lateral velocity resolution from three dimensional seismic refraction data summary a method for estimating the lateral velocity resolution from 3 d seismic refraction traveltimes is presented the method is non linear in that synthetic data calculated from checkerboard models are inverted using an iterative tomographic approach incorporating smoothness constraints and updated ray paths at each iteration two applications are presented 1 real data from the faeroe basin experiment using the ray coverage and noise level of approximately 50 000 picks and 2 ideal data from the same experiment corresponding to 100 per cent data recovery equivalent to about three times as many ray paths the refraction data constrain the sedimentary and basement structure to approximately 12 km depth a comparison of the results from the two data sets provides a qualitative check on the resolution method illustrates the pitfalls of using ray coverage as an indicator of resolution and demonstrates the potential resolution of currently feasible 3 d refraction experiments 32 checkerboard models consisting of alternating positive and negative velocity anomalies superimposed on the preferred final model of the basin were tested each with a diverent cell size position or orientation the problem of ray bending associated with the 5 per cent checkerboard anomalies is shown to be small using models with four cell sizes allowed for resolution estimates from 3 to 15 km this range being imposed by the frequency content of the data and the areal dimensions of the study area 44 26 km an operator centred on each model node measures the local semblance between the known and recovered checkerboard models a determination of the smallest well resolved cell size at each model node is then made using a 0 7 semblance threshold to provide a spatially dependent lateral resolution estimate resolution for the real data set varies from an average of just over 3 km at 2 km depth to 10 km at 10 km depth where there is a local maximum in ray coverage due to a concentration of ray bottoming points for the ideal data set the average lateral resolution drops ov roughly linearly from better than 3 km at 2 km depth to 6 3 km at 10 km depth the largest diverence between the resolution associated with the ideal and real data occurs at 7 2 km depth where it averages 4 6 km for the former and 10 8 km for the latter due to the near absence of ray bottoming points between 7 and 9 km the results show that the final model for the faeroe basin obtained by regularized inversion with smoothness constraints is consistent with the resolution estimates in the sense that the model does not contain structure with a wavelength smaller than the resolution estimate at any point the results for the ideal data show that good quality 3 d refraction experiments can provide a lateral resolution of no worse than the receiver spacing shot spacing for land data throughout most of the sampled volume

lithospheric uplift in eastern gondwana evidence for a long lived mantle plume system recent work has demonstrated that modern drainage systems on rifted continental margins may be the product of ancient lithospheric doming in areas of flood basalt magmatism this uplift is thought to reflect the existence of widespread mafic underplating and excess melt generation at rift zones here i show that uplift due to the initiation of a thermal anomaly plume head in the asthenosphere may be evidenced by the formation of discrete rift structures and persistent paleodrainage sediment dispersal patterns for up to 150 m y prior to the commencement of igneous activity and final continental breakup volcanism thus occurs as a comparatively late stage geochemical response to the long term residence of hot plume material beneath continental mantle lithosphere

upper limit of docking time for stikinia and terrane i paleomagnetic evidence from the eocene ootsa lake group british columbia the middle eocene ootsa lake group is exposed in the central portion of the stikine terrane where it was sampled along the shoreline of tahtsa reach and whitesail reach the group consists of dominantly subaerial flows which range in composition from basalt to rhyolite that unconformably overly the jurassic hazelton group detailed alternating field and thermal stepwise demagnetizations were done on all specimens from the 21 sites collected the presence of a normal and reversed polarity remanence a positive fold test and high coercivities and unblocking temperatures indicate that a prefolding primary remanence has been isolated the mean tilt corrected direction of d 002 2 i 69 2 95 7 4 from 13 sites for which paleohorizontal is well known yields a pole position at 354 6 e 88 0 n a95 11 5 which is statistically indistinguishable from published 50 ma reference poles for cratonic north america this evidence demonstrates that the proposed large scale northward displacement of stik

error analysis for the determination of tectonic rotation from paleomagnetic data appendix b is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 001 2 50 payment must accompany order recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25 this has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence 95 an analytical calculation which is valid when error bars are small shows that the correct error bars are 75 to 80 of the values derived in previous studies i have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used the analytical approximation may be used whenever 95 is less than 10 and the mean inclination is not too close to vertical confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20 25 narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data

a crustal density model across the cambay basin india and its relationship with the aravallis abstract the cambay rift basin in the northwest indian platform shows high bouguer gravity anomaly values as compared to the values outside the basin the seismic results show the presence of a large thickness of sediments which should have produced a gravity low of about 50 60 mgal the deep seismic sounding results along the strike of the basin show the presence of a velocity of 7 3 km s at 23 25 km depth followed by the moho at a depth of 31 33 km two dimensional density modelling of the seismic structure across the cambay basin shows that the crust is thinner underneath the basin and is associated with a high density lower crust a comparison of the gravity anomaly across the cambay basin with that across the proterozoic aravalli delhi trends suggests crustal thinning in both the regions on either side of which the moho deepens rapidly

faunal evidence for a cool boundary current and decoupled regional climate cooling in the permian of western laurentia abstract deglaciation following the late paleozoic ice age was a complex climate transition marked by smaller glacial interglacial transitions superimposed on an overall permian warming trend in contrast to this growing recognition of a dynamic climate system few studies have investigated geographic variations in postglacial climate changes which may have been as dynamic as the temporal variations because temperature and ocean circulation are important controls on the biogeography of marine invertebrates many permian taxa were characteristic of either paleotropical or high latitude faunal realms this study takes advantage of that temperature sensitivity using a database of more than 5000 permian fossil collections from northwestern pangea to reconstruct spatial variations in permian climate changes throughout the region cluster analysis demonstrates that localities in the phosphoria basin nevada idaho wyoming were taxonomically distinct from coeval faunas in texas and occupied the transitional zone between paleoequatorial and boreal realms quantitative scoring of the faunal affinity of all taxa in 1 by 1 paleolatitudinal bins suggests that the western margin of laurentia was affected by a persistent southward flowing cool boundary current and that the effects of the current became more pronounced throughout the permian in conjunction with cooling in the boreal ocean the increasingly cool water nature of phosphoria basin faunas contrasted with warming in texas and implies that northern and northwest pangea were decoupled from the broader global warming trend following the late paleozoic ice age

geology of southern prince of wales island southeastern alaska geologic mapping on southern prince of wales island combined with u pb zircon geochronologic geochemical and conodont studies yield new information about the early paleozoic geologic framework and evolution of the southern alexander terrane the oldest rocks recognized consist of arc type meta volcanic and metasedimentary rocks wales metamorphic suite which were derived from late proterozoic cambrian protoliths these rocks were deformed metamorphosed and uplifted during the middle cambrian early ordovician wales orogeny beginning soon after this orogenic event and continuing into early silurian time basaltic to rhyolitic rocks and marine clastic strata descon formation were deposited and dioritic to granitic plutons were emplaced a variety of geochemical data suggest that these ordovician early silurian rocks formed in a volcanic arc environment the middle silurian earliest devonian klakas orogeny marked the end of arc type magmatism it is manifest on southern prince of wales island by regional imbrication on southwest vergent thrust faults penetrative brecciation and pervasive hydrothermal alteration of pre devonian rocks in some areas and at least several kilometres of structural uplift lower devonian strata karheen formation record the waning stages of this event conglomeratic redbeds low in the section were deposited in topographically rugged subaerial to shallow marine environments whereas strata high in the section were deposited in more distal marine environments post devonian rocks on southern prince of wales island include the bokan mountain granite jurassic basaltic to dacitic dike swarms and mid cretaceous granodiorite and diorite bodies prior to emplacement of the mid cretaceous plutons the devonian and older rocks were offset along several sets of strike slip faults and then downdropped along the keete inlet fault

northward motion of the whitehorse trough paleomagnetic evidence from the upper cretaceous carmacks group reply the upper cretaceous carmacks group 70 4 2 4 ma comprises gently dipping basaltic and andesitic lava flows overlying volcaniclastic deposits of the intermontane belt in the whitehorse trough the sampling area is in southern yukon and northern british columbia it lies west of the tintina northern rocky mountain trench fault and teslin suture zone and east of the denali shakwak fault volcanic sections were sampled in three regions spread over 300 km providing the first paleomagnetic data from pre tertiary volcanic rocks in the northern canadian cordillera alternating field and thermal demagnetization revealed stable magnetization for 18 of the 27 sites collected the overall mean direction d 166 7 i 71 4 k 53 95 4 8 n 18 sites is pre folding and is most probably primary latest cretaceous this gives a paleopole at 109 4 e 82 1 n k 21 a95 7 8 a critical evaluation of north american cratonic data yields a reference paleopole for the latest cretaceous at 185 8 e 77

shallow bias of paleomagnetic inclinations in the paleozoic and precambrian abstract an updated analysis of the global paleomagnetic database shows that the frequency distributions of paleomagnetic inclinations for the cenozoic and mesozoic eras younger than 250 ma are compatible with a random geographical sampling of a time averaged geomagnetic field that closely resembles that of a geocentric axial dipole in contrast the frequency distributions of paleomagnetic inclinations for the paleozoic and precambrian eras prior to 250 ma are over represented by shallow inclinations after discounting obvious secondary causes for the bias such as from data averaging sedimentary inclination error inhomogeneous lithological distributions and tropical remagnetization we show that the anomalous inclination distributions for the paleozoic and precambrian can be explained by a geomagnetic field source model which includes a relatively modest 25 contribution to the axial dipole from a zonal octupole field and an arbitrary zonal quadrupolar contribution the apparent change by around 250 ma to a much more axial dipolar field geometry might be due to the stabilization of the geodynamo from growth of the inner core to some critical threshold size a gross speculation which would imply that either the threshold size was rather large or the inner core nucleated rather late in earth history alternatively if a geocentric axial dipole model is assumed or can eventually be demonstrated independently the anomalous inclination distributions for the paleozoic and precambrian may reflect a tendency of continental lithosphere to be cycled into the equatorial belt perhaps because geoid highs associated with long term continental aggregates influence true polar wander

paleomagnetism of the late triassic hound island volcanics revisited the collision and accretion of the alexander terrane profoundly influenced the geologic history of alaska and western canada however the terrane s displacement history is only poorly constrained by sparse paleomagnetic studies we studied the paleomagnetism of the hound island volcanics in order to evaluate the location of the alexander terrane in late triassic time we collected 618 samples at 102 sites in and near the keku strait alaska from the late triassic hound island volcanics the permian pybus formation and 23 ma gabbroic intrusions we found three components of magnetization in the hound island volcanics the high temperature component component a resides in hematite and magnetite and was found only in highly oxidized lava flows in a geographically restricted area we think it is primary or acquired soon after eruption of the lavas principally because the directions pass a fold test the paleolatitude indicated by this component 19 2 10 3 is similar to those determined for various portions of wrangellia consistent with the geologic interpretation that the alexander terrane was with the wrangellia terrane in late triassic time we found two overprint directions in the hound island volcanics component b was acquired 23 m y ago due to intrusion of gabbroic dikes and sills this interpretation is indicated by the similarity of upper hemisphere directions in the hound island volcanics to those in the gabbro component c found in both the hound island volcanics and the permian pybus formation is oriented northeast and down fails a regional fold test and was acquired after regional deformation around 90 to 100 ma this overprint direction yields a paleolatitude similar to but slightly higher than slightly older rocks from the coast plutonic complex suggesting that the alexander terrane was displaced 17 in early late cretaceous time the occurrence of these two separate overprinting events provides a satisfying explanation of the earlier puzzling results from the hound island volcanics hillhouse and gromme 1980 finally great circle analysis of the paleomagnetic data from the pybus formation suggests the alexander terrane may have been in the northern hemisphere in permian time

late palaeozoic palaeomagnetic data for wrangellia resolution of the polarity ambiguity wrangellia and the alaska peninsula terrane are two of the many allocthonous terranes making up southern alaska today palaeomagnetic data from these areas clearly indicate low palaeolatitudes in the early mesozoic because of local and regional rotations the mean declination of the palaeomagnetic vectors is uncertain this in addition to incomplete temporal coverage has led to an ambiguity in the polarity of the vector and hence an ambiguity in whether the palaeolatitudes determined are in the northern or southern hemisphere the data considered here have resolved the ambiguity for wrangellia and by implication for the alaska pennisula terrane and also allow the construction of a highly speculative apparent polar wander curve

modeling of sh and p sv wave fields and seismic microzonation based on response spectra for talchir basin india abstract the p sv and sh wave field in the talchir basin is simulated along eight profiles four profiles strike across the basin and the other four along the basin the hybrid method which combines two computational techniques modal summation and finite differences is used to produce multiphase synthetic seismograms an m 6 earthquake is considered with hypocenter along the north orissa boundary fault nobf at a depth of 5 km and with the focal mechanism parameters dip 90 strike 160 and rake 180 the peak acceleration a max along each profile is determined considering the maximum acceleration obtained at the horizontal components the response spectra ratio rsr as a function of frequency is computed for the eight profiles and the higher amplification is seen to increase in correspondence with the thicker sedimentary cover especially for the radial component higher site amplification for all the profiles is observed in the frequency range from 0 5 to 1 4 hz to validate the obtained site effects the sources for the profiles across the basin are placed near to the southern end of the profile and the site amplifications are recomputed even if the spatial distribution of a max is mainly controlled by the epicentral distance i e the geometrical spreading prevails on local soil effects the rsr shows a pattern that can be easily correlated with the local site conditions the rsr at the intersection of the profiles is dependent not only upon the local lithology and sediment thickness but also upon the epicentral distance the talchir basin is classified into three zones based on the rsr values low rsr zone 1 6 1 9 intermediate rsr zone 2 0 2 8 and high rsr zone 2 9 5 2 the pga estimated for the bedrock model by bhatia et al 1999 for the study region is around 0 05 to 0 10 g while the indian seismic zonation map estimated it to be in the range from 0 1 to 0 2 g in the present study that considers the effects of lateral variations in the structure the pga is estimated to be as high as 0 55 g along the ba profile

geodynamically consistent inferences on the uniform sampling of earth s paleomagnetic inclinations abstract paleomagnetism is a key method to reconstruct the earth s paleogeography and thus essential for understanding tectonic evolution but it assumes that the earth s magnetic field structure has always averaged to a geocentric axial dipole gad the gad hypothesis may be tested using the observed inclination frequency distribution but only if continents sampled all of the earth s latitudes uniformly which is not known here we provide new insight into the uniform sampling problem by employing a suite of 3d spherical mantle convection models that feature the self consistent evolution of mantle convection plate tectonics and continental drift over timescales of 2 gyr or more our results suggest that continents unlikely sampled latitudes uniformly during the phanerozoic consistent with previous suggestions this finding is robust for a variety of geodynamic evolutions with different mantle and lithosphere structures at least in the absence of true polar wander for longer sampling durations uniform sampling typically becomes more feasible but may only be achieved with confidence after time scales of minimum 1 3 gyr this time scale depends on the structure of the mantle and lithosphere and may be shortest when upper mantle viscosity is small such that reduced resistive drag at the cratonic base allows for faster continental drift weak plates low plastic yield strength promote more dispersed continent configurations which tends to facilitate uniform sampling if these conditions are not met the uniform sampling time scale can easily exceed several billion years even the minimum estimate of 1 3 gyr challenges the validity of using the phanerozoic inclination frequency distribution to infer the past average magnetic field structure the approach could however still be applicable using the precambrian inclination record

seven devils terrane is it really a piece of wrangellia the seven devils terrane of eastern oregon and western idaho has been correlated with the wrangellia terrane which is best exposed on vancouver island british columbia and in the wrangell mountains alaska however geologic histories of these two terranes are different the triassic section of the seven devils terrane represents a volcanic arc of probable calc alkalic affinity it is characterized by abundant volcaniclastic lithologies a complete suite of flow rocks and fractionated rare earth element distribution patterns the sequence of similar age in wrangellia includes thick accumulations of tholeiitic pillow basalts subaerial basalt flows tuffs and breccias the tectonic environment in which these rocks were extruded is not known but they are not an island arc assemblage paleomagnetic and faunal evidence suggests that the seven devils terrane and wrangellia could have formed close to one another this evidence does not override the geologic arguments against correlation of these terranes lithologic geochemical and stratigraphic data all militate against consanguinity of the seven devils arc and wrangellia

rare gas abundances in morb abstract data are presented for he ar and xe in a number of morb glasses and crystalline rocks the rare gases in the latter group are characterized by atmospheric ar and xe coupled with loss of trapped mantle he ar and xe the glass data also show clear signs of an atmospheric like component for ar and xe a reasonably constant world wide average trapped mantle gas value of 40 ar 36 ar 15000 3000 is suggested anomalies are seen in the 129 xe 132 xe ratio but at least part of this might be due to mass fractionation rather than to 129 i decay correlating the data with those from oceanic island basalts in ar xe space indicates that the latter samples are probably also greatly influenced by an atmospheric component rather than or in addition to trapping gases from an undepleted mantle source

the metallogeny of late triassic rifting of the alexander terrane in southeastern alaska and northwestern british columbia a belt of unusual volcanogenic massive sulfide vms occurrences is located along the eastern margin of the alexander terrane throughout southeastern alaska and northwestern british columbia and exhibits a range of characteristics consistent with a variety of syngenetic to epigenetic deposit types deposits within this belt include greens creek and windy craggy the economically most significant vms deposit in alaska and the largest in north america respectively the occurrences are hosted by a discontinuously exposed 800 km long belt of rocks that consist of a 200 to 800 m thick sequence of conglomerate limestone marine clastic sedimentary rocks and tuff intercalated with and overlain by a distinctive unit of mafic pyroclastic rocks and pillowed flows faunal data bracket the age of the host rocks between anisian middle triassic and late norian late late triassic this metallogenic belt is herein referred to as the alexander triassic metallogenic belt the vms occurrences show systematic differences in degree of structural control chemistry and stratigraphic setting along the alexander triassic metallogenic belt that suggest important spatial or temporal changes in the tectonic environment of formation at the southern end of the belt felsic volcanic rocks overlain by shallow water limestones characterize the lower part of the sequence in the southern and middle portion of the belt a distinctive pebble conglomerate marks the base of the section and is indicative of high energy deposition in a near slope or basin margin setting at the northern end of the belt the conglomerates limestones and felsic volcanic rocks are absent and the belt is composed of deep water sedimentary and mafic volcanic rocks this northward change in depositional environment and lithofacies is accompanied by a northward transition from epithermal like structurally controlled discontinuous vein and pod shaped pb zn agba cu occurrences with relatively simple mineralogy to sulfosalt enriched vms occurrences exhibiting characteristics of vein diagenetic replacement and exhalative styles of mineralization and finally to cu zn co au occurrences with larger and more clearly stratiform orebody morphologies occurrences in the middle of the belt are transitional in nature between structurally controlled types of mineralization that formed in a shallow water near arc setting to those having a more stratiform appearance formed in a deeper water rift basin setting the geologic setting in the south is consistent with shallow subaqueous emplacement on the flanks of the alexander terrane northward the setting changes to an increasingly deeper back or intra arc rift basin igneous activity in the alexander triassic metallogenic belt is characterized by a bimodal suite of volcanic rocks and a previously unrecognized association with mafic ultramafic hypabyssal intrusions immobile trace and rare earth element ree geochemical data indicate that felsic rocks in the southern portion of the belt are typical calc alkaline rhyolites which give way in the middle of the belt to peralkaline rhyolites rhyolites are largely absent in the northern part of the belt throughout the belt the capping basaltic rocks have transitional geochemical signatures radiogenic isotope data for these rocks are also transitional basalts and gabbros end 4 9 and 87sr 86sr initial at 215 ma 0 7037 0 7074 together these data are interpreted to reflect variable assimilation of mature island arc crust by more primitive melts having the characteristics of either mid ocean ridge morb or intraplate within plate basalts wpb the ore and host rock geochemistry and the sulfosalt rich mineralogy of the deposits are strikingly similar to recent descriptions of active sea floor hydrothermal white smoker systems in back arcs of the southwest pacific ocean these data in concert with existing faunal ages record the formation of a belt of vms deposits and occurrences in a propagating intra to back arc rift tectonic setting during the late triassic a modern analogue having similar tectonic and metallogenic features is the southward projection of the lau basin from the active sea floor hydrothermal vents of the valu fa ridge to the taupo volcanic zone of the north island new zealand

geology of the duncan canal shear zone evidence for early to middle jurassic deformation of the alexander terrane southeastern alaska the duncan canal shear zone central southeastern alaska disrupts and penetratively deforms devonian pennsylvanian and upper triassic strata of the lexander terrane the deformation is interpreted to have occurred within a regime of right lateral shear during early or middle jurassic time clastic and volcanic rocks correlated with the upper jurassic and lower cretaceous strata of the gravina belt lack the fabrics characteristic of the deformed alexander terrane strata and are inferred to unconformably overlie the shear zone deformational fabrics of mid cretaceous age observed in the gravina rocks locally truncate and deform the shear zone strata dextral strike slip faults of late cretaceous and or tertiary age that disrupt and modify fabrics of the shear zone are interpreted to accommodate some of the displacement observed on the clarence strait fault the duncan canal shear zone extends southward to the mosman inlet region on etolin island where it projects beneath jura cretaceous gravina strata in duncan canal and mosman inlet the shear zone evolved within the alexander terrane and does not coincide with any recognized terrane boundaries since the duncan canal shear zone is restricted to the currently exposed eastern margin of the alexander terrane the shear zone is interpreted to reflect deformation along the eastern margin of the alexander terrane

tomographic imaging of the shallow crustal structure of the east pacific rise at 9 30 n compressional wave travel times from a seismic tomography experiment at 9 30 n on the east pacific rise are analyzed by a new tomographic method to determine the three dimensional seismic velocity structure of the upper 2 5 km of oceanic crust within a 20 18 km2 area centered on the rise axis the data comprise the travel times and associated uncertainties of 1459 compressional waves that have propagated above the axial magma chamber a careful analysis of source and receiver parameters in conjunction with an automated method of picking p wave onsets and assigning uncertainties constrains the prior uncertainty in the data to 5 to 20 ms the new tomographic method employs graph theory to estimate ray paths and travel times through strongly heterogeneous and densely parameterized seismic velocity models the nonlinear inverse method uses a jumping strategy to minimize a functional that includes the penalty function horizontal and vertical smoothing constraints and prior model assumptions all constraints applied to model perturbations are normalized to remove bias we use the tomographic method to reject the null hypothesis that the axial seismic structure is two dimensional three dimensional models reveal a seismic structure that correlates well with cross and along axis variations in seafloor morphology the location of the axial summit caldera and the distribution of seafloor hydrothermal activity the along axis segmentation of the seismic structure above the axial magma chamber is consistent with the hypothesis that mantle derived melt is preferentially injected midway along a locally linear segment of the rise and that the architecture of the crustal section is characterized by an en echelon series of elongate axial volcanoes approximately 10 km in length the seismic data are compatible with a 300 to 500 m thick thermal anomaly above a midcrustal melt lens such an interpretation suggests that hydrothermal fluids may not have penetrated this region in the last 103 years asymmetries in the seismic structure across the rise support the inferences that the thickness of seismic layer 2 and the average midcrustal temperature increase to the west of the rise axis these anomalies may be the result of off axis magmatism alternatively the asymmetric thermal anomaly may be the consequence of differences in the depth extent of hydrothermal cooling

tomographic imaging of large volcanic province due to intense magmatism in the mahanadi delta of eastern india abstract we have derived a 2d tomographic p wave velocity image of the shallow upper crust 8 0 km depth using first arrival seismic refraction data along the 120 km e w trending profile in the mahanadi delta of eastern india the tomographic image depicts smooth velocity variations of quaternary 1 8 2 0 km s and gondwana sediments 3 5 4 5 km s lying above the basement 5 9 6 1 km s which is highly distorted due to the presence of deep basinal faults these faults reach maximum 6 0 km depth forming the cuttack depression gondwana graben in which thick 3 0 km column of gondwana sediments are deposited the basement was uplifted forming chandikhol ridge in the west and bhubaneswar ridge in the east of the profile with anomalous high velocity 5 5 km s dyke intrusions imaged on either side of the graben having high velocity 6 5 km s magmatic intrusives lying below the basement these dykes are volcanic in nature and fed by the large volcanic activity during early cretaceous period due to rifting of the indian plate from east gondwana this event is followed by the formation of an early cretaceous volcanic province ecvp in the rift zones of the mahanadi and lambert grabens which were conjugate to each other before breakup of india and antarctica the signature of ecvp is imaged as magmatic intrusives below the basement and intruded to shallow depths through the conduits and deep basinal faults

bokan mountain peralkaline granitic complex alexander terrane southeastern alaska evidence for early jurassic rifting prior to accretion with north america the circular bokan mountain complex bmc on southern prince of wales island southernmost alaska is a jurassic peralkaline granitic intrusion about 3 km in diameter that crosscuts igneous and metasedimentary rocks of the alexander terrane the bmc hosts significant rare metal rare earth elements y u th zr and nb mineralization related to the last stage of bmc emplacement u pb zircon and 40ar 39ar amphibole and whole rock geochronology indicates the following sequence of intrusive activity i a paleozoic basement composed mainly of 469 4 ma granitic rocks ii intrusion of the bmc at 177 1 ma followed by rapid cooling through ca 550 c at 176 1 ma that was synchronous with mineralization associated with vertical wnw trending pegmatites felsic dikes and aegirine fluorite veins and late stage sinistral shear deformation and iii intrusion of crosscutting lamprophyre dikes at 150 ma and again at ca 105 ma the peralkaline nature of the bmc and the wnw trend of associated dikes

evidence from neodymium isotopes for mantle contributions to phanerozoic crustal genesis in the canadian cordillera sm nd and rb sr isotopic studies of several phanerozoic orogenic belts have shown that much of the crust in these belts is composed of reworked pre existing continental crust in contrast isotopic data collected from two of the largest terranes in the canadian cordilleran orogenic belt the alexander and stikine terranes show that these tectonic fragments are composed of mantle derived continental crust in many respects the style and rate of crustal accretion of these and related terranes appear similar to those of some proterozoic crustal regions

seismicity and geodynamics of the low to high grade transition zone of peninsular india abstract a new seismogenetic belt has been identified in the well known transition zone of peninsular india lat 12 14 n and long 74 80 e based on a critical analysis of historical and recent seismic data the diffused and weak seismicity in the area could be attributed to the reactivation of adjoining shear zones a seismic energy release map has been prepared for southern peninsular india which depicts the long term seismicity in this region temporal variations in seismic activity in space indicate migration of seismic activity from east to west within the transition zone this phenomenon is discussed in relation to the mobility of shear belts adjacent to the dharwar craton further the maximum horizontal principal compressive stress directions on the indian continent are also briefly discussed these stresses may be mainly responsible for the mobility of the major shear zones adjacent to the dharwar craton the analysis of the seismic data in space and time thus suggests two possible geodynamic models viz 1 a stick slip model and 2 a crustal shortening model

mid cretaceous paleomagnetic reference field for north america restudy of 100 ma intrusive rocks from arkansas we have obtained new paleomagnetic data from three middle cretaceous alkalic intrusive complexes in central arkansas magnet cove and potash sulfur springs intrusions about 100 ma 20 sites and granite mountain stock about 88 ma 6 sites out of 254 specimens measured 147 from 20 sites were accepted the remainders have scattered magnetization directions which experiments indicate were caused by lightning occurring during frequent electrical storms in this region the geologic evidence indicates that the intrusions have not been tilted since emplacement the directions of magnetization calculated with respect to present day horizontal yield a mean paleopole at 192 5 e 74 1 n n 20 k 34 a95 5 7 this does not differ at 95 confidence from that determined previously by scharon and hsu but the error is diminished by half the angular dispersion sd of site virtual geomagnetic poles vgps is 13 6 similar to the value of sd about 15 predicted by current paleosecular variation models the revised arkansas paleopole is combined with four other cratonic north american paleopoles of mid cretaceous age to yield a revised middle cretaceous reference pole at 196 e 71 n a95 4 9 the mean of nine paleopoles derived from overprints in rock units from the craton and craton margin which range in age from proterozoic to jura cretaceous is 202 e 68 n a95 8 2 this is not different from the reference paleopole indicating that overprinting was very widespread across north america in mid cretaceous time

finite difference calculation of travel times the travel times of the first arriving seismic waves through any velocity structure can be rapidly computed on a two or three dimensional numerical grid by finite difference extrapolation from point to point wavefronts rather than the traditional rays are tracked head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 03 per cent are found between the results of this technique and raytracing for a complex two dimensional model this scheme is useful for the windowing of finite difference calculations to increase computing speed and promises to aid in earthquake location tomographic inversion and kirchhoff migration in structures that have strong lateral velocity gradients

tomographic inversion for three dimensional velocity structure at mount st helens using earthquake data tomographic inversion is applied to 17 659 p phase observations at 21 stations from 2023 earthquakes in the vicinity of mount st helens to study the three dimensional velocity structure block size for the inversion is 2 km horizontally and 2 km or more vertically locations of hypocenters are assumed known and are based on a reference one dimensional layered velocity structure a conjugate gradient technique lsqr is used to invert the large sparse system of equations augmented by regularization with a laplacian roughening matrix resolution is estimated by computing the impulse response of the inversion for various critical locations and uncertainties of the estimates are determined by a jackknife approach the results of the inversion show a remarkable correlation with known geological and geophysical features the spirit lake and spud mt plutons are characterized by high velocity regions extending to approximately 9 km depth the st helens seismic zone a band of diffuse seismicity extending nnw from the volcano is evident as a prominent low velocity lineation the change in character of the velocity anomalies south of st helens corresponds well with the near cessation of seismic activity there a low velocity anomaly beneath the crater from 6 to 16 km depths may represent modern magma accumulations

tectonic implications of paleomagnetic poles from lower tertiary volcanic rocks south central alaska we have determined the paleolatitude of lower tertiary volcanic rocks in southern alaska to measure possible poleward translation of the wrangellia and the peninsular terranes after 50 m y ago previous paleomagnetic studies have shown that in triassic and jurassic time these terranes were located near the equator and have moved at least 3000 km poleward relative to the north american craton our sample localities are in the northern talkeetna mountains in mildly deformed andesite and dacite flows 50 4 51 3 53 9 and 56 3 m y by k ar that overlap lower cretaceous flysch lower permian volcanic rocks of wrangellia and upper triassic pillow basalt of the susitna terrane results from 26 cooling units 23 of reversed polarity and 3 of normal polarity give a mean paleomagnetic pole at 69 5 n 179 6 e 95 12 2 stratigraphic sections from opposite limbs of a syncline yield directional paths that pass the fold test satisfying a necessary condition for primary origin of the magnetization the corresponding mean paleolatitude 76 n of the northern talkeetna mountains is 8 10 higher than the latitude predicted from the eocene reference pole for north america therefore northward drift of the talkeetna superterrane which is the amalgamation of the wrangellia and peninsular terranes during and after middle jurassic time was probably complete by 50 m y ago our results are consistent with paleomagnetic poles from uppermost cretaceous and paleocene volcanic sequences in denali national park the lake clark region northern bristol bay region and near mcgrath these poles generally lie south of the cratonic poles suggesting that the region between the kaltag bruin bay and castle mountain faults has rotated counterclockwise relative to north america since the early eocene

on the low inclination bias of the precambrian geomagnetic field abstract a long tradition has emerged in applying the inclination frequency analysis to estimate the functionality of the geocentric axial dipole gad hypothesis in paleomagnetism in the present study the theory has been tested to the database with 3246 records of the precambrian geomagnetic field to find the best fitting inclination distribution different combinations of geocentric axial dipolar gad quadrupolar g2 and octupolar g3 spherical harmonics have been analyzed the influence of various factors on the inclination distribution has been studied including the geologic age rock type magnetic polarity quality of data and their spatiotemporal distribution by two dimensional chi square analysis on crystalline rocks only which avoid problems associated with remanence shallowing among sedimentary rocks the most plausible estimates for the zonal non dipolar contributions of the field have been determined as 2 for g2 and 5 for g3 values much lower than in previous estimates using the same method however the inherent non uniqueness of the inclination frequency analysis and the uneven spatiotemporal sampling of the field around the globe during the precambrian necessitate other independent methods of testing the gad hypothesis

is 600 myr long enough for the random palaeogeographic test of the geomagnetic axial dipole assumption summary a simple analysis suggests that a time interval of more than about 5000 myr is needed for the random palaeogeographic test to be effective in assessing the geomagnetic axial dipole assumption this time is so long as to render the test ineffective consequently intervals of 600 myr should be recognized as providing only a small sample that is likely to be biased towards low inclination values

seismic tomographic interpretation of paleozoic sedimentary sequences in the southeastern north sea seismic wide angle data were recorded to more than 300 km offset from powerful airgun sources during the mona lisa experiments in 1993 and 1995 to determine the seismic velocity structure of the crust and uppermost mantle along three lines in the southeastern north sea with a total length of 850 km we use the first arrivals observed out to an offset of 90 km to obtain high resolution models of the velocity structure of the sedimentary layers and the upper part of the crystalline crust seismic tomographic traveltime inversion reveals 2 8 km thick paleozoic sedimentary sequences with p wave velocities of 4 5 5 2 km s these sedimentary rocks are situated below a mesozoic cenozoic sequence with variable thickness 2 3 km on the basement highs 2 4 km in the horn graben and the north german basin and 6 7 km in the central graben the thicknesses of the paleozoic sedimentary sequences are 3 5 km in the central graben more than 4 km in the horn graben up to 4 km on the basement highs and up to 8 km i

paleomagnetism of the flores volcanics vancouver island in place by eocene time the eocene flores volcanics of the insular belt of southwestern vancouver island have u pb zircon ages of 51 50 ma and a mean direction of magnetization d i of 349 8 69 6 12 collecting sites

assessment of crustal velocity models using seismic refraction and reflection tomography two tomographic methods for assessing velocity models obtained from wide angle seismic traveltime data are presented through four case studies the modelling inversion of wide angle traveltimes usually involves some aspects that are quite subjective for example 1 identifying and including later phases that are often difficult to pick within the seismic coda 2 assigning specific layers to arrivals 3 incorporating pre conceived structure not specifically required by the data and 4 selecting a model parametrization these steps are applied to maximize model constraint and minimize model non uniqueness however these steps may cause the overall approach to appear ad hoc and thereby diminish the credibility of the final model the effect of these subjective choices can largely be addressed by estimating the minimum model structure required by the least subjective portion of the wide angle data set the first arrival times for data sets with moho reflections the tomographic velocity model can be used to invert the pmp times for a minimum structure moho in this way crustal velocity and moho models can be obtained that require the least amount of subjective input and the model structure that is required by the wide angle data with a high degree of certainty can be differentiated from structure that is merely consistent with the data the tomographic models are not intended to supersede the preferred models since the latter model is typically better resolved and more interpretable this form of tomographic assessment is intended to lend credibility to model features common to the tomographic and preferred models four case studies are presented in which a preferred model was derived using one or more of the subjective steps described above this was followed by conventional first arrival and reflection traveltime tomography using a finely gridded model parametrization to derive smooth minimum structure models the case studies are from the se canadian cordillera across the rocky mountain trench central india across the narmada son lineament the iberia margin across the galicia bank and the central chilean margin across the valparaiso basin and a subducting seamount these case studies span the range of modern wide angle experiments and data sets in terms of shot receiver spacing marine and land acquisition lateral heterogeneity of the study area and availability of wide angle reflections and coincident near vertical reflection data the results are surprising given the amount of structure in the smooth tomographically derived models that is consistent with the more subjectively derived models the results show that exploiting the complementary nature of the subjective and tomographic approaches is an effective strategy for the analysis of wide angle traveltime data

inversion of three dimensional wide angle seismic data from the southwestern canadian cordillera seismic refraction wide angle reflection data were recorded on a triangular array in southwestern british columbia centered on the boundary between the coast belt to the southwest and the intermontane belt to the northeast the experiment part of the lithoprobe southern cordillera transect enabled determination of the three dimensional 3 d velocity structure of the crust and upper mantle an algorithm for the inversion of wide angle seismic data to determine 3 d velocity structure and depth to reflecting interfaces is developed the algorithm is based on existing procedures for the inversion and forward modeling of first arrival travel times and forward modeling of reflection travel times including 1 forward modeling using a 3 d finite difference algorithm and 2 a simple velocity model parameterization for the inversion which eliminates the need to solve a large system of equations the existing procedure is extended to allow 1 the inversion of reflection times to solve for depth to a reflecting interface and or velocity structure 2 the inversion of first arrival travel times to solve for depth to a refracting interface and 3 layer stripping application of the algorithm to southern cordillera data uses pg to constrain upper crustal velocity structure pmp to constrain lower crustal velocity structure and depth to moho and pn to constrain upper mantle velocities and depth to moho the 3 d velocity model for the southwestern canadian cordillera is characterized by 1 significant lateral velocity variations at all depths that do not in general correlate with surface geological features or gravity data 2 a relatively high velocity middle and lower crust in the southwestern part of the study area which correlates with a strong relative gravity high and outlines the eastern extent of lower wrangellia an accreted terrane forming the insular belt to the west 3 a narrow zone of slower velocity in the lower crust and change in crustal thickness associated with the fraser fault system lending additional support to the view that it is a crustal penetrating fault 4 an average upper mantle velocity of 7 85 km s and 5 a depth to moho of 33 36 km in the intermontane belt and 36 38 km throughout most of the coast belt decreasing in the west to 33 km near the insular coast contact horizontal velocity structure slices and an interpreted cross section based on these and other results show the complexity of crustal structure in the region

northward translation of baja british columbia along the late cretaceous to paleocene margin of western north america paleomagnetic data suggest that a large part of british columbia and southern alaska was at the latitude of baja california mexico in early late cretaceous time this allochthonous block baja bc includes the insular talkeetna in alaska and intermontane composite terranes the miniterranes between them and possibly part of the omineca metamorphic belt assuming that baja bc was adjacent to north america by middle cretaceous an analysis using plate kinematics of the pacific basin produces a model of northward movement of baja bc which is compatible with the known geology of the reconstructed margin of western north america baja bc moved north 2400 km as part of the kuia plate along a transform margin with north america from 85 to 66 ma this time interval is marked by the cessation of igneous activity in the sierra nevada development of wrench faults and basins along coastal california and initiation of laramide style tectonics to the east within baja bc diminished igneous activity to the west was coeval with possible wrench tectonic basins along its eastern margin by the end of the cretaceous baja bc was positioned from oregon to northern british columbia between 66 and 56 ma baja bc became attached to north america in a dextral transpressive stage during which the kula north america plate boundary expanded to encompass the whole block with a series of northwest trending strike slip faults a fold and thrust belt in the east and large scale uplifts and the formation of an incipient volcanic arc on the west southern alaska was the site of convergence during the whole of the period of northward translation of baja bc a late cretaceous subduction zone in alaska evolved into a major compressional belt as the basin and miniterranes on the northern margin of baja bc collided with interior alaska the collision culminated in the paleocene with formation of the mckinley granites and coeval dextral slip on the denali fault system

paleomagnetism of eocene volcanic rocks talkeetna mountains alaska previous paleomagnetic studies of eocene rocks deposited in the talkeetna mountains on the northern edge of the combined peninsular wrangellia and alexander terranes the southern alaska superterrane sas yield a paleolatitude of 76 n while late cretaceous rocks on the southern edge of the sas record a 32 n paleolatitude at face value these drastically different paleolatitudes imply that wrangellia moved 44 to the north in only 20 m y requiring a northward component of velocity of about 24 cm yr alternatively the discrepancy might be explained by postulating an unrecognized tectonic boundary separating the two localities allowing different emplacement ages we have tested the unrecognized tectonic boundary hypothesis allowing post eocene emplacement of the sas by sampling eocene volcanic rocks deposited near the southern edge of the sas in the talkeetna mountains our data set comprises measurements from 97 oriented cores collected from 26 lava flows and two tuffs distributed over a distance of 50 km a primary remanence is indicated by positive fold tests and penecontemporaneous intrusion tests age control is provided by nine new k ar whole rock age determinations ranging from 38 8 to 53 6 ma the mean paleomagnetic pole calculated for this study is not significantly different from the pole derived from the eocene flows to the north and gives a paleolatitude of 78 thus ruling out the hypothesis of an unrecognized suture accommodating major post eocene displacement this result has prompted a reevaluation of the data which suggest rapid movement including the precision of the age control the adequacy of averaging of secular variation and the possibility of sedimentary inclination error the bulk of the evidence suggests northward displacement from lower latitudes however the error limits are such that the dramatic velocities previously postulated are not required

crustal structure based on gravity magnetic modelling constrained from seismic studies under lambert rift antarctica and godavari and mahanadi rifts india and their interrelationship modelling of gravity and magnetic anomalies along selected profiles across the lambert glacier antarctica constrained from the results of deep seismic sounding dss along a profile provide i 6 7 km thick sediments of low density 2 35 g cm3 which represent permian triassic sediments as they are exposed along the margins of this basin ii high density 2 75 g cm3 and high susceptibility rocks with a stable natural magnetization of 0 025 0 035 gauss as the basement iii a high density body 2 80 g cm3 along the western shoulder which may represent mid crustal granulite and charnockite rocks exposed in the prince charles mountains iv a thin crust of 25 km with a high density 3 05 g cm3 and high velocity underplated lower crust these signatures suggest that the lambert rift and the amery ice shelf represent an active continental rift valley with signatures of magmatism the adjoining prydz bay basin along the east coast of antarctica is also characterized by 5 6 km thick permian triassic cretaceous sediments with seaward dipping reflectors of basic volcanic rocks the godavari and the mahanadi gondwana rift valleys of almost same period and length 500 600 km across the east coast of india are also characterized by permian triassic cretaceous sediments of almost same density and thickness the mahanadi basin depicts a thin crust 33 34 km with a high density 3 00 g cm3 underplated lower crust similar to the lambert rift and a low density 2 65 g cm3 layer in the upper crust it indicates substantial rift magmatism in the coastal part of the mahanadi basin and adjoining continental shelf which forms a part of the early cretaceous volcanic province of east india as the east coast of india and antarctica were juxtaposed together at the time of evolution of these rift valleys the above similarities between them might be due to their common heritage the signatures of magmatism in the lambert rift of antarctica and the mahanadi basin of india indicate that the thermal source at the time of their evolution was located under the eastern part of east antarctica which might be responsible for its uplift supplying sediments of same provenance and fossil record in these basins

discordant paleomagnetic poles from the canadian coast plutonic complex regional tilt rather than large scale displacement stratigraphic petrologic and isotopic data indicate that parts of the coast plutonic complex and the north cascade range have been tilted northeast side up by angles of approximately 30 degree about north northwest trending axes these tilts can account for discordant paleomagnetic directions observed in mid cretaceous plutons from these regions without large scale displacement relative to north america

an 40 ar 39 ar investigation of the contact effects of a dyke intrusion kapuskasing structural zone ontario an 40ar 39ar study on biotite and hornblende from baked tonalite gneiss adjacent to a 14 m wide kapuskasing dyke in the chapleau block of the kapuskasing structural zone employed resistance furnace step heating of bulk mineral separates single grain laser step heating and laser spot dating of individual grains in thick section all three methods yielded concordant results for the biotite indicating that it has been uniformly overprinted at ca 2 05 ga this date is probably a close estimate of the age of the dyke intrusion the argon data from the hornblende are more difficult to interpret all three methods suggest that the hornblende was partially overprinted and then affected by a later influx of excess argon from the external environment laser spot dates record an apparent age gradient extending radially outward from 2 25 ga in the grain cores to 2 4 2 7 ga in the grain rims corresponding 37ar 39ar values show no systematic variation across the hornblende although 38ar 39ar ratios display a similar pattern to the age distribution with lower values in the cores than in the rims the bulk separate and single grain step heating spectra mimic a volume diffusion ar loss profile that would appear to be inconsistent with the laser spot dates three possible interpretations to explain this apparent discrepancy may involve either experimental difficulties an effective radius of diffusion which is much smaller than the actual grain radius or an effective radius of diffusion that is of the order of the grain radius it is clear that for the 40ar 39ar dating of samples that have experienced complex geologic histories it is important to utilize both resistance furnace and laser techniques

northward displacement and accretion of wrangellia new paleomagnetic evidence from alaska wrangellia is a vast terrane that accreted along the pacific margin of north america from oregon to central alaska previous paleomagnetic studies of the triassic volcanic rocks of wrangellia in south central alaska and british columbia have given paleolatitudes of 10 17 which are anomalously low in comparison to results from similar aged rocks of nuclear north america the discrepancy in paleolatitude implies substantial northward drift of the terrane relative to the craton we have extended paleomagnetic sampling to the northern boundary of the terrane in the mount hayes and healy quadrangles of the central alaska range this collection of 46 triassic lava flows yields a mean paleolatitude of 13 9 95 3 8 the polarity of the data and hence the hemisphere of origin are not resolved with certainty in contrast triassic paleomagnetic poles from the stable part of north america predict a paleolatitude of 42 1 n 95 4 6 for the region the northernmost fragment of wrangellia probably reached mainland alaska in the middle cretaceous when upper jurassic and lower cretaceous flysch within the intervening basin underwent severe deformation tertiary transcurrent faulting has modified the positions of wrangellian fragments in alaska but such displacements are probably minor as indicated by paleomagnetic data from early tertiary volcanic rocks that overlap the terrane

paleomagnetic results for eocene volcanic rocks from northeastern washington and the tertiary tectonics of the pacific northwest the direction of remanent magnetization for 102 sites in eocene volcanic and volcaniclastic rocks of the o brien creek formation sanpoil volcanics and klondike mountain formation suggests approximately 25 of clockwise rotation of a 100 by 200 km area in northeastern washington the volcanic rocks consist chiefly of rhyodacite and quartz latite flows with intercalated ash flow tuff and volcaniclastic layers these rocks have been sampled at 102 sites distributed among five volcanotectonic depressions the toroda creek republic keller and first thought grabens and the spokane enterprise lineament the volcanic rocks probably range in age from 55 m y to about 48 m y and the 50 to 48 m y old volcanic rocks within this suite appear to be rotated as much as the older rocks previous investigators have shown that 40 m y old and younger plutonic rocks of northwestern washington are not rotated hence we infer that the north central washington rocks were rotated to their present declination between 48 and 40 m y b p during the middle and or late eocene during early eocene time this region was extended in a westward direction through crustal necking gneiss doming diking and graben formation internal deformation of the region related to this crustal extension was extreme but most bedrock units that were formed concurrent with the crustal extension were probably in place prior to the rotation hence we infer that the rotation was chiefly accommodated by movement on faults peripheral to the sampled area faults active during paleogene time appear to define boundaries of a triangular crustal block the sanpoil block encompassing much of northeastern washington northern idaho northwestern montana and adjacent parts of british columbia the faults include the laramide thrusts of the rocky mountain thrust belt the strike slip faults of the lewis and clark line and strike slip faults of the straight creek fraser zone we suggest that during early eocene time the sanpoil block was extended westward through crustal necking and dilation and then during the middle eocene was rotated clockwise and thrust over the craton in a final stage of laramide thrusting the motor driving these deformations presumably was interaction of north america with oceanic lithosphere off its western margin such interaction probably involved right oblique underthrusting and dextral shear

precambrian continental crust of india and its evolution the indian precambrian continental crust exhibits a variety of geological features fashioned at different times by different geotectonic processes the bulk of this crust was formed prior to 2600 m y ago and remobilized at least twice between 2600 2000 m y ago early proterozoic mobile belt epmb and 2000 1500 m y ago middle proterozoic mobile belt mpmb three early precambrian nucleii karnataka kn jeypore bastar jbn and singhbhum sn appear to have survived in the craton and are characterized by low grade supracrustals and tonalitic trondhjemite gneisses formed 3800 2600 m y ago the epmb event involved sedimentation amphibolite granulite facies metamorphism and co 2 k metasomatism and produced amphibolite facies rocks and k granites in the north and charnockite and other granulite facies rocks in the south gold sporadically distributed in the supracrustal rocks of the craton was remobilized during the epmb event k granites form a garland around the central dharwar craton sugg

statistical significance of the fold test in palaeomagnetism summary the significance of the change in direction dispersion on applying the fold test in palaeomagnetism is used as the criterion for deciding whether or not the fold test has statistical significance a set of tables is compiled which can be used for quick application of the test

non dipole fields and inclination bias insights from a random walk analysis abstract the paleomagnetic assumption that the earth s magnetic field is reduced to a geocentric axial dipole gad when sufficiently sampled has been called into question for mesozoic and earlier times it has been suggested for example that modest contributions from axial quadrupolar 10 and octupolar 25 fields are resolvable using inclination only data from paleomagnetic studies the underlying assumption in inclination only studies is that considerable continental drift has occurred over a sufficiently long period of time to render paleomagnetic sampling random in a paleogeographic sense this assumption was stated in all previous studies dating back to 1976 but was never tested we have developed a random walk model designed to test this assumption our model uses three different configurations for the continents in the random walk and allows the user to vary parameters such as maximum velocity sampling distribution sampling frequency and frequency of directional change the model generates large sample sizes that cannot be adequately evaluated using the standard 2 statistical test and therefore we introduce two statistical parameters used in structural equation models our models indicate that the random paleogeographic sampling assumption used in the previous studies is not valid due primarily to the lack of an adequate sample size and temporal distribution we show for example that even the most robust dataset compiled in 1998 is severely undersampled a series of model runs on a gad earth with sampling over a 600 myr period demonstrates that detailed sampling will on average produce a gad like distribution only 30 of the time other model runs demonstrate that inadequate sampling can produce false quadrupolar and octupolar effects it is our conclusion that time averaged inclination only studies using the extant paleomagnetic database should be viewed with extreme caution

a new late cretaceous paleomagnetic pole from the adel mountains west central montana north america s apparent polar wander path has been poorly defined between the mid cretaceous and paleocene reference pole positions existing data allowed 13 of apparent polar motion over about 22 m y 87 65 ma roughly coinciding with the beginning of laramide deformation 80 ma we report on a paleomagnetic study of the adel mountain volcanic rocks to refine the north american apparent polar wander path for this interval the shonkinite rocks of the adel mountain volcanic field are on the eastern edge of the cretaceous paleocene fold and thrust belt some of these structures disturb the western edge of the volcanic pile we obtained two new k ar dates from the adel rocks one date on biotite from a shonkinite dike that crosscuts most of the volcanic rocks is 71 2 2 7 ma the other a whole rock date from a flow deep in the volcanic pile is 81 1 3 5 ma we collected six to nine paleomagnetic samples from each of 34 sites in roadcuts and natural outcrops of flows dikes and laccoliths positive fold and conglomerate tests along with alternating field and thermal demagnetization indicate that our characteristic remanent directions are primary magnetizations acquired before late cretaceous to paleocene thrust belt deformation averaging the virtual geomagnetic poles from 26 reliable sites all of normal polarity yields a paleopole at 82 2 n 209 9 e 95 6 80 k 18 38 this pole is concordant with the paleocene reference pole 82 0 n 170 2 e 95 3 5 k 18 6 diehl et al 1983 and is 11 6 from the globerman and irving 1988 mid cretaceous pole at 71 n 196 e the youngest information in the cretaceous stillstand pole is from the niobrara formation shive and frerichs 1974 at about 85 89 ma if we take the average age of the adel mountain volcanics to be 76 ma then 12 of apparent polar motion occurred between 87 ma and 76 ma thus rapid apparent polar motion correlates well with the onset of laramide deformation

northward motion of the whitehorse trough paleomagnetic evidence from the upper cretaceous carmacks group discussion discordant paleomagnetic directions in mid cretaceous plutons of the coast plutonic complex and north cascades have been interpreted as indicating mid cretaceous paleolatitudes 2000 2500 km south relative to north america beck et al 1981 irving et al 1985 umhoefer 1987 but this magnitude of northward transport contradicts interpreted pre midcretaceous links between the north american miogeocline and westerly parts of the cordillera price and charmichael 1986 because paleohorizontal cannot be directly determined in plutonic rocks the discordant paleomagnetic directions may also be explained by tilting of the plutons about a horizontal axis primarily because of the consistent nature of the paleomagnetic discordance over a large region beck et al 1981 and irving et al 1985 have preferred the interpretation of northwards transport however butler et al 1989 recently pointed out geological evidence indicating that widespread northeast side up tilting may have affected the western portion of the coast plutonic complex and the north cascades resolution of this debate is of obvious importance to understanding the tectonic evolution of the canadian cordillera marquis and globerman reported results of a paleomagnetic study of the upper cretaceous carmacks group in the whitehorse trough the carmacks group is composed of gently dipping basaltic and andesitic lava flows that have been dated at 70 4 2 4 ma these rocks lie west of the tintina northern rocky mountain trench fault and the teslin suture zone because paleohorizontal can be unambiguously determined for these volcanic rocks the paleomagnetic data presented by marquis and globerman are potentially very important in resolving the tilt versus translation question marquis and globerman collected paleomagnetic samples in three regions spread over 300 km based on detailed laboratory studies they successfully isolated characteristic remanent magnetization chrm from 18 of 27 paleomagnetic sites flows the acquired paleomagnetic data pass a fold test of stability and contain sites of both normal and reversed polarity and magnetic properties indicate that the chrm is a primary thermoremanent magnetization the observed angular dispersion of virtual geomagnetic poles vgp s indicates some degree of required sampling of geomagnetic secular variation marquis and globerman determined a paleomagnetic pole from the carmacks group at 82 n 109 e with 95 confidence region ag5 of radius 7 8 using a 70 ma north american reference paleomagnetic pole of 77 7 n 185 8 e ag5 7 7 and the methods of beck 1980 as modified by demarest

testing the geomagnetic dipole and reversing dynamo models over earth s cooling history abstract we use the inclination test of the geocentric axial dipole hypothesis to interpret observational magnetic field models and earthlike polarity reversing numerical dynamo models with uniform and latitudinally variable buoyancy flux boundary conditions dynamo models with uniform buoyancy flux represent three convective states of the mantle and core 1 present era earth driven thermochemically at the inner core boundary 2 mantle overturn with elevated heat flux at the core mantle boundary and 3 ancient earth prior to inner core nucleation with buoyancy production solely at the core mantle boundary by verifying the effect of latitudinal heat flux variation on reversal frequency we show that polar cooling at the core mantle boundary is likely to yield only small inclination anomalies instead we find that radial heat flow structure can explain magnetic field morphologies over broad eras of earth s cooling history consistent with earth s present magnetic field dynamos driven by buoyancy due to inner core growth are nearly dipolar in contrast elevated core mantle boundary heat flow yields small to moderate inclination flattening due to a persistent octupole that reverses synchronously with the dipole we find that reversal frequency and inclination anomalies can indicate the convective state of the mantle core system in particular our results along with evidence of a young inner core are consistent with a previous suggestion that an entirely liquid core contributed to shallow inclinations in precambrian time

paleomagnetism and geochronology of 23 ma gabbroic intrusions in the keku strait alaska and implications for the alexander terrane samples of tertiary gabbro from 24 sites in the keku strait alaska help constrain the displacement history of the alexander terrane step heating experiments on a plagioclase separate from these previously undated intrusions indicate a discordant 40ar 39ar age of 23 1 1 7 ma the characteristic magnetization resides in magnetite is easily isolated by thermal and alternating field demagnetization and has both normal and reversed polarities the mean paleomagnetic pole with no structural correction is latitude 87 1 n longitude 141 6 e a95 10 1 with n 20 sites this pole indicates insignificant tectonic displacement 0 5 8 2 southward and rotation 0 6 15 2 counterclockwise we therefore conclude that any northward displacement or vertical axis rotation of the alexander terrane and or tilting in the vicinity of the keku strait must have occurred before 23 ma

paleomagnetism and mesozoic tectonics of the seven devils volcanic arc in northeastern oregon upper triassic volcanogenic rocks of the seven devils group and huntington formation of brooks 1979b in northeastern oregon record two episodes of magnetization the older magnetization which predates late jurassic deformation of the volcanic arc terrane comprising the seven devils group was probably acquired in late triassic time the inclinations of both normal and reversed polarity magnetizations indicate that the triassic rocks originated approximately 18 from the paleoequator the results are consistent with paleomagnetic data from similar triassic rock assemblages on vancouver island and in south central alaska we interpret these assemblages as displaced fragments of wrangellia a once coherent late paleozoic volcanic arc and early mesozoic volcanic plateau that was torn apart and carried north by plate motion along the continental margin we interpret the younger magnetization which fails the fold test as a widespread thermochemical overprint caused by late jurassic and early cretaceous plutonism this magnetization yields a paleomagnetic pole at 60 n 295 e 95 7 and is similar to the pole obtained from plutons that intrude the seven devils group the angular difference between the pole from the plutons and the late jurassic early cretaceous reference pole for stable north america suggests that the seven devils volcanic arc has undergone a large clockwise rotation 66 21 the rotation may reflect the last stages of the arc s accretion to the continental margin or later large scale block movements in the pacific northwest

argon retentivity of hornblendes a field experiment in a slowly cooled metamorphic terrane hornblende from samples of amphibolite and granitic gneiss collected within a single outcrop in the central adirondacks yield significantly different sup 40 ar sup 39 ark sub k dates of 948 plus minus 5 and 907 plus minus 5 ma assuming that this terrane cooled slowly following high grade metamorphism and that the samples have experienced the same thermal history the difference in dates apparently reflects a corresponding difference in blocking temperature for diffusion of radiogenic argon in these hornblende samples the fe fe mg mn of the hornblende samples are 0 8 and 0 6 the higher ratio corresponding to the younger sup 40 ar sup 39 ar sub k date transmission electron microscopy observations indicate that both hornblende samples are homogeneous and devoid of any exsolution but contain zones of fibrous phyllosilicates approximately 0 1 to 2 mu m wide parallelling 100 and 110 these alteration zones probably formed during post metamorphic cooling as a result of the migration of fluids through the hornblendes and are obvious pathways for argon escape from hornblende as these features are more abundant in the hornblende sample with the younger sup 40 ar sup 39 ar date and higher fe fe mg mn they may influence the argon blocking temperature by effectively partitioning the hornblende grains intomore diffusion domains of varying size biotite from the granitic gneiss yields an sup 40 ar sup 39 ar sub k date of 853 plus minus 2 ma with a mildly discordant step heating spectrum that in part reflects the degassing of submicroscopic inclusions precipitated during alteration of the host biotite plagioclase from the amphibolite yields a sup 40 ar sup 39 ar sub k integrated date of 734 plus minus 3 ma less

paleomagnetic evidence for displacement from the south of the coast plutonic complex british columbia the mid cretaceous spuzzum and porteau plutons of the coast plutonic complex of british columbia have two magnetizations a and b the a magnetization eight sites 83 specimens d 30 3 i 56 7 95 4 9 paleolatitude 37 5 n paleopole 65 0 n 14 9 w a95 6 2 is considered to have been acquired in the age range 105 90 ma this result differs from the field established for cratonic north america in this time range the difference could be caused either by previously undetected tilting about a horizontal axis of the plutons or by their rotation about a vertical axis and lateral displacement relative to the craton previously observed mid cretaceous magnetizations from other rock units from the western canadian cordillera and the cascades of washington united states are similarly discordant with respect to the craton this similarity over such a large area indicates that although local undetected tilting could be partly responsible it is unlikely to be the prime cause and we argue ther

on determining paleohorizontal and latitudinal shifts paleomagnetism of spences bridge group british columbia the spences bridge group of late albian age palynology and u pb zircon date of 104 5 ma is situated in the intermontane belt of the southern canadian cordillera zeolite grade andesite lavas and interbedded volcaniclastic rocks have been sampled through a thickness of about 2900 m sampling has been confined to localities whose bedding attitudes are well defined and to rocks that have not undergone internal deformation of the 27 sites studied 25 gave single component magnetizations with well grouped directions high coercivities and high unblocking temperatures applying corrections for geological tilt to all 25 sites yielded minimum dispersion after 65 untilting analysis indicates that this is a consequence of the interaction of two modes of behavior one exhibited by y magnetizations at 8 sites in which minimum dispersion occurs after 35 untilting and one x magnetizations at 17 sites in which minimum dispersion occurs after 100 correction actually the dispersion is essentially constant in the range 88 to 100 correction the y magnetizations are uninterpretable in terms of the paleofield because no estimate of paleohorizontal can be made because of their positive response to the tilt test the x magnetizations were acquired before tilting and it is argued that geologically determined bedding planes are good estimators of paleohorizontal at the time of remanence aquisition there may be errors because of paleoslope but arguments are presented for believing that these are small on this basis the best estimate of the mean direction of the paleofield in spences bridge time is estimated to be d i declination inclination 38 7 63 9 17 sites k 44 compared with k 11 in situ 95 5 4 with a paleopole at 64 4 n 321 0 e k 24 compared with k 5 in situ a95 7 5 when compared with the cratonic reference field for mid cretaceous time this pretilting magnetization indicates a clockwise rotation of the study area of 66 3 11 8 p 0 05 and an apparent displacement from the south of 15 5 7 1 1725 790 km when these and other data previously obtained from cretaceous rocks of the pacific northwest are compiled they show an increase in apparent displacement from east to west across the cordillera this result is discussed in terms of three hypotheses 1 no translation but systematic tilts of about 30 to the southwest 2 translation of a substantial part of the pacific northwest referred to as baja british columbia and is defined more exactly in the text by variable amounts up to more than 2000 km from the south together with clockwise rotations and 3 lesser translation of baja british columbia by about 1500 km combined with variable tilting predominantly to the south it is concluded that paleomagnetic data as they are presently known are consistent with hypotheses 2 and 3 but unless serious sources of error are present are not consistent with hypothesis 1 the most likely candidate for such errors is the uncertainty in estimating initial dip of beds paleoslope in volcanic rock units

three dimensional seismic refraction tomography a comparison of two methods applied to data from the faeroe basin this paper presents a comparison of two tomographic methods for determining three dimensional 3 d velocity structure from first arrival travel time data the first method is backprojection in which travel time residuals are distributed along their ray paths independently of all other rays the second method is regularized inversion in which a combination of data misfit and model roughness is minimized to provide the smoothest model appropriate for the data errors both methods are nonlinear in that a starting model is required and new ray paths are calculated at each iteration travel times are calculated using an efficient implementation of an existing method for solving the eikonal equation by finite differencing both inverse methods are applied to 3 d ocean bottom seismometer obs data collected in 1993 over the faeroe basin consisting of 53 479 travel times recorded at 29 obss this is one of the most densely spaced large scale 3 d seismic refraction experiments to date different starting models and values for the free parameters of each tomographic method are tested a new form of backprojection that converges more rapidly than similar methods compares favorably with regularized inversion but the latter method provides a simpler model for little additional computational expense when applied to the faeroe basin data bounds on two model features are assessed using regularized inversion with combined smoothness and flatness constraints an inversion of synthetic data corresponding to 100 data recovery from the real experiment shows a marked improvement in lateral resolution at deeper depths and demonstrates the potential of currently feasible 3 d refraction experiments to provide well resolved long wavelength velocity models the similarity of the final models derived from the two tomographic methods suggests that the results from the new form of backprojection can be relied on when limited computational resources rule out regularized inversion

evaluation of indian coastal response an integrated approach abstract since its separation 130 ma from the mainland the indian subcontinent has experienced major evolutionary changes a comparison of the structure evolution and tectonic elements of the east and west coast reveals characteristic differences thus while the eastern coast shows significant anomalous behaviour in all geophysical observations in contrast the corresponding observations on the western coast are relatively devoid of such anomalies in this investigation available magsat gravity electromagnetic gds and heatflow data have been utilized to search for a possible coherent cause for these different anomalies and thereby some inferences are sought to relate these causes to the geodynamics of this part of the indian plate

paleomagnetism of the eocene kamloops group and the cratonization of terrane i of the canadian cordillera the lower middle eocene 49 4 2 4 ma kamloops group is exposed in the middle of the quesnellia subterrane of terrane i the group consists of the siliciclastic tranquille beds and the overlying dewdrop flats plateau basalts and andesites detailed alternating field af and thermal step demagnetization was carried out on 282 specimens from 26 flow sites and one conglomerate site and saturation isothermal remanent magnetization sirm tests were performed to examine the remanence carriers the petrology of the gently dipping flows the presence of antiparallel normal and reverse remanence the conglomerate test and the fold test all indicate that a primary remanence has been isolated it resides in both magnetite and hematite over a broad range of af coercivities blocking temperatures and domain sizes its mean direction of 355 0 73 4 95 6 9 gives a pole position of 138 4 w 81 4 n dp 11 0 dm 12 3 that is statistically indistinguishable from the 50 ma reference pole for the north a

paleomagnetism of eocene rocks of the kelowna and castlegar areas british columbia studies in determining paleohorizontal the middle eocene marron volcanics mean age 52 2 ma of the kelowna outlier form the upper part of the hanging wall of the westerly dipping okanagan valley fault ovf in south central british columbia they overlie quesnellia the ovf is currently interpreted as the westernmost member of a network of low angle extension faults in the southern omenica belt the ovf was active in the middle eocene at much the same time that the marron volcanics were cooling relative to present horizontal the magnetizations are widely scattered fisher s precision parameter k 8 and after correction for bedding attitudes there is no significant improvement k 9 evidently some magnetizations were acquired before referred to as category 1 and others after category 2 tilting that is the horizontal plane at the time of magnetization sometimes did and sometimes did not coincide with the bedding plane partial unfolding experiments carried out on the two categories separately yield a precision comparable to t

a mesoproterozoic supercontinent evidence from the indian shield abstract geophysical signatures of late paleoproterozoic collisional sutures fold belts between the deccan protocontinent and bundelkhand craton and between the bundelkhand and mewar cratons of the north indian shield are presented the aravalli and satpura orogenies evolved during these continent continent collision episodes both the orogenic episodes are associated with granulite facies metamorphism and metallogeny serpentinised ultramafic breccia in ophiolitic melange is present in both regions structural discontinuities and oppositely dipping reflection fabrics with high conductivity anomalies are the characteristic features of these collision zones a major gravity high is also observed all along the strike of these fold belts to a distance of 700 1000 km paleoproterozoic 1 8 2 1 ma ages corresponding to compressional activity have also been observed in the southern granulite terrain and other gondwana fragments extensive granites and gneisses of 1900 ma age are observed in the lower himalayas and nanga parbat ranges late paleoproterozoic orogenic activity is also observed in all of the earth s shields intense orogenic activity of this period observed in various widely separated cratonic blocks is related to global plate tectonic processes and supercontinental episodes the available geophysical evidences in various parts of the globe suggest the presence of a supercontinent referred to as columbia during the early mesoproterozoic period

early tertiary paleomagnetic evidence and the displacement of southern alaska paleomagnetic data from early paleocene lavas a few kilometers north of lake clark just inboard of the peninsular terrane indicate modest post early tertiary latitudinal displacement and about 55 sup 0 of counterclockwise rotation of this region with respect to north america a nearby middle eocene sequence of lava flows however gives no indication of displacement or rotation south of lake clark the paleomagnetism of five paleogene 30 to 40 ma volcanic sequences that are a part of the peninsular terrane proper shows no northward translation and no rotation a major part of the northward movement documented by other studies in the gulf of alaska region may be accounted for by cumulatively significant intraterrane displacements 32 references 3 figures 1 table

the evolution of the geothermal gradient from lambert graben and mahanadi basin a contribution to the indo antarctic rift debate abstract the east antarctic lambert graben and the indian mahanadi basin are considered to represent segments of an intra gondwanan rift structure that was active at least since the paleozoic fission track analyses of apatites from a comprehensive data set across the shoulders of both grabens were used to compare their low temperature history and to estimate the paleo geothermal gradients before the onset of the last denudation rifting stage late jurassic the paleo geothermal gradients of both juxtaposed gondwana margins similarly increase from the basement towards the respective rift shoulder from 15 20 ckm 1 to 25 30 ckm 1 this trend of increasing paleo geothermal gradients together with a denudation episode commencing in the early cretaceous and coeval igneous activity indicates a common rifting stage accompanying the breakup of gondwana in the india antarctica sector

40ar 39ar constraints on the emplacement uplift and cooling of the coast plutonic complex sill southeastern alaska a 700 km long and 5 to 15 km wide aggregation of sill like tertiary plutons separates low to medium grade greenschist to amphibolite facies metamorphic rocks to the west from medium to high grade upper amphibolite to granulite facies gneisses and granitic plutons to the east along the western boundary of the coast plutonic complex in southeastern alaska and british columbia 40ar 39ar analyses of hornblende biotite and plagioclase samples from a sea level traverse and a 1 to 2 km elevation traverse across one of the sills near holkham bay southeastern alaska yield dates that vary systematically with position samples collected on the eastern margin of the sill are 3 to 6 m y younger than samples of the same mineral collected at the same elevation from the western margin of the sill indicating that the western margin cooled earlier samples collected at sea level yield 40ar 39ar dates on hornblende biotite and plagioclase that are consistently younger than those collected at 1 and 2 km elevation dates from the center and eastern margin of the sill define isotherm migration rates km m y of 0 8 1 3 at 60 56 ma hornblende 0 8 0 6 at 56 53 ma biotite and 0 4 at 54 50 ma plagioclase 40ar 39ar dates from a single sea level sample collected 15 km farther south indicate no significant variation in dates parallel to the southeasterly strike of the sill for at least 30 km the younger dates calculated for the east side relative to the west side and metamorphic temperatures suggest that the country rock was hotter to the east than to the west when the pluton cooled through mineral blocking temperatures the rapid isotherm migration rates calculated from the vertically distributed samples and the suggestion of hotter country rock to the east during cooling is consistent with an interpretation that the pluton intruded during uplift of the rocks to the east because the relative precision of the 40ar 39ar technique is sufficient to detect subtle difference in the cooling ages its application to spatially distributed samples within a homogeneous plutonic body as in this study may yield new insights into the timing of metamorphism magmatism and tectonism of young mountain belts

estimation of the regional bouguer gravity field over the indian peninsula using two dimensional filtering abstract the regional bouguer gravity field for india has been obtained objectively by digitising the gravity data at 0 25 interval and by subjecting it to two dimensional symmetrical filtering with cutoff of 0 004 cycles km 200 km the data at continental edges were appended with co sinusiods of appropriate wave number to produce a uniform rectangular grid this process also avoids gibb s phenomenon the data set thus obtained was divided into four overlapping independent rectangles for the purpopse of filtering this process also provided a check on the filtered values the low pass filtered deeper and the residual shallower maps so obtained are correlated with geology and tectonic regimes of the india subcontinent the high gradients in the contours of the residual map and the gentle gradients in the regional map reflect the effects of shallower and mainly deeper structures respectively the regional field indicates 1 1 a characteristic difference between the eastern and the western coasts 2 2 a central high encompassing the godavari graben and satpura fold belt which bends syntaxially towards aravalli trend and 3 3 a nw se trending low in southern peninsular india the salient observations from the residual map are 1 1 a clear rift like structural signature with negative gravity values in the centre flanked by positive gravity values over the godavari graben 2 2 the ne sw aravalli trend merges with the e w satpura trend in the south 3 3 the gravity signature of the narmada son lineament is not very conspicuous and 4 4 an isolated patch of negative gravity values south of bangalore is prominent

paleomagnetic reconnaissance of early mesozoic carbonates from williston lake northeastern british columbia canada evidence for late mesozoic remagnetization three classic sections of middle and late triassic fossiliferous limestones cropping out around williston lake in british columbia canada were sampled for paleomagnetic study the objective was to test the suitability of these units for detailed magnetobiostratigraphic study with the aim of improving the reference triassic geomagnetic polarity time scale the williston lake characteristic magnetizations differ however from any triassic north america cratonic reference directions a satisfactory agreement is found instead with cretaceous early cenozoic north america cratonic reference directions the exclusive occurrence of normal polarity suggests that remagnetization likely occurred during the cretaceous long normal superchron remagnetizations may have been triggered by connate brines which moved along aquifers of porous sandstones and carbonates in the early stages of laramide folding resume trois sections fossiliferes classiques de calcaires trias moyen et tardif affleurant autour du lac williston en colombie britannique ont ete echantillonnees pour une etude paleomagnetique l objectif etait de s assurer de la pertinence de ces unites pour une etude magneto bio stratigraphique detaillee visant a ameliorer l echelle referentielle de temps de la polarite geomagnetique au trias les magnetisations caracteristiques du lac williston different toutefois de toute direction cratonique referentielle au trias en amerique du nord toutefois une correlation satisfaisante est etablie avec les directions cratoniques referentielles du cretace cenozoique precoce en amerique du nord l occurrence exclusive d une polarite normale suggere qu il y eut probablement une remagnetisation au cours du long superchron normal du cretace les remagnetisations peuvent avoir ete declenchees par des saumures connees qui se sont deplacees le long d aquiferes de gres et de carbonates poreux dans les premieres phases du plissement laramide

regional tectonic framework structure and evolution of the western marginal basins of india abstract the kutch saurashtra cambay and narmada basins are pericontinental rift basins in the western margin of the indian craton these basins were formed by rifting along precambrian tectonic trends interplay of three major precambrian tectonic trends of western india dharwar nnw sse aravalli delhi ne sw and satpura ene wsw controlled the tectonic style of the basins the geological history of the basins indicates that these basins were formed by sequential reactivation of primordial faults the kutch basin opened up first in the early jurassic rifting was initiated in late triassic along the delhi trend followed by the cambay basin in the early cretaceous along the dharwar trend and the narmada basin in late cretaceous time along the satpura trend the evolution of the basins took place in four stages these stages are synchronous with the important events in the evolution of the indian sub continent its breakup from gondwanaland in the late triassic early jurassic its northward drifting during the jurassic cretaceous and collision with the asian continent in the early tertiary the most important tectonic events occurred in late cretaceous time the present style of the continental margins of india evolved during early tertiary time the saurashtra arch the extension of the aravalli range across the western continental shelf subsided along the eastern margin fault of the cambay basin during the early cretaceous it formed an extensive depositional platform continuous with the kutch shelf for the accumulation of thick deltaic sediments a part of the saurashtra arch was uplifted as a horst during the main tectonic phase in the late cretaceous the present high thermal regime of the cambay bombay high region is suggestive of a renewed rifting phase

a deep structural ridge beneath central india a joint inversion of magnetic satellite magsat and free air gravity data has been conducted to quantitatively investigate the cause for bouguer gravity anomaly over central indian plateaus and possible fold consequences beside himalayan zone in the indian sub continent due to collision between indian and eurasian plates the appropriate inversion with 40 km crustal depth model has delineated after discriminating high density and magnetisation models for the first time about 1500 km long hidden ridge structure trending nw se the structure is parallel to himalayan fold axis and the indian ocean ridge in the arabian sea a quantitative relief model across a representative anomaly profile confirms the ridge structure with its highest point nearly 6 km higher than the surrounding crustal level in peninsular india the ridge structure finds visible support from the astro geoidal contours

a palaeomagnetic test of the axial dipole assumption and implications for continental distribution through geological time the distribution of the magnetic inclination i in the global palaeomagnetic data is examined over broad windows of geological time extending from the cenozoic back to the archaean distributions are compared with the theoretical predictions from the uniform global sampling of dipolar and higher order geomagnetic field sources the distributions are found to accord more closely with a dipolar source than with a multipolar source over most of geological time reinforcing a conclusion by evans based on 10 of the present data base the exception is the interval 600 300 ma broadly coincident with palaeozoic times when the data critically fail to accord with any simple multipole assumption we interpret this to be a symptom of the remagnetisation effect the sedimentary derived and to a much lesser extent the igneous and metamorphic derived record from these times has been extensively remagnetised in late palaeozoic times as the result of a unique incidence of diagenetic tectonic and thermal processes otherwise we conclude that the palaeomagnetic data interpreted on the basis of the axial dipole assumption can be used reliably to evaluate tectonic models for older 600 ma and younger 250 ma parts of geological time models for the global distribution of the continental crust for those cases that have postulated episodic or long term accretion of the crust into quasi rigid supercontinents are also evaluated the proterozoic supercontinent yields the most acceptable solution to the i distribution observed over the long interval of precambrian times 3000 600 ma it also explains the variable ratios of high latitude palaeomagnetic poles from the ancient shield segments of continental crust owing to the failure of the palaeozoic data set to accord with the dipole assumption the i distributions cannot be used to test the relationship between the supercontinent of pangaea and the geoid symmetry during the earlier part of its history but after 250 ma it was most closely constrained to the form of the geoid positive over geological time as a whole the continental crust has shown a marked preference for the i distribution defined by the present geoid positive rather than the distribution defined by the geoid negative we therefore conclude that the deep mantle effects responsible for the geoid have on average had a surface response which has tended to move the continental crust towards this sector of the globe

denali fault system of southern alaska an interior strike slip structure responding to dextral and sinistral shear coupling the denali fault system dfs extends for 1200 km from southeast to south central alaska the dfs has been generally regarded as a right lateral strike slip fault along which post late mesozoic offsets of up to 400 km have been suggested the offset history of the dfs is relatively unconstrained particularly at its western end for this study we calculated relative motion vectors at discrete points along the length of the dfs based on the well understood kinematic interaction between the north american pacific and kula plates and the following assumptions 1 the arcuate geometry of the dfs has existed essentially unchanged since the late cretaceous 2 the yukon tanana terrane and other terranes north of the dfs were fixed in situ prior to the accretion of the southern alaskan terranes and 3 tangential and normal relative motion vector components calculated for points along the dfs using the plate model of kelley 1993 describe the plate kinematics of the dfs since the late cretaceous the consequent kinematic model for the dfs predicts that left lateral stresses have acted upon the western end of the dfs for much of its history and conflicting senses of shear exist between the eastern and western ends of the system the offset history of the western end of the denali fault system should be significantly different than the history of the central and eastern sections consequently individual crustal blocks in southeast and southwest alaska may have undergone respectively clockwise and counterclockwise rotations the sense of rotation predicted by our model is in agreement with rotations determined by paleomagnetic studies and provides an alternative model to the alaskan orocline hypotheses

limits to northward drift of the paleocene cantwell formation central alaska volcanic rocks of the paleocene cantwell formation in central alaska apparently originated at a paleolatitude of 83 n 95 9 7 as indicated by paleomagnetic results when compared with the paleocene pole for the north american craton the 95 confidence limits of the results suggest that terranes north of the denali fault have moved no more than 550 km northward relative to the north american craton since paleocene time

phanerozoic history of the mahanadi region india the mahanadi basin is a nw se stretching gondwana basin perpendicular to the east coast of india the metamorphic basement to the basin mainly comprises rocks of the eastern ghats mobile belt egmb which experienced a complex thermotectonic evolution involving several episodes of deformation and magmatism it can be inferred from 40ar 39ar ages on hornblende 845 ma and biotite 622 and 593 ma from basement rocks that the last high grade metamorphism in the western part of the egmb i e the western khondalite zone occurred during the late grenvillian with any pan african overprint only reaching anchimetamorphic conditions only pseudotachylite intrusions and their related thermal imprint with ages around 515 ma display a direct pan african signature fission track analyses on nine zircon 419 313 ma and 49 apatite 443 119 ma samples indicate a rift related formation of the mahanadi basin as an asymmetrical half graben in two stages during the permo carboniferous and the middle triassic moreover the gondwana breakup in eastern india is signified by dyke intrusions and the thermal overprint of thrusts and faults in the mahanadi region and the onset of ocean floor production in the mahanadi offshore basin at around 120 ma

tomographic imaging of sub basalt mesozoic sediments and shallow basement geometry for hydrocarbon potential below the deccan volcanic province dvp of india summary a shallow 2d tomographic p wave velocity image is obtained in the deccan volcanic province dvp of india using first arrival seismic data along a 90 km long n s trending profile in the deccan syneclise region the tomographic image depicts smooth velocity variations of quaternary 2 0 2 5 km s sediments basalt 5 0 5 5 km s and sub basalt mesozoic sediments 4 0 4 5 km s as well as basement 5 8 6 0 km s configuration down to a maximum depth of 5 0 km due to late cretaceous volcanism with outpouring of basaltic lava flows this region is affected by numerous dyke intrusions and thick basaltic trap forming sills with the presence of horst and graben structures due to deep basinal faults although imaging below the basalt is a major challenge for the oil industry with the help of tomographic imaging technique of long offset seismic refraction data we are able to image sporadic sub and intra trappean mesozoic sediments 0 75 km deposited below thick columns 0 3 km of basalt flows above the basement the imaged mesozoic sediments are expected to contain hydrocarbon due to their wide extension and suitable trapping mechanism of basaltic sills the robustness of the velocity image is assessed through numerous tests like velocity perturbations 2 estimates rms residuals of traveltime fit resolution and uncertainty estimates through computation of ray density or hits

teleseismic receiver functions modeling of the eastern indian craton abstract we estimate receiver functions rfs through the time domain deconvolution using three component broadband data of 100 teleseismic events 30 90 from 15 seismographs in the eastern indian craton estimated radial rfs show a positive phase at 4 6 5 8 s delay time corresponding to the crustal thicknesses of 37 46 km through the differential evolution de waveform inversion modeling of radial receiver functions we delineate the crustal structure at 15 broadband stations on an average the archean singhbhum odisha craton soc is characterized by a thick crust of 43 3 km in comparison to a relatively thin crust of 41 1 km underlying the proterozoic chotanagpur granite gneissic terrain cggt while a thin crust of 38 1 km characterizes the younger eastern ghats mobile belt egmb the main results of our modeling reveal a 46 km thick archean crust underlying the singhbhum granite sg of 3 6 ga which is characterized by a 3 km crustal thickening probably resulted from the archean subduction process our modeling also detects a 2 3 km crustal thinning with the thinnest crust of 37 km below the region near south singhbhum shear zone which could be attributed to the 1 6 ga plume activity associated with dalma volcanic our modeling also led to the delineation of a crustal thinning of 2 3 km underlying the region in egmb which was influenced by a much younger 117 ma rajmahal magmatism associated with the gondwana break up episode however our study could not detect any age dependent variation of crustal thicknesses in the eastern indian craton the main result of our modeling suggests a two phase crustal evolution process for the soc viz older e w crustal thickening due to e w plate compression and later crustal thinning episodes associated with the dalma volcanism in the north and the rajmahal volcanism in the south

evidence of underplating from seismic and gravity studies in the mahanadi delta of eastern india and its tectonic significance 1 we have imaged the crustal configuration of the mahanadi delta of eastern india by modeling both wide angle seismic and bouguer gravity data this delta has undergone several stages of rifting subsidence sedimentation and uplift it is one of the important gondwana sedimentary basins of india where widespread volcanic activity occurred during the early cretaceous along the rift zones this volcanic activity corresponds to the breakup of greater india from east gondwana e g present antarctica and australia we have derived a five layer crustal model with velocities of 6 0 6 5 6 0 7 0 and 7 5 km s having estimated densities of 2 7 2 8 2 65 2 9 and 3 05 g cm3 respectively the velocity density relation along with heat flux and other geological geochronological information indicates typical rift related evolution of the delta with a midcrustal low velocity 6 0 km s and low density 2 65 g cm3 zone and a 10 km thick high velocity 7 5 km s and high density 3 05 g cm3 layer at the base of the crust presumably due to underplating the moho upwarping or crustal thinning in the rift zone and emplacement of thick high velocity material at the base of the crust strongly suggests basaltic underplating probably due to the kerguelen hot spot activity these activities are synchronous with 117 ma rajmahal volcanism in northeast india and the lambert graben in east antarctica and are closely associated with the gondwana breakup in the india antarctica sector the rifting stages are closely correlated with the sedimentation phases of the lower and upper gondwana deposits in the mahanadi delta which evolved parallel to the lambert graben in east antarctica

testing the geomagnetic dipole hypothesis palaeolatitudes sampled by large continents summary an analysis is presented of the way in which a continental spherical cap samples geographic latitude bands as it drifts over the earth s surface this is necessary to establish the likely success of the random palaeogeographic test of the geocentric axial dipole hypothesis and to gauge how much geological time is required it appears that a few hundred million years may suffice