chemical evolution of magmas in the proterozoic terrane of the st francois mountains southeastern missouri 1 field petrographic and major element data major element chemical data for plutonic and volcanic rocks of the eastern st francois mountains indicates that ring plutons of the butler hill caldera are low silica granites and chemically distinct from the high silica butler hill granite and grassy mountain ignimbrite additionally the ring plutons silvermine stono knoblick and slabtown granites are characterized by abundant mafic inclusions whereas these are almost completely absent in butler hill granite silvermine granite is mostly of a medium grained variety but cliffs along the st francis river expose gradations upward to a high silica fine grained cap rock least squares mixing calculations indicate that the cap rock could have formed by the fractional crystallization of plagioclase hornblende biotite magnetite and sphene all of which are liquidus phases in the medium grained silvermine granite the chemical similarity of the fine grained cap rock to butler hill granite and grassy mountain ignimbrite suggested that silvermine granite might be a sample of the parental magma from which the high silica rocks were derived least squares mixing calculations indicate that butler hill granite magma could have been derived from the composition of the average medium grained silvermine by fractional crystallization of the same liquidus phases indicated above the major element composition of grassy mountain ignimbrite can be derived from the average butler hill granite magma by fractional crystallization of plagioclase alkalic feldspar quartz biotite and magnetite all of which are liquidus phases in butler hill granite textural and chemical variations in butler hill granite from northeast to southwest suggest that the body was internally differentiated and that the present distribution of rock types is due to tilting and erosion the internal zonation of butler hill granite can be accounted for by fractional crystallization of plagioclase alkalic feldspar quartz biotite sphene and magnetite the chemical data and calculations indicate that the primary magma emplaced at the beginning of igneous activity was of silvermine type the magma crystallized fractionally so that the magma chamber became compositionally zoned with high silica magma in the upper part the top most part of this material was erupted to form grassy mountain ignimbrite during formation of the butler hill caldera subsequently more of the high silica magma was emplaced into the volcanic material in the caldera during resurgent doming after formation and resurgent doming of the butler hill caldera some of the parental magma from deeper in the chamber leaked upward into the ring fracture or into arcuate faults within the caldera forming the ring plutons the abundant mafic inclusions in the ring plutons may be earlier crystallized magma carried upward during intrusion one of the ring plutons silvermine granite apparently formed a sill like intrusion and underwent in situ differentiation a body of foliated granodiorite exposed as a giant inclusion in granite at hawn state park east of the principal exposures of the st francois mountains has the mineralogy major element chemistry and trace element chemistry to be a sample of the cumulate resulting from the processes in the magma chamber

mineral chemistry and geochemical behavior of hydrothermal alterations associated with mafic intrusive related au deposits at the atud area central eastern desert egypt abstract vein type gold deposits in the atud area are related to the metagabbro diorite complex that occurred in gabal atud in the central eastern desert of egypt this gold mineralization is located within quartz veins and intense hydrothermal alteration haloes along the nw se brittle ductile shear zone as well as along the contacts between them by using the mass balance calculations this work is to determine the mass volume gains and losses of the chemical components during the hydrothermal alteration processes in the studied deposits in addition we report new data on the mineral chemistry of the alteration minerals to define the condition of the gold deposition and the mineralizing fluid based on the convenient geothermometers two generations of quartz veins include the mineralized grayish to white old vein trending nw se and the younger non mineralized milky white vein trending ne sw the ore minerals associated with gold are essentially arsenopyrite and pyrite with chalcopyrite sphalerite enargite and goethite forming during three phases of mineralization first second main ore and third supergene phases three main hydrothermal alteration zones of mineral assemblages were identified zones 1 3 placed around mineralized and non mineralized quartz veins in the underground levels the concentrations of au ag and cu are different from zone to zone having 25 790 ppb 0 7 69 6 ppm and 6 93 8 ppm 48 6 176 1 ppb 0 9 12 3 ppm and 39 6 118 2 ppm and 53 9 155 4 ppb 0 7 3 4 ppm and 0 2 79 ppm for zones 1 2 and 3 respectively the mass balance calculations and isocon diagrams calculated using the geoiso windows program revealed the gold to be highly associated with the main mineralized zone as well as sericitization kaolinitization and muscovitization in zone 1 more than in zones 2 and 3 the sericite had a higher muscovite component in all analyzed flakes average x ms 0 89 with 0 10 0 55 phengite content in wall rocks and 0 13 0 29 phengite content in mineralized quartz veins wall rocks had higher calcite caco 3 contents and lower mgco 3 and feco 3 contents than the quartz veins the chlorite flakes in the altered wall rocks were composed of pycnochlorite and ripidolite with estimated formation temperatures of 289 295 c and 301 312 c respectively albite has higher albite content 95 08 99 20 which occurs with chlorite in zone 3

tectonically buried continent ocean boundary ouachita mountains arkansas a seismic reflection sequence observed at depth beneath the south flank of the benton uplift in arkansas is interpreted to mark the early paleozoic continent ocean boundary the sequence is similar in both external and internal geometry to wedge shaped features commonly observed in the narrow zone separating continental from oceanic basement on modern passive margins and interpreted as layered volcanic sequences it is suggested that slope and rise sediments now exposed in the benton uplift were thrust in a thin skinned fashion over the edge of the north american continent during initial stages of the carboniferous ouachita orogeny but that later uplift and slight northward translation of the continental margin basement occurred in a more thick skinned fashion preserving the boundary wedge intact beneath the earlier thrust sheets

cocorp seismic profiling of the appalachian orogen beneath the coastal plain of georgia a southeastward extension onto the coastal plain of an earlier cocorp traverse which confirmed large scale thin skinned thrusting of crystalline rocks of the southern appalachians has provided some of the most spectacular reflections yet seen in crustal seismic data most of the reflectors can be interpreted as either fault surfaces or as metamorphosed strata of late precambrian early paleozoic age they are consistent with the hypothesis that a major detachment extends eastward beneath this part of the orogen although other interpretations with a more complex pattern of detachments or sutures are also possible large scale overthrusting provides a mechanism for incorporating sedimentary rocks into the lower crust and may help to explain many of the layered features on crustal seismic data reflections from deep beneath the coastal plain indicate that the structural configuration of the rocks is complex and that the remains of a collision zone are being observed several east dipping horizons which bear strong similarities to thrust faults in valley and ridge sedimentary rocks are seen in the basement at shallow and mid crustal levels beneath the coastal plain the augusta fault for example displays a reflection which extends at a low angle some 80 km or more southeast of its surface position in conjunction with surface geologic information these new data demonstrate that late paleozoic compressive deformation was pervasive and resulted in lateral movements in the upper crust extending from the valley and ridge to the crystalline rocks beneath the coastal plain a distance of 400 km or more a large antiform cresting at about 2 3 sec or about 6 km below the surface and other structures beneath the coastal plain of georgia deserve further consideration for petroleum exploration although metamorphism may have eliminated petroleum from these rocks refracted arrivals and fault geometries indicate two triassic rift basins beneath coastal plain sedimentary rocks one of which has apparently not been recognized previously

on isostasy at atlantic type continental margins cross spectral techniques have been used to analyze the relationship between free air gravity anomalies and topography of approximately 100 profiles over the continental margins of eastern north america southwest africa and the coral sea lord howe rise the resulting admittance functions for these margins have been interpreted in terms of the flexure model of isostasy the relatively young coral sea lord howe rise margin is associated with the lowest value of the effective elastic thickness te 5 km while the relatively old eastern north american margin is associated with the highest value 10 te 20 km thus te appears to increase with the age of the margin these results which are consistent with flexure studies from the major ocean basins have been interpreted in terms of a simple model in which the flexural strength of the lithosphere increases with age the model suggests that a low rigidity plate or airy type model is most applicable early in rifting history while a high rigidity plate is more applicable later in margin evolution thus sediments at a margin initially load a relatively thin and hot plate while sediments later in margin evolution load a relatively thick and cold plate the model explains a number of features of the tectonic stratigraphic evolution of margins these include a change in flexural style from a narrow and deep basin to a broad and shallow basin coastal plain onlap and uplift in the continental interior furthermore the flexure model of isostasy contributes in a major way to observed free air gravity anomalies at continental margins in particular the model explains the existance of airy isostatic gravity anomalies at margins the amplitude and wavelength of these anomalies are a function of the sediment supply the type of mechanisms controlling sediment deposition up building and or out building and the thermal structure history and age of the margin

seismic reflections from the basal contacts of batholiths although seismic reflection data in batholithic terranes are scarce datasets available to us from three different regions show surprising similarities all three profiles one across precambrian rocks in texas and two across mesozoic batholiths in california and nevada show strong subhorizontal reflections at depths of 6 10 km these reflections are interpreted to originate from the base of the batholiths and indicate that these batholiths are tabular in shape the large amplitudes of the reflections require a marked contrast in acoustic impedance product of velocity and density between the batholith and the underlying rocks two of the profiles show reflective zones having appreciable thicknesses 0 5 2 5 km while the third profile shows a single continuous reflection the four most favored candidates for sub batholithic reflections are 1 underplated gabbro 2 cumulate layers 3 migmatites and 4 thrust faults cumulate layering is the preferred interpretation where reflectors constitute a thick package and are laminated in appearance

subsurface exploration via satellite structure visible in seasat images of north sea atlantic continental margin and australia satellite observations of the sea surface height are sensitive to crustal structure overcoming the inability of previous remote sensing techniques to penetrate the earth s surface seasat gravity images have been compared with the structure of offshore sedimentary basins in northwestern europe seasat images display primary structural features e g the anglo dutch and smaller basins down to the scale of the manx furness basin and numerous secondary structural features e g the halibut horst additionally because of their synoptic viewpoint seasat images reveal features hard to distinguish i cite as an example a basement ridge extending from scotland to norway in position to have controlled marine circulation during deposition of the principal source rock wh ch is equivalent to the kimmeridge clay in north america and south america seasat images have been used to examine the relation of the atlantic fracture zones to the structure of the continental margin of the united states and brazil in australia known basins of the northwest continental margin and the bass strait have been compared with seasat projections of shallow structure in comparison with reconnaissance seismic magnetics and ship borne gravity surveys seasat images are valuable in discovering the presence of thick sediment accumulations seasat images are potentially more valuable than seismic and magnetics in the presence of basement masking by intrasectional volcanics or massive carbonates in known basins seasat data deliver a uniform synoptic view permitting the identification of large structural features in this form these images give the structural geologist a tool like that given the stratigrapher by regional seismic sections in remote unexplored regions seasat images provide a means of optimizing the layout of reflection seismic the gravity data from seasat offer the first of increasingly accurate structural images to be expected from altimetric satellites

crustal structure beneath the southern appalachians nonuniqueness of gravity modeling gravity models computed for a profile across the long wavelength paired negative positive bouguer anomalies of the southern appalachian mountains show that the large negative anomaly can be explained by a crustal root zone whereas the steep gradient and positive anomaly east of the root may be explained equally well by three different geometries a suture zone a mantle upwarp or a shallow body seismic data support the existence of a mountain root but are inadequate to resolve differences among the three possible geometries for the positive anomaly the presence of outcropping mafic and ultramafic rocks in the southern appalachians and the inferred tectonic history of the appalachian orogen are most consistent with the suture zone model crust similar to continental crust probably exists beneath the coastal plain and inner continental shelf where the gravity anomalies return to near zero values

geophysics of proposed proterozoic sutures in canada abstract sutures have been identified or proposed at several sites in the canadian shield although many authors have suggested that some form of primitive plate tectonic processes have played a major role in the formation of the archean crust of the superior structural province proterozoic sutures resembling phanerozoic examples have been proposed only within the younger churchill and grenville provinces and at or near the boundaries between the older superior and slave provinces and the younger provinces gravity magnetic seismic and paleomagnetic data related to three proposed sutures in the latter category have been interpreted and generally support the view that the shield comprises a mosaic of once separated but now joined cratonic blocks the sutures vary in complexity from wide zones containing distinctive rocks formed at ancient accreting and consuming plate margins to cryptic sutures marked only by abrupt changes in metamorphism and zones of cataclasis paired negative and positive gravity anomalies associated with these sutures have been interpreted in terms of juxtaposed crustal blocks of different density and thickness magnetic anomaly patterns change abruptly at structural province boundaries available seismic results support the concept of thickened crust in the younger province as derived from gravity modelling most authors prefer interpretation of proterozoic pole positions in terms of one plate models but in some cases the interpretation of paleomagnetic data is equivocal and two plate models have been proposed

timing and chemistry of igneous events associated with the southern oklahoma aulacogen abstract igneous activity in the southern oklahoma aulacogen of north america was concentrated in the early rifting stages of aulacogen development the time span over which liquids rose may not have exceeded 50 m y and certainly terminated before the upper cambrian igneous activity began with three basaltic liquids stratigraphically identifiable but perhaps not all distinct genetically this was followed by one large rhyolitic granitic episode of a type character one final basaltic event ended the activity all the basaltic types seem to be tholeiitic showing more kinship with either the older proterozoic north american midcontinental rift or the northern part of the cenozoic rio grande rift than the cenozoic east african rift two major uplifts occurred one between the earlier basalts and the rhyolite and one much later after all igneous activity was over in the pennsylvanian

implications of selected subsurface data on the structural form and evolution of some basins in the northern basin and range province nevada and utah

geophysical and isotopic mapping of preexisting crustal structures that influenced the location and development of the san jacinto fault zone southern california we examine the role of preexisting crustal structure within the peninsular ranges batholith on determining the location of the san jacinto fault zone by analysis of geophysical anomalies and initial strontium ratio data a 1000 km long boundary within the peninsular ranges batholith separating relatively mafic dense and magnetic rocks of the western peninsular ranges batholith from the more felsic less dense and weakly magnetic rocks of the eastern peninsular ranges batholith strikes north northwest toward the san jacinto fault zone modeling of the gravity and magnetic field anomalies caused by this boundary indicates that it extends to depths of at least 20 km the anomalies do not cross the san jacinto fault zone but instead trend northwesterly and coincide with the fault zone a 75 km long gradient in initial strontium ratios sr i in the eastern peninsular ranges batholith coincides with the san jacinto fault zone here rocks east of the fault are characterized by sr i greater than 0 706 indicating a source of largely continental crust sedimentary materials or different lithosphere we argue that the physical property contrast produced by the peninsular ranges batholith boundary provided a mechanically favorable path for the san jacinto fault zone bypassing the san gorgonio structural knot as slip was transferred from the san andreas fault 1 0 1 5 ma two historical m6 7 earthquakes may have nucleated along the peninsular ranges batholith discontinuity in san jacinto valley suggesting that peninsular ranges batholith crustal structure may continue to affect how strain is accommodated along the san jacinto fault zone

model for the tectonic evolution of the mississippi embayment and its contemporary seismicity aeromagnetic and gravity surveys of the northern mississippi embayment have delineated a northeast trending basement depression about 70 km wide and more than 300 km long the feature the mississippi valley graben is remarkably linear over its known length has nearly parallel sides and has estimated depths relative to surrounding basement of more than 2 km both gravity and magnetic anomalies show an alignment of dense magnetic plutons along the boundaries of the graben estimated magnetization directions for the plutons indicate a mesozoic age the graben of probable earliest paleozoic age underlies the northwest side of the embayment and contains the area of major seismicity in particular much of the instrumentally located contemporary seismicity and the estimated locations of the large magnitude 1811 1812 earthquakes are in the central part of the graben we interpret the early paleozoic graben the mesozoic plutonism and the sedimentation cycles as evidence of stretching events seismicity is attributed to strain in and below the central graben area which would appear to be a focal point of deformation in the stretching model we suggest that the sequence of events can be explained by the action of a rotating horizontal stress field on a crustal flaw that persisted well back in pre cambrian time the postulated rotating stress field is presumably controlled by the system that drives the global plates

tectonic setting of the southern cascade range as interpreted from its magnetic and gravity fields we have compiled and analyzed aeromagnetic data from the southern cascade range and compared them with residual gravity data from the same region in order to investigate regional aspects of these young volcanic rocks and of basement structures beneath them various constant level aeromagnetic surveys were mathematically continued upward to 4 571 m and numerically mosaicked into a single compilation extending from lat 40 10 n to lat 44 20 n these data were reduced to the pole upward continued an additional 10 km and compared with a magnetic topographic model and with residual gravity data upward continued to the same level several intriguing regional features are suggested by these data 1 the trinity ophiolite complex that is exposed west of mount shasta probably dips at a shallow angle to the east and continues in the subsurface at least 10 km east of mount shasta 2 mount shasta lassen peak and medicine lake volcanoes are located in a widespread magnetic low possibly caused by an upwarp of the curie temperature isotherm 3 crater lake caldera is located at the intersection of various linear anomalies interpreted to be related to structure in basement rocks below the cascade range 4 three sisters volcanoes and newberry crater are connected to each other by an arcuate magnetic source 5 the high cascades from lat 40 10 n to at least lat 44 30 n are marked by a residual gravity low which includes the three sisters volcanoes mount shasta medicine lake volcano mount mcloughlin and crater lake we believe this gravity feature represents a major structural depression beneath the high cascades 6 except for newberry crater every major volcano of the study area is located on the perimeter of a local gravitational low we suggest that the gravity lows reflect subsidence of low density volcanic material relative to denser country rock and that the major volcanoes have developed over structures at the perimeters of their respective depressions

gravity interpretation of the southern wind river mountains wyoming the wind river range is the largest laramide uplift in wyoming and is cored by precambrian rocks thrust over sedimentary rocks of the green river basin to the southwest the origin of these uplifts is in dispute it has been ascribed to horizontal compression along thrusts or vertical movement along high angle reverse faults therefore the attitude of the bounding fault is critical to understanding the mechanics of deformation a gravity study based on 1800 stations has been carried out to attack this question and to complement a cocorp deep crustal reflection study smoothed free air gravity anomalies are highly positive over the wind river range and near zero over the adjacent margins bouguer gravity anomalies range from 252 and 225 mgal in the green river basin and wind river basin respectively to 150 mgal in the basement core of the wind river range the bulk density of sedimentary rocks in the adjacent basins is the critical parameter for gravity interpretation of the deep structure and ranges

mechanism of subsidence of ancient cratonic rift basins cratonic basins commonly occur over ancient rift zones these inactive rift basins are recognizable by a positive linear bouguer gravity anomaly that may correspond to the axial gravity high found in modern rift valleys many of these basins undergo discrete periods of increased subsidence rates or reactivations long after the mass excess responsible for the linear gravity high was emplaced furthermore the reactivation of many cratonic basins occurs simultaneously with large scale compressional tectonics it is suggested that the driving force for subsidence is the isostatically uncompensated ancient mass excess the subsidence of these basins is modelled by a lithospheric flexure model with a nonlinear maxwell viscoelastic rheology solutions to this model indicate that basins may experience a low subsidence rate throughout geologic time the subsidence of a basin will stop only when isostatic compensation of the mass excess is achieved since ancient rift mass excesses may be uncompensated over long geologic time intervals the early thermal and structural evolution of rifts may not significantly influence later basin subsidence the models suggest that basins may be reactivated by any mechanism which lowers the effective viscosity of the lithospheric material allowing the uncompensated basin to settle toward an isostatic compensation depth faster than normal since viscosity is a strong function of temperature reactivation by a world wide increase in heat flow is suggested as a possible mechanism for the synchroneity of basin subsidence throughout a continent an increase of 15 in the geothermal gradient for example from 16 5 18 9 k km will cause about a 5 increase in subsidence this increase in heat flow however seems unlikely of producing by itself the magnitude of basin subsidence during a reactivation phase that is observed in the geologic record where up to 100 increase in subsidence might occur since the rheology of the lithosphere is nonlinear effective viscosity is also a strong nonlinear function of stress the presence of a regional compressive stress during periods of tectonism of 1 1 108 pa about 2 8 of the buckling strength of the lithosphere produces a short period of reactivated subsidence 105 yr during the reactivated subsidence the newly imposed regional stress relaxes sufficiently in the lower lithosphere to restore the effective viscosity to values similar to that before reactivation this suggests that reactivated subsidence caused by regional compression can be maintained as long as the stress level remains high in the lower lithosphere this may be accomplished by an intermittent application of the regional stress over time

geological development origin and energy mineral resources of williston basin north dakota the williston basin of north dakota montana south dakota and south central canada manitoba and saskatchewan is a major producer of oil and gas lignite and potash located on the western periphery of the phanerozoic north american craton the williston basin has undergone only relatively mild tectonic distortion during phanerozoic time this distortion is largely related to movement of precambrian basement blocks sedimentary rocks of cratonic sequences sauk through tejas are present in the basin sauk cambrian lower ordovician tippecanoe ordovician silurian and kaskaskia devonian mississippian sequence rocks are largely carbonate as are the major oil and gas producing formations absaroka pennsylvanian triassic and zuni jurassic tertiary rocks have more clastic content but carbonates are locally important clastics of the zuni sequence fort union group contain abundant lignite tejas tertiary quaternary sequence rocks are not significant in the production of minerals or energy although glacial sediments cover much of the region oil exploration and development in the united states portion of the williston basin since 1972 have given impetus to restudy basin evolution and geologic controls for energy resource locations consequently oil production in north dakota for instance has jumped from a nadir of 19 million bbl in 1974 compared to a previous zenith of 27 million in 1966 to 32 million bbl in 1979 and 40 million bbl in 1980 geologic knowledge of carbonate reservoirs has expanded accordingly depositional environments throughout sauk tippecanoe and kaskaskia deposition were largely shallow marine subtidal and even basinal environments were developed in the basin center but sabkha deposits were abundant near the basin periphery evidence of subaerial weathering was commonly preserved in structurally high areas and on the basin periphery especially in upper kaskaskia rocks some pinnacle reefs were developed in kaskaskia deposition morphologically similar to the silurian pinnacle reefs of the michigan basin clastic sediments were transported into the southern part of the basin during absaroka sequence deposition a product of erosion of ancestral rocky mountain orogenic structures continental and shallow marine clastic sediments were deposited during zuni sedimentation until cretaceous deeper marine environments were established laramide orogenesis to the west provided detritus that was deposited in fluvial deltaic and marginal marine environments regressing to the east major lignite deposits are part of this postorogenic regressive rock body major structures in the basin and the basin itself may result from left lateral shear along the colorado wyoming and fromberg zones during pre phanerozoic time deeper drilling in the basin has revealed several major structures and given indications of others most structures probably resulted from renewed movement or tensing of pre phanerozoic faults meteorite impact events have been suggested as the origin for one or more structures

geological significance of surface gravity measurements in the vicinity of the illinois deep drill holes a gravity study of northeastern illinois and southwestern wisconsin was undertaken to determine the three dimensional geometry of the precambrian granitic body encountered in the illinois deep drill holes gravity data acquired during this study and existing observations show a marked negative gravity anomaly in the vicinity of these drill holes which is interpreted as originating from a negative density contrast between the granitic body and the intruded crystalline rocks a fifteenth order polynomial surface fitted to the bouguer anomaly data which includes only wave lengths greater than 35 km was chosen as best displaying the effects of the granitic body and profiles of this surface were modeled employing a variety of techniques and assumptions the preferred model of the causitive body has a depth to the top of 2 km a density contrast of 100 kg m 3 and a maximum vertical extent of 5 km an isopach map of the body constructed from the modeling places the illinois deep drill holes on the edge of an oval shaped mass

gravity anomalies and flexure of the lithosphere at mountain ranges the bouguer gravity anomaly over the himalayan alpine and appalachian mountains is characterized by a generally asymmetric gravity low which spans the mountains and associated foreland basins the minimum of the gravity low is generally systematically displaced from the region of greatest topographic relief and shows no obvious relationship to surface geology in addition the alps and appalachians are associated with a generally symmetric gravity high that is unrelated to the topographic relief together the gravity low and high form a characteristic positive negative anomaly couple the steep gravity gradient between the positive and negative couple correlates with the insubric line and its equivalents in the alps and the brevard zone in the southern appalachians this gravity anomaly couple is interpreted as evidence for flexure of the continental lithosphere by subsurface buried and surface topographic loading the magnitude of the subsurface load is estimated from the integrated excess mass represented by the positive anomaly and the magnitude of the surface load from the topography by combining these loads the flexure of the lithosphere and the associated bouguer gravity anomaly are calculated for different assumed values of the elastic thickness te of continental lithosphere the best fitting value of te for the himalayas is 80 100 km for the alps 25 50 km and for the appalachians 80 130 km the model calculations suggest that the main contribution to the gravity couple in the alps and appalachians arises not from surface loads such as thrust sheets and nappes but from subsurface loads however in the case of the himalayas the topography appears to be a sufficient load to explain the observed asymmetric gravity low when subsurface loads do exist therefore we would not expect a close correlation between mountain topographic relief and depth to the m discontinuity as required by classical models of isostasy rather the m discontinuity is expected to relate to the center of mass of all the loads acting on the lithosphere the surface and subsurface loads inferred from the gravity anomaly play a major role in the development of mountains and foreland basins the emplacement of large subsurface loads represents the primary event in the alps and appalachians while in the himalayas the emplacement of the surface load is the primary event the emplacement of the subsurface load probably marks the initiation of foreland basin development the regional characteristics of the basin are controlled by the primary load while local variations are controlled by the secondary thrust sheet loads subsequent movement of either load will produce migration of the basin depocenter the origin of the subsurface load is not clear but its association with the insubric line in the alps and the brevard zone in the appalachians suggests that it may be related to the obduction of crustal blocks flakes onto the underlying plate during continental collision or possibly to the preloaded crustal structure of the underlying plate

proterozoic structural provinces in the north central united states geophysical and radiometric age data now permit attempts to outline precambrian structural provinces in the united states there are two archean terrains the superior and wyoming provinces the churchill province 1 700 m y and older splits around the wyoming block one branch extends southwest across montana the other south and southeast between the wyoming and superior cratons an east northeast trending terrain probably accreted along the southern margins of the wyoming churchill superior block beginning about 1 800 m y ago the churchill province may have been truncated by this accretionary terrain or may merge into it southeast geophysical trends in missouri and kansas suggest southwestward accretion in that region 1 600 1 700 m y ago

geologic and geochemical evidence for the nature and development of the middle proterozoic keweenawan midcontinent rift of north america abstract general models of active continental rifting must take into account the large plateau basalt provinces many of which have been precursors of or associated with continental breakup the keweenawan middle proterozoic midcontinent rift mcr of north america 2300 km long is one such major continental rift but it aborted before significant crustal separation was achieved these plateau basalt rifts including the mcr differ from the east african type in being dominated by broad subsidence rather than rift valleys tholeiitic flood basalts rather than alkalic central volcanoes and large positive gravity anomalies geologic and geophysical evidence is reviewed which contradicts some recent models which have been proposed for the development of the mcr the basic causes of the pre rift subsidence the u shaped nature of the structure and the abrupt termination of its activity are obscure a triple junction in the usual sense does not appear to have been formed

cocorp seismic reflection profiling in the ouachita mountains of western arkansas geometry and geologic interpretation cocorp seismic reflection profiling in the ouachita mountains of western arkansas indicates that 1 carboniferous foreland basin deposits within the arkoma basin thicken dramatically toward the south reaching an aggregate thickness in excess of 12 km 4 5 s two way travel time beneath the southern edge of the frontal thrust zone 2 evidence for northward directed low angle thrusting within this clastic sequence is prominent with a probable decollement surface lying at or near the contact with underlying early paleozoic shelf carbonates 3 beneath the benton uplift a sequence of discontinuous events defines a broad antiform cresting at approximately 7 km 2 8 s this structure appears to mimic the anticlinorial shape of the benton uplift indicated by the surface outcrop pattern 4 beneath the southern ouachitas south dipping stratified events are observed to depths in excess of 14 km 5 0 s 5 deeper in the section a prominent gently north dipping reflection occurs at approximately 22 km depth 7 6 s beneath the northern coastal plain southern ouachitas extrapolation of data along strike in the ouachita belt and consideration of large scale structure observed in other collisional orogenic belts suggest that the reflection events observed beneath the benton uplift represent early paleozoic shelf carbonates correlative with those that floor the arkoma basin to the north this interpretation requires that the early paleozoic deep water sediments exposed in the core of the ouachitas be allochthonous and also implies significant basement uplift beneath the core zone to the south the prominent package of layered reflections occurring beneath the southern ouachitas and northern coastal plain indicates that a significant portion of the crust in that region is composed of imbricate sedimentary and or metasedimentary strata at relatively shallow levels these strata are correlative with the carboniferous flysch cropping out in the southern ouachitas and at somewhat deeper levels they are correlative with the early to middle paleozoic deep water sediments exposed on the benton uplift

rb sr and u pb geochronology and distribution of rock types in the precambrian basement of missouri and kansas most of what we know about basement rocks in kansas and missouri is derived from cores and cuttings from deep drilling these rocks may be divided into a northern terrane underlain by rocks consisting of abundant granite commonly showing cataclastic textures and by metavolcanic to metasedimentary rocks and a southern terrane underlain almost exclusively by rhyolitic flows and ash flow tuffs and epizonal granite plutons the northern terrane is interrupted in central kansas by mafic igneous rocks and flanking arkosic sedimentary rocks of the central north american rift system data from 63 rb sr whole rock analyses yield ages ranging from 1 153 to 1 748 m y but these are considered to represent only the minimum ages of the rocks ages derived from u pb analyses of suites of cogenetic zircons from 22 rock samples indicate that some rocks of the northern terrane were formed 1 610 to 1 650 m y ago these are apparently intruded by younger granite plutons formed 1 450 1 470 m y and 1 340 1 380 m y ago rocks of the southern terrane were formed 1 460 1 480 m y ago in the st francois mountains terrane of southeastern missouri and its buried equivalents but these are about 1 380 m y old in southwestern missouri and southern kansas rocks of the central north american rift system in kansas are assumed to be about 1 100 m y old by geophysical and drill hole extension to their outcrop in the lake superior region where they have been dated both the northern and southern terranes are notable for the great abundance of granitic rocks and the scarcity of intermediate to mafic igneous rocks quartzite is the most abundant metamorphic rock although the decrease in ages from north to south in the mid continent region suggests their sequential accretion at the edges of a pre existing continent the rock assemblages are not consistent with convergent plate margin suites of the andean type the great volumes of rhyolite and epizonal granite of the southern terrane may represent melting of thickened somewhat older crust following accretion at the continental margin major element chemical data are presented for basement rocks from kansas oklahoma and texas and these are compared with the compositions of similar rocks from the st francois mountains names and locations are given for wells from which samples were obtained

mapping the upper mantle three dimensional modeling of earth structure by inversion of seismic waveforms a method is presented for the inversion of waveform data for the three dimensional distribution of seismic wave velocities the method is applied to data from the global digital networks international deployment of accelerometers global digital seismograph network the selected data set consists of some 2000 seismograms corresponding to 53 events and 870 paths the moment tensors of the earthquakes are determined through an iterative procedure which minimizes the corrupting influence of lateral heterogeneity a global model is constructed for shear wave velocity expanded up to degree and order 8 in spherical harmonics and described by a cubic polynomial in depth for the upper 670 km of the earth s mantle although no a priori information is incorporated the model predictions reproduce much of what is known about the dispersion of mantle waves for example high phase velocities for shields low velocities at ridges and a strong degree 2 pattern for rayleigh waves since the method makes use of complete waveforms overtone data are also included it is shown that the model is reproducible in that substantially the same model can be constructed from each half of the total data set considered independently the model shows that shields and ridges are major features in the depth interval 25 250 km the ridges of the southern pacific and the larger shields persist to 350 km but the southeast indian rise is underlain by a high velocity anomaly at this depth as is much of the mid atlantic ridge at 450 650 km the major features are a broad region of high velocities incorporating south america much of the south atlantic and parts of west africa a broad region of low velocities in the central and eastern pacific high velocities in the western pacific and a low velocity anomaly beneath the red sea and the gulf of aden in the absence of a crustal correction degrees 2 and 3 show a high positive correlation with the geoid paradoxically this is largely destroyed when the distribution in crustal thickness is taken into account spherical harmonic degrees 4 7 show a significant negative correlation

three dimensional geometry of the gorda plate beneath northern california the isostatic residual gravity field over northern california displays a gravity gradient interpreted to reflect the south edge of the gorda plate where it is subducted eastward beneath the north american plate the locus of points of maximum slope defines a line trending s60 e from a coastal point approximately 20 km south of cape mendocino a point where the buried plate boundary is inferred from magnetic and seismicity data southeast from the coast for a distance of 120 km the gravity anomaly parallels the strike of the blanco fracture zone and the present direction of relative motion between the pacific and northern gorda plates calculations from the form of the anomaly yield depth estimates that fit an east southeast plunge of approximately 9 for the top of the gorda south edge the sense of the anomaly higher gravity to the south supports the hypothesis that a window developed in the subducted slab east of the san andreas fault and south of the gorda plate south of the gorda boundary the base of the north american plate is thus in contact with hot material from the asthenosphere that invaded the window because the overlying north american plate has been moving relatively south across the gorda boundary the north american plate beneath the coast ranges east of the san andreas fault in central california may be decoupled from the underlying material at a depth slightly deeper than the depth to the top of the boundary at the time the north american plate passed over it

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

depth constraints on azimuthal anisotropy in the great basin from rayleigh wave phase velocity maps abstract we present fundamental mode rayleigh wave azimuthally anisotropic phase velocity maps obtained for the great basin region at periods between 16 s and 102 s these maps offer the first depth constraints on the origin of the semi circular shear wave splitting pattern observed in central nevada around a weak azimuthal anisotropy zone a variety of explanations have been proposed to explain this signal including an upwelling toroidal mantle flow around a slab lithospheric drip and a megadetachment but no consensus has been reached our phase velocity study helps constrain the three dimensional anisotropic structure of the upper mantle in this region and contributes to a better understanding of the deformation mechanisms taking place beneath the western united states the dispersion measurements were made using data from the usarray transportable array at periods of 16 s and 18 s which mostly sample the crust we find a region of low anisotropy in central nevada coinciding with locally reduced phase velocities and surrounded by a semi circular pattern of fast seismic directions away from central nevada the fast directions are n s in the eastern great basin nw se in the walker lane region and they transition from e w to n s in the northwestern great basin our short period phase velocity maps combined with recent crustal receiver function results are consistent with the presence of a semi circular anisotropy signal in the lithosphere in the vicinity of a locally thick crust at longer periods 28 102 s which sample the uppermost mantle isotropic phase velocities are significantly reduced across the study region and fast directions are more uniform with an e w fast axis the transition in phase velocities and anisotropy can be attributed to the lithosphere asthenosphere boundary at depths of 60 km we interpret the fast seismic directions observed at longer periods in terms of present day asthenospheric flow driven deformation possibly related to a combination of juan de fuca slab rollback and eastward driven mantle flow from the pacific asthenosphere our results also provide context to regional sks splitting observations we find that our short period phase velocity anisotropy can only explain 30 of the sks splitting times despite similar patterns in fast directions this implies that the origin of the regional shear wave splitting signal is complex and must also have a significant sublithospheric component

rift structure of the northern mississippi embayment from the analysis of gravity and magnetic data gravity and magnetic data have been simultaneously inverted to derive a crustal model of the northern mississippi embayment which is compatible with geologic and seismic refraction data the results indicate that this region is a site of a late precambrian early paleozoic rift system an anomalously dense layer is present at the base of the crust and thickens beneath a broad northeast trending graben that formed during the initial phases of rifting the thickest part of anomalous crust occurs beneath the region of greatest seismic activity which lies within the geographic limits of the graben it is suggested from this correlation that strain accumulates within the anomalous crust layer which acts as an inhomogeneity in a relatively homogeneous lithosphere based on the results of this study and previous geophysical studies the complex tectonic evolution of the upper mississippi embayment since late precambrian time is rift formation possibly along a pre existing shear zone graben development in cambrian time reactivations in permian and cretaceous time and subsequent formation of the present mississippi embayment and fault zones with large influences from rift structures

stratigraphy and tectonics of the virginia north carolina blue ridge evolution of a late proterozoic early paleozoic hinge zone late proterozoic 690 570 ma through ordovician rocks of the virginia north carolina blue ridge and valley and ridge record the evolution of an eastward facing passive margin and its subsequent deformation during continent arc collision taconic orogeny comparison of blue ridge stratigraphy with modern rifts and passive margins suggests that late proterozoic rifting involved substantial crustal attenuation in the eastern blue ridge the axial zone of the blue ridge from central virginia southward is occupied by a major fault system hayesville fries rock fish valley fault contrasts between rocks deposited east and west of this fault suggest that it is a reactivated hinge zone separating highly attenuated continental crust to the east from continental crust of more normal thickness to the west during rifting normal faulting and rapid subsidence of attenuating crust in the eastern blue ridge produced rift basins in which deep water clastic and volcanic rocks accumulated lynchburg group ashe formation landward of the hinge zone in the western blue ridge mainly terrestrial to shallow water clastic and volcanic rocks were deposited in a series of subaerial rift basins swift run catoctin grandfather mountain mount rogers formations the hinge zone not only controlled depositional patterns throughout the rift and passive margin history but it was also reactivated during the taconic collision when attenuated crust and its cover were thrust westward over the shelf edge turbidite dominated rift sequences of the eastern blue ridge are unlike typical rift deposits which are characterized by predominantly subaerial and shallow water sedimentation deep water rifts may be common on attenuated crust as in the bay of biscay however they are poorly known from the stratigraphic record because of their susceptibility to destruction during orogeny

the cheyenne belt analysis of a proterozoic suture in southern wyoming abstract the cheyenne belt of southeastern wyoming is a major shear zone which separates archean rocks of the wyoming province to the north from 1800 1600 ma old eugeoclinal gneisses to the south miogeoclinal rocks 2500 2000 ma old unconformably overlie archean basement immediately north of the shear zone and were deposited under transgressive conditions along a rift formed continental margin intrusive tholeiitic sills and dikes are interpreted as rift related intrusions and a date of 2000 ma on a felsic differentiate of these intrusions gives the approximate age of rifting there are no known post 2000 ma felsic intrusions north of the cheyenne belt volcanogenic gneisses and abundant syntectonic calc alkaline plutons of the southern terrane are interpreted as island are volcanic and plutonic rocks the volcanics are a bimodal basalt rhyolite assemblage plutons include large gabbroic complexes and quartz diorite 1780 ma syntectonic granitoids 1730 1630 ma and post tectonic anorthosite and granite 1400 ma there is no evidence for archean crust south of the cheyenne belt structural data thrusts in the miogeoclinal rocks vertical stretching lineations and the same fold geometries north and south of the shear zone suggest that juxtaposition of the two terranes took place by thrusting of the southern terrane island arc over the northern terrane craton and miogeocline probably as a continuation of the south dipping subduction which generated calc alkaline plutons of the southern terrane a metamorphic discontinuity across the shear zone with greenschist facies rocks to the north and upper amphibolite facies rocks and migmatites to the south also suggests thrusting of the southern terrane deeper crustal levels over the northern terrane shallower levels the cheyenne belt may be a deeply eroded master decollement perhaps analogous to a ramp in the master decollement in the southern appalachians this interpretation of the cheyenne belt as a proterozoic suture zone provides an explanation for the geologic geochronologic geophysical metallogenic and metamorphic discontinuities across the shear zone

a terrane of 1 350 to 1 400 m y old silicic volcanic and plutonic rocks in the buried proterozoic of the mid continent and in the wet mountains colorado much of the precambrian basement of the mid continent region of the united states is underlain by silicic volcanic rocks and related silicic epizonal granite bodies in the st francois mountains of southeastern missouri and in the buried basement of illinois indiana and eastern ohio these rocks mainly yield ages of 1 450 to 1 480 m y previously published u pb ages from zircons showed that rhyolite and granite in the subsurface of southwestern missouri southern kansas and parts of oklahoma were formed 1 350 to 1 400 m y ago new u pb age data from zircons presented here show that this terrane makes up much of the basement of eastern and central oklahoma where it includes both rhyolite and granite and extends into the texas panhandle where the panhandle rhyolite terrane which was thought to be 1 200 m y old yields zircon u pb ages of 1 350 to 1 400 m y zircons from the san isabel batholith of the southern wet mountains colorado also yield u pb ages of 1 360 m y and are thus coeval with the granite and rhyolite of the southern mid continent region the san isabel batholith intruded crust formed at least 1 700 m y ago a possible model is that the granite and rhyolite of the mid continent are essentially a veneer emplaced upon and within an older possibly 1 700 m y old crust the mesozonal san isabel batholith is a body formed during this igneous activity that is exposed in contact with older crust because of later uplift in the southern rocky mountains the tectonic setting of the emplacement of these rocks is obscure but appears to be continent marginal because only younger rocks are known to the south

plate tectonics model for proterozoic continental accretion in the southwestern united states proterozoic crustal provinces in the southwestern united states decrease in age south from the archean wyoming province supracrustal rocks define three major age provinces from north to south 1 72 to 1 80 1 65 to 1 72 and 1 1 to 1 2 b y successions within each province are similar and define two assemblages a bimodal volcanic assemblage commonly overlain by a quartzite shale assemblage precambrian granitic plutons intrude all three provinces geochemical and sr isotope data indicate a variably depleted upper mantle source for basalts and a short lived these observations can be explained by a plate tectonics model involving successive marginal basin closures and andean type orogenies associated with southward migrating arcs the net result is to accrete 1 300 km of continental crust to the southwestern margin of north america between 1 8 and 1 1 b y ago

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

identification of a precambrian rift through missouri by digital image processing of geophysical and geological data digital free air and bouguer gravity anomaly images have been constructed from approximately 287 000 station readings for the region bordered by 25 49 n latitude and 80 110 w longitude the technique used to interpolate between station locations was based on a two dimensional spatial filter where the average of the anomaly values located within the filter area was computed the images contain as many as 256 contours values in byte variable so that subtle anomaly patterns can be identified and traced with much greater certainty than on most contour maps a newly discovered feature in the midcontinent is a gravity low that begins at a break in the midcontinent gravity high in se nebraska extends across missouri in a nw se direction and intersects the mississippi valley graben to form the pascola arch the anomaly varies from 120 to 160 km in width extends about 700 km and is best expressed in southern missouri where it has a bouguer amplitude of approximately 34 mgal the magnitude of the anomaly cannot be explained on the basis of a thickened section of paleozoic sedimentary rock the gravity data and the sparse seismic refraction data for the region are consistent with an increased crustal thickness beneath the gravity low some of the discrete positive magnetic anomalies in missouri are located along the borders of the gravity low digitally enhanced thermal infrared images from the heat capacity mapping mission show a distinct alignment of linear structures with the gravity feature the linear features in some cases correspond to mapped high angle normal faults to drape folds over relief within the precambrian basement and in some cases to extensions of mapped structures the gravity anomaly also cuts across the major precambrian boundary in se missouri marking the change from older sheared granites and metasedimentary rocks to younger granites and rhyolites given the cumulative evidence the gravity anomaly is probably the present expression of a failed arm of a rifting event perhaps one associated with the spreading that led to or preceded formation of the granite and rhyolite terrain of southern missouri

evidence for a major late precambrian tectonic event rifting in the eastern midcontinent region united states recently acquired gravity and aeromagnetic data delineate a large linear gravity anomaly which extends through eastern kentucky and tennessee and coincides with a zone of complex high amplitude magnetic anomalies basement lithologies in the area can be interpreted as a bimodal volcanic suite which is locally peralkaline in nature these volcanics appear to be metamorphosed where they lie east of the grenville front suggesting they predate the grenville metamorphic event the available gravity aeromagnetic seismic refraction and petrologic data along with regional correlations suggest that the best tectonic interpretation of these data is that a keweenawan rift zone extended through the area this rift can be roughly outlined by the gravity high which is locally offset suggesting the presence of transform faults the boundaries of this rift have been locally reactivated and in fact a recent earthquake was located along its western boundary in northern kentucky

a new isostatic residual gravity map of the conterminous united states with a discussion on the significance of isostatic residual anomalies to display more clearly the gravity anomalies caused by geologic bodies in the upper parts of the crust a new colored isostatic residual gravity map of the conterminous united states has been prepared using the gravity data set compiled for the gravity anomaly map of the united states society of exploration geophysicists 1982 the new isostatic residual gravity map is based on an airy heiskanen model of local compensation in which the surface load requiring compensation is defined by 5 min topographic and bathymetric data sets a colored first vertical derivative map of isostatic residual gravity further enhances the short wavelength anomalies produced by bodies at or near the surface and emphasizes the regional fabrics and trends in the gravity field for the purpose of displaying gravity anomalies caused by shallow bodies of geologic significance the nature of the isostatic model and the values of its parameters are of lesser importance than the application of an isostatic correction of some sort most isostatic models result in residual gravity maps that appear nearly identical in their main patterns and features anomalies on isostatic residual gravity maps should not be casually interpreted in terms of undercompensation or overcompensation because large amplitude anomalies can be produced by crustal bodies in complete local isostatic equilibrium many isostatic residual gravity anomalies less than several hundred kilometers wide can be related to known geologic bodies we present here a classification scheme that attempts to categorize such anomalies on the basis of tectonic environment in general highs correlate with intruded or accreted mafic material or with upthrusted crustal sections whereas lows occur over low density sedimentary or volcanic sections felsic intrusive bodies or down warped crustal sections although some longer wavelength anomalies such as the broad gravity high centered over montana could be manifestations of density contrasts deep in the mantle many such anomalies can also be modeled by geologically reasonable density contrasts that are isostatically compensated and confined to depths of less than several hundred kilometers so that their source bodies need not be deep the fact that certain of these broader anomalies have well defined boundaries which correlate with near surface geologic features increases the likelihood that their sources lie entirely within the lithosphere if so then the density contrasts required to explain the gravity data imply fundamental anomalies in chemical composition or thermal state for the crustal and upper mantle columns under these regions we have investigated spectral analysis as a method to quantitatively characterize regional anomaly patterns contoured plots of normalized amplitude spectra were prepared for various areas of the isostatic residual gravity field of the united states these fourier domain representations show characteristic patterns that can be interpreted in terms of the trends and wavelengths of anomalies and may help to more objectively distinguish geologic basements with different origins or tectonic histories

approximating edges of source bodies from magnetic or gravity anomalies cordell and grauch 1982 1985 discussed a technique to estimate the location of abrupt lateral changes in magnetization or mass density of upper crustal rocks the final step of their procedure is to identify maxima on a contoured map of horizontal gradient magnitudes we attempt to automate their final step our method begins with gridded magnetic or gravity anomaly data and produces a plan view of inferred boundaries of magnetic or gravity sources the method applies to both local surveys and to continent wide compilations of magnetic and gravity data e g zietz 1982 simpson et al 1983a kane et al 1982

florida a jurassic transform plate boundary magnetic gravity seismic and deep drill hole data integrated with plate tectonic reconstructions substantiate the existence of a transform plate boundary across southern florida during the jurassic on the basis of this integrated suite of data the pre cretaceous florida bahamas region can be divided into the pre jurassic north american plate jurassic marginal rift basins and a broad jurassic transform zone including stranded blocks of pre mesozoic continental crust major tectonic units include the suwannee basin in northern florida containing paleozoic sedimentary rocks a central florida basement complex of paleozoic age crystalline rock the west florida platform composed of stranded blocks of continental crust the south georgia rift containing triassic sedimentary rocks which overlie block faulted suwannee basin sedimentary rocks the late triassic jurassic age apalachicola rift basin and the jurassic age south florida bahamas and blake plateau marginal rift basins the major tectonic units are bounded by basement hinge zones and fracture zones fz the basement hinge zone represents the block faulted edge of the north american plate separating paleozoic and older crustal rocks from jurassic rifted crust beneath the marginal basins fracture zones separate mesozoic marginal sedimentary basins and include the blake spur fz jacksonville fz bahamas fz and cuba fz bounding the blake plateau bahamas south florida and southeastern gulf of mexico basins the bahamas fz is the most important of all these features because its northwest extension coincides with the gulf basin marginal fault zone forming the southern edge of the north american plate during the jurassic the limited space between the north american and the south american african plates requires that the jurassic transform zone connecting the central atlantic and the gulf of mexico spreading systems was located between the bahamas and cuba fz s in the region of southern florida our plate reconstructions combined with chronostratigraphic and lithostratigraphic information for the gulf of mexico southern florida and the bahamas indicate that the gulf was sealed off from the atlantic waters until callovian time by an elevated florida bahamas region restricted influx of waters started in callovian as a plate reorganization and increased plate separation between north america and south america africa produced waterways into the gulf of mexico from the pacific and possibly from the atlantic

decompensative gravity anomalies reveal the structure of the upper crust of antarctica as antarctica is almost entirely covered by thick ice shields impeding in situ measurements information about upper crustal structures and sedimentary basins is still sparse and the analysis of the gravity anomalies offers new insights isostatic gravity anomalies are often used to investigate upper crust structures however compensating masses significantly reduce the gravity effect of unknown sedimentary and upper crustal structures to separate these effects we apply so called decompensative corrections to the isostatic anomalies for the antarctic continent which reach values of up to 70 mgal the obtained decompensative anomalies well correspond to the known sedimentary basins such as in the areas of the filchner ronne ice shelf and lambert graben and also suggest the existence of other large sedimentary deposits both in west and east antarctica which are not or only sparsely mapped by existing seismic surveys e g in coastal dronning maud land and enderby land a dipole like structure exists at the transantarctic mountains and the wilkes subglacial basin suggesting the presence of isostatic disturbances linked to the dynamic uplift of the transantarctic mountains and thick sedimentary accumulations in the east extended positive anomalies in east antarctica are likely related to the old and dense cratonic crust as well as to isostatic disturbances caused by the transition from local to regional compensation around the lambert graben

isostatic response of the lithosphere with in plane stress application to central australia the relation between gravity and topography in central australia cannot be described by a conventional isostatic response function whether it assumes local or regional compensation or whether the crust behaves as an elastic or viscoelastic medium in particular the response function exhibits considerable anisotropy hence a theory for isostatic response has been developed for a crust in mechanical equilibrium of surface loads buoyancy and in plane forces and subject to erosion sedimentation and stress relaxation the observed north south response function exhibits extreme isostatic overcompensation at some wavelengths and this can be fully explained by the viscoelastic plate model with in plane compression the observed east west response function is consistent with models of regional or local isostatic compensation the response function implies an effective flexural rigidity of 5 10 1022 n m an effective maxwell relaxation time of 25 50 m y an in plane compression of 125 150 mpa and an erosion time constant of about 200 m y these parameters assume that the model evolved over a period of about 700 m y and are wholly consistent with the evolutionary model developed from geological observations

mapping the lower mantle determination of lateral heterogeneity in p velocity up to degree and order 6 the data from the international seismological centre bulletins for the years 1964 1979 are used to derive a three dimensional model of lateral variations of the p velocity in the lower mantle unlike previous studies in which such perturbations were represented by a blocklike parameterization we seek the solution in the form r k 0k l 0l m 0l k r kalm cos m kblm sin m plm cos some 500 000 travel time residuals for teleseismic distances from 5000 earthquakes are used in an iterative procedure to derive the coefficients a and b the maximum k and l used are 4 and 6 respectively in each iteration in addition to solving for the three dimensional structure we also relocate all the earthquakes perturb the average travel time curve and determine station corrections for over 1000 stations particular attention is given to the problem of weighting the individual observations in order to avoid as much as possible the bias due to the uneven distribution of sources and receivers the resulting model shows a high level of perturbations just below the 670 km discontinuity and just above the core mantle boundary where the maximum perturbations reach 1 1 5 of the average velocity even for this highly smoothed model at a depth of 2000 km the rms perturbations are some 3 to 4 times lower than at either of the two boundaries the model predicts well the large scale pattern of observed travel time residuals for various source regions except for the distinct effects of the subduction zones the most striking large scale three dimensional feature of the model is a ring of high velocities circumscribing the pacific basin within a depth range from 1000 km to the core mantle boundary if the proportionality between perturbations in the velocity and temperature is assumed the derived pattern of anomalies could be consistent with a two boundary layer model of convection in the lower mantle some caution however should be exercised in interpreting the results near the 670 km discontinuity where the resolving power of our data set is limited by the lack of adequate control of the upper mantle structure at the same time a significant global increase in the level of perturbations in the lowermost mantle is established beyond question the results of dziewonski et al 1977 on the correlation between the equivalent geoid and the observed geoid for the angular order numbers 2 and 3 is confirmed but there is no significant correlation for degrees from 4 to 6 these calculations are made for the rigid earth richards and hager this issue show that introduction of finite viscosity can change the sign of a geoid anomaly in addition recent studies of lateral variations in the shear velocity reveal the presence of large scale anomalies in the upper mantle thus while the inferences made here with respect to the origin of the gravest orders of the geopotential field are not conclusive they point the way in which results from seismology can be used to address some of the basic questions of geodynamics

quantitative investigations of the missouri gravity low a possible expression of a large late precambrian batholith intersecting the new madrid seismic zone analysis of gravity and magnetic anomaly data helps characterize the geometry and physical properties of the source of the missouri gravity low an important cratonic feature of substantial width about 125 km and length 600 km filtered anomaly maps show that this prominent feature extends nw from the reelfoot rift to the midcontinent rift system geologic reasoning and the simultaneous inversion of the gravity and magnetic data lead to an interpretation that the gravity anomaly reflects an upper crustal 11 km thick batholith with either near vertical or outward dipping boundaries considering the modeled characteristics of the batholith structural fabric of missouri and relations of the batholith with plutons and regions of alteration a tectonic model for the formation of the batholith is proposed the model includes a mantle plume that heated the crust during late precambrian and melted portions of lower and middle crust from which the low density granitic rocks forming the batholith were partly derived the batholith called the missouri batholith may be currently related to the release of seismic energy in the new madrid seismic zone earthquake concentrations occur at the intersection of the missouri batholith and the new madrid seismic zone three qualitative mechanical models are suggested to explain this relationship with seismicity

precambrian basement geology of north and south dakota combined analysis of drill hole gravity and magnetic data indicates that the buried precambrian basement rocks of the dakotas can be divided into several lithotectonic terranes eastern north dakota and northeastern south dakota are underlain by archean gneiss except for the black hills region of south dakota where archean rocks are also exposed the western third of both dakotas is underlain mainly by early proterozoic gneiss and metasedimentary rocks part of this region is underlain by archean crust with an early proterozoic tectonic overprint a broad transition zone of strongly overprinted archean crust occurs between the proterozoic rocks to the west and the archean rocks to the east south central south dakota is underlain by an early proterozoic batholith early proterozoic felsic volcanic rocks occur in southeast south dakota the bootheel portion of south dakota contains a diverse assemblage of basement rocks that are partly archean in age churchill province rocks of the trans hudson foldbel

bouguer gravity in northeastern georgia a buried suture a surface suture and granites a bouguer gravity map of an area of northeastern georgia encompassing parts of the inner piedmont charlotte belt and carolina slate belt has been constructed from gravity stations spaced 2 km apart anomalies in the charlotte carolina slate belts are due to metamorphosed mafic rocks in the upper crust depth extent

on a possible connection between three major earthquakes in california and oil production abstract the 1983 coalinga m 6 5 the 1985 kettleman north dome m 6 1 and the 1987 whittier narrows m 5 9 earthquake sequences each occurred beneath major producing oil fields and caused an aftershock sequence whose epicentral extent coincides approximately with that of the overlying oil field despite this coincidence a causal relationship between these sequences and oil production has been discounted because the earthquakes were located at depths of the order of 10 km whereas oil production is confined to the topmost several km moreover because all three oil fields coalinga kettleman north dome and montebello are situated on anticlines that have undergone recent uplift as a consequence of horizontal tectonic compression of the crust natural earthquakes are not unexpected a mechanical connection between oil production and the earthquake sequence is suggested however by the observation that in each case the total seismic deformation was just that required to offset the force imbalance caused by oil production specifically for each of the three sequences m 0 2 m c where m 0 is the sum of the seismic moments is the modulus of rigidity m is the mass of the fluid removed due to oil production is the fraction of the upper crust that is seismogenic and c is the average upper crustal density in essence net extraction of oil and water reduces slightly the average density of the upper crust causing an isostatic imbalance the ductile lower crust deforms in response to this imbalance thus increasing the load on the seismogenic layer which fails seismically to thicken the crust so as to restore static equilibrium locally accordingly earthquakes near the base of the upper crust may be an expected outcome of major oil production from growing anticlines irrespective of the depths of the producing formations

analogous tectonic evolution of the ordovician foredeeps southern and central appalachians new stratigraphic data suggest that the diachronous evolution of the ordovician foredeeps in the southern and central appalachians was remarkably similar stratigraphic features that characterize the middle ordovician sevier basin in tennessee and the middle and late ordovician martinsbuirg basin in pennsylvania are in identical ascending order 1 disconformity on the knox group beekmantown group 2 shelf carbonates 3 slope deposits 4 submarine fan turbidites and 5 contourites and muddy turbidites we propose that diachronous attempted subduction of the north american craton beneath southeastern microplates and or volcanic arcs resulted in uplift and erosion of the western shelf followed by its rapid subsidence basinward migration of eastern and northeastern terrigenous source areas and associated submarine fan turbidites resulted from continued convergence and filled the basins finally tectonic stabilization and lowering of the source area is recorded by contourites and muddy turbidites the evolutionary model proposed for the sevier and martinsburg basins closely resembles present day tectonics of the timor foredeep and the adjoining sahul shelf north of australia similar comparisons have also been made for ordovician basins in the northern appalachians analogous tectonic mechanisms therefore appear to have operated diachronously along the eastern margin of north america from tennessee to new england and possibly to newfoundland during ordovician time

laramide crustal thickening event in the rocky mountain foreland and great plains the rocky mountain foreland and great plains of the western united states were formerly part of a continental platform adjusted by erosion and deposition in cambrian through jurassic time to near mean sea level and therefore to a near uniform crustal thickness of approximately 33 km today the region stands at regional elevations up to 2 km isostatically supported by a crust exceeding 50 km in thickness reasonable estimates of tertiary sedimentation and laramide strain do not account for more than 15 of the implied thickening however from approximately 70 40 m y b p this region was underlain by a horizontally subducting slab of farallon plate lithosphere moving northeast this slab could have been the cause of the thickening finite difference thermal models with specified kinematics show only a temporary cooling of the base of the north american lithosphere by this slab the excess weight of the slab would have depressed the region plate bending calculations show a quantitative agreement of predicted depression with upper cretaceous isopachs since depression by this slab lasted until the eocene at least the latest cretaceous regression was probably caused by a laramide crustal thickening event the farallon slab might have caused crustal thickening in two ways its excess weight would have drawn in ductile lower crust from surrounding regions however calculations show that this effect is too slow too local and too reversible to explain most of the crustal thickening therefore it seems likely that ductile lower crust was transported from sw to ne by shear stresses which the farallon plate exerted on the base of the north american lithosphere a preliminary finite element calculation based on this hypothesis shows the correct general pattern of crustal thickening an unexpected but encouraging result is that predicted principal compression directions are orthogonal to many laramide basement uplifts

truncation of the appalachian piedmont beneath the coastal plain of alabama evidence from new magnetic data a new aeromagnetic survey of a part of southern alabama reveals that magnetic signatures of the appalachian piedmont are truncated by a major magnetic lineament beneath the gulf coastal plain mylonitic rocks have been recovered from a drillhole along this lineament which is probably a fault zone of late paleozoic and or triassic jurassic age we suggest that this fault zone may initially have been the alleghanian convergent suture between the north american craton and accreted terranes to the southeast the zone may have been locally reactivated as part of an extensive buried triassic jurassic graben system

rift structure beneath the michigan basin from cocorp profiling cocorp consortium for continental reflection profiling surveys across the mid michigan gravity anomaly near the mcclure sparks no 1 deep well delineate a thick layered sequence of relatively strong reflectors that define a structural trough within the precambrian beneath the michigan basin these deep reflections which correlate spatially with the mid michigan gravity and magnetic high are interpreted to be a pile of basaltic extrusive rocks intercalated with clastic rocks and correlative with keweenawan volcanics exposed in the lake superior region to the northwest a reflection poor zone between the base of the paleozoic as found in the deep well and the top of the deep layered sequence probably corresponds to the upper keweenawan clastic assemblage direct evidence of faulting within the inferred volcanic sequence is lacking although sharp flexures structural benches and dips of as much as 10 suggest subsidence controlled by fault block motions reflections from beneath the inferred volcanic pile are rare the deepest clear event being at about 18 km there is no unequivocal evidence on these sections to indicate whether precambrian structures were reactivated to affect the overlying paleozoic strata of the michigan basin proper although data quality may be insufficient to detect such effects

proterozoic basin in the southern midcontinent of the united states revealed by cocorp deep seismic reflection profiling cocorp deep crustal seismic profiles in southwestern oklahoma show strong persistent continuous and undeformed layering in the basement over an area probably very much greater than 2 500 km 2 such layering is very unusual judging by cocorp experience with basement rocks elsewhere in the united states the data can be interpreted as representing a proterozoic basin filled with clastic sedimentary and felsic volcanic rocks 7 to 10 km thick whose base lies 10 to 13 km deep these rocks are believed on the basis of sparse evidence from regional geology to have been deposited or extruded about 1 200 to 1 400 m y ago and some of them may now be metamorphosed this basin lies on the south side of the wichita mountains under the paleozoic hardeman basin and is similar in depth to the paleozoic anadarko basin north of the mountains the deep basement layering is truncated on the south side of the wichita mountains probably by precambrian faults in conjunction with granitic intrusions pennsylvanian compression probably reactivated these precambrian trends extensive precambrian basin deposits in this area were unexpected on the basis of evidence from sparse well control and reports of other layered basement reflections elsewhere in the southern midcontinent suggest that precambrian basins may be an important feature of this region simple models for the evolution of southwestern oklahoma as an aulacogen must be reformulated in the light of these new data

stability analysis of rock slopes subjected to seepage forces using the modified hoek brown criterion abstract the paper deals with the stability analysis of a water saturated rock slope by means of the kinematic approach of limit analysis theory particular emphasis is first given to the effects of pore water pressure on the global stability of geotechnical structures the rock strength properties being formulated in terms of effective stresses it is shown how the effect of seepage flow may be accounted for such an analysis through driving body forces derived from the gradient of excess pore pressure distribution is shown the latter is obtained as the solution to a hydraulic boundary value problem and then incorporated as external loading in the subsequent stability analysis the rock strength properties are modeled by a modified hoek brown failure criterion for which closed form expressions of the support functions have been derived in a previous paper the approach is then applied to investigate the stability of rock slopes computational results are given providing ample evidence of the destablizing effects induced by the seepage forces the influence of relevant geometrical strength and loading parameters is also discussed

geoid anomalies in a dynamic earth in order to obtain a dynamically consistent relationship between the geoid and the earth s response to internal buoyancy forces we have calculated potential and surface deformation love numbers for internal loading these quantities depend on the depth and harmonic degree of loading they can be integrated as green functions to obtain the dynamic response due to an arbitrary distribution of internal density contrasts spherically symmetric self gravitating flow models are constructed for a variety of radial newtonian viscosity variations and flow configurations including both whole mantle and layered convection we demonstrate that boundary deformation due to internal loading reaches its equilibrium value on the same time scale as postglacial rebound much less than the time scale for significant change in the convective flow pattern by calculating relaxation times for a series of spherically symmetric viscous earth models for uniform mantle viscosity the geoid signature due to boundary deformations is larger than that due to internal loads resulting in net negative geoid anomalies for positive density contrasts geoid anomalies from intermediate wavelength density contrasts are amplified by up to an order of magnitude geoid anomalies are primarily the result of density contrasts in the interior of convecting layers density contrasts near layer boundaries are almost completely compensated layered mantle convection results in smaller geoid anomalies than mantle wide flow for a given density contrast viscosity stratification leads to more complicated spectral signatures because of the sensitivity of the dynamic response functions to model parameters forward models for the geoid can be used to combine several sources of geophysical data e g subducted slab locations seismic velocity anomalies surface topography to constrain better the structure and viscosity of the mantle

united states crustal thickness abstract the thickness of the crust the thickness of the basal intermediate or lower crustal layer and the average velocity at the top of the mantle p n have been mapped using all available deep penetrating seismic refraction profiles in the conterminous united states and surrounding border areas these profiles are indexed to their literature data sources the more significant long wavelength anomalies on the three maps are briefly discussed and analyzed an attempt to use bouguer gravity to validate mantle structure was inconclusive