the tectonic evolution of the south atlantic from late jurassic to present an improved tectonic database for the south atlantic has been compiled by combining magnetic anomaly geosat altimetry and onshore geologic data we used this database to obtain a revised plate kinematic model starting with a new fit reconstruction for the continents around the south atlantic we present a high resolution isochron map from chron m4 to present fit reconstructions of south america and africa that require rigid continental plates result in substantial misfits either in the southern south atlantic or in the equatorial atlantic to achieve a fit without gaps we assume a combination of complex rift and strike slip movements 1 along the south american parana chacos basin deformation zone 2 within marginal basins in south america salado colorado basin and 3 along the benue trough niger rift system in africa these faults are presumed to have been active before or during the breakup of the continents our model describes a successive unzipping of rift zones starting in the southern south atlantic between 150 ma tithonian and approximately 130 ma hauterivian rifting propagated to 38 s causing tectonic movements within the colorado and salado basins subsequently between 130 ma and chron m4 126 5 ma the tip of the south atlantic rift moved to 28 s resulting in intracontinental deformation along the parana chacos basin deformation zone between chron m4 and chron mo 118 7 ma rifting propagated into the benue trough and niger rift inducing rift and strike slip motion after chron mo the equatorial atlantic began to open while rifting and strike slip motion still occurred in the benue trough and niger rift since chron 34 84 ma the opening of the south atlantic is characterized by simple divergence of two rigid continental plates

1600 1500 ma hotspot track in eastern australia implications for mesoproterozoic continental reconstructions mesoproterozoic a type magmatic rocks in the gawler craton curnamona province and eastern mount isa inlier form a palaeo curvilinear belt for reconstructed plate orientations the oldest igneous rocks in the gawler craton are the hiltaba granite suite c 1600 1575 ma the youngest in the mount isa inlier are the williams naraku batholiths c 1545 1500 ma the belt is interpreted as a segment of a hotspot track that evolved between c 1600 and 1500 ma this hotspot track may define a quasilinear part of australia s motion between 1636 and 1500 ma and suggests that australia drifted to high latitudes an implication of this interpretation is that australia and laurentia may not have been fellow travellers leading to the formation of rodinia a hotspot model for a type magmatism in australia differs from geodynamic models for this style of magmatism on other continents this suggests that multiple geologic processes may be responsible for the genesis of proterozoic a type magmas

parana flood basalt volcanism primarily limited to 1 myr beginning at 135 ma new 40 ar 39 ar ages for rocks from rio grande do sul and critical evaluation of published radiometric data abstract 40 ar 39 ar step heating analyses were carried out on seven rocks five basalts an andesite and a rhyolite from the southern parana province 28 s 30 s they yield plateau isochron ages of 135 134 ma in good agreement with published step heating data on rocks from the same area critical review of laser spot isochron ages for rocks from the province ranging from 140 to 130 ma are shown to be unreliable estimates of crystallization ages as the rocks were substantially altered step heating results on three of these rocks thought to yield good plateau ages are shown to be incorrect as a result of a technicality in dating procedures followed u pb ages on zircon and baddeleyite separated from a variety of rock types 30 s 23 s fall in the range 135 to 134 ma all reliable 40 ar 39 ar and u pb ages indicate volcanism was sharply focused initiated at 135 ma and 1 myr in duration no variation of age with either latitude or longitude is noted scrutiny of published 40 ar 39 ar ages on the florianopolis dykes shows they cannot be used as reliable crystallization ages u pb work shows that this dyke swarm was formed coevally with the main part of the parana province most of the published 40 ar 39 ar ages on the ponta grossa dyke swarm are unreliable a few ages appear reliable and suggest the magmatic event in this area may have postdated the main parana pulse by 1 2 myr a single 40 ar 39 ar age from a high nb basalt in the southernmost part 34 s of the parana at 135 ma highlights the need for further radiometric work on other areas of this flood basalt province the parana province postdates the time of the jurassic cretaceous bound ary by 10 myr

rocas verdes ophiolites southernmost south america remnants of progressive stages of development of oceanic type crust in a continental margin back arc basin

crustal origin for the parana rhyolites discussion of description and petrogenesis of the parana rhyolites southern brazil by garland et al 1995 our criticisms of the garland et al 1995 paper are introduction three fold and concern 1 the interpretation of oxygen the south atlantic ocean divides what was once a single isotope data on the rhyolites 2 radiogenic isotope large igneous province into the larger parana flood basalt fingerprinting of potential source material 3 the major province of south america and the smaller etendeka element composition of melts produced from potential flood basalt province of namibia the term flood basalt sources in the lower crust province is in this case something of a misnomer because a substantial thickness of silicic volcanic rocks is preserved on both sides of the atlantic the parana rhyolites can be divided into two main types the chapeco high ti oxygen isotopes type which have a high concentration of high field one of the problems of using oxygen isotopes to solve strength elements and low sr isotope ratios sr sr at problems of magma genesis is that oxygen isotope ratios 130 ma 0 705 0 708 and the palmas low ti type are changed during low temperature alteration processes which has lower concentrations of high field strength it is well known that large shifts in do value can occur elements and elevated sr isotope ratios sr sr at 130 in glassy volcanic rocks such as the parana rhyolites e g ma 0 714 0 727 mantovani et al 1985 bellieni et al taylor 1968 cerling et al 1985 these problems can 1986 peate et al 1992 garland et al 1995 suggested be circumvented by analysing fresh phenocrysts such as that the former were produced by partial melting of pyroxene which are unaffected by the alteration process high ti parana basalts whereas the more voluminous harris et al 1990 used the pyroxene do values to palmas rhyolites were produced by prolonged asestimate the magma do values of the palmas low ti similation accompanying crystal fractionation of low ti type rhyolite of the parana to be between 10 1 and parana basalt magmas previous workers have suggested 10 7 with the equivalent low ti magma type in the that the palmas type rhyolites are derived from melting etendeka having do values between 9 7 and 11 2 of pre mesozoic middle to lower crust piccirillo et al these high values led harris et al 1990 to suggest an 1988 harris et al 1990 and we believe that garland et origin by crustal melting al 1995 have overlooked several important lines of garland et al 1995 acknowledged the high do evidence in rejecting the crustal melting hypothesis in values and went on to state that the high do values this comment we will draw on data from the etendeka were used by harris et al 1990 to invoke an entirely province of namibia which was once part of the same crustal origin but it has been argued elsewhere that post eruptive alteration can lead to an increase in do continental flood basalt province

evolution of the walvis ridge rio grande rise hot spot system implications for african and south american plate motions over plumes crystallization ages of volcanic rocks dredged or drilled from the walvis ridge ten sites and the rio grande rise one site have been determined by the 40ar 39ar incremental heating technique the fundamentally age progressive distribution of these basement ages suggests a common hot spot source for volcanism on the island of tristan da cunha along the walvis ridge and rio grande rise and for the formation of the continental flood basalts located in namibia africa and brazil south america the walvis ridge rio grande rise volcanic system evolved along a section of the south atlantic spreading axis as the african and south american plates migrated apart astride or in close proximity to an upwelling plume reconstructions of the spatial relationship between the spreading axis the tristan hot spot and the evolving walvis ridge rio grande rise volcanic feature show that at about 70 ma the spreading axis began to migrate westward away from the hot spot the resulting transition to intraplate hot spot volcanism along the walvis ridge and associated termination of rio grande rise formation also involved a northward migration of previously formed african seafloor over the hot spot rotation parameters for african motion over fixed hot spots i e absolute motion have been recalculated such that the predicted trail of the tristan hot spot agrees with the distribution of radiometric and fossil basement ages along the walvis ridge african absolute motion has been extended to the south and north american plates by the addition of relative motion reconstruction poles

the lebombo monocline and associated feeder dyke swarm diagnostic of a successful and highly volcanic rifted margin abstract as part of the karoo large igneous province lip the 700 km long and 182 ma old lebombo monocline outcrops inland of a younger mozambique lowland cover that separates it from the indian ocean detailed mapping of a particularly well exposed olifant river traverse across this monocline correlates basaltic and rhyodacitic lava units within the lebombo group to at least three hosted generations of intrusions within the monocline parallel northern lebombo dyke swarm implying that a much denser feeder dyke swarm is covered by younger flows measured attitudes and thicknesses of up to 197 lavas and 791 dykes statistically quantify a structural cross section from which a properly weighted lava log of nearly 5 km basalts inter bedded with an overlying 3 5 km of rhyodacites is constructed the geological section is compatible with a slightly deeper sub lava section through the innermost 30 km s of a relatively narrow east greenland margin to which the lebombo monocline can be rigorously compared in both cases field relationships are consistent with early tectonic extension being overwhelmed by dyke dilation within margin parallel swarm segments that may have been injected laterally from central magma chambers thus it is argued that the lebombo monocline is part of a successfully rifted margin that furthermore belongs to a distinct class of highly volcanic and relatively narrow margins which only form proximal to classical triple rift lip centers where an elevated magma flux sustains a magmatic mode of extension for the karoo lip this conforms with 1 a tight gondwana reconstruction of africa with antarctica 2 successful breakup during intense 182 ma emplacement of the lebombo group 3 triple rift formation together with a short lived 181 178 ma okavango dyke swarm and 4 anomalously thick igneous crust formation during continued 2 5 cm yr separation into a loose 155 ma plate configuration of africa and antarctica

integration of lunar polar remote sensing data sets evidence for ice at the lunar south pole in order to investigate the feasibility of ice deposits at the lunar south pole we have integrated all relevant lunar polar data sets these include illumination data arecibo ground based monostatic radar data newly processed clementine bistatic radar data and lunar prospector neutron spectrometer measurements the possibility that the lunar poles harbor ice deposits has important implications not only as a natural resource for future human lunar activity but also as a record of inner solar system volatiles e g comets and asteroids over the past billion years or more we find that the epithermal neutron flux anomalies measured by lunar prospector are coincident with permanently shadowed regions at the lunar south pole particularly those associated with shackleton crater furthermore these areas also correlate with the 0 circular polarization ratio cpr enhancements revealed by new processing of clementine bistatic radar echoes which in turn are colocated with areas of anomalous high cpr observed by arecibo observatory on the lower sun shadowed wall of shackleton crater estimates of the extent of high cpr from arecibo observatory and clementine bistatic radar data independently suggest that 10 km2 of ice may be present on the inner earth facing wall of shackleton crater none of the experiments that obtained the data presented here were ideally suited for definitively identifying ice in lunar polar regions by assessing the relative merits of all available data we find that it is plausible that ice does occur in cold traps at the lunar south pole and that future missions with instruments specifically designed to investigate these anomalies are worthy

a mariner 10 color study of mercurian craters 1 we have used recalibrated mariner 10 color image data uv and orange to examine spectral trends associated with crater features on the incoming hemisphere of mercury spacecraft images of the moon galileo and clementine and lunar sample laboratory spectra provide a framework for interpreting the mercurian data two spectral parameter images were constructed one reveals opaque mineral abundance and the other provides a map of variations related to optical maturity and feo content the crater kuiper and its rays are bright mainly because they are fresh immature but also because kuiper has excavated material with a lower opaque content than the surroundings under the influence of space weathering kuiper will eventually lose its rays and evolve into a bright floored crater like lermontov lermontov and nearby smaller craters are examples of craters that are likely mature but remain bright because of the low opaque material exposed on their floors in this portion of the planet it appears that a surface layer with moderate opaque abundance overlies low opaque material found at depth on the basis of estimates of crater excavation depths we suggest that the moderate opaque surface layer is 3 4 km thick this two layer stratigraphy is reminiscent of the lunar highland crust and may have originated in similar magma ocean processes we also describe a feature that exposes dark material with a relatively high opaque content this feature could be a mercurian analog to a lunar dark halo impact crater or perhaps a pyroclastic deposit this implies that opaque rich possibly mafic magmas were generated in the interior and reached the surface through effusive or explosive activity

mantle plumes and flood basalt stratigraphy in the parana south america geochemical studies of the parana continental flood basalts in brazil have led to the recognition of distinct magma types which have been used to infer the internal stratigraphy of the lava pile the overstepping of stratigraphic units toward the north and a similar compositional change in sills within the underlying parana basin sedimentary rocks imply that the site of magnatism migrated 750 km toward the north during this volcanic event this has important implications for recent plume related models that have argued whether fithospheric rifting must accompany the presence of a mantle plume in order to generate continental flood basalt magmatism it is difficult to ascribe the magnitude and direction of the shift in the locus of the parana magmatism to movement of the brazilian lithosphere relative to the underlying tristan mantle plume instead it is suggested that the observed lava distribution and internal structure of the parana etendeka flood basalt province were imposed by the intracacies of the rifting process during the initial opening of the south atlantic ocean

active tectonics of the south chilean marine fore arc 35 s 40 s the south chilean marine fore arc 35 s 40 s is separated into four tectonic segments concepcion north concepcion south nahuelbuta and tolten from north to south these are each characterized by their individual tectonic geomorphology and reflect different ways of mechanical and kinematic interaction of the convergent nazca and south american plates splay faults that cut through continental framework rock are seismically imaged in both concepcion segments and the tolten segment additionally the concepcion south segment exhibits prominent upper plate normal faults normal faults apparently relate to uplift caused by sediment underthrusting at depth this has led to oversteepening and gravitational collapse of the marine fore arc there is also evidence for sediment underthrusting and basal accretion to the overriding plate in the tolten segment there uplift of the continental slope has created a landward inclined seafloor over a latitudinal distance of 50 km in the nahuelbuta segment transpressive upper plate faults aligned oblique to the direction of plate motion control the seafloor morphology based on a unique acoustic data set including 90 of bathymetric coverage of the continental slope we are able to reveal an along strike heterogeneity of a complexly deformed marine fore arc which had escaped attention in previous studies that only considered the structure along transects normal to the plate margin

mercury evidence for anorthosite and basalt from mid infrared 7 3 13 5 μm spectroscopy abstract spectroscopic observations 7 3 13 5 m of three locations on the surface of mercury are reported the observed spectral radiance emanated from equatorial and low latitude regions between 12 and 32 mercurian longitude on 8 december 1990 from the longitudinal region 22 44 on 10 december 1990 and from the longitudinal region 110 130 on 12 july 1992 all locations are primarily intercrater plains spectra indicate compositional differences among these three locations the emissivity maximum or christiansen emission peak occurs at 8 1 m in the 8 december 1990 spectra but at shorter wavelengths in the data of 10 december 1990 and 12 july 1992 emission peaks near 8 m indicate rocks of intermediate or mafic composition spectra from 22 to 44 longitude resemble spectra of terrestrial basalt and diorite with sio2 content between 49 and 55 the christiansen feature in spectra from near 110 130 longitude strongly suggests the presence of plagioclase in particular labradorite while the overall spectrum resembles anorthosite the spectra from all three locations on mercury show distinct and recognizable features the principal christiansen emission peak being the most prominent but they also contain features that we have not yet identified the general indication from the spectra is that mercury s surface consists of minerals more depleted in oxidized iron than those on the moon we also explore the theoretical and observational complexities of ground based mid infrared spectroscopy of airless bodies in general and mercury in particular a spectroscopic study of quartzite in both reflectance and emittance illustrates the practical spectral validity of kirchhoff s law

thermal infrared spectroscopy of experimentally shocked anorthosite and pyroxenite implications for remote sensing of mars 1 the feldspar and pyroxene mineralogies on mars revealed by the thermal emission spectrometer tes on mars global surveyor likely record a variety of shock effects as suggested by petrologic analyses of the martian meteorites and the abundance of impact craters on the planet s surface to study the effects of shock pressures on thermal infrared spectra of these minerals we performed shock recovery experiments on orthopyroxenite and anorthosite samples from the stillwater complex montana over peak pressures from 17 to 63 gpa we acquired emissivity and hemispherical reflectance spectra 350 1400 cm 1 7 29 mm of both coherent chips and fine grained powders of shocked and unshocked samples these spectra are more directly comparable to remotely sensed data of mars e g tes than previously acquired absorption or transmission spectra of shocked minerals the spectra of experimentally shocked feldspar show systematic changes with increasing pressure due to depolymerization of the silica tetrahedra for the spectra of chips this includes the disappearance of small bands in the 500 650 cm 1 region and a strong band at 1115 cm 1 and changes in positions of a strong band near 940 cm 1 and the christiansen feature near 1250 cm 1 spectra of the shocked powders show the gradual disappearance of a transparency feature near 830 cm 1 fewer changes are observed in the pyroxene spectra at pressures as high as 63 gpa spectra of experimentally shocked minerals will help identify more precisely the mineralogy of rocks and soils not only from tes but also from future mars instruments such as minites and themis index terms 3944 mineral physics shock wave experiments 3934 mineral physics optical infrared and raman spectroscopy 5420 planetology solid surface planets impact phenomena includes cratering 5460 planetology solid surface planets physical properties of materials keywords anorthosite pyroxenite shock spectra thermal infrared remote sensing mars

basaltic volcanism mantle plumes and the mechanics of rifting the parana flood basalt province of south america dynamic modeling of continental extension between south america and africa shows that the mechanics of rifting played an important role in determining the pattern of volcanism within the parana and etendeka flood basalt provinces on the brazilian and namibian margins the key feature of the model is the development of a horizontal pressure gradient in the lower crust during the early stages of extension which provided a mechanism for transporting magma generated beneath the incipient sea floor spreading axis into the parana province 100 200 km distant the horizontal pressure gradient developed as a consequence of the dynamic interaction of preexisting weaknesses in the middle crust and upper mantle during rifting the model accounts for the large quantity of basalt the asymmetric distribution of basalt on the conjugate margins and the northward migration of the eruptive center with time the rapidity of magma genesis is in agreement with models of decompression melting during rifting the model indicates that although elevated asthenosphere temperatures associated with the tristan plume account for the volume of melt generated the mechanics of rifting control the location and style of emplacement the model suggests that extension in the region began ca 150 155 ma and lasted about 25 m y

tectonic significance of the florianopolis dyke swarm parana etendeka magmatic province a reappraisal based on precise u pb dating abstract the first precise id tims u pb baddeleyite zircon dating of dykes representative of the main activity in the florianopolis dyke swarm parana etendeka magmatic province south brazil reveals a restricted age range 134 7 0 3 to 133 9 0 7 ma this is at least 5 ma older than indicated by previous 40 ar 39 ar stepwise heating dating and precludes a direct relationship with syn rifting magmatism connected with opening the south atlantic ocean instead as indicated by their age and geochemical signature these dykes are feeders to the nearby urubici lavas and coeval khumib lavas in etendeka these new geochronological results also cast doubt on a large 10 ma time span for the emplacement of the florianopolis dyke swarm implied by 40 ar 39 ar dates but the determination of its upper age limit still requires accurate dating of younger low volume fine grained dykes with distinct low ti chemistry crystals suitable to u pb dating were not obtained thus far from these dykes

modeling of continental weathering under high co2 atmospheres during precambrian times batch experiments were conducted to simulate abiotic weathering of the continental crust under high co2 atmospheric conditions during precambrian times i e corresponding to the general archaean conditions as well as to the immediate aftermath of the neoproterozoic snowball earth ice ages three types of rock basalt granodiorite and tonalite representative of the archaean to proterozoic continental crust were reacted in the form of powders for 1 year at 40 c with pure water under various water rock ratios in oxic and anoxic atmospheres containing 10 co2 that is under conditions assumed to characterize the greenhouse effect which prevailed at that time chemical and mineralogical data collected during the course of the experiments reflect alteration phenomena occurring in two steps 1 rapid leaching of the fresh surfaces probably related to the proton donor capacity of the co2 and 2 a steady state reaction under near neutral conditions these observed dissolution rates can be satisfactorily modeled by kinetic data available in the literature using a 1 d weathering model reproducing the two steps we evaluate the atmospheric co2 consumption rate at the snowball earth aftermath focusing on the relative contribution of surface leaching versus steady state reaction the results show that the process is dominated by surface exchange

listric normal faulting on the cascadia continental margin analysis of multichannel seismic reflection profiles reveals that listric normal faulting is widespread on the northern oregon and washington continental shelf and upper slope suggesting e w extension in this region fault activity began in the late miocene and in some cases has continued into the holocene most listric faults sole out into a subhorizontal d6collement coincident with the upper contact of an eocene to middle miocene m61ange and broken formation mbf known as the hoh rock assemblage onshore whereas other faults penetrate and offset the top of the mbf the areal distribution of extensional faulting on the shelf and upper slope is similar to the subsurface distribution of the mbf evidence onshore and on the continental shelf suggests that the mbf is overpressured and mobile for listric faults which become subhorizontal at depth these elevated pore pressures may be sufficient to reduce effective stress and to allow downslope movement of the overlying stratigraphic section along a low angle 0 1o 2 5 o detachment coincident with the upper mbf contact mobilization extension and unconstrained westward movement of the mbf may also contribute to brittle extension of the overlying sediments no pliocene or quaternary extensional faults have been identified off the central oregon or northernmost washington coast where the shelf is underlain by the rigid basaltic basement of the siletzia terrane quaternary extension of the shelf and upper slope is contemporaneous with active accretion and thrust faulting on the lower slope suggesting that the shelf and upper slope are decoupled from subduction related compression

lunar pure anorthosite as a spectral analog for mercury plans are underway for spacecraft missions to the planet mercury beginning in the latter part of this decade nasa s messenger and esa s bepicolombo mercury is an airless body whose surface is apparently very low in ferrous iron much of the mercurian surface material is expected to be optically mature a state produced by the space weathering process from direct exposure to the space environment if appropriate analog terrains can be identified on the moon then study of their reflectance spectra and composition will improve our understanding of space weathering of low fe surfaces and aid in the interpretation of data returned from mercury by the spacecraft we have conducted a search for areas of the lunar surface that are optically mature and have very low ferrous iron content using clementine uvvis image products several regions with these properties have been identified on the farside these areas representing mature pure anorthosites 90 plagioclase feldspar are of interest because only relatively immature pure anorthosites have previously been studied with earth based spectrometry a comparison of mercury with the lunar analogs reveals similarities in spectral characteristics and there are hints that the mercurian surface may be even lower in feo content than the lunar pure anorthosites we also investigate the potential for use of spectral features other than the commonly studied 1 m mafic mineral absorption band as tools for compositional assessment when spacecraft spectral measurements of mercury become available most low fe minerals plausibly present on mercury lack absorption bands but plagioclase possesses an iron impurity absorption at 1 25 m detection of this diagnostic band may be possible in fresh crater deposits

mesozoic plate tectonic reconstructions of the central south atlantic ocean the role of the west and central african rift system abstract the mesozoic opening history of the central equatorial and south atlantic oceans is closely linked in time and space to the development of the cretaceous rift system in west and central africa geological and geophysical studies of the west and central african rift system indicate it to be dominated by a set of sinistral and dextral strike slip zones diverging from the gulf of guinea and terminating as extensional rift basins in niger and sudan plate tectonic studies of the opening history of the atlantic basins indicate four stages of development stage 1 the jurassic opening of the central atlantic stage 2 the early cretaceous 130 119 ma opening of the south atlantic with rifting propagating deep into africa via the benue trough stage 3 the opening of the equatorial atlantic 119 105 ma and stage 4 the linkage of these oceanic basins and development of a single opening mid oceanic rift system as seen today the west and central african rift system continued to develop until the end of the cretaceous i e into stage 4 when rifting was terminated by a compressional shear deformation event causing the folding of sediments in the benue trough and reactivation of the central african shear zone and extensional tectonics in the basins of southern sudan this event correlates with an important change in the relative opening of the atlantic basins during tertiary recent times the west african rift system has not been active however during this period domal uplift and volcanism have occurred in central cameroon and western sudan and a very active phase of volcanism has occurred within the last 10 ma along the present day cameroon volcanic line although these uplift and volcanic events indicate thermal processes in the upper mantle there is as yet no clear link between their development and plate tectonic processes

precise 40ar 39ar age for the initiation of palaeogene volcanism in the inner hebrides and its regional significance sanidine crystals from two tuff layers at the base of the palaeocene eigg lava formation on the isle of muck inner hebrides yield step wise ar release plateau ages of 62 8 0 6 2 and 62 4 0 6 ma using the laser 40 ar 39 ar dating technique these determinations which are in accord with but significantly more precise than earlier results provide a definitive age for the inception of igneous activity in the small isles from stratigraphical inferences the muck ages provide precise maximum constraints on the possible age of skye magmatism and possibly most of the british tertiary volcanic province the new ages significantly predate the oldest ages determined so far for tertiary magmatism in ne greenland but are similar to ages obtained from w greenland and offshore se greenland and support the concept that the initiation of regional volcanism was primarily controlled by lithospheric thin spots the muck sanidine age appears to precisely date the first manifestation of the impingement of the iceland mantle plume on the lithosphere beneath nw scotland

identification of mercurian volcanism resolution effects and implications for messenger the possibility of volcanism on mercury has been a topic of discussion since mariner 10 returned images of half the planet s surface showing widespread plains material these plains could be volcanic or lobate crater ejecta an assessment of the mechanics of the ascent and eruption of magma shows that it is possible to have widespread volcanism no volcanism on the surface whatsoever or some range in between it is difficult to distinguish between a lava flow and lobate crater ejecta based on morphology and morphometry no definite volcanic features have been identified on mercury however known lunar volcanic features cannot be identified in images with similar resolutions and viewing geometries as the mariner 10 dataset examination of high resolution low sun angle mariner 10 images reveals several features which are interpreted to be flow fronts it is unclear if these are volcanic flows or ejecta flows this analysis implies that a clear assessment of volcanism on mercury must wait for better data messenger will take images with viewing geometries and resolutions appropriate for the identification of such features

mafic volcaniclastic deposits in flood basalt provinces a review flood volcanic provinces are assumed generally to consist exclusively of thick lavas and shallow intrusive rocks mostly sills with any pyroclastic rocks limited to silicic compositions however mafic volcaniclastic deposits mvds exist in many provinces and the eruptions that formed such deposits are potentially meaningful in terms of potential atmospheric impacts and links with mass extinctions the province where mvds are the most voluminous the siberian traps is also the one temporally associated with the greatest phanerozoic mass extinction a lot remains to be learned about these deposits and eruptions before a convincing genetic link can be established but as a first step this contribution reviews in some detail the current knowledge on mvds for the provinces in which they are better known i e the north atlantic igneous province including greenland the faeroe islands the british isles and tephra layers in the north sea basin and vicinity the ontong java plateau the ferrar and the karoo we also provide a brief overview of what is known about mvds in other provinces such as the columbia river basalts the afro arabian province the deccan traps the siberian traps the emeishan and an archean example from australia the thickest accumulations of mvds occur in flood basalt provinces where they underlie the lava pile faeroes n1 km ferrar province z400 m siberian traps 700 m in the faeroes case the great thickness of mvds can be attributed to accumulation in a local sedimentary basin but in the ferrar and siberian provinces the deposits are widespread n310 5 km 2 for the latter on the ontong java plateau over 300 m of mvds occur in one drill hole without any overlying lavas where the volcaniclastic deposits are sandwiched between lavas their thickness is much less in most of the cases reviewed primary mvds are predominantly of phreatomagmatic origin as indicated by the clast assemblage generally consisting of basaltic clasts of variable vesicularity dominantly non to poorly vesicular mixed with abundant country rock debris the accidental lithic components often include loose quartz particles derived from poorly consolidated sandstones in underlying sedimentary basins east greenland ferrar karoo these underlying sediments or sedimentary rocks were not only a source for debris but also aquifers that supplied water to fuel phreatomagmatic activity in the

age of mesozoic igneous rocks in northwesternnamibia and their relationship to continental breakup mesozoic igneous rocks of northwestern namibia comprise continental flood basalts and quartz latites of the parana etendeka province and a suite of central sub volcanic intrusions the damaraland complexes conventional k ar ages published for these rocks range from 377 88 ma with the majority of samples yielding ages of 150 110 ma this large spread of ages is interpreted as being due to disturbance of the k ar system by low temperature alteration which is ubiquitous among the volcanic rocks rb sr and 40 ar 39 ar dating methods which are less susceptible to such disturbance yield a narrower range of ages 132 124 ma with one outlier at 149 ma new rb sr internal isochron ages of 129 8 3 8 ma 126 8 1 3 ma and 126 1 7 3 ma have been determined for an etendeka quartz latite and nepheline syenite intrusions in the messum and okorusu complexes respectively in addition 40 ar 39 ar release spectrum ages of 137 0 0 7 ma 135 0 0 7 ma and 126 9 0 6 ma have been determined for samples from the paresis comendite cape cross nepheline syenite and okorusu nepheline syenite complexes respectively a 13 ma period between 137 and 124 ma is considered to be the best estimate of the timing of mesozoic igneous activity in northwestern namibia the continental flood basalt volcanism was probably limited to the early part of this period and is inferred to have been relatively short lived 1 2 ma magmatic activity among the damaraland complexes was contemporaneous with the onset of flood basalt volcanism but continued for approximately 15 ma and terminated after the onset of sea floor spreading in this area 130 125 ma both the flood basalt volcanism and the magmatic activity of the damaraland complexes are thought to have occurred in response to rifting across the upwelling tristan mantle plume the linear distribution of intrusions in damaraland is interpreted as being due to magmatism focused along a structural discontinuity between the pan african damara terrain to the south and proterozoic cratonic basement to the north

high precision 40ar 39ar dating of oligocene rhyolites from the mogollon datil volcanic field using a continuous laser system replicate 40ar 39ar analyses of sanidine from seven oligocene 28 2 ma rhyolite flows using a continuous laser dating system routinely yield analytical precisions of less than 0 45 1s on samples smaller than 0 5 mg this is as good or better than has been obtained by any other method even though the sample mass is several orders of magnitude smaller the system has the potential of resolving differences in apparent age of as little as 0 2 for high k ca samples as young as a few ma the analyses suggest that the seven volcanic units analyzed were erupted in an interval of about 0 1 m y or less

anorthosite formation by plagioclase flotation in ferrobasalt and implications for the lunar crust abstract the sept iles layered intrusion quebec canada is dominated by a basal layered series made up of troctolites and gabbros and by anorthosites occurring 1 at the roof of the magma chamber 100 500 m thick and 2 as cm to m size blocks in gabbros of the layered series anorthosite rocks are made up of plagioclase with minor clinopyroxene olivine and fe ti oxide minerals plagioclase displays a very restricted range of compositions for major elements an 68 an 60 trace elements sr 1023 1071 ppm ba 132 172 ppm and sr isotopic ratios 87 sr 86 sr i 0 70356 0 70379 this compositional range is identical to that observed in troctolites the most primitive cumulates of the layered series whereas plagioclase in layered gabbros is more evolved an 60 an 38 the origin of sept iles anorthosites has been investigated by calculating the density of plagioclase and that of the evolving melts the density of the feo rich tholeiitic basalt parent magma first increased from 2 70 to 2 75 g cm 3 during early fractionation of troctolites and then decreased continuously to 2 16 g cm 3 with fractionation of fe ti oxide bearing gabbros plagioclase an 69 an 60 was initially positively buoyant and partly accumulated at the top of the magma chamber to form the roof anorthosite with further differentiation plagioclase 60 became negatively buoyant and anorthosite stopped forming blocks of anorthosite autoliths even fell downward to the basal cumulate pile the presence of positively buoyant plagioclase in basal troctolites is explained by the low efficiency of plagioclase flotation due to crystallization at the floor and or minor plagioclase nucleation within the main magma body dense mafic minerals of the roof anorthosite are shown to have crystallized from the interstitial liquid the processes related to floating and sinking of plagioclase in a large and shallow layered intrusion serve as a proxy to refine the crystallization model of the lunar magma ocean and explain the vertically stratified structure of the lunar crust with gabbro noritic rocks at the base and anorthositic rocks at the top we propose that the lunar crust mainly crystallized bottom up this basal crystallization formed a mafic lower crust that might have a geochemical signature similar to the magnesian suite without kreep contamination while flotation of some plagioclase grains produced ferroan anorthosites in the upper crust

a comparison of mercurian reflectance and spectral quantities with those of the moon we have examined previously published earth based telescopic spectra of mercury with the goal of obtaining clues to the planet s surface composition the study has three parts first because of suggestions that mercury s crust may be anorthositic we compare near infrared spectra of mercury with spectra of lunar anorthosites the spectral slopes and lack of strong mineralogic absorption features make the mercurian spectra similar to spectra of lunar anorthosites second we compare albedo quantities for mercury and the moon computed from telescopically derived photometric parameters with the laboratory albedos of returned lunar samples the albedos calculated from the photometric parameters are much lower than those of lunar samples measured in the laboratory third the mercurian spectra are interpreted in terms of the current model for the reflectance of an iron bearing silicate regolith on an airless body recent steps toward quantifying this model permit estimates of the feo content and opaque mineral abundance of the surface to be made we find that the mercurian spectra least affected by telluric absorptions imply feo tio2 4

paleomagnetism and dating of the early cretaceous florianopolis dike swarm santa catarina island southern brazil abstract the florianopolis fl basaltic dikes are well exposed on santa catarina island close to the present southeastern border of the parana magmatic province in southern brazil and their intrusion was related to the tectonic and magmatic evolution of rifting in the southern atlantic the dikes are vertical or subvertical and cut the crystalline basement rocks mainly late proterozoic granites most of the dikes exhibit ne trends corresponding to structural trends of the crystalline basement a few nw trending dikes can also be observed occasionally crosscutting the ne dikes samples from 73 dikes were collected for paleomagnetic work and subjected to both alternating field and thermal demagnetization characteristic magnetizations of normal and reversed polarities interpreted as original thermoremanences are carried by ti poor magnetites showing remanent coercive forces in the range 18 45 mt the corresponding paleomagnetic pole fl pole is located at 3 3 e 89 1 s n 65 95 2 6 k 47 samples for 40 ar 39 ar dating were collected at the same paleomagnetic sites plagioclase samples from nine dikes were analyzed by stepwise heating the apparent age spectra are strongly discordant due to excess 40 ar contamination but five samples yielded plateau ages ranging from 119 0 0 9 ma to 128 3 0 5 ma one additional sample yielded an isochron age of 121 5 0 5 ma the inferred ages apparently define two modes at 119 122 ma and 126 128 ma although the data set is probably insufficient to determine whether dike intrusion was continuous or episodic the ages of 128 119 ma are coeval with the final stages of the rifting at this latitude suggesting that the emplacement of the fl dikes is related to extension of the continental crust just prior to the formation of oceanic crust by comparing the fl pole with the existing paleomagnetic poles for the nw ponta grossa dike swarm mainly 130 5 ma to the north the parana volcanics mainly 133 132 ma and the central alkaline province 127 130 ma on the western side of the parana province it is concluded that the differences in pole positions reflect plate displacements and therefore the fl pole is probably dominated by the dikes with ages younger than 127 ma

analysis of normal fault populations in the kumano forearc basin nankai trough japan 1 multiple orientations and generations of faults from 3 d coherency mapping 1 analyses of normal faults in the kumano forearc basin of the nankai trough reveal multiple normal fault populations in a region generally thought to be under compression most faults have offsets of less than 20 m and dips of 60 70 and show no growth structures indicating that the faults were active for short periods of time the oldest generation of faults is older than 0 9 ma and strikes 50 60 the next oldest faults strike 160 170 are older than 0 44 ma and are related to local uplift along the western edge of the region the youngest faults cut the seafloor shallow faults near the se margin of the basin curve from 100 in the middle of the survey area to 145 at the se corner of the area the pattern of the two youngest fault populations is consistent with the regional stress pattern maximum horizontal stress subparallel to the trench orientations of older fault populations are caused by uplift of the underlying accretionary prism implying that the forearc basin region is not as stable as previously thought reconstruction of displacements on the youngest faults shows that the overall horizontal extension is less than 2 concentrated near the seaward edge of the basin the active normal faults distributed throughout the basin support the idea that the horizontal stress parallel to the plate convergence direction does not reach the critical stress to activate or form thrust faults and produce horizontal shortening within the shallow portion of the inner wedge

protracted felsic magmatic activity associated with the opening of the south atlantic felsic magmatism is often observed in the final stages of the eruption of continental large igneous provinces the occurrence of these felsic extrusive igneous rocks potentially provides key information on the complex interplay of magmatism and tectonics in this paper we investigate the timing of eruption of early cretaceous felsic magmas associated with the parana etendeka flood basalt province using the 40 ar 39 ar technique and interpret the results in terms of the recently proposed petrogenetic histories and dominant stress factors there appears to be a link between lithospheric thinning and a change in the origin of the felsic magmas from open system fractional crystallization to mid lower crustal melting particularly in southern uruguay

permanent fore arc extension and seismic segmentation insights from the 2010 maule earthquake chile 1 geologists have long known that young normal faults are an important structural element of the andean coastal cordillera but their relationship to the subduction seismic cycle is still unclear some of the largest aftershocks of the 2010 mw 8 8 maule earthquake in central chile were nucleated on upper plate normal faults including the mw 6 9 and 7 0 events of the pichilemu earthquake sequence we use the available coseismic gps displacements moment tensor sums and slip distribution models for the maule earthquake to compute the static strain and stress fields imposed on the upper plate by slip on the subduction interface the extensional strains calculated from coseismic gps and from a moment tensor sum of the pichilemu events have similar orientations and orders of magnitude the normal coulomb stress increment csi on the pichilemu fault has maximum positive stresses as high as 4 9 mpa regionally the maule event produced a semi elliptical radial pattern of static extension and deviatoric tension csi 1 5 mpa along the coastal cordillera enclosing the rupture area this elliptical pattern mimics the trends of the major upper crustal structures the static deformation field produced by a great subduction earthquake is an effective mechanism for generating permanent extension above the seismogenic zone reactivating suitably oriented long lived normal faults we suggest that the semi elliptical outline of the first order structures along the coastal cordillera may define the location of a characteristic long lived megathrust segment this observation implies a persistence at least over the quaternary of great subduction ruptures along the maule segment

accommodation of lithospheric shortening on mercury from altimetric profiles of ridges and lobate scarps measured during messenger flybys 1 and 2 the mercury laser altimeter on the nasa messenger mission has ranged to several ridges and lobate scarps during two equatorial flybys of the planet mercury the tectonic features sampled like others documented by spacecraft imaging and earth based radar are spatially isolated and have vertical relief in excess of 1 km the profiles also indicate that the faulting associated with their formation penetrated to tens of kilometers depth into the lithosphere and accommodated substantial shortening to gain insight into the mechanism s of strain accommodation across these structures we perform analytical and numerical modeling of representative dynamic localization mechanisms we find that ductile localization due to shear heating is not favored given our current understanding of thermal gradients and shallow thermal structure of mercury at the time of ridge and scarp formation and is likely to be of secondary importance at best brittle localization associated with loss of resistance during fault development or with velocity weakening during sliding on mature faults is weakly localizing but permits slip to accumulate over geological time scales the range of shallow thermal gradients that produce isolated faults rather than distributed fault sets under the assumption of modest fault weakening is consistent with previous models for mercury s early global thermal history to be consistent with strain rates predicted from thermal history models and the amount of shortening required to account for the underlying large offset faults ridges and scarps on mercury likely developed over geologically substantial time spans

cosmochemical and spectroscopic properties of northwest africa 7325 a consortium study this work is part of a project to build an infrared database in order to link ir data of planetary materials and therefore possible mercury material with remote sensing observations of mercury which will probably be obtained by the mertis instrument on the forthcoming bepicolombo mission the unique achondrite northwest africa nwa 7325 which has previously been suggested to represent the first sample from mercury was investigated by optical and electron microscopy and infrared and raman spectroscopy in addition the oxygen strontium xenon and argon isotopes were measured and the abundance of selected trace elements determined the meteorite is a cumulate rock with subchondritic abundances of hfse and ree and elevated sr contents which underwent a second heating and partial remelting process oxygen isotope measurements show that nwa 7325 plots in the ureilite field close to the alm a trachyandesitic fragment found in the unique almahata sitta meteorite breccia on the other hand mineralogical investigations of the pyroxenes in nwa 7325 provide evidence for similarities to the lodranites and acapulcoites furthermore the rock is weakly shocked and argon isotope data record ancient 4 5 ga plateau ages that have not been reset the sample records a cosmogenic exposure age of 19 ma systematics of rb sr indicate an extreme early volatile depletion of the precursor material similar to many other achondrite groups however despite its compositional similarities to other meteorite groups our results suggest that this meteorite is unique and unrelated to any other known achondrite group an origin for nwa 7325 as a sample from the planet mercury is not supported by the results of our investigation in particular the evidence from infrared spectroscopy indicates that a direct relationship between nwa 7325 and the planet mercury can be ruled out no acceptable spectral match between laboratory analyses and remote sensing observations from mercury has been obtained however we demonstrate that infrared spectroscopy is a rapid and nondestructive method to characterize mineral phases and thus an excellent tool for planetary surface characterization in space missions

tectonic controls on magmatism associated with continental break up an example from the parana etendeka province the high and low ti basalts of the parana etendeka province were primarily derived from old trace element enriched source regions in the lithospheric mantle and they are associated with dyke swarms of different orientations these swarms appear to reflect different amounts of extension and it is inferred that the high and low ti magma types were characterised by different melt generation rates of 0 15 km3 yr 1 and 0 4 km3 yr 1 respectively stewart et al earth planet sci lett 143 1996 95 109 there is probably a gap of 2 myr between the end of the main phase of cfb magmatism and the oldest rocks on the adjacent ocean floor a simple numerical model has been used to constrain the amounts and rates of melt generated from the continental lithosphere and asthenosphere under finite duration extension melting in the mantle is assumed to be controlled by the dry peridotite solidus in the asthenosphere and the hydrous 0 2 h2o peridotite solidus in the lithosphere for a maximum of 4 and a duration of extension of 10 myr the derivation of melt from the asthenosphere by dry peridotite melting depends primarily on potential temperature tp and is relatively insensitive to the thickness of the mbl while the converse is the case for melt derived from the lithosphere by hydrous peridotite melting for a tp of 1450 50 c inferred from the crustal thickness estimates along the rio grande rise and walvis ridge the model successfully generates 2 4 km of lithosphere derived melt before producing significant volumes of asthenosphere derived melt it is concluded that increases of melt volume with time can be generated by decompression melting of the mantle lithosphere critically in areas of significant melt generation within the mantle lithosphere during extension and break up there is likely to be a gap in the volcanic record between the end of melt generation in the lithosphere and the onset of melting in the underlying asthenosphere no such gap is present if all melts are generated within the mantle plume and thus these models may in principle be tested in the geologic record

short lived magmatic activity in an anorogenic subvolcanic complex 40ar 39ar and ion microprobe u pb zircon dating of the erongo damaraland namibia abstract the erongo subvolcanic center in namibia is the largest composite bimodal complex in the mesozoic etendeka igneous province of namibia this study of 40ar 39ar and high spatial resolution u pb zircon dating demonstrates that emplacement of the various igneous units at erongo took place within a time span equivalent to or shorter than geochronologically resolvable age differences ca 2 ma and at the peak of regional flood basalt activity in the etendeka parana province the erongo complex comprises a series of felsic volcanic and intrusive units that overlie or intrude basaltic lavas attributed to the etendeka group the stratigraphically oldest rhyodacite erongorus unit yielded u pb zircon ages of 131 8 1 1 ma 1 the overlying ombu rhyodacite was previously dated at 135 0 1 6 ma and our new ages for a resurgent intrusion of the same magma type ombu granodiorite in the vent area are 132 6 1 1 ma 40ar 39ar and 132 3 1 9 ma u pb zircon respectively the youngest felsic magmas at erongo are the high silica ekuta rhyolite and compositionally equivalent erongo granite these units yielded u pb zircon ages of 131 9 2 9 and 130 3 1 4 ma respectively concordant 40ar 39ar biotite ages from two samples of the granite average 132 2 0 8 ma the final stage of magmatism at erongo involved emplacement of basic alkaline plugs and dikes phlogopite and kaersutite from foidite plugs of this series yielded 40ar 39ar ages of 130 8 1 0 and 132 0 1 0 ma in a regional context these age results indicate that silicic magmatism in the largest complexes of the damaraland erongo brandberg paresis messum began simultaneously with the peak of flood basalt effusion at about 132 ma throughout the etendeka province and ceased by about 130 ma the silicic magmas are hybrid with varying contributions of crustal and mantle derived melts and the age constraints suggest that crustal melting was caused by a short lived thermal pulse related to the main flood basalt event basic magmatism in the damaraland complexes continued sporadically thereafter to about 123 ma but lacked the power to create further crustal melting

the influence of inherited structures on dike emplacement during gondwana breakup in southwestern africa abstracta kinematic analysis of cretaceous and pre cretaceous structures was carried out along the west coast of southern africa extending from the cape town area into namibia with the aim of investigating whether inherited structural discontinuities controlled gondwana breakup and the associated opening of the south atlantic in the cretaceous this was augmented by map and satellite image analyses of mainly cretaceous mafic dikes exposed from sw angola to the southern tip of africa the combined results provide consistent evidence of e w to ne sw directed extension during the cretaceous e w extension dominated from southern angola to the namibian south african border and ne sw directed extension occurred in an area extending from west central namibia to the southwestern coast of south africa these two extension directions overlap from west central namibia to southernmost namibia the kinematic analysis of pan african structures provides evidence of ene wsw directed shortening in the western saldania

analysis of normal fault populations in the kumano forearc basin nankai trough japan 2 principal axes of stress and strain from inversion of fault orientations 1 we use a high resolution 3 d seismic survey to map a population of recent normal faults within the kumano basin of the nankai subduction zone in order to quantify patterns of strain and stress state over the last 0 44 myr we identify distinct fault populations that define three phases of extension phases 1 and 2 comprise nw se striking faults located along the western basin edge and in the northwestern portion of the study area respectively the ne sw striking faults of phase 3 comprise the largest population and extend 20 km landward from the basin s seaward edge phase 2 faults typically terminate within a few reflectors of the seafloor whereas most phase 3 faults form seafloor scarps inversion of the fault populations documents ne sw extension during phases 1 and 2 and nw se extension during phase 3 consistent with both core scale structures and horizontal stress orientations observed at integrated ocean drilling program iodp boreholes slip on phase 3 faults accommodates strain of up to 1 2 concentrated near the basin s seaward edge inversion for a best fit stress tensor yields a subvertical 1 and subhorizontal 2 and 3 for all faulting phases we find that during phase 3 in most portions of the basin 2 3 shmax shmin reflecting widely varying fault strikes this contrasts with distinct shmax and shmin magnitudes inferred from iodp borehole data these observations may be reconciled if the orientation of maximum horizontal stress fluctuates due to variation of subduction parallel compression through the seismic cycle

modern sediment dispersal and accumulation on the outer poverty continental margin abstract the poverty margin was sampled in 2005 and 2006 as part of an international initiative to examine the terrestrial and marine sedimentary response to natural and human impacts on dispersal systems at mud dominated coasts the nsf margins source to sink initiative 210pb accumulation rates generally decrease from 1 g cm2 y in the mid shelf depocenter on the outer shelf to 0 1 g cm2 y on the mid slope plateau range 0 04 2 53 g cm2 y higher accumulation rates are observed all along the outer poverty shelf extending over the shelf break onto the upper poverty slope in canyons and gullies rates are fast in gullies that incise into the shelf edge 0 75 1 g cm2 y particularly in the area between waipaoa canyon and poverty canyon and in the axis of poverty canyon 1 29 1 89 g cm2 y below 1200 m water depth rates in the axis of poverty canyon are no more rapid than those found at open slope cores in similar water depths 0 12 0 15 g cm2 y excess 210pb profiles generally exhibit steady state characteristics except in the axis of poverty canyon where non steady state 210pb profiles are observed 137cs was not found above minimum detection limits or above statistical background levels in any cores from the poverty margin 7be was found distributed widely along and across the margin during both summer and winter periods observed to depths of up to 4 cm widespread presence of 7be on the margin in both winter and summer suggests that terrestrially derived sediment is reaching the outer shelf and upper slope throughout the year a sediment budget based on 210pb accumulation rate data shows that approximately 13 18 of the waipaoa annual discharge is accumulating on the outer poverty margin as a whole and approximately 11 15 of the annual discharge is accumulating on the continental slope alone notably approximately 28 of the sediment annually accumulating in the outer margin is being sequestered in slope gullies and canyons although they represent only 6 of the area of the outer margin

magmatism and continental break up in the south atlantic high precision40ar 39ar geochronology abstract a detailed ar ar study of the parana etendeka continental flood basalts cfb has been undertaken using laser spot analysis data provide information not only on the age of the samples but also on the variability of the non radiogenic ar component and state of alteration the results indicate the parana etendeka cfb were erupted over 10 million years between 137 and 127 ma providing a minimum overall eruption rate of 0 1 km3 yr 1 this is an order of magnitude less than that previously proposed for this the deccan and siberian cfb provinces but it is similar to estimates for hawaii iceland and the columbia river cfb in detail the new ar ar analyses indicate that magmatism within the parana etendeka province migrated from nw to se prior to and during the opening of the south atlantic providing an explanation for the asymmetry of the cfb lavas about the south atlantic moreover chemically defined magma types were erupted at different times in different places and so within the paranathey may not be used as reliable chronostratigraphic units rather such magmatic units may reflect the extent of compositionally distinct source regions in the uppermost mantle and indicate that partial melting took place over a wide area under the paranabasin the nw se migration of the onset of magmatism might be interpreted as a plume trace but the inferred rate of movement is 3 times faster than that inferred from subsequent magmatism on the rio grande rise it is argued that the onset of magmatism reflected extensional tectonics normal to the ponta grossa dyke swarm given that magmatism preceded the main phase of rifting and that extension across the ponta grossa was moderate current plume models have difficulty in predicting sufficient melting within the asthenosphere

age of the ponta grossa dike swarm brazil and implications to parana flood volcanism the ponta grossa dike swarm pgds occurs in a nw trending 200 km wide zone exposed just east of the parana basin in southeastern brazil the predominantly basaltic dikes intrude crystalline basement paleozoic mesozoic sediments and rarely flows of the parana etendeka flood volcanic province pep the pgds resembles the failed arm of a rift rift rift triple junction related to the separation of south america and africa detailed geochemical studies of the dikes including major minor trace element and sr nd isotopic analyses indicate that they probably represent feeders for the voluminous phase of flood volcanism represented by relatively uncontaminated predominantly high tio2 lavas of the northern pep where lava accumulations reach 1700 m thick 40ar 39ar stepwise degassing data using both laser and radiofrequency induction furnace on plagioclase separates from eighteen dikes and one sill yield seventeen plateau ages three are between 120 7 1 3 ma and 125 8 0 6 ma and fourteen are clustered between 129 2 0 4 ma and 131 4 0 5 ma isochron ages are not significantly different from the plateau ages and plateau ages are adopted in all but two cases as being the most accurate age estimates the age probability distribution for the dominant pulse 131 4 0 4 to 129 2 0 4 ma shows a pronounced peak at 130 5 ma this distribution probably reflects the magma production history in the region the new geochronologic data are consistent with conclusions based on paleomagnetic and chemical stratigraphic data that the pgds is younger than the volumetrically dominant majority of volcanism in the southern pep which occurred at 133 132 ma the younger commonly ne trending dikes may reflect the initiation of full drift which was coincident with major basin development in the adjacent continental borderland at 125 120 ma the pgds may indeed represent the failed arm of a rift rift rift triple junction but the triple junction did not coincide exactly in time or space with the site that would be inferred for plume impact

tectonics of the hellenic trench a synthesis of sea beam and submersible observations abstract detailed bathymetric investigations were performed in four zones of the hellenic trench system in 1978 using the multi narrow beam echo sounder sea beam on r v jean charcot in 1979 a field study using the 3000 m submersible cyana was conducted in three of the four sites covered by sea beam we summarize here the principal tectonic results of these studies and publish the corresponding sea beam maps it is shown that compressional structures affect the sedimentary cover of the outer wall and trench whereas extensional tectonics is present on the inner wall in addition there is evidence that most of the upper sedimentary cover is not subducted but rather piles up in front of the trench to form the mediterranean ridge we suggest that the surface limit of the zone of mechanical decoupling occurs near the base of the inner wall we show that the gross structure of the trench as well as the strain pattern observed is controlled by the kinematics of subduction and we discuss its evolution since its initiation about 13 m y ago

a revised control network for mercury an improved control net for mercury has been completed by utilizing images acquired during the three mariner 10 flybys in 1974 1975 relative positional errors within the net are 1 km on average and absolute locations are estimated to be better than 25 km the analytical triangulation resulted in new values for focal lengths camera a 1493 6 mm camera b 1500 1 mm w0 329 548 and camera orientation angles for 811 images acquired during all three flybys of mercury

testing early cretaceous africa south america fits with new palaeomagnetic data from the etendeka magmatic province namibia abstract we provide an independent test for the proposed pre breakup fits between the south american and african continental margins based on new palaeomagnetic data in the course of modelling the opening of the south atlantic ocean several reconstructions have been proposed which correspond to different estimates of the amount of pre drift extension in the south american and african continental margins and amounts of displacement along intracontinental accommodation zones the parana etendeka large igneous province erupted during a brief time interval between 136 and 131 ma just prior to opening of the south atlantic and has magmatic products on both the african etendeka lavas and south american parana lavas plates here we provide new palaeomagnetic constraints on the etendeka volcanic rocks these yield a declination of d dx 314 4 7 0 and inclination of i ix 42 7 8 3 plat 47 5 plon 88 9 k 26 1 a95 6 3 n 21 when our results are combined with another recently published dataset we obtain an etendeka palaeopole at 49 1 s 87 6 e d dx 315 9 3 8 i ix 43 9 4 4 k 23 4 a95 3 5 n 75 we compare three recent models for south atlantic ocean opening discuss which of the models best fits the palaeomagnetic data and present a revised reconstruction for west gondwana during the early cretaceous

the emissivity of a fine grained labradorite sample at typical mercury dayside temperatures abstract analyzing the surface composition of mercury s regolith from remote sensing measurements is a challenging task in support of the national aeronautics and space agency s mercury surface space environment geochemistry and ranging messenger mission and especially in preparation for the mercury radiometer and thermal infrared spectrometer mertis instrument on the bepicolombo mission of the european space agency and the japan aerospace exploration agency we are developing a planetary emissivity laboratory at deutsches zentrum fur luft und rahmfahrt dlr in berlin here we present the first measurements of labradorite as a mercury surface analog obtained with a test setup at a temperature of 420 c appropriate for the dayside of mercury because of limitations on the current test apparatus we present here data only for mid infrared wavelengths the spectra show strong indications of significant changes in spectral features in the mid infrared with changing temperature this result introduces an additional complication to the analysis of spectra obtained from the surface of mercury as the surface temperature and the thermal history of an observed area must be taken into account in the interpretation of the spectra this inference implies that the latitude and possibly also the time of day must be considered when analyzing a spectrum in particular the surface at the equator might exhibit different spectral characteristics than the surface at high latitudes even for the same composition these observations while still preliminary underscore strongly the need for temperature dependent measurements of emissivity of candidate planetary surface materials

rb sr age determinations of rocks from the okenyenya igneous complex northwestern namibia the okenyenya igneous complex is one of a suite of intrusions which define a prominent northeast trending linear feature in damaraland northwestern namibia precise rb sr internal isochron ages range from 128 6 1 to 123 4 1 4 ma for the major phases of intrusion identified within the complex the tholeiitic gabbros forming the outer rings of the complex and the later alkali gabbros which form the central hills cannot be distinguished in terms of rb sr ages although field relations clearly indicate the younger age of the latter the intrusionsof nepheline syenite and essexite comprising the mountain of okenyenya bergon the northern edge of the complex give ages of 123 4 1 4 and 126 3 1 ma respectively and form the final major phase of intrusion the ages obtained for early and late intrusive phases define a minimum magmatic life span of approximately 5 ma for the complex the determined age of the okenyenya igneous complex 129 123 ma when taken together with the few reliable published ages for other damaraland complexes 130 134 ma suggests that these sub volcanic complexes were emplaced contemporaneously with the widespread etendeka volcanics 130 ma and relate to magmatism associated with the breakup of southern africa and south america with the opening of the south atlantic ocean the linear distributionof intrusions in damaraland is interpreted to be due to magmatism resultingfrom the upwelling tristan plume being focused along a structural discontinuity between the pan african damaran terrain to the south and proterozoiccratonic basement to the north

3 d 40ar39ar geochronology in the parana continental flood basalt province new 40ar39ar analyses of borehole samples from the parana etendeka continental flood basalt province provide critical evidence of eruption rates in unexposed regions when combined with surface samples the new data clarify the duration of this major volcanic province new ages from both surface and borehole samples confirm a 10 12 million year duration of magmatism which post dates the tithonian mass extinction and provide a unique 3 dimensional picture of the spatial and temporal eruption of the lava pile chemically defined magma types are diachronous and the onset of magmatism occurred 500 1000 km inland migrating southeast towards the incipient south atlantic ocean calculated eruption rates increased from 0 03 km3 yr 1 between 138 and 135 ma through 0 13 km3 yr 1 between 135 and 133 ma to 0 21 km3 yr 1 between 133 and 131 ma supporting the notion that the rate of magmatism increased with time and proximity to the developing south atlantic after 131 ma the continental eruption rates dropped dramatically to 0 01 km3 yr 1 presumably because rifting had occurred and eruption centred in the oceanic rio grande rise with eruption rates of around 0 3 0 5 km3 yr 1 given the available constraints on the potential temperature of the plume thickness of the continental lithosphere and degrees of extension through time predicted melt production rates within the plume seriously underestimate the calculated eruption rates on the continent however the eruption rates can be reconciled if melting occurred by conductive heating of volatile enriched mantle the diachronous nature of the magma types supports the idea that melting occurred over a wide area and that the different magma types reflect different source regions rather than the temporal evolution of magmas from a single source given that 10 million years is appropriate to the timescales for conductive heating above the mantle plume it is suggested that these distinct source regions were located within the lithospheric mantle consistent with geochemical studies

lithofacies analysis of basic lava flows of the parana igneous province in the south hinge of torres syncline southern brazil abstract the parana igneous province records the volcanism of the earlier cretaceous that preceded the fragmentation of the gondwana supercontinent historically investigations of these rocks prioritized the acquisition of geochemical and isotopic data considering the volcanic pile as a monotonous succession of tabular flows this work provides a detailed analysis of the emplacement conditions of these basic volcanic rocks applying the facies analysis method integrated to petrographic and geochemical data the torres syncline is a nw se tectonic structure located in southern brazil where a thick sequence of the parana etendeka volcanic rocks is well preserved this study was performed in the south hinge of the syncline where the basaltic lava flows are divided into three lithofacies associations early compound pahoehoe early simple pahoehoe and late simple rubbly the first lavas that erupted were more primitive compound pahoehoe flow fields composed of olivine basalts with higher mgo contents and covered the sandstones of the botucatu formation the emplacement of compound pahoehoe flow fields is possibly related to intermittent low effusion rates whereas the emplacement of simple pahoehoe is related to sustained low effusion rates with continuous supply the thick simple rubbly lavas are associated with high effusion rates and were formed during the main phase of volcanism in the area the absence of paleosoils between the lavas and lithofacies associations suggests that the successive emplacement of the lava flows occurred in a relatively short time gap geochemically the lithofacies associations are low tio2 and belong to gramado magma type the lavas of the south hinge of the torres syncline have a similar evolution when compared to other continental basaltic provinces with earlier compound flows at the base and thicker simple flows in the upper portions

a subducted oceanic ridge influencing the nankai megathrust earthquake rupture megathrust earthquake rupture is highly influenced by seafloor bathymetric features such as oceanic fracture zones seamounts and ridges in convergent plate margins where these features subduct recent seismic reflection and refraction data reveal a trough parallel subducted oceanic ridge that is attached to the descending philippine sea plate beneath the accretionary wedge of the overriding eurasian plate in the eastern nankai trough subduction zone consisting of the tonankai and tokai segments the seismic survey data confirm the existence of subducted paleo zenisu ridge that was suggested by just magnetic data and as a result expands its distribution to the west the newly constrained subducted ridge that spans the two segments is estimated to be a maximum of 2 5 km high 20 30 km wide and 200 km long spatial mapping of the ridge shows that it is located roughly at the seaward edge of the coseismic rupture zone of the 1944 tonankai earthquake m 8 1 this ridge appears to be in close contact with the seaward end of the rigid backstop in the tonankai segment both the spatial correlation and the ridge backstop collision geometry suggest that the subducted ridge might be strongly mechanically coupled and may thus play a significant role as a seaward barrier inhibiting the 1944 earthquake rupture from propagating farther seaward

hikurangi margin tsunami earthquake generated by slow seismic rupture over a subducted seamount abstract tsunami earthquakes generate much larger tsunami than their surface wave magnitude would suggest and are a problem for tsunami warning systems they are often not accompanied by intense or even strong ground shaking and hence do not provide a natural warning for self evacuation the lesser known 1947 offshore poverty bay and tolaga bay earthquakes along the east coast of the north island new zealand share many characteristics with other well known tsunami earthquakes including low amplitude shaking long durations and anomalously large tsunami however these two new zealand events are rare in that their source area has been imaged directly by long offset 2d seismic reflection profiles in this contribution we propose a source model for the 1947 offshore poverty bay tsunami earthquake recognising that the hypocentre occurs in a region where seismic reflection and magnetic data support the existence of a shallow m 7 earthquakes with complex low velocity rupture scenarios that enhance tsunami waves and their role in seismic hazard should not be under estimated

investigating the shock histories of lunar meteorites miller range 090034 090070 and 090075 using petrography geochemistry and micro ftir spectroscopy fourier transform infrared ftir spectroscopy and cathodoluminescence cl imaging techniques combined with electron microprobe analyses have been used to determine the physical state of feldspathic phases that have been subject to varying levels of shock in the grouped lunar meteorites miller range 090034 090070 and 090075 six feldspathic phases have been identified based on spectral textural and chemical properties a specific infrared wavelength band ratio 1064 932 cm 1 equivalent to 9 40 10 73 m chosen because it can distinguish between some of the feldspathic phases can be used to estimate the pressure regimes experienced by these phases in addition ftir spatial mapping capabilities allow for visual comparison of variably shocked phases within the samples by comparing spectral and compositional data the origin and shock history of this lunar meteorite group has been determined with each of the shocked feldspathic phases being related to events in its geological evolution as such we highlight that ftir spectroscopy can be easily employed to identify shocked feldspathic phases in lunar samples estimate peak shock pressures and when compared with chemical data can be used to investigate their shock histories

evidence for a basalt free surface on mercury and implications for internal heat microwave and mid infrared observations reveal that mercury s surface contains less feo tio2 and at least as much feldspar as the lunar highlands the results are compatible with the high albedo brightness of mercury s surface at visible wavelengths in suggesting a rock and soil composition that is devoid of basalt the primary differentiate of terrestrial mantles the occurrence of a basalt free highly differentiated crust is in accord with recent models of the planet s thermal evolution and suggests that mercury has retained a hot interior as a result of a combination of inefficient mantle convection and minimal volcanic heat loss

spectra of extremely reduced assemblages implications for mercury we investigate the possibility that mercury s crust is very reduced with feo concentrations of less than 0 1 wt we believe that such a surface could have a composition of enstatite plagioclase diopside and sulfide similar to the mineral assemblages found in aubritic meteorites to test this hypothesis we investigated the spectra of aubrites and their constituent minerals as analogs for the surface of mercury we found that some sulfides have distinctive absorption features in their spectra shortwards of 0 6 m that may be apparent in the spectrum of such an object determination of the surface composition of mercury using orbital x ray spectroscopy should easily distinguish between a lunar highlands and enstatite basalt composition since these materials have significant differences in concentrations of al mg s and fe the strongest argument against mercury having an enstatite basalt composition is its extreme spectral redness significant reddening of the surface of an object such as mercury is believed to require reduction of feo to nanophase iron thus requiring a few percent feo in the material prior to alteration

dynamic processes controlling evolution of rifted basins abstract the extension of the lithosphere controlling the development of rifted basins is driven by a combination of plate boundary forces frictional forces exerted on the base of the lithosphere by the convecting asthenosphere and deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system although mantle plumes are not a primary driving force of rifting they play an important secondary role by weakening the lithosphere and by controlling the level of rift related volcanic activity a distinction between active and passive rifting is only conditionally justified the extension of the lithosphere depending on its rate and magnitude and the potential temperature of the asthenosphere can cause by adiabatic decompression partial melting of the lower lithosphere and upper asthenosphere in rift systems the level and timing of volcanic activity is highly variable the lack of volcanic activity implies passive rifting an initial passive rifting stage can be followed by a more active one during which magmatism plays an increasingly important role magmatic destabilization of the moho may account for the frequently observed discrepancy between upper and lower crustal extension factors combined with evidence for thermal thinning of the mantle lithosphere this suggest that the volume of the lithosphere is not necessarily preserved during rifting as advocated by conventional stretching models the structural style of rifts is controlled by the rheological structure of the lithosphere the availability of crustal discontinuities that can be tensionally reactivated the mode orthogonal or oblique and amount of extension and the lithological composition of pre and syn rift sediments simple shear extension prevails in rifts that subparallel the structural grain of the basement pure shear extension is typical for rifts cross cutting the basement grain pre existing crustal and mantle lithospheric discontinuities contribute to the localization of rift systems the duration of the rifting stage of extensional basins is highly variable stress field changes can cause abrupt termination of rifting in major rift systems progressive strain concentration on the zone of future crustal separation entails abandonment of lateral rifts depending on constraints on lateral block movements crustal separation can be achieved after as little as 9 my and as much as 280 my of rifting activity syn rift basin subsidence is controlled by isostatic adjustment of the crust to mechanical stretching of the lithosphere its magmatic inflation and thermal attenuation of the mantle lithosphere post rift basin subsidence is governed by thermal reequilibration of the lithosphere asthenosphere system deep seated thermal anomalies related to syn rift pull up of the asthenosphere lithosphere boundary have decayed after 60 my by about 65 and after 180 my by about 95 the magnitude of post rift subsidence is a function of the rift induced thermal anomaly and crustal density changes the potential temperature of the asthenosphere and initial water depths intraplate stresses can have an overprinting effect on post rift subsidence stretching factors derived from post rift subsidence analyses must be corrected for such effects

geological geochemical and isotope diversity of 134 ma dykes from the florianopolis dyke swarm parana magmatic province geodynamic controls on petrogenesis abstract the florianopolis dyke swarm fds one of the major dyke swarms belonging to the early cretaceous 135 131 ma parana magmatic province is largely dominated by high sr ti p basalts that are confirmed here as feeders of the unique urubici khumib lavas of the parana and edendeka lava piles on the basis of their age and geochemistry our study integrates field petrographic whole rock geochemistry and sr nd pb isotope geochemistry of representative samples from three main areas of exposition santa catarina island garopaba and pinheira beaches thus encompassing the whole extension of the fds compared to the urubici lavas the dykes have usually higher contents of lile and lree more radiogenic sr and pb and more unradiogenic nd features attributed to a more pronounced interaction with melts derived from the country rocks registered in the basic magmas that remained in the conduits some of these dykes show strongly interactive contacts that must be part of a wider zone of crustal melting probably more developed at greater depths small volumes of intermediate to acidic rocks form the cores of some composite dykes and correspond to products of fractional crystallization from urubici basalts contaminated with high rb sr and u th crustal melts probably derived from neoproterozoic granites as indicated by geochemical and sr nd pb isotope data the chemical and isotope signatures of the less contaminated fds basalts and related urubici lavas do not show clear evidence of inputs from primitive mantle and seem heavily influenced by enriched mantle this suggests that the mantle wedge that was affected by subduction during the neoproterozoic may have been frozen and coupled to the base of the lithospheric plate where the early cretaceous magmatism occurred a control of previous tectonic limits on the sources of the urubici basalts seems evident since they seem to be related to the younger lithosphere from the south domain related to the florianopolis batholith and no influence from the older cratonic lithosphere of the central domain can be identified in their feeders

geodetic evidence of back arc spreading in the mariana trough 1 repeated gps surveys in the mariana islands show that the mariana block is moving apart from the philippine sea plate the velocities of the islands relative to the philippine sea plate range from about 15 mm yr in the northern islands to about 45 mm yr near guam the data also suggest convergence rates for the mariana forearc with respect to the pacific plate of 35 45 mm yr at 19 n increasing to 55 70 mm yr at 13 5 n in addition the velocity vectors show a slight north south expansion of the arc the estimated location of the euler pole of the mariana forearc with respect to the philippine sea plate is well south of the geographical point where the back arc basin narrows to zero width

widespread seismicity excitation throughout central japan following the 2011 m 9 0 tohoku earthquake and its interpretation by coulomb stress transfer 1 we report on a broad and unprecedented increase in seismicity rate following the m 9 0 tohoku mainshock for m 2 earthquakes over inland japan parts of the japan sea and izu islands at distances of up to 425 km from the locus of high 15 m seismic slip on the megathrust such an increase was not seen for the 2004 m 9 1 sumatra or 2010 m 8 8 chile earthquakes but they lacked the seismic networks necessary to detect such small events here we explore the possibility that the rate changes are the product of static coulomb stress transfer to small faults we use the nodal planes of m 3 5 earthquakes as proxies for such small active faults and find that of fifteen regions averaging 80 by 80 km in size 11 show a positive association between calculated stress changes and the observed seismicity rate change 3 show a negative correlation and for one the changes are too small to assess this work demonstrates that seismicity can turn on in the nominal stress shadow of a mainshock as long as small geometrically diverse active faults exist there which is likely quite common