temporal changes in fault strike to 90 and extension directions during multiple episodes of extension an example from eastern nevada in areas with prolonged extensional histories or multiple episodes of extension temporally distinct extensional fault systems including dip slip normal oblique slip and or strike slip faults may form these fault systems may be spatially distinct overlapped or completely superimposed and different fault systems may have different fault strikes and extension directions although fault strike is not a very sensitive indicator of principal stress directions changes through time in fault strike of as much as 90 exceed the uncertainty overprinted extensional fault systems of different ages with significantly different to 90 fault strikes and extension directions provide strong evidence of changes in the regional stress field with time tens of millions of years of extension occurred in the basin and range province of the united states and many temporally distinct fault sets formed through that time we document at least four extensional episodes and the fault kinematics of each where possible in the central hiko range eastern nevada the four cenozoic extensional events are prevolcanic synvolcanic postvolcanic miocene pliocene and postvolcanic pliocene quaternary the prevolcanic faults strike north south and are interpreted as footwall faults to the regional eocene oligocene snake stampede extensional system which caused approximately east west extension the synvolcanic faults are late oligocene miocene age and strike east west although they are rare and have minor offset they accommodated north south directed extension postvolcanic faults part of the timpahute lineament are approximately east west striking normal and northeast striking oblique slip faults they also accommodated approximately north south extension during miocene time the northeast striking left normal oblique slip fault set formed as new faults and records a change between north south and east west extension in miocene time later east west extension occurred along the pliocene to quaternary hiko fault zone these four overprinted extensional systems show two separate changes in normal fault strike of 90 from north south to east west and from east west back to north south consequently the extension direction similarly changed through time we suggest that causes for the changes in direction of upper crustal extension include a change from stresses created by 1 plate interactions which generated approximately east west extension to 2 mantle motions which generated approximately north south extension and 3 back again the extension related to mantle motions is associated with a band of volcanism that passed across the northern basin and range province moving generally from north to south during the protracted period of extension therefore a change from plate boundary or interaction derived stresses to mantle or volcanism related stresses and back again is possible

morphostructural development of the eritrean rift flank southern red sea inferred from apatite fission track analysis 1 the red sea is one of the best exposed young rift basins in the world its flanks on both the african and arabian sides are characterized by basement uplifts parallel to the margins and by active erosion through the integration of 37 new apatite fission track ft analyses and regional geology we elucidate the uplift and denudational history of the eritrean continental margin along the southern red sea and in particular the development timing and past and present morphostructural features of its onshore portion ft data indicate that at around 20 ma the eritrean margin was affected by a crustal cooling event due to a postrifting accelerated phase of denudation this cooling has the same age as those already detected on the conjugate arabian margin yemen and saudi arabia ft ages increase from 10 20 to 300 400 ma with increasing distance and elevation from the coastal areas toward the interior this trend indicates a diminishing amount of eroded section in the same direction we use ft and structural data to define the position of the main border fault along the margin where the original scarp was located we estimate rates of vertical denudation of 190 200 m myr at the border fault and of 60 70 m myr on the plateau on the basis of these new data we argue against an oligocene tectonic unroofing of the margin through a low angle detachment assumed by previous authors but we maintain the essential role of the erosional denudation in the development of present margin morphology

evolution of large normal faults evidence from seismic reflection data recent advances have been made in understanding how extensional faults and basins develop as faults propagate and link evidence for these linkage patterns in seismic reflection data can be seen in data from east africa early fault linkage patterns for boundary faults can follow three possible paths fault linkage and propagation occur either 1 prior to significant basin formation 2 after minor faulting has created an extensive area of subsidence or 3 during basin development the data from east africa show examples mainly of paths 1 and 2 transverse anticlines anticlines developed parallel to and in the hanging wall of the strike of faults associated with boundary faults are common features they represent either the sites of old synthetic transfer zones or a region of low fault displacement along the strike of a fault where two or more depocenters of different ages overlap as fault activity decreases over time displacement tends to be concentrated on progressively narrower parts of the fault this pattern is developed particularly well in continental rifts and may help discriminate late synrift sedimentation from postrift sedimentation where strike lines across the hanging wall of the fault are observable

rift kinematics during the incipient stages of continental extension evidence from the nascent okavango rift basin northwest botswana high resolution aeromagnetic data from the nascent okavango rift in northwest botswana provide an unprecedented view of rift kinematics during the incipient stages of continental extension crosscutting relationships between west northwest trending 180 ma karoo dikes and reactivated northeast trending proterozoic basement faults are used to document the kinematics of cenozoic faulting during the initial stages of rifting depth estimates to the top of the dikes using three dimensional euler deconvolution solutions have produced the following interpretations 1 the okavango rift is a half graben with a downthrow of 200 300 m 2 the width of the okavango rift 100 20 km is similar to that of more mature continental rifts such as the tanganyika and baikal rifts this suggests that the width of continental rifts is acquired early in their evolution and reflects neither the age and maturity of the rift basin nor the amount of extension it is suggested that the cumulative downthrow sediment infill included and subsidence may be a better indicator of the relative maturity of rift basins 3 preexisting basement faults exert a major control during rifting and reactivation processes do not occur synchronously along the entire length of preexisting faults 4 the okavango rift is defined by normal faults there is no evidence of major strike slip faults thus excluding a pull apart tectonic model for this nascent continental rift stage 5 the preexisting sekaka shear zone terminates the okavango rift to the south suggesting that such shear zones represent major barriers during longitudinal propagation of rifts this probably explains why such shear zones commonly evolve into accommodation or transfer zones during further evolution of continental rifts

a continuum model for water movement in an unsaturated fractured rock mass the movement of fluids in a fractured porous medium has been the subject of considerable study this paper presents a continuum model that may be used to evaluate the isothermal movement of water in an unsaturated fractured porous medium under slowly changing conditions this continuum model was developed for use in evaluating the unsaturated zone at the yucca mountain site as a potential repository for high level nuclear waste thus its development has been influenced by the conditions thought to be present at yucca mountain a macroscopic approach and a microscopic approach are used to develop a continuum model to evaluate water movement in a fractured rock mass both approaches assume that the pressure head in the fractures and the matrix are identical in a plane perpendicular to flow both approaches lead to a single flow equation for a fractured rock mass the two approaches are used to calculate unsaturated hydrologic properties i e relative permeability and saturation as a function of pressure head for several types of tuff underlying yucca mountain using the best available hydrologic data for the matrix and the fractures rock mass properties calculated by both approaches are similar

experimental determination of the effect of dissolved co2 on the dissolution kinetics of mg and ca silicates at 25 c dissolution rates of diopside forsterite fo100 and fo92 wollastonite and hornblende were measured at 25 c in nacl solutions as a function of ph 1 ph 12 pco2 0 and 1 atm and activity of hco3 and co32 10 5 co2 0 1 m dissolution rates of diopside forsterite and horneblende are not affected within the experimental uncertainty by 1 atm pco2 at ph 4 and by the presence of bicarbonate up to 0 1 m and carbonate up to 0 01 m ions at ph of 8 and 11 12 respectively although wollastonite dissolution rates are unaffected by pco2 at ph 4 its rates are increased by the presence of hco3 at 0 01 hco3 0 1 m and ph 7 8 and decreased slightly by co32 at co32 0 0001 m and ph 12 these results should be useful for modeling the effect of silicate rock weathering on the global biogeochemical cycle of carbon for the first time we were able to distinguish the net effect of pco2 versus that of ph it is observed in general accord with previous results that the direct effect of co2 on basic silicates dissolution rates is very weak as a result the major parameter governing basic silicates dissolution is ph of soil solutions in contact with minerals

fault kinematics and stress fields in the rwenzori mountains uganda the rwenzori mountains in western uganda form an active rift transfer zone in the western branch of the east african rift system here we quantify local stress fields in high resolution from field observations of fault structures to shed light on the complex polyphase tectonics expected in transfer zones we apply the multiple inverse method which is optimized for heterogeneous fault slip data to the northern and central rwenzori mountains observations from the northern rwenzori mountains show larger heterogeneity than data from the central rwenzori including unexpected compressional features thus the local stress field indicates polyphase transpressional tectonics we suggest that transpression here is linked to rotational and translational movements of the neighboring victoria block relative to the rwenzori block that includes strong overprinting relationships stress inversions of data from the central rwenzori mountains indicate two distinct local stress fields these results suggest that the rwenzori block consists of smaller blocks

long term cooling history of the albertine rift new evidence from the western rift shoulder d r congo to determine the long term landscape evolution of the albertine rift in east africa low temperature thermochronology was applied and the cooling history constrained using thermal history modelling acquired results reveal 1 old cooling ages with predominantly devonian to carboniferous apatite fission track ages ordovician to silurian zircon u th he ages and jurassic to cretaceous apatite u th sm he ages 2 protracted cooling histories of the western rift shoulder with major phases of exhumation in mid palaeozoic and palaeogene to neogene times 3 low palaeozoic and neogene erosion rates this indicates a long residence time of the analysed samples in the uppermost crust with the current landscape surface at a near surface position for hundreds of million years apatite he cooling ages and thermal history models indicate moderate reheating in jurassic to cretaceous times together with the cooling age distribution a possible albertine high with a distinct relief can be inferred that might have been a source area for the congo basin

spatio temporal trends in normal fault segmentation recorded by low temperature thermochronology livingstone fault scarp malawi rift east african rift system the evolution of through going normal fault arrays from initial nucleation to growth and subsequent interaction and mechanical linkage is well documented in many extensional provinces over time these processes lead to predictable spatial and temporal variations in the amount and rate of displacement accumulated along strike of individual fault segments which should be manifested in the patterns of footwall exhumation here we investigate the along strike and vertical distribution of low temperature apatite u th he ahe cooling ages along the bounding fault system the livingstone fault of the karonga basin of the northern malawi rift the fault evolution and linkage from rift initiation to the present day has been previously constrained through investigations of the hanging wall basin fill the new cooling ages from the footwall of the livingstone fault can be related to the adjacent depocentre evolution and across a relay zone between two palaeo fault segments our data are complimented by published apatite fission track aft data and reveal significant variation in rock cooling history along strike the centre of the footwall yields younger cooling ages than the former tips of earlier fault segments that are now linked this suggests that low temperature thermochronology can detect fault interactions along strike that these former segment boundaries are preserved within exhumed footwall rocks is a function of the relatively recent linkage of the system our study highlights that changes in ahe and potentially aft ages associated with the along strike displacement profile can occur over relatively short horizontal distances of a few kilometres this is fundamentally important in the assessment of the vertical cooling history of footwalls in extensional systems temporal differences in the rate of tectonically driven exhumation at a given location along fault strike may be of greater importance in controlling changes in rates of vertical exhumation than commonly invoked climatic fluctuations

tracing the exhumation history of the rwenzori mountains albertine rift uganda using low temperature thermochronology the rwenzori mtns form a striking feature within the albertine rift of the east african rift system they are made up of a dissected precambrian metamorphic basement block reaching heights of more

sand supply to the lake albert basin uganda during the miocene pliocene a multiproxy provenance approach a multi proxy provenance approach heavy mineral analysis u pb zircon geochronology and pb isotopic analysis of k feldspar has constrained sediment supply within the upper nile drainage system in the miocene pliocene provenance data from sandstones were obtained from three exploration wells two situated on the north eastern margin and one on the eastern flank of the lake albert basin nw uganda data suggest that high to low grade metamorphic rocks and granitoids have variably supplied the heavy mineral assemblages around the lake albert basin during the miocene pliocene with contributions from the isotopically heterogeneous archean cratons including the local ugandan craton tanzanian and congo cratons and the pan african rocks the mozambique belt with possible contributions from the neoproterozoic and paleoproterozoic rocks these data also highlight clear differences between supply to the eastern basin margin compared with the northeast which is reconcilable with current models for miocene pliocene drainage in the region supply to northeastern lake albert during the miocene pliocene appears to have been through a proto albert nile draining from ne to sw and from a proto victoria nile or similarly oriented palaeo river systems draining from the east in contrast the eastern flanks of the basin were likely supplied via the palaeo nkusi river tapping local hinterland sources and more distal basement to the far east mozambique belt this study highlights the importance of utilizing a multi proxy approach in provenance analysis as no one signal is capable of distinguishing the different source lands and constraining the evolving drainage patterns

erosion of the rwenzori mountains east african rift from in situ produced cosmogenic10be 1 high relief and steep topography are thought to result in high erosion rates in the rwenzori mountains of the albert rift east africa where more than 3 km of relief have formed during uplift of the rwenzori fault block overall low denudation rates prevail we measured in situ derived cosmogenic denudation rates of 28 2 to 131 mm kyr in mountainous catchments and rates of 7 8 to 17 7 mm kyr on the adjacent low relief east african plateau these rates are roughly an order of magnitude lower than in other settings of similar relief we present an extensive geomorphological analysis and find that denudation rates are positively correlated with relief hillslope gradient and channel steepness indicating that river incision controls erosional processes in most upper headwater reaches above quaternary ela levels 4500 m a s l glacial imprinting inherited from several older and recent minor glaciation stages prevails in regions below 4500 m a s l however mild climatic conditions impede frost shattering favor dense vegetation and minimize bare rock areas and associated mass wasting we conclude that erosion of the rwenzori mountains is significantly slower than corresponding rates in other mountains of high relief due to a combination of factors extremely dense mountain cloud forest vegetation high rock strength of gneiss and amphibolite lithologies and low internal fracturing due to the extensional tectonic setting this specific combination unique to this extensional tropical setting leads to unexpected low erosion rates that cannot outpace post pliocene ongoing rock uplift of the rwenzori fault block

post palaeozoic evolution of weathered landsurfaces in uganda by tectonically controlled deep weathering and stripping a model for the evolution of weathered landsurfaces in uganda is developed using available geotectonic climatic sedimentological and chronological data the model demonstrates the pivotal role of tectonic uplift in inducing cycles of stripping and tectonic quiescence for cycles of deep weathering it is able to account for the development of key landforms such as inselbergs and duricrust capped plateaux which previous hypotheses of landscape evolution that are based on climatic or eustatic controls are unable to explain development of the ugandan landscape is traced back to the permian following late palaeozoic glaciation a trend towards warmer and more humid climates through the mesozoic enabled deep weathering of the jurassic mid cretaceous surface in uganda during a period of prolonged tectonic quiescence uplift associated with the opening south atlantic ocean terminated this cycle and instigated a cycle of stripping between the mid cretaceous and early miocene deep weathering on the succeeding miocene to recent african surface has occurred from miocene to present but has been interrupted in the areas adjacent to the western rift where development of a new drainage base level has prompted cycles of stripping in the miocene and pleistocene

apatite fission track analysis apatite fission track aft analysis has been widely used during the past 30 years to constrain the low temperature thermal histories of many igneous metamorphic and sedimentary rocks in a wide range of geological settings applicable geological settings include orogenic belts rifted margins faults sedimentary basins cratons and mineral deposits the types of geologic problems that can be addressed include the timing and rates of tectonic events sedimentary basin evolution the timing of hydrocarbon generation and ore mineralization the absolute age of volcanic deposits the effects of major climatic changes on the near surface geothermal gradient and long term landscape evolution early work by naeser 1967 and wagner 1968 1969 first established the basic procedures that enabled the fission track dating method to be applied routinely to these geologic problems fleischer et al 1975 summarized the early studies of the broader discipline of nuclear track detection in different solid state materials more recent comprehensive overviews of fission track applications have been provided by naeser and mcculloh 1989 wagner and van den haute 1992 gallagher et al 1998 van den haute and de corte 1998 dumitru 2000 and gleadow et al 2002 etched natural fission tracks in several apatite grains are shown in figure 1 successful aft analysis is limited by the following 1 the availability of apatite from which useful aft data can be obtained often due to a lack of apatite of sufficient grain size and quality within available rock types or alternatively the lack of available rock samples due to minimal outcrop exposure or other reasons 2 economic considerations in terms of the time and money required to obtain sufficient aft data 3 the inherent limitations of aft data to resolve geological thermal history information often related to limited numbers of accumulated spontaneous fission tracks due to

influence of wildfires on apatite and zircon u th he ages low closure temperatures of the apatite and zircon u th he thermochronometers allow valuable constraints on timing and rates of bedrock exhumation through shallow crustal depths but raise the possibility that shallow level processes other than exhumation related cooling may also influence he ages a simple he diffusion model predicts that wildfires can completely or partially reset apatite he ages as much as 3 cm below rock surfaces and partially reset zircon he ages in the outermost 1 cm measured he ages in bedrock and sediments from the washington cascades that were exposed to extensive wildfires in 2001 show strong agreement with these model predictions apatite he ages decrease from a regionally consistent age of 19 5 1 2 ma at a distance 3 cm from the rock surface to as low as 1 9 ma in the outermost 1 cm whereas zircon he ages decrease from 65 to 55 ma over the same distance thin 3 cm flakes shed from a nearby boulder during or after the most recent fire have apatite he ages ranging from 9 7 0 6 to 17 2 1 0 ma the partial resetting profiles are best explained by model thermal histories involving at least one short duration 5 10 min high temperature 575 650 c event and at least one longer 30 40 min lower temperature 350 450 c event age depth profile data may be useful in determining wildfire intensities or locations and also suggest that he ages of detrital apatites from some environments may be subject to bias from the thermal effects of wildfires

evidence for high frequency cyclic fault activity from high resolution seismic reflection survey rukwa rift tanzania a high resolution seismic survey of the rukwa rift has imaged the upper 300 m of the pliocene holocene section the sediments are characterized by aggradational deposition in shallow lacustrine conditions that episodically dried out overall the reflection package expands northeastwards towards the lupa fault half graben bounding fault isopach maps for nine intervals 6 m to 65 m thick show four periods of marked expansion of the section towards the lupa fault interpreted to represent times of fault activity or infilling of fault created topography during the early stage of fault quiescence there are three periods of little or much reduced expansion interpreted to represent infilling of the accommodation space between the lake floor and the lake surface during periods of fault inactivity cyclicity of such high frequency has not been widely reported for faults which penetrate the crust the periods of activity show remarkably similar displacement patterns with no lateral migration of the main depocentre implying that seismic and aseismic slip has persistently clustered in the same parts of the fault identification of high frequency fault cyclicity has implications for sequence stratigraphy in tectonically active areas since there is a tendency to assume faults do not have cyclic effects on the scale of tens of thousands of years

an active fracture model for unsaturated flow and transport in fractured rocks the unsaturated zone at yucca mountain a potential repository site of high level nuclear waste is a complex hydrologic system in which a variety of important flow and transport processes is involved to quantify these processes as accurately as possible is a theoretically challenging and practically important issue in this study we propose a new formulation for modeling flow and transport in unsaturated fractured rocks the formulation is mainly based on a hypothesis that only a portion of connected fractures are active in conducting water analysis of the relevant data with the new formulation suggests that about 18 27 of the connected fractures in the topopah spring welded tsw unit the potential repository unit of yucca mountain are active under ambient conditions the relatively high percentage of active fractures is consistent with field observations from a variety of sources sensitivity analyses are performed to investigate effects of the activity of connected fractures on flow and transport behavior in unsaturated rocks

holocene explosive eruptions in the rungwe volcanic province tanzania abstract the holocene explosive eruptive record of rungwe and ngozi volcanoes of the rungwe volcanic province in tanzania was reconstructed based on detailed stratigraphic field evidence combined with whole rock major and trace element analyses of tephra samples this reconstruction is supported by 25 new radiocarbon dates on palaeosols that provide additional constraints on the holocene tephro chronostratigraphy we show evidence of two catastrophic ngozi eruptions and five rungwe pumice fallout deposits and also identify several more intercalated poorly preserved pumice and ash deposits the ngozi eruptions probably played a role in shaping the present day caldera the rungwe record includes a ca 2 2 km3 deposit of a plinian style eruption dated at ca 4 ka a sub plinian one at ca 2 ka and at least three additional smaller scale fallout deposits the rungwe explosive eruptive record shows that the volcano has been frequently active in its late holocene past this study highlights the need for volcanic monitoring in the region and presents herewith the first basis of future volcanic hazard assessment

the relationship between track shortening and fission track age reduction in apatite combined influences of inherent instability annealing anisotropy length bias and system calibration abstract the relationship between reduced track density 0 and reduced mean confined track length l l 0 during annealing has been studied for both spontaneous and induced tracks in four apatite samples with different chemical composition compositional range and origin and with spontaneous tracks giving mean confined track lengths varying from 14 5 m down to 12 3 m in all four apatites induced tracks show a relationship between l l 0 and 0 composed of two regions at higher values the two appear to be equal while at lower values l l 0 is greater than 0 this can be understood qualitatively in terms of the anisotropy of annealing and the bias against revelation of short tracks data from three apatites of uniform composition show near identical relationships in a sample showing a range of apatite compositions the point of divergence from the 1 1 relationship is somewhat higher than in the uniform composition apatites because individual grains anneal at different rates and the presence of highly annealed grains affects the track density more strongly than the mean track length spontaneous tracks show similar relationships to those observed for induced tracks in each sample when the data are renormalised to allow for the natural annealing present in the spontaneous tracks previous studies which led to the idea of length reduction without density reduction for spontaneous tracks did not renormalise density data as well as length data thereby distorting the apparent relationship data for the multi compositional apatite sample where annealing has taken place in geological conditions also agrees with the laboratory annealing data for both spontaneous and induced tracks this consistency suggests that a single process is responsible in all three cases implying that a laboratory based description of annealing can be applied to geological conditions with confidence recognition that spontaneous tracks in apatite are commonly annealed suggests that in general the fission track age should be interpreted as a reflection of both the time over which tracks have been retained and the distribution of etchable track lengths since the latter is a manifestation of the thermal history the fission track age should be interpreted in similar terms

using stained glass windows to understand the durability of toxic waste matrices abstract using stained glasses sampled from french and german cathedrals and from different archaeological sites this work presents an estimation of the effect of weathering conditions and glass composition on glass dissolution due to accurate dating we also show that stained glass windows allow the determination of the average dissolution kinetics of many toxic elements contained in the glass including transition and heavy metals actinides lanthanides therefore a significant advantage of using stained glass windows over other natural glasses is that they permit the direct study of non stoichiometric elemental dissolution rates under natural conditions of alteration averaged over more than a thousand years and hence to constrain the long term behaviour of vitrified waste matrices

3d seismic analysis of the ak fault orange basin south africa implications for hydrocarbon leakage and offshore neotectonics abstract a high resolution 3d reflection seismic dataset was used to achieve a detailed description of the half graben bounded ak fault in the ibhubesi gas field of the offshore orange basin seismic analysis of the fault involved the combination of both the conventional seismic interpretation and seismic attributes the main objective of the study was to provide insights into hydrocarbon leakage and neotectonic activity along the west coast shelf of south africa the ak fault is among the several half graben bounded faults that formed in the late jurassic to early cretaceous during the separation of the south american and the african continents the ak fault consists of five seismically detectable segments whose evolution reflects characteristics of both the coherent and the isolated end members model of segmented fault origin diapiric structures that are direct indicators of vertical fluid movement were also observed along the ak fault system providing evidence of significant roles that the fault plays in promoting the vertical migration of hydrocarbon in the orange basin recent reactivation of the fault has also occurred along an oblique extensional regime that is dominated by normal dip slip faulting with minor shear components along the tips

late cretaceous coeval multi directional extension in south zealandia implications for eastern gondwana breakup abstract gondwana continental crust in south zealandia was deformed by cretaceous rift faults with multiple orientations our interpretation of 2d and 3d seismic reflection lines tied to wells in the canterbury basin of south zealandia shows three coeval sets of faults with trends of ne sw e w and nw se geometric and displacement analyses of segmented faults from each set suggests that they mainly accommodated normal dip slip producing multi directional stretching growth strata indicate that the three fault sets were active synchronously over 20 myr from 105 to 85 ma the fault sets are present throughout zealandia and roughly parallel to spreading centres produced during gondwana breakup primarily defining the present day margins of zealandia faults trending ne sw and e w are parallel to the mid ocean ridge separating south zealandia and western antarctica 800 1000 km from the canterbury basin while nw se rift faults are sub parallel to the spreading centre between australia and north zealandia sub parallelism between spreading centres and rift faulting is consistent with a model in which the multiple directions of extension produced mild 500 km from these centres ceased or continued at much diminished rates

episodic denudation in east africa a legacy of intracontinental tectonism apatite fission track data from samples collected over an altitude range of 2 5 km in western and central kenya reveal three elevation intervals where the apparent apatite age varies linearly with elevation the mean ages of these intervals are 64 1 ma 112 2 ma and 180 6 ma and they result from cooling of rocks below 110 10 c by erosional denudation the form of the apatite age elevation profile and forward modeling of the fission track length distributions suggests that discrete periods of exhumation commenced at 70 75 ma 120 130 ma and 220 ma these times correspond with periods of intracontinental deformation of the african lithosphere including the separation of madagascar from east africa opening of the south atlantic ocean and changes in the geometry of plate spreading in the south atlantic

paleogene initiation of the western branch of the east african rift the uplift history of the rwenzori mountains western uganda abstract the two branches of the east african rift system ears are believed to have initiated diachronously however a growing body of work continues to suggest the onset of rifting in the western branch occurred in the paleogene coeval to the eastern branch due to a lack of pre miocene stratigraphy attempts to resolve the geological history of the western branch must study the uplift and erosional histories of the modern rift topography in this study the rock uplift history of the rwenzori mountains western uganda is resolved to better our understanding of the tectonic history of the western branch of the ear through the application of low temperature thermochronology mapping and the modelling of river profiles we show that rock uplift of the rwenzori dates back to the oligocene with thermal history models suggesting uplift induced exhumation may date back as far as the eocene this provides tangible evidence that extension began in the region in the paleogene coeval with the eastern branch and not the late neogene these results have broad implications for the tectonic evolution of the entire east african rift system and suggest our current understanding of the region s rift history remains incomplete

dip analysis as a tool for estimating regional kinematics in extensional terranes abstract in basin analyses that rely almost solely on seismic reflection data to configure the underlying structure it has become common practice to use extensional fault geometries to infer kinematic histories this procedure relies on the existence of fault geometries predicted by ideal orthogonal extension and assumes the structure is well constrained in areas where data are sparse or of equivocal quality this often leads to circular analysis that is ideal orthogonal extension is assumed to have been operative then under constrained fault geometries are mapped according to model predictions fault geometries so derived are subsequently used as evidence that orthogonal extension was operative and that the direction of extension is determinable we develop an alternative tool for inferring kinematics which is independent of fault geometries the method relies on the empirical evidence that the dip of the hangingwall s is systematic and predictable in extensional systems even when fault geometries are non ideal although not without significant limitations dip analysis can provide additional constraints for structural and kinematic interpretations especially where seismic reflection data are the main source of information and dip domains define discrete crustal blocks

influence of crystal size on apatite u th he thermochronology an example from the bighorn mountains wyoming near surface tectonic and geomorphic processes involve cooling of rocks through low temperatures 50 200 c because rates of helium diffusion in apatite titanite and zircon are sensitive to temperature variations in this range uranium thorium helium thermochronometry u th he dating is well suited to establishing the timing and rates of these processes in the geologic record however because fractional loss of he is controlled by crystal size such that larger crystals retain a larger fraction of radiogenic he u th he ages must vary not only with thermal history but also with crystal size here we present crystal size correlated he ages from co existing apatites from the bighorn mountains wyoming that range from 100 to 350 ma these correlations are a sensitive indicator of the rock s thermal history in a temperature range below the system s nominal closure temperature t c 70 c for apatite and are consistent with a thermal history involving residence in the upper 2 3 km of crust since the precambrian with maximum temperatures of 65 80 c just prior to laramide orogenic exhumation the influence of crystal size on he ages will be most apparent in rocks where temperatures have been in the range of partial he retention for long periods of time 30 70 c for 10 7 years in such cases accurate interpretation of u th he ages must incorporate the effect of crystal size and this method may provide insights to thermal histories of rocks in previously inaccessible low temperature ranges

apatite u th he thermochronometry using a radiation damage accumulation and annealing model helium diffusion from apatite is a sensitive function of the volume fraction of radiation damage to the crystal a quantity that varies over the lifetime of the apatite using recently published laboratory data we develop and investigate a new kinetic model the radiation damage accumulation and annealing model rdaam that adopts the effective fission track density as a proxy for accumulated radiation damage this proxy incorporates creation of crystal damage proportional to production from u and th decay and the elimination of that damage governed by the kinetics of fission track annealing the rdaam is a version of the helium trapping model hetm shuster d l flowers r m and farley k a 2006 the influence of natural radiation damage on helium diffusion kinetics in apatite earth planet sci lett 249 148 161 calibrated by helium diffusion data in natural and partially annealed apatites the chief limitation of the hetm now addressed by rdaam is its use of he concentration as the radiation damage proxy for circumstances in which radiation damage and he are not accumulated and lost proportionately from the crystal by incorporating the rdaam into the hefty computer program we explore its implications for apatite u th he thermochronometry we show how u th he dates predicted from the model are sensitive to both effective u concentration eu and details of the temperature history the rdaam predicts an effective he closure temperature of 62 c for a 28 ppm eu apatite of 60 m radius that experienced a 10 c ma monotonic cooling rate this is 8 c lower than the 70 c effective closure temperature predicted using commonly assumed durango diffusion kinetics use of the rdaam is most important for accurate interpretation of u th he data for apatite suites that experienced moderate to slow monotonic cooling 1 0 1 c ma prolonged residence in the helium partial retention zone or a duration at temperatures appropriate for radiation damage accumulation followed by reheating and partial helium loss under common circumstances the rdaam predicts u th he dates that are older sometimes much older than corresponding fission track dates nonlinear positive correlations between apatite u th he date and eu in apatites subjected to the same temperature history are a diagnostic signature of the rdaam for many but not all thermal histories observed date eu correlations in four different localities can be explained with the rdaam using geologically reasonable thermal histories consistent with independent fission track datasets the existence of date eu correlations not only supports a radiation damage based kinetic model but can significantly limit the range of acceptable time temperature paths that account for the data in contrast these datasets are inexplicable using the durango diffusion model the rdaam helps reconcile enigmatic data in which apatite u th he dates are older than expected using the durango model when compared with thermal histories based on apatite fission track data or other geological constraints it also has the potential to explain at least some cases in which u th he dates are actually older than the corresponding fission track dates

low long term erosion rates and extreme continental stability documented by ancient u th he dates zircon and apatite crystals from the western canadian shield yield u th he dates that are the oldest yet reported for terrestrial rocks zircon dates from 1 73 to 1 58 ga are consistent with independent geological and thermochronological constraints and indicate that the rocks were at temperatures 180 c and crustal depths 7 10 km since ca 1 7 ga apatite dates from 0 95 to 0 55 ga suggest residence of rocks at temperatures 40 50 c and crustal depths 1 5 2 km for 1 0 0 6 b y when interpreted using conventionally accepted apatite he diffusion kinetics and considering the proposed effect of radiation damage on apatite he retentivity our analysis implies long term integrated unroofing rates of 2 5 m yr since ca 1 7 ga these rates are significantly lower than the long term rates suggested by previous thermochronological data sets in continental interiors but are within the range of short term erosional estimates the results are consistent with the extreme stability of this region since the proterozoic

large scale geomorphology classical concepts reconciled and integrated with contemporary ideas via a surface processes model linear systems analysis is used to investigate the response of a surface processes model spm to tectonic forcing the spm calculates subcontinental scale denudational landscape evolution on geological timescales 1 to hundreds of million years as the result of simultaneous hillslope transport modeled by diffusion and fluvial transport modeled by advection and reaction the tectonically forced spm accommodates the large scale behavior envisaged in classical and contemporary conceptual geomorphic models and provides a framework for their integration and unification the following three model scales are considered micro meso and macroscale the concepts of dynamic equilibrium and grade are quantified at the microscale for segments of uniform gradient subject to tectonic uplift at the larger meso and macroscales which represent individual interfluves and landscapes including a number of drainage basins respectively the system response to tectonic forcing is linear for uplift geometries that are symmetric with respect to baselevel and which impose a fully integrated drainage to baselevel for these linear models the response time and the transfer function as a function of scale characterize the model behavior numerical experiments show that the styles of landscape evolution depend critically on the timescales of the tectonic processes in relation to the response time of the landscape when tectonic timescales are much longer than the landscape response time the resulting dynamic equilibrium landscapes correspond to those envisaged by hack 1960 when tectonic timescales are of the same order as the landscape response time and when tectonic variations take the form of pulses much shorter than the response time evolving landscapes conform to the penck type 1972 and to the davis 1889 1899 and king 1953 1962 type frameworks respectively the behavior of the spm highlights the importance of phase shifts or delays of the landform response and sediment yield in relation to the tectonic forcing finally nonlinear behavior resulting from more general uplift geometries is discussed a number of model experiments illustrate the importance of fundamental form which is an expression of the conformity of antecedent topography with the current tectonic regime lack of conformity leads to models that exhibit internal thresholds and a complex response

tectonic inheritance and continental rift architecture numerical and analogue models of the east african rift system 1 the western branch of the east african rift is composed of an arcuate succession of elongate asymmetric basins which differ in terms of interaction geometry fault architecture and kinematics and patterns of uplift subsidence and erosion sedimentation the basins are located within proterozoic mobile belts at the edge of the strong tanzanian craton surface geology suggests that the geometry of these weak zones is an important parameter in controlling rift development and architecture although other processes have been proposed in this study we use lithosphere scale numerical models and crustal scale analogue experiments to shed light on the relations between preexisting structures and rift architecture results illustrate that on a regional scale rift localization within the mobile belts at the curved craton s western border results in an arcuate rift system which implies that under a constant extensional stress field part of the western branch experienced orthogonal extension and part oblique extension largest depocenters are predicted to form mostly orthogonal to the extension direction and smaller depocenters will form along the oblique parts of the rift the varying extension direction along the rift zone furthermore results in lengthwise varying rift asymmetry segmentation characteristics and border fault architecture trend length and kinematics analogue models predict that discrete upper crustal fabrics may influence the location of accommodation zones and control the architecture of extension related faults at a local scale models support that fabric reactivation is responsible for the oblique slip kinematics on faults and for the development of z shaped or arcuate normal faults typically documented in nature

horst faulting in continental rifts abstract horsts in continental rifts are classified into three types on the basis of their structural characteristics type 1 large horsts bounded by near vertical bifurcating faults at rift termini or structural nodes a subset of this type includes horsts comprising an exceptionally uplifted rift shoulder type 2 generally smaller than type 1 type 2 horsts are commonly associated with graben formed in broad zones of crustal extension and thinning in particular they are a feature of both volcanic and sedimentary basins where they are bounded by listric faults type 3 small horsts occurring close to the margins of rift zones formed in a transtensional regime and bounded by near vertical faults the genesis of type 2 and some type 3 horsts is explained by the vening meinesz isostatic model but type 1 horsts require an additional hydraulic force acting at the base of thick fractured crust small scale convection in the asthenosphere could provide this force

active transsection faults in rift transfer zones evidence for complex stress fields and implications for crustal fragmentation processes in the western branch of the east african rift new structural and seismologic evidence from the rwenzori mountains uganda indicate that continental rifts can capture and rotate fragments of the lithosphere while rift segments interact in a manner analogous to the interaction of small scale fractures the rwenzori mountains are a basement block within the western branch of the east african rift system that is located at the intersection of two rift segments and is apparently rotating clockwise structural data and new seismological data from earthquake epicentres indicate a large scale 20 km long transsection fault is currently detaching the rwenzori micro plate on its northern margin from the larger victoria plate tanzania craton whereas it is already fully detached in the south we propose that this fault develops due to the rotation of the rwenzori block in a numerical model we show how rift segment interaction block rotation and the development of transsection faults faults that cut through the rwenzori mountains evolve through time the model suggests that uplift of the rwenzori block can only take place after the rift has opened significantly and rotation leads to the development of transsection faults that connect two rift segments so that the block is captured within the rifts our numerical model suggests that the majority of the uplift has taken place within the last 8 ma

alteration rate of medieval potash lime silicate glass as a function of ph and temperature a low ph dependent dissolution abstract potash lime silicate glasses were extensively used during the middle ages as components of stained glass windows their external faces have been subjected to alteration in the atmospheric medium for centuries leading to the development of thick alteration layers on the surfaces this alteration is mainly caused by water in liquid rainfall or gaseous vapor form in order to implement geochemical models simulating the long term behavior of these glasses by coupling the different atmospheric factors it is necessary to study mechanisms and kinetics of alteration induced by these different hygroscopic situations rainfall and relative humidity the aim of this study is here to evaluate the impact of liquid water on stained glass windows and especially to determine alteration rate laws of dissolution and interdiffusion as a function of ph and temperature for that a medieval model glass was altered in aqueous solution in batch experiments at different ph values from 2 to 11 temperatures from 5 to 70 c and saturation states of the solution from the elemental concentrations in solution the release rate of elements was calculated and the ph and t dependencies were determined the dissolution and interdiffusion rate laws can now be used to simulate the alteration of a medieval glass submitted to a rainfall event moreover this study highlights that the si k ca medieval glass presents a very low ph dependency at alkaline ph which can be explained by its composition

variability of apatite fission track annealing kinetics ii crystallographic orientation effects a method is presented that permits the length of any horizontal confined fission track inclined at a specified angle to the crystallographic c axis in apatite to be converted to an equivalent track length parallel to the crystallographic c axis the model is based on the results of annealing experiments for six selected apatites five calcian fluorapatites and durango apatite representing a subset of the 15 total apatite specimens studied an iterative process of calculation is required to project fission track lengths onto the c axis and computer source code implementing the solution to this problem is presented this method of projecting apatite fission track lengths onto the crystallographic c axis is shown to remove effectively fission track length variation within single fission track populations due to anisotropic track length reduction for all 15 apatites studied in addition a model is developed that offers predictions that closely reproduce published experimental data concerning the relationship between fission track density etched fission tracks per unit area of apatite surface and the arithmetic mean fission track length finally it is shown that natural fission track populations exhibit fission track length anisotropy similar to that of fission track populations created and annealed in the laboratory this observation implies that the same process by which apatite fission tracks anneal in the laboratory is responsible for annealing of apatite fission tracks in the geological environment

the late quaternary glaciation of africa a regional synthesis abstract there are numerous mountain ranges in africa which carry evidence of quaternary glaciation though few have glaciers now and these are rapidly disappearing the best studied sites are in east africa and ethiopia in east africa the three highest mountains kilimanjaro kenya and the rwenzori each have evidence of 3 5 major glaciations dating back to an estimated 400 000 bp one of which is dated to the last glacial maximum lgm by radiocarbon or cosmogenic radionucleide crn methods two other mountains have similar large moraine systems inferred to date to the lgm in ethiopia three mountains have clear evidence of glaciation and on two of them radiocarbon dates give minimum ages which suggest that these are lgm features in north africa the atlas and other groups of mountains have evidence of glaciation but in the absence of any dating it is unclear whether these sites were glaciated at the lgm although geomorphic features resembling moraines and protalus ramparts have been described from the drakensberg mountains south africa it is still unclear whether these features indicate glaciation and can be attributed to the lgm thus although circumstances for fieldwork on african mountains are very difficult in many cases investigations for more than a century provide an extensive picture of quaternary glaciation in some cases there has been an increasingly detailed mapping of field evidence and the first crn dates were carried out recently thus improving the global picture considerably particularly in east africa and ethiopia there is sufficiently reliable field information to make useful estimates of equilibrium line altitude changes at the provisionally identified lgm but this age needs confirmation by further crn dating by making some hypothetical assumptions inferences can be drawn that indicate a temperature lowering of 5 6 c during the lgm elsewhere in africa further field surveys are required

a finite difference technique to incorporate spatial variations in rigidity and planar faults into 3 d models for lithospheric flexure summary we present a finite difference formulation for 3 d elastic flexure of the lithosphere which is solved by a direct matrix method to incorporate the effect of spatial variations in rigidity additional terms for the bi harmonic 3 d flexure equation have been derived from a variational displacement formulation as used in finite element methods additionally planar faults are treated as discontinuities these are implemented by an additional degree of freedom for fault heave and a coupled continuum equation for zero differential tilting across the fault the 3 d finite difference results have been tested for line loads point loads and disc loads by analytical solutions and for spatial variation in effective elastic thickness eet by 2 d finite difference solutions fault related flexure patterns are compared to the 2 d analytical broken plate model developed by vening meinesz 1950 we subsequently apply the 3 d fault model to investigate fault controlled 3 d basement geometries in lake tanganyika east africa we show that our model is capable of predicting 3 d basement geometries characteristically observed in rifted basins the modelling results indicate that fault controlled upper crustal flexure patterns are associated with low values for eet a comparison with regional scale model studies showing a superposition of high eet flexure effects supports a multilayered rheological control on continental rifting

shaping the rwenzoris balancing uplift erosion and glaciation the rwenzori mountains in africa represent an extremely uplifted basement fault block at the eastern edge of the western branch of the east african rift system a large scale rift system controlled by extensional stresses the rugged alpine topography reaches an altitude of up to 5109 m and the highest parts are ice covered glacial landforms and moraines proof repeated more extensive glaciations during the last glacial cycles in order to elucidate magnitudes and the varying role of erosional processes in shaping the relief of the rwenzori mountains over the past 2 mill years we performed numerical simulations with the landscape evolution programme ultima thule it is controlled by a climate driver with temperature as a master variable as well as changing precipitation and evapotranspiration over time the morphological processes considered are fluvial erosion hillslope diffusion and glacial abrasion and the latter controlled by the simulated glaciation of the landscape we provide three sets of model runs the first one starting from the present day topography and running for approx 800 ka the second one extending the modelling period to 2 ma and the third one starting from a peneplain and evolving for 2 ma our results provide constraints on the temperature history of the rwenzori mountains the interplay of morphological degradation and tectonic uplift and a time frame for the formation of the mountain chain from a peneplain to the present relief the modelled landscape evolves from a peneplain 2 ma ago to a rwenzori type mountain range when the fairly strong average rock uplift of 1 2 mm year 1 is compensated by a strong fluvial erosion component the rock uplift rate is needed to obtain elevations above the equilibrium line altitude around 500 ka bp and results in surface uplift over time around that time a periodic ice cap appears in the models and glacial abrasion then limits the height of the rwenzori mountains to its present elevation

rift nucleation rift propagation and the creation of basement micro plates within active rifts abstract in this contribution we study the dynamics of rift nucleation and the interaction of propagating rift segments we use the east african rift system ears as a case study with special emphasis on the albertine rift system situated within the western branch of the ears with the 5000 m high rwenzori horst a basement block that was captured by two rift segments fieldwork in the rwenzori mountains shows that the range is not only affected by range parallel faults but that major fault sets cut across the rwenzoris in order to understand these complex fault patterns and the dynamics of the process that leads to the capturing of basement blocks within the extending rift system we developed a two dimensional visco elasto plastic spring model the model shows that propagating rift faults commonly capture micro plates with shapes similar to the rwenzori mountains these micro plates rotate during successive rift opening in a clockwise or anti clockwise sense depending on the direction of the rift segment overstep a left lateral overstep as is the case for the albertine rift system produces a clockwise rotation of the associated micro plate indicating that the rwenzori mountains rotate clockwise during the rotation of the micro plate the local stress field at the rift segment tips is perturbed so that the rifts turn 70 to 90 and propagate towards each other to capture the plates completely this perturbed stress field produces an extensional fault set that is oriented almost parallel to the far field extension direction and explains the range cutting faults that we find in the rwenzori mountains the large scale geometry of the rift system seems to be strongly controlled by rigid cratons in the lithosphere leading to a curvature of the western branch of the ears and a sinistral wrench component in the lake albert rift north of the rwenzori mountains capturing of micro plates of various sizes within the rift system is a common phenomenon and leads to strong perturbations of the local stress field during micro plate rotation the resulting structures are normal and strike slip faults that cut through the micro plates and local reverse faults

implications of structural inheritance in oblique rift zones for basin compartmentalization nkhata basin malawi rift ears abstract the cenozoic east african rift system ears is an exceptional example of active continental extension providing opportunities for furthering our understanding of hydrocarbon plays within rifts it is divided into structurally distinct western and eastern branches the western branch comprises deep rift basins separated by transfer zones commonly localised onto pre existing structures offering good regional scale hydrocarbon traps at a basin scale local discrete inherited structures might also play an important role on fault localisation and hydrocarbon distribution here we consider the evolution of the central basin of the malawi rift in particular the influence of pre existing structural fabrics integrating basin scale multichannel 2d and high resolution seismic datasets we constrain the border mlowe nkhata fault system mnf to the west of the basin and smaller mbamba fault mf to the east and document their evolution intra basin structures define a series of horsts which initiated as convergent transfers along the basin axis the horsts are offset along a ne sw striking transfer fault parallel to and along strike of the onshore karoo permo triassic ruhuhu graben discrete pre existing structures probably determined its location and oriented obliquely to the extension orientation it accommodated predominantly strike slip deformation with more slowly accrued dip slip to the north of this transfer fault the overall basin architecture is asymmetric thickening to the west throughout while to the south an initially symmetric graben architecture became increasingly asymmetric in sediment distribution as strain localised onto the western mnf the presence of the axial horst increasingly focussed sediment supply to the west as the transfer fault increased its displacement so this axial supply was interrupted effectively starving the south east while ponding sediments between the western horst margin and the transfer fault this asymmetric bathymetry and partitioned sedimentation continues to the present day overprinting the early basin symmetry and configuration sediments deposited earlier become increasingly dissected and fault juxtapositions changed at a small 10 100 m scale the observed influence of basin scale transfer faults on sediment dispersal and fault compartmentalization due to pre existing structures oblique to the extension orientation is relevant to analogous exploration settings

a study of thermally induced convection near a high level nuclear waste repository in partially saturated fractured tuff we present simple estimates for velocities of buoyancy driven flow of liquid and gas phases near a hypothetical high level nuclear waste repository in partially saturated fractured tuff these estimates indicate that gas phase convection could take place with appreciable velocity of the order of 20 m year while liquid convection is expected to be very slow of the order of 0 1 mm year detailed numerical simulations using an effective continuum approximation to represent fracture effects confirm these estimates and demonstrate a sensitivity of gas convection to the strength of binary diffusion

geomorphologic proxies for bedrock rivers a case study from the rwenzori mountains east african rift system geomorphic proxies yield useful insights into understanding long term endogenic and exogenic response to erosion and or rock uplift rates by evaluating areal proxies including asymmetry factor af mountain front sinuosity smf hypsometric integral hi geophysical relief and shape factor shp and linear proxies including normalized steepness index ksn length gradient index slk and chi gradient m the erosion and or rock uplift rates can be quantified we carried out morphotectonic analysis in the rwenzori mountains which represents an anomalously uplifted precambrian horst within the western branch of the east african rift system ears this study aims to 1 evaluate the relationship between geomorphic proxies and drainage basin s maturity 2 evaluate the usefulness of geomorphic proxies as recorders of erosion and or rock uplift rates 3 evaluate the sensitivity of each geomorphic proxy to the drainage basin size and geometry stream order glaciers extent and local structures 4 explore internal correlation within the geomorphic proxies and 5 contribute to the understanding of morphotectonic evolution of the rwenzori mountains for this we computed the stream s good of fitness r2 an indicator of the drainage basin s maturity and geomorphic proxies for the drainage basins and their streams in the rwenzori mountains from the advanced spaceborne thermal emission and reflection radiometer aster digital elevation model dem subsequently we correlated the areal geomorphic proxies with each other and with r2 also we correlated the linear geomorphic proxies with each other and with published erosion rates obtained from cosmogenic 10be analysis our results show that the areal geomorphic proxies af smf hi relief and shp considering the drainage basin size and geometry stream order glacier extent and local structures can be applied to locally evaluate the maturity of the drainage basin we also found that the linear geomorphic proxies ksn slk and m have strong positive correlation with the erosion rates they are effective in detecting fault activities and some of them ksn and m are highly correlated with each other additionally our geomorphic proxy results suggest that the north and south sectors of the rwenzori mountains are in relative tectonic quiescence but that the central sector is experiencing higher levels of erosion and rock uplift

african stress pattern from formal inversion of focal mechanism data abstract the kinematic models and the associated orientation of extensional stress of the east african rift system have been subjected to much debate since a long time in the past decades the proposed models relied on the interpretation of the overall rift geometry geological fault slip data and the few focal mechanisms available these models generally suffer of a poor time control and an underestimation of the possible changes in the stress field and geodynamic regime with time and space in the recent years there has been a significant increase in the number of focal mechanisms available for the entire rift system and it is now possible to estimate the present day stress field in relative detail based on seismotectonic data alone we compile 347 focal mechanism data from the global harvard cmt catalogue and various other sources and grouped 332 of them in 24 distinct regions boxes on the basis of their geographical proximity kinematic homogeneity and tectonic setting for each box and for the same data set reduced stress tensors have been obtained by formal stress inversion using both the tensor program delvaux sperner 2003 and the slick method michael 1984 1987 both inversion methods show in comparable results in terms of horizontal stress axes orientations and tectonic stress regimes which stem for the robustness of the approach the obtained stress pattern reflects a complex interaction between 1st order effects as different driving forces including plate boundary forces and 2nd and 3rd order effects as gravitational potential of topography intra lithospheric processes and the influence of structural heterogeneities of the rift structures the evidence present in the stress orientations of the 2nd and 3rd order stress pattern as the variations in the horizontal stress axes along the axis of the rift are of particular interest as they were not yet captured or shown in earlier numerical models of the stress field additional sources of tectonic forces in supplement to the gravitational potential energy forces as considered in the earlier models are necessary to explain the observed patterns

reproducibility of apatite fission track length data and thermal history reconstruction abstract the ability to derive detailed thermal history information from apatite fission track analysis is predicated on the reliability of track length measurements however insufficient attention has been given to whether and how these measurements should be standardized in conjunction with a fission track workshop we conducted an experiment in which 11 volunteers measured 50 track lengths on one or two samples one mount contained durango apatite with unannealed induced tracks and one contained apatite from a crystalline rock containing spontaneous tracks with a broad length distribution caused by partial resetting results for both mounts showed scatter indicative of differences in measurement technique among the individual analysts the effects of this variability on thermal history inversion were tested using the hefty computer program to model the spontaneous track measurements a cooling only scenario and a reheating scenario more consistent with the sample s geological history were posed when a uniform initial length value from the literature was used results among analysts were very inconsistent in both scenarios although normalizing for track angle by projecting all lengths to a c axis parallel crystallographic orientation improved some aspects of congruency when the induced track measurement was used as the basis for thermal history inversion congruency among analysts and agreement with inversions based on data previously collected was significantly improved further improvement was obtained by using c axis projection differences among inversions that persisted could be traced to differential sampling of long and short track populations among analysts the results of this study while demonstrating the robustness of apatite fission track thermal history inversion nevertheless point to the necessity for a standardized length calibration schema that accounts for analyst variation

from source to sink in central gondwana exhumation of the precambrian basement rocks of tanzania and sediment accumulation in the adjacent congo basin apatite fission track aft and u th he ahe thermochronometry data are reported and used to unravel the exhumation history of crystalline basement rocks from the elevated 1000 m a s l but low relief tanzanian craton coeval episodes of sedimentation documented within adjacent paleozoic to mesozoic basins of southern tanzania and the congo basin of the democratic republic of congo drc indicate that most of the cooling in the basement rocks in tanzania was linked to erosion basement samples were from an exploration borehole located within the craton and up to 2200 m below surface surface samples were also analysed aft dates range between 317 33 ma and 188 44 ma alpha ft corrected ahe dates are between 433 24 ma and 154 20 ma modelling of the data reveals two important periods of cooling within the craton one during the carboniferous triassic 340 220 ma and a later less well constrained episode during the late cretaceous the later exhumation is well detected proximal to the east african rift 70 ma thermal histories combined with the estimated geothermal gradient of 9 c km constrained by the aft and ahe data from the craton and a mean surface temperature of 20 c indicate removal of up to 9 2 km of overburden since the end paleozoic the correlation of erosion of the craton and sedimentation and subsidence within the congo basin in the paleozoic may indicate regional flexural geodynamics of the lithosphere due to lithosphere buckling induced by far field compressional tectonic processes and thereafter through deep mantle upwelling and epeirogeny tectonic processes

tectonic and lithological constraints on the evolution of the karoo graben of northern malawi east africa the results of a lithostratigraphic tectonic and kinematic study of the karoo deposits of northern malawi are reported the objective of the lithostratigraphic study is to correlate the deposits of the karoo basins of northern malawi with the well known deposits of southern tanzania thus establishing a stratigraphic framework through which the timing of faulting can be constrained the kinematic analysis of faulting constrains the opening direction for the karoo graben in this area and provides basic data to discuss the karoo graben development within the regional tectonic framework of south eastern africa the studied adults are defined by moderately to steeply dipping cataclastic zones with a width of up to 15 m and are characterized by an array of slickensided fault surfaces with different orientations and slip directions in this study small faults offset 10 m but generally not exceeding 30 40 m have been distinguished methods used to analyse the kinematic data include the pressure tension pt method which estimates the principal axes for the bulk brittle strain and the internal rotation axis ira method which estimates the axis of bulk internal rotation and the overall sense of slip at the faults a mass balance calculation reveals a volume increase of up to 16 during cataclastic deformation in the fault zones the pt method shows an approximately east trending extension direction for faults that occur only in the latest carboniferous and early permian strata whereas the fault kinematics from faults that cut middle permian to early triassic rocks is characterized by a ese to se trending extension direction the small faults yield essentially the same kinematic results as the meso scale faults in a transport parallel cross sectional view the principal extension axes are at an acute angle of approximately 60 to the major fault planes given the moderate fault density the relatively high angle between the orientation of the principal extension axis and the fault planes suggest only a moderate amount of horizontal extension across the karoo graben of northern malawi riedel structures in the fault zones formed within two conjugate sets of localized shear zones slip on one set was top to the w nw and on the other top to the e se the two conjugate sets of riedel structures have an acute angle about the regional shortening axes implying that no pronounced rotation of the strain axes occurred the internal rotation axes for the riedel structures reveal a largely bimodal distribution and inferred weakly monoclinic to orthorhombic symmetry therefore the overall deformation during karoo rifting in northern malawi is interpreted to be close to a coaxial deformation with a limited amount of horizontal extension p

surface uplift uplift of rocks and exhumation of rocks uplift of the surface of mountain belts requires forces that are comparable in magnitude to those associated with plate motion and therefore determination of rates of surface uplift could provide important information on the dynamics of mountain ranges rates of uplift of the surfaces of mountain ranges have not however been quantified sufficiently well that they provide useful constraints on those processes many reports off surface uplift in mountain ranges are based on mistaking exhumation of rocks or uplift of rocks for surface uplift and provide no information whatsoever on the rates of surface uplift

thermal and exhumation history of the central rwenzori mountains western rift of the east african rift system uganda the rwenzori mountains mtns in west uganda are the highest rift mountains on earth and rise to more than 5 000 m we apply low temperature thermochronology apatite fission track aft and apatite u th sm he ahe analysis for tracking the cooling history of the rwenzori mtns samples from the central and northern rwenzoris reveal aft ages between 195 0 8 4 ma and 85 3 5 3 ma and ahe ages between 210 0 6 0 ma to 24 9 0 5 ma modelled time temperature paths reflect a protracted cooling history with accelerated cooling in permo triassic and jurassic times followed by a long period of constant and slow cooling than succeeded by a renewed accelerated cooling in the neogene during the last 10 ma differentiated erosion and surface uplift affected the rwenzori mtns with more pronounced uplift along the western flank the final rock uplift of the rwenzori mtns that partly led to the formation of the recent topography must have been fast and in the near past pliocene to pleistocene erosion could not compensate for the latest rock uplift resulting in oligocene to miocene ahe ages

geodynamic significance of the trm segment in the east african rift w tanzania active tectonics and paleostress in the ufipa plateau and rukwa basin abstract the tanganyika rukwa malawi trm rift segment in western tanzania is a key sector for understanding the opening dynamics of the east african rift system ears in an oblique opening model it is considered as a dextral transfer fault zone that accommodates the general opening of the ears in an nw se direction in an orthogonal opening model it accommodates pure dip slip normal faulting with extension orthogonal to the rift segments and a general e w extension for the entire ears the central part of the trm rift segment is well exposed in the ufipa plateau and rukwa basin within the paleoproterozoic ubende belt it is also one of the most seismically active regions of the ears we investigated the active tectonic architecture and paleostress evolution of the ufipa plateau and adjacent rukwa basin and in order to define their geodynamic role in the development of the ears and highlight their pre rift brittle tectonic history the active fault architecture fault kinematic analysis and paleostress reconstruction show that the recent to active fault systems that control the rift structure develop in a pure extensional setting with extension direction orthogonal to the trend of the trm segment two pre rift brittle events are evidenced an older brittle thrusting is related to the interaction between the bangweulu block and the tanzanian craton during the late pan african early paleozoic it was followed by a transpressional inversion during the early mesozoic this inversion stage is the best expressed in the field and caused dextral strike slip faulting along the fault systems that now control the major rift structures it has been erroneously interpreted as related to the late cenozoic ears which instead is characterized by pure normal faulting our third and last stress stage

normal vs strike slip faulting during rift development in east africa the malawi rift kinematic analysis of neogene and quaternary faults demonstrates that the direction of extension in the malawi rift rotated from east northeast to southeast rift development commenced with the formation of half grabens bounded by northwest north and northeast striking normal faults owing to slightly oblique rifting the northwest striking faults in the northernmost rift segment show a small dextral oblique slip component whereas north and northeast oriented faults in the central part of the rift display a sinistral oblique slip component this first event resulted in block faulting and basin subsidence which is largely responsible for the present day basin morphology of lake malawi a major change in fault kinematics occurred because of rotation of the extension direction and permutation of the principal compressive and intermediate axes the structural pattern inherited from the first rifting phase was no longer suitably oriented to accommodate extensional deformation and strike slip faulting assumed a major role the strike slip regime amplified uplift of basement ridges within the rift in regions of local transpression but it also created alluvial basins because of local transtension this new kinematic style is compatible with the recent seismicity older faults that show mainly the first deformational increment are restricted to the outermost parts of the rift toward the center the faults depict an increase in strike slip components of movement suggesting deformation propagation toward the rift center which results in a narrowing of the active rift environments with time

the influence of preexisting structure on the evolution of the cenozoic malawi rift east african rift system this paper analyzes the importance of preexisting structure for the evolution of the cenozoic malawi rift which constitutes the southernmost part of the western branch of the east african rift system kinematic analyses demonstrate that the regional extension direction rotated clockwise from ene to se during rifting cenozoic rift faults of dip oblique and strike slip character rejuvenated crustal structures whenever those were properly oriented with respect to the extension directions depending on these boundary conditions the rift attempts to find the easiest pathway in order to develop in a mechanically modest way proterozoic basement structures represent the basic anisotropy which already influenced pre cenozoic rifts and also the cenozoic malawi rift amongst these basement structures pan african retrograde to cataclastic strike slip shear zones have the strongest influence on later rift structures rift related transfer faults appear as differently striking passive structures and their orientation seems to be controlled by a variety of steep basement structures retrograde shear zones foliation planes and fracture arrays

the influence of natural radiation damage on helium diffusion kinetics in apatite abstract stepwise degassing diffusion experiments on 39 different apatite samples using radiogenic 4he and proton induced 3he reveal a range in closure temperature tc from 50 to 115 c for a cooling rate of 10 c myr there is no correlation between helium diffusion and apatite chemistry including f cl ratio but the closure temperature is positively correlated with the radiogenic 4he concentration 4he in each sample we argue that 4he is a proxy for a sample s natural exposure to actinide radioactivity below the closure temperature and that helium diffusion in apatite is impeded by radiation induced damage to the apatite structure the kinetics must therefore be an evolving function of time measured diffusivities thus reflect a snapshot in time and cannot alone be applied to the thermochronometric interpretation of a given sample the effect of radiation damage on helium diffusion appears to far exceed other known controls on helium diffusivity including grain size our diffusion data are well described by a previously proposed quantitative model that consists of two arrhenius relations one for volume diffusion through undamaged mineral structure and one for release of helium from radiation damage traps the unknown parameters in this trapping model were determined from the diffusion experiments and allow us to develop a tentative mathematical function that relates diffusivity to temperature and 4he by inserting this function into a 4he production diffusion model we have explored how these results affect the interpretation of apatite u th he thermochronometry the model predicts that the effective 4he closure temperature of apatite will vary with cooling rate and effective u concentration eu and may differ from the commonly assumed tc of 70 c by up to 15 c the 4he partial retention zone will look similar to previous expectations but its depth will depend on accumulation time and on eu most notably samples subjected to reheating after accumulation of substantial radiation damage will be more retentive than previously expected these predictions are consistent with recent observations of unexpected apatite u th he ages in some settings

improved measurement of fission track annealing in apatite using c axis projection apatite fission track length data are used extensively for thermal history inversion however several studies have documented instances of poor reproducibility of length data we address this problem by using c axis projection to normalize track lengths for crystallographic angle in the extensive laboratory annealing data set acquired by barbarand et al 2003a 2003b a new simplification reduces the c axis projection model from six to four fitted parameters normalizing for track angle using c axis projection improves every aspect of length measurement reproducibility examined it accelerates convergence of mean length in single analyses increases consistency among replicate measurements by a single analyst enhances consistency of measurements of the same mounts by different analysts and improves the match between analyses conducted with and without cf irradiation c axis projection is also shown to enhance the thermal sensitivity of length data based on these results we assert that c axis projection is a good means of compensating for observer bias although it does not overcome differences caused by experimental error

escarpment evolution on high elevation rifted margins insights derived from a surface processes model that combines diffusion advection and reaction experiments with a surface processes model of large scale 1 1000 km long term 1 100 m y erosional denudation are used to establish the controls on the evolution of a model escarpment that is related to the rifting of a continent the model describes changes in topographic form as a result of simultaneous short and long range mass transport representing hillslope diffusive processes and fluvial transport advection respectively fluvial entrainment is modeled as a first order kinetic reaction which reflects the credibility of the substrate and therefore the fluvial system is not necessarily carrying at capacity one dimensional and planform models demonstrate that the principal controls on the evolution of an initially steep model escarpment are 1 antecedent topography drainage 2 the timescale or equivalently a length scale in the fluvial entrainment reaction 3 the flexural response of the lithosphere to denudation and 4 the relative efficiencies of the short and long range transport processes when rainfall and substrate lithology are uniform a significant amount of discharge draining over the escarpment top causes it to degrade only when the top of the model escarpment coincides with a drainage divide can escarpment retreat occur for these conditions an additional requirement for retreat of a model escarpment without decline is a long reaction time scale for fluvial entrainment this corresponds to a substrate that is hard to detach by fluvial erosion and therefore to fluvial erosion that is not transport limited continuous backtilting of an escarpment due to flexural isostatic uplift in response to denudational unloading helps maintain the scarp top as a divide it is essential if the escarpment gradient is to be preserved during retreat in a uniform lithology low flexural rigidities promote steep and slowly retreating escarpments for given rainfall and substrate conditions the morphology of a retreating model escarpment is determined by the ratio of the short range diffusive and long range advective transport efficiencies a low ratio which is interpreted to correspond to a relatively arid climate and weathering limited conditions promotes steep sharp topped escarpments with straight main slopes and escarpment retreat occurs over a wide range of height scales a high ratio interpreted to correspond to a more humid temperate climate produces a convex upper slope and concave lower slope morphology and only major escarpments are predicted to preserve a high scarp gradient lithological contrasts in the model produce more complex morphologies and predict the formation of scarps crowned by an erosionally resistant caprock however resistant caprocks are not an essential requirement for model scarps to retreat we conclude that the inferred controls and model behavior are both consistent with the present day morphology of rifted continental margins and with modern conceptual models of landscape evolution

a semianalytical solution for steady infiltration in unsaturated fractured rock a semi analytical solution is developed for one dimensional steady infiltration in unsaturated fractured rock the differential form of the mass conservation equation is integrated to yield an analytical expression relating elevation to a function of capillary pressure and relative permeability of the fracture and rock matrix constitutive relationships for unsaturated flow in this analysis are taken from van genuchten 1980 and mualem 1976 but alternative relations can also be implemented in the integral solution expressions are presented for the liquid saturations and pore velocities in the fracture matrix and effective continuum materials as a function of capillary pressure and elevation results of the analytical solution are applied to examples of infiltration in fractured rock consisting of both homogeneous and composite layered domains the analytical results are also compared to numerical simulations to demonstrate the use of the analytical solution as a benchmarking tool to address computational issues such as grid refinement

on thermohydrologic conditions near high level nuclear wastes emplaced in partially saturated fractured tuff 2 effective continuum approximation this paper presents an effective continuum approximation for modeling of fluid and heat flow in fractured porous media the approximation is based on the thermohydrologic behavior observed in detailed simulations with explicit consideration of fracture effects see the companion paper part 1 the crucial concept in the development of an effective continuum approximation is the notion of local thermodynamic equilibrium between rock matrix and fractures where applicable it provides a substantial simplification of the description of fluid and heat flow in fractured porous media we derive formulas for effective continuum characteristic curves relative permeabilities and capillary pressures in terms of the properties of fracture and matrix continua respectively numerical simulations demonstrate that under favorable conditions the effective continuum approximation closely matches predictions obtained from an explicit modeling of fracture effects it is also demonstrated that the approximation breaks down under unfavorable conditions very tight rock matrix a simple criterion for the applicability of an effective continuum approximation is derived from consideration of diffusive processes a quantitative evaluation shows the criterion to be consistent with results of our numerical simulations

the role of pre existing faults in basin evolution constraints from 2d finite element and 3d flexure models the lithospheric driving forces which cause intraplate basin deformation are relatively constant over large areas consequently lateral variations in deformation and stress and strain concentrations seem to be primarily caused by pre existing heterogenities in the rheological signature of the continental lithosphere underlying the sedimentary basins in this paper we explore the weak character of upper crustal faults and their control on basin shape for a number of case studies on intraplate phanerozoic basin settings using a 2d finite element and a 3d flexure model of key importance is the integration of seismic data and field observations with the tectonic modelling allowing the investigation of deformation processes and their expressions on different scales operating on different levels of the lithosphere finite element models for sub basin scales incorporating weak upper crustal faults predict strong control of these weak zones on local stress distributions and subsequent deformation in agreement with observed deformation patterns slip along upper crustal faults control stress distribution and subsequent faulting in overlying sedimentary rocks the effect of weak upper crustal fault movements on basin wide regional upper crustal flexure is looked at in two case studies on 1 extensional tectonics in the lake tanganyika rift zone east africa and 2 compressional tectonics in the central system and tajo basin central spain both settings indicate that basement warpings are controlled by large amounts of slip along so called weak crustal scale border faults which are mostly planar adopting border fault displacements in the 3d flexure model the results indicate low effective elastic thickness eet values in a range of 3 7 km for induced basement deflection patterns in accordance with observations the low eet values most likely reflect a partly decoupling of upper crustal and subcrustal deformation facilitated by the weak lower crust and in agreement with standard rheological assumptions for phanerozoic lithosphere in contrast the inferred weakness of upper crustal faults relative to surrounding rock is not evident from uniform rheological assumptions however observations of reactivations of faults which are not preferably aligned with the stress field and reactivations of basin deformation on long time scales are in support of this feature

rift induced delamination of mantle lithosphere and crustal uplift a new mechanism for explaining rwenzori mountains extreme elevation with heights of 4 5 km the topography of rwenzori mountains a large horst of old crustal rocks located inside a young passive rift system poses the question why are the rwenzori mountains so high the cenozoic western rift branch of the east african rift system is situated within the late proterozoic mobile belts between the archean tanzania craton and congo craton the special geological setting of the massif at a rift node encircled by the ends of the northern western rift segments of lake albert and lake edward suggests that the mechanism responsible for the high elevation of the rwenzoris is related to the rifting process our hypothesis is based on the propagation of the rift tips surrounding the stiff old lithosphere at rwenzori region thereby triggering the delamination of the cold and dense mantle lithosphere ml root by reducing viscosity and strength of the undermost lower crust as a result this unloading induces fast isostatic pop up of the less dense crustal rwenzori block we term this rid rift induced delamination of mantle lithosphere the physical consistency of the rid hypothesis is tested numerically viscous flow of 2d models is approximated by a finite difference method with markers in an eulerian formulation the equations of conservation of mass momentum and energy are solved for a multi component system based on laboratory data of appropriate rock samples a temperature pressure and stress dependent rheology is assumed assuming a simple starting model with a locally heated ml the ml block between the weakened zones becomes unstable and sinks into the asthenosphere while the overlying continental crust rises up thus rid seems to be a viable mechanism to explain geodynamically the extreme uplift important conditions are a thermal anomaly within the ml a ductile lower crust with visco plastic rheology allowing significant strength reduction and lateral density variations the special situation of a two sided rifting or offset rift segments to decouple the ml laterally from the surrounding continental lithosphere seems to be most decisive further support for the rid mechanism may come from additional crustal thickness and an extensive stress field some parameters such as the excess temperature and yield stress are very sensitive small changes determine whether delamination takes place or not

a users guide to fission track dating calibration abstract misconceptions regarding the fission track dating method prompt the description of five handling procedures and the derivation of commonly used system calibrations problems of registration geometry inhomogeneous uranium distribution accumulated radiation damage and anisotropic etching largely invalidate all but the population and external detector methods and impose strict limitations upon even their routine application complexities of unique absolute thermal neutron dosimetry are illustrated by the calibration and use of the nbs pre irradiated glass srm 962 the 20 disparity in measured f decay constant values cannot be isolated from neutron fluence measurement and calibration frequently a calibration ratio f is evaluated against an age standard and then split into its component parts to illustrate this interdependence of f and zircon fission track ages in agreement with independent k ar ages are obtained by calculating the same track count data with each of the preferred values of f f 7 03 10 17 yr 1 and8 46 10 17 yr 1 together with appropriate selected neutron dosimetry schemes an alternative approach is presented formally relating unknown ages of samples to known ages of standards either by direct comparison of standard and sample track densities or by the repeated calibration of a glass against age standards practical recommendations are given for the reporting and critical assessment of fission track data

interpretation of u th he single grain ages from slowly cooled crustal terranes a case study from the transantarctic mountains of southern victoria land low temperature thermochronologic techniques e g apatite fission track aft thermochronology and u th he dating constrain near surface t t paths and are often applied to uplift denudation and landscape evolution studies samples collected in vertical profiles from granitic walls on either side of the ferrar glacier southern victoria land antarctica were analyzed using aft thermochronology and apatite u th he dating to further constrain the lowest temperature thermal history of this portion of the transantarctic mountains aft central ages vary systematically with elevation and together with track length information define a multi stage cooling denudation history in the cretaceous and early tertiary apatite u th he single grain age variation with elevation is not as systematic with considerable intra sample age variation although many complicating factors e g u and th rich micro inclusions fluid inclusions variation in crystal size particle ejection correction zonation and particle ejection correction implantation of he into a crystal or impediment of he diffusion out of a crystal and 147sm derived particles may contribute to age dispersion we found that variation in single grain ages correlated with cooling rate samples that cooled relatively quickly have less variation in single grain ages whereas samples that cooled relatively slowly 3 c m y or resided within an u th he partial retention zone heprz prior to more rapid cooling have a comparatively greater variation in ages decay of u and th via particle emission creates a 4he concentration profile dependent upon the initial parent u th within a crystal variation of single grain ages for samples with non homogeneous u th distributions will be enhanced with long residence time in the partial retention zone i e slow cooling because of the relative importance of loss via volume diffusion and loss via particle ejection with respect to the u th zonation and the grain boundary correction of ages for particle ejection ft correction factor typically assumes uniform u and th distribution within the crystal and when applied to a population of crystals with different u and th distributions will enhance the variation in ages most complicating factors listed above for apatite u th he ages result in ages that are too old we propose that if considerable variation in u th he single grain ages exists that a weighted mean age is determined once outlier single crystal ages are excluded using the criterion of chauvenet or a similar approach we suggest that the true age or most representative age for that age population lies between the minimum u th he age and the weighted mean age we apply this approach coupled with composite age profiles to better constrain the t t history of the profiles along the ferrar glacier significant intra sample variation in single crystal apatite u th he ages and other minerals dated by the u th he method should be expected especially when the cooling rate is slow the variation of u th he single crystal ages is therefore another parameter that can be used to constrain low temperature thermal histories

geometry of the livingstone mountains border fault nyasa malawi rift east africa we use multifold seismic reflection profiles to constrain the geometry and slip sense of a half graben border fault and the extension direction in the adjacent rift basin our data are from the western branch of the east african rift system often cited as an analog of early stage passive continental margin formation in this case study classic tilt block half graben clearly occur along a documented strike slip fault the livingstone mountains border fault bounds the northernmost basin in the nyasa malawi rift and is a series of straight dextral oblique slip fault segments which mimic the characteristics of a parallel underlying prerift ductile shear zone structures adjacent to the border fault show extension slightly oblique to the border fault indicating that the border fault is predominantly a strike slip fault consistent with studies of border fault exposures data indicate overall extension in the adjacent livingstone basin is at a much higher angle to the strike slip border fault consistent with the tilt block half graben architecture this characteristic is similar to basins bounded by weak strike slip faults some aspects of livingstone basin fault geometry resemble pull apart strike slip duplex and extensional imbricate fan fault geometries however the overall inexact match suggests the livingstone basin may be an incipient form of these classic strike slip basin models

tectonic controls on rift basin morphology evolution of the northern malawi nyasa rift radiometric k ar and ar 40 ar 39 age determinations of volcanic and volcaniclastic rocks combined with structural gravity and seismic reflection data are used to constrain the age of sedimentary strata contained within the seismically and volcanically active northern malawi nyasa rift and to characterize changes in basin and flank morphologies with time faulting and volcanism within the tukuyu karonga basin began at approximately 8 6 ma when sediments were deposited in abroad initially asymmetric lake basin bounded on its northeastern side by a border fault system with minor topographic relief extensions primarily by a slip along the border fault and subsequent regional isostatic compensation led to the development of a 5 km deep basin bounded by broad uplifted flanks along the low relief basin margin opposite border fault younger stratigraphic sequences commonly onlap older wedge shaped sequences although their internal geometry is often progradational intrabasinal faulting flankuplift and basaltic and felsic volcanism from centers at the northern end of the basin became more important at about 2 5 ma when cross rift transfer faults developed to link the tukuyu karonga basin to the rukwa basin local uplift and volcanic construction at the northern end of the basin led to a southeastward shift in the basin s depocenter sequence boundaries are commonly erosional along this low relief hanging wall margin and conformable in the deep lake basin the geometry of stratigraphic sequences and the distribution of the erosion indicate that horizontal and vertical crustal movements both across and along the length of the rift basin led to changes in levels of the lake irrespective of paleoclimatic fluctuations