u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

potential of pressure solution for strain localization in the baccu locci shear zone sardinia italy abstract the mylonites of the baccu locci shear zone blsz sardinia italy were deformed during thrusting along a bottom to top strain gradient in lower greenschist facies the microstructure of metavolcanic protoliths shows evidence for composite deformation accommodated by dislocation creep within strong quartz porphyroclasts and pressure solution in the finer grained matrix the evolution of mylonite is simulated in two sets of numerical experiments assuming either a constant width of the deforming zone model 1 or a narrowing shear zone model 2 a 2 5 mm y 1 constant external velocity boundary condition is applied on the basis of geologic constraints inputs to the models are provided by inverting paleostress values obtained from quartz recrystallized grain size paleopiezometry both models predict a significant stress drop across the shear zone however model 1 involves a dramatic decrease in strain rate towards the zone of apparent strain localization in contrast model 2 predicts an increase in strain rate with time from 10 14 to 10 12 s 1 which is consistent with stabilization of the shear zone profile and localization of deformation near the hanging wall extrapolating these results to the general context of crust strength suggests that pressure solution creep may be a critical process for strain softening and for the stabilization of deformation within shear zones

geodynamic evolution of the sw europe variscides 1 the early evolution of sw europe variscides started by opening of the rheic ocean at 500 ma splitting avalonia from armorica iberia subduction on the se side of rheic generated the paleotethys back arc basin 430 390 ma splitting armorica from iberia with development of porto tomar ferreira do alentejo ptfa dextral transform defining the boundary between continental armorica and finisterra microplate to the w obduction of paleotethys was followed by armorica iberia collision and emplacement of nw iberian allochthonous units at 390 370 ma whereas toward the west of ptfa there was antithetic ophiolite obduction beja acebuches and rheic ophiolites plus finisterra continental slices on top of ossa morena zone with simultaneous development of eclogites and orogenic magmatism under a flake double wedge tectonic regime continued convergence 370 ma proceeded by intracontinental deformation with progressive tightening of the ibero armorican arc through dextral transpression on the cantabrian indentor from iberia to armorica the proposed model is discussed at the light of the driving mechanism of soft plate tectonics

estimating locations and magnitudes of earthquakes in southern california from modified mercalli intensities modified mercalli intensity mmi assignments instrumental moment magnitudes m and epicenter locations of thirteen 5 6 m 7 1 training set events in southern california were used to obtain the attenuation relation mmi 1 64 1 41 m 0 00526 h 2 63 log h where h is the hypocentral distance in kilometers and m is moment magnitude intensity magnitudes m i and locations for five 5 9 m 7 3 independent test events were consistent with the instrumental source parameters fourteen historical earthquakes between 1890 and 1927 were then analyzed of particular interest are the m i 7 2 9 february 1890 and m i 6 6 28 may 1892 earthquakes which were previously assumed to have occurred near the southern san jacinto fault a more likely location is in the eastern california shear zone ecsz these events and the 1992 m 7 3 landers and 1999 m 7 1 hector mine events suggest that the ecsz has been seismically active since at least the end of the nineteenth century the earthquake catalog completeness level in the ecsz is m 6 5 at least until the early twentieth century online material figures and table of empirical mmi site corrections

pre alpine variscan inheritance a key for the location of the future valaisan basin western alps the boundary between the helvetic and the penninic brianconnais zones has long been recognized as a major fault penninic front in the western alps a narrow oceanic domain has been postulated at least along part of this boundary the valaisan ocean however the information provided by the pre triassic basement has not been fully exploited and will be discussed here in detail the igneous and metamorphic history of the pre triassic basement shows significant differences between the external massifs from the helvetic zone with abundant late carboniferous granites and the basement of the brianconnais zone including the internal massifs dora maira gran paradiso monte rosa devoid of carboniferous granites a major coal bearing basin the zone houillere opened along this boundary this limnic intramontane basin has never been properly investigated the zone houillere is not comparable with the external paralic flexural basins on both sides of the variscan belt but shows similarities with the saar saale basin like the latter we interpret the zone houillere as a transtensional basin opened along a major crustal scale fault zone namely the east variscan shear zone the permian magmatism and sedimentation displays contrasting distributions being absent or very localized in the helvetic zone and widespread in the penninic zone the above data indicate that the structural inheritance from the variscan belt plays a major role in defining the future location of the valaisan basin i e the boundary between the european palaeomargin and the brianconnais microcontinent

reply to comment on revisiting the 1872 owens valley california earthquake by susan e hough and kate hutton by william h bakun bakun 2009 argues that the conclusions of hough and hutton 2008 are wrong because the study failed to take into account the sierra nevada attenuation model of bakun 2006 in particular bakun 2009 argues that propagation effects can explain the relatively high intensities generated by the 1872 owens valley earthquake using an intensity attenuation model that attempts to account for attenuation through the sierra nevada bakun 2006 infers the magnitude estimate m w 7 4 7 5 that is currently accepted by national earthquake information center neic

age of amphibolitic metamorphism in the ophiolitic unit of the morais allochthon portugal implications for early hercynian orogenesis in the iberian massif structurally lower portions of the allochthonous morais complex are comprised of an ophiolitic unit exposed in two contrasting structural units separated by a ductile thrust fault the lower unit is dominated by variably foliated amphibolite whereas the upper unit is largely comprised of metaperidotite and metagabbro most of the ophiolitic unit has recrystallized synkinematically under amphibolite facies metamorphic conditions c 500 c and 5 kbar amphiboles formed during this metamorphism are typically magnesio hornblende an amphibole concentrate from the lower structural unit records a 39 ar 40 ar plateau age of 397 3 4 ma an amphibole concentrate from the upper structural unit displays an internally discordant 39 ar 40 ar age spectrum corresponding to a total gas age of 431 1 27 3 ma gas fractions evolved from each amphibole concentrate at intermediate and high experimental temperatures are characterized by similar apparent k ca ratios and yield well defined 36 ar 40 ar v 39 ar 40 ar isotope correlations which result in similar ages of 384 2 5 2 and 392 4 7 3 ma these are considered to be more reliable than the plateau ages and are interpreted as dating the last cooling through those temperatures required for intracrystalline retention of argon within constituent amphibole grains the associated tectonothermal event represented a first stage of hercynian orogenesis in the iberian massif it was probably related to oceanic closure and associated obduction of granulitic eclogitic massifs which at present structurally overlie the ophiolitic unit

seismic anisotropy tracing plate dynamics in the mantle elastic anisotropy is present where the speed of a seismic wave depends on its direction in earth9s mantle elastic anisotropy is induced by minerals that are preferentially oriented in a directional flow or deformation earthquakes generate two seismic wave types compressional p and shear s waves whose coupling in anisotropic rocks leads to scattering birefringence and waves with hybrid polarizations this varied behavior is helping geophysicists explore rock textures within earth9s mantle and crust map present day upper mantle convection and study the formation of lithospheric plates and the accretion of continents in earth history

experimental resetting of the u th pb systems in monazite abraded fragments 200 400 m of a large chemically homogeneous and non metamict brazilian monazite crystal characterised by a concordant u pb ages of 474 1 ma 208pb 206pb 19 5 were hydrothermally treated at varying temperatures with solutions of different compositions product monazites were analysed with scanning electron microscope sem electron microprobe emp secondary ion mass spectrometer sims and isotope dilution thermal ionisation mass spectrometer id tims experiments with pure water over a temperature range of 800 1200 c at 700 mpa and durations ranging from 5 to 60 days showed that even at 1200 c any dissolution and recrystallization of new monazite is confined to the outermost surface of the grain neither pb diffusion at the emp scale nor significant discordancy were observed we performed experiments at 800 and 1000 c for different durations using different fluid compositions at quartz saturation a 10 wt cacl2 fluid a 10 wt srcl2 fluid a 10 wt nacl fluid and a fluid containing nbs 982 pb standard 208pb 206pb 1 for all runs emp traverses revealed no pb diffusion profiles significant overgrowths of newly formed monazite are documented by sem analyses they occurred only in the 1000 c experiments when either cacl2 or pb bearing fluids were present in the cacl2 experiment two zones could be distinguished within the crystal a core possessing the initial monazite composition and a rim consisting of newly formed monazite produced by dissolution precipitation which was enriched in ca and pb free id tims dating of single grains treated with srcl2 and cacl2 solutions at 1000 c are significantly discordant experiments employing the nbs pb standard produced sub concordant monazite for which the 207pb 206pb apparent age has become older than prior to the experiment 544 ma at 800 c and 495 ma at 1000 c the newly grown monazite rim had obviously incorporated pb from the fluid none of our reaction products contained a detectable diffusion profile the only resetting mechanism we detected involved dissolution precipitation the extent of the dissolution precipitation process depends on fluid composition and is a more efficient mechanism than diffusion for controlling the resetting of monazite in natural rocks

discussion on middle jurassic shear zones at cap de creus eastern pyrenees spain a record of pre drift extension of the piemonte ligurian ocean journal of the geological society london 174 289 300 vissers et al 2017 performed muscovite 40ar 39ar geochronology on shear zones from the northern cap de creus shear belt variscan basement of the eastern pyrenees they propose a model according to which these shear zones formed during middle jurassic extension associated with the opening of the piemonte ligurian ocean to the east we argue that this model contradicts structural and tectonic evidence and is not sufficiently supported by geochronological data

orogen parallel tectonic transport in the variscan belt of northeastern sardinia italy implications for the exhumation of medium pressure metamorphic rocks a transpressive crustal scale dextral shear zone is documented in the variscan basement of northeastern sardinia it indicates the presence of a shear deformation parallel to the belt overprinting previous d1 structures related to nappe stacking and top to the s and sw thrusting the l2 stretch ing lineation points to an orogen parallel stretching and to a general change in the tectonic transport from d1 to d2 phase d1 developed during initial frontal collision whereas the d2 deformation was characterized by dextral shearing during the increasing curvature of the ibero armorican arc transpressional deformation developed in a regime of decreasing pressure it caused telescoping of the barrovian isograds and the exhumation of the low to medium grade metamorphic rocks in this sector of the variscan belt exhumation is due to continuing compression with an increasing compo nent of horizontal displacement the overall change of the shortening direction in a large sector of an orogenic belt with the occurrence of increasing orogen parallel displacement may be regarded as a general mechanism affecting the exhumation of rocks and preventing the thickened collisional crust from undergoing a generalized gravitational collapse

strain analysis and vorticity of flow in the northern sardinian variscan belt recognition of a partitioned oblique deformation event abstract a field example of strain partitioning has been analysed along the nurra asinara transect of the nw sardinian variscan chain italy the section in the nurra asinara area is in a continuous sequence of tectono metamorphic complexes made of low to high grade metamorphic rocks affected by a polyphase tectonic history the principal fabric of the area is controlled by a d2 progressive deformation phase in which the strain is partitioned into folds and shear zone domains the d2 stretching lineation and shear sense show a clear change from south to north the principal meso and micro structures vorticity gauges and a quantitative kinematic analysis of local strain suggest that the d2 kinematic history could be envisaged as an oblique heterogeneous deformation similar to the transpressive systems described in ancient and modern settings elsewhere using a simple kinematic model we also propose that both a transpressive system followed by thrusting or a partitioned transpressive system could be responsible for the fabric distribution and strain accumulation described in the study transect

are some of the deep crustal conductive features found in sw iberia caused by graphite abstract recent results obtained from two dimensional inversion of magnetotelluric mt data acquired in sw iberia reveal high conductive features at the middle lower levels of the crust the top of these anomalous structures correlates very well with the depth 10 13 km of an important seismic interface that has been interpreted as a regional detachment horizon very shallow and relatively narrow conductors in the ossa morena zone appear to correspond to small scale fluid deposited graphite systems in the preorogenic metasedimentary sequences some of the midcrustal conductors can be ascribed to graphite bearing thrust zones the formation of graphite with variable crystallinity being a consequence of variscan shearing processes deep seated conductors are tentatively interpreted as a result of relatively continuous highly crystalline grain boundary graphite films presumably preserved in basement granulite rocks assuming that graphite occurs as interconnected films calculations indicate that a fraction of 0 006 0 02 of this accessory mineral is enough to explain the range of the electrical resistivity estimated on the basis of mt models the role of graphite on the thermal behaviour of the crust is also discussed the results show that low contents of graphite do not significantly change the thermal behaviour of earth materials

early permian 90 clockwise rotation of the maures esterel corsica sardinia block confirmed by new palaeomagnetic data and followed by a triassic 60 clockwise rotation

the armorica microplate fact or fiction critical review of the concept and contradictory palaeobiogeographical data abstract the problem of an armorica microplate detached from gondwana and having had during part of the palaeozoic an independent latitudinal evolution is reconsidered in terms of a critical review of the palaeomagnetic data that were the very roots of this concept and through an alternative approach based on palaeoclimatic and palaeobiogeographical data palaeomagnetic data for the silurian and the devonian of the south european regions supposedly constituting the armorica microplate remain rare and ambiguous those from gondwana are more numerous but contradictory enough to give rise to diverging models regarding the latitudinal evolution of this continent consequently the reality of an armorica microplate cannot be considered as established on the contrary lithological indicators of palaeoclimate and palaeobiogeographical data are in total harmony and indicate that in actual fact the southern european regions remained permanently closely connected with gondwana of which they composed the northern margin although repeatedly maintained for more than 20 years the concept of an armorica microplate can thus be considered a fiction this conclusion should lead to the dismissal of geodynamical models proposed for the variscan belt to which this concept was integrated and are contradicted by inescapable palaeobiogeographical constraints

kinematic and geochronological constraints on shear deformation in the ferriere mollieres shear zone argentera mercantour massif western alps implications for the evolution of the southern european variscan belt in the western alps a steeply dipping km scale shear zone the ferriere mollieres shear zone cross cuts variscan migmatites in the argentera mercantour external crystalline massif structural analysis joined with kinematic vorticity and finite strain analyses allowed to recognize a high temperature deformation associated with dextral transpression characterized by a variation in the percentage of pure shear and simple shear along a deformation gradient u th pb dating of syn kinematic monazites was performed on mylonites the oldest 340 ma ages were obtained in protomylonites whereas ages of 320 ma were found in mylonites from the core of the shear zone these ages indicate that the ferriere mollieres shear zone is a still preserved variscan shear zone ages of 320 ma obtained in this work are in agreement with ages of the dextral transpressional shear zones occurring in the maures tanneron massif and corsica sardinia however transpression in the argentera mercantour massif started earlier than in other sectors of the southern variscan belt this is possibly caused by the curvature of the belt triggering the progressive migration of shear deformation our data allow a correlation between the argentera mercantour massif and other segments of the southern european variscan belt in particular with maures tanneron massif and corsica sardinia and contribute to fill a gap in the age of activity and in the kinematics of the flow of the system of dextral shear zones of the southern portion of the evsz

variscan accretionary complex of northwest iberia terrane correlation and succession of tectonothermal events the allochthonous terranes of northwest iberia can be correlated with specific paleogeographic realms of the continental masses and intervening oceans involved in the variscan collision assuming that the existing ophiolites represent the suture formed by the closure of the rheic ocean the units in the footwall to the suture correspond to the outer edge of the gondwana continental margin and the units in the hanging wall are interpreted as fragments of the conjugate margin represented by the meguma terrane this correlation establishes a precise link between circum atlantic terranes and makes it possible to draw a relatively simple scenario of the successive tectonothermal events recorded following the amalgamation of avalon to laurentia the remaining outboard terranes underwent a progressive accretion to this continent that ended with the collision between laurentia and gondwana

evolution of the corsica sardinia batholith and late orogenic shearing of the variscides the corsica sardinia batholith formed in the late carboniferous permian along the northern gondwana margin one or more of the following processes may have raised the variscan geotherm enhancing melting of the crust and uppermost mantle such as i break off and detachment of the rheic oceanic slab ii mantle delamination due to gravitational re equilibration of thickened crust iii shear heating iv advection of mantle derived melts and v concentration of heat producing elements in this paper we present a simple one dimensional thermal model to explain the origin of the batholith in a geodynamic setting consistent with intraplate shearing between gondwana and cadomian avalonian blocks input parameters and boundary and initial conditions of the model crust are derived from a careful re examination of large petrological geochronological and structural datasets all parameters are varied within a range of geologically realistic values to reproduce thermal histories related to different tectonic processes and the reliability of models is quantitatively evaluated by comparing the simulated geotherms with a large dataset of pressure temperature time p t t constraints the best fit to p t t constraints is obtained for i break off in the late devonian early carboniferous ii decreasing exhumation rates from 340 to 280 ma and iii transpression at high average strain rates mostly coeval with the peak of high t low p metamorphism

thermal and mechanical evolution of an orogenic wedge during variscan collision an example in the maures tanneron massif se france synthesis of structural petrological and geochronological data for the maures tanneron massif and its integration in the framework of adjacent massifs i e sardinia and corsica has allowed us to propose a new model of evolution for the southern variscan belt after siluro devonian subduction associated with high pressure low temperature hp lt metamorphism m 0 c 10 15 8c km 21 and subsequent carboniferous nappes stacking the belt underwent strong reworking related to back thrusting nappes stacking and back thrusting were associated with typical barrovian metamorphism m 1 c 20 30 8c km 21 starting at 360 ma that progressively evolved to higher temperature metamorphisms m 2 c 40 60 8c km 21 and m 3 c 60 80 8c km 21 during 330 300 ma in the internal part of the belt progressive increase of the thermal gradient is interpreted as a consequence of gravitational instabilities triggered in the partially molten orogenic root continuous compressive forces applied to the belt allowed vertical extrusion of the orogenic root in fold dome structures the mass transfer is accommodated by orogen parallel transpressive shearing synchronous with m 3 during late carboniferous time the orogenic wedge is characterized by two main tectono metamorphic units decoupled by a major shear belt an internal zone with migmatites and syntectonic granitoids where hp relicts have been exhumed and an external zone that escaped the late ht event and preserved precious structures

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

metallogenetic evolution of the east alpine paleozoic basement paleo metallogenetic sketch maps of the east alpine area are presented for the ordovician silurian the devonian lower carboniferous and the upper carboniferous permian mineralization and tectonic evolution are then discussed in the frame of a plate tectonic model compatible with recent results in western europe

das phanomen trias kurze ubersicht uber palaogeographische palaotektonische und palaoklimatologische probleme der trias unter ausklammerung des germanischen beckens allgemeiner rahmen zu den referaten der trias tagung 1982 in wurzburg

geophysical evidence of a mantle derived intrusion in sw iberia 1 deep seismic reflection data acquired as part of the sw iberia europrobe project across the transpressional variscan orogen sample three tectonic terranes the south portuguese zone the ossa morena zone and the central iberian zone the seismic data reveal the existence of a mid crustal reflective body 140 km long and of variable thickness up to 5 km the iberian reflective body the conductivity image provided by coincident mt soundings the amplitude characteristics of the seismics mineralization studies related to magmatic ore deposits and the surface geology suggest that the irb is a mantle derived mafic intrusion the geophysical geological and petrological data suggest that the irb is most probably an early carboniferous approximately at 350 340 ma mantle derived intrusion possibly linked to plume activity that took place in europe in the carboniferous and permian

petrology and geotectonic development of the western ecuadorian andes the basic igneous complex abstract the basic igneous complex bic consists of several volcanic formations which form the ecuadorian western cordillera and coastal cordillera the rocks differ significantly from other cordillerian successions and form an isolated tectnic block the upper cretaceous to lower tertiary volcanic series of the ecuadorian basic igneous complex are of island arc type according to their petrography and geochemistry an outer tholeiitic island arc series parts of the pinon formation can be distinguished from a central calc alkalic series depleted in potassium macuchi celica and silante formations the chontal yunguilla and zapotillo formations as well as the upper cayo formation guayaquil cherts are interpreted to represent back arc sediments and sediments of small intra arc basins within a largely submarine island arc the cayo formation consists almost entirely of volcaniclastic and epiclastic rocks deposited on the oceanward side of the island arc which was situated close to the south american continent volcanic activity continued into early eocene times as revealed by k ar dating of the cayo and silante formations 52 ma a small back arc basin was filled with the sediments of the yunguilla formation similar to present day volcanically active island arcs e g aleutians and kuriles kamchatka the bic island arc was mostly underlain by oceanic crust although parts of it built on continental crust celica formation southern ecuador in the early tertiary the island arc was accreted to the south american continent cessation of volcanic activity and accretion to the continent may be explained by increased drift velocities of the lithospheric plates accompanied by a change in the geometry and stress in the subduction zone

structural evolution of zonal metamorphic sequences in the southern chinese altai and relationships to permian transpressional tectonics in the central asian orogenic belt the chinese altai in the central asian orogenic belt is characterized by the occurrence of kyanite and andalusite type metamorphic zones however the tectonic setting associated with these metamorphic zones is still debated here we present structural observations from the southern chinese altai aletai area where a relict kyanite zone occurs within an andalusite type metamorphic zonal sequence of sillimanite andalusite sillimanite andalusite staurolite staurolite garnet and biotite we document two major phases of folding in this area the earlier structures d 1 developed synchronously with kyanite growth and are represented by an axial planar fabric of tight folds d 1 fabric is mostly obliterated by later deformation but is inferred to be penetrative as indicated by a profound intersection lineation l 21 the second generation of deformation d 2 is associated with superimposed f 2 folds and a dominant axial planar foliation s 2 with similar to nw se orientation microstructural observations show that d 2 structures are associated with the andalusite type metamorphic series which is characterized by an early d 2 occurrence of sillimanite staurolite garnet and biotite zonal sequence overprinted by the late d 2 growth of andalusite combining with previous metamorphic data we consider that these two folding events d 1 and d 2 correspond with the development of kyanite and andalusite type metamorphic zones respectively which records the burial and exhumation history of metamorphic rocks possibly in response to devonian carboniferous accretion collision and permian transpressional tectonics the latter was associated with oblique convergence between the chinese altai and the intra oceanic arc system of the east west junggar c 2015 elsevier b v all rights reserved

the fast evolution of a crustal hot zone at the end of a transpressional regime the saint tropez peninsula granites and related dykes maures massif se france granite and dolerite occurring in the eastern unit of the maures massif southern france variscan belt were emplaced in the waning stage of the evolution of a dextral shear zone along the gondwana border with laurasia we present a geochronological and petro geochemical study major and trace elements sr nd isotopes of 1 the foliated moulin blanc cordierite granite and related foliated dykes 2 the equant camarat granite and dyke and 3 the andesite basalt dolerite dykes frequently mingled with leucogranite material at pinet and capon tips and cross cutting leucogranitic dykes zircon and monazite dating of the moulin blanc granite gives a crystallization age of 301 2 ma shrimp dating on zircons from the leucogranite component of a pinet composite dyke only reveals zircon core ages of mainly 310 10 ma age of migmatization of the host gneiss undulating contacts of dolerite dykes within the c 300 ma old camarat granite suggest penecontemporary relationships between the two intrusions the age interval between the moulin blanc granite and the dolerite dykes would thus be very short the moulin blanc pluton is formed by mingling and mixing of melts of granitic and dioritic composition the latter occurring mainly as microgranular mafic enclaves two other types of diorites are also identified in the pluton the foliated dykes are similar in composition to the moulin blanc granite which confirms their relationship to the main pluton the moulin blanc granite is a typical example of a pluton formed by repeated recharges coming from a deeper magma chamber formation of the camarat dykes is controlled by hydrothermal fluids that are responsible for the non charac behavior of trace elements the pinet and capon leucogranite components of the dolerite dykes have distinct geochemical signatures different from the camarat granite and dykes the cross cutting leucogranitic dykes have much geochemical similarities with the capon granite the age of similar dykes in the tanneron massif supports the contemporaneity of camarat granite and dolerites the dolerite geochemistry was deeply modified by alteration processes but an andesite basalt composition is still recognizable by using immobile element ratios in a end vs sri diagram all rocks plot close to a mixing hyperbola between a depleted mantle component and a crustal component characteristic of the variscan crust in the french massif central there is a broad correlation between the ndtdm 2 stage model ages and the degree of fractionation of the rocks modeling of dehydration melting in a range of p t conditions reveals that a variety of lithologies were the source of the saint tropez magmas high alumina and alkali basalt amphibolites metagraywackes metapelites and meta arkoses these sources were melted almost simultaneously at different levels in the crust the calculated critical dyke width required to transport the granitic melt by buoyancy is far too large compared to the observed granitic dyke thicknesses therefore tectonic processes and or melt overpressure must have played a role in the emplacement mechanism thermo mechanical constraints on mixing between mafic and felsic melts suggest that in the pinet composite dykes both melts should have mixed the observed lack of mixing is interpreted as due to quick cooling the region was affected by a hot zone at the end of a transpressional regime

conditions for flow in the continental crust 1 earthquake focal depth distributions and effective elastic thickness variations on the continents suggest that in old cold regions such as shields the lower continental crust can be stronger than the upper mantle and imply that the upper mantle can flow even when the lower crust does not however in other places substantial flow in the lower crust does occur particularly near extensional metamorphic core complexes we suggest that the lower crust only flows when its viscosity is significantly reduced either by heating from igneous intrusions or by the addition of water in both cases the important characteristics of the flow are a consequence of the limited time over which the viscosity reduction occurs the timescale is controlled either by conductive cooling of intrusions or by the separation of melt from its matrix which removes the water

crustal thickening and deformation sequence in the footwall to the suture of the variscan belt of northwest spain abstract the sequence of formation and propagation of structures as well as the evolution of crustal thickening in the variscan belt of northwest spain are proposed in this paper the foreland of this mountain belt shows thin skinned tectonics a polyphase deformation with associated regional metamorphism is recognized in the hinterland zones two important culminations the narcea and lugo antiforms are present in an e w cross section through the footwall to the suture of this orogen such culminations are derived from the stacking of several thrust systems generated during two successive periods the sequence of thrust emplacement helps to clarify the formation of different types of structures at the same time in different places in the orogen

the ibero armorican arc a collision effect against an irregular continent the ibero armorican arc is the main variscan macrostructure in western europe although it was recognized in the 1920s its genesis is still debatable and the proposed models for its generation questionable however even if some doubts persist it seems that most agree on the general geodynamic evolution of this virgation in both branches in the middle to late devonian iberia was deformed by a sinistral transpressive regime while in the northern branch thrusting events were predominant in the carboniferous a dextral transpression begins to predominate in the armorican branch while the southern branch was deformed by southward thrusting in an attempt to correlate these events we propose that during the late devonian a cantabrian indentor moved northward producing the oblique closure of the southern part of the rheic ocean and an almost orthogonal closure in central europe in the carboniferous the collision with the irregular margin of laurasia induced a rotation of the indentor the intracontinental deformation was then achieved by dextral transpression in the northern branch and thrusting in the southern one

kinematic vorticity and strain rate patterns associated with ductile extrusion in the chelmos shear zone external hellenides greece abstract kinematic vorticity strain rate finite strain and structural data have been used to describe the heterogeneous nature of ductile deformation in an extruding crustal scale shear zone the chelmos shear zone csz of the external hellenides the csz has a curved shape which is marked by a frontal flat and a steep zone the spatial variation in the kinematic vorticity number is strongly correlated with the geometry and the structural depth of the shear zone the pure shear component of deformation decreases from back to front and from top to bottom the ductile thinning normal to the shear zone s boundaries is higher in the steep zone and decreases westwards quartz grain size and strain rate in the csz is distributed into two approximately homogeneous domains the higher strain rate is recorded in the steep zone and the base of the frontal flat zone with lower rates in the upper parts of the frontal flat zone based on mesoscopic structural data and the spatial distribution of the shear strain rate the existence of a west dipping back flow zone in the boarder zone between these two domains is inferred the overall pattern of deformation in the csz implies that tectonic extrusion from the steep zone occurred at the final stages of ductile deformation and was associated with the formation of the backflow zone in the front of the shear zone ductile deformation of rocks probably started close to simple shear on a planar low angle shear zone progressively became penetrative and more general shear occurred as the csz steepened up this progressive increase in pure shearing may have occurred in a heterogeneous manner and was higher close to both the eastern margin of the steep zone and the top of the frontal flat zone solid state extrusion in the csz was also associated with a heterogeneous and progressive increase in strain rate

mmi attenuation and historical earthquakes in the basin and range province of western north america earthquakes in central nevada 1932 1959 were used to develop a modified mercalli intensity mmi attenuation model for estimating moment mag nitude m for earthquakes in the basin and range province of interior western north america m is 7 4 7 5 for the 26 march 1872 owens valley california earthquake in agreement with beanland and clark s 1994 m 7 6 that was estimated from geologic field observations m is 7 5 for the 3 may 1887 sonora mexico earthquake in agreement with natali and sbar s 1982 m 7 4 and suter s 2006 m 7 5 both estimated from geologic field observations mmi at sites in california for earthquakes in the nevada basin and range appar ently are not much affected by the sierra nevada except at sites near the sierra nevada where mmi is reduced this reduction in mmi is consistent with a shadow zone produced by the root of the sierra nevada in contrast mmi assignments for earthquakes located in the eastern sierra nevada near the west margin of the basin and range are greater than predicted at sites in california these higher mmi values may result from critical reflections due to layering near the base of the sierra nevada

archean protoliths within early proterozoic granulitic crust of the west european hercynian belt possible relics of the west african craton granulites from the bay of biscay area have been studied by using u pb and sm nd methods results on four granulites suggest two major events of crustal growth and high grade metamorphism the first is archean 2 7 ga and the second is early proterozoic 1 9 ga in the hercynian belt the oldest basement remnants known so far are about 2 0 ga and are located in the northern armorican massif the 2 7 ga orthogneiss protolith age from the bay of biscay is the first direct evidence of archean crust involved in the hercynian belt major events at 2 7 1 8 and 0 6 ga in the hercynian belt suggest that european precambrian microplate s were part of the west african craton where the same sequence of events is observed

on the modified mercalli intensities and magnitudes of the 1811 1812 new madrid earthquakes we reexamine original felt reports from the 1811 1812 new madrid earthquakes and determine revised isoseismal maps for the three principal mainshocks in many cases we interpret lower values than those assigned by earlier studies in some cases the revisions result from an interpretation of original felt reports with an appreciation for site response issues additionally earlier studies had assigned modified mercalli intensity mmi values of v vii to a substantial number of reports that we conclude do not describe damage commensurate with intensities this high we investigate several approaches to contouring the mmi values using both analytical and subjective methods for the first mainshock on 02 15 lt december 16 1811 our preferred contouring yields m 7 2 7 3 using the area moment regressions of johnston 1996 for the 08 00 lt on january 23 1812 and 03 45 lt on february 7 1812 mainshocks we obtain m 7 0 and m 7 4 7 5 respectively our magnitude for the february mainshock is consistent with the established geometry of the reelfoot fault which all evidence suggests to have been the causative structure for this event we note that the inference of lower magnitudes for the new madrid events implies that site response plays a significant role in controlling seismic hazard at alluvial sites in the central and eastern united states we also note that our results suggest that thrusting may have been the dominant mechanism of faulting associated with the 1811 1812 sequence

geochronological constraints on post collisional shear zones in the variscides of sardinia italy terra nova 24 42 51 2012 abstract the post collisional tectonics in the inner zone of the variscan belt of corsica sardinia recorded a dextral transpression developed with an orogen parallel tectonic transport recent works documented the presence of nearly parallel dextral and sinistral crustal scale shear zones responsible for the exhumation of the metamorphic complexes to constrain for the first time the timing of both shear zones and the timing of the transpressional shearing zircons and monazites were collected for u th pb ages results indicate that the shear zones have been active at c 320 ma in a transpressional setting widespread in the southern european variscan belt

early ordovician orogenic event in galicia nw spain evidence from u pb ages in the uppermost unit of the ordenes complex new u pb data zircon monazite and rutile obtained from rocks of the uppermost allochthon in the variscan belt of nw spain indicate that the hangingwall to the suture includes an allochthonous unit with a pre variscan tectonothermal evolution this evolution is characterised by an early ordovician 498 500 ma bimodal magmatism followed almost immediately 493 498 ma by a barrovian style metamorphism up to the granulite facies the metamorphism subsequent to the igneous intrusions requires convergence and crustal thickening in order to generate the barrovian facies pattern shown by the lithologies of the uppermost allochthon the almost coeval magmatism and metamorphism and the chemistry of the metabasites suggest an accretionary complex probably related to a volcanic arc as the most probable setting for the origin of the uppermost unit the implications of the existence of early ordovician convergent plate boundaries are discussed in the context of gondwana laurentia avalon interactions

the east variscan shear zone geochronological constraints from the capo ferro area ne sardinia italy abstract the exhumation of the variscan high grade metamorphic complex of sardinia was driven by large scale asymmetric shear zones which reached middle crustal levels at 325 1 3 u pb monazite ma exhumation was accompanied by heating and partial melting the migmatization process was followed by the emplacement of syntectonic granites dated with u pb on zircon at 318 3 ma and 317 2 ma which produced contact metamorphism at 315 1 3 ma u pb monazite in mylonitic silurian orthogneisses the nw se dextral shear zone of the capo ferro area ne sardinia is affected by partial melting by granite emplacement and shows remarkable petrotectonic analogies to and can thus be related to the east variscan shear zone a major intra continental shear zone which was active in the southern european variscides from 325 ma to 315 ma and that has been precisely dated with the u pb monazite system the emplacement of migmatitic gneisses along large shear zones within this time interval is not restricted to sardinia but has also been observed in southern corsica in the french massif central in the western alps and in central spain this pattern of shear zones is related to a complex geodynamic scenario linked to the oblique convergence of the gondwana and laurussia plates during the middle upper carboniferous

an outline of shear sense analysis in high grade rocks ductile shear zones are important in tectonic reconstructions as a source of information on the relative motion of large crustal blocks or plates in the geological past methods to interpret fabric in ductile shear zones were mostly developed for low grade rocks where overprinting relations are usually well preserved however high grade shear zones are common and dominate in many precambrian terrains high grade shear zones should be analysed in a different way from low grade zones the plane on which shear sense markers should be observed the vorticity profile plane is more difficult to find than in low grade shear zones the most reliable shear sense markers in high grade shear zones are shear bands mineral fish mantled porphyroclasts sigmoids and asymmetric boudins 2006 published by elsevier b v on behalf of international association for gondwana research

magnetotelluric measurements in sw iberia new data for the variscan crustal structures 1 thirty new mt soundings were carried out in sw iberia allowing the investigation of the lateral continuity of some major structures and lithological features that are believed to control decisively the deep architecture of the ossa morena zone belts results of the two dimensional inversion of the data reveal high conductivity zones in the middle lower crust interpreted as the signal of a granulitic basement with grain boundary interconnected films of graphite the resistive features located preferentially in the upper middle crust are ascribable to igneous complexes of variable age

tectonics of the beja acebuches ophiolite a major suture in the iberian variscan foldbelt the beja acebuches ophiolitic complex baoc south portugal spain corresponds to a high grade metamorphic belt along the boundary between ossa morena and the south portuguese zones and comprises a lithostratigraphic sequence including from top to bottom metabasalts metamorphosed multiple dyke intrusions in gabbro flasergabbros and metaserpentinites it is affected by three deformation phases d1 affects the ophiolite lower stratigraphic units and is represented by a mylonitic cleavage with a stretching lineation where shear criteria indicates the sense of shear to be towards the north north east this deformation event can be related to the ophiolite emplacement above the crystalline footwall of the serpa antiform affecting precambrian basement and cambrian cover rocks the obduction polarity ist north eastwards similar to the subduction polarity that generates the beja gabbroic complex bgc implying a flake geometry the second deformation phase d2 is represented by an intense wnw ese sinistral shear event which is responsible for the shattered appearance of the suture d2 is reactivated later by a more brittle d3 event involving thrusting to the south west again with a sinistral component 40ar 39ar isotopic ages were obtained for metamorphosed multiple dyke intrusions in the baoc s gabbro 342 6 1 4 ma for metagabbroic cumulates 340 7 1 9 ma and for the undeformed unmetamorphosed bgc 341 1 1 3 ma occurring to the north of the ophiolitic suture these ages reflect a last regional cooling event in the area which post dates the ophiolite emplacement and the intrusion of the bgc through this oceanic sequence

fluid induced disturbance of the monazite th pb chronometer in situ dating and element mapping in pegmatites from the rhodope greece bulgaria abstract the monazite th pb chronometer is widely used in a variety of geological contexts but may be affected by interaction with fluids in the central part of the rhodope massif greece bulgaria synfolial pegmatite veins were emplaced within two major synmetamorphic shear zones in the chepelare shear zone czs which displays no evidence of post metamorphic fluid flow consistent monazite th pb ages of 36 ma are interpreted as dating vein emplacement higher in the metamorphic pile zircon and monazite give u th pb ages of 42 ma for vein emplacement in the nestos shear zone nsz pegmatites show evidence of post emplacement interaction with fluids during greenschist facies ductile deformation evidence for this includes the precipitation of calcite in microscale tension gashes within feldspar porphyroclasts and the oxygen isotope disequilibrium between quartz and feldspar aqueous carbonic fluids deriving from carbonate lithologies in the footwall of the nsz are identifiable in the pegmatites by the high 18 o value of quartz up to 21 8 and the presence of calcium in calcite veinlets in these samples some of the monazite grains show large intragrain scattering of th pb age spanning up to 12 ma with intergrain scattering reaching 16 ma from 39 to 55 ma within individual grains age domains correlate with chemical heterogeneities and some show a characteristic ca excess these chemical and isotopic alterations are interpreted to be caused by interaction with fluids derived from carbonate lithologies complementary u pb data on zircons from the nsz pegmatites yield mostly mesozoic ages related to an older metamorphic cycle and an age of 48 ma for one grain with typical magmatic zoning the th pb ages of 49 3 1 6 to 54 9 1 7 ma probably relate to the emplacement of the pegmatites the spread of younger th pb ages from 38 6 1 1 to 46 2 1 6 ma probably reflects the period of fluid circulation during progressive cooling to greenschist facies conditions or variable perturbations of the monazite isotope system during fluid assisted greenschist facies deformation this is consistent with 39 ar 40 ar mica ages of 32 to 34 ma interpreted as a tighter constraint for the timing of the greenschist facies metamorphism this study illustrates the capacity of monazite to record distinct events in a single rock and highlights the need for identifying the potential involvement of fluids in order to interpret monazite ages

magnetotelluric survey of the electrical conductivity of the crust across the ossa morena zone and south portuguese zone suture abstract the transition between the south portuguese zone spz and the ossa morena zone omz is made up of a major geosuture which is indicated by the beja acebuches ophiolite several geological data suggest that this suture is the result of an oblique collision with a northward propagation between the spz and omz from the structural point of view the omz is much more complex than the adjacent central iberian zone and the spz the frequently complicated internal structure is the result of superposition of various structural elements developed during different tectonic events with the aim of providing new constraints to this complex deep structure a magnetotelluric mt survey was designed by the universities of lisbon and barcelona in the scope of the bilateral cooperation and the europrobe programs a first nne ssw profile across the spz and omz was carried out in september 1997 the profile is 40 km long and consists of nine deep mt soundings with periods ranging from 0 0039 to 4000 s the time series were processed using a robust algorithm after visual inspection the study of the dimensionality and directionality of the regional electrical structures is based on the analysis of the induction arrows and the groom bailey decomposition a strike of n125e was determined in accordance with regional geological features 2 d inversion was undertaken on both te and tm modes using the rri method the main feature of the model is a large conductive body

high resolution 5 μm u th pb isotope dating of monazite with excimer laser ablation ela icpms abstract monazite lree po 4 a common accessory mineral in magmatic and metamorphic rocks is complementary to zircon in u th pb geochronology because the mineral can record successive growth phases it is useful for unravelling complex geological histories a high spatial resolution is required to identify contrasted age domains that may occur at the crystal scale bulk mineral techniques such as id tims applied to single monazite grains recording multiple overgrowths or isotope resetting can result in partly scattered discordant analytical points that produce inaccurate intercept ages laser ablation la icpms has been demonstrated to be a useful technique for u th pb dating of zircons and this study tests its analytical capabilities for dating monazite a sector field high resolution icpms coupled with a 193 nm arf excimer laser ablation microprobe is capable of achieving a high spatial resolution and producing stable and reliable isotope measurements the u th pb systematic was applied to monazite grains from several samples a lower palaeozoic lens from high grade terrains in southern madagascar neogene hydrothermal crystals from the western alps a palaeoproterozoic very high temperature granulite from central madagascar and a variscan leucogranite from spain directly on a polished thin section the major aim was to compare and or reproduce tims and emp ages of monazite from a variety of settings and ages the three independent 206 pb 238 u 207 pb 235 u and 208 pb 232 th ratios and ages were calculated isotope fractionation effects mass bias laser induced fractionation were corrected using a chemically homogeneous and u pb concordant monazite as external standard this study demonstrates that excimer laser ablation ela icpms allows u th pb dating of monazite with a high level of repeatability accuracy and precision as well as rapidity of analysis a spatial resolution almost comparable to that of emp in terms of crater width 5 m produced precise 208 pb 232 th 206 pb 238 u and 207 pb 235 u ratios for dating palaeozoic to precambrian monazites the advantages of ela icpms isotope dating are precision accuracy and the ability to detect discordance in the case of late miocene hydrothermal monazites from the alps a larger spot size of 25 m diameter is required and precise and accurate ages were obtained only for 208 pb 232 th systematics results from the variscan granite show that in situ u th pb dating of monazites with ela icpms is possible using a 5 m spot directly on thin sections so that age data can be placed in a textural context

the geomagnetic field in permo triassic time summary a least squares fit of the 500 fathom or 1000 metre contour of the principal continental masses has been constructed which may approximate to the arrangement of the continents in permo triassic time reliable permotriassic palaeomagnetic data have been plotted on this reassembly mean pole positions for the larger fragments have been calculated and grand mean poles for permian and triassic time have been estimated to a first approximation results are consistent with a geocentric dipole field but small systematic departures have been detected these may be attributed to superimposed multipole components coaxial with a dipole field although other explanations such as anomalies originating in the crust cannot be completely excluded tests for the existence of multipole components are suggested the effect of such components on the accuracy of continental reassemblies based on palaeomagnetism is briefly noted

late palaeozoic palaeomagnetic and tectonic constraints for amalgamation of pangea supercontinent in the european variscan belt abstract this paper provides a review of the evolution of palaeomagnetic directions and sequence of tectonic events in the european variscan belt during late palaeozoic times these data are correlated with palaeomagnetic records from gondwana and baltica in order to provide a large scale geodynamic framework during the carboniferous to permian the early carboniferous palaeomagnetic records reported mainly from the rhenohercynian and saxothuringian magmatic arcs indicate a 70 anticlockwise rotation while late carboniferous to permian magnetic directions from various rocks of western and central europe are consistent with 120 clockwise rotation of the eastern and central parts of the variscan belt our review of the chronology of tectonic events is based on a robust database of geochronologically constrained deformation metamorphic and magmatic events which allow discretizing the 80 ma long orogenic evolution into five principal events in this scenario the variscan belt can be regarded as a linear sub plate isolated from gondwana and laurussia by the rhenohercynian and palaeotethysian oceans during devonian times this linear composite belt was segmented by transform faults and boundaries in the late devonian to early carboniferous times 360 335 ma during progressive e w closure of rhenohercynian ocean synchronously with collision between saxothuringian and moldanubian blocks subsequent relocation of subduction to the northern boundary palaeotethysian ocean was responsible for n s shortening almost orthogonal to the ancient sub plate n s elongation at around 335 325 ma this deformation resulted in dextral reactivation of transform boundaries associated with anticlockwise rotation of intermittent blocks at the end of this rotation the faults were parallelized to the teysseire tornquist zone the southern margin of baltica while the lozenge shaped blocks of the former variscan sub plate were further shortened during continuous contraction this evolution can fully explain the 70 anticlockwise rotation from cn3 to cn2 palaeomagnetic directions which are regarded as successive carboniferous magnetizations subsequently the variscan belt suffered a giant transtensional event from 325 to 310 ma that was related to the development of extensional syn magmatic core complexes over the whole belt and significant dextral reactivation of earlier nw se trending transform faults this extensional event was associated with important tilts recorded by cp magnetic overprint resulting from a major thermally induced remagnetization during this event new sets of sinistral transfer nne ssw trending faults originated that partly reactivated boundaries of the principal tectonic zones blocks delimited by second order sinistral and first order dextral faults then rotated in a clockwise manner by 80 the whole system subsequently suffered a period of nne ssw shortening that affected the variscan belt namely along the former laurussian and former variscan sub plate contact in the north and in the south where the giant cantabrian orocline developed at around 310 300 ma due to hard collision with gondwana this deformation is associated with the clockwise rotation of laurussia together with the accreted northern sector of the variscan belt and the anticlockwise of gondwana this clockwise 30 rotation is achieved by a1 remagnetizations while the southern part of iberia suffered anticlockwise rotation the final stage of rapid clockwise rotation affecting the northern limb of the iberian arc including corsica and sardinia is attributed to giant n s extension affecting the whole variscan belt at the onset of permo triassic opening of the tethys ocean this complex evolution is regarded as a result of the readjustment of inhomogeneous and thermally and mechanically instable mobile space in between reorganizing the gondwana and laurussia megaplates and the opening of the palaeotethys ocean during the final stages of formation of the pangea supercontinent finally the palaeomagnetically constrained rotations and tectonic evolution of the variscan belt are explained using a pinned model of internal and external rotations of blocks driven by activation of dextral shear zones by n s compression and e w transtension in particular

the atlas east variscan elbe shear system and its role in the formation of the pull apart late palaeozoic basins the variscan orogeny occurred as a result of the late devonian to late carboniferous collision and accretion of gondwana derived microcontinents and continental masses with those of laurussia the irregular boundaries of the colliding continents caused isochronous transpressional and transtensional tectonics accompanied by a complex pattern of intracontinental shear zones at the scale of the southern european variscides these shear zones and their configuration controlled the subsequent evolution of permian to middle triassic paleogeography the geographic distribution from morocco to the eastern alps of the late carboniferous permian up to triassic basins most of which are considered as pull apart basins was related with the development of the late palaeozoic intracontinental shear network our analysis of the stratigraphic tectonic and volcanic features of the late carboniferous permian continental basins across the laurussia gondwana boundary reveals the role of the east variscan shear zone during this time as a precursory lineament for the development of the permian to triassic rifting of pangaea and the following opening of oceanic basins e g the neothetyan ocean

kinematics and vorticity of flow associated with post collisional oblique transpression in the variscan inner zone of northern sardinia italy abstract quartz c axis fabrics vorticity of flow and deformation temperatures were analyzed in samples collected in the central sector of the variscan inner zone in sardinia italy in order to constrain the kinematics of exhumation of medium and high grade metamorphic rocks during d2 post collisional deformation we investigated two shear belts with opposing shear senses these shear belts are developed parallel to the trend of the inner zone which marks the boundary between two metamorphic complexes our data suggest that the sinistral shear belt initiated during the early stage of d2 deformation in a simple shear dominated regime whereas the dextral shear belt initiated later in a pure shear dominated regime we document a general non coaxial flow regime with contemporaneous pure and simple shear components geometrical relationships reveal the operation of oblique extrusion during a compressive regime where the coaxial component acted sub horizontal and orthogonal to the trend of the axial zone changes in the kinematics vorticity of flow and strain geometry that occurred during d2 deformation may be related to post collisional orogen parallel displacement caught up in the variscan orogeny

electromagnetic imaging of variscan crustal structures in sw iberia the role of interconnected graphite abstract the western part of the iberian peninsula iberian massif is the best exposed fragment of the variscan orogen in europe its southern half was generated by an oblique collision between three continental terranes belonging to the margins of laurassia avalonia the south portuguese zone spz and gondwana the ossa morena zone omz and the central iberian zone ciz the boundaries between them are considered to be suture zones a 200 km long magnetotelluric profile across the three variscan terranes was done in a nne direction approximately perpendicular to the main tectonic features the results of two dimensional inversion of the mt dataset reveal high conductivity zones coinciding with the transitions spz omz and omz ciz these conductive bodies related to the sutures at depth were interpreted as graphite enrichments along shear planes formed due to the overall transpressive regime a high conductivity layer extending along the whole omz was found at a depth of 15 25 km the top of which spatially correlates with a broad reflector detected by a recently acquired deep seismic reflection profile the high conductivity was interpreted as caused by the precambrian serie negra graphite rich rocks carbon and oxygen x ray mapping with electron microprobe on polished sections of serie negra samples from omz revealed the presence of interconnected graphite which supports the hypothesis that graphite is determinant for the high conductivity two graphite types which help to record the geological evolution were identified graphite accumulations in the schistosity surfaces produced by folding and metamorphism and metallic films of graphite developed along late faults the conductive layer shows blobs of higher conductivity suggesting macro anisotropy additional mylonitisation and shearing produced by thrusting at depth can be the origin of these zones of enhanced conductivity given that the detachment level is located within the serie negra several high resistivity features were found in the upper crust related to devonian and carboniferous successions and probably to some unexposed plutons in the spz and the palaeozoic series of omz plus some granitic intrusions in the ciz a high resistivity zone extending to the whole crust is correlated with extensive late variscan granite intrusions

lower crustal rejuvenation and growth during post thickening collapse insights from a crustal cross section through a variscan metamorphic core complex the variscan lower crust of europe acquired its main features seismic layering intrusion of mantle derived magmas granulite grade metamorphism when the upper crust underwent post thickening extension through gravitational collapse we suggest that post thickening collapse is a geodynamic process that contributes to the growth and differentiation of continental lithosphere in collapsed mountain belts

a mid triassic paleomagnetic age of the kupferschiefer mineralization in poland based on a revised apparent polar wander path for europe and russia extensive mineralization occurs in southwest poland as zoned blankets of cu ag pb zn sulphides in the kupferschiefer and zechstein limestone across the base of the late permian zechstein restricted marine sequence and in the weissliegendes sandstone at the top of the early permian rotliegendes continental volcanic and siliciclastic sequence the rote faule rf zone an oxidized hematitic equivalent of the normally pyritic basal zechstein directly underlies the copper ore and is part of the metal zoning although the origin has generally been considered to be syngenetic geologic controls of the rf ore systems in poland suggest a late diagenetic origin by converting metalliferous brines migrating through the rotliegendes clastic redbeds up along basement highs oxidizing the pyritic kupferschiefer to form the rf and precipitating base and precious metals on the reduced side of this oxidation reduction front a paleomagnetic study of the ore zone in five mines in poland the rf zone in three mines and of the barren pyritic basal zechstein from outcrops and quarries in poland germany and england was undertaken to determine the absolute age of the mineralization in the barren pyritic rocks and the copper sulphide ore thermal demagnetization and vector subtraction revealed an unstable low temperature first remanence component 76 9 n 12 2 e a63 1 6 parallel to the ambient field and a second higher temperature component 82 6 n 145 9 e a63 1 9 which could be miocene 10 20 ma or less likely cretaceous 110 ma or jurassic 180 ma this second component is erased at 350 400 c and is thought to be carried by a product of weathering after alpine age uplift and erosion no primary magnetization could be isolated in the copper or pyritic zones the rf hematite however contained a very stable third reversed component 49 0 n 157 2 e a63 2 2 which was revealed typically between 500 and 620 c and is considered to represent a secondary chemical remanent magnetization acquired during the mineralizing process in order to date the isolated magnetic components carboniferous to neogene apparent polar wander apw paths for europe and russia were constructed using paleopole catalogs mean paleopoles every 10 ma from 350 to 0 ma with a63 circles of confidence were calculated as running averages using overlapping 20 and 30 ma time intervals the rf pole corresponding to the third remanence component coincides extremely well with a middle triassic age of 240 ma on the 20 ma apw path and a late triassic age of 230 ma on the 30 ma apw path statistical analysis yields upper and lower age limits of 250 and 220 ma this triassic paleomagnetic age supports the proposed late diagenetic origin of the kupferschiefer ore deposits and suggests that the rf ore systems were formed immediately after triassic continental rifting associated with the opening of the tethys ocean to the south a likely source of the thermal anomaly necessary to initiate adequate convective velocities within the rotliegendes clastic basins

comment on revisiting the 1872 owens valley california earthquake by susan e hough and kate hutton hough and hutton 2008 have reconsidered macroseismic observations and geologic observations for the 1872 owens valley california earthquake their preferred moment magnitude m is 7 8 7 9 and they suggest that the 1872 owens valley earthquake was larger than the 1906 san francisco m 7 8 earthquake hough and hutton 2008 also concluded that m 7 5 8 0 earthquakes can occur over much of california i disagree with these conclusions and the interpretation and analysis of the macroseismic observations on which they are based hough and hutton s 2008 estimate of m for the 1872 event is biased high because they assumed an intensity attenuation model that does not account for the significant effects of the sierra nevada they then lowered their m estimate considerably from m 8 5 9 to m 7 8 7 9 by reassigning modified mercalli intensities mmi using revised criteria that they characterized as conservatively as reasonably possible bakun 2006b analyzed an edited version of toppozada et al s 1981 mmi assignments for the 1872 event wherein mmi assignments based on

the permo triassic paleotethys in mainland southeast asia and adjacent parts of china aspects of the geodynamic evolution of mainland southeast asia and china are discussed in consideration of the permo triassic paleotethys problem it is shown that all sutures in thailand vietnam and yunnan already closed during the paleozoic era and that the entire region must be regarded as part of the northern continents the permo triassic paleotethys must be expected south of tibet and in burma

diachronous variscan tectonothermal activity in the nw iberian massif evidence from 40ar 39ar dating of regional fabrics abstract multigrain concentrates of hornblende and muscovite together with whole rock slate phyllite samples have been dated 27 analyses using 40ar 39ar incremental release methods along a systematic traverse across the various lithotectonic structural elements which comprise northwestern sectors of the variscan iberian massif hornblende concentrates from amphibolites in the allochthonous ordenes complex yield plateau isotope correlation ages of 425 ma and 377 ma muscovite concentrates and whole rock slate phyllite from this and the cabo ortegal complex yield plateau ages which range from 367 ma to 295 ma analyses of similar material from the relative autochthon yield plateau ages between 359 ma and 275 ma muscovite concentrates from three late to post kinematic granitic stocks yield plateau ages between 309 ma and 274 ma at least seven of the 40ar 39ar analyses from metamorphic rocks record variable thermal rejuvenation of intracrystalline argon systems associated with emplacement of proximal granitic stocks the remaining analyses may be used to constrain the local age of various variscan tectonothermal events the oldest fabric ages are recorded in allochthonous units whereas the youngest fabric ages occur along the boundary between internal and external zones middle devonian ages are recorded in the allochthon and suggest a chronological continuity with deformational events in the relative autochthon where variscan deformation initiated in the upper devonian and diachronously prograded eastward the first deformational events recorded in the limit within the internal and external zones occured ca 20 25 ma later lower namurian variscan deformation systematically prograded diachronously eastward across the orogen as new crustal material was added along the front of the developing orogenic wedge however the entire orogen remained tectonically active with different structural features forming at different times and at different places an average propagation rate of ca 5 km m y is suggested by consideration of a 20 25 ma difference in correlative fabric ages and present separations

the structure of a major suture zone in the sw iberian massif the ossa morena central iberian contact abstract we have investigated the stratigraphy structure and metamorphism of the boundary between the ossa morena zone omz and the central iberian zone ciz two significant continental portions of the variscan iberian massif the omz ciz contact is marked by a strongly deformed and metamorphosed nw se trending narrow band namely the central unit in which partially retrogressed eclogites are included during the middle late devonian the ciz overthrust the omz and in the footwall km scale recumbent folds and thrusts developed with decoupling and underplating of the lower crust at the same time in the hanging wall there took place intense though localized back folding and back shearing in the early carboniferous a transtensional tectonic regime sank the overthrust block resulting in the exhumation of eclogites these eclogites probably came from the underthrust omz lower crust and they are at present included in the suture zone central unit of this continental collision the extension is responsible for the origin of a basin and bimodal magmatism on the southern border of the ciz a late episode of folding and fracturing significantly contributed to the final complex picture of this suture boundary

development of high thermal gradients by coeval transpression and magmatism during the variscan orogeny insights from the cap de creus eastern pyrenees abstract the cap de creus massif in the eastern pyrenees is a fragment of the variscan mid crust geological mapping and meso and micro scale analysis of the relationships between fold structures metamorphic mineral assemblages and fabrics affecting igneous bodies suggest the interaction of three processes deformation metamorphism and magmatism low pressure prograde metamorphism grades laterally from greenschist to amphibolite facies over a distance of 5 km zones of intense metamorphism coincide with transpressional deformation and with the intrusion of small sheets of diorite and granitoid sillimanite schists and small migmatite pods are distributed around the small intrusions of basic intermediate composition the inferred maximum thermal gradient is 80 c km this thermal pattern is similar to patterns observed in other variscan massifs in the pyrenees where high metamorphic gradients are interpreted in different ways in the ne cap de creus the type and distribution of prograde metamorphism was caused or at least enhanced by the intrusion of diorites and granitoids alternatively deformation developed at prograde conditions and synchronously with magmatism displays a highly heterogeneous distribution producing a sub vertical zone of high strain that coincides with domains of highest grade metamorphism and igneous activity this zone of high strain developed in a broad transpressive regime could have favoured the injection of magmatic sheets from deep to mid crustal levels progressive folding from prograde to retrograde metamorphic conditions might account for the narrowing of metamorphic zones

tillites et tilloides du massif armoricain abstract a glacial origin had been suggested for the upper brioverian pebble mudstones of the armorican massif the lack of any true evidence of glacial abrasion on the pebbles the turbiditic sedimentary environment and the great amount of resedimentation figures would rather plead for a deposition by debris flow or mass flow the late ordovician diamictites which contain striated pebbles are inferred to be of glacio marine origin the new palaeomagnetic data are suggesting that the periglacial deposits of southeastern europa are located close to the late ordovician pole for gondwanaland

accumulation of organic carbon rich sediments in the late jurassic and cretaceous atlantic ocean a synthesis abstract organic carbon rich sediments black shales were deposited at some 40 deep sea drilling project dsdp sites in the atlantic ocean during late jurassic and cretaceous times in order to reconstruct the depositional environments of black shale formation we have studied in detail changes in the occurrence of black shales as well as the type and amount of organic matter in relation to changes in paleogeography paleooceanic circulation and paleoclimate we have calculated accumulation rates of marine and terrigenous organic carbon and combined these with organic carbon content sedimentation rate relationships to obtain information about paleoenvironments main factors causing the preservation and accumulation of organic carbon are 1 the supply of terrestrial organic matter 2 the supply of marine organic matter 3 the oxygenation of the deep water and 4 the bulk sedimentation rate changes in the importance of these different factors and changes in the interrelationships among them determine the complex system of black shale formation and their cyclic variations increased supply of terrestrial organic matter due to increased river discharge and humid climate in northeast america southwest europe and south africa were important in the hauterivian albian northwest and northern northeast atlantic and in the late jurassic aptian south atlantic in the latter anoxic deep water conditions due to restricted circulation in the nascent largely isolated basin as well as higher oceanic productivity may have reinforced the high preservation rates of marine organic matter during that time off northwest africa higher flux rates of marine organic matter were recorded in areas of higher productivity which may have been caused by fluvial nutrient supply during the hauterivian and by intensified upwelling during albian turonian times in general the depositional environment was oxygenated and the content of marine organic matter was a function of the sedimentation rate furthermore at some places redeposition by turbidity currents or slumps may have been important for the accumulation of organic matter in the deep sea basins the only episode of widespread anoxia in the western and eastern basins of the north atlantic may have occurred in the cenomanian turonian when high amounts of marine organic matter were also preserved in areas of low productivity and low sedimentation rates

paleogeographic maps of the continents bordering the indian ocean since the early jurassic paleomagnetic data from the african plate from the early jurassic onward are critically re evaluated and updated resulting in a set of 30 paleopoles from which an apparent polar wander path is derived using 20 ma intervals this path is then used in conjunction with kinematic models of the indian and atlantic oceans to place other continents bordering the indian ocean eurasia india islands and plateaus madagascar seychelles mascarene and blocks involved in the collision of india with eurasia south tibet indochina in an absolute paleogeographic frame fixed with respect to africa paleogeographic maps are presented for the early jurassic pre breakup the jurassic cretaceous boundary anomaly ml5 the early cretaceous m2 the late cretaceous a 34 the cretaceous tertiary boundary a 29 the early eocene a 24 the middle eocene a 21 and the oligocene miocene boundary a 6 observed and predicted paleomagnetic directions for landmasses other than africa i e for western europe china korea india and australia which are not used in deriving the maps are found to be in very satisfactory agreement the quality of reconstructions as measured by rms discrepancies between observed and predicted paleolatitudes is of the order of 5 the main features of the paleogeographic maps are discussed with the aim of relating oceanic and on land geodynamic events the constraints provided by tibetan data on the india asia collision and the 20 ma reorganization of geology and plate boundaries in a band extending from the south china sea through tibet to the red sea system are discussed in more detail we emphasize the episodic nature of the breakup of gondwanaland and formation of the indian ocean and subsequent interaction with eurasia in which six major discrete events are identified separated by 30 ma phases of rather uniform plate motion

strain deformation temperatures and vorticity of flow at the top of the greater himalayan slab everest massif tibet this paper presents quantitative data on strain deformation temperatures and vorticity of flow at the top of the greater himalayan slab the data were collected from the tibetan side of the everest massif where two low angle normal faults bound the upper surface of the greater himalayan slab the earlier and structurally lower lhotse detachment and the later and structurally higher qomolangma detachment greenschist to sillimanite grade quartz rich metasedimentary rocks exposed in the rongbuk to north col region of the everest massif are characterized by cross girdle quartz c axis fabrics indicating approximate plane strain conditions fabric opening angles progressively increase with depth beneath the overlying lhotse detachment and indicate progressively rising deformation temperatures of 525 625 50 c at depths of 300 600 m beneath the detachment deformation temperatures of c 450 c are indicated by fabric opening angles in epidote amphibolite facies mylonites located closer to the overlying detachment a top down to the north normal shear sense is indicated by the asymmetry of microstructures and c axis fabrics but the degree of asymmetry is low at distances greater than 400 m beneath the detachment and sillimanite grains are drawn into adjacent conjugate shear bands but still appear pristine indicating that deformation occurred at close to peak metamorphic temperatures these quenched fabrics and microstructures indicate rapid exhumation in agreement with previous isotopic dating studies mean kinematic vorticity numbers w m were independently calculated by three analytical methods calculated w m values range between 0 67 and 0 98 and indicate that although a simple shear component is generally dominant particularly in greenschist facies mylonites located between the lhotse and overlying qomolangma detachments there is also a major component of pure shear in samples located at 400 600 m beneath the lhotse detachment pure and simple shear make equal contributions at w k 0 71 our integrated strain and vorticity data indicate a shortening of 10 30 perpendicular to the upper surface of the greater himalayan slab and confirm that the upper surface of the slab is a stretching fault with estimated down dip stretches of 10 40 assuming plane strain deformation measured parallel to the flow plane transport direction

crustal exhumation during ongoing compression in the variscan maures tanneron massif france geological and thermo mechanical aspects abstract the maures tanneron massif mtm together with corsica and sardinia represent the southeastern branch of the huge european variscan belt a continuous evolution from continental collision to exhumation is described from ca 350 ma to ca 320 ma based on an extended compilation of available geological and geochronological data this geological compilation is complemented with thermo mechanical modelling that tests the conditions under which deep seated hp units can melt and exhume massively during continued convergence we simulate in two dimensions the development of gravitational instabilities in a partially melting crust depending on rheological layering and heat sources and accounting for temperature dependent elasto visco plastic rheologies in order to reproduce pt exhumation patterns in the mtm over the appropriate time scale 20 my and spatial extent 100 km a best fit was obtained with a convergence rate of 0 5 cm yr and moderate diffusive surface processes the contribution of radiogenic heating in the pre stacked felsic crustal units is crucial a lateral alternation with mafic units is also required in order to prevent lateral spreading of the orogen a low viscosity partially molten crust 1020 pa s thus rather felsic accompanies crustal exhumation in a decoupled mode from the deeper mantle a shallow asthenosphere below the orogen lab at 70 km depth e g shallow slab break off produces too warm and sudden exhumation as opposed to a competent mantle lithosphere 120 km depth lab that restrains it the mtm witnesses the typical competition between far field plate convergence and internal body forces and our study pleads for a progressive evolution of transpression towards perpendicular extension from ca 320 ma

polyphase variscan emplacement of exotic terranes morais and braganca massifs onto iberian successions evidence from 40ar 39ar mineral ages abstract the braganca and morais nappe complexes comprise internally imbricated continental tectonic units including schists deformed metagranite granulite and local eclogite and a structurally underlying ophiolite nappe high pressure assemblages in some of the continental sequences were variably retrogressed during an amphibolite facies metamorphic overprint by contrast rocks within the ophiolite nappe display prograde amphibolite facies assemblages hornblende concentrates from both tectonic units exposed in the braganca nappe complex record 40 ar 39 ar plateau and isotope correlation ages of c 385 390 ma a similar date is recorded by hornblende within mylonitic amphibolite ophiolite protolith along a ductile thrust which separates the two major nappe units muscovite within blastomylonitic metagranite lagoa augen gneiss in upper structural levels of the morais massif records 370 375 ma 40 ar 39 ar plateau ages muscovite within phyllinite developed along a ductile thrust separating the gneiss from the structurally underlying ophiolite nappe records a 375 ma age together the 40 ar 39 ar data suggest that a regionally significant early variscan amphibolite facies metamorphism occurred prior to c 385 390 ma early devonian this was associated with ductile imbrication of continental and ophiolite structural units with resultant formation of a composite nappe complex the composite nappe complex must have been maintained at relatively high structural levels throughout the remaining variscan evolution because there is no evidence for post 370 ma rejuvenation of intracrystalline argon systems in any of crystalline structural units examined this is in marked contrast to a significant late devonian c 330 ma locally high pressure metamorphism which is recorded in structurally underlying elements of the lower allochthon part of a foreland thrust complex composed of metasedimentary units with iberian protolith affinities together available chronologic constraints indicate emplacement of the previously assembled composite nappe complex into its present relative structural position at c 330 ma with resultant deformation and metamorphism of structurally underlying components of the iberian thrust complex

reply to discussion on middle jurassic shear zones at cap de creus eastern pyrenees spain a record of pre drift extension of the piemonte ligurian ocean journal of the geological society london 174 289 300 we welcome the discussion by elena druguet jordi carreras and jochen mezger druguet et al 2017 of our paper vissers et al 2017 b in which we report jurassic 40ar 39ar ages from shear zones at cap de creus nw spain the authors argue that our interpretation of these results and the inferred development of the cap de creus shear zones during jurassic stretching and opening of the piemonte ligurian ocean contradicts structural and tectonic evidence and that it is not sufficiently supported by geochronological data below we first consider their structural and tectonic arguments from the tectonic palaeogeographical scale down to the smaller field scale and then proceed to discuss our ar ar results druguet et al 2017 note that prior to the oligo miocene rifting event the corso sardinian block was fixed east of ne iberia hence that the rocks at cap de creus were not located at the jurassic pre drift continental margin and that the northern cap de creus shear zones could not have formed as normal faults related to continental rifting this palaeogeographical argument builds on a previous hypothesis by stampfli borel 2002 stampfli et al 2002 and stampfli hochard 2009 in which iberia the corso sardinian block and the brianconnais domain formed one single microcontinent this hypothesis however must be discarded on the basis of recent palaeomagnetic work on sardinia by advokaat et al 2014 showing that between late jurassic and eocene time sardinia underwent no vertical axis rotations relative to eurasia and was fixed to europe in a rotated position that results from post eocene deformation whereas all available evidence indicates that iberia was located much further west and underwent some 35 counterclockwise rotation during the cretaceous van hinsbergen et al 2017 vissers et al 2017 a sardinia

revisiting the 1872 owens valley california earthquake the 26 march 1872 owens valley earthquake is among the largest historical earthquakes in california the felt area and maximum fault displacements have long been regarded as comparable to if not greater than those of the great san andreas fault earthquakes of 1857 and 1906 but mapped surface ruptures of the latter two events were 2 3 times longer than that inferred for the 1872 rupture the preferred magnitude estimate of the owens valley earthquake has thus been 7 4 based largely on the geological evidence reinterpreting macroseismic accounts of the owens valley earthquake we infer generally lower intensity values than those estimated in earlier studies nonetheless as recognized in the early twentieth century the effects of this earthquake were still generally more dramatic at regional distances than the macroseismic effects from the 1906 earthquake with light damage to masonry buildings at nearest fault distances as large as 400 km macroseismic observations thus suggest a magnitude greater than that of the 1906 san francisco earthquake which appears to be at odds with geological observations however while the mapped rupture length of the owens valley earthquake is relatively low the average slip was high the surface rupture was also complex and extended over multiple fault segments it was first mapped in detail over a century after the earthquake occurred and recent evidence suggests it might have been longer than earlier studies indicated our preferred magnitude estimate is mw 7 8 7 9 values that we show are consistent with the geological observations the results of our study suggest that either the owens valley earthquake was larger than the 1906 san francisco earthquake or that by virtue of source properties and or propagation effects it produced systematically higher ground motions at regional distances the latter possibility implies that some large earthquakes in california will generate significantly larger ground motions than san andreas fault events of comparable magnitude

zooming on northern cap de creus shear zones abstract the northern cap de creus shear belt provides fine examples of shear zones developed at different scales in rocks bearing pre existing penetrative foliations the shear zones developed under retrograde metamorphic conditions and they are preferentially located in crystalline schists lower grade metasedimentary rocks accommodated deformation by folding the shear zones are linked in an anastomosing framework with self similar properties they are interpreted to have formed as the result of a progressive wrench dominated deformation affecting anisotropic rocks a progressive non coaxial deformation regime is considered responsible for the development of the complex kinematic pattern wherein shear zones with opposite senses of shear may come to lie in close parallelism in such a framework individual shear zones are characterised by marked strain gradients not only across the shear zone but also along it these features together with the geometrical analysis of shear zone marginal domains indicate that these structures developed with a component of shortening across the shear zones the shear zones are interpreted to have nucleated as buckling instabilities that gradually evolved to become shear zones approaching the simple shear model microstructural and quartz c axis fabric analyses indicate that shear zone formation is associated with strong strain partitioning on all scales

the relationship between tectono metamorphic evolution and argon isotope records in white mica constraints from in situ 40ar 39ar laser analysis of the variscan basement of sardinia the basement of sardinia represents a nearly complete section of a segment of the variscan belt that experienced a polyphase tectonometamorphic evolution and barrovian metamorphism this basement is well suited to investigate the relationship between tectono metamorphic evolution and argon isotope records in white mica micaschists from the garnet zone maximum t of up to 520 560 c contain two texturally and chemically resolvable generations of white mica 1 deformed celadonite rich flakes defining a relict s1 foliation preserved within the main s2 foliation or enclosed in rotated albite porphyroblasts 2 celadonite poor white micas aligned along the main s2 foliation the s1 foliation developed earlier and at a deeper crustal level with respect to that at which the thermal peak was reached from the staurolite zone t of up to 590 625 c to the sillimanite k feldspar zone white mica is nearly uniform in composition muscovite and is predominantly aligned along the s2 foliation or is of later crystallization in situ ar ar laser analyses of white mica yielded ages of 340 315ma in the garnet zone and 320 300ma in the staurolite and sillimanite k feldspar zones results highlight a close link between argon isotope records in white mica and both textures and structure forming major elements the oldest ages were detected in samples where the earlier syn d1 white mica generation did not texturally and chemically re equilibrate at upper crustal levels this study suggests that the white micas that escaped recrystallization retain argon isotope records of an earlier metamorphic stage that survived a later event characterized by temperatures higher than 500 c

electromagnetic imaging of a transpressional tectonics in sw iberia forty one magnetotelluric soundings were carried out along a 200 km long profile approximately nne ssw across the three major geotectonic units in sw iberia a model obtained from two dimensional inversion of the magnetotelluric data set reveals high conductivity zones in the middle lower crust 10 30 km two of these zones correspond to the transition between the main geotectonic units one between the south portuguese zone and the ossa morena zone interpreted as having been caused by metasediments and the other between the ossa morena zone and the central iberia zone associated with a shear zone and metasediments another high conductivity anomaly related to black shales with major graphite impregnation was detected within the ossa morena zone the resistive features located preferentially in the upper crust 1 10 km coincide with gabbroic and granitic complexes

metamorphic response in orogens of different obliquity scale and geometry abstract investigation of material flow within transpressional orogens must involve integration of structural and metamorphic datasets to illustrate the problems in documenting flow vectors we present integrated structural metamorphic datasets from two transpressional systems the kaoko belt in namibia and the kalinjala shear zone in south australia these orogens experienced widely differing metamorphic responses to transpressional deformation integration of kinematic and metamorphic datasets from the kaoko belt indicate shallow up plunging extrusion trajectories in the orogen core and show that the maximum stretching direction pattern matches the inferred flow vectors high grade domains 800 840 c and 7 0 8 0 kb in the orogen core developed low angle upward verging maximum stretching direction trajectories whereas a low grade domain 575 600 c and 5 0 5 5 kb in the orogen core has downward verging lineation trajectories the barometric differential between these high grade and low grade domains is entirely consistent with the angle of plunge of maximum stretching directions within the high grade domains that were extruded obliquely for the amount of lateral shear estimated for the orogen core the kalinjala shear zone in south australia contrasts strongly with the kaoko belt in this example the high grade and high strain shear zone core of the orogen experienced high t high p metamorphism with low thermal gradients of 21 26 c km and steep decompressive p t paths the lower grade external domains experienced lower t lower p metamorphism with high thermal gradients of 35 37 c km sub horizontal maximum stretching directions do not match the vertical extrusional flow in the high grade core that is indicated by the metamorphic data this comparison shows that in general and on a gross scale maximum stretching directions do not necessarily correlate with the real flow vectors experienced during orogenesis in some cases maximum stretching direction recorded by deformation structures is to some degree decoupled from the vertical component of material flow consequently information pertaining to flow is often partitioned into information derived from deformation structures and information derived from the metamorphic record these two datasets must be used in concert to obtain realistic constraints on first order material flow trajectories at orogenic scales the horizontal component of flow is typically best recorded by structural fabrics maximum stretching direction and sense of shear whereas the vertical component is typically best recorded by metamorphic information such as p t paths temperature over depth ratio g and metamorphic field gradients i e t p and g across the orogen

palaeobiogeography of ordovician calcareous algae abstract from the study of ordivician algal microfloras the following can be concluded during the early ordovician the algal microflora is scantly but cosmopolitan and different from that of the upper cambrian its cosmopolitan character seems to denote the absence of a climatic gradient during that time by middle ordovician times new algal taxa appear the algal palaeographic distribution suggests a possible climatic zonation no algae are known from gondwana which was located at high latitudes for the same reason calcaareous algae are absent or not recorded from south europpe in contrast the occurence of calcareous algae and stromatoporoids of middle ordovician age provides support for inferring that the north europe plate has reached low latitudes by middle ordovician times during the upper ordovician some groups of calcareous algae crossed the proto atlantic or iapetus ocean and migrated towards north european and north american regions according to the general pattern of oceanicsurface currents this crossing and this migration are the result of the narrowing of the proto atlantic or iapetus ocean at thattime like other pelagic and benthic organisms many groups of calcareous algae are declining during the middle and late ordovician the migration and the distribution of some groups of calcareous algae resulted from asynchronous oceanic surface currents close to the palaeoequator probably mainly in an eastward direction in the late middle ordovician from north america to euroasia and mainly in a westward direction in the late ordovician lower silurian from euroasia to north america

nature and age of pre variscan eclogite protoliths from the low to medium grade metamorphic complex of north central sardinia italy and comparisons with coeval sardinian eclogites in the northern gondwana context u pb zircon data on retrogressed eclogites sampled in the giuncana locality from the sardinian medium grade metamorphic complex yielded a weighted age of 454 6 ma this is in agreement with u pb zircon ages of 453 460 ma obtained from eclogites from the high grade metamorphic complex the giuncana eclogites are very similar to the other well known sardinian eclogites all of the sardinian eclogites show positive k rb ba u and pb anomalies and negative nb la ce and sr anomalies th is depleted in the giuncana eclogites and enriched in those from punta de li tulchi and punta tittinosu all these data reveal clear crustal contamination of the sardinian ordovician mantle ree patterns typical of normal mid ocean ridge basalt n morb characterize all of the sardinian eclogites the supply of crustal and calc alkaline materials to the sardinian mantle during the ordovician is further confirmed by the fact that most sardinian eclogites plot on the left side and well above the mantle array in a th yb v nb yb diagram in the general variscan framework of northern gondwana the sardinian eclogites are witness to the most recent back arc basins generated by the northward opening of the rheic ocean

the cretaceous tertiary boundary chronostratigraphic position and sequence of events the cretaceous tertiary boundary has always been considered to be exceptional and linked to special events and mechanisms where catastrophes have often been advocated the boundary stratotype stevns klint in denmark has been paleomagnetically analysed the c t boundary is now given an absolute age of 66 7 m y the boundary is shown to have occurred under stable geomagnetic conditions about 165 000 years before the polarity reversal of the base of anomaly 29 and about 0 7 1 0 m y before the extinction of dinosaurs in new mexico significant disturbances in the earth s rate of rotation and direction of spin axis at the passing from the cretaceous into the tertiary are strongly contradicted the cat boundary events cannot be explained by a single large catastrophe a sequence of different events and some gradualism need also to be included in present and future scenarios of the boundary events

structural setting of a transpressive shear zone insights from geological mapping quartz petrofabric and kinematic vorticity analysis in ne sardinia italy

permo triassic fluvial systems in the southeastern karoo basin south africa abstract a prominent clastic wedge of latest permian to early triassic age katberg sandstone prograded northwestwards into the main karoo basin from a southerly source area as a sedimentary response to renewed tectonism associated with the cape fold belt proximal to distal relationships within this clastic wedge and its relationships with underlying and overlying formations reveal a variety of stream types which reflect changing tectonic and climatic conditions towards the close of permian times the depositional area of the southeastern karoo basin was crossed by meandering river channels cutting through earlier formed floodplain deposits balfour formation tectonic rejuvenation of the source area in early triassic times led to steeper gradients and a sharp increase in the supply of coarser grained detritus as a result alluvial fans developed in areas adjacent to the source terrane and the river channels became braided depositing only sands katberg sandstone with muds and silts being carried down into the most distal parts of the floodplain burgersdorp formation subsequent denudation of the source area gradually reduced slopes and allowed the source ward shift of the distal facies until it eventually overstepped the earlier formed braided stream deposits accompanying the changes in tectonic conditions was a change in climatic which also influenced stream type the late permian to early triassic period records a general change to warmer climatic conditions following the widespread glaciation of the early to middle permian thus the lowest beaufort group sediments were probably deposited in warm temperate to humid conditions with later deposits being laid down under an increasingly arid regime

continental tectonics in the aftermath of plate tectonics the success of plate tectonics required an acceptance of continental drift and thus a reinterpretation of the large scale geological history of most of the earth but the basic tenet of plate tectonics rigid body movements of large plates of lithosphere fails to apply to continental interiors where buoyant continental crust can detach from the underlying mantle to form mountain ranges and broad zones of diffuse tectonic activity

phanerozoic equal area maps this paper outlines a method used to make paleocontinental maps and outlines its limitations 15 pairs of new lambert equal area paleocontinental maps are presented drawn at 40 ma intervals from the present day back to 560 ma early cambrian time

on the permian biogeographic boundary between gondwana and eurasia in tibet china as the eastern section of the tethys abstract as is well known glossopteris as a cool climate flora and the bivalve eurydesma the rugose coral lytvolasma the brachiopod globiella and the fusulinid monodiexodina as a cold cool water fauna often occur with tillites and tilloids of glacial glaciomarine origin or else closely following such rocks in the northern margin of the godwanaland in early permina times asselian sakmarian therefore they are usually considered as the indicators of the gondwana tethys facies the case of the gondwana tethys is quite different from that of the cathayan tethys as the southern margin of eurasia the biota of south china and the kunlun mountains belonging to the cathayan tethys is characterized by tropical flora and warm water fauna evidently south china and the kunlun mountains area were covered by a warm water sea during the whole permian period and perhaps from the late middle permian stage onwards the seawater became tropical the indus yarlung suture is the biogeographic boundary between gondwana and eurasia based on the discoveries of glossopteris flora and lytvolasma fauna in qubu and kangmar southern tibet recently with the rapid advance of geological and palaeontological data the cool climate flora and the cold cool water fauna with glaciomarine deposits have been found in early permian rocks in many localities of the gangdise nyainqentanglha and qiantang blocks therefore they definitely belong to the epeiric sea of the marginal gondwanaland the biogeographic boundary between gondwana and eurasia should be moved from the yarlung suture and the bangongco nujiang suture northwards to the longmuco yushu suture i e the boundary passes between the songban ganze block and the qiangtang block it is very interesting that the permian biota of tibet such as that of the salt range shows the three developmental stages the cold cool water type the mixture type and the tropical warm water type the change of flora and fauna might be caused by the gondwanaland glacial action in early permian times asselian sakmarian the worldwide warming of climate and the rifting and rapidly northwards drifting of gondwanaland during the early middle permian artinskian in the late middle permian the gondwana tethys became still warmer and even tropical and the typical warm tropical fauna of neoschwagerina and verbeekina quickly replaced the cool water one