magnetite authigenesis and diagenetic paleotemperatures across the northern appalachian basin the concentration of magnetite in the lower and middle devonian helderberg and onondaga carbonate formations varies in a distinct pattern along an east west profile across new york state magnetite concentrations are uniformly low in the western half of the profile increase sharply eastward to a peak value near syracuse and decrease farther east the pattern strongly resembles previously reported variations in the extent of clay mineral diagenesis that reflect differences in paleotemperatures along the profile previous paleomagnetic studies have documented that the magnetite carries a well defined magnetization of pennsylvanian permian age and magnetite authigenesis is therefore no younger than late paleozoic we attribute the correlation between high magnetite concentration and high illite content to temperature dependent diagenesis triggered by orogenic fluids the large proportion of secondary magnetite indicates that over most of the area the mechanism of late paleozoic remagnetization was principally chemical thermal effects played a less direct role in the remagnetization of the strata by controlling the extent of the chemical processes that resulted in overprinting of the original remanence

paleomagnetic evidence for tectonic rotations in the variscan mountain belt a critical review of devonian and carboniferous paleomagnetic results from the hercynian basement of western and central europe reveals large rotations their geographic distribution suggests a tectonic origin for large scale curved structures such as the ibero armorican arc or the variscan v together with a coherence of the hercynian domain with time geodynamic models which can have produced the observed rotation pattern are discussed it appears that lateral compression due to the accommodation of continental collision as in the india asia convergence is a plausible mechanism although variscan and himalayan mountain belts have different characteristics

plate and intraplate processes of hercynian europe during the late paleozoic abstract it is speculated that until late carboniferous time the region of hercynian europe was occupied by an elongated island arc system underlain by a segment of continental crust in the upper carboniferous two subduction zones are assumed to have extended from the north and south beneath hercynian europe an extensive zone of hot partially molten upper mantle lay above and between these and diapiric uprise of portions of this material led to separation of mafic magmas widespread partial melting in the lower and middle crust high temperature low pressure metamorphism in crustal rocks and regional uplift and extension of the crust as indicated by intermontane troughs and their associated volcanic rocks in visean to westphalian time hercynian europe collided with both the large neighbouring plates north america europe and africa during these diachronous collisions and owing to reduced rigidity of the relatively hot island arc crust the irregular continental margins of the larger and thicker continental plates induced oroclinal bending of hercynian europe after the collision processes had been terminated processes of upper mantle activity continued causing further crustal uplift and even enhanced crustal extension for several tens of million years into the lower permian decline of the upper mantle activity beneath hercynian europe is indicated by crustal subsidence and formation of a peneplain in permian time followed by the upper permian transgression of both the zechstein sea and the tethys sea which mark the end of the hercynian geodynamic cycle

origin of a thrust related fold geometric vs kinematic tests abstract geometric tests to determine the origin of fault related folds are common but as is typical in structural geology more than one fold origin may yield the final natural geometry thus the results of geometric tests are usually non unique in contrast kinematic tests of origin which employ both geometry and data about deformation commonly yield more constrained if not unique results unfortunately the necessary data collection requires much more work than for a geometric test in this study the thrust related barclay anticline is analyzed both geometrically and kinematically to determine which test is more effective geometric tests using angular relationships indicate three possible origins fault bend fault arrest and break thrust for the kinematic test predicted deformations for interlayer and flexural slip flexural flow simple shear and bending strains are compared to micro and mesostructural distributions solution strain and finite strain from the anticline strain measurements indicate that microscale deformation is uniformly distributed through the structure and is lithification dominated the microscale deformation does not match kinematic predictions and did not accommodate fold formation fold growth was achieved primarily through layer parallel slip restricted mostly to the forelimb and absent in the hinge which eliminates fault bend and fault arrest origins that require material transport from forelimb to backlimb the barclay anticline is therefore interpreted to be a break thrust structure interestingly a suite of contraction faults in the forelimb and hinge indicates material transport from forelimb to backlimb such transport has been discounted for break thrust folds the most important result of this study is that a kinematic test was a necessary step for distinguishing fold origin geometric testing alone was insufficient given that the barclay anticline has geometric characteristics typical of many thrust related folds kinematic testing appears necessary to determine their structural origin

palaeomagnetism of the early carboniferous deer lake group western newfoundland no evidence for mid carboniferous displacement of acadia early carboniferous visean to possibly earliest namurian sedimentary rocks of the deer lake group of western newfoundland rest unconformably on grenvillian basement rocks of the canadian shield which form the western border of the early palaeozoic appalachian orogen in addition to magnetically soft magnetizations directed along the present field three families of magnetization directions are found two of them referred to as n north and s south are antiparallel and prefolding and were probably acquired during the depositional process n and s are roughly of equal frequency they have a mean direction irrespective of sign of 0 7 35 2 k 40 95 8 9 and a palaeopole 21 5 n 121 8 e 10 3 6 0 corresponding to a palaeolatitude of 20 6 s this agrees with the palaeolatitude 17 5 s determined from the somewhat older early carboniferous tournaisian terenceville formation of the avalon platform on the eastern side of the appalachian orogen in newfoundland the third magnetization referred to as h roughly horizontal has a mean direction 156 8 13 3 k 37 95 10 1 and a palaeopole 45 4 n 140 3 e 10 3 5 3 corresponding to a palaeolatitude of 7 4 n we interpret this to be an early kiaman latest carboniferous to early permian overprint probably acquired chemically the palaeolatitude determined from the h overprint agrees with that determined from early carboniferous rocks of cratonic north america west of the appalachians therefore we argue early carboniferous palaeofield determinations for cratonic north america have been strongly biased by unremoved kiaman overprints because of this and because of the good agreement between early carboniferous palaeolatitudes obtained from opposite margins of the appalachian orogen we suggest that there is at present no palaeomagnetic evidence for the previously proposed 1500 km displacement from the south of an eastern portion of the appalachians acadia relative to cratonic north america during the carboniferous

diagenetic sources of stable remanence in remagnetized paleozoic cratonic carbonates a rock magnetic study hysteresis measurements lowrie fuller tests cisowski tests and low temperature demagnetization experiments on samples of the knox dolomite trenton limestone and onondaga limestone indicate that the remanence in these remagnetized carbonate units resides principally in single domain magnetite in contrast to previous interpretations volumetrically however a significant fraction of the magnetite is coarser grained as previous scanning electron microscopic work on magnetic extracts has shown these conclusions are based mainly on the characteristic wasp waisted shape of the hysteresis loops and on the anomalously high ratios of remanent coercivity to bulk coercivity both of which clearly indicate a bimodal distribution of coercivities grain sizes the magnetization is controlled mainly by cubic magnetocrystalline anisotropy consistent with the spheroidal particles commonly observed in extracts hysteresis trends show that especially the finest particles lack shape anisotropy in view of these conclusions the two components of natural remanence in these rocks are interpreted as 1 a late paleozoic kiaman chemical remanence carried by single domain diagenetic magnetite and 2 a cenozoic thermoviscous overprint residing principally in coarser pseudosingle domain and multidomain magnetite also of late paleozoic diagenetic origin

magnetic studies of the pendleside limestone evidence for remagnetization and late diagenetic dolomitization during a post asbian normal event the pendleside formation a sequence of bioclastic wackestones and grainstones has been extensively dolomitized and silicified especially towards the top and near the axis of the clitheroe anticline undolomitized limestones are characterized by the presence of magnetite and primary style magnetic fabrics and an exclusively reversed nrm with mean direction of dec 204 inc 21 the magnetization is considered to be a depositional postdepositional remanent magnetization acquired before any significant alteration of the limestones dolomitized and silicified limestones are characterized by hematite some silicified limestones show the intermediate state of co existing hematite and magnetite these rocks have non primary style magnetic fabrics and a complex nrm with normal components the best estimate of this component is dec 22 inc 20 almost antiparallel to the primary remanence the magnetization is considered to be a chemical remanent magnetization crm generated during the dolomitization of the pendleside formation the origin of the dolomitization may be associated with mudrock dewatering as the sequence was buried the normal crm indicates that the remagnetization spanned a post asbian normal event possibly in the westphalian coincident with the maximum burial of the pendleside formation

displacement transfer at thrust terminations the saltville thrust and sinking creek anticline virginia u s a abstract the saltville thrust terminates in the core of the sinking creek anticline changes in the amplitude of the anticline may preserve profile shortening as the thrust displacement decreases towards the fault termination consequently thrust displacement would be transferred into the fold we combine newly published maps and new strain and mesostructural data from the tuscarora sandstone to argue that the sinking creek anticline did perform this kinematic role we propose that the anticline developed as a fault propagation fold that experienced both decollement and anticlinal breakthrough by the saltville thrust thrust displacement was accommodated by the development of the modified fault propagation fold and by transfer to a roof flat from the decollement breakthrough new strain and mesostructural data indicate no fixed pin lines in either the hinge or backlimb throughout folding consequently layer slip occurred through the hinge during folding and the forelimb deformed internally during breakthrough towards the plungeout of the sinking creek anticline the displacement on the saltville thrust is transferred to a floor thrust we apply this interpretation to the termination of the st clair thrust which has a similar surface geometry where the thrust terminates into an anticline a testable implication of this proposed interpretation for the st clair thrust is that the appalachian plateau must absorb over 20 km of thrust displacement as horizontal shortening

thin skinned tectonics in the northern rhenish massif germany a network of profiles in the hercynian mountain system geo traverse rhenoherzynikum is needed to determine details of the geological structure and find the reasons for tectonic movements outstanding features within the mountain system such as the large fault zone along the southern boundary of the hunsruck1 and the left flank of the rhinegraben have already been investigated by reflection seismology2 3 we describe here a new reflection refraction experiment carried out in 1978 in the most northern part of the rhenish massif across the stavelot venn massif near aachen fig 1 while layers in the lower crust dip to the nnw a strong upper reflector at 3 4 km depth shows a slight dip to the sse and indicates a prominent fault zone near aachen known in belgium as the faille du midi here a comparable and apparently identical reflector was found some 100 and 175 km to the west of our profile we interpreted this reflector as a prominent and well lubricated thrust fault along which a huge horizontal nappe displacement took place during the last stages of the variscan orogeny

the problem of strain marker centers and the fry method abstract the fry method and its variants will underestimate finite strain when applied to strain markers deformed heterogeneously at marker scale despite homogeneous behavior at a population scale this difference results from the post deformation centers not coinciding with the original pre deformation centers the amount of error may rapidly approach 50 for small strains in ideal cases but is likely to be in the range of 20 30

the fry method applied to an augen orthogneiss problems and results the application of the fry method to measure strain in deformed porphyritic granites is discussed this method requires that the distribution of markers has to satisfy at least two conditions it has to be homogeneous and isotropic statistics on point distribution with the help of a morishita diagram can easily test homogeneity isotropy can be checked with a cumulative histogram of angles between points application of these tests to undeformed mte capanne granite elba and to deformed randa orthogneiss alps of switzerland porphyritic granite reveals that their k feldspars phenocrysts both satisfy these conditions and can be used as strain markers with the fry method other problems are also examined one is the possible distribution of deformation on discrete shear bands providing several tests are met we conclude that the fry method can be used to estimate strain in deformed porphyritic granites

structural and strain analyses of the middle part of the talassian alatau ridge middle asia kirgizstan abstract the middle part of the talassian alatau ridge is composed of upper precambrian lower paleozoic greenschists the usunakhmatian sheet which were overthrust on unmetamorphosed rocks of nearly the same age the talassian and kumishtagian sheets three phases of deformation can be recognized in the usunakhmatian sheet the most intensive of which was the late silurian early devonian d 2 deformation in the usunakhmatian sheet the axis of maximum extension is subhorizontal and parallel to f 2 fold axes r xz varies from 2 4 to 7 5 and k from 0 03 to 1 75 flattening is widespread but the strain of many samples is nearly plane strain in the talassian sheet r xz varies from less than 1 8 to 4 4 and k from 0 to 0 35 r xz and k values increase near large faults r xz values determined by quartz and chert grains were found to be linearly related a three stage strain history is related to d 2 deformation the earliest was simple shear parallel to the axial surfaces of folds following this the fold and thrust system was transformed by pure shear with an orogen parallel axis of extension some minor increments of strain are locally represented by pressure shadows around pyrite crystals

strain variations and three dimensional strain factorization at the transition from the southern to the central appalachians abstract the transition between the southern and the central appalachian foreland thrust system is a region subjected to at least two non coaxial deformations where typical two dimensional analyses such as cross section balancing or plane strain retro deformation cannot be applied three dimensional factorization can simulate the non coaxial strain history simulation results for compaction and layer parallel shortening lps were compared to finite strains of 16 quartz arenite samples from the tuscarora sandstone successful simulations indicate in the southern appalachians 30 35 total compaction by volume loss including a 4 5 9 e w horizontal shortening during lithification and 5 lps along 150 330 by plane strain or axial symmetric flattening in the central appalachians 30 35 total compaction by volume loss including a 3 7 n s horizontal shortening during lithification and 10 lps along 120 300 by plane strain or axial symmetric flattening in the transition zone 30 compaction by volume loss including a 3 e w horizontal shortening during lithification 5 southern lps along 150 330 by axial symmetric flattening and 5 10 central shortening along 130 or 140 by plane strain or axial symmetric flattening at 10 to the normal central trend from the simulations horizontal shortening in the southern appalachians during lithification in the mississippian was greater than the later alleghanian deformation this shortening probably represents a very early lps developed in unlithified sand that required less deviatoric stress to deform than did the cemented quartz arenites during later tectonic deformation also the oblique and weaker central tectonic lps in the transition zone probably represents behavior at the limits of central appalachian deformation finally the lps simulations indicate that axial symmetric flattening oblate strain is more successful than plane strain as the representative behavior so caution is recommended in assuming plane strain as the dominant strain behavior in orogenic forelands

quantitative assessment of low temperature deformation mechanisms in a folded quartz arenite valley and ridge province west virginia abstract analysis of the grain scale deformation mechanisms in folded tuscarora sandstone from the western margin of the appalachian foreland near keyser wv reveals that deformation was accomplished by a combination of pressure solution microfracturing and crystal plastic mechanisms the finite strain was assessed using the normalized fry method and showed a predominant apparent flattening strain regime resulting from diagenetic compaction partitioning of the strain between operative deformation mechanisms revealed that layer normal shortening 25 and layer parallel shortening 10 by pressure solution were the greatest contributors to the finite strain followed by microfracturing and crystal plastic strains respectively although the crystal plastic contribution to the finite strain is small 1 0 layer normal shortening strain partitioning allowed detection of layer parallel extension consistent with the structural position of the samples on the outer arc of a fold the variation in inferred stress directions throughout the progressive deformation indicates that local stratigraphic and structural control on the stress field is important in the development of certain microstructures only the earliest stages of deformation record far field alleghanian compression whereas later stages reflect local stress fields controlled by bedding orientation and position on developing folds similarities in the microstructure evolution between the western and eastern margins of the appalachian foreland indicate that the entire belt underwent a similar deformation history

three dimensional 3 d finite strain at the central andean orocline and implications for grain scale shortening in orogens three dimensional 3 d finite strain analyses from across the central andes are used to document the contribution of grain scale strain in quartzites and sandstones to the total shortening budget the results are compared to thermal stratigraphic and strain data from other fold and thrust belts to determine the influence of lithologic strength and deformation temperature on strain accommodation during orogenic evolution in the central andes 3 d best fit ellipsoids are inconsistently oriented relative to structural trends have short axes at high angles to bedding z mean plunge 78 21 and have bedding parallel long axes x mean plunge 6 24 ellipsoid shapes are dominantly oblate x y z indicate low natural octahedral shear strains e s 0 03 0 19 and have axial ratios that range from 1 02 1 0 81 e s 0 19 to 1 02 1 0 97 e s 0 03 highly variable r f data r f 1 0 5 0 fluctuations exceeding 100 indicate detrital grain shapes may overwhelm any measurable tectonic strain fabric recorded by grain geometry the best fit ellipsoids may reflect either weak compaction strain or they may be related to a depositional fabric at a minimum granular strain was insufficient to reset the detrital grain fabric and therefore grain scale strain in quartzites and sandstones is not a significant factor in deformation we suggest that the nonstrained nature of these stiffer lithologies indicates a lack of regional penetrative strain in the central andes like that quantified in similar lithologies in other orogens the regional lack of strain may be due to deformation temperatures

paleozoic evolution of the armorica plate on the basis of paleomagnetic data new paleomagnetic data support the contention that armorica and gondwana formed a coherent block until ordovician time however by early devonian time armorica collided with the north american northern european assembly to form the old red continent and the collision itself was responsible for the acadian orogeny not for the earlier taconic orogeny as postulated previously the carboniferous collision of gondwana with the old red continent subsequently formed pangaea and produced the appalachian hercynian orogeny

the remagnetization of devono carboniferous sediments from the ardenno rhenish massif abstract a paleomagnetic study was conducted on the paleozoic sedimentary rocks of the ardenno rhenish massif and the brabant massif lower carboniferous and middle devonian carbonates in the dinant synclinorium and inde syncline carry a thermoviscous remagnetization residing in magnetite lower devonian red beds in the dinant synclinorium carry a chemoremanent remagnetization residing in hematite the sediments were remagnetized by burial metamorphism during the asturian phase of the variscan orogeny with a few exceptions in all formations the magnetizations are of inverse polarity corresponding to the kiaman paleofield in the permo carboniferous the virtual geomagnetic poles vgps of the inde syncline agree with the permo carboniferous pole position of europe vgps of the dinant synclinorium which deviate from the permo carboniferous reference direction are due to syntectonic remagnetizations and subsequent 10 20 maximum 40 dextral shear deformations these deformations can be correlated with the northwards overthrusting of the dinant nappe for the inde syncline post asturian rotational deformations can be excluded should a n s transport of this part of the dinant nappe not have occurred the declination pattern indicates that the western part of the dinant nappe was transported further north than the eastern part

finite strain determination of gneiss application of fry s method to porphyroid in the southern massif central france abstract we present an attempt to apply fry s method of finite strain ellipsoid determination to deformed gneissic rocks several finite strain ellipsoids have been measured on different porphyroid gneiss samples from the cevennes area southern massif central france the ellipsoids are characterized by a k parameter close to 1 plane strain or k 1 constriction field the method appears to be powerful and we show that in some cases effects of a pretectonic fabric can be eliminated from these results and microstructural data a strain path can be inferred

primary structure influence on compositional banding in psammites examples from the puncoviscana formation north central argentina abstract we investigate the influence of sedimentary structures in development of foliations in banded psammites and pelites in the meta turbiditic pampean belt eastern sierras pampeanas northwest argentina these anchizone to upper greenschist facies rocks were exhumed from near surface to mid crustal levels and preserve sedimentary structures and gradual changes in tectonic fabrics and microstructures primary variations in clay content affected the degree of development of a compaction related pressure solution cleavage s p resulting in enhancement of the original sedimentary banding the resulting compositional banding was overprinted by a tectonic cleavage s 1 related to mid cambrian chevron folding in banded sandstones pressure solution and mica growth that define s p and s 1 foliations are increasingly better developed in phyllosilicate rich domains with increasing structural depth and metamorphic grade clay rich laminae in the protolith acted as the locus for mica nucleation and growth favoring dissolution of quartz up to 42 causing passive concentration of mica pressure solution thus produced banded sandstones with a spaced bedding parallel s p cleavage in shallower sections and compositionally banded psammites and schists at depth these observations demonstrate that formation of compositional banding does not require tectonic transposition of an older planar fabric similar processes may produce compositional banding in a wide range of meta sedimentary rocks

structural deformation of the leoville chondrite foliations defined by alignment of elongated chondrules have been noted previously in chondrites but none displays this effect so well as leoville cv3 the shapes of leoville chondrules were produced by deformation in situ as indicated by inclusions and clasts with similar shapes and preferred orientations to those of chondrules similarities in the aspect ratios of apparent strain ellipses measured for chondrules alone 1 9 and 2 0 by several methods and for the whole meteorite 2 0 indicate either that leoville deformed homogeneously or that it deformed as a framework of touching chondrules this amount of strain corresponds to approximately 33 uniaxial shortening assuming constant volume because the strain ellipse was measured in only one orientation this strain value is a minimum estimate for the meteorite lack of correlation between foliation and either shock or thermal effects argues that impact or metamorphism are unlikely to have produced this deformation compaction due to overburden from progressive accretion on the chondrite parent body is suggested to have been its cause estimates of maximum deviatoric stresses in the interiors of asteroid sized bodies and constraints on maximum temperatures for cv3 chondrites are consistent with diffusional flow as the deformation mechanism for olivine in these chondrules diffusional flow is also suggested by the scarcity of observed lattice dislocations deformation of leoville olivines by this mechanism at geologically reasonable strain rates appears to require higher temperatures than those believed to have been experienced by this meteorite 600 c however differences in olivine grain size the presence of water or a more complex deformation history might explain this discrepancy

strain partitioning across a fold and thrust belt the rhenish massif mid european variscides abstract strain data across the entire western rhenish massif illustrate the pattern of deformation partitioning within a fold and thrust belt from the foreland on the northern front of the european variscides to the suture with the internal zones of higher metamorphic grade shortening perpendicular to cleavage increases from 16 to 27 constant volume assumed in the northern rhenish massif to 51 at the southern border these values correspond to a weak general layer parallel shortening of the basin filling in cross section on the order of about 5 15 and more in the case of volume loss the geometry of apparent finite strain is nearly plane strain except in the venn anticlinorium where strain is apparent prolate and near the southern margin of the massif where it is apparent oblate net orogenic shortening in the upper crust of the fold and thrust belt is approximately 42 it is mainly achieved by folding and tectonic stacking of the deformed basin filling which was apparently detached from the lower crust distribution of strain in the orogenic wedge is controlled by different factors at different scales heterogeneity is caused by changes in lithology and average grain size by the structural position of samples in folds and their distance from thrusts in the hangingwall at deeper crustal levels and by the regional distribution of the metamorphic grade i e by the dominant deformation mechanism this last factor causes an exponential increase of strain with approach to the ductile lower crust the superposition of strain partitioning mechanisms on different scales creates a complex regional strain pattern the southern part of the thrust belt apparently suffered late westward motion in gently dipping imbricates or strike parallel motion in steeply dipping imbricates which is probably due to oblique convergence between the rhenohercynian and saxothuringian microplates

review of major shale dominated detachment and thrust characteristics in the diagenetic zone part i meso and macro scopic scale there is considerable variability in the thickness structure and macroscopic deformation style within shale prone major thrust zones which also impacts the structural style in the overlying and sometimes the underlying section the properties and percentages of clay minerals and percentages of organic material quartz and calcite affect fault zone weakness and behaviour the distribution of competent units within a shale prone zone can also significantly affect fault zone architecture even over short strike distance of 10s to 100s of metres other influences include high pore fluid pressures and associated veins deformation of the fault zone may disrupt the original principal displacement zones and force them to relocate including switching lithologies calcite veins can become concentrated locally in the fault zone and become a focus of fault zone weakness we still need many more descriptions of shale prone fault zones to understand the potential variations in thrust sheet structural style caused by differences in shale detachment characteristics the variations in structural style in the hanging walls of shale detachments are broad and range between the sledge runner style in many appalachian examples and the extensive closely spaced imbricate style of the osen roa detachment southern norway such contrasts are related to several factors including thrust sheet mechanical stratigraphy thickness of the overburden lateral continuity of the detachment detachment mechanical properties composition pore fluid pressures presence absence of higher level detachments and strain rate in many structural locations and at many scales shale prone sequences will pile up to produce over thickened masses cleavage duplexes s c fabrics duplexes imbricates pseudo duplexes and vein formation can all contribute to the thickening at nested scales a mixture of brittle and solution diffusion processes overall cause the affected unit to exhibit flow like characteristics while the broad principles are now well established considerable work remains to be done understanding the full range of structural geometries associated with shale detachments what factors regarding the nature of the detachments affect overlying structural styles rates of deformation and the controls on deformation mechanisms within the thrust zones

strain patterns of crescentic granitoid plutons in the archean greenstone terrain of ontario abstract granitoid plutons of crescentic outcrop pattern in the archean terrain of northwestern ontario are dominated by constrictive strains that are interpreted to be due to intrusion rather than regional deformation the strain patterns of antiformal plutons are characterized by subvertical lineations in contrast the strain patterns of synformat plutons are characterized by subhorizontal lineations presumably only the last flow increments were recorded by the deformed mineral aggregates employed as strain gauges and occurred when the granitoid magma was largely crystalline simple kinematic models are capable of explaining many but not all elements of the natural strain patterns

polyphase development of chocolate tablet boudins in the sat zone kumaun lesser himalaya india abstract a combination of two groups of boudins exposed in orthogonal sections is observed as chocolate tablet structures in the south almora thrust sat zone kumaun lesser himalaya evidence from boudin associated veins fractures folds and microstructures suggest that the chocolate tablet boudins were developed as a consequence of layer parallel stretching in two orthogonal directions during two successive phases of ductile shearing in the sat zone the early phase of top to nw shearing is speculated as a pre himalayan deformation during which the early boudins were developed this deformation was superposed by a late phase of top to sw himalayan shearing and development of the attendant late boudins it is concluded that the interference of two phases of successively developed boudins has resulted into development of the chocolate tablet structures in the sat fluid inclusion and microstructures indicate a low temperature about 300 c growth of the chocolate tablet boudins the finite strain analysis reveals that a maximum strain ratio rs of 1 7 was accumulated during the pre himalayan phase of deformation whereas rs was of the order of 2 1 in the himalayan phase of deformation

diagenetic magnetite carries ancient yet secondary remanence in some paleozoic sedimentary carbonates many sedimentary carbonate rocks carry stable magnetizations that can be shown to reside in magnetite when such magnetizations are observed it is often argued or demonstrated that the magnetite was incorporated into the sediment during deposition however paleomagnetic and rock magnetic studies in conjunction with analyses of magnetic extracts from the helderberg and bonneterre carbonates united states indicate that the magnetite present in these rocks is most likely of diagenetic i e postdepositional origin

some guidelines for the practical application of fry s method of strain analysis abstract a systematic analysis of patterns in artificial and natural point distributions is presented in order to demonstrate how the nature and degree of anticlustering of point distributions affect the character of fry diagrams derived from them ideally anticlustered artificial point distributions illustrate the nature of the dependence between the degree of anticlustering and the minimum number of points required in such a distribution if it is to reveal strain accurately by fry s method in addition these distributions provide straightforward results which aid in the interpretation of less ideally anticlustered natural point distributions the results from the artificial distributions may be used to estimate the minimum number of points required for strain determination in natural distributions of varying degrees of anticlustering as long as the anticlustering is sufficiently strong however application of the results from the artificial distributions to less strongly anticlustered natural point distributions shows not only that the existence of a vacancy field is an insufficient criterion for determining whether or not fry s method is appropriate for strain determination but also that the nature of the anticlustering in these natural distributions limits the effectiveness of enhancing strain definition by increasing the number of points composing the distribution

strain cleavage and microstructure variations in sandstone implications for stiff layer behaviour in chevron folding sandstone layers within tightly chevron folded quartz rich turbidites of the lachlan fold belt southeastern australia show a fanning spaced cleavage overprinted by a non fanning grain alignment cleavage low internal strains recorded by quartz grain separations xz 1 4 1 8 and low fold flattening strains xz 1 8 angular relations between spaced cleavage and bedding in folds with varying tightness suggest that sandstone layers initially act as stress supporting members with spaced cleavage development due to pressure solution taking place at limb dips up to 15 during fold development sandstone layers undergo rigid body rotation involving interlayer slip without marked internal deformation except at fold hinges after fold lock up bedding spaced cleavage angles change due to inner arc collapse by pressure solution and subsequent flattening with formation of a grain alignment cleavage and mica beard growth internal deformation of sandstone is characterized by microstructures related to solution redeposition processes and growth of mica chevron folds form by homogeneous shortening and pressure solution due to layer parallel shortening tangential longitudinal strain due to buckling fold tightening by inner arc collapse due to pressure solution and fold flattening at metamorphic peak with growth of mica geometric modelling of internal strains and bedding cleavage angles requires a significant component of interlayer slip during chevron fold development under greenschist facies conditions

ability of the fry method to characterize pressure solution deformation abstract the applicability of the fry method to marker distributions deformed by pressure solution is examined four types of pressure solution deformation were modeled they differ by the spacing of the dissolution surfaces relative to the distance between marker centers the amount of volume loss and whether or not markers in contact with dissolution surfaces are dissolved completely the results show that the fry method correctly characterizes each type of deformation but without additional information such as the amount of volume loss it can be used only where the deformation is constant volume and where dissolution surfaces are more closely spaced than marker centers

strain partitioning and factorization in a quartz arenite abstract most rocks deform by multiple grain scale deformation mechanisms in order to assess completely the contributions of each mechanism the strain associated with that mechanism must be known and compared to the finite strain this study describes the partitioning of strain between three mechanisms pressure solution dislocation creep and microfracturing in a quartz arenite deformed at low temperatures pressure solution which occurred primarily during compaction dominates the finite strain with an average of 24 shortening normal to bedding dislocation creep which occurred during layer parallel shortening accounts for about 2 shortening parallel to bedding dip microfractures which occur in three orthogonal sets resulted in 2 4 extension normal to bedding parallel to bedding strike and parallel to bedding dip the validity of the mechanism strains was tested using strain factorization models were constructed using the mechanism strains in an order determined by their relative ages because porous well sorted quartz arenites such as the one studied are likely to undergo some mechanical compaction in the early stages of diagenesis variable amounts of mechanical compaction were included in factorization models models using the measured deformation mechanism strains with an initial 5 10 mechanical compaction yield finite strains in close agreement with the measured finite strains this suggests that the mechanisms identified the strain associated with each mechanism and the deformation sequence are plausible

comparison of fry strain ellipse and ams ellipsoid trends to tectonic fabric trends in very low strain sandstone of the appalachian fold thrust belt abstract in moderately to highly strained sandstones both the long axis of the bedding parallel finite strain ellipse as calculated by the normalized fry method and the projection of the long axis of the ams ellipsoid on the plane of bedding align well with local structural grain trends of cleavage folds and faults this relationship implies that results of both 2d fry and ams analyses represent the local layer parallel tectonic strain component do both methods provide comparable results for very low strain sandstone e g

two types of chemical remanent magnetization during the oxidation of magnetite abstract the chemical remanent magnetization crm m crm resulting from oxidation of equidimensional magnetite to a mixture of 90 hematite and 10 cation deficient magnetite is jointly controlled by the initial anhysteretic remanent magnetization m arm of the parent magnetite and the field h crm applied during oxidation m crm lies in the plane defined by h crm and m arm which were perpendicular in our experiments in an intermediate orientation determined by the relative strengths of h crm 0 05 0 1 and 0 2 mt and m arm 0 002 0 12 saturation remanence the crms are univectorial remanences that remain fixed in these stable intermediate directions throughout the course of stepwise alternating field and thermal demagnetization similar experiments in which acicular single domain cation deficient magnetite was oxidized to maghemite gave quite different results m crm was always parallel to m arm its direction was unaffected by field strengths as high as h crm 1 5 mt the essential factors controlling crm direction seem to be the number and crystallographic structure of oxidation products for single phase oxidation without change in crystal lattice crm remains parallel to the initial remanence of the parent mineral for multiphase oxidation to a mixture of rhombohedral and spinel phases both initial remanence and the field acting during oxidation influence the crm direction which therefore has an intermediate orientation that parallels neither m arm nor h crm

on the lag time between internal strain and basement involved thrust induced exhumation the case of the colombian eastern cordillera abstract thrust sheets accumulate internal strain before they start moving along discrete fault planes however there are no previous studies evaluating the time difference between initiation of strain and fault displacement in this paper we use observations from the eastern cordillera of colombia to evaluate this interval we utilize multiple thermochronometers and paleothermometers to refine the timing of deformation based on these new data we build time temperature path estimates that together with geometric outcrop based structural analysis and fluid inclusions allow us to assign relative timing to features associated with strain such as cleavage veins and certain types of fractures and compare that with the timing of thrusting we find that cleavage was only formed close to maximum paleotemperatures almost coeval with the onset of thrust induced denudation by the late oligocene the corresponding structural level of fold related veins suggest that they were formed later but still when the country rocks were at temperatures higher than 160 c mostly during the early miocene and still coexisted with the latest stages of cleavage formation our data show that the main period of strain hardening was short probably a few million years and occurred before first order basement thrusting was dominant but was associated with second order folding

fry spacing of deformed and undeformed modeled and natural salt domes fry s center point spacing strain analysis is applied to experiments of gravity driven overturn of horizontal fluid layers on scales of centimeters or decimeters and to natural salt diapirs on scales of kilometers to tens of kilometers laboratory experiments in which about 100 diapirs formed from initially plane horizontal layers resulted in laterally isotropic spacing and yielded an open circle on a fry plot with a radius equal to the wavelength w lateral deformation and initial departures from horizontal layers with uniform properties may influence w sample experiments of diapirism with and without lateral deformation are compared to natural equivalents the spacing of post late triassic diapirs in the zechstein salt of germany and of post jurassic diapirs in the hormuz salt of arabia yields circles on fry plots which indicate that these gravity structures developed without additional lateral forces fry plots of the spacing of salt diapirs in the zagros mountains define a strain ellipse with the long axis w in arabia parallel and the short axis perpendicular to post miocene regional fold axes the ratio of the zagros strain ellipse is about 1 7 which suggests a northeast southwest shortening of 41 rather than 10 21 as estimated from folding alone this mismatch may be amore result of unquantified shortening due to thrusts and or layer parallel shortening center point spacing of post late triassic domes of zechstein salt in the central north sea also gives an ellipse here the long axis coincides with the direction of middle jurassic to cretaceous extension the axial ratio of the north sea strain ellipse is about 2 8 compared to estimates of about 1 5 2 0 by sclater and christie and 1 8 by wood and barton less

semi automatic delimitation of volcanic edifice boundaries validation and application to the cinder cones of the tancitaro nueva italia region michoacan guanajuato volcanic field mexico abstract the shape and size of monogenetic volcanoes are the result of complex evolutions involving the interaction of eruptive activity structural setting and degradational processes morphological studies of cinder cones aim to evaluate volcanic hazard on the earth and to decipher the origins of various structures on extraterrestrial planets efforts have been dedicated so far to the characterization of the cinder cone morphology in a systematic and comparable manner however manual delimitation is time consuming and influenced by the user subjectivity but on the other hand automatic boundary delimitation of volcanic terrains can be affected by irregular topography in this work the semi automatic delimitation of volcanic edifice boundaries proposed by grosse et al 2009 for stratovolcanoes was tested for the first time over monogenetic cinder cones the method based on the integration of the dem derived slope and curvature maps is applied here to the tancitaro nueva italia region of the michoacan guanajuato volcanic field mexico where 309 plio quaternary cinder cones are located the semiautomatic extraction allowed identification of 137 of the 309 cinder cones of the tancitaro nueva italia region recognized by means of the manual extraction this value corresponds to the 44 3 of the total number of cinder cones analysis on vent alignments allowed us to identify ne sw vent alignments and cone elongations consistent with a ne sw max and a nw se min constructing a vent intensity map based on computing the number of vents within a radius r centred on each vent of the data set and choosing r 5 km four vent intensity maxima were derived one is positioned in the nw with respect to the volcano tancitaro one in the ne one to the s and another vent cluster located at the se boundary of the studied area the spacing of centroid of each cluster 24 km can be related to the thickness of the crust 9 10 km overlying the magma reservoir

the significance of microgranitoid enclave shapes and orientations abstract enclaves are often incorrectly used to measure magmatic strains in plutons we emphasize that microgranitoid enclaves are not like other ellipsoidal markers used to determine strain for the following reasons 1 adjacent enclaves may form at different times and different places and initially have non spherical shapes with axial ratios up to 2 7 2 the final shapes and orientations of enclaves are a complex function of a initial shape and temperature of enclaves b subtle changes in composition melt percents volatiles grain sizes and thus temporally variable viscosity contrasts between the enclave and magma c a competition between strain and interfacial energies and d deformation path which may include internal strain and rigid rotations caused by magma flow during ascent convection expansion chamber boundary processes and tectonism 3 enclaves spend much of their time in magma as relatively rigid objects and thus rigidly rotate and potentially break apart rather than strain at matrix strain rates 4 in some instances enclaves do not record or track finite strain because of the above final enclave populations are heterogeneous and the use of single enclaves or enclave populations as strain markers violates many assumptions needed to complete strain analyses on the other hand a comparison of the preserved characteristics of igneous layering mineral fabrics and carefully evaluated enclave fabrics including internal mineral alignment in enclaves may provide qualitative data on the changing magnitude and kinematics of magmatic strains

paleozoic paleogeography of north america gondwana and intervening displaced terranes comparisons of paleomagnetism with paleoclimatology and biogeographical patterns new paleomagnetic data have become available in the past 5 yr that require modifications in previously published paleogeographic reconstructions for the silurian and devonian in this paper the new paleopoles are compared to published paleogeographic models based on paleoclimatologic and biogeographic data the data from the three fields of paleomagnetism paleoclimatology and biogeography are generally in excellent agreement and an internally consistent paleogeographic evolutionary picture of the interactions between north america gondwana and intervening displaced terranes is emerging during the interval of the ordovician silurian and devonian north america stayed in equatorial paleoposition while rotating counter clockwise the northwest african part of gondwana was in high southerly latitudes during the late ordovician and was fringed by peri gondwanide terranes such as southern europe armorica and avalonian basement blocks now found in eastern newfoundland nova scotia the boston basin the appalachian piedmont and northern florida subsequently gondwana and the peri gondwanide terranes displayed rapid drift with respect to the pole this drift translates into the following pattern of movement for northwest africa during the latest ordovician early silurian this area moved rapidly northward from polar to subtropical latitudes followed by equally rapid southward motion from subtropical to intermediate about 50 s paleolatitudes during the late silurian middle devonian it is likely that significant east to west motion accompanied the latter shift in paleolatitudes with the caledonian acadian orogeny the result of silurian to early devonian convergence and collision between gondwana and north america this collision sandwiched several of the intervening displaced terranes between gondwana and north america subsequent to this collision gondwana was separated in the late devonian by a medium width ocean from north america and the avalonian and southern european blocks which were left behind adjacent to north america this new ocean closed during the carboniferous and the resulting convergence and collision were the cause of the hercynian alleghanian orogenic belt problems remaining for future research besides the further gathering of reliable paleopoles involve the uncertain pre devonian position of the southern british isles in this scenario and the very rapid velocity with respect to the pole that results from the rapid late ordovician silurian apparent polar wander for gondwana

constraints on deformation mechanisms during folding provided by rock physical properties a case study at sheep mountain anticline wyoming usa summary the sheep mountain anticline wyoming usa is a well exposed asymmetric basementcored anticline that formed during the laramide orogeny in the early tertiary in order to unravel the history of strain during folding we carried out combined anisotropy of magnetic susceptibility ams anisotropy of p wave velocity apwv and fry strain analyses the results are compared to previously published stress strain data from calcite twins at the microscopic scale and from fracture sets at the mesoscopic scale and are used to discuss the kinematics and mechanics of forced folding the results obtained in sandstone and carbonate lithologies demonstrate a good agreement between 1 the principal axes of the ams and apwv tensors 2 stress strain tensors derived from calcite twins 3 fry strain axes and mesoscopic fracture sets furthermore these tensors are coaxial with the main structural trends of the anticline the differences between ams and apwv fabrics on one hand and the differential stress values of the forelimb and the backlimb on the other hand emphasize how the macroscopic asymmetry of sheep mountain anticline affects the strain pattern at the microscopic scale the data set presented in this paper offers a consistent mechanical scenario for the development of sheep mountain anticline