elastic wave velocity structure of the crust and upper mantle beneath the north china basin travel time residuals from locally and teleseismically recorded earthquakes are used to determine the three dimensional elastic wave velocity structure beneath the central north china basin and surrounding mountains we calculated p wave and s wave travel time residuals for 200 aftershocks of the 1976 tangshan earthquake and p wave travel time residuals for 194 teleseisms recorded by the beijing network between 1976 and 1983 we performed iterative one and three dimensional block inversions for crust and uppermost mantle structure based solely on the aftershock travel time residuals we also performed iterative three dimensional block inversions for crust and upper mantle structure based solely on the teleseismic travel time residuals and based on the combined aftershock and teleseismic travel time residuals we determined that the average thickness of the crust beneath the north china basin and surrounding mountains is about 35 km beneath the onshore portion of the north china basin however the crust is closer to 32 or 33 km thick the velocity structure of the upper 20 km or so of the crust is well correlated with the surface geology and parallels the short wavelength a few tens of kilometers features of the surface topography rocks with low seismic velocities lie in or beneath the basin rocks with high seismic velocities lie in or beneath the mountains the uppermost mantle and possibly even the lower crust appears to be a zone of transition in velocity structure between that of the upper crust and the mantle the trends of relative highs and lows in the velocity structure of the mantle are nearly opposite that of the crust the mantle material underlying most of the north china basin has higher than average seismic velocities while the material beneath the western mountains has lower than average velocities the velocity structure of the mantle parallels the long wavelength tens to hundreds of kilometers features evident in the gravity and topography of north china the high velocity mantle material presently beneath the basin probably represents mantle processes not directly related to the formation of the north china basin we infer that the transition zone between the short wavelength and long wavelength velocity features represents a zone of decoupling between the lithosphere and asthenosphere specifically the lithosphere beneath the north china block is moving eastward with respect to the asthenosphere

crustal structure across the northern cordillera yukon territory from seismic wide angle studies omineca belt to intermontane belt a seismic refraction wide angle reflection experiment shot in 1997 in the southern yukon territory crosses the omineca belt which includes the strike slip tintina fault and terminates within the intermontane belt of the northern canadian cordillera lithospheric structure is interpreted from two dimensional forward and inverse modelling of traveltimes combined with forward amplitude modelling and from 2 5 dimensional modelling of gravity data beneath the cassiar terrane and the north america miogeocline average velocities in the upper 20 km of crust are 6 1 km s in the west beneath the accreted cache creek slide mountain and yukon tanana terranes average velocities increase to 6 3 km s in the upper crust the velocity model beneath these terranes thus correlates with more mafic accreted material and not with a subsurface extension of the cassiar terrane the tintina fault is a crustal scale structure across which significant structural differences occur a mid crustal reflector terminates

the miocene great basin of western north america as an extending back arc region abstract a narrow continental volcanic arc has lain near the western margin of the north american continent since mid miocene time its south end began retreating northward 18 m y ago the active remnant of this andesitic arc today is the cascade range of oregon washington and british columbia a large part of the great basin and region to its north was situated throughout this period first wholly then partially in a back arc position relative to the arc this region had and still has most of the characteristics of back arc regions elsewhere in the world a thinned faulted and extended crust and lithosphere limited development of a geophysical bilateral symmetry a mid to late miocene symmetry axis is marked by narrow swarms of basaltic dikes with associated positive linear magnetic anomalies that are parallel to the contemporary trench high heat flow low bouguer gravity and seismic wave velocity values notable seismic attenuation in the mantle and a fault plane solution record dominated by normal fault mechanisms this back arc region differs somewhat from most others those composed of oceanic crust because of its continental setting and recent geologic history despite its name the great basin stands high nearly 2 km above sea level this high elevation owes in part to the fact that extension and heating of the continental lithosphere are still underway though not as a back arc phenomenon the present lithospheric stretching results from oblique extension near the transform boundary of the north american plate the san andreas fault system the latest stretching has probably maintained the physical state of the lithosphere first developed during back arc spreading

sea level records at 80 ka from tectonically stable platforms florida and bermuda studies from tectonically active coasts on new guinea and barbados have suggested that sea level at 80 ka was significantly lower than present whereas data from the atlantic and pacific coasts of north america indicate an 80 ka sea level close to that of the present we determined ages of corals from a shallow submerged reef off the florida keys and an emergent marine deposit on bermuda both localities are on tectonically stable platforms distant from plate boundaries uranium series ages show that corals at both localities grew during the 80 ka sea level highstand and geologic data show that sea level at that time was no lower than 7 9 m below present florida and may have been 1 2 m above present bermuda the ice volume discrepancy of the 80 ka sea level estimates is greater than the volume of the greenland or west antarctic ice sheets comparison of our ages with high latitude insolation values indicates that the sea level stand near the present at 80 ka could have been orbitally forced

deep crustal electrical signatures of eastern dharwar craton india abstract wide band magnetotelluric mt investigations were carried out along a profile from kavali in the east to anantapur towards west across the eastern ghat granulite terrain eggt eastern dhanvar craton edc and a proterozoic cuddapah basin this 300 km long profile was covered with 20 stations at an interval of 12 18 km the mt data is subjected to robust processing decomposition and static shift correction before deriving a 2 d model the model shows a resistive crust 10 000 30 000 ohm m to a depth of 8 10 km towards west of the cuddapah basin the mid crust is less resistive about 500 ohm m and the lower crust with a slight increase in resistivity about 1 500 ohm m in the depth range of 20 22 km the resistivity picture to the east of the cuddapah basin also showed a different deep crustal structure the resistivity of upper crust is about 5 000 ohm m and about 200 ohm m for mid and lower crust the sediment resistivity of cuddapah basin is of the order of 15 20 ohm m mt model has shown good correlation with results from other geophysical studies like deep seismic sounding dss gravity and magnetics the results indicate that the lower crustal layers are of intermediate type showing hydrous composition in eastern dhanvar craton

quaternary reef response to sea level and environmental change in the western atlantic the tropical subtropical western atlantic realm harbours abundant fringing reefs two major barrier reefs belize florida several atolls and large reef fringed carbonate platforms bahamas data from belize and south florida indicate that major barrier reefs were established during the high sea levels of the long and warm marine isotope stage 11 reefs formed during pleistocene sea level highstands whereas erosion non deposition and the formation of subaerial exposure surfaces prevailed during lowstands of sea level florida bahamas exposed reefs and platforms with siliciclastic hinterlands were characterized by deposition of clays and sands on fluvial plains belize elevated terraces and outcrops of reefs of the last interglacial of marine isotope stage 5e are evidence of higher than present sea level and temperature that sustained large reef fringed carbonate platforms and ramps in the florida bahamas region and central america belize mexico including ooid deposition in warm and shallow waters during the highstands of marine isotope stages 5c and 5a at sea levels several metres lower than today fringing reefs developed in parts of the caribbean for example in south florida outlier reefs data from the island of barbados support the contention that the postglacial sea level rise from about 120 m was rapid and punctuated by two meltwater pulses ia and ib which bracket the beginning and end of the younger dryas respectively after reflooding of shelves in the early holocene reefs developed preferentially on highs of the underlying karstified pleistocene limestone surface and on former river channel bars many reefs in the western atlantic region kept pace with holocene sea level rise whereas some were drowned reasons for reef drowning are debated and include sea level rise elevation of antecedent topography and deterioration of environmental conditions such as high nutrient input apart from sea level and antecedent topography factors such as exposure to waves and currents storms and sedimentation have controlled reef development the historical decline of acroporid corals in the caribbean realm has been attributed to cooling following the holocene thermal optimum however historical die offs of reef building corals and other reef organisms suggest that some if not all recent declines were unprecedented during the holocene rising temperatures decreasing carbonate saturation increased frequency of major storms high nutrient concentrations and sedimentation overfishing and disease certainly played a role in decline but systematic studies relating these environmental factors to holocene reef building are only in the early stages

high energy carbonate sand accumulation the quicksands southwest florida keys abstract high resolution seismic reflection profiles of the quicksands located along a broad ridge on the platform shelf west of key west florida indicate a significant deposit of non oolitic carbonate sand occurs in a belt 47 km long by 28 km wide the surface of the belt is ornamented by large 5 m migrating tidal bars oriented in a north south direction on which sand waves oriented in an east west direction are superimposed some of the sand waves are awash at low tide the sand waves are formed by strong reversing tidal currents flowing between the gulf of mexico and the straits of florida the waves migrate directly over pleistocene bedrock to the east but the deposit thickens to the west and sand waves there overlie non oolitic holocene accumulations as thick as 12 m westward ipping accretionary bedding indicates that net migration of the sands is to the west despite north south movement of tidal currents the westward edge of the accumulation has accreted over deeper muddier deposits although tidal currents and resultant bedforms appear identical to those of active ooid deposits in the bahamas and elsewhere no oolitically coated grains were found in this study thin section analyses show the principal component average 48 of the sands is fragmented plates of species of the green alga halimeda followed by particulate coral average 17 which increases off the flanks of the main sand body short vibracores confirm the presence of cross bedding

deglacial sea level record from tahiti corals and the timing of global meltwater discharge the timing of the last deglaciation is important to our understanding of the dynamics of large ice sheets1 and their effects on the earth s surface2 3 moreover the disappearance of the glacial ice sheets was responsible for dramatic increases in freshwater fluxes to the oceans which probably disturbed the ocean s thermohaline circulation and hence global climate4 7 sea level increases bear witness to the melting of continental ice sheets but only two such records from barbados8 9 and new guinea10 11 corals have been accurately dated but these corals overlie active subduction zones where tectonic movements are large and often discontinuous especially in new guinea so the apparent sea level records may be contaminated by a complex tectonic component here we date fossil corals from tahiti which is far from plate boundaries and thus is likely to be tectonically relatively stable and remote from the locations of large former ice sheets the resulting record indicates a large sea level jump shortly before 13 800 calendar years bp which corresponds to meltwater pulse 1a in the barbados coral records8 9 the timing of this event is more accurately constrained in the tahiti record revealing that the meltwater pulse coincides with a short and intense climate cooling event12 15 that followed the initiation of the bolling allerod warm period12 16 but preceded the younger dryas cold event by about 1 000 years

observations of regional phase propagation across the tibetan plateau we present observations of regional phase velocity and propagation characteristics using data recorded during a 1 year deployment of broadband digital seismic stations across the central tibetan plateau along the qinghai tibet highway from golmud to lhasa previous seismological studies within this region have had to rely on earthquakes recorded almost exclusively at stations outside of the plateau we have the opportunity to study numerous source receiver paths confined entirely within the tibetan plateau our analysis concentrates on travel time amplitude and frequency content measurements of the pg pn and sn phases pn can be clearly picked for all observed paths and propagates at an average velocity of 8 16 0 07 km s within the tibetan plateau sn however shows dramatic variations in propagation efficiency across the tibetan plateau that is strongly dependent on frequency we observe that sn rapidly decreases in frequency and amplitude as it passes through the northern portion of the plateau we show that in general sn propagation efficiency decreases with increasing frequency content we use 122 events from outside of the plateau and 61 from within to refine the boundaries of a region of inefficient high frequency sn propagation specifically we show that a larger portion of the northern tibetan plateau attenuates sn energy than was previously suggested in the southern plateau where high frequency sn is observed we computed an average velocity of 4 59 0 18 km s we also observed that the pn velocity within this region of inefficient high frequency sn propagation is nearly 4 slower than the pn velocity computed for paths restricted to the southern plateau and that the crust is about 10 km thinner than in the south the coincident locations of inefficient sn propagation and slow pn velocity is commonly observed in regions of active tectonics our results add constraints to the velocity structure of the lithosphere beneath the tibetan plateau and require first order lateral variations in the uppermost mantle structure despite the relatively uniform topography of the plateau

pn velocities beneath continental collision zones the turkish iranian plateau summary tomographic imaging of the uppermost mantle velocity in the middle east shows normal pn velocities 8 0 8 2 kms beneath the black sea and the southern caspian sea in contrast low pn velocities 7 9 km s i are found beneath the aegean sea and much of the turkish iranian plateau a region of exceptionally low pn velocities c7 6 km s beneath the lesser caucasus mountains and along the turkish iranian border coincides with a region of high sn attenuation and extensive neogene volcanism all these features suggest that near solidus conditions exist within the uppermost mantle beneath the turkish iranian plateau such conditions may result either from the decompression melting associated with upwelling convection cells or by the infiltration of water released from subducted lithosphere into the mantle above the presence of a partially melted uppermost mantle weakens the lithosphere beneath the turkish iranian plateau thus allowing it to become the locus of deformation in the arabian eurasian collision zone

the paradox of drowned reefs and carbonate platforms shallow water carbonate platforms and reefs are drowned when tectonic subsidence or rising sea level outpaces carbonate accumulation and benthonic carbonate production ceases drowned platforms are common in the geologic record but they present a paradox if one considers rates of processes involved during the early holocene when sea level rose at rates of 6 000 to 10 000 m yr mm 1 000 yr most reefs and platforms were outpaced by the rising sea during the late holocene with sea level rising 500 to 3 000 m yr in the atlantic caribbean area reefs and platforms started to recover built to sea level and prograded seaward 1 000 m yr is thus a conservative estimate of the average growth potential of modern reefs and platforms independently accumulation rates of prograding platforms in the geologic record suggest growth potential in excess of several hundred microns per year the growth potential of 1 000 m yr exceeds any relative rise of sea level caused by long term processes in the geologic record newly formed ocean crust subsides at a maximum of 250 m yr basin subsidence averages 10 to 100 m yr and sea level rises due to increased sea floor spreading amount to less than 10 m yr rapid pulses of relative rise of sea level or reduction of benthic growth by deterioration of the environment remain the only plausible explanations of drowning the geologic record shows examples of both of these processes global mass extinctions of reefs and platforms occurred in the middle cretaceous eustatic rise due to submarine volcanism or desiccation of a small ocean basin and the late devonian global crisis of ocean environment extraterrestrial cause drowning controlled by regional tectonics prevailed in the jurassic and early cretaceous of the tethyan realm and the drowning of mesozoic platforms in the western north atlantic seems to have been dictated by plate tectonic drift to higher latitudes

changes in planktic foraminiferal faunas temperature and salinity in the gulf stream during the last 30 000 years influence of meltwater via the mississippi river abstract variations in the distribution of planktic foraminiferal faunas temperature and salinity in the surface water of the gulf stream during the last 30 000 years have been reconstructed based on a sediment core from the blake ridge subtropical western atlantic ocean temperatures and salinities were calculated using planktic foraminiferal transfer functions and previously published oxygen isotope values productivity was estimated using the 106 m size fraction of planktic foraminifera the reconstructed values show that temperature variations have been small during the investigated period with a total range of only 3 c the highest temperatures about 1 5 c above the present temperature in the area occurred during the late glacial heinrich event 1 and the mid holocene time periods the lowest temperatures about 1 5 c below the present temperatures occurred during the bolling interstadial the early holocene period was relatively cool the low temperatures during the bolling interstadial and during the early holocene are in contrast to the northeastern atlantic where the bolling interstadial was the warmest period of the deglaciation and the early holocene the warmest during the holocene we attribute the lower temperatures during the bolling and early holocene periods to colder meltwater from the laurentide ice sheet flowing into the gulf of mexico through the mississippi river system and carried to the blake ridge via the gulf stream

the holocene drowned reef of les saintes plateau as witness of a long term tectonic subsidence along the lesser antilles volcanic arc in guadeloupe abstract resulting from the interplay between tectonics and eustatism reef terraces are powerful markers of vertical movements at a scale of 1000 to 100 000 years in the lesser antilles they grow around every island of the archipelago and record both local and subduction related tectonics the recent acquisition and interpretation of very high resolution bathymetry of les saintes submarine plateau french west indies together with seismic reflection profiles are a unique opportunity to study one of these submarine structures at metric to kilometric scales addressing the questions of its nature age and growth environment but also of the control of active tectonics on its formation the 20 km wide les saintes reef plateau lies at about 45 mbsl it is crosscut by nw se striking north dipping normal faults that belong to les saintes fault system and graben which produced a mw 6 3 earthquake in 2004 the plateau is composed of four 20 m thick reef units piled up in layer cake morphology down to 110 mbsl the upper unit has a fresh morphology and presents typical reef features like barrier and lagoon spurs and grooves pinnacles etc from its morphology we propose that it grew during the holocene last transgression below the three other units likely formed during pleistocene sea level highstands and were eroded during the low stands as evidenced in seismic reflection profiles this scenario would imply that les saintes plateau formed in a context of subsidence with a rate we evaluate to be of the order of tenths of mm yr probably linked to local tectonics we believe that this deformation is also related to plate scale subduction processes similarly to deformations occurring on the other islands of the guadeloupe archipelago

compressional velocity structure and anisotropy in the uppermost mantle beneath italy and surrounding regions travel times of about 39 000 pn arrivals recorded from regional earthquakes by the italian telemetered seismic network and by stations of nearby countries are inverted to image lateral variations of seismic velocity and anisotropy at subcrustal depth in italy and surrounding regions this method allows simultaneous imaging of variations of pn velocity and anisotropy as well as crustal thickness variations the po plain the adriatic sea and the ionian sea have normal to high pn velocities in contrast lower velocities 7 9 8 0 kin s are imaged in italy beneath the western alps the northern apennines and eastern sicily and nearby calabria as well as in northern albania and beneath the pannonian basin low pn velocities beneath the northern apennines correlate with present day extension and may have resulted from thermal anomalies in the uppermost mantle possibly due to delamination processes low velocities are consistent with the high attenuation zone inferred in the uppermost mantle beneath the internal apennine units and the tyrrhenian margin of the peninsula by mele et al 1996 1997 on the contrary low velocities beneath the western alps may be an apparent effect due to the abrupt thickening of the crustal roots pn velocity is anisotropic in the study area with a maximum amplitude of 0 2 km s the largest anisotropic velocity anomalies are observed along the major arc structures of italy i e the northern apennines and the calabrian arc indicating that these features are controlled by uppermost mantle processes the anisotropy anomaly along the calabrian arc extends as far as albania but ends abruptly north of this area suggesting that a lithospheric discontinuity is present along the northern albanian border

vp vs ratios in the upper crust of the southern korean peninsula and their correlations with seismic and geophysical properties abstract the p and s velocity ratio v p v s is a seismic property that is convertible to poisson s ratio a widely analyzed physical property of elastic materials the lateral variation in the v p v s ratios of the upper crust of the southern korean peninsula is investigated based on the travel times of p and s waves of local events with epicentral distances of 50 km or less and focal depths of 25 km or less the dense seismic networks in the korean peninsula allow us to investigate the regional variation of v p v s ratios these v p v s ratios are estimated to be 1 60 1 91 with an average of 1 69 the v p v s ratios are as high as 1 73 1 91 in the gyeongsang basin jeju island and offshore region of the eastern okcheon belt but have relatively low values of 1 64 1 72 in precambrian massif regions the stability of the v p v s ratios is tested with randomly resampled travel time data the results of which suggest nearly constant v p v s ratios with increasing depth in the upper crust the influences of plausible errors on origin times and arrival times are quantified to verify the v p v s estimates the v p v s ratios are correlated with geological and tectonic structures comparisons with known seismic and geophysical properties suggest that structural features observed on the surface may extend at least to the mantle lid

upper mantle velocity temperature conversion and composition determined from seismic refraction and heat flow 1 we compile upper mantle pn velocities from seismic refraction wide angle reflection surveys in the southern superior province of the canadian shield and compare them with temperatures at the moho deduced from heat flow data calculated moho temperatures and pn velocities correlate well showing that in this area pn depends primarily on temperature the obtained values of v pn t depend weakly on the assumed value of moho heat flow and are on the order of 6 0 10 4 10 km s 1 k 1 within the range of temperature derivatives obtained in laboratory studies of ultramafic rocks comparison between observed pn velocities and predicted values for several mineralogical models at moho temperatures allows constraints on both the moho heat flow and the shallow mantle composition for all moho heat flows undepleted clinopyroxene rich mantle compositions do not allow a good fit to the data for depleted mantle compositions temperatures consistent with the observed pn velocities correspond to values of moho heat flow larger than 12 mw m 2 for our preferred moho heat flow of 15 mw m 2 the best fit mantle composition is slightly less depleted than models for average archean subcontinental lithospheric mantle this may be due to rejuvenation by melt related metasomatism during the keweenawan rifting event the similarity in pn t conversion factors estimated from this empirical large scale geophysical study and those from laboratory data provides confidence in the absolute temperature values deduced from heat flow measurements and seismic studies

platforms of the nicaraguan rise examples of the sensitivity of carbonate sedimentation to excess trophic resources the nicaraguan rise is an active tectonic structure in the western caribbean carbonate accumulation on its platforms has not kept pace with relative holocene sea level rise despite a tropical location remote from terrigenous sedimentation trophic resources apparently exceed levels favoring coral reef development because sponge algal communities dominate the drowning western platforms in contrast to mixed coral algal benthos on pedro bank and well developed coral reefs along the north coast of jamaica concentrations of biotic pigments in sea surface waters show a corresponding west east gradient oceanic waters flowing over the western banks carry nearly twice as much biotic pigment as oceanic waters north of jamaica sources enriching the western caribbean are terrestrial runoff upwelling off northern south america and topographic upwelling over the nicaraguan rise that relatively modest levels of trophic resources can suppress coral reef development holds important implications for understanding carbonate platform drownings in the geologic record

flexural isostasy and uplift of the sierra nevada of california the sierra nevada of california has experienced several kilometers of uplift in the last 10 m y but its present elevation is isostatically compensated by a crustal root that has probably existed since late cretaceous flexural isostasy can reconcile these observations basin and range faulting and extension starting 10 m y ago allowed rapid uplift of an eroded and regionally compensated magmatic arc underlain by an excess crustal root a quantitative model of an unbroken elastic plate buoyed up by the excess root provides an explanation for the estimated 10 m y paleotopography of the range the same model fits the present topography when broken shear and fiber stresses not transmitted at the location of the owens valley fault zone the broken plate model predicts the appropriate amount of westward tilt of the sierra nevada and can explain the pronounced eastward tilt of the white and inyo mountains the position where paleo and present drainages in the sierra cross over in elevation is the most sensitive indicator of the elastic thickness required of the lithosphere an effective elastic thickness of 50 km best fits the observed crossover distances and is consistent with the amount of post 10 m y uplift calculated fiber and shear stresses suggest that actual extension of the crust is more important in permitting the uplift than the mere presence of normal faults part of the residual isostatic gravity anomaly pattern high gravity over the eastern great valley and western sierra nevada low residual gravity over the high sierra may be explained by the continuing flexural support of departure from local isostatic equilibrium the model implies that before 10 m y ago the sierra should have been marked by a negative isostatic gravity anomaly of more than 100 m gal flanked by isostatic gravity highs such large isostatic anomalies have not been observed to date over eroded arcs but the sierra has uniquely low heat flow and presumably therefore high lithospheric strength and the sierran root is at the upper limit of size that could be isostatically suppressed

multiple outer reef tracts along the south florida bank margin outlier reefs a new windward margin model high resolution seismic reflection profiles off the lower florida keys reveal a multiple outlier reef tract system approximately 0 5 to 1 5 km seaward of the bank margin the system is characterized by a massive outer main reef tract of high 28 m unburied relief that parallels the margin and at least two narrower discontinuous reef tracts of lower relief between the main tract and the shallow bank margin reefs the outer tract is approximately 0 5 to 1 km wide and extends a distance of approximately 57 km a single pass divides the outer tract into two main reefs the outlier reefs developed an antecedent low gradient to horizontal offbank surfaces interpreted to be pleistocene beaches that formed terracelike features radiocarbon dates of a coral core from the outer tract confirm a pre holocene age these multiple outlier reefs represent a new windward margin model that presents a significant unique mechanism for progradation of carbonate platforms during periods of sea level fluctuation infilling of the back reef terrace basins would create new terraced promontories and would extend or step the platform seaward for hundreds of meters subsequent outlier reef development would produce laterally accumulating sequences

crust mantle structure of the central north island new zealand based on seismological observations abstract two geophysical features characterise the anomalous crust mantle structure beneath central north island of new zealand the apparent thinning of the felsic crust by a factor of at least 50 and upper mantle p wave velocities pn phase about 10 less than normal 7 3 7 4 km s pn velocities increase slowly to a maximum of 7 8 km s at a depth of about 80 100 km above a depth of 15 km seismic p wave velocities vp are 6 km s or less representing what is likely to be a mix of the original greywacke crust that has been stretched along with an unknown volume of intruded igneous rocks that are in various stages of cooling defining the moho is problematic in this dynamic region below a depth of 15 km there is a continuous increase in vp such that velocities 7 km s are reached at a depth as shallow as 20 km on a east west line and 7 4 km s on a north south line we interpret these wave speeds as a transition zone into low pn velocities rather than high lower crustal values of vp pn wave speeds as low as 7 4 km s are detected by tomographic methods to depths of at least 65 km and laterally over an area of at least 10 000 km2 a 1 4 partial melt in the mantle wedge beneath the central north island is estimated based on qp seismic anisotropy low values of pn and amplitudes of deep reflections these upper mantle features are more indicative of asthenosphere than lithosphere and coupled with the geological evidence for rapid uplift here at 5 ma are consistent with convective removal of the mantle lithosphere from western and central north island in late miocene times

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

quaternary cave levels in peninsular florida abstract the hypothesis that caves in the florida peninsula are tied to quaternary sea levels was proposed by hydrogeologists without data some 40 years ago the hypothesis is a version of glacial control of cave levels which is the logical combination of the water table theory of speleogenesis and the concept that base level positions the water table at the usa type example of glacial control of cave levels mammoth cave in the paleozoic rocks of kentucky the intermediary is base level determined by rivers by hypothesis the intermediary for florida is glacioeustatic sea level this paper presents elevation data that supports this hypothesis recent cave surveys in the air filled caves and spot elevations from archived maps reveal prominent levels of passages centered at 5 12 21 and 30 m above sea level over broad areas they do not follow the large scale structure of the floridan aquifer instead they align with nearby coastal marine terraces identified as modal peaks on frequency plots from various topographic data bases levels matching with the three highest terraces wicomico penholoway and talbott are particularly clear lower levels if they accord with sea level stands are likely composites data from cavities encountered in drilled wells e g bit drops and spot elevations from archived underwater cave maps demonstrate passage levels at depths of 15 30 70 and 90 120 m below the modern water table the depths below water table are similar to the depths below sea level of distant submerged terraces and paleoshoreline features identified using multibeam bathymetric data in the gulf of mexico the cave bit drop and terrace data are all consistent with the concept that quaternary sea level is the fundamental control on the cave scale porosity within the floridan aquifer this conclusion does not rule out the possibility that lithologically favored positions paleokarst features and confining units and mixing zones are also involved in the location of caves levels in this near coastal environment

pn velocity and cooling of the continental lithosphere average upper mantle p sub n velocities and heat flow were computed within continental physiographic provinces in north america from published data p sub n velocity and heat flow data display an inverse relationship and were found to correlate with a statistically significant correlation coefficient temperatures at the crust mantle boundary were estimated from the heat flow values and these were used to demonstrate a correlation between p sub n velocity and temperature of upper mantle material the value of partialp sub n partialt p thus obtained 4 4 x 10 sup 4 to 8 1 x 10 sup 4 km s sup 1 sup 0 c sup 1 is within the range of temperature derivatives determined from laboratory studies of ultramafic rocks this dependence of p sub n velocity on temperature implies that one possible explanation for the observed geographical distribution in upper mantle seismic velocity is that the variation in p sub n velocity is primarily a temperature effect combined with the relationship between heat flow and crustal age for continents the p sub n versus heat flow correlation suggests a relationship between p sub n velocity and crustal age probably due to progressive cooling of the continental lithosphere after a thermotectonic event

accreted terranes of northwestern british columbia canada lithospheric velocity structure and tectonics 1 lithospheric structure beneath the accreted terranes of the northwestern canadian cordillera is interpreted from analyses of p wave refraction and wide angle reflection data the 535 km long profile extends from the deformed and displaced ancestral north american margin across the omineca belt suture zone and intermontane superterrane to the coast belt this region is interpreted to have grown westward through a progression from thin skinned to thick skinned accretionary tectonics large lateral variations of velocities 2 0 6 4 km s within the upper 15 km correlate with the mapped geological and terrane elements fault offsets and the imbricated base of an overlap basin are clearly identified inboard of the stikine accreted terrane a zone of slow upper crustal velocities thins to the east consistent with an interpretation of thin skinned accretion leaving slivers of terranes overlying a wedge of metamorphosed proterozoic continental margin sediments however in the lower crust the proposed underlying cratonic ramp is not distinguished from outboard or overlying accreted crust by lateral p velocity contrasts or wide angle reflections the moho remains nearly horizontal beneath the intermontane and omineca belts gradually thinning to the east from 36 5 to 35 km to the southwest below the coast belt and to the northeast beneath the tintina fault the crust thins to 32 km upper mantle velocities across the coast intermontane and omineca belts are 7 8 7 9 km s the slow upper mantle and lower crustal velocities are indicative of high temperatures that may permit the flat moho or weak lower crust to partially decouple the crust from the mantle reflections in the upper mantle between 50 70 km depth may be related to the base of the lithosphere or to heterogeneities introduced through melting or accretionary subduction events

relationships between the petrophysical properties density seismic velocity heat generation and mineralogical constitution the petrophysical properties density p compressional wave velocity v p and heat generation a exhibit considerable variation with rock type to characterize a given rock quantitatively a single parameter the cation fraction expressed by the k value has been found to be suited best 100 crystalline rocks have been sampled to cover the widest possible range in k value from silicic types like granophyres and granites through granodiorites tonalites diorites gabbros amphibolites hornblendites pyroxenites to ultrabasic rocks p k a and v p were determined experimentally the latter at 0 5 kbar 50 mpa the interrelations between these properties were established by regression analysis the relation k and v p k show a linear dependence whereas a k a and a v p follow exponential laws in a w m 3 12 20 230 5 k mole cm 3 in a 22 5 8 15 g cm 3 and in a 16 5 2 74 v p km s all correlation coefficients are better than 0 9

the yellowstone snake river plain seismic profiling experiment crustal structure of the eastern snake river plain seismic refraction profiles recorded along the eastern snake river plain esrp in southeastern idaho during the 1978 yellowstone snake river plain cooperative seismic profiling experiment are interpreted to infer the crustal velocity and attenuation q 1 structure of the esrp travel time and synthetic seismogram modeling of a 250 km reversed refraction profile as well as a 100 km detailed profile indicate that the crust of the esrp is highly anomalous approximately 3 to 6 km of volcanic rocks with some interbedded sediments overlie an upper crustal layer compressional velocity approx 6 1 km s which thins southwestward along the esrp from a thickness of 10 km near island park caldera to 2 to 3 km beneath the central and southwestern portions of the esrp an intermediate velocity approx 6 5 km s layer extends from approx 10 to approx 20 km depth a thick approx 22 km lower crust of compressional velocity 6 8 km s a total crustall thickness of approx 42 km and a p sub n velocity of approx 7 9 km s is observed in the esrp similar to the western snake river plain and the rocky mountains provinces high attenuation is evident on the amplitude corrected seismic data due to low q values in the volcanic rocks q sub p more 20 to 200 and throughout the crust q sub p 160 to 300 based on these characteristics of the crustal structure and volcanic age progression data it is suggested that the esrp has resulted from an intensitive period of intrusion of mantle derived basaltic magma into the upper crust generating explosive silicic volcanism and associated regional uplift and caldera collapse this activity began about 15 m y ago in southwestern idaho and has migrated northeast to its present position at yellowstone subsequent cooling of the intruded upper crust results in the 6 5 km s velocity intermediate layer crustal subsidence and periodic basaltic volcanism as represented by the esrp complete the sequence of crustal evolution less

regional quaternary submarine geomorphology in the florida keys high quality seismic reflection profiles fill a major gap in geophysical data along the south florida shelf allowing updated interpretations of the history of the quaternary coral reef system incorporation of the new and existing data sets provides the basis for detailed color maps of the pleistocene surface and thickness of overlying holocene accretions the maps cover the florida keys to a margin wide upper slope terrace 30 to 40 m deep and extend from the elbow reef north key largo to rebecca shoal gulf of mexico the data indicate that pleistocene bedrock is several meters deeper to the southwest than to the north east yet in general holocene sediments are 3 to 4 m thick shelf wide the pleistocene map demonstrates the significance of a westward dipping bedrock surface to holocene flooding history and coral reef evolution seismic facies show evidence for two possible holocene stillstands aerial photographs provide information on the seabed surface much of which is below seismic resolution the photographs define a prominent regional nearshore rock ledge that extends 2 5 km seaward from the keys9 shoreline they show that bands of rock ridges exist along the outer shelf and on the upper slope terrace the photographs also reveal four tracts of outlier reefs on the terrace one more than had been documented seismically seismic and photographic data indicate the tracts are 200 km long nearly four times longer than previously thought new interpretations provide insights into a youngest possible terrace age ca 175 ka and the likelihood that precise ages of oxygen isotope substage 5e ooid tidal bar and coral reef components may differ the tidal bar reef complex forms the florida keys

marine evidence for episodic holocene megafloods in north america and the northern gulf of mexico hemipelagic muds deposited during the past 5 3 cal kyr in the northern gulf of mexico orca basin contain seven intervals punctuated by relatively coarse siliciclastic grain size peaks planktonic faunal turnovers and negative 13c excursions we believe these episodes represent megaflood deposits reflecting historically unprecedented outfall of north american floodwater and terrigenous mud plumes into the gulf resulting in collapse of the open ocean pelagic ecosystem the deposits record multidecadal episodes of high continental precipitation and large mississippi river floods at 4 7 3 5 3 0 2 5 2 0 1 2 and 0 3 cal ka 500 1200 year recurrence interval variations in tropical plankton frequencies define submillenial warming intervals that culminate in these fluvial episodes strengthened tropical currents in the gulf at these times appear to have increased sea surface temperatures and associated flow of moist gulf air to the midwest terrestrial paleohydrologic records support the marine evidence for millennial scale changes in recurrence of large midwest flood episodes

development of small carbonate banks on the south florida platform margin response to sea level and climate change abstract geophysical and coring data from the dry tortugas tortugas bank and riley s hump on the southwest florida margin reveal the stratigraphic framework and growth history of these carbonate banks the holocene reefs of the dry tortugas and tortugas bank are approximately 14 and 10 m thick respectively and are situated upon pleistocene reefal edifices tortugas bank consists of the oldest holocene corals in the florida keys with earliest coral recruitment occurring at 9 6 cal ka growth curves for the tortugas bank reveal slow growth

loop current boundary variations the periodicity of warm ring separation from the loop current and the characteristics of cold perturbations found along the loop current boundary were studied using a long term data set made up of noaa satellite infrared images the results showed that the period between separation of warm rings from the loop current was highly variable the average period was about 10 9 months the minimum period was 6 months and the maximum period was 17 months for the data set used ring separations occurred most often in the winter the study of the cold perturbations indicated that these perturbations were most pronounced along the northern and eastern portions of the loop current boundary and were less pronounced south of 27 n along the western boundary the data suggested that the perturbations either formed or intensified in the northwestern portions of the boundary the large effects along the eastern boundary of the loop current were thought to be associated with the movement of the intense perturbations that are initially found along the northwestern portions of the boundary

millennial scale paleoclimate cycles recorded in widespread palaeozoic deeper water rhythmites of north america rhythmically interbedded limestone and shale or limestone and chert rhythmites are a common feature of many deep water phanerozoic carbonate marine deposits seventeen different palaeozoic rhythmite successions from across north america 8 studied in detail 9 studied in reconnaissance are described and summarized individual rhythmite couplets 4 20 cm thick are composed of fine grained laminated to massive detrital limestone alternating with shale or marl or laminated spiculitic chert stratigraphic and primary depositional features within rhythmites and associated facies indicates that the carbonate rich and carbonate poor interbedding is the result of repetitive changes in sediment input primary rather than due to diagenetic redistribution of calcareous material secondary the average duration of individual rhythmite couplets is calculated using spectral analysis to determine couplet thickness combined with biostratigraphically calibrated undecompacted average sediment accumulation rates the duration of rhythmite couplets from the eight well studied successions ranges between 150 and 4900 yrs with the majority lying between 1000 and 3000 yrs i e millennial scale the paucity and low diversity of skeletal and trace fossils preserved suspension laminae lack of sediment reworking and facies associations indicate rhythmite accumulation was favored by 1 deposition below storm wave base which limited reworking by currents 2 dysaerobic bottom waters which restricted bioturbation and intermixing of interbedded lithologies and 3 proximity to nearshore carbonates supplying abundant fine grained detrital carbonate these combined conditions were best met along flooded subtropical continental shelves or epeiric seas during my scale 3rd order sealevel rises the rhythmic alternation between carbonate rich and carbonate poor layers is interpreted to represent millennial scale paleoclimatic changes related to 1 wet dry climate cycles which influenced the amount of continent derived eolian and or fluvial sediment input 2 variations in offshore transport via storm generated or density currents of nearshore derived terrigenous or carbonate sediments and or 3 changes in wind driven upwelling and availability of recycled biogenic silica the various rhythmite successions accumulated under dramatically different paleoenvironment and paleogeographic conditions including active to passive tectonic settings equatorial to subtropical latitudes long term icehouse through greenhouse climatic conditions calcite versus aragonitic seas variable atmospheric co2 concentrations before and after land plant and animal evolution and across widely varying ocean basin configurations if our short term paleoclimatic interpretations for the rhythmites are correct then it is apparent that millennial scale climate changes occurred over a very wide spectrum of paleoceanographic paleogeographic paleoclimatic tectonic and biologic conditions and over time periods from the cambrian to the quaternary given this it is difficult to invoke models of internally driven thermohaline oceanic oscillations or continental ice sheet instabilities to explain their origin instead we suggest that

coral reef complexes at an atypical windward platform margin late quaternary southeast florida major coral reef complexes rim many modern and ancient carbonate platforms their role in margin evolution is not fully understood particularly when they border a margin atypical of the classic model classic windward margins are steeply inclined the windward margin of southeast florida is distinct with a very low gradient slope and a shelf edge ringed with 30 m high quaternary outlier reefs on a shallow upper slope terrace a newly developed synthesis of temporally well constrained geologic events is used with surface and subsurface seismic reflection contours to construct morphogenetic models of four discontinuous reef complex sequences the models show uneven subsurface topography upward and landward buildups and a previously unreported rapid holocene pro gradation the terms backstepped reef complex margin backfilled prograded margin and coalesced reef complex margin are proposed for sections exhibiting suitable signatures in the stratigraphic record the models have significant implications for interpretation of ancient analogues the florida record chronicles four kinds of geologic events 1 thirteen transgressions high enough for marine deposition occurred between ca 325 ka and the present six gave rise to stratigraphically successive coral reef complexes between ca 185 and ca 77 8 ka the seventh reef ecosystem is holocene 2 two primary coral reef architectures built the outer shelf and margin producing respective ridge and swale and reef and trough geometries of very different scales 3 massive outlier reefs developed on an upper slope terrace between ca 106 5 and ca 80 ka and are inferred to contain corals that would date to highstands at ca 140 and 125 ka 4 sea level remained below elevation of the shelf between ca 77 8 and ca 9 6 ka

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

collapse of a hercynian tibetan plateau into a late palaeozoic european basin and range province the large width the importance of crustal shortening and the likelihood of thick crust in the late palaeozoic hercynian or variscan belt of southern europe suggest a tectonic evolution analogous to the evolution of the tibetan plateau and its neighbouring himalaya1 and of the altiplano and other high plateaux of the andes here we extend dewey and burke s analogy1 of the hercynian chain with tibet to include the subsequent stage of pervasive normal faulting and crustal extension and thinning which characterize the active tectonics of tibet parts of the high andes and the basin and range province of western north america it appears that normal faulting and rifting also characterize the late palaeozoic tectonic activity of the hercynian chain

heat flow and lithospheric thermal regime in the northeast german basin abstract new values of surface heat flow are reported for 13 deep borehole locations in the northeast german basin negb ranging from 68 to 91 mw m 2 with a mean of 77 3 mw m 2 the values are derived from continuous temperature logs measured thermal conductivity and log derived radiogenic heat production the heat flow values are supposed free of effects from surface palaeoclimatic temperature variations from regional as well as local fluid flow and from thermal refraction in the vicinity of salt structures and thus represent unperturbed crustal heat flow two d numerical lithospheric thermal models are developed for a 500 km section along the dekorp basin 9601 deep seismic line across the basin with a north eastward extension across the tornquist zone a detailed conceptual model of crustal structure and composition thermal conductivity and heat production distribution is developed different boundary conditions for the thickness of thermal lithosphere were used to fit surface heat flow the best fit is achieved with a thickness of thermal lithosphere of about 75 km beneath the negb this estimate is corroborated by seismological studies and somewhat less than typical for stabilized phanerozoic lithosphere modelled moho temperatures in the basin are about 800 c heat flow from the mantle is about 35 to 40 mw m 2 in the southernmost part of the section beneath the harz mountains higher moho temperatures up to 900 to 1000 c are shown while the relatively high level of surface heat flow in the negb obviously is of longer wave length and related to lithosphere thickness changes in crustal structure and composition are responsible for short wave length anomalies

sequential and climatic framework of the growth and demise of a carbonate platform implications for the peritidal cycles late jurassic north eastern france the middle oxfordian of the eastern paris basin constitutes a remarkable example of the growth and demise of a carbonate platform fischer plots sedimentary and diagenetic features allow the identification of four depositional cycles s5 to s8 in the transversarium zone they are inserted in a lower frequency cycle of increased decreased accommodation space soiii the long term period of accommodation creation occurred during the older s5 and s6 cycles the maximum accommodation zone being located in the lower part of the s6 cycle this high accommodation period was tectonically controlled and was coeval with local distensive activity of a hercynian fault a major minimum accommodation zone exists during the s8 cycle at that time the platform was isolated and presented both a windward and a leeward margin the growth of the platform was favoured by a warm and arid climate oligotrophic conditions and reduced siliciclastic input during a highstand in relative sea level these palaeoenvironmental features favoured the proliferation of phototrophic organisms producing carbonate material the death of the platform was generated by a reduction in the carbonate production surface during a lowstand in relative sea level and by the appearance of mesotrophic conditions induced by the increase in siliciclastic inputs at the beginning of a period with a cooler and more humid climate in the eastern paris basin during the middle oxfordian the parasequences are ordered and present greenhouse characteristics in contrast at the beginning of the s8 cycle the randomness in the thickness of contiguous parasequences increased decreased carbonate production during the lowstand caused by a transition from photozoan to heterozoan benthic communities certainly favoured this randomness and the appearance of catch down parasequences

crustal structure of nw british columbia and se alaska from seismic wide angle studies coast plutonic complex to stikinia crustal structure beneath the transition from the coast belt to the intermontane superterrane of the northern canadian cordillera is interpreted from the inversion of refraction and wide angle reflection seismic data the profile traverses an accretionary suture zone coast plutonic complex to continental crust deformed by the transpressive collision stikine terrane using data acquired by the accrete onshore offshore experiment and by a partially overlapping lithoprobe onshore experiment p wave travel time inversion and forward amplitude modeling are employed to determine crustal velocity structure the model exhibits a well defined transition throughout the crust that distinguishes the coast plutonic complex cpc from stikinia average crustal velocities beneath the cpc 6 45 km s are considerably faster than those beneath stikinia 6 25 km s crustal thickness also changes across the transition thin crust beneath the coast belt 30 32 km thickens beneath stikinia 35 37 km the observations within the coast belt are consistent with a tectonic history most recently dominated by extensional deformation primary structural control could be associated with either neogene extension and or the processes that are responsible for exhuming the coast belt during the early paleogene and have been inferred from geological studies slow mantle velocities 7 8 7 9 km s beneath the entire profile are indicative of high upper mantle temperatures comparison with the southern cordilleran coast belt reveals similar velocity structure within the massive plutonic complexes however substantial differences between the northern and southern coast belts emphasize along strike variations in terranes orogen geometry and postorogenic tectonics

first local seismic tomography for red river shear zone northern vietnam stepwise inversion employing crustal p and pn waves abstract the 900 km long red river shear zone rrsz lends a compelling support to the continental extrusion model for the tectonic evolution of southeastern asia but has been challenged by many of views as some new records mainly from northern vietnam suspecting the dimensions of rrsz neither in depth nor in displacement are as large as we expected before however compared to the northwestern half of the rrsz in yunnan province better studied by many fields the southeastern half in northern vietnam is relatively poorly constrained by seismic study due to insufficient stations and data in the past this study using a newly deployed portable broadband seismic network obtained the first local seismic tomography with a stepwise inversion using p and p n phases surface geology major structures and rock properties are well correlated and identified in our model suggesting the rrsz is a lithospheric structure at least penetrating to the uppermost mantle with mantle thermal anomalies in general the crust of northern vietnam appears to be weak and sits on a relatively hot uppermost mantle showing a long and complex thermo tectonic history a mid lower crustal segmentation of rrsz is also proposed to compromise the discrepancies recently observed between yunnan province and northern vietnam

the role of nutrient availability in bioerosion consequences to carbonate buildups abstract zooxanthellate organisms which are among the major carbonate producers on coral reefs are highly adapted to nutrient deficient conditions and tend to be outcompeted by filamentous or fleshy algae if nutrients are abundant reef dwelling bioeroding organisms on the other hand seem to increase in abundance with increasing availability of nutrient and food resources maximum rates of calcium carbonate production in a reef system are comparable in magnitude to maximum rates of bioerosion the dynamic interplay between accretion destruction of coral reefs is therefore likely to be strongly influenced by nutrient availability nutrient availability may also influence development of carbonate facies in reef systems that develop in oligotrophic waters skeletal sands and framework should dominate carbonate buildups in systems that develop in mesotrophic waters enhanced bioerosion should reduce much of the skeletal material to carbonate muds increasing the prevalence of mud rich facies eutrophic conditions should result in bioeroded hardgrounds and carbonate depositional hiatuses when carbonate accumulation rates fail to equal or exceed rates of local relative sea level rise reefs drown bored and lithified pavements with crusts of ferromanganese oxides phosphates or glauconite often separate shallow reef from overlying deeper water deposits nondeposition bioerosion and evidence for low redox potentials at the sediment water interface all implicate excess nutrient availability as a factor in reef drowning some carbonate depositional hiatuses and bioerosional surfaces are local phenomena evidence of responses to local increases in nutrient availability resulting from submergence of soils runoff or topographic upwelling others appear to reflect regional or worldwide events because many carbonate producing organisms have life history strategies specialized to nutrient deficient conditions they are unable to compete when nutrients become plentiful events that triggered ocean turnover or sharp increases in the rate of deep ocean circulation would have increased sea surface nutrient availability worldwide such eutrophication could have caused widespread extinctions in reef communities because eutrophication promotes bioerosion details of the fossil records of those events are probably lost in bioerosional hiatuses

structure of the crust mantle transition beneath the variscan foreland in sw poland from coincident wide angle and near vertical reflection data abstract lower crustal and moho reflectivity vary significantly across the transition from trans european suture zone to the variscan orogen of central europe along the sudetes 2003 s01 profile one 50 km segment of the 270 km profile was a detailed reflection refraction survey called grundy 2003 in which the shot points were also recorded along the polish part of the s01 profile to produce dense recordings of wide angle reflections processed near vertical incidence nvi data recorded along the grundy 2003 g01 line revealed highly reflective laminated lower crust starting at ca 7 s twt 20 23 km depth moho was inferred as the base of the crustal reflectivity at ca 11 5 12 s 33 37 km depth wide angle data recorded along the s01 profile processed and stacked using cdp method with up to 41 fold revealed reflectivity associated with the lower crust and the moho in two areas east of the odra fault zone and close to the dolsk fault moho boundary deduced from both the nvi and wide angle stacks fits well to the refraction moho constrained by the ray tracing modelling and joint refraction reflection tomography of pg and pmp arrivals performed along the s01 profile part of the moho boundary imaged along the s01 profile close to the dolsk fault zone is proposed to represent a metamorphic front of lower crustal rocks partly undergone eclogite transformation this observation is consistent with the proposed southward underthrusting of baltica lower crust below the avalonian acretionary prism the postulated thin up to 10 km high velocity eclogite layer overlying normal velocity mantle would also help to reconcile the apparent contradiction of a high heat flow observed over the area with the high pn velocities

sediments of the dry tortugas south florida usa facies distribution on a ramp like isolated carbonate platform abstract four sedimentary facies may be delineated based on quantitative analysis of texture and composition of modern surface sediments on the dry tortugas carbonate platform these include 1 mollusk halimeda wackestone 2 mollusk packstone to grainstone 3 coralgal halimeda grainstone and 4 coralgal grainstone even though the tortugas platform is characterized by an open circulation due to deep broad marginal channels and a lack of a continuous surface breaking marginal reef facies are not distributed at random and show bathymetrical zonation also facies appear to cover wide belts rather than forming a mosaic mollusk halimeda wackestone occurs in protected platform interior areas ca 10 18 m deep mollusk packstone to grainstone occurs in more open platform interior settings adjacent to channels and in deeper outer reef areas of 14 25 m water depth coralgal halimeda grainstone is found on shallow marginal shoals 1 11 m deep and coralgal grainstone on the somewhat deeper 3 16 m seaward edges of these shoals however there is bathymetrical overlap of facies in intermediate depths of ca 5 17 m this limitation has implications for the interpretation of the fossil record because changes in water depth are commonly thought to be reflected in facies changes e g in sequence stratigraphy comparison with previous sediment studies of the 1930s 1960s and 1970s in the area exhibit a decrease in coral fragments and increases in coralline algal and mollusk shell fragments these observations might be a result of environmental changes such as coral decline and die outs during temperature events disease and the increase in macroalgae due to the ecological extinction of the echinoid diadema the results suggest that more long term studies are needed that further explore the influence of environmental change on reef sediment composition dry tortugas surface sediments consist of lower portions of halimeda plates and mollusk shell fragments and higher percentages of coralline algal fragments as compared to sediments of the adjacent florida reef tract possibly as a consequence of differences in platform morphology and exposure to waves and currents

high velocities in the uppermost mantle of the siberian craton abstract anomalous high seismic velocities up to 8 8 km s have been reported suvorov et al 1985 in the mantle beneath the kimberlite areas of siberia high mantle velocities were also observed on some geon center profiles carried out using nuclear explosions egorkin et al 1987 to obtain clear general signatures of the mantle structure in siberia and the role of the high velocity blocks the geon data were analyzed and compared with other geophysical information this analysis showed that the largest blocks of 300 400 km width with velocities up to 8 6 km s are observed beneath the tunguss depression and in the southern part of the craton near its boundary with the baikal rift zone the blocks penetrate to depths of 100 110 km where they are underlain by low velocity zones or a horizontally homogeneous layer there is no evidence of these blocks in the gravity field velocity anisotropy is proposed as a main cause of the anomalous high velocities although the crossing profiles show that such velocities are observed in different directions the yakutia kimberlite fields are located in the region with average sub moho velocity of 8 3 8 4 km s but with higher velocity at depths of 70 80 km

tomography of the western united states from regional arrival times first arrival times from regional distances 200 1200 km in the western united states extracted from the international seismological centre isc data set were inverted in a tomographic study to map the laterally varying pn velocity structure of the uppermost mantle i e the mantle lid and to estimate the crustal static delay at each seismograph station synthetic data were used to evaluate resolution results correlate well with major tectonic features we find low uppermost mantle velocities v 7 9 km s these low apparent pn velocities are primarily due to the effects of isostasy hotspot volcanism and crustal extension we interpret the low apparent pn velocities beneath the sierra nevada 7 6 7 7 km s to be the result of time delays introduced by ray paths tunneling through the deep crustal root the yellowstone hotspot migrating along the snake river plain has left low velocity 7 6 7 8 km s hot upper mantle in its wake lower pn velocities 7 8 km s beneath the extensional basin and range are also presumably due to hotter uppermost mantle based on station static delays for the basin and range we infer that moho depths there do not vary significantly from 30 km normal continental pn velocities 8 0 km s as well as thicker crust 40 50 km underlying the rocky mountains wyoming basin province colorado plateau and the northern columbia plateau indicate the presence of relatively cool uppermost mantle beneath these regions

north atlantic surface circulation during the 1990 s observed with satellite tracked drifters a new compilation of lagrangian velocity observations describes the state of the north atlantic surface circulation during the 1990s gridded fields of velocity and eddy kinetic energy eke are constructed from trajectories of more than 1500 15 m drogued satellite tracked surface drifters in service between january 1990 and december 1999 this time period overlaps a coordinated field study of circulation and variability in the north atlantic completed between 1996 2000 as part of the world ocean circulation experiment we describe the construction of a self consistent drifter climatology present decadal mean quasi eulerian velocity and eke fields computed on a 1 grid and compare these results with contemporary satellite measurements detailed discussion of the inferred surface circulation is focused on three regions 1 the gulf stream and north atlantic current 2 the labrador sea and subpolar gyre and 3 the caribbean sea the swiftest drifter motions were found in the equatorial region and along the tropical subtropical and subpolar western boundaries the maximum instantaneous speed determined from a single quality controlled and filtered drifter observation was 273 cm s 1 in the gulf stream southeast of cape cod the highest eke value in the north atlantic 2790 cm2 s 2 was found in the gulf stream just downstream of the new england seamounts over most of the atlantic basin drifter derived eke values were found to be o 100 cm2 s 2 higher than corresponding values derived from satellite altimetry in the labrador sea a region of sharply elevated eke appears to be geographically related to the localized ejection of drifters and by extension mass and kinetic energy from the energetic west greenland current between 60 and 62 n when compared to drifter measurements made in the late 1970s our results suggest but do not statistically confirm an enhancement and slight northward shift of the zonal gulf stream extension such a shift is consistent in sign with expectations based on observed interdecadal variations in wind stress and subtropical gyre potential energy associated with the north atlantic oscillation

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

pn anisotropy in the northern apennine chain italy pn travel times recorded by the stations of the italian seismic network have been used to image the azimuthal variations of seismic velocity in the uppermost mantle beneath the northern part of the apennine chain the azimuthal variation of pn velocity is interpreted here in terms of seismic anisotropy we have found that about 5 anisotropy characterizes the uppermost mantle of the studied area and that the fastest direction of pn follows the arcuate trend of the chain this suggests that seismic anisotropy is strongly related to the tectonic regime that originated the northern apennine arc

an analysis of p wave amplitudes recorded by seismological stations in the ussr summary short period p wave amplitudes at soviet seismic stations are analysed to determine an amplitude distance curve in the range 0 180 from the ussr station amplitude terms corrected for crustal amplification effects are determined from seismic sources located between 30 and 90 a relationship between these station terms and regional p wave speed and regional heat flow is determined and is found to be in close agreement with similar observations in north america station amplitude terms were found to be reasonably uniform across the continental portion of the ussr but the baikal rift valley is characterized by very low p wave amplitudes and is the most significant feature discernible from the p wave amplitude terms

the p wave velocity structure near the base of the mantle travel times from 16 events recorded at two temporary networks of short period seismometers deployed across australia have been used to study the p wave velocity structure of the mantle below depths of 1700 km the slowness distance curve derived from these times shows small deviations from the slownesses predicted from the preliminary reference earth model of dziewonski and anderson 1981 that may be significant the most important feature is a rapid decrease of more than 0 5 s degree between distances of 85 and 88 velocity filtering of seismograms from 5 nuclear explosions in kazakh recorded at these australian networks indicates the presence of two arrival branches at distances between 83 and 91 whose slownesses differ by 0 40 to 0 55 s degree and which intersect at a distance of 87 processed p wave seismograms from the yellowknife array in canada also indicate the presence of a similar phenomenon in the same distance range in two separate regions the results can be explained by a widespread rapid or sharp increase in velocity of 2 5 to 3 0 about 200 km above the core mantle boundary

a crustal seismic refraction study in west central arizona a reversed seismic refraction profile was recorded in the southern basin and range province between parker and globe arizona interpretation of both refracted and reflected phases indicates a two layered crust 23 25 km thick and a low pn velocity 7 67 km s pmp pn amplitude analysis on individual traces and synthetic seismogram modeling indicate that a positive velocity gradient may exist in the upper mantle the low pn velocity is indicative of partial melting in the upper mantle and implies that the crust is identical with the lithosphere the heat flow data suggest that the lithosphere was about 40 km thick approximately 5 m y ago on the basis of this and a negative free air gravity anomaly we propose that the lithosphere has been thinned from about 40 to 24 km over the past 5 m y and will probably thin and uplift further in the southern basin and range in arizona

heat generating radioelements in granitic magmas abstract spatial variation trends of the radioelements u th and k in granitic units show distinct regularities zonation patterns of u and th at fairly constant k contents are common in individual granitic bodies it is shown that radioelement abundance and distribution result from a combined migration diffusion process in the fluid phase during and after solidification of the granite magma the process is mainly governed by pressure and temperature gradients as well as by chemical potentials model calculations yield numerical values for the intrinsic diffusion coefficient 10 4 cm 2 s and migration velocity 0 2 2 cm s which are in agreement with laboratory experiments on comparable systems the process distributing the radioelements is of limited duration it is also clearly related to the cooling history of the granite analogies to the overall distribution of crustal radioactivity are discussed

extracting a climate signal from the skeletal geochemistry of the caribbean coral siderastrea siderea 1 the first bimonthly time series of paired 18o and sr ca from the slow growing coral siderastrea siderea from the dry tortugas florida has been generated that documents that robust proxy climate records of the tropical atlantic and intraamerican seas can be produced from this massive coral the time series contain a 20 year long calibration window 1973 1992 for both 18o and sr ca and a 73 year long verification window 1900 1972 for sr ca these time series permit the quantification of the relationship between coral 18o sst and sr ca sst and the assessment of the stability of the proxy relationships over time both coral geochemical records are highly correlated with the augmented instrumental sst record through the calibration period and sr ca remains highly correlated through the verification period both at the bimonthly r 0 97 and annual average resolution r 0 72 coral 18o and sr ca are highly reproducible within the same core and sr ca exhibits no extension related vital effects this study sets the stage for generating multicentury scale climate records from the tropical atlantic ocean using the skeletal geochemistry of this massive but slow growing coral

tectonics and thermal structure of western satpura india abstract increased seismicity and occurrences of hot springs having surface temperature of 36 58 c are observed in the central part of india 74 81 e 20 25 n where the ne trending middle proterozoic aravalli mobile belt meets the ene trending satpura mobile belt earlier deep seismic sounding dss studies along thuadara sendhwa sindad profile in the area has showed mesozoic sediments up to around 4 km depth covered by deccan trap and the moho depth with a boundary velocity p n of 8 2 km s in the present study surface heat flow of 48 4 mw m 2 has been estimated based on p n velocity which agrees with the value of heat flow of 52 4 mw m 2 based on curie point isotherms estimates the calculated temperature depth profile shows temperature of 80 120 c at the basement which is equivalent to oil window temperature in mesozoic sediments and around 570 635 c at moho depth of 38 43 km and the thermal lithosphere is about 110 km thick which is comparatively higher than those of adjoining regions the present study reveals the brittle ductile transition zone at 14 41 km depth temperature around 250 600 c where earthquake nucleation takes place

sub crustal lvz below dharwar craton india an evidence for mantle metasomatism and tectonothermal activity in the archean crust abstract improved knowledge of seismic structure of the continental crust provides deep insights into the mechanism of the crustal growth since the archean times reprocessing of deep seismic sounding data acquired over western dharwar craton india reveals presence of i a high velocity magmatic layer 7 30 km s at the base of the crust ii a higher pn velocity 8 35 km s at a deeper depth of 45 km than in an usual archean crust and iii an unusually low velocity layer 7 40 km s at a depth range of 48 60 km in the upper mantle such shallow subcrustal lithospheric lvzs are rarely found in a stable archean craton which is associated with low surface heat flow and devoid of recent tectonic activities considering various geological tectonic and geothermal scenarios occurrence of such a low velocity zone has been ascribed to chemical alteration caused by mantle metasomatism rather than mafic partial melts or high temperature sub solidus the derived velocity structure and distribution of volcanic rocks of different ages in the studied area indicate multiple crust mantle thermal interaction in the archean crust

the separation of the yucatan current from the campeche bank and the intrusion of the loop current into the gulf of mexico data collected in the eastern gulf of mexico during 1974 1975 and 1976 show that the penetration of the loop current into the gulf is strongly correlated with the location of the yucatan current on the campeche bank the loop does not penetrate far into the gulf when the yucatan current separates from the bank in the vicinity of the catoche tongue i e the eastern campeche bank deep loop penetrations are correlated with separations farther west on the bank the angle of the yucatan current at separation is also correlated with the location of separation i e smaller angles relative to due east are correlated with separations from farther east on the bank thus small angles at separation are correlated with shallow intrusions of the loop historical temperature data collected in the eastern gulf are reviewed and support these correlations simple conservation of potential vorticity considerations can explain the correlation between the angle at separation and the penetration of the loop

seismological mapping of fine structure near the base of the earth s mantle the earth s core mantle boundary cmb juxtaposes liquid iron and crystalline silicates and is a region of large vertical thermal gradients the d region which extends up to 200 300 km above the cmb often has elevated shear wave velocity and suggestions of lateral variations in structure1 recent improvements in our ability to assemble and analyse records from regional seismic networks have allowed us to examine long profiles of travel times amplitudes and waveforms from more than a thousand short period seismometers2 we observe across canada and the united states p waves that have grazed the cmb from the powerful nuclear test in lop nor china on 21 may 1992 first arrival travel times and large secondary arrivals are consistent with a 1 5 compressional velocity increase with depth 130 km above the cmb about half the thickness of d in this locality3 our observations together with evidence for the absence of such a thin fast layer in neighbouring regions suggest the presence of lateral heterogeneity in composition or phase at the base of the mantle

the 1978 yellowstone eastern snake river plain seismic profiling experiment crustal structure of the yellowstone region and experiment design in 1978 a major seismic profiling experiment was conducted in the yellowstone eastern snake river plain region of idaho and wyoming fifteen shots were recorded that provided coverage to distances of 300 km in this paper travel time and synthetic seismogram modeling was used to evaluate an average p wave velocity and apparent q structure of the crust from two seismic profiles reversed across the yellowstone national park region this area includes the well known hydrothermal features of yellowstone national park geysers fumeroles etc a large collapse caldera and extensive silicic volcanism of quaternary age features attributed to shallow crustal sources of magma the averaged crustal structure for this region as interpreted from the seismic data consists of 1 a highly variable near surface layer approximately 2 km thick with variable velocities of 3 0 to 4 8 km s and a low apparent q of 30 that is interpreted to be composed of weathered rhyolites and sedimentary infill 2 an upper crustal layer 3 to 4 km thick with variable velocities of 4 9 to 5 5 km s and apparent q of 50 to 200 that is thought to represent the accumulation of the pleistocene quaternary rhyolite flows ash flow tuffs and possible paleozoic and precambrian metamorphic equivalents 3 the crystalline upper crust that is characterized by a laterally inhomogeneous layer that varies in velocity from 4 0 to 6 1 km s averaging 5 km thick with a q of 300 this layer appears to be a cooling but still hot body of granitic composition beneath the yellowstone caldera it is thought to be a remnant of the magma chambers that produced the quaternary silicic volcanic rocks of the yellowstone plateau and may still be a major contributor to the high heat flow 4 a laterally homogeneous intermediate crustal layer 8 to 10 km thick with a velocity of 6 5 km s and apparent q of 100 to 300 5 a homogeneous 25 km thick lower crust with a velocity of 6 7 to 6 8 km s and an apparent q of 300 and 6 a total crustal thickness of 43 km the upper crustal layer 5 5 to 6 0 km s is thought to be the thermally altered equivalent of the continental crystalline basement that is normally 15 to 20 km thick in the surrounding thermally undisturbed archean crust an interpretation from these results suggests that mafic melts from the mantle have penetrated the lower crust without significant variations in the velocity structure but produce the main source of heat that drives the volcanic and hydrothermal systems of yellowstone the high apparent attenuation and large lateral velocity variations in the upper crust are consistent with a model in which partial fractionation partial melting and metamorphism differentiate the original upper crust to produce silicic melts that were extruded as rhyolites and ash flow tuffs across the yellowstone plateau this seismic model is consistent with the evidence for a systematic northeastward propogation of silicic volcanic centers along the eastern snake river plain to their present location beneath the yellowstone hydrothermal system while these findings do not bear directly on the origin and source of the heat i e mantle lumes lithospheric fractures mantle radiogenetic heat basal lithospheric shearing etc they provide a constraint on the configuration and lateral extent of crustal layers that reflect thermal and compositional boundaries

pn anisotropic tomography under the entire tienshan orogenic belt abstract we present a new anisotropic tomography of the uppermost mantle under the tienshan orogenic belt and surrounding regions using a number of pn arrival time data hand picked from portable seismic stations and chosen from the xinjiang provincial observation bulletins and the ehb datasets our results exhibit prominent lateral heterogeneities in the study region distinct low velocity anomalies are visible under the tectonically active regions such as the tienshan orogenic belt and western kunlun mountains whereas pronounced high velocity anomalies are imaged beneath the stable blocks such as the kazakh shield the junggar tarim qaidam and turpan hami basins and the tajik depression most strong earthquakes ms 7 0 are mainly distributed along the transition zone of high to low velocity anomalies suggesting a possible correlation between the strong earthquakes and the upper mantle structure the fast directions of pn anisotropy beneath the tienshan orogenic belt are generally parallel to its striking orientation whereas those beneath pamir show a northward arc shaped distribution the pn fast velocity directions on the boundaries of the kazakh shield and the tarim and junngar basins are approximately perpendicular to the strike of the tienshan orogenic belt by integrating with previous findings our results suggest that the tarim and kazakh lithospheric materials could have underthrusted beneath the tienshan orogenic belt that leads to the hot mantle material upwelling under the tienshan orogenic belt which is attributable to the indo asian collision these dynamic processes could play important roles in the tienshan mountain building

tides on the west florida shelf the principal semidiurnal m2 and s2 and diurnal k1 and o1 tidal constituents are described on the west florida continental shelf wfs using a combination of in situ measurements and a three dimensional primitive equation numerical model the measurements are of sea level and currents along the coastline and across the shelf respectively the model extends from west of the mississippi river to the florida keys with an open boundary arcing between it is along this open boundary that the regional model is forced by a global tide model standard barotropic tidal analyses are performed for both the data and the model and quantifiable metrics are provided for comparison based on these comparisons the authors present coamplitude and cophase charts for sea level and velocity hodographs for currents the semidiurnal constituents show marked spatial variability whereas the diurnal constituents are spatially more uniform apalachicola bay is a demarcation point for the semidiurnal tides that are well developed to the southeast along the wfs but are minimal to the west the largest semidiurnal tides are in the florida big bend and florida bay regions with a relative minimum in between just to the south of tampa bay these spatial distributions may be explained on the basis of local geometry a lagrangian stokes drift coherently directed toward the northwest is identified but is of relatively small magnitude when compared with the potential for particle transport by seasonal and synoptic scale forcing bottom stressinduced tidal mixing is examined and estimates are made of the bottom logarithmic layer height by the m2 tidal currents

linear relationships between petrophysical properties and mineralogical constitution preliminary results abstract a direct way for tightly relating the mineralogical constitution to density and seismic velocity is demonstrated in an attempt to characterize rocks by a single quantity the oxygen ions are considered as a closely packed structure thus the distribution of cations including silicon ought to correlate with the density as well as with the seismic velocity the cation fraction for each mineral phase is defined by the value the ratio of the number of cations per mole to the molar volume normalized by avogadro s number it is shown experimentally that linear relationships exist between the value and both density and seismic velocity

subduction and back arc activity at the hikurangi convergent margin new zealand the hikurangi margin is a region of oblique subduction with northwest dipping intermediate depth seismicity extending southwest from the kermadec system to about 42 s the current episode of subduction is at least 16 20 ma old the plate convergence rate varies along the margin from about 60 mm a at the south end of the kermadec trench to about 45 mm a at 42 s the age of the pacific lithosphere adjacent to the hikurangi trench is not known

mapping high pn velocity beneath the colorado plateau constrains uplift models the massive international seismological centre data set of the past 20 years and the two station method are used to determine pn velocities in the mantle lid beneath the colorado plateau in this method the event is located at distances where pn is the first arrival 2 16 and the path is in or very near the azimuth of the two station pair and crosses the plateau this technique to a large extent minimizes the hypocenter mislocation effect and possible errors due to variations in the crustal structure near the source since only the difference in travel times at the two stations is used however this technique has a few underlying assumptions and possible sources of errors such as the quality of the pn data base and station delays caused by varying crustal structure that require an extremely careful application of the method a detailed study of the source of errors and a methodology of selection of the data are presented application of this method to the colorado plateau using all possible two station pairs from 53 stations located within or along the margin of the plateau yields an average high pn velocity of 8 12 0 09 km s this value is considerably larger than the average value of 7 83 km s based on available but very limited seismic refraction profiles but is remarkably similar to the average value of 8 1 km s for the relatively stable midcontinent region our new pn velocity for the colorado plateau eliminates the paradox in the literature that emphasizes the rather close similarity between average pn velocities beneath the colorado plateau and the basin and range province while their tectonic and magmatic cenozoic history is dramatically different previous models for the structure and evolution of the plateau have used the low pn velocity as an important constraint on density and thermal state of the lithosphere hence such models should be reexamined on the basis of this new uppermost mantle pn velocity determination there are two main models that have been proposed to explain the 2 km uplift of the colorado plateau one is based on a combination of thermal thinning of the lithosphere and crustal thickening and the other involves a combination of the delamination of the subducted subhorizontal farallon oceanic plate from the overriding north american plate and crustal thickening we show that the delamination model is more readily consistent not only with our velocity determination and the elevation of the plateau but also with varied geological observations reported in the literature that concern the cenozoic evolution of western north america

shallow water marl limestone alternations in the late jurassic of western france cycles storm event deposits or both abstract the contribution of event deposits to various basin fills can be very significant higher than 90 in some cases events may lead to the formation of marl limestone alternations which can also result from cyclic changes in sea level or climate for example the marl limestone alternations of the late jurassic of western france contain abundant coarse grained accumulations that resemble storm deposits described in other western european successions the detailed analysis of facies evolution and hierarchical high frequency stacking pattern of depositional sequences of the phare de chassiron section ile d oleron western france allows the controls on marl limestone formation to be defined this section contains nearshore and shallow marine mud deposits that were exposed to high energy events elementary small and medium scale depositional sequences are defined the stacking pattern and the duration of these sequences suggest an orbital control on sedimentation precession 20 ka cycles notably controlled the formation of elementary sequences that correspond to marl limestone alternations the deposition of marly or carbonate mud occurred in this storm dominated system because of muddy sea beds the gentle slope of the shelf and the great amount of particles in suspension which reduced water energy resulting from storms sediment supply was also sufficient to limit bioturbation and favour the preservation of numerous storm deposits the production of carbonate mud was localised on positive structures and partly controlled by milankovitch scale sea level cycles transport by storms of carbonate mud to the adjacent marly depressions during high carbonate production periods led to the formation of calcareous beds marl limestone alternations in the late jurassic of western france therefore result from the combined effects of cyclic changes in carbonate production and high energy episodic events

lithospheric structure of the marmara and aegean regions western turkey we have simulated the waveforms of three aftershocks of the izmit 17 august 1999 earthquake with magnitudes greater than m 5 0 to determine the lithospheric structure of the gulf of izmit marmara region using the discrete wavenumber technique bouchon 1981 different layered crustal models have been tested for the simulation of the waveforms and the best crustal model was determined by a best fit criterion between the observed and simulated seismograms our results indicate that the average thickness of the crust is 32 km and that pn and s velocities are 8 0 and 4 60 km sec respectively in the gulf of izmit marmara region we also computed synthetic waveforms of the 21 april 2000 denizli honaz and the 9 july 1998 izmir doganbey earthquakes in the aegean in order to compare the lithospheric structures of the marmara and the aegean regions the aegean region has an average crustal thickness of 33 km and pn and s velocities of 7 85 and 4 53 km sec respectively although thicknesses of the crusts are comparable and suggest an approximately equal amount of e w stretching in the marmara and n s stretching in the aegean regions a patchy midcrustal low velocity zone exists in the aegean the upper mantle pn velocity variation between the marmara and the aegean regions is interpreted as the effect of a thinning continental lithosphere toward the active aegean arc and the establishment of a consequent upper mantle temperature gradient increasing from the north to the south additionally the black sea oceanic lithosphere that steeply dips southward beneath the marmara region as evidenced by the seismic tomographic results obtained by gulen and kuleli 1995 can contribute to the advective cooling of the upper mantle that causes relatively high pn velocities in the marmara region manuscript received 20 august 2000

the lateral variation of pn velocity gradient under eurasia mantle lid p wave velocity gradient or pn velocity gradient reflects the depth and lateral variations of thermal and rheological state of the uppermost mantle mapping the pn velocity gradient and its lateral variation helps us gain insight into the temperature composition and dynamics of the uppermost mantle in addition because pn velocity gradient has profound influence on pn propagation behavior an accurate mapping of pn velocity gradient also improves the modeling and prediction of pn travel times and amplitudes in this study i used measured pn travel times to derive path specific pn velocity gradients i then inverted these velocity gradients for two dimensional 2 d pn velocity gradient models for eurasia based on the assumption that a path specific pn velocity gradient is the mean of laterally varying pn velocity gradients along the pn path result from a monte carlo simulation indicates that the assumption is appropriate the 2 d velocity gradient models show that most of eurasia has positive velocity gradients high velocity gradients exist mainly in tectonically active regions most tectonically stable regions show low and more uniform velocity gradients strong velocity gradient variations occur largely along convergent plate boundaries particularly under overriding plates

the velocity structure of the crust and upper mantle under china from broad band p and pp waveform analysis summary we model 45 p and pp waveforms at epicentral distances of 14 to 40 primarily sampling western and central china to determine regionalized p wave velocity variations for the crust and upper mantle in this region observed p and pp waveforms are matched by 1 d forward modelling using the reflectivity technique our approach is to first find a homogeneous average velocity model for china which can match the observed broad band waveforms filtered with a long period instrument response this model is then used as a starting model for the matching of the broad band waveforms to investigate the lateral velocity variations our preferred average model wch has a 50 km thick crust with a velocity of 6 4 km s a p velocity of 8 15 km s and a low velocity zone between 100 km and 175 km depth it has intermediate characteristics between an active tectonic region and a stable shield compatible with previous s wave modelling results modelling of the broad band waveforms indicates significant lateral deviations from model wch for the tibetan plateau we observe larger crustal thickness 55 70 km and p velocity 8 15 8 25 km sc in combination with a low velocity zone that varies in magnitude throughout the tibetan region lower velocities than our average model may be present below 200 km depth our modelling indicates a thin lid of about 30 45 km under tibet we infer from our results and previous work that crustal shortening and thickening is the most likely process responsible for the central and northern tibetan plateau s thick crust and high average elevation and that the indian plate has probably only underplated the southernmost part of the plateau mantle convection associated with the crustal shortening process has led to strong lateral heterogeneity in the upper mantle under tibet higher p velocities in western tibet might indicate that this region is presently undergoing crustal shortening paths traversing eastern china require a constant velocity lid of about 100 km thickness a p velocity of 8 12 8 14kms and a crustal thickness between 27 and 35 km a preliminary model for northern china suggests a crustal thickness between 43 and 50km a p velocity of about 8 0kms and a 100km thick lid with a positive velocity gradient for this region

inhomogeneity and anisotropy of the lithosphere for the territory of the u s s r the dss data obtained for the territory of the u s s r have shown some characteristic features in the distribution of the crustal and mantle inhomogeneities the crustal thickness and average velocities in its consolidated part correlate with the geostructure age its position relative to the continent margin depth to the basement and the surface relief a strong dependence is observed between the lithosphere structure and the heat flow higher temperature regimes are accompanied by a thinner crust and lower uppermost mantle velocities thin layering and q factor structure are typical for the upper mantle of both tectonically active and stable regions it is difficult to determine the lithosphere asthenosphere system the s waves give information on the vpvs ratio and on seismic anisotropy derived from the splitting of differently polarized waves the vpvs value depends on the depth the geostructure age and the temperature regime the seismic anisotropy was observed more in the crust than in the mantle

is extension in death valley accommodated by thinning of the mantle lithosphere beneath the sierra nevada california while relatively rapid and large scale extension occurred in the death valley region of california during neogene time little or no shallow extension occurred in the adjacent sierra nevada this contrast in tectonic history has often been extrapolated to include the entire lithosphere but geophysical and geologic observations indicate that more extension of the mantle lithosphere has occurred under the sierra than under the death valley region upper mantle seismic velocities observed beneath the high sierra are lower than those observed in other regions with comparable surface heat flow this discrepancy could be resolved if the mantle lithosphere beneath the high sierra had become warmer presumably by tectonic thinning in the last 10 m y upper mantle seismic velocities averaged topography and bouguer gravity anomalies all are consistent with the presence of thinner mantle lithosphere beneath the high sierra than beneath the california portion of the basin and range province to the east this suggests that extension of the crust near death valley might be accommodated at a deeper level by thinning of the mantle lithosphere beneath the sierra nevada the extension in the crust of the california basin and range province and the thinning of the mantle lithosphere under the high sierra appear to share the same bounds in time and space the uplift of the high sierra occurred over the past 9 m y which coincides with most of the extension that occurred in the california basin and range province because the orientation of extension in the california basin and range province is inferred to be approximately n60 w from geologic geodetic and in situ stress measurements the northern and southern edges of the death valley extensional subprovince may extend n60 w from the inferred northern and southern limits of west dipping low angle normal faults of the death valley region pronounced changes in the averaged topography and bouguer gravity anomaly across these two bounds both in the basin and range province and in the sierra nevada support a connection between the tectonics of both regions the geomorphic history of the southern sierra suggests an up to the north warp of the sierra across this southern bound during latest cenozoic time hence extension near death valley may be localized in the crust and may be laterally connected to thinning of the mantle lithosphere beneath the sierra nevada this geometry requires

crustal structure in tibet high seismic velocity in the lower crust a least squares procedure is used to extract pure path tibetan group velocities in the period range 8 100 s from fundamental mode rayleigh and love waves recorded along 50 mixed paths the crustal model derived from these dispersion data reveals that the lower half of the very thick 74 10 km tibetan crust has a shear velocity similar to those found in the lower crust beneath stable continental and shield regions a second interesting feature of the model is the presence of a prominent low velocity channel at midcrustal depth the uniqueness of the tibetan crustal structure places the plateau region in a class by itself among tectonic provinces characterized by high heat flow and or wide spread recent volcanism setting stringent constraints on tectonic models of the tibetan plateau

why is it downhill from tonopah to las vegas a case for mantle plume support of the high northern basin and range a major abrupt southward decline in elevation 800 m with an accompanying increase in bouguer gravity anomaly 80 mgal crosses a seismically active region of southern nevada at about 37 n the steepness of the gravity gradient requires significant crustal density contrast some of which is probably caused by the plutonic roots of voluminous tertiary volcanism to the north but the amplitude of the anomaly requires additional contrast within the mantle the topographic step probably developed in mid miocene time coincident with the arrival of the starting head of the yellowstone thermal plume at the base of the lithosphere a plausible combination of crustal and deep buoyancy sources related to the heat and melt input of an anomalously hot asthenospheric source is consistent with gravity seismic heat flow and isotopic observations and explains the origin of the topographic step

strange bedfellows a deep water hermatypic coral reef superimposed on a drowned barrier island southern pulley ridge sw florida platform margin abstract the southeastern component of a subtle ridge feature extending over 200 km along the western ramped margin of the south florida platform known as pulley ridge is composed largely of a non reefal coastal marine deposit modern biostromal reef growth caps southern pulley ridge spr making it the deepest hermatypic reef known in american waters subsurface ridge strata are layered lithified and display a barrier island geomorphology the deep water reef community is dominated by platy scleractinian corals leafy green algae and coralline algae up to 60 live coral cover is observed in 60 75 m of water although only 1 2 of surface light is available to the reef community vertical reef accumulation is thin and did not accompany initial ridge submergence during the most recent sea level rise the delayed onset of reef growth likely resulted from several factors influencing gulf waters during early stages of the last deglaciation 14 kyr b p including cold low salinity waters derived from discrete meltwater pulses high frequency sea level fluctuations and the absence of modern oceanic circulation patterns currently reef growth is supported by the loop current the prevailing western boundary current that impinges upon the southwest florida platform providing warm clear low nutrient waters to spr the rare discovery of a preserved non reefal lowstand shoreline capped by rich hermatypic deep reef growth on a tectonically stable continental shelf is significant for both accurate identification of late quaternary sea level position and in better constraining controls on the depth limits of hermatypic reefs and their capacity for adaptation to extremely low light levels

heat flow heat generation and crustal temperatures in the northern canadian cordillera thermal control of tectonics 1 the heat flow in the northern canadian cordillera between 59 and 64 n is very high 105 22 mw m2 the average upper crust heat generation is 4 6 2 4 microw m3 giving a reduced deep heat flow of 59 mw m2 similar to the southern canadian cordillera 49 53 n estimated moho temperatures of 800 1000 c and lithosphere only 50 60 km thick are substantiated by slow upper mantle seismic velocities and by high elevations associated with crust only 32 35 km thick the new data support the conclusion that all of the north and south america cordilleras are hot the paleoproterozoic wopmay orogen to the east has a remarkably high heat flow of 90 15 mw m2 for such an old and stable region however basement samples indicate high heat generation and estimated deep temperatures are intermediate between the cordillera and the slave craton further to the east heat flow from the yellowknife area of the slave craton 53 mw m2 implies low lithosphere temperatures in agreement with kimberlite xenolith p t data the high temperatures in the cordillera result in a very thin and weak lithosphere and may allow a lower crust detachment the north and central cordillera have been inland of the coastal queen charlotte transform fault since subduction stopped in the eocene thus some process has maintained high temperatures in the former back arc for more than 40 ma

measurements of coda buildup and decay rates of western pacific p po and so phases and their relevance to lithospheric scattering we use empirical functions to characterize the shape of band passed p po and so coda envelopes of 93 earthquakes the data sets consist of earthquake data from the western and northwestern pacific recorded by arrays of several stations and include both hydrophone and geophone recordings on analog and digital instruments each earthquake was band passed into six octave wide frequency bands between 0 8 and 50 hz the empirical function a poisson function pn t a n 1 tn e t n has parameters a a measure of energy falloff rate and nc time of the maximum the study showed that these parameters varied in systematic ways po so area a ratios were found to be considerably higher for hydrophone data in the 20 35 range than for geophone and hydrophone data in the 8 20 range the difference was significantly larger than between vertical and hydrophone data in the same range in some instances a strong frequency dependence or reverse dispersion of the time of maximum amplitude n was observed with the lower 1 2 hz frequencies arriving up to 25s later than the higher 10 20 hz frequencies p po and so average falloff rates a vary systematically with frequency where the lowest frequencies most commonly have the minimum falloff rates and the middle frequencies have the maximum falloff rates the variation in falloff between earthquakes is large with the trends only becoming apparent upon averaging many measurements a strong increase in falloff rates at about 3 hz may be associated with a decrease in the amount of scatterers with scale lengths of about one half the wavelength of a 3 hz compressional wave which is about 1 3 km falloff rates were seen to decrease with epicentral range reflecting an increased scattering with length of propagation path the scattering that produces the coda is thus not concentrated in the source region but distributed along the propagation path falloff rates were seen to decrease with increasing age of crust the difference may be due to the different average age of the lithosphere in these two geographical areas earthquakes at a depth of 100 300 km have systematically lower po falloff values than shallower events this effect is clearly seen in trench earthquakes and indicates that significant scattering occurs within or near the deeper buried part of the subducting plate the so po falloff ratio is a factor of 2 lower for the hydrophone data than for the geophone measurements this difference is most likely due to conversion of shear waves at or below the seafloor if we assume that the coda decay is controlled by intrinsic attenuation then measurements of coda decay can be converted into estimates of the quality factor q at 10 hz our falloff rates give qpo 1250 which is a reasonable number for a compressional wave quality factor in the uppermost mantle and qso qpo 2 the low snear wave q is in disagreement with inferences made by other observers however the fact that the falloff rates are fairly constant in the frequency range 5 30 hz but vary with propagation distance suggests that attenuation cannot be the only phenomenon determining the decay of the coda other factors such as reverberations in the water column may play an important part in the coda formation as well and may bias these estimates q

crustal thickness and upper mantle velocities in the tibetan plateau region from the inversion of regional pnl waveforms evidence for a thick upper mantle lid beneath southern tibet the crustal thickness and upper mantle velocity variation in tibet and surrounding regions was mapped through the inversion of 130 individual long period pnl waveforms the 130 paths crisscross the tibetan plateau hindu kush karakoram southwest china burma and northern india regions we regionalized these areas on the basis of topographic and geologic expression and determined the average upper mantle velocity and crustal thickness value for each block or region through a linear least squares inversion of the 130 parameter values crustal thicknesses in the tibetan plateau karakoram and hindu kush regions range from 63 to 72 km and pn velocities are 0 10 0 20 km s faster beneath these regions than those beneath northern india we observed impulsive first arrivals evidence for positive upper mantle gradients on seismograms recorded at distances between 1000 and 1400 km the paths of these waves crossed portions of tibet the hindu kush and east india regions simple modeling of the long period pn wave form and amplitude requires fairly high upper mantle gradients of 0 18 0 25 km s per 100 km beneath the pamir hindu kush region and requires an upper mantle lid in excess of 100 km thickness the southern half of tibet also requires a lid of about 100 km thickness a thinner upper mantle lid 50 km provided a poor match to the tibetan observations even with a very high gradient of 0 2 km s per 100 km the upper mantle structure of east india appears to be similar to the southern half of tibet 100 km thick positive gradient zone both the increase in pn velocity from india into tibet and the evidence for a thick upper mantle lid beneath tibet are consistent with the model in which indian lithosphere has underthrust southern tibet due to poor path coverage the structure beneath northern tibet cannot be resolved and thus underthrusting beyond central tibet cannot be constrained the rapid increase in upper mantle velocity from northern india into tibet can be explained by the pressure increase alone produced by the double thick crust and indicates that the indian lithosphere which underlies tibet is out of thermal equilibrium

new perspectives on the geology and origin of the florida middle ground carbonate banks west florida shelf usa abstract bathymetric and seismic surveys of the florida middle ground area on the west florida shelf aid in identifying the structure and origin of these unusual banks recent cores recovered from the area by other workers provide a useful context for interpreting geophysical data data indicate that high relief banks may be largely biohermal whereas low relief banks are likely composed of clastic sediments with a vermetid gastropod carbonate caprock the most seaward ridges reveal subtle internal aggradational stratal geometries indicating a possible origin as a subtidal shoal or paleoshoreline deposited during or immediately following the younger dryas bank locations appear to be related to the antecedent geology specifically the occurrence of the highly weathered miocene surface high resolution multibeam bathymetry and backscatter data reveal varied morphologies that suggest the influence of karst processes linear trends suggestive of wave reworked intertidal hardbottom poorly lithified and benthic fauna mantled beach ridges or sand waves or perhaps surface water flow patterns during lowstand exposure circular depressions on low banks are of the same scale and morphology as modern shallow karst features in the everglades an episodically active sand wave field occurs in the northern study area and provides sediment to the system scoured depressions surrounding carbonate banks also indicate intense episodic current activity originating from shelf and wave currents a partial modern analog for some of the morphology of this system may be the ten thousand islands and portions of the florida everglades data indicate that the banks represent a brief period of reef growth possibly mixed coral and vermetid constrained by two rapid sea level rise events and impacted by changing water quality and trophic conditions