miocene stable isotopic stratigraphy and magnetostratigraphy of buff bay jamaica previously reported biostratigraphic relationships from middle upper miocene sections exposed near buff bay jamaica 18 n tropical bioprovince differ from the subtropical north atlantic sites 563 and 558 time scales for this interval rely on correlations established at these subtropical sites and the differences with the tropical section have implications to global correlations planktonic foraminiferal zones n13 and n15 are thick at buff bay but are virtually absent at sites 563 and 558 nannofossil zone nn9 is associated with zone n15 and uppermost zone n14 at buff bay but is associated with zone n16 at the other sites magnetostratigraphic data presented here further complicate the interpretation zone nn9 is associated with a thick normal magnetozone at sites 563 and 558 at buff bay it is associated with a thick reversed magnetozone although a secondary magnetization at buff bay makes it difficult to identify confidently miocene normal magnetozones the thick reversed magnetozone most likely represents the paleomagnetic field and correlates with chron c5r the magnetobiostratigraphic relationships require either diachrony of taxa or two mutually exclusive hiatuses in jamaica and the north atlantic we address this problem by analyzing benthic foraminiferal 18 o and 13 c from the buff bay section these isotopic data allow us to evaluate three hypotheses that reconcile the magneto bio and isotopic stratigraphic data and conclude that the first and last occurrences of five taxa were diachronous by 0 3 0 5 m y between tropical and subtropical locations this requires revised age estimates for late middle to early late miocene biostratigraphic datum levels we suggest that the ranges of several taxa are useful for endemic tropical or subtropical zonations but correlations between the low and midlatitudes were affected by an increase in latitudinal thermal gradients during the late middle miocene however we admit that further studies are needed before this issue is resolved

evolving geographic patterns of cenozoic magmatism in the north american cordillera the temporal and spatial association of magmatism and metamorphic core complexes four maps are presented here that show the location and extent of magmatic fields between eastern alaska and northern mexico during the successive time intervals of 55 40 40 25 25 10 and 10 0 ma and four others show the distribution of metamorphic core complexes during the same cenozoic time intervals the maps are based on u s geological survey and canadian cordilleran data bases contining about 6000 isotopic dates and extensive literature review for nearly 60 ma the development of metamorphic core complexes has coincided with the locus of a really extensive and voluminous intermediate composition magmatic fields the association is suggestive of a close link between magmatism and core complex formation namely that magma directly and indirectly lowers the strength of the crust magmatism thus controls the location and timing of core complex formation the stresses responsible may be inherited from mesozoic crustal thickening locally created by uplift and magmatic thickening of the crust and imposed by the global pattern of plate motions and driving forces since the miocene rates of magmatism extension and core complex formation have declined the modern basin and range province is not a suitable model for the situation that existed during major magmatic culminations the singular event of early miocene time the merging of two large magmatic fields extinguishing the laramide magmatic gap explains several disconnected observations the hyperextension episode of the colorado river corridor rapid reorientation of stress patterns across much of western north america and subsequent rapid tectonic movements in california magma triggered breakup of western north america lithosphere coincided with development of the san andreas transform system thermal destruction of the laramide magmatic gap created a california microplate about 22 ma ago that moved rapidly away from north america thus two plate tectonic processes thermal destruction of the lithosphere bridge and northward growth of a transform system interacted to produce miocene and later tectonic patterns and events

monazite xenotime garnet equilibrium in metapelites and a new monazite garnet thermometer prograde suites of pelitic rocks were examined with electron microprobe introduction and laser ablation inductively coupled plasma mass spectrometry to monazite ce la th po4 plays an important role in determine the systematics of element partitioning between coexisting studies of igneous and metamorphic petrogenesis foremonazite xenotime and garnet monazite grains that grew in most monazite is used to date specific events in a equilibrium with xenotime are enriched in y and dy compared with petrogenetic sequence e g parrish 1990 harrison et al monazite that grew in xenotime absent assemblages y and heavy 1997 hawkins bowring 1999 foster et al 2000 rare earth element contents of monazite coexisting with xenotime monazite may contain a large percentage of the sample increase with rising temperature monazite xenotime y gd and rare earth element ree budget and thus exert an y dy partitioning is systematic within a metamorphic grade and important influence on the evolution of melt composition increases slightly with increasing metamorphic grade suggesting wark miller 1993 bea 1996 the thermobarometric that monazite xenotime pairs approached partitioning equilibrium potential of monazite coexisting with xenotime has been garnet and monazite in both xenotime bearing and xenotime absent recognized leading to calibration of monazite geoassemblages show a strong r 0 94 systematic relationship thermometers gratz heinrich 1997 1998 heinrich between inverse temperature and ln keq for the net transfer equiet al 1997 andrehs heinrich 1998 in addition the librium yag oh ap 25 4 qtz 5 4 grs 5 4 an recognition that nearly all lead in monazite is radiogenic 3ypo4 mnz 1 2h2o suggesting that garnet and monazite parrish 1990 montel et al 1994 has led to development crystallized in compositional equilibrium the following temand application of electron microprobe monazite dating perature keq relationship for the equilibrium above has been derived techniques suzuki adachi 1991 montel et al 1996 williams et al 1999 t c 1 45p bars 447772 32052 567 40 rln keq 273 15 full realization of monazite as a thermochronometer requires detailed understanding of the specific reactions responsible for its formation previous studies of monazite with a precision of some 30 c for temperature estimates our petrogenesis have focused on monazite compositional observations suggest that major and accessory phases interact in a zonation zhu o nions 1999a 1999b monazite coupled fashion during metamorphism and also approach a state growth kinetics ayers et al 1999 behavior of monazite of compositional equilibrium as reactions proceed during hydrothermal alteration poitrasson et al 1996 crowley ghent 1999 and melting events watt 1995 or to a limited extent the influence of major phase mineral assemblage in monazite reactivity zhu

basin evolution in oceanic volcanic plateaux seismic reflection evidence from the kerguelen plateau south indian ocean abstract multi channel seismic reflection data from the kerguelen plateau south indian ocean are used to describe the evolution of sedimentary basins on this large feature a number of sedimentary basins are present the largest of which the raggatt basin is over 150 km wide all the sedimentary basins are underlain by basement which shows internal reflection indicative of volcanic flows these reflections define basement wedges and synforms overlain by a marked erosional unconformity which developed early in the history of the plateau intra basement synforms acquired their initial shape during the emplacement of the plateau before the erosional episode the lateral continuity of the reflectors across most of the basement synforms suggests that the volcanic sources of the plateau lie outside of them the basement synforms may represent low areas between elevated volcanoes subsidence of large volcanoes may have caused the subsidence of these basins as flexural moats if so superposition of the subsidence of several volcanoes on the plateau has resulted in the variable shape of the basins it also induced subsidence of basins relative to some adjacent volcanoes this relative subsidence seems to have been active throughout the lifetime of this early cretaceous plateau as indicated by onlaps within the sediments and the small bathymetric deeps associated with the basins the observation that the sources of the volcanic flows lie outside the basement synforms is not consistent with existing models which explain the origin of dipping basement reflectors along passive margins

the mesozoic cenozoic tectonothermal evolution of the ruby mountains east humboldt range nevada a cordilleran metamorphic core complex a combined k ar and 40ar 39ar geochronological study of the ruby mountains east humboldt range nevada a cordilleran metamorphic core complex has revealed a complex tectonothermal history the three structural subdivisions of the core complex migmatitic core mylonitic zone and cover share magmatic and deformational histories in part but also display important contrasts in structural style metamorphic paragenesis and thermal history during the late jurassic ca 160 ma miogeoclinal rocks were polyphase deformed metamorphosed under amphibolite facies conditions and pervasively intruded by peraluminous granitic magmas thereby forming an igneous and metamorphic complex late jurassic muscovite granite porphyry plugs also intruded unmetamorphosed cover rocks late cretaceous cooling of the igneous and metamorphic complex is locally suggested by poorly resolved cretaceous incremental release 40ar 39ar hornblende spectra and by u pb monazite ages from pelitic schist exposed in the northern ruby mountains between ca 45 ma and 20 ma the igneous and metamorphic rocks experienced an episode of complex extensional tectonism which involved the development of low angle normal sense simple shear zones as a result rocks metamorphosed at depth in the mesozoic were translated to higher crustal levels which resulted in cooling through temperatures required for the intracrystalline retention of argon within minerals near concordance of hornblende and biotite plateau ages from the migmatitic core of the complex suggest relatively rapid exhumation however it is not certain if this cooling followed a prolonged maintenance at elevated temperatures after the ca 160 ma metamorphism or if distinct locally superposed late mesozoic and or middle tertiary thermal overprints had affected the cooling history a major extensional simple shear zone is marked by an approximately 1 kilometer thick mylonite zone that experienced late kinematic superposed brittle deformation a geometric consequence of the development of low angle extensional fault zones is the eventual translation of colder rocks over hotter rocks such a history is supported by the 40ar 39ar data for example mylonitic mafic orthogneisses from a major allochthon record 40ar 39ar hornblende plateau ages between 44 and 48 ma and biotite plateau ages between 32 and 33 ma and are structurally positioned above mylonitic rocks of the northern ruby mountains which record biotite cooling ages of ca 22 24 ma such relations suggest either multiple episodes of tertiary mylonitization or a protracted multiphase history of mylonitization however after ca 20 ma all of the presently exposed mylonitic rocks were maintained below 100 c and have only experienced upper crustal brittle deformation this article contains supplementary material

cenozoic and late mesozoic stratigraphy and subsidence history of the norwegian margin the stratigraphy of the passive continental margin off norway is mapped utilizing both deep sea drilling project dsdp data at five sites on the outer margin and multichannel seismic mcs data collected by lamont doherty geological observatory distinct units in the acoustic stratigraphy are tied to biostratigraphic and lithostratigraphic units in the drill data and mapped throughout most of the margin stratigraphy of the pre lower eocene section not encountered in drilling is inferred from depositional patterns and correlation with the continental shelf sequences the stratigraphy of which has been developed independently from detailed surveys the margin can be divided into three physiographic provinces two broad shelf and slope regions separated by the jan mayen fracture zone and the steep narrow shelf and slope off the lofoten islands each experienced a separate cenozoic subsidence history the voring plateau is partly underlain by oceanic crust it subsided nearly 1 000 m less than expected from thermal contraction but it did so following the development of a relatively deep water environment in the paleocene north of the voring plateau the margin may have subsided a few hundred metres more than thermal studies had predicted there is no evidence for a phase of extensional deformation associated with the inception of spreading the youngest faulted level occurs at the late jurassic level preceding sea floor spreading by nearly 100 ma and deformation in the cretaceous section is characterized by large uplift structures that may be of halokinetic origin the physiographic provinces as well as the cenozoic subsidence patterns along the margin appear to be related to the style and extent of mesozoic basin development and tectonics along the margin

time transgressive late cenozoic radiolarian events of the equatorial indo pacific a biostratigraphic study of late cenozoic radiolaria in the equatorial indo pacific shows an asymmetrical distribution between synchronous and diachronous events a majority of synchronous events 15 out of 19 are last occurrences the majority of diachronous events 10 out of 13 are first occurrences extinctions may therefore be preferable to first occurrences in the selection of datum levels for the definition of biostratigraphic zonations and for correlation control within global time scales diachronous equatorial radiolarian events span 1 to 2 million years several orders of magnitude longer than the nominal mixing time of the oceans suggesting that the biological and physical exchange processes associated with speciation events may not follow simple advective mixing models

vertical coupling and decoupling in the lithosphere recent advancements in the understanding of mountain belts have focused on vertical coupling of the lithospheric layers this volume describes the extent to which observed or inferred sub horizontal coupling or attachment zones provide vertical kinematic linkage between rheologically distinct layers in the continental lithosphere a common theme is whether the deformation which partitioned differently in each layer is linked kinematically across attachment zones and driven by flow in the deeper crust and mantle lithosphere the papers are divided into six sections the first analyses the extent to which mantle flow controls deformation of the overlying layers the vertical axis block rotations section uses geological and palaeomagnetic data to constrain the role and magnitude of basal shear across mid crustal attachment zones the lower crustal flow and topography section addresses the time dependent development of orogenic plateaux and their role in the orogenic cycle multiple examples of the spatial and temporal development of lithospheric coupling are given in both the orogenic examples and subduction examples sections finally rheological constraints of vertical coupling in the lithosphere are investigated

treitel ridge a unique inside corner hogback on the west flank of extinct aegir spreading ridge norway basin abstract detailed shipborne mapping sidescan sonar swath bathymetry seismic reflection 3 5 khz profiling gravity and magnetics of the extinct aegir spreading axis ca 55 25 ma in the norway basin revealed a highly unusual in a mor setting narrow 130 km long asymmetrical basement hogback treitel ridge tr on the southern west flank of the extinct rift close to its junction with the iceland faeroe aseismic ridge and the putative paleo iceland hotspot center the ridge seems to continue north another 130 km in the form of a wider more irregular basement rise for a combined length of ca 260 km the southern narrow part of tr approximately following or slightly younger than chron 18n ca 39 ma attains a basement relief of ca 1000 m in the south its summit descending gradually 1 7 100 towards the ne although an east flank conjugate appears generally lacking along both northern and southern parts of tr asymmetry of the southern portion steeper slope facing towards the extinct rift valley suggests volcanic formation and axially dipping normal faulting at or near the active plate boundary the possibility of continental crust extending close to the southern end of tr may also be important because some of this crust may have been remelted at depth contributing magma to feed tr eruptions southern tr may record a brief 0 1 1 m y inside corner tectonic event probably a flexurally compensated normal faulted offset in effect a mega abyssal hill we suggest tr was probably not an expression of change in iceland hotspot activity or somehow related to nearby continental crust but rather represents the volcano tectonic response of the aegir accretion axis at the time it became the eastern edge of the new jan mayen micro plate to extinction of the mid labrador sea spreading center as proposed by nunns 1983 the annexation of the greenland plate by the north american plate forced a plate boundary reorganization east of greenland with northward propagation of the mid atlantic ridge rift along the greenland margin to form the jan mayen micro plate simultaneously two ca 107 trending transforms developed from the previously 146 trending faeroes fracture zone both tr formation and fracture zone trend changes may record aegir ridge response to this major plate kinematic reorganization because tr was formed during a short interval spreading along the mid labrador sea ridge may have ceased abruptly as well

allochthonous archean basement in the northern east humboldt range nevada archean basement rocks occur in the core of a fold nappe in the northern east humboldt range nevada the archean rocks consist primarily of plutonic orthogneiss and metasedimentary paragneiss both units contain numerous amphibolitic bodies interpreted as mafic intrusions u pb zircon studies indicate that the orthogneiss unit is at least 2520 110 ma and therefore is an example of an archean granitoid these data extend the known distribution of precambrian basement rocks into northeastern nevada and provide new constraints on the delineation of the archean wyoming province the basement rocks are surrounded by upper amphibolite facies metasedimentary rocks correlated with late precambrian to devonian rocks of the eastern great basin miogeoclinal sequence the contact between the archean basement rocks and the metasedimentary mio geoclinal sequence is interpreted as a premetamorphic and prefolding low angle fault the age and prefolding geometry of this low angle fault are currently poorly constrained but it may record important mesozoic crustal shortening in the hinterland of the sevier orogenic belt the age of nappe formation is not yet fully established but at least some isoclinal folds in the area were formed in the tertiary and are kinematically linked to a major extensional deformation

thermobarometric modelling of zircon and monazite growth in melt bearing systems examples using model metapelitic and metapsammitic granulites u pb age data collected from zircon and monazite are used to draw fundamental inferences about tectonic processes in the earth despite the emphasis placed on zircon and monazite ages the understanding of how to relate the timing of growth of zircon and monazite to an evolving rock system remains in its infancy in addition few studies have presented large datasets of geochronological data from zircon and monazite occurring in the same metamorphic rock sample such information is crucial for understanding the growth of zircon relative to monazite in a systematic and predictive manner as per this study the data that exist support the generally held conception that zircon ages tend to be older than monazite ages within the same rock here experimental data for zircon and monazite saturation in melt bearing rocks are integrated with phase diagram calculations the calculations constrain the dissolution and growth behaviour of zircon and monazite with respect to evolving pressure temperature and silicate mineral assemblages in high grade melt bearing metasedimentary rocks several key results emerge from this modelling first that in aluminous metapelitic rocks i e garnet cordierite sillimanite assemblages zircon ages are older than monazite ages in the same rock second that the growth rate of accessory minerals is nonlinear and much higher at and near saturation than at lower temperatures and third that the difference in zircon and monazite ages from the same rock may be ascribed to differences in the temperature s at which zircon and monazite grow rather than differences in closure temperature systematics using our methodology the cooling rate of granulites from the reynolds range central australia have been constrained at 4 c myr 1 this study serves as a first pass template on which further research in applying the technique to a field study can be based

history of the cretaceous osbourn spreading center the osbourn trough is a fossil spreading center that rifted apart the manihiki and hikurangi plateaus during cretaceous time previous models of the osbourn spreading center are based on data collected near the trough axis and therefore only constrain the history of the osbourn spreading center during the last few ma of spreading our data set includes multibeam data collected northward to the manihiki plateau allowing us to examine seafloor morphology created during the entire active period of the osbourn spreading center as well as several additional multibeam data sets that provide the opportunity to examine the relationship between the osbourn paleospreading center and the cretaceous pacific phoenix ridge the axial gravity of the trough is similar to the gravity found at other extinct slow intermediate spreading rate ridges magnetic field measurements indicate that spreading at the trough ceased during chron c34 abyssal hill trends indicate that spreading during the early history of the osbourn spreading center occurred at 15 20 the east west component of this spreading explains the modern east west offset of the manihiki and hikurangi plateaus spreading rotated to 2 5 prior to extinction abyssal hill rms amplitudes show that a decrease in spreading rate from 7 cm yr to 2 6 cm yr full spreading rate occurred 2 6 ma prior to ridge extinction our data analysis is unable to determine the exact spreading rate of the osbourn spreading center prior to the slowing event the temporal constraints provided by our data show that the osbourn spreading center ceased spreading prior to 87 ma or 93 ma depending on whether the manihiki and hikurangi plateaus rifted at 115 ma or 121 ma our model resolves the conflict between regional models of osbourn spreading with models based on trough characteristics by showing that spreading at the osbourn spreading center was decoupled from pacific phoenix spreading

early cenozoic crust at the norwegian continental margin and the conjugate jan mayen ridge multichannel seismic profiles at the voring plateau margin off norway and the northern jan mayen ridge have provided a framework for the early tertiary evolutionary history of these areas which were adjacent prior to the opening of the norwegian sea we propose that their paleogene evolution is quite similar and that the two acoustic basement reflectors ee and jo were formed in the early eocene by extrusion of flow basalts from the time of opening to about anomaly 23 time oceanic seafloor was generated by icelandic type spreading during that time the central jan mayen fracture was the main transform the seaward dipping reflector sequences are flow basalts generated by the icelandic spreading the existence of less well developed dipping sequences seaward of the primary ones probably reflects that these regions submerged later because of higher initial elevation during the early eocene subaerial spreading episode which is related to the north atlantic volcanic province basalt flows also covered the adjacent thinned and heavily intruded continental crust it is proposed that the continent ocean boundary is located just landward of anomaly 24b a short distance seaward of the voring plateau escarpment reflector k is only recognized landward of this boundary in a region of transitional crust the escarpment experienced syndepositional faulting and the most elevated parts of the voring marginal high did not subside below shallow water depths before oligocene miocene time we suggest that the voring margin and the jan mayen ridge represent volcanic passive margins the evolution of this margin type may relate to initial uplift due to rifting in previously thinned crust

the oceanic crustal structure at the extinct slow to ultraslow labrador sea spreading center two seismic refraction lines were acquired along and across the extinct labrador sea spreading center during the seismic investigations off greenland newfoundland and labrador 2009 cruise we derived two p wave velocity models using both forward modeling rayinvr and traveltime tomography inversion tomo2d with good ray coverage down to the mantle slow spreading paleocene oceanic crust has a thickness of 5 km while the eocene crust created by ultraslow spreading is as thin as 3 5 km the upper crustal velocity is affected by fracturation due to a dominant tectonic extension during the waning stage of spreading with a velocity drop of 0 5 to 1 km s when compared to paleocene upper crustal velocities 5 2 6 0 km s the overall crustal structure is similar to active ultraslow spreading centers like the mohns ridge or the south west indian ridge with lower crustal velocities of 6 0 7 0 km s an oceanic core complex is imaged on a 50 km long segment of the ridge perpendicular line with serpentinized peridotites 7 3 7 9 km s found 1 5 km below the basement the second ridge parallel line also shows extremely thin crust in the extinct axial valley where 8 km s mantle velocity is imaged just 1 5 km below the basement this thin crust is interpreted as crust formed by ultraslow spreading which was thinned by tectonic extension

monazite behaviour and age significance in poly metamorphic high grade terrains a case study from the western musgrave block central australia abstract integrated in situ textural chemical and electron microprobe age analysis of monazite grains in a migmatitic metapelitic gneiss from the western musgrave block central australia has identified evidence for multiple events of growth and recrystallisation during poly metamorphism in the mesoproterozoic garnet sillimanite bearing metapelite underwent partial melting and segregation to palaeosome and leucosome during metamorphism between 1330 and 1296 ma with monazite grains in leucosome recording crystallisation at 1300 ma monazite breakdown during melting is inferred to have occurred in the palaeosome during a subsequent granulite facies event at 1200 ma deformation and metamorphism of leucosome and palaeosome resulted in partial disturbance of ages and potential minor growth on 1300 ma monazite in leucosome growth of new high y hree monazite in palaeosome domains occurred during garnet breakdown in the presence of sillimanite to cordierite and spinel as a result of post peak isothermal decompression diffusive enrichment of resorbed garnet rims in y hree suggests garnet breakdown occurred slower than volume diffusion of ree monazite in both palaeosome and leucosome were subsequently partially to penetratively recrystallised during a retrogression event that is suggested to have occurred at 1150 1130 ma the intensity of recrystallisation and disturbance of ages appears linked to proximity to retrogressed garnet porphyroblasts and their occurrence in the relatively reactive or fertile local environments provided by the palaeosome mesosome volumes which caused localised changes in retrogressive fluids towards compositions more aggressive to monazite like reaction textures it is apparent that domainal equilibrium and reaction may control or at least strongly influence monazite ree and u th pb chemistry and hence ages

five generations of monazite in langtang gneisses implications for chronology of the himalayan metamorphic core monazite grains from greater himalayan sequence gneisses langtang valley nepal were chemically mapped and then dated in situ via th pb ion microprobe analysis correlation of ages and chemistry reveals at least five different generations of monazite ranging from c 9 to 300 ma petrological models of monazite chemistry provide a link between these generations and the thermal evolution of these rocks yielding an age for the melting of greater himalayan rocks within the main central thrust sheet c 16 ma and for the timing of thrust sheet emplacement that are younger than commonly viewed chemical characterization of monazite is vital prior to chronological microanalysis and many ages previously reported for monazite from the greater himalayan sequence are interpretationally ambiguous

dynamic models for metamorphic core complex formation and scaling the role of unchannelized collapse of thickened continental crust abstract metamorphic core complexes at collisions between cratons and softer terranes such as in the northern north american cordillera share a set of characteristic features including spatial and temporal association of ductile mid crustal deformation with brittle normal faulting spatial coincidence with prior crustal thickening characteristic spatial scaling and limited duration and extent of deformation these properties are reproduced in numerical solutions for gravity driven collapse of a viscous crustal region under conditions where vertical stress is continuous through thickened lithosphere rigid deformable conditions such solutions allow inversion for effective mechanical properties and crustal geometry from direct observations of aspect ratio and exhumation velocity in the northern rockies core complex geometry is consistent with a twofold decrease in viscosity of the thickened cordilleran crustal welt

the oligocene miocene boundary in the south pacific qualitative and quantitative planktonic foraminiferal trends have been examined across the oligocene miocene transition in three deep sea drilling project sites dsdp 289 208 206 in the south pacific ranging from the equator to temperate regions primary objectives were to determine main changes in planktonic foraminiferal assemblages over the oligocene miocene transition of available planktonic foraminiferal datums in the south pacific sequences globoquadrina dehiscens is the most useful for designation and correlation of the oligocene miocene boundary the stratotype aquitanian stage earliest miocene in france contains gq dehiscens although the base of the aquitanian stratotype is marked by an unconformity biostratigraphic data elsewhere within the basin suggest that the evolutionary appearance of gq dehiscens occurred close to the time of the oldest stratotypic sediments therefore the evolutionary appearance of gq dehiscens seems to represent one of the most reliable datums for interregional correlation of the oligocene miocene boundary one that occurs within the biostratigraphic range of globorotalia fohsella kugleri and marks the boundary between zones n4a and n4b globigerinoides first evolved during the late oligocene and is not coincident with the oligocene miocene boundary unlike the eocene oligocene boundary the oligocene miocene boundary is not marked by a crisis in the oligocene planktonic foraminiferal assemblages most oligocene forms continue their range upward into the early miocene where most are replaced by typical neogene forms the only important oligocene phylogenetic lineage to be essentially eliminated during the oligocene miocene transition is catapsydrax the last important forms of which disappeared during earliest miocene evolution of gq dehiscens effectively heralds the beginning of major evolutionary radiations in planktonic foraminifera that led to the distinctive neogene phylogenies appearance of this form was followed by evolutionary radiation of globigerinoides into a number of species initial evolution of globorotalia globoconella incognita which forms the earliest ancestral form of globoconella evolution of the globigerina woodi group sphaeroidinellopsis disjuncta the ancestral stock of the sphaeroidinellopsis sphaeroidinella lineage globorotalia fohsella peripheroronda from gr f kugleri representing the continued development toward the classic fohsella forms and typical forms of the dentoglobigerina stock represented by dentoglobigerina altispira these evolutionary radiations are reflected by a general increase in simple species diversity through early miocene at all three sites species diversity is lowest in the interval near the oligocene miocene boundary among the three sites examined diversity is highest in warm subtropical site 208 because at this latitude faunas include both tropical and temperate forms the earliest miocene is marked by high frequencies of gr f kugleri in tropical areas and gq dehiscens in warm subtropical to temperate areas

the aegir ridge structure of an extinct spreading axis in 1988 a reflection seismic survey was carried out by the alfred wegener institute for polar and marine research to study structure and development of the aegir ridge an extinct spreading axis in the norway basin a number of volcanolike structures were detected in the central ridge the emerging lava flows dip away from their source in ridge strike perpendicular to ridge strike we observed lava flows dipping towards the ridge center this pattern might represent a more general flow pattern at mid ocean ridges after spreading ceased sedimentation resulted in the deposition of 800 ms two way time twt to 1300 ms twt about 900 1400 m sediments in the central ridge distribution of the sediments indicate subsidence of the central ridge probably due to cooling after termination of spreading this subsidence started no later than late oligocene or early miocene we detected an area where migration of a fluid phase led to obstruction of the sedimentary layering and the formation of mounds and faults at the ocean bottom several sources for the fluid phase are discussed

early opening history of the north atlantic i structure and origin of the faeroe shetland escarpment summary marine geophysical surveys show that the escarpment is the buried feather edge of a thick pile of flood basalts of early eocene age overlying a thinner more widespread layer of basalts of late palaeocene age the escarpment does not therefore define the continent ocean boundary in the southern norwegian sea but instead marks the contemporary shoreline separating terrestrially erupted basalt flows in the north from a restricted shallow water shelf to the south the basalts overlie 5 6 km of mesozoic sediments which have completely buried a large conical flat topped seamount of similar dimensions to anton dohrn and rosemary bank we call this newly postulated body the brendan seamount the mesozoic sediments are at least as old as early cretaceous in age therefore precluding the possibility that the mid cretaceous seafloor spreading episode supposed by some to have created the rockall trough could have also created the thin crust inferred to underlie the faeroe shetland trough and mbre basin

a rare earth element study of complex zircons from early archaean amitsoq gneisses godthabsfjord south west greenland we present high spatial resolution ion microprobe rare earth element ree data for discrete growth phases of complex polyphase zircons from early archaean amitsoq gneisses outer godthabsfjord sw greenland in matsuda diagrams the two major growth phases 3 8 ga cores and ca 3 65 ga rims have steep positive slopes from la to lu prominent positive ce anomalies and negative eu anomalies that are consistent with growth in a melt exceptions to this are non cathodolurnmescent zircon developed between the cores and rims sometimes truncating zoning in the cores and late archaean prismatic tip overgrowths both of which exhibit flatter light ree lree patterns and have small or no eu anomaly which we interpret as the result of metamorphism and or small degree isolated partial melting our data support previous interpretations that the ca 3 65 ga zircon phase was generated in a melt with the 3 8 ga phase representing either original protolith zircons in a large degree partial melt or inherited zircons in an introduced magma regardless which of these two interpretations is correct for these and similar rocks in the outer godthabsfjord the 3 65 ga event will have profoundly affected isotopic systems and obscured beyond recognition any earlier igneous features such as cross cutting relationships which may only be assigned a minimum 3 65 ga age c 2003 elsevier science b v all rights reserved

the norway basin revisited from continental breakup to spreading ridge extinction abstract we re evaluate the structure and spreading evolution of the southeastern norway basin nb based on a new high resolution aeromagnetic survey nb 07 the survey covers a complete oceanic spreading segment from the continental oceanic transition of the more margin off norway to the aborted aegir ridge the new survey documents a transform margin an orthogonally rifted segment and an oblique sheared volcanic margin formed during the onset of breakup observed from the east jan mayen fracture zone to the faroe platform the detailed fabric of the nb revealed by the new data indicates that two distinct tectonic phases have reshaped the basin before the cessation of seafloor spreading and abortion of the aegir ridge in the late oligocene after continental breakup phase i from c24 to c21r 52 to 49 ma marks the earliest phase of spreading probably initiated in the central and outer part of the more basin during this period competing oceanic segments led to the formation of overlapping systems and pseudo fault development we observe a significant change in the nb s oceanic spreading system in the late early eocene and based on observations from surrounding areas we suggest that this is a record of a major tectonic event in the norwegian greenland sea around c21r 49 47 9 ma during phase ii from c21r to c10 48 to 28 ma of nb s development spreading rates decreased spreading direction changed and the number of faulting with large displacement increased leading to the formation of unexpected n s oriented oceanic fracture zones the fan shaped development of the spreading system initiated around c21r 49 47 9 ma instead of c18 c17 40 38 ma or c24 53 3 52 3 ma as previously proposed these new observations were used to re evaluate the tectonic evolution of the norwegian greenland sea and discuss some implications on the syn and post breakup development of the surrounding continental margins and the evolution of the jan mayen microcontinent

petrogenesis of garnet two mica granites in the ruby mountains nevada garnet two mica granites of jurassic 160 3 m y and cretaceous 83 1 3 m y ages intrude amphibolite facies metasedimentary rocks of precambrian age and lower paleozoic sedimentary rocks in the northern part of the ruby mountains nevada high initial 87sr 86sr and high values of 18o for minerals suggest a dominantly sedimentary source for the magmas total rare earth contents are low and less fractionated cen ybn 10 than granitic rocks of similar composition in the sierra nevada and show a negative eu anomaly eu eu 0 57 these data are consistent with melting in the stability field of plagioclase with little contribution to the melt from refractory minerals such as zircon sphene and apatite minor euhedral garnets are manganese rich and magnesium poor al60sp34py3 6gr2 8 compared to garnet al79sp6py13gr2 in the intruded metamorphic rocks and show an increase in manganese and a decrease in calcium in the outer few microns fractionation of magnesium iron between garnet and biotite and of sodium potassium between k feldspar and plagioclase in the granites suggest submagmatic equilibration temperatures in the range of 365 to 505 c whereas oxygen isotope equilibration temperatures for two granites are higher at 480 and 570 c the compositions of the granites plot near minimum melting compositions in the water saturated quartz albite orthoclase system comparison of muscovite plus quartz stability the water saturated granite solidus and experimental garnet melt equilibria suggests pressures of crystallization no lower than about 3 5 kbar this pressure is compatible with that estimated from garnet plagioclase sillimanite quartz equilibria for the intruded metasedimentary rocks

hinterland drainage closure and lake formation in response to middle eocene farallon slab removal nevada u s a abstract hinterland basins can accumulate high resolution archives of orogenic processes and continental climate but are challenging to reconstruct due to tectonic overprinting and the inherent complexity of their lithofacies assemblages the cordilleran hinterland of northeast nevada has been interpreted to have overlain a flattened farallon slab from the late cretaceous to eocene slab removal and advection of asthenospheric mantle beneath nevada have been invoked to explain a southwestward migrating wave of eocene to oligocene volcanism and proposed as a driver for topographic uplift however the timing of slab removal and possible subsequent delamination of north american lithospheric mantle can only ambiguously be related to the surface record subsequent neogene extension and basin filling has complicated the correlation and interpretation of strata that record these events here we apply single crystal sanidine 40ar 39ar geochronology to 26 ash beds in northeast nevada to reconstruct paleogene geographic and hydrologic evolution we use these ages and legacy geochronology to compare lithofacies and isotope proxy records of meteoric waters to regional tectonics and global climate and assess competing tectonic interpretations for lake basin formation lakes formed locally prior to ca 48 7 ma in northeast nevada coeval with foreland lakes of the green river formation the most expansive phase of lacustrine deposition resulted in onlap onto locally derived fluvial deposits and folded paleozoic bedrock and occurred between ca 43 4 and ca 40 8 ma elko formation strata exhibit a basin wide transition from fluvial lacustrine to fluctuating profundal lithofacies at ca 42 7 ma suggesting a shift towards regional hydrologic closure the stromatolitic upper elko formation is intercalated with ash fall tuffs and several partially welded to unwelded ignimbrites from increasingly proximal volcanism elko formation deposition ended by ca 40 4 ma 40ar 39ar ages for seven ash beds in the dead horse formation at copper basin in northern elko county indicate intermittent ash bed deposition between 45 2 ma and 38 6 ma and an episode of lacustrine deposition between 39 8 ma and 38 6 ma that post dates the main phase of lake elko d values of volcanic glass sampled from dated ash beds reflect changes in the hydrogen isotope compositions of local eocene waters and systematically vary by 80 102 according to their depositional environment the elko formation and overlying volcanic strata are overlain regionally by a pronounced unconformity of 20 m y in the copper basin area deposition continued locally into the oligocene in the hanging wall of a ductile detachment the geochronologic and isotopic framework presented here permits reanalysis of the pinon range carbonate proxy record that was previously interpreted to record both regional uplift and the middle eocene climatic optimum new data suggest instead that isotope values of hydration waters within the elko formation were strongly influenced by evaporation and a change from lacustrine to non lacustrine conditions can account for the 18 o shift that was interpreted to reflect regional uplift moreover the end of elko formation deposition predated the middle eocene climatic optimum we interpret the overall record of drainage ponding and paleovalley inundation progressively more evaporative lacustrine conditions increasingly proximal volcanism and subsequent prolonged unconformity to reflect the surface effects of progressive ne to sw removal of the farallon slab

monazite allanite phase relations in metapelites abstract calculations of the transition from allanite to monazite bearing assemblages in typical pelitic bulk compositions have been made using thermodynamic data estimated from oxide sums and inferred from natural parageneses calculations in the cfashpce and mnnckfmashpce systems place the allanite to monazite transition in the middle amphibolite facies 525 600 c for a bulk composition similar to shaw s average pelite the temperature of the transition is pressure dependent and strongly dependent on the bulk rock cao content consistent with inferences from natural parageneses the transition is also a function of the bulk al 2 o 3 content although the calculated result is opposite to that inferred from natural samples comparison with published results in the la mg system suggest that the nature of the ree phosphate does not greatly influence the conditions of the transition

the impact of zircon garnet ree distribution data on the interpretation of zircon u pb ages in complex high grade terrains an example from the rauer islands east antarctica abstract the responsiveness of zircon to granulite metamorphism and post peak fluid infiltration in polymetamorphic terranes has been investigated utilising a case study from the rauer islands of east antarctica imaging bse cl textural analysis in situ ion microprobe trace element analysis of zircon garnet orthopyroxene plagioclase and apatite and ion microprobe oxygen isotope analysis of zircon have been integrated to establish the event significance of c 510 ma zircon age domains that occur within and on c 2840 ma protolith magmatic zircons initial magmatic zircon is characterised by moderate th u 0 4 0 6 and elevated steep hree yb n gd n 20 c 510 ma zircon domains feature lobate cuspate and planar bse and cl zones that have lower th u to 0 05 and either decreased mree or decreased mree and hree metamorphic garnet initially characterised by flat hree shows extensive replacement by orthopyroxene and plagioclase accompanied by near rim increases in hree to values that are consistent with equilibrium between garnet rims and product orthopyroxene zircon garnet mree hree distribution coefficients d ree calculated using compositions of modified zircon domains coupled with both initial metamorphic garnet cores and rims are far removed from presently accepted equilibrium values with the zircon mree being strongly depleted relative to garnet these d ree indicate that the observed modification of zircon did not occur in response to the major high grade metamorphic events recorded in the studied rock mineralogy but instead occurred after the high grade metamorphism the zircon was blind to these metamorphic events but instead was responsive to a later post peak fluid infiltration episode that shifted zircon 18 o values downwards by some 3 the likelihood of blind or unresponsive behaviour of zircon to high grade metamorphism must be considered when reconstructing crustal histories and terrane scenarios based on u pb age data and tested for using an integrated approach that includes in situ ree and other trace element analysis of zircon and its potentially coexisting minerals

numerical ages of cenozoic biostratigraphic datum levels results of south atlantic leg 73 drilling six sites were drilled in the south atlantic during the leg 73 cruise of the deep sea drilling project dsdp hydraulic piston coring at five of the six sites obtained a nearly complete sequence of undisturbed cenozoic samples the magnetostratigraphy at those sites was investigated by close sampling representing time intervals of about 10 4 yr most of the cenozoic nannofossil and many of the foraminiferal zonal boundaries were accurately determined and magnetostratigraphically calibrated at those five leg 73 boreholes their numerical ages have been computed assuming a linear spreading rate and a radiometric age of 66 5 m y for the cretaceous tertiary boundary alternative magnetostratigraphic ages were obtained with the adoption of a 63 5 m y age for the cenozoic our data confirm previous determinations of the pleistocene pliocene boundary at 1 8 1 7 m y b p and of the pliocene miocene boundary at 5 1 5 0 m y b p the miocene oligocene boundary is placed within chron c 6 c with a magnetostratigraphic age of 23 8 24 0 22 7 22 9 m y the oligocene eocene age is also very precisely located within chron c 13 r with a magnetostratigraphic age of 37 1 37 2 35 5 35 6 m y the eocene paleocene boundary should be located within an uncored interval of chron c 24 with a magnetostratigraphic age of 59 0 55 4 0 2 m y a general accord of the magnetostratigraphic ages from leg 73 sites and the radiometric ages published in the literature for the various zonal boundaries validates the assumption of a linear sea floor spreading rate during the cenozoic

late miocene early pliocene stratigraphy and paleoceanography of the south atlantic and southwest pacific oceans a synthesis the stratigraphy and paleoceanography of the late miocene and early pliocene have been examined at six sites in the south atlantic and southwest pacific oceans deep sea drilling project dsdp sites 284 516a 519 588 and 590 and two piston cores from chain cruise 115 a consistent stratigraphy was developed among sites using graphic correlation which resulted in age models for all sites that are tied to the revised paleomagnetic time scale of berggren et al 1985 applying these chronologies we assessed latitudinal and interocean contrasts in the stratigraphic ranges of late miocene early pliocene planktonic foraminiferal and nanno fossil datums salient stratigraphic results include 1 the last appearance datum lad of globoquadrina dehiscens is a late miocene 6 4 ma event in the subtropics and is not useful for the placement of the miocene pliocene m p boundary in this biogeographic province 2 the first appearance datum fad of globorotalia crassaformis occurred at 5 1 ma in the south atlantic near the m p boundary suggesting that gr crassaformis may have first evolved in the south atlantic and later migrated to other regions 3 in the southwest pacific the fads of gr margaritae 5 97 ma gr puncticulata 5 09 ma and gr crassaformis 4 87 ma are significantly time transgressive between temperate and warm subtropical regions time lags of 1 0 m y were required for these species to adapt to physical and or biotic conditions peripheral to their endemic biogeographic provinces 4 between the subtropics of the south atlantic and southwest pacific many planktonic foraminiferal datums fad of dentogloboquadrina altispira gr cibaoensis gr conomiozea gr margaritae and gq dehiscens and lad of gr cibaoensis markedly depart from the correlation suggested by magnetostratigraphy indicating that these datum levels are unreliable for correlation between these ocean basins 5 in contrast available calcareous nannofossil datum levels fall on or near the paleomagnetic correlation line indicating synchroneity of events within the subtropics 6 biostratigraphic magnetic and 87sr 86sr correlation between sites 588 and 519 and the m p neostratotype at capo rossello sicily suggests that the base of the zanclean stratotype occurs at 5 1 5 0 ma in the lower reversed subchron of the gilbert about 2 3 105 years above the gilbert chron 5 boundary oxygen isotopic results from dsdp sites 284 519 and ch115 piston cores confirm a prolonged benthic 18o increase in the latest miocene between 5 6 and 5 0 ma as originally proposed by shackleton and kennett 1975 at dsdp site 588 the benthic 18o record in the latest miocene is marked by high frequency fluctuations with amplitude variations of 0 5 and a long period wavelength component of 400 000 years maximum 18o values however occurred during the late miocene kapitean stage between 5 5 and 5 1 ma the late miocene 18o changes resulted from mid and high latitude cooling and pulses of ice sheet expansion and contraction glacial events were most intense during the latest miocene kapitean stage and occurred at 5 50 5 35 ma and at 5 10 ma glacial events are estimated to have lowered sea level by 40 to 60 m and contributed to the isolation and desiccation of the mediterranean basin during the late messinian interglacial conditions prevailed at 5 2 ma and between 5 0 and 4 1 ma ihe early pliocene the beginning of the pliocene was marked by changes in many proxy climatic indicators at all sites suggesting a prolonged interval of warm interglacial conditions between 5 0 and 4 1 ma during the earliest pliocene supplementary tables are available with entire article onmicrofiche order from american geophysical union 2000florida avenue n w washington dc 20009 documentp86 001 2 50 payment must accompany order

cretaceous to present kinematics of the indian african and seychelles plates an iterative inverse model of seafloor spreading data from the mascarene and madagascar basins and the flanks of the carlsberg ridge describes a continuous history of indian african plate divergence since 84 ma visual fit modelling of conjugate magnetic anomaly data from near the seychelles platform and laxmi ridge documents rapid rotation of a seychelles plate about a nearby euler pole in palaeocene times as the euler pole migrated during this rotation the amirante trench on the western side of the plate accommodated first convergence and later divergence with the african plate the unusual present day morphology of the amirante trench and neighbouring amirante banks can be related to crustal thickening by thrusting and folding during the convergent phase and the subsequent development of a spreading centre with a median valley during the divergent phase the model fits fracture zone trends in the north arabian and east somali basins suggesting that they formed in india africa plate divergence seafloor fabric in and between the basins shows that they initially hosted a segmented spreading ridge that accommodated slow plate divergence until 71 69 ma and that upon arrival of the deccan re union plume and an increase to faster plate divergence rates in the period 69 65 ma segments of the ridge lengthened and propagated ridge propagation into the indian continental margin led first to the formation of the laxmi basin which accompanied extensive volcanism onshore at the deccan traps and offshore at the saurashtra high and somnath ridge a second propagation episode initiated the ancestral carlsberg ridge at which seychelles india and india africa plate motions were accommodated with the completion of this propagation the plate boundaries in the mascarene basin were abandoned seafloor spreading between this time and the present has been accommodated solely at the carlsberg ridge

monazite zircon and garnet growth in migmatitic pelites as a record of metamorphism and partial melting in the east humboldt range nevada monazite and zircon thermometry and geochronology were applied to anatectic pelites of the northern east humboldt range nevada the study area is an exhumed portion of the sevier orogenic root and is characterized by two crustal blocks with different prograde metamorphic histories the winchell lake nappe and the underlying lizzies basin block u th pb secondary ion mass spectrometry shrimp results from zircon and monazite indicate that in the lizzies basin block prograde metamorphism began by 96 5 8 0 ma cooling and melt crystallization was initiated by 80 1 1 4 ma possible reheating and a monazite growth event occurred again at 76 2 2 8 to 68 2 2 0 ma in the upper limb of the overlying winchell lake nappe prograde metamorphism began by 82 8 1 3 ma cooling and melt crystallization recorded by zircon and monazite growth recrystallization at 77 4 12 4 to 58 9 3 6 ma melt may have been present in the winchell lake nappe upper limb for a protracted period or may have formed during several unresolved melting events thin eocene oligocene 40 32 ma high y monazite rims are found in both crustal blocks these probably represent a phase of heating during eocene oligocene magmatism and extensional deformation u th pb geochronology results are consistent with differential burial heating and exhumation of different crustal blocks within the east humboldt range significantly fast exhumation of upper limb rocks of the winchell lake nappe and somewhat slower exhumation of lizzies basin block rock occurred prior to the core complex phase of the exhumation of the ruby mountains east humboldt range metamorphic core complex differences in the timing and tectonic significance of exhumation episodes within the east humboldt range may reflect how the localized presence of partial melt can affect the evolution of an exhumed orogenic terrane

a gravity and magnetic anomaly study of the extinct aegir ridge norwegian sea in 1990 the u s naval research laboratory mapped the extinct aegir ridge in the norwegian sea with seamarc ii side scan sonar hydrosweep multibeam seismics gravity and magnetics onboard r v maurice ewing we analyzed the 92 center beam bathymetry gravity and magnetic profiles in this study to examine slow spreading ridge processes and cessation of spreading about 25 ma in bathymetric expression the rift valley deepens and narrows southward while the rift mountain summits become higher south of 67 n the western rift mountains and flanks stand higher than the eastern mountains and flanks free air anomalies along the ridge axis show undulating intermediate wavelength 100 km anomalies superimposed on a regional decrease of the rift valley negative from 0 mgal in the north to 55 mgal in the south after removal of sediment effects results from three dimensional gravity modeling imply a variation of crustal thickness a new rotation pole for the movement of the jan mayen microplate relative to the eurasia plate from 50 ma to 25 ma was found at 64 34 n 13 w based on magnetic lineations and inferred discontinuity traces a plot of angular separation between significant anomaly peaks versus absolute age suggests an opening rate of 2 0 m y 36 ma followed by gradual deceleration to 0 at 25 ma full spreading rate between chrons c13n and c21n ranged from 13 mm yr in the north to 8 mm yr in the south

tertiary magmatism and mylonitization in the ruby east humboldt metamorphic core complex northeastern nevada u pb geochronology and sr nd and pb isotope geochemistry u pb analyses of 48 zircon and or monazite fractions from ten intrusive rocks that range in composition from quartz diorite to leucogranite yield crystallization ages between 40 and 29 ma the new data indicate that tertiary intrusive rocks constitute a widespread and significant magmatic component of the ruby east humboldt metamorphic core complex some of the intrusive rocks are gneissic to mylonitic and have been penetratively deformed in a shear zone that can be traced for 100 km along the west flank of the core complex these data when coupled with previously published geochronologic data indicate that an important episode of mylonitization occurred between 29 and 23 ma late oligocene some field relations and geochronologic data however also suggest an earlier history of mylonitization in parts of the core complex this earlier mylonitic history may have occurred in the eocene or even late mesozoic time although more detailed geochronologic data are needed to clearly define the exact time interval sr nd and pb isotopic data from the dated intrusive units indicate that a relatively sharp approximately east west striking crustal boundary separating predominantly archean basement in the north from proterozoic basement in the south transects the complex we speculate that the crustal boundary represents the continuation of the cheyenne belt into this part of northeastern nevada these new data cast doubt on the existence of the proposed 2 0 2 3 ga crustal province in this part of the northern great basin the isotopic data from the plutonic rocks coupled with published data from at least in part coeval volcanic rocks indicate that mantle derived magmas were important in their genesis collectively these data indicate that significant crustal growth via the addition of mantle derived magmas occurred during large magnitude extension in this part of the northern great basin

the role of mantle delamination in widespread late cretaceous extension and magmatism in the cordilleran orogen western united states extension during plate convergence and mountain building is widely recognized yet the causes of synconvergent extension remain controversial here we propose that delamination of lithospheric mantle aided by decoupling of the crust from the mantle via a reduction in the viscosity of the lower crust through heating incursion of fl uids and partial melting explains many enigmatic yet prevalent aspects of the metamorphic magmatic and kinematic history of the sevier laramide orogen of the western united states during the late cretaceous extension heating anatexis magmatism and perhaps rock uplift were widespread during a restricted time interval in the late cretaceous 75 67 ma along the axis of maximum crustal thickening within the mojave sector of the sevier orogen and to a lesser extent within the interior of the idaho utah wyoming sector to the north similar processes may have been active in the peninsular range sierran western mojave and salinian segments of the mesozoic cordilleran arc these processes are viewed as predictable consequences of the thermal rheological and dynamic state of the overlying crust following delamination of mantle lithosphere beneath isostatically compensated mountain belts the proposed delamination would have occurred immediately prior to eastward propagation of lowangle subduction of the farallon plate during the inception of the laramide orogeny following delamination extension and anatexis of the north american crust were aided locally by egress of slab derived fl uids from the low angle farallon slab we suggest that lithospheric delamination may have aided in the shallowing of the slab to achieve lowangle subduction geometry delamination has been proposed to be common in areas of thickened continental lithosphere in the terminal phase or late in orogenesis the late cretaceous delamination event proposed here for the sevier laramide orogen occurred during protracted plate convergence and was synchronous with and followed by continued shortening in the external part of the orogen

yttrium zoning in garnet coupling of major and accessory phases during metamorphic reactions the concentration of yttrium in pelitic garnets as a function of metamorphic grade has been examined in relation to the distribution of xenotime ypo 4 in samples from new england and british columbia samples with xenotime present only as inclusions in garnet generally possess high y cores and concentrations that drop off discontinuously along zoning shoulders of variable width to low y outboard regions samples with matrix xenotime are restricted to the garnet zone y concentration of these garnets generally decreases smoothly from core to rim xenotime may also be present in reaction zones around garnet in xenotime bearing samples y g r t is strongly temperature dependent and ranges from 5000 ppm in the garnet zone to 150 ppm in the sillimanite zone measured yttrium zoning profiles in xenotime absent samples are reproduced with both rayleigh fractionation and diffusion models but p t histories of the samples examined favor the rayleigh model with garnet volume bulk rock yttrium and mode of y hree accessory phases controlling the profile shape high yttrium annuli in staurolite zone samples may form by garnet overgrowth of proximal matrix enriched in yttrium due to garnet consumption during discontinuous staurolite forming reactions an increase in y g r t and hree g r t in garnet from anatectic samples is related to dissolution of phosphates in vapor absent peraluminous melt with partitioning of highly compatible y and hree into garnet grown during anatexis textural analysis reveals that phosphates are absent from regions of garnet grown in equilibrium with melt a main result of this study is identification of an intimate coupling between major pelite phases and accessory phases during reaction progress this coupling is of great advantage in that it may be used to 1 calibrate sensitive geothermometers and geobarometers 2 identify particular regions of garnet grown in different garnet producing reactions over a range of grades and 3 reveal portions of pelite reaction history invisible to major elements

new calibration of neogene radiolarian events in the north pacific abstract new age models for twelve deep sea drilling project sites in the north pacific have been produced based on in order of importance in our dataset a recompilation of previously published diatom calcareous nannofossil and foraminifer first and last occurrences and magnetostratigraphy the projected ages of radiolarian first and last occurrences derived from the line of correlation of the age depth plots have been computed from these sites and 28 radiolarian events have thereby been newly cross calibrated to north pacific diatom and other stratigraphy several of the north pacific radiolarian events are older than in previously published equatorial pacific calibrations and some may be diachronous within the north pacific these patterns may be due to complex latitudinal patterns of clinal variation in morphotypes within lineages or to migration events from the north pacific towards the equator

thermochronologic evidence of major tectonic denudation associated with detachment faulting northern ruby mountains east humboldt range nevada fission track ft thermochronology studies on igneous and metamorphic rocks from the ruby mountains east humboldt range metamorphic core complex provide important constraints on the timing and nature of major mid tertiary extension in northeast nevada rocks from within the ruby east humboldt detachment brittle ductile normal sense shear zone were analyzed the dominant lithology studied was variably mylonitic mafic orthogneiss nonmylonitic amphibolite from the top of the structurally lower migmatitic core and porphyritic biotite granodiorite from the oligocene circa 36 ma harrison pass pluton were also studied ft ages on apatite zircon and sphene except one 18 4 ma apatite from the basal portion of the detachment zone are concordant and range in age from 26 5 ma to 23 6 ma all ages overlap at 1 between 25 4 ma and 23 4 ma these data suggest that rocks of the upper and middle portions of the detachment zone cooled rapidly from temperatures above 285 c sphene closure temperature to below 70 c lower temperature limit of track stability in apatite near the beginning of the miocene minimum cooling rate of 40 c ma the length distribution of confined fission tracks in apatites from these exhumed middle crustal rocks is strikingly similar to those of rocks known to have cooled rapidly fish canyon ash flow tuff the lower part of the detachment zone as well as the underlying migmatitic core also cooled through sphene and zircon closure temperatures during this time interval but apparently only cooled to a temperature within the zone of partial annealing of apatite 70 130 c residence of these rocks in this zone led to a partial retention of fission tracks thus resulting in a reduced age confined track length distributions from apatite provide independent evidence of very rapid cooling but also indicate that the rocks of the lower detachment zone experienced a more protracted cooling history remained hotter longer than structurally higher levels of the zone ft data firmly establish the lower limit on the timing of mylonitization during detachment faulting in this area as 23 4 ma rapid cooling of the region is considered to reflect large scale tectonic denudation intracrustal thinning the vertical complement to crustal extension rocks originating in the middle crust 10 15 km were quickly brought near the surface along the ruby east humboldt detachment fault brittle ductile simple shear zone and juxtaposed against brittlely extended rocks deformed under upper crustal conditions

margin to craton expansion of late ordovician benthic marine invertebrates a biostratigraphic survey of 57 late ordovician marine shelly invertebrates from the climacograptus manitoulinensis zone of eastern canada supports suggestions that throughout the early phanerozoic benthic marine speciations occurred preferentially in marginal marine environments the species subsequently spread onto the craton there is no obvious positive correlation between the times of first appearance of new associations or novel communities along the continental margin and the first appearance on the craton of the species making up these communities taxonomic similarities between marine communities that occupied both marginal and cratonic regimes may reflect a more static local ecology than the evolutionary dynamics of a piecemeal species by species reassembly

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

evolutionary changes in the biometry of the fossil radiolarian stichocorys peregrina lineage in the eastern equatorial and eastern north pacific abstract stichocorys peregrina riedel contributes to paleoceanography as a valuable stratigraphic tool for correlation of deep sea siliceous sediments in world oceans and as a paleoceanographic indicator this paper documents the evolutionary morphologic changes of the s peregrina lineage from the middle miocene to late pliocene in the eastern equatorial and eastern north pacific iodp site u1335 and odp site 887 respectively the size and shape changes show that there are at least two geographical variations in the s peregrina lineage in the eastern equatorial pacific two significant morphological shifts took place about 11 0 and 7 0 ma but the morphology remained relatively stable from 7 0 to 3 0 ma coincident with morphological changes in the s peregrina lineage were changes in biogenic productivity these changes suggest that there is a close correspondence between major paleoceanographic events in the late neogene and evolutionary changes of the s peregrina lineage in the low latitudes in the high latitudes the ratio of the third segment to fourth segment in width and height showed a maximum from 9 5 to 8 0 ma and decreased stepwise at about 8 0 and 6 5 ma however the radiolarian data in the high latitudes do not show a clear relationship between the paleoceanographic events and size variation of this lineage the evolutionary transition from stichocorys delmontensis to s peregrina can be used as a primary biostratigraphic marker in the low latitudes however it is not easy to use the evolutionary transition for biostratigraphic correlation and age determination in the high latitudes because there is no obvious change in the ratio of the third to fourth segment widths that can be used to distinguish s delmontensis from the descendant s peregrina

an empirical garnet yag xenotime thermometer a pronounced negative correlation between the yttrium concentration in garnet y grt and temperature has been observed in xenotime ypo4 bearing metapelites from central new england usa the y grt decreases roughly two orders of magnitude 5500 to less than 100 ppm y over a 150 c interval a regression of ln y grt against estimated reciprocal temperature yields the following relationship

micro zircon origin and evolution during metamorphism the size and distribution of zircon within a garnet mica schist from the scottish highlands were assessed using scanning electron microscopy the study reveals that abundant 0 2 3 0 m sized zircon is preferentially concentrated within garnet and biotite porphyroblasts the micro zircon has grown during regional metamorphism and represents 90 of the total number of zircon in the schist it is texturally distinct from a few larger detrital zircon grains in the schist that commonly preserve evidence of dissolution and more rarely small metamorphic outgrowths the sequential incorporation of zircon in porphyroblasts allows prograde changes in the morphology of the zircon population to be identified zircon is reactive and soluble and responds to medium grade metamorphism in a series of dissolution and crystallization events linked to possible changes in fluid composition deformation also has a significant influence on the distribution of zircon allowing inclusions previously trapped within biotite to react about 8 106 micro zircon occur as inclusions within a typical individual 5 mm garnet porphyroblast and their presence must be considered prior to trace element or isotopic analysis of such metamorphic phases

thermal evolution of a portion of the sevier hinterland the northern ruby mountains east humboldt range and wood hills northeastern nevada conventional thermobarometry and gibbs method modeling have been used to obtain new information pertaining to the mesozoic tertiary thermal evolution of the ruby mountains east humboldt range and wood hills metamorphic complexes northeastern nevada kyanite grade assemblages from clover hill in the extreme northeastern part of the east humboldt range were produced at temperatures of 780 810 k and depths in excess of 35 km previously published geochronologic data suggest an early cretaceous or late jurassic age for this event the clover hill area subsequently experienced a major unroofing interval prior to late cretaceous time during which it was brought roughly 20 km closer to the surface the lack of stratigraphic evidence for substantial cretaceous erosion in the region and the topology of the pressure temperature paths for the clover hill samples suggest that the unroofing mechanism may have been tectonic denudation related to the relaxation of large topographic gradients produced by crustal thickening in the hinterland of the sevier orogen metamorphic rocks at clover hill and within the wood hills core complex equilibrated at 820 900 k and 500 640 mpa 18 24 km in cretaceous time prior to 115 ma other parts of the ruby mountains east humboldt range metamorphic complex presently structurally separated from the clover hill sequence were intruded extensively by late cretaceous granitic magmas and attained substantially higher temperatures than the clover hill wood hills block sillimanite grade assemblages from these areas apparently underwent roughly 10 km of unroofing and 130 k of cooling prior to final equilibration at 820 920 k and 360 430 mpa 13 16 km during oligocene extension

intraplate seismicity in the atlantic ocean basin a teleseismic catalog a comprehensive compilation and reassessment of intraplate seismicity in the atlantic ocean basin is presented for the years 1918 through 1990 of the 846 earthquakes whose original bulletin epicenters were located in atlantic ocean crust and were at least 2 from active plate boundaries 289 were found to be reliably intraplate a significant number 237 of the original epicenters were actually mislocated interplate earthquakes that located to active plate boundaries upon relocation or whose 95 confidence locations determined through monte carlo simulations intersected seismically active boundaries many of these erroneous locations particularly in regions surrounding the mid atlantic ridge frequently appear in global seismicity maps the resulting reliable catalog of intraplate seismicity is not randomly distributed but shows significant clustering in space and time much of the seismicity is associated with the locations of hot spots such as the bermuda canary and cape verde islands seismicity is also clustered around other tectonic features such as the diffuse plate boundary between north america and south america and the fossil spreading ridge between greenland and north america while we did not examine intraplate earthquakes within 2 of the mid atlantic ridge and some of our unreliable near ridge earthquakes may be truly intraplate we do not find an indication within our reliable data set of an increase in intraplate seismicity with increased proximity to the mid atlantic ridge beyond our 2 limit

trace element distributions in silicates during prograde metamorphic reactions implications for monazite formation to assess the petrogenetic relationship between monazite and major silicates during prograde metamorphism ree were measured across coexisting zoned silicates in garnet through kyanite grade pelitic schists from the great smoky mountains western blue ridge terrane southern appalachians to establish ree concentrations and distributions before and after the monazite in isograd and to identify the role major silicates play in the formation of monazite results indicate significant scavenging of light rare earth elements lree from silicates during the monazite in isograd reaction however the absolute concentration of lree hosted in the silicates was insufficient to produce monazite in the quantity observed in these schists monazite must have formed mainly from either the dissolution of allanite or some other source of concentrated lree possibly adsorbed onto grain boundaries even though direct evidence for allanite is lacking in a majority of the samples laser ablation icp ms analyses and theoretical thermodynamic calculations show that monazite may have formed as a result of contributions from both allanite and major silicates allanite breakdown initially formed monazite and monazite production drew lree liberated from allanite major silicates and possibly from crystal boundaries in many rocks the reaction was further promoted by the staurolite in reaction allowing for rapid isogradic monazite growth

monazite ages in the chesham pond nappe sw new hampshire u s a implications for assembly of central new england thrust sheets four distinct generations of monazite growth have been identified in samples from the chesham pond nappe and three monazite compositional domains 2 3 and 4 have been correlated with both temperature and mineral assemblage domain 1 cores were interpreted previously to be detrital relics or vestiges of an earlier acadian metamorphism the four monazite domains have been dated by in situ isotope and chemical methods the following are chemical ages of each domain weighted average 2 standard errors of the mean 400 10 ma domain 1 381 8 ma domain 2 372 6 ma domain 3 352 14 ma domain 4 heating and cooling rates derived from combining monazite ages monazite thermometry and 40ar 39ar closure temperatures are approximately 10 15 c m y for heating from 470 to 740 c approximately 8 c m y for cooling from 740 to 375 c and approximately 1 2 c m y for cooling from 375 to 150 c temperature time paths calculated with monazite ages and monazite thermometry indicate that 1 plutonism at ca 400 ma was the likely heat source for the formation of monazite domain 1 and 2 monazite domains 2 4 were produced during a regional low pressure high temperature metamorphism active between 380 350 ma the regional metamorphism is ascribed to lithospheric mantle delamination followed by asthenospheric mantle upwelling which heated a wide area of the merrimack basin southwestern new hampshire central massachusetts central connecticut to temperatures in excess of 725 c monazite ages in the chesham pond nappe and adjacent structural units to the west constrain the commencement of nappe overthrusting to roughly 355 ma