palaeoclimate perturbations before the sheinwoodian glaciation a trigger for extinctiond during the ireviken event telychian to sheinwoodian conodonts pterospathodus eopennatus ssp n 1 zone through the ozarkodina sagitta rhenana superzone were studied in detail for their oxygen isotope composition the upper kockelella ranuliformis and ozarkodina sagitta rhenana zones represent the peak interval of the sheinwoodian positive 18oapatite excursion reflecting the coolest conditions with marine sea water temperatures decreased by more than 6 c in the subtropics the 18oapatite data cover pre through post ireviken event strata the study includes also material from younger levels up to the late wenlock ctenognathodus murchisoni zone however this record is incomplete with one major hiatus reflecting the regression associated with the middle silurian mulde event the stratigraphic levels of the analyzed conodont samples from estonia can be correlated with high precision to the datum points of the ireviken event strata in the lower and upper visby formations on gotland sweden inferred short term climatic changes in the upper telychian part of the studied interval may have been caused by phases of major volcanic activity along the caledonian front across the ireviken event we observe variations in oxygen isotope ratios which allow a direct comparison to the stepwise extinctions and faunal reorganisations in different groups our data suggest that faunal extinctions are connected to time intervals of warming before the establishment of more stable and cooler conditions during the main sheinwoodian glacial the most severe extinctions and faunal turnovers at datum points 2 and 4 occurred during the early warmer interval of the event after the shift into the sheinwoodian icehouse in the lower kockelella ranuliformis zone reflected by rapidly decreasing sea water temperatures faunas restabilized and reefal communities started to flourish sedimentary evidence for the glaciation is discussed for different palaeolatitudes less

dynamic uplift of the himalaya it has become widely accepted that the himalaya is the product of collision between the indian landmass and the eurasian continent holmes1 conjectured that the indian shield was carried northwards by sub lithospheric mantle flow to cause a double thickening of crust beneath the himalayas and the tibetan plateau wager2 hypothesized that the extra height of the great peaks of the himalaya over tibet was due to isostatic readjustment following the removal of load by intense erosion molnar et al 3 relate the uplift to the shear stress on the boundary faults beneath the himalayas relevant data have recently become available from the northern flank of the himalaya and the tibet plateau of which especially important are 1 a gravity profile across central tibet 2 seismic crustal structures of the region from explosion seismology 3 seismicity 4 geothermal activities 5 geodetic palaeogeographical and radiometric data bearing on the uplift rate of the region and 6 field observation of the surface fault patterns the combined data set has imposed strong constraints on theories of the dynamic processes in this region we summarize here the relevant observations and discuss the results and implications of our model analysis details of the modelling procedure are given elsewhere4

the sacta limestone member early wenlock cool water temperate carbonate deposition at the distal foreland of gondwana s active margin bolivia abstract the sacta limestone member is located towards the base of the silurian kirusillas formation in central bolivia and forms part of the sedimentary record of the peru bolivia palaeozoic basin the early wenlock sheinwoodian age of this thin carbonate unit has been established by conodont and foraminifer biostratigraphy during the silurian a thick siliciclastic wedge was deposited in the foredeep of the basin related to tectonism along the active margin of gondwana strong subsidence and downslope resedimentation slides slumps debris flows turbidites dominated towards the west of the basin during the llandovery and early wenlock in contrast a much thinner record was deposited at the distal margin of the foreland basin pinching out towards the northeast and onlapping the stable precambrian basement the sacta member represents cool water temperate carbonate deposition at mid to high latitude on the distal stable margin of this foreland basin its sedimentation took place towards the end of a phase of important tectonic instability and the carbonate bed was itself also partially resedimented late ashgill llandovery glaciation of nearby reliefs along the active margin of gondwana was followed by brief and local cool water temperate carbonate deposition in the early wenlock and subsequently by the development of a storm and wave dominated shallow clastic shelf during the rest of the silurian the sacta member records carbonate deposition in western gondwana during a brief and singular event with important palaeogeographic and palaeoclimatic implications

on the tectonics of the himalaya and the tibet plateau new data for the gravity anomalies and the uplift rate in the himalaya and the tibet plateau have imposed strict constraints on acceptable models for present tectonic processes in this region through two dimensional finite element modeling incorporating realistic rheology for the crust and upper mantle a model is found which satisfactorily predicts both the observed uplift rate and the changes of the gravity anomalies this model shows that 1 the himalaya is dynamically supported 2 the northern margin of the indian plate underthrusts the himalaya and at the same time the entire tibet thickens at a rate of 2 mm yr 3 the partition of the total shortening across the region is about 15 in the ganges basin 30 in the himalaya and 55 in tibet and 4 the extent of deformation reaches beyond the northern boundary of tibet

mountain belt growth inferred from histories of past plate convergence a new tectonic inverse problem past plate motions display a range of variability including speedups and slowdowns that cannot easily be attributed to changes in mantle related driving forces one key controlling factor for these variations is the surface topography at convergent margins as previous modeling shows that the topographic load of large mountain belts consumes a significant amount of the driving forces available for plate tectonics by increasing frictional forces between downgoing and overriding plates here we use this insight to pose a new tectonic inverse problem and to infer the growth of mountain belts from a record of past plate convergence we introduce the automatic differentiation method which is a technique to produce derivative code free of truncation error by source transformation of the forward model we apply the method to a publicly available global tectonic thin shell model and generate a simple derivative code to relate nazca south america plate convergence to gross topography of the andes mountain belt we test the code in a search algorithm to infer an optimal paleotopography of the andes 3 2 m y ago from the well known history of nazca south america plate convergence our modeling results are in excellent agreement with published estimates of andean paleotopography and support the notion of strong feedback between mountain belt growth and plate convergence 2007 elsevier b v all rights reserved

mantle convection with a brittle lithosphere thoughts on the global tectonic styles of the earth and venus summary plates are an integral part of the convection system in the fluid mantle but plate boundaries are the product of brittle faulting and plate motions are strongly influenced by the existence of such faults the conditions for plate tectonics are studied by considering brittle behaviour using byerlee s law to limit the maximum stress in the lithosphere in a mantle convection model with temperature dependent viscosity when the yield stress is high convection is confined below a thick stagnant lithosphere at low yield stress brittle deformation mobilizes the lithosphere which becomes a part of the overall circulation surface deformation occurs in localized regions close to upwellings and downwellings in the system at intermediate levels of the yield stress there is a cycling between these two states thick lithosphere episodically mobilizes and collapses into the interior before reforming the mobile lid regime resembles convection of a fluid with temperature dependent viscosity and the boundary layer scalings are found to be analogous this regime has a well defined nusselt number rayleigh number relationship which is in good agreement with scaling theory the surface velocity is nearly independent of the yield stress indicating that the plate motion is resisted by viscous stresses in the mantle analysis suggests that mobilization of the earth s lithosphere can occur if the friction coeycient in the lithosphere is less than 0 03 0 13 lower than laboratory values but consistent with seismic field studies on venus the friction coeycient may be high as a result of the dry conditions and brittle mobilization of the lithosphere would then be episodic and catastrophic

early palaeozoic cooling events peri gondwana and beyond the short lived end ordovician hirnantian glaciation allied to marine mass extinction is variously considered as a short lived event or as the peak of long drawn out climatic cooling through at least late ordovician early silurian times evidence from early palaeozoic facies faunas and stable isotope excursions used to interpret climatic cooling events ranges farther from late mid cambrian to late silurian times glacigenic sediments structures and geomorphology provide direct evidence of glacial episodes cool water carbonate deposition which is particularly widespread during the late ordovician boda event in high latitude peri gondwana gondwana and beyond into mid low palaeolatitudes is interpreted as indicating global cooling not warming as has been proposed such carbonates also characterize mid latitude continents widely at horizons earlier in the ordovician and more locally in the mid silurian in high latitude gondwana cool water carbonate mounds have distinctive facies controlled mound faunas across palaeocontinents other facies evidence for palaeoclimates includes black shale deposition including deglacial organic rich hot shales which indicate transgression in epeiric seas and sea level curves interpreted from facies and faunal successions correlation is shown between facies evidence and positive c isotope excursions from which cyclicities are apparent the possible interface of orbitally controlled rhythms is considered against evolving palaeobiogeography and changes in global sea level and in pco2 facies and faunal evidence from peri gondwanan terranes armorica central europe alborz is assessed with that from gondwana mostly north africa south america and correlatives in avalonia baltica and laurentia to establish a wider picture of early palaeozoic cooling events

early silurian glaciation along the western margin of gondwana peru bolivia and northern argentina palaeogeographic and geodynamic setting the record of the ordovician silurian transition within the southern peru bolivia palaeozoic basin extending across peru bolivia and northern argentina is characterized by a diamictite bearing unit which overlies several different ordovician units and underlies mid silurian shales we review the lithostratigraphy sedimentology and biostratigraphy of this unit revising stratigraphic relationships and several ideas and interpretations previously proposed analysis of facies and lithofacies associations allow for a reinterpretation of sedimentary processes and environments involved during its deposition turbidite and shale interbeds within the diamictites indicate a deep marine environment with interbedded mud flows debris flows slumps and large displaced slabs providing evidence for sediment instability and resedimentation chitinozoan biostratigraphy indicates a llandovery age in contrast with previous proposals of a hirnantian age and suggests the need for more detailed sedimentologic and biostratigraphic studies and a reassessment of this diamictite unit in relation to adjacent regions in south america the resedimented character of the deposit explains some of the ordovician fauna previously described which must be considered as recycled from underlying units in situ fauna needs to be reassessed as it may be endemic diachronic and or indicative of migrations within gondwana glacially faceted and striated clasts as well as large granitoid boulders within the resedimented materials provide evidence for glaciation of the source area and are interpreted as recycled from former glacigenic deposits the evidence found in the central andes indicates a glaciomarine origin for the diamictites and corroborates glaciation of a western source area prior to the late llandovery telychian the precise age of glaciation in this part of gondwana cannot be confirmed do to a lack of true tillites tectonic deformation and the resulting relief are respectively identified as the origin for sediment instability and for local glaciation along the active margin of western gondwana during the late ordovician and early silurian the new data imply that the ordovician silurian boundary was not preserved in some areas of the basin and that an erosional hiatus disconformity with variable regional significance is present between different ordovician units and the diamictite unit late ordovician late caradoc early ashgill euxinic black shales locally present in the central altiplano and eastern cordillera of bolivia may represent the youngest ordovician unit in the region and more research should be focused on them paleogeographic reconstruction of the early silurian peru bolivia basin depicts the extension of the diamictite unit southwards into northern argentina and western paraguay connecting with the parana basin through the asuncion arch to the north it continued into northern peru and ecuador connecting with the silurian record in colombia and venezuela and to the northeast into brazil solimoes and amazonas basins the sedimentary record of glaciation in the peru bolivia basin was due to the development of local ice fields to the west of the basin related with tectonism and resulting reliefs along the active margin of gondwana and probably did not coincide with the development of the north african late ashgillian ice cap

tectonics and topography for a lithosphere containing density heterogeneities the purpose of this paper is to clarify the dynamic role of lithospheric density heterogeneities in particular with respect to mountain building and other processes of intraplate deformation density anomalies within or just beneath the lithosphere constitute major sources for tectonic stress fields the product of their magnitude by their depth is shown to characterize their ability to induce deformation this rule of the density moment directly yields the lithospheric thickening or thinning rate when applied to structures of large lateral extent for anomalies of lateral extent that is small in comparison with their depth the deformation is vertically inhomogeneous and has been computed with the help of simple physical models of a stratified viscous newtonian lithosphere the analytical treatment is based on fourier transform continent continent collision thickens not only the crust but the entire lithosphere the cold root underlying a mountain chain induces strong regional compressive stresses able to sustain the mountain bulding process without further help from forces transmitted from far away thus the continental lithosphere is in a somewhat metastable mechanical state adiabatic i e rapid thickening or thinning tends to grow further once initiated tectonic phases of strong compression correspond to the climax of such instabilities the response of models with cold lithospheric roots of various intensities has been computed both in two and three dimensions they yield velocity distributions and stress fields instructive comparisons are made with earthquake focal mechanisms and in situ stress measurements in the alpine and appalachian regions in the presence of lateral variations of the mechanical properties of the lithosphere the tectonic style is not only function of the local topography and of the nature of its compensation deformations in neighbouring provinces are coupled as shown by 3 dimensional models for example thickening sustained by a cold lithospheric root may generate extension in peripheral zones of weakness these last results illustrate the point that the computation of regional tectonic stresses requires the knowledge of the density anomalies within the lithosphere on the one hand and of geometrical constraints related to lateral mechanical heterogeneities on the other

the role of a heterogeneous inclusion during continental collision abstract we study a simple model of the deformation due to collision between two continental plates the model is patterned after the geometry of the india asia collision but it may be applicable to more general situations the collision is modelled by the indentation of a viscoplastic plate asia by a rigid punch india a finite element approach capable of dealing with finite deformation heterogeneity and non linear rheologies is used to study the role of an inclusion of increased rigidity on the viscoplastic flow of the plate the heterogeneous zone was chosen to approximate the shape and position of the tarim basin behind tibet the lateral boundaries of the plate were defined so as to approximate those that are believed to apply for the asian plate i e fixed boundaries to the west and north free boundaries to the west and south several geometries and contrasts of rheology were analyzed to verify the general validity of our results the purpose was to determine the main kinematic features of the flow not to model the particular case of tarim it is found that the heterogeneity affects the flow in a drastic way even for small contrasts of rigidity and for small size heterogeneities the main effect of the heterogeneity is to create two important shear zones or internal boundary layers the most important is in front of the inclusion separating the main flow from the inclusion as the flow proceeds the inclusion rotates and the shear zone progressively bends in qualitative agreement with the bending of the large strike slip faults like the altyn tagh that separates the tarim from tiber our model of continuous deformation does not allow for the formation of faults or fractures but the shear zones may be understood as the initial stages for the generation of fractures or velocity discontinuities another shear zone develops to the left of the inhomogeneity near the fixed boundary and in the wake of the punch the role of these shear zones is to isolate the heterogeneity from the main flow to facilitate its rigid rotation inside the flow of the softer plate a final observation is that in the presence of the heterogeneity the lower left hand corner rotates almost like a rigid block producing a sharp bend on the left hand side of the plate as the inclusion rotates pushed by the punch this rigid block rotates clockwise to open up the space necessary for the homogeneity

thermo mechanical modeling of subduction of continental lithosphere abstract we consider two dimensional thermo chemical mantle convection models to investigate the deformation of the continental lithosphere that follows the oceanic lithosphere into the subduction zone the models account for the compositional buoyancy forces by considering lithospheric plates with distinct crustal layers continental convergence results in crust mantle detachment in the subducting plate and crustal thickening in both subducting and overriding plates the depth of detachment ranges from 90 to 200 km depending on the strength of the lithosphere and the density of the continental crust as the crust thickens the convergence velocity in the collision zone decreases and the locus of subduction gradually shifts toward the interior of the subducting plate in models with greater viscosity the subducting mantle lithosphere maintains its integrity and does not break up in models with weaker rheology the positive buoyancy of the thickened crust can overcome the strength of the subducting lithosphere and causes the oceanic slab to break off and sink into the mantle the breakoff occurs over a time interval of 10 20 million years which is roughly the time needed for the crust to reach its maximum thickness crustal detachment takes place over a wide range of lithospheric strength suggesting that crustal buoyancy has an important role in the dynamics of continental subduction

simulation of collision zone segmentation in the central mediterranean abstract collision processes between the african and european plates in the ionian sea offshore calabria and in sicily take place in several ways to the east normal subduction of oceanic crust underlying the ionian sea occurs underneath the calabrian arc whereas to the west continental collision between the pelagian block and the calabrian arc which extends from calabria to the northeastern tip of sicily and offshore of the island to the north gives rise to the maghrebian thrust belt on mainland sicily this belt consists mostly of stacked basinal and platform carbonates of mesozoic early tertiary age deposited along the africa passive margin no clear subduction characterizes the area north of the hybleandashmalta plateau where it collides with the maghrebian chain the origin of the hybleandashmalta plateau is unclear geophysical data indicate however that its crustal structure is different from that of the surrounding pelagian foreland crustal structure variations at the edge of the subducted african plate may thus cause the observed segmentation of the collisional arc system a computer model of the region was created to learn about the deformation and faulting processes in the region the simulation starts prior to the collision of the pelagian block with the calabrian arc with time the hybleandashmalta plateau collides with the northern crustal block and as a result a new simulated topography a new distribution of velocities and most important a new distribution of fracture zones are created a shear fracture zone is formed east of the hybleandashmalta plateau malta escarpment extending to the subduction zone at the calabrian arc a less active strike slip fracture zone is created west of the hybleandashmalta plateau an additional e w trending active fracture zone is creating in the southern part of the simulated area this feature may correspond to a fault zone which runs from the strait of sicily to the ionian sea

the san nicolas batholith of coastal peru early palaeozoic continental arc or continental rift magmatism major and trace element u pb zircon geochronology and pb isotopic data are presented on plutons from the san nicolas batholith of central and southern peru the major oxides of 9 samples indicate a high k calc alkaline trend and trace element discriminant diagrams reveal affinities to volcanic arc or late to post collision calc alkaline intrusions the apparent contradiction implied by the rb hf ta triangular diagram within plate strongly suggests loss of rb an interpretation supported by the extensive deuteric alteration and sericitization of feldspar ta zr ta hf and ta nb ratios are considerably higher than those of volcanic arc intrusions and favour the late or post collision calc alkaline classification u pb zircon geochronology shows that the san nicolas batholith was emplaced in two short lived magmatic events at approximately 425 ma and 394 388 ma ago the presence of an inherited component of radiogenic pb of precambrian age in the zircons suggests xenocrystic cores which were most likely scavenged from precambrian basement rocks of the arequipa massif time adjusted pb isotopic compositions of feldspars have the following ranges 206 pb 204 pb 17 798 18 272 207 pb 204 pb 15 571 15 626 and 208 pb 204 pb 38 182 38 291 indicating that precambrian basement was incorporated into the magmas of the batholith other components are believed to have originated in an enriched upper mantle or lower crustal wedge pervasively metasomatized by fluids extracted largely from subducted sediments u pb zircon age distribution trace element characteristics and the linear nature of the plutons firmly establish the san nicolas batholith as the plutonic substructure of an early palaeozoic continental arc which may extend northwards in peru and southwards

early silurian paired δ13ccarb and δ13corg analyses from the midcontinent of north america implications for paleoceanography and paleoclimate abstract paired 13ccarb and 13corg samples from well preserved marine carbonates of the wayne formation in central tennessee were analyzed to test a previously proposed model of relative changes in atmospheric pco2 during the well known ireviken early wenlock positive carbon isotope 13ccarb excursion our investigation provides the first detailed 13corg stratigraphy through this interval and documents a negative excursion in 13corg associated with the positive 13ccarb excursion this is consistent with the notion that the ireviken positive 13ccarb excursion was associated with elevated global temperatures that began during an icehouse greenhouse transition although our data are consistent with predictions made by the silurian oceanographic model jeppsson l 1990 an oceanic model for lithological and faunal changes tested on the silurian record j geol soc lond 147 663 674 a link to global changes in atmospheric pco2 remains tenuous until it can be shown that the 13corg trends shown herein can be reproduced globally

intraplate deformation due to continental collisions a numerical study of deformation in a thin viscous sheet abstract one of the more successful conceptual models of the deformations associated with continental collisions involves the penetration of a rigid punch into a deformable sheet we have used the england and mckenzie 1982 mathematical description of this model to compute the magnitude and rate of change of crustal thickness mass point velocities strain rates and their principal axes modes of deformation areal changes and stress particular attention is payed to the various geophysical and mathematical assumptions that are used in the model finite element calculations show that the choice of lateral boundary conditions can dictate whether the shearing that occurs diagonally in front of the advancing punch terminates in compression or extension however the deformation in front of the punch is insensitive to the specific choice of lateral boundary conditions whenever the diameter of the viscous sheet is three times that of the punch or more for smaller sheets or wider punches crustal thickening in front of the punch is larger when the lateral boundary is fixed than when it is free for a rigid punch which maintains sharply defined edges deformation is concentrated near the punch corners conversely when the punch is not rigid the attendant shearing results in the deformation being concentrated near the leading point center of the punch the variation in compression rate with distance from the punch is sensitive to the choice of theological law and magnitude of tectonic stress for newtonian viscous flow in the lithosphere with viscosities greater than 10 23 pa sec the axial distance from the middle of the punch to the point of most rapid contraction is relatively invariant with time this maximum occurs much farther from the punch than is predicted from the recently published approximate analytical expressions of england et al 1985 for nonlinear viscous flow power law with n 3 coupling between horizontal tectonic stress and isostatic relaxation results in a propagation of the location of the maximum away from the punch sheet boundary

orogeny and orography the effects of erosion on the structure of mountain belts a numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes crustal deformation is calculated by a two dimensional finite element model of deformation in response to subduction and accretion of continental crust erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power the relative importance of the tectonic and erosion processes is given by a dimensionless erosion number relating convergence velocity rock erodibility and precipitation rate this number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction these characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt the two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior models of the latter case predict a focused zone of exhumation at the margin of the orogen and at high erosion number a reversal in the topographic asymmetry natural examples of these two cases are presented the southern alps of new zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate the asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry the contrasting example of dominant wind in the direction of subduction motion is provided by the olympic mountains of washington state in this case exhumation of deep levels of the cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation

large scale deformation associated with ridge subduction summary continuum models are used to investigate the large scale deformation associated with the subduction of aseismic ridges formulated in the horizontal plane using thin viscous sheet theory these models measure the horizontal transmission of stress through the arc lithosphere accompanying ridge subduction modelling was used to compare the tonga arc and louisville ridge collision with the new hebrides arc and d entrecasteaux ridge collision which have disparate arc ridge intersection speeds but otherwise similar characteristics models of both systems indicate that diffuse deformation low values of the effective stress strain exponent n are required to explain the observed deformation deformation is somewhat insensitive to the vertically integrated strength of the arc inversely proportional to the argand number ar but indicates that the arc lithosphere is not extremely weak ar 100 low values of both ar and n suggest that the thermal structure is typical of cold or normal arcs and that deformation is dominated by flow in the lower crust and mantle in addition low values of n approaching newtonian flow may indicate that specific deformation mechanisms dictate deformation of the arc lithosphere possible mechanisms include low stress grain size dependent creep pyroxenite controlled rheology and mechanisms associated with water weakening changes in the boundary conditions greatly affect deformation within island arcs high rates of arc ridge intersection speed tonga louisville system yield arc parallel tension and crustal thickening in the wake of ridge subduction in contrast low rates of arc ridge intersection speed new hebrides d entrecasteaux system yield compressional deformation directly arcward of the collision zone and transverse strike slip faulting adjacent to the region of compressional deformation localized regions of extensional deformation along the frontal part of the arc adjacent to the collision zone may contribute to the formation of re entrants

new finite element techniques for modeling deformation histories of continents with stratified temperature dependent rheology previous techniques for modeling anelastic deformation of the lithosphere have included plane strain models restricted to two dimensional problems and quasi three dimensional plane stress or thin plate models that did not accurately include the effects of the shallow frictional layer or of kinematic detachment of crust from mantle this paper presents techniques to remedy these deficiencies of thin plate models an iteration strategy in which the rheology is linearized using artificial prestress and a particular effective viscosity tensor causes the calculation of horizontal velocities to converge monotonically even with a frictional layer at the top of the lithosphere a technique using two planar grids allows deformation and displacement to be different at the crust and mantle levels at far less cost than that of a three dimensional grid a finite element technique is developed for computing the changes in thickness of these layers caused by pure shear simple shear and pressure gradients a technique based on relaxation of perturbation eigenfunctions solves the heat equation in the lithosphere during deformation accuracy of component numerical methods is good for simple test problems but in realistic nonlinear problems utilizing all components only the precision can be determined because of the lack of analytic solutions precision of the combined program is tested with a realistic sample problem and presented as a function of the number of iterations in each velocity solution convergence factor 0 73 to 0 88 size of time step in the predictor corrector time integration convergence as t0 8 and number of degrees of freedom in the finite element grid convergence as n 0 5 to 0 8 for most variables overall cost of a simulation varies with the fractional precision as 3 3 a new consequence of kinematic detachment a moving wave of crustal thickness is found unfortunately the form of the wave depends on the finite element size this means that element size must be chosen to approximate the smoothing by flexural rigidity effects that were neglected because of cost

oblique convergence of the crust driven by basal forcing implications for length scales of deformation and strain partitioning in orogens summary we investigate the effects of oblique convergence on crustal thickening by modelling the crust as a thin viscous sheet crustal deformation is driven by basal velocity boundary conditions representing the kinematics of the underlying convergent mantle lithosphere which is assumed to detach and subduct the crustal sheet is coupled to the basal velocity field by a weak simple shear layer which exerts a traction on the base of the sheet the use of basal velocity boundary conditions with a thin sheet model is shown to be valid provided coupling with the base is weak and deformation extends over a horizontally wide zone in comparison to the thickness of the sheet the effect of the mantle lithosphere on crustal deformation can be shown to depend on the non dimensional ampferer number am which represents the relative strengths of the crust and the weak basal shear zone analytical and numerical model results for small amounts of normalized convergence show that the ratio of horizontal length scales for normal and transcurrent crustal deformation is 2 for both linear and non linear viscous rheologies the length scales depend on the relative strength of the crust and the basal shear zone convolved with a forcing length scale from the boundary conditions for obliquely incident velocities the length scale ratio is between 2 and 1 the exact value depending on the power law exponent of the rheology and the relative magnitude of the normal and transverse velocity components driving deformation the model results for the ratio of normal to transcurrent deformation length scales are different from those found by using a side driven viscous thin sheet model england houseman sonder 1985 sonder england houseman 1986 we propose that this difference may be used as a test to distinguish the primary driving mechanism for large scale orogenic deformation if the deformation mechanism corresponds to pure shear of the whole lithosphere without significant basal tractions we expect the deformation length scale to be determined by the scale of the applied boundary conditions and buoyancy forces in the crust the normal transcurrent length scale ratio for this case in early stages of deformation will be approximately 4 alternatively if the results presented here are more appropriate then we expect the deformation length scale to be determined by the intrinsic relative strengths of the crust and underlying basal shear zone the corresponding normal transcurrent length scale ratio for this case in early stages of deformation will be between 2 and 1 on the basis of a comparison of the predicted length scales of deformation with the length scale over which the velocity boundary conditions are applied we suggest that long narrow linear orogens such as the andes are controlled by basal boundary conditions whereas the himalayan tibetan orogen has dimensions consistent with either basally or side driven deformation

determining rheology from deformation data the case of central italy 1 the study of geodynamics relies on an understanding of the strength of the lithosphere however our knowledge of kilometer scale rheology has generally been obtained from centimeter sized laboratory samples or from microstructural studies of naturally deformed rocks in this study we present a method that allows rheological examination at a larger scale utilizing forward numerical modeling we simulated lithospheric deformation as a function of heat flow and rheological parameters and computed several testable predictions including horizontal velocities stress directions and the tectonic regime to select the best solutions we compared the model predictions with experimental data we applied this method in italy and found that the rheology shows significant variations at small distances the strength ranged from 0 6 0 2 tn m within the apennines belt to 21 6 tn m in the external adriatic thrust these strength values correspond to an aseismic mantle in the upper plate and to a strong mantle within the adriatic lithosphere with respect to the internal thrust we found that strike slip or transpressive but not compressive earthquakes can occur along the deeper portion of the thrust the differences in the lithospheric strength are greater than our estimated uncertainties and occur across the adriatic subduction margin using the proposed method the lithospheric strength can be also determined when information at depth is scarce but sufficient surface data are available

paleozoic terranes of eastern australia and the drift history of gondwana abstract critical assessment of paleozoic paleomagnetic results from australia shows that paleopoles from locations on the main craton and in the various terranes of the tasman fold belt of eastern australia follow the same path since 400 ma for the lachlan and thomson superterranes but not until 250 ma or younger for the new england superterrane most of the paleopoles from the tasman fold belt are derived from the lolworth ravenswood terrane of the thomson superterrane and the molong monaro terrane of the lachlan superterrane consideration of the paleomagnetic data and geological constraints suggests that these terranes were amalgamated with cratonic australia by the late early devonian the lolworth ravenswood terrane is interpreted to have undergone a 90 clockwise rotation between 425 and 380 ma although the tamworth terrane of the western new england superterrane is thought to have amalgamated with the lachlan superterrane by the late carboniferous geological syntheses suggest that movements between these regions may have persisted until the middle triassic this view is supported by the available paleomagnetic data with these constraints an apparent polar wander path for gondwana during the paleozoic has been constructed after review of the gondwana paleomagnetic data the drift history of gondwana with respect to laurentia and baltica during the paleozoic is shown in a series of paleogeographic maps

numerical modeling of intraplate deformation simple mechanical models of continental collision we study a model of continental collision in which one of the continents acts as a rigid die indenting the other plate which flows as an incompressible viscoplastic medium we consider two extreme cases of plane deformation 1 plane strain which corresponds to an infinitely thick lithospheric plate and 2 plane stress corresponding to a very thin plate deformation of the lithosphere a thick plate should be intermediate between those extremes we found that the flow in the plane strain case is quite similar to that obtained by slip line theory the plane stress results are quite different since in this case most of the plate shortening is taken up by the thickening of the lithosphere we also explored the role of boundary conditions on the flow in particular the role of the side walls containing the flow of the lithosphere in the case of a free lateral boundary the main feature is a flow of matter toward this free wall and a s like pattern for the horizontal stress field for a rigid wall on the other hand the plane strain and the plane stress results are quite different in the first case there is a large return flow on the sides of the punch the material being extruded along the only free surface available in the plane stress case the return flow disappears and the material displaced by the penetration of the die tends to thicken the plate the role of a nonlinear constitutive relation is studied for power law creep as the power of the flow limit increases the flow retains its general features but the deformation localizes creating sharper contrasts between high and low strain rate areas in plane stress the effect of nonlinearity is to increase the contrasts in vertical motion available data for asia are discussed in the light of the new results

a thin viscous sheet model for continental deformation summary for the purposes of describing its large scale and long term deformation the continental lithosphere is regarded as a continuum obeying a newtonian or a power law rheology the flow of a thin sheet of power law material overlying an inviscid substrate is studied under the assumption that vertical gradients of the horizontal velocity are negligible a numerical model is used to investigate the deformation of such a sheet under conditions approximating those of continent continent collision the material flows in response to forces applied to its boundaries for example the indenting of one continent by another and to forces in its interior arising from gradients in crustal thickness the horizontal divergence of the flow produces changes in the crustal thickness and hence a time dependent form to the flow itself for a given set of boundary conditions the flow depends on the stress exponent in the power law rheology n and on the argand number which is a measure of the ratio between the stress arising from crustal thickness contrasts and the stress required to deform the material at the ambient strain rates when the effective viscosity of the medium is very high ar o crustal thickness variations do not influence the flow if the material is newtonian n l the deformation associated with an influx of material approximating an indenter is of much greater lateral dimension than the width of the indenter whereas when material has a power law rheology n 3 5 are used the deformation is confined to a region of lateral extent comparable to that of the indenter as ar increases the forces arising from crustal thickness contrasts exert more influence on the flow and the maximum crustal thickness that can be sustained by a given influx of material is related by a simple expression to the effective viscosity of the medium at the ambient strain rates in the limit of a very weak medium ar 10 the lithosphere is unable to sustain appreciable crustal elevation contrasts the results of these numerical experiments show that systems in which the effective viscosities are such that the maximum deviatoric stresses are between 1 kbar

reply to bonev n spikings r and marchev p 2016 comment on georgiev et al structure and u pb zircon geochronology of an alpine nappe stack telescoped by extensional detachment faulting kulidzhik area eastern rhodopes bulgaria of the four superposed tectonic units in the study area the uppermost unit iv is lithologically similar to the lowermost unit i both are formed dominantly by orthogneiss in an earlier article bonev et al 2010 had shown that the orthogneisses of units i and iv display similar trace element patterns and therefore interpreted unit iv as an equivalent of unit i emplaced by an overthrust on top of the other units our new u pb zircon dating however showed that the orthogneiss of unit i was formed from a late carboniferous protolith and the orthogneiss of unit iv from a neoproterozoic protolith these rocks are of different ages and their lithological and geochemical similarity is a mere coincidence in their comment bonev et al 2016 again demonstrate the geochemical similarity their fig 1 and write that this allows some caution to be expressed relative to the age of 581 ma determined by us we do not share this opinion on the contrary we think that more caution would be necessary in interpreting geochemical data to illustrate this we show in fig 1 the compositions of the three orthogneisses that bonev et al 2016 show in their fig 1b in order to demonstrate their similarity together with the average composition of the upper continental crust thick blue line after wedepohl 1995 and the composition of jg 1 thick red line a common granodiorite standard from japan after ando et al 1971 the similarity between the patterns of the three bulgarian orthogneisses is not larger than the similarity between any of them and the granodiorite from japan and all of them are similar to the average composition of the upper continental crust these concentrations reflect fundamental differentiation of our planet and are not capable of fingerprinting a certain origin introduction

correction to a thin viscous sheet model for continental deformation summary the governing equation derived by england mckenzie for the deformation of a thin viscous shell contains an error we give the correct derivation here and correct those figures where the change makes a visible difference the correct results differ quantitatively from those of england mckenzie but their conclusions are not affected

application of continuum models to deformation of the aleutian island arc continuum models were constructed to describe large scale deformation of the aleutian island arc over the past 5 m y these models consider the island arc as a continuum in the horizontal plane with the velocity boundary condition at the pacific edge stated as a fraction of pacific plate convergence transferred to the arc first a simple model of uniformly distributed strain is formulated to illustrate the mechanics of continuous deformation lineaments along the arc massif rotated about a vertical axis are matched by small element rotation calculated from the model however this model does not predict across arc variations in deformation and produces an unrealistic amount of crustal thickening after 5 m y a physically more meaningful model of deformation is the thin viscous sheet model based on averages of stress and rheology throughout the lithosphere the amount of motion transferred from the pacific plate to the arc is constrained by the rotated lineaments while the effective stress strain exponent n and the ability the lithosphere has to sustain crustal thickness contrasts the argand number are independent variables primarily bathymetry earthquake focal mechanisms and styles of faulting are used to evaluate the models the preferred model is one where the amount of motion transferred from the pacific plate is greater in an arc parallel direction than in an arc normal direction producing stresses consistent with strike slip faulting at the far western end of the arc and tensional stresses consistent with transverse normal faulting elsewhere in the arc massif this model agrees with observations of slip vectors by ekstr6m and engdahl 1989 who conclude that a portion of the arc parallel component of relative plate motion is taken up in the overriding plate this model implies that compressive stress transferred to the arc is small in comparison to along arc shear stress and that stresses conducive to strikeslip faulting are prevalent throughout the arc

role of lithospheric strength heterogeneities in the tectonics of tibet and neighbouring regions unlike rigid oceanic plates continents show diffuse deformation for example the tibetan plateau that has accommodated much of the convergence of india with asia we interpret the tertiary deformation of asia as the three dimensional strain of a non linear viscous continental lithosphere lateral strength contrasts modify this deformation stress concentration around the relatively strong tarim basin gives rise to local crustal thickening in the tien shan to the north and to strike slip in the altyn tagh along its southern margin

the role of rheology in extensional basin formation modelling the rheology of the lithosphere determines its deformation under given initial and boundary conditions this paper presents a critical discussion on how rheological properties are taken into account in extensional basin modelling since strength envelopes are often used in models we review the uncertainties in temperature and rheological parameters and assumptions in type of rheology and mode of deformation involved in their construction models of extensional basins are classified into three groups kinematic kinematic with rheological constraints and dynamic rheology enters kinematic models only implicitly in the assumption of an isostatic compensation mechanism we show that there is a critical level of necking that reconciles local isostasy with the finite strength of the lithosphere which requires a flexural response kinematic models with rheological constraints make use of strength envelopes to assess the initial lateral variations of lithospheric strength and its evolution with time at the site of extension dynamic models are the only ones to explicitly introduce rheological constitutive equations usually in plane strain or plane stress they usually however require the presence of an initial perturbation thickness variations pre existing faults thermal inhomogeneities rheological inhomogeneities the mechanical boundary conditions kinematic and dynamic and the thermal boundary conditions constant temperature or constant heat flux at the lower boundary of the lithosphere may result in negative positive feedbacks leading to cessation acceleration of extension we conclude that while kinematic models with rheological constraints if possible are very successful in accounting for the observed characteristics of sedimentary basins dynamic models are necessary to gain insight into the physical processes underlying basin formation and evolution

large scale deformation related to the collision of the aleutian arc with kamchatka the far western aleutian island arc is actively colliding with kamchatka westward motion of the aleutian arc is brought about by the tangential relative motion of the pacific plate transferred to major right lateral shear zones north and south of the arc early geologic mapping of cape kamchatka a promontory of kamchatka along strike with the aleutian arc revealed many similarities to the geology of the aleutian islands later studies support the notion that cape kamchatka is the farthest west aleutian island and that il has been accreted to kamchatka by the process of arc continent collision deformation associated with the collision onshore kamchatka includes gravimetrically determined crustal thickening and formation of a narrow thrust belt of intensely deformed rucks directly west of cape kamchatka the trend of the thrust faults is concave toward the collision zone indicating a radial distribution of maximum horizontal compressive stress offshore major crustal faults trend either oblique to the kamchatka margin or parallel to major aleutian shear zones these offshore faults are complex accommodating both strike slip and thrust displacements as documented by focal mechtmisms and seismic reflection data earthquake activity is much higher in the offshore region within a zone bounded to the north by the northernmost aleutian shear zone and to the west by an apparent aseismic front analysis of focal mechanisms in the region indicate that the present day arc continent contact zone is located directly east of cape kamchatka in modeling the dynamics of the coiiision zone using thin viscous sheet theory the rhealogical parameters are only partially constrained to values of n the effective power law exponent 2 3 and ar the argand number s 30 these values are consistent with a fnrcarc thermal profile of kamchatka previously determined from heat flow modeling the thin viscous sheet modeling also indicates that onshore thrust faulting is a consequence not only of compressive stresses resulting from the west directed collision but also of sediment induced coupling of the subducting pacific plate

fault growth in brittle ductile experiments and the mechanics of continental collisions using laboratory models of continental collisions we study the mechanisms responsible for large scale deformation and the nature of the penetrative deformation localized or homogeneous and its characteristic scales and structure in order to focus the study one of the most spectacular cases of continental collisions namely the india asia collision is considered different models with varying rheologies are analyzed which attempt to respect the brittle ductile stratification corresponding to the crust and mantle structure in the earth each experiment is quantified by studying the strain field and the fault pattern as a function of the position p of the indenter within the system the strain field is characterized by 1 the second invariant of the two dimensional strain tensor and its variation with position for a fixed p the evolution of its average as a function of p 2 the evolution with p of the participation ratio s2 which quantifies the fraction of the system surface area which participates in the deformation 3 the evolution of the escape ratio calculated as the surface increase gained by the system during its eastward lateral escape divided by the surface covered by the indenter penetration qualitatively we find that the deformation first spreads out but later on undergoes localization once the fault pattern is mature the cumulative fault length does not increase anymore we observe that the strain field is essentially controlled by the kinematics of the larger faults each fault pattern is quantified by studying 1 the histograms of fault orientations 2 the capacity and barycenter fractal dimensions 3 the multifractal generalized dimensions and 4 the distribution of fault lengths the fractal dimension d is found to almost constant within experimental uncertainty d 1 7 0 1 and thus appears rather insensitive to the particular chosen rheology we find a correlation between the generalized multifractal dimensions and two exponents the barycenter fractal dimension b and the exponent a of the fault length distribution this shows that the scaling properties of fault patterns can be characterized by the knowledge of only two exponents b and a of the spatial and length distributions of faults our main results are 1 observation of the growth of self similar fault patterns 2 complete characterization of the fault patterns with two scaling exponents a and b 3 wide distribution of undeteriorated domains and large heterogeneity of the deformation field 4 maturation of the fault structure corresponding to a localization of the deformation on a few large faults in the late stage of the deformation

slip rate depth distribution for active faults in central italy using numerical models abstract slip rate is a critical parameter for describing geologic and earthquake rates of known active faults although faults are inherently three dimensional surfaces the paucity of data allows for estimating only the slip rate at the ground surface and often only few values for an entire fault these values are frequently assumed as proxies or as some average of slip rate at depth evidence of geological offset and single earthquake displacement as well as mechanical requirements show that fault slip varies significantly with depth slip rate should thus vary in a presumably similar way yet these variations are rarely considered in this work we tackle the determination of slip rate depth distributions by applying the finite element method on a 2d vertical section with stratification and faults across the central apennines italy in a first step we perform a plane stress analysis assuming visco elasto plastic rheology and then search throughout a large range of values to minimize the rms deviation between the model and the interseismic gps velocities using a parametric analysis we assess the accuracy of the best model and the sensitivity of its parameters in a second step we unlock the faults and let the model simulate 10 kyr of deformation to estimate the fault long term slip rates the overall average slip rate at depth is approximately 1 1 mm yr for normal faults and 0 2 mm yr for thrust faults a maximum value of about 2 mm yr characterizes the avezzano fault that caused the 1915 mw 7 0 earthquake the slip rate depth distribution varies significantly from fault to fault and even between neighbouring faults with maxima and minima located at different depths we found uniform distributions only occasionally we suggest that these findings can strongly influence the forecasting of cumulative earthquake depth distributions based on long term fault slip rates

carbonate clumped isotope constraints on silurian ocean temperature and seawater δ 18 o much of what we know about the history of earth s climate derives from the chemistry of carbonate minerals in the sedimentary record the oxygen isotopic compositions 18 o of calcitic marine fossils and cements have been widely used as a proxy for past seawater temperatures but application of this proxy to deep geologic time is complicated by diagenetic alteration and uncertainties in the 18 o of seawater in the past carbonate clumped isotope thermometry provides an independent estimate of the temperature of the water from which a calcite phase precipitated and allows direct calculation of the 18 o of the water the clumped isotope composition of calcites is also highly sensitive to recrystallization and can help diagnose different modes of diagenetic alteration enabling evaluation of preservation states and identification of the most pristine materials from within a sample set critical information for assessing the quality of paleoproxy data generated from carbonates we measured the clumped isotope composition of a large suite of calcitic fossils primarily brachiopods and corals sedimentary grains and cements from silurian ca 433 ma stratigraphic sections on the island of gotland sweden substantial variability in clumped isotope temperatures suggests differential preservation with alteration largely tied to rock buffered diagenesis complicating the generation of a stratigraphically resolved climate history through these sections despite the generally high preservation quality of samples from these sections micro scale observations of calcite fabric and trace metal composition using electron backscatter diffraction and electron microprobe analysis suggest that only a subset of relatively pristine samples retain primary clumped isotope signatures these samples indicate that silurian tropical oceans were likely warm 33 7 c and similar in oxygen isotopic composition to that estimated for a modern ice free world 18 o vsmow of 1 1 1 3 this result joins the growing body of evidence that suggests the 18 o of earth s ocean waters has remained broadly constant through time

finite strain calculations of continental deformation 1 method and general results for convergent zones a finite element method is used to calculate stresses and strain rates in a thin viscous sheet representing the continental lithosphere that is bordered in one part by an indenting boundary we calculate solutions for velocity fields crustal thickness distributions strain rates rotation time integrated vorticity and finite strain ellipsoids to show the dependence of these quantities on the controlling parameters the stress strain exponent n and the strength of the sheet relative to the gravitational forces which may be expressed as a dimensionless number the argand number ar for n 3 and ar between 1 and 10 a plateau of thickened crust is formed in front of the indenter the strain histories of individual elements within the viscous sheet may be quite complex and the finite strain at the end of deformation may bear no obvious relation to the principal stress orientations during the deformation the strain rate fields are interpreted in terms of the style of faulting that would be expected in the brittle upper crust if it were coupled to the ductile but stronger upper mantle in general the length scale of deformation increases with ar and time since the start of the collision and decreases with n for ar 3 the increase in length scale is accompanied by a change in the style of deformation from predominantly crustal thickening with thrust faulting in the brittle layer to predominantly transcurrent deformation strike slip faulting in the brittle layer

early silurian glaciations in brazil abstract the opening of the brazilian paleozoic basins during the late ordovician was followed by a tectonic stabilization in the silurian at the time of the early silurian glaciations deposition of continental sandstones prevailed in the basins offshore environments were present in the amazonas basin and to a smaller extent in the parana basin the ice sheets advanced over fluviatile and littoral sediments except for the south margin of the amazonas basin where the glacial diamictites immediately overlie marine facies the ice produced much meltwater even when advancing and this was probably climatically controlled the ice movements were from the northwest at the northern margin of the amazonas basin and towards the northeast at the southern margin in the parnaiba basin and its outliers the jatoba basin and cariri valley the ice came from southeast and probably also from east further to the south in the parana basin it came from the north and northeast four glaciations have been dated to the early silurian the oldest is dated as latest ashgill and or earliest llandovery in the parana basin as evidenced from tillites in the upper rio ivai and lower vila maria formations including the former iapo formation it is contemporaneous with a glaciation in the cape basin south africa the next glaciation is dated as early middle llandovery gregarius graptolite zone and is present in the amazonas basin and the andean belt and probably also in the parnaiba basin and its outliers the two youngest tillites were widespread in the northern brazilian basins amazonas parnaiba and jatoba basins and cariri valley as well as in the andean belt and are dated as early late llandovery possibly starting already in the late middle llandovery and latest llandovery to earliest wenlock the latter tillite is the most widespread one assuming these glaciations are related to polar positions the apparent migration of the south pole ice cap must have been quite rapid from central north africa in late ashgill towards central brazil in the middle late llandovery the glaciation in the parana basin took place up to 45 from the late ashgillian south pole the cool climate continued during the silurian but no new glaciations are known before latest devonian

the cretaceous tertiary deformation of the lhasa block and its implications for crustal thickening in tibet the strong tertiary deformation of the northern margin of the indian plate and of the tsangpo suture zone is not matched by comparable deformation within the lhasa block yet the southern margin of the block was the site of large scale intrusive activity the gangdese or transhimalayan batholith throughout the late cretaceous and was therefore presumably weaker than its surroundings this poses a problem for any approach that interprets the deformation of tibet in terms of diffuse crustal thickening however there is evidence that the lhasa block was in elevation as well as in magma type an andean margin by the end of the cretaceous and we suggest that the buoyancy force associated with the elevated crust was sufficient to inhibit compressional deformation within the southern lhasa block this suggestion is tested quantitatively by treating the continental lithosphere as a thin viscous sheet containing an isostatically compensated elevation contrast when subjected to boundary conditions representing the collision between india and asia the sheet accommodates convergence by diffuse deformation over a region comparable in size with the tibetan plateau in the absence of an elevation contrast this deformation consists of approximately 100 thickening strain over the region a precollision elevation contrast of 3000 m in the lhasa block would have resulted in its experiencing roughly 40 rather than 100 postcollisional thickening smaller elevation contrasts would have resulted in greater thickening and vice versa these calculations have implications for the mode of crustal thickening in northern tibet as the available evidence suggests that the topographic expression of the earlier mesozoic tectonic activity in southern asia was erased before the india asia collision we expect that there has been widespread and relatively homogeneous crustal shortening migrating northward from the lhasa block during the tertiary

fluctuations in epeiric sea carbonate production during silurian positive carbon isotope excursions a review of proposed paleoceanographic models positive excursions in the carbon isotopic ratio of marine carbonates 13ccarb are commonly attributed to an increase in the burial ratio of organic versus carbonate carbon in the global oceans regardless of whether this increased deposition of organic carbon was oceanographically or biologically induced the fact that high carbon isotope values are most often attributed to the sequestration and burial of organic carbon remains the notion that high 13ccarb values should be the result of organic rich deposition in epeiric sea settings is not surprising considering that the overwhelming majority of organic carbon burial in the modern oceans occurs in continental shelf and margin settings what is surprising is that major positive carbon isotope excursions during the silurian are recorded in clean carbonate successions with little to no appreciable black shale deposition in epeiric seas in fact silurian 13ccarb excursions occurred during intervals of prolific reef development and an overall expansion of carbonate platform environments globally this discrepancy has sparked considerable debate among silurian carbon isotope specialists but has offered a valuable opportunity to re examine the intricate connections among nutrients primary productivity carbonate sedimentology and the global carbon cycle in the lower paleozoic here we consider several sedimentological and geochemical features of one of these silurian positive carbon isotope 13ccarb excursions and discuss the broader implications of current silurian 13ccarb models to the interpretation of lower paleozoic carbonate sequences

kinematics and time of emplacement of the upper allochthon of the rhodope metamorphic complex evidence from the rila mountains bulgaria the rhodope metamorphic complex is a nappe pile of four large allochthonous megaunits referred to as lower middle upper and uppermost allochthons these were originally derived from different paleogeographic provinces and were stacked on top of each other in the course of convergent processes along the european continental margin between the late jurassic and the late eocene the direction and age of thrusting as well as the degree and age of alpine metamorphism vary between individual allochthons and are altogether controversial the affiliation of individual tectonic units to allochthons is often unclear however for the area of the eastern rhodope mountains there is agreement that the thrusting of the upper allochthon onto the middle allochthon happened before ca 68 70 ma since upper cretaceous magmatic bodies crosscut the metamorphic fabric of the rocks for the northwestern parts of the rhodope metamorphic complex rila mountains the age of thrusting was considered to predate the late cretaceous here we present the results of mapping structural observations and la icp ms dating of zircons and monazites from the northwest rila mountains and revise the tectonic architecture and history of the area there the boundary between the upper and middle allochthons is the dodov vrah shear zone this regional scale structure represents a top to the southeast amphibolite facies thrust along which numerous syn kinematic intrusive bodies were emplaced u pb la icp ms zircon and monazite dating of the latter as well as dating of pre and post kinematic plutonic rocks from the study area showed that the time of activity of the dodov vrah shear zone can be bracketed between ca 58 ma and 50 48 ma thus at least for the northwestern parts of the rhodope metamorphic complex the thrusting of the upper onto the middle allochthon happened after the latest palaeocene and before the end of the lower eocene

the effect of early alpine thrusting in late stage extensional tectonics evidence from the kulidzhik nappe and the pelevun extensional allochthon in the rhodope massif bulgaria abstract in the northeastern rhodope massif the kulidzhik nappe exposes a unique juxtaposition of a high grade basement allochthon onto a low grade mesozoic unit and the counterpart pelevun extensional allochthon belonging to the same unit the kulidzhik nappe tectonostratigraphy comprises structurally upward i a lower unit consisting of high grade basement orthogneisses ii a low grade greenschist phyllite unit consisting of jurassic extrusive rocks and metasedimentary rocks iii the nappe allochthon built by the lower high grade basement unit orthogneisses and iv eocene sedimentary rocks and oligocene volcanic cover rocks the pelevun extensional allochthon is heterogeneous and consists of mesozoic low grade unit marbles and greenschists and the upper high grade basement unit we have combined structure and kinematics with lithological information and 40 r 39 r geochronology to constrain the tectonic evolution and regional significance of the kulidzhik nappe and the pelevun extensional allochthon mineral chemistry reveals igneous phases of the granitic protolith of the allochthonous orthogneisses with textures related to ductile deformation and metamorphism higher than 500 c their trace element patterns are indistinguishable from the high grade basement orthogneisses in the eastern rhodope mineral chemistry of the metamorphic assemblage in the underlying greenschists is consistent with medium grade greenschist facies metamorphism at temperatures well below 450 c whose geochemistry defines transitional morb to iat affinities with a strong arc imprint the greenschist s composition is extremely similar to the composition of a supra subduction zone jurassic arc extrusive suite that occurs in the low grade unit of the eastern rhodope the structural elements in all of the tectono metamorphic units and the nappe surface indicate top to the nne tectonic transport deformation evolved from ductile to brittle conditions coevally with a progressive decrease from lower amphibolite to weak greenschist facies metamorphism towards the structural top two klippen gave plateau 40 r 39 r mica ages of 154 23 0 66 ma and 156 70 0 81 ma respectively which reflect cooling following the greenschist facies event below 350 c during shallow crustal level thrust emplacement and exhumation of the kulidzhik allochthon the hanging wall pelevun extensional allochthon preserves an internal nne directed ductile tectonic transport trend but is underlain by a ssw directed ductile brittle extensional detachment and has experienced ne sw brittle extension on high angle normal faults an amphibole 40 r 39 r inverse isochron age of 156 58 0 60 ma constrains a late jurassic upper greenschist facies tectono metamorphic event whereas a white mica plateau age of 39 66 0 47 ma provides evidence for middle eocene cooling and exhumation of the pelevun extensional allochthon in the hanging wall of the detachment we relate the kulidzhik nappe to late jurassic crustal deformation during arc continental margin collision that involved nne directed nappe staking and metamorphism of continental margin basement and island arc units the nappe shares a tectono metamorphic history with the nappes of the adjacent strandzha massif implying a region wide early alpine orogenic system our results reveal a record of early alpine thrust tectonics and show the significance of crustal accretion related assembly for the tectonic evolution of the rhodope massif both the late jurassic thrusting event and the subsequent cretaceous thrusting event thickened the rhodope crust creating crustal instability which influenced tertiary crustal extension

the strength of faults in the crust in the western united states abstract the strength of secondary faults within plate boundary zones and of master faults like the san andreas has been controversial for decades we use a global finite element code with a variable resolution grid and global plate driving forces to determine whether the effective friction on the san andreas fault is high 0 6 1 intermediate 0 3 0 5 or low 0 2 whether a single value of can be used for all mapped faults within california and whether weakening of the ductile lower crust associated with faulting is important we compare our model results with existing data on fault slip rates gps velocities stress field and earthquake depth distribution the comparison indicates that all faults are weak 0 2 and that additional weakening of major faults is important all viable solutions also indicate that weakening of the lower crust below major faults is necessary the strongest faults in the region have in the range 0 2 0 05 the san andreas fault is a very weak fault among weak faults with

paleoenvironmental changes in the silurian indicated by stable isotopes in brachiopod shells from gotland sweden ratios of stable carbon and oxygen isotopes in brachiopod shells more than 370 specimens esp atrypa reticularis from the silurian of gotland sweden have been analysed preservation of biological skeletal ultrastructures observed in sem micrographs and cathodoluminescence analyses indicate that usually no diagenetical alteration occurs the silurian of gotland consists of 440 m carbonate deposits spanning the late llandovery to late ludlow epochs 431 411 m y repeatedly uniform sequences of micritic limestones and marks are interrupted by complex structured reefs and adjacent platform sediments previously the alteration of facies is interpreted as the result of sea level fluctuations caused by a gradual regression with superimposed minor transgressive pulses the silurian sequence of gotland exhibits principally parallel carbon and oxygen isotope records corresponding closely to the topostratigraphic units lower values occur in periods dominated by deposition of marly sequences higher values are observed in periods dominated by reefs and extended carbonate platforms the isotope ratios are influenced by local as well as global factors the oxygen isotope ratios are interpreted to reflect paleosalinity changes due to varying freshwater input rather than changes in paleotemperature consequently the facies distribution of gotland is interpreted as resulting from changes in terrigenous input caused by different rates of continental weathering and freshwater runoff rather than by sea level fluctuations periods of arid climate and therefore anti estuarine downwelling of oxygenated surface water appear as short episodes of reef growing 1 5 m y in an epoche characterized by a tropic humid climate which causes an estuarine circulation and the upwelling of co2 rich deep water carbon isotope ratios are obviously connected to these changes in circulation by the advection of 13c rich surface water arid episodes or upwelling of 13c depleted deep water humid episodes of a silurian ocean which itself reveals generally euxinic deep water conditions due to the burial of organic carbon in black shales

comment on georgiev et al structure and u pb zircon geochronology of an alpine nappe stack telescoped by extensional detachment faulting kulidzhik area eastern rhodopes bulgaria in this discussion we evaluate the field chemical sedimentary structural and metamorphic data related to the kulidzhik area tectonic proposal

steady propagation of delamination events delamination of the lithospheric thermal boundary from overlying continental crust propagates laterally from the line of initiation accelerating as the sinking slab of detached lithosphere grows longer this propagation has been numerically modeled with steady state equations in a moving reference frame by matching an interior finite element solution to flexible boundary conditions which represent the mechanical and thermal response of the surroundings the form of the solution depends on the shear coupling of intruding asthenosphere to the top of the sinking slab across a thin layer of crustal material without coupling the tip of the intrusion cools and stiffens to form a wedge dividing the crust cold mode with coupling the intrusion is forced to convect and remains ductile hot mode the cold mode can propagate at all velocities the hot mode has a lower limiting velocity of 1 2 cm year but offers less resistance at higher speeds resistance to delamination includes a constant term from the buoyant crustal downwarp plus a velocity proportional term representing viscous deformation however the proportionality constant of the latter term is only weakly dependent on crust and lithosphere viscosities matching this resistance to loading lines of 100 to 800 km slabs sinking in a mantle of 1022 p velocities of 0 3 8 0 cm year are obtained changes in viscosity affect this rate but cold mode delamination is unstoppable except at continental margins or by failure in the sinking slab the surface expression of delamination is a leading outer rise followed by a submarine trough with a large negative free air anomaly which finally evolves into a 1 km plateau if crustal viscosity and velocity are both low however there is a montonic crustal uplift with no trough thus the present lack of linear supracontinental oceans does not preclude delamination at up to 4 cm year driven by slabs up to 400 km in length

structure and u pb zircon geochronology of an alpine nappe stack telescoped by extensional detachment faulting kulidzhik area eastern rhodopes bulgaria the rhodope metamorphic complex is a stack of allochthons assembled during obduction subduction and collision processes from jurassic to paleogene and overprinted by extensional detachment faults since middle eocene in the study area the following nappes occur in superposition from base to top an orthogneiss dominated unit unit i garnet bearing schist with amphibolite and serpentinite lenses unit ii greenschist phyllite and calcschist with reported jurassic microfossils unit iii and muscovite rich orthogneiss unit iv u pb dating of zircons from a k feldspar augengneiss unit i yielded a protolith age of ca 300 ma garnet bearing metasediment from unit ii yielded an age spectrum with distinct populations between 310 and 250 ma detrital ca 150 ma and ca 69 ma the last two of high grade metamorphic origin an orthogneiss from unit iv yielded a wide spectrum of ages the youngest population gives a concordia age of 581 5 ma interpreted as the age of the granitic protolith unit i represents the lower allochthon byala reka kechros dome unit ii the upper allochthon krumovitsa kimi unit unit iii the uppermost allochthon circum rhodope belt and unit iv a still higher far travelled unit of unknown provenance telescoping of the entire rhodope nappe stack to a thickness of only a few 100 m is due to late eocene north directed extensional shearing along the newly defined kulidzhik detachment which is part of a major detachment system along the northern border of the rhodopes older top to the south mylonites in unit i indicate that tertiary extension evolved from asymmetric top to the south to symmetric top to the south and top to the north bivergent unroofing

constraints on extension of continental lithosphere the extension of the continental lithosphere is considered in terms of the deformation of a thin viscous sheet with a temperature dependent rheology which is governed primarily by the strength of the upper mantle the lithosphere is subjected either to a constant strain rate or to a strain rate decreasing with time corresponding to the stretching of a region of fixed volume with a constant velocity boundary condition the progress of the extension is influenced by the decrease in strength of the lithosphere owing to its attenuation and by an increase in strength resulting from cooling the relative importance of these two effects depends on two parameters a dimensionless strain rate and the total strain for a given strain rate the extending lithosphere goes through two phases as the strain increases in the first the effect of thinning the lithosphere is predominant and the average strength of the lithosphere remains close to and perhaps slightly below its original strength in the second the effect of cooling of the lithosphere leads to very rapid increase in strength as a function of strain resulting presumably in an effective limit on the degree of stretching that the lithosphere experiences for the case of strain rate decreasing with time when the initial strain rates are of order 10 15s 1 this maximum extension is between 100 and 150 for a strain rate of 10 l4 s 1 the maximum lies around 400 to 500 at higher strain rates than this very large extension may occur before cooling significantly affects the strength of the lithosphere this model accords well with the constraints available from the continental margins of the north atlantic

gravitational spreading of high terrain in ishtar terra venus magellan altimetry measurements indicate that the mountain belts and plateau margins of ishtar terra have topographic slopes in the range 2 30 magellan radar images reveal that numerous sets of narrow closely spaced troughs and lineations which we interpret as graben and normal faults are located in many of these regions of high slope the orientation of many of these graben sets perpendicular to the downslope direction suggests that they formed as a result of gravitational spreading in several of the mountain belts the extensional structures are parallel to the apparent direction of shortening consistent with the interpretation that gravitational spreading occurred while mountain building was still active to explore the implications of this spreading for the tectonic evolution of ishtar terra we model the process by means of a finite element algorithm for viscoelastic deformation in a vertical section of the crust at the margin of a plateau or broad mountain range flow in the crust is assumed to be governed by a nonlinear depth dependent rheology we predict brittle failure and relaxation of topographic relief as functions of time for ranges of crustal thickness 10 30 km and thermal gradient 5 25 k km and for observed ranges in topographic elevation 1 6 km and slope 1 30 this parameter range predicts a large spread in the time scales for initial failure and significant relaxation of relief for a crustal thickness greater than 10 km a thermal gradient of 15 k km or more and average values of relief 3 km and slope 3 the topography relaxes to 25 of its original height in less than 10 m y although initial failure in the form of normal faulting on the highlands and margin and occasional shallow thrusting in the lowlands is predicted to occur much earlier values of horizontal surface strain that are likely to be observable 1 do not accumulate until significant relaxation of relief begins given the predicted rate of topographic relaxation either the crust of ishtar terra must be anomalous in some way either much stronger than predicted by standard flow laws or very thin or cold or the topographic relief and high slopes have been actively built and maintained until times significantly younger than the average crater retention age for venus of 500 ma the large apparent depth of compensation the abundance of volcanism and the uncertainty in the local age favor the view that mountain building in ishtar terra has occurred until times at least as recent as 10 ma

fault slip rates for the active external dinarides thrust and fold belt 1 we present estimates of slip rates for active faults in the external dinarides this thrust and fold belt formed in the adria eurasia collision zone by the progressive formation of ne dipping thrusts in the footwalls of older structures we calculated the long term horizontal velocity field slip rates and related uncertainties for active faults using a thin shell finite element method we incorporated active faults with different effective fault frictions rheological properties appropriate geodynamic boundary conditions laterally varying heat flow and topography the results were obtained by comparing the modeled maximum compressive horizontal stress orientations with the world stress map database the calculated horizontal velocities decrease from the southeastern external dinarides to the northwestern parts of the thrust and fold belt this spatial pattern is also evident in the long term slip rates of active faults the highest slip rate was obtained for the montenegro active fault while the lowest rates were obtained for active faults in northwestern slovenia low slip rates influenced by local active diapirism are also characteristic for active faults in the offshore central external dinarides these findings are contradictory to the concept of adria as an internally rigid aseismic lithospheric block because the faults located in its interior release a part of the regional compressive stress we merged the modeling results and available slip rate estimates to obtain a composite solution for slip rates

stress magnitude strain rate and rheology of extended middle continental crust inferred from quartz grain sizes in the whipple mountains california knowledge of the magnitude of differential stress and strain rate during the formation of mylonitic shear zones in metamorphic core complexes provides constraints on the mechanical behavior of the middle continental crust during extension we analyzed the differential flow stress during the mylonitization of quartzofeldspathic rocks in the whipple mountains california using grain size piezometers and kinetic laws for grain growth mylonitic gneisses collected from two widely separated transects have grain sizes that cluster in a range from 32 to 61 m analysis of grain growth kinetics indicates that mylonitization of the gneisses continued during cooling to temperatures 500 c compatible with estimates from two feldspar thermometry quartz grain size piezometers suggest that the mylonitization occurred under differential stresses 1 3 of 40 150 mpa or maximum shear stresses of 20 75 mpa extrapolation of quartzite flow laws to 500 c indicates that the mylonitization occurred at strain rates faster than 10 14 s 1 these estimates suggest that the mylonitic zone within the whipple mountains had an effective viscosity of the order of 1018 4 1020 4 pa s these low viscosities and rapid strain rates combined with seismic reflection data showing that continental crust is layered suggest that more realistic physical models of extension of the continental lithosphere should treat the lithosphere as a heterogeneous distribution of high viscosity regions separated by low viscosity zones

laramide crustal thickening event in the rocky mountain foreland and great plains the rocky mountain foreland and great plains of the western united states were formerly part of a continental platform adjusted by erosion and deposition in cambrian through jurassic time to near mean sea level and therefore to a near uniform crustal thickness of approximately 33 km today the region stands at regional elevations up to 2 km isostatically supported by a crust exceeding 50 km in thickness reasonable estimates of tertiary sedimentation and laramide strain do not account for more than 15 of the implied thickening however from approximately 70 40 m y b p this region was underlain by a horizontally subducting slab of farallon plate lithosphere moving northeast this slab could have been the cause of the thickening finite difference thermal models with specified kinematics show only a temporary cooling of the base of the north american lithosphere by this slab the excess weight of the slab would have depressed the region plate bending calculations show a quantitative agreement of predicted depression with upper cretaceous isopachs since depression by this slab lasted until the eocene at least the latest cretaceous regression was probably caused by a laramide crustal thickening event the farallon slab might have caused crustal thickening in two ways its excess weight would have drawn in ductile lower crust from surrounding regions however calculations show that this effect is too slow too local and too reversible to explain most of the crustal thickening therefore it seems likely that ductile lower crust was transported from sw to ne by shear stresses which the farallon plate exerted on the base of the north american lithosphere a preliminary finite element calculation based on this hypothesis shows the correct general pattern of crustal thickening an unexpected but encouraging result is that predicted principal compression directions are orthogonal to many laramide basement uplifts

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

formation of the rocky mountains western united states a continuum computer model one hypothesis for the information of the rocky mountain structures in late cretaceous through eocene time is that plate of oceanic lithosphere was underthrust horizontally along the base of the north american lithosphere the horizontal components of the motion of this plate are known from paleomagnetism and the edge of the region of flat slab can estimated from reconstructed patterns of volcanism new techniques of finite element modeling allow prediction of the thermal and mechanical effects of horizontal subduction on the north american plate a model that has a realistic temperature dependent rheology and a simple plane layered initial condition is used to compute the consequences of horizontal underthrusting in the time interval 75 million to 30 million years before present successful prediction of this model include i the location amount and direction of horizontal shortening that has been inferred from laramide structures ii massive transport of lower crust from southwest to northeast iii the location and timing of the subsequent extension in metamorphic core complexes and the rio grande rift and iv the total area eventually involved in basin and range style extension in a broad sense this model has predicted the belt of laramide structures the transport of crust from the coastal region to the continental interior the subsequent extension in metamorphic core complexes and the rio grande rift and the geographic region of late tertiary basin and range extension its principal defects are that i many events are predicted about 5 million to 10 million years too late and ii the wave of crustal thickening does not travel far enough to the east reasonable modifications to the oceanic plate kinematics and rheologies that were assumed may correct these defects the correspondence of model predictions to actual geology is already sufficiently close to show that the hypothesis that horizontal subduction caused the laramide orogeny is probably correct the rocky mountain thrust and reverse faults formed in an environment of east west to northeast southwest compressive stress that was caused by the viscous coupling between the oceanic plate and the base of the north american crust nonuniform crustal thickening by simple shear transport also caused relative uplifts therefore this model is consistent with both of the range forming mechanisms that have been inferred 1 a new proposal that arises from this simulation is that horizontal subduction also caused the subsequent extensional basin and range taphrogeny by stripping away the mantle lithosphere so that the crust was exposed to hot asthenosphere after the oceanic slab dropped away

continuum calculations of continental deformation in transcurrent environments a thin viscous sheet approximation is used to investigate continental deformation near a strike slip boundary the vertically averaged velocity field is calculated for a medium characterized by a power law rheology with stress exponent n driving stresses include those applied along boundaries of the sheet and those arising from buoyancy forces related to lateral differences in crustal thickness calculations are performed with either velocity or stress prescribed on the transcurrent boundary exact and approximate analytic solutions for a region with a sinusoidal strike slip boundary condition are compared with solutions for more geologically relevant boundary conditions obtained using a finite element technique the across strike length scale of the deformation is approximately 1 4 n 1 2 times the dominant wavelength of the imposed strike slip boundary condition for both the analytic and the numerical solutions this result is consistent with length scales observed in continental regions of large scale transcurrent faulting an approximate linear relationship between displacement and rotation is found that depends only on the deformation length scale and the rheology calculated displacements finite rotations and distribution of crustal thicknesses are consistent with those observed in the region of the pacific north america plate boundary in california

the strength of large scale plate boundaries constraints from the dynamics of the philippine sea plate since 5ma abstract friction controls most of the strength of tectonic plate boundaries and thus the force mutually exchanged between plates estimates of the plate boundary friction coefficient are therefore of paramount importance to our understanding of the lithosphere torque balance however several lines of evidence indicate that the friction coefficient of plate margins is significantly lower than is measured in laboratory experiments on fractured rocks this poses a significant limit to extrapolation from the laboratory scale to the large scale of earth s lithosphere the record of past and present day plate motions represents an efficient probe into the torque balance of plate tectonics because plate motions readjust virtually instantaneously to temporal changes in driving and resisting forces here i derive inferences on the strength of large scale plate boundaries by focusing on the convergence of the fast moving philippine sea plate towards eurasia since subduction initiation 5 myr ago because at the present day the philippine slab reaches depths shallower than the 410 km transition zone in the upper mantle its weight is unlikely to provide sufficient driving force to shear the trailing plate over the viscous mantle at the observed rates using global models of the coupled mantle lithosphere system i argue that frictional coupling with the pacific and eurasia plates is the main driver for the dynamics of the philippine sea plate this allows me to infer that the friction coefficient of large scale plate boundaries is in range 0 01 0 07 and that the amount of sediments entering plate margins efficiently modulates the friction coefficient contributing significantly to its lateral variations importantly i corroborate my conclusions by looking at the present day vertical deformation of the philippine sea eurasia margin evident from observed trench parallel gravity anomalies

the ireviken event in the lower silurian of gotland sweden relation to similar palaeozoic and proterozoic events abstract for a long time the silurian was thought to represent a time of stable environmental conditions in the greenhouse period that followed the late ordovician glaciation during the past decade knowledge about the silurian has increased markedly and today it is known that the conditions in the silurian were much more variable than previously assumed detailed isotopic investigations have revealed several distinct positive excursions in both carbon and oxygen isotope values in low latitudes these periods of high c and o isotope values are in many cases characterised by the growth of reefs and the formation of extended carbonate platforms the sediments deposited during these excursions contain impoverished fossil assemblages especially with respect to conodonts graptolites and trilobites a conspicuous isotope excursion coincident with facies changes and a marked mass extinction is observed near the llandovery wenlock boundary this event is called the ireviken event after its type locality on the island of gotland sweden jeppsson l 1987 in palaeobiology of conodonts ellis horwood ltd chichester pp 129 145 here isotope data from nine sections at the nw coast of gotland are presented that cover the time interval of the ireviken event the 13c mean values rise from 1 4 to 4 5 and the 18o values increase from 5 6 to 5 0 the relative timing of stable isotope development extinctions and facies development is discussed it is shown that first extinctions precede the isotope excursion this indicates that extinction events and stable isotope development are only indirectly connected but might reflect the same causes other events characterised by similar relationships between positive isotope excursions mass extinctions and facies development are found in younger parts of the silurian late wenlock and ludlow in the late ordovician the late cambrian and with some reservations in the proterozoic the similarities between these events indicate analogous controlling mechanisms for the silurian climatic changes between humid and arid conditions in low latitudes were postulated in an earlier study of the authors a palaeoceanographic climatic model was postulated which is consistent with most of the sedimentological palaeontological and geochemical data here we demonstrate that this model may be applicable also for the older events

early silurian carbonate platform ostracods from iran a peri gondwanan fauna with strong laurentian affinities abstract early silurian aeronian carbonate platform deposits of the lower niur formation of east central iran yield an ostracod fauna of fourteen species dominated by podocopes with rare palaeocopes and binodicopes the podocope rich fauna is unusual in that early palaeozoic carbonate platforms are typically dominated by palaeocopes despite its mid palaeolatitude peri gondwanan setting and isolation from low palaeolatitudes the ostracod assemblage of the lower niur formation shows striking affinities to species level with late ordovician and early silurian ostracod faunas of palaeo equatorial laurentia biogeographically the lower niur formation ostracod fauna also shows species and genus links with late ordovician and early silurian faunas of low palaeolatitude 0 30 s baltica and mid palaeolatitude peri gondwanan uzbekistan dispelling the notion of strict endemism for early silurian ostracods two new species bulbosclerites jamshidis sp nov and steusloffina fravashi sp nov are described

fault friction regional stress and crust mantle coupling in southern california from finite element models ongoing deformation of southern california is simulated in a set of 63 models in an attempt to empirically determine fault friction to a previous code for simulation of anelastic crustal strain we introduce a new finite element to represent a curved fault of arbitrary rheology we also add several selfconsistent patterns of horizontal drag on the base of the crust caused by independent upper mantle flow a grid of isoparametric finite elements represents the regional fault geometry and the known variations in elevation and heat flow plate tectonic velocity boundary conditions are applied solutions include the time averaged flow velocities and fault slip rates and vertically averaged stresses each model is scored by comparing its slip rate predictions with 56 rates from dated offset geologic features in membrane stress models the optimal fault friction is 0 4 0 6 frictional self heating of faults is not a major factor determining strength addition of crust mantle coupling improves results if convergence of north american and pacific lithospheres is assumed to occur under the transverse ranges among such models those with fault friction of about 0 3 score best by most norms of the prediction error these have over 80 of regional strain in faults and only a small amount of block deformation however one model with friction of 0 85 scores better by the percent of correct predictions norm this model has only half the regional strain as fault slip and matches slow rates well but underpredicts slip rates over 1 cm yr furthermore its predictions are degraded considerably if hydraulic lubrication of thrust faults is assumed the low friction models are unaffected we conclude that some source of variation in fault friction remains unmodeled but that at least the fastslipping faults in california have low friction similar to that of hydrated clay gouges appendix is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 001 2 50 payment must accompany order