emergence of modern continental crust about 3 billion years ago the continental crust provides a record of earth s evolution analysis of the geochemical signature of continental crust formed since the hadean points to the initiation of plate tectonics about 3 billion years ago

the volcano seismic crisis in afar ethiopia starting september 2005 we describe the seismo volcanic crisis that occurred in northern afar in late 2005 which involved 15 earthquakes greater than m5 and a small explosive silicic eruption from a vent called da ure at 12 651 n 40 519 n close to dabbahu volcano the purpose is to pull together the different sources of information into a coherent preliminary interpretation of what happened the main geophysical data are the locations of the largest earthquakes and a radar interferogram of unusually high quality that reveals injection of a 60 km long dyke with surface deformation expressed as normal faulting subsidence occurred around the dabbahu volcanic edifice most of the dyke is likely to have been basaltic rather than silicic although the eruption was silicic the volume of the subsidence represents at most 25 of the magma injected into the dyke the silicic eruption was possibly triggered by interaction of basaltic magma with a shallow silicic reservoir at about the same time as the eruption and dyking episode some activity appears also to have taken place at the lava lake at erta ale volcano some 150 km to the north of the eruption site we evaluate the possibility that there may be some link by calculating stresses associated with opening of the fissure and looking at the activity of the lava lake as revealed by the thermal anomaly seen by weather satellites

clay mineral evolution changes in the mechanisms of formation and global distribution of phyllosilicate clay minerals through 4 567 ga of planetary evolution in our solar system reflect evolving tectonic geochemical and biological processes clay minerals were absent prior to planetesimal formation 4 6 billion years ago but today are abundant in all near surface earth environments new clay mineral species and modes of clay mineral paragenesis occurred as a consequence of major events in earth s evolution notably the formation of a mafic crust and oceans the emergence of granite rooted continents the initiation of plate tectonics and subduction the great oxidation event and the rise of the terrestrial biosphere the changing character of clay minerals through time is thus an important part of earth s mineralogical history and exemplifies the principles of mineral evolution

continental growth and convergence related arc plutonism in the mesoarchaean evidence from the barberton granitoid greenstone terrain south africa u pb zircon ages from trondhjemitic gneisses of the badplaas domain located in the southwestern parts of the mesoarchaean barberton granitoid greenstone terrain of south africa document a hitherto largely unrecognized plutonic event the geochronological results indicate the addition of juvenile felsic crust over a period of 60 myr between ca 3290 and 3230ma and prior to the main collisional event in the granitoid greenstone terrain at 3230 ma the timing and duration of plutonism together with the structural and compositional heterogeneity of the badplaas domain suggest that the badplaas domain represents part of a convergence related magmatic arc on a regional scale the spatial and temporal relationships between plutonism metamorphism and deformation are interpreted to preserve an almost complete inventory of a mesoarchean arc trench system this includes 1 the 3290 3230ma convergence related magmatic arc 2 a largely coeval deposition of the back arc type volcano sedimentary 3260 3225ma fig tree group of the barberton greenstone belt 3 relics of the underplated crust preserved in high p low t rocks to the immediate east of the magmatic arc juxtaposed against 4 high t medium p rocks of the overlying plate along the major crustal structure of the inyoni shear zone these findings corroborate the notion that thermal and rheological conditions on the early earth supported at least locally crustal growth and the extraction of buoyant felsic crust along convergence related magmatic arcs

remnants of arc related mesoarchaean oceanic crust in the tartoq group of sw greenland the tartoq group located in sw greenland consists of supracrustal rocks of mainly tholeiitic basaltic composition including pillow lavas sills dykes and gabbros as well as ultramafic rocks metamorphic grade ranges from greenschist fades to granulite facies the tartoq group crops out as a series of blocks and slivers that are imbricated with originally intrusive mesoarchaean ttg orthogneisses the supracrustal rocks form part of a se vergent fold and thrust belt consistent with the imbrication of ttg gneisses and supracrustal rocks along a convergent margin la icp ms u pb zircon dating of an intrusive ttg sheet yields a minimum age of 2986 4 ma for the tartoq group this age is consistent with mc icp ms lu hf and sm nd isotopic whole rock data for mafic samples from different blocks of the tartoq group which yield errorchron ages of 3189 65 ma and 3068 220 ma respectively the mafic supracrustal rocks of the tartoq group have chondrite normalized ree patterns with la cn sm cn of 0 67 1 96 and rather flat primitive mantle normalized multi element patterns except for scattered lile contents and generally negative nb anomalies with nb nb of 0 26 1 31 th yb varies between 0 06 and 0 47 and nb yb between 0 45 and 4 4 indicative of an arc affinity when compared to rocks from modern settings the similar geochemistry of the different lithological units together with their coeval formation as evident from trace element geochemical trends supports a co magmatic origin for the rock assemblage and their formation as imbricated relics of oceanic crust accordingly we propose that the tartoq group represents remnants of mesoarchaean oceanic crust which formed in a suprasubduction zone geodynamic environment c 2011 international association for gondwana research published by elsevier b v all rights reserved less

tectonic implications of new late cretaceous paleomagnetic constraints from eastern liaoning peninsula ne china 1 a paleomagnetic study has been carried out in the east of the tan lu fault in liaoning province ne china to understand the timing of tan lu fault activity samples aging from early paleozoic to late mesozoic from 51 sites have been analyzed paleozoic and late permian early triassic rocks are remagnetized by the recent geomagnetic field however late cretaceous between 118 and 83 ma red tuffaceous sandstone passes a positive fold test shows no present earth field characteristic remanent magnetization carried by both magnetite and hematite presents a solo normal polarity and thus provides the only reliable paleomagnetic data in this study the paleomagnetic pole calculated from these rocks p 59 4 n p 205 5 e and a95 7 3 is statistically undistinguishable from all available cretaceous paleomagnetic data from eastern liaoning korean peninsula indicating that these areas may belong to a single tectonic unit here named the east liaoning korea elk block at least since the late cretaceous conversely a significant discrepancy between the elk block and chinese block i e north and south china blocks characterized by a differential rotation 22 5 10 2 with a negligible latitudinal displacement 0 8 6 1 is demonstrated this result indicates first that the left lateral displacement along the tan lu fault if any must have occurred before the late cretaceous and second that the korean block can not be considered as a rigid part of north china block sedimentological and structural evidence show that the meso cenozoic triangle shaped plain consisting of songliao xialiaohe sanjiang zeya and other smaller basins developing in northeast china and southeast russia south of the mongol okhotsk belt experienced a heterogeneous rifting from the late jurassic to tertiary the variable amount of extension larger in the northeast 300 km than in the southwest 80 km is probably related to the clockwise rotation of the elk block centered at the south of the bohai bay basin this result shows that a significant segment of the east eurasian margin more than 1000 km long experienced differential rotation in cenozoic times

zircon geochronology and hf isotopes of the dwalile supracrustal suite ancient gneiss complex swaziland insights into the diversity of palaeoarchaean source rocks depositional and metamorphic ages abstract we report the results of combined u pb and lu hf isotope analyses of detrital and metamorphic zircon grains from a variety of metavolcano sedimentary rocks of the dwalile supracrustal suite dss from the ca 3 66 to 3 20 ga ancient gneiss complex agc in swaziland the results indicate that the dss is made up by palaeo and mesoarchaean rocks which have been affected by a polymetamorphic overprint and became tectonically assembled during the neoarchaean the oldest dwalile rocks were formed during a phase of felsic to intermediate volcanism at ca 3 46 ga and are derived from the same magma source as the tsawela gneisses based on the combined u pb hf isotope data two groups of metasedimentary rocks can be distinguished an older dwalile group i that is similar in age to the hooggenoeg and kromberg formations 3 46 3 40 ga but with a different source area a younger dwalile group ii is coeval with the fig tree and possibly moodies groups the u pb hf isotope data and petrological observations demonstrate that the dss was affected by three stages of metamorphic overprint an event at 3 23 ga pervasive amphibolite facies metamorphism at 3 15 ga and low p granulite facies metamorphism at 2 99 ga metamorphic zircon in the amphibolite facies rocks resulted from new growth in aqueous fluids at 3 15 ga whereas in the granulite facies rocks new zircon was formed in the presence of melt the 3 23 ga event coincides with the last significant metamorphic overprint documented in the barberton greenstone belt bgb whereas as two younger events are restricted to the agc the data support the model suggesting amalgamation between the agc and bgb terranes at ca 3 23 ga in addition they require crustal thickening in the agc terrane at ca 3 15 ga causing barrovian type metamorphism in the dss and granulite facies metamorphism in the luboya kubuta terrane low p granulite facies metamorphism at ca 2 99 ga is most likely related to crustal extension during the onset of pongola basin rifting causing mantle upwelling mafic magmatism and heat transfer from the mantle into the thinned crust

fluid flow triggered migration of events in the 1989 dobi earthquake sequence of central afar during the august 1989 sequence of earthquakes beneath the dobi graben in central afar principal events including 10 shocks with ml 5 5 propagated 50 km north westwards in 50 h the sequence is analyzed in terms of triggering by propagation of a critical pore pressure wave in a compressible fluid through a fractured medium the macroscopic fluid diffusion tensor d in the crust beneath the graben is estimated its principal eigenvector strikes n115 e nearly parallel to the graben axis with a value it is about 10 times larger than the other two consistent with fissure anisotropy in the graben the order of magnitude of the permeability 10 8 m is compatible with characteristic fissure widths ranging from 1 mm to 3 cm estimating such parameters in other extensional regions might help to understand better the temporal evolution of seismic swarms

probabilistic seismic hazard assessment for eritrea to date little is known about seismic hazard in eritrea despite its location in a volcanically and tectonically active region and the gathering pace of major infrastructure projects in response we report the findings of a comprehensive probabilistic seismic hazard assessment for eritrea and adjacent areas seismic source and ground motion models are constructed separately we use an adaptive spatiotemporal smoothing method to map expected patterns of seismicity to construct a consistent earthquake catalog from different data sets we use orthogonal regression to convert and unify different magnitude scales a sensitivity analysis of the different input parameters helps constrain them and disaggregation of site specific hazard estimates yields insights into the relative contribution from seismic sources of different magnitudes and distances the results highlight seismic hazard in proximity to the red sea gulf of aden afar depression and along the boundaries of the danakil microplate we estimate a 10 chance over 50 years of observing pseudospectral accelerations psas at 0 2 s exceeding 0 16g in the port city of massawa population 32 000 and the town of bada population 4000 for the capital asmara population 520 000 we calculate a psa of 0 11g at 0 2 s compared with previous studies our results provide greater spatial resolution use more recent ground motion models and benefit from a smoothed seismicity method our aims are to stimulate further studies and contribute to the safe development of the region in light of its exposure to seismic hazards

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

rhenium variations in molybdenite mos2 evidence for progressive subsurface oxidation abstract temporal trends in earth s near surface mineralogy correlate with major events in earth s geochemical and tectonic history new and published analyses of 422 molybdenite mos2 specimens from 135 localities with known ages from 2 91 billion years ga to 6 3 million years ma reveal two statistically significant trends first systematic increases in average and maximum trace concentrations of re in molybdenite since 3 0 ga point to enhanced oxidative weathering by subsurface fluids in addition episodic molybdenum mineralization correlates with five intervals of supercontinent assembly from 2 7 ga kenorland to 300 ma pangaea

complex calc alkaline volcanism recorded in mesoarchaean supracrustal belts north of frederikshab isblink southern west greenland implications for subduction zone processes in the early earth we present new geochemical data for three mesoarchaean supracrustal belts ravns storo bjornesund and perserajoorsuaq situated north of frederikshab isblink in southern west greenland for which we propose the collective name the ikkattup nunaa supracrustal association they comprise mainly amphibolites of tholeiitic basalt composition and leucoamphibolites of calc alkaline andesite composition both lithological units are cut by aplite sheets of tonalite trondhjemite granodiorite ttg composition with u pb zircon ages of c 2900 ma lu hf and sm nd isochrons based on whole rock amphibolite and leucoamphibolite samples yield ages of 2990 41 ma and 3020 78 ma respectively which are within error of the age of the fiskenaesset complex situated 5 25 km to the north leucoamphibolites from the three supracrustal belts show apparent geochemical mixing trends between tholeiitic amphibolites and ttg gneisses as the end members by assimilation fractional crystallisation afc modelling we can show that one group of leucoamphibolites can indeed be explained by contamination of the parental melts by a ttg like end member and another group of high p2o5 la and nb leucoamphibolites can be explained by contamination involving a hypothetical low silica adakite end member however the leucoamphibolites are juvenile with epsilon nd 2970 ma from 2 1 to 3 5 and epsilon hf 2970 ma of 3 5 to 4 3 thus the mafic source of the felsic contaminant melts must have been derived from a depleted mantle source more or less at the same time 60 ma as the volcanism took place our preferred interpretation of the geochemical and isotope data is that the protoliths of the supracrustal rocks formed in an island arc setting where early tholeiitic volcanism gave way to calc alkaline volcanism in a maturing arc the apparent afc trends are thus explained by in situ partial melting of basaltic arc crust to form juvenile ttg and adakite melts that mixed with mafic magmas or contaminated their mantle source to produce the calc alkaline leucoamphibolite protolith this model has important implications for the general interpretation of other archaean supracrustal belts because afc and geochemical mixing trends towards a ttg end member are not uniquely diagnostic of crustal contamination but may rather reflect processes operating at lower crustal or source levels in arcs such as melting assimilation storage homogenisation mash or slab melt low silica adakite metasomatism of the mantle source respectively c 2012 elsevier b v all rights reserved less

tectonic implications of the 1989 afar earthquake sequence in august 1989 a sequence of m b 5 6 normal faulting earthquakes occurred in the area between the ghoubbet asal manda inakir and manda haroro abhe bad rifts in central afar relocation of the events suggest they occurred on major faults in the dobi graben located midway between the principal rift zones the earthquakes are evidence that extensional strain is still occurring in the area the bookshelf tectonic model for central and southern afar proposed by tapponnier et al 1990 is modified to account for ongoing extension in the area between the principal rifts as well as distributed shear

capturing magma intrusion and faulting processes during continental rupture seismicity of the dabbahu afar rift continental rupture models emphasize the role of faults in extensional strain accommodation extension by dyke intrusion is commonly overlooked a major rifting episode that began in 2005 september in the afar depression of ethiopia provides an opportunity to examine strain accommodation in a zone of incipient plate rupture earthquakes recorded on a temporary seismic array 2005 october to 2006 april direct observation of fault patterns and geodetic data document ongoing strain and continued dyke intrusion along the 60 km long dabbahu rift segment defined in earlier remote sensing studies epicentral locations lie along a 3 km wide 50 km long swath that curves into the se flank of dabbahu volcano a second strand continues to the north toward gab ho volcano considering the 8 m of opening in the september crisis we interpret the depth distribution of microseismicity as the dyke intrusion zone the dykes rise from 10 km to the near surface along the 60 km long length of the tectono magmatic segment focal mechanisms indicate slip along nnw striking normal faults perpendicular to the arabia nubia plate opening vector the seismicity insar continuous gps and structural patterns all suggest that magma injection from lower or subcrustal magma reservoirs continued at least 3 months after the main episode persistent earthquake swarms at two sites on dabbahu volcano coincide with areas of deformation identified in the insar data 1 an elliptical northwestward dipping zone of seismicity and subsidence interpreted as a magma conduit and 2 a more diffuse 8 km radius zone of shallow seismicity 2 km above a shadow zone interpreted as a magma chamber between 2 5 and 6 km subsurface insar and continuous gps data show uplift above a shallow source in zone 2 and uplift above the largely aseismic gab ho volcano the patterns of seismicity provide a 3 d perspective of magma feeding systems maintaining the along axis segmentation of this incipient seafloor spreading segment

seismotectonics in the republic of djibouti linking the afar depression and the gulf of aden abstract the arta seismological array around the gulf of tadjurah monitored earthquake activity for over 15 years epicentral locations and focal mechanisms are compared with tectonic evidence from surface breaks traces of active faults and the spatial distribution of swarms of aftershocks strike slip motion on near vertical faults predominates in the gulf left lateral strike slip on n150 e oriented faults is suggested at variance with the emphasis in previous models on either 1 a continuous plate boundary with spreading centres connected by ene trending right lateral transform faults 2 a propagating rift oriented approximately east west or 3 a few broad zones of extension between n150 e normal faults left lateral strike slip motion occurring on a series of parallel faults results in clockwise rotation of the blocks between the faults the type of deformation has similarities to that further west but is not obviously controlled by a distinct pair of overlapping rifts as proposed inland by tapponnier et al 1990 it is suggested that the stresses induced by the northeasterly drift of arabia alone may deform the southern part of the more easterly oriented zone of lithosphere derived of plume and ridge which separates arabia from africa

thickness and geothermal gradient of neoarchean continental crust inference from the southeastern north china craton abstract the thickness and geothermal gradient of archean continental crust are critical factors for understanding the geodynamic processes in earth s early continental crust archean tonalite trondhjemite granodiorite ttg gneisses provide one of the potential indicators of paleo crustal thickness and geothermal gradient because crust derived ttg melts are generally thought to originate from partial melting of mafic rocks at the crustal root in the western shandong province wsp of the north china craton ncc two episodes of neoarchean ttg magmatism are recognized at 2 70 ga and 2 55 ga which were sourced from partial melting of juvenile crustal components the 2 70 ga ttg gneisses show highly fractionated rare earth element ree patterns average la yb n 39 whereas the 2 55 ga ttg gneisses have relatively less fractionated ree patterns average la yb n 18 petrogenetic evaluation suggest that the magmatic precursors of the ttg gneisses of both episodes originated from partial melting of juvenile crustal materials at different crustal depths with residual mineral phases of grt cpx amp pl and ilm together with the garnet proportion in the residue the p t pseudosections of equilibrium mineral assemblages and the temperature calculated from titanium in zircon thermometer we estimate the neoarchean crustal thicknesses as 44 51 km with geothermal gradients of 17 to 20 c km for the 2 70 ga ttg gneisses whereas the 2 55 ga ttg gneisses show lesser crustal thicknesses of 35 43 km with geothermal gradients of 19 to 26 c km with an approximately 10 km difference in crustal thickness our estimates on the thicknesses and geothermal gradients of the neoarchean crust are similar to those 41 km 20 c km of the modern average continental crust indicating that a modern style plate tectonic regime may have played an important role in the formation and evolution of the neoarchean continental crust in the ncc

a microseismic study in the western part of the gulf of corinth greece implications for large scale normal faulting mechanisms summary we present the results of a dense seismological experiment in the western part of the gulf of corinth psathopyrgos aigion area one of the most active rifts in the aegean region for which we have precise tectonic information the network included 51 digital stations that operated during july and august 1991 covering a surface of 40 x 40 km2 among the 5000 recorded events with ml ranging between 1 0 and 3 0 we precisely located 774 events we obtained 148 well constrained focal mechanisms using p wave first motions of these 60 also have mechanisms obtained by combining the p wave first motions with the s wave polarization directions the observed seismicity is mainly located between 6 and 11 km depth most of the fault plane solutions correspond to e w striking normal faulting in agreement with the geological evidence most of the well determined mechanisms indicate a nodal plane dipping 10 25 due north and a steep south dipping plane a similar asymmetry is also seen in the seismicity distribution and in the overall geological structure of the corinth rift we discuss this evidence and the inference of a deep detachment zone a structure where the major faults seen at the surface appear to root a large part of the microseismic activity appears to cluster in regions near the junctions of the main faults with the proposed detachment zone this feature of the microseismicity is interpreted in terms of stress transfer and stress concentration in regions of probable nucleation of future large earthquakes

post rifting relaxation in the afar region ethiopia 1 crustal accretion at divergent plate boundaries typically occurs via the periodic intrusion of dikes but their emplacement and the associated deformation are rarely observed the few existing observations at subaerial rifts show that these diking events are followed by a decadal scale period with extension rates faster than the secular divergent plate motion this transient accelerated deformation has been explained by continued subsurface magma injection or by relaxation in the viscoelastic mantle of the stress changes imparted by dike opening for the first time gps measurements were collected within a few months of a rifting event at a major plate boundary the september 2005 60 km long dike intrusion in the dabbahu segment afar ethiopia extension rates for the first 3 years greatly exceed the plate motion nubia arabia secular divergence rate even at sites located more than 60 km from the rift axis here we show that these observations are consistent with stress relaxation in a viscoelastic upper mantle with a viscosity of about 5 1018 pa s overlain by a 12 14 km thick elastic crust the alternative model of continued diking requires continuous opening well below the moho and is therefore unlikely instead magma injection in afar since june 2006 has taken the form of smaller discrete diking events tapping into a mid crustal melt reservoir under the segment center

september 2005 manda hararo dabbahu rifting event afar ethiopia constraints provided by geodetic data faults the volume of the 2005 dike 1 5 2 0 km 3 is not balanced by sufficient volume loss at dabbahu and gabho volcanoes 0 42 and 0 12 km 3 respectively taking into account the deflation of a suspected deep midsegment magma chamber simultaneously to dike intrusion produces a smoother opening distribution along the southern segment above the dike faults slipped by an average 3 m yielding an estimated geodetic moment of 3 5 10 19 nm one order of magnitude larger than the cumulative seismic moment released during the earthquake swarm between dabbahu and ado ale volcanic complexes significant opening occurred on the western side of the dike the anomalous location of the dike at this latitude offset to the east of the axial depression may explain this phenomenon a two stage intrusion scenario is proposed whereby rifting in the northern manda hararo rift was triggered by magma upwelling in the dabbahu area at the northern extremity of the magmatic segment although vigorous dike injection occurred during the september 2005 event the tectonic stress deficit since the previous rifting episode was not fully released leading to further intrusions in 2006 2009

slip accumulation and lateral propagation of active normal faults in afar we investigate fault growth in afar where normal fault systems are known to be currently growing fast and most are propagating to the northwest using digital elevation models we have examined the cumulative slip distribution along 255 faults with lengths ranging from 0 3 to 60 km faults exhibiting the elliptical or bell shaped slip profiles predicted by simple linear elastic fracture mechanics or elastic plastic theories are rare most slip profiles are roughly linear for more than half of their length with overall slopes always 0 035 for the dominant population of nw striking faults and fault systems longer than 2 km the slip profiles are asymmetric with slip being maximum near the eastern ends of the profiles where it drops abruptly to zero whereas slip decreases roughly linearly and tapers in the direction of overall aden rift propagation at a more detailed level most faults appear to be composed of distinct shorter subfaults or segments whose slip profiles while different from one to the next combine to produce the roughly linear overall slip decrease along the entire fault on a larger scale faults cluster into kinematically coupled systems along which the slip on any scale individual fault or fault system complements that of its neighbors so that the total slip of the whole system is roughly linearly related to its length with an average slope again 0 035 we discuss the origin of these quasilinear asymmetric profiles in terms of initiation points where slip starts and barriers where fault propagation is arrested in the absence of a barrier slip apparently extends with a roughly linear profile tapered in the direction of fault propagation

bookshelf faulting and horizontal block rotations between overlapping rifts in southern afar lateral slip on initially rift parallel normal faults may be a particularly efficient mechanism to accomodate strain between overlapping oceanic rifts it occurs in southern afar where clockwise block rotations result from distributed dextral shear between the overlapping ghoubbet asal manda inakir and manda hararo abhe bad rifts faulting observed during the 1969 serdo earthquakes and on spot images is consistent with the shear being taken up by left lateral slip on steep nw se striking faults which formed as normal faults before extensional strain became localized in the two rifts this bookshelf faulting accounts quantitatively for the 14 5 7 5 rotation documented by paleomagnetism in the 1 8 0 4 ma old afar stratoid basalts given the 17 5 5 mm yr rate of separation between arabia and somalia

how feasible was subduction in the archean as the asthenospheric mantle rises at oceanic spreading centres it undergoes partial melting producing oceanic crust and depleted mantle both of which have lower intrinsic density than the asthenospheric mantle from which they were derived with a warmer asthenosphere in the archean these effects are enhanced leading to the possibility that subduction was no longer feasible i investigate the density of the oceanic crust and underlying mantle for a mantle with temperatures 200 c higher than today using models of the chemistry of melting and the mineralogy of the ensuing rocks for the melting model used crustal thicknesses are 21 km and the depth to which the mantle is partially melted is 114 km compared with 7 and 54 km for a comparable model of modern earth two thermal evolution models for archean oceanic lithosphere are examined one assumes twice the heat flow into the base of the plates which severely restricts the depths to which the plates can cool with age a second assumes the plates c

length and timescales of rift faulting and magma intrusion the afar rifting cycle from 2005 to present although fault and magmatic processes have achieved plate spreading at mid ocean ridges throughout earth s history discrete rifting episodes have rarely been observed this paper synthesizes ongoing seismic structural space based geodetic and petrologic studies from the subaerial red sea rift in ethiopia where a major rifting episode commenced in september 2005 our aims are to determine the length and timescales of magmatism and faulting the partitioning of strain between faulting and magmatism and their implications for the maintenance of along axis segmentation most of the magma for the initial and subsequent 12 intrusions was sourced from the center of the dabbahu manda hararo rift segment strain is accommodated primarily by axial dike intrusions fed from mid segment magma chamber s these findings show that episodic approximate century interval rapid opening of discrete rift segments is the primary mechanism of plate boundary deformation the scale 65 km 8 km and intensity of crustal

evidence for mesoarchean 3 2ga rifting of the pilbara craton the missing link in an early precambrian wilson cycle abstract a sequence of c 3 2 ga low grade sedimentary rocks bimodal volcanic rocks and subvolcanic layered mafic ultramafic sills and dykes occur in greenstone belts that flank the margins of the 3 51 3 24 ga east pilbara terrane of the pilbara craton western australia u pb shrimp zircon data and sm nd model age data suggest that these rocks represent juvenile addition of material to the margins of the craton during a period of major extension accompanied by the local emplacement of granitic rocks this extensional event is also represented in adjacent terranes by thick basalt flows with c 3 2 ga zircon and sm nd model ages and by granitic rocks based on the available age data geochemistry of basaltic rocks and the distribution of this event in space and time within the craton we interpret the c 3 2 ga rocks to reflect widespread rifting of the protocraton and associated thinning of the lithosphere this event resulted in separation of the flanking kurrana and karratha terranes from their parent east pilbara terrane which represents the ancient nucleus of the craton these terranes were then reassembled during subsequent accretion at 3 07 2 905 ga combined the data suggest the preservation of a mesoarchean wilson cycle in the pilbara craton

u pb hf isotope systematics of detrital zircons in high grade paragneisses of the ancient gneiss complex swaziland evidence for two periods of juvenile crust formation paleo and mesoarchaean sediment deposition and 3 23 ga terrane accretion abstract we present the first results of combined u pb and lu hf isotope analyses of detrital zircon grains in high grade paragneisses luboya kubuta granulites from the ca 3 66 3 22 ga ancient gneiss complex agc in swaziland uranium pb ages demonstrate that sedimentation along the se margin of the proto kaapvaal craton took place during two time intervals during the paleoarchaean ca 3 53 and 3 43 ga and mesoarchaean ca 3 22 and 3 16 ga the hf isotope data show that the hinterland to the paleoarchaean metasediments experienced juvenile crust formation from a depleted mantle source at ca 3 53 ga hf 3 53ga values between 0 7 and 4 1 and by reworking of more ancient crust t dm 3 8 3 6 ga in contrast detrital zircons in mesoarchaean metasediments record juvenile crust formation at 3 32 ga hf 3 32ga values between 1 5 and 6 0 t dm 3 45 3 32 ga followed by internal reworking of this crust until 3 22 ga a small number of detrital zircons indicate limited input from older paleoarchaean agc crust compared to sediments of the barberton greenstone belt i e fig tree and moodies group the mesoarchaean metasediments of swaziland are more juvenile in character and show greater complexity in their detrital zircon age spectra pointing towards a different provenance our new u pb hf isotope data together with field and petrological information from the study area suggest that the paleoarchaean metasediments were largely derived from the agc whereas the mesoarchaean metasediments were derived from a ca 3 45 3 32 ga juvenile island arc terrane located se of the agc which became accreted onto the kaapvaal craton at ca 3 23 ga i e the onset of burial and high grade metamorphism in the area

paleomineralogy of the hadean eon a preliminary species list the hadean eon encompassing earth9s first 550 million years was a time of significant planetary evolution nevertheless prebiotic earth9s near surface environment may have held no more than approximately 420 different rock forming or accessory mineral species that were widely distributed and or volumetrically significant this relative hadean mineralogical parsimony is a consequence of the limited modes of mineral paragenesis prior to 4 ga compared to the last 3 0 billion years dominant hadean eon mineralizing processes include the evolution of a diverse suite of intrusive and extrusive igneous lithologies hydrothermal alteration over a wide temperature range notably serpentinization authigenesis in marine sediments diagenesis and low grade metamorphism in near surface environments and impact related processes including shock mineralization creation of marginal hydrothermal zones and excavation of deep metamorphosed terrains on the other hand the hadean eon may have been notably lacking in mineralization generated by plate tectonic processes such as subduction zone volcanism and associated fluid rock interactions which result in massive sulfide deposition convergent boundary orogenesis and consequent extensive granitoid rooted continental landmasses and the selection and concentration of incompatible elements in complex pegmatites with hundreds of accompanying minerals the dramatic mineralogical consequences of life are reflected in the absence of hadean biomineralization for example the lack of extensive carbonate deposits and the associated restricted development of skarn and cave minerals prior to 4 ga most importantly it was not until after the establishment via photosynthesis of significant near surface redox gradients that supergene alteration redox controlled ore deposition and subaerial weathering in an oxidizing environment could diversify earth9s near surface mineralogy these post hadean processes may be responsible for over 4000 of the more than 4800 approved mineral species any scenario for life9s origins that invokes minerals as agents of molecular synthesis selection protection or organization must take into account the limited mineralogical repertoire of the time

global ionospheric response to the 2009 sudden stratospheric warming event using ionospheric data assimilation four dimensional ida4d algorithm a data assimilation algorithm is used to delineate the time dependent three dimensional ionospheric response to the 2009 sudden stratospheric warming ssw event we use the ionospheric data assimilation four dimensional ida4d algorithm to study the global ionospheric response to the 2009 ssw this is the first study to utilize global ionospheric measurements in a data assimilation framework to unambiguously characterize atmosphere ionosphere coupling via tidal modifications during the 2009 ssw event model results reveal that the dominant mode of ionospheric variability during the 2009 ssw is driven by the enhancements in westward propagating semidiurnal tide with zonal wave number 1 the ida4d results completely characterize the tidal perturbation during the 2009 ssw for the first time and show the global 3 d structure of the tide in total electron content tec and electron density the largest ionospheric responses were seen at low latitudes where ionospheric plasma is extremely sensitive to the zonal electric field and susceptible to modifications by tidal winds in the lower thermosphere the ionospheric response to the warming was characterized by an increase in tec in the morning early afternoon sector and a decrease during the late afternoon evening period the effects of coupling between the stratosphere and ionosphere were strongest between 220 km and 380 km the ida4d results also show a reversal of asymmetry in the equatorial ionization anomaly crests occurring several days after the peak of the 2009 ssw event we suggest that this could be a result of the equatorial fountain effect being further modified by the summer to winter meridional neutral winds

preserved and modified mid archean crustal blocks in dharwar craton seismological evidence abstract we report significant lateral variability in shear wave velocity and moho depth in the archean crust beneath the dharwar craton using earthquake waveform data recorded over 50 broadband seismographs the craton is a continuously exposed archean continental fragment divided into the west dharwar craton wdc of age 2 7 3 36 ga and the east dharwar craton edc of age dominantly 2 5 ga the craton progressively transitions into the southern granulite terrain sgt with age of metamorphism around 2 6 ga the inversion and modeling of receiver function data reveal significant variation of moho depth viz 38 54 km in the wdc and 40 46 km in the sgt and 32 38 km in the edc the average shear wave velocity vs of crust beneath the wdc is 3 85 km s as compared to 3 6 km s in the edc we infer highly variable thickness 16 30 km of mafic cumulate vs 4 0 km s and vp 7 0 km s beneath the wdc in contrast with a thin one 50 km as compared to felsic to intermediate composition for the edc crust with almost flat moho down to 38 km these results suggest preserved mafic crustal root beneath the middle archean terrain in the wdc that has remained inert since then on the other hand felsic to intermediate composition of crust with a nearly flat moho beneath the late archean edc could be a consequence of regional delamination of lower crust the preserved distinct moho topography across the archean terrains suggests it as compositional boundary considering the surface exposure of 15 20 km crust based on p t condition in the granulite segment of the wdc we speculate a himalaya like crustal thickness 50 70 km beneath the middle archean crust pointing toward a plate tectonic like scenario at 3 0 ga

high resolution tephra and u pb chronology of the 3 33 3 26 ga mendon formation barberton greenstone belt south africa abstract the mendon formation in the barberton greenstone belt of south africa marks the boundary between the onverwacht and fig tree groups these groups are characterized by mafic to ultramafic volcanism and felsic volcanism with related epiclastic sedimentation respectively this transition marks the end of komatiitic volcanism in the barberton greenstone belt and is accompanied by numerous impact related spherule layers this study characterizes the upper mendon formation texturally and geochemically over a wide areal extent and across structure and facies changes in an attempt to better understand the evolution of tectonic processes at this boundary a suite of whole rock and handheld x ray fluorescence analyses are presented in conjunction with textural information stratigraphic relationships and u pb ages to create a temporal and chemostratigraphic framework for the mendon formation local and regional stratigraphic variations including absence of distinctive layers and variation in layer thickness seen across the mendon preclude ascription of a single stratigraphy that accurately describes the 1 2 km of section present in this formation these variations indicate diachronous deposition of the mendon formation over a wide areal extent and into multiple basins or sub basins by more than one magmatic source 204 pb corrected 206 pb 238 u and 207 pb 235 u concordia model ages of 3279 9 1 ma and 3287 3 2 9 ma for two samples from upper portions of the mendon formation provide temporal context for deposition two samples from the basal 10 m of the fig tree group above the s2 spherule bed that marks the boundary between the onverwacht and fig tree groups give model ages of 3267 8 6 9 ma and 3261 18 ma these ages provide added constraints for the onverwacht fig tree boundary and confirm that the weltevreden formation is roughly age correlative with the uppermost mendon formation while the mendon and weltevreden formations are in part age correlative and have similar lithologies they do not appear to be genetically related the dominance of ultramafic volcanic rocks and the paucity of felsic volcanic and terrigenous sedimentary rocks within the mendon and weltevreden formations indicate that the primary mode of crustal formation was likely plume related magmatic accretion and not subduction the relatively sharp transition within the barberton greenstone belt from ultramafic volcanic sequences to more felsic volcanic and epiclastic sedimentary sequences is everywhere marked by impact related spherule layers which suggest that major impacts may have played a role in the evolution of early tectonics to more modern subduction related styles

relocation of m 2 events of the 1989 dobi seismic sequence in afar evidence for earthquake migration the ethiopian side of central afar was struck in august 1989 by the largest seismic sequence three 6 1 ms 6 3 events 15 with ms or mb 5 0 since that of serdo in 1969 using the djibouti seismological network we relocated 297 of the events of that sequence as most of the large events took place outside the network we assessed the accuracy and stability of earthquake relocations by using three different velocity models and two relocation codes to try to relate individual shocks to distinct faults and surface breaks a majority of the events apparently occurred underneath the floor of the dobi graben an area about 45 km long and 15 km wide rupturing boundary and inner floor faults in agreement with the surface cracks and scarps that we mapped in the area the relocation shows that the principal events propagated about 50 km northwestwards along the graben in the first 40 hr a day and a half after the beginning of the sequence smaller events m 4 started to propagate more than 55 km eastwards towards asal lake using two three component stations installed near the ethiopian border we could determine reliable depths for 21 events the depths are compatible with a seismogenic crust about 14 km thick in the dobi and hanle graben area although the dobi sequence ruptured about 50 km of the fault array extending from serdo to asal where the regional stress was released by earthquakes in 1969 and 1978 respectively a seismic gap about 50 km long still subsists along the northern part of the gaggade region der ela half graben

hafnium isotope evidence for a transition in the dynamics of continental growth 3 2 gyr ago earth s lithosphere probably experienced an evolution towards the modern plate tectonic regime owing to secular changes in mantle temperature radiogenic isotope variations are interpreted as evidence for the declining rates of continental crustal growth over time with some estimates suggesting that over 70 of the present continental crustal reservoir was extracted by the end of the archaean eon patterns of crustal growth and reworking in rocks younger than three billion years gyr are thought to reflect the assembly and break up of supercontinents by wilson cycle processes and mark an important change in lithosphere dynamics in southern west greenland numerous studies have however argued for subduction settings and crust growth by arc accretion back to 3 8 gyr ago suggesting that modern day tectonic regimes operated during the formation of the earliest crustal rock record here we report in situ uranium lead hafnium and oxygen isotope data from zircons of basement rocks in southern west greenland across the critical time period during which modern like tectonic regimes could have initiated our data show pronounced differences in the hafnium isotope time patterns across this interval requiring changes in the characteristics of the magmatic protolith the observations suggest that 3 9 3 5 gyr old rocks differentiated from a 3 9 gyr old source reservoir with a chondritic to slightly depleted hafnium isotope composition in contrast rocks formed after 3 2 gyr ago register the first additions of juvenile depleted material that is new mantle derived crust since 3 9 gyr ago and are characterized by striking shifts in hafnium isotope ratios similar to those shown by phanerozoic subduction related orogens these data suggest a transitional period 3 5 3 2 gyr ago from an ancient 3 9 3 5 gyr old crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3 2 gyr ago that involved juvenile crust generation by plate tectonic processes

normal faulting during the august 1989 earthquakes in central afar sequential triggering and propagation of rupture along the dobi graben in august 1989 an earthquake sequence including ten events with 6 3 m 5 5 in the first two days produced widespread ground deformation in the dobi graben of central afar numerous surface breaks with complex geometry including fresh scarplets with vertical throws up to 30 cm high and open fissures up to 30 cm wide were observed coseismic slip incremented the deformation normal faulting block tilting and counterclockwise rotation of basaltic slices accumulated in the last 2 m y in the transfer zone between the dobi and hanle grabens by combining maps of surface ruptures relative event relocations with the local djibouti network published focal mechanisms and source sizes we tentatively relate most of the main shocks of the sequence to slip on individual faults the largest shocks at 11h16 on 20 august 1989 ms 6 2 and at 1h09 on 21 august 1989 ms 6 3 ruptured southern segments of the southwestern bounding fault of the graben a dozen other faults also slipped along the edges of and inside the graben on average triggered seismic fault ing propagated about 35 km northwestward along the graben in about 20 hr slip on the main faults was coupled with slip on secondary antithetic faults branching from them at depth although the dobi earthquakes ruptured part of the fault array between the asal rift 1978 sequence and the serdo region 1969 sequence an approximately 50 km long gap subsists along the der ela half graben we infer the patterns of sur face faulting in the dobi sequence which coinvolved bookshelf faulting about both horizontal and vertical axes to typify the complexity of coseismic stress release in central afar and in other active zones of distributed extension e g iceland abruzzi basin and range online material additional photos and descriptions of surface effects of the dobi earthquake sequence

along rift propagation of pleistocene holocene faults from a central volcano abstract the mechanisms of rift propagation are still not fully understood especially at mid oceanic ridges owing to the inherent difficulty in collecting submarine data here we investigate the fault slip profiles of the 60 km long theistareykir rift northern iceland that may suggest the direction of along axis rift propagation this is one of the few places on earth where rifting processes and mid oceanic ridge formation can be studied directly moreover this rift hosts an active central volcano and this enables to fully understand the relations between rift propagation and magma systems we reconstructed the slip profiles of all the 281 main pleistocene holocene faults that compose this n s rift by merging measurements performed in the field collected by unmanned aerial vehicle surveys and derived from digital surface models results indicate that north of the volcano 75 of the asymmetric faults propagated northward the value increases to 82 if the cumulated fault length is considered south of the volcano 47 of the asymmetric faults propagated southward 54 if the cumulated fault length is considered these data point to a dominant mechanism of along axis propagation of the rift outward from the volcano suggesting a genetic link with the underlying magma chamber two possible processes are suggested which can also occur in combination i faults develop following lateral dyke propagation outward from the magma chamber and ii faults nucleate near the volcano as a consequence of the different crustal rock rheology produced by a higher heat flux the rift architecture is complicated by the presence of tectonic zones with different dominant fault dips separated by transversal accommodation zones the latter also play the role of barriers guiding local fault propagation

aftershock patterns of ms 7 earthquakes in the india asia collision belt anomalous results from the muzaffarabad earthquake sequence kashmir 2005 the seismicity rate triggered by the muzaffarabad kashmir 2005 m w 7 6 m s 7 7 earthquake is above the average when analyzing the aftershock rate of the 18 m s 7 0 events in the india asia collision belt between 1973 and 2008 all 17 m s 7 0 events occurred within 10 latitude and 20 longitude distance from the 2005 muzaffarabad earthquake epicenter we determined the huge aftershock productivity for this earthquake is not driven by a specific omori law parameter this anomaly in aftershock rate value corresponds to a relatively higher rate decay as measured by the p value and a larger density value in a higher background seismicity rate setting prestress conditions than the other 17 sequences respectively after normalizing by the mainshock size and by the magnitude range of observation we resolved the anomalies with a 95 confidence level for five aftershock sequences for either the duration the productivity the density or the background seismicity rate when we did not include the local processes that drive these specific anomalies two other global results emerged for the averaged data from the 18 m s 7 0 aftershock sequences first for all sequences background seismicity and duration are anticorrelated graphic 1 within the framework of rate and state friction law this result supports the assertion that the observed background seismicity rate can be used as a proxy for the stressing rate dieterich 1994 second we robustly resolve the dependence of aftershock productivity on the mainshock faulting styles for example the aftershock productivity for strike slip mainshocks is on average four times smaller than the productivity of thrust faulting mainshocks 1 embed inline graphic 1 gif

palaeomagnetic constraints from granodioritic plutons jiaodong peninsula new insights on late mesozoic continental extension in eastern asia mechanism and kinematics of the late mesozoic continental extension event of eastern asia are still debated in order to better constrain its evolution two granodioritic plutons of the jiaodong peninsula have been chosen as targets for a time constrained palaeomagnetic study indeed plutons are devoid of visible deformation did not experience rotation along horizontal axis and are precisely dated by u pb and 40ar 39ar methods multidomain md magnetite has been identified as the principal magnetic remanent carrier the interpolation of existing and new u pb and 40ar 39ar ages revealed that characteristic remanent magnetisation was acquired in a narrow range of 116 2 ma twenty out of 27 sites present stable magnetic directions calculated from high temperature or high coercive components the observations of the solo normal magnetic polarity for this palaeomagnetic collection and of the magnetic remanent age consistent with the cretaceous normal superchron cns argue that the characteristic magnetic remanence may be considered as primary detailed field observations of the intrusive relationship between the plutons and country rocks and anisotropy of magnetic susceptibility ams study reveal the absence of the subsequent deformation of plutons or rigid rotation of plutons along a horizontal axis two palaeomagnetic poles have been therefore calculated from these plutons among 12 out 15 cretaceous palaeomagnetic poles including the two new poles provided by this study from the jiaodong peninsula and on both sides of tan lu and muping jimo faults are statistically consistent as a result the jiaodong peninsula behaved as a rigid block as internal deformation appears negligible the remaining three derived poles are probably due to the secular variation or and the vicinity of fault zones near of the palaeomagnetic sampling site thus they can not be applied to the peninsula scaled tectonics comparison of these time52 constrained cretaceous palaeomagnetic results of the jiaodong peninsula with those of north china block ncb indicate that the jiaodong peninsula was rigidly attached to ncb since 3 at least the cretaceous as residual difference is clearly insignificant with respect to the error brackets these new palaeomagnetic results confirm that the relative clockwise cw rotation of eastern liaoning korea block elkb with respect to ncb does not affect the jiaodong peninsula as a rigid block nevertheless some tectonic structures have been reactivated since cenozoic and local rotations could be produced this palaeomagnetic study reveals that the late mesozoic extension in eastern asia is heterogeneously expressed in time and space the complete understanding of the mechanism s of this extensional event needs further multidisciplinary investigations

mesoarchean geodynamic regime evidenced from diverse granitoid rocks in the anshan benxi area of the north china craton abstract the petrogenesis and inner genetic relations of mesoarchean granitoid rocks have been poorly researched in the anshan benxi ab area of the north china craton these mesoarchean granitoids are mainly composed of 3 14 ga lishan trondhjemitic gneisses lstg and 2 95 3 01 ga donganshan jinjialing trondhjemitic to monzogranitic gneisses dtmg and tiejiashan gongchangling high k granitoid gneisses tgkg which show essentially consistent ages of magmatic crystallization the lstg are characterized by sio2 70 71 73 72 wt and al2o3 14 25 15 99 wt contents and low mgo 0 46 1 12 wt contents with low mg values 32 49 and k2o na2o values of 0 43 to 0 66 and total rare earth elements tree of 97 to 151 ppm with la yb n ratios of 14 19 to 83 15 integrated with hf o isotope characteristics from previous research and partial melting simulation the geochemical characteristics of lstg indicate that the trondhjemitic magma was generated by the partial melting of the source consisting of dominant 3 3 ga juvenile crustal material and some paleo to eoarchean sediments at the lower crust level in contrast with the lstg the dtmg shows higher sio2 73 10 75 99 wt contents slightly lower al2o3 13 41 15 00 wt and mgo 0 21 0 61 wt contents with mg values 18 48 higher k2o na2o values 0 53 1 26 and similar tree contents of 85 to 162 ppm la yb n ratios 30 78 125 02 and ehf t2 values from 5 38 to 3 16 detailed petrogenetic investigations suggest that the magma of the dtmg was derived from the partial melting of the source comprising main 3 3 ga basaltic material and more mesoarchean sediments at the lower crust level then experienced slight plagioclase fractionation crystallization the tgkg is characterized by the highest sio2 72 86 77 57 wt concentrations but the lowest al2o3 11 96 14 47 wt and mgo 0 01 0 54 wt contents with k2o na2o values of 0 86 to 2 53 and mg values of 2 to 41 the wide tree contents 142 766 ppm and la yb n ratios 6 41 86 87 ehf t2 value range of 9 71 to 2 14 suggesting that the magma of tgkg formed from the partial melting of abundant eo and paleoarchean recycled sediments and igneous rocks this petrogenesis of tgkg magma indicates that eo to paleoarchean supracrustal material may exist under the tiejiashan gongchangling area integrating previous studies on the ab area these diverse granitoids record two episodes of major crust growths at 3 65 3 8 ga and 3 3 ga subsequently the geological processes were dominated by the recycling of 3 3 ga basaltic material and eo to paleoarchean crustal materials during 3 14 2 95 ga the petrogenesis of mesoarchean diverse granitoids indicates that voluminous supracrustal materials with different ages were transferred into the lower crust level during 2 95 3 14 ga revealing that pre subduction buckling convergence may be the most probable mesoarchean geodynamic regime in the ab area of the north china craton

are aftershocks of large californian earthquakes diffusing we analyze 21 aftershock sequences of california to test for evidence of space time diffusion aftershock diffusion may result from stress diffusion and is also predicted by any mechanism of stress weakening here we test an alternative mechanism to explain aftershock diffusion based on multiple cascades of triggering in order to characterize aftershock diffusion we develop two methods one based on a suitable time and space windowing the other using a wavelet transform adapted to the removal of background seismicity both methods confirm that diffusion of seismic activity is very weak much weaker than reported in previous studies a possible mechanism explaining the weakness of observed diffusion is the effect of geometry including the localization of aftershocks on a fractal fault network and the impact of extended rupture lengths which control the typical distances of interaction between earthquakes

earthquake distribution patterns in africa their relationship to variations in lithospheric and geological structure and their rheological implications we use teleseismic waveform inversion along with depth phase analysis to constrain the centroid depths and source parameters of large african earthquakes the majority of seismic activity is concentrated along the east african rift system with additional active regions along stretches of the continental margins in north and east africa and in the congo basin we examine variations in the seismogenic thickness across africa based on a total of 227 well determined earthquake depths 112 of which are new to this study seismogenic thickness varies in correspondence with lithospheric thickness as determined from surface wave tomography with regions of thick lithosphere being associated with seismogenic thicknesses of up to 40 km in regions of thin lithosphere the seismogenic thickness is typically limited to 20 km larger seismogenic thicknesses also correlate with regions that have dominant tectonothermal ages of 1500 ma where the east african rift passes around the archean cratons of africa through the older proterozoic mobile belts these correlations are likely to be related to the production affected by method and age of basement formation and preservation affected by lithospheric thickness of a strong anhydrous lower crust the congo basin contains the only compressional earthquakes in the continental interior simple modelling of the forces induced by convective support of the african plate based on long wavelength free air gravity anomalies indicates that epeirogenic effects are sufficient to account for the localization and occurrence of both extensional and compressional deformation in africa seismicity along the margins of africa reflects a mixture between oceanic and continental seismogenic characteristics with earthquakes in places extending to 40 km depth

the afar depression interpretation of the 1960 2000 earthquakes summary we studied the seismic activity of the afar depression ad and adjacent regions during the period 1960 2000 we define seven distinct seismogenic regions using geological tectonic and seismological data based on the frequency magnitude relationships we obtain b values of about 1 for the different regions the pattern of the distribution of the location of epicentres fits with the known active fault zone in the ad and the axial volcanic ridges the bab el mandab area and the danakil aysha a blocks are less active for 125 intermediate to strong earthquakes the seismic moment and source parameters were calculated the results of the fault plane solutions for the afar depression indicate mainly strike slip and normal sense of movement originating from fault planes striking nw se these results indicate a clockwise block rotation described previously as a bookshelf model in central ad there are a few rightlateral faults east of massawa with e w striking fault planes at the southern red sea north of the danakil block the mixed focal mechanisms with axial plane striking nw se comprise several reverse faulting strike slip motion and normal faulting right lateral movement was also calculated for a cluster of seismic events between the manda hararo and alyata volcanic ridges along nw se striking faults along the n s striking faults in the escarpment at the western afar margins there are two distinct clusters of epicentres the strong earthquakes at the southern cluster exhibit normal or strike slip motions the intermediate to small earthquakes in the northern cluster exhibit reverse and strike slip motions mainly normal faults were calculated along ne sw striking faults of the ethiopian east african rift estimates of the seismic efficiency suggest that the maximal values are about 50 per cent or less implying that most of the motion is taken aseismically

onset of plate tectonics the further back we look into the geological past the more obscured the view masked by an increasingly fragmentary geological record this has resulted in a controversy on the rates and mechanisms of early continental crust formation and whether plate tectonics the dominant crust forming process over at least the past 2 5 billion years operated the same way or even at all during early earth history 1 2 on page 434 of this issue shirey and richardson 3 shed light on this issue by looking at it from a new angle from the bottom up they investigated the composition of a suite of minerals occurring as inclusions in diamonds dredged up by young kimberlite volcanoes these diamonds derive from great depths 125 to 175 km within the ancient lithospheric mantle keels that underpin the stable continental crustal regions known as cratons because the minerals can be precisely dated they can provide a snapshot of the subcontinental lithospheric mantle sclm composition at the time when the crust was being formed

archaean andesite petrogenesis insights from the graedefjord supracrustal belt southern west greenland we present new whole rock major trace and platinum group element data as well as sm nd and lu hfisotope data for meta volcanic rocks from the mesoarchaean graedefjord supracrustal belt gsb locatedwithin the tasiusarsuaq terrane southern west greenland we also present new in situ zircon u pbisotope data by la icp ms for associated felsic rocks this region has experienced amphibolite to lowergranulite facies metamorphism causing re equilibration of most mineral phases including zircon an intrusive tonalite sheet with a zircon u pb age of 2888 6 8 ma yields a minimum age for the gsb the sm nd and lu hf isotope data do not provide meaningful isochron ages but the isotope compositionsof the mafic rocks are consistent with the ca 2970 ma regional volcanic event which is documented in pre vious studies of the tasiusarsuaq terrane the major and trace element data suggest a significant crustalcontribution in the petrogenesis of andesitic volcanic rocks in the gsb the trace element variation of theseandesitic leucoamphibolites cannot be explained by bulk assimilation fractional crystallisation afc processes involving local basement rather the observed patterns require binary mixing between basalticand felsic end member magmas with between 50 and 80 contributions from the latter depending onthe assumed felsic composition hf isotope constraints point to contamination with pre existing conti nental crust with an age of ca 3250 ma basement gneisses of this age were previously described at twolocalities in the tasiusarsuaq terrane which supports the mixing hypothesis thus the felsic end memberlikely represents melts derived from the local basement ultramafic rocks 18 35 22 80 wt mgo in gsb have platinum group element pge patterns that aresimilar to magmas derived from high degree melting of mantle but they have relatively enriched traceelement patterns we propose that the ultramafic rocks represent arc related picrites or alternativelywere derived by melting of metasomatised sub continental lithospheric mantle overall these new geochemical data from the mesoarchaean graedefjord supracrustal belt and thepetrogenetic mixing model in particular are similar to observations from modern continental subductionzone environments which also require large degrees of mixing with felsic basement melts therefore we propose that the metavolcanic rocks formed in a modern style subduction zone geodynamic setting which due to the hotter archaean mantle conditions allowed for substantial amounts of partial meltingand magma mixing rather than assimilating pre existing continental crust less

source parameters and tectonic origin of the 1996 june 1 tianzhu mw 5 2 and 1995 july 21 yongden mw 5 6 earthquakes near the haiyuan fault gansu china summary the 1996 june 1 tianzhu mw 5 2 ms 4 9 and the 1995 july 21 yongden mw 5 6 ms 5 4 earthquakes are the two largest events recorded in the last 10 years between the 1990 october 20 ms 5 8 and the recent 2000 june 6 ms 5 6 earthquakes near the tianzhu seismic gap on the haiyuan fault in northeastern tibet we use frequency time analysis ftan to extract the fundamental modes of love and rayleigh waves from digital records a joint inversion of their amplitude spectra and of p wave first motion polarities is then performed to calculate the source parameters focal mechanisms depths and seismic moments of these two ms 5 earthquakes such a joint inversion is tested for the first time we use iris and geoscope network records for period ranges of 20 40 s for the former event and 35 70 s for the latter the inversion of the tianzhu earthquake yields nodal planes with strike dip and slip of 282 72 and 3 and 191 87 and 162 respectively a focal depth around 12 km and a seismic moment of 0 56 1017 n m consistent with the harvard cmt calculation and the alignment and depths of the aftershocks recorded by a local network we propose two possible tectonic interpretations for this off fault event the solution for the yongden earthquake is consistent with a thrust with strike dip and slip of 105 45 and 75 respectively a focal depth around 6 km and a seismic moment of 2 4 1017 n m also in agreement with the harvard cmt mechanism the distribution of the aftershocks recorded by a regional network and the general tectonic setting that we refine

dynamic source parameters of the march may 1969 serdo earthquake sequence in central afar ethiopia deduced from teleseismic body waves long period teleseismic p and s waveforms for the largest four earthquakes in the serdo sequence are compared with synthetic seismograms to infer mechanisms focal depths source time functions and dynamic energy release for all four events studied the mechanisms are predominantly strike slip depth estimates obtained for the four earthquakes by matching synthetic seismograms to the observations are found to lie between 5 and 8 km the azimuthal variation of observed body wave duration for the main event indicates that the rupture propagated from ne to sw average seismic moments obtained from body wave synthetics vary from 4 6 1017 to 2 5 1018 n m average final displacements stress drops and lengths of the fault are found to lie between 0 16 and 0 56 m 0 45 and 1 56 mpa and 10 and 16 km respectively the low stress drop value obtained may indicate the presence of softer materials near the source the energy release shows that the mode of energy release during the sequence took place in two steps mainly by the two largest shocks

lithological structural and geochemical characteristics of the mesoarchean târtoq greenstone belt southern west greenland and the chugach prince william accretionary complex southern alaska evidence for uniformitarian plate tectonic processes the mesoarchean t rtoq greenstone belt southern west greenland consists of tectonically imbricated slices of metamorphosed basalt gabbro peridotite and sedimentary rocks and is intruded by fel

neoproterozoic active margin in the northwestern yangtze block south china new clues from detrital zircon u pb geochronology and geochemistry of sedimentary rocks from the hengdan group the yangtze block in south china constitutes an important precambrian landmass in the present east asian continent the neoproterozoic sedimentary successions of the hengdan group in the nw yangtze block record essential information for deciphering the neoproterozoic tectonics along the nw margin however its depositional age provenance and tectonic properties remain uncertain here a combined analysis of detrital zircon u pb dating and geochemistry is performed on representative samples from the hengdan group concordant dating results of samples from the bottom and upper parts constrain the maximum depositional age at c 720 ma detrital zircon age patterns of samples reveal a uniformly pronounced age peak at c 915 720 ma which is consistent with the magmatic pulses in domains at the nw end of the yangtze block in addition these samples display left sloping post archaean australian shale paas normalized rare earth element patterns and variable trace element patterns resembling sediments accumulated in a basin related to an active continental margin geodynamic setting provenance analysis reveals that the main sources featured intermediate to felsic components which experienced rapid erosion and sedimentation these integrated new investigations along with previous compilations indicate that the hengdan group might have been deposited in a fore arc basin controlled by subduction beneath the bikou terrane thus such interpretation further supports proposals for subduction related tectonics along the western margin of the yangtze block during the early neoproterozoic

a change in the geodynamics of continental growth 3 billion years ago models for the growth of continental crust rely on knowing the balance between the generation of new crust and the reworking of old crust throughout earth s history the oxygen isotopic composition of zircons for which uranium lead and hafnium isotopic data provide age constraints is a key archive of crustal reworking we identified systematic variations in hafnium and oxygen isotopes in zircons of different ages that reveal the relative proportions of reworked crust and of new crust through time growth of continental crust appears to have been a continuous process albeit at variable rates a marked decrease in the rate of crustal growth at 3 billion years ago may be linked to the onset of subduction driven plate tectonics

block versus continuum deformation in the western united states abstract the relative role of block versus continuum deformation of continental lithosphere is a current subject of debate continuous deformation is suggested by distributed seismicity at continental plate margins and by cumulative seismic moment sums which yield slip estimates that are less than estimates from plate motion studies in contrast block models are favored by geologic studies of displacement in places like asia a problem in this debate is a lack of data from which unequivocal conclusions may be reached in this paper we apply the techniques of study used in regions such as the alpine himalayan belt to an area with a wealth of instrumental data the western united states by comparing plate rates to seismic moment release rates and assuming a typical seismogenic layer thickness of 15 km it appears that since 1850 about 60 of the pacific north america motion across the plate boundary in california and nevada has occurred seismically and 40 aseismically the san francisco bay area shows similar partitioning between seismic and aseismic deformation and it can be shown that within the seismogenic depth range aseismic deformation is concentrated near the surface and at depth in some cases this deformation can be located on creeping surface faults but elsewhere it is spread over a several kilometer wide zone adjacent to the fault these superficial creeping deformation zones may be responsible for the palaeomagnetic rotations that have been ascribed elsewhere to the surface expression of continuum deformation in the lithosphere our results support the dominant role of non continuum deformation processes with the implication that deformation localization by strain softening must occur in the lower crust and probably the upper mantle our conclusions apply only to the regions where the data are good and even within the western united states i e the basin and range deformation styles remain poorly resolved nonetheless we maintain that block motion is the deformation style of choice for those continental regions where the data are best

coupling volcanism and tectonics along divergent plate boundaries collapsed rifts from central afar ethiopia magma along divergent plate boundaries is erupted from fissures or vents from central volcanoes with limited impact on rift architecture here i summarize the geological and structural features accompanying the eruption of part of a km thick volcanic sequenc e stratoids along the red sea divergent boundary in central afar in the area of tendaho and dobi grabens more than 4700 km 3 ma per 100 km of rift length of magma have been produced by repeated fissure eruptions from within tendaho graben the graben sides show distinctive structural features as steep topographic gradients coinciding with inward tilted blocks forming dominoes coeval to the emplacement of the km thick volcanic sequence similar features are observed also in the dobi graben this allows proposing an original mechanism where the distinctive structure of the grabens results from the collapse at the surface induced by magma withdrawal during the emplacement of the volcanic sequence this portion of afar shows how rift architecture is shaped by voluminous fissure eruptions forming collapsed rifts these occur in continental domains during the breakup stage central afar and in oceanic domains where rifts narrow east pacific rise collapsed rifts represent an end member type of volcano tectonic activity where the width of the erupting reservoir balances that of the active rift zone along divergent boundaries the width of the reservoir influences the style of surface deformation a progressively higher ratio of the width of the reservoir emptied re to that of the active rift zone ri generates in sequence axial grabens calderas and collapsed rifts

seismicity during lateral dike propagation insights from new data in the recent manda hararo dabbahu rifting episode afar ethiopia seismicity released during lateral dike intrusions in the manda hararo dabbahu rift afar ethiopia provides indirect insight into the distribution and evolution of tensile stress along this magma assisted divergent plate boundary in this paper 5 dike intrusions among the 14 that form the 2005 present rifting episode are analyzed with local and regional seismic data during dike intrusions seismicity migrates over distances of 10 15 km at velocities of 0 5 3 0 km h away from a single reservoir in the center of the rift segment confirming the analogy with a slow spreading mid ocean ridge segment comparison with geodetic data shows that the reservoir is located 7 km down rift from the topographic summit of the axial depression dikes emplaced toward the north are observed to migrate faster and to be more voluminous than those migrating southward suggesting an asymmetry of tension in the brittle elastic lithosphere seismicity during dike injections is concentrated near the propagating crack front in contrast faults and fissures in the subsurface appear to slip or open aseismically coeval with the intrusions the seismic energy released during dike intrusions in the manda hararo rift appears to be primarily modulated by the local magnitude of differential tensile stress and marginally by the rate of stress change induced by the intrusion the low level of seismic energy accompanying dike intrusions despite their significant volumes is likely an indicator of an overall low level of tension in the lithosphere of this nascent plate boundary

modelling aftershock migration and afterslip of the san juan bautista california earthquake of october 3 1972 abstract the san juan bautista earthquake of october 3 1972 m l 4 8 located along the san andreas fault in central california initiated an aftershock sequence characterized by a subtle but perceptible tendency for aftershocks to spread to the northwest and southeast along the fault zone the apparent dimension of the aftershock zone along strike increased from about 7 10 km within a few days of the earthquake to about 20 km eight months later in addition the mainshock initiated a period of accelerated fault creep which was observed at 2 creep meters situated astride the trace of the san andreas fault within about 15 km of the epicenter of the mainshock the creep rate gradually returned to the preearthquake rate after about 3 yrs both the spreading of the aftershocks and the rapid surface creep are interpreted as reflecting a period of rapid creep in the fault zone representing the readjustment of stress and displacement following the failure of a stuck patch or asperity during the san juan bautista earthquake numerical calculations suggest that the behavior of the fault zone is consistent with that of a material characterized by a viscosity of about 3 6 10 14 p although the real rheology is likely to be more complicated in this model the mainshock represents the failure of an asperity that slips only during earthquakes aftershocks represent the failure of second order asperities which are dragged along by the creeping fault zone

the evolution of the gulf of corinth greece an aftershock study of the 1981 earthquakes summary a preliminary study of the aftershocks of three earthquakes that occurred near to corinth greece in 1981 is combined with observations of the morphology and faulting to understand the evolution of the eastern gulf of corinth the well located aftershocks form a zone 60km long and 20km wide they do not lie on the main fault planes and are mostly located between the north dipping faulting on which the first two earthquakes occurred and the south dipping faulting associated with the third event a cluster of aftershocks also lies in the footwall of the eastern end of the south dipping fault of the third event morphologically it is observed that in the evolution of the eastern gulf of corinth antithetic faulting apparently predates the appearance of the main faulting at the surface this evolution can be explained by motion on a deep seated shallow angle aseismic normal fault a model based on such a fault also accounts for the aftershock distribution of the 1981 earthquakes