balanced section in thrust belts part 1 construction balanced geological cross sections are an important aid to understanding thrust belt structures and to estimating their hydrocarbon bearing potential two approaches to section balancing have been taken in the past construction of retrodeformable sections from raw data and modification of previous interpretations after an evaluation procedure these approaches may be unified by use of a langrangian grid projection of datum locations dips and stratigraphic thicknesses onto the section plane is performed directly by calculation or graphically using plunge lines spline interpolation and parallel fold modeling are the simplest ways of filling gaps in the sectional data but isogon interpolation yields rheologically more realistic results faults are interpolated using cutoff geometries as constraints fault tips are located from distance displacement plots whereas the depth to detachment is obtained by a modified chamberlin construction interpolations made on the section are directed back to the geologic map via plunge lines although these techniques of section construction do not guarantee a balance especially when complexities such as growth structures diapirs or strike slip faults are present they eliminate many potential errors

structural traverse across the sierra madre oriental fold thrust belt in east central mexico the exposed structural geometry of the sierra madre oriental foreland fold thrust belt is quantified for an 130 km long strip along the moctezuma river 21 n lat east central mexico the strip extends from the belt s leading edge in the east to its most internal continuous outcrops in the west where the cordilleran structures disappear beneath cenozoic volcanic rocks the style of deformation is controlled mainly by the geometry and lithology of two major cretaceous carbonate banks el doctor and valles san luis potosi the long dimensions of which parallel the cordilleran structural trend the exposed shortening is concentrated on the platform margins 5 km on the eastern edge of the el doctor bank and 10 to 12 km on either edge of the valles san luis potosi platform and is characterized by thrust and fold nappes the thrust faults cut across the platform margin carbonates on 20 inclined tectonic ramps that were partly steepened by imbrication or by subsequent folding but pass in the overlying mechanically weak rocks nearly parallel to bedding the interiors of the platforms appear relatively undeformed whereas the regions between the two platforms zimapan basin and between the eastern edge of the valles san luis potosi platform and the foreland are composed of flexural slip folds resulting from decoupling along several incompetent layers the exposed bulk shortening measures 41 4 km or 25 for the entire traverse retrodeformable sections of the subsurface structure down to basement result in a bulk shortening of 90 1 km or 40 the subsurface deformation model for the western part of the traverse is thin skinned and has geometry similar to that on the surface the model is very speculative in the eastern part where the areal shortening calculated for thin skinned models is much larger than is the exposed linear shortening balance was attained by inferring a blind thrust on an upper detachment in the late cretaceous which has a shortening of 20 to 30 km it could also be attained however by intrabasement decoupling in a depth of 10 to 20 km deformation is brittle in the eastern part of the belt east of the valles san luis potosi platform whereas igneous intrusion controlled higher ductility characterizes the western part of the foreland fold thrust belt

backward modeling of the rifting kinematics in the upper rhine graben insights from an elastic perfect contact law on the restoration of a multi bloc domain this paper addresses the problem of volume restoration for 3 d sedimentary basin kinematic deformation the primary purpose is methodological and concerns the use of contact mechanics with the finite element method in order to deform a geological multi bloc domain this approach is applied to backward model the later stage of rifting of a segment of the southern upper rhine graben france germany border preliminary results from our modeling demonstrate the ability of the method not only to handle complex geometries but also to successfully perform retro deformation of a complex geological domain in addition they provide or confirm crucial information on the rifting evolution and tectonic features of this segment of the upper rhine graben such as the distribution of deformation the asymmetry of the graben and a significant left lateral strike slip component of displacement

application of fourier analysis for computer representation of fold profiles abstract stabler 1968 and hudleston 1973 described single surface fold profiles in terms of a fourier harmonic series this paper describes a development and application of this technique for representing profile sections by computer each fold profile is characterized by values of the equation coefficients that can be used to generate grid coordinates of the model fold surface a curve fitting regression allows evaluation of large numbers of data points and equation terms for improved accuracy evaluation of errors shows that the position of a point on a fold profile can be predicted to an accuracy of 98 1 or better some applications are illustrated including representation of a cross section of a folded orebody extrapolated from a small number of contact positions exposed in the mine another example illustrates a series of profile sections that were linked and intermediate profiles extrapolated to produce a three dimensional representation of a complexly folded surface

two dimensional modeling of the p t t paths of regional metamorphism in simple overthrust terrains quantitative modeling of metamorphic p t t paths in thrust belts has so far been limited to one dimensional simplification we have developed a two dimensional time dependent finite element procedure capable of tracing and updating fault and strata boundaries during faulting for modeling thermal evolution of overthrust terrains we show that the two dimensionality of an overthrust and the speed and duration of thrusting have significant effects on the p t t paths of rocks in orogenic belts the results suggest that petrological studies of p t paths of rocks from different lateral positions in an orogenic belt along with two dimensional modeling procedure can put strong constraints on models of regional tectonic processes

three dimensional kinematic modeling of reversible fault and fold development in the current context of continuous supply of energy the discovery and development of new prospects will rely on our ability to detect reserves in deeper and structurally more complex formations these exploration areas stretch the capabilities of currently available three dimensional 3 d exploration software which cannot accommodate a realistic geometrical description of present day geological structures and the tectonic deformation steps correctly handling the kinematics of structural deformation and evaluating the pressure regime and temperature history at the scale of exploration will remain as challenges for several years to come in this article we focus on geometric aspects using a reversible kinematic approach to deform and restore faulted and folded structures kinematic modeling is a good alternative to the complexity of a mechanical approach and is sufficiently representative of the natural processes involved sedimentation erosion and compaction its reversibility ensures that the basin parameters need to be defined only once for both the restoration and the deformation steps the model describes the incremental development of the basin in space and time it is based on a hexahedral discretization process that is fully adapted and appropriate for thermal and fluid transfer different deformation modes flexural slip and vertical shear are mixed to integrate natural deformation more effectively the algorithm is validated using different geological examples of growing complexity up to curved normal and thrust faults the approach offers various prospects for improvement integrating both kinematic and mechanical constraints considering the challenges that the industry needs to overcome in future exploration the results of this approach are very encouraging and can be considered as a solution for solving the structural part of 3 d basin modeling in complex areas

a model for low amplitude detachment folding and syntectonic stratigraphy based on the conservation of mass equation abstract this paper presents a model for the structural evolution of low strain detachment folds in which rock is treated as an incompressible deformable material and its kinematics is governed by the continuity equation the model also considers the following boundary conditions i the vertical flux across the core of the fold is described by a cosine function ii the horizontal flux due to transport along a basal detachment remains constant with depth and iii the depth of the detachment remains fixed upon finding analytical expressions for the velocity field of this deformation process a model for the accumulation of growth strata and the degradation of the topography created by folding is then derived these processes are approximated by means of the transport diffusion equation solutions of these two models are in excellent agreement with geometric and stratigraphic relations documented in fold and thrust belts and analog experiments moreover the model indicates that fold growth in this class of structures is a self affine process a compilation of detachment folds around the world indicates that these structures share a common profile moreover fold amplitude and wavelength of these folds are scaled by different amounts confirming this result the stratigraphy obtained with the growth strata model exhibits the typical thinning and truncation of timelines toward the core of the anticline cross cutting relations often observed in seismic cross sections and field data

strains developed in the hangingwalls of thrusts due to their slip propagation rate a dislocation model reply abstract fault planes propagate radially from a point source and this failure propagation process is very similar to the movement of a dislocation through a crystal lattice an elastic strain represented by an extra half plane is necessarily imparted to the lattice at an edge dislocation and this is equivalent to the ductile bead which accompanies the propagating tip of a thrust fault this ductile bead migrates with the fault tip and imparts a characteristic internal strain to the thrust sheet a two dimensional model is presented to illustrate the inter relationship between fault plane slip fault tip propagation and internal strain in multilayered sequences internal strain is usually represented by asymmetric folds verging in the thrust transport direction a simple technique the displacement distance plot utilizes the fact that displacement dies out towards the fault tip this plot can be used to quantify the relative stretch measured parallel to the fault movement which is dependent on the slip propagation rate and it may be used to define exactly the position of the fault tip on a cross section examples of fold thrust structures from devon england and pembrokeshire wales are used to illustrate the technique

planetary shock waves abstract a number of general circulation models have recently been proposed that compute the steady state structure of the general circulation observation of 18 c water formation on the other hand suggest the need for a study of the time dependent large scale structure of the oceans in this paper the planetary geostrophic equations are used to compute the evolution of large thermal anomalies with a view toward understanding the variability in the general circulation caused by water mass formation events the evolution of a thermal anomaly is considered in the absence of wind forcing in this case the planetary geostrophic equations can be reduced to a first order equations the planetary geostrophic wave equation pgwe arbitrary initial conditions governed by the pgwe tend to steepen and under an assumed diffusive closure from shock waves the evolution of an initially columnar eddy is obtained and four different phases of shock propagation are identified the implications for heat transport p

a general harmonic coordinate transformation to simulate the states of strain in inhomogeneously deformed rocks in order to investigate the possible states of strain that may exist in an inhomogeneously deformed body a general deformation is proposed in the form of a harmonic coordinate transformation this deformation differs from those previously considered by jaeger ramsay graham and hobbs in that it is capable of representing a wide variety of deformations previous efforts had inbuilt assumptions regarding the mechanism of deformation the transformation contains adjustable terms all of which have distinct geometrical significance some represent a homogeneous deformation some represent inhomogeneous shortening some represent inhomogeneous shear and others correspond to a pinch and swell type of deformation by combining these terms with different degrees of emphasis many kinds of deformation may be simulated in this paper two constraints are developed in conjunction with the general harmonic transformation these are the conditions for constant volume deformation both locally and generally and the condition for zero shear strain of lines initially normal to the distorted layer again both locally and generally

k ar dating constraints on the tectonothermal evolution of the external humber zone southern quebec appalachians in the external humber zone the emplacement episodes of allochthonous nappes and related low grade metamorphism are mainly based on paleontological ages from the st lawrence lowlands in fact late ordovician flysches are affected by thrusting and it is not clear if the nappe imbrication is the result of the taconian or the acadian deformation along the chaudiere river transect fourteen shales and phyllites of cambrian to ordovician age were sampled for determination of the k ar values of their 2 m size fractions to interpret the k ar data mineral composition and clay mineral crystallinity of the 2 m size fractions and maturation of the organic matter were determined in addition the intensity of deformation was characterized by describing the rock texture at macroscopic and microscopic scale the combined approach used in this study emphasized that nearly all k ar dates are mixed or partly reset values however it can be stated that in the quebec promontory nappe and part of the chaudiere

comparison of tectonic styles in the mexican and canadian rocky mountain fold thrust belt despite the fact that most fold thrust belts around the world share many features successfully explained by the critical wedge model the details of their geometric evolution and tectonic style development are poorly understood in the classic section of the southern canadian rocky mountains the dominant tectonic style consists of imbricate thrust sheets with relatively little internal deformation of the individual slices in the mexican fold thrust belt central mexico the age of deformation the overall structural pattern and the total amount of shortening are similar but the individual thrust sheets exhibit much more internal deformation as manifest by metre scale buckle folds one of the differences between these localities is the lateral variation of facies resulting in massive platform limestone separated by thinly bedded basinal limestone in the central mexico section strain is concentrated toward the margins between platforms and basins in canada thick platform carbonates form continuous resistant units across the front range possible reasons for the differences in tectonic style between the two sections include the dominant lithology distribution of lithologies taper angle of the tectonic wedges amount of friction along the basal detachment and the degree of anisotropy of the basin facies rocks fold thrust belts are common features that developed on the edges of orogenic belts worldwide many have been well studied and their characteristic features established dahlstrom 1970 chapple 1978 boyer elliott 1982 price fermor 1985 mcclay 1992 these features include the classic notions of forward propagation of thrusts piggyback style a basal decollement and decreasing intensity of deformation towards the foreland most fold thrust belts are wedge shaped and since the mid 1980s the critical taper model of their development davis et al 1983 dahlen et al 1984 dahlen 1990 has become generally accepted analogue and numerical models have been developed that are able to capture many of the basic features of fold thrust belts and that are broadly consistent with the critical taper theory e g huiqi et al 1992 stockmal et al 2007 from early studies it has been clear that most of the large folds in fold thrust belts are intimately related genetically to the thrusts and extensive studies of thrust related folds in the field and in analogue and numerical models have established the nature of this relationship e g dahlstrom 1969 1970 elliott 1976 suppe 1983 mcclay 1992 wilkerson et al 2002 spratt et al 2004 despite the many similarities among fold thrust belts in general there are significant differences among belts and along strike within individual belts these reflect among other things differences in the strength of the basal detachment and the composition and distribution of the rocks in the fold thrust wedge in this contribution we compare basic elements of the structures developed roughly synchronously in two widely separated parts of a single orogenic belt the north american cordillera the sections we compare are a well known section across the southern canadian rocky mountains and a new section across the sierra madre oriental in central mexico the dimensions of the fold thrust belts in these two locations are similar and both sections are dominated by carbonates the structural style however is quite different we compare structures in the two locations and speculate on what may be responsible for the differences

deformation mechanisms in the southeastern ramp region of the pine mountain block tennessee a field and petrofabric study of the southwestern part of the pine mountain thrust sheet shows that most deformation features may be attributed to one of two processes the first layer parallel compression is responsible for a a layer parallel maximum shortening direction in the mechanically twinned fabric of the limestone and dolomite country rock b transport parallel extension fractures and joints c transport normal stylolite surfaces and d some small thrusts the second bending at thrust ramp hinges is responsible for a non bedding parallel maximum shortening directions in mechanically twinned calcite from fracture fillings b transport normal extension fractures and joints and c small normal faults possible explanations for the failure of calcite in the country rock to twin in response to bending at the ramp include both 1 an early imposed pre thrusting calcite twin strain hardening and 2 deformation of the country rock by competing mechanisms such as pressure solution small normal faults are parallel to the strike of the thrust sheet and seem to have evolved from strike parallel extension fractures in response to continued extension both small thrusts and stylolite surfaces are rare in outcrop there seems to be no major change in deformation of rocks which have traveled through different fractions of the total ramp length so long as the rocks have passed onto the ramp

numerical modeling of trishear fault propagation folding in contrast to kink band migration modeling methods trishear numerical models produce fault propagation folds with smooth profiles and rounded hinges modeled fold hinges tighten and converge downward within a triangular zone of distributed deformation which is focused on the fault tip such features have been reported from field studies and are also seen in analogue models of compressional deformation however apart from its initial application to laramide folds little quantitative work has been undertaken on trishear fault propagation folding in other settings in addition no study has been undertaken into the growth strata which might be associated with such structures this paper uses an equivalent velocity description of the geometric model of trishear together with models of erosion and sedimentation to investigate trishear fault propagation folding of both pregrowth and growth strata the trishear model is generalized to include a variety of fault propagation to slip ratios and fault propagation from a flat decollement the models show continuous rotation of the forelimb with the characteristic development of cumulative wedges within growth strata when total slip on a structure is high the model predicts overturned pregrowth and growth strata during the initial stages of deformation beds in the forelimb thicken but later thin when they become steep or overturned the effect of variations in fault propagation to slip ratios on two dimensional finite strain in the models is assessed by the use of initially circular strain markers high fault propagation to slip p s ratios lead to narrow zones of high finite strain while lower p s ratios lead to more ductile deformation and broader zones of high strain in all cases hanging wall anticlines and footwall synclines originate as early ductile folds which are later cut by the propagating fault modeled structures are compared with natural examples

cordilleran deformation along the eastern edge of the valles san luis potosi carbonate platform sierra madre oriental fold thrust belt east central mexico the eastern margin of the cretaceous valles san luis potosi carbonate platform hidalgo queretaro and san luis potosi states became activated during the formation of the sierra madre oriental fold thrust belt a series of thrust sheets with a minimum linear shortening of 3 to 4 km each and 10 to 12 km in total formed where the platform edge was approximately normal to the greatest principal stress trajectories as deduced from fold axes a tear fault developed where the platform edge formed an acute angle with the same stress trajectories the overthrusts cut across the competent layers of the lower cretaceous on tectonic ramps that dip 10 and became partly steepened by subsequent imbrication or folding in the mechanically weak upper cretaceous rocks the thrust faults are nearly parallel to bedding for several kilometres the location of the thrusts is controlled mainly by the lithological and thickness change of the middle cretaceous rocks from an orthotropically layered basin facies into a two to five times thicker homogeneous platform edge assemblage the bank margin was a zone of stress concentration the magnitude of the horizontal tectonic components which caused the deformation may have changed by a factor of two to five across the platform margin due to the change in cross sectional area oblique and layer parallel discrete shears and subordinate tectonic stylolites appear to have been the dominant deformational mechanisms the rocks did not suffer any measurable penetrative ductile deformation ooids present at the base of the xilitla thrust are unflattened but are marked by a closely spaced stylolitic cleavage perpendicular to bedding the deformations are bracketed by the paleontological age of the youngest strata affected by the overthrusts globotruncana contusa planktonic foraminiferal zone and by the isotopic age of a post tectonic pluton 62 2 ma and are thus of late maastrichtian paleocene age

the metamorphic history of the damara orogen based on k ar data of detrital white micas from the nama group namibia abstract detrital white micas from molasse sediments of the nama group were dated by the k ar method the kuibis subgroup and the underlying pre damara sinclair sequence contain detrital muscovite with ages of 1100 1000 ma and thus cannot be derived from the damara orogen but prove to have originated from a basement probably affected by the kibaran orogeny muscovite size fractions of 250 100 m from the upper nama group upper schwarzrand and fish river subgroups yielded k ar ages 670 570 ma because these sediments are regarded as molasse series of the damara orogen these k ar ages can be interpreted as cooling ages of a first early tectono thermal event in the damara orogen a later damaran metamorphic overprint which peaked at about 530 ma is not documented in the detrital content of the nama group at the present level of erosion because detrital micas with k ar ages over 600 ma have been found up to the uppermost fish river subgroup it can be concluded that at least two tectono metamorphic events followed by uplift and erosion have taken place during the development of the damara orogen thermal alterations of the nama sediments were detected by means of illite crystallinity and k ar dating of clay size fractions these data as well as the almost exclusive occurrence of the 2m mica polytype in these size fractions point to a diagenetic to very low grade metamorphic alteration of the area investigated at about 530 500 ma and younger the deposition of the upper nama group is younger than the 570 ma detrital white micas but occurred before the 530 500 ma thermal alteration of the nama deposits the deposition of the lower nama group probably took place between 635 and 570 ma

mineralogical kar stable and sr isotope systematics of k white micas during very low grade metamorphism of limestones helvetic nappes western switzerland abstract mineralogical k ar rb sr and stable isotope analyses have been carried out on k white micas from helvetic malm limestones in order to examine their evolution during very low to low grade alpine metamorphism associated with intense ductile deformation metamorphic temperatures were estimated at 300 c from stable isotopes quartz calcite thermometry occurrence of chloritoid and epizonal illite crystallinity index k white micas consist of variable mixtures of 2m 1 phengite and muscovite as revealed by detailed x ray diffraction analyses using peak decomposition of the 060 331 spectra k ar apparent ages display a strong grain size dependence in which mainly fine grained size fractions m record alpine ages 37 15 ma however these ages provide a relative rather than an absolute chronology of the diachronous alpine metamorphic evolution of the helvetic nappes the resetting of the k ar isotopic system of k white micas to alpine metamorphic conditions reflects an apparent combination of crystallization recrystallization and radiogenic 40 ar diffusion loss the oxygen isotope compositions of micas 15 to 22 are intermediate between detrital and 18 o enriched values expected for micas neoformed within an abundant marine carbonate matrix no isotopic equilibrium has been reached between calcite and micas the variable depletion of hydrogen isotope compositions 126 to 82 is influenced by the interaction with organic matter under closed system conditions organic matter if not removed may also represent a serious source of error in k ar age determination by introducing radiogenic 40 ar contamination 87 sr 86 sr isotope ratios of micas range from 0 70879 to 0 70902 with one outlier at 0 71794 the low values reflect sr exchange with calcite occurring during crystallization recrystallization of micas under closed system conditions

k ar dating of magmatic sericite crystallites for determination of cooling paths of metamorphic overprints a combined optical and x ray characterisation of fine grained micas in granodioritic to tonalitic dikes and intrusions of the eastardennes eifel nappe linksrheinisches schiefergebirge germany belgium was completed with k ar dating of separated size fractions from these rocks in each sample the k ar ages decrease with decreasing size fraction but the same size fractions of the different samples yield different ages application of the closure temperature concept to the k ar ages of the various size fractions indicates that the crystallite size expressed as cis fwhmser001ad and coherent scattering domain csd thickness acts as the effective radius of ar diffusion a linear model may describe the correlation between k ar ages and crystallite thickness the ages younger than the model prediction are caused by the occurrence of microcline as an accessory k bearing mineral phase the obtained k ar ages were interpreted as representative of the cooling path of the magmatic components in the eastardennes eifel nappe below 330 c the k ar age of the largest size fraction of the tonalitic dike v11 313 6 7 2 ma is the closest value for the age of the variscan cleavage development the oldest age of the hydrothermally altered tonalitic dike 300 2 8 4 ma might represent an episode of hydrothermal fluid migration after cleavage development it represents at least a minimum age the microcline age 242 8 6 0 ma of the hill intrusion may represent the closure temperature of this mineral at about 140 c

how normal faulting and sedimentation interact to produce listric fault profiles and stratigraphic wedges abstract in a series of scale experiments we generated listric normal faults by extending models composed of colored sand layers deposited at regular time intervals throughout deformation during syntectonic sedimentation progressive bulk extension along a single set of faults caused domino type tilting of fault blocks underlying tilted faults propagated upwards with constant dips into newly sedimented layers hence fault profiles become progressively more listric tilting of the free surface resulted in asymmetrical depressions and in fan shaped deposits stepwise or continuous geometric models of domino tilting and synchronous sedimentation yield the same structural and stratigraphic features as observed in the sand models the continuous geometric model shows that a critical parameter governing fault curvature is the ratio r between rates of bulk strain and sedimentation both types of models reveal that synchronous sedimentation and bulk extension are capable by themselves of generating listric normal faults

timing of low temperature metamorphism and cooling of the paleozoic and mesozoic formations of the bukkium innermost western carpathians hungary k ar ages of illite muscovite and fission track ages of zircon and apatite were determined from various lithotypes of the bukkium which forms the innermost segment of the western carpathians the stratigraphic ages of these dinaric type formations cover a wide range from the late ordovician up to the late jurassic the grade of the orogenic dynamo thermal metamorphism varies from the late diagenetic zone through the anchizone up to the epizone chlorite maximally biotite isograd of the greenschist facies the k ar system of the illite muscovite in the 2 m grain size fraction approached equilibrium only in epizonal and high temperature anchizonal conditions the orogenic metamorphism culminated between the eo hellenic 160 120 ma phase connected to the beginning of the subduction in the dinarides and the austrian 100 95 ma phase characterized by compressional crustal thickening no isotope geochronological evidence was found for proving any hercynian recrystallization the stability field of fission tracks in zircon was approached using the thermal histories of the different tectonic units a temperature less than 250 c and effective heating time of 20 30 ma had only negligible effects on the tracks whereas total annealing was reached between 250 and 300 c apatite fission track ages from the paleozoic and mesozoic formations show that the uplift of the bukk mountains occurred only in the tertiary not earlier than ca 40 ma ago thermal modeling based on apatite fission track length spectra and preserved paleogene sediment thickness data proved that the late neogene burial of the recently exhumed plateau of the bukk mountains exceeded 1 km

time constraints for the tectono thermal evolution of the cantabrian zone in nw spain by illite k ar dating abstract contrasting sedimentary facies and tectonic structures diversely distributed and affected by diagenetic to low grade metamorphic activity form a complex pattern in the northwestern spanish cantabrian area of the variscan orogen the distribution of these metamorphic features is based on field observations clay mineralogy mineral crystallinity indices vitrinite reflectance data and observation of rock microtextures k ar dating of fine grained illite rich size fractions provides a complete set of ages for the episodic deformation and thermal overprints in the southeastern cantabrian area the usual k ar isochron plotting was combined with the bench type age distribution to unravel the successive illite crystallization episodes several tectono thermal episodes occurred in the southeastern pisuerga carrion province initially during the early permian at 293 4 ma in the ruesga line valsurvio dome and sierra del brezo area a further mid permian event was detected at 268 6 ma in the palentine nappes and during the mid triassic at 243 5 ma again in the valsurvio and sierra del brezo areas and along the ruesga line a later reactivation of the ruesga line was detected during mid jurassic time at 175 6 ma

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

rb sr systematics of vendian cambrian claystones from the east european platform implications for a multi stage illite evolution a combined mineralogical chemical morphological and rb sr isotopic study of blue clays from the lower cambrian lontova unit shows that these minerals contain three generations ofillite the oldest detrital 2m illite particles are typicauy round with sharp borders sometimes rolled up and have a minimum isotopic age of 722 13 ma this generation of illite is mainly concentrated in the 0 6 2 zm size fractions of the samples the second generation of illite is represented by well crystallized lath type particles of 0 1 zm size with an age of 533 8 ma considered to be the sedimentation age faulting and or regional emergence during the ordovician to early silurian time probably induced extensive circulations of subsurface waters which precipitated an additional 0 1 gm generation of low temperature lath type illite particles at 430 480 ma ago the precambrian cambrian boundary may be set at 530 10 ma based on the data presented here leaching of different clay subfractions by ammonium acetate shows that illite particles of different generations crystallizcd in environments with different svsr z6sr ratios the varied environmental sr isotopic ratios were at least in part due to local interactions between the interstitial waters and the sedimentary mass additionally the initial stsr s6sr ratios of the clay particles from argillaceous sedimentary rocks were mostly higher than that of marine st

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

kinematic and metamorphic aspects of cleavage formation in very low grade metamorphic slates abstract fabric studies on first cleavages in very low grade metamorphic slates from the rheinisches schiefergebirge and other very low grade metamorphic fold belts reveal two types of slaty cleavage 1 1 cleavage without syntectonic phyllosilicate recrystallization these are generally crenulation cleavages 2 2 cleavage with syntectonic phyllosilicate recrystallization with increasing syntectonic metamorphism the crenulation fabric is modified by solution of grains along cleavage planes and precipitation of material within the cleavage lamellae with further increase of metamorphism the crenulation fabric is gradually obliterated the finite fabric results from progressive solution crystallization and recrystallization superimposed with bending kinking and internal rotation achieved by slip parallel to the basal planes of phyllosilicates

identification of remagnetization processes in paleozoic sedimentary rocks of the northeast rhenish massif in germany by k ar dating and ree tracing of authigenic illite and fe oxides 1 this study combines mineralogical chemical rare earth elemental ree and isotopic k ar data of clay minerals as well as chemical compositions major and ree of fe oxide leachates from remagnetized palaeozoic sedimentary rocks from ne rhenish massif in germany for which the causes of remagnetization are not yet clear the dominant carrier of the syntectonic pervasive carboniferous magnetization is magnetite the middle devonian clastic rocks record an illitization event at 348 7 ma probably connected to a major magmatic event in the mid german crystalline rise whereas a second illitization episode at 324 3 ma is coeval to the northward migrating deformation through the rhenish massif being only detected in upper devonian and lower carboniferous rocks the age of that younger illitization is not significantly different from that of the remagnetization which however is not restricted to the upper part of the orogenic belt but affects also the middle devonian strata the ree patterns of the fe enriched leachates support two mineralization episodes with varied oxidation reduction conditions outlined by varied eu and ce anomalies this is not compatible with a unique pervasive migration of orogenic fluids on a regional scale to explain the remagnetization in the studied region while clay diagenesis and remagnetization are time equivalent in upper devonian and lower carboniferous rocks they are not so in middle devonian rocks transformation of smectite into illite cannot therefore account for the growth of associated authigenic magnetite which must have been triggered by a different process since remagnetization and deformation ages are similar the mechanism could relate to local physical conditions such as pressure solution and changing pore fluid pressure due to tectonic stress as well as to chemical conditions such as changing composition of the pore fluids

dynamics of the gulf stream deep western boundary current crossover part i entrainment and recirculation abstract a regional primitive equation model is applied to the study of the interaction between the gulf stream and the deep western boundary current dwbc where they cross at cape hatteras it is found that for a wide range of forcing parameters the upper core of the dwbc is split into two mean paths at the crossover one flowing toward the south along the western boundary and the other flowing toward the cast under the gulf stream the eastward branch is entrained into the southern recirculation gyre and after being diverted into the interior for up to 1500 km eventually returns to the western boundary current and continues to flow southward this recirculation and mixing is shown to have a significant impact on the separation point and mean path of the gulf stream the basin scale stratification and the properties of the dwbc south of the crossover for most configurations the lower dwbc remains largely on the western boundary and interacts only weakly with the interior the entrainment of the upp

the illite crystallinity technique a critical appraisal of its precision analysis of the precision of the illite crystallinity technique shows that machine errors are 0 8 it is necessary that contours should be at intervals of 0 1 2 which is equivalent to subdivision of the anchizone into upper and lower units where previous interpretations have relied upon an isocryst method of contouring at less than 0 1 2 the conclusions must be regarded as unsubstantiated centrifuge separation of clay fractions based on a stokes law application gives separations in which a significant but variable percentage of grains have long axes greater than the size calculated for the typical 2 m fraction utilized some 20 of grains lie in the 2 4 m range although the proportion is not believed to have a significant effect upon crystallinity values the formula is applicable for grain sizes down to 0 5 m illite crystallinity values on samples prepared by an ultrasonic disaggregation method show a small increase on those prepared by ball mill crushing the differences are minimal at the epi anchizone level but increase to some 10 at the anchizone diagenetic level the effect on grade determinations is again thought to be minimal and indicates that concern over unsuitability of the ultrasonic disaggregation method is unfounded

age of the late paleozoic tectonothermal activity in northcentral mauritanide west africa abstract late proterozoic sedimentary successions unconformably overlie each other on the crystalline basement in the northcentral mauritanide foreland clay sized m fractions from allochthonous metasedimentary samples collected close to the basal thrust are characteristic of an upper anchizone very low grade lower epizone low grade thermal development these fractions record conventional k ar ages ranging between 301 8 and 312 7 ma which are interpreted to date structural emplacement of the allochthon clay fractions separated from the underlying autochthonous sedimentary sequence record a diagenetic to low middle anchizonal very low grade thermal development these fractions yield k ar values between 337 8 and 472 12 ma which probably reflect variable diffusional rejuvenation processes of detrital and or diagenetic precursors 40 ar 39 ar incremental release analyses of hornblende and biotite concentrates from crystal line basement beneath the sedimentary sequence suggest that the respective closure temperatures last occurred at 2400 and 1850 ma ago as no record of any other thermal overprint was observed the results suggest that emplacement of the allochthon occurred in the late carboniferous and was not associated with a regional metamorphism of the north central mauritanide foreland

40 ar 39 ar dating of cleavage formation in tuffs during anchizonal metamorphism 40ar 39ar incremental release analyses were carried out on whole rock and constituent white mica illite rich size fractions 0 63 1 to 6 3 20 m within two very low grade penetratively cleaved metatuffs of contrasting anchizonal metamorphic grade northeastern rheinisches schiefergebirge federal republic of germany one sample from the upper anchizone displays internally concordant 40ar 39ar spectra with plateau ages ranging between ca 316 and 325 ma these are similar to conventional k ar ages determined for the whole rock and size fractions together the isotopic results suggest that cleavage formed at ca 320 ma during a concomitant very low grade metamorphism this is consistent with biostratigraphic controls which suggest that metamorphism and cleavage formation occurred during the westphalian

neodymium and strontium isotopic dating of diagenesis and low grade metamorphism of argillaceous sediments the behaviour of the rb sr and sm nd isotopic systems with increasing degree of hercynian metamorphic overprint was studied along a transect in cambrian shales of northwestern morocco clay fractions of 0 2 to 2 6 m size from five samples were investigated representing a range from nonmetamorphic to epizonal metamorphic conditions the samples were washed in cold l n hc1 prior to digestion to separate soluble exchangeable rb sr sm and nd from amounts of these elements fixed in the crystallographic sites of the minerals and to analyze both components separately the results reveal that the rb sr isotopic system is dominated by sr hosted by clay mineral phases both detrital and authigenic illite and chlorite and carbonate hosted soluble sr isotopic homogenization of sr occurred during hercynian metamorphism yielding ages between 309 and 349 ma the sm nd isotopic system on the other hand is dominated by cogenetic apatite and fe oxide hydroxide both having high contents of leachable rees the leachates yield a sm nd isochron age of 523 72 ma indicating diagenetic equilibrium between apatite and fe oxide hydroxide fine grained clay fractions of 0 2 m show inheritance of a detrital nd component the study demonstrates that the diagenesis of the investigated argillaceous sediments can be dated by the sm nd chronometer in authigenic cement phases the isotopic system of these minerals apatite fe hydroxide oxide was homogenized during authigenic mineral growth in a sediment that was flushed by diagenetic fluids and had abundant primary or secondary interconnected pore space the hercynian metamorphic overprint caused partial isotopic rehomogenization of the adsorbed and clay hosted portion of the sr as well as of the carbonate hosted sr the sm nd system in the cement phases survived this metamorphism this results in decoupling of the two isotopic systems and allows the dating of diagenesis on the one hand sm nd and metamorphism on the other hand rb sr

interpreting structural geometry in fold thrust belts why style matters abstract structural interpretation in fold thrust belts has become reliant on a few idealized geometric models i e fault bend fault propagation and detachment folding and their quantitative methods for section construction and validation we couple historical review with selected outcrops to show that there is a substantially greater range of solutions available for interpreting the geometry and evolution of thrust belt structures than implied by these idealized models examples are documented and lessons drawn from comparing structural interpretations developed in the foothills of the canadian rockies with those in the western alps both show a range of structural geometries with regional variations that reflect variations in the pre kinematic stratigraphic template locally fold thrust development can localize on pre existing structures thus consideration of the precursor geology is essential for structural interpretation using a case study from the papuan fold belt we show that even with seismic data assessing the role of basement in structural development can be uncertain the idealized models offer only a narrow range of possible geometries for constructing cross sections and developing structural understanding in fold thrust systems failure to consider alternatives and the inherent interpretation uncertainty has biased understanding of thrust systems leading in turn to over optimistic risk assessment and repeated drilling surprises

implications of k ar ages of whole rock and grain size fractions of metapelites and intercalated metatuffs within an anchizonal terrane white mica bearing fractions ranging in grain size from 0 4 m to 6 3 20 m were separated from metapelites and intercalated metatuffs of the eastern rheinisches schiefergebirge frg the stratigraphic age of these rocks is middle devonian eifelian and they contain detrital material of northwestern provenance old red continent probably mainly derived from the caledonian orogen folding in the carboniferous was associated with cleavage formation and an apparently synkinematic anchizonal metamorphism apparent k ar ages of metapelite fractions display a marked positive correlation with grain size illustrating the detrital influence which is diminished with decreasing grain size and increasing metamorphism determined by illite crystallinity contrasting grain morphologies observed by sem enable the interpretation of apparent age grain size relationship for coarse fractions the anticipated lack of detrital mica in metatuffs is confirmed by the fairly consistent apparent k ar ages determined for the coarser than 0 63 m size fractions which date the anchizonal metamorphism at ca 330 ma comparison of metatuff and metapelite apparent ages suggests that the extent of rejuvenation in the latter was largely dependent on grain size rejuvenation was also somewhat controlled by the degree of anchizonal metamorphism as suggested by differences in k ar results of metapelites which were metamorphosed at variable anchizonal conditions fractions 0 63 m from upper anchizonal metapelites record ca 330 ma ages similar to those of the 0 63 20 m sizes in metatuffs together those results confirm the limited applicability of conventional k ar dating on bulk clay fractions 2 m of very low grade anchizonal metamorphic rocks in dating metamorphic events and concomitant cleavage formation

the geometry of listric normal faults and deformation in their hangingwalls abstract in cross sections containing listric extensional faults area balancing techniques for depth to decollement are usually based on either bed length conservation or displacement conservation listric fault geometry may be constructed from a hangingwall roll over profile using the chevron construction this construction based on conservation of heave necessitates a reduction in fault displacement with decreasing fault dip a modification of this construction utilizing conservation of fault displacement predicts a listric fault that detaches at a shallower depth a new construction based on slip lines uses fault perpendicular displacement segments to generate listric fault shape fault propagation strain may be responsible for anomalous hangingwall geometries and these can be predicted by forward modelling using either a modified chevron construction or a slip line construction

a kinematic model for the formation of duplex systems with a perfectly planar roof thrust we present a cross sectional kinematic forward model for the formation of duplexes with a perfectly planar roof thrust the major assumptions are a constant dip and constant spacing of the ramps in the undeformed state and sequential deformation in the direction of tectonic transport with equal displacement along each ramp the model is based on a coordinate transformation that simulates flexural slip parallel to the active fault surface this causes angular parallel folds and keeps the layer thickness constant except in the forelimbs of the horses attempts by previous workers to simulate the formation of duplexes with a perfectly planar roof thrust on the other hand were based on the assumptions of constant bed thickness and bed length or a different topology of the axial planes delimiting the forelimbs of the horses and resulted in corrugated roof thrusts we show that it is not possible to form a flat roof duplex type and preserve the forelimb thickness of the horses under flexural slip parallel to the active fault we describe duplexes by three parameters which are the separation s between ramps the ramp length l and the displacement u along the ramps in a us vs ls diagram duplexes with a perfectly planar roof thrust resulting from numerical experiments with our kinematic algorithm occupy specific families of straight lines our results are independent of the dip or internal geometry of the thrust ramps

balanced cross section construction from seismic sections in areas of extensional tectonics abstract the application of the technique of balanced section construction initially developed for areas of compressional folding and faulting is reviewed with reference to extensional tectonics a number of examples are discussed where these techniques have been successfully applied in the north sea the interpretation of geoseismic sections is considered to be greatly assisted by careful application of geometrical balance and a consideration of strain even in areas of low crustal extensions the nature of seismic sections however places limitations on the validity of balancing which must be borne in mind with such interpretations and wherever possible the balancing of a geoseismic section should be confirmed by complete depth conversion the rapid testing of the integrity of the geoseismic section by attempting to balance the section at the interpretational phase can eliminate many problems as well as allowing the fullest use to be made of the geophysical information

orogenic wedge growth during collision constraints on mechanics of a fossil wedge from its kinematic record rhenohercynian ftb central europe abstract the kinematic evolution of the rhenohercynian fold and thrust belt of the central european variscides can be reconstructed from incremental strain and palaeostress data synkinematic metamorphism geochronology synorogenic sediments and the geometry of shortening this data set allows to infer the mechanical development with respect to the mohr coloumb theory of critically tapered orogenic wedges growth of the rhenohercynian orogenic wedge was kinematically partitioned into a broad orogen normal moving and a narrow obliquely moving domain the ratio of the final widths of these domains varies from 1 6 to 2 3 this low ratio indicates strong basal coupling growth averages are 14 km ma of shortening and a strain rate of 6 10 16 s 1 the mode of mass transfer from lower to upper plate which controlled the accretion geometry and the particle paths evolved in the long term from dominantly basal to frontal accretion this evolution was accompanied by a decrease in taper from precollisional 18 28 to final 5 8 4 6 accretionary subsystems assemble the wedge each is characterised by a sequence of basal to frontal accretion both rooting in a common detachment the development of the subsystems is supported by the observation of stepwise migration of a foreland bulge ponding flysch subbasins these features imply shorter termed ca 2 ma cyclic mass transfer above a high friction detachment with episodes of basal accretion followed by episodes of a quickly forward propagating thrust front transfer of the basal detachment from ductile mid crustal to brittle conditions via a crustal ramp and particle flow across the ramp provoked large scale adjustment of wedge geometry internal wedge thickening is spatially and chronologically related to the formation of the ramp and was supported by surficial erosion of ca 3 kbar for keeping a critical taper three growth stages of the wedge can be reconstructed the high taper of the precollisional stage plots in the tectonically erosive field in a taper stability diagram then taper decreases to 10 after emplacement of a flysch nappe stack during the final stage of contraction an eastern and a western branch develop with geometries fluctuating around a critically stable state the two branches link with differences in large scale wedge architecture bulk contraction and detachment geometry the latter being controlled by the former basin geometry

kar systematics of clay to mica minerals in a multi stage low grade metamorphic evolution the behavior of the ar isotopic system of clay to mica fractions was studied in cambrian greywacke samples along a transect of northwestern morocco these rocks were affected by a progressively increasing metamorphic overprint spanning from diagenetic to epizonal metamorphic conditions k ar isotopic data of 0 2 10 m size fractions recovered from seven samples allowed to recognized three episodes of illite crystallization 1 during an upper ordovician thermal event at 445 ma which is characterized by an illite chlorite mineral paragenesis in the northernmost part of the transect 2 during a carboniferous low grade overprint at 305 ma which is characterized by a similar mineral paragenesis and 3 during a triassic to liassic hydrothermal activity at 205 ma which generated a smectite dominated paragenesis the ages were examined in combining x ray mineralogic and k ar isotopic data in terms of linear arrays in both isochron and harper diagrams to differentiate true isochrons from mixing lines it could also be shown that mixtures of two generations of micaceous minerals can induce different trends in the k ar dates depending whether the first generation of minerals was either or not affected by the event inducing the later generation

neoproterozoic metamorphism and deformation at the southeastern margin of the east european craton uralides russia the eastern margin of the east european craton eec has a long lasting geological record of precambrian age archaean and proterozoic strata are exposed in the western fold and thrust belt of the uralides and are known from drill cores and geophysical data below the palaeozoic cover in the uralides and its western foredeep in the southern uralides sedimentary metamorphic and magmatic rocks of riphean and vendian age occur in the bashkirian mega anticlinorium bma and the beloretzk terrane in the eastern part of the bma yamantau anticlinorium and the beloretzk terrane k ar ages of the 2 m size fraction of phyllites potassic white mica and slates illite give evidence for a complex pre uralian metamorphic and deformational history of the precambrian basement at the southeastern margin of the eec interpretation of the k ar ages considered the variation of secondary foliation and the diagenetic to metamorphic grade in the yamantau anticlinorium the greenschist facies metamorphism of the mesoproterozoic siliciclastic rocks is of early neoproterozoic origin about 970 ma and the s1 cleavage formation of late neoproterozoic about 550 ma the second wide spaced cleavage is of uralian origin in the central and western part of the bma the diagenetic to incipient metamorphic grade developed in late neoproterozoic time in post uralian time proterozoic siliciclastic rocks with a cleavage of uralian age have not been exhumed to the surface of the bma late neoproterozoic thrusts and faults within the eastern margin of the eec are reactivated during the uralian deformation

detrital spinels from alpinotype source rocks in middle devonian sediments of the rhenish massif middle devonian sandy siltstones of the gummersbach area rhenish massif show increased cr contents up to 675 ppm the chromium is mainly associated with detrital cr al spinels the spinels are characterized by a wide spectrum of chemical composition in more than 50 of 134 analyzed spinel grains the cr2o3 content exceeds 50 wt the a12o3 content which is negatively correlated to cr2o3 reaches 40 wt the mgo contents are with few exceptions lower than the feo contents both elements are negatively correlated the fe2o3 contents which were calculated from a quarter of the grains rarely exceed 5 wt besides cr al fe mg tio2 contents were determined the chemical composition of the detrital spinels indicates provenance from peridotites of an ophiolite suite provenance analysis shows that the source area of the middle devonian clastic sediments was situated in the north

40ar 39ar ages of detrital muscovite and whole rock slate phyllite narragansett basin ri ma usa implications for rejuvenation during very low grade metamorphism late pennsylvanian sedimentary rocks in the narragansett basin were metamorphosed lower anchizone to sillimanite grade during late paleozoic regional metamorphism at ca 275 280 ma twenty five variably sized concentrates of detrital muscovite were prepared from samples collected within contrasting low grade areas diagenesis lower greenschist facies microprobe analyses suggest that the constituent detrital grains are not chemically internally zoned however some grains within several concentrates display very narrow 25 m compositionally distinct low grade epitaxial peripheral overgrowths detrital muscovite concentrates from the lower anchizone are characterized by internally concordant 40ar 39ar age spectra which define plateau ages of ca 350 360 ma these are interpreted to date post devonian acadian cooling within proximal source areas concentrates from lower grade sectors of the middle anchizone display slightly discordant spectra in which apparent ages systematically increase from ca 250 275 ma to define intermediate and high temperature plateaus of ca 360 400 ma detrital muscovite within samples from higher grade sectors of the middle anchizone and the upper anchizone are characterized by systematic low age discordance throughout both low and intermediate temperature increments high temperature ages only range up to ca 330 ma six size fractions of detrital muscovite from a sample collected within the lower greenschist facies have similarly discordant spectra in which apparent ages increase slightly throughout the analyses from ca 250 ma to 275 ma the detrital muscovite results are interpreted to reflect variable affects of late paleozoic regional metamorphism however it is uncertain to what extent the systematic low age spectra discordance reflects intracrystalline gradients in the concentration of 40ar and or experimental evolution of gas from relatively non retentive epitaxial overgrowths however low age discordance occurs regardless of the extent of epitaxial overgrowth intermediate temperature increments evolved during 40ar 39ar whole rock analyses of five slate phyllite samples are characterized by internally consistent apparent k ca ratios these are attributed to gas evolved from constituent very fine grained white mica samples from lower grade portions of the middle anchizone are characterized by intermediate temperature apparent ages which systematically increase from ca 275 300 ma to ca 360 375 ma before evolution of a high temperature contribution from detrital plagioclase feldspar this age variation may reflect partial late paleozoic rejuvenation of very fine grained detrital material with a source age similar to that for the detrital muscovites slate phyllite samples from upper sectors of the middle anchizone and from the upper anchizone were completely rejuvenated during late paleozoic metamorphism and record intermediate and high temperature plateau ages of ca 270 290 ma these data document that metamorphic conditions of the lower to middle biotite zone ca 325 350 c are required to completely rejuvenate intracrystalline argon systems of detrital muscovite therefore the 40ar 39ar dating method may be useful in determination of detrital muscovite provenance and in resolution of the metamorphic evolution of low grade terranes

the evolution of illite to muscovite mineralogical and isotopic data from the glarus alps switzerland thirty five illite and muscovite concentrates were extracted from triassic and permian claystones shales slates and phyllites along a cross section from the diagenetic alpine foreland tabular jura and borehole samples beneath the molasse basin to the anchi and epimetamorphic helvetic zone of the central alps concentrates and thin sections were investigated by microscopic x ray infrared mossbauer thermal dta and tg wet chemical electron microprobe k ar rb sr 40ar 39ar and stable isotope methods with increasing metamorphic grade based on illite crystallinity data xrd and ir the following continuous changes are observed i the 1md 2m1 polymorph transformation is completed in the higher grade anchizone ii k2o increases from 6 8 wt diagenetic zone to 8 5 10 anchizone to 10 11 5 epizone reflecting an increase in the total negative layer charge from 1 2 to 2 0 iii a decrease of the chemical variation of the mica population with detrital muscovite surviving up to the anchizone epizone boundary iv a shift of an endothermic peak in differential thermal curves from 500 to 750 c v k ar and rb sr apparent ages of the fraction 2 m decrease from the diagenetic zone to the epizone k ar ages being generally lower than rb sr ages the critical temperature for total ar resetting is estimated to be 260 30 c k ar and rb sr ages become concordant when the anchizone epizone boundary is approached the stable isotope data on the other hand show no change with metamorphic grade but are dependent on stratigraphic age these results suggest that the prograde evolution from 1 md illite to 2m1 muscovite involves a continuous lattice restructuration without rupture of the tetrahedral and octahedral bonds and change of the hydroxyl radicals however this is not a recrystallization process this restructuration is completed approximately at the anchizone epizone boundary the isotopic data indicate significant diffusive loss of 40ar and 87sr prior to any observable lattice reorganization the restructuration progressively introduces a consistent repartition of ar and k in the mineral lattices and is outlined by the 40ar 39ar age spectra concordant k ar and rb sr ages of around 35 30 ma with concomitant concordant 40ar 39ar release spectra are representative for the main phase of alpine metamorphism calanda phase in the glarus alps a second age group between 25 and 20 ma can probably be attributed to movements along the glarus thrust ruchi phase while values down to 9 ma in regions with higher metamorphic conditions suggest thermal conditions persisting at least until the middle tortonian

kinematic modeling of cross sectional deformation sequences by computer simulation we derive a kinematic algorithm which permits simulation of postulated cross sectional deformation sequences in sedimentary rocks affected by faulting fault related folding and simple shear the transformations are to a more deformed state and are formulated analytically in terms of the less deformed configuration forward modeling lagrangian description the medium is subdivided into domains of constant dip and homogeneous displacement vector fields which are delimited by the axial planes of the fault inflections the displacement trajectory is of constant length for all the displaced particles throughout the medium and parallel to the underlying active fault segment as a consequence the deformation path is continuous but not smooth and causes an angular style of parallel folding except at the deformation front where the layer thickness is not conserved the inhomogeneity of the displacement vector field across axial planes introduces longitudinal and angular shear strains whereas the area of the medium remains constant these strains which are superposed on the externally applied simple shear make the mapping transformation unconformal we define the stratigraphic layering of the undeformed medium by an approximating body of finite quadrilateral elements evaluate the defined displacement functions for the grid nodes and graph the deformed mesh eventually we create with the algorithm synthetic deformation sequences for simple boundary conditions and try in a specific case study to match the resulting synthetics with the available observational and experimental data

self determining numerical modeling of compressional fault bend folding we present the application of the hca hybrid cellular automata numerical modeling technique to the evolution of compressional fault bend anticlines involving layers that have competency contrasts and a mechanical stratigraphy the new modeling results show that for competent hanging wall layers 1 the fault style is not preserved in the overlying fold 2 fold development is not self similar and rotational growth wedges develop in the forelimb when syntectonic sedimentation occurs and 3 balanced fault bend anticlines can develop for ramp cutoff angles as high as 60 these results compare favorably with field examples and laboratory analogues and dramatically broaden the applicability of fault bend folding to model the evolution of thrust related anticlines

the meridional flow of source driven abyssal currents in a stratified basin with topography part ii numerical simulation abstract a numerical simulation is described for source driven abyssal currents in a 3660 km 3660 km stratified northern hemisphere basin with zonally varying topography the model is the two layer quasigeostrophic equations describing the overlying ocean coupled to the finite amplitude planetary geostrophic equations describing the abyssal layer on a midlatitude plane the source region is a fixed 75 km 150 km area located in the northwestern sector of the basin with a steady downward volume transport of about 5 6 sv sv 106 m3 s 1 corresponding to an average downwelling velocity of about 0 05 cm s 1 the other parameter values are characteristic of the north atlantic ocean it takes about 3 2 yr for the abyssal water mass to reach the southern boundary and about 25 yr for a statistical state to develop time averaged and instantaneous fields at a late time are described the time averaged fields show an equatorward flowing abyssal current with distinct up and downslope groundings with decr

a 1 5d model of the steady state thermal structure of detachment folds abstract we present a 1 5d steady state model of thermal perturbations in the core of detachment anticlines the model incorporates the effects of heat conduction advection by folding radiogenic sources and conversion of work into heat by viscous dissipation we work out analytical solutions of model s equations for heat flow and temperature field for the case of low amplitude detachment folds growing in a thick homogeneous stratigraphy our analytical solutions for the simplest case of no heat sources show that detachment folds develop a thermal boundary layer conduction of heat dominates inside the layer whereas advection of heat by folding prevails outside the boundary layer we also find that heat flow in a growing detachment fold decays exponentially with the square of the thickness of the detached stratigraphic section when heat sources are included into the problem we find that radiogenic heat generates thermal anomalies at the crest of the anticlines whereas viscous dissipation produces thermal anomalies at the core of the detachment fold we analyze three cases of geological interest detachment folds growing in evaporitic sequences marine carbonates and shaley sediments for a typical uplift rate of 2 mm yr conduction dominates in the first class of rocks viscous dissipation dominates in folds growing in limestone due to its high viscosity advection and the generation of heat by radiogenic isotopes dominate the growth of detachment folds in shale additionally the cooling histories of lagrangian particles embedded in the fold for the last two cases are analyzed the core of the fold is characterized by rapid cooling whereas the limbs cool at slower rates than the core thus the development of detachment anticlines may either accelerate maturation of source rocks and expand the maturation window or halt it by removing source rocks from the generating window

late stage evolution of fault propagation folds principles and example abstract fault propagation folding commonly occurs near the fronts of mountain belts this type of folding may even represent in some thrust systems the major mode of deformation however fault propagation folds are frequently altered by two kinds of late stage evolution breakthrough thrusting or transport on the flat we model the geometric features of these kinds of fold in particular the late stage modification and present the algorithms and equations used in an original program an application based on an example from the atlas mountains of algeria illustrates the main features of the model

40 ar 39 ar mineral age record of early caledonian tectonothermal activity in the baltoscandian miogeocline central scandinavia in central portions of the scandinavian caledonides greenschist facies volcanosedimentary successions within the koeli nappe complex have been thrust several hundred kilometers eastward onto the baltoscandian platform these were derived from eugeoclinal terranes situated outboard west of the baltica continent during the early paleozoic the koeli nappe complex is tectonically underlain by higher grade units within the seve nappe complex these are composed of amphibolite and granulite facies rocks and locally contain eclogites the seve nappes tectonically separate koeli units from structurally lower allochthons derived from more inboard environments along the baltoscandian miogeocline previous mineral isotopic age determinations from seve and koeli units have been in the 430 to 390 ma range and have been interpreted to presumably date cooling following scandian middle silurian to early devonian metamorphism however incremental release sup 40 ar sup 39 ar dates recorded by minerals within some of the koeli and seve nappes exposed in jaemtland sweden taennforsen and are districts provide evidence of earlier tectonothermal activity hornblendes from the seve and koeli nappe complexes display variably discordant age spectra as a result of low temperature experimental evolution of loosely bound extraneous argon components however in most analyses plateau ages of 510 to 475 ma koeli and 465 to 455more ma seve are defined in contrast muscovite and biotite from all tectonic units record scandian cooling ages between 245 and 410 ma the older events recorded by hornblende within these seve and koeli units are evidence of early caledonian tectonothermal activity and subsequent diachronous cooling during the early middle ordovician less

postdepositional evolution of platform claystones based on a simulation of thermally induced diffusion of radiogenic 40ar from diagenetic illite abstract the lower cambrian lontova claystones contain well crystallized authigenic 1m illite mixed with detrital 2m illite part of the authigenic illite probably precipitated during sedimentation and subsequently experienced a temperature up to 140 c despite a limited burial to 500 1000 m the difference between the rb sr and k ar dates of this authigenic illite are attributed to preferential loss of radiogenic 40ar induced by a short lasting thermal pulse this interpretation is based on an ar diffusion model suggesting that the lontova blue clays were heated to around 140 c for 2 5 myr this thermal pulse could have occurred in relation with hot fluid migration induced by either tectonic dislocations in the basement rocks during the late silurian early devonian or rifting conditions during the permian by comparing rb sr and k ar data evaluation of radiogenic 40 ar losses by small authigenic illite particles at low temperatures could be a powerful tool for reconstruction of evolutionary histories of sedimentary successions that were either shallowly buried or percolated by migrating fluids released during tectono thermal activities

the tectonic significance of k ar illite fine fraction ages from the san luis formation eastern sierras pampeanas argentina the sierra de san luis forms the southern tip of the eastern sierras pampeanas in central argentina two narrow belts of low grade phyllites and quartz arenites i e the san luis formation have accommodated part of the strain related differential exhumation of the medium to high grade metamorphic domains that constitute to the basement complex of the sierra eleven phyllite samples were subjected to the k ar fine fraction dating technique results are interpreted in relation to the kubler index of the illites which indicate epimetamorphic conditions for the majority of the samples obtained ages between 330 and 290 ma cover a period of compressional tectonics in the late mississippian visean serpukhovian boundary followed by the subsidence during the formation of the paganzo basin in the provinces of la rioja and san luis these tectonic movements are coincident with the toco orogeny in northern chile and southern bolivia this suggests that the older k ar ages document the compressional stage and that younger ages record the cooling of the basement during the subsequent extensional uplift of the basement

finite difference modelling of hangingwall deformation abstract modelling of hangingwall deformation during extension or compression may be achieved using a finite difference approach under the assumption of incompressible flow equivalent to area balancing this method enables calculation of a particle velocity field giving the trajectory of any point in the hangingwall during deformation this technique is more flexible than graphical algorithms because it is not restricted to simple shear cases inputs to the algorithm are the bounding fault and the particle displacement directions simple examples are used to illustrate the method

potassium argon timing of episodic mica and illite crystallization in highly indurated hassi messaoud algeria hydrocarbon bearing sandstones nanometer to micrometer mica and illite separates of indurated cambrian and ordovician oil bearing sandstones from the hassi messaoud field algeria were extracted x rayed observed by scanning and transmission electron microscopy and k ar dated electron microscope observations revealed typical euhedral shapes for the mica to illite particles of most size fractions almost no odd shaped detrital crystals were detected the combined results document several generations of mineralogical and morphological identical mica to illite crystals that could not be differentiated by the traditional identification methods illite and mica genesis was multiphased with crystallization episodes at 340 10 ca middle mississippian 280 10 ma ca early permian and 170 10 ma ca middle jurassic younger than the stratigraphic age of the host rocks which is incompatible with a detrital origin the two older mica ages confirm that the hydrocarbon generation and emplacement had to start after the variscan tectonothermal event and before exhumation of the meta sediments the younger k ar ages at 135 to 110 ma ca early cretaceous relate to further crystallization episodes whereas those at circa 295 265 and 210 ma probably correspond to variable mixtures of the older and younger mica to illite end members three average k ar values are statistically significant the oldest at 340 10 ma corresponds to the start of the variscan tectonic activity and the intermediate at 280 10 ma sets its end both episodes probably modifying the reservoir capacities of the potential hydrocarbon host rocks the ages at 170 10 ma identify a further diagenetic activity characterized by illitization of dickite type precursors in local reservoirs these younger ages could correspond to the hydrocarbon charge into reservoirs which stopped diagenetic illitization at a present day depth of approximately 4000 m 13 000 ft

variation of illite muscovite 40 ar 39 ar age spectra during progressive low grade metamorphism an example from the us cordillera 40ar 39ar step heating data were collected from micron to submicron grain sizes of correlative illite and muscovite rich cambrian pelitic rocks from the western united states that range in metamorphic grade from the shallow diagenetic zone zeolite facies to the epizone greenschist facies with increasing metamorphic grade maximum ages from 40ar 39ar release spectra decrease as do total gas ages and retention ages previous studies have explained similar results as arising dominantly or entirely from the dissolution of detrital muscovite and precipitation recrystallization of neo formed illite while recognizing the importance of these processes in evaluating our results we suggest that the inverse correlation between apparent age and metamorphic grade is controlled primarily by thermally activated volume diffusion analogous to the decrease in apparent ages with depth observed for many thermochronometers in borehole experiments our results suggest that complete resetting of the illite muscovite ar thermochronometer occurs between the high anchizone and epizone or at roughly 300 c this empirical result is in agreement with previous calculations based on muscovite diffusion parameters which indicate that muscovite grains with radii of 0 05 2 m should have closure temperatures between 250 and 350 c at high anchizone conditions we observe a reversal in the age grain size relationship the finest grain size produces the oldest apparent age which may mark the stage in prograde subgreenschist facies metamorphism of pelitic rocks at which neo formed illite muscovite crystallites typically surpass the size of detrital muscovite grains it is also approximately the stage at which neo formed illite muscovite crystallites develop sufficient ar retentivity to produce geologically meaningful 40ar 39ar ages results from our sampling transect of cambrian strata establish a framework for interpreting illite muscovite 40ar 39ar age spectra at different stages of low grade metamorphism and also illuminate the transformation of illite to muscovite at frenchman mtn nv where the cambrian bright angel formation is at zeolite facies conditions illite muscovite 40ar 39ar data suggest a detrital muscovite component with an apparent age 967 ma the correlative carrara fm is at anchizone conditions in the panamint and resting spring ranges of eastern california and in these locations illite muscovite 40ar 39ar data suggest an early permian episode of subgreenschist facies metamorphism the same type of data from equivalent strata at epizone conditions greenschist facies in the footwall of the bullfrog fluorspar canyon detachment in southern nevada reveals a period of slow to moderate late cretaceous cooling