tectonothermal evolution of the badajoz cordoba shear zone sw iberia characteristics and 40ar 39ar mineral age constraints field characteristics indicate that the badajoz cordoba shear zone has had a long polyphase tectonothermal evolution 40ar 39ar cooling ages recorded within central ductile sectors of the badajoz cordoba shear zone range from ca 370 to 360 ma amphibole and 340 to 330 ma muscovite these are interpreted to date post metamorphic cooling through contrasting temperatures required for intracrystalline retention of argon following a regionally significant variscan tectonothermal overprint these results combined with field relationships indicate that the badajoz cordoba shear zone experienced at least 15 km of uplift relative to adjacent areas during late paleozoic sinistral transpression the data also indicate that individual structural units within the shear zone experienced contrasting uplift histories both in total amount and rate the 40ar 39ar cooling ages indicate complete variscan rejuvenation of all older intracrystalline argon systems within central sectors of the badajoz cordoba shear zone

s type granite generation and emplacement during a regional switch from extensional to contractional deformation central iberian zone iberian autochthonous domain variscan orogeny zircon grains extracted from s type granites of the meda escalhao penedono massif central iberian zone variscan orogen constrain the timing of emplacement and provide information about potential magma sources simple and composite zircon grains from three samples of s type granite were analyzed by la icp ms new u pb data indicate that granites crystallized in the bashkirian 318 7 4 8 ma overlapping the proposed age range of ca 321 317 ma of the nearby s type granitic rocks of the carrazeda de anciaes lamego and ucanha vilar massifs the timing of emplacement of such s type granites seems to coincide with the waning stages of activity of a d2 extensional shear zone i e pinhel shear zone developed in metamorphic conditions that reached partial melting and anatexis ca 321 317 ma dykes of two mica granites resembling diatexite migmatite are concordant and discordant to the compositional layering and s2 main foliation of the high grade metamorphic rocks of the pinhel shear zone much of the planar fabric in these dykes was formed during magmatic crystallization and subsequent solid state deformation field relationships suggest contemporaneity between the ca 319 317 ma old magmatism of the study area and the switch from late d2 extensional deformation to early d3 contractional deformation inherited zircon cores are well preserved in these late d2 early d3 s type granite plutons u pb ages of inherited zircon cores range from ca 2576 to ca 421 ma the spectra of inherited cores overlap closely the range of detrital and magmatic zircon grains displayed by the ediacaran to silurian metasedimentary and metaigneous rocks of the iberian autochthonous and parautochthonous domains this is evidence of a genetic relationship between s type granites and the host metamorphic rocks there is no substantial evidence for the addition of mantle derived material in the genesis of these late d2 early d3 s type granitic rocks the end arrays of heterogeneous crustal anatectic melts may be just inherited from the source probably reflecting mixing of a range of crustal materials with different ages and primary isotopic signatures the generation of the bashkirian s type granites has been dominated by continental crust recycling rather than the addition of new material from mantle sources

gravity changes before and after the 2008 m w 7 9 wenchuan earthquake at pixian absolute gravity station in more than a decade absolute gravity repeated measurements at pixian station about 35 km away from the epicenter of the 2008 wenchuan mw 7 9 earthquake have been carried out over more than a decade four pre earthquake measurements at the pixian station show 30 gals increase from 2002 to 2008 but 13 measurements since 2008 shows that the trend of gravity increase ended after the wenchuan earthquake we analyzed the gravity effects from ground vertical motions using data from continuous gps stations collocated with the pixian absolute gravimetry station and surficial and hydrological processes using local hydrological data we found that these effects are much smaller than the observed gravity increase before the earthquakes the continuously gravity increase before the 2008 wenchuan earthquake cannot be explained by the groundwater level change and vertical deformation and related superficial reasons all absolute gravity was measured by the fg 5 gravimeter in this study the high precision gravity change revealed that the pre earthquake gravity increase may be caused by strain and mass fluid transfer in broad seismogenic source regions further studies are needed to validate such pre earthquake gravity changes which however are difficult to be resolved from space based gravity models

the palaeozoic variscan oceans revisited abstract geological evidence supported by biogeographical data and in accord with palaeomagnetic constraints indicates that one ocean models for the variscides should be discarded and confirms instead the existence of three gondwana derived microcontinents which were involved in the variscan collision avalonia north armorica franconia and thuringia subdivided by a failed vesser rift and south armorica central iberia armorica bohemia all divided by small oceans in addition parts of south eastern europe including adria and apulia are combined here under the new name of palaeo adria which was also peri gondwanan in the early palaeozoic oceanic separations were formed by the break up of the northern gondwana margin from the late cambrian onwards most of the oceans or seaways remained narrow but much like the alpine cenozoic oceans gave birth to orogenic belts with hp uhp metamorphism and extensive allochthons the saxo thuringian ocean between north and south armorica and the galicia moldanubian ocean between south armorica and palaeo adria only the rheic ocean between avalonia and peri gondwana was wide enough to be unambiguously recorded by biogeography and palaeomagnetism and its north western arm closed before or during the emsian in europe ridge subduction under the northernmost part of armorica in the emsian created the narrow and short lived rheno hercynian ocean it is that ocean and not the rheic whose opening and closure controlled the evolution of the rheno hercynian foldbelt in south west iberia south west england germany and moravia czech republic devonian magmatism and sedimentation set within belts of early variscan deformation and metamorphism are probably strike slip related the first arrival of flysch on the forelands and or the age of deformation of foreland sequences constrains the sequential closure of the variscan seaways galicia moldanubian in the givetian saxo thuringian in the early famennian rheno hercynian in the tournaisian additional mid to late devonian and partly early carboniferous magmatism and extension in the rheno hercynian saxo thuringian and galicia moldanubian basins overlapped with variscan geodynamics as strictly defined the early carboniferous episode was the start of episodic anorogenic heating which lasted until the permian and probably relates to tethys rifting

determination of earthquake epicentroids by inversion of gravity variation data in the bttz region china abstract during the last decade and a half precise measurements of gravity variations have been carried out in the beijing tianjin tangshan zhangjiakou bttz region in china a possible seismogenic mechanism of a combined dilatancy model cdm for the bttz region has been proposed by kuo and sun 1993 the model consists of the dilatancy diffusion the dilatancy instability and the fault zone dilatancy models gravity variations in space and time associated with the model involve a tectonically stressed volume surrounding a small volume of an impending rupture fault zone they refer to the center of effective mass of the total stress volume which is defined as hypocentroid the projection of which on the earth s surface is defined as epicentroid this paper attempts to confirm the above stated concept of epicentroid associated with the fourteen earthquakes of magnitude mainly 4 5 which occurred in 1981 95 in the bttz region by means of a proposed least squares inversion method using observed gravity variation data on the basis of the 1976 tangshan earthquake the 1989 datong earthquake and these fourteen earthquakes an empirical relationship between the maximum variations of gravity and magnitude was first established iterative inversions of observed gravity variation data were then carried out inversion results show that 1 the separation of the epicentroids and epicenters range from 0 to about 40 km 2 the epicenters of these fourteen earthquakes are generally located at the terminations of the faults or at the intersections of the faults whereas the epicentroids are inevitably located within intact tectonic blocks and 3 the epicentroid of an earthquake at least in the bttz region as attested appears to be determinable

magnetotelluric imaging of the lithosphere across the variscan orogen iberian autochthonous domain nw iberia a new magnetotelluric mt survey comprising 17 mt soundings throughout a 30 km long n30 w transect in the iberian autochthons domain of nw iberia central iberian zone is presented the 2 d inversion model shows the resistivity structure of the continental crust up to 10 km depth heretofore unavailable for this region of the variscan orogen the mt model reveals a wavy structure separating a conductive upper layer underlain by a resistive layer thus picturing the two main tectonic blocks of a large scale d2 extensional shear zone i e pinhel shear zone the upper layer represents a lower grade metamorphic domain that includes graphite rich rocks the lower layer consists of high grade metamorphic rocks that experienced partial melting and are associated with granites more resistive emplaced during crustal thinning the wavy structure is the result of superimposed crustal shortening responsible for the development of large scale d3 folds e g marofa synform later deflected and refolded by a d4 strike slip shear zone i e juzbado penalva do castelo shear zone the later contribution to the final structure of the crust is marked by the intrusion of postkinematic granitic rocks and the propagation of steeply dipping brittle fault zones our study demonstrates that mt imaging is a powerful tool to understand complex crustal structures of ancient orogens in order to design future prospecting surveys for mineral deposits of economic interest

contrasting isotopic sources sm nd of late ediacaran series in the iberian massif implications for the central iberian ossa morena boundary abstract in the iberian massif westernmost sector of the variscan orogen the nature of the boundary between the central iberian zone and the ossa morena complex has been largely discussed peri gondwanan ediacaran series similar in age c 565 ma but different in composition occur at both sides of that boundary the lower alcudian series central iberian zone is located to the north while the serie negra ossa morena complex can be found to the south of the boundary major and trace element compositions and sm nd isotopic systematics of siliciclastic rocks from both series suggest a common active margin setting for their deposition the serie negra likely occupying a more external fore arc position within the arc system relative to the lower alcudian series which would have been deposited within a large back arc domain tdm ages for these two series are notably different yielding values of 1421 2040 ma and 1256 1334 ma for the serie negra and lower alcudian respectively these values suggest higher input from old cratonic sources in the ossa morena complex with probable participation of the west african craton which would be less prominent in the central iberia zone the whole data set indicate deposition of the ediacaran series in separated basins probably far away from each other along across the margin of gondwana current juxtaposition of these basins is explained by variscan and probably also pre variscan tectonics similar conclusions could be also be extended to other similar boundaries in the variscan orogen

the late devonian variscan suture of the iberian massif a correlation of high pressure belts in nw and sw iberia correlation of three high p and low to intermediate t belts from nw and sw iberia allows recognition of a single terrane affected by late devonian continental subduction during the variscan orogeny this terrane is located in the lower part of the allochthonous complexes of nw iberia and in a lower position in most of the ossa morena zone that therefore emerges as another allochthonous complex of iberia the ossa morena complex together with the allochthonous complexes of nw iberia form a huge rootless nappe that was transported towards the variscan foreland during final assembly of pangea

geochemical and isotopic smnd provenance of ediacaran cambrian metasedimentary series from the iberian massif paleoreconstruction of the north gondwana margin the geochemical and isotopic smnd features of the ediacaran cambrian siliciclastic series from the central iberian zone and the iberian allochthonous domains indicate a significant paleogeographic change at the northern margin of gondwana associated with the development of the avalonian cadomian arc during the neoproterozoic the opening of a peri gondwanan back arc basin and its subsequent widening during the cambrian led to a progressive separation of the sedimentary basins from the main focus of the probably waning cadomian magmatic arc activity this work presents a paleogeographic reconstruction of the distribution of the iberian terranes along the north gondwanan margin during ediacaran and cambrian times based on the geochemical and nd isotope data from the iberian allochthonous and autochthonous domains these results suggest a location close to the west africa craton for both the allochthonous and autochthonous terranes but they would be distributed laterally along the northern margin of gondwana occupying arc or continental ward positions within a wide back arc basin in this context the peri gondwanan volcanic arc acted as the main supplier of the abundant juvenile material as reflected in positive values of end and young tdm model ages 720 1215 ma of the middle cambrian siliciclastic series from nw iberia upper allochthonous units on the other hand the rather equivalent geochemical and isotopic features of the sedimentary series of the nw and sw iberia basal allochthonous units suggest a common paleolocation for those series yet within the back arc basin but closer to the mainland and older isotope sources the sedimentary series of these terranes present the oldest tdm ages 1499 2156 ma clearly greater than the tdm ages that characterize coeval sedimentary series of the iberian autochthonous domain 1256 1334 ma this finding allows us to place iberian basal allochthonous units very close to the west africa craton from where they received a dominant contribution of old crustal materials whereas the iberian autochthonous domain occupied a more eastern paleoposition closer to the sahara metacraton

time space distribution of silicic plutonism in a gneiss dome of the iberian variscan belt the evora massif ossa morena zone portugal abstract in the iberian variscan belt polyphasic deformation has been recognized as comprising an early phase of crustal thickening followed by an intermediate phase of crustal extension and doming and a later phase of shortening the evora massif is a gneiss dome of the westernmost domains of the ossa morena zone sw iberia which provides a remarkable insight into the late paleozoic deep crustal structure of the variscan continental crust of northern gondwana in this study we bring new structural and geochronological u pb data for the northern hanging wall of the evora massif we describe the existence of low dipping d2 extensional shear zones associated with buchan type metamorphism m2 this enables three tectono metamorphic units to be distinguished the lower gneiss unit the intermediate schist unit and the upper slate unit d2 m2 structures experienced subhorizontal shortening d3 and were transposed by low plunging folding thrusting and strike slip faulting zircon grains extracted from pavia quartz feldspathic gneiss of the lower gneiss unit yielded a crystallization age of ca 521 ma cambrian stage 2 3 which establishes a correlation with tectono metamorphic units of the footwall and southern hanging wall of the evora massif u pb zircon dating of divor foliated quartz diorite 339 7 ma malarranha weakly foliated biotite rich granite 322 3 ma and undeformed porphyritic granite of the pavia pluton 314 4 ma constrain the timing of emplacement of granitic magmas synchronously with doming carboniferous magmatism initiated with doming me1 ca 343 335 ma continued through d2 m2 me2 ca 328 319 ma and lasted until the waning stage of crustal extension me3 ca 317 313 ma the evora massif gneiss dome probably formed as result of the combined effect of gravitational collapse of the thickened crust and buoyancy driven gravitational instability developed in the partially molten continental crust influenced by the transfer of heat from rising mantle derived i e dioritic gabbroic magmas rocks found in the footwall of the evora massif

correlation of the nappe stack in the ibero armorican arc across the bay of biscay a joint french spanish project a correlation between allochthonous units exposed in the nw iberian massif and the southern armorican massif is carried out based on lithological associations structural position age and geochemistry of protoliths and tectonometamorphic evolution the units on both sides of the bay of biscay are grouped into upper middle and lower allochthons whereas an underlying allochthonous thrust sheet identified in both massifs is referred to as the parautochthon the lower allochthon represents a fragment of the outermost edge of gondwana that underwent continental subduction shortly after the closure of a palaeozoic ocean which in turn is represented by the middle allochthon the latter consists of supra subduction ophiolites and metasedimentary sequences alternating with basic mid ocean ridge basalt morb type volcanics with inheritances suggesting the proximity of a continental domain seafloor spreading began at the cambro ordovician boundary and oceanic crust was still formed during the late devonian covering the lifetime of the rheic ocean which is possibly represented by the middle allochthon the opening of the oceanic domain was related to pulling apart the peri gondwanan continental magmatic arc which is represented by the upper allochthon

comment on the late devonian variscan suture of the iberian massif a correlation of high pressure belts in nw and sw iberia tectonophysics 654 96 100 by r fernandez and r arenas abstract a vast variscan nappe spreading from the allochthonous complexes of nw iberia to the ossa morena zone in sw iberia has been proposed by diez fernandez and arenas 2015 based on the correlation of different outcrops of high pressure and morb type rocks this interpretation conflicts with well known geological and geophysical data regarding the sw iberian variscides which point to i major kinematic and geometric inconsistencies come up when interpreting the ossa morena zone as an allochthonous unit ii the outcrops of high pressure and morb type rocks belong to two different sutures rather than a single one and iii sw iberia is characterized by transpressional deformation related to a very oblique left lateral continental collision while nw iberia features a rather frontal collision

the last stages of the avalonian cadomian arc in nw iberian massif isotopic and igneous record for a long lived peri gondwanan magmatic arc abstract the upper allochthonous units of nw iberian massif contain an extensive cambrian magmatism c 500 ma covering felsic to mafic compositions the magmatic activity generated large massifs of granitoids and gabbros with calc alkaline and tholeiitic compositions respectively petrological and geochemical features of these massifs are characteristic of volcanic arc the plutons intruded siliciclastic sedimentary series deposited in the periphery of the west africa craton u pb hf isotopic compositions of detrital zircon in the siliciclastic host series indicate continental arc activity between c 750 ma and c 500 ma it was characterized by a large variety of isotopic sources including from very old continental input even archean to the addition of a significant amount of juvenile mafic material these isotopic sources experienced an extensive mixing that explains the composition and isotopic features ehft from 50 until 15 of the represented cambrian plutons the cambrian igneous rocks of the upper units of nw iberia are related to the latest activity of the avalonian cadomian arc from the middle cambrian arc activity in the periphery of gondwana was replaced by pronounced extension associated with the development of continental rifting which finally led to separation of the microcontinent avalonia subsequent drifting of avalonia to the north caused progressive opening one of the main paleozoic ocean the rheic ocean

extensional flow produces recumbent folds in syn orogenic granitoids padron migmatitic dome nw iberian massif this contribution provides a case example on the generation of large scale recumbent folds in syn orogenic granitoids we analyze the progressive reworking of extension related structures into later ones after a period of crustal thickening the padron migmatitic dome formed after the climax of the gondwana laurussia collision in the late paleozoic petrostructural analysis carried out in the eastern flank of this dome reveals that extensional flow resulted in progressive exhumation of mainland gondwana which rested under peri gondwanan allochthonous terranes and a suture zone during maximum crustal thickening exhumation proceeded up to upper crust levels andalusite stability field along with partial melting of the middle lower crust and with the generation of granitoid laccoliths during an early extensional stage newly formed lithological and mechanical anisotropies such as the presence of variably sized sheet shaped bodies of syn orogenic granitoids provided a favorable rheological setting for fold nucleation during the intermediate stages of extension in extending orogenic crust whether recumbent folds occur after significant melt production depends on the lateral vertical flow ratio and on the orientation of deforming bodies with regard to kinematic strain axes we suggest that subhorizontal extensional flow dominated over vertical flow during the early and intermediate stages of the evolution of the padron dome a component of vertical diapiric flow caused progressive tilting of the sheet like bodies and obliquity respect to strain axes this resulted in the development of regional scale folds at the expense of syn orogenic granitoids such as in the case of the portomouro recumbent synform extensional ductile flow was oblique to the trend of the orogen during the whole process and directed to the nnw during the formation of recumbent folds non coaxial shearing favored an nnw sse elongate shape for the syn kinematic granitic massifs as well as the subsequent nucleation of recumbent folds deformation concentrated along discrete detachments during the late stages of extension

imaging the crustal structure of the central iberian zone variscan belt the alcudia deep seismic reflection transect alcudia is a 230 km long vertical incidence deep seismic reflection transect acquired in spring 2007 across the southern central iberian zone part of the pre mesozoic gondwana paleocontinent of the variscan orogen of spain the carefully designed acquisition parameters resulted in a 20 s twtt deep 60 90 fold high resolution seismic reflection transect the processed image shows a weakly reflective upper crust the scarce reflectivity matching structures identified at surface a thick highly reflective and laminated lower crust and a flat moho located at 10 s twtt 30 km depth the transect can be divided into three segments with different structural styles in the lower crust in the central segment the lower crust is imaged by regular horizontal and parallel reflectors whereas in the northern and southern segments it displays oblique reflectors interpreted as an important thrust north and tectonic wedging involving the mantle south the alcudia seismic image shows that in an intracontinental orogenic crust far from the suture zones the upper and lower crust may react differently to shortening in different sectors which is taken as evidence for decoupling the interpreted structures as deduced from surface geology and the seismic image show that deformation was distributed homogeneously in the upper crust whereas it was concentrated in wedge thrust structures at specific sectors in the lower crust the seismic image also shows the location of late variscan faults in spatial association with the lower crustal thickened areas

tectonic evolution of the boundary between the central iberian and ossa morena zones variscan belt southwest spain three tectonometamorphic units can be differentiated in the boundary between the central iberian and ossa morena zones of the iberian massif variscan belt the three units have been named the northern unit central unit and southern unit the northern unit corresponds to the border of the central iberian zone it evolved at low temperature and intermediate or low pressure metamorphic conditions and was affected by top to the se ductile shearing the central unit placed under the northern unit preserves high pressure silurian eclogitic assemblages retrograded by high to medium temperature and finally low temperature ductile shearing with top to the nw sense of movement oblique left lateral the central unit is superposed on the southern unit the latter corresponds to the border of the ossa morena zone and underwent low pressure intermediate temperature metamorphism synchronous with right lateral top to the se ductile shearing the envisaged tectonic evolution is as follows after a stage of lower paleozoic rifting subduction of the central unit under the northern unit took place in silurian times as a result of the crustal thickening a gravitational instability developed giving way to left lateral extensional shearing that affected the entire central unit the combined action of oblique thrusting at the front of the wedge and oblique extensional shearing at the rear caused the exhumation of the high pressure rocks of the central unit this tectonic evolution reveals that the boundary between the central iberian and ossa morena zones is a suture of the variscan belt

birth and demise of the rheic ocean magmatic arc s combined u pb and hf isotope analyses in detrital zircon from sw iberia siliciclastic strata paleozoic continental reconstructions indicate that subduction of rheic oceanic lithosphere led to collision between laurussia and gondwana which was a major event in the formation of the ouachita appalachian variscan orogenic belt and the amalgamation of pangea however arc systems which record rheic ocean subduction are poorly preserved the preservation of devonian detrital zircon in late devonian early carboniferous siliciclastic rocks of sw iberia rather than arc related igneous rocks indicates that direct evidence of the arc system may have been largely destroyed by erosion here we report in situ detrital zircon u pb isotopic analyses of late devonian early carboniferous siliciclastic rocks from the pulo do lobo zone which is a reworked late paleozoic suture zone located between laurussia and gondwana detrital zircon age spectra from the pulo do lobo zone frasnian formations show striking similarities revealing a wide range of ages dominated by neoproterozoic and paleoproterozoic grains sourced from rocks typical of peri gondwanan terranes such as avalonia the meguma terrane and the ossa morena zone pulo do lobo rocks also include representative populations of mesoproterozoic and early silurian zircons that are typical of avalonia and the meguma terrane which are absent in the ossa morena zone the famennian tournaisian formations from the pulo do lobo zone however contain more abundant middle late devonian zircon indicating the contribution from a previously unrecognized source probably related to the rheic ocean magmatic arc s the middle late devonian to early carboniferous zircon ages from the siliciclastic rocks of sw iberia south portuguese pulo do lobo and ossa morena zones have a wide range in ehft values 8 2 to 8 3 indicating the likely crystallization from magmas formed in a convergent setting the missing rheic ocean arc was probably built on a meguma avalonia type basement we propose for the pulo do lobo zone that the frasnian sedimentation occurred through the opening of a back arc basin formed along the laurussian active margin during rheic ocean subduction as has been recently proposed for the rhenohercynian zone in central europe detrital zircon ages in the frasnian siliciclastic rocks indicate provenance in the meguma terrane avalonia and devonian rheic ocean arc s as a result of back arc basin inversion the frasnian formations underwent deformation metamorphism and denudation and were unconformably overlain by famennian to visean siliciclastic strata including the phyllite quartzite formation of the south portuguese zone the latest devonian early carboniferous detritus were probably shed to the pulo do lobo zone represa and santa iria formations by recycling of devonian siliciclastic rocks from the south portuguese zone meguma terrane and from a new distinct source with baltica laurentia derivation preserved in the horta da torre formation and alajar melange

tectonic evolution of variscan iberia gondwana laurussia collision revisited an integrated interpretation of the late paleozoic structural and geochronological record of the iberian massif is presented and discussed under the perspective of a gondwana laurussia collision giving way to the variscan orogen compressional and extensional structures developed during the building of the variscan orogenic crust of iberia are linked together into major tectonic events operating at lithosphere scale a review of the tectonometamorphic and magmatic evolution of the iberian massif reveals backs and forths in the overall convergence between gondwana and laurussia during the amalgamation of pangea in late paleozoic times stages dominated by lithosphere compression are characterized by subduction both oceanic and continental development of magmatic arcs over and under thrusting of continental lithosphere and folding variscan convergence resulted in the eventual transference of a large allochthonous set of peri gondwanan terranes the iberian allochthon onto the gondwana mainland the iberian allochthon bears the imprint of previous interaction between gondwana and laurussia including their juxtaposition after the closure of the rheic ocean in lower devonian times stages governed by lithosphere extension are featured by the opening of two short lived oceanic basins that dissected previous variscan orogenic crust first in the lower middle devonian following the closure of the rheic ocean and then in the early carboniferous following the emplacement of the peri gondwanan allochthon an additional major intra orogenic extensional event in the early middle carboniferous dismembered the iberian allochthon into individual thrust stacks separated by extensional faults and domes lateral tectonics played an important role through the variscan orogenesis especially during the creation of new tectonic blocks separated by intracontinental strike slip shear zones in the late stages of continental convergence

100 myr cycles of oceanic lithosphere generation in peri gondwana neoproterozoic devonian ophiolites from the nw african iberian margin of gondwana and the variscan orogen

two stage collision exploring the birth of pangea in the variscan terranes abstract the variscan suture exposed in nw iberia contains a stack of terranes including two allochthonous units with continental affinity and gondwanan provenance upper and basal units separated by an ophiolite belt where the most common units show protolith ages at c 395 ma recent lu hf zircon data obtained from these ophiolites indicate interaction between the gabbroic magmas and old continental crust hence the ophiolites could not have originated in a deep ocean basin associated with a mature mid ocean ridge or intraoceanic subduction the tectonothermal evolution of the continental terranes bounding the suture zone records two consecutive events of deep subduction the upper units record an initial high p ultra high p metamorphic event that occurred before 400 390 ma while the basal units were affected by a second high p low to intermediate t metamorphic event dated at c 370 ma continental subduction affected the most external margin of gondwana and developed in a setting of dextral convergence with laurussia development of the two high p events alternated with the opening of an ephemeral oceanic basin probably of pull apart type in early devonian times this ephemeral oceanic domain is suggested as the setting for the protoliths of the most common ophiolites involved in the variscan suture current ideas for the assembly of pangea advocate a single collisional event between gondwana and laurussia in the carboniferous however the new evidence from the allochthonous terranes of the variscan belt suggests a more complex scenario for the assembly of the supercontinent with an interaction between the colliding continental margins that started earlier and lasted longer than previously considered based on modern analogs of continental interaction the development of complex collisions as here suggested for gondwana and laurussia during the assembly of pangea could have been the norm rather than the exception throughout earth history

extensional orogenic collapse captured by strike slip tectonics constraints from structural geology and upb geochronology of the pinhel shear zone variscan orogen iberian massif abstract the late paleozoic collision between gondwana and laurussia resulted in the polyphase deformation and magmatism that characterizes the iberian massif of the variscan orogen in the central iberian zone initial continental thickening d1 folding and thrusting was followed by extensional orogenic collapse d2 responsible for the exhumation of high grade rocks coeval to the emplacement of granitoids this study presents a tectonometamorphic analysis of the trancoso pinhel region central iberian zone to explain the processes in place during the transition from an extension dominated state d2 to a compression dominated one d3 we reveal the existence of low dipping d2 extensional structures later affected by several pulses of subhorizontal shortening each of them typified by upright folds and strike slip shearing d3 d4 and d5 as identified by superimposition of structures the d2 pinhel extensional shear zone separates a low grade domain from an underlying high grade domain and it contributed to the thermal reequilibration of the orogen by facilitating heat advection from lower parts of the crust crustal thinning decompression melting and magma intrusion progressive lessening of the gravitational disequilibrium carried out by this d2 shear zone led to a switch from subhorizontal extension to compression and the eventual cessation and capture of the pinhel shear zone by strike slip tectonics during renewed crustal shortening high grade domains of the pinhel shear zone were folded together with low grade domains to define the current upright folded structure of the trancoso pinhel region the d3 tamames marofa satao synform new dating of syn orogenic granitoids shrimp u pb zircon dating intruding the pinhel shear zone together with the already published ages of early extensional fabrics constrain the functioning of this shear zone to ca 331 311 ma with maximum tectonomagmatic activity at ca 321 317 ma the capture and apparent cessation of movement of the pinhel shear zone occurred at ca 317 311 ma

constitutive relations for fault slip and earthquake instabilities constitutive relations for fault slip are described and adopted as a basis for analyzing slip motion and its instability in the form of earthquakes on crustal faults the constitutive relations discussed include simple rate independent slip weakening models in which shear strength degrades with ongoing slip to a residual frictional strength and also more realistic but as yet less extensively applied slip rate and surface state dependent relations for the latter the state of the surface is characterized by one or more variables that evolve with ongoing slip seeking values consistent with the current slip rate models of crustal faults range from simple single degree of freedom spring slider systems to more complex continuous systems that incorporate nonuniform slip and locked patches on faults of depth dependent constitutive properties within elastic lithospheric plates that may be coupled to a viscoelastic asthenosphere

the galicia ossa morena zone proposal for a new zone of the iberian massif variscan implications abstract correlation of a group of allochthonous terranes referred to as basal ophiolitic and upper units exposed in the nw and sw of the iberian massif is used to propose a new geotectonic zone in the southern branch of the variscan orogen the galicia ossa morena zone recent advances in sw iberia identify most of the former ossa morena zone as another allochthonous complex of the iberian massif the ossa morena complex equivalent to the cabo ortegal ordenes malpica tui braganca and morais complexes described in nw iberia the new geotectonic zone and its counterparts along the rest of the variscan orogen constitute an internal variscan zone with ophiolites and units affected by high p metamorphism the galicia ossa morena zone includes a variscan suture and pieces of continental crust bearing the imprint of ediacaran cambrian events related to the activity of peri gondwanan magmatic arcs cadomian orogenesis in the iberian massif the general structure of this geotectonic zone represents a duplication of the gondwanan platform the outboard sections being juxtaposed on top of domains located closer to the mainland before amalgamation this interpretation offers an explanation that overcomes some issues regarding the differences between the stratigraphic and paleontological record of the central and southern sections of the iberian massif also equivalent structural relationships between other major geotectonic domains of the rest of the variscan orogen are consistent with our interpretation and allow suspecting similar configurations along strike of the orogen a number of issues may be put forward in this respect that potentially open new lines of thinking about the architecture of the variscan orogen

metamorphism and kinematics of the early deformation in the variscan suture of sw iberia the early devonian collisional stage in sw iberia has been investigated through the analysis of deformation in the cubito moura schists the main lithology of an allochthonous complex putatively rooted in the suture between the ossa morena and south portuguese zones the first deformation in these schists d1 is recorded as a s1 l1 mylonitic fabric well preserved in early quartz veins subsequent d2 deformation caused the main folds and the main s2 foliation after restoration the stretching lineation l1 trends at a small angle with the ossa morena south portuguese suture this trend together with the top to the east kinematics determined from quartz microfabric is indicative of an oblique left lateral collisional scenario in sw iberia chlorite white k mica quartz chloritoid multi equilibrium calculations yield p t conditions in the range 0 9 1 2 gpa and 300 400 c during the first collisional stage p t conditions during d2 were 0 3 0 8 gpa and 400 450 c thus indicating an important stage of exhumation of the allochthonous complex during these two collisional events after subduction of the ossa morena zone margin under the south portuguese zone continental crust in the general context of the variscan orogen dominated by dextral collision the left lateral convergence in sw iberia can be explained in terms of the avalonian salient represented by the south portuguese zone which would impinge between iberia and morocco

physical models of earthquake instability and precursory processes this paper selectively reviews physical models of earthquake instability in these models instability arises as a result of interaction of a fault constitutive relation with deformation of the surrounding material that occurs in response to remote tectonic loading in contrast to kinematic models in which the fault slip is imposed it is calculated in physical models and consequently these models are essential for understanding precursory processes some kind of weakening behavior for the fault constitutive relation is required to produce an instability analogous to an earthquake two commonly employed idealizations discussed here are rate independent slip weakening and rate state dependent friction when these constitutive models are employed on surfaces embedded in elastic half spaces or layers possibly coupled to a viscoelastic substrate the results are capable of simulating realistically some aspects of earthquake occurrence common to all models is the prediction that earthquake instability is preceded by precursory slip which produces a departure of surface strain rate from the background level near the epicenter of a moderate to large earthquake the magnitude of this departure appears to be well within the range of current geodetic measurement accuracy and its duration is of the order of months to years however details depend on a variety of factors including the modelling of the constitutive relation near peak stress coupling of elastic crust to the asthenosphere and coupling of deformation with pore fluid diffusion

allochthonous terranes involved in the variscan suture of nw iberia a review of their origin and tectonothermal evolution abstract nw iberia includes a rather complete section of a variscan suture where different terranes with continental or oceanic affinities appear with clear structural relationships three groups of terranes namely upper ophiolitic and basal units and a frontal tectonic melange appear in galicia in cabo ortegal ordenes and malpica tui complexes they constitute a huge allochthonous pile thrust over the iberian parautochthonous and autochthonous domains which represent the section of the gondwanan margin that escaped continental subduction during the variscan cycle considering the allochthonous character of the nappe pile and the strong deformation associated to the variscan collision there are problems to identify the original tectonic setting of the terranes and thence it is difficult to reconstruct the paleogeographic context during the variscan and pre variscan times in detail key features to perform any model for the variscan convergence should consider the existence of two different high p metamorphic events dated at c 400 and 370 ma respectively separated in time for the generation of mafic ultramafic sequences at c 395 ma which constitute the most common ophiolites described in the variscan suture some dynamic models developed in nw iberia have important problems to explain the observed tectonothermal evolution especially the older high p metamorphic event and the exhumation of deeply subducted transitional type sections on the other hand the recently discovered participation of an older continental crust in the generation of different protoliths of the middle devonian ophiolites makes difficult the interpretation of these ophiolites in relation to open wide oceanic domains this paper describes the distribution structure lithologies geochronology and chemical composition of the terranes involved in the variscan suture of nw spain the scope of this description ranges from detailed regional aspects to the discussion of the development of the variscan orogen in the context of the assembly of pangea a two stage collisional model affecting a wide gondwanan platform may explain most of the evidences in nw iberia the generation of a long pull apart basin probably occurred after the first collision where the devonian ophiolites were formed these ophiolites have been in general described in the context of the rheic ocean but according to the new existing data they do not seem related to this ocean but rather they were formed after its closure

statistical evaluation of efficiency and possibility of earthquake predictions with gravity field variation and its analytic signal in western china this paper aimed at assessing gravity variations as precursors for earthquake prediction in the tibet xizang qinghai xinjiang sichuan region western china we here take a statistical approach to evaluate efficiency and possibility of earthquake prediction we used the most recent spatiotemporal gravity field variation datasets of 2002 2008 for the region that were provided by the crustal movement observation network of china cmonc the datasets were space sparse and time discrete in 2007 2010 13 earthquakes ms 6 0 occurred in the region the observed gravity variations have a statistical correlation with the occurrence of these earthquakes through the molchan error diagram tests that lead to alarms over a good fraction of space time the results show that the prediction efficiency of amplitude of analytic signal of gravity variations is better than seismicity rate model and thd and absolute value of gravity variation implying that gravity variations before earthquake may include precursory information of future large earthquakes

single subduction zone for the generation of devonian ophiolites and high p metamorphic belts of the variscan orogen nw iberia within the variscan orogen early devonian and late devonian high p belts separated by mid devonian ophiolites can be interpreted as having formed in a single subduction zone early devonian convergence nucleated a laurussia dipping subduction zone from an inherited lithospheric neck peri gondwanan cambrian back arc slab retreat induced upper plate extension mantle incursion and lower plate thermal softening favouring slab detachment within the lower plate and diapiric exhumation of deep seated rocks through the overlying mantle up to relaminate the upper plate upper plate extension produced mid devonian suprasubduction ocean floor spreading devonian ophiolites while further convergence resulted in plate coupling and intraoceanic ophiolite imbrication accretion of the remaining cambrian ocean heralded late devonian subduction of inner sections of gondwana across the same subduction zone and the underthrusting of mainland gondwana culmination of nw iberian allochthonous pile oblique convergence favoured lateral plate sliding and explained the different lateral positions along gondwana of terranes separated by palaeozoic ophiolites

reply to comment on the late devonian variscan suture of the iberian massif a correlation of high pressure belts in nw and sw iberia the existence of a large allochthonous nappe pile emplaced onto the hinterland of the iberian massif and covering most of the basement of nw and sw iberia proposed by diez fernandez and arenas has been called into question on the basis of structural geophysical and regional data from sw iberia the discussion provided here is based on a revision of the fundamentals of the variscan geology of nw iberia and on a more detailed presentation of the geology of sw iberia our counterarguments reveal the reasoning of simancas et al against the verisimilitude of the large allochthonous pile as inconclusive incomplete or erroneous in some cases therefore we maintain that our structural correlation and its implications remain fully valid for the case of the iberian massif

modeling gravity variations caused by dilatancies abstract one postulated earthquake mechanism is the dilatancy model the dilatations due to deformation microfracture fracture and stress relaxation either accompanied by fluid invasion or not change the density the geometrical configuration in space and the porosity and the contents of the stressed volume of a tectonically active region observed gravity therefore is expected to respond to the sequence of events in the deformation cycle of dilatation in this paper a formulation of the gravity variations associated with the dilatation processes based on a combined dilatancy model is given the combined dilatancy model consists of the dilatancy diffusion the dilatancy instability and the fault zone dilatancy models for numerical calculations the total tectonically active region involved is represented by a vertical finite right circular cylinder within which the fault zone volume of the potential hypocentral region represented by a disk is embedded the cylinder has a radius of 40 km and thickness of 15 km with the bottom of the cylinder coinciding with the depth of a tectonically detached zone the disk is 2 km thick and 11 km in radius with the bottom of the disk at a depth of 15 km this representation is compatible with that of earthquakes of magnitude 4 5 that frequently occurred through the last decade or so in the beijing tianjin tangshan zhangjiakou bttz region in china the results of the calculations show that the variation forms of gravity changes are a function of space and time depending upon the size and shape of both the total tectonically active region and the fault zone volume therefore the calculation of theoretical gravity variations for a specific region comparable to that of the bttz region may be made on the basis of a thorough prior knowledge of the geological setting and the tectonic stress distribution of that region

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

cadomian subduction collision and variscan transpression in the badajoz cordoba shear belt southwest spain abstract the tectonothermal evolution of the badajoz cordoba shear belt southwest iberian massif spain involved the build up of a cadomian upper proterozoic lower paleozoic subduction collision orogenic wedge complex made up of five stacked tectonic slices and its deformation under transpression during hercynian late paleozoic intracontinental shearing episodes cadomian metamorphism in the high grade gneissic units of the wedge complex is characterized by peak metamorphic conditions of 720 c 8 kbar for the lowermost tectonic slice higuera de llerena mylonite gneiss and 670 c 10 kbar in the overlying gneissic slice blastomylonitic migmatic gneisses the p t conditions recorded by eclogites structurally overlying the previous gneisses enhance a scheme of inverted metamorphism since these eclogites equilibrated at 685 700 c 15 kbar during peak metamorphic conditions after the high pressure event the highest temperatures attained by the eclogites correspond to a period of nearly isothermal decompression with a drastic decrease in pressure later on the retrogressed eclogites experienced a pressure increase before they began to reequilibrate under progressively lower pandt conditions similar to those recorded by the retrogression of gneisses such a p increase in eclogites might be related to a period of underthrusting below continental crust responsible for the amalgamation of the different tectonic units of the badajoz cordoba shear belt the present results unravel that the models concerned with the geotectonic evolution of the ibero armorican arc and the respective correlations among hercynian and pre hercynian cadomian for our aims terranes across the north atlantic must be reconsidered

constraints on the usefulness of gravimetry for detecting precursory crustal deformations abstract the detectability of gravity changes arising from preseismic processes in the source region of an earthquake depends on the nature of the process the magnitude of the impending event the distance from the hypocentre and the background noise level using typical background spectra from recording spring type gravimeters superconducting gravimeters and long term gravity network surveys we have calculated the feasibility of detecting gravity precursors to thrust type earthquakes as a result of the 1 f 2 nature of the gravity noise power spectral density the detectability of precursors by recording gravimeters depends on the rate of build up amplitude over rise time as well as on the ultimate amplitude of the preseismic anomaly ruling out site specific fluid effects the maximum possible amplitude of the precursory gravity signal can be estimated for both preseismic dilatant volume expansion and preseismic dip slip as a function of earthquake magnitude and depth combining these results shows that detection of gravity precursors over a wide range of possible rise times minutes to years would require a network of ten to fifteen recording superconducting gravimeters tied by regular bi weekly high precision gravity measurements distributed over a 30 30 km area where there was a reasonable expectation of seismic events in the range m 6 to 7 at depths of no more than 10 20 km if monitoring were restricted to a search for short term precursors minutes to hours over a subduction zone an unconnected network of superconducting gravimeters could be expected to detect gravity effects resulting from precursory aseismic slip prior to an m 6 event under certain conditions

the elusive nature of the rheic ocean suture in sw iberia the rheic ocean suture resulted from pre carboniferous oceanic subduction followed by late devonian carboniferous variscan collision in sw iberia this suture has been classically located along the boundary between the ossa morena and south portuguese zones based on the presence of three units i a conspicuous metamafic unit beja acebuches that crops out along this boundary and has been interpreted as a pre carboniferous rheic ocean ophiolite ii a low grade metasedimentary unit with minor mid ocean ridge basalt like lithologies pulo do lobo unit thought to represent a rheic ocean subduction related accretionary prism and iii the allochthonous cubito moura unit that contains high pressure and ophiolitic like rocks we report new structural and geochronological data that allow us to reinterpret the origin and internal structure of the beja acebuches and the pulo do lobo units thus both the beja acebuches protoliths and the pulo do lobo metabasalts would have been formed in the context of an intracollisional extensional stage that interrupted the variscan collision at early carboniferous time after the rheic ocean consumption and the first continental collision later on collision was resumed in an oblique left lateral regime that gave way to coeval frontal folds and thrusts and lateral shear zones and strike slip faults structures with variable pressure temperature conditions and space distribution along time as a consequence of the superposition of transtension and complex transpression the rheic suture in sw iberia has an obscure nearly cryptic appearance

variscan exhumation of a subducted paleozoic continental margin the basal units of the ordenes complex galicia nw spain a structural and metamorphic study was carried out in the basal units of the ordenes complex in spain thought to represent a subducted part of the paleozoic margin of gondwana according to their metamorphic evolution this part of the margin was subducted at the onset of the variscan orogeny becoming part of an accretionary complex developed below a colliding element built previously variations in the pt conditions of the first high pressure metamorphic event along the units indicate a polarity of the subduction to the west subsequent underthrusting of more continental material blocked the subduction and triggered the ascent and exhumation of the basal units whereas the convergence continued recumbent folds and thrusts developed along with successive normal detachments compressional and extensional structures were synchronous or alternated in time and together induced the thinning and tapering of the orogenic wedge and its lateral spreading the unroofing took place locally under an inverted temperature gradient caused by a detachment which carried a part of the hot mantle wedge above the subduction zone over the subducted units

regional significance of kilometric scale north east vergent recumbent folds associated with east to south east directed shear on the southern border of the central iberian zone hornachos oliva region variscan belt iberian peninsula on the southern border of the central iberian zone there are two sectors with different styles of deformation to the south west in the hornachos sector large scale recumbent folds associated with ductile shearing can be seen this shearing is characterized by a direction of movement parallel to the fold axes and can be correlated for 150 km along strike the k values of the strain ellipsoid range from 0 8 to 2 0 stretching in the x direction parallel to the recumbent fold axes is more than 100 to the north east in the oliva sector first phase folds are upright and the strain intensity is lower than in the hornachos sector metamorphic geometric and kinematic considerations lead us to conclude that the shearing in the hornachos sector is better explained as conjugate to a main shear zone along which the southern border of the central iberian zone is moved onto the ossa morena zone this main thrust is at present obliterated by a left lateral extensional shear zone that affects a high pressure exotic unit located between the central iberian and the ossa morena zones this high pressure unit constitutes a suture of the variscan belt in the iberian peninsula

magmatism and early variscan continental subduction in the northern gondwana margin recorded in zircons from the basal units of galicia nw spain in situ uranium lead dating la sf icpms and sims and lu hf isotope analyses la mc icp ms of zircon from eclogite facies rocks from the basal units of the variscan belt in galicia constrain their magmatic and metamorphic evolution and give some clues about the nature and origin of the involved basement the samples studied are two felsic gneisses two eclogites and one eclogitic gneiss of intermediate composition metatonalite oscillatory zoned zircon cores from the felsic samples gave a main clustering of u pb ages at 493 2 and 494 2 ma and some older ages that represent inherited cores zircon grains from the intermediate and one of the mafi c rocks show no inherited cores and yielded ages of 494 3 and 498 6 ma respectively interpreted as time of protolith crystallization variably developed homogeneous zircon rims in one felsic gneiss yielded an age of 372 3 ma and very tiny zircons of one eclogite gave 350 2 ma both of which we interpret as metamorphic ages the new age data demonstrate that the calc alkaline magmatic suite described in the basal unit is ca 20 ma older than the alkaline to peralkaline plutonic suite of the same unit dated at 472 2 ma rodriguez et al 2007 and thus probably represents a distinct geologic event overgrowth rims are interpreted as metamorphic on the basis of their lu hf and th u ratios the 372 3 age is considered as dating the high pressure high p metamorphism and is essentially in agreement with previous ar ar and rb sr data this high p metamorphism marks the initial early variscan subduction of the gondwana margin the inherited zircon ages and hf isotopic composition of zircons point to a considerable input of crustal material with west african craton provenance to the felsic magma

rheic ocean ophiolitic remnants in southern iberia questioned by shrimp u pb zircon ages on the beja acebuches amphibolites 1 the rheic ocean was a major oceanic domain between avalonia and gondwana in ordovician silurian times most of the paleozoic plate reconstructions assume that the rheic ocean suture lies within southern iberia coinciding with the contact between the south portuguese zone and the ossa morena zone this paper reports four sensitive high resolution ion micro probe shrimp u pb zircon ages from mid ocean ridge basalt morb featured rocks of the beja acebuches amphibolite unit which crops out along the boundary between the ossa morena and the south portuguese zone and is considered its most conspicuous suture unit the obtained ages range from 332 3 to 340 4 ma corresponding to the crystallization of the mafic protoliths these early carboniferous ages for the beja acebuches amphibolites imply that this unit can no longer be viewed as an ophiolite belonging to the rheic ocean suture since this oceanic domain was presumably closed in devonian times tectonic reconstructions joining in a single suture line the beja acebuches amphibolite unit in southern iberia to either the devonian lizard ophiolite in southern england or the root zone of the devonian ordovician ophiolitic units in northwest iberia must be therefore reconsidered because of the age difference we interpret the beja acebuches amphibolite unit to represent a narrow and very ephemeral realm of oceanic like crust that opened in early carboniferous times after total consumption of the rheic ocean we suggest that a mantle plume underneath southern iberia in early carboniferous times is the most plausible large scale geodynamic scenario for the formation of these morb featured rocks

geochemistry of the ediacaran early cambrian transition in central iberia tectonic setting and isotopic sources abstract a complete ediacaran early cambrian stratigraphic transition can be observed in the southern part of the central iberian zone iberian massif two different stratigraphic units underlying ordovician series display geochemical and sm nd isotopic features in agreement with an evolving geodynamic setting pusa shales early cambrian rest unconformably on greywackes of the lower alcudian formation late ediacaran both sequences present minor compositional variations for major and trace element contents and similar ree patterns close to those of paas post archean australian shale trace element contents and element ratios suggest mixed sources with intermediate to felsic igneous contributions for both units tectonic setting discrimination diagrams for the ediacaran greywackes indicate that these turbiditic series were deposited in a sedimentary basin associated with a mature active margin volcanic arc however the compositions of the cambrian shales fit better with a more stable context a back arc or retro arc setting end t and tdm ages are compatible with dominance of a similar cratonic source for both sequences probably the west africa craton end 565 values for the ediacaran greywackes 3 0 to 1 4 along with tdm ages 1256 1334 ma imply a significant contribution of juvenile material probably derived from the erosion of the volcanic arc however end 530 values in the cambrian shales 5 2 to 4 0 together with older tdm ages 1444 1657 ma suggest a higher contribution of cratonic isotopic sources probably derived from erosion of the adjacent mainland coeval with the progressive cessation of arc volcanism along the peri gondwanan realm during the cambrian there was a period of more tectonic stability and increasing arrival of sediments from cratonic areas the geochemistry of the ediacaran cambrian transition in central iberia documents a tectonic switch in the periphery of gondwana from an active margin to a more stable context related to the onset of a passive margin

using distributions and stable isotopes of n alkanes to disentangle organic matter contributions to sediments of laguna potrok aike argentina when using biomarkers such as n alkanes as tools for paleo environmental reconstructions it is imperative to determine their specific sources for each setting toward that goal we analyzed a set of various potential organic matter om sources such as aquatic and terrestrial plants dust and soils from laguna potrok aike lpa and surrounding areas in southern patagonia we determined chain length distributions and hydrogen d and carbon 13c isotopic compositions of n alkanes of different om sources in order to quantify their relative contributions to lake sediments our results reveal that the mid chain n alkane n c23 is predominantly produced by submerged aquatic plants whereas long chain n alkanes n c29 n c31 are derived from various terrestrial sources we estimated their relative contributions to the sediment using two approaches i e based on the n alkane distributions and their d and 13c values both approaches result in similar estimates of aquatic and terrestrial contributions for mid and long chain n alkanes to the sediment 62 73 of the mid chain n c23 alkanes originate from aquatic sources while 66 77 of the long chain n alkanes originate from dust and 14 30 from terrestrial plants our study shows that mid chain n alkanes such as the n c23 alkane in lpa are derived mainly from aquatic macrophytes and thus have the potential to record changes in lake water isotopic composition in contrast the n c29 alkane reflects the isotopic signal of various terrestrial sources from southern patagonia

origin of cordierite bearing monzogranites from the southern central iberian zone inferences from the zoned sierra bermeja pluton extremadura spain abstract voluminous peraluminous biotite biotite cordierite and cordierite muscovite bearing granitic plutons were emplaced in the southern part of the central iberian zone iberian massif during the waning stages of the variscan orogeny the role of the mantle and the nature of the crustal sources involved in the generation of these remarkable granites are still a matter of discussion to address these issues a detailed study of the sierra bermeja pluton a 60 km2 cordierite bearing monzogranite intrusion representative of the regional nisa alburquerque los pedroches magmatic alignment has been carried out this pluton was formed by an inward sequence of three distinct but near contemporaneous magma pulses outer unit ou middle unit mu and inner unit iu the nested pluton is cut by a number of syn plutonic vaugneritic dykes while lamprophyres were distinctly later the age of the pluton has been established at ca 306 ma 309 3 to 304 3 ma by la icp ms u pb dating of zircon and monazite the peraluminosity a cnk 1 01 1 33 and p2o5 contents 0 19 0 35 wt of the monzogranitic units increase inwards ou mu iu this is accompanied by an overall decrease in tio2 mgo cao and feot and most trace elements including ree the three units display similarly low 87sr 86sr306ma ratios 0 7033 0 7046 but decreasing end306ma ou 2 7 mu 3 8 iu 7 5 to 4 7 vaugneritic rocks have homogeneous composition with similar 87sr 86sr306ma ratios 0 7043 to those of the monzogranites but higher end306ma 1 1 values whole rock geochemical data imply a partial melting of mainly cambro ordovician felsic metaigneous rocks ou with increasing proportion of neoproterozoic lower cambrian metasediments of the schist greywacke complex mu that formed almost exclusively the source of the iu thermobarometric data suggest that melting processes progressed upward through the geological record and magma emplacement took place at bt ap mnz xtm the present results could be extensible to other cordierite bearing granites of the southern central iberian zone although forthcoming experimental work and detailed comparisons are required to confirm such extrapolation

the boundary between the ossa morena and southportuguese zones southern iberian massif major suture in the european hercynian chain the ossa morena zone omz and the southportuguese zone spz represent the southernmost domains of the iberian massif european hercynian belt lithostratigraphic petrographic and structural differences exist between both zones the boundary between the omz and the spz is marked by a narrow belt of amphibolites of tholeiitic oceanic affinity bard mmunha et al 1986 and associated serpentinites flaser gabbros pillowed metabasalts castro et al 1987 which are highly sheared along their southern border these beja acebuches ophiolites op cit are exposed for over 200 km in the southernmost part of the omz and the northernmost area of the spz several zones have been distinguished on the basis of structural and lithological criteria the southernmost area of the omz which bounds the acebuches amphibolites is composed of high grade metamorphic rocks and an another important shear zone mark the omz northern boundary the boundary between the omz and the spz is considered to be a suture zone between two continental areas represented by the northern omz and the southern spz sinistral transpression associated with large scale recumbent folds are characteristic of the suture which can be tentatively correlated through the ibero armorican arc with the lizard complex sw england

from opening to subduction of an oceanic domain constrained by la icp ms u pb zircon dating variscan belt southern armorican massif france in the variscan belt of western europe the lifetime and evolution of the oceanic domain is poorly constrained by sparse outdated and unreliable multigrain id tims u pb zircon dating in this article we present a complete in situ la icp ms dataset of about 300 u pb zircon analyses obtained on most of the ophiolitic and eclogitic outcrops of southern brittany comprising new dating of previously published zircon populations and newly discovered rock samples in situ dating and cathodo luminescence imaging of each zircon grain yields new absolute time constraints on the evolution of the galicia moldanubian ocean the new results confirm that the opening of this oceanic domain is well defined at about 490 ma in contrast the generally quoted 400 410 ma age for the high pressure event related to the subduction of the oceanic crust is definitely not recorded in the zircons of the eclogites in light of these new data we propose that the obduction of oceanic rocks occurred at about 370 380 ma while the high pressure event is recorded at 355 ma in only a few zircon grains of some eclogite samples additionally this large scale dating project demonstrates that the zircons from eclogites do not systematically recrystallise during the high pressure event and consequently their u pb systems do not record that metamorphism systematically these zircons rather preserve the isotopic memory of the magmatic crystallization of their igneous protolith another example of an eclogite sample from the french massif central illustrates the frequent mistake in the interpretation of the ages of the early hydrothermal alteration of zircons in the oceanic crust versus partial or complete recrystallization during eclogite facies metamorphism

gravity increase before the 2015 mw 7 8 nepal earthquake the 25 april 2015 nepal earthquake mw 7 8 ruptured a segment of the himalayan front fault zone four absolute gravimetric stations in southern tibet surveyed from 2010 2011 to 2013 and corrected for secular variations recorded up to 22 40 1 11 gal yr of gravity increase during this period the gravity increase is distinct from the long wavelength secular trends of gravity decrease over the tibetan plateau and may be related to interseismic mass change around the locked plate interface under the himalayan tibetan plateau we modeled the source region as a disk of 580 km in diameter which is consistent with the notion that much of the southern tibetan crust is involved in storing strain energy that drives the himalayan earthquakes if validated in other regions high precision ground measurements of absolute gravity may provide a useful method for monitoring mass changes in the source regions of potential large earthquakes