mean magnetic contrasts between oceans and continents abstract the magnetic contrast between continents and oceans is an important constraint for understanding the regional distribution of magnetic sources in the crust and upper mantle this magnetic difference was investigated using satellite 2 c scalar magnetic anomalies collected by nasa s magsat mission over the atlantic and eastern pacific oceans north and south america europe and africa reducing the magnetic anomalies differentially to vertical polarization yields mean anomaly values which correlate with the tectonic ages of the various continental and oceanic regions the range of magnetic anomaly values of tectonically older lithosphere is greater than that of younger lithosphere statistically at the 99 9 confidence level the mean radially polarized magnetic anomaly of the continents is greater than that of the oceans a careful inversion was performed for regional effective susceptibility contrasts which accurately fit the magsat data assuming a model in which the moho represents a boundary between crustal magnetic and predominantly non magnetic mantle rocks the root mean square of the magnetic properties of oceans compares favorably with magnetic properties predicted from models constrained by magnetic studies of the oceanic rocks and which incorporate viscoremanent components from rocks near the base of the crust the mean magnetic anomaly and property contrasts of the oceans are negative and show large variability indicating widespread deviations from the standard oceanic crust the effective susceptibility contrasts obtained facilitate quantitative testing of inferences concerning large scale petrologic variations crustal thickness and thermal perturbations and other geologic processes which control the regional distribution of magnetic properties within the lithosphere

scalar magnetic anomaly maps of earth derived from pogo and magsat data a new pogo scalar magnetic anomaly map at 400 km altitude is presented which consists of spherical harmonics of degree 15 60 on the basis of the common features of this map with two new magsat anomaly maps dawn and dusk two scalar magnetic anomaly maps of the earth are presented using two selection criteria with different levels of stringency these selection criteria suppress the noncrustal components of the original maps by different amounts the more stringent selection criteria seek to eliminate as much contamination as possible at the expense of suppressing some anomaly signal this map is represented by spherical harmonics of degree 15 60 the less stringent selection criteria seek to retain as much crustal signal as possible at the expense of also retaining some contaminating fields this map is represented by spherical harmonics of degree 15 65 the resulting two maps are highly correlated with degree correlation coefficients greater than 0 8

sources of aeromagnetic anomalies over cement oil field oklahoma simpson oil field alaska and the wyoming idaho utah thrust belt geochemical and rock magnetic studies undertaken to determine the cause of magnetic anomalies over cement oil field anadarko basin oklahoma simpson oil field north slope basin alaska and the wyoming idaho utah thrust belt have revealed different magnetic sources developed under different sedimentologic geochemical and structural settings at cement ferrimagnetic pyrrhotite fe7s8 typically intergrown with more abundant nonmagnetic pyrite fes2 formed as a result of hydrocarbon seepage sulfur isotopic data indicate that sulfur in the fe s minerals was probably derived from two different sources 1 isotopically heavy thermochemical h2s in petroleum and 2 isotopically light h2s generated by sulfate reducing bacteria that derived metabolic energy from leaking hydrocarbons or organic compounds derived from hydrocarbons although pyrrhotite may make a minor contribution to the reported total magnetic field anomalies at cement the measured anomalies are probably mostly caused by man mad

east pacific rise 8 10 30 n evolution of ridge segments and discontinuities from seamarc ii and three dimensional magnetic studies magnetic bathymetric and seamarc ii side scan sonar data from the ridge flanks adjoining the overlapping spreading center osc at 9 03 n on the east pacific rise epr are used to establish the evolution of this ridge axis discontinuity and adjacent ridge segments within the past 2 4 m y a three dimensional inversion for magnetization shows the discontinuity evolved from one of small offset 2 km to an 8 km offset osc within the past approximately 1 0 m y the osc has left a v shaped discordant zone indicating an average southward migration of 42 km m y since the beginning of anomaly 2 time 1 8 m y the west flank discordant zone consists of discrete abandoned overlap basins bounded by highly magnetized ridge tips with anomalous lineations while the east flank discordant zone is a broad swath of high magnetization anomalous lineations and greater depths kinematic modelling indicates that southward migration has not been steady but has been accomplished by a series of episodic propagation events with rates ranging from 500 mm yr ridge tips have been abandoned at intervals of approximately 50 000 100 000 years both by linkage of neighboring ridge segments and by self decapitation magnetic modelling suggests rotations of overlap basins of 25 consistent with predictions of kinematic modelling orientation of magnetic isochrons indicate a 3 6 counterclockwise change in pacifie cocos plate motion within the past 1 m y assuming orthogonal spreading throughout this time although the discontinuity currently located at 9 03 n existed prior to this change in plate motion its growth into a right stepping 8 km offset osc may be causally related to this change the current configuration of intratransform spreading within the siqueiros has developed since onset of the brunhes anomaly 0 73 m y possibly in response to this recent counterclockwise change in plate motion in addition to the osc there is evidence for several small discontinuities offsetting the ridge during anomaly 2 time which have since migrated along the ridge and vanished an axial magnetization high is observed along the crest of the epr change in the magnitude of the axial magnetization high or shifts in its location relative to the bathymetric axis coincide with several devals within the area magnetization maxima are found at ridge transform intersections and the osc these highs may reflect presence of highly fractionated feti basalts erupted from magma chambers with restricted supply located adjacent to both large and short offset ridge axis discontinuities discrete magnetization highs are found off axis adjacent to clipperton and siqueiros fracture zones indicating that production of feti basalts at ridge transform intersections is episodic magnetizations are higher north of clipperton than south of it possibly reflecting a long lived starvation in magma supply north of clipperton and more abundant supply to the south most seamounts within the area have the same magnetization polarity as surrounding seafloor indicating that seamount formation occurs near the ridge axis the exceptions lie near magnetic reversal boundaries and are consistent with seamount formation as close as 10 km from the ridge axis few pronounced magnetization highs are associated with seamounts indicating that the magnetized source layer is no thicker beneath them than surrounding seafloor or that seamount basalts are substantially less magnetic than normal mid ocean ridge basalts

magnetic and tectonic studies of the dueling propagating spreading centers at 20 40 s on the east pacific rise evidence for crustal rotations we present the results of a magnetic study of a 225 km by 240 km area centered on the dueling propagating spreading centers located at 20 40 s on the east pacific rise a majority of the data used were collected during a cruise aboard the r v moana wave during which continuous seamarc ii coverage was obtained these data were combined with additional data to produce an anomaly map which extends to anomaly 2 aged crust a three dimensional inversion in the presence of bathymetry was carried out for the area the resulting magnetization distribution was interpreted and compared to side scan sonar and bathymetry data sets in order to determine the recent history of the discontinuity the results indicate consistent asymmetric spreading faster to the east discontinuous high magnetizations in the discordant zone associated with the discontinuity and southward migration of the feature at a rate of 90 100 mm yr between jaramillo and brunhes time 0 95 to 0 73 ma with slowing during the brunhes to less than 10 mm yr there is also an overlapping jaramillo isochron on the west flank and a gap in that isochron on the east flank indicating a transfer of crust during this time period from the nazca to the pacific plate in addition areas of oblique lineations possibly representing rotated crust were modelled using an inverse method which enables the specification of a nonuniform magnetization unit vector results from this second model support the presence of highly rotated pre brunhes nazca crust within brunhes pacific crust which has been deformed by bookshelf faulting this indicates at least two episodes of crustal transfer from the nazca plate to the pacific plate the discontinuity appears to mark the boundary between rigid plate tectonics to the north and deformation within the nazca plate between the discontinuity and the easter microplate to the south the detailed history of the discontinuity involves dueling propagation with a great deal of variation in the amount of overlap of the two ridges as well as inward and outward cutting and abandonment of the tips of both ridges

distribution and structure of hydrocarbons in four organic rich ordovician rocks abstract organic matter in four north american rocks of ordovician age has been analysed using geochemical and optical techniques including reflected light and fluorescence microscopy rock eval pyrolysis elemental analysis of isolated kerogens gas chromatography gc and gas chromatography mass spectrometry gc ms of hydrocarbons in the solvent extracted bitumens the samples analysed were guttenberg oil rock and black shales from the vinini womble and big fork formations in the united states the origin of some of the biomarkers found in the bitumen and the vitrinite particles observed in reflected light are discussed briefly with reference to possible algal and bacterial precursors the results indicate that the four samples are potentially excellent oil source rocks although the guttenberg oil rock is too immature to have generated a substantial quantity of hydrocarbons

storm tracks in the southern hemisphere abstract from 1979 to 1989 global european centre for medium range weather forecasts ecmwf analyses of many meteorological variables important in baroclinic storms have been bandpass filtered to highlight fluctuations on 2 to 8 day time scales in order to illuminate the relationships among the variables in baroclinic storm track regions because of the greater zonal symmetry in the southern hemisphere sh versus the northern hemisphere nh it is possible to examine these relationships over a zonal sector without being dominated by local effects associated with jet stream entrance and exit regions of the storm tracks the bandpassed variance of geopotential height is used to define the storm track and its meridional profile has a pronounced midlatitude maximum in the sh the locations of the jet stream and the variance maxima of meridional and zonal velocity components vorticity vertical motion specific humidity and temperature all bear a distinctive relationship to the storm track but the lat

bedrock topography from magnetic anomalie an aid for groundwater exploration in hard rock terrains the role of the magnetic method in groundwater exploration is to locate structures such as dikes faults fractures etc that control the accumulation and movement of groundwater in hard rock terrains the thickness of the weathered layer is an important parameter that determines the quantity of groundwater accumulated in the unconfined aquifer above the basement the basement rock in the process of weathering loses its magnetic properties and becomes much less magnetic therefore the magnetic response is mostly due to the unweathered hard basement rock and the depths of magnetic sources obtained from the analysis give us the top of the basement information about the thickness of the weathered layer would help in assessing the groundwater potential of the region

deep structure of the u s atlantic continental margin offshore south carolina from coincident ocean bottom and multichannel seismic data we present the results of a combined multichannel seismic reflection mcs and wide angle ocean bottom seismic profile collected in 1988 across the carolina trough on the u s atlantic continental margin inversion of vertical incidence and wide angle travel time data has produced a velocity model of the entire crust across the continent ocean transition the margin consists of three structural elements 1 rifted continental crust comprising 1 4 km of post rift sedimentary rocks overlying a 30 34 km thick subsedimentary crust 2 transitional crust a 70 to 80 km wide zone comprising up to 12 km of postrift sedimentary rocks overlying a 10 to 24 km thick subsedimentary crust and 3 oceanic crust comprising 8 km of sedimentary rocks overlying an 8 km thick crystalline crust the boundary between rifted continental and transitional crust marked by the brunswick magnetic anomaly represents an abrupt change in physical properties with strong lateral increases in seismic velocity density and magnetic susceptibility the transitional crust contains mid crustal seaward dipping reflections observed on the mcs section and has seismic velocities of 6 5 6 9 km s in the midcrust and 7 2 7 5 km s in the lower crust modeling of potential field data shows that transitional crust also produces the prominent margin parallel gravity anomaly and the brunswick and east coast magnetic anomalies these observations support the interpretation that the transitional crust was formed by magmatism during continental breakup the prodigious thickness up to 24 km of igneous material rivals that interpreted on continental margins of the north atlantic e g hatton bank and voring plateau which formed in the vicinity of the iceland hotspot these observations when combined with other transects across the margin confirm previous suggestions that the u s atlantic margin is strongly volcanic and further imply that the magmatism was not the result of a long lived mantle plume

considerations of variations in ionospheric field effects in mapping equatorial lithospheric magsat magnetic anomalies summary longitudinal seasonal and altitude dependent variability of magnetic fields is investigated in equatorial latitudes to determine their effect on the isolation of lithospheric magsat magnetic anomalies an estimate of ionospheric effect was compiled by averaging the total intensity magsat anomalies as a function of dip latitudes called dip latitude averages from dawn and dusk data sets grouped according to longitudes time months and altitudes unanticipated seasonal variations were observed in dusk magsat data over the indian sector that may contribute to an improved understanding of the equatorial electrojet the amplitudes of the dawn dip latitude averages are small in comparison to the dusk averages and they are of opposite sign as anticipated from the westward and eastward flowing currents at the dip equator at dawn and dusk time respectively however longitudinal variation in the equatorial amplitudes of the dawn diplatitude averages is not entirely consistent with present knowledge of the electrojet field in the past dawn magsat anomalies have been considered to be largely free of ionospheric fields however in the preparation of the lithospheric component maps neglecting to remove the dawn dip latitude averages from dawn magsat data set leaves conspicuous e w trending anomaly artifacts in certain regions of the world removal of both the dawn and the dusk dip latitude averages from their respective data sets substantially improves the agreement between the separately prepared dawn and dusk lithospheric component magsat maps despite the agreement between the resultant lithospheric anomaly maps dip latitude averages do appear to contain some contribution from the lithospheric fields thus processing of the data with the aid of the dip latitude averages unavoidably removes a small degree of e w oriented lithospheric signal as well

discrimination of bitumen sources in precambrian and lower palaeozoic rocks southern u k by gas chromatography mass spectrometry abstract two distinct sources for bitumen shows in lower palaeozoic and precambrian rocks in the welsh borderland can be successfully discriminated using gas chromatography mass spectrometry bitumens are widespread in the region in rocks from late precambrian longmyndian to silurian age previous geological studies have suggested that some bitumens originated from pre devonian source rocks while others migrated downwards laterally from carboniferous rocks this possibility has now been confirmed by organic geochemical studies which have also clarified the origin of some bitumens whose affinities had not been clear those bitumens concluded to have migrated from the carboniferous have also been correlated with bitumens within carboniferous rocks further east they are characterized by relatively high levels of hopanes c 27 c 29 to c 35 max c 29 or c 30 the bitumens with an assumed pre devonian source contain n alkylcyclohexanes range c 15 to c 32 max c 18 or c 19 which are absent in the other bitumens and they can be matched with a longmyndian black shale although detailed correlations are limited by the very high maturity of the samples the pre devonian bitumens also show several affinities with the distinctive ordovician oils

biochemical evolutionary significance of ordovician oils and their sources abstract geochemical data from several ordovician oils and kerogens from distinct basins in canada united states estonia and australia show some unique compositional characteristics the oils analyzed by gas chromatography and gas chromatography mass spectrometry are characterized by an odd dominance in the c9 c19 molecular weight range of normal paraffins and an odd dominated homologous series of cyclohexyl substituted normal paraffins the oils contain an unusually small amount of c20 components and pristane and phytane are conspicuous by their virtual absence the compositions of the oils and kerogens correlate closely and display characteristics that have not been observed in younger sediments the predominant organism found in the source kerogens for these unique oils is gloeocapsamorpha prisca a primitive prokaryotic organism data from the analysis of gloeocapsamorpha rich kerogens and the association of this organism with ordovician source rocks indicate a genetic relationship exists between the organism and the unusual oil chemistry the chemistry of the oils and kerogens suggests that gloeocapsamorpha was either non photosynthetic or that it fixed light using a chlorophyll that did not contain a phytyl side chain e g a bacteriochlorophyll the widespread occurrence of oils and kerogens possessing such a uniform composition appears unique to the ordovician certainly to the early paleozoic this suggests that paleocological conditions of ordovician anoxic environments were very similar and g prisca was particularly well adapted to these conditions

comparison of michigan basin crude oils abstract michigan basin oils from the ordovician trenton silurian niagaran and devonian dundee formations have been geochemically compared by gc gc ms and carbon isotope mass spectrometry one oil from each formation was selected for detailed analysis which included measurement of individual n alkane 13 c values the ordovician and devonian oils are strikingly similar to one another yet clearly different from the silurian oil this pattern is unexpected because ordovician and devonian reservoirs are physically separated by the silurian strata from time temperature considerations the devonian oil probably was formed in older strata and has migrated to its present location our analyses suggest a common source for the devonian and ordovician oils

middle ordovician organic matter assemblages and their effect on ordovician derived oils two distinct middle ordovician oil prone organic matter assemblages occur in thermally immature to marginally mature petroleum source rocks from iowa the substantially more oil prone assemblage a generates the unique ordovician oil fingerprint which has been associated with the organic walled microfossil gloeocapsamorpha prisca the second assemblage b generates a more ordinary signature the two assemblages which are mixed or interbedded in many ordovician sediments of north america explain the variations in oils derived from ordovician source rocks these mixtures also aid in interpretation of the chemistry of g prisca core samples examined are from three wells in iowa penetrating the primarily carbonate sequence from the guttenberg member of the decorah formation down to the st peter sandstone microscopy rock eval pyrolysis and capillary gas chromatography on source rock bitumen extracts and nearby oils were used to evaluate the two assemblages solvent extracts from rocks containing predominantly assemblage a have the diagnostic ordovician gas chromatographic signatures with an odd preference in c11 c19 normal alkanes low amounts of heavier normal alkanes and virtual absence of isoprenoids including pristane and phytane although assemblage a occurs only in very thin beds it is potentially so oil generative that thicknesses in inches may be significant extracts from r cks containing predominantly assemblage b show reduced odd preference a full profile of normal alkanes and conspicuous occurrence of isoprenoids including pristane and phytane these samples from iowa are representative of middle ordovician source rocks found in most paleozoic basins of north america as well as baltic europe and australia therefore the conclusions drawn from these middle ordovician rock samples from iowa provide a basis to explain variations in ordovician derived oils found in many other areas

teleconnections in the geopotential height field during the northern hemisphere winter abstract contemporaneous correlations between geopotential heights on a given pressure surface at widely separated points on earth referred to as teleconnections in this paper are studied in an attempt to identify and document recurrent spatial patterns which might be indicative of standing oscillations in the planetary waves during the northern hemisphere winter with time scales on the order of a month or longer a review of existing literature on the subject reveals the existence of at least four such patterns the north atlantic and north pacific oscillations identified by walker and bliss 1932 a zonally symmetric seesaw between sea level pressures in polar and temperature latitudes first noted by lorenz 1951 and what we will refer to as the pacific north american pattern which has been known to operational long range forecasters in this country since the 1950 s a data set consisting of nmc monthly mean sea level pressure and 500 mb height analyses for a 15 year period is used as a basis fo

sequence stratigraphy of the red river formation williston basin usa stratigraphic signature of the ordovician katian greenhouse to icehouse transition abstract the upper katian upper ordovician red river formation is a carbonate evaporite supersequence that formed in the epicratonic williston basin of western north america this study focuses on the upper part of the formation in the southern us part of the basin the studied succession is made up of three transgressive regressive sequences rr1 rr2 and rr3 each 0 5 1 m y in duration and composed of relatively few meter scale high frequency cycles parasequences sequence boundaries are characterized by intraclastic breccia laminated dolomite successions that have been truncated and capped by evaporite deposits subaqueous anhydrites within each sequence represent evaporite lowstand systems tracts lsts that formed during relative sea level lows that lead to the precipitation of gypsum later recrystallized into anhydrite in a quiet salina type environment transgressive tst and early highstand systems tracts hst are composed of subtidal facies of predominantly skeletal mudstone to wackestone packstone with abundant burrow mottling late hsts are predominantly composed of peritidal porous laminated dolomite the study illustrates how the sequence stratigraphic framework of carbonate evaporite deposits from the williston basin relates to the late ordovician glacio eustasy transitional climates and low basin subsidence in an arid setting

analysis of magsat magnetic contrasts across africa and south america abstract comparisons of magsat magnetic contrasts and geology across the mesozoic assembly of africa and south america provide new insight into the interpretation of the long wavelength magnetic anomalies near the present continental margins across continental africa and south america the magsat magnetic contrasts can be correlated with geologic provinces formed before the mesozoic separation of the continents on the continents areas affected by significant mesozoic hotspot tectonism display negative magnetic contrasts suggesting a causative relationship between hotspot tectonism and the origin of the observed magnetic contrasts the magnetic characteristics of a portion of the lower crust in these areas appear to have been significantly altered during the mesozoic hotspot epeirogeny by analogy with the processes of continental rifting it is suggested that the magnetic mineralogy of the intruded lower crust may be dominated by weakly to non magnetic titanomagnetites oceanic magnetic contrast comparisons show that the positions of magnetic anomaly highs over the rio grande walvis ridge system of the south atlantic are consistent with the interpretation of their evolution over the walvis hotspot however only the parts of these ridges that were formed during the cretaceous normal polarity geomagnetic epoch bear strong magnetic contrasts remanent magnetization thus appears to be an important contributor toward the magsat magnetic anomalies of these features

separation of lithospheric external and core components of the south polar geomagnetic field at satellite altitudes we present a new approach to producing scalar magsat magnetic anomaly maps based on correlation coefficient filtering and the use of almost all of the available orbits our method differs from earlier techniques with respect to the following 1 passes are selected based on their variance properties rather than planetary indices such as kp 2 the core field model is least squares fit to individual passes and subsequently removed instead of subtracting the model directly this technique replaces band pass filtering or polynomial trend removal methods 3 each selected pass is sorted geographically and by local time placed into one of four different altitude bands and correlation coefficient filtered with the two adjacent passes the filtering is the second step toward isolating the static lithospheric signal from the more dynamic external field signals 4 least squares collocation is used to grid the correlated passes subsequently the dawn and dusk maps are also correlation filtered providing another step toward removal of external fields 5 the four resultant total field maps are continued to a common altitude and again correlation filtered for the static lithospheric anomalies 6 the filtered results are then averaged together to provide a new total field map of the lithosphere south of 40 s latitude our total field map differs from previous efforts over the crustal blocks of west antarctica we obtained a positive anomaly over edward vii peninsula extending into the byrd subglacial basin and obtained a negative anomaly over the ellsworth mountains and parts of the byrd subglacial basin also a positive anomaly extending from the ross sea to offshore wilkes land is present in our map however this feature is absent in other maps positive anomalies marking the weddell sea in previous efforts are not present in our map prominent external field anomalies in the quadrant offshore of wilkes land are present in all previous efforts however these signatures are reduced in our total field map this map significantly advances efforts for the identification and interpretation of magnetic anomalies of the south polar lithosphere

tectonic setting of the portland vancouver area oregon and washington constraints from low altitude aeromagnetic data seismic activity in the portland vancouver metropolitan area may be associated with various mapped faults that locally offset volcanic basement of eocene age and younger this volcanic basement is concealed in most places by young deposits vegetation and urban development the u s geological survey conducted an aeromagnetic survey in september 1992 to investigate the extent of these mapped faults and possibly to help identify other seismic and volcanic hazards in the area the survey was flown approximately 240 m above terrain along flight lines spaced 460 m apart and over an area about 50 50 km these magnetic data indicate a pronounced northwest striking magnetic lineation east of the willamette river in downtown portland associated with a fault concealed beneath quaternary sedimentary deposits and previously inferred from shallow well data the magnetic lineation confirms the existence of the fault and suggests that it has had a prolonged history 1 although well data indicate 200 m of vertical offset of underlying volcanic basement models based on the aeromagnetic data from downtown portland suggest reverse faulting with up to 1 km of offset deeper in the section 2 the magnetic lineation associated with this fault extends southeast to the clackamas river drainage a distance of 50 km and considerably beyond the mapped extent of the fault a northwest striking magnetic anomaly located southwest of the tualatin mountains corresponds closely with another mapped fault and with mixed reverse and strike slip faulting during a seismic swarm m 3 in 1991 we believe these and other anomalies in the aeromagnetic data reflect the portland hills fault zone believed to be the southwestern boundary of a structural basin now occupied by portland and vancouver the postulated northeastern boundary of the basin the frontal fault zone is also evident although less well represented in the aeromagnetic data aeromagnetic anomalies geologic mapping and earthquake focal plane solutions demonstrate a complex deformational history in the portland vancouver area since middle miocene time that includes elements of compression extension and dextral slip these complexities reflect portland vancouver s unique position within a north south transition in tectonic styles along the cascadia margin from compressional in the north to extensional in the south

an overlapping propagating spreading center at 87 30 w on the galapagos spreading center abstract in september of 1987 we completed a seamarc ii smii survey of the propagating spreading center located at 87 30 w on the galapagos spreading center gsc the spreading rate in the area is intermediate with a full rate of approximately 70 mm yr and the spreading axis is marked by an axial high analysis of bathymetry sidescan sonar and surface magnetic data indicates 90 100 mm yr eastward propagation of the 26 km offset the morphology of the feature is that of an overlapping spreading center pair osc it fits a kinematic model of overlapping ridges with cyclic failure of the dying rift there are marked differences in the morphology and kinematics of this propagator compared to the propagator at 95 30 w where the spreading rate and offset are comparable the 87 30 w propagator is marked by an axial high and greater than 40 km of overlap in contrast to the axial rift valley and small amount of overlap associated with the 95 30 w propagator near field stresses associated with variable axial topography contribute to decrease crack propagation forces at the 95 30 w propagator and increase those at the 87 30 w propagator differences in crack propagation forces and morphology may be due to a fundamental difference in the amount of available magma this difference is evident in dominance of volcanism near 87 30 w as opposed to the dominance of tectonism near 95 30 w

the dynamics of large scale cyclogenesis over the north pacific ocean abstract earlier studies of persistent large scale flow anomalies have been extended with the aim of identifying the primary mechanisms for persistent anomaly development in this study the focus is on wintertime cases of persistent cyclonic flow anomalies over the north pacific these cases are typically manifested by an abnormally intense cyclonic circulation extending over the north pacific basin an unusually strong and eastward extended east asian jet and a well defined pacific north american teleconnection pattern we have conducted extensive diagnostic analyses in order to determine the mechanisms responsible for development in particular these diagnostics examine the processes influencing the time evolution of eddy potential enstrophy and potential vorticity anomalies the cases are preceded by a buildup of anomalously high potential vorticity air at upper levels over eastern asia this high potential vorticity air is initially advected eastward in association with synoptic scale cyclogenesis

lithospheric cross sections of the european cenozoic rift system abstract the lithospheric structure of the european cenozoic rift system ecris is presented in transects through the southern rhine graben and the rhenish massif hessen depression emphasizing the geophysical structure of the lithosphere based on seismic refraction reflection investigations teleseismic tomography electromagnetic depth sounding models and gravity aeromagnetic earthquake uplift subsidence and heat flow data the rift is clearly expressed in the rhine graben but is not evident at the surface in the area of the rhenish massif where its existence is indicated by seismicity it is characterized by abnormal crustal and upper mantle structures which vary considerably in horizontal direction for example under the rhine graben the crust is thinned to 25 km but at 40 km depth anomalously high velocities are observed in contrast beneath the rhenish massif the crust is thickened to 35 37 km and under its eastern part a high velocity thin upper mantle slice is seen at 30 km depth within the lower crust which towards the hessen depression is gradually replaced by normal variscan mantle with the moho near 30 km depth under the western part of the rhenish massif p and s wave velocities are reduced below 50 km depth which is not seen east of the rhine river under the rhine graben the existence of a low velocity upper mantle above 100 km cannot be generalized but is restricted to confined regions

crustal magnetic anomalies this quadrennial report departs in format from its predecessors earlier reports provided brief descriptions of the entire range of topics on crustal magnetic anomalies following guidelines established by the agu we instead focus our discussion on only a few selected topics our abbreviated list of references reflects this restricted focus a more comprehensive bibliography can be obtained from the authors as noted subsequently the past quadrennium was distinguished by a significant change in the scale and scope of crustal magnetic anomaly research advances in instrumentation navigation and data processing techniques resulted in increasing use of two dimensional anomaly maps rather than profile data although maps are commonly used in aeromagnetic studies of continental areas their use in the ocean basins has been limited until recently the other important change has been a trend towards increased integration of magnetic anomaly data with gravity seismic geochemical remote sensing and geologic data to enhance the interpretation and reduce the ambiguity of magnetic field data

mixing of thermogenic natural gases in northern appalachian basin stable isotopic studies were undertaken to investigate the origin of 20 oil associated and non associated gases from western and central new york state in the northern appalachian basin gas wetness c2 c5 c1 c5 methane dgr13c and methane dgrd values range from 0 8 to 26 9 47 9 to 30 7 pmil and 305 to 143 pmil respectively corresponding ethane and propane dgr13c values range from 40 9 to 32 0 pmil and 34 9 to 27 9 pmil these data are interpreted to indicate a thermogenic origin with source rock maturities ranging from early mature equivalent vitrinite reflectance ro approximately 0 5 to post mature equivalent ro 2 0 inferred gas maturities generally in rease with increasing reservoir age from late to middle devonian marcellus and overlying rocks middle devonian to latest silurian oriskany onondaga akron silurian lockport herkimer medina to ordovician queenston and black river oil and gas in many late to middle devonian reservoirs may have migrated from interbedded organic rich shales in other reservoirs particularly those of ordovician and silurian age unusual isotopic reversals methane dgr13c ethane dgr13c suggest that gas generated locally from equivalent age rocks within the oil window mixed with post mature gas migrated from deeply buried sources to the south and southeast the post mature gas was probably sourced from middle to late ordovician shales regional migration of middle ordovician gas may have occurred along the middle ordovician knox unconformity gas was presumably channeled into overlying silurian and devonian reservoirs along faults and fractures

variability of the observed midlatitude storm tracks in relation to low frequency changes in the circulation pattern abstract the principal modes of month to month variability of the wintertime storm tracks over the north pacific and north atlantic are identified by empirical orthogonal function analysis of the root mean square statistics of bandpass 2 5 6 day filtered geopotential height data for 19 yr one of the two leading modes depicts fluctuations in the level of synoptic scale activity without any noticeable spatial displacement of the storm track axes whereas the other mode is associated with meridional shifts of the storm tracks from their time averaged positions higher order modes are indicative of diversion or truncation of cyclone tracks in particular geographical regions it is demonstrated that the leading storm track modes are linked to some of the best known monthly averaged teleconnection patterns the dipolar western pacific and western atlantic patterns for the monthly mean flow are seen to be accompanied by marked changes in the intensity of the storm tracks over the western oceans whereas the m

life cycles of persistent anomalies ii the development of persistent negative height anomalies over the north pacific ocean abstract the present study extends our previous work on the life cycles of persistent anomalies by providing more comprehensive analyses of the synoptic and dynamical characteristics associated with the developments of the anomalies we focus here on the developments of major cases of persistent negative height anomalies over the extratropical central north pacific pac region during wintertime these ewes are generally manifested at the surface by an anomalously intense and eastward displaced aleutian low and at upper levels by an abnormally strong zonal jet that extends across most of the western and central pacific at midlatitudes the associated flow anomalies usually resemble particularly strong realizations of the pacific north american pna teleconnection pattern the large scale flow anomalies are typically preceded by a buildup of anomalously cold air over asia and an intensification of the upper level jet over southeastern asia and the fox western pacific a few days prior to the larger scal

variations in oceanic crustal magnetization systematic changes in the last 160 million years the amplitudes of marine magnetic anomalies show a clear worldwide pattern of systematic variation when viewed as a function of age globally the amplitudes decrease with age over the first 20 to 30 million years a phenomenon that has been attributed to the low temperature oxidation of the magnetic mineral titanomagnetite in the upper extrusive rocks in oceanic crust older than 40 million years however the amplitudes of the magnetic anomalies increase with increasing age and remain at elevated levels for the entire period between 80 and 160 million years in order to examine the processes responsible for this elevated crustal magnetization in older oceanic crust we compiled all of the existing rock magnetic data from relevant deep sea drilling project and ocean drilling program sites that sampled extrusive rocks from normal ocean crust this compilation shows that the laboratory measurements of the magnetization of the upper basement rocks closely reflect that of the anomaly amplitudes both in the decrease due to oxidation in the first 20 to 30 million years and in the increase in magnetization in older 80 million years crust this positive correlation between anomaly amplitudes and upper crustal magnetization supports the argument that a major source of the marine magnetic anomalies is in the upper extrusive volcanic rocks further the curie temperature data show that oxidation of the magnetic minerals in oceanic basalts occurs largely within the first 30 million years and does not increase significantly beyond that point finally the strong positive correlation between the intensity of the magnetization of the drill core samples and the saturation magnetization argues that the higher magnetic anomaly amplitudes over older crust are related to a change in an intrinsic property of the upper crust specifically we propose that the observed elevated magnetization of older ocean crust is due primarily to an increased abundance of magnetic feti oxides in the older crustal rocks this large increase in abundance of the feti oxides may be related to a systematic increase in bulk feti content in older tholeiitic basalts or more likely is due to a difference in the partitioning of the iron and titanium content between the silicate and oxide phases

hydrocarbon biomarkers from ordovician sediments and the fossil alga gloeocapsomorpha prisca zalessky 1917 numerous ordovician oils worldwide are known to show unusual and distinctive distributions of hydrocarbons which it has been suggested are derived from a unique benthonic mat forming non photosynthetic prokaryotic organism gloeocapsomorpha prisca zalessky 1917 which is the major contributor of organic matter organic matter rich sediments from the canning and amadeus basins of australia known to contain g prisca fossils and to have the characteristic predominance of odd carbon number c13 c19 n alkanes were investigated to determine other hydrocarbon distributions sediments from both basins contained abundant n alkylcyclohexanes with odd carbon number predominance c13 c19 and methyl n alkylcyclohexanes c14 c20 which in immature sediments showed an even carbon number predominance the isomer distribution of these latter compounds was determined by direct comparison with synthetic standards the sediments from both basins also contained very similar distributions of steranes and pentacyclic triterpanes and the derived kerogens had a characteristically light carbon isotope signature pyrolysis of a g prisca rich kerogen yielded a hydrocarbon mixture with a similar composition to the sediment extract except that there was a marked increase in the relative abundance of pristane phytane alkylcyclohexanes steranes and hopanes relative to n alkanes we argue on the basis of the geological geochemical and palynological data that g prisca was probably planktonic photosynthetic and very possibly eukaryotic and that the striking character of ordovician oils and sediments derive from bacterial and other diagenetic imprints superimposed on the primary signature of this organism

seismicity and aeromagnetic features of the gulf of aqaba elat region recent seismicity in the gulf of aqaba region has been examined in relation to pull apart tectonics and structures indicated by bathymetry surface geology and new analyses of aeromagnetic data forty earthquakes occurring in the region from 1985 to 1989 were located using data from a network of five telemetered seismographs in the midyan region of saudi arabia east of the gulf of aqaba and from a network north of the gulf in israel interpretation of aeromagnetic data from the gulf of aqaba and the adjacent midyan region confirms previous estimates of offsets across the zone of transform faulting in the gulf aeromagnetic anomaly patterns obtained by reduction to the pole techniques and earthquake locations provide evidence for continuation of the faulting regime from the gulf northeastward into the midyan region the vicinity of the elat deep primarily the southwest flank probably is undergoing active normal faulting other seismicity and first motion polarity data confirm the regional pattern of nne trending sinistral strike slip faulting

the occurrence of petroleum in sedimentary rocks of the meteor impact crater at lake siljan sweden abstract during the devonian large parts of sweden were covered by sandstones shales and limestones of precambrian to upper silurian age at the end of the devonian 360 m yr b p a meter struck causing a large impact crater at lake siljan with a diameter of 50 60 km annular depression was formed which is still preserved today this depression contains undisturbed as well as tilted paleozoic rocks numerous sedimentary occurrences of asphalts and seep oils are reported from quarries in the sedimentary rocks and some shallow boreholes contain a thin layer of free oil floatingon arterian groundwater a representative set of paleozoic shales and limestones found in the siljanarea was analyzed with respect to organic richness type and thermal maturity three intervals of organic rich and oil prone source rocks 2 10 toc were distinguished and extracts of these rocks were geomically compared with the seep oils using stable carbon isotopes and terpenoid biomarkers even though these low maturity oils were altered by biodegradation a correlation with ordovician source rocks was established all the analyzed organic rich rocks are on the verge of oil generation as determined by 20s 20r 5 h c 29 sterane ratios in addition burial history indicates insufficient overburden available for oil generation the seep oils however are more mature and within the oil window yet below peak generation levels thus it appears that the heat effect of meteor impact locally matured the source rock allowing for geologically instantaneous hydrocarbon generation and expulsion resulting in the formation of seep oils

paleozoic oil source rock correlations in the tarim basin nw china we studied a suite of 40 oils and extracts of purported source rocks from the tarim basin in nw china the main group of oils comes from tazhong and tabei wells which sample the largest known petroleum accumulations in the basin these oils can be statistically correlated with extracts of ordovician rocks based upon high relative concentrations of 24 isopropylcholestanes and low relative concentrations of dinosteranes triaromatic dinosteroids and 24 norcholestanes in contrast extracts from cambrian rocks have low relative concentrations of 24 isopropylcholestanes with high relative concentrations of dinosteranes triaromatic dinosteroids and 24 norcholestanes although some tarim basin cambrian rocks yield high total organic carbon contents we see little evidence in the analyzed oil samples to suggest that they came from cambrian source rocks

the global continent ocean magnetization contrast abstract a schematic global model for the distribution of magnetization in the lithosphere is devised this model is characterized by a uniform stepwise change of radially integrated susceptibility at continent ocean boundaries and an axial dipolar inducing field a spherical harmonic expansion of the magnetic field produced by this source distribution is computed then the standard method of truncation at degree n 13 is used to determine the respective contribution of this field to the main field and to the lithospheric anomaly field the latter is shown to display significant anomalies near continental margins as well as within continents and oceanic basins the energy density spectrum of this field is shown to be very similar to the spectrum of the magsat derived lithospheric anomaly field furthermore several features of this field are shown to account for unexplained anomalies of the magsat field this supports the hypothesis that such an abrupt magnetization variation may occur at several continental margins and contribute to the signal in the magsat derived long wavelength anomaly maps this hypothesis provides an order of magnitude estimate for the magnetization difference between oceanic and continental lithospheres finally it is argued that the synthetic anomaly field produced by this model provides an upper bound to the energy density spectrum of the lithospheric field at low harmonic degrees and concluded that the lithospheric field does not contribute significantly to the earth s field below degree 13

aeromagnetic evidence for a buried early cretaceous magmatic arc northeast japan positive aeromagnetic anomalies recent drilling and models constructed from these data delineate the plutonic roots of the early cretaceous kitakami magmatic arc in northeast japan buried plutons mostly offshore produce belts of positive magnetic anomalies these anomalies and magnetotelluric data suggest that the plutons form a batholith 70 120 km wide nearly 800 km long and 10 15 km thick the batholith may mark the location of the main kitakami arc most of the exposed kitakami plutons are 2 20 km in diameter some are 3 km thick the small plutons line up along nw trending faults some may have been satellite vents that tapped into the magma supply of the main arc the batholithic roots of the main arc now compose almost half of the modern japan forearc basement steep magnetic gradients offset anomalies and basin stratigraphy portray extensive faulting of the kitakami batholith during oblique subduction in the late cretaceous and rifting in the miocene the eastern boundary of the kitakami batholith lies between 90 and 140 km west of the modern trench much closer than the 300 km distance between the active arc and trench the early cretaceous forearc basin and accretionary prism may underlie the modern forearc basin east of the batholith but clear evidence is lacking much of the early cretaceous margin including most of its forearc therefore is missing how the material was removed is unknown it could have been strike slip faulted eroded by subduction related processes or both

low frequency variability in the northern hemisphere winter geographical distribution structure and time scale dependence abstract low frequency variability in wintertime 500 mb height is examined with emphasis on its structure geographical distribution and frequency dependence a 39 year record of 500 mb geopotential height fields from the nmc analyses is time filtered to partition the fluctuations into frequency bands corresponding to periods of 10 60 days 60 180 days and 180 days winter is defined as the six month period november through april variance teleconnectivity and anisotropy fields and selected loading vectors derived from orthogonal and oblique rotations of the eigenvectors of the temporal correlation matrix for each band are shown and discussed the variability in all frequency bands exhibits substantial anistropy with meridionally elongated features arranged as zonally oriented wave trains prevailing over the continents and zonally elongated features organized in the form of north south oriented dipole patterns prevailing over the oceanic sectors of the hemisphere the wave trains are most pronounc

morphology and tectonics of the mid atlantic ridge 25 27 30 s from sea beam and magnetic data high resolution sea beam bathymetry and sea surface magnetic data have been collected between 25 s and 27 30 s on the mid atlantic ridge analysis of the morphology and structure of the ridge axis and flanking terrain as well as the results of a three dimensional inversion of sea surface magnetic data are presented this 227 km long portion of the mid atlantic ridge is offset 42 km by the right lateral rio grande transform at 25 40 s and 9 5 km by the left lateral nontransform moore discontinuity at 26 30 s a third nontransform discontinuity the midway discontinuity exists near 26 12 s and is defined by a small displacement of the axial magnetization distribution and the disruption in the along strike continuity of ridge parallel terrain elements although the detailed survey presented in this paper extends out to only 5 to 7 m y old crust a regional compilation of magnetic data from this area by cande et al 1988 indicates that the relative positions and dimensions of the spreading cells have characterized this part of the mid atlantic ridge for at least the past 55 m y the offsets dimensions and geometries however have changed markedly because of prolonged differential asymmetric spreading between segments between and within the accretionary units that are spreading at uniform rates 35 mm yr total opening rate the morphology of the ridge axis varies dramatically from a well defined rift valley 20 25 km wide 1 1 5 km deep to an axial swell that rises 1500 m above the adjacent valley floor the axial magnetic structure is also highly variable along strike the inversion solution shows a series of generally linear short wavelength 10 km magnetization highs within the central brunhes anomaly over each of the four segments in the survey area these anomalies are interpreted to be the central anomaly magnetization high that is observed over the neovolcanic zones of many of the world s mid ocean spreading centers the distinct axial magnetization highs can extend out to 2 m y old crust at ridge discontinuity intersections suggesting that in some instances they may also be the product temporal variations in the geochemistry of the extrusive rocks e g highly fractionated and strongly magnetized basalts enriched in iron and titanium the magnetic inversion shows that the central anomaly is locally attenuated at the minimum depth points of three of the four spreading segments in the survey area both inversion and forward modeling of magnetic data show that topography alone cannot produce the anomalously low values observed in the case of an axial swell centered at 26 s attenuation of the central anomaly is attributed to the effective thinning of the source layer by demagnetization effects of a crustal thermal anomaly and or a zone of substantial alteration taken together the bathymetric data and magnetic modeling document strong along axis gradients in crustal and upper mantle properties

induced magnetization of the oceanic lithosphere and ocean continent magnetization contrast inferred from magsat anomalies a petromagnetic forward model of induced magnetization of iceland plateau is presented magnetic susceptibilities of icelandic rocks from the literature are placed in a vertical stratigraphic context according to a model of igneous and metamorphic layering of the icelandic crust that is compatible with seismic and other geophysical data curie isotherm depths for the magnetic forward model are based on the geothermal gradients measured in drill holes and those derived from magnetotelluric surveys the petromagnetic model is used to predict the satellite altitude 400 km magnetic field of iceland plateau comparison of calculated and magsat anomalies shows that the total induced magnetization the vertically integrated induced magnetization in the surrounding oceanic lithosphere is 2300 a this value is very similar to independent estimates based on rock magnetic models of the oceanic crust rockall plateau is a precambrian microcontinent largely surrounded by oceanic crust a few hundred kilometers south and east of iceland the size age and location of this plateau are ideally suited to characterize the ocean continent magnetization contrast the total induced magnetization contrast between the plateau and its surroundings estimated from the magsat anomaly of rockall plateau is 14 000 a

magsat magnetic anomaly contrast across labrador sea passive margins many passive margins not complicated by nearby anomalous crustal structure have satellite elevation crustal magnetic anomaly contrasts across them that are recognizable in reduced to pole versions of the magsat and pogo data in the labrador sea region this contrast is particularly well developed with strong positive anomalies overlying the continental crust of greenland and eastern canada and prominent negative anomalies situated over the labrador sea and baffin bay we use forward modeling of the large scale crustal bodies in this region continental oceanic passive margin several anomalous structures to show that the magsat anomaly contrast is due simply to the change in crustal susceptibility and thickness at the continental oceanic crustal transition because the thickness varies more than the average susceptibility from continental to oceanic crust the strong anomaly contrast is essentially an edge effect due mostly to the change in crustal structure

geochemical indicators of depositional environment and source rock potential for the upper ordovician maquoketa group illinois basin two depositional cycles are recognized within the upper ordovician maquoketa group of the illinois basin in a core from the new jersey zinc 1 parrish well of fulton county illinois organic carbon corg total sulfur 13c content of the organic carbon dgr13corg hydrogen and oxygen indices hi and oi from rock eval pyrolysis and yields of extractable organic matter eom vary through the cycles dark brown to black laminated shales are present in the lower portion of each cycle and have high values of corg 1 0 3 0 hi 500 1000 mg hydrocarbon hc g total organic carbon toc and eom 500 2500 ppm and more negative dgr13corg values dgr13corg 30 to 30 5 mil gray to greenish gray bioturbated shales are present in the upper portion of each cycle and have low values of corg 1 0 hi 500 mg hc g toc and eom 500 ppm and more positive dgr13corg values 28 5 to 29 5 pmil compared to the laminated shales although thermally immature to marginally mature in this core the laminated shales represent potential source rocks for petroleum because they contain good to excellent quantities of oil prone organic matter and are more deeply buried in other areas of the basin kerogen elemental data and rock eval data suggest that the source of organic matter in the maquoketa was uniform with the notable exception of graptolite rich layers distributions of saturated hydrocarbons for maquoketa samples resemble those derived from amorphous organic matter the organic walled microfossil gloeocapsomorpha prisca was not identified in kerogens of the maquoketa shale and its absence is most likely due to an unfavorable depositional environment and or climate variations in bulk geochemical data and carbon isotopic compositions within the maquoketa group indicate substantial reworking and degradation of organic matter associated with bioturbation and oxygenated depositional conditions organic rich shale was deposited in an environment where circulation of the water column was sluggish and dissolved oxygen concentrations were low resulting in the preservation of organic matter organic poor shale was deposited in an environment where circulation of the water column was strong dissolved oxygen concentrations were high and burrowing organisms thoroughly reworked the sediment resulting in the degradation of organic matter a maximum difference in carbon isotopic ratios of 2 pmil between organic rich and organic poor units of the maquoketa roup suggests that heterotrophic reworking of organic matter was important in the depositional environment the carbon isotopic composition and h c ratio of kerogen for graptolite rich shale suggests that organic productivity may have played an intermittent role in the accumulation of organic matter in the maquoketa shale

european tectonic features observed by magsat abstract regional three dimensional magnetic models have been developed to characterize the principal european long wavelength magnetic anomalies represented on the improved magsat magnetic anomaly map of europe the magnetic models were constrained by regional variations in geology and geophysical parameters e g geologic boundaries crustal thickness heat flow because only limited measurements of magnetization are available on lower crustal and uppermost mantle rock samples our results are useful in constraining and understanding the overall magnetization of these regions illustrations of these include 1 geologic provinces across the tornquist teisseyre tectonic zone 2 regions of thin crust and high mantle heat flow in south central europe 3 the kursk voronezh magnetic anomaly and 4 the ladoga gulf of bothnia zone the region of the tornquist teisseyre tectonic zone that marks the boundary between the fennoscandian baltic shield and metastable europe is a major magnetic discontinuity in south central europe the regional magnetic variations appear to be directly related to variations in the lower crustal thickness and possibly also to heat flow in addition the famous kursk ukraine iron ore deposit produces a prominent bullseye anomaly at satellite altitude the kiruna magsat anomaly is modelled as having a large deep body as its source the high p wave velocity basal crustal layers encountered in rift e g the tornquist teisseyre tectonic zone itself and continental arc e g the ladoga gulf of bothnia zone settings of europe appear to be nearly non magnetic

crustal structure across the brunswick magnetic anomaly offshore georgia from coincident ocean bottom and multi channel seismic data we present results from a coincident wide angle and deep penetrating multichannel reflection seismic experiment conducted on the continental shelf of the southeast georgia embayment over 5000 air gun shots were recorded by a 6 km long towed streamer and six ocean bottom seismic instruments along a 250 km north south profile crossing the brunswick magnetic anomaly bma these data indicate a transition in seismic properties across the bma including higher seismic velocities south of the bma particularly in the upper crust 7 15 km depth and a transition from reflective to transparent crust from north to south we interpret this transition to indicate an increased mafic content in crust south of the bma magnetic modeling based on our seismic results indicates that the bma may be explained as an edge effect anomaly of the more mafic upper crust we suggest that the increased mafic content toward the south is due to rift related mafic intrusion an alternative interpretation is discussed in which the seismic transition across the bma is related to an alleghanian suture athough there is no direct indication in the data for the association of the bma and an alleghanian suture along our profile such an association cannot be ruled out the southern portion of the profile crosses the margin hinge zone where we observe seaward dipping reflectors similar to those observed along the northern blake plateau basin and carolina trough hinge zones which have been interpreted as volcanic sequences our seismic data include no evidence for very high velocity 7 0 km s mid crust to lower crust inboard of the hinge zone such as is observed in the carolina trough outboard of the hinge zone this indicates that the volume of emplaced mafic material changes dramatically across the hinge zone and suggests that rift related magmatic processes along the margin were highly focused

interpretation of the magnetic anomaly over the omaha oil field gallatin county illinois a 40 nanotesla nt magnetic anomaly identified in an aeromagnetic survey over southern illinois contours as a localized magnetic high on the west flank of a regional magnetic low this magnetic anomaly is generally coincident with the omaha oil field in northwest gallatin county illinois it was initially assumed that cultural sources of steel associated with this oil field were the primary source of the magnetic feature however similar oil fields overflown by the survey do not exhibit magnetic anomalies in the data set the luther rister et ux number sign 1 well drilled near the apex of the omaha structural dome encountered two zones of ultramafic intrusive rock containing 9 0 by volume magnetite these intrusives were identified to be alnoeites which are a class of mantle derived ultramafic rock that can be associated with the incipient stages of crustal rifting a ground magnetic survey verified the presence of the anomaly and provided detailed data for 3 d modeling of the source petrophysical evaluations magnetic susceptibility measurements and thin section modal analysis were made on drill cuttings from the ultramafic intrusives encountered in the luther rister number sign 1 well these measurements were made to constrain the 3 d magnetic modeling by the petrophysical characteristicsmore of the source after removal of the regional magnetic field the resulting 140 nt residual magnetic anomaly was successfully modeled using two ultramafic sills with an igneous feeder plug the two igneous sills adequately account for the structural closure exhibited in the omaha oil field and raise the interesting possibility of other hydrocarbon trapping structures generated by intrusives emplaced into the sedimentary section less

a response climatology of idealized midlatitude thermal forcing experiments with and without a storm track several gcm studies indicate that eddy activity may have a considerable influence on the atmospheric response to midlatitude sea surface temperature anomalies the effect of eddy activity on the atmospheric equilibrium response to idealized midlatitude thermal forcing is analyzed for an atmosphere with or without an idealized storm track experiments using a simplified global circulation model forced by thermal anomalies of different sign and location are discussed consistent with the linear theory the geopotential height field displays a baroclinic response with a shallow low high somewhat downstream of the warm cold anomaly farther downstream an equivalent barotropic response occurs with positive negative amplitude increasing with height eddy feedbacks have weak impact on the baroclinic part but the equivalent barotropic response is strongly enhanced if the bandpass filtered streamfunction tendency is in phase with the linear geopotential height response this is the case in an experiment with a warm anomaly near 408n located southwesterly of the idealized storm track in the corresponding experiment with a cold anomaly the two patterns are out of phase and the equivalent barotropic response is slightly reduced it is weakened strengthened if a warm cold anomaly is shifted about 108 poleward or equatorward relative to the idealized storm track midlatitude heat sources generate wave trains that extend equatorward and poleward developing large scale correlations between the flow at remote locations teleconnections the space time variability can be changed considerably by eddy feedbacks developing stronger variance for large scale retrogressive traveling and standing waves partially blocking like events develop

regional study of mineral resources in nevada insights from three dimensional analysis of gravity and magnetic anomalies a three dimensional interpretation of the basins of nevada was developed based on statewide data bases of digital gravity magnetic geologic well and topographic information an iterative technique was applied to isostatic residual gravity anomalies in nevada in order to define the location and shape of pre tertiary basement and to produce a gravity map that reflects variations of density within the pre tertiary basement the dominant feature of the basement gravity of nevada is an enormous area of low gravity that spans the entire state between latitudes 37 n and 40 5 n this regional low strongly correlates with the distribution of middle and late tertiary volcanic rocks and may reflect silicic intrusions within the mid crust and upper crust that are the counterparts of volcanic rocks at the surface although 80 of nevada is covered by cenozoic deposits these deposits are thicker than 1 km over only about 20 of the state the remaining 60 of nevada may have pre tertiary basement rocks within reach of current mineral exploration techniques aeromagnetic profiles from the national uranium resource evaluation nure were analyzed in order to produce a map showing the location of shallow magnetic sources in nevada this analysis shows that 46 of the state has magnetic sources generally mesozoic and cenozoic igneous rocks within 1 km of the surface a linear magnetic anomaly in north central nevada has been interpreted by others as a rift zone active during middle miocene time the rift also is evident in nure magnetic profiles but our interpretation suggests that the magnetic expression of the rift continues south southeast with similar strike to at least 38 n and perhaps to the amagmatic zone lat 37 n the survival since the middle miocene of this narrow crustal feature essentially linear over a distance of 500 km is difficult to interpret in light of later basin and range deformation our analysis of gravity anomalies shows that many deep cenozoic basins are located near the rift yet only two basins cut across it and at least five others change strike near the rift as if to avoid it the rift may have remained linear because it is associated with crustal structures that acted to resist subsequent deformation

barotropic wave propagation and instability and atmospheric teleconnection patterns abstract a global barotropic model linearized about the 300 mb climatological mean january flow is perturbed by applying a series of localized forcings distributed throughout the tropics and subtropics structures which resemble the observed pacific north american and east atlantic teleconnection patterns noted by wallace and gutzler 1981 tend to recur in the responses similar patterns are found to result from the dispersion of isolated initial perturbations placed at a variety of locations in the tropics and midlatitudes it is shown that these structures are related to the most rapidly growing mode associated with barotropic instability of the zonally varying climatological basic state in the absence of damping this mode has an e folding time of about a week and a period close to 50 days in localized regions the instantaneous growth rates can be competitive with those of baroclinic instability these episodes of rapid local barotropic growth are interspersed with intervals in which the local

teleconnections and low frequency variability in idealized experiments with two storm tracks low frequency variability and teleconnection patterns induced by two storm tracks in idealized experiments depend on their zonal distance a separation of about 150 which resembles the northern hemisphere storm track distribution reveals teleconnection patterns which are similar to the pacific north american pna pattern and the north atlantic oscillation nao the nao like structure is associated with retrograde travelling rossby waves barotropic stream function tendencies show that the interaction of the stationary eddies with the low frequency flow and the high frequency eddies contribute to the amplification of the pattern its decay is due to their interaction with the zonal mean flow and the low frequency contribution to the divergence term the pna like pattern has a longer memory and is linked to a quasi stationary wave the stationary wave activity flux shows the dominating influence of baroclinic processes and the other storm track further upstream the correlation between the teleconnection indices is significant for storm track distances between 180 and 160 and vanishes between 150 and 130

persistent anomaly forcing in a two level global circulation model abstract perpetual january data from a simple global two level model are used to diagnose the forcing of the model s low frequency lf large scale flow the forcing is considered within the context of a hypothetical low order model that resolves only the slowly varying planetary scale modes thus the forcing is partitioned into resolved components consisting of lf planetary scale i e autonomous processes and unresolved components that involve high frequency hf transient eddy and lf synoptic scale effects spatial patterns of the net tendencies of low pass filtered planetary scale barotropic streamfunction and vertically averaged temperature are compared with corresponding patterns of resolved and unresolved forcing during the onset maintenance and decay stages of composited persistent anomalies growth stages of positive and negative persistent anomalies that are found in both the atlantic atl and pacific pac regions are dominated by autonomous processes for positive anomalies the n

the dynamics of low frequency variability in a simple model of the global atmosphere abstract we examine the dynamics of low frequency variability in a simple global model with zonally homogeneous boundary conditions low frequency structure comprises trains of nearly equivalent barotropic rossby waves with weak eastward phase velocities and strong eastward group velocities these low frequency eddies are strongly coupled to synoptic activity the flux of vorticity by synoptic waves acts both to reinforce the low frequency eddies and to retard their eastward propagation mechanistic experiments with an imposed forcing show that the synoptic scale vorticity fluxes are organized by the low frequency flow the behavior is generally similar to that found in previous studies of blocks the present work shows that strong mutual interactions between the low and synoptic frequencies also characterize less dramatic low frequency flows

light hydrocarbon gasoline range parameter refinement of biomarker based oil oil correlation studies an example from williston basin abstract we evaluated geochemical compositions of 189 crude oils produced from paleozoic reservoirs across the williston basin emphasis is placed on compositional variations in the gasoline range i c 5 h 12 n c 8 h 18 to verify the biomarker based classification of oil families the oils belong to four distinct compositional oil families a b c and d broadly confined to specific stratigraphic intervals the unique character of each oil family evident from their n alkane and biomarker signatures is supported by distinctive gasoline range characteristics in general and c 7 mango parameters in particular an invariance in the k1 parameter among oils from a single compositional group is observed for most of the oils the k1 ratio although relatively constant within each suite of oils is different for each oil family clearly indicating their compositional distinction other mango parameters n2 p2 p3 show a similar reflection of the oil families however while c 7 parameters provide excellent evidence for distinct familial association of oils from families a b and d family c often overlaps with the latter two families perhaps indicating greater genetic and source heterogeneity in the family c oils nevertheless differences in the gasoline range composition suggest that the existing biomarker based classification of oil families can be more universally applied throughout the entire williston basin moreover because the light hydrocarbon parameters prove very useful in refining oil oil correlations routine gasoline range analysis shows good potential as a supplementary component in geochemical correlation of crude oils especially when high levels of thermal maturity decrease the usefulness of biomarker compounds

the seafloor spreading rate dependence of the anomalous skewness of marine magnetic anomalies summary we have determined the anomalous skewness of marine magnetic anomalies other than anomaly 33 34 in slow spreading regimes in the arctic atlantic and indian oceans and found a significant dependence of the anomalous skewness on the spreading rate the anomalous skewness is highest for slow spreading and insignificant for spreading above 6 cm yr 1 also the anomalous skewness of anomaly 33 34 is significantly larger than that of anomalies 25 26 and 20 21 whether or not anomalies m2 m4 exhibit anomalous skewness is uncertain because of the large variability in their shapes to facilitate the study of anomalous skewness of marine magnetic anomalies we have developed an analytic method based on functions calculated directly from the magnetic profiles that does not require a priori knowledge about the exact locations of magnetization contrasts

empirical observations relating near surface magnetic anomalies to high frequency seismic data and landsat data in eastern sheridan county montana our empirical synergistic correlations of aeromagnetic and seismic data and a landsat lineament interpretation revealed lineations on the magnetic map that have expression on seismic sections we observed a conjugate set of northwest southeast and northeast southwest trending magnetic lineaments zones which offset and truncate near surface magnetic anomalies we believe these ozs offset zones represent lateral faults in a wrench fault system lateral offsets appear to be 100s of meters to a few kilometers fractions of a mile to a few miles we observed a direct correlation between ozs and vertical faults in seismic data faults on seismic sections extend from near the surface to near the seismic basement the faults are most pronounced in the upper cretaceous reflectors and seem to disappear with depth fault throws are inconsistent reversing throw across faults ozs trend northeast southwest in the north half of the study area and both northeast southwest and northwest southeast in the south half

organic geochemistry of impactites from the haughton impact structure devon island nunavut canada organic matter in impactites from the 24 km wide and 39 ma old haughton impact structure canadian high arctic is a mixture of fossil and modern biological components the fossil component represents a conventional oil that was generated from lower palaeozoic marine source material before impact and permeates bedrock dolomites biomarker maturity parameters record the thermal effect of the mid tertiary impact maturity influenced sterane rearranged hopanoid and triaromatic steroid ratios all increase towards the centre of the impact structure where thermal alteration was greatest the heating was probably dominated by an impact related hydrothermal system as such systems last long enough for kinetically based thermal alteration to occur kinetically related biomarker data suggest that the hydrothermal heating lasted for c 5000 years biomarkers are also preserved in dolomite clasts within impact melt breccia and indicate strong thermal alteration modern biological contamination of the rocks is responsible for the superposition of two geochemical signatures which could be cyanobacteria non marine algae or higher plant matter onto the fossil component but they can be recognized and distinguished the data show that the impact structure system holds a record of both the pre impact organic signature and the thermal signature of the impact and thereby indicates that organic geochemistry is a valuable tool in documenting the response of rocks to impacts

fine scale magnetic anomaly field over the southern juan de fuca ridge axial magnetization low and implications for crustal structure sea surface deep tow and submersible magnetic surveys show that the spreading axis of the cleft segment of the southern juan de fuca ridge sjdf is overlain by a distinctive magnetic anomaly low in contrast to the more typical magnetic anomaly high found over the adjacent vance segment and most mid ocean ridge spreading centers forward iterative thickness modeling based on the near bottom magnetic data shows that this axial magnetic low can be produced by a thin magnetic source layer beneath the cleft axial valley thick crust is required to produce the magnetic anomaly high over the vance axial valley this magnetic modeling demonstrates that variations in the base of the magnetic source layer which is assumed to be equivalent to the base of the extrusive lavas can produce a significant proportion of the observed short wavelength magnetic anomaly signal these magnetic layer thickness models also correlate quite well with nearby seismic results which show that seismic layer 2a thins beneath the axial valley of the cleft and is thicker beneath the vance axial valley recent seismic results over other fast and medium spreading rifts also show rapid changes in the thickness of the uppermost crust near the axis of spreading which could produce significant magnetic contrasts the results of this study and the correlation with independent seismic measurements suggest that while magnetization may vary due to geomagnetic behavior or because of petrology or alteration first order change in upper crustal thickness is one of the most important variables to consider in the production of fine scale magnetic anomalies in young ocean crust

a unified three dimensional instability theory of the onset of blocking and cyclogenesis ii teleconnection patterns abstract the instability characteristics of three dimensional northern hemisphere average winter tropospheric flow are examined in a two layer spherical quasi geostrophic model all the growing modes from the fastest down to the first stationary but growing mode have been examined for three cases 1 2a and 3 having increasingly larger static stability parameters comparisons with observations and with the results of the corresponding barotropic model are presented it is found that the modes for cases 1 2a and 3 can be divided into six approximately distinct classes class a modes are rapidly propagating monopole cyclogenesis disturbances with largest amplitudes in the geographical locations of the storm tracks in the pacific and atlantic oceans class b and c modes are respectively pacific and pacific and atlantic onset of blocking dipole modes with longer periods than class a modes and which like class a disturbances have westward tilts with height they also have largest amplitudes upstream of the

comment on comparison of michigan basin crude oils by vogler et al abstract data is presented for michigan basin oils that indicate in addition to the three distinct oil types oils of mixed composition intermediate between the silurian and ordovician types also exist these mixed oils which occur in devonian detroit river reservoirs are identified primarily by their isoprenoid and normal paraffin compositions

source rock characterization and oil to source rock correlation of a cambrian ordovician fold and thrust belt petroleum system western newfoundland abstract in this study we use biomarker and isotope data to address the complexities of petroleum system analysis in fold thrust systems specifically related to hydrocarbon generation and migration such as variable thermal maturities of source rocks complex migration pathways and mixing source rocks extracts and oil samples are taken from paleozoic rocks along the appalachian structural front in western newfoundland oil seeps along this fold and thrust belt have motivated episodic exploration efforts over the last 150 years however economic development has been unsuccessful to date in part because of the complex nature of oil to source relationships pyrolysis analyses identify promising source intervals in lower ordovician floian formations with an excellent source potential containing type i ii organic matter toc up to 9 35 hi up to 840 a second good source interval identified within the late cambrian furongian continental slope and rise sediments contains type ii iii organic matter toc up to 2 34 hi 380 source rock samples from outcrops are marginally mature to mature with tmax values between 436 and 447 geochemical analyses of source rock extract imply a clastic shale dominated source rock with a minor contribution from carbonate source rocks chemometric analyses yield 3 extract groups groups 1 and 2 contain lower ordovician samples with organic matter derived from oxidized micro plankton with varying amounts of gleocapsomorpha prisca extract group 3 originates from cambrian samples and shows evidence of a bacterial derived organic matter similar genetic relationships among 10 oil samples from natural oil seeps and abandoned well sites are indicated by high resolution biomarker analyses seven oil samples originated from ordovician source rocks that produced high api oils with low bisnorhopane hopane tricyclic hopane ratios and high sterane concentrations supporting interpretation of a source containing algal derived organic matter three oils originated from cambrian shales these high api oils show high bisnorhopane hopane and tricyclic hopane ratios low sterane concentrations and low carbon isotope values all indicative of organic matter derived from bacteria generated over a wide range of maturities 0 98 1 26 ro thermal maturity sensitive biomarkers and naphthalene ratios show that source rocks generated oils with a wide range of maturities related to their dipping character in the imbricated thrust stack the cambrian source generated oils that locally experienced secondary cracking during acadian inversion whereas the ordovician source generated oil with low maturities after the acadian inversion oil operaters exploring fold and thrust belt petroleum systems may expect extreme changes of oil properties for example api gravity in spacially restricted areas

long term variations in magnetization intensity with crustal age in the northeast pacific atlantic and southeast indian oceans the magnetization intensity of oceanic crust in the northeast pacific atlantic and southeast indian oceans was calculated from marine magnetic anomaly data to investigate long term variations in magnetization intensity with crustal age the magnetization intensity was derived by two dimensional inversion of the upward continued marine magnetic anomalies the principal magnetization intensity variations versus age are summarized as follows 1 in all the oceans a high magnetization intensity exists at the ridge crest followed by a sharp decrease within 8 my and a minimum is reached at 8 20 ma 2 in the northeast pacific and atlantic oceans the magnetization intensity gradually increases up to at least 70 ma this variation pattern is consistent with the previous results of nrm measurements of drilled rocks and inversion of marine magnetic anomalies in the south atlantic

organic geochemistry and oil source correlations paleozoic of ohio routine analytical methods and new kerogen kerogen pyrolyzate techniques used for detailed petroleum geochemical studies permit the correlation of oils to their respective source rocks in ohio in the paleozoic section four good to excellent source units were identified the mississippian sunbury shale the devonian ohio shale and olentangy shale and the shales of the ordovician point pleasant formation these marine shales are dominantly oil prone with maturation levels that span the immature to peak oil generation range no source rocks examined exceeded an ro of 1 0 1 1 or equivalent the reservoired oils in ohio are mostly supermature based on normal alkanes pristane n c17 to phytane n c18 ratios low asphaltene and sulfur contents and high paraffin indices three basic oil groups were identified using oil characterization procedures group 1 consists of cambrian ordovician and some silurian oils group 2 consists of some silurian devonian one mississippian and some pennsylvanian oils and group 3 consists of oil from the mississippian berea sandstone oil source rock correlation techniques permitted the assignment of probable source rocks to each oil group group 1 oil was probably derived from the point pleasant shales group 2 oil was derived from the ohio shale and olentangy shale and group 3 oil came from the sunbury shale because the supermature oils in ohio occur in reservoirs with maturities generally under 0 8 ro the oils would have had to migrate from deep within the appalachian basin migration could have followed fracture zones unconformities or extensive and permeable units such as the silurian clinton sands

degradation et lessivage des hydrocarbures de la formation ordovicienne de thumb mountain encaissant le gite zn pb de polaris territoires du nord ouest canada carbonate rocks of the ordovician thumb mountain formation host to the zn pb polaris deposit contain hydrocarbons that show types of alteration classically attributed to biodegradation and water washing the hydrocarbons of the upper part of this formation and of the overlying irene bay formation indicate alterations due to water washing only the hydrocarbons of the impermeable shales of the overlying cape phillips formation display indices of only higher thermal maturity than the underlying units contrary to classical concepts of hydrocarbon biodegradation n alkanes cyclohexylalkanes even isoprenoids and perhaps also steranes seem to have been degraded simultaneously and not successively in this sequence this alteration process is mainly responsible for a log linear decrease of the amounts of n alkanes and cyclohexylalkanes with increasing depth the severe and uniform alteration of aromatic hydrocarbons throughout the interval which is opposite to the progressive alteration of associated n alkanes can be attributed solely to water washing this conclusion necessitates a reconsideration of previous interpretations attributing the loss of short side chain substituted polyaromatic compounds to microbial activity hopanes tri and tetra cyclic terpanes as well as aromatic steroids and hopanoids seem to have been unaffected by the alteration phenomena the increase in the degree of alteration of the hydrocarbons with increasing depth implies that the responsible migrating fluid circulated per ascensum

low frequency variability in a simplified atmospheric global circulation model storm track induced spatial resonance the interaction between two storm tracks is analysed by using idealized experiments with a simplified global circulation model a zonal distance of 180 between the storm tracks drives a retrograde travelling zonal wave number two wave with an amplitude modulation of about 50 days the stream function tendency equation demonstrates that the amplitude modulation is excited in the storm track regions by the high frequency periods between two and eight days eddy forcing spatial resonance of a retrograde propagating zonal wave number two rossby wave and high frequency eddy forcing are proposed as the amplifying mechanisms while friction and self interaction between the low frequency eddies ensure damping sensitivity experiments indicate that the pronounced modulation of 50 days vanishes with decreasing zonal distance between the storm tracks thereby enhancing the intensity and duration of blocking events

molecular indicators of redox and marine photoautotroph composition in the late middle ordovician of iowa u s a abstract saturated and aromatic hydrocarbons were used to evaluate depositional redox conditions and marine photoautotroph contributions to middle caradocian strata of the central united states ia at the base of the spechts ferry member of the decorah formation 13c enriched aryl isoprenoids derivatives of green sulfur bacteria become abundant indicating the development of photic zone anoxia this is coincident with the disappearance of the organic walled microfossil gloeocapsomorpha prisca and a marked decrease in the relative abundances of cyanobacterial biomarkers the development of dysoxic to anoxic conditions and or associated changes in basin circulation potentially affected the distributions and abundances of these organisms in the overlying guttenberg member g prisca derived organic matter becomes dominant but relative cyanobacteria abundances remain low in addition the percentage total organic carbon is greater than 20 even though selected biomarker ratios pristane phytane ratios greater than 3 and homohopane indices less than 0 5 and the presence of bioturbation indicate that bottom waters were oxygenated it is suggested that deposition of g prisca affected both organic matter preservation and depositional redox conditions observed variations in redox indicators and marine photoautotroph contributions are associated with changes in siliciclastic deposition reported macrofauna turnover and with evidence for oceanic cooling and a change in circulation patterns documented in the eastern united states

geochemistry and possible origin of crude oils from bashituo oil field tarim basin the entire range of hydrocarbons in bashituo oils from devonian and carboniferous reservoirs were investigated to determine their origin as well as their alteration mixing and maturity using gas chromatography gas chromatography mass spectrometry and stable carbon isotope analysis these geochemical studies indicate that the crude oils in the devonian and carboniferous reservoirs share similar c15 molecular marker compositions of marine organic matter origin whereas their light hydrocarbon compositions show distinct differences the carboniferous reservoired oils are characterized by a relatively greater abundance of cycloalkanes and methylcyclohexane which indicate a terrestrial organic matter om contribution based on biomarker and carbon isotope analysis cambrian ordovician j o marine rocks are assumed to be the main source for both the devonian and carboniferous reservoired oils whereas carboniferous rocks with terrestrial om input also contributed to the carboniferous reservoired oils the coexistence of 25 norhopanes evident humps from unresolved complex mixture and intact n alkanes in devonian reservoired oils indicate a mixture of early charged biodegraded oils with late fresh oils corresponding to at least two oil generation episodes by the j o rocks light hydrocarbon indicators suggest a relatively high maturity beyond peak oil generation for the j o sourced late fresh oil whereas c15 molecular marker parameters indicate a maturity equivalent to early peak oil generation for the j o sourced early charged biodegraded oil the maturity of the carboniferous sourced oil is equivalent to peak oil generation the application of the entire range of hydrocarbons is essential when assessing a mixed or altered oil system because light hydrocarbons and biomarkers may yield different source oil correlations and maturities

mechanisms of low frequency variability in a simple model with orography abstract mechanisms of low frequency variability are examined in an extended run of a two level nonlinear global model with a single isolated mountain the presence of the mountain modifies the low frequency behavior compared with that in a zonally homogeneous model in two ways first slow variations in the zonally averaged circulation the zonal index are manifest as variability in the train of stationary waves generated by the orography second trains of low frequency rossby waves are modified by their propagation through the model s zonally asymmetric time mean flow these eddies become meridionally elongated at the jet entrance and zonally elongated at its exit allowing them to extract energy barotropically from the mean flow in the jet exit

storm track organization and variability in a simplified atmospheric global circulation model a storm track is simulated in a coarse resolution multilevel primitive equation model with linear surface friction and heating terms a restoration temperature distribution consisting of a dipole embedded in a zonally symmetric profile forces the model to simulate the surface heating three simulations each with a different dipole orientation are performed to test the sensitivity of storm track organization to the external forcing field the climatological time mean circulation and the transient disturbances of the reference simulation agree well with observations of northern hemisphere wintertime storm tracks local energetics show that baroclinic instability is responsible for the enhanced eddy kinetic energy downstream of the jet and the downstream end of the storm track results from the barotropic conversions of eddy kinetic energy to the time mean flow low frequency fluctuations with a period of about 50 days associated with a retrograding large scale wave pattern are identified by a complex empirical orthogonal function analysis of the vertically averaged stream function a composite life cycle of the low frequency variability reveals the growth and decay of a blocking anticyclone downstream of the storm track a cyclogenesis initiated by an eastward propagating wave train is observed a few days before the amplitude of the low frequency anomaly attains its peak the net forcing of the high frequency eddies contributes to the growth and decay of the blocking anticyclone the results suggest that the occurrence of the blocking like event is part of the storm track dynamics

aeromagnetic study of the island of hawaii an aeromagnetic study of the island of hawaii provides new insight on magnetic properties of subsurface rock and geologic structure on a regional scale spectral depth analysis delineates two shallow magnetic zones each roughly 1 5 km thick lying at a depth of 1 km one zone of unknown origin lies in the center of the island and correlates with a regional magnetic high the other zone coincides with pronounced magnetic lows paralleling kilauea s active east rift zone these magnetic lows probably depict rocks chemically altered by hydrothermal fluids in which titanomagnetite has been destroyed analysis of magnetic terrain effects indicates that magnetization also decreases with depth within mauna kea and mauna loa we estimate that magnetization is reduced by about half at a depth of 1 km the magnetic method is particularly useful in delineating the lateral extent of local shield structures such as rifts summit calderas pit craters and vent fissures rifts possess characteristic magnetic patterns primarily long wavelength linear magnetic low zones we propose alteration processes reduce magnetizations along the flanks of rifts on the other hand along young rifts e g kilauea s east rift zone short wavelength magnetic anomalies probably reflect slowly cooled unaltered intrusions altered rock may also produce magnetic lows that help define buried summit calderas

seismotectonic significance of the 29 january 1989 etne earthquake sw norway the integrated analysis of geological seismological and field observations with lineament data derived from satellite images allows the identification of a possible seismogenic fault zone for an earthquake which occurred near etne in southwestern norway on 29 february 1989 the hypocentre of the earthquake was located at the mid crust at a depth of 13 8 0 9 km which is typical of small intraplate earthquakes the etne earthquake occurred as a result of normal faulting with a dextral strike slip component on a nw se trending fault available geological and lineament data indicate correlation of the inferred seismogenic fault with the nw se trending etne fault zone an aeromagnetic anomaly related to the etne fault zone forms a regional feature intersecting both precambrian basement and allochthonous caledonian rocks based on these associations the occurrence of the etne event is ascribed to the reactivation of a zone of weakness along the etne fault zone slope instabilities developed in the superficial deposits during the etne event demonstrate the existence of potentially hazardous secondary effects of such earthquakes even in low seismicity areas such as southwestern norway

the continuum and dynamics of northern hemisphere teleconnection patterns this study presents an alternative interpretation for northern hemisphere teleconnection patterns rather than comprising several different recurrent regimes this study suggests that there is a continuum of teleconnection patterns this interpretation indicates either that 1 all members of the continuum can be expressed in terms of a linear combination of a small number of real physical modes that correspond to basis functions or 2 that most low frequency patterns within the continuum are real physical patterns each having its own spatial structure and frequency of occurrence daily ncep ncar reanalysis data are used that cover the boreal winters of 1958 97 a set of nonorthogonal basis functions that span the continuum is derived the leading basis functions correspond to well known patterns such as the pacific north american teleconnection and north atlantic oscillation evidence for the continuum perspective is based on the finding that 1 most members of the continuum tend to have similar variance and autocorrelation time scales and 2 that members of the continuum show dynamical characteristics that are intermediate between those of the surrounding basis functions the latter finding is obtained by examining the streamfunction tendency equation both for the basis functions and some members of the continuum the streamfunction tendency equation analysis suggests that north pacific patterns basis functions and continuum are primarily driven by their interaction with the climatological stationary eddies and that north atlantic patterns are primarily driven by transient eddy vorticity fluxes the decay mechanism for all patterns is similar being due to the impact of low frequency period greater than 10 days transient eddies and horizontal divergence analysis with outgoing longwave radiation shows that tropical convection is found to play a much greater role in exciting north pacific patterns a plausible explanation for these differences between the north atlantic and north pacific patterns is presented

dynamics of low frequency variability barotropic mode the dynamics of low frequency variability is investigated in terms of sensitivity to the dependence of its relative zonal position of a single storm track in a simplified global circulation model a streamfunction tendency equation is derived that explicitly distinguishes between barotropic and baroclinic components one of the two low frequency patterns located in the center of the storm track grows barotropically and its decay is accomplished by low frequency eddy vorticity fluxes and the baroclinic contribution to the divergence term the growth and decay of the second pattern located at the downstream end of the storm track is dominated by nonlinear processes this indicates that the dynamical processes leading to growth and decay of low frequency patterns depend on the zonal position of the low frequency pattern relative to a storm track

skewness of marine magnetic anomalies created between 85 and 40 ma in the indian ocean we have performed a detailed analysis of the skewness of marine magnetic anomalies in indian ocean basins created between 85 and 40 ma as a result of the northward motion of india visual and semiautomated methods of skewness determination were applied to the data both provide consistent results but the visual method is prefered for its ability to deal with noisy data plots of apparent effective remanent inclination or skewness corrected for present geomagnetic field inclination versus time for conjugate basins display the combination of three effects a gradual increase with time related to the northward motion of the ridges attached to india in the geomagnetic reference frame a gap between conjugate curves which represents anomalous skewness senso stricto and short period fluctuations which represent the sequence effect i e the effect of neighboring magnetic sources on the skewness of a given anomaly the anomalous skewness decreases with faster spreading rate and completely disappears above 50 km m y an observation which negates geomagnetic field behavior as a possible cause of the observed anomalous skewness

organic geochemistry of mid continent middle and late ordovician oils ordovician oils in mohawkian and cincinnatian reservoirs of the united states mid continent retain the biochemical imprint of middle and upper ordovician oceanic life before the evolution of land plants and most vertebrates thus these oils have some geochemical features that distinguish them from younger oils these features include 1 a predominance of n c15 n c17 and n c19 alkanes in the saturated hydrocarbon fraction 2 relatively low amounts of longer chain n alkanes 3 low amounts of chlorophyll derived isoprenoids such as pristane and phytane and 4 abundant c29 sterane relative to c27 with rearranged forms diasteranes predominant over normal steranes ordovician oils also generally contain little sulfur and have a somewhat variable light stable carbon isotopic composition with dgr13csat and dgr13caro values of 28 to 31 pmil pdb but these features are typical of many marine oils the unusual chemistry of these ordovician oils supports the interpretation of reed illich and horsfield 1986 that prokaryotic organisms provided the organic matter for most ordovician oils although their claim for gloeocapsamorpha a problematic unicellular prokaryote possibly a blue green alga or an unusually large bacterium cannot be proven from oil chemistry alone knowing that indigenous mid continent ordovician oils were derived from prokaryotic organisms may aid in future exploration for these reservoirs

high magnetization zones near spreading center discontinuities abstract three dimensional inversions of the magnetic field reveal the presence of zones of high magnetization over the propagating limb of large spreading center discontinuities the presence of high magnetization zones is confirmed by rock magnetic measurements where available magnetization highs are associated with basalts which tend to have high fe contents and low mg numbers these data suggest that high magnetization zones observed over large ridge axis discontinuities are associated with highly differentiated basalts enriched in iron following christie and sinton 1 such highly evolved basalts may be the result of shallow level crystal fractionation in small magma bodies with a low supply rate these small magma bodies are postulated to correspond to the first stages in the development of a sub axial magmatic system as a result of the propagation of one of the limbs of the offset into older lithosphere because high magnetization zones at large ridge axis discontinuities often correspond to gaps in the along axis extent of a seismically detectable magma chamber these magma bodies may be smaller than a few hundred meters rock magnetic measurements suggest that the enrichment in iron associated with increased differentiation may be accompanied in a few cases by an increase in the concentration of titanomagnetite within the basalts and in the magnetization of the rocks however the exact relationship between high magnetization intensities and iron enrichment is complex and unclear and may be significantly affected by factors such as magnetic mineralogy and crystallization history

deducing anomalous wave source regions during the life cycles of persistent flow anomalies abstract anomalous wave source regions are identified during the life cycles of persistent flow anomalies occurring over the north pacific and north atlantic oceans during boreal winter these cases project strongly upon the pacific north american and eastern atlantic teleconnection patterns respectively and represent two of the primary modes of intraseasonal low frequency variability in the northern hemisphere wintertime circulation in the upper troposphere these cases are manifested by wave trains of large scale and large amplitude flow anomalies extending downstream from key regions near the aleutian islands and south of iceland respectively the occurrence of persistent flow anomalies is closely linked to persistent extremes in surface weather and to variations in the skill of extended range weather forecasts black and dole performed a detailed diagnostic analysis of the onset of persistent cyclonic flow anomalies over the north pacific a key result of their study was that wave activity flux di