tectonic stress within the new madrid seismic zone refraction data indicate a significant high density rift pillow beneath the new madrid seismic zone we present results of linear and nonlinear viscoelastic finite element modeling to determine whether support of the rift pillow may contribute significantly to the total present day stress field and we consider the implications for intraplate seismicity these models were run for a loading time of 100 m y to account for relaxation and transfer of stress since the last reactivation of the rift in the mid mesozoic results indicate that the nonlinear viscoelastic model with rheological stratification based on composition and temperature agrees well with the observed deformation within the seismic zone and with estimates of regional stress magnitudes the model predicts a maximum compression of 30 40 mpa above the rift pillow in the center of the rift axis if the magnitude of local compression predicted by the nonlinear model produces the inferred clockwise rotation of the order of 10 30 in the direction of shmax maximum horizontal compression near the rift axis the magnitude of regional compression is a factor of 1 to 2 times the magnitude of local compression and consistent with an origin due to ridge push forces the addition of the local stress associated with the rift pillow however results in an approximately 30 reduction in the resolved maximum horizontal shear stress thus while the stress associated with the rift pillow can rotate the stress field into an orientation favorable for failure reduction in the resolved shear stress requires a separate mechanism for strength reduction results of the modeling indicate that stresses from the load of the rift pillow may still be present in the upper crust even after 100 m y and may still play a role in present day deformation and seismicity of the new madrid seismic zone local stress fields of significant tectonic magnitudes may also occur around other ancient rift pillows and help explain the observed correlation between intraplate seismicity and failed rift zones

ferroan alkalic volcanism associated with calymmian rifting in the paramirim aulacogen sao francisco craton brazil new insights from lithofacies analysis and evidence of mantle derived alkaline h2o rich metasomatic fluids affecting ancient crustal materials abstract rift related volcanism is widely used in models of supercontinent reconstruction while the associated metasomatic fluids provide information about the mantle crust process during intracrustal melting events the paleoproterozoic mesoproterozoic paramirim aulacogen corresponds to multiple intracratonic rift cycles that register many basin developing events intense volcanic activity is recorded in some rift events but the volcanic lithofacies and their petrogenesis are poorly constrained herein we present lithofaciological characterization whole rock and mineral chemistry and u pb and lu hf isotope data from the 1 58 ga volcanic rocks trachytic and trachy andesitic lava flows and primary to secondary volcaniclastic lithotypes characterize this volcanism the primary volcaniclastic rocks resulted from direct volcanic activity and can be divided into pyroclastic and hydroclastic lithofacies associations the pyroclastic lithotypes correspond to air fall density currents and block and ash flow deposits formed in explosive eruptions hydroclastic lithotypes are represented by hyaloclastites and peperites reflecting effusive eruptions and interactions between magma water and sediments the secondary volcaniclastic lithotypes resulted from late stage reworking processes and consist of debris flows ephemeral streams associated with high energy and gravitational flow deposits the lava flows show well preserved primary volcanic features such as magma flow textures and euhedral phenocrysts based on petrographic and chemical characteristics associated with negative hf t values we infer that high melt rate of an ancient crust previously metasomatized by h2o rich alkaline mantle derived fluids generated the alkalic ferroan lavas these fluids promoted potassic and sodic metasomatism that affected all the volcanic rocks modifying the chemistry of the feldspars to pure orthoclase or96 98 and albite ab98 100 in some lavas a higher concentration of na rich fluids synthesized aegirine at a minimum temperature of 200 c and promoted its peralkaline character the potassic sodic metasomatism influenced the volcano sedimentary sequence of the pajeu group and the oldest rocks of the basement but did not affect the overlying sedimentary sequences indicating that fluids ceased with the end of volcanism we suggest that the h2o rich alkaline fluids arose by focused degassing in response to a rising asthenospheric mantle during the calymmian rift of the paramirim aulacogen

statistical tests of additional plate boundaries from plate motion inversions we have investigated the application of the f ratio test a standard statistical technique to the results of relative plate motion inversions the method tests whether the improvement in fit of the model to the data resulting from the addition of another plate to the model is greater than that expected purely by chance this approach appears to be useful in determining whether additional plate boundaries are justified we confirm previous results favoring separate north american and south american plates with a boundary located between 30 n and the equator using chase s global relative motion data we show that in addition to separate west african and somalian plates separate west indian and australian plates with a best fitting boundary between 70 e and 90 e can be resolved these results are generally consistent with the observation that the indian plate s internal deformation extends somewhat westward of the ninetyeast ridge the relative motion pole is similar to minster and jordan s and predicts the nw se compression observed in earthquake mechanisms near the ninetyeast ridge

a magmatic hydrothermal transition in arkaroola northern flinders ranges south australia from diopside titanite pegmatites to hematite quartz growth a set of palaeozoic diopside titanite veins are present in mesoproterozoic metagranites and metasediments that constitute the basement mt painter inlier of the adelaide fold belt south aus tralia these massive veins up to 1 m of pegmatitic nature contain large crystals of diopside lree y enriched titanite up to 40 cm in length and minor amounts of quartz they can be used to trace the sys tem s development from a high temperature magmatic stage through to a massive hydrothermal event the pegmatitic origin of these veins is evident from a complex fluid melt inclusion assemblage consisting of a highly saline inhomogeneous fluid and relicts of melt immiscibility of melt and heterogeneous highly saline fluids exceeding 61 eq mass nacl is preserved in primary inclusions in diopside and secondary inclusions in titanite indicating relatively shallow conditions of formation 510 20 c and 130 10 mpa graphic intergrowth of diopside and albite occurs at the contact with granitic pegmatites the system evolved into hydrothermal conditions which can be deduced from a later population of only fluid inclusions homogeneous and less saline 40 eq mass nacl trapped around 350 20 c and 80 10 mpa during quartz crystallization the conditions moved across the halite liquidus resulting in a heterogeneous mixture of brine and halite crystals which were trapped at 200 20 c and 50 10 mpa brecciation and a palaeo geother mal system overprinted the pegmatitic veins with an epithermal hematite quartz assemblage and lesser amounts of bladed calcite and fluorite in an intermit tently boiling hydrothermal system of fairly pure h2o at 100 140 c and 1 5 mpa remobilization of lrees and y from titanite and or the granitic host rock is evidenced by precipitation of apatite allanite and wakefieldite in an intermediate stage occasional incorporation of radioactive elements or minerals presumably u rich in the fluorite is responsible for radiolysis of h2 ot o h 2

monument motion and measurements of crustal velocities it is usually assumed in geodetic studies that measurement errors are independent from one measurement to the next and that the rate of deformation velocity is constant over the duration of the experiment any temporal correlation between measurements can substantially affect the uncertainty in this velocity estimate when it is determined from the time series of measurements one source of possible long term correlation is motion of the geodetic monument with respect to the deep crust available measurements suggest that this motion introduces errors that have the form of a random walk process we show how such errors affect the uncertainty of velocity estimates for a geodetic experiment of set duration we calculate the velocity uncertainty as a function of the number of observations and of the relative amount of correlated and uncorrelated noise we find that 1 neglecting long term temporal correlations makes the uncertainty in the estimated velocities much too small and that 2 when the correlated and independent noise sources are of similar magnitude the expected improvement in uncertainty from having more measurement is not realized there is almost no improvement in some cases we have also examined the effect of outliers blunders on the velocity uncertainty for a frequency of outliers typical of geodetic field campaigns the previous two conclusions remain unchanged these results suggest that long term correlations have a large effect on estimating deformation rates unless these correlations are small frequent observations give little advantage if frequent observations are planned the amount of correlated noise due to monument instability must be kept small if the full capabilities of the measurement technique are to be realized

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

rapid intraplate strain accumulation in the new madrid seismic zone remeasurement of a triangulation network in the southern part of the new madrid seismic zone with the global positioning system has revealed rapid crustal strain accumulation since the 1950s this area experienced three large moment magnitudes 8 earthquakes in 1811 to 1812 the orientation and sense of shear is consistent with right lateral strike slip motion along a northeast trending fault zone as indicated by current seismicity detection of crustal strain accumulation may be a useful discriminant for identifying areas where potentially damaging intraplate earthquakes may occur despite the absence of large earthquakes during historic time

the determination of displacement fields from geodetic data along a strike slip fault observations of angles or distances between stations of a geodetic network are commonly used to infer information about the movement of the surface of the earth the absence of any observations external to the network leads to an ambiguous displacement field existing techniques for eliminating this ambiguity are all unsatisfactory in some respect the best technique an inner coordinate solution is not appropriate for networks located in a strike slip fault environment the inner coordinate solution zeros the rotation of all stations about their center of mass along a strike slip fault like the san andreas however motion normal to the fault is less likely than motion parallel to the fault the solution presented here an outer coordinate solution finds the rotation of the network that minimizes the components of displacement normal to the fault since motion along a strike slip fault is generally expected to be parallel to the fault the displacements obtained with the outer coordinate solution are more reasonable than those obtained with other techniques examination of a trilateration network near san francisco bay demonstrates the large effect that the choice of adjustment technique can have on the inferred relative motion of the two sides of the fault the inner coordinate solution gave a rate of about 1 mm yr whereas the preferred outer coordinate solution rate was 36 mm yr

an introduction to the global positioning system and some geological applications receivers equipped to measure dual frequency carrier phase signals from satellites of the global positioning system gps have been capable under special conditions of determining relative horizontal positions among stations separated by one to a few hundred kilometers with a precision of one to several millimeters since the early 1980s the major obstacles to making this capability routine extending it to all parts of the globe and extending it to longer station separations have been equipment cost limitations in the gps satellite constellation arduous data analysis uncertainties in satellite orbits uncertainties in propagation delays associated with variable tropospheric water vapor and difficulties in resolving carrier phase cycle ambiguities recent improvements have occurred in all these areas the increasing ease and reduced cost of gps data acquisition and analysis are having a significant impact on studies of near fault crustal deformation and earthquake processes until recently the province of conventional terrestrial geodetic techniques the enhanced satellite constellation improved models and establishment of global tracking networks have extended several millimeters horizontal positioning capability to station separations of 1000 km or more in virtually all parts of the world this enables study of new classes of tectonic problems that previously were difficult to attack with any geodetic technique examples include a complete kinematic description of ongoing crustal deformation in broad complex continental plate boundary zones and measurement of relative plate motion at convergent boundaries where global models may be poorly constrained

can the okhotsk plate be discriminated from the north american plate the plate geometry in northeast asia has been a long standing question with a major issue being whether the sea of okhotsk and northern japanese islands are better regarded as part of the north american plate or as a separate okhotsk plate this question has been difficult to resolve because earthquake slip vectors along the kuril and japan trenches are consistent with either pacific north america or pacific okhotsk plate motion to circumvent this difficulty we also use slip vectors of earthquakes along sakhalin island and the eastern margin of the japan sea and compare them to the predicted eurasia okhotsk and eurasia north america motions for a model with a separate okhotsk plate we invert 10 eurasia okhotsk and 255 pacific okhotsk slip vectors with pacific north america and eurasia north america nuvel 1 data alternatively for a model without an okhotsk plate those eurasia okhotsk and pacific okhotsk data are regarded as eurasia north america and pacific north america data respectively the model with an okhotsk plate fits the data better than one in which this region is treated as part of the north american plate because the improved fit exceeds that expected purely from the additional plate the data indicate that the okhotsk plate can be resolved from the north american plate the motions on the okhotsk plate s boundaries predicted by the best fitting euler vectors are generally consistent with the recent tectonics the eurasia okhotsk pole is located at northernmost sakhalin island and predicts right lateral strike slip motion on the nne striking fault plane of the may 27 1995 neftegorsk earthquake consistent with the centroid moment tensor focal mechanism and the surface faulting along the northern boundary of the okhotsk plate the north america okhotsk euler vector predicts left lateral strike slip consistent with the observed focal mechanisms on the nw boundary of the okhotsk plate the eurasia okhotsk euler vector predicts e w extension discordant with the limited focal mechanisms and geological data this misfit may imply that another plate is necessary west of the magadan region in southeast siberia but this possibility is hard to confirm without further data such as might be obtained from space based geodesy

gravitational viscoelastic postseismic relaxation on a layered spherical earth viscoelastic relaxation of a ductile asthenosphere underlying a purely elastic plate is a strong candidate process for explaining anomalous rates of crustal deformation observed following large earthquakes the nongravitational treatment of pollitz 1992 which is valid on a global scale and includes the effects of compressibility is here extended to permit the calculation of gravitational viscoelastic relaxation for a specified spherically layered viscoelastic rheology following an earthquake in an elastic layer the simple approximations we adopt make the resulting treatment particularly suitable for near field calculations for an asthenosphere with a maxwell rheology the effect of gravitational coupling is manifested only many relaxation times after the earthquake as obtained by previous investigators its effect is generally to speed up the long wavelength component of the relaxation process and attenuate the overall vertical displacement pattern several subtle features common to the relaxation behavior from several different fault types thrust rift and strike slip are identified the effect of gravitational coupling on horizontal displacements is consistent with flexure of the upper elastic plate driven by the corresponding effects on the vertical displacement stress diffusion away from the source region generally exhibits pulse like behavior which is dispersive in both space and time if the asthenosphere is confined to a relatively narrow channel then the dispersion branches governing relaxation are radically altered and stress diffusion effects far from the coseismic rupture zone exhibit a complicated time dependence reflecting the competing tendencies of toroidal and spheroidal mode relaxation

estimation of intraplate strain accumulation in the new madrid seismic zone from repeat gps surveys the new madrid seismic zone nmsz a region within the eastern portion of the north american plate where great earthquakes occurred in 1811 1812 provides a type example of large earthquakes within a relatively stable continental interior assessing the scale and causes of deformation in such regions as well estimation of the recurrence interval for the large earthquakes within them has long been recognized as an important but challenging issue for which the advent of space based geodesy provides an important new tool here we report initial results from two global positioning system gps surveys of a geodetic network spanning the new madrid seismic zone because the expected strain signal is small the data were analyzed in three different ways first site velocities were estimated and analyzed for spatial gradients parallel and perpendicular to the trend of the major strike slip faults inferred from seismicity this analysis suggests but does not require a few millimeters per year of right lateral shear across the southern portion of the seismic zone consistent with the earthquake focal mechanisms second we divided the network into near field subnetworks and found possibly significant shear associated with approximately e w shortening within the southern nmsz and possible n s shortening across the left step connecting the presumed strike slip faults third we compared the strain observed along individual baselines to that predicted from the results of earlier near field surveys which combined gps and triangulation data in the southern part of the seismic zone we found general agreement with the earlier study although our data suggest lower strain rates hence after only 2 years of measurements the gps data suggest but do not yet require detectable strain accumulation however given that great earthquakes occur in the area and that our results are reasonably consistent with the results of other geodetic studies using different data and with earthquake mechanisms it seems likely that we are seeing strain accumulation in the southern nmsz a simple interpretation of the data as showing about 3 to 5 mm yr 1 of right lateral fault parallel motion implies a 1000 to 3300 year recurrence for great earthquakes with 5 to 10 m slip assuming all the accumulating strain is released seismically this recurrence interval may be an underestimate if the observed strains are largely postseismic effects of the 1811 1812 earthquakes a possibility suggested by the observation that plate wide data show less motion across the nmsz future surveys should resolve this issue as the longer measurement spans yield more precise velocity estimates

tectonic development of the new madrid rift complex mississippi embayment north america abstract geological and geophysical studies of the new madrid seismic zone have revealed a buried late precambrian rift beneath the upper mississippi embayment area the rift has influenced the tectonics and geologic history of the area since late precambrian time and is presently associated with the contemporary earthquake activity of the new madrid seismic zone the rift formed during late precambrian to earliest cambrian time as a result of continental breakup and has been reactivated by compressional or tensional stresses related to plate tectonic interactions the configuration of the buried rift is interpreted from gravity magnetic seismic refraction seismic reflection and stratigraphic studies the increased mass of the crust in the rift zone which is reflected by regional positive gravity anomalies over the upper mississippi embayment area has resulted in periodic subsidence and control of sedimentation and river drainage in this cratonic region since formation of the rift complex the correlation of the buried rift with contemporary earthquake activity suggests that the earthquakes result from slippage along zones of weakness associated with the ancient rift structures the slippage is due to reactivation of the structure by the contemporary nearly e w regional compressive stress which is the result of plate motions

carrier phase ambiguity resolution for the global positioning system applied to geodetic baselines up to 2000 km the global positioning system gps carrier phase data are biased by an integer number of cycles a successful strategy has been developed and demonstrated for resolving these integer ambiguities for geodetic baselines of up to 2000 km in length resulting in a factor of 3 improvement in baseline accuracy and giving centimeter level agreement with coordinates inferred by very long baseline interferometry in the western united states for this experiment a method using pseudorange data is shown to be more reliable than one using ionospheric constraints for baselines longer than 200 km an automated algorithm exploits the correlations between the many phase biases of a gps receiver network to enable the resolution of ambiguities for very long baselines a method called bias optimizing has been developed which unlike traditional bias fixing does not require an arbitrary confidence test bias optimizing is expected to be preferable to bias fixing for poorly configured networks in order to enable ambiguity resolution for long baselines it is recommended that future gps networks have a wide spectrum of baseline lengths ranging from 1000 km and that gps receivers be used which can acquire dual frequency p code data

anomalous alkaline rocks of soustov kola evidence of mantle derived metasomatic fluids affecting crustal materials the intrusive complexes of gremiakha vyrmes and soustov represent the two extremes of the early proterozoic alkaline plutons of kola predominantly composed of feldespathoidal syenites gremiakha vyrmes rocks zircon age 1 884 6 ma have trace element and isotope signatures 87sr 86srt 0 704 endt 3 1 3 compatible with an ultimate mantle origin soustov syenites zircon age 1 872 8 ma are totally different and show an acute crustal imprint they have sodaline and analcite instead of nepheline contain a plethora of ree hfse rich accessories and are characterised by elevated contents of f cl ree y th u zr hf nb ta sn be li rb tl pb and cs negative eu anomalies k rb 190 160 nd th 3 and nb ta 12 with extremely high 87sr 86srt 0 720 and at the same time relatively high endt 1 6 1 7 in this paper we explore the idea that the anomalous features of soustov syenites can be explained if we assume they are derived from a metasomatic agent initially an h2o co2 supercritical fluid released by alkaline mafic magmas that was profoundly contaminated during percolation through crustal materials as percolation advanced the bulk composition of the fluid solute changed from alkali halides and carbonates to a silica undersaturated alkaline melt when the fluid cooled to a temperature of 550 600 c it reached the point at which vapor and melt were no longer miscible and split into two components a vapour phase and a cl and f rich silica undersaturated silicate melt that crystallised to produce soustov syenites to study this process we have developed a numerical method for modelling the solute composition of the fluid during the infiltration metasomatism our results using the lree abundances and the sr and nd isotope composition of a gremiakha vyrmes pegmatite as the starting solute composition of the fluid and the mode and mineral trace element and isotope composition of a common kola gneiss as representative of percolated materials indicate that the fluid would have acquired a signature closely matching soustov s even in the case of nd isotopes if the gneiss age is 2 9 ga near its real age this model is still a mere working hypothesis that needs further refinements but may represent a reasonable explanation of the genesis of anomalous alkaline rocks with high 87sr 86srt and endt 0 either saturated or undersaturated which are difficult to understand in terms of magmatic fractionation contamination

interaction of biotite amphibole gneiss with h2o co2 k na cl fluids at 550 mpa and 750 and 800 с experimental study and applications to dehydration and partial melting in the middle crust to constrain effects of chloride bearing h2o co2 fluids on complex natural assemblages during high grade metamorphism and anatexis we report the results of experiments on the interaction of biotite hornblende tonalitic gneiss from the sand river formation limpopo complex south africa with h2o co2 h2o co2 kcl h2o co2 nacl and h2o co2 k na cl fluids at 550mpa 750 and 8008c and varying chloride h2o co2 ratios with molar co2 co2 h2o 1 4 0 5 heating of solid cylinders of gneiss at both temperatures in the absence of a free fluid phase produced no changes in the gneiss phase assemblage the equimolar h2o co2 fluid at 7508q also did not significantly influence the phase assemblage addition of kcl to the fluid at 7508q resulted in formation of the clinopyroxene k feldspar ilmenite titanite assemblage after biotite hornblende and plagioclase orthopyroxene accompanied by amphibole appeared only at 8008c as a result of biotite breakdown in the presence of h2o co2 and low salinity h2o co2 kcl fluids increase in the kcl content in the fluid at 8008q resulted in the production of a clinopyroxenebearing assemblage increase of the nacl content stabilized amphibole in an assemblage with either orthopyroxene at low nacl concentrations or clinopyroxene nevertheless clinopyroxene albite is stable only at high salt concentrations comparison of the experimental results with the results of thermodynamic modeling using the gibbs free energy minimization method perple x software showed that mineral reactions and assemblages in the run products were governed by the activities of alkali components imposed by kcl and nacl in the h2o co2 fluids and decrease of the water activity served as an additional factor stabilizing anhydrous assemblages no melts formed at 7508c in the presence of the h2o co2 kcl fluids these fluids provoked melting only at 8008c with formation of rhyolitic melts with increasing kcl content of the fluid the melt composition changed to potassic rhyolitic with al2o3513 5 wt cao52 wt k2o na2o47 wt feo feo mgo 40 8 k2o na2o41 and moderate enrichment in cl 0 2 0 6 wt increasing nacl content caused melting at 7508q and shifted the melt composition towards trachytic and trachyandesitic compositions at both 750 and 8008q the experiments support a model for the formation of ferroan a type granite syenite complexes via crustal melting in the presence of h2o co2 salt fluids in extensional tectonic settings they demonstrate a possible link between a type granitoids and mid crustal dehydration zones in amphiboliteto granulite facies terrains and allow a new interpretation of mineral assemblages within these zones in terms of variations in fluid salinity

multiple late holocene earthquakes along the reelfoot fault central new madrid seismic zone the reelfoot fault is an east vergent reverse fault underlying the lake county uplift a low amplitude late holocene anticline bordered on the east by the 32 km long reelfoot scarp fluvial deposits across the scarp define an 8 m high east facing monocline most near surface deformation along the scarp is accommodated via folding rather than faulting we interpret the scarp as a fault propagation fold developed over a wcst dipping reverse fault interpreted from shallow seismic reflection data trench exposurcs provide evidence for three episodes of deformation along the reelfoot fault within thc past approximately 2400 years between a d 780 and 1000 between a d 1260 and 1650 and during a d 1812 our best estimate of the average recurrence interval for deformation along the scarp is 400 500 years each episode of deformation had a slightly different style the third most recent event produced a small graben a few tens of centimeters deep in the hanging wall of the reverse fault the second most recent earthquake produced about 1 3 m of throw in the graben as well as folding along the updip projection of the reverse fault and development of the scarp thesc relations suggest that graben development increased through time concomitant with growth of the monocline or that the events are of different magnitude the 1811 1812 episode of deformation produced abundant liquefaction prominent folding of fluvial strata along the scarp and minor faulting in the graben

reconciling short recurrence intervals with minor deformation in the new madrid seismic zone at least three great earthquakes occurred in the new madrid seismic zone in 1811 and 1812 estimates of present day strain rates suggest that such events may have a repeat time of 1000 years or less paleoseismological data also indicate that earthquakes large enough to cause soil liquefaction have occurred several times in the past 5000 years however pervasive crustal deformation expected from such a high frequency of large earthquakes is not observed this suggests that the seismic zone is a young feature possibly as young as several tens of thousands of years old and no more than a few million years old

a diffuse plate boundary model for indian ocean tectonics it is suggested that motion along the virtually aseismic owen fracture zone is negligible so that arabia and india are contained within a single indo arabian plate divided from the australian plate by a diffuse boundary the boundary is a zone of concentrated seismicity and deformation commonly characterized as intraplate the rotation vector of australia relative to indo arabia is consistent with the seismologically observed 2 cm yr of left lateral strike slip along the ninetyeast ridge north south compression in the central indian ocean and the north south extension near chagos

high frequency paleoclimate signals from foulden maar waipiata volcanic field southern new zealand an early miocene varved lacustrine diatomite deposit abstract a freshwater diatomite deposit near dunedin south island new zealand provides an exceptional archive of mid latitude seasonal climate variation during latest oligocene early early miocene time the diatomite accumulated in a small 1 5 km diameter maar floored lake that formed during basaltic volcanism except for exposures of basanite dated at 23 2 ma by 40 ar 39 ar and remnants of a paleogene sandstone and conglomerate cover the diatomite body is surrounded by otago schist geophysical profiles indicate that the total diatomaceous sediment thickness may exceed 100 m two depositional facies are described from 15 5 m of weakly consolidated fresh diatomite accessible in two pits a thinly laminated facies comprising 60 of the section consists of dark brown and white couplets of average thickness 0 5 mm both brown and white laminae are composed essentially of frustules of a single pennate diatom along with 1 2 of siliceous sponge spicules dark laminae also contain abundant 5 7 5 m diameter siliceous chrysophycean stomatocysts and organic matter decalcified complete skeletons of galaxias a southern hemisphere group of freshwater or diadromous fish are found throughout diverse well preserved leaves some with attached scale insects are dominated by lauraceae rare flowers are also present trace fossils preserved in the laminated comprise 1 1 5 mm wide pale diatomaceous strings of probable faecal origin and isolated 20 40 mm diameter sand lenses interpreted as faecal remains of swimming waterfowl the second diatomite facies group comprises dark brown speckled beds interpreted as sediment gravity flows up to 14 cm thick and interspersed throughout the study section they incorporate abundant laminated diatomite flecks and leaves and minor woody plant matter and terrigenous silt almost all are capped by 1 8 mm of white diatomite that is interpreted as post flow fall out of resuspended diatom frustules associated breccia and swirly beds up to 8 cm thick are composed of variably contorted clasts of laminated diatomite in a speckled matrix nodules spheroids and films of authigenic framboidal pyrite are commonly associated with plant and fish remains as in modern lakes white laminae likely formed from diatom blooms when light flux temperature or nutrients were optimal during spring early summer dark laminae accumulating during autumn winter months laterally continuous pinstripe lamination absence of bioturbation and excellent fish and plant preservation indicate that the profundal lake and floor were anoxic periodic variations in couplet thickness of 3 10 years compare with quaternary records of el nino southern oscillation variability and indicate that the early early miocene low altitude new zealand climate was seasonal and strongly ocean influenced as it is today