magnetism on the angrite parent body and the early differentiation of planetesimals angrites are among the oldest known pristine basaltic meteorites and record the earliest stages of planet formation and differentiation our paleomagnetic analysis of three angrites found that they record a past magnetic field of 10 microteslas on the angrite parent body extending from 4564 to at least 4558 million years ago because the angrite paleomagnetic record extends beyond the expected lifetime of the early circumstellar disk these paleofields were probably generated internally on the angrite parent body possibly by an early dynamo in a rapidly formed metallic core

accelerated extension of tibet linked to the northward underthrusting of indian crust the tibetan plateau is extending numerical simulations suggest that regional scale extension is caused by gravitational collapse of the plateau whereas rapid extension in the south is caused by underthrusting of the indian slab

present day deformation of northern pakistan from salt ranges to karakorum ranges episodic gps measurements are used to quantify the present day velocity field in the northwestern himalaya from the southern pamir to the himalayan foreland we report large postseismic displacements following the 2005 kashmir earthquake and several mm yr thrusting of the central segment of the salt ranges and potwar plateau over the foreland westward thrusting of nanga parbat above the kohistan plateau and 12 mm yr sse velocities of the karakorum ranges and of the deosai and kohistan plateaus relative to the indian plate numerical simulations allow to determine a first approximation of slip along active faults 1 substantial creep of 87 mm yr between 2006 and 2012 along the flat northeast of the balakot bagh thrust affected by the 2005 earthquake 2 5 mm yr slip of the central segment of the salt ranges and potwar plateau whereas their western boundaries are clearly inactive over the time span covered by our measurements 3 13 mm yr ductile slip along the main himalayan thrust modeled by a dislocation dipping 7 northward locked at a depth of 15 km and 4 20 mm yr slip along the shear zone forming the western boundary of nanga parbat between depths of 1 6 and 6 5 km residuals velocities suggest the existence of left lateral strike slip along the jhelum fault

the paleomagnetism of single silicate crystals recording geomagnetic field strength during mixed polarity intervals superchrons and inner core growth 1 the basic features of the geomagnetic reversal chronology of the last 160 million years are well established the relationship between this history and other features of the field however has been elusive the determination of past field strength paleointensity is especially challenging commonly accepted results have come from analyses of bulk samples of lava historic lavas have been shown to faithfully record the past field strength when analyzed using the thellier double heating method data from older lavas however tend to show effects of in situ and laboratory induced alteration here we review an alternative approach single plagioclase crystals can contain minute magnetic inclusions 50 350 nm in size that are potential high fidelity field recorders thellier experiments using plagioclase feldspars from an historic lava on hawaii provide a benchmark for the method rock magnetic data from older lavas indicate that the feldspars are less susceptible to experimental alteration than bulk samples this resistance is likely related to the lack of clays in addition magnetic minerals are sheltered by the encasing silicate matrix from natural alteration that can otherwise transform the well defined thermoremanent magnetization into an irresolute chemical remanent magnetization if there is a relationship between geomagnetic reversal frequency and paleointensity it should be best expressed during superchrons intervals with few or no reversals thellier data sets based on single plagioclase crystals from lavas erupted during the cretaceous normal polarity superchron 83 120 million years ago suggest a strong 12 1022 am2 stable field consistent with an inverse relationship between reversal frequency and paleointensity superchrons may represent times when the pattern of core mantle boundary heat flux allows the geodynamo to operate at peak efficiency as suggested in some numerical models thellier data from single plagioclase crystals formed during times of moderate 10 reversals million years reversal occurrence suggest a weaker and more variable field these paleointensity data together with a consideration of paleomagnetic directions suggest that geomagnetic reversals field morphology secular variation and intensity are related the linkages over tens of millions of years imply a lower mantle control on the geodynamo on even longer timescales the magnetization held by plagioclase and other silicate crystals can be used to investigate the proterozoic and archean geomagnetic field during the onset of growth of the solid inner core data from plagioclase crystals separated from mafic dikes together with directional data from whole rocks indicate a dipole dominated field similar to that of the modern 2 5 2 7 billion years ago older archean rocks are of great interest for paleomagnetic and paleointensity investigations because they may record a time when the compositionally driven convection of the modern dynamo may not have been operating and a solid inner core did not play its current role in controlling the geometry of outer core flow most rocks of this age have been affected by low grade metamorphism investigations using single silicate grains provide arguably our best hope of seeing through secondary geologic events and reading the early history of the geodynamo absolute paleointensity measurements of the oldest rocks on the planet will require the further development of methods to investigate silicate crystals with exsolved magnetic minerals that address the uncertainties posed by thermocrystallization remanent magnetization anisotropy and slow cooling fortunately prior work in rock magnetism together with advances in analytical equipment and techniques provides a solid foundation from which these frontier issues can be approached

electrical and magnetometric fields in a layered earth containing buried electrodes the number of analytical magnetometric resistivity mmr results available for basic earth geometries is limited compared to that of electrical resistivity methods which is unfortunate since mmr has advantages for certain classes of problems this paper extends the list of mmr results by deriving the response for the homogeneous half space and the multilayered earth both are calculated for arbitrary source and receiver positions we show how the result for the layered earth reduces to that of the half space when there is only one layer necessary procedures for successful numerical evaluation of results for the layered case are given sample borehole logs of electric and magnetic fields for an example of a two layer model illustrate an advantage of the magnetic field measurement namely that it is sensitive to the position of layer boundaries rather than to the position of transmitter electrodes the algorithm is also applied to interpretation of mmr field data from two boreholes drilled near massive s

a whole rock absolute paleointensity determination of dacites from the duffer formation ca 3 467 ga of the pilbara craton australia an impossible task abstract we have conducted a whole rock type magnetic and absolute paleointensity determination of the red dacite of the duffer formation from the pilbara craton australia the age of the dated rock unit is 3467 5 ma 95 confidence vector analyses results of the step wise alternating field demagnetization nrm up to 100 mt and thermal demagnetization from nrm up to 650 c yield three components of magnetization curie point determinations indicate three characteristic temperatures one at 150 200 c a second one at 450 c and a third one at 580 c magnetic grain size experiments were performed on small specimens with a variable field translation balance vftb the coercivity of remanence hcr suggests that the nrm is carried by low coercivity grains that are associated with a magnetite fraction as is shown by the high temperature component with blocking temperatures above 450 c and up to at least 580 c the ratios of the hysteresis parameters plotted as a modified day diagram show that most grain sizes are scattered within the single domain sd and the superparamagnetic and single domain sp sd domain ranges in addition to the rock magnetic experiments we have performed absolute paleointensity experiments on the samples using the modified thellier coe double heating method to determine the paleointensities partial trm p trm checks were performed systematically to document magnetomineralogical changes during heating the temperature was incremented by steps of 50 c between room temperature and 590 c the paleointensity determinations were obtained from the slope of arai diagrams our paleointensity results indicate that the paleofield obtained was 6 4 0 68 n 11 micro teslas with a virtual dipole moment vdm of 1 51 0 81 1022 am2 from a medium to high temperature component ranging from 300 to 590 c that has been interpreted to be the oldest magnetization yet recorded in paleomagnetic studies of the duffer formation the absolute paleointensity is relatively low and we interpret this low paleofield bias a result of a thermochemical remanent magnetization tcrm process that indicates a possible underestimate of the paleofield by a factor of four for the red dacite of the duffer fm

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

first results of the moses experiment sea sediment conductivity and thickness determination bute inlet british columbia by magnetometric offshore electrical sounding a static electrical method has been developed to determine the electrical resistivity of crustal rock beneath the sea the transmitter is a vertical long wire bipole extending from the sea surface to the sea floor a commutated current generated on the ship is fed to two large electrodes one near the sea surface the other at the end of a long insulated wire the return path for the current is through the sea and the subjacent crust the receiver is a self contained remote microprocessor controlled magnetometer which is deployed from the ship to the sea floor and subsequently recovered the data are measurements of the azimuthal component of the magnetic field as a function of transmitter receiver horizontal separation the acronym moses has been coined for the method the choice of the name moses is appropriate because the system geometry is carefully arranged to remove many of the adverse effects of the relatively conductive sea water in particular accurate estimates of sea floor resistivity ar

magnetic properties and opaque mineralogy of drilled submarine intrusive rocks summary measurements of natural remanent magnetization nrm curie temperature ferrimagnetic and paramagnetic susceptibility saturation induced and remanent magnetizations coercive forces alternating field properties and viscous magnetization are reported for 50 submarine intrusive rocks drilled during legs 30 37 and 45 of dsdp the collection includes doleritic sill rocks fresh and serpentinized cumulate gabbros and serpentinized cumulate peridotites and serpentinized lherzolites believed to have originated in layers 2b 3b and 4 respectively magnetite with a curie temperature between 520 and 580 c is the principal magnetic mineral in all samples there is no indication of maghemitization or of metamorphism to greenschist facies or above the magnetite in the doleritic and cumulate gabbros is a product of deuteric alteration of titanomagnetite and pyroxene the stable nrm is a primary trm the magnetite in the serpentinized rocks is a secondary product of serpentinization the stable nrm is a crm in most rock types the magnetite is of single domain or pseudo single domain size and soft components of nrm are small the magnetite grain size in some of the doleritic gabbros is much coarser these rocks acquire large viscous magnetizations which however are readily removed by alternating field cleaning the cleaned nrms of legs 30 and 45 rocks have approximately dipole inclinations but the nrms of most of the mutually intruded leg 37 units have been dispersed by tectonic rotations the doleritic gabbros and the serpentinized peridotites have stable directionally coherent nrms 10 3emu cm 3 in intensity their counterparts in layers 2b and 3b 4 are likely contributors to oceanic magnetic anomalies layer 3b cumulate gabbros and tectonic serpentinites in the middle and upper crust are less likely anomaly sources

strain and rotation rate from gps in tibet anatolia and the altiplano 1 deformation measured by regional gps networks in continental plateaus reflects the geologic and tectonic variability of the plateaus for two collisional plateaus tibet and anatolia and one noncollisional the altiplano we analyze the regional strain and rotation rate by inverting gps velocities to calculate the full two dimensional velocity gradient tensor to test the method we use gridded velocities determined from an elastic block model for the eastern mediterranean middle east region and show that to a first order the deformation calculated directly from the gps vectors provides an accurate description of regional deformation patterns principal shortening and extension rate axes vertical axis rotation and two dimensional 2 d volume strain dilatation are very consistent with long term geological features over large areas indicating that the gps velocity fields reflect processes responsible for the recent geologic evolution of the plateaus differences between geological and gps descriptions of deformation can be attributed either to gps networks that are too sparse to capture local interseismic deformation or to permanent deformation that accrues during strong earthquakes the altiplano has higher internal shortening magnitudes than the other two plateaus and negative 2 d dilatation everywhere vertical axis rotation changes sign across the topographic symmetry axis and is due to distributed deformation throughout the plateau in contrast the collisional plateaus have large regions of quasi rigid body rotation bounded by strike slip faults with the opposite rotation sense from the rotating blocks tibet and anatolia are the mirror images of each other both have regions of positive dilatation on the outboard sides of the rotating blocks positive dilatation in the aegean correlates with a region of crustal thinning whereas that in eastern tibet and yunnan province in china is associated with an area of vertical uplift rollback of the hellenic trench clearly facilitates the rotation of anatolia rollback of the sumatra burma trench probably also enables rotation about the eastern syntaxis of tibet

evidence for a 3 45 billion year old magnetic remanence hints of an ancient geodynamo from conglomerates of south africa 1 paleomagnetic and rock magnetic analyses of 3445 million year old dacite conglomerate clasts and parent body rocks from the barberton greenstone belt south africa define two dominant components of magnetization one component unblocked at low temperature is an overprint acquired 180 million years ago the other component is unblocked at high temperatures and passes a conglomerate test indicating that this component is older than the depositional age of the conglomerate 3416 ma the high unblocking temperature component shows scatter in the parent body rocks that can be explained by the effects of modern lightning strikes archean overprinting and the presence of multidomain magnetic grains that are conducive to carrying secondary magnetizations alternatively this scatter can be explained by exotic magnetization scenarios in the absence of a dynamo including magnetization by an external field related to solar wind interaction with the atmosphere such exotic mechanisms can be tested with the acquisition of paleointensity data while more scattered than paleomagnetic data recording the more recent geomagnetic field the high unblocking temperature component in the dacite parent body shows some consistency and the simplest explanation of the data is that they reflect a geodynamo 3445 million years ago

preliminary palaeomagnetic results of an archaean dolerite dyke of west greenland geomagnetic field intensity at 2 8 ga summary the geomagnetic field intensity during archaean times is evaluated from a palaeomagnetic and chronological study of a dolerite dyke intruded into the 3000 ma nuuk gneisses at nuuk 64 2 n 51 7 w west greenland plagioclase from the dolerite dyke yields a mean k ar age of 2752 ma palaeomagnetic directions after thermal demagnetization of the dyke and the gneiss reveal a positive baked contact test indicating that the high temperature component magnetization of the dyke is primary thellier experiments on 12 dyke specimens yield a palaeointensity value of 13 5 4 4 pt the virtual dipole moment at ca 2 8 ga is 1 9 0 6 x 10 am2 which is about onequarter of the present value the present study and other available data imply that the earth s magnetic field at 2 7 2 8 ga was characterized by a weak dipole moment and that a fairly strong geomagnetic field similar to the present intensity followed the weak field after ca 2 6 ga

paleointensity of the early geodynamo 2 45 ga as recorded in karelia a single crystal approach we report thellier paleointensity results from single plagioclase crystals derived from ca 2 45 ga border dikes of the burakovka layered intrusion karelia russia rock magnetic and transmission electron microscope analyses indicate that these plagioclase crystals contain pseudosingle to single domain magnetite inclusions fifteen determinations from four dikes meet reliability criteria and yield an average paleointensity of 43 2 10 8 t which corresponds to a virtual dipole moment of 8 43 2 11 x 10 2 2 am 2 although our result is unlikely to adequately represent the time averaged field the mean and range of values are similar to those of the present day field these values suggest that the inner core which may stabilize the geodynamo had started to grow by early proterozoic time

time variations in geomagnetic intensity 1 after many years spent by paleomagnetists studying the directional behavior of the earth s magnetic field at all possible timescales detailed measurements of field intensity are now needed to document the variations of the entire vector and to analyze the time evolution of the field components a significant step has been achieved by combining intensity records derived from archeological materials and from lava flows in order to extract the global field changes over the past 12 kyr a second significant step was due to the emergence of coherent records of relative paleointensity using the remanent magnetization of sediments to retrace the evolution of the dipole field a third step was the juxtaposition of these signals with those derived from cosmogenic isotopes contemporaneous with the acquisition of records new techniques have been developed to constrain the geomagnetic origin of the signals much activity has also been devoted to improving the quality of determinations of absolute paleointensity from volcanic rocks with new materials proper selection of samples and investigations of complex changes in magnetization during laboratory experiments altogether these developments brought us from a situation where the field changes were restricted to the past 40 kyr to the emergence of a coherent picture of the changes in the geomagnetic dipole moment for at least the past 1 myr on longer timescales the field variability and its average behavior is relatively well documented for the past 400 myr section 3 gives a summary of most methods and techniques that are presently used to track the field intensity changes in the past in each case current limits and potential promises are discussed the section 4 describes the field variations measured so far over various timescales covered by the archeomagnetic and the paleomagnetic records preference has always been given to composite records and databases in order to extract and discuss major and global geomagnetic features special attention has been devoted to discussing the degree of confidence to be put in the data by considering the integration of multiple data sets involving different techniques and or materials

present day crustal thinning in the southern and northern tibetan plateau revealed by gps measurements gps measurements from sites within the tibetan plateau show not only east southeast west northwest extension but also more importantly horizontal dilation throughout the interior of the plateau assuming conservation of volume vertical thinning strain rates equal horizontal dilation rates and they 8 9 0 8 nanostrain a 1 and 7 4 1 2 nanostrain a 1 in northern and southern tibet and 12 0 3 2 nanostrain a 1 in its southwestern part suggest no measureable difference principal extensional strain rates also are similar in magnitude and orientation if crustal thinning began at 10 15 ma and the current rates of horizontal dilation applied both to the entire crust and to that period the crust should have thinned by 5 5 8 5 km if isostatic equilibrium applied the mean elevation of the plateau would have dropped 1 km the similar rates for northern southern and southwestern tibet suggest that the processes dictating crustal extension normal faulting and crustal thinning in the three regions differ little from one another

basin formation in the high himalaya by arc parallel extension and tectonic damming zhada basin southwestern tibet 1 the late miocene pleistocene zhada basin in southwestern tibet provides a record of subsidence and basin formation within an active collisional thrust belt the 800 m thick basin fill is undeformed and was deposited along an angular unconformity on top of tethyan strata that were previously shortened in the himalayan fold thrust belt modal sandstone petrographic data conglomerate clast count data and detrital zircon u pb age spectra indicate a transition from detritus dominated by a distal northern source to a local southern source this transition was accompanied by a change in paleocurrent directions from uniformly northwestward to basin centric at the same time the depositional environment in the zhada basin changed from a large braided river to a closed basin lake sedimentation in the zhada basin was synchronous with displacement on the qusum and gurla mandhata detachment faults which root beneath the basin and exhume midcrustal rocks along the northwestern and southeastern flanks of the basin respectively these observations indicate that accommodation for zhada basin fill was produced by a combination of tectonic subsidence and damming as midcrustal rocks were evacuated from beneath the zhada basin in response to arc parallel slip on crustal scale detachment faults

palaeomagnetism a more ancient shield earth s magnetic field has protected our atmosphere from erosion by the solar wind ever since it started up silicate crystals from some of earth s oldest rocks date that event to more than 3 billion years ago

clockwise rotation of the brahmaputra valley relative to india tectonic convergence in the eastern himalaya naga hills and shillong plateau gps data reveal that the brahmaputra valley has broken from the indian plate and rotates clockwise relative to india about a point a few hundred kilometers west of the shillong plateau the gps velocity vectors define two distinct blocks separated by the kopili fault upon which 2 3 mm yr of dextral slip is observed the shillong block between longitudes 89 and 93 e rotating clockwise at 1 15 myr and the assam block from 93 5 e to 97 e rotating at 1 13 myr these two blocks are more than 120 km wide in a north south sense but they extend locally a similar distance beneath the himalaya and tibet a result of these rotations is that convergence across the himalaya east of sikkim decreases in velocity eastward from 18 to 12 mm yr and convergence between the shillong plateau and bangladesh across the dauki fault increases from 3 mm yr in the west to 8 mm yr in the east this fast convergence rate is inconsistent with inferred geological uplift rates on the plateau if a 45 n dip is assumed for the dauki fault unless clockwise rotation of the shillong block has increased substantially in the past 4 8 myr such acceleration is consistent with the reported recent slowing in the convergence rate across the bhutan himalaya the current slip potential near bhutan based on present day convergence rates and assuming no great earthquake since 1713 a d is now 5 4 m similar to the slip reported from alluvial terraces that offsets across the main himalayan thrust and sufficient to sustain a mw 8 0 earthquake in this area

geomagnetic field strength 3 2 billion years ago recorded by single silicate crystals analysis of silicate rocks from the archaean kaapvaal craton is used to obtain palaeodirections and intensities of the earth s magnetic field some 3 2 billion years ago and finds that the field strength was within 50 per cent of the present day value indicating that a viable magnetosphere sheltered the early earth s atmosphere from solar wind erosion

a comparison of receiver technologies in borehole mmr and em surveys a series of downhole magnetometric resistivity dhmmr and downhole electromagnetic dhem surveys were conducted near broken hill new south wales australia and at zinkgruvan sweden to determine how probe and receiver equipment choices affect the amount of noise visible in borehole mmr and em data noise analyses performed on the data using the standard deviation to gauge the relative noise levels between different probes and receiver systems indicate that high noise levels in mmr data result primarily from the use of a three component em probe which has a reduced effective area and hence a higher noise floor compared with a single component em probe high noise attributable to cultural sources such as nearby power lines in either mmr or em data can be reduced through the use of full waveform multipurpose receiver systems these systems allow for the use of tapered stacking which is a more effective method of eliminating coherent noise associated with power line transients than the boxcar stacking method used by traditional receiver systems

optimal interpolation of spatially discretized geodetic data abstract we present an algorithm to calculate horizontal strains or strain rates through interpolation of geodetically derived displacements or velocities this is an underdetermined inverse problem to derive smoothly distributed strains or strain rates using spatially discretized geodetic observations a priori information in the form of weighted smoothing is critical to facilitate the solution at a given site the horizontal displacement or velocity field in its vicinity is approximated by a bilinear function and represented by rigid block translation rotation and strains or their rates the weighted displacement or velocity data in the neighborhood are used to estimate the field parameters through a least squares inversion procedure optimal weightings are prescribed for the neighboring data based on their distances to the interpolation site and their spatial coverage nonelastic strains resulted from surface fault rupture and creep may also be excluded from the solution we apply this method to the southern california earthquake center crustal motion map version 4 0 velocity field and derive the strain rate field in southern california our result shows that 1 distance dependent weighting can be optimally achieved by employing either a gaussian or quadratic decay function with the former offering a slightly sharper result than the latter 2 spatially dependent weighting is important to improve the interpolation and can be done by invoking either an azimuthal weighting or a voronoi cell areal weighting function 3 the strain rate pattern in southern california is dominated by dextral shear of the san andreas fault saf system and the secondary faults surrounding the big bend of the saf strike at oblique angles with respect to the maximum shear direction suggesting that tectonic deformation field on and off the saf is dominated by mechanic processes of the saf

thermochemical remanent magnetization in precambrian rocks are we sure the geomagnetic field was weak 1 thellier paleointensity determinations from two dikes of the early proterozoic 2 46 ga matachewan dike swarm canada yield field values of 2 14 0 18 and 9 81 1 65 t corresponding values of virtual axial dipole moment are very low 0 54 0 05 1022 and 2 49 0 42 1022 am2 respectively when compared with the modern field the characteristic remanent magnetization chrm was isolated over a narrow range of high unblocking temperatures 520 580 c detailed rock magnetic analyses indicate that the chrm is carried by nearly stoichiometric pseudo single domain magnetite scanning electron microscopy sem reveals that the magnetite is in the form of fine intergrowths with ilmenite formed by oxyexsolution during cooling the high temperature oxidation defined in the sem images could have continued at temperatures below the curie point of magnetite in this case the chrm would be a thermochemical remanent magnetization tcrm rather than a thermal remanent magnetization trm estimates of the tcrm trm ratio show that the thellier data could underestimate the true field value by a factor of 4 without violating experimental selection criteria this uncertainty in trm fidelity translates into a potential range of field values that spans that defined by the modern field 8 1022 am2 and proposed low precambrian levels 2 1022 am2 therefore understanding further how tcrm is acquired and records the field represents a major challenge if these and many other similar rocks are to be used in thellier studies aimed at defining the strength of the precambrian field

middle to late miocene extremely rapid exhumation and thermal reequilibration in the kung co rift southern tibet 1 the kung co rift is an approximately nnw striking wsw dipping normal fault exposed in southern tibet and is part of an extensive network of active approximately ns striking normal faults exposed across the tibetan plateau detailed new and published u th he zircon and apatite thermochronometric data from the footwall of the early miocene kung co granite provide constraints on the middle miocene to present day exhumation history of the footwall to the kung co fault inverse modeling of thermochronometric data yield age patterns that are interpreted as indicating 1 initiation of normal fault slip at 12 13 ma and rapid exhumation of the footwall between 13 and 10 ma 2 acceleration of normal fault slip at rates of 21 9 6 9 mm yr at 10 ma 3 rapid thermal reequilibration between 10 and 9 ma and 4 slow exhumation and or quiescence from 9 ma to the present day hanging glacial valleys in the footwall and fault scarps that cut late quaternary till and moraine deposits indicate that fault slip continues today middle to late miocene initiation of extension across the kung co rift is broadly the same as the documented initiation of ew extension across the south central tibetan plateau eastward flow of middle or lower crust from beneath tibet accommodated by northward underthrusting of indian crust beneath tibet provides a plausible explanation for the onset of ew extension across the tibetan plateau

intensity of the earth s magnetic field in late archean obtained from diabase dikes of the slave province canada abstract the geomagnetic field intensity in late archean times is evaluated from archean diabase dikes intruded into the yellowknife greenstone belt of the slave province n w t canada the dike set used is referred to as the 8a dikes and their age is bracketed between 2620 and 2642 ma by crosscutting relationships between the 8a dikes and adjacent granitic rocks dated by u pb method paleomagnetic directional data after thermal demagnetization thd and results of some field tests suggest that the characteristic remanent magnetizations chrm of the dikes have been acquired at ca 2 6 ga several rock magnetic studies indicate that the dike samples are suitable for paleointensity determinations thelliers method with consistency checks was used for the paleointensity determinations and two of the 8a dikes yielded the mean paleointensities of 30 9 1 1 and 43 9 1 2 t respectively the corresponding virtual dipole moments vdm of 6 3 0 2 1022 a m2 and 9 0 0 2 1022 a m2 are similar to the present day value suggesting that in the earth s core the dynamo process of comparable activity to that of the present day has already existed at ca 2 6 ga

an intermediate depth earthquake beneath tibet source characteristics of the event of september 14 1976 this paper presents the results of a detailed study of an intermediate depth earthquake beneath southern tibet september 14 1976 0643 51 6 ut 29 8l n 89 57 e focal depth 90 10 km mb 5 4 from the differences in arrival times of p pp and sp and from rayleigh wave spectral densities the focus is estimated to be at depth of 90 10 km and therefore in the uppermost mantle from the polarities of the first motions of p waves inversion of the relative amplitudes of p pp and sp and synthesis of their waveforms the source mechanism is constrained to be predominantly normal faulting with the t axis approximately horizontal and trending east west this mechanism is similar to those of shallow earthquakes in tibet the occurrence of an earthquake at depth of 90 km is consistent with rapid deformation at a temperature of about 700 to 800 c in the mantle beneath the tibetan crust the seismic moment of an equivalent double couple point source is estimated to be about 9 1024 dyne cm with an uncertainty factor of 1 3 from the sp amplitudes in the period range near 10 s and 5 1024 dyne cm also with an uncertainty factor of 1 3 from the spectral density of rayleigh waves with periods of 50 s

nailing down the slip rate of the altyn tagh fault 1 previous estimates of the geodetic and geologic slip rates of the 1500 km long altyn tagh fault bordering the northern edge of the tibetan plateau vary by a factor of five proposed reasons for these discrepancies include poor gps geometry interpretative errors in terrace morphology and changes in fault slip rate over time here we present results from a new dense gps array orthogonal to the fault at 86 2 e that indicates a velocity of 9 0 3 2 4 4 mm yr in close agreement with geomorphologic estimates at the same location our estimated geodetic slip rate is consistent with recent geological slip rates based on terrace offsets the resulting mean combined geological and geodetic slip rate 9 0 4 0 mm yr is remarkably uniform for the central 800 km of the altyn tagh fault significantly lower than early kinematic estimates and consistent with deformation elsewhere in tibet and central asia

the intensity of the geomagnetic field at 3 5 ga paleointensity results from the komati formation barberton mountain land south africa abstract paleointensity measurements have been carried out on 3 5 ga samples from the komati formation type locality using both the thellier and van zijl methods these samples contain a single steeply directed negative trm component acquired during metamorphism of the komati lavas thellier experiments yielded values ranging from 12 to 37 t but an average paleofield intensity for the four best determinations is 20 3 t a slightly lower average paleointensity of 15 3 t was obtained using van zijl experiments preheating was used to chemically stabilize seven samples used in van zijl determinations and these produced nearly ideal plots with an average paleointensity of 13 2 t a single basaltic komatiite sample gave a nearly ideal van zijl plot indicating about 21 t nearly the same paleointensity as the peridotitic komatiite samples even though its nrm intensity was several orders of magnitude lower since the komati characteristic remanence was acquired during a slow cooling the data must be reduced by a factor of 1 55 to account for the difference between laboratory and natural cooling rates calculation of an equivalent equatorial paleointensity using the paleolatitude implied by the steep komati characteristic remanence then gives value of 5 t for the intensity of the geomagnetic field at 3 5 ga lower than the present value of about 30 t

static stress interactions in extensional earthquake sequences an example from the south lunggar rift tibet 1 an extensional earthquake sequence occurred in 2004 8 across a graben in the south lunggar rift on the tibetan plateau we use insar data to determine the location fault geometry and slip distribution of these earthquakes and to test whether the sequence is compatible with static stress triggering the mw 6 2 and 6 3 earthquakes in 2004 and 2005 both ruptured west dipping faults on the east side of a graben in 2008 a mw 6 7 earthquake ruptured a pair of east dipping fault segments on the other side of the graben offset from the earlier ruptures we compute first order dislocation models of stress change and demonstrate that the order and spatial configuration of this sequence of events is compatible with triggering by static stress transfer a continuation of the sequence would be most likely to occur on the northern extension of the 2008 rupture although variable slip rate along the rift may mean that the sequence has run its course the insar data for the 2008 earthquake also reveal slip on a fault that cuts the graben at a highly oblique angle we suggest that this is a release fault accommodating differential throw in the hanging wall and associate the deformation with a mw 6 0 aftershock activity on such a release fault has not been directly imaged before the zhongba sequence is one of several examples of recent clustered normal fault earthquakes on the plateau and may be an example of phase locking of similar faults

gps velocity field for the tien shan and surrounding regions 1 measurements at 400 campaign style gps points and another 14 continuously recording stations in central asia define variations in their velocities both along and across the kyrgyz and neighboring parts of tien shan they show that at the longitude of kyrgyzstan the tarim basin converges with eurasia at 20 2 mm yr nearly two thirds of the total convergence rate between india and eurasia at this longitude this high rate suggests that the tien shan has grown into a major mountain range only late in the evolution of the india eurasia collision most of the convergence between tarim and eurasia within the upper crust of the tien shan presumably occurs by slip on faults on the edges of and within the belt but 1 3 mm yr of convergence is absorbed farther north at the dzungarian alatau and at a lower rate with the kazakh platform to the west the tarim basin is thrust beneath the tien shan at 4 7 mm yr with respect to eurasia the ferghana valley rotates counterclockwise at 0 7 myr 1 about an axis at the southwest end of the valley thus gps data place a bound of 4 mm yr on the rate of crustal shortening across the chatkal and neighboring ranges on the northwest margin of the ferghana valley and they limit the present day slip rate on the right lateral talas ferghana fault to less than 2 mm yr gps measurements corroborate geologic evidence indicating that the northern margin of the pamir overthrusts the alay valley and require a rate of at least 10 and possibly 15 mm yr

extension on the tibetan plateau recent normal faulting measured by insar and body wave seismology we use insar and body wave modelling to determine the faulting parameters for a series of five mw 5 9 7 1 normal faulting earthquakes that occurred during 2008 including the march 20 yutian earthquake mw 7 1 one of the largest normal faulting events to have occurred recently on the continents we also study three earlier normal faulting earthquakes that occurred in southern tibet between 1992 and 2005 coseismic deformation for each of these eight events is measured with ascending and descending interferograms from envisat ers and alos sar data elastic dislocation modelling of the line of sight insar displacements and body wave seismological modelling of p and sh waves are used to estimate fault parameters and are found to be in good agreement for all the events studied the use of insar to measure deformation allows a relatively precise determination of the fault location in addition to resolving the focal plane ambiguity only five of the eight events are associated with a clear surface topographic feature suggesting that an underestimation of the amount of extension would result from using the surface expressions of normal faulting alone the observations in all cases are consistent with slip on planar surfaces with dips in the range 40 50 that penetrates the uppermost crust to a depth of 10 15 km we find no evidence for active low angle dip less than 30 normal faulting the contribution of the normal faulting to overall extension estimated by summing seismic moments over earthquakes for the past 43 yr is 3 4 mm yr 1 or 15 20 per cent of the rates of extension measured across the plateau using gps 85 per cent of the moment release in normal faulting over the past 43 yr has occurred in regions whose surface height exceeds 5 km this observation adds weight to the suggestions that the widespread normal faulting on the plateau is the result of variations in the gravitational potential energy of the lithosphere

cross hole magnetometric resistivity mmr the last decade has seen a growing acceptance of the magnetometric resistivity mmr method as a viable exploration technique in various geologic environments until recently mmr exploration was carried out with both current electrodes and recording magnetometer located on the surface of the earth significant improvements in anomaly amplitude can be achieved by lowering the recording magnetometer inside a drill hole in contrast the lowering of current electrodes beneath the surface does not always improve surface mmr responses the advantages of locating the magnetic detector in a drill hole are illustrated numerically anomaly calculations being carried out with a novel yet simple integral equation technique for a plate like body the practicality of the cross hole mmr technique is demonstrated with a successful case history massive sulfide mineralization is mapped at a depth exceeding 500 m

geophysical mapping by stationary electric and magnetic field components a combination of potential gradient mapping and magnetometric resistivity mmr methods a combined application of two independent direct current geoelectric methods potential gradient mapping pm and magnetometric resistivity mmr is proposed for the investigation of high resistivity basement structures from a one dimensional relation between horizontal electric and magnetic fields due to currents originating from point electrodes placed on the surface of a two layered half space having a high resistivity basement several advantages of the combined pm mmr method are shown for two dimensional and three dimensional basement structures three dimensional models have been studied by analogue modeling experiments this new mapping method embraces two parameters the horizontal electric conductance map derived from the ratio of the related electric and magnetic components and the gradient map of their product a close relation between structural forms and anomaly maps is obtained by using two orthogonal source electrode layouts based on model results a two channel measuring instrument was developed the method and the instrument were tested near sopron where a gradual deepening of and faults in the paleozoic basement were found

low paleointensities recorded in 1 to 2 4 ga proterozoic dykes superior province canada abstract paleointensity studies have been conducted on six mafic dyke swarms from the superior province in canada with u pb ages between 1141 ma and 2473 ma buchan et al tectonophysics 319 2000 167 198 the mean direction of the characteristic magnetization for each dyke swarm coincides with results of earlier studies in which the primary origin of the magnetization was established on the basis of contact and secular variation tests this primary component was isolated over a very narrow range of high unblocking temperatures usually 550 580 c indicating that it is carried by magnetite or low titanium titanomagnetite magnetic mineralogy and grain size experiments suggest that the magnetization is dominated by pseudo single domain or single domain grains taken together the characteristics of this component suggest that it is thermal in origin paleointensity experiments were conducted with a specially designed oven using a revised approach of the thellier coe method eighty five successful determinations of paleointensity were obtained for 20 sites from six dyke swarms doubling the number of existing well constrained precambrian data the paleofield estimates for the dyke swarms vary between 5 and 11 2 t yielding virtual dipole moments between 0 85 0 1 and 2 8 0 87 1022 am2 the overall convergence between these new results and the rest of the database strongly reinforces the existence of a weaker geomagnetic field during the 1000 2400 ma period than has been recorded between 0 3 and 400 ma however additional determinations are required particularly for the 400 2000 ma interval in order to establish whether a long term evolution in the time averaged field intensity can be linked to the onset of the growth of the earth s inner core

paleomagnetic constraints on the archean geomagnetic field intensity obtained from komatiites of the barberton and belingwe greenstone belts south africa and zimbabwe abstract the intensity of the geomagnetic field during archean is an important source of information about the evolution of the earth s core hale earth planet sci lett 86 1987b 354 reported a very low equatorial paleointensity of 5 t at ca 3 5 ga obtained from komatiites of the barberton greenstone belt south africa and regarded their remanences as thermal overprints due to a metamorphic event however this result has a major drawback since remanences of the barberton komatiites are carried by magnetites formed during serpentinization that typically results in acquisition of chemical remanent magnetization crm we report the results of detailed paleomagnetic rock magnetic and paleointensity investigations using komatiite samples from the barberton ca 3 5 ga and belingwe ca 2 7 ga greenstone belts in south africa and zimbabwe respectively paleodirectional data for barberton are consistent with results of previous work and the characteristic remanent magnetization chrm of the belingwe samples reveals a positive fold test suggesting that samples from both areas record the archean geomagnetic field the main carriers of chrm are ti free magnetites presumed to be akin to single domain grains and their rock magnetic properties are very stable when subjected to laboratory heating up to 600 700 c our microscopic observations provide support that these are secondary minerals formed during serpentinization thellier thellier paleointensity experiments for the barberton and belingwe samples yielded very low mean virtual dipole moment vdm estimates of 1 8 1 3 1022 a m2 and 1 1 0 9 1022 a m2 respectively these correspond to about 24 and 15 of the present day value considering the low metamorphic grade for the sampling areas however it seems to be somewhat difficult to regard the entire ranges of komatiite chrm as thermal overprints acquired by reheating during metamorphic events we hence propose as the most likely scenario the possibility that most fractions of chrm are survivors of primary grain growth crm taking the theoretical and experimental estimates of the ratio of crm to thermoremanent magnetization trm into account this suggests that these low mean vdm values can provide constraints on the lower limits of the geomagnetic field intensities in both greenstone belts at ca 3 5 and 2 7 ga

evidence for an early archean geomagnetic field a paleomagnetic study of the komati formation barberton greenstone belt south africa 3 5 ga old peridotitic komatiites of the komati formation barberton greenstone belt south africa have a characteristic nrm natural remanent magnetization that is stable during alternating field and thermal demagnetization the thermal nature of this remanence is demonstrated by successful thellier type paleo intensity determinations on some samples in the temperature range 350 to 575 c the characteristic nrm is an almost total thermal overprint acquired during unroofing and cooling following pervasive greenschist metamorphism at 3470 20 ma the mean direction of magnetization after cleaning is d 326 i 81 k 60 95 8 n 7 sites with a paleopole at 11 s 21 e p m 15 the paleopole falls in southern africa ancestral southern africa was therefore in polar latitudes around 3 5 ga the same position it occupied in the late archean and early proterozoic paleofield intensities are about one half present day intensities for polar latitudes a vigorous earth s magnetic field was already in existence in the early archean implying that the core had a suitable size composition and convective flow to support dynamo action

crustal radial anisotropy across eastern tibet and the western yangtze craton 1 phase velocities across eastern tibet and surrounding regions are mapped using rayleigh 8 65 s and love 8 44 s wave ambient noise tomography based on data from more than 400 program for array seismic studies of the continental lithosphere and chinese earthquake array stations a bayesian monte carlo inversion method is applied to generate 3 d distributions of vsh and vsv in the crust and uppermost mantle from which radial anisotropy and isotropic vs are estimated each distribution is summarized with a mean and standard deviation but is also used to identify highly probable structural attributes which include 1 positive midcrustal radial anisotropy vsh vsv across eastern tibet spatial average 4 8 1 4 that terminates abruptly near the border of the high plateau 2 weaker 1 0 1 4 negative radial anisotropy vsh vsv in the shallow crust mostly in the songpan ganzi terrane 3 negative midcrustal anisotropy 2 8 0 9 in the longmenshan region 4 positive midcrustal radial anisotropy 5 4 1 4 beneath the sichuan basin and 5 low vs in the middle crust 3 427 0 050 km s of eastern tibet midcrustal vs 3 4 km s perhaps consistent with partial melt is highly probable only for three distinct regions the northern songpan ganzi the northern chuandian and part of the qiangtang terranes midcrustal anisotropy provides evidence for sheet silicates micas aligned by deformation with a shallowly dipping foliation plane beneath tibet and the sichuan basin and a steeply dipping or subvertical foliation plane in the longmenshan region near vertical cracks or faults are believed to cause the negative anisotropy in the shallow crust underlying tibet

normal faulting in central tibet since at least 13 5 myr ago tectonic models for the evolution of the tibetan plateau interpret observed east west thinning of the upper crust to be the result of either increased potential energy of elevated crust1 or geodynamic processes that may be unrelated to plateau formation2 3 4 5 6 a key piece of information needed to evaluate these models is the timing of deformation within the plateau the onset of normal faulting has been estimated to have commenced in southern tibet between about 14 myr ago7 and about 8 myr ago8 and in central tibet about 4 myr ago9 here however we report a minimum age of approximately 13 5 myr for the onset of graben formation in central tibet based on mineralization ages determined with rb sr and 40ar 39ar data that post date a major graben bounding normal fault these data along with evidence for prolonged activity of normal faulting in this and other tibetan grabens support models that relate normal faulting to processes occurring beneath the plateau thinning of the upper crust is most plausibly the result of potential energy increases resulting from spatially and temporally heterogeneous changes in thermal structure and density distribution within the crust and upper mantle beneath tibet this is supported by recent geophysical and geological data10 11 12 13 14 15 16 17 which indicate that spatial heterogeneity exists in both the tibetan crust and lithospheric mantle

nyainqentanglha shear zone a late miocene extensional detachment in the southern tibetan plateau a major low angle ductile shear zone containing s c mylonites involves metamorphosed granitic rocks at the southeastern edge of the nyainqentanglha mountain range southern tibet prominent triangular facet geomorphology is developed by valley erosion of the detachment surface defined by the top of the southeast dipping mylonitec shear zone kinematic criteria consistently indicate a top to southeast sense of shear the ductile shearing deformation is inferred from isotopic cooling ages to have occurred during the interval 11 5 ma late miocene we interpret the shear zone as a regional extensional detachment its development indicates when the extensional tectonics in this area started which in turn may mark the time when the maximum sustainable surface elevation and perhaps crustal thickness was reached in southern tibet

paleomagnetism of late archean rocks of hamersley basin western australia and the paleointensity at early proterozoic we report the results of paleomagnetic and rock magnetic measurements of late archean rocks from hamersley basin western australia and the paleointensity determination for early proterozoic basalts and banded iron formations have two to four components of remanent magnetization and have consistent directions for different localities indicative of geomagnetic origin rock magnetic measurements of basalts reveal that the main magnetic mineral is a fine grained magnetite present in 10 ppm in mass which do not alter when heated up to its blocking temperature we interpret the stable component up to 390 c as the post tilting thermal overprint from uplift at 2 0 ga and the higher temperature component as the pre tilting thermoviscous remanence during burial metamorphism from the thellier type paleointensity experiments using the thermal overprint component we obtain a mean virtual dipole moment vdm estimate of 1 8 3 6 10 22 am 2 this suggests that early proterozoic was characterized by a 1 weak geomagnetic field of less than one half of the present 2001 elsevier

oldest reliable 40ar 39ar ages for terrestrial rocks barberton mountain komatiites we report here the first 40ar 39ar ages for komatiites and komatiitic basalts from the barberton mountain greenstone belt on the transvaal swaziland border in southern africa1 both rock types display remarkable argon retentivity while some variation is found among the samples the best argon age estimate for the time of metamorphism is in the 3 450 3 490 myr range this age is only slightly less than that found in komatiites from the same area by sm nd dating 3 540 15 myr 2 within the uncertainties of the decay constants involved the two age estimates are not different the results show that the principal pervasive greenschist metamorphism1 in the area must have occurred within 100 myr of the eruption of the komatiite analytically precise ages were obtained despite potassium contents as low as 80 p p m in the presence of high calcium concentrations 3 5 ca to our knowledge the results represent by far the oldest reliable ages obtained for terrestrial rocks using the k ar system