the influence of thermochemical convection on the fixity of mantle plumes abstract a general feature of both isochemical and thermochemical studies of mantle convection is that horizontal plume velocities tend to be smaller than typical convective velocities however it is not clear which system leads to a greater fixity of mantle plumes we perform two and three dimensional numerical calculations and compare both thermochemical and isochemical cases with similar convective vigor to determine whether presence of a dense component in the mantle can lead to smaller ratios of horizontal plume velocity to surface velocity we investigate different viscosity and density contrasts between chemical components in the thermochemical calculations and we perform isochemical calculations with both free slip and no slip bottom boundary conditions we then compare both visually and quantitatively the results of the thermochemical and isochemical calculations to determine which leads to greater plume fixity we find that horizontal plume velocities for thermochemical calculations are similar to those from isochemical calculations with no slip bottom boundary conditions in addition we find that plumes tend to be more fixed for isochemical cases with free slip bottom boundary conditions for two dimensional calculations however in three dimensions we find that plume fixity is similar to that observed in thermochemical calculations

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

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

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

geographic correlation between hot spots and deep mantle lateral shear wave velocity gradients abstract hot spot volcanism may originate from the deep mantle in regions exhibiting the earth s most pronounced lateral s wave velocity gradients these strong gradient regions display an improved geographic correlation over s wave velocities to surface hot spot locations for the lowest velocities or strongest gradients occupying 10 of the surface area of the core mantle boundary cmb hot spots are nearly twice as likely to overlie the anomalous gradients if plumes arise in an isochemical lower mantle plume initiation should occur in the hottest thus lowest velocity regions or in the regions of strongest temperature gradients however if plume initiation occurs in the lowest velocity regions of the cmb lateral deflection of plumes or plume roots are required the average lateral deflections of hot spot root locations from the vertical of the presumed current hot spot location ranges from 300 to 900 km at the cmb for the 10 30 of the cmb covered by the most anomalous low s wave velocities the deep mantle may however contain strong temperature gradients or be compositionally heterogeneous with plume initiation in regions of strong lateral s wave velocity gradients as well as low s wave velocity regions if mantle plumes arise from strong gradient regions only half of the lateral deflection from plume root to hot spot surface location is required for the 10 30 of the cmb covered by the most anomalous strong lateral s wave velocities we find that strong gradient regions typically surround the large lower velocity regions in the base of the mantle which may indicate a possible chemical in addition to thermal component to these regions

experiments on the interaction of thermal convection and compositional layering at the base of the mantle accumulation and survival of compositional heterogeneity at the base of the convecting mantle is investigated using laboratory experiments with temperature dependent viscous fluids two models of heterogeneity in the d layer are considered 1 a reservoir of dense material above the core mantle boundary and 2 continuous recharge of the d layer by subduction of cold negatively buoyant high viscosity slabs of oceanic lithosphere survival of a dense reservoir depends on rp the ratio of stabilizing compositional buoyancy to destabilizing thermal buoyancy rapid mixing occurs for rp 1 for rp 1 mixing occurs by slow entrainment only the reservoir model predicts d layer heterogeneity is concentrated beneath lower mantle upwellings differentiation of composite slabs consisting of dense crustal and light depleted components occurs in a hot thermal boundary layer composite slabs pile up in the basal thermal boundary layer and upon heating the light component is ejected by rayleigh taylor instabilities the dense component accumulates in the boundary layer arrayed in a pattern of interconnected spokes and is slowly entrained into thermal plumes according to these experiments the time required for conversion of slabs to plumes in d is 1 2 gyr the correlation between lateral variations in d layer thickness and lateral variations in lower mantle seismic velocity suggests that d may consist of dense material perhaps former oceanic lithosphere which has sunk through the lower mantle

some mineral physics constraints on the rheology and geothermal structure of earth s lower mantle we explore the implications of recent mineral physics measurements of diffusion coefficients and melting temperatures of lower mantle materials on the rheological and geothermal structure of earth s lower mantle we show that mgsio3 perovskite is significantly stronger than mgo periclase and therefore the rheology of the lower mantle depends strongly on the geometry of a weaker phase periclase we calculate viscosities of the lower mantle for two cases 1 where periclase occurs as isolated grains and 2 where periclase occurs as continuous films using mineral physics data and models of two phase rheology we find that the effective viscosity for the former is about 10 1000 times higher than the latter we therefore suggest that the rheology of the lower mantle is structure and hence strain dependent leading to weakening at large strains due to the formation of continuous films of periclase overall depth variation of viscosity depends not only on the pressure dependence of creep but also on the geothermal gradient both mgsio3 perovskite and periclase have relatively small activation energies e g r t m with g 10 14 where r is the gas constant and t m is melting temperature and therefore the depth variation of viscosity is rather small even for a nearly adiabatic temperature gradient however the geothermal gradients consistent with the geodynamical inference of nearly depth independent viscosity are sensitive to the pressure dependence of viscosity which is only poorly understood a superadiabatic gradient of up to 0 6 k km is also consistent with mineral physics and geodynamical observations

the denudation history of the onshore continental margin of se brazil inferred from apatite fission track data a suite of 68 new apatite fission track analysis results from the onshore rift margin in southeast brazil are presented some of the results reflect resetting related to the eruption of the parana continental flood basalt 130 ma and four others are directly related to late cretaceous magmatism 80 ma the majority of the results were obtained from precambrian basement rocks and sediments from the parana basin the fission track ages broadly increase inland from 60 90 ma on the coastal plain to 300 ma in the continental hinterland the track length distributions show a characteristic trend being negatively skewed at the coast and showing increasing bimodality inland except for the oldest samples which are more unimodal these trends do not reflect rift related heating and resetting but are the result of protracted denudation since the opening of the south atlantic the results are consistent with more than 3 km of exhumation on the coastal plain but little more than 1 km in the hinterland the topographic morphology in the northern region of the study area is complex and is clearly influenced by structure and bedrock lithology for the region further south model calculations of the isostatic response to the denudation suggest that isostatic rebound makes a minor contribution to the long wavelength topography unless the flexural rigidity is more than 1025 n m in this case the initial elevation of the rift margin would need to be 2 5 km in order to accommodate the amount of denudation inferred from the fission track data however it is possible that the effective flexural rigidity varies across the margin becoming greater in the continental interior alternatively the broad scale wavelength topography may be the result of magmatic underplating related to the parana continental flood basalt event if so estimates of the volume of magma generated would need to be revised upward by as much as a factor of 5

influence of a basal thermal anomaly on mantle convection abstract we perform laboratory experiments to study the effects of a variable basal thermal anomaly on convection in the regime of cellular convection ra max around the lower thermal boundary layer in the absence of thermal anomaly we estimate the possible temperature variation which can be generated by a partially molten region at the cmb we find that assuming that max for mantle is the same as that obtained from the experiment a region less viscous than the surrounding region by an order of magnitude can generate a hotspot

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

effect of mantle plumes on the growth of d by reaction between the core and mantle the core mantle boundary is a fundamental compositional discontinuity in the earth where molten iron alloy from the core meets solid silicate minerals from the mantle heat flow from the core to the mantle creates a thermal boundary layer at the base of the mantle at the same time chemical reactions may create a layer of different composition and density than the overlying mantle using a finite element model of convection in a spherical shell that includes formation of dense material at the base of the shell we investigate how a layer forms at the base of the mantle development of a layer at the base of the mantle by this method depends both on the composition of the material forming at the core mantle boundary and on the rate at which the material diffuses into the mantle if the material is less than 3 6 denser than the overlying mantle assuming reasonable choices of lower mantle thermodynamic parameters the reactant material will be swept away by upwelling plumes more dense material on the other hand forms an internally convecting layer that entrains material from above the varying thickness and composition of the layer is consistent with geophysical observations of the characteristics of the deep mantle

northwest to north northwest extension direction in the ethiopian rift deduced from the orientation of extension structures and fault slip analysis northeast to north northeast trends of eruptive centers some of which formed at the termination of faults and en echelon extension fractures the largest one of which is 0 1 km wide up to 1 km long and at least 50 m deep exist between lat 7 n and 9 n in the ethiopian rift on systeme probatoire d9observation de la terre spot imagery the extension fractures form scars in the rift floor border faults of the rift are generally oriented northeast to north northeast two fault systems observed on land sat multi spectral scanner mss cross the rift the bilate weyira fault with a north northwest strike and the wonji fault belt with a north northeast trend north northwest striking faults cutting across the rift are interpreted to be transfer faults that cut the rift into rhomboid shaped basins extension fractures and rows of eruptive centers collectively referred to as extension structures are used in the paper to infer the direction of plate motion in the ethiopian rift northwest to north northwest extension of the rift is deduced by assuming that the direction of opening of extension structures is also the orientation of the minimum principal stress 3 fault slip inversion on faults that cut post miocene volcanic rocks confirms this interpretation

combined effects of eurasia sunda oblique convergence and east tibetan crustal flow on the active tectonics of burma it is widely accepted that deformation of the india sunda plate is the result of partitioned hyper oblique convergence presently sub meridian dextral strike slip faulting accommodates this india sunda motion in a buffer zone the burma platelet this wide dextral strike slip shear zone is complicated by the side effect of the tibet plateau collapse that can be described in term of crustal flow and gravity tectonics the loss of potential energy related to this plateau collapse affects most of the burmese platelet particularly in its northernmost part interaction of these two distinct geodynamic processes is recorded in the gps based regional strain field the analysis of seismic focal mechanism but also from direct geologic observations both onshore and offshore myanmar and bangladesh we propose the apparent e w shortening component of this so called partitioned hyper oblique subduction is only the effect of regional gravitational forces related to the tibet plateau collapse whereas the ns strike slip faulting accommodates the india sunda motion

distribution of quaternary deformation in the central main ethiopian rift east africa 1 the main ethiopian rift mer is a narrow continental rift characterized by an along axis variation in rift evolution with early stages in the south evolving to incipient breakup in the north although distribution and style of quaternary volcanotectonic deformation is well known in the northern rift sector knowledge of these characteristics is comparatively less constrained southward in this paper we present the results of a field structural study carried out to better constrain the time space distribution of faulting in the central sector of the mer central mer the new field structural data coupled with new 14c radiometric dating of faulted rocks suggest a localization of faulting at both rift margins of the central mer where radiometric dating of faulted material has allowed establishing a late pleistocene holocene activity of border faults conversely in rift faulting wonji fault belt wfb is subordinate highlighting a major difference with the northern sector of the mer where deformation is essentially accommodated in the axial zone this is consistent with an along axis variation in rift evolution showing the central mer less evolved than the northern rift sector inversion of cumulative fault slip data reveals a variation in the extension direction between the rift margins n105 110 e and the rift floor n90 95 e which accords well with the current nubia somalia plate kinematics the variation in extension direction across the rift could manifest a slip partitioning between the boundary faults and in rift wfb faults

high resolution estimates of nubia somalia plate motion since 20 ma from reconstructions of the southwest indian ridge red sea and gulf of aden

shear wave speeds at the base of the mantle we inverted 4864 scs s and 1671 s diff sks residual travel times for shear wave speed anomalies at the base of the earth s mantle we applied ellipticity corrections accounted for mantle structure outside of the basal layer using mantle tomography models and employed finite size sensitivity kernels the basal layer thickness was set to 290 km however the data allow thicknesses between 200 and 500 km the residuals were inverted using a spherical harmonic basis set of degree 30 for a model that both is smooth and has a small euclidean norm which limited spectral leakage of higher order structures into low order wavelengths hotspots dominantly overlay slow wave speed regions nonsightings of ultralow velocity zones ulvzs most frequently appear in fast regions suggesting that slow regions at the base of the mantle are associated with ulvzs however ulvz sightings appear in both slow and fast regions recently active subduction zones do not correlate with velocity anomalies however the locations of subduction zones active prior to 90 ma correlate extremely well with fast anomalies implying that slabs descend as fast as 2 cm yr through the lower mantle the correlation continues through the historical subduction record to 180 ma suggesting that slabs remain in the deep mantle at least 90 myr fast anomalies reach 2 percent while slow anomalies extend to 5 percent if we assume that the anomalies are thermal and anharmonic in origin and apply a wave speed thermal anomaly conversion the temperature deviations would be over 500 k cold in fastest regions and over 1000 k hot in the slowest regions which would initiate plumes much hotter than those observed at the surface author

topography river network and recent fault activity at the margins of the central main ethiopian rift east africa abstract along its length the main ethiopian rift mer in east africa records a transition from early fault dominated morphology in the south to axial magma assisted rifting typical of continental break up in the north it is one of the few locations on earth offering a complete picture of the evolution of continental rifting and thus provides a unique opportunity to directly analyze how the drainage network reorganizes under extensional tectonic forcing in this paper we present a new analysis of the river network and relative landforms complemented with a summary of recent geological data at both rift margins of the central mer a key sector of the rift capturing the phase of drainage reorganization between incipient and mature rifting this analysis shows that hydrography is strongly influenced by recent tectonics rectangular drainage patterns windgaps and lacustrine swampy areas formed by structural dams document that the rivers are in continuous competition with fault activity the irregular longitudinal profiles with knickpoints knickzones in correspondence with faults also suggest that rivers are in a transient state of disequilibrium related to recent tectonic activity at rift margins in agreement with previous geological and seismological data a more regional analysis extended to the adjoining northern and southern mer indicates that rifting evolves from initial stages characterized by margins poorly incised by rivers with gentle channel gradients except in correspondence with faults to mature phases in which rift margins are highly incised by a well organized fluvial network composed by concave and steep rivers our regional analysis also indicates a stronger and or more recent tectonic activity at the rift margins proceeding to the south in line with previous models of rift development

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

miocene to present differential exhumation in the western alps insights from fission track thermochronology 1 analysis of cooling age patterns yielded by low temperature thermochronometers provides key information about the role played by tectonic discontinuities during the late stages of exhumation of metamorphic belts in the western alps fission track data published so far are heterogeneously scattered and concentrated in few structural domains preventing analyses at the scale of the whole belt the new apatite fission track data reported in this work obtained with the external detector method as well as the population method in very low u content samples fill this gap they constrain the postmetamorphic evolution of the western alps along two transects from the foreland to the retroforeland unraveling the effective role played by some major faults during the exhumation of the belt at shallow crustal levels a clear regional pattern characterized by decreasing ages moving from the axial sector to the european external sector of the belt and by an along strike gradient with increasing ages from north to south has been unraveled evident breaks in this age pattern have been detected in correspondence of faults that are near parallel to the trend of the belt pointing to the occurrence of active tectonics during and after exhumation the most apparent breaks have been observed in the axial sector of the belt where the postmetamorphic deformation would have been negligible according to classic tectonic models faults located in the axial sector split the belt into two major blocks eastern and western since the miocene the western block experienced higher exhumation rates than the eastern one such differential exhumation was accommodated in the northern portion of the belt thanks to reverse motion along the internal houiller fault which occurred in a convergent transcurrent framework to the south it was accommodated instead by normal reactivation of the brianconnais front and by activity of the longitudinal fault system which occurred in a divergent transcurrent framework the tectonic activity affecting the axial sector of the belt in a context of regional dextral strike slip is coeval with the forward propagation of the external thrusts and of similar magnitude we suggest that the contrasting kinematic regimes i e convergent versus divergent transcurrence observed in the western alps moving along strike were responsible of the increasing exhumation rates toward the north revealed in both blocks by the along strike age gradient the higher exhumation rates recognized northward would be related to an increasing importance of crustal shortening that promoted erosion during the late stages of exhumation of the belt

partial melting in a thermo chemical boundary layer at the base of the mantle abstract seismological detections of complex structures in the lowermost mantle boundary layer the d region motivate a conceptual model of a compositionally stratified thermo chemical boundary layer tcbl within which lateral temperature variations sustained by large scale mid mantle flow cause variations of partial melt fraction partial melt fractions of from 0 to 30 in the tcbl occur due to the eutectic of the boundary layer material lying below the high temperature at the core mantle boundary cmb coupled with the presence of steep thermal gradients across the boundary layer regions of the tcbl with the highest temperatures have extensive partial melting producing lateral chemical variations and strong effects on seismic velocities and boundary layer dynamics this tcbl concept provides a relatively simple framework that can plausibly account for diverse seismological observations such as predominance of large scale volumetric elastic wave velocity heterogeneity in the boundary layer laterally extensive but intermittent abrupt shear and compressional velocity increases and decreases at the top of the d region small scale topography of d velocity discontinuities thin ultra low velocity zones at the cmb widespread shear wave anisotropy in d bulk sound velocity anomalies detected in low shear velocity regions of d and large scale upwellings from the most extensively melted regions of the boundary layer neutral or negative buoyancy of the partial melt in the tcbl is required along with an increase in bulk modulus and some density increase of the boundary layer material relative to the overlying mantle more complex models in which the partially melted chemical boundary layer is laterally displaced by mid mantle downwellings are also viable but these appear to require additional special circumstances such as a phase change efficient segregation of slab crustal material or abrupt onset of anisotropy in order to account for rapid velocity increases at the top of d the precise nature of the compositional anomaly or anomalies in d and the eutectic composition of this zone are yet to be determined but it appears likely that partial melting within the lowermost mantle plays a paramount role in the observed seismic velocity heterogeneity increased resolution geodynamic calculations of dense boundary layer structures with partial melting and attendant viscosity reductions and chemical heterogeneity are needed to quantify this tcbl concept

the origin of along rift variations in faulting and magmatism in the ethiopian rift the geological record at rifts and margins worldwide often reveals considerable along strike variations in volumes of extruded and intruded igneous rocks these variations may be the result of asthenospheric heterogeneity variations in rate and timing of extension alternatively pre existing plate architecture and or the evolving kinematics of extension during breakup may exert first order control on magmatism the main ethiopian rift mer in east africa provides an excellent opportunity to address this dichotomy it exposes along strike several sectors of asynchronous rift development from continental rifting in the south to incipient oceanic spreading in the north here we perform studies of volcanic cone density and rift obliquity along strike in the mer by synthesizing these new data in light of existing geophysical geochemical and petrological constraints on magma generation and emplacement we are able to discriminate between tectonic and mantle geodynamic controls on the geological record of a newly forming magmatic rifted margin the timing of rift sector development the three dimensional focusing of melt and the ponding of plume material where the rift dramatically narrows each influence igneous intrusion and volcanism along the mer however rifting obliquity plays an important role in localizing intrusion into the crust beneath en echelon volcanic segments along strike variations in volumes and types of igneous rocks found at rifted margins thus likely carry information about the development of strain during rifting as well as the physical state of the convecting mantle at the time of breakup

a refinement of the chronology of rift related faulting in the broadly rifted zone southern ethiopia through apatite fission track analysis to reconstruct the timing of rift inception in the broadly rifted zone in southern ethiopia we applied the fission track method to basement rocks collected along the scarp of the main normal faults bounding i the amaro horst in the southern main ethiopian rift and ii the beto basin in the gofa province at the amaro horst a vertical traverse along the major eastern scarp yielded pre rift ages ranging between 121 4 15 3 ma and 69 5 7 2 ma similarly to two other samples one from the western scarp and one at the southern termination of the horst 103 4 24 5 ma and 65 5 4 2 ma respectively more interestingly a second traverse at the amaro northeastern terminus released rift related ages spanning between 12 3 2 7 and 6 8 0 7 ma in the beto basin the ages determined along the base of the main northwestern fault scarp vary between 22 8 3 3 ma and 7 0 0 7 ma we ascertain through thermal modeling that rift related exhumation along the northwestern fault scarp of the beto basin started at 12 2 ma while in the eastern margin of the amaro horst faulting took place later than 10 ma possibly at about 8 ma these results suggest a reconsideration of previous models on timing of rift activation in the different sectors of the ethiopian rift extensional basin formation initiated more or less contemporaneously in the gofa province 12 ma and northern main ethiopian rift 10 12 ma at the time of a major reorganization of the nubia somalia plate boundary i e 11 2 ma afterwards rift related faulting involved the southern mer amaro horst at 8 ma and only later rifting seemingly affected the central mer after 7 ma