oroclines thick and thin an orocline is a thrust belt or orogen that is curved in map view due to it having been bent or buckled about a vertical axis of rotation two distinct types of oroclines are recognized progressive and secondary progressive oroclines are restricted to the scale of a thrust sheet to thrust belt are thin skinned and develop during thrust sheet emplacement secondary oroclines are larger occurring at the scale of an orogen and are plate scale features that affect crust and lithospheric mantle unlike progressive oroclines which develop during initial orogenesis and in response to the same orogen perpendicular stress responsible for thrust sheet emplacement secondary oroclines are extra orogenic developing after initial orogenesis and in response to an orogen parallel principal compressive stress that is oriented at a high angle to the stress responsible for orogen development we present case studies of the wyoming salient a progressive orocline that characterizes the sevier thrust belt of the western united states and the coupled cantabrian and central iberian oroclines which are linked secondary oroclines affecting the variscan orogen of iberia the vertical axis rotations involved in progressive and secondary orocline formation are most readily quantified through paleomagnetic analysis detailed three dimensional palinspastic restoration that incorporates translation rotation and strain can distinguish the role if any of primary curvature in progressive oroclines the use of tectonic vectors such as paleocurrent directions provides a means of recognizing and characterizing the initial geometry of secondary oroclines because secondary oroclines involve the entire lithosphere detailed studies of coeval metamorphism and magmatism provide a means of constraining the fate of the mantle lithosphere during oroclinal buckling

three dimensional p wave velocity image under the carpathian arc abstract an inversion of p wave travel time residuals from selected earthquakes in the distance range 30 98 to two seismic station networks was used to model p wave velocity anomalies down to 250 km depth in the first inversion experiment a region between 43 5 47 5 n and 21 29 e was modelled using 35 seismic stations while in the second one a region between 44 47 n and 25 29 e was modelled using 19 seismic stations the 4 layer block model of the first inversion offers 19 reduction in residual variance while the 5 layer block model of the second one offers 26 reduction the rest being explained by noise and smaller scale heterogeneities the obtained velocity anomalies correlate remarkably well with the gravity anomalies and with the tectonic model for the vrancea region of fuchs et al 1979

tiltmeter measurements in long valley caldera california in 1983 the u s geological survey instrumented long valley caldera near mammoth lakes california with an array of borehole tiltmeters to detect the rapid deformation that might accompany incipient volcanic activity such as the intrusion of a dike at shallow depth the array consists of seven sites within and surrounding the zone of the 1983 earthquake swarm five of these sites have instruments installed at a depth of 6 m and consist of two closely spaced installations one containing a rockwell kinemetrics instrument and the other an instrument designed by j westphal of the california institute of technology the sixth site consists of two of westphal s instruments and one rockwell kinemetrics instrument and the seventh site has two of the rockwell kinemetrics instruments only installed at a depth of 2 m since july 1985 all but one site have been equipped with a telemetry system that uses the geostationary operational environmental satellite goes most sites are also equipped with a telemetry system based on telephone lines and radio links this tiltmeter array affords a unique opportunity to compare the relative merits of different instrument types and installation techniques under similar settings in general the results are similar for the two different instruments and installations but local geologic conditions substantially affect the performance at each site the records from the 6 m deep boreholes have better stability and lower noise than those from the 2 m deep boreholes at one site the tilt record appears to be dominated by perturbations related to local geothermal production performance at most other sites indicates that sudden deformation with time spans shorter than about 1 week and amplitudes of about 5 10 rad or greater should easily be detected by the instruments within the array

nature of the vrancea seismic zone eastern carpathians new constraints from dispersion of first arriving p waves abstract the vrancea region of the southeastern carpathians is one of the most active seismic zones in europe and it is well known for its strong intermediate depth earthquakes seismic tomography had revealed a high velocity body beneath vrancea and the moesian platform that extends to a depth of at least 350 km and can be interpreted as descending lithosphere the strong earthquakes occur within the northeastern part of this high velocity body in a very limited seismogenic volume at intermediate depth 70 180 km several geodynamic models have been proposed for this area they can be split into two main categories in terms of the nature of the high velocity anomaly which may a be associated with descending relic oceanic lithosphere beneath the bending zone of the se carpathians either attached or already detached from the continental crust or b it may represent continental lithosphere that has been delaminated after continental collision and orogenic thickening based on currently available information it appears difficult to distinguish between these two types of models in this paper we attempt to shed more light on the nature of the seismic anomaly as well as that of the origin of the intermediate depth seismicity in the vrancea zone by investigating the waveform character of p waves excited by local earthquakes beneath this area and in particular the dependence of group arrival times on frequency we present observations of such a dispersion from stations situated at the bending zone of the se carpathians on the other hand signals from the same earthquakes but observed at reference stations outside of the anomalous zone do not show that frequency dependence a natural explanation for these observations is that it is caused by the presence of a low velocity channel at the top of the seismic anomaly which is too thin to be resolved by classical seismic tomographic techniques similar observations of dispersed first arriving p waves have been made above subduction zones around the world in which low velocity layers with a thickness of several kilometers are known to exist this suggests that a tabular slab of subducted oceanic crust is present within the seismic anomaly under the vrancea region and that the anomaly consists of subducted oceanic lithosphere rather than continental lithosphere at least at depths shallower than the seismically active zone

lateral variations in lithosphere strength in the romanian carpathians constraints on basin evolution lithospheric strength profiles constructed through extrapolation of rock mechanics data and constrained by crustal geophysics data demonstrate the existence of important spatial variations of thermomechanical properties in different domains of the romanian carpathians the pannonian basin and the transylvanian depression these models show important lateral variations in lithospheric rigidity along the southern and eastern carpathian foreland compatible with inferences from flexural modelling studies the modelling predicts an absence of mantle strength and the presence of weak lithosphere in the pannonian and transylvanian basins a nw se trending rheological transect is presented connecting the moesian platform the vrancea region of the carpathians the transylvanian basin and the apuseni mountains with the romanian part of the pannonian basin this transect demonstrates the existence of a close correlation of the inferred spatial variations in mechanical properties of the romanian lithosphere with observed patterns in large scale gravity anomalies and with tectonic units a notable feature of the rheological models is the presence of an intra carpathian weak zone the combination of a strong foreland lithosphere with a weak lithosphere in the inner part of the carpathian transylvanian pannonian system sheds light on a number of key features observed in the evolution of sedimentary basins in these areas remarkable differences in rheology predictions between flexural models and depth dependent rheology models indicate that the crustal lithosphere is detached from the mantle part of the lithosphere in the carpathian foreland

neogene kinematics of the transylvanian basin romania the transylvanian basin is a major sedimentary basin located in the eastern part of the european alpine orogenic system it has a sedimentary fill more than 5 km up to 8 km in some small areas thick of upper cretaceous to upper miocene deposits it represents the main gas producing province in romania and central eastern europe in this article we analyze seismic lines and structural data from the basin as well as from its northern eastern and southeastern borders these data indicate a tectonic origin for the neogene structures from the transylvanian basin these structures include 1 northeast and southwest dipping thrust faults and 2 east northeast and west northwest striking strike slip faults having normal or reverse slip secondary structures such as salt diapirs and folds are related to these faults structural data from the basin and its border indicate a neogene overall compressional transpressional regime having maximum principal stress oriented north to northeast daniel ciulavu received a diploma in geology and geophysics at the university of bucharest in 1989 and his ph d in geology at the vrije universiteit amsterdam netherlands in 1999 he worked two years for geasol s a romania in hydrogeology coal and alluvial gold exploration since 1991 he has been on the staff of the university of bucharest where he teaches the courses on structural geology in the department of geophysics and kinematic analysis for master s students corneliu dinu received a diploma in geology in 1966 and his ph d in 1976 at the university of bucharest romania after graduation he worked as field geologist for prospectiuni s a romania since 1970 he has been on the staff of the university of bucharest where he is professor of structural geology and basin analysis since 1990 he has served as the dean of sciences at the faculty of geology and geophysics university of bucharest alexandru szakacs received a diploma in geology in 1974 and his ph d in 2000 at the university of bucharest romania he is senior geologist at the romanian institute of geology he has 25 years of experience in studying the neogene volcanism of the carpathian pannonian area he has also worked on the neogene volcanism of the cordillera betica spain dorin dordea received a diploma in geology in 1974 at the university of cluj romania his field of activity is remote sensing he has worked in several areas in africa and romania since 1999 he has served as the chief geologist of prospectiuni s a bucharest romania he is now working on the application of remote sensing in mining exploration

geochemistry and tectonic development of cenozoic magmatism in the carpathian pannonian region this review considers the magmatic processes in the carpathian pannonian region cpr from early miocene to recent times as well as the contemporaneous magmatism at its southern boundary in the dinaride and balkans regions this geodynamic system was controlled by the cretaceous to neogene subduction and collision of africa with eurasia especially by adria that generated the alps to the north the dinaride hellenide belt to the east and caused extrusion collision and inversion tectonics in the cpr this long lived subduction system supplied the mantle lithosphere with various subduction components the cpr contains magmatic rocks of highly diverse compositions calc alkaline k alkalic ultrapotassic and na alkalic all generated in response to complex post collisional tectonic processes these processes formed extensional basins in response to an interplay of compression and extension within two microplates alcapa and tisza dacia competition between the different tectonic processes at both local and regional scales caused variations in the associated magmatism mainly as a result of extension and differences in the rheological properties and composition of the lithosphere extension led to disintegration of the microplates that finally developed into two basin systems the pannonian and transylvanian basins the southern border of the cpr is edged by the adria microplate via sava and vardar zones that acted as regional transcurrent tectonic areas during miocene recent times major trace element and isotopic data of post early miocene magmatic rocks from the cpr suggest that subduction components were preserved in the lithospheric mantle after the cretaceous miocene subduction and were reactivated especially by extensional tectonic processes that allowed uprise of the asthenosphere changes in the composition of the mantle through time support geodynamic scenarios of post collision and extension processes linked to the evolution of the main blocks and their boundary relations weak lithospheric blocks i e alcapa and western tisza generated the pannonian basin and the adjacent styrian transdanubian and z rand basins which show high rates of vertical movement accompanied by a range of magmatic compositions strong lithospheric blocks i e dacia were only marginally deformed where strike slip faulting was associated with magmatism and extension at the boundary of adria and tisza dacia strike slip tectonics and core complex extension were associated with small volume miocene magmatism in narrow extensional sedimentary basins or granitoids in core complex detachment systems along older suture zones sava and vardar accommodating the extension in the pannonian basin and afterward pliocene quaternary inversion magmas of various compositions appear to have acted as lubricants in a range of tectonic processes

slab breakoff a critical appraisal of a geological theory as applied in space and time abstract the idea that prominent geological phenomena observed in the crust and at the surface may be caused by the detachment of subducting dense lithosphere has been anticipated by tectonic models in the sixties and seventies and imaged with improved geophysical techniques in the eighties in the mid nineties the model of slab breakoff was defined formally by davies and von blanckenburg initially proposed as a thought provoking working hypothesis the theory rapidly received wide a priori acceptance and was applied to virtually every mountain range around the world and even in orogens as old as paleoproterozoic and archean to explain a range of different phenomena these include magmatic flare ups rapid topographic uplift increasing sediment supply fast exhumation of metamorphic rocks ore mineralizations anomalous distribution of seismicity changes in stress regimes and inversion of subduction polarity even multiple breakoff events were assumed to have occurred at different times in the same orogenic belt or subduction zone in the last 20 years slab breakoff has been invoked in so many settings and time frames that it could have hardly taken place in each and every case in which it was called upon this article does not critically examine the theory but critically examines its use the extensive review of the vast literature presented here on the subject reveals how the model has been often applied to provide ad hoc explanations for a range of poorly understood observations based on incomplete evidence of deep seated processes our aim is to illustrate a paradigmatic example of how earth scientists in the face of evidence that challenges our capacity of understanding often recur to hypotheses based on other hypotheses such an approach may induce researchers to look for confirmation in the absence of compelling constraints or even in the face of conflicting evidence the faith in models should not lead us to confuse speculative theories with axiomatic truths and to build upon them theoretical edifices that are vulnerable and exposed to the risk of circular reasoning

petrogenesis of the early paleozoic strongly peraluminous granites in the western south china block and its tectonic implications abstract strongly peraluminous sp granites have a cnk molecular al 2 o 3 cao na 2 o k 2 o ratios 1 1 indicating a predominant origin from the partial melting of metasedimentary rocks however an increasing number of studies have documented that mantle derived magmas can also be involved in the petrogenesis of some sp granites this is the case in the dulong batholith in the southeastern yunnan province southwestern south china block which is typically composed of sp granitic rocks with high a cnk values 1 1 zircon u pb dating of four samples from this batholith yielded consistent crystallization ages of ca 430 ma synchronous with the widespread late orogenic magmatism including s and i type granites and mafic igneous rocks in the wuyi yunkai orogen south china block scb all the granites have fractionated ree patterns la yb n 1 28 12 3 and conspicuous negative eu anomalies eu eu 0 07 0 43 with a similar depletion in hfse nb zr hf p ba and sr suggesting that these granitic magmas had a dominantly crustal source likely the neoproterozoic metasedimentary rocks i e the danzhou or banxi groups that are dominated by pelitic rocks with minor interlayered siltstones in the western yangtze block however their zircon hf isotopic results also reveal an important input of the mantle derived melts into their parental magma taken together their geochemical and isotopic compositions reflect a derivation by magma mixing between the crust and mantle derived 10 in volume magmas their chemical variations resulted from the fractionation process during the emplacement from the magma chamber after magmatic mixing in conjunction with regional data within the scb it is most likely that the magmatism younger than 435 ma was generated in a post collisional extensional regime related to the partial delamination of an overthickened lithospheric mantle root without lower crust

tertiary palaeomagnetic data for the carpatho pannonian region in the light of miocene rotation kinematics abstract the tectonic interpretation of the palaeomagnetic data for the neogene history of the carpatho pannonian region has usually been based on the palaeomagnetic data for the pre cenozoic complexes here a review of the tertiary palaeomagnetic data is given for the sites with two or more sets of data new site mean directions are calculated for most areas new area mean directions are given with a uniform criterion for the limitation of the normally distributed sub populations based on radiometric data some neogene volcanic sequences are subdivided into two complexes with separate area mean directions the possible influence of the areal tilts on the area mean palaeomagnetic directions is analysed and conclusions on the horizontal rotations are drawn the palaeomagnetic rotations are interpreted in the light of the author s kinematic model for the neogene history of the carpatho pannonian region the rotation of both the southeastern unit by about 100 clockwise and the northwestern unit by about 35 anti clockwise are supported by palaeomagnetic data in addition some rotations predicted by the model are documented by palaeomagnetic data as follows 1 the anti clockwise rotation of the flanks of the darno line gemer bukk area with sinistral displacements in the early miocene 2 the anti clockwise rotations along the southeastern margin of the northwestern unit with sinistral displacements before 10 ma 3 the anti clockwise rotation of the ukrainian carpathians before 12 ma some palaeomagnetic rotations have not been predicted by the kinematic model but occur in the regions where deformations are quite probable these unexpected palaeomagnetic rotations throw a new light on some details of the kinematics 1 the advance of the outer west carpathian nappes later than 14 ma is documented by the increased palaeomagnetic inclination of the pieniny andesites 2 the movement of the central and inner west carpathians later than 12 ma is recorded in the palaeomagnetic rotations of the central and east slovakian volcanites this movement seems to reflect a bay of thin crust within the underthrust european plate margin or more precisely the reaction of the eastern contour of the bay on the drag of the northwestern unit towards the northeast by the still rotating southeastern unit the rotation and deformation of the northwestern unit in the miocene have to be taken into account in the reconstructions of the pre neogene situations since they intensely influenced the arrangement of all older palaeotectonic and palaeogeographic zones the southeastern unit also suffered intense rotation and deformation in the miocene unfortunately there are almost no palaeomagnetic data available for its tertiary complexes

cretaceous to miocene thrusting and wrenching along the central south carpathians due to a corner effect during collision and orocline formation field studies in the romanian south carpathians longitude 22 5 to 24 2 e and latitude 45 2 to 45 6 n demonstrate 1 cretaceous top to ne shearing parallel to the present strike of the thrust system connected with coaxial flattening within the generally northwest dipping foliation 2 paleogene ductile brittle dextral wrenching e w compression 1 87 15 and basin formation petrosani basin along the cerna jiu fault system 3 large scale miocene dextral wrenching along the northern margin of moesia 1 143 16 and 4 probably pliocene early pleistocene n s compression 1 205 25 we discuss the tectonics of the south carpathians stressing the corner effect of the moesian foreland promontory during convergence and formation of the carpathian orocline up to the late early cretaceous subduction of oceanic crust was active between europe moesia on one side and east carpathia rhodopia on the other side collision and intracontinental deformation occurred during the late early and late cretaceous the pinning of the thrust front at the western tip of moesia and the foreland recess north of it caused superposition of thrusting and wrenching during collision and lateral translation tangential stretching during orocline formation and spreading into the recess further convergence during the early tertiary resulted in dislocation of the previously welded east carpathian rhodopian and moesian fragments along the cerna jiu fault system and the further northeast translation of the western segment the intramontane petrosani basin opened as a northeasterly propagating transient pull apart structure along the cerna jiu fault system which acquired a curved northwesterly convex transtensional trace due to the shape of the moesian promontory tightening of the carpathian orocline and or rearrangement of the microplate geometry during the formation of the pannonian basin system led to large scale dextral wrenching along the northern margin of moesia pliocene n s compression reflects final shortening in the carpathian system before ongoing convergence between europe and africa was transferred to the mediterranean rotation of material lines around the moesian corner is corroborated by paleomagnetic studies

tertiary tectonic evolution of the external east carpathians romania abstract paleostress calculation and analysis of mesoscopic structures are integrated with depth interpreted geological profiles based on seismic studies and well correlation to derive a tertiary tectonic model for the east carpathians following early miocene and older orogenic phases the first tectonic event that affected the studied area is characterised by a wsw ene oriented shortening of middle miocene late burdigalian in age resulting deformations induced ene ward thrusting of tarcau and marginal units as well as the internal part of the subcarpathian nappe a second shortening event with an e w to wsw ene contraction direction took place in the late miocene sarmatian characterised by further foreland thrusting of the subcarpathian nappe and out of sequence deformation in the tarcau and marginal folds nappes along strike differences in deformation mechanisms are controlled by the friction coefficients along the main detachment layers by the lateral variations in the wedge thickness and by the involvement in the northern part of the thrusting system of the thick competent east european platform tear faulting occurred in both tectonic events the main resulting structure being the triangle zone developed south of the trotus valley the latest miocene latest sarmatian early pliocene is characterised by a strike slip stress field with nne ssw compression and wnw ese tension axis left lateral faults being dominant the last deformation which affected the studied area is characterised by nnw sse shortening during the pliocene major deformations taking place mainly in the sw most bending zone

the east carpathians indications of phase transitions lithospheric failure and decoupled evolution of thrust belt and its foreland abstract an analysis is presented of the tectonic development of the east carpathians since the late oligocene the geological and geomorphological data show that five epochs of convergence and nappe emplacement took place with the last one occurring 11 12 m y b p after these compressional events the nappe surface remained near sea level the crustal uplift which formed the present mountains began 3 m y b p and after 99 of the shortening of the carpathians had been completed the pre carpathian foreland basin up to 3 5 km deep formed on the margin of the east european platform in the middle miocene analysis of the evolution of the basin shows that it deepened away from the nappe stack and most of the subsidence occurred when there was no convergence this precludes lithospheric flexure due to convergence as a mechanism for basin formation at 11 12 m y b p the inner part of the basin 50 km wide was overridden by the carpathian nappe up to 12 14 km thick at that time a slight crustal uplift took place rather than subsidence in the shallow remnant outer basin part the present foredeep which emerged above sea level an additional 7 8 km of subsidence then occurred in the foredeep of the southeastern carpathians in the late miocene early pleistocene when very little convergence took place the evolution of the carpathian foredeep shows that lithospheric flexure of 2 5 km occurred in regions 20 40 km wide similar amounts of flexure and large basement faults exist under the nappe this style of deformation indicates flexural rigidity of the lithosphere that is several orders of magnitude smaller than in stable cratonic regions under a low flexural rigidity the crust should be close to local isostasy then the isostatic response to superposition of a 10 13 km thick nappe on the platform margin which was near sea level would form high mountains the crustal surface however remained at a low altitude we interpret this to be the result of contraction of gabbro in the lower crust with the formation of dense garnet granulites during nappe emplacement a layer of these dense rocks 20 km thick with p wave velocities 7 5 7 6 km s exists in the lower crust of the east carpathians according to the geophysical data there is no cool high velocity slab now subducting towards the intra carpathian area an increase in the mean height of the carpathians by only 1 km since the termination of convergence precludes a recent slab detachment rapid metamorphism of mafic rocks in the lower crust requires the infiltration of volatiles from the asthenosphere infiltration of volatiles into mantle lithosphere and the upper crust could strongly reduce their viscosity these phenomena might be the cause of an abrupt loss of lithospheric rigidity on the margin of the east european platform in the neogene lithospheric failure allowed the incorporation of this region into the carpathian fold belt after a long period of cratonic development

the neogene evolution of the outer carpathian flysch units poland ukraine and romania kinematics of a foreland fold and thrust belt system abstract various techniques of geometric and kinematic analyses including cross section balancing and forward modeling have been applied successfully to the outer carpathian flysch units along regional geologic profiles crossing the belt from its undeformed foreland to the pieniny klippen belt western carpathians poland or to the central east carpathian allochthon eastern carpathians ukraine and romania the amounts of neogene shortening as well as the mode and the location of basement consumption are discussed on the basis of balanced cross sections compiled along the three most representative regional traverses in addition the complete kinematic evolution of complex carpathian structures such as the fore dukla triangle zone southeastern poland the borislav pokut duplexes ukraine and the marginal folds romania are described in more detail by using a forward modeling approach major results concern the tectonic heritage of laramian inverted structures mesozoic normal faults reactivated as reverse faults during the late cretaceous paleocene compression or transpression in the development of the neogene outer carpathian edifice pre eocene deformation and erosion effectively induce discontinuities in the mid cretaceous blackshale detachment horizon and help to localize the neogene deformations in complex triangle zones the thrust sequence itself evolves alternatively in a piggyback or in an out of sequence mode clearly the critical taper of the outer carpathians is preserved by a constant balance between erosion and thrust reactivations in the inner part of the wedge and sedimentation and tectonic accretion at its front

crustal constraints on the origin of mantle seismicity in the vrancea zone romania the case for active continental lithospheric delamination abstract the vrancea zone of romania constitutes one of the most active seismic zones in europe where intermediate depth 70 200 km earthquakes of magnitude in excess of m w 7 0 occur with relative frequency in a geographically restricted area within the 110 bend region of the southeastern carpathian orogen geologically the vrancea zone is characterized by a a laterally restricted steeply nw dipping seismogenic volume 30 70 200 km situated beneath b thickened continental crust within the highly arcuate bend region of the carpathian orocline and c miscorrelation of hypocenters with the position of known or inferred suture zones in the carpathian orogenic system geologic data from petroleum exploration in the eastern carpathians published palinspastic reconstructions and reprocessing of industry seismic data from the carpathian foreland indicate that 1 crust of continental affinity extends significantly westward beneath the external thrust nappes sub carpathian marginal folds and tarcau of the eastern carpathians 2 cretaceous to miocene strata of continental affinity can be reconstructed westward to a position now occupied by the transylvanian basin and 3 geologic structure in the carpathian foreland including the moho is sub horizontal directly to the east and above the vrancea seismogenic zone taken together these geologic relationships imply that the vrancea zone occupies a region overlain by continental crust and upper mantle and does not appear to originate from a subducted oceanic slab along the length of the carpathian orogen accordingly the vrancea zone appears to potentially be an important place to establish evidence for active lithospheric delamination

tertiary evolution of the intra carpathian area a model abstract the outer carpathian flysch nappes encircle an intra carpathian domain which can be divided into two megatectonic units north pannonian and tisza mostly on the basis of contrasting mesozoic and palaeogene facies development we see two major kinematic problems to be solved 1 1 the present distribution of the mesozoic and palaeogene facies is mosaic like and some belts form exotic bodies within realms of austroalpine affinity 2 2 late eocene palinspastic reconstruction of the outer carpathian flysch nappes suggest that the entire intra carpathian area must have been located several hundreds of kilometres to the south and to the west of its present position neogene extension can account for shortening in the external carpathian nappes but is unable to explain mesozoic facies anomalies and offsets of palaeogene formations we suggest that evolution of the intra carpathian area involved first late palaeogene early miocene juxtaposition of the north pannonian and tisza megatectonic units accompanied by the closure of the external carpathian flysch troughs thereafter extension of this amalgamated unit occurred which was compensated by thrusting of flysch nappes onto the european foreland and formation of molasse foredeeps eastward escape of the north pannonian unit from the alpine collisional belt involved left lateral shear along the pieniny klippen belt and right lateral shear along the mid hungarian zone parts of the late palaeogene basin and an early miocene volcanic edifice were dissected offset and elongated by several 100 kms the driving mechanism of the eastward escape of the intra carpathian area can be related to the collision of apulia and europe and the subduction of the external carpathian crust under the pannonian units the escape ceased gradually in the early miocene when oblique collision between the north pannonian unit and european continent occurred neogene extension of the pannonian region was an areal deformation the extension at locally variable rate resulted in the break up of the heterogenous floor of the neogene basin the driving mechanism of basin extension and contemporaneous compressional deformation of the external carpathians is thought to be related to ongoing subduction involving the marginal part of the attenuated european continental crust tectonic activity in the carpathians and basin subsidence and volcanism shifted in time and in unison from the west toward the east southeast

earthquake precursors in japan precursor time and detectability abstract earthquake precursor data amounting to 418 in number accumulated in japan are analyzed most data are obtained under the nation wide earthquake prediction programme the precursor time t versus main shock magnitude m relationship is different for different groups of precursors a linear relation such as log t a bm where a and b are constants seems to hold for precursor disciplines such as land deformation revealed by geodetic survey and tide gauge observation ground tilt observed by a water tube tiltmeter anomalous seismic activity and geomagnetic field change although a similar relationship seems to hold for the disciplines of earth currents radon concentration underground water etc the precursor time is substantially smaller for these disciplines than for the former disciplines the above characteristics of precursor time suggests that precursors closely connected to the dilatancy generation seem to be observed at an earlier stage of seismogenetic activity precursors probably associated with diffusion of underground water into the dilatant region seem to follow for a number of precursor disciplines such as foreshock ground tilt observed by a pendulum tiltmeter resistivity by a variometer and so on no dependence of t on m is found the relationship between m and the logarithmic epicentral distance d measured in kilometers is different from discipline to discipline relying on empirical and theoretical studies of coseismic strain step it turns out that precursors found by geodetic and geomagnetic observations seem to reflect a crastal strain of the order of 10 7 10 6 precursors obtained by a resistivity variometer seem to represent a strain of the order of 10 9 10 8 precursors of other disciplines seem to correspond to a strain of 10 8 10 7 it is clear that precursors can be detected further from the epicenter as the main shock magnitude becomes larger in some favourable cases a precursor can be observed even where the epicentral distance is several times larger than that of the epicentral area

geophysical constraints on geodynamic processes at convergent margins a global perspective abstract convergent margins being the boundaries between colliding lithospheric plates form the most disastrous areas in the world due to intensive strong seismicity and volcanism we review global geophysical data in order to illustrate the effects of the plate tectonic processes at convergent margins on the crustal and upper mantle structure seismicity and geometry of subducting slab we present global maps of free air and bouguer gravity anomalies heat flow seismicity seismic vs anomalies in the upper mantle and plate convergence rate as well as 20 profiles across different convergent margins a global analysis of these data for three types of convergent margins formed by ocean ocean ocean continent and continent continent collisions allows us to recognize the following patterns 1 plate convergence rate depends on the type of convergent margins and it is significantly larger when at least one of the plates is oceanic however the oldest oceanic plate in the pacific ocean has the smallest convergence rate 2 the presence of an oceanic plate is in general required for generation of high magnitude m 8 0 earthquakes and for generating intermediate and deep seismicity along the convergent margins when oceanic slabs subduct beneath a continent a gap in the seismogenic zone exists at depths between ca 250 km and 500 km given that the seismogenic zone terminates at ca 200 km depth in case of continent continent collision we propose oceanic origin of subducting slabs beneath the zagros the pamir and the vrancea zone 3 dip angle of the subducting slab in continent ocean collision does not correlate neither with the age of subducting oceanic slab nor with the convergence rate for ocean ocean subduction clear trends are recognized steeply dipping slabs are characteristic of young subducting plates and of oceanic plates with high convergence rate with slab rotation towards a near vertical dip angle at depths below ca 500 km at very high convergence rate 4 local isostasy is not satisfied at the convergent margins as evidenced by strong free air gravity anomalies of positive and negative signs however near isostatic equilibrium may exist in broad zones of distributed deformation such as tibet 5 no systematic patterns are recognized in heat flow data due to strong heterogeneity of measured values which are strongly affected by hydrothermal circulation magmatic activity crustal faulting horizontal heat transfer and also due to low number of heat flow measurements across many margins 6 low upper mantle vs seismic velocities beneath the convergent margins are restricted to the upper 150 km and may be related to mantle wedge melting which is confined to shallow mantle levels

revised crustal architecture of the southeastern carpathian foreland from active and passive seismic data 1 integration of active and passive source seismic data is employed in order to study the nature of the relationships between crustal seismicity and geologic structures in the southeastern se carpathian foreland of romania and the possible connection with the vrancea seismogenic zone vsz of intermediate depth seismicity one of the most active earthquake prone areas in europe crustal epicenters and focal mechanisms are correlated with four deep industry seismic profiles the reprocessed danube and carpathian integrated action on process in the lithosphere and neotectonics dacia plan profile and the deep reflection acquisition constraining unusual lithospheric activity ii and iii dracula profiles in order to understand the link between neotectonic foreland deformation and vrancea mantle seismicity projection of crustal foreland hypocenters onto deep seismic profiles identifies several active crustal faults in the se carpathian foreland and suggests a mechanical coupling between the mantle located vsz and the overlying foreland crust the coupled associated deformation appears to take place on the trotus fault the sinaia fault and the newly detected ialomita fault seismic reflection imaging reveals the absence of west dipping reflectors in the crystalline crust and a slightly east dipping to horizontal moho in the proximity of the vrancea area these findings argue against previously purported mechanisms to generate mantle seismicity in the vsz including oceanic lithosphere subduction in place and oceanic slab break off furthermore suggesting that the vrancea seismogenic body is undetached from the overlying crust in the foreland

a seismotectonic picture of the inner southern western alps based on the analysis of anomalously deep earthquakes abstract the anomalously deep seismicity beneath the western po plain is here analyzed to shed light on the complex and still poorly understood tectonic configuration of the internal side of the western alps area the original dataset including 590 earthquakes deeper than 20 km recorded during the last 25 years has been accurately relocated with hypodd using both catalog and cross correlation differential times we found that the distribution of seismic events faithfully mirrors the presence of two distinct tectonic domains axial belt domains 1 and 2 originally belonging to the paleogene alpine wedge and now anomalously juxtaposed beneath the sedimentary infill of the western po plain shallow low magnitude earthquakes 8 km and accommodated in correspondence of the fault could be linked to its neogene activity coeval with the northward translation of the retreating adriatic slab

vrancea99 the crustal structure beneath the southeastern carpathians and the moesian platform from a seismic refraction profile in romania the vrancea99 seismic refraction experiment is part of an international and multidisciplinary project to study the intermediate depth earthquakes of the eastern carpathians in romania as part of the seismic experiment a 300 km long refraction profile was recorded between the cities of bacau and bucharest traversing the vrancea epicentral region in nne ssw direction the results deduced using forward and inverse ray trace modelling indicate a multi layered crust the sedimentary succession comprises two to four seismic layers of variable thickness and with velocities ranging from 2 0 to 5 8 km s the seismic basement coincides with a velocity step up to 5 9 km s velocities in the upper crystalline crust are 5 9 6 2 km s an intra crustal discontinuity at 18 31 km divides the crust into an upper and a lower layer velocities within the lower crust are 6 7 7 0 km s strong wide angle pmp reflections indicate the existence of a first order moho at a depth of 30 km near the southern end of the line and 41 km near the centre constraints on upper mantle seismic velocities 7 9 km s are provided by pn arrival times from two shot points only within the upper mantle a low velocity zone is interpreted travel times of a plp reflection define the bottom of this low velocity layer at a depth of 55 km the velocity beneath this interface must be at least 8 5 km s geologic interpretation of the seismic data suggests that the neogene tectonic convergence of the eastern carpathians resulted in thin skinned shortening of the sedimentary cover and in thick skinned shortening in the crystalline crust on the autochthonous cover of the moesian platform several blocks can be recognised which are characterised by different lithological compositions this could indicate a pre structuring of the platform at mesozoic and or palaeozoic times with a probable active involvement of the intramoesian and the capidava ovidiu faults especially the intramoesian fault is clearly recognisable on the refraction line no clear indications of the important trotus fault in the north of the profile could be found in the central part of the seismic line a thinned lower crust and the low velocity zone in the uppermost mantle point to the possibility of crustal delamination and partial melting in the upper mantle

thermal evolution and geometry of the descending lithosphere beneath the se carpathians an insight from the past abstract mantle heterogeneities imaged by seismic tomography in the se carpathians contain information on the present thermal state of the mantle based on p wave seismic velocity anomalies we develop a model of the present mantle temperature beneath the region and combine the model with a model of crustal temperature constrained by heat flow data the modeled temperatures are assimilated into the geological past using the information on the regional movement in the early and middle miocene prominent thermal states of the lithospheric slab descending in the region are restored from its diffuse present state in miocene times the slab geometry clearly shows two portions of the sinking body the northwest southeast oriented portion of the body is located in the vicinity of the boundary between the east european and scythian platforms and this portion of the sinking body may be a relic of cold lithosphere that has traveled eastward another portion has a northeast southwest orientation and is related to the present descending slab above a depth of 60 km the slab had a concave thermal shape confirming the curvature of the carpathian arc and a convex surface below that depth the slab maintained its convex shape until it split into two parts at a depth of about 220 km we propose that this change in the slab geometry which is likely to be preserved until the present can cause stress localization due to the slab bending and subsequent stress release resulting in large mantle earthquakes in the region also we hypothesize that either the processes of dehydration and partial melting of the descending lithospheric slab or dragging down of hotter rocks from the adjacent uppermost mantle by the slab are possible causes of the reduction in seismic velocities beneath the transylvanian basin

a unified view of craton evolution motivated by recent deep seismic reflection and refraction results summary an attempt is made to integrate recent continental seismic reflection refraction and geologic observations into a unified scheme of craton evolution 1 nascent continental crust of basaltic bulk composition is produced in island arcs as with oceanic crust moho in this setting lies within the magmatic edifice and corresponds to a downward transition from dominantly mafic cumulate rocks above to dominantly ultramafic cumulates below the petrologic crust mantle boundary lying 25 per cent deeper in the lithosphere has no seismic expression and corresponds to a depositional magmatic contact between cumulate ultramafic rocks above and residual mantle ultramafic tectonite below both contacts are presumably extensively disrupted by magmatic injection 2 island arcs are amalgamated into continents at collision zones delamination of the lower mafic ultramafic portion of the crust along with the mantle lithosphere during hard collisions acts as a mechanical refining process that pushes the bulk crust toward intermediate composition 3 subsequent extensional collapse of the overthickened crust together with thermal re equilibration of the lithosphere results in youthful stable continental crust which is 30 km thick with its top surface awash at sea level and has the bulk composition of andesite this crust characteristically exhibits extensional features half grabens in the upper portion and is prominently laminated in the lower portion due to the injection of modest amounts of basaltic magma in the form of sills during delamination the moho and petrologic base of the crust coincide in this setting and can correspond to either a ductile high strain zone or intrusive contact separating mafic intermediate composition material above from fertile mantle lherzolite below 4 subsequent cratonization is the cumulative effect of episodic injection of the crust by mafic magma at intervals of hundreds of myr as manifest by the ubiquitous overlapping dike swarms of the precambrian shields this results in net long term thickening of the crust by underplating shifts the bulk crust back toward mafic composition periodically produces a new deeper moho and petrologic crust mantle boundary that do not coincide and through repeated dike injection disrupts pre existing laminated reflectors the first magmatic injection event sweats out the light melting fraction in the pre existing andesitic crust producing voluminous granitic magmatism typified by the great proterozoic anorogenic granite rhyolite provinces subsequent injection events into refractory crust produce uplift with attendant erosional denudation of supracrustal sequences the cumulative result is shield type crust in which intermediate crustal levels are exposed over wide areas crustal thickness commonly exceeds 40 km average crustal velocity is somewhat higher than in younger stable crust and the crust is commonly complexly reflective diffractive throughout

delamination or slab detachment beneath vrancea new arguments from local earthquake tomography 1 vrancea located at the southeastern carpathians arc bend is one of the areas in the alpine himalayan belt that features strong earthquakes occurring at intermediate depths 60 200 km in this study we investigated the crustal and lithospheric structure beneath the vrancea seismic area using a local earthquake tomography approach we used an updated and revised catalog spanning from 1982 to 2006 that uses data from both permanent and temporary networks in the target area simultaneous tomographic inversion for the vp and vs anomalies and the vp vs ratio and source locations was done using the lotos code the reliability and robustness of the results were rigorously checked using various tests e g by studying the role of different parameters on the results of the inversion performing the inversion using random data subsets and synthetic modeling the tomography results clearly indicate the presence of a high velocity material beneath vrancea at a depth interval of about 60 200 km that coincides with the distribution of intermediate depth seismicity this result agrees generally with previous tomographic studies we compare two scenarios leading to this structure 1 subduction and slab detachment and 2 drop forming or delamination the latter mechanism presumes that the thickening of the crust due to continent continent collision causes transformation of the mafic lower crust into denser eclogite this material accumulates until it reaches a critical mass at which point it forms a large drop that begins to fall down we propose that the high velocity anomaly we observe in our tomogram might represent the descending eclogitic lower crust material enveloped by the entrained lithosphere it is possible that a similar delamination process can be observed in other parts of the alpine himalayan belt such as in the pamir hindu kush area

kinematic constraints on buckling a lithospheric scale orocline along the northern margin of gondwana a geologic synthesis abstract the paleozoic variscan orogeny was a large scale collisional event involving amalgamation of multiple continents and micro continents existing data suggests oroclinal buckling of an originally near linear convergent margin during the last stages of variscan deformation in the late paleozoic closure of the rheic ocean resulted in e w shortening present day coordinates in the carboniferous producing a near linear n s trending east verging belt subsequent n s shortening near the carb permian boundary resulted in oroclinal buckling this late stage orogenic event remains an enigmatic part of final pangea amalgamation the present day arc curvature of the variscan has inspired many tectonic models with little agreement between them while there is general consensus that two separate phases of deformation occurred various models consider that curvature was caused by dextral transpression around a gondwana indentor strike slip wrench tectonics or a change in tectonic transport direction due to changing stress fields more recent models explain the curvature as an orocline with potentially two opposite facing bends caused by secondary rotations deciphering the kinematic history of curved orogens is difficult and requires establishment of two deformation phases an initial compressive phase that forms a relatively linear belt and a second phase that causes vertical axis rotation of the orogenic limbs historically the most robust technique to accurately quantify vertical axis rotation in curved orogens is paleomagnetic analysis but recently other types of data including fracture geochemical petrologic paleo current and calcite twin data have been used to corroborate secondary buckling a review of existing and new variscan data from iberia is presented that argues for secondary buckling of an originally linear orogenic system together these data constrain oroclinal buckling of the cantabrian orocline to have occurred in about 10 ma during the latest carboniferous which agrees well with recent geodynamical models and structural data that relate oroclinal buckling with lithospheric delamination in the variscan

postcollisional lithospheric evolution of the southeast carpathians comparison of geodynamical models and observations seismic evidence and thermal and topographic transients have led to the interpretation of lithospheric removal beneath the southeast carpathians region a series of numerical geodynamic experiments in the context of the tectonic evolution of the region are conducted to test the surface crustal response to lithosphere delamination and slab break off the results show that a delamination type removal plate like migrating instability causes a characteristic pattern of surface uplift subsidence and crustal extension shortening to occur due to the lithospheric deformation and dynamic thermal forcing of the sublithospheric mantle these features migrate with the progressive removal of the underlying lithosphere model results for delamination are comparable with observables related to the geodynamic evolution of the southeast carpathians since 10 ma the mantle structure inferred by seismic tomography migrating patterns of uplift 1 5 km and subsidence 2 km in the region crustal thinning in the carpathian hinterland and thickening at the focsani depression and regional extension in the carpathian corner e g opening of brasov basin correlating with volcanism e g harghita and persani volcanics in the last 3 myr

full waveform seismic tomography of the vrancea romania subduction region abstract the vrancea region is one of the few locations of deep seismicity in europe seismic tomography has been able to map lithospheric downwelling but has not been able yet to clearly discriminate between competing geodynamic interpretations of the geological and geophysical evidence available we study the seismic structure of the vrancea subduction zone using adjoint based full waveform tomography to map the 3d vp and vs structure in detail we use the database that was built during the calixto carpathian arc lithosphere x tomography temporary experiment restricted to the broadband sensors and local intermediate depth events we fit waveforms with a cross correlation misfit criterion in separate time windows around the expected p and s arrivals and perform 17 iterations of vp and vs model updates altogether requiring about 16 million cpu hours before reaching stable convergence among other features our resulting model shows a nearly vertical high velocity body that overlaps with the distribution of seismicity in its northeastern part in its southwestern part a slab appears to dip less steeply to the nw and is suggestive of ongoing or recently concluded subduction geodynamic processes joint inversion for vp and vs allow us to address the vp vs ratio distribution that marks high vp vs in the crust beneath the focsani sedimentary basin possibly due to high fluid pressure and a low vp vs edge along the lower plane of the subducting lithosphere that in other similar environment has been attributed to dehydration of serpentine in the slab in spite of the restricted amount of data available and limitations on the usable frequency pass band full waveform inversion reveals its potential to improve the general quality of imaging with respect to other tomographic techniques although at a sensible cost in terms of computing resources our study also shows that re analysis of legacy data sets with up to date techniques may bring new useful information

tectonics and sedimentation during convergence of the alcapa and tisza dacia continental blocks the pienide nappe emplacement and its foredeep n romania the juxtaposition of the alcapa and tisza dacia continental blocks although one of the key issues in the evolution of the carpathians is not well known in terms of associated effects on the sedimentary systems during frontal foreland development most of the contact between alcapa and tisza dacia being covered by post tectonic deposits these effects can best be observed in northern romania sedimentological data on facies palaeocurrents and modal composition of sandstones combined with micropalaeontological data and 2d well calibrated seismic lines constrain the tectonic history of the contact zone between alcapa and tisza dacia pervasive deposition of sand dominated siliciclastics beginning in late early oligocene late rupelian times is interpreted to reflect the onset of convergence between alcapa and tisza dacia in the study area the depocentre of coarse siliciclastic material migrates southward finally forming a southeastward thinning clastic wedge in the transylvanian basin this burdigalian age clastic wedge is interpreted as fill of a flexural foreland basin that formed in response to the coeval thrusting of parts of alcapa pienides over tisza dacia a shift from an e w to se nw striking basin axis during oligocene times towards a wsw ene oriented basin axis during burdigalian times is interpreted as a result of clockwise rotation of tisza dacia during basin formation the miocene geological history of the carpathians is characterized by the emplacement of continental blocks alcapa and tisza dacia into the so called carpathian embayment a large scale bight in the european margin between the moesian and bohemian promontories fig 1 emplacement of these continental blocks and their subsequent extensional collapse was coeval to thin skinned nappe stacking at the exterior of the thrusted chain e g sa undulescu 1988 horvath 1993 lithospheric slab retreat of the distal parts of the carpathians foreland royden 1993 is con sidered to represent the principal driving force for the invasion of the two continental blocks fig 1 they were separated by a broad zone of defor mation the mid hungarian fault zone e g fodor et al 2005 along this fault zone substantial strike slip movements and block rotations are documented for palaeogene to early miocene

subsidence analysis and tectonic evolution of the external carpathian moesian platform region during neogene times abstract the miocene pliocene subsidence of the tectonic platforms in the romanian carpathians foreland is analysed using standard 1d backstripping techniques for individual wells combined in two regional sections and six contour maps the subsidence patterns were integrated together with previous paleostress and kinematic studies in order to derive the tertiary kinematics of the buried faults in the carpathians lower plate the study revealed accelerated subsidence during the early miocene in the western part of the moesian platform getic depression in direct relationship with the opening of a wsw ene trending extensional basin the largest subsidence recorded in the front of the carpathians took place during the late miocene due to final e ward emplacement of the thrust sheets the late miocene subsidence showed anomalous high values between the intramoesian and trotus faults as a result of the orogenic collision with the east european platform northward and acceleration of the subduction process in the se carpathians corner further pliocene subsidence continued only in the latter region the depocenter being shifted southward near the junction with the south carpathians foreland

architecture of the focsani depression a 13 km deep basin in the carpathians bend zone romania 1 in front of the se carpathians bend a very deep basin focsani depression developed in miocene to recent times an important part of its subsidence occurred after the main stages of thrusting in the carpathians apparently the basin lies in the wrong place and evolved in the wrong time in this study we constrain its architecture and evolution by analyzing a large database consisting of more than 1000 km two dimensional seismic lines and more than 60 wells around 13 km thick badenian to quaternary 16 5 myr sediments were deposited in the central part of the focsani depression during the badenian 16 5 13 myr the foreland south of trotus fault underwent ne sw directed extension and nw trending basins opened in the eastern moesian platform a nw se oriented area of subsidence stretched from the transylvania basin through the focsani depression to the se of the moesian platform while thrusting was going on in the east european scythian platform east carpathians and getic depression starting with the sarmatian 13 10 myr the focsani depression depocenter moved out of the carpathian belt coeval with the exhumation of the south and the east carpathians north of the trotus fault the basin became wider and was tilted toward the belt tilting was accompanied by dextral shearing mainly along the intramoesian and peceneaga camena faults after sarmatian times subsidence occurred practically only sse of trotus fault during meotian pontian 10 5 myr subsidence slowed down stronger pliocene quaternary subsidence is coeval with normal faulting and shearing in moesian platform the western margin of the focsani depression was then tilted eastward coeval with the exhumation of the bend zone and opening of the intramontane basins in the inner part of the belt

stress induced late stage subsidence anomalies in the pannonian basin abstract subsidence sedimentation and tectonic quiescence of the pannonian basin was interrupted a few million years ago by tectonic reactivation this recent activity has manifested itself in uplift of the western and eastern flanks and continuing subsidence of the central part of the pannonian basin low to medium magnitude earthquakes of the carpathian pannonian region are generated mostly in the upper crust by reverse and wrench fault mechanisms there is no evidence for earthquakes of extensional origin 2 d model calculation of the subsidence history shows that a recent increase in magnitude of horizontal compressional intraplate stress can explain fairly well the observed quaternary uplift and subsidence pattern we propose that this stress increase is caused by the overall europe africa convergence in late pliocene consumption of subductible lithosphere at the eastern margin of the pannonian basin was completed and the lithosphere underlying the pannonian basin became locked in a stable continental frame consequently extensional basin formation has come to an end and compressional inversion of the pannonian basin is in progress

slab in the wrong place lower lithospheric mantle delamination in the last stage of the eastern carpathian subduction retreat a model with a new type of delamination of the lower lithospheric mantle is proposed to explain the pliocene to recent tectonic evolution of the eastern carpathians we suggest that after the continental collision in middle miocene time break off of the west dipping subducting slab occurred at a depth of 70 km slab break off propagated horizontally toward the east inducing lithospheric delamination and movement of the vrancea slab into its present position delamination was followed by rapid asthenospheric rise resulting in magma generation and the explosive alkaline basaltic magmatism of the persani mountains contamination of the former subduction related magmatic reservoirs with the alkalic basaltic material and differentiation processes produced the harghita calc alkaline and shoshonitic rocks the asthenospheric rise induced crustal uplift which is the triggering mechanism for extension and formation of the brasov gheorghieni hinterland basins the extension was accommodated by shortening and folding in the foreland the vertical vrancea slab appears in our model as a segment of delaminated lower lithospheric mantle that is seismically active due to the ongoing pull of the eclogitized oceanic crust the model explains the displaced and shallow position of the slab relative to the eastern carpathian suture zone

crustal structure and evolution of the trans hudson orogen results from seismic reflection profiling a 400 km long deep seismic reflection transect across northeastern montana and northern north dakota reveals the crustal scale structural fabric of the early proterozoic trans hudson orogen beneath the williston basin comparison with deep seismic reflection data across the canadian portion of the same orogen 700 km to the north reveals first order similarities in crustal architecture but documents significant along strike variation in orogenic evolution both transects display a broad crustal scale antiform axial to the orogen in the north geologic data suggest that this antiform is cored by an archean microcontinent in the south west dipping reflections on the western flank of the antiform extend from the upper crust to the uppermost mantle and truncate prominent subhorizontal lower crustal reflections of the archean wyoming craton within the wyoming craton the eastern limit of east dipping midcrustal reflections coincides with the subsurface age boundary between the craton and the early proterozoic trans hudson orogen as interpreted from potential field and drill core data on the basis of subsurface geochronologic data from the crystalline basement and by analogy with the glennie domain within the exposed trans hudson orogen in canada we suggest that the southern antiform is cored by an archean crustal fragment that was caught up in the terminal collision of the wyoming and superior cratons during hudsonian orogeny the eastern side of the trans hudson orogen is characterized on both seismic transects by predominantly east dipping crustal penetrating reflections we interpret the easterly dip of these reflections as evidence that the superior province was thrust westward over the interludes of the orogen during terminal collision although juvenile early proterozoic terranes characterize the exposed segment of the trans hudson orogen in canada limited drill core information within the dakota segment of the orogen shows a predominance of granulitic archean age crust this difference in basement lithologies along strike within the orogen may indicate the following either juvenile crust comparable to that exposed in the northern trans hudson was never present in the south or it was removed by progressive over thickening erosion and or faulting postorogenic extensional collapse may be responsible for preservation of juvenile terranes in the north

deep seismic reflectivity beneath an intracratonic basin insights into the behavior of the uppermost mantle beneath the illinois basin abstract reflectivity images of the lower crust and uppermost mantle beneath the paleozoic illinois basin have been derived from reprocessing of several hundred kilometers of industry seismic reflection data using extended vibroseis recorrelation the recorrelation was based on extending an originally 4 s correlated record acquired with a 16 s sweep from 14 to 126 hz to the absolute limit of the full 20 s 70 km listening traveltime sub moho reflections are recorded to 18 s two way traveltime unmigrated and are observed on intersecting profiles apparently dipping to the southwest and striking northwest southeast occasional moho reflections are also observed across the profiles 12 s or 38 km while reflectivity in the lower crust is generally marked by intermittent horizontal packages and short gently dipping reflections and diffraction segments the presence of newly observed mantle reflectivity beneath the illinois basin indicates significant uppermost mantle heterogeneity relative to other parts of the usa studied using reflection methods the mantle reflectivity effectively defines an area of anomalous mantle that rests beneath the depocenter of the early paleozoic illinois basin which is itself superimposed over a deeper proterozoic basinal depocenter filtering and inversion of geopotential field data reveal that this anomalous mantle also partially correlates with a positive density anomaly in the upper mantle these correlations suggest that the anomalous mantle zone exerted an influence on the early paleozoic subsidence of the basin perhaps as a buried load due to eclogitization of former lower crust the relatively isolated occurrence of sub moho reflections beneath the basin makes it difficult to uniquely infer their origin however available geologic and geophysical constraints especially from geochemical and geochronological studies of drilled and exposed basement rocks limit the possibilities to 1 remnants or scars of sub crustal processes associated with lithospheric extension or delamination related to the melting of the proterozoic crust that led to the emplacement of the granite rhyolite province that underlies much of usa midcontinent or 2 deformation caused by plate subduction associated with the hypothetical accretion of a juvenile island arc to the pre 1 6 ga southern margin of the laurentian continent

near vertical seismic reflection image using a novel acquisition technique across the vrancea zone and foscani basin south eastern carpathians romania the dacia plan danube and carpathian integrated action on process in the lithosphere and neotectonics deep seismic sounding survey was performed in august september 2001 in south eastern romania at the same time as the regional deep refraction seismic survey vrancea 2001 the main goal of the experiment was to obtain new information on the deep structure of the external carpathians nappes and the architecture of tertiary quaternary basins developed within and adjacent to the seismically active vrancea zone including the focsani basin the seismic reflection line had a wnw ese orientation running from internal east carpathians units across the mountainous south eastern carpathians and the foreland focsani basin towards the danube delta there were 131 shot points along the profile with about 1 km spacing and data were recorded with stand alone reftek 125s also known as btexansq supplied by the university texas at el paso and the passcal institute the entire line was recorded in three deployments using about 340 receivers in the first deployment and 640 receivers in each of the other two deployments the resulting deep seismic reflection stacks processed to 20 s along the entire profile and to 10 s in the eastern focsani basin are presented here the regional architecture of the latter interpreted in the context of abundant independent constraint from exploration seismic and subsurface data is well imaged image quality within and beneath the thrust belt is of much poorer quality nevertheless there is good evidence to suggest that a thick 10 km sedimentary basin having the structure of a graben and of indeterminate age underlies the westernmost part of the focsani basin in the depth range 10 25 km most of the crustal depth seismicity observed in the vrancea zone as opposed to the more intense upper mantle seismicity appears to be associated with this sedimentary basin the sedimentary successions within this basin and other horizons visible further to the west beneath the carpathian nappes suggest that the geometry of the neogene and recent uplift observed in the vrancea zone likely coupled with contemporaneous rapid subsidence in the foreland is detached from deeper levels of the crust at about 10 km depth the moho lies at a depth of about 40 km along the profile its poor expression in the reflection stack being strengthened by independent estimates from the refraction data given the apparent thickness of the meta sedimentary supracrustal units the crystalline crust beneath this area is quite thin b20 km supporting the hypothesis that

deep and near surface consequences of root removal by asymmetric continental delamination abstract the occurrence of continental delamination has been proposed for a number of areas characterized by highly variable geodynamic settings in this study we present results of numerical simulations considering different initial setups representative for geodynamic scenarios where delamination could potentially develop to mimic a post collisional orogenic scenario we have designed an initial state characterized by the presence of an area of orogenic lithosphere with both crustal and lithospheric roots in a second setup we have considered a lithospheric root representative of a remnant slab with a flat overlying crust we focus on predicted evolution of surface and near surface observables namely the crustal structure surface heat flow and isostatic and dynamic topography evolution our results show that a high density orogenic lower crust likely related to the presence of eclogite significantly accelerates the sinking of the lithospheric mantle the pattern of local isostatic elevation is characterized by laterally migrating surface uplift subsidence this pattern is shown to be little sensitive to lower crust density variations in contrast predicted dynamic topography is more sensitive to these changes and shows surface subsidence adjacent to the delaminating lithospheric mantle for the model with a high density lower crust and surface uplift above the slab for a model with a less dense lower crust the reason for uplift in this second model is that the effect of the positive buoyancy of the thickened crust overwhelms the effect of negative buoyancy of the slowly sinking lithospheric mantle we infer from our modeling that there is not a specific characteristic pattern of topography changes associated with delamination but it depends on the interplay between highly variable factors as slab sinking velocity asthenospheric upwelling and changes in crustal thickness

partial detachment of a lithospheric root under the southeast carpathians toward a better definition of the detachment concept the southeast corner of the carpathian mountains romania provides a unique opportunity to study the very final and shortlived stage of plate convergence when a piece of subducted lithosphere finally detaches from the overriding plate and begins to sink into the mantle research efforts commonly focus on trying to image the cold lithospheric slab tomographic techniques and on trying to understand the tearing process from the earthquakes it generates here this issue is tackled from a different perspective emphasizing that the southeast carpathians are extremely low considering crustal thickness the crust there is almost as thick as the crust of the alps 50 55 km but the elevation is less than onehalf of the elevation of the alps it is interesting that the apennines italy are just the opposite they are rebounding after slab breakoff to reach elevations higher than expected from their crustal thickness 30 35 km like the crust of most passive margin coasts it is thus proposed that although seismologic studies indicate that the cold mantle body under the southeast carpathians is already partly detached from the overriding lithosphere it is still viscously coupled to the lithosphere and pulls it downward by utilizing residual topography analysis observed topography minus the contribution of the crust to it the subcrustal load is quantified and interpreted in terms of mantle lithosphere thickness extending to a depth of 300 km a particularly interesting finding is that the seismically active region is shifted 50 km east of the place of maximum pull down indicating that lithospheric tearing is now occurring at one side of the detaching root and not above it this interpretation is consistent with the idea of lateral delamination of the lower lithosphere

delamination in collisional orogens during continental collision the weight of the downgoing slab may lead to extensional shearing in the descending plate permitting separation of the downdip dense oceanic lithosphere from the adjacent lighter continental lithospher separation may occur by means of a lithosphere penetrating normal sense shear zone by means of pure shear necking of the lithosphere or by some combination of these mechanisms during separation the continental crust attached to the lower plate may undergo an episode of extensional tectonics the proximity of asthenosphere to crust may result in syntectonic regional metamorphism and plutonism if the continents continue to converge following delamination a compressive tectonic regime will be reestablished in the crust the subsequent cooling of asthenosphere at the base of the crust may lead to the establishment of a continuous flat moho the likely imbrication of crustal and mantle lithologies during delamination and during the subsequent compressive tectonic regime as well as underplating associated with syntectonic plutonism may account for the seismic complexities observed at moho depths beneath some mountain belts

large scale deformation in a locked collisional boundary interplay between subsidence and uplift intraplate stress and inherited lithospheric structure in the late stage of the se carpathians evolution 1 the interplay between slab dynamics and intraplate stresses in postcollisional times creates large near surface deformation particularly in highly bent orogens with significant lateral variations in mechanical properties this deformation is expressed through abnormal foredeep geometries and contrasting patterns of vertical movements intraplate folding is often the controlling mechanism particularly when the orogenic belt is locked the study of these tectonic processes in the se carpathians indicates a generalized subsidence period during latest miocene pliocene times driven by the slab pull and an intraplate folding due to an overall quaternary inversion the latter accommodates 5 km ese ward movement of this area with respect to the neighboring units which creates complicated three dimensional deformation patterns potentially driven at a larger scale by the interaction between the adriatic indentor and the entire carpathians system the lithospheric anisotropy inherited from the subduction times concentrates strain and induces large scale deformation far away from the active plate margins this anisotropy is dynamic because of deep mantle processes related to the subducted slab during postcollisional times such as thermal reequilibration or increase in slab dip

lithospheric gravitational instability beneath the southeast carpathians abstract the southeast corner of the carpathians known as the vrancea region is characterised by a cluster of strong seismicity to depths of about 200 km the peculiar features of this seismicity make it a region of high geophysical interest in this study we calculate the seismic strain rate tensors for the period 1967 2007 and describe the variation of strain rate with depth the observed results are compared with strain rates predicted by numerical experiments we explore a new dynamical model for this region based on the idea of viscous flow of the lithospheric mantle permitting the development of local continental mantle downwelling beneath vrancea due to a rayleigh taylor instability that has developed since the cessation of subduction at 11 ma the model simulations use a lagrangean frame 3d finite element algorithm solving the equations of conservation of mass and momentum for a spatially varying viscous creeping flow the finite deformation calculations of the gravitational instability of the continental lithosphere demonstrate that the rayleigh taylor mechanism can explain the present distribution of deformation within the downwelling lithosphere both in terms of stress localisation and amplitude of strain rates the spatial extent of the high stress zone that corresponds to the seismically active zone is realistically represented when we assume that viscosity decreases by at least an order of magnitude across the lithosphere the mantle downwelling is balanced by lithospheric thinning in an adjacent area which would correspond to the transylvanian basin crustal thickening is predicted above the downwelling structure and thinning beneath the basin

a non plume model for the permian protracted 266 286 ma basaltic magmatism in the beishan tianshan region xinjiang western china abstract the convenient mantle plume model for the permian protracted mafic ultramafic intrusions and mafic dykes 266 286 ma in the beishan tianshan region northern xinjiang western china can be rejected because their temporal spatial distribution does not show a hotspot track predicted by such model new zircon u pb ages reveal that two small mafic dyke clusters podong 280 5 2 ma luodong 266 2 3 2 ma that are separated by only 20 km in the pobei area the southernmost part of the beishan tianshan region have a large age difference of 18 ma the older mafic dykes are characterized by nearly flat mantle normalized rare earth element patterns pronounced negative nb ta anomalies and positive e nd t values from 5 5 to 7 5 similar to the majority of the permian mafic ultramafic intrusions in the region the younger mafic dykes are characterized by significant light rare earth element enrichments as well as pronounced negative nb ta anomalies plus lower e nd t 1 2 to 2 6 values and higher initial 87 sr 86 sr ratios than the older mafic dykes the observed compositional variations can be explained by source mantle heterogeneity plus different degrees of crustal contamination overall the permian mafic ultramafic rocks in the beishan tianshan region are geochemically consistent with the products of basaltic magmatism induced by lithospheric delamination and asthenosphere upwelling in a convergent tectonic zone

constraints from moho geometry and crustal thickness on the geodynamic origin of the vrancea seismogenic zone romania abstract reprocessing of industry deep seismic reflection data ramnicu sarat and braila profiles from the se carpathian foreland of romania provides important new constraints on geodynamic models for the origin of the intermediate depth vrancea seismogenic zone vsz mantle 70 200 km earthquakes of the vsz are characterized by high magnitudes greater than 6 5 frequent occurrence rates approximately 25 years and confinement in a very narrow 30 70 200 km 3 near vertical zone atypical for a wadati benioff plane located in front of the orogen these two deep 20 s seismic reflection profiles 70 km length across the foreland reveal 1 a high amplitude gently east dipping reflection across most of the section from what we interpret to be the moho at 15 s 40 42 km on the ramnicu sarat line to 16 s 47 48 km on the braila line 2 a thick sedimentary cover increasing in thickness from east 1 s 800 m to west 7 5 s 14 km 3 an eastward increase in crustal thickness from 38 km near vsz to 45 km 4 seismic and topographic evidence for a newly imaged possibly seismically active basement fault with a surface offset of 30 m observed on the ramnicu sarat line 5 a lack of notable west dipping structures in the crust and across the moho and 6 variable displacements on peceneaga camena fault of 5 km at moho and 200 m at the basement sedimentary cover contact these observations appear to argue against recent models for west dipping subduction of oceanic lithosphere at or in the vicinity of the vrancea seismogenic zone given the lack of west dipping fabrics in the lower crust and across the crust mantle boundary consequently one possible explanation for the geodynamic origin of vsz could be partial delamination of the continental lithosphere in an intra plate setting along a sub horizontal lithospheric interface in the carpathian hinterland that likely involves remnant lithospheric coupling between the crust and uppermost mantle in the foreland

geodynamics and intermediate depth seismicity in vrancea the south eastern carpathians current state of the art abstract the vrancea region of the south eastern carpathians is a remarkable site of intra continental intermediate depth seismicity a large set of geological geophysical and geodetic observations has been accumulated for the last few decades and utilised to improve our knowledge of the shallow and deep structures beneath vrancea the crustal and mantle dynamics and the linkage between deep and surface processes in the region in this article we review geology and tectonics of the vrancea region including post collisional to recent deformations syn to post collisional magmatism and orogenic exhumation along the east and south carpathians the regional seismicity is analysed and the recent seismic studies including reflection refraction body and surface wave tomography are reviewed we discuss new geodetic measurements of horizontal and vertical movements in the region geoelectric studies density gravity and thermal modelling qualitative and quantitative including retrospective geodynamic models developed for vrancea are analysed the knowledge of regional tectonics geodynamics seismicity lithospheric deformation and stress regime in the vrancea earthquake prone region assists in an assessment of strong ground motion seismic hazard and risk the earthquake simulation seismic hazard and earthquake forecasting models have also been reviewed providing a link between deep geodynamic processes and their manifestation on the surface finally we discuss unresolved problems in vrancea in order to improve our understanding of the regional evolution present tectonics mantle dynamics intermediate depth seismicity and surface manifestations of the lithosphere dynamics and to enhance our ability to forecast strong earthquakes in the vrancea region the problems to be solved include i the origin of the high velocity body revealed by seismic tomography studies oceanic versus continental ii the lithospheric scale mechanism driving the miocene subsidence of the transylvania basin iii sub crustal structure between 40 and 70 km iv contemporary regional horizontal and vertical movements and v a comprehensive seismic hazard assessment in the region

kinematics of retreating subduction along the carpathian arc romania kinematic axes and resultant vectors of displacement along the carpathian arc and the apuseni mountains aid in the reconstruction of retreating subduction along the carpathians the recent vertical position of the subducted slab below the vrancea area of the eastern carpathians represents the final rollback stage of a small fragment of oceanic lithosphere formerly situated between the moesian and east european plates the ages and locations of the eruption centers of the andesitic volcanic chain along the carpathian arc in the overriding plate as well as the thrust directions are used as markers to reconstruct the rollback area of the subducted slab between the moesian and east european plates

eclogite formation and dynamics of crustal roots under continental collision zones field observations on deep parts of the root zone of the caledonian orogen are combined with the geophysical data available for recent continental collision zones such as the alps to provide a model for dynamics of crustal root zones rocks descending into the root zone are locally infiltrated by fluids which cause eclogitization along shear zones and fluid pathways the eclogite facies rocks vary in density from 3 1 to 3 6 g cm 3 reflecting the compositional variation in their protoliths where the amount of highly ductile eclogitic material reaches about 40 the descending crustal segment loses its coherence and disintegrates into a breccia where blocks of granulite facies rocks float in a matrix of eclogite material a density stratified root zone develops where light unreacted or partly eclogitized material floats on an ocean dominated by eclogite facies rocks the top of which may correspond to the moho catastrophic decent of this eclogitic layer may 1 transport blocks of light material to depths of 90 km which upon further disintegration due to eclogitization are released and return by buoyancy forces to the crust 2 result in extension and uplift of the partly eclogitized lighter material

seismological evidence for a delamination process in the lithosphere under the alboran sea summary the continental collision between the african and eurasian plates in the iberian peninsula is solved with a nw se compression with an extensional basin the alboran sea to explain this paradox several models have been proposed including the subduction process of a lithospheric plate in this study a relocation of the seismicity of the area using a jhd algorithm for the period 1950 1995 has been performed which shows a specific hypocentre pattern that is not compatible with plate subduction in addition the direction of the stresses acting in the area has been determined through a comprehensive study of all existing focal mechanisms together with calculations for 19 new earthquakes with sufficient data to determine the focal solution after considering all these data the absence of intermediate shocks in the central part of the alboran basin and the predominantly thrusting mechanisms at the borders of the basin with pressure axes perpendicular to the front we consider a delamination process as a possible mechanism acting in the area

unveiling seismic and density structure beneath the vrancea seismogenic zone romania an edited version of this paper was published by agu copyright 2009 american geophysical union

tomographic imaging of deep velocity structure beneath the eastern and southern carpathians romania implications for continental collision the vrancea seismic zone located beneath the elbow of the eastern and southern carpathians is characterized by intermediate depth seismicity associated with subduction and plate collision among many strong earthquakes that have occurred in the region the 1977 vrancea earthquake march 4 1977 ms 7 1 depth of 86 km caused extensive damage and fatalities the vrancea seismic zone is defined by a tabular region of seismicity that extends from the surface to a depth of 200 km the small size about 80 km in length and 40 km in width and geometry of the seismic zone have made it difficult to interpret the kinematics of subduction and continental collision in the romanian carpathians to better understand the collision subduction environment body wave seismic tomography was used to image the deep structure beneath the eastern and southern carpathians in romania a total of 7837 p and 2292 s arrival times from 433 shallow and intermediate depth earthquakes recorded by local and regional seismic stations were inverted for the three dimensional 3 d seismic velocity structure the travel times and ray paths were calculated by an efficient 3 d ray tracing technique by utilizing the lsqr algorithm p and s wave velocity perturbations were determined from the surface down to a depth of 200 km the high resolution p wave tomographic images reveal a pattern of broad heterogeneity in the velocity structure of the region one of the most important features is a high velocity body existing beneath the carpathian arc at depths between 100 and 170 km this high velocity body outlines a slab dipping near vertically where most of the intermediate depth earthquakes occur beneath the transylvanian basin a high velocity zone is dipping toward the se and may represent the base of lithosphere which is in convergence with the subducted slab low velocity bodies are found along the forearc of the carpathian foredeep down to depths of 70 to 80 km these low velocity bodies may be correlated with the shallow sedimentary layer in the foreland and may represent crustal materials wedged and underthrust along with the subducted lithosphere