the leka opholite complex central norwegian caledonides field characteristics and geotectonic significance the leka ophiolite complex loc of lower ordovician age u pb zircon age of 497 2 ma contains the following components 1 a lower strongly deformed harzburgite unit representing the depleted upper mantle this unit becomes progressively richer in dunite towards 2 a sequence of ultramafic cumulates unconformably overlying the harzburgite tectonite the crystallization sequence in the ultramafic cumulates is olivine chromite clinopyroxene orthopyroxene in the south western part of the ultramafic cumulate sequence layers of metagabbro appear interlayered with banded dunite wehrlite the above mentioned rock sequences may be in tectonic contact with 3 layered metagabbros which give way upwards to laminated and vari textured metagabbro within the laminated vari textured metagabbro is the first appearance of 4 metabasalt dykes which increase upwards in abundance from scattered single dykes to a 100 sheeted dyke swarm associated with the vari textured metagabbro and metabasalt dykes are minor acid intrusions within the upper part of the dyke swarm is the first appearance of 5 pillow lavas which are associated with thick volcaniclastic metasediments at apparently higher stratigraphic levels geochemically the metabasalt dykes and associated pillow lavas range from predominant island arc tholeiite iat to morb character and typical metaboninites are also present this range of geochemical composition as well as the crystallization sequence of the cumulates strongly suggests that the first stages in the formation of the loc was by spreading above a subduction zone the pillow lavas associated with volcaniclastic rocks are of morb or wpb type and these probably represent the later spreading stages of the loc in a back arc basin but at sufficient distance to be unaffected by the geochemical fingerprints of the subduction process

localized slip controlled by dehydration embrittlement of partly serpentinized dunites leka ophiolite complex norway abstract dehydration of partly or completely serpentinized ultramafic rocks can increase the pore fluid pressure and induce brittle failure a process referred to as dehydration embrittlement however the extents of strain localization and unstable frictional sliding during deserpentinization are still under debate in the layered ultramafic sections of the leka ophiolite complex in the central norwegian caledonides prograde metamorphism of serpentinite veins led to local fluid production and to the growth of mg rich and coarse grained olivine with abundant magnetite inclusions and 18o values 1 0 1 5 below the host rock embrittlement associated with the dehydration caused faulting along highly localized

residence of ree y th and u in granites and crustal protoliths implications for the chemistry of crustal melts a systematic study with laser ablation icp ms scanning electron microscopy and electron microprobe revealed that 70 95 wt of ree except eu y th and u in granite rocks and crustal protoliths reside within reeythu rich accessories whose nature composition and associations change with the rock aluminosity the accessory assemblage of peraluminous granites migmatites and high grade rocks is composed of monazite xenotime in low ca varieties apatite zircon thorthosilicate uraninite and betafite pyrochlore metaluminous granites have allanite sphene apatite zircon monazite and thorthosilicaie peralkaline granites have aeschinite fergusonite samarskite bastnaesite fluocerite allanite sphene zircon monazite xenotime and th orthosilicate granulite grade garnets are enriched in nd and sm by no less than one order of magnitude with respect to amphibolite grade garnets granulitegrade feldspars are also enriched in lree with respect to amphibolite grade feldspars accessories cause non henrian behaviour of ree y th and u during melt solid partitioning because elevated fractions of monazite xenotime and zircon in common migmatites are included within major minerals their behaviour during anatexis is controlled by that of their host settling curves calculated for a convecting magma show that accessories are too small to settle appreciably being separated from the melt as inclusions within larger minerals biotite has the greatest tendency to include accessories thereby indirectly controlling the geochemistry of ree y th and u we conclude that ree y th and u are unsuitable for petrogenetical modelling of granitoids through equilibrium based trace element fractionation equations

pull apart formation and strike slip partitioning in an obliquely divergent setting leka ophiolite norway abstract the leka ophiolite complex loc is located on the island of leka norway and belongs to the uppermost allochthon of the scandinavian caledonides the rocks of the adjacent mainland and most of the surrounding islands are basement gneisses and supracrustal rocks not related to the ophiolite complex paleostress analysis gravity inversion and regional geology support a fault bounded rhombochasm geometry for the loc the paleostress inversions revealed two types of tensors interpreted as small strains 1 horizontal extension generally e w to ne sw and 2 horizontal extension in the same direction with an added component of perpendicular horizontal contraction a strong positive gravity anomaly 25 mgal is centered on leka and gravity inversion indicates that the loc lies directly below its surface exposures with steep sided walls and a flat bottom located at 7 km depth the faults bounding the loc probably initiated during postorogenic extension in the scandinavian caledonides the faults are regional in scale and are parallel to other ne sw trending en echelon faults along the norwegian coastline and on the adjacent mainland a pull apart structure explains the down dropping and subsequent preservation of the loc as it is surrounded by rocks from lower structural positions within the nappe stack the paleostress directions from leka support a sinistral component of shear along these faults the gravity inversion is consistent with a fault bounded geometry this pull apart structure as uniquely recorded by the dense ophiolitic rocks suggests that strike slip partitioning was active in an obliquely divergent setting

southern san andreas fault seismicity is consistent with the gutenberg richter magnitude frequency distribution the magnitudes of any collection of earthquakes nucleating in a region are generally observed to follow the gutenberg richter gr distribution on some major faults however paleoseismic rates are higher than a gr extrapolation from the modern rate of small earthquakes would predict this along with other observations led to the formulation of the characteristic earthquake hypothesis which holds that the rate of small to moderate earthquakes is permanently low on large faults relative to the large earthquake rate wesnousky et al 1983 schwartz and coppersmith 1984 we examine the rate difference between recent small to moderate earthquakes on the southern san andreas fault ssaf and the paleoseismic record hypothesizing that the discrepancy can be explained as a rate change in time rather than a deviation from gr statistics we find that with reasonable assumptions the rate changes necessary to bring the small and large earthquake rates into alignment agree with the size of rate changes seen in epidemic type aftershock sequence modeling where aftershock triggering of large earthquakes drives strong fluctuations in the seismicity rates for earthquakes of all magnitudes the necessary rate changes are also comparable to rate changes observed for other faults worldwide these results are consistent with paleoseismic observations of temporally clustered bursts of large earthquakes on the ssaf and the absence of m 7 earthquakes on the ssaf since 1857

analysis of the completeness magnitude and seismic network coverage of japan abstract a reliable estimate of completeness magnitude m c above which all earthquakes are considered to be detected by a seismic network is vital for seismicity related studies we show a comprehensive analysis of m c in japan we use the catalog maintained by the japan meteorological agency jma and also available information on seismic stations that report to jma for computing m c we adopt a commonly used method based on the gutenberg richter frequency magnitude law presently m c 1 0 might be typical in the mainland but to have a complete catalog one needs to use earthquakes with magnitudes of 1 9 or larger comparison with the southern california seismic network scsn suggests that the recent event detectability in the mainland generally shows similar completeness levels to that in the authoritative region of scsn we argue that the current m c of japan is due to the success of network modernization over time particularly we show that the spatiotemporal change of m c closely matches the addition of the hi net borehole stations to enhancing seismic station density it started in october 1997 in southwestern japan continuing to northeastern japan until 2002 as suggested from this matching we confirm that m c inversely correlates with station density further we find that irrespective of the network change after 1997 this correlation is unchanged in time demonstrating that the influence on m c from factors beyond station density does not vary in time contrary to alaska and california wiemer and wyss 2000 our results do not attribute such factors simply to anthropogenic noise because this is due to the borehole stations that reduce ambient noise we conclude that in japan the anthropogenic noise has an insignificant effect on m c

magma development of the leka ophiolite complex central norwegian caledonides abstract the magmatic products of the leka ophiolite complex of lower ordovician age indicate formation in different tectonic settings and generation from different mantle sources harzburgites of the mantle tectonite clinopyroxenes from wehrlites of the ultramafic cumulates the metabasalts of the dyke complex and earliest pillow lavas iat morb boninites all show characteristics compatible with formation above a subduction zone in an intra oceanic setting nd isotopes indicate that some of the iat and boninites may have been derived from a source contaminated by continental material and a cab source differ significantly from that of the iat and boninites the later pillow lavas of morb composition show only minor influence of subduction related processes minor or no negative ta anomalies and the supposed latest volcanic sequence of an alkaline oib type none at all the morb and oib type magmas are thought developed by spreading in a back arc setting in which the latter magma type developed in a remote position from the subduction zone

the younger dryas main line on leka norway as determined from a high resolution digital elevation model derived from airborne lidar data abstract the main line is a conspicuous late glacial erosive feature along parts of the norwegian coastline although the shoreline has been much studied the genesis of the ancient raised sea level is still somewhat unaccounted for in this study the main line on leka norway is illuminated through a high resolution 1 m pixel size digital elevation model dem derived from airborne lidar data the dem allows a detailed mapping of the shoreline s dimensions and actual distribution and hence provides an improved foundation for interpreting the pronounced features a new method for estimating cliff degradation during shoreline formation is presented the main line is found from 106 112 m asl on leka varying platform morphology across the island points to different shore eroding mechanisms the shoreline is thought to be developed through the interaction of sea ice processes and freeze thaw mechanisms where the former agent is considered to be of particular importance where the platforms are most levelled a bulge on the outermost part of the shoreline features is described for the first time and interpreted as a consequence of freeze on plucking mean platform widths range from 9 29 m while mean cliff degradation is estimated to be 140 m3 m 1 by adapting the age span of main line formation reported elsewhere maximum erosion rates over 1 ma totals to 58 km the study sheds light on the main line s potential as a reference level when assessing postglacial landscape development and highlights the significance of episodic erosional events in cold climate regimes

the gutenberg richter or characteristic earthquake distribution which is it abstract paleoearthquake and fault slip rate data are combined with the cit usgs catalog for the period 1944 to 1992 to examine the shape of the magnitude frequency distribution along the major strike slip faults of southern california the resulting distributions for the newport inglewood elsinore garlock and san andreas faults are in accord with the characteristic earthquake model of fault behavior the distribution observed along the san jacinto fault satisfies the gutenberg richter relationship if attention is limited to segments of the san jacinto that are marked by the rupture zones of large historical earthquakes or distinct steps in fault trace the observed distribution along each segment is consistent with the characteristic earthquake model the gutenberg richter distribution observed for the entirety of the san jacinto may reflect the sum of seismicity along a number of distinct fault segments each of which displays a characteristic earthquake distribution the limited period of instrumental recording is insufficient to disprove the hypothesis that all faults will display a gutenberg richter distribution when averaged over the course of a complete earthquake cycle but given that 1 the last 5 decades of seismicity are the best indicators of the expected level of small to moderate size earthquakes in the next 50 years and 2 it is generally about this period of time that is of interest in seismic hazard and engineering analysis the answer to the question posed in the title of the article at least when concerned with practical implementation of seismic hazard analysis at sites along these major faults appears to be the characteristic earthquake distribution

shisha pangma leucogranite south tibetan himalaya field relations geochemistry age origin and emplacement the shisha pangma pluton forming most of the xixabangma 8027 m massif in south tibet is one of the 20 larger leucogranite intrusives that mark the highest structural levels of the himalayan metamorphic core the pluton occurs immediately below the shisha pangma detachment a strand of the south tibetan detachment std system a low angle 30 north dipping normal fault placing paleozoic black slates atop sillimanite grade pelites and calc silicate rocks k feldspar augen gneisses containing fibrolite and sillimanite paragneisses along the footwall show strong internal s c fabrics indicative of down to the north extension the shisha pangma leucogranite is a heterogeneous polyphase intrusion with an earlier foliated biotite rich phase and a later tourmaline muscovite rich phase typically containing the assemblage kfs pl qtz ms tur gt bt sil ap the highly peraluminous granites have high 87sr 86sr ratios 0 738 0 750 typical of pelite derived anatectites nd depleted mantle model

a u pb age for the sulitjelma gabbro north norway further evidence for the development of a caledonian marginal basin in ashgill llandovery time the sulitjelma gabbro in the central norwegian caledonides has been dated with the u pb zircon sphene method to 437 2 ma geological relationships as well as trace element geochemistry and nd and sr isotopic compositions suggest that the gabbro and associated dyke swarms and pillow lava formed during the initial stages of back arc spreading the age constrains the timing of juxtaposition of an older island arc terrane and continental margin lithologies exposed within the caledonian nappes the age of the gabbro is similar to that of the solund stavfjord ophiolite in west norway and confirms that spreading related magmatism and marginal basin development took place along the margin of iapetus in late ordovician early silurian times

repeating earthquake activities associated with the philippine sea plate subduction in the kanto district central japan a new plate configuration revealed by interplate aseismic slips abstract we detect repeating earthquakes associated with the philippine sea plate subduction to reveal the plate configuration in the kanto district we find 140 repeating earthquake groups with 428 events by waveform similarity analysis most repeating earthquakes in the eastern part of the kanto district occur with a regular time interval they have thrust type focal mechanisms and are distributed near the upper surface of the philippine sea plate these observations indicate that the repeating earthquakes there occur as a repetition of ruptures on the isolated patches distributed on the plate boundary owing to the concentration of stress caused by aseismic slips in the surrounding areas this shows that the distributions of repeating earthquakes suggest the aseismic slips in the surrounding areas of small patches we determine spatial distributions of repeating earthquakes in the eastern part of the kanto district and find that they correspond to the upper boundary of the philippine sea plate that is the upper boundary of the oceanic crust layer of the philippine sea plate the plate geometry around choshi is newly constrained by repeating earthquake data and a rather flat geometry in the eastern part of the kanto district is revealed the obtained geometry suggests uplift of the philippine sea plate due to the collision with the pacific plate beneath choshi repeating earthquakes in the western part of the kanto district have extremely shorter recurrence times and their focal mechanisms are not of the thrust types these repeating earthquakes are classified as burst type activity and likely to occur on the preexistent fault planes which are distributed around the collision zone between the philippine sea plate and the inland plate the variation among the repeating earthquake activities in the kanto district indicates that regular repetition of repeating earthquakes is possible only on the plate boundary with a smooth and simple geometry

earthquake frequency distribution and the mechanics of faulting the level of intraplate seismicity in japan generally shows a positive correlation with the density of quaternary faulting in southwest japan where intraplate seismicity is concentrated on land rates of seismic moment release m 0 are similar when calculated from either the 400 year historical record of seismicity or geologically determined slip rates of quaternary faults a data set of 18 earthquakes with seismic moments m0 ranging from 0 01 to 3 1027 dyn cm shows a relationship between rupture lengthl and m0 log m0 23 5 1 94 log l when seismic moment on each quaternary fault is assumed to occur in discrete events every t m0 m 0g years where m0 is estimated for a rupture extended over the entire fault length and m 0g is proportional to the slip rate of each quaternary fault the moment frequency distribution of earthquakes log n a b log m0 predicted from the geologic record is virtually identical to that seen with the 400 year record of seismicity in contrast if it is assumed that earthquakes on each fault occur according to the gutenberg richter relation we obtain poor agreement with the observed seismicity thus while regional seismicity satisfies the relation log n a b log m0 or equivalently log n a b log m where m is magnitude it appears that seismicity on individual faults does not this further implies that the primary factor that leads to the magnitude frequency distribution in regional seismicity studies is the relative distribution of the slip rates and lengths of preexisting faults

applicability of the gutenberg richter relation for major active faults in new zealandapplicability of the gutenberg richter relation for major active faults in new zealand

rodingitization and hydration of the oceanic lithosphere as developed in the leka ophiolite north central norway abstract ophiolite complexes in mountain chains may give supplementary information on the hydration of the oceanic lithosphere to that obtained from dredged and drilled samples from the ocean floor the ultramafic mantle and the layered ultramafic to anorthositic crustal sequences of the cambrian 497 ma leka ophiolite are variably serpentinized and chloritized grossular rodingite rodingite s s has been found over a c 500 m long and tens of meters wide zone in the layered crustal section of the complex and is developed in both pyroxenites and gabbro anorthosite layers shear zones and meter wide fracture zones where the rock has developed a fracture cleavage are oriented at high angel to the layering and these zones were the main conduits for transport of fluid and solute between the various lithologies some 5 15 cm thick layers of anorthosite or leucogabbro have been rodingitized around such a fractures zone with the development of three distinct metasomatic zones along the plagioclase layer a central grossular dominated zone with clinopyroxene clinozoisite prehnite chlorite and minor titanite rodingite zone extends for up to 3 m along strike and gives way to a clinozoisite dominated zone typically 0 5 m wide with additional grossular clinopyroxene and chlorite which is followed outward by a lile enriched zone lile zone with clinozoisite phlogopite k feldspar plagioclase and preiswerkite the lile zone extends more than 3 m out from the clinozoisite dominated zone clz zone assuming constant volume the rodingite formed from the plagioclase layer by addition of 20 g of cao per 100 g of rock all na 2 o c 2 g was removed from both rodingite and clz zones ti and v increase almost 10 in the rodingite compared to its protolith k ba rb and cs are strongly enriched in the lile zone compared to the protolith and suggest interaction with sea water the lithologies alternating with the plagioclase layers clinopyroxenite wehrlite websterite and dunite display textures indicating a number of ca releasing cpx chl cpx srp cpx amph and ca consuming opx cpx2 ol cpx2 cpx1 cpx2 reactions the replacement textures are distributed around fracture and shear zones with the ca releasing reactions in the core and the ca consuming reactions in distal parts forming a metasomatic column out from the fluid pathways serpentinization and chloritization of clinopyroxene was the main ca source for the rodingitization process this first description of rodingite in a layered sequence of an ophiolite complex indicates that the hydration of the oceanic lithosphere occurred at various structural levels and was associated with ca metasomatism also in places where rodingite s s is lacking the different lithologies exchanged elements through transport on shear and fracture zones

tectono magmatic evolution of the west philippine mariana region and the origin of boninites re examination of published data on the tectomagmatic evolution of the west philippine mariana region indicate that arc magmatism and back arc extensional pulses are not synchronous but are largely asynchronous arc volcanism ceases within a few million years of the development of a back arc basin and recommences oceanward on a new arc during the final stages in the development of the back arc basin following karig s model we believe that ascent beneath the arc axial chain of a line of diapirs of morb source mantle interferes with processes of arc magma generation and arc magmatism wanes partial melting of the diapirs produces voluminous morb type tholeiites which split the arc and form the crust of a widening back arc basin we show for both the west philippine mariana region and occurrences of boninite like lavas in ophiolites that boninites appear to be erupted after arc magmatism and immediately before eruption of morb type lavas passage of a diapir of hot morb source mantle through refractory sub arc peridotite which has been enriched in lile and h2o will result in limited partial melting of the hydrous peridotite and the restricted production of boninitic magmas these are more likely to erupt modified or only slightly modified by crystal fractionation through the thin forearc crust continued ascent of the morb source mantle diapir will result in it partially melting to yield morb tholeiites which erupt shortly after and in considerably more voluminous amounts than the boninites finally we suggest that forearcs regions are likely to be incorporated into foldbelts and that boninite or low ti lava bearing ophiolites may characterize such regions

transfer of olivine crystallographic orientation through a cycle of serpentinisation and dehydration our ability to decipher the mechanisms behind metamorphic transformation processes depends in a major way on the extent to which crystallographic and microstructural information is transferred from one stage to another within the leka ophiolite complex in the central norwegian caledonides prograde olivine veins that formed by dehydration of serpentinite veins in dunites exhibit a characteristic distribution of microstructures the outer part of the veins comprises coarse grained olivine that forms an unusual brick like microstructure the inner part of the veins surrounding a central fault is composed of fine grained olivine where the fault movement included a dilational component optically clear equant olivine occurs in the centre electron backscatter diffraction mapping reveals that the vein olivine has inherited its crystallographic preferred orientation cpo from the olivine in the porphyroclastic host rock however misorientation is weaker and associated to different rotation axes we propose that prograde olivine grew epitaxially on relics of mantle olivine and thereby acquired its cpo growth towards pre existing microfractures along which serpentinisation had occurred led to straight grain boundaries and a brick like microstructure in the veins when dehydration embrittlement induced slip a strong strain localisation on discrete fault planes prevented distortion of the cpo due to cataclastic deformation grain size reduction did not significantly modify the olivine cpo this illustrates how a cpo can be preserved though an entire metamorphic cycle including hydration dehydration and deformation processes and that the cpo and the microstructures e g grain shape of one phase do not necessarily record the same event

package fluids 1 computer programs for analysis of fluid inclusion data and for modelling bulk fluid properties the computer package fluids contains five sets of computer programs written in c for the calculation of fluid properties both model fluids and data from fluid inclusions can be analysed in the programs bulk density molar volume and composition of fluid inclusions are calculated with the program bulk which uses microthermometric data micro raman spectroscopic data cation ratios and volume fraction estimations as input the program offers a broad variety of equations of state which are selected according to the fluid system or the temperature and pressure conditions of interest the isochores that correspond to the output or test v x properties of fluid inclusions are calculated by the program isoc in calculating the isochores this program takes account of the compressibility and expansion of selected host minerals the program test allows different equations of state to be compared directly with experimental data thereby facilitating selection of the most adequate model in the programs bulk and isoc loner is a group of programs that handle individual equations of state eos for all fluid systems presented in the programs bulk and isoc aqso is a group of programs that allows purely empirical modelling of electrolyte bearing aqueous solutions henry s law for dilute gas bearing aqueous solutions and osmotic coefficients for dilute electrolyte bearing aqueous solutions the programs are available on the university of leoben web site at http www unileoben ac at buero62 minpet ronald

mantle fragments in the scandinavian caledonides abstract mantle fragments of ultramafic composition are widespread in the scandinavian caledonides sc lenses and boudins of alpine type peridotites in the scandinavian caledonides represent parts of dismembered ophiolite sequences and fragments of sub continental upper mantle metaperidotites of nappes in internal positions are generally isofacial with the metamorphic envelope usually caledonian metasediments but in places also precambrian metagranitoids forming the basement cores of the nappes caledonian metamorphism strongly modified the texture and mineralogy of the peridotites and resulted in a systematic metamorphic pattern which is consistent with the pattern observed in the envelope metaperidotites of the external massifs display at least a two stage metamorphic history an early caledonian high pressure high temperature phase related to early crustal stacking and a late caledonian regional metamorphic overprint which produced a regular barrovian type metamorphic pattern of in situ metamorphism metaperidotites from nappes in intermediate positions iapetus ocean ophiolites and ultramafic rocks from island arc environments show strongly diverging histories metaperidotites from internal ophiolites oceanic ophiolites koli lack any evidence of subduction metamorphism are serpentinized to various degrees show abundant primary mantle relic mineralogies and the caledonian metamorphic overprint is low metaperidotites from external island arc ophiolites and other associations seve often show relic high pressure metamorphism related to the finnmarkian phase of the caledonian orogeny the seve metaperidotites are occasionally associated with eclogites and show a weak overprint of late caledonian regional metamorphism alpine type peridotites are absent in the foreland of the baltic shield and in the innermost nappes lofoten the metamorphic characteristics and evolution recorded by the metaperidotites in the scandinavian caledonides allow a general reconstruction of the dynamics of collision belt formation

statistics of small earthquakes and frequency of occurrence of large earthquakes along the mexican subduction zone abstract the validity of the gutenberg richter g r frequency magnitude relation over a wide range of magnitudes is usually accepted and the statistics of small earthquakes is often extrapolated to predict the frequency of occurrence of large earthquakes we take noaa and pde data from 1963 to 1981 related to the mexican subduction zone h 65 km and convert m b to m s in the range of 3 3 m b 5 7 for this conversion we use regression of m s on m b for mexican and central american events as well as an m s m b regression based on worldwide data wyss and habermann 1982 we also take into account the standard error in m s this may be important since the number of events decreases as m b increases the m s data of 18 5 yr is normalized to 75 5 yr and is compared to the statistics of observed large earthquakes m s 6 4 for the period 1906 to 1981 it is found that the g r relation in the range 4 5 m s 6 0 when extrapolated grossly underestimates the observed frequency of large earthquakes m s 6 5 for the oaxaca and jalisco regions there is strong evidence that the plate interface in the oaxaca region consists of a few 3 or 4 large fault zones each of which cyclically ruptures over its entire dimension without giving rise to smaller events in the manner implied by the g r relation the result is of profound importance in seismic risk analysis and in the understanding of fault mechanics this phenomenon has been reported for only a few regions e g turkey bath 1981 japan wesnousky et al 1983 it may have escaped detection in other regions due to a variety of causes the most important being the long recurrence periods of large earthquakes

formation of uppermost oceanic crust the uppermost portion of oceanic crust extrusive basalts underlain and fed by diabasic dikes represents the chilled roof zone or lid to mid oceanic spreading center magma chambers this rigid lid forms and evolves through a steady state process of flexure subsidence and internal rotation all occurring within a couple of kilometers of the spreading axis the lid accommodates lateral spreading with dikes emplaced along a unique axis dikes tap underlying magma to feed overlying flows basalts accumulating along the axis of the lid load the lid which responds by flexing down and away from the intrusive axis this subsidence affects lid structure in the following ways emplaced dikes are tilted away from and basalt flows are tilted toward the intrusive axis lid subsidence and section rotation extend the base of the lid more rapidly at the axis than elsewhere consequently dikes preferentially intrude at the existing axis to maintain existing axis shape and relative position through continued spreading loading and flexure of the lid plus faster accretion at the base of the lid as a result of lid rotation cause the lid to be uplifted distal to the axis this creates flanking marginal highs and generates extension structures fissuring and faulting flanking and paralleling the loading extrusive axis the crustal structure and geometry produced by this process is compatible with structure and topography observed at present day mid oceanic spreading centers and with structure observed in ophiolites particularly the bay of islands ophiolite

the nature and significance of magma chamber margins in ophiolites examples from the norwegian caledonides abstract large scale intrusive contacts with associated marginal series have been encountered within norwegian ophiolite complexes at karmoy solund and leka the contacts limit individual magma chambers and are found at different structural levels of the plutonic suites examples of magma chamber margins adjacent to interlayered ultramafic and gabbroic rocks modally layered gabbros high level gabbros and sheeted dykes are described the nature of the intrusive boundaries and the presence of partially resorbed xenoliths in the vicinity of the intrusive margins suggest that stoping and assimilation have been important mechanisms during the development of the magma chambers characteristic marginal series are developed along the intrusive boundaries the thicknesses and appearance of these series vary with depth in the complexes whereas the marginal series are well developed within the uppermost levels of the plutonic complexes exhibiting rock types such as microgabbro massive gabbro and magnetite gabbro the marginal series observed at lower levels are thinner and also devoid of chilled facies rocks and magnetite gabbros the marginal series may be subdivided into border and roof series the latter are characterized by an intimate relationship with sheeted dykes which comprise dyke swarms formed both prior to during and subsequent to crystallization of the roof series based on these relationships the dykes can be subdivided into rooted and rootless dykes a multiple magma chamber model with magma chambers migrating from a low to a high level within the oceanic crust is proposed on the basis of the observed features

terminology for layered intrusions the terminology of igneous cumulates was originally formulated on the concept that the rocks were concentrations of minerals accumulated by crystal settling it is now well established for several major intrusions that plagioclase that had been typed as cumulus should in fact have floated in its parental magmatic liquid nevertheless the terminology has been valuable in the classification of the rocks of these and other intrusions and it has been useful in certain aspects of their genetic interpretation the name cumulate is therefore redefined so that crystal settling is a possible but not essential process in the origins of the rocks to which it is applied the overall nomenclature of layered intrusions is then reviewed and definitions of names are refined and systematized to clarify their applications careful distinction is made between descriptive and genetic names a cumulate is defined as an igneous rock characterized by a framework of touching mineral crystals and grains that evidently were concentrated through fractional crystallization of their parental magmatic liquids a layer is a sheetlike unit of cumulate that is a distinctive entity in its compositional and or textural features a lamina is a thin 2 5 cm sharply defined layer igneous lamination is a general name for planar structure that is pervasive through cumulates on the scale of their grain size layering is the overall structure and fabric manifest through combinations of layers laminae and lamination the difference between layers and layering is essentially the difference between individuals and a population and the terms that describe one are not necessarily appropriate to the description of the other but the fundamental variants of all of layering lamination grading in layers and layer contacts can essentially be described by one or more of the four modifiers grain size modal textural and cryptic where the last refers to cumulus miner al compositions a system for subdividing layered intrusions is proposed in which distinction is made between general divisions ranked as series zones and subzones and prominent units that are called groups members rhythmic units and cyclic units applications to the skaergaard bushveld muskox stillwater and other intrusions are briefly described

tectonic environment and geodynamic significance of the neo cimmerian donqiao ophiolite bangong nujiang suture zone tibet the bangong nujiang suture zone is underlain by a discontinuous and roughly ew ophiolite belt around donqiao gyanco ophiolitic and metasedimentary palaeozoic series form nappes thrusted over jurassic flysch series all these series have undergone a late symmetrical folding accompanied by generally northward reverse faulting although strongly dismembered a complete ophiolitic sequence comprising harzburgites with large podiform type chromite deposits wehrlites cumulate gabbros sheeted dolerite dykes foliated garnet bearing amphibolites and pillow lavas has been found

water in granite and pegmatite forming melts article i nfo in this paper we have compiled water concentration data obtained from melt inclusion in minerals mostly quartz and topaz from granites and pegmatites a small number of data come from analysis of natural glasses about 4600 melt inclusions are from granites and about 2100 melt inclusions are from pegmatites another 4159 values are from melt inclusions and glasses of rocks with more than 66 g g sio2 are added belonging to a recent compilation the total of 10 859 melt inclusions provides a statistically significant basis from which to derive some conclusions generally the water concentration in granite melt inclusion is b10 g g with three maxima at 4 0 1 3 5 9 1 2 and 8 1 1 1 g g water for pegmatites a larger range of water contents was found from 2 to more than 50 g g water with two distinct maxima at 10 5 8 1 and 26 2 14 3 g g we show that the great spread and the generally high water content of the pegmatite forming melts as sampled by melt inclusions are the result of melt melt immiscibility pro cesses connected with the formation of conjugate melt fraction with contrasting physical and chemical char acteristics which have also a major influence on the element partitioning between such fractions in this paper we also discuss viscosity as an important transport property of natural silicate melts and we show that due to high primary water concentration in the pegmatite forming melts their viscosity is very low over a wide range of temperatures in the case of water rich peralkaline melt fractions the viscosity can decrease still further together these suggest that liquidus undercooling as the major cause of pegmatite textures is redundant furthermore we demonstrate that the effect of boron carbonates and bicarbonates on pegmatite melts these two last mostly ignored in the past have a large influence on the melt structure and they enable pegmatite formation in the absence of the usual discussed fluxing elements such as f and p

crustal deformation processes and the stability of the gutenberg richter relationship abstract global and regional surveys of earthquakes show that empirically determined b values of the gutenberg richter distribution are remarkably stable generally limited to values of 1 0 2 here i interpret observations from california new zealand and japan to suggest that the stability of the b value is a manifestation of a physical process specifically the tendency of crustal strains to organize along relatively discrete zones given a portion of the earth9s crust subject to a displacement field displacement is accommodated by a system of fault lengths that obey a power law distribution with continued displacement longer faults develop at the expense of shorter faults and take up an increasingly greater portion of the displacement budget shifts in regional displacement directions lead to reversals of these trends the changes in the ratio of long faults to short faults in a region is thus accompanied by changes in the relationship between fault length and fault slip rate because the recurrence of earthquakes on faults is a function of both fault length and fault slip rate the deformation process may result in a system of feedback that inhibits changes in the b value as fault populations change a corollary to the idea is that the magnitude frequency statistics of seismicity may be attributed to the same physical processes that are responsible for the development of plate tectonic boundaries and the gross spatial distribution of seismicity around the globe

transformation induced jointing as a gauge for interfacial slip and rock strength various theoretical and numerical models have been proposed in order to explain joint formation and spacing in layered rock series however most of these models assume that the interfaces between the rock layers are perfectly welded i e no slip occurs and that all the layers are subjected to the same remote strain due to various processes e g tectonic processes other factors may also induce extensional strain in rocks e g phase transformations however such processes may induce different amounts of strain on the layers in a rock series leading to a strain mismatch between these layers in this paper we present a 1 d finite difference linear elastic model which allows joint formation within the middle layer in a three layer rock series and is induced by a strain mismatch between the fractured central layer and the surrounding matrix furthermore the central layer in our model is not necessarily welded to the matrix layers and is allowed to slip along the interfaces between these layers if the shear strength of the material at the interface is reached we find that the final fracture spacing to layer thickness ratio s tf in such layered systems is directly proportional to the ratio of the tensile and shear strength of the material changes in the material properties such as the shear modulus or young s modulus do not affect these results a natural analog of joint formation driven by phase transformations is found in the orthopyroxenite dykes of the leka ophiolite complex loc norway joint formation in orthopyroxenite dykes results from serpentinization driven expansion of the surrounding dunite matrix detailed field studies and measurements 583 sample points yield s tf ratios between 0 1 and 1 0 with a mean value of 0 45 0 20 we demonstrate that the strain mismatch driven joint formation associated with interfacial slip explains the low s tf ratios obtained from field measurements and may also help us constrain rock strength

the caledonian heilhornet pluton north central norway geological setting radiometric age and implications for the scandinavian caledonides the heilhornet pluton is a medium sized c 50 km 2 sub circular discordant intrusion which penetrates the thrust zone separating the helgeland nappe complex from paragneisses in the vestranden region north central norway the intrusion consists of biotite hornblende granite and ranges in composition from 61 5 to 72 5 wt si0 2 the initial sr ratio c 0 7070 shows that crustal sources have been involved in the petrogenesis of the granite a u pb zircon age of 444 11 ma for the pluton indicates that juxtaposition of the nappe complex with metasedimentary rocks in vestranden took place prior to or during the late ordovician a consequence of this is that the general correlation of the metasedimentary rocks in vestranden with the seve nappes must be revised final eastward thrusting of the complex together with a yet unknown part of the vestranden supracrustal sequence probably took place during the middle silurian to early devonian scandian orogenic event

ordovician faunas island arcs and ophiolites in the scandinavian caledonides ordovician faunal data from the scandinavian caledonides is tested with new geochemical information from zircons to give u pb ages and source origins of volcanic arc and ophiolite sequences early ordovician arenig llanvirn low latitude toquima table head faunas from the upper upper allochthon are associated with an island arc system formed adjacent to laurentia contemporaneous mafic magmas were contaminated by crustal material during subduction and associated granites contain inherited zircons of archaean age the nearest source for such rocks is on the laurentian rather than the baltic side higher latitude celtic province faunas from the upper upper allochthon are from one insular site accessible to forms from both laurentia and baltica the late ordovician low latitude holorhynchus and subtropical hirnantia faunas occur in overstep sequences above deeply eroded early ordovician arc complexes the transgression appears to be coeval with a second generation of spreading related complexes single detrital zircons from sediments show sources from archaean proterozoic and early ordovician terranes this suggests that deposition was in a basin situated along the same continental margin laurentia to which the early ordovician ophiolite arc sequences had already become accreted the late ordovician faunas link both laurentia and baltica at a time of narrowing of lapetus the new geochemical data together with the faunal information is supported by recent palaeomagnetic studies

the sources of energy for crustal melting and the geochemistry of heat producing elements crustal melting to produce granite magmas requires a tremendous amount of energy in principle there are three main mechanisms of heating that can extensively melt a fertile crust radiogenic heat caused by the decay of 40k 230th 235u and 238u increased subcrustal heat flux caused by the upwelling of deeper therefore hotter mantle materials advection of heat caused by the emplacement and crystallization of hot mantle magmas two dimensional finite elements modeling reveals that a fertile crust thickened to 65 70 km would produce copious granite magmatism after 30 40 m y if its average heat production is a 1 2 w m 3 but it would scarcely melt if a 0 65 w m 3 increasing the subcrustal heat flow from the normal value of qm 0 025 w m 2 to qm 0 04 w m 2 may also lead to extensive crustal melting especially if the crust does not thin to less than 30 35 km very high qm 0 06 w m 2 affecting the continental crust is unlikely but the combination of moderately high qm 0 04 w m 2 and a thick fertile crust with a 1 2 w m 3 such as often happens in the volcanic and back arc areas of subduction zones is ideal to produce copious granite magmatism lastly the emplacement of hot mantle magmas in a fertile crust can produce crustal melts in just a few thousand years but the volume of these is equal to or less than the volume of the intruding magma a clue for understanding the relative importance of each of these three mechanisms comes from the radiogenic heat production of granite rocks calculated from the concentration of 40k 230th 235u and 238u at the time of their formation this parameter estimated on more than 3400 granites samples of different ages and provenance reveals a strongly asymmetric distribution peaking around 2 4 w m 3 a value much higher than the average continental crust about 1 1 2 w m 3 and certainly much higher than the average lower continental crust about 0 4 0 8 w m 3 only those granite rock types that are clearly connected with mantle heat sources such as the archean ttg post archean subduction related trondhjemites and recent adakites have a heat production equal to or smaller than the lower continental crust since the bulk melt solid partition coefficient of the heat producing elements hpe k th and u is k 1 the elevated hpe contents of granites indicates that most of them have been derived from hpe rich sources we conclude that radiogenic heating is often essential and always advantageous for generating large volumes of granite magmas and that granite magmatism is the main cause of the accumulation of hpe in the upper crust

analytic models for orogenic collapse abstract orogenic plateaus and collisional mountain belts are formed by tectonic driving forces such as those associated with a continental collision and magmatism when the horizontal driving forces are removed e g at the conclusion of an orogeny the elevation of the plateau or collisional orogen will relax by one or both of two processes the first of these is erosion which removes the near surface rocks and allows uplift the second is the gravitational collapse of elevated topography in this paper we consider a simplified analysis in order to quantify the relative importance of erosion and gravitational collapse in the removal of topography we treat the erosion problem using the culling diffusion equation approach the basic assumption is that the material transport down slope is proportional to the slope with the equivalent coefficient of diffusion k the constant of proportionality the parameter k quantifies the rate of erosion it is applicable whether or not the topography is compensated for the gravitational collapse problem we use the thin viscous sheet approximation and assume a linear viscous rheology to simplify the analysis we assume the thickness of the continental crust approximates the thickness of the highly viscous lithosphere and that the crust overlies a much lower viscosity mantle we also assume the topography is harmonic with a wavelength the viscosity quantifies the thinning of the crust due to the buoyancy driven lateral flow our results indicate an exponential decay of topography with time and show that the relative role of erosion versus gravitational collapse for compensated topography is controlled by the value of the non dimensional collapse number c gh o 2 c 16 k 2 where g is the acceleration due to gravity h o is a reference thickness of the crust c is the density of the crust and k and are as defined above if c is large compared with unity gravitational collapse dominates if c is small compared with unity erosion dominates

rhyacian evolution of subvolcanic and metasedimentary rocks of the southern segment of the mineiro belt sao francisco craton brazil abstract the tiradentes suite in conjunction with the coeval estacao tiradentes metasedimentary sequence makes up a distinct tectonic element within the paleoproterozoic framework of the mineiro accretionary belt in the southern sao francisco craton this suite is the second occurrence of volcanic and subvolcanic rocks in the mineiro belt and shows petrographic and geochemical similarities with the nearby serrinha suite 2 23 2 21 ga the first reported rhyacian volcanism in the mineiro accretionary belt the tiradentes suite is represented by mafic andesites dacites granophyres and tonalites and shows petrographic features of volcanic rocks such as spherulites amygdales granophyric intergrowths and porphyritic textures with plagioclase phenocrysts the rocks of this suite are metaluminous to peraluminous enriched in alkaline elements compatible with low al2o3 trondhjemites and show primary fractionation of plagioclase apatite hornblende biotite and ilmenite high hree content coupled with incipient negative eu anomaly and high y values are consistent with a garnet amphibolite protolith with plagioclase and hornblende as present in the residual phases the samples of the tiradentes suite show geochemical signatures of magmatic arcs which are also observed in rocks from the serrinha suite rocks of the tiradentes suite yield the following u pb zircon crystallization ages i mafic andesite 2217 23 ma ii dacitic dike 2204 11 ma iii subvolcanic tonalite 2213 9 ma the deposition of the estacao tiradentes metasedimentary sequence probably took place after ca 2088 12 ga although it may be as young as 1990 39 ma according to the u pb detrital zircon geochronology of selected samples diamictite ferruginous quartzite and carbonaceous phyllite the tiradentes rocks show three tdm model ages ranging between 2 4 and 2 3 ga indicating short crustal residence for the magmatic protoliths this inference is also supported by the nd 2 2 ga values 1 1 to 2 3 whereas the respective 87sr 86sr t ratios 0 7018 0 7032 are consistent with the involvement of low rb sr material in the magma genesis such as a paleoproterozoic oceanic lithosphere from a tectonic perspective the above data are consistent with the idea that a paleoproterozoic seafloor developed outboard of the archean siderian passive margin setting minas basin of the sao francisco proto craton the 2 23 2 21 ga volcanic subvolcanic rocks of the tiradentes and serrinha suites were associated with a rhyacian oceanic arc the estacao tiradentes metasedimentary sequence turbidites and distal phyllites probably formed in back arc setting concomitant with the tiradentes suite as indicated by detrital zircon geochronology

origin of pegmatites and fluids at ponta negra rj brazil during late to post collisional stages of the gondwana assembly the ponta negra pegmatites pnp part of a pegmatitic province in rio de janeiro state brazil crop out along an intensely deformed medium to high grade metamorphic area that is proximal to a crustal scale thrust zone developed during the brasiliano pan african orogeny fieldwork shows that the pegmatites formed in two distinct stages i syn collisional leucosome veins group i conformable with the tectonic foliation of the gneissic host rocks and ii late to post collisional dykes group ii that cross cut the same tectonic foliation at a high angle in this paper we use geochemistry of whole rock and mineral separates alkali feldspar and biotite fluid inclusion microthermometry and stable isotopic 18o d 13c determinations on minerals quartz alkali feldspar biotite and magnetite and fluid inclusions to provide insights into the composition of the pegmatite forming melts associated fluids and their geotectonic significance u pb shrimp dating of the caju syenogranite was performed to evaluate and compare the timing of magmatic events along the cabo frio tectonic domain as this is the closest post collisional pluton to the studied pegmatites the calculated temperature for the group i syn collisional veins 740 c is similar to previous estimates for the peak metamorphic conditions in the study area variations in the temperature of the group ii pegmatite dykes obtained from stable isotopes 380 to 720 c and microthermometric data from primary fluid inclusions with traces of n2 th 280 to 360 c may reflect the thermodynamics of the pegmatite crystallization exsolution textures and isotopic exchange the composition of fluids in equilibrium within the pegmatite dykes consists of magmatic and metamorphic components the minimum pressures calculated for the emplacement of the pegmatites are equivalent to a shallow crustal depth between 1 7 and 3 5 km which corresponds to the exhumation of the orogen since the emplacement of the pegmatites a linear trend of decreasing co2 content and 13cco2 is consistent with mixtures between i carbon derived from organic matter or volatilization of skarns and ii inorganic carbon carbonate based on the data obtained we propose that the pegmatites of ponta negra are close to an lct type geochemical signature highly peraluminous magmas with normative corundum and originated by partial melting of the metasedimentary palmital succession at depth during the waning stages of the buzios orogeny the primary melts of the pnp cross cut both the neoproterozoic supracrustals and the paleoproterozoic orthogneissic basement during its ascent and emplacement at higher crustal levels variable melt sources explain the slight differences in geochemical compositions among the studied rocks within the metasedimentary succession which probably include mn bearing exhalites as well as differentiation processes the 454 5 ma u pb zircon age of the caju syenogranite overlaps previous geochronological data of 440 11 ma obtained on a pegmatite dyke at ponta negra bracketing and extending the time interval for the gondwana assembly collapse magmatism in the region the heat that triggered this magmatic event could still be a consequence of the collisional orogeny increasing contents of heat producing elements or a large intraplate extension that followed the gondwana amalgamation and initiated the formation of paleozoic basins

what can physical source models tell us about the recurrence time of earthquakes abstract earthquake prediction no matter what the timescale has been and continues to be a contentious subject and it is indubitably a prominent challenge for modern seismology and earthquake physics indeed few natural events can have the catastrophic consequences of earthquakes earthquakes today account for about 60 of natural fatalities a physical description of an earthquake represents an amenable approach to the prediction but it suffers of some limitations basically due to the notorious ignorance about the initial state of a given fault and about the physical law controlling its traction evolution independent on those intrinsic epistemic limitations the concept of the earthquake recurrence based upon the idea of the cyclic or characteristic earthquake has been often invoked to describe and thus to predict subsequent instability events on a seismogenic structure in this paper by using the simplest analog fault model the one degree of freedom mass spring system we quantitatively show that the concepts of the recurrence time and the earthquake cycle have limitations even making them meaningless we will discuss in a compendious synopsis all the possible physical mechanisms which can dramatically affect the recurrence time our conclusions emphasize again that the competing mechanisms potentially occurring during faulting even in the simplest and idealized condition of an isolated fault can significantly complicate the regular cyclicity of earthquakes predicted by the analog fault system these conclusions can contribute to the debate about the role of the physical modeling of earthquakes in the contest of seismic hazard assessment

variability of displacement at a point implications for earthquake size distribution and rupture hazard on faults abstract to investigate the nature of earthquake magnitude distributions on faults we compare the interevent variability of surface displacement at a point on a fault from a composite global data set of paleoseismic observations with the variability expected from two prevailing magnitude frequency distributions the truncated exponential model and the characteristic earthquake model we use forward modeling to predict the coefficient of variation cv for the alternative earthquake distributions incorporating factors that would effect observations of displacement at a site the characteristic earthquake model with a characteristic magnitude range of 0 25 produces cv values consistent with the data cv 0 5 only if the variability for a given earthquake magnitude is small this condition implies that rupture patterns on a fault are stable in keeping with the concept behind the model this constraint also bears upon fault rupture hazard analysis which for lack of point specific information has used global scaling relations to infer variability in average displacement for a given size earthquake exponential distributions of earthquakes from m 5 to the maximum magnitude give rise to cv values that are significantly larger than the empirical constraint a version of the model truncated at m 7 however yields values consistent with a larger cv 0 6 determined for small displacement sites although this result allows for a difference in the magnitude distribution of smaller surface rupturing earthquakes it may reflect in part less stability in the displacement profile of smaller ruptures and or the tails of larger ruptures online material global surface displacement data and identified sources of uncertainty

low temperature alteration of serpentinized ultramafic rock and implications for microbial life abstract alteration serpentinization of ultramafic rocks is known to generate hydrogen through oxidation of ferrous iron in olivine or pyroxene and reduction of water hydrogen oxidation can act as a primary source for chemolithotrophic microbial communities and it is suggested that hydrogen availability is essential for the existence of a deep subsurface biosphere in this study we focus on ongoing low temperature water rock processes in partially serpentinized dunite from the leka ophiolite complex mid norway we aim to determine whether these reactions release reduced compounds and nutrients that can be utilized by the associated microbial communities the study is based on the chemical and physical alteration features of a 51 m deep rock core as well as the chemical composition of groundwater from the core s borehole the borehole intersects three main fracture zones observations during drilling indicated that water infiltration largely takes place along the deepest zone the rock in the fracture zones is nearly completely altered to serpentine brucite magnetite and minor calcite an increase in ph and decrease in mg si and alkalinity of the groundwater along the borehole from the deepest to the outermost fracture zone reflect ongoing water rock reactions where dissolution of brucite serpentine and calcite and precipitation of chrysotile hydromagnesite and iron hydroxide are most likely to occur detection of both h 2 and o 2 in the groundwater influenced by the outermost fracture zone suggests that the local development of highly reducing conditions in the narrow fractures is due to low water rock ratios and to dissolution of fe ii containing brucite and serpentine resulting in the formation and successive diffusion of h 2 into the oxic borehole oxidation of h 2 reduced fe and mn nh 4 and ch 4 are all potential energy yielding reactions that could be utilized by chemolithotrophic microorganisms using dissolved organic or inorganic carbon as possible carbon sources methane could result from microbial oxidation of h 2 and reduction of co 2 or from the degradation of organic matter the results demonstrate that low temperature water rock reactions in serpentinized ultramafic rock lead to production of reduced chemical species that could support primary producers in diverse microbial communities

granitic melt viscosity and dike formation melt viscosity is an important variable controlling the rates of granitic melt separation and transport from its source region viscosities of granitic melts are significantly affected by changes in temperature and volatile content addition of 2 wt h2o to anhydrous peraluminous and metaluminous granitic melts reduces viscosity by six orders of magnitude at 800 c which is equivalent to the effect of raising the temperature by 500 c addition of 10 wt more h2o only decreases melt viscosity by approximately 1 5 additional orders of magnitude at higher temperatures the effect of h2o addition on melt viscosity is less significant at lower temperatures more significant the effect of pressure on hydrous granitic melt viscosities at crustal conditions is insignificant in comparison between 0 5 and 1 5 gpa the viscosity at 1000 c increases by 0 4 log units or less this effect can be ignored in most petrological calculations using newly acquired viscosity data at low h2o contents the baker model for the calculation of metaluminous and peraluminous granitic melt viscosities has been updated and expanded the new model can predict hydrous 0 3 12 3 wt h2o granitic 69 77 wt sio2 melt viscosities between 577 and 1200 c at crustal pressures the new granitic melt viscosities have been used to investigate the possibility of granitic melt transport via diking diking can be an efficient mechanism of damp granitic melt transport through the crust for granitic melts containing a minimum of 2 wt h2o if the lower crust has been sufficiently heated modeling the thermal history of basaltic intrusions and surrounding lower crust demonstrates that a sheet like intrusion 5 km thick or a suite of intrusions with the same aggregate thickness that were rapidly emplaced can provide enough heat to allow granitic magmas to migrate through much of the crust in dikes the generation of granitic dikes associated with thinner intrusions appears unlikely

u pb ages of ophiolites and arc related plutons of the norwegian caledonides implications for the development of iapetus u pb zircon ages are reported for four ophiolites and three crosscutting arc related plutons from the norwegian caledonides plagiogranite differentiated from gabbro of the karmoy ophiolite is dated at 493 7 4 ma whereas arc related trondhjemite cutting this ophiolite crystallized at 485 2 ma a crosscutting clinopyroxene phyric gabbro intrusion is dated at 470 9 5 ma by near concordant magmatic titanite sphene and discordant u rich 2903 6677 ppm zircon lower intercepts of 247 68 and 191 70 ma defined by the plagiogranite and clinopyroxene phyric gabbro best fit lines may reflect a real low t alteration rift related event

the relationships between late ordovician sea level changes and faunal turnover in western baltica geochemical evidence of oxic and dysoxic bottom water conditions abstract the late sandbian to the early katian late ordovician in southeastern norway is dominated by marine mudstones that contain an exceptional spectrum of macrofauna and trace element geochemistry recording both abrupt and gradual faunal changes during major sea level and environmental shifts this study investigates the relationships between sea level changes and their influence on the source of clastic material oxygen levels in the bottom water and faunal changes with special emphasis on the brachiopod fauna trace element ratios indicate that nearly stable upper dysoxic bottom water conditions prevailed in the northwestern part of the baltoscandian sea the exception is during the major shallowing in the earliest katian when there was an abrupt shift to oxic conditions as the sea bottom came within a normal storm wave base nonetheless the pre shallowing epibenthic fauna though not the shelly endobenthic in this area was rich and diverse and two major immigration phases of new brachiopod taxa are seen well before the shallowing this indicates that the immigration of new taxa was not the result of an increase in oxygen content in the bottom water more brachiopod genera stayed during the abrupt shallowing and increased oxic level than did during the following gradual return to deeper dysoxic environments the major brachiopod immigration phases took place markedly earlier in this northwestern part of the baltoscandian sea than in the central part eastern baltic and possibly also the comparable faunal turnover in laurentia the following disappearance of taxa during an early katian transgressive event coincided with the faunal turnover in the shallow water environments of the east baltic the depositional history was consistent within the investigated mudstone dominated offshore facies of the oslo region which are comparable to the central baltoscandian confacies belt of baltoscandia the composition of the siliciclastic material which was deposited in the basin was nearly constant through the sequence though the land area expanded to include ophiolites expressed by cr enrichment coinciding with the major sea level drop the cr enrichment indicates that by that time at least the ophiolite complexes north of the oslo region became subaerially exposed this enrichment thus forms a potential geochemical marker horizon representing the basal katian stage

implications of fault slip rates and earthquake recurrence models to probabilistic seismic hazard estimates abstract increasingly fault slip rates are being used to constrain earthquake recurrence relationships for site specific probabilistic seismic hazard ground motion assessments this paper shows the sensitivity of seismic hazard assessments to variations in recurrence models and parameters that incorporate fault slip rates two models are considered to describe the partitioning of the slip rate or seismic moment rate into various magnitude earthquakes an exponential magnitude distribution and a characteristic earthquake distribution assuming an exponential distribution the activity rate n m 0 is constrained by the upper bound magnitude m u the b value for the region and the fault slip rate s for a given s variations in m u and b value have significant effects on recurrence and computed hazard depending on whether the assumption is made that the seismicity rate is constant or the moment rate is constant there is increasing evidence that the characteristic earthquake model is more appropriate for individual faults than the exponential magnitude distribution based on seismicity data from areas having repeated large earthquakes a generalized recurrence density function is developed and the resulting recurrence relationship requires only m u b value and s a comparison of the recurrence relationships from this model with the historical seismicity and paleoseismicity data on the wasatch and san andreas faults shows a good match the computed hazard based on the characteristic earthquake model differs from that obtained for the exponential model as a function of the fault to site distance and the acceleration level one check on the recurrence models using slip rate is to compare over large regions activity rates based on seismicity data and slip rate data such a comparison in the western transverse ranges shows a reasonable match for both the exponential and characteristic earthquake models in the moderate to large magnitude range

reaction assisted hierarchical fracturing during serpentinization abstract hierarchical fracture patterns are the result of a fracturing process that successively divides rocks into smaller domains in a 2d outcrop section such fracture patterns are characterized by four sided domains and t junctions where new fractures stop growing at right angles to pre existing fractures here we describe fracturing of mm to dm thick serpentinized orthopyroxenite dykes in a dunite matrix from the leka ophiolite complex in nord trondelag norway the fracturing process is driven by volume changes associated with the serpentinization of the dunites during this process the orthopyroxenite dykes are extended by the expanding dunite and undergo considerable fracturing the resulting fracture patterns are hierarchical the cumulative distributions of fracture lengths show a scaling behavior that lies between a log normal and power law fractal distribution this is consistent with a simple fragmentation model in which domains are divided according to a top hat distribution of new fracture positions within unfractured domains reaction assisted hierarchical fracturing is also likely to be responsible for other structures commonly observed in serpentinized ultramafic rocks including the mesh textures observed in individual olivine grains and the high abundance of rectangular domains at a wide range of scales this mechanism may provide first order controls on fluid migration rates in a wide variety of geological settings

fault trace complexity cumulative slip and the shape of the magnitude frequency distribution for strike slip faults a global survey summary we examine whether the shape of the magnitude frequency distribution for strike slip faults is described by the gutenburg richter relationship log n a bm or by the characteristic earthquake model by analysing a data set of faults from california mexico japan new zealand china and turkey for faults within regional seismic networks curves of the form log n yrpl a bm where n yr is the number of events per year equal to magnitude m are fit to the instrumental record of seismicity and geological data are used to estimate independently the size and recurrence rate of the largest expected earthquakes that would rupture the total length of the fault extrapolation of instrumentally derived curves to larger magnitudes agrees with geological estimates of the recurrence rate of the largest earthquakes for only four of the 22 faults if uncertainties in curve slope are considered and significantly underestimates the geological recurrence rates in the remaining cases also if we predict the seismicity of the faults as a function of fault length and slip rate and the predicted seismicity is distributed in accord with the gutenburg richter relationship we find the predicted recurrence rate to be greater than the observed recurrence rates of smaller earthquakes along most faults if individual fault zones satisfy the gutenburg richter relationship over the long term our observations imply that during the recurrence interval of the largest expected earthquakes the recurrence of lesser sized events is not steady but rather strongly clustered in time however if the instrumental records provide an estimate of the long term rate of small to moderate earthquakes along the faults our observations imply that the faults generally exhibit a magnitude frequency distribution consistent with the characteristic earthquake model also we observe that the geometrical complexity of strike slip faults is a decreasing function of cumulative strike slip offset the four faults we observe to be consistent with the gutenburgrichter relationship are among those characterized by the least amount of cumulative slip and greatest fault trace complexity we therefore suggest that the ratio of the recurrence rate of small to large earthquakes along a fault zone may decrease as slip accumulates and the fault becomes smoother

characteristic small earthquake sequence off sanriku northeastern honshu japan 1 we found eight similar earthquakes of m4 8 regularly occurring from 1957 to 1995 on the plate boundary off sanriku northeastern japan the mean recurrence interval of the events is 5 35 years and its standard deviation is only 0 53 years these events have the same focal mechanisms of low angle thrust fault type and their source sizes are estimated to be around 1 km the slip of each event is comparable to the cumulative amount of relative plate motion in the inter seismic period thus these events are probably caused by repeating slip at a small but strong asperity surrounded by creeping zone on the plate boundary the next event was expected to occur by the end of november 2001 with 99 probability and actually m4 7 event occurred on november 13 2001

spatial separation of large earthquakes aftershocks and background seismicity analysis of interseismic and coseismic seismicity patterns in southern california we associate waveform relocated background seismicity and aftershocks with the 3 d shapes of late quaternary fault zones in southern california major earthquakes that can slip more than several meters aftershocks and near fault background seismicity mostly rupture different surfaces within these fault zones major earthquakes rupture along the mapped traces of the late quaternary faults called the principal slip zones pszs aftershocks occur either on or in the immediate vicinity of the pszs typically within zones that are 2 km wide in contrast the near fault background seismicity is mostly accommodated on a secondary heterogeneous network of small slip surfaces and forms spatially decaying distributions extending out to distances of 10 km from the pszs we call the regions where the enhanced rate of background seismicity occurs the seismic damage zones one possible explanation for the presence of the seismic damage zones and associated seismicity is that the damage develops as faults accommodate bends and geometrical irregularities in the pszs the seismic damage zones mature and reach their finite width early in the history of a fault during the first few kilometers of cumulative offset alternatively the similarity in width of seismic damage zones suggests that most fault zones are of almost equal strength although the amount of cumulative offset varies widely it may also depend on the strength of the fault zone the time since the last major earthquake as well as other parameters in addition the seismic productivity appears to be influenced by the crustal structure and heat flow with more extensive fault networks in regions of thin crust and high heat flow

mean recharge times and chemical modelling transfers from shallow groundwater to mineralized thermal waters at montrond les bains eastern massif central france summary a re appraisal of interactions between basinal waters and hydrothermal fluids within the silici clastic series of the intracontinental cenozoic forez basin is proposed using major element and isotope geochemistry together with thermodynamic models recharge processes and shallow sedimentary units in the forez basin were compared using major elements and isotopic data 87 sr 86 sr 18 o 2 h 13 c tritium and 14 c for waters sedimentary carbonates and hercynian basement rocks waters derived from late pleistocene to early holocene surface recharge followed by mixing and chemical evolution through percolation into deep groundwater horizons are investigated isotopic ratios and chemical speciation computed for deeper waters from 0 to 200 m depth indicate that mineralised waters were produced by initial surficial dissolution of calcite in calcareous mudstone providing its sr isotopic signature at about 200 m depth na h 3 o cation exchange with clayey sand rich levels provides additional ca na and oh contents for carbonate saturated water na h 3 o cation exchange can not contribute large na hco 3 concentrations geochemical changes such as increasing 13 c dic values observed from 200 to 500 m depth were modelled by addition of thermal water and geogenic co 2 in increasing proportions with depth 500 mbs thermodynamic calculations of mineral proportions indicate water rock interactions with primary minerals in the metamorphic basement and secondary minerals observed in the basin water temperature and its meteoric origin suggest a basin wide recharge system with water rock interaction driven by heat flow rather than through deep crustal fractures

effect of seismic source model parameters on the probabilistic seismic hazard assessment results a case study for the north anatolian fault zone the inputs to the probabilistic seismic hazard analysis psha contain large uncertainties regarding the seismic source model parameters therefore results may vary significantly due to subjective judgment and interpretation of the limited data the objective of this study is to show the effect of seismic source models on the hazard results by quantifying the differences in the design ground motions for different hazard levels at different locations around the north anatolian fault analysis showed the variances in the hazard results obtained by different seismic source models are closely correlated with the location of the site and hazard level additionally sensitivity of the hazard results to the uncertainties involved in each source parameter especially the source zone boundaries annual slip rate maximum and minimum magnitudes fault width b value and scenario weights are analyzed and presented to provide insight on the relative contribution of source or fault parameters to the psha results

geochemical signature of provenance in sand size material in soils and stream sediments near the tobacco root batholith montana u s a abstract trace element geochemistry of sandstones are being used to determine provenance we have conducted preliminary and limited experiments to determine to what extent daughter sands retain the geochemical signature of parent rocks six sets of first order stream sediments soils from adjacent slopes and a variety of parent rocks were collected from southwestern montana u s a sampling in a low relief area ensured that climate and residence time of soils on slopes could be eliminated as variables sand size fractions of stream sediments and soils and the corresponding parent rocks granodiorite quartz monzonite granite gneiss biotite tonalite gneiss and amphibolite were analyzed for most major elements and selected trace elements petrologic modal analysis of the parent rocks and the 0 25 0 50 mm fraction of each sand was done to monitor major mineralogic control if any on chemical compositions of the samples our data show that the abundances of the si and al in sediments do not discriminate provenance abundances of ca mg fe and ti may broadly distinguish between sands derived from metamorphic and igneous source rocks at least in the area studied differences in abundances of the ba and th and the ratio of la lu between granitic tonalitic and amphibolitic parent rocks are preserved in the daughter sediments that we studied however the size of the eu anomaly in the ree patterns of different daughter sediments is not diagnostic of parent rocks abundances of co and sc distinguish between sediments derived from felsic and mafic rocks a better provenance discrimination is obtained if the ratios la sc th sc la co ba sc and ba co are used petrologic modal data show that mineral contents and chemical compositions of parent rocks are compatible with each other the chemical composition of the sands may be roughly correlated to the petrological modal data but the abundances of some minor and trace elements of sediments cannot be inferred from modal mineralogy this is expected because these elements may concentrate in accessory minerals and or may weather out into aqueous or clay mineral fractions it is also compatible with conclusions of previous studies that some of these elements do not reside in sand size fractions of siliciclastic sediments

characteristic earthquake model and seismicity around late quaternary active faults in japan a total of 172 late quaternary active fault zones in japan are examined to determine whether the gutenberg richter relationship or the characteristic earthquake model more adequately describes the magnitude frequency distribution during one seismic cycle by combining seismicity data for more than 100 active fault zones at various stages in their seismic cycles we reduced the short instrumental observation period compared to the average recurrence interval in only 5 of the active fault zones were the number of observed events equal to or larger than the number of events expected by the gutenberg richter relationship the average and median frequency ratios of the number of observed events to the number of expected events from the gutenberg richter relationship are only 0 33 and 0 06 respectively suggesting that the characteristic earthquake model more appropriately describes the magnitude frequency distribution along the late quaternary active faults during one seismic cycle moreover the larger an average slip rate is or the shorter an average recurrence interval the larger the gap in magnitude tends to be between the characteristic earthquake and the largest among other events a fault zone with a shorter average recurrence interval and or a larger average slip rate has generally produced more earthquakes in the past or is likely to be at a more mature or developed stage thus these tendencies may reflect a change in the magnitude frequency distribution related to the maturity or development of fault zones

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

nonlinearity of the magnitude frequency relation in the hellenic arc trench system and the characteristic earthquake model magnitude frequency g r relations of shallow mainshocks have been examined in five separate segments of the hellenic arc trench ha t system in its western and eastern megasegments as well as in the ha t system considered as an entity data sets of ms 4 5 and ms 6 0 covering the 22 year and 85 year intervals of 1964 1985 and 1901 1985 respectively have been used frequencies of the 22 year interval have been normalized to the 85 year interval so that to obtain frequencies n compatible with the real frequencies nr of the 85 year interval for magnitudes of ms 6 0 straight g r lines determined for n of small and moderate shocks as a rule do not fit when extrapolated the larger magnitude frequencies n and nr magnitude gaps bulge effects and trends toward the vertical are the geometrical expressions of the nonlinear structure of g r in the large magnitude range magnitude gaps ranging from 0 5 to 0 9 in the nr plots and in a wider magnitude spectrum in the n plots of four out of five separate segments indicate that the stress fails to release with ruptures of moderate size thus implying that the characteristic earthquake model may provide an adequate explanation for the g r nonlinearity the ionian islands segment constitutes an obvious exception no magnitude gap in nr appears and the g r linearity is well shaped characteristic events may not occur in that segment when two or more segments are examined together as megasegments the magnitude gap disappears because of overlapping of separate gaps the nonlinear structure of g r shows the general need for properly designed models of seismic hazard determination

the legacy of crystal plastic deformation in olivine high diffusivity pathways during serpentinization crystal plastic olivine deformation to produce subgrain boundaries composed of edge dislocations is an inevitable consequence of asthenospheric mantle flow although crystal plastic deformation and serpentinization are spatio temporally decoupled we identified compositional readjustments expressed on the micrometric level as a striped fe enriched bar x text fe 0 24 0 02 zones 0 12 0 02 bulk or fe depleted bar x text fe 0 10 0 01 zones 0 13 0 01 bulk zoning in partly serpentinized olivine grains from two upper mantle sections in norway focused ion beam sample preparation combined with transmission electron microscopy tem and aberration corrected scanning tem enabling atomic level resolved electron energy loss spectroscopic line profiling reveals that every zone is immediately associated with a subgrain boundary we infer that the zonings are a result of the environmental fe2 mg 1 exchange potential during antigorite serpentinization of olivine and the drive toward element exchange equilibrium this is facilitated by enhanced solid state diffusion along subgrain boundaries in a system which otherwise re equilibrates via dissolution reprecipitation fe enrichment or depletion is controlled by the silica activity imposed on the system by the local olivine orthopyroxene mass ratio temperature and the effect of magnetite stability the fe mg exchange coefficients k text d text atg text ol between both types of zoning and antigorite display coalescence toward exchange equilibrium with both types of zoning mn is enriched and ni depleted compared with the unaffected bulk composition nanometer sized heterogeneously distributed antigorite precipitates along olivine subgrain boundaries suggest that water was able to ingress along them crystallographic orientation relationships gained via electron backscatter diffraction between olivine grain domains and different serpentine vein generations support the hypothesis that serpentinization was initiated along olivine subgrain boundaries

thickness of the seismic slip zone this article reviews geologic and other evidence constraining the thickness of the principal slip zone psz that accommodates the bulk of coseismic shear displacement during an individual rupture event surface deformation from rupturing may occupy swaths tens of meters or more in width but trenches across active faults generally reveal that incremental slip is accommodated by a psz that is tens of centimeters or less in thickness geomorphic evidence coupled with the observations from trenching suggest a psz may stay well localized for distances of several kilometers through many rupture episodes mine exposures and exhumed fault zones demonstrate that pszs separating different lithologies within the fault core although contained within damage zones of variably fractured rock ranging up to hundreds of meters in thickness often comprise just a few centimeters of gouge ultracataclasite that have accommodated large finite displacements 1 km microstructural studies demonstrate incremental slip localized still further down to 1 10 mm as do other fault rock assemblages slickensides and slickenfibers fault veins of pseudotachylyte friction melt intravein septa in hydrothermal fault infills the accumulated evidence indicates that localization of coseismic shearing to less than 10 cm on planar faults is widespread throughout the crustal seismogenic zone with extreme localization to less than 1 cm not uncommon however some distributed coseismic shear may also develop especially at rupture irregularities coseismic reduction of shear resistance from friction melting t 1000 c or from transient thermal pressurization of aqueous fluids t 100 c requires slip during moderate to large earthquakes u 1 m to be restricted to narrow zones respectively a few centimeters or tens of centimeters in thickness given the evidence for slip localization the apparent scarcity of pseudotachylyte suggests either that seismic friction melting is a rare phenomenon or that pseudotachylyte is only rarely preserved in recognizable form within mature hydrated fault zones

early ordovician terne linkages between oceanic and continental ternes in the central scandinavian caledonides the presence of a first order unconformity in the central part of the scandinavian caledonides linking together early ordovician late cambrian outboard oceanic ternes and the continental margin of baltic is of considerable importance for interpreting the geodynamic framework of the orogen this and other criteria for terne linkage are described together with implications for the regional tectonostratigraphic development in this segment of the caledonides

mantle unrooting in collisional settings abstract we present a two dimensional numerical model to study the thermo mechanical evolution of the lithosphere under a convergence regime in order to define the conditions that lead to lithospheric mantle break up and consequent unrooting a newtonian rheology with a temperature dependent viscosity is considered the system is not closed and horizontal flow through lateral boundaries is permitted a horizontal velocity is imposed at the top of the model to simulate compression whereas velocity vanishes at the bottom of the model the initial conditions correspond to a homogeneous lithosphere with a constant heat production in the crust the analysis of variations of maximum shear stress strain rate and total kinetic energy allowed us to define four major stages during the mantle unrooting process orogenic growth initiation of gravitational instability until lithospheric failure sinking of the detached lithosphere and relaxation of the system numerical results also show that the conditions for lithospheric unrooting strongly depend on the convergence velocity the wideness of the deformation zone and the imposed rheology

statistics of characteristic earthquakes statistical methods are used to test the characteristic earthquake hypothesis several distributions of earthquake size seismic moment frequency relations are described based on the results of other researchers as well as my own tests evidence of the characteristic earthquake hypothesis can be explained either by statistical bias or statistical artifact since other distributions of earthquake size provide a simpler explanation for available information the hypothesis cannot be regarded as proven

frequency moment distribution of earthquakes in the aleutian arc a test of the characteristic earthquake model the frequency moment distribution of earthquakes is derived for discrete regions of the entire aleutian arc both for segments which have ruptured in historic times and those regions which have been identified as seismic gaps this distribution is then compared to the moment of the largest event in the respective region or for gaps the largest event which is likely to occur based on a plate kinematic model of accumulated moment data for this study are based upon three data sources instrumental data are utilized for the 18 yr subsequent to the establishment of the world wide standardized seismograph network wwssn data for events of magnitude 7 and greater have been carefully collected and are presumed to be complete for the time period 1899 to present the last data set is comprised of the largest event in each region either an actual earthquake or the moment estimate based on current plate slip rates data were normalized to the recurrence time of the largest event in the respective zone in all cases the data seriously underestimate the largest moment event indicating that a characteristic earthquake model is appropriate for individual fault zones however if the data are combined for the entire arc the largest event is closely predicted this indicates that a b value model may be utilized only for regions in which there is a distribution in the size of earthquake producing faults

early magnetite amphibole plagioclase alteration mineralization in the island copper porphyry copper gold molybdenum deposit british columbia the initial hydrothermal event in the evolution of the ca 377 mt 0 41 percent cu island copper porphyry cu au mo deposit northern vancouver island was the formation of an extensive alteration mineralization facies dominated by magnetite calcic amphibole and intermediate to sodic plagioclase early stage quasipervasive magnetite rich alteration and associated magnetite rich veinlets in this middle jurassic island arc hosted hydrothermal system developed in both an axial dacitic porphyry dike and contiguous basaltic flows and pyroclastic strata the magnetite rich alteration zone 500 to 700 m wide and originally extending over at least 450 m vertically exhibits an outward zonation from quartz magnetite albite an sub or 6 amphibole trace apatite trace through quartz magnetite amphibole albite an sub 6 10 or oligoclase andesine an sub 15 36 apatite trace or scapolite trace to amphibole magnetite oligoclase or sodic andesinc an sub 15 39 or quartz or apatite trace assemblages definition of early stage mineral assemblages impeded by unusually intense later intermediate argillic alteration and more local phyllic and advanced argillic alteration was facilitated by the use of novel petrographic techniques including incident light nomarski differential interference contrast microscopy early stage mineralization comprises several well defined vein veinlet types which were demonstrably emplaced prior to the main stage chalcopyrite pyrite stockwork and associated k silicate alteration minor chalco pyrite and pyrite occur in some early stage veins and magnetite is a constituent of many later main stage sulfide rich veins but the distinction between these two major vein systems is clear mass exchange calculations demonstrate that early alteration involved intense iron metasomatism of both felsic and mafic country rocks with an fe enrichment approaching 450 percent 20 g 100 cc in the dacite dike and lesser and more variable na enrichment with both ti and al behaving as mobile constituents paragenetically early fluid inclusions in the quartz phenocrysts of the dike are tentatively correlated with the magnetite rich alteration mineralization and imply that this event was initiated above 650 degrees c and at pressures of at least 1 kbar and was generated by fecl 2 rich brines at first single phase and moderately saline avg 15 wt nacl equiv but latterly boiling at ca 560 degrees to 645 degrees c and or ca 0 55 kbars the main stage potassic alteration zone has the configuration of an annulus ca 100 to 150 m in width which was entirely superimposed on the more extensive early alteration mineralization zone at ca 430 degrees to 575 degrees c and below 450 bars gold although correlated overall with cu and potassic alteration was probably extensively introduced in the early stage it is inferred that the scarcity of sulfides in the early alteration assemblages reflects the highly oxidized nature of the initial fluids in which so super 2 exceeded both h 2 s and so super 2 4 the deposition of abundant magnetite and the widespread and intense fe metasomatism which define the early stage record the high solubility of fe as fecl 2 in high temperature brines in equilibrium with magnetite bearing quartzofeldspathic rocks as well as the retrograde solubility of magnetite between ca 550 degrees and 750 degrees c whereas the high so 2 h 2 o so 4 ratios of the initial hydrothermal fluids at island copper and in other similar deposits may have resulted from the high inherent oxygen fugacity of the parental dacitic magmas they may have directly reflected the composition of supercritical fluids expelled on quenching of underplating s rich mafic melts the mineral assemblages of the early stage veins and associated quasi pervasive alteration cannot be assigned to the accepted alteration mineralization facies of porphyry systems such as a veins and potassic alteration and record specific and distinct conditions of fluid rock interaction

branching sheeted dikes and seafloor spreading within an early proterozoic intra arc basin the payson ophiolite 1 73 ga is exposed as a shallow dipping pseudostratigraphic sequence of gabbro sheeted dikes and submarine volcanic rocks with screens and roof pendants of older arc crust the sheeted dike complex is particularly well exposed and analysis of measured sections reveals a threefold increase in the average dike width with depth in the complex magmatic temperatures and slow cooling at the base of the sheeted dike complex indicated by gabbro dike mingling and mutual intrusion and coarse textures may have facilitated the growth of wide dikes in contrast extreme alteration of the top of the sheeted dike complex suggests that hydrothermal circulation dominated and caused rapid cooling as thin dikes the connection between wide dikes at depth with thinner dikes toward the top requires a new model where dikes branch upward in response to the strong vertical gradient in the rate of cooling and solidification the branching dike model has important implications for the timing and spatial distribution of seafloor phenomena in the neovolcanic zone of spreading centers

magma in forearcs implication for ophiolite generation abstract forearc areas non volcanic arcs of contemporary island arcs at convergent plate boundaries contain magmatic rocks geological evidence seismic profiles heat flow data density considerations and petrological and geochemical arguments suggest that a forearc tholeiitic association fat containing high mg calc alkaline andesites is present in non volcanic arcs at some stage of island arc development the fractionated as well as primitive magma is unable to penetrate low density sediments and underplates thick piles of unconsolidated accreting rocks the underplating causes upwelling the occurrence of magma in forearcs provides an alternative interpretation for the tectonic setting of some ophiolitic masses rather than ocean ridge formation and later obduction it offers an autochthonous island arc bound and geologically substantiated interpretation for the ophiolite suite

origin of rodingite forming fluids constrained by calcium and strontium isotope ratios in the leka ophiolite complex abstract alteration of oceanic lithosphere plays an important role in the global element cycling due to its dominant abundance on the earth s surface the formation of rodingites a ca rich lithology that occurs intimately associated with ca depleted serpentinite complexes is a widespread process during strong alteration of ocean floor lithologies and may represent a key process that influences the global ca cycle stable ca and radiogenic sr isotope compositions of rodingites and associated lithologies from the leka ophiolite complex norway preserve mantle signatures and thus provide evidence that the ca and sr sources for rodingitisation are dominantly mantle derived with no detectable seawater input the difference of 0 7 in 44 40ca of different phases found in the rocks is overall consistent with inter mineral fractionation at high temperatures related to the cao bonding environment but there are also indications for isotopic disequilibria between non cogenetic mineral phases while the water required for rodingite formation may originate from seawater the dissolved ca and sr involved in the mineral reactions during rodingitisation plg ca gros clz al can be balanced at a local scale being derived from the serpentinisation of mantle peridotites cpx1 h2o al srp chlorite ca cpx2 consequently rodingites do not act as a sink for oceanic ca instead they are a sink for ca released during serpentinisation of the oceanic lithospheric mantle

displacement and geometrical characteristics of earthquake surface ruptures issues and implications for seismic hazard analysis and the process of earthquake rupture there now exist about three dozen historical earthquakes for which in vestigators have constructed maps of earthquake rupture traces accompanied by de scriptions of the coseismic slip observed along the fault strike the maps and slip distributions are compiled here to place observational bounds on aspects of seismic hazard analysis and fault mechanics analysis leads to an initial statistical basis to predict the end points of rupture and the amount of surface slip expected at sites along the strike during earthquakes on mapped faults the observations also give support to the ideas that there exists a process zone or volume of about 3 4 km in dimension at the fronts of large laterally propagating earthquake ruptures within which stress changes may be sufficient to trigger slip on adjacent faults and that the ultimate length of earthquake ruptures is controlled primarily by the geometrical complexity of fault traces and variations in accumulated stress levels along faults that arise due to the location of past earthquakes to this may be added the observation that the form of earthquake surface slip distributions is better described by asymmetric rather than symmetric curve forms and that earthquake epicenters do not appear to correlate in any systematic manner to regions of maximum surface slip observed along strike online material maps of surface ruptures digitized values and curve fits to surface slip distributions and notes and references for tables 1 and 2

the velay dome french massif central melt generation and granite emplacement during orogenic evolution this paper is a synthesis of available data on the velay dome that include both small and large scale lithologic and structural mapping strain analysis isotope geochemistry geochronology and pressure temperature estimates the velay dome one of the largest granite migmatite domes of the variscan belt formed during orogenic collapse at around 300 ma its study allows an assessment of the thermal and geodynamic context leading to voluminous crustal anatexis of the variscan orogenic crust a first melting stage developed in connection with south verging thrust zones during the early carboniferous leading to a crustal thickening estimated at 20 km minimum the involvement of fertile lithologies and the intrusion of plutons of deep origin contributed to the development of water saturated melts the volume of biotite granite extracted from melt during this period was limited the second phase of melting corresponded to generalized melting of gneiss achieved by biotite dehydration melting reactions and accompanied by the generation of cordierite bearing granites at this stage crustal scale detachment faults were activeandpartiallyobliteratedtheearlierstructures thenewstructureswereprogressivelytiltedtotheverticalatthemarginofthe velay dome due to the southward and lateral ballooning of the granitic dome the reconstructed p t path indicate that the large volume ofmelt produced was a consequence ofa significant increase in temperature at the onset of biotite dehydration melting at the base of the crust this melting event is coeval with granulite facies metamorphism associated to underplating of mantle derived magmas as suggested by the geochemical signature of late paleozoic lower crustal xenoliths sampled by cenozoic volcanoes and withtheisotopicsignatureofthelategraniticintrusions accordingly itisproposedthatasthenosphericupwellingwasresponsible for the temperature increase favoring melting of hydrous minerals d 2001 published by elsevier science b v

mantle flow at oceanic spreading centers models derived from ophiolites abstract the internal structures of eleven ophiolite complexes have been structurally investigated the attitude of the foliation and the lineation in the peridotites is related to asthenospheric flow in the uppermost mantle at an oceanic spreading center from the orientation of these structures with respect to the paleohorizontal plane two oceanic situations are distinguished the horizontally spreading asthenosphere illustrated by the table mountain massif newfoundland is characterized by 1 a homogeneous regular and sub horizontal plastic flow pattern over large distances 2 b flat paleo mono and absence of large scale folding 3 c rare economical chromite deposits and depleted mantle the diapirically spreading asthenosphere illustrated by the acoje type philippines displays 1 a inhomogeneous and sub vertical plastic flow pattern 2 b deformed paleo moho and large scale folds 3 c important economical chromite deposits and less depleted mantle presumably due to secondary magmatic impregnation this leads to vogt s model of asthenospheric flow beneath an oceanic ridge the direction of which being equated with the dike swarm in this model the horizontally spreading asthenosphere is derived from the diapirically spreading one in the following way the vertical mantle flow at the diapir center diverges horizontally in every direction including that parallel to the ridge at about 50 km from the diapir center the mantle flow becomes progressively perpendicular to the ridge due to the damming effect of transform faults an important economical consequence of this model is that large chromite deposits are expected preferentially in the diapiric type of ophiolite because chromite is preferentially precipitated at the diapir center from pristine magma