heterogeneity in geochemical expression of subaerial exposure in limestones and its implications for sampling to detect exposure surfaces abstract replicate sampling along stratigraphic horizons in one outcrop of ordovician limestones of the nashville dome reveals considerable variation in 13c 18o and sr concentration among seemingly identical samples ten vertical transects were sampled each consisting of samples taken at 10 cm intervals from 2 m below to 1 m above the m4 m5 sequence boundary of holland and patzkowsky 1997 1998 the 10 transects were each 2 m apart the resulting 10 replicate and thus seemingly interchangeable samples from each horizon have ranges of 13c as great as 2 4 and ranges of 18o as great as 2 8 and sr concentrations that vary by as much as a factor of four variance is great both just below and just above surfaces of subaerial exposure as a result maxima in variance in 13c as well as or in some cases instead of minima in mean 13c can be used to test such surfaces for exposure maxima in variance and minima in mean 13c may be found either below a surface of subaerial exposure in the meteorically altered sediment or above that surface in the transgressive lag where reworked pre exposure sediment is mixed with unaltered post exposure sediment the large variances and ranges found in this study and the dissimilarity of geochemical profiles from the 10 vertical transects indicate that single samples of stratigraphic horizons and thus single vertical transects can yield misleading characterizations of stratigraphic variation of geochemical parameters instead replicate sampling along horizons better characterizes isotopic and elemental variation through stratigraphic sections than does existing single sample strategies and it will be more useful in future testing of surfaces of subaerial exposure for example the replicate geochemical data used in this project confirm the presence of three surfaces of subaerial exposure whereas only two had been recognized as exposure surfaces before data from replicate samples also make unnecessary the use of arbitrary criteria for inferring exposure such as the predetermined numerical cutoffs used by railsback et al 2003

lithostratigraphy of the upper turinian lower chatfieldian upper ordovician foreland succession and a u pb id tims date for the millbrig volcanic ash bed in the ottawa embayment three stages of carbonate platform development are preserved in the upper turinian lower chatfieldian succession of the ottawa group in the ottawa embayment and represent deposition along the late ordovician taconic foreland interior of paleo southern laurentia compared with contemporary stratigraphy in the adjacent northern appalachian southern ontario new york state and western quebec basins the intermediate stage 2 succession which brackets the turinian chatfieldian boundary preserves embayment specific stratigraphic patterns these include i dramatic west to east thickening of the upper turinian watertown formation that defines differential subsidence along the present axis of the embayment ii post watertown base level fall defined by appearance of shoreface siliciclastics iii early chatfieldian marine transgression represented by the proposed l orignal formation that is coeval with but lithologically distinct from the selby formation in the northern appalachian basin and iv platf

high resolution sequence stratigraphy of lower paleozoic sheet sandstones in central north america the role of special conditions of cratonic interiors in development of stratal architecture well known difficulties in applying sequence stratigraphic concepts to deposits that accumulated across slowly subsiding cratonic interior regions have limited our ability to interpret the history of continental scale tectonism oceanographic dynamics of epeiric seas and eustasy we used a multi disciplinary approach to construct a high resolution stratigraphic framework for lower paleozoic strata in the cratonic interior of north america within this framework these strata proved readily amenable to modern sequence stratigraphic techniques that were formulated based on successions along passive margins and in foreland basins settings markedly different from the cratonic interior parasequences parasequence stacking patterns systems tracts maximum flooding intervals and sequence bounding unconformities can be confidently recognized in the cratonic interior using mostly standard criteria for identification the similarity of cratonic interior and foreland basin successions in size geometry constituent facies and local stacking patterns of nearshore parasequences is especially striking this similarity indicates that the fundamental processes that establish shoreface morphology and determine the stratal expression of retreat and progradation were likewise generally the same despite marked differences in tectonism physiography and bathymetry between the two settings our results do not support the widespread perception that paleozoic cratonic interior successions are so anomalous in stratal geometries and constitute such a poor record of time that they are poorly suited for modern sequence stratigraphic analyses the particular arrangement of stratal elements in the cratonic interior succession we studied is no more anomalous or enigmatic than the variability in architecture that sets all sedimentary successions apart from one another thus paleozoic strata of the cratonic interior are most appropriately considered as a package that belongs in a continuum of variable stratigraphic packages reflecting variable controls such as subsidence and shelf physiography special conditions of exceptionally slow subsidence rate shallow bathymetry and nearly flat regional shelf gradient are manifest mostly by the presence of individual systems tracts of relatively long duration that extend for much greater distances across depositional strike than those that characterize successions deposited in more dynamic tectonic and physiographic settings these results suggest that if other cratonic interior successions are as anomalous as reported a low sediment supply may have played a primary role in development of their apparently condensed stratal architecture the results also lead us to suggest that a nonvegetated lower paleozoic landscape played a relatively insignificant role in the development of what are commonly perceived to be enigmatic stratigraphic features of sheet sandstones particularly their widespread yet thin geometry and a scarcity of shale and siltstone

evidence of a tectonically driven sequence succession in the middle ordovician taconic foredeep the late mohawkian through early cincinnatian ordovician trenton group carbonates and coeval siliciclastic succession of the central mohawk valley new york state were deposited in a foreland basin during the taconic orogeny development of this facies mosaic records changes in stacking patterns that took place during episodic basin deepening and simultaneous carbonate ramp shallowing along a ramp to basin transect located on the distal slope of the foreland basin differential rates of subsidence across the region produced accommodation rates that increased by an order of magnitude from west to east high rates of siliciclastic sediment supply and even higher subsidence rates resulted in a thick deep water succession in the eastern part of the study region the coeval trenton group succession in the west formed under conditions of moderate sediment supply and lower subsidence rate resulting in a net shallowing upward succession these relationships suggest that active margin tectonism was the proximate cause of the mohawk valley facies architecture

kinematics of the western africa eurasia plate boundary from focal mechanisms and gps data summary the western mediterranean displays a complex pattern of crustal deformation distributed along tectonically active belts developed in the framework of slow oblique plate convergence we used earthquake and global positioning system gps data to study the present day kinematics and tectonics of the africa eurasia plate boundary in this region crustal seismicity and focal mechanisms analysed in terms of seismic moment release and seismic deformation outline the geometry of major seismic belts and characterize their tectonics and kinematics continuous gps data have been analysed to determine euler vectors for the nubian and eurasian plates and to provide the global frame for a new mediterranean gps velocity field obtained by merging continuous and campaign observations collected in the 1991 2005 time span gps velocities and displacements predicted by the nubia eurasia rotation pole provide estimates of the deformation accommodated across the tectonically active belts the rather simple deformation occurring in the atlantic region characterized by extension about perpendicular to the middle atlantic and terceira ridges and right lateral motion along the gloria transform fault turns into a complex pattern of deformation occurring along broader seismic belts where continental lithosphere is involved our analysis reveals a more complex fragmentation of the plate boundary than previously proposed the roughly e w trending mainly compressive segments i e southwestern iberia northern algeria and southern tyrrhenian where plate convergence is largely accomodated across rather localized deformation zones and partially transferred northward to the adjacent domains i e the algero balearic and tyrrhenian basins are interrupted by regions of more distributed deformation i e the rif alboran betics tunisia libya and eastern sicily or limited seismicity i e the strait of sicily which are characterized by less homogeneous tectonics regimes mainly transcurrent to extensional in correspondence of the observed breaks tectonic structures with different orientation interfere and we find belts with only limited deformation i e the high and middle atlas the tunisian atlas and the offshore tunisia libya belt that extends from the plate boundary into the nubian plate along pre existing tectonic lineaments our analysis suggest that the sicilian pelagian domain is moving independently from nubia according to the presence of a right lateral and extensional decoupling zone corresponding to the tunisia libya and strait of sicily deformation zone despite the space variability of active tectonic regimes plate convergence still governs most of the seismotectonic and kinematic setting up to the central aeolian region in general local complexities derive from pre existing structural features inherited from the tectonic evolution of the mediterranean region on the contrary along calabria and the apennines the contribution of the subducted ionian oceanic lithosphere and the occurrence of microplates i e adria appear to substantially modify both tectonics and kinematics finally gps data across the gibraltar arc and the tyrrhenian calabria domain support the hypothesis that slab rollback in these regions is mostly slowed down or stopped

source process and tectonic implications of the great 1975 north atlantic earthquake summary the atlantic segment of the africa europe plate boundary has usually been interpreted as a transform boundary on the basis of the bathymetric expression of the gloria fault and dextral strike slip first motion mechanisms aligned along the azores gibraltar line of seismicity the 1975 may 26 earthquake ms 7 9 was assumed to fit into this framework because it occurred in the general area of this line and has a similar first motion focal mechanism strike 288 dip 72 slip angle 184 however several anomalies cast doubt on this picture the event is abnormally large for an oceanic transform event a sizeable tsunami was excited the aftershock area is unusually small for such a large event and most significantly the epicentre is 200 km south of the presumed plate boundary the rayleigh wave radiation pattern indicates a change in focal mechanism to one with a significant dip slip component the short duration of the source time history 20 s as deconvolved from long period p waves the lack of directivity in the rayleigh waves and the small one day aftershock area suggest a fault length less than 80 km one nodal plane of the earthquake is approximately aligned with the trace of an ancient fracture zone we have compared the pasadena 1 90 record of the 1975 earthquake to that of the 1941 north atlantic strike slip earthquake 200km to the nnw and confirmed the large size of the 1941 event m 8 2 the non colinear relationship of the 1975 and 1941 events suggests that there is no well defined plate boundary between the azores and gibraltar this interpretation is supported by the intraplate nature of both the 1975 event and the large 1969 thrust event 650 km to the east this study also implies that the largest oceanic strike slip earthquakes occur in old lithosphere in a transitional tectonic regime

seismic velocity model of the central united states version 1 description and simulation of the 18 april 2008 mt carmel illinois earthquake abstract we have developed a new three dimensional seismic velocity model of the central united states cusvm that includes the new madrid seismic zone nmsz and covers parts of arkansas mississippi alabama illinois missouri kentucky and tennessee the model represents a compilation of decades of crustal research consisting of seismic aeromagnetic and gravity profiles geologic mapping geophysical and geological borehole logs and inversions of the regional seismic properties the density p and s wave velocities are synthesized in a stand alone spatial database that can be queried to generate the required input for numerical seismic wave propagation simulations we test and calibrate the cusvm by simulating ground motions of the 18 april 2008 m w 5 4 mt carmel illinois earthquake and comparing the results with observed records within the model area the selected stations in the comparisons reflect different geological site conditions and cover distances ranging from 10 to 430 km from the epicenter the results based on a qualitative and quantitative goodness of fit gof characterization indicate that both within and outside the mississippi embayment the cusvm reasonably reproduces 1 the body and surface wave arrival times and 2 the observed regional variations in ground motion amplitude cumulative energy duration and frequency content up to a frequency of 1 0 hz in addition we discuss the probable structural causes for the ground motion patterns in the central united states that we observed in the recorded motions of the 18 april mt carmel earthquake online material simulated and observed waveforms and response spectral acceleration ground motion prediction equation comparison

revised sequence stratigraphy of the upper katian stage cincinnatian strata in the cincinnati arch reference area geological and paleontological implications abstract the upper ordovician cincinnatian series is a critical upper katian reference succession previously six 3rd order depositional sequences c1 to c6 have been recognized and frequently used as regional time slices recent research however indicates a need for revision and additions including the recognition of additional 3rd and 4th order sequences we propose amendments based upon high resolution correlations using a combination of regional disconformities cycle stacking patterns bioevents marker beds and chemostratigraphy several disconformities including a large regionally angular mid richmondian unconformity are newly recognized or reinterpreted in addition distinctive depositional patterns aid in the recognition of systems tracts with a thick proximal laminated peritidal facies representing early transgressive systems tracts tsts or lowstands in part lsts b shell rich limestone packages representing later tsts and overlain by c more shale prone highstand systems tracts hsts themselves overlain locally by d siliciclastic dominated falling stage systems tract fsst deposits that are commonly removed beneath substantial unconformities sequence boundaries at present we recognize eight 3rd order cincinnatian sequences designated c1 through c8 and 16 4th order subdivisions and their component systems tracts a younger hirnantian sequence may be present at some localities as the enigmatic centerville formation and or basal belfast member of the brassfield formation the implications of these new correlations and sequence interpretations include a new standard for relative sea level changes and insights into basin dynamics this framework also permits a more nuanced history of biotic changes that include geologically brief pulses of immigration associated with the transgressions of the c1 lower kope formation c5c clarksville member of waynesville formation and c7a basal whitewater formation

petrology of ultrahigh pressure rocks from the southern su lu region eastern china abstract in the su lu ultrahigh p terrane eastern china many coesite bearing eclogite pods and layers within biotite gneiss occur together with interlayered metasediments now represented by garnet quartz jadeite rock and kyanite quartzite in addition to garnet omphacite rutile coesite other peak stage minerals in some eclogites include kyanite phengite epidote zoisite talc nyboite and high al titanite the garnet quartz jadeite rock and kyanite quartzite contain jadeite quartz garnet rutile zoisite apatite and quartz kyanite garnet epidote phengite rutile omphacite assemblages respectively coesite and quartz pseudomorphs after coesite occur as inclusions in garnet omphacite jadeite kyanite and epidote from both eclogites and metasediments study of major elements indicates that the protolith of the garnet quartz jadeite rock and the kyanite quartzite was supracrustal sediments most eclogites have basaltic composition some have experienced variable crustal contamination or metasomatism and others may have had a basaltic tuff or pyroclastic rock protolith the su lu ultrahigh p rocks have been subjected to multi stage recrystallization and exhibit a clockwise p t path inclusion assemblages within garnet record a pre eclogite epidote amphibolite facies metamorphic event ultrahigh p peak metamorphism took place at 700 890 c and p 28 kbar at c 210 230 ma the symplectitic assemblage plagioclase hornblende epidote biotite titanite implies amphibolite facies retrogressive metamorphism during exhumation at c 180 200 ma metasedimentary and metamafic lithologies have similar p t paths several lines of evidence indicate that the supracrustal rocks were subducted to mantle depths and experienced in situ ultrahigh p metamorphism during the triassic collision between the sino korean and yangtze cratons

prograde metamorphism decompressional partial melting and subsequent melt fractional crystallization in the weihai migmatitic gneisses sulu uhp terrane eastern china abstract in order to decipher fluid melt activity during subduction and subsequent exhumation of felsic continental crust we carried out a combined study of zircon internal structure zircon u pb age zircon trace element composition and whole rock geochemistry on migmatitic gneisses from the weihai region in the sulu ultra high pressure uhp metamorphic terrane eastern china the weihai migmatitic gneisses are composed of intercalated compositional layers of melanosome and plagioclase pl rich leucosome and k feldspar kfs rich pegmatite veins whole rock geochemistry suggests that mgo fe2o3t cao tio2 p2o5 rare earth elements ree and large ion lithophile elements hfse are nearly completely retained in the melanosome whereas significant amounts of lile e g rb ba k sr pb and u are partitioned into the pl rich leucosome and the kfs rich pegmatite veins the kfs rich pegmatite vein has much higher k2o u pb and rb but lower cao and na2o contents than those of the pl rich leucosome layer protolith magmatic zircon domains with an upper intercept age of about 780 ma are common in the cores of zircon grains from the melanosome and the pl rich leucosome layers indicating that the protolith of the migmatitic gneiss is mid neoproterozoic magmatic rock metamorphic zircons with concordant ages ranging from 243 to 256 ma occur as overgrowth mantles on the protolith magmatic zircon cores they are characterized by remarkably flat heavy rare earth element hree patterns without obviously negative eu anomalies and have low y and th contents low th u and high hf y ratios indicating a prograde metamorphism in the hp eclogite facies during subduction recrystallized rims of zircons 228 2 ma in the pl rich leucosome layer are similar to magmatic zircons in terms of their hree enriched patterns with negative eu anomalies high y and low th contents and low hf y and th u ratios this indicates that the gneiss has experienced decompressional partial melting and the pl rich leucosome layers were formed by fractional crystallization of the primary melt during the early stage hot exhumation zircon grains 219 2 ma in the kfs rich veins are characterized by aqueous fluid melt related signatures with strongly hree enriched patterns high u pb and low th contents and low th u and high lu hf ratios indicating that the pegmatite vein was formed by crystallization of residual aqueous melt after the crystallization of pl rich leucosome layers during later stage cooling during exhumation our results suggest that the migmatitic gneiss successively recorded information concerning the protolith prograde hp metamorphism during subduction fractional crystallization of primary melt and crystallization of the residual melt during exhumation

sea level curve for the middle to early late ordovician in the armorican massif western france icehouse third order glacio eustatic cycles abstract this contribution deals with the middle to early late ordovician of the armorican massif western france the aim is to reconstruct a multi order eustatic curve from the stratigraphic record the studied section crozon peninsula area shows sediments deposited in tidal to storm dominated shelf environments a sequence analysis facies and stacking pattern analyses combined with a gamma ray record and a backstripping procedure was carried out chitinozoan assemblages provide the biostratigraphic framework including time calibration the stratigraphic succession is dominated by the stacking of sequences representing different orders of sea level change i e very high formosa and pissotensis biozones of the darriwilian the reconstructed sea level curve reveals that most of the third order cycles comprise three fourth order cycles of assumed 400 kyr duration this 1 2 myr duration of the ordovician third ordersequences suggests that they may be related to long period obliquity cycles an orbital forcing characteristic of icehousesequences in the late cenozoic and the quaternary these results imply that the major part of the middle and early late ordovician was characterized by icehouse conditions and third orderglacio eustatic cycles a time calibration of the chitinozoan biozones was developed from the sequential analysis it suggests that the early darriwilian biozones from henryi to calix biozones have short durations of several 10 to several 100 kyr whereas the pissotensis biozone has a temporal extent of several myr extending into the early sandbian

a palaeoproterozoic dolomite vempalle formation cuddapah basin india showing phanerozoic type dolomitisation abstract the palaeoproterozoic vempalle formation of the cuddapah basin india significantly adds to our understanding of the evolution of precambrian marine carbonate systems and the redox state of the earth s early oceans a facies microfacies diagenetic geochemical examination of samples from a 1000 m long exposure in a freshly cut canal section shows that 10 15 of precursor limestone is still preserved in the vempalle formation in the form of remnant patches of calcimicrite and ooids with calcite spar cement the ooids preserving primary radial and concentric fabrics and radial fractures are considered to have been originally precipitated as calcite which may have been low mg in places the preserved calcite spar that is partially replaced by fabric destructive dolomite shows type i calcite twin lamellae petrographic observations demonstrate that vempalle formation dolomite formed through very early precipitation which in stromatolites preserved microbial filaments as well as through fabric destructive dolomitization during shallow to moderate burial vempalle formation dolomite is characterized by micritic dolomite crystals which suggest rapid early dolomitization of lime mud and micritic calcite from a supersaturated mg ca rich solution probably near surface or during shallow burial depletion of na and sr contents of vempalle formation dolomite along with negative 18o values indicate dolomite recrystallisation during burial and further replacement dolomite 13c values of 0 5 to 2 are likely inherited original marine values geochemical proxies trace elements and rare earths imply that cuddapah basin seawater and dolomitizing fluids were anoxic and ferruginous but not euxinic geochemical analyses also indicate that the burial diagenetic fluids evolved from eu enriched seawater that probably resulted from continental rifting around 1 9 2 0 ga this probable ocean chemistry is in contrast with the anoxic ferruginous and extremely high mg ca dolomite oceans that prevailed during proterozoic time the vempalle dolomite shows more similarities with dolomitised phanerozoic platform carbonates than typical precambrian dolomite with its well preserved textures and burial dolospar cements

toward the reconciliation of seismological and petrological perspectives on oceanic lithosphere heterogeneity the character of the high frequency seismic phases po and so observed after propagation for long distances in the oceanic lithosphere requires the presence of scattering from complex structure in 3 d current models use stochastic representations of seismic structure in the oceanic lithosphere the observations are compatible with quasi laminate features with horizontal correlation length around 10 km and vertical correlation length 0 5 km with a uniform level of about 2 variation through the full thickness of the lithosphere such structures are difficult to explain with petrological models which would favor stronger heterogeneity at the base of the lithosphere associated with underplating from frozen melts petrological evidence mostly points to smaller scale features than suggested by seismology the models from the different fields have been derived independently with various levels of simplification fortunately it is possible to gently modify the seismological model toward stronger basal heterogeneity but there remains a need for some quasi laminate structure throughout the mantle component of the oceanic lithosphere the new models help to bridge the gulf between the different viewpoints but ambiguities remain

sequence stratigraphy and relative sea level history of the middle and upper ordovician of the nashville dome tennessee abstract eleven depositional sequences are present in the middle and upper ordovician of the nashville dome all but one lack lowstand systems tracts and many display evidence of subaerial exposure such as paleokarst or regional erosional truncation at their sequence boundaries which are combined with transgressive surfaces features formed during the transgression such as hardgrounds are more prominent at most sequence boundaries than are those features related to subaerial exposure all sequences have maximum flooding zones with aggradational stacking rather than discrete maximum flooding surfaces condensation features are much more common at transgressive surfaces than within the maximum flooding zone the expression of these parasequences and sequences changes with the onset of the taconic orogeny although parasequences from equivalent depositional environments can be recognized before and after the onset of the orogeny their facies composition changes completely sequences following the orogeny display fewer paleokarst features more pyritized and iron stained hardgrounds at flooding surfaces and transgressive surfaces and more phosphatic lags and mineralization at transgressive surfaces than those preceding the orogeny a relative sea level curve for the nashville dome has a long term 5 m y component presumably dominated by subsidence and a short term 1 3 m y component consisting of 10 50 m fluctuations that is primarily the result of eustasy

multiphase solid inclusions in uhp rocks su lu china remnants of supercritical silicate rich aqueous fluids released during continental subduction abstract primary multiphase solid ms inclusions without preserved fluid are found within peak minerals in kyanite quartzite topaz and kyanite phengite epidote eclogite from donghai area su lu terrane typical mineral association in inclusions is paragonite muscovite anhydrite corundum alunite type sulphate zircon calcite chlorite sio 2 barite pyrite apatite in quartzites and paragonite rutile apatite amphibole zn staurolite magnetite plagioclase zircon pyrite alunite type sulphate zn mg fe al ti spinel in eclogites on the basis of the fluid inclusion textures and of the daughter phase assemblage calculated fluid composition is as follows in quartzites 24 wt sio 2 30 wt al 2 o 3 9 wt cao 5 wt k 2 o 3 wt na 2 o 11 wt so 3 18 wt h 2 o with traces of tio 2 fe 2 o 3 feo mgo bao p 2 o 5 cl f and co 3 2 and in eclogites 26 wt sio 2 21 wt tio 2 20 wt al 2 o 3 2 wt mgo 4 wt feo 6 wt fe 2 o 3 7 wt cao 3 wt na 2 o 3 wt p 2 o 5 7 wt h 2 o 1 wt cl and traces of zno mno k 2 o so 3 and f originally trapped chloride poor aqueous fluids contained very high amounts in the order of tens of wt of si 4 al 3 and ti 4 solute species reflect the chemical composition of the host rocks mg 2 fe 2 ti 4 p 5 and na are abundant in the fluids present in eclogites while fluids are enriched in al 3 k na so 4 2 co 3 2 in quartzites we propose that multiphase solid inclusions represent remnants of high density supercritical silicate rich aqueous fluids that were in equilibrium with peak minerals at uhp conditions these fluids show characters which are transitional between aqueous fluids and silicate melts and were probably produced by dehydration reactions of the host rocks during the latest stages of subduction

controls on geochemical expression of subaerial exposure in ordovician limestones from the nashville dome tennessee u s a abstract stable isotope and trace element data from 301 samples in six stratigraphic sections largely support previous inferences of subaerial exposure at third order sequence boundaries in mohawkian and cincinnatian limestones of the nashville dome they also reveal the presence of previously unrecognized surfaces of subaerial exposure for example evidence of subaerial exposure at the tops of parasequences suggests that at least some of the latter are better interpreted as high frequency sequences clusters of surfaces of subaerial exposure near some previously interpreted sequence boundaries suggest that the latter may represent sequence boundary zones some surfaces of subaerial exposure are at neither recognized sequence boundaries nor parasequence boundaries and may represent missed beats in which sea level fluctuations are preserved geochemically but not recognized on stratigraphic criteria recognition of these unexpected surfaces of subaerial exposure may in part depend on the close spacing of samples used in this study different sequence boundaries exhibit different degrees of geochemical alteration and more extensive alteration appears to characterize sequence boundaries of greater temporal significance overprinting of exposure surfaces is controlled by accommodation rate in that meteoric alteration as suggested by low c isotope composition and sr concentration is seemingly pervasive in an interval of slow accommodation but not in an interval of rapid accommodation 18o values show little relationship to exposure surfaces in these strata so that sr 13c plots are more useful than 18o 13c plots in recognizing meteoric diagenesis in aggregated data in this data set 13c excursions interpreted to result from pedogenic processes are at most 2 4 and extend at most only 1 5 meters below exposure surfaces less than the extent of such signatures in younger carbonates such minima are best developed in paleo lowland and paleo coastal settings and they are essentially unrecognizable in paleo inland settings where more section was removed by lowstand and transgressive erosion

the source scanning algorithm mapping the distribution of seismic sources in time and space summary we introduce a new method which we call the source scanning algorithm ssa for imaging the distribution of seismic sources in both time and space using trial locations and origin times the method calculates the brightness function by summing the absolute amplitudes observed at all stations at their respective predicted arrival times the spatial and temporal distribution of sources is then identified by a systematic search throughout the model space and time for the maximum brightness the greatest advantages of this method are that 1 it exploits waveform information both arrival times and relative amplitudes without the need to calculate highfrequency synthetic seismograms and 2 it requires neither pre assembled phase picking data nor any a priori assumptions about the source geometry a series of tests using synthetic data have shown that this method is robust and can faithfully recover the input source configuration to within 1 grid interval finally we demonstrate the value of the algorithm by locating a typical tremor event with emergent waveforms that occurred during the recent episodic tremor and slip ets sequence in the northern cascadia subduction zone

introduction to tectonics of china abstract the continental crust of china is a mosaic of cratonic blocks and orogenic belts containing small cratons and terranes with various tectonic settings they have diverse origins and complex histories of amalgamation and often suffered repeated reworking after multiple episodes of amalgamation in the last three decades extensive geological geochemical and geophysical investigations have been carried out on these cratonic blocks and intervening orogenic belts producing an abundant amount of new data and competing interpretations this provides important insights into understanding the formation and evolution of the chinese continents the papers assembled in this volume present a timely and comprehensive overview on major advancements and controversial issues related to the formation and evolution of continental crust in china complex tectonic histories were experienced not only by the large scale cratonic blocks and orogenic belts but also by small scale terranes and orogens between and inside these blocks nevertheless our understanding of lithotectonic units and geological processes has been greatly advanced by recent studies of zirconology and geochemistry for various rock types from major petrotectonic units in china it has been further advanced from integrated interpretations of geochemical and petrological data for petrogenesis of magmatic rocks an overview of these observations and interpretations provides new insights into understanding the continental plate tectonics and the chemical geodynamics of subduction zones

seismotectonics of portugal and its adjacent atlantic area abstract a study of focal mechanisms of earthquakes and stress indicators are presented for portugal and its adjacent atlantic margins recently riberio et al 1996 published a work on this subject in this study we introduce new focal mechanisms of nine earthquakes we use these results together with 12 other focal mechanisms and the main tectonic features in the region to define more accurately the type of mechanism and the stress pattern of the region all the focal mechanisms used in this study differ partially but are compatible with the regional stress field the stress indicators resulting from focal mechanisms and other geological and geophysical data show that the portugal continental and its atlantic margins are under horizontal pressure in nnw sse direction with a greater proportion of strike slip and reverse oblique mechanisms for the whole area

the relative motion between africa and eurasia as derived from itrf2000 and gps data 1 studies of intra and inter plate deformation typically need a model describing the motions of the stable part of the tectonic plates for reference purposes we have developed deos2k a model for the current motion of seven major tectonic plates derived from space geodetic observations this paper focuses on relative motion between africa and eurasia in the past this motion has been poorly established because of poor data coverage for africa deos2k is based on itrf2000 altamimi et al 2002 and new african gps observations it is an improvement over the nuvel 1a model for predicting the present day relative motions of these two plates deos2k predicts in northeastern africa that africa eurasia relative motion is about 40 smaller in magnitude than nuvel 1a and trends more to the northwest this is consistent with independent local geodetic observations a similar shift in orientation clockwise is observed at the western tip of the plate boundary

measuring of clock drift rates and static time offsets of ocean bottom stations by means of ambient noise marine seismology usually relies on temporary deployments of stand alone seismic ocean bottom stations obs which are initialized and synchronized on ship before deployment and re synchronized and stopped on ship after recovery several months later in between the recorder clocks may drift and float at unknown rates if the clock drifts are large or not linear and cannot be corrected for seismological applications will be limited to methods not requiring precise common timing therefore for example array seismological methods which need very accurate timing between individual stations would not be applicable for such deployments we use an obs test array of 12 stations and 75 km aperture deployed for 10 months in the deep sea 4 5 5 5 km of the mid eastern atlantic the experiment was designed to analyse the potential of broad band array seismology at the seafloor after recovery we identified some stations which either show unusual large clock drifts and or static time offsets by having a large difference between the internal clock and the gps signal skew we test the approach of ambient noise cross correlation to synchronize clocks of a deep water obs array with km scale interstation distances we show that small drift rates and static time offsets can be resolved on vertical components with a standard technique larger clock drifts several seconds per day can only be accurately recovered if time windows of one input trace are shifted according to the expected drift between a station pair before the cross correlation we validate that the drifts extracted from the seismometer data are linear to first order the same is valid for most of the hydrophones moreover we were able to determine the clock drift at a station where no skew could be measured furthermore we find that instable apparent drift rates at some hydrophones which are uncorrelated to the seismometer drift recorded at the same digitizer indicate a malfunction of the hydrophone

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

role of partial melting in the evolution of the sulu eastern china ultrahigh pressure terrane strongly deformed potassium feldspar rich dikes are widely distributed in the northern part of the sulu ultrahigh pressure uhp metamorphic terrane eastern china the fact that the crystallization ages of these dikes overlap with the age of peak uhp metamorphic conditions implies the presence of melt during metamorphism sr isotopic ratios of the dikes are compatible with their origin as partial melts of the dominant felsic sulu gneiss partial melting may be the key to solving several unusual features of the sulu and other uhp terranes such as the almost complete lack of mineralogical evidence for uhp conditions and the limited growth of zircon during uhp conditions in the dominant felsic gneiss in addition because partial melting will cause a drastic reduction in the strength of the uhp gneisses the most likely exhumation mechanism is diapiric rise of a low viscosity partially molten mass containing entrained blocks of eclogite and not a thin sheet as usually proposed

comparative sequence stratigraphy of two classic upper ordovician successions trenton shelf new york ontario and lexington platform kentucky ohio implications for eustasy and local tectonism in eastern laurentia abstract comparison of the classic upper ordovician mohawkian to lower cincinnatian caradoc to lower ashgill black river and trenton groups in new york state southern ontario with the tyrone lexington formations exposed in the jessamine dome southern cincinnati arch in north central kentucky and southern ohio reveals striking similarities previous emphasis on complex local facies mosaic has obscured widespread regional patterns biostratigraphy and k bentonites provide broad constraints on inter regional correlations however an allostratigraphic approach permits higher resolution correlations and a partial test of eustatic vs strictly local tectonic models to explain stratigraphic patterns upper mohawkian to lower cincinnatian 455 449 ma depositional sequences previously recognized for the jessamine dome and nashville dome areas are correlated between the two main study areas and further refined chatfieldian rocklandian to shermanian or cobourgian of traditional terminology sequences m5 and m6 of previous workers are interpreted to be composite sequences and are each subdivided into three smaller scale sequences which also have counterparts in the new york ontario strata in turn these correlations indicate at least partial allocyclic control on sedimentary cycles complex lateral variations within depositional sequences especially in the late shermanian to edenian indicate that tectonically controlled patterns of basinal subsidence and uplift of crustal blocks perhaps reflecting forebulge migration exerted a strong influence on the local facies and motif of depositional sequences these tectonic features however did not obliterate the underlying allocyclic pattern indeed high resolution sequence stratigraphy enables detailed resolution of shifting patterns of minor uplift and subsidence

record of microendoliths in different facies of the upper ordovician in the cincinnati arch region usa the early history of light related microendolithic zonation abstract this study provides the first documentation of microendolithic borings made by cyanobacteria algae and fungi in invertebrate skeletons from the late ordovician of eastern north america samples from the upper mohawkian to cincinnatian series katian stage caradoc to lower ashgill of traditional usage limestones and calcareous shales of ohio indiana and kentucky were selected for study because of the excellent preservation of fossil shell material the substrates mostly are brachiopods and corals strata of the mohawkian lexington limestone and cincinnatian kope formation maysville and richmond groups have been recently studied in terms of facies sequence stratigraphy and paleoenvironments which facilitates a reliable comparison of the distribution of microborings and inferred environmental settings especially relative paleobathymetry this paper reports the first occurrence of 10 ichnospecies plus various conchocelis traces in late ordovician cincinnatian marine shells these traces have been previously observed in shells from younger geological periods their modern counterparts are produced by species of cyanobacteria green algae red algae and heterotrophic microendoliths considering the bathymetric index ichnocoenoses of microendoliths defined for the later phanerozoic it appears that only one or two light related bathymetric zones can be recognized in a few samples from the upper ordovician cincinnatian of ohio and kentucky but not in all deposits where they might have been expected the shallow euphotic zone iii and with less certainty the deep euphotic zone on the contrary our samples from the underlying lexington formation contain only borings of worms macrotubular form and some forms with unknown producers although they were collected in facies realms comparable to the younger samples given the present state of knowledge the best explanation for the appearance of these 10 ichnospecies and the first bathymetric index ichnocoenoses within the late ordovician is that they represent the early evolution of microendolithic ichnospecies and early emplacement of ichnocoenoses

dating of zircon from ti clinohumite bearing garnet peridotite implication for timing of mantle metasomatism garnet peridotites from the kokchetav ultrahigh pressure uhp massif contain abundant volatile and high field strength element hfse bearing minerals such as ti clinohumite and zircon these characteristics are interpreted to be related to fluid infiltrated mantle metasomatism from the oceanic lithosphere that had been subducted below the area the zircons from the peridotites were dated by using sensitive high resolution ion microprobe shrimp and yielded apparent u pb ages of 554 494 ma weighted mean age 528 ma that are mostly consistent with the timing of uhp metamorphism deduced from diamond bearing country rocks in this massif these zircons have an almost flat rare earth element ree pattern and very low ree concentrations these characteristics are similar to those observed in kimberlitic zircons inherited zircon cores although only rarely preserved yielded apparently proterozoic ages and have different trace element characteristics compared to the overgrowth rims these features indicate that the mantle metasomatic events and the recrystallization of ti clinohumite and zircon were due to hfse enriched fluid infiltration during the uhp metamorphism at great depths the metasomatized mantle may have been transported farther into the deep upper mantle and contributed to the source of intraplate magmas such as kimberlites and alkali basalts because these rocks have characteristically high volatile and hfse concentrations such as those of the ti clinohumite bearing garnet peridotites

small scale heterogeneities in the oceanic lithosphere inferred from guided waves 1 we analyze seismic waveforms from deep focus earthquakes occurring in the subducting slab beneath japan recorded by broadband ocean bottom seismometers bbobss installed on the northwestern pacific ocean seafloor the data reveal waveforms with a low frequency direct p onset followed by large amplitude high frequency long duration po and so waves from the analysis of the bbobs records and a numerical finite difference method simulation of seismic wave propagation we elucidate the generation and propagation processes of such guided waves we demonstrate that the low frequency direct p and s waves propagate in the asthenosphere and that the following high frequency long duration po and so waves are developed by multiple forward scattering of p and s waves the scattering occurs due to laterally elongated heterogeneities in both the subducting and horizontal parts of the oceanic lithosphere with the apparent velocities vp 8 1 km s vs 4 6 km s being close to the velocities of oceanic lithosphere

records of crustal metasomatism in the garnet peridotites of the ulten zone upper austroalpine eastern alps peridotites we infer that the metasomatizing hydrous fluids could peridotites in the ulten zone upper austroalpine eastern alps represent the residual fluids left after the crystallization of leucosomes occur as small bodies within lower crustal rocks gneisses and starting from water undersaturated melts produced during migmigmatites subducted at eclogite facies conditions during the vamatization of the host gneisses riscan orogeny they record a complex metamorphic and deformation evolution as indicated by the transition from coarse grained spinelbearing peridotites to fine grained garnet amphibole bearing peridotites and are interpreted as portions of mantle wedge that

hyposat an enhanced routine to locate seismic events a program package called hyposat has been under development that attempts to use the maximum information possible to estimate the hypocenter of a seismic source the standard input parameters can be used arrival times of first and later onsets with backazimuths and ray parameters or apparent velocities in addition travel time differences between different phases observed at the same station can be optionally used the observed standard deviations are used to weight all input parameters and the inversion is done with a generalized matrix inversion code

ultrahigh pressure metamorphism in the forbidden zone the xugou garnet peridotite sulu terrane eastern china the xugou garnet peridotite body of the southern sulu ultrahigh pressure uhp terrane is enclosed in felsic gneiss bounded by faults and consists of harzburgite and lenses of garnet clinopyroxenite and eclogite the peridotite is composed of variable amounts of olivine fo91 enstatite en92 93 garnet alm20 23prp53 58knr6 9grs12 18 diopside and rare chromite the ultramafic protolith has a depleted residual mantle composition indicated by a high mg number very low cao al2o3 and total ree contents compared to primary mantle and other sulu peridotites most garnet prp44 58 clinopyroxenites are foliated except for rare kyanite bearing eclogitic bands most eclogites contain a simple assemblage of garnet alm29 34prp32 50grs15 39 omphacite jd24 36 minor rutile clinopyroxenite and eclogite exhibit lree depleted and lree enriched patterns respectively but both have flat hree patterns normalized la sm and yb contents indicate that both eclogite and garnet clinopyroxenite formed by high pressure crystal accumulation variable trapped melt from melts resulting from two stage partial melting of a mantle source recrystallized textures and p t estimates of 780 870 c 5 7 gpa and a metamorphic age of 231 11 ma indicate that both mafic and ultramafic protoliths experienced triassic uhp metamorphism in the p t forbidden zone with an extremely low thermal gradient 5 c km 1 and multistage retrograde recrystallization during exhumation develop of prehnite veins in clinopyroxenite eclogite felsic blocks and country rock gneiss and replacements of eclogitic minerals by prehnite albite white mica and k feldspar indicate low temperature metasomatism

interaction between the mid atlantic ridge and the azores hot spot during the last 85 myr emplacement and rifting of the hot spot derived plateaus 1 multiple and single beam bathymetric data are compiled over the azores plateau to produce a 1 km 1 km grid between latitudes 32 n and 49 n and longitudes 22 w and 43 w mantle bouguer anomalies are then calculated from this grid and the satellite derived gravity these grids provide new insights on the temporal and spatial variations of melt supply to the ridge axis the elevated seafloor of the azores plateau is interpreted as resulting from the interaction of a mantle plume with the mid atlantic ridge mar the presence of a large region of elevated seafloor associated with a thick crust between the great meteor seamounts and the azores platform on the africa plate and less developed conjugate structures on the north america plate favors genetic relations between these hot spot derived structures this suggests that a ridge hot spot interaction has occurred in this region since 85 ma this interaction migrated northward along the ridge axis as a result of the sse absolute motion of the africa plate following a direction grossly parallel to the orientation of the mar kinematic reconstructions from chron 13 35 ma to the present allow a proposal that the formation of the azores plateau began around 20 ma and ended around 7 ma a sharp bathymetric step is associated with the beginning of important melt supply around 20 ma the excess of melt production is controlled by the interaction of the ridge and hot spot melting zones the geometry and distribution of the smaller scale features on the plateau record episodic variations of the hot spot melt production the periodicity of these variations is about 3 5 myr following the rapid decrease of widespread volcanism the plateau was subsequently rifted from north to south by the mid atlantic ridge since 7 ma this rifting begins when the mar melting zone is progressively shifted away from the 200 km plume thermal anomaly these results bear important consequences on the motion of the africa plate relative to the azores hot spot they also provide an explanation to the asymmetric geochemical signature of the azores hot spot along the mid atlantic ridge

dehydration melting of ultrahigh pressure eclogite in the dabie orogen evidence from multiphase solid inclusions in garnet several types of multiphase solid ms inclusions are identified in garnet from ultrahigh pressure uhp eclogite in the dabie orogen the mineralogy of ms inclusions ranges from pure k feldspar to pure quartz with predominance of intermediate types consisting of k feldspar quartz silicate plagioclase or epidote barite the typical ms inclusions are usually surrounded with radial cracks in the host garnet similar to where garnet contains relict coesite barite aggregates display significant heterogeneity in major element composition with total contents of only 57 73 and highly variable sio2 contents of 0 32 25 85 that are positively correlated with bao and so3 contents the occurrence of ms inclusions provides petrographic evidence for partial melting in the uhp metamorphic rock the occurrence of barite aggregates with variably high sio2 contents suggests the coexistence of aqueous fluid with hydrous melt under hp eclogite facies conditions thus local dehydration melting is inferred to take place inside the uhp metamorphic slice during continental collision this is ascribed to phengite breakdown during hot exhumation of the deeply subducted continental crust as a consequence the aqueous fluid is internally buffered in chemical composition and its local sink is a basic trigger to the partial melting during the continental subduction zone metamorphism

metamorphic growth and recrystallization of zircon distinction by simultaneous in situ analyses of trace elements u th pb and lu hf isotopes in zircons from eclogite facies rocks in the sulu orogen simultaneous in situ analyses of trace elements u th pb and lu hf isotopes were carried out on distinct domains of zircons in ultrahigh pressure uhp eclogite facies metamorphic rocks from the main hole of the chinese continental scientific drilling ccsd in the sulu orogen for the first time trace elements are directly linked to lu hf isotopes in metamorphic zircons with reference to their u pb dates this enables methodological integration to distinguish four types of metamorphic zircon solid state replacement and dissolution recrystallizations of protolith zircons and new growth from the aqueous fluid metamorphically grown zircons are characterized by concordant u pb ages for the metamorphism flat hree patterns typical of the garnet effect low contents of ree especially hree y nb ta and th u high contents of hf low lu gd n lu hf and th u 0 1 ratios and elevated 176hf 177hf ratios relative to solid state recrystallized zircons this suggests the effects of both garnet and fluid on the growth of metamorphic zircons in contrast metamorphic recrystallization has reset the u th pb isotope system of protolith zircons to different extents depending on the extents of fluid action during metamorphism solid state recrystallized zircons exhibit the lowest degrees of resetting and thus almost inherit all geochemical features from the protolith zircons which are characterized by discordant u pb ages close to or below the protolith age steep mree hree patterns typical of magmatic origin high contents of trace elements and their ratios and low 176hf 177hf ratios on the other hand dissolution recrystallized zircons show the highest degrees of reworking and thus have concordant or nearly concordant u pb ages for the metamorphism steep mree hree patterns lowered contents of trace elements such as ree th u y nb ta and ti relative to the protolith zircons and almost unchanged hf isotope ratios replacement recrystallized zircons display intermediate degrees of reworking and thus have their many features of elements and isotopes in between while the metamorphic growth in the presence of both garnet and fluid is characterized by both depletion of hree with flat pattern and the low contents of trace elements the metamorphic recrystallization in the presence of aqueous fluid is indicated by gradual decreases of mree to hree without the flat hree pattern therefore the simultaneous in situ analyses of metamorphic zircons have the advantage over single term analyses in making distinction between the new growth and the different types of recrystallization

the middle caradoc facies and faunal turnover in the late ordovician baltoscandian palaeobasin abstract a late ordovician episode of remarkable biotic climatic sea level and facies changes named here as the middle caradoc facies and faunal turnover took place in the baltoscandian area this paper presents an integrated overview of these changes in the critical middle caradoc interval data are given on carbonate rock composition distribution and grain size composition of the siliciclastic material and the carbon isotopic composition of whole rock carbonates in cores of estonia and sweden the middle caradoc facies and faunal turnover can be described as a succession of related environmental changes the turnover began with a positive excursion in carbonate 13c and continued with sea level changes that led to a sedimentary hiatus on the shelf and a change from carbonate dominated to siliciclastic dominated sedimentation in the basin the turnover ended with an extinction event and associated microfaunal crisis the middle caradoc turnover in baltoscandia is comparable to a similar succession of changes in north america the turnover affected two palaeocontinents and reflects a widespread possibly global environmental change onset of glaciation on gondwana and or increased orogenic activity might have initiated the changes in ocean circulation and led to the initial carbon isotope excursion the following sea level rise and faunal changes affected several different continents

an extensive erosion surface of a strongly deformed limestone bed in the gushan and chaomidian formations late middle cambrian to furongian shandong province china sequence stratigraphic implications abstract in order to understand sequence development and sea level fluctuations during the late middle cambrian to early furongian on the north china epeiric platform the present study focuses on a unique subtle erosion surface of an extensive approx 100 km strongly deformed limestone bed in the uppermost part of the gushan formation shandong province china the gushan formation and the overlying chaomidian formation consist mainly of shales and a variety of carbonates that were deposited in subtidal environments e g deep subtidal shallow subtidal shoreface shoal subtidal microbial flat and restricted platform interior three third order depositional sequences s1 3 are identified each of which comprises a thin transgressive systems tract tst and a relatively thick highstand systems tract hst each sequence is bounded by a drowning unconformity sb1 a subaerial unconformity sb2 or a surface of submarine erosion sb3 the upper sequence boundary sb2 of sequence 1 s1 is represented by a subtle erosion surface of an extensive deformed limestone bed with a wide variety of soft sediment deformation structures e g lime mudstone breccias chaotic wacke packstone laminae and fragments homogenized oolites and clastic dykes and is overlain by small sporadic microbial buildups and an extensive bioclastic grainstone bed the deformed limestone was formed during early diagenesis by differential deformation processes brecciation liquefaction fluidization and injection which were most likely induced by pore water overpressure during the period of rapid sea level fall despite the lack of subaerial exposure features e g paleokarst paleosol etc the characteristics of the erosion surface cutting well lithified sediment below the missing of a significant geological record the prochuangia biozone and the worldwide correlatable positive carbon isotope excursion collectively indicate that the erosion surface developed under conditions of subaerial exposure after contemporaneous marine cementation of the deformed sediment the missing of the prochuangia biozone is most likely due to non deposition at a subaerial hiatal surface the erosion surface was submerged as a result of subsequent rise in sea level where sporadic microbial buildups formed under suitable conditions freshly deposited winnowed shell dominated transgressive lag deposits containing chuangia trilobite fragments brachiopod shells and abundant glauconite grains formed with continued rise in sea level which became in turn overlain by shale dominated facies the unique combination of the subtle erosion surface sensu stricto a subaerial unconformity and the underlying deformed limestone bed provides an important criterion for recognizing the subtle changes in relative sea level on shallow epeiric platforms

structure of the oceanic lithosphere and upper mantle north of the gloria fault in the eastern mid atlantic by receiver function analysis receiver functions rf have been used for several decades to study structures beneath seismic stations although most available stations are deployed on shore the number of ocean bottom station obs experiments has increased in recent years almost all obss have to deal with higher noise levels and a limited deployment time 1 year resulting in a small number of usable records of teleseismic earthquakes here we use obss deployed as mid aperture array in the deep ocean 4 5 5 5 km water depth of the eastern mid atlantic we use evaluation criteria for obs data and beam forming to enhance the quality of the rfs although some stations show reverberations caused by sedimentary cover we are able to identify the moho signal indicating a normal thickness 5 8 km of oceanic crust observations at single stations with thin sediments 300 400 m indicate that a probable sharp lithosphere asthenosphere boundary lab might exist at a depth of 70 80 km which is in line with lab depth estimates for similar lithospheric ages in the pacific the mantle discontinuities at 410 km and 660 km are clearly identifiable their delay times are in agreement with prem overall the usage of beam formed earthquake recordings for obs rf analysis is an excellent way to increase the signal quality and the number of usable events

sequence stratigraphy in the middle ordovician carbonate successions yeongheung formation mid east korea paleogeographic implications for a land detached ramp type platform abstract this study reveals the origin of the stratigraphic discrepancy between the yeongweol and taebaek groups and reconstructs paleogeographic model for the taebaeksan basin on the sino korean craton skc the global sea level fall in the earliest middle ordovician would have subaerially extensively exposed the lower ordovician peritidal successions forming the solution collapsed breccia zones on the sauk tippecanoe bounding surface the successive rise in sea level generated the fine grained siliciclastic successions in the taebaek group during early transgression since the carbonate production of the group was hampered by the strong input of siliciclastic sediments from terrestrial sources however such successions are absent in the yeongweol group because the yeongweol platform was distant from the siliciclastic sources and sustained carbonate production due to isolation from terrestrial influences the yeongweol group is interpreted as a land detached isolated ramp type platform as it responded to sea level fluctuations with restricted terrestrial influence whereas the taebaek group can be described as a land attached ramp type platform affected by a large input of siliciclastic sediments during lowstand and early transgression based on the comparison of stratigraphic patterns and configurations of land attached and detached ramp type successions this study concludes that the yeongweol sequence would have been formed on a land detached isolated carbonate ramp type platform located at the skc along the northeastern gondwana margin

imaging active faulting in a region of distributed deformation from the joint clustering of focal mechanisms and hypocentres application to the azores western mediterranean region abstract the matching between linear trends of hypocentres and fault planes indicated by focal mechanisms fms is frequently used to infer the location and geometry of active faults this practice works well in regions of fast lithospheric deformation where earthquake patterns are clear and major structures accommodate the bulk of deformation but typically fails in regions of slow and distributed deformation we present a new joint fm and hypocentre cluster algorithm that is able to detect systematically the consistency between hypocentre lineations and fms even in regions of distributed deformation we apply the method to the azores western mediterranean region with particular emphasis on western iberia the analysis relies on a compilation of hypocentres and fms taken from regional and global earthquake catalogues academic theses and technical reports complemented by new fms for western iberia the joint clustering algorithm images both well known and new seismo tectonic features the azores triple junction is characterised by fms with vertical pressure p axes in good agreement with the divergent setting and the iberian domain is characterised by nw se oriented p axes indicating a response of the lithosphere to the ongoing oblique convergence between nubia and eurasia several earthquakes remain unclustered in the western mediterranean domain which may indicate a response to local stresses the major regions of consistent faulting that we identify are the mid atlantic ridge the terceira rift the trans alboran shear zone and the north coast of algeria in addition other smaller earthquake clusters present a good match between epicentre lineations and fm fault planes these clusters may signal single active faults or wide zones of distributed but consistent faulting mainland portugal is dominated by strike slip earthquakes with fault planes coincident with the predominant nne ssw and wnw ese oriented earthquake lineations clusters offshore sw iberia are predominantly strike slip or reverse confirming previous suggestions of slip partitioning

techniques for assessing spatial heterogeneity of carbonate δ13c values implications for craton wide isotope gradients the sedimentary record of carbonate carbon isotopes 13ccarb provides one of the best methods for correlating marine strata and understanding the long term evolution of the global carbon cycle this work focuses on the late ordovician guttenberg isotopic carbon excursion a ca 2 5 positive 13ccarb excursion that is found in strata globally substantial variability in the apparent magnitude and stratigraphic morphology of the guttenberg excursion at different localities has hampered high resolution correlations and led to divergent reconstructions of ocean chemistry and the biogeochemical carbon cycle this work investigates the magnitude spatial scale and sources of isotopic variability of the guttenberg excursion in two sections from missouri usa centimetre scale isotope transects revealed variations in 13ccarb and 18ocarb greater than 2 across individual beds linear 13ccarb to 18ocarb mixing lines together with petrographic and elemental abundance data demonstrate that much of the isotopic scatter in single beds is due to mixing of isotopically distinct components these patterns facilitated objective sample screening to determine the least altered data a 18ocarb filter based on empirical 18ocarb values of well preserved carbonate mudstones allowed further sample discrimination the resulting least altered 13ccarb profile improves the understanding of regional as well as continental scale stratigraphic relations in this interval correlations with other laurentian sections strongly suggest that i small scale variability in guttenberg excursion 13ccarb values may result in part from local diagenetic overprinting ii peak guttenberg excursion 13ccarb values of the midcontinent are not distinct from their taconic equivalents and iii no primary continental scale spatial gradient in 13ccarb for example arising from chemically distinct aquafacies is required during guttenberg excursion time this study demonstrates the importance of detailed petrographic and geochemical screening of samples to be used for 13ccarb chemostratigraphy and for enhancing understanding of epeiric ocean chemistry

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

structure and evolution of the volcanic rift zone at ponta de sao lourenco eastern madeira ponta de sao lourenco is the deeply eroded eastern end of madeira s east west trending rift zone located near the geometric intersection of the madeira rift axis with that of the desertas islands to the southeast it dominantly consists of basaltic pyroclastic deposits from strombolian and phreatomagmatic eruptions lava flows and a dike swarm main differences compared to highly productive rift zones such as in hawai i are a lower dike intensity 50 60 dikes km and the lack of a shallow magma reservoir or summit caldera 40ar 39ar age determinations show that volcanic activity at ponta de sao lourenco lasted from 5 2 to 4 ma early madeira rift phase and from 2 4 to 0 9 ma late madeira rift phase with a hiatus dividing the stratigraphy into lower and upper units toward the east the distribution of eruptive centers becomes diffuse and the rift axis bends to parallel the desertas ridge the bending may have resulted from mutual gravitational influence of the madeira and desertas volcanic edifices we propose that ponta de sao lourenco represents a type example for the interior of a fading rift arm on oceanic volcanoes with modern analogues being the terminations of the rift zones at la palma and el hierro canary islands there is no evidence for ponta de sao lourenco representing a former central volcano that interconnected and fed the madeira and desertas rifts our results suggest a subdivision of volcanic rift zones into 1 a highly productive endmember characterized by a central volcano with a shallow magma chamber feeding one or more rift arms and 2 a less productive endmember characterized by rifts fed from deep seated magma reservoirs rather than from a central volcano as is the case for ponta de sao lourenco

the enigma of crustal zircons in upper mantle rocks clues from the tumut ophiolite southeast australia we suggest a new explanation for the presence of crustally derived zircons in the upper mantle rocks of ophiolitic complexes as an alternative to subduction related models integrated isotopic u pb hf and o isotopes and trace element data for zircons from the tumut ophiolitic complex southeast australia indicate that these grains are related to granitic magmatism and were introduced into the mantle rocks after their emplacement into the crust these observations emphasize that a clear understanding of the origin of individual zircon populations and their relationship to the host rock is essential to interpretations of the tectonic history of upper mantle rocks and the dynamics of crust mantle interactions

po so synthetics for a variety of oceanic models and their implications for the structure of the oceanic lithosphere summary polso synthetic seismograms were computed for a variety of oceanic structures in order to model data from an t zb 5 7 earthquake recorded during the oss iv seismic experiment satisfactory modelling of polso waveshapes and frequency content was achieved with no lateral heterogeneity but with a small random vertical heterogeneity in the mantle superimposed on a mean velocity structure which is consistent with seismic refraction data the random heterogeneity in the p and s velocities appears to be about 2 per cent with a scale length of 5 km this kind of heterogeneity is easily achieved by varying slightly the major mineral components in either the peridotite or eclogite mantle mineralogy and can be another cause of observed upper mantle anisotropy using a step function source time behaviour our numerical modelling also indicates that in the upper mantle qp is roughly proportional to rising from about 450 at f 1 hz to about 800 at f 15 hz and that q is roughly proportional to rising from about 900 at f 1hz to about 1800 at f 15hz a slight decrease in q from these values in any small zone of the upper mantle reduces so amplitudes drastically and we believe this is why so is sometimes missing from observed seismograms the computed synthetic record sections were also used to examine the relation between intrinsic q and the apparent q inferred from data in earlier studies apparent q was found to be lower than intrinsic q below about 5hz and higher than intrinsic q above 5 hz when these differences are accounted for the mantle q s of this study agree with the mantle q s of butler et al 1987 to within 10 per cent there can no longer be any doubt that oceanic upper mantle q s are very large above 5 hz and that q is about twice as large as qp

stratification of the earth beneath the azores from p and s receiver functions abstract seismic recordings of iris ida gsn station cmla and of several temporary stations in the azores archipelago are processed with p and s receiver function prf and srf techniques contrary to regional seismic tomography these methods provide estimates of the absolute velocities and of the vp vs ratio up to a depth of 300 km joint inversion of prfs and srfs for a few data sets consistently reveals a division of the subsurface medium into four zones with a distinctly different vp vs ratio the crust 20 km thick with a ratio of 1 9 in the lower crust the high vs mantle lid with a strongly reduced vp vs velocity ratio relative to the standard 1 8 the low velocity zone lvz with a velocity ratio of 2 0 and the underlying upper mantle layer with a standard velocity ratio our estimates of crustal thickness greatly exceed previous estimates 10 km the base of the high vs lid the gutenberg discontinuity is at a depth of 80 km the lvz with a reduction of s velocity of 15 relative to the standard iasp91 model is terminated at a depth of 200 km the average thickness of the mantle transition zone tz is evaluated from the time difference between the s410p and sks660p seismic phases that are robustly detected in the s and sks receiver functions this thickness is practically similar to the standard iasp91 value of 250 km and is characteristic of a large region of the north atlantic outside the azores plateau our data are indicative of a reduction of the s wave velocity of several percent relative to the standard velocity in a depth interval from 460 to 500 km this reduction is found in the nearest vicinities of the azores in the region sampled by the prfs but as evidenced by srfs it is missing at a distance of a few hundred kilometers from the islands we speculate that this anomaly may correspond to the source of a plume which generated the azores hotspot previously a low s velocity in this depth range was found with srf techniques beneath a few other hotspots

upper mantle s wave speed heterogeneity and anisotropy beneath the north atlantic from regional surface wave tomography the iceland and azores plumes summary we present a high resolution sv wave velocity and azimuthal anisotropy model for the upper mantle beneath the north atlantic and surrounding region derived from the analysis of over 3000 fundamental and higher mode rayleigh waveforms much of the data set comes from global and national digital seismic networks but to improve the path coverage we have also deployed a number of instruments at coastal sites in northwest europe iceland and eastern greenland the dense path coverage wide azimuthal distribution substantial higher mode content and the relatively short path lengths in the data set have enabled us to build an upper mantle model for the region with a horizontal resolution of a few hundred kilometres extending to 400 km depth three major hotspots iceland azores and eifel exist within the region of the model and slow upper mantle velocities are associated with each of these areas the best depth resolution in our model occurs in nw europe and in this area low sv velocities in the vicinity of the eifel hotspot extend to approximately 400 km depth major negative velocity anomalies exist in the north atlantic upper mantle beneath both iceland and the azores hotspots both anomalies are above 200 km depth 5 7 per cent slow with respect to prem and are elongated along the mid atlantic ridge low velocities extend to the south of iceland beneath the reykjanes ridge where other geophysical and geochemical observations have indicated the presence of hot plume material a similar but somewhat weaker feature exists beneath the kolbeinsey ridge north of iceland where there is also supporting evidence for the presence of hot plume material this observation might also be associated with a plume beneath jan mayen the low velocity structure beneath the azores hotspot is also elongated along the mid atlantic ridge the fast propagation direction of horizontally propagating sv waves in the atlantic south of iceland correlates well with the east west ridge spreading direction at all depths and changes to a direction close to ns in the vicinity of iceland

revised stratigraphy of the middle simcoe group ordovician upper sandbian katian in its type area an integrated approach the upper ordovician bobcaygeon formation of southern ontario is a widespread unit that spans the sandbian katian stage boundary and contains exceptionally preserved invertebrate fossil assemblages

observations of flexure and the rheology of oceanic lithosphere the principal evidence for the long term i e 1 myr mechanical behaviour of the oceanic lithosphere has come from studies of how it deforms in response to large loads such as volcanoes and sediments a model widely used to explain the deformation is an elastic plate in which flexural rigidity depends on the thermal age of the lithosphere at the time of loading an elastic model however is time invariant and does not take into account temporal changes that may occur in the flexural rigidity as a consequence of loading there is evidence for example that the flexural rigidity of the oceanic lithosphere also depends on load age being large at short times and small at long times thus competing effects may exist between thermal cooling which strengthens the lithosphere and some form of load induced stress relaxation which weakens it in order to investigate the relative roles of these processes we have developed a multilayered viscoelastic model which is based on the results of experimental rock mechanics that creep in the lithosphere is a thermally activated process and on a thermal structure that is given by the plate cooling model by comparing the predictions of the model with a new compilation of flexural rigidity estimates we have found that if the upper mantle viscosity is 1020 pa s the activation energy that best describes the long term mechanical behaviour of the oceanic lithosphere is 120 kj mol 1 this parameter pair explains the dependence of flexural rigidity on both plate and load age it also helps account for the subsidence and uplift history of oceanic islands and the stratigraphic patterns that develop in the flexural moats that flank them at atolls a multilayered viscoelastic model explains the rapid subsidence that follows shield building and does not require that the age of the lithosphere that supports a volcano is thermally re set to a younger value our studies suggest that while the oceanic lithosphere has viscoelastic properties the viscosity of its upper layers is so much higher than that of its lower layers that in effect it behaves as a thin elastic plate on long timescales there will therefore be a certain permanence to the observations of oceanic flexure such that they may be used with some confidence to evaluate the tectonic setting of individual features of the seafloor and in some cases their age

frictional behavior of oceanic transform faults and its influence on earthquake characteristics 1 we use a three dimensional strike slip fault model in the framework of rate and state dependent friction to investigate earthquake behavior and scaling relations on oceanic transform faults otfs gabbro friction data under hydrothermal conditions are mapped onto otfs using temperatures from 1 a half space cooling model and 2 a thermal model that incorporates a visco plastic rheology non newtonian viscous flow and the effects of shear heating and hydrothermal circulation without introducing small scale frictional heterogeneities on the fault our model predicts that an otf segment can transition between seismic and aseismic slip over many earthquake cycles consistent with the multimode hypothesis for otf ruptures the average seismic coupling coefficient c is strongly dependent on the ratio of seismogenic zone width w to earthquake nucleation size h c increases by four orders of magnitude as w h increases from 1 to 2 specifically the average c 0 15 0 05 derived from global otf earthquake catalogs can be reached at w h 1 2 1 7 further in all simulations the area of the largest earthquake rupture is less than the total seismogenic area and we predict a deficiency of large earthquakes on long transforms which is also consistent with observations to match these observations over this narrow range of w h requires an increase in the characteristic slip distance dc as the seismogenic zone becomes wider and normal stress is higher on long transforms earthquake magnitude and distribution on the gofar and romanche transforms are better predicted by simulations using the visco plastic model than the half space cooling model

late ordovician glaciation initiated by early land plant evolution and punctuated by greenhouse mass extinctions abstractlate ordovician hirnantian glaciation indicated by periglacial paleosols tillites and glacial pavements in saharan africa has been attributed to advances in weathering and carbon sequestration due to evolution of early land plants in the same way that devonian permian glaciation has been attributed to evolution of forests and quaternary glaciations to evolution of grasslands two problems for carbon cycle explanations of this glaciation are an hirnantian co2 greenhouse estimated at 4035 ppmv from mass balance models and possibly related hirnantian mass extinctions this estimate of high co2 is a 10 m yr average here evaluated against high resolution sequences of paleosols that give estimates not only of co2 but also of paleoclimate and vegetation through the late ordovician katian paleosols of the eastern united states show increases in depth and in degree of carbon consumption and storage comparable with those preceding glaciation of the devonian permian and quaternary early thalloid l

the 40ar 39ar age dating of the madeira archipelago and hotspot track eastern north atlantic the 40ar 39ar ages for 35 volcanic rocks and 14c ages for two charcoal samples from the madeira archipelago and ampere seamount eastern north atlantic are presented the volcanic evolution of madeira can be divided into a voluminous shield stage 4 6 0 7 ma and a subsequent low volume posterosional stage 70 myr old hotspot which formed porto santo island 11 1 14 3 ma seine ampere 31 ma corral patch and ormond 65 67 ma feraud et al 1982 1986 seamounts and the serra de monchique 70 72 ma mcintyre and berger 1982 complex in southern portugal age and spatial relationships result in a calculated absolute african plate motion above the hotspot of 1 2 cm yr around a rotation pole located at 43 36 n 24 33 w

detection and characterization of seismic phases using continuous spectral estimation on incoherent and partially coherent arrays summary seismic arrays are employed in the global monitoring of earthquakes and explosions because of their superior ability to detect and estimate the direction of incident seismic arrivals traditional beamforming and f k analysis require waveform semblance over the full array aperture and cannot be applied in many situations where signals are incoherent between sensors the norsar and mjar arrays are two primary ims stations where this is the case for high frequency regional phases large intersite distances and significant geological heterogeneity at these arrays result in waveform dissimilarity which precludes coherent array processing in the frequency bands with optimal snr multitaper methods provide low variance spectral estimates over short time windows and seismic arrivals can be detected on single channels using a non linear spectrogram transformation which attains local maxima at times and frequencies characterized by an energy increase this detection procedure requires very little a priori knowledge of the spectral content of the signal the transformed spectrograms can be beamformed over large aperture arrays or networks according to theoretical time delays resulting in an incoherent detection system which does not require waveform semblance at any frequencies we outline a real time automatic detection system for regional phase arrivals on the norsar array and demonstrate how stable and accurate slowness and azimuth estimates can be obtained for quite marginal signals in the case of partially coherent arrays the procedure described may provide stable if low resolution estimates which can subsequently be refined using coherent processing over subsets of sensors in particular we illustrate how the spectrogram beamforming method facilitates a stable and accurate slowness estimate for the incoherent high frequency pn arrival at the mjar array in japan from the 2006 october 9 underground nuclear test in north korea

oceanic lithospheric s wave velocities from the analysis of p wave polarization at the ocean floor our knowledge of the absolute s wave velocities of the oceanic lithosphere is mainly based on global surface wave tomography local active seismic or compliance measurements using oceanic infragravity waves the results of tomography give a rather smooth picture of the actual s wave velocity structure and local measurements have limitations regarding the range of elastic parameters or the geometry of the measurement here we use the p wave polarization apparent p wave incidence angle of teleseismic events to investigate the s wave velocity structure of the oceanic crust and the upper tens of kilometres of the mantle beneath single stations in this study we present an up to our knowledge new relation of the apparent p wave incidence angle at the ocean bottom dependent on the half space s wave velocity we analyse the angle in different period ranges at ocean bottom stations obs to derive apparent s wave velocity profiles these profiles are dependent on the s wave velocity as well as on the thickness of the layers in the subsurface consequently their interpretation results in a set of equally valid models we analyse the apparent p wave incidence angles of an obs data set which was collected in the eastern mid atlantic we are able to determine reasonable s wave velocity depth models by a three step quantitative modelling after a manual data quality control although layer resonance sometimes influences the estimated apparent s wave velocities the apparent s wave velocity profiles are well explained by an oceanic prem model in which the upper part is replaced by four layers consisting of a water column a sediment a crust and a layer representing the uppermost mantle the obtained sediment has a thickness between 0 3 km and 0 9 km with s wave velocities between 0 7 km s 1 and 1 4 km s 1 the estimated total crustal thickness varies between 4 km and 10 km with s wave velocities between 3 5 km s 1 and 4 3 km s 1 we find a slight increase of the total crustal thickness from 5 km to 8 km towards the south in the direction of a major plate boundary the gloria fault the observed crustal thickening can be related with the known dominant compression in the vicinity of the fault furthermore the resulting mantle s wave velocities decrease from values around 5 5 km s 1 to 4 5 km s 1 towards the fault this decrease is probably caused by serpentinization and indicates that the oceanic transform fault affects a broad region in the uppermost mantle conclusively the presented method is useful for the estimation of the local s wave velocity structure beneath ocean bottom seismic stations it is easy to implement and consists of two main steps 1 measurement of apparent p wave incidence angles in different period ranges for real and synthetic data and 2 comparison of the determined apparent s wave velocities for real and synthetic data to estimate s wave velocity depth models

seismicity source mechanisms and tectonics of the azores gibraltar plate boundary abstract a study and partial revision of the seismicity and source mechanisms of earthquakes are presented for the oceanic plate boundary of eurasia and africa from the azores islands to the strait of gibraltar focal mechanisms of 36 earthquakes wave form modelling of 4 and seismic moments and dimensions of 28 have been calculated these results indicate that the boundary is divided into three sections with different characteristics the azores the central section and the east section the azores section has high seismicity of moderate magnitude and a low slip rate 0 76 cm yr the predominant mechanism is normal or transform faulting with horizontal tensions on average in a n25 e direction in the central section large earthquakes occur with strike slip right lateral motion along faults in an e w direction there is a high slip rate 3 39 cm yr the east section is of more complex nature large earthquakes also occur but the predominant mechanism is reverse faulting with a horizontal pressure axis in the n30 w direction resulting in underthrusting of africa from the slip vectors the pole of rotation for the african plate is located at 28 2 n 21 1 w nearer to the boundary than previous locations close to the continent there is a continuation in the ocean crust of some structural features of the iberian peninsula

high resolution sequence stratigraphy of a mixed carbonate siliciclastic cratonic ramp upper ordovician kentucky ohio usa insights into the relative influence of eustasy and tectonics through analysis of facies gradients abstract detailed facies analysis and event stratigraphy of an upper ordovician rocklandian edenian cratonic ramp succession in eastern north america yields insights into eustatically driven sequence architecture and localized tectonic instability seven predominantly subtidal mixed carbonate siliciclastic depositional sequences 3rd order are identified and correlated across the length of a 275 km ramp to basin profile within the larger depositional sequences 3rd order at least two smaller orders 4th and 5th of cyclicity are recognizable three systems tracts occur within each sequence transgressive tst highstand hst regressive rst and are considered in terms of their component parasequences 5th order generally tsts are composed of skeletal grainstone rudstone facies hsts are dominated by shaly nodular wacke packstone facies and rsts are mostly calcarenite facies systems tracts sequence boundaries and their correlative conformities maximum flooding surfaces and forced regression surfaces were traced from shallow shelf to basinal settings this high resolution framework also provides insight into the timing of tectonic fluctuations on this cratonic ramp during the taconic orogeny and documents the relative influence of tectonism on lateral facies distributions and eustatically derived cyclicity

quantification of macroscopic subaerial exposure features in carbonate rocks abstract the macroscopic features that characterize subaerial exposure surfaces in carbonates are well known but their significance has not been quantitatively evaluated this study presents such an analysis in the lower oligocene suwannee limestone of west central florida eleven cores were point counted on a foot by foot basis for the abundance of caliche rhizoliths karst breccia open vugs infiltrated sediment fractures and pedogenic alteration these features occur at and below intraformational exposure surfaces which represent hiatuses estimated at 104 to 105 years and an uppermost sequence bounding unconformity representing 0 5 myr as revealed by sr isotope data statistical analyses of the point count data reveal only a few significant relationships 1 the hierarchy of exposure surfaces and by inference duration of exposure is differentiated only at a marginally significant level by sediment filled vugs preferentially associated with the sequence boundary duration of exposure did not have a significant impact on the relative abundance of all other features 2 proximity 5 ft to both types of surfaces fractures open vugs and infiltrated sediment show no proximal or distal preference for either type of surface 3 depositional texture has no statistically significant affect on the presence or abundance of the exposure features with the exception that rhizoliths and open vugs are preferentially more abundant in packstones relative to grainstones this is interpreted to be the result of a soil moisture effect factor analysis defines four factors that explain 46 to 52 of the total variance in the abundance data relative to the sequence boundary and the intraformational surfaces respectively the loading of each exposure feature on each factor is the same with respect to both types of surfaces which is further evidence that the abundance of exposure features is independent of duration of exposure factor 1 is interpreted to be the amplitude of base level changes and controls the abundance of karst breccia factor 2 is interpreted to be abundance of vegetation and relates to the abundance of rhizoliths and fractures factor 3 is interpreted to be a combination of soil zone pco2 and the availability of water and affects the abundance of pedogenic overprinting caliche and open vugs factor 4 is stratigraphic proximity to the sequence boundary which controls the presence of sediment filled voids the amount of uncorrelated unique variance associated with infiltrated sediments pedogenic overprinting caliche and open vugs is large 60 meaning that feature abundance is also influenced by other unidentified site specific factors these results demonstrate that quantifying the abundance of macroscopic subaerial exposure features in limestones has the potential to yield more insight into the significance of those features than a mere qualitative assessment this is particularly true when assessing the potential role of the many variables that can affect the development of these features

carbonate platform growth and cyclicity at a terminal proterozoic passive margin infra krol formation and krol group lesser himalaya india the infra krol formation and overlying krol group constitute a thick 2 km carbonate rich succession of terminal proterozoic age that crops out in a series of doubly plunging synclines in the lesser himalaya of northern india the rocks include 18 carbonate and siliciclastic facies which are grouped into eight facies associations 1 deep subtidal 2 shallow subtidal 3 sand shoal 4 peritidal carbonate complex 5 lagoonal 6 peritidal siliciclastic carbonate 7 incised valley fill and 8 karstic fill the stromatolite rich peritidal complex appears to have occupied a location seaward of a broad lagoon an arrangement reminiscent of many phanerozoic and proterozoic platforms growth of this complex was accretionary to progradational in response to changes in siliciclastic influx from the south eastern side of the lagoon metre scale cycles tend to be laterally discontinuous and are interpreted as mainly autogenic variations in the number of both sets of cycles and component metre scale cycles across the platform may result from differential subsidence of the interpreted passive margin apparently non cyclic intervals with shallow water features may indicate facies migration that was limited compared with the dimensions of facies belts correlation of these facies associations in a sequence stratigraphic framework suggests that the infra krol formation and krol group represent a north to north westfacing platform with a morphology that evolved from a siliciclastic ramp to carbonate ramp to peritidal rimmed shelf and finally to open shelf this interpretation differs significantly from the published scheme of a basin centred on the lesser himalaya with virtually the entire infra krol krol succession representing sedimentation in a persistent tidal flat environment this study provides a detailed neoproterozoic depositional history of northern india from rift basin to passive margin and predicts that genetically related neoproterozoic deposits if they are present in the high himalaya are composed mainly of slope basinal facies characterized by fine grained siliciclastic and detrital carbonate rocks lithologically different from those of the lesser himalaya

reevaluation of magnetic chrons in the north atlantic between 35 n and 47 n implications for the formation of the azores triple junction and associated plateau 1 in this paper we present a new magnetic compilation for an area of the north atlantic located between 35 n and 47 n and up to anomaly 33r we also present a strategy to pick magnetic isochrones and compute finite rotation poles this technique is based on a continuous reduction to the pole technique and some basic assumptions regarding the direction of the remanent magnetization vector a cost function that measures the misfit between interpreted and rotated isochrones and the systematic exploitation of the parameter space is used to compute the best set of finite eulerian rotations for the chrons 5 6 6c 11 12 13 18 20 21 22 23 24 25 26 28 29 30 32 33 and 33r this set of chrons and poles is used to discuss the evolution of the north atlantic close to iberia and in particular the onset and early development of the azores triple junction area we show that the relative motion between the eurasian and the african plates can be coherently described in terms of rigid plate kinematics respecting both the anomalies shapes and the precise location of the main structural elements of the area the pico fracture zone the east azores fracture zone and the gloria fault we distinguish from the magnetic point of view two different areas of the azores plateau the south azores domain where almost undisturbed nnw magnetic lineations can be found and the azores domain close to the topographic highs and with no systematic magnetic stripping with the exception of a few recent lineations probably matuyama and brunhes we present an approximate reconstruction of the plate configuration after chron 18 to conclude that the attachment of iberia to eurasia was younger than previously thought lower miocene triggering the formation of the azores domain in which stretching took place essentially in the last 20 ma at an average rate of 3 8 mm a and progressively attaching the south azores domain to the african plate by a northward progression of the triple junction

metamorphic growth and recrystallization of zircons in extremely 18o depleted rocks during eclogite facies metamorphism evidence from u pb ages trace elements and o hf isotopes abstract a combined in situ sims and la mc icpms study of u pb ages trace elements o and lu hf isotopes was conducted for zircon from eclogite facies metamorphic rocks in the sulu orogen the two microbeam techniques sampled various depths of zircon domains revealing different element and isotope relationships between residual magmatic cores and new metamorphic rims and thus the geochemical architecture of metamorphic zircons which otherwise cannot be recognized by the single microbeam technique this enables discrimination of metamorphic growth from different subtypes of metamorphic recrystallization magmatic cores with u pb ages of 769 9 ma have positive 18o values of 0 1 10 1 high th u and 176lu 177hf ratios high ree contents and steep mree hree patterns with negative eu anomalies they are interpreted as crystallizing from positive 18o magmas during protolith emplacement in contrast newly grown domains have concordant u pb ages of 204 4 to 252 7 ma and show negative 18o values of 10 0 to 2 2 low th u and 176lu 177hf ratios low ree contents and flat hree patterns with weak to no eu anomalies they are interpreted as growing from negative 18o fluids that were produced by metamorphic dehydration of high t glacial hydrothermally altered rocks during continental subduction zone metamorphism differences in 18o between different domains within single grains vary from 0 8 to 12 5 suggesting different degrees of o isotope exchange between the positive 18o magmatic core and the negative 18o metamorphic fluid during the metamorphism the magmatic zircons underwent three subtypes of metamorphic recrystallization depending on their accessibility to negative 18o fluids the zircons recrystallized in solid state maintained positive 18o values and ree and lu hf isotopes of protolith zircon but their u pb ages are lowered the zircons recrystallized through dissolution exhibit negative 18o values similar to the metamorphic growths almost completely reset u pb ages and partially reset ree systems the zircons recrystallized through replacement show variably negative 18o values and partially reset ree and u pb and lu hf isotopic systems therefore this study places robust constraints on the origin of metamorphic zircons in eclogite facies rocks and provides a methodological framework for linking the different types of metamorphic zircons to petrological processes during continental collision