petrophysical and paleomagnetic data of drill cores from the bosumtwi impact structure ghana physical properties from rocks of the bosumtwi impact structure ghana central africa are essential to understand the formation of the relatively young 1 07 ma and small 10 5 km impact crater and to improve its geophysical modeling results of our petrophysical studies of deep drill cores lb 07a and lb 08a reveal distinct lithological patterns but no depth dependence the most conspicuous difference between impactites and target lithologies are the lower bulk densities and significantly higher porosities of the suevite and lithic breccia units compared to meta graywacke and metapelites of target lithologies magnetic susceptibility shows mostly paramagnetic values 200 500 10 6 si throughout the core with an exception of a few metasediment samples and correlates positively with natural remanent magnetization nrm and q values these data indicate that magnetic parameters are related to inhomogeneously distributed ferrimagnetic pyrrhotite the paleomagnetic data reveals that the characteristic direction of nrm has shallow normal in a few cases shallow reversed polarity which is in agreement with the lower jaramillo n polarity chron direction and is carried by ferrimagnetic pyrrhotite however our study has not revealed the expected high magnetization body required from previous magnetic modeling furthermore the lb 07a and lb08 a drill cores did not show the predicted high content of melt in the rocks requiring a new interpretation model for magnetic data

the fold test in palaeomagnetism summary the fold test suggested by mcelhinny is shown to be inappropriate more correctly one should determine whether the mean direction of a group of sites from one limb of a fold may be distinguished statistically from the mean direction of a group of sites from another limb details of the application of this test are given with examples it is shown that the test indicates significance under far less severe folding than does the test suggested by mcelhinny

a multichannel seismic study of lithospheric flexure across the hawaiian emperor seamount chain existing models assume that the thickened crust beneath seamounts is the result of a surface volcanic load flexing an elastic plate new results suggest that flexed oceanic crust beneath the hawaiian emperor seamount chain is underlain by a 4 km thick deep crustal body we intepret the body as a deep crustal sill complex associated with the tholeiitic stage of volcano building along the chain

rapid high temperature formation of large scale rheomorphic structures in the 2 06 ma huckleberry ridge tuff idaho usa in the teton river canyon eastern idaho the ca 2 06 ma 130 m thick huckleberry ridge tuff exhibits large scale rheomorphic fold geometries defined by eutaxitic fabrics parallel to both the primary internal zonation and the basal contact with older strata paleomagnetic data from a large amplitude 150 m northwest trending overturned fold near the failed teton dam indicate folding above maximum magnetization unblocking temperatures 580 c the in situ characteristic remanent magnetization chrm direction is indistinguishable from previous studies of undeformed huckleberry ridge tuff and a fold test is negative k minimized at 100 unfolding anisotropy of magnetic susceptibility data reveal magnetic foliation planes that dip northeast roughly parallel to the axial surface of the fold because deformed and undeformed huckleberry ridge tuff exposures preserve the same anomalous chrm direction large scale rheomorphic structures in the tuff must have formed rapidly at high temperatures shortly after development of compaction fabrics post welding high temperature deformation is consistent with field evidence indicating rapid plastic secondary deformation of much of the tuff prior to devitrification

detection of a meteoritic component in ivory coast tektites with rhenium osmium isotopes measurement of rhenium re and osmium os concentrations and os isotopic compositions in ivory coast tektites natural glasses with upper crustal compositions that are ejected great distances during meteorite impact and rocks from the inferred source crater lake bosumtwi ghana show that these tektites incorporate about 0 6 percent of a meteoritic component analysis of elemental abundances of noble metals alone gives equivocal results in the detection of meteoritic components because the target rocks already have relatively large amounts of noble metals the re os system is ideally suited for the study of meteorite impacts on old continental crust for three reasons i the isotopic compositions of the target rocks and the meteoritic impactor are significantly different ii closed system mixing of target rocks and meteorites is linear on re os isochron diagrams which thus permits identification of the loss of re or os iii osmium isotopic compositions are not likely to be altered during meteorite impact even if re and os are lost

crust and upper mantle structure beneath the pacific northwest from joint inversions of ambient noise and earthquake data 1 we perform a joint inversion of phase velocities from both earthquake and ambient noise induced rayleigh waves to determine shear wave velocity structure in the crust and upper mantle beneath the pacific northwest we focus particularly on the areas affected by mid miocene to present volcanic activity the joint inversion combined with the high density seismic network of the high lava plains seismic experiment and data from the earthscope transportable array provides outstanding resolution for this area in oregon we find that the pattern of low velocities in the crust and uppermost mantle varies between the high lava plains physiographic province and the adjacent northwestern basin and range these patterns may be due to the presence of the brothers fault zone which separates the clockwise rotating northwest basin and range from the relatively undeformed areas further north further to the east the owyhee plateau snake river plain srp and northeastern basin and range are characterized by high crustal velocities though the depth extent of these fast wave speeds varies by province of particular interest is the mid crustal high velocity sill previously only identified within the srp we show this anomaly extends significantly further south into utah and nevada we suggest that one possible explanation is lateral crustal extrusion due to the emplacement of the high density mafic mid crustal sill structures within the srp

intraplate extensional tectonics of the eastern basin range inferences on structural style from seismic reflection data regional tectonics and thermal mechanical models of brittle ductile deformation the general lack of a correlation between earthquakes and surface faulting in the interior of the western u s cordillera has motivated our efforts to evaluate the geometry structural style and mechanism of normal faulting characteristic of this region of intraplate extension to address the problem we have interpreted over 1500 km of seismic reflection data and constructed detailed cross sections of the upper crust rheological models of the continental crust were calculated to examine the possibility of shallow quasi plastic flow and its influence on faulting in the eastern basin range the western colorado plateau and the middle rocky mountains our data and interpretations have revealed the following styles of cenozoic deformation 1 steep to low angle dip normal faulting along the wasatch fault 2 low angle dip and listric normal faulting possibly associated with movement on preexisting thrusts 3 the occurrence of asymmetric mostly eastward tilted tertiary basins that are bounded by low to moderate dipping planar and listric faults and 4 at least three vertically stacked en echelon low angle reflections in the mid to upper crust that dip gently westward from 3 km beneath the wasatch plateau to over 15 km at the utah nevada border these reflections are interpreted as normal detachment faults the structural style of the pervasive low to moderate angle dipping faults cannot be easily reconciled with classic brittle failure theory but the interpreted termination of normal faults at or above the deeper low angle reflections suggests the presence of shallow zones of ductile deformation that may have accommodated slip an important observation based on interpretations of seismic reflection profiles is that normal fault zones dip more gently in the subsurface than their associated scarps in unconsolidated surficial deposits segment boundaries of the wasatch fault zone apparently coincide with the positions of east trending displacement transfer structures in thrust sheets and the position of the precambrian uinta aulacogen suggesting that preexisting crustal structure partly controls the geometry of extension to examine the influence of ductile deformation quasi plastic flow was modeled for appropriate geotherms of the basin range and colorado plateau crusts these models provide constraints on the depth to the frictional quasi plastic transition that occurs as shallow as 8 km in the eastern basin range this depth also corresponds to the approximate depth above which most accurately determined earthquake foci for small earthquakes m 5 5 appear to nucleate at 10 15 km in or near the brittle ductile transition the rheological modeling suggests that a thermally deforming continental lithosphere plays an important role in the evolution of this extending intraplate region large scale low angle reflections may represent decollements and normal faults at the top of upper crustal zones of reduced shearing strength and higher ductility

ascent of a granitoid diapir in a temperature varying medium a relationship for the ascent velocity of a granitoid diapir is derived and used in conjunction with a crustal heat flow model which incorporates a depth dependent temperature to predict emplacement histories ascent rate is strongly dependent on the width of a deformation layer of variable viscosity that is formed by the transfer of heat to the country rock and on the buoyancy force which propels the diapir through it ascent velocity is described by the expression u 2 gd3z 0r where is the density contrast between diapir and country rock g is gravitational acceleration d is the deformation zone width z is a parameter which describes the increase in viscosity across the zone 0 is the country rock viscosity at the contact and r is the diapir radius the width of the deformation zone as dictated by the temperature distribution and rheological function is the point at which the viscosity increases by a factor f from the contact value although ascent rates vary with choice of this parameter ascent distances are not strongly dependent on f results of simulations which incorporate a spectrum of input parameters reveal that ascent distance is strongly dependent on both the rheology and ambient temperature of the encompassing rock but only weakly dependent on the initial diapir temperature ascent velocity varies with the diapir size with larger bodies moving more slowly and ascending for longer periods relative to smaller plutons as a result of the proportionately larger amount of thermal energy available for ascent larger bodies will rise greater distances than smaller diapirs however the additional ascent appears nominal relative to the contrast in available energy modeling diapirs using a no slip boundary condition decreases ascent velocity by a factor of about 3 relative to results incorporating a stress free boundary condition the time scale of granitoid diapirism over crustal distances cannot be less than 104 years due to the rate limiting effect of thermal conduction above the body and not greater than 105 years reflecting solidification times of kilometer sized plutons a consequence of ascent by a deformation zone mechanism is the generation following emplacement of a mechanically weakened crust which extends from beneath the diapir to near the source perhaps explaining the relatively common occurrence of synmagmatic to postmagmatic mafic dikes within granites believed to be derived from interaction of basalt with the lower crust

detailed structural analysis of the rim of a large complex impact crater bosumtwi crater ghana the 1 ma bosumtwi crater in ghana is an 11 km diameter presumably complex well preserved impact structure that is associated with the ivory coast tektite strewnfield detailed structural geologic studies along a complete traverse through the northwestern rim section indicated four zones characterized by distinct deformation styles from just outside of the crater rim to near the crater floor zone 1 is dominated by thick deposits of lithic impact breccia intercalated in places with products of local mass wasting zone 2 contains inward dipping thrust planes conjugate radial fractures isoclinal folding and overturned stratigraphic sequences zone 3 represents a megabreccia zone in which block size decreases upward and outward toward the rim crest the innermost zone 4 is dominated by intense thrust faulting of multiple orientations resulting in complex duplex and lens shaped bodies these deformation styles generally correspond to those previously reported from the rims of simple bowl shaped meteorite impact craters and appear to be characteristic of impact structures in general

kinematic analysis of fractures in the great rift idaho implications for subsurface dike geometry crustal extension and magma dynamics 1 extension across the southern great rift of the eastern snake river plain esrp idaho was measured to calculate the dimensions of underlying dikes and interpret magmatic and extensional processes cumulative rift perpendicular extension ranges from 0 64 to 4 50 m along the 14 km long kings bowl segment from 1 33 to 4 41 m along the 14 km long new butte segment and from 0 74 to 1 57 m along the 4 km long minidoka segment along strike of each segment extension increases toward coeval vents each rift segment is interpreted to be underlain by a subsurface dike whose dimensions are calculated using buoyancy equilibrium and boundary element models dikes are calculated to have tops that are 950 530 m deep bottoms that are 23 31 km deep and widths that taper to zero from a maximum of 2 21 m modeling suggests that the kings bowl dike has a maximum probable width of 8 m and a volume of 2 km3 about 400 times the volume of its coeval lava flow dike widths and ages at the southern great rift provide evidence for a holocene esrp strain rate of about 1 to 3 10 16 s 1 which is as much as an order of magnitude slower than strain rates in the adjacent seismically active basin and range province eruptive fissures are present where rift width is 1650 m this corresponds to a depth to dike top of 700 m which we propose was the depth where vesiculation initiated thus increasing magma pressure and inducing eruption

the early proterozoic birimian supergroup of ghana and some aspects of its associated gold mineralization abstract rocks of the birimian supergroup of early proterozoic age sm nd isochron age 2166 66 ma form part of the west african craton ghana contains the eastern portion of the birimian terrane five parallel evenly spaced several hundred kilometre long volcanic belts consisting mainly of low grade metamorphic tholeiitic lavas morb chemistry characterize the birimian supergroup of ghana the belts are separated by basins containing isoclinally folded dacitic volcaniclastics wackes and argillitic sediments as well as granitoids lavas and sedimentary rocks were deposited contemporaneously as lateral facies equivalents within the sedimentary basins various lithofacies are distinguished in the transition zone between belt volcanics and basin sediments a chemical facies occurs this is defined by cherts manganiferous and carbon rich sediments fe ca mg carbonates and sulphide mineral disseminations after their emplacement birimian lavas and sediments were folded in the course of the eburnean tectono thermal event intruded by various types of granitoids uplifted and eroded the erosion products were deposited as sediments of the tarkwaian group in long narrow intramontane grabens which formed due to rifting preferentially in the central portions of all five birimian volcanic belts tarkwaian sediments were deformed by gravity tectonics two major types of eburnean granitoids in ghana are characterized as follows basin granitoids cape coast type occur as synorogenic foliated batholiths chiefly in the central portions of birimian sedimentary basins they are peraluminous and generally granodioritic in composition belt granitoids dixcove type mostly form late orogenic unfoliated intrusions in volcanic belts they possess a metaluminous character and commonly are of tonalitic composition bongo granitoids are of post tarkwaian age and k rich in contrast to these granitoid types which have strong mantle affinities the winneba type had an archean sialic precursor a model involving small scale equidimensional parallel and contemporaneously operating convection cells in the upper mantle which caused rifting of a highly attenuated protocrust as well as linear eruptions of tholeiitic magmas is proposed recent sm nd pb pb and rb sr data of volcanic and plutonic rocks are incorporated most ghanaian birimian gold occurrences and mines are concentrated in narrow corridors of 10 15 km width in the transition zone between volcanic belts and sedimentary basins as are the chemical facies and regionally extensive shear zones at the volcanics sediment interface birimian gold in ghana is present as two major types 1 the disseminated sulphide type which is generally lithofacies controlled i e controlled by chemical sediments and to a lesser extent by selvages of gold quartz veins and 2 the quartz vein type which is exclusively structure controlled data on geochemical underground traverses across both types of gold mineralization are briefly summarized

dating recent hawaiian lava flows using paleomagnetic secular variation hawaiian paleomagnetic secular variation sv is defined from samples at 67 sites on lava flows of known age paleomagnetic directions range through 40 of inclination and 30 of declination angular dispersion within sites is commonly dating precision is limited by dispersion of 4 5 among sites of apparently similar age the main dispersion sources are in 14 c dates 3 0 intra flow deformations 2 0 and local magnetic anomalies 1 5 results from 68 sites on undated flows show that 95 of kilauea9s surface is younger than 1 000 yr a hiatus in volcano growth 1 000 1 500 yr ago coincides with the filling of a large caldera averaging of available data yields an sv reference curve which is fairly reliable to 1 500 yr ago contains gaps and ambiguities 1 500 3 000 yr ago and remains highly uncertain 3 000 6 000 yr ago the sv dating method can be precise but is limited by the need for a calibrated sv history it can be a powerful correlation tool even if a history is unavailable

drill core lb 08a bosumtwi impact structure ghana petrographic and shock metamorphic studies of material from the central uplift during a recent drilling project sponsored by the international continental scientific drilling progam icdp two boreholes lb 07a and lb 08a were drilled into the crater fill of the bosumtwi impact structure and the underlying basement into the deep crater moat and the outer flank of the central uplift respectively the bosumtwi impact structure in ghana west africa which is 10 5 km in diameter and 1 07 myr old is largely filled by lake bosumtwi here we present the lithostratigraphy of drill core lb 08a recovered between 235 6 and 451 33 m depth below lake level as well as the first mineralogical and petrographic observations of samples from this core this drill core consists of approximately 25 m of polymict clast supported lithic breccia intercalated with suevite which overlies fractured brecciated metasediment that displays a large variation in lithology and grain size the lithologies present in the central uplift are metasediments composed dominantly of fine grained to gritty meta graywacke phyllite and slate as well as suevite and polymict lithic impact breccia the suevites principally present between 235 6 and 240 5 m and between 257 6 and 262 2 m display a fine grained fragmental matrix about 39 to 45 vol and a variety of lithic and mineral clasts that include meta graywacke phyllite slate quartzite carbon rich organic shale and calcite as well as melt particles fractured quartz unshocked quartz unshocked feldspar quartz with planar deformation features pdfs diaplectic quartz glass mica epidote sphene and opaque minerals the crater fill suevite contains calcite clasts but no granite clasts in contrast to suevite from outside the northern crater rim the presence of melt particles in suevite samples from the uppermost 25 meters of the core and in suevite dikelets in the basement is an indicator of shock pressures exceeding 45 gpa quartz grains present in suevite and polymict lithic impact breccia abundantly display 1 to rarely 4 sets of pdfs per grain the shock pressures recorded by the pdfs in quartz grains in the polymict impact breccia range from 10 to 30 gpa we also observed a decrease of the abundance of shocked quartz grains in the brecciated basement with increasing depth meta graywacke samples from the basement are heterogeneously shocked with shock pressures locally ranging up to 2530 gpa suevites from this borehole show a lower proportion of melt particles and diaplectic quartz glass than suevites from outside the northern crater rim fallback impact breccia as well as a lack of ballen quartz which is present in the external breccias similar variations of melt particle abundance and shockmetamorphic grade between impact breccia deposits within the crater and fallout impact breccia outside the crater have been observed at the ries impact structure germany

geochemistry of impactites and basement lithologies from icdp borehole lb 07a bosumtwi impact structure ghana in 2004 a drilling project by the international continental scientific drilling program icdp at the bosumtwi impact crater ghana 1 07 myr old and 10 5 km in diameter obtained drill core lb 07a which sampled impactites and underlying metasediments in the crater moat surrounding the small central uplift of the structure the lb 07a core consists of three sequences 82 29 m of an upper impactite sequence of alternating polymict lithic and suevitic impact breccias overlying 54 88 m of so called lower impactite of monomict impact breccia with several suevite intercalations and 74 53 m of meta graywacke and altered shale of the basement also containing a number of suevite intercalations major and trace element characteristics of all three sequences have been determined to investigate breccia formation and the role of the respective basement lithologies therein compositions of polymict impact breccias of the crater fill revealed by core lb 07a are compared with the compositions of the ivory coast tektites and the fallout suevites the impactites of the lb 07a borehole appear well homogenized with respect to the silicate component and little change in the ranges of many major and trace element differences is seen along the length of the borehole except for fe2o3 mgo and cao contents much scatter is observed for a number of elements and in many cases this increases with depth it is proposed that any variability in composition is likely the function of clast population differences i e also of relatively small sample sizes no systematic compositional difference between polymict lithic and suevitic impact breccias is evident an indication of carbonate enrichment due to hydrothermal alteration is observed in samples from all lithologies the impactites of the borehole generally show intermediate compositions to previously defined target rocks the fallout suevites have comparable major element abundances except for relatively lower mgo contents the ivory coast tektites are generally similar in composition to the lb 07a suevites but broader ranges in mgo and cao contents are observed for the lb 07a suevites

chromium isotopic studies of terrestrial impact craters identification of meteoritic components at bosumtwi clearwater east lappajarvi and rochechouart chromium isotopic compositions and elemental abundances in impact melt rock and impact glass samples from four terrestrial impact craters were measured to verify the presence of an extraterrestrial component and to identify the meteorite type of the impactor all meteorite classes have cr isotopic signatures that are different from those of terrestrial rocks thus precise measurements of cr isotopic abundances can unequivocally distinguish terrestrial from extraterrestrial materials for all four studied craters bosumtwi ghana clearwater east canada lappajarvi finland and rochechouart france we found positive 53cr excesses that eliminate carbonaceous chondrite projectiles because those would show apparent negative excesses and enstatite chondrites because of the magnitude of the excess in all four cases ordinary chondrites have been identified as the best fit for the data in the case of lappajarvi interelement correlations together with the cr isotope data make an h chondrite the most likely projectile whereas in the case of clearwater east both l or h chondrites are possible for bosumtwi and rochechouart the high indigenous contents of the siderophile elements and disturbances of the elemental abundances by weathering and hydrothermal alteration respectively do not allow further constraints to be placed on the type of ordinary chondrite involved in the impact

chemical variation within fragments of australasian tektites in previous studies intersample variation between compositions of different tektites from one particular group were studied and in a few cases major element variations within single tektites no data for intra sample trace element variations existed thus we sectioned a muong nong type tektite fragment from vietnam and a splash form tektite fragment from the philippines into eleven and six pieces respectively and analyzed the individual fragments for major and trace element contents the compositions obtained agree well with those found in previous studies supporting argument that tektites have been derived from terrestrial upper crustal sediments chemical variations within the tektite fragments are present but do not show any systematic trends probably reflecting incomplete mixing of parent rocks the intra sample heterogeneity of the muong nong type tektite is more pronounced than that in the philippinite for the muong nong type tektite the intra sample variation in the trace element contents is higher than that for the major elements again reflecting target rock properties for the philippinite the intra sample variations mostly do not exceed the limits imposed by the precision of the analytical data confirming that the splash form tektites are indeed well homogenized

geochemical studies of the subo 18 enkingen drill core and other impact breccias from the ries crater germany suevite and melt breccia compositions in the boreholes enkingen and polsingen are compared with compositions of suevites from other ries boreholes and surface locations and discussed in terms of implications for impact breccia genesis no significant differences in average chemical compositions for the various drill cores or surface samples are noted compositions of suevite and melt breccia from southern and northeastern sectors of the ries crater do not significantly differ this is in stark contrast to the published variations between within crater and out of crater suevites from northern and southern sectors of the bosumtwi impact structure ghana locally occurring alteration overprint on drill cores especially strong on the carbonate impregnated suevite specimens of the enkingen borehole does affect the average compositions overall the composition of the analyzed impact breccias from ries are characterized by very little macroscopically or microscopically recognized sediment clast component the clast populations of suevite and impact melt breccia are dominated consistently by granitic and intermediate granitoid components the polsingen breccia is significantly enriched in a dioritic clast component overall chemical compositions are of intermediate composition as well with dioritic granodioritic silica contents and relatively small contributions from mafic target components selected suevite samples from the enkingen core have elevated ni co cr and ir contents compared with previously analyzed suevites from the ries crater which suggest a small meteoritic component platinum group element pge concentrations for some of the enriched samples indicate somewhat elevated concentrations and near chondritic ratios of the most immobile pge consistent with an extraterrestrial contribution of 0 1 0 2 chondrite equivalent

single crystal u pb zircon age and sr nd isotopic composition of impactites from the bosumtwi impact structure ghana comparison with country rocks and ivory coast tektites the 1 07 myr old bosumtwi impact structure ghana excavated in 2 1 2 2 gyr old supracrustal rocks of the birimian supergroup was drilled in 2004 here we present single crystal u pb zircon ages from a suevite and two meta graywacke samples recovered from the central uplift drill core lb 08a which yield an upper concordia intercept age of ca 2145 82 ma in very good agreement with previous geochronological data for the west african craton rocks in ghana whole rock rb sr and sm nd isotope data of six suevites five from inside the crater and one from outside the northern crater rim three meta graywacke and two phyllite samples from core lb 08a are also presented providing further insights into the timing of the metamorphism and a possibly related isotopic redistribution of the bosumtwi crater rocks our rb sr and sm nd data show also that the suevites are mixtures of meta greywacke and phyllite and possibly a very low amount of granite a comparison of our new isotopic data with literature data for the ivory coast tektites allows to better constrain the parent material of the ivory coast tektites i e distal impactites which is thought to consist of a mixture of metasedimentary rocks and possibly granite but with a higher proportion of phyllite and shale than the suevites i e proximal impactites when plotted in a rb sr isochron diagram the sample data points n 29 including literature data scatter along a regression line whose slope corresponds to an age of 1846 160 ma with an initial sr isotope ratio of 0 703 0 002 however due to the extensive alteration of some of the investigated samples and the lithological diversity of the source material this age which is in close agreement with a possible metamorphic age of 1 8 1 9 ga tentatively derived from our u pb dating of zircons is difficult to consider as a reliable metamorphic age it may perhaps reflect a common ancient source whose rb sr isotope systematics has not basically been reset on the whole rock scale during the bosumtwi impact event or even reflect another unknown geologic event

archaeabacterial lipids in drill core samples from the bosumtwi impact structure ghana meteorite impacts are associated with locally profound effects for microorganisms living at the terrestrial surface and the subsurface of the impact zone the bosumtwi crater in ghana west africa is a relatively young 1 07 myr structure with a rim to rim diameter of about 10 5 km in a preliminary study targeting the subsurface microbial life in the impact structure seven samples of the impact breccia from the central uplift of the bosumtwi crater were analyzed for the presence of typical archaeal membrane lipids gdgts these have been detected in four of the samples at a maximum depth of 382 m below the lake surface which is equivalent to 309 m below the surface sediment the concentration of the gdgts does not show a trend with depth and their distribution is dominated by gdgt 0 possible origins of these lipids could be related to the soils or rocks predating the impact event the hydrothermal system generated after the impact or due to more recent underground water transport

history and dynamics of the greater yellowstone glacial system during the last two glaciations abstract the greater yellowstone glacial system gygs covered about 20 000 km2 at its maximum pleistocene extent the initiation culmination and ultimate decay of the gygs involved complex interactions between several coalescent ice masses flowing from glacial source areas adjoining and including the yellowstone plateau here we present an updated review of the history and dynamics of the gygs during the penultimate bull lake and last pinedale glaciations drawing upon an integration of glacial geologic mapping with 130 cosmogenic 10be and 3he exposure ages bull lake glacial deposits in greater yellowstone are dated to ca 150 140 ka and correlate with marine isotope stage 6 the bull lake glaciation extended well beyond the pinedale along the southern and western gygs margins but pinedale glaciers overrode bull lake ice limits on the north and east sides the northeastward shift of the center of ice mass from bull lake to pinedale time may be explained by uplift on the leading edge of the yellowstone hotspot and subsidence on the trailing margin in early pinedale time 22 18 ka ice buildup culminated in the high terrain of the beartooth uplift and high absaroka range glaciers from these source regions flowed onto the northeastern margin of the yellowstone plateau and advanced to terminal moraines beyond clarks fork canyon and in jackson hole by middle pinedale time 18 16 ka the yellowstone plateau ice cap surface had risen above the equilibrium line altitude stimulating orographic glacial buildup nourished by storms funneled eastward through the snake river plain the plateau ice cap eventually thickened to 1000 m and joined glaciers from the beartooth uplift and gallatin range to form the northern yellowstone outlet glacier terrain east and downwind of the ice cap crest was placed in a precipitation shadow resulting in glacial recession in these regions during the late pinedale 16 13 ka the plateau ice cap prograded southwestward toward the direction of moisture supply leading to advances along the southern and western margins of the gygs northern sectors of the plateau ice cap were nearly stagnant at this time the yellowstone region experienced widespread deglaciation ca 15 14 ka in response to warming climate unloading of the 1 km thick plateau ice cap and consequent release of pressure on the magmatic system beneath yellowstone was not accompanied by volcanism indicating that the magma chamber was not primed to erupt via decompression during the last deglaciation

a dynamical model of lithosphere extension and sedimentary basin formation convection in the upper mantle can give rise to normal stresses on the base of the continental lithosphere that may cause localized elevation of its surface and extensional deviatoric stresses this possibility is investigated assuming homogeneous biaxial strain in a continental lithosphere whose strength is governed by a combination of brittle failure and power law creep in the crust and by brittle power law or dorn law behavior in the mantle for a given geotherm and for rheologies based on current laboratory data the lithosphere responds to convection induced uplift in one of three ways depending primarily on the magnitude of the driving stresses these regimes are as follows 1 when the driving stress exceeds a threshold value the resulting strain rates are high enough greater than about 3 10 16 s 1 that the lithosphere strains almost isothermally its effective rheology is strain thinning and it undergoes accelerating extension the magnitude of the dynamic uplift required to produce the threshold value of the driving stress depends on the geotherm characterized here by the moho temperature and varies from about 1 km for a moho temperature of 700 c to around 3 km for a moho temperature of 550 c an uncertainty of at least 100 c is attached to these temperatures owing to the inaccuracies in extrapolating laboratory data to geological strain rates 2 at stress levels due to less than about half of the critical uplift extensional strain is negligible 3 at intermediate stress levels moderate uplift produces extension that is bounded by strength increases due to cooling of the ductile portion of the lithosphere during extension

seismicity of the rocky mountains and rio grande rift from the earthscope transportable array and crest temporary seismic networks 2008 2010 we developed a catalog of small magnitude ml 0 1 to 4 7 seismicity across colorado and new mexico from the earthscope usarray transportable array and crest colorado rocky mountains experiment and seismic transects seismic networks from 2008 to 2010 to characterize active deformation in the rio grande rift we recorded over 900 earthquakes in the rio grande rift region not including induced earthquakes and mine blasts and find that the rift is actively deforming both broadly and in distinct regions seismic events that are likely induced mostly in the raton basin make up 66 of the catalog 1837 earthquakes neogene faults in the northern rift in north central colorado are seismically active in the north park basin and northwestern colorado the central rift from the san luis basin southern colorado to south of the socorro magma body is the most seismically active rift region and seismicity delineates the deformation in the colorado plateau transition zone which is spatially correlated with volcanic vents dikes and faults within the western jemez lineament the eastern jemez lineament is nearly aseismic and surrounded by a halo of seismicity culminating in boundaries defined by recent moderate mw 3 9 and mw 3 3 earthquakes the southern rift is characterized by diffuse seismicity in texas and mexico this study provides an updated seismic catalog built with uniformity in seismometer coverage and low epicentral uncertainties 2 km that allows for regional evaluation of seismicity during this time period clusters of seismicity and moderate magnitude earthquakes characterize deformation in a low strain rate extensional environment

geochemistry and age of ivory coast tektites and microtektites abstract ivory coast tektites were first reported in 1934 from a geographically restricted area at ivory coast west africa although some additional specimens have been found later the total number remains small a few hundred the bosumtwi impact crater in ghana is most likely the source crater for the ivory coast tektites based on the finding that the tektites and the crater have the same age as well as similar isotopic and chemical compositions in addition to tektites on land microtektites were found in so far eleven deep sea cores off the west african coast between about 9 n and 11 s and 0 and 23 w defining the extent of the ivory coast tektite strewn field in this study we analyzed eleven ivory coast tektites for their major and trace element composition studied their petrographical characteristics provided major element data for 111 microtektites and major and trace element data for four microtektites we determined the 40ar 39ar step heating age of five ivory coast tektites and four microtektites and obtained fission track dates for ten tektites and one bosumtwi impact glass the tektites have very small intersample and intrasample variations of their major and trace element composition 111 ivory coast microtektites from eleven cores were analyzed for their major element compositional range their compositional range is significantly wider than that of the ivory coast tektites but the majority of all microtektites have compositions very similar to those of the tektites within a factor of 1 2 trace element compositions of the tektites also show little variation between samples the samples do not show any distinct eu anomaly in the ree patterns this characteristic as well as the high absolute ree abundances and la nybn ratios of about 8 indicate that archean rocks are plausible source rocks the major and trace element contents of four individually analyzed ivory coast microtektites show compositions that are very similar to those of the ivory coast tektites however the microtektites contain 20 rel higher abundances of some of the lithophile and siderophile trace elements such as sc cr co ni sr zr ba hf ta th and the rees these differences are probably due to incorporation of a higher abundance of accessory trace minerals with the microtektite forming melt the ivory coast microtektites also have a uniform internal composition duplicate 40ar 39ar step heating age analyses were performed on five tektites the best age estimate for the formation age of the tektites was calculated by taking a weighted average of the ages from the plateau portions of the runs resulting in an age of 1 1 0 05 ma we also tried to date four microtektites by 40ar 39ar age analyses but their young age and small sample size makes it impossible to assign a reliable age to the microtektites one run yielded satisfactory results that were similar to the tektite age in addition we determined the fission track ages for ten individual ivory coast tektite samples and for one impact glass sample from the bosumtwi crater the track size corrected ages for the ivory coast tektites ranged from 0 91 to 1 18 ma resulting in an average fission track age of 1 05 0 11 ma this age is within errors identical to that of the bosumtwi impact glass at 1 03 0 11 ma and to the 40ar 39ar age of 1 1 0 05 ma the preferred age of the ivory coast tektite event is 1 07 ma

ignimbrites of the eastern snake river plain evidence for major caldera forming eruptions the eastern snake river plain is a predominantly rhyolitic province analogous to the present day yellowstone plateau volcanic field but older and further evolved the widespread quaternary basaltic lavas that now blanket the eastern snake river plain appear to have erupted after major rhyolitic activity ceased and represent less than 20 of the total volume of volcanic rock near the north central margin of the plain the rhyolites show a general age progression that becomes younger northeastward toward yellowstone three major ignimbrite sheets comparable in magnitude to those of the yellowstone plateau field have been correlated on both sides of the eastern snake river plain from the arco and pocatello areas to margins of the yellowstone plateau these 4 3 to 6 5 ma old ignimbrites are referred to herein as the heise volcanic field studies of facies changes and lateral variations in the three major ignimbrites of the heise field permit identification of related calderas that are now buried beneath the plain petrologic studies suggest that although the ignimbrites overlap compositionaliy each was derived from an isotopicaliy distinct magma that contained a significant component of crustal material

a deep low velocity body under the yellowstone caldera wyoming delineation using teleseismic p wave residuals and tectonic interpretation in previously published papers iyer 1975 iyer and stewart 1977 iyer 1979 evidence was presented for the presence of a large body of anomalous material with compressional velocities considerably lower than normal from the surrounding rock in the crust and upper mantle under yellowstone national park wyoming in this paper we give a detailed analysis of p residual data from the 26 element telemetered seismic array and three groups of portable stations operated by the u s geological survey in the yellowstone region using a variety of analytical techniques we delineate the shape of the low velocity body determine the velocity structure inside the body and interpret the nature of the material that constitutes the body we will also outline some tectonic implications of our model the spectacular surface geothermal phenomena for which yellowstone is famous result from deep seated volcanism the volcanic evolution of yellowstone is discussed by christiansen and blank 1972 and summarized by eaton and others 1975 briefly the geothermal manifestations in yellowstone are results of three cycles of quaternary volcanism each culminating in a devastating pyroclastic eruption and resulting in large collapse calderas and predominantly rhyolitic flows the oldest cycle ended about 1 9 m y ago forming a caldera from island park to perhaps the central part of the present yellowstone caldera the second cycle climaxed about 1 2 m y ago and was confined to the island park caldera the third cycle began soon after the second and is responsible for the 75 by 45 km yellowstone caldera as it is outlined today this caldera is the result of a catastrophic eruption approximately 600 000 yr ago the most recent episode of volcanism probably began about 150 000 yr ago in the western half of the caldera near old faithful geyser and the youngest flows in yellowstone about 70 000 yr old are probably associated with this episode

variation of depth to the brittle ductile transition due to cooling of a midcrustal intrusion the depth to the brittle ductile transition in the crust is often defined by the intersection of a shear resistance relation in the brittle upper crust that increases linearly with depth and a power law relation for ductile flow in the lower crust that depends strongly on temperature transient variation of this depth caused by a magmatic intrusion at a depth near the regional transition can be modeled by a heat conduction model for a rectangular parallelepiped superimposed on a linear geothermal gradient when parameters appropriate for the southeastern united states are used a moderate sized intrusion is found to decrease the transition depth by as much as 7 km significant variations last about 10 m y since the base of the seismogenic zone is identified with the brittle ductile transition and since significant depth variations provide loci for strain energy concentration these results imply that intrusions of late tertiary age or younger could be important sources of clustered seismicity

onset timing and slip history of the teton fault wyoming a multidisciplinary reevaluation the dramatic relief of the teton range in northwestern wyoming results from motion along the teton normal fault new apatite u th he ahe and fission track aft ages of samples collected in the footwall yield fundamental constraints on contrasting models of teton fault activity and consequent relative footwall uplift low elevation samples in the immediate footwall of the fault range from 12 5 to 6 5 ma ahe ages of samples from subvertical transects range from 57 8 to 6 5 ma rendezvous 54 0 to 6 8 ma grand and 20 5 to 7 1 ma moran with all three transects yielding an expected trend of decreasing age with decreasing elevation aft ages obtained from the three transects range from 55 4 to 45 7 ma rendezvous 43 2 to 13 3 ma grand and 38 0 to 11 7 ma moran inverse thermal history modeling indicates that the onset of relatively rapid cooling as a proxy for relative footwall uplift initiated first in the northern part of the range 15 13 ma and then migrated south as fault displacement continued 7 ma to present at rendezvous thus this work suggests that although the teton fault likely initiated due to basin and range extension this structure remains active normal fault displacement models also suggest a minimum of 6 km displacement for footwall uplift of 2 km if evaluated in the context of established normal fault length displacement relationships this yields a length of 180 km substantially longer than current estimates of teton fault length and putting the northern end of the fault well into and potentially beyond the bounds of yellowstone national park

regional uplift associated with continental large igneous provinces the roles of mantle plumes and the lithosphere the timing and duration of surface uplift associated with large igneous provinces provide important constraints on mantle convection processes here we review geological indicators of surface uplift associated with five continent based magmatic provinces emeishan traps 260 million years ago ma siberian traps 251 ma deccan traps 65 ma north atlantic phase 1 61 ma and phase 2 55 ma and yellowstone 16 ma to recent all five magmatic provinces were associated with surface uplift surface uplift can be measured directly from sedimentary indicators of sea level in the north atlantic and from geomorpholocial indicators of relative uplift and tilting in yellowstone in the other provinces surface uplift is inferred from the record of erosion in the deccan north atlantic and emeishan provinces transient uplift that results from variations in thermal structure of the lithosphere and underlying mantle can be distinguished from permanent uplift that results from the extraction and emplacement of magma transient surface uplift is more useful in constraining mantle convection since models of melt generation and emplacement are not required for its interpretation observations of the spatial and temporal relationships between surface uplift rifting and magmatism are also important in constraining models of lip formation onset of surface uplift preceded magmatism in all five of the provinces biostratigraphic constraints on timing of uplift and erosion are best for the north atlantic and emeishan provinces where the time interval between significant uplift and first magmatism is less than 1 million years and 2 5 million years respectively rifting post dates the earliest magmatism in the case of the north atlantic phase 1 and possibly in the case of siberia the relative age of onset of offshore rifting is not well constrained for the deccan and the importance of rifting in controlling magmatism is disputed in the emeishan and yellowstone provinces in these examples rifting is not a requirement for onset of lip magmatism but melting rates are significantly increased when rifting occurs models that attempt to explain emplacement of these five lips without hot mantle supplied by mantle plumes often have difficulties in explaining the observations of surface uplift rifting and magmatism for example small scale convection related to craton or rift boundaries edge driven convection cannot easily explain widespread 1000 km scale transient surface uplift emeishan deccan north atlantic and upper mantle convection initiated by differential incubation beneath cratons the hotcell

hydrothermal alteration in the bosumtwi impact structure evidence from 2m1 muscovite alteration veins and fracture fillings drill core samples from the bosumtwi impact structure 1 07 myr old and 10 5 km in diameter in ghana exhibit mineralogical evidence for post impact hydrothermal alteration nine samples of drill core obtained through the 2004 international continental scientific drilling project icdp were studied including an uppermost fallback layer overlying impactite breccias and partly deformed massive meta graywacke bedrock the petrographic study revealed alteration veins containing secondary sericitic muscovite comparable to 2m1 muscovite crosscutting original bedding in meta graywacke and forming a matrix between clasts in impactite breccias x ray diffraction xrd shows that these impactite samples are rich in 2m1 muscovite consistent with post impact fluid deposition and alteration optical analysis indicates the presence of a pre impact stratiform chlorite in meta graywacke samples and a secondary alteration chlorite occurring in all samples secondary illite was detected in upper impactites of drill core lb 08a and samples containing accretionary lapilli the lower temperature constraint for the hydrothermal event is given by 2m1 muscovite secondary chlorite and illite all of which form at temperatures greater than 280 c an absence of recrystallization of quartz and feldspar indicates an upper temperature constraint below 900 c the presence of alteration materials associated with fractures and veins in the uppermost impactites of drill cores lb 07a and lb 08a indicates that a post impact hydrothermal system was present in and adjacent to the central uplift portion of the bosumtwi impact structure a sample containing accretionary lapilli obtained from drill core lb 05a exhibits limited evidence that hydrothermal processes were more widespread within the impactites on the crater floor

a 12 000 year record of vertical deformation across the yellowstone caldera margin the shorelines of yellowstone lake the 600 ka yellowstone caldera exhibits several signs of unrest the most evident of which is historic ground deformation including both uplift and subsidence we document deformation in the area of the southeastern caldera across 12 000 years using the postglacial shoreline terraces of yellowstone lake raised shoreline elevations were interpreted from 230 leveling profiles surveyed across flights of terraces with an accuracy of 0 5 m of about 11 recognizable terraces the five most continuous raised shorelines were correlated around the lake basin to reveal deformation patterns shoreline ages are based on minimum and maximum limiting radiocarbon and obsidian hydration dates each terrace is interpreted as representing an episode of uplift 1 kyr 1 of the caldera interior and subsequent subsidence with little net volume change this cyclic behavior may result from magma emplacement and subsequent withdrawal or cooling and crystallization and or episodic trapping and release of magmatic fluids evolved during cooling and crystallization in a self sealing reservoir as hypothesized by dzurisin et al 1990 early postglacial shoreline deformation reveals substantial intracaldera subsidence possibly reflecting cooling and loss of trapped hydrothermal fluids extension along north south trending normal faults and related structures is also apparent in early to middle postglacial deformation both inside and outside of the caldera margin including downwarping and local faulting of hot weak intracaldera crust net deformation over the past 3 kyr has been dominantly up within the caldera interior and slightly down along the caldera rim relative to the extracaldera region this uplift is roughly similar to the historic pattern and may largely represent the effects of the most recent inflation episode subtraction of the total estimated magnitude of inflation in this episode suggests that the overall trend of postglacial deformation has been subsidence the cause of this trend is undetermined but is most likely related to the effects of regional extension and long term cooling within the yellowstone caldera

the 1978 yellowstone eastern snake river plain seismic profiling experiment data and upper crustal structure of the yellowstone region eleven in line refraction profiles recorded to distances of 300 km and one azimuthal fan plot were constructed from data recorded with a 150 station array in the yellowstone national park area during the 1978 yellowstone snake river plain seismic experiment interpretations of the data suggest that the crustal p wave velocity model for the yellowstone region is characterized by 1 an averaged 10 km thick upper crustal layer v sub p 6 0 km s 2 an average crustal velocity of 6 3 km s and 3 a total crustal thickness of 44 km velocity models are presented for profiles that emphasize the upper crust and show 1 a decrease in the depth to the top of the upper crustal crystalline basement from 5 km in southwestern yellowstone near island park to 1 km at the northeast side of the yellowstone plateau that is interpreted as a progressive thinning of the silicic surface volcanic layer to the northeast and 2 evidence for a large lateral inhomogeneity interpreted to be a low velocity body with a decrease of at least 10 in p wave velocity located beneath the northeast corner of the yellowstone plateau the low velocity zone coincides with a local 30 mgal residual gravity anomaly and is located beneath partmore of the sour creek resurgent dome and part of the hot springs basin the largest hydrothermal system in yellowstone the low velocity body has a maximum depth to the top of 3 km and a minimum depth to the bottom of 9 km and may represent a zone of partial melt in comparison to the thermally undisturbed upper crust of the surrounding rocky mountains the upper crust of the northeastern yellowstone plateau appears laterally inhomogeneous in velocity and layer thickness suggesting effects of thermal and magma intrusion whereas the lower crust appears relatively homogeneous less

the basin and range province origin and tectonic significance the basin and range province is a vast arid tract of regionally corrugated angular topography of high relief in the western cordillera it is characterized by evenly spaced parallel mountain ranges and intervening desert basins fig ure la the range flanks are marked by poorly sorted gravel aprons that slope smoothly basinward interrupted here and there by low fault scarps that paral lel the range front faults and by alluvial fans at the mouths of canyons draining the ranges thermal springs located at or near range bounding faults attest to vigorous hydrothermal circulation within zones of fracture porosity created and maintained by faulting in the southern part of the province especially in southeastern california and southwestern arizona range fronts have been worn back by erosion leaving a thin veneer of gravel on an erosion cut bedrock surface that slopes gently outward the range bounding faults of these mountain blocks are buried at the outer edge of such pediments often at considerble distances from the erosional remnants of the ranges themselves the american physiographer n m fenneman 1928 1931 named the basin and range province and defined its general boundaries as thus circum scribed the province includes some 800 000 km2 of area in eight western states later students pardee 1950 lawrence 1976 reynolds 1979 eaton 1979b have observed that many of the fundamental geological and geo physical characteristics of the province are found well beyond the boundaries drawn by fenneman which were based on physiography alone as a tec tonophysical entity its areal extent is greater than 1 million km2 more than 10 of the area of the united states figure 1 fenneman 1931 subdivided the province into five physiographic sections the largest of which is the great basin see figure 1b it is not as its name implies a single regional depression with a common topographic center but is characterized instead by isolated networks of interior drainage divisible into

bosumtwi impact structure ghana geochemistry of impactites and target rocks and search for a meteoritic component major and trace element data including platinum group element abundances of representative impactites and target rocks from the crater rim and environs of the bosumtwi impact structure ghana have been investigated for the possible presence of a meteoritic component in impact related rocks a comparison of chemical data for bosumtwi target rocks and impactites with those for ivory coast tektites and microtektites supports the interpretation that the bosumtwi structure and ivory coast tektites formed during the same impact event high siderophile element contents compared to average upper crustal abundances were determined for target rocks as well as for impactites chondrite normalized and iron meteorite normalized abundances for target rocks and impactites are similar they do not however allow the unambiguous detection of the presence or identification of the type of a meteoritic component in the impactites the indigenous siderophile element contents are high and possibly related to regional gold mineralization although mineralized samples from the general region show somewhat different platinum group element abundance patterns compared to the rocks at bosumtwi the present data underline the necessity of extensive target rock analyses at bosumtwi and at impact structures in general before making any conclusions regarding the presence of a meteoritic component in impactites

stratigraphy paleomagnetism and magnetic fabric of the toba tuffs constraints on the sources and eruptive styles the toba depression in north central sumatra is a complex of several overlapping calderas resulting from three major ignimbrite forming eruptions within the depression the upland masses of samosir and northern uluan consist of welded ignimbrite capped by coarse breccia and lacustrine sediment hitherto interpreted to be two parts of a single resurgent dome this study has demonstrated that the welded tuffs of samosir and uluan have different magnetic polarities and therefore at least two different ignimbrites are present the samosir uluan massif may consist of parts of two resurgent domes the first ignimbrite eruption occurred at 0 84 ma and produced a very thick 400 m densely welded unit having a reversed polarity anisotropy of magnetic susceptibility ams flow direction and lithic size data indicate that the source lies in the southern part of the toba depression and the thick deposit of uluan is thought to have ponded in a 40 km wide caldera the second ignimbrite is normally magnetized ams flow direction data indicate two separate source vents one to the north in the haranggaol caldera and another to the south the thick deposit at samosir is thought to have ponded in the southern caldera coarse sediments then accumulated over samosir and northern uluan and were capped by lacustrine deposits a renewed episode of resurgence then uplifted samosir island and possibly the northern part of uluan at approximately 0 075 ma the last and apparently largest ignimbrite eruption occurred from calderas in the north and south parts of the toba depression this ignimbrite is mostly nonwelded and normally magnetized part of the uluan dome was destroyed by collapse of the sibandung caldera and latung graben and concomitant with renewed subsidence of the haranggaol and porsea calderas

10be content in clasts from fallout suevitic breccia in drill cores from the bosumtwi impact crater ghana clues to preimpact target distribution rocks from drill cores lb 07a crater fill and lb 08a central uplift into the bosumtwi impact crater ghana were analyzed for the presence of the cosmogenic radionuclide 10be the aim of the study was to determine the extent to which target rocks of various depths were mixed during the formation of the crater filling breccia and also to detect meteoric water infiltration within the impactite layer 10be abundances above background were found in two out of 24 samples from the lb 07a core and in none of five samples from the lb 08a core after excluding other possible explanations for an elevated 10be signal we conclude that it is most probably due to a preimpact origin of those clasts from target rocks close to the surface our results suggest that in crater breccias were well mixed during the impact cratering process in addition the lack of a 10be signal within the rocks located very close to the lake sediment impactite boundary suggests that infiltration of meteoric water below the postimpact crater floor was limited this may suggest that the infiltration of the meteoric water within the crater takes place not through the aerial pore space but rather through a localized system of fractures

an analytic model for a mantle plume fed by a boundary layer summary an approximate analytical solution for flow in a mantle plume of constant radius viscosity and density contrast is obtained in cylindrical coordinates the differential equations for vertical velocity of the mantle surrounding the plume and for topography are homologous to the equation for flexure of an elastic plate although the model is too simple to be fully applicable to the earth one can conclude that the vertical velocity in the mantle changes significantly away from plumes that the viscosity of the plume is important for controlling flow rate and that the long wavelength geoid anomalies are sensitive to the viscosity of the surrounding mantle the first induced upwelling away from a plume is quite weak and unlikely to control the spacing of plumes

regional structural synthesis wyoming salient of western overthrust belt surface geologic mapping regional and high density reflection seismic data and information from approximately 370 wells have been combined to describe geometrically that area of the western overthrust belt between the snake river plain and the uinta uplift particular care has been taken to verify two dimensional interpretations of multiple thrust sheets by linear restoration by establishing equivalence of pre thrust lengths of affected beds one can gain confidence in interpreted structural geometries as well as generate data in regard to internal shortening and deformation intensity because of irresoluable geometric problems within sections 47 cross sections were developed at roughly 6 mi 10 km spacing to help verify changes in displacement or placement of key st uctural elements displacements of 0 to 32 mi 0 to 51 km have been demonstrated jurassic to paleocene sevier thrusts were active across a previously deformed cratonic shelf terrane and interacted with active structural elements such as the uinta uplift gros ventre range and moxa arch thrusts are progressively younger to the east except for the darby prospect pair in which the darby system is younger

drill core lb 08a bosumtwi impact structure ghana geochemistry of fallback breccia and basement samples from the central uplift the 1 07 myr old bosumtwi impact structure in ghana west africa which measures 10 5 km in diameter and is largely filled by lake bosumtwi is associated with one of four currently known tektite strewn fields two boreholes were drilled to acquire hard rock samples of the deep crater moat and from the flank of the central uplift lb 07a and lb 08a respectively during a recent icdp sponsored drilling project here we present results of major and trace element analysis of 112 samples from drill core lb 08a this core which was recovered between 235 6 and 451 33 m depth below lake level contains polymict lithic breccia intercalated with suevite which overlies fractured brecciated metasediment the basement is dominated by meta graywacke from fine grained to gritty but also includes some phyllite and slate as well as suevite dikelets and a few units of a distinct light greenish gray medium grained meta graywacke most of the variations of the major and trace element abundances in the different lithologies result from the initial compositional variations of the various target rock types as well as from aqueous alteration processes which have undeniably affected the different rocks suevite from core lb 08a fallback suevite and fallout suevite samples from outside the northern crater rim display some differences in major mainly in mgo cao and na2o contents and minor mainly cr and ni element abundances that could be related to the higher degree of alteration of fallback suevites but also result from differences in the clast populations of the two suevite populations for example granite clasts are present in fallout suevite but not in fallback breccia and calcite clasts are present in fallback breccia and not in fallout suevite chondrite normalized rare earth element abundance patterns for polymict impact breccia and basement samples are very similar to each other siderophile element contents in the impact breccias are not significantly different from those of the metasediments or compared to target rocks from outside the crater rim so far no evidence for a meteoritic component has been detected in polymict impact breccias during this study in agreement with previous work

crustal structure of the eastern snake river plain determined from ray trace modeling of seismic refraction data ray trace travel time modeling of the seismic refraction record sections for a profile from near soda springs idaho to near mckay idaho was used to derive a crustal model across the eastern snake river plain esrp the derived crustal model is consistent with the velocity structure interpreted from a profile along the axis of the esrp the interpretation also indicates that significant lateral inhomogeneities exist in the upper crust beneath the esrp when compared with the upper crust beneath the adjoining northern rocky mountain and basin and range provinces the most prominent features of the crustal structure inferred by the ray trace modeling are as follows 1 the northwest margin of the esrp was modeled as a fault structure downthrown on the srp side se with an offset of greater than 4 km the southeast margin conversely appears to be downwarped with possible minor faulting paleozoic sedimentary rocks are present beneath the cenozoic volcanics of the esrp possibly to and beyond 40 km from the southeastern margin of the esrp near blackfoot idaho 2 the modeling indicates no abrupt variation in the depth to the top of the lower crustal layer near the margins of the esrp 3 an intermediate 6 5 km s layer occurring beneath the esrp interpreted from a refraction profile located along the axis of the esrp was found to be localized within the esrp margins this layer is interpreted as a pervasive intrusion of higher velocity material from the upper mantle into the highly fractured upper crustal layer in this region 4 a density model of the crust across the esrp was prepared with the densities selected using the interpreted seismic velocities as a constraint the gravity field calculated from this model resulted in a good match to observed gravity data over the eastern snake river plain

platinum group elements provide no indication of a meteoritic component in icdp cores from the bosumtwi crater ghana in an attempt to identify the type of projectile 14 samples from the bosumtwi crater in ghana were analyzed for platinum group element pge concentrations by nickel sulfide fire assay inductively coupled plasma mass spectrometry icp ms the majority of the samples come from the impactite material recovered by cores lb 07a and lb 08a which were drilled by the international continental scientific drilling program icdp one sample originates from the fallback material found at the contact between the impactite and the overlying lake sediment in core lb 05b no clear signature of a meteoritic contamination was identified in the 13 impactite samples the target rock apparently dominates the pge contribution in the impactites these results agree with the pge concentrations reported for the suevites collected at the crater rim and in other parts of the bosumtwi icdp cores however based on cr and os isotopic signatures a meteoritic component could be present in the sample of fallback material supporting the reports of the existence of meteoritic material in the ivory coast tektites further analyses of the fallback material from the bosumtwi drill cores should confirm or not this first result

crustal structure of mauna loa volcano hawaii from seismic refraction and gravity data abstract in october 1978 the u s geological survey established a 100 km long seismic refraction profile normal to the kona coast of hawaii island analysis of these data along with available gravity data suggest that the oceanic crust dips about 3 landward under the submerged flank of the island increasing to 8 5 under the kona coast maximum vertical deflection of the base of crust from beneath the deep ocean to a point beneath the summit of mauna loa is about 9 km high velocity high density rocks vp about 7 1 km sec density about 2 9 gm cm 3 comprise the bulk of the volcanic edifice and reach to within a few kilometers of the surface beneath the north flank of mauna loa the data also suggest that an elongate high velocity high density body lies parallel to the kona coast just below the surface this body probably represents an extinct buried rift zone

effects of hydrothermal alteration on the magnetization of the oligocene carpenter ridge tuff bachelor caldera san juan mountains colorado intracaldera oligocene carpenter ridge tuff fills the bachelor caldera in the central san juan caldera complex and hosts mineral deposits of the creede mineral district the carpenter ridge tuff and unaltered portions of its intracaldera bachelor mountain member have strong high coercivity reverse magnetizations with average magnetic susceptibility ms and natural remanent magnetization nrm of 6 10 3 volume si and 8 a m respectively oxide phenocrysts in these rocks are titanomagnetite and ilmenite however magnetization appears to be controlled by microcrystic titanomaghemite based on thermal demagnetization unblocking temperatures and curie temperatures between 580 c and 620 c much of the intracaldera tuff was affected by potassic metasomatism a type of hydrothermal alteration characterized by addition of k and loss of ca and na between 27 3 ma and 25 1 ma potassic metasomatism resulted in the replacement of original feldspars by potassium feldspar and quartz oxidation of the original oxide phenocrysts to hematite and rutile and consequent suppression of ms and nrm by a factor of 5 also present however are metasomatized rocks that have high magnetizations even though their original oxide phenocrysts were destroyed values of ms and nrm are similar to those of outflow tuff such rocks are suspected of containing secondary magnetite on the basis of 1 curie and thermal unblocking temperatures at and below 580 c indicating that magnetization is carried by magnetite but that primary microcrysts of maghemite were destroyed and 2 observation in one sample of magnetite rimming cores of hematite rutile the presence of secondary magnetite in metasomatized rocks requires a local shift to more reducing fluid chemistry although we are unable to determine whether this shift occurred during potassic metasomatism or during later alteration or ore deposition these altered rocks may record a change from a pervasive alteration by alkaline oxidizing fluids to a vein controlled alteration dominated by more reduced fluids in which magnetite was stable

the lake bosumtwi impact crater ghana the 1 m y old bosumtwi crater ghana has a nearly circular shape with a rim diameter of 11 km north south and 10 km east west it is surrounded by a circular depression and an outer ridge of diameter 20 km polymict breccias averaging at least 20 m thick with clasts as much as 5 m long occur on the outer ridge and the crater rim shows in situ shattered rock patches of suevite have been found in the circular depression north and south of the crater analogy with better known craters suggests that bosumtwi has a central uplift rising to 200 m beneath the lake floor an aeromagnetic anomaly of amplitude 50 nanotesla nt over the northern half of the lake is interpreted as due to a layer of magnetized fallback breccia beneath the lake sediments the normal polarity of the breccia shows that the crater was formed during the normal jaramillo event of 0 97 to 0 85 m y ago which agrees with the magnetic stratigraphy of the related ivory coast microtektites a regional gravity survey indicates a negative bouguer anomaly over the crater there is some geochemical evidence that the meteorite was an iron and its mass and energy are suggested as about 108 tons and 3 1019 joules or 7 3 103 megatons

roter kamm impact crater namibia geochemistry of basement rocks and breccias abstract the roter kamm crater in the southern namib desert has previously been identified as an impact structure on the basis of crater morphology and the presence of impact melt breccias which contain shock metamorphosed quartz and lithic clasts to better define the variety of target rocks and breccias we studied the petrography and chemical composition of a new suite of twenty eight basement and breccia samples from the roter kamm crater based on chemical data for target lithologies and breccias we suggest that the crater was formed in a two layer target region an upper layer of gariep metasediments schist marble quartzite and sandstone overlying the crystalline basement of the namaqualand metamorphic complex the basement was also heavily intruded by coarse grained quartz veins and quartz and quartz feldspar pegmatites the clast population in the melt breccias indicates that impact induced melting involved mainly metasedimentary target rocks with rarely detected contributions from pegmatite and granite granodiorite three varieties of melt breccias can be defined 1 schistose 2 quartzitic melt breccias 3 true impact melt breccias these melt breccia types are chemically heterogeneous and even the impact melt breccias may have been produced in situ and not from a coherent melt body the shapes of the schistose melt breccias previously thought to be ejected impact breccias are most likely caused by erosion and these breccias are now interpreted to be locally derived the crater basement as exposed at the rim was structurally severely affected and at least locally considerable thermal energy was generated during formation of large volumes of cataclastic mylonitic and pseudotachylitic breccias analyses of mylonite and pseudotachylites from the crater rim as well as their respective host rocks show that these breccias were mainly formed from local material analyses of pseudotachylite like breccias indicate that these possible friction melts are generated by preferential melting of hydrous ferromagnesian minerals and feldspar similar to their tectonically produced counterparts although no significant fluid effects resulting from formation of mylonites or pseudotachylites are indicated several breccias compared to their host rocks do show evidence of severe chemical alteration chloritisation and sericitisation the presence of large vesicles filled with hydrothermal mineral assemblages in some schistose breccias and other petrographic and chemical data support the hypothesis of an impact induced hydrothermal event in the crater area

the icdp lake bosumtwi impact crater scientific drilling project ghana core lb 08a litho log related ejecta and shock recovery experiments the 1 07 myr old lake bosumtwi impact crater in ghana was drilled within the framework of the international continental scientific drilling project icdp hole lb 08a drilled into the outer flank of the central uplift and with a total depth of 451 m yielded 215 71 m of impact related rocks this paper summarizes observations of the lithological logging on core lb 08a between a depth of 235 6 and 260 m the section consists of a melt bearing allochthonous polymict and mostly clast supported impact breccia down to 418 m the section comprises a rather uniform unit of metagraywacke alternating with phyllite to slate lower greenschist facies few par autochthonous impact breccia bodies and rare impact dike breccias are present the lowermost part of the section contains several centimeter to decimeter thick melt bearing breccia dikes in country rocks identical to those occurring above omnipresent fracturing was mapped in a qualitative manner most prominent shock effects in the uplifted target rocks comprise planar fractures and deformation elements in quartz and polysynthetic twinning in carbonate minerals the maximum shock pressure as evidenced by quartz is below 26 gpa the allochthonous breccias occasionally contain a few vol of melt particles suevites occur outside the crater rim carrying diaplectic crystals coesite and ballen quartz as well as true melt glasses and a variety of lithic clasts among those spectacular staurolite rich mica schists the recorded shock level in the uplifted target rocks is lower than expected and modeled shock recovery experiments with analogue carbonaceous graywackes at 34 and 39 5 gpa yielded nearly complete transformation of quartz into diaplectic glass we therefore exclude a specific shock behavior of the soft fluid rich target material carbonaceous graywackes shales slates in core lb 08a as the prime or only reason for the melt deficit and the generally low shock levels recorded inside the lake bosumtwi impact crater

pronounced central uplift identified in the bosumtwi impact structure ghana using multichannel seismic reflection data the lake bosumtwi impact structure is the youngest and best preserved complex terrestrial impact crater and serves as an important reference site for the study of cratering processes because the impacting body struck continental crystalline target rocks and not a submerged sedimentary platform no significant backwash processes have modified the crater morphology not only may bosumtwi contain the best preserved central uplift structure on earth but it is the most accessible relatively large young crater in the solar system generated in a large gravity field there is a well established link between the lake bosumtwi impact structure and the ivory coast tektite field and the lacustrine sediments within the crater contain a unique 1 m y record of paleoclimate in the continental tropics south of the sahel eight profiles of marine type multichannel seismic reflection mcs data were acquired from the 8 km diameter similar to75 m deep lake that fills much of the crater these were augmented by wide angle seismic data acquired with ocean bottom hydrophones mcs data reveal a well defined central uplift near the northwest central part of the lake and a maximum postimpact lacustrine sediment thickness of similar to310 m the central uplift structure has a diameter of 1 9 km and a maximum height of 130 m above the annular moat inside the crater an intermediate velocity layer 3200 m s beneath the lacustrine sediment is interpreted as fallback breccia or a breccia melt horizon the measured apparent depth of the crater d a is 500 m implying a slightly higher aspect ratio for the structure than predicted from published empirical relationships the bosumtwi structure is a small complex crater that deviates slightly from trends predicted from classical scaling laws perhaps because of the effects of a large gravity field

a fixed sublithospheric source for the late neogene track of the yellowstone hotspot implications of the heise and picabo volcanic fields the heise and picabo volcanic fields of eastern idaho are part of the more extensive time transgressive yellowstone snake river plain hotspot track calderas associated with these two silicic volcanic fields are buried under 1 to 3 km of younger basalt so their locations and eruption record histories have been based on analysis of silicic units along the margins of the eastern snake river plain along with some limited geophysical data a 1 5 km borehole penetrating through basalt into underlying silicic rocks provides new data we used to reassess caldera locations and the timing of eruptions of these volcanic fields using these new caldera locations we calculate an extension adjusted rate of 2 35 cm yr for the north american plate over the last 6 66 m y and a velocity of 2 30 cm yr over the 10 27 m y recalculation of a previously determined plate velocity based migration of the deformation field surrounding the eastern snake river plain yields an extension adjusted rate of 2 38 0 21 cm yr these migration rates all fall within the previously published range of north american plate velocities of 2 2 0 8 cm yr 2 4 cm yr and 2 68 0 78 cm yr based on a global hot spot reference frame the consistency of these rates suggest that over the last 10 m y the yellowstone hot spot is fixed with respect to the motion of the north american plate and therefore consistent with a classical deep sourced hotspot model

diachroneity of basin and range extension and yellowstone hotspot volcanism in northwestern nevada some of the earliest volcanic rocks attributed to the yellowstone hotspot erupted from the mcdermitt caldera and related volcanic centers in northwestern nevada at 17 15 ma at that time extensional faulting was ongoing to the south in central nevada leading some to suggest that the nascent hotspot caused or facilitated middle miocene basin and range extension regional geologic relationships indicate that the total magnitude of extension in northwestern nevada is low compared to the amount documented in central nevada and that extension was accommodated almost entirely by the widely spaced high angle normal fault systems that formed the modern ranges the santa rosa range is the closest major range to the mcdermitt caldera it was tilted 15 east and uplifted as an intact crustal block by 8 km of slip along a west dipping normal fault on the west side of the range apatite fission track data from the santa rosa footwall block indicate that faulting and extension was ongoing ca 7 5 ma and began as early as 10 ma data from the pine forest range to the west indicate a similar 8 10 ma age for extensional faulting basin and range extension in northwestern nevada is therefore significantly younger than 17 15 ma hotspot volcanism this timing argues against a direct link between the yellowstone hotspot and the initiation of extension casting additional doubt on the role of the hotspot in extension across the broader basin and range province

a paleomagnetic study of the structural deformation in the yerington district nevada 2 mesozoic basement units and their total and pre oligocene tectonism

petrographic studies of fallout suevite from outside the bosumtwi impact structure ghana field studies and a shallow drilling program carried out in 1999 provided information about the thickness and distribution of suevite to the north of the bosumtwi crater rim suevite occurrence there is known from an 1 5 km 2 area its thickness is 15 m the present suevite distribution is likely the result of differential erosion and does not reflect the initial areal extent of continuous bosumtwi ejecta deposits here we discuss the petrographic characteristics of drill core samples of melt rich suevite macroscopic constituents of the suevites are melt bodies and crystalline and metasedimentary rock granite graywacke phyllite shale schist and possibly slate clasts up to about 40 cm in size shock metamorphic effects in the clasts include multiple sets of planar deformation features pdfs diaplectic quartz and feldspar glasses lechatelierite and ballen quartz besides biotite with kink bands basement rock clasts in the suevite represent all stages of shock metamorphism ranging from samples without shock effects to completely shock melted material that is indicative of shock pressures up to 60 gpa

uppermost impact fallback layer in the bosumtwi crater ghana mineralogy geochemistry and comparison with ivory coast tektites in 2004 an international continental scientific drilling program icdp drilling project at the bosumtwi impact crater ghana 10 5 km in diameter 1 07 myr old was performed to study the sediments that fill the lake as well as the underlying impactites in one lb 05 of 16 cores drilled into the lake sediments the zone between the impact breccias and the post impact sediments was penetrated preserving the final fine grained impact fallback layer this 30 cm thick layer contains in the top 10 cm accretionary lapilli microtektite like glass spherules and shocked quartz grains glass particlesmostly of splash form less than 1 mm size make up the bulk of the grains 70 78 by number in the coarser size fraction 125 m of the top of the fallback layer about one third of all quartz grains in the uppermost part of the layer are shocked with planar deformation features pdfs almost half of these grains are highly shocked with 3 or more sets of pdfs k feldspar grains also occur and some show shock deformation the abundance of shocked quartz grains and the average shock level as indicated by the number of sets of pdfs for both quartz and k feldspar decrease with depth into the layer the well preserved glass spherules and fragments are chemically rather homogeneous within each particle and also show relatively small variations between the various particles on average the composition of the fallback spherules from core lb 5b is very similar to the composition of ivory coast tektites and microtektites with the exception of cao contents which are about 1 5 to 2 times higher in the fallback spherules this is a rare case in which the uppermost fallback layer and the transition to the post impact sediments has been preserved in an impact structure its presence indicates that the impactite sequence at bosumtwi is complete and that bosumtwi is a very well preserved impact crater

deformation from 1973 to 1987 in the epicentral area of the 1959 hebgen lake montana earthquake ms 7 5 a 40 km aperture trilateration network centered on the 1959 hebgen lake earthquake epicenter has been surveyed in 1973 1974 1976 1978 1981 1984 and 1987 the deformation inferred from those surveys is described roughly by a uniaxial 0 266 0 014 strain yr n15 e 1 extension that is uniform in both time and space that extension is orthogonal to the strike n78 w 5 of the 1959 rupture plane inferred from focal mechanism solutions the absence of strain accumulation in the n75 w direction suggests that the source of deformation must be greatly elongated in that direction looked at in greater detail the deformation is found to be concentrated in the northern part of the network the zone of concentrated deformation coincides with a part of a well defined trend in seismicity that extends at least 100 km n75 w from the sour creek resurgent dome in the yellowstone caldera extension perpendicular to that trend is indicated by focal mechanism solutions thus the zone of rifting identified within the hebgen lake network apparently extends east southeast to the yellowstone caldera about 8 mm yr extension is observed across a 20 km width within the zone

petrology and geochemistry of target rocks from the bosumtwi impact structure ghana and comparison with ivory coast tektites the 10 5 km diameter bosumtwi crater in ghana west africa is the most likely source crater for the ivory coast tektites as the tektites and the crater have the same age 1 07 ma and there are close similarities between the isotopic and chemical compositions of the tektites and crater rocks the crater is excavated in 2 1 2 2 ga old metasediments and metavolcanics of the birimian supergroup here we present the first integrated petrographic and geochemical study of rocks from the bosumtwi impact crater a variety of target rocks from the bosumtwi impact structure were selected to represent the major rock types that have been described before resulting in four groups shale phyllite graywacke and two different types of granites from dispersed dikes and from the so called pepiakese intrusion at the northeastern side of the crater these rocks were analyzed for their major and trace element composition and their petrographic characteristics in addition representative samples were also analyzed for their o sr and nd isotopic compositions the target rocks do not show any unambiguous evidence of shock metamorphism i e planar deformation features pdfs distinct impact characteristic shock effects pdfs were identified only in clasts within suevite derived melt fragments the compositional range of the target rocks is significantly wider than that of the ivory coast tektites but overlap the tektite compositions a best fit line for the bosumtwi crater rocks in a rb sr isotope evolution diagram yields an age of 1 98 ga and an initial 87sr 86sr ratio of 0 701 which is close to results previously obtained for granitoid intrusions in the birimian of ghana our nd isotopic data yield depleted mantle model ages ranging from 2 16 to 2 64 ga and nd values of 17 2 to 25 9 harmonic least squares hmx mixing calculations were able to reproduce the composition of ivory coast tektites from a mixture of bosumtwi country rocks that include about 70 phyllite graywacke 16 granite dike and 14 pepiakese granite the oxygen isotopic composition of the metasedimentary rocks and granite dikes 18o 11 3 13 6 and the tektites 18o 11 7 12 9 also this work agree fairly well while the pepiakese granites have lower values 18o 8 6 9 0 indicating that only minor amounts of these rocks were incorporated in the formation of the ivory coast tektites the large variation in sr and nd isotopic compositions of the target rocks do not allow the unambiguous identification of distinct endmember compositions but in both a 87sr 86sr vs 1 sr plot and an sr vs nd diagram the tektites plot within the area occupied by the metasedimentary and granitic bosumtwi crater rocks thus our data support the interpretation that the composition of the ivory coast tektites is similar to that of rocks exposed at the bosumtwi impact structure indicating formation during the same impact event

chemical analyses of bosumtwi crater target rocks compared with the ivory coast tektites abstract the bosumtwi crater ghana was excavated in phyllites and greywackes with subordinate microgranite dykes and quartz veins of the 2000 ma old lower birimian system with a granodiorite intrusion at pepiakese on its northeastern side new major and trace element analyses are presented for 7 phyllites 5 greywackes 2 microgranites 3 pepiakese intrusion rocks and 1 suevite using xrf and inna means and standard deviations were calculated using all available modern analyses for each element in the bosumtwi target rocks bosumtwi suevite glasses and ivory coast tektites good agreements between the means were found for the three groups with the suevite glasses and tektites having more limited compositional ranges than the target rocks least squares mixing between target rock types shows that the best fits to the tektite and suevite glass compositions require components of about a third or a quarter from the pepiakese intrusion and some extra silica derived from quartz veins as well as the metasediments the new data provide evidence for vapour phase fractionation of p2o5 and na2o in the tektites in addition to the previously reported pb and rb evidence for a meteoritic component in the tektites was found to be equivocal since the target rocks are probably a sufficient source of the meteorite indicator elements ni and ir

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

thermoelastic stress in oceanic lithosphere due to hotspot reheating we investigate the effect of hotspot reheating on the intraplate stress field by modeling the three dimensional thermal stress field produced by nonuniform temperature changes in an elastic plate temperature perturbations are calculated assuming that the lithosphere is heated by a source in the lower part of the thermal lithosphere a thermal stress model for the elastic lithosphere is calculated by superposing the stress fields resulting from temperature changes in small individual elements the stress in an elastic plate resulting from a temperature change in each small element is expressed as an infinite series wherein each term is a source or an image modified from a closed form half space solution we apply the thermal stress solution to midplate swells in oceanic lithosphere with various thermal structures and plate velocities our results predict a stress field with a maximum deviatoric stress on the order of 100 mpa 1 kbar covering a broad area around the hotspot plume the predicted principal stress orientations show a complicated geographical pattern with horizontal extension perpendicular to the hotspot track at shallow depths and compression along the track near the bottom of the elastic lithosphere although stress data near oceanic swells are limited the source parameters of intraplate earthquakes near several hotspots are consistent with the thermal stress model these results indicate that thermal stress due to reheating may be an important contributor to stress fields near hotspots in old oceanic lithosphere

paleomagnetic directional dispersion produced by plastic deformation in a thick miocene welded tuff southern nevada implications for welding temperatures two voluminous rhyolitic to quartz latitic ash flow sheets the tiva canyon and topopah spring members of the paintbrush tuff were erupted from the miocene claim canyon cauldron in southern nevada although lithologically similar these units differ greatly in their recording of the ancient geomagnetic field the reversely magnetized tiva canyon member yields remanent directions that are well grouped both within and between sites the normally magnetized topopah spring member however yields remanent directions that vary both laterally and vertically remanent directions from a 100 m thick section of nonwelded to partially welded topopah spring are well grouped precision parameter k 200 and give a mean direction d 293 i 69 that is probably a good approximation of the geomagnetic field at the time of emplacement and cooling in three 300 m thick sections of more densely welded tuff inclinations from the upper parts of the topopah spring are steep 70 but flatten with depth to about 20 and then steepen abruptly near the base of the unit in general the directional dispersion increases with the thickness of the section k 30 for sections that are about 300 m thick much of the directional variation in the topopah spring can be attributed to an inclination error produced by subblocking temperature rotation of the magnetic carriers during welding the possibility of subblocking temperature deformation in an ash flow sheet is enhanced by 1 high blocking temperature spectra of the primary magnetic phases 2 relatively low emplacement temperatures and 3 great stratigraphic thickness thermal demagnetization results from the topopah spring member indicate that welding may have continued to temperatures as low as 475 c in the basal vitrophyre of the ash flow sheet remanent directions from the tiva canyon member were unaffected by the welding process because this unit possesses significantly lower blocking temperatures and does not attain the great thicknesses of the topopah spring

investigating the lake bosumtwi impact structure insight from numerical modeling 1 the current topography of the lake bosumtwi crater and some of its structural dimensions have been determined by geophysical methods we combine these data with sophisticated numerical models to evaluate the cratering process itself for example melt and tektite generation as well as to test the modeling code the geophysical maps show some asymmetry in plan view with the main anomaly north of the crater center the simulations of the early stage show asymmetric patterns only in ejecta and tektite distributions while the late stage is modeled for the vertical impact without any asymmetry we estimate the projectile size from scaling laws and then varying material properties reproduce a crater which is similar to the bosumtwi but too deep bulking allows us to reconcile differences between the model results and the observed topography shock melt estimates are in good agreement with the bosumtwi magnetic signature modeled distribution of tektites assumes an impact angle of 30 45 and an impact direction from the n ne the combination of numerical models and field evidence not only provides necessary information for upcoming scientific drilling of the structure but also suggests interesting and well suited drill sites besides the central uplift and the annular moat with a suggested thick breccia cover drilling at the location of the geophysical anomalies and comparison of downrange and transversal locations will provide new insight into preimpact and impact induced asymmetries

estimates of flow direction for calc alkaline welded tuffs and paleomagnetic data reliability from anisotropy of magnetic susceptibility measurements central san juan mountains southwest colorado abstract flow directions are estimated from the measurement of the magnetic fabric of 106 samples collected at 18 sites in four welded tuff units in the central san juan mountains of southern colorado the estimates assume that the tuffs generally flowed directly away from the extrusive vents and that the lineations of magnetic grains within the tuffs represent the flow direction at individual sites errors in the estimation may arise from topographic variation rheomorphism post emplacement mass flow within the tuff and other factors magnetic lineation is defined as the site mean anisotropy of magnetic susceptibility maximum azimuth a test on the flow directions for individual units is based on the projection of lineation azimuths and their intersection within or near the known source caldera for the tuff this test is positive for the four units examined paleomagnetic results for these tuffs are probably reliable indicators of the geomagnetic field direction in southwest colorado during the time 28 2 26 5 ma of emplacement

paleomagnetic results from the snake river plain contribution to the time averaged field global database 1 this study presents paleomagnetic results from the snake river plain srp in southern idaho as a contribution to the time averaged field global database paleomagnetic samples were measured from 26 sites 23 of which 13 normal 10 reverse yielded site mean directions meeting our criteria for acceptable paleomagnetic data flow ages on 21 sites range from 5 ka to 5 6 ma on the basis of 40ar 39ar dating methods the age and polarity for the 21 dated sites are consistent with the geomagnetic reversal time scale except for a single reversely magnetized site dated at 0 39 ma this is apparently the first documented excursion associated with a period of low paleointensity detected in both sedimentary and igneous records combining the new data from the srp with data published from the northwest united states between the latitudes of 40 and 50 n there are 183 sites in all that meet minimum acceptability criteria for legacy and new data the overall mean direction of 173 normally magnetized sites has a declination of 2 3 inclination of 61 4 a fisher concentration parameter of 58 and a radius of 95 confidence 95 of 1 4 reverse sites have a mean direction of 182 4 declination 58 6 inclination of 50 and 95 of 6 9 normal and reversed mean directions are antipodal and indistinguishable from a geocentric axial dipole field at the 95 confidence level virtual geomagnetic pole dispersion was found to be circularly symmetric while the directional data were elongate north south an updated and corrected database for the northwestern u s region has been contributed to the magnetics information consortium magic database at http earthref org

a model of rift basin evolution constrained by first order stratigraphic observations the stratigraphy of many nonmarine rift basins records a common sedimentological history consisting of a relatively abrupt increase of water depth followed by a slower shoaling upward trend we explore in this paper the origin of these sequences through a three dimensional physical model that incorporates the affects of fault growth flexure erosion sedimentation and isostasy fault growth is assumed to occur in a self similar manner and is treated as dislocations inducing flexure in a thin elastic plate topography generated by faulting and flexure is allowed to undergo diffusion to simulate erosive and sedimentary processes isostatic response to the redistribution of mass results in additional flexure of the thin plate the model is tested using two models of fault growth and we compare the numerical results with the stratigraphy observed in nonmarine rifts basins generated using a constant lengthening rate mode of fault growth fail to reproduce the stratigraphy of continental rift basins we find that the commonly observed stratigraphic succession of lacustrine to fluvial conditions requires the displacement rate of the bounding fault to decrease with time consistent with opening at a relatively constant areal extension rate

upper mantle origin of the yellowstone hotspot fundamental features of the geology and tectonic setting of the northeast propagating yellowstone hotspot are not explained by a simple deep mantle plume hypothesis and within that framework must be attributed to coincidence or be explained by auxiliary hypotheses these features include the persistence of basaltic magmatism along the hotspot track the origin of the hotspot during a regional middle miocene tectonic reorganization a similar and coeval zone of northwestward magmatic propagation the occurrence of both zones of magmatic propagation along a first order tectonic boundary and control of the hotspot track by preexisting structures seismic imaging provides no evidence for and several contraindications of a vertically extensive plume like structure beneath yellowstone or a broad trailing plume head beneath the eastern snake river plain the high helium isotope ratios observed at yellowstone and other hotspots are commonly assumed to arise from the lower mantle but upper mantle processes can explain the observations the available evidence thus renders an upper mantle origin for the yellowstone system the preferred model there is no evidence that the system extends deeper than 200 km and some evidence that it does not a model whereby the yellowstone system reflects feedback between upper mantle convection and regional lithospheric tectonics is able to explain the observations better than a deep mantle plume hypothesis

metalliferous sediments from the h m s challenger voyage 1872 1876 abstract the legendary cruise of h m s challenger 1872 1876 around the globe must always occupy an eminent place in the annals of oceanography as being the first systematic attempt made on a global scale to explore the ocean this expedition made fundamental discoveries in biology and geology which have not been surpassed by any later scientific cruise sediment with high content of metals later called metalliferous was among the enigmatic findings taken onboard although the nature of metalliferous sediments is well known today the very first sampled sediments of this type have not been studied to date motivated by the historical value of challenger s metalliferous sediment collection we undertook an investigation addressing two questions 1 the composition of sediments from seafloor for which we have very limited data 2 sr nd pb fe zn isotope signature of these sediments collected before the substantial human impact on the ocean during the 20th century the se pacific metalliferous sediments sampled by the challenger s explorers are of 2 types 1 metalliferous oozes blanketing ridge crests and flanks down to the calcite compensation depth ccd and 2 stripped of caco 3 metalliferous sediments located beneath the ccd in the deeps near the mid ocean ridges the abiogenic part of these sediments is composed mainly of poorly crystalline to x ray amorphous fe mn oxyhydroxides and an amorphous silicate phase these sediments have geochemical features similar to those of all the other metalliferous sediments very high fe and mn content on abiogenic basis very low al al fe mn and high content on abiogenic basis of as ba be bi cd co cu mo ni pb sb th tl u v w y zn and zr their ree distribution patterns are similar to that of deep seawater and show weak signs of hydrothermal imprint weak positive or no eu anomaly seawater and or terrigenous input from south america control the sr nd pb isotope signature of the challenger metalliferous sediments and have almost completely obliterated any original morb derived hydrothermal signal zn isotopes are mainly contributed from seawater although other zn sources hydrothermal fluid and detrital aluminosilicates barite and volcanic glass are necessary to fully explain zn isotope ratios fe isotopes indicate relatively slow fe 2 to fe 3 oxidation in the non buoyant plume thus producing relatively lighter fe isotope signature of the feooh particles that formed the studied metalliferous sediments

quaternary tectonic setting of the 1983 borah peak earthquake central idaho the 1983 borah peak earthquake was accompanied by extensive surface faulting along a part of the lost river fault that has abundant evidence of latest quaternary last 15 000 yr offset this fault and two similar range front normal faults along the lemhi range and beaverhead mountains lie in an area of basinand range structure in central idaho that is part of a roughly v shaped belt of latest quaternary surface faulting that extends from the wasatch fault through the yellowstone area to the lost river fault the position of this belt may be related to the outward migration of a thermal front associated with the northeastward progression of late cenozoic silicic volcanism along the yellowstonesnake river plain axis the central segments of the lost river lemhi and beaverhead faults have been active more recently and probably more active throughout quaternary time than the southern and northern segments the main 1983 surface faulting occurred in an area of high structural relief along a central segment of the lost river fault that has ruptured in latest quaternary time which suggests that comparable areas along other range fronts in the area should be regarded as likely sites of future surface faulting other perspective of fault behavior suggest additional possible sites and all segments of the range front faults are regarded as capable of surface faulting

mesozoic evolution hinterland of the sevier orogenic belt west of the mesozoic foreland sevier orogenic belt in northern utah and southern idaho a regional detachment of inferred mid jurassic age occurs in upper mississippian shale and can be traced westward to the margin of the coeval magmatic arc represented by spaced calc alkalic intrusions the detachment may be the oldest and structurally highest intrusions the detachment may be the oldest and structurally highest thrust of the idaho wyoming thrust belt if so it bridges the apparent spatial and temporal gap between magmatism and thrusting thus suggesting a direct genetic link thin skinned deformation is interpreted here to be driven from the heated magmatic arc toward the colder craton by lateral tectonic compression created by lateral volume increase due to intrusion elevated crustal geotherms and possible thickening in the arc the metamorphic complexes occur at the crustal interface between these two thermal tectonic regimes and may themselves be allochthonous

seismicity and late quaternary faulting of the northern basin and range province montana and idaho abstract examination of seismicity and late quaternary faults in montana and idaho north of the snake river plain shows a geographic correspondence between high seismicity and 24 faults that have experienced surface rupture during the late quaternary the lewis and clark zone delineates the northern boundary of this tectonically active extensional region earthquakes greater than magnitude 5 5 and all identified late quaternary faults are confined to the montana idaho portion of the basin and range province south of the lewis and clark zone furthermore all 12 holocene faults are confined to a seismologic zone centennial tectonic belt parallel to the northern flank of the snake river plain in extreme southwestern montana and adjacent idaho fault trends strain data and focal mechanisms suggest that both late quaternary faulting and seismicity in the region are primarily products of two distinct stress regimes 1 overall basin and range extension along a s45 w direction relative to the mid continent and 2 localized effects of the yellowstone hot spot which appears to have the least principle stress axis oriented horizontally at about s4 w the three largest historic earthquakes north of the snake river plain are indicative of three different stress provinces the 1983 borah peak earthquake occurred in the montana idaho basin and range the 1959 hebgen lake earthquake is associated with the yellowstone hot spot area and the 1925 clarkston valley event appears to be associated with the stress field of the united states mid continent

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

lithostratigraphic and petrographic analysis of icdp drill core lb 07a bosumtwi impact structure ghana lithostratigraphic and petrographic studies of drill core samples from the 545 08 m deep international continental scientific drilling program icdp borehole lb 07a in the bosumtwi impact structure revealed two sequences of impactites below the post impact crater sediments and above coherent basement rock the upper impactites 333 38 415 67 m depth comprise an alternating sequence of suevite and lithic impact breccias the lower impactite sequence 415 67 470 55 m depth consists essentially of monomict impact breccia formed from meta graywacke with minor shale as well as two narrow injections of suevite which differ from the suevites of the upper impactites in color and intensity of shock metamorphism of the clasts the basement rock 470 55 545 08 m depth is composed of lower greenschist facies metapelites shale schist and minor phyllite meta graywacke and minor meta sandstone as well as interlaminated quartzite and calcite layers the basement also contains a number of suevite dikelets that are interpreted as injection veins as well as a single occurrence of granophyric textured rock tentatively interpreted as a hydrothermally altered granitic intrusion likely related to the regional pre impact granitoid complexes impact melt fragments are not as prevalent in lb 07a suevite as in the fallout suevite facies around the northern crater rim on average 3 6 vol of melt fragments is seen in the upper suevites and up to 18 vol in the lower suevite occurrences shock deformation features observed in the suevites and polymict lithic breccias include planar deformation features in quartz 1 to 3 sets rare diaplectic quartz glass and very rare diaplectic feldspar glass notably no ballen quartz which is abundant in the fallout suevites has been found in the within crater impact breccias an overall slight increase in the degree of shock metamorphism occurs with depth in the impactites but considerably lower shock degrees are seen in the suevites of the basement rocks which show similar features to each other the bulk of the suevite in lb 07a appears to have been derived from the 35 gpa shock zone of the transient crater

the role of magma overpressure in suppressing earthquakes and topography worldwide examples in an extending terrane basaltic magma supplied at a pressure greater than the least principal stress overpressure may be capable of suppressing normal faulting and the earthquakes and topographic relief that commonly accompany normal faulting as vertical dikes intrude they press against their walls in the direction opposite the least principal stress and increase its magnitude the emplacement of tabular intrusions causes the internal magma pressure to act selectively in opposition to tectonic stresses this process tends to equalize the stresses and thus diminishes the deviatoric stress difference between maximum and minimum stresses that creates faults and causes earthquakes observations of the pattern of seismicity and magmatism worldwide indicate that magmatism commonly supplants large earthquakes as the primary mechanism for accommodating tectonic extension recognizing the extent of magmatic stress accommodation is important in assessing seismic and volcanic risks

geology and geochemistry of shallow drill cores from the bosumtwi impact structure ghana the 1 07 ma well preserved bosumtwi impact structure in ghana 10 5 km in diameter formed in 2 ga old metamorphosed and crystalline rocks of the birimian system the interior of the structure is largely filled by the 8 km diameter lake bosumtwi and the crater rim and region in the environs of the crater is covered by tropical rainforest making geological studies rather difficult and restricted to road cuts and streams in early 1999 we undertook a shallow drilling program to the north of the crater rim to determine the extent of the ejecta blanket around the crater and to obtain subsurface core samples for mineralogical petrological and geochemical studies of ejecta of the bosumtwi impact structure a variety of impactite lithologies are present consisting of impact glass rich suevite and several types of breccia lithic breccia of single rock type often grading into unbrecciated rock with the rocks being shattered more or less in situ without much relative displacement autochthonous and lithic polymict breccia that apparently do not contain any glassy material allochtonous the suevite cores show that melt inclusions are present throughout the whole length of the cores in the form of vesicular glasses with no significant change of abundance with depth twenty samples from the 7 drill cores and 4 samples from recent road cuts in the structure were studied for their geochemical characteristics to accumulate a database for impact lithologies and their erosion products present at the bosumtwi crater major and trace element analyses yielded compositions similar to those of the target rocks in the area graywacke phyllite shale and granite graywacke phyllite and granite dikes seem to be important contributors to the compositions of the suevite and the road cut samples fragmentary matrix with a minor contribution of pepiakese granite the results also provide information about the thickness of the fallout suevite in the northern part of the bosumtwi structure which was determined to be 15 m and to occupy an area of 1 5 km2 present suevite distribution is likely to be caused by differential erosion and does not reflect the initial areal extent of the continuous bosumtwi ejecta deposits our studies allow a comparison with the extent of the suevite at the ries another well preserved impact structure

the bosumtwi meteorite impact structure ghana a magnetic model abstract a magnetic model is proposed for the bosumtwi meteorite impact structure in ghana africa this relatively young 1 07 ma structure with a diameter of 10 5 km is exposed within early proterozoic birimian tarkwaian rocks the central part of the structure is buried under postimpact lake sediments and because of lack of drill cores geophysics is the only way to reveal its internal structure to study the structure below and beyond the lake a high resolution low altitude 70 m airborne geophysical survey across the structure was conducted which included measurements of the total magnetic field electromagnetic data and gamma radiation the magnetic data show a circumferential magnetic halo outside the lakeshore 12 km in diameter the central north part of the lake reveals a central negative magnetic anomaly with smaller positive side anomalies north and south of it which is typical for magnetized bodies at shallow latitudes a few weaker negative magnetic anomalies exist in the eastern and western part of the lake together with the northern one they seem to encircle a central uplift our model shows that the magnetic anomaly of the structure is presumably produced by one or several relatively strongly remanently magnetized impact melt rock or melt rich suevite bodies petrophysical measurements show a clear difference between the physical properties of preimpact target rocks and impactites suevites have a higher magnetization and have low densities and high porosities compared to the target rocks in suevites the remanent magnetization dominates over induced magnetization koenigsberger ratio 3 preliminary palaeomagnetic results reveal that the normally magnetized remanence component in suevites was acquired during the jaramillo normal polarity epoch this interpretation is consistent with the modelling results that also require a normal polarity magnetization for the magnetic body beneath the lake the reverse polarity remanence component superimposed on the normal component is probably acquired during subsequent reverse polarity events

an international and multidisciplinary drilling project into a young complex impact structure the 2004 icdp bosumtwi crater drilling project an overview the bosumtwi impact crater in ghana arguably the best preserved complex young impact structure known on earth displays a pronounced rim and is almost completely filled by lake bosumtwi a hydrologically closed basin it is the source crater of the ivory coast tektites the structure was excavated in 2 1 2 2 gyr old metasediments and metavolcanics of the birimian supergroup a drilling project was conceived that would combine two major scientific interests in this crater 1 to obtain a complete paleoenvironmental record from the time of crater formation about one million years ago at a near equatorial location in africa for which very few data are available so far and 2 to obtain a complete record of impactites at the central uplift and in the crater moat for ground truthing and comparison with other structures within the framework of an international and multidisciplinary drilling project led by the international continental scientific drilling program icdp 16 drill cores were obtained from june to october 2004 at six locations within lake bosumtwi which is 8 5 km in diameter the 14 sediment cores are currently being investigated for paleoenvironmental indicators the two impactite cores lb 07a and lb 08a were drilled into the deepest section of the annular moat 540 m and the flank of the central uplift 450 m respectively they are the main subject of this special issue of meteoritics planetary science which represents the first detailed presentations of results from the deep drilling into the bosumtwi impactite sequence drilling progressed in both cases through the impact breccia layer into fractured bedrock lb 07a comprises lithic in the uppermost part and suevitic impact breccias with appreciable amounts of impact melt fragments the lithic clast content is dominated by graywacke besides various metapelites quartzite and a carbonate target component shock deformation in the form of quartz grains with planar microdeformations is abundant first chemical results indicate a number of suevite samples that are strongly enriched in siderophile elements and mg but the presence of a definite meteoritic component in these samples cannot be confirmed due to high indigenous values core lb 08a comprises suevitic breccia in the uppermost part followed with depth by a thick sequence of graywacke dominated metasediment with suevite and a few granitoid dike intercalations it is assumed that the metasediment package represents bedrock intersected in the

constraints on north american plate velocity from the yellowstone hotspot deformation field oceanic hotspot tracks generally indicate that the mantle plumes that give rise to them are fixed in position relative to a lower mantle reference frame for timespans of 100 myr1 3 it has not been clear whether continental hotspots are equally stable for example there is evidence that the velocity of the yellowstone hotspot has been quite irregular with respect to the north ameri can plate over the past 16 myr4 7 but measurements of its velocity based on caldera locations have been too uncertain and irregular to be definitive here i describe a new method of obtaining the plume velocity which uses the broader lithospheric effects of the hotspot rather than the locations of individual calderas for the past 10 myr the results indicate a constant north american plate velocity of 2 2 cm yr 1 which agrees with independent estimates from global plate motion inversions8 9 this velocity is considerably lower than previous estimates obtained from caldera locations and clearly differs from the much higher but less well constrained velocities of the previous 6 myr refs 5 7