magma mixing in boninite sequence of chichijima bonin islands abstract boninite from chichijima can be divided into two major types a porphyritic type phenocryst mode exceeds 1 and b aphyric type phenocryst mode is less than 1 both types are further subdivided by means of microphenocryst assemblages into five subtypes i olivine ol clinoenstatite cen bronzite brz ii ol brz iii brz iv brz pigeonite pig augite aug v cen brz there is a systematic correlation between these rock types and the bulk composition of aphyric boninites the most magnesian boninites are of type i and the least magnesian ones are of types iii and v type ii is slightly higher in cao than type i at a given feo mgo ratio type iv is distinct from the others for its high cao and low incompatible element contents at a given feo mgo ratio type iv cannot be derived from the others by crystallization differentiation compositional data for the other types of boninite define rectilinear compositional variations which lie on a tie line connecting fe rich bronzite and primitive boninite microphenocrysts in equilibrium with boninite magma are more magnesian so it would appear that the fe rich bronzite was in equilibrium not with boninite magma but with some evolved descendant e g bronzite andesite magma the variations of aphyric boninites are then best explained by a combination of mixing of primitive boninite and evolved descendants together with equilibrium crystallization processes in order to account for this combination of processes a density stratified open system magma chamber of the type proposed for oceanic ridge systems seems appropriate the upper part of the magma chamber is composed of differentiated bronzite andesite magma and the lower part consists of hot dense and primitive boninite magma crystallization and sporadic overturn and mixing give rise to the spectrum of magma types erupted at the surface

composition of back arc basin volcanics valu fa ridge lau basin evidence for a slab derived component in their mantle source abstract samples dredged from 2 localities near the crest of the valu fa ridge an active back arc basin spreading centre in the lau basin consist of highly vesicular lava fragments of andesitic composition the samples are characterized by rare euhedral an 85 plagioclase phenocrysts in a hypocrystalline groundmass of an 60 plagioclase laths brown glass and rare subhedral clinopyroxene samples from within and to a lesser extent between the dredge hauls show remarkable isotopic and chemical homogeneity with 87 sr 86 sr 0 70330 2 143 nd 144 nd 0 51303 2 206 pb 204 pb 18 65 2 207 pb 204 pb 15 55 1 208 pb 204 pb 38 34 4 sr 165 ppm rb 7 ppm cs 0 17 ppm k 3300 to 4200 ppm ba 96 ppm and ree lree depleted with 12 18 chondritic abundances on sr nd pb pb and sr pb plots the volcanics lie just within or on the edge of the morb fields overlapping with island arc volcanics from the marianas and tonga compared with morb and ocean island basalts the samples show alkali element enrichment relative to ree and higher cs relative to rb the isotopic and geochemical characteristics of the valu fa ridge volcanics clearly indicate a minor but significant slab derived component in the back arc basin mantle source

middle to late jurassic tectonic evolution of the klamath mountains california oregon the geochronology stratigraphy and spatial relationships of middle and late jurassic terranes of the klamath mountains strongly suggest that they were formed in a single west facing magmatic arc built upon older accreted terranes a middle jurassic arc complex is represented by the volcanic rocks of the western hayfork terrane and consanguineous dioritic to peridotitic plutons new u pb zircon dates indicate that the middle jurassic plutonic belt was active from 159 to 174 ma and is much more extensive than previously thought this plutonic belt became inactive just as the 157 ma josephine ophiolite which lies west and structurally below the middle jurassic arc was generated late jurassic volcanic and plutonic arc rocks rogue formation and chetco intrusive complex lie outboard and structurally beneath the josephine ophiolite u pb and k ar age data indicate that this arc complex is coeval with the josephine ophiolite both the late jurassic arc complex and the josephine ophiolite are overlain by the galice formation a late jurassic flysch sequence and are intruded by 150 ma dikes and sills the following tectonic model is presented that accounts for the age and distribution of these terranes a middle jurassic arc built on older accreted terranes undergoes rifting at 160 ma resulting in formation of a remnant arc back arc basin island arc triad this system collapsed during the late jurassic nevadan orogeny 150 ma and was strongly deformed and stacked into a series of east dipping thrust sheets arc magmatism was active both before and after the nevadan orogeny but virtually ceased at 140 ma

formation and emplacement of the josephine ophiolite and the nevadan orogeny in the klamath mountains california oregon u pb zircon and 40ar 39ar geochronology cordilleran ophiolites typically occur as basement for accreted terranes in the klamath mountains ophiolitic terranes were progressively accreted by underthrusting beneath north america the josephine ophiolite is the youngest of the klamath ophiolites and forms the basement for a thick late jurassic flysch sequence galice formation this ophiolite flysch terrane forms an east dipping thrust sheet sandwiched between older rocks of the klamath mountains above and a coeval plutonic volcanic arc complex below the outcrop pattern of the roof orleans thrust indicates a minimum displacement of 40 km and geophysical studies suggest 110 km of displacement the basal madstone cabin thrust is associated with an amphibolitic sole and has a minimum displacement of 12 km a rapid sequence of events from ophiolite generation to thrust emplacement has been determined using 40 ar 39 ar and pb u geochronology ophiolite generation occurred at 162 164 ma a thin hemipelagic sequence was deposited from 162 to 157 ma and flysch deposition took place between 157 and 150 ma tight age constraints on thrusting and low grade metamorphism associated with ophiolite emplacement nevadan orogeny are provided by abundant calc alkaline dikes and plutons ranging in age from 151 to 139 ma deformation and metamorphism related to the nevadan orogeny appears to have extended from 155 to 135 ma most of the crustal shortening took place by thrusting constrained to have occurred from 155 to 150 ma on both the roof and basal thrusts minimum rates of displacement are 2 4 and 3 6 mm year for the basal and roof thrusts respectively but correlations with coeval thrusts yield rates of 8 4 and 22 mm year within the range of plate velocities the high displacement rates and synchronous movement along the basal and roof thrusts suggest that the ophiolite may have behaved as a microplate situated between western north america and an active arc from 155 to 150 ma a steep thermal gradient was present in the josephine galice thrust sheet from 155 to 150 ma with amphibolite facies conditions developed along the basal thrust after accretion of the ophiolite by underthrusting the ophiolite and overlying flysch underwent low grade dynamothermal regional metamorphism from 150 to 135 ma the upper age limit is tightly constrained by a 135 ma k feldspar cooling age syntectonic plutons as young as 139 ma and a lower cretaceous angular unconformity very rapid exhumation is indicated by the late valanginian to hauterivian age 130 ma of the unconformably overlying strata suggesting unroofing by extensional tectonics

petrology of the axial ridge of the mariana trough backarc spreading center abstract the axial ridge of the mariana trough backarc basin between 17 40 n and 18 30 n rises as much as 1 km above the floor of a 10 15 km wide rift valley physiographic segmentation with minor ridge offsets and overlaps coincides with a petrologic segmentation seen in trace element and isotope chemistry analyses of 239 glass and 40 aphyric basalt samples collected with alvin and by dredging show that the axial ridge is formed largely of olivine hypersthene normative tholeiitic basalt about half of these are enriched in both lil elements and volatiles but are depleted in hfs elements like other rocks found throughout much of the mariana trough the lil enrichments distinguish these rocks from n morb even though nd and sr isotope ratios indicate that much of the crust formed from a source similar to that for n morb in addition to lil enriched basalt there is lil depleted basalts even more closely resembling n morb in major and trace elements as well as sr nd and pb isotopes both basalt varieties have higher al and lower total fe than morb at equivalent mg level mg ranges from relatively primitive e g mg 65 70 to more highly fractionated e g mg 45 50 highest parts of the axial ridge are capped by pinnacles with elongated pillows of basaltic andesite e g 52 56 sio 2 these are due to extreme fractional crystallization of basalts forming the axial ridge active hydrothermal vents with chimneys and mats of opaline silica barite sphalerite and lesser amounts of pyrite chalcopyrite and galena formed near these silicic rocks the vents are surrounded by distinctive vent animals polychaete worms crabs and barnacles isotope data indicate that the mariana trough crust was derived from a heterogeneous source including mantle resembling the morb source and an arc source component the latter was depleted in hfs elements in previous melting events and later modified by addition of h 2 o and lil elements

flow rate dictates permeability enhancement during fluid pressure oscillations in laboratory experiments flow rate dictates permeability enhancement during fluid pressure oscillations in laboratory experiments thibault candela 1 2 emily e brodsky 2 chris marone 1 derek elsworth 1 1 dept geosciences penn state univ university park pa united states 2 dept earth sciences uc santa cruz santa cruz ca united states

neogene foreland basin deposits erosional unroofing and the kinematic history of the himalayan fold thrust belt western nepal sedimentological and provenance data from the lower miocene pliocene dumri formation and siwalik group in western nepal provide new information about the timing of thrust faulting and the links between erosional unroofing of the himalaya and the cenozoic 87 sr 86 sr record of the ocean in western nepal the dumri formation is an 750 1300 m thick fluvial sandstone and overbank mudstone unit the siwalik group is 4200 m thick and consists of a lower member 850 m of 2 12 m thick fluvial channel sandstones and oxidized calcareous paleosols a middle member 2400 m of very thick 20 m channel sandstones and mainly organic rich histosols and an upper member 1000 m composed of gravelly braided river deposits paleocurrent data indicate that middle miocene pliocene rivers in western nepal flowed southward transverse to the thrust belt throughout deposition of the siwalik group no evidence was found for an axial fluvial trunk system i e the paleo ganges river in siwalik group sandstones a major increase in fluvial channel size is recorded by the transition from lower to middle siwalik members at 10 8 ma probably in response to an increase in seasonal discharge modal petrographic data from sandstones in the dumri formation and the siwalik group manifest an upsection enrichment in potassium feldspar carbonate lithic fragments and high grade metamorphic minerals modal petrographic analyses of modern river sands provide some control on potential source terranes for the miocene pliocene sandstones the dumri formation was most likely derived from erosion of sedimentary and low grade metasedimentary rocks in the tibetan tethyan himalayan zone during early miocene emplacement of the main central thrust the presence in dumri sandstones of plagioclase grains suggests exposure of crystalline rocks of the greater himalayan zone perhaps in response to tectonic unroofing by extensional detachment faults of the south tibetan detachment system during deposition of the lower siwalik group 15 11 ma emplacement of the dadeldhura thrust sheet one of the synformal crystalline thrust sheets of the southern himalaya on top of the dumri formation supplied abundant metasedimentary lithic fragments to the foreland basin a steady supply of plagioclase grains and high grade minerals was maintained by deeper erosion into the main central thrust sheet from 11 ma to the present k feldspar sand increased steadily suggesting that granitic source rocks became widely exposed during deposition of the upper part of the lower siwalik group this provenance change was caused by erosion of passively uplifted granites and granitic orthogneisses in the dadeldhura thrust sheet above a large duplex in the lesser himalayan rocks since the onset of deposition of the conglomeratic upper siwalik group 4 5 ma fault slip in this duplex has been fed updip and southward into the main boundary and main frontal thrust systems we obtained 113 u pb ages on detrital zircons from modern rivers and siwalik group sandstones that cluster at 460 530 ma 850 1200 ma 1 8 2 0 ga and 2 5 ga an abundance of cambrian ordovician grains in the siwalik group suggests sources of siwalik detritus in the granites of the dadeldhura thrust sheet and possibly the greater himalayan orthogneisses the older ages are consistent with sources in the greater and lesser himalayan zones an overall upsection increase in zircons older than 1 7 ga suggests increasing aerial exposure of lesser himalayan rocks none of the detrital zircons even in the modern river samples yielded a cenozoic age that might suggest derivation from the cenozoic greater himalayan leucogranites but this may be attributable to the inheritance problems that characterize the u pb geochronology of the leucogranites when compared with recent studies of the 87 sr 86 sr composition of paleosol carbonate nodules and detrital carbonate in paleosols from the siwalik group the provenance data suggest that erosion and weathering of metamorphosed carbonate rocks in the lesser himalayan zone and cambrian ordovician granitic rocks of the crystalline thrust sheets in central and eastern nepal may have played a significant role in elevating the 87 sr 86 sr ratio of middle miocene synorogenic sediments in the indo gangetic foreland basin and the bengal fan as well as global seawater

a new observation of dynamically triggered regional seismicity earthquakes in greece following the august 1999 izmit turkey earthquake the m w 7 4 izmit turkey earthquake triggered widespread regional seismicity in greece over a study region extending from 400 km to nearly 1000 km away from the epicenter small events began immediately after the passage of the mainshock surface waves suggesting that the transient stresses of the seismic waves were the trigger the increase in cataloged earthquakes in ordinary continental crust is a new observation and is statistically significant at the 95 level unlike the previous example of distant triggering during the landers earthquake the activated seismicity occurred entirely in non volcanic areas the greek sites were triggered by waves with amplitudes at least a factor of 3 lower than the observed triggering threshold for imperial valley we speculate that dynamic triggering on a regional scale results in countrywide episodes of increased seismicity or superswarms in regions with low triggering thresholds such as greece

petrology of mariana trough and lau basin basalts the crust of actively opening back arc basins has close chemical and mineralogic similarities to normal mid ocean ridge basalts n morb but some subtle but distinctive differences exist these are largely due to differences in volatiles fe oxidation ratio abundances of large ion lithophile elements lil and some high field strength hfs elements isotope data for sr pb nd he and o are variable but broadly are morb like we present data contrasting mariana trough mtb and lau basin basalts lbb showing how they differ in certain trace element and isotopic signatures and compare both to n morb relative to n morb and lbb the mtb are enriched in alkalis alkaline earths light rare earth elements ree h2o and fe fe even for primitive samples while having morb type he 3 4 7 8 to 8 4 ra sr 87 86 mean 0 70293 and nd e 10 2 the lau basin is zoned with a central area having n morb characteristics for h2o lil ree nd e 8 1 to 9 0 and o but a depletion in hfs elements sr is more radiogenic mean 87 86 0 70347 and 3he 4he 3 4 7 2 to 11 0 ra is high relative to n morb the lau basin has a zone bordering the adjacent island arcs which is like mtb in both back arc basins relatively primitive unfractionated basalts with mg 100 mg fe 62 are found which resemble postulated primitive n morb calculated parental magmas obtained by adding xenocrystic olivine plot near or on 3 and 4 phase boundaries at 1 1 5 gpa for the ol di si system low p fractional crystallization and magma mixing explain most of the observed rock types possible explanations for differences between mtb and lbb include differences in the extent of previous melting episodes of their mantle sources and effects of source metasomatism through dehydration and partial melting of subducted lithosphere the long and complex history of mariana trough mantle sources led to a metasomatized mantle which gives rise to the distinctive mtb in contrast the lau basin is dominated by basalts derived from a source depleted in volatiles lil and hfs elements a previous melting history is implied mixing with a mantle component enriched in 87sr and 3he may be due to mantle counterflow driven by the subduction process

geochemical evolution and tectonic significance of boninites and tholeiites from the koh ophiolite new caledonia the central chain ophiolites in new caledonia are fragments of a supra subduction zone ssz ophiolite now preserved from the upper layered gabbros through to volcanics and overlain by pelagic cherts and a thick middle triassic to upper jurassic volcaniclastic sequence most of the fragments were formed by a single tholeiitic magmatic episode but one of these the koh ophiolite was formed by two tholeiitic magmatic episodes separated by boninites the first event in the koh ophiolite formed cumulate gabbros dolerites plagiogranites and the first pillow lava sequence from a tholeiitic magma with strong depletion in the light rare earth elements lree and abnormally low tio2 0 5 at mg 60 shortly after their eruption these tholeiitic lavas were overlain by a high ca boninitic unit with a basal section of boninite pillows flows and breccias and an upper section of boninitic dacites and tuffs the last magmatic phase involved eruption of evolved tholeiitic basalts as pillows above the boninites and as dykes and sills intruding the older plutonic and volcanic sections of the ophiolite this second phase of tholeiitic magmatism is compositionally distinct from the first and is closest to back arc basin basalts babb erupted during the early rifting history of modern back arc basins the boninitic volcanics belong to a high ca series with slightly lower sio2 al2o3 and tio2 compared to those from modern island arc systems and they lack the positive zr spike relative to adjacent rare earth elements ree in normalised element variation patterns these boninites were formed shortly after the production of back arc basin crust represented by the depleted tholeiites and shortly before a second spreading event which caused 40 60 extension of the initial basin crust and eruption of the upper tholeiites the dominance of babb like tholeiites throughout the central chain ophiolites in new caledonia the restricted occurrence of boninites and the stratigraphy and chemistry of the koh ophiolite suggest that the boninites were erupted in response to an exceptional tectonic situation we suggest that this boninite generation event was driven by adiabatic decompression of hot depleted mantle residual from the production of the lower tholeiites during initiation of rifting of young oceanic crust intimately associated with propagation of a back arc basin spreading centre the occurrence of a thick blanket of calc alkaline volcaniclastic sediments above the ophiolite indicates proximity to a mature arc and suggests that the koh boninites were not associated with the initiation of subduction a close modern analogy for the koh ophiolite exists on the hunter ridge protoisland arc between southernmost vanuatu new hebrides island arc and the fijian islands there high ca boninites lacking positive zr spikes occur together with low ti tholeiites and more typical babb tholeiites where the southern spreading centre of the north fiji basin is propagating into the protoarc crust of the hunter ridge

petrology and origin of primitive lavas from the troodos ophiolite cyprus parental magmas to the troodos ophiolite are characterised by low tio2 and al2o3 and high sio2 extremely fresh and chemically primitive high mgo rocks are found within the upper pillow lavas and along the arakapas fault belt of cyprus and contain forsteritic olivine enstatite and groundmass clinopyroxene set in glass or plagioclase with accessory magnesiochromite and sometimes hornblende they are quartz normative and may have originally contained up to 3 wt h2o geochemically there are three distinct groups of primitive lavas based on tio2 and zr contents but also reflected by cao na2o and ree abundances these groups cannot be related by crystal fractionation and are considered to have been generated by incremental melting of a variably depleted source region the parental magma to the least depleted group group i was that of the major portion of the troodos plutonic complex and is similar to those postulated for other low ti ophiolites chemically it has close affinities with komatiitic basalts the most depleted lavas group iii all have u shaped ree profiles and variable 143nd 144nd ratios interpreted in terms of metasomatism of the source region by an incompatible element enriched component which was probably derived from a subducted slab these lavas represent an intermediate step in the development of boninite series rocks

metabasalts axial hot springs and the structure of hydrothermal systems at mid ocean ridges knowledge of the chemical transfer and mineralogical transformations that occur when sea water reacts with basalt at elevated temperatures and pressures can be used along with geological and geophysical data to deduce the typical structure and evolutionary sequence for hydrothermal systems within the oceanic crust along the axis of a mid ocean ridge studies of metabasalt and metadiabase dredged from fault scarps along the axial valley of the mid atlantic ridge reveal consistent relationships among the bulk chemistry of the altered rocks and their secondary mineralogy mineral abundances and mineral compositions especially for chlorite these relationships can be interpreted in terms of the distribution of alteration with respect to time temperature and water rock ratio in young crust assemblages of chl ab ep act chl ab ep act qtz chl ab qtz and chl qtz are produced at successively higher effective sea water to rock ratios within the temperature range of the greenschist facies 250 to 450 c toward higher ratios chlorites tend to become more mg rich pillow basalts of layer 2 are commonly altered under these conditions by sea water on the descending rather than the ascending limb of a convection system this type of alteration which reflects moderately high water rock ratios may characteristically occur above a still partially molten magma chamber as the magma chamber solidifies and is then penetrated more pervasive alteration of the deeper part of layer 2 and of layer 3 occurs at lower water rock ratios producing solutions such as those recovered from springs on the galapagos rift and the east pacific rise at 21 n localized upwelling limbs of the convection system can produce veins filled with quartz sulfides and fe rich chlorite

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

evidence for large scale rotations at spreading centers from the josephine ophiolite abstract repetition of the josephine ophiolite by large scale open folding and imbricate faulting provides an excellent opportunity to study the geometric relationships between sheeted dikes igneous layering and bedding measurements of over 600 dike margins from the sheeted dike complex and high level gabbro indicate that dikes are oriented at 20 40 to bedding in metasedimentary rocks overlying the ophiolite in addition igneous layering is discordant to gross lithologic layering and is oriented at a large angle to both sheeted dikes and bedding similar relationships may occur in several other ophiolites the inclined sheeted dikes and discordant igneous layering are best explained by a 45 rotation of new oceanic crust ophiolite immediately following formation at a spreading center rotation could have been accomplished by either subsidence or faulting further evidence for large scale rotations of oceanic crust include frequent anomalous magnetic inclinations in drill cores of layer 2 from the atlantic

petrologic consequences of rift propagation on oceanic spreading ridges abstract the production of anomalously differentiated lava compositions at several mid ocean spreading centers can be attributed to magmatic processes associated with propagating rifts the degree of differentiation attained by magmas beneath oceanic spreading ridges depends mainly on the balance between cooling rate and the supply rate of new magma to shallow chambers low supply rates and moderate cooling rates allow advanced degrees of closed system fractionation to occur high supply rates result in open systems in which magma compositions are buffered by frequent replenishment with new hot magma propagating rift tips are a special class of ridge transform intersection in which the balance between cooling and supply rates is conducive to the development of advanced degrees of differentiation over an expanded length of ridge this balance is affected by the spreading rate the propagation rate of the rift the length of the bounding transform and proximity to hotspots maximum compositional variability and maximum degree of differentiation occur within 50 km of propagating rift tips and subsequently diminish with increasing distance rifts that propagate through plates in directions approximating their absolute motion relative to the lower mantle are characterized by the presence of anomalously differentiated lavas over longer ridge segments than are rifts that propagate against their absolute motion geochemical anomalies may persist though changing in degree and extent for several million years on ridge segments that stop propagating the concept of magnetic telechemistry is generally supported by our study but in the vicinity of hotspots magnetic anomaly amplitude may be controlled more by bathymetric and or thickened magnetic layer effects than by geochemistry

earthquake triggering at alaskan volcanoes following the 3 november 2002 denali fault earthquake the 3 november 2002 m w 7 9 denali fault earthquake provided an excellent opportunity to investigate triggered earthquakes at alaskan volcanoes the alaska volcano observatory operates short period seismic networks on 24 historically active volcanoes in alaska 247 2159 km distant from the mainshock epicenter we searched for evidence of triggered seismicity by examining the unfiltered waveforms for all stations in each volcano network for 1 hr after the m w 7 9 arrival time at each network and for significant increases in located earthquakes in the hours after the mainshock we found compelling evidence for triggering only at the katmai volcanic cluster kvc 720 755 km southwest of the epicenter where small earthquakes with distinct p and s arrivals appeared within the mainshock coda at one station and a small increase in located earthquakes occurred for several hours after the mainshock peak dynamic stresses of 0 1 mpa at augustine volcano 560 km southwest of the epicenter are significantly lower than those recorded in yellowstone and utah 3000 km southeast of the epicenter suggesting that strong directivity effects were at least partly responsible for the lack of triggering at alaskan volcanoes we describe other incidents of earthquake induced triggering in the kvc and outline a qualitative magnitude distance dependent triggering threshold we argue that triggering results from the perturbation of magmatic hydrothermal systems in the kvc and suggest that the comparative lack of triggering at other alaskan volcanoes could be a result of differences in the nature of magmatic hydrothermal systems

oceanic faulting and fault controlled subseafloor hydrothermal alteration in the sheeted dike complex of the josephine ophiolite based upon detailed mapping 1 10 and 1 100 of a large water polished outcrop of the sheeted dike gabbro transition zone in the josephine ophiolite of nw california and sw oregon the following history of alternating episodes of magmatic structural and hydrothermal events has been documented using crosscutting relationships petrography geochemistry and strontium and oxygen isotopic data 1 crystallization of gabbro and later subvertical mafic dike injection 2 amphibolite facies metamorphism 3 extensional faulting and tilting of dikes 4 continued faulting tilting and dike injection associated with retrograde metamorphism at greenschist facies conditions 5 continued extensional faulting and tilting synchronous with the development of a variety of hydrothermal veins at decreasing temperature and increasing fluid rock ratios and 6 subvertical injection of a highly fractionated dike which truncates all previous features features 1 5 trace element geochemistry indicates the highly fractionated dike feature 6 is genetically related to the other dikes and thus was intruded at or near the paleospreading axis hence all previous events features 1 5 can be constrained to have occurred at the rift axis including large scale tilting 50 of the sheeted dikes and extensional faulting the 87sr 86sr initial ratios from recrystallized whole rocks and from hornblende epidote and prehnite separates from veins display a systematic increase with relative age from 0 7033 for altered gabbro screens and mafic dikes to 0 7049 for prehnite in the youngest oceanic fault rock calculated oxygen isotope fluid compositions for the same suite of samples range from 18ofluid 5 to 1 with time indicating a change to a seawater dominated hydrothermal system with time consistent with observed increased permeability due to seafloor extensional faulting the crosscutting relationships alteration mineral assemblages and isotopic data suggest 1 an early stage of high temperature 450 c hydrothermal alteration with low permeability i e grain scale flow followed by 2 a decrease in temperature 350to 200 c and an increase in permeability due to faulting and accompanied by tectonic tilting at the rift axis the consistency of these crosscutting relationships at similar pseudo stratigraphic levels at different localities in the josephine ophiolite suggests that alternating magmatic and structural extension with synchronous retrograde alteration is common in crust formed at similar rates of spreading slow to intermediate spreading centers such as the modern mid atlantic ridge

timescales of orogeny jurassic construction of the klamath mountains an electronic supplement of this material may be obtained on a diskette or anonymous ftp from kosmos agu org login to agu s ftp account using anonymous as the username and guest as the password go to the right directory by typing cd apend type ls to see what files are available type get and the name of the file to get it finally type exit to leave the system paper 94ycj2454 timescales of orogeny jurassic construction of the klamath mountains b r hacker m m donato c g barnes m o mcwilliams and w g ernst diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 15 00 payment must accompany order classical interpretations of orogeny were based on relatively imprecise biostratigraphic and isotopic age determinations that necessitated grouping apparently related features that may in reality have been greatly diachronous isotopic age techniques now have the precision required to resolve the timing of orogenic events on a scale much smaller than that of entire mountain belts forty five new 40ar 39ar ages from the klamath mountains illuminate the deformation metamorphism magmatism and sedimentation involved in the jurassic construction of that orogen leading to a new level of understanding regarding how preserved orogenic features relate to ancient plate tectonic processes the new geochronologic relationships show that many jurassic units of the klamath mountains had 200 ma or older volcanoplutonic basement subsequent formation of a large 170 ma arc was followed by contractional collapse of the arc collision with a spreading ridge may have led to large scale nw se extension in the central and northern klamaths from 167 to 155 ma coincident with the crystallization of voluminous plutonic and volcanic suites marked cooling of a large region of the central klamath mountains to below 350 c at 150 ma may have occurred as the igneous belt was extinguished by subduction of colder material at deeper structural levels these data demonstrate that the klamath mountains and perhaps other similar orogens were constructed during areally and temporally variant episodes of contraction extension and magmatism that do not fit classical definitions of orogeny

sedimentation metallogenesis and tectonic origin of the basal galice formation overlying the josephine ophiolite northwestern california basal sediments overlying the late jurassic josephine ophiolite consist of interbedded siliceous argillite radiolarian chert and rare graywacke overlain by a thick flysch sequence together the pelagic rocks and flysch comprise the galice formation in northwestern california the transition between pelagic rocks and overlying flysch is comprised of radiolarian argillites containing abundant silt size detritus deposition of many of the argillites and cherts by turbidity currents is indicated by sedimentary structures such as graded bedding some of the cherts are tuffaceous metalliferous rocks up to 22 fe 2 o ast 3 and 6 mno occur in some sections and have geochemical characteristics indicative of a substantial hydrothermal component formed by precipitation from heated seawater circulated through the underlying ophiolite these rocks are found approximately 8 21 m above the base of the sequence suggesting an origin from off axis hydrothermal vents a massive sequence of metalliferous sed

petrogenesis and implications for tectonic setting of cambrian suprasubduction zone ophiolitic rocks in the central beishan orogenic collage northwest china abstract the hongliuhe ophiolite is a fragment of the early paleozoic oceanic crust situated in a suture zone central to the beishan orogenic collage which is a subset to the central asian orogenic belt in northwest china it has lithologic geochemical and chronological similarities to the yueyashan xichangjing ophiolite 400 km east along strike allowing for correlative interpretations of marginal seafloor spreading processes the hongliuhe ophiolite is explored in this study with complete petrologic geochemical and structural descriptions the ultramafic body comprises cumulate layered lherzolite and gabbro plutonic lithologies tectonized with syn magmatic extension a u pb age of 520 3 5 8 ma was obtained by shrimp analysis of zircons from a pegmatitic gabbro the ultramafic rocks are variably depleted and some show refertilization of lile s consistent with slab dehydration in a supra subduction mantle wedge mineral chemistry shows high cr 36 82 populations of spinel and wide ranging lower mg 8 50 consistent with known forearc peridotite chemistry partially re equilibrated by tectonism or through staged magmatism olivine and pyroxene chemistries are consistent with this interpretation a thin volcanic cover is interlayered with red cherts and has a bimodal set of geochemical signatures both island arc tholeiite and calc alkaline they bear distinct nb ta anomalies consistent with slab derived fluid initiated melting in a supra subduction environment coarse volcaniclastic turbidites infilled paleo topography along the ophiolite and they are interbedded with quartzofeldspathic sandstones indicating an initial proximity to an arc the sandstones contain detrital spinel with analyses showing relation to both the ophiolite and possibly a secondary alkalic volcanic source the supracrustal sequence continues with a thick contiguous covering of margin related rocks including interlayered limestone pyroclastic and siliciclastic rocks and basalts as a constraint on emplacement we provide an u pb zircon shrimp date 413 6 3 5 ma from an undeformed granitoid body intruded within the overlying stratigraphy the entire analysis paints a picture of subduction rollback related extension affecting the margin of an emerging or mature arc just prior to a known magmatic cycle that ended with a collision in the devonian at the latest this suture is one of many in the beishan and our interpretation provides evidence in a long running debate supporting accretionary orogenesis in an island arc archipelago as the main system for growing sialic crust in the paleozoic central asia

origin of the amphibolite sole of the josephine ophiolite emplacement of a cold ophiolite over a hot arc the amphibolite sole that occurs along the basal thrust of the josephine ophiolite madstone cabin thrust is distinct from those described for ophiolites such as the bay of islands and oman antigorite bearing serpentinite mylonites occur directly above the amphibolite and the presence of antigorite brucite indicates formation between 300 and 400 c the antigorite mylonites also contain relict lizardite chrysotile suggesting formation of antigorite by prograde metamorphism along the base of the ophiolite further evidence for prethrusting low temperature serpentinization comes from a lizardite chrysotile shear zone within fresh peridotite directly above the thrust that is cut by undeformed diabase dikes having geochemical affinities to the upper pillow lavas of the josephine ophiolite the prograde metamorphism of the serpentinite at the base of the ophiolite was apparently synchronous with greenschist facies retrogressive metamorphism of the underlying amphibolite the amphibolite is chemically diverse and has been divided into two groups group 1 amphibolite appears to be derived from mafic plutonic rocks and has immobile trace element abundances indicating affinities to volcanic arc magmas group 2 amphibolite is metavolcanic contains some quartzite and has magmatic affinities to mid ocean ridge basalt and within plate basalt an association that is distinctive of older basement rocks elsewhere in the western klamath terrane we suggest the amphibolite was derived from the footwall of the madstone cabin thrust including country rocks for the chetco complex group 2 and possibly older intrusive phases of the chetco complex itself group 1 previous workers have shown that the base of the amphibolite is intruded by gneissic gabbro of the chetco complex that intrusion of the chetco complex overlapped in time with deformation of the amphibolite and that during and after emplacement the josephine ophiolite and overlying flysch were intruded by abundant arc related dikes and plutons emplacement of the josephine ophiolite appears to have occurred by underthrusting of a cold ophiolite by an active magmatic arc

an expanded view of jurassic orogenesis in the western united states cordillera middle jurassic pre nevadan regional metamorphism and thrust faulting within an active arc environment klamath mountains california basaltic to basaltic andesitic volcaniclastic rocks and their contemporaneous mafic ultramafic intrusive complexes delineate a middle jurassic arc terrane within the klamath mountain province of northern california exposures of the supracrustal volcaniclastic rocks are restricted to a single fault bounded terrane but the deeper level intrusive complexes were emplaced into most if not all the pre late jurassic terranes of the klamath mountain region the pre late jurassic terranes thus constitute the basement of the middle jurassic arc u pb zircon analyses of 39 zircon fractions from 12 intrusive complexes plus k ar dating of the volcaniclastic strata demonstrate magmatic activity over the interval of 177 159 ma the active arc and its basement were imbricated by a compressive deformational event the signature of which included thrust faulting isoclinal folding and regional metamorphism several diverse lines of evidence including k ar dating of metamorphic rocks crosscutting relations of dated intrusive complexes to thrust faults and u pb dating of synmetamorphic intrusive complexes establish a distinctly pre nevadan middle jurassic age ongoing at 169 ma and over by at least 161 ma for this compressive deformational episode outboard and structurally beneath the middle jurassic arc and its basement are several terranes that collectively comprise the western jurassic belt these terranes were deformed and regionally metamorphosed during the late jurassic nevadan orogeny which occurred within the time interval of 157 150 ma as upper jurassic plutons with 150 to 142 m y old zircon ages have contact aureoles that overprint the nevadan fabric and the 157 m y old rogue formation was deformed in the nevadan event the middle and late jurassic compressive deformational events were thus distinct and separated by as much as 15 20 m y the relation between middle and late jurassic magmatism and deformation suggests that the klamath mountain province records the evolution of a considerably long lived arc system that evolved above an east dipping subduction zone in addition we suggest that this are system may represent an oceanic continuation of the long recognized early mesozoic arc terrane of the western u s cordillera

the devils elbow ophiolite remnant and overlying galice formation new constraints on the middle to late jurassic evolution of the klamath mountains california the josephine ophiolite and its sedimentary cover galice formation of the western jurassic belt klamath mountains northern california are the expression of a late jurassic rift basin which developed between an active late jurassic arc to the west and a remnant middle jurassic are to the east the devils elbow ophiolite remnant deo and its sedimentary and volcanic cover represent the southern continuation of the josephine ophiolite and overlying galice formation the deo consists of a dike complex volcanic sequence and ophiolitic breccia overlying strata consist of terrigenously derived turbidites and andesitic to dacitic volcaniclastic rocks the following relations suggest formation of the deo in a marginal basin setting in accordance with the setting inferred for the josephine ophiolite basin 1 dikes and lavas are characterized by an abundance of clinopyroxene phenocrysts by crystallization of clinopyroxene before plagioclase and by trace element geochemistry which is transitional between iat and morb 2 overlying sedimentary rocks appear to have been derived from erosion of older klamath mountain terranes and were deposited into the deo basin shortly after and or during its formation 3 the texture and composition of the volcanic deposits interlayered with sedimentary strata indicate the presence of a nearby active volcanic arc source the period of rifting which culminated in the formation of the josephine ophiolite deo basin began by at least 164 ma the age of the deo but is unlikely to have begun much earlier as the region was experiencing a major period of compression just prior to this time rifting began at least 5 m y prior to cessation of magmatism in the rifted middle jurassic arc the overlap in rifting and arc magmatism can be explained if spreading in the josephine ophiolite deo basin was asymmetric relative to the axis of middle jurassic are magmatism and or if transform motion approximately parallel to the middle jurassic are was important in the spreading geometry of the basin the western boundary fault of the western jurassic belt and klamath mountain province is the south fork fault previously referred to as part of the coast range thrust in the southern western jurassic belt a serpentinite matrix meelange smm occurs along this fault blocks in the smm consist largely of greenstones with the trace element geochemistry of morb other block types include gabbroic to dioritic intrusive rocks diabase harzburgite amphibolite chert sandstone and shale generally similar serpentinite matrix melanges have been described from other locations along the south fork fault this spatial association suggests that the south fork fault provided a pathway for the bouyant rise of serpentinite or serpentinite matrix melange from a source at depth obvious sources for the smm include parts of the franciscan complex the josephine ophiolite deo or possibly the coast range ophiolite the geochemistry and relative abundances of different lithologies in the smm however indicate that none of these could represent the source in contrast the smm is nearly identical in block content and block geochemistry to serpentinite matrix melange of the rattlesnake creek terrane which structurally overlies the western jurassic belt to the east this suggests that a previously unrecognized part of the rattlesnake creek terrane may underlie the josephine ophiolite deo at depth as the rattlesnake creek terrane formed the western part of the basement for the rifted middle jurassic arc the part of the rattlesnake creek terrane which underlies the josephine ophiolite deo may represent a fragment of crust which was rifted oceanward with the opening of the josephine ophiolite deo basin and then subsequently thrust beneath the basin during the late jurassic nevadan orogeny

oceanic versus emplacement age serpentinization in the josephine ophiolite implications for the nature of the moho at intermediate and slow spreading ridges we present field petrographie and geochemical evidence for oceanic serpentinization in the 162 ma josephine ophiolite of nw california and sw oregon undeformed and unrodingitized dikes that intruded into serpentinized shear zones provide time markers for serpentinization and deformation the dikes intruding serpentinites are of two types 1 fe ti enriched dikes with normal mid ocean ridge basalt nmorb magmatic affinity which are geochemically linked to the uppermost lavas and a late fe ti dike within the crustal sequence of the ophiolite and 2 hornblende bearing calc alkaline dikes intruded during ophiolite emplacement between 150 and 146 ma based on crosscutting relationships between dikes and serpentinites serpentinization of upper mantle peridotites took place at or near the ridge axis during periods of amagmatic structural extension in the absence of a magma chamber lizardite bearing seipentinites in the northern peridotite are constrained to be oceanic and indicate temperatures of 350 c within the oceanic upper mantle oceanic lizardite bearing serpentinized shear zones from the ultramafic cumulate section to the basal sole indicate that the entire josephine peridotite may have been transected by serpentinized shear zones prior to ophiolite emplacement the ultramafic cumulate sequence was completely serpentinized prior to ophiolite emplacement and we suggest that the paleo moho in the josephine ophiolite is a serpentinization boundary the basal sole is interpreted to be a reactivated oceanic fault along which antigorite mylonites formed from a preexisting serpentinite during ophiolite emplacement oceanic serpentinization in the josephine ophiolite took place beneath a 2 3 km crustal sequence and suggests that serpentinization may be an important feature at intermediate spreading rate mid ocean ridges as well as at slower spreading ridges

remotely triggered microearthquakes and tremor in central california following the 2010 mw 8 8 chile earthquake 1 we examine remotely triggered microearthquakes and tectonic tremor in central california following the 2010 mw 8 8 chile earthquake several microearthquakes near the coso geothermal field were apparently triggered with the largest earthquake ml 3 5 occurring during the large amplitude love surface waves the chile mainshock also triggered numerous tremor bursts near the parkfield cholame section of the san andreas fault saf the locally triggered tremor bursts are partially masked at lower frequencies by the regionally triggered earthquake signals from coso but can be identified by applying high pass or matched filters both triggered tremor along the saf and the ml 3 5 earthquake in coso are consistent with frictional failure at different depths on critically stressed faults under the coulomb failure criteria the triggered tremor however appears to be more phase correlated with the surface waves than the triggered earthquakes likely reflecting differences in constitutive properties between the brittle seismogenic crust and the underlying lower crust

the setting style and role of magmatism in the formation of volcanogenic massive sulfide deposits throughout earth s history all volcanogenic massive sulfide vms hosting environments are associated with specific assemblages of mafic and felsic rocks with distinct petrochemistry petrochemical assemblages indicative of formation at anomalously high temperatures within extensional geodynamic environments in mafic dominated juvenile ophiolitic vms environments there is a preferential association with mafic rocks with boninite and low ti tholeiite mid ocean ridge basalt morb and or back arc basin basalt affinities representing forearc rifting or back arc initiation mid ocean ridges or back arc basin spreading or back arc basins respectively felsic rocks in juvenile oceanic arc environments in archean terrains are high field strength element hfse and rare earth element ree enriched in post archean juvenile oceanic arc terrains felsic rocks are commonly hfse and ree depleted and have boninite like to tholeiitic signatures in vms environments that are associated with continental crust i e continental arc and back arc and dominated by felsic volcanic and or sedimentary rocks evolved environments felsic rocks are the dominant hosts to mineralization and are generally hfse and ree enriched with calc alkalic a type and or peralkalic affinities representing continental arc rifts continental back arcs and continental back arcs to continental rifts respectively coeval mafic rocks in evolved environments have alkalic within plate ocean island basalt like and morb signatures that represent arc to back arc rift versus back arc spreading respectively the high temperature magmatic activity in vms environments is directly related to the upwelling of mafic magma beneath rifts in extensional geodynamic environments e g mid ocean ridges back arc basins and intra arc rifts underplated basaltic magma provides the heat required to drive hydrothermal circulation extensional geodynamic activity also provides accommodation space at the base of the lithosphere that allows for the underplated basalt to drive hydrothermal circulation and induce crustal melting the latter leading to the formation of vms associated rhyolites in felsic dominated and bimodal vms environments rifts also provide extensional faults and the permeability and porosity required for recharge and discharge of vms related hydrothermal fluids rifts are also critical in creating environments conducive to preservation of vms mineralization either through shielding massive sulfides from seafloor weathering and mass wasting or by creating environments conducive to the precipitation of subseafloor replacement style mineralization in sedimented rifts subvolcanic intrusions are also products of the elevated heat flow regime common to vms forming environments shallow level intrusive complexes i e within 1 3 km of the seafloor may not be the main drivers of vms related hydrothermal circulation but are likely the manifestation of deeper seated mantle derived heat i e 3 10 km depth that drives hydrothermal circulation these shallower intrusive complexes are commonly long lived i e millions of years and reflect a sustained thermally anomalous geodynamic environment such a thermally anomalous environment has the potential to drive significant hydrothermal circulation and therefore multi phase long lived subvolcanic intrusive complexes are excellent indicators of a potentially fertile vms environment the absence of intrusive complexes however does not indicate an area of low potential as they may have been moved or removed due to post vms tectonic activity in some cases shallow level intrusive systems contribute metals to the vms hydrothermal system

subduction zone infancy examples from the eocene izu bonin mariana and jurassic california arcs a new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the izu bonin mariana ibm arc system and then applied to late jurassic ophiolites of cailfornia the model accounts for several key observations about the earliest stages in the evolution of the ibm system 1 subduction nucleated along an active transform boundary which separated younger less dense lithosphere in the west from older more dense lithosphere to the east 2 initial arc magmatic activity occupied a much broader zone than existed later 3 initial magmatism extended up to the modern trench over a region now characterized by subnormal heat flow 4 early are magmatism was characterized by depleted tholeiitic and ultra depleted boninitic magmas indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth 5 early arc magmatism was strongly extensional with crust forming in a manner similar to slow spreading ridges and 6 crust production rates were 120 to 180 km 3 km ma several times greater than for mature arc systems these observations require that the earliest stages of subduction involve rapid retreat of the trench we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault this resulted in strong extension and thinning of younger more buoyant lithosphere to the west this extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere addition of water and asthenospheric upwelling led to catastrophic melting which continued until lithosphere subsidence was replaced by lithospheric subduction application of the subduction zone infancy model to the late jurassic ophiolites of california provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger sierran magmatic arc and the franciscan subduction complex provides a mechanism for the formation and subsidence of the great valley forearc basin and explains the limited duration of high t high p metamorphism experienced by franciscan melanges

the lau havre taupo back arc basin a southward propagating multi stage evolution from rifting to spreading abstract the lau havre taupo lht system is an active 2000 km long back arc basin associated with pacific australian plate convergence in the sw pacific data from the lht back arc system include odp drillhole swath bathymetric and side scan imagery seismic reflection rock geochemistry magnetic anomaly heat flow shallow seismicity gravity and seismic refraction data the lht forms a longitudinal transect from southward propagating true oceanic spreading in the lau basin near tonga similarly southward propagating arc rifting in the havre trough and southward propagating continental rifting in the taupo volcanic zone tvz in northern new zealand four transitional tectonic settings are identified along the lht north to south between the continuum of spreading to rifting and comprise 1 oceanic spreading of the central lau spreading centre clsc propagating into oceanic crust formed by the eastern lau spreading centre elsc 2 oceanic spreading of the elsc propagating into rifted lau havre arc crust 3 rifting within havre arc crust propagating into rifting new zealand continental crust and 4 rifting of continental crust propagating into as yet non rifted but flexurally subsiding continental crust the enhanced spatial and temporal resolution of observed tectonic and magmatic processes along the strike of the lht is used to constrain a model for the evolution of back arc rifting to back arc spreading a sequence of five stages is identified and comprise 1 a combination of incipient simple and pure shear 2 early distributed and spatially heterogeneous half graben rifting with low angle synthetic and antithetic structures associated with rapid graben sediment infilling ephemeral early morb like rift magmatism and the initial trenchward migration of the arc front 3 a latter phase of half graben rifting mostly confined to a contiguous axial zone with continued ephemeral morb like rift magmatism and further trenchward migration of the arc front 4 initial oceanic spreading with evolving steady state morb magmatism along the axial rift graben system and 5 the reconfiguration of ridge geometry in response to changes in opening orientation leading to the development of ridge propagation into newly created oceanic crust rifting stages 2 and 3 can produce at least 120 km wide back arc crust by a balance of crustal extension with an estimated factor of 3 with rift and constructional arc magmatism in the lht system depending on latitude rates of widening during stages 2 and 3 vary between 15 and 45 mm yr 1 whilst stages 4 and 5 have full spreading rates of 90 mm yr 1

the zhaheba ophiolite complex in eastern junggar nw china long lived supra subduction zone ocean crust formation and its implications for the tectonic evolution in southern altaids abstract the composite zhaheba ophiolite complex exposed in eastern junggar in the southern altaids records an unusually long record of oceanic crust and magmatic arc evolution the zhaheba ophiolite complex consists of ultramafic rocks gabbro diorite basalt and chert intruded by diabase dikes and diorite porphyry these rocks are overlain by a several km thick section of tuffaceous rocks volcaniclastic sedimentary rocks and intermediate volcanic rocks the igneous rocks of the ophiolite complex show negative nb and ta anomalies and lree enrichment relative to hree suggesting the influence of fluids derived from a subducting oceanic slab the la icpms u pb age of zircons from gabbro is 495 1 3 5 ma zircon ages from diorite and basalt are 458 3 7 2 ma and 446 6 6 0 ma respectively the basalt is locally overlain by bedded chert diabase dikes and diorite porphyry yield the u pb ages of 421 5 4 1 ma and 423 7 6 5 ma respectively the age of stratigraphically lower part of the overlying volcanic volcaniclastic section is constrained to be about 410 ma the maximum depositional age of the tuffaceous sandstone from u pb detrital zircon ages late rhyolite at the top of the stratigraphic section yielded a u pb zircon age of 280 3 3 7 ma the age and stratigraphic relationships for the zhaheba ophiolite complex and related rocks suggest that the period of 70 ma of initial supra subduction magmatism was followed by construction of a mature island arc that spanned an additional 140 ma many other ophiolites in the southern altaids appear to record similar relationships and are represented as substrates of oceanic island arcs covered by island arc volcanism in supra subduction zone the occurrence of the zhaheba ophiolite complex with tuffaceous and intermediate to felsic volcanic rocks is different from the rock association of classic tethyan ssz ophiolites but similar to some ophiolites in north america although the zhaheba ophiolite belt is flanked by the dulate arc in the north and yemaquan arc in the south it cannot stand a suture between two arcs it is suggested that devonian carboniferous dulate arc was built on the late cambrian middle ordovician zhaheba supra subduction oceanic crust the late carboniferous rocks and early permian rocks in dulate arc are interpreted to form in the extensional process within zhaheba dulate arc composite system

dacitic ocelli in mafic lavas 3 8 3 7 ga isua greenstone belt west greenland geochemical evidence for partial melting of oceanic crust and magma mixing abstract mafic volcanic rocks in the 3 8 3 7 ga isua greenstone belt southern west greenland contain randomly distributed 1 to 10 centimeters long white spheroidal structures in this study these structures are called ocelli in the western part of the belt ocelli bearing lavas are enclosed in basaltic to picritic flows mgo 9 21 wt with a subduction zone geochemical signature the ocelli are composed predominantly of polycrystalline na plagioclase and quartz with minor hornblende and biotite whereas the surrounding amphibolite matrix basaltic host is composed mainly of hornblende ca plagioclase and quartz the ocelli are devoid of radial or concentric internal structure and display all stages of coalescence contacts between the ocelli and surrounding amphibolite matrix are sharp to gradational compositionally the ocelli are calc alkaline dacites sio2 62 9 72 0 wt mgo 0 60 3 50 wt ni 58 143 ppm cr 250 510 ppm whereas the surrounding matrix is tholeiitic basalt sio2 46 6 50 6 wt mgo 8 70 12 30 wt ni 119 175 ppm cr 330 600 ppm in terms of major element composition the isua ocelli closely resemble plagiogranites in phanerozoic supra subduction zone ophiolites field and petrographic observations and geochemical data sio2 54 2 60 7 wt mgo 3 95 7 72 wt ni 127 158 ppm cr 500 570 ppm on the transitional areas between the ocelli and the matrix suggest magma mixing between dacitic and basaltic melts on a chondrite normalized diagram the basaltic host is characterized by variably depleted lree patterns la smcn 0 30 0 94 gd ybcn 1 03 1 45 whereas the dacitic ocelli display lree enriched patterns la smcn 1 30 2 60 gd ybcn 1 32 2 58 the strongly depleted ree patterns in the basaltic host are attributed to lree loss during carbonate alteration partial melting of a forearc mantle wedge is favoured for the origin of the protolith of the basaltic host the geochemical characteristic of the ocelli cannot be explained by post magmatic alteration slab melting fractional crystallization of tholeiitic melts or liquid immiscibility we suggest that the dacitic ocelli might have been derived from hydrous melting of the fragments of oceanic crust high mg volcanic rocks that fell into the magma chamber suggesting magma crust interaction in the early earth formation of dacitic volcanic rocks by partial melting of altered oceanic crust may have played an important role in the generation of felsic crust in the early archean

a model for induced seismicity caused by hydrocarbon production in the western canada sedimentary basin over the past three decades a significant number of small magnitude and shallow earthquakes have occurred in the western canada sedimentary basin and are located along its western flank near areas of oil and gas production one of the better documented examples is the swarm of earthquakes associated with the strachan field in the alberta foothills a model based on segall s poroelastic theory is developed to account for the occurrence of earthquakes below the strachan reservoir using this methodology we show that the earthquake of 19 october 1996 underneath the strachan field was most probably triggered by gas extraction the numerical model also implies that gas extraction would cause subsidence and localized changes in in situ stress magnitudes there is a strong correlation between rates of production and the number of seismic events but the onset of major seismic activity postdates the commencement of production by approximately 5 years poroelastic modelling can account neatly for this observe

a field chemical and stable isotope study of subseafloor metamorphism of the josephine ophiolite california oregon the josephine ophiolite is a large complete ophiolite generated in a late jurassic back arc basin along western north america the ophiolite underwent subseafloor metamorphism under a steep thermal gradient as indicated by downward changes in mineralogy and 18o values characteristic mineral zonation includes mica orthoclase and hematite in the lower extrusives greenschist facies assemblages in the upper sheeted dikes and amphibolite facies assemblages in the lower sheeted dikes and high level gabbro cumulate gabbros are only incipiently altered but are depleted in 18o indicating high temperature alteration with fluids that evolved at low water w rock r ratios a second off axis circulation system is recorded by metalliferous sediments that occur 8 21 m above the ophiolite alteration in the upper sheeted dike complex and especially the extrusive sequence is heterogeneous at outcrop scale as indicated by large variations in mineralogy chemistry and 18o values within individual pillows and dikes the heterogenity is due to 1 variations in w r ratio and starting material during alteration by downwelling seawater e g pillow cores versus glassy rims and 2 localized discharge mineralization mostly epidosites superimposed on the background recharge alteration epidosites occur largely as dike parallel stringers irregular replacement of pillow and massive lava and replacement of interpillow and pillow breccia matrices the chemistry of most extrusives and sheeted dikes is characterized by loss of ca and gain in na and mg consistent with alteration by downwelling fluids in addition the sheeted dikes are depleted in k2o and the lower sheeted dikes and high level gabbros are depleted in zn and cu the 18o and d values for most samples from the extrusive sequence indicate alteration by seawater at 50 rotation of the entire crustal sequence at the spreading axis suggests that the ophiolite formed at a slow spreading center where magma chambers were episodic

oxygen isotopes in subducted oceanic crust a new perspective from siberian diamondiferous eclogites 1 there are three methods of investigating the oxygen isotope composition of oceanic crust 1 data from drilling obtained samples 2 study of obducted ophiolites and 3 from crustal derived xenoliths however each approach has limitations drilling capabilities are limited to the upper 1 3 km of the oceanic crust ophiolites are segments of oceanic crust and underlying mantle exposed at the earth s surface but their origin and emplacement are still debated nonetheless the study of ophiolite sequences has enabled many of the fundamental observations regarding ancient oceanic crust and the oxygen isotope signature of the upper mantle analyses of ophiolite sequences e g samail reveal 18o variations ranging from isotopically enriched in relation to morb 18o morb to isotopically depleted low 18o morb interpreted to show the effects of seawater interaction at different temperatures mantle derived xenoliths also display compelling geochemical evidence suggesting they are remnants of subducted ocean crust and may in fact be more representative samples some kimberlite hosted diamondiferous eclogite xenoliths with crustal protoliths reveal a marked oxygen isotope disparity between oceanic crust and the upper mantle i e a bimodal distribution of both mantle like 18o 5 6 and isotopically enriched oxygen 18o 6 8 the isotopically high 18o values are often observed in eclogite xenoliths with ultramafic protoliths which form beyond the low temperature alteration zone this finding is in contrast to traditional oxygen isotope versus depth curves of ophiolite sequences here we present a compiled data set from siberian eclogites and pyroxenites that illustrates this bimodality using magnesium number mg as a depth proxy and investigate the conditions that could potentially result in the observed abnormal oxygen isotope variations

hydraulic fracturing and the crooked lake sequences insights gleaned from regional seismic networks within central alberta canada a new sequence of earthquakes has been recognized as of 1 december 2013 in a region of previous seismic quiescence near crooked lake 30 km west of the town of fox creek we utilize a cross correlation detection algorithm to detect more than 160 events to the end of 2014 which is temporally distinguished into five subsequences this observation is corroborated by the uniqueness of waveforms clustered by subsequence the crooked lake sequences have come under scrutiny due to its strong temporal correlation 99 99 to the timing of hydraulic fracturing operations in the duvernay formation we assert that individual subsequences are related to fracturing stimulation and despite adverse initial station geometry double difference techniques allow us to spatially relate each cluster back to a unique horizontal well overall we find that seismicity in the crooked lake sequences is consistent with first order observations of hydraulic fracturing induced seismicity

the absence of remotely triggered seismicity in japan remote dynamic triggering occurred in the western united states and greece following the following the denali hector mine landers and izmit main shocks in seismically active largely geothermal areas that experienced shaking from the mainshock typically exceeding 0 2 cm sec because triggered earthquakes are small often below mw 2 investigations of whether remote triggering is a generally occurring phenomena must be concentrated in well instrumented locations as a seismically active geothermal well instrumented location japan constitutes an ideal location for such study we systematically examine whether remote dynamic trig gering occurs following mainshocks in japan producing amplitudes in the study region above 0 2 cm sec at distances greater than two estimated fault lengths from 1998 to 2004 we use both catalog data and filtered waveforms to search for seis micity increases one significant regional increase follows a mw 7 6 mainshock in march 2000 largely due to a volcanic swarm that began the day before and increased in intensity in the following days two localized increases in cataloged seismicity occur in kyushu which is the only area in the study region with onshore tectonic extension two factors possibly inhibit regional triggering in japan 1 compres sional tectonics and 2 the frequent occurrence of large mainshocks the ambient seismicity in japan combined with the lack of observable widespread triggering ren ders rate state friction and stress corrosion to be improbable triggering mechanisms fracture unclogging as a triggering mechanism is consistent with the observations

isolated cases of remote dynamic triggering in canada detected using cataloged earthquakes combined with a matched filter approach here we search for dynamically triggered earthquakes in canada following global main shocks between 2004 and 2014 with ms 6 depth 0 2 cm s we use the natural resources canada nrcan earthquake catalog to calculate statistical values in 1 1 bins in 10 day windows before and after the main shocks the statistical analysis suggests that triggering may occur near vancouver island along the border of the yukon and northwest territories in western alberta western ontario and the charlevoix seismic zone we also search for triggering in alberta where denser seismic station coverage renders regional earthquake catalogs with lower completeness thresholds we find remote triggering in alberta associated with three main shocks using a matched filter approach on continuous waveform data the increased number of local earthquakes following the passage of main shock surface waves suggests local faults may be in a critically stressed state

coseismic damage and softening of fault rocks at seismogenic depths abstract elastic stiffness a critical property for stress orientation propagation of earthquake ruptures and associated seismic waves and the capability of crustal rocks to store strain energy is expected to be highly variable throughout the seismic cycle due to complex sequences of damage and healing post seismic healing and exhumation related alteration render it impossible to assess how well rock stiffness as measured in the laboratory on samples collected from fault zones represents in situ coseismic rock stiffness at seismogenic depths here we estimate the in situ coseismic stiffness of fault rocks from the pseudotachylyte bearing gole larghe fault zone italian southern alps using aspect ratio measurements of pseudotachylyte injection veins and numerical simulations aspect ratios of injection veins cutting across tonalite and cataclasite exhibit a maximum vein aperture positively correlating with vein length to model vein opening fault and injection veins are assumed to be filled with pressurized melt consistent with recent results from studies of melt lubrication we assume that the magnitude of the melt pressure is in equilibrium with the fault normal stress and the fault vein approximately maintains constant thickness during slip the numerical simulations of injection vein opening due to pressurized frictional melt indicate that the average in situ coseismic stiffness of the wall rocks is 5 50 times smaller than the stiffness obtained from laboratory measurements on the same rocks in their present day state the disagreement between laboratory measurements and simulations brings into question the appropriateness of using laboratory derived values for rock stiffness to model coseismic processes at depth

petrology and geochemistry of boninite series volcanic rocks from the mariana trench boninite series volcanic rocks have been recovered from three dredge hauls on the inner slope of the mariana trench these hauls included olivine boninites boninites boninitic andesites and boninitic dacites as well as island arc tholeiitic basalts and andesites the boninite series volcanics range from 52 to 68 sio2 and are characterized by very low abundances of high field strength cations and heavy rare earth elements boninites and olivine boninites have phenocrysts of olivine and orthopyroxene the andesites phenocrysts of orthopyroxene and clinopyroxene and the dacites orthopyroxene clinopyroxene and plagioclase most of the major and trace element variation in the series from boninite to boninitic dacite can be modelled by fractionation of olivine orthopyroxene clinopyroxene and plagioclase in the proportions 2 5 4 1 2 leaving 47 residual liquid the fractionation must be in part open system reverse zoned phenocrysts resorbed olivine and plagioclase xenocrysts and bulk rock compositions which cannot be fit by simple closed system crystallization indicate some magma mixing and phenocryst accumulation two boninitic magma stems can be identified with similar high field strength element abundances but different amounts of ca na al and light rare earth elements there is also evidence for a magma stem transitional in chemistry from the boninites to arc tholeiites the compositions of these boninites are consistent with hypotheses for boninite formation by partial melting of a depleted mantle mixed with an incompatible element enriched fluid the mariana forearc boninite series lacks a strong iron enrichment but produces andesites with lower ti al and y zr and higher mg ni and cr than typical calcalkaline arc andesites and dacites boninites in the mariana system were erupted only in the earliest phases of subduction zone activity

geochemistry and tectonic setting of the ophiolitic ingalls complex north cascades washington implications for correlations of jurassic cordilleran ophiolites abstract this study synthesizes new and existing chemical data to interpret the tectonic setting of the late jurassic ingalls complex of the northwest cascades washington and to compare it with other cordilleran ophiolites of similar age mafic rocks of the ingalls complex represent a spectrum of magma types including normal type and enriched mid ocean ridge basalt within plate basalt and island arc basalt based on mafic magma chemistry compositions of cr spinels in associated peridotite and field relations the ingalls complex is a suprasubduction zone ophiolite formed in a back arc basin cut by an oceanic fracture zone regional tectonic relations also support this hypothesis our synthesis of available chronologic geochemical and stratigraphic data for all jurassic cordilleran ophiolites shows that the ingalls complex has similarities and differences with each the most compelling likeness is between the ingalls complex and josephine ophiolite both of which exhibit a wide range of magmatic af

mineralization alteration and hydrothermal metamorphism of the ophiolite hosted turner albright sulfide deposit southwestern oregon the turner albright sulfide deposit part of the josephine ophiolite formed on and below the seafloor during late jurassic volcanism at a back arc spreading center ore fluids were probably localized by faults which were active on the seafloor at the time of sulfide deposition the uppermost massive sulfide formed on the seafloor at hydrothermal vents similar to sulfide depositing hot springs on modern spreading centers the bulk of the sulfide mineralization formed below the seafloor within olivine basalt hyaloclastite erupted near the time of mineralization infiltration of hydrothermal fluid into the hyalo clastite altered the rock to quartz fe chlorite and quartz sericite fe chlorite intense alteration resulted in replacement of the protolith by quartz sulfide the fluid responsible for the hydrothermal alteration was evolved seawater with low ph and mg and high fe sulfide 534s values average 4 7 and indicate contribution of sulfur from both basalt and seawater sulfate the average value of sulfide 34s and the difference between sulfide and contemporaneous seawater sulfate 34s values are similar to ophiolite hosted sulfide deposits in cyprus chlorite rich hydrothermally altered basalt has 18o values of 3 0 6 8 due to high temperature 250 350 c reaction with a seawater derived ore fluid quartz in silicified basalt has higher 18o values of 12 3 15 5 probably reflecting lower temperature recrystallization of amorphous silica formed during initial alteration of basalt mudstone and clinopyroxene basalt above the sulfide horizons were not altered by the ore transporting hydrothermal fluid but these rocks were hydrothermally metamorphosed by altered seawater heated by deep circulation into hot oceanic crust this subseafloor metamorphism produced a mineral assemblage typical of prehnite pumpellyite facies metamorphism exchange with altered seawater increased the whole rock 18o of the basalts to values of 9 4 11 2

rare earth element geochemistry of the betts cove ophiolite newfoundland complexities in ophiolite formation the betts cove ophiolite includes the components of typical ocean crust pillow lavas sheeted dikes gabbros and ultramafics however the trace element geochemistry of basaltic rocks is unusual three geochemical units are recognized within the lava and dike members within the pillow lavas the geochemical units correspond to stratigraphic units upper lavas have normal i e typical for ocean floor basalts tio2 contents 0 75 to 2 0 wt heavy rare earth elements hree values in the range 6 20 chondrites and chondrite normalized ree patterns with relative lree depletion intermediate lavas have tio2 contents between 0 30 and 0 50 wt hree contents from 4 7 chondrites and extreme relative lree depletion lower lavas have anomalously low tio2 contents 0 30 wt and unusual convex downwards ree patterns with ree abundances around 2 5 chondrite these geochemical differences can be explained if the three groups were derived from different mantle sources independent mantle sources for the three units are consistent with their different 143nd144nd ratios varying at 480 m y b p from 0 51222 in a lower lava to 0 51238 in an upper lava the upper lavas may be partial melts of a source similar in composition to that of modern morb the intermediate lavas may be from a very depleted oceanic mantle second stage melt and the lower lavas may have formed by melting an extremely depleted mantle that had been invaded by a lree enriched fluid a possible tectonic environment where these different sources could be juxtaposed is a back arc or inter arc basin

boninitic and low ti subduction related lavas from intraoceanic arc backarc systems and low ti ophiolites a reappraisal of their petrogenesis and original tectonic setting a comprehensive review of boninitic and other low ti subduction related lavas from modern intraoceanic arc backarc systems and of low ti ophiolites shows a complete compositional range from boninite to island arc basalts transitional low ti types distinguishable from the main magma types are well represented and defined here as 1 transitional boninites mainly from low ti ophiolites and 2 low ti island arc basalts major compatible and incompatible elements are clearly decoupled in all these low ti subduction related magmas in general their petrogenesis reflects a high degree of hydrous but not necessarily water saturated melting at pressures lower than 15 20 kbar of severely depleted mantle sources enriched in lile and h2o which are residual after the earlier extraction of basalt s as the residues after the extraction of low ti subduction related magmas very likely have a dunitic clinopyroxenefree harzburgitic composition it is proposed that the cao al2o3 ratios of the primary melts reflect those of their mantle sources particularly for the boninites and transitional boninites in this model the source of boninites sensu stricto would show a variable degree of depletion from clinopyroxene bearing to clinopyroxene free harzburgite whereas those of transitional boninites low ti island arc basalts would have clinopyroxene bearing harzburgite clinopyroxene poor lherzolite compositions the metasomatizing components involved in the genesis of low ti island arc basalts have a very close affinity with those involved with the sources of normal island arc tholeiites the common eruptive setting indicates that these low ti lavas represent extreme magma types of an otherwise normal island arc tectonomagmatic regime the petrogenesis of tertiary boninites and transitional boninites and the comparable magma types of the low ti ophiolites require at least these components 1 a variably depleted mantle source probably a subarc mantle residual after island arc basalt extraction 2 a hydrous fluid probably derived from dehydration of the subducted altered oceanic crust which carries most of sr ba k and rb in the subarc source and 3 a lree zr and nb enriched component with a low 143nd 144nd ratio probably derived from an ocean island type source it is proposed that this third component also carried significant amounts of rb and k which totally obliterated the ba spike typical of island arc tholeiites iats and produced the distinctive rb spike typical of the boninite transitional boninite magmas this component is hypothetically identified in liquids produced by partial melting of small scale large magnitude k richerite phlogopite bearing heterogeneities within the hot rising asthenospheric diapirs required in the model of crawford et al 1981 for the genesis boninite transitional boninite magmas geochemical thermal and geological constraints are best explained in a model which restricts the genesis of boninite transitional boninite magmas to the early stages of intraoceanic interarc spreading and to important rifting phases of the arc and or early interarc basin structures connected with hot rising asthenospheric mantle diapirs

the rattlesnake creek terrane klamath mountains california an early mesozoic volcanic arc and its basement of tectonically disrupted oceanic crust the rattlesnake creek terrane in the south western klamath mountains consists of a serpentinite matrix melange basement unconformably overlain by a coherent cover sequence of upper triassic and lower jurassic volcanic hemipelagic and clastic sedimentary rocks both the meelange and cover sequence are intruded by a suite of ca 193 207 ma u pb zircon gabbroic to quartz dioritic plutons these rocks collectively record a complex multistage evolution through several distinct tectonic environments blocks in the meelange basement include peridotite greenstone amphibolite pillow basalt various generally mafic plutonic rocks metachert and limestone all suspended in a matrix of sheared serpentinite trace element chemistry indicates that basalt amphibolite and greenstone blocks were derived from normal to enriched mid oceanic ridge basalt n morb to e morb and within plate basalt wpb protoliths there is no geochemical evidence that any blocks were derived from a magmatic arc nor do any of the sedimentary rocks contained within the meelange reflect deposition near a terrigenous sediment source collectively these relations indicate that the basement meelange was derived by disruption of oceanic crust and upper mantle in a setting far removed from either an arc or a terrigenous landmass and are most consistent with an interpretation that meelange formation oc curred in an oceanic fracture zone the overlying cover sequence is divisible into a stratigraphically lower salt creek assemblage and a stratigraphically higher dubakella mountain assemblage both deposited in a marine setting volcanic rocks in the lower assemblage are basaltic pillow lava and pillow breccia with the trace element characteristics of a primitive island arc tholeiite suite whereas those in the upper assemblage are predominantly chnopyroxene phyric basalt to basaltic andesite and compositionally similar volcaniclastic rocks with the geochemical characteristics of a more evolved cale alkaline to shoshonitic volcanic arc suite the geochemical character of these volcanic rocks is indistinguishable from that of the coeval early mesozoic plutons leading to the conclusion that these plutons represent the intrusive roots of the volcanic succession collectively these rocks reflect construction of a late triassic to early jurassic island arc and indicate that subduction had initiated within or near the under lying fracture zone assemblage by at least the late triassic the latest stages of crustal disruption in the basement thus probably occurred in a subduction zone environment both cover sequence units also contain inter layered chert and argillite as well as coarse epiclastic rocks with an abundance of quartzose sedimentary and metasedimentary grains these elastic rocks provide the first evidence for terrigenous input into the rattle snake creek terrane and indicate that the early mesozoic arc developed in proximity to a terrigenous sediment source probably paleozoic rocks of the western north american cordillera several lines of evidence suggest that early mesozoic arc magmatism was accompanied by an episode of extensional tectonism these include structural evidence for an early mesozoic phase of high angle faulting that appears to have localized cover sequence deposition in fault bounded grabens sedimentologic evidence indicating that both the basement and terrigenous sediment sources experienced episodic pulses of uplift and erosion during cover sequence deposition and widespread intrusion of arc plutons into the upper crustal levels of the are carapace both the basement complex and the early mesozoic arc assemblage of the rattlesnake creek terrane were then deformed by thrust faulting folding and penetrative foliation development during over thrusting from the east by the western hayfork terrane in the middle jurassic regional de formation was in response to northeast south west directed shortening and was accompanied by metamorphism to subgreenschist grade insofar as the western hayfork terrane represents part of a middle jurassic arc whose plutonic roots intrude most klamath mountain terranes east of and including the rattle snake creek terrane it is clear that the rattlesnake creek terrane formed part of the klamath mountain province by middle jurassic time the paleogeographic and tectonic relation of the rattlesnake creek terrane to the rest of the klamath mountains prior to the middle jurassic however is unclear

remote triggering of tremor along the san andreas fault in central california received 30 august 2008 accepted 16 april 2009 published 18 july 2009 1 we perform a systematic survey of triggered tremor along the san andreas fault in central california for the 31 teleseismic earthquakes with mw 7 5 since 2001 we identify 10 teleseismic events associated with clear triggered tremor about 52 of the tremor is concentrated south of parkfield near cholame where ambient tremor has been identified previously and the rest is widely distributed in the creeping section of the san andreas fault north of parkfield tremor is generally initiated and is in phase with the love wave particle velocity however the pattern becomes complicated with the arrival of the rayleigh waves and sometimes tremor continues after the passage of the surface waves we identify two cases in which tremor is triggered during the teleseismic pkp phase these results suggest that while shear stress from the passage of the love waves plays the most important role in triggering tremor in central california other factors such as dilatational stresses from the rayleigh and p waves also contribute we also examine the ambient tremor occurrence rate before and after the teleseismic events and find a transient increase of stacked tremor rate during the passage of the teleseismic surface waves this observation implies that the occurrence time of tremor is temporally advanced by the dynamic stresses of the teleseismic waves the amplitude of the teleseismic waves correlates with the occurrence of triggered tremor and the inferred tremor triggering threshold is 2 3 kpa the relatively low triggering threshold indicates that the effective stress at the tremor source region is very low most likely due to near lithostatic fluid pressure

time relations and structural stratigraphic patterns in ophiolite accretion west central klamath mountains california new geochronological data and published structural and stratigraphic data show that two distinctly different ophiolitic assemblages formed in general proximity to one another at nearly the same time and were subsequently imbricated along a regional thrust zone the josephine ophiolite constitutes a complete oceanic crust and upper mantle sequence which lies within the western jurassic belt of the klamath province within the study area the josephine ophiolite was formed by seafloor spreading at about 157 m y before present it was immediately covered by a thin pelagic and hemipelagic sequence which grades into a thick flysch sequence both of which comprise the galice formation the galice flysch was derived from volcanic arc and uplifted continental margin orogenic assemblages a major nonvolcanic source for the galice flysch appears to have been the western paleozoic and triassic belt of the klamath province exposed to the east proximal volcanic arc activity migrated to the site of the josephine galice section by 151 m y and is represented by numerous dikes and sills which intrude the ophiolite and galice formation the preston peak ophiolite is a polygenetic assemblage consisting of 1 a pre mid jurassic tectonitic peridotite amphibolite substrate which represents disrupted and unroofed basement of the western paleozoic and triassic belt and 2 an upper mafic complex which was intruded through and constructed above the tectonite substrate at about 160 m y the mafic complex consists primarily of diabase hypabyssal rocks that are overlain by diabase clast breccia and hemipelagic deposits a major arc plutonic complex was emplaced into the preston peak ophiolite in at least two pulses at 153 and 149 m y major phases of this complex consist of wehrlite gabbro pyroxene diorite and hornblende diorite the josephine ophiolite is interpreted as the remnants of interarc basin crust the preston peak ophiolite is interpreted as either a primitive remnant arc complex or a rift edge facies for the josephine interarc basin the galice formation represents a submarine fan complex that was built on juvenile crust of the josephine basin floor during the time interval of 153 to 149 m y the locus or arc magmatism migrated to an area which included the interarc basin floor and the remnant arc or basin edge the basin shortly thereafter closed by convergent tectonics during the nevadan orogeny resulting in the imbrication of the josephine and preston peak ophiolites and their superimposed arc assemblages the transition from seafloor spreading generation of josephine ophiolite to its tectonic accretion by convergence and basin closure occurred within 5 to 10 m y the process of rifting and ophiolite formation in series with convergence and ophiolite accretion is considered an important mechanism for generating and displacing allocthonous terranes in the klamath mountains sierra nevada region and perhaps throughout the western cordillera

subsurface configuration of the orleans fault implications for deformation in the western klamath mountains california geologic and geophysical evidence from western klamath mountains california provides quantitative information on the shape and regional subsurface extent of the orleans fault this major jurassic thrust fault lies beneath all exposures of the western paleozoic and triassic belt within the study area and mainly dips gently or is subhorizontal the regional extent of the juxtaposition of different units across the fault implies a minimum horizontal displacement of 110 km due to westward overthrusting the fault cuts the wooley creek batholith which indicates that thrusting took place after intrusion at 163 3 ma the shape of the fault near condrey mountain is consistent with post thrusting neogene uplift of the condrey mountain dome and suggests a minimum of 6 km of differential uplift

geochemical evidence in clinopyroxenes from gabbroic sequence for two distinct magmatisms in the oman ophiolite abstract in the oman ophiolite one of the best preserved and most studied ophiolites in the world two distinct petrogenetic suites of gabbroic rocks from the layered gabbro sequence of the wadi haymiliyah section is established using trace element chemistry of ca rich clinopyroxenes the earlier gb1 suite is characterized by plagioclase with lower an ca ca na content and clinopyroxene with low large ion lithophile elements lile concentrations the later dwgb2 suite contains plagioclase with rather high an content and clinopyroxene with high lile this difference in clinopyroxene chemistry can be extended to the extrusive rocks in this section lower earlier hv1 suite with low lile clinopyroxene and upper later hv2 suite with high lile clinopyroxene difference in lile concentration of clinopyroxenes is essentially due to geochemical difference in parental magmas the gb1 hv1 suites formed at fast spreading mor setting and dwgb2 hv2 suites at ssz setting supporting a model of transition from mid oceanic ridge to supra subduction zone settings of the oman ophiolite our results indicate that geochemical signature of clinopyroxene is a very strong tool for identification of tectonic setting of ophiolites

episodic magma chambers and amagmatic extension in the josephine ophiolite geochemical and structural aspects of the josephine ophiolite indicate that it formed by alternating cycles of magmatic and structural amagmatic extension episodic magma chambers are indicated by the presence of very primitive lavas in the lower pillow lavas which would have mixed into a magma chamber fe ti basalts that occur in the uppermost extrusive rocks and plagiogranites that occur along the sheeted dike gabbro contact represent highly fractionated magmas produced during the last stages of crystallization of a magma chamber during periods of amagmatic extension breccias accumulated and the entire crustal sequence was tilted 50 probably by faulting above an upper mantle detachment more than 1000 m of uplift and down dropping between magmatic cycles is suggested by the presence of out of place screens in the sheeted dike complex

geochemistry of lau basin volcanic rocks influence of ridge segmentation and arc proximity side scan sonar imaging of the central lau basin sw pacific has revealed a central lau spreading centre clsc propagating southwards at the expense of an eastern lau spreading centre elsc with a small intermediate lau spreading centre ilsc forming a relay between the two volcanic rocks sampled along these spreading centres and from two adjacent seamounts are glassy to fine grained pillow lavas and sheet flows of basalts ferrobasalts and andesites the evolved rocks are mostly confined to the propagating tip of the clsc and can be explained by a high rate of cooling relative to magma supply as invoked for magma genesis at propagating ridges elsewhere compared with equivalent rocks from the eastern pacific the most evolved members of the clsc suite require similarly high degrees 90 of fractional crystallization from their basaltic parents fractional crystallisation cannot however account for the compositional differences between clsc and elsc lavas whereas the composition of the clsc lavas lies just within the compositional spectrum of typical n morb the elsc lavas are distinctly enriched in alkali and alkaline earth elements reach oxide and apatite saturation at lower fe ti and p concentrations and generally show greater vesicularity despite slightly greater depths of eruption all indicative of a water rich subduction component they also have lower contents of ni sc na and fe and higher contents of si at a given mgo concentration that indicate a more depleted and more hydrous mantle beneath the elsc compared with the clsc these results provide further evidence that beneath the central lau basin the source composition changes progressively from morb type to island arc tholeiite type as the subduction zone is approached both eastwards from the clsc to elsc and southwards along the elsc to the valu fa ridge they also indicate that the composition of the subduction component may vary systematically away from the arc with th lree ba rb and h as h2o all present close to the arc only ba rb and h2o present at intermediate distances and just h2o perceptible at the furthest distances

an investigation of seismicity clustered near the cordel field west central alberta and its relation to a nearby disposal well historically seismicity documented in the western canada sedimentary basin has been relatively quiescent and earthquakes are usually restricted to the foreland belt of the rocky mountains however exceptional clusters of events which have remained active for decades are recognized in alberta in this study we investigate the seismicity in this region using data obtained from recently established regional arrays emphasizing the relationship between a disposal well in the cordel field and a nearby cluster of previously reported earthquakes we explore temporal correlations of wastewater pumping rates and local seismic activity dating back to 1960 we find that the first statistically significant increase in seismicity lags the onset of wastewater injection october 1991 by 3 33 years in particular the waveform similarity of 32 events are analyzed from continuous data recorded at nor a nearby 30 km station operated by the university of alberta starting in september of 2006 results from this analysis suggest that many events are well correlated in the characteristics of the waveforms and thus are likely to share a similar origin and source mechanism the most prolific of these multiplets repeats more than 10 times sporadically throughout the entire duration of recorded data from october 2006 to march 2012 despite the limited availability of nearby stations which adversely affects the resolution of our analysis hypocenter depths could be relatively accurately determined from waveform synthesis and double difference methods the results of our analysis provide first order evidence that the seismicity is consistent with fluid injection induced events

boninitic volcanism in the oman ophiolite implications for thermal condition during transition from spreading ridge to arc the discovery of boninite a typical high mgo andesite in the oman ophiolite is reported the boninites in the oman ophiolite occur as lavas and dikes of the alley volcanic sequence that overlie or crosscut the spreading ridge derived lavas geotimes volcanic sequence and sheeted dikes the phenocryst mineral assemblage and the major and trace element compositions observed for these boninites resemble those of the izu mariana forearc boninites indicating that the alley boninites represent primitive melt generated by partial melting of hydrous peridotite the occurrence of boninite provides strong thermal and chemical constraints on the formation of the oman ophiolite that require hot hydrous shallow mantle 1250 c at 30 km depth to have underlain the proto oman ophiolite at the time of boninite generation the initiation of subduction of the young hot oceanic lithosphere and obduction of the future oman ophiolite near the spreading ridge and the resultant melting of the highly depleted shallow mantle wedge metasomatized by slab derived fluid represent the most favorable mechanism for the genesis of the alley boninites

fe ti basalts and propagating rift tectonics in the josephine ophiolite the jurassic josephine ophiolite of northwest california and southwest oregon is one of the largest and most complete ophiolites in north america and clearly formed in a suprasubduction zone setting the geochemistry of lavas and dikes is highly variable because of a range in magma types and different degrees of fractionation basalts rich in fe and ti occur in the uppermost part of the extrusive sequence and as late dikes cutting oceanic serpentinite they have morb mid oceanic ridge basalt affinities and appear to be unrelated to the rest of the ophiolite that has affinities dominantly transitional between iat and morb iat island arc tholeiite this late fe ti morb suite was erupted while the ophiolite was undergoing the last half of 50 of tilting of the entire crustal sequence and after widespread serpentinization of ultramafic cumulates and uppermantle peridotite related to freezing of the axial magma chamber fe ti basalts are characteristic of propagating spreading centers on mid ocean ridges and in at least one backarc basin geochemical structural stratigraphic and regional geologic constraints suggest formation of the josephine ophiolite by propagation of a spreading center into rifted island arc lithosphere that is preserved along the margin of the ophiolite another possible indication of propagating rift tectonics is the presence of off axis metalliferous sedimentary rocks that may have been deposited as hydrothermal plume fall out from a second propagating spreading center the lau basin provides modern analogues including spreading centers propagating into rifted arc lithosphere into older backarc ocean crust and across the arc into the forearc

earthquakes near rocky mountain house alberta and their relationship to gas production facilities a 1980 field survey of earthquake activity near rocky mountain house alberta using one digital and six conventional seismographs recorded 146 microearthquakes m 3 4 in 23 days and located 67 of them the located events and probably the unlocated events as well all occurred in a small zone approximately 4 km north south by 4 km east west by 1 km thick centred near 52 12 5 n 115 15 w at a depth of 4 0 km with an uncertainty of 2 km on each of the location coordinates the activity occurred below and or in the strachan d 3a sour gas reservoir which is a devonian aged limestone reef complex in a section of nearly flat lying unfaulted sediments the earthquakes had a composite thrust faulting mechanism with generally north trending intermediate dipping planes in the presence of a regional deviatoric stress field that was horizontally compressive and oriented approximately east west the earthquake activity may be related to the extraction of the natural gas and related fluids from the reservo

seismic triggering by rectified diffusion in geothermal systems widespread seismicity was triggered by the june 28 1992 landers california earthquake at a rate which was maximum immediately after passage of the exciting seismic waves rectified diffusion of vapor from hydrothermal liquids and magma into bubbles oscillating in an earthquake can increase the local pore pressure to seismically significant levels within the duration of the earthquake in a hydrothermal system modeled as a two component h 2o co 2 fluid in porous rock the pressure initially increases linearly with time the rate of pressure buildup depends sensitively on the mean bubble radius and is large for small bubbles the diffusion induced pressure is relaxed by percolation and resorption of vapor into the liquid solution the induced seismicity itself also relieves stress values of parameters used in the present calculations give results consistent with observations of triggered seismicity at long valley caldera after the landers earthquake for one representative condition at 250 c and 5 6 km depth oscillating strain acting on 10 m diameter bubbles increases pore pressure at the rate of 151 pa s resulting in a pressure increase of 12 kpa in the 80 s duration of the landers earthquake the elevated pressure induced by a single 26 m diameter cloud of bubbles in saturated rock relaxes by percolation through soil of 0 2 mdarcy permeability in 53 6 hours observations of earthquake swarms at other locations suggest that self induced buildup of pore pressure by rectified diffusion can provide a positive feedback mechanism for amplifying seismicity

caribbean island arc rifting and back arc basin development in the late cretaceous geochemical isotopic and geochronological evidence from central hispaniola this work froms part of the mcyt projects bte 2002 00326 and cgl2005 02162 bte and also received aid from the cartographic project of the dominican republic funded by the sysmin program of the european union careful reviews from j geldmacher a c kerr and an anonymous reviewer are much appreciated

the josephine ophiolite northwestern california the 157 m y old josephine ophiolite is a complete ophiolite consisting of the following units harzburgite tectonite cumulate ultramafics cumulate gabbro interlayered with cumulate ultramafics cumulate gabbro high level gabbro and diorite a 1 5 km thick sheeted dike complex and 0 4 km of pillow lava and breccia the ophiolite is conformably overlain by the galice formation consisting of a thin siliceous pelagic sequence overlain by a thick sequence of turbidites having both volcanic arc and continental margin sources the ophiolite and overlying galice formation were intruded by numerous 150 to 151 m y old calc alkaline dikes and sills followed shortly by deformation and regional metamorphism under conditions of prehnite pumpellyite to lower greenschist facies the josephine peridotite makes up the base of the ophiolite and covers 800 km2 it is an alpine type peridotite that represents upper mantle material and the residue of partial melting ultramafic rocks overlying the josephine peridotite are olivine spinel clinopyroxene cumulates that grade upward into clinopyroxene and orthopyroxene olivine plagioclase cumulates and finally to clinopyroxene plagioclase cumulates high level gabbros and diorites are characterized by highly variable textures complex intrusive relationships strongly zoned plagioclase and abundant mafic dikes plagiogranites occur at this level as dikes small intrusions and screens between mafic dikes but they are rare the sheeted dike complex consists of mafic dikes averaging 0 6 m in thickness chilled margins are locally well preserved and many of the dikes have been split and intruded by younger dikes resulting in half and marginless dikes the dike complex is composed of 100 sheeted dikes near the center but contains gabbro screens near the base and pillow lava and breccia screens in its upper parts the dikes are inclined at 40 to bedding after correcting for folding and are inferred to have been rotated shortly after formation the extrusive sequence consists predominantly of pillow lavas but contains abundant pillow breccias and some massive lavas some of the breccias formed as talus deposits whereas others are graded and matrix supported indicating transport and redeposition possibly in an area of rugged submarine topography the dikes and lavas have geochemical affinities to island arc tholeiites but are in some aspects transitional to mid ocean ridge basalts the dikes and lavas are cogenetic with the underlying plutonic sequence as indicated by similarity in crystallization sequence contradictory intrusive relationships between sheeted dikes and high level plutonic rocks and lack of diabase dikes in the lowermost cumulates and harzburgite the upper portions of the ophiolite were hydrothermally metamorphosed at the spreading center resulting in downward changes in mineralogy texture and 18o in addition metalliferous sediments and massive sulfides that occur overlying and within the pillow lavas probably were formed by precipitation from thermal springs a geothermal gradient of 250 350 c km is inferred from mineral assemblages on the basis primarily of its regional setting the ophiolite is inferred to have formed in a back arc basin in particular it is within a thrust sheet sandwiched between a coeval magmatic arc complex and a slightly older arc complex remnant arc this interpretation is consistent with abundant volcanic arc detritus in graywackes overlying the ophiolite the island arc magmatic affinities of the lavas and dikes may be the result of formation of the ophiolite during the earliest stages of back arc spreading

implications of new jurassic stratigraphic geochronometric and paleolatitudinal data from the western klamath terrane smith river and rogue valley subterranes combined biostratigraphic chronostratigraphic and geochronometric studies of the rogue and galice formations rogue valley subterrane southwestern oregon indicate that the oxfordian kimmeridgian boundary should be placed at 154 1 5 ma rather than at 156 6 ma as advocated in the decade of north american geology 1983 geologic time scale in the smith river subterrane northwestern california southwestern oregon well preserved radiolaria were recovered not only from strata overlying the josephine ophiolite but also from within the volcanic member of the ophiolite u pb geochronometric data constrain the age of the mirifusus first occurrence event to after 162 1 ma and to being 157 1 5 ma or slightly older radiolarian faunal data indicate that the josephine ophiolite originated at central tethyan paleolatitudes during the latest callovian 162 ma and was carried northward to northern tethyan and southern boreal paleolatitudes during the oxfordian