the petrogenesis of alkaline intrusives from arabia and northeast africa and their implications for within plate magmatism abstract trace element data from alkaline intrusions on both sides of the red sea indicate a within plate origin rb sr geochronology however demonstrates that alkaline magmatism in arabia was restricted to a post island arc emplacement of alkaline granite 650 500 ma whereas in northeast africa alkaline magmatism began with a similar phase but continued sporadically until the opening of the red sea at 30 ma with magmas becoming progressively undersaturated in silica with time isotope geochemistry of the earlier alkaline granites indicates a rb and lree depleted source region for the magmas and is consistent with a source in crustal rocks which had an age of less than 800 ma it is proposed that the time dependent undersaturation in silica resulted from the increasing role of asthenospheric volatiles and possibly magma in the petrogenesis of the younger alkaline magmas comparison between apparent polar wandering paths for africa inferred from recently published palaeomagnetic data and rb sr isochron ages from northeast africa and other alkaline provinces in africa active between 8 and 30 ma supports the theory that within plate magmatic emplacement is suppressed during periods of rapid lithospheric movement

numerical simulations of impact crater formation with dilatancy impact induced fracturing creates porosity that is responsible for many aspects of the geophysical signature of an impact crater this paper describes a simple model of dilatancy the creation of porosity in a shearing geological material and its implementation in the isale shock physics code the model is used to investigate impact induced dilatancy during simple and complex crater formation on earth simulations of simple crater formation produce porosity distributions consistent with observations dilatancy model parameters appropriate for low quality rock masses give the best agreement with observation more strongly dilatant behavior would require substantial postimpact porosity reduction the tendency for rock to dilate less when shearing under high pressure is an important property of the model pressure suppresses impact induced dilatancy in the shock wave at depth beneath the crater floor and in the convergent subcrater flow that forms the central uplift consequently subsurface porosity distribution is a strong function of crater size which is reflected in the inferred gravity anomaly the bouguer gravity anomaly for simulated craters smaller than 25 km is a broad low with a magnitude proportional to the crater radius larger craters exhibit a central gravity high within a suppressed gravity low lower crustal pressures on the moon relative to earth imply that impact induced dilatancy is more effective on the moon than earth for the same size impact in an initially nonporous target this difference may be mitigated by the presence of porosity in the lunar crust

a strain based porosity model for use in hydrocode simulations of impacts and implications for transient crater growth in porous targets numerical modelling of impact cratering has reached a high degree of sophistication however the treatment of porous materials still poses a large problem in hydrocode calculations we present a novel approach for dealing with porous compaction in numerical modelling of impact crater formation in contrast to previous attempts e g p alpha model snowplow model our model accounts for the collapse of pore space by assuming that the compaction function depends upon volumetric strain rather than pressure our new e alpha model requires only four input parameters and each has a physical meaning the model is simple and intuitive and shows good agreement with a wide variety of experimental data ranging from static compaction tests to highly dynamic impact experiments our major objective in developing the model is to investigate the effect of porosity and internal friction on transient crater formation we present preliminary numerical model results that suggest that both porosity and internal friction play an important role in limiting crater growth over a large range in gravity scaled source size

numerical modelling of impact melt production in porous rocks abstract melting of rocks by impact is a fundamental geologic process that results from shock wave compression and subsequent release from high pressure shock melted material is found in many terrestrial craters however most impact craters in sedimentary targets which are typically porous have been reported as containing significantly less melt than craters in nonporous crystalline targets to investigate the effect of porosity on shock melting we present a numerical modelling approach that combines a porosity compaction model with an equation of state for the solid component to demonstrate that our model accurately calculates shock pressures in porous materials we compare these results with hugoniot data for porous quartzite coconino sandstone and calcite our results show that the presence of porosity significantly reduces the critical pressure required for melting which enhances melt production we show that this effect is reduced but not totally diminished by faster shock wave decay in porous materials so that all other things being equal impact melt production is greater when target porosity is higher we conducted a series of simulations of impacts into targets of different material composition quartzite calcite dunite and porosity of up to 50 and varied the impact velocity between 6 and 50 km s we found a significant increase in the production of impact melt with increasing velocity in agreement with several previous studies and quantify the net increase in impact melt due to target porosity for the nonporous case our results agree well with estimated melt volumes at terrestrial impact craters over a range in size 7 90 km in diameter in porous targets our results contradict the widely cited observation of a melt deficiency in sedimentary target craters however our model results agree well with recently revised estimates of melt volumes at terrestrial craters formed in sedimentary or mixed sedimentary and crystalline targets this further strengthens the view that substantial volumes of impact melt should be produced during impacts into porous sedimentary targets and that the apparent paucity of impact melt in some sedimentary target craters may be due to difficulties in recognizing impact melts derived from sedimentary rocks

normal faulting origin for the cordillera and outer rook rings of orientale basin the moon 1 orientale basin is the youngest and best preserved large impact basin on the moon with at least four topographic rings contained within the topographic rim marked by the cordillera ring diameter 930 km its well exposed interior makes this basin a prime location to study basin formation processes forward mechanical modeling of basin ring topography shows that the outermost rings the cordillera ring cr and outer rook ring orr are large scale normal faults with displacements d of 0 8 to 5 2 km fault dip angles of 54 to 80 and vertical depth of faulting t between 19 and 37 km with most faults having t 30 5 km these faults and the distribution of maria inside the basin suggest that the transient crater important for determining many impact related characteristics such as projectile size was contained entirely within the orr and likely had a diameter between 500 and 550 km the difference in crustal thickness between the western and eastern sides of the basin is not a result of the basin forming event which indicates the formation of the hemispheric crustal thickness asymmetry was likely before the formation of orientale basin 3 68 to 3 85 ga

trace elements in continental margin magmatism part iii alkali granites and their relationship to cratonization summary alkali granites form as the final tectonic magmatic event in shield areas and in many phanerozoic orogenic belts they occur mostly as discordant post tectonic plutons plutons of various ages and geographic locations are lithologically extremely similar with an average composition of 72 5 sio 2 0 3 tio 2 14 1 a1 2 o 3 2 2 total fe as fe 2 o 3 0 4 mgo 1 3 cao 3 6 na 2 o and 4 8 k 2 o the alkali granite suites differ from the felsic rocks of most calcalkalic batholiths in having higher k 2 o mgo ratios and few rocks with less than 65 sio 2 alkali granite magmas may form by normal crystallization differentiation from calc alkalic adamellites quartz monzonites although not all alkali granite magmas have originated by that process most alkali granites do not show evidence of having been derived from a crustal source in some occurrences pre existing sial apparently was not present in the area at the time that the alkali granite magmas evolved thus alkali granites may be part of the process that causes development of stable cratons by addition of new sialic material to thicken the continental crust the higher concentration of k 2 o in younger shield areas and compensatory changes in abundances of other elements are caused by the higher abundance of alkali granites in younger cratons

the formation of lunar mascon basins from impact to contemporary form positive free air gravity anomalies associated with large lunar impact basins represent a superisostatic mass concentration or mascon high resolution lunar gravity data from the gravity recovery and interior laboratory spacecraft reveal that these mascons are part of a bulls eye pattern in which the central positive anomaly is surrounded by an annulus of negative anomalies which in turn is surrounded by an outer annulus of positive anomalies to understand the origin of this gravity pattern we modeled numerically the entire evolution of basin formation from impact to contemporary form with a hydrocode we simulated impact excavation and collapse and show that during the major basin forming era the preimpact crust and mantle were sufficiently weak to enable a crustal cap to flow back over and cover the mantle exposed by the impact within hours with hydrocode results as initial conditions we simulated subsequent cooling and viscoelastic relaxation of topography using a finite element model focusing on the mare free freundlich sharonov and mare infilled humorum basins by constraining these models with measured free air and bouguer gravity anomalies as well as surface topography we show that lunar basins evolve by isostatic adjustment from an initially subisostatic state following the collapse stage the key to the development of a superisostatic inner basin center is its mechanical coupling to the outer basin that rises in response to subisostatic stresses enabling the inner basin to rise above isostatic equilibrium our calculations relate basin size to impactor diameter and velocity and they constrain the preimpact lunar thermal structure crustal thickness viscoelastic rheology and for the humorum basin the thickness of its postimpact mare fill

asymmetric thermal evolution of the moon 1 the moon possesses a clear dichotomy in geological processes between the nearside and farside hemispheres the most pronounced expressions of this dichotomy are the strong concentration of radioactive heat sources on the nearside in a region known as the procellarum kreep terrane pkt and the mare basaltic lava flows that erupted in or adjacent to this terrane we model the thermochemical evolution of the moon using a 3 d spherical thermochemical convection code in order to assess the consequences of a layer enriched in heat sources below the pkt on the moon s global evolution we find that in addition to localizing most of the melt production on the nearside such an enriched concentration of heat sources in the pkt crust has an influence down to the core mantle boundary and leaves a present day temperature anomaly within the nearside mantle moderate gravitational and topographic anomalies that are predicted in the pkt but not observed may be masked either by crustal thinning or gravitational anomalies from dense material in the underlying mantle our models also predict crystallization of an inner core for sulfur concentrations less than 6 wt

nonuniform cratering of the moon and a revised crater chronology of the inner solar system abstract we model the cratering of the moon and terrestrial planets from the present knowledge of the orbital and size distribution of asteroids and comets in the inner solar system in order to refine the crater chronology method impact occurrences locations velocities and incidence angles are calculated semi analytically and scaling laws are used to convert impactor sizes into crater sizes our approach is generalizable to other moons or planets the lunar cratering rate varies with both latitude and longitude with respect to the global average it is about 25 lower at 65 n 90 e and larger by the same amount at the apex of motion 0 n 90 w for the present earth moon separation the measured size frequency distributions of lunar craters are reconciled with the observed population of near earth objects under the assumption that craters smaller than a few kilometers in diameter form in a porous megaregolith varying depths of this megaregolith between the mare and highlands is a plausible partial explanation for differences in previously reported measured size frequency distributions we give a revised analytical relationship between the number of craters and the age of a lunar surface for the inner planets expected size frequency crater distributions are calculated that account for differences in impact conditions and the age of a few key geologic units is given we estimate the orientale and caloris basins to be 3 73 ga old and the surface of venus to be 240 ma old the terrestrial cratering record is consistent with the revised chronology and a constant impact rate over the last 400 ma better knowledge of the orbital dynamics crater scaling laws and megaregolith properties are needed to confidently assess the net uncertainty of the model ages that result from the combination of numerous steps from the observation of asteroids to the formation of craters our model may be inaccurate for periods prior to 3 5 ga because of a different impactor population or for craters smaller than a few kilometers on mars and mercury due to the presence of subsurface ice and to the abundance of large secondaries respectively standard parameter values allow for the first time to naturally reproduce both the size distribution and absolute number of lunar craters up to 3 5 ga ago and give self consistent estimates of the planetary cratering rates relative to the moon

crustal accretion in the pan african nd and sr isotope evidence from the arabian shield abstract nd and sr isotope determinations on late precambrian to early palaeozoic igneous and sedimentary rocks from the arabian shield are used to investigate the proportion of reworked older crust and the rate at which new crust was generated during the pan african event eight rb sr whole rock isochrons on igneous suites yield ages in the range 770 590 ma and initial 87 sr 86 sr ratios of 0 7038 0 7023 these data confirm that magmatism in this area was largely restricted to the period 850 550 ma and the initial ratios are sufficiently low to preclude significant contributions from a long lived upper crustal source the initial 143 nd 144 nd ratios of a variety of lithologies including several samples of possible basement are all higher than the contemporaneous values for chur e nd 1 6 to 6 9 suggesting that many were derived directly from the upper mantle and that any inferred crustal source regions for the remainder could not have separated from likely lree depleted mantle reservoirs before 1200 ma the arabian shield therefore provides an example of rapid crustal growth during the late proterozoic and contrasts with the damara intracratonic belt of namibia where nd and sr isotopes provide strong evidence for extensive reworking of older continental crust during the same period

investigating the onset of multi ring impact basin formation multi ring basins represent some of the largest oldest rarest and therefore least understood impact crater structures various theories have been put forward to explain their formation there is currently however no consensus here numerical modeling is used to investigate the onset of multi ring basin formation on the moon using two thermal profiles suitable for the lunar basin forming epoch various multi ring basin formation hypotheses are discussed compared and evaluated against target deformation and strain distribution in the models as well as geological and geophysical observations the mechanism that most closely resembles the numerical models in terms of basin formation and structure as well as observations appears to be the ring tectonic theory whereby ring formation is dependent on transient cavities penetrating entirely through the moon s lithosphere into the asthenosphere below the numerical models suggest that all lunar basins larger than schrodinger 320 km diameter should be capable of forming multiple rings as their transient cavities penetrate into the asthenosphere for both thermal profiles additionally the models demonstrate that the target s thermal profile starts to influence basin formation and structure when impact energy exceeds that of the schrodinger event

ancient multiring basins on the moon revealed by clementine laser altimetry analysis of laser altimetry data from clementine has confirmed and extended our knowledge of nearly obliterated multiring basins on the moon these basins were formed during the early bombardment phase of lunar history have been filled to varying degrees by mare lavas and regional ejecta blankets and have been degraded by the superposition of large impact craters the mendel rydberg basin a degraded three ring feature over 600 kilometers in diameter on the lunar western limb is about 6 kilometers deep from rim to floor only slightly less deep than the nearby younger and much better preserved orientale basin 8 kilometers deep the south pole aitken basin the oldest discernible impact feature on the moon is revealed as a basin 2500 kilometers in diameter with an average depth of more than 13 kilometers rim crest to floor this feature is the largest deepest impact crater yet discovered in the solar system several additional depressions seen in the data may represent previously unmapped ancient impact basins

u pb dating of precambrian rocks from northern cameroon orogenic evolution and chronology of the pan african belt of central africa abstract northern cameroon has been classically divided into an old basement complex a middle proterozoic cover series intermediaires and granites emplaced during the pan african orogeny new u pb geochronological data on different rock units from this region are presented a rhyolite from the poli schists series intermediaires gives an age close to 800 ma migmatitic gneisses of the basement complex seem to derive from different detrital or magmatic sources the metadiorites and foliated granites of the basement complex give ages around 630 ma the 800 ma age is interpreted as dating volcanic activity probably related to the initial stage of the pan african evolution ages around 630 ma date a syn tectonic plutonism dominated by a regional scale basic to intermediate plutonic suite bip zircons of the bip suite are almost concordant and show no evidence of inheritance the suite shows calc alkaline affinity and relatively low 87 sr 86 sr initial ratios thus the bip suite represents material newly added to the crust during the panafrican orogeny these data are at variance with previous interpretations of this part of the mobile belt as old precambrian crust reactivated during the pan african orogeny the previous distinction between the series intermediaires and a regional basement complex must be reappraised similar ages being found in both units a chronological succession of the pan african evolution is established and tentatively compared with that of the trans saharan belt in nigeria and hoggar

formation of the orientale lunar multiring basin multiring basins large impact craters characterized by multiple concentric topographic rings dominate the stratigraphy tectonics and crustal structure of the moon using a hydrocode we simulated the formation of the orientale multiring basin producing a subsurface structure consistent with high resolution gravity data from the gravity recovery and interior laboratory grail spacecraft the simulated impact produced a transient crater 390 kilometers in diameter that was not maintained because of subsequent gravitational collapse our simulations indicate that the flow of warm weak material at depth was crucial to the formation of the basin s outer rings which are large normal faults that formed at different times during the collapse stage the key parameters controlling ring location and spacing are impactor diameter and lunar thermal gradients

geochronology geochemistry and tectonics of the ne bayuda desert n sudan implications for the western margin of the late proterozoic fold belt of ne africa abstract five suites of rocks collected from the precambrian basement in the ne bayuda desert of central northern sudan give late proterozoic whole rock rb sr isochron ages the abu harik complex thought by some previous workers to be an older basement gives an age of 898 51 ma upper amphibolite facies metasediments give a metamorphic age of 761 22 ma the supposedly younger greenschist facies el koro volcanic series were erupted 800 83 ma ago these are chemically similar to the volcanics which unconformably overlie the sol hamed ophiolite in the red sea hills of ne sudan and to some modern island are volcanics the metasediments were intruded 678 43 ma ago by the diefallab granite which is itself deformed the younger weakly deformed amaki series with a basal conglomerate containing basement clasts overlain by purple grits is probably equivalent to the molasse type hammamat group of the eastern desert of egypt which was deposited between 616 and 596 ma ago finally the post tectonic shallal granite with within plate geochemistry was intruded 549 12 ma ago geochemical data suggest that the abu harik complex the el koro volcanic series and the diefallab granite are arc related magmatic rocks they were intruded into or thrust onto shallow water shelf sediments during subduction and then collision between c 900 and 550 ma the data presented here give no support to previous views that the high grade metasediments were metamorphosed prior to the late proterozoic events that they unconformably overlie a still older perhaps archaean basement or that they are unconformably overlain by younger late proterozoic low grade volcanics the precambrian rocks along the e side of the bayuda desert must now all be assigned to the late proterozoic and the boundary between this late proterozoic fold belt and an older craton known to crop out at jebel uweinat must lie farther to the w

gravity field of the orientale basin from the gravity recovery and interior laboratory mission the orientale basin is the youngest and best preserved major impact structure on the moon we used the gravity recovery and interior laboratory grail spacecraft to investigate the gravitational field of orientale at 3 to 5 kilometer km horizontal resolution a volume of at least 3 4 0 2 106 km3 of crustal material was removed and redistributed during basin formation there is no preserved evidence of the transient crater that would reveal the basin s maximum volume but its diameter may now be inferred to be between 320 and 460 km the gravity field resolves distinctive structures of orientale s three rings and suggests the presence of faults associated with the outer two that penetrate to the mantle the crustal structure of orientale provides constraints on the formation of multiring basins

estimating transient crater size using the crustal annular bulge insights from numerical modeling of lunar basin scale impacts 1 the transient crater is an important impact cratering concept its volume and diameter can be used to predict impact energy and momentum impact melt volume and maximum depth and volume of ejected material transient crater sizes are often estimated using scaling laws based on final crater rim diameters however crater rim estimates especially for lunar basins can be controversial here we use numerical modeling of lunar basin scale impacts to produce a new alternative method for estimating transient crater radius using the annular bulge of crust observed beneath most lunar basins using target thermal conditions appropriate for the lunar imbrian and nectarian periods we find this relationship to be dependent on lunar crust and upper mantle temperatures this result is potentially important when analyzing lunar basin subsurface structures inferred from the grail mission

age and chemical characteristics of some pre pan african rocks in the egyptian shield abstract u pb ages on zircons from several types of sialic rocks together with a variety of geochemical data on one sample of early plutonic activity within the egyptian shield are presented the earliest autochthonous granitic rocks in the egyptian shield are probably quartz diorites tonalites emplaced just prior to the main peak of pan african igneous activity 550 650 ma the studied example has an age of 711 ma 7 ma and is characterized by very low initial sr ratio 87sr 86sr 0 7026 and very low abundances of k2o and related large ion lithophile trace elements ree contents are low less than 20 chondritic abundances abundance patterns show only moderate light rare earth enrichment la yb 4 5 these characteristics are inconsistent with any models requiring fractional fusion of pre existing continental crustal material granitic and arkosic cobbles occur in rare conglomerate beds within volcanoclastic and greywacke sedimentary sequences the cobbles show a wide range in ages 1 1 2 3 ga and have no obvious source within the egyptian shield it is suggested that they are not indicative of any sialic plutons in the egyptian shield which might predate deposition of the enclosing immature sediments more probably they were derived from adjacent continental areas such as the uweinat region to the west and were deposited in an evolving arc ocean basin complex

projectile remnants in central peaks of lunar impact craters unusual minerals observed in lunar craters were thought to originate from beneath the moon s surface numerical simulations show that rather than being vaporized much of the impactor material can survive in the crater implying that the unusual minerals come from the impactor and may not be indigenous to the moon

elliptical structure of the lunar south pole aitken basin abstract the south pole aitken basin sp a is the largest and oldest basin on the moon the basin has usually been interpreted to exhibit a degraded circular structure but here we demonstrate that the topography iron and thorium signatures of the basin are well described by ellipses with axes measuring 2400 by 2050 km and centered at 53 191 e topography abundances of iron thorium and the distribution of mare basalts are all elevated in the northern halves of the ellipses we also identify an outer topographic ellipse whose semiminor axis scales with the main topographic ellipse by approximately 2 taken together these data imply that the basin was created by an oblique impact along an azimuth of approximately 19 measured counterclockwise from longitude 191 e the geometry of the elevated central farside topography surrounding sp a suggests that it predates the impact the elliptical ring structures of sp a and their scaling relationships will help to understand the formation of large and elliptical basins elsewhere in the solar system this refined basin shape will also inform local geology geochemistry and geophysics of the region

a simple mechanical model of valhalla basin callisto the valhalla basin on callisto is a multiringed structu e that extends over much of the satellite s surface although its appearance differs in detail from lunar multiringed basins its origin may also be attributed to lithospheric fragmentation accompanying collapse of the transient crater formed by an impact event this paper explores the mechanics of the collapse process by treating the lithosphere as a thin elastic von mises plastic sheet plane geometry or shell spherical geometry flow of the underlying asthenosphere inward toward the crater cavity induces plastic failure of the lithosphere and produces a characteristic pattern of faults in the disrupted lithosphere the pattern and extent of faulting is a function of a single dimensionless parameter that involves the strength and thickness of the lithosphere the crater depth and diameter and the surface gravity of the planet the tectonic structures of valhalla correspond well with the failure pattern expected for a large crater produced in a thin circa 30 km weak strength 100 bar lithospheric shell

lead isotope systematics of some igneous rocks from the egyptian shield lead isotope data on late precambrian igneous rocks from the eastern desert of egypt are presented previous work has indicated that this igneous suite is characterized by uniformly low initial ratios of 37sr86sr 0 7035 the pb data define three groups loosely corresponding to age an older tonalite to granodiorite otg suite with ages in the range 610 710 ma has pb isotope characteristics similar to modern mantle derived oceanic mafic rocks the age corrected initial pb isotope ratios of the otg group lie near the ocean regression line in correlation diagrams of 206pb204pb vs 207pb204pb and 203pb204pb the isotope data imply an origin for the otg group analogous to calc alkalic igneous rocks in modern intra oceanic island arcs a younger suite of k rich plutonic rocks 570 595 ma has similar 206pb204pb and 208pb204pb initial ratios relative to the otg suite but have higher 207pb204pb initial ratios when calculated from measured whole rock u and pb contents however k feldspar and galena associated with this suite indicate initial pb isotope ratios close to the otg suite we interpret this to indicate that the younger granites could have originated by anatexis of otg type material but they probably experienced an episode of metasomatism after emplacement a suite of volcanic rocks the dokhan volcanics from the central eastern desert are intermediate between the younger granites and otg in both age 610 ma and pb isotope characteristics limited data on initial 208pb204pb ratios suggest that all the egyptian samples originated from a source with anomalously low thpb ratios although the relative abundance of granitic rocks in the egyptian shield decreases to the south no evidence of north south heterogeneity in the pb isotope composition of the respective source regions is apparent in the data however the westernmost sample studied from the aswan area contains distinctly more radiogenic pb relative to the mean this location may mark the boundary between the late precambrian ensimatic orogen now outcropping in the egyptian and saudi arabian shields and an older sialic craton to the west

pan african structures ophiolites and melange in the eastem desert of egypt a traverse at 26 n a detailed structural traverse across the basement rocks of the eastern desert of egypt shows that they consist apart from intrusions of four broadly recumbent tectonic units the lowest of arkosic metasediments of continental shelf facies is exposed in a dome this unit is overlain by an allochthonous ophiolitic melange containing complete and dismembered ophiolitic masses in a matrix of deep oceanic graphitic pelites and turbidites a near horizontal schistose to mylonitic fabric most intense near and below the base of the melange indicates extensive lateral tectonic transport of the ophiolitic melange over the shelf metasediments but the melange originated as an olistostrome before being tectonically transported the melange extends over an area of at least 10 000 km 2 it is locally overlain by calc alkaline volcanics in which deformation is less intense but increases and schistosity flattens downwards indicating some translation over the melange unconformable on the melange and calc alkaline volcanics is a molasse facies series itself also locally strongly deformed late tectonic granites preceded and locally post date the molasse facies sediments still later diapiric peralkaline riebeckite granites locally up domed the recumbent structures the tectonic evolution is related to a late proterozoic subduction zone to the se of the region

distribution of oceanic and continental leads in the arabian nubian shield new common lead data for feldspar whole rock and galena samples from the arabian nubian shield together with data from previous work can be divided into two main groups group i leads have oceanic mantle characteristics whereas group ii leads have incorporated a continental crustal component of at least early proterozoic age the group i leads are found in rocks from the red sea hills of egypt and the western and southern parts of the arabian shield group ii leads are found in rocks from the northeastern and eastern parts of the arabian shield as well as from the southeastern shield near najran they are also found in rocks to the south in yemen to the east in oman and to the west at aswan egypt this distribution of data suggests that the arabian nubian shield has an oceanic core flanked by rocks that have developed at least in part from older continental material two mechanisms are suggested by which this older lead component could have been incorporated into the late proterozoic rocks and each may have operated in different parts of the shield the older lead component either was derived directly from an underlying early proterozoic basement or was incorporated from subducted pelagic sediments or sediments derived from an adjacent continent

ejection of rock fragments from planetary bodies the recent discovery of alha81005 a small meteorite that undoubtedly came from the moon has raised questions about how an intact rock fragment could survive acceleration to planetary escape velocities this acceleration could only have been produced by a large impact on the lunar surface a small amount of material 0 005 to 0 02 projectile volume may be ejected from an impact crater at speeds exceeding lunar escape velocity without suffering petrographically detectable shock damage the ejected material is protected by stress wave interference close to the free surface the existence and size of this zone depend upon parameters such as the rise time of the stress pulse produced by an impact the fragment size is a function of ejection velocity the lunar meteorite was ejected during an impact event that produced a crater at least 3 6 km in diameter other meteorites may have originated in a similar way

using landscape spatial relationships to improve estimates of land cover area from coarse resolution remote sensing abstract a two stage modeling strategy significantly improves land cover area estimates from low spatial resolution remote sensing by correcting measurements of class proportions within large blocks of pixels vegetation class type information is developed through supervised classification of thematic mapper spectral data at both fine 30 m and coarse 1020 m resolutions stage 1 models use measurements of landscape spatial properties to estimate the slopes and intercepts of proportion transition relationships between fine and coarse resolution classes within randomly located pixel blocks following this step a stage ii model uses a linear estimator to predict true class proportions based on measured coarse scale proportions and the slope and intercept estimates from the stage i models model development and testing on a training site is followed by testing and inversion for a validation site model inversion involves using spatial variables measured only at the coarse resolution as input to the stage i models for both the training and the validation data the procedure results in a statistically significant reduction in error when estimating land cover area by class type within the sampling blocks

significance of contrasting magmatism in north east africa and saudi arabia the red sea is a north south rift that split the arabian nubian shield into eastern arabian and western egypt and sudan segments by seafloor spreading less than 30 myr ago1 because geological studies in the region have generally have confined by national frontiers a comparative study of magmatic evolution on either side of the red sea has not so far as we are aware previously been undertaken and current models for arabian nubian shield evolution suggest broadly similar histories for the basement on both sides of the red sea however although the igneous rock geochronology on both sides of the red sea indicate that precambrian magmatic histories were broadly similar since then they have been quite distinct in that the african crust has been invaded episodically by quartz syenites nepheline syenites and peralkaline granites between 500 and 80 myr no such igneous activity is evident in arabia fig 1 we propose here that the red sea line has had structural significance from at least the end of the cambrian though whether it defines a line of contrasting upper mantle or lower crust geochemistry or of contrasting crustal rigidity cannot yet be established

proterozoic deformation of the east saharan craton in southeast libya south egypt and north sudan abstract the basement areas in southeast libya south egypt and north sudan west of the nile between gebel uweinat and the bayuda desert are part of an approximately 1000 km wide complexly folded polymetamorphic zone with a regional n nne ne ene trend of foliation and fold axis since this belt extends southwestward into the area of zalingei in the southern darfur block west sudan it is named the northern zalingei fold zone sr and nd isotopic studies suggest that this zone is older than pan african and further indicate that apart from archean rocks in the gebel uweinat area this belt is of early middle proterozoic age an early middle proterozoic three stage deformational and anatectic event established the present day fold and fault geometry in the western parts of this zone in the gebel uweinat gebel kamil area the pan african tectono thermal episode was most effective in the eastern part of the belt near the boundary with the nubian shield volcano sedimentary ophiolite granitoid assemblages it caused migmatization granite emplacement mylonitization and large scale wrench faulting which was related to late proterozoic accretionary and collisional events of the arabian nubian shield with the margin of the east saharan craton

numerical modeling of the formation and structure of the orientale impact basin 1 the orientale impact basin is the youngest and best preserved lunar multi ring basin and has thus been the focus of studies investigating basin forming processes and final structures a consensus about how multi ring basins form however remains elusive here we numerically model the orientale basin forming impact with the aim of resolving some of the uncertainties associated with this basin by using two thermal profiles estimating lunar conditions at the time of orientale s formation and constraining the numerical models with crustal structures inferred from gravity data we provide estimates for orientale s impact energy 2 9 1025 j impactor size 50 80 km diameter transient crater size 320 480 km excavation depth 40 55 km and impact melt volume 106 km3 we also analyze the distribution and deformation of target material and compare our model results and orientale observations with the chicxulub crater to investigate similarities between these two impact structures

subsurface morphology and scaling of lunar impact basins impact bombardment during the first billion years after the formation of the moon produced at least several tens of basins the gravity recovery and interior laboratory grail mission mapped the gravity field of these impact structures at significantly higher spatial resolution than previous missions allowing for detailed subsurface and morphological analyses to be made across the entire globe grail derived crustal thickness maps were used to define the regions of crustal thinning observed in centers of lunar impact basins which represents a less unambiguous measure of a basin size than those based on topographic features the formation of lunar impact basins was modeled numerically by using the isale 2d hydrocode with a large range of impact and target conditions typical for the first billion years of lunar evolution in the investigated range of impactor and target conditions the target temperature had the dominant effect on the basin subsurface morphology model results were also used to update current impact scaling relationships applicable to the lunar setting based on assumed target temperature our new temperature dependent impact scaling relationships provide estimates of impact conditions and transient crater diameters for the majority of impact basins mapped by grail as the formation of lunar impact basins is associated with the first approximately 700 myr of the solar system evolution when the impact flux was considerably larger than the present day our revised impact scaling relationships can aid further analyses and understanding of the extent of impact bombardment on the moon and terrestrial planets in the early solar system

modeling damage and deformation in impact simulations numerical modeling is a powerful tool for investigating the formation of large impact craters but is one that must be validated with observational evidence quantitative analysis of damage and deformation in the target surrounding an impact event provides a promising means of validation for numerical models of terrestrial impact craters particularly in cases where the final pristine crater morphology is ambiguous or unknown in this paper we discuss the aspects of the behavior of brittle materials important for the accurate simulation of damage and deformation surrounding an impact event and the care required to interpret the results we demonstrate this with an example simulation of an impact into a terrestrial granite target that produces a 10 km diameter transient crater the results of the simulation are shown in terms of damage a scalar quantity that reflects the totality of fragmentation and plastic strain both total plastic strain the accumulated amount of permanent shear deformation regardless of the sense of shear and net plastic strain the amount of permanent shear deformation where the sense of shear is accounted for damage and plastic strain are both greatest close to the impact site and decline with radial distance however the reversal in flow patterns from the downward and outward excavation flow to the inward and upward collapse flow implies that net plastic strains may be significantly lower than total plastic strains plastic strain in brittle rocks is very heterogeneous however continuum modeling requires that the deformation of the target during an impact event be described in terms of an average strain that applies over a large volume of rock large compared to the spacing between individual zones of sliding this paper demonstrates that model predictions of smooth average strain are entirely consistent with an actual strain concentrated along very narrow zones furthermore we suggest that model predictions of total accumulated strain should correlate with observable variations in bulk density and seismic velocity

the transition from complex crater to peak ring basin on the moon new observations from the lunar orbiter laser altimeter lola instrument impact craters on planetary bodies transition with increasing size from simple to complex to peak ring basins and finally to multi ring basins important to understanding the relationship between complex craters with central peaks and multi ring basins is the analysis of protobasins exhibiting a rim crest and interior ring plus a central peak and peak ring basins exhibiting a rim crest and an interior ring new data have permitted improved portrayal and classification of these transitional features on the moon we used new 128 pixel degree gridded topographic data from the lunar orbiter laser altimeter lola instrument onboard the lunar reconnaissance orbiter combined with image mosaics to conduct a survey of craters 50 km in diameter on the moon and to update the existing catalogs of lunar peak ring basins and protobasins our updated catalog includes 17 peak ring basins rim crest diameters range from 207 km to 582 km geometric mean 343 km and 3 protobasins 137 170 km geometric mean 157 km several basins inferred to be multi ring basins in prior studies apollo moscoviense grimaldi freundlich sharonov coulomb sarton and korolev are now classified as peak ring basins due to their similarities with lunar peak ring basin morphologies and absence of definitive topographic ring structures greater than two in number we also include in our catalog 23 craters exhibiting small ring like clusters of peaks 50 205 km geometric mean 81 km one humboldt exhibits a rim crest diameter and an interior morphology that may be uniquely transitional to the process of forming peak rings a power law fit to ring diameters d ring and rim crest diameters dr of peak ring basins on the

a unified description of localization for application to large scale tectonics 1 localized regions of deformation such as faults and shear zones are ubiquitous in the earth s lithosphere however we lack a simple unified framework of localization that is independent of the mechanism or scale of localization we address this issue by introducing the effective stress exponent ne a parameter that describes how a material responds to a local perturbation of an internal variable being tested for localization the value of ne is based on micromechanics a localizing regime has a negative ne indicating a weakening behavior and localization is stronger for more negative 1 ne we present expressions for the effective stress exponent associated with several mechanisms that trigger localization at large scale brittle failure with loss of cohesion elastoplasticity rate and state dependent friction shear heating and grain size feedback in ductile rocks in most cases localization does not arise solely from the relation between stress and deformation but instead requires a positive feedback between the rheology and internal variables brittle mechanisms failure and friction are generally described by ne of the order of 100 shear heating requires an already localized forcing which could be provided by a brittle fault at shallower levels of the lithosphere grain size reduction combined with a transition from dislocation to diffusion creep leads to localization only if the grain size departs significantly from its equilibrium value because either large scale flow moves rocks through different thermodynamic environments or new grains are nucleated when shear heating or grain size feedback produce localization 1 ne can be extremely negative and can control lithospheric scale localization

samarium neodymium data on two late proterozoic ophiolites of saudi arabia and implications for crustal and mantle evolution whole rock and mineral samples from the jabal al wask and jabal ess ophiolites northwestern saudi arabia yield sm nd isochron ages of 743 24 ma and 782 38 ma respectively these formation ages which provide maximum limits for possible obduction ages are in broad but not precise agreement with the previously known geologic history of the arabian shield they indicate that the ophiolitic rocks are roughly coeval with nearby volcanic and plutonic rocks supporting a back arc origin for the two ophiolites we suggest that the jabal al wask and jabal ess ophiolites were parts of the same northeast southwest trending ophiolite belt now offset along the najd fault system initial nd values range from 6 6 to 7 6 indicating derivation from a mantle source that has been lil depleted for at least 2 ga reported nd values from the arabian shield that are lower than this suggest the presence of older reworked continental crust

the late proterozoic ophiolite of sol hamed ne sudan abstract the sol hamed complex a sequence from ultramafics through gabbros thin sheeted dykes to pillow lavas is an ophiolite it was obducted tilted nearly to vertical eroded and covered unconformably by the nafirdeib series this is a volcaniclastic series dated at 712 58 ma the lower part of which includes conglomerates with ophiolitic clasts olistostromes one with large oolitic limestone slabs felsitic and dacitic tuffs turbidites and black shales with andesites above some of the structures in the ophiolite are attributed to sub oceanic deformation deformation after the deposition of the nafirdeib series produced folds and cleavage trending about ne sw and more locally a cleavage trending nw in a shear zone where the ophiolite was thrust northeastwards over the nafirdeib series the ophiolite and the nafirdeib series were intruded by the batholithic granite dated elsewhere at ca 669 ma the ophiolite is thought to occur on a suture with the nafirdeib series representing part of an island arc sequence