control of pre rift lithospheric structure on the architecture and evolution of continental rifts insights from the main ethiopian rift east africa we investigate the along axis variations in architecture segmentation and evolution of the main ethiopian rift mer east africa and relate these characteristics to the regional geology lithospheric structure and surface processes we first illustrate significant along axis variations in basin architecture through analysis of simplified geological cross sections in different rift sectors we then integrate this information with a new analysis of ethiopian topography and hydrography to illustrate how rift architecture basin symmetry asymmetry is reflected in the margin topography and has been likely amplified by a positive feedback between tectonics flexural uplift and surface processes fluvial erosion unloading this analysis shows that 70 of the 500 km long mer is asymmetric with most of the asymmetric rift sectors being characterized by a master fault system on the eastern margin we finally relate rift architecture and segmentation to the regional geology and geophysical constraints on the lithosphere we provide strong evidence that rift architecture is controlled by the contrasting nature of the lithosphere beneath the homogeneous strong somalian plateau and the weaker more heterogeneous ethiopian plateau differences originating from the presence of pre rift zones of weakness on the ethiopian plateau and likely amplified by surface processes the data provided by this integrated analysis suggest that asymmetric rifts may directly progress to focused axial tectonic magmatic activity without transitioning into a symmetric rifting stage these observations have important implications for the asymmetry of continental rifts and conjugate passive margins worldwide

early mis 3 occupation of mochena borago rockshelter southwest ethiopian highlands implications for late pleistocene archaeology paleoenvironments and modern human dispersals abstract between 70 and 50 ka bp anatomically modern humans dispersed across and out of africa to eventually populate all inhabitable continents knowledge of paleoenvironments and human behavioral patterns in africa prior to and during these dispersals is crucial for understanding how and why hunter gatherers were able to adapt rapidly to the new environments they encountered however few well dated sites from this time period are known from the horn of africa one of the purported staging areas for population movements into southern arabia and asia excavations at mochena borago rockshelter situated on the western slopes of a dormant volcano where the sw ethiopian highlands meet the ethiopian rift have yielded the first securely dated archaeological sequence for later periods of the dispersal three major lithostratigraphic groups incorporating occupational episodes have yielded charcoal radiocarbon ages 53 38 ka calbp deeper deposits have been tested but remain undated archaeological assemblages consist mainly of obsidian flaked stone artifacts manufactured from small minimally prepared single to multi platform flake cores radially prepared cores are rare and blade cores are absent small unifacial to bifacial points from non radial cores dominate the earliest shaped tool assemblages and backed pieces first appear by 45 ka calbp by 43 ka calbp scrapers and backed pieces are predominant rather than points however there is little evidence for technological change other than the appearance of bipolar technology mochena borago s archaeological sequence thus cannot be neatly classified as middle stone age later stone age or transitional and calls into question some of the principles by which archaeologists have attempted to classify african toolmaking traditions

geology geochronology and rift basin development in the central sector of the main ethiopia rift based on stratigraphic relationships and k ar dating of volcanic rocks from both of the escarpments flanking plateaus and from the rift floor of the central sector of the main ethiopian rift six major volcanic episodes are recognized in the rift9s development over a time span from the late oligocene to the quaternary using the k ar data correlation of volcanic units from the six periods of activity throughout the study area forms the basis for establishing six time stratigraphic chronozones for the central sector that are related to volcanism in the ethiopian cenozoic volcanic province the oldest basalt and rhyolite flows exposed along the rift margins of the central sector are time correlative to or older than those in river canyons for example blue nile on the adjacent northwest plateau a thinned mesozoic stratigraphic sequence along the guraghe western rift margin suggests that doming may have preceded volcanism and rifting of the cenozoic by late miocene time at least by 8 3 ma and 9 7 ma the eastern and western faulted margins respectively of the rift had formed at guraghe and at agere selam as indicated by containment of flows of that age within the rift wall during eruption a paroxysm of calc alkaline ignimbrite activity produced voluminous flows nearly fully contained within the rift during the pliocene epoch the munesa crystal tuff 3 5 ma a prominent marker tuff exposed on both rift margins is present at depth in a geothermal well beneath the rift floor and indicates a minimum of 2 km of downthrow in the central sector since its eruption structural and stratigraphic relationships in the central sector indicate a two stage rift development this is characterized by an early phase late oligocene or early miocene of a series of alternating opposed half grabens along the rift with alternating polarity such as that in the present gregory and western rifts of east africa and symmetrical rifts that evolved from these grabens in late miocene or early pliocene time thus evolution from alternating half graben to a full symmetrical graben with a medially located neovolcanic zone that is bifurcated to marginal grabens in the northern part of the study area may be a fundamental part of the rifting process the study indicates that there are major petrologic and tectonic differences between the main ethiopian rift and the gregory kenyan rift

latest pleistocene and holocene river network evolution in the ethiopian lakes region abstract river network geomorphologic paleohydrologic stratigraphic and sedimentologic analyses document a dramatic reorganization of the drainage pattern in the northern part of the main ethiopian rift mer during latest pleistocene and early holocene the river network modification was induced by tectonic deformation volcanic activity and by the arid conditions connected with the last glacial maximum lgm this arid phase triggered the shrinking of a pleistocene megalake that formerly flooded large part of the main ethiopian rift the northern tributaries paleo awash and paleo mojo rivers extended following the lake shore retreat and incised the fesesa koye and cheleleka sulula hafa paleovalleys through the pleistocene deposits at the beginning of the holocene humid conditions induced a water level rise in the lacustrine basin ziway shala basin supplied from the north by the large awash mojo meki fluvial system a well exposed cross section of the cheleleka paleovalley at the confluence with the meki river and the use of paleohydrological methods allowed to infer the bankfull paleo discharge of the larger awash mojo river system tectonic events allowed the awash and mojo rivers to divert their courses to the east toward the afar depression depriving the ziway shala lacustrine basin of large volumes of water supply this and the further increase in aridity during the late holocene led to the separation of the ziway shala paleolake into the present four lakes ziway langano abjiata shala this study indicates that in the main ethiopian rift climatic changes cannot be inferred from lake level variations alone because changes in water supply are also influenced by the tectonic induced rearrangement of the fluvial drainage networks

formation of the japan sea basin reassessment from ar ar ages and nd sr isotopic data of basement basalts of the japan sea and adjacent regions abstract many studies have examined the japan sea basalts recovered during ocean drilling program odp leg127 128 of these the 40ar 39ar dating undertaken is important in constraining the timing of the formation of the japan sea however the implications of their results do not appear to be fully appreciated by the geological community in this paper i reassess the 40ar 39ar age data of the basalts with reference to nd sr isotopic data the 40ar 39ar dating was performed on basalts somewhat enriched in large ion lithophile elements and recovered from odp sites 794 795 and the lower part of 797 yielding the plateau ages of 21 2 17 7 ma these basalts show the nd sr isotopic signature of a moderately depleted mantle source end 0 6 6 9 in contrast the basalts from the upper part of site 797 have yet to be dated due to their low k content although their nd isotopic compositions are similar to that of morb end 8 4 10 4 by analogy to the secular nd sr isotopic trends reported for sikhote alin and northeast japan the age of the upper basalts at site 797 may be inferred to be younger than the lower basalts probably around 16 ma the nd sr isotopic compositions of the japan sea basalts have been interpreted in terms of eastward asthenospheric flow as have the lavas of the sikhote alin and northeastern japan the timing of volcanic activity in the japan sea region i e from 21 2 to 14 86 ma is consistent with the timing of rotational crustal movements inferred from paleomagnetic studies of the japanese islands i e 14 8 4 2 ma for southwest japan and 16 5 14 4 ma for northeast japan

late mesozoic and cenozoic volcanism of the east sikhote alin area russian far east a new synthesis of geological and petrological data abstract the geotectonic framework and the evolutionary history of the east sikhote alin area esa need revision in accordance with various geological and petrological evidence gathered in recent years it has been suggested that the investigated region resulted from the subduction of the izanagi kula and pacific oceanic plates that lasted from late cretaceous to late tertiary however only late cretaceous intermediate and silicic volcanics enriched in lile and depleted in hfse can be interpreted as typical subduction complexes changes in the paleogene paleocene to early miocene magmatic style of the esa area reflect variation in the pacific plate eastern eurasian convergence parameter the termination late cretaceous subduction slab tearing and injection of the oceanic asthenosphere of pacific morb type into subcontinental lithosphere the sea of japan opening began in the eocene with formation of small rift basins in the tatar strait which accumulated coastal facies from eocene to early miocene the extension zone affected by oceanic asthenosphere gradually moved eastward i e to the modern sea of japan the effect of oceanic asthenosphere on the continental margin ended in the late miocene after the sea of japan had opened and new subduction initiated east of the japan islands em i signature of the late miocene pliocene within plate basalts is evidence for eastward asthenospheric flow

the mid proterozoic mporokoso group of northern zambia stratigraphy sedimentation and regional position abstract the mporokoso group formerly plateau series pro parte redefined is a sequence of fluvial aeolian and lacustrine sedimentary rocks consisting predominantly of sandstones with conglomerate mudstone and tuff intercalations the group overlies granites and volcanics of the crystalline basement of the bangweulu block in northern zambia dated at ca 1820 ma and is intruded by the lusenga syenite dated at 1134 ma it was probably deposited during a period close to the older age limit the source area of clastics was to the south of the sedimentary basin and the ubendian belt did not contribute sedimentary material to the mporokoso group which is subdivided into four formations the older subdivisions lower and upper plateau series and the abercorn sandstone were established in a non typical local area and cannot be followed across the whole basin and their use is discontinued similarities of the mporokoso group and the coeval waterberg group of southern africa are discussed the sedimentary basins of both groups have a similar relation to centres of extrusive volcanism preceding subsidence

the eruptive history and magmatic evolution of aluto volcano new insights into silicic peralkaline volcanism in the ethiopian rift the silicic peralkaline volcanoes of the east african rift are some of the least studied volcanoes on earth here we bring together new constraints from fieldwork remote sensing geochronology and geochemistry to present the first detailed account of the eruptive history of aluto a restless silicic volcano located in a densely populated section of the main ethiopian rift prior to the growth of the aluto volcanic complex before 500 ka the region was characterized by a significant period of fault development and mafic fissure eruptions the earliest volcanism at aluto built up a trachytic complex over 8 km in diameter aluto then underwent large volume ignimbrite eruptions at 316 19 ka and 306 12 ka developing a 42 km2 collapse structure after a hiatus of 250 ka a phase of post caldera volcanism initiated at 55 19 ka and the most recent eruption of aluto has a radiocarbon age of 0 40 0 05 cal ka bp during this post caldera phase highly evolved peralkaline rhyolite lavas ignimbrites and pumice fall deposits have erupted from vents across the complex geochemical modelling is consistent with rhyolite genesis from protracted fractionation 80 of basalt that is compositionally similar to rift related basalts found east of the complex based on the style and volume of recent eruptions we suggest that silicic eruptions occur at an average rate of 1 per 1000 years and that future eruptions of aluto will involve explosive emplacement of localised pumice cones and effusive obsidian coulees of volumes in the range 1 100 106 m3

quaternary oblique extensional tectonics in the ethiopian rift horn of africa abstract the ethiopian rift extends in a northeasterly direction from southern ethiopia to the afar region it shows a complex fault pattern characterised by the interplay of a n30 e n40 e trending border fault system with the quaternary wonji fault belt which is constituted by right stepping en echelon n s to n20 e trending faults the wonji fault belt affects mainly the rift floor but it also overlaps with some segments of the margins its en echelon arrangement indicates a left lateral component of displacement along the rift trend the general fault pattern of the ethiopian rift as well as mesoscopic fault analyses and structural features of some key areas indicate the occurrence of a roughly e w extension which is compatible with the sinistral shear component of motion along the rift this paper proposes that oblique rifting related to the e w direction of extension has been active since the beginning of the quaternary and that it came after an earlier phase of roughly pure extension orthogonal to the rift trend

correlation of plio pleistocene tephra in ethiopian and kenyan rift basins temporal calibration of geological features and hominid fossil records abstract the 200 m thick fossiliferous konso formation and overlying terrace deposits which crop out at the end of the southern sector of the main ethiopian rift mer contain more than 30 distal tephra layers local and regional tephra correlations of more than 20 tephra units were established using major and trace element data of discrete and purified bulk glass samples within the konso study area eleven correlative marker tuffs were recognized in stratigraphic sections of both the konso formation and the omo turkana basin sediments in southern ethiopia and northern kenya the turoha hope ivory bright white and boleshe tuffs in the konso formation and the upper white tuff in the overlying terrace deposits are securely correlated with the kbs akait lokapetamoi chari lower nariokotome and silbo tuffs of the omo turkana basin using least mobile major elements cao fe2o3 and tio2 and geochronology preliminary correlations are also suggested between the konso formation distal tephra and proximal units of the quaternary caldera forming silicic centers in the central sector of the mer the strongly peralkaline tuffs of the konso formation are compositionally similar to proximal eruptions mostly located along the quaternary axial rift zone of the southern central and northern sectors of the mer the tephra correlation provides information about the temporal and spatial features of the volcanic and tectonic processes recorded in the evolving basins thickness and sedimentation rate were determined for both the konso formation and the omo turkana basin sections measured between the turoha kbs 1 91 0 03 ma and the bright white chari 1 40 0 02 ma tuffs although the lithostratigraphic sequence at the konso study area is younger sedimentation rate within the konso formation was comparable to that of the koobi fora formation higher in the nachukui formation and lower in the shungura formation local and regional differences in thickness and sedimentation rates within the plio pleistocene deposits of the konso shungura nachukui and koobi fora formations may be attributed to erosion associated with increased topographic gradients maintained by continued subsidence mostly along axial fault zones of basins and uplift along the margins of the broadly rifted zone of the omo turkana basin and the southern sector of the mer

geochronology of the mkushi gneiss complex central zambia abstract rb sr and k ar ages are reported from the mkushi gneiss complex and its intrusives in central zambia of the rb sr whole rock data of the mkushi gneisses three suites 22 samples only one suite six samples defines an isochron of 1777 89 ma with initial 87sr 86sr of 0 713 0 0006 errors 95 confidence level 87rb 1 42 10 11 a 1 the other samples scatter below this isochron but above a 1480 ma line four hornblendes from amphibolitic dykes yield k ar ages between 864 and 804 ma whole rocks of the red facies of the intrusive mtuga granite nine samples define an isochron of 607 39 ma with initial 87sr 86sr of 0 730 0 005 the white facies of the mtuga granite four samples and probably also the copper bearing aplitic veins seven samples are of the same age but with even higher initial 87sr 86sr ratios seven biotites from the mkushi gneisses yield rb sr and k ar ages between 469 ma and 444 ma these data are interpreted to signal 1 about 1780 ma ago emplacement of the granitoid precursors of the mkushi gneisses derived from older continental material 2 between about 1480 ma and 860 ma ago formation of the mkushi gneisses and the amphibolites probably during the irumide orogeny about 1350 ma ago 3 between about 860 ma and 800 ma ago closure of the hornblendes to k ar 490 550 c after resetting probably in relation to the c 860 ma old tectono thermal event widespread in eastern and eastern central africa 4 about 600 ma ago intrusion of the mtuga granite and the copper bearing splites post tectonic pan african possibly derived from the mkushi gneisses and 5 about 460 450 ma ago closure of the biotites to rb sr and k ar approximately 400 c reflecting the final regional cooling termination of the pan african events some regional implications are discussed

asthenospheric injection and back arc opening isotopic evidence from northeast japan nd and sr isotopic compositions were determined for the tertiary volcanics from the back arc side of the ne japan arc sr isotopes show a linear trend through time from an enriched signature 87sr86sr 0 705437 to a depleted signature 87sr86sr 0 70270 in a complementary fashion nd isotopes start at low value nd 0 80 and show a gradual increase nd 8 3 with decreasing age isotopic change of nd and sr from the enriched signature to the depleted one is synchronous with the opening of the japan sea at 15 ma this synchronism indicates that the opening of the japan sea was initiated by the injection of the asthenosphere during the pre opening stage the mantle wedge was composed of a two layered structure the sub continental lithosphere and the underlying asthenosphere the volcanics of this stage characterized by an enriched isotopic signature were derived from a source with a higher proportion of sub continental lithosphere the sub continental lithosphere of the back arc side was thinned by the injection of the depleted asthenosphere which accelerated the growth of the morb source within the mantle wedge of the back arc side and resulted in magma generation with the depleted isotopic signature of nd and sr at the post opening stage

characteristics of a tonian reef rimmed shelf before the onset of cryogenian insights from neoproterozoic kunihar formation simla group lesser himalaya abstract the kunihar formation records evolution of a unique rimmed shelf during the tonian before the cryogenian marinoan glaciation of krol formation lesser himalaya the mixed siliciclastic carbonate system of neoproterozoic kunihar formation provokes interest for outcrop based facies analysis and sequence stratigraphy in order to frame a better understanding on the mechanism and control of its deposition the formation records a 450 m thick mixed siliciclastic carbonate rimmed shelf five facies associations have been identified peritidal fa1 lagoonal fa2 barrier island ooidal shoal fa3 reef complex fa4 and fore reef fa5 facies associations two systems tracts were delineated transgressive tst and highstand hst systems tracts tst is marked by dominant aggradation of stromatolitic carbonates while siliciclastic incursion surpassed carbonate deposition during highstand it conforms to the lower carbonate upper clastic model of mixed siliciclastic carbonates during the precambrian reef rimmed shelves were considered more abundant in the archaean and palaeo mesoproterozoic this study determines that development of rimmed shelves was more common during the neoproterozoic than previously speculated neoproterozoic reef rimmed shelves are concentrated more within the cryogenian and ediacaran period stromatolitic reef rimmed shelf of kunihar formation is the only rimmed shelf recorded in the tonian period till date increased microbial stromatolitic accretion was propelled by surge in nutrients due to chemical weathering of underlying tholeiitic basalt darla volcanics associated with the rifting of supercontinent rodinia with increasing interest of tonian cryogenian transition as a key period in both planetary and organic evolution kunihar formation represents a terminal reef rimmed shelf system that evolved due to microbial stromatolites in the absence of complex organisms subsequent to this cryogenian and ediacaran biota took over as the dominant reef builders

hydrogeochemical study in the main ethiopian rift new insights to the source and enrichment mechanism of fluoride the central main ethiopian rift suffers a severe water quality problem characterized by an anomalously high fluoride f content that causes an endemic fluorosis disease the current study conducted in the ziway shala lakes basin indicates that the f content exceeds the permissible limit for drinking prescribed by the world health organization who 1 5 mg l in many important wells up to 20 mg l with even more extreme f concentration in hot springs and alkaline lakes up to 97 and 384 mg l respectively the groundwater and surface water from the highlands typically characterized by low total dissolved solids tds and ca mg hco3 hydrochemical facies do not show high f content the subsequent interaction of these waters with the various rocks of the rift valley induces a general increase of the tds and a variation of the chemical signature towards na hco3 compositions with a parallel enrichment of f the interacting matrixes are mainly rhyolites consisting of volcanic glass and only rare f bearing accessory minerals such as alkali amphibole comparing the abundance and the composition of the glassy groundmass with other mineral phases it appears that the former stores most of the total f budget this glassy material is extremely reactive and its weathering products i e fluvio volcano lacustrine sediments further concentrate the fluoride the interaction of these weathered reworked volcanic products with water and carbon dioxide at high ph causes the release of fluoride into the interacting water this mainly occurs by a process of base exchange softening with the neo formed clay minerals i e ca mg uptake by the aquifer matrix with release of na into the groundwater this is plausibly the main enrichment mechanism that explains the high f content of the local groundwater as evidenced by positive correlation between f ph and na and inverse correlation between f and ca mg saturation indices si have been calculated using phreeqc 2 for the different water groups highlighting that the studied waters are undersaturated in fluorite in these conditions fluoride cannot precipitate as caf2 and so mobilizes freely without forming other complexes these results have important implications for the development of new exploitation strategies and accurate planning of new drilling sites

quaternary volcano tectonic activity in the soddo region western margin of the southern main ethiopian rift 1 we present an analysis of the distribution timing and characteristics of the volcano tectonic activity on the western margin of the southern main ethiopian rift in the soddo area latitudes between 7 10 n and 6 30 n the margin is characterized by the presence of numerous normal faults with limited vertical offset and often sigmoidal in shape which accommodate a gentle transition from the rift floor to the ethiopian plateau new radiocarbon dating indicates post 30 ka fault activity pointing to a significant late pleistocene holocene tectonic activity of the soddo margin comparison of the fault architecture with analog models suggests that deformation has been controlled by a sub e w roughly n100 e extension direction resulting in an oblique extension with respect to the roughly ne sw trending rift this well accords with inversion of fault slip data collected on faults with pleistocene holocene activity and is also in good agreement with recent gps data from the southern main ethiopian rift our data support a close correlation between the recent volcanic activity and deformation in the study area with eruptive vents located along the recent border faults the axial tectono magmatic activity is subordinate in the area these findings support a transition from axial tectono magmatic deformation in the northern main ethiopian rift to marginal deformation in the central and southern main ethiopian rift in turn indicating an along axis north to south decrease in rift maturity

field evidence for active normal faulting in tibet crustal thickening does not now occur in southern tibet field observations made during the chinese french expedition of 1980 support an earlier hypothesis based on a combined analysis of satellite images and fault plane solutions of earthquakes immediately north of the highest peaks of the himalayas the tectonic regime is dominated by east west extension we have mapped a large number of north south normal faults which sharply cut the glacial and post glacial morphology and followed several recent earthquake breaks along faults detected on satellite and aerial photographs microtectonic measurements in the quaternary suggest that the maximum principal stress is vertical

ensialic spreading subsidence in the mesozoic and palaeogene andes of central chile an oceanic plate subducting eastwards below a continental margin and resulting in crustal shortening can be inferred to have been active in central chile during the palaeozoic however the geological record for the mesozoic and palaeogene shows evidence of a persistent tensional regime and absence of crustal shortening although lateral asymmetry inherent to a subducting model is shown by this record a bilateral symmetry is also well substantiated a coupled action of plate subduction and spreading subsidence the latter taking place within the margin of a continental plate is suggested to explain these features

the role of silica and iron oxide mobility in the formation of gold bearing fluvial sediments in the proterozoic mporokoso basin northern zambia

sedimentology of proterozoic cratonic sheet sandstones of the eastern canadian shield sims formation labrador canada abstract the upper lower proterozoic to middle proterozoic sims formation rests unconformably on lower proterozoic strata of the knob lake group of the labrador trough sequence the 700 m thick sims formation consists of basal fluvial arkoses and conglomerates about 100 m and an upper sheet like sand body about 600 m thick composed of quartz arenites and minor quartz clast conglomerates this upper unit is interpreted as beach shoreface tidal and shallow shelf deposits the lowest 70 m of the quartz arenite unit appear to be of fluvial origin but probably were formed by reworking of quartzose sands produced in coastal environments during a period of elevated sea level the lower sims formation probably represents molasse shed from either the hudson or labrador orogens rivers flowed northwest away from these upland areas the upper part of the sims formation records deposition during a period of slow steady sea level rise in an ocean that deepened to the west the sims formation outliers are remarkably similar to outliers of the sakami formation of northern quebec a unit that has a poorly constrained early to middle proterozoic age if these units are correlative a hypothesis supported mainly by identical facies and facies sequences then the transgressive quartz arenites would originally have covered an area of at least 200 000 km2

tectonic control of complex slope failures in the ameka river valley lower gibe area central ethiopia implications for landslide formation abstract even though major faults represent important landslide controlling factors the role the tectonic setting in actively spreading rifts plays in the development of large complex landslides is seldom discussed the ameka complex landslide area is located on the eastern scarp of the gibe gorge approximately 45 km to the west of the main ethiopian rift and 175 km to the southwest of addis ababa investigation of the complex landslide failures required a combination of satellite and airborne data based geomorphology geological field survey complemented with structural analysis radiocarbon geochronology and vertical electric sounding the obtained observations confirmed the multiphase evolution of the landslide area we have documented that apart from climatic and lithological conditions the main triggering factor of the ameka complex landslide is the tectonic development of this area the e w extension along the nne ssw trending main ethiopian rift is associated with the formation of numerous parallel normal faults such as the gibe gorge fault and the almost perpendicular scissor faults the geometry of the slid blocks of coherent lithology have inherited the original tectonic framework which suggests the crucial role tectonics play in the fragmentation of the compact rock masses and the origin and development of the ameka complex landslide area similarly the main scarps were also parallel to the principal tectonic features the local tectonic framework is dominated by faults of the same orientation as the regional structures of the main ethiopian rift such parallel tectonic frameworks display clear links between the extension of the main ethiopian rift and the tectonic development of the landslide area the ameka complex landslide developed in several episodes over thousands of years according to the radiocarbon data the last of the larger displaced blocks representing only 2 of the total area most likely slid down in the seventh century ad the main scarps namely the high scarps in the western part are unstable over the long term and toppling and falling type slope movements can be expected here in the future

control of rift obliquity on the evolution and segmentation of the main ethiopian rift the main ethiopian rift has evolved in two stages with successive activation of differently oriented fault systems analogue modelling of the earth s lithosphere demonstrates that such a rift evolution requires neither magma weakening nor a change in plate kinematics the main ethiopian rift is an active rift in the break up stage and it marks the incipient boundary between the nubia and somalia plates1 rifting started with the activation of large boundary faults and diffuse volcanism followed by focused magmatism and faulting in the rift floor along step like segments oblique to the rift axis that now act as a protoridge for future seafloor spreading2 this concentration of volcano tectonic activity has been thought to be either magma assisted2 3 or controlled by a change in rift kinematics with a late oblique rifting phase that would have caused the development of the step like fault segments that focused magma upwelling4 5 geodetic6 7 seismic8 and stress field9 data confirm current oblique rifting kinematics but plate kinematics models do not predict a change in nubia somalia motion in the past 11 million years10 here i use lithospheric scale analogue models of oblique rifting to analyse the development of the main ethiopian rift i find that neither magma weakening nor a change in plate kinematics are required to simulate a two phase evolution with successive activation of differently oriented fault systems i conclude that rift evolution and segmentation are controlled by rift obliquity independent of magmatic processes

spatial relationship between earthquakes and volcanic vents in the central northern main ethiopian rift abstract during the breakup of continents extension is commonly accommodated by connected networks of fluid filled fractures dykes and by faults despite the importance of these two extension mechanisms their spatial relationship in three dimensions is poorly understood primarily because it is difficult to quantify the subsurface distribution of faulting and intrusion in order to address this problem we conduct a quantitative analysis of the spatial distribution and clustering of earthquakes and volcanic vents in the main ethiopian rift in east africa in order to understand how extension by faulting and intrusion is distributed throughout the volcanic rift we use fractal analysis of earthquake epicentres in order to infer the 2d characteristics of the subsurface fault network and directly test our model results against the 3d distribution of earthquake hypocentres our results show that fractal analysis of these features is a reliable means to characterise the 3d properties of the fault network in addition the strong similarity between the properties of the fault network derived from earthquakes and properties of the magma filled fracture network derived from fractal analysis of volcanic vents strongly suggests that these are genetically linked we then explore their spatial link using computation of earthquake and vent density which shows that the zone of seismicity is generally around 20 30 km wide while the zone of vents is narrower and centred within the zone of seismicity this spatial relationship suggests that the faults which form rift axial grabens are induced above a narrower and central zone of diking we also demonstrate significant along rift variation in degree of magmatism and faulting with regions of increased degree of diking inferred from a higher cone density characterised by reduced degree of faulting

a new kinematic approach for the danakil block using a digital elevation model representation abstract data from the literature are integrated in a regional digital elevation model dem in order to analyse the motion of the danakil block with regard to the arabian and somalian plates the application of the poles and angles of rotation taken from the literature induces a superposition of the danakil block on the arabian plate and the formation of a gap in the afar region the determination of new poles of rotation using a best fitting procedure allows one to avoid these drawbacks in all cases calculations are carried out keeping the nubian plate stationary the present approach shows that the danakil block is an independent entity and is not related to the nubian and arabian plates between the oligocene and the miocene it has been submitted to a n20 e sinistral strike slip motion anterior to the rotation itself that was started in the middle miocene this rotation is directed by a mechanical couple due to the combination of the red sea and north afar extensions

early poststabilization sedimentation and later growth of shields subsidence and shallow water sedimentation occurring within a few tens of millions of years after cratonic stabilization of shields can be shown by at least three sedimentary suites the wajid formation of southwestern saudi arabia the roraima formation of venezuela and neighboring countries and the baraboo interval of wisconsin and nearby states this subsidence indicates that complete attainment of present crustal thickness of the shield areas did not occur at the time of crustal stabilization instead crustal growth and uplift were passive processes extending perhaps for several hundred million years after stabilization

quaternary extension in southern tibet field observations and tectonic implications we summarize evidence for quaternary and active faulting collected in the field during three sino french expeditions to southeastern tibet 1980 1982 detailed mapping of quaternary and active faults as well as microtectonic measurements indicate that normal faulting has been the dominant tectonic regime north of the himalayas in the last 2 0 5 m y the maximum horizontal principal stress in south tibet appears to be only the intermediate principal stress 2 1 being vertical south of the chord joining the eastern and western syntaxes of the himalayan arc extensional strains are principally localized within seven regularly spaced rift zones three of which have been studied in some detail the extension direction is determined to be n96 e 7 mainly from statistical averaging of strikes of newly formed normal faults throw rates on normal faults are evaluated for different time spans 2 0 5 m y 60 40 kyr and 10 2 kyr b p using structural and topographic reliefs as well as synglacial and postglacial vertical offsets the rate of quaternary extension is about 1 m y 1 along an 1100 km long ese traverse across south tibet this corresponds to a spreading rate of 1 0 6 cm yr this rate and the divergent horizontal projections of slip vectors of earthquakes along the himalayan front constrain the rate at which rigid india underthrusts southern tibet to be 2 1 cm yr although most of the normal faults appear to be independent of and nearly orthogonal to the e w mesozoic tertiary tectonic fabric the yadong gulu rift appears to be guided for over 130 km by the older oblique ne sw nyainqentanglha range and fault zones along it this reactivated zone is the most prominent left lateral strike slip fault system in se tibet excepting this zone and the vicinity of the se extremity of the karakorum fault quaternary strike slip faulting is rare in south tibet i e south of the chord between the syntaxes of the himalayan arc north of the chord the tectonic style is different there minor conjugate strike slip faulting is widespread and appears to control quaternary normal faulting which is more diffuse and subdued than in south tibet along the chord the presence of a major zone of active right lateral en echelon strike slip faults karakorum jiali fault zone probably reflects the greater facility of eastward extrusion in north central tibet in response to the northward push of india the eastern himalayan syntaxis may be an obstacle to such extrusion movements south of the chord

a thin viscous sheet model for continental deformation summary for the purposes of describing its large scale and long term deformation the continental lithosphere is regarded as a continuum obeying a newtonian or a power law rheology the flow of a thin sheet of power law material overlying an inviscid substrate is studied under the assumption that vertical gradients of the horizontal velocity are negligible a numerical model is used to investigate the deformation of such a sheet under conditions approximating those of continent continent collision the material flows in response to forces applied to its boundaries for example the indenting of one continent by another and to forces in its interior arising from gradients in crustal thickness the horizontal divergence of the flow produces changes in the crustal thickness and hence a time dependent form to the flow itself for a given set of boundary conditions the flow depends on the stress exponent in the power law rheology n and on the argand number which is a measure of the ratio between the stress arising from crustal thickness contrasts and the stress required to deform the material at the ambient strain rates when the effective viscosity of the medium is very high ar o crustal thickness variations do not influence the flow if the material is newtonian n l the deformation associated with an influx of material approximating an indenter is of much greater lateral dimension than the width of the indenter whereas when material has a power law rheology n 3 5 are used the deformation is confined to a region of lateral extent comparable to that of the indenter as ar increases the forces arising from crustal thickness contrasts exert more influence on the flow and the maximum crustal thickness that can be sustained by a given influx of material is related by a simple expression to the effective viscosity of the medium at the ambient strain rates in the limit of a very weak medium ar 10 the lithosphere is unable to sustain appreciable crustal elevation contrasts the results of these numerical experiments show that systems in which the effective viscosities are such that the maximum deviatoric stresses are between 1 kbar

geodynamic and magmatic stages on a traverse through the andes between 20 and 24 s n chile s bolivia nw argentina metamorphic precambrian basement is exposed in the chilean pre cordillera in the form of a narrow horst and also in the argentinian puna miogeosynclinal pelites and psammites of devonian age dominate in the western part of the andes while early palaeozoic clastics occur in the e insignificant triassic sediments were followed by wide spread shallow marine jurassic deposition cretaceous and tertiary strata are predominantly terrestrial deposits whereas the precambrian and palaeozoic series were intensely deformed by folding block faulting dominates in the volcano sedimentary cover magmatic activity has been significant since the precambrian while s type intrusives and extrusives occurred mainly in the palaeozoic i type magmatic rocks predominated in the mesozoic the cenozoic volcanism in the cordillera occidental originated in the lower and upper crust magnetotelluric and seismic investigations on the traverse show extremely low electric resistivity values coinciding with a strong absorption of seismic waves below the chilean pre cordillera and the cordillera occidental this phenomenon can best be explained by the existence of partial melts at depths of more than 10 km the bouguer values decrease by about 430 mgal from the coast to the cordillera occidental in the precambrian and palaeozoic the continent apparently extended farther to the w the change from a passive to an active continental margin took place at the palaeozoic mesozoic boundary with the break up of pangea in order to grasp better the tectonic and magmatic evolution as well as the recent structure of the crust in the central andes a combined geological geophysical investigation has been made along a traverse through the andes between 20 and 24 s fig 1 the following pages present a review of the geological development together with the first results of work from the chilean section of the traverse

reconstructing prehistoric settlement models and land use patterns on mt damota sw ethiopia abstract although high altitude mountain habitats are often regarded as unfavorable for human occupation e g aldenderfer 2014 on the other hand tropical highlands in africa are suggested as potential refugia during times of environmental stress e g basell 2008 brandt et al 2012 archaeological investigations on mount damota 2908 m a s l located on the boundary between the southwest ethiopian highlands to the west and the southern main ethiopian rift valley to the east yielded a large number of archaeological sites from the middle stone age period until historical times in this paper we try to reconstruct settlement models for the late pleistocene and holocene occupation in this area and speculate about potential land use patterns such complex topics demand a landscape archaeological approach that includes open air sites and rock shelters the results from our excavations at mochena borago rock shelter and evidence from open air sites that were recorded during intensive surveys on the slopes and plateau of the mountain allow a first reconstruction of the settlement history of the area

environmental changes in a tropical lake lake abiyata ethiopia during recent centuries lake abiyata is a small closed saline alkaline lake located in the central part of the ethiopian rift valley east africa a multi proxy study of a sediment core 116 cm long and with undisturbed mud water interface was performed to test the sensitivity of the lake system and of different proxies to the changes in climate and human activities that occurred in the catchment during the past few centuries the 210pb analyses suggest that the upper 80 cm of the core represent the past 200 years this study complements millennial scale environmental records available in the region the main freshwater climatic and biological features of the modern lake system and their variations over the past decades as known from observations are first summarised results derived from individual proxies analysed along the core are then presented successively major physical and chemical properties of bulk sediments diatoms and pollen uncertainties on the chronological framework are discussed major limnological stages are finally identified based on the multi proxy interpretation of our record our record shows large variations in the lake water and salt balances in the sediment sources and in the vegetation distribution in the basin using our 210pb chronology major changes observed in the core are tentatively compared with environmental events known from instrumental and historical records the upper 41 cm of the core 210pb age 1940 1998 ad reveal several fluctuations in diatom inferred water depth and salinity which seem to be consistent with known changes in water level human impact on vegetation clearly appears since about 30 years the interval 85 41 cm suggests a period of overall water deficit lake abiyata experienced episodes shallower and more saline than over the past decades especially around 68 66 cm 210pb dated at ca 1890 ad this level may coincide with one of the worst droughts known in the ethiopian history during 1888 1892 the lower part of the core includes a stage 108 85 cm of lake level much higher than today and which ended before 1800 ad although its base is undated so far this stage suggests that conditions much wetter than today have prevailed in the region during at least part of the 18th century lake abiyata appears to be a suitable site for a detailed environmental reconstruction over the recent past although further work is needed to reduce the uncertainties on our record as discussed in the conclusions

bayesian noise reduction in arabia somalia and nubia arabia finite rotations since 20 ma implications for nubia somalia relative motion knowledge of nubia somalia relative motion since the early neogene is of particular importance in the earth sciences because it i impacts on inferences on african dynamic topography and ii allows us to link plate kinematics within the indian realm with those within the atlantic basin the contemporary nubia somalia motion is well known from geodetic observations precise estimates of the past 3 2 myr average motion are also available from paleo magnetic observations however little is known of the nubia somalia motion prior to 3 2 ma chiefly because the southwest indian ridge spread slowly posing a challenge to precisely identify magnetic lineations this also makes the few observations available particularly prone to noise here we reconstruct nubia somalia relative motions since 20 ma from the alternative plate circuit nubia arabia somalia we resort to trans dimensional hierarchical bayesian inference which has proved effective in reducing finite rotation noise to unravel the arabia somalia and arabia nubia motions we combine the resulting kinematics to reconstruct the nubia somalia relative motion since 20 ma we verify the validity of the approach by comparing our reconstruction with the available record for the past 3 2 myr obtained through antarctica results indicate that prior to 11 ma the total motion between nubia and somalia was faster than today furthermore it featured a significant strike slip component along the nubia somalia boundary it is only since 11 ma that nubia diverges away from somalia at slower rates comparable to the present day one kinematic changes of some 20 might have occurred in the period leading to the present day but plate motion steadiness is also warranted within the uncertainties

the time bound character of uranium mineralising processes with special reference to the proterozoic of gondwana abstract it is more useful to consider the time bound character of uranium mineralising processes than the time bound character of uranium deposits this is because present uranium deposits are the result of a series of mineralising steps which occurred over hundreds of millions of years we have identified five periods which are characterised by moderately distinct mineralising processes 1 1 before 2800 ma uranium was fractionated into the upper crust during late archaean early proterozoic cratonisation by magmatic and metamorphic processes the granitic terranes were an important source of uranium for subsequent mineralising events 2 2 2800 2200 ma although the atmosphere was probably not reducing at this time the level of oxygen was sufficiently low for uraninite to be transported and concentrated in sedimentary basins cratonisation was a prerequisite for the formation and preservation of long lived basins rapid burial and reducing conditions during diagenesis were important for preserving the uranitite the uranium was mobilised during diagenesis and metamorphism 3 3 2200 1200 ma after 2000 ma when the oxygen level in the atmosphere increased markedly the movement of uranium in solution as uranyl ions became more important the first marine shelf sequences containing chemical and organic sediments to be deposited on the cratons formed excellent sites for the syn sedimentary diagenetic or metamorphic concentration of uranium in some cases this mineralisation was economic in other cases it may have been just a step in a succession of mineralising events continental clastic sediments which overlie these shelf sediments were excellent hosts for large hydrothermal circulation systems which could remobilise the uranium from either granites or sediments the lack of organic and argillaceous material in these continental sequences resulted in the precipitation of uranium near the basal unconformity where there were reducing sediments 4 4 1200 400 ma with the start of modern plate tectonics there was greater potential for the thermally activated remobilisation of uranium the period 1200 900 ma marked the final mineralisation in some important deposits the pan african and brazilian events 700 400 ma mobilised uranium of uncertain origin and concentrated it in previously deposited marginal marine sediments 5 5 400 0 ma the incorporation of organic material into continental clastic sediments allowed uranyl ions to be precipitated before they reached the sea to form the important sandstone type of deposits any time bound mineralising process may be either the final or an intermediate step in forming an economic uranium deposit which of these it is depends on the subsequent history of the area on whether the uranium is preserved dispersed or reconcentrated that many major uranium deposits occur on late archaean early proterozoic cratons emphasises the importance of the long term preservation of sedimentary sequences even though the uranium may be remobilised several times within the sediments

three dimensional seismic imaging of a protoridge axis in the main ethiopian rift models of continental breakup remain uncertain because of a lack of knowledge of strain accommodation immediately before breakup our new three dimensional seismic velocity model from the main ethiopian rift clearly images mid crustal intrusions in this active transitional rift setting supporting breakup models based on dike intrusion and magma supply the most striking features of our velocity model are anomalously fast elongate bodies velocity v p 6 5 6 8 km s extending along the rift axis interpreted as cooled mafic intrusions these 20 km wide and 50 km long bodies are separated and laterally offset from one another in a right stepping en echelon pattern approximately mimicking surface segmentation of quaternary volcanic centers our crustal velocity model combined with results from geologic studies indicates that below a depth of 7 km extension is controlled by magmatic intrusion in a ductile middle to lower crust whereas normal faulting and dike intrusion in a narrow zone in the center of the rift valley control extension in the brittle upper crust this zone is inferred to be the protoridge axis for future seafloor spreading

focal depths and fault plane solutions of earthquakes under the tibetan plateau we compare synthetic and recorded p wave forms to place constraints on the focal depths and fault plane solutions of 16 crustal earthquakes beneath the highest parts 4000 m of the tibetan plateau fault plane solutions for all 16 events show combinations of normal and strike slip faulting with t axes oriented approximately east west none of these solutions show thrust faulting thus the data corroborate previous inferences that the active tectonics are dominated by east west extension focal depths for all 16 events are less than 15 km and appear to be between 5 and 10 km this style of deformation and these depths of faulting are similar to those in the basin and range province of the western united states two intermediate depth events below the crust of southern tibet also show primarily normal faulting with east west t axes the solution for one discussed by chen et al 1981 is unambiguous the solution for the other the event of august 1 1973 27 59 n 89 17 e 85 10 km mb 4 9 is less certain both apparently occurred in the mantle beneath a thick aseismic lower crust and their occurrence suggests that brittle deformation occurs there in response to a stress field similar to that operating at shallow depths beneath tibet

structural and geochronological implications of the fentale volcanics at a nascent passive margin of the main ethiopian rift constraints from magnetostratigraphy study at the kereyou lodge ethiopia abstract we sampled twenty four cooling units for magnetostratigraphic study from a 110 m thick early pleistocene volcanic section near kereyou lodge in the main ethiopian rift seven samples were used from each site for analysis of remanent magnetization using thermal and alternating field demagnetization techniques the natural remanent magnetization nrm direction comprises two vector components in most samples the first low stability component is isolated generally by heating to 100 c 250 c or by demagnetization using an alternating field intensity of 10 15 mt after these steps most magnetization directions define straight lines directed towards the origin and record the primary nrm or chrm characteristic remanent magnetization magnetization decay curve analyses show the possible magnetic minerals to be titanomagnetite magnetite titanohematite and hematite paleomagnetic site mean directional analyses revealed that upper 18 flows are of reversed polarity and the bottom 4 flows are normal using available age information the normal interval is correlated to the olduvai subchron on the geomagnetic polarity time scale gpts of cande kent 1995 and the flows at the polarity transition are dated to be 1 77 ma the overall declination and inclination differences calculated d 4 8 4 4 i 0 8 4 2 are consistent with the 7 counterclockwise reported kidane et al 2009 site mean inclinations show a systematic change with flow position inclinations get progressively shallower with depth for the top 17 volcanic flows and progressively steeper for the bottom 4 flows consistent with progressive backward and rift ward block tilting mechanisms respectively

opening of the kuril basin deduced from the magmatic history of central hokkaido north japan abstract the kuril basin is a fan shaped back arc basin in the northwestern pacific rim the basin is separated by the kuril arc from the pacific ocean to the southeast and is bounded by the subsided okhotomorsk block to the north and by the late eocene to early miocene hidaka magmatic arc to the west the clastic sediments constituting the late cretaceous to early eocene proto kuril arc which underlies the modern kuril arc were transported from the okhotomorsk block between them the kuril basin now intervenes this suggests that the kuril basin had not opened and that the proto kuril arc had been located in the southern margin of the okhotomorsk block at least until early eocene time it also indicates that the kuril basin was not entrapped by the proto or modern kuril arc the tertiary hidaka magmatic arc in central hokkaido was formed along a n s trending subduction zone where the pacific plate was subducting beneath the eurasian plate therefore the kuril basin was not yet present on the east of central hokkaido during the period when the hidaka magmatism was taking place based on the fact that the hidaka arc magmatism ceased at 17 16 ma and coevally along its n s trending extension it is suggested that the kuril basin opened just after 16 ma by the southward retreat of the proto kuril arc trench system in central hokkaido an extensional stress regime during the middle miocene is inferred from the occurrence of a type magmatism and graben like sedimentary basins this stress field can be attributed to subsidence of the pacific slab covered by the retreated proto kuril arc during the opening of the kuril basin the occurrence of an undeformed and horizontal sedimentary cover on the rugged basement of the kuril basin is consistent with rather rapid opening at 16 15 ma during the middle miocene the kuril basin and japan basin were formed by a similar process i e rotation and oceanward retreat of an arc trench system opening of the two back arc basins along different plate boundaries i e the pacific north american boundary for the kuril basin and the pacific eurasian boundary for the japan basin may have been caused by the migration of a hot region across the overriding plate boundary

have wet and dry precambrian crust largely governed cenozoic intraplate magmatism from arabia to east africa to explain cenozoic continental volcanism between arabia and east africa the existing model infers that a plume impinged beneath ethiopia between 30 ma and 20 ma and volcanism extruded within a 1000 km radius because relative motion of the afro arabian plate was about northeast in the last 120 ma we infer that at 84 ma a plume originated from the core mantle boundary impinged beneath nubia arabia and is now under the tanzania craton this plume caused uplift afro arabian swell and magma under plating after fyfe s idea 1992 the conceptual model proposed herein suggests that following plume impact there was in nubia arabia only intrusion of mafic dykes because the crust was largely unprocessed wet at about 50 ma the plume was under ethiopia and coeval volcanism extruded because the crust was highly recycled dry in zaire burundi and tanzania volcanism is explained to be coeval with the arrival of the plume because there also the crust is recycled in arabia and yemen ethiopia continental flood basalts younger than 30 ma formed because lithospheric extension along the red sea gulf of aden was the cause of or the result of plume s probably originated from the upper mantle

geochemistry of the quaternary volcanic rocks of the northeast japan arc major and trace element data are presented for a series of lavas from 17 volcanic centres in the ne japan arc these represent a transect of the quaternary arc from its volcanic front type a volcanoes in the east across its central zone type b to its western margin type c rocks range from basalt to dacite in composition and the variation is attributed to fractionation of plagioclase mafic silicates and titanomagnetite for comparative purposes trace element data for each volcano are presented on a 55 wt sio2 normalised basis the volcanoes display the characteristic features of arc volcanism viz enrichment in large ion lithophile elements rb sr ba k and depletion in high field strength elements zr nb hf ta using these data and derived primary magma compositions tatsumi et al 1983 mixing calculations give the percentages of fractionating mineral phases necessary to derive the 55 wt sio2 liquids these data together with published distribution coefficients allow the calculation of trace element abundances in the primary magmas reciprocal trace element plots indicate that irrespective of position with respect to the volcanic front the elements la ba k sr nb y and zr of the primary magmas all lie on a common line strongly suggesting that the melts are the products of varying degrees of melting of a common homogeneous source however those melts close to the volcanic front type a show anomalous rb th and pb values suggesting that their mantle source was enriched in these elements further calculations suggest that the hfs depletion is not due to retention by residual mineral phases but is a feature of the source thus the data suggest a that the source of the arc volcanism was homogeneous and b that the characteristic chemical features were an inherent feature of the source since zr nb ratios in the source melts are close to those found in n type morb it is suggested that a major component of arc volcanic source rocks was a depleted mantle of the type which has given melts of n type morb composition lil elements added to such a depleted source are derived by the loss of silica rich aqueous fluids from the descending subducting slab such losses occur within the first 100 km and are fixed in the overlying mantle wedge this contaminated material is transported down by drag induced convection producing a homogeneous but metasomatised mantle within the melting zone diapiric uprise produced by small scale melting produces further melting with the major control of melt chemistry being the degree of partial melting superimposed on this chemistry is a localised addition of pb rb and th in the source of those volcanoes nearest the volcanic front

petrogenesis and tectonic implications of the triassic volcanic rocks in the northern yidun terrane eastern tibet abstract triassic paleo tethyan subduction related volcanic rocks are extensively distributed in the xiangcheng and changtai regions northern yidun terrane eastern tibet the 228 ma volcanic rocks in the xiangcheng region have intermediate felsic composition 60 1 to 64 7 wt sio 2 with relatively low y 14 18 ppm and high sr 630 1830 ppm yielding high sr y ratios 43 94 they have lree enriched and hree depleted ree patterns la yb 19 26 typical of adakites sr nd isotopic compositions of these volcanic rocks are similar to those of the contemporaneous high silica adakitic rocks in the southern yidun terrane the 231 230 ma volcanic rocks in the changtai region are composed of basalts andesites dacites and rhyolites with sio 2 ranging from 47 7 to 81 0 wt basalts in the changtai region have initial 87 sr 86 sr ratios from 0 7052 to 0 7058 and e nd t from 1 1 to 1 5 they display oib like geochemical signatures and were likely derived from low degrees of partial melting of an oib like mantle source with subordinate input of subduction components indicated by increasing th content andesites and dacites show elemental and isotopic compositions that evolved from the basalts indicative of derivation via crustal contamination and fractional crystallization afc of basaltic magmas rhyolites have lower initial 87 sr 86 sr ratios and higher e nd t values than andesites dacites inconsistent with an afc process and are suggested to have been derived from anatexis of crustal materials emplacement of volcanic rocks in the changtai and xiangcheng regions could be attributed to the subduction of the ganzi litang ocean a branch of the paleo tethys ocean volcanic rocks in the changtai and xiangchang regions have zircon u pb ages about 4 6 myr older than that of arc granites in the eastern yidun terrane the spatio temporal distribution of the volcanic rocks indicates that the subduction was initiated under the southern yidun terrane in the middle triassic and extended to the northern yidun terrane at the beginning of the late triassic

the deposits internal structure and geometry in six alluvial fan fan delta bodies devonian norway a study in the significance of bedding sequence in conglomerates discussion and reply

garba iii melka kunture ethiopia a msa site with archaic homo sapiens remains revisited garba iii in the upper awash valley of ethiopia is one of the many sub sites of melka kunture where the overall archaeological record starts at c 1 8 ma garba iii was excavated over several years in the 1970s under the direction of francis hours who was able to publish only preliminary reports before his untimely death at the base of the sequence acheulean layers were discovered and above then msa layers characterized by ferruginous concretions where three cranial fragments of homo sapiens were also found an age close to 150 ka was suggested for the human remains and associated industry of the upper layers which are the focus of the current re assessment at the time however no clear cut distinction was made between acheulean and msa in 2011 the original site was re located geological trenches were dug the stratigraphic sequence was documented in detail and a geomorphological reconstruction was prepared lithic collections and anthropological remains kept in addis ababa were also re studied new field research highlights complex site formation processes including cyclic phases of erosion and re deposition of pre existing soils and deposits however pedogenetic processes which developed twice also point to prolonged phases of stability in good accordance with the state of preservation of the lithic industry which is neither rolled nor fragmented small sized obsidian pebbles were knapped which were available locally in the alluvium deposits the technological and typological analysis confirms that the lithic industry is early msa a variety of knapping methods were in use including the levallois method well established and mostly recurrent points were produced as well as scrapers denticulates and scaled pieces circumstantial evidence points to an age not later than an early phase of mis 5e the anthropological remains one of which was so far undescribed are fragments of a right parietal bone of a parietal or more likely of a frontal bone and of an occipital bone they contribute to the still scanty fossil record available at the key time of the archaic h sapiens emergence and early spread

effect of improper water and land resource utilization on the central main ethiopian rift lakes the ethiopian rift is characterized by a chain of lakes varying in size hydrological and hydrogeological setting some of the lakes and tributary rivers are used for irrigation soda abstraction commercial fish farming and recreation and they support a wide variety of endemic birds and wild animals few lakes shrunk due to excessive abstraction of water others expanded due to increase in surface runoff and groundwater flux from percolated irrigation water excessive land degradation deforestation and over irrigation resulted in sedimentation in lakes and increase in soil salinity the chemistry of some of the lakes has also been changed dramatically this paper addresses the major environmental changes in the last few decades in the central main ethiopian rift lakes that resulted mainly from anthropogenic factors the methods employed include field hydrogeological mapping supported by aerial photograph and satellite imagery interpretations hydro meteorological data analysis water balance estimation catchment hydrological and groundwater flow modelling and hydrochemical analysis a converging evidence or integrated approach has been adapted to reconstruct the temporal and spatial variations of lake levels and hydrochemistry the result revealed that the major changes in the rift valley are related mainly to recent improper utilization of water and land resources in the lakes catchment and direct lake water abstraction aggravated intermittently by climatic changes the terminal lakes show dramatic reduction in level and increase in salinity these changes appear to have grave environmental consequences on the fragile rift ecosystem which demands extremely urgent integrated basin wide water management practice

collision rotation and back arc spreading in the region of the okhotsk and japan seas the india eurasia collision caused extensive deformation in the eurasia continent since late eocene time we propose that back arc spreading in the okhotsk and japan seas was related to the movement of microplates relevant to the india eurasia collision the kuril basin of the okhotsk sea and the basins of the japan sea were formed as back arc basins simultaneously their late oligocene to middle miocene age is constrained by sediment stratigraphy basement depth and heat flow data the movements of two microplates the okhotsk and amuria plates are significant for the tectonics of the okhotsk and japan seas simultaneous retreat of both microplates from trench hinge lines caused back arc spreading in the kuril basin and the japan sea the amuria plate moved north northeastward due to the india eurasia collision this caused pull apart opening of the baikal rift along a transform boundary between the siberia and amuria plates and a collision along the stanovoy range at its northern margin this movement triggered a clockwise rotation of the okhotsk plate with a component of dextral collision between the okhotsk and amuria plates which is well observed along the central zone of the sakhalin hokkaido islands between the two back arc plates the clockwise rotation of the okhotsk plate is suggested by the southwestward fan shaped opening of the kuril basin the kuril basin narrows to the northeast and terminates south of the kamchatka peninsula where a relative rotation pole between the okhotsk and kuril forearc plates is located the kamchatka peninsula was a zone of collision on the other side of the clockwise rotating okhotsk plate during the opening of the kuril basin the sredinny range in the kamchatka peninsula is one of the results of the collision between the okhotsk plate and the forearc plate of the kuril arc the bending of the japanese islands occurred in association with the back arc spreading of the yamato basin of the japan sea

the mid proterozoic mporokoso basin northern zambia sequence stratigraphy tectonic setting and potential for gold and uranium mineralisation abstract the mporokoso basin was formed on a dying silicic magmatic arc between 1800 and 1100 ma the basin consists of up to 5000 m of fluvial and shallow marine siliciclastic sediments the stratigraphy and tectonic setting resembles that of mid proterozoic cratonic basins elsewhere the mporokoso basin is the result of crustal extension rather than compression the gravitational collapse of crust thickened by tectonic and magmatic processes may have caused the extension and subsidence in this and other mid proterozoic basins fluvial sediments in the lower part of the basin fill are known to contain traces of gold analysis has shown up to 1 3 ppm gold in conglomerates gold concentrations are most likely to occur along subtle unconformities which form the boundaries of depositional sequences these sequence boundaries can be identified through sedimentary facies relationships the underlying silicic igneous rocks are unlikely to have been the source of the gold the lupa goldfields of tanzania lie to the east of the mporokoso basin and are the nearest probable source however only the lowermost sequence in the basin was derived from the east the basement and sediments of the mporokoso basin are enriched in uranium the critical factor missing for an uranium deposit of the unconformity type is a reducing lithology the only known possible reductants are carbonaceous mudstone within the basin and graphite in the basement 200 km away

chronological and behavioral contexts of the earliest middle stone age in the gademotta formation main ethiopian rift there is a general consensus that our species emerged first in africa currently the best known skeletal evidence for the earliest anatomically modern homo sapiens amhs derives from sites in the rift valley in ethiopia however archaeological evidence from cave sites in southern and northern africa largely dominates discussions on behaviors that characterize the dawn of modern humans later middle pleistocene open air sites in east africa present unparalleled geochronological control and thus a better chance to study hominin behavior here we present results of a multidisciplinary investigation of the archaeology and geochronology of one of the oldest known middle stone age msa occupations from the gademotta formation fm in the main ethiopian rift renewed excavation lithic analysis fracture mechanics and tephrochronological investigations at gademotta provide a better understanding of the chronological and behavioral contexts during the critical period immediately preceding the origin of our species

accretion of continental crust thermal and geochemical consequences the irreversible chemical differentiation of the earth s mantle to produce sialic crust over the past 3900 ma has most probably occurred during widely separated but short lived accretion episodes these episodes involved the massive addition of juvenile sialic magma to the earth s surface thickening pre existing crust simple numerical simulations based on tectonic petrological and geochemical observations on archaean high grade orthogneiss terranes have been used to explore the metamorphic and geochemical consequences of massive thickening of sialic crust during short lived accretion episodes the location of the main sites of magmatic addition within the crust exert a profound influence on the thermal regimes geochemical differentiation of the continental crust by partial melt and vapour phase controlled processes and the development of granulite facies mineral assemblages can be integrated with the simple numerical models finally the survival of thick archaean continental crust imples the contemporaneous stabilization of thick lithospheric substructures to the newly formed continental masses

dispersion of the afar plume implications from the spatiotemporal distribution of the late miocene to recent volcanics southwestern arabian peninsula abstract forty three new k ar dates from the yemen inland volcanic fields amran sana a dhamar rada and marib sirwah field in the southwestern arabian peninsula reveal a progression that can be divided into four stages stage 1 6 4 5 2 ma stage 2 3 6 2 9 ma stage 3 2 2 1 7 ma and stage 4

crustal structure of the main ethiopian rift from gravity data 3 dimensional modeling abstract a three dimensional interpretation of the newly compiled bouguer anomaly map of the main ethiopian rift is presented a high resolution 3 d model constrained with the seismic results reveals a possible crustal thickness and density distribution beneath the graben the bouguer anomalies along the axial portion of the rift floor as deduced from the results of the regional and residual separation are mainly caused by the deep seated structures the inferred zone of intrusion which is the main subject of the present study coincides with the maximum gravity anomaly of the rift floor the intrusion is displaced at several sectors along the east west direction and the two major displacements coincide with the locations of the major rift offsets on the surface because of the asthenospheric uplift the crust under the main ethiopian rift is slightly thinned the zone of crustal thinning 31 km coincides with the location of the intrusion beneath the rift floor and the maximum of which is attained in the northern and central sectors of the graben the trend of the crustal thinning zone which is from south to north is the same as the one obtained in the afar depression the southeastern and western plateaus on the other hand show by far the largest crustal thickness in the region 38 51 km in contrast to the afar depression where the crust is partly oceanized the thickness and density of the crust suggest that the main ethiopian rift is underlain by a purely continental crust the deep and relatively large nature of the intrusion leads to the conclusion that a large scale asthenospheric upwelling might be responsible for the thinning of the crust and subsequent rifting of the graben

scaling of differentially eroded surfaces in the drainage network of the ethiopian plateau differentially eroded regions selected over the ethiopian plateau northeast africa were statistically analyzed using satellite images of various electromagnetic spectrum regions and resolutions landsat tm and ers 1 the power spectrum exponent values for the landsat tm2 visible and ers 1 images over the same surface are associated to the intrinsic properties of the different sensor type and postprocessing of the data differences in the values were observed between eroded area blue nile canyon bnc and relatively noneroded area plateau pla for all data sets these differences are associated to the mechanical erosion of the plateau the remotely sensed data fields show scaling from 35 m to 15 km with no break at 1 5 km and are highly multifractal analyses of the landsat tm bands over each area demonstrated something particular values for bands in the shortwave infrared swir range differed from values for bands in the visible spectrum range in the plateau area by about 0 48 while in the drainage area this difference is around 0 13 landsat tm swir bands are sensitive to spectral signature of clay minerals while data in the visible spectrum range mostly depict topography gradients two concurrent processes are highlighted mechanical erosion and chemical erosion deposition which interact to produce the observed differences in the drainage area where cliffs and steep slopes are present and mechanical erosion intensively occurs the alteration minerals are remobilized quickly whereas in the plateau area mechanical erosion is low and alteration mineral deposition is less disturbed such new statistical highlights of topographic versus chemical surfaces will have to be taken into account in landforming models

dynamics of magma emplacement in centrifuge models of continental extension with implications for flank volcanism a set of experimental small scale models of continental extension investigated the emplacement of orthogonal and oblique magma chambers initially underplated at the base of the crust the models are driven by a centrifugal body force which simulates the role of gravity in nature the models represent crustal conditions analogous to relatively mature continental rifts and consider both symmetric and asymmetric extension the experimental results suggest that magma emplacement is controlled by the interactions between tectonics and rheology of the crustal layers in particular extension is mainly accommodated by lateral flow and ductile doming in the viscous layer simulating the lower crust and by listric normal faults in the overlying brittle layer associated with the ductile domal uplift stretching of the continental crust induces a reactive migration of viscous layers into the footwall of major normal faults where magma accumulation at the core of the domes takes place this behavior suggests that widespread magmatism is expected to localize in the footwall of major normal faults a situation that is often observed in core complex structures although the models presented here are intended to simulate magma emplacement during extension of a two layer brittle ductile system lateral flow and migration of magma initially underplated at the base of the crust beneath narrow rifts e g the ethiopian rift may also provide a similar mechanism and a possible explanation for the occurrence of important volcanoes on the plateaus flanking the rift zones in the suggested model such volcanoes are related to large basaltic magma reservoirs located at the base of the crust to accommodate the space vacated by lateral flow and thinning of the ductile crust the similarity in structure of the models with natural examples of continental extension modes such as rift systems and core complexes may suggest a close similarity of dynamic processes

the central main ethiopian rift is younger than 8 ma confirmation through apatite fission track thermochronology an isolated block of precambrian basement rocks and mesozoic sediments is exposed at kella along the western margin of the central main ethiopian rift mer surrounded by tertiary to quaternary volcanic rocks apatite fission track thermochronology on two basement samples yielded ages of 7 2 1 0 ma and 6 7 3 0 ma and a long mean track length 14 5 lm rapid late miocene cooling is attributed to denudation related to rifting despite the paucity of data due to the absence of suitable lithologies in the area our data confirm that the central mer is younger than 8 ma as recently proposed on the basis of field evidence and radiometric dating of volcanics this implies that the central mer formed after the northern mer indicating a diachronous development of this third arm of the red sea gulf of aden ethiopian rift system

new age constraints on the timing of volcanism and tectonism in the northern main ethiopian rift southern afar transition zone ethiopia abstract forty new k ar and 40 ar 39 ar isotopic ages from the northern main ethiopian rift mer southern afar transition zone provide insights into the volcano tectonic evolution of this portion of the east african rift system the earliest evidence of volcanic activity in this region is manifest as 24 23 ma pre rift flood basalts transition zone flood basalt activity renewed at approximately 10 ma and preceded the initiation of modern rift margin development bimodal basalt rhyolite volcanism in the southern afar rift floor began at approximately 7 ma and continued into recent times in contrast post subsidence volcanic activity in the northern mer is dominated by mio pliocene silicic products from centers now covered by quaternary volcanic and sedimentary lithologies unlike other parts of the mer mio pliocene silicic volcanism in the mer afar transition zone is closely associated with fissural basaltic products the presence of pliocene age ignimbrites on the plateaus bounding the northern mer whose sources are found in the present rift indicates that subsidence of this region was gradual and that it attained its present physiography with steep escarpments only in the plio pleistocene large 7 5 ma silicic centers along the southern afar and northeastern mer margins apparently formed along an e w oriented regional structural feature parallel to the already established southern escarpment of the afar the addis ababa rift embayment and the growth of 4 5 3 ma silicic centers in the addis ababa area are attributed to the formation of a major cross rift structure and its intersection with the same regional e w structural trend this study illustrates the episodic nature of rift development and volcanic activity in the mer afar transition zone and the link between this activity and regional structural and tectonic features

seismicity associated with magmatism faulting and hydrothermal circulation at aluto volcano main ethiopian rift the silicic volcanic centres of the main ethiopian rift mer play a central role in facilitating continental rifting many of these volcanoes host geothermal resources and are located in heavily populated regions insar studies have shown several are deforming but regional seismic networks have detected little seismicity a local network of 12 seismometers was deployed at aluto volcano from 2012 to 2014 and detected 2142 earthquakes within a 24 month period we locate the events using a 1d velocity model that exploits a regional model and information from geothermal boreholes and calculate local magnitudes b values and focal mechanisms event depths generally range from the near surface to 15 km with most of the seismicity clustering in the upper 2 km a significant amount of seismicity follows the artu jawa fault zone which trends in alignment with the wonji fault belt nne ssw and is consistent with previous studies of strain localisation in the mer focal mechanisms are mostly normal in style with the mean t axes congruent to the orientation of extension in the rift at this latitude some show relatively small left lateral strike slip components and are likely associated with the reactivation of ne ene structures at the southern tip of the aluto gedemsa segment events range from 0 40 to 2 98 in magnitude and we calculate an overall b value of 1 40 0 14 this relatively elevated value suggests fluid induced seismicity that is particularly evident in the shallow hydrothermal reservoir and above it subdividing our observations according to depth identifies distinct regions beneath the volcanic edifice a shallow zone 2 0 km of high seismicity and high b values that corresponds to the hydrothermal system and is influenced by a high fluid saturation and circulation a relatively aseismic zone 0 2 km with low b values that is impermeable to ascending volatiles a region of increased fluid induced seismicity 2 9 km that is driven by magmatic intrusion from below and a deeper zone below 9 km that is interpreted as a partially crystalline magmatic mush these observations indicate that both the magmatic and hydrothermal systems of aluto volcano are seismically active and highlight the need for dedicated seismic monitoring at volcanoes in the mer

low frequency earthquakes beneath tullu moye volcano ethiopia reveal fluid pulses from shallow magma chamber abstract the active magmatic processes beneath volcanoes in continental rifts is poorly understood for example until recently in the east african rift ear the majority of the young volcanoes were thought to be inactive more recent studies have shown that numerous volcanoes in the ear are seismically active and deforming rapidly however an unambiguous sign of actively degassing magma hosted in shallow magma bodies has eluded most investigators here we present detailed analysis of the first low frequency lf earthquake swarms to be observed in the main ethiopian rift the earthquakes locate to beneath tullu moye volcano and are directly related to the presence of a shallow magma body with a high fluid content using spectral modelling we show that the lf earthquakes appear to have low stress drops 1 50 kpa which we interpret in terms of low rupture velocities and high pore fluid pressure careful relocation of the lf earthquakes place them approximately 4 km below the surface within one of two possible clusters however analysis of the correlation between earthquake waveforms show that each swarm contains a range of earthquake families and as such a diversity of earthquake source mechanisms to explain these observations we propose the seismicity is induced by h2o co2 fluid pulses from the shallow magma body into a highly fractured region fluid pulses cause high pore fluid pressures which also cause the low rupture velocities

constraining the african pole of rotation abstract in the absence of well defined transform faults in the east african rift system for constraining the plate kinematic reconstruction the pole of relative motion for the african nubian and somalian plates has been determined from residual motion if africa and somalia are to continue to drift apart along the east african rift system which would then evolve into a series of ridges offset by transform faults then incipient transform faults that may reflect the direction of relative motion should already be in place along the east african rift system the incipient transforms along the east african rift system are characterized by shear zones such as the zambezi shear zone in the south and the aswa and hamer shear zones in the north some of these shear zones have been associated with recent strike slip faulting in the nw se direction during periods of earthquakes provided that these consistently nw se oriented strike slip movements in the shear zones give the direction of relative motion of the adjacent plates then they can be used to constrain the position of the africa somalia euler pole due to the fact that identifying transform faults in the east african rift system is difficult and because the genesis of transform faults characterizing a plate boundary at an inception stage is not well known the discussion here is limited to the northern segment of the east african rift system where shear zones are better characterized by the existing geophysical data the characterizing features vary with latitude indicating the complexity of the problem of the genesis of transform faults i believe however that the relatively well defined intra continental transform fault in the northern east african rift system which is characterized by strike slip faulting and earthquakes constrains the pole of relative motion for the african and somalian plates to a position near 1 5 s and 29 0 e

secular variation of magma source compositions beneath the northeast japan arc abstract sr and nd isotopic compositions were determined for tertiary volcanic rocks with ages of 22 16 and 8 ma along the volcanic front in the ne japan arc in order to examine the secular variation of magma source compositions in the mantle wedge these rocks show no through time variation in both sr and nd isotope ratios 0 70412 0 704759 and 2 09 4 12 units respectively it follows that the subduction components do not increasingly pollute the magma source region during the continuous subduction of oceanic lithosphere the degree of pollution by the subduction components is controlled by the amount of clinopyroxene and aluminous phases in mantle wedge peridotites because h 2 o which causes partial melting of the mantle wedge beneath a volcanic front is supplied by the breakdown of both amphibole and chlorite in the dragged hydrated peridotite formed by the addition of slab derived fluid beneath the fore arc region

heterogeneous initial sr isotope compositions of highly evolved volcanic rocks from the main ethiopian rift ethiopia many individual mineral sr isotope studies have revealed the complex evolution of highly evolved rocks in upper crustal magmatic systems casting doubts on the meaning of whole rock sr isotopes in such samples in this paper whole rock sr isotope measurements were replicated three to 13 times on six highly evolved peralkaline rhyolites from the main ethiopian rift mer to appraise their internal heterogeneity and thus the significance of such data these rocks were all fresh samples of pumice obsidian and lava their maximum sr contents and ages were 15 g g and 1 7 ma respectively significant small scale heterogeneities of both sr isotopes and rb and sr contents were observed in most samples although not necessarily associated with petrological characteristics suggesting possible inheritance processes only two almost crystal free obsidian give fairly homogeneous sr isotope ratios these results outline the ambiguity of a single whole rock sr isotope determination on highly evolved peralkaline rocks especially when no simultaneous accurate determination of the rb sr is performed they also suggest that the limitations of sr whole rock analyses are not restricted to phenocryst rich samples as phenocryst poor obsidian and almost aphyric pumices and lava are also concerned these data further underscore that unreplicated whole rock sr isotope measurements should always be used with great caution in the petrogenetic modeling of highly evolved rocks however multiple determinations of whole rock 87sr 86sr in the same samples combined with other geochemical and isotopic data may provide constraints on the shallow level evolution of these magmas it is suggested that selective upper crustal contamination and or interactions with halogen bearing hydrous fluids typical of evolved peralkaline magmas were probably involved in the late magmatic evolution of these mer rhyolites the more pervasive character of fluid interaction processes would probably better account for the small scale association of uncontaminated and contaminated signatures in a single sample thus even fresh samples may have their rb sr isotopic system significantly modified by fluid interactions not as a secondary process but at the late magmatic stage

geochemistry of miocene basaltic rocks temporally straddling the rifting of lithosphere at the akita yamagata area northeast japan abstract voluminous basaltic rocks with arc geochemical signatures were produced in the akita yamagata area the back arc side of ne japan in the middle miocene temporally straddling the opening of the japan sea eleven of these rocks were studied for trace elements among these seven whole rocks and five clinopyroxene separates were analyzed for 87 sr 86 sr and four clinopyroxene separates were analyzed for 143 nd 144 nd the results suggest that the lithosphere has been partly broken up and eliminated or thinned beneath the back arc side of ne japan since 16 ma the new analyses and existing data collectively show that the basaltic magmas have decreased in 87 sr 86 sr and increased in 143 nd 144 nd since 16 ma coincidently the magmas have changed their geochemical signature from a continental margin type to an oceanic arc type on average the basaltic rocks younger than 16 ma are likely to be products of varying degrees of mixing between subduction related basaltic magmas with low 87 sr 86 sr 0 703 0 7035 and partial melts of subarc lithospheric mantle with high 87 sr 86 sr 0 7055 at least in part and arc tholeiitic geochemical characteristics the basaltic rocks older than 16 ma seem to have been more intensely contaminated by crustal material

geodynamic evolution of the bangweulu block northern zambia abstract the bangweulu block is a cratonic unit underlying most of northern zambia and adjacent parts of tanzania and zaire its geodynamic evolution is shown to be entirely proterozoic except for cainozoic rifting and related sedimentation the block has a crystalline basement of schists acid metavolcanics and granitoids which formed during a deformational and magmatic event of eburnian age between 2000 and 1800 ma ago the metavolcanics have affinities with high k calc alkaline suites from modern subduction controllled tectonic regimes suggesting an eburnian plate margin to the northwest of the block the polyphase sedimentary cover of the bangweulu block comprises the mporokoso group a 5000 m thick sequence of fluvial aeolian and lacustrine sediments deposited in the intracratonic mporokoso basin in response to the final phase of the eburnian tectogenic cycle the kasama formation a sequence of fluvial sediments which is 100 300 m thick on the bangweulu block but increases in thickness to nearly 400 m in the downfaulted pericratonic irumide basin formed in the initial phase of the kibaran tectogenic cycle the luitikila beds a 3000 m thick unit of post orogenic molasse of the kibaran irumide belt which encroaches with reduced thickness on the bangweulu block the luapula beds deposited on the margins of the bangweulu block and representing parts of the kundelungu sedimentary sequence accumulated in the pan african pericratonic basin faults and shear zones bordering the bangweulu block developed in sequence during the eburnian kibaran and pan african orogenies the older ones are invariably reactivated during the younger tectogenetic events resulting in substantial lateral and rotational movement of the bangweulu block with respect to adjacent structural terranes

sr and nd isotopic compositions of the magma source beneath north hokkaido japan comparison with the back arc side in the ne japan arc abstract most middle miocene to quaternary basaltic rocks younger than about 12 ma from north hokkaido show a narrow range in initial 87 sr 86 sr sri and initial 143 nd 144 nd ndi isotope ratios which are slightly undepleted compared to morb sri ranges from 0 70296 to 0 70393 and ndi from 0 51288 to 0 51307 but there is no temporal variation in either sri or ndi an exceptional basalt from ohmu area which is more undepleted in terms of both sri and ndi than other north hokkaido nh basaltic rocks most of the nh basaltic rocks are isotopically similar to basaltic rocks found on the back arc side of the ne japan arc ba basaltic rocks younger than about 15 ma whereas basaltic rocks from the ohmu area are isotopically similar to ba basaltic rocks older than 15 ma the generation of both nh and ba basaltic rocks appears to be closely related to mantle evolution this involves upwelling of asthenosphere into the continental lithosphere beneath north hokkaido and the back arc side of the ne japan arc at the same time as spreading of the kurile back arc basin and japan sea back arc basin the magma source for the ohmu area basalts and the ba basaltic rocks prior to 15 ma may correspond to the subcontinental lithospheric mantle thinned by the upwelling of asthenospheric mantle material from which most of the nh basaltic magmas and the ba basaltic magmas younger than 15 ma were derived this asthenospheric mantle is slightly different from morb source in terms of major and trace element characteristics such as tio 2 k 2 o and zr y as well as sr and nd isotopes which may be due to involvement of em ii like component

morphological comparison of distributed volcanic fields in the main ethiopian rift using high resolution digital elevation models abstract distributed volcanic fields of scoria cones maars tuff cones and lava flows represent a crucial expression of the interplay between magmatism tectonics and surface processes in continental rifts we compare the cone morphology of two fields butajira and east ziway in the main ethiopian rift mer using high resolution digital elevation models dems to investigate volumes surface environment and age of emplacement volumes of individual cones are similar in both fields mean 4 106 m3 and lower than in other distributed volcanic fields located in extensional environments volcanism at butajira has created linear clusters of superimposed cones suggesting a small number of larger 107 108 m3 fissure eruptions compared to the isolated events at east ziway the presence of maars and deep craters within cones at butajira indicates phreatomagmatic activity not seen at east ziway where cones may have been emplaced during a dry climate phase shape parameters including curvature of cone rims and ratios of cone height width and crater depth suggest younger eruption ages at butajira compared to east ziway and the possibility of ongoing activity in this area a broader survey of cone fields across the mer using the aster global digital elevation model fails to identify systematic variation in height width and volume from cone morphology limited by low resolution high resolution topography provides a useful insight into rift environments at the time of emplacement and further surveys across the main ethiopian rift may advance our understanding of distributed volcanism in extensional settings and the hazards they present

3 d analysis and interpretation of magnetotelluric data from the aluto langano geothermal field ethiopia the main ethiopian rift valley encompasses a number of volcanoes which are known to be actively deforming with reoccurring periods of uplift and setting one of the regions where temporal changes take place is the aluto volcanic complex it hosts a productive geothermal field and the only currently operating geothermal power plant of ethiopia we carried out magnetotelluric mt measurements in early 2012 in order to identify the source of unrest broad band mt data 0 001 1000 s have been acquired at 46 sites covering the expanse of the aluto volcanic complex with an average site spacing of 1 km based on this mt data it is possible to map the bulk electrical resistivity of the subsurface down to depths of several kilometres resistivity is a crucial geophysical parameter in geothermal exploration as hydrothermal and magmatic reservoirs are typically related to low resistive zones which can be easily sensed by mt thus by mapping the electrical conductivity one can identify and analyse geothermal systems with respect to their temperature extent and potential for production of energy 3 d inversions of the observed mt data from aluto reveal the typical electrical conductivity distribution of a high enthalpy geothermal system which is mainly governed by the hydrothermal alteration mineralogy the recovered 3 d conductivity models provide no evidence for an active deep magmatic system under aluto forward modelling of the tippers rather suggest that occurrence of melt is predominantly at lower crustal depths along an off axis fault zone a few tens of kilometres west of the central rift axis the absence of an active magmatic system implies that the deforming source is most likely situated within the shallow hydrothermal system of the aluto langano geothermal field

gold bearing fluvial and associated tidal marine sediments of proterozoic age in the mporokoso basin northern zambia abstract the structurally defined mporokoso basin contains up to 5000 m of continental and marine clastic sediments and minor silicic volcanics which together form the mporokoso group these rocks overlie unconformably a basement of silicic intermediate igneous rocks and accumulated within the interval 1830 1130 ma this sedimentological study was restricted to the eastern end of the basin and was part of an assessment of the potential for palaeoplacer gold in the mporokoso group at the base of the mporokoso group the mbala formation consists of 1000 1500 m of purple sandstones and conglomerates deposited in a braided stream system overlain by 500 1000 m of mature quartz arenites deposited in a tidal marine setting a general coarsening upward trend exists within the fluvial sediments sandy distal braided stream facies passes upwards into more proximal conglomeratic facies in proximal sections poorly sorted conglomerates form the top of the coarsening up sequence which is 500 700 m thick the overlying fluvial sediments fine upwards the tidal marine sandstones at the top of the mbala formation resulted from reworking of fluvial sediments during a marine transgression well exposed sections with fluvial conglomerates were studied in detail individual conglomerate bodies form sheets extending for hundreds of metres downstream and at least one hundred metres across stream with little sign of deep scouring or channelling they are generally matrix supported the whole fluvial sequence is characterised by a paucity of mud or silt these conglomerates were deposited by large velocity sheet flows of water which transported a bed load of pebbles and sand most fine material settling out from suspension was eroded by the next flow the great lateral and vertical extent and the uniformity of the fluvial sediments suggest that the sediments accumulated over an unconfined alluvial plain and that the tectonic evolution of the source area was relatively continuous and not episodic these features are characteristic of other proterozoic fluvial sequences there are no distinctly channelised fluvial conglomerates nor angular unconformities within the fluvial sequence both of which would have been potential sites for economic gold concentrations reworking of the fluvial sands during the marine transgression may have concentrated gold locally within the marine sandstones