deformation of the pacific north america plate boundary at queen charlotte fault the possible role of rheology published 2018 this article is a u s government work and is in the public domain in the usa the definitive version was published in journal of geophysical research solid earth 123 2018 4223 4242 doi 10 1002 2017jb014770

crustal modeling from spectrally correlated free air and terrain gravity data a case study of ohio we investigate the use of spectral correlation theory to analyze terrain gravity effects and free air gravity anomalies of ohio for possible constraints on the thickness variations and neotectonics of the crust terrain gravity effects are computed from the topography by gauss legendre quadrature integration and are compared against independent free air gravity anomaly observations for their wavenumber correlation spectrum spectral correlation filters are designed accordingly to extract terrain correlated free air gravity anomalies that are subtracted from the terrain gravity effects for estimates of the compensated terrain gravity effects these effects are used to model the moho by inversion assuming they predominantly reflect crustal thickness variations our results characterize the middle third of ohio as a broad zone of thickened precambrian crust which also may include rifted regions where the thickness of the prerift crust has been reduced greatly furthermore we find that about 83 of the ins

collision induced rifting the grenville orogeny and the keweenawan rift of north america abstract we propose that the keweenawan rift formed as a result of convergence related to the synchronous grenville orogeny this proposal is based on 1 1 isotopic ages in the keweenawan rift and the grenville province are similar according to the best u pb and rb sr dates the grenville orogeny existed from 1150 to 1100 ma and keweenawan rifting occurred 1120 1100 ma 2 2 a similar modern orogenic belt and rift relationship in the middle east serves as an actualistic model the red sea gulf of aden rift system has formed as a continuous series of coalesced pull apart basins and is opening parallel to the orogenic front of the zagros and bitlis mountains rifting began in the early miocene while the collision began in the eocene suggesting that rifting is related to the collision 3 3 many other ancient rift collision pairs have a similar age relationship and geometrical configuration based on comparisons of these tectonic regimes we have derived a model whereby these rifts formed as a result of strike slip faults forming in either the foreland or the hinterland of orogenic zones

the lackawanna synclinorium pennsylvania a salt collapse structure partially modified by thin skinned folding the lackawanna synclinorium is a 110 km long structural trough in the appalachian foreland of northeastern pennsylvania in map view its hinge displays a concave to the foreland i e concave toward the west curve historically the entire synclinorium was thought to be an alleghanian thin skinned contractional structure that formed similarly to the fold trains of the central appalachian valley and ridge province interpretation of seismic reflection data across the structure however suggests that the synclinorium formed primarily by the removal of salt the trend of the central and northern synclinorium reflects the location of upper silurian salina salt in the southern synclinorium the salt collapse structure translated northwestward over a thrust ramp joining detachments in the cambrian waynesboro formation and the salina group interpretation of seismic reflection data also suggests that the basement cover contact deepens westward and that basement penetrating reverse faults and a neoproterozoic rift basin are present beneath the synclinorium we speculate that 1 partial barriers due to sedimentation and or reef formation and 2 localized subsidence over a neoproterozoic rift zone may have formed a late silurian restricted basin that allowed thick accumulations of salina salt regional scale salt shale flow and or foreland directed i e westward fluid migration during the alleghanian orogeny might have removed as much as 700 m of salt the removal of this salt caused the overlying strata to collapse downward and form the synclinorium only in the southern synclinorium was the collapse structure modified significantly by decollement tectonics thus the map view curvature of the structure reflects greater tectonic shortening and westward translation toward the foreland in the southern synclinorium compared to the northern synclinorium

an ancient rift complex and its relation to contemporary seismicity in the new madrid seismic zone gravity magnetic geologic and seismicity data have been combined in a seismotectonic analysis of the new madrid seismic zone previous studies have presented evidence for several rift zones in this area upper mississippi embayment including the reelfoot rift a late precambrian early paleozoic failed arm which extends n ne from the ancient continental margin in this study we propose that the northern terminus of the reelfoot rift forms a rift complex which appears to correlate well with the seismicity in the area this correlation suggests that faults associated with this rift complex are being reactivated in the contemporary stress field ene compression if this interpretation is valid it represents a seismotectonic model which can be used to predict the extent of future seismicity in the new madrid seismic zone the proposed rift complex also provides a coherent model for the tectonic development of this region of the north american midcontinent

paleorift structure constrained by gravity and stratigraphic data the statherian arai rift case abstract gravimetric and stratigraphic data were used to investigate the paleoproterozoic arai paleorift a failed statherian continental rift located in the western margin of the sao francisco craton where basement and cover were affected by the neoproterozoic brasiliano orogeny euler deconvolution tilt total horizontal gradient amplitude and upward continuation technics were applied to terrestrial gravimetric data in order to investigate the rift s main faults location direction and depth allowing to identify its main horsts grabens volcanic and plutonic centers we found that rift faults occur to a maximum depth of ca 38 km but major fault throw occurs from 4 to 8 km deep and attenuates from 8 to 12 km probably the brittle ductile transition zone at the time of rifting practically disappearing at 20 km stratigraphic data and basement mapping were used in order to constraint gravimetric results we classify the arai rift as a passive three armed failed rift narrow to divergent type that produced preferably anorogenic rapakivi related magmas most of it still lodged in the crust from surface down to ca 19 km deep and subsidiary mafic magmatism the results indicate the deep occurrence of low density magmas beneath the rift s main axis detected up to 20 km deep correlation to other global statherian rifts show that the sao francisco craton was strongly affected by taphrogenesis during the statherian together with siberia north america and north china cratons finally by comparing our results to recent rifts we found that the ethiopian rift s morphology is quite similar to the arai surrounding the tanzanian craton the cenozoic east africa rift system morphology is compared to the arai espinhaco rift system which surrounds the sao francisco craton the major contribution of this paper is the recognition of arai paleorift surface and subsurface morphology up to now unknown over an area of ca 45 000 km2

models of lithospheric thinning lithospheric thinning denotes a gcodynamieal process that is associated with extensional tectonic phenomena in plate tectonics the lithosphere is a layer of strength compared with the underlying asthenosphere its thick ness is commonly defined by the depth to a particular isothermal or by a fraction of the melting temperature of rocks a typical thickness is of the order 100 km and includes the earth s crust and parts of the upper mantle this definition is equivalent to that used in seismology but it does not necessarily correspond to the elastic thickness derived from flexural defor mations in response to loading according to its thermal regime state of stress and lithology we expect the lithosphere to contain layers of predominantly brittle and ductile deformation the oceanic and continental lithospheres have distinctive origins structures and compositions while the age of the oceanic litho sphere is only of order 108 myr the continental lithosphere contains the oldest rocks known on earth the latter has therefore the most com prehensive record of past geodynamical processes lithospheric thinning is expressed by a variety of tectonic and epeiro genic manifestations such as domal uplifts and hotspot swells crough 1983 continental rifts morgan baker 1983 platform basins sleep et ai 1980 and plateau uplifts on the other hand our understanding of the thinning mechanisms is based on physical concepts in association with observations reheating of the lithosphere b uoyantly driven diapiric pene tration or stretching under isostatic conditions are the most frequently discussed models they implicitly define the nature of the source of thin ning

determination of radiogenic isotopes rbsr smnd and pbpb in fluid inclusion waters an example from the bluebell pb zn deposit british columbia canada abstract rb sr pb nd and sm isotopes were determined in fluid inclusion waters extracted from quartz samples of the bluebell pb zn deposit british columbia the results demonstrate that radiogenic isotopes in fluid inclusion waters can constitute useful geochronological and genetic tools when applied in conjunction with geological geochemical and other geochronological studies rb sr isotopes of fluid inclusion waters and of mineralized marbles hosting the deposit indicate a tertiary mineralization age of 19 2 5 9 ma and an initial 87 sr 86 sr ratio of 0 7143 this rb sr date is in general agreement with a cenozoic age suggested by geologic field relations described by irvine 1957 and fyles 1967 pb isotope data suggest two isotopically distinct fluids one less radiogenic and similar to values measured on galenas and the other more radiogenic this is in agreement with ohmoto and rye s 1970 hypothesis of two meteoric hydrothermal fluids and lecouteur s 1973 convection cell model nd and sm isotope also suggest mixing of fluids both pb pb and nd sm data suggest very old lower and upper crustal materials as possible source of the metals

state of stress and intraplate earthquakes in the united states recently compiled data on the state of stress have been used to define stress provinces in the conterminous united states in which the orientation and relative magnitude of the horizontal principal stresses are fairly uniform the observed pattems of stress constrain mechanisms for generating intraplate lithospheric stresses coupled with new information on geologic structure and tectonism in seismically active areas of the midcontinent and east these data help to define some characteristics common to these areas and to identify key questions regarding why certain faults seem to be seismically active

physical changes in the lithosphere associated with thermal relaxation after rifting we define the terms modem rift and paleorift with respect to the thermal state of the lithosphere the former being rifted lithosphere not in thermal equilibrium and the latter being thermally stabilized rifted lithosphere a comparison of the physical characteristics of modern rifts with paleorifts shows that they share many of the same geomorphic structural and magmatic features but have important differences in their geophysical signatures using a simple model for the thermal relaxation of the lithosphere in the transition from modern rift to paleorift we show that differences in upper mantle seismic velocities and density surface heat flow thermal structure and curie point depth and magnetotelluric properties of the lithosphere between modern and paleorifts can be explained by lithospheric cooling cooling alone cannot explain the complex subsidence histories associated with and following rifting however nor the presence of high velocity crustal layers and sometimes thickened crust in paleorifts these features can be explained by consideration of lithospheric flexural strength and magmatic thickening of the crust associated with rifting we speculate that the anomalous upper mantle beneath many modern rifts may be associated with the process of magmatic thickening of the rifted crust

deep rifts as sources for alkaline intraplate magmatism in eastern north america intracontinental magmatism in eastern north america does not exhibit the regular time progression associated with mantle plume activity instead the magmatism is sporadic but locally persistent suggesting that the magmas rise from a mantle source that drifts with the continental plate a spatial association with ancient rifts further suggests that this source is fertile9 melt generating material emplaced in the mantle lithosphere during the early stages of rifting

reviewing the configuration and extent of the midcontinent rift system abstract uncertainty exists in the configuration and extent of the midcontinent rift system mrs because of deficiencies in geophysical data and limited information from outcrops and basement drill holes additional ambiguity is caused by misunderstanding the definition of continental rifts six principal problematic regions in mapping the mrs are described gravity and magnetic data supported by drill hole and seismic reflection data show that the eastern lake superior rift segment of the mrs continues south from lake superior and connects to a much narrower rift in northern lake michigan the eastern margin of this transition is ill defined because of the lack of definitive anomalies and supporting seismic and drill hole data but is interpreted to occur near the u s canada border the rift segment in southeastern michigan intersects the grenville front and likely continues eastwards in modified form to near the boundary with canada cross cutting gravity and magnetic signatures may reflect grenvillian overthrusts near the terminus of the mrs in michigan the proposed southerly extensions of both branches of the rift system into oklahoma and ohio are based primarily on positive gravity anomalies but neither postulated extension appears to be associated with rifted troughs rather the gravity anomalies of the western branch are related to intrusive mafic rocks and those of the eastern branch are most likely related to deep crustal metamorphic rocks thrust into juxtaposition with less dense crust by grenville orogenesis recent paleomagnetic investigations in conjunction with high resolution radiometric dating imply that the mrs developed during the rapid southward movement of laurentia during a quiescent period along its eastern continental margin massive magmatic activity accompanying the rifting was likely due to rising mantle material that was displaced by subducted lithosphere along the southern margin the heated crust was made more ductile fostering rifting due to extensional stresses the nipigon embayment remains as a possible candidate for an early third branch of the mrs but current evidence is insufficient to include the fort wayne rift as part of the mrs future studies of the mrs would be well served by new age dating and high resolution seismic studies of the lithosphere

fossil flat slab subduction beneath the illinois basin usa abstract the illinois basin is one of several well studied intracratonic sedimentary basins within the north american craton whose formational mechanisms and subcrustal structure are not well understood we study the s velocity structure of the upper mantle beneath the illinois basin and its surrounding area through seismic tomography we utilize continental scale waveform data of seismic s and surface waves enhanced by regional earthquakes located near the illinois basin our 3d tomographic model il05 confirms the existence of a slow s velocity structure in the uppermost mantle beneath the illinois basin region this anomalously slow region exists from the base of the crust to depths of 90 km and is slower than the north american cratonic average by about 200 m s this anomalous uppermost mantle beneath the illinois basin is underlain by a faster lithosphere typical of the surrounding craton to depths of 200 km excluding the formation of the reelfoot rift this area of north american has been stable for over 1 0 gy thus we do not expect thermal anomalies from before that time to persist into present day s velocity anomalies and we consider a delamination origin as an explanation of illinois basin subsidence unlikely we cannot rule out that the slow mid lithosphere beneath the illinois basin is caused by an uppermost mantle enriched by a deep but weak plume we attribute the slow mid lithosphere to the presence of either oceanic hydrous crust or a relatively cool mantle wedge with preserved hydrous minerals in the illinois basin s uppermost mantle related to a fossilized flat subduction zone

horizontal gravity gradient an aid to the definition of crustal structure in north america a map of the magnitude of the horizontal bouguer gravity gradient over the north american continent is used to delineate lateral discontinuities in upper crustal density and or thickness associated with such processes as suturing and rifting the usefulness of gradient trends in mapping major structural boundaries which are sometimes poorly exposed or completely buried is demonstrated by examples such as the buried southward extension of the grenville front and buried boundaries of the superior province gradient trends also draw attention to poorly known structures which may have major tectonic significance and to a continent wide structural fabric which may provide a record of the tectonic growth of the north american continent

a possible southeastern extension of the midcontinent rift system located in ohio in 1988 a stratigraphic test core drilled in southwestern ohio penetrated a previously unknown structure and sedimentologic unit core analyses disclosed a sequence of lithic arenite and siltstone of probable pre phanerozoic age deposited within an alluvial fluvial environment a seismic reflection profile across the core site shows a sequence of strong horizontal paleozoic reflectors unconformably overlying eastward dipping layered units of poor reflectivity beyond the core hole total depth a regime of excellent reflectors is seen the pre mount simon upper cambrian sedimentary sequence encountered has been defined as the type section of the middle run formation point counter analyses of typical middle run formation samples indicate that this sequence is similar in composition to middle proterozoic units associated with the midcontinent rift system in the lake superior region the newly discovered structure in ohio is also similar to half graben structures composing the lake superior basin portion of the midcontinent rift on the basis of similarities in lithology stratigraphy structure and proximity to regional gravity anomalies the middle run formation and its apparent half graben basin is proposed as evidence for the probable extension of the midcontinent rift system southward from its normally accepted termination in southeastern michigan into southwestern ohio

the role of rifting in the tectonic development of the midcontinent u s a abstract recent studies have proposed the existence of several major ancient rift zones in the midcontinent region of north america although the dating of some of these rifts and even the rift interpretations are subject to question an analysis of these paleo rifts reveals three major episodes of rifting keweenawan 1 1 b y b p eocambrian 600 m y b p and early mesozoic 200 m y b p the extent of these events documents that rifting has played a major role in the tectonic development of the midcontinent region this role goes well beyond the initial rifting event because these features display a strong correlation with paleozoic basins and a strong propensity for reactivation for example the eocambrian reelfoot rift was reactivated in the mesozoic to form the mississippi embayment and is the site of modern seismicity which suggests reactivation in a contemporary stress field of ene compression even though the importance of rifting can be established recognition of rifts and delineation of their complexities remain a major problem which requires more study

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

tectonic development of the new madrid rift complex mississippi embayment north america abstract geological and geophysical studies of the new madrid seismic zone have revealed a buried late precambrian rift beneath the upper mississippi embayment area the rift has influenced the tectonics and geologic history of the area since late precambrian time and is presently associated with the contemporary earthquake activity of the new madrid seismic zone the rift formed during late precambrian to earliest cambrian time as a result of continental breakup and has been reactivated by compressional or tensional stresses related to plate tectonic interactions the configuration of the buried rift is interpreted from gravity magnetic seismic refraction seismic reflection and stratigraphic studies the increased mass of the crust in the rift zone which is reflected by regional positive gravity anomalies over the upper mississippi embayment area has resulted in periodic subsidence and control of sedimentation and river drainage in this cratonic region since formation of the rift complex the correlation of the buried rift with contemporary earthquake activity suggests that the earthquakes result from slippage along zones of weakness associated with the ancient rift structures the slippage is due to reactivation of the structure by the contemporary nearly e w regional compressive stress which is the result of plate motions

lithospheric discontinuities beneath the u s midcontinent signatures of proterozoic terrane accretion and failed rifting abstract seismic discontinuities between the moho and the inferred lithosphere asthenosphere boundary lab are known as mid lithospheric discontinuities mlds and have been ascribed to a variety of phenomena that are critical to understanding lithospheric growth and evolution in this study we used s to p converted waves recorded by the usarray transportable array and the oiink ozarks illinois indiana kentucky flexible array to investigate lithospheric structure beneath the central u s this region a portion of north america s cratonic platform provides an opportunity to explore how terrane accretion cratonization and subsequent rifting may have influenced lithospheric structure the 3d common conversion point ccp volume produced by stacking back projected sp receiver functions reveals a general absence of negative converted phases at the depths of the lab across much of the central u s this observation suggests a gradual velocity decrease between the lithosphere and asthenosphere within the lithosphere the ccp stacks display negative arrivals at depths between 65 km and 125 km we interpret these as mlds resulting from the top of a layer of crystallized melts sill like igneous intrusions or otherwise chemically modified lithosphere that is enriched in water and or hydrous minerals chemical modification in this manner would cause a weak layer in the lithosphere that marks the mlds the depth and amplitude of negative mld phases vary significantly both within and between the physiographic provinces of the midcontinent double or overlapping mlds can be seen along precambrian terrane boundaries and appear to result from stacked or imbricated lithospheric blocks a prominent negative sp phase can be clearly identified at 80 km depth within the reelfoot rift this arrival aligns with the top of a zone of low shear wave velocities which suggests that it marks an unusually shallow seismic lab for the midcontinent this boundary would correspond to the top of a region of mechanically and chemically rejuvenated mantle that was likely emplaced during late precambrian early cambrian rifting these observations suggest that the lithospheric structure beneath the reelfoot rift may be an example of a global phenomenon in which mlds act as weak zones that facilitate the removal of cratonic lithosphere that lies beneath

three dimensional electrical resistivity of the north central usa from earthscope long period magnetotelluric data abstract we present initial results from three dimensional inversion of long period earthscope magnetotelluric mt transportable array data from 232 sites covering the north central us the study area covers the 1 1 ga mid continent rift mcr system which cuts across a series of archean and paleoproterozoic lithospheric blocks the western arm of the mcr is clearly evident in shallow depth sections with a narrow resistive core flanked by elongate conductive basins other prominent upper crustal features mapped include the moderately conductive michigan and illinois basins and extremely high conductivities in foreland basin rocks at the southern margin of the superior craton the most prominent conductive anomalies in an otherwise relatively resistive mid lower crust are two elongate east west oriented structures which are closely aligned with previously inferred continental sutures the first underlies the southern margin of the superior craton just north of the niagara fault nf and can be associated with the 1 85 ga penokean orogeny a second further south beneath iowa and western wisconsin lies just south of the spirit lake tectonic zone sltz and can be identified with yavapai accretion at 1 75 ga both of these conductive sutures are cleanly cut by the mcr which is otherwise not clearly evident in the deeper parts of the resistivity model the break in the anomalies is narrow comparable to the surface expression of the mcr indicating that rifting impacts on the entire crustal section were highly localized both suture related anomalies are imaged as extending into and perhaps through the lithosphere as dipping diffuse zones of reduced mantle resistivity sense of dip of these structures southward for the nf anomaly northward for sltz agrees with previously inferred models for subduction and accretion suggesting that a conductive phase most likely carbon has been thrust deep into the lower crust and uppermost mantle providing a marker of the three dimensional boundary between lithospheric blocks resistivities drop below 100 m below 200 km depth in rough agreement with the seismically determined lithosphere asthenosphere boundary lab there are modest lateral variations in this deep low resistivity layer but the reliability and significance of these are not yet clear

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

thermal maturation by vitrinite reflectance of woodford shale anadarko basin oklahoma the woodford shale upper devonian lower mississippian a black shale widely regarded as an important hydrocarbon source rock has attained thermal maturity to postmaturity with respect to the generation of liquid hydrocarbons in most of the anadarko basin in western oklahoma untreated whole rock well cuttings and core material from selected 10 ft intervals of the woodford shale in 28 wells were examined under reflected white light thermal maturation was determined by measuring the reflectance in oil immersion ro of first generation vitrinite particles a minimum of 45 reflectance measurements were recorded for each well representing one or more sample intervals the woodford shale was sampled at depths from 5 060 ft 1 542 m in the northeastern shelf to 25 115 ft 7 655 in the deepest part of the basin a systematic increase in mean vitrinite reflectance meanro with depth was observed over much of the anadarko basin from northeast to southwest across the basin mean ro increases from 0 48 to 2 61 high temperatures have raised the woodford shale to anthracite rank in two wells 4 29 and 4 89 ro in the deep anadarko basin an isoreflectance map for the woodford shale indicates those areas of the basin where the thermal history of the shale is optimum for preserving liquid hydrocarbons mean ro of 0 5 to 2 0 computer generated plots of vitrinite reflectance versus depth provide a regression equation that predicts the vitrinite reflectance of the woodford shale at any depth in the anadarko basin and determines the amount of erosion

thermal evolution of venus abstract the theory of three dimensional and finite amplitude convection in a viscous spherical shell with temperature and pressure dependent physical parameters is developed on the basis of a modified boussinesq fluid assumption the lateral dependences of the variables are resolved through their spherical harmonic representations whereas their radial and time dependences are determined by numerical procedures the theory is then applied to produce thermal evolution models for venus the emphasis is on illustrating the effects of certain physical parameters on the thermal evolution rather than proposing a specific thermal history for the planet the main conclusions achieved in this paper are 1 a significant portion of the present temperature in the mantle and heat flux at the surface of venus is probably owing to the decay of a high temperature established in the planet at the completion of its core formation 2 the effective rayleigh number of the mantle is so high that even the lower order modes of convection cool the planet sufficiently and maintain an almost adiabatic temperature gradient in the convecting region and high temperature gradients in the thermal boundary layers 3 the convection is oscillatory with avalanche type properties which induces oscillatory features to the surface heat flux and the thickness of the crustal layer and 4 a planetary model with a recycling crust cools much faster than those with a permanently buoyant crust the models presented in this paper suggest that venus has been highly convective during its history until 0 5 ga ago the vigorous convection was bringing hot and fresh material from the deep interior to the surface and dragging down the crustal slags floating on the surface in to the mantle the rate of cooling of the planet was so high that its core has solidified in the last 0 5 ga the vigour of convection diminished considerably and the crustal slags developed into a global and permanently buoyant crustal layer the tectonic style on venus has consequently changed from the recycling of crustal plates to hot spot volcanics at the present time the planet is completely solid except in the upper part of its mantle where partial melting may occur

an interpretation of magnetic signatures of aulacogens and cratons in africa and south america abstract magnetic susceptibility anomalies associated with the lithosphere in africa and south america show that the lithosphere underlying sedimentary basins within aulacogens which originated during the rupture of the gondwana continent have lower magnetic susceptibility than the lithosphere underlying the cratons this can be explained by the demagnetization effects of high temperature asthenospheric material possibly intruded into the aulacogens during the rupture thermal evolution of different intrusion models implies that a thermal demagnetization may last for about 300 ma b there is no close relationship between the early subsidence rate of a basin and the magnetization of the underlying lithosphere c surface heat flow anomalies do not necessarily correlate with the magnetic susceptibility anomalies and d once an intrusion of more magnetic material cools to the ambient temperature the total magnetic susceptibility of the lithosphere may increase high magnetic susceptibilities associated with sedimentary basins over old crustal rifts can be explained by this phenomenon

possible extension of the midcontinent rift in west texas and eastern new mexico the midcontinent rift system forms one of the most prominent gravity features in north america the recognized geophysical anomaly extends in an arc from southern oklahoma to lake superior and then into southern michigan the midcontinent rift system was active between 1185 1010 ma as indicated in the lake superior region by age determinations on intrusive igneous rocks we suggest that the period of formation of the midcontinent rift was also a time of extensive igneous activity in texas and new mexico this activity is represented by intrusions beneath the central basin platform texas and new mexico intrusions which crop out at the pajarito mountain in the sacramento mountains new mexico a basaltic debris flow in the franklin mountains texas basalt flows at van horn texas and the crosbyton geophysical anomaly east of lubbock texas these bodies and other bodies located by geophysical anomalies and wells drilled into mafic precambrian rocks may be related to the midcontinent rift system

evidence for paleoproterozoic grenvillian and pan african age gondwanan crust beneath northeastern mexico abstract a number of previous studies including two recent reconstructions of rodinia have considered the grenvillian basement of mexico to be continuous with the laurentian basement of texas isotopic studies and u pb dating of zircons from precambrian granitoids that occur as cobbles and boulders in a paleozoic conglomerate exposed in the state of coahuila northeastern mexico indicate the region is underlain by gondwanan rather than laurentian basement essentially nothing was known of the basement before this study because the region is blanketed with phanerozoic sedimentary and volcanic deposits nine out of ten analyzed samples have grenvillian crystallization ages 1232 7 to 1214 2 ma this phase of magmatic activity in northeastern mexico was coeval with the construction of volcanic arcs in east central and southern mexico 1220 1150 ma but was later than the arc magmatism of laurentia e g elzevirian arcs 1230 ma in texas and the grenville province a subset of the grenvillian samples contains xenocrystic 1 85 ga zircons which are the first evidence of paleoproterozoic crust south of the paleozoic ouachita suture in mexico the remaining sample has a pan african crystallization age 580 4 ma and this is the first sample of pan african age basement found in mexico the pan african orogeny marked the assembly of gondwana and laurentia was unaffected by that event two lines of evidence argue that some of the grenvillian rocks of northeastern mexico were affected by a pan african age event first two of the grenvillian clasts have u pb lower intercept ages within error of 580 ma second common pb and nd isotopic compositions of the pan african granite strongly suggest that it was derived either by melting of grenvillian basement or by assimilating massive amounts of that basement thus far no other grenvillian terrane is known to have the exact combination of nd pb isotopic compositions and absolute ages as the coahuila samples although there are some similarities with the santa marta massif of colombia and the cordillera de merida of venezuela

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

zircon u pb geochronology of two basement cores kentucky usa implications for late mesoproterozoic sedimentation and tectonics in the eastern midcontinent abstractbasement cores from two wells drilled west and east of the grenville front consist of feldspathic litharenite and granitic orthogneiss respectively detrital zircon u pb ages for the litharenite define a broad dominant u pb age mode at ca 1115 ma the dominant mode matches that for the type locality of the middle run formation in the ohio subsurface and is interpreted to consist of detrital zircons sourced from east continent rift volcanic sources ca 1100 ma and grenville shawinigan granites gneisses 1120 1180 ma the youngest detrital zircon ages ca 1020 ma require a maximum depositional age that is at least 70 my younger than the time of midcontinent and east continent rifting and magmatism we propose that the litharenite is correlative with the middle run formation in ohio and was deposited in an evolving late grenville rift foreland basin adjacent to the exhuming grenville orogen zircon u pb secondary ion mass spectrometry ages from orthogneiss define a discordant array with interc

geophysical evidence for deep basin in western kentucky the rough creek fault zone is a major element of the 38th parallel lineament in western kentucky and southern illinois near the head of the mississippi embayment gravity magnetic and subsurface data suggest that this fault zone marks the northern boundary of a large graben in the precambrian basement for which we propose the name rough creek graben this graben is a major structural feature which probably formed initially in late precambrian to early paleozoic time and has been reactivated perhaps several times during the late paleozoic and possibly the mesozoic the graben is as much as 5 5 km deep and the large volume of deeply buried sediments favors further exploration

analysis and use of magsat satellite magnetic data for interpretation of crustal structure and character in the u s midcontinent abstract magnetic anomaly data acquired by nasa s magsat satellite have been processed analyzed and interpreted in terms of causative crustal sources major structures tectonic development geologic provinces crustal composition and magnetic properties in the central u s midcontinent an optimum data set was developed by cleaning correcting and editing and then reduced to a 1 1 latitude longitude grid at a common altitude of 400 km then wavelength filtered and reduced to the magnetic pole the resulting satellite anomalies have a magnitude range up to 15 nt in the region the relationship between these long wavelength anomalies and crustal sources is investigated a magnetic high over kentucky tennessee is associated with more mafic and thicker lower crust and a high over texas oklahoma and trending northeast through missouri may be due to a geologic province and anomalously magnetic lower crust a magnetic low over the mississippi embayment aulacogen can be explained by crustal thinning and less magnetic crust and one over georgia by less magnetic lower crust the magnetic anomaly gradient pattern generally follows and reflects the southern ancient rifted continental margin of the north american craton and there is a lack of apparent magnetic expression of the central north american rift system midcontinent geophysical anomaly

the midcontinent rift system one of the most prominent features on gravity and aeromagnetic maps of the united states is a series of major generally linear anomalies that extend from central kansas to lake superior and then turn southward into central michigan figures 1 2 this system of anomalies clearly reflects major geological features and as such it must represent a very significant episode in the history of the north american continental lithosphere the geologic features associated with these anomalies particularly axial basins filled with basalt and immature clastic rocks along with evidence of crustal extension indicate that the episode was probably one of incipient rifting