early proterozoic ties between two suspect terranes and the mojave crustal block of the southwestern u s southern california and adjacent areas contain two suspect or exotic terranes comprised largely of ancient continental crust namely the tujunga san gabriel and joshua tree terranes that have been considered part of a larger displaced terrane the santa lucia orocopia allochthon paleomagnetic data for the allochthon indicate northward transport in excess of 2000 km and thus an origin extraneous to north america however early proterozoic plutons of the mojave crustal block and the joshua tree and tujunga terranes have strikingly comparable features including 1 crystallization ages of 1 63 to 1 68 ga 2 biotite sphene magnetite pm hornblende pm garnet mineralogy 3 high lil and enriched hfs elemental composition 4 wpg within plate granite trace element chemistry 5 similar and unique oxygen isotopic compositions and 6 distinct pb and nd isotopic signatures these features of the mojave block which clearly originated as part of native north america nevertheless distingui

crustal growth associated with anorogenic mid proterozoic anorthosite massifs in northeastern north america abstract mid proterozoic anorthosite massifs north of the grenville front tectonic zone in labrador are believed to have been emplaced in an anorogenic setting chemistry of the adirondack anorthosite mangerite charnockite suite suggests that it is bimodal and was also emplaced anorogenically by inference it is assumed that the intervening massifs of the grenville province are of anorogenic derivation as well geochronologic data indicates that these complexes formed during an interval of 500 m y from 1600 1100 ma with most magma emplaced during the last 300 m y the currently exposed area of the anorthosite fraction of the massifs represents 13 of the area into which they were emplaced if this proportion is continued downward throughout a 35 40 km thick crust its volume yields a computed maximum crustal growth rate of 5 10 km 3 ma per kilometer of magmatic belt if the anorthosites are continued only to 5 km the growth rate becomes 1 2 km 3 ma per kilometer of magmatic belt although considerably less than rates of crustal addition in volcanic arcs the computed rates are substantial and indicate that mid proterozoic anorthositic magmatism made significant local contributions to continental crust during the complicated evolution of northeastern north america

the transition of the monsoonal and the n atlantic climate system off nw africa during the holocene 1 the northward extent of the influence of the w african monsoon during humid periods of interglacials is subject to investigations highlighting feedback mechanisms such as vegetation to detect this regional variation and the climate system acting farther to the north will be the aim of this paper focussing on the holocene we present two very high resolution holocene sediment records off nw africa located at 31 n and 27 n the well known mid holocene climate change from the african humid period to present arid conditions is well reflected by the terrigenous input in the southern core in contrast in the northern core spectral and wavelet analyses indicate a periodic oscillation of about 900 years of the terrigenous input throughout the last 9000 years b p we conclude that the w african monsoonal influence characterized by the abrupt climatic change at 5000 years b p can be separated from the influence of the n atlantic climate system reflected by a periodic oscillation throughout the holocene

small scale heterogeneity of phanerozoic lower crust evidence from isotopic and geochemical systematics of mid cretaceous granulite gneisses san gabriel mountains southern california an elongate belt of mid cretaceous compositionally banded gneisses and granulites is exposed in cucamonga terrane in the southeastern foothills of the san gabriel mountains of southern california banded gneisses include mafic granulites of two geochemical types type 1 rocks are similar to high al arc basalts and andesites but have higher hfse high field strength element abundances and extremely variable lile largeion lithophile element abundances while type 2 rocks are relatively low in al and similar to alkali rich mor midocean ridge or intraplate basalts intercalated with mafic granulites are paragneisses which include felsic granulites aluminous gneisses marble and calc silicate gneisses type 1 mafic granulites and calcic trondhjemitic pegmatites also oceur as cross cutting synmetamorphic dikes or small plutons small scale heterogeneity of deep continental crust is indicated by the lithologic and isotopic diversity of intercalated ortho and paragneisses exposed in cucamonga terrane geochemical and isotopic data indicate that k rb and u depletion and sm nd fractionation were associated with biotite muscovite dehydration reactions in type 1 mafic granulites and aluminous gneisses during high grade metamorphism field relations and model initial isotopic ratios imply a wide range of protolith ages ranging from early proterozoic to phanerozoic

nature and origin of proterozoic a type granitic magmatism in the southwestern united states of america the mountain ranges of arizona and adjacent california and nevada contain large areas underlain by proterozoic anorogenic granites comprising the southwesternmost portion of a transcontinental belt of 1 4 1 5 ga old anorogenic complexes that extends across north america northeast into labrador of these a two mica monazite bearing granitic suite resides in central and southeastern arizona as part of a peraluminous subprovince that is bordered on the south southern arizona to sonora and west western arizona and adjacent portions of california and nevada by marginally metaluminous granites bearing biotite sphene hornblende and fluorite all of these 1 4 ga granites are distinctly more potassic iron enriched relative to mg and depleted in ca mg and sr in contrast to typical orogenic granitoids in general the large ion lithophile element enriched composition is a consequence of limited melting of a water deficient crustal source at depths greater than 25 37 km for the peraluminous granites this contrast is less extreme perhaps resulting from a larger degree of melting as a consequence of a greater metasedimentary component and water in its crustal source the anorogenic granitic magmas intruded into the upper crust at depths of 8 17 km or shallower at temperatures up to 790 c the most dramatic variation in the crystallization intensive parameters resides in the oxygen fugacity which spans three orders of magnitude relative to other anorogenic suites all of the magmas crystallized at elevated levels of o2 as reflected in their assignment to the anorogenic magnetite series yet a regionally significant rise in primary o2 levels unmatched elsewhere in the transcontinental belt occurs for plutons in western arizona including the holy moses and hualapai granites the most extreme case is the hualapai granite whose biotite fe fe mg ratios drop due to high o2 to a low of 0 27 down from more typical levels of 0 54 to 0 75 such extreme variations in primary levels of oxygen fugacity must be an indirect imprint of regional changes of the level of oxidation of the lower crust the high fo2 holy moses and hualapai plutons have intruded near the regional boundary between the metaluminous and peraluminous granites and appear to be imaging a major change in the level of oxidation of the lower crust this boundary is also approximately equivalent to significant changes in the nd and pb isotopic compositions of these granites and the metamorphic and magmatic character of the older orogenic terrane on a global scale the crust forming orogenies ended by 1 6 ga ago and the continents entered a long lived era dominated by localized extension and transcontinental intrusion of anorogenic potassic rapakivi granite mafic dike swarms charnockite and anorthosite the absence of orogenic deformation implies that plate consumption became intraoceanic during this time the profuse and widespread nature of the igneous activity has no phanerozoic analogue and is considered to be unique to the proterozoic a crustal overturn model ties the magmatism to heating within a largely undepleted subcontinental mantle the eventual rise of mantle plumes and the transfer of heat into the youthful undifferentiated proterozoic crust subsequent melting and rise of potassic granitic magmas from the lower crust leads to considerable crustal reorganization a process that would continue until both the mantle and crust reached a stable configuration

a petrologic comparison of triassic plutonism in the san gabriel and mule mountains southern california triassic magmatism in the southwest u s cordillera forms a semicontinuous magmatic arc extending from northwestern nevada to southeastern california quartz monzodioritic and quartz monzonitic rocks and associated diorites and granites are widespread in southeastern california and we suggest that these rocks represent exposure of a structurally deeper part of the triassic arc where it was emplaced into comparatively thick proterozoic crust elemental and isotopic data suggest that triassic quartz monzodiorites and quartz monzonites in the mule and san gabriel mountains were derived from a relatively undepleted nonradiogenic mafic lithospheric source with virtually no upper crustal interaction very limited data for associated triassic diorites indicate a wide range in composition and a surprisingly radiogenic isotopic signature younger triassic granites record a strong geochemical signature of interaction with continental crust including inherited zircon and high initial sr ratios but comparatively less radiogenic pb isotopic compositions the major and trace element geochemistry of late triassic plutonic rocks in southeastern california is similar in many respects to alkalic components of the triassic arc in the mojave desert however contemporaneous rocks farther north have a calc alkalic signature perhaps reflecting the variation in age and composition of lithosphere across which the triassic arc was constructed

1 08 ga diabase sills in the pahrump group california implications for development of the cordilleran miogeocline u pb baddeleyite ages of 1087 3 and 1069 3 ma for twodiabase sills that intrude the crystal spring formation of the pahrump group southeastern california provide a minimum estimate for the depositional age of the crystal spring formation the new ages high light the potential range in deposition ages for the pahrump group from 1087 ma to possibly as young as 600 ma for kingston peak glaciogenic sediments consequently there probably are significant disconformities within the pahrump group that separate unrelated tectonically distinct episodes of basin subsidence the crystal spring and beck spring formations are considered to be platform cover rocks preserved as a result of block faulting during deposition of the upper kingston peak formation the age of the pahrump sills is in excellent agreement with the age of sills that intrude the apache group central arizona and the unkar group grand canyon and supports earlier correlations of those units

young high temperature granulites from the base of the crust in central mexico characterization of the lower continental crust is essential to understanding processes of crustal formation and evolution many workers have concluded that the lower crust is composed of granulite facies rocks see for example refs 1 2 but many exposed granulite facies terranes seem to have mid crustal rather than lower crustal origins3 5 here we present new data from direct samples of the lower continental crust in the form of metapelitic rock fragments brought to the surface by quaternary volcanism in central mexico these xenoliths contain exsolved ternary feldspars that record extremely high minimum metamorphic temperatures of 950 1 100 c the highest yet known to be preserved in deep seated metamorphic rocks the metamorphism seems to be the result of underplating by basaltic magma and to be regional in extent our barometry on these samples demonstrates that granulite facies metamorphic rocks are present in the lower crust and provides important constraints on tectonic models and interpretations of geophysical data chemical and text lira i evidence suggest that the xenoliths were not able to cool to below 850 900 c before being entrained in their host magmas indicating that the granulite facies metamorphism occurred no more than 30 myr ago

spatio temporal patterns of groundwater depths and soil nutrients in a small watershed in the ethiopian highlands topographic and land use controls abstract soil and water conservation structures promoted by local and international development organizations throughout rural landscapes aim to increase recharge and prevent degradation of soil surface characteristics this study investigates this unexamined relationship between recharge water table depths and soil surface characteristics nutrients in a small sub watershed in the northwestern ethiopian highlands these highland watersheds have high infiltration rates mean 70 mm hr 1 median 33 mm hr 1 recharging the shallow unconfined hillslope aquifer with water transport occurring via subsurface pathways down the slope the perched water tables reflect the subsurface flux and are deep where this flux is rapid in the upland areas 138 cm below surface soil saturation and overland flow occur when the subsurface flux exceeds the transport capacity of the soil in the lower downslope areas near the ephemeral stream 19 cm below surface land use is directly related to the water table depth corresponding to grazing and fallowed saturated land in the downslope areas and cultivated unsaturated land in the middle and upper parts where the water table is deeper kjeldahl total nitrogen tn bray ii available phosphorus ap and exchangeable potassium k averages exhibit different behaviors across slope land use transects or saturation conditions tn was moderate to low 0 07 0 04 in various land uses and slope regions bray ii ap had very low concentrations 0 25 mg kg 1 0 26 among the different slope regions with no significant differences throughout p 05 the exchangeable cation k ca 2 mg 2 concentrations and ph however were greater in non cultivated seasonally saturated lands and in a downslope direction p

chronologic and isotopic framework for early proterozoic crustal evolution in the eastern mojave desert region se california the early proterozoic geologic evolution of the eastern mojave desert region as defined by characteristics of its supracrustal rocks granitoids metamorphism structural history and pb and nd isotopic signature contrasts sharply with other proterozoic provinces of the southwestern united states the oldest supracrustal rocks of the mojave desert region contain zircons over 2 0 ga corroborating nd isotopic evidence for a much older crust here than elsewhere in the southwestern united states granitoids widely emplaced within these supracrustal rocks range from 1 76 to 1 64 ga the earlier plutons and surrounding supracrustal rocks were metamorphosed to granulite and high amphibolite facies throughout the province at about 1705 ma in a migmatite producing event that we term informally the ivanpah orogeny subsequent granitoids emplaced from 1 69 to 1 67 ga were voluminous along a north trending belt in the middle of the mojave province younger plutons were emplaced at about 1 66 ga in several places and at about 1 64 ga along the extreme southern part of the province commonalities between the proterozoic evolutions of the mojave and arizona crustal provinces do not conclusively establish the time that the provinces were juxtaposed the data only suggest that the juxtaposition occurred between about 1 76 and 1 64 ga

palaeoclimate africa on the edge saharan humidity has varied dramatically throughout the pleistocene era a new deep sea sediment record reveals large and rapid hydrological shifts that are linked to the competing influences of low and high latitude climate processes

geochronology of the proterozoic basement of southwesternmost north america and the origin and evolution of the mojave crustal province the proterozoic baldwin gneiss in the central transverse ranges of southern california a part of the mojave crustal province is composed of quartzofeldspathic gneiss and schist augen and granitic gneiss trondhjemite gneiss and minor quartzite amphibolite metagabbro and metapyroxenite sensitive high resolution ion microprobe shrimp data indicate that augen and granitic gneisses comprise a magmatic arc intrusive suite emplaced between 1783 12 and 1675 19 ma adjacent to or through thinned archean crust high u th rims on zircons in most samples suggest an early metamorphic event at 1741 ma but peak amphibolite facies metamorphism and penetrative west vergent deformation occurred after 1675 ma the baldwin gneiss is part of a regional allochthon emplaced by west vergent deformation over a proterozoic shelf slope sequence joshua tree terrane we hypothesize that emplacement of this regional allochthon occurred during a late early or middle proterozoic arc continent collision along the western margin of laurentia

theoretical phase relations involving cordierite and garnet revisited the influence of oxygen fugacity on the stability of sapphirine and spinel in the system mg fe al si o the theoreticalp t grid for stability relations of the phases cordierite cd sapphirine sa hypersthene hy garnet ga spinel sp sillimanite si and quartz qz of hensen 1971 has proved useful in the interpretation of metamorphic mineral assemblages formed at low oxygen fugacity both experimental data and evidence from natural rocks indicate that at high oxygen fugacity compatability relations change as a result of the enlargement of the stability field of spinel which causes a topological inversion and the stabilisation of the invariant points sa ga and cd this implies the stable existence of the univariant equilibria for f text o text 2 buffered conditions sp qz ga hy si o2 sa cd cd sp qz hy si o2 sa ga and sa sp qz hy si o2 ga cd and the divariant reaction sp qz hy si o2 sa ga cd these redox equilibria are restricted to conditions of high oxygen fugacity the proposed theoreticalp t grids for both low and high oxygen fugacity satisfactorily explain all experimental data and metamorphic mineral assemblages so far found in granulites

petrology of biotite cordierite garnet gneiss of the mccullough range nevada i evidence for proterozoic low pressure fluid absent granulite grade metamorphism in the southern cordillera proterozoic migmatitic paragneisses exposed in the mccullough range southern nevada consist of cordierite almanditic garnet biotite sillimanite plagioclase k feldspar quartz ilmenite hercynite this assemblage is indicative of a low pressure facies series at hornblende gr anulite grade textures record a single metamorphic event involving crystallization of cordierite at the expense of biotite and sillimanite thermobarometry utilizing cation exchange between garnet biotite cordierite hercynite and plagioclase yields a preferred temperature range of 590 750 c and a pressure range of 3 4 kb equilibrium among biotite sillimanite quartz garnet and k feldspar records ahlo between 0 03 and 0 26 the low ah o together with low fol qfm and optical properties of cordierite indicate metamorphism under fluid absent conditions preserved mineral compositions are not consistent with equilibrium with a melt phase earlier limited partial melting was apparently extensive enough to cause desiccation of the pelitic assemblage the relatively low pressures attending high grade metamorphism of the mccullough range paragneisses allies this terrane with biotite cordierite garnet granulites in other orogenic belts closure pressures and temperatures require a transient apparent thermal gradient of at least 50 c km during part of this proterozoic event in the southern cordillera

fast teleconnections to the tropical atlantic sector from atlantic thermohaline adjustment 1 we examine the first decade of adjustment of the north atlantic climate following an abrupt freshening of the high north atlantic and resulting slowdown of the atlantic meridional overturning circulation amoc in a coupled model with reference to previously proposed teleconnection mechanisms after an initial ocean driven cooling at the subtropical subpolar gyre boundary subsequent equatorward progression is dominated by an atmosphere surface ocean mechanism driving evaporative cooling in the tropical north atlantic and resulting in an anomalous southward itcz displacement by year 2 the tropical cooling is countered by fast ocean baroclinic adjustment to the reduced overturning that increases the volume of tropical surface waters as well as ocean circulation changes to the increased northeasterly trades however it is the atmosphere surface ocean teleconnection that explains the hemispheric asymmetric nature of the tropical atlantic surface climate impact to amoc slowdown underlining its crucial importance in high to low latitude teleconnections

chapter 3 mid crustal emplacement ofmesozoic plutons san gabriel mountains california and implicationsfor the geologic history of the san gabriel terrane

the origins of granulites a metamorphic perspective although many recent reviews emphasize a uniformity in granulite pressure temperature p t conditions and paths granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation a thorough survey of over 90 granulite terranes or occurrences reveals that over 50 of them record p t conditions outside the 7 5 1 kbar and 800 50 c average granulite regime preferred by many authors in particular an increasing number of very high temperature 900 1000 c terranes are being recognized both on the basis of distinctive mineral assemblages and geothermobarometry petrogenetic grid and geothermobarometric approaches to the determination and interpretation of p t histories are both evaluated within the context of reaction textures to demonstrate that the large range in p t conditions is indeed real and that both near isothermal decompression itd and near isobaric cooling ibc p t paths are important amphibolite granulite transitions promoted by the passage of co 2 rich fluids as observed in southern india and sri lanka are exceptional and not representative of fluid related processes in the majority of terranes it is considered on the contrary that fluid absent conditions are typical of most granulites at or near the time of their recorded thermal maxima itd granulites are interpreted to have formed in crust thickened by collision with magmatic additions being an important extra heat source erosion alone is not however considered to be the dominant post collisional thinning process instead the itd paths are generated during more rapid thinning 1 2 mm yr exposure related to tectonic exhumation during moderate rate or waning extension ibc granulites may have formed in a variety of settings those which show anticlockwise p t histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion with or without additional crustal extension ibc granulites at shallow levels very rapid 5 mm yr extensional thinning subsequent to collision it is suggested that the preservation of ibc paths rather than itd paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust and that hybrid itd to ibc paths should also be observed most ibc granulites and probably many itd granulites have not been exposed at the earth s surface as a result of the tectonic episodes which produced them but have resided in the middle and lower crust for long periods of time 100 2000 ma following these events the eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation

revisiting nitic horizon properties of nitisols in sw ethiopia abstract nitisols are a soil group whose nature and genesis are relatively poorly documented in order to better understand the development of nitic horizons characterized by a blocky structure and shiny ped faces soil profiles in a major nitisol region in ethiopia were analysed using physico chemical mineralogical and micromorphological methods as well as mossbauer spectroscopy factors that are identified as contributing to nitisol development are i the presence of open 2 1 clays besides dominant kaolinite ii seasonal cycles of wetting and drying contributing to strong blocky soil structure development and iii exceptional fe oxide characteristics particularly extremely small particle size together with the presence of abundant clay coatings these fe oxide characteristics may explain the shiny aspect of ped faces that is characteristic of nitic horizons the profiles are affected by an admixture of surface derived volcanic ash components which might be common in nitisol regions and which puts into question the use of silt clay ratio as a requirement for this wrb reference soil group

the role of phytolith formation and dissolution in controlling concentrations of silica in soil solutions and streams abstract by integrating the results of recent studies on the concentration and flux of si in european forest soils podzols and acid brown soils with published evidence on the amounts of si cycled through forest vegetation we conclude that phytoliths must be the principal immediate source and sink of silica in soil solution although mineral weathering is the ultimate source during the growing season forest vegetation takes up most of the si made available by weathering of soil minerals and by phytolith dissolution and converts it into phytoliths phytoliths will be the principal source of si leached from the soil during winter rains and spring snowmelt this work accounts for the plateau concentrations of si observed in laboratory equilibrations of bs horizon soils for the maximum si concentration reported for b horizons in the field for a deep sink for si in bs horizons and for the fact that the concentrations of si in soil drainage water and in streams all fall in or below the range expected for phytolith solubility concentrations of si in surface horizons can exceed these levels during droughts the common assumption that the concentration of si in soil solution is a result of a dynamic balance between si released by mineral weathering and that lost by leaching is untenable for bs horizons

u th pb rb sr and ar ar mineral and whole rock isotopic systematics in a metamorphosed granitic terrane southeastern california mesozoic structural domes are developed in an older proterozoic crystalline basement of granitic to granodioritic foliate metaplutonic rocks in the halloran hills southeastern california isotopic analyses of whole rocks and mineral separates from these rocks by u th pb rb sr and ar ar techniques yield a complex pattern of discordance that is the result of a fairly simple geologic history individual mineral isotopic systems have variably equilibrated with each other in response to mesozoic regional metamorphism and locally to later heating during mesozoic batholith emplacement discordant u th pb zircon data indicate that the granitic core rocks are 1 710 ma and that one dioritic phase may be slightly older rb sr whole rock model dates scatter about 1 700 ma rb sr amphibole whole rock and u th pb amphibole dates are also proterozoic potassium feldspars retain a 207 pb 206 pb signature of their proterozoic age ar ar amphibole spectra from the flank of the main dome reveal disturbed dates of 1 450 ma to 1 100 ma and the dates become younger toward the structurally deeper core of the dome all remaining isotopic determinations yield mesozoic or younger dates for mineral whole rock systems rb sr whole rock apatite feldspar biotite analyses show nonequilibration of strontium isotopes with resultant mineral pair dates from 4 foliate plutonic rocks ranging from 200 to 50 ma no single metamorphic age is indicated by the rb sr data rb sr whole rock biotite dates are consistently younger than any other determinations and may be reduced by weathering or gain of nonradiogenic strontium from ground water u pb sphene and apatite analyses from rocks that yield 1 710 ma zircon dates are nearly concordant at 140 ma an amphibole from the structurally deepest rocks of the main dome that yield 140 to 150 ma u pb sphene dates has an ar ar plateau date of 144 ma the u pb sphene and ar ar amphibole analyses are believed to be the best age estimate for the end of the highest temperature phase of regional metamorphism th pb sphene and apatite dates and ar ar biotite dates cluster at 90 5 ma as a consequence of regional cooling during late cretaceous time following extensive mesozoic plutonism in the region at 97 to 90 ma we interpret the discordant mineral date patterns to have resulted from metamorphism of 1 700 ma plutonic rocks during the jurassic 140 50 ma and subsequent uplift and cooling to 200 c at about 90 ma on the basis of this study the isotope dating systems ranked in decreasing order of resistance to resetting are u th pb zircon concordia intercept rb sr whole rock rb sr amphibole u th pb amphibole pb pb whole rock ar ar amphibole rb sr sphene u pb sphene and apatite rb sr plagioclase potassium feldspar apatite th pb sphene and apatite ar ar biotite u pb feldspars rb sr biotite

intrusion of horizontal dikes tectonic significance of middle proterozoic diabase sheets widespread in the upper crust of the southwestern united states initially horizontal sheet intrusions of middle proterozoic diabase are abundant in a region 650 by 300 km across in arizona and california the diabase forms discordant sheets in basement granite and gneiss and sills in overlying shelf sedimentary sequences massive granite is the most common basement host for the sheets probably because it fractured more easily than foliated hosts during sheet emplacement steep feeder dikes are rare compared to the sheets the diabase in many places is exposed in fault blocks that were tilted during tertiary tectonic extension structure sections restored from the map patterns of upended blocks show that the sheets were intruded at levels throughout the upper crust to depths of at least 13 km sheet intrusion implies a vertical orientation of the least compressive stress so i conclude that the crust was under tectonic compression or in an isotropic state of stress at the time of diabase intrusion about 1 1 ga magma overpressures water encountered by rising magma vertical changes in the crustal stress regime and flotation of low density granite all may be important factors for sheet intrusion the stress conditions suggested by the presence of the sheets argue against an extensional tectonic regime earlier proposed for the diabase event the diabase province contrasts structurally with similar age provinces of basaltic magmatism elsewhere in north america that show evidence of tectonic extension such as the midcontinent rift the consistent orientations of the sheets commonly allow them to be used as structural markers for postdiabase deformation of basement blocks sheet intrusions in the geologic record may be seriously underreported their recognition is important for the interpretation of seismic reflection profiles of continental crust sheets in arizona may be responsible for the bagdad reflection sequence which extends to depths of at least 15 km

southern extension of the independence dike swarm of eastern california new geochronologic data show that jurassic dikes in the western mojave desert and eastern transverse ranges are part of the independence dike swarm of eastern california these occurrences extend the length of the dike swarm to more than 500 km u pb zircon concordia intercept ages for dikes near stoddard well in the western mojave desert and big wash in the eagle mountains are identical within analytical uncertainties to 148 ma zircon ages for independence dikes north of the garlock fault recognition of this widespread magmatic and tectonic event with a restricted age has important ramifications 1 crosscutting relations between the dikes and pre upper jurassic and cretaceous rocks help establish relative ages 2 persistence of dikes across several inferred terrane boundaries constrains terrane accretion ages 3 the dike rocks represent magmatic processes in a wide range of wall rock settings 4 the dikes are structural markers that record translations and rotationsof crustal flakes 5 regional persistence and dilational emplacement of the independence dike swarm suggest that it is the last extension in a series of alternating extensional and compressional events during the jurassic the tectonic regime responsible for the independence dikes may be related to arc normal extension changes in plate motions or oblique subduction with left lateral shear

geochemistry and petrogenesis of proterozoic diabase in the southern death valley region of california diabase sills and dikes of proterozoic age intrude crystalline basement and the overlying crystal spring formation in the southern death valley region of california despite pervasive deuteric alteration analyses of relict plagioclase an66 45 titaniferous augite and ilmenite permit the calculation of initial crystallization temperatures of 1 165 25 c for plagioclase and 1 110 25 c for augite with an oxygen fugacity of 10 11 atm the early crystallization of plagioclase is consistent with the generally subophitic texture of the diabase

shrimp rg u pb zircon geochronology of mesoproterozoic metamorphism and plutonism in the southwesternmost united states abstract mesoproterozoic intrusive and granulite grade metamorphic rocks in southern california have been inferred to be exotic to north america on the basis of perceived chronologic incompatibility with autochthonous cratonal rocks ion microprobe geochronology indicates that zircons in granulite grade gneisses dated at 1 4 ga using conventional methods are composed of 1 68 1 80 ga cores and 1 19 ga rims these early proterozoic gneisses were metamorphosed at extremely high temperatures and moderate pressures during emplacement of the 1 19 ga san gabriel anorthosite complex the lack of a 1 4 ga metamorphic event suggests that proterozoic rocks in this region rather than being exotic to north america may in fact be a midcrustal window into mesoproterozoic crustal evolutionary processes in southwestern north america

petrogenetic modeling and stable isotopic evaluation of anorthositic and jotunitic to syenitic magma series in the san gabriel anorthosite complex southern california abstract two magmatic series have been recognized in the 1 12 ga san gabriel anorthosite syenite complex in the western san gabriel mountains of southern california one anorthositic and the other jotunitic to syenitic the anorthosite series consists of anorthosite avg an48 gabbroic anorthosite and gabbro anorthositic rocks are bordered on their southern and western margins by a mafic jotunite syenite series that forms slightly curved trends on harker variation diagrams volumetrically minor ultramafite occurs as dikes and sheets that are highly enriched in fe ti and p and have geochemical signatures suggesting affinity with the jotunite syenite series although high grade regional metamorphism has not affected the complex it has experienced post emplacement deformation and most rocks exhibit variable hydrothermal alteration alteration includes extensive sericitization in the anorthosite rocks and near complete replacement of primary pyroxene by actinolitic aggregates in all rock units oxygen isotopic fractionations between coexisting secondary calcite and host feldspar indicate disequilibrium it is concluded that 18o values of primary minerals are not far shifted from magmatic values and that the effects of hydrothermal alteration were generally localized moreover the alteration has not affected the elemental composition of the two series allowing their use in evaluation of magmatic evolution the anorthosite series and the jotunite syenite series are modeled both as 1 comagmatic and descending from a broadly dioritic parent and 2 as non comagmatic and descending from a gabbroic anorthosite parent for the anorthosite series and from a jotunitic parent for the jotunite syenite series in both cases rocks of the anorthosite series are considered to be entirely cumulate in origin in the jotunite syenite series ultramafite is regarded as an early formed sorted cumulate similar in composition to mafic jotunite but lacking feldspar and strongly layered mafic jotunite is also regarded as a cumulate felsic jotunite and syenite may more closely approximate magmas but the small variation in mg number suggests that crystal sorting may be responsible for much of the compositional variation in the series high k ba rb and eu eu in some syenite may indicate accumulation of potassium feldspar in these rocks during anorthosite crystallization magmas are envisioned to follow an sio2 depletion feenrichment trend that is reversed by the crystallization of mafic jotunite in the comagmatic case but may eventually result in the formation of ore bodies in the non comagmatic model major element models are applied successfully for both hypotheses however the ree data cannot be successfully modeled in the comagmatic case possibly due to the strong effect of apatite and uncertainty in apatite partition coefficients although the unconstrained choice of parent magma s composition is a deficiency in both models the failure of the comagmatic model to describe ree variations is not related to choice of parent magma composition the non comagmatic model to describe ree variations is not related to choice of parent magma composition the non comagmatic model can best explain all compositional variations largely as a result of the additional freedom of choice of parent magma composition in addition the limited spatial occurrence of the jotunite syenite unit and the lack of jotunite dikes in the complex may support the intrusion of jotunite syenite along the anorthosite country rock contact as a pluton after or possibly during the final stages of anorthosite crystallization on the basis of geochemical modeling and field relations we favor the crystallization of the two magma series as non comagmatic with anorthositic rocks forming as cumulates from a gabbroic anorthosite parent magma and ultramafite jotunite and syenite forming from a jotunitic parent magma

triassic plutonism in southern california southward younging of arc initiation along a truncated continental margin earliest cordilleran magnatism in the southwestern united states is recorded by a belt of triassic plutons that inme protemzoic basemint of the mojave crustal province and its cratona miogeoclinal cover the belt extends from the western mojave desert through the trausverse ranges to the colorado river trough triassic plutons are predominantly alkali calcic fe and sr mriched quartz mzrdior tes and monzonites the northern part of the belt is comlx sed of two older plutonic mites 241 231 ma which are high k to shoshonitic the southern part of the belt is a younger 218 213 ma sodic alkalic suite the plutonic record in southern california suggests a short lived southward younging continental margin arc sang for eraplacement of triassic plutons sulim on a continental margin modified by s mistral tranqform faulting triassic plutonism in this region was followed by a magmatic lull prior to the onset of voluminous middle to late jurassic cordilleran arc magmatism

pressure temperature time paths and a tectonic model for the evolution of granulites petrologic studies and application of well calibrated mineralogic thermometers and barometers reveal several important features common to many granulite terrains 1 peak metamorphic pressure and temperature conditions cluster around values of 7 5 pm 1 kbar and 800 pm 50 c implying average geothermal gradients of 30 35 c km 2 in comparison with upper amphibolite facies rocks especially in paired amphibolite granulite terrains it is evident that granulites are distinguished from amphibolites by the former s higher metamorphic temperatures but not higher pressure 3 initial cooling of granulite terrains from peak conditions is nearly isobaric for garnet closure temperatures of 600 pm 50 c the dp dt retrograde paths inferred from compositionally zoned garnet rims are 2 8 b c in most granulites 4 primary coarse sillimanite is found in most granulites and retrograde kyanite is known in a few these data suggest that granulites form as a result of anomalous thermal gradients

spatial patterns causes and consequences of landslides in the gilgel gibe catchment sw ethiopia abstract the gilgel gibe catchment in sw ethiopia is one of the areas in east africa affected by landslides to better understand the patterns and the causal factors of these landslides all landslides in a small study area 14 km in the hilly parts of the gilgel gibe catchment were mapped and analyzed in total 60 landslides were mapped these landslides caused a displacement of 1 million m slope material which corresponds to a mean displaced volume of 50 ton ha 1 y 1 in the last 20 years moreover many landslides deliver directly sediment to the rivers and hence increase the sediment load in the rivers this soil loss to the rivers was estimated at 11 ton ha 1 y 1 during the same period high annual rainfall ca 2000 mm y 1 lithological and pedological properties and to a lesser extent steep 16 slopes turn the area into an inherent unstable situation and can be indicated as preconditions for the landslides in the study area distance to rivers is significantly the most important precondition as slopes near rivers are less stable than slopes further away from the rivers this is mainly caused by river incision and bank erosion which often occur in the area and which can be attributed to increased runoff due to deforestation over the past 20 years therefore recent deforestation caused more shallow landslides but also indirectly more deep seated landslides close to the rivers heavy rainfall is indicated as the main triggering factor for almost all landslides

origin of gneisses in the aureole of the san gabriel anorthosite complex and implications for the proterozoic crustal evolution of southern california granulite facies gneisses along the southwestern margin of the san gabriel anorthosite complex include compositionally banded locally migmatitic felsic gneisses and mafic granulites with rare interlayered augen gneiss and garnet hypersthene spinel aluminous gneiss previous workers accepted a 1 4 ga age for these granulites and a 1 2 ga age for the adjacent anorthosite complex and hence that this proterozoic basement is allochthonous with respect to the southwestern united states part of the north american craton solvus geothermometry in felsic and mafic granulites yields minimum metamorphic temperatures of 900 950 c and new u pb zircon data yield a minimum metamorphic age of 1186 ma coincident with that of the adjacent anorthosite we reinterpret these rocks as a 1190 ma anorthosite complex and its granulite grade thermal aureole geochemical data suggest that the anorthosite complex intruded early proterozoic rocks correlative with the mojave crustal province representing the north american craton exposed to the northeast we conclude that these proterozoic basement rocks represent the southwestern margin of the north american craton and not an exotic terrane accreted in mesozoic or tertiary time

metamorphism of the scourian complex nw scotland the rocks of the scourian complex have been intensively studied but there is still no consensus as to the conditions of the granulite facies metamorphism preserved in these rocks recent estimates of these conditions fall into two groups one at 820 920 c and ca 11 kbar and the second at ca 1000 c and 12 kbar investigation of a variety of rocks shows that the recorded conditions vary with grain size with higher grade conditions recorded by the cores of coarser ca 10 mm crystals and lower grade conditions recorded by the rims of coarser grains and by finer grains this observation suggests that re equilibration during recovery of these rocks to the surface has been important which may account for the discrepancy in estimated p t conditions revised estimates of the equilibration conditions of the scourian complex of t 1000 c and p 8 5 kbar are presented the conditions suggested for the peak of metamorphism mean that the role of anatexis in the genesis of these rocks must be considered and the nature of the fluid phase thoroughly investigated

the metamorphic evolution of granulites at fyfe hills implications for archaean crustal thickness in enderby land antarctica granulites at fyfe hills in enderby land antarctica crystallized at temperatures in excess of 850 c and possibly as high as 1000 c and at pressures of 8 10kbar during the mid to late archaean a number of features including repeated retrograde metamorphism at 5 5 8kbar retrograde reaction textures and rimward zoning in pressure sensitive systems suggest that following peak metamorphism the granulites stabilized at a depth of 18 26 km after stabilization the granulites cooled near isobarically to temperatures of 600 700 c assuming a total crustal thickness of 35 40 km during this late archaean interval of isobaric cooling the peak metamorphic crustal thickness is estimated at 35 56 km this estimate is significantly less than the 60 70 km obtained by summing the depths of the present levels of exposure 26 34 km and the thickness of the crust presently beneath fyfe hills approxi mately 35km and is therefore consistent with independent evidence for extensive post archaean thickening of the enderby land crust

plio pleistocene paleosols loess paleosol sequence studied in the beregovoye section the crimean peninsula abstract the loess paleosol sequence exposed in the beregovoye section southwest of the crimean peninsula has been thoroughly studied detailed analysis of the paleosol morphology physical properties and chemical composition micromorphology and other characteristics made it possible to identify types of paleosols formed in the pliocene and pleistocene the results obtained gave us an insight into the climate and environments of the past according to them the climate of the pliocene optimum was extremely hot and wet and the soil zonality was hardly pronounced practically over entire central and eastern europe at the final phase of the pliocene the climate became drier though the rainfall amount was still greater than in the holocene during the early pleistocene the soil development followed the mediterranean type the climate being still warm though less humid than in the pliocene there are two peaks of catastrophic changes in the evolution of the climate and environments distinctly traceable in the beregovoye section first corresponding to the global change recorded between the pliocene and pleistocene and second falling on the initial phase of the modern holocene soil formation

late cretaceous juxtaposition of metamorphic terranes in the southeastern san gabriel mountains california new structural petrological and geochronological data from the southeastern san gabriel mountains define the amalgamation history of suspect terranes in this region four metamorphic terranes constitute the area the cucamonga san antonio san gabriel and baldy terranes and were juxtaposed in two discrete episodes during the late cretaceous the cucamonga san antonio and san gabriel terranes were juxtaposed along subparallel east west trending left lateral mylonitic fault zones during the first episode the cucamonga terrane consists of lower cretaceous granulite facies gneissic rocks these rocks were intruded in the late cretaceous by granitic rocks as the cucamonga terrane was faulted ductilely against the southern margin of the san antonio terrane mylonitized units were subjected concurrently to retrograde amphibolite facies metamorphism metasedimentary pendants in the san antonio terrane were metamorphosed under upper amphibolite facies conditions as plutonic rocks were emplaced syntectonically along its northern boundary the san antonio terrane was juxtaposed concurrently against the san gabriel terrane a terrane consisting of precambrian gneiss and precambrian and mesozoic intrusive rocks the boundary is represented by middle fork complex a ductile zone intermingling rocks of the two terranes after palinspastic restoration of local cenozoic fault displacements the left lateral mylonitic fault zones separating the terranes appear as parts of a major sinistral transform system along the southern margin of the san gabriel mountains displacement for each of the mylonitic fault zones was approximately several tens of kilometers and was part of a broad left lateral shear system this system is modeled as a lateral ramp or tear fault to a west directed synplutonic ductile thrust system underlying the san gabriel terrane crosscutting syn and post tectonic intrusives indicate that most of the displacement occurred between 88 and 78 m y ago no lithologic isotopic or structural data support previous assertions that these terranes were accreted to north america during the tertiary the late cretaceous synplutonic juxtaposition of the cucamonga san antonio and san gabriel terranes was followed by underthrusting of oceanic rocks of the baldy terrane that is pelona schist along the vincent thrust precise timing of this episode is as yet uncertain

th pb ion microprobe dating of allanite allanite which is a common accessory mineral in a wide variety of rock types typically contains high concentrations of th and u thus an in situ method of u th pb dating of this phase would have broad application we describe a method to permit th pb ages of allanite to be determined with approximately 10 accuracy using a high resolution ion microprobe knowledge of the composition and substitution mechanisms of this complex mineral is key to understanding the relative ion ization efficiencies of th and pb the chemical compositions of three allanite samples used as age standards cima d asta pluton 275 5 1 5 ma atesina volcanic complex 276 3 2 2 ma la posta pluton 94 2 ma were determined using an electron microprobe permitting an assessment of matrix effects on ionization an ion microprobe calibration curve involving elemental and oxide species of th and pb i e 208 pb th vs tho 2 th yields highly scattered apparent ages when allanite age standards with different fe contents are used however a three dimensional plot of 208 pb th vs tho 2 th vs feo sio improves the accuracy of the calibration to about 10 even though this level of uncertainty is substantially greater than that expected for u th pb ion microprobe analyses of zircon or monazite th pb ages of allanite can still be used to address important geologic questions we used this method to date two metamorphic allanite grains from the footwall of the main central thrust nepal himalaya and an allanite grain from the pacoima canyon pegmatite california allanite inclusions in garnet from nepal yield significantly older ages than the coexisting monazite indicating that allanite formation in these rocks records a previous metamorphic cycle that predates slip along the fault the pacoima canyon allanite grain yields a younger age than that reported for zircon implying pb loss during cooling of the pegmatite

soil magnetic susceptibility and its relationship with naturally occurring processes and soil attributes in pedosphere in a tropical environment abstract soil magnetic susceptibility has potential to be used as a pedoenvironmental indicator from which mineralogy pedogeochemical pedogeomorphological and pedogenic processes can be inferred it can be used in pedosphere studies as an auxiliary information for appropriate and sustainable soil use and management this research aimed to analyze how pedogenesis and geochemical processes affect the and some of its attributes as well as its potential use in discriminating soil great groups following the digital soil mapping approach the study area is located in sao paulo state brazil soil samples were collected for physical chemical analysis from 79 locations 0 20 cm depth at these sites magnetic susceptibility was measured with a portable field instrument and analyzed in terms of geology relief and soil class the results showed that geology strongly affects mainly in diabase derived soils followed by metamorphosed siltstone and siltstone in fluvial sediments the exhibits different behaviors due to different sediments deposited by the capivari river in less evolved soils such as cambisols lithology is a more important contributor to than pedogenesis in more evolved soils pedogenesis increases whereas argilluviation ferralitization reduces it the values did not decrease significantly or even increase downslope due to the presence of diabase on the lower parts differences in where observed between diabase bedrock located in different parts of the study area indicating more of an influence by geomorphic processes rather than lithology with respect to soil attributes positive correlations between and base saturation cation exchange capacity organic matter and iron and clay content were found whereas a negative correlation was found between and sand content the correlates with changes in lithology and soil class demonstrating its application as a potential tool for the discrimination of soil great groups and digital soil mapping