3 2 b y of organic compound formation near sea floor hot springs sea floor hydrothermal vent sites as modern equivalents of the earliest ecosystems on earth is an intriguing hypothesis but one that has no supporting evidence from the geologic record we have therefore analyzed the organic content of samples representative of sea floor hydrothermal activity from 3 2 ga as well as samples up to the present including active sea floor hydrothermal deposits the mass spectroscopy results show that a great variety of organic compounds is present in all the samples the data are essentially indistinguishable between the various samples irrespective of their age these findings imply among other things that organic productivity associated with sea floor hydrothermal environments was already vigorous as early as the middle archean supporting the premise that life may have started near vent sites however abundant organic material in concomitant pelagic sediment shale together with supporting carbon isotopic evidence shows that a photosynthetic marine biota was probably also active at this time thus ecosystems related to sea floor hydrothermal activity and photosynthetic microorganisms in the ambient ocean coexisted at 3 2 ga indicating that these earliest ecosystems must have been in existence well before this time

the nature and genesis of marginal cu pge au sulphide mineralisation in paleogene macrodykes of the kangerlussuaq region east greenland the kangerlussuaq region of east greenland hosts a variety of early tertiary extrusive and intrusive igneous rocks related to continental break up and the passage of the ancestral iceland plume these intrusive bodies include a number of gabbroic macrodykes two of which the miki fjord macrodyke and the newly discovered togeda macrodyke contain cu pge au sulphide mineralisation along their margins sulphides occur as disseminated interstitial blebs and rounded globules of chalcopyrite and pyrrhotite with some fe ti oxides and platinum group minerals comprising largely pd bismuthides and tellurides the globules are interpreted to have formed from fractionation of trapped droplets of an immiscible cu and pd rich sulphide melt and show geopetal indicators sulphur isotopes imply a local crustal source of s in these from pyritic sediments of the kangerlussuaq basin thus generation of these sulphide occurrences was controlled by local country rock type low ni cu and pt pd ratios also present in the platinova reefs in the skaergaard intrusion indicate that early fractionation of olivine may have depleted the magma of ni and suggest the likely presence of a large magma chamber at depth xenoliths of ni rich olivine cumulates in the miki fjord macrodyke may have been sourced from such a body the location of thus far unidentified conduit or feeder zones to the macrodykes beneath the present day surface may represent potential targets for more massive sulphide orebodies

release fault a variety of cross fault in linked extensional fault systems in the sergipe alagoas basin ne brazil abstract two types of cross faults are herein recognized transfer faults and the newly termed release faults transfer faults form where cross faults connect distinct normal faults and horizontal displacements predominate over vertical ones in contrast release faults form where cross faults associated with individual normal faults die out within the hangingwall before connecting to other normal faults and have predominantly vertical displacements release faults are geometrically required to accommodate variable displacements along the strike of a normal fault thus they form to release the bending stresses in the hangingwall and do not cut normal fault planes nor detachment surfaces at depth release faults have maximum throws adjacent to normal faults and may be nearly perpendicular or obliquely oriented to the strike of the latter such geometry appears not to depend upon pre existing weaknesses but such variable orientation to normal faults is an inherent property of release faults release faults commonly appear as simple normal faults in seismic sections without implying extension along the strike of rift and basins three dimensional strain deformation occurs in the hangingwall only between the terminations of an individual normal fault but regionally release faulting is associated with plane strain deformation in linked extensional fault systems

the least radiogenic terrestrial leads implications for the early archean crustal evolution and hydrothermal metasomatic processes in the isua supracrustal belt west greenland abstract vein type galenas from within metasomatized tonalitic gneiss sheets in a metamorphosed oceanic crustal sequence of the arc outbound isua supracrustal belt isb west greenland reveal the hitherto least radiogenic terrestrial lead isotope compositions these leads deviate from commonly applied average continental crust evolution curves for lead and can be modeled by a single stage growth with starting parameters 238u 204pb 7 70 and 232th 238u 4 37 inferring model ages of 3 81 and 3 76 ga for these galenas the initial constraints are indicative of higher u pb and th u in the source of the hydrothermal fluids and arguably in the magmatic precursors of the gneisses they support contentions regarding a time integrated terrestrial pre history with an elevated th u in the early earth s mantle necessary to reconcile the difference between the observed th u and the time integrated th u recorded by pb isotope ratios of present depleted morb mantle derived rocks in addition they are in line with the requirement of preferential enrichment of u relative to pb in a proto continental crust relative to depleted upper mantle in the early earth s history the age equivalency between the galena generations and abundant tonalitic gneiss sheets and their mutual spatial relationship within the isb favors an interpretation of the least radiogenic terrestrial leads to closely approximate the initial pb of 3 75 ga and pre 3 8 ga old tonalitic protoliths and therefore prompts a close genetic relationship between early archean fluid flow and the magmatic events the inferred 3 81 ga model age of the least radiogenic galena pb particularly agrees with previous u pb zircon geochronological results on tonalite sheets within the supracrustals which constrain a ca 3 79 3 81 ga minimum age for the formation to the earth s oldest oceanic crustal sequence in combination with 3 74 ga pb pb ages of post deformational hydrothermal metasomatic minerals our results imply that early archean metasomatic activity is indistinguishable in time from the earliest metamorphic overprint and from the ca 3 75 ga major pulse of tonalite intrusions in the isua area

depositional setting and paleogeographic implications of earth s oldest supracrustal rocks the 3 7 ga isua greenstone belt west greenland abstract new structural and stratigraphic mapping in the isua greenstone belt west greenland has revealed that the exposed lithologic succession is quite different to that depicted in earlier studies the key to a better understanding of the stratigraphy has been the recognition of intense strain and metasomatic effects combined with ductile fault structures that segment the belt into a number of tectonic slices in some of these slices deformation and metasomatism are somewhat lower than in surrounding slices though still significant which permit the recognition of primary depositional features that may be used to compare with lithologies from elsewhere in the belt the belt is dominated by amphibolite that in a number of places show well defined pillows pillow breccias and basaltic debris flows also occur within this package strongly recrystallized ultramafic bodies that occur in the belt are interpreted as intrusions or komatiitic flows the most common sedimentary rock type is chert banded iron formation these lithologies have been strongly affected by brittle and ductile deformation in combination with coarse recrystallization siliciclastic detrital rocks such as conglomerate and sandstone are much less common in the belt and where present have intrabasinally derived sources highly deformed quartzo feldspathic schist crops out in a number of places in the belt and most likely has multiple origins previously these schists have been considered to be felsic volcaniclastic rocks though recent mapping and geochemical and isotopic studies support the hypothesis that at least some of these schists are highy deformed and carbonated tonalitic gneiss sheets or replaced pillow lava successions carbonate rocks in the belt are now considered to be mostly or entirely replacement in origin where metasomatizing fluids have particularly utilized amphibolite chert contacts additionally some carbonate may represent the products of early sea floor alteration we envision a temporarily emergent below wave base sea floor origin for the succession based on the assemblage of lithologies

reassessing the evidence for the earliest traces of life the isotopic composition of graphite is commonly used as a biomarker in the oldest 3 5 gyr ago highly metamorphosed terrestrial rocks earlier studies on isotopic characteristics of graphite occurring in rocks of the approximately 3 8 gyr old isua supracrustal belt isb in southern west greenland have suggested the presence of a vast microbial ecosystem in the early archean this interpretation however has to be approached with extreme care here we show that graphite occurs abundantly in secondary carbonate veins in the isb that are formed at depth in the crust by injection of hot fluids reacting with older crustal rocks metasomatism during these reactions graphite forms from the disproportionation of fe ii bearing carbonates at high temperature these metasomatic rocks which clearly lack biological relevance were earlier thought to be of sedimentary origin and their graphite association provided the basis for inferences about early life the new observations thus call for a reassessment of previously presented evidence for ancient traces of life in the highly metamorphosed early archaean rock record

late pleistocene glacial events in the central apennines italy the study of glacial evidence in the gran sasso massif of the central apennines italy has allowed the last maximum advance and the subsequent stadial phases to be dated and the mean annual temperature and quantity of precipitation in the form of snow to be assessed for a number of periods the glaciers probably reached their maximum extension campo imperatore stade ca 22 60014c yr b p and started to retreat ca 21 000 yr b p leaving behind three recessional moraines after a first interstade fornaca interstade the fontari stade appears to have taken place shortly after 16 000 yr ago ca 15 000 yr ago the glacier started retreating leaving behind four more recessional moraines an interstade venacquaro interstade preceded the mount aquila stade datable at ca 11 000 yr b p a strong correlation is evident between the glacial phases on land and the isotopic variations in cores from the tyrrhenian sea

setting and origin for problematic rocks from the 3 7 ga isua greenstone belt southern west greenland earth s oldest coarse clastic sediments abstract whether or not coarse detrital sedimentary rocks occur within the 3 7 ga isua greenstone belt igb southern west greenland has been debated for some time repeated regional metamorphic deformational and metasomatic events have obscured most protolith lithologies leading to misunderstandings about the stratigraphy and environments of deposition rocks here interpreted as meta conglomerate crop out in a fault bounded structural domain that is lower in strain relative to adjacent domains the meta conglomerate has a strike length of 1 km and is 10 m thick bed thickness ranges from 10 cm to more than 1 m and beds may be either framework or matrix supported a poorly sorted and variably rounded polymict assemblage of framework clasts consisting of meta chert bif and a variety of mafic volcanic rock fragments are set into a matrix of biotite quartz garnet schist clast compositions indicate reworking of adjacent stratigraphic units a lack of structural similarity in framework clasts range of grain sizes range of rounding and polymict composition demand a primary sedimentary origin for the deposit inferred depositional processes include traction and debris flow which would be consistent with subaerial or shallow subaqueous environments although the limited extent of the meta conglomerate warrants interpretational caution

u rich archaean sea floor sediments from greenland indications of 3700 ma oxygenic photosynthesis abstract 3700 ma metamorphosed pelagic shale from west greenland contains up to 0 4 wt reduced carbon with 13c values down to 25 6 pdb peedee belemnite the isotopic signature and mode of occurrence suggest that the carbon derived from planktonic organisms the pb isotopic composition shows that the shale had high primary u th this indicates that organic debris produced a local reducing environment which precipitated u transported to the site of sedimentation by oxidized ocean water the existence of highly productive plankton that fractionated c isotopes strongly and set up oxidation contrast in the environment suggests that oxygenic photosynthesis evolved before 3700 ma

relay ramp forms and normal fault linkages canyonlands national park utah fieldwork within a series of mesoscale grabens in southeast utah has revealed a particularly well exposed system of interlinked extensional faults a series of down faulted grabens are developed within a 460 m thick brittle layer of upper paleozoic sandstone and shale which overlies a ductile layer with a high gypsum content all the major grabens consist of two or more overlapping elements which are composed of fault segments these segments may be hard linked fault surfaces are joined or soft linked fault surfaces are isolated but linked by ductile strain of the rock volume between them in map view relay structures are defined as zones connecting the footwalls and hanging walls of overlapping fault segments representing soft linkage of fault segments in the canyonlands grabens the transfer of displacement between soft linked fault segments is characterized by well de fined dipping relay ramps commonly rotated and extended to accommodate the ductile strain between the overlapping fault segments relay ramps develop as ephemeral structures eventually becoming breached by hard linkage of the fault segments breakdown of ramps by breaching is part of the process of fault growth by segment linkage within the canyonlands graben system four orders of segmentation are present this may be a consequence of different scales of mechanical heterogeneity but evidence from the canyonlands grabens and elsewhere points to a fundamental process of discontinuous fault growth by segment linkage

microstructural evidence for seismic and aseismic slips along clay bearing carbonate faults in this multi methodological study microstructural observations of fault rocks are combined with micromechanical property analyses contact resonance atomic force microscopy cr afm and with rotary friction experiments shiva apparatus to find evidence of seismic to aseismic slip and understand the nanoscale rheology of clay bearing carbonate hosted faults fluidized structures truncated clasts pores and vesicles and phyllosilicate nano sized spherules and tubes suggest fast deformation events occurred during seismic slip whereas clay assisted pressure solution processes clumped clasts foliation surfaces and mantled clasts indicate slow deformation events occurred during postseismic interseismic periods cr afm measurements show that the occurrence of 5 wt of clay within the carbonate hosted gouges can significantly reduce the fault core stiffness at nanoscale in addition during high velocity friction experiments simulating seismic slip conditions the presence of ultra thin phyllosilicate bearing 3 wt layers within calcite gouges as those observed in the natural fault show faster dynamic weakening than that of pure calcite gouges the weak behavior of such layers could facilitate the upward propagation of seismic slip during earthquakes thus possibly enhancing surface faulting microstructural observations and experimental evidence fit some well known geophysical and geodetic observations on the short to long term mechanical behavior of faults such as post seismic interseismic aseismic creep interseismic fault locking and seismic slip propagation up to the earth s surface

segmentation pattern and structural complexities in seismogenic extensional settings the north matese fault system central italy abstract we investigated the northern slope of the matese mts molise central italy with the aim of characterizing the n to ne dipping active normal fault system in the bojano basin a sector of primary importance from a seismic hazard perspective we collected field data to define the geometry and segmentation pattern of two sub systems patalecchia colle di mezzo and bojano campochiaro new evidence of late quaternary faulting was obtained by exploiting well log interpretations kinematic analysis revealed the interaction of pre quaternary inherited mainly e w striking and newly formed nw se striking normal faults slip accommodation through linkage was clearly noted in the case of the patalecchia colle di mezzo sub system detailed topographic profiles across the active fault segments provided post lgm 15 3 kyr slip rates up to 2 mm yr which agree with the high deformation rates based on different approaches in the literature finally the instrumental seismicity analysis constrained the bottom of the seismogenic layer to depths of 13 14 km and the gathered information allowed us to reconstruct the north matese seismogenic source its 3d geometry and dimensions agree with both the dimension magnitude relationships and macroseismic information available for the 1805 earthquake mw 6 6 the main historical earthquake to have struck the bojano basin

considerations on the significance of some post glacial fault scarps in the abruzzo apennines central italy abstract in this paper the tectonic and palaeoseismological significance of the fault scarps present in the massifs of the velino and of mount greco and in the plains adjacent to these massifs forming part of the abruzzo apennines is discussed the basic features of post glacial fault scarps already mentioned in the literature are illustrated found in the fucino plain in the ovindoli piani di pezza area on the mount magnola in the valle majelama alluvial fan and in the piano di aremogna others not previously mentioned surveyed by the author in the mount ocre cefalone area in the piana di corvaro and on the mount greco toppe del tesoro are also discussed on the basis of the characteristics of these scarps and of the chronological framework of the last phases of activity of the faults which gave rise to the scarps described the reliability of the assumptions made for palaeoseismological studies in other places is discussed for the abruzzo area the studies performed in the area of the velino and greco massifs suggest that to establish the possible co seismical activity of the faults the use of morphological indices and also of the results obtained with the excavation of trenches has to be very cautiously applied it is considered that the following can be stated it is probable that the majority of the mentioned fault scarps are co seismic in view of the height of some fault scarps it cannot be excluded that they were formed during the course of earthquake sequences lasting not more that 2 3000 years it is possible that some of the above mentioned scarps were produced during seismic events of great magnitude it does not seem possible at present to calculate a precise slip rate the amount of the offset often changes along the same fault even over short distances moreover there are many horst and graben type structures produced by local effects which contribute towards making an assessment of the fault offsets complicated it is also clear that erosion can change the aspect of the scarps an evaluation of the displacements caused by the faults producing them must therefore be made extremely cautiously caution is needed in assessing the magnitude of past earthquakes and in assuming recurrence intervals the dating of the phases of fault activity is approximate and there is great difficulty in recognizing with any certainty the duration the sort of activity and the length of the fault segment concerned in the activity moreover the existence in the past of aseismic creep and of movements linked to post glacial isostatic rebound cannot be excluded with any certainty these considerations must be regarded for the two massifs examined since there have been the epicentres of rare earthquakes of low intensity the only large earthquake known in history which caused surface ruptures occurred in the fucino area in 1915 the fault scarps reported in this paper seem however interpretable in general as indications of post glacial tectonic activity the intensity of which was unsuspected until a few years ago

archaean stromatolites from the ngesi group belingwe greenstone belt zimbabwe preservation and stable isotopes preliminary results abstract the petrology and stable isotope chemistry of cyanobacterial stromatolites of archaean age 2 7 ga from the cheshire and manjeri formations of the belingwe greenstone belt in zimbabwe have been examined palaeomagnetic data suggest that the stromatolites formed in tropical to subtropical latitudes the cheshire formation shows little evidence of either anion or cation exchange during metamorphism and the stable carbon and oxygen isotope ratios suggest a formation at temperatures perhaps considerably below 80 c the manjeri formation only slightly older but overlain by a thick volcanic sequence shows a low grade of metamorphism and isotope ratios that are consistent with a metamorphic temperature of around 200 c

pb and nd isotope systematics of stromatolitic limestones from the 2 7 ga ngezi group of the belingwe greenstone belt constraints on timing of deposition and provenance abstract pb pb isochrons have been obtained for stromatolitic limestones from the late archaean belingwe greenstone belt of zimbabwe providing direct age constraints on the deposition of these shallow water marine sediments samples from the manjeri formation and stratigraphically higher cheshire formation yield age estimates of 2706 49 ma mswd 11 5 and 2601 49 ma mswd 0 93 respectively these data are in agreement with published u pb zircon and pb pb whole rock ages of associated volcanics and we therefore interpret our pb pb ages as representing the timing of early diagenesis thus providing a minimum age for carbonate precipitation a 2543 70 ma age mswd 5 1 for one sample from the cheshire formation is considered to reflect either late stage diagenesis a craton wide thermal chemical disturbance or tectonic activity along the crustal scale mtshingwe fault calculated model 1 values for manjeri and cheshire limestones are 8 40 0 02 and 9 02 0 01 similar to values for approximately 3 5 and 2 9 ga archaean basement units adjacent to the belingwe belt negative end tdeposition and fsm nd suggest derivation of the ree from old lree enriched continental crust two stage nd model ages for the carbonates indicate that precursor rocks were extracted from the mantle at around 3 5 ga cheshire formation and 3 3 ga manjeri formation in good agreement with mantle extraction ages for local basement units model tdm 3 5 3 3 ga

ion microprobe analysis of graphite from ca 3 8 ga metasediments isua supracrustal belt west greenland relationship between metamorphism and carbon isotopic composition in situ ion microprobe measurements of carbon isotopic compositions of graphite were made in seven metasediments and two carbonate rocks from the ca 3 8 ga isua supracrustal belt west greenland the 13c values of micron scale graphite globules in the metasediments and the carbonate rocks vary from 18 to 2 and from 7 to 3 respectively the maximum 13c value of graphite globules in the metasediment rises from 14 to 5 as the metamorphic grade increases from epidote amphibolite to upper amphibolite facies in a single hand specimen the 13c values of graphite inclusions in garnet are 7 lower on average than those outside garnet similarly graphite armored by quartz apparently shows a few permil lower 13c values than those on grain boundaries between noncarbonate minerals the fact that early crystallized minerals include relatively 13c depleted graphite indicates that the regional metamorphism increased the 13c values of the isua graphite this is consistent with the regional trend of 13c enrichment accompanied by the increase of metamorphic grade the minimum fractionation between graphite and carbonate is consistent with the equilibrium fractionation at about 400 to 550 c these observations indicate that isotopic exchange with isotopically heavy carbonate caused 13c enrichment of isua graphite the 13c values of graphite reported here 13c 18 were produced either as a metamorphic modification of organic carbon with initially much lower 13c values or as an abiological reaction such as decomposition of carbonate if the isotopic exchange between carbonate and graphite during regional metamorphism controlled the 13c enrichment of isua graphite previously reported large 13c depletion of graphite especially armored by apatite mojzsis et al 1996 was probably premetamorphic in origin this supports the existence of life at isua time ca 3 8 ga

sulfur isotope evidence for the origin of 3 4 to 3 1 ga pyrite at the princeton gold mine barberton greenstone belt south africa abstract the princeton mine is one of several mesothermal gold deposits found in the barberton greenstone belt the host rocks of the ore bodies are carbonaceous and pyritic shales of the fig tree group 3 4 to 3 2 ga in age the methothermal gold mineralization has occurred during the latest thrusting and deformation events at 3 1 ga micro analyses of sulfur isotopes of pyrite were performed on four representative samples collected from the princeton mine the 34 s values were determined in situ on individual pyrite crystals or aggregates of fine grained pyrite by the laser microprobe method 58 analyses and also on aggregates of separated pyrite crystals by the conventional cu 2 o combustion method 11 analyses trace element concentrations of pyrite were also determined using an electron microprobe based on their grain sizes and chemical compositions the pyrite samples examined are divided into two groups 1 fine grained pyrite and 2 coarse grained pyrite the fine grained group includes disseminated pyrite and pyrite laminae concordant to sedimentary bedding their textures suggest that the fine grained pyrite was formed during sedimentation of the carbonaceous shale at 3 4 to 3 2 ga the 34 s values of the fine grained pyrite range from 0 8 to 4 4 within a micro scale area 1 mm 2 mm 29 laser microprobe analyses suggesting that the microbial reduction of seawater sulfate was responsible for the formation of fine grained pyrite this further suggests that the 3 4 to 3 2 ga ocean already contained an appreciable amount of sulfate which is in contrast to the previously popular theory that the archean ocean was h 2 s rich and pyrite in archean sedimentary rocks was formed by an inorganic process coarse grained pyrite is associated with auriferous quartz veins and often contains gold grains trace element concentrations of coarse grained pyrite such as as concentrations are different from these of fine grained pyrite the petrographic features suggest that the most coarse grained pyrite was directly precipitated from high temperature hydrothermal fluids coarse grained pyrite has homogeneous 34 s values ranging from 1 1 to 3 6 29 laser microprobe analyses suggesting that source of sulfur for the coarse grained pyrite was reduced sulfur species in hydrothermal fluids

geometry and development of relay ramps in normal fault systems relay ramps occur between normal fault segments that overstep in map view the geometry and evolution of exposure scale relay ramps are described from the somerset coast england and are compared with larger scale ramps from elsewhere relay ramps can be classified into four groups based on the degree of interaction and linkage between the overstepping segments these groups are interpreted as being evolutionary stages in stage 1 the segments do not interact stage 2 involves the reorientation of bedding between two interacting faults to produce a relay ramp in stage 3 connecting fractures start to break the relay ramp stage 4 is when the relay ramp is destroyed to produce a single fault that has an along strike bend these evolutionary stages can develop through ti e but they can also be seen spatially a branch line between normal faults or an along strike bend may represent a stage 4 relay with progressively earlier stages occurring updip or downdip characteristic variability in displacement distance profiles for fault segments and linked faults accompanies the interaction and linkage processes displacement transfer by relay ramps is accompanied by steep displacement gradients along fault segments at oversteps relay ramps often contribute to a minimum in total fault displacement at a linkage point

the orestes melt zone mcmurdo dry valleys antarctica spatially distributed melting regimes in a contact melt zone with implications for the formation of rapakivi and albite granites

sedimentary and tectonic evolution of plio pleistocene alluvial and lacustrine deposits of fucino basin central italy abstract the fucino basin was the greatest lake of the central italy which was completely drained at the end of 19th century the basin is an intramontane half graben filled by plio quaternary alluvial and lacustrine deposits located in the central part of the apennines chain which was formed in upper pliocene and in quaternary time by the extensional tectonic activity the analysis of the geological surface data allows the definition of several stratigraphic units grouped in lower units and upper units the lower units upper pliocene are exposed along the northern and north eastern basin margins they consist of open to marginal lacustrine deposits breccia deposits and fluvial deposits the upper units lower pliocene holocene consist of interbedded marginal lacustrine deposits and fluvial deposits thick coarse grained fan delta deposits are interfingered at the foot of the main relief with fluvial lacustrine deposits most of the thickness of the lacustrine sequences more than 1000 m thick is buried below the central part of the fucino plain the basin is bounded by e w wsw ene and nw se fault systems velino magnola fault e w and tremonti celano aielli fault wsw ene and s potito celano fault nw se in the north the trasacco fault the pescina celano fault and the serrone fault nw se in the south east the geometry and kinematic indicators of these faults indicate normal or oblique movements the study of industrial seismic profiles across the fucino basin gives a clear picture of the subsurface basin geometry the basin shows triangular shaped basin fill geometry with the maximum deposits thickness toward the main east boundary fault zones that dip south westward serrone fault trasacco fault pescina celano fault on the basis of geological surface data borehole stratigraphy and seismic data analysis it is possible to recognize and to correlate sedimentary and seismic facies the bottom of the basin is well recognized in the seismic lines available from the good and continuous signals of the top of meso cenozoic carbonate rocks the shape of sedimentary bodies indicates that the filling of the basin was mainly controlled by normal slip along the nw se boundary faults in fact the continental deposits are frequently in on lap contact over the carbonate substratum several disconformable contacts occurred during the sedimentary evolution of the basin the main faults with antithetic and synthetic fault planes displace the whole sedimentary sequence up to the surface indicating a recent faults activity 1915 avezzano earthquake ms 7 0 the stratigraphic and tectonic setting of the fucino basin and neighboring areas indicates that the extensional tectonic events have had an important role in driving the structural sedimentary evolution of the plio quaternary deposits the geometry of the depositional bodies of the fault planes and their relationships indicate that the fucino basin was formed as a half graben type structure during plio quaternary extensional events some internal complexities are probably related to the fold and thrust structures of the apenninic orogeny formed in messinian time in this area and to a different activity timing of the e w and wsw ene fault systems and the nw se fault systems we believe based on the similarity of the surface characteristics that the structural setting of the fucino basin can be extrapolated to the other great intramontane basins in central italy e g rieti l aquila sulmona sora isernia basins

fault slip directions in central and southern greece measured from striated and corrugated fault planes comparison with focal mechanism and geodetic data fault slip directions recorded by outcropping striated and corrugated fault planes in central and southern greece have been measured for comparison with extension directions derived from focal mechanism and global positioning system gps data for the last 100 years to test how far back in time velocity fields and deformation dynamics derived from the latter data sets can be extrapolated the fault slip data have been collected from the basin bounding faults to plio pleistocene to recent extensional basins and include data from arrays of footwall faults formed during the early stages of fault growth we show that the orientation of the inferred stress field varies along faults and earthquake ruptures so we use only slip directions from the centers of faults where dip slip motion occurs to constrain regionally significant extension directions the fault slip directions for the peloponnese and gulfs of evia and corinth are statistically different at the 99 confidence level but statistically the same as those implied by earthquake focal mechanisms for each region at the 99 confidence level they are also qualitatively similar to the principal strain axes derived from gps studies extension directions derived from fault slip data are 043 047 for the southern peloponnese 353 for the gulf of corinth and 015 014 for the gulf of evia extension on active normal faults in the two latter areas appears to grade into strike slip along the north anatolian fault through a gradual change in fault slip directions and fault strikes to reconcile the above with 5 myr 1 clockwise rotations suggested for the area we suggest that the faults considered formed during a single phase of extension the deformation and formation of the normal fault systems examined must have been sufficiently rapid and recent for rotations about vertical axes to have been unable to disperse the fault slip directions from the extension directions implied by focal mechanisms and gps data thus in central and southern greece the velocity fields derived from focal mechanism and gps data may help explain the dynamics of the deformation over longer time periods than the 100 years over which they were measured this may include the entire deformation history of the fault systems considered a time period that may exceed 1 2 myr

continental extensional setting for the archean belingwe greenstone belt zimbabwe the tectonic setting of the archean ngezi group greenstones in the belingwe greenstone belt zimbabwe is controversial both autochthonous continental and oceanic settings have been proposed the sedimentary facies of the basal manjeri formation have been logged in surface sections and in drill cores deposition occurred under fluviatile and shallow marine conditions followed by alluvial fans and fan deltas detrital mineralogy rare earth element patterns and sm nd depleted mantle model ages between 2 9 and 3 7 ga are consistent with derivation from very local sources the facies and geochemical association imply sedimentation in an extensional continental setting

geodetic investigation of the 13 may 1995 kozani grevena greece earthquake the ms 6 6 13 may 1995 kozani grevena earthquake struck a region of low historical seismic activity which includes a 10 year old triangulation network in northern greece after the earthquake monuments from this network were occupied with gps to measure co seismic displacements inversion of the co seismic displacement field to yield a source mechanism is achieved by use of a hybrid simplex monte carlo method which requires no a priori constraints the model focal mechanism agrees well with the global cmt solution and locally observed aftershocks but implies a significantly higher scalar moment than do seismological or sar interferometry studies and has a longer fault length than the model based on sar interferometry

apatite in early archean isua supracrustal rocks southern west greenland its origin association with graphite and potential as a biomarker abstract rare earth element ree abundances in individual apatite crystals in banded iron formations bifs metacherts metacarbonates and mafic dykes in the isua supracrustal belt isb have been determined by laser ablation inductively coupled plasma mass spectrometry the results together with petrographic observations on the distribution of graphite have been used to track the origin of the different compositional types of apatite and to evaluate the potential proposed in earlier studies for use of the apatite graphite association as a biomarker the chondrite normalized distribution patterns of apatite in metasedimentary bifs and metacherts fall into three groups relatively flat profiles with distinct positive eu anomaly are interpreted as characterizing sedimentary diagenetic apatite that carry the ree signature of the archean ocean secondary apatite in isua metasdiments with either middle ree enriched profiles or with light ree depleted profiles is interpreted to have crystallized from percolating carbonate rich metasomatic fluids or from fluids derived from cross cutting mafic dykes respectively the occurrence together of these different genetic types of apatite with distinct ree signatures within cm scale samples shows the immobility of ree in preexisting apatite during metamorphic episodes apatite crystals in isua rocks of uncontested chemical sedimentary origin bif and metachert samples do not have graphite inclusions or coatings graphite inclusions and coatings on the other hand characterize apatite in secondary metacarbonate rocks in these rocks graphite is produced by thermal metamorphic reduction of carbonate ion derived from dissociation of the metasomatic ferrous carbonate where iron serves as electron donor oxidizing to form magnetite in view of the non sedimentary metasomatic origin of isua metacarbonates and the abiogenic source of graphite the apatite graphite assemblage can not be considered as a biomarker and does not provide information on early archean life in the isb

distinguishing biological from hydrothermal signatures via sulphur and carbon isotopes in archaean mineralizations at 3 8 and 2 7 ga

orogenic gold mineralisation hosted by archaean basement rocks at sortekap kangerlussuaq area east greenland a gold bearing quartz vein system has been identified in archaean basement rocks at sortekap in the kangerlussuaq region of east greenland 35 km north northeast of the skaergaard intrusion this constitutes the first recorded occurrence of au mineralisation in the metamorphic basement rocks of east greenland the mineralisation can be classified as orogenic style quartz vein hosted au mineralisation two vein types have been identified based on their alteration styles and the presence of au mineralisation mineralised type 1 veins occur within sheared supracrustal units and are hosted by garnet bearing amphibolites with associated felsic and ultramafic intrusions gold is present as native au and au rich electrum together with arsenopyrite and minor pyrite and chalcopyrite in thin alteration selvages in the immediate wall rocks the alteration assemblage of actinolite clinozoisite muscovite titanite scheelite arsenopyrite pyrite is considered to be a greenschist facies assemblage the timing of mineralisation is therefore interpreted as being later and separate event to the peak amphibolite facies metamorphism of the host rocks type 2 quartz veins are barren of mineralisation lack significant alteration of the wall rocks and are considered to be later stage fluid inclusion microthermometry of the quartz reveals three separate fluids including a high temperature th 300 350 c h2o co2 ch4 fluid present only in type 1 veins that in interpreted to be responsible for the main stage of au deposition and sulphidic wall rock alteration it is likely that the carbonic fluids were actually trapped at temperatures closer to 400 c two other fluids were identified within both vein types which comprise low temperature 100 200 c brines with salinities of 13 25 wt eq nacl and at least one generation of low salinity aqueous fluids the sources and timings of the secondary fluids are currently equivocal but they may be related to the emplacement of paleogene mafic intrusions the identification of this occurrence of orogenic style au mineralisation has implications for exploration in the underexplored area of east greenland between 62 and 69 n where other similar supracrustal units are known to be present

remnants of early archaean hydrothermal methane and brines in pillow breccia from the isua greenstone belt west greenland abstract fluid inclusions containing high density methane and saline waters brines associated with carbonates have been found in undeformed annealed quartz bearing vesicles from pillow breccia at isua west greenland massive quartz veins cementing the pillow fragments contain the same type of carbonate bearing saline aqueous inclusions as the pillows but different gaseous inclusions either trails of low density methane close to the boundary of the pillow fragment or isolated high density co2 rich inclusions in the centre of the veins high density methane and carbonate bearing aqueous inclusions brines are presumed to represent remnants of a sea floor type hydrothermal alteration system subsequently re equilibrated in terms of fluid density during the metamorphic evolution low density methane has been expelled from pillow fragments during post metamorphic uplift whereas high density co2 probably represents remnants of the peak metamorphic fluids some present day sea floor hydrothermal systems notably the off axis hydrothermal vent field near the mid atlantic ridge at 30 n appear to be quite comparable with this hypothetical early archaean fluid this setting represents a highly favourable environment for the development of life on earth

steep rock lake sedimentology and geochemistry of an archean carbonate platform abstract the 2800 ma mosher carbonate formation at steep rock lake in south central canada is one of earth s oldest limestone deposits it is both thick up to 500 m and relatively well preserved and was one of the first precambrian successions to be noted for evidence of early life it continues to provide information on the antiquity of such processes as photosynthesis the carbonate contains a variety of fabrics and facies from apparently fine grained bedded limestones with domal and columnar stromatolites atikokania radial fans and sheet cracks to closely packed metric scale hybrid domes of originally aragonite seafloor crust interlayered with fenestral fabric here we review previous work provide new observations concerning these sediments and their geochemistry and discuss the inception overall development and ultimate demise of the steep rock platform we divide the mosher carbonate into the lower hogarth member and upper elbow point member these units constitute a shallow water carbonate platform succession the hogarth member overlies eroded 3000 ma tonalite and is at least 120 m thick thin horizons of domal pseudocolumnar columnar and fenestral stratiform stromatolites and occasional layers of atikokania radial crystal fans occur within relatively fine grained thin bedded limestone with iron carbonate horizons in the upper part meter scale domes occur locally with sheet cracks and stromatactis the irregular laminae that build the small domal and columnar stromatolites are interpreted to be lithified mats and biofilms presumably cyanobacteria dominated fine grained well bedded facies of the lower hogarth member and its varied association of small stromatolites is interpreted to have accumulated in a relatively protected inner platform environment numerous very thin horizons of iron carbonate in the lower and upper parts of the hogarth member suggest temporally limited influence of iron rich offshore waters vertical trends in fe mn ba sr ce and 13 c probably reflect progressive restriction of circulation with at least one abrupt but short lived change back to more open seawater conditions the upper hogarth member with large banded domes and fenestral crusts and stromatolites is interpreted as back margin facies the overlying elbow point member is at least 70 m thick and appears to be entirely composed of innumerable juxtaposed elongate smooth and thickly layered domes lacking intervening sediment each dome is typically several meters across and composed of centimetric alternations of cuspate fenestral fabric possibly microbial and crystal fan fabric that is probably abiotic seafloor aragonite precipitate adjacent cuspate fenestral and crystal fan fabric show significant differences in concentration of 13 c sr ba fe and mn this giant dome facies is interpreted as a platform margin deposit in which alternating calcite aragonite mineralogies within each dome reflect a laterally fluctuating offshore redox boundary negative ce anomalies positive gd anomalies and reduced positive eu anomalies relative to laterally correlative deep water iron formation occur in the majority of crystal fan samples and in some cuspate fenestral fabric samples ca carbonate precipitation at the margin of an anoxic iron rich sea would have been promoted by oxygenic removal of iron from seawater otherwise fe carbonate would have been favored over ca carbonate precipitation the mosher carbonate formation is interpreted as an early marine oxygen oasis a shallow platform that favored cyanobacterial productivity was sufficiently isolated from open marine circulation for the seawater to become relatively oxygenated and where carbonate sediment aggradation was a positive feedback that sustained these shallow water conditions limestone accumulation terminated as transgressive suboxic and anoxic seawater deposited first manganese oxide rich and then manganese oxide poor iron formation sediments

contrasting geochemical patterns in the 3 7 3 8 ga pillow basalt cores and rims isua greenstone belt southwest greenland implications for postmagmatic alteration processes abstract pillow basalts from the early archean 3 7 to 3 8 ga isua greenstone belt west greenland are characterized by well preserved rims and concentric core structures the pillow rims and cores have different mineral assemblages and chemical and isotopic compositions the rims have systematically higher contents of fe2o3 mgo mno k2o rb ba ga y and transition metals than the cores in contrast the cores possess higher concentrations of sio2 na2o p2o5 sr pb u nb and the light rare earth elements rees than the rims these compositional variations in the rims and cores are likely to reflect the mobility of these elements during posteruption alteration variations of many major and trace element concentrations between the rims and cores of the isua pillow basalts are comparable to those of modern pillow basalts undergoing seafloor hydrothermal alteration al2o3 tio2 th zr and the heavy rees display similar values in both rims and cores suggesting that these elements were relatively immobile during postemplacement alteration in addition the rims and cores have distinctive sm nd and rb sr isotopic compositions in that the rims are characterized by higher 143nd 144nd and 87sr 86sr ratios than the cores the pillow basalts yield 2569 170 ma and 1604 170 ma errorchron ages on 143nd 144nd vs 147sm 144nd and 87sr 86sr vs 87rb 86sr diagrams respectively the sm nd errorchron age may correspond within errors to a late archean tectonothermal metamorphic event recorded in the region the sm nd errorchron may have resulted from a combination of isotopic homogenization and preferential loss of nd relative to sm during late archean metamorphism although the rb sr errorchron age overlaps with the timing of an early to mid proterozoic tectonothermal metamorphic event recorded in the region because of a considerably large mean square of weighted deviates value and scatter in 86sr 87sr and 87rb 86sr ratios this age may not have a precise geological significance the 1 6 ga rb sr errorchron is likely to have resulted from the loss of radiogenic 87sr collectively the sm nd and rb sr data obtained from the 3 7 3 8 ga isua pillow basalt rims and cores are consistent with disturbances of the sm nd and rb sr systems by tectonothermal metamorphic events long after their eruption in contrast to the sm nd and rb sr systems the lu hf system appears to be largely undisturbed by metamorphism five core samples and three rim samples yield a 3935 350 ma age within error of the approximate age of eruption 3 7 to 3 8 ga two rim samples that have gained lu give an age of 1707 140 ma within error of the rb sr errorchron age initial 176hf 177hf ratios of the undisturbed samples at 3 75 ga lie within 1 e unit of the chondritic value suggesting no long term depletion in the mantle source of the basalts

the geometry kinematics and rates of deformation within an en echelon normal fault segment boundary central italy abstract the geometry kinematics and rates of deformation have been investigated within an en echelon fault segment boundary between two left stepping major normal faults in the central apennines italy examination of faulted post glacial sediments and geomorphic features attributed to glacial retreat 18 ka reveals that the two major faults dominate the recent slip with rates that are 5 10 times greater than the other faults at the centres of the major faults slip is parallel to the regional extension direction ne sw defined by focal mechanisms and borehole breakouts slip vector azimuths defined by striations on faults in the stepover zone indicate ne sw extension on nw se faults sub parallel to the regional extension direction together with significant and contemporaneous nw se extension along the strike of the apennines on ese wnw and ene wsw faults thus distributed three dimensional strain occurs in the stepover between the two major bounding faults extension rates summed across the stepover are 0 68 mm y parallel to the regional extension compared to 2 mm y at the centre of one of the major faults extension rates along strike of the fault zone are 0 14 mm y within the stepover and zero at the centres of the two major bounding faults the above information is used to discuss how extension is partitioned between different structures during the growth and linkage of normal faults

coseismic and postseismic subsurface displacements and strains for a dip slip normal fault in a three layer elastic gravitational medium a three layer elastic gravitational fault displacement model has been developed using elastic dislocation and employed to examine the effects of rigidity layering gravity and stress relaxation on surface and subsurface displacements fields for a dip slip normal fault within our three layer model layer 1 represents the upper crust layer 2 the lower crust and layer 3 the upper mantle the fault is embedded in the upper crust horizontal as well as vertical displacement components have been determined displacement field changes due to postseismic stress relaxation have been calculated using both the correspondence principle and relaxed rigidity moduli and results are in close agreement the relaxed rigidity method provides an accurate and computationally efficient method of examining postseismic relaxation postseismic relaxation within the lower two layers only gives surface uplift increasing footwall uplift and decreasing hangingwall subsidence and also increases the wavelength of surface vertical displacement postseismic stress relaxation within all three layers i e uniform half space produces a shorter wavelength of surface vertical deformation with respect to the coseismic response moho topography created during coseismic deformation is initially amplified during postseismic relaxation the relaxed moho topography is dependent on the strength of the upper crust as well as the strength of the lower crust and mantle gravity has a significant influence on displacements only at the postseismic stage when the effective rigidity of the lower layers is small coseismic and postseismic normal strains associated with dip slip normal faulting have been examined for a large normal basement fault intersecting the free surface the coseismic horizontal surface strain perpendicular to fault strike is compressive adjacent to the fault and in the footwall and tensile in the hangingwall away from the fault coseismic stress redistribution may generate significant tensile brittle failure of the upper crust adjacent to large basement faults

controversial pb pb and sm nd isotope results in the early archean isua west greenland oxide iron formation preservation of primary signatures versus secondary disturbances pb stepwise leaching pbsl determinations on two magnetite enriched fractions of a bif sample from the northeastern part of the isua supracrustal belt west greenland yield an isochron of 3691 22 ma mswd 0 4 in combination with previously published geochronological constraints for a minimum deposition age of 3 71 ga for volcanic sequences nutman et al 1997 comprising the studied bifs and supported by microtextural observations this demonstrates an early archean amphibolite facies metamorphic event in the supracrustal pbsl data on magnetite from slightly discordant veins within the same iron stone together with bulk pb isotope values of paragenetically late pyrite yield a 3 63 0 07 ga mswd 8 6 errorchron with significantly different pb isotopic compositions from those obtained from the main magnetite layers and this suggests that the discordant layers of magnetite and sulfides crystallized when externally derived fluids passed through the formation a few tens of million of years later the fluids controlling this redistribution of magnetite can be tentatively correlated with metasomatic alteration features produced during the widespread intrusion of 3 6 ga granitic into 3 7 ga tonalitic amitsoq gneisses enclosing the belt tremolite rich layers in the bif are characterized by sm nd ratios close to chondritic end 3800 of 1 7 and 2 1 within error of published results from adjacent basic units in the supracrustal suite in contrast the magnetite dominated layers yield unrealistically high end 3800 of 14 8 and 14 4 indicative of sm nd ratios resembling ree fractionated continental sources these high end 3800 together with radiogenic sr leached from the magnetite enriched separates is ascribed to secondary hydroxyapatite which predominantly forms as crystal overgrowths in the magnetite rich bands the timing of the hydrothermal event during which apatite was deposited within the bif remains uncertain but a tchur model age of 1 85 ga from the apatite dominated hcl leachate may point to a close genetic relationship with local proterozoic metamorphism and granite formation by crustal remelting in the isukasia area step leaching of magnetite from a similar silicate oxide facies bif from the western part of the isua supracrustal belt yield a pb pb isochron age of 2 84 0 05 ga mswd 1 43 the agreement between the pbsl age and previously published field and isotopic evidence for a major late archean metamorphic event affecting the western area suggests there was widespread equilibration accompanying the intrusion of the 2 75 2 83 ga granodioritic ikkattoq gneisses west and southwest of the western limb of the isua supracrustal belt we argue that the pbsl isochron ages date the peak of amphibolite facies metamorphic events which respectively affected the eastern and western sections of the isua supracrustal belt during the early and late archean our results give additional support to the suggestion that the scatter on published end t values from the isua supracrustal belt and adjoining gneisses can be assigned to post formational hydrothermal processes and underline the need for care in the interpretation of sm nd bulk data from polymetamorphic rocks to constrain isotopic models of early earth s evolution

recognizable primary volcanic and sedimentary features in a low strain domain of the highly deformed oldest known 3 7 3 8 gyr greenstone belt isua west greenland a low strain domain has been identified in the metamorphosed mostly highly deformed volcanic and sedimentary rocks of the early archaean isua supracrustal belt this domain contains well preserved volcanic and sedimentary features including basaltic pillow lavas pillow breccia heterogeneous volcanic breccia amygdules in metabasalt and polymict conglomerate dominated by recrystallized chert and volcanic clasts the low strain domain is bounded by highly deformed rocks mostly derived from basalt chert and banded iron formation these discoveries demonstrate that some primary features have escaped the pervasive metasomatism dominant in other parts of the belt and furthermore strengthen the characterization of the isua supracrustals as a typical greenstone belt

geomorphic significance of postglacial bedrock scarps on normal fault footwalls the existence of well preserved holocene bedrock fault scarps along active normal faults in the mediterranean region and elsewhere suggests a dramatic reduction in rates of rock weathering and erosion that correlates with the transition from glacial to interglacial climate we test and quantify this interpretation using a case study in the italian central apennines holocene rates are derived from measurements of weathering pit depth along the magnola scarp where previous cosmogenic cl 36 analyses constrain exposure history to estimate the average hillslope erosion rate over similar to 10 5 years we introduce a simple geometric model of normal fault footwall slope evolution the model predicts that the gradient of a weathering limited footwall hillslope is set by fault dip angle and the ratio of slip rate to erosion rate if either slip or erosion rate is known the other can be derived applying this model to the magnola fault yields an estimated average weathering rate on the order of 0 2 0 4 mm yr more than 10 times higher than either the holocene scarp weathering rate or modern regional limestone weathering rates a numerical model of footwall growth and erosion in which erosion rate tracks the oxygen isotope curve reproduces the main features of hillslope and scarp morphology and suggests that the hillslope erosion rate has varied by about a factor of 30 over the past one to two glacial cycles we conclude that preservation of carbonate fault scarps reflects strong climatic control on rock breakdown by frost cracking

trench investigations of the 1915 fucino earthquake fault scarps abruzzo central italy geological evidence of large historical events earthquake hazard analyses in italy are mainly based on historical seismic catalogs however the completeness of the italian catalog which spans more than 2000 years progressivcly decreases going back in time in the fucino basin historical research has not produced evidence of large earthquakes before the january 13 1915 event m s 7 0 33 000 people killed this event generated surface faulting along at least two parallel quaternary normal faults bounding the eastern side of the basin the celano gioia and parasano cerchio faults over an end to end length of about 23 km with a downthrow of 30 90 cm toward southwest to improve geological aspects of the seismic history of this area we trenched onc of the coseismic scarps along the celano gioia fault at a trench site near the village of san benedetto de marsi the fault traverses the area formerly covered by the fucino lake drained for farming in 1875 a d the evidence collected in the trenchcs suggests at least two large events in historical times before 1915 radiocarbon dates constrain the age of event b between the 6th and the 9th centuries a d possibly coinciding with the cvent felt in rome in 801 a d molin arid guidoboni 1989 event a probably occurred after the 10th ccntury a d and almost certainly before the 1349 a d earthquake because historical studies show 1 that event a was not produced during the earthquake sequence that occurred in central italy in 1349 a d molin and guidoboni 1989 and 2 that the post 1349 seismic catalog can be regarded as complete for surface faulting earthquakes surface displacements determined for events a and b compared to the 1915 surface faulting indicate that m 6 5 earthquakes were associated with each rupturc these obscrvations suggest that the return period for large earthquakes in the fucino basin is shorter than that indicated by the historical record alone the fucino example clearly shows that thc cxceptionally long historical record in italy is still inadequate for a comprehcnsive seismic hazard characterization

non biogenic graphite in 3 8 ga metamorphic rocks from the isua district greenland abstract a 3 8 ga old metamorphic rock that contains up to 5 5 wt reduced carbon as graphite was found in the isua district greenland this rock exhibits a strong schistosity comprises mostly talc contains high mgo 28 0 wt ni 553 ppm and cr 267 ppm and low al2o3 2 13 wt and bears a high fe 2 o 3 feo ratio 0 94 the original rock was probably a submarine ultramafic tuff the carbon content and the 13c value of the rock vary on a microscopic scale a few mm3 scale from 0 7 to 5 5 wt and from 15 to 12 respectively the second rock a quartzite contains 0 13 wt c as reduced carbon with a 13c value of 11 25 these 13c values fall within the range 27 to 10 reported by previous investigators for graphite from the isua district the 13c values of the isua graphite have been interpreted by previous investigators e g schidlowski and aharon 1992 as metamorphic modification of organic matter with initial 13c values of 30 an important implication of this model has been that the 3 8 ga ocean already supported some biological activity however an examination of the carbon content vs 13c relationship of the graphite bearing rocks from isua has revealed that the 13c value generally increases with increasing carbon contents the rocks with 13c values less than 20 contain extremely small amounts of carbon the mineralogical and elemental characteristics of the rocks of this study a thermodynamic consideration of the temperature and composition of metamorphic fluids and carbon isotope mass calculations suggest that the major component of graphite with 13c values around 12 was formed by an inorganic rather than by a biological process the graphite appeared to have formed at temperatures between 700 and 400 c by one or both of the following two processes 1 reactions between co2 and ch4 co2 ch4 2c 2h2o during cooling of fluids with a co 2 ch 4 molar ratio of 1 and or 2 reactions between the feo component in the ultramafic rock with co2 rich fluids 4 feo co2 2 fe2o3 c the 13c c values for the fluids for both i and 2 were probably between 12 and 5 suggesting that the carbon bearing fluids could have been derived from the mantle

influence of strike slip fault segmentation on drainage evolution and topography a case study the palomares fault zone southeastern betics spain abstract the palomares fault zone pfz is one of the main sinistral strike slip faults in the betics se spain with approximately 16 km of north south displacement the pfz initiated during the tortonian messinian as a transfer structure linking areas subject to nw se shortening during the plio quaternary the fault zone lengthened cutting previous fold structures and widened towards the east displacing the active mountain front to the western border of sierra almagrera sierra almenara these sierras which show moderate uplift rates of 0 05 0 15 m ka formed in response to the oblique slip regime of the pfz the drainage system in the vicinity of the pfz is asymmetric with respect to a main axial valley that runs parallel to the pfz on the downthrown fault block in this block the drainage density is lower and the streams are longer than in the uplifted block furthermore west of the main axial valley the streams describe eastward directed deflections indicating the progressive eastward migration of the main axial valley during the pleistocene the drainage system on the uplifted ranges shows a consequent pattern indicating recent uplift and folding under nnw sse convergence recent activity along segments of the pfz has increased topographical gradients favouring dissection of previous streams by headward erosion of streams transverse to the active fault segments

fault behavior and characteristic earthquakes examples from the wasatch and san andreas fault zones paleoseismological data for the wasatch and san andreas fault zones have led to the formulation of the characteristic earthquake model which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum analysis of scarp derived colluvium in trench exposures across the wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes at all of the sites studied the displacement per event has been consistently large measured values range from 1 6 to 2 6 m and the average is about 2 m on the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry six major segments are recognized along the wasatch fault on the basis of the most likely number of surface faulting events 18 that have occurred on segments of the wasatch fault zone during the past 8000 years an average recurrence interval of 400 666 years with a preferred average of 444 years is calculated for the entire zone geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south central segment of the san andreas fault suggest that the m 8 1857 earthquake is a characteristic earthquake for this segment comparisons of earthquake recurrence relationships on both the wasatch and san andreas faults based on historical seismicity data and geologic data show that a linear constant b value extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes the characteristic earthquake appears to be a fundamental aspect of the behavior of the wasatch and san andreas faults and may apply to many other faults as well

geometric pattern rupture termination and fault segmentation of the dixie valley pleasant valley active normal fault system nevada u s a abstract active fault systems of both interplate and intraplate settings clearly show patterns of segmentation fault segmentation therefore is very important for understanding fault behavior and assessing seismic hazard the complexity of geometric patterns along fault systems has a strong influence on the propagation of earthquake ruptures because it can form structural heterogeneities that tend to inhibit rupture propagation this implies that there should be relations between fault segmentation and the complexity of fault geometry the active normal faults of the dixie valley pleasant valley system provide an excellent example to study such relations and to establish the geometric method of normal fault segmentation in the basin and range province studies of fault segmentation in the dixie valley pleasant valley fault system indicate that there are changes of both geometric pattern and geomorphic character in the area of the segment boundary the characteristics of segment boundaries on normal faults of the dixie valley pleasant valley system can be explained in terms of the nature of rupture propagation and termination these changes may be helpful criteria for distinguishing normal fault segmentation at least in the basin and range province

a healing reloading feedback control on the growth rate of seismogenic faults spatial and temporal variations in the growth rates of faults are explained in terms of a stress feedback mechanism operating in the seismogenic upper crust it is based on the idea that seismic rupture of a fault perturbs the surrounding stress field advancing the occurrence of future earthquakes on some faults that are optimally oriented while relaxing stress levels on others if post slip healing is geologically rapid then the earthquakes that are thus induced will contribute to reloading along the earlier rupture zone because of the symmetry of the optimal geometry a positive feedback is set up so that even in areas that are undergoing uniform tectonic straining some faults develop higher displacement rates and grow more rapidly while others experience reduced rates or become inactive using a thin plate elastic model for lithospheric scale faulting it is shown that this healing reloading feedback mechanism drives rapid localisation and the formation of major through going faults moving at plate boundary velocities enhanced displacement rates compared to an isolated fault develop shortly after the onset of deformation along those faults which are optimally positioned in the overall fault population thus the formation of a new plate boundary fault zone is predetermined and is a consequence of rather than the precursor of preferentially high displacement rates also fault segments located at points of rupture symmetry e g the central portion of a fault zone are reloaded more frequently and develop higher displacement rates and consequently have longer segment lengths and or larger displacement to length ratios episodic fault movement through time is a general feature of the model these predictions are consistent with available field observations over a wide range of scales thus elastic brittle failure and healing appear to be important rheological components of the lithosphere on long time scales 104 106 y as well as on the time scale of earthquake recurrence

protoliths of the 3 8 3 7 ga isua greenstone belt west greenland abstract the isua greenstone belt fig 1 contains the oldest known relatively well preserved metavolcanic and metasedimentary rocks on earth the rocks are all deformed and many were substantially altered by metasomatism but both the deformation and metasomatism were heterogeneous transitional stages can be seen from relatively well preserved primary volcanic and sedimentary structures to schists in which all primary features have been obliterated likewise different kinds and different episodes of metasomatic alteration can be seen that produced a diversity of different compositions and metamorphic mineral assemblages from similar protoliths new geological mapping has traced out gradations between the best preserved protoliths and their diverse deformed and metasomatised equivalents by this means the primary nature of the schists that make up most of the isua greenstone belt was reinterpreted and a new map that better portrays the primary nature of the rocks has been produced the previously mapped stratigraphy was found to be of little value in understanding the geology stratigraphic units were defined by different and diverse criteria such as current composition structure metamorphic texture and inferred protoliths much of this stratigraphy represents a misinterpretation of the primary nature of the rocks the new work indicates that most of the isua greenstone belt consists of fault bounded rock packages mainly derived from basaltic and high mg basaltic pillow lava and pillow lava breccia chert bif and a minor component of clastic sedimentary rocks derived from chert and basaltic volcanic rocks a previously mapped extensive unit of felsic volcanic rocks was found to be derived from metasomatised basaltic pillow lava and pillow breccia intruded by numerous sheets of tonalite

inversion of source parameters from near and far field observations an application to the 1915 fucino earthquake central apennines italy we use a nonlinear inversion approach for determining source parameters of the fucino m s 6 9 earthquake that occurred along central apennines italy in 1915 taking into account both near field surface deformations and far field first motion polarities this event was one of the most destructive in italy this century causing over 30 000 casualties altitude and leveling measurements were performed before 1862 and after 1917 the event along a circular route consisting of 18 benchmarks polarities of first arrivals do not constrain a unique fault plane solution while the one obtained by inverting geodetic data with relevant constraints provided by neotectonic data disagrees with p wave polarities our joint inversion based on a robust fitting technique has resulted in only one well defined solution satisfying geophysical data as well as surface faulting distribution and paleoseismological information

overlapping faults intrabasin highs and the growth of normal faults normal fault systems bounding extensional basins are typically adjoined by a series of subbasins separated by intrabasin highs the strata within these basins form syndepositional anticlines and synclines whose axes are transverse to the strike of the main bounding fault one possible explanation for these intrabasin highs is that they result from persistent along strike deficits in fault displacement such deficits are incompatible with scaling relationships observed between fault displacement and length based on large populations of faults we present data from active normal faults within the basin and range province and from inactive normal faults of the newark basin of eastern north america demonstrating a clear correlation between the along strike position of overlapping splay faults and the location of intrabasin highs as well as syndepositional transverse folds summed displacements for all faults within an intrabasin high are comparable to the displacements on faults bounding flanking subbasins o

displacement rates of normal faults previous estimates of displacement rates on individual faults have been limited to neotectonic faults and averaged over time intervals of about 200 kyr or less1 2 3 4 5 these estimates have been highly variable which has led to a belief that longer term displacement rates on individual faults are likely to be variable as well here we report estimates of long term normal fault displacement rates averaged over time intervals ranging from 1 to 40 myr and based on observed decreases in displacement of progressively younger horizons intersected by syn sedimentary faults we find that displacement rates are remarkably stable over these longer time periods and within a given fault system the rates are strongly dependent on the relative size of the fault as measured by cumulative vertical displacement taken together these results indicate that faults become large relative to nearby faults by having higher displacement rates even when small rather than as a consequence of having been active for longer our analyses also show that high regional strain rates tend to be accommodated by high fault displacement rates rather than high fault densities

carbon isotopes as biogeochemical recorders of life over 3 8 ga of earth history evolution of a concept ever since pioneering studies in the late 1930s had shown that the conversion of inorganic carbon into biogenic substances entails sizeable redistributions of the stable carbon isotopes biologically mediated 13c 12c fractionations have come to be recognized as a common corollary of biochemical reactions meanwhile it is firmly established that the universal bias in favour of 12c characterizing biological materials primarily derives from a kinetic isotope effect that is imposed on the first carbon fixing enzymatic carboxylation reaction in the primary metabolism of co2 fixing autotrophic organisms this preference for 12c has turned out to be one of the most enduring relics of the ordered state of the biological precursor substances that may be preserved in fossil organics over billions of years with the currently known sedimentary record at hand it can be stated with confidence that biological carbon isotope fractionations have persisted throughout 3 8 ga of recorded earth history indicating that microbial prokaryotic and archaeoprokaryotic ecosystems had been prolific already on the archean earth while for the time span 3 5 ga the isotopic evidence is unequivocal the information encoded in the preceding record is commonly blurred by a metamorphic overprint this holds particularly for the metasediments of the 3 8 ga old isua supracrustal belt of west greenland which apart from widespread metasomatism have suffered amphibolite grade metamorphism it is known that 13c 12c exchange occurs in organic kerogenous rock constituents during both amphibolite and granulite facies metamorphism if a second carbon partner is available as either fluids or carbonate with isotopic re equilibration often only partially achieved due to the sluggish kinetics of the exchange reaction thermodynamic equilibria predict however that 13c 12c ratios in kerogen and graphite increase during this process hence high t exchange equilibria are always bound to drive 13c values in positive direction the lowermost values encountered being consequently the least exchanged and most pristine with the lowest values of reduced graphitic carbon obtained in early isua studies falling into the range 22 to 28 pdb we had straightforward evidence since the late 1970s that carbon constituents with the isotopic composition of biogenic matter were indeed present in the pre metamorphic isua suite it was therefore by no means surprising that the results of recent isotope work performed on apatite hosted carbonaceous microdomains in isua banded iron formation utilizing advanced techniques of instrumental microanalysis had prompted similar conclusions hence the mainstream of the sedimentary carbon isotope record can be best interpreted as the geochemical manifestation of the isotope discriminating properties of the principal co2 fixing reactions s in biological carbon assimilation suggesting an extreme degree of evolutionary conservatism in the biochemistry of autotrophic carbon fixation as a consequence biological modulation of the geochemical carbon cycle had been established at least 3 8 ga ago having been fully operative by the time of formation of the earth s oldest sediments

paleoseismicity of the ovindoli pezza fault central apennines italy a history including a large previously unrecorded earthquake in the middle ages 860 1300 a d geomorphic and trenching investigations along the ovindoli pezza fault show that this normal fault is one of the major seismogenic faults in the central apennines we found clear geological evidence for three holocene surface faulting earthquakes on this fault the most recent earthquake occurred in the middle ages between 860 and 1300 a d the penultimate occurred about 1900 b c or shortly after and the oldest probably occurred between 3300 and 5000 b c although the most recent surface faulting earthquake occurred during historical time no evidence for it have been found in the historical record slip per event ranges between 2 and 3 m and the length of the rupture is at least 12 20 km suggesting m 6 5 7 0 for the paleoearthquakes the dip slip rate determined from trenching is 0 7 1 2 mm yr and is consistent with the long term slip rate of 0 9 2 5 mm yr lower values preferred obtained from displaced geomorphic features trench data combined with long term slip rate estimates suggest the recurrence interval is longer than a millennium and possibly as long as 3300 years the time elapsed since the most recent earthquake is 700 1130 years the seismic behavior of the ovindoli pezza fault is consistent with other well known seismogenic faults of the central and southern apennines the lack of mention or mislocation of the most recent event on the ovindoli pezza fault in the historical record of the past two millennia should be attributed mainly to the unsettled cultural and social conditions and poor economic state that characterized the middle ages especially in thinly populated regions such as the central apennines this example highlights an intrinsic limit of the historical data and raises the possibility that other regions considered seismically quiet on the basis of a long historical record may in reality have had large earthquakes that were not recorded

geometry kinematics and rates of deformation in a normal fault segment boundary central greece the geometry kinematics and rates of deformation within a fault segment boundary between the ends of two major active normal fault segments have been investigated through examination of a faulted 126 ka marine terrace slip vector azimuths defined by striations on the faults indicate n s extension on c e w faults sub parallel to those from earthquake focal mechanisms together with significant and contemporaneous e w extension on c n s faults summed rates of e w extension along a c 550 m transect 0 17 mm yr are comparable with those for n s extension 0 20 mm yr along a c 350 m transect our observations show that distributed non plane strain extension occurs in fault segment boundaries and this should be noted when studying fault tip fracture toughness and regional deformation rates