effects of topography on the curvature of fold and thrust belts during shortening of a 2 layer model of continental lithosphere abstract we have used analogue experiments to investigate the effects of surface topography on the curvature of fold and thrust belts under conditions of 1 initial relief but no erosion and 2 no initial relief but differential erosion sedimentation and transport in experiments where a 2 layer model lithosphere shortened and thickened in front of an advancing straight piston the geometry of the developing thrust wedge was very sensitive to variations in surface topography in models with an initially flat horizontal surface and in the absence of erosion and sedimentation thrusts were straight propagated forwards and nucleated at buckle folds far in front of an advancing piston around an initial topographic high plateau or cone thrusts tended to be arcuate forming salients towards the foreland initial plateaux and cones tended to behave rigidly while arcuate thrust slices formed around them to accommodate differential slip transfer zones developed on both sides of initial highs fault blocks rotated about vertical axes and thrusts moved in oblique slip within transfer zones in models with initially horizontal surfaces which were subject to differential erosion sedimentation and transport thrusts initially were straight but then progressively rotated around non eroded thickened and stronger areas these worked as indenters in front of which new thrusts nucleated at curved buckle folds these thrusts were also curved their apices being in front of the thickened non eroded areas in nature arcuate structural patterns are to be found around the altiplano of the central andes and around the tromen volcanic ridge in the neuquen basin of northern patagonia we infer that these areas behaved in quasi rigid fashion protected as they were by their high elevations and that differential erosion at the scale of the entire andes may have contributed to oroclinal bending

relative ages of loess and till in two quaternary palaeosols in gorges valley mount kenya east africa two compound palaeosol profiles formed in loess and till were studied in the oldest early quaternary end moraine gorges age system on mount kenya although both tills appear to have similar weathering histories the overlying loessic sediments serving as secondary parent materials for the palaeosols are considered to have different ages on the basis of colour presence or absence of clay films and field texture especially percentage of silt palaeomagnetic determinations showed both tills to have reversed remanent magnetism suggesting that they were emplaced during the matuyama chron the overlying loessic sediments in profiles gor55 and gor58 show normal magnetism and grade upward into the lower zone of bioturbation in the a horizon complex however in gor58 the surface loessic sediments have normal remanent magnetism with considerable secular variations suggesting it was emplaced over a longer time frame during the brunhes chron i e 0 78 ma profile gor55 contains larger amounts of gibbsite indicating more aggressive leaching over time and an absence of secular magnetic variations suggests deposition over a shorter time interval the study of volcanic feldspars and ash using scanning electron microscopy in the two profiles shows that sand grain 63 250 m weathering is slightly different in the two tills relative weathering effects e g corrosion of grain surfaces and neoformation of clay minerals are greater in profile gor55 than in profile gor58 overall the data indicate that some sites in the lower tropical afroalpine timberline environment may have aeolian covers that are considerably younger than the deposits they overlie 1997 by john wiley sons ltd

epicenter distribution and magnitude of earthquakes in fold thrust belts insights from sandbox models scaled analogue models are used to illustratethe eect of basal friction and erosion on fault activity andhence on epicentre distribution and magnitude of earth quakes in the sedimentary cover of active fold thrust belts model results suggest that in fold thrust belts shortenedabove low friction ductile decollements rock salt or over pressured mudstone low to moderate magnitude earth quakes mw 5 3 5 6 distributed over a wide area occuralong several long lived thrust faults conversely in areasshortened above high friction decollements large magnitudeearthquakes mw 6 6 6 8 distributed over a narrowzone are likely to occur along few short lived thrust ramps calculated magnitude of earthquakes from models and theirdistribution are in agreement with recorded earthquake pat tern from the zagros mountain belt which is partially short ened above a ductile decollement of hormuz salt formation model results also showed that erosion reactivates older in active thrusts and promotes formation of out of sequencethrusts

late neogene volcanics and interbedded palaeosols near mount kenya abstract two lava flows with interbedded palaeosols outcrop c 40 km sw of mount kenya near the amboni river north of mweiga kenya along the nyeri thompson falls road at 0 18 s 37 48 e these flows overlain by loess are principally trachyandesite and form the base of the mount kenya volcanic series which in the early literature is described as being of probable miocene pliocene age here we report 39 ar 40 ar dates c 5 2 5 5 ma and reversed magnetizations which establish a latest miocene to earliest pliocene age for these flows weathering characteristics of palaeosols interbedded with the lavas indicate generally dry climatic conditions during the late miocene punctuated with humid events during the pliocene and quaternary these late miocene quaternary palaeosols depict a relatively long and complex weathering history followed by loess deposition the palaeosols appear to have been episodically deflated initially in phase with the deposition of lavas when surfaces were devoid of vegetation and later during periods of climatic deterioration when wind systems intensified such weathering histories within palaeosol profiles are also documented on nearby mount kenya where well weathered lower palaeosol horizons developed on matuyama age tills are overlain by much younger less weathered horizons developed on brunhes age loess the geochronology of late miocene lavas reported here provides maximum ages for weathering histories of palaeosols formed in a xeric tropical highland climate

lithospheric scale folding numerical modelling and application to the himalayan syntaxes we describe the eastern and western himalayan syntaxes which are large scale antiforms situated at geodynamically similar locations and the metamorphic evolution of which is coeval in the india asia collisional history to understand the mechanical plausibility of the structural interpretation we present two dimensional finite element modelling of lithospheric folding the models reveal the coeval development of adjacent synformal basins analogous to the peshawar and kashmir basins on both sides of the western syntaxis similarities between geological data and calculated models indicate that lithospheric buckling is a basic response to large scale continental shortening and an efficient mountain building process

do faults trigger folding in the lithosphere a number of observations reveal large periodic undulations within the oceanic and continental lithospheres the question if these observations are the result of large scale compressive instabilities i e buckling remains open in this study we support the buckling hypothesis by direct numerical modeling we compare our results with the data on three most proeminent cases of the oceanic and continental folding like deformation indian ocean western gobi central asia and central australia we demonstrate that under reasonable tectonic stresses folds can develop from brittle faults cutting through the brittle parts of a lithosphere the predicted wavelengths and finite growth rates are in agreement with observations we also show that within a continental lithosphere with thermal age greater than 400 my either a bi harmonic mode two superimposed wavelengths crustal and mantle one or a coupled mode mono layer deformation of inelastic folding can develop depending on the strength and thickness of the lower crust

crystallographic preferred orientation magnetic and seismic anisotropy in rocks from the finero peridotite ivrea verbano zone northern italy interplay of anisotropy contributions from different minerals abstract mineral alignment can provide valuable information on a rock s geological history many structural tectonic or geodynamic studies rely on determining the preferred orientation of rock forming minerals either by direct texture determination or using anisotropy measurements as proxies robust interpretations of anisotropy require detailed understanding of its relationship with texture this study investigates the texture magnetic and seismic anisotropies of ultramafic rocks from the ivrea verbano zone ivz northern italy with a special focus on the interplay between anisotropy contributions carried by different minerals texture was obtained from electron backscatter diffraction magnetic anisotropy from low and high field anisotropy of susceptibility measurements and seismic anisotropy from p and s wave velocities along the three principal directions of the macroscopic fabric using 22 mm diameter cores texture based models of magnetic anisotropy agree well with corresponding measurements larger variability is observed for seismic anisotropy possible explanations are 1 velocities were measured in three directions insufficient to determine the full elasticity tensor and 2 seismic anisotropy depends on grain boundaries pores and cracks in addition to crystallographic preferred orientation the contributions to magnetic and seismic anisotropy of each constituent mineral can interfere positively leading to a larger overall anisotropy or negatively resulting in a weaker anisotropy seismic anisotropy is mostly controlled by olivine whereas magnetic anisotropy can be dominated by olivine pyroxene or hornblende this study illustrates that although the relationship between texture and anisotropy can be complex it is possible to quantitatively predict magnetic and to a lesser degree seismic anisotropy from texture data even though both magnetic and seismic anisotropies are common proxies for texture they are not coaxial and no simple relationship between the degree of magnetic and seismic anisotropy was found these results underline the importance of understanding different minerals contributions to anisotropy in upper mantle rocks

intraplate lithosphere strength and heat flow the magnitude levels of intraplate stresses and the strength of intraplate lithosphere are critical for understanding the mechanisms of intraplate deformation the decay of stress in the ductile lower lithosphere and its resulting amplification in the brittle upper lithosphere constitute an important mechanism for providing stress levels in the upper brittle lithosphere sufficient for fracture such stress transfer and the subsequent lithosphere deformation are controlled by lithosphere rheology a model is examined here for the deformation of viscoelastic lithosphere with temperature and stress dependent rheology subjected to laterally applied stress for different lithosphere thermal structures the geotherm is found to be of critical importance in determining the strength of the lithosphere and the extent of the resulting lithosphere deformation for lithosphere with heat flow 75 mw m 2 0 25 kbar tensile stress is sufficient to cause geologically significant deformation while for compressional deformation either greater stress levels or a hotter lithosphere are necessary

experiments on shortening of a 4 layer model of the continental lithosphere davy ph and cobbold p r 1991 experiments on shortening of a 4layer model of the continental lithosphere in p r cobbold editor experimental and numerical modelling of continental deformation tecronophysics 188 l 25 we present an experimental method for modelhng lithospheric deformation the method is based on a simplified profile of lithosphere rheology where only four layers are considered brittle crust ductile crust brittle mantle and ductile mantle the strengths of the four layers calculated from laboratory determined flow laws depend strongly on geothenns and slightly on strain rates and deformation regimes the rheological profiles of the lithosphere have been recreated in experimental models built with analog materials sand silicone putty and golden syrup which flow under their own weight during lithospheric shortening the style of deformation depends on the relative strengths of the four layers especially those in the mantle of special importance is the existence or non existence of a brittle mantle the initial stages of compression often produce flexures or buckles without local isostatic compensation buckling exerts a strong control on the localisation of thrust faults in later stages the thrusts tend to form at the inflexion points of buckles the style of crustal thickening depends on the relative thicknesses and resistances of the lithospheric layers in 2 layer lithospheric models without resistant mantle shortening was concentrated upon a small number of thrusts widely spaced in 3 layer models shortening was concentrated into a large number of reverse faults more closely spaced and more steeply dipping than before in clayer models a single thrust formed in the brittle mantle motion on this thrust dragged the crustal layers down forming an asymmetric crustal root with steep imbricate slices

horizontal lithosphere compression and subduction constraints provided by physical modeling physical modeling of the subduction is performed with a two layered mantle model elastico plastic lithosphere and low viscosity asthenosphere and is governed by the criteria of similarity compression of the lithosphere in the area of a passive continental margin has been shown to produce a buckling instability in the oceanic plate with wavelengths of 200 km on the average later a localization of deformation occurs in sagging at some distance from the margin where a strongly dislocated linear ridge is formed due to the thrusting the plate then experiences a failure along the inclined zone and subduction starts the inner trench slope which forms therefore has a scraped structure and include a block of crushed and dislocated oceanic crust and sediments in the lower section if there is an old inclined fault striking across the compression of an oceanic lithosphere it is on that fault that a subduction zone is initiated the inner trench slope has then a different structure and forms due to normal faulting in the frontal part of the overriding plate the formation of a subduction zone requires a compression that is smaller than that in the preceding case by a factor of 2 or 3 the subducting plate experiences an elastico plastic bending and under specific conditions thrusting along the zone dipping from under the overriding plate oceanward and crossing the entire lithosphere the best agreement between generalized relief in subduction zone in the model and nature is achieved when a shear yield limit s 1 3 108 pa modulus of elasticity e about a few times 1011 pa and a thickness h 60 km are adopted for the real lithosphere

sedimentary basins and crustal thickening abstract we consider the development of sedimentary basins in a tectonic context dominated by horizontal shortening and vertical thickening of the crust well known examples are foreland basins others are ramp basins and buckle basins we have reproduced various styles of compressional basins in experiments properly scaled for gravity a multilayered model lithosphere with brittle and ductile layers floats on a model asthenosphere a computer driven piston provides shortening and thickening synchronous with erosion and sedimentation after a first stage of lithospheric buckling thrust faults appear mainly at inflection points slip on an isolated reverse fault is accompanied by flexure footwall flexure results in a foreland basin and becomes accentuated by sedimentation hangingwall flexure is less marked but may become accentuated by erosion motion on a fault leads to hangingwall collapse at the surface either footwall sedimentation or hangingwall erosion tends to prolong the active life of a reverse fault slip on any pair of closely spaced reverse faults of opposite vergence results in a ramp basin simultaneous slip produces a symmetric ramp basin whereas alternating slip results in a butterfly shaped basin with superposed foredeeps some well developed ramp basins become pushed down until bounding faults meet at the surface and the basin disappears from view at this stage the basin depth is equivalent to 15 km or more slip on any pair of widely spaced reverse faults of opposite vergence results in a pronounced central anticline between two distinct foredeeps in central asia and in western europe cenozoic crustal thickening is due to continental collision for central asia western china kyrgyzstan uzbekistan tajikistan we have compiled a regional structure contour map on the base of the tertiary as well as 4 regional sections foreland basins and ramp basins are numerous and associated with cenozoic thrusts large basins tarim junggar fergana tajik occur around and between mountain ranges but smaller basins issyk kul naryn occur within them in western europe the alps and pyrenees are surrounded by foreland basins ramp basins or intermediate styles in the andes and its foreland neogene thrusts and compressional basins are due to subduction of oceanic lithosphere in colombia they account for much of the cordillera oriental in nw argentina for the altiplano in west central argentina for the sierras pampeanas compressional basins are also common in other areas of older crustal thickening

stratigraphy of the gorges moraine system mount kenya palaeosol and palaeoclimate record moraines marking an early pleistocene glaciation on mt kenya are known from several valleys along the eastern and southeastern flanks of the mountain the most prominent group of end moraines delimiting the lowermost extent of the early pleistocene gorges glaciation 2900 m above sea level overlie either older till or weathered bedrock the latter composed of thin pedostratigraphic remnants covered with locally derived aeolian sediments truncated by the ingress of ice the loess tills and interbedded palaeosols of the gorges glaciation crop out adjacent to the nithi river which drains the gorges valley and abut the upper timberline of montane forest the lowermost palaeosols at these sites were formed either in till predating the gorges glaciation or in weathered bedrock of trachytic textured lavas similar to the lithology forming the base of the mt kenya volcanic series of miocene or pliocene age palaeomagnetism and weathering characteristics are here used to refine the age of sediments assigned to the gorges glaciation these normally magnetized deposits carry a persistent reversed overprint suggesting that they were deposited during one of the normal subchrons within the matuyama reversed chron they are underlain either by a reversely magnetized till or by weathered bedrock and lower palaeosol the latter exhibiting normal magnetization gauss with reversed overprint matuyama the sediments of the gorges glaciation are overlain at all three sites by normally magnetized loesses and palaeosols presumably of brunhes age but carrying a high percentage of well weathered recycled grains the normal magnetization and reversed overprint recorded in sediments of the gorges glaciation most probably span a considerable portion of the olduvai subchron 1 78 1 950 ma which persisted for sufficient time to accommodate an extensive montane glaciation followed by a prolonged period of weathering and soil formation the importance of these sediments in global palaeoclimate reconstruction and insolation changes is discussed the stratigraphic evidence contradicts the cosmogenic exposure ages presented by previous workers attempting to obtain true ages of deposits using exhumed coarse clastic sediment without analysis of complex pedostratigraphic complexes supplementary materials tables 1s 5s are available at www geolsoc org uk sup18592

topography as a major factor in the development of arcuate thrust belts insights from sandbox experiments abstract we have used sandbox experiments to investigate and to illustrate the effects of topography upon the development of arcuate thrust belts in experiments where a sand pack shortened and thickened in front of an advancing rectilinear piston the geometry of the developing thrust wedge was highly sensitive to variations in surface topography in the absence of erosion and sedimentation the surface slope tended to become uniform as predicted by the theory of critical taper under these conditions the wedge propagated by sequential accretion of new thrust slices in contrast where erosion or sedimentation caused the topographic profile to become irregular thrusts developed out of sequence for example erosion throughout a hinterland caused underlying thrusts to remain active and inhibited the development of new thrusts in the foreland where initial topography was irregular in plan view accreting thrusts tended to be arcuate they were convex towards the foreland around an initially high area concave towards the foreland around an initially low area initial plateaux tended to behave rigidly while arcuate thrust slices accreted to them thrust motions were radial with respect to each plateau within transfer zones to each side fault blocks rotated about vertical axes and thrust motions were oblique slip at late stages of deformation the surface slope of the thrust wedge tended towards a uniform value initial mountains of conical shape representing volcanoes also escaped deformation except at depth where they detached arcuate thrust slices accreted to front and back where a developing thrust wedge was subject to local incision accreting thrust slices dipped towards surrounding areas of high topography forming vs across valleys arcuate structural patterns are to be found around the three highest plateaux on earth tibet pamirs and altiplano and around the tromen volcanic ridge in the neuquen basin of northern patagonia we infer that these areas behaved in quasi rigid fashion protected as they were by their high topography

thrust initiation and propagation during shortening of a 2 layer model lithosphere analogue modelling was used to investigate thrust initiation and propagation in the lithosphere the model comprised a top layer of light sand model brittle crust overlying a layer of heavy silicone putty model ductile lower crust both resting upon higher density and lower viscosity natural honey model ductile mantle the experiments show the following 1 thrust initiation may occur in two main modes fold first or fault first they seem to be the result of presence or absence of an initial perturbation close to the piston respectively the fault first mode eventually goes into a fold first mode after a certain amount of deformation 2 in both modes thrusts localized around the inflection points of both fore and back limbs of buckles as a result of tectonic horizontal gravitational vertical and elastic forces 3 thrust propagation in the fault first mode occurred by thrust wedge thickening and downward flexure due to isostatic compensation which led to forward formation of bulges and subsequent amplification by buckling continued shortening increased the stresses that reached the brittle yield of sand and new thrusts originated 4 bulging ahead of thrusts tended to develop into asymmetric buckles with the shorter limb back limb steeper towards the piston 5 as a whole thrust initiation and propagation seem to depend mostly on how stresses evolve and propagate through the model and how material strength varies with progressive deformation the main stresses at play are born at the moving piston model horizontal tectonic force from thickening and isostatic compensation in the model gravitational forces and from elastic resistance to buckling

stress in the indo australian plate cloetingh s and wortel r 1986 stress in the ind austr ian plate in b johnson and a w bally editors intraplate deformation characteristics processes and causes tectonophysics 132 49 67 we modelled the state of stress in the indo australian plate in order to investigate quantitatively variations observed in tectonic style the numerical procedure incorporates the dependence of slab pull and ridge push on the age of the oceanic lithosphere estimates are presented for the average net resistive forces at the himalayan collision zone the suction force acting on the overriding indo australian plate segment at the tonga kermadec trench and the drag at the base of the lithosphere our modelhng shows a concen ation of compressive stresses of the order of 3 5 lcbar in the ninetyeast ridge area the effects of the compressive resistance associated with himalayan collision and subduction of young lithosphere off the northern part of the sunda arc are focused in this region the stress field as calculated gives a consistent explanation for the observed concentration of seismic activity stein and okal 1978 and significant deformation in the oceanic crust weissel et al 1980 mcadoo and sandwell 1985 in the area the calculated stress field in the area adjacent to the southeast and central india ridges is characterized by tension parallel to the spreading axis this explains the concentration of near ridge normal faulting seismicity with t axes subparaile1 to the spreading ridge in the indian ocean as recently observed by bergman et al 1984 and wiens and stein 1984 the regional stress field along the strike of the sunda arc varies from compression seaward of and parallel to the sumatra trench segment to tension perpendicular to the java flores segment this explains the selective occurrence of well developed grabens seaward off the java flores segment of the trench observed by hilde 1983 our modelling shows that the observed rotation of the stress field denham et al 1979 in the australian continent is mainly the consequence of its geographic position relative to the surrounding trench segments and the variations of the forces acting on the down going slab in each of these the state of compression in west and central australia is induced by the action of resistive forces at the himalayan and banda arc collision zones the joint occurrence of high levels of compression in the plate s interior and normal faulting seismicity in the near ridge areas is a transient feature unique to the present dynamic situation of the indo australian plate

control of folding by gravity and matrix thickness implications for large scale folding 1 we show that folding of a non newtonian layer resting on a homogeneous newtonian matrix with finite thickness under influence of gravity can occur by three modes 1 matrix controlled folding dependent on the effective viscosity contrast between layer and matrix 2 gravity controlled folding dependent on the argand number the ratio of the stress caused by gravity to the stress caused by shortening and 3 detachment folding dependent on the ratio of matrix thickness to layer thickness we construct a phase diagram that defines the transitions between each of the three folding modes our priority is transparency of the analytical derivations e g thin plate versus thick plate approximations which permits complete classification of the folding modes involving a minimum number of dimensionless parameters accuracy and sensitivity of the analytical results to model assumptions are investigated in particular depth dependence of matrix rheology is only important for folding over a narrow range of material parameters in contrast strong depth dependence of the viscosity of the folding layer limits applicability of ductile rheology and leads to a viscoelastic transition our theory is applied to estimate the effective thickness of the folded central asian upper crust using the ratio of topographic wavelength to moho depth phase diagrams based on geometrical parameters show that gravity does not significantly control folding in the jura and the zagros mountains but does control folding in central asia applicability conditions of viscous and thin sheet models for large scale lithospheric deformation derived in terms of the argand number have implications for the plate like style of planetary tectonics

shortening of analogue models of the continental lithosphere new hypothesis for the formation of the tibetan plateau initial stages of compression produce periodic buckling of models analogous to a four layer continental lithosphere with further shortening amplification of the buckles occurs by thrust faulting at inflection points in both brittle layers of the upper crust and upper mantle the rescaled results suggest that a continental lithosphere under compression will buckle with a first order wavelength of about 200 km and a second order wavelength of 20 30 km continued shortening results in the amplification of undulations troughs sink and become compressional basins while deformation keeps going on major thrusts within the upper mantle thus giving rise to an irregular topography of the moho fifty percent shortening results in a 60 km thick continental crust we argue that this model history may explain the elevation and deep crustal structure of the tibetan plateau

lithospheric scale structures from the perspective of analogue continental collision abstract analogue models were employed to investigate continental collision addressing the roles of 1 a suture zone separating different crustal blocks 2 mid crustal weak layers and 3 mantle strengths these models confirmed that low amplitude lithospheric and crustal buckling is the primary response to shortening with a wavelength mainly controlled by the strong upper crust and or lithospheric mantle with increasing shortening bivergent thrusts formed across the brittle layers equivalent to upper crust at inflection points of the buckles while ductile layers lower crust and mantle continued being folded the resulting geometries displayed pop ups and pop downs above the boundaries sutures between blocks further shortening led to wider thrust bounded deformation zones belts in front of the stronger blocks identified as effective indenters during shortening upward extrusion into belts allowed exhumation of deep material weak layers enhanced strain localisation these modelling results have similarity in large scale structures of modern orogens and are relevant to the understanding of natural collisional processes

sand coatings in paleosols evidence of weathering across the plio pleistocene boundary to modern times on mt kenya abstract a comparison of younger post olduvai paleosol horizons with older buried horizons of plio pleistocene age shows that primary and secondary minerals and organic molecules in sand coatings in these paleosols remain intact and provide a record of past weathering events from ir spectra concentrations of organic and some inorganic constituents within these coatings reveal supporting evidence of variable weathering trends from late pliocene to holocene time with some variability hematite and goethite concentrations in sand coatings parallel similar variable concentrations of secondary fe extractable concentrations previously determined for pre and post olduvai paleosols in these mt kenya sections negative correlations between metahalloysite and gibbsite previously determined to result from aggressive leaching of si from metahalloysite in paleosols tend to follow similar distributions in the sand coatings reported here similarly additions of gibbsite in selected paleosol horizons are followed by reductions of lesser concentrations of metahalloysite suggesting that individual horizons within paleosols have independent leaching histories different from the entire paleo pedon as with these paleosols analyzed previously quartz exists in greater proportion within sands recovered from ah and ahb horizons and this mineral has previously been considered to be sourced from the older surrounding african crust rather than from local neogene volcanic sources the presence of vermiculite in sand coatings previously not detected in mt kenya paleosols may be related to the presence of cedar stands juniperus procera within the timberline zone of the mountain a forest species clay mineral correlate that may relate to past forest biome evolution specifically in the pre olduvai paleosols the occurrence of vermiculite in these older paleosols a clay species with a known affinity for conifers birkeland 1999 may indicate a higher timberline during pre olduvai time the outcome of this analysis shows that certain horizons act as a singular open weathering system responding to the throughput of either meteoric or groundwater in selective ways to produce a range of weathering products sourced from primary minerals and selective allochthonous mineral inputs from near and far more importantly sand coatings are shown to provide important paleoenvironmental data parallel to that recovered from macro paleosol systems moreover the wealth of paleoenvironmental data recovered from sand coatings rivals that in weathering rinds and the combined data are applicable to other paleoenvironments both on earth and mars

measurements of friction coefficients and cohesion for faulting and fault reactivation in laboratory models using sand and sand mixtures abstract simple laboratory tests have been made on dry quartz sand and on mixtures of sand and clay and sand and cement tests were made on both low and high density specimens of each mixture as determined by physical handling techniques the tests included measurements of density estimates of the coefficient of internal friction based on fault orientation and estimates of the friction coefficients and cohesion values for fault initiation and reactivation in a shear frame apparatus which was also used to test artificial fault zones made from pure clay glass microspheres and wheat flour the results suggest that density is determined more by handling technique than by composition and that friction coefficients and perhaps cohesion both increase with increasing density the shear testing revealed a consistent drop in cohesion for fault reactivation in all densely packed mixtures tested although small this drop may be significant for some types of laboratory models

loessic silt deposits in the western himalayas their sedimentology genesis and age abstract loess is abundant in the foreland basins of the himalayas however it is poorly developed within the intermontane basins of the western himalayas and karaloram selected sections were studied from the swat himalaya and the karakoram mountains fieldwork and sedimentological analysis including grain size mineralogy and microfabric showed that the loessic sediments have been reworked and some have undergone weak pedogeneses these silts are not true loess rather a loessic colluvium thermoluminescence tl dates from the loessic sediments in sections from the swat valley provided younger ages than earlier workers had suggested this is consistent with reworking of the loess in these areas it is concluded that care must be taken when using loessic silts for tl in similar mountainous terranes

on the gravitational potential of the earth s lithosphere the mean potential energy of the lithosphere is useful for defining the tectonic reference state trs of the earth and can be used to constrain the ambient state of stress in the plates in the absence of external forces applied at the base or along plate boundaries a lithospheric column with the potential energy of the trs would remain undeformed thus the difference between the potential energy of a lithospheric column and the trs determines whether the column is in an extensional joeutral or compressional state of stress we evaluate and intraplate variations about this mean using a simple first order lithospheric density model this model assumed that the continental geotherm is linear and density variations below a depth of 125 km have negligible influence on and is consistent with observed geoid anomalies across continental margins is estimated to be 2 379 1014 n m 1 which is equivalent to the potential energy of both near sea level continental lithosphere 160 to 220 m for an assumed crustal density c in the range 2800 2700 kg m 3 and cooling oceanic lithosphere at a depth of 4 3 km with the exception of eurasia which has anomalously high mean potential energy 2 383 1014 n m 1 the mean potential energies of the continental plates are nearly identical to the global mean the mean potential of the oceanic plates was found to be a strong function of the mean age of the oceanic lithosphere both the global and plate mean potential energies are relatively insensitive to a wide range in c the potential of the mid ocean ridges 2 391 1014 n m 1 is greater than the global mean which is consistent with the divergent nature of the ridges elevated continental lithosphere with a height of about 70 m has an equivalent potential energy to suggesting that in the absence of external forces continental regions will be in a slightly extensional state of stress the importance of our potential energy formulation is substantiated by the strong correlation between the torque poles associated with the potential energy distributions and the observed plate velocity poles for the south american nazca indo australian and pacific plates

weathering rinds archives of paleoenvironments on mount kenya east africa abstractweathering rind thicknesses on pebble and cobble size sediment have been used for the past half century at least as an age indicator of postdepositional time following a geologic event in mountainous terrain rind thickness is taken as a measurement of weathering over time frames of 0 5 m yr variable thicknesses are used to discriminate relative ages of glacial deposits the effects of chemical and physical weathering that together produce rinds are only rarely considered and most research objectives have centered on lichen alteration of clast surfaces recent microscopic analyses of weathering rinds on volcanic clasts of 70 0 ka to 2 0 m yr age produced new data on weathering products as well as unexpected incorporated biotic materials undergoing diagenesis the question as to how much physical mineral chemical biotic paleoenvironmental data might be archived in rinds is discussed the character and classification of organic materials undergoing diagenesis are also discussed

mineralogy and chemistry of late pliocene early pleistocene paleosols on mount kenya weathering indices of relative age and paleoenvironmental reconstruction abstract iron and al extracts as weathering indices in paleosols have been used in many localities to determine relative age transformation of total chemical element concentrations to secondary forms translocation of organically complexed al and long standing inundation of soils with groundwater on mt kenya a succession of paleosols straddling the olduvai subchron are here analyzed to determine the degree to which fe al extracts assist paleoenvironmental reconstruction especially relative age determination genesis and translocation of alteration products and the effect of paleoclimatic deterioration with the advent of glaciation ca 2 0 ma and possibly before warmer humid climate from the onset of the plio pleistocene to the olduvai subchron thereafter reverting to a long episode of oscillating drier wetter ice age perturbations is written into the profile morphologies mineralogies and chemistries supporting earlier stratigraphic interpretations

periodic instabilities during compression of the lithosphere 2 analogue experiments we have modeled the behavior of the continental and oceanic lithospheres under compression using materials with analogous properties in laboratory experiments to study the development of lithospheric buckling periodic instabilities which are a major deformation process during the compression of the lithosphere have already been described by several authors using an analytical perturbation method at small strains analogue experiments corroborate most of the results obtained by the perturbation method 1 the deformation modes geometrical relationships of interfaces and related wavelengths are mainly dependent on the spatial distribution of the brittle layer s and 2 the amplitude of buckling is an exponential function of the horizontal strain some departure from the perturbation method occurs when there are two instabilities growing concurrently the breakdown of the exponential growth occurs for strains of about 5 and is concomitant with the appearance of thrust faults in experiments including one brittle layer which model the compression of the oceanic lithosphere faults are regularly located at the inflection points of the folds in experiments including two brittle layers which model continental lithosphere faults form more complicated patterns with an asymmetrical deep thrust overlain by a fan shaped symmetrical thrust system in the upper brittle layer such fault geometries give some new highlights on typical compressive geological structures such as those encountered in central asia

some examples and mechanical aspects of continental lithospheric folding abstract recent theoretical and observational advances in understanding the dynamics of lithosphere processes have yielded strong evidence in favour of the existence of large scale lithosphere folds intraplate compressional stresses generated by plate tectonic forces can reach magnitudes that approach the plastic buckling strength of mechanical models of lithosphere with brittle ductile rheological stratification the models predict lower plastic buckling stresses for continental lithosphere than for oceanic lithosphere intraplate folding of relatively strong oceanic lithosphere documented extensively in the northeastern indian ocean occurs where stresses are concentrated by restricted geometric and dynamic conditions resulting in unusually high stress levels several hundreds mpa on continents where geophysical data suggest that lithosphere folding may have occurred during the development of sedimentary basins in northern canada and central australia buckling stresses can have magnitudes that are of a similar order to those thought typically to occur in the lithosphere several tens mpa folding of continental lithosphere although more difficult to document is likely to be an important mechanism of large scale continental deformation in particular thinned continental lithosphere with a history of superposition of thick sedimentary successions might be the preferential locus of such folding lithosphere folding could also be a controlling factor in the near surface deformation of rifted basins as reflected by tectonic basin inversion

tectonic stresses in the lithosphere various types of observables earthquake focal mechanisms in situ measurements and geological deformations give information about the large scale lithospheric stress field the latter has often been explained by postulating appropriate forces acting at the edges and beneath the plates this approach ignores the role of mass heterogeneities within the lithosphere here we analyze the effect of both boundary and internal forces on the stress pattern and show that both contributions are of comparable magnitude the presence of internal sources makes the problem three dimensional we show however that it can be reduced to a two dimensional plane stress formulation whereby the edge forces are expressed by the non hydrostatic stresses and the basal shear is increased by the addition of a term proportional to the gradient of the mean vertical stress for the oceanic lithosphere we derive a compression that increases with age the comparison with geophysical observables yields an upper bound of a few bars on the magniude of the basal drag for the continents we infer the existence of an underlying upper mantle somewhat denser than under oceans

distributions of halloysite metahalloysite and gibbsite in tropical mountain paleosols relationship to quaternary paleoclimate abstract paleosol profile available water varied during the quaternary and when high was accompanied by increased leaching and loss of sio 2 this mechanism controlled the formation of gibbsite and halloysite metahalloysite in paleosols on mount kenya during the time they were exposed to subaerial weathering the available data for distributions of weathering products in paleosols support a reconstruction of high energy paleoclimate and wetness during the first two interglacials lake ellis gorges naro moru lake ellis followed by reduced energy lower precipitation in the teleki naro moru interglaciation lower brunhes and ending with increased paleoclimatic energy during the liki teleki interglaciation isotope stage 5 based on the data available weathering during the present postglacial interval fails to achieve even the magnitude of a low energy interglaciation using relative amounts of gibbsite halloysite it is possible to contruct a model of relative paleoclimatic energy for each of the interglacial on mount kenya