a microearthquake study of the mygdonian graben northern greece during march and april 1984 a temporary network of 29 portable stations was operated in the region of the mygdonian graben near thessaloniki northern greece where a destructive earthquake ms 6 5 had occurred in the summer of 1978 during a period of six weeks we recorded 540 earthquakes with magnitudes ranging from 0 2 to 3 0 from this set of data 254 events are selected which according to us have a precision in epicenter and depth better than 1 5 km a total of 54 single event focal mechanisms have been determined the seismicity and focal mechanisms show a rather complex pattern there are no clear individual faults but the e w and nw se striking zones show n s extension zones striking nne ssw show dextral strike slip motion but nw se zones with sinistral strike slip are also observed in the center of the graben where the 1978 earthquake was located we observe several thrust mechanisms distributed in two groups showing either nw se or e w compression these earthquakes seem to be located 2 km above the earthquakes showing normal mechanisms the mean direction of the t axes found from the focal mechanisms trends n15 and dips sub horizontal we propose a model for the formation and evolution of a complex graben system comprising several stages in the initial stage the deformation occurs along pre existing nw se or nne ssw faults with normal or strike slip movements in the second stage a new e w trending group of normal faults is formed over the ancient fault network these new faults have a direction perpendicular to the mean t axis and accommodate better the actual state of stress at this stage the initial faults adjust to the deformation produced by the e w trending new faults and may constitute geometric barriers to the evolution of the new normal faults

neotectonics of the central parts of the balkan peninsula basic features and concepts the neotectonic movements on the balkan peninsula occurred after the last intense thrusting early miocene and after the early middle miocene planation they were controlled by extensional collapse of the late alpine orogen and by extension behind the aegean arc and were influenced by the complicated vertical and horizontal movements in the pannonian region the stara planina and dinarian hellenic linear neotectonic morphostructures inherited the alpine orogenic zones balkanides and dinarides hellenides and bounded the central balkan neotectonic region the linear morphostructures were tilted towards the pannonian and euxinian basins and the north aegean trough the central balkan neotectonic region has a complicated block structure horst and graben pattern dominated by the nnw sse struma and vardar lineaments the wnw ese sava and marica lineaments and the middle mesta and north anatolian fault zones the dominating serbo macedonian neotectonic swell was rifted and subsided along the struma and vardar lineaments the range of the vertical neotectonic displacements reached a maximum of 3 4 km and even up to 6 km at the edges of the pannonian and aegean basins the general doming of the region was controlled by the isostatic uplift of a thickened crustal lens rhodope massif in the southern margin of the eurasian plate the collapse of the complicated domal structure began along the main struma vardar and marica lineaments in the central parts of the dome and continued in the pliocene and quaternary along a more external contour bounded by the stara planina and dinarian hellenic linear morphostructures

geology of the argentine continental shelf and margin from aeromagnetic survey new aeromagnetics show a complex of magnetic patterns and source depths of the north and central argentine continental and adjacent oceanic crust the 40 km wide positive magnetic g anomaly reflects an end rift associated volcanic edifice at the continental margin m4 mo and cretaceous quiet zone oceanic crust are identified the colorado magnetic discontinuity forms the southern boundary of the precambrian la plata craton to the south the continental crust of the patagonia platform is composed of amalgamated late precambrian through end paleozoic weakly magnetic sedimentary prisms the sw patagonia platform crust shows a strong high frequency magnetic pattern associated with mesozoic volcanics while zones of mid crustal level intrusives and some volcanics on the outer shelf are expressed as a broad higher amplitude magnetic pattern

euro seistest determination of the geological structure of the volvi basin and validation of the basin response the volvi basin northern greece is a 5 km wide graben of neogene age it has been chosen by european union directorate general xii for science research and development in climatology and natural hazards as a test site euro seistest for studying site effects in one of europe s most seismic active areas the site is located in the epicentral area of the 1978 ms 6 5 earthquake that struck the city of thessaloniki the geometry velocity and stratigraphy of the basin were not well known consequently a number of geophysical and geotechnical studies were performed to determine the 2d geometry and the dynamic characteristics vp vs qs of the valley investigations included 78 refraction tests 20 surface wave inversions and 8 borehole seismic tests as well as geotechnical tests this article presents the methodology used for the geophysical investigation and the results in its center the graben depth reaches 190 m and at least four normal faults have been located along the section the overall graben structure and the fault locations have been confirmed by the records of a shot generated at one edge of the valley and by the measurements of a local earthquake wave propagation appears to be strongly influenced by the thickness of the sediments which in turn is controlled by fault displacements simple 2d modeling of the basin response was performed and the results are in good agreement with weak motion data confirming the validity of the seismic structure inferred from seismic prospecting

the development of a residual soil profile from a mudstone in a temperate climate in the paper the development of an expansive residual clay soil profile weathering from a permian mudstone is described the characteristics considered include soil mineralogy soil chemistry soil texture and engineering parameters including expansive potential the effects of weathering are shown to be most evident in the development of a series of texturally distinct soil horizons it is shown that the chemical and mineralogical characteristics of the parent mudstone are very similar to those of its derived expansive clay soils even though the expansive potential in the clay soil is almost twice as great as in the parent mudstone the origins of an enhanced expansive potential in the b horizon are explored and it is found that for the weathering of a mudstone in a temperate climate the effects of weathering on structure are more important than the effects of weathering on mineralogy it is concluded that most of the smectite clays in the weathered clay soils were inherited from the parent mudstone with little or no transformation and that these clay components were likely to have been present in the sediments from which the mudstone formed the enhanced expansive potential in the residual clays is attributed to the physical effects of weathering in breaking down structure and or bonding in the parent rock which reduces the potential expansiveness of the clays contained in the parent rock this proposal is supported by experimental evidence

the 1978 earthquake sequence near thessaloniki northern greece summary this paper presents a detailed study of teleseismically and locally recorded foreshocks and aftershocks of the 1978 thessaloniki earthquake sequence and discusses the relations between the locations and mechanisms of these shocks and the three dimensional geometry of faulting in the source region sixteen teleseismically recorded events were relocated using a relative relocation method and positioned geographically using the accurate locally determined location of the largest aftershock all these shocks had depths shallower than 15 km the spatial distribution of the foreshocks showed that the activity was initiated at the central part of the epicentral region progressed westwards and for about 20 days clustered in the region of the imminent mainshock no teleseismically recorded foreshock was located west of the mainshock location in less than 1 hr after the mainshock large aftershocks occurred west of it and in later stages spanned the entire western part of the epicentral region no teleseismically recorded aftershocks were located east of the mainshock locally recorded aftershocks were accurately located and provided a more detailed picture of the seismic deformation they occurred at depths between 3 and 12 km and in three distinct clusters separated by well defined gaps the largest foreshock on may 23 had a moment of 5 6 1024dyne cm and a mean displacement of 31cm the mainshock on june 20 had a moment of 5 2 1025dyne cm a strain drop of 4 0 10 5 static stress drop of 12 bar and a mean displacement of 64 cm these values are for one of the two possible interpretations of the aftershock distribution which suggests that the largest earthquakes of the sequence occurred on two different but adjacent fault segments the first event 1 on may 23 occurred along the central fault segment and was followed 28 days later by the second event 2 on june 20 along a western fault segment finally 61 days later another smaller earthquake event 3 occurred along another fault east of event 1

the northeastern brazil and gabon basins a double rifting system associated with multiple crustal detachment surfaces analysis of structural stratigraphic and gravimetric data from the reconcavo tucano and sergipe alagoas basins in northeastern brazil and the gabon basin in africa shows that these basins originated in a double rifting system associated with multiple crustal detachment surfaces and that the direction of dip of the detachment surfaces was reversed at the vaza barris fault system this geometry is in agreement with models and data from modern rifts but it requires the existence of a predominantly extensional stress regime in the northern part of the south atlantic during the early stages of oceanic evolution evidence for the existence of such a stress regime is given by application of kinematic constraints to early cretaceous reconstructions of the south atlantic ocean interpretation of the regional geologic setting suggests that the double rifting system and the point of final continental rupture were controlled by the preexisting structural orientation of the tectonic provinces in the precambrian basement

crustal p and s velocity structure of the serbomacedonian massif northern greece obtained by non linear inversion of traveltimes summary in the present study the p and s velocity structure of the crust and uppermost mantle in the area of central macedonia northern greece is presented as derived from the inversion of traveltimes of local events an appropriate preconditioning of the final linearized system is used in order to reduce ray density evects on the results the study focuses mainly on the structure of the broader area of the serbomacedonian massif interesting features and details of the crustal structure can be recognized in the final tomographic images the crustal thickness shows strong variations under the serbomacedonian and western rhodope massifs the crust has a thickness that exceeds 30 km on the other hand the north aegean trough exhibits a fairly thin crust 25 27 km moreover the serbomacedonian massif is bounded by two regions that trend parallel to the axios river thermaikos gulf and the strymon river orfanou gulf respectively which show significant crustal thinning 25 28 km the observed match between the direction of this crustal thinning and the basins axes indicates that they have been generated by the same extensional deformation episode

late cenozoic development of the strouma and mesta fluviolacustrine systems sw bulgaria and northern greece abstract the strouma and mesta are two of the largest rivers that drain across sw bulgaria and northern greece into the northern aegean sea their modern valleys flanked by quaternary river terraces are incised into a diverse landscape which records the region s complex tectonic history a network of lacustrine basins existed in the region in the late oligocene to earliest miocene but was disrupted by thrusting and folding related to early miocene transpression this deformation was followed by a period of erosion covering most of the early and middle miocene which probably marked the initiation of the strouma and mesta fluviolacustrine system with geometries unrelated to the older systems the first clear evidence of these river systems dates from the middle miocene late badenian to sarmatian the systems evolved in the late miocene maeotian to early pontian when lakes existed characterized by diatomaceous algae and by occasional burial of abundant plant fragments and coal formation areas in the south south of the kerkini fault were intermittently submerged beneath the aegean sea at this time intense localized uplift of horst blocks in late pontian and pliocene associated with crustal extension resulted in deposition of thick alluvial fans with tilting of sedimentary successions in adjacent grabens evident by the end of the pliocene the highest horsts osogovo rila pirin and belasitsa experienced additional uplift in the pleistocene in part as a result of regional uplift and in part through continued normal faulting pleistocene climate change also resulted in influxes of glacial and fluvioglacial systems the present form of the strouma and mesta fluviolacustrine systems is thus the result of interplay between crustal extension regional uplift and global climate change

mesozoic plate tectonic reconstructions of the central south atlantic ocean the role of the west and central african rift system abstract the mesozoic opening history of the central equatorial and south atlantic oceans is closely linked in time and space to the development of the cretaceous rift system in west and central africa geological and geophysical studies of the west and central african rift system indicate it to be dominated by a set of sinistral and dextral strike slip zones diverging from the gulf of guinea and terminating as extensional rift basins in niger and sudan plate tectonic studies of the opening history of the atlantic basins indicate four stages of development stage 1 the jurassic opening of the central atlantic stage 2 the early cretaceous 130 119 ma opening of the south atlantic with rifting propagating deep into africa via the benue trough stage 3 the opening of the equatorial atlantic 119 105 ma and stage 4 the linkage of these oceanic basins and development of a single opening mid oceanic rift system as seen today the west and central african rift system continued to develop until the end of the cretaceous i e into stage 4 when rifting was terminated by a compressional shear deformation event causing the folding of sediments in the benue trough and reactivation of the central african shear zone and extensional tectonics in the basins of southern sudan this event correlates with an important change in the relative opening of the atlantic basins during tertiary recent times the west african rift system has not been active however during this period domal uplift and volcanism have occurred in central cameroon and western sudan and a very active phase of volcanism has occurred within the last 10 ma along the present day cameroon volcanic line although these uplift and volcanic events indicate thermal processes in the upper mantle there is as yet no clear link between their development and plate tectonic processes

geomorphological and structural modification of the serbomacedonian massif during the neotectonic stage abstract the serbomacedonian massif is an old geotectonic zone strongly modified during the neogene and the quaternary this modification resulted from intensive tectonic activity two main sets of faults crossed almost at right angles and contributed to the breakage of the pre alpidic crust of the massif while the subsequent vertical movements of the broken blocks contributed to its morphostructural differentiation geomorphological and structural features reveal the formation of three distinct morphotectonic units in the continental part of this zone the northern unit is dominated by the morava basin the central unit forms a mountainous asymmetric ridge the southern unit forms a complex area of horst and graben structures the central and the southern units are bounded by the struma strymon basin and the vardar axios basin in terms of global tectonics the northern unit was affected by a miocene extension in the pannonian basin it continued after the miocene as a long term slow process as indicated by the present low seismicity the central unit bears evidence of compression almost transverse to the long axis of the zone the tectonic processes are still active as is indicated by high seismicity in the struma basin and recent uplift of the crust in the area of the ridge the southern unit was affected by an early miocene extension and a late strong quaternary extension in the aegean region the high seismicity and mobility of this unit indicate that very active tectonic processes are still continuing

petrogenesis of the gross spitzkoppe topaz granite central western namibia a geochemical and nd sr pb isotope study abstract the early cretaceous gross spitzkoppe granite is one of several igneous complexes that formed in the damaraland of namibia during an episode of continental rifting and magmatism associated with opening of the south atlantic this is a classic setting of rift related anorogenic magmatism showing the typical association of mantle derived mafic and alkaline rocks along with silicic units the gross spitzkoppe granite is an important example of the latter because its peraluminous composition and crustal isotopic signature makes it well suited to characterize the crustal contribution to magmatism the gross spitzkoppe stock gss is composed of three texturally distinct topaz bearing biotite monzogranites average 76 wt sio 2 30 wt rb sr and 0 5 wt f iron rich biotite is the only mafic silicate in proximity to the stock are numerous felsic dikes rhyodacites to rhyolites the elevated high field strength elements hfse contents and high ga al ratios of the granites and felsic dikes are typical of a type granites the granites and dikes show similar geochemical differentiation trends and the compositions of the more evolved dikes overlaps with the granites this and similar initial isotope ratios suggest that the dikes and granites have a common origin major and trace element modeling confirms that the composition of the gross spitzkoppe granite can be produced by fractional crystallization from magma with the average felsic dike composition rb sr isotope data for nine whole rock samples of the gss yield an age for emplacement of the complex at 125 1 ma initial 87 sr 86 sr ratios vary from 0 710 to 0 716 neodymium isotope compositions of the granites and dikes yield initial e nd 125 values from 5 6 to 6 4 the pb isotopic ratios k feldspar or age corrected whole rock fall in the range 206 pb 204 pb 18 70 18 82 207 pb 204 pb 15 66 15 71 and 208 pb 204 pb 38 57 39 07 these geochemical and isotopic characteristics are consistent with derivation of the gross spitzkoppe granitic magma from a granulite facies metasedimentary source batch melting calculations suggest that low 10 20 degree partial melting of a felsic granulitic source is capable of producing melt compositions similar to those of the felsic dikes with further fractional crystallization being required to produce the granites the source rocks are thought to be damara belt metasediments that were melt depleted in the early paleozoic by extraction of the s type damara granites the most likely scenario for crustal melting is by basaltic underplating of the source region in the lower crust the range in initial nd and sr values is attributed to minor mixing of basaltic magma with the crustal melts although sr was also affected by minor contamination from the radiogenic damara granites near the level of emplacement

age of mesozoic igneous rocks in northwesternnamibia and their relationship to continental breakup mesozoic igneous rocks of northwestern namibia comprise continental flood basalts and quartz latites of the parana etendeka province and a suite of central sub volcanic intrusions the damaraland complexes conventional k ar ages published for these rocks range from 377 88 ma with the majority of samples yielding ages of 150 110 ma this large spread of ages is interpreted as being due to disturbance of the k ar system by low temperature alteration which is ubiquitous among the volcanic rocks rb sr and 40 ar 39 ar dating methods which are less susceptible to such disturbance yield a narrower range of ages 132 124 ma with one outlier at 149 ma new rb sr internal isochron ages of 129 8 3 8 ma 126 8 1 3 ma and 126 1 7 3 ma have been determined for an etendeka quartz latite and nepheline syenite intrusions in the messum and okorusu complexes respectively in addition 40 ar 39 ar release spectrum ages of 137 0 0 7 ma 135 0 0 7 ma and 126 9 0 6 ma have been determined for samples from the paresis comendite cape cross nepheline syenite and okorusu nepheline syenite complexes respectively a 13 ma period between 137 and 124 ma is considered to be the best estimate of the timing of mesozoic igneous activity in northwestern namibia the continental flood basalt volcanism was probably limited to the early part of this period and is inferred to have been relatively short lived 1 2 ma magmatic activity among the damaraland complexes was contemporaneous with the onset of flood basalt volcanism but continued for approximately 15 ma and terminated after the onset of sea floor spreading in this area 130 125 ma both the flood basalt volcanism and the magmatic activity of the damaraland complexes are thought to have occurred in response to rifting across the upwelling tristan mantle plume the linear distribution of intrusions in damaraland is interpreted as being due to magmatism focused along a structural discontinuity between the pan african damara terrain to the south and proterozoic cratonic basement to the north

attenuation of coda waves in northern greece the single scattering model has been applied for the estimation of codaq values for local earthquakes that occurred in northern greece during the period 1983 1989 and recorded by the telemetered network of the geophysical laboratory of the university of thessaloniki codaq estimations were made for four frequency bands centered at 1 5 hz 3 0 hz 6 0 hz and 12 0 hz and for the lapse time windows 10 20 sec 15 30 sec 20 45 sec 30 60 sec and 50 100 sec the codaq values obtained show a clear frequency dependence of the formq c q 0 f n whileq 0 andn depend on the lapse time window q 0 was found equal to 33 andn equal to 1 01 for the time window of 10 to 20 sec while for the other windowsq 0 increased from 60 to 129 withn being stable close to 0 75 this lapse time dependence is interpreted as due to a depth dependent attenuation the high attenuation and the strong frequency dependence found are characteristic of an area with high seismicity in agreement with studies in other seismic regions

transform faulting extension and subduction in the carpathian pannonian region the carpathian arc formed during the cretaceous to miocene continental collision of europe with a smaller continental fragment following southward and westward subduction of an oceanic terrane during and after the last stages of thrusting in the outer carpathians a set of discrete basins opened up inside the carpathian loop these basins are regions of local extension which appear to be associated with strike slip faults in general northeast and northwest trending sets of conjugate shears reflect east west extension of the intra carpathian region during the middle and late miocene palinspastic reconstruction of the basins indicates approximately 75 to 100 km of extension comparable to the magnitude of synchronous crustal shortening in the east carpathians extension may have occurred to accommodate continued westward a type subduction after plate convergence was prohibited by geometrical constraints and suggests that in this region miocene subduction is driven by forces acting on the downgoing plate

neotectonic development of the struma kraistid lineament southwest bulgaria and northern greece the struma kraistid lineament is a part of a fault belt of regional importance it strikes nnw sse and cuts through different alpine tectonic zones along the whole balkan peninsula normal and strike slip faults occurred in environments of extension and graben formation during collapse after or between collision epochs in the palaeogene and early neogene and in a back arc extensional environment during the neotectonic end of middle miocene quaternary stage the last alpine compression phase occurred in the beginning of the miocene and early middle miocene planation formed the initial peneplain new intense faulting marked the beginning of the neotectonic stage late badenian and the neotectonic development including sedimentation proceeded in four regional macrocycles badenian sarmatian maeotian pontian dacian and eopleistocene pleistocene the neotectonic development was marked by formation of the serbo macedonian swell as well as by rifting the vardar and struma rifts

recent vertical crustal movements of the carpatho balkan region abstract joo i csati e jovanovic p popescu m somov v i thurm h thury j totomanov i n vanko j and wyrzykowski t 1981 recent vertical crustal movements of the carpatho balkan region in p vyskocil r green and h malzer editors recent crustal movements 1979 tectonophysics 71 41 52 the academies of science and the geodetic offices of the countries in the carpatho balkan region cbr have been examining the recent vertical crustal movements mostly by geodetic methods the total length of the levelling network used in the examinations amounts to 40 000 km within this are 389 levelling traverses 249 nodal points and 133 polygons the time elapsed between the first and the second levellings is 42 years on average the adjustment was carried out using the correlation method for the calculation of the velocity values the data of eight mareographs located at the adriatic baltic and black sea were used the accuracy of the mareographic data is 0 1 1 2 mm yr and that of the nodal points 0 3 1 4 mm yr eustatical effects were neglected the map of the crustal movements at the scale 1 1 000 000 was compiled together with its planimetry and hydrography the map presents the mareographs the repeated levelling network the isovelocity lines and the relevant tectonic characteristics as well as the earthquake epicentres of greater intensity the examination shows relatively little vertical crustal movement ranging from 4 to 6 mm yr for the cbr an uplift was found for the carpathians in those parts outside the carpathians minor a slight uplift was also found for the carpathian basin for most of the yugoslav coast subsidence was recorded otherwise uplifting and subsiding zones alternate in the balkan region

comment on a new scheme for the opening of the south atlantic ocean and the dissection of an aptian salt basin by trond h torsvik sonia rousse cinthia labails and mark a smethurst p torsvik et al recently presented a revised model for the opening of the south atlantic ocean according to these authors this new plate tectonic model is internally consistent and consistent with globally balanced plate motion solutions and takes into account realistic intraplate deformation however this model shows a number of kinematic and geological problems that we underline here together with some comparisons with the new kinematic model recently proposed by moulin et al

magmatism and continental break up in the south atlantic high precision40ar 39ar geochronology abstract a detailed ar ar study of the parana etendeka continental flood basalts cfb has been undertaken using laser spot analysis data provide information not only on the age of the samples but also on the variability of the non radiogenic ar component and state of alteration the results indicate the parana etendeka cfb were erupted over 10 million years between 137 and 127 ma providing a minimum overall eruption rate of 0 1 km3 yr 1 this is an order of magnitude less than that previously proposed for this the deccan and siberian cfb provinces but it is similar to estimates for hawaii iceland and the columbia river cfb in detail the new ar ar analyses indicate that magmatism within the parana etendeka province migrated from nw to se prior to and during the opening of the south atlantic providing an explanation for the asymmetry of the cfb lavas about the south atlantic moreover chemically defined magma types were erupted at different times in different places and so within the paranathey may not be used as reliable chronostratigraphic units rather such magmatic units may reflect the extent of compositionally distinct source regions in the uppermost mantle and indicate that partial melting took place over a wide area under the paranabasin the nw se migration of the onset of magmatism might be interpreted as a plume trace but the inferred rate of movement is 3 times faster than that inferred from subsequent magmatism on the rio grande rise it is argued that the onset of magmatism reflected extensional tectonics normal to the ponta grossa dyke swarm given that magmatism preceded the main phase of rifting and that extension across the ponta grossa was moderate current plume models have difficulty in predicting sufficient melting within the asthenosphere

age of the ponta grossa dike swarm brazil and implications to parana flood volcanism the ponta grossa dike swarm pgds occurs in a nw trending 200 km wide zone exposed just east of the parana basin in southeastern brazil the predominantly basaltic dikes intrude crystalline basement paleozoic mesozoic sediments and rarely flows of the parana etendeka flood volcanic province pep the pgds resembles the failed arm of a rift rift rift triple junction related to the separation of south america and africa detailed geochemical studies of the dikes including major minor trace element and sr nd isotopic analyses indicate that they probably represent feeders for the voluminous phase of flood volcanism represented by relatively uncontaminated predominantly high tio2 lavas of the northern pep where lava accumulations reach 1700 m thick 40ar 39ar stepwise degassing data using both laser and radiofrequency induction furnace on plagioclase separates from eighteen dikes and one sill yield seventeen plateau ages three are between 120 7 1 3 ma and 125 8 0 6 ma and fourteen are clustered between 129 2 0 4 ma and 131 4 0 5 ma isochron ages are not significantly different from the plateau ages and plateau ages are adopted in all but two cases as being the most accurate age estimates the age probability distribution for the dominant pulse 131 4 0 4 to 129 2 0 4 ma shows a pronounced peak at 130 5 ma this distribution probably reflects the magma production history in the region the new geochronologic data are consistent with conclusions based on paleomagnetic and chemical stratigraphic data that the pgds is younger than the volumetrically dominant majority of volcanism in the southern pep which occurred at 133 132 ma the younger commonly ne trending dikes may reflect the initiation of full drift which was coincident with major basin development in the adjacent continental borderland at 125 120 ma the pgds may indeed represent the failed arm of a rift rift rift triple junction but the triple junction did not coincide exactly in time or space with the site that would be inferred for plume impact

glacial geologic confirmation of an intraplate boundary in the parana basin of brazil the predrift fit of south america and africa remains problematic given the inability to match all parts of the coastlines simultaneously early opening of the south atlantic ocean may have been facilitated by intraplate deformation in either south america or africa but the location of such deformation is controversial and poorly constrained broad scale remote sensing has suggested an intraplate boundary in south america stretching from the cochabamba bend in the andes to the rio grande walvis rise but the inferred boundary crosses the paranaa basin of brazil where any evidence of deformation is obscured by late jurassic flood basalts a subsurface study of the upper paleozoic glaciogenic infill of the paranaa basin the 1300 m thick itarare group ca 300 260 ma identifies repeated episodes of intracratonic rifting on either side of an accommodation zone crossing the basin along the trend of the inferred intraplate boundary renewed influence of this zone during the opening of the south atlantic is suggested by the asymmetric distribution of successive magma units within the serra geral lava pile and by the trend of dike swarms the intraplate boundary identified within the paranaa basin may extend westward along the northern margin of the chaco basin in bolivia

a premonitory pattern of earthquakes in northern greece on 20 june 1978 at 20 h 03 min 23 s an earthquake of surface wave magnitude 6 5 occurred in the migdonian basin between the lagada and volvi lakes and 26 km from thessaloniki the main shock was preceded by a seismic sequence which started on 8 may the largest earthquake of that sequence occurred on 23 may and had a surface wave magnitude of 5 8 the long duration of this sequence before the main shock of 20 june had tremendous psychological effects on the whole country greek seismologists were in a difficult position during that time 8 may 20 june especially after the earthquake of 23 may when pressed to state whether these earthquakes were foreshocks or aftershocks long lasting seismic sequences with earthquakes of magnitude 6 are common in the aegean area because these sequences are sometimes followed by large earthquakes it is important to see whether the latter can be predicted the may june 1978 seismic sequence provided the opportunity for testing hypotheses about statistical properties which it has been suggested could distinguish between foreshocks and aftershocks this particular sequence has been investigated extensively1 4 and more accurate data are available than for any other sequence around greece focal mechanism studies5 6 and field observations2 showed that the largest earthquakes of the sequence were produced by horizontal extension in a nearly north south direction and that the main rupture occurred on a normal fault which strikes almost east west and dips to the north the fault length of the main shock of 20 june was estimated7 to be 32 km accurate determination of earthquake foci using portable seismographs showed that the earthquakes of this sequence were shallow h 17 km while the distribution of the epicentres had a very clear east west elongation4 5 a migration of the seismic activity from east to west has also been observed between 8 may and 14 july we investigate here the time variation of the average magnitude and the frequency of all the shocks with m 3 0 of this sequence the time variations of these seismic parameters are examined in connection with the distribution of well determined epicentres of 38 earthquakes with m 4 3 in order to search for any premonitory pattern of the 23 may and 20 june major earthquakes

seismic velocity constraints in the thessaloniki and chalkidiki areas northern greece from a 3 d tomographic study abstract three dimensional tomographic inversion of p wave travel time data is used to investigate the seismic velocity structure of the crust in thessaloniki and chalkidiki northern greece local earthquakes recorded by two networks operating in the area are used as natural seismic sources two different target volumes defined on the surface by 39 50 41 50 n and 21 25 24 20 e and 40 10 41 00 n and 22 45 23 50 e are investigated the first dataset is recorded by 12 stations and the second by 29 the size of the blocks used to parameterize the areas is 10 10 km and 3 3 km in the horizontal respectively with varying depth layering the major seismic velocity anomalies within the crust obtained by the tomographic inversion are resolved with a horizontal spatial resolution of about 20 and 7 km for the first and second target volume respectively our particular interest is to illuminate velocity anomalies and more detailed characteristics of the two main neogene quaternary basins in the area vardar axios and struma strymon these basins are identified as low velocity features overlying relatively higher p wave velocity structures in the lower crust the complex mygdonian area reveals a similar pattern of low velocity basin overlying higher velocity basement overall the velocity patterns correlate well with the location and strike of the main geological and tectonic units in the area as well as the basic assumptions on basin development this highlights the utility of local tomography to illuminate structural tectonic and rheological properties within the crust

3 d 40ar39ar geochronology in the parana continental flood basalt province new 40ar39ar analyses of borehole samples from the parana etendeka continental flood basalt province provide critical evidence of eruption rates in unexposed regions when combined with surface samples the new data clarify the duration of this major volcanic province new ages from both surface and borehole samples confirm a 10 12 million year duration of magmatism which post dates the tithonian mass extinction and provide a unique 3 dimensional picture of the spatial and temporal eruption of the lava pile chemically defined magma types are diachronous and the onset of magmatism occurred 500 1000 km inland migrating southeast towards the incipient south atlantic ocean calculated eruption rates increased from 0 03 km3 yr 1 between 138 and 135 ma through 0 13 km3 yr 1 between 135 and 133 ma to 0 21 km3 yr 1 between 133 and 131 ma supporting the notion that the rate of magmatism increased with time and proximity to the developing south atlantic after 131 ma the continental eruption rates dropped dramatically to 0 01 km3 yr 1 presumably because rifting had occurred and eruption centred in the oceanic rio grande rise with eruption rates of around 0 3 0 5 km3 yr 1 given the available constraints on the potential temperature of the plume thickness of the continental lithosphere and degrees of extension through time predicted melt production rates within the plume seriously underestimate the calculated eruption rates on the continent however the eruption rates can be reconciled if melting occurred by conductive heating of volatile enriched mantle the diachronous nature of the magma types supports the idea that melting occurred over a wide area and that the different magma types reflect different source regions rather than the temporal evolution of magmas from a single source given that 10 million years is appropriate to the timescales for conductive heating above the mantle plume it is suggested that these distinct source regions were located within the lithospheric mantle consistent with geochemical studies

development of the pannonian basin basement through the cretaceous cenozoic collision a new synthesis abstract a geodynamic analysis was carried out going back in time from the present the collision took place during the cretaceous and cenozoic based on the subduction origin of the neogene calc alkalic volcanites three microplates are delineated the southern and the northern ones are approximately the same as in previous plate tectonic syntheses but the third one is outlined here for the first time as the basement of a miocene island arc restoring the former position of microplates a basin of oceanic crust is outlined on the site of the present pannonian basin of continental crust at the beginning of the neogene the present northern microplate was located somewhere west of this basin and the present southern one was located on its southern shelf the reconstructed cretaceous paleogene histories of these microplates were different during the whole barremian to oligocene time interval the southern microplate was located on the passive margin of an oceanic basin which was coming into being through continental rifting of the european plate margin in the valanginian to hauterivian the northern microplate located on the african plate margin was undergoing collision with an island arc in the early cretaceous the oceanic basin of marginal see type behind this island arc was closed along a southerly directed subduction zone in the eocene this resulted in the appearance of a calc alkalic volcano chain along this microplate and in a new basin opening behind this marginal sea basin divided the present northern microplate from the african plate and joined the oceanic basin north of the present southern microplate the present pannonian basin was assembled as a lithospheric mosaic through convergent and transcurrent movements of the defined microplates during the neogene sediments of the former oceanic basin now strike along the contacts of continental microplates as flysch belts