ordovician conodonts from the thong pha phum area western thailand abstract the ordovician conodont biostratigraphy of a limestone sequence in the tha manao formation exposed in the thong pha phum area western thailand comprises four early to middle ordovician conodont zones these are the juanognathus variaviris zone the walliserodus comptus zone the juanognathus jaanussoni histiodella holodentata zone and the plectodina onychodonta zone in ascending order these conodont faunas are similar to those from the midcontinent province north china australia and argentina

the dav and periadriatic fault systems in the eastern alps south of the tauern window alpine deformation of austroalpine units south of the tauern window is dominated by two kinematic regimes prior to intrusion of the main periadriatic plutons at 30 ma the shear sense was sinistral in the current orientation with a minor north side up component sinistral shearing locally overprints contact metamorphic porphyroblasts and early periadriatic dykes direct rb sr dating of microsampled synkinematic muscovite gave ages in the range 33 30 ma whereas pseudotachylyte locally crosscutting the mylonitic foliation gave an interpreted 40ar 39ar age of 46 ma the transition from sinistral to dextral transpressive kinematics related to the periadriatic fault occurred rapidly between solidification of the earlier dykes and of the main plutons subsequent brittle ductile to brittle faults are compatible with n s to nnw sse shortening and orogen parallel extension antithetic riedel shears are distinguished from the previous sinistral fabric by their fine grained quartz microstructures with local pseudotachylyte formation one such pseudotachylyte from speikboden gave a 40ar 39ar age of 20 ma consistent with pseudotachylyte ages related to the periadriatic fault the magnitude of dextral offset on the periadriatic fault cannot be directly estimated however the jump in zircon and apatite fission track ages establishes that the relative vertical displacement was 4 5 km since 24 ma and that movement continued until at least 13 ma

the kinematics of movements along the insubric line and the emplacement of the ivrea zone schmid sm zingg a and handy m 1987 the kinematics of movements along the insubric line and the emplacement of the ivrea zone in h j zwart m martens i van der molen c w passchier c spiers and r l m vissers editors tectonic and structural processes on a macro meso and micro scale tecta a hys 135 47 66 the insubric line west of locamo is characterized by a 1 km thick greenschist facies mylonite belt east of locamo these mylonites are overprinted by a discrete brittle fault the mylonites are derived from basement units of the central alps sesia zone and the southern alps ivrea zone as well as from the permo mesozoic cover of the southern alps canavese sense of shear criteria indicate that the mylonites accomm ated backt s g followed by dextral strike slip motion mylonitization during backtbrusting was synchronous with backfolding of the central alpine nappes under higher metamorphic conditions a horizontal temperature gradient resulted from the rapid juxtaposition of the warm central alpine block against the cold southern alpine block mylonites formed during the later dextral strike slip event are related to large transcurrent displacements in the central alps deduced from regional kinematic considerations thus both mylo t ation events are contempor eous with deformation to the north and south of the insubric line insubric phase extensively modifying the pre insubric crustal configuration of the alps the insubric phase post dates the bergell intrusion 30 m y the emplacement of the geophysical ivrea body is a combined effect of vertical uplift due to e w directed crustal thinning during the early jurassic and underplating by continental crust associated with late cretaceous compression a deep crustal normal fault pogallo line subsequently rotated during tertiary alpine orogenesis separates deeper parts of the southern alpine crust ivrea zone from intermediate crustal levels strona ceneri zone the rigid ivrea body localir ed large strains within the insubric mylonite belt and is responsible for the present curvature of the insubric line in uetion

carbon isotope and magnetic polarity evidence for non depositional events within the cambrian ordovician boundary section near dayangcha jilin province china carbon isotope and magnetic polarity stratigraphic results from the cambrian ordovician boundary section at xiaoyangqiao near dayangcha jilin province china in comparison to a contemporaneous section at black mountain australia indicate strata equivalent to major portions of the australian sequence are either absent or are restricted to highly condensed intervals these intervals are correlative with regressive sea level events identified in australia and western north america suggesting regional or eustatic sea level changes strongly influenced deposition of the xiaoyangqiao sequence these results also suggest the xiaoyangqiao section is unfavourable as the site of the cambrian ordovician boundary global stratotype section and point

conodonts as biostratigraphic tools for redefinition and correlation of the cambrian ordovician boundary the cambrian ordovician boundary is recognized at different horizons on various continents by utilizing several fossil groups conodonts are abundant diverse and less provincial and facies controlled than other fossils in this interval many species are widespread and some are cosmopolitan strata representing the cambrian ordovician boundary interval from asia australia and north america can be correlated easily when those strata are from cratonal or shelf environments strata from slope facies have fewer conodonts fewer taxa and taxa may have different ranges compared with cratonal and shelf facies it may be easier to correlate intercontinentally within cratonal shelf facies than to correlate intracontinentally from cratonal shelf facies to slope facies a new hierarchical arrangement of conodont interval zones and subzones is proposed for western north america these include the proconodontus tenuiserratus zone the p posterocostatus zone new the p muelleri zone new the eoconodontus zone new with eoconodontus notchpeakensis and cambrooistodus minutus subzones the cordylodus proavus zone emended with hirsutodontus hirsutus fryxellodontus inornatus and clavohamulus elongatus subzones the cordylodus intermedius zone new with hirsutodontus simplex and clavohamulus hintzei subzones the cordylodus lindstromi zone new and cordylodus angulatus zone new the boundary point for the base of the ordovician system will be chosen so as to coincide with a correlatable conodont zonal boundary other fossil groups will support correlation of this boundary point three alternative horizons are being considered the base of the cordylodus proavus zone is the most distinctive but is regarded by some as older than is appropriate for the base of the ordovician the base of the cordylodus intermedius zone is recognizable by faunal changes in several evolutionary lineages at a level that is slightly younger than the presently recognized boundary in australia north america and parts of asia but slightly older than the base of the tremadoc series of europe the base of the c lindstromi zone can be recognized in most areas by the lowest occurrence of the nominate species which is of questioned taxonomic validity and uncertain biostratigraphic utility this horizon is closest to the base of the tremadoc series of europe choice of a stratotype section has been narrowed to the cow head group in newfoundland eastern canada and to the fengshan and yehli formations in jilin province northeastern china the cow head group was deposited on and near the base of the continental slope erosion by debrisslide breccias resulted in a significant hiatus near the boundary interval in the broom point sections a condition contrary to guidelines for choice of boundary sections this may be less of a problem in other sections such as green point the dayangcha section in china was deposited on the outer part of a continental shelf and has acritarchs conodonts graptolites and trilobites additional study of the china section is needed to document fully the ranges of critical conodont taxa

variable preservation of middle tertiary coarse grained nearshore to outer shelf storm deposits in southern california abstract the middle tertiary oligocene early miocene strata of the san emigdio range in southern california were deposited along a shoreline oriented north northeast to south southwest adjacent to a tectonically active highland that formed the southern margin of the san joaquin basin abundant storm deposits sandstone and conglomerate are preserved in the san emigdio pleito and lower temblor formations extensive lateral exposures allow recognition of fair weather and storm dominated facies in contemporaneous nearshore inner shelf and outer shelf deposits the nearshore deposits consist of coarse grained progradational sequences that are similar to those described by clifton 1981 in miocene rocks of the nearby caliente range each complete sequence consists of in ascending order a basal transgressive lag followed by inner shelf surf zone longshore trough and beach foreshore facies the inner shelf deposits consist of fair and foul weather deposits the former deposits comprise massive locally pebbly very fine grained sandstone whereas the latter deposits consist of alternating intervals of cross stratified landward oriented conglomerate and hummocky cross stratified fine grained sandstone the outer shelf deposits are composed of massive very fine grained to silty sandstone and sequences of mollusk beds and hummocky cros stratified sandstone the massive outer shelf sandstones were deposited during periods of dominantly fair weather the sequences of hummocky cross stratified sandstone were deposited during and immediately after storms the outer shelf storm deposits apparently were preserved on an event by event basis whereas the inner shelf storm deposits were preserved in amalgamated packages unmodified storm deposits are rare in the nearshore deposits owing to the reworking capabilities of high energy nearshore processes reconstructions of wave conditions based on airy wave theory paleogeographic constraints and storm facies suggest that peak storm waves were more than 5 m high with periods of 7 9 seconds the study area was situated at the end of a long fetch in a favorable position to receive the full brunt of northwesterly wind driven waves

middle cenozoic depositional tectonic and sea level history of southern san joaquin basin california the eocene to lower miocene fill of the southern san joaquin basin contains three complete depositional sequences the tejon san emigdio and pleito each of which corresponds to a formal formation the tejon sequence lower to middle eocene is marine and incorporates nearshore shelf slope and basinal deposits the san emigdio upper eocene to lower oligocene and pleito upper oligocene sequences intertongue eastward with alluvial fan deposits of the tecuya formation the lower part of the san emigdio sequence was deposited by a westward prograding gilbert type delta the upper part of the san emigdio sequence and lower part of the pleito sequence were deposited by a system of shelf fan deltas that prograded at least 10 km to the west the middle and upper parts of the pleito sequence were deposited by a slope fan delta in relatively deep hundreds of meters water regional transgression during the early eocene initiated deposition in the southern san joaquin basin the lower san emigdio gilbert type delta prograded from the shelf edge during a lowstand in eustatic sea level at approximately 40 ma relative highstand deposits in the san emigdio and pleito formations consist of widespread progradational shallow marine and nonmarine facies the eocene to early miocene tectonic history of the southern san joaquin basin included three distinct periods of increasingly intense activity rapid early to middle eocene subsidence of the basin was associated with the emplacement of the salinian block late eocene to early oligocene uplift and subsidence occurred in two discrete pulses and a seismically generated submarine slumping episode triggered the change from shelf fan deltas to slope fan deltas during the late oligocene to early miocene the southern san joaquin basin was disrupted by major uplift proximal volcanism and syndepositional faulting as the mendocino triple junction migrated past the study area and san andreas fault related tectonism was initiated in general local tectonic events rather than eustatic ea level events seem to have exerted the predominant control on middle cenozoic sedimentation in the southern san joaquin basin

the evolution of illite to muscovite an example of the behaviour of isotopes in low grade metamorphic terrains abstract a review of the possibilities and limitations of the use of radiogenic isotopes in low grade terrains i e in the temperature range below the blocking temperature of potassic white mica is discussed using a background of a data base of over 500 isotope analyses on more than 250 illitic muscovitic samples published in various papers special stress is laid on the main problems 1 distinction between detrital and neoformed minerals 2 the slow evolution of minerals without pronounced breaks and 3 retention problems of radiogenic daughter products a brief description of low grade metamorphism is given and criteria enabling a subdivision into low to very low grade metamorphism are discussed comprising illite crystallinity mineralogy fluid inclusions and coal rank measurements the useful type of lithologies for this kind of work ranges from shales slates through volcaniclastic tuffs with high primary glass contents and limestone dolomites marbles to mylonites it can be concluded that carefully selected illitic muscovites yield geologically meaningful ages provided that more than one isotopic method is applied

three new upper cambrian stages for north america the existing upper cambrian stages for north america are obsolete three new stratotype defined stages are equivalent to and replace three successive biomeres marjuman stage replaces marjumiid biomere lower boundary at base ptychagnostus atavis zone steptoean stage replaces pterocephaliid biomere lower boundary at base aphelaspis zone sunwaptan stage replaces ptychaspid biomere lower boundary at base irvingella major subzone the base of the overlying ibexian series is placed at the base of the eurekia apopsis subzone

high resolution strontium isotope stratigraphy across the cambrian ordovician transition we have analyzed 214 fossil apatite samples from nine stratigraphic sections worldwide that cover the lower cambrian to lower ordovician for their strontium isotope ratios of these samples 180 from six sections that cover the cambrian ordovician transition 5 ma reveal how the extent of isotopic alteration can differ greatly according to lithology and sample type from three limestone sections 126 euconodont samples yield the most consistent 87sr 86sr ratios and are used in this article to constrain seawater 87sr 86sr across the cambrian ordovician boundary by contrast protoconodonts paraconodonts and inarticulate brachiopods are far less likely to retain a primary 87sr 86sr signature and appear to be of only limited value for chemostratigraphy seawater 87sr 86sr fell from maximum values for the phanerozoic of more than 0 7092 during the early late cambrian to 0 7090 by the earliest ordovician high resolution sampling has permitted the recognition of sinusoidal variations with a wavelength on the order of 1 ma that are superimposed on the overall fall in 87sr 86sr these variations may be caused by high order cycles in seawater 87sr 86sr or diagenetic alteration

lowermost ordovician basal tremadoc radiolarians from the little port complex western newfoundland lowermost ordovician basal tremadoc cherts from the little port complex western newfoundland contain a distinctive moderately well preserved radiolarian assemblage the fauna differs from those reported from the arenig suggesting that some of the earliest radiolarian forms may have bios tratigraphic potential the abundance of radiolarians in chert and the ease with which they can be extracted suggest that they are a potentially valuable tool for use in investigations of the timing and development of early palaeozoic orogenic systems a previously undescribed radiolarian beothuka terranova sp nov belonging to a new genus beothuka gen nov and of uncertain higher level affinity is formalized herein

microstructurally constrained versus bulk fault gouge k ar dating abstract structural mineralogical and geochronological data were collected from two brittle fault zones fault i and ii deforming the ordovician rolvsnes granodiorite in sw norway microstructurally constrained k ar data from cohesive damage zone samples are compared to bulk k ar data from fault gouges from the adjacent fault cores in fault i fluid ingress along the fault core caused pervasive alteration of the host granodiorite plagioclase and biotite are almost completely altered to kaolinite quartz illite and smectite in fault ii a complex deformation history is recorded by multiple fault rock microdomains including an ultracataclasite with 50 authigenic illite and an illite jarosite bearing pseudotachylyte microdomains hosting authigenic illite jarosite from both studied faults were separated characterized mineralogically and dated by k ar results confirm that each domain contains only one generation of authigenic illite jarosite and provide robust time constraints on fault initiation in the carboniferous 339 8 ma and reactivation in the permian 289 6 ma k ar geochronology of the adjacent fault gouges yields inclined age versus grain size relationships the ages of the coarsest grain size fractions are similar to the age range of the damage zone data whereas the ages of the finer fractions are significantly younger our study provides new and important insights into the dynamics of long lived fault zones isotopic signatures and structural features representative of fault initiation are preserved in the damage zones while they are progressively overprinted due to cumulative strain localization in the fault cores finally our study further clarifies the significance of age plateaus in age versus grain size plots which allows for a robust and geologically meaningful interpretation of k ar data

apatite fission track analysis of oligocene strata in south texas u s a testing annealing models analysis of apatite fission track data from forty seven surface and subsurface samples of fine grained sandstone to pebbly conglomerate from the oligocene catahoula gueydan frio and vicksburg formations documents the response of fission track ages and track length distributions to simple post depositional thermal histories characterized by monotonic heating data from outcrop samples document that the apatite component was derived from source areas characterized by rapid cooling through the temperature range from 120 to 50 c during the early tertiary the apatite population is characterized by a log normal chlorine distribution mean 0 3 wt 0 3 wt consisting predominantly of low cl apatites 80 is 0 5 wt mean track lengths from core samples decrease systematically with increasing depth and temperature the decrease in apatite fission track ages with increasing depth and temperature is less systematic due to the scatter in inherited ages at the time of deposition no tracks were observed in apatite from samples at temperatures above 130 c mean track lengths for the south texas data agree well with those for a similar data set from the otway basin southeastern australia for present day temperatures of 70 c at higher temperatures mean lengths are 1 to 9 m longer than those observed from the otway basin this discrepancy is interpreted to reflect up to 40 ma longer residence times at maximum burial temperatures for the otway basin samples standard deviations about mean lengths are also lower than observed in the otway basin study consistent with the wider variation of cl contents mean 0 8 wt 0 7 wt that characterize apatite from the otway basin recently published apatite fission track annealing models predict significantly different mean track length vs present day temperature profiles for thermal histories believed to be representative of those experienced by sampled south texas strata because of the large uncertainties attributed to present day temperatures 10 c the predicted mean length vs temperature trends for these models essentially fall within or on the uncertainty envelope for the observed trend residuals of predicted minus observed mean lengths suggest that the previously published carlson composite parameter model using an initial mean length of 15 8 m instead of 16 5 m or the crowley et al model for track length reduction down to 11 m provide the best estimates of mean track length reduction over geologic time scales for near end member f apatite populations

fission track dating of faulting events accommodating plastic deformation of biotites we performed fission track ft analyses to constrain the timing of fault slip events associated with plastic deformation of biotite grains formed at high temperature higher than 200 300 c apatite ft ages of 49 6 16 7 ma were obtained from late cretaceous granites near the fault with younger ages near a basaltic dike that intruded at 19 ma by comparing the distribution of apatite ft ages using forward thermal and ft annealing calculations we interpreted the younger ages as being explained by basaltic intrusion we concluded that the plastic deformation of the biotite grains could have occurred associated with the following events 1 primal cooling of the granitic body at 68 50 ma and or 2 basaltic intrusion at 19 ma a combination of detailed observation of microscale deformation structures and dense ft measurements successfully constrained timing of fault events distributed in bedrock without covering layers

microbialite morphostratigraphy as a tool for correlating late cambrian early ordovician sequences abstract microbialite morphostratigraphy is a new tool for intrabasinal correlation using diverse microbialite structures morphotypes the recognition of the succession of morphotypes over constrained temporal intervals and broad areas is a function of the complex interactions that operate to create the structure because so many nonlinked variables e g biotic sedimentological physicochemical are involved similar morphotypes do not reoccur over long temporal intervals to demonstrate the technique the upper cambrian lowermost ordovician shelf strata of the great basin united states were correlated using both morphostratigraphy and standard lithostratigraphy six morphozones and one morphosubzone were recognized as were four main lithologic successions because the boundaries between the morphozones and lithologic successions did not coincide it is inferred that the characteristics of the various microbialite structures are not solely controlled by physical factors the principles for establi

combined high precision chronometry and geochemical tracing using accessory minerals applied to the central alpine bergell intrusion central europe abstract the behaviour of nd u th and pb isotopes in granitic rocks is usually controlled by accessory minerals in this study an approach is adopted which uses the initial 143 nd 144 nd as a geochemical tracer together with high precision u th pb geochronology for accessory phases and their respective whole rock this combination of techniques is applied to zircon allanite sphene apatite and thorite from both a tonalite and a granodiorite from the tertiary alpine bergell intrusion inherited isotopic signatures have been identified in bulk zircon fractions by inverse discordance in the u pb system in the case of the granodiorite the initial nd in the zircon is 1 77 units lower than the bulk rock this result indicates that nd isotopes in zircon may serve as a useful tool for tracing crustal precursors in silicic rocks all other accessories yield an initial nd equal to their respective whole rock and u th pb ages equal to or lower than the respective intrusion ages this indicates either the absence of inherited cores or that such cores have exchanged their nd and pb in the case of apatite also sr by diffusion where available experimental mineral melt equilibria and diffusion models are consistent with this behaviour u pb geochronology of carefully selected core free zircons yields a precise intrusion age of 31 88 0 09 ma for the tonalite in contrast granodioritic zircons are partially reset due to radiation damage caused by high u concentrations however an alternative means of dating young intrusions is to use 232th 208pb ages of allanite in the case of allanite the presence of a large excess of 206pb derived from 230th indicates that no total pb loss has occurred since crystallization and the nd isotopes indicate the absence of inheritance using both the allanite age and a 238u 206pb age for sphene an intrusion age of 30 13 0 17 ma is indicated for the granodiorite taken together with an allanite th pb age for the tonalite these results strongly support the high closure temperatures for the allanites studied here 700 800 c at high post magmatic cooling rates all other accessory mineral ages are reset to values similar to hornblende 39ar 40ar ages and reflect the effect of post magmatic cooling superimposed on post metamorphic cooling

active deformation in the inner western alps inferred from comparison between 1972 classical and 1996 gps geodetic surveys abstract eighteen geodetic points surveyed in 1972 by the french national geographic institute ign were remeasured by gps in 1996 in the brianconnais and piemont zones east of the pelvoux massif french western alps a displacement vector set was determined for the two surveys common points calculations of the strain rate tensors associated with 15 triangular cells of the network have been performed only four of them show a strain rate significant at a 95 level of confidence these data suggest an e w extension of about 2 4 mm yr between the western and eastern part of the network pelvoux external crystalline massif and queyras blueschists respectively associated with n s shortening this active deformation agrees with neotectonic and seismotectonic data the measured tectonic motion seems to be distributed throughout the central part of the brianconnais zone where the seismic activity is concentrated the local seismicity has been precisely surveyed since 1989 it is moderate ml

tempo of earliest ordovician graptolite faunal succession conodont based correlations from the tremadocian of quebec successive tremadocian planktic dendroid graptolite assemblages from continental slope sequences in quebec can be correlated with north american platform biozonations on the basis of conodonts anisograptid bearing assemblage 2 middle tremadocian matane faunas are associated with early ordovician rossodus manitouensis zone new designation conodonts younger middle tremadocian faunas with adelograptids assemblage 3 are no younger than the rossodus manitouensis zone key dendroid evolutionary immigration events take place within the lower conodont fauna b interval rooted dendroids near cap des rosiers quebec and in eastern new york state occur with lower fauna b conodonts and the trilobites pareuloma and borthaspidella however the earliest tremadocian and earliest ordovician dendroid immigration event represented by the local lowest occurrence of faunas with dictyonema flabelliforme s l at localities in western newfoundland eastern new york state norway and eastern china also lies with

dating incipient metamorphism using 40 ar 39 ar geochronology and xrd modeling a case study from the swiss alps six samples of a single carbonate rich unit of the swiss prealpes progressively metamorphosed from diagenesis to deep anchizone yield 40ar 39ar spectra with variably developed staircase patterns consistent with mixtures of detrital mica and neocrystallized mixed layer illite smectite the lowest temperature heating steps for different size fractions 2 6 m and 6 20 m converge to 40 ma providing an imprecise maximum age of regional metamorphism a method is described for distinguishing and quantifying the amount of pre existing detrital mica versus neoformed illite layer in the illite smectite formed during tertiary alpine metamorphism by comparison of x ray diffraction patterns with newmod simulations in the least metamorphosed samples the illite smectite contains 65 neoformed illite and this illite accounts for approximately 17 of all dioctahedral phyllosilicate minerals in the rock e g detrital mica and illite smectite in contrast the illite smectite from the more strongly metamorphosed samples contains 97 neoformed illite which accounts for 70 to 90 of all dioctahedral phyllosilicate minerals phyllosilicate morphologies viewed by scanning electron microscopy are consistent with these estimates a process of dissolution reprecipitation is inferred as a mechanism for the growth of the neoformed phyllosilicates a plot of neoformed illite content versus 40ar 39ar total fusion age yields a near linear curve with an extrapolated age of 27 ma for 100 neoformed dioctahedral phyllosilicates this age is interpreted as the time of incipient metamorphism and is consistent with independent biostratigraphic constraints model 40ar 39ar age spectra constructed with the xrd simulation results correspond well to the experimental data and illustrate the changes in degassing properties of progressively metamorphosed mixtures of detrital mica and neoformed illite

timing of alpine fault gouges k ar ages from clay rich fault gouges in the european alps are consistent internally with established field constraints and with fission track ages demonstrating the applicability of this method for direct dating of brittle deformation illite grown by retrograde hydration of granitic and or high grade metamorphic protoliths is the major k bearing mineral in the fractions that have been separated 0 1 to 6 10 m the ages obtained are bracketed by apatite and zircon fission ages from adjacent localities this together with the interpreted cooling curve for one well constrained sample from the simplon fault indicates that illite grew or the k ar isotopic system in illite closed at temperatures in the range 120 150 c consistent with the stability of illite alone rather than illite smectite average ages obtained for fractions finer than 2 m were 1 5 2 2 6 ma 2 for 11 analyses from the detachment and footwall of the simplon fault 2 8 8 0 6 ma for four analyses from the centovalli fault at trontano 3 19 9 2 6 ma for two analyses from a major riedel fault related to the periadriatic fault offsetting the tertiary bergell tonalite at gesero 4 21 5 5 5 ma for two analyses from a zone parallel to the periadriatic fault in the adjacent southern alps in val morobbia and 5 14 5 3 9 ma for two analyses from a gouge zone on the northern rim of the tertiary mauls tonalite in the eastern alps the ages from the simplon centovalli and periadriatic faults are not identical and do not lend support to models linking these three faults as a single coeval structure

two dimensional thermal modelling of normal faulting the simplon fault zone central alps switzerland abstract two dimensional finite difference thermal modelling of the major extensional simplon fault zone in the central alps establishes that the shape of cooling curves for samples from the footwall and hanging wall at various distances from the detachment are very sensitive to the actual displacement history if the displacement had occurred within a very limited time span 18 15 ma samples from the hanging wall close to the detachment should have significantly increased in temperature with a peak around 14 ma this is not discernible in the cooling curves interpreted from the mineral ages if the rate of normal fault displacement had been lower but constant from 18 ma to present the cooling curves for the footwall should show a correspondingly steady decrease in temperature the interpreted cooling curves from mineral ages are steep after 18 ma becoming shallower in more recent times this shape can be reproduced by the thermal modelling for a variable rate of fault displacement initially rapid total relative displacement 10 mm a across the fault zone between 18 and 15 ma but reducing to a more steady 0 4 mm a between 15 ma and 3 ma the corresponding exhumation rate of the footwall was 4 6 mm a from 18 to 15 ma and 0 6 mm a from 15 to 3 ma whereas the hanging wall was exhumed at a constant rate of 0 4 mm a the difference in cooling curves for samples with increasing horizontal distance into the footwall reflects a distributed ductile shear with the sample at 20 km distance initially at 25 km depth at 25 ma whereas that at 70 m distance was only at 19 km depth at the same time samples in the hanging wall were at 10 km depth total relative displacement parallel to the fault plane was around 36 km the thermal modelling demonstrates that during rapid fault displacement the apparent exhumation rate calculated assuming a constant geothermal gradient is too low whereas the apparent rate calculated for the subsequent period of slower displacement is too high

active tectonism in the central alps contrasting stress regimes north and south of the rhone valley an integrated interpretation of seismicity fault plane solutions and deep seismic reflection data suggests that the ne sw to nw se trending rhone simplon fault zone and the gently s dipping basal penninic thrust separate fundamentally different stress regimes in the western swiss alps north of the rhone simplon fault zone strike slip earthquakes on steep dipping faults within the helvetic nappes are a consequence of regional nw se compression and ne sw extension to the south vertical maximum stress and n s extension are responsible for normal mechanism earthquakes that occur entirely within the penninic nappes above the basal penninic thrust such normal faulting likely results from extension associated with southward movements collapse of the penninic nappes and or continued uplift and relative northward displacements of the underlying alpine massifs geological mapping and fission track dating suggest that the two distinct stress regimes have controlled tectonism in the western swiss alps since at least the neogene

salt diapir influenced shallow marine sediment dispersal patterns insights from outcrop analogs unique outcrop exposures of two salt diapirs a secondary salt weld and associated syndiapiric strata in northeast mexico offer an important perspective on salt influenced petroleum reservoirs by allowing recognition and description of salt related sandstone depocenters spectacular progressive unconformities and halokinetic sequences coupled with laterally continuous exposures permit accurate correlation and interpretation of syndiapiric units analysis of the syndiapiric upper cretaceous delgado sandstone member potrerillos formation delineates regional shoreline sediment dispersal locally impacted by diapiric relief and the distribution and internal character of salt diapir proximal sandstone depocenters sequence stratigraphic correlation defines striking relationships between highstand hst and transgressive systems tracts tst stratal thinning trends and salt diapir relief transgressive systems tract and highstand systems tract strata show thinning and lithofacies shoaling trends toward diapirs however the latter is more pronounced in the hst and occurs at a greater distance from salt diapirs within 1 2 km 0 6 1 2 mi sandstone depocenters roughly 0 5 1 0 km 0 3 0 6 mi wide and 0 5 0 2 km 0 3 0 1 mi thick are present in both tst and hst strata and consist of sandier shallower water facies however depocenters are better developed in tst strata as thicker stratigraphic sections on updip diapir margins we propose that sandstone depocenters formed by preferential sediment reworking and shelf ridge development on landward diapir margins during marine transgression elevated diapir relief and higher subsidence rates adjacent to salt diapirs likely enhanced this process additionally depocenters adjacent to el papalote diapir are smaller and contain deeper water facies than the age equivalent depocenters adjacent to el gordo diapir suggesting that it had higher broader sea floor relief

provenance and stratigraphy of the eocene tejon formation western tehachapi mountains san emigdio mountains and southern san joaquin basin california abstract the eocene tejon formation of the western tehachapi and san emigdio mountains of california provides a unique east west trending outcrop belt of otherwise deeply buried strata that underlie the san joaquin basin to the north its stratigraphy and framework mineralogy have been used to define offsets along the san andreas fault a major neogene plate boundary transform fault that has translated coastal california northward more than 300 km by right slip the tejon formation was deposited directly on crystalline basement rocks as a result of an eastward transgression of at least 45 km from the early to the late middle eocene followed by a major regression in the late middle eocene the basal uvas conglomerate member consists entirely of coarse grained transgressive system tract tst deposits characterized by abundant current formed sedimentary structures it is overlain by offshore marine shales of the liveoak shale member which consist of lower tst deposits overlain by highstand system tract hst deposits the overlying regressive metralla sandstone member consists mostly of highly bioturbated fine grained shallow marine hst sandstones with local thin bedded turbidites present in some western exposures the quartzofeldspathic sandstones grand mean qm51 12f42 12lt7 4 of the tejon formation suggest a provenance consisting of plutonic metamorphic and volcanic rocks such as those present in the basement of the san emigdio and techachapi mountains northwestern mojave desert and southern sierra nevada petrologic parameters however suggest variable contributions from these source areas through time three distinctive petrofacies coincide with the three principal lithostratigraphic subdivisions the shallow marine tst sandstones from the uvas conglomerate member are more quartzose than those of the hst metralla sandstone member possibly because higher energy deposition preferentially preserved quartz thin bedded turbidites in the western exposures of the metralla sandstone in contrast have more abundant feldspar and volcanic lithic fragments than the uvas and liveoak members the composition of the tejon formation suggests derivation from a basement uplift provenance with progressive upsection changes from qm60 f33 lt8 uvas to qm46 f44 lt10 liveoak and qm 43 f52 lt5 metralla the composition and provenance of the tejon formation is different from that of the coeval butano sandstone and point of rocks sandstone submarine fan deposits to the north and west

geochronological constraints on the evolution of the periadriatic fault system alps fault rocks from various segments of the periadriatic fault system paf alps have been directly dated using texturally controlled rb sr microsampling dating applied to mylonites and both stepwise heating and laser ablation 40ar 39ar dating applied to pseudotachylytes the new fault ages place better constraints on tectonic models proposed for the paf particularly in its central sector along the north giudicarie fault oligocene e se directed thrusting 29 32 ma is currently best explained as accommodation across a cogenetic restraining bend within the oligocene dextral tonale pustertal fault system in this case the limited jump in metamorphic grade observed across the north giudicarie fault restricts the dextral displacement along the kinematically linked tonale fault to 30 km dextral displacement between the tonale and pustertal faults cannot be transferred via the peio fault because of both late cretaceous fault ages 74 67 ma and sinistral transtensive fault kinematics in combination with other pseudotachylyte ages 62 58 ma widespread late cretaceous paleocene extension is established within the austroalpine unit coeval with sedimentation of gosau group sediments early miocene pseudotachylyte ages 22 16 ma from the tonale pustertal jaufen and passeier faults argue for a period of enhanced fault activity contemporaneous with lateral extrusion of the eastern alps this event coincides with exhumation of the penninic units and contemporaneous sedimentation within fault bound basins

conodonts from the cambrian ordovician boundary interval in the southeast margin of the sichuan basin china abstract conodonts from the cambrian ordovician transition at the liangcun section in xishui county guizhou and at the huangcao section in wulong county chongqing are examined for the first time both sections are located at the southeast margin of the sichuan basin a total of 1367 specimens were recovered representing 30 species and 15 genera based on the ranges of conodonts generalized from these two sections and another six sections previously studied in the same region three conodont zones cordylodus proavus monocostodus sevierensis and cordylodus angulatus zones are recognized the index species of the cambrian ordovician boundary at the global stratotype section and point gssp iapetognathus fluctivagus and its substitute in china iapetognathus jilinensis are not observed in the study sections therefore it is impossible to determine the cambrian ordovician boundary exactly however it probably lies within the lower part of m sevierensis zone the upper part of the loushanguan group correlating with the gssp in canada and the dayangcha section in china chronological sequences of the fad first appearance datum of c angulatus chosonodina herfurthi and rossodus manitouensis are not obvious in the study so the c angulatus zone here is correlated with zones defined by c angulatus ch herfurthi and r manitouensis in the lower yangtze platform

terminal cambrian and lowest ordovician succession of mexican west gondwana biotas and sequence stratigraphy of the tinu formation the tinu formation of oaxaca state is the only fossiliferous lower palaeozoic unit between the laurentian platform in northwest mexico and gondwanan successions in andean south america the tinu traditionally has been referred to the lower ordovician tremadoc and regarded as having a provincially mixed fauna with laurentian avalonian and gondwanan elements bio and lithostratigraphic re evaluation demonstrates that the tinu is a gondwanan passive margin succession it includes a lower thin to 16 m condensed uppermost cambrian yudachica member new the yudachica nonconformably overlies middle proterozoic basement as a result of very high late late cambrian eustatic levels the yudachica changes from storm dominated but slightly dysoxic shelf facies fossil hash limestone and shale in the south to an upper slope facies with debris flows 50 km to the north three biostratigraphically distinct depositional sequences comprise the yudachica the yudachica has gondwanan aspect trilobites with low diversity conodonts characteristic of unrestricted marine temperate facies the upper tinu or rio salinas member new is a lower ordovician tremadoc depositional sequence that records strong early but not earliest tremadoc eustatic rise marked by graptolite and olenid bearing dysoxic mudstones higher strata shoal upward into shell hash limestones and proximal tempestite sandstones with upper lower tremadocian unrestricted marine temperate conodonts new taxa include orminskia rexroadae landing gen et sp nov from the cordylodus andresi zone this euconodont is related to hyaline coniform genera best known from ordovician tropical platform successions cornuodus clarkei landing sp nov resembles the coeval upper lower tremadoc tropical species scalpellodus longipinnatus ji barnes

review of trilobite biofacies distributions at the cambrian ordovician boundary detailed analysis of trilobite assemblages at the cambrian tremadoc transition reveals the presence of four possibly five biogeographic provinces baltic north china southeast china north american and possibly siberian one or more discrete biofacies characterize the transition in each province when plotted palaeogeographically these appear to be largely temperature controlled interdigitation of provincialized trilobite biofacies is recognized by the representation of dominant types of trilobite olenaceans ceratopygaceans remopleuridaceans dikelocephalaceans and leio stegiaceans in particular on key boundary sections these sections at nochixtlan mexico batyrbay southern kazakhstan broken skull river district of mackenzie canada and jiangshan chang shan western zhejiang china permit the correlation of biogeographic provinces globally as a result of the application of trilobite biostratigraphy relatively few problems prevent the maintenance of a cambrian ordovician boundary stratotype section in its traditional baltic biogeographic setting

miocene phosphate accumulation in the cuyama basin southern california abstract middle and upper miocene phosphatic strata in the cuyama basin of southern california contain approximately 6 4 10 9 metric tons 2 1 10 14 mol of phosphorus p that accumulated in slope and shelf strata at rates between 0 9 and 3 8 mol p cm 2 year 1 from 15 5 to 10 8 ma an estimated 63 of all phosphorus lies in a single third order sequence deposited between 14 8 and 13 ma the main impetus for this brief phosphogenic event was a 200 m relative sea level rise that resulted from strike slip faulting immediately east of the basin minor element and petrologic analyses suggest the overall increase in phosphorus accumulation correlates with less reducing conditions in the shallowly buried marine sediments on the slope and the expansion of phosphogenesis to the outer shelf both events were probably related to landward shifts of the pacific oxygen minimum zone and coastal upwelling centers during the relative sea level rise whereas these tectonic events were apparently local the phosphatic deposits of the cuyama basin clearly demonstrate that tectonism can strongly influence phosphate accumulation rates in marginal marine environments

new conodont records of the los sombreros formation cambrian ordovician from the western precordillera argentina biostratigraphic and palaeoenvironmental implications

the cuyama strike slip basin california u s a an exemplar of contrasting syntectonic and post tectonic strata abstract miocene strata in the cuyama basin of southern california exemplify syntectonic and post tectonic stratigraphic development in a strike slip basin integrated seismic well log and outcrop data provide a detailed portrayal of the seismic stratigraphy and sedimentary facies in the previously unstudied middle and upper miocene sequences results show sharp contrast among syntectonic and post tectonic strata from composition and texture of sediments to lateral distribution and thickness of facies seven post tectonic sequences were mapped across the basin sequences contain eolian to fluvial terrestrial facies wave and tide dominated nearshore facies biosiliceous dolomitic and phosphatic outer shelf and slope mudstones and shale rich submarine fans in basinal environments authigenic dolomites and phosphates are pervasive in outer shelf and slope facies and indicate the possibility of restricted sediment supply post tectonic sequences are thin but laterally continuous and they exhibit little depositional asymmetry minor erosion is distributed equally among regressive and transgressive relative sea level changes asynchronous changes in tectonism sediment supply and eustatic sea level change created dramatic differences among syntectonic and post tectonic sequences interbasinal tectonic quiescence led to the abrupt end of stratigraphic mismatches and the accumulation of sediment during brief episodic periods multiple points of progradation persisted through several million years of post tectonic basin development and highlight the importance of extrabasinal sediment supply to the development of all sequences the eventual loss of most accommodation space promoted the importance of eustatic sea level change and reduced the prominence of asymmetric facies and abrupt lateral facies changes

early carboniferous transgression on a passive continental margin deposition of the kekiktuk conglomerate northeastern brooks range alaska the base of the ellesmerian sequence in the northeastern brooks range is a widespread terrigenous clastic succession assigned to the kekiktuk conglomerate late tournaisian visean the kekiktuk is situated above a regional angular unconformity and is the base of a transgressive succession that is thought to record coastal onlap associated with the formation of a passive continental margin six depositional units have been recognized in the kekiktuk conglomerate unit a consists of clast supported and rare matrix supported pebble and cobble conglomerate with minor interbeds of pebbly sandstone it records deposition at the base of incised paleovalleys from high sediment concentration flows and debris flows units b c and d consist of multistory fining upward cycles that record deposition within incised paleovalleys from low sinuosity bed load dominated streams low to moderate sinuosity bed load dominated streams and high sinuosity mixed load streams respectively unit e consists of multistory fining upward cycles and laminated sandstone with thin conglomerate stringers it records deposition in the distal regions of incised paleovalleys in a variety of marginal mari e settings including fluvial channel channel mouth bar beach ridge and coastal swamp environments unit f consists of horizontally bedded sandstone interbedded sandstone and organic rich mudstone it records deposition over paleotopographic highs on the sub mississippian unconformity from sheetfloods and in associated coastal swamp and mud flat settings relative sea level rise is thought to have initiated fluvial deposition in the northeastern brooks range by the latest tournaisian earliest visean coal bearing fluvial channel fill and flood basin successions and palynologic data suggest that this part of arctic alaska was situated in a humid climatic zone during the early carboniferous the stratigraphic and sedimentologic organization of the kekiktuk conglomerate indicates that once base level rise began to affect the northeastern brooks range local and regional variations in paleotopography along the sub mississippian unconformity exerted primary control on accommodation and consequent distribution of kekiktuk depositional environments the stratigraphic position of the kekiktuk conglomerate above a regional angular unconformity nd below marginal marine and shallow marine shales combined with its limited thickness widespread distribution and suite of paleoenvironments suggests deposition in an upland region situated landward of the tectonic hinge zone along a steadily subsiding passive continental margin thermal subsidence combined with eustatic sea level rise were probably responsible for relative sea level rise in arctic alaska and the resulting regional transgression end page 679

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

annealing kinetics of fission tracks in zircon an experimental study a series of laboratory annealing experiments on zircon fission tracks has been carried out under heating conditions of 350 750 c for 10 1 103 hr i e 4 5 min 40 days variation in the confined lengths of spontaneous fission tracks was determined using zircon grains from nisatai dacite the fading contours of normalized mean track length r on the arrhenius diagram showed as sets of straight lines we performed a series of model fittings called the parallel and fanning models in order to describe the decrease in r with increasing temperature or heating time the lowest temperature limit of the zircon partial annealing zone zpaz was defined as r 0 95 and the highest as r 0 4 which approximately corresponded to the total fading of surface tracks extrapolation of the results of the laboratory experiments to the geological timescale gives for a heating duration of 106 yr estimated values of the zpaz of 210 320 c 60 c 2 with the parallel model 190 350 c 50 c 2 with the fanning model critical temperature t0 and 170 390 c 50 c 2 with the fanning model t0 the temperatures of the zpaz decrease by 20 c for an annealing duration that is an order of magnitude longer because the estimated closure temperature of zircon fission track analysis approximately corresponds to the middle of the zpaz these results support the previously estimated closure temperature of 240 c by varying the etching time it was revealed that significant removal of radiation damage occurs at r 0 93

the proposed cambrian ordovician global boundary stratotype and point gssp in western newfoundland canada sections exposing the cambrian ordovician boundary interval at broom point in western newfoundland have been proposed earlier for a global systemic boundary stratotype these lie within the cow head group a late middle cambrian to early middle ordovician allochthonous unit of limestone shale and conglomerate deposited at the toe of the ancient continental slope and on the adjacent continental rise several recent studies have further investigated the stratigraphy sedimentology and palaeontology of the cow head group and others are under way on magnetostratigraphy and chemostratigraphy these aspects are reviewed for six key boundary sections representing proximal to distal facies cow head ledge broom point south broom point north st pauls inlet quarry martin point and green point in particular new data are presented from 260 conodont samples that yielded 15500 conodonts this intense sampling has allowed the discrimination of minor hiatuses in the proximal to intermediate facies where conglomerates have eroded and cannibalized underlying strata new conodont data from broom point north have lowered the base of the c lindstromi zone into unit 74 conglomerates thereby making this section unsuitable as a boundary stratotype new collections from green point have yielded abundant conodonts and over 9400 conodonts have been recovered from 77 samples the conodont graptolite and trilobite biostratigraphy through the boundary interval is documented allowing accurate correlation between sections and more precisely revealing small hiatuses in the proximal and intermediate facies the sequence of conodont zones is eoconodontus notchpeakensis cordylodus proavus c caboti c intermedius c lindstromi and c angulatus these can be correlated with trilobite zones established from both in situ and clast faunas from the proximal to intermediate facies and with graptolite assemblages of cooper 1979 especially in the intermediate to distal facies three new species of cordylodus are described c andresi c hastatus and c tortus and the full apparatus of iapetognathus preaengensis is illustrated the criteria for selecting a global boundary stratotype and point gssp are reviewed in terms of the cow head sections the green point section is shown to meet and largely surpass the prerequisites required of a stratotype the green point section is proposed to be the global boundary stratotype with the base of the ordovician system defined at the base of unit 23 which is the base of the broom point member green point formation at a level coincident with the base of the cordylodus lindstromi zone in addition to an abundant and superbly preserved conodont fauna this section preserves the best sequence of earliest planktic graptolites through a 40 m interval the first nematophorous graptolites of assemblage 1 occur in unit 25 6 9 m above the base of the c lindstromi zone this level can be readily correlated into the proximal facies where both deep and shallow water trilobites in situ and in clasts respectively show the base of the c lindstromi zone to lie within the symphysurina brevispicata trilobite subzone

the arc of the western alps in the light of geophysical data on deep crustal structure recently two international deep seismic campaigns in the western and central alps etude continentale et oceanique par reflexion et refraction sismique progetto strategico crosta profonda ecors crop roure et al 1996b and national research program 20 nrp 20 pfiffner et al 1997 have been completed here we present a synoptic interpretation of the wealth of geophysical data about deep crustal structure of the alps collected during the past 40 years including the two above mentioned geophysical projects the tectonic reinterpretation of the ecors crop high resolution seismic transect is based on an evaluation of the deep crustal structure by integrating new and literature data on surface geology combined with previously published interpretations regarding the central alps schmid et al 1996 this study reveals substantial differences in geometry and kinematics between transects across the western and central alps respectively at depth the transition between the central and western alps is marked by the western limits of an adriatic lower crustal wedge shaped structure and the northeastern limits of a similar structure made of european lower crustal material at the surface it coincides with a corridor of dextral strike slip along the tonale and simplon lines in addition the thickness of the seismogenic zone shows a remarkable variation from over 40 km beneath the penninic realm of the western alps to less than 20 km in the central alps the formation of the western alpine arc was initiated during convergence and collision before 35 myr ago when the adriatic micro plate moving northward with respect to the european foreland caused sinistral transpression in the western alps while the central and eastern alps underwent head on convergence and collision during the post collisional stage i e after 35 myr ago the arcuate shape of the western alps was accentuated by wnw directed movement and anticlockwise rotation of the adriatic microplate decoupled from the central and eastern alps along the tonale simplon dextral shear zone this led to wedging of lower crustal slices both in the western alps and in the central alps the new tectonic interpretation of the ecors crop transect allows a kinematic model to be established for crustal shortening in the western alps during the past 35 myr the ivrea mantle plays the role of a backstop in our tectonic model we distinguish three episodes of post collisional crustal shortening along the ecors crop transect from 35 to 30 ma the brianconnais basement was back thrusted over the gran paradiso units leading to 30 km of crustal shortening first episode in the early miocene movements concentrated mainly along the penninic frontal thrust and resulted in about 60 km shortening second episode post 12 ma shortening within the external massifs is associated with folding in the jura mountains when the crust was shortened by an additional 30 km third episode

the cambrian ordovician transition in dysoxic facies in baltica diverse faunas and carbon isotope anomalies the cambrian ordovician boundary interval in scandinavia is characterized by largely endemic trilobite species and fossil meager intervals within the alum shale formation previous investigations of this interval in scandinavia based on drill cores are rather sketchy in order to characterize the faunal signature in a largely dysoxic setting during this time interval as well as providing biostratigraphic and chemostratigraphic data valuable for intercontinental correlation a small strip in the outskirts of the village sodra sandby in scania southern sweden was excavated nearly 5 m of the cambrian ordovician boundary strata largely represented by alum shale were exposed and the profile was meticulously investigated for fossil content and lithological characteristics and sampled for 13corg analyses the uppermost cambrian in sweden has previously been described as barren of fossils however the present study reveals a rather diverse fauna including lingulid brachiopods trilobites protoconodonts paraconodonts and euconodonts in the uppermost 1 6 m of the furongian the first appearance datum of planktic graptolites is represented by a single shale surface covered by specimens of rhabdinopora flabelliforme parabola at 1 74 m above the base of the section and roughly corresponds to the cambrian ordovician boundary the conodont fauna includes several cordylodids important for intercontinental correlation the sodra sandby section 13corg data were coupled with isotope data from two scanian drill cores haslov 1 and tosterup 2 in order to compile a composite isotope curve spanning the uppermost ctenopyge linnarssoni trilobite zone in the furongian to the upper r flabelliforme parabola graptolite zone in the lower ordovician lower tremadocian two isotope shifts from baseline values observed at the base of the peltura paradoxa trilobite zone and the lower part of the peltura transiens trilobite zone respectively can be correlated with contemporaneous shifts in other parts of the world the former a negative shift of approximately 0 4 0 7 corresponds to the widely documented top of cambrian carbon isotope excursion toce and the latter a positive shift of approximately 1 corresponds to an as yet unnamed excursion at the base of the cordylodus proavus conodont zone in terms of faunal content and isotopic signals the present study represents the first detailed description of the cambrian ordovician boundary interval in baltica the relatively diverse fauna recorded suggests that the dysoxic environment was not a serious inhibitor for marine life globally no isotope values significantly different from background values have been reported at the cambrian ordovician boundary this is confirmed by the present study however as the base of the ordovician gssp lacks meaningful isotope data global correlation of this important boundary is problematic

cambrian ordovician conodonts from slump deposits of the argentine precordillera new insights into its passive margin development fil voldman gustavo gabriel consejo nacional de investigaciones cientificas y tecnicas centro cientifico tecnologico conicet cordoba centro de investigaciones en ciencias de la tierra universidad nacional de cordoba facultad de ciencias exactas fisicas y naturales centro de investigaciones en ciencias de la tierra argentina

conodont biostratigraphy of the latest cambrian early ordovician upper mckay group southeastern british columbia the age of the upper mckay group based on conodont biostratigraphy is latest cambrian cordylodus proavus zone to late early ordovician middle floian oepikodus communis zone a collection of 12 940 conodont elements was recovered from 306 samples of upper mckay group strata exposed in the western main ranges of the southern canadian rocky mountains southeastern british columbia the conodont fauna is assigned to 53 species representing 30 genera twelve zones are recognized two of which are cosmopolitan cordylodus proavus zone and iapetognathus zone seven midcontinent realm zones in ascending order include polycostatus falsioneotensis rossodus tenuis rossodus manitouensis low diversity interval scolopodus subrex and acodus kechikaensis zones and tropodus sweeti subzone of the oepikodus communis zone three atlantic realm zones in ascending order include cordylodus angulatus acodus deltatus and paroistodus proteus zones the zonation for the upper mckay group establishes correlation w

very coarse grained beaches as a response to generalized sea level drops in a complex active tectonic setting pleistocene marine terraces at the cadiz coast sw spain abstract the studied pleistocene deposits at the cadiz coast are located on the current intertidal zone or at cliffs parallel to the actual coast 6 50 m a s l from the localities of el puerto de santa maria to chiclana five sequences of very coarse grained beaches have been recognized and dated they unconformably overlie pliocene units and are formed by clast supported conglomerate and coarse grained sandstone chronological ages 87 sr 86 sr and ams 14 c analysis and stratigraphic correlation stand that the oldest marine terrace is the one placed topographically higher and towards the se of cadiz city dated as early pleistocene 1 3 ma after it the sea retreatment produces subaerial exposure of sequence 1 and the development of several calcrete profiles and a paleokarst subsequent sea level rises allow the sedimentation of sequences 2 3 and 4 during early middle pleistocene 1 to 0 8 ma in areas close to cadiz city with subaerial exposure recorded at the top of sequences 3 and 4 during the very upper pleistocene sedimentation of sequence 5 occurs towards the se of cadiz city based on the spatial position of sequence 5 we can conclude that this area constituted a downthrown block during the latest pleistocene in relation to neotectonic processes and diapirism sandstone framework hybrid arenites reveals that main source areas for these marine terraces include units from the betic orogene betic external zones triassic and jurassic rocks and the gibraltar arc the guadalquivir basin passive margin iberian massif the guadalquivir foreland basin marine miocene and pliocene record and local inputs of pleistocene units the siliciclastic supplies from sources were mainly constant during the sedimentation of all set of sequences suggesting maturation processes as reworking and recycling of homogenized sediments during transport early diagenetic processes have been identified as carbonate cementation neomorphism and dissolution outlining the deduced sea level fluctuations during deposition of pleistocene sequences the marine terraces arrangement and its comparison with regionally adjacent areas allows to deduce that the southern and central portuguese coast iberian massif and of the southern and southeastern spanish coast betic orogene were predominantly tectonically raising during pleistocene however the cadiz gulf coast formerly guadalquivir foreland basin was mainly sinking during the same time interval

a composite tectonic eustatic origin for shelf sandstones at the cambrian ordovician boundary in north greenland shelf sandstones deposited at the cambrian ordovician boundary in central and western north greenland have a sheet like large scale geometry and extend for over 300 km along strike within a shallow marine carbonate dominated platform sequence conodont faunas from units above and below the most extensive sand sheet the kap coppinger member of the cass fjord formation and the permin land formation indicate that deposition of the sands spanned the proavus intermedius and lindstromi conodont biozones sand was derived by reworking of older quartzite dominated formations following the uplift and unroofing of a platform bulge to the east during the cambrian this sediment was transported northwards from the uplifted area across an unstable platform into a deep water trough however during sea level lowstands within the cambrian ordovician boundary interval this clastic material was deflected westwards and transported across the stable platform by progradation of tide and storm dominated shorelines facies sequences in the thickest part of the permin land formation suggest at least three such regressive cycles combined to form a composite sand body each regressive cycle comprises a shallowing upward transition from tide and storm deposited marine sediments to clean sandstones of marine or terrestrial origin the cycles are separated by thin but laterally extensive dolomites interpreted to represent drowning events thinner sequences further to the east are attributed to reduced accommodation space as a consequence of lower subsidence rates close to the uplifted source area whilst thin sequences in the west are ascribed to the effect of increasing distance from the sediment source so that only the youngest of the three regressive cycles is present

post nappe brittle tectonics and kinematic evolution of the north western alps an integrated approach abstract data from remote sensing structural geology and thermochronology provide the basis for this integrated reconstruction of the oligocene to present kinematic evolution of the north western alpine nappe stack two brittle tectonic phases post date the cretaceous eocene ductile deformation a nw se extension developed in the oligocene d1 along three main conjugate fault systems arranged in orthorhombic symmetry n nw and se dipping cooling rate contour maps from published apatite and zircon fission track ages and rb sr biotite ages highlight the differential exhumation of large fault bounded blocks during this phase whilst synkinematic hydrothermal veins and calc alkaline dykes 29 32 ma help to constrain its age from the miocene onwards a general rearrangement of the strain pattern led to sw directed lateral extrusion d2 of the pennine graian alps block bounded by a network of seismogenic shear zones the most important being the ospizio sottile simplon rhone and chamonix faults the internal deformation of the pennine graian alps block is characterised by an overall more or less homogeneous ne sw extension the approach undertaken integrating remote sensing structural analysis on different scales and thermochronology with the cooling rate map representation is therefore effective in reconstructing the late orogenic extensional tectonic evolution of metamorphic nappe stacks

dating fault gouges geological faults are fractures in the earth s crust along which there has been some displacement and are thus zones of weakness which are of importance in planning civil engineering projects constructing underground caverns and conducting earthquake research to assess the potential for future displacements and fluid circulation along faults it is helpful to establish the age of the latest episode of mineral growth in fault zones however there have been few attempts to date recrystallized and newly grown minerals developed in fault zones at low temperature1 4 here we report k ar and rb sr ages for clay fractions from two fault zones in the eastern alps and compare them with ages of the same fine fractions from the undeformed host rock the results suggest that the bulk of the clays along these faults were formed during early cretaceous and tertiary times respectively no major displacements seem to have taken place in recent times

microfabric memory of vein quartz for strain localization in detachment faults a case study on the simplon fault zone abstract this manuscript deals with the adaptation of quartz microfabrics to changing physical deformation conditions and discusses their preservation potential during subsequent retrograde deformation using microstructural analysis a sequence of recrystallization processes in quartz ranging from grain boundary migration recrystallization gbm over subgrain rotation recrystallization sgr to bulging nucleation blg is detected for the simplon fault zone sfz from the low strain rim towards the internal high strain part of the large scale shear zone based on i the retrograde cooling path ii estimates of deformation temperatures and iii spatial variation of dynamic recrystallization processes and different microstructural characteristics continuous strain localization with decreasing temperature is inferred in contrast to the recrystallization microstructures crystallographic preferred orientations cpo have a longer memory cpo patterns indicative of prism and rhomb glide systems in mylonitic quartz veins overprinted at low temperatures 400 c suggest inheritance of a high temperature deformation in this way microstructural textural and geochemical analyses provide information for several million years of the deformation history the reasons for such incomplete resetting of the rock texture is that strain localization is caused by change in effective viscosity contrasts related to temporal large and small scale temperature changes during the evolution of such a long lived shear zone the spatially resolved quantitative investigation of quartz microfabrics and associated recrystallization processes therefore provide great potential for an improved understanding of the geodynamics of large scale shear zones

a middle ordovician drowning unconformity on the northeastern flank of the okcheon ogcheon belt south korea abstract the maggol limestone of ordovician age was deposited in the taebaeksan taebacksan basin which occupies the northeastern flank of the okcheon ogcheon belt of south korea carbonate facies analysis in conjunction with conodont biostratigraphy suggests that an overall regression toward the top of the maggol limestone probably culminated in subaerial exposure of platform carbonates in the early middle ordovician earliest darriwilian elsewhere this subaerial exposure event is manifested as a major paleokarst unconformity at the sauk tippecanoe sequence boundary beneath the middle ordovician succession and its equivalents most in notably north america and north china due to its global extent this paleokarst unconformity has been viewed as a product of second or third order eustatic sea level fall during the early middle ordovician the sauk tippecanoe sequence boundary in south korea however appears to be a discrete marine flooding surface in the upper maggol limestone strata beneath this surface represent by a thinning upward stack of exposure capped tidal flat dominated cycles that are closely associated with multiple occurrences of paleokarst related solution collapse breccias this marine flooding surface is onlapped by a thick succession of thin bedded micritic limestone that is eventually overlain by a middle ordovician condensed section this physical stratigraphic relationship suggest that second and third order eustatic sea level fall may have been significantly tempered by regional tectonic subsidence near the end of maggol deposition the tectonic subsidence is also evidenced by the occurrence of coeval off platform lowstand siliciclastic quartzite lenses as well as debris flow carbonate breccias i e the yemi breccia in the basin with continued tectonic subsidence a subsequent rise in the eustatic cycle caused drowning and deep flooding of the carbonate platform forming a discrete marine flooding surface that may be referred to as a drowning unconformity this tectonic interpretation contrasts notably with the slowly subsiding carbonate platform model for the basin as has been previously suggested thus it is proposed that the taebaeksan basin in the northeastern flank on the okcheon belt evolved from a slowly subsiding carbonate platform to a rapidly subsiding intracontinental rift basin during the early middle ordovician

widespread post nappe normal faulting in the internal western alps a new constraint on arc dynamics abstract field data collected in the brianconnais and piemont zones at the centre of the western alpine arc demonstrate a high density of normal faults normal displacements are pervasive from the millimetric to the kilometric scales they occur both parallel and at high angles to the arc overprinting all the alpine folds and thrusts and the associated schistosities the orientation of fault striae implies that the pile of metamorphic nappes se of the pelvoux massif has undergone a general extension extension tends to be multidirectional in the brianconnais zone orogen parallel extension prevails in most of the piemont schistes lustres to the west in brianconnais zone brittle extension is associated with a reversal of movement along the brianconnais frontal thrust that started before the end of the oligocene in the piemont schistes lustres the onset of normal faulting occurred early in the miocene and continued the ductile extension that accompanied the exhumation of hp lt metamorphic thrust sheets during the palaeogene in both zones extension remains currently active resulting in the regional seismicity since more than 20 ma ago this normal faulting in the internal arc coexists with an outward propagation of folds and thrusts in the external arc we discuss its possible significance in the context to the dynamic evolution of the alps

staircase 40 ar39ar spectra of fine grained white mica timing and duration of deformation and empirical constraints on argon diffusion staircase 40 ar 39 ar spectra of 2 to 6 m white micas in ductile carbonate mylonites morcles doldenhorn nappe switzerland record ages between 32 and 13 ma these ages are within independent geochronologic and biostratigraphic time limits for the beginning and ending of deformation of this fold thrust nappe we interpret these data to record protracted white mica neocrystallization and growth during low temperature ductile deformation such that the differences between the maximum and minimum ages in the 40 ar 39 ar spectra record the time span over which mica formed and grew we have modeled the argon concentrations in these micas taking into account the thermal history of the nappe mica growth and diffusional loss of argon our results indicate that available experimental data underestimate the activation energy for argon diffusion in muscovite and that the retentivity of argon in white mica is greater than generally thought