mineral parageneses in metamorphic rocks exposed along tailuko gorge central mountain range taiwan field petrographic and microprobe investigations of metaclastic rocks calcareous schists marbles chloritic calcareous meta volcanic units and schists paragneisses which crop out along the eastern portion of the central east west cross island highway in taiwan demonstrate that metamorphic intensity gradually increases eastward the lower greenschist facies slate formation on the w contains completely recrystallized pure albitic plagioclase but at least some of the white micas chlorites probably represent relict detrital flakes neo blastic biotite and epidote occur sporadically in the pihou formation and increase dramatically eastward concomitantly the abundance of carbonaceous matter decreases to zero in the eastern tailuko zone and the amount of chlorite white mica diminishes somewhat epidote becomes more aluminous at higher metamorphic grade eastward phengites change progressively to more muscovitic compositions as the proportion of biotite increases a close approach to chemical equilibrium for the pre cenozoic complexly deformed metamorphic basement assemblages is suggested by regular systematic major and minor element partitioning between analysed coexisting phases fractionation is less pronounced on the e reflecting higher temperatures estimated physical conditions of recrystallization with h2o and co2 moderate are t 325 75 c p 3 kbar w t 425 75 c p 4kbar e the gradual eastward increase in metamorphic intensity from the slate formation through the pihou formation and the three tailuko zones as well as the relict precursor textures in the pre cenozoic layered basement rocks indicate that the observed paragenetic sequence could represent a synchronous neogene recrystallization event probably accompanying the plio pleistocene collision of the asiatic continental margin and the luzon coastal range andesitic arc

closure of the tornquist sea constraints from mona lisa deep seismic reflection data deep seismic reflection profiles west of denmark across the suture between baltica and eastern avalonia reveal weak southward dipping reflectors within the crystalline basement these reflectors are interpreted as thrusts resulting from emplacement of ea

short paper low grade metamorphism of the welsh basin lower palaeozoic succession an example of diastathermal metamorphism the lower palaeozoic succession of the welsh basin has suffered the effects of low grade metamorphism which shows features that do not accord with classical models of regional or burial metamorphism textural evidence demonstrates the development of greenschist facies metamorphism prior to deformation and cleavage development while mineralogical characters indicate a low pressure facies series in order to accommodate these features it is suggested that the area has suffered diastathermal metamorphism in which an enhanced thermal flux develops early in the tectonic cycle in response to an extensional setting temperatures remained relatively high during final closure associated with deformation of the basin sediments and consequent cleavage development such a model adequately resolves previously conflicting interpretations of burial and deformation related metamorphism

permo carboniferous magmatism of the northeast german basin abstract variscan continent continent collision in central europe was followed by a period of syn to late tectonic magmatism involving the extrusion of large volumes of permosilesian age volcanic rocks particularly in the northeast german basin the volcanic suite up to ca 2000 m thick comprises rhyolites acid and basic andesites and tholeiitic and alkali basalts tectono geochemical analysis in conjunction with volume calculations and petrography reveals changes in volcanic activity over time a total of five eruptive stages are recognised beginning in the stephanian and extending up into the upper rotliegend permian these include the stephanian late carboniferous andesite pre ignimbrite stage i lower rotliegend permian age explosive ignimbrite stage ii post ignimbrite stage iii late rhyolite stage iv and upper rotliegend permian age late basalt stage v in addition several volcanic centres including at least two large caldera like structures are recognised the total calculated volume for the northeast german basin volcanics is ca 48 000 km3 comprising rhyolitic rocks and ignimbrites ca 70 andesitic rocks ca 26 and basalts with cogenetic dolerites ca 4 regional variations in composition and geochemistry are related to diverse crust mantle regimes which correlate with differing positions relative to the heterogeneous basement of the variscan foredeep or within the rhenohercynian orogenic zone the crustal thickening and increasing generation depth of the mafic magmas from north to south may reflect the presence of a pre existing subduction influenced basaltic magma source

incipient metamorphism in the lower palaeozoic marginal basin of wales a diagenctic through anchizone to epizone transition is demonstrated in pelitic rocks of the lower palaeozoic marginal basin of wales by examination of variations in phyllo silicate mineralogy illite crystallinity and bo parameter of white micas this transition represents a temperature range from 150 c to 400 c and the metamorphism is of a low pressure facies series type with a geothermal gradient of 40 ckm 1 variations in grade can be correlated largely with the original basin and shelf form suggesting a depth related metamorphism however in areas closer to the site of caledonian plate collision an increasingly syn tectonic metamorphic event is apparent correlation of pelite data with metabasite assemblages is variable the most consistent relationship being between epizone crystallinity values andepidote actinolite greenschist facies assemblages diagenetic clay mineral assemblages are found associated with prehnite pumpellyite assemblages in metabasites and it is suggested that the latter represent non buffered and therefore non diagnostic assemblages

development of slaty cleavage and degree of very low grade metamorphism a review illite crystallinity ic and other indicators of the grade of very low grade metamorphism associated with the appearance of various stages of slaty cleavage in phyllosilicate rich rocks have been compiled from a wide variety of terranes ic values have been converted to a kubler equivalent standard scale but the diverse characterizations of the cleavage fabrics in published descriptions do not always allow an unequivocal identification of equivalent stages of cleavage development nevertheless there exists a distinct relationship between grade and the appearance of various stages of cleavage development 1 indications of incipient slaty cleavage such as s0 s1 pencil structure appearance of primary s1 crenulation cleavage and of closely spaced cleavage without parallel fabric in the microlithons is associated with a wide range of mostly medium and high grade diagenetic ic values 2 the appearance of smooth cleavage with a strong parallel fabric in the microlithons and or quartz mica beards and the chlorite mica stacks shortened at a high angle to 001 and of irregular cleavage in sandy beds is associated with a much narrower range of predominantly low and medium grade anchimetamorphic grades rarely high grade diagenetic the first appearance of these stages of cleavage development with higher grades can often be related to post kinematic magmatic heating polymetamorphism pre cleavage metamorphism or static recrystallization without cleavage formation for example in low strain zones there exists a relationship between finite strain fabric and metamorphic grade in mudstones and slates in coarser clastic rocks the same finite strain fabric relationship occurs at appreciably higher grades a relationship between finite strain in carbonate rocks and ic in the nearby rocks has been reported from the helvetic zone of the swiss alps the earlier stages of cleavage formation are associated with little improvement in ic the narrow range of ic associated with smooth cleavage is concluded to represent recrystallization and grain growth concurrent with cleavage formation

comment on geochronologic evidence for a cold arc continent collision the taiwan orogeny by r p wintsch h j yang x h li k a tung lithos 125 2011 236 248

potential field imaging of palaeozoic orogenic structure in northern and central europe abstract the trans european suture zone tesz is the most fundamental lithospheric boundary in europe separating the ancient crust of the fennoscandian shield east european craton from the younger crust of central europe and extending deep into the mantle geophysical potential field images provide an overview of the entire palaeozoic orogenic system of northern and central europe for the first time the tesz is largely concealed by sedimentary basins of permian cenozoic age geological observations are largely restricted to local basement highs and deep boreholes and the coverage of deep seismic surveys is widely spaced despite experiments recently acquired within the europrobe programme by contrast the potential field data offer a relatively detailed coverage of standardised observations throughout the tesz while some features of the images may be sourced in the near surface particularly in the gravity image much of their content reflects the structure of the underlying palaeozoic basement at the scale presented the images highlight the most fundamental features of the crustal structure of the tesz these include the strong contrast between the highly magnetic crust of the east european craton and the less magnetic palaeozoic accreted terranes of central europe the lateral continuity of terranes and their internal structure particularly where arc magmatic complexes are involved and the location and geometry of the terrane boundaries oceanic sutures and strike slip zones that separate them

evolution of the lower saxony basin abstract the lower saxony basin is a 300 km long and 65 km wide late jurassic and early cretaceous trough which parallels the northern margin of the rhenish massif this basin became inverted during the senonian and paleocene the lower saxony basin is the most important oil province of germany past production amounts to 200 10 6 t of oil 1 5 10 9 bbl the lower saxony basin is superimposed on the late carboniferous variscan foredeep basin and the frontal structures of the variscan fold belt during the permian to middle jurassic the area formed part of the northwest european basin differential subsidence of the lower saxony basin was initiated during the late jurassic and persisted till albian time this involved reactivation of permo carboniferous fracture systems by divergent wrenching a major aptian deep seated laccolith is evident in the western part of the lower saxony basin during the senonian and paleocene sinistral convergent wrenching induced uplift of the basin floor and folding of its sedimentary fill the margins of the inverted lower saxony basin are thrust over the adjacent stable platform areas the differential subsidence of the lower saxony basin can be related to a period of major crustal extension in the north sea rift basin inversion coincides with important orogenic events in the alpine system and is thought to have been induced by intra plate compressional stresses

evolution of the northeast german basin inferences from a 3d structural model and subsidence analysis abstract a 3d structural model of the northeast german basin was evaluated with special emphasis on its evolution as an intracontinental depression the study includes investigations on subsidence history and structural setting of the basin thickness evolution and calculated tectonic subsidence volumes of permian to quaternary sediments in the northeast german basin indicate that the subsidence history was related to five stages of basin evolution which differ in their subsidence mechanisms for the initial rift phase in the late carboniferous to early permian a dominant thermal event and subordinate horizontal stresses were indicated by thickness variation evolution and by structural evidence the main part of basin subsidence occurred in a nw se oriented basin in the subsequent phase of thermal relaxation with maximum subsidence from early permian rotliegend to middle triassic muschelkalk from middle triassic the thermal subsidence pattern was superposed by further tectonic events in the middle triassic regional extension led to a reconfiguration of the southern part of the basin where new nne ssw trending troughs rheinsberg and gifhorn troughs developed in the jurassic the northwestern part of the basin was uplifted while in the south the keuper subsiding areas continued to sink and nw se trending depressions related to salt margins became important differentiation continued into cretaceous times when regional compression caused uplift of the southeastern part of the basin and basin margins a final subsidence phase occurred in the cenozoic this was accompanied by intensive salt movement recent basin configuration reflects the superposition of structural elements resulting from different evolution stages the main structural characteristics of the basin are 1 a vertical tectonic zonation in a pre zechstein succession which lacks significant internal structures and a strongly deformed post zechstein succession which was decoupled due to the thick zechstein salt and 2 a marked asymmetry of the basin with a shallow northern slope and a steep bounding fault at the southern margin elbe fault system the northwestern part of the basin shows the structural properties of an intracratonic sag basin with persisting subsidence and with minor salt mobilisation in contrast initial structures in the southeastern part are strongly overprinted by younger tectonic events including middle triassic to jurassic extension late cretaceous inversion and late to post cretaceous salt movements tectonic elements that deform the whole sedimentary succession are restricted to the basin s southern eastern and northeastern margins where salt thickness decreases combined volumetric and backstripping investigations show that 2 3 of the total subsidence was induced by the sediment load and 1 3 was caused by tectonics the tectonic subsidence history varies laterally across the basin while tectonic subsidence curves in the northwestern part of the basin show fast subsidence during the permian and early triassic decreasing exponentially with time subsidence curves in the southeastern part indicate repeated tectonic activity however the tectonic subsidence volume created during permian to late triassic is significantly higher than the tectonic subsidence during younger phases of basin history

strain patterns and shortening in a folded thrust sheet an example from the southern appalachians abstract total strain patterns estimated across the pulaski thrust sheet of the southwest virginia appalachians show an approximately homogeneous plane strain deformation associated with folding and distortion above a subsurface decollement estimated strains are low 1 2 x z

on the origin of oriented rutile needles in garnet from uhp eclogites although oriented rutile needles in garnet have been reported from several ultrahigh pressure uhp rocks and considered to be important uhp indicators their crystallographic features including growth habit and lattice correspondences with garnet host have never been properly characterized this paper presents a detailed analytical electron microscopic aem study on evenly distributed oriented rutile needles in garnet of two eclogitic rocks from sulu some garnet in one uhp diamondiferous quartzofeldspathic rock from the saxonian erzgebirge and in one high pressure hp felsic granulite from bohemia also contain a few unevenly distributed oriented rutile needles they have also been studied for the purpose of comparison despite different distribution patterns aem revealed that all rutile needles are oriented along the 111 directions of garnet with their lateral sides surrounded by the 110 planes of garnet and that the growth directions of most needles are close to the normal of the 101 planes of rutile no other specific crystallographic orientation relationships between rutile and garnet host were observed and there is no pyroxene associated with rutile as necessitated by the precipitation reaction of rutile in garnet as previously proposed a simple solid state precipitation scenario for the formation of the rutile needles in garnet in these two eclogitic rocks is not justified three alternative mechanisms are considered for the formation of oriented rutile needles i the rutile needles may be inherited from precursor minerals ii the rutile needles may be formed by a dissolution reprecipitation mechanism and iii the rutile needles may be formed by cleaving and healing of garnet with rutile deposition none of these mechanisms can fully explain the observations although the first one is less likely and the third one is preferred this study presents an example where the presence of oriented aligned inclusions in minerals does not necessarily imply a precipitation origin

the legacy of the european geotraverse abstract the european geotraverse egt created a 4600 km profile across europe from north cape norway to tunisia not only did this produce the first comprehensive cross section to a depth of 450 km of continental lithosphere covering an eighth of the earth s circumference it covered the geological history of europe from the archaean to the present egt built up a detailed knowledge of the crust and upper mantle of europe by integrating geological and geophysical information in a coherent way continuously along a single profile it illuminated the dramatic contrast between the thickness and complexity of the lithosphere of western europe and that of fennoscandia which remain in isostatic equilibrium and elucidated the multilayered elements of thickened alpine crust and lithosphere and the dynamics of the western mediterranean nine years on the legacy of egt is the platform it provided for major scientific advances that have stemmed from it the greatest being through europrobe projects the pancardi project has discovered the subducted slab beneath the carpathian arc in the process of tearing apart from the lithosphere uralides has discovered the urals orogen is almost perfectly preserved since the palaeozoic and the tesz project has elucidated the complex evolution of the transitional region between the baltic shield and the caledonian and variscan crustal terranes of western europe the scientific advances of egt were matched by its achievement in mobilising an international workforce from every discipline of the earth sciences friendships made in that endeavour are another legacy of egt and many of those who started their research careers in egt are now leading europrobe projects and other collaborative ventures we all owe a great debt to professor stephan mueller who founded and led the european geotraverse and was its enduring inspiration

chlorite mica stacks in low strain rocks from central wales weakly deformed low grade lower palaeozoic metasediments from central wales contain abundant stack like intergrowths of chlorite and white mica that closely resemble stacks described from the devonian hunsruckschiefer of west germany the ordovician martinsburg slate new jersey u s a and elsewhere several theories have been proposed to explain the origin of such stacks including a detrital origin strain controlled growth of chlorite on a detrital mica nucleus and strain controlled intergrowth during metamorphism none of these satisfactorily explains the central wales stacks a detrital origin is precluded by the presence of many stacks with shapes too delicate to have survived transportation and a lack of hydrodynamic equivalence between the stacks and the clastic host grains features inconsistent with strain controlled growth are constant alignment parallel to bedding but non systematic orientation with respect to tectonic cleavage their common occurrence in undeformed rocks and petrographic evidence that they precede the tectonic cleavage it is proposed that the stacks formed during diagenesis and low grade metamorphism and before the onset of deformation through mimetic growth on a primary bedding fabric composed of clay minerals

evidence for syntectonic crystallization for the mudstone to slate transition at lehigh gap pennsylvania u s a data primarily for phyllosilicates have been obtained for the continuous transitional sequence from mudstone to slate with well developed slaty cleavage at lehigh gap pennsylvania and for slates from quarries in the same area samples were studied by optical microscopy powder x ray diffraction and electron microprobe analysis with emphasis on transmission and analytical electron microscopy mineral grains are virtually free of deformation induced strain concomitant with the gradual development of cleavage normal to bedding the following changes are observed or confirmed 1 the orientation of phyllosilicate grains changes discontinuously from being preferentially parallel to bedding to being parallel to cleavage 2 crystal imperfections as expressed in layer terminations low angle grain boundary like features and other defects decrease in density 3 complex mixed layering is replaced by homogeneous packets of layers of single phases and 4 illite transforms to muscovite with increase in k a1 and change from a lmd to 2m polytype slaty cleavage apparently develops due in part to pressure solution of phyllosilicates oriented parallel to bedding mass transport of components and crystallization to form new grains parallel to cleavage it reflects transitions from imperfect metastable phases toward ordered stable phases in a low temperature 225 c metamorphic environment

on the origin of the southern permian basin central europe abstract a detailed study of the structural and stratigraphic evolution of the southern permian basin during latest carboniferous to early jurassic times supported by quantitative subsidence analyses and forward basin modelling for 25 wells leads us to modify the conventional model for the rotliegend zechstein development of this basin the late permian early jurassic tectonic subsidence curves are typical for a permian to early triassic extensional stage that is followed by thermal subsidence however a purely extensional model is extremely problematic because active faulting during this time is minor and generally hard to document using inverse techniques to model the subsidence curves we quantitatively show that a significant component of late permian and triassic tectonic subsidence can be explained by thermal relaxation of early permian lithospheric thinning and by delayed infilling of paleo topographic depressions that developed during the early permian in this interpretation stephanian autunian wrenching resulted in thermal destabilisation of the lithosphere deep fracturing of the crust disruption and erosion of its sedimentary cover and regional uplift of the area of the future southern permian basin upon termination of wrench tectonics and associated volcanism towards the end of the autunian the southern permian basin began to subside in response to thermal contraction of the lithosphere the evolving basin was isolated from the world oceans and had subsided possibly up to some 700 m below their level at the beginning of upper rotliegend sedimentation after catastrophic flooding of this paleo topographic depression at the beginning of the zechstein changing sea level sedimentation and subsidence rates remained essentially in balance although the effects of triassic rifting overprinted parts of the southern permian basin its overall subsidence pattern persisted well into the jurassic in contrast to the remainder of the southern permian basin permian and triassic crustal extension contributed significantly to the subsidence of the polish trough

the southern margin of the east european craton new results from seismic sounding and potential fields between the north sea and poland abstract the extension of eastern avalonia from britain through the ne german basin into poland is in some sense a virtual structure it is covered almost everywhere by late paleozoic and younger sediments evidence for this terrane is only gathered from geophysical data and age information derived from magmatic rocks during the last two decades much geophysical and geological information has been gathered since the european geotraverse egt which was followed by the babel lt 7 mona lisa dekorp basin 96 and polonaise 97 deep seismic experiments based on seismic lines a remarkable feature has been observed between the north sea and poland north of the elbe line el the lower crust is characterised by high velocities 6 8 7 0 km s a feature which seems to be characteristic for at least a major part of eastern avalonia far eastern avalonia in addition the seismic lines indicate that a wedge of the east european craton eec or baltica continues to the south below the southern permian basin spb a structure which resembles a passive continental margin the observed pattern may either indicate an extension of the baltic crust much farther south than earlier expected or oceanic crust of the tornquist sea trapped during the caledonian collision in either case the data require a reinterpretation of the docking mechanism of eastern avalonia and the elbe odra line eol as well as the elbe fault system together with the intra sudedic faults appear to be related to major changes in the deeper crustal structures separating the east european crust from the paleozoic agglomeration of middle european terranes

palaeozoic terranes and their lithospheric boundaries within the trans european suture zone tesz a review the trans european suture zone tesz is a broad and complex zone of terrane accretion separating ancient lithosphere of the baltic shield and east european craton eec from the younger lithosphere of western and southern europe there is debate about the number of terranes involved and their origins the most significant terrane boundaries originally oceanic sutures are poorly exposed and are defined using faunal provinciality prominent geophysical lineaments may represent sub terrane boundaries within composite terranes the avalonia composite terrane was amalgamated to baltica in latest ician time the provenance of the malopolska and moravo silesian terranes whose neoproterozoic basement is suspect with respect to baltica but with which they nonetheless share faunal affinities is still debated the extent of the variscide rhenohercynian zone defines the southern limit of the early palaeozoic accreted terranes terranes within the bohemian massif exhibit hp metamorphism recording subduction driven collision prior to incorporation in the tesz collage itself associated with a hp event in romania the saxothuringian zone and rheic suture are directly juxtaposed with the eec and the crustal structure of the tesz has been much modified by post palaeozoic tectonism deep seismic reflection data where available show that the oceanic sutures are frequently associated with inclined reflectivity zones irz in the lithospheric mantle here inferred to mark relict subduction zones interpreting the age of subduction is complicated by offset of originally coplanar mantle and crustal segments of the irz resulting from early orogenic lithospheric delamination at the moho interface and or post orogenic reordering processes the mantle irz appear to indicate a n directed polarity for early palaeozoic subduction apparently incompatible with the geological evidence which indicates subduction of the tornquist sea beneath avalonia and a large amount of underthrusting of avalonia by baltica possible explanations for these apparently incompatible observations are evaluated

biotite percussion figures in naturally deformed mylonites abstract under experimental conditions characteristic fracture patterns can be produced on cleavage plates on mica by using a blunt tool if stress is applied rapidly by striking the surface in a controlled way a pattern known as the percussion figure is produced when the stress is applied by steady pressure on the tool a different but complementary pattern of fracture is formed in sum these induced fractures constitute the pressure figure the orientation of each of these two sets of fractures with respect to the optical axial plane oap of mica is different and therefore diagnostic of the manner in which they are produced these patterns are distinct from those formed as a result of exsolution of fe ti oxides which are commonly visible in sections of biotite cut parallel to the basal plane 001 a description is given of percussion figures produced by natural deformation in biotites from mylonite belts cutting the proterozoic metasediments of the feidong group in eastern anhui province and another from yunnan province china the principal fracture of the natural percussion figure evidently is parallel to the oap of the biotite and the other two sets are quite distinct as well thus identifying it really as a percussion figure microscopic inclusions of sphene also are located along the crystallographically controlled fracture planes of the percussion figures the data indicate that high strain rates would be required to form these natural percussion figures and that a special history of deformation must have affected the mylonites in which they occur it is proposed that the homogeneous deformation of the mylonite in a ductile regime was complicated by strain hardening which led to episodes of abrupt stress itself relief stick slip at rates of strain high enough to induce the formation of percussion figures in the biotites

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

relations between stages of diagenetic and metamorphic evolution and the development of a primary cleavage in the northwestern moroccan meseta abstract the metamorphic evolution of the studied region is determined by the illite crystallinity index and by the evolution of the other phyllosilicates essentially chlorite and mixed layer minerals several zones are described at a regional scale characterized respectively by a diagenetic anchimetamorphic and epimetamorphic evolution the estimation of total strain is approached by the type of the cleavage in greywackes mapping of all these parameters reveals at the scale considered a close relationship between metamorphism and strain which is briefly discussed

the rb sr isotopic record in taiwan gneisses and its tectonic implication abstract a rb sr isotopic study of six taiwan gneiss bodies has revealed three thermal events at ca 90 ma ca 40 ma and

seismic structure across the caledonian deformation front along mona lisa profile 1 in the southeastern north sea abstract seismic data from the mona lisa marine and onshore north sea acquisition for lithospheric seismic analysis project in the southeastern north sea image the caledonian deformation front cdf which is the collisional suture between baltica to the north and east and avalonia to the south and west the n s trending mona lisa normal incidence reflection profile 1 was recorded to 26 s twt coincident wide angle data were acquired on nine ocean bottom hydrophones and several onshore mobile seismographs along and off the profile the model of compressional seismic velocity shows three different crustal types a a typical three layered shield type crust below the ringkobing fyn high to the north b a highly complex transitional crust in the central part and c a two layered crust of caledonian origin to the south sharp and strong normal incidence and wide angle reflections from moho were recorded south of the caledonian deformation front in contrast to less distinct reflections further north s dipping crustal reflections from 4 to 11 s twt over 70 km horizontal distance terminate at moho and coincide with a change in the seismic velocity structure this indicates northward obduction of avalonian crust non migrated normal incidence seismic sections show crossing weak n dipping and stronger s dipping reflections to 20 s twt from the uppermost mantle we propose a tectonic model where the closure of the tornquist sea took place along a n dipping subduction zone which was later overprinted by a late or post caledonian s dipping shear zone sub moho velocities are 7 8 7 9 km s under 34 35 km thick baltica crust and 8 1 8 3 km s under 25 26 km thick caledonian crust the sub horizontal moho across the caledonian collision zone implies late or post caledonian re equilibration of the seismological moho we interpret the low velocity upper mantle 7 8 8 1 km s to the north as former baltica lower crust in eclogite facies after pressure induced metamorphism as a result of lithospheric flexure during the caledonian orogeny these rocks today appear as upper mantle that was uplifted to their present position during the middle devonian collapse of the north german polish caledonides

geophysical characteristics of the tornquist fan area northwest trans european suture zone indication of late carboniferous to early permian dextral transtension

reflection seismic constraints on paleozoic crustal structure and moho beneath the ne german basin abstract a 330 km long ne sw trending seismic transect across the ne german basin combined with a 75 km long perpendicular cross line in the basin centre reveals the structure of this intracontinental basin between the stable precambrian baltic shield and the weaker mid european variscides the combination of high fold vibroseis and low fold explosive seismic profiling images both upper and lower crustal structures in detail distinct faults related to a rift stage are not observed below the permian sedimentary basin but there is evidence of lower crustal thinning a moho pattern less reflective below the basin centre than at the margins may be related to magmatic underplating the rather flat lying moho dips slightly towards the basin margins structures further north suggest a southern extent of baltica below the ne german basin the caledonian accretionary wedge may be outlined by sw dipping mid crustal reflectors beneath the basin

some comparisons of the structure and evolution of the southern appalachian ouachita orogen and portions of the trans european suture zone region abstract many aspects of the tectonic history of the appalachian ouachita orogen and the trans european suture region are similar in a tectonic sense they are geographically linked because at the end of the paleozoic one could travel from texas to poland following a continuous orogenic belt much of each of these orogenic belts is however buried by younger strata and thus geophysical studies are a key element of efforts to better understand them many more geophysical data are needed to provide a comprehensive picture of the deep structure but in several areas new models of the lithospheric structure are emerging in this paper we review some of these models and introduce two new ones for the southern appalachians in addition we explore the implications of these models for the evolution of these orogenic belts drawing attention to similarities and differences in their structure and evolution the appalachian and caledonian orogens represent temporally parallel development along irregular late proterozoic continental margins but the crustal structure is different because of different accretionary elements and complications in the geometry of the original margins lateral transport via thrusting can confuse efforts to tie geologic structures identified on the surface with deep structures for example in the appalachian ouachita orogen the thrust related transport distance for major features such as the blue ridge piedmont sheet and the benton uplift is at least 200 km on a crustal scale deformation in the appalachian ouachita orogen varies greatly in style and intensity in the ouachita orogen the late proterozoic continental margin seems to be preserved while in the northern appalachians crustal scale deformation is intense along the us appalachians the major crustal structure variation is the thin dense crust of avalonia abutting the relatively thick crust of laurentia along the ouachita orogen several large crustal blocks can be identified that are unlike anything seen along the appalachians

relationship of concretions and chlorite muscovite porphyroblasts to the development of domainal cleavage in low grade metamorphic deformed rocks from north central wales great britain abstract spaced or domainal cleavage is widespread in deformed rocks of low metamorphic grade this study presents evidence on the origin of spaced cleavage in deformed pelites from silurian turbidite sequences in north central wales the variable development of cleavage folia is related to the presence of concretions which served to concentrate strain effects of well developed cleavage folia in the matrix in some zones and to produce strain shadows few or no cleavage folia in others compositional differences between matrix narrow transition zones and concretions also influenced the development of cleavage folia chlorite muscovite porphyroblast growth was initiated before cleavage formation but its further growth and development took place during cleavage formation the relationship of cleavage to the concretions and to the chlorite muscovite porphyroblasts indicates that cleavage developed in a matrix that must have been in an advanced state of lithification anchimetamorphic to lowermost greenschist facies of metamorphism and not during soft sediment deformation

bundled slaty cleavage in laminated argillite north central minnesota abstract exceptional bundled slaty cleavage defined herein has been found in drill cores of laminated folded weakly metamorphosed argillite at several localities in the early proterozoic animikie basin of north central minnesota the cleavage domains are more closely spaced within the cleavage bundles than outside them the mean tectosilicate grain size of siltstone layers measured normal to cleavage is less in the cleavage bundles than outside them and the cleavage bundles are enriched in opaque phases and phyllosilicates relative to extra bundle segments these facts suggest that pressure solution was a major factor in bundle development if it is assumed that opaque phases have been conserved during pressure solution the modal differences in composition between intra bundle and extra bundle segments of beds provide a means for estimating bulk material shortening normal to cleavage argillite samples from the central part of the animikie basin have been shortened a minimum of about 22 as estimated by this method these estimates are similar to the shortening values derived from other strain markers in other rock types interbedded with the argillite and are also consistent with the regional pattern of deformation

crustal structure along the central segment of the egt from seismic refraction studies abstract the seismic refraction line between the gulf of genova and the baltic sea was carried out in 1986 as part of the european geotraverse egt 86 the central segment of the egt also known as eugemi european geotraverse middle part has a total length of 825 km and runs from the northern margin of the alps to the baltic sea traversing mainly the variscan mountain area of western germany the p wave data of nine shotpoints were interpreted in detail and a two dimensional p velocity model of the crust was developed between the molasse basin and the southern end of the north german basin for the north german basin the data quality justifies only the construction of a simplified model the internal crustal structure is quite complex in the upper crust of the moldanubian and saxothuringian zones of the variscan orogen the velocities are lower than in the rhenohercynian area the velocity of 6 0 km s representing consolidated seismic basement is reached at depths of as much as 10 12 km northern margin of alps and north german basin and of only 3 km under the mid german crystalline rise mid crustal velocities range from 5 8 to 6 1 km s forming weak velocity reversals in some parts in the lower crust velocities as low as 6 2 km s are seen near the alps in contrast to velocities between 6 8 and 7 2 km s in the exposed variscan orogen north of shotpoint e between hannover and the river elbe the velocity of the lower crust is about 6 4 km s north of the elbe line it increases abruptly to 6 9 km s the crust mantle boundary rises from 34 km depth under the northern margin of the alps to 29 km under the swabian jura under the variscan orogen to the north the crustal thickness varies only slightly around an average depth of about 29 1 k but drops to about 30 km under the north german basin between the elbe line and the ringkobing fyn high the moho rises to a depth of about 26 km and the data indicate at least one fault like step of 1 km before the crust thickens toward the basement high the velocities in the uppermost mantle show considerable variation the highest velocity of 8 4 km s is observed beneath the saxothuringian zone and the lowest values of 7 8 7 9 km s are recorded under the hessen depression

seismic structure of the palaeozoic platform along polonaise 97 profile p1 in northwestern poland abstract the deep seismic profile p1 was conducted in northwestern poland as part of the polonaise 97 project data acquisition involved 100 mobile seismographs which recorded seismic waves from 11 dispersed chemical controlled explosions along this 300 km long profile a variety of seismic modelling techniques were employed to derive a velocity model along the profile this model indicates that the mesozoic and palaeozoic sediments are 6 km thick in the northwest and 3 km thick in the southeast with the main thickness variation taking place in the palaeozoic succession an undulating basement surface indicates tectonic activity during deposition of the lower palaeozoic sequence the crust is 30 33 km thick along the profile thickening gradually to the southeast the p wave seismic velocities are low 1 down to a depth of 20 km along the whole profile which we interpret as evidence for their being of sedimentary or volcanic origin the lower crust has a high p wave velocity 6 7 7 5 km s 1 high vertical velocity gradient and strong ringing reflectivity the velocity of the sub moho mantle p n is high 8 2 km s 1 features similar to the ones below this profile have been interpreted for the area south of the caledonian deformation front in northern germany and southern denmark these similarities indicate that the crust between the southeastern north sea and poland has a common origin we therefore suggest that the deep crust beneath the profile is not of variscan origin despite its location south of the variscan deformation front we propose that variscan tectonism in the area was thin skinned while the deeper crust may originate from the caledonian microcontinent east avalonia or possibly from baltica

seismic images of caledonian lithosphere scale collision structures in the southeastern north sea along mona lisa profile 2 abstract the unexposed suture between baltica and eastern avalonia is imaged by coincident normal incidence reflection and wide angle reflection refraction seismic data of the mona lisa project we present new results of the upper lithospheric seismic structure from the n s striking profile 2 across the caledonian deformation front which represents the crustal collision suture between baltica and eastern avalonia that formed after closure of the tornquist sea during the caledonian orogeny in late ordovician times three different crustal types are identified with great similarities to the nearby profile 1 1 a three layered crust typical of shields to the north 2 a transitional crust suture zone in the central part and 3 a two layered crust of caledonian origin to the south characterized by very low velocities throughout the crust the crustal thickness varies from 38 35 km under the northern margin of the ringkobing fyn high baltica crust to 28 27 km beneath the north german basin in the caledonian crust to the south the suture zone is imaged by s dipping crustal reflections from 1 9 to 10 6 s two way travel time twt over a horizontal distance of 70 km within the transitional crust the reflection suture zone terminates in a 60 km wide reflective lens with velocities of 6 6 6 8 km s in the lowermost crust the reflective lens may be interpreted as a remnant of oceanic or island arc crust that was accreted to the leading edge of baltica during closure of the tornquist sea alternatively the reflective lens may represent an indentor of baltica crust into the avalonian terrane the change in lower crustal reflectivity and the abrupt transition from a three to a two layered crust further south suggest that the trans european fault and the elbe lineament continue into the north sea the upper mantle reflectivity is dominated by bands of strong amplitude s dipping reflections from 13 5 to 21 8 s twt and a weaker band of n dipping reflections from 12 to 16 s twt reversed wide angle refractions and reflections indicate the presence of a s dipping high velocity layer 8 65 8 8 km s in the uppermost mantle the sub moho high velocity layer partially coincides with the strong s dipping normal incidence mantle reflections we propose that the s dipping mantle structure may represent a late caledonian or late carboniferous to early permian mantle shear zone

grain growth and re orientation of phyllosilicate minerals during the development of slaty cleavage in the south kitakami mountains northeast japan abstract using x ray diffraction analysis the mutual relations among illite crystallinity degree of preferred orientation of chlorite grain size change of chlorite and illite during metamorphism and development of slaty cleavage have been investigated for argillaceous rocks in the south kitakami mountains northeast japan the metamorphic grade of ic illite crystallinity index 0 29 is a critical one beyond which the homogenization of chlorite composition coarsening of chlorite and illite grains and degree of preferred orientation of chlorite are abruptly advanced grain coarsening is also promoted by the development of slaty cleavage especially in the range of coarser grain size the oriented growth by the effects of both the anisotropy of intrinsic growth rate of mineral grains and that of the environment in which grains grow is considered to bring about the preferred orientation of chlorite and illite

titanium aluminum and interlayer cation substitutions in biotite from high grade gneisses west greenland detailed electron microprobe study pelitic to mafic metamorphic rocks from two localities higher ti k k na and possible k na similar al iv and lower al vi in granulite vs amphibolite grade samples modified journal abstract

caledonian and proterozoic terrane accretion in the southwest baltic sea abstract a marine seismic reflection survey was carried out in 1996 by dekorp potsdam and bgr hannover in the sw baltic sea several tectonic lineaments were crossed nearly perpendicularly for example the caledonian deformation front which is assumed to mark the northern boundary of the terrane avalonia which accreted to baltica ca 400 ma here a bivergent collision is clearly observed confirming early ideas from the babel survey the ne dipping reflections in the uppermost mantle are interpreted as signs of the subducted tornquist ocean a similar tectonic style of compression and indentation is observed in the proterozoic crust northeast of bornholm where in addition a remarkable crustal thickening and a strong increase of reflection power is observed the three dekorp lines in this area provide a certain three dimensional control and allows extension of similar observations from the babel line a southward this proterozoic terrane accretion seems to be connected to major tectonic lineaments in southern sweden either to the gothian thrust or the sveconorwegian front

strain partitioning and crack seal growth of chlorite muscovite aggregates during progressive noncoaxial strain an example from the slate belt of taiwan abstract chlorite muscovite aggregates from the slate belt of southeastern taiwan have an ellipsoidal geometry k 1 on a flinn diagram with an aspect ratio a c ranging from about 7 1 to 12 1 and a crystallographic preferred orientation with the basal planes of the chlorite and muscovite at a high angle to the slaty fabric in the rock the principal axes of the aggregates are approximately coaxial with the principal finite strain directions determined from fibrous pressure shadows in cleavage parallel sections straight fibers parallel to the downdip mineral lineation record plane strain in cleavage perpendicular sections however pressure shadows have curved fibers that are consistent with w directed thrusting we propose a model in which the chlorite mica aggregates grow syntectonically by cracking of detrital chlorite grains parallel to the basal planes and precipitation of secondary chlorite and mica in the open cracks this model involves 1 rotation of preexiting detrital chlorite grains during progressive noncoaxial deformation 2 cracking of these grains parallel to the basal plane when the 001 planes rotate into the incremental shortening field with nucleation and growth of secondary chlorite and muscovite in the open microcracks as the aspect ratio increases the grains stabilize in orientation with the long axes normal to the basal planes at a small angle to the shear direction extension normal to 001 is also accompanied by some shortening parallel to 001 but with an overall increase in size of the aggregates the final grain size is controlled by the cumulative displacement associated with crack seal events microprobe analyses of both detrital and secondary chlorite grains show distinct compositional differences within individual grains perpendicular to 001 particularly in the detrital grains which we interpret as primary and secondary chlorite consistent with a primary and secondary origin for the aggregates

thermotectonic evolution of the western margin of the yilgarn craton western australia new insights from 40 ar 39 ar analysis of muscovite and biotite abstract the post cratonisation history of the yilgarn craton is somewhat fragmentary due to incomplete or absent stratigraphic records for overlying sediments however the exposed archean crystalline rocks can provide important constraints on the missing thermotectonic history if appropriate thermochronological methods are used previous thermochronology studies from the southwestern yilgarn craton suggest that it was subjected to late pan african tectonism and east west compression resulting in tectonic loading thrusting of sediments onto basement rocks along its western margin however this proposed tectonic model is based largely on bulk rb sr biotite analyses with minimal petrological and geochemical control this study across a 80 km e w transect along the western yilgarn craton employed laser 40 ar 39 ar incremental heating experiments on muscovite and single biotite grains from well documented sample sites broadly comparable to those sampled previously for rb sr biotite analysis muscovites record relatively consistent 40 ar 39 ar ages ranging from 2220 to 2450 ma coexisting biotite 40 ar 39 ar results however reveal significant age variations at both intra and inter grain scales single grain biotite 40 ar 39 ar dating results show consistent ages of 2480 2580 ma across much of the yilgarn craton with ages decreasing abruptly to 1000 1100 ma over a narrow transitional zone 30 40 km wide before decreasing further to 620 630 ma along the western margin of the craton petrographic observations indicate that most biotite grains especially in the transitional zone and western margin are severely chloritized and contain abundant titanite inclusions that may have resulted from fluid induced recrystallisation under greenschist facies metamorphism the transitional zone 40 ar 39 ar biotite ages are temporally compatible with metamorphism of the adjacent northampton and mullingarra complexes suggesting either their juxtaposition along the western margin of the yilgarn craton during the mesoproterozoic pinjarra orogeny or coincidental partial re equilibration at this time during younger thermal events the westernmost biotite recrystallisation ages of 620 630 ma support palaeomagnetic indications of oblique collision between great india and the australian continent during gondwana amalgamation

an integrated study of the ne german basin abstract the ne german basin is part of the southern permian basin south of the transeuropean suture zone tesz here we report an attempt to integrate a variety of geological and geophysical data in order to reveal the present day deep crustal structure of the ne german basin special focus is taken on detailed geological information available reflection seismic data wide angle refraction seismic data and gravity data based on this integrative approach a concise crustal model is developed which can be subject to further evaluation furthermore it is shown that the ne german basin is an outstanding feature quite different from classical basin types as the moho is flat below the basin neither a simple shear nor a pure shear extension model can be applied instead a thick high velocity lower crustal layer is present below the basin centre this high velocity lower crust is also observed in the western part of the basin and can be taken as characteristic for east avalonia results of gravity modelling indicate the presence of a high density lower crust thus supporting the wide angle data furthermore changes in crustal structure can be derived from changes in reflectivity pattern and from the gravimetric signature along the elbe fault system indicating the presence of different crustal domains north and south of the fault system an elastic plate model where the forces applied are the sediment load a vertical load at the southern margin accounting for the harz moutains and a nne ssw directed compression indicates a buckling of the crust during late cretaceous to tertiary inversion however the correlation between the observed crustal structure and the tectonic events that have affected the area remains a subject of discussion a permian thermal event affected the upper mantle and the lower crust and therefore may have modified the crustal memory with regard to caledonian and variscan events additional tectonic events affected the area during the mesozoic in conclusion it remains an open question whether the high velocity and high density lower crustal structure represents a remnant of east avalonia whether it is related to the formation of the permo triassic basin or is the cumulative result of a series of events during the palaeozoic and mesozoic

geophysical evidence for early permian igneous activity in a transtensional environment denmark abstract integrated interpretation of gravity magnetic and seismic data in the area of the strong silkeborg gravity high in central denmark has led to a structural geological model involving a transtensional rift feature which probably formed in connection with permo carboniferous wrench faulting and the development of the oslo horn graben system reinterpretation of the deep seismic data from profile 2 of the eugeno s project in the area of the silkeborg gravity high detected a new thin high velocity layer 6 25 km s at about 7 km deep within the palaeozoic sedimentary sequence in the crystalline basement a separate body of high velocity 6 9 7 0 km s was found at 11 km deep beneath this body the moho reaches its shallowest position in the area the seismic results compare well with an interpretation based on magnetism of a shallow volcanic or subvolcanic layer at a depth of 6 8 km this layer shows strong remanent and induced magnetization with a southward inclination of about 10 gravity reinterpretation of the area allows the separation of an upper body of density contrast 0 1 g cm3 from a lower body of density contrast 0 2 g cm3 the upper body roughly coincides with the magnetic body the lower body the major one coincides with the deep seismic body and it is responsible for the main gravity anomaly in the area

gradational development of domainal slaty cleavage its origin and relation to chlorite porphyroblasts in the martinsburg formation eastern pennsylvania abstract slaty cleavage frequently consists of narrow folia or domains of phyllosilicates alternating with microlithons richer in quartz within cleavage domains the 001 planes of phyllosilicates are usually parallel or subparallel to the domain boundaries but they may be at a small but persistent angle in microlithons the phyllosilicates are commonly not parallel to the cleavage domains domainal cleavage may grade into pervasive slaty cleavage with few or no microlithons and a parallel orientation of all the phyllosilicates or may grade within distances as short as a few millimeters into pelite with no cleavage and with the phyllosilicates either with no preferred orientation or a weak to strong orientation parallel to bedding study of cleavage specimens mainly from the martinsburg formation of pennsylvania and new jersey indicates that cleavage development is related to strain variation and to volume loss at least as measured by deformed quartz calcite veins in the xz plane of a fold mass transfer of the more soluble constituents under conditions of low grade metamorphism chlorite zone and under the influence of deviatoric stress leads to the development of solution ways initiated at heterogeneities in the fabric which result in solution cleavage that is extended by terminal and lateral propagation stress gradients result in gradational development of solution cleavage with minimal development in strain shadows the solution ways develop into cleavage domains by accumulation of inert materials particularly carbonaceous grains iron ores such as framboidal pyrite and phyllosilicates however the preferred orientation of phyllosilicates is mainly the result of crystallization of new grains in the domains because of the breakdown of detrital or diagenetic phyllosilicates and of other minerals such as feldspar or epidote under low grade metamorphic conditions this interpretation is supported by lack of bent or deformed phyllosilicates in and along the boundaries of cleavage domains larger detrital phyllosilicates with 001 at a high angle to cleavage commonly show deformation further support is provided by the occurrence of chlorite grains with their 001 planes approximately parallel to the cleavage in strain shadows on competent grains such as framboidal pyrite and by the shape change of diagenetic chlorite porphyroblasts as traced from zones with no domainal cleavage into zones with well developed domainal cleavage mechanical rotation appears to be of minimal importance in the development of cleavage domains that are unrelated to a crenulation structure there will be grain boundary sliding and other adjustments as a consequence of removal of material along the solution ways deformation of a rock possessing a strong preferred orientation of phyllosilicates parallel to bedding or to a previous cleavage may result in the development of a crenulation structure and thenec a differentiated crenulation cleavage mechanical re orientation will in this case be a more important element in formation of cleavage although mass transfer and crystallization are still necessary to produce a domainal cleavage

fine scale chlorite muscovite association in low grade metapelites from nurra nw sardinia and the possible misidentification of metamorphic vermiculite chlorite muscovite porphyroblasts occur in the low grade metapelites from nurra nw sardinia although actually heterogeneous and built up by two different intermixed minerals the porphyroblasts strongly simulate optical homogeneity and produce microprobe analyses that are typical for metamorphic vermiculite transmission and scanning electron microscopy combined with analytical facilities leads to a deeper characterization of the porphyroblasts that actually consist of a random association of chlorite and muscovite

upb and sr isotopic studies on granitoids from taiwan and chinmen lieyu and tectonic implications abstract u pb zircon monazite dating and sr isotope results for granitoids from taiwan and chinmen lieyu se china place important constraints for the tectonic evolution and the timing of felsic magmatism associated with mid cretaceous subduction process the chinmen gneiss 139 4 ma and granite 100 9 ma represent the oldest granitic units and belong to the previously recognized late yanshanian phase of magmatism a younger suite of granitoids occurs in the tananao metamorphic complex of taiwan and includes the yuantoushan and tachoshui granites 87 3 ma and the kanagan gneiss 90 2 ma these mid cretaceous ages for the younger granitoid suite mitigate against the existence of the hypothetical late paleozoic dongnanya microcontinent in the study area as proposed by other investigators u pb zircon inheritance ages ranging between 1 67 and 2 09 ga indicate the presence of a precambrian basement beneath much of coastal se china and taiwan however there is no evidence of archean crust as indicated by zircon inheritance patterns for rocks west of the present study area on a 87 sr 86 sr vs 87 rb 86 sr plot the available data for the granitoids in chinmen and coastal fujian se china however define pseudoisochrons with apparent ages of 158 167 ma although the pseudoisochron could be interpreted as a result of a two end member mixing process with subsequent fractional crystallization the apparent age of the pseudoisochron substantiated by the circumstantial evidence is postulated as the approximate time of sr isotope homogenization of the crustal protoliths of the granitoids during the previous early yanshanian tectonic event in this region such a sr isotope homogenization process must have occurred over a large area possibly facilitated by a metamorphic fluid phase or by large scale anatexis the cretaceous granitoids in this area were probably produced by partial melting of this crust in a more restricted scale the cretaceous calc alkaline granitoids in coastal fujian and taiwan are of i type or contaminated i type a subduction component is inferred during the magma genesis although the taiwan granitoids are suggested to be subduction related the possible tectonic environment for the genesis of coastal fujian granitoids is yet to be solved

the origin and sedimentary diagenetic and metamorphic evolution of chlorite mica stacks in llandovery sediments of central wales u k chlorite mica stacks in the llandovery sedimentary rocks of central wales have been studied both within cleaved turbidite mudrocks and within early diagenetic apatite carbonate and monazite concretions stacks show a graded distribution within turbidite units indicating an origin as detrital particles concretions provided an arrested diagenetic environment which restricted subsequent deformation and so revealed the nature of the stack precursor minerals these included detrital mica and also probable pyroxenes amphiboles and volcanic rock particles relic textures of which can be seen preserved within the concretions diagenetic alteration to chlorite probably with some white mica intergrowth occurred most probably via smectite and then mixed layer or swelling chlorite intermediates subsequent modification during the caledonian cleavage forming episode determined the present shape and mineralogy of the stacks outside concretions chlorite pseudomorphs were rotated by tectonic compression crushed at their margins and pulled apart along 001 cleavage and slip planes growth of at least two generations of secondary mica infillings has been recognized and this was probably accompanied by the growth of secondary chlorite this resulted in barrel shaped stacks elongated parallel with cleavage and with the 001 plane rotated from the bedding towards the perpendicular to the cleavage within early diagenetic concretions it is evident that some stacks had formed prior to lithification however these have a relatively simple morphology and the original bedding parallel grain shapes are largely preserved

polonaise 97 an international seismic experiment between precambrian and variscan europe in poland abstract the trans european suture zone tesz comprises a series of sutures that developed by palaeozoic amalgamation of the phanerozoic central europe onto the proterozoic baltica and east european plate during the formation of pangea within tesz the permian basin in poland forms the easternmost part of the permian central european basin which is bordered on the east by the east european craton eec and on the southwest by the bohemian massif the axis of the basin the mid polish trough parallels the edge of the eec the teisseyre tornquist zone ttz is a geological inversion zone in the polish trough a large seismic experiment the polonaise 97 project was conducted in poland during may of 1997 and targeted the deep structure of the tesz and the complex series of upper crustal features associated with it it included contributions from the geophysical communities in poland denmark the usa lithuania germany finland sweden and canada this large lithospheric seismic experiment deployed 613 instruments to record 64 shots along five profiles with a total length of about 2000 km moreover five multichannel seismic reflection stations 90 and 120 channels recorded all signals from shots two dimensional velocity models were derived by p wave tomographic inversion these models provide an initial interpretation of this massive data set and allow us to present a few significant seismic and tectonic observations one of the most important is a very distinct asymmetry between the maximum thickness of the sedimentary cover in the polish trough 16 20 km and the crustal root 50 km associated with tesz ttz

tem and aem constraints on the origin and significance of chlorite mica stacks in slates an example from central wales u k chlorite mica stacks grains of intergrown chlorite and white mica in a matrix of fine grained white mica and chlorite have been studied using xrd sem empa tem and aem methods the stacks occur in a weakly cleaved llandoverian mudstone central wales which has a white mica illite crystallinity index of 0 35 a20 corresponding to the lower anchizone white mica occurs as packets 100 to 8 m thick interleaved with dominant chlorite packets in stacks with both apparent coherent interfaces or cross cutting small angle boundaries with chlorite layers it is well crystallized 2m polytype with phengitic composition and low paragonite component na na k 0 0 07 chlorite in stacks is fe rich and relatively homogeneous tio2 crystals surround stacks and occur within chlorite and white mica is ti rich compared to matrix white mica fine grained white mica and chlorite in the matrix have two different orientations one parallel to bedding and one parallel to cleavage which is approximately 30 50 to bedding matrix white mica is predominantly a 2m1 polytype but some cleavage parallel white mica is 3 t and some bedding parallel white mica is 1md it is na rich na na k 0 14 0 42 and relatively heterogeneous some discrete paragonite and phengitic muscovite are observed to coexist in the cleavage orientation matrix white mica and chlorite each contain less fe than corresponding white mica and chlorite in stacks both matrix and stack chlorite are one layer polytypes the data imply that chlorite in the stacks is largely derived from the replacement of volcanogenic biotite and other ferromagnesian minerals probably via intermediate expandable trioctahedral phyllosilicates most intergrowths of chlorite and mica in stacks formed by mica replacement of chlorite and altered biotite along cleavage fissures subsequent deformation caused further modification of pre existing chlorite mica stacks whereas partial dissolution of stacks and bedding parallel matrix phyllosilicates resulted in the formation of cleavage parallel phyllosilicates

nature of crustal reflectivity along the dekorp profiles in germany in comparison with reflection patterns from different tectonic units worldwide a review reflectivity of the continental crust displays many different patterns the dekorp lines are used as a basis for comparing and reviewing reflectivity in different tectonic units the brittle upper crust generally exhibits only two types of reflectivity it is either rather transparent preferably in some extensional provinces or and it shows traces of thrust and shear zones of former or present ruptures as these zones have a low impedance interior with few exceptions their first reflection onsets have a negative polarity and evince strong but short signals which sometimes can be correlated over several kilometers the generally ductile lower crust displays a completely different reflectivity in warm extensional and thin crusts the lower part is full of reflecting lamellae it is suggested that this type of reflectivity has a thermo rheological origin the creation of lamellae must take place in a ductile material with contrasting impedance under extensional stresses it can be associated with mineral alignment and corresponding seismic anisotropy destruction of lamellae may take place by a cooling process transforming parts of the lower crust into a brittle regime small stresses might deform or break the lamellae and leave a certain dispersed reflectivity like that in some old and cold shields there are no observations of reflecting lamellae in the upper crust or in the upper mantle in all areas the moho is the last reflecting band reflection moho which most often is identical with the classical refraction moho there are isolated mostly dipping reflections in the uppermost mantle in zones where the last tectonic event a delamination or subduction was not succeeded by a heating process the uppermost mantle is brittle again in most areas and may keep the memory of a cold collision over billions of years

rejuvenation of kar systems for minerals in the taiwan mountain belt the late mesozoic amphibolites and granitic rocks in the basement complex of the taiwan mountain belt are overprinted by late tertiary greenschist facies metamorphism the kar isotope systems of late mesozoic horn blende muscovite biotite and microcline have been reset by this metamorphic event as reflected in three systematic variations 1 a regional decrease in40ar 39ar dates with increasing metamorphic overprinting 2 a unique sequence of decreasing dates among minerals i e hornblende muscovite biotite microcline in the same sample or within closely spaced samples and 3 a positive correlation between the grain size fractions and their40ar 39ar dates e g smaller grain size fractions of a given mineral yield younger dates in the lower greenschist facies area hornblendes and coarse grained muscovites still retain most of their radiogenic argon and display fairly flat40ar 39ar age spectra with plateau dates of 82 95 ma which record a rapid cooling during the nanao orogeny a late mesozoic tectonothermal event in contrast all microclines are completely reset and yield young plateau dates of 1 6 and 1 7 ma reflecting the rapid uplift associated with the penglai orogeny most of the partially reset minerals e g muscovites from the upper greenschist facies area and biotites from the lower greenschist facies area yield geologically meaningless40ar 39ar integrated dates that fall between the ages of the two tectonothermal events and exhibit disturbed age spectra which are caused mainly by mixing of argon released from two generations of phases excess argon and partial argon loss after isolating possible recrystallization and mixing effects the regional variation in the40ar 39ar dates for the earlier generation phases could be simulated by a volume diffusion model coupled with a thermal model for the taiwan mountain belt

titanite inclusions in altered biotite from granitoids of taiwan microstructures and origins biotites with three sets of titanite inclusions i e sagenitic biotites have been reported from both igneous and metamorphic rocks two formation mechanisms have been postulated the precipitation model for the sagenitic biotite of igneous origin shau y h yang h y peacor d r 1991 on oriented titanite and rutile inclusions in sagenitic biotite am mineral 76 1205 1217 and the percussion figure model for the sagenitic biotite of metamorphic origin xu s ji s 1991 biotite percussion figures in naturally deformed mylonites tectonophysics 190 373 380 sagenitic biotites in granitoids of the tananao metamorphic complex of taiwan were studied especially with regard to the variable degrees of superimposed collision induced deformation metamorphism mass balance considerations transmission electron microscopic observations and regional geological relations exclude the simple intra biotite precipitation model as the possible mechanism instead the formation of titanite inclusions in these igneous biotites is suggested more likely to be facilitated by inward diffusion of ca and outward diffusion of ti along the basal cleavage planes and the fracture surfaces induced by shear deformation i e the percussion figure in biotites under greenschist facies temperature conditions interdiffusion may also account for the formation of rutile particles with varied size and varied crystallographic orientation in some titanite laths q 2001 elsevier science ltd all rights reserved

microstructures of intergrown phyllosilicate grains from verrucano metasediments northern apennines italy interleaved phyllosilicate grains ipg of various compositions are widespread in low grade verrucano metasediments of the northern apennines italy they are ellipsoidal or barrel shaped up to 300 400 m long and they are often kinked and folded phyllosilicate packets occur as continuous lamellae or as wedge shaped layers terminating inside the grain using electron microscopy techniques sem tem six types of ipg have been distinguished on the basis of their mineralogical composition 1 chl ms kln 2 chl ms pg kln 3 ms prl pg 4 ms prl su 5 ms prl chl su 6 su ms types 1 and 2 are mainly composed of chlorite with ms and pg as minor phases kln grows on ms in highly weathered samples types 3 4 5 and 6 are composed of muscovite with intergrown prl chl su and new formed muscovite the ipg show all kinds of contacts from coherent grain boundaries with parallel basal planes and along layer transitions to low and high angle grain boundaries the ipg formed on pristine minerals such as chlorite and muscovite the transformations took place during the prograde and retrograde metamorphic path of the rocks they were facilitated by deformation and they occurred in equilibrium with a fluid phase which allowed cation diffusion prograde reactions chl ms or pg ms prl ms chl involve dehydration and sometimes a decrease in volume whereas retrograde reactions ms kln ms su involve hydration and an increase in volume these transformations do not simply occur through an interchange of cations but often involve deep structural changes transitions from one phyllosilicate to another generally proceed through dissolution recrystallization reactions in conclusion verrucano ipg represent microstructural sites which have not completely equilibrated with the whole rock and whose mineral assemblage depends on the original composition of the microstructural sites

a seismic velocity model for the sw baltic sea derived from basin 96 refraction seismic data abstract within the basin 96 project basin analysis and seismic investigation in north germany 1996 a marine survey gathered seismic data in the southwest baltic sea to study the caledonian collisional zone between baltica to the north and avalonia to the south additionally the airgun pulses have been observed by several land stations special attention was paid to record good quality data on rugen island aiming at linking offshore and land seismic wide angle observations therefore a complex geophone array the star of rugen was designed to enhance the signal quality the star of rugen data and other onshore observations in ne germany and on bornholm an island in the baltic sea have been used for p wave velocity modelling important structures of the transition zone from palaeozoic western europe to the precambrian baltic shield are imaged in the resulting three dimensional velocity model a pronounced basement high north of rugen island indicates the location of the caledonian deformation front cdf velocities also reflect other prominent features such as the ronne graben a pull apart structure of the tornquist zone and the basement outcrops around bornholm island in the investigated area the moho depth varies between 27 and 33 km a high velocity layer in the lower crust below the ne german mainland may indicate genetic relations to baltica the structural continuity however is disturbed by a velocity decrease in an area north of the cdf sporadic but strong reflections from the upper mantle below the cdf seem to be consistent with the reflection seismic data dekorp basin research group 1999 geology 27 1 55 58

formation of corrensite chlorite and chlorite mica stacks by replacement of detrital biotite in low grade pelitic rocks transmission and scanning electron microscopy were utilized to investigate the nature and mechanisms of alteration of abundant detrital biotite of volcanic origin and progressive modification of phyllosilicate aggregates in a prograde sequence of pelitic rocks illite crystallinity index 0 19 0 58 a28 from the gasp6 peninsula in quebec detrital biotite has been diagenetically altered to form corrensite and chlorite through two mechanisms 1 layer by layer replacement gave rise to interstratification of packets of layers and complex mixed layering via several kinds of layer transitions between biotite and chlorite corrensite or smectite 2 dissolution transport precipitation resulted in the formation of relatively coarse grained aggregates of randomly orientated corrensite rich flakes and fine grained corrensite intergrown with chlorite and illite in the matrix the data show that stacks consisting of alternating packets of trioctahedral and dioctahedral phyllosilicates originated during early diagenesis when lenticular fissures in strained altering biotite were filled by dioctahedral clays subsequent prograde evolution of dioctahedral clays occurred through deformation dissolution and crystallization and overgrowth illite evolved to muscovite with k in part provided through biotite alteration and corrensite chlorite to homogeneous chlorite the alteration of detrital biotite is closely related to the formation of titanite and magnetite in diagenetic rocks and pyrite calcite and anatase or rutile in the higher grade rocks the observations demonstrate that detrital biotite of volcanic origin may be the principal precursor of chlorite in chlorite rich metapelites originating in marginal basins the mineral parageneses suggest that the transitions from corrensite to chlorite and illite to muscovite may be a function of local chemistry and time