msec data sets record glacially driven cyclicity examples from the arrow canyon mississippian pennsylvanian gssp and associated sections abstract here we report magnetic susceptibility ms analyses from three mainly marine sections the mississippian pennsylvanian mid carboniferous global boundary stratotype section and point gssp in arrow canyon se nevada and two secondary sections nearby one in arrow canyon 200 m distant and the second in battleship wash 2 km south of the gssp all three sections are easily correlated using the magnetosusceptibility event and cyclostratigraphy method msec cyclicity is clearly apparent in all three sections and time series analysis independently verifies the ms zonation developed from the smoothed data set the periods determined in the three sections support the argument that deposition of these mid carboniferous rocks resulted from climate controlled glacio eustatic fluctuations that were driven by gondwana glaciation given the published average thicknesses of t r cycles 50 m and the estimates for timing of glacial interglacial cyclicity during the mid carboniferous we assign the strong ft peak observed for the three ms data sets a value of 400 000 years corresponding to the milankovitch e1 eccentricity band we then calculate a floating point time scale fpts for the sampled sequence the fpts results indicate that 1 sediment accumulation rates bracketing the mid carboniferous boundary averaged 0 7 cm 1000 years and 2 were relatively constant for these reference sections

observed tidal currents outside block island sound offshore decay and effects of estuarine outflow 1 an array of moored profiling current meters on the inner continental shelf 15 65 m deep is used to investigate tidal currents outside the block island sound and long island sound estuarine system the m2 constituent dominates due to near resonant semi diurnal estuarine response vertical mean m2 currents rotate clockwise in time in ellipses elongated toward the estuary mouth with little sensitivity to complex local bathymetry semi major axes decay sharply offshore 55 to 20 cm s over 10 km in agreement with the nearly inverse square radius dependence of a kinematic theory estimated tidal volume exchange out of block island sound to the south is 2 9 105 m3 s observed vertical structure of m2 ellipses in the deepest 10 20 m amplitude decay ellipse flattening major axis turning clockwise phase advance is generally captured well by optimally fit frictional solutions despite the fact that they omit bathymetry and include stratification only indirectly through its influence on eddy viscosity in the upper water column vertical structure varies seasonally mid depth ellipses enlarge in spring near surface ellipses are larger smaller than optimal fit solutions in fall winter spring observed seasonal average estuarine outflow includes surface intensified coriolis deflected mean flow that strengthens and spreads farther offshore in spring richardson numbers based on moored hydrographic profiler records suggest spring stratification suppresses deep eddy viscosities which in frictional solutions can explain mid depth ellipse enlargement an inviscid theory for mean flow modifications due to ambient vorticity of estuarine outflow currents is shown to be the most plausible explanation for observed seasonal changes in near surface tidal ellipses

muscovite recrystallization and saddle shaped 40 ar 39 ar age spectra example from the blond granite massif central france muscovites from the blond granite west french massif central were dated by the 40ar 39ar single grain method the 40ar 39ar ages obtained vary from 305 5 0 3 ma to 311 3 0 5 ma and most of the age spectra are slightly saddle shaped the analyzed muscovites show phengitic recrystallization under optical microscope observations sem images and electron microprobe chemical analyses it is proposed that the saddle shaped age spectra result from a partial recrystallization which produced three different isotopic reservoirs in the analyzed white mica single grains domains of early muscovite domains of neocrystallized muscovite formed by phengitic and al fe substitutions and low activation energy sites

proterozoic metamorphism and cooling in the southern lake superior region north america and its bearing on crustal evolution abstract metamorphism along the southern margin of the archean superior province has been historically attributed to the penokean orogeny a narrow corridor of amphibolite facies rocks north of the main suture does record 1 83 1 80 ga metamorphic monazite u th pb ages that mark the culmination of arc accretion however subsequent widespread amphibolite facies metamorphism and associated magmatism is recorded along the regions of greatest penokean crustal thickening the tectonically buried archean proterozoic continental margin in minnesota new monazite geochronology reveals a profound midcrustal metamorphic imprint caused by emplacement of the 1 775 ga east central minnesota batholith at moderate depths in northern wisconsin and upper peninsula michigan metamorphic monazite growth at 1 78 1 745 ga and far from geon 17 intrusions reflect a previously little recognized regional amphibolite facies metamorphic event associated with ca 1 76 ga yavapai interval accretion not solely penokean induced crustal collapse south of the penokean suture penokean terrane rocks were twice metamorphosed to upper greenschist facies first during yavapai accretion and again during geon 16 mazatzal accretion geon 16 overprinting also affected a small part of the continental margin in the northeast orogen the peavy metamorphic node south directed basement thrusts there likely accommodated substantial mazatzal foreland shortening suggesting thick skinned deformation mazatzal amphibolite facies metamorphism occurred throughout iowa and southernmost wisconsin south of the baraboo quartzite 40 ar 39 ar mineral cooling ages from eastern wisconsin reveal a limited metamorphic aureole associated with the intrusion of the 1 47 ga wolf river batholith in part reflecting its rapid emplacement at shallow crustal levels a local area of anomalously young 40 ar 39 ar mica ages away from known midcontinent rift exposures probably reflect shallow crustal level reheating during rifting not exhumation the pattern and degree of proterozoic metamorphism preserved across the northern interior of the north american midcontinent is highly variable and is a reflection of multiple accretionary and intrusive events between 1 87 and 1 47 ga collectively the geo and thermochronologic data reveal a progressive tectonic younging toward the south and overall waning of proterozoic metamorphism in this region these results demonstrate a changing tectonic environment from active margin to continental interior as laurentia grew southward and stabilized over a few hundred million years

quantification of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetisation abstract a new method of analysing isothermal remanent magnetisation irm acquisition curves based on cumulative log gaussian analysis robertson and france phys earth planet inter 82 1994 223 234 is proposed it is based on the curve fitting of the irm acquisition curve versus the logarithm of the applied field with i the acquisition curve on a linear scale ii the acquisition curve expressed as a gradient and iii the acquisition curve on a probability scale even when a sample is not saturated its magnetic properties can be defined although with less certainty the number of magnetic components required for an optimal fit to a measured irm acquisition curve is evaluated statistically the method discriminates on the basis of different mineral coercivity therefore additional rock magnetic tests are still required to separate minerals with similar coercivities

tectonic implications of metamorphism and gravity field in the penokean orogen of northern michigan published geologic data together with new metamorphic pressure and temperature data and gravity data provide strong evidence for northward directed thrusting of lower proterozoic rocks in northern michigan and wisconsin metamorphic studies indicate pressures of 7 kbar at watersmeet node and 4 kbar at the peavy node the republic metamorphic node located 30 km to the north of peavy attained pressures of 2 3 kbar all three metamorphic highs reached temperatures in the range of 600 650 c and are correlated with areas of negative gravity anomalies relative to surrounding regions in the peavy high the center of the gravity anomaly lies on the sillimanite isograd and we suggest that the source of this negative anomaly may be remobilized underlying archean gneiss the highest grade rocks of the republic node lie well within a residual gravity anomaly which is interpreted to have a very shallow source and the watersmeet high lies above gneiss domes which show negative gravity anomalies a north south geologic cross section constructed from a two dimensional gravity model which extends northward from the dunbar dome in the magmatic arc terrane of wisconsin across a terrane boundary interpreted as a suture and marked by the niagara fault and across the peavy metamorphic node suggests that the dunbar dome was transported northward onto the continental foreland during the penokean orogenic event these north verging fold thrust sheets or nappes produced prominent tectonic thickening at the peavy and watersmeet as indicated by the relatively high metamorphic pressures there the thrust sheets evidently thinned to the north as suggested by the relatively low pressures at republic we suggest that the archean continental edge defined by the steep gradient in the regional northeast trending gravity high lies beneath the magmatic rocks of wisconsin and thus extends south of the niagara fault suture

40ar 39ar and rbsr analyses from ductile shear zones from the atacama fault zone northern chile the age of deformation abstract the influence of deformation on the k ar and the rb sr isotope system is investigated it is assumed that due to the diffusion processes involved deformation has a similar effect on isotopic equilibrium as has temperature in order to examine the influence of deformation on the k ar and the rb sr isotope systems two shear zones from the atacama fault zone afz situated in the north chilean coastal cordillera have been investigated the afz which was active as a sinistral strike slip fault during the mesozoic has two sets of shear zones one formed under amphibolite sz1 one under greenschist facies conditions sz2 rb sr and 40ar 39ar age determinations were conducted on samples from cross sections of each set in sz1 the hornblendes and bioties from a weakly deformed sample reveal cooling ages of 153 152 and 150 1 ma respectively biotite from the center of the shear zone of sz1 gave an isochron of 143 9 0 3 ma mswd 0 04 which is interpreted as the age of deformation which produced resetting of the mineral system in sz2 hornblendes yielded 40ar 39ar plateau cooling ages of 138 ma biotites from undeformed samples gave rb sr and 40ar 39ar total degassing ages of 130 1 ma whereas biotite from the mylonitic rocks yielded 126 125 ma which dates the time of deformation sr isotope homogenization occurred in the mylonitic rocks and is most likely a result of deformation the formation of sz1 can be correlated to the araucanian nevadan phase the deformation in sz2 is related to the onset of uplift and cooling of the coastal cordilleran magmatic arc

argon diffusion in hypogene and supergene alunites implications to geochronology and thermochronometry on earth and mars abstract argon release mechanisms and diffusivity were quantified for coarsely crystalline hypogene alunite kal3 so4 2 oh 6 from marysvale utah and microcrystalline supergene alunite aggregates from coober pedy south australia prior to diffusivity studies all alunite samples sorted in the 500 200 200 100 100 50 50 10 and 460 c isothermal holding time and agesme modelling using our calculated diffusion parameters indicate that alunite should preserve ar quantitatively for long periods 4 0 ga at earth and mars surface conditions and both hypogene and supergene alunite should preserve original formation ages independently of precipitation mechanism

a new chalcolithic era tsunami event identified in the offshore sedimentary record of jisr al zarka israel abstract tsunami field evidence is a critical resource for determining coastal risk however in many cases there is low potential for long term preservation of such deposits on land a growing body of evidence suggests that tsunami deposits can be present in the shallow offshore realm and may present a largely untapped but important worldwide sedimentological reference set for investigating past tsunami events here different proxies for tsunami sediment identification and differentiation granulometry xrd xrf ft ir were used to determine the presence or absence of such deposits in a core collected offshore at jisr al zarka midway between tel aviv and haifa the study aimed to recognize anomalous deposits and test different analytical methods to define their origins five anomalous sedimentological horizons were identified four of which were interpreted as tsunamigenic by comparison to previously identified poorly sorted shell rich tsunamites found nearby in caesarea and contrast to the fine sand nile derived background sediments the remaining anomaly was distinctive but not determined to be tsunamigenic due to a lack of coarse shell in the matrix the oldest tsunamigenic layer coinciding with the chalcholithic cultural period 5 7 ka is the oldest such deposit identified offshore israel to date and the non tsunamigenic anomalous horizon may correspond with a transitional climatic phase 5 5 ka this research reinforces the importance of offshore investigations as a means of improving tsunami catalogues and identifying environment altering events

tsunami sedimentation associated with the lisbon earthquake of 1 november ad 1755 boca do rio algarve portugal a description is given of a distinctive marine deposit at boca do rio on the algarve coast of portugal it is proposed here that the sediment accumulation was deposited by the tsunami generated by the lisbon earthquake of 1 november ad 1755 the deposit exhibits sedimentary characteristics quite unlike other coastal sediment accumulations that are deposited by more moderate wave regimes these include laterally continuous sand layers chaotic pebble horizons large amounts of gravel sized shell debris and distinctive assemblages of marine microfossils a preliminary attempt is made here to use the empirical field and laboratory data to define more clearly the processes of coastal sedimentation that characterize tsunami runup the use of tsunami sediments as time synchronous marker horizons is also discussed

sedimentological record of tsunamis on shallow shelf areas the case of the 1969 ad and 1755 ad tsunamis on the portuguese shelf off lisbon one of the aims of the sedport project esf euromargins eurocores program was to investigate the depositional features on the shelf and upper slope of the portuguese margin tagus system during the holocene to fulfill the objectives two shallow box cores and two sedimentary sequences were studied po287 26b and the spliced sequence of sites d13902 po287 26g recovered on the portuguese continental shelf ssw off the tagus river mouth and the box po287 28b and gravity core geob8903 collected towards the w of the river mouth magnetic susceptibility grain size and xrf fe data as well as 210 pb and ams 14 c dating of these sedimentary sequences have allowed the identification of an instantaneous deposit at about 20 cm in the box cores downcore we could establish a hiatus corresponding to 355 yr of sedimentation at both sites a 39 cm instantaneous deposit of coarse material including carbonate broken shells on the ssw site and a 1 5 m instantaneous deposit of fine material on the w site both hiatus and instantaneous deposits are considered to be of tsunamigenic origin the box cores deposit has an estimated age of 1969 ad and it is attributed to the 1969 earthquake caused tsunami the hiatus and deposits found downcore are due to the age limits encountered believed to represent the tsunami related to the 1755 lisbon major earthquake these data further indicate a sediment deposit similar in type and thickness on both ssw and w sites in 1969 while the 1755 ad record indicates a higher energy backwash along the ssw and a preferential deposition of fine suspended matter towards the w c 2007 elsevier b v all rights reserved

rifting in sw china structural and sedimentary investigation of the initial crustal response to emplacement of the permian emeishan lip detailed structural volcanic and sedimentary investigations of the crustal response to the emplacement of the middle late permian emeishan large igneous province show that a rifting system developed prior to the main stage of flood basalt eruptions in the form of sedimentary breccias clastic sedimentary deposits and mafic hydromagmatic units detrital zircon grains from sandstones yield 750 800 ma la icp ms 206 pb 238 u age clusters showing that material was sourced from the yangtze crystalline basement gabbros and pegmatites intruded along the normal faults of the rift system yield zircon ages of 264 260 ma and thus constrain the timing of rifting n s trending rift zones developed along the western flank of the pan xi palaeo uplift with ne sw and nne ssw trending rifts on the eastern side and along the western and eastern margins of the yangtze block the rifting progressed in pulses with an initial phase of normal faulting followed by rapid deposition of breccias later there was lower energy deposition of sandstone with accompanying rhyolitic eruptions this was followed by low energy sedimentation of mudstones and dolomites with accompanying hydromagmatic deposits rift system formation was constrained by a combination of far and near field tectonic stresses due to plate motions and lithospheric interaction with initial emeishan volcanism

tsunami geomorphology erosion and deposition from the 15 november 2006 kuril island tsunami the 15 november 2006 kuril earthquake mw 8 1 8 4 and tsunami enabled us to collect a compelling data set of coastal geomorphic change in the kuril islands from 3 months before to 9 and 21 months after the tsunami our pre tsunami and post tsunami surveys of the islands including four topographic profi les measured in 2006 and reoccupied in 2007 allow us the confi dence to attribute many changes to the tsunami in spite of an absence of eyewitness accounts in the central islands areas with low runup 15 m underwent massive erosion that dramatically altered the coastline tsunami deposits roughly corresponded with the extent of tsunami runup and inundation the amount of sediment eroded by the tsunami far outweighed the amount deposited on land in all cases studied the tsunami was dominantly erosive in the kuril islands because the high relief topography of the coastline accelerated tsunami outfl ow

using magnetic fabric to reconstruct the dynamics of tsunami deposition on the sendai plain japan the 2011 tohoku oki tsunami abstract the magnetic fabric and grain size of sand deposits emplaced during the 2011 tohoku oki tsunami were studied in five trenches along a 1800 m long shore normal transect on the sendai plain as well as in a near shore sedimentary infill of a scour depression the magnetic susceptibility in all deposits is due to ferromagnetic minerals mainly magnetite making the anisotropy of magnetic susceptibility magnetic fabric suitable for fabric analyses the dominant magnetic fabric is planar in all trenches and stronger in finer grained inland deposits than in the coarser sediments this planar fabric is related to tractive shearing of the bedload basal portion of the tsunami flow that led to the deposition of traction carpet layers among the various fabric parameters used for this study the vertical evolution of the shape factor q of the magnetic ellipsoid in each trench follows the evolution of the magnetic lineation l and foliation f these parameters provide information on the hydrodynamic energy flow speed fluctuations during the emplacement of the tsunami deposit for the most proximal deposits characterized by well sorted reworked beach sand with minor fluctuations in grain size distribution the magnetic fabric is sensitive to hydrodynamic energy variations during sedimentation reconstruction of tsunami flow orientation in the sediments based on the orientation of the mean k max calculated for each trench appears to be unambiguous only for the sandy infills of small topographic depressions the variations in flow direction indicators elsewhere could be related to local variation of the flow and to micro topographic effects these findings are encouraging for the use of the magnetic fabric proxy in the study of paleotsunami deposits

shallow marine sediment cores record climate variability and earthquake activity off lisbon portugal for the last 2000 years abstract sea surface temperature sst river discharge and biological productivity have been reconstructed from a multi proxy study of a high temporal resolution sedimentary sequence recovered from the tagus deposition center off lisbon portugal for the last 2000 years sst shows 2 c variability on a century scale that allows the identification of the medieval warm period mwp and the little ice age lia high iron fe and fine sediment deposition accompanied by high n alkane concentrations and presence of freshwater diatoms during the lia 1300 1900 ad science 292 2001 662 suggest augmented river discharge whereas higher total alkenone concentrations point to increased river induced productivity during the mwp 550 1300 ad science 292 2001 662 larger mean grain size and low values of magnetic susceptibility and concentrations of fe n alkanes and n alcohols are interpreted to reflect decreased runoff at the same time increased benthic and planktonic foraminifera abundances and presence of upwelling related diatoms point to increased oceanic productivity on the basis of the excellent match found between the negative phases of the north atlantic oscillation nao index and the intensified tagus river discharge observed for the last century it is hypothesized that the increased influx of terrigenous material during the lia reflects a negative nao like state or the occurrence of frequent extreme nao minima during the milder few centuries of the mwp stronger coastal upwelling conditions are attributed to a persistent positive nao like state or the frequent occurrence of extreme nao maxima the peak in magnetic susceptibility centered at 90 cm composite core depth ccd is interpreted as the result of the well known 1755 ad lisbon earthquake the lisbon earthquake and accompanying tsunami are estimated to have caused the loss of 39 cm of sediment 355 years of record most of the lia and the instantaneous deposition of a 19 cm sediment bed

the interpretation of inverse isochron diagrams in 40 ar 39 ar geochronology abstract the concepts involved in the construction and interpretation of inverse isochron diagrams used in 40 ar 39 ar geochronology are reviewed the diagrams can be useful as a means of recognising atmospheric argon and excess 40 ar incorporated in the mineral lattice which cannot be recognised from 40 ar 39 ar spectra the age established using an inverse isochron plot unlike that yielded by a spectrum is not affected by trapped argon 40 ar 36 ar ratios that are different from the atmospheric argon ratio e g due to excess 40 ar and may contribute to a better age interpretation however a heterogeneous distribution of excess 40 ar or heterogeneous argon loss can cause false isochrons with axial intercepts indicating an incorrect age and an incorrect trapped argon composition inconsistency between the ages from a spectrum and from the associated inverse isochron plot may indicate the degree of inaccuracy of isochrons however it is possible that both the spectrum and inverse isochron yield the same incorrect age the importance of considering all possible interpretations before assigning an age to a specimen is stressed

the goodman swell a lithospheric flexure caused by crustal loading along the midcontinent rift system rb sr biotite ages of archean and early to middle proterozoic crystalline rocks in northern wisconsin and adjacent upper peninsula of michigan describe a regionally systematic pattern related to differential uplift an age low occurs in northern wisconsin where values range from 1070 1172 ma for rocks with crystallization ages of 1760 to 1865 ma these values overlap with the main episode of mafic igneous activity 1090 to 1120 ma along the midcontinent rift system mrs we interpret these low biotite ages as registering closure due to cooling below the 300 c isotherm as a consequence of uplift and rapid erosion of an area that we are informally naming the goodman swell the position of the goodman swell is independent of the archean and early proterozoic regional geologic framework but closely linked with the curvilinear concave to the south mrs we interpret the swell to be a forebulge imposed on an elastic crust by loading of mafic igneous rocks along and within the axis of the mrs the southward concavity of load axes resulted in flexural interaction that localized the swell because of rapid uplift the swell may have been a source area for clastic sediments in the rift sequence

transport and resuspension of fine particles in a tidal boundary layer near a small peninsula the authors present a theory on the transport and resuspension of fine particles in a tidal boundary layer when the ambient tidal flow is nonuniform due to a peninsula along the coastline as a first step toward better physical understanding the authors adopt the model of constant eddy viscosity and a horizontal seabed attention is focused on a peninsula whose horizontal dimension is much smaller than the tidal wave length but larger than the tidal excursion length general expressions of shear induced dispersivities and convection velocity are derived in terms of the ambient flow field nonuniformity in the horizontal flow is found to have profound effects on the spatial variation of the dispersion tensor as well as the horizontal convection velocity two numerical examples are given for a semicircular peninsula one for a particle cloud released near the peninsula over a nonerodible seabed and one for the resuspension over an erodible belt around the peninsula

the chengwatana volcanics wisconsin and minnesota petrogenesis of the southernmost volcanic rocks exposed in the midcontinent rift the southernmost exposed rocks of the north american midcontinent rift system 1100 ma consist of 3000 m of mafic volcanic flows and minor interflow sediment exposed along the st croix river in minnesota and wisconsin the flows are mostly high fe tholeiitic basalt with plagioclase phenocrysts and ophitic to subophitic clinopyroxene abundant secondary chlorite epidote and actinolite indicate the group was metamorphosed to greenschist facies 350 c low sodium m4 site and tetrahedral aluminum aliv contents of actinolite indicate low pressure metamorphism 0 25 gpa and imply a geothermal gradient of 45 50 c km low magnesium mg 0 37 0 58 and ni contents 36 185 ppm indicate the basalts have undergone significant fractionation and are not primary mantle melts incompatible element abundances are inversely correlated with mg and most samples plot within either high or low trace element groups e g ti p zr the basalts are enriched in the light rare earth elements and th and are v

detrital remanent magnetization in haematite bearing neoproterozoic puga cap dolostone amazon craton a rock magnetic and sem study summary neoproterozoic cap dolostones of the araras group mato grosso brazil deposited on top of glacial diamictites puga formation 630 ma yielded a dual polarity component providing al ow palaeolatitude for the amazon craton because palaeomagnetic negative directions are very close to the present day dipolar field a rock magnetic and sem study was conducted in order to check the primary nature of the remanent magnetization and to identify the nature and origin of the remanence carriers results show that detrital specular haematite grain size 10 m is the main magnetic carrier in the araras dolostones and was likely inherited from the underlying puga diamictites in some sites a mixture of haematite and magnetite carries the remanence of both polarities the primary nature of the magnetization is supported by the detrital character of the magnetic carriers in addition the remanence anisotropy is weak pointing to the lack of significant shallowing due to compaction these results reinforce the low palaeolatitude of the amazon craton at the time of the marinoan ice age

a comparison of different magnetic methods for determining the relative grain size of magnetite in natural materials some results from lake sediments abstract a new rapid method for identifying relative grain size variations in magnetic involves the parameter anhysteretic susceptibility arm i e specific arm obtained in a 1 oe steady field which is particularly sensitive to the single domain sd and small pseudo single domain psd grains of the finer magnetite fraction a second parameter low field susceptibility is relatively more sensitive to the coarser magnetite fraction larger psd and smaller multidomain md grains we can then obtain a measure of the ratio of coarse to fine grain magnetite for large numbers of samples by plotting arversus a simple idealized model based on sized magnetite samples is proposed to explain the use of the armversus plot for detecting relative grain size changes in the magnetic content of natural materials the sediments of three lakes that contain magnetite or a similar magnetic carrier and have a wide range of values of arm and are used to test the model the model is used to interpret the magnetic variations observed and the interpretations are supported by high field hysteresis measurements of the same sediments the combination of the high field hysteresis method of day et al 1 and the arm vs method is a powerful technique allowing the rapid identification of both the relative grain size and domain state for large numbers of samples containing magnetite the armvs method should be used as an intial means of identifying distinct groups of samples the high field hysteresis method should then be applied to a few representative samples from each group to confirm the initial interpretation

the k ar and 40ar 39ar methods revisited for dating fine grained k bearing clay minerals abstract the strengths and weaknesses of the two ar isotopic methods k ar and 40ar 39ar were evaluated on the basis of respective recent applications mainly on low temperature k bearing illite type clay minerals this review includes a presentation of basic analytical and technical aspects for both methods as well as a discussion of varied claims on the two methods and of requests about sample preparation and characterization whenever possible the advantages and weaknesses of each method were compared on coeval results obtained by both methods on the same mineral separates the comparative review examines stratigraphic dating of glauconites indirect dating of low temperature ore deposits dating of burial related illitization and dating of polyphased tectono thermal activity more specifically of fault gouges some pending questions such as the necessary encapsulation due to 39ar recoil and its restoration into step heating patterns are also raised together with the new potential of ar dating of nanometric illite crystals weakness of the k ar method is in its pioneering status that makes many believe that it is no longer accurate because of its traditional analytical aspects and of the k determinations leading to somewhat large uncertainties however precise evaluation of varied applications points to a k ar method having probably larger applicability in sedimentary to diagenetic environments than the 40ar 39ar method the drawbacks become less important if the method is applied to nanometer sized clay minerals in diagenetic to low grade metamorphic environments in this instance the extracted size fractions are generally homogeneous and the relative uncertainty given by the age calculations if mathematically justified can be reduced by duplicate analyses weakness of the 40ar 39ar method is in its basics such as the 39ar recoil the necessary encapsulation the reintegration of the 39ar into the step heating patterns and the meaning of the step heating patterns that are more suggestive of variable 39ar reservoirs created among the clay particles by irradiation than of meaningful geologic ages if the k ar method is the preferred method for dating diagenetic clay processes such as glauconitization illite crystal nucleation and growth or low temperature hydrothermal activities then the 40ar 39ar method has more potential in dating low temperature tectono thermal activities and in detailing mixtures of multi generation illite

on the origin of early proterozoic gneiss domes and metamorphic nodes northern michigan biotite 40 r 39 r cooling ages from medium pressure 500 600 mpa rocks in the watersmeet district northern michigan suggest significant cooling uplift and concomitant deformation during gneiss dome formation at 1755 ma well after the close of the 1870 1830 ma penokean orogeny however an 1822 ma hornblende plateau date indicates that the isograds surrounding the dome are penokean in age we attribute gneiss dome formation and doming of penokean aged isograds to an episode of orogenic collapse superimposed on an earlier history of crustal shortening this contrasts with the compressional origin for gneiss domes preserved in the low pressure 200 300 mpa republic district the different origins may reflect the fact that collapse was localized along the overthickened region of the orogenic belt in contrast to the watersmeet area hornblende and biotite 40ar 39ar ages obtained from the republic area are 1720 1680 ma given the relatively shallow depth of this region it is unlikely that temperatures re

single grain argon laser probe dating of phengites from the blueschist to greenschist transition on sifnos cyclades greece measurements of 40ar 39ar age spectra on single white mica crystals of tertiary age is a new means of obtaining detailed geochronological information of alpine blueschists internal consistency of the data set as well as excellent agreement with previous conventional k ar results demonstrates that this new technique can be used with confidence to obtain information on the thermal evolution of young metamorphic belts

determining the extent and nature of mazatzal related overprinting of the penokean orogenic belt in the southern lake superior region north central usa abstract twenty one hornblende and mica 40ar 39ar dates from central and northwest wisconsin usa provide important information on the timing spatial extent and intensity of mazatzal age metamorphism and deformation which overprinted the paleoproterozoic 1870 1820 ma penokean orogenic belt in the southern lake superior region 1760 1750 ma mica plateau ages from bedrock beneath undeformed 1750 1630 ma quartzites are interpreted as the time of lower temperature 300 350 c cooling and crustal stabilization after the penokean orogeny six mica ages from bedrock underlying deformed paleoproterozoic quartzites cluster around 1600 ma 1576 1614 ma the complete absence of 1760 1750 ma mica ages beneath regions of deformed quartzites suggests widespread heating to temperatures above 300 350 c during mazatzal related deformation and metamorphism the 1600 ma mica ages are interpreted to date the cooling phase of this metamorphism two anomalously young biotite dates are interpreted to indicate partial resetting associated with mesoproterozoic rifting at 1100 ma ten hornblende 40ar 39ar dates obtained in this study address the higher temperature overprinting effects in the southern lake superior region one latest archean age of 2503 18 ma and two ages of 1853 and 1830 ma are interpreted as remnant evidence of archean and penokean age amphibolite metamorphic events respectively the majority of hornblende ages are younger than the penokean orogeny scattering between 1796 and 1638 ma microtextural analysis indicates that similar microstructures exist in samples yielding highly discordant hornblende ages this suggests that shearing and recrystallization did not play an important role in the retention or loss of argon the 1638 ma hornblende age is concordant with the mazatzal orogeny to the south and is interpreted as representing complete thermal or fluid related resetting associated with that event six other post penokean ages scatter over a 70 million year interval 1796 1723 ma and probably reflect variable retention of radiogenic argon they are interpreted to indicate variable degrees of partial intermediate temperature 350 500 c resetting of hornblende argon systematics at 1650 1630 ma collectively these data suggest that the effects of the mazatzal orogeny in the southern lake superior region involved 350 500 c metamorphism and penetrative deformation

postcollisional cooling of the penokean orogen in east central minnesota new 40ar 39ar thermochronologic data from the internal zone of the penokean orogen in east central minnesota indicate rapid cooling from mid crustal temperatures early proterozoic metamorphosed cover rocks yield concordant hornblende and biotite plateau ages of 1755 ma underlying archean basement gneiss yields concordant muscovite and biotite plateau ages of 1705 ma these are some of the oldest cooling ages recorded in rocks buried during the 1830 1860 ma penokean collisional orogeny cooling was coeval with regional rhyolitic volcanism and granite emplacement suggesting a period of tectonic activity in this region during mid to late 1700 ma based on similarities with phanerozoic extensional regimes we suggest preliminarily that these data might be explained by a previously unrecognized major extensional episode in the lake superior region

the thermal significance of potassium feldspar k ar ages inferred from age spectrum results 40ar39ar age spectrum analyses of three microcline separates from the separation point batholith northwest nelson new zealand which cooled slowly 5 c ma 1 through the temperature zone of partial radiogenic 40ar accumulation are characterized by a linear age increase over the first 65 percent of gas release with the lowest ages 80 ma corresponding to the time that the samples cooled below about 100 c the last 35 percent of 39ar released from the microclines yields plateau ages 103 99 and 93 ma which reflect the different bulk mineral ages and correspond to cooling temperatures between about 130 to 160 c theoretical calculations confirm the likelihood of diffusion gradients in feldspars cooling at rates 5 c ma 1 diffusion parameters calculated from the 39ar release yield an activation energy e 28 8 1 9 kcal mol 1 and a frequency factor grain size parameter d0l2 5 6 3 9 14sec 1 this arrhenius relationship corresponds to a closure temperature of 132 13 c which is very similar to the independently estimated temperature from the observed diffusion compensation correlation this d0l2 implies an average diffusion half width of about 3 m similar to the half width of the perthite lamellae in the feldspars the range in microcline k ar ages from the separation point batholith is the result of relatively small temperature differences within the pluton during cooling comparison of the diffusion laws determined for microcline with those for anorthoclases and other homogeneous k feldspars e 40 to 52 kcal mol 1 reveals that ar diffusion is more highly temperature dependent in the disordered structural state than in the ordered structural state previously published u shaped age spectra are probably the result of the superimposition of excess 40ar upon diffusion profiles of the kind described here

the effect of the instability of muscovite during in vacuo heating on 40ar 39ar step heating spectra abstract the goal of this study was to ascertain the nature of the ar release mechanism s for a single well characterized muscovite heated incrementally in vacuo 1 at temperatures and time intervals typical of 40ar 39ar step heating experiments and 2 isothermally at 500 c and 700 c for time intervals much longer than typical 40ar 39ar experiments the rate of delamination was monitored by sem and the rate of dehydroxylation was determined by xrd analyses the rate of in vacuo ar loss was compared to that for muscovite heated at 700 c and 2 kbar h2o pressure for all experiments the rate of ar loss was found to be consistent with the rates of dehydroxylation and delamination at 500 c ar loss behavior for a 3000 m diameter muscovite grain was approximated by volume diffusion but the ar release pattern became increasingly non fickian with decreasing grain size we propose that the cause of the non fickian behavior is delamination the portions of the isothermal runs exhibiting diffusion like profiles also yielded discordant 40ar 39ar ages and low atmospheric ar contamination we attribute this phenomenon to the preferential concentration of atmospheric ar in interlayer extended defects and of 40ar in interlayer vacancies whereas 39ar is uniformly distributed throughout the muscovite structure by recoil the 40ar 39ar age spectrum of the muscovite heated under hydrothermal conditions has a staircase shape comparable to that usually interpreted as due to 40ar diffusive loss whereas 40ar 39ar laser spot analyses did not allow this interpretation the in vacuo ar release rate for this sample however indicates that it had been altered during the experiment therefore we propose that this 40ar 39ar age spectrum reflects the differential release of ar from the alteration phases vs muscovite for our sample dehydroxylation and delamination control the in vacuo ar release at temperatures less than 900 c whereas ar diffusion from thin dehydroxylated slabs may dominate the ar release mechanism for temperatures greater than 900 c this model would predict that the recovery of fossil 40ar concentration gradients from muscovite by typical incremental heating schedules is not possible the rates of delamination in vacuo and the rate of ar diffusion for muscovite in nature however both depend upon its the chemical composition structural integrity and grain size consequently for samples with either a naturally occurring broad range of grain sizes such as detrital muscovite and authigenic clay from sedimentary rocks or a compositionally or structurally heterogeneous population of grains such as muscovite from shear zones 40ar 39ar incremental heating may be able to detect intergranular variations in the k ar age

transport and fate of river discharge on the continental shelf of the southeastern united states the authors analyzed 8 years of wind and salinity data from a frontal zone in a region of the inner continental shelf off the southeastern united states the changes in low salinity water stored in the frontal zone have been parameterized by analyzing the monthly rate of change in freshwater content when the rate of change in freshwater content was negative this was interpreted as a loss of low salinity water from the frontal zone when this parameter was compared with seasonally averaged alongshore wind stress the rate of loss was independent of the alongshore wind stress magnitude until threshold of about 0 1 dyne cm sup 2 was reached above the threshold there was a clear relationship between northward alongshore wind stress and rate of loss of freshwater from the inner shelf experimental evidence suggests that horizontal currents in the inner shelf frontal zone have cyclonic shear with increasing depth when wind stress is northward and offshore near surface low salinity water is transported offshore by ekman transport while near bottom high salinity water is transported shoreward 9 figures 2 tables

runup of solitary waves on a circular island this is a study of the interactions of solitary waves climbing up a circular island a series of large scale laboratory experiments with waves of different incident height to depth ratios and different crest lengths is described detailed two dimensional run up height measurements and time histories of surface elevations around the island are presented a numerical model based on the two dimensional shallow water wave equations including runup calculations was developed numerical model predictions agreed very well with the laboratory data and the model was used to study wave trapping and the effect of slope under certain conditions enhanced runup and wave trapping on the lee side of the island were observed suggesting a possible explanation for the devastation reported by field surveys in babi island off flores indonesia and in okushiri island japan

erratum to identification of tsunami induced deposits using numerical modeling and rock magnetism techniques a study case of the 1755 lisbon tsunami in algarve portugal physics of the earth and planetary interiors volume 182 issues 3 4 pages 187 198 october 2010 abstract storm and tsunami deposits are generated by similar depositional mechanisms making their discrimination hard to establish using classic sedimentologic methods here we propose an original approach to identify tsunami induced deposits by combining numerical simulation and rock magnetism to test our method we investigate the tsunami deposit of the boca do rio estuary generated by the 1755 earthquake in lisbon which is well described in the literature we first test the 1755 tsunami scenario using a numerical inundation model to provide physical parameters for the tsunami wave then we use concentration ms sirm and grain size arm arm b1 2 arm sirm sensitive magnetic proxies coupled with sem microscopy to unravel the magnetic mineralogy of the tsunami induced deposit and its associated depositional mechanisms in order to study the connection between the tsunami deposit and the different sedimentologic units present in the estuary magnetic data were processed by multivariate statistical analyses our numerical simulation show a large inundation of the estuary with flow depths varying from 0 5 to 6 m and run up of 7 m magnetic data show a dominance of paramagnetic minerals quartz mixed with lesser amount of ferromagnetic minerals namely titanomagnetite and titanohematite both of a detrital origin and reworked from the underlying units multivariate statistical analyses indicate a better connection between the tsunami induced deposit and a mixture of units c and d all these results point to a scenario where the energy released by the tsunami wave was strong enough to overtop and erode important amount of sand from the littoral dune and mixed it with reworked materials from underlying layers at least 1 m in depth the method tested here represents an original and promising tool to identify tsunami induced deposits in similar embayed beach environments

seismic moment assessment of earthquakes in stable continental regions iii new madrid 1811 1812 charleston 1886 and lisbon 1755 summary the sizes of three major or great historical earthquakes are reassessed using the isoseismal area regression tools developed in parts i and ii of this study of stable continental region scr seismicity the earthquakes are 1811 new madrid central united states and its following sequence 1886 charleston coastal south carolina and 1755 lisbon oceanic intraplate off the continental shelf of portugal the analysis confirms the large size of these events and for the first time places constraints on the uncertainty of their seismic moment release because of the exceptionally low seismic wave attenuation of eastern north america ena a separate north american regression of seismic moment on isoseismal area was developed additionally the unknown western extents of the new madrid isoseismal areas were calibrated with the patterns of the m 6 3 6 6 1843 and 1895 earthquakes application of part ii analysis procedures with these corrections yields new madrid size estimates expressed as moment magnitude of m 8 1 0 31 for the 1811 december 16 m 7 8 0 33 for the 1812 january 23 and m 8 0 0 33 for the 1812 february 7 principal events the charleston earthquake s magnitude decreases from m 7 4 to m 7 3 0 26 after compensation for the effect of coastal plain sediments on its inner isoseismals intensity regressions for lisbon are calibrated against the isoseismal pattern of the nearly co located m 7 8 1969 st vincent earthquake which in this case increases the predicted size of lisbon from m 8 4 to m 8 7 0 39 these size estimates are supported by data from independent phenomena extent and severity of liquefaction the maximum distance of induced landslides and for lisbon tsunami wave amplitudes estimated source parameters are controlled by crustal or lithospheric temperature which governs the depth extent of brittle faulting using estimated continental and oceanic geotherms viable fault lengths are 30 80 km for charleston 120 180 km for 1811 new madrid and 180 280 km for lisbon for average displacements of 2 4 m 8 11 m and 10 14 m respectively and for average static stress and strain drops at the estimated seismic moments of this study the 1811 new madrid and the 1755 lisbon events are respectively the largest known scr and oceanic lithosphere earthquakes

an assessment of 40 ar 39 ar dating of incompletely degassed xenoliths the possibility of measuring the age of eruption of pleistocene lavas by 40 ar 39 ar analysis of entrapped ancient potassic xenoliths is demonstrated by a study of model systems upon inclusion in the hot magma such xenoliths are commonly only partially degassed of radiogenic 40 ar which has accumulated in them since their original crystallization the residual 40 ar will increase the apparent k ar age of the xenolith however if a xenolith is of cretaceous age or younger then a plateau in its 40 ar 39 ar age spectrum giving the age of eruption is expected to extend over 25 50 of the total 39ar released if degassing of the xenolith in the magma exceeded 90 and if the phases in the xenolith are characterized by sufficiently different diffusion dimensions or activation energies if diffusion was from a bimodal population of spheres then the radii must differ by a factor of 10 or more or the diffusion coefficients by a factor of 100 or more or if the spheres were equal in size and in diffusion coefficients then the activation energies must differ by a factor of at least 1 5 that such requirements may be realized in real xenoliths containing k feldspars is expected from published activation energies for microcline and from data determined on a granitic xenolith which was degassed in an early pleistocene basalt flow the experimental results appear to establish that old xenoliths may contain ar in distinctive phases which degas at sufficiently different temperatures as to permit determination of the age of degassing or eruption

isotopic dating in metamorphic belts isotopic dating in metamorphic rocks is concerned with the timing of processes whose operation is controlled by continually changing physical conditions of these temperature has a pervasive influence and the use of physical models to elucidate the ways in which temperature is likely to vary is briefly reviewed the central concept of closure temperature is critically discussed and quantitative estimates for biotite and hornblende are evaluated in order to illustrate the factors which control closure temperatures and limit their application in practice the possibility of dating crystallization using minerals which crystallize below their respective closure temperatures is examined taking examples involving k ar rb sr sm nd and u pb analyses the indirect dating of metamorphism via structurally dated plutons is briefly considered and the problems of interpreting both discordant u pb zircon ages and rb sr whole rock isochrons in metamorphosed granites are discussed the value of near concordant zircon ages is emphasized the extent of isotopic equilibration during metamorphism specifically of sr isotopes in metasediments is shown not to be a simple function of metamorphic grade the flow of fluids produced by metamorphic reactions appears to be crucial and there is a potential for dating metamorphism using whole rock samples where such flow can he demonstrated independently the concluding discussion briefly examines existing data for the dalradian and indicates the potential for further progress using both well established and novel approaches

tectono stratigraphic evolution of the early proterozoic wisconsin magmatic terranes of the penokean orogen the early proterozoic penokean orogen developed along the southern margin of the archean superior craton the orogen consists of a northern deformed continental margin prism overlying an archean basement and a southern assemblage of oceanic arcs the wisconsin magmatic terranes the south dipping niagara fault suture zone separates the south facing continental margin from the accreted arc terranes the suture zone contains a dismembered ophiolite the wisconsin magmatic terranes consist of two terranes that are distinguished on the basis of lithology and structure the northern pembine wausau terrane contains a major succession of tholeiitic and calc alkaline volcanic rocks deposited in the interval 1860 1889 ma and a more restricted succession of calc alkaline volcanic rocks deposited about 1835 1845 ma granitoid rocks ranging in age from about 1870 to 1760 ma intrude the volcanic rocks the older succession was generated as island arcs and or closed back arc basins above the south dipping subducti

rb sr biotite and whole rock data from the kapuskasing uplift and their bearing on the cooling and exhumation history rb sr isotopic ratios were measured on biotite and whole rocks from a suite of 21 granitoid rocks from the southern kapuskasing uplift results of 18 of the whole rock analyses fall on an isochron with an age of 2 677 0 057 ga and sri 0 70080 biotite model ages range from 2 50 to 1 93 ga in a general spatial relationship of decreasing age with depth in the structural section as calibrated with the igneous hornblende barometer for tonalitic rocks and garnet pyroxene barometers for granulites near the michipicoten belt in the west biotite ages of 2 ga reflect disturbances also recorded by paleomagnetic and whole rock rb sr systems to the east in a 70 km long northwest southeast transect toward the ivanhoe lake fault zone ages decrease from 2 50 to 1 95 ga in a southern transect ages are in the range 2 30 2 38 ga to within 15 km of the fault reflecting consistent erosion levels corresponding to 0 53 mpa but decrease to 1 93 ga to the east in concert with paleopressures in the 0 7 1 0 gpa ra

variability of current temperature and bottom pressure across the west florida continental shelf winter 1981 1982 observations are analyzed from four current meter moorings deployed on the broad continental shelf in the northeastern gulf of mexico from november 29 1981 to february 8 1982 71 days consistent with recent modeling studies the shelf circulation responds within an inertial period to the alternating up and down coast synoptic scale wind forcing average response to a 0 5 dyn cm 2 alongshore wind stress as measured at the coast is 20 cm s 1 off cedar key and 40 cm s 1 in the north where the shelf narrows off cape san bias lower layer currents veer counterclockwise with depth as in a bottom ekman layer e folding scale 8 m the pressure field decays offshore e folding scale 160 km and yields a geostrophic current in good agreement with the observed alongshelf flow cross shelf and vertical variations of the tidal signals compare nicely with results from barotropic theory calculations indicate that most of the shoreward propagating tidal energy must be dissipated in depths of 20 m within 50 km of shore

constraining the thermal history of the north american midcontinent rift system using carbonate clumped isotopes and organic thermal maturity indices abstract the midcontinent rift system mrs is a late mesoproterozoic 1 1 ga sequence of volcanic and sedimentary rocks exposed in the lake superior region of north america the mrs continues to be the focus of much research due to its economic mineral deposits as well as its archive of precambrian life and tectonic processes in order to constrain the post depositional thermal history of the mrs samples were analyzed for carbonate clumped isotope composition and organic thermal maturity clumped isotope values from sedimentary early diagenetic samples were partially reset during burial to temperatures between 68 and 75 c solid state reordering models indicate that maximum burial temperatures of 125 155 c would reset the clumped isotope values to the observed temperature range prior to the onset of regional cooling and uplift clumped isotope results from late stage veins in the white pine mine encompass a greater temperature range 49 116 c indicative of spatially variable hydrothermal activity and vein emplacement after burial temperatures fell below 100 c during regional cooling and uplift clumped isotope and organic thermal maturity data do not indicate significant spatial differences in thermal history along the mrs observed variability in bulk organic matter composition and biomarker indices are therefore more likely a result of shifts in primary productivity or early degradation processes these results demonstrate that the mrs experienced a spatially consistent relatively mild thermal history 125 155 c and is therefore a valuable archive for understanding the late mesoproterozoic environment

crustal scale thrusting and origin of the montreal river monocline a 35 km thick cross section of the midcontinent rift in northern michigan and wisconsin a structurally simple 35 km thick north facing stratigraphic succession of late archean to middle proterozoic rocks is exposed near the montreal river which forms the border between northern wisconsin and michigan this structure the montreal river monocline is composed of steeply dipping to vertical sedimentary rocks and flood basalts of the keweenawan supergroup middle proterozoic along the south limb of the midcontinent rift and disconformably underlying sedimentary rocks of the marquette range supergroup early proterozoic these rocks lie on an archean granite greenstone complex about 10 km of which is included in the monocline this remarkable thickness of rocks appears to be essentially structurally intact and lacks evidence of tectonic thickening or repetition tilting to form the monocline resulted from southward thrusting on listric faults of crustal dimension the faults responsible for the monocline are newly recognized components of a well known regional fault system that partly closed and inverted the midcontinent rift system resetting of biotite ages on the upper plate of the faults indicates that faulting and uplift occurred at about 1060 20 ma and followed very shortly after extension that formed the midcontinent rift system

crustal structure of the midcontinent rift system results from glimpce deep seismic reflection profiles interpretation of great lakes international multidisciplinary program on crustal evolution glimpce seismic reflection profiles indicates that the midcontinent keweenawan 1100 ma rift system of volcanic rocks and postvolcanic and interbedded sedimentary rocks extends to depths as great as 32 km about 10 5 s reflection time along profiles crossing western central and eastern lake superior and the northern end of lake michigan this area may overlie the greatest thickness of intracratonic rift deposits on earth times to moho reflections vary along strike from 11 5 to 14 s about 37 46 km depth in the west to 17 s about 55 km in the center and 13 to 15 s about 42 49 km in the eastern end of lake superior the prerift crust however was thinned 25 30 km beneath the central rift compared with its flanks providing evidence for crustal extension by factors of about 3 4 the midcontinent rift system differs from phanerozoic rifts in having total crustal thicknesses equal to or greater than the surrounding presumably unextended regions

vertical structure of tidal currents over the camarinal sill at the strait of gibraltar an acoustic doppler current profiler adcp is used to estimate the vertical structure of currents at the camarinal sill at the strait of gibraltar the most important tidal components namely m2 s2 o1 k1 m4 m3 msf and mm are examined the semidiurnal tides are the most energetic with along strait speeds of up to 120 cm s 1 for m2 and 48 cm s 1 for s2 the diurnal tides have along strait speeds of 15 35 cm s 1 m4 increases from 4 cm s 1 in the upper layer to 15 cm s 1 below 220 m the fortnightly component msf reaches speeds in excess of 30 cm s 1 at the upper 120 m the vertical structure of the tidal signal is complicated differs between semidiurnal and diurnal tides and varies between spring and neap tides plausible assumptions about the density profile and the bottom friction yield tidal components similar to those observed the effects of friction are described by the use of an analytical constant eddy viscosity model and the effects of stratification by estimating the internal modes at each tidal frequency the major part of the tidal signal is barotropic with the internal tide modes and the frictional effects contributing 20 cm s 1 for the semidiurnal tides nevertheless individual modes reach 50 cm s 1 the internal tides for the diurnal components are 5 cm s 1 with individual modes reaching 8 cm s 1 the vertical profile of msf can be explained by the use of internal modes reaching amplitudes of 40 cm s 1 the fitting of the internal wave modes to the vertical profiles also explains the observed phase change with depth of the semimajor axis the comparison with pressure difference across the strait is consistent with previous studies this report is the first on the adcp time series on the camarinal sill

time series analysis of magnetic susceptibility variations in deep marine sedimentary rocks a test using the upper danian lower selandian proposed gssp spain abstract it has been clearly demonstrated that past climate fluctuations are recorded in marine sedimentary rocks however for many reasons extracting the climate signature is difficult initial low field mass specific magnetic susceptibility ms data can potentially provide a measure of climate variability and thus become a proxy characterizing climate cyclicity in a wide range of marine sediments this is due to the fact that climate change warm wet versus cold dry drives cyclic weathering and erosional variations that are recorded as the detrital components of marine sediment that dominates the ms to test the utility of ms to yield climate proxies in marine sediments showing major changes in lithology we have sampled the well studied danian selandian boundary interval lower paleocene at zumaia zumaya spain this interval represents a dramatic rapid lithologic change from danian carbonate dominated limestone marl couplets to a detrital dominated marl shale sequence in the selandian indicating onset of a major regression erosional event beginning in the lowest selandian sampling included a continuous sequence from the uppermost danian stage 3 71 m into the lowermost selandian stage 5 2 m a suite of 175 samples collected at 5 cm intervals our results indicate that ms measurements reflect changes in detrital sediment at the site first by closely tracking high frequency limestone marl couplets second with a large rapid shift toward higher ms values beginning at the danian selandian boundary resulting from a major regression and third by tracking low frequency climate controlled variations known to have occurred during deposition of these sediments ms zones developed from the cyclicity observed throughout the sequence supported by time series analysis using fourier transform ft methods applied to the ms results exhibit milankovitch cyclicities in the precessional 19 24 kyr obliquity 41 54 kyr and eccentricity bands 100 kyr this is in excellent agreement with previous ft work on the section using measured variations in cyclic bed thicknesses with the new ms data set and ft results we then developed a floating point time scale fpts for the sequence sampled covering 550 kyr through the danian selandian boundary interval yielding a time scale resolution for the uppermost danian to 10 000 years however only the 100 000 year eccentricity band for the selandian is sufficiently well developed for an fpts estimate and yields a time scale resolution of 50 000 years our test of the utility of ms data sets in this varying depositional setting demonstrated that these data can provide a climate proxy that is not disrupted by large lithologic changes

a well verified multiproxy reconstruction of the winter north atlantic oscillation index since a d 1400 abstract a new well verified multiproxy reconstruction of the winter north atlantic oscillation nao index is described that can be used to examine the variability of the nao prior to twentieth century greenhouse forcing it covers the period a d 1400 1979 and successfully verifies against independent estimates of the winter nao index from european instrumental and noninstrumental data as far back as 1500 the best validation occurs at interannual timescales and the weakest at multidecadal periods this result is a significant improvement over previous proxy based estimates which often failed to verify prior to 1850 and is related to the use of an extended reconstruction model calibration period that reduced an apparent bias in selected proxies associated with the impact of anomalous twentieth century winter nao variability on climate teleconnections over north atlantic sector land areas although twentieth century nao variability is somewhat unusual comparable periods of persistent positive phase

rock magnetism of remagnetized paleozoic carbonates low temperature behavior and susceptibility characteristics we have conducted a new set of rock magnetic experiments on samples of remagnetized paleozoic carbonates of eastern north america these experiments were designed to investigate the origin of the unusual hysteresis behavior of these rocks by evaluating 1 the importance of ferrimagnetic pyrrhotite as a remanence carrier and 2 the sources of low field susceptibility low temperature measurements of saturation isothermal remanent magnetization sirm indicate that the pyrrhotite magnetic transition at 32 k is absent in the trenton and onondaga limestones of new york this transition is observed but poorly expressed in the knox group of east tennessee room temperature sirms cooled to 10 k in zero field lose a small fraction of their intensity between 30 and 35 k samples from all three formations show a broad peak in irm intensity at about 200 k which is typical of pyrrhotite in the trenton and onondaga samples the ferrimagnetic component of low field susceptibility is significantly larger than the ratio mrs hcr and is thus probably due dominantly to magnetite in some of the knox samples the reverse is true suggesting an important pyrrhotite contribution for all of the samples the ferrimagnetic susceptibility normalized by the saturation magnetization is anomalously high about a factor of 5 or 10 higher than the typical value for magnetite we believe that this indicates a very substantial contribution from superparamagnetic particles strong frequency dependence of susceptibility and very high ratios of anhysteretic to saturation remanence confirm the importance of ultrafine particles spanning the superparamagnetic single domain boundary all three of these chemically remagnetized carbonates units exhibit the following properties which have not previously been found together for any rock or synthetic analog and which therefore appear to constitute a diagnostic set of rockmagnetic criteria for recognizing chemically remagnetized rocks mrs ms 0 89 hcr hc 0 6 k ms 50 m a arm sirm 20 ka mrs hcr 50

retrograde evolution of eclogites evidences from microstructures and 40ar 39ar white mica dates munchberg massif northern bavaria phengites from eclogites and pegmatites 3t 2m1 coarse grained and recrystallized of the munchberg massif weissenstein and oberkotzau have been dated by the 40ar 39ar method 3t micas from the eclogites yielded plateau and isochron ages of 365 7 ma 2m1 micas show disturbed degassing spectra micas from pegmatites show a slight excess ar component with an isochron age of 353 to 351 3 ma an age component of approximately 300 ma was also detected in combination with age values from the literature the cooling history of the munchberg massif from eclogite facies conditions 390 ma to cooling below 350 c 350 ma is documented the age component of 300 ma is attributed to a low grade stage of mineral growth accompanied by a transitional ductile brittle deformation the petrological effects include formation of pumpellyite prehnite facies minerals frequently precipitated in microcraks and cleavage planes of earlier formed minerals this stage has to be seen in conjunction with the intrusions of the fichtelgebirge granite

new model of the midcontinent rift in eastern minnesota and western wisconsin the midcontinent rift contains a central zone of mostly volcanic and associated intrusive rocks flanking and in places overlying these mafic igneous rocks are clastic sedimentary basins which range in thickness from less than 1 5 km southwest of the west end of lake superior to more than 3 0 km near the minnesota iowa border new gravity and magnetic modeling of the rift has shown that most of the volcanic and intrusive rocks are contained within a central block which in minnesota and wisconsin extends to a depth of at least 10 to 20 km the geometry of this central block which ranges from fairly symmetric to very asymmetric appears to reflect a change from predominantly a graben to only a half graben development the modeling has also suggested the presence of a major prerift intrusive body along the minnesota iowa border which could have locally controlled the rift s development

cooling history of the nw himalaya pakistan fission track and 40ar 39ar cooling ages indicate that the late tertiary cooling history of the himalayan ranges of northern pakistan is largely a function of uplift and erosion interpretation of cooling ages which range from under 0 5 ma to over 80 ma suggests that during the late tertiary long term uplift rates at least doubled from under 0 2 mm yr to in some cases well over 0 5 mm yr uplift rates show strong and systematic regional variations as well which reflect the greater uplift of eastern and northern regions the association of very rapid uplift and erosion with the nanga parbat haramosh massif can be explained by a locally vigorous collision of india with eurasia near a promontory of indian crust the resultant rapid uplift of the nanga parbat haramosh massif reactivated the main mantle thrust melange zone with a reversed sense of motion discontinuities in the coolingage distribution along the main mantle thrust in the southern swat hazara region may be the result of the thermal effects of overthrusting

storms versus tsunamis dynamic interplay of sedimentary diagenetic and tectonic processes in the cambrian of montana the upper cambrian deadwood formation of north central montana is composed mainly of fine grained calcareous sandstone and intercalated silty shale and contains scattered intraclastic flat pebble conglomerate these rocks typical of many mixed carbonate siliciclastic subtidal shelf successions of proterozoic through early ordovician age are distal storm deposits in which the conglomerates are conventionally thought to record occasional very high energy events that scoured incipiently cemented layers in the deadwood formation most sandstone beds have a linear to crudely reticulate pattern of vertical cracks filled with injected silt and clay the cracks exhibit a consistent northwest southeast trend and are very shallow burial deformation structures caused by the passage of seismic waves possibly originating from strong earthquakes in idaho most conglomerates are composed of angular to subangular sandstone intraclasts the polygonal plan view of which is similar in outline to the areas between cracks in the sandstones the deformation implied by the cracks sporadic distribution of conglomerates high degree of scouring indicated and angularity of the intraclasts argue that rather than storms the conglomerates were generated by occasional tsunamis the sweep of these tsunamis across the shallow intracratonic sea created extraordinarily strong oscillating bottom currents at the deeper reaches of storm wave base tsunamites are therefore identified with confidence virtually for the first time in shallow subtidal shelf deposits

the occurrence and origin of late paleozoic remagnetization in the sedimentary rocks of north america although it has been known for 25 years that some paleozoic sedimentary rock units in europe and north america were remagnetized during pennsylvanian or permian time it is only very recently that the complex and widespread nature of the late paleozoic remagnetization phenomenon has been generally acknowledged it is now recognized that many paleozoic paleomagnetic poles for north america that were once considered reliable are in fact the result of remagnetization and as a consequence the paleomagnetic data base for the paleozoic is undergoing rapid and drastic revision the causes of late paleozoic remagnetization in north america are currently the focus of much interest and active research the remagnetization can reside in either hematite or magnetite and different remagnetization mechanisms have been important in different settings chemical remagnetization processes some related to specific diagenetic events are dominant in hematite bearing sandstones and carbonates in magnetite bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important but it is difficult to determine which process is the dominant one in some settings some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain building events that affected much of the continent during the late paleozoic paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids

a rapid method for magnetic granulometry with applications to environmental studies we have developed a rapid method of characterizing the fine scale variation sub micron to hundreds of microns m in the grain size of magnetite in long sediment cores this method is a new rock magnetic tool and can be readily applied to the following disciplines 1 environmental studies 2 hydrology 3 petrology and 4 geomagnetism the method consists of a comparative plot of the depth variation of two magnetic parameters the first parameter low field less than 0 1 millitesla mt or 1 oe susceptibility in a steady field is plotted on the abscissa the second parameter anhysteretic remanent magnetization arm imparted by a smoothly decreasing af of peak value 100 mt 1 000 oe in the presence of a weak steady field 0 01 0 1 mt or 0 1 1 oe is plotted on the ordinate for a sediment column in which the magnetic mineral is magnetite a plot of the two magnetic parameters shows transitions in the average magnetic grain size with depth as clearly discernible changes in the slope of a regression line fit to the individual points arm

variations in sediment provenance during the past 3000 years off the tagus river portugal marine sediments from the portuguese shelf are influenced by environmental changes in the surrounding continental and marine environment these are largely controlled by the north atlantic oscillation but additional impacts may arise from episodic tsunamis in order to investigate these influences a high resolution multi proxy study has been carried out on a 5 4 m long gravity core and five box cores from the tagus prodelta on the western portuguese margin incorporating geochemical c org n total ratios delta c 13 org delta n 15 delta o 18 c org and caco3 content and physical sediment properties magnetic susceptibility grain size subsurface data of the five box cores indicate no major effect of early postdepositional alteration surface data show a higher fraction of terrigenous organic material close to the river mouth and in the southern prodelta gravity core geob 8903 covers the last 3 2 kyrs with a temporal resolution of at least 0 1 cm yr very high sedimentation rates between 69 and 140 cm core depth indicate a possible disturbance of the record by the ad1755 tsunami although no evidence for a disturbance is observed in the data sea surface temperature and salinity on the prodelta the local budget of marine no3 as well as the provenance of organic matter remained virtually constant during the past 3 2 kyrs a positive correlation between magnetic susceptibility ms and north atlantic oscillation nao is evident for the past 250 years coinciding with a negative correlation between mean grain size and nao this is assigned to a constant riverine supply of fine material with high ms which is diluted by the riverine input of a coarser low ms component during nao negative high precipitation phases end member modelling of the lithic grain size spectrum supports this revealing a third coarse lithic component the high abundance of this coarse end member prior to 2 kyr bp is interpreted as the result of stronger bottom currents concentrating the coarse sediment fraction by winnowing as continental climate was more and prior to 2 kyr bp subboreal the coarse end member may also consist of dust from local sources a decrease in grain size and caco3 content after 2 kyr bp is interpreted as a result of decreasing wind strength the onset of a fining trend and a further decrease in caco3 around ad900 occurs simultaneous to climatic variations reconstructed from eastern north atlantic records a strong increase in ms between ad1400 and ad1500 indicates higher lithic terrigenous input caused by deforestation in the hinterland c 2008 elsevier b v all rights reserved

keweenaw hot spot geophysical evidence for a 1 1 ga mantle plume beneath the midcontinent rift system the proterozoic midcontinent rift system of north america is remarkably similar to phanerozoic rifted continental margins and flood basalt provinces like the younger analogues the volcanism within this older rift can be explained by decompression melting and rapid extrusion of igneous material during lithospheric extension above a broad asthenospheric thermal anomaly which the authors call the keweenaw hot spot great lakes international multidisciplinary program on crustal evolution seismic reflection profiles constrain end member models of melt thickness and stretching factors which yield an inferred mantle potential temperature of 1 500 1 570c during rifting combined gravity modeling and subsidence calculations are consistent with stretching factors that reached 3 or 4 before rifting ceased and much of the lower crust beneath the rift consists of relatively high density intruded or underplated synrift igneous material the isotopic signature of keweenawan volcanic rocks presented in a companion paper by nicholson and shirey this issue is consistent with the model of passive rifting above an asthenospheric mantle plume

middle proterozoic uplift events in the dunbar dome of northeastern wisconsin usa isotopic ages of granitic and metamorphic rocks exposed in the dunbar structural dome of northeastern wisconsin identify a protracted series of tectonic and hydrothermal events that culminated in major regional uplift during middle proterozoic keweenawan ca 1 100 ma continental rifting and volcanism the major rock forming events and the structural development of the dome occurred during the interval 1 862 4 ma to 1 836 6 ma whole rock rb sr ages are partly reset in response to a widely recognized but cryptic event in wisconsin and michigan at about 1 630 ma the scale and systematic character of the whole rock resetting strongly suggests the presence of a fluid phase derived in situ from water dissolved in the silicates or externally from a subthrust plate of low grade metamorphic rocks the regional nature of the 1 630 ma disturbance possibly indicates that it is related to a major tectonic event such as an active plate margin far to the south rb sr biotite ages for the dunbar dome this study the southern complex of the marquette district van schmus and woolsey 1975 and the felch trough area aldrich and others 1965 provide a remarkably coherent pattern that reflects multiple episodes of differential uplift younger events superimposed on a regional 1 630 ma imprint are recorded at 1 330 ma and 1 140 ma the 1 330 ma disturbance could reflect stabilization following intrusion of the wolf river batholith at 1 485 ma the 1 140 ma uplift event occurred during keweenawan rifting and volcanism as a result of stresses imposed on a mosaic of fault bounded blocks with possible subcrustal influence the remarkably small variance in the 1 140 ma biotite age peak argues for rapid uplift and cooling and hence rapid erosion detritus from the uplift probably was being shed into nearby tectonic basins most of which did not survive subsequent uplift and erosion

cross shore variation of wind driven flows on the inner shelf in long bay south carolina united states and exceeded 0 1 n m 2 vertical mixing occurred the two layered flow pattern disappeared and currents were directed alongshore with the wind at all sites and throughout the water column in the fall near bed flows close to the shore water depth 7 m were often reduced compared to or opposed those measured farther offshore under southwestward winds a simplified analysis of the depth averaged alongshore momentum balance illustrates that the alongshore pressure gradient approached or exceeded the magnitude of the alongshore wind stress at the same time that the nearshore alongshore current opposed the wind stress and alongshore currents farther offshore in addition the analysis suggests that the wind stress is reduced closer to shore so that the alongshore pressure gradient is large enough to drive the flow against the wind

hydro and morpho dynamic modeling of breaking solitary waves over a fine sand beach part i experimental study abstract the objectives of this work are 1 to examine the mechanisms that influence tsunami erosion and deposition mechanisms in the littoral zone via physical simulations of breaking solitary waves over a fine sand beach and 2 to provide experimental data for validation of numerical models to predict tsunami erosion and deposition processes the experiments were carried out in a 48 8 m 2 16 m 2 1 m wave flume the flume was instrumented to observe free surface elevations cross shore velocities suspended sediment concentrations vertical and cross shore pore pressure gradients near the shoreline and morphological changes in addition wave sediment interactions were observed via underwater video cameras the results are systematically analyzed to investigate the roles of wave breaking bore runup wave drawdown and wave induced pore pressure variations on tsunami erosion and deposition processes the studies showed that the wave plunging on a thin layer of water prior to reaching the shoreline did not cause much sediment suspension while the waterjet impinging directly on the beach entrained substantial amounts of sand the suspended sediments were subsequently pushed up the slope by fluid momentum as the broken wave transformed to a turbulent bore a small net deposition region was observed near the maximum runup point where both the flow velocity and the water depth were near zero a significant amount of the sediment transport occurred during the wave drawdown in the form of thick sheet flow which resulted in net erosion of the shore face and the beach a hydraulic jump formed near the wave breaking region toward the end of the drawdown which caused most of the suspended sand to deposit in the wave breaking region consequently breaking solitary waves over a sloping fine sand beach led to net erosion of the shore face and the beach net deposition in a small region immediately seaward of the max excursion point and net deposition in the wave breaking zone

use of magnetic signatures to correlate tephra layers in holocene loessial soil profiles from a small region se iceland rock magnetic measurements have been applied to two adjacent loessial soil sections from a small region se iceland the soils are composed of reworked locally derived sediment including airfall tephra and also contain several visible discrete airfall tephra layers the main magnetic minerals in the airfall tephras are ferrimagnetic e g magnetite with paramagnetic minerals also present the main magnetic grain sizes in the tephras are pseudo single domain the results show that individual tephras do not have unique magnetic signatures that can be used for identification and correlation between sedimentary sequences however a correlation of tephra layers was achieved through the application of statistical techniques to a comprehensive dataset of magnetic parameters similarity coefficients and euclidian distance measures were used to identify the best correlation between tephra layers in the two soil profiles the technique works well providing some tephras present within both profiles have been formally identified e g oraefajokulla ad 1362 using electron microprobe analyses given this initial framework the statistical analyses of the magnetic parameters can help in the identification and correlation of unknown tephras between two soil profiles

some observations on the interpretation of 40ar 39ar age spectra several recent applications of the 40ar 39ar age spectrum technique have utilized the ability of this technique to resolve the internal ar isotope distribution in crystals for the purpose of placing constraints on the timing of thermal events additionally a few of these studies have attempted to use certain properties of the age spectrum to estimate the temperature and duration of heating experienced by the sample during the disturbance although these successful applications are encouraging the interpretation of age spectra of disturbed samples is in many ways still an art form with the beholder apparently relatively free of constraints that might limit an assessment in fact there are a number of restrictions on interpretation imposed by 1 multiple episodic loss 2 slow cooling 3 mixed phases 4 resolution of the age spectrum 5 excess 40ar 6 recoil artifacts 7 grain distribution and size and 8 phase changes while in the vacuum extraction system these effects are discussed with a view to understanding possible ambiguities in ar release patterns in most cases only with independent geochronological knowledge an understanding of the release systematics and complete sample characterization can these constraints be successfully dealt with in a quantitative framework

constraints on the age of heating at the fenton hill site valles caldera new mexico subsurface samples and temperature data from the fenton hill drilling site adjacent to the valles caldera new mexico allow an investigation of the thermal history of this site through analyses of the behavior of isotopic dating systems in changing thermal regimes and through thermal modeling estimates of the age of the heating event at fenton hill responsible for the current elevated temperatures vary from about 1 to 4 ma however new 40ar 39ar analyses and thermal modeling indicate that the age of heating may be much younger than previously believed the 40ar 39ar analyses of microcline separates from five samples ranging in drill hole depth from 1 13 to 4 56 km and in situ temperatures from 110 c to 313 c indicate thermal events at the site at around 1030 and 870 ma and a very recent thermal event related to the thermal development of the caldera the maximum estimates of peak heating duration during this recent thermal event are between 3 and 60 ka two distinct thermal perturbations are recognized in the subsurface temperature data a shallow disturbance probably related to heating from a lateral flow of hot water just above the precambrian surface at a depth of about 730 m and a deep thermal disturbance the source of which is unknown model studies indicate the age of the shallow thermal disturbance to be about 10 ka with reasonable constraints on the source of the deep thermal disturbance it must lie within a few kilometers of the site and its maximum age is estimated to be less than about 40 ka thus heating at the fenton hill site appears to be much younger than the main caldera event at about 1 ma and is probably related to a magmatic and or hydrothermal event very close to fenton hill during the last few tens of thousands of years

new39ar 40ar ages of hercynian and alpine thermotectonic events in grande kabylie algeria abstract the grande kabylie massif is situated within the internal zone of the alpine chain of northern algeria new 39 ar 40 ar ages which have been obtained from several key areas within the massif are compared with previous u pb and rb sr data the distinct isotopic signatures obtained lead to the recognition of several units with different tectonometamorphic histories that were intimately imbricated during the alpine orogeny in the north three structurally bound zones occur that form a core complex which is tectonically overlain by a nappe of low grade and non metamorphosed rocks from north to south the core complex is composed of 1 1 the naciria massif composed of high temperature paragneisses intruded by peraluminous granites measurements on biotites and feldspars from these series indicate that high temperature post kinematic recrystallization of these minerals occurred ca 80 ma ago before a low temperature event that produced some argon loss at ca 27 ma ago 2 2 structurally underlying the naciria zone is the sidi ali bou nab massif which consists of biotite bearing micaschists intruded by a heterogeneously mylonitized hercynian granite biotites and muscovites yield convergent 39 ar 40 ar and rb sr ages at 25 ma for this mylonitization synchronous with southeasterly overthrusting 3 3 further south orthogneisses anatectic paragneisses marbles and minor kinzigites form a second high temperature gneissic complex as for the naciria massif 39 ar 40 ar data on biotites and muscovites indicate that isotopic closure occurred 80 ma ago above the core complex is a thick series of epizonal phyllites that are unconformably overlain by unmetamorphosed permo triassic red beds of the dorsale calcaire low grade metamorphism in the phyllitic series is dated at 295 315 ma and thus reflects the influence of hercynian metamorphism from these results we consider the phyllitic series and its dorsale calcaire cover to be allochthonous units thrust upon a high temperature metamorphic basement during the early miocene

excess40ar in metamorphic rocks from broken hill new south wales implications for40ar 39ar age spectra and the thermal history of the region 40ar 39ar age spectrum analyses of samples from broken hill new south wales indicate that the region has experienced a complex thermal history following high grade metamorphism 1660 ma ago the terrain cooled slowly 3 c ma 1 until about 1570 ma ago when the temperature fell below about 500 c following granitoid emplacement 1500 ma ago the region remained relatively cold until affected by a thermal pulse 520 40ma ago causing temperatures to rise to 350 c in some places during this event accumulated40ar was released from minerals causing a significant ar partial pressure to develop laboratory ar solubility data combined with the40ar 39ar age spectra gives a local estimate of this partial pressure of 10 4atm the region finally cooled below 100 c about 280 ma ago 40ar 39ar age spectrum analyses of hornblende plagioclase and clinopyroxene containing excess40ar are characterized by saddle shaped age spectra detailed analysis of plagioclase samples reveals a complex diffusion behaviour which is controlled by exsolution structures this effect in conjunction with the presumed different lattice occupancy of excess40ar with respect to radiogenic40ar appears to be responsible for the saddle shaped age spectra

vertical structure of bottom ekman tidal flows observations theory and modeling from the northern adriatic 1 from september 2002 to may 2003 fifteen bottom mounted acoustic doppler current profilers measured currents of the northern adriatic basin tidal fluctuations at all seven of the major adriatic frequencies were synthesized from a response tidal analysis of these measurements most observed tidal current ellipses were nearly reversing but near the bottom tidal current ellipses all shortened and broadened semidiurnal currents led upper water column currents and diurnal tidal current ellipse orientations rotated counterclockwise toward the bottom theoretical solutions for a tidally forced bottom ekman layer with vertical eddy viscosity of the form az z k were least squares fit to the observations average values were 3 10 4 m s and k 5 10 4 m2 s the value of k was important in matching tidal orientation and phase changes and a nonzero was important in matching tidal amplitude changes the navy coastal ocean model ncom and the quoddy model were also compared to the observations the average rms errors for the bottom ekman layer were 0 22 cm s for the best fit theory 0 35 cm s for ncom and 0 36 cm s for quoddy az structures from ncom and quoddy show that time variation in az is relatively unimportant for adriatic tides the bottom shear stresses from theory were larger in magnitude than those from the bottom drag formulations in ncom and quoddy

implications of low temperature cooling history on a transect across the colorado plateau basin and range boundary west central arizona fission track ages of apatite and zircon from metamorphic plutonic and sedimentary rocks along a 80 km transect across the colorado plateau basin and range boundary in west central arizona show differences in the low temperature cooling histories between the provinces the transect extends from cypress mountain in the colorado plateau transition zone to the eastern buckskin mountains in the basin and range along the northeast margin of the basin and range province metamorphic rocks exposed in the footwall of a major detachment fault system yield zircon and apatite fission track ages of 16 10 ma these ages are similar to k ar fusion ages of biotite and age minima of k feldspar on 40ar 39ar age spectra and collectively indicate rapid cooling one k feldspar age spectrum has an age maximum of about 22 8 ma an age minimum of 11 8 ma and a spectrum whose shape is suggestive of reheating possibly in middle miocene time the heating event was probably related to hydrothermal activity during emplacement of cu and mn deposits in and above the detachment fault zone effects of this heating are only locally detected in rocks above the detachment fault in the poachie range fission track ages of apatite and zircon are 60 50 and 80 70 ma respectively the disparity between the apatite and zircon ages indicates that the rocks cooled slowly in late cretaceous and early tertiary time probably due to gradual uplift and erosion total uplift and denudation in the area of the poachie range since cretaceous time is 6 km or more to the northeast of the range fission track ages of apatite and zircon increase and diverge indicating that apparent uplift decreased in that direction the apatite ages from the poachie range are concordant with early tertiary hornblende ages determined in other studies of lower plate rocks near the southwest end of the transect the ages represent cooling of crystalline rocks after cretaceous regional metamorphism and magmatism near bagdad 20 km northeast of the poachie range 2 km or less of erosion has occurred since intrusion of high level plutons and dikes and caldera formation in late cretaceous time remnants of an erosion surface that developed in middle tertiary time are preserved in the transition zone volcanic and sedimentary rocks at least as old as early miocene were deposited on the erosion surface and filled valleys cut into it dissection of these deposits began about 8 ma we interpret these data combined with those from other studies to indicate that in cretaceous time southward thrusting and later extensive magmatism in the middle crust led to thickening and heating of the crust the cretaceous igneous rocks at bagdad are high level manifestations of this magmatism uplift and slow cooling occurred in late cretaceous and early tertiary time in late oligocene and early miocene time during northeast southwest extension middle crustal rocks moved southwest put from beneath the southwest margin of the transition zone tectonic denudation rapidly exposed the crust that had been brought up from a depth of 10 km or more and rapidly cooled in the eastern buckskin and harcuvar mountains middle miocene reheating occurred locally in the lower plate along the detachment and in nearby parts of the upper plate

coercivity analysis of magnetic phases in sapropel s1 related to variations in redox conditions including an investigation of the s ratio 1 abstract the most recent sapropel s1 in the eastern mediterranean has been extensively investigated with geochemical and rock magnetic techniques different redox conditions prevailed in different zones of the sediment through time the oxidized sapropel zone is particularly interesting because earlier studies indicated that new magnetic material was formed including possible magnetosomes here we utilize component analyses of isothermal remanent magnetization irm acquisition curves and the analysis of first order reversal curves forc to further investigate the magnetic mineralogy in the entire box core abc26 the original input of eolian dust consisted of both magnetite and hematite in the oxidized sapropel and in the active oxidation zone an additional magnetite component is present this magnetite component has a higher coercivity than the eolian magnetite and a very small coercivity dispersion suggesting a narrow grain size distribution this is a strong indication that magnetosomes which are formed in the active oxidation zone are the magnetic carriers of this coercivity fraction forc diagrams support these findings the s ratio is forwardly modeled for mixed magnetic mineralogies with varying coercivity distributions to explain the down core s ratio behavior close to 1 in the top sediment and in the oxidized sapropel a drop in values in the active oxidation zone and again close to 1 in the syn sapropel the observed s ratio pattern in the oxidized sapropel and in the active oxidation zone can be explained by the recovered irm components whereas both oxidized sapropel and active oxidation zone contain the extra magnetite component their s ratios are different because of the differences in coercivity characteristics coercivity and dispersion of the two magnetites in these zones thus in these zones the s ratio does not reflect variations in the relative contributions of hematite to magnetite but variations in the characteristics of the individual magnetite assemblages also in the rest of the core the s ratio depends on the coercivity characteristics of the magnetite component rather than on the relative contributions of high versus low coercivity minerals the s ratio thus appears to be an unsuitable parameter to describe variations in the magnetic mineralogy especially when more than two components are present therefore the classical interpretation of the s ratio should be treated with caution

sedimentation and hydrodynamic processes associated with the tsunami generated by the 1755 lisbon earthquake abstract an unusual sedimentary horizon believed to have been deposited by the tsunami associated with the 1755 lisbon earthquake has been identified at boca do rio in southern portugal this horizon marks a distinct sedimentological and micropalaeontological break from the deposits enclosing it the sedimentary characteristics of the horizon which consists of several distinct sub units vary greatly in both vertical and horizontal section detailed textural analysis through vertical sections of these sub units shows abrupt changes in particle size characteristics the highly variable nature of the horizon appears to be due to rapid variation in the hydrodynamic characteristics of the depositional event the lowermost section of the deposit appears to have been deposited from a highly turbulent water mass which was able to transport gravel sized clasts as well as gravel sized mud balls eroded from underlying material the water mass rapidly lost energy leading to the deposition of predominantly sand silt and clay particles sharp breaks in textural characteristics are likely to be due to changes in current direction caused by wave reflection or swash backwash interference the main sources of sand sized material within the deposit are the seaward barrier beach foredune system the tsunami deposit at boca do rio shares many of the sedimentary characteristics of tsunami deposits from other sites around the world

rapid early miocene acceleration of uplift in the gangdese belt xizang southern tibet and its bearing on accommodation mechanisms of the india asia collision abstract five samples from a biotite hornblende granodiorite phase of the 42 5 ma quxu pluton gangdese batholith southern tibet have been collected at 250 m vertical intervals biotite from these rocks yields monotonically decreasing40ar 39ar isochron ages with decreasing elevation of 26 8 0 2 23 3 0 5 19 7 0 3 18 4 0 4 and17 8 0 1ma tc 335 c coexisting k feldspars have virtually identical minimum apparent40ar 39ar ages of 17 0 0 4ma tc 285 c these data indicate parts of southern tibet experienced a pulse of uplift in the early miocene with the rate of uplift rising from 0 07 to 4 4 mm year in the interval 20 to 17 ma an apatite fission track age of 9 9 0 9ma from this locality constrains the average uplift rate at this site to about 0 81 mm year between 17 and 9 9 ma and 0 30 mm year from 9 9 ma to present k feldspar from the dagze granite 30 km to the east near lhasa yields a minimum apparent40ar 39ar age of 35 9 0 9ma tc 227 c which indicates an average uplift rate there of 0 21 mm year since then the marked pulse of uplift of the quxu granodiorite and the difference in uplift history between the dagze and quxu plutons suggests southern tibet has experienced discrete pulses of uplift variable in both space and time these data are not consistent with models which require a large proportion of uplift of the tibetan plateau to have occurred in the last 2 ma the data support the suggestion that convergence between india and asia was largely accommodated by tectonic escape during the opening of the south china sea 32 to 17 ma ago and permit distributed shortening as a mechanism for crustal thickening and uplift of this part of the tibetan plateau subsequent to 20 ma

what do the hirm and s ratio really measure in environmental magnetism the hard isothermal remanent magnetization hirm and the s ratio are widely used in environmental magnetism to quantify the absolute and relative concentrations respectively of antiferromagnetic minerals hematite and goethite in mineral mixtures we demonstrate that synthetic al substituted hematite and goethite exhibit a wide range of coercivities which significantly influences the hirm and s ratio these parameters are therefore not necessarily straightforward indicators of the absolute and relative concentrations of hematite goethite to circumvent this problem we propose a new parameter the l ratio which is the ratio of two remanences after alternating field af demagnetization of an irm imparted in a 1 t field with a peak af of 100 mt and 300 mt irmaf 300mt irmaf 100mt these parameters are easily measured using modern vibrating sample or alternating gradient magnetometers changes in hirm only reflect changes in the absolute concentration of hematite and or goethite if the l ratio is relatively constant conversely l ratio fluctuations indicate variable coercivities that possibly reflect changes in the source of hematite goethite corresponding hirm and s ratio variations should be interpreted with caution in such cases the l ratio can be determined using equivalent terms depending on available instrumentation and measurement protocols for example the hirm is equivalent to irmaf 300mt likewise 0 5 sirm irm 100mt where irm 100mt represents the remanent magnetization obtained by first saturating the sample in a high field and then applying a back field of 100 mt is equivalent to irmaf 100mt the hirm 0 5 sirm irm 100mt ratio is therefore a suitable substitute for the l ratio when measurements are made with a long core magnetometer the newly proposed l ratio is straightforward to measure on a wide range of instruments and can provide significant new insights and reduce ambiguities associated with interpretation of two widely used parameters in environmental magnetism the hirm and s ratio

turbulent bottom ekman boundary layer measured over a continental shelf 1 an acoustic doppler current profiler was deployed in summer and autumn in 2007 2009 at two stations on the east china sea shelf clear velocity spirals that basically correspond to theoretical ekman spirals were identified for both mean and tidal currents from these spirals and the corresponding ekman equation we estimated the time averaged eddy viscosity profiles the estimated was largest 2 3 10 3 m2 s 1 around 5 m from the bottom and decreased almost exponentially with height a qualitatively similar profile of the eddy diffusivity was also inferred from the acoustic doppler current profiler data and microstructure profiler data the flux richardson number was estimated as 0 11 0 10 0 46 0 17 indicating relatively large buoyancy contribution to the turbulent kinetic energy budget

first 40ar 39ar geochronology of lateritic manganiferous pisolites implications for the palaeogene history of a west african landscape abstract a pisolitic superficial formation deriving from the geochemical degradation of a previous massive manganiferous crust has been investigated in west africa using 40 ar 39 ar laser probe analysis on different generations of cryptomelane from fe mn pisolites and their embedding lateritic matrices prior to the 40 ar 39 ar geochronological analysis a detailed petrographical study has allowed the identification of successive assemblages from mn pisolites to fe mn pisolites embedded in mn rich and fe rich matrices respectively the age spectra obtained for pisolites including cores cortices and matrices enables the definition of three age clusters around 56 59 44 47 and 24 27 ma the first two age clusters with the petrographical and geochemical results obtained on the different assemblages indicate that the development of the pisolitic crust first induced mn leaching and fe al accumulations due to warmer and wetter climatic conditions in the age interval 56 47 ma that characterizes the tertiary greenhouse effect period propitious to bauxite formation in west africa the absence of 40 ar 39 ar ages between 44 and 27 ma only means that geochemical conditions for cryptomelane crystallisation were not fulfilled but could still be favourable to fe and or al oxy hydroxides formation before drier climatic conditions became rather effective at the oligocene period to sustain mechanical erosion rather than chemical weathering processes the age cluster 24 27 ma indicates however a reactivation of the manganiferous lateritic weathering late oligocene the 40 ar 39 ar dating results are discussed in terms of climatic condition changes during the palaeogene that were favourable to the genesis of either al and fe or mn oxy hydroxides in the course of development and evolution of the pisolitic formation the 40 ar 39 ar dating also provides a new highlight to large scale geomorphological patterns of west africa

the tsunamite problem the genetic term tsunamite is used for a potpourri of deposits formed from a wide range of processes overwash surges backwash flows oscillatory flows combined flows soft sediment deformation slides slumps debris flows and turbidity currents related to tsunamis in lacustrine coastal shallow marine and deep marine environments tsunamites exhibit enormous variability of features e g normally graded sand floating mudstone clasts hummocky cross stratification etc these sedimentary features may also be interpreted as deposits of turbidity currents turbidites debris flows debrites or storms tempestites however sedimentary features play a passive role when these same deposits are reinterpreted as tsunamites on the basis of historical evidence for tsunamis and their triggering mechanisms e g earthquakes volcanic explosions landslides and meteorite impacts this bipartite sedimentological vs historical approach which allows here classification of the same deposit as both turbidite and tsunamite has blurred the distinction between shallow marine and deep marine facies a solution to this problem is to classify deposits solely by a descriptive sedimentological approach the notion that tsunami waves can directly deposit sediment in the deep sea is unrealistic because tsunami waves represent transfer of energy and they are sediment starved during tsunamis and major storms submarine canyons serve as the physical link between shallow water and deep water environments for sediment transport tsunami related deposition involves four progressive steps 1 triggering stage offshore 2 tsunami stage incoming waves 3 transformation stage near the coast and 4 depositional stage outgoing sediment flows in this progression deep water deposition can commence only after the demise of incoming tsunami waves due to their transformation into outgoing sediment flows deposits of these sediment flows already have established names e g debrite and turbidite therefore the term tsunamite for these deposits is obsolete

sedimentary features of tsunami deposits their origin recognition and discrimination an introduction the occurrence of numerous recent large tsunami suggests that they are a common natural event yet tsunami deposits in the geological record are rare this apparent anomaly may be due to a real infrequency of events and or poor preservation potential but alternatively may be due to misidentification robust criteria for the discrimination of tsunami deposits from other sources such as storms is still lacking establishing these criteria is essential if we are to improve our understanding of tsunami frequency over geological timescales by this means we will improve the identification of tsunami hazard and improve risk assessment this special issue focuses on tsunami deposits identifying their characteristics and their discriminating features from other sources such as storms although perhaps not as common as other event sediments it is considered likely that a reappraisal of the geological record is required to place them in their representative context

analysis of current observations on the georgia shelf abstract an analysis is made of hf radar measurements of surface currents in shallow water near the georgia coast and also of vertical profile measurements of current and density the dominant structure is found to be a clockwise ellipse of semidiurnal periodicity the ellipses becoming smaller narrower and turning clockwise with depth there is a definite phase lead of the bottom currents with respect to the upper currents and some evidence of veering of the currents in the non ekman sense in order to explain the observed vertical variations a simple analytical expression is developed for periodic rotary currents in a barotropic ocean of constant eddy viscosity and depth h when the free surface elliptic motion is known the solution depends on the ratios f and h hekman and also on the sense of turning of the free surface ellipse the model is able to explain several features of the observed vertical variations as frictional effects

tsunamis as geomorphic crises lessons from the december 26 2004 tsunami in lhok nga west banda aceh sumatra indonesia abstract large tsunamis are major geomorphic crises since they imply extensive erosion sediment transport and deposition in a few minutes and over hundreds of kilometres of coast nevertheless little is known about their geomorphologic imprints the december 26 2004 tsunami in sumatra indonesia was one of the largest and deadliest tsunamis in recorded human history we present a description of the coastal erosion and boulder deposition induced by the 2004 tsunami in the lhok nga bay located to the west of banda aceh northwest sumatra the geomorphological impact of the tsunami is evidenced by beach erosion some beaches have almost disappeared destruction of sand barriers protecting the lagoons or at river mouths numerous erosion escarpments typically in the order of 0 5 1 5 m when capped by soil and more than 2 m in dunes bank erosion in the river beds the retreat along the main river is in the order of 5 15 m with local retreats exceeding 30 m large scars typically 20 50 cm deep on slopes dislodgement of blocks along fractures and structural ramps on cliffs the upper limit of erosion appears as a continuous trimline at 20 30 m a s l locally reaching 50 m the erosional imprints of the tsunami extend to 500 m from the shoreline and exceed 2 km along riverbeds the overall coastal retreat from lampuuk to leupung was 60 m 550 000 m 2 and locally exceeded 150 m over 276 000 m 3 of coastal sediments were eroded by the tsunami along the 9 2 km of sandy coast the mean erosion rate of the beaches was 30 m 3 m of coast and locally exceeded 80 m 3 m the most eroded coasts were tangent to the tsunami wave train which was coming from the southwest the fringing reefs were not efficient in reducing the erosional impact of the tsunami the 220 boulders measured range from 0 3 to 7 2 m large typically 0 7 1 5 m with weights from over 50 kg up to 85 t we found one boulder less than 1 m large at 1 km from the coastline but all the others were transported less than 450 m 30 m and the moderate tsunamis height

the midcontinent rift system one of the most prominent features on gravity and aeromagnetic maps of the united states is a series of major generally linear anomalies that extend from central kansas to lake superior and then turn southward into central michigan figures 1 2 this system of anomalies clearly reflects major geological features and as such it must represent a very significant episode in the history of the north american continental lithosphere the geologic features associated with these anomalies particularly axial basins filled with basalt and immature clastic rocks along with evidence of crustal extension indicate that the episode was probably one of incipient rifting