision zone and its more northerly strike and the orientations of nodal planes from fault plane solutions indicate a 25 o bend in the subducted slab to a depth of at least 150 km because the convergence rate and the timing of collision from geologic observations on timor suggest that the oart of the slab now at 150 km depth was at the trench when australia first collided with timor we infer that continental crust or at least rifted continental margin crust caused the bend in the convergent margin and has been subducted to 150 km depth fault plane solutions of several events show nearly vertical nodal planes that trend parallel to the strike of the seismic zone along which a northwest side down sense of dis placement is indicated these events are concentrated at the southern top side of the seismic zone and suggest that the subducted lithosphere is presently detaching in the 50 100 km depth range beneath the eastern savu sea fault plane solutions for earthquakes within the upper part of the detaching slab 70 100 km depth have p and t axes whose trends lie parallel to the strike of the seismic zone and horizontal southeast trending b axes these earthquakes are interpreted as due to stresses caused by the bend in the subducting plate

the marine187os 186os record of the past 80 million years we report new187os 186os data and re and os concentrations in metalliferous sediments from the pacific to construct a composite os isotope seawater evolution curve over the past 80 m y analyses of four samples of upper cretaceous age yield187os 186os values of between 3 and 6 5 and187re 186os values below 55 mass balance calculations indicate that the pronounced minimum of about 2 in the os isotope ratio of seawater at the k t boundary probably reflects the enormous input of cosmogenic material into the oceans by the k t impactor s following a rapid recovery to187os 186os of 3 5 at 63 ma data for the early and middle part of the cenozoic show an increase in187os 186os to about 6 at 15 ma variations in the isotopic composition of leachable os from slowly accumulating metalliferous sediments show large fluctuations over short time spans in contrast analyses of rapidly accumulating metalliferous carbonates do not exhibit the large oscillations observed in the pelagic clay leach data these results together with sediment leaching experiments indicate that dissolution of non hydrogenous os can occur during the hydrogen peroxide leach and demonstrate that os data from pelagic clay leachates do not always reflect the os isotopic composition of seawater new data for the late cenozoic further substantiate the rapid increase in the187os 186os of seawater during the past 15 ma we interpret the correlation between the marine sr and os isotope records during this time period as evidence that weathering within the drainage basin of the ganges brahmaputra river system is responsible for driving seawater sr and os toward more radiogenic isotopic compositions the positive correlation between87sr 86sr and u concentration the covariation of u and re concentrations and the high dissolved re u and sr concentrations found in the ganges brahmaputra river waters supports this interpretation accelerating uplift of many orogens worldwide over the past 15 ma especially during the last 5 ma could have contributed to the rapid increase in187os 186os from 6 to 8 5 over the past 15 ma prior to 15 ma the marine sr and os record are not tightly coupled the heterogeneous distribution of different lithologies within eroding terrains may play an important role in decoupling the supplies of radiogenic os and sr to the oceans and account for the periods of decoupling of the marine sr and os isotope records

a complete os excursion across a terrestrial cretaceous paleogene boundary at the west bijou site colorado with evidence for recent open system behavior abstract the few previously reported values of 187os 188os ratios from non marine k pg boundary sections are distinctly higher than the range of 187os 188os ratios measured in chondrites and the range of ratios predicted by models of physical mixing between chondrites and upper crust here re os data from the west bijou continental k pg boundary site located within the denver basin are used to better constrain the os isotopic composition of fallout from the chicxulub ejecta plume for the first time full vertical profiles of 187os 188os ratios and re and os concentrations across a continental k pg boundary section are reported within this section of lignite the lowest measured 187os 188os ratio 0 182 coincides with the k pg boundary interval as previously determined by palynology and the distribution of shocked quartz in a nearby outcrop however sediments with elevated os concentrations and measured 187os 188os below 0 23 extend over a 30 cm interval from 5 cm above the clay rich boundary interval 5 cm thick to 25 cm below maximum ir and os concentrations occur 3 cm and 10 cm below the k pg boundary respectively demonstrating greater diagenetic mobility of os relative to ir low 187os 188os above the k pg boundary suggests that accumulation of impact derived os at this site persisted after the impact event perhaps due to redistribution of impact debris in the surrounding area importantly calculated initial 187os 188os ratios throughout most of the section are impossibly low and require open system behavior likely in the form of re addition within the last 10 million years within the boundary interval re concentrations are among the lowest measured at bowring pit and calculated initial 187os 188os ratios are very similar to those previously reported from the western interior of the united states k pg boundary material 0 14 suggesting super chondritic 187os 188os is characteristic of the chicxulub ejecta plume beyond implications for our understanding of os as an impact tracer this study emphasizes the dramatic effect diagenesis can have on the re os system in organic rich environments especially through post depositional addition of re

187os 186os in marine manganese nodules and the constraints on the crustal geochemistries of rhenium and osmium during their formation marine manganese or more properly ferromanganese oxide nodules include and commonly concentrate elements dissolved in seawater this attribute makes them especially valuable recorders of the isotopic composition of elements supplied to the oceans having radiogenic isotopes for these can be used in exploring crustal composition and evolution1 5 from measurements of 187os 186os ratios in marine manganese nodules and comparison with the 143nd 144nd ratios of the same nodules we have calculated the os isotopic composition and the osmium and rhenium concentrations of the weatherable portion of the continental crust together with the proportions of ultra mafic and granitic rock that constitute this crustal mass

the moldanubian zone in the french massif central vosges schwarzwald and bohemian massif revisited differences and similarities in order to portray the main differences and similarities between the northeastern variscan segments french massif central fmc vosges black forest and bohemian massif bm we review their crustal scale architectures the specific rock associations and lithotectonic sequences as well as the ages of the main magmatic and metamorphic events this review demonstrates significant differences between the moldanubian domains in thebmand the fmc on this basis we propose distinguishing between the eastern and western moldanubian zones while the vosges black forest mountains are an intermediate section between the bm and the fmc the observed differences are the result of first the presence in the french segment of an early large scale accretionary system prior to the main variscan collision and second the duration of saxothuringian armorican subduction which generated long lived magmatic arc and back arc systems in the bohemian segment while the magmatic activity in the fmc was comparably short lived

back arc thrusting in the eastern sunda arc indonesia a consequence of arc continent collision the structure of the eastern sunda back arc region is dominated by two large north directed thrusts the wetar and flores thrusts and one or more minor thrusts which may represent early stages of subduction polarity reversal of the arc the relationships between thrusting and magmatic activity surface slopes cross arc faulting uplift forearc structure and collision by the australian continental margin provide constraints on the likely driving mechanisms of thrust formation driving mechanisms are considered in two broad categories one set of possible mechanisms originates within the upper plate and includes gravity sliding gravity spreading and magmatic intrusion the other set focuses on stress propagation between the upper and lower plates and for this arc system the major mechanism is that of continental collision while no one mechanism explains the range of observations we see in the back arc region all of these aspects play some role in the development of the thrusts we can eliminate upper plate mechanisms as primary driving forces because slope effects are similar in areas of both thrusting and nonthrusting in addition the wetar thrust shows a negative relation between thrusting and volcanism and side looking airborn radar images show no evidence of postvolcanic rifting that could accompany intrusion without volcanism collision between the arc and the australian continent provides the clearest driving mechanism but it does not explain the discontinuous nature or the particular location of the thrusts we suggest a sequence that appears to account for the data and to provide insight into the controversies often associated with other such thrust belts thrusting was initiated in those areas where the crust of the forearc region was thick a factor that facilitated stress propagation across the arc and where the crust of the back arc was thin which facilitated back arc thrusting thrusting concentrated at the base of the slope where the slope stress is maximum the early thrusting may have been helped also by thermal weakening of the crust due to volcanism as convergence proceeded volcanism waned then ceased in the eastern part of the arc the crust then strengthened and thrusting required higher stress to maintain and propagate this later stage should show higher seismic energy release which appears to be the case for the wetar thrust region displacement on the flores thrust appears to be matched by deflection of the volcanic arc we have estimated shortening in the back arc from interpretation of reflection profiles and for the flores thrust the value of approximately 30 km is consistent with that estimated for deflection of the arc compressional deformation continues west of the collision zone across the bali basin and into java it is reported in ne sumatra as well this deformation involves much less convergence than the eastern sunda thrusts and it may be related to magmatic processes to subduction of oceanic plateaus such as the roo rise or in the case of the bali basin to efficient lateral propagation of the flores thrust

inferences on sediment strength and fault friction from structures at the aleutian trench the structural relations of conjugate thrust faults and the master decollement imaged in a research level processed multichannel seismic reflection record across the eastern aleutian trench allow quantitative estimates of the sediment strength and friction across thrust boundaries of the subduction zone these estimates are derived from geometric relations between faults and from the overall geometry of accreted prisms they suggest the presence of high pore fluid pressures that result in low levels of friction along a master decollement and on the main bounding thrust faults in the accreted sediment therefore coupling in the immediate trench area apparently does not contribute significantly to the generation of horizontal compression upslope

re evaluation of the cablac limestone at its type area east timor revision of the miocene stratigraphy of timor abstract the cablac limestone widely recorded in timor has its type area on cablac mountain where it was regarded as a lower miocene shallow marine carbonate platform succession the bahaman like facies placed in the cablac limestone are now known to belong to the upper triassic lower jurassic rather than the lower miocene on the northern slopes of cablac mountain a crush breccia formerly regarded as the basal conglomerate of the formation is now considered to have developed along a high angle fault separating banda terrane units of asian affinity from an overthrust limestone stack containing units belonging to the gondwana and australian margin megasequences the cablac breccia includes rock fragments that were probably derived locally from these tectonostratigraphic units after terrane emplacement and overthrusting clasts include peloid and oolitic limestones of the upper triassic lower jurassic derived from the gondwana megasequence deep water carbonate pelagites of the cretaceous and paleogene derived from the australian margin megasequence upper oligocene lower miocene te letter stage shallow water limestone derived from the banda terrane and a younger neogene calcarenite containing clasts of mixed tectonostratigraphic affinity there is no evidence for significant sedimentary or tectonic transport of clasts that form the breccia the clast types and the present understanding of the geological history of timor suggest that the crush breccia formed late in the plio pleistocene uplift history of timor it is not the basal conglomerate of the cablac limestone however the clasts of an upper oligocene lower miocene limestone found in the breccia suggest that a shallow marine limestone unit of this age either outcrops in the region and has not been detected in the field or has been eroded completely during late neogene uplift the clasts are similar in age and lithology to an upper oligocene lower miocene formation that unconformably overlies a metamorphic complex in the booi region of west timor similar to the lolotoi metamorphic complex banda terrane that is juxtaposed against the crush breccia of cablac mountain the cablac limestone at its type area includes a mixed assemblage of carbonate rock units ranging in age from triassic to plio pleistocene and representing diverse facies as a formation the name cablac limestone should be discarded for a cenozoic unit the upper oligocene lower miocene shallow water limestone unit that is typified by outcrops in the booi region of west timor and that has contributed to clasts in the cablac breccia is informally named the booi limestone it is considered part of the allochthonous banda terrane of asian affinity and represents the only shallow marine lower miocene unit known from timor the only other miocene sedimentary unit known from timor includes carbonate pelagites designated the kolbano beds probably deposited on an australian continental terrace at water depths between 1000 and 3000 m on the northeastern edge of cablac mountain oolitic limestone and associated units of the gondwana megasequence the kolbano beds of the australian margin megasequence and the booi limestone and associated metasediments of the banda terrane were juxtaposed by a plio pleistocene high angle fault along which the cablac crush breccia formed

osmium tungsten and chromium isotopes in sediments and in ni rich spinel at the k t boundary signature of a chondritic impactor it is now established that a large extraterrestrial object hit the earth at the end of the cretaceous period about 65 ma ago we have investigated re os hf w and mn cr isotope systems in sediments from the cretaceous and the paleogene in order to characterize the type of impactor within the cretaceous tertiary k t boundary layer extraterrestrial material is mixed with terrestrial material causing a dilution of the extraterrestrial isotope signature that is difficult to quantify a phase essentially composed of ni rich spinel formed in the atmosphere mainly from melted projectile material is likely to contain the extraterrestrial isotopic signature of the impactor we show that the analysis of spinel is indeed the best approach to determine the initial isotope composition of the impactor and that w and cr isotopes confirm that the projectile was a carbonaceous chondrite