present day stress field changes along the baikal rift and tectonic implications intraplate extension in a frame of a global compressional stress field seems linked to local lithospheric perturbations lithospheric thinning or thickening able to modify the resulting state of stress zoback 1992 the baikal rift zone brz siberia is located north of the india asia collision zone and exhibits no direct communication with any oceanic domain it can thus be fully considered as an area of continental extension dominated by the global compressional intraplate stress field resulting from plate driving forces in order to address the problem of its dynamics and kinematics and their links with the india asia collision a comprehensive stress tensor analysis is presented based on 319 focal mechanisms of earthquakes located along the whole baikal rift the stress field is varying at different scales of observation when looking at central asia several thousands kilometers the maximum horizontal stress shmax directions remain rather constant with a fan shape geometry when the tectonic regime goes from compressional himalayas to extensional baikal when observing the baikal rift about 1000 km long clear variations of the stress regime are observed from an extensional regime in the central part of the rift to wrench ones in its northern and southern ends finally at the scale of 100 km systematic shmax reorientations occur close to major rift faults we thus infer that the interaction between collisional processes and inherited structures may have a strong influence on rift dynamics we then use computed stress tensors to predict slip vectors on major rift faults deformation patterns show two distinct parts of the rift the south baikal rift sbr is characterized by a constant trending around n100 e slip vector meanwhile the north baikal rift nbr exhibits a complex block rotation behavior involving at least three crustal blocks we propose to interpret these surficial structures and motions as the result of an interaction between the regional compression coming from the india asia collision and the geometry of the hardly deformable siberian platform this particular setting can explain most of the surficial deformation patterns which suggest a large scale cracking of the lithosphere in the baikal region other possible sources of stress could also be considered like deep mantellic upwelling or trench suction linked to the pacific subduction

did the growth of tibetan topography control the locus and evolution of tien shan mountain building the tien shan in central asia are the pre eminent example of intraplate mountain building and are typically considered an indirect result of the india eurasia collision via the northward indentation of the relatively strong precambrian lithosphere of the tarim plate into the relatively weak paleozoic lithosphere of the tien shan region this hypothesis fails to fully explain the concentration of tien shan mountain building along a tibet parallel axis located 500 700 km from tibet and also fails to explain the close synchronization of tien shan mountain building events with mountain building in tibet i suggest that bending stresses from lithospheric flexure in central asia exerted a critical influence on the locus and evolution of the tien shan models of paleogene to middle miocene flexure indicate that the growth of northeast tibet may have superimposed a strong bending stress field on the regional stress field the resultant stress field is characterized by strong topography normal tensile stress in the tarim basin and an 15 increase in compressive stress in the vicinity of the tien shan this stress field severely reduces the likelihood of mountain building south of the tien shan and increases the likelihood of mountain building within the tien shan this result suggests that the generation of bending stresses from lithospheric flexure is a viable and important mechanism for the propagation of stress and hence deformation into continental interiors

tectonic feedback intraplate orogeny and the geochemical structure of the crust a central australian perspective the geological record of intraplate deformation in central australia implies that past tectonic activity basin formation deformation and erosion has modulated the response of the lithosphere during subsequent tectonic activity in particular there is a correspondence between the localization of deformation during intraplate orogeny and the presence of thick sedimentary successions in the preserved remnants of a formerly widespread intracratonic basin this behaviour can be understood as a kind of tectonic feedback effected by the longterm thermal and mechanical consequences of changes in the distribution of heat producing elements induced by earlier tectonism from a geochemical point of view one of the most dramatic effects of intraplate orogeny in central australia has been the exposure in the cores of the orogens of deep crustal rocks largely depleted in the heat producing elements the geochemical structuring of the crust associated with the erosion of the heat producing upper crust resulted in long term cooling of the deep crust and upper mantle with associated lithospheric strengthening this is illustrated here by mapping the consequences of deformation and associated tectonic responses onto the h qc plane where h is the characteristic length scale for heat production distribution and qc is the total crustal heat production because rates of intraplate deformation in central australia appear to be much slower than that typical of plate margin orogens it is possible that the ongoing geochemical structuring of the crust has played an important role in terminating intraplate orogeny in central australia by providing a thermal lock the diagnostic geophysical signature of this lock may be the extraordinary gravity anomalies of the central australian intraplate orogens central australia provides an intriguing record of intraplate deformation resulting in the development of a number of extraordinary orogens that expose deep crustal rocks in their cores forman 1971 teyssier 1985 goleby et al 1989 the spatial distribution of intraplate deformation during the alice springs and petermann orogenies suggests that the mechanical response of this region has varied through time consequently this response is important for our understanding of the processes controlling reactivation in intraplate settings in previous contributions it has been argued that the distribution of deformation in central australia has been strongly influenced by prior tectonic processes such as basin formation and erosion sandiford hand 1998a hand sandiford 1999 this paper focusses on the way in which intraplate deformation modifies the geochemical structure of the crust in this region in particular the distribution of heat sources recognizing that the distribution of heat sources provides a first order control on the thermal structure of the crust the consequences of intraplate deformation for the longterm thermal and mechanical evolution of the crust is then explored our aim is to demonstrate the relevance of a novel style of tectonic feedback which seems to have modulated the longterm tectonic behaviour of the central australian region and which may be relevant to continental interiors in general the notion of tectonic feedback is compatible with a highly temperature dependent lithospheric rheology as suggested by many experimental studies of natural rocks such feedback has profound implications for the longterm tectonic evolution of continental interiors particularly their potential for reactivation the record of intraplate deformation in

active thrusting and folding along the northern tien shan and late cenozoic rotation of the tarim relative to dzungaria and kazakhstan we have studied geometries and rates of late cenozoic thrust faulting and folding along the northern piedmont of the tien shan mountain belt west of urumqi where the m 8 3 manas earthquake occurred on december 23 1906 the northern range of the tien shan rising above 5000 m overthrusts a flexural foredeep filled with up to 11 000 m of sediment of the dzungarian basement our fieldwork reveals that the active thrust reaches the surface 30 km north of the range front within a 200 km long zone of neogene quaternary anticlines fault scarps are clearest across inset terraces within narrow valleys incised through the anticlines by large rivers flowing down from the range in all the valleys the scarps offset vertically the highest terrace surface by the same amount 10 2 0 7 m inferring an early holocene age 10 2 kyr for this terrace which is continuous with the largest recent fans of the piedmont yields a rate of vertical throw of 1 0 0 3mm yr on the main active thrust at the surface a quantitative morphological analysis of the degradation of terrace edges that are offset by the thrust corroborates such a rate and yields a mass diffusivity of 5 5 2 5 m 2 kyr a rather fresh surface scarp 0 8 0 15 m high that is unlikely to result from shallow earthquakes with 6 m 7 in the last 230 years is visible at the extremities of the main fold zone we associate this scarp with the 1906 manas earthquake and infer that a structure comprising a deep basement ramp under the range gently dipping flats in the foreland and shallow ramps responsible for the formation of the active fault propagation anticlines could have been activated by that earthquake if so the return period of a 1906 type event would be 850 380 years the small size of the scarp for an earthquake of this magnitude suggests that a large fraction of the slip at depth 2 3 is taken up by incremental folding near the surface comparable earthquakes might activate flat detachments and ramp anticlines at a distance from the front of other rising quaternary ranges such as the san gabriel mountains in california or the mont blanc aar massifs in the alps we estimate the finite cenozoic shortening of the folded dzungarian sediments to be of the order of 30 km and the cenozoic shortening rate to have been 3 1 5 mm yr assuming comparable shortening along the tarim piedmont and minor additional active thrusting within the mountain belt we infer the rate of shortening across the tien shan to be at least 6 3 mm yr at the longitude of manas 85 5 e a total shortening of 125 30 km is estimated from crustal thickening assuming local airy isostatic equilibrium under the same assumption serial n s sections imply that cenozoic shortening across the belt increases westwards to 203 50 km at the longitude of kashgar 76 e as reflected by the westward increase of the width of the belt this strain gradient implies a clockwise rotation of tarim relative to dzungaria and kazakhstan of 7 2 5 around a pole located near the eastern extremity of the tien shan west of hami 96 e 43 5 n comparable to that revealed by paleomagnetism between tarim and dzungaria 8 6 8 7 a 6 mm yr rate of shortening at the longitude of manas would imply a rate of rotation of 0 45 m y and would be consistent with a shortening rate of 12 mm yr north of kashgar taking such values to be representative of late cenozoic rates would place the onset of reactivation of the tien shan by the india asia collision in the early to middle miocene 16 22 9 m y in accord with the existence of particularly thick late neogene and quaternary deposits such reactivation would thus have started much later than the collision roughly at the time of the great mid miocene changes in tectonic regimes denudation and sedimentation rates observed in southeast asia the himalayas and the bay of bengal and of the correlative rapid change in seawater sr isotopic ratio 20 to 15 ma like these other changes the rise of the tien shan might be a distant consequence of the end of indochina s escape

dynamics of the india eurasia collision zone we present simple new dynamic calculations of a vertically averaged deviatoric stress field over a depth average of 100 km for asia from geodetic geologic topographic and seismic data a first estimate of the minimum absolute magnitudes and directions of vertically averaged deviatoric stress is obtained by solving force balance equations for deviatoric stresses associated with gravitational potential energy differences within the lithosphere plus a first order contribution of deviatoric stresses associated with stress boundary conditions this initial estimate of the vertically averaged deviatoric stress field is obtained independent of assumptions about the rheology of the lithosphere absolute magnitudes of vertically averaged deviatoric stresses vary between 5 and 40 mpa assuming bulk viscous behavior for the lithosphere the magnitudes of deviatoric stresses together with the magnitudes of strain rates inferred from quaternary fault slip rate and gps data yield vertically averaged effective viscosities for tibet of 0 5 5 1022 pa s compared with 1 2 5 1023 pa s in more rigid areas elsewhere in the region a forward modeling method that solves force balance equations using velocity boundary conditions allows us to refine our estimates of the vertically averaged effective viscosity distribution and deviatoric stress field the total vertically averaged deviatoric stress and effective viscosity field are consistent with a weak lower crust in tibet they are consistent with some eastward motion of tibet and south china lithosphere relative to eurasia and they confirm that gravitational potential energy differences have a profound effect on the spatially varying style and magnitude of strain rate around the tibetan plateau our results for the vertically averaged deviatoric stress argue for a large portion of the strength of the lithosphere to reside within the seismogenic upper crust to get deviatoric stress magnitudes there to be as high as 100 300 mpa in accord with laboratory and theoretical friction experiments indicating that stress drops in earthquakes are small fractions of the total deviatoric stress

coupled crust mantle dynamics and intraplate tectonics two dimensional numerical and three dimensional analogue modeling 1 tectonic deformation of some intraplate regions may be caused by the rayleigh taylor rt instability of dense subcrustal lithosphere mantle lithosphere as it descends into the mantle we report on a series of scaled three dimensional 3 d analogue and 2 d numerical experiments of coupled crust mantle dynamics that study such a process the models investigate the effects of two geometries of rt instability linear versus axisymmetric and use different rheological stratifications of the crust the growth of the rt instability is strongly influenced by crustal rheology while rt growth rates are not greatly influenced by the geometry of the mantle lithosphere instability itself the surface is experiments having a viscous only crust are characterized by significant crustal contraction and thickening above the mantle downwelling and extension and thinning in adjacent regions in experiments with a brittle upper crust the surface deformation is much more subdued however strong mantle flow induced deformation still occurs in the ductile lower crust beneath the quiescent brittle upper crust when the lower crust is relatively strong localized styles of extensional and contractional structure develop in the brittle upper crust owing to the greater degree of coupling between upper crust and the mantle instability in all the experiments a portion of the ductile lower crustal material is entrained within the downgoing rt instability deep into the mantle the interplay between dynamic topography and crustal thickening thinning induced by the underlying mantle flow governs various time dependent phases of subsidence and uplift of the model surface the experiments help to account for a number of first order tectonic behaviors of the lithospheres of earth and other terrestrial planets for example intraplate orogens and basins complex localized tectonic structure in intraplate settings and deep crustal seismic fabrics may arise as the variable crustal response to underlying mantle lithosphere rt instabilities

magma compensated crustal thinning in continental rift zones continental rifts are elongated depressions in the earth s surface often more than 1 000 km long 20 km wide and 10 km deep filled with sediments and volcanic rocks conventional models of rift zones include three characteristic features surface manifestation as an elongated topographic trough moho shallowing due to crustal thinning and reduced seismic velocity in the uppermost mantle hans thybo and christoffer nielsen present a new seismic profile that demonstrates that the baikal rift zone in siberia displays only one of these characteristic features the elongated surface trough there is a localized zone in the lower crust with exceptionally high seismic velocity and high reflectivity the expected moho uplift appears to have been compensated by magmatic intrusion into the lower crust producing the observed high velocity zone this finding points to a previously unknown role for magmatism in rifting this study presents a seismic profile across southern lake baikal in siberia demonstrating that only a surface manifestation is observed there these findings may point toward a new role for magmatism in the rifting process continental rift zones are long narrow tectonic depressions in the earth s surface where the entire lithosphere has been modified in extension1 rifting can eventually lead to rupture of the continental lithosphere and creation of new oceanic lithosphere or alternatively lead to formation of wide sedimentary basins around failed rift zones conventional models of rift zones include three characteristic features surface manifestation as an elongated topographic trough moho shallowing due to crustal thinning and reduced seismic velocity in the uppermost mantle due to decompression melting or heating from the earth s interior2 3 4 here we demonstrate that only the surface manifestation is observed at the baikal rift zone whereas the crustal and mantle characteristics can be ruled out by a new seismic profile across southern lake baikal in siberia instead we observe a localized zone in the lower crust which has exceptionally high seismic velocity and is highly reflective we suggest that the expected moho uplift was compensated by magmatic intrusion into the lower crust producing the observed high velocity zone this finding demonstrates a previously unknown role for magmatism in rifting processes with significant implications for estimation of stretching factors and modelling of sedimentary basins around failed rift structures

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

regional primitive equation studies of the gulf stream meander and ring formation region abstract primitive equation and quasi geostrophic eddy resolving open ocean models are used for hindcast studies in the gulf stream meander and ring formation region a feature model approach is used to initialize the models based on one month of observations during november to december 1984 flat bottom and topographic calculations are carried out using an initial gulf stream velocity profile based on the pegasus dataset all of the major events observed in the upper thermocline are reproduced by both numerical models the addition of bottom topography is shown to significantly alter the character of the deep velocity fields large basin scale circulations found near the bottom in both flat bottom calculations were replaced by energetic jets and eddies associated with the dominant spatial scales of the bottom topography use of the quasi geostrophic model to dynamically adjust the initial conditions for the primitive equation model is shown to reduce the growth of large scale meanders on time scales o

the kapuskasing uplift a geological and geophysical synthesis over the past decade the kapuskasing uplift has been the subject of intense geological and geophysical investigation as lithoprobe s window on the deep crustal structure of the archean superior province enigmatic since its recognition as a positive gravity anomaly in 1950 the structure has been variably interpreted as a suture rift transcurrent shear zone or intracratonic thrust diverse studies including geochronology geothermobarometry and various geophysical probes provide a comprehensive three dimensional image of archean 2 75 2 50 ga crustal evolution and proterozoic 2 5 1 1 ga cooling and uplift the data favour an interpretation of the structure as an intracratonic uplift related to hudsonian collision eastward across the southern kapuskasing uplift erosion levels increase from 10 km in the michipicoten greenstone belt through the wawa gneiss domain 10 20 km into granulites 20 30 km of the kapuskasing structural zone juxtaposed against the low grade swayze greenstone belt al

control of the rheological structure of the lithosphere on the inward migration of tectonic activity during continental rifting analogue models are used to analyse the parameters controlling the evolution of extensional deformation in continental rifts models are deformed in a centrifuge and simulate the continental lithosphere floating and extending above a low viscosity asthenosphere model results reproduce the typical evolution of deformation during continental narrow rifting with early activation of large boundary faults and basin subsidence followed by their abandonment and localization of tectonic activity in internal faults near the centre of the rift the experiments document the strong influence exerted by the thickness of both brittle and ductile crustal layers and syn rift sediment accumulation on the evolution of deformation namely on the amount of bulk extension preceding inward fault migration thin upper and or lower crust and absent or low syn rift sedimentation promote a rapid abandonment of boundary faults and a transition to in rift fault development for low amounts of extension conversely thick upper and or lower crust and high syn rift sediment accumulation favour prolonged slip on boundary faults and delayed development of internal faulting the experimental results suggest that the inward migration of faulting during extension of continental lithosphere results from the interplay between the ductile stresses acting at the base of the upper crust and the total resistance of this brittle layer

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

india asia convergence driven by the subduction of the greater indian continent the most spectacular example of plate convergence on earth was the motion of the indian plate towards eurasia and the subsequent collision density estimates of the greater indian continent after its upper crust is scraped off at the himalayan front suggest that this continental plate is readily subductable potentially explaining why the convergence did not halt on collision the most spectacular example of a plate convergence event on earth is the motion of the indian plate towards eurasia at speeds in excess of 18 cm yr 1 ref 1 and the subsequent collision continental buoyancy usually stalls subduction shortly after collision as is seen in most sections of the alpine himalayan chain however in the indian section of this chain plate velocities were merely reduced by a factor of about three when the indian continental margin impinged on the eurasian trench about 50 million years ago plate convergence accompanied by eurasian indentation persisted throughout the cenozoic era1 2 3 suggesting that the driving forces of convergence did not vanish on continental collision here we estimate the density of the greater indian continent after its upper crust is scraped off at the himalayan front and find that the continental plate is readily subductable using numerical models we show that subduction of such a dense continent reduces convergence by a factor similar to that observed in addition an imbalance between ridge push and slab pull can develop and cause trench advance and indentation we conclude that the subduction of the dense indian continental slab provides a significant driving force for the current india asia convergence and explains the documented evolution of plate velocities following continental collision

controls on the locus of intraplate deformation in central australia abstract the locus of intraplate deformation in central australia changed from the presently preserved southern margin of the amadeus basin during the late neoproterozoic to early phanerozoic petermann orogeny to the northern margin in the late phanerozoic alice springs orogeny immediately prior to each event the thickness of the amadeus basin sediments varied from as little as 1 5 km in the central parts of the basin to as much as 7 km in the vicinity of the presently preserved margins of the basin with the locus of deformation mimicking the locus of maximum sedimentary thicknesses at the onset of each orogenic event as such these orogenies represent extreme examples of basin inversion a process observed in many intracratonic basins worldwide we show that differential burial of the central australian proterozoic basement complexes beneath the amadeus basin is capable of producing variations in moho temperature of up to 30 c prior to the petermann orogeny and up to 110 c prior to the alice springs orogeny for a brace goetze model of lithospheric rheology the variations in moho temperature equate to variations in effective lithospheric strain rate of 1 4 orders of magnitude implying that variations in thickness of the sedimentary blanket may have played a primary role in localising phanerozoic intraplate deformation in central australia an appealing aspect of this model lies in its corollary that the removal of the sedimentary blanket during denudation of the orogen will be accompanied by dramatic cooling and strengthening of the lithosphere this may provide a plausible explanation for the long term persistence of the extraordinary gravity anomalies 150 mgal developed during these central australian intraplate orogenies

crustal anisotropy from moho converted ps wave splitting analysis and geodynamic implications beneath the eastern margin of tibet and surrounding regions abstract in addition to crustal thickening distinctly different mechanisms have been suggested to accommodate the huge convergences caused by the continental collision between india and eurasia as the transition zone between the two grand tectonic domains of asia the tethys and the pacific east tibet and its surrounding regions are the ideal places to study continental deformation pervasive rock deformation may produce anisotropy on the scale of seismic wavelengths thus seismic anisotropy provides insight into the deformation of the crust and mantle beneath tectonically active domains in this study we calculated receiver function pairs of radial and transverse components at 98 stations located in sichuan and yunnan provinces china we selected 7423 pairs with high signal to noise ratio snr and unambiguous moho converted ps phases pms to measure the pms splitting owing to the crustal anisotropy both the crustal thickness and the average crustal vp vs ratio were calculated simultaneously by the h k stacking method the geodynamic implications were also investigated in relation to surface geological features gps velocities absolute plate motion apm sks skks splitting and other seismological observations in addition to the fast polarization directions fpds of the crustal anisotropy we observed a conspicuous sharper clockwise rotation around the eastern himalayan syntaxis than was revealed by gps velocities the distributed fpds within and near the main active fault zones also favored the directions parallel to the faults this implied that the deformation of a continuous medium revealed by gps motions is a proxy for the deformation of the brittle shallow crust only while the main active faults and the deep crustal interiors both play important roles in the deep deformation our results suggest that the deformation between the crust and upper mantle within the northernmost section of the indochina block is decoupled due to the large difference in the directions between the observations related to the crust gps and crustal anisotropy and mantle apm and mantle anisotropy focusing on the transition zone between the plateau and the south china and indochina blocks we suggest that the motion of the central yunnan sub block is a southeastward extrusion by way of tectonic escape there is less deformation in the deep crust and the motion is controlled by the active boundary faults of the ailaoshan red river shear zone to the west and the xianshuihe xiaojiang fault to the east the lower crustal flow within the plateau southeastward reached the lijiang xiaojinhe fault but further south it was obstructed by the central yunnan sub block

continental scale rheological heterogeneities and complex intraplate tectono metamorphic patterns insights from a case study and numerical models abstract continental plates are built over long periods of time through successive extensional and compressional cycles they are therefore rheologically heterogeneous this heterogeneity should significantly influence the mechanical response of the continental lithosphere during collision processes the study of the neoproterozoic borborema shear zone system of northeast brazil highlights a systematic link between marked changes in its tectono metamorphic pattern and the pre existing structure of the plate that is characterized by juxtaposition of continental domains either comprising an old basement palaeo to eoproterozoic or accreted during an extensional event between 1 0 and 0 7 ga in neoproterozoic time when the shear zone system was developed these domains displayed different geotherms and lithospheric thicknesses and therefore contrasted rheological behaviours we use numerical models simulating the mechanical evolution of a continental plate comprising multiple thermally induced rheological heterogeneities submitted to compression to investigate how these heterogeneities may affect strain localization and the distribution of deformation regimes and vertical strain within the plate from the very beginning of the deformation weak and stiff heterogeneities induce strain localization due to a lower initial effective viscosity or to stress concentrations at their tips respectively shear zones propagate from the heterogeneities and finally coalesce forming a network of high strain zones bounding almost underformed blocks within this network shear zones transfer strain between the different heterogeneities and model boundaries the evolution of the system depends essentially on the geometrical distribution of heterogeneities and on their strength contrast relative to the surrounding lithosphere the resulting finite strain field is heterogeneous and displays rapid lateral variations in vertical and or rotational deformation such a heterogeneous strain distribution may induce contrasted magmatic metamorphic and uplift evolutions within an orogenic belt as observed in the borborema shear zone system and other collisional belts

ediacaran intracontinental channel flow migmatitic lower crustal rocks in the ediacaran intracontinental petermann orogen central australia are bounded by regional scale low angle shear fabrics that record opposing shear senses new sensitive high resolution ion microprobe shrimp titanite geochronology suggests that the contrasting shear systems developed synchronously we argue that the macro scopic structural and metamorphic architecture satisfies the diagnostic criteria outlined by recently proposed channel flow models and thus the remarkably well preserved petermann orogen is potentially an example of ancient intracontinental channel flow

crust and mantle structure across the basin and range colorado plateau boundary at 37 n latitude and implications for cenozoic extensional mechanism we present new evidence on the seismic velocity and density of the crust and upper mantle along a 200 km long transect across the eastern basin and range and western colorado plateau at 37 n latitude receiver functions computed from the p waveforms recorded with 10 portable broadband stations deployed along the transect were used to estimate crustal thickness variations the crust is 30 35 km thick within the eastern basin and range and increases over a distance of 100 km at the western edge of the colorado plateau reaching a maximum of approximately 45 km east of the hurricane fault the timing of crustal multiples within the reciever functions were used to estimate the v p v s of the crust along the profile and we found that the western colorado plateau crust is characterized by a high poisson s ratio 0 28 0 29 indicative of a crust with an average mafic composition we estimated the upper mantle lid thickness along our profile based on teleseismic p wave travel times and constraints provided by gravity data our data and available geophysical constraints are most consistent with a lithosphere that thickens from an average thickness of 60 km beneath the basin and range to 100 km beneath the western colorado plateau although the basin and range lithosphere may have significant thickness variations the thick strong mafic crust and thicker mantle lid under the colorado plateau can account for the relative geologic stability and subdued magmatism of the plateau during laramide compression and cenozoic extension compared to surrounding regions the crustal and lithospheric thinning across the tectonic boundary occurs over a short distance 100 km suggesting it is a geologically young feature produced by a predominantly mechanical response to late cenozoic extension our new lithosphere model at 37 n latitude is consistent with the existence in early cenozoic time of a flat subducted slab at 100 km depth and a relict sevier laramide 50 60 km thick crustal welt and 60 100 pure shear extension values of 1 6 2 0 during the late cenozoic

geophysical evidence for thick skinned crustal deformation in central australia until now there has been no clear evidence from deep seismic reflection profiles of crustal faults that displace the moho new data from central australia show such a structure in a region characterized by large thrust structures large gravity anomalies and abrupt changes in teleseismic travel times here the principal thrust the redbank deformed zone cuts through the crust and the moho is significantly displaced across it the geological and geophysical evidence leads to a model of thick skinned crustal deformation which occurred in an intra cratonic environment in late devonian time

the flow of surface derived fluids through alice springs age middle crustal ductile shear zones reynolds range central australia the south east reynolds range central australia is cut by steep north west trending alice springs age c 334 ma shear zones that are up to hundreds of metres wide and several kilometres long with reverse senses of movement amphibolite facies 550 600 c 500 600 mpa shear zones cut metapelites while greenschist facies shear zones 420 535 c 400 650 mpa cut metagranites the sheared rocks commonly underwent metasomatism implying that the shear zones were the pathways of significant fluid flow altered granites within greenschist facies shear zones have gained si and k but lost ca and na relative to their unsheared counterparts suggesting that the fluid flowed down temperature and hence probably upward through the shear zones time integrated fluid fluxes calculated from silica addition are up to 2 1 1010 mol m 2 c 4 2 105 m3 m 2 similar time integrated fluid fluxes are also estimated from changes in k and na the sheared granitic rocks locally have 18o values as low as 0 which is much lower than the 18o values of the adjacent unsheared granites 7 to 9 implying that the fluid which flowed through these shear zones was derived from the surface for the estimated time integrated fluid fluxes the fluids would be able to retain their isotopic signature for many tens to hundreds of kilometres the flow of surface derived fluids into the ductile middle crust with subsequent expulsion upwards through the shear zones may have been driven by seismic activity accompanying the alice springs deformation

accommodating large scale intracontinental extension and compression in a single stress field a key example from the baikal rift system the baikal rift system in southern siberia is one of the main intracontinental extensional features on earth the rift system represents the northwestern boundary of the amuria plate and in that respect can be considered as an evolving plate boundary the baikal rift system has been widely studied both in terms of geology and geophysics and many models have been proposed for its formation and evolution however the age of the initiation of deformation and the mechanism driving this deformation are still largely debated while major extension has occurred since the late miocene pliocene the onset of extension seems older than the india asia collision implying that several driving mechanisms may have acted together or in relay through time in this work we review the available data and models for deformation in an area encompassing the baikal rift system the sayan ranges to the west and the transbaikal to the east using a synthesis of this data and our own field and mapping observations we show that the baikal rift system along with transpressional deformation in the sayan ranges and transtension in the transbaikal area can be explained through major left lateral strike slip systems the deformation is strongly controlled by inherited crustal and lithospheric structures and is distributed over a wide area within the western amuria plate that consequently cannot be considered as a rigid block such distributed deformation is likely to have a strong effect on the structure of the future continental margin if extension evolves towards the formation of oceanic crust

simultaneous miocene extension and shortening in the himalayan orogen the south tibetan detachment system separates the high grade metamorphic core of the himalayan orogen from its weakly metamorphosed suprastructure it is thought to have developed in response to differences in gravitational potential energy produced by crustal thickening across the mountain front geochronologic data from the rongbuk valley north of qomolangma mount everest in southern tibet demonstrate that at least one segment of the detachment system was active between 19 and 22 million years ago an interval characterized by large scale crustal thickening at lower structural levels these data suggest that decoupling between an extending upper crust and a converging lower crust was an important aspect of himalayan tectonics in miocene time

active intracontinental transpressional mountain building in the mongolian altai defining a new class of orogen abstract mountain ranges that are actively forming around the western and northern perimeter of the indo eurasia collisional deformational field such as the mongolian altai comprise a unique class of intracontinental intraplate transpressional orogen with structural and basinal elements that are distinct from contractional and extensional orogens late cenozoic uplift and mountain building in the mongolian altai is dominated by regional scale dextral strike slip faults that link with thrust and oblique slip faults within a 300 km wide deforming belt sandwiched between the more rigid junggar basin block and hangay precambrian craton dominant orogenic elements in the mongolian altai include double restraining bends terminal restraining bends partial restraining bends single thrust ridges thrust ridges linked by strike slip faults and triangular block uplifts in areas of conjugate strike slip faults the overall pattern is similar to a regional strike slip duplex array however the significant amount of contractional and oblique slip displacement within the range and large number of historical oblique slip seismic events renders the term transpressional duplex more accurate intramontane and range flanking basins can be classified as ramp basins half ramp basins open sided thrust basins pull apart basins and strike slip basins neither a classic fold and thrust orogenic wedge geometry nor a bounding foredeep exists the manner in which upper crustal transpressional deformation is balanced in the lower crust is unknown however crustal thickening by lower crustal inflation and northward outflow of lower crustal material are consistent with existing geological and geodetic data and could account for late cenozoic regional epeirogenic uplift in the russian altai and sayan regions

dynamics of intracontinental extension in the north baikal rift from two dimensional numerical deformation modeling we model the recent deformation along a lithospheric scale cross section perpendicular to the northern part of the baikal rift zone brz with two dimensional finite element models using realistic lithospheric structure and rheological properties and imposing extension as a far field boundary condition we find a model that matches reasonably well the topography observed deformation pattern gravity anomalies and age of formation of the northern brz our results suggest that 1 extensional strain can occur away from the main rift basin in the sayan baikal range and create depending on the rheological properties of the lithosphere a large off rift basin analogous to the barguzin basin or a series of basins and ranges as observed in the southern part of the sayan baikal range 2 anelasticity plays a major role by participating in the uplift of rift shoulders and hanging wall deformation and allowing the subsidence and tilting of the hanging wall along high angle planar normal faults 3 the lower crust accommodates differential strain between the brittle upper crust and upper mantle by horizontal shear and lateral flow towards the regions of crustal and mantle thinning 4 far field extensional stress and a lithospheric discontinuity inherited from paleozoic tectonic events are sufficient to initiate rifting and basin subsidence and 5 there is no or very little dynamic contribution of a hypothetic asthenospheric plume under the brz at least in the recent phase of its evolution fast rifting stage 3 5 myr

proterozoic history of the borborema province ne brazil correlations with neighboring cratons and pan african belts and implications for the evolution of western gondwana 1 geological and geochronological correlations between borborema province ne brazil and neighboring cratons and brasiliano pan african belts indicate that the amazonian west african and sao francisco congo cratons and the basement of the araguaia borborema nigerian and cameroon provinces were part of the atlantica supercontinent this continent was established at the end of the transamazonian eburnean cycle 2 0 ga and apart from ubiquitous taphrogenesis in the 1 8 1 7 ga interval remained largely unaffected for the following 1 ga around 1 ga an important magmatic event in borborema province correlates with rifting episodes and anorogenic magmatism in the sao francisco congo and amazonian cratons these events are interpreted as failed attempts to break up atlantica which at this time may have been part of the larger rodinia supercontinent renewed extensional conditions in borborema province during the middle and late neoproterozoic are attributed to far field stresses transmitted to the interior of atlantica by outwardly dipping subduction zones that encircled its northern present day coordinates portion the rarity of petrotectonic assemblages typical of subduction zone environments indicates that extension did not evolve enough to form large oceans basins and thus that the borborema province essentially includes reworked intracontinental domains regional deformation and metamorphism starting at 650 640 ma and shear zone development beginning at 590 595 ma were continuously developed through time and were synchronous throughout most of the borborema araguaia cameroon and nigerian provinces postorogenic conditions were reached 540 530 myr ago while active deformation was still occurring in other belts that accreted around atlantica to form western gondwana

role of lithosphere in intra continental deformation central australia abstract since the proterozoic there has been a set of deformation cycles in central australia culminating in the alice springs orogeny around 400 ma these events occurred away from plate boundaries and involved extension as well as compression although their precise history remains difficult to unravel from the geologic record much evidence of deformation is left in the central australian crust which features significant moho topography and an associated gravity signal in the past several mechanical models invoked crustal thickening and considerable compression to explain these geophysical characteristics however it is hard to envisage extensive deformation affecting the crust alone but leaving no deformation record in the sub crustal lithosphere in recent seismic tomography studies there is continuous seismically fast lithosphere in central australia below depths of about 100 km in this region the uppermost lithospheric mantle is seismically slow but exhibits no significant attenuation of seismic waves these new constraints make simple crustal thickening unlikely to be the main mechanism to generate variations of the moho depth in central australia here we propose a mechanical model of deformation that involves the entire lithosphere we make no strong assumptions about the history of deformation cycles our model does not require lithospheric thickening at any stage of the deformation cycle and results in a present day scenario compatible with shallow as well as deep constraints on the lithosphere structure

thermomechanical structure of european continental lithosphere constraints from rheological profiles and eet estimates summary the eet equivalent elastic thickness of the lithosphere is a measure of the integrated lithospheric strength it is directly related to the mechanical thickness and rheology of the crustal and mantle lithosphere we present a comparison of eet estimates and strength profiles based on the extrapolation of rock mechanics data for different parts of the european and eurasian continental lithosphere we discuss the temporal and spatial variations in the mechanical thickness and strength inferred from data for precambrian segments of europe s lithosphere variscan europe and the alpine collision belt this analysis demonstrates important spatial and temporal variations in lithospheric rigidity for orogenic belts and sedimentary basins in eastern and western europe and asian parts of eurasia the eet estimates based on synthetic rheological profiles constrained by newly available geophysical data are consistent with the estimates of eet derived from flexural studies of sedimentary basins forelands and orogenic belts these rheological profiles suggest weakening of the major parts of the european and eurasian continental lithosphere by decoupling of the crustal and upper mantle parts a comparison with the seismicity depth distribution for some selected sites suggests that the intra plate seismicity is essentially restricted to the upper crustal parts of europe s lithosphere providing additional support to the notion of the decoupled lithosphere the presence of intra plate stress fields can explain a significant part of the observed variations in eet estimates within individual thermotectonic age groups a comparison of wavelengths of crustal and lithospheric folding with observations shows these wavelengths to be consistent with estimates of eet inferred from the rheological response of basins and orogens at more moderate levels of intra plate stress

moho geometry gravity inversion experiment moggie a refined model of the australian moho and its tectonic and isostatic implications abstract at the continent scale models of moho depth based on seismic estimates alone can be inadequate due to irregular or sparse data gravity based moho modelling provides better coverage however the methods used are typically hampered by an inability to explicitly honour seismic constraints and are also limited by over simplistic model conditions e g laterally homogenous layering i present a new method to generate a continent scale moho model based on the constrained inversion of free air gravity data this method explicitly honours seismic moho estimates and accounts for a laterally heterogeneous crust and mantle resolution and sensitivity testing shows that for wavelengths greater than 200 km crustal density and moho depth are recovered with reasonable accuracy 30 kg m 3 and 3 km respectively moggie uses a six layer model incorporating ocean sedimentary basin upper crust lower oceanic crust eclogitised crust and mantle inversion variables were the density of the crustal layers constrained by a standard density model and the depths to intra crustal boundaries and the moho constrained by 230 seismic depth estimates the results demonstrate that a balanced approach to seismically constrained gravity inversion has the capability to generate detailed and well constrained models of the moho and crustal density at the continent scale for australia this is a clear improvement on the sparse and irregular resolution of the moho provided by seismic estimates of crustal thickness which fail to resolve short wavelength features newly defined tectonic features include extensive magmatic underplates crustal scale shear zones and the boundaries between tectonic blocks isostatic analysis reveals that little of the continent is close to isostatic equilibrium with isostatic disequilibria preserved at multiple scales from hundreds of kilometres to the entire continent these disequilibria are interpreted to indicate long wavelength flexure of highly competent continental lithosphere

the intraplate character of supercontinent tectonics abstract for several decades geoscientists have recognised intraplate tectonic activity far from plate margins both from modern and ancient examples this apparent disconnect with the drivers of plate tectonics does not necessarily imply unconnected processes but rather an uncertainty in understanding exactly how these systems operate are the driving forces derived locally or do they propagate from plate margins how do these forces interact with a complex tectonic inheritance to generate the observed tectonism furthermore what novel approaches have been applied to understand these processes here we review the general literature and the contents of this special issue to develop some partial answers to these questions key observations include the critical importance of local lithospheric heterogeneities as a control on the mode of orogenesis and also the role of locally derived forces from mantle upwelling or from depositing thick piles of magmatic or sedimentary rocks these processes happen within the overarching tectonic setting provided by far field plate margins which show intimate links in both time and observed processes with many intraplate regions these insights would not be possible without a growing arsenal of geological geophysical and numerical methods that can be applied to intraplate regions and this is reflected in the varied approaches within the issue we envisage that this special issue will provide a stimulus for further progress in understanding intraplate tectonics

subduction dynamics and the origin of andean orogeny and the bolivian orocline the building of the andes results from the subduction of the oceanic nazca plate underneath the south american continent however how and why the andes and their curvature the bolivian orocline formed in the cenozoic era 65 5 million years myr ago to present despite subduction continuing since the mesozoic era 251 0 65 5 myr ago is still unknown three dimensional numerical subduction models demonstrate that variations in slab thickness arising from the nazca plate s age at the trench produce a cordilleran morphology consistent with that observed the age dependent sinking of the slab in the mantle drives traction towards the trench at the base of the upper plate causing it to thicken thus subducting older nazca plate below the central andes can explain the locally thickened crust and higher elevations here we demonstrate that resultant thickening of the south american plate modifies both shear force gradients and migration rates along the trench to produce a concave margin that matches the bolivian orocline additionally the varying forcing along the margin allows stress belts to form in the upper plate interior explaining the widening of the central andes and the different tectonic styles found on their margins the eastern and western cordilleras the rise of the central andes and orocline formation are directly related to the local increase of nazca plate age and an age distribution along the margin similar to that found today the onset of these conditions only occurred in the eocene epoch this may explain the enigmatic delay of the andean orogeny that is the formation of the modern andes

intraplate deformation in central australia the link between subsidence and fault reactivation abstract central australia has experienced two intraplate orogenic events involving significant north south shortening the late neoproterozoic to early cambrian petermann orogeny and the devonian to carboniferous alice springs orogeny in each event pre existing structures inherited from mesoproterozoic terrain amalgamation were reactivated and basement rocks exhumed from beneath thick sedimentary successions accumulated in the centralian superbasin the pattern of fault reactivation during these events shows a striking similarity to the pattern of subsidence in the overlying basin immediately prior to the petermann orogeny the centralian superbasin was thickest in the vicinity of the musgrave block the region in which deformation was subsequently localised at the same time crustal scale faults elsewhere in central australia that were covered by a relatively thin sheet of sediment remained inactive despite being favourably oriented to accommodate the north south shortening between the petermann and alice springs orogenies subsidence patterns shifted such that fault systems in the arunta block and also those in the southern musgrave block were buried by significant thicknesses of sediment whereas the major structures that were exhumed during the petermann orogeny were not significantly buried during the alice springs orogeny reactivation once again occurred along the most deeply buried faults even in the instances where those faults had remained inactive during the earlier petermann orogeny importantly the major petermann aged structures that were not buried during renewed subsidence remained inactive during the alice springs orogeny the record of reactivation implies that the presence of pre existing crustal scale faults alone was insufficient to localise deformation rather fault reactivation appears to have required a priming process that modulated the strength of the lithosphere on a regional scale the correspondence between the distribution of basement fault reactivation and subsidence patterns during both the petermann and alice springs orogenies implies a link between relatively thick sedimentation and long term lithospheric weakening we show that this link is compatible with the thermal effects of a thick sedimentary blanket in the context of central australia the mechanical impact of basin formation is likely to be enhanced by the presence of regionally elevated heat production in the proterozoic basement

constraints from diffusion profiles on the duration of high strain deformation in thickened crust diffusion profiles in garnet are used to determine the duration of movement on shear zones in the musgrave block central australia grenville age granulites were overprinted at depths of 40 km by high strain zones during the intracratonic petermann orogeny ca 550 ma ago in these zones quartzofeldspathic mylonites contain garnet from the early granulite facies event that developed compositional gradients during the high pressure overprint the diffusion of ca into garnet is linked with the release of ca in plagioclase the only calcic mineral in the rock during neocrystallization of sodic feldspar during high strain deformation available diffusion data on garnet directly define the duration of high strain deformation at eclogite facies our results indicate that the integrated time scales of movement in individual high strain zones were short lived between 0 07 ma and 1 4 ma and that wide shear zones were active longer than narrow ones in addition we find that relict k feldspar in one of the narrow eclogite facies shear zones yields a 40 ar 39 ar age that is partially reset indicating that the thermal event was also short lived these short time scales support the hypothesis that shear heating was the dominant source of heat for the eclogite facies overprint

dynamic causes for the opening of the baikal rift zone a numerical modelling approach abstract previous dynamic models of the baikal rift zone brz are mostly two dimensional on vertical plane in this study a numerical model of neotectonics in the region on map view was constructed using the adapted plates program the present work is an attempt to test different mechanisms for opening baikal rift by comparing the modelled and observed stress and strain rate fields the following rifting scenarios were tested 1 pure northwest southeast extension 2 pure northeast southwest compression 3 oblique rift opening and 4 combined northwest southeast extension and northeast southwest compression the models are calibrated using geologically and gps derived strain rates and stress tensor determinations from fault slip data and earthquake focal mechanisms the most successful model requires a combination of ne sw compression and orthogonal extension the model results indicate that the present extensional regime in brz can be explained by combining the india plate indentation northward into eurasia east west convergence between the north america and eurasia plates and southeastward extrusion of the amur plate in northeastern asia predicted fault slip rates for the best fit model are consistent with the observed holocene fault slip rates in the lake baikal region the generally accepted rotation of the amur and mongolia microplates are used as independent constraints for the choice of the best fit model these data correlate well with the predicted direction of rotation in our best model

regional high pressure metamorphism during intracratonic deformation the petermann orogeny central australia the petermann orogeny is a late neoproterozoic to cambrian c 560 520 ma intracratonic event that affected the musgrave block and south western amadeus basin in central australia in the mann ranges within the central musgrave block mesoproterozoic granulite facies gneisses granites and mafic dykes have been substantially reworked by deep crustal non coaxial strain of late neoproterozoic to early cambrian age dolerite dykes have recrystallized to garnet granulite facies assemblages associated with the development of a mylonitic fabric at p 12 13 kbar and t 700 750 c migmatization is restricted to discrete shear zones which represent conduits for hydrous fluids during metamorphism peak metamorphism was followed by decompression to c 7 kbar reflecting exhumation of the terrane along the south dipping woodroffe thrust in scattered outcrops north of the mann ranges peak metamorphism occurred at p 9 10 kbar and t c 700 c the woodroffe thrust separates these deep crustal mylonites from granites that were metamorphosed during the petermann orogeny at p c 6 7 kbar and t c 650 c the similarity in peak temperatures at different crustal levels implies an unusual thermal regime during this event the existence of a relatively elevated geotherm corresponding with th and k enriched granites that were in the mid crust during the petermann orogeny suggests that radiogenic heat production may have substantially contributed to the thermal regime during metamorphism this potentially has implications for the mechanisms by which intra plate strain was localized during this event

tethyan subducted slabs under india abstract tomographic imaging of the mantle under tibet india and the adjacent indian ocean reveals several zones of relatively high p wave velocities at various depths under the hindu kush region in northeastern afghanistan and southern tajikistan a regional northward dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the indian plate under northern pakistan this same slab shows a roll over structure with the deeper portion overturned and dipping southward as can also be seen in the distribution of earthquake hypocenters farther east southeast e g in the vicinity of nepal a well resolved anomaly below 450 km depth is connected to the slab under the hindu kush but seems to be separated from the lithosphere above 350 km these upper mantle anomalies are interpreted as the remnants of delaminated sub continental lithosphere that went down when greater india continued to converge northward with asia after 45 ma the deeper high velocity anomalies under the indian sub continent appear clearly separated from the shallower ones as well as from each other and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the indian plate they occur at depths between 1000 and 2300 km and occasionally descend down to the core mantle boundary the anomalies form three parallel wnw ese striking zones we interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the neo tethys ocean closed between india and tibet in the cretaceous and earliest tertiary the northern deep mantle zone under northern afghanistan the himalayas and the lhasa block in southern tibet may represent the last subducted remnant of the paleo tethys ocean which is thought to have closed before the hauterivian stage of the early cretaceous the middle zone continues southeastward as a rather straight high velocity zone towards sumatra where it becomes convex southward and parallel to the subduction zone under the sunda arc comparison of this straight middle zone near india with the shallower upper 600 1000 km northern zone which displays a cusp like shape near the yunnan sw china syntaxis of the eastern himalayas supports the notion that the shallow northern zone represents later subduction than the deeper middle zone the suggestion of a counterclockwise rotation 20 of the indian plate during tertiary indentation of asia is supported by these features the present day latitudes of 5 35 n of the deep slabs under india and adjacent areas correspond to the approximate paleolatitudes of the cretaceous subduction zones the slab remnants in the middle mantle occur therefore near the ancient locations where they started their downward journey which implies that lateral movements in the deeper mantle were not large

far field lithospheric deformation in tibet during continental collision 1 crustal deformation along the present day northern margin of the tibetan plateau has occurred since mid to late eocene time soon after india collided with eurasia assuming that on these distance and time scales the lithosphere can be approximated using a thin viscous sheet we show that far field lithospheric deformation caused by an indenting boundary is expected to start shortly after collision begins when a strong region analogous to cratonic lithosphere like that of the tarim basin is included in the calculations shear and vertical strain rates concentrate along the southern edge of the strong region resulting in enhanced deformation there in calculations scaled to the dimensions of india and the tibetan plateau shear and vertical strain rates on the northern edge of the plateau increase soon after collision begins crustal thickening accelerates later in the calculations we propose that the apparent increased exhumation and perhaps surface uplift around 10 ma may signal the arrival of the thickening front

intraplate tectonics feedback between radioactive thermal weakening and crustal deformation driven by mantle lithosphere instabilities abstract two dimensional viscous plastic numerical experiments show that weak radioactive crust may be susceptible to significant tectonic activity by the effect of rayleigh taylor type instability of the sub crustal lithosphere below a continental interior in particular crust having a localized or regional enrichment of radioactive elements can respond to an underlying mantle lithosphere downwelling by initial subsidence then thickening and uplift owing to the positive feedback among thickening heating and weakening such models may later undergo orogenic deflation as spontaneous crustal extension thinning and surface subsidence occur while mantle downwelling continues strong homogeneous crust subsides dynamically above mantle downwellings and undergoes essentially no internal deformation a uniform distribution of radioactive elements through the crust or a concentration at depth is much more effective in causing thermal weakening of the crust and subsequent intraplate deformation than with radioactive material concentrated nearer the surface for wide regions of high heat production in the crust the length scale of the intracrustal tectonic zone is controlled by the width of the mantle downwelling in contrast with narrow heat production regions which control their own length scale in general the numerical experiments demonstrate that the presence of radioactive elements may make the crust vulnerable to intraplate tectonic deformation and represent a primary control on the way the strength of the crust evolves during sub crustal forcing the results may explain first order features of intraplate tectonics including the subsidence mechanism for intracontinental basins orogenic crustal contraction thickening and surface uplift and subsequent extension and crustal deflation the processes have particular implications for regions of high radioactive heat production and juvenile crust which would more likely have a higher concentration of radioactive elements in the lower crust

dynamic processes controlling evolution of rifted basins abstract the extension of the lithosphere controlling the development of rifted basins is driven by a combination of plate boundary forces frictional forces exerted on the base of the lithosphere by the convecting asthenosphere and deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system although mantle plumes are not a primary driving force of rifting they play an important secondary role by weakening the lithosphere and by controlling the level of rift related volcanic activity a distinction between active and passive rifting is only conditionally justified the extension of the lithosphere depending on its rate and magnitude and the potential temperature of the asthenosphere can cause by adiabatic decompression partial melting of the lower lithosphere and upper asthenosphere in rift systems the level and timing of volcanic activity is highly variable the lack of volcanic activity implies passive rifting an initial passive rifting stage can be followed by a more active one during which magmatism plays an increasingly important role magmatic destabilization of the moho may account for the frequently observed discrepancy between upper and lower crustal extension factors combined with evidence for thermal thinning of the mantle lithosphere this suggest that the volume of the lithosphere is not necessarily preserved during rifting as advocated by conventional stretching models the structural style of rifts is controlled by the rheological structure of the lithosphere the availability of crustal discontinuities that can be tensionally reactivated the mode orthogonal or oblique and amount of extension and the lithological composition of pre and syn rift sediments simple shear extension prevails in rifts that subparallel the structural grain of the basement pure shear extension is typical for rifts cross cutting the basement grain pre existing crustal and mantle lithospheric discontinuities contribute to the localization of rift systems the duration of the rifting stage of extensional basins is highly variable stress field changes can cause abrupt termination of rifting in major rift systems progressive strain concentration on the zone of future crustal separation entails abandonment of lateral rifts depending on constraints on lateral block movements crustal separation can be achieved after as little as 9 my and as much as 280 my of rifting activity syn rift basin subsidence is controlled by isostatic adjustment of the crust to mechanical stretching of the lithosphere its magmatic inflation and thermal attenuation of the mantle lithosphere post rift basin subsidence is governed by thermal reequilibration of the lithosphere asthenosphere system deep seated thermal anomalies related to syn rift pull up of the asthenosphere lithosphere boundary have decayed after 60 my by about 65 and after 180 my by about 95 the magnitude of post rift subsidence is a function of the rift induced thermal anomaly and crustal density changes the potential temperature of the asthenosphere and initial water depths intraplate stresses can have an overprinting effect on post rift subsidence stretching factors derived from post rift subsidence analyses must be corrected for such effects

initiation and propagation of shear zones in a heterogeneous continental lithosphere continental plates represent rheologically heterogeneous media in which complex finite strain fields may develop due to interaction of plate tectonic processes with intraplate heterogeneities such a deformation pattern is displayed by the borborema shear zone system in northeastern brazil it involves continental scale curvilinear e w trending right lateral transcurrent shear zones that branch off from a major ne trending right lateral strike slip deformation zone formed at the northern termination of the sao francisco craton and that finally terminate in transpressive metasedimentary belts we suggest that this strain field results from the compressional deformation of a highly heterogeneous continental lithosphere composed of a stiff domain craton and rheologically weaker domains basins the effect of a low viscosity domain on the deformation of a continental plate and in particular the perturbation induced by this domain on the development of a shear zone formed at the termination of a stiff block were investigated using numerical modeling the low strength domain induces an enhanced strain localization and the geometry of the shear zone is significantly modified it is either split forming a branched shear zone system in which one branch maintains its original orientation while the other rotates toward the low viscosity domain or completely rotated toward the weak block the perturbation of the finite strain field depends on the ability of the weak domain to accommodate deformation which is controlled by its initial viscosity contrast relative to the surrounding lithosphere its orientation relative to the convergence direction and its distance from the shear zone initiated at the termination of the stiff block the interaction between imposed boundary conditions tectonic forces and plate geometry and the internal structure of the plate may give rise to highly heterogeneous strain fields as exhibited by the borborema shear zone system in which intraplate rheological heterogeneities induce the development of branched or sinuous shear zones a heterogeneous continental plate subjected to a normal convergence may therefore display significant lateral variations in strain intensity with shear zones bordering nearly undeformed blocks and in deformation regimes and vertical strains that would result in contrasting metamorphic and uplift histories

on the structure and mechanical behaviour of the extending lithosphere in the baikal rift from gravity modelling abstract the crustal structure and lithospheric flexure of the baikal rift zone siberia are examined by means of gravity modelling we model the bouguer anomaly ba along five 1200 km long gravity profiles we first evidence that continuous elastic plate flexure due to surface loading cannot explain the observed ba then we introduce plate discontinuities coupled with a realistic brittle elasto ductile plate rheology which allow us to model of external tectonic forces acting on the plates and determination of the moho geometry we show that the clearest expression of extensional processes occurs in the central part of the rift which exhibits the highest crustal thinning it evolves southwards to a rapidly increasing compression resulting in an overthickening of the southern plate s crust and in the long wavelength flexure of the siberian plate north of the central rift crustal thinning which is always less than 7 km gives way to a more diffuse zone of deformation inside the sayan baikal folded belt based on plate flexure models we propose that the rift shoulders surrounding the central and north baikal basins are not supported by upward bending plate but have a deep crustal root caused by a downward flexure the other parts of the rift depict two adjacent plates with antithetic flexures we also infer that the axial mantle material upwarping is not related to a large scale asthenospheric upwelling since the lithosphere rheological interfaces are not significantly disturbed our results favour the role of horizontal forces and motions resulting from the india asia collision combined with the effect of inherited tectonic structures and especially the paleozoic suture bounding the siberian craton for explaining the crustal structure and plate flexure modeled

topo oz insights into the various modes of intraplate deformation in the australian continent abstract as the fastest lowest flattest and amongst the most arid of continents australia preserves a unique geomorphic record of intraplate tectonic activity evidencing at least three distinct modes of surface deformation since its rapid northward drift commenced around 43 million years ago at long wavelengths several 1000s km systematic variations in the extent of neogene marine inundation imply the continent has tilted north down southwest up at intermediate wavelengths several 100s km several undulations of 100 200 m amplitude have developed on the 1 10 myr timescale at still shorter wavelengths several 10s km fault related motion has produced local relief at rates of up to 100 m myr over several million years the long wavelength north down tilting can be related to a dynamic topographic effect associated with australia s northward drift from the geoid low dynamic topography low now south of the continent to the geoid high dynamic topography low centred above the south east asian and melanesian subduction zones the short wavelength fault related deformation is attributed in time to plate wide increases in compressional stress levels as the result of distant plate boundary interactions and in space in part to variations in the thermal structure of the australian lithosphere at the intermediate wavelengths transient low amplitude undulations can be ascribed to either lithospheric buckling or the development of instabilities in the thermal boundary layer beneath the lithosphere in the latter case topographic asymmetries suggest the australian lithosphere is moving north with respect to the mantle beneath providing a unique attribution to the progressive alignment of seismic anisotropy and absolute plate motion observed near the base of the australian lithosphere

new insights into coronae evolution mapping on venus coronae are geologically and geophysically important features on venus because they are thought to contribute to planetary heat loss it is thus necessary to understand their role in space and time in the evolution of venus detailed mapping of five coronae in guinevere and sedna planitia illustrates that although previous models invoked initial uplift not all coronae can be explained in such a simple way we show that the formation of corona annuli can be multistaged and that the position of the annulus does not always coincide with the main topographic ridges and troughs that outline the feature the magnitude and timing of volcanism are not necessarily the same at each corona and coronae can have long and complex histories in contrast with the stratigraphic relations suggested by other workers we demonstrate that coronae do not all have the same relative ages with respect to adjacent units these results suggest that corona formation was is not confined to a single time period in the history of venus and that detailed mapping is a reliable method of establishing the relative timing of corona formation