hatton bank northwest u k continental margin structure summary the continent ocean transition near hatton bank was studied using a dense grid of single ship and two ship multichannel seismic mcs profiles extensive oceanward dipping reflectors in a sequence of igneous rocks are developed in the upper crust across the entire margin at the landward shallowest end the dipping reflectors overlie continental crust while at the seaward end they are formed above oceanic crust beneath the central and lower part of the margin is a mid crustal layer approximately 5 km thick that could be either stretched and thinned continental crust or maybe newly formed igneous crust generated at the same time as the dipping reflector sequence beneath this mid crustal layer and above a well defined seismic moho which rises from 27 km continental end to 15 km oceanic end across the margin the present lower crust comprises a 10 15 km thick lens of material with a seismic velocity of 7 3 to 7 4 kds we interpret the present lower crustal lens as underplated igneous rocks left after extraction of the extruded basaltic lavas a considerable quantity of new material has been added to the crust under the rifted margin the present moho is a new boundary formed during creation of the margin and cannot therefore be used to determine the amount of thinning

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

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

petrology phase equilibria and monazite geochronology of granulite facies metapelites from deep drill cores in the ordos block of the north china craton abstract among the various precambrian crustal blocks in the north china craton ncc the geology and evolution of the ordos block remain largely enigmatic due to paucity of outcrop here we investigate granulite facies metapelites obtained from deep penetrating drill holes in the ordos block and report petrology calculated phase equilibria and in situ monazite la icp ms geochronology the rocks we studied are two samples of cordierite bearing garnet sillimanite biotite metapelitic gneisses and one graphite bearing two mica granitic gneiss the peak metamorphic age from la icp ms dating of monazite in all three samples is in the range of 1930 1940 ma the u pb th chemical ages through epma dating reveals that monazite occurring as inclusions in garnet are older than those in the matrix calculated metamorphic phase diagrams for the cordierite bearing metapelite suggest peak p t conditions ca 7 9 kbar and 775 825 c followed by decompression and evolution along a clockwise p t path our petrologic and age data are consistent with those reported from the khondalite belt in the inner mongolia suture zone in the northern part of the ordos block suggesting that these granulite facies metasediments represent the largest paleoproterozoic accretionary belt in the ncc

simulation and analysis of 1991 u s geological survey refraction data from the new madrid seismic zone two orthogonal wide aperture seismic refraction lines were acquired in the new madrid seismic zone by the u s geological survey in 1991 these n s and e w oriented lines cross the blytheville arch and adjacent portions of the reelfoot rift ten shots were fired along the e w line eight shots were fired along the n s line iterative numerical modeling including scattering and viscoelasticity has been performed for all shots on both lines using a new eighth order staggered grid two dimensional finite difference solution based on superposition of relaxation mechanisms the main features in the data can be explained by the existence of a strong 15 km wide low velocity cap lying beneath the center of the blytheville arch between 1 5 and 2 5 km depth the data do not allow estimation of its extent along the axis of the arch but it is at least 30 km long and appears to become less prominant toward the southwest the arch itself has a low velocity background that extends to at least 5 km depth an additional low velocity layer the elvins shale lies generally between 3 and 4 km depth is upwarped at the edges of the arch and thins toward the edges of the reelfoot rift there is no evidence for anomalously low intrinsic q but the data may be interpreted to indicate strong local scattering that occurs west of the low velocity cap of the blytheville arch at depths that are similar to and slightly shallower than the cap additional strong scattering occurs at the western edge of the reelfoot rift an alternative interpretation attributes the apparent scattering at the blytheville arch to instrument noise

deep seismic reflection profiling in the swiss alps explosion seismology results for line nfp 20 east in september 1986 a 120 km long seismic line was recorded through the swiss alps the line traverses major units involved in thin and thick skinned alpine tectonics our preliminary interpretation indicates that 1 lithologic boundaries such as basement cover contacts although severely deformed during the alpine orogeny can be identified on the seismic sections 2 the top of the aar massif an external basement massif extends deep into the alpine orogen 3 the upper and lower parts of the crust are more or less transparent but are separated by a swarm of reflectors at mid crustal level 4 these mid crustal reflectors might be related to trapped fluids from alpine metamorphism and 5 the moho appears as a bright reflection that steepens from the north toward the south and terminates abruptly near the center of the alpine chain perhaps because of alpine deformation

results of recent cocorp profiling in the southeastern united states summary recently acquired cocorp profiles in the southeastern united states show that 1 reflections associated with the appalachian detachment are prominent beneath the inner piedmont of western georgia but do not extend further southeast beneath the pine mountain belt 2 the brunswick magnetic low is associated with a broad zone of crustal penetrating dipping reflections that probably marks the alleghanian suture in the southeastern u s 3 the south georgia basin is a composite feature consisting of several half graben locally containing 5 km of triassic e jurassic basin fill these basins occur within the interior of the alleghanian orogen but are not specifically associated with alleghanian suture 4 across strike crustal thickness variation and distribution and character of lower crust and moho reflections in the southern appalachians is grossly similar to that observed in other parts of the appalachiadhercynian orogenic belt global comparisons suggest that these regional variations are a consequence of post collisional extensional tectonics rather than a primary palaeozoic or older feature of the orogenic belt

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

seismic velocity model of the central united states version 1 description and simulation of the 18 april 2008 mt carmel illinois earthquake abstract we have developed a new three dimensional seismic velocity model of the central united states cusvm that includes the new madrid seismic zone nmsz and covers parts of arkansas mississippi alabama illinois missouri kentucky and tennessee the model represents a compilation of decades of crustal research consisting of seismic aeromagnetic and gravity profiles geologic mapping geophysical and geological borehole logs and inversions of the regional seismic properties the density p and s wave velocities are synthesized in a stand alone spatial database that can be queried to generate the required input for numerical seismic wave propagation simulations we test and calibrate the cusvm by simulating ground motions of the 18 april 2008 m w 5 4 mt carmel illinois earthquake and comparing the results with observed records within the model area the selected stations in the comparisons reflect different geological site conditions and cover distances ranging from 10 to 430 km from the epicenter the results based on a qualitative and quantitative goodness of fit gof characterization indicate that both within and outside the mississippi embayment the cusvm reasonably reproduces 1 the body and surface wave arrival times and 2 the observed regional variations in ground motion amplitude cumulative energy duration and frequency content up to a frequency of 1 0 hz in addition we discuss the probable structural causes for the ground motion patterns in the central united states that we observed in the recorded motions of the 18 april mt carmel earthquake online material simulated and observed waveforms and response spectral acceleration ground motion prediction equation comparison

intracrustal complexity in the united states midcontinent preliminary results from cocorp surveys in northeastern kansas unusually clear indications of complex structure in the mid to lower crust is revealed by seismic reflection surveys in northeastern kansas this complexity contrasts markedly with the layer cake simplicity of both the overlying sedimentary cover and most previous crustal models for the central united states seismic sections collected by cocorp consortium for continental reflection profiling as part of a major east west traverse across the neniaha ridge and midcontinent geophysical anomaly indicate that below a thin relatively flat layered paleozoic sedimentary section the deep crust is characterized by numerous dipping and arcuate reflections and diffractions in many places layered and crosscutting these reflections suggest convoluted three dimensional folded faulted and intruded structures specific identification of these deep features may be possible if future surveys can trace them to accessible depths the basement above these reflection complexes contains significantly fewer reflections consistent with but not necessarily diagnostic of the granitic terrane that dominates basement drill hole samples in the region among the events at these shallower basement depths are several east dipping reflections some of which may be major faults travel times corresponding to expected moho depths about 36 km are characterized less by specific reflections than by an apparent decrease in the density and number of reflections while evidence of crustal heterogeneity is common among deep reflection studies the kansas seismic results outlined in this brief report stand out as being unusually clear representations of such

stream networks and long term surface uplift in the new madrid seismic zone stream networks are sensitive to low rates of surface uplift and can be used to decipher the history of large earthquakes even where faults do not rupture the surface as in intraplate seismic zones statistical analysis of alluvial network data from topographic maps in the new madrid seismic zone in the central united states shows that stream segment gradients deviate the most from an estimated natural stream profile where surface uplift is greatest evidence of cumulative deformation distilled from stream network patterns represents at least several meters of differential surface uplift during holocene time which suggests that more than one cycle of surface deformation occurred

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

local earthquake wave propagation through mississippi embayment sediments part ii influence of local site velocity structure on qp qs determinations elastic near surface structure and receiver depth effects are seen to control body wave amplitude spectra in the band from 2 to 50 hz for local earthquake observations in the new madrid seismic zone p and s wave spectral peaks are created by resonance in shallow low velocity zones and spectral nulls are created by interference of direct and free surface reflections due to receiver depth these wave propagation effects dominate sp s spectral ratios and mimic anelastic attenuation effects by making it appear that s waves are deficient in higher frequencies relative to sp waves there is no evidence from sp s spectral ratios that unconsolidated sediments of the mississippi embayment are highly attenuating dominance of p and s wave resonance suggests that the unconsolidated sediments amplify rather than attenuate seismic waves at high frequency this has important implications for estimating hazards from strong ground motions in the area in the absence of any identifiable anelastic attenuation effect in the spectral ratio the only allowable q p q s relationship for embayment sediments is that q s 3 q p based on consideration of wave travel times manuscript received 11 march 2003

geodynamics of rifting and implications for hydrocarbon habitat abstract rifting activity is probably mainly governed by frictional forces exerted on the base of the lithosphere by the convecting sub lithospheric upper mantle combined with deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system and by far filed stresses controlling the interaction of lithospheric plates moving relative to each other mantle plumes rising from the deep mantle do not appear to be a major driving force of rifting therefore it is questioned whether a distinction between active and passive rifts is still justified lithospheric extension triggers by decompression partial melting of the lower lithosphere and upper asthenosphere magmatic processes contribute materially to thinning of the subcrustal lithosphere and probably also of the lower crust this could explain the frequently observed discrepancy between upper crustal extension by faulting and lower crustal attenuation as defined by refraction and deep reflection seismic data the duration of the rifting stage of palaeorifts and passive margins is highly variable depending on plate interaction crustal separation can be achieved after as little as 7 ma and as much as 280 ma of rifting activity the availability of crustal discontinuities does not appear to play a significant role in the time required for crustal separation to be achieved the structural style of rifts is controlled by the physical state of the lithosphere the mode and amount of its extension and the lithological composition of pre and syn rift sediments subsidence of rift basins is controlled by the balance between isostatic adjustment of the crust to lithospheric stretching and uplift induced by mechanical and thermal attenuation of the subcrustal lithosphere post rift basin subsidence is mainly governed by the decay of rift induced thermal anomalies densification of the lower crust and intra plate stresses provide overprinting effects in view of this it is unlikely that quantitative analyses of post rift basin subsidence give a direct measure of lithospheric extension tectonically active rifts palaeorifts and passive margin basins contain major hydrocarbon provinces their hydrocarbon charge can rely exclusively on pre rift syn rift or post rift sedimentary sequences or a combination thereof maturation of source rocks can be achieved during the syn and or post rift stage of basin evolution during rifting conductive and convective heat transfer accounts for elevated geothermal gradients these play an important role in the maturation of pre and syn rift source rocks thermal gradients decrease asymptotically during the post rift stage

local earthquake wave propagation through mississippi embayment sediments part i body wave phases and local site responses p and s body waves from microearthquakes in the new madrid seismic zone nmsz are investigated at selected sites in an effort to understand wave propagation from future large earthquakes earthquake body waveforms display distinctive features that constrain the nature of p and s wave local site responses and wave propagation within the unconsolidated mississippi embayment sediments modeling of the waveforms demonstrates that a near surface low velocity zone is characteristic of structure within the upper 60 m of the sedimentary column and produces large p and s wave resonance effects that can be used to infer near site conditions site resonance effects change because of velocity heterogeneity between individual receivers but imply that embayment sediments will substantially amplify ground motions at high frequencies site resonance affects p and s wave amplitude spectra and can bias estimates of source and anelastic attenuation parameters travel times of observed body wave phases such as p ppphp the first p wave reverberation within the entire sedimentary column ps sp s and ssphp can be used to estimate the average wave slownesses and poisson s ratio within the embayment sediments an average poisson s ratio of 0 44 is obtained for the central nmsz under station pebm detailed s wave velocities are derived for a nafe drake sediment model using acoustic well logs and the travel time constraints of observed seismic phases v p v s ratios vary from 5 5 near the surface to approximately 2 4 at the base of the sediments use of the well log data in wave calculations also explains much of the nature of p and s wave coda within the waveforms and shows that 1d heterogeneity is a first order influence on seismic wave propagation within the mississippi embayment manuscript received 11 march 2003

late cretaceous and cenozoic geology of the new madrid seismic zone structure contour maps constructed from well seismic reflection and outcrop data of the tops of the paleozoic section upper cretaceous section paleocene midway group and eocene section illustrate the post paleozoic structure of the new madrid seismic zone region isopach maps of the late cretaceous section midway group and eocene section help constrain the timing of the structural events these maps which encompass much of the northern mississippi embayment reveal reactivation of the underlying late precambrian cambrian reelfoot rift during midway group deposition but no reactivation during late cretaceous or eocene deposition the structure contour maps also indicate a subtle south plunging depression on the tops of the paleozoic upper cretaceous and midway group along the axis of the northern mississippi embayment that we have called a trench this trench is 50 km wide has a maximum depth of 100 m and appears to have formed during the eocene the trench s western boundary coincides with the blytheville arch lake county uplift and its southeastern margin underlies memphis tennessee the blytheville arch lake county uplift is the structure responsible for the new madrid seismic zone and thus it is possible that the southeastern margin of the trench is also a fault zone northern mississippi embayment post paleozoic stratigraphy consists of sands silts and clays that thicken from 477 m at new madrid missouri to 987 m near memphis tennessee the uniformity of these sediments indicates their elastic properties and therefore seismic velocities are very similar however variations in cementation and unconformities within the section may influence seismic wave propagation manuscript received 17 june 1999

structure of the new madrid seismic source zone in southeastern missouri and northeastern arkansas new seismic reflection data in the southwestern part of the new madrid seismic zone nmsz show that a major zone of disrupted reflectors and a large antiform coincide with a prominent trend of earthquake epicenters between caruthersville missouri and marked tree arkansas we speculate that the antiform may be caused by igneous intrusions the strong correlation between the seismicity in this trend and the extent of the disrupted zone provides a geologic basis for defining the source zone for large earthquakes in this part of the nmsz

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

the relocation of microearthquakes in the northern mississippi embayment three component seismograms recorded by a small array of digital instruments in the northern mississippi embayment consistently show a high amplitude phase on the vertical component that arrives approximately 0 8 s before the shear wave on the basis of its timing and apparent velocity this phase is identified as an s p conversion from the boundary between the unconsolidated cenozoic sediments and the underlying paleozoic rocks synthetic models of ground motion in the mississippi embayment indicate that vertical displacement amplitudes are higher for s p conversions than for transmitted shear waves at all angles of incidence the models and their agreement with observations of the three component seismograms suggest that true shear waves cannot be reliably identified from vertical component seismograms recorded in this area the travel times of converted phases are used together with crustal velocity models derived from a recent seismic refraction survey to relocate approximately 500 microearthquakes recorded by the central mississippi valley regional seismic network since the network data are recorded by vertical component seismographs we assume that the s phases observed at the stations sited on unconsolidated sediments are s p conversions this assumption significantly reduces the average rms residual and provides well constrained hypocentral locations for 350 earthquakes the relocated microearthquakes cluster tightly along previously identified epicentral trends focal depths range from 0 5 to 22 km but events deeper than 14 km are rare and events shallower than 3 km are confined almost exclusively to the area between ridgely tennessee and new madrid missouri focal mechanisms are generally consistent with the results of previous studies the axial seismicity trend which extends from caruthersville missouri to marked tree arkansas is characterized by right lateral strike slip motion on a northeast trending nearly vertical fault plane the events between ridgely tennessee and new madrid missouri commonly show strike slip focal mechanisms with minor components of both normal and reverse dip slip

ductile extrusion of the higher himalayan crystalline in bhutan evidence from quartz microfabrics quartz textures measured from deformed quartz tectonites within the lesser himalaya and higher himalaya crystalline of bhutan show similar patterns orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense the asymmetry of c axis and a axis patterns consistently indicates top to the south shearing the obliquity of the texture and the inferred finite strain plane strain to moderately constrictional suggest the strain regime had a combination of rotational and irrotational strain path in most of the samples from the bhutan himalaya the inferred deformation mechanisms suggest moderate to high temperature conditions of deformation that produced the observed crystallographic preferred orientation much higher temperature of deformation is indicated in the quartz veins from a leucogranite the observed ductile deformation is pervasively developed in the rocks throughout the investigated area the intensity of deformation increases only slightly in the vicinity of the main central thrust simultaneous southward shearing within a large part of the higher himalaya crystalline near and above the main central thrust and normal faulting across the south tibetan detachment is explained by the tectonically induced extrusion of a ductily deforming wedge the process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes channel flow accounts for some observed phenomena in the himalayan orogen such as inverted metamorphic sequences near the main central thrust not related to an inversion of isotherms and the syntectonic emplacement of leucogranites into the extruding wedge locally leading to an inversion of isotherms due to heat advection

seismic reflection coefficients from mantle fault zones summary several bright reflections from structures within the mantle can be seen on birps deep seismic reflection profiles we have calculated apparent reflection coefficients for the brightest of these events and obtain values around 0 1 it is not possible to produce such large reflections by either compositional layering or seismic anisotropy if olivine and pyroxene are the only significant minerals in the mantle these large reflections can be produced by a mafic layer or a partially hydrated layer within normal peridotite the brightest reflections seem to be best explained as major faults or shear zones within the mantle

structure of the lower crust beneath the carolina trough u s atlantic continental margin data from three large offset seismic profiles provide information on the crustal structure beneath the carolina trough the profiles obtained by the u s geological survey the naval oceanographic research development agency and the scripps institution of oceanography in 1985 were oriented parallel to the trough and were located 1 seaward of the east coast magnetic anomaly ecma which is generally thought to represent the boundary between oceanic and continental crust 2 along the axis of the trough between the ecma and the hinge zone which is thought to reflect the landward limit of highly stretched and altered transitional crust and 3 along the carolina platform landward of the basement hinge zone on crust thought to have been thinned only slightly during rifting these data constrain the velocity structure of the lower crust and provide evidence for a thick lens of high velocity 7 1 km s lower crustal material that extends beneath the carolina trough and the adjacent ocean basin this lens reaches a maximum thickness of about 13 km beneath the deepest part of the trough thins to about 5 km seaward of the ecma and is either very thin or absent landward of the hinge zone it is interpreted to represent material that was underplated beneath and or intruded into the crust during the late stage of continental rifting and that led to an anomalously thick plutonic layer during the early seafloor spreading phase these data thus support the recent conclusions of white et al 1987b and mutter et al 1988 that the initiation of seafloor spreading is attended in many if not most cases by the generation of an anomalously large volume of melt

the kinematic evolution of the nepalese himalaya interpreted from nd isotopes neodymium nd isotopes from the himalayan fold thrust belt and its associated foreland basin deposits are useful for distinguishing between himalayan tectonostratigraphic zones and revealing the erosional unroofing history as controlled by the kinematic development of the orogen neodymium isotopic data from the himalayan fold thrust belt in nepal n 35 reveal that the lesser himalayan zone consistently has a more negative nd 0 value than the greater and tibetan himalayan zones our data show the average nd 0 value in the lesser himalayan zone is 21 5 whereas the greater and tibetan himalayan zones have an average nd 0 value of 16 these consistently distinct values throughout nepal enable the use of nd isotopes as a technique for distinguishing between lesser himalayan zone and greater himalayan zone rock the less negative nd 0 values of the greater himalayan rocks support the idea that the greater himalayan zone is not indian basement but rather a terrane that accreted onto india during early paleozoic time neodymium isotopic data from eocene through pliocene foreland basin deposits n 34 show that sediment provenance has been dominated by greater and tibetan himalayan detritus across nepal the nd t values in the synorogenic rocks in western and central nepal generally show an up section shift toward more negative values and record the progressive unroofing of the different tectonostratigraphic zones at 10 ma in khutia khola and 11 ma in surai khola a shift in nd t values from 16 to 18 marks the erosional breaching of a large duplex in the northern part of the lesser himalayan zone this shift is not seen in eastern nepal where the nd t values remain close to 16 throughout miocene time because there has been less erosional unroofing in this region

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

neotectonics of the upper mississippi embayment abstract although the upper mississippi embayment is an area of low relief the region has been subjected to tectonic influence throughout its history and continues to be so today tectonic activity can be recognized through seismicity patterns and geological indicators of activity either those as a direct result of earthquakes or longer term geomorphic structural and sedimentological signatures the rate of seismic activity in the upper mississippi embayment is generally lower than at the margins of tectonic plates the embayment however is the most seismically active region east of the rocky mountains with activity concentrated in the new madrid seismic zone this zone produced the very large new madrid earthquakes of 1811 and 1812 geological and geophysical evidence of neotectonic activity in the upper mississippi embayment includes faulting in the benton hills and thebes gap in missouri paleoliquefaction in the western lowlands of missouri subsurface faulting beneath and tilting of crowley s ridge in northeastern arkansas and southeastern missouri subsurface faulting along the crittenden county fault zone near memphis tennessee faulting along the east flank of the tiptonville dome and numerous indicators of historic and prehistoric large earthquakes in the new madrid seismic zone paleoearthquake studies in the new madrid seismic zone have used trenching seismic reflection shallow coring pedology geomorphology archaeology and dendrochronology to identify and date faulting deposits of liquefied sand and areas of uplift and subsidence the cause of today s relatively high rate of tectonic activity in the mississippi embayment remains elusive it is also not clear whether this activity rate is a short term phenomenon or has been constant over millions of years ongoing geodetic and geological studies should provide more insight as to the precise manner in which crustal strain is accumulating and perhaps allow improved regional neotectonic models

stratigraphy and structure of the eocene memphis sand above the eastern margin of the reelfoot rift in tennessee mississippi and arkansas usa in total 1249 geophysical well logs were interpreted to map the stratigraphy and structure of the eocene memphis sand in west tennessee northwest mississippi and crittenden county arkansas where it overlies the southeastern margin of the reelfoot rift the memphis sand is a 300 m thick reworked fluvial sand deposited on a low relief subsiding plain with limited accommodation space the memphis sand has experienced syn and postdepositional faulting of four types in the region 1 west trending grabens were active during eocene claiborne deposition these grabens also appear to control modern drainage and thus may extend to the surface 2 the n33 e shelby faults pass through shelby county tennessee into northwest mississippi and extend to or near the surface they were right lateral normal faults during eocene memphis sand deposition but they became inverted reverse faults in the late eocene 3 the n67 e striking gibson faults are high angle normal and reverse faults with a component of right lateral strike slip these faults were active during memphis sand through lower cockfield deposition north of the hatchie graben and appear to be due to faulting above an outboard fault of the reelfoot rift 4 the n43 e striking lauderdale faults are high angle normal and reverse faults that reflect right lateral strike slip reactivation of the southeastern margin of the reelfoot rift the structure and stratigraphy revealed in this mapping indicate that the geology of the memphis sand is more complex than previously recognized faulting of the memphis sand and overlying sediments reveals potentially seismogenic faults in western tennessee eastern arkansas and northwestern mississippi this study provides fundamental data for future groundwater modeling and seismic hazard analysis

reflection and transmission imaging of the upper crust using local earthquake seismograms we utilize the characteristic features of primary p and sh wave coda and sp waveforms from local microearthquake data and perform prestacking 1d migration in an attempt to image prominent reflectors in the upper crust of the new madrid seismic zone nmsz this methodology applies to data from broadband stations parm and penm of the cooperative new madrid seismic network cnmsn nearby exploration well log acoustic data of the wilson 2 14 and dow chemical wilson number 1 wells are used to constrain the upper 4 km of p wave velocity model despite polarity differences among p sh and sp waveforms this technique demonstrates that consistent reflectors in the deep sedimentary section can be imaged commonly among the three wave types there are excellent correlations associated with the base of the upper cretaceous holocene mississippi embayment supergroup and the base of knox group which were also reported in a study of nearby seismic reflection profiles by hamilton and zoback 1982 the bonne terre formation seems to be a prominent seismic stratigraphic marker associated with an interface displayed in the profiling we find that earthquake event s p times must be 3 sec or more to resolve reflectors at about 4 km depth possible basement at about 4 0 to 4 5 km appears on the reflected p wave image for parm and sh wave image for penm we conclude that the velocity structure of the upper 4 km crust beneath parm can be represented by a constant velocity of graphic 1 and graphic 2 for the unconsolidated sediments and graphic 3 and graphic 4 for the paleozoic sedimentary rocks a constant velocity of graphic 5 and graphic 6 for the unconsolidated sediments and graphic 7 and graphic 8 for the paleozoic sedimentary rocks can represent structure beneath penm 1 embed inline graphic 1 gif 2 embed inline graphic 2 gif 3 embed inline graphic 3 gif 4 embed inline graphic 4 gif 5 embed inline graphic 5 gif 6 embed inline graphic 6 gif 7 embed inline graphic 7 gif 8 embed inline graphic 8 gif

imaging the new madrid seismic zone using double difference tomography 1 three dimensional p and s wave velocity vp and vs models and high resolution earthquake relocations are determined for the new madrid seismic zone using double difference local earthquake tomography the data set consists of arrival times and differential times recorded by the cooperative new madrid seismic network cnmsn from 2000 to 2007 and the 1989 1992 portable array network and data acquisition deployment waveform cross correlation derived differential times for the cnmsn data are also incorporated the velocity solutions are compatible with previous solutions centered on the active arms of seismicity and cover a broader area including mafic intrusions along the margin of the reelfoot rift major features include elevated vp and vs associated with the mafic plutons and reduced vp and vs along and southeast of the axial fault af a major arm of seismicity trending along the rift axis low vp extends to a depth of at least 20 km along the portion of the af that extends south of the missouri bootheel a locally high vp vs anomaly imaged along the central portion of the reelfoot fault is spatially correlated with a significant change in fault trend and is interpreted as a region containing high pore pressure and or water filled microcracks

effects of irregular structure of the mississippi embayment on ground motion amplification the objective of this study is to evaluate effects of the presently available 3d velocity model of the mississippi embayment structure on the amplification of seismic waves by using simulated finite difference seismograms effects of both 2d and 3d embayment basement structures were considered the 3d model included information of the near surface velocities that were derived from the existing 1d velocity models the 2d crustal model was taken from catchings 1999 which extended from saint louis missouri to memphis tennessee finite difference seismograms were simulated for point sources embedded at both ends of the 2d structure these seismograms were examined to distinguish features like peak amplitude amplifications and duration of seismograms when the seismic waves propagated from the mississippi embayment toward the illinois basin and vice versa to establish a working 3d structure model of the embayment we compiled geologic information of the region on material properties of the shallow structure and used the 3d model developed at the center of earthquake research institute ceri memphis as the starting model the 3d model was used to generate finite difference seismograms along several profiles for a m w 7 2 scenario earthquake occurring on the new madrid fault zone an equivalent 1d model which included the basin materials was also used to compare the 3d versus equivalent 1d ground motions simulated using the finite difference method to establish the amplification factors due to the surface sediments in the 3d model finite difference seismograms were also computed for a 1d hard rock reference model these 1d and 3d responses of the mississippi embayment were used for estimating ground motion amplification at sites where the depth to the basement is deeper than 500 m our investigation suggests that the deep structure of the mississippi embayment has little impact on long period ground motion amplitudes t 2 sec for large earthquakes that rupture in the central part of the basin this supports the hypothesis that for engineering purposes ground motions simulated based on the equivalent 1d crustal model are adequate for representing ground motions from future large earthquakes m w 7 occurring in the new madrid seismic zone

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

preserved zr temperatures and u pb ages in high grade metamorphic titanite evidence for a static hot channel in the himalayan orogen abstract titanite grains from 800 c gneisses of the c 6 km thick greater himalayan sequence of central nepal were analyzed for zr in titanite temperatures and u pb ages to investigate the formation and evolution of a former weak mid crustal channel in the himalaya laser ablation icp ms depth profiles and spot analyses of titanite zr and u pb isotopic compositions are inconsistent with diffusional reequilibration on scales greater than 1 2 m and instead data appear to faithfully record both temperature and age titanite records protracted heating from 700 750 c at 37 ma to 775 c at 24 ma and possibly slight cooling to 765 c at 20 ma such temperatures exceed partial melting reactions in associated metapelites and imply profoundly weak rheologies i e a hot channel yet predate initiation of the bounding main central thrust and south tibetan detachment systems by as much as 15 myr evidently thick but weak crustal channels may remain stationary with respect to the convergence direction for 10 myr even during one of earth s biggest continent continent collisions

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

crustal structure of the grenville front and adjacent terranes under the auspices of the great lakes international multidisciplinary program on crustal evolution approximately 320 km of deep seismic reflection data were collected in lake huron along a profile that extends east from the manitoulin terrane across the grenville front to the interior of the grenville orogen the manitoulin terrane is characterized by a series of gently east dipping reflections at about 20 km depth that separate a highly reflective lower crustal layer from a markedly less reflective upper layer imaged by strong reflections at the western end of a spectacular band of moderately east dipping reflections the grenville front clearly truncates manitoulin terrane structures to the west these data are interpreted in terms of a speculative two stage model involving 1 creation of a major decollement during northward collision of an allochthonous terrane with the southern superior cratonic margin 1 83 1 89 ga penokean orogeny and 2 northwest directed stacking of microterranes at the southeastern margin of the craton followed by crust penetrating ductile imbrication under high pressure high temperature conditions leading to the ramping of deeply buried rocks to the near surface 1 0 1 3 ga grenvillian orogeny

an evaluation of the inverted metamorphic gradient at langtang national park central nepal himalaya the crystalline core of the himalayan orogen in the langtang area of nepal located between the annapurna manaslu region and the everest region contains middle to upper amphibolite grade pelitic gneisses and schists these rocks are intimately associated with the main central thrust mct one of the major compressional structures in the northern indian plate which forms a 3 7 km wide zone containing rocks of both footwall and hangingwall affinity an inverted metamorphic gradient is noticeable from upper footwall through hangingwall rocks where metamorphic conditions increase from garnet grade near the mct zone to sillimanite k feldspar grade in the upper hangingwall petrographic data distinguish two metamorphic episodes that have affected the area a high pressure moderate temperature episode m1 and a moderate pressure high temperature episode m2 comparison with appropriate reaction boundaries suggests that conditions for m1 in the hangingwall were approximately 900 1200 mpa and 425 525 c thermobarometric results for 24 samples from the footwall mct zone and hangingwall reflect p t conditions during the m2 phase of 400 1200 mpa and 490 660 c the decrease in estimated palaeopressures from footwall to hangingwall approximate a lithostatic gradient of 27 mpa km 1 with slight fluctuations in the mct zone reflecting structural discontinuities in contrast to the palaeopressures palaeotemperatures are indistinguishable across the entire area sampled although field evidence suggests the presence of the inverted palaeothermal gradient well known in the himalaya quantitative thermobarometry indicates that temperatures of final equilibration were all within error of each other across 17 km of section at langtang change in pressure is responsible for the presence of the sequence of index minerals through the section i interpret these data to reflect diachronous attainment of equilibrium temperature conditions in a lithostatic palaeopressure profile after ductile faulting of the sequence

major proterozoic basement features of the eastern midcontinent of north america revealed by recent cocorp profiling cocorp profiling in the eastern midcontinent of north america has 1 traced an extensive sequence of precambrian layered rocks beneath southern illinois indiana and western ohio 2 detected a broad zone of east dipping basement reflectors associated with the grenville front beneath western ohio and 3 discovered a wide region of west dipping reflectors penetrating most of the crust beneath eastern ohio the precambrian layered assemblage may be as much as 11 km thick beneath southern illinois extends at least 170 km in an east west direction and contains several strong reflectors that have a lateral continuity of 80 km or more industry seismic data indicate that the layering is extensive in a north south direction as well possible explanations for the layering include the silicic igneous rocks of the ca 1 48 ga eastern granite rhyolite province which are penetrated by basement drill holes throughout the region perhaps intermixed or underlain by mafic igneous or sedimentary rocks the 40 50 km wide zone of strong east dipping 25 30 reflectors beneath west central ohio corresponds to the position of the grenville front as determined from potential field and drill hole data these dipping reflectors in the upper and middle crust are interpreted to result from ductile deformation zones mylonites like those exposed at the grenville front in canada and imaged on the glimpce seismic reflection lines in lake huron both the cocorp and glimpce lines show a remarkably similar reflection geometry despite the more than 500 km separating the two profiles easternmost ohio appears to be underlain by pronounced west dipping 40 reflectors in the middle and lower crust which are also interpreted as marking a region of pervasive ductile deformation 80 km or more in width analogy with similar reflection packages elsewhere suggests that these reflections may mark a major collision zone the west dipping reflectors may be correlative with similar reflectors imaged on another cocorp survey in northern alabama the correlations suggested by these new results though tentative imply that the eastern midcontinent is composed of a relatively simple assemblage of crustal blocks

origin and age of the manila high and associated big lake sunklands in the new madrid seismic zone northeastern arkansas uplift of the manila high and subsidence of the south flowing little river during the great new madrid earthquakes of 1811 1812 formed the present lake in big lake basin of northern arkansas this is the most recent deformation of the 19 km long manila high that began between 11 000 and 5400 yr ago and is time transgressive toward the south at least 4 m of holocene uplift occurred on the northern portion of the manila high prior to initiation of the little river distributary in mid holocene time the distributary is deflected around the high and overbank sediment derived from it thins and is locally absent across the northern highest portion of the high deformation along the southern end of the high began between 3500 and 2000 yr ago since 3500 yr b p 2 m of uplift appears to have caused a reduction in gradient anastomosing upstream of the uplift and incision of the little river across the uplift north of the manila high in the big lake basin the little river has an increased aggradation rate 10 times that of the preuplift rate cores collected in and adjacent to big lake indicate that the little river was ponded twice apparently in response to deformation in 1811 1812 and between 90 b c and a d 1640

timing of india asia collision geological biostratigraphic and palaeomagnetic constraints 1 a range of ages have been proposed for the timing of india asia collision the range to some extent reflects different definitions of collision and methods used to date it in this paper we discuss three approaches that have been used to constrain the time of collision the time of cessation of marine facies the time of the first arrival of asian detritus on the indian plate and the determination of the relative positions of india and asia through time in the qumiba sedimentary section located south of the yarlung tsangpo suture in tibet a previous work has dated marine facies at middle to late eocene by far the youngest marine sediments recorded in the region by contrast our biostratigraphic data indicate the youngest marine facies preserved at this locality are 50 6 52 8 ma in broad agreement with the timing of cessation of marine facies elsewhere throughout the region double dating of detrital zircons from this formation by u pb and fission track methods indicates an asian contribution to the rocks thus documenting the time of arrival of asian material onto the indian plate at this time and hence constraining the time of india asia collision our reconstruction of the positions of india and asia by using a compilation of published palaeomagnetic data indicates initial contact between the continents in the early eocene we conclude the paper with a discussion on the viability of a recent assertion that collision between india and asia could not have occurred prior to 35 ma

a petrogenetic grid for pelitic schists in the system sio 2 al 2 o 3 feo mgo k 2 o h 2 o a quantitative petrogenetic grid for pelitic schists in the system kfmash that includes the phases garnet chlorite biotite chloritoid cordierite staurolite talc kyanite andalusite sillimanite and pyrophyllite with quartz h2o and muscovite or k feldspar in excess is presented the grid is based on thermodynamic data of berman et al 1985 and berman 1988 for endmember kfash and kmash equilibria and natural fe mg partitioning for the kfmash system calculation of p t slopes and the change in fe fe mg along reactions in the kfmash system were made using the gibbs method in addition the effect on the grid of mno and cao is evaluated quantitatively the resulting grid is consistent with typical buchan and barrovian parageneses at medium to high grades at low grades the grid predicts an extensive stability field for the paragenesis chloritoid biotite which arises because of the unusual facing of the reaction chloritoid biotite quartz h2o garnet chlorite muscovite which proceeds to the right with increasing t in the kfmash system however the reaction proceeds to the left with increasing t in the mnkfash system so the assemblage chloritoid biotite is restricted to bulk compositions with high fe fe mg mn typical metapelites will therefore contain garnet chlorite at low grades rather than chloritoid biotite

crustal channel flows 2 numerical models with implications for metamorphism in the himalayan tibetan orogen 1 results from a thermal mechanical model ht1 that includes midcrustal channel flow are compatible with many features of the himalayan tibetan system radioactive self heating and rheological weakening of thickened model orogenic crust lead to the formation of a hot low viscosity midcrustal channel and a broad plateau channel material corresponding to the greater himalayan sequence ghs flows outward from beneath the plateau in response to topographically induced differential pressure at the plateau flank it is exhumed by focused surface denudation and juxtaposed with cooler newly accreted material corresponding to the lesser himalayan sequence lhs the model channel is bounded by coeval thrust and normal sense ductile shear zones interpreted to represent the main central thrust mct zone and south tibetan detachment system respectively inverted metamorphism associated with the model mct zone results from distributed ductile shear along the mct and extrusion of the hot channel a variety of model p t t path styles resembling those observed in the ghs and lhs are produced for points traveling through contrasting tectonic regimes that coexist in different parts of the model predicted times of peak metamorphism cooling and erosion of metamorphic facies are generally compatible with observations although model ghs cooling ages are too young the times of m1 and m2 metamorphic events observed in the ghs correspond to model times of maximum burial and maximum heating respectively the results highlight the need to integrate tectonics and metamorphism in continental collision models and demonstrate the importance of lateral transport of both heat and material in large hot orogens

deep structure of the texas gulf passive margin and its ouachita precambrian basement results of the cocorp san marcos arch survey this cocorp deep seismic survey provides a comprehensive image of the southeast texas part of the gulf passive margin and its accreted ouachita arc foundation beneath the updip limit of the cenozoic sediment wedge a prominent antiformal structure is imaged within the interior zone of the buried late paleozoic ouachita orogen the structure appears to involve precambrian grenville basement the crest of the antiform is coincident with the cretaceous tertiary luling mexia talco fault zone some of these faults dip to the northwest counter to the general regional pattern of down to the basin faulting and appear to sole into the top of the antiform suggesting that the ouachita structure has been reactivated as a hingeline to the subsiding passive margin the antiform may be tied via this fault system and the ouachita gravity gradient to the similar devils river waco and benton uplifts interpreted as precambrian basement cored massifs above the paleozoic sequence a possible rift related graben is imaged near the updip limit of jurassic salt paleoshelf edges of the major tertiary depositional sequences are marked by expanded sections disrupted by growth faults and shale diapirs within the wilcox formation the transect crosses the mouth of the 900 m deep yoakum canyon a principal pathway of sediment delivery from the laramide belt to the gulf beneath the wilcox the comanchean lower cretaceous shelf edge capped by the stuart city reef is imaged as a pronounced topographic break onlapped by several moundy sediment packages because this segment of the line parallels strike the topographic break may be interpreted as a 2000 m deep embayment in the cretaceous shelf edge and possibly a major submarine canyon older and deeper than the yoakum canyon

petrologic case for eocene slab breakoff during the indo asian collision the greater himalayan sequence is the metamorphic core of the himalaya and has been a focus of considerable study yet its petrologic evolution remains controversial pre oligocene metamorphism was nearly obliterated by miocene metamorphism and melting which many workers ascribe to shear heating unusually high concentrations of radioactive elements accompanying burial and or several kilometers of miocene exhumation due to extensional faulting sparse oligocene to eocene ages are often assigned to a tectonically unspecified event an alternative slab breakoff model subduction of greater himalayan rocks to 100 km depth in the eocene followed by buoyant extrusion due to decoupling of the oceanic lithosphere has also been proposed based on rare eocene eclogites but without explanation of miocene melting or metamorphic petrogenesis we argue that slab breakoff readily explains the eocene eclogites miocene partial melts and late eocene k rich magmas in southeastern tibet and that metamorphic and plutonic ages help define the timing and rates of breakoff and extrusion this model implies that 1 much of the greater himalayan sequence was subducted to depths greater than commonly considered 2 fluid absent decompression melting at 30 35 km depth was the consequence of as much as 100 km of extrusion rather than radioactive heating or a smaller crustal level extensional or erosional event and 3 eruption of eocene k rich tibetan plateau lavas has no implication for topography of the tibetan plateau

the origin of crowley s ridge northeastern arkansas erosional remnant or tectonic uplift the geomorphology of crowley s ridge apparent quaternary reactivation of the ridge bounding faults and orientation of the faults to the present stress field suggest that the faults bounding crowley s ridge and the western margin of the reelfoot rift may be seismogenic fifteen high resolution seismic reflection lines were acquired across and adjacent to the margins of crowley s ridge to test the hypothesis that corwley s ridge is fault bounded eleven of the reflection lines were 24 channel mini sosie data with 1 sec record length and four lines were 24 channel shotgun source data with 0 25 sec record length nine lines traversed the margins and adjacent areas of the northern segment of the ridge near jonesboro arkansas and the other six were located south of jonesboro on the southern segment the reflection data show that paleozoic through eocene wilcox sections have been uplifted beneath crowley s ridge from 60 to 30 m across ridge bounding faults however eocene or pliocene pleistocene near surface strata appear to be displaced a maximum of 7 5 m based on reflection and geomorphologic data we believe that reactivation of the bounding faults of crowley s ridge occurred concurrently with and strongly influenced wisconsin denudation of the mississippi valley

quaternary deformation and fault structure in the northern mississippi embayment as imaged by near surface seismic reflection data seismicity in the new madrid seismic zone nmsz in the central united states constrains the location of present deformation at depth along four main distinct arms while the surface expression of the ongoing deformation is still unclear to better constrain the surface deformation in the nmsz we integrate existing seismic reflection data with a new 300 km long high resolution seismic reflection profile acquired along the mississippi river from cape girardeau mo to caruthersville mo based on the data we interpret the reelfoot thrust and the new markham fault as upward splays of a blind master fault defined by the seismicity and extending at depth farther north to the south two faults the axial fault and the cottonwood grove fault are imaged above the southern arm of the nmsz both fault display deformation of the paleozoic through the tertiary sediments and a relief of 20 25 m at the base of the quaternary alluvium which we interpret as the result of strike slip motion along a complex fault plane geometry we propose two alternative interpretations for the relationship between the shallow faults and the seismicity in this area 1 the faults merge at depth and are presently both active and 2 the faults are distinct at depth and were active during the quaternary and only the axial fault is presently deforming geological structures mapped at the surface as part of this study show that quaternary deformation is accommodated along a fault network that is more complex than the simple four arm system illuminated by the seismicity a behavior predicted by analog and computer models

intrusions and anomalous vp vs ratios associated with the new madrid seismic zone 1 detailed p wave velocity vp and s wave velocity vs models and vp vs ratios were determined for a major portion of the new madrid seismic zone using arrival times recorded by the new madrid seismic network and portable array for numerical data acquisition panda stations we performed a simultaneous inversion for p and s wave velocities and hypocentral locations yielding the most detailed tomographic image of the upper portion of the crust to date low vp and high vs anomalies resulted in low vp vs ratios that correspond to the major arms of seismicity north of the intersection of the cottonwood grove blytheville arch cg ba fault with the reelfoot fault the unusual low vp vs values can be attributed to the presence of quartz rich rocks two regions contain anomalous vp and vs values and vp vs ratios that cannot be attributed to variations in rock composition and are probably produced by overpressured fluids one region is located on the hanging wall of the northern portion of the reelfoot fault and is aseismic the other region corresponds to a portion of the southern reelfoot fault that experiences swarm activity a distinct velocity contrast exists across the cg ba fault at depths exceeding 4 65 km basement rocks southeast of the fault have vp values that are 4 6 slower than values for rocks located to the northwest the most logical explanation is that the fault follows a preexisting structural feature or lithologic change in basement rocks

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

seismic imaging of extended crust with emphasis on the western united states understanding of the crust has improved dramatically following the application of seismic reflection and refraction techniques to studies of the deep crust this is particularly true in areas where the last tectonic event was extensional such as the basin and range province of the western united states and much of western europe in these regions a characteristic reflective pattern has emerged whereby the lower crust is highly reflective and the upper crust and upper mantle are either poorly reflective or strikingly nonreflective in the metamorphic core complex belt in the western united states where extension can be as much as an order of magnitude greater than in the more classic continental rift zones the lower crustal reflectivity thickens and rises yielding a picture of a crust that is reflective throughout synthetic seismic studies have documented that the reflectivity in these regions can be modeled by numerous laminae tens of meters thick and hundreds of meters across characterized by inter layered high and low velocities two geologic factors are interpreted as contributing to this layered character ductile strain responding to stress in the thermally weakened middle and lower crust and intrusive layering corresponding to injection of subhorizontal sheets of mantle derived magmas these two processes yield a variety of geologic structures including transposed compositional layering mylonitic ductile shear zones and intrusive mafic sheets all of which occur at the proper scales to cause the prominent reflectivity observed if metamorphic core complexes are representative of extended continental crust world wide then these results suggest that magmatism and ductile flow have also contributed to the evolution of the middle and lower crust in many other areas around the world

synthrusting metamorphism cooling and erosion of the himalayan kathmandu complex nepal in this paper we tackle some of the outstanding problems of the himalaya in particular the external zone in the kathmandu complex using an integrated approach involving field mapping microstructure thermobarometry and geochronology the result is a new model showing the evolution of one major main central thrust therefore we refute suggestions that the kathmandu complex is a klippe or separate thrust sheet compared to the main central thrust sheet in the high himalaya the kathmandu complex shows differences in deformational and metamorphic features and timing of metamorphism that are consistent with its position some 100 km south of the high himalaya fairly near the leading edge unless there was substantial volume loss between the time of peak metamorphism and the beginning of thrusting then our geobarometry results indicate that the main central thrust wedge was 40 km thick on the northern side of the kathmandu complex and 20 km thick on the southern margin initiation of the main central thrust occurred at 22 ma possibly during the closing stages of peak amphibolite facies metamorphism slip at elevated temperature 500 600 c continued until 14 ma this is a little longer than has previously been proposed in marked contrast to the famous inverted metamorphism on the main central thrust in the high himalaya the metamorphic zonal scheme in the kathmandu complex is right way up with the exception of a thin zone of greenschist facies thrust related dynamically metamorphosed rocks at the base these mylonites postdate the high grade regional amphibolite metamorphism and give an illusion of inverted metamorphism a likely reason for the contrast is that the main central thrust cut up section toward the foreland and therefore at kathmandu carries high levels in the metamorphic structure our model involves reactivation of the main central thrust at 7 8 ma as inferred from published monazite and mica ages but because the kathmandu rocks show no evidence for high temperature reactivation at this time we presume that the late reactivation involved only the internal high himalaya zone while the main central thrust was inactive in the external kathmandu zone we attempt to quantify rates of cooling exhumation and thrusting during time period 22 ma to the present

deep seismic reflection characteristics of the continental crust four types of seismic reflection fabrics appear to characterize the continental crust 1 a cratonic fabric dominated by diffractions and dipping reflections but lacking a pronounced reflection moho 2 regions of thin skinned foreland deformation with abundant shallow but sparse deep reflections 3 a unidirectional fabric that dips toward the hinterland and may represent a crustal scale ramp zone and 4 zones of highly layered lower crust and high amplitude reflection moho the first three types are commonly observed in transects across single mountain belts but the fourth appears to correspond to regions of elevated lower crustal temperatures such as those encountered in rift provinces and perhaps elsewhere

dynamic processes controlling evolution of rifted basins abstract the extension of the lithosphere controlling the development of rifted basins is driven by a combination of plate boundary forces frictional forces exerted on the base of the lithosphere by the convecting asthenosphere and deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system although mantle plumes are not a primary driving force of rifting they play an important secondary role by weakening the lithosphere and by controlling the level of rift related volcanic activity a distinction between active and passive rifting is only conditionally justified the extension of the lithosphere depending on its rate and magnitude and the potential temperature of the asthenosphere can cause by adiabatic decompression partial melting of the lower lithosphere and upper asthenosphere in rift systems the level and timing of volcanic activity is highly variable the lack of volcanic activity implies passive rifting an initial passive rifting stage can be followed by a more active one during which magmatism plays an increasingly important role magmatic destabilization of the moho may account for the frequently observed discrepancy between upper and lower crustal extension factors combined with evidence for thermal thinning of the mantle lithosphere this suggest that the volume of the lithosphere is not necessarily preserved during rifting as advocated by conventional stretching models the structural style of rifts is controlled by the rheological structure of the lithosphere the availability of crustal discontinuities that can be tensionally reactivated the mode orthogonal or oblique and amount of extension and the lithological composition of pre and syn rift sediments simple shear extension prevails in rifts that subparallel the structural grain of the basement pure shear extension is typical for rifts cross cutting the basement grain pre existing crustal and mantle lithospheric discontinuities contribute to the localization of rift systems the duration of the rifting stage of extensional basins is highly variable stress field changes can cause abrupt termination of rifting in major rift systems progressive strain concentration on the zone of future crustal separation entails abandonment of lateral rifts depending on constraints on lateral block movements crustal separation can be achieved after as little as 9 my and as much as 280 my of rifting activity syn rift basin subsidence is controlled by isostatic adjustment of the crust to mechanical stretching of the lithosphere its magmatic inflation and thermal attenuation of the mantle lithosphere post rift basin subsidence is governed by thermal reequilibration of the lithosphere asthenosphere system deep seated thermal anomalies related to syn rift pull up of the asthenosphere lithosphere boundary have decayed after 60 my by about 65 and after 180 my by about 95 the magnitude of post rift subsidence is a function of the rift induced thermal anomaly and crustal density changes the potential temperature of the asthenosphere and initial water depths intraplate stresses can have an overprinting effect on post rift subsidence stretching factors derived from post rift subsidence analyses must be corrected for such effects

the appalachian ouachita connection paleozoic orogenic belt at the southern margin of north america late paleozoic orogenic belt structures exposed in the appalachian mountains of alabama and in the ouachita mountains of arkansas plunge from opposite directions beneath postorogenic mesozoic cenozoic strata in the mississippi embayment of the gulf coastal plain figure 1 the paleozoic stratigraphic sequence and details of structural style in the ouachita outcrops contrast strongly with those in the appalachian outcrops furthermore straight line projection of structural strike from the outcrops does not lead to a simple connection of structures beneath the coastal plain concealed by the coastal plain cover are answers to questions such as the following are the orogenic belts continuous discontinuous offset or intersecting how are along strike changes in stratigraphy and structural style expressed what is the sequence of tectonic evolution early attempts to interpret the subsurface relationships between the appalachian and ouachita structures were of necessity based on the first few scattered wells that were drilled through coastal plain strata into paleozoic rocks subsequent studies have enjoyed the availability of progressively more numerous deep wells and a growing accumulation of geophysical data various stages in the evolution of thought were outlined by king 1950 1961 1975 and by thomas 1973 1976 this review summarizes available subsurface data in a pre mesozoic paleogeologic

early ordovician u pb metamorphic ages of the eclogite bearing seve nappes northern scandinavian caledonides eclogite grade metamorphism of the seve nappe complex snc in norrbotten sweden records the attempted subduction of the baltic continental margin during the early palaeozoic evolution of the iapetus ocean metamorphic titanite sampled from several calcsilicate gneisses of the snc in norrbotten occurs as part of a prograde eclogite facies metamorphic mineral assemblage and yields concordant to nearly concordant u pb ages of 500 475 ma later structural disruption of these rocks occurred during the siluro devonian scandian phase of the caledonide orogeny but the u pb systematics show no evidence of a second generation metamorphic or recrystallized of titanite or of post early ordovician disturbance through pb loss hence the u pb ages are believed to record the time of prograde mineral growth during eclogite facies metamorphism of the snc these results support earlier sm nd and 40ar 39ar studies indicating an early ordovician metamorphic age for the eclogitic norrbotten snc and confirm the early ordovician destruction of at least this segment of the palaeozoic passive margin of baltica these results indicate that the snc in the northern scandinavian caledonides was subducted and metamorphosed to high grade some 50 70 myr prior to the high grade metamorphism of the snc in the central scandinavian caledonides this result requires significantly different early palaeozoic tectonic histories for rocks mapped as snc in the northern caledonides and those in the central caledonides despite a seemingly similar tectonostratigraphic position and broadly similar high grade metamorphism

aeromagnetic and gravity anomaly constraints on the crustal geology of ohio the u s geological survey and the ohio geological survey recently compiled aero magnetic anomaly maps and revised gravity anomaly data for crustal studies of ohio updated structural and lithologic maps of the precambrian basement surface provide additional constraints to limit interpretational ambiguities of the geopotential anomaly data the combined modeling of five east west gravity and magnetic anomaly profiles supports a generalized crustal model for ohio involving a lower zone of intermediate composition a more felsic and relatively homogeneous middle zone and an upper zone consisting of intrusive and extrusive igneous rock in the western third of the state and medium grade grenville metamorphic rocks to the east modeling of the geopotential field anomalies indicates the presence of large dense lower crustal bodies in western and south central ohio that may be related to a paleo rift system large possibly anorthositic masses in southern ohio post tectonic granitization in northwestern ohio and a volcanic source for igneous rocks in southwestern ohio may also be interpreted from the combined anomaly analysis

tectonic geomorphology of the mississippi valley between osceola arkansas and friars point mississippi abstract a geomorphic study of the mississippi river and its alluvial valley between osceola arkansas and friars point mississippi has identified anomalous surface features that can be linked to known geological structures for example relatively recent deformation along big creek fault zone white river fault zone bolivar mansfield tectonic zone blytheville arch crittenden county fault zone and reelfoot rift margins is suggested by river and topographic anomalies faults and plutons appear to affect drainage networks and the morphology of crowleys ridge suggests significant fault control the many anomalies probably reflect a fractured suballuvial surface although movement along these fractures will most likely occur in seismically active areas the probability of movement elsewhere