liquefaction evidence for strong earthquakes of holocene and latest pleistocene ages in the states of indiana and illinois usa abstract sand and gravel filled clastic dikes of seismic liquefaction origin occur throughout much of southern indiana and illinois nearly all of these dikes originated from prehistoric earthquakes centered in the study area in this area at least seven and probably eight strong prehistoric earthquakes have been documented as occurring during the holocene and at least one during the latest pleistocene the recognition of different earthquakes has been based mainly on timing of liquefaction in combination with the regional pattern of liquefaction effects but some have been recognized only by geotechnical testing at sites of liquefaction most paleo earthquakes presently recognized lie in indiana but equally as many may have occurred in illinois studies in illinois have not yet narrowly bracketed the age of clastic dikes at many sites which sometimes causes uncertainty in defining the causative earthquake but even in illinois the largest paleo earthquakes probably have been identified prehistoric magnitudes were probably as high as about moment magnitude m 7 5 this greatly exceeds the largest historic earthquake of m 5 5 centered in indiana or illinois the strongest paleo earthquakes struck in the vicinity of the concentration of strongest historic seismicity elsewhere paleo earthquakes on the order of m 6 7 have occurred even where there has been little or no historic seismicity both geologic and geotechnical methods of analysis have been essential for verification of seismic origin for the dikes and for back calculating prehistoric magnitudes methods developed largely as part of this study should be of great value in unraveling the paleoseismic record elsewhere

intercalibration of 40ar39ar dating standards a number of standards used for k ar and 40ar39ar dating have been intercalibrated multiple splits 15 40 mg each of mmhb 1 sb 3 biotite lp 6 biotite 40 60 ga1550 biotite fish canyon tuff 3 biotite taylor creek rhyolite sanidine bern 4 muscovite bern 4 biotite and pennsylvania state university orthoclase 1a were irradiated in capsules fused and analyzed in replicate analyses most of these standards proved to be homogeneous in 40ar 39ark ratio age k ca and 36ar contents mmhb 1 is inhomogeneous in age at the 15 mg level indicating it is unsuitable for use as an interlaboratory standard using material taken straight from the bottle the orthoclase sample is also inhomogeneous at the 30 mg level quoted relative to an age of 162 9 ma for the sb 3 biotite standard total fusion ages are lp 6 biotite 40 60 128 1 ma ga1550 biotite 97 8 ma fish canyon tuff 3 biotite 27 95 ma taylor creek rhyolite sanidine 28 0 ma bern 4 muscovite 18 51 ma bern 4 biotite 17 21 ma these ages are in good agreement with the absolute k ar ages of these minerals different preparations of biotite and sanidine standards from a welded tuff such as the fish canyon tuff may not be of identical ages tests on submilligram splits reveals that sb 3 biotite ga1550 biotite fish canyon tuff 3 biotite and taylor creek rhyolite sanidine are suitable for use as monitors in laser work the sanidine specimen appears to be the most suitable for use as the primary monitor for 40ar39ar dating

reply to discussion is spreading prolong episodic or incipient in the andaman sea evidence from deepwater sedimentation by j r curray 2015 abstract we consider the presence of thick section in the axial trough indicates that the pliocene recent continuous spreading model proposed by curray 2015 is not appropriate for the andaman sea the high frequency of eustatic sea level changes during the plio pleistocene is incompatible with large rapid influxes of sediment widely separated in time but related to eustacy as proposed by curray 2015 instead spreading is probably of miocene age and was only renewed recently if at all the nature of the crust adjacent to the spreading centre remains uncertain however industry seismic data from the east andaman basin strongly suggests if the western part of the basin is underlain by oceanic crust it must be oligocene or older it is not early miocene this in turn suggests the volcanics dredged from the alcock rise were extruded onto older crust whether it is oceanic island arc or continental and do not represent the age of oceanic crust formation gravity and seismic reflection data point to the east andaman basin being dominantly underlain by extended to hyper extended continental crust and transitional crust the alcock and sewell rises are interpreted as representing the western side of the strongly necked zone of continental crust

petrology and geochronology of muscovite age standard b4m muscovite b4m distributed in 1961 as an age standard was ground under ethanol five grain size fractions were obtained and characterized by x ray diffraction they display a mixing trend between a phengitic enriched in the fraction 20 m high pressure phengite is preserved as a relict in retrograde muscovite electron microprobe analyses of the distributed mineral separate reveal at least four white mica populations based on si al mg na fe and f rb k ratios vary by one order of magnitude rb sr analyses link the mineralogical heterogeneity to variable rb sr and 87sr 86sr ratios the grain size fractions define no internal isochron relict fine grained phengite gives older ages than coarse grained retrograde greenschist facies muscovite the inverse grain size age relationship also characterizes 39ar 40ar analyses cl k anticorrelates with step ages cl rich coarse muscovite is younger than cl poor fine relict phengite sr and ar preserve a similar isotopic inheritance despite peak metamorphism reaching 635 20 c a suitable mineral standard requires that its petrological equilibrium first be demonstrated relicts and retrograde reaction textures are a guarantee of isotopic disequilibrium and heterogeneous ages within single crystal at the micrometre scale

diachronous initiation of post collisional magmatism in the arabia eurasia collision zone abstract the continental collision between arabia and eurasia which gave rise to the caucasus iran anatolia cia volcanic province provides a unique opportunity for understanding collisional zone magmatism this study reports a comprehensive dataset of ages and geochemical compositions of volcanic rocks formed during the initial phase of post collisional magmatism in the cia province the age data indicate a diachronous onset of volcanism that began 17 ma in se anatolia and propagated northward from 11 to 9 ma toward ne anatolia and nw iran the rocks are characteristically bimodal with dominantly basic sio2 48 52 wt and silicic sio2 58 71 wt components that feature significant isotopic variations end 6 to 5 suggesting two principal magma sources 1 a juvenile mantle derived component and 2 an older continental crust component we therefore attribute the volcanic initiation to a migrating post collisional extension regime caused by the successive breakoff of subducted neo tethyan slabs subsequent volcanism that began from 6 5 ma resulted in a wide spectrum of calc alkaline and alkaline rocks with mafic to felsic lithologies in the entire province from 2 ma volcanism ceased in the western cia province and started propagating eastward and southeastward to se iran following the urumieh dokhtar magmatic belt consistent with the notion of an oblique continental collision between arabia and eurasia

recognizing and dating prehistoric liquefaction features lessons learned in the new madrid seismic zone central united states the new madrid seismic zone nmsz which experienced severe liquefaction during the great new madrid missouri earthquakes of 1811 and 1812 as well as during several prehistoric earthquakes is a superb laboratory for the study of world class earthquake induced liquefaction features and their use in paleoseismology in seismically active regions like the nmsz frequent large earthquakes can produce a complex record of liquefaction events that is difficult to interpret lessons learned studying liquefaction features in the nmsz may help to unravel the paleoseismic record in other seismically active regions soil characteristics of liquefaction features as well as their structural and stratigraphic relations to native american occupation horizons and other cultural features can help to distinguish prehistoric liquefaction features from historic features in addition analyses of artifact assemblages and botanical content of cultural horizons can help to narrow the age ranges of liquefaction features future research should focus on methods for defining source areas and estimating magnitudes of prehistoric earthquakes from liquefaction features also new methods for dating liquefaction features are needed

eocene north south trending dikes in central tibet new constraints on the timing of east west extension with implications for early plateau uplift east west extension has been a prominent feature of cenozoic tectonics in central southern tibet and the onset of this extension has been interpreted as indicating the surface uplift of the tibetan plateau to a critical level previous studies suggested that extension started in the neogene between 18 13 ma and 8 ma based on dating results from north south trending normal faults grabens or rifts and dikes we report here the discovery of widespread north south trending diabase and andesitic porphyry dikes in and around the shuang hu graben central tibet where crustal extension has previously been argued to start from 13 5 ma based on mineral ages from graben bounding faults our results show that dikes in this area were mainly formed in the eocene 47 38 ma geochemical and nd sr isotopic data suggest that the diabase dikes originated from partial melting of an enriched lithospheric mantle source metasomatized by melts from subducted continental crust and the andesitic porphyry dikes were probably derived from interactions between the subducted continental crust derived melts and the mantle given that north south trending eocene dikes are found widely dispersed in central tibet we argue that the onset of east west extension and likely regional uplift of the tibetan plateau began much earlier than previously thought our study implies that a large part of the tibetan plateau had already attained near maximum elevation in the eocene and is consistent with recent investigations regarding the middle late eocene 47 34 ma initiation of aridification in asia evidence from contemporary global oceanic o sr isotope curves and temperatures and the carbon cycle 2010 elsevier b v

stress field constraints on intraplate seismicity in eastern north america focal mechanisms of 32 north american midplate earthquakes mo 3 8 6 5 were evaluated to determine if slip is compatible with a broad scale regional stress field derived from plate driving forces and if so under what conditions stress regime pore pressure and frictional coefficient using independent information on in situ stress orientations from well bore breakout and hydraulic fracturing data and assuming that the regional principal stresses are in approximately horizontal and vertical planes 10o the constraint that the slip vector represents the direction of maximum resolved shear stress on the fault plane was used to calculate relative stress magnitudes defined by the parameterb s2 s3 s s3 from the fault stress geometry as long as the focal mechanism has a component of oblique slip i e the b axis does not coincide with the intermediate principal stress direction this calculation identifies which of the two nodal planes is a geometrically possible slip plane gephart 1985 slip in a majority of the earthquakes 25 of 32 was found to be geometrically compatible with reactivation of favorably oriented preexisting fault planes in response to the broad scale uniform regional stress field slip in five events was clearly inconsistent with the regional stress field and appears to require a localized stress anomaly to explain the seismicity significantly all five of these events occurred prior to 1970 when many regional networks were installed and their focal mechanisms are inconsistent with more recent solutions of nearby smaller events the frictional likelihood of the geometrically possible slip on the selected fault planes was evaluated in the context of conventional frictional faulting theory the ratio of shear to normal stress on the fault planes at hypocentral depth was calculated relative to an assumed regional stress field regional stress magnitudes were determined from 1 s s3 ratios based on the frictional strength of optimally oriented faults the basis for the linear brittle portion of lithospheric strength profiles 2 the computed relative stress magnitude b values and 3 a vertical principal stress assumed equal to the lithostat two end member possibilities were examined to explain the observed slip in these less than optimally oriented fault planes first the frictional coefficient was held constant on all faults hydrostatic pore pressure was assumed regionally and the fault zone pore pressure was determined since pore pressure is a measurable quantity with real limits in the crust p0 s3 this end member case was used to determine which of the geometrically possible slip planes were frictionally likely slip planes alternately pore pressure was fixed at hydrostatic everywhere and the required relative lowered frictional coefficient of the fault zone was computed slip in 23 of the 25 geometrically compatible earthquakes was determined to also be frictionally likely in response to an approximately horizontal and vertical regional stress field derived from plate driving forces whose magnitudes are constrained by the frictional strength of optimally oriented faults assuming hydrostatic pore pressure regionally the conditions for slip on these 23 relatively well oriented earthquake faults were determined relative to this regional crustal strength model and require only moderate increases in pore pressure between about 0 4 0 8 of lithostatic hydrostatic is about 0 37 of lithostatic or alternately moderate lowering 50 of the frictional coefficient on the faults which slipped superlithostatic pore pressures are not required focal mechanisms for the two other earthquakes with slip vectors geometrically consistent with the regional stress field however did require pore pressures far exceeding the least principal stress or extremely low coefficients of friction these events may reflect either local stress rotations undetected with current sampling or poorly constrained focal mechanisms the analysis also confirmed a roughly north to south contrast in stress regime between the central eastern united states and southeastern canada previously inferred from a contrast in focal mecha nisms between the two areas most central eastern united states earthquakes occur in response to a strike slip stress regime whereas the southeastern canadian events require a thrust faulting stress regime this contrast in stress regime with a constant maximum horizontal stress orientation determined by far field plate driving forces requires a systematic lateral variation in relative stress magnitudes superposition of stresses due to simple flexural models of glacial rebound stresses are of the correct sense to explain the observed lateral variation but maximum computed rebound related stress magnitude changes are quite small about 10 mpa and do not appear large enough to account for the stress regime change if commonly assumed stress magnitudes determined from frictional strength apply to the crust at seismogenic depths

new insights into the tectonic evolution of the andaman basin northeast indian ocean multibeam swath bathymetry data acquired over an area of about 30 000 km2 together with magnetic and single channel seismic data have been analyzed to understand the tectonic evolution of the andaman basin northeast indian ocean swath bathymetry data revealed several morphotectonic features that divide the basin into a complex western part comprising arc parallel seamount chains and n s trending fault systems and a relatively smooth eastern part a sw ne trending spreading ridge bisects the basin an offset of about 11 8 km at 94 21 e longitude divides the spreading ridge into sw and ne segments high resolution bathymetry and magnetic data enabled us to identify two finer scale segments over the sw part of the ridge that display distinct topographic fabric compared to the ne segment thin sediment cover and presence of axial seamounts characterize sw segments of the ridge whereas the ne segment has thick sediment cover and well maintained rift valley kinematics of basin evolution derived from the identification of the seafloor spreading magnetic anomalies suggest very slow initial opening rates of about 1 6 cm yr and an increase in the spreading rate to about 3 8 cm yr from anomaly 2 to present although this interpretation relies only on magnetic anomalies in the two westernmost segments it is suggested that the true seafloor spreading started about 4 ma in the andaman backarc basin rather than the 11 ma postulated previously we infer based on the integrated analysis of swath bathymetry magnetic and seismological data that the basin has evolved as a consequence of extrusive tectonics that prompted extension and rifting along the plane joining the sagaing and semengko fault systems extension by seafloor spreading during the past 4 myr resulted in the formation of the deep andaman backarc basin this phase has also experienced westward propagation of the spreading center

structural evolution of the yushu nangqian region and its relationship to syncollisional igneous activity east central tibet field mapping geochronological analyses and cross section construction reveal a protracted deformation history and a minimum of 61 km of cenozoic ne sw shortening in present coordinates across the yushu nangqian thrust belt in northern tibet cenozoic contraction started prior to 51 ma and was followed first by northwest striking right slip faulting and later by northwest striking left slip faulting renewed ne sw contraction is expressed by folding of neogene strata and thrusting which again was followed by northwest striking left slip faults late neogene deformation is expressed by local north striking normal faults shortening across the yushu nangqian belt appears to be accommodated by thin skinned thrusting which raises the question of how the deformation was accommodated in the lower crustal levels to resolve this problem we perform geochemical analysis of igneous rocks dated as 51 49 and 38 37 ma the rocks exhibit geochemical signatures characteristic of subduction which implies that coeval crustal thickening in northeastern tibet was most likely induced by continental subduction

tibetan tectonic evolution inferred from spatial and temporal variations in post collisional magmatism cenozoic magmatism on the tibetan plateau shows systematic variations in space and time that must be considered in models concerning tibetan tectonic evolution after the india asia collision which started in the early tertiary and terminated the gangdese arc magmatism in the lhasa terrane made of the southern tibetan plateau widespread potassium rich lavas and subordinate sodium rich basalts were generated from f50 to 30 ma in the qiangtang terrane of northern tibet subsequent post collisional magmatism migrated southwards producing ultrapotassic and adakitic lavas coevally between f26 and 10 ma in the lhasa terrane then potassic volcanism was renewed to the north and has become extensive and semicontinuous since f13 ma in the western qiangtang and songpan ganze terranes such spatial temporal variations enable us to elaborate a geodynamic evolution model which depicts when and how the indian continental lithospheric mantle started thrusting under asia by involving rollback and breakoff of the subducted neo tethyan slab followed by removal of the thickened lhasa lithospheric root we propose that only after the lithospheric removal which occurred at f26 ma could the indian mantle lithosphere have commenced its northward underthrusting and henceforth served as a pivotal control to the himalayan tibetan orogenesis consequently the tibetan plateau is suggested to have risen diachronously from south to north whereas the southern part of the plateau may have been created and maintained since the late oligocene the northern plateau would have not attained its present day elevation and size until the mid miocene when the lower part of the western qiangtang and songpan ganze lithospheres began to founder owing to the push of the underthrust indian mantle lithosphere d 2004 elsevier b v all rights reserved

seismotectonic implications of sand blows in the southern mississippi embayment we explore seismically induced sand blows from the southern mississippi embayment and their implications in resolving the question of near or distal epicentral source region this was accomplished using aerial photography field excavations and cone penetration tests our analysis shows that three sand blow fields exhibit a distinct chronology of strong ground motion for the southern embayment 1 the ashley county arkansas sand blow field near the arkansas louisiana state border experienced four holocene sand venting episodes 2 to the north the desha county field experienced at least three episodes of liquefaction and 3 the lincoln jefferson counties field experienced at least one episode cone penetration tests cpt conducted in and between the sand blow fields suggest that the fields may not be distal liquefaction associated with new madrid seismic zone earthquakes but rather are likely associated with strong earthquakes on local faults this conclusion is consistent with the differences in timing of the southern embayment sand venting episodes and those in the new madrid seismic zone these results suggest that active tectonism and strong seismicity in intraplate north america may not be localized at isolated weak spots but rather widespread on fault systems that are favorably oriented for slip in the contemporary stress field

intercalibration of the hb3gr 40ar 39ar dating standard abstract the 40 ar 39 ar dating technique is based on the knowledge of the age of neutron fluence monitors standards recent investigations have improved the accuracy and precision of the ages of most of the phanerozoic aged standards e g fish canyon tuff sanidine fcs alder creek sanidine ga1550 biotite and lp 6 biotite however no specific study has been undertaken on the older standards i e hb3gr hornblende and nl 25 hornblende generally used to date precambrian high ca k and or meteoritic rocks in this study we show that hb3gr hornblende is relatively homogenous in age composition ca k and atmospheric contamination at the single grain level the mean standard deviation of the 40 ar 39 ar k f value derived from this study is 0 49 comparable to the most homogeneous standards the intercalibration factor which allows direct comparison between standards between hb3gr and fcs is r fcs hb3gr 51 945 0 167 using an age of 28 02 ma for fcs the age of hb3gr derived from the r value is 1073 6 5 3 ma 1 internal error only and 8 8 ma including all sources of error this age is indistinguishable within uncertainty from the k ar age previously reported at 1072 11 ma turner g huneke j c podosek f a wasserburg g j 1971 40 ar 39 ar ages and cosmic ray exposure ages of apollo 14 samples earth planet sci lett 12 19 35 the r value determined in this study can also be used to intercalibrate fcs if we consider the k ar date of 1072 ma as a reference age for hb3gr we derive an age of 27 95 0 19 ma 1 internal error only for fcs which is in agreement with the previous determinations altogether this shows that hb3gr is a suitable standard for 40 ar 39 ar geochronology

explosive craters and soil liquefaction curious dry craters formed in the aftermath of a disastrous earthquake are explained the bhuj earthquake in january 2001 with a moment magnitude of 7 7 was one of the most disastrous in india s recorded history although its fault plane did not rupture the surface the earthquake caused ground failure including the formation of sand blow craters and sand fissures indicative of liquefaction of subsurface sediment over an area of about 15 000 square kilometres1 here we investigate the two large dry craters that formed during the earthquake and estimate the likely subsurface pressure at the time from a trajectory analysis of the ejected clasts we conclude that the craters were created by explosive deformation of the ground that probably resulted from the delayed effects of soil liquefaction

an intercalibration study of the fish canyon sanidine and biotite 40ar 39ar standards and some comments on the age of the fish canyon tuff the age of the fish canyon tuff fct has been the subject of intense study due to its potential significance as a geochronological test to establish age concordance between the k ar and u pb isotopic systems minerals from the fct notably sanidine and biotite are in widespread use as 40ar 39ar fluence monitors standards and have been employed as both primary and intercalibrated standards in attempting to constrain the true age of fct the distinction between primary vs intercalibrated ages is important as only the primary age determinations are valid an investigation of both fish canyon sanidine fcs and fish canyon biotite fct 3 was undertaken to assess the chemical and isotopic homogeneity of the minerals on a grain to grain scale electron microprobe results show that fcs is chemically homogeneous verifying previous studies although there are some very small zones of minor k depletion fct 3 biotite commonly contains numerous and small mineral inclusions 40ar 39ar experiments further show that fcs is remarkably homogeneous confirming previous observations whereas fct 3 shows much greater isotopic variability due to contamination from the mineral inclusions by setting the age of fcs equal to 27 98 0 15 ma 2 as intercalibrated to the u pb system by villeneuve et al 2000 the resultant intercalibration factor between fcs and fct 3 is 1 005 0 017 yielding an age of 28 13 0 47 ma 2 for fct 3 although the significant intragrain chemical and isotopic variability precludes its use as an effective 40ar 39ar fluence monitor the true age of the fct is problematic a survey of all primary ages for fct shows that mean k ar ages for plagioclase hornblende biotite and sanidine are consistently younger than mean u pb ages from titanite and zircon in conjunction with recently published petrological results which indicate that the fct contains material from multiple crystallization events this significant discrepancy between the k ar and u pb data sets demonstrates that the fish canyon tuff cannot serve as a golden standard between the k ar and u pb systems

thermoluminescence dating of sand dunes in rajasthan india we report here a new application of thermoluminescence tl for dating sand dunes this application assumes that exposure to sunlight causes bleaching of the geological tl of a sediment to a residual value and that tl accumulation starts again when the sediment is buried under fresh deposit and is thus shielded from the sun although the tl method has been applied to loess1 2 and ocean sediments3 4 the application of this technique to sand dunes provides the first reliable dating control for dune dynamics palaeoclimatology and spread of deserts

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

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

ybcs sanidine a new standard for 40ar 39ar dating article i nfo ar 39 ar geochronology new standard ybcs sanidine the 40 ar 39 ar dating technique is based on neutron fluence monitors standards recent investigation demon 40 ar 39 ar dating aliquots were distributed to four international laboratories for analysis and intercalibration the results show that ybcs crystals are homogenous in k content 40 ar 39 ark f value and age at the single grain level the standard deviations of the f value and age have small ranges from 0 29 to 0 53 and from 0 42 to 0 52 respectively these show that ybcs is a suitable standard for 40 ar 39 ar geochronology the

quaternary faulting in the southern mississippi embayment and implications for tectonics and seismicity in an intraplate setting the recently recognized saline river fault zone in the southwestern mississippi embayment strike 135 is characterized by moderate historic seismicity we document paleoseismicity possibly strong on the saline river fault zone that is outside the known region of neotectonism in the northern embayment six surface faults were excavated within the saline river fault zone and all faults displace marine eocene units and fluvial pliocene pleistocene units five show post wisconsin loess movement thermoluminescence tl age 23 6 3 ka and three deform middle to late holocene eolian silt tl age 5 1 0 6 ka and 3 6 0 5 ka as fault tip flexures along the principal excavated fault fault plane braiding and plunging drag folding suggest a strong component of left slip and subsidiary faults show both normal and reverse displacement 1m of pliocene pleistocene wisconsin and holocene units we interpret these faults as components of a left slip flower structure a shallow seismic reflection profile acquired across the principal fault and a subsidiary fault shows eocene stratigraphy separated in a reverse sense below 120 m depth and separated in a normal sense above 120 m supporting a strike slip interpretation in addition the only focal plane mechanism available in the area indicates left slip with a small reverse component we conclude that the saline river fault zone is a paleozoic mesozoic basement fault zone reactivated as a left slip system in a quaternary east west compressive stress field field evidence suggests that earthquakes with several meters of strike slip may be characteristic and the width of a liquefaction field on the saline river fault zone suggests an event of magnitude 5 5 6 considering a similarity of strike and sense of holocene movements of the meers fault of southern oklahoma and of the saline river fault zone there may be additional active fault zones that pose seismic hazards concealed by sediments in southern north america

evidence of strong earthquake shaking in the lower wabash valley from prehistoric liquefaction features earthquake induced liquefaction features in holocene sediments provide evidence of strong prehistoric shaking magnitude m b 6 2 to 6 7 in the wabash valley bordering indiana and illinois the source of the one or more earthquakes responsible was almost certainly in or near the wabash valley the largest event is interpreted to have occurred between 7500 and 1500 years ago on the basis of archeological pedological and stratigraphic relations

liquefaction at oceano california during the 2003 san simeon earthquake the 2003 m 6 5 san simeon california earthquake caused liquefaction induced lateral spreading at oceano at an unexpectedly large distance from the seismogenic rupture we conclude that the liquefaction was caused by ground motion that was enhanced by both rupture directivity in the mainshock and local site amplification by unconsolidated fine grained deposits liquefaction occurred in sandy artificial fill and undisturbed eolian sand and fluvial deposits the largest and most damaging lateral spread was caused by liquefaction of artificial fill the head of this lateral spread coincided with the boundary between the artificial fill and undisturbed eolian sand deposits values of the liquefaction potential index in general were greater than 5 at liquefaction sites the threshold value that has been proposed for liquefaction hazard mapping although the mainshock ground motion at oceano was not recorded peak ground acceleration was estimated to range from 0 25 and 0 28 g on the basis of the liquefaction potential index and aftershock recordings the estimates fall within the range of peak ground acceleration values associated with the modified mercalli intensity vii reported at the u s geological survey usgs did you feel it web site

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

paleoseismic features as indicators of earthquake hazards in north coastal san diego county california usa abstract new road cut and mass grading excavations in the north coastal area of san diego county california expose heretofore generally unrecognized probable late holocene tsunami deposits and paleoseismically deformed sediments remnant tsunami deposits occur up to 100 m in elevation around the margins of modern coastal lagoons and estuaries and combined with local mima mounds of possible sand blow origin provide indirect but compelling evidence for the late quaternary activity of onshore and offshore faults in the immediate study area probable paleoliquefaction features are regionally widespread and range from fissures filled with sediments derived from overlying marine terrace sand and soil to the more traditional sand filled injection dikes and sills lateral spreads and filled craterlets the source of most liquefied sediment is underlying tertiary bedrock sand and local quaternary marine terrace deposits a paleoseismic liquefaction origin rather than soft sediment loading is deduced for these features based on morphology internal stratigraphy field setting and near proximity to known seismogenic sources some paleoseismic events impacted late holocene indian middens and burial sites the last seismic event probably occurred within the past 1 to 3 ka and possibly even records the historic earthquakes of either november 22 1800 or may 27 1862 the liquefaction features also affect marine terrace sediments tens of meters above modern regional water levels inferentially recording paleoseismic events in this mediterranean type climatic region during winter rains when high level perched water saturates the several meter thick source sediments based on their regional extent the paleoseismic features were likely caused by m 7 tectonic events inferentially generated by the nearby offshore newport inglewood rose canyon fault system or possibly by smaller recently exposed related and localized faults accordingly the seismic hazard of the north coastal area of san diego county may be substantially higher than previously assumed and hence of concern owing to the rapid ongoing and projected population increase

liquefaction during the 1977 san juan province argentina earthquake ms 7 4 liquefaction effects generated by the 1977 san juan province argentina earthquake ms 7 4 are described the larger and more abundant effects were concentrated in the 60 km long band of the lowlands in the valle del bermejo and in an equally long band along the rio san juan in the valle de tulum fissures in the valle del bermejo were up to several hundred meters long and up to several meters wide sand deposits from boils that erupted through the fissures covered areas up to tens of square meters fissures generally parallelled nearby stream channels because the valle del bermejo is undeveloped these large features caused no damage liquefaction in the valle del tulum caused important or unusual damage at several localities including the following five sites 1 at the barrio justo p castro a subdivision of caucete liquefaction of subsurface sediments decoupled overlying unliquefied stiff sediments producing a form of ground failure called ground oscillation the associated differential ground movements pulled apart houses and pavements in extension while shearing curbs and buckling canal linings in compression at the same locality 2 at the escuela normal in caucete the roof of a 30 m long single story classroom building shifted westward relative to the foundation that displacement fractured and tilted columns supporting the roof the foundation was fractured at several places leaving open cracks as wide as 15 mm the cumulative width of the open cracks was 48 mm an amount roughly equivalent to the 63 mm of offset between the roof and foundation at the east end of the building the ground and foundation beneath the building extended or spread laterally opening cracks and lengthening the foundation while the roof remained in place 3 the most spectacular damage to structures at the community of san martin was the tilting of a 6 m high water tower and the toppling of a nearby pump house into a 1 m deep crater similarly a small crater developed beneath a hand pump in an open area and a large 6 m diameter crater formed nearby the following sequence of events created the craters and toppled the pump structures during the earthquake ground shaking generated excess pore pressures which were dissipated by upward flow of groundwater free drainage was restricted by an impermeable plastic silt layer water apparently accumulated below the plastic silt layer and then burst to the surface through several holes and cracks including holes around well casings 4 at the san isidro winery nine storage tanks tilted 2 to 5 five reinforced concrete tanks were dismantled but four steel tanks were repaired by placing new footings and jacking the structures into an upright position 5 at escuela j j pasos differential settlement beneath building fractured several columns and walls the largest settlements were about 60 mm and the maximum settlement of footings supporting columns was about 40 mm in spite of the damage the buildings were in no danger of collapse

is spreading prolonged episodic or incipient in the andaman sea evidence from deepwater sedimentation abstract the central andaman basin cab is generally accepted to be a site of continuous sea floor spreading since the early pliocene 4 0 ma the adjacent alcock and sewell rises and part of the east andaman basin have been interpreted as probable miocene oceanic crust published seismic lines across the eastern half of the spreading centre show that 100 s m thickness of sediment are present right up to the central trough the central trough margins are faulted uplifted and tilted away from the central trough the youngest sediment is ponded and onlaps the tilted central trough margin while older faulted sediment lies within the trough such a configuration is incompatible with continuous spreading instead either spreading in the central basin was episodic probably comprising a late miocene early pliocene phase of spreading followed by extension accommodated in the alcock and sewell rise area by faulting and dike intrusion and then a recent quaternary return to spreading in the central trough or the central trough marks an incipient spreading centre in hyper thinned continental or possibly island arc crust to resolve these possibilities regional satellite gravity data was inverted to determine crustal type and thickness the results indicate the cab is oceanic crust however the adjacent regions of the alcock and sewell rises and the east andaman basin are extended continental crust these regions were able to undergo extension before seafloor spreading and when seafloor spreading ceased unpublished seismic reflection data across the east andaman basin supports the presence of continental crust under the basin that thins drastically westwards towards the spreading centre episodic seafloor spreading fits with gps data onshore that indicate the differential motion of india with respect to se asia is accommodated on widely distributed structures that lie between the trench and the sagaing fault

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

geologic evidence for recurrent moderate to large earthquakes near charleston south carolina multiple generations of earthquake induced sand blows in quaternary sediments and soils near charleston south carolina are evidence of recurrent moderate to large earthquakes in that area the large 1886 earthquake the only historic earthquake known to have produced sand blows at charleston probably caused the youngest observed blows older late quaternary sand blows in the charleston area indicate at least two prehistoric earthquakes with shaking severities comparable to the 1886 event

new 40ar 39ar and u th he dating for the zhunuo porphyry cu deposit gangdese belt southern tibet implications for pulsed magmatic hydrothermal processes and ore exhumation and preservation understanding the magmatic hydrothermal cooling and exhumation history is fundamental for investigating porphyry deposit formation and preservation in the zhunuo porphyry copper deposit of southern tibet hydrothermal biotite related to early potassic alteration likely formed from 14 7 0 3 ma zircon u pb age to 14 23 0 13 ma molybdenite re os age and cooled below the closure temperature for 40ar 39ar system at 13 91 0 05 ma hydrothermal biotite 40ar 39ar plateau age the 40ar 39ar spectra of another hydrothermal biotite sample that was selectively replaced by chlorite record the timing of late phyllic alteration at 13 53 0 08 ma zhunuo hydrothermal alteration and mineralization likely ceased at 13 09 0 07 ma based on the 40ar 39ar plateau age of the fresh magmatic biotite crystals the biotite 40ar 39ar systematics have not been reset by 12 0 ma post mineralization granite porphyry within ore district which is likely due to rapid cooling and or absence of fluid exsolution for the granite porphyry in combination with previous zircon u pb and molybdenite re os data two pulses of magmatic hydrothermal activity from 14 78 0 11 to 14 44 0 05 ma and from 14 23 0 13 to 13 53 0 08 ma can be recognized at zhunuo u th he thermochronological data on zircon and apatite from the inter mineralization monzogranite porphyry suggest three cooling events characterized by first stage magmatic cooling at a rate of 1850 c m y and two later phases of uplift and exhumation at 70 c m y and 10 c m y approximately 1 4 km thickness of materials have been removed from the zhunuo miocene monzogranite porphyry during uplift and erosion including a large volume of ore

paleoliquefaction evidence for a strong holocene earthquake in south central indiana earthquake induced paleoliquefaction features in holocene sediments in south central indiana are evidence for strong shaking in the recent prehistoric past in an area where only infrequent low magnitude earthquakes have occurred during the 200 yr of the historic record distribution of liquefaction features and geotechnical analyses of liquefied sediments are the basis for estimating that an earthquake with a magnitude of 6 9 was centered near vallonia indiana radiometric archaeologic and stratigraphic relations bracket the event to between 3 7 ka and 4 2 ka b p evidence for a local source for strong shaking requires reassessment of earthquakes hazards in the southern indiana northern kentucky southwestern ohio area

tectonics and history of the andaman sea region abstract the andaman sea is an active backarc basin lying above and behind the sunda subduction zone where convergence between the overriding southeast asian plate and the subducting australian plate is highly oblique the effect of the oblique convergence has been formation of a sliver plate between the subduction zone and a complex right lateral fault system the late paleocene collision of greater india and asia with approximately normal convergence started clockwise rotation and bending of the northern and western sunda arc the initial sliver fault which probably started in the eocene extended through the outer arc ridge offshore from sumatra through the present region of the andaman sea into the sagaing fault with more oblique convergence due to the rotation the rate of strike slip motion increased and a series of extensional basins opened obliquely by the combination of backarc extension and the strike slip motion these basins in sequence are the mergui basin starting at 32 ma the conjoined alcock and sewell rises starting at 23 ma east basin separating the rises from the foot of the continental slope starting at 15 ma and finally at 4 ma the present plate edge was formed alcock and sewell rises were separated by formation of the central andaman basin and the faulting moved onshore from the mentawai fault to the sumatra fault system bisecting sumatra

alluvial river response to neotectonic deformation in louisiana and mississippi repeat geodetic surveys show uplift of the monroe and wiggins anticlines in louisiana and mississippi there are deformed quaternary terraces which indicate long term deformation in the valleys of the alluvial rivers that cross these structures and there are floodplain and channel convexities that provide evidence of modern deformation in addition the channels show significant variations of morphology sinuosity gradient and depth and behavior appropriate to reaches of increased and decreased valley slope these alluvial rivers are adjusting to modern deformation and their adjustment confirms two geodetic leveling anomalies

analysis of drainage basin symmetry as a rapid technique to identify areas of possible quaternary tilt block tectonics an example from the mississippi embayment there is a coincidence of seismicity and major river valleys in the southwestern mississippi embayment that suggests control of the drainage pattern by northwest trending faults pleistocene river terraces reveal a strong southwestward preferred river migration in this region that may be a result of fault block tilting a rapid new technique of drainage basin analysis is herein presented that quantifies transverse topographic basin symmetry these data permit discrimination of random stream migrations and regionally preferred stream migrations and identify the direction of maximum migration if bedrock dip can be shown to be a negligible influence on stream migration then the direction of regionally preferred migration implies a period of ground tilting in that direction drainage basin asymmetry vectors bearing and degree of asymmetry calculated from transverse topographic profiles of 271 4 km basin segments in the study area suggest preferred stream migration in response to west southwest ward ground tilting bearing 259 the eastward regional dip of bed rock horizons only locally influences migration river terrace distributions reveal a chronology of stream migration episodes southwestward migration in the ouachita saline and arkansas river basins occurred during early to middle pleistocene time in the southwestern mississippi embayment a wisconsin early holocene southwestward migration episode is evidenced along southeast flowing reaches of the ouachita and arkansas rivers but not along the southeast flowing reach of the intervening saline river suggesting that these rivers overlie separate northwest trending tilt blocks although this technique does not provide direct evidence of ground tilting it provides a quick look method of identifying possible tilting elements in neotectonic regions using only topographic maps this technique is of greatest utility where active faults are concealed or poorly exposed such as in the mississippi embayment

preliminary assessment of sand blows in the southern mississippi embayment trenching of sand blows in two liquefaction fields on late to mid holocene alluvium within the southern mississippi embayment was conducted to determine the origin of the sand blows and their possible relationship to new madrid seismic zone events trench stratigraphy and crosscutting relationships show evidence of multiple events of seismically induced sand venting these two fields of moderate to intense liquefaction are 175 km and 240 km southwest of the southern limit of similarly intense sand blows within the new madrid seismic zone nmsz liquefaction field the southernmost field ashley county arkansas is 25 km in diameter and sand blows are densely spaced sand blows are more sparsely spaced in the larger 30 km by 45 km northern field desha county arkansas various correlations of liquefaction events between these two fields and between each of these fields and the nmsz can be accommodated by our radiocarbon and infrared stimulated luminescence irsl ages of sediments predating and postdating vented sand deposits in our trenches these ages show a major liquefaction event at both the ashley and desha county fields between 4600 and 5500 years ago and a major event at the ashley county field approximately 700 years ago the early venting episode is older than the documented nmsz chronology and so a nmsz seismic source cannot be assessed the age of the later venting episode falls between two postulated nmsz events so it may reflect seismicity from a different source zone our trench logs show at least three other moderate to minor sand venting episodes at the ashley county field and two at the desha county field and one may be the same event at both fields about 2200 years ago if these sand blows formed during large nmsz events 175 km northeast the area of strong ground motion for this seismic source has been underestimated alternatively if these liquefaction fields formed due to local seismicity then additional significant seismic source zones are present in the mississippi embayment outside the nmsz each of the two fields sits astride seismicity alignments associated with the margins of a previously identified active fault block between the arkansas river fault zone that projects through the desha county field and the saline river fault zone that projects through the ashley county field

ouachita appalachian juncture a paleozoic transpressional zone in the southeastern u s a the northern margin of gondwana collided with the southern margin of the north american craton in the late paleozoic 330 265 ma near normal compressional stresses generated where the trend of the north american margin was nearly perpendicular to gondwana s plate motion vector closure direction caused extensive crustal shortening and decollement thrusting within the sedimentary section the appalachian and ouachita fold belts are the erosional remnants of this collision on the north american margin transecting the ouachita fold belt are several zones of late paleozoic transcurrent faulting the val verde ardmore anadarko and reelfoot zones they occur where the precollision margin of the north american craton was oriented oblique or parallel to the closure direct on with gondwana stresses generated within these zones during the collision were transpressional rather than simply compressional and gave rise to high angle faulting high amplitude vertical displacements and the emplacement of positive flower structures in the val verde and ardmore anadarko basins these features have proven to be major structural traps for oil and gas on the basis of seismic and well data this study identifies a zone of complex structures including positive flower structures in the subsurface paleozoic section of kemper county in east central mississippi the region is considered to be another transpressional zone analogous to the val verde ardmore anadarko and reelfoot zones the kemper county zone separates and truncates the appalachian valley and ridge fo d belt in the subsurface from the mississippi ouachita deformed belt to the west and is therefore also identified as the juncture between the ouachita and appalachian fold belt systems in the southeastern u s a

structure and evolution of the central gulf of mexico continental margin and coastal plain southeast united states as in many continental margins mesozoic rifting in the gulf of mexico trended subparallel to older tectonic structures created by opening and closure of a prior ocean basin unlike the situation in most other margins however extension in the gulf of mexico did not involve widespread reactivation of these older tectonic features this difference may be related to the unusual facts that 1 the antecedent passive continental margin was a transform rather than a rift margin and 2 the prior orogeny did not substantially thicken the crust in order to characterize the interaction between mesozoic rifting and prior tectonic events two geophysical transects were constructed across the central gulf of mexico coastal plain and continental shelf the transects show that the early paleozoic passive continental margin buried beneath the late paleozoic ouachita orogen was little affected by compressional tectonism or mesozoic extension during opening of the gulf of mexico the thickness of the crust on this ancient margin decreases from 35 km to 10 km over a distance of only 50 km consistent with previous interpretations that this was a transform margin the ouachita fold and thrust belt is shown to be a thin skinned doubly vergent orogen that formed above a well preserved southward dipping subduction system relict cambrian oceanic crust may exist beneath the southern part of the orogen but its presence is not required by the data the thin crust and shallow mantle in the ouachita suture created a zone of relatively high strength that acted as a barrier to mesozoic extension which is restricted to regions south of the orogen extension was initially distributed over a broad area that included the mississippi interior salt basin and regions south of the wiggins arch extension became progressively more focused south of the wiggins arch with time culminating in the onset of seafloor spreading beneath the modern continental rise mesozoic extension factors are 1 4 1 9 beneath the mississippi interior salt basin 2 9 immediately south of the wiggins arch and 4 adjacent to the oldest oceanic crust in the central gulf of mexico

ultra high precision 40ar 39ar ages for fish canyon tuff and alder creek rhyolite sanidine new dating standards required abstract the 40 ar 39 ar dating technique is a high precision 40 ar 39 ar standards include sanidine from the ca 28 ma fish canyon tuff fct and the ca 1 2 ma alder creek rhyolite acr despite common usage the ages of fct and acr sanidine have proven contentious with reported values varying by 2 well outside the 0 1 aspiration of earthtime www earth time org the current study presents ultra high precision multi collector 40 ar 39 ar results for fct and acr sanidine using a new generation argusvi mass spectrometer these analyses demonstrate the significantly higher by a factor of 10 level of precision achievable using the argusvi system compared to most previous studies significantly 40 ar 39 ar step heating analyses resolve distinct age gradients for both fct and acr sanidine possible explanations for these gradients include combinations of recoil loss of 39 ar instrumental bias isotopic fractionation during step heating thermally induced argon loss and or extraneous argon contamination minor recoil loss of 39 ar and partial retention of inherited argon during extended magma residence are deemed the most plausible causes of the age discordance given this assumption we calculate revised maximum eruption ages of 28 01 0 04 ma 2 for fct and 1 178 0 002 ma 2 for acr relative to the astronomically calibrated age of a1 tephra sanidine crete rivera et al 2011 earth planet sci lett 311 420 426 nonetheless the argusvi data demonstrate that fct and acr sanidine are non ideal as high precision 40 ar 39 ar standards with different heating protocols likely to induce inter laboratory bias the current findings account for at least some of the scatter in inter calibration ages reported previously for various 40 ar 39 ar standards in broader terms the results mandate a re evaluation of the astronomically calibrated ages for these and other sanidine samples while the new generation of multi collector mass spectrometers provides the means to further evaluate 40 ar 39 ar ages used to define new decay constants and standard ages

paleoseismology of the southeastern reelfoot rift in western tennessee and implications for intraplate fault zone evolution 1 the southeastern reelfoot rift margin is expressed as a series of collinear scarps across western tennessee and shallow s wave reflection profiles coring and trenches show late quaternary faulting along this lineament at the north end our seismic profile and coring suggest 1 5 m of wisconsin down to the nw faulting coring and trenches across the central lineament show 3 m of up to the nw and 8 15 m of right lateral late wisconsin holocene faulting averaged late quaternary right lateral slip has been 0 85 0 37 mm yr on one fault plane at this locality suggesting that total slip across all southeast rift margin fault planes may accommodate much of the regional strain our seismic profile at this site shows up to the west faulting of eocene strata folded down to the west indicating structural inversion seismic profiles electrical surveys coring and a trench across the southern lineament segment show holocene faulting trenching exposed a vertical fault with 0 5 m of late holocene down to the nw displacement nearby coring and electrical surveys show 0 5 m of late holocene up to the nw faulting we conclude the southeastern reelfoot rift margin is a right lateral system with high angle faults showing both up to the nw and down to the nw separations age constraints permit an earthquake circa 2500 to 2000 years b p on the southwestern central segment of this fault system that ruptured 80 km we suggest the northeastern segment of the southeastern rift margin turned off in holocene when the reelfoot step over thrust turned on

three dimensional gravity inversion for moho depth at rifted continental margins incorporating a lithosphere thermal gravity anomaly correction summary this paper describes a method for determining moho depth lithosphere thinning factor 1 1 and the location of the ocean continent transition at rifted continental margins using 3 d gravity inversion which includes a correction for the large negative lithosphere thermal gravity anomaly within continental margin lithosphere the lateral density changes caused by the elevated geotherm in thinned continental margin and adjacent ocean basin lithosphere produce a significant lithosphere thermal gravity anomaly which may be in excess of 100 mgal and for which a correction must be made in order to determine moho depth accurately from gravity inversion we describe a method of iteratively calculating the lithosphere thermal gravity anomaly using a lithosphere thermal model to give the present day temperature field from which we calculate the lithosphere thermal density and gravity anomalies for continental margin lithosphere the lithosphere thermal perturbation is calculated from the lithosphere thinning factor 1 1 obtained from crustal thinning determined by gravity inversion and breakup age for thermal re equilibration time for oceanic lithosphere the lithosphere thermal model used to predict the lithosphere thermal gravity anomaly may be conditioned using ocean isochrons from plate reconstruction models to provide the age and location of oceanic lithosphere a correction is made for crustal melt addition due to decompression melting during continental breakup and seafloor spreading we investigate the sensitivity of the lithosphere thermal gravity anomaly and the predicted moho depth from gravity inversion at continental rifted margins to the methods used to calculate and condition the lithosphere thermal model using both synthetic models and examples from the north atlantic

exhumed mantle forming transitional crust in the newfoundland iberia rift and associated magnetic anomalies transitional zones located between iberia and north america formed during continental rifting and mostly consist of exhumed mantle in this study we show that ages of exhumed mantle at ocean drilling program odp sites 1068 and 1070 in the iberia abyssal plain and site 1277 in the newfoundland basin are similar to ages determined from magnetic lineations created by serpentinization during mantle exhumation on the basis of paleomagnetic and geological data and a comparison with a fossil ocean continent transition in the alps we envisage a first episode of mantle serpentinization during which a strong component of magnetization was acquired followed by a second episode occurring at the contact with cold seawater and which only affects the upper tens of meters of the exhumed mantle the inversion of magnetic data euler deconvolution shows that magnetic sources are n s trending horizontal cylindrical bodies located within the highly serpentinized upper crust therefore the serpentinization process is able to produce magnetic lineations formed in a similar way to those formed by seafloor spreading within transition zones sequences of magnetic anomalies can provide information concerning the timing of the emplacement of crust but not on its nature oceanic versus exhumed mantle this discovery enables us to date the exhumation of mantle rocks in transition zones and allows kinematic reconstructions of the final stages of continental rifting during rifting the deep distal continental margins and the adjacent transitional zones in the newfoundland iberia rift system were formed by ultraslow extension from early berriasian to late valanginian early hauterivian and by slow extension from early hauterivian to the late aptian early albian boundary therefore transitional zones share many similarities with slow and ultraslow spreading midoceanic ridges

alder creek sanidine acs 2 a quaternary 40ar 39ar dating standard tied to the cobb mountain geomagnetic event accurate 40ar 39ar dating of quaternary and holocene samples requires a standard neutron fluence monitor of appropriate age this standard should be isotopically homogenous at the scale of analysis and should be well intercalibrated with other standards several such standards have been proposed including sanidine from the 1 19 ma alder creek rhyolite acs turrin et al 1994 turrin b d donnely nolan j m hearn b c jr 1994 40ar 39ar ages from the rhyolite of alder creek california age of the cobb mountain normal polarity subchron revisited geology 22 251 254 this sanidine is satisfactorily homogeneous well intercalibrated with fish canyon sanidine fcs renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 and is linked to the astronomically calibrated magnetic polarity time scale since the alder creek rhyolite lava is the type occurrence of the cobb mountain event cm we collected 408 kg of this rock from which we anticipate being able to extract more than 8 kg of 425 m sanidine crystals analyses of sanidine phenocryts from the acs 2 from three separate irradiations and two distinct grain sizes yield ages ranging from 1 190 0 004 to 1 194 0 003 ma1 with a weighted mean age of 1 193 0 001 ma mswd 0 74 n 225 based on fcs 28 02 ma renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 the value of the intercalibration factor renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 is rfcsacs 2 0 04229 0 00012 n 8 identical with that of the original acs measurements 0 04229 0 00012 renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 the age of acs 2 is in agreement with the recent astronomical calibration of the cm event 1 215 to 1 190 ma channell j e t mazaud a sullivan p turner s raymo m e 2002 geomagnetic excursions and paleointensities in the matuyama chron at ocean drilling program sites 983 and 984 iceland basin j geophys res 107 6 10 1029 2001jb000491 the apparent absence of xenocrysts and excess argon suggests that multigrain analyses are feasible and therefore the amount of material can be tailored to experimental requirements aliquots from various grain sizes will be available upon request zircon is also potentially available which may be of interest to u th he researchers