model for the evolution of the avalonian cadomian belt the inception and termination of the ca 670 550 ma avalonian cadomian orogenic cycle are interpreted in terms of the amalgamation and breakup of a late precambrian supercontinent synorogenic arc related volcano sedimentary and granitoid rocks are attributed to oblique southward directed subduction along the margin of the supercontinent after its amalgamation breakup of the supercontinent and inception of the iapetus cycle led to termination of subduction and its replacement by transform activity the resulting closure of late precambrian arc related basins and the development of early cambrian platform successions with acado baltic fauna accompanied the formation of iapetus but took place on the active margin of a preexisting ocean and do not record the evolution of its rifted european flank

the development of the central european continental crust since the early archaean based on conventional and ion microprobe dating of up to 3 84 b y old detrital zircons abstract archaean components up to 3 84 b y old have been detected by ion microprobe dating of zircon from various sediments and metasediments of the european hercynides moldanubian of northeastern bavaria and the massif central the early archaean age of 3 84 b y was obtained within the euhedral core of a detrital grain from a moldanubian paragneiss the same detrital zircon grain reveals at least three metamorphic overprints at 2 59 b y b p 1 94 b y b p and 460 m y b p and at least two cycles of mechanical abrasion during surface recycling in the early proterozoic and after the pan african cycle it is thus the crystal with the most complex geological history ever detected further archaean ages for the crystallization of primary magmatic zircons probably in the course of continental crust formation in the first cycle provenances of the two studied areas were found at 3 15 2 9 2 76 and 2 65 b y b p and in a few cases between 2 6 and 2 5 b y b p early proterozoic crust forming events between about 1 75 and 2 15 b y b p play an important role especially for the sediments and metasediments of the montagne noire at the southern rim of the massif central middle and late proterozoic ages cluster around 1 0 b y b p and 600 m y b p for all analysed samples therefore the deposition of the metasedimentary precursors took place after the pan african cycle which is in line with the rb sr whole rock systematics of central european metasediments the ages of metamorphism in the different source areas could be inferred in some cases by lower intercept ages obtained on single detrital grains the following sequence coinciding with times of magmatic events has been so far established 2 59 1 94 and 1 0 b y b p and 600 m y b p thus ion probe dating is also capable of defining secondary effects in the respective provenances which are typical for orogenic cycles the ubiquitous presence of pan african detrital zircons in european metasediments is a strong indication that the detrital material was derived from gondwana however from the presence of metamorphic units including eclogitefacies rocks which probably formed at different continental margins and at various times between about 500 m y and 330 m y ago it is probable that various gondwana derived microcontinental plates contributed to the different basin fillings the gondwana derived detritus is also supported by the observation of two periods of relative quiescence or non activity around 2 3 b y ago and especially around 1 5 b y ago which is a typical feature observed in gondwana cahen et al 1984 but not in laurasia the other possible source of the detritus the lack of zircons crystallized between 3 84 and 3 15 b y ago might have different causes thus the continental crust of the european hercynides probably developed since the early archaean and grew via many crustal accretion events to its present state

thin section and s e m textural criteria for the recognition of cement dissolution porosity in sandstones cement porosity relationships are described from the lower triassic sherwood sandstone group and the middle jurassic ravenscar group in the united kingdom calcite cemented sandstones display a variety of replacement textures with preferential replacement of grains and of overgrowth faces with high free surface energy dolomite and siderite cemented sandstones display similar textures but replacement is less specific and euhedral overgrowth surfaces are commonly embayed by carbonates examination of the more porous sandstones with the scanning electron microscope reveals a range of pitting and embayment textures in authigenic overgrowths and in detrital grains these range from small v shaped notches and pits through regular and irregular shaped embayments into large depressions these textures appear to be morphologically similar to the quartz surfaces seen in thin sections of carbonate cemented sandstones and are interpreted to have been formed by the dissolution of pore filling and grain replacive authigenic carbonates this is confirmed by examination of experimentally exhumed overgrowth surfaces from carbonate cemented sandstones these textures indicate that part of the intergranular porosity in these sediments is secondary in origin and has been generated by the dissolution of carbonate cements the identification of such textures may lead to a more confident interpretation of the nature of intergranular porosity in the subsurface

arcades products of stress controlled and discontinuity related weathering abstract weathering patterns on sandstone cliff faces and sloping surfaces were studied from the viewpoint of gravity induced stress combined with the presence of planar discontinuities in the rock critical evaluation of hundreds of photos taken in various regions around the world numerical modelling using finite element method including automatic stress controlled element removal technique of erosion modelling and physical in situ modelling of friable sandstone revealed the formation of a specific sub group of weathering forms related to planar discontinuities due to their striking similarity with elements known from architecture these pits are introduced as arcades or arcade like forms these pits and cavities vary in size but they are similar in shape coalesced into trains along the discontinuities and separated from each other by hourglass shaped pillars in their origin and geometry these forms differ from widely known cavernous weathering forms such as honeycombs and tafoni we have shown that arcade shape and size are controlled by the redistribution of gravity induced stress along planar discontinuities and the resulting stress shadows the stress shadow zones along the discontinuities are characterized by material loss owing to various weathering and erosional mechanisms although overlooked as yet stress controlled weathering along discontinuities has a strong effect on rock surface morphology and arcades can be taken as its principal example

the sholl shear zone west pilbara evidence for a domain boundary structure from integrated tectonostratigraphic analyses shrimp upb dating and isotopic and geochemical data of granitoids abstract the pilbara block provides a record of archaean continental growth involving the tectonic accretion of outboard island arcs and collisions with other continental scale fragments this record of continental growth is balanced by breakup and strike slip dismemberment of the continent new shrimp u pb in zircon ages and sm nd data provide evidence in the west pilbara which demonstrates that subduction related and tectonic accretion processes at the western margin of that ancestral continent between 3 15 2 78 ga were coeval with and genetically related to crustal scale tectonics and basin formation inboard of that margin the tectonic division of the west pilbara is defined by integrated tectonic analyses geochronology geochemistry and isotopic analyses geochronological studies clearly indicate that the western pilbara comparises two domains with different recorded geohistories whereas geochemistry and isotopic systematics reflect the changing tectonic regimes through time in combination these studies allow the development of a reconstruction of the relative positions of the domains through time on the western margin of the pilbara block the supracrustal rocks of the northern roebourne lithotectonic complex domain 6 in a pilbarawide scheme were formed in an island arc setting facing an ocean to the north west prior to 3260 ma the time of emplacement of voluminous granitoids into the complex in contrast the supracrustal rocks of the southern sholl belt pilbara domain 5 were formed in a back arc setting behind a north west facing arc between 3125 and 3112 ma with more or less synchronous granite emplacement at about 3115 ma the two domains were tectonically juxtaposed between 2991 and 2925 ma by the sholl shear zone a largely sinistral shear zone with subsequent volcanism in both domains to about 2925 ma the roebourne lithotectonic complex domain 6 is interpreted to be an allochthonous terrane which formed north east relative to its present position but indigenous to the pilbara block rather than an exotic terrane the east pilbara is interpreted to have acted as a cratonic hinterland during the convergent margin tectonics that affected the two west pilbara domains

the presumed tepla barrandian moldanubian terrane boundary in the orlica mountains sudetes bohemian massif structural and petrological characteristics the sudetes in southwestern poland and northern bohemia expose a palaeozoic collage of the northeastermost extremity of the variscan belt one of presumed terrane boundaries occurs in the orlica mts along the contact between the phyllite amphibolite complex neoproterozoic of the nove m sto unit of probable tepla barrandian affinities in the sw and the orthogneiss early ordovician protolith and mica schist neoproterozoic complexes of the orlica nieznik unit in the ne most likely representing the moldanubian terrane the synmetamorphic structural evolution of both adjacent units comprised four deformation events the nove m sto rocks have recorded an early stage of ductile top to the se thrusting d1 which must have resulted in an early juxtaposition of the both terranes within a nappe pile the original overthrust contact was subsequently modified by a dextral shear dominated event d2 which produced a 1 2 km wide shear zone at the boundary of both units in which earlier fabric elements were overprinted and mostly obliterated this deformation brought the lower grade nove m sto rocks into present day strike slip contact with those of the regionally uplifted higher grade orlica nieznik unit postdating this juxtaposition are e w trending folds f3 which affect both the adjacent units the contact shear zone also contains record of a late top to the sw semi brittle normal slip displacement d4 downthrowing the nove m sto unit the peak amphibolite facies metamorphic conditions were attained in the orlica nieznik unit and in the contact shear zone prior to the d2 event they corresponded to maximum temperatures and pressures of ca 600 c and 10 kbar respectively the d1 thrusting event in the phyllites of the nove m sto unit took place under greenschist facies conditions ca 350 c and 4 5 kbar and its effects mostly survived the later amphibolite facies metamorphism these structural relationships are equivalent to those observed elsewhere in the bohemian massif on the tepla barrandian moldanubian terrane boundary the characteristic features of this boundary are a high metamorphic gradient but lack of metamorphic inversion the occurrence of mid carboniferous stitching plutons and the importance of mid carboniferous deformation that resulted in significant downwards movement of the low grade tepla barrandian terrane relative to the uplifted high grade and hot lower to middle crust of the moldanubian terrane the downthrow of the tepla barrandian occurred on ductile shear zones showing either down dip slip kinematics or that of transfer strike slip as was the case with the orlica mts both kinematics being associated within the same regional scale linked shear zone extensional system the extensional crustal collapse must have been preceded by a se directed crustal stacking achieved through overthrust emplacement of the tepla barrandian on top of the moldanubian terrane

u pb zircon and sm nd model ages of high grade moldanubian metasediments bohemian massif czechoslovakia we report single grain u pb ion microprobe as well as conventional bulk size fraction ages for zircons from 3 metasediment samples of the moldanubian complex bohemian massif one of the largest crystalline complexes of the hercynian foldbelt in europe these are complemented by whole rock sm nd model ages the metasediments are of upper amphibolite to granulite grade and come from the bory massif in moravia nw of brno sample aa 1 and from the varied group aa 2 and monotonous group aa 3 in the ceske budejovice region of sw bohemia

transition from island arc to passive setting on the continental margin of gondwana u pb zircon dating of neoproterozoic metaconglomerates from the se margin of the tepla barrandian unit bohemian massif abstract sedimentological petrological and geochemical data combined with shrimp and la icp ms u pb ages of pebbles of granitic rocks and detrital zircons from neoproterozoic vletice metaconglomerates of the se margin of the tepla barrandian unit were used to characterize the regional geotectonic setting at the proterozoic paleozoic boundary the metaconglomerates form several metres thick intercalations in metasediments of the svrchnice formation roof pendants of central bohemian plutonic complex which is considered to have been deposited in neoproterozoic cambrian times a sedimentological study indicates that the metaconglomerates are products of debris flows deposited in submarine canyons or proximal parts of submarine fans geochemistry of metaconglomerate matrix and petrography of pebbles revealed an obvious difference from the western and central part of the tepla barrandian unit proterozoic where pebbles eroded from uplifted parts of the basin and island arc sources are most common the abundance of quartz quartzite granite and crystalline limestone in pebbles together with the more mature composition of host metagreywackes indicate the presence of relatively evolved continental crust in the source area for the vletice metaconglomerates and shallowing of the neoproterozoic basin to the se in the present geographic co ordinate system the neoproterozoic to lowermost cambrian age of the svrchnice formation suggested by its comparison with similar rocks in the central part of the tepla barrandian unit was confirmed by u pb dating ages of detrital zircons from the metaconglomerate matrix cluster around two values archaean to paleoproterozoic u pb ages between ca 2580 and 1780 ma and neoproterozoic to early cambrian ages between 660 and 520 ma with a significant peaks at 600 and 544 ma suggesting local volcanic activity extending shortly beyond the proterozoic paleozoic boundary zircons separated from two trondhjemite pebbles from the metaconglomerate yielded a mean u pb age of 610 17 ma arguing for active arc magmatism during the latest neoproterozoic the overall age distribution of detrital zircons and the absence of grenvillian zircon ages favour a location of the tepla barrandian unit close to the west african craton at the neoproterozoic paleozoic boundary the rapid onset of a transtensional geotectonic regime and maturing of sediments is evidenced by the overlying cambrian shallow water clastics alternating with predominantly felsic calc alkaline to alkaline volcanics during the cambrian oblique plate convergence the active continental margin was transformed into belts of horsts and narrow pull apart or small rift basins after the ordovician break off of armorican terrane assemblage blocks from gondwana the extensional regime in the tepla barrandian unit continued until the beginning of variscan orogeny in the middle devonian

ordovician passive continental margin magmatism in the central european variscides u pb zircon data from the se part of the karkonosze izera massif sudetes sw poland approximately 500 ma old orthogneisses are widespread in the eastern part of the variscan belt and are commonly interpreted to have intruded mica schist series of assumed neoproterozoic age new shrimp zircon ages of quartzofeldspathic metavolcanogenic rocks of the mica schist series in the eastern part of the karkonosze izera massif sw poland indicate that they are late cambrian early ordovician rather than neoproterozoic in age based on the zircon age spectra distributed mainly between ca 500 and 660 ma with a few proterozoic inherited minimum ages of ca 970 and 1 825 ma younger zircon dates dispersed between ca 412 and 464 ma are interpreted as a result of pb loss likely caused by subsequent metamorphism consequently the felsic metavolcanogenic rocks appear to be roughly contemporaneous with the intrusion of ca 500 ma old orthogneiss protoliths with the pooled concordia age of 487 8 ma interpreted as the best approximation of the protolith intrusive age field relationships petrological and geochemical features of the felsic and mafic rocks studied support a model in which the accompanying mica schist series are not the original country rocks to the ca 500 ma granite intrusions and indicate that their recent close proximity is the result of tectonic juxtaposition however both the mica schists enclosing the bimodal metavolcanic rocks and the orthogneisses are interpreted to represent a cambro ordovician passive continental margin sequence being part of the saxothuringian domain they are tectonically overlain to the east by hp t metamorphic units comprising morb type metaigneous rocks and delineating a tectonic suture separating the saxothuringian block in the west from an assumed continental block tepla barrandian to the south east

shrimp u pb zircon geochronology of the strzelin gneiss sw poland evidence for a neoproterozoic thermal event in the fore sudetic block central european variscides zircon ages recorded in gneissic rocks have recently been used as criteria to define and correlate various tectonic units and crustal blocks in the central european variscides a shrimp u pb zircon geochronological study of the strzelin gneiss in the fore sudetic block sw poland indicates the presence of 1 inherited zircon cores of palaeo to mesoproterozoic 206pb 238u ages between ca 2 000 and 1 240 ma and 2 zoned rims of neoproterozoic age with two distinct means of 600 7 and 568 7 ma the proterozoic age range of the cores suggests that different precambrian crustal elements were the source for the protolith of the gneiss a likely scenario is the erosion of various proterozoic granites and gneisses sedimentation after 1 240 ma and partial resistance of the original components to subsequent metamorphic dissolution and or anatectic resorption in neoproterozoic times the zoned zircon rims of both of the younger neoproterozoic ages are indistinguishable in the cathodoluminescence images the data are interpreted in terms of two different thermal events inducing zoned zircon overgrowth at ca 600 and 568 ma in general the new results confirm earlier assumptions of the proterozoic age of the gneiss protoliths and indicate their similarity to orthogneisses in the east sudetes tectonic domain e g the velke vrbno and desna gneisses the neoproterozoic dates are different from the age of 504 3 reported earlier for the go ciecice gneiss from a neighbouring locality in the strzelin massif the new data strongly indicate a moravo silesian bruno vistulian affinity for the strzelin gneiss and support the hypothesis that the strzelin massif lies within the tectonic boundary zone between the west and east sudetes domains which represents the northern continuation of the moldanubian thrust

rock cities and ruiniform relief forms processes terminology abstract spectacular rock cut landscapes consisting of closely spaced residual rock blocks separated by narrow intersecting corridors are variously termed as rock cities and ruiniform relief the former term has apparently never been formally defined except in the local czech literature whereas the latter was applied in various contexts making the terminology confusing this paper provides a global overview of forms processes and controls behind such rock city ruiniform landscapes and develops geomorphological terminology for such rock cut landscapes including the proposal that rock city becomes a legitimate scientific term rock cities and ruiniform relief are two different types of rock erosional morphology typified by the presence of a street pattern in the former and highly weathered surfaces of rock residuals in the latter although there is some overlap between the two examples of rock cities or ruiniform relief may be found in different lithologies and in a wide range of climates from hyper arid to humid classic localities include northern czechia and adjacent parts of poland and germany australia central sahara and the gran sabana in venezuela lithologies particularly suitable to the development of this type of erosional morphology include mainly sandstones followed by limestones dolomites conglomerates tuffs but igneous rocks such as granites or rhyolites are also known to support this type of relief the combination of sufficient rock strength to support steep rock faces and the presence of discontinuities which preferentially eroded turn to streets and squares in the rock cities appears as the basic prerequisite a wide range of processes are involved in their formation including surface and subsurface weathering mass movements salt tectonics karst processes in both carbonate and silicate rocks piping fluvial erosion and aeolian abrasion despite being spectacular landscapes very few models of long term evolution of rock city and ruiniform relief have been presented in the literature and it is unlikely that any one scenario would apply to all situations rather rock cities are examples of geomorphological convergence

selective weathering of cross bedded layers forming shelters and small caves on akopan tepui venezuela field laboratory and experimental evidence about diagenesis and weathering of the mataui formation arenites roraima supergroup middle proterozoic abstract south american table top mountains tepuis are formed by quartzites and sandstones of mataui formation belonging to the roraima supergroup lower to middle proterozoic field observations in the northern part of akopan tepui in venezuela revealed that unlike the beds with flat bedding the cross bedded layers are commonly affected by selective weathering expressed by depressions shelters and even small caves all the arenites are fine grained most of them are likely of aeolian origin hardness measurements confirmed that the cross bedded layers are softer than the flat bedded or structureless beds occurring above and below them petrographic analysis showed that the rocks are mostly fine grained quartzites to quartz sandstones with original sand grains overgrown by syntaxial quartz cement and bearing signs of compaction manifested by local pressure solution some intergranular spaces are filled by newly formed phyllosilicates paragonite locally intergrown with muscovite in the overlying beds no porosity was observed by microscopic study and under sem whereas in the underlying rocks signs of slight weathering and etching of phyllosilicates was observed together with locally widened spaces along the grain to grain boundaries the samples from cross bedded layers display weathering the degree of which depends on the present access of flowing water where water has periodical access to the arenite it shows dissolution of quartz cement arenization together with complete leaching and removal of the phyllosilicates due to lateritization the intergranular spaces are considerably widened and quartz grains bear signs of strong skeletal etching and pitting samples from the sites where the water access ceased show mostly leaching of phyllosilicates with weak skeletal dissolution of quartz grains without pitting to imitate the origin of structures observed in the field experimental modelling was performed using artificially layered and compacted fine grained sands on which sodium silicate solution was dripped during soaking downward preferential penetration of the solution along the cross bedding laminae funnel flow was observed in the cross bedded layer the resulting structures display good lithification of horizontally layered sands and relatively poorer lithification of the cross bedded sediment between them showing triangular shaped overhangs and shelters which mimic those on the akopan tepui differential weathering of otherwise petrologically very similar arenites indicates that there had to be a difference in the initial porosity in the time of the rock denudation a preserved porosity in the cross bedded layers was necessary for the water rock contact to perform dissolution

solution weathering rate and origin of karst landforms and caves in the quartzite of auyan tepui gran sabana venezuela abstract the paper reports the results of sio 2 analyses in the aonda cave system located on the auyan tepui one of the widest table mountains of the gran sabana south venezuela characterised by karst landforms developed in siliceous rock chemical analyses underline the very low concentration of sio 2 of the surface water percolation and cave drip waters have a sio 2 concentration of about 1 mg l the mean silica load of the cave stream is 184 mg s mainly derived from surface solution removal in the allogenic recharge area in the aonda cave system the mean sio 2 dissolved load is 40 mg s in part from surface solution 15 and mainly from underground processes 85 the low solubility of sio 2 in slightly acidic water implies the importance of the time factor in the formation of cave systems with the present dissolution rate about 10 ma would be necessary to form the known karst system this estimation can be significant only if we assume that climate has been stable in the last few tens of millions of years furthermore this age can be taken as a minimum estimate while according to the geomorphic evolution of the area the origin of the aonda cave system could be reasonably dated back to at least 20 30 ma that is to the oligocene

a hybrid model to evaluate subsurface chemical weathering and fracture karstification in quartz sandstone abstract a hybrid numerical finite differences and semi analytical empirical model has been developed to evaluate the spatial and time scale of chemical weathering along a fracture in quartz sandstones the model is based on the diffusion transport occurring from the bedrock mass toward the fracture walls driven by the sio2 concentration gradient between water in the bedrock pores high silica content and water flowing in the fracture low silica content because of molecular diffusion intergranular water becomes undersaturated with respect to silica this promotes dissolution of quartz at grain boundaries and results in a porosity profile decreasing from the fracture walls toward the rock interior bedrock individual grains are released and gradually removed by the water flowing into the fracture when a critical value of porosity is reached grain release threshold grt this process drastically increases the fracture enlargement rate the model outcomes establish a minimum time of 90 ka to reach the grt this time is independent of length aperture and dip of the fracture but is controlled by parameters such as the initial fracture aperture water temperature quartz grain size and initial bedrock porosity it is attested that the water flowing in the fracture remains undersaturated with respect to silica over very long timescales in the order of 105 years and over very long flow paths in turn this suggests that the extremely slow reaction between quartz and water is the key factor for the formation of subterranean karst like features in quartz sandstone otherwise silica saturation would be reached after short distances and deep weathering in this lithology would be prevented finally the model outcomes were compared to field data from the gran sabana caves venezuela and other quartz sandstone weathering landforms elsewhere showing that dissolution diffusion in the rock matrix is a reliable mechanism for the formation of these peculiar karst like features

underground erosion and sand removal from a sandstone tableland stolowe mountains sw poland abstract models of geomorphic evolution of tablelands vary as far as the patterns of cliff line degradation and hence scarp retreat are concerned among surface processes rock slope failures from caprock cliffs often induced by preferential seepage from rock faces are emphasized in this paper evidence is provided that mechanical removal of sand from inside the caprock through underground erosion may also play an important role in the long term evolution of cliffed escarpments in sandstone tablelands more than 140 sandy cones and nearly 80 sand sheets emerging from fissures in the sandstone cap were mapped and measured along six cliff sections of variable length in the stolowe mountains tableland in sw poland the density of sandy accumulation forms varies along the cliff length with cones preferentially associated with intact cliffs and sand sheets being more common along degraded cliff sections further redistribution of sand downslope results in widespread sandy covers below cliff lines which extend for a few hundred metres and are 1 3 m thick as demonstrated by electrical resistivity tomography sand removal from caprock accounts for progressive non catastrophic degradation of tableland margins through gradual subsidence of blocky compartments and the development of ruiniform relief behind the cliff lines thus it may be an important component of long term evolution of stepped relief in sedimentary terrains

birth of a volcanic passive margin in cambrian time rift paleogeography of the ossa morena zone sw spain abstract mapping at 1 10000 scale of both north n and south s flanks of the variscan monesterio antiform in the ossa morena zone southern spain in association with detailed petrographic and major element analyses of magmatic rocks has revealed the building of a passive but volcanically active oceanic margin in cambrian time this margin was located in northern gondwana and represents the beginning of an extensional tectonic regime that concluded with the opening of the rheic ocean at time of eruption middle cambrian the n flank was located on the northern coastline of a marine basin in the process of passive rifting the vents mainly located in its western part were subaerial continental and or shallow marine on the s flank eruptions and deposits were shallow marine occurring as lava flows and pillows interstratified within shallow marine sediments on both flanks intrusives were emplaced after the extrusives lateral thickness variations of the sedimentary deposits testify to the building during four distinct periods of a rifted passive margin with a shifting locus of volcanic activity over time the composition of magmas was bimodal the mafic magmas are silica saturated alkaline to sub alkaline and somewhat ti rich perhaps originating from partial melting of a subcontinental mantle that was less metasomatized with time the youngest magmatism with an affinity close to that of morb likely had an asthenospheric source the alkaline felsic products are diverse and may have originated from a very heterogeneous continental crust in each set partial melting relationships prevail suggesting that the melts rose along open fractures as soon as they formed in the context of lithospheric thinning it is possible that mantle and crustal melting was triggered by decoupling between rheologically different materials

the moldanubian thrust zone a terrane boundary in the central european variscides refined based on lithostratigraphy and u pb zircon geochronology abstract the zircon age populations of metavolcano sedimentary successions in the orlica nieznik dome osd stare m sto belt smb and velke vrbno dome the sudetes poland and czech republic have been used to refine the location of the moldanubian thrust zone mtz which is a boundary between the saxothuringian terrane of gondwana descent and the brunovistulian terrane being a promontory of laurussia in the northern continuation of the mtz a set of multiply activated regional scale thrusts developed and brought into contact rocks of different ages and geological histories metarhyolites in the orlica nieznik dome and the stare m sto belt have similar geochemistry and u pb isotopic zircon records which is taken in favour of their coeval formation and common saxothuringian affinity felsic metavolcanic rocks from the osd and from the thrust bounded upper and lower units of the smb yielded protolith ages of 500 3 ma and 493 4 ma to 498 5 ma respectively which indicates that metavolcano sedimentary successions in the osd and smb were deposited in late cambrian times structurally below these rocks there are the highly sheared brousek quartzites with detrital zircons that yielded a maximum depositional age of 530 ma the mylonitic quartzites accommodate deformation induced by tectonic transport on the east n znerov thrust which is interpreted as the easternmost margin of saxothuringia this fault separates palaeozoic rocks of the saxothuringian terrane from neoproterozoic bimodal volcanogenic succession in the velke vrbno dome of brunovistulia dated at 558 ma the late variscan thermal events in the early carboniferous left imprints in the form of u rich rims around the zircons of the metavolcanic rocks from the stare m sto belt and only a very minor overprint in the zircons from the velke vrbno dome and orlica nieznik dome

how hydrofractures become arrested fluids in the earth s crust are commonly transported by hydrofractures such as dykes and mineral veins many of which become arrested at various crustal depths hydrofractures are commonly arrested some showing blunt tips at contacts between soft low young s modulus and stiff high young s modulus layers for example many dyke tips are arrested at contacts between soft pyroclastic rocks and stiff basaltic lava flows and vein tips at contacts between soft marl and stiff limestone theoretical models indicate that overpressured buoyant hydrofractures in homogeneous isotropic host rocks should normally reach the surface in layered host rocks however abrupt changes in young s moduli horizontal discontinuities and layers with unusually high fracture perpendicular stresses encourage hydrofracture arrest it is proposed that for layer parallel loading stiff layers favour hydrofracture arrest during active compression but soft layers during extension it is concluded that for hydrofracture propagation to occur the stress field along its potential pathway must be essentially homogenous

tectonics and plate tectonics model for the variscan belt of europe abstract a plate tectonics model is presented to explain the tectonometamorphic characteristics of the european variscides after the closing of two oceanic domains by two sided subduction 500 420 ma and obduction 420 380 ma collision of the european and african continental plates occurred we propose that the subsequent complex intracontinental deformation 380 290 ma is the result of a double subduction of the continental lithosphere accompanied by crust mantle decollement this mechanism explains the progressive crustal thickening and migration of the deformation through time from the sutures toward the external parts of the variscan belt accounting for this model and for the relationships between the european variscides and the other paleozoic peri atlantic belts caledonides appalachian mauritanides and morocco we infer the relative positions of africa america and europe between the silurian and the permian

u pb ages from sw poland evidence for a caledonian suture zone between baltica and gondwana faunal and palaeomagnetic evidence suggests the existence of a c 3000 km wide tornquist sea between gondwana and baltica in early ordovician times which narrowed to 50 2 ma an unmetamorphosed ophiolite gives a 420 20 2 ma age whilst in two other terranes amphibolite facies island arc lavas have fossiliferous lower ludlow 424 415 ma sequences all are unconformably overlain by unmetamorphosed upper devonian conglomerates a sinistral transpressive regime is observed in regionally extensive mylonite zones titanite and zircon ages 338 2 3 and 339 4 ma record variscan magmatism the data suggest considerable narrowing of the tornquist sea during the ordovician continuing ocean floor and island arc activity in the silurian and final sinistral transpressive closure by the mid devonian

source and genesis of sulphate and phosphate sulphate minerals in a quartz sandstone cave environment gypsum caso4 2h2o alunite kal3 so4 2 oh 6 and rare phosphate sulphate sanjuanite al2 po4 so4 oh 9 h2o and rossiantonite al3 po4 so4 2 oh 2 h2o 14 have recently been identified as secondary mineral deposits in different quartz sandstone caves in the gran sabana region venezuela due to the extended time scale required for speleogenesis in the hard and barely soluble quartz sandstone lithology these caves are considered to be as old as 20 to 30 my the study of these peculiar secondary mineral deposits potentially reveals important insights for understanding the interaction between deep superficial and atmospheric processes over thousands to perhaps millions of years in this study chemical and petrographic analyses of potential host rock sources sulphur and oxygen isotope ratios and meteorological hydrological and geographical data are used to investigate the origin of sulphates and phospho sulphates the results suggest that the deposition of sulphates in these caves is not linked to the quartz sandstone host rock rather these mineral deposits originate from an external atmospheric sulphate source with potential contributions of marine non sea salt sulphates terrestrial dimethyl sulphide and microbially reduced h2s from the forests or peatbogs within the watershed air currents within the caves are the most plausible means of transport for aerosols driving the accumulation of sulphates and other secondary minerals in specific locations moreover the studied sulphate minerals often co occur with silica speleothems of biological origin although this association would suggest a possible biogenic origin for the sulphates as well direct evidence proving that microbes are involved in their formation is absent nonetheless this study demonstrates that these quartz sandstone caves accumulate and preserve allogenic sulphates playing a yet unrecognized role in the sulphur cycle of tropical environments

escarpment retreat in sedimentary tablelands and cuesta landscapes landforms mechanisms and patterns abstract although the general concept that escarpments in layered rocks retreat through time was offered more than 150 years ago recognizing the exact mechanisms patterns and rates of retreat remains a challenge in this paper we provide the state of the art of the theme of escarpment retreat reviewing both the classic contributions often forgotten but important case studies and the latest research in separate parts of the paper we discuss 1 terminology applied to the geomorphology of escarpments 2 the main assumptions and pathways of long term evolution of escarpments including conceptual models presented so far 3 processes involved in cliff retreat mainly weathering mass movements and erosion 4 the role of spring erosion and fluvial processes leading to fluvial dissection and development of canyons present in many tablelands 5 the possible role of climate change in escarpment evolution 6 rates of escarpment retreat in conclusions we summarize the major advances in our understanding of escarpment evolution of the last two three decades such as the recognition of the key role of emerging groundwater in non uniform escarpment retreat of an important role of silica dissolution and underground removal of dissolved solids increasing appreciation of rock properties in governing the patterns of escarpment retreat the proposal that cliffs do not necessarily retreat via catastrophic rock slope failures but may disintegrate in situ and the first attempts to date escarpment retreat using cosmogenic isotopes finally the remaining gaps and possible research priorities for the future are identified

a global review of solutional weathering forms on quartz sandstones abstract solutional landforms in limestone have been described for over a hundred years but landforms of similar morphology on highly siliceous sandstones and quartzites have also been identified in a wide variety of environments and generally termed pseudokarst these include large bedrock pinnacles and towers caves corridors grikes solution basins and runnels and even silica speleothems quartzites and quartz sandstones have been held to be amongst the most chemically resistant of rocks but the similarity both in morphology and genetic process of many landforms developed from them to features of known solutional origin on limestone has prompted some authors to refer to these quartzose landforms as true karst the most detailed studies of quartzose karst landforms have been in present day tropical regions or areas believed to have been tropical in the geologically recent past this concentration of research in hot wet areas allied with the long held assertion of the insolubility of silica especially quartz has led to a belief that tropical climatic conditions are necessary for karstic solution of these rocks however the existence of quartzose karst landforms in temperate and even sub polar latitudes especially where there is no evidence of prior tropical conditions suggests that the requirement of tropical weathering is no longer tenable the reports of these quartzose solutional landforms are widely scattered through the geomorphological and geological literature but a comprehensive world wide review of the range of solutional landforms on quartzose rocks has not previously been published because of the increasing awareness in this karst type such a summary is sorely overdue

late cambrian ordovician magmatic arc type volcanism in the southern gemericum basement western carpathians slovakia u pb shrimp data from zircons the low grade crystalline basement of the southern gemeric unit inner western carpathians consists mainly of siliciclastic deep water turbidite sequence associated with a huge mass of redeposited rhyolite dacite volcanics and their volcaniclastics in situ u pb shrimp zircon dating applied to metavolcanic rocks contributed to specification of the southern gemeric crystalline basement stratigraphy the gelnica group zircon populations from ten samples of three lithostratigraphic units of the gelnica group rock complex have been analysed the zircons presented significant magmatic zoning part of the magmatic zircons contained inherited older cores the vlachovo formation gives the 494 1 6 ma zircon average age the bystr potok formation gives 465 8 1 5 ma and the drnava formation 463 9 1 7 ma average age the geochemical data are compatible with an orogenic geodynamic setting dominant rhyolite dacite volcanites belong to the peraluminous calc alkaline high k magmatic series the age data let us presume two phase longlasting magmatic activity starting in the late cambrian and continuing to the middle late ordovician thus the studied late cambrian volcanic sequence of the gelnica group reflects the active continental margin setting the second middle ordovician volcanic phase documents an assumed rifting stage accompanied by a strong volcanic activity with inherited features of the late cambrian magmatic arc the inherited cores of magmatic zircons have proterozoic 600 760 1 000 1 400 2 000 2 700 ma and archean 3 099 3 164 ma ages

analysis of microtremors within the provadia region near a salt leaching mine by analysis of microtremors recorded with digital seismological monitoring equipment near the provadia sait diapir bulgaria two groups of events showing different characteristics have been detected in the vicinity of the salt production area the first group of events has low magnitudes and is located at a distance of about 1 km from the top of the salt diapir these events show low stress drops the second group of tremors is located outside the salt diapir the corresponding magnitudes and stress drops are larger the first class of events seems to be related to processes at the contour of the salt leaching caverns whereas the origin of the second group seems to be connected with stress redistribution processes around the salt body based on this analysis the tectonic model of the provadia salt diapir has been modified

the importance of along margin terrane transport in northern gondwana insights from detrital zircon parentage in neoproterozoic rocks from iberia and brittany abstract detrital zircons from late neoproterozoic rocks of the peri gondwanan cadomian belt of sw iberia and north armorican domain of brittany record neoproterozoic ca 860 550 ma palaeoproterozoic ca 2300 1800 and archaean ca 3300 2600 ma u pb ages the absence of mesoproterozoic zircons suggests that these terranes evolved in a peri w african realm this is in contrast to other western european terranes that preserve mesoproterozoic zircons and are likely to have evolved in a peri amazonian realm such a contrast in detrital zircon populations coupled with the presence of mesoproterozoic zircons in the ordovician armorican quartzite deposited in a peri african platform is interpreted to record along margin terrane transport the change in provenance suggests that subduction was replaced by transform faults that juxtaposed amazonia derived terranes against w africa derived terranes to form the avalonia and armorica microcontinents subsequent extension along the margin resulted in the birth of the rheic ocean and the outboard drift of avalonia

the margins of laurussia in central and southeast europe and southwest asia the continuation of the laurussian margin into central and southeast europe and southwest asia is defined by the rheic suture which becomes harder to trace to the southeast because of complex post variscan strike slip displacements and tectonic stacking linked to the cimmerian and alpine cycles the uncertain extent of the neoproterozoic baikalian orogeny fringing baltica ambiguous early paleozoic paleobiogeographic data and strike slip activity complicate the distinction of the tornquist suture from the transeuropean suture zone there is growing evidence for the incorporation of the brunovistulian malopolska and west moesian terranes into the late neoproterozoic cambrian baltican margin the istanbul zonguldak terrane along with the bitesh and east moesia terranes may have been part of the avalonian terrane assemblage although an arabian nubian shield or baltica provenance cannot be excluded the terranes of the armorica terrane assemblage bordering the rheic suture include the outer carpathian tatra getic and balkan terranes in southeast europe and sakarya and the anatolide tauride terrane in turkey further east the rheic suture may be present in the caucasus in all probability between the greater caucasus and the scythian platform or alternatively between the greater caucasus and transcaucasia alborz in northern iran is interpreted as a distal part of transcaucasia the location of the rheic suture in the southeast part of the turan domain of central asia is poorly constrained

assembly of the armorica microplate a strike slip terrane delivery evidence from u pb ages of detrital zircons abstract detrital zircons separated from the ordovician armorican quartzite in the foreland of the variscan belt of iberia were dated by laser ablation inductively coupled plasma mass spectrometry the age groups found correspond to proterozoic and archean crust formation events the proterozoic events reflected by zircon ages are avalonian cadomian pan african 550 800 ma grenvillean sunsas 900 1300 ma and eburnian transamazonian icartian 1800 2200 ma archean zircon ages are comprised between 2500 and 2800 ma central amazonian liberian events the presence of mesoproterozoic grenvillean zircons in the armorican quartzite which was deposited in a shallow platform bordering the nw african craton north gondwana margin suggests that the nw iberian terrane drifted from the south american margin where it was located during the neoproterozoic toward africa the drift of the iberian block likely achieved via transform faults was arrested by its docking and amalgamation with the peri nw africa

detrital zircons from the ordovician rocks of the pyrenees geochronological constraints and provenance abstract the first la icp ms u pb detrital zircon ages from quartzites located below three samples and above one sample the upper ordovician unconformity in the central pyrenees the rabassa dome andorra were investigated the maximum depositional age for the jujols group below the unconformity based on the youngest detrital zircon population is around 475 ma early ordovician whereas for the bar quartzite fm above the unconformity the presence of only two zircons of 442 and 443 ma precludes obtaining a precise maximum sedimentation age a time gap of 20 million years for the upper ordovician unconformity in the pyrenees can be proposed similar to that of the sardic unconformity in sardinia the similar age patterns obtained on both sides of the upper ordovician unconformity suggest that there was no change in the source area of these series while the absence of a middle ordovician age population may be due to a lack of sedimentation at that time the four study samples present very similar u pb age patterns the main age populations correspond to neoproterozoic ediacarian cryogenian ca 550 750 ma grenvillian tonian stenian ca 850 1100 ma paleoproterozoic orosirian ca 1900 2100 ma and neoarchean ca 2500 2650 ma the similarity with the sardinian age distribution suggests that these two terranes could share the same source area and that they were paleogeographically close in ordovician times in front of the arabian nubian shield

long lasting cadomian magmatic activity along an active northern gondwana margin u pb zircon and sr nd isotopic evidence from the brunovistulian domain eastern bohemian massif manuscript published version available in international journal of earth sciences 2016 doi 10 1007 s00531 016 1416 y

speleogenesis of the world s longest cave in hybrid arenites krem puri meghalaya india abstract meghalaya nw india is one of the regions in the world with highest recorded rainfalls because of these peculiar climatic conditions and intense solutional weathering karstic caves are widely reported in the numerous limestone areas of this part of india likewise the extremely high rainfall and the tropical monsoon climate have fostered the formation of several caves in silicate rich lithologies the most important of these caves is krem puri over 25 5 km long characterized by a fracture controlled labyrinth forming a bi dimensional maze with mostly phreatic morphologies and some recent still active vadose entrenchments a detailed petrographic study of the host rocks has shown this cave to be formed along a set of quartzarenite hybrid arenite and calcarenite layers in a mostly siliciclastic sedimentary succession xrf elemental analysis of the different rock strata guiding the speleogenesis confirms that the early stage of cave development is confined mainly in the calcium carbonate rich layers geochemical analyses carried out on water flowing in the cave drips streams have shown that carbonate dissolution is the dominant chemical weathering process contributing to krem puri water solutes whereas the dissolution of quartz feldspars and clays play only a secondary role in rock weathering the morphology of the cave points to a lithology guided dissolution of the carbonate rich layers and piping of the residual quartz sand in phreatic conditions when the surface river network was not yet evolved and the water table was at the level of or higher than the cave this phreatic speleogenetic phase predates the start of river network entrenchment that started not earlier 5 ma ago and probably accelerated around 3 5 ma which deactivated the karst maze with a low denudation rate of 0 15 mm yr this active phreatic cave level which currently lies beneath a more or less 120 m overburden was originally covered by a sedimentary sequence at least 1 2 km thick after its decoupling from the underground drainage system denudation slowly brought the surface close to the maze system gradually introducing infiltration of vadose waters into the conduits this caused the formation of the vadose entrenchments probably

palaeozoic terrane amalgamation in central europe a ree and sm nd isotope study of the pre variscan basement ne bohemian massif

an updated global review of solutional weathering processes and forms in quartz sandstones and quartzites abstract quartz is considered one of the less soluble minerals of the earth s crust and thus hardly affected by chemical weathering despite this for more than forty years it is clear that the formation of caves and peculiar solutional weathering dominated landforms in quartz rich lithologies is common and shares several similarities to the well known karstic ones in carbonate rocks in the last thirty years great strides have been made in furthering our knowledge of the distribution of these forms around the world and the geochemical processes involved these studies have clearly shown that solutional weathering is a fundamental process acting through intergranular dissolution of quartz increasing the rock porosity and decreasing the rock strength to erosion this process has been described in the concepts of arenization and phantomization and the widespread evidences of the fundamental role of quartz solution in landform genesis has even developed to the extent of several geomorphologists reassessing the definition of the term karst and its application to these peculiar lithologies nonetheless the process is complicated by several factors related both to environmental conditions water chemistry and availability as well as to the compositional and textural characters of the lithology presence of clays iron hydroxides carbonate cement etc all these aspects have to be taken carefully in consideration in order to understand if solution is a dominant or accessory process in the landscape evolution in this review the state of knowledge on the relevant chemical processes weathering mechanisms and speleogenesisk involved in the surface and underground karstification and clear examples of quartz solution and solutional landforms from different world locations are outlined and discussed

deducing the ancestry of terranes shrimp evidence for south america derived gondwana fragments in central europe we present here an example of how the sensitive high resolution ion microprobe shrimp zircon dating method can provide a terrane specific geochronological fingerprint for a rock and thus help to reveal major tectonic boundaries within orogens this method applied to inherited zircons in a ca 580 ma metagranitoid rock from the eastern bohemian massif has provided for the first time in the central european variscan basement unequivocal evidence for mesoproterozoic and late paleoproterozoic geologic events ca 1 2 ga 1 5 ga and 1 65 1 8 ga the recognition of such zircon ages has important consequences because it implies that parts of the precambrian section of variscan central europe were originally derived from a grenvillian cratonic province as opposed to the common assumption of an african connection a comparison with previously published shrimp data suggests however that these mesoproterozoic and late paleoproterozoic zircon ages may be restricted to the moravo silesian unit in the eastern variscides whereas the saxothuringian and moldanubian zones appear to contain a typical north african i e neoproterozoic plus eburnian inherited zircon age spectrum this finding supports new tectonic concepts according to which variscan europe is composed of a number of completely unrelated terranes with extremely different paleogeographic origins the moravo silesian unit can be best interpreted as a peri gondwana terrane which was situated in the realm of the amazonian cratonic province by the late precambrian comparable to the avalonian terranes of north america and the united kingdom

evidence for temporally changing mechanical stratigraphy and effects on joint network architecture previous studies of jointing in sedimentary rocks have shown that joint network archi tecture is controlled by mechanical stratigraphy which is described by 1 the thickness and rigidity of stratigraphic units and 2 the nature of interfaces between beds using joint pattern to infer rigidity contrast between beds is complicated by the possibility that material properties of the beds may change between fracturing events observation of both an early bed contained joint set and a differently oriented late throughgoing joint set in the same beds of growth strata in the oliana anticline in the spanish pyrenees may reflect changing mechanical properties i e via diagenesis during the folding process using a schmidt hammer we assess the rigidity contrast between the individual units and show that the present day contrast is not great enough to terminate joints at interfaces this result is consistent with an interpretation that the late stage throughgoing joints formed in strata with conditions similar to those of the present day and that the early bed contained joints formed when the rigidity contrast between beds was significantly greater than that of the present day for example differential diagenesis rates between layers of differing grain size would produce temporally changing mechanical stratigraphy we propose that changes in mechanical stratigraphy may have previously been unrec ognized in other stratigraphic sections and that these changes affect joint network archi tecture and subsequent fluid flow pathways

pb ages of detrital zircons in relation to geodynamic evolution paleozoic of the northern gemericum western carpathians slovakia abstract new u pb sensitive high resolution ion microprobe shrimp detrital zircon ages from early paleozoic to permian siliciclastic metasediments are used to constrain provenance and paleogeographic affinity of the northern gemericum unit ngu in the western carpathians central europe the samples span the time period from 480 ma to 260 ma and permit the recognition of provenance changes that reflect the realm s geodynamic evolution the detrital zircon age spectrum of one early paleozoic sample resembles the neoproterozoic and archean ages of the cadomian belt 550 650 ma and the west african craton provenances 1 8 2 7 ga the origin of the early paleozoic ngu rakovec basin is inferred to be the rifting as followed by back arc spreading which occurred at the cambrian ordovician boundary ca 480 ma as continental pieces started to split off from the gondwanan margin the deformation and closure of the early paleozoic basin is coincident with the 355 ma peak of the post tournaisian detrital zircon age assemblage the detrital zircon age spectra of two post mississippian and two post artinskian samples indicate high exhumation and erosion rates in the variscan collision belt pennsylvanian permian sedimentary troughs formed between 318 and 275 ma record contemporaneous unroofing of the variscan collision belt and magmatic activity during post orogenic transtensional tectonics zircon age spectra reflect these three paleozoic 480 419 ma 340 380 ma 320 275 ma tectono thermal events and permit the recognition of provenances both of which supporting the interpreted changes in geotectonic regime from ordovician rifting to variscan compression and the subsequent extension the zircon age data constrain the provenance linkage of the low grade ngu basement to the cadomian fragments within the west european variscides

gondwana derived microcontinents the constituents of the variscan and alpine collisional orogens abstract the european variscan and alpine mountain chains are collisional orogens and are built up of pre variscan building blocks which in most cases originated at the gondwana margin such pre variscan elements were part of a pre ordovician archipelago like continental ribbon in the former eastern prolongation of avalonia and their present day distribution resulted from juxtaposition through variscan and or alpine tectonic evolution the well known nomenclatures applied to these mountain chains are the mirror of variscan resp alpine organization it is the aim of this paper to present a terminology taking into account their pre variscan evolution at the gondwana margin they may contain relics of volcanic islands with pieces of cadomian crust relics of volcanic arc settings and accretionary wedges which were separated from gondwana by initial stages of rheic ocean opening after a short lived ordovician orogenic event and amalgamation of these elements at the gondwanan margin the still continuing gondwana directed subduction triggered the formation of ordovician al rich granitoids and the latest ordovician opening of palaeo tethys an example from the alps external massifs illustrates the gradual reworking of gondwana derived pre variscan elements during the variscan and alpine tertiary orogenic cycles

new palaeomagnetic constraints on the position of central bohemia during early ordovician times summary apparently conflicting data within the palaeomagnetic database suggest that bohemia was at much shallower palaeolatitudes than the armorican and iberian massifs in ordovician times in order to resolve this controversy a palaeomagnetic study of arenig and llanvirn volcanic and sedimentary rocks from the barrandfan basin central bohemia has been carried out this basin comprises a sequence of unmetamorphosed lower palaeozoic rocks the main folding of which occurred in late devonian to early carboniferous times the samples collected l sites 153 samples were subjected to stepwise thermal and alternating field treatment eleven sites yielded reliable magnetic directions with analysis of the results using standard principal component and great circle analyses up to three directions of magnetiza tion can be identified in these rocks the first termed a is generally removed below 200 c although in some cases it persists up to 450 oc in the volcanic rocks it is approximately parallel to the present day earth s magnetic field in the study area dec inc 360 67 and is thus thought to be of recent origin the second direction identified termed b is isolated at intermediate blocking temperatures 150 350 c and yields an overall in situ mean direction of 195 8 k 22 3 a 95 13 1 seven sites these directions fail the fold test of mcfadden 1990 and correspond to a palaeopole position of 34 s 356 e this coincides with the late carboniferous sector of the european apparent polar wander path and thus the b direction is interpreted as being a secondary overprint of this age the highest blocking temperature direction termed c is identified at temperatures between 350 and 450 600 c it passes the fold test resulting in an overall mean direction of 312 83 k 21 9 a 95 14 6 six sites after bedding correction this direction is interpreted as being representative of the early ordovician palaeomagnetic field direction in the bohemian massif and yields a palaeopole position of 58 n 355 e this translates into palaeolatitudes of 76 s for bohemia thus demonstrating that the bohemian massif was at similar peri polar latitudes to the rest of armorica during early mid ordovician times and formed part of the northern margin of gondwana

sequential granite emplacement a structural study of the late variscan strzelin intrusion sw poland the strzelin massif in sw poland central european variscides records a protracted igneous evolution with three main magmatic stages 1 tonalitic i 2 granodioritic and 3 tonalitic ii granitic in the northern part of this massif the strzelin intrusion proper comprises three successively emplaced rock types a medium grained biotite granite 303 2 ma a fine grained biotite granite 283 8 ma and a fine grained biotite muscovite granite based on field evidence the third variety postdates both types of the biotite granites the structural data from the three granites including their parallel approximately e w striking and steeply dipping lithological contacts and ene wsw trending subhorizontal magmatic lineations suggest that the emplacement of all three successive granite varieties was controlled by an active long lived strike slip fault striking ese wnw with a dextral sense of movement after the emplacement of the youngest biotite muscovite granite the intrusion underwent brittle extension which produced q joints striking nnw sse to n s and dipping at 55 70 wsw to w and showing evidence of broadly n s directed sinistral displacements the structural observations supported by new geochronological data indicate that the internal structure of the composite granitoid intrusion including the faint magmatic foliation and lineation formed in a long lived strike slip setting different from the subsequent post emplacement extensional tectonics that controlled the development of brittle structures

detrital zircon provenance of carboniferous sandstones of the variscan externides sw poland record of the eastern variscides exhumation the variscan externides in the area of south western poland the basement of the fore sudetic monocline are considered an extension of the rheno hercynian zone of the variscan orogen the externides are made of carboniferous siliciclastic sediments the exact stratigraphic age and provenance of which are not known in detail to constrain the sedimentary evolution and detritus provenance of the variscan external belt eight sandstone samples from boreholes located in the area of sw poland were selected for petrography whole rock geochemistry and detrital zircon u pb dating the results show abrupt change in detrital zircon u pb provenance between strata that have been classified as mississippian l carboniferous and pennsylvanian u carboniferous based on previously published miospore zonation the pennsylvanian rocks from the boreholes of wrze nia ig1 and siciny ig1 bear a significant population of zircon grains with lower carboniferous ages 350 330 ma while such a zircon population is absent in lower carboniferous rocks from the boreholes of paproc 29 and kobylin 1 the absence of moscovian to asselian zircons that are otherwise known from the adjacent areas with zircon bearing volcanic activity suggests that the studied pennsylvanian sediments represent the lower part of upper carboniferous the presented data indicate abrupt provenance change that occurred in the source areas at the mississippian pennsylvanian turn and that was probably caused by changes in the tectonic architecture of the variscan orogen this tectonic changeover resulted in exhumation and erosion of the lower carboniferous rock complexes and manifested itself by recycling of the lower carboniferous zircon into upper carboniferous strata

sandstone landforms shaped by negative feedback between stress and erosion the formation and preservation of sandstone landforms such as pillars and arches is enigmatic experiments and numerical modelling show that load bearing material weathers more slowly and thus the internal stress field can shape and stabilize sandstone landforms

reply to the comment on sandstone caves on venezuelan tepuis return to pseudokarst abstract in aubrecht et al 2011 we brought new data showing that the arenitic caves in the venezuelan tepuis did not necessarily originate through quartz cement dissolution arenization but the main portion of the caves originated because of poor lithification prior to erosion an additional important process is lateritization which is responsible for at least 30 of the caves volume sauro et al 2012 in their comment support the arenization theory quartz cement dissolution and provide several arguments against some points of our research their main objections and arguments relate to the following topics 1 the validity of the arenization theory 2 the origin and importance of the finger flow pillars 3 the importance of the schmidt hammer measurements and 4 metamorphism of the mataui formation in our reply we present further documentation that 1 the quartz cement dissolution theory of cave evolution was far less documented petrographically than the new theory presented by us although the presence of quartz dissolution is evident there is no current evidence that it is widespread or that it plays a trigger role many of the presented examples of corrosion on quartz grains could have been caused by local alkalization in addition the hydrogeochemical data presented in the comment to support the arenization theory do not in fact contradict our speleogenetic interpretations 2 finger flow pillars are speleogenetic indicators and not speleogenetic factors their absence in some caves provides no evidence against our theory 3 schmidt hammer measurements only objectivize the data on contrasting hardness in the mataui formation beds they do not say anything about the origin of this difference 4 the arguments of sauro et al 2012 involve the metamorphic overprint of the mataui formation shown by the presence of pyrophyllite and quartz mobilizations because quartz mobilization also occurs in hypergenic conditions it may be neglected as a metamorphic indicator pyrophyllite is mentioned as a metamorphic indicator in the literature anchizone to epizone but it does not explain why there is still a vast quantity of kaolinite that did not undergo this reaction in the soft arenitic beds there is an evidence that pyrophyllite may originate at lower temperatures some phases of the alumino silicate mineral phases existing at atmospheric pressure and 25 c temperature show that h 4 sio 4 concentration is a critical factor in kaolinite pyrophyllite transition an increase in this concentration can theoretically cause transformation of kaolinite to pyrophyllite without increased temperature or pressure

speleogenesis geometry and topology of caves a quantitative study of 3d karst conduits karst systems are hierarchically spatially organized three dimensional 3d networks of conduits behaving as drains for groundwater flow recently geostatistical approaches proposed to generate karst networks from data and parameters stemming from analogous observed karst features other studies have qualitatively highlighted relationships between speleogenetic processes and cave patterns however few studies have been performed to quantitatively define these relationships this paper reports a quantitative study of cave geometries and topologies that takes the underlying speleogenetic processes into account in order to study the spatial organization of caves a 3d numerical database was built from 26 caves corresponding to 621 km of cumulative cave passages representative of the variety of karst network patterns the database includes 3d speleological surveys for which the speleogenetic context is known allowing the polygenic karst networks to be divided into 48 monogenic cave samples and classified into four cave patterns vadose branchwork vb water table cave wtc looping cave lc and angular maze am eight morphometric cave descriptors were calculated four geometrical parameters width height ratio tortuosity curvature and vertical index and four topological ones degree of node connectivity and graph indices and ramification index respectively the results were validated by statistical analyses kruskal wallis test and pca the vb patterns are clearly distinct from am ones and from a third group including wtc and lc a quantitative database of cave morphology characteristics is provided depending on their speleogenetic processes these characteristics can be used to constrain and or validate 3d geostatistical simulations this study shows how important it is to relate the geometry and connectivity of cave networks to recharge and flow processes conversely the approach developed here provides proxies to estimate the evolution of the vadose zone to epiphreatic and phreatic zones in limestones from the quantitative analysis of existing cave patterns

shrimp dating of zircons from high grade metasediments of the schwarzwald sw germany and implications for the evolution of the moldanubian basement four metasedimentary zircon populations from different tectonometamorphic units of the central and the northern schwarzwald variscan belt sw germany were investigated using sem cathodoluminescence and shrimp dating despite partially strong modifications of primary internal morphologies during variscan metamorphism at amphibolite 750 c 0 4 0 6 gpa and granulite facies conditions 950 1 000 c 1 4 1 8 gpa many grains show well preserved protolith ages the detritus indicates a northern gondwana origin and different palaeozoic episodes of sediment deposition and consolidation two of the studied sediments were deposited in cambrian early ordovician times and consolidated in positions close to northern gondwana late ordovician and rare devonian detritus from sediments of two other tectonometamorphic units indicates much later sedimentation close to the leading edge of gondwana or a terrane assemblage during northern drift towards laurussia subsolidus growth of new zircon due to variscan granulite facies metamorphism of one of the tectonometamorphic units is precisely dated at 335 2 ma

development of a deep karst system within a transpressional structure of the dolomites in north east italy abstract the piani eterni karst system is one of the longest and deepest caves of italy situated in the southern sector of the dolomiti mountain range the area where the cave was formed displays peculiar structural settings confined in a tectonic transpressive corridor between two regional thrusts belluno and valsugana during miocene uplift of the range the inheritance of mesozoic structures led to the formation of a deep and wide upward branching flower or palm tree structure cutting the carbonate sequence and exposing the surrounding surface to karst processes after erosion the relative lowering of the hydrologic base level due both to the uplift of the area and then to the carving of deep glacial valleys in the quaternary allowed the formation of paleo phreatic conduits at subsequently deeper levels interconnected by vadose shafts and canyons this work gives a detailed tectonic interpretation of the transpressive structure and picks out the tectonic features most favorable to the karst development a detailed statistical analysis of the distribution and orientation of the karst conduits was performed using 31 km of 3d surveys showing that the development of the cave was strictly guided by a few favorable surfaces of stratigraphic and tectonic origin these features are known in the literature as inception horizons and tectonic inception features respectively cave levels are usually related to lithologic favorable conditions associated with standings of the paleo water table here we suggest that some tectonic surface geometries could have led to the opening of voids in the active tectonic phase leading to the formation of the original proto conduit network different types of tectonic inception features identified in the cave were described in terms of geometry and kinematics tensional fractures as well as fault plane undulations and flexural slip surfaces between beds are described as the most favorable tectonic surfaces for the development of the conduits finally we discuss why transpressional settings and related flower structures in soluble rocks can enhance the karst process allowing the formation of huge and deep karst systems

sandstone caves on venezuelan tepuis return to pseudokarst abstract venezuelan table mountains tepuis host the largest arenite caves in the world the most frequently used explanation of their origin so far was the arenization theory involving dissolution of quartz cement around the sand grains and subsequent removing of the released grains by water new research in the two largest arenite cave systems churi tepui system in chimanta massif and ojos de cristal system in roraima tepui showed that quartz dissolution plays only a minor role in their speleogenesis arenites forming the tepuis are not only quartzites but they display a wide range of lithification and breakdown including also loose sands and sandstones speleogenetic processes are mostly concentrated on the beds of unlithified sands which escaped from diagenesis by being sealed by the surrounding perfectly lithified quartzites only the so called finger flow pillars testify to confined diagenetic fluids which flowed in narrow channels leaving the surrounding arenite uncemented another factor which influenced the cave forming processes by about 30 was lateritization it affects beds formed of arkosic sandstones and greywackes which show strong dissolution of micas feldspars and clay minerals turning then to laterite barro rojo the main prerequisite to rank caves among karst phenomena is dissolution as the dissolution of silicate minerals other than quartz appears to play not only a volumetrically important role but even a trigger role these arenitic caves may be ranked as karst

geodynamic evolution of the european variscan fold belt palaeomagnetic and geological constraints the variscan fold belt of europe resulted from the collision of africa baltica laurentia and the intervening microplates in early paleozoic times over the past few years many geological palaeobiogeographic and palaeomagnetic studies have led to significant improvements in our understanding of this orogenic belt whereas it is now fairly well established that avalonia drifted from the northern margin of gondwana in early ordovician times and collided with baltica in the late ordovician early silurian the nature of the gondwana derived armorican microplate is more enigmatic geological and new palaeomagnetic data suggest armorica comprises an assemblage of terranes or microblocks palaeobiogeographic data indicate that these terranes had similar drift histories and the rheic ocean separating avalonia from the armorican terrane assemblage closed in late silurian early devonian times an early to mid devonian phase of extensional tectonics along this suture zone resulted in formation of the relatively narrow rhenohercynian basin which closed progressively between the late devonian and early carboniferous in this contribution we review the constraints provided by palaeomagnetic data compare these with geological and palaeobiogeographic evidence and present a sequence of palaeogeographic reconstructions for these circum atlantic plates and microplates from ordovician through to devonian times

small scale modelling of cementation by descending silica bearing fluids explanation of the origin of arenitic caves in south american tepuis abstract geoscientific research was performed on south american table mountains tepuis and in their sandstone cave systems to explain speleogenesis in these poorly soluble rocks two theories were introduced a arenization theory implying selective weathering of quartz along grain boundaries and releasing of sand grains b selective lithification theory implying cementation by descending silica bearing fluid flow the latter theory presumes that the descending fluid flow becomes unstable on the interface between two layers with different porosity and splits to separate flow channels so called finger flow the arenites outside these channels remain uncemented to verify the latter theory small scale modelling was performed using layered sands and sodium silicate solution fine to medium sand was used 0 08 0 5 mm along with a coarse sand fraction 0 5 1 5 mm the sands were layered and compacted in a transparent plastic boxes three liters of sodium silicate solution so called water glass were left to drip for several hours to the top of the sediment the fine grained layers were perfectly laterally impregnated whereas the descending fluid flows split to fingers in the coarse grained layers due their higher hydraulic conductivity this small scale laboratory simulation mimics the real diagenesis by descending silica bearing fluids and matches the real phenomena observed on the tepuis the resulting cemented constructions closely mimic many geomorphological features observed on tepuis and inside their caves e g finger flow pillars overhangs imperfectly formed aborted pillars in forms of hummocks hanging from ceilings locally also thicker central pillars that originated by merging of smaller fluid flow channels the modelling showed that selective lithification theory can explain most of the geomorphological aspects related to the speleogenesis in tepuis

numerical modeling of the evolution of arcades and rock pillars abstract arcades i e lenticular and other specifically shaped hollows controlled by discontinuities have recently been recognized as a weathering form typical for sandstones weathered quartzites granites or tuffs they are produced by accelerated weathering and erosion in stress shadows related to the redistribution of gravity induced stress along planar discontinuities in the rock these forms occur worldwide in various settings inland humid arid and coastal the origin of arcades has been demonstrated via physical experiments and supported by a relatively simplistic numerical modeling however details on their shaping and the evolution of related forms have not been explained we performed an advanced numerical modeling to produce various shapes of arcades and rock pillars during the erosion of rock masses dissected by discontinuities we demonstrate that the erosion model in which erosion takes place when the maximum principal stress is below a certain critical value can adequately describe the formation of arcades in the modeling we set higher critical values for stresses at discontinuities than in a homogeneous material representing a rock mass to represent the higher tendency for disintegration of the discontinuity material which was weakened by the discontinuity formation processes by applying various discontinuity geometries and values of critical stresses we were able to reproduce the formation of various arcade shapes and complex three dimensional clusters of arcade cavities with rock pillars discontinuities and stress controlled erosion weathering are the only necessary conditions for arcade formation

geomorphology of the gran sabana guayana shield southeastern venezuela abstract the forms of the gran sabana have been developed mainly on rocks of the gently folded precambrian roraima group a sequence of quartzites and siliceous conglomerates with intercalated jasper chert arkose siltstone and shale locally intruded by diabase dikes and sills two planation surfaces are preserved on the gran sabana the auyan tepui 2000 2900 m elevation and the wonken 900 1200 m surfaces with a speculative mesozoic initial age they are still extending table mountains dominate the landscape they comprise pediments foothills precipitous scarps hundreds of meters high a rim and a flattish summit the table mountain topography is controlled by joint systems water percolates along joints and bedding planes dissolves the siliceous cement the quartzite disaggregates and large blocks are isolated and collapse accumulating at the foot of the scarps and on top of the table mountain summits the percolating water forms large and intricate cave systems which frequently emerge in the scarp zone as high waterfalls these mark the lowest level of erosion of the table mountains karstic structures are prominent in the summit zone the evolution of the table mountain topography has been controlled by a combination of tectonic structural stratigraphic climatic and temporal factors

geochemistry of surface and subsurface waters in quartz sandstones significance for the geomorphic evolution of tepui table mountains gran sabana venezuela summary in situ measurements of discharge ph electric conductivity ec temperature and sio2 content have been carried out during five expeditions in the last 20 years on the summit plateaus inside caves and along the rivers of the surrounding lowlands of three tepui massifs in venezuela auyan roraima and chimanta additionally detailed chemical analyses were performed on waters sampled in a newly discovered extensive quartz sandstone cave system on the auyan tepui rock samples of the quartz sandstone bedrock from different locations have been analysed to obtain their chemical composition with a wavelength dispersive x ray fluorescence spectrometer these data show that the majority of silica present in surface and subsurface water comes from dissolution of quartz and only in minor amount from hydrolysis of other silicate minerals probably the presence of a hardened crust of iron hydroxides limits the dissolution of silica on the top surface of tepuis dissolution in the subsurface instead is more significant and causes in the long term the arenisation of the quartz sandstone and its subsequent removal by mechanical erosion on the other hand waters flowing on the arkosic rock outcropping on the lowland below the tepuis obtain their high dissolved silica content mainly from hydrolysis of silicates the morphological evolution of these table mountains appears thus to be controlled mainly by the underground weathering of the quartz sandstone with the opening of deep fractures grietas and the collapse of large underground horizontal cave systems scarp retreat instead seems to be related to the higher weathering rate of the more arkosic formations underlying the quartz sandstones

structural and lithological guidance on speleogenesis in quartz sandstone evidence of the arenisation process a detailed petrographic structural and morphometric investigation of different types of caves carved in the quartz sandstones of the tepui table mountains in venezuela has allowed identification of the main speleogenetic factors guiding cave pattern development and the formation of particular features commonly found in these caves such as funnel shaped pillars pendants and floor bumps samples of fresh and weathered quartz sandstone of the mataui formation roraima supergroup were characterised through wds dispersive x ray chemical analyses picnometer measurements edax analyses sem and thin section microscopy in all the caves two compositionally different strata were identified almost pure quartz sandstones with content of silica over 95 and high primary porosity around 4 and phyllosilicate rich quartz sandstone with contents of aluminium over 10 and low primary porosity lower than 0 5 phyllosilicates are mainly pyrophyllite and kaolinite sem images on weathered samples showed clear evidence of dissolution on quartz grains to different degrees of development depending on the alteration state of the samples grain boundary dissolution increases the rock porosity and gradually releases the quartz grains suggesting that arenisation is a widespread and effective weathering process in these caves the primary porosity and the degree of fracturing of the quartz sandstone beds are the main factors controlling the intensity and distribution of the arenisation process weathering along iron hydroxide or silt layers which represent inception horizons or a strata bounded fracture network predisposes the formation of horizontal caves in specific stratigraphic positions the loose sands produced by arenisation are removed by piping processes gradually creating anastomosing open fracture systems and forming braided mazes geometric networks or main conduit patterns depending on the local lithological and structural guidance on the weathering process this study demonstrates that all the typical morphologies documented in these quartz sandstone caves can be explained as a result of arenisation which is guided by layers with particular petrographic characteristics primary porosity content of phyllosilicates and iron hydroxides and different degrees of fracturing strata bounded fractures or continuous dilational joints

the process of ghost rock karstification and its role in the formation of cave systems abstract this paper presents an extensive review of the process of ghost rock karstification and highlights its role in the formation of cave systems the process integrates chemical weathering and mechanical erosion and extends a number of existing theories pertaining to continental landscape development it is a two stage process that differs in many respects from the traditional single stage process of karstification by total removal the first stage is characterised by chemical dissolution and removal of the soluble species it requires low hydrodynamic energy and creates a ghost rock feature filled with residual alterite the second stage is characterised by mechanical erosion of the undissolved particles it requires high hydrodynamic energy and it is only then that open galleries are created the transition from the first stage to the second is driven by the amount of energy within the thermodynamic system the process is illustrated by detailed field observations and the results of the laboratory analyses of samples taken from the karstotype area around soignies in southern belgium thereafter a series of case studies provide a synthesis of field observations and laboratory analyses from across western europe these studies come from geologically distinct parts of belgium france italy and the united kingdom the process of ghost rock karstification challenges a number of axioms associated with the process of karstification by total removal on the basis of the evidence presented it is argued that it is no longer acceptable to use karst morphologies as a basis with which to infer specific karstogenetic processes and it is no longer necessary for a karst system to relate to base level as ghost rock karstification proceeds along transmissive pathways in the rock there is also some evidence to suggest that ghost rock karstification may be superseded by karstification by total removal and vice versa according to the amount of energy within the thermodynamic system the proposed chemical weathering and subsequent mechanical erosion of limestone suggest that the development of karst terrain is related far more closely to the geomorphological development of aluminosilicate and siliceous terrains than is generally supposed it is now necessary to reconsider the origin of many karst systems in light of the outlined process of ghost rock karstification

first u pb geochronology on detrital zircons from early middle cambrian strata of the torgau doberlug syncline eastern germany and palaeogeographic implications la icp ms u pb data from detrital zircons of the ediacaran to cambrian siliciclastic sequence of the torgau doberlug syncline tds saxo thuringia germany are reported for the first time the majority of 203 analysed zircon grains is proterozoic with minor amount of archean and palaeozoic grains the u pb ages fall into three groups 2 8 2 4 ga 3 neoarchean to earliest palaeoproterozoic 2 3 1 6 ga 46 early to late palaeoproterozoic 1 0 0 5 ga 47 neoproterozoic to cambrian this age distribution is typical for the west african craton as the source area and for cadomian orogenic events in northwestern gondwana the samples show an age gap between 1 6 and 1 0 ga which is characteristic for west african provenance and diagnostic in distinguishing this unit from east avalonia and baltica the dataset shows clusters of palaeoproterozoic ages at 2 2 1 7 ga that is typical for western gondwana which was affected by abundant magmatic intrusions ca 2 2 1 8 ga during the eburnean orogeny west african craton neoarchean zircon ages 3 point to recycling of magmatic rocks formed during the liberian and leonian orogenies ediacaran to earliest cambrian rocks of the tds originated in an active margin regime of the gondwanan shelf the following early palaeozoic overstep sequence was deposited within rift settings that reflects instability of the west gondwanan shelf and the separation of terranes from ordovician onward the results of this study demonstrate distinct northwestern african provenance of the cambrian siliciclastics of the tds due to th u ratios from concordant zircon analysis igneous origin from felsic melts is concluded as the source of these grains

phreatic drainage conduits within quartz sandstone evidence from the jurassic precipice sandstone carnarvon range queensland australia abstract discrete underground drainage conduits in quartz sandstones are far less common than in limestones this paper provides field evidence from the quartzose precipice sandstone in the carnarvon range of south central queensland australia for tubular underground drainage networks similar in many ways to limestone conduits diameters range from less than 1 or 2 cm to over 1 5 m most display a near circular to oval cross section that seems to suggest phreatic or epiphreatic development and the internal surfaces of many are case hardened by secondary silica deposits a number of the region s perennial springs appear to be fed by such tubes the dominant vertical jointing of the quartz sandstone and relatively high permeability of the sandstone are important controls on tube formation solutional weathering of the sandstone is widespread and is followed by the removal of loosened sand grains by flowing underground water the process of arenisation tube development would appear to have been happening for a very long time and may still be occurring a model for tube network formation is proposed these findings highlight our potentially poor understanding of groundwater flow within some quartz sandstones and may have important groundwater management implications

the oldest rocks of greece first evidence for a precambrian terrane within the pelagonian zone the pelagonian zone in greece represents the westernmost belt of the hellenide hinterland internal hellenides previous geochronological studies of basement rocks from the pelagonian zone have systematically yielded permo carboniferous ages in this study we demonstrate for the first time the existence of a precambrian crustal unit within the crystalline basement of the pelagonian zone the u pb single zircon and shrimp ages of these orthogneisses vary from 699 7 ma to 713 18 ma which identify them as the oldest rocks in greece these late proterozoic rocks which today occupy an area of c 20 100 km are significantly different from the neighbouring rocks of the pelagonian zone they are therefore interpreted as delineating a terrane named here the florina terrane during the permo carboniferous florina was incorporated into an active continental margin where it formed part of the basement for the pelagonian magmatic arc the activity of this arc was dated in this study by single zircon pb pb ages as having taken place at 292 5 ma and 298 7 ma during the alpine orogeny florina together with the pelagonian zone eventually became a constituent of the hellenides geochemically the florina orthogneisses represent granites formed at an active continental margin because of the late proterozoic ages this late proterozoic active continental margin can be correlated to a pan african or cadomian arc as the gneisses contain inherited zircons of late to middle proterozoic age the original location of florina was probably at the northwestern margin of gondwana similar to other gondwana derived terranes such as east avalonia florina approached the southern margin of eurasia during palaeozoic times where it became part of an active continental margin above the subducting palaeotethys these interpretations further indicate that terrane accretion was already playing an important role in the early pre alpine evolution of the hellenides

quantification of rock control in geomorphology abstract rock control the influence of rock properties on landform development has for long been a relatively under researched topic but numerous techniques have been developed especially by geotechnical engineers and stone conservationists that are now being applied to geomorphological issues by geomorphologists these techniques range in scale from the quantification of major discontinuities through to analysis of micro pores rock control studies that have taken place over the last few decades have contributed important insights into our understanding of such diverse phenomena as 1 river terraces bedrock channel forms and gradients pot hole development drainage density and gully badland development 2 slope forms and failures especially with respect to rock mass strength 3 the form of glacial troughs and of subglacial erosional landforms 4 weathering controls on rock durability and the association of weathering with porosity 5 classic karst features including various types of karren 6 sandstone landforms 7 inselbergs and tors and 8 cliffs and shore platforms

genesis of giant sinkholes and caves in the quartz sandstone of sarisarinama tepui venezuela abstract quartz sandstone of the sarisarinama massif in venezuela hosts the world biggest collapse dolines in quartz rich lithologies with volumes up to some millions of cubic meters due to extremely complex logistics required to reach the massif the genesis of these depressions and of the underlying caves has never been studied in detail the lack of field campaigns and extended data has fostered a decade long scientific debate on whether their origin was due to epigenic or hypogenic processes this study integrates petrological structural and hydrochemical observations including analyses of silica concentration ph conductivity of surface and cave waters ec to investigate the speleogenetic processes acting underground petrographic and compositional analyses of the host rock mataui formation show that in the sarisarinama region quartz sandstones are regularly characterized by clay interlayers with significant content of pyrophyllite and kaolinite and minor amount of iron hydroxides compared to surface waters subsurface infiltration along vertical fractures and fault planes show enrichment in silica higher ph and lower ec confirming that chemical weathering is effective underground provoking intergranular silica dissolution along structural discontinuities the weathering of the clay and iron hydroxide interlayers guides the speleogenesis weakening specific stratigraphic levels and causing the collapse and fragmentation of the more resistant quartz sandstone strata the initial void created by piping of the loose sand released by quartz sandstone weathering can migrate upwards by means of roof and wall breakdown this chain of events eventually triggers a collapse at the surface which generates a circular or squared sinkhole the weathering acts mainly along the dominant fracture networks showing a clear guidance by regional tectonics these speleogenetic controls rule out the hypothesis of a hypogenic origin of the simas suggesting a primary role of long term epigenic chemical weathering and mechanical erosion guided by joints weak clay and iron hydroxide interlayers followed by subsequent massive collapses

eastern variscan fold belt paleomagnetic evidence for oroclinal bending the rhenohercynian zone of variscan europe trends from southern portugal around the ibero armorican arc into southern britain belgium northern germany and the moravo silesian zone of the east czech republic although the geodynamic evolution of the central and western segments of this arcuate belt is now fairly well understood the scenario for the more easterly parts remains enigmatic a paleomagnetic study of upper devonian rocks from the moravo silesian zone has been undertaken two ancient directions of magnetization have been identified the first the a magnetization is secondary in origin and relates to a remagnetization of permian carboniferous age the b magnetization passes the fold test is considered late devonian in age and agrees in terms of inclination to previously published paleomagnetic results from coeval rocks of the harz mountains and franconian forest central europe the declination however differs by about 90 restoring the moravo silesian zone to its original devonian orientation as defined by the new paleomagnetic data brings the structural trends of central and eastern europe into agreement because they were previously part of the same tectonostratigraphic unit as suggested by the stratigraphic record this implies that rotational oroclinal bending around the northern and eastern flanks of the bohemian massif occurred sometime between the late devonian and late carboniferous

diagnostic tests for conceptualizing transport in bedrock aquifers transport in bedrock aquifers is complex because there is often substantial flow through fractures and the apertures and interconnectivity of these fractures are usually uncertain single porosity numerical models often give satisfactory results for simulating flow however simulating transport is more challenging and results based on single porosity assumptions can yield inaccurate results seven cases are reviewed where travel times were found to be unexpectedly short results show that dual porosity flow is common with advective flow through fracture networks and immobile storage in the matrix however in some cases a dual or multiple permeability or porosity approach provides better simulations of aquifer behavior fracture porosity of bedrock aquifers is usually 1 resulting in rapid groundwater velocities in many aquifers overestimation of the effective porosity is the most common reason for the overestimation of travel times residence times of artificial tracers in bedrock aquifers are typically two to three orders of magnitude less than residence times of environmental tracers because the latter are retarded by matrix diffusion fourteen diagnostic tests for determining the appropriate conceptual model for bedrock aquifers are described

early palaeozoic initial rift volcanism in the central european variscides the kaczawa mountains sudetes sw poland evidence from sims dating of zircons early palaeozoic volcanic suites are widespread throughout the variscan belt and have commonly been ascribed to incipient rifting along the northern periphery of gondwana in cambrian to ordovician times their distribution across europe defines the present day extent of gondwana derived terranes and constrains the timing of their separation from the gondwanan margin the kaczawa mountains in the west sudetes at the eastern termination of the variscan belt include bimodal rift related rocks but their protolith age and hence their significance have been highly uncertain we have applied secondary ionization mass spectrometry zircon geochronology to a metarhyodacite and a metatrachyte from this suite yielding ages of 502 4 2 6 ma and 485 7 1 6 ma respectively this constrains the initial rift magmatism to c 500 485 ma in this part of the european variscides the rift related early palaeozoic volcanism thus seems to have been broadly synchronous throughout the peri gondwanan terranes of europe

evidence of inception horizons in karst conduit networks this paper outlines the conclusion from an analysis of 18 large cave systems around the world comprising more than 1500 km of conduits the 3d geometry of complex cave systems have been analysed in relation to their geological context as well as their hydrogeological boundary conditions the methodology allowed for the first time statistical evidence of the inception horizon concept thereby it confirms that the development of karst conduits under phreatic conditions is strongly related to a restricted number of so called inception horizons an inception horizon is a part of a rock succession that is particularly susceptible to the development of karst conduits because of physical lithological or chemical deviation from the predominant carbonate facies within the limestone sequence it passively or actively favours the localisation of dissolutional voids

the moldanubian zone in the french massif central vosges schwarzwald and bohemian massif revisited differences and similarities in order to portray the main differences and similarities between the northeastern variscan segments french massif central fmc vosges black forest and bohemian massif bm we review their crustal scale architectures the specific rock associations and lithotectonic sequences as well as the ages of the main magmatic and metamorphic events this review demonstrates significant differences between the moldanubian domains in thebmand the fmc on this basis we propose distinguishing between the eastern and western moldanubian zones while the vosges black forest mountains are an intermediate section between the bm and the fmc the observed differences are the result of first the presence in the french segment of an early large scale accretionary system prior to the main variscan collision and second the duration of saxothuringian armorican subduction which generated long lived magmatic arc and back arc systems in the bohemian segment while the magmatic activity in the fmc was comparably short lived

new paleomagnetic results from ordovician sedimentary rocks from nw anatolia tectonic implications for the paleolatitudinal position of the istanbul terrane abstract the istanbul terrane classically known as the paleozoic of istanbul is geologically one of the important continental components of nw turkey the terrane comprises an early ordovician to early carboniferous transgressive sedimentary sequence and appears as an exotic unit with respect to its present surroundings the paleogeographical position of the ordovician rocks is unknown we have therefore conducted a paleomagnetic study from a total of 56 sites in red fluvial clastics of the kurtkoy formation and shallow marine quartzites of the aydos formation to determine the paleolatitude of the istanbul terrane during the ordovician the lower ordovician group mean direction calculated from 17 reliable sites provides a mean inclination of i 19 4 95 2 3 k 146 0 accepting only inclination data a paleolatitude of 16 4 is obtained after considering an inclination correction factor of f 0 6 due to the e i results the early ordovician paleolatitude of the istanbul terrane shows a lower paleolatitudinal position closer to the equatorial zone than previously inferred