uncertainties in the relative positions of the australia antarctica lord howe and pacific plates since the late cretaceous we determined parameters that describe finite rotations and their uncertainty regions for relative plate motion at the spreading centers between the pacific and antarctica plates between australia and antarctica and between the lord howe rise and australia we combined these to yield a range of possible finite rotations describing the relative positions of the pacific australia antarctica and lord howe plates since the late cretaceous if the pacific australia plate boundary has had its present trend since anomaly 18 time reconstructions show 330 110 km of motion of the pacific plate relative to the lord howe rise since anomaly 5 time 9 8 m y 420 110 km since anomaly 6 time 19 5 m y 770 330 km since anomaly 13 time 35 6 m y and 820 260 km since anomaly 18 time 43 0 m y we examined two cases for times prior to anomaly 18 assuming a late cretaceous age of australia antarctica separation if a plate boundary existed between the lord howe rise and pacific plates since the late cretaceous with no plate boundary in antarctica reconstructions with the lord howe rise fixed predict 610 200 km of westward motion of the pacific plate between the times of anomalies 31 and 22 followed by 260 100 km of northward motion between the times of anomalies 22 and 18 if the lord howe rise was fixed to the pacific plate until the eocene but a plate boundary existed between east and west antarctica reconstructions show very little motion across this boundary between the times of anomalies 31 and 22 followed by convergence between the times of anomalies 22 and 18 this second case also brings 70 80 m y paleomagnetic poles from the pacific and east antarctica plates into better agreement than the first case but large uncertainties in the reconstructions do not allow the first case to be conclusively eliminated

comparison between the hot spot and geomagnetic field reference frames hot spots are located on the earth s surface and are caused by hot plumes of upwelling mantle material they manifest themselves by volcanic activity which changes its location in response to the movement of the lithosphere over the mantle plume it has been suggested1 3 that hot spots are relatively stationary with respect to each other and also that they represent a frame of reference with which to measure the absolute movement of the lithospheric plates over the surface of the earth another frame of reference can be partially defined by the position of the mean magnetic dipole field of the earth as the dipolar field is believed to be closely related to the spin axis this frame of reference automatically gives information about the palaeolatitude and orientation of the lithospheric plates but since the position of the ancient dipole field of the earth does not give any information about palaeolongitude and since in many cases the definition of the field is relatively poor due to less than adequate rock collections during the time of interest hot spot reference frames have been favoured by many people we now report a comparison of the two frames of reference and show that over the past 200 myr there has been relative movement between the two frames of reference of about 20 and that this motion has not been constant with time

late mesozoic and cenozoic palaeomagnetism of australia i a redetermined apparent polar wander path summary palaeomagnetic measurements have been carried out on one cretaceous and five cenozoic sedimentary sequences of australia of those one is in the carnarvon basin of north western australia and the others are in the otway basin of south eastern australia the new results are used together with those published previously to define a revised late mesozoic cenozoic apparent polar wander path for australia this path differs from earlier basalt derived paths by the absence of a large westward excursion and zig zag irregularities it is characterized instead by a substantially straight cenozoic trajectory a sharp bend in the late cretaceous and a non uniform rate of apparent polar wander the early tertiary segment of the new path lies north of the original paths thus eliminating a discrepancy that has been noted previously between the indian and australian palaeomagnetic data luyendyk rennick peirce and klootwyk peirce a re examination of the original data suggests that this discrepancy as well as two others in the australian results may have been caused principally by incomplete time averaging of remanence directions because the volcanics on which the results were based had been extruded episodically

low permeability rocks laboratory measurements and three dimensional microstructural analysis traditional techniques for measuring permeability and traditional models relating permeability to porosity are inadequate for low permeability or 10 sup 18 m sup 2 geologic materials special techniques have been developed for measuring fluid flow through such rock samples and for quantifying their three dimensional pore microstructure permeability is measured by a modified transient pulse technique pore structure is quantified using a computerized microgeometric modeling system and portable three dimensional display is produced by an optical imaging system results of applying this processing to a low porosity tight gas sandstone from the cotton valley formation agree with the well documented diagenetic history of these samples we find that the porosity distribution is very inhomogeneous and skewed the pore system is slightly anisotropic which is consistent with the differences in permeability measured in two orthogonal directions we indicate how such data could be used to develop more geologically and geometrically realistic models relating porosity and permeability in low permeability gas formations

lateral velocity variations in southern california i results for the upper crust from pg waves the plate boundary and major crustal blocks in southern california are imaged by a tomographic backprojection of the pg first arrivals recorded by the southern california array the method formulated specifically for local earthquake arrival times is a fast iterative alternative to direct least squares techniques with it we solve the combined problem of determining refractor velocity perturbations and source and station delays resolution and variance are found empirically by using synthetic examples a map showing lateral velocity variations at a depth of approximately 10 km is presented the results show a strong correlation with surface tectonic features clear velocity contrasts exist across the san andreas the san jacinto and the garlock faults the mojave region has the slowest velocities while the peninsula ranges have the highest the san jacinto block has velocities intermediate between mojave and peninsula range velocities and also has early station delays this may indicate that the san jacinto block has overridden mojave material on a shallow detachment surface no velocity variations are found associated with the transverse ranges which we interpret to mean that the surface batholithic rocks in this area do not extend to pg depths

marine magnetic anomalies palaeomagnetism and the drift history of gondwanaland abstract the results of palaeomagnetic surveys of mesozoic and tertiary rocks from gondwanaland can be reconciled with the results of modelling the evolution of oceanic floors from analyses of marine magnetic anomalies previous inconsistencies were mainly due to errors apparent in the australian cenozoic palaeomagnetic data an alternative tertiary apparent polar wander path apwp has been constructed from an analysis of all published laterite and weathered profile data palaeomagnetic results for africa antarctica australia india madagascar and south america are compared for rotations corresponding to marine magnetic anomalies 16 22 28 34 and m1 and for fit india has been selected as the reference continent since it provides the most detailed apwp having drifted about 50 of latitude since breakup

geophysical and isotopic mapping of preexisting crustal structures that influenced the location and development of the san jacinto fault zone southern california we examine the role of preexisting crustal structure within the peninsular ranges batholith on determining the location of the san jacinto fault zone by analysis of geophysical anomalies and initial strontium ratio data a 1000 km long boundary within the peninsular ranges batholith separating relatively mafic dense and magnetic rocks of the western peninsular ranges batholith from the more felsic less dense and weakly magnetic rocks of the eastern peninsular ranges batholith strikes north northwest toward the san jacinto fault zone modeling of the gravity and magnetic field anomalies caused by this boundary indicates that it extends to depths of at least 20 km the anomalies do not cross the san jacinto fault zone but instead trend northwesterly and coincide with the fault zone a 75 km long gradient in initial strontium ratios sr i in the eastern peninsular ranges batholith coincides with the san jacinto fault zone here rocks east of the fault are characterized by sr i greater than 0 706 indicating a source of largely continental crust sedimentary materials or different lithosphere we argue that the physical property contrast produced by the peninsular ranges batholith boundary provided a mechanically favorable path for the san jacinto fault zone bypassing the san gorgonio structural knot as slip was transferred from the san andreas fault 1 0 1 5 ma two historical m6 7 earthquakes may have nucleated along the peninsular ranges batholith discontinuity in san jacinto valley suggesting that peninsular ranges batholith crustal structure may continue to affect how strain is accommodated along the san jacinto fault zone

a seismic refraction survey of the imperial valley region california the u s geological survey conducted an extensive seismic refraction survey in the imperial valley region of california in 1979 the imperial valley is located in the salton trough an active rift between the pacific and north american plates forty shots fired at seven shot points were recorded by 100 portable seismic instruments at typical spacing of 0 5 1 km more than 1300 recording locations were occupied and more than 3000 usable seismograms were obtained we analyzed five profiles using a standard ray tracing program constructed a contour map of reduced travel times from our most widely recorded shot point and modeled an existing gravity profile across the salton trough results are itemized 1 all models have in common a sedimentary layer vp 1 8 5 0 km s a transition zone vp 5 0 5 65 km s a basement vp 5 65 km s in the imperial valley 5 9 km s on the bordering mesas and subbasement vp 7 2 km s 2 the sedimentary layer ranges in thickness along the axis of the salton trough from 3 7 km salton sea to 4 8 km u s mexican border on the bordering mesas it is quite variable in thickness 3 the transition zone is about 1 km thick in most places in the imperial valley there are no marked velocity discontinuities in this zone between the sedimentary layer and basement on the bordering mesas however there is a discontinuity at the top of this zone 4 there are apparently two types of basement on the bordering mesas basement is crystalline igneous and metamorphic rocks in the imperial valley basement is mostly lower greenshist facies sedimentary rocks based primarily on the smooth transition in character from sediment to basement arrivals the low value of basement velocity and the fact that deep 4 km wells in the valley penetrate only the upper part of the known cenozoic stratigraphic column for the salton trough 5 the subbasement or intermediate crustal layer ranges in depth along the axis of the salton trough from 16 km salton sea to 10 km u s mexican border gravity modeling requires that this layer deepen and or pinch out beneath the bordering mesas and mountain ranges based on its high velocity and the presence of intrusive basaltic rocks in the sedimentary section in the imperial valley the subbasement is thought to be a mafic intrusive complex similar to oceanic middle crust 6 several structures are seen that affect basement transition zone and deeper parts of the sedimentary layer they include a scarp along the imperial fault as much as 1 km down to the northeast and a scarp passing roughly along the topographic boundary between the imperial valley and the bordering mesa to the west as much as 3 km down to the east we interpret the latter scarp to be the suture or rift boundary between the older crystalline basement on the mesa and the younger metasedimentary basement in the imperial valley 7 on a contour map of reduced travel time from our most widely recorded shot point subtle patches of early arrivals among otherwise late arrivals in the central imperial valley correlate well with known geothermal resource areas having reservoir temperatures of more than 150 c apparently the salton trough is a location where new crust is being generated as the rift opens mafic intrusive rocks fill it from below as sedimentary rocks fill it from above rifting and intrusion produce high heat flow that metamorphoses the sedimentary rocks to shallow depth metasedimentary basement in the imperial valley and thus consolidates the new crust

new palaeomagnetic results from the kerguelen islands summary new palaeomagnetic analyses have been carried out in the kerguelen islands on 32 lava flows of well established age 20 22 ma combined with previous studies they yield a reliable pole for the lower miocene for the antarctic plate n 59 flows 349 9 e 83 5 s a 6 1 a reversal sequence reversed normal has been identified in the port jeanne d arc section

trapping of aqueous fluids in the deep crust free aqueous fluids in the deep ductile crust have residence times there which are very short in geological terms if not consumed by retrograde metamorphic reactions they empty very rapidly upwards this can be shown by applying the theory of magma percolation in a ductile host to the problem of aqueous fluid percolation below the brittle ductile transition thermal activation of ductility is strong enough to facilitate the rapid evolution of porosity and consequently allow rapid fluid transport in the cooler crust above the brittle ductile transition however this mechanism vanishes as a consequence fluid overpressures resulting from the accumulation of fluid in the transition zone must induce hydraulic fracturing rather than pore inflation in regions of horizontally compressive regional stress the commonest situation in stable continental crust these fractures will be horizontal aqueous fluids rising through or liberated from the lower crust will therefore probably accumulate at least initially at the brittle ductile transition in horizontally extensive reservoirs with high horizontal permeability but low vertical permeability

late mesozoic and cenozoic tectonic history of south central california the late mesozoic cenozoic history of south central california is punctuated by at least five major tectonic events the oldest of these involves late cretaceous drag folding and thrusting of epizonal shelf sediments westward over and to the north along the rising eastern margin of the peninsular ranges batholith these overthrusts with associated drag folds and nappes form imbricate stacks of metasediments slices of plutons and mylonites which overlie a regional core of cretaceous granitic plutons of the peninsular ranges batholith a second distinct kind of epizonal low temperature crustal displacement includes an episode of mid tertiary detachment faulting and folding this represents the westerly extension of the detachment terrane of eastern california and western arizona and utah superimposed upon the cretaceous structural features the mid tertiary motions involve major e w extension and n s compressive strain with formation of east trending antiformal core complexes remarkably parallel to a pervasive set of fold axes and b lineations formed in the cretaceous thrusting and folding all of these features are repeatedly segmented and displaced by step faulting as sial in the salton trough is fragmented and depressed 5 to 10 km below sea level at intervals throughout the cenozoic the irregular step faulted interface of the peninsular ranges batholith and salton trough is accentuated by a series of ne to se striking left lateral and dipslip faults with displacements of several to 10 kilometers these appear to both predate and occur during movements along the san andreas system pliocene pleistocene right lateral faulting a younger extension of the san andreas displacements north of the transverse ranges has broken and displaced all older structural features with total net slip in south central california of perhaps 300 km total right lateral slip along the several strands of the san andreas system are measurable via reconstruction of the mosaic of numerous older predominantly e w trending linear elements mid tertiary antiforms and cretaceous and older rocks right lateral strain on the san andreas appears to either alternate with or to be a byproduct of n s compressional strain from the pleistocene to the present a resultant effect is a large family of e w trending folds and fault scarps in plio pleistocene sediments

late mesozoic and cenozoic palaeomagnetism of australia ii implications for geomagnetism and true polar wander summary new palaeomagnetic results from australia indicate that throughout the cenozoic era the continent lay further south than suggested by hot spot data moreover while hot spots give a uniform rate of drift during most of the cenozoic the drift rate obtained from apparent polar wander varies considerably the discrepancies between the palaeomagnetic and hot spot results are analysed by comparing the australian data with those of europe and the central pacific the analysis suggests that the discrepancies are due to 1 departures of the earth s magnetic field from the geocentric axial dipole model and 2 either true polar wander or a non axial inclined dipole component it is found that since the mid tertiary the dominant non dipole component has been a quadrupole and that during this period both the quadrupolar field and the true polar displacement non axial dipole component decreased progressively during the quaternary and also at the earliest tertiary the non dipole components appear to have been moderate or small the comparison of data sets demonstrates that considerable errors may be incurred when cenozoic and presumably earlier poles from one geographic region are used to derive those of another widely separated region the results also imply that absolute plate velocities estimated from palaeomagnetic data can contain substantial errors and that hot spot data may need significant adjustments for true polar wander to yield correct palaeolatitudes finally the new early tertiary pole for australia is used in conjunction with updated early tertiary poles from other lithospheric plates to reapply the mckenzie test for true polar wander the results indicate a small true polar displacement since the beginning of the tertiary the amount and direction of the displacement however differ from those generally obtained from hot spot data

hotspot spin axis motion or magnetic far sided effect palaeomagnetic data from central pacific sediments and from the hawaiian emperor volcanic chain indicate that during the tertiary either the derived magnetic pole was located alternately on the far and near side of the geographical pole or that the hotspot coordinate systems thought to be fixed relative to the earth s mantle oscillated relative to the earth s spin axis

a weighted least squares fit of the australian apparent polar wander path for the last 100 myr summary recent versions of the australian apparent polar wander path apwp for the late mesozoic and tertiary show considerable variation re examination of the australian igneous data suggests that they are more reliable than assumed by some recent authors the trajectory of the australian apwp is defined by fitting the position of a set of poles including both igneous and laterite overprint data this allows the dated igneous poles to be used to determine age as a function of distance along the trajectory both the trajectory and the age are fitted by means of weighted least squares regression and are given approximate confidence limits age is best fitted in the australian case as a linear function of distance along the apwp this result contrasts with that of idnurm who suggested a variable rate of polar wander during the tertiary the new apwp is in better agreement with hot spot data dating of new caledonian laterites by the new apwp gives a result consistent with geological evidence while dating by reference to idnurm s path does not large non dipole components or significant true polar wander are not needed to explain the australian tertiary apwp

plate motions and the geomagnetic field i quaternary and late tertiary summary we have tested the hypothesis that the apparent increase in the northward offset of the axial dipole i e the ratio g gy with age for the last 25 ma wilson mcelhinny is due to the failure to correct for plate motions spherical harmonic analyses were performed on two types of data palaeomagnetic poles from continents islands and seamounts and magnetic inclinations from deep sea cores after returning the sampling sites to their predrift locations in fixed hotspots coordinates the results show that the quadrupole term g maintained a value of about 0 05 gy throughout the last 35 ma and that the axial octupole had a small value of about 0 02 gy for the last 5 ma sea core inclinations analysed separately gave essentially the same results as continental palaeopoles for the last few million years independent data sets for the past 2 ma and for the period 2 6 ma gave nearly identical solutions showing that the persistence of the axial terms is not a phenomenon of the more densely sampled quaternary field alone nor is it an artifice of predominantly northward motion of the plates returning all 0 5 ma data to a 5 ma reconstruction failed to eliminate the quadrupole the non zonal coefficients for the 0 5 ma field are typically an order of magnitude less than the zonal terms with the exception of h and are probably insignificant suggesting that longitudinal drift is generally effective in averaging out these components the present distribution of data is inadequate to determine coefficients of third and higher degree prior to 5 ma but there is evidence that g may have changed little during the last 35 ma in addition there has been very little relative motion between the palaeomagnetic dipole axis as defined by the first degree terms and the axis of the hotspots reference frame

magnetotelluric evidence for crustal suture zones bounding the southern great valley california a geoelectric section inferred from a regional magnetotelluric study across the coast ranges the great valley and the sierra nevada reveals significant variations in electrical resistivity zones of lower resistivity interpreted at depths from 10 km to at least 30 km lie near mapped geologic boundaries between the coast ranges and the great valley and beneath the eastern side of the great valley the former boundary is inferred by others to separate the subduction complex of the coast ranges from the mafic basement of the great valley the lower resistivities are most likely associated with metasediments trapped between the coast ranges ophiolite and the former oceanic crust beneath the great valley the latter boundary is problematic but may be evidence for a deep metasedimentary section trapped between the ophiolites beneath the great valley and the granitic rocks of the sierra nevada the lack of change in the magnetotelluric phase across the great valley indicates that a suture zone marked by lower resistivities is unlikely to be present beneath the valley however this does not preclude the existence of a resistive suture zone

palaeomagnetic dating of late cretaceous to early tertiary weathering in new england n s w australia abstract palaeomagnetic dating of australia s regolith is becoming more refined with advances in techniques and better definition of the late mesozoic and cenozoic apparent polar wander path of australia the suspected great antiquity of parts of the regolith is being confirmed through the application of palaeomagnetic dating to authigenic iron oxides which form during weathering earlier palaeomagnetic investigation tentatively suggested that deeply weathered volcanics in the new england area formed in the mesozoic implying that ideas regarding the palaeoenvironment of new england may need considerable revision published dates for other unweathered volcanics from this area are k ar ages of 21 8 ma 32 8 ma and 25 1 ma the earlier palaeomagnetic conclusions were based on results from only one the sugarloaf of the seven separate localities nine sites originally sampled these results being the only results that were internally consistent follow up work using a high sensitivity cryogenic magnetometer has now yielded consistent results from the six other localities which support those from the sugarloaf but a revised weathering age of about 60 ma seems more likely this age is virtually identical to that determined for the morney profile in southwestern queensland

hotspot tracks and the early rifting of the atlantic abstract many hotspot tracks appear to become the locus of later rifting as though the heat of the hotspot weakens the lithosphere and tens of millions of years later the continents are split along these weakened lines examples are the west coast of greenland east coast of labrador madeira hotspot the south coast of mexico north coast of honduras guyana hotspot and the south coast of west africa north coast of brazil st helena hotspot a modern day analog of a possible future rift is the snake river plain where the north american continent is being pre weakened by the yellowstone hotspot track this conclusion is based on reconstructions of the motions of the continents over hotspots for the past 200 million years the relative motions of the plates are determined from magnetic anomaly isochrons in the oceans and the motion of one plate is chosen ad hoc to best fit the motions of the plates over the hotspots however once the motion of this one plate is chosen the motions of all the other plates are prescribed by the relative motion constraints in addition to the correlation between the predicted tracks and sites of later continental breakup exposed continental shields correlate with the tracks their exposure may be the result of hotspot induced uplift which has led to erosion of their former platform sediment cover

cenozoic latitudinal shift of the hawaiian hotspot and its implications for true polar wander abstract pacific plate equatorial sediment facies provide estimates of the northward motion of the pacific plate that are independent of paleomagnetic data and hotspot tracks analyses of equatorial sediment facies consistently indicate less northward motion than analyses of the dated volcanic edifices of the hawaiian emperor chain the discrepancy is largest 60 70 ma b p the 60 to 70 ma equatorial sediment facies data agree with recent paleomagnetic results from deep sea drilling on suiko seamount 1 and from a northern pacific piston core 2 equatorial sediment facies data and paleomagnetic data combined with k ar age dates along the emperor chain 3 indicate a position of the spin axis at 65 ma b p of 82 n 205 e in the reference frame in which the pacific ocean hotspots are fixed this pole agrees well with the position of the spin axis in the reference frame in which the atlantic ocean hotspots and the indian ocean hotspots are fixed 4 5 supporting the joint hypotheses that 1 the pacific ocean hotspots are fixed with respect to the hotspots in other oceans 2 the hotspots have shifted coherently with respect to the spin axis and 3 the time average of the earth s magnetic field 65 ma b p was an axial geocentric dipole global neogene paleomagnetic data suggest that a shift of the mantle relative to the spin axis has been occurring during the neogene in the same direction as the shift between 65 ma b p and the present all data are consistent with a model in which the hotspots and by inference the mantle have shifted with respect to the spin axis about a fixed euler pole at a constant rate of rotation for the last 65 ma

magnetotelluric evidence for a brittle ductile transition peninsular ranges batholith southern california a magnetotelluric traverse of the peninsular ranges in southern california has revealed a pervasive zone of lower resistivity beginning at a uniform depth of 10 km and extending to depths of 60 90 km resistivities above 10 km depth are similar to those found in batholiths very high values correspond to outcrops of crystalline basement because seismicity below 11 12 km is sparse others have concluded that the brittle ductile transition is shallow beneath the range the zone of low resistivity in the lower crust corresponds well to the ductile region and we conclude that the lower values are caused by fluids trapped below the transition because the range has experienced vertical uplifts during the pliocene era and the top of the low resistivity zone is flat the present brittle ductile transition must have been formed in the last 5 m y a possible source for the fluids is the rift to the east in the salton trough

depth of seismicity in the imperial valley region 1977 1983 and its relationship to heat flow crustal structure and the october 15 1979 earthquake focal depths from over 1000 earthquakes occuring between 1977 and 1983 in the imperial valley southern peninsular ranges are used to study relationships between the depth of seismicity heat flow and crustal structure this study used relocated a and b quality events from the california institute of technology catalog that were carefully selected to insure focal depth precision of 2 km regional variations in focal depth appear to be related to regional heat flow variation whereas local variations in focal depth especially in the central imperial valley may be related to crustal structure these variations are studied by rheologic modeling a comparison of focal depths of earthquakes occurring before and after the october 15 1979 m 6 6 earthquake indicates that aftershocks during the first 2 months of the sequence were 2 3 km deeper than earthquakes occurring in other time periods the deepest earthquakes in imperial valley are spatially associated with a subbasement dome near the northern end of the imperial fault this dome coincides with the region where the imperial fault undergoes a transition from stick slip behavior to aseismic fault creep models of slip during the 1979 mainshock are also compared with premainshock and postmainshock seismicity a relocation of the 1940 m 7 1 mainshock suggests that this sequence began by rupturing the same portion of the fault that experienced maximum slip during the 1979 mainshock

late cretaceous apparent polar wander of the pacific plate evidence for a rapid shift of the pacific hotspots with respect to the spin axis paleomagnetic data from the pacific plate show that the average azimuth of the pacific plate decreased by 12 between 90 ma and 81 ma whereas trends of late cretaceous seamount chains suggest that the pacific plate moved northwestward relative to the hotspots with little change in azimuth during the same time interval this difference is used to infer a 13 4 shift of the spin axis relative to the hotspots in the pacific ocean basin this shift occurred within 5 m y of the occurrence of anomalous behavior of the paleomagnetic field recorded in anomalies 33 and 34 this coincidence in timing suggests there may be a causal link between these three phenomena

true polar wander abstract a re evaluation of the existence of true polar wander tpw since the late cretaceous and a comparison among the various approaches are made using updated paleomagnetic hotspot and relative motion datasets previous attempts to determine the existence of tpw had resulted in different conclusions comparison of hotspot locations and paleomagnetic poles required significant pole motion although lithospheric plate displacement analysis yielded insignificant motion however these earlier determinations cannot be directly compared to find the reason for the discrepancies because each used different datasets for this study the different approaches are applied to a single updated model with three alternative relative motions of east and west antarctica although the results are model dependent in general there was not significant motion of the pole relative to the lithosphere 1 5 since the early tertiary but a large motion 10 12 relative to the hotspot framework it is unlikely that errors in the determinations could account for this disagreement the a95 of the plate reconstruction is about 3 the uncertainty in antarctica motion is estimated to no larger than 3 and cumulative errors in the relative plate motions may also amount to 3 only if all these errors are present in the maximum estimated amount and in the same direction could they account for the 10 12 gap between the two approaches this conclusion of pole motion relative to the hotspots but not the lithosphere may indicate an independent shift of the mesosphere relative to the lithosphere or mantle roll of hargraves and duncan

early tertiary subduction zones and hot spots hot spots and subduction zones may control the position of the earth s rotation axis paleomagnetic reconstructions in the hot spot framework require a relative motion of about 10 between the hot spot reference frame and the earth s spin axis since the early tertiary if the viscosity of the mantle permits a changing distribution of mass anomalies can control the position of the earth s rotation axis as it tracks the maximum principal axis of the perturbations the present plate geometry should give the best indication of what features are associated with the controlling density anomalies for the present geoid anomalies indicate that subducting slabs are major mass contributions removing these effects a simpler residual geoid remains with highs strongly correlated with hot spot locations although neither the hot spots alone nor subducting slabs alone have maximum principal axes near the present pole adding the contributions of the two gives a combined axis within a degree of the pole suggesting that these two effects control the location of the spin axis to establish whether changes in plate geometry could account for the observed polar motion the locations of subduction zones are reconstructed in the hot spot framework for the early tertiary a time of significantly different plate motions for this reconstruction the combined maximum principal axis of the subductionhot spot systems is not at the present pole but about 10 away in a location similar to the pole position required by paleomagnetic studies this suggests that changes in subduction geometry may cause a shift in the position of the rotation pole and furthermore that inertia tensor effects may be the link between plate velocities and the spin axis

hotspots in the southern oceans an absolute frame of reference for motion of the gondwana continents abstract the geometry and geochronology of aseismic ridges and oceanic islands in the southern oceans provide a good test of the proposition that hotspots remain fixed over long periods of time that is motion of an order of magnitude less than the relative motion between plate pairs in most cases it is concluded that inter hotspot movement cannot be discerned for the period 100 m y to present and that widely distributed hotspots in the atlantic and indian oceans provide a frame of reference for plate motions following the disintegration of gondwanaland which is independent of paleomagnetism this frame of reference is absolute in that it gives the motion of the lithosphere with respect to the mantle hotspots the absolute motion model indicates that africa and antarctica are now moving only very slowly that there has been significant relative movement between east and west antarctica since the cretaceous and prescribes the relative motion between the somali and african plates

plates and their motions the last reviews which covered research in plate motions were those of herron and pitman 1975 on marine magnetic anomalies and dewey 1975 on plate tectonics both reporting on research for the period 1971 1974 in the last 12 years there has been considerable refinement of relative motion models as well as further development of global models also major effort has gone into defining reference frames for the study of plate motions and discriminating between them more recently the role of errors in individual plate rotations and the associated uncertainties in reconstructions have received attention in this report i concentrate on work published in the last 4 years 1983 1986 citing a minimum of publications from previous years however some papers in press for early 1987 have been included this report first reviews work on relative plate motions for the present and past and reconstructions and then models for these motions global ones for driving forces and local ones determining subduction history and back arc spreading the problem of reference frames both hotspot and paleomagnetic is discussed next i end with a review of work on the errors in plate motions

apparent polar wander of the mean lithosphere reference frame summary apparent polar wander in the mean lithosphere no net rotation no net torque uniform drag reference frame is compared with apparent polar wander in the hotspot reference frame over the past 100 myr palaeomagnetic poles and plate rotations previously used to determine an apparent polar wander path for the hotspot reference frame are here used to determine an apparent polar wander path in the mean lithosphere reference frame we find that the two paths are similar especially for late cretaceous time when a 10 20 shift of the pole occurred to first order the hotspots and lithosphere as a whole moved in unison relative to the palaeomagnetic axis during late cretaceous time a non dipole field explanation for the apparent shift can probably be excluded however either motion of the time averaged geomagnetic axis relative to the spin axis or polar wandering could have caused this shift the latter being the more likely explanation

latitudinal shift of pacific hotspots during the late cretaceous and early tertiary two frames of reference the geomagnetic field and hotspots have been widely used to measure the past motions and interactions of lithospheric plates several studies have found that these reference frames have undergone as much as 19 22 of relative motion often called true polar wander tpw in the past 200 myr1 4 moreover it also appears that this offset did not accumulate at a uniform rate some evidence suggests that spurts of tpw can occur at rates as high as 1 2 per myr3 5 rates even an order of magnitude less could pose serious problems for tectonic studies based on the hotspot reference frame because velocities of this magnitude cannot be considered insignificant compared to those of the plates unfortunately the imprecision of most palaeomagnetic data is such that large error bounds are common for many estimates of tpw rates in this report we present a record of tpw derived from pacific palaeomagnetic data that are relatively well constrained in age and palaeolatitude they show that offset between the two reference frames accumulated at 0 3 per myr 1 from late cretaceous to early tertiary

late mesozoic and cenozoic palaeomagnetism of australia iii bias corrected pole paths for australia antarctica and india summary palaeomagnetic results from part i of this study and their analysis in part ii are combined to eliminate bias from the cenozoic apparent polar wander path for australia a bias due to non dipole components in past geomagnetic fields or for poles calculated from hot spot data due to the motion of hot spots relative to the earth s rotational axis this path is extended in approximately bias free form to the late mesozoic and indicates a significant change in the drift direction of the continent between 26 and about 60 ma the bias corrected australian path is used first with seafloor spreading data for the southern ocean to derive a corresponding late mesozoic cenozoic pole path for antarctica the latter shows that the antarctic drift direction reversed in the early tertiary it is suggested that the early tertiary directional changes of both australia and antarctica are part of a global reorganization of plates during the eocene postulated by rona richardson cande mutter and patriat achache next the australian path is compared with hot spot data from the african and australian plates indicating a movement of the hot spots relative the earth s rotational axis during the cenozoic the direction of this movement is found to be consistent with previous results from other parts of the world finally the australian path is used together with non dipole components in the geomagnetic field to explain a prominent westward displacement of the mid and late cenozoic poles of india relative to those of australia because of uncertainties in the original poles and in the analysis the present results are likely to contain appreciable errors nevertheless their consistency with independent findings supports the dipole quadrupole model of part ii for mid and late cenozoic geomagnetic fields

a revised identification of the oldest sea floor spreading anomalies between australia and antarctica we propose that magnetic anomalies south of australia and along the conjugate margin of antarctica that were originally identified as anomalies 19 to 22 may be anomalies 20 to 34 the original anomaly identification has two troublesome aspects 1 it does not account for an extra anomaly between anomalies 20 and 21 and 2 it provides no explanation for the rough topography comprising the diamantina zone with our revised identification there is no extra anomaly and the diamantina zone is attributed to a period of very slow spreading 4 5mm yr half rate between 90 and 43 m y the ages bounding the interval of slow spreading 90 and 43 m y correspond to times of global plate reorganizations our revised identification opens up the possibility that part of the magnetic quiet zone south of australia formed during the cretaceous long normal polarity interval breakup of australia and antarctica probably occurred sometime between 110 and 90 m y b p the breakup unconformity identified by falvey in the otway basin may correspond to a eustastic sea level change

constraints on crustal structure and complex moho topography beneath pinon flat california from teleseismic receiver functions abstract we use teleseismic p waves recorded at pinon flat observatory pfo to constrain the three dimensional crustal and upper mantle velocity structure beneath the station by forward modeling radial receiver function waveforms we construct a one dimensional crustal model that includes a significant shear velocity inversion at 9 km in depth arrivals on the tangential components indicate dip of at least the uppermost discontinuity complicated moho topography deepening to the northwest of pfo is suggested by azimuthal dependence of travel times and amplitudes of the receiver functions and travel times of crustal p wave reverberations although fine details cannot be resolved each of those sets of observations plus mislocation vectors provide strong indications of abrupt moho topography possibly including step offsets of several kilometers this is not only consistent with gravity data in implying airy isostasy with compensation at moho depth but extends that model to a much finer length scale than had been resolved

horizontal plate motion a key allocyclic factor in the evolution of the great barrier reef the great barrier reef complex of northeastern australia thins dramatically and becomes younger from north to south these variations are a consequence of the cenozoic northward movement of the indian australian plate the temperate climatic conditions that applied off northeast australia during the early tertiary were progressively replaced by tropical conditions the present day south to north facies distribution along the eastern australian continental margin mimics the cenozoic vertical facies sequence through the northern part of the great barrier reef region

global plate motion frames toward a unified model 1 plate tectonics constitutes our primary framework for understanding how the earth works over geological timescales high resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals mid ocean ridges transform faults and seafloor spreading cementing the plate tectonic paradigm however so called absolute plate motions describing how the fragments of the outer shell of the earth have moved relative to a reference system such as the earth s mantle are still poorly understood accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent ocean distributions change with time a fundamental problem with deciphering absolute plate motions is that the earth s rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 ma and ideally would require knowledge about the motions within the convecting mantle in contrast models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of earth s history but cannot provide paleolongitudes because of the axial symmetry of the earth s magnetic dipole field we analyze four different reference frames paleomagnetic african fixed hot spot african moving hot spot and global moving hot spot discuss their uncertainties and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a hybrid model for the time period from the assembly of pangea 320 ma to the present for the last 100 ma we use a moving hot spot reference frame that takes mantle convection into account and we connect this to a pre 100 ma global paleomagnetic frame adjusted 5 in longitude to smooth the reference frame transition using plate driving force arguments and the mapping of reconstructed large igneous provinces to core mantle boundary topography we argue that continental paleolongitudes can be constrained with reasonable confidence

geologic and palaeomagnetic constraints on the formation of weathered profiles near inverell eastern australia abstract we use geologic and palaeomagnetic observations to constrain the timing and duration of weathering of the oakwood bauxite which outcrops in the new england region of new south wales australia the bauxite is developed on and overlain by mafic volcanic flows prior geochronologic results from surrounding regions indicate that age of these flows and thus the age of the bauxite is most likely between 18 and 26 ma though volcanics elsewhere in the central volcanic province have a much greater range of between 6 and 56 ma similarly comparison of a new palaeomagnetic pole computed from the bauxite pole latitude 75 4 n longitude 311 3 e n 58 a95 4 4 with the australian apparent polar wander path gives estimated ages of 21 27 ma individual samples of the bauxite have both normal and reversed polarity components of magnetization which beside providing a positive palaeomagnetic reversal test suggests that the duration of weathering exceeded 10 000 years and more likely lasted several million years our results suggest that the oakwood region had a palaeoclimate and palaeoenvironment that was conducive for deep weathering over a significant period in a the time interval from the mid oligocene to the early miocene

lithospheric and upper mantle structures results of electromagnetic soundings in europe abstract information on electrical conductivity of the subsurface at a crustal scale is inferred mainly from magnetotelluric mt and magnetometer data the latter include both magnetometer array studies mv and geomagnetic depth soundings gds locally controlled source data e g data from d c and very low frequency vlf resistivity surveys frequency soundings and airborne electromagnetic mappings provide information on near surface structures e g on exposures of deeper conductors the variety of electromagnetic methods provides an efficient zooming ability for structures from a regional scale of hundreds of kilometres to small local details of some metres thus both the mapping of large scale crustal conductivity structures and a detail study of the anomalous structures are possible the electrical conductivity of the crust and upper mantle is described within the plate tectonic framework examples of structures which become electrically conductive at different phases of the process are shown using the results of electromagnetic soundings in various tectonic environments in europe it is shown how the crustal conductors can be interpreted as ancient and modern tectonic markers these include structures from the fennoscandian shield precambrian extensional basins and collisional zones the carpathians scotland and ireland suture zones and the pannonian basin extensional basin and thrust zones as well as conductors detected along the european geotraverse e g the terrain boundaries and the decollement of the variscan thrusting an example which considers the lower crustal conductivity from the fennoscandian shield shows rather large lateral variations in conductance of the lower crust varying from 1 s to several hundreds of siemens estimates of the depth of the asthenospheric conducting layer within the european territory also vary considerably shallowest depths are detected beneath extensional regions the pannonian basin and the rhinegraben 40 80 km whereas depths of 200 km or over or the absence of a conducting layer are detected beneath the alps and old precambrian cratons among the key areas where electromagnetic investigations should play an essential role are the deep structure of the east european platform boundaries of archaean and proterozoic terrains the structure of the teisseyre tornquist zone the nature of the present day continental lower crust and the characteristics of the electrical asthenosphere beneath europe