measuring winds from underneath the ocean surface by upward looking acoustic doppler current profilers upward looking acoustic doppler current profilers were deployed in the gulf of lions in the western mediterranean for winter convection studies during early 1987 the near surface doppler data which are dominated by sidelobe echo returns from the surface and which are therefore usually discarded for the analysis of subsurface current profiles were found to contain information useful for determining surface winds the horizontal doppler directions were highly correlated with wind direction measured by a research vessel in the vicinity wind speed while not correlated with the size of the doppler shifts was significantly correlated with the acoustic backscatter energy at 150 khz frequency and 20 main beam angle the wind speed exponent was determined as 1 32 and at 75 khz and approximately 24 sidelobe reception angle it was 0 76 at near vertical incidence where more and more energy is scattered sideways with increasing wind speed the exponent was found to be negative the wind data were too limited to determine whether a saturation limit of the wind dependence existed while bragg scattering seems to be primarily responsible for the surface doppler shifts their magnitude cannot be explained by the bragg effect alone the modulation of the bragg scattering ripple waves by longer waves of the surface wave spectrum also appears to be important

geophysical and geological evidence of neotectonic deformation along the hovey lake fault lower wabash valley fault system central united states high resolution seismic shear wave reflection profiles were collected over a segment of the hovey lake fault a known paleozoic fault within a system of faults in the southernmost wabash river valley of the central united states although the fault zone called the wabash valley fault system lies in an area of recognized prehistoric and contemporary seismicity the seismogenic potential of this system remains poorly defined consequently the objectives of this study are to assess the hovey lake fault one of the more prominent fault strands in the system for neotectonic reactivation and if present collect sediment samples for dateable material from the disrupted horizons to provide an age constraint for the movement the resultant stacked profiles show high angle deformation extending above the paleozoic bedrock and into upper quaternary sediment time displacement calculations from the data show approximately 10 5 m of offset on the top of bedrock horizon and 2 m of inverted displacement along the earliest arriving quaternary soil reflector preliminary correlative coring and carbon 14 age dating of a disrupted soil horizon located 7 7 m below ground surface suggest fault movement at this site as late as approximately 37 000 years before present

models for midcontinent tectonism an update during the past decade significant progress has been made in defining the nature of midcontinent tectonism as a result of an improved geophysical geologic data base which permits an integration of seismicity data crustal structures and current stress directions critically placed microseismic networks have shown that many earthquakes with common focal mechanisms occur in specific spatial patterns the direction of the maximum horizontal compressive stress is generally directed east northeast which supports an origin associated with ridge push forces related to movement of the north american plate crustal structures have been mapped locally by seismic studies and regionally by extensive gravity and magnetic anomaly data these data have been used to relate current seismicity and associated tectonism to one of two models increasing evidence from across the midcontinent supports reactivation of preexisting zones of crustal weakness appropriately oriented with respect to the prevailing stress field the zone of weakness model as the dominant control on contemporary tectonism and the local basement inhomogeneity model as a mechanism for minor low energy release earthquake activity

detailed crustal thickness variations beneath the illinois basin area implications for crustal evolution of the midcontinent we present high resolution imaging results of crustal and upper mantle velocity discontinuities across the illinois basin area using both common conversion point stacking and plane wave migration methods applied to p wave receiver functions from the earthscope oiink ozark illinois indiana and kentucky experiment the images reveal unusually thick crust up to 62 km throughout the central and southeastern illinois basin area a significant moho gradient underlies the nw trending ste genevieve fault zone which delineates the boundary between the illinois basin and ozark dome relatively thinner crust 45 km underlies most of the precambrian highlands surrounding the illinois basin and beneath the rift related structures of the reelfoot rift and the rough creek graben we consider four hypotheses to explain the presence of thick crust under the central and southeastern illinois basin crustal thickening may have been produced 1 prior to its accretion to north america around 1 55 ga and is an inherited characteristic of this crustal province 2 by underthrusting or shortening during proterozoic convergent margin tectonics around 1 55 1 35 ga 3 by late precambrian magmatic underplating at the base of older crust associated with the creation of the eastern granite rhyolite province around 1 3 ga and 4 through crustal relamination during an episode of proterozoic flat slab subduction beneath the illinois basin possibly associated with the grenville orogeny

tectonic stress within the new madrid seismic zone refraction data indicate a significant high density rift pillow beneath the new madrid seismic zone we present results of linear and nonlinear viscoelastic finite element modeling to determine whether support of the rift pillow may contribute significantly to the total present day stress field and we consider the implications for intraplate seismicity these models were run for a loading time of 100 m y to account for relaxation and transfer of stress since the last reactivation of the rift in the mid mesozoic results indicate that the nonlinear viscoelastic model with rheological stratification based on composition and temperature agrees well with the observed deformation within the seismic zone and with estimates of regional stress magnitudes the model predicts a maximum compression of 30 40 mpa above the rift pillow in the center of the rift axis if the magnitude of local compression predicted by the nonlinear model produces the inferred clockwise rotation of the order of 10 30 in the direction of shmax maximum horizontal compression near the rift axis the magnitude of regional compression is a factor of 1 to 2 times the magnitude of local compression and consistent with an origin due to ridge push forces the addition of the local stress associated with the rift pillow however results in an approximately 30 reduction in the resolved maximum horizontal shear stress thus while the stress associated with the rift pillow can rotate the stress field into an orientation favorable for failure reduction in the resolved shear stress requires a separate mechanism for strength reduction results of the modeling indicate that stresses from the load of the rift pillow may still be present in the upper crust even after 100 m y and may still play a role in present day deformation and seismicity of the new madrid seismic zone local stress fields of significant tectonic magnitudes may also occur around other ancient rift pillows and help explain the observed correlation between intraplate seismicity and failed rift zones

long lived deformation in the southern mississippi embayment revealed by high resolution seismic reflection and sub bottom profiler data three high resolution seismic reflection profiles and two sub bottom profiler sections acquired along the mississippi river in southern central u s image deformation in post paleozoic sediments the northernmost profile images two faults offsetting cretaceous through at least eocene cane river reflectors interpreted to strike northwest and to be part of the arkansas river fault zone the central profile shows a down to the north fault displacing cretaceous and paleocene midway group reflectors by 210 m and 160 m respectively the fault is interpreted as the northern edge fault of the monroe uplift a late cretaceous uplift associated with igneous intrusions the southernmost profile displays a down to the south fault offsetting cretaceous and paleocene eocene wilcox group reflectors by 125 m and 32 m respectively tilted reflectors in the first 80 m indicate eocene oligocene activity of the fault although quaternary activity cannot be ruled out quaternary tectonic activity is proposed for a series of faults that offset shallow 40 m depth eocene sequences and the base of the quaternary alluvium as imaged on two sub bottom profiler sections these shallow faults are imaged in the vicinity of holocene earthquake induced liquefaction fields corroborating the evidence for recent tectonic activity in the area the spatial coincidence of the imaged faults with the inferred location of the alabama oklahoma transform strongly argues toward a long lived influence of this precambrian continental margin in focusing tectonic activity in the southern u s by controlling the reactivation of triassic jurassic syn rift basement structures and guiding the emplacement of late cretaceous igneous intrusions and the location of cenozoic deformation

half year long measurements with a buoy mounted acoustic doppler current profiler in the somali current a self contained upward looking acoustic doppler current profiler adcp mounted in the top float of a subsurface mooring was deployed in the somali current at 2 14 n 45 55 e from september 17 1985 to april 25 1986 the instrument operated at a frequency of 150 khz with a vertical beam angle of 20 vector averaged profiles of horizontal and vertical velocity were recorded every 4 hours using 200 pings per ensemble at a vertical bin length of 8 7 m the mooring was deployed in very rough topography settling in a trough at 337 m depth with the adcp located at 267 m depth data retrieval over the entire recording period was complete with doppler biasing from side lobe reception of vertically traveling rays affecting only the top 20 m below the surface over the 7 month deployment the instrument recorded current profiles encompassing the end of the 1985 summer monsoon and entire winter monsoon and also through the spring transition into the early onset phase of the 1986 summer monsoon significant echo amplitude variations of week to month long duration were observed which were only partially related to horizontal flow variations associated with the monsoons projection of the strong horizontal currents exceeding 150 cm s at times into the vertical component was not observed attesting to fairly exact orientation of the four beams and tilt meters this indicates that the vertical current measurement from adcps can be potentially useful for phenomena with vertical velocities exceeding a few millimeters per second however an analysis of echo amplitude and vertical current variations at the diurnal period suggests that the measured vertical velocity is at least at that period probably dominated by active vertical migration of biological scatterers through the water column

offshore wind forcing in the gulf of tehuantepec mexico the asymmetric circulation since the early surveys carried out by the eastern tropical pacific eastropic and scripps tuna oceanographic research stor projects in the tropical pacific off mexico the northerly winds which blow over the gulf of tehuantepec were described as an important factor controlling the dynamics of this coastal ocean in january february 1989 an international team carried out the experiment tehuano designed to study the response of the gulf to these wind pulses the complete evolution of the coastal ocean after an event of moderate intensity was observed the forcing is characterized by a mostly symmetric fan shaped offshore wind jet which in turn produces a remarkably asymmetric upper ocean response while analytical results based on ekman theory forced by a symmetric offshore wind predict the formation of a symmetric dipolar circulation the observed flow consists mainly of a large 200 km in diameter anticyclonic warm core eddy in the western gulf with only a weak cyclonic counterpart in the eastern gulf intense surface cooling under the wind jet is caused by entrainment of subsurface water into the upper layer the thermocline in the west deepens with the development of the eddy which is formed initially by the advection of warm surface waters from west of the gulf east of the axis of the wind the mixed layer deepens due to wind induced entrainment while at the same time shoaling and compression of the deeper isotherms by curl induced upwelling ekman pumping strengthen the thermocline

cocorp profiling across the southern oklahoma aulacogen overthrusting of the wichita mountains and compression within the anadarko basin cocorp consortium for continental reflection profiling deep reflection profiles recorded across the wichita mountains and anadarko basin suggest that significant crustal shortening occurred in the final stages of the evolution of the southern oklahoma aulacogen the crystalline rocks of the wichita mountains were thrust in pennsylvanian time northeastward over sedimentary rocks of the anadarko basin along a series of faults with moderate average 30 to 40 and southwesterly dips these faults can be traced possibly as deep as 20 to 24 km listric thrust faults and hanging wall anticlines developed in the sedimentary rocks of the basin these features contrast with conventional interpretations of pennsylvanian structures as the result of predominantly vertical movements along high angle faults and they suggest that pennsylvanian downwarping of the anadarko basin was at least partially due to thrust loading truncations of reflections from cambrian ordovician rocks in the deepest part of the basin suggest normal faulting which would support ideas of an early extensional stage in the aulacogen cycle the distinctive precambrian layering seen on earlier cocorp data recorded south of the wichita mountains cannot be recognized under the anadarko basin and the proterozoic basin containing that layering may have been bounded on its north side by a precambrian fault this inferred fault was probably twice reactivated during formation of the southern oklahoma aulacogen once during late precambrian early cambrian extension and again during pennsylvanian compression the popular view that aulacogens originated from radial rifting of updomed homogeneous continental crust is probably too simplified and a more important constraint on their location and development may be the nature of pre existing lines of weakness

fluid migration and associated diagenesis in the greater reelfoot rift region midcontinent united states the sources and pathways of basinal fluid flow on the north american craton have been the subjects of intense debate regional dolomitization and the distribution of petroleum and sulfide mineralization on the southern midcontinent are believed to be related to migration of warm basinal fluids from and within the arkoma and illinois basins because of its hydrologic connection with these two basins the reelfoot rift has been postulated as a potential source and or conduit for one or more of these fluids cathodoluminescence petrography has revealed nine distinct carbonate cement microstratigraphies in the reelfoot rift and on its adjacent carbonate platforms in missouri western platform and tennessee eastern platform temporal geochemical trends within each of the three areas are distinct eastern platform carbonate cements and their fluid inclusions exhibit decreasing 13c 18o homogenization temperatures t h and last ice melting t m values with decreasing age reelfoot rift dolomite and calcite cement and their fluid inclusions define a trend of decreasing 13c and 18o values and increasing t h and t m values western platform dolomite cements including those in the nearby southeast missouri mississippi valley type mineral district display a paragenetic trend of increasing 13c and 18o values followed by a return to lower values during the final stage of cement deposition distinct geochemical patterns indicate that stratigraphically controlled fluid flow through carbonate aquifers in the reelfoot rift was restricted within individual fault blocks that had limited communication with the adjacent carbonate platforms lateral fluid migration between fault blocks and vertical migration within fault blocks in the reelfoot rift likely was restricted by faults stratigraphic offset and impermeable units if significant fluid movement occurred from the reelfoot rift onto the adjacent western carbonate platform it likely occurred along northwest striking strike slip faults

age estimation of the cottage grove fault system illinois u s a from high resolution seismic reflection studies of coal measures abstract geologic and high resolution seismic reflection studies of pennsylvanian coal deposits have provided new insights concerning the cottage grove fault system cgfs a major east west oriented wrench fault system in southern illinois the age of faulting has previously been estimated to be late pennsylvanian to early permian based upon k ar dating of igneous intrusive rocks associated with subsidiary faults although these dates are accurate for intrusions and faulting at certain locations the duration of cottage grove master fault zone or subsidiary faulting is still unclear high resolution seismic reflection profiles were collected approximately 6 4 km north of the master fault zone in the vicinity of mapped subsidiary faulting to study coal deposits several small offset high angle normal and reverse faults were interpreted from the seismic records two small offset fault zones observed on the seismic records interpreted to be subsidiary faults of the cgfs bound a gentle asymmetric anticline several small sandstone channels located on the down thrown sides of these faults are interpreted from the seismic data vertical stacking of these sandstone channels and other sands within the coal measures is also observable in the correlated well data location of channel deposits suggests depositional control by these faults indicating that subsidiary faulting was contemporaneous with sand deposition thus subsidiary faulting of the cgfs occurred as early as middle pennsylvanian time

the ordovician sebree trough an oceanic passage to the midcontinent united states the sebree trough is a relatively narrow shale filled sedimentary feature extending for several hundred kilometers across the middle and late ordovician carbonate platform of the midcontinent united states the dark graptolitic shales within the trough stand in contrast to the coeval bryozoan brachiopod echinoderm rich limestones on the flanking platforms we infer from regional stratal patterns thickness and facies trends and temporal relations established by biostratigraphy and k bentonite stratigraphy that the sebree trough initially began to develop during late turinian to early chatfieldian time mohawkian series as a linear bathymetric depression situated over the failed late precambrian early cambrian reelfoot rift rising sea level and positioning of a subtropical convergence zone along the southern margin of laurentia caused the rift depression to descend into cool oxygen poor phosphate rich oceanic waters that entered the southern reaches of the rift from the iapetus ocean the trough apparently formed in a system of epicontinental estuarine circulation marked by a density stratified water column trough formation was accompanied by cessation of carbonate sedimentation deposition of graptolitic shales development of hardground omission surfaces substrate erosion and local phosphogenesis the carbonate platforms on either side of the trough are dominated by bryozoan brachiopod echinoderm grainstones and packstones that were deposited in zones of mixing where cool nutrient rich waters encountered warmer shelf waters concurrently lime mudstone and wackestone were deposited shoreward northern illinois wisconsin iowa minnesota michigan in warmer more tropical shallow seas coeval upward growth of the flanking carbonate platforms sustained and enhanced development of the trough shale facies five widespread diachronous late mohawkian and cincinnatian omission surfaces are present in the carbonate facies of the midcontinent these surfaces include sub deicke k bentonite ds1 top of black river limestone ds2 base and top of the guttenberg limestone member of the decorah formation ds3 and ds4 and top of the trenton limestone ds5 some of the surfaces correspond to previously described depositional sequence boundaries all five surfaces which embody deepening phases on top of highstand systems tracts converge in the sebree trough indicating that the trough was a long lived feature and was the source of eutrophic waters that episodically spread across the adjacent platforms terminating carbonate production late turinian and early chatfieldian incipient drowning episodes were followed by a final drowning event that began in the sebree trough during the late chatfieldian climacograptus spiniferus zone and reached southernmost minnesota and other regions far within the platform interior by richmondian time amorphognathus ordovicicus zone

slow deformation and lower seismic hazard at the new madrid seismic zone global positioning system gps measurements across the new madrid seismic zone nmsz in the central united states show little if any motion these data are consistent with platewide continuous gps data away from the nmsz which show no motion within uncertainties both these data and the frequency magnitude relation for seismicity imply that had the largest shocks in the series of earthquakes that occurred in 1811 and 1812 been magnitude 8 their recurrence interval should well exceed 2500 years longer than has been assumed alternatively the largest 1811 and 1812 earthquakes and those in the paleoseismic record may have been much smaller than typically assumed hence the hazard posed by great earthquakes in the nmsz appears to be overestimated

the seasat a satellite scatterometer the geophysical evaluation of remotely sensed wind vectors over the ocean on the seasat a satellite a microwave scatterometer was used to determine the vector wind over the world s oceans the technique is based on the sensitivity of microwave radar backscatter to the centimeter length ocean waves created by the action of the surface wind this paper describes the algorithm used to convert the scatterometer s measurements of ocean normalized radar cross section to the neutral stability vector wind at 19 5 m height and the comparison of these winds with high quality surface observations the wind vector algorithm used an empirical model function to describe the dependence of the ocean 0 on the 19 5 m neutral stability wind vector two model functions developed from a limited base of aircraft and satellite measurements were evaluated by using an independent set of in situ surface wind observations from the joint air sea interaction experiment jasin although these model functions were found to have some weaknesses the results of these comparisons produced better results than the seasat specifications of wind speed accuracy of 2 m s and wind direction accuracy of 20 over the 0 16 m s range of winds observed during jasin an improved model function was later developed by tuning to these jasin data so that the remaining biases between the observed surface winds and the scatterometer derived winds were minimized results are presented for this model function compared against other surface wind observations from the gulf of alaska seasat experiment and the seasat storms hurricane experiment

the not so simple relationship between seismicity and oil production in the permian basin west texas we have relocated seismicity occurring in the permian basin of west texas between 1975 and 1979 using three dimensional velocity models constructed from well log information and compared the locations to detailed geological and geophysical models of specific oil fields the seismicity appears to be related to a variety of causes including migration of naturally occurring overpressured fluids tectonic activity reservoir production and enhanced recovery operations many earthquakes may represent a combination of these effects although the permian basin is the site of numerous oil and gas fields only a limited number of fields appears to be associated with seismicity we find that there are similarities in the structural setting in the fields associated with seismicity furthermore fields within the delaware basin that are associated with seismicity are located in regions of high vertical and lateral fluid pressure gradients related to an overpressured zone within the delaware basin

lithospheric waveguide beneath the midwestern united states massive low velocity zone in the lower crust variations in seismic velocities are essential in developing a better understanding of continental plate tectonics fortunately the usarray has provided an excellent set of regional phases from the recent m5 6 oklahoma earthquake 6 november 2011 table 1 that can be used for such studies its strike slip mechanism produced an extraordinary set of tangential recordings extending to the northern edge of the usarray the crossover of the crustal slow s to the faster s n phase is well observed s ms has a critical distance of around 2 and its first multiple sms 2 reaches critical angle near a distance of about 4 and so on until s ms n merges with the stronger crustal love waves these waveforms are modeled in the period band of 2 100 s by assuming a simple three layer crust and a two layer mantle which allows a grid search approach our results favor a 15 km thick low velocity zone lvz in the lower crust with an average shear velocity of less than 3 6 km s the short period lg waves s waves at periods of 0 5 2 s travel with velocities near 3 5 km s and decay with distance faster than high frequency s n 5 0 hz which travels at a velocity of 4 6 km s and persists to large distances although these short period waveforms are not modeled their amplitude and travel times can be explained by adding a small velocity jump just below the moho with essentially no attenuation p n is equally strong but is complicated by the interference produced by the depth phase sp but well modeled the p velocities appear normal with no definitive lvz while these observations of s n and p n are common beneath most cratons the lower crustal lvz appears to be anomalous and maybe indicative of hydrous processes possibly caused by the descending farallon slab

estimation of surface winds from upward looking acoustic doppler current profilers three upward looking acoustic doppler current profilers adcp were deployed beneath meteorological buoys in the gulf of tehuantepec mexico during winter 1988 1989 hourly averaged wind speed data from the buoys and from ship when in the vicinity were compared with surface acoustic backscatter intensity recorded at the adcps the backscatter was found to be a significant predictor of wind speeds from both buoy and ship the latter when within 50 km of the mooring site there was no apparent saturation of the backscatter signal at the maximum wind speeds 15 m s 1 the results cast doubt on the ability of near surface doppler directional information to provide reliable estimates of wind direction

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

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

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

observations with moored acoustic doppler current profilers in the convection regime in the golfe du lion abstract in the golfe du lion south of france favorable conditions for deep winter convection exist and were documented by the medoc experiments during 1969 75 a renewed investigation of that regime with upward looking moored acoustic doppler current profilers adcps was carried out during 24 january 5 march 1987 to record the three dimensional currents associated with the deep mixing while in the earlier studies initial deep convection did not begin until fairly late in the winter season a very strong mistral around 10 january 1987 had already generated a 1arge deep mixed patch homogeneous down to around 2000 m at deployment time three adcps two working at 150 khz and one at 75 khz were moored in a triangle of 15 km sidelength at 550 780 m depth full records at 1 hour ensemble time intervals 400 pings per ensemble 8 m bin lengths were obtained by the 75 khz and one of the 150 khz adcps in mid february a second mistral hit the region with the onset of strong winds and surface cooling the

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

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

hotspot origin of the mississippi embayment and its possible impact on contemporary seismicity abstract previous authors have related the late cretaceous early tertiary subsidence of the mississippi embayment to the opening of the gulf of mexico but the gulf opened earlier in triassic jurassic time we offer an alternative hypothesis that development of the embayment was coeval with the passage of the mississippi valley graben system over the bermuda hotspot about 90 ma several lines of evidence of significant uplift of the embayment axis accompanying mid cretaceous magmatism and prior to late cretaceous subsidence support this proposal first reactivation of the pascola arch in the northern embayment is recorded by flanking deposits of basal upper cretaceous gravel second beneath a regional mid cretaceous unconformity subcrops of jurassic and early cretaceous strata define a pronounced southwest plunging arch in the southern embayment this arch is collinear with an arch revealed in paleozoic rocks after restoration to mid cretaceous structural geometries third a deep weathering profile on mid cretaceous alkalic plutons along the western embayment margin is nonconformably overlain by paleocene sediments and rapid mid cretaceous cooling of these intrusions has been interpreted from apatite fission tracks moreover exploratory holes along the embayment axis encountered similar weathered alkalic intrusions nonconformably overlain by basal upper cretaceous strata fourth there was an anomalous influx of clastic sediment into the northern gulf of mexico during mid cretaceous time and subsequent clastic facies patterns suggest the mississippi river drainage began to enter the gulf in the late cretaceous passage of the mississippi valley graben over the bermuda hotspot during elevated hotspot activity of cretaceous time may have significantly weakened the previously rifted lithosphere rifted continental margin at charleston south carolina also passed over this hotspot in latest cretaceous time similarly the st lawrence rift system passed over the great meteor hotspot during the cretaceous it is important to note that these rift systems are the principal loci of strong seismicity in eastern north america and thus weakening by increased cretaceous hotspot activity may be an important common factor for these seismic rift zones

sinking mafic body in a reactivated lower crust a mechanism for stress concentration at the new madrid seismic zone we propose a geodynamic model for stress concentration in the new madrid seismic zone nmsz the model postulates that a high density mafic body situated in the deep crust directly beneath the most seismically active part of the nmsz began sinking several thousands of years ago when the lower crust was suddenly weakened based on the fact that deformation rates in the nmsz have accelerated over the past 9 k y we envision the source of this perturbation to be related to the last north american deglaciation excess mass of the mafic body exerts a downward pull on the elastic upper crust leading to a cycle of primary thrust faulting with secondary strike slip faulting after which continued sinking of the mafic body reloads the upper crust and renews the process this model is consistent with the youth of activity the generation of a sequence of earthquakes and the velocity evolution during interseismic periods which depend upon the density contrast of the mafic body with respect to the surrounding crust its volume and the viscosity of the lower crust

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

deep faulting and structural reactivation beneath the southern illinois basin abstract the investigation of deep fault structure and seismogenesis within stable continental interiors has been hindered by the paucity of detailed subsurface information and by low levels of seismicity outstanding seismotectonic questions for these areas include whether pre existing structures govern the release of seismic energy as earthquakes can reactivation of such structures be recognized and to what extent have precambrian basement structures exerted long lived controls on the development of overlying phanerozoic features the southern portion of the illinois basin provides a premier area in which to study the relation between contemporary seismicity and pre existing structures due to the frequency of seismic events the concentration of available geophysical data and the wealth of borehole information we have integrated the study of this information in order to create a 2 5 dimensional picture of the earth for local seismogenic depths 0 15 km for a study area of moderate 20th century earthquake activity the area is located along the western flanks of two of the major structures within the illinois basin the wabash valley fault system wvfs and the la salle anticlinal belt lsa the results of reprocessing seismic reflection profiles combined with earthquake hypocenter parameters suggest three distinct seismotectonic environments in the upper crust first we have delineated a fault pattern that appears to correspond to the steep nodal plane of a strike slip mechanism event 1974 04 03 mb 4 7 the fault pattern is interpreted to be a deeply buried rift zone or zone of intense normal faulting underpinning a major paleozoic depocenter of the illinois basin fairfield basin second a similar event 1987 06 10 mb 5 2 and its well located aftershocks define a narrow zone of deformation that occurs along and parallel to the frontal thrust of the lsa third the hypocenter of the largest event in the study area 1968 11 09 mb 5 5 may be spatially associated with a prominent zone of dipping middle crustal reflections just west of the wvfs which have been interpreted as a deeply buried blind thrust the proposed correlation of pre existing structures with earthquakes having consistently oriented structural parameters supports the reactivation of old deformation zones by contemporary stresses as previously proposed by earlier workers however the degree to which deformation has propagated upward from precambrian basement into the paleozoic rocks varied significantly even over a small study area the societal value of associating an earthquake with a specific pre existing deformation zone in the seismogenic crust is to improve the assessment of seismic hazard or to assess the integrity of a stratigraphic formation being considered as a target for natural gas storage or carbon sequestration

a cocorp deep reflection profile across the buried reelfoot rift south central united states abstract a cocorp deep reflection survey across the northern mississippi embayment reveals essential features of the late precambrian early paleozoic reelfoot rift stratified reflections delineating the sedimentary fill of the rift basin are of variable thickness the base ranging from 2 0 to a maximum of 3 5 s twt 4 to 8 km depth in contrast the base of phanerozoic the sedimentary section immediately east of the rift is at 1 4 s 2 8 km depth correlation with a basement penetrating well indicates that the base of the upper cretaceous holocene mississippi embayment supergroup base of the cambrian ordovician knox group and the lower upper cambrian bonneterre formation are basin wide seismic stratigraphic markers clastic sedimentary strata of the lamotte formation which immediately overlie precambrian basement show clear angular discordance with the overlying subhorizontal bonneterre formation indicating that the base of the bonneterre marks the transition from active extension to passive subsidence within the reelfoot rift palinspastic restoration of the basement suggests relatively modest extension associated with rift formation 17 the blytheville arch an axial antiformal feature as well as a number of lesser structures indicative of multiple episodes of fault reactivation are also evident on the cocorp profile the crystalline basement beneath the reelfoot rift is relatively devoid of reflections down to 5 0 s 12 15 km depth beyond which it passes gradationally into a complexly reflective diffractive middle and lower crust the lower crust however does not exhibit the distinctly laminated character commonly observed in mesozoic cenozoic extensional provinces the crust mantle transition beneath the rift is characterized by a gradational downward cessation of crustal reflectivity at 12 13 s 40 km depth followed by an unreflective zone which in turn is followed by a distinct horizontal reflection at 14 5 s the latter is not observed on existing profiles outside the rift at present it is not possible to determine strictly whether the deep horizon represents moho or alternatively a discrete feature sill within the uppermost mantle however its spatial association with the reelfoot rift suggests that the two are genetically related regional relations together with the existence of dipping fabric in the deep crust beneath the rift suggest the possibility that the reelfoot rift formed along the southern extension of the circa 1 ga grenville front

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

geophysical constraints on understanding the origin of the illinois basin and its underlying crust abstract interpretation of reprocessed seismic reflection profiles reveals three highly coherent layered unconformity bounded sequences that overlie or are incorporated within the proterozoic granite rhyolite province beneath the paleozoic illinois basin and extend down into middle crustal depths the sequences which are situated in east central illinois and west central indiana are bounded by strong laterally continuous reflectors that are mappable over distances in excess of 200 km and are expressed as broad basinal packages that become areally more restricted with depth normal fault reflector offsets progressively disrupt the sequences with depth along their outer margins we interpret these sequences as being remnants of a proterozoic rhyolitic caldera complex and or rift episode related to the original thermal event that produced the granite rhyolite province the overall thickness and distribution of the sequences mimic closely those of the overlying mt simon late cambrian clastic sediments and indicate that an episode of localized subsidence was underway before deposition of the post cambrian illinois basin stratigraphic succession which is centered farther south over the new madrid rift system i e reelfoot rift and rough creek graben the present configuration of the illinois basin was therefore shaped by the cumulative effects of subsidence in two separate regions the proterozoic caldera complex and or rift in east central illinois and west central indiana and the new madrid rift system to the south filtered isostatic gravity and magnetic intensity data preclude a large mafic igneous component to the crust so that any proterozoic volcanic or rift episode must not have tapped deeply or significantly into the lower crust or upper mantle during the heating event responsible for the granite rhyolite

quaternary deformation along the meeman shelby fault near memphis tennessee imaged by high resolution marine and land seismic reflection profiles 1 a series of high resolution seismic reflection surveys was carried out in 2008 2010 and 2011 providing a total of five new seismic profiles constraining the location and character of the meeman shelby fault msf about 9 km west of memphis tennessee in the central u s the msf is the best documented fault closest to memphis yet discovered and shows a recurrent fault history the fault as imaged by the reflection profiles is 45 km long strikes n25 e and dips west northwest 83 exhibiting an up to the west sense of motion with a possible right lateral strike slip component the data show that on average the msf offsets the paleozoic unit 77 m and folds the top of the cretaceous unit and the paleocene eocene wilcox group 44 and 25 m respectively one seismic profile acquired along the mississippi river images the bottom of the quaternary alluvium warped up 28 m indicating recent activity of the msf calculated vertical slip rates of the msf during the deposition of the upper cretaceous paleocene eocene and quaternary sediments are 0 0022 0 0010 0 0004 and 0 2154 mm yr respectively suggesting an increase in fault activity during the quaternary consistent with the present stress field and the deformation of the new madrid seismic zone fault system we interpret the msf as a p shear fault in the context of a left stepping right lateral constraining strike slip fault system under a nearly east west oriented compressional stress field source scaling estimates indicate that the msf is capable of generating a m6 9 earthquake if rupturing in one event

contour mapping of relic structures in the precambrian basement of the reelfoot rift north american midcontinent a new contour map of the basement of the reelfoot rift constructed from drill hole and seismic reflection data shows the general surface configuration as well as several major and minor structural features the major features are two asymmetric intrarift basins bounded by three structural highs and the rift margins the basins are oriented normal to the northeast trend of the rift two of the highs appear to be ridges of undetermined width that extend across the rift the third high is an isolated dome or platform located between the basins the minor features are three linear structures of low relief oriented subparallel to the trend of the rift two of these located within the rift basins may divide the rift basins into paired subbasins these mapped features may be the remnants of initial extensional rifting half graben faulting and basement subsidence the rift basins are interpreted as having formed as opposing half graben and the structural highs are interpreted as having formed as associated accommodation zones some of these features appear to be reactivated seismogenic structures within the modern midcontinent compressional stress regime a detailed knowledge of the geometries of the reelfoot rift s basement features therefore is essential when evaluating their seismic risk potential

estimation of intraplate strain accumulation in the new madrid seismic zone from repeat gps surveys the new madrid seismic zone nmsz a region within the eastern portion of the north american plate where great earthquakes occurred in 1811 1812 provides a type example of large earthquakes within a relatively stable continental interior assessing the scale and causes of deformation in such regions as well estimation of the recurrence interval for the large earthquakes within them has long been recognized as an important but challenging issue for which the advent of space based geodesy provides an important new tool here we report initial results from two global positioning system gps surveys of a geodetic network spanning the new madrid seismic zone because the expected strain signal is small the data were analyzed in three different ways first site velocities were estimated and analyzed for spatial gradients parallel and perpendicular to the trend of the major strike slip faults inferred from seismicity this analysis suggests but does not require a few millimeters per year of right lateral shear across the southern portion of the seismic zone consistent with the earthquake focal mechanisms second we divided the network into near field subnetworks and found possibly significant shear associated with approximately e w shortening within the southern nmsz and possible n s shortening across the left step connecting the presumed strike slip faults third we compared the strain observed along individual baselines to that predicted from the results of earlier near field surveys which combined gps and triangulation data in the southern part of the seismic zone we found general agreement with the earlier study although our data suggest lower strain rates hence after only 2 years of measurements the gps data suggest but do not yet require detectable strain accumulation however given that great earthquakes occur in the area and that our results are reasonably consistent with the results of other geodetic studies using different data and with earthquake mechanisms it seems likely that we are seeing strain accumulation in the southern nmsz a simple interpretation of the data as showing about 3 to 5 mm yr 1 of right lateral fault parallel motion implies a 1000 to 3300 year recurrence for great earthquakes with 5 to 10 m slip assuming all the accumulating strain is released seismically this recurrence interval may be an underestimate if the observed strains are largely postseismic effects of the 1811 1812 earthquakes a possibility suggested by the observation that plate wide data show less motion across the nmsz future surveys should resolve this issue as the longer measurement spans yield more precise velocity estimates

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

receiver function and gravity constraints on crustal structure and vertical movements of the upper mississippi embayment and ozark uplift the upper mississippi embayment ume where the seismically active new madrid seismic zone resides experienced two phases of subsidence commencing in the late precambrian and cretaceous respectively to provide new constraints on models proposed for the mechanisms responsible for the subsidence we computed and stacked p to s receiver functions recorded by 49 usarray and other seismic stations located in the ume and the adjacent ozark uplift and modeled bouguer gravity anomaly data the inferred thickness density and vp vs of the upper and lower crustal layers suggest that the ume is characterized by a mafic and high density upper crustal layer of 30 km thickness which is underlain by a higher density lower crustal layer of up to 15 km those measurements in the background of previously published geological observations on the subsidence and uplift history of the ume are in agreement with the model that the cretaceous subsidence which was suggested to be preceded by an approximately 2 km uplift was the consequence of the passage of a previously proposed thermal plume the thermo elastic effects of the plume would have induced wide spread intrusion of mafic mantle material into the weak ume crust fractured by precambrian rifting and increased its density resulting in renewed subsidence after the thermal source was removed in contrast the ozark uplift has crustal density thickness and vp vs measurements that are comparable to those observed on cratonic areas suggesting an overall normal crust without significant modification by the proposed plume probably owing to the relatively strong and thick lithosphere

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

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

receiver function study of the crustal structure of the southeastern caribbean plate boundary and venezuela 1 we have investigated crustal thickness and composition across the southeastern caribbean plate boundary with the receiver function technique we used teleseismic data recorded by a temporary broadband array deployed under the bolivar project and the permanent national seismic network of venezuela we used the primary p to s conversion and crustal reverberations to estimate crustal thickness and average crustal vp vs ratio over the region we observe large variations in crustal thickness and poisson s ratio estimated moho depth ranges from 16 km beneath the southeastern caribbean sea to 52 km beneath northeastern venezuela and the venezuelan andes there is a good correlation between crustal structure and tectonic terranes data from the precambrian guayana shield suggest that the underlying crustal structure is relatively uniform with a moderate thickness 37 km and an intermediate composition a thick crust is found below the foreland basins the two mountain systems in northern venezuela the serrania del interior and the serrania del falcon have a thin crust with arc composition and are likely dynamically supported by elastic rebound or underthrusting of the oceanic plateau that characterizes the southern caribbean on the other hand the venezuelan andes and perija range on the western side of the country are probably isostatically balanced by thick crustal roots

seismic reflection profiling studies of a buried precambrian rift beneath the wabash valley fault zone sixty eight kilometers of 12 fold seismic reflection data were collected in the wabash river valley of southwestern indiana and southeastern illinois to investigate the configuration of a basement structure inferred from regional gravity and magnetic anomaly data the seismic profiles were also positioned to cross faults of the wabash valley fault system in a number of locations interpretation of the seismic reflection profiles and detailed gravity and magnetic profile data provides evidence for a series of northeasterly trending grabens in the basement these grabens are filled with pre mt simon layered rocks and are overlain by paleozoic sedimentary rocks of the illinois basin beneath the wabash river near grayville illinois an interpreted graben the grayville graben is approximately 15 km wide and contains about 3 km of fill individual boundary faults for the graben cut prominent reflectors within pre mt simon rocks and display offsets of up to 500 m the interpreted configuration of basement

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

geothermal structure in a seismoactive region of central india the geothermal structure beneath a seismoactive region of central india has been determined by using two independent geophysical parameters i e p n velocity and curie point depth which are temperature dependent p n velocity has been determined by reanalyzing deep seismic sounding dss records obtained for this region surface heat flow has been computed using the p n velocity data from this region and found to be 55 5 mw m 2 curie point depths have been derived from the magnetic satellite data in this region and reveal an identical value of heat flow of around 56 4 mw m 2 we estimated temperatures of 600 700 c at a moho depth of 39 km by using surface heat flow ranging from 50 to 60 mw m 2 and a simple 1 d geothermal model the present study suggests the presence of a brittle ductile transition high strength zone ranging in depth from 20 to 35 km where the temperatures range from 350 to 550 c we also suggest that the nucleation of earthquakes in this region took place in a relatively high strength zone this is supported by the fact that the focal depths of all the recent earthquakes from 1978 to 2000 of magnitude 3 5 lie in this zone of high strength

the basement revealed tectonic insight from a digital elevation model of the great unconformity usa cratonic platform across much of north america the contact between precambrian basement and paleozoic strata is the great unconformity a surface that represents a 0 4 b y long hiatus a digital elevation model dem of this surface visually highlights regional scale variability in the character of basement topography across the united states cratonic platform specifically it delineates phanerozoic tectonic domains each characterized by a distinct structural wavelength horizontal distance between adjacent highs and or structural amplitude vertical distance between adjacent lows and highs the largest domain the midcontinent domain includes long wavelength epeirogenic basins and domes as well as fault controlled steps the pronounced change in land surface elevation at the rocky mountain front coincides with the western edge of the midcontinent domain on the basement dem in the rocky mountain and colorado plateau domains west of the rocky mountain front structural wavelength is significantly shorter and structural amplitude significantly higher than in the midcontinent domain the bordering basins domain outlines the southern and eastern edges of the midcontinent domain as emphasized by the basement dem several kilometers of structural relief occur across the boundary between these two domains even though this boundary does not stand out on ground surface topography a plot of epicenters on the basement dem supports models associating intraplate seismicity with the midcontinent domain edge notably certain changes in crustal thickness also coincide with distinct changes in basement depth

crustal signatures of the tectonic development of the north american midcontinent abstract the stable eastern portion of the north american continent offers an excellent environment to study the tectonic development of intra continental structures the midcontinent of north america formed by the accretion of proterozoic terranes and has since experienced episodes of deformation during the subsidence of the illinois basin and uplift of the ozark plateau rifting also initiated in eastern north america but extension did not continue and arms of failed rifts extend across the region the new madrid seismic zone situated within a portion of the failed reelfoot rift represents an active zone of intraplate seismicity analyzing the structure of the crust and upper mantle within the midcontinent will therefore provide insight into the factors that lead to intraplate deformation using data from over 180 transportable array seismic stations we calculate receiver functions to investigate the crust and upper mantle of the midcontinent at close to 40 km thick the crust of the new madrid seismic zone is thinner than in the surrounding areas outside of the reelfoot rift and rough creek graben the illinois basin cannot be characterized by a single crustal structure as crust near 50 km thick in the central portion of the basin thins to between 40 and 45 km thick towards the northern and southern portions of the basin discontinuities within the crust and upper mantle are prominent in and around the new madrid seismic zone and mark locations of crustal modification and underplating comparing changes in crustal structure to the distribution of bouguer gravity anomalies the presence of positive gravity anomalies suggests that higher density crust plays a role in maintaining low surface elevations within the reelfoot rift conversely a negative gravity anomaly in an area of thinner crust within the ozark plateau supports the need for low density crustal material to influence the uplift of the plateau

pre mesozoic paleomagnetism and plate tectonics although the arguments perhaps did not convince everyone at the time the paleomagnetic polar wander paths for europe and north america plotted by runcom 1956 certainly convinced me as a young graduate student that continental drift as we called it in those pre plate tectonic days deserved serious attention then throughout the 1960s the geoscience world seemed busy documenting and refining the models of seafloor spreading and plate tectonics that governed the opening of today s oceans as the number of publications espousing plate tectonics seemed to grow exponentially an often tacit assumption was that the continental breakup was a unique event and that pangea was a quasi primordial continental configuration ahead of his time however j tuzo wilson 1966 queried in a now classical paper did the atlantic close and then re open the 1973 penrose conference in vail colorado on pre mesozoic plate tectonics dewey spall 1975 showed that by then many earth scientists were preoccupied with proving that the tacit assumption was wrong and that wilson was right but compared to mesozoic and later plate tectonics the task of quantifying pre mesozoic continental movements is much more difficult since there are no seafloor magnetic anomalies to work with and much of the potential evidence e g ophiolites diagnostic supracrustal sequences high pressure low temperature metamorphic rocks has been destroyed by that ultimate plate tectonic process the continent continent collision and by sub sequent uplift and erosion nevertheless a multitude of observations facts interpretations reinterpretations proposals and hypotheses can now be found in the literature for virtually any pre mesozoic period and any geographic

geologic structures related to new madrid earthquakes near memphis tennessee based on gravity and magnetic interpretations new inversions of gravity and magnetic data in the region north of memphis tennessee and south of latitude 36 define boundaries of regional structures and igneous complexes in the upper crust microseismicity patterns near interpreted boundaries suggest that igneous complexes influence the locations of microseismicity a weak seismicity cluster occurs near one intrusion covington pluton at the intersection of the southwest margin of the missouri batholith and the southeast margin of the reelfoot rift a narrow seismicity trend along the reelfoot rift axis becomes diffuse near a second intrusion osceola intrusive complex and changes direction to an area along the northwest flank of the intrusion the axial seismicity trend also contains a tight cluster of earthquakes located just outside the osceola intrusive complex the mechanical explanation of the two seismicity patterns is uncertain but the first cluster may be caused by stress concentration due to the high elastic stiffness and strength of the covington intrusion the spatially changing seismicity pattern near the osceola complex may be caused by the preceding factors plus interaction with faulting along the rift axis the axial seismicity strand itself is one of several connected and interacting active strands that may produce stress concentrations at strand ends and junctions the microseismicity clusters at the peripheries of the two intrusions lead us to conclude that these stress concentrations or stressed volumes may be locations of future moderate to large earthquakes near memphis

clastic pipes and mud volcanism across mars terrestrial analog evidence of past martian groundwater and subsurface fluid mobilization abstract clastic pipes are cylindrical injection features that vertically crosscut bedding with sharp contacts terrestrial pipes have cylindrical morphologies massive or radially graded interiors and raised outer rims increased grain size and subsequent cementation along the more porous edges makes the rims more resistant to weathering pipes have crosscutting relationships with other pipes due to multiple formation events or migrating eruption centers and are associated with other soft sediment deformation features terrestrial clastic pipes form via liquefaction and fluidization which require a near surface groundwater system to initiate i e sand sized sediments must be saturated for pipes to form clastic pipes cm to 1000 m diameters occur on mars across multiple time periods and terrains and are recognized in both rover and orbital imagery a conical or cylindrical geometry and massive interior core characterize these features the pipes on mars show various geomorphic expressions and are classified into five types by the presence or absence of a raised outer rim or a central pit these types are interpreted to represent the erosional evolution of pipes at different stages from their initial surface eruptive structures to their weathered remnants eroded conical hills martian pipes formed via fluidization as evidenced by 1 raised outer rims possibly indicating textural differences along the pipe edges from traction interactions of fluid suspended sediment 2 crosscutting relationships with other pipes suggesting reactivated fluidization pathways or migrating eruptive vents and 3 close spatial association between pipes and other features thought to form by soft sediment deformation e g contorted bedding integration of terrestrial analog data with the mars observations provides evidence for past martian groundwater and subsurface fluid mobilization

using gravity and magnetic data for insights into the mesoproterozoic st francois terrane southeast missouri implications for iron oxide deposits gravity and magnetic data in conjunction with available drill hole data were used to decipher the nature of the volcanic lithologies and their relationship to iron oxide ore deposits within the northwest portion of the mesoproterozoic st francois terrane in southeast missouri analyses of the data included creating a deep seated gravity anomaly map by removing the gravitational effect of the paleozoic sedimentary units residual gravity and magnetic anomaly maps and two and one half dimensional gravity and magnetic models the residual gravity and magnetic anomaly maps highlighted a series of short wavelength maxima and minima anomalies that could be interpreted to be granitic within collapsed caldera systems surrounded later by ring intrusions the gravity and magnetic models illustrate that the sources of the anomalies are situated mainly within the upper 5 km of the crust and help to constrain the location of suspected calderas the iron oxide deposits some of which contain significant quantities of ree minerals are rarely associated with both residual gravity and magnetic maxima but may be on the edge of these maxima and or occur over gravity minima the ree rich pea ridge deposit is located over a gravity minimum where modeling shows the main ore body extending down to 1 5 km with a low density region extending to 5 km below the ore body the cause of this low density body is unknown but may be associated with the formation of the ore deposit potential iron oxide deposits may occur to the south and southwest of pea ridge based on our analysis while to the northeast of the pea ridge deposit an anomaly pattern similar to that over pea ridge was found

seismic reflection images of shallow faulting northernmost mississippi embayment north of the new madrid seismic zone high resolution seismic reflection surveys document tectonic faults that displace pleistocene and older strata just beyond the northeast termination of the new madrid seismic zone at the northernmost extent of the mississippi embayment these faults which are part of the fluorspar area fault complex in southeastern illinois are directly in line with the northeast trending seismic zone the reflection data were acquired using an elastic weight drop source recorded to 500 msec by a 48 geophone array 24 fold with a 10 ft 3 0 m station interval recognizable reflections were recorded to about 200 msec 100 150 m the effects of multiple reflections numerous diffractions low apparent velocity i e steeply dipping noise and the relatively low frequency content of the recorded signal provided challenges for data processing and interpreting subtle fault offsets data processing steps that were critical to the detection of faults included residual statics post stack migration deconvolution and noise reduction filtering seismic migration was crucial for detecting and mitigating complex fault related diffraction patterns which produced an apparent folding of reflectors on unmigrated sections detected individual offsets of shallow reflectors range from 5 to 10 m for the top of paleozoic bedrock and younger strata the migrated sections generally indicate vertical to steeply dipping normal and reverse faults which in places outline small horsts and or grabens tilting or folding of stratal reflectors associated with faulting is also locally observed at one site the observed faulting is superimposed over a prominent antiformal structure which may itself be a product of the quaternary deformation that produced the steep normal and reverse faults our results suggest that faulting of the paleozoic bedrock and younger sediments of the northern mississippi embayment is more pervasive and less localized than previously thought

widespread buried precambrian layered sequences in the u s mid continent evidence for large proterozoic depositional basins large regions of the north american mid continent are underlain by precambrian layered rocks buried beneath phanerozoic sedimentary strata south of the wichita mountains published seismic reflection profiles show a precambrian layered assemblage extending for at least 40 km in both the north south and east west directions and industry data show that it may continue 150 km to the southeast seismic reflection data in the illinois region show a precambrian layered assemblage extending 320 km in an east west direction and 200 km in a north south direction in both cases the layered rocks are as much as 12 km thick apparent sequence boundaries onlap downlap within these assemblages suggest they are parts of large depositional basins with diffractions and dipping strat due to faulting the layered sequences correlate with regions of relatively long wavelength and low amplitude magnetic anomalies the extent of this magnetic signature suggests that about 200 000 km 2 of illinois indiana and western ohio about 50 000 km 2 of southernmost oklahoma and north central texas and about 32 000 km 2 of southern missouri and northern arkansas may be underlain by similar precambrian strata drill holes indicate that the top of the mid continent precambrian basement is composed largely of silicic igneous rocks such material may comprise a large part of the layered sequences alternatively these igneous rocks could be intermixed with or underlain by nonvolcanic meta sedimentary strata the strong reflectivity of some layers suggest that minor mafic flows and or sills may also be present analysis of u pb and nd sm isotopes within the granites and rhyolites imply that the layered sequences postdate crustal formation at 1 7 2 0 ga and predate or are contemporaneous with the 1 3 1 5 ga crystallization ages of the granites and rhyolites though these layered rocks have a spatial association with igneous rocks and thus have likely been metamorphosed the possibility tha they contain precambrian hydrocarbons that escaped heating is as yet untested

estimation of the coda wave attenuation and geometrical spreading in the new madrid seismic zone using the single backscattering method coda quality factor functions through coda window lengths of 20 30 40 50 and 60 s have been estimated for the new madrid seismic zone nmsz furthermore geometrical spreading functions for distances less than 60 km have been determined in this region at different center frequencies exploiting the coda normalization method a total of 284 triaxial seismograms with good signal to noise ratios snr 5 from broadband stations located in the nmsz were used the database consisted of records from 57 local earthquakes with moment magnitudes of 2 6 4 1 and hypocentral distances less than 200 km q factor values were evaluated at five frequency bands with central frequencies of 1 5 3 6 12 and 24 hz vertical components were utilized to estimate vertical coda q factor values horizontal coda q factor values were determined using the average amount of the q factor values estimated from two orthogonal horizontal components the coda q factor increases with increasing of the coda window length implying that with increasing the depth the coda q factor increases the intermediate values of the q factor and intermediate values of the frequency dependency indicate that the earth s crust and upper mantle beneath the entire nmsz is tectonically a moderate region with a moderate to relatively high degree of heterogeneities the geometrical spreading factors of s wave amplitudes are frequency dependent and determined to be 0 761 0 991 1 271 1 182 and 1 066 for center frequencies of 1 5 3 6 12 and 24 hz respectively at hypocentral distances of 10 60 km the geometrical spreading factors for lower frequencies are not recommended to be used due to the greater impact of the radiation pattern and directivity effect on low frequencies as well as the greater sensitivity of band pass filtered seismograms of small earthquakes to the noise in low frequencies

crustal structure beneath the wabash valley seismic zone from the joint inversion of receiver functions and surface wave dispersion implications for continental rifts and intraplate seismicity

mini sosie high resolution reflection survey of the cottonwood grove fault in northwestern tennessee abstract the cottonwood grove fault is located within a portion of the new madrid seismic zone in northwestern tennessee focal mechanism studies indicate that this area is a seismic transition zone to the southwest is a southwest northeast seismic trend in which movements along deeper seated faults is predominantly right lateral strike slip to the north is a southeast northwest seismic trend in which reverse and normal faulting predominate the cottonwood grove fault is buried beneath the poorly consolidated sediments of the mississippi embayment the fault as identified by an earlier vibroseis survey is a northeast southwest trending eastward dipping reverse fault with approximately 75 m 245 ft of displacement on the paleozoc cretaceous boundary a mini sosie high resolution seismic reflection survey was conducted through the village of cottonwood grove along the previously surveyed vibroseis line to improve estimates of the age geometry and displacements of the cottonwood grove fault results of the mini sosie survey reveal that displacements across the major fault are relatively consistent within cretaceous paleocene and middle eocene sedimentary rocks in upper eocene and younger rocks however there is no evidence for faulting our interpretation includes a previously undetected secondary fault at the boundary between upper cretaceous and paleocene rocks also included in our interpretation of the subsurface profile through cottonwood grove is an eocene age channel feature located 2 km east of the cottonwood grove fault in addition the paleozoic cretaceous boundary is interpreted to be an erosional surface with no intrusives included in the paleozoic rocks synthetic seismogram modeling detailed gravity survey data and theoretical gravity calculations support this interpretation and indicate that shallow intrusive bodies within paleozoic rocks are not needed to explain the observed data seismic reflections which would be expected if the intrusives were present are not observed and the observed bouguer gravity anomaly can be explained by use of irregularities on the erosional paleozoic bedrock surface along with sedimentary features within the post paleozoic sediments these data suggest that cottonwood grove fault formed during middle eocene time and that since that time any major movement on deeper faults has been predominantly strike slip with little or no vertical reactivation this interpretation is consistent with the prevailing hypotheses relating current seismicity of the new madrid seismic zone to the contemporary regional compressive stress field acting on zones of weakness associated with the precambrian reelfoot rift complex registered trademark of conoco inc registered trademark of societe nationale elf aquitane production

a kinematic model for the collision and complete suturing between gondwanaland and laurussia in the carboniferous maps of middle to late carboniferous strike slip faults in western europe northern africa and eastern north america have been compiled from geological gravity and magnetic onshore and offshore data a large majority of these faults can be interpreted in terms of a logical syn to post collisional pattern superimposed on the initial suture between the african i e gondwana and north american european laurussian continental margins the pre collision positions of the major continents are derived from paleomagnetic data and when combined with the general distribution of the strike slip faults lead to a kinematic model of the complex middle late carboniferous collision in this model the west african precambrian craton acted as a rigid promontory or die with respect to the partially rigid plastic zones between it and the large north american craton these partially rigid plastic zones comprise the external appalachian and canadian maritime margins the mauritanide and hercynian margins of north

on the determination of wind direction using an upward looking acoustic doppler current profiler schott 1989 demonstrated that velocity estimates of the near surface ocean made with an upward looking acoustic doppler current profiler adcp could be used to estimate wind directions subsequent studies have attempted to repeat schott s results with limited success brown et al 1992 ruiz 1993 visbeck and fischer 1995 considering the behavior of this near surface velocity with changes in wind speed and other observations of near surface acoustic scattering we conclude that the acoustic backscatter is caused by subsurface bubbles generated by wave breaking data sets from both the equator and the north pacific are examined the north pacific data were collected in a region where geostrophic flows are relatively small typically a few centimeters per second the adcp measured surface currents in this region are shown to be directed on average at 12 23 i e to the right of the wind direction the amount of deflection increases with wind speed we interpret this effect to be a consequence of ekman dynamics and increased bubble concentration in the surface range bin at high wind speeds in principle the wind speed dependence of the surface current deflection can be accounted for although additional data will be required to demonstrate that such a correction is generally applicable at the equator currents driven by large scale zonal winds contribute to the observed surface velocity we estimate the regional current by using adcp velocity observations from 24 m depth and calculate the locally wind driven surface current as a residual the results obtained using this technique for equatorial data show an average difference between wind direction and wind driven surface current direction to be 1 4 8

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

stressing of the new madrid seismic zone by a lower crust detachment fault a new mechanical model for the cause of the new madrid seismic zone in the central united states is analyzed the model contains a subhorizontal detachment fault which is assumed to be near the domed top surface of locally thickened anomalous lower crust rift pillow regional horizontal compression induces slip on the fault and the slip creates a stress concentration in the upper crust above the rift pillow dome in the coseismic stage of the model earthquake cycle where the three largest magnitude 7 8 earthquakes in 1811 1812 are represented by a single model mainshock on a vertical northeast trending fault the model mainshock has a moment equivalent to a magnitude 8 event during the interseismic stage corresponding to the present time slip on the detachment fault exerts a right lateral shear stress on the locked vertical fault whose failure produces the model mainshock the sense of shear is generally consistent with the overall sense of slip of 1811 1812 and later earthquakes predicted rates of horizontal strain at the ground surface are about 10 7 year 1 and are comparable to some observed rates the model implies that rift pillow geometry is a significant influence on the maximum possible earthquake magnitude