earthquake mechanisms of the adriatic sea and western greece implications for the oceanic subduction continental collision transition summary we present 21 focal solutions magnitude 5 5 reliably computed by body wave modelling for the western hellenic arc from yugoslavia to the southern peloponnese mechanisms located within the aegean show normal faulting the t axis trending n s in the centre and parallel to the active boundary in the external part mechanisms associated with the keffalinia fault are consistent with dextral strike slip motion reverse mechanisms located along the active boundary are remarkably consistent and do not depend on the nature of the active boundary continental collision or oceanic subduction the consistency in azimuth of the slip vectors and of the gps velocity relative to africa all along the active boundary suggests that the deformation is related to the same motion the discrepancy between seismic energy release and the amount of shortening confirms that the continental collision is achieved by seismic slip on faults but the oceanic subduction is partially aseismic the northward decrease in velocity between continental collision and oceanic subduction suggests the continental collision to be a recent evolution of the active subduction

waves of bubbles in basaltic magmas and lavas initially homogeneous suspensions of bubbles in basaltic magmas and lava flows may become unstable and form rising waves or layers of bubbles we derive a set of model equations for the two phase bubble liquid system and present the results of a linear stability analysis and numerical simulations periodic vesicle layers are preserved in columbia liver flows with a spacing of 1 m the prediction of the stability analysis combined with a simple model for solidification of the lava suggests that vesicle layers may represent waves of bubbles formed within the flow the spacing of layers is determined by a balance between the growth of bubbles and hydrodynamic self diffusion vesicular layers preserved in columbia liver flows may thus form by hydrodynamic processes and do not require the repeated injection of fresh volatile rich magma or a periodic nucleation of bubbles waves of bubbles may also develop in magma chambers and conduits and could play a role in eruption dynamics

the flin flon paleosol and the composition of the atmosphere 1 8 bybp within the 1800 to 1900 my old flin flon snow lake greenstone belt amisk group volcanics are overlain by missi group fluvial sediments several localities along the missi amisk contact the volcanics show evidence of subaerial weathering field relationships mineralogical evidence and chemical analyses confirm that this alteration zone is a paleosol pedogenic fabrics and mineralogy were somewhat obscured by greenschist grade metamorphism associated with the hudsonian orogeny 1750 my this is especially true in the upper meter of the paleosol where metamorphic paragonite and sericitic micas developed in a crenulated fabric this metamorphism did not however obliterate the imprint of weathering on the amisk volcanics features characteristic of well drained modern soils are evident in the paleosol corestones of spheroidally weathered pillow lavas occur at depth within the paleosol cr horizon the corestones decrease in size upward and eventually disappear into a hematite rich horizon at the top of the paleosol these macroscopic changes are accompanied by a decrease in cao and mgo and by an increase in al2o3 tio2 and total iron toward the paleosol missi contact ferrous iron decreases upward toward the contact feo was apparently oxidized to ferric iron and retained within the paleosol during weathering the oxidation and retention of iron within the flin flon paleosol indicates that po2 was probably or 10 2 p a l at the time of weathering the behavior of iron in the flin flon paleosol contrasts sharply with its behavior in the 2200 my hekpoort paleosol which is strongly depleted in iron this difference suggests that a significant increase in the ratio of po2 pco2 in the atmosphere took place between 2200 and 1800 mybp

preservation of a magnetization carried by magnetite inclusions during amphibolite grade metamorphic conditions an example from the sarek dykeswarm northern swedish caledonides an outcrop consisting of a 608 myr old sheeted dyke complex and dyke intruded metasediments has been investigated palaeomagnetically it is located in the interior of a nappe the basal and upper parts of which suffered penetrative deformation and amphibolite grade metamorphism during the caledonian orogeny the only trace from the orogeny at the outcrop is a widely spaced network of brittle and ductile shear zones crosscutting dyke contacts foliated amphibolite consisting of hornblende quartz plagioclase garnet and sphene occur locally within them the opaque mineralogy of the dykes differs from that commonly found in continental dykes with ilmenite and pyrrhotite being the major visible opaque phases and there are indications that magnetite inclusions exsolved from silicates carry a primary remanence even though ar 39 40 ar mica ages were reset at ca 430 myr during the orogeny a secondary component carried by pyrrhotite is less prominent but falls on the baltican apw path at 430 myr the inferred primary component has a large directional uncertainty but indicates low palaeolatitudes for the baltscandian margin during rifting less

depth determination for shallow teleseismic earthquakes methods and results the depths of moderate sized shallow earthquakes can be routinely determined to an accuracy of several kilometers using teleseismic data alone making it possible to determine the vertical positions of earthquakes within the lithosphere this capability provides a valuable tool for plate tectonic studies since earthquake depths can be used to constrain lithospheric thermal and mechanical structure at plate boundaries and in intraplate regions two basic techniques are used for depth analyses surface wave spectra can be used to determine focal depths as mode excitation reflects the behavior of the eigenfunctions with depth the primary limitation on such studies stems from lateral heterogeneity of velocity structure the waveforms of body waves are also diagnostic of focal depth which controls the time separation between the direct arrival and near source surface reflections these studies can be limited by a partial trade off between depth and source time function duration the reliability of the body wave modeling approach now in general use is shown by excellent agreement between depths determined by different investigators using different algorithms the depth determination is robust to typical uncertainties in focal mechanism and near source structure the trade off with source time function duration generally does not produce major difficulties for small events ms 6 5 for larger events the trade off can be resolved using a approach in which for a given depth the time function yielding the best fit to a suite of seismograms is found by deconvolution deconvolution at a series of depths yields a misfit function whose minimum indicates the focal depth tests with synthetic data show that the deconvolution process yields reliable depth estimates even given uncertainties in focal mechanism and near source structure it successfully handles multiple sources moderate sized finite sources and horizontally propagating ruptures good results are obtained for vertically propagating ruptures until the fault area becomes so large that waveforms differ significantly from the point source approximation a number of important tectonic results have emerged from studies using these depth determination methods the maximum depth of oceanic intraplate seismicity increases with lithospheric age and is approximately bounded by the 750 c isotherm this depth roughly equals the flexural thickness of the lithosphere but is much less than the depth to the low velocity zone indicated by surface wave dispersion the maximum depth of continental intraplate seismicity is also temperature limited but by a lower temperature these observations are consistent with the rapid weakening of the appropriate rocks with depth predicted by standard temperature dependent rheologies the maximum depth of oceanic transform seismicity corresponds to an isotherm of about 400 c this temperature is lower than would be predicted from the intraplate results and may indicate that transforms are either hotter or weaker than expected the depths of ridge crest earthquakes large enough for teleseismic analysis are extremely shallow 0 8 km the depth distribution of seismicity in continental normal faulting regions provides constraints on the nature of the extension process at subduction zones precise depth determinations can resolve the upper and lower flexural regions in the bending downgoing plate precisely locate the plate interface and map the vertical extent and position of faulting in large earthquakes

life associated with a 2 76 ga ephemeral pond evidence from mount roe 2 paleosol dark sericitic material at and near the top of the 2 765 0 01 ga mount roe 2 paleosol in western australia contains 0 05 0 10 wt organic carbon with delta 13c values between 33 and 51 pdb peedee belemnite such negative isotopic values strongly indicate that methanotrophs once inhabited this material the textures and the chemical composition of the dark sericitic material indicate that the methanotrophs lived in or at the edges of ephemeral ponds that these ponds became desiccated and that heavy rains transported the material to its present sites the discovery of methanotrophs associated with the mount roe 2 paleosol may extend their geologic record on land by at least 1 5 b y methanotrophy in this setting is consistent with the notion that atmospheric methane levels were or 20 mu atm during the late archean the radiative forcing due to such high atmospheric methane levels could have compensated for the faint younger sun and helped to prevent massive glaciation during the late archean

sedimentary provenance and weathering processes in the 1 1 ga midcontinental rift of the keweenaw peninsula michigan usa abstract fluvial alluvial lacustrine and debris flow sedimentary rocks from the keweenaw peninsula michigan usa are geochemically characterized to examine weathering processes and provenance these sedimentary rocks result from the erosion weathering and redistribution of effusive and plutonic igneous lithologies during periods of volcanic quiescence associated with the failed 1 1 ga north american midcontinental rift three units comprising the sedimentary igneous mixed portage lake volcanics coarseness and source varied copper harbor conglomerate and the quartz rich post rift fill unit of the jacobsville sandstone were studied over a geographically wide and lithologically varied range of localities across the keweenaw peninsula michigan provenance indicators such as ti zr and ti al ratios and la th sc ternary diagrams reveal that the portage lake volcanic conglomerates are near source and that the copper harbor conglomerate is primarily derived from felsic uplifted basement and recycled orogen lithologies to the east the jacobsville sandstone is a combination of pulses of sources from the top to the bottom of the stratigraphic section 1 felsic arkosic arenites sourced from the erosion and uplift of basement orogenic volcanic and plutonic rocks top 2 felsic arkosic subarkosic arenites sourced from the erosion of uplifted basement recycled orogeny and reworking of primary sedimentary and metasedimentary rocks middle and 3 quartz arenites eroded from the cratonic interior highly mature this work can consequently aid in structuring a stratigraphic nomenclature for the jacobsville sandstone both petrographic data and weathering indices such as the chemical index of alteration cia and the paleosol hydrolysis metric al ca na mg k indicate low degrees of weathering during syn rift deposition at 1 1 ga but increases with evolution of the rift in post rift units toward the late mesoproterozoic early neoproterozoic values for the cia throughout the mesoproterozoic from this and other localities have the same trend to that of proposed atmospheric co2 concentrations derived from paleosol data demonstrating that characterizing the weathering trends of floodplain deposits can be a useful addition to deciphering past paleoclimatic conditions in the precambrian geologic record

equation for compaction of paleosols due to burial existing empirically derived compaction curves for marine sediments fail to estimate the compaction of paleosols flood plain sediments and peats as indicated by deformation of clastic dikes dinosaur footprints and sandstone paleochannels yet compaction estimates are vital to reconstructing paleoclimate and geochemical profiles of paleosols to modeling sediment accumulation rates of nonmarine rocks and to tectonic flexural modeling of molasse facies this paper presents a compaction equation and physically derived constants that make geologically realistic estimates of compaction of paleosols and other nonmarine sediments a protocol for the application of the equation is suggested that would allow the following equation to solve for burial compaction c as a fraction of original thickness given depth of burial d in km as c s i f e d k 1 this equation can be applied to nonmarine sedimentary rocks or paleosols given appropriate empirical data on the physical constants in the equation such as initial solidity s i initial porosity f and the corresponding curve fitting constant k data on physical constants useful for this equation are compiled here for a range of paleosol and nonmarine sediment types

mineral deposits of the mesoproterozoic midcontinent rift system in the lake superior region a space and time classification abstract the mesoproterozoic midcontinent rift system mrs of north america hosts a diverse suite of magmatic and hydrothermal mineral deposits in the lake superior region where rift rocks are exposed at or near the surface historically hydrothermal deposits such as michigan s native copper deposits and the white pine sediment hosted stratiform copper deposit were major mrs metal producers on going exploration for and potential development of copper nickel sulfide deposits hosted by the duluth complex of minnesota and the opening of the eagle nickel mine in michigan indicate an expanding interest in mrs magmatic deposits mrs hydrothermal and magmatic mineral deposits many of which are significant past present and likely future providers of critical minerals here are placed into a space and time metallogenic framework to construct this framework regional mrs mineral deposits extracted from the u s geological survey mineral resources data system mrds and the ontario ministry of energy northern development and mines mineral deposit inventory mdi were supplemented by other known and recently recognized mineral deposits described in the literature all mineral deposits were classified by deposit type host rock age and type and estimated timing of mineralization deposits were then put into a tectonic evolutionary framework stages for the mrs which shows that deposits formed within discrete spatial and temporal stages of rift evolution each stage of rift evolution is characterized by specific mineral deposit types that are largely confined both by their physical location in rift rocks and type and timing of mineralization examples include mrs nickel rich conduit type magmatic sulfide deposits which are restricted to an early magmatic stage mrs history when magma compositions were characterized by ni rich picrites and high mg basalts in contrast contact type magmatic sulfide deposits with cu ni were derived from more evolved al rich tholeiitic magmas that dominated a later time of voluminous magmatic activity hydrothermal sediment hosted stratiform chalcocite mineralization along the margins of western lake superior in michigan is economic only where fluid flow through red beds was concentrated by structures along the margins of a post volcanic mrs sedimentary basin widespread native cu and native ag mineralization occurred about 40 million years after formation of host basalt lava flows and interflow sedimentary rocks these descriptions of the diverse mineral deposits in the lake superior region cataloged within the space of the local and regional geology and over the more than 60 million year mineralizing history of the mrs are indicative of the complex and at times overlapping magmatic and hydrothermal mineral systems that operated within this major large igneous province

paleointensity determination on the late precambrian tudor gabbro ontario paleointensity determination by the coe modified thellier method was carried out on the 1100 ma tudor gabbro of southern ontario the tudor has a bivectorial natural remanent magnetization nrm the c nrm is a grenvillian uplift thermoviscous remagnetization which overprints the a nrm the surviving primary thermoremanent magnetization trm of the tudor intrusion a sharp junction at 530 c between the a and c vectors in laboratory thermal demagnetization matches the peak metamorphic reheating temperature of 480 500 c in nature when the difference in time scales is taken into account the unblocking temperature distributions of the a nrm and of laboratory trm also match a is carried by single domain or small pseudo single domain grains probably magnetite rods exsolved in plagioclase or pyroxene at temperatures above 580 c arai plots of nrm versus partial trm are linear from 520 c or 530 c to 580 c for 45 specimens of 19 samples from 9 sites the 45 reliable paleointensity values from these specimens give a virtual axial dipole moment vadm of 4 6 0 8 1022 a m2 around 1100 ma this is about one half of the modern field intensity but is similar to the mean cretaceous cenozoic intensity and to most archean and proterozoic vadms keweenawan rocks whose ages are within 20 myr of the tudor gabbro have a vadm of 11 4 1022 a m2 the difference between the tudor and keweenawan vadms is similar to differences recorded over similar time intervals in the cretaceous and seems to represent normal variation in geomagnetic field strength throughout earth s history

the significance of u pb zircon dates in lower crustal xenoliths from the southwestern margin of the kaapvaal craton southern africa the combination of high precision id tims u pb geochronology with cathodoluminescence cl imaging and spatially resolved microsampling of zircon provides a high fidelity record of the temporal evolution of samples of the middle and lower crust a suite of xenoliths exhumed in cretaceous kimberlites at the southwestern margin of the kaapvaal craton including eclogite granulite amphibolite and greenschist facies plutonic rocks are the subject of these detailed zircon studies cl imaging demonstrates the complexity of chemical and growth structure in single zircons and highlights the importance of sampling on an intragrain scale to extract end member ages for zircon growth events for these diverse metamorphic rocks linking the zircon chronology to geologic events and processes requires an understanding of the kinetics and thermodynamics of zircon stability during metamorphism using an empirical approach we attempt to relate zircon crystallization dissolution and regrowth to metamorphic conditions through a detailed analysis of zircon chemical zonation patterns mineral inclusions and petrography these lines of evidence indicate the crystallization of zircon under p t conditions ranging from eclogite through greenschist facies we discuss four examples of this approach 1 at the lowest grade of metamorphism examined primary igneous zircon 1900 ma in a syenite xenolith from the klipfontein 08 kimberlite underwent two episodes 1015 and 964 ma of probable fluid mediated resorption and reprecipitation in the greenschist facies 2 an upper amphibolite facies xenolith from the markt kimberlite contains two distinct populations of blocky and fir tree sector zoned zircon which are suggested to date prograde hornblende breakdown reactions 1107 ma and melt infiltration under retrograde metamorphic conditions 1092 ma respectively 3 at granulite facies metamorphic zircon crystallized at 1114 ma during prograde reactions involving hornblende breakdown in a granulite xenolith also from the markt kimberlite later 960 ma zircon resorption followed by minor secondary crystallization occurred in both the amphibolite and granulite xenoliths during retrograde greenschist facies reactions possibly enhanced by fluids 4 in a much older episode of eclogite facies metamorphism recorded in a xenolith from the lovedale kimberlite zircon crystallized with the peak metamorphic mineral assemblage or possibly early on the cooling path in the late archean 2630 ma each of these examples provides unique information regarding the evolution of the kaapvaal craton while the links to metamorphic reactions and p t conditions clarify the interpretation of zircon dates obtained from these samples of the middle to lower crust

the mesoproterozoic midcontinent rift system lake superior region usa abstract exposures in the lake superior region and associated geophysical evidence show that a 2000 km long rift system developed within the north american craton 1109 1087 ma the age span of most of the volcanic rocks this system is characterized by immense volumes of mafic igneous rocks mostly subaerial plateau basalts generated in two major pulses largely by a hot mantle plume a new ocean basin was nearly formed before rifting ceased perhaps due to the remote effect of the grenville continental collision to the east broad sagging subsidence combined with a system of axial half grabens separated along the length of the rift by accommodation zones provided conditions for the accumulation of as much as 20 km of volcanic rocks and as much as 10 km of post rift clastic sediments both along the rift axis and in basins flanking a central post volcanic horst pre rift mature quartzose sandstones imply little or no uplift prior to the onset of rift volcanism early post rift red bed sediments consist almost entirely of intrabasinally derived volcanic sediment deposited in alluvial fan to fluvial settings the exception is one gray to black carbon bearing lacustrine unit this early sedimentation phase was followed by broad crustal sagging and deposition of progressively more mature red bed fluvial sediments with an extra basinal provenance

emplacement and rheomorphic deformation of a large lava like rhyolitic ignimbrite grey s landing southern idaho the miocene grey9s landing ignimbrite reaches 70 m thick and covers at least 400 km 2 in the central region of the snake river plain it shows particularly intense welding and rheomorphic deformation and although parts are eutaxitic most is lava like with flow banding and no fiamme a near ubiquitous penetrative flow lamination associated with a well developed elongation lineation is folded into small intrafolial tight to isoclinal oblique and sheath folds which are refolded by larger folds in the upper parts structural and kinematic analysis reveals that welding and early deformation occurred rapidly during deposition from a very hot 1000 c high mass flux pyroclastic density current that flowed westward across a graben faulted landscape as hot particles were deposited they rapidly agglutinated and coalesced and underwent noncoaxial shear in a subhorizontal ductile shear zone close to the current deposit interface the shear zone is interpreted to have been less than 2 m thick it produced and deformed the rheomorphic fabric and it migrated upward with the rising current deposit interface during aggradation so that it transiently affected all levels of the resultant thick ignimbrite deformation was progressive and after the density current had dissipated viscous spreading and downslope flow continued and involved an increasingly thick portion of the sheet this folded the flow banding and f 1 intrafolial isoclines into larger sheath folds and into more upright periclines near the top of the ignimbrite we demonstrate that structural and kinematic analysis can elucidate the emplacement history of rheomorphic ignimbrites

a procedure for modelling shallow dislocation sources summary expressions for displacements on the surface of a layered half space due to an arbitrary oriented shear dislocation are given in terms of generalized ray expansions useful approximations of these expressions for shallow events as recorded at teleseismic distances for realistic earth models are presented the results of this procedure are used to generate synthetic p sv and sh waveforms for various assumptions of stress drop

in situ formation of welded tuff like textures in the carapace of a voluminous silicic lava flow owyhee county sw idaho the badlands rhyolite on the owyhee plateau of southwestern idaho can be demonstrated to be a large lava flow on the basis of its geometry of large and small flow lobes its well exposed near vent features and its response to pre existing topography however samples of the dense upper vitrophyre of the unit reveal a range of annealed fragmental textures including material which closely resembles the compressed welded glass shards which are characteristic of ignimbrites formation of these tuff like textures involved processes probably common to emplacement of most silicic lava flow units decompression upon extrusion causes inflation of pumice at the surface of the lava flow some of this pumice is subsequently comminuted producing loose bubble wall shards bits of pumice chips of dense glass and fragments of phenocrysts this debris sifts down around loose blocks and into open fractures deeper in the flow where it can be reheated compressed and annealed to varying degrees the end result is a dense vitrophyre layer beneath the true upper non welded carapace breccia which can be extremely texturally heterogeneous with areas of flow foliated lava occurring very near lava which in many aspects looks like welded ignimbrite complete with flattened pumices identical textures in other silicic units have been cited by previous workers as evidence that those units erupted as pyroclastic flows which then underwent sufficient rheomorphism to create a flow foliated rock which otherwise appears to be lava the textures described herein indicate that lava flows can come to mimic rheomorphic ignimbrites at least at scales ranging from thin sections to outcrops voluminous silicic units with scattered fragmental textures but with otherwise lava like features are probably true effusive lava flows

review reservoir triggered seismicity rts at koyna india over the past 50 yrsreview reservoir triggered seismicity rts at koyna india over the past 50 yrs

distinguishing strongly rheomorphic tuffs from extensive silicic lavas high temperature silicic volcanic rocks including strongly rheomorphic tuffs and extensive silicic lavas have recently been recognized to be abundant in the geologic record however their mechanisms of eruption and emplacement are still controversial and traditional criteria used to distinguish conventional ash flow tuffs from silicic lavas largely fail to distinguish the high temperature versions we suggest the following criteria ordered in decreasing ease of identification to distinguish strongly rheomorphic tuffs from extensive silicic lavas 1 the character of basal deposits 2 the nature of distal parts of flows 3 the relationship of units to pre existing topography and 4 the type of source as a result of quenching against the ground basal deposits best preserve primary features can be observed in single outcrops and do not require knowing the full extent of a unit lavas commonly develop basal breccias composed of a variety of textural types of the flow in a finer clastic matrix such deposits are unique to lavas because the chilled base of an ashflow tuff generally does not participate in secondary flow primary pyroclastic features are best preserved there massive flow banded bases are more consistent with a lava than a pyroclastic origin lavas are thick to their margins and have steep abrupt flow fronts ashflow tuffs thin to no more than a few meters at their distal ends where they generally do not show any secondary flow features lavas are stopped by topographic barriers unless the flow is much thicker than the barrier ash flow tuffs moving at even relatively slow velocities can climb over barriers much higher than the resulting deposit lavas dominantly erupt from fissures and maintain fairly uniform thicknesses throughout their extents tuffs commonly erupt from calderas where they can pond to thicknesses many times those of their outflow deposits these criteria may also prove effective in distinguishing extensive silicic lavas from a postulated rock type termed lava like ignimbrite the latter have characteristics of lavas except for great areal extents up to many tens of kilometers these rocks have been interpreted as ash flow tuffs that formed from low boiling over eruption columns based almost entirely on their great extents and the belief that silicic lavas could not flow such distances however we interpret the best known examples of lava like ignimbrites to be lavas this interpretation should be tested through additional documentation of their characteristics and research on the boiling over eruption mechanism and the kinds of deposits it can produce flow bands flow folds ramps elongate vesicles and probably upper breccias occur in both lavas and strongly rheomorphic tuffs and are therefore not diagnostic pumice and shards also occur in both tuffs and lavas although they occur throughout ash flow tuffs and generally only in marginal breccias of lavas dense welding secondary flow and intense alteration accompanying crystallization at high temperature commonly obliterate primary textures in both thick rheomorphic tuffs and thick lavas high temperature silicic volcanic rocks are dominantly associated with tholeiitic flood basalts extensive silicic lavas could be appropriately termed flood rhyolites

constitutive mass balance relations between chemical composition volume density porosity and strain in metasomatic hydrochemical systems results on weathering and pedogenesis relations characterizing the chemical physical and mechanical changes resulting from metasomatic hydrochemical processes are developed using mass balance models which formally link chemical composition to bulk density mineral density volumetric properties porosity and amount of deformation strain rigorous analysis of aqueous solute transport effects is then made possible in a variety of porous media flow environments including chemical weathering pedogenesis soil formation diagenesis ore deposition and enrichment and metamorphism application of these linear constitutive relations to chemical weathering profiles shows that immobile and locally mobile chemical elements with masses conserved on the scale of soil profiles can be accurately identified from analysis of appropriate data arrays and then used as natural geochemical tracers to infer the nature and extent of hydrochemical weathering processes and volume changes during pedogenesis assumptions commonly made in the past about the supposed immobility of certain elements e g ti and zr become unnecessary quantitative differentiation between the effects of residual and supergene fractionation is then easily made these methods are applied to ni rich laterites developed by weathering of ultramafic rocks showing that during ordinary residual enrichment ni is concentrated by as much as 4 protolith peridotite concentrations this occurs simply by silicate mineral dissolution and removal of chemical elements other than ni e g mg with a corresponding reduction in saprolite density and increase in bulk porosity without significant deformation in contrast laterites with mineable concentrations of ni which are similarly undeformed such as the nickel mountain mine in riddle oregon have experienced in addition to residual enrichment strong supergene enrichment by fractionation of ore elements between a leached zone from which ni is extracted and a complementary enriched zone positioned farther along the direction of ground water flow soil forming processes in podzol chronosequences developed on sandy beach terraces of the mendocino coast of california involved soil column collapse of 60 percent by dissolution of silicate minerals in the albic horizon of al and fe leaching and 70 percent dilation expansion in the overlying organic rich layer by root growth the amount of erosion based upon paleosurface reconstructions using the excess mass of fe al pb ga and cu in the zone of supergene enrichment spodic horizon below the ground water table indicates that subsurface erosion by dissolutional collapse is three times that of surficial erosion finally using published chemical data for ti zr and cr on major bauxite deposits in australia where erosion rates are thought to be low we infer that there may have been major amounts of dissolutional collapse to explain the upwards increase of detrital zircon and rutile in weathering profiles

the sturgeon falls paleosol and the composition of the atmosphere 1 1 ga bp a paleosol is exposed along the north bank of the sturgeon river some 25 km sw of baraga michigan the paleosol was developed on hydrothermally altered keweenawan basalt and is overlain by the jacobsville sandstone textures mineralogy and chemical composition change gradually upwards from unweathered metabasalt through the paleosol to the contact of the paleosol with the jacobsville sandstone many of these changes are similar to those in modern soils developed on basaltic rocks however k has clearly been added to the paleosol probably by solutions which had equilibrated with k feldspar in the jacobsville sandstone the keweenawan basalt was oxidized quite extensively during its conversion to greenstone during weathering the remaining fe2 was oxidized to fe3 and was retained in the paleosol the composition of the parent greenstone and its change during weathering can be used to define an approximate lower limit to the ratio of the o2 pressure to the co2 pressure in the atmosphere during the formation of the paleosol formula see text free o2 must have been present in the atmosphere 1 1 ga ago but its partial pressure could have been 10 3 times lower than in the atmosphere today

pedogenic translocation of fe in modern and ancient vertisols and implications for interpretations of the hekpoort paleosol 2 25 ga abstract many interpretations of oxygen levels in the precambrian atmosphere use interpretations of fe distributions measured in paleosols this article addresses the current lack of knowledge concerning baseline fe translocations in modern soil analogs for the 2 25 ga hekpoort paleosol at waterval onder south africa as well as the lack of understanding of diagnostic petrographic features related to fe translocation preserved in the precambrian paleosols petrographic features related to fe translocations include redox related fe depletions and enrichments and possibly fe mn nodules which compare favorably with similar pedogenic features in paleozoic paleo vertisols and modern analog vertisols additionally preservation of sepic plasmic microfabrics in the hekpoort paleosol which are characteristic of clay soils experiencing shrink swell supports previous paleo vertisol interpretations total fe losses in vertisols are 10 50 losses increase with increasing soil age and with increasing mean ann

reappraisal of a 2200 ma old paleosol near waterval onder south africa abstract a fossil soil paleosol about 2200 ma old in a deep roadcut between the towns of waterval boven and waterval onder transvaal here called the waterval onder clay paleosol was formerly reported to have formed on the hekpoort basalt it is here thought to have developed on a thick formerly at least 5 5 m upward fining and thinning sequence of sandstone siltstone and shale derived from pre existing soils on hekpoort basalt within the basal portion of the fluvial dwaal heuvel formation in contrast to paleosols developed on hekpoort basalt the waterval onder clay paleosol is little deformed lacks corestones and shows compositional variation and berthierine rich laminae interpreted as relict bedding the paleosol has a ridge and swale topography linear or wavy gilgai and a subsurface structure mukkara typical of modern vertisols since there is considerable relict bedding it probably formed over a short period of time less than 7000 years and was not self plowing to the extent seen in modern vertisols clastic dikes filled with sandstone from the overlying rocks are evidence that the profile was periodically dry to depths of at least 2 m more drab colored rocks below that may represent the level of permanent water table iron and manganese stain of layers and claystone granules within the surface horizon of the profile have some characteristics of modern rock varnish and could be taken as evidence for life in the soil the high ratio of ferric to ferrous iron near the surface of the paleosol together with evidence from clastic dikes that this horizon was not waterlogged is an indication of an oxidizing atmosphere although probably one in which oxygen was much less abundant than it is today the very low amounts of acid titratable bases in the shaly parent material of this paleosol and surficial loss of these bases within the paleosol appear to have been produced by decarbonation reactions comparable to those found today in soils of vascular land plants a likely source of weathering acid during the precambrian is an elevated abundance of atmospheric carbon dioxide considering these likely atmospheric differences together with the distribution of comparable modern vertisols and other geological evidence the climate under which this soil formed was probably semiarid to subhumid seasonally dry and temperate

a preliminary survey of relict shoreface attached sand ridges in western lake superior abstract high resolution multibeam and seismic reflection surveying in western lake superior near the outflow of the amnicon river wisconsin has shown that the nearshore lake floor appears to consist of till with no recent sediment cover this surface is partially obscured 3 km offshore by a series of low relief elongate sand ridges that are arranged in an en echelon pattern at an oblique angle to the present day shoreline they are extensively gullied suggesting that they are not presently active two sets of elongate features can be identified the features in first group group i are over 0 5 km wide rise 3 5 m above the surrounding lake floor and are over 2 km long the features belonging to the second set group ii are not as well developed and are typically smaller than those in group i group ii features are typically 0 2 0 3 km wide 0 4 1 6 km long and rise 1 3 m above the surrounding lake floor the most striking difference between the groups is the difference in their orientation with respect to the shoreline the group i features run ne sw while the group ii features are directed approximately n s the shoreline runs approximately ene wsw so the features in both cases open northeast the group i features are believed to be relict drowned shoreface attached sand ridges that formed when the lake level lay over 10 m below its present day level these sand ridges formed as the result of currents associated with strong northeasterly gales moving the abundant sediment eroded off the shore the smaller group ii features are believed to be sub aqueous sand dunes that developed transverse to bottom currents produced by the northeasterly gales no dates have been obtained for the sand ridges based upon their depth and the lake level history curve for lake superior we suggest that the ridges formed either 6500 6000 bp prior to the nipissing highstand

physical volcanology of mid proterozoic plateau lavas the keweenawan north shore volcanic group minnesota the north shore volcanic group a thick sequence of plateau lavas of keweenawan proterozoic 1 100 m y age was erupted during the development of the midcontinent rift of north america it contains many well exposed physical features that imply a range of eruptive styles and flow mechanisms these include a high volume and high effusion rate floods of basalt and basaltic andesite which ponded and crystallized largely after flow had ceased b moderate to small volume and effusion rate flows and small floods some probably building low shields and c small flows to very large flows and ignimbrites of icelandite and rhyolite lacking visible vent areas viscosities calculated from 183 chemical analyses correlate well with the physical features the style of volcanism combined characteristics of the columbia plateau and the snake river plain plains volcanism and was very similar to that of the tertiary of eastern iceland

geology and geochemistry of paleosols developed on the hekpoort basalt pretoria group south africa the hekpoort paleosols comprise a regional paleoweathering horizon developed on 2 224 0 021 ga basaltic andesite lavas at the top of the hekpoort formation of the pretoria group transvaal supergroup south africa in five separate profiles from outcrops along road cuts near waterval onder and the daspoort tunnel and in three drill cores from the bank break area bb3 bb8 and bb14 the top of the paleosol is a sericite rich zone the sericite zone grades downward into a chlorite rich zone in core bb8 and in the road cut at the daspoort tunnel we sampled the underlying or parent basaltic andesite into which the chlorite zone grades we did not obtain samples of the parent material at waterval onder and in cores bb3 and bb14 but chemical analyses indicate that the chlorite and sericite zones in these profiles derive from underlying lavas similar to the ones we sampled in core bb8 and at the daspoort tunnel the presence of apparent rip up clasts of the paleosol in the overlying ironstones of the strubenkop formation in the cores from bank break makes it very unlikely that most of the alteration was a result of interactions with hydrothermal fluids desiccation cracks at the top of the paleosol that were filled with sand during the deposition of the overlying sediments at waterval onder point to a subaerial weathering origin very little if any al ti zr v or cr moved a discernible distance during weathering of any of the five profiles the vertical distribution of fe mg mn ni and co indicates that these elements were largely removed from the top of the soil during weathering the overall abundance of these elements in each of the profiles indicates that a significant fraction of the complement lost from the top subsequently reprecipitated in the lower portion of the soil as constituents of an fe 2 rich smectite the loss of fe from the top of the soil during weathering of the hekpoort paleosols indicates that atmospheric p o 2 was less than 8 x 10 4 atm about 2 22 ga fe 2 rich smectite should only precipitate during soil formation if atmospheric p co 2 is less than or equal to 2 x 10 2 atm rye kuo and holland 1995 ca and na were largely lost during weathering some na was apparently added to the sericite zone in cores bb3 bb8 and bb14 after weathering all five profiles are enriched in k and rb and most are enriched in ba the distribution of these elements indicates that they all were added during post weathering hydrothermal metasomatism rb sr analysis of the paleosol at the daspoort tunnel indicates that metasomatism last affected that profile 1 925 0 032 ga macfarlane and holland 1991

snake river sr type volcanism at the yellowstone hotspot track distinctive products from unusual high temperature silicic super eruptions a new category of large scale volcanism here termed snake river sr type volcanism is defined with reference to a distinctive volcanic facies association displayed by miocene rocks in the central snake river plain area of southern idaho and northern nevada usa the facies association contrasts with those typical of silicic volcanism elsewhere and records unusual voluminous and particularly environmentally devastating styles of eruption that remain poorly understood it includes 1 large volume lithic poor rhyolitic ignimbrites with scarce pumice lapilli 2 extensive parallel laminated medium to coarse grained ashfall deposits with large cuspate shards crystals and a paucity of pumice lapilli many are fused to black vitrophyre 3 unusually extensive large volume rhyolite lavas 4 unusually intense welding rheomorphism and widespread development of lava like facies in the ignimbrites 5 extensive fines rich ash deposits with abundant ash aggregates pellets and accretionary lapilli 6 the ashfall layers and ignimbrites contain abundant clasts of dense obsidian and vitrophyre 7 a bimodal association between the rhyolitic rocks and numerous coalescing low profile basalt lava shields and 8 widespread evidence of emplacement in lacustrine alluvial environments as revealed by intercalated lake sediments ignimbrite peperites rhyolitic and basaltic hyaloclastites basalt pillow lava deltas rhyolitic and basaltic phreatomagmatic tuffs alluvial sands and palaeosols many rhyolitic eruptions were high mass flux large volume and explosive vei 6 8 and involved h2o poor low 18o metaluminous rhyolite magmas with unusually low viscosities partly due to high magmatic temperatures 900 1 050 c sr type volcanism contrasts with silicic volcanism at many other volcanic fields where the fall deposits are typically plinian with pumice lapilli the ignimbrites are low to medium grade non welded to eutaxitic with abundant pumice lapilli or fiamme and the rhyolite extrusions are small volume silicic domes and coulees sr type volcanism seems to have occurred at numerous times in earth history because elements of the facies association occur within some other volcanic fields including trans pecos texas etendeka parana lebombo the english lake district the proterozoic keewanawan volcanics of minnesota and the yardea dacite of australia

mantle temperature and lithospheric thinning beneath the midcontinent rift system evidence from magmatism and subsidence the tectono magmatic history of the midcontinent rift system can be explained by the presence of a mantle plume bringing elevated temperature mantle beneath the rift system at about 1110 ma huge volumes of extrusive and intrusive igneous rocks were generated as abnormally hot mantle decompressed beneath the lithospheric rift geochemical and isotopic data from the keweenawan volcanics show that the earliest melts were derived from small degree melting of primitive plume mantle coupled with enriched metasomatic melts derived from the continental lithosphere as rifting progressed the main bulk of the volcanics was generated primarily from the plume mantle with the melting starting at depths of about 120 km and extending to as shallow as the base of the stretched lithosphere at 45 km depth elevated mantle temperatures of 1500 1560 c approximately 150 200 c above normal are inferred from the rare earth element concentrations in the volcanic rocks further constraints on the mantle temperature come f

the 1966 kremasta reservoir earthquake sequence the ten month february november 1966 earthquake sequence thought to be associated with the filling of the kremasta reservoir in northern greece can be divided into two distinct segments the first consisting of the main shock m b 5 6 m 5 8 and its aftershocks in the following nine months indicate normal faulting primarily on a northwest striking southwest dipping fault plane to a depth of 35 45 km the second which began in late october with another large m b 5 8 m s 5 3 event consists of thrust faulting not on the previous fault plane the kremasta sequence occurs at the transition between extensional and compressive faulting in a distributed plate boundary region the main shock occurred near the top of an aftershock zone much larger than the expected fault area in accord with a model in which the main shock and many of its aftershocks were induced by pore water diffusion along a preexisting feature alternatively the main shock may be background seismicity and the aftershocks are located near the lake due to pore pressure effects the later thrust aftershocks are less likely to be directly reservoir related

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

lamellar pyroxenes and their petrogenetic significance three examples from the czech massif the first event in the crystallization of pyroxenite in nodules from bory was the growth of clinopyroxene at an uncertain 1 190 30 c and 2 5 2 gpa lamellae of garnet and orthopyroxene in cpx nucleated slightly above 1 000 c and equilibrated at 940 20 c and 3 9 6 gpa the growth of lamellae was followed by crystallization of groundmass that corroded lamellar clinopyroxene phenocrysts of clinopyroxene and orthopyroxene in olivine pyroxenite from t i studn crystallized at 1 290 100 c and 2 3 1 4 gpa the lamellae in both phases nucleated between 1 400 and 1 200 c and equilibrated at 860 40 c and 0 2 3 gpa before regional metamorphism the temperatures of nucleation mentioned rest on uncertain assumptions in the phase boundary theory and should be viewed with caution the serpentinized peridotite pyroxenite from destna contains clinopyroxene relics the only primary phase with lamellae of a 1 42 nm sheet silicate topotactically pseudomorphing enstatite possibly a chlorite or vermiculite all phases in lamellae are crystallographically oriented in the hosts the present cases suggest that when applying the theory of phase boundaries a modified procedure of robinson et al should also be considered in which identity of adjacent areas rather than just vectors is the basis of calculation spinel lamellae in cpx are bounded not only by faces parallel to coherent interfaces but also by faces formed by accumulation of incoherent edges of growth ledges

weathering and paleosol formation in the 1 1 ga keweenawan rift abstract at 1 1 ga the north american craton began to rift resulting in flood style basaltic volcanism during the phase of active tectonism some of that basalt was weathered and redistributed by fluvial processes new observations near the northwestern shore of lake superior have also revealed the presence of seven paleosols formed on weathered immature sediments derived from the basalt that are described here for the first time only three other paleosols have previously been described from this part of the geologic record so these new paleosols represent a significant new discovery the paleosols are weakly developed and exhibit physical and chemical characteristics similar to phanerozoic entisols in contrast to many precambrian paleosols the keweenawen paleosols have been subject to minimal post burial alteration among the paleosols three distinct pedofacies are recognized 1 waterlogged gleyed fluvial proximal paleosols 2 dry fluvial distal paleosols and 3 cumulative fluvial proximal paleosols despite these facies differences using a combination of physical and chemical measures of weathering it is found that the paleosols are all derived from the same parental basalt with one of the paleosols showing an additional extra basinal rhyolitic component by considering the mass balance behaviour of a variety of alkali and alkaline earth elements it is possible to rule out significant potassium metasomatism suggesting that these paleosols may be particularly useful for paleoatmospheric and paleobiologic reconstructions and that they form an important new source of data about mesoproterozoic weathering

seismic attribute inversion for velocity and attenuation structure using data from the glimpce lake superior experiment a simultaneous inversion for velocity and attenuation structure using multiple seismic attributes has been applied to refraction data from the 1986 glimpce lake superior experiment the seismic attributes considered include envelope amplitude instantaneous frequency and travel time of first arrival data instantaneous frequency is converted to t using a matching procedure which approximately removes the effects of the source spectra the derived seismic attributes are then used in an iterative inversion procedure referred to as aft inversion for amplitude instantaneous frequency and time uncertainties and resolution of the velocity and attenuation models are estimated using covariance calculations and checkerboard resolution maps a simultaneous inversion of seismic attributes from the glimpce data results in a velocity model similar to that of previous studies across lake superior a central rift basin and a northern basin are the most prominent features with an increase in velocity near the isle royale fault although there is an indication of the central and northern basins in the attenuation model for depths greater than 4 km the separation is not evident for shallower depths this may result from microfractures masking compositional variations in the attenuation model for shallower depths attenuation q values range from approximately 60 near the surface to near 500 at 10 km depth a relationship between inverse q and velocity of q 0 0210 0 0028 v was found with a correlation coefficient of 0 96 this suggests a nearly linear inverse relationship between q l and velocity beneath lake superior which supports previous laboratory results the inverted velocity and attenuation models provide important constraints on the lithology and physical properties of the midcontinent rift beneath lake superior

interpretations of reservoir induced seismicity may not always be valid the case of seismicity during the impoundment of the kremasta dam greece 1965 1966 the kremasta seismic sequence in western greece is one of the most commonly cited examples of reservoir induced seismicity ris here we show that this sequence is a result of normal tectonic activity and that only some small unrelated microseismic events are reservoir induced shortly after the beginning of the impoundment of the kremasta dam in 1965 the then newly established seismic monitoring network in greece recorded two ms 6 0 events and numerous small shocks spread over a 120 km wide region these were interpreted as a single seismic sequence namely the kremasta seismic sequence and assumed to be reservoir induced we revisit the epicenter locations of these events and interpret them in the framework of the regional tectonic context and the local hydrogeology placing these events into the local context shows that they represent an amalgamation of separate ordinary tectonic seismic sequences further the regional rocks are highly fragmented by small faults and the spatial distribution of seismic events is not consistent with a model of stress transfer from reservoir loading in addition it is not likely that events at such long 20 30 km distances from the reservoir could be induced by an initial reservoir load head of 30 m whilst the larger magnitude events are tectonic after impoundment local residents reported an unusual frequency of small microseismic events felt only within 10 km of the dam we provide evidence that these are a result of the collapse of numerous shallow karstic cavities adjacent and beneath the reservoir due to increased water load locally 100 150 m depth this study has significant implications for interpretation of seismic triggering mechanisms in other regions earthquake occurrence within the proximity of reservoirs during and after impoundment time cannot be assumed to be ris unless supported by seismological geological and hydrogeological evidence

keweenaw hot spot geophysical evidence for a 1 1 ga mantle plume beneath the midcontinent rift system the proterozoic midcontinent rift system of north america is remarkably similar to phanerozoic rifted continental margins and flood basalt provinces like the younger analogues the volcanism within this older rift can be explained by decompression melting and rapid extrusion of igneous material during lithospheric extension above a broad asthenospheric thermal anomaly which the authors call the keweenaw hot spot great lakes international multidisciplinary program on crustal evolution seismic reflection profiles constrain end member models of melt thickness and stretching factors which yield an inferred mantle potential temperature of 1 500 1 570c during rifting combined gravity modeling and subsidence calculations are consistent with stretching factors that reached 3 or 4 before rifting ceased and much of the lower crust beneath the rift consists of relatively high density intruded or underplated synrift igneous material the isotopic signature of keweenawan volcanic rocks presented in a companion paper by nicholson and shirey this issue is consistent with the model of passive rifting above an asthenospheric mantle plume

crustal structure of the central graben of the midcontinent rift beneath lake superior abstract travel times and amplitudes of refracted and reflected arrivals observed during glimpce great lakes international multidisciplinary program on crustal evolution from the midcontinent rift in lake superior have been modeled using two dimensional ray tracing techniques forward modeling was used to iteratively refine an initial velocity model that was constructed from results of tau sum analysis of the travel time data at each station combined with information from a coincident common midpoint cmp reflection profile when converted to time the resulting model agrees quite well with the cmp reflection profile within the central graben of the midcontinent rift approximately 2 km of sedimentary rock 2 8 4 6 km s overlie an 8 km thick sequence of volcanics and interflow sediments 5 0 6 5 km s within the central rift graben observed on the reflection data beneath this sequence is a 6 8 km thick sequence of 6 6 7 0 km s material interpreted to represent metamorphosed volcanics the velocity of the material at the base of the rift graben is not well constrained approx 7 0 7 2 km s but probably comprises an additional 10 12 km of metavolcanic rocks and intrusions that extend to the base of the graben as imaged on the coincident seismic reflection profile the central graben is ranked by reverse faults the isle royal and keweenawan faults that probably represent normal faults associated with graben formation that were reactivated during the late precambrian a 50 100 ms time advance anomaly associated with the isle royal fault is observed at every station this anomaly has been modeled as shallow high velocity blocks located directly beneath the fault the blocks correlate well with the walls of a steep sided bathymetric trough and are believed to represent highly indurated upper keweenawan sediments resulting from hydrothermal alteration a marked decrease in the apparent velocity and amplitude of the first arrivals is observed on reversed sections at offsets exceeding 100 km and has been modeled as pre rift continental crustal rocks with a velocity of about 6 5 km s adjacent to the 7 0 km s material in the graben calculation of the gravity response of the seismic model demonstrates that the gravity high centered over the rift can be entirely attributed to high density rocks occupying the central half graben imaged on the cmp profile wide angle reflections from about 15 30 km depth beneath the flanks of the graben indicate the presence of velocity discontinuities that may represent rift related detachment surfaces and or pre rift structures within the pre rift crust additional wide angle reflections define the top and bottom of high velocity lower crustal layer the high velocity lower crustal layer is thickest beneath the rift graben and is interpreted to represent magmatic material underplated to the crust during rifting as discussed by trehu et al 1991 total crustal thickness beneath the lake varies from about 55 km beneath the graben to about 35 km beneath the southern shore and 40 km beneath the northern shore

gabbroic rocks from the mathematician ridge failed rift plutonic rocks from the ocean crust provide direct evidence on the petrological processes leading to its formation and evolution the nature of such rocks is important for interpreting the origin of on land ophiolites so far oceanic plutonic rocks have been mainly recovered from slow spreading ridges and their associated fractures1 13 and from trenches14 15 here we report the results of petrological studies of a suite of gabbros and related plutonic and volcanic rocks dredged from the mathematician ridge mr which was a fast spreading ridge 43 mm yr 1 until it was abandoned 3 5 myr ago16 textures and mineral chemistry of the rocks together with structural considerations indicate that before abandonment the mr produced fractionated mid ocean ridge basalt morb and fractionated gabbros the gradual decrease of spreading rate during abandonment resulted in morphological change eruption of more primitive morb and possibly tectonic emplacement of the older ferrogabbros alternatively the latter could have been tectonically emplaced after abandonment

seismicity focal mechanisms and tectonics in the last four years there has been a tremendous increase in the detail of tectonic processes reported in the typical seismotectonic study new data from many local and regional networks of seismographs established during the previous four year period plus the now routine application of improved hypocentral location procedures long period body wave modelling and moment tensor inversion using digitally recorded data have increased resolution of both the hypocentral location and the nature of the faulting process consequently research emphasis has been on local tectonics or those aspects of regional and global tectonics requiring high resolution i briefly describe below some of the highlights of these studies

petrogenesis of orthopyroxene magnetite ilmenite intergrowths from an ultramafic layer a new occurrence of myrmekitic orthopyroxene magnetite ilmenite intergrowth from a thin ultramafic layer within a heterogeneous noritic rock from the frederiksdal area south greenland was investigated chemically and texturally compositionally the intergrown ilmenite and orthopyroxene show the lowest recorded mgo content from pyroxene oxide mineral intergrowths the magnetite ilmenite intergrowth has skeletal morphology consisting of sets of thinly tabular plates parallel to the 111 planes of the spinel structure the orthopyroxene oxide intergrowth is considered to be the result of a simultaneous magmatic crystallization that of the oxide minerals being faster

seismicity and associated strain of central greece between 1890 and 1988 summary we examined the seismicity of central greece between 1890 and 1988 using macroseismic and instrumental data to ask two questions 1 does the seismicity of this period reveal all the major tectonic structures that are known to be active and 2 what are the likely strains associated with the seismicity over this period many known active structures have been effectively aseismic for the last hundred years and even the inclusion of all known large events earlier than 1890 reveals no activity associated with the ne coast of evia gulf of argos or graben ne of mt parnassos it is clear that even 100 years data are inadequate for either a reasonable assessment of seismic risk or for a confident estimation of maximum magnitude however we are aware of no earthquakes in central greece during the last 200 yr that were larger than ms 7 0 it is probable that the maximum magnitude is restricted by the maximum length of fault segments which appears to be around 15 20 km the earthquakes of ms 5 8 during 1890 1988 can account for a n s displacement of around 45 70 cm with maximum and minimum estimates a factor of two greater and smaller than this across part of a 1890 1900 triangulation network in central greece that was resurveyed in 1988 the contribution of smaller events may increase this displacement by about 50 per cent this cumulative seismic displacement is similar to that estimated from the geodetic work about 100 cm but a detailed comparison of the two sets of observations will be reported elsewhere a re evaluation of all the important earthquakes of 1890 1988 in central greece is presented in the appendix which summarizes information of use to both earth scientists and engineers

calcium carbonate cement caliche in keweenawan sedimentary rocks 1 1 ga upper peninsula of michigan abstract a complex caliche profile and numerous widespread zones of carbonate cement occur in a thick sequence of fluvial siliciclastic sandstones and conglomerates that were deposited in the lake superior basin during and after continental basaltic volcanism as interflow conglomerates of the portage lake volcanics and as the oronto group in conglomerates the carbonate cement forms discontinuous zones up to 60 cm thick and several meters in length consisting of up to 25 vol carbonate whereas the caliche horizon at the centennial mine shows typical upward grading features characteristic of pedogenic caliche the widespread sparry carbonate cement is less diagnostic it is interpreted also as caliche for the following reasons 1 it predates regional epidote metamorphism 2 a caliche profile has been described from the same strata 3 textures characteristic of caliche such as carbonate rimmed pebbles pendulose calcite on pebbles sand grains floating in a carbonate matrix and exfoliated chlorite 4 possible vadoids 5 the carbonate rich zones are widespread stratigraphically and geographically in basinward inclined alluvial fans and 6 the distribution of these zones and the total quantity of contained carbonate requires other than a point source for the origin of the carbonate several lines of evidence suggest that the prevailing climate may have been arid with a fluctuating perhaps seasonal rainfall pattern required for caliche 1 a paleolatitude about 30 north 2 carbonate filled shrinkage cracks denoting high evaporation 3 intermittent flashy or ephemeral streams recorded locally in fluvial beds by numerous superimposed horizons of mud chip conglomerate enormous previously cemented mudstone rip ups and desiccation cracked mud drapes interpretation of the cement as vadose caliche is compatible with a sedimentation model for the region as a semi arid intermontane basin flanked by alluvial fans and with a central saline lake

a catalogue of seismicity in greece and adjacent areas summary a new earthquake catalogue for greece has been formed to cover the instrumental period 1901 78 in particular 605 earthquakes for the period 1917 63 inclusive are relocated using first arrival data from the international seismological summary these relocations incorporate macro seismically and other well controlled master events into an ensuing joint epicentre determination technique the largest annual average shift is 165 km for 78 earthquakes during the decade after 1917 decreasing to 17 km for the later years 1957 63 magnitudes are redetermined mainly using readings from the swedish network and uppsala since as early as 1908 catalogue completeness exists for magnitudes around 5 5 for at least the last 60 years but magnitudes of about 4 7 are completely reported only during the most recent 15 years the new epicentral positions lead to a better delineation of the seismic zones than has previously been achieved the majority of shallow earthquakes are contained in a belt parallel to the hellenic arc which extends north into albania in the south east the epicentral locations give a more diffuse extension of this zone into the west coast of turkey depths of intermediate earthquakes along the hellenic arc tend to relocate to shallower depths in the south west part of crete no earthquake focus deeper than 100 km is found although in the south eastern section of the arc a tendency to increased depths is observed a second zone starts at leukas island in the west and extends through central greece to near volos on the east coast where it divides into two branches which are less well defined but which eventually join the seismicity of western turkey a third zone follows the saronikos and corinth gulfs

keweenawan age caliche paleosol in the lower part of the calumet and hecla conglomerate centennial mine calumet michigan the calumet and hecla conglomerate a keweenawan interflow sedimentary unit that is 1 1 b y in age consists at the centennial mine of a felsitic conglomerate upper part and of 1 2 m of siltstone and shale along the base in these fine grained sediments there is a complex submature caliche soil profile represented by an upward increase in calcite and corresponding changes in calcite structures there is also caliche in conglomerate all of this calcite predates the period of regional epidote alteration the caliche is interpreted to have developed in slowly aggrading sediments that were deposited on a rock floored fluvial plain and took place during the initial stages of slope reversal of the lava plain from southerly to northerly this kind of carbonate accumulation in sediments requires a highly seasonal temperate or tropical climate and represents 15 000 yr of weathering the corresponding keweenawan paleolatitude has been reported to be 30 n

an aem study of garnet clinopyroxenite from the sulu ultrahigh pressure terrane formation mechanisms of oriented ilmenite spinel magnetite amphibole and garnet inclusions in clinopyroxene optical microscopy secondary electron microscopy and analytical electron microscopy were used to characterize crystallographic orientation relationships between oriented mineral inclusions and clinopyroxene cpx host from the hujialing garnet clinopyroxenite within the sulu ultrahigh pressure uhp terrane eastern china one garnet clinopyroxenite sample 2hj 2c and one megacrystic garnet bearing garnet clinopyroxenite rz 11d were studied porphyroblastic clinopyroxene from sample 2hj 2c contains oriented inclusions of ilmenite ilm spinel spl magnetite and garnet whereas clinopyroxene inclusions within megacrystic garnet from sample rz 11d contain oriented inclusions of ilmenite and amphibole specific crystallographic relationships were observed between ilmenite spinel plates and host clinopyroxene in sample 2hj 2c and between ilmenite plates and host clinopyroxene in sample rz 11d i e 1 bar 1 00 ilm 0 bar 1 0 cpx 0001 ilm 100 cpx and 110 spl 0 bar 1 0 cpx bar 1 11 spl 100 cpx these inclusions are suggested to be primary precipitates via solid state exsolutions most of the needle like magnetite spinel inclusions generally occur at the rims or along fractures of clinopyroxene within sample 2hj 2c despite the epitaxial relation with host clinopyroxene these magnetite spinel needles would have resulted from fluid melt infiltrations non epitaxial garnet lamellae in clinopyroxene of sample 2hj 2c were formed via fluid infiltration deposition primarily along 010 and subordinately along 100 partings epitaxial amphibole plates with a thickness 1 m and lamellae with a thickness 1 10 m in host clinopyroxene of sample rz 11d were probably results of hydration processes although amphibole plates could otherwise be interpreted as exsolution products temporal relations between mineral inclusions in each sample can be established and a semi quantitative p t path for this garnet clinopyroxenite body was derived accordingly the present results show that the hujialing garnet clinopyroxenite may not have subducted to mantle depths as deep as 250 km during uhp metamorphism as suggested by previous studies this study demonstrates that the crystallographic and temporal spatial relationships between aligned inclusions and host minerals are essential to a correct genetic interpretation of metamorphic rocks

the 1 1 ga midcontinent rift system central north america sedimentology of two deep boreholes lake superior region abstract the midcontinent rift system mrs of central north america is a 1 1 ga 2500 km long structural feature that has been interpreted as a triple junction rift developed over a mantle plume as much as 20 km of subaerial lava flows mainly flood basalts are overlain by as much as 10 km of sedimentary rocks that are mostly continental fluvial red beds this rock sequence known as the keweenawan supergroup has been penetrated by a few deep boreholes in the search for petroleum in this paper two deep boreholes in the upper peninsula of michigan are described in detail for the first time both the amoco production 1 29r test herein referred to as the st amour well and the nearby hickey creek well drilled by cleveland cliffs mining services were 100 cored the former is 7238 ft 2410 m deep and the latter is 5345 ft 1780 m deep the entirety of the stratigraphic succession of the hickey creek core correlates very well with the upper portion of the st amour core as determined by core description and point counting of 43 thin sections selected out of 100 studied thin sections two lower paleozoic units and two keweenawan red bed units the jacobsville sandstone and the underlying freda sandstone are described the jacobsville is largely a feldspatholithic sandstone and the freda is largely a lithofeldspathic sandstone below the freda the remaining footage of the st amour core consists of a thick quartzose sandstone unit that overlies a heterogenous unit of intercalated red bed units of conglomerate sandstone siltstone and shale black shale individual basalt flows and a basal ignimbritic rhyolite this lower portion of the st amour core presents an enigma as it correlates very poorly with other key boreholes located to the west and southwest while a black shale sequence is similar to the petroleum bearing nonesuch formation farther west there is no conglomerate unit to correlate with the copper harbor conglomerate other key boreholes are distributed over a 1300 km distance along the better known southwest arm of the triple junction mrs and can be correlated rather well with the units that are exposed in the lake superior region however a definitive explanation of the anomalous deeper st amour stratigraphy is elusive and any explanation is tenuous a possible explanation for this anomalous stratigraphy may be the geographic proximity of the st amour borehole to the keweenawan hot spot mantle plume the suggested thermal force behind the development of the mrs similarly a drastic change in structural architecture may be explained by this geographic relationship thus within the locale of this rifting center complexities of expansion tectonics may well be responsible for igneous and sedimentary sequences that differ considerably from those found farther west along the rift arm

the present status of reservoir induced seismicity investigations with special emphasis on koyna earthquakes abstract the status of reservoir induced seismicity ris has been reviewed periodically rothe 1968 1973 gupta and rastogi 1976 simpson 1976 packer et al 1979 in the present paper the significant work carried out during the last three years on ris is reviewed an earthquake of magnitude 5 1 2 occurred on november 14 1981 in the vicinity of aswan lake egypt 17 years after the filling started in 1964 this event occurred 4 days after the seasonal maximum in the reservoir water level and was followed by a long sequence of aftershocks another event of magnitude 4 1 2 occurred in the vicinity of aswan lake on august 20 1982 results of preliminary investigations indicate that this seismic activity is reservoir induced recent analyses of induced seismic events at nurek reservoir u s s r show that the second stage of filling during august to december 1976 increasing the maximum depth from 120 m to 200 m was accompanied by an intense burst of shallow seismic activity an outward migration from the centre of the reservoir possibly associated with diffusion of pore pressure is revealed by the temporal distribution of earthquake foci a variety of investigations including the in situ measurement of tectonic stress pore pressure permeability distribution of faults etc in addition to monitoring seismicity have been undertaken in the vicinity of the monticello reservoir south carolina the largest reservoir induced earthquake is predicted not to exceed magnitude 5 the koyna reservoir india continues to be the most outstanding example of ris three earthquakes of magnitude 5 occurred in september 1980 earthquakes of magnitude 4 occur frequently in the vicinity of koyna the latest being on february 5 1983 events that occurred during the period 1967 1973 have been relocated using better procedures and are found to be much shallower and the epicentres less diffused location of 12 earthquakes of ms 4 0 their foreshocks and aftershocks that occurred during 1973 1976 composite focal mechanism solutions and related studies are consistent with the delineation of a n s trending fault through the reservoir area in a couple of interesting studies it has been demonstrated that earthquakes of magnitude 5 0 in the koyna region are usually preceded by several magnitude 4 earthquakes in the preceding fortnight also a rate of loading of koyna reservoir of at least 40 ft week appears to be a necessary although not sufficient condition for the occurrence of magnitude 5 earthquakes smooth filling emptying appears to be the key to reduce the hazard of ris a map and a table of the reported cases of reservoir induced changes in seismicity through 1982 have been compiled

regional and local patterns of low grade metamorphism in the north shore volcanic group minnesota usa the north shore volcanic group in northern minnesota is part of the middle proterozoic keweenawan sequence one of the largest plateau lava provinces in the world the primary geochemistry of the basalts suggests that volcanism occurred in an intracontinental rift environment the subaerial lava flows mainly amygdaloidal olivine tholeiites and tholeiites have undergone low grade metamorphism from zeolite to lower greenschist facies on the basis of alteration phases replacing the primary magmatic minerals infilling amygdales and veins and replacing secondary minerals the following zones have been distinguished 1 thomsonite scolecite smectite 2 heulandite stilbite smectite 3 laumontitechlorite albite 4 laumontite chlorite albite prehnite pumpellyite and 5 epidote chlorite albite actinolite zone in addition to the overall zonation based on mineral parageneses zonations in the composition of the ab content of the newly formed albite replacing primary ca rich plagioclase and of the newly formed mafic phyllosilicates are observed within the sequence and within single flows mafic phyllosilicates in the upper part of the sequence mainly smectites and mixed layer smectite chlorites display high si and ca na k contents whereas in the lower part of the sequence the amounts of si and ca na k are markedly lower mainly chlorites and mixed layer chlorite smectites similar zonations are observed within the individual flows the albite content of the newly formed plagioclase is highest and the si and ca na k content of the phyllosilicates lowest in the amygdaloidal flow top while the opposite is true for the massive flow interior the above features suggest that the overall pattern is one of burial type metamorphism associated with extension in the rift setting in detail the mineral assemblages are controlled not only by the stratigraphic position but also by the flow morphology controlling permeability whose effect on the assemblages is most pronounced in the stratigraphically upper parts this suggests that at the first stages of alteration lowest grade the patterns of fluid flow were important effects in controlling the assemblages at greater burial depth assemblages are more homogeneous perhaps representative of a more even and pervasive flow pattern using the observed assemblages at face value to define grade and or facies different conditions would be assigned within the different morphological flow portions thus at low grade metamorphic conditions it is essential to integrate assemblages from different morphological flow portions in order to define satisfactorily the overall metamorphic conditions