generation of continental rifts basins and swells by lithosphere instabilities 1 continents may be affected simultaneously by rifting uplift volcanic activity and basin formation in several different locations suggesting a common driving mechanism that is intrinsic to continents we describe a new type of convective instability at the base of the lithosphere that leads to a remarkable spatial pattern at the scale of an entire continent we carried out fluid mechanics laboratory experiments on buoyant blocks of finite size that became unstable due to cooling from above dynamical behavior depends on three dimensionless numbers a rayleigh number for the unstable block a buoyancy number that scales the intrinsic density contrast to the thermal one and the aspect ratio of the block within the block instability develops in two different ways in an outer annulus and in an interior region in the outer annulus upwellings and downwellings take the form of periodically spaced radial spokes the interior region hosts the more familiar convective pattern of polygonal cells in geological conditions such instabilities should manifest themselves as linear rifts striking at a right angle to the continent ocean boundary and an array of domal uplifts volcanic swells and basins in the continental interior simple scaling laws for the dimensions and spacings of the convective structures are derived for the subcontinental lithospheric mantle these dimensions take values in the 500 1000 km range close to geological examples the large intrinsic buoyancy of archean lithospheric roots prevents this type of instability which explains why the widespread volcanic activity that currently affects western africa is confined to post archean domains

mantle transition zone thickness beneath cameroon evidence for an upper mantle origin for the cameroon volcanic line summary the thickness of the mantle transition zone beneath cameroon has been mapped using data from the 2005 2007 cameroon broadband seismic experiment to evaluate models for the originof the cameroon volcanic line cvl some 2200 receiver functions have been stacked using a 3 d velocity model revealing ps conversions from the mantle transition zone discontinuities at depths of 410 and 660 km results yield a nearly uniform transition zone thickness 251 10 km that is similar to the global average implying that any thermal anomalies in the upper mantle beneath the cvl do not extend as deep as the transition zone this finding when combined with regional p and s velocity models of the mantle supports an explanation attributing the origin of the cvl to upper mantle processes such as an edge flow convection cell in the upper mantle along the northwestern side of the congo craton lithosphere

effects of baroclinicity on the cloud pattern and structure of polar lows a high resolution numerical experiment 1 high resolution numerical experiments using a three dimensional non hydrostatic model with horizontal grid size of 5 or 2 km are performed to clarify what determines the structure and cloud pattern of polar lows an axisymmetric initial vortex is imposed in a baroclinic basic state for which thermal wind balance holds it is found that when the baroclinicity is absent a hurricane like vortex with spiral cloud bands a cloud free eye and a warm core develops when the baroclinicity is strong on the other hand a vortex with a comma shaped cloud and slightly larger horizontal scale develops the former appears to develop due to cisk wishe mechanism while the latter due to baroclinic instability modified by latent heating

mantle temperature as a control on the time scale of thermal evolution of extensional basins abstract extension of the lithosphere the thermo mechanical boundary layer above the convecting mantle is followed by cooling and subsidence the timescale of oceanic basin subsidence is 100 myr whereas basins of the continental interior often subside continuously for more than 200 myr after rifting using numerical modelling we show how these diverse rifting scenarios are unified when accounting for varying mantle potential temperature at a temperature of 1300 c cooling is plate like with nearly exponential subsidence as observed in oceanic basins at 1200 c subsidence is almost linear and continues for more than 800 myr the longevity of basin subsidence in the continental interior can therefore be explained by variation of mantle temperature an additional cause of the longevity of subsidence is related to the equilibrium thickness of the lithosphere which is increased by the local reduction of heat producing elements due to crustal thinning consequently the thermally mature lithosphere of an extensional basin can be thicker than the surrounding lithosphere this mechanism contributes to additional thermal subsidence compared to the case where the base of the lithosphere is assumed held at a constant depth

continental tectonics and continental kinetics abstract we present a model of continental growth which combines the results of geochemical studies and tectonic ideas about the evolution of continents through geological time the process of continental growth is mainly controlled by surface phenomena continental material is extracted from the mantle along subduction zones at the periphery of oceans and is destroyed in collision zones where it is remobilized and made available for subduction we derive an equation for s the portion of the earth s surface occupied by continents which reads as follows ds dt a 1 s b s coefficients a and b depend on the geometry of plates on their number and on their velocities we assume that they decrease exponentially with time with the same time scale this model satisfies both geochemical and tectonic constraints and allows the integration of several current observations in a single framework

temperature and rheological properties of the mantle beneath the north american craton from an analysis of heat flux and seismic data 1 we combine heat flux data and seismic velocity models for the north american lithosphere to derive constraints on thermal conditions and deformation mechanisms in the underlying convecting mantle local heat flux averages that are not affected by shallow crustal heat production contrasts allow calculation of reliable lithospheric geotherms and uncertainty ranges for consistency with the seismic data the mantle potential temperature beneath north america must lie within a 1290 c 1450 c range close to that for the oceanic mantle sampled at mid ocean ridges the heat flux at the base of the lithosphere varies laterally from 11 3 mw m 2 beneath the 250 km thick archean core of the superior province to 15 3 mw m 2 beneath the thinner younger appalachians province it is shown that the most likely cause of such rates of heat supply into the north american continent is small scale convection in an unstable boundary layer beneath the rigid mechanical lithosphere this allows useful constraints on the mantle rheological properties we show that the most likely deformation mechanism is dislocation creep in wet mantle rocks ranges for the mantle temperature water content and rheological parameters could be tightened very significantly once strong constraints are obtained on radiogenic heat production in the lithospheric mantle

the development of magmatism along the cameroon volcanic line evidence from teleseismic receiver functions 1 the cameroon volcanic line cvl in west africa is a chain of cenozoic volcanism with no clear age progression the reasons for its existence are unclear and the nature of its magmatic plumbing system is poorly understood specifically whether or not the cvl crust presently contains melt and or mafic intrusions as is often observed at hot spots and rifts elsewhere is presently unknown to address this issue we present a receiver function study of crustal structure using earthquakes recorded by the cameroon broadband seismic experiment in regions of the cvl unaffected by cretaceous extension associated with the breakup of gondwana e g the garoua rift vp vs ratios are markedly low network average 1 74 compared to hot spots elsewhere providing no evidence for either melt or cooled mafic crustal intrusions due to cvl magmatism the character of p to s conversions from beneath the cvl also indicates that lower crustal intrusions often termed underplate are not present beneath the region our observations thus corroborate earlier petrological studies that show cvl alkaline magmas fractionate in the mantle not the crust prior to eruption hypotheses for the formation of the cvl should not include markedly elevated upper mantle potential temperatures or large volumes of partial melt both of which can explain observations at hot spots and rifts worldwide the protracted yet sporadic development of small volume alkali melts beneath the cvl may instead be explained better by lower melt volume mechanisms such as shear zone reactivation or lithospheric delamination

coupling patterns between para glacial and permafrost degradation responses in antarctica the recently deglaciated environments in maritime permafrost regions are usually affected by very active paraglacial processes elephant point is an ice free area of 1 16 km2 located in the sw of livingston island south shetland islands antarctica between 1956 2010 the retreat of the ice cap covering most part of this island has exposed 17 3 of the land surface in this peninsula two geomorphological units were identified in this new ice free area a moraine extending from the western to the eastern coastlines and a relatively flat proglacial surface the glacier in 1956 sat in contact with the northern slope of the moraine but its accelerated retreat in parallel to the warming trend recorded in the antarctic peninsula left these areas free of glacier ice subsequently the postglacial evolution was controlled by the relaxation phase typical of paraglacial systems the typology and intensity of geomorphological processes show a significantly different dynamics between the southern and northern slopes of the moraine this pattern is related to the different stage of paraglacial adjustment in both slopes in the southern side on coarser sediments pronival ramparts debris flows and alluvial fans are distributed with a low to moderate activity of slope processes in the northern side mass wasting processes are extremely active on fine grained unconsolidated sediments ice rich permafrost is being degraded by thermokarst processes landslides and mudflows transfer large amounts of sediments down slope the surface affected by retrogressive thaw slumps in the moraine has been quantified in 24 172 m2 which accounts for 9 6 of its surface the abundance of kettle lakes is also indicative of the degradation of the ground ice paraglacial processes are expected to continue in the moraine and proglacial area in the near future although their intensity and duration will depend on the magnitude and rate of future climate trends in the northern antarctic peninsula copyright 2015 john wiley sons ltd

timing the breakup of a proterozoic supercontinent evidence from australian intracratonic basins in the late proterozoic several broad shallow intracratonic depressions appeared across a vast area of central australia the basins which all contain shallow marine to fluvial successions appear to have been tenuously interconnected through much of their history analysis of their fill by means of tectonic subsidence curves suggests that they are the product of two separate and distinct periods of crustal extension one at about 900 ma and a second at about 600 ma these extensional episodes were probably the result of failed rifting events that almost fragmented the australian continent during the late proterozoic the second period of extension almost certainly relates to the breakup of a proterozoic supercontinent the results suggest that sediments preserved in the relatively protected environment of interior basins may provide a more subtle record of major tectonic events than continental margin sequences that were exposed to major continental interactions

a physically based 3 d model of ice cliff evolution over debris covered glaciers we use high resolution digital elevation models dems from unmanned aerial vehicle uav surveys to document the evolution of four ice cliffs on the debris covered tongue of lirung glacier nepal over one ablation season observations show that out of four cliffs three different patterns of evolution emerge i reclining cliffs that flatten during the ablation season ii stable cliffs that maintain a self similar geometry and iii growing cliffs expanding laterally we use the insights from this unique data set to develop a 3 d model of cliff backwasting and evolution that is validated against observations and an independent data set of volume losses the model includes ablation at the cliff surface driven by energy exchange with the atmosphere reburial of cliff cells by surrounding debris and the effect of adjacent ponds the cliff geometry is updated monthly to account for the modifications induced by each of those processes model results indicate that a major factor affecting the survival of steep cliffs is the coupling with ponded water at its base which prevents progressive flattening and possible disappearance of a cliff the radial growth observed at one cliff is explained by higher receipts of longwave and shortwave radiation calculated taking into account atmospheric fluxes shading and the emission of longwave radiation from debris surfaces the model is a clear step forward compared to existing static approaches that calculate atmospheric melt over an invariant cliff geometry and can be used for long term simulations of cliff evolution and to test existing hypotheses about cliffs survival

properties of natural supraglacial debris in relation to modelling sub debris ice ablation as debris covered glaciers become a more prominent feature of a shrinking mountain cryosphere there is increasing need to successfully model the surface energy and mass balance of debris covered glaciers yet measurements of the processes operating in natural supraglacial debris covers are sparse we report measurements of vertical temperature profiles in debris on the ngozumpa glacier in nepal that show i conductive processes dominate during the ablation season in matrix supported diamict ii ventilation may be possible in coarse surface layers iii phase changes associated with seasonal change have a marked effect on the effective thermal diffusivity of the debris effective thermal conductivity determined from vertical temperature profiles in the debris is generally 30 higher in summer than in winter but values depend on the volume and phase of water in the debris surface albedo can vary widely over small spatial scales as does the debris thickness measurements indicate that debris thickness is best represented as a probability density function with the peak debris thickness increasing down glacier the findings from ngozumpa glacier indicate that the probability distribution of debris thickness changes from positively skewed in the upper glacier towards a more normal distribution nearer the terminus although many of these effects remain to be quantified our observations highlight aspects of spatial and temporal variability in supraglacial debris that may require consideration in annual or multi annual distributed modelling of debris covered glacier surface energy and mass balance copyright 2012 john wiley sons ltd

distribution and growth of thaw slumps in the richardson mountains peel plateau region northwestern canada abstract retrogressive thaw slumps are one of the most active geomorphic features in permafrost terrain this study investigated the distribution and growth of thaw slumps in the richardson mountains and peel plateau region northwestern canada using tasseled cap tc trend analysis of a landsat image stack based on the tc linear trend image more than 212 thaw slumps were identified in the study area of which 189 have been active since at least 1985 the surface area of the slumps ranges from 0 4 to 52 ha with 10 slumps exceeding 20 ha the thaw slumps in the region are all situated within the maximum westward extent of the laurentide ice sheet based on relations between frequency distribution of slumps and that of terrain factors in the landscape the slumps are more likely to occur on the ice rich hummocky rolling moraines at elevations of 300 350 m and 450 500 m and along east facing slopes slope aspects of 15 to 180 with gradients of 8 to 12 pixel level trend analysis of the tc greenness transformation in the landsat stack allowed calculating headwall retreat rates for 19 thaw slumps the 20 year average retreat rates 1990 2010 period for 19 slumps ranged from 7 2 to 26 7 m yr 1 with the largest slumps having higher retreat rates at the regional scale the 20 yr headwall retreat rates are mainly related to slope aspect with south and west facing slopes exhibiting higher retreat rates and large slumps appear to be generating feedbacks that allow them to maintain growth rates well above those of smaller slumps overall the findings presented in this study allow highlighting of key sensitive landscapes and ecosystems that may be impacted by the presence and growth of thaw slumps in one of the most rapidly warming region in the arctic

structure of the crust beneath cameroon west africa from the joint inversion of rayleigh wave group velocities and receiver functions the cameroon volcanic line cvl is a major geologic feature that cuts across cameroon from the south west to the north east it is a unique volcanic lineament which has both an oceanic and a continental sector and consists of a chain of tertiary to recent generally alkaline volcanoes stretching from the atlantic island of pagalu to the interior of the african continent the oceanic sector includes the islands of bioko formerly fernando po and sao tome and principe while the continental sector includes the etinde cameroon manengouba bamboutos oku and mandara mountains as well as the adamawa and biu plateaus in addition to the cvl three other major tectonic features characterize the region the benue trough located northwest of the cvl the central african shear zone casz trending n70 degrees e roughly parallel to the cvl and the congo craton in southern cameroon the origin of the cvl is still the subject of considerable debate with both plume and non plume models invoked by many authors e g deruelle et al 2007 ngako et al 2006 ritsema and allen 2003 burke 2001 ebinger and sleep 1998 lee et al 1994 dorbath et al 1986 fairhead and binks 1991 king and ritsema more 2000 reusch et al 2010 crustal structure beneath cameroon has been investigated previously using active stuart et al 1985 and passive dorbath et al 1986 tabod 1991 tabod et al 1992 plomerova et al 1993 source seismic data revealing a crust about 33 km thick at the south western end of the continental portion of the cvl tabod 1991 and the adamawa plateau and thinner crust 23 km thick beneath the garoua rift in the north stuart et al 1985 figure 1 estimates of crustal thickness obtained using gravity data show similar variations between the garoua rift adamawa plateau and southern part of the cvl poudjom et al 1995 nnange et al 2000 in this study we investigate further crustal structure beneath the cvl and the adjacent regions in cameroon using 1 d shear wave velocity models obtained from the joint inversion of rayleigh wave group velocities and p receiver functions for 32 broadband seismic stations from the 1 d shear wave velocity models we obtain new insights into the composition and structure of the crust and upper mantle across cameroon after briefly reviewing the geological framework of cameroon we describe the data and the joint inversion method and then interpret variations in crustal structure found beneath cameroon in terms of the tectonic history of the region less

geoid anomalies and the structure of continental and oceanic lithospheres geoid anomalies in the wavelength range corresponding to spherical harmonies 6 to 30 are used to bring new constraints on the deep structure of the continental lithosphere on the nature of upper mantle seismic anomalies and on the thermal evolution of the oceanic lithosphere for the shallow lithospheric density anomalies considered here the geoid anomaly is shown to follow approximately the moment law derived from isostasy by a procedure of least squares inversion the geoid over old cratons is found to be equal to that over relatively young 50 myr oceanic lithosphere the oceanic geoid as a function of age follows the expected linear trend with flattening at old ages these results are independent of the degree at which the spherical harmonics are truncated if the tectosphere consisted only of a thick thermal boundary layer marked geoid lows with an amplitude larger than 20 m should be present over cratons in disagreement with the present observation the predicted geoid agrees with the observations if the negative density anomalies associated with a depleted peridotitic layer are present to the bottom of the lithosphere this analysis of the geoid confirms the petrological inferences that the deep roots observed seismically in the top upper mantle form a chemically differentiate reservoir the density of these cold roots is similar to that of mantle material elsewhere because the increase in density resulting from their lower temperature is largely balanced by the decrease in density caused by their depletion analysis in wavelength range corresponding to harmonics 6 to 30 in the oceans confirms that the geoid flattens at old ages and thus supports the view that an important amount of heat is transferred at the base of the lithosphere by small scale convection

backwasting rate on debris covered koxkar glacier tuomuer mountain china a physically based energy balance model with improved parameterization of solar radiation for a sloped ice surface has been developed to estimate the backwasting rate of an ice cliff in a debris covered area the model has been tested against observations between 5 august and 5 september 2008 on 38 ice cliffs in the debris covered area of koxkar glacier tuomuer mountain china we calculated that the energy balance model gave a good estimate of the backwasting rates with errors in the range 1 96 cm d 1 and root mean square errors of 0 99 cm d 1 errors arising from setting of surface albedo and turbulent flux parameterization were limited we found that shortwave radiation is the most important heat source for ice cliff ablation contributing about 76 of the total heat available for ice melt while the sensible heat flux provides nearly 24 of the total heat for ice cliff wastage the latent heat flux and net longwave radiation are comparatively small according to the model calculation the mean backwasting rate of ice cliffs in the debris covered area of koxkar glacier is estimated at 7 64 m a 1 when the winter ablation is neglected with this annual backwasting rate and given a mean slope angle of 46 48 the backwasting of ice cliffs produces about 1 60 10 6 m 3 of meltwater accounting for about 7 3 of the total melt runoff from the debris covered area

plate boundary forces at subduction zones and trench arc compression abstract viscoelastic finite element modelling has been used to study the state of stress in the overriding and subducting plates meeting at a subduction zone the subduction fault is included using the dual node technique it is demonstrated that substantial horizontal deviatoric compressive stress occurs in the trench arc region as a result of the downpull of the dense slab and the associated surface depression including the trench and other downflexing of the plates this may be masked at the trench by bending stress it is the lack of significant shearing stress along an unlocked subduction fault in the presence of this compressive stress that gives rise to the slab pull and trench suction plate boundary forces slab pull and trench suction were found to be of comparable magnitude within the range 1 0 to 4 0 10 12 n m in models studied with vertical subduction and there are indications that this may also apply when the slab dips at 45 as a result of viscous flow induced by rollback when the slab dips beneath the arc backarc region it is shown that horizontal deviatoric compression can occur in this region contemporaneous with plate interior tension produced by trench suction this suggests that backarc tension associated with marianas type trench arch systems may be related to the nearly vertical slab whereas backarc compression in the chilean type may result from the low dip and small downpull of the slab it is also shown that successive locking and unlocking of the subduction fault may give rise to large variations of stress in plate interiors

evolution of neoarchaean and proterozoic basins of australia abstract this study correlates basin initiation sedimentation and major tectonic events on an australia wide scale over the neoarchaean and proterozoic an interpretation of a depth to basement surface and faults based on magnetic and gravity data was calibrated with limited well data magnetic depth models geological maps and depth data from cross sections and literature tectonic events are correlated to basin formation and deposition of basin units event maps show the interpreted active faults a principal stress direction has been assigned to basin forming tectonic events the results of this study can be used as a kinematic stratigraphic and temporal framework to examine the development of individual basins and the regional influence of tectonic events tectonic events are grouped into four major time intervals the neoarchaean and early palaeoproterozoic 2775 2000 ma the mid late palaeoproterozoic to early mesoproterozoic 2000 1400 ma the mid late mesoproterozoic 1400 1000 ma and the neoproterozoic 1000 570 ma during the neoarchaean and early palaeoproterozoic basins formed along the southern pilbara and the northern yilgarn margins mainly in response to differential movements between these and other archaean cratons events in the mid late palaeoproterozoic to early mesoproterozoic were related to the assembly and subsequent intracontinental extension of the australian continent mid late mesoproterozoic events were due to interactions between australia and other continents during assembly of the supercontinent rodinia preservation of sediments from this time period is limited neoproterozoic basin forming events mainly related to the break up of rodinia and led to the formation of the centralian superbasin sedimentation in this basin was terminated at the end of the neoproterozoic due to compression associated with the amalgamation of the gondwana supercontinent

the tectonic factor in sea level change a countervailing view it is now demonstrable that many unconformity bounded assemblages of strata the sequences and groupings of sequences of seismic stratigraphy can be correlated subglobally the facies tracts that are identifiable within sequences clearly represent depositional environments that bear a systematic relationship to the strandline and to water depths and thus to sea levels lacking a more suitable explanation for the intercontinental correlation of sequences it has become widely accepted that eustatic changes of sea level are responsible for the interruptions in sedimentation that separate sequences and for the shifts of coastal onlap and of facies tracts implicit in sequence analysis students of cratonic basins are convinced that continental interiors are subject to a tectonic evolution of positive and negative vertical movements and that these control relative sea level and the distribution of sedimentary environments proof of this control is buried hi the sedimentary fills of cratonic basins but it has been difficult to produce such proof in a form digestible by neo neptunist doubters digitization of isopach maps of successive stratal units produces thickness data amenable to numerical study for example the logarithms of thicknesses of a stratal unit commonly exhibit a uniform areal distribution the hallmark of a bivariate normal distribution this revelation makes available a number of gaussian statistical measures as descriptors of time variable basin geometry extraction and analysis of geometric parameters lead to the realization that basin evolution necessarily including relative sea levels and the transgression and progradation of strandlines and sedimentary environments has a strong tectonic component that gives evidence of synchrony of widely separated basins on separate cratons secular variation in radial heat flow leading to episodic inflation and deflation of continents is suggested as a possible globally effective mechanism

implications of deformation following the 2002 denali alaska earthquake for postseismic relaxation processes and lithospheric rheology 1 during the first 2 years following the 2002 mw 7 9 denali alaska strike slip earthquake a large array of global positioning system gps receivers recorded rapid postseismic surface motions extending at least 300 km from the rupture and at rates of more than 100 mm yr in the near field here we use three dimensional 3 d viscoelastic finite element models to infer the mechanisms responsible for these postseismic observations we consider afterslip both from an inversion of gps displacements and from stress driven forward models poroelastic rebound and viscoelastic flow in the lower crust and upper mantle several conclusions can be drawn 1 no single mechanism can explain the postseismic observations 2 significant postseismic flow below a depth of 60 km is required to explain observed far field motions best explained by a weak upper mantle with a depth dependent effective viscosity that ranges from 1019 pa s at the moho 50 km depth to 3 4 1018 pa s at 100 km depth 3 shallow afterslip within the upper crust occurs adjacent to and beneath the regions of largest coseismic slip 4 there is a contribution from deformation in the middle and lower crust from either lower crustal flow or stress driven slip afterslip is preferred over broad viscoelastic flow owing to the existence of seismic velocity discontinuities across the fault at depth though our modeling does not favor either mechanism if the process is viscoelastic relaxation the viscosity is a factor of 3 greater than the inferred mantle viscosity 5 poroelastic rebound probably contributed to the observed postseismic deformation in the immediate vicinity of the denali totschunda junction these conclusions lead us to infer an alaskan mechanical lithosphere that is about 60 km thick overlying a weak asthenosphere and a denali fault that cuts through the entire lithosphere with shear accommodated by faulting in the top 20 km and time dependent aseismic shear below

a lithospheric instability origin for the cameroon volcanic line the cameroon volcanic line cvl is an enigmatic structure that defies common dynamic models of melt generation and volcanic activity on earth there magma generation and intrusion has been sustained for more than 70 myr over a 1600 km long chain straddling the ocean continent boundary with no detectable spatial age progression the chain is nearly perpendicular to the coastline and terminates in a y shaped structure that has not been affected by absolute plate motions implying that the mantle upwelling that feeds magmatic activity is attached to the continent we propose that this form of volcanism is due to a new type of instability that may develop within the subcontinental lithospheric mantle at the edge of a continent laboratory experiments document how lithosphere beneath a continental block of finite size can become unstable due to cooling from above the instability pattern is made of linear upwellings and downwellings that converge radially towards the center of the continent in an outer region and an array of polygonal cells in a central region the pattern is characterized by branching structures that are reminiscent of the strike and y shaped outline of the cvl the instability develops over long timescales with small rates of upwelling and melting and is attached to the continent by construction downwellings adjacent to upwellings induce compression in the crust which may account for deformation in the benue trough just before the onset of cvl magmatism

crustal refraction surveys across the trans hudson orogen williston basin of south central canada eight reversed seismic refraction profiles have been recorded across the phanerozoic williston sedimentary basin that overlies the margins of the superior and wyoming archean cratons and the proterozoic trans hudson orogen these data have been modeled using two dimensional ray tracing techniques to match the times and amplitudes of primary and coherent secondary arrivals within the southern extension of the superior craton margin thompson belt a high velocity 6 4 km s lid overlies a narrow region of high electrical conductivity and a zone of westerly dipping reflections is nearly coincident with a weak velocity discontinuity that seems to delineate the western edge thompson fault of the craton margin complex zones of low velocity material and the north american central plains electrical conductivity anomaly are concluded to lie within the southern extension of the reindeer south indian lakes terrain a component of the trans hudson orogen a high velocity 7 0 km s lower crustal layer which represents a zone of complexity separating normal crust from the upper mantle extends throughout the region the crust is relatively thick with depths to moho varying smoothly between 41 and 48 km there is no evidence in the relatively low resolution seismic refraction data for abrupt changes in the level of the moho but there may be a minor 3 4 km thinning of the crust near the center of the williston basin

the driving mechanism of plate tectonics relation to age of the lithosphere at trenches sea floor age depth and age trench depth relations suggest that the oceanic lithosphere continues to thicken and subside with age beyond 80 ma if so slab pull fs and ridge push fr forces can be calculated from the age of the sea floor at trenches t and if the shear stress acting on the base of the plate increases with absolute velocity a the approximate force balance equation is dva2 svat32 rt where d s and r are constants related to drag slab pull and ridge push respectively the relation between velocity and age is va2 s vat12 r we have tested this model by linear regression using rate and age data for 15 trenches va mm a 2 9 20 32 vat12 26 250 the strength of this correlation r 0 98 is strong evidence in favor of the validity of this simple model these results suggest that ridge push is just sufficient to overcome drag at the base of the plate and does not contribute significantly to the motions of oceanic plates though the value of r is consistent with the motions of plates not attached to subducted slabs

origin of cratonic basins tectonic subsidence curves show that the illinois michigan and williston basins formed by initial fault controlled mechanical subsidence during rifting and by subsequent thermal subsidence thermal subsidence began around 525 ma in the illinois basin 520 460 ma in the michigan basin and 530 500 ma in the williston basin in the illinois basin a second subsidence episode middle mississippian through early permian was caused by flexural foreland subsidence in response to the alleghanian hercynian orogeny resurgent permian rifting in the illinois basin is inferred because of intrusion of well dated permian alnoites such intrusive rocks are normally associated with rifting processes the process of formation of these cratonic basins remains controversial past workers have suggested mantle phase changes at the base of the crust mechanical subsidence in response to isostatically uncompensated excess mass following igneous intrusions intrusion of mantle plumes into the crust or regional thermal metamorphic events as causes of basin initiation cratonic basins of north america europe africa and south america share common ages of formation around 550 to 500 ma histories of sediment accumulation temporal volume changes of sediment fills and common dates of interregional unconformities their common date of formation suggests initiation of cratonic basins in response to breakup of a late precambrian super continent this supercontinent acted as a heat lens that caused partial melting of the lower crust and upper mantle followed by emplacement of anorogenic granites during extensional tectonics in response to supercontinent breakup intrusion of anorogenic granites and other partially melted intrusive rocks weakened continental lithosphere thus providing a zone of localized regional stretching and permitting formation of cratonic basins almost simultaneously over sites of intrusion of these anorogenic granites and other partially melted intrusive rocks

headwall retreat of ground ice slumps banks island northwest territories the relationship between ice face ablation and headwall retreat is described for three ground ice slumps in the sand hills moraine southwest banks island retreat exceeds ablation by a geometric factor that depends on the slope of the ice and the gradient of the surrounding terrain amounts of retreat predicted from ice ablation calculations compare favourably with field measurements except at the start of thermokarst activity in the spring or when the ground ice remains covered by debris for long periods long term headwall retreat for slumps in southern banks island with different orientations and ice contents can be estimated using a model based on meteorological information the model predicts headwall recession of 11 m a for a ground ice slump facing south and 8 8 9 3 m a for one facing north with inputs of a 35 ice face a ground slope of 5 and a volumetric latent heat of 270 mj m3 these predictions are close to the maximum rates of retreat over a 10 year period as measured from air photographs

geophysics a third way to rift continents rifting of continents is usually explained by one of two mechanisms based on effects that originate far from the zone of rifting laboratory experiments show that this geodynamic process can also be caused by local effects

modelling ice cliff backwasting on a debris covered glacier in the nepalese himalaya ice cliffs have been identified as a reason for higher ablation rates on debris covered glaciers than are implied by the insulation effects of the debris this study aims to improve our understanding of cliff backwasting and the role of radiative fluxes in particular an energy balance model is forced with new data gathered in may and october 2013 on lirung glacier nepalese himalaya observations show substantial variability in melt between cliffs between locations on any cliff and between seasons using a high resolution digital elevation model we calculate longwave fluxes incident to the cliff from surrounding terrain and include the effect of local shading on shortwave radiation this is an advance over previous studies that made simplified assumptions on cliff geometry and radiative fluxes measured melt rates varied between 3 25 and 8 6cm d 1 in may and 0 18 and 1 34 cmd 1 in october model results reproduce the strong variability in space and time suggesting considerable differences in radiative fluxes over one cliff in october the model fails to reproduce stake readings probably due to the lack of a refreezing component disregarding local topography can lead to overestimation of melt at the point scale by up to 9

patterns and rates of riverbank erosion involving ice rich permafrost yedoma in northern alaska abstract yedoma a suite of syngenetically frozen silty ice and organic rich deposits with large ice wedges that accumulated during the late pleistocene is vulnerable to thermal degradation and erosion because of the extremely high ice contents this degradation can result in significant surface subsidence and retreat of coastal bluffs and riverbanks with large consequences to landscape evolution infrastructure damage and water quality we used remote sensing and field observations to assess patterns and rates of riverbank erosion at a 35 m high active yedoma bluff along the itkillik river in northern alaska the total volumetric ground ice content including wedge segregated and pore ice was estimated to be 86 the process of riverbank erosion and stabilization include three main stages typical of the areas with ice rich permafrost 1 thermal erosion combined with thermal denudation 2 thermal denudation and 3 slope stabilization active riverbank erosion at the main study site started in july 1995 when the itkillik river changed its channel the total retreat of the riverbank during 1995 2010 within different segments of the bluff varied from 180 to 280 m the average retreat rate for the most actively eroded part of the riverbank was almost 19 m y from august 2007 to august 2011 the total retreat varied from 10 to almost 100 m the average retreat rate for the whole 680 m long bluff was 11 m y for the most actively eroded central part of the bluff 150 m long it was 20 m y ranging from 16 to 24 m y more than 180 000 m3 of ground ice and organic rich frozen soil or almost 70 000 metric tons t of soil solids including 880 t of organic carbon were transported to the river from the retreating bank annually this study reports the highest long term rates of riverbank erosion ever observed in permafrost regions of eurasia and north america

the initiation of rifting at constant tectonic force role of diffusion creep we have investigated the effect of diffusion creep on lithospheric extension using a one dimensional numerical model that assumes a constant force is available to drive extension the model is motivated by the fact that continental areas with average heat flow should be too strong to rift using standard estimates of lithospheric strength however such areas do rift and diffusion creep is a mechanism by which lithosphere may deform at a lower stress level than is required for the usually assumed dislocation creep we consider the evolution of strain rate and temperature at the center of an idealized pure shear rift the strain rate will either increase or decrease with time depending on whether lithospheric weakening or strengthening dominates as extension progresses given an initial thermal condition and an assumed lithospheric rheology the applied force must be such that the initial strain rate is greater than some critical value for weakening to dominate the force at which this condition is met we will term the critical force diffusion creep is more efficient at smaller grain sizes and the model results indicate that mantle grain sizes would have to be less than 1 mm for diffusion creep to significantly reduce the critical force we speculate that during the initial stages of continental rifting grain size reduction and diffusion creep deformation mechanisms may have an important effect on lithospheric strength

upper mantle structure beneath cameroon from body wave tomography and the origin of the cameroon volcanic line the origin of the cameroon volcanic line cvl a 1600 km long linear volcanic chain without age progression that crosses the ocean continent boundary in west central africa is investigated using body wave tomography relative arrival times from teleseismic p and s waves recorded on 32 temporary seismic stations over a 2 year period were obtained using a multichannel cross correlation technique and then inverted for mantle velocity perturbations the p and s wave models show a tabular low velocity anomaly directly beneath the cvl extending to at least 300 km depth with perturbations of 1 0 to 2 0 for p and 2 0 to 3 0 for s the s wave velocity variation can be attributed to a 280 k or possibly higher thermal perturbation if composition and other effects on seismic velocity are negligible the near vertical sides of the anomaly and its depth extent are not easily explained by models for the origin of the cvl that invoke plumes or decompression melting under reactivated shear zones but are possibly consistent with a model invoking edge flow convection along the northern boundary of the congo craton lithosphere if edge flow convection in the sublithospheric upper mantle is combined with lateral flow channeled along a fracture zone beneath the oceanic sector of the cvl then the oceanic sector can also be explained by flow in the upper mantle deriving from variations in lithospheric thickness

interactions of 3d mantle flow and continental lithosphere near passive margins abstract we investigate the time evolution of 3d numerical models of convection in the upper mantle which incorporate both plate motions and thick continental lithosphere the resultant flow in the upper mantle is driven by a combination of bottom heated convection and applied shear velocity boundary conditions that represents plate motion both the plate velocity and continental lithosphere topography are varied in a way to assess the general influence of 3d geometry as well as a more specific tectonic analogue of the australian plate transient thermal events offshore of the trailing passive margin are observed and include plume migration boundary layer instability growth at the passive margin and variations in surface heat flux the geometry and plate velocity both play a significant role in controlling the magnitude and duration of these transient features in particular there are large differences between the different models in the oceanic region downstream of the trailing edge of the continent at near stationary plate speeds cold linear downwelling sheets propagate away from the 3d edge of the continent with regions offshore of the continents central axis localising hot cylindrical upwelling plumes at very fast plate speeds the shear flow is dominated by the plate motions this causes regions neighbouring the trailing edge of the continent to produce broad hot upwellings and the cold linear sheets to migrate around the continent at moderate 2 cm yr plate speeds oceanic lithosphere neighbouring the passive margin along the trailing edge of the continent is buffered by cold downwelling instabilities sinking along the edges of the continental lithosphere such neighbouring regions are subjected to larger heat flux than for regions distant to the passive margin yet also record smaller and less variable vertical surface velocities these dynamics have implications for volcanism and surface topography for which 3d aspects play a significant role

trans hudson orogen and williston basin in montana and north dakota new cocorp deep profiling results cocorp consortium for continental reflection profiling deep reflection profiling across the williston basin and underlying trans hudson orogen shows the following 1 there is no evidence for a precursor rift basin beneath the axis of the williston basin with the exception of small scale structures e g nesson and cedar creek anticlines the basement surface beneath the williston basin is smooth and has a dish shaped geometry 2 crustal penetrating reflections within the eastern trans hudson orogen dip eastward in apparent contradiction to the conventional view that the superior province underthrusts the eastern side of the orogen 3 basement reflections beneath the center of the orogen define a broad crustal scale antiform the axis of which coincides approximately with the depocenter of the overlying williston basin midcrustal reflections defining the crest and western flank of this antiform also coincide generally with the location and shape of the source of the north american central plains conductivity anomaly 4 crustal penetrating reflections beneath the western side of the orogen dip westward supporting the current view that subduction polarity was to the west beneath the wyoming province 5 the eastern edge of the wyoming province is marked by east dipping upper crustal reflections and subhorizontal lower crustal reflections that are truncated by the west dipping reflectivity of the trans hudson orogen

mobility of continental mantle evidence from postseismic geodetic observations following the 1992 landers earthquake the crust around the rupture zone of the 1992 landers earthquake has continued to deform in the years following the earthquake at rates 3 times greater than pre earthquake rates we use a combination of global positioning system gps and synthetic aperture radar insar data collected during a 3 year epoch following the earthquake in order to investigate postseismic mechanisms responsible for the high transient velocities in order to maximize the potential signal from viscoelastic relaxation we evaluate and model postseismic relaxation following the first few months of documented accelerated deformation the combination of gps and insar data allows us to establish viscoelastic relaxation of the lower crust and upper mantle as the dominant postseismic process and to discriminate among possible viscoelastic models the data particularly require the presence of a highly ductile uppermost mantle beneath the central mojave domain with temperature between the wet and dry basalt solidus this is consistent with independent seismic and geochemical inferences of a regionally warm uppermost mantle further consideration of seismic velocity variations in conjunction with faulting patterns within the mojave desert suggests that the primary faulting characteristics of the mojave desert namely the pervasive late cenozoic deformation within the eastern california shear zone versus the near absence of faults in the western mojave domain are controlled by the rheology of the uppermost mantle

assessing ice cliff backwasting and its contribution to total ablation of debris covered miage glacier mont blanc massif italy continuous surface debris cover strongly reduces the ablation of glaciers but high melt rates may occur at ice cliffs that are too steep to hold debris this study assesses the contribution of ice cliff backwasting to total ablation of miage glacier mont blanc massif italy in 2010 and 2011 based on field measurements physical melt models and mapping of ice cliffs using a high resolution 1 m digital elevation model dem short term model calculations closely match the measured melt rates a model sensitivity analysis indicates that the effects of cliff slope and albedo are more important for ablation than enhanced longwave incidence from sun warmed debris or reduced turbulent fluxes at sheltered cliff bases analysis of the dem indicates that ice cliffs account for at most 1 3 of the 1 m pixels in the glacier s debris covered zone but application of a distributed model indicates that ice cliffs account for 7 4 of total ablation we conclude that ice cliffs make an important contribution to the ablation of debris covered glaciers even when their spatial extent is very small

interactions of scales of convection in the earth s mantle abstract the existence of undulations of the geoid gravity and bathymetry in ocean basins as well as anomalies in heat flow point to the existence of small scale convection beneath tectonic plates the instabilities that could develop at the base of the lithosphere are sufficiently small scale 60 ma they form networks that are shaped by the plate evolution slabs plumes and the geometry of continental boundaries plumes impacting the boundary layer from below have a particular influence through rejuvenating the thermal lithosphere they create a wake in which new instabilities form downstream these wakes form channels that are about 1000 km wide and thus are possibly detectable by seismic tomography beneath fast plates cold sinking instabilities are tilted in the direction opposite to plate motion while they sink vertically for slow plates these instabilities are too small to be detected by usual seismic methods since they are about 200 km in lateral scale however this preferred orientation of instabilities below fast plates could produce a pattern of large scale azimuthal anisotropy consistent with both plate motions and the large scale organisation of azimuthal anisotropy obtained from recent surface wave models

upper mantle anisotropy of the borborema province ne brazil implications for intra plate deformation and sub cratonic asthenospheric flow abstract the geological record of the borborema province northeast brazil documents tectonic events that characterised the precambrian formation and mesozoic breakup of gondwana large scale shear zones and associated granitic plutons that developed during the neoproterozoic brasiliano pan african orogeny and major sedimentary basins of mesozoic age indicate significant deformation across the region however whether or not the shear zones resulted from precambrian terrane accretion or are simply the result of episodes of subsequent intra plate deformation is debated also poorly understood is the effect of the thick sao francisco mantle keel on present day asthenospheric flow to address these issues we have performed a teleseismic shear wave splitting study of mantle seismic anisotropy from a new broadband seismograph network centred on the borborema province shear wave splitting parameters t reveal a lack of plate scale anisotropic fabrics associated within the continental interior perhaps supporting models of formation and evolution of the borborema province involving minimal deformation of the lithospheric mantle delamination of anisotropic lithosphere during the development of cenozoic volcanism that eroded older fossil lithospheric fabrics is unlikely because the widespread cenozoic magmatism required to achieve this is absent in the geological record instead the apparently low levels of seismic anisotropy observed in the interior of the borborema province may simply reflect depth dependent anisotropy nulls low t observations may be the subtractive result of orthogonal fast directions in the lithosphere and asthenosphere towards the brazilian coast t 1 s and fast directions are sub parallel to stretching fabrics formed during the opening of the south atlantic this may imply that the mantle lithosphere was deformed but not completely destroyed during gondwana breakup however a more complete backazimuthal coverage of splitting measurements across the region is needed to confirm or refute these hypotheses unambiguously

on the relationships between slab dip back arc stress upper plate absolute motion and crustal nature in subduction zones 1 statistical analysis of modern oceanic subduction zone parameters such as the age of a downgoing plate or the absolute plate motions is performed in order to investigate which parameter controls the dip of a slab and conversely what the influence of slab geometry is on upper plate behavior for that purpose parameters have been determined from global databases along 159 transects from all subduction zones that are not perturbed by nearby collision or ridge plateau seamount subduction on the basis of tomographic images slabs that penetrate through or lie on the 670 km discontinuity are also identified the results of the statistical analysis are as follows 1 back arc stress correlates with slab dip i e back arc spreading is observed for deep dips deeper than 125 km larger than 50 whereas back arc shortening occurs only for deep dips less than 30 2 slab dip correlates with absolute motion of the overriding plate the correlation is even better when the slab lies on or even more penetrates through the 670 km discontinuity 3 slabs dip more steeply by about 20 on average beneath oceanic overriding plates than beneath continental ones 4 slabs dip more steeply on average by about 10 near edges 5 slab dip does not correlate with the magnitude of slab pull the age of subducting lithosphere at the trench the thermal regime of the subducting lithosphere the convergence rate or the subduction polarity east versus west the present study provides evidence that the upper plate absolute motion plays an important role on slab dip as well as on upper plate strain retreating overriding plates are often oceanic ones and thus may partially explain the steeper slab dips beneath oceanic upper plates one can infer that low slab dips correlate well with compression in continental advancing upper plates whereas steep dips are often associated with extension in oceanic retreating upper plates excess weight of old slabs is often counterbalanced by other forces probably asthenospheric in origin such as lateral mantle flow near slab edges or anchor forces to determine slab dip

the thermal regime of a retrogressive thaw slump near mayo yukon territory the development of a retrogressive thaw slump near mayo yukon territory has been traced from initiation by bank erosion 1949 of the stewart river to stabilization in 1993 1994 the stabilized headwall of the slump is 450 m from the river and the slope of the slump floor is 3 a transect of the slump from the river to the stabilized headwall was drilled in july 1995 to determine the extent and rate of permafrost degradation in the slump floor thermistors were placed in access tubes to 12 m depth at five sites four near the transect and one in undisturbed terrain to determine the magnitude of thermal disturbance due to slump development data loggers at the sites recorded the ground temperature at 1 m depth for two years from august 1995 the annual mean ground temperatures measured by the data loggers varied between 1 2 and 1 8 c in the slump compared with 2 4 c in undisturbed ground indicating a disturbance of about 4 c due to slumping the depth of thaw in the slump floor is consistent wit

india asia collision and the cenozoic slowdown of the indian plate implications for the forces driving plate motions the plate motion of india changed dramatically between 50 and 35 ma with the rate of convergence between india and asia dropping from 15 to 4 cm yr this change is coincident with the onset of the india asia collision and with a rearrangement of plate boundaries in the indian ocean on the basis of a simple model for the forces exerted upon the edges of the plate and the tractions on the base of the plate we perform force balance calculations for the precollision and postcollision configurations we show that the observed euler poles for the indian plate are well explained in terms of their locations and magnitudes if 1 the resistive force induced by mountain building in the himalaya tibet area is 5 6 10 12 n m 2 the net force exerted upon the indian plate by subduction zones is similar in magnitude to the ridge push force 2 5 10 12 n m and 3 basal tractions exert a resisting force that is linearly proportional to the plate velocity in the hot spot reference frame the third point implies an asthenospheric viscosity of 2 5 10 19 pa s assuming a thickness of 100 150 km synthetic euler poles show that crustal thickening in the tibetan plateau was the dominant cause of the cenozoic slowdown of the indian plate

seismic anisotropy beneath the incipient okavango rift implications for rifting initiation abstract this study represents the first shear wave splitting investigation of the okavango rift zone orz an incipient continental rift belonging to the east african rift system in northern botswana analysis of broadband seismic data recorded along a 750 km long profile of 22 stations traversing the orz and adjacent congo and kalahari cratons and several precambrian orogenic zones reveals dominantly ne sw fast orientations which are parallel to both the absolute plate motion direction based on the nnr nuvel 1a model and the trend of most tectonic boundaries including that of the orz spatial coherence analysis of the splitting parameters and correspondence between the observed fast orientations and the trend of tectonic features indicate that the main source of observed anisotropy is most likely in the upper asthenosphere probably due to simple shear associated with the relative movement of the lithosphere against the asthenosphere the presence of consistently rift parallel fast orientations and normal splitting times in the orz and most parts of southern africa implies that neither an upper mantle plume nor small scale convection is the dominant source for rift initiation and development the first shear wave splitting measurements in the vicinity of the orz favor a model in which continental rifting develops in response to intra plate relative movement of continental blocks along zones of weakness produced by ancient tectonic events

fifty years of coastal erosion and retrogressive thaw slump activity on herschel island southern beaufort sea yukon territory canada abstract patterns of coastal erosion in the arctic differ dramatically from those coasts in more temperate environments thick sea ice and shore fast ice limit wave based erosional processes to a brief open water season however despite this permafrost coasts containing massive ice ice wedges and ice bonded sediments tend to experience high rates of erosion these high rates of erosion reflect the combined thermal mechanical processes of thawing permafrost melting ground ice and wave action climate change in the arctic is expected to result in increased rates of coastal erosion due to warming permafrost increasing active layer depths and thermokarst rising sea levels reduction in sea ice extent and duration and increasing storm impacts with the most ice rich permafrost in the canadian arctic the southern beaufort sea coast between the tuktoyaktuk peninsula and the alaskan border is subject to high rates of erosion and retrogressive thaw slump activity under many climate change scenarios this area is also predicted to experience the greatest warming in the canadian arctic this paper presents results of a remote sensing study on the long term patterns of coastal erosion and retrogressive thaw slump activity for herschel island in the northern yukon territory using orthorectified airphotos from 1952 and 1970 and an ikonos image from 2000 corrected with control points collected by kinematic differential global positioning system and processed using softcopy photogrammetric tools mean coastal retreat rates of 0 61 m yr and 0 45 m yr were calculated for the periods 1952 1970 and 1970 2000 respectively the highest coastal retreat rates are on north west facing shorelines which correspond to the main direction of storm related wave attack during the period 1970 2000 coastal retreat rates for south to south east facing shorelines displayed a distinct increase even though these are the most sheltered orientations however south to south east facing shorelines correspond to the orientations where the highest densities of retrogressive thaw slumps are observed differences in rates of headwall retreat of retrogressive thaw slumps and coastal erosion results in the formation of larger thermokarst scars and the development of polycyclic thaw slumps on south to south east exposures the number and the total area of retrogressive thaw slumps increased by 125 and 160 respectively between 1952 and 2000 as well the proportion of active retrogressive thaw slumps increased dramatically polycyclic retrogressive thaw slumps appear to develop in a periodic fashion related to retrogressive thaw slump stage and maximum inland extent

calculating ice melt beneath a debris layer using meteorological data generalized numerical models of sub debris ice ablation are preferable to empirical approaches for predicting runoff and glacier response to climate change as empirical methods are site specific and strongly dependent upon the conditions prevailing during the measurement period we present a modified surface energy balance model to calculate melt beneath a surface debris layer from daily mean meteorological variables despite numerous simplifications the model performs well and modelled melt rates give a good match to observed melt rates suggesting that this model can produce reliable estimates of ablation rate beneath debris layers several decimetres thick this is a useful improvement on previous models which are inappropriate for thick debris cover

climate driven thaw of permafrost preserved glacial landscapes northwestern canada ice marginal glaciated landscapes demarcate former boundaries of the continental ice sheets throughout circumpolar regions permafrost has preserved relict ground ice and glacigenic sediments delaying the sequence of postglacial landscape change that transformed temperate environments millennia earlier here we show that within 7 10 6 km 2 of glaciated permafrost terrain extensive landscapes remain poised for major climate driven change across northwestern canada 60 100 km wide concentric swaths of thaw slump affected terrain delineate the maximum and recessional positions of the laurentide ice sheet these landscapes comprise 17 of continuous permafrost terrain in a 1 27 10 6 km 2 study area indicating widespread preservation of late pleistocene ground ice these thaw slump relict ground ice and glacigenic terrain associations are also evident at the circumpolar scale recent intensification of thaw slumping across northwestern canada has mobilized primary glacial sediments triggering a cascade of fluvial lacustrine and coastal effects these geologically significant processes highlighted by the spatial distribution of thaw slumps and patterns of fluvial sediment mobilization signal the climate driven renewal of deglaciation and postglacial permafrost landscape evolution