rise in frequency of surface melting at siple dome through the holocene evidence for increasing marine influence on the climate of west antarctica 1 a new melt layer history from siple dome west antarctica indicates notable late holocene summertime warming visual stratigraphic analyses of the 1004 m ice core identified 62 years with melt layers melting events began around 11 7 ka followed by a period of no melting from 8 8 6 6 ka melt layer frequency increased from 6 6 ka to the present with the 1000 year average melt layer frequency reaching a maximum of 2 at 0 8 ka we use our millennial scale archive of melt events as a unique seasonal paleothermometer to elucidate changes in west antarctic holocene summer climate our calibration suggests the change in melt frequency from 0 to 2 may represent a summer temperature increase of 2 c from the middle to late holocene this temperature change cannot be explained entirely by local change in ice elevation or summer insolation and is in contrast to east antarctic climate records which show peak warmth in the early holocene followed by stable or decreasing temperature we interpret the rise in melt frequency as evidence of an increasing marine influence on the ross sea sector of west antarctica although the surface elevation of siple dome has not changed greatly the continued lateral retreat of the west antarctic ice sheet from its last glacial maximum configuration across the outer continental shelf and the delayed drawdown in ice thickness from the adjacent coastal marie byrd land region in conjunction with periods of increased cyclogenesis perhaps related to variations in enso would allow a moderated maritime climate to more easily reach west antarctica

nature of mantle source contributions and the role of contamination and in situ crystallisation in the petrogenesis of proterozoic mafic dykes and flood basalts labrador proterozoic tholeiitic dyke swarms share many compositional features with and pose similar petrogenetic problems to phanerozoic continental flood basalts but there are few extrusive equivalents of such swarms the mesoproterozoic 1 27 ga harp dyke swarm in labrador is one where possible extrusive equivalents exist in the seal lake group but are slightly displaced in space and time and can probably be related by models of progressive crustal extension here we try to evaluate the roles of crystal differentiation in situ crystallisation crustal assimilation and the relative contributions of asthenosphere and lithosphere derived melts in the petrogenesis of the mafic magmas modelling of the major and trace element variations both within individual dykes and between dykes and within the lava sequence does not suggest an important role for continental crust involvement while in situ crystallisation processes could account for some of the compositional variations the most successful models invoke mixing or contamination of asthenospheric magmas with by veined material in the lower lithosphere upper asthenosphere which carries the continental characteristics the results imply an important role for hydrous phases such as phlogopite and hornblende in the sub lithosphere mantle much of the low mgo character of mafic dykes may result from significant removal of mafic phases during in situ crystallisation within the lithosphere

reverse and normal drag along a fault an analysis of the theoretical displacement field around a single dip slip fault at depth reveals that normal and reverse fault drag develop by perturbation flow induced by fault slip we analytically model the heterogeneous part of the instantaneous displacement field of an isolated two dimensional mode ii fault in an infinite homogeneous elastic body in response to fault slip material on both sides of the fault is displaced and opposing circulation cells arise on opposite sides of the fault with displacement magnitudes increasing towards the center of the fault both normal and reverse drag can develop at the fault center depending on the angle between the markers and the fault normal drag develops there for low angles 30 408 and reverse drag for higher angles a comparison of the theoretical results with published models and natural examples reveals that the characteristics of normal and reverse fault drag are largely insensitive to the scale and rheology of the faulted rocks and that drag forces generated by frictional resistance need not be the primary cause of fault drag fault drag has some interesting geometric implications for normal and reverse fault terminology emphasizing the importance for discrimination of vertical separation and throw furthermore our results lead us to propose an alternative model for the formation of rollover anticlines above normal faults

northland ophiolite new zealand and implications for plate tectonic evolution of the southwest pacific the northland ophiolite and coeval ocean floor sedimentary rock form the northland allochthon obducted from the northeast onto new zealand in the late oligocene the ophiolite was probably emplaced as a single sheet and separated into individual massifs during subse quent movement of the allochthon chemically the bulk of the igneous rocks are normal mid ocean ridge basalts but the ophiolite also includes a younger suite of hornblende modal within plate alkalic rocks believed to represent seamounts two alternative models one involving a subduction flip the other involving continuous westward subduction are proposed to account for the obduction of the upper part of the northland ophiolite the oceanic crust from which the ophiolite originated was formed simultaneously with tasman sea spreading on the western rim of a once much larger south fiji plate assemblage

magma systems related to the red sea opening abstract four distinct magma environments can be recognized in association with the red sea basin 1 1 mid ocean ridge basalts within the present actively spreading axial trough 2 2 basaltic dikes and layered gabbros within the extended precambrian crust on the arabian margin 3 3 granites and rhyolites developed in the extended precambrian crust and 4 4 plateau basalts asymmetrically distributed on the arabian flanks of the red sea these magmas developed as the result of the initial break up of africa and the establishment of an oceanic spreading center within the red sea original asymmetric continental extension occurred on east dipping detachment faults in the oligocene and caused morb like magmas to invade an extending precambrian crust contemporaneous small scale mantle convection initiated limited partial melting under the arabian plate to form the undersaturated alkali olivine plateau basalts slow extension through the miocene allowed an evaporite basin to develop in the red sea while small volumes of rift and plateau basalts formed on the arabian plate a small change in plate motion in the pliocene initiated the seafloor spreading in the axial trough where morb magmas formed oceanic crust rise of the asthenosphere produced small scale thermal perturbations under the arabian plate controlled by an east dipping master detachment fault small scale mantle convection extended partial melting 2 5 eastward under the arabian plate producing more undersaturated diverse alkali olivine plateau basalts distinct chemical and isotopic signatures of these four magma types lead to obvious actualistic correlations with the geodynamic setting

tectonic and gravity extensional collapses in overpressured cohesive and frictional wedges two modes of extensional collapse in a cohesive and frictional wedge of arbitrary topography finite extent and resting on an inclined weak decollement are examined by analytical means the first mode consists of the gravitational collapse by the action of a half graben rooting on the decollement and pushing seaward the frontal part of the wedge the second mode results from the tectonics extension at the back wall with a similar half graben kinematics and the landward sliding of the rear part of the wedge the predictions of the maximum strength theorem equivalent to the kinematic approach of limit analysis and based on these two collapse mechanisms not only match exactly the solutions of the critical coulomb wedge theory once properly amended but generalizes them in several aspects wedge of finite size composed of cohesive material and of arbitrary topography this generalization is advantageous to progress in our understanding of many laboratory experiments and field cases for example it is claimed from analytical results validated by experiments that the stability transition for a cohesive triangular wedge occurs with the activation of the maximum length of the decollement it is shown that the details of the topography for the particular example of the mejillones peninsula north chile is however responsible for the selection of a short length scale dynamic instability corresponding to a frontal gravitational instability a reasonable amount of cohesion is sufficient for the pressures proposed in the literature to correspond to a stability transition and not with a dynamically unstable state

structure and development of the southeast newfoundland continental passive margin derived from screech transect 3 new seismic reflection data from the grand banks of newfoundland and the newfoundland basin add to the growing knowledge of the composition structure and history of this nonvolcanic margin geophysical imaging is now approaching the extent of that done previously on the conjugate margin along iberia providing a valuable database for the development of rifting models two parallel profiles over the shelf platform image deep crustal fabric representing precambrian or possibly appalachian deformation as well asmesozoic extension progressively more intense extension of continental crust is imaged oceanwards without the highly reflective detachments frequently seen on profiles off galicia a landward dipping event l is imaged sporadically and appears to be analogous to a similar event on the iberian iam9 profile the transition zone is probably exposed serpentinized mantle as interpreted off the iberian margin although there appears to be a difference in the character of ridge development and reflectivity the distinctive u reflection identified previously at the base of the newfoundland basin deep water sedimentary section and recently identified as one or more thin basalt sills is imaged on newly presented profiles that connect previously published profiles scr3 and scr2 showing that u is highly regular and continuous except where interrupted by basement highs u is also seen to have a major impact on the ability to image underlying basement a full transect beginning over completely unextended continental crust through to oceanic crust has provided a data set from which estimates of extension and the pre rifting location of the present continental edge can be made two estimates were obtained 85 km based on faulting and 120 km based on crustal thickness

archean greenstone belts are not oceanic crust archean greenstone belts with a preponderance of mafic volcanic rocks often preserved in tectonically complex sequences are obvious candidates in the search for remnants of archean ocean crust we review the tectonic setting and stratigraphy of a number of greenstone sequences previously interpreted as archean ophiolites and conclude on the basis of basal unconformities presence of xenocryst zircons geochemical and isotopic evidence for crustal contamination intrusive relationships with older basement and their internal stratigraphy that none of these examples is derived from archean oceanic crust thermal tectonic and isostatic constraints imply that archean oceanic basins did exist and were covered with several kilometers of water we consider it possible indeed probable that relict archean oceanic crust is preserved in granite greenstone terrains or other archean tectonic settings but that an unequivocal archean ophiolite has not yet been recognized

implications of selected subsurface data on the structural form and evolution of some basins in the northern basin and range province nevada and utah

an interlinked system of folds and ductile shear zones late stage svecokarelian deformation in the central fennoscandian shield finland abstract the svecokarelian deformation of the karelian and svecofennian provinces in finland involved four main phases isoclinal folding and associated thrusting were the main processes of d 1 which was followed by more open folding in d 2 subsequent deformation was localized during d 3 and d 4 this paper focuses on the late stage d 4 svecokarelian deformation that created most of the ductile shear zones in the central fennoscandian shield and was the last penetrative deformational event to affect the entire palaeoproterozoic domain deformation during d 4 occurred approximately from 1 85 1 80 ga and left a greater imprint in the rocks than has commonly been supposed the shear zones formed during d 4 are referred to collectively as the finlandia shear system the karelian province is composed of archaean basement and its autochthonous allochthonous palaeoproterozoic sedimentary cover svecokarelian tectonics split the basement into separate complexes the rheological properties of which differed markedly from those of the adjacent proterozoic supracrustal units on the northern and southern margins of the archaean complexes folding with steep e w trending axial surfaces was the main response to d 4 deformation whereas on the eastern and western margins of the complexes the response to d 4 was the development of sinistral and dextral shear zones trending ne sw and nw se respectively on a large scale these areas of folding and of shearing form an interlinked system the changes from one style of deformation to the other being gradual the principal d 4 elements in the area of juvenile svecofennian crust are folds with e w trending axial surfaces

mechanics of extensional wedges the extensional wedges that overlie low angle basal detachment faults are characterized by a tapered cross section similar to that exhibited by a fold and thrust belt but with an opposite sense of shear on the basal fault such wedges are common in zones of crustal extension such as the basin and range province of the north american cordillera the north sea and the u s gulf coast this paper examines the mechanics of these upper crustal extensional wedges using a modification of the compressional coulomb critical taper model developed for fold and thrust belts and accretionary wedges if a compressional wedge is mechanically analogous to a wedge of soil being pushed up an incline by a moving bulldozer then an extensional wedge is analogous to the same wedge with the bulldozer moving in reverse gear down the incline a critically tapered extensional wedge is one that is on the verge of coulomb failure everywhere a wedge whose taper is narrower may be slid stably down the same incline without internal deformation a wedge whose taper is greater will fail by normal faulting and reduce its taper until it is critical after testing the quantitative predictions of this extensional critical taper theory in a series of laboratory experiments on dry sand wedges we apply the theory to a pair of active extensional wedges in the brazos area offshore texas both wedges are underlain by well imaged basal detachment faults dipping at 17 o down to depths of 10 km one wedge exhibits relatively little internal deformation whereas the other is pervasively faulted along a series of moderately dipping synthetic normal faults direct and indirect estimates of the pore fluid pressure distribution within the two wedges suggest that the undeformed wedge is stable whereas the pervasively faulted wedge is unstable but nearly critical regional values for the coefficients of basal and internal friction in excess of 0 3 together with vertical cohesion gradients in excess of 1 kpa m are consistent with the observed pore pressures and the geometry of the wedges as well as with in situ stress data acquired during drilling and the dips of active synthetic normal faults such values are comparable to values inferred from mechanical and thermal models of compressional wedges and they are consistent with laboratory rock mechanics data

nature of the early proterozoic outokumpu assemblage eastern finland abstract the 1970 ma old outokumpu assemblage consists of serpentinites gabbro and mafic metavolcanites with pelagic and chemogenic sediments lying in the early proterozoic 2200 1950 ma karelides of eastern finland its formation occurred after the development of the jatulian 2200 2000 ma quartzite dominated shelf sediments and prior to the deposition of the kalevian flysch the outokumpu assemblage has been compared to an ophiolite and though the lithologies are all present sheeted dykes excepted the internal stratigraphy suggests otherwise the mafic metavolcanites have a more depleted chemistry than n morb and were erupted into shallow water along with the deposition of pelagic shales carbonates and chemogenic sediments including metalliferous exhalites ultramafic sills intruded these rocks at a later stage prior to rapid subsidence and flysch deposition the recently discovered jormua ophiolite complex may constitute part of the depositional basement to the outokumpu assemblage and the kalevian flysch which formed over stretched crust in an ensialic basic

mid ocean ridge or marginal basin origin of the east taiwan ophiolite chemical and isotopic evidence the east taiwan ophiolite eto occurs as allochthonous fragments embedded in the plio pleistocene lichi melange in the coastal range of eastern taiwan it is probably the youngest among the known ophiolitic complexes of the world the overall geochemical and isotopic characteristics of the representative rock types are very oceanic and strongly argue for a mid ocean or marginal basin origin of the eto the best criteria include 1 major element compositions 2 ree distribution patterns 3 nd isotopic ratios some have nd 12 4 pb isotopic ratios and 5 the presence and nature of the red shale this conclusion is supported by previous studies of metamorphic evolution of the eto

the impact of analogue material properties on the geometry kinematics and dynamics of convergent sand wedges abstract simulation of geodynamic processes in sandbox experiments requires analogue materials with a deformation behaviour that reproduces the deformation mechanisms of typical crustal rocks we present data on the frictional strength of different sand types employing static and dynamic shear tests the sand types analysed are characterised by an elastic frictional plastic mechanical behaviour with a transient strain hardening and strain softening phase prior to transition to stable sliding this is in conflict with the standard assumption of an ideal cohesionless coulomb material with constant frictional properties the influence of the identified transient material properties on the kinematics growth mechanisms and internal deformation patterns of convergent sand wedges results in characteristic wedge segments which vary depending on material compaction between wedges with well defined segments i e frontal deformation zone frontal imbrication zone and internal accumulation zone with straight slopes and wedges with a continuous convex topographic profile for most materials only the frontal part of the wedge is critical during experimental runs taper and strength of the wedge segments can be shown to be controlled by the frictional properties of active faults wedge segmentation is controlled by a bulk strength increase toward the rear of the wedge due to fault rotation in mechanically less favourable orientations and plastic material hardening the limit between the frontal critical parts of a wedge and internal stable parts is largely controlled by a critical state of stress upon which either renewed failure or fault inactivation occurs on this basis we suggest that critical taper analysis of wedges must be restricted to specific kinematic segments comparison of the experimental results with the nankai accretionary wedge suggests that our interpretation also applies to natural convergent wedges moreover we provide constraints for the selection of adequate granular analogue materials to simulate typical crustal rocks in natural convergent wedges

a new strategy for discrete element numerical models 2 sandbox applications 1 here we present a series of numerical experiments using a new formulation of the discrete element method dem that improves performance in modeling faults and shear zones in the new method named the stress based discrete element method sdem which is introduced in the companion paper by egholm stress tensors are stored at each circular particle further sdem includes rotational resistivity of particles and elastoplastic constitutive rules for governing particle deformation when combining these new features the sdem is capable of reproducing the friction properties of rocks and soils without the need for the ad hoc calibration routines normally associated with dem in contrast to the conventional dem the friction properties of a sdem particle system are in agreement with the mohr coulomb constitutive model with friction angles specified on a particle level benchmark sandbox models show that unlike most commonly used numerical methods sdem faults and shear zones develop at angles in agreement with general observations from structural geology and analogue modeling studies citation egholm d l m sandiford o r clausen and s b nielsen 2007 a new strategy for discrete element numerical models 2 sandbox applications j geophys res 112 b05204 doi 10 1029 2006jb004558

control of fluid pressures on the formation of listric normal faults abstract listric normal faults are widespread in the extension of the upper crust despite major advances in understanding the formation of listric faults through various experiments the mechanical conditions that allow their formation are highly debated in particular anderson s faulting theory predicts that newly formed normal faults are planar and are dipping at least at 45 and in practice at 60 65 for most rock types here we develop limit analysis to investigate the formation of a listric fault at the onset of slip linking a deep detachment to the topographic surface we find that listric normal faults can occur in the brittle upper crust without appealing to viscous or ductile behaviours nor to flexural stresses the disequilibrium compaction fluid pressures typically observed in many sedimentary basins are essential for the formation of listric faults the fluid pressure is hydrostatic down to the fluid retention depth z f r d and sustains a higher gradient below this depth parametric studies show that the surface slope is also essential for the formation of listric faults even with a gently dipping surface slope 4 whereas a flat topographic surface leads to a simple andersonian geometry at the onset of slip the method is applied to two field examples in order to determine fluid overpressures that best match the fault shapes interpreted from seismic data for the offshore niger delta the simulated normal faults match the observed listric faults with a very shallow z f r d 0 5 0 75 km and below z f r d the fluid pressure has a lithostatic gradient consistent with the observed fluid pressure profiles to reproduce a series of listric faults joining on one common low angle detachment in the nw gulf of mexico we demonstrate that a shallow z f r d 0 7 1 1 km is required below which the fluid pressure increases to the lithostatic pressure on the detachment in agreement with the fluid pressure observations

structural control of coalbed methane production in alabama abstract thin skinned structures are distributed throughout the alabama coalbed methane fields and these structures affect the production of gas and water from coal bearing strata extensional structures in deerlick creek and cedar cove fields include normal faults and hanging wall rollovers and area balancing indicates that these structures are detached in the pottsville formation compressional folds in gurnee and oak grove fields by comparison are interpreted to be detachment folds formed above decollements at different stratigraphic levels patterns of gas and water production reflect the structural style of each field and further indicate that folding and faulting have affected the distribution of permeability and the overall success of coalbed methane operations area balancing can be an effective way to characterize coalbed methane reservoirs in structurally complex regions because it constrains structural geometry and can be used to determine the distribution of layer parallel strain comparison of calculated requisite strain and borehole expansion data from calliper logs suggests that strain in coalbed methane reservoirs is predictable and can be expressed as fracturing and small scale faulting however refined methodology is needed to analyze heterogeneous strain distributions in discrete bed segments understanding temporal variation of production patterns in areas where gas and water production are influenced by map scale structure will further facilitate effective management of coalbed methane fields

the internal structure of fault zones in basaltic sequences abstract normal faults in basalt along mid ocean ridges and continental rifts play a major role in the formation of topography advection of fluids and the dynamics of biosphere however almost nothing is known of the internal structure of volcanic growth faults at depth where many key processes take place in this paper we present the results of scaled model experiments validated against outcrop studies of a volcanic growth fault system on hawai i the analogue model uses cohesive hemihydrate powder with carefully characterized physical properties a cohesion of 62 pa friction angle of 0 71 and tensile strength of 33 pa imply a length scaling ratio of 1 5000 50 000 between the model and the upper part of extending oceanic crust with a basalt column above a normal fault time lapse imagery high resolution particle image velocimetry and field observations show ground flexure in an initial elastic stage before subvertical mode i fissures propagate from the surface downwards some of the mode i fractures become inactive while others develop into massively dilatant normal faults the dominant processes within the faults are brecciation block rotation and gravitational transport of fragments along the fault maintaining meter sized cavities to a depth of several hundred meters even when these volcanic growth faults are resurfaced a pervasive dilatancy persist within the system if circulating seawater provides a sufficient oxygen supply the large cavities in normal faults at mid ocean ridges could harbour large ocean floor life forms to several hundred meters depth

normal faults and their hanging wall deformation an experimental study we have used clay models to study the effects of fault shape and displacement distribution on deformation patterns in the hanging wall of a master normal fault the experimental results show that fault shape influences the style of secondary faulting and folding mostly antithetic normal faults form above concave upward fault bends whereas mostly synthetic normal faults form above low angle fault segments and convex upward fault bends beds dip toward the master normal fault above concave upward fault bends and away from the master normal fault above low angle fault segments and convex upward fault bends generally secondary faulting and folding are youngest at fault bends and become progressively older past fault bends hanging wall deformation patterns differ significantly when a basal plastic sheet imposes a constant magnitude displacement distribution on the master normal fault in models without a plastic sheet numerous secondary normal faults form in the hanging wall of the master normal fault most secondary normal faults propagate upward and consequently have greater displacement at depth in models with a plastic sheet few visible secondary normal faults develop most of these faults propagate downward and consequently have less displacement at depth hanging wall folding is wider and bedding dips are gentler in models without a plastic sheet than in identical models with a plastic sheet the observed particle paths displacement distributions bedding dips and orientations of the principal strain axes in our physical models with and without a basal plastic sheet are compatible with the assumption that homogeneous inclined simple shear accommodates the hanging wall deformation not all of our modeling observations however are consistent with this assumption specifically the observed variability with depth of the distribution and intensity of deformation is incompatible with homogeneous inclined simple shear as the hanging wall deformation mechanism

prediction of thrusting sequences in accretionary wedges 1 the main objective is to determine the three stages of the life of a thrust in an accretionary wedge which are the onset of thrusting along its ramp the development with the construction of the relief and the arrest because of the onset of another thrusting event a simple kinematics is proposed for the geometry of the developing thrust fold based on rigid regions separated by velocity discontinuities along which work is dissipated according to the coulomb criterion the evolution of the thrust fold satisfies mechanical equilibrium and is optimized at every time of the three stages to provide the least upper bound in tectonic force according to the maximum strength theorem the development of the thrust or its arrest because of the initiation of another thrust is decided by selecting the event which leads to the least upper bound in tectonic force the approach is first validated by proving that the critical slope angle c for the classical triangular wedge is properly captured it is shown that a perturbation in the form of an extra relief in this perfectly triangular geometry leads to the onset of thrusting with the ramp or the back thrust outcropping either at the back or to the front of the perturbation respectively for a range of slope dip close to the critical angle c the study of normal thrust sequences from the rear to the front in the wedge toe reveals that weakening of the ramp accounted for by changing its friction angle from an initial to a smaller final value is necessary for each thrust to have a finite life span this life span is longer with a larger relief buildup for more pronounced weakening decreasing the decollement friction angle results in an increase in the number of thrusts in the sequence each thrust creating milder relief the normal sequence is ended with the first out of sequence thrust which occurs earlier for smaller weakening over the ramp the proposed methodology is partly used to construct an inverse method proposed to assess the likeliness for the transfer of activity from the active to the incipient thrust in a section of nankai s accretionary wedge the inverse method provides the initial friction angle over the incipient ramp and the final friction angle over the fully active ramp from the geometry of the corresponding thrusts and the topography it is shown that the friction angle over the incipient ramp is most likely to be larger then the one over the active ramp justifying a key hypothesis needed to predict discrete sequences of thrusting

predicting orogenic wedge styles as a function of analogue erosion law and material softening 1 the evolution of a compressive frictional wedge on a weak frictional and planar decollement subjected to frontal accretion is predicted with a two step method called sequential limit analysis the first step consists in finding with the kinematic approach of limit analysis the length of the active decollement and the dips of the emerging ramp and of the conjugate shear plane composing the emerging thrust fold the second step leads to a modification of the geometry first because of the thrust fold development due to compression and second because of erosion erosion consists in removing periodically any material above a fictitious line at a selected slope as done in analogue experiments this application of sequential limit analysis generalizes the critical coulomb wedge theory since it follows the internal deformation development with constant frictional properties the deformation is mostly diffuse a succession of thrust folds being activated so that the topographic slope reaches exactly the theoretical critical value frictional weakening on the ramps results in a deformation style composed of thrust sheets and horses applying an erosion slope at the critical topographic value leads to exhumation in the frontal central or rear region of the wedge depending on the erosion period and the weakening erosion at slopes slightly above or below the critical value results in exhumation toward the foreland or the hinterland respectively regardless of the erosion period exhumation is associated with duplexes imbricate fans antiformal stacks and major backthrusting comparisons with sandbox experiments confirm that the thickness dips vergence and exhumation of thrust sheets can be reproduced with friction and erosion parameters within realistic ranges of values

petrology and geochemistry of metabasalts from the 1 95 ga jormua ophiolite northeastern finland the jormua ophiolite exposes a unique fragment of red seatype oceanic crust formed in a setting related to continental break up 1950 ma ago two distinct types of basalt are represented the early dykes and the main basalt suite early dykes have fractionated h ree patterns oib like trace element patterns low 2j nb 6 and ejwfyss ga 0 6 indicative of their derivation from an enriched source the remaining dykes and all lavas belong to the second e morb like main basalt suite which is characterized by high mg number and cr contents flat ree patterns zr b 6 17 chondritic th ta and only moderately depleted isotopic signatures zjufi gs a 4 1 9j most main suite samples cannot be related solely by fractional crystallization to a common parental magma rather they represent distinct melt fractions that underwent variable amounts of chromite olivine plagioclase fractionation during ascent a significant part of the compositional diversity of the main basalt suite can be explained by mixing a depleted source with a relatively uniform proportion of an enriched component similar to that represented by the oib like early dykes it is probable that during the latest stages of continental rifting the oib type melts metasomatized the upper part of the depleted asthenospheric mantle which became the source of the main basalt suite soon after the old continental lithosphere was ruptured the complete absence of any evidence for a subduction related component in the basalts implies that jormua is not a back arc ophiolite

geochronological and geochemical constraints on the origin of the yunzhug ophiolite in the shiquanhe yunzhug namu tso ophiolite belt lhasa terrane tibetan plateau abstract the formation of the shiquanhe yunzhug namu tso ophiolite melange zone snmz within the lhasa terrane tibetan plateau is key to understanding the mesozoic tectonic evolution of this terrane which remains controversial we show that the yunzhug ophiolite in the central segment of the snmz formed at 150 ma based on u pb dating of zircons from a gabbroic sample in a well developed sheeted dike complex geochemically these mafic rocks are dominated by e morb type compositions along with minor amounts of rocks with p morb type compositions the samples also exhibit high end t values and lack negative nb and ta anomalies data for all the samples plot within the morb array on a th yb nb yb diagram therefore these mafic rocks most likely formed in either a slow spreading oceanic setting or an embryonic ocean and not in a back arc basin as has been previously assumed taking into account the regional geology we propose that the yunzhug ophiolite is part of a distinct ophiolitic belt and represents material formed in an embryonic ocean within the lhasa terrane which provides new insights into the jurassic tectonic evolution of the lhasa terrane

a microseismic study in the western part of the gulf of corinth greece implications for large scale normal faulting mechanisms summary we present the results of a dense seismological experiment in the western part of the gulf of corinth psathopyrgos aigion area one of the most active rifts in the aegean region for which we have precise tectonic information the network included 51 digital stations that operated during july and august 1991 covering a surface of 40 x 40 km2 among the 5000 recorded events with ml ranging between 1 0 and 3 0 we precisely located 774 events we obtained 148 well constrained focal mechanisms using p wave first motions of these 60 also have mechanisms obtained by combining the p wave first motions with the s wave polarization directions the observed seismicity is mainly located between 6 and 11 km depth most of the fault plane solutions correspond to e w striking normal faulting in agreement with the geological evidence most of the well determined mechanisms indicate a nodal plane dipping 10 25 due north and a steep south dipping plane a similar asymmetry is also seen in the seismicity distribution and in the overall geological structure of the corinth rift we discuss this evidence and the inference of a deep detachment zone a structure where the major faults seen at the surface appear to root a large part of the microseismic activity appears to cluster in regions near the junctions of the main faults with the proposed detachment zone this feature of the microseismicity is interpreted in terms of stress transfer and stress concentration in regions of probable nucleation of future large earthquakes

nd and pb isotopic constraints on the origin of the purtuniq ophiolite and early proterozoic cape smith belt northern quebec canada abstract we report pb isotopic compositions for 35 samples and sm nd isotopic data for 44 volcanic rocks from the purtuniq ophiolite the continental povungnituk and oceanic chukotat groups in the cape smith belt pb pb whole rock dating of the sheeted dike complex and cumulate rocks of the ophiolite which are preserved in separate thrust sheets yield identical isochron ages of 1 97 0 04 and 1 97 0 11 ga respectively supporting a contemporaneous origin combined data for layered cumulates sheets dikes and pillow basalts constrain the age of the ophiolite to 1 98 0 03 ga 1 8 0 in good agreement with a previously reported 1 998 ga u pb zircon age in contrast pb isotopic data of the 1 96 1 92 ga continental povungnituk and related transitional continental oceanic chukotat group basalts yield young dates of 1 89 and 1 72 ga respectively which may record tectonothermal events related to continental collision sm nd isotopic data for the ophiolite indicate highly heterogeneous sources nd t 1 0to 6 1 and form a mixing array corresponding to an anomalously old age of 2 2 ga a similar sm nd isotopic array with an age of 2 3 ga is displayed by the data for lree enriched continental povungnituk volcanic rocks and the associated more depleted oceanic basalts of the chukotat group magma contamination by late archean basement can be ruled out and we attribute the excessively old sm nd ages to melting and mixing of two distinct mantle sources the dominant magma source with nd t values of 3 to 5 is similar to the depleted early proterozoic mantle a subordinate mantle component displays nd t values as low as 1 and enrichment in lree relative to the depleted early proterozoic mantle it may have resulted from a relatively young enrichment event that produced a source with high nd sm and moderately radiogenic nd at 2 ga at the other extreme it may reflect a much older event with little source enrichment in lree in this case the strong lree enrichment in some volcanics would be due to lree fractionation during melting and it can be assumed that the isotopically evolved component had been stored outside the depleted mantle for a long time in order to avoid its destruction by convection the isotopic evidence is compatible with a magma origin in a plum pudding type mantle or a heterogeneous highly depleted mantle plume the isotopic heterogeneity in the ophiolite suggests an origin at a slowly spreading ridge in an immature ocean basin

a sedimentation device to produce uniform sand packs abstract in physical modelling using sand the construction of a sand pack is one of the main experimental difficulties existing solutions are generally based either on delicate manipulations pouring and scraping or on motorized sedimentation devices and the qualities of packs thus produced are known to affect experimental outcomes however uniformity planarity reproducibility and efficiency can be achieved with a simple sedimentation device without motors a rectangular reservoir pierced with holes rests on a support plate pierced with wider holes sand flows when the reservoir is displaced so that its holes match those of the support plate sand jets are diffused by planar horizontal sieves below which the sand sets into the experimental box tests on a 250 m median grain size sand show that the density is at its maximum value reproducible and uniform the spatial variations are only of 0 4 of the mean density thickness of the sand layers shows spatial variations around 2 of the mean thickness and 6 near the side walls very fine grains 90 m median grain size produce less uniform and less planar packs because of their greater sensitivity to air currents caused by the sedimentation according to direct shear tests the sand pack has a well defined static friction coefficient decreasing to a lower dynamic value after about 3 mm of slip with dilatancy in contrast poured sand packs which are initially less dense develop only the dynamic friction coefficient no peak shear stress during slip without dilatancy hole diameters hole spacing and the number and openings of the sieves are the parameters controlling the qualities of the sand pack for a given grain size distribution

the large scale inflow layer structure of hurricane frederic 1979 abstract aircraft rawinsonde satellite ship and buoy data collected over a 40 h period were composited to analyze the inflow layer structure of hurricane frederic 1979 within a radius of 10 latitude of the storm center to improve the quality of the composite analyses the low level cloud motion winds cmws employed in this study were assigned a level of best fit lbf an lbf was assigned to each cmw by determining the level at which the closest agreement existed between cmw and ground truth wind data e g rawinsonde aircraft ship and buoy the cmws were then adjusted vertically to uniform analysis levels combined with ground truth wind data and objectively analyzed these objectively analyzed wind fields were used to obtain kinematically derived fields of vorticity divergence and vertical velocity an angular momentum budget was also computed to obtain estimates of surface drag coefficients the low level cmws in this study were found to have lbfs ranging from 300 to 4000 m it was sho

spatial arrangement and size distribution of normal faults buckskin detachment upper plate western arizona abstract fault arrays typically include a wide range of fault sizes and those faults may be randomly located clustered together or regularly or periodically located in a rock volume here we investigate size distribution and spatial arrangement of normal faults using rigorous size scaling methods and normalized correlation count ncc outcrop data from miocene sedimentary rocks in the immediate upper plate of the regional buckskin detachment low angle normal fault have differing patterns of spatial arrangement as a function of displacement offset using lower size thresholds of 1 0 1 0 01 and 0 001 m displacements range over 5 orders of magnitude and have power law frequency distributions spanning four orders of magnitude from less than 0 001 m to more than 100 m with exponents of 0 6 and 0 9 the largest faults with 1 m displacement have a shallower size distribution slope and regular spacing of about 20 m in contrast smaller faults have steep size distribution slopes and irregular spacing with ncc plateau patterns indicating imposed clustering cluster widths are 15 m for the 0 1 m threshold 14 m for 0 01 m and 1 m for 0 001 m displacement threshold faults results demonstrate normalized correlation count effectively characterizes the spatial arrangement patterns of these faults our example from a high strain fault pattern above a detachment is compatible with size and spatial organization that was influenced primarily by boundary conditions such as fault shape mechanical unit thickness and internal stratigraphy on a range of scales rather than purely by interaction among faults during their propagation

lip reading recognizing oceanic plateaux in the geological record basaltic oceanic plateaux are important features in the geological record not only do they record ancient mantle plume activity but they also are believed to be important building blocks in the formation of the continental crust in this paper we review the salient features of two cretaceous oceanic plateaux the ontong java and the caribbean colombian thick sequences of predominantly homogeneous basalt the occurrence of high mgo basalt including komatiites and an apparent absence of sheeted dyke complexes in addition pyroclastic deposits may be scarce we then explore ways of distinguishing plateaux from basaltic sequences erupted in different tectonomagmatic settings continental flood basalt provinces island arcs back arc basins ocean islands and mid ocean ridges using these criteria potential archaean and proterozoic oceanic plateaux are reviewed and identified finally we explore how these remnant oceanic plateaux became incorporated into the continents by reviewing the proposed accretion mechanisms for the cretaceous caribbean colombian oceanic plateau on the basis of evidence from south america and the tonalites of the southern caribbean island of aruba

noncohesive critical coulomb wedges an exact solution active fold and thrust belts or submarine accretionary complexes can be modeled as critically tapered wedges of material on the verge of coulomb failure everywhere overlying a basal decollement where frictional sliding is occurring ignoring cohesion the four strength parameters needed to describe a critical coulomb wedge are its internal and basal coefficients of friction and b and its internal and basal hubbert rubey fluid pressure ratios and b an exact relation between surface slope and basal dip of a noncohesive critical wedge with uniform properties is derived the state of stress within such a wedge has the same orientation everywhere and is constant if is and vice versa a coefficient of internal friction 1 1 is consistent with the known surface slope basal dip and pore fluid pressures in the active fold and thrust belt of western taiwan assuming that byerlee s law b 0 85 is valid on the base the wide variety of tectonic styles observed to occur along convergent margins including subduction erosion active accretion subduction without accretion and even extension and normal faulting may be controlled by relatively small spatial or temporal variations in either b or b

origin of rollover rollover is the folding of hanging wall fault blocks by bending or collapse in response to slip on nonplanar commonly listric normal faults the shapes of rollover folds are controlled by a number of variables including 1 the shape of the fault 2 the total fault slip after a bed is deposited 3 the direction of relative particle motion in hanging wall collapse 4 the history of sedimentation rate relative to fault slip rate and 5 compaction the importance and role of each of these variables is illustrated by a two dimensional balanced structural modeling technique that treats continuously curved faults as though composed of a large number of straight fault segments in this modeling an active axial surface oriented parallel to the direction of relative p rticle motion in hanging wall collapse emanates from each fault bend and is the instantaneous locus of folding the quantitative correctness of this theory of rollover is tested by modeling natural structures from the gulf of mexico for which both fault shape and fold shape are known from high quality seismic and well sections the direction of hanging wall collapse commonly is in the antithetic or synthetic normal fault or coulomb shear orientations although sliding along weak bedding planes also is an important collapse mechanism in some regions collapse is in the antithetic shear direction for concave fault bends and in the synthetic shear direction for convex bends these collapse directions can be observed directly in some high quality seismic images as axial surfaces emanating from fault bends the shapes of rollovers within growth strata depend strongly on the sedimentation rate relative to fault slip rate as well as the total slip after a bed is deposited the crests of classic gulf coast rollovers are growth axial surfaces along which are abrupt changes in sedimentation rate within the growth st atigraphic interval these changes are produced by deformation of the sediment water interface along active axial surfaces compaction can substantially modify the relationship between fault shape and rollover shape however under certain common conditions the history of compaction can be neglected if the folding is modeled in the compacted state

structure of the outokumpu ore district and ophiolite hosted cu co zn ni ag au sulfide deposits revealed from 3d modeling and 2d high resolution seismic reflection data abstract the outokumpu district within the north karelia schist belt in eastern finland hosts a number of cu co zn ni ag au sulfide deposits that are associated with palaeoproterozoic ophiolitic metaserpentinites derived from depleted mantle peridotites that were subsequently tectonically interleaved with allochthonous metaturbidites the metaperidotites have been extensively metasomatized to quartz carbonate calc silicate rocks of the outokumpu assemblage the outokumpu area has been affected by a multiple phase tectonic history comprising various phases of folding and shearing followed by several faulting events future exploration has to expand the search into deeper areas and requires knowledge of the subsurface geology in order to unravel the complex structure 3d geologic models of different scales have been built using a variety of information including geological aeromagnetic and gravity maps digital terrain models and mine cross sections as well as data like drill core logs combined with observations from underground mine galleries structural measurements aeromagnetic data and seismic surveys for crustal structures data from seismic surveys lines have been reprocessed for our purpose both deposit scale and regional scale models allow the reconstruction of a sequence of structural events the mined ore has formed during remobilization of a proto ore and is closely related to shear zones thrusts that truncate the outokumpu assemblage later faults dismembered the ore explaining the variable depth of the different ore bodies along the outokumpu ore zone on larger scale at least four km scale thrust sheets separated by major listric shear zones can be identified in the ore belt which are internally further imbricated by subordinate shear zones these thrusts separate a number of lens shape metaperidotite bodies that are probably surrounded by outokumpu assemblage rocks thrust stacking was followed by at least three stages of faulting that divided the ore belt into fault bounded blocks with heterogeneous displacements i faulting along nw dipping faults with unresolved kinematics ii reverse faulting along c 50 60 se dipping faults and iii sw ne to ssw nne striking faults which may have formed at an earlier stage and have been reactivated the specific outokumpu alteration assemblage around metaperidotite bodies combined with shear zones acting as path ways for fluids are the main vectors to mineralization seismic reflection data do not provide a simple tool to directly detect the sites of outokumpu assemblage bodies at depth but they identify strong reflector zones which are characteristic for though not exclusive to the assemblage shear zones can be recognized as curved dislocations in the seismic lines our study shows that 3d modeling when used in combination with surface geology and other geophysical data and good knowledge about the structural evolution clearly improves the interpretation of reflectors and enables the identification of strong reflector packages as outokumpu assemblage that due to absent geological control have first been mapped as unknown reflector it thus enhances the chances for locating potentially economic horizons at depth and to delineate target areas for detailed exploration

dem simulation of normal faults in cohesive materials abstract we performed discrete element method dem simulations of the formation of normal faults in a brittle cohesive material above a basement fault focusing on the formation of open fractures and cavities in areas of low normal stress i e near the surface of the model the simulations produce a wide range of fundamental structures similar to those observed in nature and in analogue models comparisons with these sandbox experiments show that the fundamental structures generated are similar between the analogue and the numerical models in a parameter study of the material properties and the model geometry we have investigated the effect of these parameters far beyond what is possible in analogue models over a range of parameters we show that the evolving structural style is robust i e all the models develop similar structures in a similar sequence results show a systematic dependence of some structural features such as location an style of faults on the simulation parameters in particular we observe that the maximum depth of the open fractures varies linearly with the cohesion suggesting that we can predict the depth of open cavities formed by tensile failure in natural fault systems if the material properties are known

svecofennian detrital zircon ages implications for the precambrian evolution of the baltic shield metasediments intruded by 1 90 1 87 ga old plutonic rocks form the oldest major proterozoic crustal component in the svecofennian domain of the baltic fennoscandian shield their ndtdm model ages and conventional multigrain zircon u pb ages between 2 4 and 2 1 ga have previously been interpreted either as mixing ages between 1 9 ga old juvenile materials and a minor archaean component or as actual rock and protolith ages to resolve the ensuing controversy 120 individual detrital zircons from svecofennian metasediments in sweden and finland were analysed using the shrimp ion microprobe the oldest materials in this array are a 3 44 ga old zircon from the tampere schist belt in finland and a 3 32 ga old crystal from southeastern sweden about 30 of the analysed crystals are 2 97 2 60 ga old while 65 have ages between 2 12 and 1 88 ga thus there is no evidence of 2 6 2 1 ga old protoliths but the age range of the proterozoic zircons indicates that a major area of 2 1 1 9 ga old crust was in erosional position 1 9 ga ago this implies that the formation of palaeoproterozoic crust in the baltic shield or its one time close neighbourhood must have commenced 100 200 ma earlier than hitherto assumed in conjunction with previously obtained isotopic data the youngest detritus ages of the present study constrain the age of svecofennian sedimentation it can also be concluded that the archaean zircons found in quartzites from southern sweden may have been derived from source areas to the southwest of the central svecofennian marine depositional basin the so called bothnian basin separating southern sweden from the archaean craton in the northeastern part of the shield

loss of isotopic nd o and chemical ree memory during metamorphism of komatiites new evidence from eastern finland komatiites are often considered to depict the chemical and isotopic composition of their source rocks in the archean mantle however a weakness of these rocks in tracking the initial compositional heterogeneity of the earth s mantle is the ubiquitous presence of metamorphic recrystallization which casts some doubt about the preservation of primary chemical and isotopic characteristics two spinifex textured komatiite flows from the 2 75 ga old kuhmo greenstone belt siivikkovaara area of eastern finland document this weakness both flows have experienced low to medium grade metamorphism t 450 50 c and now consist entirely of secondary metamorphic assemblages of amphibole chlorite plagioclase with minor proportions of magnetite and ilmenite mgo contents range from 25 to 8 which suggests that low pressure differentiation was likely controlled by olivine and clinopyroxene fractional crystallization however neither major nor trace elements fall on olivine and or clinopyroxene control lines this is particularly well illustrated by the ree as there is an overall 60 variation of ce sm n ratios 0 38 to 0 91 which far exceeds that expected from olivine and clinopyroxene fractionation alone in fact careful evaluation of petrographic including mineral composition data and chemical characteristics shows that most elements of geological interest including the reputedly immobile ree were mobile on a whole rock scale during metamorphic recrystallization of these two flows this view is fully supported by sm nd isotopic data since both whole rock and mineral amphibole and plagioclase samples lie on a single isochron relationship at t 1800 ma an age which corresponds to the time of regional metamorphism thus the meta komatiite flows from siivikkovaara document a case of komatiite flow units in which metamorphism has induced whole rock scale resetting of primary ree patterns and sm nd isotope systematics as regards the nature of the mechanism responsible for this resetting it is suggested here that the secondary mineralogy played an important role as there are correlations between whole rock 147sm 144nd ratios major and trace element chemistry oxygen isotopic ratios and modal proportions of secondary minerals consideration of oxygen isotopic data as well as previous results from a komatiite flow from the nearby tipasjarvi belt further enable us to propose that the ree carrier was likely a co2 rich fluid the process of secondary ree redistribution prevents estimation of the true initial nd isotopic compositions of the two flows taken as a whole the results presented in this paper show that great care should be be exercised in the use of meta komatiites as probes of archean mantle composition

the evolution of the mantle s chemical structure abstract the geochemistry of flood basalts and their associated picrites and of komatiites and their associated basalts combined with a theoretical model for the structure of mantle starting plumes can be used to decipher key elements of the geochemical structure of the deep mantle and show how it has varied through time we argue that the thermal boundary layer above the core consisted mainly of depleted mantle similar to the present morb source during the archaean and this was largely replaced between 2 7 and 2 0 billion years ago by enriched mantle to form the oib source we suggest that this change in the nature of the hotspot source reflects a fundamental change in the dominant component of downward convection from cold plumes breaking away from beneath a stable lithosphere during the pre archaean to subduction of lithosphere in the archaean and post archaean mantles

an early proterozoic ophiolite the jormua mafic ultramafic complex northeastern finland abstract the 1 96 ga old jormua complex comprises a fault bounded unit 2 5 20 km in the centre of the kainuu schist belt at the eastern margin of the early proterozoic svecokarelides the complex was dismembered and metamorphosed to lower amphibolite facies during the svecokarelian orogeny in the intact complex the basal unit was formed by serpentinites the middle unit of mafic dykes and the uppermost extrusive unit of basaltic pillow lavas gabbroic rocks are present as intrusive bodies in the serpentinites and narrow interdyke screens within the dyke unit trondhjemitic rocks occur as minor segregations in the gabbroic rocks or as cross cutting narrow dykes in the mafic dyke unit the dyke unit is locally composed exclusively of subparallel metabasalt and metadolerite dykes in places interdyke metagabbro and serpentinite screens are abundant well preserved chilled margins are common and many dykes are split and intruded by younger dykes the chemical composition of the serpentinites indicates derivation from harzburgitic dunitic mantle peridotites the gabbroic rocks which represent obvious tholeiitic cumulates show extreme differentiation similar to gabbros of the high ti ophiolites and gabbros dredged from present midocean ridges in chemistry the trondhjemitic rocks are close to granitic rocks from mid ocean ridges and high ti ophiolites the similar chemical composition of the dykes and lavas of the complex indicates that they are cogenetic both being tholeiitic metabasalts with chondrite normalized ree patterns ranging from slightly lree depleted to slightly lree enriched ones compared with modern rocks the jormua metabasalts most closely resemble transitional midocean ridge basalts it is suggested that the jormua complex and related mafic ultramafic bodies in eastern finland originated as divergent margin ophiolites concomitantly with the break up of the archaean craton in the extensional stage of the svecokarelian orogeny which culminated in the opening of a major ocean after 1 97 ga the tectonic emplacement of these ophiolites into their present locations occurred in connection with thrusting during the compressional stage of the orogeny about 1 9 ga ago

half graben structures balanced models of extensional fault bend folds one common structural style in extensional regimes is a half graben bounded on one side by a master normal fault and on the other side by a domain of beds dipping toward the master fault this geometry is modeled as being caused by a bend in the master fault the hanging wall beds dipping toward the master fault are bounded by axial surfaces formed as the hanging wall moves past the bend resulting in an extensional fault bend fold the footwall beds remain undeformed and unrotated the major assumptions used in the derivation are that the geometry is area balanced and that the axial surfaces in the hanging wall have dips equal and opposite to the dip of the master fault above the bend important consequences of the model include a relationship between master fault dip above and below a bend and the amount of hanging wall dip the horizontal width of the dipping beds in the hanging wall is twice the heave on the master fault above the bend the asymmetry of the axial surfaces in the hanging wall requires that the rotated beds be strained hanging wall beds dipping toward the master fault are predicted for a fault bend wherein the dip decreases downward hanging wall beds dipping away from the master fault are predicted for a bend wherein the dip increases downward the model fits measurements taken from a published ramp and flat clay model by e cloos and is used to develop a balanced and restorable cross section of the schell creek fault a basin and range structure in nevada

mechanical behaviour of granular materials used in analogue modelling insights from grain characterisation ring shear tests and analogue experiments abstract the mechanical behaviour of several dry granular materials is investigated through ring shear tests grain characterisation and simple analogue experiments analysed by x ray computed tomography an improved knowledge of granular materials is essential to determine their suitability as analogues for upper crustal rocks in experimental models and to compare analogue and numerical experiments the ring shear tests show that the granular materials have an elastic frictional plastic behaviour with strain hardening preceding failure at peak strength followed by strain softening until a dynamic stable value is reached this is similar to the behaviour exhibited by experimentally deformed rocks the physical characteristics of the grains determine the amount of diffuse deformation before failure the percentage of strain softening and act on the thickness of the shear zones before broadening initial shear zone width in extensional and contractional experiments is between 11 and 16 times the mean grain size the angle of internal friction defining one of the mechanical properties of granular materials and thus fault dip is not only related to physical characteristics of the grains and to the handling technique used e g sieving or pouring but also to the overburden and to the experimental setup used

inverse method applied to a sand wedge estimation of friction parameters and uncertainty analysis we examine the fundamental mechanism of frontal accretion in a sand wedge from the occurrence of a forward thrust ramp evolving into a fault bend fold to the jump of deformation to a new frontal ramp ahead we use inverse problem theory to extract quantitative information on friction parameters from the systematic comparison of experimental observations and theoretical predictions the observables are locations dips and lifetime of thrust ramps hinge and associated compression the experimental values observed data are cast into statistical models describing the error bars the theory of limit analysis provides calculated data requiring five parameters material density friction coefficient of the decollement plane friction coefficient of the bulk material and the variation of friction with slip on the ramp as well as the distance for this variation the misfit between observed data and calculated data is determined for all physically admissible values of the parameters values yielding a small misfit are interpreted as highly probable the mean misfit per observable is within their error bars and therefore application of the theory reproduces the observables bulk and decollement friction coefficient values with high probability are compared to independent measurements the inversion also reveals systematic discrepancies the frictional weakening on the ramps is overestimated while the force is underestimated the calculated thrust sheet is longer than observed and the calculated jump to a second ramp occurs earlier than observed these conclusions allow us to identify necessary improvements for the experimental set up and of the theoretical assumptions

purtuniq ophiolite cape smith belt northern quebec canada a reconstructed section of early proterozoic oceanic crust the crustal part of a dismembered ophiolite is preserved in the structurally highest thrust sheets of the early proterozoic cape smith belt in northern quebec canada the rocks of the ophiolite represent two magmatic suites each with a mantle source that is nd isotopically distinct the older suite comprises pillowed and massive volcanic rocks sheeted dikes dominantly gabbroic layered cumulate rocks and rare plagiogranite sills and dikes in the volcanic sequence this 5 km thick tholeiitic suite is ca 1998 ma and is compositionally and petrographically similar to rocks formed at modern oceanic spreading ridges the younger suite 4 km comprises sheeted mafic dikes and layered ultramafic to mafic cumulate rocks that are similar to tholeiitic rocks found in modern plume generated oceanic islands such as hawaii the composite section is in excess of 9 km although the total thickness of the purtuniq ophiolite may not be typical of that for oceanic crust formed at early proterozoic spreading ridges elsewhere it does suggest that ancient ridge formed crust was at least as thick as that in modern oceans

low angle normal faults and seismicity a review although large low angle normal faults in the continental crust are widely recognized doubts persist that they either initiate or slip at shallow dips 30 because 1 global compilations of normal fault focal mechanisms show only a small fraction of events with either nodal plane dipping less than 30 and 2 andersonian fault mechanics predict that normal faults dipping less than 30 cannot slip geological reconstructions thermochronology paleomagnetic studies and seismic reflection profiles mainly published in the last 5 years reinforce the view that active low angle normal faulting in the brittle crust is widespread underscoring the paradox of the seismicity data for dip slip faults large enough to break the entire brittle layer during earthquakes m w 6 5 consideration of their surface area and efficiency in accommodating extension as a function of dip suggests average recurrence intervals of earthquakes r tan assuming stress drop rigidity modulus and thickness of the seismogenic layer do not vary systematically with dip if the global distribution of fault dip normalized to total fault length is uniform the global recurrence of earthquakes as a function of dip is shown to be r tan sin this relationship predicts that the frequency of earthquakes with nodal planes dipping between 30 and 60 will exceed those with planes shallower than 30 by a factor of 10 in good agreement with continental seismicity assuming major normal faults dipping more than 60 are relatively uncommon revision of andersonian fault mechanics to include rotation of the stress axes with depth perhaps as a result of deep crustal shear against the brittle layer would explain both the common occurrence of low angle faults and the lack of large faults dipping more than 60 if correct this resolution of the paradox may indicate significant seismic hazard from large low angle normal faults

low angle detachment origin for the red sea rift system abstract the tectonic and magmatic history of the jizan coastal plain tihama asir southwest arabia suggests a two stage evolution a first stage of extension began during the oligocene and ended with uplift of the arabian graben shoulder which began about 14 ma ago it was followed by a period of approximately 10 ma characterized by magmatic and tectonic quiescence a second stage of extension began roughly contemporaneously with the onset of seafloor spreading in the southern red sea some 4 5 ma ago and is still active today the geometry of faulting in the jizan area supports a wernicke model of simple shear for the development of the southern red sea regional asymmetries of the red sea area such as the distribution of volcanism the marginal topography and asymmetries in the geophysical signatures are consistent with such a model available seismic profiles allow a rough estimate for values of the arabian red sea margin and were used to simulate subsidence history and heat flow of the red sea for classical two layer stretching models neither finite uniform nor finite non uniform stretching models can account for observed subsidence and heat flow data thus two model scenarios of whole lithosphere normal simple shear are presented for the geological history of the southwestern arabian margin of the red sea these models are limited because of the serravallian rearrangement in the kinematics of the red sea

analogue and numerical modeling of normal fault patterns produced due to slip along a detachment zone abstract in this paper we investigate normal fault patterns produced by the sliding motion along a gently dipping normal fault by using analogue model tests and numerical modeling the motivation for this study was offered by microseismic test data that indicate the existence of an active low angle shear zone at a depth of 9 11 km in the extensional region of high seismic activity of the gulf of corinth greece both modeling techniques seem to support the hypothesis that the system of high angle normal faults that are responsible for the final asymmetrical graben formation initiate at the tip of the active basal detachment nearest to the free surface the normal faults propagate upwards with progressive sliding of the inclined basal plane resulting in a first phase of symmetrical graben configuration that is delimited by a main synthetic fault and an antithetic fault forming a rankine zone subsequent sliding on the inclined base induces a family of secondary antithetic normal faults which are responsible for the asymmetry of the failure pattern and the diffusive character of deformation in that area shear deformation is more intense and localized along the synthetic normal fault than along the antithetic faults elaboration on the analogue test results has led to the phenomenological relations among four main parameters that describe the geometry of grabens namely i the width and ii the maximum subsidence of the graben iii the dip angles of the conjugate normal faults and iv the amount of sliding along the low angle normal fault however analogue models do not produce the system of synthetic faults that is observed in the gulf of corinth the effects of both friction angle variation along the detachment base and of the constitutive behavior of the model material on the configuration of the final structural pattern were also studied with a series of numerical continuum models it was found that a the fault pattern of the gulf of corinth may be reproduced with either a strain softening material with low elastic modulus or a constant strength material and b two consecutive grabens such as those of gulfs of corinth and evia may also be reproduced by an appropriate combination of variation of dip and frictional properties along the hypothesized detachment zone

sm nd and pb isotopic study of mafic rocks associated with early proterozoic continental rifting the peropohja schist belt in northern finland the perapohja schist belt in northern finland rests unconformably on archaean granitoids and marks the early stages of proterozoic crustal evolution in the fennoscandian baltic shield 2440 ma old layered mafic intrusions predate the supracrustal and ca 2200 ma old sills of the gabbro wehrlite association intrude the lowest quartzites and volcanics runkaus of the sequence the sm nd mineral isochron of the penikat layered intrusion gives an age of 2410 64 ma the initial nd values of the penikat intrusion nd 2440 1 6 and the runkausvaara sill nd 2200 0 suggest that these mafic magmas were contaminated by older crustal material the sm nd and pb isotopic results on the 2 44 2 2 ga old runkaus volcanics indicate mobility of pb fractionation of sm nd during late greenschist facies metamorphism and crustal contamination the pb pb data provide an age of 1972 80 ma with a high initial 207pb 204pb ratio 1 8 49 while scattered sm nd data result in an imprecise age of 2330 180 ma with an initial nd value of about zero secondary titanite gives an u pb age of ca 2250 ma the jouttiaapa basalts in contrast ascended from the mantle without interaction with older crust these lree depleted tholeiites mark a break in continental sedimentation and yield a sm nd age of 2090 70 ma their initial nd 4 2 0 5 implies that the subcontinental early proterozoic mantle had been depleted in lree for a long period of time the first lava flows are strongly depleted in lree suggesting that their source was significantly more depleted than the source of mid ocean ridge basalts today

petrology and texture of the ultramafic rocks of the xigaze ophiolite tibet constraints for mantle structure beneath slow spreading ridges abstract the xigaze ophiolite southern tibet exhibits an unusual ophiolite sequence characterized by 1 1 the limited thickness less than 3 km of its crustal unit which is mostly composed of dolerite intrusives the gabbros being very scarce 2 2 the scarcity of harzburgites and dunites the ultramafic rocks consisting mostly of cr diopside bearing harzburgites and iherzolites although there are some textural and chemical variations from the basal iherzolites to the upper harzburgite and dunites no sharp discontinuity exists between the peridotite series of the xigaze ophiolite which all belong to the same peridotite suite the peridotites have been equilibrated at relatively low temperatures and pressures 1155 c 0 5 0 6 gpa for the cr diopside bearing harzburgites and 1255 c 1 0 1 1 gpa for the stratigraphically 2 km deeper iherzolites indicative of a low thermal gradient at the xigaze paleo ridge accordingly the peridotites have locally been deformed under low stress 0 2 gpa conditions and at decreasing temperatures from up to 1000 c to 900 800 c as evidenced by the activation of the 010 100 ok1 100 and 001 100 olivine glide systems the equilibrium temperatures and pressures estimated for the peridotites the inferred shape of the melting zone beneath the xigaze paleo ridge the low temperature low stress conditions inferred for the deformation of the peridotites and the pattern of the mantle structure determined on the basis of structural and microstructural data are in good agreement with a slow spreading ridge origin for the xigaze ophiolite

the ophiolitic ingalls complex north central cascade mountains washington the ingalls complex is the largest and most complete of several middle to late jurassic ophiolites in northwestern washington it structurally overlies the skagit metamorphic core of the north cascades and is intruded by the late cretaceous mount stuart batholith the last truncates major structures of the ophiolite and divides it into two outcrop areas a main area south of the batholith and a smaller area in a roof pendant in the main area the internal structure of the ophiolite is dominated by two wide fault zones characterized by steeply dipping serpentinite melange displacement probably was a combination of strike slip and dip slip with the latter apparently related to diapiric rise of serpentinite deformation probably occurred in an oceanic transform fault zone ultramafic tectonites predominate in the main outcrop area including from south to north 1 harzburgites and dunites 2 mylonitic iherzolites and hornblende peridotites and 3 iherzolites unit 2 represents a high temperature shear zone that may record early movement in the deeper levels of the postulated oceanic fracture zone mafites generally occur as tectonic blocks in the southern fault zone they underwent ocean floor type metamorphism but to the north metagabbros experienced dynamothermal amphibolite facies metamorphism prior to incorporation into the serpentinite melange gabbros in the southern fault zone locally intrude ultramafites and in places pass upward into a diabase dike complex gabbros and diabases intrude partly pillowed basaltic flows and pillow breccias with interbeds of argillite chert and ophiolite derived sedimentary breccia another sequence of basalt argillite chert and minor limestone is penetratively deformed and may be exotic in the pendant on the mount stuart batholith the north directed windy pass thrust wpt places the ingalls on the pelitic chiwaukum schist of the skagit metamorphic core the ingalls is imbricated with amphibolite facies metasedimentary and metaplutonic slices that may be a part of the skagit core not exposed in the lower plate imbrication was synchronous with amphibolite facies metamorphism of the ophiolite in the pendant but the postmetamorphic sole thrust wpt truncates both the imbricate slices and fabric elements in the chiwaukum schist thrusting was mid cretaceous apparently postdating high angle faulting within the main portion of the ingalls the wpt marks a major cretaceous boundary between an oceanic and a metamorphic terrane skagit core and its upper plate represents the only rocks preserved above the skagit core the ingalls is similar in age to ophiolites in california oregon and british columbia as well as those in northwestern washington the california and oregon ophiolites coast range smartville josephine apparently originated in small short lived marginal basins whereas the ingalls was deformed in a fracture zone in a wider longer lived marginal basin or large ocean

geochemistry of 1 9 ga morb and oib like basalts from the amisk collage flin flon belt canada evidence for an intra oceanic origin subaqueously erupted basalts that occur in kilometre scale allochthons within the 1 9 ga flin flon belt canada appear to have been generated at oceanic ridges and possibly oceanic plateaus remote from archean cratons the ocean floor basalts fall into two categories 1 n type resembling n morbs and mariana type back arc basin basalts depleted to flat ree patterns high zrnb variable thnb and initial nd 3 3 to 5 4 2 e type resembling transitional and plume morbs slightly enriched ree patterns lower zrnb initial nd 3 1 to 4 5 in the largest and best studied allochthon the elbow athapapuskow assemblage mixing between depleted n morb and enriched oib sources or melts coupled with variable addition of a subduction lile component can explain the chemical variations in the basalts zircon u pb dates of 1904 ma for a syn volcanic diabase sill and 1901 6 5 ma for a gabbro peridotite cumulate complex demonstrate that crystallization of the ocean floor basalts overlapped with in part eruption of the tectonically juxtaposed 1 90 1 88 ga arc volcanic rocks the elbow athapapuskow allochthon is interpreted as back arc basin crust that developed simultaneously with flin flon arc magmatism subaerially erupted basalts that chemically resemble tholeiitic oibs 8 14 mgo relative hree depletion initial nd 2 2 to 3 4 occur in tectonic contact with the elbow athapapuskow assemblage the oibs may have been generated by deeper garnet residue melting of enriched mantle tapped during extension in the elbow athapapuskow back arc basin and were possibly erupted onto a remnant arc deeper mantle melting is also indicated by the presence of the lree enriched oceanic plateau like basalts of the sandy bay assemblage the back arc 01b and plateau volcanic assemblages were jux taposed against ca 1 9 ga arc assemblages in a philippines like intraoceanic accretionary complex by 1 87 ga

sources of svecofennian granitoids in the light of ion probe upb measurements on their zircons abstract the presence of 1 91 1 93 ga old granitoids at the archean proterozoic boundary along the raahe ladoga zone in finland has been demonstrated on various occasions these rocks have been considered to represent juvenile crustal material as their e nd values are markedly positive however as svecofennian metasediments contain detrital zircons derived from a ca 2 ga old source the possibility has existed that the 1 92 ga age may have been a mixture between 2 and 1 89 ga old zircon populations as such mixing would not markedly affect their neodymium isotopic properties also some syntectonic 1 89 ga old svecofennian granitoids contain heterogeneous zircon populations but it has been impossible to determine the age and origin of the older zircons by conventional methods nordsim ion probe results on three samples from the 1 92 ga age group confirm the earlier conclusions especially important is that no zircons older than 1 95 ga were detected in the 1 92 ga group samples thus the 1 92 ga event was the beginning of the formation of new continental crust in the primitive svecofennian island arc and these granitoids formed by partial melting of basaltic magmas derived from a depleted mantle source one sample also contains a younger zircon population formed during the orogenic culmination at 1 89 ga in contrast one grain from a sample representing the 1 89 ga age group contains an archean core which is considered to represent sedimentary detritus assimilated during either magma formation or intrusion while the results prove the true igneous nature of the 1 92 ga event they also rule out these rocks as a possible provenance for the ca 2 ga old zircons encountered in the svecofennian metaturbidites thus there is still no direct evidence from granitoid rocks for an extensive svecofennian protocrust the existence of which has been postulated on the basis of geochemical and sm nd isotopic data

evidence for the emplacement of ca 3 0 ga mantle derived mafic ultramafic bodies in the ukrainian shield abstract the archean bedrock of the ukrainian shield is divided into western and central blocks by a n w trending tectonic zone known as the golovanevsk suture zone within this zone there are numerous mafic ultramafic bodies including the chromitite bearing lipovenki and kapitanov massifs the chromitites of these massifs possess mineralogical and textural features and platinum group element systematics similar to those typically observed in ophiolitic podiform chromitites although there is no field evidence for an ophiolitic structure the geochemical evidence strongly suggests an origin in the oceanic lithosphere the re os ratios of the chromites and whole rock chromitites are extremely low and consequently their 187 os 188 os ratios have increased little since crystallization osmium isotope model ages indicate formation at approximately 3 0 ga suggesting that the massifs represent portions of the archean oceanic lithosphere

comparison of observed and predicted gravity profiles over aphrodite terra venus the authors compare observed pioneer venus orbiter pvo gravity profiles over aphrodite terra to profiles predicted from models of thermal isostasy mantle convection and airy compensation similar approaches are used in order to investigate how well the models can be distinguished with the pvo data topography profiles across aphrodite are compared to model spreading ridge profiles in order to further assess this model airy compensation depths and convection layer thicknesses are greater under eastern aphrodite than western aphrodite compensation depths in the east are greater than most estimates of lithospheric thickness suggesting that this part of the ridge is dynamically supported in parts of western aphrodite the spreading ridge model gravity provides a better fit to the data than either airy compensation or mantle convection best fit spreading rates are between 0 3 and 1 6 cm yr airy compensation and mantle convection cannot be distinguished in most places using only pvo data

characterization of sulfide deposits by airborne magnetic and gamma ray responses in eastern finland abstract integrated interpretation of aerogeophysical data permits geophysical characterization of sulfide deposits in eastern finland the structural control and the hydrothermal alteration of the rocks can be discerned from airborne magnetic and radiometric data the cu co zn deposits associated with serpentinite dolomite calc silicate rock siliceous rock black shale settings of the outokumpu type are locally characterized by reduced intensity of magnetization and electrical conductivity and reduced eth and k radiation enhanced eu eth anomalies are commonly spatially related to major structural lineaments such as the boundaries of structurally more coherent bedrock blocks or intersections of faults and shear zones electromagnetic surveys can be used to distinguish black shales and serpentinites in the region the sulfide deposits in eastern finland can be divided into two groups on the basis of airborne geophysical characteristics 1 intense magnetization and high electrical conductivity and 2 moderate magnetization and electrical conductivity in the sulfide deposits of group 1 ore forming constituents mainly precipitated during primary sedimentation for example the black shale hosted ni cu zn ore at talvivaara group 2 is related to sulfide deposits where major ore forming processes have affected the associated black shale formation after the sedimentation phase outokumpu type cu co zn deposits in cases where the magnetization and electrical conductivity of black shales are especially weak ore formation processes probably continued during late metamorphic or post metamorphic deformation the theoretical basis for the observed differences in magnetic response between different deposits has to do with the mineralogy of the black shales pyrite changes into ferrimagnetic monoclinic pyrrhotite during metamorphism above greenschist facies especially along tectonic structures if no further alteration occurs pyrrhotite bearing black shale formations appear as highly magnetic and electrically conductive zones on geophysical maps wherever intense tectonic deformation emplacement of quartz sulfide veins hydrothermal alteration or contact metamorphism has affected the black shale formation after sedimentation both magnetic and electromagnetic responses may be weakened because of diminished amounts of the pyrrhotite bearing and graphite rich material in addition the presence of antiferromagnetic pyrrhotite of hexagonal crystal structure may decrease the magnetic response the classification presented here applies specifically to eastern finland but may be widely applicable to terranes with abundant black shale formations that have undergone regional metamorphism of upper greenschist facies or higher association of sulfide ores with black shales is a common feature in the geological record our integrated geophysical method offers a quick way of examining large areas and it should prove useful in the search for sulfide deposits in other parts of the world

active normal faulting beneath a salt layer an experimental study of deformation patterns in the cover sequence scaled experimental models show that the presence of a viscous layer such as salt facilitates the development of extensional forced folds above active normal faults the geometries of the extensional forced folds and their associated secondary fault patterns depend on the thickness and viscosity of the viscous layer the thickness of the cover sequence the strength and ductility of the cover sequence and the magnitude and rate of displacement on the underlying master normal fault increasing the thickness of the viscous layer and the cohesive strength and ductility of the overburden enhances the decoupling between the deep and shallow deformation alternatively increasing the viscosity of the viscous layer the thickness of the overburden and the magnitude and rate of displacement on the master normal fault reduces the decoupling between the deep and shallow deformation enhanced decoupling facilitates the formation of broad extensional forced folds and the development of detached secondary faults both near and far from the master normal faults the model predicted deformation patterns closely resemble those observed in the gulf of suez the haltenbanken area of offshore norway and the jeanne d arc basin of the grand banks offshore southeastern canada

deformation pattern during normal faulting a sequential limit analysis we model in 2 d the formation and development of half graben faults above a low angle normal detachment fault the model based on a sequential limit analysis accounting for mechanical equilibrium and energy dissipation simulates the incremental deformation of a frictional cohesive and fluid saturated rock wedge above the detachment two modes of deformation gravitational collapse and tectonic collapse are revealed which compare well with the results of the critical coulomb wedge theory we additionally show that the fault and the axial surface of the half graben rotate as topographic subsidence increases this progressive rotation makes some of the footwall material being sheared and entering into the hanging wall creating a specific region called foot to hanging wall fhw the model allows introducing additional effects such as weakening of the faults once they have slipped and sedimentation in their hanging wall these processes are shown to control the size of the fhw region and the number of fault bounded blocks it eventually contains fault weakening tends to make fault rotation more discontinuous and this results in the fhw zone containing multiple blocks of intact material separated by faults by compensating the topographic subsidence of the half graben sedimentation tends to slow the fault rotation and this results in the reduction of the size of the fhw zone and of its number of fault bounded blocks we apply the new approach to reproduce the faults observed along a seismic line in the southern jeanne d arc basin grand banks offshore newfoundland there a single block exists in the hanging wall of the principal fault the model explains well this situation provided that a slow sedimentation rate in the lower jurassic is proposed followed by an increasing rate over time as the main detachment fault was growing